Mechanistic insights into the long-term stabilization of Cr(VI) by modified nanoscale zero-valent iron in sediments

Industrial emissions have resulted in the excessive accumulation of hexavalent chromium (Cr(VI)) in aquatic sediments, posing severe ecological and health risks. To address this environmental concern, an amino biochar loaded sulfidized nanoscale zero-valent iron (SC) material was applied for the immobilization of Cr(VI) in bottom sediments, with a focus on investigating its effects on Cr(VI) immobilization efficiency, microbial community structure, and long-term stability. After 15 days of incubation, the SC material significantly reduced the leaching toxicity and bioavailability of Cr(VI), achieving a fixation efficiency of 95.89%. Meanwhile, the effective state and bioavailability of total chromium (Cr(T)) decreased by 84.74% and 86.64%, respectively. Specifically, the SC treatment group exhibited a 98% reduction in exchangeable Cr (EX-Cr) and a 373% increase in residual Cr (RS-Cr), which effectively lowered Cr mobility. The addition of SC increased sediment pH, cation exchange capacity (CEC), and organic matter (OM) content, while decreasing the redox potential (Eh). Among these changes, pH and Eh variations were beneficial for Cr fixation. Microbial analysis demonstrated an increase in the relative abundance of Gammaproteobacteria and other bacterial communities following toxicity reduction. In long-term experiments, the material maintained a fixation efficiency exceeding 95%, and the total iron concentration in the overlying water decreased by 54.14% compared to the nanoscale zero-valent iron (NZ) group, achieving the dual objectives of Cr stabilization and iron release control. Mechanistic investigations revealed adsorption-reduction-complexation-coprecipitation synergistically driven by XPS- quantified Cr(VI) reduction to Cr(III) via Fe⁰ oxidation and FTIR/XRD-identified Cr(III)-Fe(III) coprecipitate (FeCr₂O₄) and surface CrOOH complex formation.