[Remediation of Heavy-Metal-Contaminated Soil by EGTA Washing Enhanced with Reduction Solubilization]

This study used a reducing agent to regulate the activity of heavy metals in soil, with the aim of enhancing the washing efficiency of heavy metals in soil by ethylene glycol bis (2-aminoethyl ether) tetraacetic acid (EGTA). The effects of EGTA dosage, reductant type, liquid-solid ratio, and leaching time on washing efficiency were studied. The heavy metal concentrations in the washing solution and the chemical specification under different washing conditions were analyzed. Scanning electron microscopy/energy-dispersive X-ray spectroscopy and Fourier-transform infrared spectroscopy were used to characterize the soil samples under different treatments. The results showed that the washing efficiencies of Cu, Zn, Pb, and Cd were 85.0%, 60.7%, 88.6%, and 66.4%, respectively, when the liquid-solid ratio was 7.5:1, the leaching time was 2 h, and the dosage of EGTA and sodium thiosulfate (Na₂S₂O₃) was 5 g·L^-1. Compared with single EGTA treatment, the leaching concentration of the four heavy metals all decreased, with the Pb decreasing most significantly. The soil particles became smaller, the aggregate structure was destroyed, the O and Al content decreased, the Si content increased, and therefore the adsorption capacity of soil mineral components was supposed to decrease. The soil organic matter content decreased significantly, and the -OH vibration was significantly weakened, indicating that the organic functional groups with stronger coordination ability were reduced, and the adsorption of organic components in the soil was reduced. The decrease in content of strongly adsorbed components in soil is an important mechanism for Na₂S₂O₃ to enhance heavy metal elution by EGTA.