Microbe-mineral interactions control fluoride mobilization from aluminum-substituted goethite into groundwater.

The genesis of geogenic fluoride (F-)-contaminated groundwater is frequently attributed to abiotic processes, while the roles of biotransformation kinetics of redox-sensitive iron (Fe) oxides have largely been underestimated. Aluminum (Al) substitution alters the structure and surface properties of natural Fe oxides, driving dynamic shifts between F- sinks and source by modulating F- mobilization-immobilization during redox-driven Fe-oxide transformation, but the influence of Al substitution on microbial Fe(III)-oxide reduction and F- turnover remains unclear. Through field investigations, microcosm experiments, and new kinetic modeling, this research delineates biogeochemical effects of microbially-mediated reductive dissolution of varying Al(III)-substituted Fe oxides (goethite) on F- enrichment in groundwater. Results show that Al substitution governs F- mobilization-immobilization dynamics during dissimilatory goethite bioreduction. Low-level Al(III) substitution weakened the mineral lattice and enhanced F-bearing goethite bioreduction and F- release. Co-mobilized Al3+ facilitated F- stabilization by forming Al-F complexes. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses suggest that Al-F complexes reduced free F-, alleviating F- biotoxicity toward Fe-reducing bacteria and sustaining Fe(III)-oxide bioreduction even at high dissolved F- concentrations. High-level Al(III) substitution promoted Al(III) accumulation on mineral surfaces (surface-associated Al(III)), thereby preempting microbe-mineral contact sites and suppressing Fe(III) bioreduction and F- release. Bioreduction of Fe(III) in goethite led to partial F- release during the early stages, and then surface-bound Al(III) acted as a cation bridge to re-immobilize F- via surface complexation. This inhibiting effect explains decoupled Fe and F behaviors in aquifers where Fe(III) bioreduction proceeds but F- is re-immobilized by surface-bound Al(III). Our results highlight an underappreciated mechanism driving high-F groundwater under Fe(III)-reducing conditions and unravel the critical roles of microbe-mineral interactions for F mobilization/immobilization by Al(III)-substituted Fe oxides. The findings have broad implications for deciphering anionic toxic compounds enrichment in reducing groundwater and guiding rehabilitation of geogenic F-contaminated groundwater.