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Scientists have discovered bacteria that are able to biodegrade plastics from polymers back into ... [+]
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In March 2016, scientists in Japan published an extraordinary finding. After scooping up some sludge from outside a bottle recycling facility in Osaka, they discovered bacteria which had developed the ability to decompose, or “eat,” plastic.
The bacteria, Ideonella sakaiensis, [ https://en.wikipedia.org/wiki/Ideonella_sakaiensis ] was only able to eat a particular kind of plastic called PET, from which bottles are commonly made,
and it could not do so nearly fast enough to mitigate the tens of millions of tons of plastic waste that enter the environment every year. [ https://en.wikipedia.org/wiki/Ideonella_sakaiensis#Genetic_engineering ] 7, 8, 9
Still, this and a series of other breakthroughs in recent years mean it could one day be possible to build industrial-scale facilities where enzymes chomp on piles of landfill-bound plastic, or even to spray them on the mountains of plastic that accumulate in the ocean or in rivers.
These advances are timely. By vastly increasing our use of single-use plastics such as masks and takeaway boxes, the Covid-19 pandemic has focused attention on the world’s plastic waste crisis. Earth is on track to have as much plastic in the ocean as fish by weight by 2050, according to one estimate.
However, experts caution that large-scale commercial use of plastic-eating microorganisms is still years away, while their potential release in the environment, even if practical, could create more issues than it solves.
The scientists working to find and develop plastic-eating organisms must contend with a basic reality: evolution. Microbes have had millions of years to learn how to biodegrade organic matter - such as fruits and tree bark.
They have had barely any time at all to learn to decompose plastics, which did not exist on Earth at any scale before roughly 1950.
“Seaweed has been around for hundreds of millions of years, so there is a variety of microbes and organisms that can break it down,” said Pierre-Yves Paslier, the co-founder of a British company, Notpla, that is using seaweed and other plants to make films and coatings that could replace some types of plastic packaging. By contrast plastic is very new, he said.
Still, recent discoveries of plastic-eating microorganisms show that evolution is already getting to work. A year after the 2016 discovery of Ideonella sakaiensis in Osaka, scientists reported a fungus able to degrade plastic at a waste disposal site in Islamabad, Pakistan.
In 2017 a biology student at Reed College in Oregon analyzed samples from an oil site near her home in Houston, Texas, and found they contained plastic-eating bacteria.
In March 2020, German scientists discovered strains of bacteria capable of degrading polyurethane plastic after collecting soil from a brittle plastic waste site in Leipzig.
In order to make any of these naturally-occurring bacteria useful, they must be bioengineered to degrade plastic hundreds or thousands of times faster. Scientists have enjoyed some breakthroughs here, too. In 2018 scientists in the U.K. and U.S. modified bacteria so that they could begin breaking down plastic in a matter of days. In October 2020 the process was improved further by combining the two different plastic-eating enzymes that the bacteria produced into one “super enzyme.”
The first large-scale commercial applications are still years away, but within sight.
Carbios, a French firm, could break ground in coming months on a demonstration plant that will be able to enzymatically biodegrade PET plastic.
This could help companies - such as PepsiCo and Nestle, with whom Carbios is partnering, achieve longstanding goals of incorporating large amounts of recycled material back into their products.
They’ve so far failed to succeed because there has never been a way to sufficiently break down plastic back into more fundamental materials. (Because of this, most plastic that is recycled is only ever used to make lower-quality items, such as carpets, and likely won't ever be recycled again.)
“Without new technologies, it’s impossible for them to meet their goals. It’s just impossible,” said Martin Stephan, deputy CEO of Carbios.
Besides plastic-eating bacteria, some scientists have speculated that it may be possible to use nanomaterials to decompose plastic into water and carbon dioxide. One 2019 study in the journal Matter demonstrated the use of “magnetic spring-like carbon nanotubes” to biodegrade microplastics into carbon dioxide and water.
The challenges ahead
Even if these new technologies are one day deployed at scale, they would still face major limitations and could even be dangerous, experts caution.
Of the seven major commercial types of plastic, the plastic-eating enzyme at the heart of several of the recent breakthroughs has only been shown to digest one, PET.
To summarize, there are 7 types of plastic exist in our current modern days:
1 – Polyethylene Terephthalate (PET or PETE or Polyester) ...
2 – High-Density Polyethylene (HDPE) ...
3 – Polyvinyl Chloride (PVC) ...
4 – Low-Density Polyethylene (LDPE) ...
5 – Polypropylene (PP) ...
6 – Polystyrene (PS) ...
7 – Other. :: Jul 17, 2018 7 Types of Plastic that You Need to Know - Waste4Change
Other plastics, such as HDPE, used to make harder materials such as shampoo bottles or pipes, could prove more difficult to biodegrade using bacteria.
Nor are the bacteria able to degrade the plastic all the way back into their core elemental building blocks, including carbon and hydrogen. Instead, they typically break up the polymers out of which plastics are composed back into monomers, which are often useful only to create more plastics. The Carbios facility, for example, is intended only to convert PET plastic back into a feedstock for the creation of more plastics. [ ENTER "Rate Earth Elemts (REE) ]
Plastic garbage floats on the Iskar River near the "Svoge" hydroelectric plant in January 2021 in ... [+]
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Even if one day it becomes possible to mass produce bacteria that can be sprayed onto piles of plastic waste, such an approach could be dangerous. Biodegrading the polymers that comprise plastic risks releasing chemical additives that are normally stored up safely inside the un-degraded plastic.
Others point out that there are potential unknown side-effects of releasing genetically engineered microorganisms into nature. “Since most likely genetically engineered microorganisms would be needed, they cannot be released uncontrolled into the environment,” said Wolfgang Zimmerman, a scientist at the University of Leipzig who studies biocatalysis.
Similar issues constrain the potential use of nanomaterials. Nicole Grobert, a nanomaterials scientist at Oxford University, said that the tiny scales involved in nanotechnology mean that widespread use of new materials would “add to the problem in ways that could result in yet greater challenges.”
The best way to beat the plastic waste crisis, experts say, is by switching to reusable alternatives, such as Notpla's seaweed-derived materials, ensuring that non-recyclable plastic waste ends up in a landfill rather than in the environment, and using biodegradable materials where possible.
Judith Enck, a former regional Environmental Protection Agency (EPA) administrator in the Obama administration and the president of Beyond Plastics, a non-profit based in Vermont, pointed to the gradual spread of bans on single-use plastics around the world, from India to China to the EU, U.K. and a number of U.S. states from New York to California. These are signs of progress, she said, although more and tougher policies are needed. “We can’t wait for a big breakthrough.”
"... Morgan Vague, a biology student at Reed College in Oregon, may have found a solution to one of the most urgent environmental crises in the world. She discovered a breeding bacteria that can "eat" plastic and break it down into by-products that are harmless. One of the most common plastics is PET or polyethylene terephthalate. It is used in bottles, clothing, and food packaging. This type of plastic takes hundreds of years to break down, and it causes damage to the environment. [ https://www.nature.com/articles/2181205a0 < "breeding bacteria"]
Ms. Vague stated that the process - that will be done by the plastic "eating" bacteria could have a big part in solving the world's plastic problem.
Every year, billions of tonnes ( of ?????) are dumped in oceans and landfills. Around 300 million tonnes of plastic are thrown away every year, and 10% of it are recycled.
Because of the statistics about all of the "plastic waste", it shows that we have a serious problem, and it needs to be addressed immediately. Ms. Vague said that after she learned about bacterial metabolism, she decided to find out if there were microbes that can degrade plastic.
She started looking for microbes that are adapted to degrade plastic in water and soil around the refineries in Houston, her hometown. She then took some samples back to Portland, Oregon, where she began testing 300 strains of bacteria for a fat-digesting enzyme called lipase, which is capable of breaking down plastic and making it "edible" for the bacteria.
Out of the 300 strains, there were 20 that produced lipase, and three of those have high levels of the enzyme.
She then put the three microbes on a diet of PET that she got from water bottles. [ ???????? ] That was when she discovered that the bacteria digested the plastic.
Even though her discovery can help the world with its ongoing battle with plastic trash, she said that there is still a long way to go before we can see the microbes eating plastic at the rate that will be useful enough to dispose of plastics.
Professor Jay Mellies, a microbiologist [who supervised the thesis of Ms. Vague], suggested to speed things up and to improve the treatments on the PET to make it more "edible" for the bacteria to eat and eventually train the bacteria to work on different kinds of plastics.
With this discovery, there is a silver lining in this huge plastic problem that we are all facing. People are becoming more aware of the ongoing problem. Although this plastic-eating bacteria is not the total solution, it can still be a part of the solution.
Working Group of Environmental Microbiology, Department of Evolutionary Biology and Environmental Studies, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
Working Group of Environmental Microbiology, Department of Evolutionary Biology and Environmental Studies, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
Received: 04 May 2016 Accepted: 05 June 2016 Published: 06 June 2016
In the light of an expected supply shortage of rare earth elements (REE) measures have to be undertaken for an efficient use in all kinds of technical, medical, and agricultural applications as well as—in particular—in REE recycling from post-use goods and waste materials. Biologically- based methods might offer an alternative and supplement to physico-chemical techniques for REE recovery and recycling. A wide variety of physiologically distinct microbial groups have the potential to be applied for REE bioleaching form solid matrices. This source is largely untapped until today. Depending of the type of organism, the technical process (including a series of influencing factors), the solid to be treated, and the target element, leaching efficiencies of 80 to 90% can be achieved. Bioleaching of REEs can help in reducing the supply risk and market dependency. Additionally, the application of bioleaching techniques for the treatment of solid wastes might contribute to the conversion towards a more sustainable and environmental friendly economy.
1. Rare Earth Elements as Resource
Rare earth elements [REE; lanthanides, atomic numbers 57 to 70 usually excluding promethium (61) due to its instable isotopes, and including also scandium (21) and yttrium (39)] have unique chemical and physical properties and are indispensable for a huge variety of technical application fields. They are often referred to as the “seeds of technology” since they are an important part of many electronical devices such as smartphones, computers, TV sets, and many more [1,2]. Especially in the fast-growing energy sector where REE are used for e.g., catalytic converters, fluorescent phosphors, rechargeable batteries, and permanent magnet in wind turbines, REEs can contribute in the view of many to a greener economy [3]. However, geogenic REE mineral resources are not infinite and limited as much as fossil resources. This might lead in the future to REE shortages and increased market prices. Therefore, measures have soon to be taken to circumvent these constraints thereby mainly focusing on efficient REE use in technical applications as well as on increased recycling efforts [4].
Even though REE are not “rare” per se in the Earth crust, they are very difficult to mine and to purify [2]. Mining also causes environmental problems, because of water pollution, high energy consumption, and radioactive by-products (such as e.g., thorium), which generate radioactive waste [5]. The market of REE gets more and more important and is predicted to increase 8 to 12% per year by 2020 [6]. Furthermore, most operational mines are located in China and the market dependency on China (with 86% of the annual global mining production in 2014) is huge [2,7,8]. Also in the future, China is expected to remain the world’s principal rare earth supplier [5]. Worldwide, there is a growing interest to recycle REE from waste to scale down the supply risk. Nevertheless, current rates of recycling REEs by physico-chemical techniques are in many cases below 1% compared to iron and steel which are recycled with rates between 70% to 90%, one of the highest among industrially-used metals [9]. This is mainly due to technological difficulties and, until recently, low prices, and the lack of incentives. The technological issue can be explained by the rather low amounts and different forms of REE in goods. This makes it very difficult to establish a general approach for REE recycling. Rather, the development of product specific recycling schemes is recommended [10]. In addition, due to the small quantities in a wide variety of technical devices and medical applications, REE dissipation after disposal of goods is very critical and REE are irrevocably lost in the environment as e.g., in the case of cerium as additive of diesel or gadolinium used as contrasting agent in magnetic resonance imaging [2,11].
2. Microbe-REE-interactions
In the last decade, several studies have been published addressing the interactions of microorganisms and REE including both REE mobilization from solids through metabolic reactions and REE immobilization from liquids mainly through sorption by biomass as well as the role of REE in bacterial growth. As example, adsorption onto the cell envelope of Gram-positive and Gram negative bacteria were examined, particularly adsorption behavior of europium to Halobacterium salinarum, Pseudomonas fluorescens, and Bacillus subtilis[12,13]. Recently published work demonstrated sorption of REE (dysprosium in this case) a by the fungal strain Penidiella sp. T9 [14]. Dysprosium biosorption took place even at pH values as low as 2.5. When cells of Roseobacter sp. AzwK-3b were pre-protonated at this pH with nitric acid, heavy REE such as thulium, ytterbium , and lutetium were adsorbed to a higher degree as compared to light-group REE [15]. Summaries on REE biosorption are given in several recent review articles [16,17,18,19].
Microbially mediated mobilization of elements occurs mainly via acidolysis, redoxolysis, and complexolyis [20,21]. Acidolysis (also termed proton-induced solubilization) means the exchange and replacement of elements by from mineral surfaces by protons. Mobilization by reductive or oxidative reactions is described by the term redoxolysis whereas complexolysis is characterized by the reaction of complexing agents with mineral surfaces (also termed ligand-induced solubilization) [20]. These general mechanisms apply for all solid matrices, also those containing REE. However, it has still to be elaborated why microorganisms mobilize REE and what their ecological advantage from this interaction is. At the moment, only a very limited number of reports is available [22,23,24,25,26,27] and there is a big lack in studies on the mechanistic interactions between microorganisms and REE since the focus so far has been mainly on microbe-metal-interactions of commodity metals [21]. Microbes might mobilize REE either by pure coincidence through their metabolic reactions or by a true need for these elements. Recent studies demonstrated that some microbes are strictly growth-dependent on the presence of REE, because they act as essential cofactors for some of the microbe’s key enzymes. Methylacidiphilumfumariolicum cultivated on methane showed positively correlated growth with the different concentrations of cerium [19]. In addition, lanthanum, neodymium, and praseodymium supported growth to a high extent, whereas samarium, europium, and gadolinium were less favorable, but still supportive [19]. It remains to be investigated in future studies, if these findings can be generalized and REE promote growth of other microorganisms as well. Even more as it is discussed in this study that laboratory glassware might contain REE in trace amounts (originating from silicates used in fabrication or additionally supplied during production). REE might be mobilized and released into the medium by cultivation under acidic conditions [19].
In comparison to calcium, the addition of lanthanum and cerium increased the activity of methanol dehydrogenase in Methylobacterium radiotolerans, M. fujisawaense, and M. zatmanii by a factor of four to six suggesting the induction of latent genes [24]. Mutant strains of Methylobacterium extorquens showed a REE-dependent growth behavior [26]. Concentrations of lanthanum as low as 2.5 nM stimulated growth on methanol in comparison to the addition of only calcium due to the increased activity of a lanthanide-dependent methanol dehydrogenase. Due to this fact, it has been suggested that methylotrophic microorganisms might find an application in REE biomining and recycling [26]. How effective the organisms are in the recovery of REE, however, remains to be investigated. Also the metabolism of non-methylotrophic microorganisms such as Bradyrhizobium is influenced by REE [27]. Mainly cerium, lanthanum, and praseodymium stimulated the formation of extracellular polymeric substances, but not bacterial growth.
3. Biological Mobilization of REE from Solids
Biohydrometallurgical technologies offer an alternative to physico-chemically based methods of REE recycling (Table 1). These technologies—termed “bioleaching” —are well-known in the mining industry and especially suited at low elemental concentrations in the materials of interest where conventional techniques for metal recovery cannot be economically performed [20,28]. In this context, bioleaching of REE was investigated regarding the recovery from spent industrial catalysts and luminescent powder originating from cathode ray tubes (CRT). A heterogenic culture of sulfur oxidizing Acidithiobacilllus ferrooxidans, A. thiooxidans, and Leptospirillum ferrooxidans was grown at 30 °C and a pH of 2. The recovery rate after 16 days for yttrium from the CRT powder was 70% [29]. Leaching efficiencies were tested in the presence and absence of ferric iron, since the addition of Fe2+ was assumed to enhance bacterial resistance to high levels of metals. Experiments showed that the presence of such elements suppressed bacterial growth, resulting in lower leaching efficiency. As in many bioleaching approaches, there is a negative dose-response relationship regarding pulp density [30]. This negative effect of high powder dose was also observed when treating CRT powder suggesting either a certain toxicity of the dissolved metals [29] and/or a mechanical stress on the microorganisms.
Early bioleaching experiments to mobilize REE form solid minerals were published in the late 1980 and beginning of 1990ies [31,32,33]. Zircon containing Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu (in total 7.3 g REE per kg) was leached in suspensions of 10 g/l by Acetobacter methanolicus and Acidithiobacillus (formerly Thiobacillus)ferrooxidans resulting after experimental optimization in total REE mobilization efficiencies of 62.1% and 79.6%, respectively [31]. Zircon was first ground to a grain size of about 60 μm. Using unground zircon and applying the same experimental conditions resulted in a remarkable decrease of leaching efficiency. Only about 18% of the REE were solubilized. Mainly gluconic acid formed by the microorganisms was responsible for the leaching process [7]. Cell-free assays with gluconic acid as lixiviant showed leaching efficiencies of only 4%, thus indicating that microbial metabolic activity plays an important role in bioleaching. During microbial leaching a differentiation between light and heavy REE took place. There was a preference of mobilizing light REE rather than heavy REE. In contrast, the reaction of zircon with gluconic acid in the absence of the microorganism showed no differentiation [31]. The treatment of phosphorus furnace slag (containing in total 7 g REE per kg) by Acetobacter methanolicus gave REE rates of up to 70% [7,34].
A more detailed study on REE extraction from zircon resulted in mobilization efficiencies of approximately 80% with extraction rates of 1.1 mg per hour using Acidithiobacillus ferrooxidans and 67% at a rate of 1.4 mg per hour applying Acetobacter methanolicus, respectively [33]. However, there were distinct mechanistic differences regarding recovery efficiencies between autotrophic A. ferrooxidans and heterotrophic A. methanolicus. Most of mobilized REE were adsorbed by the biomass of A. ferrooxidans and only a minor portion was present in the cultivation fluid. Using praseodymium as a model compound, it has been demonstrated that sorption onto Acidithiobacillus biomass took place within minutes [33]. In contrast, higher amounts of REE were found in the cultivations fluids of A. methanolicus indicating that biosorption was less pronounced. For both A. ferrooxidans and A. methanolicus, leaching efficiencies (sum of REE present in the biomass and the supernatant) decreased with increasing REE atomic numbers resulting in maximum recoveries of 91.6% (for La) and 89.4% (for Pr and Nd), respectively [33].
Also anaerobic microorganisms have been applied for the mobilization REE from solid materials. Yttrium was mobilized from phosphogypsum (a by-product originating from fertilizer production) in a fixed-bed reactor by sulfate-reducing Desulfovibrio desulfuricans with efficiencies of almost 80% [32].
Gibbsite samples (4.9 g of REE per kg) in the shale beds of Um Bogma formation in South-Western Sinai (Egypt) was processed through a bioleaching procedure where cultures of Acidithiobacillus ferrooxidans were pumped through a column (1.2 m in height) filled with 1 kg of mineral [35]. Additionally, factors influencing leaching efficiency such as incubation period (1 to7 days), sulfur addition (0.1% to 0.5%), and mineral pre-treatment (sterilized or non-sterilized) was investigated. Mobilization showed a gradual increase with increasing reaction time and reached an optimum after 6 days. Further prolongation of the incubation time did not improve efficiency. Sulfur addition was observed to increase the bacterial acid formation and, therefore, positively influencing the leaching efficiency. On the other hand, pre-sterilization of the mineral decreased the leaching efficiency compared to the non-sterilized material probably due to the elimination of the endogenous microflora during sterilization. By optimization, total REE mobilization resulted in 67.6%.
Microbial REE mobilization from solid matrices can also be achieved by fungi. Red mud, a waste material from bauxite processing in aluminum mining operations, was treated with Penicillium tricolor[36]. In the study, three different bioleaching processes were performed: One-step bioleaching (fungal growth in the presence of sterilized red mud), two-step bioleaching (pre-cultivation of the microorganisms and in biomass production followed by the addition of sterilized red mud), and finally, cell-free spent medium (cultivation of the microorganisms followed by filtration to obtain cell-free medium, which was then added to sterilized red mud). As part of the study, the effect of red mud pulp density (20, 50, and 100 g/l) was determined. In general, with increasing pulp density, there was an observable decrease in REE leaching efficiencies in all leaching approaches. In the one-step process at a pulp density of 20 g/l, the highest bioleaching efficiency was achieved, whereas at 100 g/l, the two-step bioleaching method exhibits maximum leaching efficiency indicating that the two-step bioleaching process is more suitable for leaching red mud at high pulp densities. Looking at individual REE, leaching efficiency was clearly higher for heavy REE than for the light REE. Overall efficiency generally increased with increasing REE atomic numbers.
Several species of actinomycetes (among them Streptomyces fungicidicus, S. aureofaciens, S. chibaensis) have been used to mobilize REE from sandy and silty soil samples [37]. In suspensions of 10 g soil per liter (cultivated for 48 hours at 30 °C) REE leaching efficiencies of up to 37% were obtained depending on the strain applied.
Instead of pure cultures also mixed microbial cultures have been applied recently for REE mobilization. Acidophilic chemolithotrophic microbial communities leached coal-derived ash-slag waste (ASW; containing per ton: 46 g La, 39 g Nd, 31 g Y, 9.4 g Sc, 7.2 g Sm, 6.5 g Gd) collected from heaps of a heat power station [38]. Experiments were carried out in 300 ml airlift percolators which were loaded with ASW, elemental sulfur, and inoculated with 100mL of a mixed bacterial culture of A. ferrooxidans, A. thiooxidans, A. caldus, and Sulfobacillus thermosulfidooxidans. As already shown in earlier experiments [36], a high pulp density resulted in a drastic decrease of REE bioleaching efficiency. Increasing the density from 100 to 330 g per liter, bioleaching efficiency was more than halved [30]. Furthermore, other process parameters such as temperature (25, 28, and 40 °C), adjustment of initial pH (pH 3, pH 2.6, pH 2), and different ratios of ASW to elemental sulfur (1:10, 1:100) were evaluated. Rising the cultivation temperature enhanced the leaching efficiency, especially for Sc, Y, and La, whereas a temperature rise from 28 °C to 45 °C effected the increase 2.2-, 1.7-, and 2.1-fold, respectively. Reducing the amount of sulfur added turned out to have negative effect on the leaching efficiency since the energy substrate was limited and resulted in insufficient amounts of sulfuric acid formation that mainly mediates the bioleaching process. Based on all findings (45 °C cultivating temperature, initial pH 2, ASW/sulfur ratio 1:10) a recovery of scandium, yttrium, and lanthanum of 52.0, 52.6, and 59.5%, respectively, was achieved after ten days of bioleaching. Besides coal ASW also slag from municipal waste incineration has been considered as resource for REE recovery although amounts in the slag are rather low [39].
Either contact or non-contact bioleaching (termed earlier as direct and indirect bioleaching) might be the underlying leaching mechanism of metal mobilization. Contact leaching describes the direct physical contact between microorganisms and a solid whereas in non-contact leaching the biomass is physically separated from the solid to be treated. Leaching efficiencies of REE from carbonaceous shale powder were tested with several species of Aspergillus and Penicillium[40]. Results showed that contact bioleaching noticeably generated higher REE leaching efficiencies of up to 86% after 7 days than non-contact bioleaching, independent from the fungi tested. Overall, Aspergillus sp. reached higher bioleaching rates than Penicillium sp., thereof A. flavus and A. niger were the most effective ones. In accordance to previous studies [36,38] findings confirm that the amount of REEs mobilized decreases with increasing sample concentrations, because best leaching efficiency was found in suspension of 10 g/l.
Besides a series of solid REE containing waste materials (as described above), native REE bearing minerals such as monazite have also been studied as a substrate for bioleaching operations [41]. Three fungal species (Aspergillus niger, Aspergillus terreus, and Paecilomyces sp.) were cultivated in differently composed growth media and on various carbon sources. As consequence, a mixture of different organic acids such as acetic, citric, gluconic, itaconic, lactic, oxalic, and succinic acid accumulated in the cultivation fluid. Their presence and concentration depended on the fungal strain used. However, REE leaching rates of approximately 3% were rather low, probably due to the nature and type of the solid material. By comparing original REE content in monazite sand it was be concluded that neodymium, cerium, praseodymium, and lanthanum were all mobilized by the three fungal strains in the same ratio and no preferential bioleaching of a particular REE was observed [41]. However, there are indications that REE might be mobilized from solids such as e.g., ash-slag-waste at different degrees by acidophilic sulfur oxidizers. Scandium, lanthanum, cerium, and praseodymium were mobilized from the original matrix by 15 to 20%, whereas neodymium, yttrium, samarium, gadolinium, dysprosium, erbium, and europium showed mobilization rates of 25 to 30% [42]. It remains to be investigated whether this is due to the chemical characteristics of the solid material or due to the metabolic preferences of the microorganisms applied. Interestingly, REE sorption by microbial biomass was not observed under the conditions applied.
Besides Aspergillus ficuum, monazite has also been biologically treated with a heterotrophic bacterial strain, Pseudomonas aeruginosa[43]. After 9 days at a pulp density of 6 g/l, over 53% of REE were released to the medium. A. ficuum was able to mobilize approximately 75%. Also in this study, it was proven that contact bioleaching yielded in significantly higher leaching rates indicating that the presence of microorganisms is necessary to obtain recoveries as high as possible. In an earlier approach using the same microbial strains lanthanum, cerium, and yttrium were released from a thorium-uranium concentrate [44]. Cell-free supernatants obtained as filtrates after cultivation were amended with REE containing concentrates and left to react for 24 hours. A. ficuum mobilized 2.5, 20, and 33% of yttrium, lanthanum, and cerium, respectively, whereas P. aeruginosa released only 1.2, 4.3, and 5.4%.
In an effort to simulate heap leaching, a long-term study was carried out over a period of 52 months [45]. In the course of the experiments several factors such as e.g., nutrient addition, ferric iron addition, weekly or monthly flushing were tested for their influence on REE release from conglomerate ore containing approximately 400 mg of REE per kg. Endogenous acidophiles were stimulated by the addition of ferric sulfate at pH 3.5. A maximum of 45% of total REE was released.
A mixture of chemolithotrophic sulfur-oxidizing microorganisms (termed VUR-9 and not described in detail) was applied to mobilize 55 to 70% of REE from phosphogypsum after incubation times of up to 30 days at low pH values of 1.5 to 1.8. (RU 2457267 and RU 2457267 in Table 2). Also a mixed bacterial culture (“S20 bacterial group”) containing mainly Acidithiobacillusalbertiensis (99.72%), but also A. thiooxidans (0.02%), other Acidithiobacillus species (0.15%), and unspecified proteobacteria (0.02%) has been used to recover scandium, dysprosium, neodymium, and praseodymium from low-grade mine waste (WO 2014178360 in Table 2). However, the role and function of the different organisms in REE mobilization remains to be investigated.
Sulfuric acid generated by A. ferrooxidans and A. thiooxidans is the main mobilization agent for biological treatment of REE-containing phosphate minerals through a heap or column leaching process (DE 102012210991 in Table 2). A moderately thermophilic mixed microbial population community of acidophilic chemolithotrophs mobilized scandium, yttrium, and lanthanum from fly ash-slag at 45 °C (RU 2560627 in Table 2). Alternatively to sulfur-oxidizing autotrophic microorganisms, heterotrophs such as Acetobacter methanolicus, lactic acid bacteria, and fungi (Schizophyllum commune) have also been considered for REE mobilization (DD 249156, CN 105063383, DE 10213266042 in Table 2). Acetobacter methanolicus has been grown on glucose in the presence of waste slag (derived from the processing of phosphate minerals) bringing 80% of REE in solution (DD 249156 in Table 2). Gluconic acid was the main leaching agent DD 259121 in Table 2).
5. Perspectives of REE Recovery and Recycling
A promising solid matrix as starting material for a REE recycling is fluorescent powder from spent lamps. It mainly contains three of the five most critical rare earth elements, namely yttrium, europium, and terbium, often in gram quantities per kg of powder [29]. Since the separation and collection of the lamps is already mandatory in many different countries, mainly to remove toxic mercury form the waste, further recycling processes would be applicable. The state of art for REE leaching from lamp phosphors is covered so far mainly through chemical approaches [46]. To be able to recover REEs from lamp phosphors, the phosphor mixture has to be attacked chemically to bring the REEs into solution from where they can be separated by precipitation or solvent extraction. To close product life cycles, REE mobilization following recovery from spent fluorescent lamps seems to have big potential and might contribute to a more sustainable world. Doing so with the aid of microbes makes it a relatively inexpensive approach for industrial waste treatment which is, in addition, flexible enough to be applied for different leachable waste materials. Recent studies of REE mobilization from fluorescent phosphors included besides well-known A. ferrooxidansand A. thiooxidans other bacteria (Komatogateibacter xylinus, Lactobacillus casei, Corynebaterium collunae), yeasts (Yarrowia lipolytica) as well as the tea fungus Kombucha [47,48,49].
In contrast to chemical leaching which has often a high energy demand to generate high temperatures and produces chemical wastes, bioleaching is supposed to be a much cleaner, efficient, and low cost process to mobilize metals. Although bioleaching of REEs cannot solve all the objectives, it can help to reduce the supply risk and market dependency. In addition, the development of bioleaching methods in general, can contribute to the conversion towards a more sustainable and environmental friendly economy.
The awareness of potential upcoming supply shortage of REE is also affecting current politics. The European Union (EU) recently launched the European Innovation Partnership (EIP), a program aiming on the reduction of import dependency by improving supply conditions from EU and other sources and by providing resource efficiency and alternative supply [50]. Although bioleaching cannot solve the main objectives of the EIP on its own, it can certainly contribute to the future challenges of the EIP by improving extraction, processing and recycling of critical raw materials [8].
Overall, biological leaching is a relatively inexpensive approach for industrial waste treatment and quite flexible in relation to different growth conditions and different leachable wastes. Compared to traditional chemical processes, bioleaching is supposed to be a “green” technology for the recovery of valuable metals from industrial waste to close element cycles. Its applicability for an industrial REE recovery and recycling remains to be accurately evaluated in detail in the near future.
6. Conclusions
Natural microbial mechanisms that enable the mobilization of elements or metals from solids are the basis of industrial processes mimicking element cycles in nature. New evolving bioleaching and biorecovery processes involving REE can open new doors for “green” recycling strategies which might contribute to a more sustainable world and can help to reduce supply risks and market dependencies.
Conflict of Interest
All authors declare no conflict of interest.
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Microbial mobilization of rare earth elements (REE) from mineral solids—A mini review
Working Group of Environmental Microbiology, Department of Evolutionary Biology and Environmental Studies, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
Received: 04 May 2016 Accepted: 05 June 2016 Published: 06 June 2016
In the light of an expected supply shortage of rare earth elements (REE) measures have to be undertaken for an efficient use in all kinds of technical, medical, and agricultural applications as well as—in particular—in REE recycling from post-use goods and waste materials. Biologically- based methods might offer an alternative and supplement to physico-chemical techniques for REE recovery and recycling. A wide variety of physiologically distinct microbial groups have the potential to be applied for REE bioleaching form solid matrices. This source is largely untapped until today. Depending of the type of organism, the technical process (including a series of influencing factors), the solid to be treated, and the target element, leaching efficiencies of 80 to 90% can be achieved. Bioleaching of REEs can help in reducing the supply risk and market dependency. Additionally, the application of bioleaching techniques for the treatment of solid wastes might contribute to the conversion towards a more sustainable and environmental friendly economy.
