Dynamic Interplay between Biogenic Fe(III) (oxyhydr)oxides and Microbial Reduction Pathways Governs Arsenic Mobilization

Microbial reductive dissolution of As-bearing Fe(III) (oxyhydr)oxides is a primary mechanism of arsenic (As) mobilization in reducing aquifers, yet the relative contributions of mineral bioreactivity and microbial metabolic pathways remain insufficiently quantified.Here, we compared Fe(III)-reducing bacteria (Shewanella oneidensis MR-1 and Geobacter sulfurreducens) with a bacterium capable of both As(V) and Fe(III) reduction (Clostridium sp.anHT01, isolated from a high As aquifer sediment from Inner Mongolia) to resolve Fe and As release kinetics from As(V)bearing biogenic ferrihydrite-goethite (Bio-GF) and lepidocrocite (Bio-Lp) under bicarbonate-buffered reducing conditions.Biogenic mineralogy primarily governed the extent of Fe(III) reduction, with Bio-GF exhibiting higher bioreactivity than Bio-Lp, but did not dictate net As mobilization.In contrast, microbial metabolic pathways controlled the temporal relationship between Fe reduction and As release.Systems with active As(V) reduction exhibited delayed Fe(III) reduction yet rapid As(III) release, resulting in elevated As released /Fe released ratios and net As mobilization during the Fe-reduction lag phase.Secondary Fe(II) minerals modulated As partitioning and retention but did not govern the onset of As mobilization.Together, these results demonstrate that As mobilization during microbial reduction of biogenic Fe(III) (oxyhydr)oxides is jointly governed by mineral bioreactivity and microbially mediated Fe-As release relationships, providing a mechanistic basis for improving prediction of As behavior in reducing aquifer systems.