Effects of freeze-thaw leaching on physicochemical properties and cadmium transformation in cadmium contaminated soil

This study aims to investigate the effect of the combined method of freeze-thaw and leaching on the removal of cadmium (Cd) in soil and to provide a theoretical basis for the remediation of farmland soil polluted by heavy metals. The removal process and mechanism of Cd were deduced through oscillatory leaching experiments and freeze-thaw leaching simulation experiments, and the influence of the freeze-thaw leaching technology on the soil environment was evaluated. The results of oscillatory leaching showed that a mixture consisting of 0.80 mol/L citric acid and 0.80 mol/L ferric chloride in a 1:19 vol ratio effectively remove 47.75 % of Cd, indicating that the composite leaching agent could effectively remove Cd from the soil. The results of the freeze-thaw leaching simulation experiment showed that although the freeze-thaw leaching treatment increased the total Cd content in the 0-5 cm soil layer, the total Cd content in the 5-10 cm, 10-15 cm, and 15-20 cm soil layers decreased by 5.08 %, 2.39 %, and 5.68 %, respectively. The freeze-thaw leaching increased the content of exchangeable Cd (p<0.05), carbonate bound Cd, but decreased organic bound Cd and residual Cd (p<0.05), thereby increasing the bioavailability of Cd. Freeze-thaw leaching not only increased the competitive adsorption of Cd²⁺ by decreasing soil pH, cation exchange capacity, and increasing the content of exchangeable calcium and exchangeable magnesium, thus reducing the adsorption of Cd in soil. And the results of XPS and FTIR similarly showed that the freeze-thaw leaching could promote the chelation between Cd²⁺ and hydroxyl, carboxyl and carbonyl functional groups. Although the freeze-thaw leaching destroyed the large particle structure (0.05-2 mm) and large pores in the soil, and increased the clay content (<0.002 mm) and the proportion of small pores in the soil, the XRD results showed that freeze-thaw leaching had no significant effect on the minerals in the soil. In summary, this study shows that freeze-thaw leaching has a significant effect on the removal of soil heavy metals, suggesting that the synergistic effect of freeze-thaw and leaching should be considered in the process of removing soil pollutants in seasonal freeze-thaw zones, and that this method provides a new insight into the remediation of contaminated soils.