Reshaping geochemical fate of gold-mining-derived arsenic in Jiehe River sediments from molecular to field scales via an innovative iron-based mineralization strategy.

Arsenic (As) contamination in riverine sediments within global gold mining regions poses persistent ecological risks due to its easy release into the overlying water especially under anaerobic conditions. To ensure long-term stability of As in the sediment, we developed a composite material consisting of ferrous iron, schwertmannite, and calcium carbonate (FSC) for synergistic As sequestration. It was found that FSC exhibited high adsorption capacities of 107.3 mg/g for As(III) and 116.6 mg/g for As(V). Applying FSC at 3% in the contaminated sediment with total As of 317-4200 mg/kg drastically reduced acetic acid extractable As (aeAs) by 75.2% - 84.7% and maintained long-term structural stability even under flooded conditions since As partitioned toward residual and crystalline Fe-oxide fractions. Synchrotron XAFS reveals two distinct valence-dependent As immobilization pathways. As(V) forms rigid bidentate-edge-sharing (2E) complexes with Fe and exhibits As-Ca coordination as johnbaumite precursors. In contrast, As(III) undergoes Fe(II)-mediated electron transfer and is ultimately sequestered into goethite templates. Corroborating these atomic insights, dynamic flume-scale simulation trial over 720 days confirmed the robustness of the surface layer of the sediment amended with FSC, through which a 96.2% decrease of As concentrations in overlying water was observed. Furthermore, in-situ real application of FSC in the remediation engineering project of the Jiehe River ultimately validated the superior comprehensive performance of this technology with comparison to already-reported materials. This strategy of altering As geochemical fate via anthropogenically regulated iron (oxyhydr)oxide formation offers a theoretical foundation and operable template for the global remediation of metalloid-contaminated sediments.