Citation: Fabienne Barmettler, Claudio Castelberg, Carlotta Fabbri, Helmut Brandl. Microbial mobilization of rare earth elements (REE) from mineral solids—A mini review[J]. AIMS Microbiology, 2016, 2(2): 190-204. doi: 10.3934/microbiol.2016.2.190
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In the present study, the feasibility to use phosphate solubilizing bacteria (PSB) to develop a biological leaching process of rare earth elements (REE) from monazite-bearing ore was determined. To predict the REE leaching capacity of bacteria, the phosphate solubilizing abilities of 10 species of PSB were determined by halo zone formation on Reyes minimal agar media supplemented with bromo cresol green together with a phosphate solubilization test in Reyes minimal liquid media as the screening studies. Calcium phosphate was used as a model mineral phosphate. Among the test PSB strains, Pseudomonas fluorescens, P. putida, P. rhizosphaerae, Mesorhizobium ciceri, Bacillus megaterium, and Acetobacter aceti formed halo zones, with the zone of A. aceti being the widest. In the phosphate solubilization test in liquid media, Azospirillum lipoferum, P. rhizosphaerae, B. megaterium, and A. aceti caused the leaching of 6.4%, 6.9%, 7.5%, and 32.5% of calcium, respectively. When PSB were used to leach REE from monazite-bearing ore, ~5.7 mg/L of cerium (0.13% of leaching efficiency) and ~2.8 mg/L of lanthanum (0.11%) were leached by A. aceti, and Azospirillum brasilense, A. lipoferum, P. rhizosphaerae and M. ciceri leached 0.5–1 mg/L of both cerium and lanthanum (0.005%–0.01%), as measured by concentrations in the leaching liquor. These results indicate that determination of halo zone formation was found as a useful method to select high-capacity bacteria in REE leaching. However, as the leaching efficiency determined in our experiments was low, even in the presence of A. aceti, further studies are now underway to enhance leaching efficiency by selecting other microorganisms based on halo zone formation.
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/4.0/).
... Such molecules can create a low pH environment, promoting acidolysis, but can also promote mineral dissolution via complexolysis. Thus, their effectiveness in furthering mineral dissolution depends on their complex formation abilities either on whether they can generate protons (Table 1) (Ilyas & Lee, 2014;Shin, Kim, Kim, Jeong, & Lee, 2015). In general, complex stability increases with the denticity of the Acetic acid Acetic acid bacteria e.g., Acetobacter aceti ...
... However, their ability to solubilize metal phosphates seems to be dependent on the phosphate compound. While Shin et al. (2015) reported that Ca-phosphate solubilizing Acetobacter aceti was also able to solubilize REE-phosphates this was not observed for other bacteria such as Klebsiella pneumoniae and Klebsiella oxytoca (Corbett, Eksteen, Niu, Croue, & Watkin, 2017). ...
... These biohydrometallurgical processes are advantageous given the high energy efficiency (low temperature and pressure requirements) and minimized dependence on corrosive chemicals. However, the technologies still require development as the efficiency of recovering REEs varies widely with the organisms used and ore types (Qu and Lian 2013;Shin et al. 2015). ...
... Technical and technological development efforts have been concentrated on REE extraction, processing and separating REEs from primary sources, secondary mining materials (bauxite, mine tailings, ash), and electronic waste (see "Major REE Production Pathways"). While low-impact alternatives such as phytoremediation and microbialassisted extraction have been developed (Shin et al. 2015;Chour et al. 2020), these processes may not be economically profitable (Jally et al. 2021). Future development of novel extraction alternatives is vital to promote and upscale these alternatives. ...
... G. oxydans produces gluconate via oxidization of glucose using membrane-bound glucose dehydrogenase and the produced gluconate can be further oxidized to 2-ketogluconate and 5-ketogluconate during bacterial cultivation (Gupta et al., 2001;Hölscher et al., 2009). Other microorganisms such as Pseudomonas aeruginosa, Pseudomonas putida and Enterobacter intermedium have been also studied for production of gluconate and/or ketogluconates for metal solubilization from various sources, however these studies did not systematically compare the leaching efficiency of the individual compounds (Hassanien et al., 2014;Hwangbo et al., 2003;McKenzie et al., 1987;Shin et al., 2015). Depending on the cultivation conditions, different ratios of gluconate and its keto-derivatives may exist in the culture supernatants of these microorganisms. ...
... The higher REE leaching yields obtained with gluconate compared to the ketogluconates, despite having a higher pH, indicate that gluconate is more effective in forming stable complexes with REEs. The stability of metal complexes depends on the chemical structure of the ligand and its interaction with the metal ions (Ilyas and Lee, 2014;Shin et al., 2015). The stability of the complex increases with the denticity of the ligand (Janiak et al., 2012). ...
... Note that 15% of our soil samples failed to form clear zones on the plates used for counting PSM colonies within an incubation period of 7 days, of which nearly 80% were soil samples from mined lands. This is in agreement with the well-known observation that the edaphic conditions of mined lands are generally unfavourable for soil microbes responsible for soil nutrient cycling (Sheoran, Sheoran & Poonia, 2010). As a result, a total of 367 soil samples whose PSM populations could be counted after 7 days of incubation were included for further analysis. ...
... In its natural environment, G. oxydans produces biolixiviants to liberate phosphate from minerals, not metals [41][42][43] . Under phosphate-limiting conditions, the PstSCAB phosphate transporter will activate the histidine kinase, PhoR, which in turn phosphorylates the transcription factor PhoB, and activates the pho regulon, enabling phosphate solubilization and uptake 44 . ...
... Notwithstanding, these effects could be assuaged by reconditioning the microbial seed culture to gradually advancing metal concentrations (Jang and Valix, 2017) or employing a consortium of other metal tolerant organisms. Numerous investigations regarding REE bioprecipitation have been conducted in controlled environments, where the experiential effects of these experimental components have been easily analyzed, individually (Shin et al. 2015;Brisson et al. 2016;Reed et al. 2016). However, the accurate reproduction of these cardinal factors altogether on the commercial scale is far from reach, due to the harsh and erratic atmospheric conditions at most mining and mineral processing locations (Jia et al. 2019). ...
... Another noticeable point was drop in concentration of all excreted organic acid after reaching a peak which was consistent with previous reports of heterotrophic culture (do Carmo et al., 2019;Paavilainen et al., 1994;Qu et al., 2019a). The HPLC data demonstrate that current alkaliphilic bacteria have a relatively good ability of organic acids production in comparison with other heterotrophic bacteria, possibly as a result of the initial alkaline medium which can induce organic acid secretion (do Carmo et al., 2019;Paavilainen et al., 1994;Shin et al., 2015). ...
... 5.7 mg/L of Ce (0.13% of leaching efficiency) and ca. 2.8 mg/L of La (0.11%) were leached by Acetobacter aceti, and Azospirillum brasilense, Azospirillum lipoferum, Pseudomonas rhizosphaerae and Mesorhizobium cicero leached 0.5-1 mg/L of both Ce and La (Shin et al., 2015). ...
... Both autotrophic and heterotrophic microorganisms can be used to leach rare earth elements [77,78]. In particular, Pseudomonas, Enterobacter, Serratia, and Bacillus have remarkable rare earth element recovery abilities in monazite ores [79,80]. In one study, different strain types and growth media were used to extract rare earth elements from rare earth element ore in China, and the results showed that the leaching efficiency of Streptomyces sp. ...
Pursuant to Executive Order 13817, REEs are [described as] used in many components and products in various sectors.
Examples of sectors in which REE are commonly used include: petrochemicals, metals and alloys, glass and ceramics, and electronics.
Examples of components and products that contain REE include catalysts (e.g., oil refineries, automobiles), permanent magnets (e.g., cell phones, wind turbines, electric
vehicle motors), fiber optics (e.g., signal amplifiers, lasers), and lighting/displays (e.g., fluorescent lights, cell phone and
computer displays). Many components and products containing REE are used for defense applications.
China is currently the global leader in REE mining, refining, and component manufacturing.
From 2015 through 2018, the United States imported 80% of REE compounds and metals from China;
much of the imports from other countries were derived from Chinese REE inputs.
Between 2010 and 2014, China imposed export restrictions on REE.
The supply shock resulted in dramatic increases in prices and concern over securing access to REE, which led to increased global exploration for economical REE deposits.
Privately and federally funded research is seeking to lower REE extraction costs.
- Lower costs and improved extraction processes or technologies could make previously uneconomical deposits of REE available for extraction. Similarly, ongoing research is seeking means of extracting or reusing REE in components and products containing them. Lowering the costs to extract REE from other minerals, mining waste, and recycled products could potentially create economically viable sources of REE.
Typically 60% of REE consumed domestically ( USA ) is used in catalysts (e.g., oil refineries, automobiles); the remainder [ 40% ] is
typically divided (around 10% each) among metallurgical applications and alloys, ceramics and glass, polishing, and other
uses.
Prices vary greatly among REEs; two examples from 2018 include $455 per kilogram of terbium oxide and $2 per kilogram of lanthanum oxide.
Price trends for REE appear to have stabilized after China lifted its export restrictions.
( https://apnews.com/article/joe-biden-donald-trump-beijing-global-trade-tibet-a9038d1fea6606a3d52e96a12a9e4ca2 )
"... BEIJING (AP) — China’s top diplomat called Monday for new U.S. President Joe Biden’s administration to lift restrictions on trade and people-to-people contacts while ceasing what Beijing considers unwarranted interference in the areas of Taiwan, Hong Kong, Xinjiang and Tibet. Foreign Minister Wang Yi’s comments at a Foreign Ministry forum on U.S.-China relations come as Beijing presses the new administration in Washington to drop many of the confrontational measures adopted by former President Donald Trump.Trump hiked tariffs on Chinese imports in 2017 and imposed bans and other restrictions on Chinese tech companies and academic exchanges as he sought to address concerns about an imbalance in trade and accusations of Chinese theft of American technology. ..."
A range of legislation to increase U.S. access to REE has been introduced in the 116th Congress. The Senate considered a
major energy and minerals bill, S. 2657, in 2020. A Senate cloture motion on S. 2657 did not pass in March. The House
considered a major energy and minerals bill, H.R. 4447, which was passed in the House in September. Both bills would
(among other provisions) authorize funds to research the extraction of REE from coal and coal byproducts (formalizing an
ongoing research program at the National Energy Technology Laboratory). S. 2657 and H.R. 4447 would direct the U.S.
Department of Energy (DOE) to research REE extraction via a consortium. H.R. 4447 contains provisions directing DOE to
create a research program related to recycling critical minerals from energy storage systems. Three other bills, S. 3694, S.
4537, and H.R. 7812, would direct the U.S. Department of Defense (DOD) to establish a grant program to encourage the
domestic development of critical minerals, among other provisions. These bills would authorize $50 million per year for four
years.
Four bills, S. 3694, S. 4537, H.R. 7812, and H.R. 8143, would create tax incentives for firms investing in domestic REE
extraction and consumption. The incentives would include 100% expensing (immediate tax deduction) for qualified property
involved in extracting critical minerals and metals from deposits in the United States; a special allowance (100% depreciation
deduction) for nonresidential real property; and a 200% cost deduction for the purchase of critical minerals and metals
extracted within the United States.
Two bills, S. 2093 and H.R. 4410, would create a federally chartered cooperative and a federally chartered corporation to
mitigate potentially high costs associated with extracting REE from minerals that may be found with radioactive elements
(typically thorium and uranium). The cooperative would accept REE material from domestic and international partners for
processing into salable products. The corporation would accept any related materials containing uranium or thorium from the
cooperative for storage and potential sale. The federally chartered cooperative and corporation would be privately funded.
Congressional Research Service
link to page 4 link to page 4 link to page 5 link to page 6 link to page 7 link to page 10 link to page 10 link to page 11 link to page 12 link to page 13 link to page 14 link to page 15 link to page 16 link to page 17 link to page 18 link to page 18 link to page 12 link to page 13 link to page 15 link to page 6 link to page 19 An Overview of Rare Earth Elements and Related Issues for Congress
Contents
Introduction ..................................................................................................................................... 1
Critical Minerals and Rare Earth Elements Defined ....................................................................... 1
Supply of Rare Earth Elements ....................................................................................................... 2
Global Resources ...................................................................................................................... 3
Domestic Resources .................................................................................................................. 4
Improving Extraction and Recycling to Increase Supply .......................................................... 7
Extraction Technologies ...................................................................................................... 7
Recycling Technologies ...................................................................................................... 8
Demand for Rare Earth Elements .................................................................................................... 9
Domestic Consumption by Sector ........................................................................................... 10
REE Prices ............................................................................................................................... 11
Policy Topics and Legislative Activity .......................................................................................... 12
REE from Coal and Coal Byproducts ..................................................................................... 13
Additional Critical Mineral and REE Research ...................................................................... 14
Tax Incentives for the Supply and Consumption of Domestic REE ....................................... 15
REE Cooperative and Corporation .......................................................................................... 16
Figures
Figure 1. Global Demand and Domestic Consumption of REE ...................................................... 9
Figure 2. Relative Domestic Consumption of REE by Sector ...................................................... 10
Figure 3. Relative REO Price Changes ......................................................................................... 12
Tables
Table 1. Active REE Mines in 2017 ................................................................................................ 3
Contacts
Author Information ........................................................................................................................ 16
Congressional Research Service
link to page 12 An Overview of Rare Earth Elements and Related Issues for Congress
Introduction
The rare earth elements (REE) are a commonly recognized group of 17 elements included in the
list of critical minerals identified by the U.S. Geological Survey (USGS) pursuant to Executive
Order 13817.1 Although domestic resources exist for some REE, the United States is currently
reliant on imports. In 2019, the United States imported 100% of rare earth metals and compounds
it consumed, even though it exported some domestically mined rare earth element concentrate for
further processing (due in part to a lack of domestic processing facilities).2
As the United States currently imports all rare earth metals and compounds, an ongoing concern
for many is maintaining unrestricted access to these supplies. REE are used in many products and
sectors; examples of products containing REE include metal alloys, catalysts, magnets, motors,
and electronic displays. More information on the consumption of REE in representative sectors
and products is presented in the section “Demand for Rare Earth Elements” below.
This report provides an overview of REE, with a focus on domestic mineral resources and the
potential for transforming REE sources (e.g., minerals, ores, concentrate, compounds) into inputs
for other products. The United States has known deposits of REE, and one has been mined
intermittently over several decades. While some deposits offer additional potential supply
options, the United States currently does not have commercial-scale REE extraction capabilities.
Ongoing research aims to lower the cost of extracting REE from mineral deposits and from
recycled materials. Congress has shown interest in securing and enhancing the domestic supply of
rare earth elements and critical minerals through proposed legislation.
Critical Minerals and Rare Earth Elements Defined
Many discussions of critical minerals employ the definition stated in, and the resulting list
pursuant to, Executive Order (E.O.) 13817. The E.O. defines a critical mineral to be
(i) a non-fuel mineral or mineral material essential to the economic and national security
of the United States, (ii) the supply chain of which is vulnerable to disruption, and (iii) that
serves an essential function in the manufacturing of a product, the absence of which would
have significant consequences for our economy or our national security.3
The E.O. directs the Secretary of the Interior, who in turn directs the USGS, to work with other
federal agencies to produce a list of critical minerals (and to update the list periodically). The
Department of the Interior (DOI) accepted the final list from USGS, which includes the REE as
critical minerals.4
Seventeen elements are commonly considered to be REE, 15 within the lanthanoid group of
elements, as well as yttrium and scandium. REE are often discussed or categorized as light REE
(LREE) or heavy REE (HREE). LREE include lanthanum (La), cerium (Ce), praseodymium (Pr),
neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), and gadolinium (Gd).
1 Department of the Interior, “Final List of Critical Minerals 2018,” 83 Federal Register 23295, 2018.
2 U.S. Geological Survey, Mineral Commodity Summaries, 2020, pp. 132-133.
3 Executive Order 13817, “A Federal Strategy to Ensure Secure and Reliable Supplies,” 82 Federal Register 60835
(2017).
4 The full list is aluminum (bauxite), antimony, arsenic, barite, beryllium, bismuth, cesium, chromium, cobalt,
fluorspar, gallium, germanium, graphite (natural), hafnium, helium, indium, lithium, magnesium, manganese, niobium,
platinum group metals, potash, the rare earth elements group, rhenium, rubidium, scandium, strontium, tantalum,
tellurium, tin, titanium, tungsten, uranium, vanadium, and zirconium.
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HREE include terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),
ytterbium (Yb), and lutetium (Lu). Yttrium (Y) is generally considered a HREE, due to its similar
chemical and physical properties. Scandium (Sc) is not included in either subcategory.
Promethium is sometimes excluded from these subcategories because it does not occur in nature.5
Supply of Rare Earth Elements
REE are found in a variety of minerals, but not all are equally suitable for economic
development.6 While more abundant than many other elements, REE are generally found in
concentrations below what is economically viable for extraction at current prices using available
technology. Increasing the domestic supply of REE could be achieved by locating additional
geologic reserves,7 lowering the costs of extracting REE from the ores (or minerals) in which they
are found, enhancing technologies to produce REE by recycling, and increasing the
substitutability among REE for a given use.
During exploration for new deposits of REE, the grade of the deposit is commonly reported in
percent terms of the total deposit that could be produced as rare earth oxide (REO). REE mineral
deposits are typically discussed in terms of the quantity of recoverable REO (typically in tons or
metric tons). Recoverable REO from a resource is typically the product of the quantity (e.g., tons)
of the resource multiplied by the grade (percent) of the resource.
REO is a commonly traded form of refined REE. Some mine operations include the necessary
facilities to extract REE from ores and produce REO on site. Other mine operations may produce
REE concentrate, which results from subjecting the ore to a variety of physical and chemical
refining processes; REE concentrate requires additional processing to become REO.
The grade, or concentration, of REE in its host mineral may not always be a useful indicator of
economic viability. Some higher-grade deposits (e.g., ~8% REO at Mountain Pass, CA) may have
characteristics that require expensive means of extraction or are technologically infeasible to
produce. Alternatively, some lower-grade deposits (e.g., 0.05% in some Chinese clay deposits)
may have characteristics that facilitate REE extraction, resulting in an economically viable
deposit. Characteristics that affect economic viability include the type of deposit (e.g., vein,
placer), the mineralogy (crystalline structure) of the REE minerals in the deposit, accessibility to
5 Bradley S. Van Gosen, Philip L. Verplanck, and Poul Emsbo, Rare Earth Element Mineral Deposits in the United
States, U.S. Geological Survey, Circular 1454, 2019, pp. 1-2, https://doi.org/10.3133/cir1454.
6 “REE-bearing minerals are diverse and often complex in composition. At least 245 individual REE-bearing minerals
are recognized; they are mainly carbonates, fluorocarbonates, and hydroxylcarbonates.... Many of the world’s
significant REE deposits occur in carbonatites, which are carbonate igneous rocks. The REEs also have a strong genetic
association with alkaline magmatism. The systematic geologic and chemical processes that explain these observations
are not well understood. Economic or potentially economic REE deposits have been found in (a) carbonatites, (b)
peralkaline igneous systems, (c) magmatic magnetite-hematite bodies, (d) iron oxide-copper-gold (IOCG) deposits, (e)
xenotime-monazite accumulations in mafic gneiss, (f) ion-absorption clay deposits, and (g) monazite-xenotime-bearing
placer deposits.” Bradley S. Van Gosen, Philip L. Verplanck, Robert R. Seal II, Keith R. Long, and Joseph Gambogi,
Critical Mineral Resources of the United States—Economic and Environmental Geology and Prospects for Future
Supply, ed. Klaus J. Schulz, John H. DeYoung, Jr., Robert R. Seal II, Dwight C. Bradley (Reston, VA: U.S. Geological
Survey, 2017), p. O1, https://doi.org/10.3133/pp1802O.
7 Geologic deposits of minerals are commonly discussed as reserves or resources; different countries may define these
terms differently. Reserves commonly denote mineral deposits that could be profitably produced using available
technology, at current prices; sub-classifications can include marginal and sub-economic reserves. Resources
commonly denote mineral deposits that could be currently or potentially produced economically; resource sub-
classifications can include measured, indicated, and inferred (U.S. Geological Survey, Mineral Commodity Summaries,
2020, pp. 195-198).
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and infrastructure available at the deposit, specific type of REE contained in the deposit, and
presence of uranium or thorium in the deposit, which can increase production costs.8
Global Resources
From the 1960s until around 1985, the United States was the world’s largest producer of REE,
with all production originating from the Mountain Pass mine in California. Starting in the mid-
1980s, China began REE mining and extraction operations and became the largest contributor to
global REE production. By the 2010s, China was producing nearly 85% of the world’s supply of
REE and supplying 95% of processed REE.9 China imposed export restrictions on REE between
2010 and 2014, resulting in dramatic increases in REE prices during those years.10 These high
prices led to increased global exploration for REE deposits. The 40 largest exploration projects
indicate over 3,000 million metric tons (Mt) of inferred resources (at various grades) in more than
15 countries.11
In addition to various REE exploration projects globally, there were 10 active REE mining
operations in 2017 (Table 1).
Table 1. Active REE Mines in 2017
Deposit Name
Location
Resource (Mt)
Grade (REO, %)
Mount Weld
Australia
23.9
7.9
Buena Norte
Brazil
na
na
Bayan Obo
China
800
6
Daluxiang (Dalucao)
China
15.2
5
Maoniuping
China
50.2
2.89
South China clay deposits China
na
0.05 – 0.4
Weishan
China
na
na
Karnasurt Mountain
Russia
na
na
Mountain Pass
United States
16.7
7.98
Dong Pao
Vietnam
na
na
Source: Bradley S. Van Gosen, Philip L. Verplanck, Robert R. Seal II, Keith R. Long, and Joseph Gambogi, Critical
Mineral Resources of the United States—Economic and Environmental Geology and Prospects for Future Supply, ed.
Klaus J. Schulz, John H. DeYoung, Jr., Robert R. Seal II, Dwight C. Bradley (Reston, VA: U.S. Geological Survey,
2017), Table O3, p. O12, https://doi.org/10.3133/pp1802O.
8 Uranium and thorium may be considered “source material” by the U.S. Nuclear Regulatory Commission (NRC),
which states, “Source material is licensed and regulated to ensure that the material is used for safe, commercial uses
and is not used by adversaries.” Processing of these ores requires a source material license, adding to the production
costs of the REE (NRC, “Source Material,” https://www.nrc.gov/materials/srcmaterial.html).
9 Bradley S. Van Gosen, Philip L. Verplanck, and Poul Emsbo, Rare Earth Element Mineral Deposits in the United
States, U.S. Geological Survey, Circular 1454, 2019, p. 4, https://doi.org/10.3133/cir1454.
10 For background on this REE supply interruption, see CRS In Focus IF11259, Trade Dispute with China and Rare
Earth Elements, by Wayne M. Morrison.
11 Bradley S. Van Gosen, Philip L. Verplanck, Robert R. Seal II, Keith R. Long, and Joseph Gambogi, Critical Mineral
Resources of the United States—Economic and Environmental Geology and Prospects for Future Supply, ed. Klaus J.
Schulz, John H. DeYoung, Jr., Robert R. Seal II, Dwight C. Bradley (Reston, VA: U.S. Geological Survey, 2017),
Table O4, pp. O14-O15, https://doi.org/10.3133/pp1802O.
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Notes: Ordered by Location and Deposit Name; Mt: mil ion metric tons; REO: rare earth oxide; na: not
available. Reported resource information may be out of date for some deposits.
The USGS estimates that in 2019 China produced 132,000 metric tons of REO and the United
States produced 26,000 metric tons (REO equivalent) of ores and compounds; the estimated
global total REO production was 210,000 metric tons.12 In 2019, estimates of REO equivalent
production in Australia and Myanmar were approximately 22,000 metric tons in each country;
production in India, Madagascar, and Russia was estimated to be 3,000 metric tons or less for
each country in the same year.13 REO are the commonly traded form of REE to be used in
components and products. REO result from the extraction and refining processes using REE ores
and compounds (or concentrates) as inputs.
Domestic Resources
The global REE supply shock related to Chinese export restrictions between 2010 and 2014
contributed to efforts to identify domestic REE resources. In addition to ongoing private
exploration efforts, the USGS began implementation of the Earth Mapping Resources Initiative
(Earth MRI) in 2019, which “is planned as a partnership between the U.S. Geological Survey
(USGS), the Association of American State Geologists (AASG), and other Federal, State, and
private-sector organizations.”14 The first steps of Earth MRI are to:
forge cost-shared cooperative agreements between the USGS and State geological surveys;
establish contracts with private industry to conduct geophysical and lidar surveys; offer
partnership opportunities for collecting lidar data; and complete a preliminary national-
scale data inventory of geologic framework and minerals data.15
Some U.S. deposits that contain or may contain economically recoverable quantities of REE are
indicated below, followed by a list of additional locations where recoverable quantities of REE
may be found.
Mountain Pass, CA
In a 2019 report the USGS states that the Mountain Pass deposit “produced most of the REEs
mined in the United States since the late 1960s and contains proven and probable reserves
totaling 18.4 million metric tons of carbonatite ore averaging 7.98 percent rare earth oxide (REO)
using a cutoff grade of 5 percent REO.”16 The Mountain Pass mine produced all of the
domestically mined REE in 2019, which was exported for processing.17 One report indicates that
MP Materials, the company operating the Mountain Pass mine, is planning to install the necessary
equipment to process its rare earth concentrate into REO; it reportedly exports its rare earth
12 U.S. Geological Survey, Mineral Commodity Summaries, 2020, p. 133.
13 U.S. Geological Survey, Mineral Commodity Summaries, 2020, p. 134.
14 Warren C. Day, The Earth Mapping Resources Initiative (Earth MRI): Mapping the Nation’s Critical Mineral
Resources, U.S. Geological Survey, Fact Sheet 2019–3007, 2019, p. 1, https://doi.org/10.3133/fs20193007.
15 Ibid.
16 Bradley S. Van Gosen, Philip L. Verplanck, and Poul Emsbo, Rare Earth Element Mineral Deposits in the United
States, U.S. Geological Survey, Circular 1454, 2019, p. 9, https://doi.org/10.3133/cir1454.
17 U.S. Geological Survey, Mineral Commodity Summaries, 2020, p. 133.
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concentrate to Asia for processing.18 MP Materials indicates that its facilities consist “of fully
integrated and co-located mining and processing capabilities.”19
Bokan Mountain, AK
The Bokan Mountain deposit is located on federal lands in southern Alaska on Prince of Wales
Island. The surface land is managed by the U.S. Forest Service (FS), and the subsurface mineral
estate is managed by the Bureau of Land Management (BLM). Bokan Mountain lies within the
Tongass National Forest and is the site of the closed Ross Adams uranium mine; uranium, in
varying concentrations, is found in and around areas of the REE deposit.
Ucore, a Canadian company, has acquired 512 mining claims covering almost 15 square miles of
the mountain.20 Ucore filed a proposed plan of operations with FS in 2012,21 which includes a
proposed underground mining operation that aims to produce 375 metric tons of REE concentrate
per day, for a mine life of 11 years, from an estimated resource of 5.2 Mt.22 Ucore’s initial
exploration of the mineral resource indicates a LREE grade of 0.394% and a HREE grade of
0.259%, which it states is the highest grade of any HREE deposit in the United States.23 After
additional analysis, Ucore updated its LREE and HREE resource findings, indicating that six
additional critical minerals may also be recoverable from its planned mining operation (including
beryllium, hafnium, niobium, titanium, vanadium and zirconium).24 Ucore states that it is focused
on commercializing its proprietary solvent extraction technology, which it would employ at a
location in Ketchikan, AK, near the Bokan Mountain mine.25
Bear Lodge, WY
The Bear Lodge deposit is located near the town of Sundance, WY, on federal land managed by
the FS. Rare Element Resources Ltd. (RER) owns the interests in 499 mining claims and 640
acres that it intends to develop into the Bear Lodge Critical Rare Earth Project.26 The Bear Lodge
deposit is estimated to contain 18 Mt of ore at a grade of 3.05% REO, representing approximately
549,000 tons of REO over the expected 38-year life of the mine.27 In 2016, FS suspended RER’s
application process at RER’s request, which was in the process of preparing a Draft
18 NS Energy, “Mountain Pass Rare Earth Mine,” https://www.nsenergybusiness.com/projects/mountain-pass-rare-
earth-mine/.
19 MP Materials, “Our Facility,” https://www.mpmaterials.com/what-we-do/#our-facility.
20 Tetra Tech, Preliminary Economic Assessment Bokan Mountain Rare Earth Element Project, Document No.
1196000100-REP-R0001-02, 2013, p. 4-4, https://www.ucore.com/s/NI-43-101-Preliminary-Economic-Assessment-
PEA-Technical-Report-for-Bokan-Mountain-Heavy-REE-Project.
21 U.S. Forest Service, Environmental Assessment, Ucore Bokan Mountain Mining Plan of Operations, June 2013,
https://www.fs.usda.gov/nfs/11558/www/nepa/85247_FSPLT3_1447554.pdf.
22 Ibid., p. 1-2.
23 Ibid., p. 1-5, and Ucore, “Home,” https://www.ucore.com/.
24 Newsfile, “Ucore Increases Bokan Mineral Resource with Critical Co-Products,” October 15, 2019,
https://www.newsfilecorp.com/release/48774/Ucore-Increases-Bokan-Mineral-Resource-with-Critical-CoProducts.
25 Ibid. and Ucore, “Home,” https://www.ucore.com/.
26 Rare Element Resources Ltd. (RER), “Rare Element Resources Ltd.,” https://www.rareelementresources.com/
company.
27 Roche Engineering, Inc., Bear Lodge Project Canadian NI 43-101 Prefeasibility Study Report on the Mineral
Reserves and Development of Bull Hill Mine, Wyoming, 2014, p. 1-2, https://www.rareelementresources.com/bear-
lodge-project/project-related-studies-reports/2015/08/24/ni-43-101-technical-report-on-positive-pre-feasibility-results-
for-bear-lodge-project.
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Environmental Impact Statement (DEIS).28 RER would use crushing, screening, and gravity
separation at the mine site, and then truck the upgraded REE material to its planned processing
plant in Upton, WY, for extraction and separation into REO using its proprietary technology.29
Round Top, TX
The Round Top deposit is located on state lands in Hudspeth County, TX. The Texas Mineral
Resources Corporation (TMRC) holds state leases on 950 acres encompassing the deposit.30
Exploration of the Round Top deposit indicates combined measured and indicated resources to
include 304,000 metric tons of recoverable REO; other elements in the deposit that might be
recovered include niobium, hafnium, tantalum, tin, uranium and thorium.31 TMRC is pursuing an
initial 20-year mining and on-site extraction operation that aims to produce 2,212 metric tons per
year of REO (70% HREE) and 8,956 metric tons per year of lithium carbonate, among other
products.32
Elk Creek, NE
The Elk Creek deposit is located in Johnson County, NE, under privately owned land. NioCorp
has retained 100% of the mineral and/or surface rights to 4,038 acres of the deposit.33 The Elk
Creek deposit contains the REE scandium, and the critical minerals niobium and titanium, among
other elements. If sufficient funds become available, NioCorp plans to pursue mining and
extraction of these three elements over an estimated mine life of 36 years, estimating an average
annual production of 95 metric tons of scandium trioxide; 7,220 metric tons of ferroniobium; and
11,642 metric tons of titanium dioxide.34
Other Domestic Deposits
The USGS indicates other deposits or regions that could contain economical quantities of REE,35
including
Iron ore mining tailings piles in Mineville, NY, estimated to contain 9 million
tons of REE tailings;
28 U.S. Forest Service, “Bear Lodge Project—Rare Earth Mine—Suspended,” press release, January 22, 2016,
https://www.fs.usda.gov/project/?project=37875.
29 RER, “Proposed Operations,” https://www.rareelementresources.com/bear-lodge-project/proposed-operations.
30 Texas Mineral Resources Corporation (TMRC), “Rare Earths—Round Top,” http://tmrcorp.com/projects/rare_earths/
. TMRC and USA Rare Earth, LLC, entered into a joint venture in November 2018 (TMRC, “TMRC Secures
Development and Funding Partner for Round Top Rare Earth Project,” usarareearth.com/wp-
content/uploads/2019/08/TMRC_USA-Rare-Earth-Press-Release-Nov-20-2018.pdf).
31 Gustavson Associates, Amended NI 43-101 Preliminary Economic Assessment Round Top Project Sierra Blanca,
Texas, 2014, p. 13, http://tmrcorp.com/_resources/reports/Amended_TRER_NI43-101_PEA_FINAL_28April2014.pdf.
32 TMRC and USA Rare Earth, “Texas Mineral Resources and USA Rare Earth Report Significantly Upgraded
Resource and Confirm Prior Potential Economics in Updated Round Top Preliminary Economic Assessment,”
http://usarareearth.com/wp-content/uploads/2019/08/2019-PEA-Draft-Press-Release-FINAL.pdf.
33 Nordmin Engineering Ltd., NI 43-101 Technical Report Feasibility Study, Elk Creek Superalloy Materials Project,
Nebraska, Project # 18000-01, 2019, pp. 31-32, http://www.niocorp.com/wp-content/uploads/180001_FINAL_43-
101_FS_NioCorp_AS_FILED.pdf.
