Abstract

The synergistic stabilization of cadmium (Cd) and arsenic (As) in co-contaminated soils remains challenging because of their contrasting geochemical behaviors under flooded conditions. This study evaluated three sulfur–iron modified biochars (SIMBs), namely pyrite-modified biochar (PMB), iron-sulfide-based porous biochar (FSB), and a sulfur iron carbon composite (SFC), using flooding incubation and anaerobic microcosm experiments. PMB showed the fastest Cd passivation, lowering porewater Cd to 0.01 mg/L on day 1, while increasing residual Cd by 45.4% and decreasing bioavailable As by 6.0%. FSB exhibited the strongest Cd immobilization, decreasing porewater Cd by 92.2% on day 60, accompanied by an 81.8% increase in residual Cd and a 12.5% decrease in bioavailable As. SFC showed the strongest and most persistent suppression of As release, lowering porewater total As by 72.2% on day 60, while increasing residual Cd by 209% and decreasing bioavailable As by 31.3%. At the microbial level, PMB and SFC reduced arrA abundance by 46% and 50%, respectively, and decreased Geobacteraceae abundance by 15% and 45%, indicating that sulfur–iron modification not only strengthened Fe–S-coupled sequestration pathways but also restrained microbial reductive processes associated with As mobilization. Random forest modeling identified porewater Fe(II) and As(III) as the dominant predictors of Cd and As bioavailability. Sequential extraction and XPS further showed that Cd stabilization was driven mainly by sulfide/mineral precipitation, whereas As sequestration depended on Fe–S coupled transformation, secondary reactive Fe phase formation, and Fe-associated re-sequestration. Overall, sulfur–iron modification shifted biochar from a simple sorbent to a chemical and microbial regulator of soil Fe–S biogeochemistry.