By Brooke K. Mayer, PhD, PE
Rare earth elements (REEs) have increasingly entered public discourse given their expanding use in a diverse range of applications. For example, they are critical to the production of flat-panel displays, screens, some lightbulbs, camera lenses, computer hard disks, electric car batteries, and magnets in wind turbine generators. REEs include the 15 lanthanide series elements from atomic numbers 57 to 71, representing lanthanum through lutetium; yttrium (atomic number 39) and scandium (atomic number 21) are also often included due to chemical similarities.
Ironically, most REEs are relatively common in the earth’s crust. For example, on average, they are more common than silver, gold, or platinum; however, concentrated and economically minable REE deposits are rare, making extraction and separation challenging.[1,2] Uneven distribution of global deposits introduces trade concerns, given the wide use of REEs in high-tech commercial products. China controls over 50 percent of global REE resources, and, over time, China’s share of REE production has increased.[2]
In 1993, China produced 38 percent of REEs; the United States produced 33 percent, and Australia produced 12 percent. By 2015, China’s share of global production had increased to 97 percent.3 For the U.S., such shifts translated into increased reliance on imported mineral commodities (Figure 2).
In 2021, 100 percent of yttrium and more than 90 percent of the lanthanides were imported. The U.S. Department of Energy has thus designated five REEs—yttrium, dysprosium, europium, neodymium, and terbium—as critical rare-earth elements, since they are expected to be in short supply within the next two decades.[4] Accordingly, it is readily apparent that supplies of REEs and the risks of supply limitations are very important. But how does this relate to water?
Given the challenges with producing and distributing these strategic materials, developing effective and economical means of recovering these elements from secondary sources is crucial.[2] For example, acid mine waters from coal or metal mining industries can contain REEs at the milligrams-per-liter level (approximately 1,000 times higher than concentrations in natural waters).[4] Figure 3 illustrates the relative potential economic value of REE recovery from several types of wastewater streams. As shown, higher REE loading in acid mine waters translates to one to three orders of magnitude higher economic value compared to shale gas wastewater and municipal wastewater. The combination of elemental occurrence and economic value indicates that yttrium generally offers the highest potential value recovery, with lanthanum, praseodymium, and lutetium also playing major roles. In the case of shale gas wastewater, the potential value of recovered europium is also significant.
Prospects for realizing waste stream valorization via REE recovery depend on the target element, as well as the process used. Commonly used approaches to separate, concentrate, and recover REEs include precipitation, solvent extraction, electrochemical, membrane filtration, adsorption, oxidation/reduction, and ion exchange.[2] A comparison of a subset of potential technologies for recovering europium from shale gas wastewater is shown in Figure 4.
For REE recovery from wastewater, solvent extraction and ion exchange are two commonly used techniques.[4] Solvent extraction is popular in use with high REE concentrations (for example in hydrometallurgical processing), while ion exchange is effective for treating large volumes containing dilute levels of REEs, a scenario that is unfavorable for conventional methods such as solvent extraction.[7,8] For example, using an ion-exchange resin to recover REEs from acidic mine water, 260 times concentration of lanthanum and 160 times concentration of cerium were reported, boosting the value proposition of the dilute waste stream.[4] Additional advantages of ion exchange over solvent extraction include the reduction of chemical additions and technological simplicity of the method.[4,7]
Lanthanides typically occur as trivalent cations, making them susceptible to adsorption on cationic ion exchange resins.[2,7] Strongly acidic and weakly acidic cation exchange resins, as well as chelating resins (which adsorb cations via ligand bonding with iminodiacetate, aminophosphonic, or picolyamine), are often employed.[11,12] Rychkov et al. (2021)[12] reviewed REE recovery using cation exchange resins featuring sulfonic acid, carboxylic, phosphorus-based, and polyfunctional functional groups. The resins were tested in a range of environmental samples; wastewater streams, including acid mine drainage; and a number of leaching solutions, for example from uranium, phosphogypsum and phosphoric acid, and waste electronic enterprises.[12] Performance varied by metal target, resin, and wastewater matrix. While the selection of an appropriate ion exchanger varies by application, large-scale technologies for REE production rely on resins featuring appropriate selectivity, high mechanical strength and stability, and reusability during sorption/desorption cycles.[12]
Using strong acid cation resins, several studies reported greater selectivity for light REEs (atomic numbers 57 to 64 and scandium) compared to heavy REEs (atomic numbers 65 to 71 and yttrium) based on higher selectivity for smaller hydration shell radii.[7] Selectivity is also influenced by the water matrix, for example pH and the presence of competing ions.
