Interaction of chromium-sulfur-iron during Cr(VI) stabilization by polysulfide-modified nanoscale zero-valent iron for groundwater remediation: Batch experiments and numerical simulation

Sulfidated nano zero-valent iron (nZVI) is a vital technology for stabilizing hexavalent chromium (Cr(VI)) in groundwater. However, it faces challenges such as oxidation susceptibility and unclear reaction mechanisms. In this study, a novel CaSx-modified nZVI material (PS-nZVI) was synthesized through an improved preparation method by using a two-step method to investigate its interactions with chromium, sulfur and iron for Cr(VI) immobilization. Results showed that the addition of PS significantly altered the microstructure and properties of the material. As the PS content increased, the material transformed from nZVI to a flower-like FeSn structure, enhancing its reduction capability and protecting the internal nZVI. PS-nZVI exhibited higher Cr(VI) removal capacity compared to nZVI, with the best efficiency at a PS/Fe molar ratio of 1/10. The removal process followed a pseudo-second-order kinetic model, indicating that chemical adsorption dominated. Initially, PS-nZVI focused on reducing Cr(VI), differentiating it from other sulfide-modified nZVI. The formation of FeSn on the surface of material facilitated the reduction of Fe(III) and Cr(VI) and acted as an electron transfer intermediate, enabling efficient Cr(VI) stabilization. Using a reactive transport model, it can be found that iron contributed 57.4% to Cr(VI) solidification as chromite, while sulfur and iron contributed 95.9% and 4.1%, respectively, to Cr(VI) reduction to Cr(III).