Transformation of Chromium Speciation during High Hexavalent Chromium-Contaminated Soil Remediation by CPS and Biostimulation

To address the secondary pollution problem of chemical reduction and the defects of a long bioremediation period for the soil around a chromium (Cr) salt plant in China, calcium polysulfide (CPS) combined with biostimulation (adding nutrient solution with glucose and urea) was used to reduce and stabilize hexavalent chromium [Cr(VI)] in the soil. The results showed that the remediation effect of adding CPS and nutrient solution was better than that of CPS alone for Cr(VI)-contaminated soil. An amount of 2.15% CPS with the Cr(VI) concentration reduced to approximately 330.0 mg/kg was selected as the optimal reducing agent dosage. Following the addition of 11 g/kg glucose and 13 g/kg urea, the degradation rate of Cr(VI) reached over 92% after 3 days, and the water-soluble fraction decreased by 95.6% after 40 days. The stability of the Cr was significantly enhanced. The correlation and regression analyses of Cr speciation indicated that the water-soluble fraction had significant negative correlations with the iron-manganese (Fe-Mn) oxide-bound fraction and the organic matter-bound fraction. There were significant negative correlations between the exchangeable fraction and the carbonate-bound fraction as well as extremely significant positive correlations between the Fe-Mn oxide-bound fraction and the organic matter-bound fraction. It was speculated that both the Fe-Mn oxide-bound fraction and the organic matter-bound fraction were transformed from the water-soluble fraction, and the exchangeable fraction and the carbonate-bound fraction were mutually transformed. This study enables us to acquire more knowledge about the speciation transformation of Cr in soil and provides an efficient, low-cost, and low-risk technology for the remediation of high Cr(VI)-contaminated soil.