Evaluation of remediation of Cr(Ⅵ)-contaminated soils by calcium polysulfide: Long-term stabilization and mechanism studies

摘要

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 (CaS_4) 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 CaS_4 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 FeSO_4 and Na_2S, the mechanisms of the false high effectiveness and strong long-term stabilization ability of CaS_4 is revealed. In the remediation process, CaS_4 produces much elemental sulfur (S~0) which remains in the soils. During the alkaline digestion, the S~0 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 CaS_4 exhibits lowest effectiveness. The S~0 cannot be leached away from soils and oxidized by oxygen under natural conditions, this makes CaS_4 exhibit a persistent interference ability, which is mistaken for a strong long-term stabilization ability.