34 NioCorp., “Overview of the Elk Creek Project,” http://www.niocorp.com/elk-creek-project/.
35 Bradley S. Van Gosen, Philip L. Verplanck, and Poul Emsbo, Rare Earth Element Mineral Deposits in the United
States, U.S. Geological Survey, Circular 1454, 2019, pp. 11-12, https://doi.org/10.3133/cir1454. For more examples,
see Table 1 in Jane M. Hammarstrom and Connie L. Dicken, Focus Areas for Data Acquisition for Potential Domestic
Sources of Critical Minerals—Rare Earth Elements, U.S. Geological Survey, Open-File Report 2019–1023, 2019, p. 3.
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Phosphorite deposits (belonging to the Upper Ordovician horizon) in seven
states, including one measuring 2,000 square kilometers in Arkansas, estimated
to contain 1.3 million metric tons of REE;
The Lemhi Pass district along the Montana-Idaho border; and
The Wet Mountains mineral district in Colorado.
Improving Extraction and Recycling to Increase Supply
In addition to new deposits of REE, changes to existing extraction and recycling technologies
could increase the secondary supply of REE by allowing the use of otherwise unusable sources of
REE. New extraction technologies or lower cost extraction technologies can allow lower-grade
deposits to be mined economically (i.e., lower a mine’s cut-off grade). Such technologies can also
potentially be applied to new sources of REE, such as wastewater or recycled waste.
Extraction Technologies
Typical REE production follows processes common to the production of many metals. After the
ore is mined, it is crushed and milled to a desired size. The milled product may be subjected to a
variety of separation processes, including the use of surfactants, magnetics, and physical forces
(e.g., concentrators, thickeners, cyclones). The concentrated ore is typically subjected to solvent
extraction, commonly through acid leaching. Solvent extraction is the most common technology
employed to produce REO, at various concentrations. One source states that 100% of current
commercial separation is through solvent extraction.36
While solvent extraction is well understood, variations in mineralogy or chemical composition
can result in an uneconomical process. If separation is not economically viable, the REE will be
discarded as waste. The USGS notes:
Unlike many traditional metal commodities hosted in relatively simple minerals (as
examples, copper in chalcopyrite [CuFeS] or lead in galena [PbS]), most of the REEs are
hosted by minerals that have complex chemical formulas, presenting more of a challenge
to process and extract the REEs.37
One example of a low-grade REE deposit that is economically viable due in part to low-cost
separation is the South China clay deposit. According to the USGS,
Although these clay deposits typically contain modest REE concentrations (approximately
0.03 to 0.2 percent REEs), they have become economically viable deposits, because (1)
they are referentially enriched in the high-value HREEs; (2) the REEs are easily extracted
from the clays with weak acids; and (3) mining costs are low.38
In addition to exploring and developing REE deposits, some REE mining companies are
researching extraction technologies. Innovation Metals Corp. (IMC), a wholly owned subsidiary
of Ucore (the company pursing development of the Bokan Mountain deposit), developed an
enhanced solvent extraction technology, RapidSX, which it expects to provide significant
technical and economic efficiency for producing commercial-grade REO.39 To help fund the
36 Ucore, “RapidSX,” https://www.ucore.com/rapidsx.
37 Bradley S. Van Gosen, Philip L. Verplanck, and Poul Emsbo, Rare Earth Element Mineral Deposits in the United
States, U.S. Geological Survey, Circular 1454, 2019, p. 13, https://doi.org/10.3133/cir1454.
38 Ibid.
39 Newsfile, “Ucore Announces Technical Services Agreement with Innovation Metals Corp. for RapidSX(TM) Rare
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development of RapidSX, IMC received $1.8 million in assistance from the U.S. Department of
Defense’s (DOD’s) Army Research Laboratory.40
Ongoing research, supported by federal and private funds, is testing alternatives to solvent
extraction. A recent study notes that research on REE extraction from secondary resources in
aqueous solutions commonly focuses on “several techniques, including ion exchange, bio-
sorption, adsorption, solvent extraction, and precipitation.”41 In addition to critiquing these
techniques, the study proposes electrodialysis as an alternative to solvent extraction, and reports
successful application of the technology to the separation of scandium in a laboratory setting.
However, successful application in the laboratory does not ensure economic viability for the
process for an actual REE deposit.
New technologies or new applications of existing technologies could allow otherwise
uneconomical REE mineral formations or REE products to become economically viable. Coal,
which can contain REE in concentrations similar to other deposits,42 could potentially serve as a
source of REE if extraction technologies prove to be viable. Coal products, such as fly ash from
combusted coal or acid mine drainage from coal mines, could potentially become viable sources
of REE if extraction technologies can be found to economically separate and concentrate the REE
in these sources.43
Some ongoing federally funded research into the use of coal and coal products as sources of REE
focuses on creating economically viable technologies to extract the REE from these sources. One
example is the Feasibility of Recovering Rare Earth Elements program at the National Energy
Technology Laboratory (NETL), whose objectives include “Recover REEs from coal and coal
by-product streams,” and “Advance existing and/or develop new, second-generation or
transformational extraction and separation technologies.”44
Recycling Technologies
Recycling products that contain REE represents a potential source of REE. As the number of
products containing REE increases, the amount of REE potentially available from recycled
products increases. However, recycling such products also poses separation and extraction
challenges. For example, two common REE components (or powders) used in numerous products
are magnets and phosphors. REE magnets are commonly used as components in hard disk drives
(HDD), and REE phosphors are commonly used as fluorescent powders inside fluorescent light
bulbs (FB). While HDD and FB are known to contain REE, they will not be available for
recycling until the end of the products’ lives, potentially resulting in unpredictable supplies of
recyclable material. If the products enter the recycled waste stream, the REE components must be
Earth Element Separation Technology Testing,” February 14, 2020, https://www.newsfilecorp.com/release/52447/
Ucore-Announces-Technical-Services-Agreement-with-Innovation-Metals-Corp.-for-RapidSXTM-Rare-Earth-
Element-Separation-Technology-Testing.
40 Ibid.
41 Changbai Li, Deepika L. Ramasamy, and Mika Sillanpää, et al., “Separation and concentration of rare earth elements
from wastewater using electrodialysis technology,” Separation and Purification Technology, vol. 254 (2021), pp. 1-7.
42 Vladimir V. Seredin, Shifeng Dai, and Yuzhuang Sun, et al., “Coal Deposits as Promising Sources of Rare Metals for
Alternative Power and Energy-Efficient Technologies,” Applied Geochemistry, vol. 31 (2013), pp. 1-11.
43 Clint Scott and Allan Kolker, Rare Earth Elements in Coal and Coal Fly Ash, U.S. Geological Survey, Fact Sheet
2019–3048, 2019, pp. 1-4, https://doi.org/10.3133/fs20193048.
44 NETL, “REE-CM Program,” https://www.netl.doe.gov/coal/rare-earth-elements/program-overview/background.
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able to be extracted from the products and processed economically. Research into these and other
challenges is ongoing.45
Demand for Rare Earth Elements
REE are used in many components in many products; Figure 1 provides examples of sectors and
uses of REE globally and domestically. Examples of components and products that contain REE
include catalysts (for use in oil refineries and automobiles), permanent magnets (for use in cell
phones, wind turbines, and electric vehicle motors), fiber optics (for use in signal amplifiers and
lasers), and lighting/displays (for use in fluorescent lights, cell phone screens, and computer
displays). Although the United States exported some REE concentrate in 2019, it imported 100%
of REE metals and compounds.46 According to the USGS, “The estimated value of rare-earth
compounds and metals imported by the United States in 2019 was $170 million.”47
Figure 1. Global Demand and Domestic Consumption of REE
Source: National Energy Technology Laboratory (NETL), “REE-CM Program,” https://www.netl.doe.gov/coal/
rare-earth-elements/program-overview/background.
45 For an example of research on recycling REE in fluorescent powders used in lighting, see Ajay B. Patil, Mohamed
Tarik, and Rudolf P.W.J. Struis, et al., “Exploiting End-of-Life Lamps Fluorescent Powder E-waste as a Secondary
Resource for Critical Rare Earth Metals,” Resources, Conservation, and Recycling, vol. 164 (2021), pp. 1-8. For an
example of research on recycling REE in HDD magnets, see S. T. Abrahami, Y. Xiao, and Y. Yang, “Rare-Earth
Elements Recovery from Post-Consumer Hard-Disc Drives,” Mineral Processing and Extractive Metallurgy, vol. 124,
no. 2 (2015), pp. 106-115. For a life-cycle analysis of HDD, including technology options that do not require separation
of REE from host materials, see Hongyue Jin, Kali Frost, and Ines Sousa, et al., “Life Cycle Assessment of Emerging
Technologies on Value Recovery from Hard Disk Drives,” Resources, Conservation, and Recycling, vol. 157 (2020),
pp. 1-13.
46 U.S. Geological Survey, Mineral Commodity Summaries, 2020, pp. 132-133.
47 U.S. Geological Survey, Mineral Commodity Summaries, 2020, p. 132.
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Notes: The figure provides examples of uses and sectors. Source and year of data is not given in original. Light-
blue shaded elements are LREE; dark-blue shaded elements are HREE; green underscores denote “critical REE,”
as defined by NETL.
Domestic Consumption by Sector
Tracking domestic consumption and demand for REE poses challenges, due in part to the many
components and products containing REE. For example, if a foreign-manufactured cell phone is
imported, the REE contained in it (e.g., phosphors in the display, magnets in the speaker and
motor) will not be included in domestic demand figures for REE. However, if such REE-
containing components were manufactured in the United States and exported for inclusion in the
cell phone (independent of its final sale location), the REE would be included as domestic
consumption in the appropriate sector.48
Figure 2 shows, by sector, relative domestic REE consumption between 2015 and 2019 (the
values for 2019 are estimates). Domestic REE use in catalysts consumed more than 50% of the
total for each of the five years shown. Consumption in the other four sectors ranged from 5% to
15%.
Figure 2. Relative Domestic Consumption of REE by Sector
Source: U.S. Geological Survey, Mineral Commodity Summaries, 2020, p. 132.
Notes: Excludes scandium. Values for 2019 are estimates.
48 According to the Bureau of Economic Analysis (BEA), in the U.S. Department of Commerce, “The end-use
classification system for goods ... is based on the principal use rather than the physical characteristics of the
merchandise” (Concepts and Methods of the U.S. National Income and Product Accounts, 2017, p. 8-7, available at
https://www.bea.gov/sites/default/files/methodologies/nipa-handbook-all-chapters.pdf.). For more information on how
commodities and products are classified for production, consumption, and trade, see the North American Industry
Classification System (Executive Office of the President, Office of Management and Budget, 2017, available at
https://www.census.gov/eos/www/naics/2017NAICS/2017_NAICS_Manual.pdf).
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The trends shown in the figure can be interpreted differently. One view may be that the
distribution of use of REE among these sectors is relatively stable, with the greatest change
stemming from estimated data in 2019. Another view may highlight that consumption within a
sector varies considerably, as the non-catalyst series frequently change ± 5% of the total from one
year to the next (and the variation in the catalyst series is even greater). Excluding the estimated
catalyst value for 2019, none of the series appears to have definitive positive or negative trends.
Demand for components and products containing REE can change for numerous reasons, some of
which include large-scale changes in a given sector (e.g., increased demand for more electricity
produced from wind turbines containing permanent magnets made with REE), changes in product
technology (e.g., introduction of LED lights, which use less REE material than fluorescent lights),
and price-induced substitution (e.g., in permanent magnets, where the higher cost of one REE
results in the use of a lower-cost alternative, with an acceptable lower level of performance).
REE Prices
For commodities traded in markets, changes reflecting demand preferences, quantities demanded,
and availability of supply are generally captured in movements in prices. Prices among different
REOs vary considerably: in 2018 the price per kilogram of terbium oxide was $455, while the
prices per kilogram for cerium and lanthanum oxides were $2.49 To highlight recent price trends
in REOs, Figure 3 shows price indices for seven REOs between 2015 and 2019 (the price data
underlying the 2019 values are estimates). The base year of the index is 2015; the index values
are equivalent to percentage values. This presentation of the data highlights relative changes in
each series. The higher relative values for 2015 capture the then-ongoing adjustments to the
elimination of export restrictions that were still in place in 2014 in REO markets. The figure
shows that relative prices continued to fall as China increased quantities of REO available for
export.50
The price indices highlight what some may consider a return to a more stable REO market: from
2016 through 2019 prices fluctuated, but the changes do not appear to be market disrupting.
These data could reflect greater supply of REE meeting growing demand; or the high demand for
some REE during the supply disruptions may have led manufacturers to incorporate alternatives,
thus freeing REE to be used by others.
49 U.S. Geological Survey, Mineral Commodity Summaries, 2020, p. 132.
50 For more information on the history of global REE prices, see Viviana Fernandez, Resources Policy, vol. 53 (2017),
pp. 26-45.
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Figure 3. Relative REO Price Changes
Source: CRS calculations using nominal price data from U.S. Geological Survey, Mineral Commodity Summaries,
2020, p. 132.
Notes: Cerium and Lanthanum oxide values are identical. See source for purity grade of each category.
Mischmetal is an alloy of REE. Values for 2019 are estimates.
Policy Topics and Legislative Activity
Legislation has been introduced in the 116th Congress to address concerns related to the supply
and demand of REE. The House and Senate have each held at least one hearing on critical
minerals, including discussion of REE during the second session of this Congress.51 This section
discusses selected bills and policy options related to some of these bills, focusing on those that
would have a direct impact on REE. This section does not discuss bills indirectly impacting REE
through broad changes to large sectors (e.g., bills altering the proportion of electricity supplied
from renewable resources, which often employ REE components). Some discussion of critical
minerals is included, as the current definition of critical minerals includes REE. Congress has
debated whether to increase exploration and development of critical mineral resources, including
REE, on federal lands, but this report does not discuss that issue in detail.52 This section does not
consider uses of or increases in authorizations to the Defense Production Act, which can be used
51 Two hearings include U.S. Congress, House Committee on Science, Space, and Technology, Subcommittee on
Energy, Research and Innovation to Address the Critical Materials Challenge, 116th Cong., 2nd sess., December 10,
2019, and U.S. Congress, Senate Committee on Energy and Natural Resources, Full Committee Hearing on the Impact
of COVID-19 on Mineral Supply Chains, 116th Cong., 2nd sess., June 24, 2020.
52 For more information on these topics, including legislation proposed in the 116th Congress, see CRS Report R46278,
Policy Topics and Background Related to Mining on Federal Lands, by Brandon S. Tracy.
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to fund REE-related activities,53 nor does it consider authorizations for the National Defense
Stockpile to acquire certain REE compounds.54 Some additional legislative options are discussed.
In addition to the stand-alone bills discussed in this section, two comprehensive energy and
minerals bills in the 116th Congress address U.S. REE supply. S. 2657, the American Energy
Innovation Act of 2020,55 was considered by the Senate starting March 5, 2020. On March 9, a
cloture motion on the bill did not pass on a 15-73 vote. The bill incorporates language from
several energy and mineral bills reported by the Senate Committee on Energy and Natural
Resources. H.R. 4447, the Clean Economy Jobs and Innovation Act, was passed by the House on
September 24, 2020. The bill incorporates language from several energy and mineral bills
introduced and/or reported by various House committees. Among other provisions, S. 2657 and
H.R. 4447 would authorize programs related to the domestic supply of REE, discussed below.
REE from Coal and Coal Byproducts
The United States contains vast coal resources and coal waste associated with decades of coal
mining and combustion. Coal contains some amounts of various REE, coal and coal byproducts
represent a potentially economical source of REE. S. 2657 and H.R. 4447 would authorize the
U.S. Department of Energy (DOE) to develop technologies for the extraction of REE from coal
and coal byproducts. These bills would formally authorize a program, part of the Clean Coal and
Carbon Management program in the National Energy Technology Laboratory, which was
appropriated funds in FY2014 for similar purposes.56 The Senate bill would authorize
appropriations of $23 million per year from FY2021 through FY2027 to fund this program, while
the House bill would authorize appropriations of approximately $25 million per year from
FY2021 through FY2025. These bills include similar or identical text from S. 1052, S. 1317, S.
4324, S. 4775, and H.R. 3607.57
Policymakers could consider different policy options to address the supply of domestic REE
and/or domestic coal/coal byproducts’ production and use. Finding additional uses for domestic
coal appeals to some, as falling demand for coal as a fuel source has had negative economic
impacts on some coal mining-dependent communities; spurring demand for coal through REE
could help these affected communities. Additionally, producing REE from acid mine drainage
could potentially reduce the financial burden of mitigating the environmental damage associated
with such drainage. Some argue that additional uses of coal may result in more coal being
combusted, resulting in the release of additional carbon dioxide (i.e., greenhouse gas) emissions
53 The Urban Mining Company, which recycles REE magnets into new REE magnets, indicates that it has been
awarded $28.8 million from the U.S. Department of Defense, through a Defense Production Act, Title III program
(Colin Staub, “Rare Earth Recycler Draws $28 Million in Federal Funding,” E-Scrap News, September 11, 2020,
https://resource-recycling.com/e-scrap/2020/09/11/rare-earth-recycler-draws-28-million-in-federal-funding). For more
information on the Defense Production Act, see CRS Report R43767, The Defense Production Act of 1950: History,
Authorities, and Considerations for Congress, by Michael H. Cecire and Heidi M. Peters.
54 S. 1790, the National Defense Authorization Act for Fiscal Year 2020, authorizes the use of up to $37,420,000 for
the acquisition of five materials, including cerium and lanthanum compounds.
55 The American Energy Innovation Act is a substitute amendment to S. 2657, the Advanced Geothermal Innovation
Leadership (AGILE) Act of 2019, as reported by the committee. For more information on S. 2657, see CRS Report
R46372, Summary and Analysis of S. 2657, the American Energy Innovation Act, coordinated by Brent D. Yacobucci.
56 For background on the Senate bill, see S.Rept. 116-74, available at https://www.congress.gov/116/crpt/srpt74/CRPT-
116srpt74.pdf.
57 S. 1052, Rare Earth Element Advanced Coal Technologies Act; S. 1317, American Mineral Security Act; S. 4324,
Restoring Critical Supply Chains and Intellectual Property Act; S. 4775, Delivering Immediate Relief to America’s
Families, Schools, and Small Businesses Act; and H.R. 3607, Fossil Energy Research and Development Act of 2019.
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and other pollutants. Policymakers could consider funding non-coal research options related to
exploiting higher quality deposits or other sources of REE. Others question the use of federal
funds for research on activities that may have benefits limited to a few private entities.
Additional Critical Mineral and REE Research
Funding research to lower the costs of extracting REE from minerals and REE-containing
components is a potential means of increasing domestic REE supply. S. 2657 and H.R. 4447
would authorize appropriations for and direct the DOE to promote the efficient production, use,
and recycling of critical minerals throughout the supply chain. The proposed research program to
achieve these goals is similar to those articulated for the Critical Materials Institute (CMI), which
DOE established in FY2013 as one of its Energy Innovation Hubs. DOE did not request
appropriations for CMI in FY2019, FY2020, or FY2021, stating “prior year appropriations will
be used to conduct an orderly wind-down and termination of the existing institutes.”58 Congress
appropriated $167 million in FY2019 and $199 million in FY2020 for Energy Efficiency and
Renewable Energy-Research and Development (EERE-R&D) Consortia activities, which include
those of CMI.59 The House and Senate bills include similar or identical text from S. 1317, S.
4324, S. 4775, and H.R. 7061. H.R. 4447 would authorize appropriations of approximately $135
million per fiscal year from FY2021 through FY2025. S. 2657 would authorize appropriations of
$50 million per fiscal year from FY2021 through FY2029 for these and related activities; a
specific amount is not indicated for the critical minerals research program.
H.R. 4447 also contains provisions that would direct DOE to create a research program related to
recycling critical minerals from energy storage systems. Three other bills, S. 3694, S. 4537, and
H.R. 7812, would direct DOD to establish a grant program to encourage the domestic
development of critical minerals, among other provisions.60 These bills would authorize
appropriations of $50 million per year for four years.
Additional legislative options exist to complement some provisions in these introduced bills. As
research continues to investigate how recycled REE products and components may be introduced
into the REE supply chain, one type of option to reduce the risks of unstable supplies of recycled
materials could include a federal program encouraging recycling of REE products. A variety of
recycling options and programs exist throughout the domestic economy, including private sector
recycling companies and ordinances requiring separation of household recyclables. Another
example includes state-level beverage container deposit programs (sometimes called “bottle
bills”), which could serve as a model for recycling REE from discarded consumer products. The
National Conference of State Legislatures describes these programs in the following manner.
When a retailer buys beverages from a distributor, a deposit is paid to the distributor for
each container purchased. The consumer pays the deposit to the retailer when buying the
beverage, and receives a refund when the empty container is returned to a supermarket or
other redemption center. The distributor then reimburses the retailer or redemption center
the deposit amount for each container, plus an additional handling fee in most states.
58 U.S. Department of Energy (DOE), FY2019 Congressional Budget Justification, Volume 3 Part 2, 2018, p. 176,
DOE, Congressional Budget Request, Volume 3 Part 2, 2019, p. 166, and DOE, FY2021 Congressional Budget
Request, Volume 2, 2020, p. 250.
59 DOE, FY2021 Congressional Budget Request, Volume 2, 2020, p. 251.
60 S. 3694, ORE Act; S. 4537, RECOVERY Act; and H.R. 7812, ORE Act.
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Unredeemed deposits are either returned to the state, retained by distributors, or used for
program administration.61
Such programs could be initiated at a state or federal level. Programs implemented at the state
level could potentially benefit from tailoring programs to consumer behavior at a more granular
scale. A federal program might have an advantage in coordinating a limited number of large
manufacturers of REE products and an expected limited number of large REE extraction facilities
using recycled products as inputs. Authorizing and funding such a program at the federal level
could potentially mitigate problems experienced by some state container deposit programs,
including payments for products whose sales pre-dated deposit assessments. A deposit and
recycling program might require consideration of implementation aspects, such as collection and
shipping of products containing REEs, handling of potentially toxic or unsafe products, and
returning recycled REEs into commerce.
Tax Incentives for the Supply and Consumption of Domestic REE
Reducing taxes assessed on commercial activities is a potential means of stimulating investment.
Investments in sectors with high initial costs, such as mines and ore refining operations, may be
accelerated if tax incentives allow for earlier expected profitability. Among other provisions, S.
3694, S. 4537, H.R. 7812, and H.R. 8143 would provide tax incentives for domestic production
and purchase of certain domestic critical minerals (defined in the legislation). The incentives
would include full (100%) expensing (immediate deduction from taxable income) for qualified
property involved in extracting critical minerals and metals from deposits in the United States; a
special allowance (100% depreciation deduction) for nonresidential real property; and a cost
deduction (200%) for the purchase of critical minerals and metals extracted within the United
States.
Such tax incentives could potentially reduce net operating costs of firms producing and
purchasing domestic REE minerals and products. As the Congressional Budget Office has not
scored these bills, the potential impacts on federal revenue and the economy are not available.
The legislation is silent on responding to changes in the supply and demand of critical minerals.
Additional legislative options exist to complement some provisions in these introduced bills. One
option would include the imposition of a federal excise tax on some REE products. The excise tax
or taxes could be applied generally to REE products from imported sources, or the tax could
target products containing REE deemed critical for other uses. Congress could try to control the
market-distorting effects of such taxes by varying the tax rate. For example, if the supply of
erbium, which is commonly used in fiber optic amplifiers, becomes scarce, an excise tax could
target uses of erbium in products other than fiber optic amplifiers. The increased costs would be
expected to either reduce erbium demand in products other than fiber optic amplifiers or provide
revenue that could be directed to the fiber optics sector to procure erbium or erbium substitutes
from more expensive sources. Another option for the use of the revenue from such excise taxes
includes offsetting the tax incentives included in these introduced bills. Managing excise taxes on
multiple products or REE could prove challenging, especially on internationally traded
commodities subject to supply and demand volatility.
61 National Conference of State Legislatures, “State Beverage Container Deposit Laws,” March 13, 2020,
https://www.ncsl.org/research/environment-and-natural-resources/state-beverage-container-laws.aspx.
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REE Cooperative and Corporation
Among other provisions, S. 2093 and H.R. 4410 would establish a federally chartered cooperative
to process domestic and international sources of REE ores and materials containing thorium into
products for sale. A federally chartered corporation would accept and store all radiological
material (e.g., thorium) produced by the cooperative and sell any valuable materials, and could
conduct research on new uses of such materials. The cooperative and corporation would be
privately funded and operated.
The presence of thorium (or other radiological minerals) in REE deposits can add to overall costs
of production, as the thorium becomes regulated source material during processing.62 A
cooperative/corporation could create a scale of operations that could overcome these additional
costs that may render smaller operations unprofitable. In the absence of federal funding, it is not
clear whether the federally chartered cooperative and corporation would be any more viable than
a similar cooperative and corporation created by private sector entities.
These bills suggest that a federally chartered corporation could eliminate the higher costs a REE
mining operation faces if the ore body contains thorium by taking ownership of thorium materials
produced by cooperative members. It is unclear how a federally chartered cooperative could
reduce the costs associated with the presence of thorium, as thorium materials would be expected
to be regulated at the mine, where physical refining (e.g., crushing, sorting, floatation separation)
can increase thorium concentrations to those regulated as source material. If the cooperative and
corporation were to receive ore directly from the REE mine, they would likely face the higher
costs associated with transportation of the ore and disposal of the tailings.
Author Information
Brandon S. Tracy
Analyst in Energy Policy
Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan
shared staff to congressional committees and Members of Congress. It operates solely at the behest of and
under the direction of Congress. Information in a CRS Report should not be relied upon for purposes other
than public understanding of information that has been provided by CRS to Members of Congress in
connection with CRS’s institutional role. CRS Reports, as a work of the United States Government, are not
subject to copyright protection in the United States. Any CRS Report may be reproduced and distributed in
its entirety without permission from CRS. However, as a CRS Report may include copyrighted images or
material from a third party, you may need to obtain the permission of the copyright holder if you wish to
copy or otherwise use copyrighted material.
62 Uranium and thorium ores may be considered source material by the U.S. Nuclear Regulatory Commission (NRC);
processing of these ores requires a source material license, adding to the production costs of the REE (NRC, “Source
Material,” https://www.nrc.gov/materials/srcmaterial.html).
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..."
“Final List of Critical Minerals 2018” Department of the Interior "2018"
SOURCE: https://www.federalregister.gov/documents/2018/05/18/2018-10667/final-list-of-critical-minerals-2018
https://www.federalregister.gov/documents/2018/05/18/2018-10667/final-list-of-critical-minerals-2018
"... The final list includes:
SOURCE: https://museum.wales/mineralogy-of-wales/minerals-where-and-how-they-occur/ < WALES
"... Minerals occur in a wide range of geological settings. Many are found as the essential constituents of rocks: these are termed rock-forming minerals, while others are found concentrated in mineral deposits. Rocks are divided into three main categories, igneous, metamorphic and sedimentary, and each of these categories is subdivided further using internationally agreed terminology and definitions. Mineral deposits have no such single standard for classification. The Welsh mineral deposits, described in this site, have therefore been grouped into an extensive range of settings to provide a clearer picture of the geological processes that formed them. ..."
Ore Mineral "Rare" "Earth" "Element"
The content of the individual rare-earth elements varies considerably from mineral to mineral and from deposit to deposit.
Of the approximately 160 minerals that are known to contain rare earths, only four are currently mined for their rare earths:
"bastnasite" "mine" [ https://en.wikipedia.org/wiki/Bastn%C3%A4site ] [ Mtn Pass https://www.voanews.com/usa/california-mine-becomes-key-part-push-revive-us-rare-earths-processing ]
https://archive.epa.gov/epawaste/nonhaz/industrial/special/web/pdf/id4-rar.pdf
https://archive.epa.gov/epawaste/nonhaz/industrial/special/web/pdf/id4-rar.pdf "Associate Minerals" Luka Green Cove Springs, FL
https://www.nrc.gov/info-finder/decommissioning/complex/iluka-resources.html
"... The Green Cove Springs, FL-located plant has operated as Associated Minerals, Titanium Enterprises, and RGC Minerals Sands and currently as Iluka Resources, Inc. The plant processed heavily mineralized sands for the purpose of extracting specific products. The products included, but are limited to, ilminite, rutlite, zircon and leucozene. The host sand contained naturally occurring radioactive components (thorium and some uranium) which in finite portions of the process were temporarily increased to >0.05% by weight. During April 2009, production ceased and the decommissioning plan was submitted August 2009. ... The electrostatic and wet process plant was first licensed by the Florida Bureau of Radiation Control for the processing and disposal of technically-enhanced concentrations of naturally-occurring radioactive materials (TENORM) in the 1994. The specific license included specific areas and equipment within the plant where materials were temporarily concentrated. TENORM was found in actionable concentrations only in the product dry mill and its attached conveyer and dryer systems. After the products were removed, the non-product materials were slurried back together (diluted) and pumped back to the mining impoundment area. ... Dismantling, surveying and cleaning (if necessary) and removal of process equipment are in progress. Demolition of all plant structures except a few ancillary buildings will follow. Additional phased activities are planned toward termination and release. ..."
Buried alive - in jars? You can perform a "RARE EARTH ELEMENT SEPARATION EXPERIMENT"
9-8-2021 Experiment performed:
A retired Software Engineering Technical Writer - Susan - approached her kitchen spice rack AND saw a small empty jar - previously containing Tartar - she had saved.
( https://cornershopapp.com/en-us/products/1h6wd-spice-islands-cream-of-tartar-3-oz-96l-arlington-market )
She examined the clean jar - in the morning light.
Using a clean towel - she dusted the jar's exterior and placed the towel's corner inside it (also) - as if she was drying it - after a wash.
Satisfied the jar was clean and dry - she sat it onto a dry coffee filter - she had placed onto a flat clean plate.
She placed a small amount (half teaspoon) of ground coffee into the jar.
She placed 2 drops of dish soap onto the coffee grounds.
Then, she ran cold water into the jar - filling it to half volume. Bubbles appeared! She held the jar to the window and observed: Bubbles on the water surface, Clear water - in the middle - And, coffee grounds in the jar bottom.
She stirred the mixture - with a small metal spoon.
She added more water - very slowly.
Bubbles emerged from the jar's opening.
She skimmed the bubbles - to the side - and they fell down the jar.
As they met the dry clean filter - on which the jar was sitting - they burst.
At the bursting - it appeared each bubble (a "micelle") contained a coffee ground.
[ https://en.wikipedia.org/wiki/Micelle ] [ https://www.youtube.com/watch?v=fmo8HEZrFDI ]
She continued, adding water - only drops at a time - until the water was to the jar's brim - and no more bubbles were present.
She observed, the water was clear - and, some coffee grounds remained in the jar's bottom.
She threw the used coffee grounds into the trash.
She commented - to her husband (observing her) - it appeared the grounds had "seperated" - into various kinds and sizes.
She explained "surfactants" - to her husband. Hans hugged her. She made breakfast > "Oats".
