Effectiveness of various chemical leaching systems in the remediation of chromium-contaminated soil and their impact on chromium speciation.

Chemical leaching techniques have been proven effective in removing heavy metal contaminants from soil using various leaching agents. Previous research has shown that both single-agent and composite leaching systems have been applied for the remediation of chromium-contaminated soils, with varying degrees of success depending on soil type and contaminant form. However, the removal rate of total chromium (Cr) and hexavalent chromium (Cr(VI)) often fluctuates based on the chemical composition of the leaching agents, as well as the soil's physicochemical properties, such as pH and Cr speciation stability. Therefore, this study investigates the effectiveness of 20 composite leaching systems, including deionized water, lime water, calcium chloride, sodium carbonate, and sodium phosphate, through soil column leaching tests. The aim was to evaluate their impact on soil pH, total Cr, and Cr(VI) removal, and to examine the transformation of various Cr species during the leaching process. Results reveal that lime water and sodium phosphate were particularly effective in stabilizing Cr(VI) and neutralizing soil pH, while total Cr removal amount ranged from 197.4 mg/kg to 1671.6 mg/kg and Cr(VI) removal amount ranged from 113.2 mg/kg to 316.8 mg/kg. We also find that using 0.2 mol/L citric acid, 0.1 mol/L hydrochloric acid, and 1.2 mL/g lime solution adjusted soil pH to 7.37, with average removal efficiencies of 34.6 %. for total Cr and 72.7 % for Cr(VI). Overall, our results suggest that the combined use of lime water and sodium phosphate is an effective strategy for remediating chromium-contaminated soil, particularly for stabilizing unleached Cr and adjusting soil pH.