Iron-silicon modified biochar for remediation of cadmium/arsenic co-contaminated paddy fields: Is it possible to kill two birds with one stone?

Biochar-based remediation can reduce Cd pollution through adsorption but may potentially increases As accumulation in rice grains by microbial reduction. Therefore, developing materials that simultaneously immobilize Cd and inhibit As translocation to plants is crucial for co-contamination management. Iron-silicon modified biochar (Fe/Si-BC) was synthesized and evaluated in pot experiments for remediation of Cd/As co-contamination. The results demonstrated that Fe/Si-BC significantly reduced Cd accumulation in rice grains by up to 81.12 %. Mantel test and random forest importance analyses revealed that Fe and dissolved Si introduced by Fe/Si-BC were key factors in reducing Cd bioavailability, highlighting the synergistic roles of Fe and Si in increasing remediation efficiency. Notably, Fe/Si-BC exhibited dual effects on As toxicity in rice. Specifically, in the high-pollution soils, 0.35 % Fe/Si-BC significantly reduced As accumulation in rice grains by 28.21 %. In contrast, the 1.0 % Fe/Si-BC slightly increased As accumulation in rice grains. Unfortunately, Fe/Si-BC increased the risk of As release into soil porewater. Further analysis revealed that Fe/Si-BC promoted the formation of iron plaques, which possessed a pronounced affinity for As, thereby substantially diminishing As uptake by rice roots. Overall, this study highlights the potential of Fe/Si-BC for the remediation of Cd/As co-contaminated paddy soils, while emphasizing the need for application to manage As mobility risks.