Bioleaching of rare earth elements from bastnaesite-bearing rock by actinobacteria

Biological leaching of rare earth elements (REEs) from low-grade ores and mine tailings represents a potentially cost-effective green technology for REE recovery, yet the specific mechanisms of such biological process, especially by common rock-dwelling actinobacteria, remain unclear. In this study, we performed laboratory bioleaching experiments to understand the rates and mechanisms of REE bioleaching. Four actinobacterial strains, including two isolates from REE-rich rocks and two from surrounding red soil, were found to have the capability of bioleaching REEs from a bastnaesite-bearing rock and bastnaesite. Bastnaesite is one member of a REE mineral family REE(CO3)F, where REE is dominated by Ce, La or Y. In a nutrient-rich growth medium, bioleached total REE concentration ranged from 56 to 342 μg/L. In an oligotrophic medium, only one strain (Streptomyces sp.) grew in the presence of the bastnaesite-bearing rock and bastnaesite, and leached as much as 548 μg/L total REEs. The leaching efficiency of total REEs, defined as the amounts of leached REEs relative those present in the rock was 0.008-0.08%, which should be underestimated due to new REE-bearing mineral precipitation and/or sorption of leached REEs to residual rock and cell surfaces. These bioleached REE concentrations were higher than those leached by abiotic methods under a similar pH condition, suggesting that pH was not the only factor in REE bioleaching. Additional experiments demonstrated that these four actinobacterial strains secreted various organic acids, complexing ligands, and siderophores as dominant agents to extract REEs from bastnaesite-bearing rock. Some of the leached REEs were observed to accumulate on cell surfaces. Multiple lines of evidence suggest that REEs were leached primarily from primary REE mineral bastnaesite. Our results suggest that microorganisms have a potential to selectively bioleach and recover certain REEs from low-grade ores and mine tailings.