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

Abstract

The ability of FeCl 3 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 FeCl 3) 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 FeCl 3 is proton release coupled with hydroxide generation, as iron hydroxides are insoluble. Washing with FeCl 3 led to the formation of Cd–chloride complexes, enhancing Cd extraction from the soils. FeCl 3 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 FeCl 3 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 FeCl 3 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 FeCl 3, adverse effects were not crucial and could be corrected. The bench-scale test confirmed the effectiveness of FeCl 3 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.