Phase transformation of antimony-bearing Schwertmannite and redistribution mechanisms of antimony mediated by sulfate-reducing bacteria

Schwertmannite （Sch）， a typical sulfate mineral commonly found in mine drainage environment， plays a crucial role in the geochemical cycling of heavy metals. Due to its unique surface complexation sites and isomorphic substitution properties， Sch exhibits a strong affinity for immobilizing antimony （Sb）， thereby influencing Sb mobility and bioavailability in mining-impacted environment. Sulfate-reducing bacteria （SRB） play a key role in the biogeochemical transformation of sulfur and metals through sulfate reduction， a process that induces mineral phase transformations and regulates the migration and redistribution of heavy metals. However， the specific interfacial reaction mechanisms between SRB and Sb-bearing Sch （Sb-Sch） remain poorly understood， particularly under varying pH conditions. In this study， a controlled reaction system involving SRB and Sb-Sch was established to systematically investigate the phase transformation of Sb-Sch and the associated Sb redistribution mechanisms under different initial pH conditions. By integrating advanced mineralogical （such as XRD， SEM-EDS， and FTIR） and microbiology （16S rRNA） characterization techniques， this study provides a comprehensive insight into the pathways governing Sb-Sch transformation in biologically active environments. The major findings of this research were as follows： 1） SRB-driven dissolution and recrystallization processes： the coupled effects of proton secretion and sulfate reduction significantly enhanced the dissolution of Sb-Sch and the subsequent precipitation of secondary mineral phases. The primary secondary mineral formed was mackinawite， a common product of biogenic iron-sulfide precipitation. Additionally， distinct Sb-bearing mineral phases were formed under different pH conditions： in acidic environments， berthierite was the predominant Sb-bearing phase； in neutral conditions， stibnite was observed； whereas under alkaline conditions， kermesite was identified as the primary Sb mineral phase. 2） Sb speciation transformations and immobilization mechanisms： during Sb-Sch dissolution， the released Sb（V） was subjected to electron transfer processes at the microbe-mineral interface， leading to its reduction to Sb（Ⅲ）. The reduced Sb（Ⅲ） exhibited dual immobilization pathways： a portion was sequestered through surface adsorption onto newly formed iron sulfides and iron oxides， while another fraction complexed with sulfide （S²⁻）/ ferrous iron （Fe²⁺）， forming stable Fe/S-Sb minerals. 3） microbial community dynamics and functional contributions： microbial community analysis revealed significant variations in SRB composition and activity under different pH conditions， highlighting their functional roles in Sb-Sch transformation. In acidic and neutral environments， Desulfosporosinus was the dominant SRB genus， actively mediating sulfur， iron， and antimony redox transformations through enzymatic electron transfer pathways. Conversely， in alkaline conditions， Anaerostignum emerged as the most dominant genus， playing a pivotal role in Sb（Ⅲ） biomineralization processes. This pH-dependent shift in microbial dominance underscores the relationship between microbial metabolic activity， mineral phase transformations， and heavy metal sequestration in biogeochemical systems. These findings clarify microbe-mineral controls on Sb stability， guiding bioremediation to reduce contamination and provide theoretical and technical support for mining ecological restoration.