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

Biochar remediation reduces Cd pollution via adsorption but potentially increases As accumulation in rice grains by microbial reduction. Thus, developing materials that simultaneously adsorb Cd and inhibit As translocation to plants is crucial. 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 the Fe/Si-BC significantly reduced Cd accumulation in rice grains by up to 81.12%. Mantel test analysis and random forest importance analysis revealed that the Fe and dissolved Si introduced by the 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 the rice. Specifically, in the high-pollution soils, 0.35% Fe/Si-BC significantly reduced As accumulation in the rice grains by 28.21%. In contrast, the 1.0% Fe/Si-BC slightly increased As accumulation in the rice grains. Unfortunately, Fe/Si-BC increased the risk of As release into porewater. Further analysis revealed that the Fe/Si-BC promoted the formation of iron plaques, which possessed a pronounced affinity for As, thereby substantially diminishing As uptake by the rice roots. Overall, this study highlights the potential of Fe/Si-BC for the remediation of Cd/As co-contaminated soils, while emphasizing the need for application to manage As mobility risks.