Depth-dependent fate of antimony in sediment is governed by microbially driven iron transformation under redox fluctuation

Antimony (Sb) is a highly ecotoxic metalloid that frequently coprecipitates with hydroxyl ferric sulfate minerals (e.g., schwertmannite) in mining environments. The depth-dependent release patterns and governing mechanisms of Sb in sediments under redox fluctuation are not yet clear, which limits a systematic understanding of its biogeochemical behavior and associated environmental risks. This study constructed sediment column systems with different Sb doping levels, and simulated a cycle of 40-day flooding and 20-day drying treatment. Results showed that Sb migration was controlled by a microbial-mediated Fe-Sb coupling cycle with distinct vertical differentiation. During flooding, strong deep-layer reducing conditions drove reductive dissolution and Sb(III) accumulation. Consequently, dissolved Sb increased with depth, reaching 59.0 μg/L (16.3% Sb(III)) in deep layer (15 cm) in high-Sb treatment (0.10Sb_Sch: 674 mg Sb/kg sediment). This process was dominated by iron-reducing bacteria (Bacteroidales), with specific taxa like Acidimicrobiia facilitating Sb(V) reduction. After drying, surface aerobic conditions triggered the proliferation of iron-oxidizing bacteria (Gallionellaceae), promoting Fe oxidation and Sb re-fixation. In contrast, deep sediments (15 cm) maintained high Sb mobility (18.91 μg/L in 0.10Sb_Sch treatment) due to persistent reducing bacteria (Bacillota, Thermodesolufobacteria). This study revealed the hierarchical regulation of microbial-mediated Fe-Sb coupling mechanism under hydrological fluctuation, deepened the understanding of Sb environmental behavior, and provided theoretical basis for the prevention, control and restoration of Sb pollution risk in mining areas.