[ end ]
Results: DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD
21 Sc Scandium "transmutation"
39 Y Yttrium "transmutation"
57 La Lanthanum "transmutation" [ https://www.oecd-nea.org/jcms/pl_28292/chernobyl-chapter-ii-the-release-dispersion-deposition-and-behaviour-of-radionuclides ]
58 Ce Cerium "transmutation" [ https://www.oecd-nea.org/jcms/pl_28292/chernobyl-chapter-ii-the-release-dispersion-deposition-and-behaviour-of-radionuclides ]
59 Pr Praseodymium "transmutation"
60 Nd Neodymium "transmutation"
61 Pm Promethium "transmutation"
62 Sm Samarium "transmutation"
63 Eu Europium "transmutation"
64 Gd Gadolinium "transmutation"
1 of 4 https://www.oecd-nea.org/jcms/pl_17120/international-comparison-calculations-for-a-bwr-lattice-with-adjacent-gadolinium-pins-br-c-maeder-p-wydler
2 of 4 https://www.oecd-nea.org/jcms/pl_24450/used-fuel-criticality
3 of 4 https://www.oecd-nea.org/jcms/pl_27105/jeff-report-23-supplement
4 of 4 https://www.oecd-nea.org/jcms/pl_24751/icsbep-participants
65 Tb Terbium "transmutation"
66 Dy Dysprosium "transmutation"
1 of 2 https://www.oecd-nea.org/jcms/pl_20430/irphe-handbook-2019-edition-contents
2 of 2 https://www.oecd-nea.org/jcms/pl_24751/icsbep-participants
67 Ho Holmium "transmutation"
68 Er Erbium "transmutation"
1 of 2 https://www.oecd-nea.org/jcms/pl_20430/irphe-handbook-2019-edition-contents
2 of 2 https://www.oecd-nea.org/jcms/pl_24751/icsbep-participants
69 Tm Thulium "transmutation"
70 Yb Ytterbium "transmutation" [ https://www.oecd-nea.org/jcms/pl_26494/data-bank-e-newsletter-september-2011 ]
71 Lu Lutetium "transmutation"
Surface tension of pure liquid lanthanide and early actinide metals
(Article in Physics and Chemistry of Liquids · January 2012)
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Surface tension of pure liquid lanthanide and early actinide metals
Fathi Aqra a & Ahmed Ayyad a a
Department of Chemistry, Faculty of Science and Technology, Hebron University,
PO Box 40, Hebron, West Bank, Palestine
Version of record first published: 23 Dec 2011. To cite this article:
Fathi Aqra & Ahmed Ayyad (2012): Surface tension of pure liquid lanthanide and early actinide metals, Physics and Chemistry of Liquids: An International Journal, 50:3, 336-345 To link to this article: http://dx.doi.org/10.1080/00319104.2011.561349
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This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Physics and Chemistry of Liquids Vol. 50, No. 3, May–June 2012, 336–345 Surface tension of pure liquid lanthanide and early actinide metals Fathi Aqra and Ahmed Ayyad* Department of Chemistry, Faculty of Science and Technology, Hebron University, PO Box 40, Hebron, West Bank, Palestine (Received 26 December 2010; final version received 4 February 2011)
[ABSTRACT]
A systematic theoretical study of the surface tension of liquid rare earth metals and early actinides is performed. An equation, based on the theoretical considerations - suggested by Eyring, enables one to calculate the surface tension of elementary substances in a wide temperature range from melting to boiling points. The results of temperature-dependent surface tension calculations of a pure liquid terbium (1629–1880 K) are fitted as ¼ 8450.1 (T Tm) (mJ m2 ), where the surface tension decreases linearly with temperature. The surface tension was also calculated, at melting points, for all the liquid rare earth metals from La to Lu and for the first six metals of the actinide series from Ac to Pu. It is observed that the lanthanides may be divided into three groups in accordance with their electronic structure. Mostly, the calculated results agree well with available experimental data.
1. Introduction The surface tensions of liquid metals are of particular scientific and technological importance in analysing and understanding metallurgical processing operations. It is well known that surface tension determines the kinetics of vapor–liquid phase transitions. Information on the temperature dependence of the surface tension is of importance for technological applications, such as the sintering, soldering and phase distributions at high temperatures. Its dependence on temperature leads to the well-known Marangoni convection, which plays a central role in some casting and welding situations. However, liquid metals are difficult to handle due to the high temperatures and their high reactivity because they tend to oxidise which dramatically changes the optical, thermal and mechanical properties of the surface. Therefore, experimental studies of surface tension are very laborious and expensive [1,2], and often suffer from ambiguities in the interpretation of the resulting frequency spectra [3–5]. Inaccuracy of experiments has prevented scientists from fully understanding this phenomena. Information on the experimental values of the surface tension of lanthanides is poor and contradictory, and the data on some metals reported in the literature are different. The understanding of the nature and behaviour of rare earth metals in their liquid phases requires accurate values of their physical properties. However, keeping
*Corresponding author.
Email: ahmedayyad12@hotmail.com ISSN 0031–9104 print/ISSN 1029–0451 online 2012 Taylor & Francis http://dx.doi.org/10.1080/00319104.2011.561349 http://www.tandfonline.com
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samples in their liquid phases free from contamination long enough to carry out measurements represents a formidable challenge. This is due to high reactivity and melts contamination of these elements with crucibles or gaseous environment. Rare earth metals and their compounds are currently used to improve the resistance of certain glasses, to fabricate hydrogen sponges and strong magnets, and as dopants in optical amplifiers [6]. Terbium has found applications as a dopant in materials that are used in solid-state devices (e.g. photovoltaic cells, laser amplifiers) and as a stabiliser of fuel cells which operate at high temperatures. Terbium is also utilised in alloys and in the production of television tubes and fluorescent lamps [6]. To assist further material development, the knowledge of the physical properties of rare earth metals and their temperature dependences is therefore paramount. However, these metals are very reactive, oxidising rapidly when exposed to air and reacting directly with nitrogen and other elements [6]. This explains why accurate physical properties are difficult to measure above their melting points when traditional methods are used (e.g. crucible, support) and why there are no data reported in the under-cooled region. Many attempts have been made to predict the surface tension of liquid metals over the past several decades. Among these methods, computer simulation with Monte Carlo or molecular dynamics methods may be one of the methods [7]. However, relations for calculating the temperature dependences of the surface tension are important for the development of chemical thermodynamics [8], and are of the most promising methods. Many relations were established and valid in the vicinity of melting temperature [9,10]. Thus, an urgent problem is the calculation of the temperature dependences for the surface tension of various substances. This problem can be solved within the framework of Eyring approach, in which a relation was derived that can be used for determining the surface tension of elementary substances within a wide temperature range from their melting to boiling points. This article describes theoretical calculations of the surface tension of pure liquid terbium as a function of temperature, and the calculation of the surface tension, at the melting point, of all liquid lanthanide metals and early liquid actinides. The calculated results are compared with the existing experimental data.
2. Theory and calculations
The theoretical consideration, here, is based on classical statistical thermodynamics formulation of Eyring and coworkers [11–13]. We previously derived an equation for calculating the surface tension of liquid metals [14]. It was successfully applied to pure liquid gallium [14], bismuth [15] and mercury [16], in a wide temperature ranges. In this work, the equation is applied for calculating the surface tension of liquid rare earths and early actinides (f-block metallic elements). The true form of this equation (since f ¼ 0.287 for all metals) is expressed as [14–16]:
¼ ð1ÞðVs=V Þ 2 kT ½0:139Es=RT 0:053 ð1Þ where is the surface tension of pure liquid metal, k the Boltzmann constant (1.381023 J K1 ), T the absolute temperature (K), R the universal gas constant (8.31 J K1 mol1 ), V the molar volume of the liquid metal at any temperature, Vs the molar volume of the solid at the melting point, the area occupied by one atom, Physics and Chemistry of Liquids 337 Downloaded by [Hebron University] at 03:55 04 October 2012 Es the sublimation energy (its value is available in literature). The value of surface tension obtained from the above equation should be multiplied by 107 in order to get the value in mJ m2 . Using Equation (1), it possible to calculate the temperature dependence of the surface tension of any liquid metal provided the experimental temperature dependence density data of the metal is available. The f value (dimensionless constant parameter) expresses the fraction of broken bonds in liquid metals. It is a temperature independent positive fractional number that is fixed by the best fit of the results with the experimental measurements for many metals. It has been determined by taking experimental surface tension values of 10 metals at their melting points and its value was varied until the calculated surface tension matches that of the experimental one. Its average value for all metals is 0.287 and has been shown to be a good value for liquid metals. Usually, we understand that the coordination numbers of liquid metals are about 10–11, while the original crystalline structures are different. Therefore, the estimated f-value is related to the difference of coordination number of the surface atoms compared to atoms in the bulk of the liquid (i.e. it is related to the ratio of the surface to bulk coordination numbers). In general, for liquids, the surface tension decreases with increasing temperature (i.e. negative temperature coefficient d/dT) going to zero at the critical point: the variation often follows a linear function law over a limited temperature range: ðTÞ ¼ ðTmÞ þ d=dT ðT TmÞ ðmJ m2 Þ ð2Þ
3. Results and discussion
The density values, as a function of temperature, of liquid terbium show that it is decreasing and almost a linear function with temperature [17]. Calculations, using the parameters provided (Table 1) and the reported density values of terbium [17], show a decreasing function of the surface tension in the temperature range 1629– 1880 K (Figure 1). The calculated results are accurately described by ¼ 845 – 0.1(TTm) (mJ m2 ), while the reported experimental expression [17] is given by ¼ 893 0.1(TTm) (mJ m2 ). Figure 1 shows terbium surface tension behaviour observed theoretically for a wide temperature interval, and predicts how well the qualitative functional behaviour of the theory and experiment agree. It is observed that there is a similar decrease of the surface tension as a function of temperature both theoretically and experimentally [17] for liquid terbium. On the other hand, the theoretical values of the surface tension differ slightly from the experimental ones, which is within the experimental errors. The calculated and the measured [17] surface excess entropy (d/dT) from the slope is similar (0.1 mJ m2 K1 ). The surface tension of liquid rare earth metals and the early liquid actinides were calculated, at their melting points, and compared with the reported experimental data [18–25] (Table 1). The values obtained in our study compares remarkably well with the mean value published by others. Figure 2 illustrates a comparison of the calculated and the experimentally reported surface tension of liquid rare earth metals, at their melting point, with the electronic structure. It is seen that there is a great similarity between our calculated values and the existing experimental data. Many properties of the lanthanide metals, 338 F. Aqra and A. Ayyad Downloaded by [Hebron University] at 03:55 04 October 2012
Table 1. Calculated and experimental surface tension values of f-block liquid metals at their melting points, and other parameters needed for calculations.
Physics and Chemistry of Liquids 339 Downloaded by [Hebron University] at 03:55 04 October 2012 –2 0 2 4 6 8 10 12 14 16 200 300 400 500 600 700 800 900 1000 g (mJm–2) 4fn Calculated Experiment
Figure 2. Comparison of the calculated and reported surface tension values of liquid rare earth metals, at melting point.
1610 1680 1750 1820 1890 825 840 855 870 885 Calculated Experimental g (mJm–2) T (K)
Figure 1. Comparison of the calculated surface tension (squares) and reported experimental data (circles) [17] of liquid terbium in the temperature range 1629–1880 K.
340 F. Aqra and A. Ayyad Downloaded by [Hebron University] at 03:55 04 October 2012 such as molar volumes, density, thermal coefficient of volume expansion, heat of vaporisation and binding energy, change smoothly along the series, except for Eu and Yb, and occasionally Sm and Tm. The deviations occur with those lanthanides that have the greatest tendency to exist in the 2þ state; presumably these elements tend to donate only two electrons to the conduction band of the metal, thus leaving layer cores and affording lower binding forces. The periodicity of the calculated surface tension changing in the rare earth series, characteristic of the other properties of these metals. Evidently, this is connected with lanthanide compression effect. The dependence of the liquid rare earth surface tension, at the melting point, on the serial number is shown in Figure 2, where the periodic character of the surface tension variation in the rare earth series is seen. This agrees with the conclusion of Eremenko [26], where the strong applicability of a periodic law to the surface properties is shown. The character of the change of the surface tension in the rare earth series, and the dependence of the surface tension on the electronic structure of metals would be explained by the proposition of Shytil [27], who made a correlation between the surface tension and binding energy in metals in terms of Tm/(Vm) 2/3 (where Tm is the melting point and Vm the atomic volume at melting temperature). If the calculated is given as a function of (Tm/(Vm) 2/3) or 4-fn electrons (Figure 3), three straight lines are observed indicating the presence of three groups: First is Sm, Eu, Tm and Yb (with lowest values among the series) that form a straight line with a positive slope; second is Lu alone (highest value among the series) and the third group is La, Ce, 120 160 200 240 280 280 350 420 490 560 630 700 770 840 910 980 g (mJm–2) Tm/V2/3 (Km–2 mol–2/3) Lu Sm Tm Eu Yb Tb Ce La Pm Pr Nd Gd Er Ho Dy
Figure 3. Surface tension at melting point vs. the quantity Tm/(Vm) 2/3 for liquid lanthanide metals.
Physics and Chemistry of Liquids 341 Downloaded by [Hebron University] at 03:55 04 October 2012 Pr, Nd, Pm, Gd, Tb, Dy, Ho, Er, (medium values among the series) that form a straight line with a negative slope. This interesting fact may be explained in terms of the electronic structure of these metals. In case of Lu, the 4f shell is occupied with electronic structure of f14d1 , and so it may be considered as a d metal. Hence, this metal resembles Sc and Y metals, and forms a straight line with a positive slope with them. Sm (f6 d0 ), Eu (f7 d0 ), Tm(f13d0 ) and Yb (f14d0 ), in the metallic state, are divalent leading to anomalous properties in the lanthanide series. Therefore, these four metals have a divalent surface. Their values of Tm, Vs and are low, thus they are shifted to the right with respect to the other metals due to increasing (Tm/(Vm) 2/3). The negative slope of the third group may be also explained by the influence of the lanthanide compression effect. The melting point and density at Tm of every metal in this group is larger than that of the preceding one, while the molar volume is almost smaller than that of the preceding one, and so the surface tension almost decreases and (Tm/(Vm) 2/3) increases with moving from left to right in the series. It is possible to calculate surface excess entropy for all the metals using the correlation dependence by Lang [28]: d=dT ¼ m=ðTc TmÞ ð3Þ Semi-empirical predictions based on the correlation between the surface and bulk thermodynamic properties are always possible [18,29–33]. For example, correlation 0.0 2.0x108 4.0x108 6.0x108 0 100 200 300 400 500 600 700 800 900 1000 gm(mJm–2) Hv/V2/3 ( mJm–2 mol–1/3) Figure 4. Surface tension vs. Hv/V2/3 of lanthanides, from top to bottom (Lu, Tb, Ce, La, Pm, Pr, Er, Gd, Nd, Ho, Dy, Tm, Sm, Eu and Yb). The solid line is the linear least square fit to the data.
Slope ¼ 1.754 106 . 342
F. Aqra and A. Ayyad Downloaded by [Hebron University] at 03:55 04 October 2012 of surface tension to the heat of evaporation [18,29]: m ¼ cHv=ðVmÞ 2=3 ð4Þ where m is the surface tension of liquid metal at melting point (mJ m2 ), Hv the liquid–vapor transition enthalpy at boiling temperature (kJ mol1 ), Vm the atomic volume at melting temperature (m3 mol1 ) and c being an unknown constant. Since there is no suitable theoretical determination of c, this equation seems to apply only to s and d block metals [18,31,34] (c, for these metals, has been determined as 0.174 108 and 0.16 108 mol1/3, respectively). Equation (4) has existed for more than 10 decades, but attempts to theoretically determine c are scarce. Therefore, Figure 4 shows a plot of m versus (Hv/(Vm) 2/3) (J m2 mol1/3) of liquid lanthanides with a linearly regressed slope c of 1.754 106mol1/3, where the correlation coefficient of the fit is 0.999. All rare earth metals can thus be estimated by the same c value, which implies that Tm/Hv is almost a constant. Since the melting entropy is almost a constant for metallic elements, the c value as a constant is reasonable. The surface tension values calculated by our model Equation (1) and this formula Equation (4) are very close (Table 1).
Figure 5 illustrates the temperature coefficient, d/dT, (mJ m2 K1 ) of rare earth liquid metals with the quantity m/Tm (mJ m2 K1 ). In this relation, d/dT increases almost linearly with an increase in m/Tm for the metals in the same period, although some deviations appear. Since the s þ d electrons of most lanthanide elements remain constant, their surface excess entropy values (d/dT ) hardly change. This indicates that the effect of f electrons on the values of this quantity is 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 dg/d T (mJm–2 K–1) gm/Tm(mJm–2 K–1) La Ce Nd Pr Yb Tm Eu Sm Ho Dy Gd Pm Lu Tb
Figure 5. Surface tension temperature coefficient of lanthanides vs. their m/Tm values.
Physics and Chemistry of Liquids 343 Downloaded by [Hebron University] at 03:55 04 October 2012
TABLE 2
very small because f electrons do not work as valence electrons. The values of quantities plotted in Figures 3–5 are given in Table 2.
In this work, the surface tension of liquid early actinides is also theoretically studied. The calculated results are in good agreement with experiment ones. Little information on the properties of these metals are known due to their radioactivity, and thus we attempted to study theoretically an important thermophysical property (surface tension), for these metals, in a view to learn more about their behaviour. However, the calculated surface tension values of liquid early actinides, at melting temperature, are found to be higher than the corresponding lanthanides because the 5f electrons are not shielded as that of the 4f counterparts.
Various investigations have been made, in the past, to describe the surface tension of liquid metals [22,35]. However, a successful model is still not identified [36–39] because the agreement with experiment is not very satisfactory. In view of this, we introduced a simple analytical model, based on the powerful statistical thermodynamic formulation of Eyring, for calculating of the surface tension of liquid metals at any temperature. It requires only the knowledge of the parameters (Table 1) that can be calculated or found in the literature. In this article, we performed theoretical calculations of the surface tension of the melts of pure terbium, as a function of temperature, and of liquid lanthanide and actinide metals at their melting points. The accuracy of these results was shown by comparison with prior experimental results. The results reveal that the proposed theoretical model for calculating surface tension of liquid metals at any temperature is important, interesting, sounds scientifically and provides a piece of addition for the understanding of the physical and chemical properties of liquid metals. Relations of surface to bulk properties of liquid rare earth metals have been explored.
Table 2. Thermodynamic quantities of liquid lanthanides.
Metal Tm/V2/3 (K m2 mol2/3) Hv/V2/3 (108 ) (mJ m2 mol1/3) m/Tm (mJ m2 K1 ) La 146 4.42 0.64 Ce 138 4.79 0.78 Pr 155 3.96 0.57 Nd 170 3.75 0.50 Pm 174 4.02 0.53 Sm 176 2.35 0.30 Eu 115 1.59 0.25 Gd 206 3.83 0.42 Tb 215 4.86 0.52 Dy 232 3.50 0.36 Ho 237 3.54 0.35 Er 254 3.86 0.37 Tm 249 2.75 0.26 Yb 117 1.43 0.22 Lu 272 5.23 0.47 344 F. Aqra and A. Ayyad Downloaded by [Hebron University] at 03:55 04 October 2012
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In 1965, this message appeared on a Holiday Inn sign in Ames, Iowa, to greet the attendees of the 5th Rare Earth Research Conference (RERC). Rare-earth chemistry was at a fulcrum point in its history. Previously, during World War II, chemists working in Ames at Iowa State University had played a critical role in rare-earth technology. As part of the Manhattan Project in 1942, the researchers developed methods to remove traces of rare-earth metals from the uranium used to create the first nuclear chain reaction and to make the first nuclear bombs.
“Welcome Rare Earthers.”
In 1965, this message appeared on a Holiday Inn sign in Ames, Iowa, to greet the attendees of the 5th Rare Earth Research Conference (RERC). Rare-earth chemistry was at a fulcrum point in its history. Previously, during World War II, chemists working in Ames at Iowa State University had played a critical role in rare-earth technology. As part of the Manhattan Project in 1942, the researchers developed methods to remove traces of rare-earth metals from the uranium used to create the first nuclear chain reaction and to make the first nuclear bombs.
Five piles of rare earth powders.
Credit: Peggy Greb/USDA/Science Source
Rare-earth oxides (clockwise from top center): praseodymium, cerium, lanthanum, neodymium, samarium, and gadolinium.
IN BRIEF
The rare earths, a collection of 17 elements including the lanthanides—lanthanum to lutetium—along with scandium and yttrium, have become indispensable components in many essential technologies of modern life, including smart phones, LEDs, and medical imaging. But not much was known about rare-earth chemistry until the 1960s. Since then, researchers have uncovered the elements’ abundant magnetic, luminescent, electrochemical, and other properties. The Rare Earth Research Conference, held recently in Ames, Iowa, brought together researchers who discussed the evolution of rare-earth chemistry, new research trends, and the need for sound natural resource management to ensure their sustainable use for future technologies.
Not much was known about the chemistry of the rare earths in the 1940s, or even by the 1960s. This group of 17 elements, which include the lanthanides—lanthanum to lutetium—along with scandium and yttrium, seemed to consist of fairly unreactive metals that all behaved similarly. Most of the research and development involving the elements was metallurgical, not chemical. Until 1965, the major applications for the rare earths included using mixtures of rare-earth oxides for polishing lenses and mirrors, using cerium and lanthanum oxides as promoters in zeolite catalysts for petroleum refining, and using “mischmetal” rare-earth alloys with iron as flints for lighters.
Flash forward to this past June, and rare earthers were back in Ames for the first time since 1965, for the 28th RERC. The Holiday Inn sign is long gone from the still-cozy midwestern college town, and the world of rare earths has changed.
In 1965, the “counterculture revolution” was under way, and many of the world’s young adults were protesting in favor of peace over nuclear power and the nuclear weapons of the Cold War. At the same time, they were falling in love with modern conveniences, driving demand for microwave ovens, early versions of personal computers and video games, and televisions. Coincidentally, 1965 witnessed the launch of the first major commercial use for a highly purified rare-earth material: Europium-doped yttrium salts began to be used as red phosphors in the picture tube of color television sets.
Now 52 years on, the internet rules nearly everyone’s lives, and handheld devices have greater capabilities than those early bulky computers and televisions. Much of the technology evolution since the 5th RERC has come about as researchers discovered that rare earths have abundant magnetic, luminescent, electrochemical, and thermal properties that have made possible smart phones, electric cars, light-emitting diodes, wind turbines, medical imaging, and more. This year’s RERC served as a grand review of this rare-earth chemistry, highlighting how adding a dash of rare-earth metals to materials is like adding a bit of magic fairy dust—the metals help everything perform better.
“The remarkable properties of the rare earths have long engaged the imagination of the scientific community,” said rare earther Ana de Bettencourt-Dias of the University of Nevada, Reno, one of this year’s conference organizers. She marveled at the metals’ importance in applications as varied as automobile starter motors, audio speakers, lasers, and colored glass.
The conference revealed new research trends showing that rare-earth chemistry is still very much a scientific frontier of the periodic table, with further technological developments to come. The discussion in Ames also turned to the need for better environmental stewardship of rare-earth element resources to ensure their sustainable, socially responsible use in existing and future technologies.
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Although labeled as a “rare earth” conference, scientists in Ames included the actinides such as uranium on the agenda, given the kinship between lanthanides and actinides as fellow f-block elements. The actinides are increasingly being used in nuclear medicine and as catalysts.
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A Holiday Inn sign in Ames, Iowa, in 1965 says "Welcome Rare Earthers," greeting attendees to the Rare Earth Research Conference.
Credit: Vitalij Pecharsky/Ames Laboratory
A new term, “rare earthers,” was created for attendees of the 5th Rare Earth Research Conference when the Holiday Inn welcomed them to Ames, Iowa, in 1965.
The f-element relationship is actually how rare-earth chemistry got started. For the Manhattan Project, scientists needed pure uranium, lots of it, fast and at a low price. Rare-earth impurities in uranium could prevent a nuclear reactor from working by absorbing the neutrons needed to keep the chain reaction going. So the rare earths had to be removed. Yet the chemical similarity of the lanthanides makes it extremely difficult to isolate them from one another and to separate them from uranium and other actinides. Iowa State chemistry professor Frank H. Spedding, an expert on rare earths, helped guide solution processing efforts to make tons of pure uranium oxide and a new high-temperature smelting method—called the Ames process—to make tons of pure uranium metal.
After WWII, in 1947, Spedding helped create and became director of Ames Laboratory, a U.S. Department of Energy national laboratory with a mission to further develop rare earths, initially for military uses and space exploration. Building on their earlier work, Spedding and his team developed an ion-exchange method to extract rare earths from minerals, enabling better separation of the rare-earth metals on a larger scale.
“Rare-earth research became commonplace after the groundbreaking work performed in Ames in the 1940s and 1950s,” explained materials scientist Vitalij Pecharsky of Iowa State and Ames Laboratory, another RERC organizer. “Spedding and his colleagues made pure rare earths available in quantities required for broad chemistry and physics research and applications for the first time.”
Pecharsky said his own research developing new rare-earth magnetic materials was made possible by those pioneering efforts. For example, his work with one of those pioneers, Karl A. Gschneidner Jr., who passed away last year and was memorialized at the conference, led to the invention of Gd5Si2Ge2. This material’s temperature, Pecharsky explained, can be altered by exposing it to a changing magnetic field. Such magnetocaloric materials are being developed for high-efficiency refrigeration that uses a quarter of the energy of current systems.
“Rare earths remain a favorite playground for both basic and applied science,” Pecharsky said, “driven by the potential for their everyday applications.”
Kenneth N. Raymond of the University of California, Berkeley, another rare-earth-playground leader, was recognized during the conference as this year’s Spedding Award winner, a prize that honors outstanding achievement in rare-earth research and education. Raymond’s award lecture, “Lanthanides in Bondage: From Basics to Business,” which led off the conference, reviewed his 40-plus-year career studying lanthanide luminescence and developing biomedical applications for the rare earths.
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Starting out, Raymond worked with other leading researchers of the day—UC Berkeley’s Glenn T. Seaborg and Andrew Streitwieser—to determine crystal structures of new lanthanide and actinide complexes. “Although there were a number of these compounds known, there was almost no structural information about them,” Raymond said. “So I set about to change that.” Raymond developed predictive models for the structure and bonding of lanthanide and actinide compounds based on the ionic radii of the metals.
A photo shows a small ingot of Gd5Si2Ge2, a material whose temperature can be altered by exposing it to a changing magnetic field, a property that could be used in high-efficiency refrigeration.
Credit: Courtesy of Grant Luchmann
Gd5Si2Ge2, a material created at Ames Laboratory, exhibits a giant magnetocaloric effect: Its temperature can be altered by exposing it to a changing magnetic field, a property that could be used in high-efficiency refrigeration.
Subsequent studies have enabled chemists to better understand the electronic transitions possible in lanthanide complexes when they are excited by light, and helped researchers design sequestering agents for plutonium and other heavy elements used to prepare and reprocess nuclear fuels.
Although lanthanides are weaker light emitters than organic dyes such as rhodamine B and fluorescein, Raymond said, they provide a technical advantage because their luminescence lasts longer, which allows better detection. Understanding the photochemistry of these lanthanide complexes when they are attached to drugs, proteins, and antibodies has proved valuable for developing new types of diagnostic assays and medical imaging.
In 2001, Raymond cofounded Lumiphore to commercialize terbium, europium, and other lanthanide luminescent complexes. These complexes are now used in assay kits sold by other companies to test saliva, blood, or tissues for drugs of abuse, hospital bacterial infections, and fetal and neonatal diseases. The technology is also being developed for biological imaging—for example, to study protein-protein interactions in living cells.
“We have learned a lot about the fundamental coordination chemistry of lanthanides over the years, and it’s still an interesting subject for research,” Raymond said. “Maybe the biggest thing for me is that if you put these metals in bondage, that is, incorporate them inside chelating ligands, the chemistry can be used to practical effect.”
William J. Evans, another rare earther, agrees. Evans and his coworkers at the University of California, Irvine, are one of several research groups who have been designing new metal complexes that are shaking up the conventional wisdom about rare-earth element oxidation states. Their findings point to new approaches for manipulating the magnetic, optical, and catalytic properties of rare earths.
“When I started working with rare-earth metals in the 1970s, they were considered an exotic part of the periodic table—people had to stop and think about where those metals are positioned,” Evans told C&EN. “My research advisers thought I was crazy to go to work in this area, because there was no demonstrated importance in the chemistry of these elements.” But Evans said the dearth of work on lanthanides was their allure.
Rare earths at a glance
NAME SYMBOL ATOMIC NUMBER ELECTRON CONFIGURATIONa ETYMOLOGY NOTABLE APPLICATIONS
Scandium Sc 21 3d14s2 Scandia (Latin for Scandinavia) Aluminum alloys, lighting
Yttrium Y 39 4d15s2 Ytterby, Swedenb Yttrium-aluminium-garnet lasers, red phosphors, cancer drugs
Lanthanum La 57 4f05d16s2 Lanthanein (Greek, meaning to lie hidden) Refinery catalyst, camera lenses, lighter flints
Cerium Ce 58 4f15d16s2 Ceres, the dwarf planet, and Roman goddess of agriculture Car catalytic converters, glass polishing agent, lighter flints
Praseodymium Pr 59 4f35d06s2 Prasios and didymos (Greek, meaning leek-green twin) Magnets, lighter flints, greenish-yellow glass and ceramics
Neodymium Nd 60 4f45d06s2 Neo and didymos (Greek, meaning new twin) Nd2Fe14B magnets, lasers, violet glass and ceramics
Promethium Pm 61 4f55d06s2 Greek Titan Prometheus Radioactive; luminous paint, pacemaker batteries
Samarium Sm 62 4f65d06s2 Vasili Samarsky-Bykhovetsc SmCo5 magnets, cancer therapy, nuclear reactor control rods
Europium Eu 63 4f75d06s2 Europe Red phosphors for lighting and color displays
Gadolinium Gd 64 4f75d16s2 Johan Gadolind Refractive glass, MRI contrast agent, nuclear reactor shielding
Terbium Tb 65 4f95d06s2 Ytterby, Swedenb Green phosphors for lighting, magnetorestrictive alloys
Dysprosium Dy 66 4f105d06s2 Dysprositos (Greek for hard to get) Stabilizing additive in magnets, lasers
Holmium Ho 67 4f115d06s2 Holmia (Latin for Stockholm) Lasers, magnets
Erbium Er 68 4f125d06s2 Ytterby, Swedenb Lasers, fiber optics, nuclear reactor control rods
Thulium Tm 69 4f135d06s2 Mythological land of Thule Portable X-ray source, light filaments, lasers
Ytterbium Yb 70 4f145d06s2 Ytterby, Swedenb Lasers, chemical reducing agent, stainless steel additive, cancer therapy
Lutetium Lu 71 4f145d16s2 Lutetia (Latin for the city that became Paris) PET scan detectors, refractive glass, refinery catalyst
a Ground shells, NIST data. b One of four new elements in the first discovered rare-earth ore found near Ytterby, Sweden, credited to Carl Arrhenius. c Mining engineer who discovered samar- skite, a mineral containing samarium, the first element named after a person. d Gadolinium is named in honor of this rare earther who first identified yttrium in a sample sent from Carl Arrhenius.
Today, the attitude toward rare earths is much different, Evans said, because people have heard of their many uses. “So now when I say I am a rare-earth chemist, people have an idea that these are useful, interesting, and strategically important metals.”
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Conventional wisdom among chemists until recently held that most rare earths exist only in the +3 oxidation state in molecular complexes. Each of the 17 elements in the rare-earth collection have three ionizable electrons—two s electrons and one d electron. Moving across the row of lanthanides from lanthanum to lutetium, each element has an additional electron that takes up a position in a 4f orbital. Because the 4f orbitals don’t extend far enough from the atom, they tend not to directly participate in bonding, yet they still influence the elements’ magnetic and optical properties. That’s a broad, simplistic view; the real situation “is complicated,” Evans said.
Researchers found early on with rare earths that cerium readily assumes a +4 oxidation state in compounds, and later researchers began to find that other elements could take on +2 or possibly +5 oxidation states. By the late 1990s, chemists had isolated lanthanide(II) complexes for six of the elements. At the time, the researchers rationalized their results by noting the ions were stabilized by having filled, nearly filled, half-filled, or nearly half-filled 4f orbitals.
But as researchers continued to investigate, they discovered that ligands with the right electronic properties could provide additional stability, leading to additional +2 complexes. In 2013, Evans and Matthew R. MacDonald in his group reported complexes for the final seven elements, completing the lanthanide(II) series to show that it could be done. The team actually didn’t make the promethium complex, because the element is radioactive and doesn’t occur naturally. But the researchers expect it should follow suit and form a +2 complex.
The lanthanide(II) complexes are especially interesting, Evans noted, because they are kind of a hybrid between transition metals and lanthanide metals. They don’t appear to have a 6s electron, but rather a 5d electron like a transition metal, in addition to any 4f electrons. And they display one-electron redox chemistry, which is not always typical for transition metals. “It’s like they are a new type of metal,” he said.
Evans and his colleagues have since found that the dysprosium(II) and holmium(II) complexes exhibit the highest magnetic moments reported to date for any single metal ion complex. They haven’t figured out how to use this breakthrough practically yet, Evans said. One idea is that these complexes could function as single-molecule magnets and be used in thin-film coatings for computer hard drives to increase the density of data storage.
As for what’s next with rare-earth chemistry, Evans asked: Why not try to make +1 rare-earth complexes? He thinks as chemists continue to tinker with new electron configurations, they should be able to further elaborate on rare-earth technologies. “Hopefully, future research will allow us to use these metals more efficiently and reduce the tremendous demand on them for their special properties.”
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As Evans alluded, the demand for rare earths could lead to their commercial demise. Access to the metals has always been tenuous, leading to worries that one day demand for them may outstrip supply.
To head off that scenario, Gisele Azimi of the University of Toronto, another rare earther speaking at the conference, sees an opportunity for developing better strategies to ensure sustainable supplies of rare earths. These include finding new sources of the metals, developing better extraction technology for them, improving the efficiency of rare-earth-metal usage in products, and closing the loop on their use with better recycling.
