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.锑（Sb）是一种高生态毒性的类金属，在矿山环境中常与羟基铁硫酸盐矿物（如施威特曼石（schwertmannite））共沉淀。氧化还原波动条件下沉积物中锑的深度依赖性释放模式及其调控机制尚不明晰，这限制了对其生物地球化学行为及相关环境风险的系统认知。本研究构建了不同锑掺杂水平的沉积物柱系统，并模拟了40天淹水-20天风干的循环处理过程。结果表明，锑的迁移受微生物介导的铁-锑耦合循环调控，并呈现显著的垂直分异特征。淹水阶段，深层强还原环境驱动了还原性溶解与三价锑（Sb(III)）的积累。因此，溶解态锑的浓度随深度增加而升高，在高锑处理组（0.10Sb_Sch：674 mg Sb/kg沉积物）的深层（15 cm）中可达59.0 μg/L（其中三价锑占比16.3%）。该过程以铁还原菌（拟杆菌目（Bacteroidales））为主导，特定类群如酸微菌纲（Acidimicrobiia）可促进五价锑（Sb(V)）的还原。风干阶段，表层好氧环境触发了铁氧化菌（披毛菌科（Gallionellaceae））的增殖，促进了铁的氧化与锑的再固定。与之相反，深层沉积物（15 cm处）仍维持较高的锑迁移活性（高锑处理组0.10Sb_Sch中为18.91 μg/L），这归因于持久性还原菌（芽孢杆菌门（Bacillota）、热脱硫杆菌目（Thermodesolufobacteria））的作用。本研究揭示了水文波动下微生物介导的铁-锑耦合机制的层级调控规律，深化了对锑环境行为的认知，并为矿山区域锑污染风险的防控与修复提供了理论依据。