Iron-Sulfur reduction coupling Controls Cadmium and arsenic dynamics in paddy Soil-Rice Systems: Insights from biogeochemical cycling

• Fe and S passivation decreased bioavailable Cd and As in soils. • S-mediated dimethylarsenate dominated total As increases in grains. • Fe passivation simultaneously decreased rice grain Cd and As. • Fe-S coupling remediation increased the accumulation of total As in grains. • Fe-S-coupled remediation was more effective for grain Cd than Fe or S alone. It is widely recognized that iron-sulfur (Fe-S) reduction is a dominant biogeochemical process in paddy soils. Individually, Fe or S remediation techniques have been proven effective in reducing the accumulation of Cd or As in rice grains. However, the combined effects of Fe-S remediation technology on Cd/As co-contaminated paddy soil remain unclear. This study investigated the dynamic effects of Fe and S reduction on Cd and As in soil-rice systems by adding typical iron oxides (ferrihydrite) and sulfates (potassium sulfate) in pot experiments. Unexpectedly, the Fe-S reduction technique had a contrary influence on the migration behavior of Cd and As. Fe-S coupling reduction resulted in synergistic remediation, with a greater effect (61.4%) than that of either of the individual methods (46.3%, 43.2%). This was attributed to the synergistic effect of Fe-S passivators, with Fe-S-coupled reduction stabilizing the structures of amorphous iron oxides and facilitating the formation of FeS, thereby enhancing Cd immobilization. Interestingly, the biogeochemical effects of sulfur-induced arsenic reduction exhibited a double-edged effect, both decreasing total soluble As through sulfide formation and simultaneously facilitating the biotransformation and mobilization of methylarsenic species in pore water, ultimately resulting in increased dimethylarsinic acid (DMA) accumulation in rice grains. While Fe reduction remediation alone decreased As accumulation in the rice grains by 13.6%, sulfate addition resulted in a 120% increase in DMA facilitated by sulfate reduction. Given rice’s tendency to hyperaccumulate methylated substances compared to inorganic As, this led to a 41.3% increase in the total As content in the grains. Additionally, although Fe-S coupling remediation technology synergistically increased iron plaque formation on the root surface (47.8%) and enhanced the blocking effect of this iron plaque on Cd/As (57.9%/33.7%), DMA produced by sulfate-mediated methylation appeared to suppress the retention capability of this plaque. Overall, the findings emphasize that Fe-S biogeochemistry and cycling have inconsistent effects on the migration of Cd/As, suggesting that selective Fe and S remediation strategies should be applied accordingly in situations involving heavy metal pollution.