[+]Enlarge
Ames Laboratory scientist Ikenna Nlebedim holds samples of neodymium-iron-boron magnets sourced and manufactured entirely in the U.S., part of the Ames Lab’s Critical Materials Institute’s role in supporting American manufacturing.
Credit: Ames Laboratory
Ames Laboratory scientist Ikenna Nlebedim holds samples of neodymium-iron-boron magnets sourced and manufactured entirely in the U.S. The magnets are the result of the lab’s Critical Materials Institute’s role in supporting American manufacturing and national security interests.
Despite their name, rare-earth elements are not rare. All the metals except radioactive promethium are actually more abundant in Earth’s crust than silver, gold, and platinum. When the first rare earths were discovered in the late 18th century, they were found as complex mixtures of metal oxides in various minerals. The minerals were initially, and incorrectly, classified as “earths,” a geological term denoting nonmetallic substances that are insoluble in water and resistant to chemical change by heating. Furthermore, because of their geochemical properties, rare-earth elements don’t concentrate in ore deposits but are widely dispersed, so they only seemed to be scarce.
Global rare-earth reserves, at more than 130 million metric tons, appear to be ample, Azimi noted. However, most of those reserves either are too low in concentration to be extracted economically, or they are not readily accessible, such as metals locked away in deep-sea manganese-based nodules or hydrothermal deposits.
“Currently, the demand for rare earths is increasing at a rate of about 5% annually,” Azimi said. Some rare-earth applications require large amounts of the metals: Wind turbines use some 600 kg of rare-earth metals apiece. Some uses need intermediate amounts: Electric vehicle batteries use 5 to 10 kg of rare earths each. And some uses require small amounts: Phosphors in lighting may use a fraction of a gram per bulb.
The key issue at the moment is that China is home to about 25% of the world’s rare-earth reserves, yet economic forces have aligned so that China now accounts for roughly 90% of the global supply. This situation has made sourcing rare earths precarious, subject to price and supply swings that are disruptive to product manufacturers.
That leaves mining companies outside China caught in an industrial catch-22: If they want to grab a larger piece of the market, the companies must ramp up production of rare earths, but they must do so without saturating the market with supply to avoid lowering prices. While companies deal with those economic conditions, they are also increasingly being judged for their environmental stewardship and their social responsibility to the communities they operate in.
The U.S., Australia, Brazil, and Canada have large reserves of rare earths, Azimi said, but little active mining is currently going on in those countries because of the cost. The real challenge going forward, she said, is going to be scaling up supply at a rate that matches increases in demand. “Opening new mines takes time—a decade or more—for prospecting, permitting, and construction. It’s expensive and has environmental downsides.” From digging in the ground to producing pure rare-earth oxides and pure metals, the process uses a lot of water, acids, organic solvents, and extraction chemicals.
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One solution Azimi is promoting is to rescue rare-earth metals from existing stockpiles of mining wastes. For example, she is working with residue called red mud from bauxite mining to produce aluminum, and leftovers called phosphogypsum from phosphate rock mining that makes fertilizer. These materials contain 300 to 500 ppm of rare-earth metals, which is more concentrated than the roughly 0.2 to 60 ppm levels of the elements found in natural mineral deposits. With millions of tons of these waste materials piled up globally, thousands of tons of rare-earth metals could be procured, she said. Other researchers at the conference discussed coal ash and spent nuclear fuel from power plants as additional secondary sources of rare earths.
Rare earths are also plentiful in electronic waste, Azimi explained. Used fluorescent lights, computer hard drives, large permanent magnets, and batteries tend to end up in landfills. “Only 1% of their rare-earth content is being recycled,” Azimi said. “It is absolutely imperative to develop recycling processes to address the sustainability challenges associated with these critical materials.”
That’s a big challenge for e-waste because most consumer products aren’t designed to be recycled. In addition, the metals are often used in combination for specific applications and need to be separated again. For example, neodymium-based permanent magnets (Nd2Fe14B) contain some dysprosium to improve thermal performance.
Azimi’s group is working toward a designer acid-leaching process to recover rare earths from red mud and phosphogypsum. At RERC, she described how her team is further using the metals to design rare-earth-containing materials for new commercial uses. For example, the researchers are working on rare-earth oxide ceramics as transparent waterproof coatings for applications such as heat exchangers, airplanes, and solar panels. These materials use a minimal amount of rare earths and they are thermally stable, so they last longer than other types of hydrophobic coatings, which could help address the rare-earth supply challenge, she said
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Other methods being used or tested to recycle e-waste include traditional coordination and chelation chemistry, ionic liquid extraction, metal-adsorbing bacteria, and the Ames high-temperature metallurgical process. For example, Eric J. Schelter of the University of Pennsylvania and his group have been developing a new type of targeted metal separation process that uses tailored nitroxide ligands to efficiently separate mixtures of rare-earth metals when recycling magnets and lighting phosphors.
“Rare earths have become indispensable and, in many cases, irreplaceable components of materials that are essential in modern life,” said Schelter, one of the conference organizers. “Moving forward, I’m excited to see the magnetic properties of rare-earth molecules and molecular clusters reach their potential, for f-element complexes to catalytically drive multielectron transformations, and for the realization of highly efficient chemistries for recycling rare earths from consumer materials.”
Technical solutions to the long-term sustainable use of rare earths undoubtedly will come from further research and design of new materials, such as core-shell nanoparticles that use just a dash of rare earths or even thin films of single molecules that achieve the same magnetic, optical, and other magical rare-earth properties as bulkier materials. Some researchers are looking to rare earths as a guide to exploit transition- and main-group metals to mimic rare-earth properties. Those developments might be on the agenda the next time rare earthers reunite in Ames.
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COMMENTS
Mike Jarvis
(August 28, 2017 1:42 PM)
Interesting story. More info on rare earths at USGS. https://minerals.usgs.gov/minerals/pubs/commodity/rare_earths/
Steve Ritter
(August 30, 2017 2:07 PM)
Thanks for point to this reference. The U.S. Geological Survey is a great resource for U.S. production and consumption of rare earths, as well as global production and reserves.
Chris Stock
(August 29, 2017 5:10 PM)
The graphic at the top of this article is misleading. Scandium and yttrium are more similar to lutetium, and should be placed above it (and lawrencium) on the Periodic Table -- not above lanthanum.
Steve Ritter
(August 30, 2017 3:16 PM)
This is a good point, one generating a lot of debate over the years. One can consider scandium, yttrium, and all the lanthanides as being in group 3 of the periodic table--they each have (in general) three ionizable electrons, two s and 1 d. The electron configuration of Sc, Y, La, and Lu end in d1s2 (so does Ac below them in the periodic table, Lr has s2p1). The lanthanides beyond La to Lu also have f electrons. Some periodic tables only put Sc, Y. La, and Ac in group 3, leaving Ce to Lu and Th to Lr pulled out separately (and unassigned a group). Some only consider Sc and Y as truly belonging to group 3. Should the location be based on the electron configuration, or the similarity of chemical properties? Can of worms now open.
O.P.SOMANI
(August 29, 2017 11:16 PM)
For the separation of REE there is talk of RapidSX and Molecular Recognition Technology(MRT). Both claim for cheaper and quicker REE separation. What is the view of author on these techniques?
Steve Ritter
(August 30, 2017 3:46 PM)
Thanks for pointing out these technologies. Rapid SX and Molecular Recognition Technology are two rare-earth separation technologies being developed commercially. Rapid SX is an organic-aqueous liquid-liquid extraction process that appears to be an improvement over current technology by reducing the amount of solvent needed, yet it still requires a significant amount of base, acid, water, and solvent (http://www.innovationmetals.com/the-rapidsx-process). Molecular Recognition Technology relies on metal-selective ligands attached to solid particles with no organic solvent, but ligands can be expensive (http://ucore.com/Ucore2016.pdf). In the end, we need green technologies that can be adapted to different needs in mining and recycling, which means multiple technologies are needed. Fast and cheap are good, but they also need to work as effectively or better than current technology and be environmentally friendlier. Perhaps those noted in the article and Rapid SX and Molecular Recognition Technology will all be successful.
Elvis Acheampong
(August 31, 2017 4:45 PM)
I would love to receive news items from your magazine.
Rachel Pepling, Production Director
(September 1, 2017 8:27 AM)
Elvis, you can start by signing up for our weekly newsletter: http://connect.acspubs.org/CENNewsletterSignUp
NM Abrams
(September 1, 2017 1:17 PM)
The "rare earths at a glance" table is a great learning tool for those of us in academia. Any chance this would be available as a downloadable graphic/figure or infographic (with attribution)?
Jim Beall
(September 2, 2017 9:26 AM)
Could you expand a bit on rare earths in spent nuclear fuel?
How much per ton? Would they be fission products or activation products? Left over neutron poisons? Manufacturing impurities?
Any particular rare earths? Or, mostly all of them?
Steve Ritter
(September 5, 2017 4:47 PM)
Thousands of tons of spent nuclear fuel is removed from power plant reactors each year globally. This material still contains a significant amount of the nuclear fuel (uranium oxide) along with fission products that include rare earths and transition metals in concentrations higher than in typical ores. Researchers have spent quite a bit of time developing chemical processes to reprocess the spent fuel to make new nuclear fuel, which includes separating the other metals. The amount of the most abundant half dozen or so rare earths and some platinum group metals such as rhodium and ruthenium could make spent nuclear fuel a future source of pure (nonradioactive) metals, although retrieving them now with current technology isn’t economically feasible. It’s a similar situation for extracting rare earths and transition metals from coal ash as asked below, although that seems financially more promising.
Jim Beall
(September 2, 2017 9:32 AM)
I have a very similar questions on coal ash as I did on spent nuclear fuel.
Jack Lifton
(September 2, 2017 10:36 AM)
The American rare earths' industry consists primarily of purveyors (using outsourcing) and assemblers using rare earth enabled components and sub assemblies (motors, magnets, lasers,...)who market consumer products. Although there is yet an American fluid cracking catalyst (for oil refining) industry that uses light rare earths in blends of chemical salts and carriers, and this industry imports its "raw materials" from China, there is today NO commercial supply chain in North America (or South America) to process rare earth ores, separate the individual rare earths from those ores; produce rare earth metals and alloys; or fabricate those alloys into rare earth enabled end-use products. China is not only the monopoly producer of rare earths, but the monopsony user of them to produce end use products. The US military should ensure the existence of a minimalist total rare earths supply chain; but this will have to be subsidized. The US market economy will not be able to compete with China for a rare earths industry unless it is willing to capitalize security of supply, but such capitalization is a political decision not a financial one. American venture capital has abandoned the concept of total supply chains, if it ever even considered them, and government is overwhelmed by crisis management. I don't see a domestic American rare earth total supply chain revival, and therefore mining is of no value, since the mined concentrates would have to go to China for any downstream work.
William Bieri
(September 2, 2017 10:30 PM)
Hmm, this article talks repeatedly about 'sustainable' use of rare earths. I honestly find this rather confusing, as a mineral resource, aren't they by definition non-renewable? Last I checked deposits are not regenerated or refreshed on anything shorter than geologic timescales.
Any 'sustainable' approach to rare earth elements seem, to me, like it should focus on recycling efficiencies and accessing new deposits. Basically utilizing what limited resources we have to ensure continued access in the future. Be that from seabed mining, asteroid refinement, or whatever else. The focus on devices or methods that use the minimum amount of rare-earth elements seems... a distraction, to me. Though considering I've yet to complete a bachelors degree I may just be off the mark.
Steve Ritter
(September 5, 2017 5:00 PM)
Thanks for your comment. Yes, the amount of rare earths on the planet is finite, and as such nonrenewable. But that does not mean the metals can't be managed sustainably--that is, indefinitely used. While new mining will continue to provide new metals into the processing loop, by developing new technology that requires less metal per device and recycling devices at the end of their useful life, we extend the time we have to use the finite resources. New sources of metals from mining will only continue as long as it is cost-effective to process them. At some point in the future, supply and demand and cost will perhaps balance out so that no new mining or sources are needed and all metal needed for new devices will come from recycling old ones. That's a basic view of what could happen and may need to happen to continue to use rare earths as we do now.
A new approach to biomining: Bioengineering surfaces for metal recovery from aqueous solutions:
Jesica Urbina 1*,
Advait Patil2,
Kosuke Fujishima3,
IvanG. Paulino-Lima4,
Chad Saltikov1
& Lynn J. Rothschild5
Electronics waste production has been fueled by economic growth and the demand for faster, more efcient consumer electronics. The glass and metals in end-of-life electronics components can be reused or recycled; however, conventional extraction methods rely on energy-intensive processes that are inefcient when applied to recycling e-waste that contains mixed materials and small amounts of metals.
To make e-waste recycling economically viable and competitive with obtaining raw materials, recovery methods that lower the cost of metal reclamation and minimize environmental impact need to be developed.
Microbial surface adsorption can aid in metal recovery with lower costs and energy requirements than traditional metal-extraction approaches.
We introduce a novel method for metal recovery by utilizing metal-binding peptides to functionalize fungal mycelia and enhance metal recovery from aqueous solutions such as those found in bioremediation or biomining processes.
Using copper-binding as a proof-of-concept, we compared binding parameters between natural motifs and those derived in silico, and found comparable binding afnity and specifcity for Cu.
We then combined metal-binding peptides with chitin-binding domains to functionalize a mycelium-based flter to enhance metal recovery from a Cu-rich solution.
This fnding suggests that engineered peptides could be used to functionalize biological surfaces to recover metals of economic interest and allow for metal recovery from metal-rich efuent with a low environmental footprint, at ambient temperatures, and under circumneutral pH.
End-of-life electronics waste (e-waste) production has been fueled by economic growth and the demand for faster, more efcient consumer electronics.
[ https://www.te.com/usa-en/home.html ]
Te glass and metals in e-waste can be reused or recycled - however, "developed countries" tend to not recycle - due to high labor costs and strict environmental regulation1 .
Instead, "e-waste" accumulates in landflls or is exported to developing countries - where it is "recycled" - using primitive techniques such as open-air incineration or strong acid treatments for metal recovery, without regard for worker safety or environmental impact2,3 .
Additionally, many elemental components in e-waste such as the rare-earth elements (REEs) and transition metals like titanium (Ti) are emerging as new contaminants that have never before existed in concentrated quantities sufcient to produce toxicity to organisms4–6 .
These activities have led to severe heavy metal pollution in communities that handle e-waste as they are experiencing adverse health efects and toxicity to aquatic and terrestrial ecosystems7,8 .
Development of economically viable alternatives for elemental recovery is essential for sustaining a balance between technological development and environmental responsibility.
Conventional extraction methods rely on energy-intensive processes and are inefcient when applied to recycling e-waste that contains mixed materials and small amounts of metals.
Applying a biological approach to resource extraction from e-waste, which we term "urban biomining", allows for metal extraction at ambient temperatures with lower environmental impacts and energy requirements than current approaches. Microbial surface adsorption can aid in metal recovery from aqueous solutions containing metals from e-waste; however, there is limited specifcity because the surface functional groups will bind many cations with high afnity9,10.
A handful of studies have focused on the addition of metal-binding peptide tags onto bacterial surface proteins and they
1University of California Santa Cruz, Department of Microbiology and Environmental Toxicology, Santa Cruz, CA, 95064, USA.
2Universities Space Research Association, Mountain View, CA, 94043, USA.
3Tokyo Institute of Technology, Earth-Life Science Institute (ELSI), Tokyo, Japan.
4Blue Marble Space Institute of Science, Seattle, Washington, 98154, USA.
5 NASA Ames Research Center, Space Biosciences Division, Mofett Field, CA, 94035, USA.
have shown to sequester more metal than controls - however, the tags offer limited specifcity as to the metals that are adsorbed11–15.
In principle, a biological approach to metal adsorption using peptides should be exquisitely specifc because all cellular fuids contain a mixture of metal ions at diferent concentrations; yet, metal cofactors are not easily replaced from their cognate metalloproteins by competing ions in the intracellular milieu.
Metal coordination number (the ability to bind to a given number of ligands) and molecular geometry are shared properties between a ligand and its cognate metal and are proposed to be a key determinant of specifcity16. A metalloprotein [ https://en.wikipedia.org/wiki/Metalloprotein ] will bind a metal cofactor with amino acids in the primary coordination sphere that refers to the molecules that are attached directly to the metal, and that is optimal for the molecular geometry and coordination number of the cognate metal. [ https://en.wikipedia.org/wiki/TE_Connectivity ]
The effects of the second and third coordination spheres consist of molecules that interact via hydrogen and Van der Waals interactions with the primary coordination sphere and are those presumed to determine specifcity for a metal co-factor.
Computer-generated design that considers only the primary coordination sphere of a metal and the approximate steric compatibility of a scafold protein can be used to introduce a selective binding site into a protein17,18. Yet, it remains challenging - to model "metal specifcity" into de novo-designed proteins due to substantial computational requirements19,20.
An alternative to "computational design" is a "biopanning technique" [ 1, 2, 3 ] to identify metal-binding peptides that are selective for a target metal and have also shown promise in identifying natural proteins that adsorb target metals21,22,25.
For this work, we hypothesized that a metal-binding motif from the primary coordination sphere of a metalloprotein (natural or designed in silico) was sufcient to bind a metal (Cu) with high afnity and fdelity without regard for the secondary or tertiary coordination spheres.
Our objective was to develop an approach for biorecovery of metals from aqueous solutions using metal-binding peptides to adsorb various metals of economic and geopolitical interest from a metal-rich leachate solution. [ https://www.frontiersin.org/articles/10.3389/fbioe.2018.00157/full ]
First, we compare naturally-occurring metal binding motifs [ A, B, C ] to those designed in silico, to elucidate factors in binding and specifcity that can be applied to other metals. [ HERE ]
Since the role and chemistry of copper (Cu) in biology is relatively well-characterized [a, b, c ] , we use this as a proxy for metals with unknown biological roles - such as REEs or platinum group metals.
Second, we show a - pathway to implementation - by demonstrating [through pilot experiments ] that the application of our metal-binding peptides containing a chitin-binding domain can functionalize the otherwise hydrophobic surface of fungal mycelia to aid in metal recovery. [ https://pubs.acs.org/doi/10.1021/acsami.0c06138 ]
Our data [ Authors above ] indicate that natural and designer motifs are selective and effective metal chelators, and that when these are used to functionalize a mycelial surface, they increase the metal-binding capacity of an inexpensive and non-toxic substrate that can be used in metal recovery applications. [ https://en.wikipedia.org/wiki/Chelation ]
In order for e-waste recycling to be "economically viable" and competitive with raw materials, recovery methods that lower the cost of metal reclamation and minimize environmental impact need to be developed.
Intrinsic binding parameters for Cu. To compare binding afnities from natural motifs versus those derived in silico, we characterized select peptides for their binding parameters through isothermal titration calorimetry (ITC).
The natural peptides were chosen because they have been previously characterized and have amino acid residues that are highly represented in Cu-binding.
The putative Cu-binding domain CXXC, is found in metalloproteins with diverse functions such as the metal-binding domain of E. coli GTPase (HypB1,2)23,24, Arabidopsis sp. Zn- and Cu- binding peptides (CZB-7)25, and in a consensus motif represented in diferent types of Cu binding (Cu-02)26.
The rationally-designed peptide motifs we used for comparison are predicted, in silico, to bind Cu (HHTC, CHSK) or Zn (KDKD, KDTK)27.
The peptides were derived by applying quantum mechanical methods that consider complexation energies of the metal with the amino acid side chains of a primary coordination sphere in a metalloprotein, and the molecular geometry (Ni - octahedral, Zn - tetrahedral, Cu - square planar) of the cognate metal27. These are referred to according to their metal binding residues. We used a positive control, HHTC, that was previously characterized for Cu binding [ NASA ] by using matrix-assisted desorption/ionization (MALDI) and isothermal titration calorimetry (ITC), and CHSK as a negative control determined through MALDI to not bind Cu in the gas phase despite in silico predictions.
The complete list of assessed peptides and their respective amino acid sequences is in Table 1. The natural peptides showed a range of afnities for Cu as shown in Fig. 1A and Table 1. The "binding afnities" for the rationally-designed motifs HHTC and CHSK had high afnities for Cu and were comparable to the natural HypB peptides.
Binding parameters for HHTC with Cu have been previously published and were conducted at pH 7 with ACES bufer27. At this pH, thermodynamic modeling and experimental data show that Cu is mostly precipitated (>99%) into a solid mineral Cu(OH)2(s) phase10. Tus, we suspected that the reported binding parameters would be confounded by a change in enthalpy - due to Cu dissociation from a solid mineral phase, rather than from Cu binding to the HHTC peptide.
We conducted our binding experiments at pH 5.5, where 99.991% of Cu is predicted to remain as Cu2+. The published binding afnity for HHTC is Ka=(2.4±0.5) × 106M−1 .
Our observed parameters for HHTC match these within error despite diferences in experimental conditions. Additionally, the use of buffers with diferent heats of ionization, such as ACES and MES (used in this study) buffers, with ΔHion=31.4 kJ/mol and 15.5 kJ/mol, respectively, did not appear to afect the measured enthalpies for Cu binding. Tese results indicate the previously published binding parameters accurately describe binding between Cu and the peptide under these conditions and are not due to proton transfer or Cu-mineral phase changes.
Specifcity of metal binding. E-waste components have multiple metals, and these remain in solution afer removal of the scafold material. In order to determine if our constructs were specifc for Cu, we added competing metal ions to the peptide-Cu complex. Ni and Zn were chosen - as the "competing" ions and are often competitors for the same ligand due to their similar electron confguration, ionic radius, valence, and/or molecular geometry.
Similarly, Cu was added as the competing ion to solutions with other metal-peptide mixtures.
Intrinsic binding parameters were frst determined for each peptide with individual metal solutions then apparent binding parameters were obtained when a competing metal was titrated into the already-formed metal-peptide complex. Previously published binding parameters on the select peptides did not determine whether they were specifc to or showed preference for Cu when competing with other ions in solution. Figure 1B shows isotherms for competition experiments with HHTC and Cu, where Ni was titrated into the HHTC-Cu complex and resulted in no observed change in heat. Additionally, Cu was titrated into the sample cell containing HHTC and Zn and revealed Cu but no Zn binding. From this, we concluded that Cu occupied all available binding sites and was not dissociated from the peptide when competing ions were introduced into the solution. In all cases where Zn or Ni was titrated into an already-formed peptide-Cu complex, there was no change in enthalpy when the competing metals were added to the solution, indicating that Cu was not replaced by the competing ions. We then tested whether the peptides would bind to Zn or Ni only, and no isotherm was calculated in all cases, indicating the peptides did not bind these metals (Table 1).
We subsequently titrated Cu into the peptide+Zn, or peptide+Ni solutions to determine if the afnity for Cu was retained when competing ions were in solution. Te HypB peptides retained their afnity for Cu at the same or higher level when Ni or Zn was in solution while Cu afnity for the consensus motif Cu-02 was an order of magnitude lower when Zn was in solution and Cu afnity was lost when Ni was in solution (Table 1). Te CZB-7 peptide that had previously shown a weak afnity for Cu at Ka=(7.78±1.25) × 103 M−1 , did not bind Cu at all when the competing ions were in solution even though there was no appreciable binding to Zn or Ni, as no isotherm was calculated in these cases. In the case of the synthetic peptides, the afnities of HHTC and CHSK for Cu were lowered by an order of magnitude when Zn or Ni was in solution (Table 1). Additionally, the peptide CHSK showed the afnity for Cu was lost when Ni was the competing ion. Te Cu isotherm for HHTC is shown afer it was titrated into a solution containing Zn and HHTC (Fig. 1C). It is a recognized phenomenon that a determinant of peptide selectivity for a divalent metal ion follows the Irving-Williams series28, where the stability constants (i.e., strength of bonds due to electrostatic interactions between molecules) for metal complexes with any set of ligands is: Mg2+ Zn2+. If this is the case, then any peptide tested would preferentially bind Cu, and it would displace all of the other metals in the series, regardless of other metal-ligand specifcity principles. To test whether the rational design approach was efective at predicting specifc binding to other metals, or whether any peptide would bind Cu with high afnity, we assessed peptides that were designed to bind Zn. Te Zn-binding peptide KDTK had a higher afnity for Cu than it did for the cognate metal, Zn (Table 1) and KDKD did not show any binding to Zn, however did bind Cu with low afnity. Tus, with these peptides, Cu bound with higher afnity by at least an order of magnitude than for the cognate metal, Zn. Binding parameters of modifed synthetic peptides. We observed that the rationally-derived peptides had a lower afnity for Cu when competing ions were in solution, despite there being no apparent binding to the competing ions, and this led us to further examine the designer peptides. We compared primary amino acid sequences for the natural and rationally-derived peptides and found that some of the latter contained residues that are highly represented in Ni and Zn binding sites (Fig. 2). Data mining studies show KDETSY (Lys-Asp-Glu-Tr-Ser-Tyr) amino acids are favored in Zn and Ni binding sites29 while 95% of residues in Cu binding sites are HCM (His-Cys-Met)23. Not surprisingly, we found that the natural motif from the HypB protein did not contain any of the Zn/Ni binding residues, while the peptides with low Cu-binding afnities such as CZB-7 and the rationally designed peptides did (Fig. 2). To test whether the KDETSY residues were essential for retaining specifcity for Cu by accommodating for molecular geometry, we assessed the binding parameters for HHTC and CHSK that had either the KDETSY residues omitted or replaced with non-interacting residues NLGQV (Table 2). Te Cu binding afnities of the modifed motifs were comparable to HypB and unmodifed HHTC and CHSK afnities. An unpaired t-test between the original HHTC peptide and the one with KDETSY residues omitted (HHTC-Tr), revealed no signifcant difference (two-tailed P value=0.2783) in intrinsic binding afnity for Cu. Te HHTC-Tr peptide had an intrinsic Type Name Amino acid sequence Cognate metal Source Cu [M−1 ] Zn [M−1 ] Ni [M−1 ] Zn → Cu [M−1 ] Ni → Cu [M−1 ] Natural motifs HypB1 CTTCGCG Unknown Douglas et al., 2012 (2.37 ± 0.71) × 106 0 0 (1.29 ± 0.26) × 107 n/a HypB2 MCTTCGCGEG Unknown Chang et al., 2008 (1.30 ± 0.07) × 106 0 0 (1.98 ± 0.99) × 106 (3.51 ± 0.18) × 106 CZB-7 GFHGRADALLHKI Cu/Zn Yeh et al., 2010 (7.78 ± 1.25) × 103 0 0 0 0 Consensus Cu-02 HCWCHM Cu Bertini et al., 2010 (9.89 ± 2.18) × 105 0 0 (4.97 ± 0.7) × 104 0 Rational design HHTC HNLGMNHDLQG ERPYVTEGC Cu Kozisek et al., 2008 (1.74 ± 0.49) × 106 0 0 (8.64 ± 3.11) × 105 (5.02 ± 1.10) × 105 CHSK CPSEDHVSQDK Cu (1.28 ± 0.33) × 106 0 0 (2.35 ± 0.81) × 105 0 KDTK KTEYVDERSKSLTVDLTK Zn (1.05 ± 0.91) × 104 (2.44 ± 3.53) × 103 0 (6.51 ± 0.75) × 103 n/a KDKD KFFKDFRHKPATELTHED Zn (1.27 ± 0.11) × 104 0 0 (3.08 ± 2.33) × 106 (1.71 ± 1.08) × 106 Table 1. Metal-binding motifs assessed for Cu-binding parameters. Intrinsic association constants (Ka) for peptides titrated with Cu, Zn, Ni, and apparent binding constants when titrated with Cu afer Zn or Ni was in solution. Scientific Reports | (2019) 9:16422 | https://doi.org/10.1038/s41598-019-52778-2 4 www.nature.com/scientificreports/ www.nature.com/scientificreports binding afnity for Cu comparable to the unmodifed peptide and retained its afnity for Cu when Zn or Ni were in solution. Te HHTC peptide with the KDETSY residues replaced with NLGQV had a slightly higher afnity (but not quite statistically signifcant by conventional criteria, two-tailed P=0.0621) for Cu than the original peptide with and this peptide retained its afnity for Cu when Zn or Ni was in solution. Te binding afnity for CHSK, however was markedly diferent when non-binding amino acid residues were replaced or omitted. When the KDETSY residues were omitted from the peptide, CHSK-Tr Cu afnity was similar to the original CHSK, however afnity for Cu was lost when Ni or Zn were present in solution even though no binding isotherm was observed for Ni or Zn. When the CHSK peptide had the KDETSY residues replaced with the non-interacting NLGQV, binding afnity of CHSK-Re for Cu was lowered by an order of magnitude and this was only slightly less if Zn was in solution prior to the addition of Cu and completely lost if Ni was the competing ion. Figure 1. (A) Raw data and isotherms for tested peptides and Cu. Te natural HypB peptides had a range of afnities with Ka=(2.37±0.7) × 106M−1 and (1.30±0.07) × 106M−1 , for HypB1 and HypB2, respectively. Te HypB peptide was tested with and without leading and trailing residues and showed that the addition or absence of the amino acids at the N- and C- termini did not lead to an appreciable diference in Cu binding afnities. Te consensus sequence Cu-02 exhibited a mid-range afnity with Ka=(9.89±2.18) × 105M−1 and the CZB-7 peptide had low afnity with Ka=(7.78±1.25) × 103M−1 . Te designer peptides showed comparable afnities to the HypB motifs with HHTC Ka=(1.89±0.3) × 106M−1 and CHSK Ka=(1.28±0.3) × 106M−1 . (B) In this experiment, Cu was frst titrated into HHTC and an isotherm was calculated based on changes in enthalpy. Ni was then titrated into the HHTC-Cu complex and this resulted in no isotherm calculated for Ni indicating that Cu was not displaced from the peptide. Additionally, raw data and isotherm for CHSK solution containing Zn with added Cu where Zn was frst titrated into CHSK and revealed no changes in enthalpy. Cu was then titrated into the sample cell containing CHSK+Zn and this resulted in an isotherm for Cu. (C) Raw data and isotherm showing binding afnity of HHTC for Cu in the absence and presence of the competing ion, Zn. Figure 2. Primary amino acid sequences for natural peptides (HypB1, HypB2, CZB-8, Cu02) and those derived through rational design (HHTC, CHSK). Amino acids that are highly represented in Cu binding are shown in red, and those most implicated in Ni or Zn binding are shown in blue. Scientific Reports | (2019) 9:16422 | https://doi.org/10.1038/s41598-019-52778-2 5 www.nature.com/scientificreports/ www.nature.com/scientificreports Tandem metal-binding motifs and linear increase in metal binding. Previous studies and our own observations determined that the peptides we tested all had a 1:1 stoichiometry to Cu. In an applied setting it would be ideal to bind more than one metal atom per peptide molecule, thus we tested whether having more than one metal binding motif in each peptide molecule would lead to a stoichiometric increase in Cu binding. Since we ultimately seek to move away from natural proteins with native metal co-factors, we chose to continue with the rationally designed peptide, HHTC, because it remained robust when non-binding amino acids were excluded or replaced with amino acids that did not interact with competing ions Ni or Zn. We modeled30 and constructed HHTC-Re motifs designed to have tandem repeats in series as single (1x-HHTC-Re), double (2x-HHTC-Re), and triple (3x-HHTC-Re) peptide molecules and assessed them for their binding parameters. We assessed these for Cu binding parameters with ITC and binding afnity Ka reveals strong Cu binding for the all of the peptides (Table 2). Additionally, a trend is observed that suggests the 2x and 3x peptides do bind 2 and 3 times more Cu, respectively as a linear increase is observed as we add more motifs in series and thus, we can bind multiple metal ions per peptide molecule (Fig. 3). Functionalizing mycelium surface with metal-binding peptides containing chitin-binding domain. When designing our bioflter, we had two guiding questions in mind: how can we best bind metals on a molecular level, and how can we create a platform for functionalizing mycelial material? We also considered the advantages of using mycelia in comparison to previous eforts, such as fagella-based or cellulose fltration tools31,32. One of the biggest benefts of using mycelium material on this NASA-funded project is that it leverages the concept of economies of scale, and presents a feasible option for scale-up of our technology to a level that could be successfully implemented on a space mission or on Earth in developing countries with poor access to clean water. Fungi are capable of growth on diverse biomass types, and grow at a rate that is unparalleled by other biological agents used in synthetic biology applications today33. Using fungal mycelium as an immobile substrate, we designed a cost-efective, scalable, biodegradable fltration system for metal recovery from aqueous solutions using copper as a proof of concept. We assessed the feasibility of metal sequestration using a functionalized mycelia by treating the fungal surface with peptides containing metal-binding motifs in tandem repeats containing a chitin-binding domain that could bind to the solid mycelial surface of Gandoderma lucidum. G. lucidum has been described in literature for heavy metal binding, however this strain was not chosen for the innate metal-binding abilities, rather because it was shown to be a suitable candidate for the objectives of our overall project; these include ease of growth on diverse substrates, under diferent temperature regimes, and in environments that would be representative of those found in space-exploration applications. Please refer to the iGEM Stanford-Brown-RISD team website: for the characterization of this and other fungal strains: http://2018.igem.org/Team:Stanford-Brown-RISD/Experiments. Mycelium, the vegetative structure of a fungus that is analogous to the root system of most plants, can branch out and bind various substrates to fll molds in diferent shapes, thus making it a good candidate for water fltration applications. The peptides were designed to contain a chitin-binding domain (CBD), flexible linkers GSGGSG, and 2x-HHTC-Re (Fig. 4A). Afer confrming the copper binding of the individual HHTC-Re x n peptides (see previous section), we then needed to assess whether our fusion protein could bind copper and chitin, and whether it could do so when already saturated with the other substrate. Figure 5 depicts two experiments a) Raw data and b) isotherm for 2x-HHTC-Re-CBD+NaDg and Cu. In this experiment, N-acetyl D-glucosamine (“NaDg”; analogous to a chitin monomer and widely used in the literature for assessing chitin binding) was frst titrated into 2x-HHTC-Re-CBD and no isotherm was calculated because binding sites were not saturated by the ligand. Cu was then titrated into the 2x-HHTC-Re-CBD+NaDg complex and this resulted in a Cu afnity at Ka=(7.61±1.49) × 106 M−1 that is an order of magnitude higher than 2x-HHTC-Re (no CBD) with Ka=(3.73±0.53) × 105 M−1 and comparable to 2x-HHTC-Re-CBD without bound NaDg with Ka=(1.55±0.21) × 106 M−1 . Data show 20 1 µL injections. 