Mobilization and fractionation of rare earth elements during experimental bio-weathering of granites

Abstract

Weathering of parent granite is pivotal in the formation of regolith-hosted rare earth elements (REE) deposits, which provide over 90 % of global heavy rare earth elements (HREE) demand. Microbes have been recognized to control the fates of REE during weathering jointly with other geochemical factors, but the specific mechanisms of how microbes affect REE mobilization and fractionation remain poorly understood. In this study, bio-weathering experiments at room temperature were conducted with four native bacterial isolates from the Dabu regolith-hosted HREE deposit to investigate the microbial effects on REE mobilization from the parent granite. The presence of these isolates remarkably enhanced the liberation of REE by a factor of 4–21 at the end of the 30-day experiments. Concomitantly, microbes could immobilize REE through extracellular metabolites-induced precipitation and, to a lesser extent, by cell sorption, resulting in 25 %–82 % underestimations of the REE release efficiencies. The synergy of acidolysis and complexation was found to be a crucial bio-weathering mechanism, and complexation with organic acid ligands was more efficient in mobilizing REE at weakly acidic to near-neutral pH. REE fractionation during bio-weathering was mainly controlled by the properties of the host minerals but also influenced by microbes. Specifically, the preferential dissolution of synchysite-Y and parisite-Ce at the initial stage of granite weathering resulted in relatively higher release efficiencies of light rare earth elements (LREE) and middle rare earth elements (MREE). In addition, the secretion of ample organic acids, malic and tartaric acids, in particular, accounted for the preferential release of MREE and HREE mediated by the two isolates from the Bacillus genus. Our results showcase high potential microbial effects on the mobilization and fractionation of REE during granite weathering, posing new sights into the biogeochemical process during the formation of regolith-hosted REE deposits.