Ex-situ reductive stabilization for Cr(VI)-contaminated alkaline soil: engineering application and performance evaluation

Addressing the issue of medium- and high-concentration Cr(VI) contamination in typical surface-layer alkaline soil with low organic matter, this study systematically screened and optimized the reductive stabilization agents and processes to achieve efficient and long-term stabilization of Cr(VI). The results showed that under alkaline conditions (pH≥8.50), iron-based reducing agents (ferrous sulfate, ferrous chloride) failed to effectively reduce Cr(VI), even at a dosage 20 times the theoretical stoichiometric ratio (nFe(II)∶nCr(VI)=3). In contrast, solid calcium polysulfide (CPS) required only 8 times the theoretical stoichiometric ratio (nCPS∶nCr(VI)=1.5) to reduce the Cr(VI) content to below the screening value for Class 2 construction land (5.7 mg/kg) specified in the Soil Environmental Quality-Risk Control Standard for Soil Contamination of Development Land (GB 36600-2018). This performance was significantly superior to that of its aqueous solution. The optimized remediation conditions were: using CPS as the reductant at a dosage of nCPS∶nCr(VI)=12∶1, controlling soil particle size below 1 cm, maintaining moisture content between 15% and 20%, with no need for pH adjustment. In a field-scale engineering application, CPS was applied at 1.6 times the laboratory-determined optimal dosage under these optimized conditions. Following 150 days of natural curing, soil Cr(VI) content remained consistently below the regulatory limit with no rebound observed. Leachate concentrations of both Cr(VI) and total chromium were below the limits (5 and 15 mg/L, respectively) stipulated by the Identification Standards for Hazardous Waste: Identification for Extraction Toxicity (GB 5085.3-2007). The remediation system pH exhibited an initial increase followed by a decrease before stabilizing, while water-soluble calcium and sulfate levels rose initially and then remained constant. This study demonstrates that CPS alone can effectively remediate medium to high Cr(VI)-contaminated, alkaline, organically deficient surface soil, offering excellent long-term stability and promising potential for engineering application.