2x-HHTC-Re-CBD was selected as the candidate for testing because it displayed the most consistent and strong results during protein purifcation procedures and seemed most promising for downstream Type Name Amino acid sequence Cu Ka [M−1 ] Zn Ka [M−1 ] Ni Ka [M−1 ] Zn → Cu Ka [M−1 ] Ni → Cu [M−1 ] Rational design with KDETSY residues removed HHTC Truncated HNLGMNHLQGRPVTGC (4.92±2.17) × 106 0 0 (1.67±0.85) × 106 (1.50±0.69) × 106 CHSK Truncated CPHVSQK (1.08±0.10) × 106 0 0 0 0 Rational design with KDETSY residues replaced with NLGQV HHTC Replaced HNLGMNHVLQGNRPLVTQGC (1.55±0.21) × 106 0 0 (3.99±2.24) × 106 (6.42±3.76) × 106 CHSK Replaced CPNLGHVSQNK (4.86±0.83) × 105 0 0 (3.34±0.46) × 105 0 2xHHTC Replaced in tandem 2x-HHTC-Re HNLGMNHVHNLGMNHVLQGN RPLVTQGCLQGNRPLVTQGC (3.73±0.53) × 106 — — — — 3xHHTC Replaced in tandem 3x-HHTC-Re HNLGMNHVLQGNRPLVTQGCHN LGMNHVLQGNRPLVTQGCHNLG MNHVLQGNRPLVTQGC (1.50±0.05) × 106 — — — — Table 2. Intrinsic association constants (Ka) for modifed rational design peptides titrated with Cu, Zn, Ni, and apparent binding constants when titrated with Cu afer Zn or Ni was in solution. Arrows Zn → Cu, Ni → Cu, indicate the association constants for Cu afer Zn or Ni are in solution as competing ions. Peptides in tandem were not assessed for Ni or Zn afnity. Scientific Reports | (2019) 9:16422 | https://doi.org/10.1038/s41598-019-52778-2 6 www.nature.com/scientificreports/ www.nature.com/scientificreports applications (such as our flter). Future work will focus on optimizing protein binding parameters to the chitin substrate and creating new fltration prototypes. Mycelium treated with and without 0.45mM 2x-HHTC-Re-CBD Cu/chitin-binding peptides were incubated with a solution containing Cu to determine the amount of Cu sequestered by the mycelium. Te Cu concentration in our starting solution was 325 (+/−25) µM Cu and afer an incubation with the untreated mycelium we observed that the remaining solution was 250 (+/−50) µM Cu, thus the untreated mycelium adsorbed approximately 23% of the Cu available in solution (Fig. 4B,C). In contrast, afer a 30-minute incubation, the mycelium treated with the CBD-2x-HHTC-Re motifs removed Cu to below detection limits thus sequestering at least 92% Figure 3. ITC experiment to determine binding parameters for peptides 1x-, 2x-, and 3x-HHTC-Re and visual representation of the peptide molecules predicted by the Zhang lab QUARK ab initio program. Te binding afnity Ka reveals strong Cu binding for all tested peptides and a trend is observed that suggests the 2x and 3x peptides do bind 2 and 3 times more Cu, respectively. Figure 4. (A) QUARK ab initio model of 2x-HHTC-Re with chitin binding domain. Domains within the fusion protein have been annotated to display the conformation and spatial orientation. (B) Cu (µM) remaining in solution for n=3 samples afer incubation with mycelium treated or not treated with CBD-2xHHTC-Re for 30minutes and (C) afer 72hours. Te Cu concentration in the initial copper solution was 325 (+/−25) µM Cu. Control reactions contained Cu solution only. All experiments were conducted in triplicate. Te amount of Cu adsorbed was calculated by taking the diference between the initial Cu in system and the remaining Cu afer incubation with treated or untreated mycelium. Cu is below detection limits for the treated flters afer 30minutes and 72hours. Afer 30minutes of tangential fow, the untreated mycelium adsorbed about 23% of the copper in solution, while the treated mycelium (flter prototype) was able to sequester ~92% of the available Cu in solution. Scientific Reports | (2019) 9:16422 | https://doi.org/10.1038/s41598-019-52778-2 7 www.nature.com/scientificreports/ www.nature.com/scientificreports of the available Cu in solution. Incubation for 72hours revealed that the untreated mycelium removed about 70% of Cu in solution while the treated mycelium removed all Cu from solution within the detection limits of the assay. Future work in this area will focus on characterization and optimization of the fungal mycelial surfaces and metal-binding peptides. Discussion In this work, we compared binding afnity and specifcity of naturally-occurring metal-binding motifs and peptides derived in silico and found that they had comparable binding parameters. Cu is an essential element in biology with roles as a cofactor in a wide range of proteins and diferences in binding afnities are due to the function of the metalloprotein they are derived from. Copper is a highly reactive element that can cause oxidative damage if not tightly controlled and regulated by the cellular machinery thus, Cu proteins are characterized by high afnity for their cognate metal. When applying the rational design approach for peptide engineering, an important consideration is that the algorithms used to predict peptide binding partners rely on protein databases to characterize the molecular geometry of a cognate metal in a metalloprotein. Te rationally-designed motifs showed comparable afnities to the natural peptides because they themselves are derived from known Cu structures in Protein Data Bank29. Te algorithms used by previous researchers build from known metalloprotein structures to take short motifs and put them together into one peptide that fulflls the molecular geometry of the target metal27. A key feature of these previous approaches is that only metal-peptide complexes for which there is crystal structure data are used, thus limiting the computational approach to metals with known biological functions and excluding metals that have not been characterized for biological interactions. It is possible to model interacting ligands with amino acid binding partners represented by individual residue side chains and without the need for the short motifs obtained through PDB that add rigidity to the peptides predicted to bind Cu. For example, it was found that while sofer ligands such as cysteine and methionine have to be modeled using bulkier representations (e.g., the whole side chain of the residue), harder ligands can be represented by simply using carboxyl groups, amines, and alcohols in computational models34. Tis approach to peptide prediction achieves a similar level of accuracy to using the whole amino acid molecule, while greatly reducing the computational requirements. Te rationally-designed HHTC and CHSK peptides give insight into the types of binding a metal can participate in. Both fulfll the preferred geometry of Cu, square planar, but with very diferent binding residues that suggest a certain fuidity in binding principles with regard to the ligands involved. Both tested peptides have non-canonical binding partners (threonine in HHTC, and serine and lysine in CHSK) as the residues that coordinate a Cu atom yet they each bind selectively and with similar afnity despite having vastly diferent primary sequences. Teir afnity for Cu was enhanced when removing or replacing the amino acid residues (KDETSY) Figure 5. ITC raw data and isotherm for experiment to assess binding afnity of fusion protein CBD-2xHHTC-Re for chitin (represented by N-acetyl D-glucosamine) and Cu. Motifs retained afnity for chitin monomers in the presence of Cu, and retained afnity for Cu in the presence of chitin (monomers). Scientific Reports | (2019) 9:16422 | https://doi.org/10.1038/s41598-019-52778-2 8 www.nature.com/scientificreports/ www.nature.com/scientificreports that were presumed to interact with competing metal ions. It is possible that this observed phenomenon was due to a more favorable ft around the Cu ion. An interesting phenomenon was observed when the Cu-02 and modifed CHSK motifs, which lack the Zn and Ni binding residues, both lost afnity for Cu when Ni was in solution. While the peptide was designed for the square planar molecular geometry of Cu, Ni can bind in octahedral molecular geometry which can also accommodate square planar molecular geometry, thus making these peptides susceptible to competitive inhibition35. In this sense, the algorithm used to create these peptides and predict optimal binding for Cu, is limited by having to use only PDB structures. For future work in rational design, it is essential that all of the residues in a peptide, not only the ligands that interact with the metal ion, be considered for specifcity since interaction with competing ions will reduce afnity for the cognate metal. Te rational design peptides KDKD and KDTK, that were designed to bind Zn did bind Cu with more afnity than for their cognate metal and thus conform to the Irving-Williams stability series principles. While no S-containing residues are present in the peptide sequences (cysteine, methionine), both sequences tested contain the hydroxyl-containing threonine resides that can interact with Cu. It would be curious to test whether removal of these residues would afect their afnity to Zn while limiting the electrostatic interactions with Cu. Additionally, since Cu is predicted to form the most stable complexes, future approaches in metal sequestration from mixed solutions should follow the order of the stability of complexes as stated in the Irving-Williams series. To develop a pathway to implemented metal recovery, we explored the feasibility of metal attenuation with our engineered peptides and functionalized fungal mycelia and found that incubation with metal-binding peptides containing a chitin binding domain increased the amount of Cu removed from solution. Previous studies have characterized the adsorptive properties of mycelium and they found that diferences in metal-binding capacity for Cu was directly related to the cation exchange capacity at the mycelial surface of individual fungal species36. Tus, there is an inherent ability for mycelium to attenuate heavy metals despite the hydrophobicity of the chitin substrate. Tis property is further enhanced by treatment with our peptide constructs that turn an otherwise hydrophobic surface into a functionalized adsorptive surface that can interact with dissolved ions in aqueous solutions. While cellulose provides an alternative substrate and cellulose-binding motifs are well-known, the production of nanocellulose requires substantial inputs of glucose, and other forms of cellulose incur agricultural costs37. Tus, we believe fungal mycelia provide a better alternative in most situations including bioremediation. Future work in metal-binding peptide design should introduce molecular geometries for metals that have no role in biology but are of geopolitical importance such as the lanthanide series and platinum group elements. Tese metals do not have biological roles, however, by calculating association energies between the metals and amino acid residue side chains, the free energies can be approximated, and peptide partners predicted. If no known molecular binding partners in available amino acid side chains exist, then using synthetic amino acids with residues that have the ability to bind metal ligands can be another approach at targeting these metals. In conclusion, we found that motifs developed through rational design by applying quantum mechanical methods that account for complexation energies of the elemental binding partners and molecular geometry of the cognate metal, not only show high afnity for the cognate metal, but they show specifcity and discrimination against other metal ions that would-be competitors for the same binding sites. Previous computational models used to predict metal-peptide complexes use known crystal structure data, however this limits peptide models to metals with biological function and known ligands. It is possible to model interacting ligands de novo with binding partners represented by using individual residue side chains as discreet binding partners and this approach can be used to design peptides to recover other metals such as rare earth or platinum group elements for biomining/recycling. Methods Peptide synthesis. Peptides were synthesized by Elim Biopharmaceuticals (Hayward, CA, USA), purifed by HPLC with (H)Cl as the counter ion, and provided as a lyophilized powder. Purity was>98% and verifed through mass-spectrometry. Peptides were modifed to have N-terminal acetylation and C-terminal amidation to avoid having a charged peptide. Peptides were reconstituted in 10mM 2-(N-morpholino)-ethanesulfonic acid (MES) bufer pH 5.5 and concentrations determined through spectrophotometry with the Pierce™ BCA Assay. Fusion protein production and purifcation. Te DNA sequences encoding the 2x-HHTC-Re protein constructs containing an intein self-cleavage site, hexahistidine and Lumio ® detection tags, were commercially synthesized (IDT, gBlocks®) and were assembled into the PSB1C3 iGEM backbone using Gibson Assembly (NEB), transformed NEB DH5a® competent E. coli for plasmid construction and transformations plated on chloramphenicol selective LB plates. Te transformations were incubated at 37 °C overnight and DNA constructs confrmed in select colonies using verifcation primers and colony PCR. Plasmids from sequence-verifed clones were then transformed into the T7 Express® E. coli (NEB) protein production strain. His-tag purifcation on crude cell extract from our colonies using TermoFisher HisPur® Ni-NTA spin columns. Te standard protein purifcation protocol was modifed by introducing a bufer containing 50mM DTT for on-site cleavage, so the desired fusion protein could be eluted. We then performed a BCA Assay to determine our total protein concentration in each elution. Following protein de-salting with Amcon® flter spin columns, we confrmed the presence of target protein in the fnal elution using TermoFisher Scientifc Lumio® Tag Detection Kit. Mycelium biomass production. To test the peptides, the monokaryon form of Ganoderma lucidum from USDA Forest Products Laboratory culture collection was grown in plates containing liquid Potato Dextrose Yeast Agar (PDYA), composed of 2.0% dextrose, 0.10% potato extract, 0.15% yeast extract, and 97.50% water at 30°C for 14 days. Pure mycelium material was isolated, cut into strips, pressed into thin uniform sheets, dried at 120°C for three hours to kill the fungus, and then cut into uniform 1 cm2 pieces to be used as flter prototypes. We Scientific Reports | (2019) 9:16422 | https://doi.org/10.1038/s41598-019-52778-2 9 www.nature.com/scientificreports/ www.nature.com/scientificreports tested growth rates on varied substrates to determine which could be used with minimal added growth medium. Noteworthy substrates of environmental relevance were sawdust, lawn clippings, and used cofee grounds and other forms of food waste. Te standardized substrate on which we tested diferent conditions was potato dextrose yeast agar (PDYA)s which provides optimal nutrients for the fungus without providing excess nutrients that would encourage bacterial growth. Phen green SK assay. For Cu adsorption estimation, Phen Green SK dye (Invitrogen, USA) was prepared in a stock solution of 28 µM in 0.9% PBS. PGSK fuorescence is quenched in the presence of Cu and is proportional to the amount of Cu in a solution. 200 µL of this stock solution was added to each well in a 96-well plate to which 25µL of sample was added. All experiments were done in triplicate and the reported concentrations are the average of n=3 experimental replicates. A standard curve was generated for Cu and PGSK and gave approximations of the amounts of Cu in the tested solutions. Bulk adsorption experiments. All experiments were conducted in triplicate. One cm2 pieces of mycelium were either treated with purifed protein 0.45 mM CBD-2xHHTC in 10 mM MES bufer pH 5.5 or with bufer only in shake fasks for 24 hours. Treated and untreated mycelium were placed in 3 mL of 0.25 mM Cu solution and samples taken at 30minutes and at 72hours. Te incubation medium was analyzed for the remaining Cu in solution. Te amount of Cu adsorbed was calculated by taking the diference between the initial Cu in system and the remaining Cu afer incubation with treated or untreated mycelium. Isothermal titration calorimetry. Isothermal titration calorimetry (ITC) was used to determine the association equilibrium constant (Ka). Te instrument used to obtain measurements was a Malvern MicroCal iTC200 in the Space Biosciences Division at NASA Ames Research Center, Mountain View, CA. Instrument performance was verifed by running the standard Ca-EDTA titration kit available from Malvern Analytical. All binding parameters for the test were within the specifcations determined by the manufacturer. Te bufer chosen for the ITC experiments was 10mM, MES bufer. It was chosen because it has been shown to not cause metal ion interference as a result of complexation or amine oxidation, is stable through the entire range of pH 3–11, and has a stable pKa over a relatively wide temperature range (15 °C–45 °C)38,39. Bufer pH for peptides and metal solutions was 5.5 to prevent metal precipitation. Metal chloride salts were used as the source of metal ions, and speciation was verifed through thermodynamic modeling using Visual Minteq. 3.040. Concentrations for metal stock solutions were determined with the Termo iCAP 7400 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) at the University of California Santa Cruz, Marine Analytical Laboratory. To measure Cu binding to our peptide, the metal solution was prepared from cupric chloride, dihydrate, crystal, BAKER ANALYZED™ A.C.S. Reagent, J.T. Baker™. Te peptides were used without further purifcation. Te peptide solutions were prepared by dissolving a weighed amount of the lyophilized powder in 10mM MES prepared from Alfa Aesar™ MES, 0.2M bufer soln., pH 5.5. pH. Metal solutions were prepared by dissolving a weighed amount of the pure metal chloride salts into the same stock MES bufer that was used to prepare the peptide solution to minimize the efect of the heat of dilution/mixing when measuring the samples. Te ITC experiments were run at 25 °C and set to deliver 20, 0.5–1 µL injections at 150 sec intervals. Titrate and titrant solutions were de-gassed prior to loading into calorimeter cell and injection syringe. Te procedure involved titrating (Cu, Zn, Ni)-Cl2 in excess by 10–20 times the concentration of the cognate motif. Typically, the peptide solutions were prepared to 0.5mM, and the metal salts were at 2.4–6.4mM concentrations. In some cases of low afnity, or when no saturation of the metal-binding peptide was observed, up to 100 times the concentration of metal was used. Metal-chloride salts were chosen because they remain as dissolved ions with chloride as the counter ion. Tis was complementary to the peptide conditions where chloride (HCl) was used as the counter ion during purifcation. Te experiments were run such that the metal solution in the syringe was titrated into the peptide solution in the cell. Raw data were corrected by subtracting the heats of dilution. Integrated heat data were ft with a one-site binding model using the Origin-7™ sofware provided with the MicroCal iTC200. Te “best-ft” parameters resulting from the nonlinear regression ft of these data are also shown in the fgures. Controls and heat of injections, heat of dilution. Te mixing and dilution efects for the ITC experiments were minimized by using the same bufer for the peptide and metal salt preparations. Heat of dilution was determined by three titration experiments where 1) metal chloride (ligand) solution was titrated into bufer in the sample cell, 2) bufer was injected from the syringe into the peptide-bufer solution in the sample cell, and 3) bufer was titrated into bufer only in the sample cell. Metal-chloride titrations into the sample cell with blank bufer released heats comparable to blank bufer mixing where bufer-bufer titrations released 0.02 μcal/sec per injection, 5mM CuCl2=0.08 μcal/sec, 2.6mM NiCl2=0.05 μcal/sec. Heat of dilution/mixing for 6.4mM ZnCl2 was measured at 1.0 μcal/sec per injection and up to 15 μcal/sec for 64mM ZnCl2. In cases where heat of dilution/mixing caused a high background, the blank values were subtracted from the raw data prior to model isotherm fts. Te heat of ionization of the bufer due to the release or uptake of protons during binding from the bufer conjugate base was determined to be negligible thus we did not correct for heat of ionization of the MES bufer41. Received: 7 March 2019; Accepted: 26 September 2019; Published: xx xx xxxx Scientific Reports | (2019) 9:16422 | https://doi.org/10.1038/s41598-019-52778-2 1 0 www.nature.com/scientificreports/ www.nature.com/scientificreports
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Acknowledgements
We thank NASA’s Innovative Advanced Concepts (NIAC) program, and NASA Advanced Exploration Systems for funding graduate student JU and this work through grants to LJR. We would also like to thank Colin Williams and the US Geological Survey for post-doctoral support for JU during the fnal implementation stages of this project and for funding our publication. Additional analytical support was provided by John Hogan at NASA Ames Research Center Space Biosciences Division for the ITC instrument, and Rob Franks in the Marine Analytical Laboratory at the University of California Santa Cruz with the ICP-OES instrument. We would also like to thank Seth Rubin from the University of California Santa Cruz for his contributions as a thesis advisor to JU in data interpretation and experimental design. For the fungal work, we thank PhD Candidate Rolando Perez and the Drew Endy Lab at Stanford University their guidance with mycelium production. Author contributions J.U., A.P., K.F., I.G.P.L. and L.J.R designed the experiments; J.U. and A.P. performed the experiments. J.U., C.S., A.P. analyzed and interpreted data; J.U. wrote the manuscript with contributions from all authors. Competing interests Te authors declare no competing interests. Additional information Correspondence and requests for materials should be addressed to J.U. Reprints and permissions information is available at www.nature.com/reprints. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional afliations. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Te images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
Abstract
Graphical abstract
Keywords
1. Introduction
2. High-technology applications of REE
3. Distribution of REE in the Earth's crust and mineralogy
4. REE exploration
5. Metallurgy
6. Environmental impact and health effects of REE exposure
7. REE in agriculture
8. REE in medicine
9. Recycling
10. R&D studies on the reducing their usage or on substitution for REE in different applications
11. Chemical characterization of REE—Application of different instrumental analytical techniques
12. Conclusions
Acknowledgements
References
Vitae
This paper presents an update on all aspects of REE including their distribution in the Earth's crust, impact on environment, and human health.
•
Other aspects covered include different types of deposits, metallurgy, applications in agriculture and medicine, and recycling.
•
An outline of the recent advances in their precise determinations required in all these studies is also presented.
•
This is the most comprehensive review attempted on REE so far in the literature.
Abstract
Rare earth elements (REE) include the lanthanide series elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) plus Sc and Y. Currently these metals have become very critical to several modern technologies ranging from cell phones and televisions to LED light bulbs and wind turbines. This article summarizes the occurrence of these metals in the Earth's crust, their mineralogy, different types of deposits both on land and oceans from the standpoint of the new data with more examples from the Indian subcontinent. In addition to their utility to understand the formation of the major Earth reservoirs, multi-faceted updates on the applications of REE in agriculture and medicine including new emerging ones are presented. Environmental hazards including human health issues due to REE mining and large-scale dumping of e-waste containing significant concentrations of REE are summarized. New strategies for the future supply of REE including recent developments in the extraction of REE from coal fired ash and recycling from e-waste are presented. Recent developments in individual REE separation technologies in both metallurgical and recycling operations have been highlighted. An outline of the analytical methods for their precise and accurate determinations required in all these studies, such as, X-ray fluorescence spectrometry (XRF), laser induced breakdown spectroscopy (LIBS), instrumental neutron activation analysis (INAA), inductively coupled plasma optical emission spectrometry (ICP-OES), glow discharge mass spectrometry (GD-MS), inductively coupled plasma mass spectrometry (including ICP-MS, ICP-TOF-MS, HR-ICP-MS with laser ablation as well as solution nebulization) and other instrumental techniques, in different types of materials are presented.
Rare-earth elements (hereinafter referred to as REE) are a group of seventeen chemical elements in the periodic table, in particular the fifteen lanthanides as well as yttrium and scandium as defined by the International Union of Pure and Applied Chemistry (IUPAC).
Scandium and yttrium are considered REE since they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties. All REE occur in nature but not in pure metal form, although Promethium, the rarest, only occurs in trace quantities in natural materials as it has no long-lived or stable isotopes (Castor and Hendrik, 2006). In lanthanide atoms, the configuration of the valence electrons of the outermost shell is the same for all the species while the 4f orbitals are progressively filled with increasing atomic number. Screening of the 4f orbitals leads to the extremely similar physical and chemical properties of the elements. Another related consequence is the so-called "lanthanide contraction" in which the ionic radius progressively decreases from La3+ (1.06 Å) to Lu3+ (0.85 Å). REE are the elements that have become extremely important to our world of technology owing to their unique magnetic, phosphorescent, and catalytic properties. These elements are critical to technologies ranging from cell phones and televisions to LED light bulbs and wind turbines. The estimated average concentration of the REE in the Earth's crust which ranges from around 130 μg/g to 240 μg/g which is actually significantly higher than other commonly exploited elements, and much higher than their respective chondritic abundances (Zepf, 2013). Table 1 presents the average abundances of REE (in μg/g) in Earth's crust in comparison with chondritic abundances. This review aims to provide an updated understanding of the global scenario of REE, starting from their applications in high technology products, occurrence, different types of economic deposits both on land and oceans, their behavior in different geological systems, state-of-the-art chemical characterization techniques and recycling. Other aspects such as their applications in agriculture, medicine together with the environmental effects and search for alternatives to REE in different applications are also presented.
Table 1. Average abundance (in μg/g) of REE in the Earth's crust in comparison with chondritic abundances.
During the last three decades, there has been an explosion in the applications of REE and their alloys in several technology devices such as computer memory, DVDs, rechargeable batteries, autocatalytic converters, super magnets, mobile phones, LED lighting, superconductors, glass additives, fluorescent materials, phosphate binding agents, solar panels and magnetic resonance imaging (MRI) agents. These metals are being consumed this way for a variety of applications at an unprecedented rate. Since they are extremely important ingredients in all high-technology gadgets, these elements are called “The Vitamins of Modern Industry” (http://metalpedia.asianmetal.com/metal/rare_earth/application.shtml). For example, Nd is extensively applied in super magnets for disk drives, Ce is critical ingredient in autocatalyts and all REE are used in making the flat-panel TVs. Several compounds of REE are in smart-batteries that power every electric vehicles and hybrid-electric vehicles. Because of their unique physical, chemical, magnetic, luminescent properties, these elements help to make many technological advantages such as performing at reduced energy consumption, greater efficiency, miniaturization, speed, durability and thermal stability. In recent years, their demand is particularly on rise in energy efficient gadgets (green technology) which are faster, lighter, smaller and more efficient. These technologies are even helping the analytical instruments to become smaller and more efficient (Balaram, 2016a). Table 2 summarizes the extensive applications of REE in the modern technological gadgets in different areas. Because of the emerging green technologies in a big way, the REE application boom would continue in near future also. Fig. 1 presents an overview of REE production by different countries and utilization for different applications in 2017. It is very interesting to see that these applications are dominated by their use in autocatalytic converters and industrial catalysts. Dutt et al. (2016) predicted that global REE demand is slated to grow at annual rate of 5% by 2020 and Graede (2015) in a three-dimensional graphic indicated that there will be scarcity and supply risk for Eu, Dy and Er in addition to several other metals in the near future. This estimation is based on their scores in three areas namely supply risk, environmental implications, and vulnerability to supply restrictions (Fig. 2). Thus, the increasing global demand for these elements is currently generating lot of interest among exploration geochemists as well as technology developers especially because of their very important role in green applications such as hybrid cars, electric car batteries and wind turbines under the backdrop of climate change and global warming.
Television screens, computers, cell phones, silicon chips, monitor displays, long-life rechargeable batteries, camera lenses, light emitting diodes (LEDs), compact fluorescent lamps (CFLs), baggage scanners, marine propulsion systems
Manufacturing
High strength magnets, metal alloys, stress gauges, ceramic pigments, colorants in glassware, chemical oxidizing agent, polishing powders, plastics creation, as additives for strengthening other metals, automotive catalytic converters
Medical Science
Portable X-ray machines, X-ray tubes, magnetic resonance imagery (MRI) contrast agents, nuclear medicine imaging, cancer treatment applications, and for genetic screening tests, medical and dental lasers
Hybrid automobiles, wind turbines, next generation rechargeable batteries, biofuel catalysts
Others
The europium is being used as a way to identify legitimate bills for the Euro bill supply and to dissuade counterfeiting. An estimated 1 kg of REE can be found inside a typical hybrid automobile
Holmium has the highest magnetic strength of any element and is used to create extremely powerful magnets. This application can reduce the weight of many motors
Figure 2. Estimated supply risk for Eu, Dy and Er in addition to several other metals in the near future (Graede, 2015).
3. Distribution of REE in the Earth's crust and mineralogy
In nature, REE do not exist as individual native metals such as gold, copper and silver because of their reactivity, instead occur together in numerous ore/accessory minerals as either minor or major constituents. Though REE are found in a wide range of minerals, including silicates, carbonates, oxides and phosphates, they do not fit into most mineral structures and can only be found in a few geological environments. The principal economic sources of REE minerals are bastnaesite, monazite, and loparite and the lateritic ion-adsorption clays. There are over 250 minerals which contain REE as important constituents in their chemical formula and crystal structure (Dostal, 2017). Table 3 presents the list of some important REE bearing minerals associated with REE deposits.
Table 3. Names and formulae of some important REE-bearing minerals associated with REE deposits (Dostal, 2017).
Mineral
Formula
element
Allanite
(Y,Ln,Ca)2(Al,Fe3+)3(SiO4)3(OH)
Apatite
(Ca,Ln)5(PO4)3(F,Cl,OH)
Bastnaesite
(Ln,Y) (CO3)F
Eudialyte
Na4(Ca,Ln)2(Fe2+,Mn2+,Y)ZrSi8O22(OH,Cl)2
Fergusonite
(Ln,Y)NbO4
Gittinsite
CaZrSi2O7
Iimoriite
Y2(SiO4) (CO3)
Kainosite
Ca2(Y,Ln)2Si4O12(CO3)·H2O
Loparite
(Ln,Na,Ca) (Ti,Nb)O3
Monazite
(Ln,Th)PO4
Mosandrite
(Na,Ca)3Ca3Ln (Ti,Nb,Zr) (Si2O7)2(O,OH,F)4
Parisite
Ca (Ln)2(CO3)3F2
Pyrochlore
(Ca,Na,Ln)2Nb2O6(OH,F)
Rinkite (rinkolite)
(Ca,Ln)4Na(Na,Ca)2Ti(Si2O7)2(O,F)2
Steenstrupine
Na14Ln6Mn2Fe2(Zr,Th) (Si6O18)2(PO4)7·3H2O
Synchysite
Ca (Ln) (CO3)2F
Xenotime
YPO4
Zircon
(Zr,Ln)SiO4
4. REE exploration
Currently the world reserves of REE by principal countries such as China, Brazil, Vietnam, Russia and India, stand at about 130 million tonnes (U.S. Geological Survey, 2018) (Table 4). These resources are primarily from four geologic environments: carbonatites, alkaline igneous systems, ion-adsorption clay deposits, and monazite-xenotime-bearing placer deposits. China with one-third of world's REE reserves, is still the world leader in REE exploration and production. Before REE mining boom in China, the US dominated the global market. Mountain Pass initiated operations in 1965 and was the leading producer worldwide for decades (Barakos, 2017). However, mining activities stopped in 1998, mainly due to the competition from China as well as in response to environmental issues in the surrounding area of Mountain Pass (Ali, 2014, Mancheri, 2015). Apart from Mountain Pass, significant exploration projects in the United States include the Bear Lodge, the Bokan-Dotson Ridge, the Round Top and the La Paz projects (Barakos et al., 2018). While the exploration studies worldwide are mainly concentrated on Au, Ag, Cu, platinum group elements (PGE), Ni, Cu, Cr, Li, U, Zn, K and Pb, some countries in Africa and Asia Pacific showed interest in REE exploration in 2017. Apart from China and India, countries such as Kazakhstan, Kyrgyzstan, Tajikistan, Uzbekistan, and Turkmenistan have also identified significant REE-bearing mineral occurrences including alkaline igneous rocks and carbonatites (Kogarko et al., 1995, Mihalasky et al., 2018). There has been continued interest in exploration for REE worldwide. In an annual exploration review, Wilburn and Karl (2018) reported that the United States Geological Survey (USGS) conducted extensive hyperspectral surveys in Alaska and evaluated the mineral potential for REE in addition to other metal deposits throughout the State in 2017. India which account for about 5% of the world's REE reserves, currently exploits its primary resource, monazite. Significant REE minerals found in India include ilmenite, sillimanite, garnet, zircon, monazite and rutile, collectively called Beach Sand Minerals (BSM). India has almost 35% of the world's total beach sand mineral deposits. The REE minerals in India predominantly contain light REE (LREE). The Geological Survey of India (GSI) has taken up number of REE exploration projects across the country during the last 5 years. The search for REE mineralization includes identification of anomalous concentration zones on the basis systematic geological mapping (SGM) at scale 1:50,000, specialized thematic mapping (STM) at scale 1:25,000, and National Geochemical Mapping (NGCM) at scale 1:50,000 and delineation of target zones for enhanced potential through reconnaissance stage investigations (https://www.gsi.gov.in). In recent times, GSI has increased its efforts for the exploration of REE in addition to base metals and rare metals, and also for the development of cost-effective extraction and separation methods. Major targets are alkaline rocks, skarns, carbonatites, peraluminous granitoids and their derivatives like pegmatites, quartz-veins and greisens which host primary REE deposits in addition to placers and residual weathering deposits derived from primary deposits. Xenotime deposits (xenotime is a rare earth phosphate mineral which is a rich source of yttrium and heavy rare earths) in Madhya Pradesh, carbonatite-alkaline complex in Ambadongar, Gujarat, polymetallic mineralization in Siwana Ring Complex, Rajasthan (Banerjee et al., 2014) are some of the promising areas for REE exploration and exploitation. Monazite in the beach and inland placer deposits in India have been estimated to be 11.935 million tonnes in 2016 with over 30% of the resources coming from the coastal state of Andhra Pradesh (IBM Report, 2018). During the last quarter century, the demand for REE skyrocketed because of innumerable number of applications. Other countries with notable production are US, Australia, Brazil, India, Malaysia, Russia, Thailand and Vietnam. Fig. 1 presents the estimated global REE production in 2017. However, China has restricted its exports since 2010 for reasons unknown. Although there are lots of research and development efforts going on to find out a cheaper substitution for the REE (e.g. Pavel et al., 2017), there are no significant breakthroughs yet in this direction. Because of these reasons, the REE demand increased manifold and currently vigorous exploration efforts are going on to find out new REE ore deposits. However, due to their geochemical characteristics, REE are very dispersed and not usually found concentrated as rare earth minerals in economically exploitable ore deposits. Hence, it is unusual to find them in quantities significant enough to form economic ore deposits. The world's largest REE deposits are mostly bound to carbonatites or their altered equivalents (Laznicka, 2010). Detecting possible future deposits and determining the concentrations and relative enrichment of REE in carbonatites, silico-carbonatites, peralkaline granites and pegmatites are of interest for mineral exploration and mining processes. However, commercially operating mines around the world mainly extract bastnaesite, monazite and xenotime ores. Mainly, monazite from beach placers is mined in India as the principal ore mineral for REE, although xenotime holds out some prospect for the future. Of India's estimated reserve of 5 million tons of monazite, 70%–75% which occurs in beach placers and the rest in the inland and offshore areas. Monazite content of beach sands may be up to 11 wt.%. Recent studies (Gonzalez et al., 2010, Kato et al., 2011, Balaram et al., 2012) have indicated that different types of seafloor sediments which harbor high concentrations of REE. It is believed that ocean bottom-REE resources are much more promising than on-land resources (Kato et al., 2011, Takaya et al., 2018).
