Reductive Transformation of Sb(III)-Loaded Schwertmannite by Sulfate-Reducing Bacteria in a Phosphate-Rich Anaerobic Batch Microcosm.

Antimony (Sb)-bearing schwertmannite is a metastable host phase in sulfidic anaerobic environments, yet its fate during sulfate-reducing bacteria (SRB) activity remains poorly understood. This study investigated the reductive transformation of Sb(III)-loaded schwertmannite mediated by an enriched SRB consortium, using four parallel microcosms to distinguish the roles of mineral carrier and Sb exposure mode. SRB metabolism promoted Fe(III) and sulfate reduction, driving schwertmannite transformation into secondary Fe phases (mainly mackinawite, with vivianite formation under phosphate-rich laboratory conditions). In the system containing Sb-loaded schwertmannite, aqueous Sb exhibited a triphasic pattern: initial release, transient stabilization, and secondary mobilization, reflecting the dynamic competition between mineral reductive dissolution and sulfide-mediated re-immobilization. In contrast, where Sb was initially present as dissolved Sb(III), efficient removal (92.8%) was achieved via sulfide precipitation, confirming the high efficiency of biogenic sulfide for directly bioavailable Sb. Solid-phase analyses suggested the formation of stibnite-like products, though this sequestration pathway was insufficient to fully offset late-stage Sb remobilization, likely due to thioantimonite complexation under sulfidic conditions. Microbial community and functional analyses revealed that Sb exposure and mineral interfaces reshaped community composition and sulfate-reduction potential. Collectively, these findings suggested that Sb fate in anaerobic schwertmannite-rich systems was governed by the balance between reductive dissolution, sulfide precipitation, and secondary mobilization, highlighting the potential risk of SRB-mediated Sb release during mineral transformation.