by 
Thomas Brück
Demand for rare earth elements is growing and could reach 315,000 tonnes by 2030. Meanwhile, more than 40 million tonnes of e-waste – computers, mobile phones and other electronics that are junk – are generated every year. Some of the waste contains the same valuable elements that meet increasing demand.
Over the years, several notable methods have been proposed to recover used or waste-based rare earth elements, such as urban mining or nanofiltration systems in streams. A persistent idea is to use microorganisms such as bacteria to “bioabsorb” the desired substances – a passive biological process in which the organisms bind to and remove the substances from an aqueous solution. The technology has yet to be rolled out on an industrial scale, but some researchers suggest their latest findings represent a major step forward.
In a recent paper, Thomas Brück, a professor at the Technical University of Munich who studies synthetic biotechnology and sustainability, and his colleagues describe identifying 12 exotic species of cyanobacteria that are particularly good at absorbing rare earth elements. These species can be used to recover desirable elements while cleaning the land and water. “(I)t’s not something we predicted by any means,” Brück told Ars.
The unusual suspects
The research was six years in the making. The team began screening a variety of algae and bacterial species, but none of them absorbed rare earth elements particularly well. So they turned their attention to a dozen species of cyanobacteria. Some came from environments that were particularly inhospitable to most life forms. For example, Lake Natron, which is both unusually alkaline, with a pH of around 10, and sees temperatures that can reach 60º C (or 140º F). According to Brück, it is unclear how these organisms evolved to thrive in these environments contributed to their ability to devour rare earth elements.
Often the species came from incredibly specialized habitats such as dry desert soils in Namibia, the alkaline Lake Natron in Chad, rock crevices in South Africa or polluted streams in Switzerland. These were “really unique, extreme environments,” Brück said.
Most of these bacteria had not previously been assessed for bioremediation potential. In the lab, the teams exposed cultures of the different species to aqueous solutions containing the rare earth elements lanthanum, cerium and neodymium, then checked to see how well they held the elements to the surface using infrared spectroscopy.
A previously uncharacterized species of Nostoc cyanobacteria performed best. Its bioabsorption of the four rare earth elements from the solutions absorbed between 84.2 and 91.5 mg of metal per gram of biomass. The worst performance was Scytonema hyalinum at 15.5 to 21.2 mg per gram of biomass. However, the paper noted that how well each candidate performed depended on acidity and that the processes were more efficient when there were no other metals in the solution to compete with the targeted rare earth element.
The dirty, erm, clean dozen
It is also possible, and relatively easy, to get the desired rare earth elements back out of the biomass. It would simply be a matter of changing the pH of the solution – using an acid or something like lye – or salinity. The elements would functionally only “wash” out from the biomass. Returning the solution back to its previous state would then allow the process to restart, meaning the cyanobacterial cultures could be reused.
“It’s not a one-off deal where at the end of the bond you have to burn the biomass to recover your metals,” Brück said.
Researchers or industrial players can create bioreactors – specialized vessels containing microbial biomass – for several future applications. First, they could be used to collect rare earth elements from e-waste dumps, although this would require turning the e-waste into a form usable by the microorganisms. This will provide environmental benefits by removing waste from these areas and potentially create jobs in parts of the world where e-waste is regularly shipped from the Global North, places like Nigeria, Ghana and Tanzania. Then, Brück added, they could be used to clean out and recover these elements from industrial runoff, such as from mining or from the chemical industry.
According to Brück, the research could represent a major advance. From this point, the researchers hope to scale up significantly – something that has not yet happened in the field. They plan to work with partners in various industries to do so, although it’s a difficult prospect given that it’s a fairly specialized process: It’s not like growing corn in agriculture, nor is it like traditional metal refining methods. Nevertheless, the researchers are hopeful about the results.
“I think we’re at a step right now where we can say, ‘Hey, we can make this work,'” Brück said.
Frontiers, 2023. DOI: 10.3389/fbioe.2023.1130939 (About DOIs)