Although REE are relatively abundant in the Earth's crust, unlike most other metals, they are rarely concentrated into mineable ore deposits. Potential deposits of the REE can be divided into primary and secondary deposits. Primary deposits are those formed by magmatic, hydrothermal and/or metamorphic processes. These deposits are most commonly associated with alkaline igneous rocks and carbonatites, emplaced into extensional settings. Secondary deposits are those formed by erosion and weathering and may include placers, laterites and bauxites. Within these two groups REE deposits can be further subdivided depending on their genetic associations, mineralogy and form of occurrences. As REE deposits appear in a wide variety of geological environments, it is not very easy to classify them into different categories. Sowerbutts (2017) classified the economically important REE deposits into four categories, however, in view of significant quantities of these elements in coal and marine sediments, in the present study, the REE deposits have been divided into the following five categories, i.e., (i) Alkaline igneous rocks: pegmatites and carbonatites, (ii) residual deposits, (iii) heavy mineral placers, (iv) REE in coal and (v) REE in the sediments of continental shelf and Ocean bottom. These will be considered in little more detailed way in the following.
4.1.1. Alkaline igneous REE deposits
Alkaline igneous rocks are formed by the partial melting of deep mantle rocks, which subsequently rise and cool within the Earth's crust. Typically, an alkaline magma is enriched in not just REE but also Zr, Nb, Sr, Ba, and Li. As the magma ascends it changes chemically in response to a complex interaction of factors including temperature, pressure, and the chemistry of the surrounding rocks. This complex interaction results in the formation of great variety of REE deposits (Ray and Shukla, 2004). The hosts are differentiated rocks ranging from nepheline syenites and trachytes to peralkaline granites. These complexes usually occur in continental within-plate tectonic settings associated with rifts, faults, or hotspot magmatism. The REE mineralization is also found in layered alkaline complexes, granitic stocks, and late-stages dikes and rarely trachytic volcanic and volcaniclastic deposits (Dostal, 2017). Examples of REE deposits associated with alkaline igneous rocks include Mountain Pass, California; China's Bayan Obo; and Ytterby, Sweden. China's Bayan Obo deposit, the largest REE deposit, is a high-grade, igneous related carbonatite deposit that sources 80% of the world's LREE (Verplanck et al., 2014). Carbonatites of the Sung Valley ultramafic–alkaline–carbonatite Complex, West Jaintia Hills and East Khasi Hills districts of Meghalaya, North East India has been identified by Singh et al. (2014) with encouraging values for REE (ΣLREE values range from 895.17 μg/g to 1264.85 μg/g and ΣHREE values range from 60.98 μg/g to 81.92 μg/g). Cut-off grade in mining metallurgical operations are influenced by economic indicators such as market price of both the primary product and the by-product, mining and processing costs, metallurgical recovery tonnage grade distribution (Nieto and Zhang, 2013). Hence, it is very difficult to mention the cut-off grade of the ore. The Saranu deposit located in Rajasthan, is a significant carbonatite deposit within India that carries notable concentrations of REE. The deposit contains ≥5.5% REO (rare earth oxides) and they are hosted in carbonatite dikes of about 10 cm wide (Wall and Mariano, 1996). LREE minerals such as bastnäsite, parisite, and synchysite have been reported from the Amba Dongar carbonatite complex, Gujarat (Doroshkevich et al., 2009). By using geological mapping and grid channel geochemical sampling methods, Bhushan and Kumar (2013) discovered carbonatite plugs, sills and dykes hosting REE deposits at Kamthai (very close to Saranu deposit), Barmer district, Rajasthan, India, with potential REE resources of 4.91 Mt and estimated up to 84 m depth having highest value of LREE is 17.31% and the mean grade is 3.33%. In a subsequent study of these carbonatites as well as host ijolite plug of Kamthai, Bhushan (2015) attributed the genesis of this deposit to mantle ‘hot spot’ activity coeval with the Deccan volcanism. Recently Bhushan and Somani (2019) made an extensive study on the REE and Y potentials of Neoproterozoic peralkaline Siwana granite of Malani igneous suite, Barmer district, Rajasthan. They also made an extensive REE mineral characterization of the suite with well exposed and outcrops and opined that the area is well suited for open cost mining. Total REE content of Neoproterozoic Peralkaline Siwana granite of Malani igneous suit, is ranging from 2% to 2.5%. The economic viability will greatly depend upon the mineralogy and leachability and separation through metallurgical test. Even 50% extraction from the ore will be economical due to high price of HREE. As the area is well exposed and outcrops have high relief, the cost through opencast mining will be minimum (Bhushan and Somani, 2019).
4.1.2. Residual deposits
Residual deposits formed from deep weathering of igneous rocks, pegmatites, iron-oxide copper-gold deposits. Intense weathering of carbonatite and peralkaline intrusives may form concentrated residual deposits of REE minerals. Examples include REE-laterite in south China resulting from weathering of tin granite. Ion-adsorption ores which come under this category are known only from China. Appreciable area of North East India experiences climatic conditions favorable for the development of thick lateritic profile and is promising for this type of REE deposits (Singh et al., 2014). Bauxite, the principal ore of aluminum formed during the weathering of various kinds of aluminosilicate source rocks (Valeton, 1972), is also found to be a rich source for REE. Bauxite residue (red mud) is the solid residue generated in the Bayer process for aluminum production. This waste residue as well as selected bauxite deposits are potential sources of REE (Vind et al., 2018).
4.1.3. Heavy mineral placer deposits
Potentially useful concentrations of REE-bearing minerals are also found in placer deposits. Most placer accumulations with significant amounts of REE minerals are Tertiary or Quaternary deposits derived from source areas that include granitic rocks or high-grade metamorphic rocks; however, paleo placer deposits that are as old as Precambrian, also contain REE resources. Monazite [(Ce,Th)PO4], a common REE-bearing accessory mineral in igneous, metamorphic, and sedimentary rocks, may be concentrated with other heavy minerals in placer deposits. Monazite is an important REE ore and most monazite ore deposits that are economically viable REE resources. Monazite includes cerium, and associated light REE and heavy REE such as Tb, Dy and Gd tend to be rarer, smaller, and less concentrated. In India, monazite is the principal source of light REE. As a result, for HREE, India depends heavily on imports. Examples of placer deposits of REE include tin-rich river placers in Malaysia; paleo-placers in Witwatersrand, South Africa; Elliot Lake, Ontario, and beach sand deposits of Kerala, Andhra Pradesh, Tamil Nadu and Odisha in India. Though China, Malaysia and India are the countries involved at present in the processing of monazite, Brazil and Russia are also entering in to monazite processing. Palaparthi et al. (2017) have carried out a comprehensive study to determine the radioelement and REE concentrations in beach placer deposits at selected locations along the eastern coast of Andhra Pradesh in India to understand their economic potential. Small concentrations of thorium and uranium are making monazite a restricted ‘atomic’ mineral in India. As a result, private firms are not being allowed processing of monazite at present.
4.1.4. REE in coal deposits
As the gap between REE global demand and supply increases, the search for their alternative resources has become more and more important, especially for the countries which depend highly on their import. Coal fly ash, which is considered as waste, has now been regarded as the possible source for many elements, including REE (Franus et al., 2015). This is because the REE concentrations in many coals or coal ashes are equal to or higher than those found in conventional REE ores. For example, in coal samples from the Mazino Coal Mine, Tabas Coalfield, Iran, ΣREE range of 16.4–184 μg/g with average of 88.9 μg/g (Pazand, 2015). In a recent study, Sahadev et al. (2018) have observed considerably low REE in the Indian coal samples from the North Eastern Region (NER) in comparison to world average coal. On the other hand, in world coal ash ΣREE value is ∼404 μg/g (Dai et al., 2016). In fact, even much higher values up to 1358 μg/g of REE (including Y) have been reported in the literature (Hower et al., 2016). This would offer the potential to reduce our dependence on others for these critical elements and also create new industries in regions where coal has played an important economic role. The possible recovery of REE from abundant coal and coal ash is an exciting new research area, representing a dramatic paradigm shift for coal (Franus et al., 2015). Some scientists believe that the future of REE may lie with coal (Alvin et al., 2017). Currently several studies are underway to optimize the extraction of REE from coal fly ash (e.g. Franus et al., 2015, Taggart et al., 2018).
4.1.5. REE in the sediments of continental shelf and ocean bottom
Mineral resources of the continental shelf are similar to those of contiguous land. Minerals eroded from the land may be concentrated in placer deposits in beaches along the coast and similar placer deposits can be expected farther offshore, beneath the ocean. Continental shelves also contain huge deposits of phosphorite (phosphate rock or rock phosphate), a non-detrital sedimentary rock which is also an important source of REE. Recent studies of marine phosphorite deposits reveal that there are significant quantities of REE contained within the phosphorites (Mazumdar et al., 1999, Nath et al., 2000, Khan et al., 2012, Dar et al., 2014, Li and Schieber, 2015). Phosphorite deposits form as chemical precipitates on continental shelves at relatively shallower depths in oceans (Fig. 3) unlike manganese nodules and cobalt crusts. Upwelling of cold, phosphate rich waters causes warming and decreases in solubility, and are deposited. It has been recognized that REE get enriched in the francolite mineral phase of phosphorites where REE substitute for Ca in the francolite lattice (Jarvis, 1994). Some of the phosphorite deposits contain ΣREE up to 2000 μg/g, although composition of these rocks mostly depends on their type and origin. Emsbo et al. (2015) while presenting the chemical analysis data of 23 sedimentary phosphate deposits (phosphorites) with ΣREE up to 18,000 μg/g and ΣHREE up to 7000 μg/g, suggest that phosphorite deposits are highly potential resources for REE than most conventional REE deposits.
Recent studies (Hein et al., 2010, Kato et al., 2011, Balaram et al., 2012, Zhong et al., 2018) also indicate that significant REE resources are available in ocean bottom sediments. There are essentially three types of REE resource in deep oceans: polymetallic nodules, ferromanganese crusts (cobalt crusts) and deep-sea muds (Fig. 3). Polymetallic nodule deposits form on the sediment-covered abyssal plains (at water depth of 4500–6000 m) and occur in surficial seafloor sediments in abyssal plain muds, mainly in the Pacific and Indian Oceans (Nath et al., 1994). Cobalt crusts (more precisely referred to as cobalt-rich ferromanganese crusts) are found throughout the ocean basins on seamounts, ridges, and plateaus on hard-rock substrates and occur as a surface encrustation on seamounts and rock outcrops in all oceans (Fig. 3), but with the richest deposits found in the western Pacific; and seafloor massive sulfides (SMS) that are formed at seafloor hot springs along ocean plate boundaries (Paropkari et al., 2010, Zeng et al., 2015). The compositions of these bottom sediments reflect the proportions of influxes of genetically different materials: hydrogenic, biogenic, hydrothermal, volcanogenic, and lithogenic (Banakar and Borole, 1991, Nath et al., 1992). Out of these two types, cobalt crust assumes lot of significance from the REE point of view. They form by precipitation from cold ambient bottom waters at depths typically of about 1000–3000 m and the enrichment of REE is controlled mainly by the sorption of ferromanganese oxides and clay minerals in the crusts as well as nodules. The crusts grow at extremely slow rates, about 1–7 mm/Ma and contain high concentrations of strategically and economically important metal (e.g. Co, Ti, Ce, Zr, Ni, Pt, Mo, Te, Cu, W) besides containing very high amounts of REE. There are essentially two types of ferromanganese crusts; hydrogenous (normally continuous) and hydrothermal (mostly episodic) in origin. REE patters are useful in distinguishing these two types of deposits (Prakash et al., 2012). These ocean bottom sediments are found to contain surprisingly high amounts of REE (ΣREE <2511 μg/g). Especially, the enrichment rate of Ce content is high, accounting for almost 60% of the total REE (Zhong et al., 2018). Hot plumes from hydrothermal vents brought these elements out of sea water and deposited them on ocean floors, bit by bit, over tens of millions of years (Sander and Koschinsky, 2011). In an attempt to prove that the Indian Ocean also bestowed with huge resources of these valuable metals in its bottom sediments, Balaram et al. (2012) explored the seamount crust at the Afanasy Niktin Seamount (ANS) in the Indian Ocean. Subsequently, the study was extended to detailed multi-beam swath bathymetric and geochemical investigations of seamount cobalt crust (Krishna et al., 2014). Table 5 presents comparative data of ΣREE from different marine sediments of world oceans, including those of Indian Ocean.
Table 5. Overview of ΣREE range (μg/g) in marine sediments from different oceans.
S. No.
Ocean
ΣREE/ΣREY
range (μg/g)
Matrix
Reference
1
Afanasy Niktin Seamount (ANS) in the Eastern Equatorial Indian Ocean
The third significant resource for REE is the deep-sea mud which is available in abundance on the ocean bottom (Fig. 3). Yasukawa et al. (2015) analyzed 1338 deep-sea sediment samples from 19 Deep Sea Drilling Project/Ocean Drilling Program sites covering a large portion of the Indian Ocean, and constructed a new and comprehensive data set of their bulk chemical compositions, including REE, major, minor and trace elements. The resource potential of these areas particularly of the high REE-rich mud and Fe–Mn nodule fields which broadly overlap, has been highlighted. The primary challenge associated with the production of REE from these muds is the extraction process in order to make it economically feasible. Takaya et al. (2018) while reporting deep-sea mud containing over 5000 μg/g of total REE including Y from the western North Pacific Ocean near Minamitorishima Island, Japan, used an effective mineral processing method with a hydrocyclone separator which selectively separates biogenic calcium phosphate grains having high REE content (up to 22,000 μg/g). The authors also concluded that this new REE resource could be exploited in the near future. At places the Pacific mud deposit contain 100–1000 times more REE than the world's presently known land reserves of 130 million tons of REE. According to some studies, REE resources undersea are much more promising than on-land resources. One square patch of metal-rich mud 2.3 km wide might contain enough REE to meet most of the global demand for a year. Kato et al. (2011) showed that REE and Y are readily recovered from the mud by simple acid leaching, and suggested that deep-sea mud constitutes a highly promising resource for these elements.
However, with the current state of ocean mining technology, the economic viability of deep-sea mining is questionable. If the environmental and financial factors were cleared, then deep sea mining would definitely be a feasible option for the long term.
4.1.6. Extraterrestrial REE resources
According to some recent studies (McLeod and Krekeler, 2017), the REE reserves available across the world currently will be sufficient for our day-to-day needs only for <2500 years. Based on current demand, mining operations and technologies, with an expected increase in the population and technology needs, exploration of potential extraterrestrial REE resources is inevitable, with the Earth's Moon being a logical first target, after all, the Moon was once part of the Earth and got ejected in the early history of the Solar System. While REE abundances of trace phases in lunar rocks are high, their abundances are low compared to terrestrial ores which means that there is to date, no geological evidence to support mining the Moon for REE yet. While the relative concentrations and abundances of the REE in even different mineral phases of Moon, Mars and chondritic meteorites that have been characterized to date provide crucial insights into the differentiation history of planetary objects, current and future missions to the Moon, Mars, and other nearby objects in our Solar System may yet reveal one or more extraterrestrial REE resources that one day will be utilized (Haque et al., 2014).
5. Metallurgy
Due to the rarity of an economic REE deposits, they are usually mined as a co-product or a by-product to another material. For example, the primary product of Bayan Obo mine in China is iron ore, but Bayan Obo also produces much of the world's REE as by product (Philip and Anderson, 2018). Today, REE-bearing carbonates (bastnäsite) and phosphates (monazite, xenotime) are commercially processed whereas the processing technologies for REE-bearing silicates require additional Research & Development (R & D) investment to make them commercialized (Paulick and Machacek, 2017). Highly efficient separation technologies have been developed for the production of high purity REE. Gupta and Krishnamurthy (2005) provided a comprehensive review of the-state-of-the-art of extractive metallurgy of REE. According to the IBM Report (2018) Indian Rare Earth Limited (IREL), Kerala mainly extracts REE from monazite at its plants. Two of these operations are located at Aluva and Chavara in the state of Kerala. The other two are located at Manavalakurichi in Tamil Nadu and at Chatrapur in Orissa. The extraction of REE usually involves the dissolution of the ore using acidic or alkaline solutions depending on the mineralogy of the REE-containing phases and reactivity of gangue phases, typically, the use of acidic solutions is more common. Depending on mineralogy, the extraction step often involves roasting of the REE ore at 400–500 °C in concentrated sulphuric acid to remove fluoride and CO2, and to change the mineral phase to make it more water-soluble. Generally, separation techniques such as solvent extraction, ion exchange, and precipitation are often used for the recovery of REE from pregnant leach solutions (PLS) obtained from acid or alkali leaching. Solvent extraction is generally accepted as the most appropriate commercial technology for separating REE due to the need to be able to handle larger volumes. For example, ground monazite is digested with caustic sodalye to produce trisodium phosphate (TSP) and mixed hydroxide slurry. This slurry is used for production of diverse rare earth compounds. Kumari et al. (2015) reported a review on the commercial processes based on pyro-hydro or hybrid techniques as well as systematic research for process development to recover REE from monazite. Monazite concentrate obtained are processed under different condition of time, temperature and concentration using acidic or alkaline solution. They are usually processed using thermal treatment followed by REE recovery under optimized conditions of leaching and their extraction via solvent extraction, precipitation, etc. Battsengel et al. (2018) developed a method for separating LREE and HREE in apatite from pregnant leach sulfuric acid solution by employing solvent extraction and stripping techniques. Currently, the leading process for the recovery of REE from apatite would involve leaching with nitric acid. After a nitric acid leach is performed, the REE could then be precipitated through the addition of ammonia (Sandstrom and Fredriksson, 2012).
In recent times, a revolutionary technology called SuperLig® molecular recognition technology (MRT) is being increasingly used to selectively separate and recover individual REE (Izatt et al., 2017). MRT is a green chemistry method for metal separations at the molecular level utilizing nanochemistry principles. PLS derived from the dissolution of ore is treated by the SuperLig® MRT Plant, initially to separate all 16 REE from impurity metals (“gangue metals”) followed by separation of individual REE with SuperLig® resins, consisting of highly metal selective ligand attached via a tether to silica gel or another substrate. The target REE will be selectively bound to the SuperLig® resin, with remaining feed solution going to raffinate. Following the column washing, resin-bound REE will be eluted and recovered in concentrated and pure form. According to authors, the MRT process features rapid kinetics of metal-ligand binding and release, simple elution chemistry, non-use of harsh chemicals/reagents/solvents, ability to recover REE present in feed solutions at μg/mL or lower levels, and minimal generation of waste (Izatt et al., 2015). Potential applications include REE recovery from primary ore, tailings, coal ash, and spent industrial feedstock such as permanent magnets, rechargeable batteries and LED lighting systems (Izatt et al., 2018).
6. Environmental impact and health effects of REE exposure
Technological innovations in addition to modern living conditions have enhanced intake of significant quantities of toxic elements in to the human body leading to health problems. Contamination of the environment by different kinds of toxic inorganic, organic, and organometallic species is one of the most serious problems in the world today. The REE group represents important elements found in the environment and need to be studied at greater depth to understand their effects on human health. Environmental scientists have been working on the ill-effects of the well-known toxic trace elements such as As, Pb, Cd, Hg and U (Sparks, 2005, Reddy et al., 2012, Rani et al., 2013). However, many other elements, which were not commonly used in the past, e.g., the REE and PGE are increasingly being used in modern industries for the production of numerous new materials, finished products and for several technological applications. Dumping of huge amounts of e-waste is facilitating the release of significant quantities of these elements along with several other toxic elements in to the subsoils and groundwater. REE are known to be more mobile in solutions rich in F−, Cl−, HCO3−, CO32−, HPO42−, PO43− ions. In addition, large quantities of REE are also getting into the agricultural soils through the phosphate-based fertilizers. The use of advanced and powerful analytical techniques such as inductively coupled plasma mass spectrometry (ICP-MS) and high resolution-ICP-MS (HR-ICP-MS) is now helping in improving our understanding of the reactivity and mobility of these metals in near-surface environment, their bioavailability and related health effects (Wei et al., 2013, Ali, 2014, Balaram, 2016a, Balaram, 2016b). These elements which are also finding their way into different environmental pathways especially those related to the ground and surface waters, probably have their own contribution to the environmental pollution and human health. As a consequence, a new group of elements viz., REE and PGE has been added to the already existing classification of elements (Fig. 4) depending upon their function in the environment and their toxicity in terms of human health (Bott, 1995, Balaram, 2016b). Under natural conditions, REE may only become available in small amounts via the groundwater and the atmosphere, however, their increased use has enhanced the amount of REE, and has created several new routes for bioaccumulation (in plants, animals, and human beings). Background level of REE content in waters, both surface and subsoil, varies significantly and also depends mostly on the local geology. Unfortunately, maximum acceptable limits for REE in drinking water are not available from any international health organization and also there is no sufficient data available about their toxicity to human health as stated above. In a recent study, Al-Rimawi et al. (2013) have observed very high concentrations of REE and several other metals in ground water samples from south West Bank/Palestine. The authors expressed concern as most of them have no maximum acceptable limits and also there is no sufficient data about their toxicity to human health. However, Sneller et al. (2000) have reported maximum permissible concentrations for different REE in drinking water (Table 6). According authors, these limits were derived using the data on ecotoxicology and environmental chemistry.
Figure 4. Trace elements have been divided into four groups depending upon their function in the environment (Bott, 1995, Balaram, 2016b).
Table 6. Detection limits of REE obtainable by some popular instrumental analytical techniques along with maximum permissible concentrations (MPC) in drinking water.
One major contamination seen in Europe and US but not yet reported for India is related to Gd which is used as a contrasting agent in magnetic resonance imaging (MRI). After excretion from the human body with urine, it passes through waste water treatment plants (WWTPs) almost unaffected into the aquatic system. Rivers draining highly populated urban areas in countries with a highly developed health care system are, therefore, expected to carry a large positive Gd anomaly, which is supported by a growing body of data (Bau et al., 2006, Kulaksız and Bau, 2007). Such positive gadolinium anomalies have been found worldwide, in Europe (Rabiet et al., 2009, Tepe et al., 2014), USA (Verplanck et al., 2005), Asia (Zhu et al., 2004) and Australia (Lawrence et al., 2009). German researchers from the University of Münster investigated the fate of Gd-based contrast agents emitted to the environment via wastewater discharge and were able to follow it into the processed drinking water of several water plants in Germany (Birka et al., 2016a, Birka et al., 2016b). On the other hand, a 20-year record of not only Gd but also the total REE concentrations in San Francisco Bay suggests that there has been considerable increase of these elements (Hatje et al., 2016).
Intense REE mining and production activities have led to significant environmental and health impacts in countries such as China, US, India, Malaysia and Brazil. Mining activities such as cutting, drilling, blasting, transportation, stockpiling, and processing can release dust containing REE, other toxic metals and chemicals into the air and surrounding water bodies which can impact local soil, wildlife and vegetation in addition to humans. More mining of REE, however, will mean more environmental degradation and human health hazards as waste disposal areas can be exposed to weathering conditions and have the potential to pollute the air, soil and water if adequate monitoring and protection measures are not utilized (Barakos et al., 2015). Some of the REE minerals contain significant amounts of radioactive elements such as uranium and thorium, which can contaminate air, water, soil and groundwater. These problems are due to insufficient environmental regulations and controls in the mining and processing areas. One of the most significant problems is the radioactivity of some ores. For example, the Bayan Obo mine (China) employs nearly 7000 workers, of which about 3000 are exposed to thorium containing airborne dust. Elevated thoron (220Rn) concentrations in air are also found. Exposure to gamma radiation is significant in the mining areas (IAEA, 2011). The extensive use of REE in various modern technologies continually grow despite some knowledge about the environmental concerns of REE as they are getting released into the environment along with radionuclides. Most of the harmful effects of REE exposure to humans and their potential health effects come from the studies of mine workers and others who regularly deal with REE or their products, where exposure is typically much higher than what the general population would experience. Some studies indicate that the chemicals used in the ore processing, extraction and refining processes have been responsible for the health hazards of the workers and local residents, water pollution and destruction of farmland (Rim et al., 2013). Recent socio-environmental issue of the health impacts of REE ore processing (from both radioactive and non-radioactive contamination) in areas of China have been raised as a major concern. For example, an urban street dust of an industrial city, Zhuzhou in central China recorded very significant concentrations of REE (ΣREE ranged from 66.1 μg/g to 237.4 μg/g with an average of 115.9 μg/g) reveals the gravity of the REE pollution, particularly in industrial cities (Sun et al., 2017). This is best exemplified by the recent social and environmental conflict surrounding the development of the Lynas Advanced Materials Plant (LAMP) in Kuantan, Malaysia which led to international activism and claims of environmental and social injustice (Ali, 2014). There are also several reports of REE occupational exposure which resulted in bioaccumulation and adverse effects to respiratory tracts (Sabbioni et al., 1982, McDonald et al., 1995, Yoon et al., 2005, Rim, 2017).
In addition, contamination from dumping of huge amounts of e-waste releasing of REE in large quantities in to the subsoils and ground water is emerging very fast (Haxel et al., 2002). Each year, the electronics industry generates up to 41 million tons of e-waste, but as the number of consumers rises, and the lifespan of devices shrinks in response to demand for the newest and best, that figure could reach 50 million tons in 2018. Lange et al. (2017) studied the impact of a scrapyard of impounded vehicles on topsoil in São Paulo state, Brazil for several heavy metals and REE. Mass fractions of all elements including REE were much higher than the reference values. Hot spots were observed for most elements suggesting vehicular source.
Currently, there are substantial gaps in our understanding of the adverse effects of REE to human health, their anthropogenic levels and fate to their biogeochemical or anthropogenic cycling and their individual and additive toxicological effects. More studies are required to identify the anthropogenic sources, transfer mechanisms, bioaccumulation and their environmental behavior to minimize human health risks in future. Consequently, the adoption of new public policies and the development of more effective treatment technologies will determine the future adverse impacts of REE in aquatic systems. There is a great need to understand the toxicological properties of REE as there is a wide-spread use of these elements within agriculture and medicine and more studies and consolidation are needed to accurately assess the impact of these elements on human health (Gwenzi et al., 2018).
7. REE in agriculture
In addition, REE themselves are used in agriculture as fertilizer to improve crop growth and production and therefore leading to further increase in the concentrations of REE in soil (Tyler, 2004). In general, the mineral fertilizers (i.e., phosphate fertilizers) and soil conditioners contain macronutrients (Ca, Mg, N, P, and S), micronutrients (such as Fe and Si) and REE. In soils, the REE can also originate from local geological parent materials (Liu, 1988). REE containing micro-fertilizers are directly applied on a large-scale to plants in agriculture in China for improving the yield and quality (Guo et al., 1988, Diatloff et al., 1995). In general, the accumulated concentrations of REE have been reported to be very low. However, the accumulation capacity of a particular plant depends upon several factors such as plant species, their growing conditions, the REE content in the substrate soil or rock (Fu et al., 2001). For example, higher values were found in rice which indicated that rice has a higher accumulation ability for REE than corn. Following excessive application of REE in agriculture, there is a raising environmental concern that these elements may enter the food chain though plant uptake, which might be deleterious to human health. Some studies (Redling, 2006) confirmed very low concentrations of REE in cereal grains and no significant accumulation due to REE fertilization. Thus, grains and products made of them such as wheat flour are considered to be safe. Thomas et al. (2014) have indicated that countries like Russia and Nigeria where the natural abundance levels of REE are high in their soils will have more environmental threats arising from the increased input of REE. Close monitoring may be needed in countries where phosphate-based fertilizers (mined from monazite deposits) are applied in large scale.
8. REE in medicine
Their unique properties, such as radiation emission or magnetism, allow REE to be used in many different therapeutic and diagnostic applications in modern medicine. Currently there are a few major applications of REE in medicine but many more of them are on the horizon. Several studies (Zhang et al., 2000a, Zhang et al., 2000b, Wakabayashi et al., 2016) confirmed the antibacterial and antifungal activities of REE which is comparable to that of copper ions and these elements are beginning to find several pharmaceutical applications. For example, Gd has been used in a chelated form as a contrast agentin magnetic resonance imaging (MRI) measurements (Raju et al., 2010), though new research finds direct evidence of gadolinium deposition in neuronal tissues which can be harmful to patients (McDonald et al., 2015, Gulani et al., 2017). REE can also be used as nematicide as they can also inhibit the formation and germination of fungal spores and thus influence large number of organisms (Zhang et al., 2000b). Even the pharmaceutical samples when analyzed for inorganic impurity are found to contain appreciable amounts of especially LREE (La ∼25 μg/g; Ce ∼ 7 μg/g; Gd ∼ 8 μg/g) when analyzed by ICP-MS (Balaram, 2016b). The implications of the presence of this range of REE concentrations in pharmaceuticals, are not clear at present. REE have been found to disease causing and occupational poisoning of local residents in mining areas, water pollution, and farmland destruction, etc. Conversely, a body of evidence has shown REE-associated antioxidant effects in the treatment of many diseases (Rim, 2016). Recently the medical and biological properties of REE have been reviewed by Panichev (2015). New medical applications for these elements are being found at an increasing rate and emerging advancements such as nanotechnology might be used to enhance their use in medicine in the future.
