Several strains of cyanobacteria can bind to rare earth metals. In some strains, the adsorbed metals accounted for up to ten percent of the dry matter of the microorganisms, reports a German research team led by Thomas Brück from the Technical University of Munich in the specialist magazine Frontiers in Bioengineering and Biotechnology. The discovery could enable a biotechnological recycling process for rare earths.
Mobile phones, cameras, electric motors, wind turbines and lamps – these are just a few of the products for which rare earth metals are used. These 17 metals have extraordinary electromagnetic, catalytic and optical properties. However, there are only a few deposits worldwide where the mining of rare earths is economically worthwhile. In addition, the extraction of the metals is often environmentally harmful and large amounts of waste are produced. Therefore, scientists are looking for ways to recycle rare earth metals.
Cyanobacteria are known to bind to metals. Brück and colleagues conducted their experiments with four known laboratory strains and eight other strains isolated from their natural environment. They all originally come from habitats with extreme environmental conditions, such as natron lakes in Chad, deserts in Namibia, crevices in South Africa or polluted streams in Switzerland.
“These cyanobacteria could be used in future environmentally friendly processes for the recovery of rare earth metals and for the treatment of industrial wastewater at the same time,” Brück is quoted as saying in a statement from the specialist magazine.
The microbes bind metals such as lead or aluminum more efficiently than rare earths
The scientists exposed the bacterial cultures to an aqueous solution containing the rare earth metals lanthanum, cerium, neodymium and terbium. Five of the strains tested showed interesting binding properties. In nature, they live under very different conditions and come from different biological orders, but they have one thing in common: They produce larger amounts of so-called extracellular polymeric substances (EPS), which result in a biofilm.
Experiments with different pH values indicate an electrochemical binding mechanism associated with this biofilm. Cyanobacteria of the genus Nostoc brought the highest yield with 84.2 to 91.5 milligrams of metal per gram of biomass. Sometimes the reaction is very quick. In experiments with cerium, most of the metal was bound to the biomass within the first five minutes.
The problem that has not yet been solved is that the microbes bind metals such as lead or aluminum more efficiently than rare earth metals. But Brück is confident that, with the help of bacteria, a commercial recycling process can be developed at some point. A major advantage is that the binding mechanism is reversible: “That means we can wash out the metals and reuse the biomass.” In this way, a true circular economy could emerge.