Microbially mediated mobilization of arsenic from aquifer sediments under bacterial sulfate reduction

Understanding the biogeochemical processes controlling arsenic (As) mobilization under bacterial sulfate reduction (BSR) in aquifer sediments is essential for the remediation of high As groundwater. Here, we conducted microcosm experiments with shallow aquifer sediments from the Jianghan Plain (central Yangtze River Basin) under the stimulation of exogenous sulfate. Initially, co-increases of As(III) (from 0.0 to 88.5 μg/L), Fe(II) (from 0.5 to 6.0 mg/L), and S(-II) (from 0.0 to 90.0 μg/L) indicated the concurrent occurrence of sulfate, Fe(III), and arsenate reduction. The corresponding increase of the relative abundance of OTUs classified as sulfate-reducing bacteria, Desulfomicrobium (from 0.5 to 30.6%), and dsrB gene abundance indicated the strong occurrence of BSR during the incubation. The underlying mechanisms of As mobilization could be attributed to the biotic and abiotic reduction of As-bearing iron (hydro)oxides either through the iron-reducing bacteria or the bacterially generated sulfide, which were supported by the variations in solid speciation of Fe, S, and As. As the incubation progressed, we observed a transient attenuation followed by a re-increase of aqueous As, due to the limited abundance of newly-formed Fe-sulfide minerals with a weak ability of As sequestration. Moreover, the formation of thioarsenate (H₂AsS₄^-) during the mobilization of As from the sediments was observed, highlighting that BSR could facilitate As mobilization through multiple pathways. The present results provided new insights for the biogeochemical processes accounting for As mobilization from sediments under BSR conditions.