Biobased Method Could Extract Domestic Supply of Rare-Earth Elements

An eco-friendly way to extract and separate rare earth elements (REEs) from unconventional sources has been demonstrated by researchers at Penn State and Lawrence Livermore National Laboratory (LLNL). The method relies on a bacterial protein that is almost a billion times better at binding to REEs than to other metals. The protein, called lanmodulin, was previously discovered by the same research team.

According to the researchers, the new method could be scaled up to extract and separate REEs from low-grade sources, including industrial and electronic wastes, to help establish a U.S. domestic supply of REEs.

In the all-aqueous method, lanmodulin is conjugated onto porous support materials to enable tandem REE purification and separation under flow-through conditions. The protein is immobilized onto tiny beads within a column, and the liquid source material is added. The protein binds to the REEs in the sample, which allows the REEs to be retained in the column while the remaining liquid is drained off.

The immobilized lanmodulin maintains the attractive properties of the soluble protein, including REE selectivity, the ability to bind REEs at low pH, and high stability over numerous low-pH adsorption/desorption cycles.

By changing the conditions in the column — for example, by changing the acidity or adding chelators — the REEs unbind from the protein and can be drained and collected. By changing the conditions in sequence, the researchers found that it was possible to separate individual REEs. Even when a sample had very low amounts of REEs, the researchers were able to use the procedure to extract and separate heavy REEs with high purity.

A method improves the extraction and separation of rare earth elements — a group of 17 chemical elements critical for technologies such as smart phones and electric car batteries — from unconventional sources. The method could eventually be scaled up to help develop a domestic supply of rare earth metals from industrial waste and electronics due to be recycled. Courtesy of Barsamuphe, Wikimedia Commons.

The separation of REEs from other metals in essential when dealing with low-grade sources that are made up of multiple metals, said professor Joseph Cotruvo Jr. of the research team.

“Even in a very complex solution where less than 0.1% of the metals are rare earths — an exceedingly low amount — we successfully extracted and then separated a grouping of the lighter rare earths from a grouping of the heavier rare earths in one step,” Cotrovu said. Separating the REEs into individual elements is necessary for them to be put to practical use, he added.

Using the new method, the researchers separated the REE yttrium (Y) from the REE neodymium (Nd) and achieved greater than 99% purity. The team was able to transform a low-grade leachate (0.043 mol % REEs) into separate heavy and light REE fractions in a single column run while using about 90% of the column capacity. The researchers also demonstrated the ability of immobilized lanmodulin to separate the REE pair neodymium/dysprosium (Nd/Dy), commonly found in electronic waste, with greater than 99.9% purity. They said that they were able to complete the separation process 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,” LLNL researcher Ziye Dong said. “Because the protein is able to be used for many cycles, it offers an attractive eco-friendly alternative to the methods currently used.”

While the protein-based method is not expected to supplant the current liquid-liquid extraction process that is commonly used for high-volume production of lighter rare earth elements from high-grade sources, it could enable efficient use of low-grade sources, and especially for extraction and separation of the heavy REEs.

“Our process is particularly convenient because these high-value metals can be purified off the column first,” Cotruvo said.

“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,” LLNL researcher Dan Park said. “The value is really in the production of individual rare earths and especially the heavier elements.”

Low-grade sources of rare earth elements (REEs), 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 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. Courtesy of Dong et al. 2021, ACS Central Science, doi: 10.1021/acscentsci.1c00724.

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

“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,” Cotruvo said. “This includes improving the efficiency and alleviating the environmental burden of the extraction and separation processes for these metals.

“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.”

The ability to realize tandem extraction and grouped separation of REEs from complex aqueous feedstock solutions without requiring organic solvents establishes this lanmodulin-based approach as an important advancement for sustainable hydrometallurgy.

The research was published in ACS Central Science (www.doi.org/10.1021/acscentsci.1c00724).