Bacteria offer benign rare earths separation

HARVARD researchers say bacteria could be the key to the challenge of cleanly separating rare earth metals.

Rare earth metals are critical to modern technologies ranging from smartphones and wind turbines to catalysts and missiles but because the 17 elements are so similar to one another, the process of separating them requires hundreds of steps. As well as being costly, current separations rely on large volumes of hazardous chemicals – so much so that producing one ton of rare earth metals can result in as much as 2,000 t of toxic waste, the researchers explain.

“Rare earths are very desirable but only if you can separate them,” says Harvard researcher David Clarke. “It’s a big 'if' because how can you separate ions that are almost the same size and the same charge?”

Bacteria may hold the key to enabling industry to achieve cleaner separations, the team has found. Bacteria are already used to bioabsorb toxic elements from wastewater and filter metals from mine drainage systems. Armed with this knowledge, the team set about seeing whether rare earths could be filtered through bacteria.

The researchers immobilised a bacteria from marine algae on an assay filter and then passed a solution of mixed rare earth metals through it. The bacteria absorbed the elements of the solution, fixing them to their own surfaces. The team found that it could selectively release each element depending on the acidity of the solution washed through the filter.

“We found that it is possible to concentrate a solution of equal concentrations of each lanthanide to nearly 50% of the three heaviest lanthanides in just two passes,” said researcher William Bonificio. “This surpasses existing industrial practice.”

The processes have so far been conducted at laboratory scale. The team is now looking for an industrial partner to help overcome the challenges of scaling up the technology, Bonificio told The Chemical Engineer.

“This is a radically different way of doing separation,” added Clarke. “We have an opportunity to harness the diversity of bacterial surface chemistry to separate and recover these valuable metals in a way that is environmentally benign.”