Biogeochemical mechanisms of zero-valent iron and biochar for synergistically mitigating antimony uptake in rice

Antimony (Sb) contamination in paddy fields can lead to its accumulation in rice grains, posing a threat to food safety. To address this issue, the combined use of zero-valent iron (ZVI) and biochar (BC) were applied to decrease the uptake of Sb in Sb-polluted soils, and their effects on Sb uptake from soil to rice grains were investigated. Our results showed that the combination treatment of 0.05% ZVI and 0.095% BC resulted in a significant decrease (42.8%) in Sb accumulation in rice grains that was comparably more efficient than that by 0.05% ZVI (decrease of 15.8% Sb accumulation) or 0.095% BC (decrease of 12.7% Sb accumulation) alone, demonstrating the synergistic effect of ZVI and BC on mitigating Sb uptake by rice plants. ZVI presence resulted in the formation of iron oxides in the soil and on root surfaces, and the S^2-/S₂^2- ascent also increased by 58.7% on day 75 compared with that of the control, facilitating the reduction of Sb(V) to less mobile Sb(III), thereby decreasing Sb accumulation in rice plants. BC initially increased the mobility of Sb owing to its alkaline nature, whereas the electron shuttle properties of BC contributed to a decrease in Sb mobility. The abundance of the arsenite-reducing gene arrA ultimately increased by 203.2% on day 120 compared with the initial phase on day 5, and BC caused a remarkable increase in arrA gene abundance. This study revealed the synergistic mechanisms by combining ZVI and BC to mitigate Sb uptake by rice, which may be useful for the sustainable remediation of contaminated rice paddies.