Restoration of cadmium-contaminated paddy soils by washing with ferric chloride: Cd extraction mechanism and bench-scale verification

The ability of FeCl₃ to extract Cd from three paddy soils was compared with that of various irons, manganese, and zinc salts to elucidate the extraction mechanism. Manganese, zinc and iron salts (including FeCl₃) extracted 4-41%, 8-44% and 24-66% of total Cd, respectively. This difference reflected the pH of the extraction solution, indicating that the primary mechanism of Cd extraction by FeCl₃ is proton release coupled with hydroxide generation, as iron hydroxides are insoluble. Washing with FeCl₃ led to the formation of Cd-chloride complexes, enhancing Cd extraction from the soils. FeCl₃ effectively extracted Cd from all of the three soils compared to HCl that is a conventional washing chemical, when the concentrations of the two washing chemicals were between 15 and 60 mM_c (at above extraction pH 2.4), while the corresponding extraction pH of FeCl₃ was slightly higher than HCl. As HCl is the strong acid of complete dissociation, if excess amount of HCl was added to soil, it is possible to give the dissolution of clay minerals in soils. In contrast, proton release from FeCl₃ is controlled by the chemical equilibrium of hydroxide formation. While soil fertility properties were affected by a bench-scale soil washing with 45 mM_c FeCl₃, adverse effects were not crucial and could be corrected. The bench-scale test confirmed the effectiveness of FeCl₃ for removal of soil Cd. The washing had no negative effect on rice yield and lowered the Cd concentration of rice grain and rice straw in a pot experiment.