Microbial-mediated bastnaesite dissolution as a viable source of clay-adsorbed rare earth elements in the regolith-hosted deposits

Understanding the weathering processes of minerals containing rare earth elements (REE) is crucial for unraveling the genesis of regolith-hosted REE deposits. However, the weathering mechanisms of bastnaesite, a primary REE carrier in parent rocks, remain uncertain. Discrepancies between field observations and thermodynamic calculations regarding its weatherability during mineral-groundwater interactions have raised questions about the factors controlling the natural weathering of bastnaesite. Here, we propose that microbial activities significantly contribute to the dissolution of bastnaesite. To test this hypothesis, we conducted bio-weathering experiments using natural bastnaesite and a wild strain, Bacillus thuringiensis (Bt) isolated from regolith-hosted REE deposits. The results indicate that, consistent with thermodynamic predictions, bastnaesite exhibited resistance to dissolution under simulated groundwater pH conditions (∼6). However, the presence of Bt significantly enhanced bastnaesite dissolution. Bt exuded various types of organic acids, acidifying the solution during bio-weathering. Comparative biotic and abiotic experiments demonstrated that Bt could induce bastnaesite dissolution through acidolysis and ligand complexation. These effects were further strengthened by direct cell attachment to the mineral surfaces. Existing field studies suggest the rapid dissolution of bastnaesite during the very early rock weathering period, adding uncertainty about the contribution of bastnaesite to the enrichment of clay-adsorbed REE. Our results indicate that the dissolution of bastnaesite is largely pH-dependent, with bio-dissolution rates (R_Ce = 10⁻¹³ − 10⁻¹² mol·m⁻²·s⁻¹) close to or slightly lower than the lab-determined dissolution rates of feldspars and micas at weakly acidic to neutral pH levels. Since the weathering of these aluminosilicate minerals provides the dominant source of clay minerals, we infer that some REE released from bastnaesite can be retained by clay minerals in the weathering profile. These findings may provide new insights into the natural weathering of bastnaesite and advance our understanding of the REE biogeochemical cycling during the formation of regolith-hosted REE deposits.