Tests using a sulfonic acid resin showed selectivity in the order of REE ≈ thorium(IV) > aluminum(III) ≈ iron(III), whereas chelating resins demonstrated selectivity for thorium(IV) ≈ iron(III) >> REEs ≈ aluminum(III).[14] When transition metals such as iron and aluminum were present, observed recovery of REEs dropped fourfold.[13] Given that transition metals are often present at much higher levels compared to REEs, and their potential adverse impact on resin efficiency, removal of these elements enhances subsequent REE recovery.[4]
Beyond being able to selectively separate REEs from bulk matrices, the amount of REE adsorbed to the resin, or its capacity, is a critical design parameter. The resin’s capacity is influenced by the ion exchanger’s properties, including surface area, functional groups, and porosity; the sorbed ion’s charge, size, and radius of hydration; and operational parameters, such as pH, competing ions, and reaction time.[7] Hérès et al. (2018)1[3] recommended that a total ion exchange capacity greater than 2 milliequivalents per gram of resin is generally able to achieve reasonable investment costs. Other target characteristics include greater than 15 square meters per gram surface area and 2–50 nm pore diameters, the combination of which maximizes ion exchange capacity while promoting diffusion into the matrix.[13]
Ion exchange offers a viable approach to recover REEs from wastewater, which represents an important secondary source of these difficult-to-extract critical materials. Such approaches are crucial to support the production of high-tech products now and into the future.
References
1. USGS. The Rare-Earth Elements: Vital to Modern Technologies and Life. 2014. doi:10.1177/001698626601000310
2. Kolodynska D, Fila D, Gajda B, Gega J, Hubicki Z. Rare Earth Elements—Separation Methods Yesterday and Today. In: Inamuddin, Ahamed M, Asiri A, eds. Applications of Ion
Exchange Materials in the Environment. Springer Nature; 2019:161–185. doi:10.1007/978-3-030-10430-6
3. AGI. What are rare earth elements, and why are they important? Published 2022. Accessed December 3, 2022. https://www.americangeosciences.org/critical-issues/faq/what-are-rare-earth-elements-and-why-are-they-important
4. Hermassi M, Granados M, Valderrama C, Skoglund N, Ayora C, Cortina JL. Impact of functional group types in ion exchange resins on rare earth element recovery from treated acid mine waters. J Clean Prod. 2022;379(P2):134742. doi:10.1016/j.jclepro.2022.134742
5. USGS. Comparison of U.S. Net Import Reliance for Nonfuel Mineral Commodities—A 60-Year Retrospective (1954-1984-2014). 2015. https://pubs.er.usgs.gov/publication/fs20153082
6. USGS. Mineral Commodity Summaries 2022. USGS; 2022.
7. Felipe ECB, Batista KA, Ladeira ACQ. Recovery of rare earth elements from acid mine drainage by ion exchange. Environ Technol (United Kingdom). 2021;42(17):2,721-2,732. doi:10.1080/09593330.2020.1713219
8. Hermassi M, Grandados M, Valderrama C, Ayora C, Cortina JL. Recovery of rare earth elements from acidic mine waters by integration of a selective chelating ion-exchanger and a solvent impregnated resin. J Environ Chem Eng. 2021;9(105906).
9. Tian L, Chang H, Tang P, et al. Rare Earth Elements Occurrence and Economical Recovery Strategy from Shale Gas Wastewater in the Sichuan Basin, China. ACS Sustain Chem Eng. 2020;8(32):11914-11920. doi:10.1021/acssuschemeng.0c04971
10. Westerhoff P, Lee S, Yang Y, et al. Characterization, recovery opportunities, and valuation of metals in municipal sludges from U.S. wastewater treatment plants nationwide. Environ Sci Technol. 2015;49:9479−9488. doi:10.1021/es505329q
11.Meyers PS. How Chelating Resins Behave. AESF/EPA Conf. Published online 1998:22-29.
12. Rychkov V, Kirillov E, Kirillov S, et al. Rare Earth Element Preconcentration from Various Primary and Secondary Sources by Polymeric Ion Exchange Resins. Sep Purif Rev. 2022;51(4):468-483. doi:10.1080/15422119.2021.1993255
13. Hérès X, Blet V, Di Natale P, et al. Selective extraction of rare earth elements from phosphoric acid by ion exchange resins. Metals (Basel). 2018;8(9). doi:10.3390/met8090682
14. Page MJ, Soldenhoff K, Ogden MD. Comparative study of the application of chelating resins for rare earth recovery. Hydrometallurgy. 2017;169:275-281. doi:10.1016/j.hydromet.
2017.02.006
About the Author
Dr. Brooke K. Mayer is an Associate Professor in the Department of Civil, Construction and Environmental Engineering as part of the Opus College of Engineering at Marquette University. She holds Bachelors, Masters and Doctorate degrees in civil engineering with an emphasis in environmental engineering from Arizona State University. She is a registered Professional Engineer in the state of Arizona.