Microwave-enhanced reductive immobilization of high concentrations of chromium in a field soil using iron polysulfide

High concentrations of Cr(VI) are often detected in contaminated soil. Yet, cost-effective remediation technologies have been lacking. In this study, we prepared a type of FeS_x based on commercial FeSO₄.7H₂O and CaS_x and tested a microwave-assisted technology based on FeS_x for reductive immobilization of high concentrations of Cr(VI) in a field contaminated soil. The as-prepared FeS_x particles appeared as a honeycomb-like and highly porous structure. The microwave-assisted FeS_x reduction process was able to rapidly reduce the TCLP-based reachability of Cr(VI) from 391.8 to 2.6 mg·L^-1. The dosage of FeS_x, S/Fe molar ratio, initial moisture content, microwave power, and irradiation time can all affect the treatment effectiveness. After 500 days curing under atmospheric conditions, the TCLP-leached concentration of Cr remained below the regulatory limit of 5 mg·L^-1, while other treatments failed to meet the goal. S_x^2- or S^2- served as the primary electron donors, whereas Fe facilitated the microwave absorption and the formation of the stable final product of FeCr₂O₄. S and Fe are mostly precipitated in soil. The microwave-assisted FeS_x reduction was shown to be an effective approach to rapidly reduce the leachability of Cr(VI) in contaminated soil, especially in heavily contaminated soil.