Remediation of Cu-, Zn-, and Pb-Contaminated Soil Using Different Soil Washing Agents: Removal Efficiencies and Mechanisms

The influence of different pH conditions and simulated inter-root environmental solutions on the release behavior of heavy metals (Cu, Pb, and Zn) from contaminated soils was investigated. The results indicate that heavy metal leaching from the soil is more likely to occur under acidic conditions and there is a risk of sustained presence of heavy metals in the inter-root solution. To address this issue, soil washing experiments were conducted, showing that the overall heavy metals removal efficiency by ethylenediaminetetraacetic acid disodium salt (EDTA) is higher than other agents such as citric acid, malic acid, HCl, and HNO3. Under the optimal conditions, the removal rates of Cu, Zn, and Pb from the soil are 43.9%,73.1%, and 41.5%, respectively. Single-factor experimental studies show a strong positive correlation between leaching concentration and temperature. Thermodynamic calculations suggest the feasibility of EDTA as a soil washing agent for remediation, as the process is found to be spontaneous, endothermic, and entropically favorable. The combined results of X-ray photoelectron spectroscopy (XPS) analysis and density functional theory (DFT) calculations suggest that EDTA is particularly effective in removing steady-state Pb, while citric acid has an insignificant effect on Pb removal. EDTA has a stronger ability to remove lead than citric acid attributed to the higher complexing strength of EDTA with Pb and the better water solubility of EDTA metal complexes. In addition, the removal efficiency of Zn and Cu with EDTA is significantly (p < 0.05) greater compared to that with citric acid.