Chemical remediation of cadmium-contaminated paddy soils by washing with ferric chloride: Cd extraction mechanism and on-site verification

摘要

Codex Alimentarius Commission (Codex) has adopted the maximum permissible concentration of cadmium (Cd) in polished rice and other relevant crops, which requires an alleviation of the Cd contamination in rice grain. Various chemicals were tested for their Cd extraction efficiency by using three paddy soils, selecting ferric chloride (FeCl_3) as a promising chemical for on-site soil washing. The comparison of FeCl_3 extraction ability to that of various iron, manganese, and zinc salts revealed the primary extraction mechanism of FeCl_3 to be proton release coupled with hydroxide generation (hydrolysis). This indicates that proton release from FeCl_3 is controlled by the chemical equilibrium of hydroxide formation, and minimizes the negative effect orsoil properties and environment, which are different from hydrochloric acid (HC1). Washing with FeCl_3 led to the formation of Cd--chloride complexes, enhancing Cd extraction from the soils. We also developed in situ three-step washing method for Cd-contaminated paddy fields with FeCl_3. The method comprised 1) chemically washing the field soil with a FeCl3 solution; 2) washing the treated soil with water to eliminate residual Cd and FeCl_3; and 3) on-site treatment of wastewater using a portable wastewater treatment system. Concentrations of Cd in the treated water were below Japan's environmental quality standard (0.01 mg/L). The on-site soil washing confirmed the effectiveness of FeCl_3for decreasing Cd in soil and rice grains without negative effect on rice yield.