Reduction and stabilization of Cr(VI) in groundwater by using polysulfide-modified nanoscale zero-valent iron

Abstract

Hexavalent Chromium Cr(VI) is highly toxic in groundwater as a result of natural and anthropogenic sources. As a widely reported material for Cr(VI) remediation, sulfide-modified nZVI (S-nZVI) can enhance the nanoparticle selectivity and reaction rate, but has deficiencies of instability and low removal efficiency. In this study, the optimal S-nZVI used for Cr(VI) removal from groundwater was selected, and the mechanism and influence of environmental factors had been discussed. Among the selected sulfur sources including Na2S, Na2S2O3, Na2S2O4, and CaSx, the CaSx-nZVI had the highest removal efficiency. A 1.5-fold removal rate than traditional S-nZVI was observed for CaSx-nZVI during the treatment of a highly chromium-containing groundwater (200 mg/L), because of its significant inhibition on Fe0 oxidation due to the stable shell-core structure. With the sulfur-iron ratio increased from 1/20 to 1/2, the removal efficiency displayed a trend of first increase followed by a decrease. This was ascribed to the enhanced oxidation resistance with the increased ratio of S/Fe since FeSn (n > 2) were generated inside the materials. However, the excessive sulfidation of nZVI likely led to redundant FeS deposition on its surface to reduce the removal efficiency. The reaction process of Cr(VI) removal by CaSx-nZVI was composed with chemical reduction and physical adsorption. Combined with XPS and SEM-EDS analysis, the reaction mechanism was first the Cr(VI) reduction by CaSx-nZVI via a surface-covered layer of FeSn and then the Cr(III) adsorption. Hence, Sn2− was critical to promote the reduction of Fe3+ and the generation of FeS and FeS2. According to the Gibbs free energy results, the reaction occurred spontaneously with increasing temperature. The initial pH affected the removal efficiency by influencing the phase state of chromium. Cations promoted the reactivity to Cr(VI) in solution, while the influence of the anions was quite different. Our research demonstrated that CaSx-nZVI was a promising technique for the in-situ remediation of Cr(VI)-contaminated groundwater.