Evaluation of Remediation of Cr(VI)-contaminated Soils by Calcium Polysulfide: Long-term Stabilization and Mechanism Studies

In the remediation of Cr(VI)-contaminated soils, the effectiveness and long-term stability are critical qualities for the selection of a reductant. In current engineering practices, iron-based materials and sulfides are the most prevalent reductants, and calcium polysulfide (CaS4) is considered as the one with the highest effectiveness and strongest long-term stabilization ability. But this opinion is questioned by the high interference ability of CaS4 to soil Cr(VI) analysis. This study provides a pretreatment method to eliminate the interference of residual ferrous and sulfides to soil Cr(VI) analysis. By this pretreatment method and comparing with FeSO4 and Na2S, the mechanisms of the false high effectiveness and strong long-term stabilization ability of CaS4 is revealed. In the remediation process, CaS4 produces much elemental sulfur (S0) which remains in the soils. During the alkaline digestion, the S0 generates polysulfide which reduces the extracted Cr(VI), inducing serious negative analysis bias. When this negative bias is eliminated by pretreatment method, analysis results show that CaS4 exhibits lowest effectiveness. The S0 cannot be leached away from soils and oxidized by oxygen under natural conditions, this makes CaS4 exhibit a persistent interference ability, which is mistaken for a strong long-term stabilization ability.