Biochar-supported sulfurized nano-zero-valent iron for soil Cr(VI) immobilization: Mitigating migration and elucidating reduction mechanisms

Hexavalent chromium (Cr(VI)), with its high mobility and acute toxicity, seriously threatens both soil integrity and human health. Herein, we propose a material design strategy to enhance Cr(VI) detoxification by loading sulfur-modified nanoscale zero-valent iron onto phosphorus-modified biochar (S-nZVI@PBC), attaining highly efficient remediation of Cr residues in contaminated sites. Characterization results demonstrated that S modification and PBC loading effectively prevent oxidation and aggregation of primitive nZVI, thereby enhancing electron transfer and reactivity of the material. Compared to traditional nZVI, S-nZVI@PBC exhibited a 2.4 times enhancement in Cr(VI) reduction efficiency and 1.12 times improvement in electron utilization efficiency, achieving an optimal adsorption capacity of 352.44 mg/g within 60 min. After 28 d of remediation, soil Cr(VI) content decreased significantly from 2000 to 361.03 mg/kg, while the TCLP leachate concentration of Cr was reduced from 2000 to 178.71 mg/kg, effectively mitigating the biotoxicity of Cr(VI). Furthermore, S-nZVI@PBC exhibited excellent reactivity and mobility, the effect of remediation achieved nearly 100 % Cr(VI) removal in topsoil within 1 d and reduced the deep soil concentrations by 1164.70 mg/kg after 21 d, while effectively transforming exchangeable Cr into immobilized Fe/Mn oxide-bound species. The remediation mechanism of S-nZVI@PBC in Cr(VI)-contaminated soils predominantly decreased Cr bioavailability via adsorption, reduction, and co-precipitation pathways. Collectively, this study presents a highly effective nZVI modification strategy, providing theoretical and technical support for effective site remediation.