New research led by scientists at Penn State and the Lawrence Livermore National Laboratory (LLNL) demonstrates how a protein isolated from bacteria can provide a more environmentally friendly way to extract rare earth elements from unconventional sources such as mine tailings and e-waste and to separate them from other metals and from each other. An open-access paper on the work is published in the journal ACS Central Science.

Low-grade sources of rare earth elements (REE), for example from industrial waste, typically contain many rare earth elements and other metals mixed together. The new method relies on a protein called lanmodulin (LanM) that first binds to all the rare earth elements in the source. Then other metals are drained and removed. By changing the conditions of the sample, for example by changing the acidity or adding ingredients called chelators, individual types of rare earth elements become unbound and can be collected. Even when a sample has very low amounts of the rare earth elements, this new procedure successfully extracts and separates heavy rare earth elements with high purity. Credit: Dong et al. 2021, ACS Central Science

In order to meet the increasing demand for rare earth elements for use in emerging clean energy technologies, we need to address several challenges in the supply chain. This includes improving the efficiency and alleviating the environmental burden of the extraction and separation processes for these metals. In this study, we demonstrate a promising new method using a natural protein that could be scaled up to extract and separate rare earth elements from low-grade sources, including industrial wastes.

—Joseph Cotruvo Jr., assistant professor and Louis Martarano Career Development Professor of Chemistry at Penn State and co-corresponding author

Because the US currently imports most of the rare earth elements it needs, a new focus has been placed on establishing a domestic supply from unconventional sources, including industrial waste from burning coal and mining other metals as well as electronic waste from cell phones and many other materials. These sources are vast but considered low-grade, because the rare earths are mixed with many other metals and the amount of rare earths present is too low for traditional processes to work well. Furthermore, current methods for extraction and separation rely on harsh chemicals, are labor intensive, sometimes involve hundreds of steps, produce a high volume of waste, and are high cost.

The new method takes advantage of a bacterial protein called lanmodulin, previously discovered by the research team, that is almost a billion times better at binding to rare earth elements than to other metals.

The extraction and subsequent separation of individual rare earth elements (REEs) from REE-bearing feedstocks represent a challenging yet essential task for the growth and sustainability of renewable energy technologies. As an important step toward overcoming the technical and environmental limitations of current REE processing methods, we demonstrate a biobased, all-aqueous REE extraction and separation scheme using the REE-selective lanmodulin protein.

Lanmodulin was conjugated onto porous support materials using thiol-maleimide chemistry to enable tandem REE purification and separation under flow-through conditions. Immobilized lanmodulin maintains the attractive properties of the soluble protein, including remarkable REE selectivity, the ability to bind REEs at low pH, and high stability over numerous low-pH adsorption/desorption cycles.

We further demonstrate the ability of immobilized lanmodulin to achieve high-purity separation of the clean-energy-critical REE pair Nd/Dy and to transform a low-grade leachate (0.043 mol % REEs) into separate heavy and light REE fractions (88 mol % purity of total REEs) in a single column run while using ∼90% of the column capacity. This ability to achieve, for the first time, tandem extraction and grouped separation of REEs from very complex aqueous feedstock solutions without requiring organic solvents establishes this lanmodulin-based approach as an important advance for sustainable hydrometallurgy.

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The protein is first immobilized onto tiny beads within a column—a vertical tube commonly used in industrial processes—to which the liquid source material is added. The protein then binds to the rare earth elements in the sample, which allows only the rare earths to be retained in the column and the remaining liquid drained off. Then, by changing the conditions, for example by changing the acidity or adding additional ingredients, the metals unbind from the protein and can be drained and collected. By carefully changing the conditions in sequence, individual rare earth elements could be separated.

The research team separated yttrium (Y) from neodymium (Nd)—both abundant in primary rare earth deposits and coal byproducts—with greater than 99% purity. They also separated neodymium from dysprosium (Dy)—a crucial pairing that is common in electronic waste—with greater than 99.9% purity in just one or two cycles, depending on the initial metal composition.

The high-purity of the recovered neodymium and dysprosium is comparable to other separation methods and was accomplished in as many or fewer steps without using harsh organic solvents. Because the protein is able to be used for many cycles, it offers an attractive eco-friendly alternative to the methods currently used.

—Ziye Dong, a postdoctoral researcher at LLNL and first author

The researchers do not think their method will necessarily supplant the current liquid-liquid extraction process that is commonly used for high-volume production of lighter rare earth elements from high-grade sources. Instead, it will allow for efficient use of low-grade sources and especially for extraction and separation of the rarer and generally far more valuable heavy rare earths.

Other recent methods are capable of extracting rare earth elements from low-grade sources, but they typically stop at a ‘total’ product that has all the rare earths lumped together, which has relatively little value and then needs to be funneled into more conventional schemes for further purification of individual rare earth elements. The value is really in the production of individual rare earths and especially the heavier elements.

—Dan Park, staff scientist at LLNL and co-corresponding author

The researchers plan to optimize the method so fewer cycles are required to obtain the highest-purity products and so it can be scaled up for industrial use.

If we can engineer derivatives of the lanmodulin protein with greater selectivity for specific elements, we could recover and separate all 17 rare earth elements in a relatively small number of steps, even from the most complex mixtures, and without any organic solvents or toxic chemicals, which would be a very big deal. Our work shows that this goal should be achievable.

—Joseph Cotruvo Jr.

This work was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the US Department of Energy, and the DOE Office of Science.

Resources

• Ziye Dong, Joseph A. Mattocks, Gauthier J.-P. Deblonde, Dehong Hu, Yongqin Jiao, Joseph A. Cotruvo, and Dan M. Park (2021) “Bridging Hydrometallurgy and Biochemistry: A Protein-Based Process for Recovery and Separation of Rare Earth Elements” ACS Central Science doi: 10.1021/acscentsci.1c00724