9. Recycling
Strategic high-tech metals such as cobalt, lithium, PGE, hafnium, tantalum, gallium and especially REE are fundamental to the world currently for the development of efficient and high-tech and environment friendly products such as electric cars which require lithium and neodymium and wind turbines requiring neodymium and dysprosium. On one hand, the world is moving towards cleaner and greener future, it is becoming extremely difficult to meet the growing demand for the REE as most of the production is located only in few countries such as China, US, Australia and India although lots of research works are going on the replacements for REE in critical technologies such as super magnets. On the other hand, mountains of e-waste rich in REE are indeed growing across the globe, but if this waste is turned into a valuable resource, this will protect human health and protect the planet's increasingly strained REE resources. Hence, many nations realized the value of recycling of e-waste, mainly scrapped electronics which is in greater demand in regions such as the EU, China and India. Essentially two options can be considered for a secure supply of REE. First from primary resources (old mines or new deposits, ocean bed sediments, coal ash, etc.) and from secondary resources (electronic and industrial waste). Electronic waste could in theory cover a significant part of the demand for REE. It is estimated that ∼50 million metric tons of e-waste are disposed in landfills around the world each year. However, about only 12.5% of e-waste is currently being recycled for all metals.This e-waste is supposed to contain significant concentrations of REE and other precious metals such as Au, Ag, Pt, Pd and Rh. Even the recent life cycle assessments have indicated that recycling of consumer materials is a promising alternative to conventional production processes (Sprecher et al., 2014). However, recycling of REE is not easy and challenges are at every level. First of all, these elements are present in small amounts in tiny electronic parts of gadgets like mobile phones. In some materials like touch screens, these metals are evenly distributed making much more difficult to extract. Regardless of the end use, REE are not recycled in large quantities mainly because of low yield and cost, but recycling could be feasible if recycling becomes a mandate or the prices of REE go extraordinarily very high. In order to reduce future REE criticality, several studies are going on globally for cost-effective recovering REE from e-waste (Bogart et al., 2015, Bogarta et al., 2016, Fang et al., 2017, Nguyen et al., 2017). These studies include automated approaches to disassembling electronic scrap, as well as chemistry to extract REE from them. Recently Apple rolled out a Robot for iPhone dismantling which can dismantle up to 200 devices/hr. Processing 100,000 iPhones has the potential to yield 1900 kg of aluminum, 770 kg of cobalt, 710 kg of copper, 93 kg of tungsten, 42 kg of tin, 11 kg of REE, 7.5 kg of silver, 1.8 kg of tantalum, 0.97 kg of gold and 0.1 kg of palladium. The company reiterated its 2017 pledge to use only recycled materials in its supply chain (https://resource-recycling.com/e-scrap/2018/04/26/apple-rolls-out-robot-for-iphone-dismantling/).
REE separation chemistry is a big challenge and a chief barrier to possible widespread recycling activity, which is currently performed at a rate of only ∼1%. Separation and purification of individual REE is challenging due to their chemical similarities. To develop other sources of REE and reduce the environmental impact of their isolation, there is a clear-cut need for new separation technologies that reduce the cost of industrial-scale REE separations and recycling. In this context, Fang et al. (2017) developed a simple, fast, and low-cost technology to help recycle mixtures of REE. Schelter's group has synthesized new organic compounds (a ‘ligand’): tris (2-tert-butylhydroxylaminato) benzylamine (H3TriNOx) for separations. The central hypothesis of this work is that these tailored organic compounds can provide simple and effective separations for mixtures of REE, based on solubility differences of the REE complexes. The method developed by Schelter and his team is expected to contribute to reducing waste and reduce REE mining activity by adding recycled REE to the supply chain. U.S. Department of Energy's Critical Materials Institute (CMI) developed a method of using bacteria to produce acids to dissolve and separate REE from shredded electronics. Gluconobacter bacteria consumes sugars and produces acids and the method is more environmentally friendly (CMI, 2018). Further work is currently under way by these workers to develop the concepts into practical and industrially viable recycling processes. From the studies currently underway and the progress made so far, it is expected that the recycling of REE has the potential to be economical and more readily achievable than the exploitation of new mineral deposits.
10. R&D studies on the reducing their usage or on substitution for REE in different applications
Because of the environmental, cost and supply problems, lot of R & D studies have been initiated recently either to reduce the amount of REE usage in a particular application or to find out alternate and less harmful material for substitution for a particular REE. Machida et al. (2017) have designed a new autocatalyst that reduces the amount of Ce used by 30% from reference catalysts. The grafted cerium oxide, CeO2/MnFeOy, has both fast release and large storage capabilities for oxygen and has high performance capability for converting NOx, CO, and total hydrocarbon to less harmful materials. Several countries have already started working on finding substitutes to REE devices. Hitachi Metals is working on a magnet that minimizes the use of REE by employing copper alloys. U.S. Department of Energy is funding many projects to look for substitutes of REE and to create REE-free devices. Toyota is developing future hybrid vehicles that do not require REE. Some groups are trying to develop a “super magnet” by layering of iron and nickel which is designed to be synthetic form of tetrataenite, a rare magnetic extra-terrestrial iron-nickel alloy found only in meteorites. This can be a future substitute for Nd. Fluorescent light bulbs and LEDs depend on phosphors made from Tb, Eu and Yb. Organic LEDs (OLEDs) and halogen incandescent lights are REE-free and can be used as substitute lighting. As markets shift to these alternatives, demand in those REE can decrease. Studies are also going on to create cheaper magnetic materials for cars and wind turbines (Pathak et al., 2015, Pavel et al., 2017). Lots of other leading multinational electronic firms such as Samsung are vigorously working on replacing REE in their applications. On the other hand, some technologists feel that REE are allowing the miniaturization of the components of high-tech-mass-produced-consumer-devices that transform electrical signals in to motion, sound, images and light, and it may be very difficult to replace them economically in the near future.
11. Chemical characterization of REE—Application of different instrumental analytical techniques
The chemical analysis of REE in geological, industrial and environmental materials is essential for geochemical exploration studies, mining, extraction, quality checks of both raw materials and finished products in industry, and also monitoring the environment. Physical and chemical similarities of REE make their determination usually difficult and complicated. This is particularly true if a selected element among them has to be determined in the mixture of the other REE, because of numerous interferences and coincidences. In the past accurate determination of REE in geological materials at their crustal abundance levels by using classical methods such as gravimetry, titrimetric, spectrophotometry, flame atomic absorption spectrometry (F-AAS) and graphite furnace atomic absorption spectrometry (GF-AAS) was extremely difficult and time consuming (Wengert et al., 1952, Onishi et al., 1962; Saxena, 1970, Andreev et al., 1974). These techniques are not considered here. But currently with the availability of sophisticated instrumental analytical techniques, these tasks have become relatively simpler. Among the instrumental methods available currently, instrumental neutron activation analysis (INAA) and ICP-MS including HR-ICP-MS are commonly used for REE determination in different kinds of materials because of their multi-element capability, high sensitivity wide linear dynamic range, fewer interferences, ease of operation and accuracy. In addition, techniques such as X-ray fluorescence spectrometry (XRF), inductively coupled plasma optical emission spectrometry (ICP-OES), glow discharge mass spectrometry (GD-MS), laser induced breakdown spectroscopy (LIBS) and recently introduced microwave plasma atomic emission spectrometry (MP-AES) are also found to be extremely valuable in such investigations. Though techniques such as isotope dilutionthermal ionization mass spectrometry (ID-TIMS) and spark source mass spectrometry (SSMS) were used in the past for the determination of REE particularly in geological materials, currently their application is limited because of the requirement of tedious sample preparation methods and huge cost (Jochum et al., 1988, Klinkhammer et al., 1994). On the other hand, multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) is superior in terms of the analytical reproducibility compared with quadrupole ICP-MS or with ID-TIMS techniques for the determination of REE in geological materials. Precisions <0.4% can be obtained for all REE (Baker et al., 2002). Some of the important analytical techniques and their utility towards REE analysis in different applications are considered briefly in the following.
X-ray fluorescence spectrometry (XRF) is a reliable method for trace element analyses at the μg/g level although the technique is relatively insensitive to the REE (Table 6). However, XRF, as a conventional analytical technique for REE analysis, has distinct advantages over the other methods in respect to its accuracy, speed and cost. Its only disadvantage is relatively low sensitivity. It can be classified into wavelength-dispersive X-ray fluorescence spectrometry (WD-XRF) and energy-dispersive X-ray fluorescence spectrometry (ED-XRF) according to the methods of excitation, dispersion, and detection (Potts and Webb, 1992). Both forms, namely, WD-XRF and ED-XRF techniques have been successfully applied for the determination of REE in geological and environmental materials. In general, because of higher detection limits offered by this technique for all REE, many determination methods involve separation and preconcentration procedures for their accurate determination (De Vito et al., 2000). For example, Juras et al. (1987) have developed a rapid and sensitive procedure for analyzing REE in geological samples, ranging in composition from ultramafic rocks to rhyolite by XRF after all REE are separated from other constituents by an ion-exchange procedure. Wu et al. (2010) have presented review on the applications XRF analysis in Chinese rare earth industry. The applications consisted of the analysis of REE in ores and soil, concentrates, compounds, metals, alloys, functional materials, fast and online analysis in separation process, and so on. Smoliński et al. (2016) have used WD-XRF to determine the content of 16 REE in several samples of combustion ash of coals from Polish mines. In recent times portable XRF (ED-XRF form) is being used successfully for on-site in the field quantification of REE including La, Ce, Pr, and Nd, as well as typical REE pathfinders in geochemical exploration studies including Y, Th, and Nb (Balaram, 2017). These gadgets are extremely valuable in the field as they help to quickly determine the next course of action in exploration, ore grade/process control and environmental sustainability studies.
11.2. Laser-induced breakdown spectroscopy (LIBS)
LIBS is an emission spectroscopic technique using a laser-generated plasma to ablate and excite the atoms in the sample, usually in solid form, although liquid samples can also be analyzed (Radziemski and Cremers, 2013, Balaram, 2017). Laser ablation of solids or liquids can directly fingerprint elemental constituents via their characteristic optical emission spectra. Detection limits can range from between 10 μg/g and 100 μg/g for most REE (Table 6) in common applications with precisions ranging from 3% to 5% and often better than 2% for homogeneous materials (Cremers and Radziemski, 2006). Both desktop and hand-held models are available from different firms. LIBS technique has numerous advantages compared to other techniques, as it allows very fast measurements, can be employed in the field, requires no sample preparation, and consumes only tiny amounts of sample. The greatest advantage of LIBS is its ability to do real time identification of different metals including REE and also non-metals in seconds (Bhatt et al., 2018). This feature will be of special use in recycling operations and recovery of REE components from scrap electronics. The accuracy of solid measurements using LIBS is often compared to that of XRF, which can also measure elements in solid targets on site. Considering the overall accuracy, it can be said that the suitability of LIBS for in-situ analysis is comparable to XRF (Takahashi and Thornton, 2017).
INAA is a high-sensitive and versatile analytical technique for the determination of the concentration of major, minor and trace elements in a variety of matrices. A small sample of 5 to 100 mg is subjected to a neutron flux in a nuclear reactor. The stable nuclei absorb neutrons and become unstable radioactive nuclides and the resultant radioactive nuclides decay with emission of particles or, more importantly gamma rays, which are characteristic of the elements from which they were emitted. The energy of the emitted gamma rays is used to identify the nuclide and the intensity of the radiation can be used to determine its abundance. Semiconductor radiation detectors are normally used for quantitative measurement. Comparison of the intensity of these gamma rays with those emitted by a standard, permit a quantitative measure of the concentrations of the various nuclides. This technique is extremely popular because nuclear reactions and decay processes are virtually unaffected by chemical and physical structure of the material during and after irradiation. Several workers have applied INAA technique for the determination of REE at extremely low concentrations in different earth and environmental samples (Vukotić, 1983, Baidya et al., 1999; Bounouira et al., 2007, Alharbi and El-Taher, 2016). Ravisankar et al. (2006) have determined REE in beach sands from Tamil Nadu, India by INAA to understand their geochemical behavior. Table 7 presents the REE results of the Ocean bed polymetallic nodule reference sample (2388) from Indian Ocean (Pandey, 1992) determined by INAA in comparison with the data obtained by the author using other popular analytical techniques, namely ICP-MS and ICP time-of-flight MS (ICP-TOF-MS). As certified data for several elements including REE are not available for this unique Indian reference material, this data will be valuable for providing certified values for all REE in future. Though INAA is a sensitive and multielement technique, it has certain limitations. For more precise determinations, INAA needs matrix matching reference materials for calibration to allow for the emitted X-rays which are subjected to self-attenuation in the sample under the same counting conditions. Hence synthetic standards prepared by mixtures of REE are not suitable unless the matrix is also matching to that of the sample. Table 8 presents concentrations (μg/g) of REE obtained by INAA in ferromanganese crust (cobalt crust) samples crust samples collected from the Afanasy Niktin Seamount (ANS) in the Eastern Equatorial Indian Ocean, using the procedure described by Figuelredo and Marques (1989). Calibrations were performed using international polymetallic nodule reference samples, Nod-A-1 and Nod-P-1. INAA is very sensitive technique and is therefore extremely valuable for the determination of REE in geological and environmental materials. Despite these advantages, INAA is certainly not a popular analytical technique as it is time-consuming, not independent, requires a reactor nearby and involves longer cooling times for certain elements.
Table 7. Comparative concentrations (μg/g) of REE in the Indian Polymetallic Nodule Reference Material, 2388 by INAA, ICP-OES, ICP-MS and ICP-TOF-MS.
Table 8. Concentrations (μg/g) of REE in ferromanganese crust (cobalt crust) samples collected from the Afanasy Niktin Seamount (ANS) in the Eastern Equatorial Indian Ocean.
Elements
CC2-ADR24
Ferro manganese crust
CC2-ADR25
Ferro manganese crust
CC1-DR-12
Ferro manganese crust
La
217 ± 3
189 ± 3
236 ± 5
Ce
1163 ± 34
1186 ± 35
1041 ± 31
Nd
232 ± 8
122 ± 5
185 ± 6
Sm
35.4 ± 0.7
27.0 ± 0.5
39.0 ± 0.8
Eu
8.5 ± 0.3
6.4 ± 0.2
9.2 ± 0.2
Tb
6.8 ± 0.8
4.7 ± 0.6
6.5 ± 0.7
Yb
17.3 ± 1.5
15.5 ± 1.3
19 ± 2
Lu
2.4 ± 0.2
2.1 ± 0.2
2.5 ± 0.2
Sc
8.8 ± 0.4
9.0 ± 0.4
11.0 ± 0.5
Note: The samples were determined by NAA facility by the author at the Institute of Energetic and Nuclear Research, Sao Paulo, Brazil.
ICP-OES is a multi-element analytical technique used for the accurate determination of different elements at major, minor and trace concentration levels in a variety of materials. A liquid sample is converted to an aerosol and transported to the high temperature inductively coupled plasma. In the plasma the sample undergoes desolvation, vaporization, atomization and ionization. The atoms and ions then absorb energy from the plasma which causes electrons within them to move from one energy level to another. When the electrons fall back to ground state, light of wavelengths specific to each element are emitted. The measured emission intensities are then compared to the intensities of reference materials of known concentrations to obtain the respective elemental concentrations in an unknown sample (Greenfield et al., 1964, Wendt and Fassel, 1965). The technique can simultaneously measure up to 60 elements with high sensitivity and an extraordinarily wide linear dynamic range which is perhaps the most outstanding feature of the ICP-OES (Balaram et al., 1995). Bentlin and Pozebon (2010) determined low concentrations of the fourteen naturally occurring lanthanides directly by ICP-OES after selecting the most appropriate spectral lines. But in general, due to several spectral interferences encountered during the determination of REE in geological materials, it is essential to adopt a separation and preconcentration procedure before their determination by ICP-OES. Jarvis (1994) determined REE and Y in 37 international rock reference materials by ICP-OES after adopting a cationic-exchange chromatography for their separation and preconcentration followed by a conventional rock dissolution technique. It's very challenging to accurately determine REE and other trace elements in the presence of large concentrations of REE in a sample, using ICP-OES because the line-rich spectra emitted by REE. Chausseau et al. (2014) have obtained very high-quality results on the analysis of cerium oxide, gadolinium oxide and NdFeB magnetic materials matrices using high resolution ICP-OES. In general, because of the spectral complexities and relatively low concentrations of REE in different types of rocks, separation of the REE via ion exchange is required after sample digestion prior to analysis by ICP-OES making this technique less productive (Walsh et al., 1981, Jarvis and Jarvis, 1985, Balaram et al., 1995). Clarice et al. (2017) have determined REE in geological and agricultural materials using ICP-OES by measuring the emission signals in radial as well as dual viewing modes for obtaining information geochemical formation history, plant nutrition status, need for supplementation and possible contamination.
A new commercial instrument representing yet another analytical technique called the MP-AES has been introduced in 2011, which appeared to provide an attractive alternative to ICP-OES for REE determinations. The MP-AES system utilizes a relatively new design of plasma torch, utilizing nitrogen gas for generating high temperature microwave plasma as reported by Hammer (2008). With the detection limits for many REE in μg/g range (Table 6), the utility of MP-AES has been demonstrated using several different geological and environmental matrices, including industrial effluents, water, sediments, soils, rocks and ores during the last 5 years (Balaram et al., 2013a, Balaram et al., 2014, Kamal et al., 2014, Sreenivasulu et al., 2017). Helmeczi et al. (2016) developed a novel methodology for rapid digestion of REE ores and determined REE both by MP-AES and dynamic reaction cell-ICP-MS. The REE results obtained by MP-AES were identical to those obtained by ICP-MS. Tupaz et al. (2015) determined scandium in a laterite sample using MP-AES and the results favorably compare with those obtained by ICP-MS. Currently MP-AES is a potential analytical technique for inorganic contents in a variety of geological and environmental materials, because it has a number of obvious advantages over techniques such as F-AAS and ICP-OES. Given the similarity of sample introduction systems between the MP-AES and conventional ICP-OES, it is anticipated that sample introduction and sample preconcentration strategies already validated for ICP-OES will be equally robust for MP-AES, which would make MP-AES an attractive alternative technique in future as it is cheaper.
11.6. Inductively coupled plasma mass spectrometry (all forms of ICP-MS, ICP-TOF-MS, HR-ICP-MS & MC-ICP-MS)
ICP-MS occupies an invaluable position in the modern analytical laboratory due to its simplicity, excellent sensitivity, very limited interferences, precision and accuracy (Balaram, 1995). In the past, until time of the advent of ICP-MS, the determination of REE in geological samples was a difficult and expensive task involving separation of the REE by utilizing time consuming methods such as precipitation, solvent extraction and ion exchange prior to analysis by techniques such as XRF and ICP-OES. Even INAA method of determination of REE was also very slow because of sample irradiation and cooling requirements. But the successful linking of ICP to the quadrupole mass spectrometer has presented the analyst with a most valuable addition to the range of techniques available for elemental analysis (Houk et al., 1980). The ICP source converts the atoms of the elements in the sample to ions. Samples, usually in solution form reach the plasma as aerosol after passing through a nebulizer and a spray chamber/desolvator. In addition to the solution nebulization, different other sample introduction systems such as laser ablation and different chromatography techniques can be coupled to the instrument allowing direct analysis of solids and speciation analysis capability. At the high temperature attained in the plasma source, most elements including those with higher ionization potentials can be almost completely atomized and ionized. A portion of the ions generated are then separated and detected by the mass spectrometer and analyzed on the basis of m/z ratios. Over the last 35 years, ICP-MS has been well established as a powerful analytical tool for rapid multielement analysis. Extremely low detection limits (Table 6), high sample throughput, requirement of very small quantities, element versatility (major, minor, trace and ultra-trace) and isotopic detection capability are some of the very important features that have made ICP-MS an excellent analytical technique for the analysis of a variety of materials (Balaram et al., 1995). Several workers world over including from our laboratory have determined REE in different types of materials using ICP-MS during the last three decades or so (Date and Gray, 1985, Jarvis and Jarvis, 1985, Lichte et al., 1987, Balaram, 1996, Dey et al., 2018). Isotope dilution ICP-MS can provide highly precise REE data in different materials including geological materials (Tanaka et al., 2018). Currently ICP-MS (including HR-ICP-MS) technique is being very extensively applied for the accurate determination of REE in different types of materials (Fig. 5) when compared to other popular analytical techniques such as INAA, ICP-OES, XRF and UV-vis spectrophotometer. Table 9 presents a comparison of concentrations of REE (μg/g) in USGS manganese nodule reference materials, Nod-A-1 and Nod-P-1 by different popular instrumental analytical techniques including ICP-MS (Nath et al., 1992).
Figure 5. The application of different analytical techniques for REE determination in the earth and environmental materials (ICP-MS includes HR-ICP-MS also).
Table 9. Comparison of concentrations of REE (μg/g) in USGS manganese nodule reference materials, Nod-A-1 and Nod-P-1 by different popular instrumental analytical techniques.
Development of ICP-TOF-MS brought yet another dimension to the trace analysis in recent times (Mayers and Hieftje, 1993, Mahoney et al., 1997, Balaram et al., 2013b). ICP-TOF-MS offers extremely high data-acquisition speeds, high ion transmission and quasi-simultaneous measurement of all masses in packet extracted from the ion source leading to better detection limits than those offered by quadrupole ICP-MS (Table 6). Table 7 presents REE data (μg/g) in the Indian Polymetallic Nodule Reference Material, 2388 obtained by ICP-TOF-MS in comparison with the data by other analytical techniques, namely, INAA and ICP-MS.
Out of different forms of ICP-MS, HR-ICP-MS has gained confidence especially by the scientific community in recent years due to its extremely high sensitivity and capability to clearly resolve several spectroscopic interferences. HR-ICP-MS combines an ICP source with a double focusing magnetic analyzer to perform trace/ultra-trace metal analysis and/or isotope ratio measurement (Balaram, 2018, Satyanarayanan et al., 2018a, Singh et al., 2018). This instrument can be utilized in high resolution mode to resolve even most complex interferences (Bradshaw et al., 1989) or even at lower resolution to provide extremely low detection limits. Current HR-ICP-MS instruments have resolving powers up to 10,000 and will be typically operated at preset resolution settings for low, medium or high (Walder and Freedman, 1992, Satyanarayanan et al., 2018b). This is the most sensitive analytical technique available today for inorganic analysis. This is evident from the results in Table 6 where the detection limits of REE obtainable by HR-ICP-MS are presented in comparison with some contemporary popular instrumental analytical techniques. Table 6 also shows maximum permissible REE concentrations in drinking water published by Sneller et al. (2000).
Multi-collector ICP-MS (MC-ICP-MS) system combines a plasma source (ICP), an energy filter, a magnetic sector analyzer, and multiple collectors for the simultaneous measurement of different isotopes. This powerful technique measures precise isotopic ratios of number of isotopes simultaneously which has enabled significant advances in our understanding of geological, biological, nuclear, and physical processes in terrestrial and extra-terrestrial environments (Balaram, 2018). This technique is probably the best technique at present for the precise determination of REE and isotope ratios. Although isotope dilution using thermal ionization mass spectrometry (TIMS) is the best technique for discriminating and stabilizing the ion beams among REE (e.g. Masuda et al., 1973), the technique requires robust laboratory methods. MC-ICP-MS can provide highly reproducible data (<0.2%) for all REE in geological materials very rapidly (Baker et al., 2002). Extremely accurate and precise REE data, with analytical uncertainty typically >1% can be obtained using isotope dilution MC-ICP-MS (Kent et al., 2004).
11.7. Glow discharge mass spectrometry
Like most mass spectrometric techniques, glow discharge mass spectrometry (GD-MS) has also become an established analytical tool for the analysis of major, minor, trace and ultra-trace compositions over a wide dynamic range (ng/g to 100%), in solid materials including geological metallurgical and semiconductor materials (both conducting and non-conducting) with a range of applications spanning several disciplines (King et al., 1995, Becker and Dietze, 2003). The first commercial GD-MS, magnetic sector model VG 9000 (“Thermo Elemental”, Winsford, UK) was built in 1985. Taking the detection limits for various REE to sub-μg/g levels (Table 6), this technique is valuable in checking the purity levels of individual rare earth oxides.
11.8. In-situ analytical techniques for REE
The micro-analysis of elemental concentrations direct in solid samples has been an attractive frontier in the development of analytical science. Gray (1985) was the first to demonstrate the feasibility of analyzing direct solids by ICP-MS using laser ablation sampling (LA-ICP-MS). Compared with solution nebulization-ICP-MS for the bulk analysis of geological samples, LA-ICP-MS analysis has several advantages such as very low background, lower oxide and hydroxide interference levels, a simpler sample preparation procedure, faster analyses, and cost effectiveness. For example, the same fusion beads prepared for XRF analysis can be used for the determination of major, minor and some selected trace elements including REE by LA-ICP-MS. John et al. (1993) developed a rapid method for the analysis of REE and several other trace-elements on direct whole-rock fused glasses using laser sampling ICP-MS. All REE including Y and Sc can be measured at sub-μg/g levels (Table 6). In a study, Tanaka et al. (2007) determined REE concentrations in carbonate samples using LA-ICP-MS. The effect of matrix on LA-ICP-MS analysis was investigated using NIST glasses and synthetic CaCO3 doped with REE as calibration standards. The influence of the matrix on LA-ICP-MS analysis was found to be relatively small. Liu et al. (2013) have presented an excellent review of the application of LA-ICP-MS for the analysis of geological samples for major, minor and several trace elements including REE. Secondary ion mass spectrometry (SIMS) or Ion-Microprobe is one of the best techniques for the analysis of REE in geological materials (Sano et al., 1999). SIMS relies on the physical phenomenon of “sputtering” to produce analyte ions. A primary beam of ions (generally O+ or Cs+) is accelerated into a solid sample at potentials of a few kV. The impact of these primary ions gradually erodes (“sputters”) a shallow crater in the sample. A portion of the material sputtered from the sample emerges as ions, and these “secondary” ions are the analyte species that are introduced into the mass spectrometer for subsequent detection and quantification (Muir et al., 1987). These instruments are capable of both isotopic ratio analysis and sub-μg/g elemental analysis with extremely high spatial resolution. The spatial resolution of SIMS is better (5–10 mm) than that of LA-ICP-MS, which is typically >10 mm. However, SIMS is a less rapid and more complex analytical technique than LA-ICP-MS. Highly sophisticated similar analytical tool, sensitive high-resolution ion micro probe (SHRIMP) with special resolution ∼ 25 μm, which mostly applied for the determination of high precise isotopic ratios, is also used to determine REE distributions in fly ash glasses to test the REE partition into the glass, in studies related to the extraction of REE from coal ash (Ireland et al., 2008, Kolker et al., 2017).
12. Conclusions
Fast emerging green technologies ranging from electric car batteries to solar panels to wind turbines, in addition to others where REE are widely being used together with price rise, are expected to drive tremendous growth and demand for these metals in near future. There is a greater need to intensify our search for REE resources not only on land but also in ocean bottom sediments. Deep sea mining would definitely be a feasible option in near future in addition to the development of cost-effective recovery of REE from abundant coal, coal ash and red mud. There is a great need for developing a sustainable exploitation schemes for all kinds of REE ore deposits and meticulously follow to prevent further damage to the environment as it will take a long time and cost a great deal of money to restore the environment and to ensure the sustainable development of the REE industry. Instead of opening new mining ventures, extraction of REE from coal fired ash, red-mud and electronic recycling schemes are considered promising options for near future REE supply. The wide-spread application of REE in different industries as well as agriculture is alarmingly increasing leading to a constant increase of the concentrations of these elements in the environment which would not only disturb the aquatic system but also the plant and soil ecosystem leading to number of human health issues. Close monitoring may be needed at places where phosphate-based fertilizers are used, and in areas where soil conditions are favorable to REE mobility, availability and uptake by plants, and/or at e-waste dump sites where surface runoff could contaminate the local environment. Extensive utilization of REE in day-to-day life may warrant an urgent toxicological assessment of these elements from a human health perspective. Recycling of REE from e-waste has not yet taken off and more emphasis should be put on the R & D efforts on finding out substitutes for REE to draw away from reliance on REE. Though several studies have been initiated to find out good substitutes for REE in different technologies, so far there is no breakthrough in any of the critical technologies, and more studies are required. The innovative processes and designs are needed to be developed for REE extraction and process from different sources which also must adequately address the environmental safety and human health issues. In all these activities, precise and accurate determination of individual REE in different materials both in solid and liquid forms becomes essential. Currently an array of high-sensitive and selective analytical techniques is available for the accurate and precise determination of REE in different materials. Performance characteristics such as multielement nature, high sensitivity and high resolving power of most interferences, HR-ICP-MS would certainly become an important analytical tool in REE activities in future.
Acknowledgements
Prof. Ana Maria Figueiredo, the Institute of Energetic and Nuclear Research, Sao Paulo, Brazil, is thanked for extending the INAA facility for REE measurements in some geological samples during the author's visit in 2010. Prof. N.V. Chalapathi Rao, Centre of Advanced Study in Geology, Institute of Science, Banaras Hindu University, Varanasi, India is acknowledged for his valuable comments and suggestions on the first draft. This article is very much benefitted from my participation in the recently organized “International Conference on Science, Technology and Applications of Rare Earths” at Tirupati, Andhra Pradesh, India during 23–25, September, 2018. The author thanks the organizers, Prof. C.K. Jayasankar and Dr. M.L.P. Reddy, for inviting him to participate in the conference and deliver a talk.
Analysis of different rare metals, rare earth elements, and other common metals in groundwater of South West Bank/Palestine by ICP/MS-data and health aspects
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A comparative study on the trace and rare earth element analysis of an Indian polymetallic nodule reference sample by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry
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Rare earth elements and plant growth -third responses of corn and mungbean to low concentrations of cerium in dilution, continuously flowing nutrient solutions
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A novel methodology for rapid digestion of rare earth element ores and determination by microwave plasma-atomic emission spectrometry and dynamic reaction cell-inductively coupled plasma-mass spectrometry
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Simultaneous determination of 58 major and trace elements in volcanic glass shards from the INTAV sample mount using femtosecond laser ablation-inductively coupled plasma-mass spectrometry
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Rare-earth elements and stable isotope geochemistry of early cambrian chert-phosphorite assemblages from the lower tal formation of the krol belt lesser himalaya, India
Rare-earth Elements and Uranium in Phosphatic Nodules from the Continental Margins of India. Marine Authigenesis: From Global to Microbial, SEPM (Society for Sedimentary Geology) Special Publication No. 66 (2000), pp. 221-232
Rare earth elements: review of medical and biological properties and their abundance in the rock materials and mineralized spring waters in the context of animal and human geophagia reasons evaluation
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A novel extraction chromatography and MC-ICP-MS technique for rapid analysis of REE, Sc and Y: revising CI-chondrite and Post-Archean Australian Shale(PAAS) abundances
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Distribution of REE and yttrium among major geochemical phases of marine Fe-Mn-oxides: comparative study between hydrogenous and hydrothermal deposits
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Trace element geochemistry of Amba Dongar carbonatite complex, India: evidence for fractional crystallization and silicate-carbonate melt immiscibility
Journal of Earth System Science, 113 (4) (2004), pp. 519-531
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Dr. V. Balaram (DOB 1st June, 1951) received M.Sc. (1974) and Ph.D. degrees (1979) in Chemistry from the Andhra University, Visakhapatnam, India. He is Former Emeritus Scientist and Chief Scientist & Head, Geochemistry Division, CSIR - National Geophysical Research Institute, Hyderabad- 500 007, India. His research areas include trace element geochemistry, marine geochemistry, mineral exploration, spectroscopy and environmental chemistry. He has over 300 publications in international peer-reviewed journals, with ∼3100 citations (h-index 31& i10-index 84) and guided 5 PhD students, few postdoctoral and hundreds of PG students from different universities across the country. He is also recipient of several prestigious national and international awards such as "National Geoscience Award" from Government of India, New Delhi (2000), "S. Narayanaswamy Award" from Geological Society of India, Bangalore (2010); "Eminent Mass Spectrometrists Award" from Indian Society of Mass Spectrometry (ISMAS), Mumbai (2006); "Mantripragada Gold Medal” from Indian Society of Applied Geochemists (ISAG), Hyderabad (2006) and Lifetime Achievement Awards for Excellence in Science and Technology from ISAS-Kerala (2015) and Bundelkhand University, Jhansi, UP (2016). He was also the Leader of Regional Committee, Central Working Group for India for “International Geochemical Mapping Programme” (IGCP 360) during 1994–1997. He has popularized science by delivering >550 lectures in >225 academic institutions across India and abroad which also include some of the world's premier academic institutions across the globe during the last over 30 years.
Peer-review under responsibility of China University of Geosciences (Beijing).
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