A protein that binds rare-earth elements could be used to extract and separate these valuable metals from low-grade sources, such as coal ash or electronic waste (ACS Cent. Sci. 2021, DOI: 10.1021/acscentsci.1c00724).
The rare-earth elements (REEs) include the 15 lanthanides, along with scandium and yttrium, and many are in high demand for products such as electric vehicles, wind turbines, and light-emitting diodes. Although most REEs are not particularly rare, they tend to be spread thinly throughout Earth’s crust rather than concentrated in a single place, which limits opportunities for mining them at scale. And the elements are often found together in the ground and share similar chemistry, requiring separation processes that involve large amounts of energy and organic solvents.
These difficulties mean that it is neither economically feasible nor environmentally friendly to extract and separate REEs from anything but high-grade ores using existing technology. The same challenges apply to waste materials containing low but potentially valuable concentrations of REEs, including ash left by burning coal, runoff from mines, and waste electronic items.
A team led by Joseph A. Cotruvo Jr. at Pennsylvania State University and Dan M. Park at Lawrence Livermore National Laboratory has now developed an alternative process that could tap these sources without requiring organic solvents, improving access to REEs while helping to avoid industrial waste. It relies on lanmodulin, a small acid-resistant protein with a very strong affinity for lanthanides. It is produced by certain methane-digesting bacteria.
The researchers attached the protein to porous microbeads in a column. As an acidic solution of metal ions passes through the column, lanmodulin snatches the REEs out of the solution while allowing others, such as copper or zinc, to pass right through. The REEs can then be freed by changing the pH of the solution or by adding a counterion such as citrate to chelate the REEs. Acid from the process, and the columns themselves, can be reused many times, Cotruvo says.
The team tested the purification column on a solution leached from coal ash. One pass through the column was enough to produce an enriched solution in which 88% of the metal ions were REEs, a 2,000-fold improvement in purity. “It’s impressive, because it’s starting at less than 0.1% rare earths, so it’s a really large upgrading,” Cotruvo says. By carefully adjusting the pH, the researchers could draw REEs from the column in two separate batches containing lighter and heavier REEs, respectively.
The technique could also completely separate mixtures of two REEs, including neodymium and dysprosium, which are commonly found together in electronic waste containing rare-earth magnets. “If we’re going to recycle electronic waste, this is one of the most important separations,” Cotruvo says. Starting with a solution that mimicked the composition of such waste, the researchers used several separation cycles to recover more than 80% of each element, each at more than 99% purity.
“It’s a nice demonstration of how a natural system can be used, pretty much unaltered, to solve this problem of separating lanthanides,” says bioinorganic chemist Lena J. Daumann of Ludwig Maximilian University Munich, who studies lanthanide-binding proteins and was not involved in the research. “This protein-based approach, once scaled up and optimized for a continuous process, could be a really promising technique for low-grade sources.”
At a larger scale, Cotruvo says that multiple separation columns could be linked to run continuously, and tweaking conditions such as pH, flow rate and other factors should also improve the column’s performance.