Advanced Risk-Alleviation-Technologies for Rice Paddy Fields Contaminated with Metals and Radioactive Material = Advanced Risk-Alleviation-Technologies for Rice Paddy Fields Contaminated with Metals and Radioactive Material

다국어 초록 (Multilingual Abstract)

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 chloClride (FeCl3) as a promising chemical for on-site soil washing. The comparison of FeCl3 extraction ability to that of various iron, manganese, and zinc salts revealed the primary extraction mechanism of FeCl3 to be proton release coupled with hydroxide generation (hydrolysis). This indicates that proton release from FeCl3 is controlled by the chemical equilibrium of hydroxide formation, and minimizes the negative effect on soil properties and environment, which are different from hydrochloric acid (HCl). Washing with FeCl3 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 FeCl3. The method was comprised of 1) chemically washing the field soil with a FeCl3 solution; 2) washing the treated soil with water to eliminate residual Cd and Cl ; 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-1). The on-site soil washing confirmed the effectiveness of FeCl3 for decreasing Cd in soil and rice grains without negative effect on rice yield. After the radioactive leakage from the Tokyo Electric Power Company’s Fukushima Dai-ichi nuclear power plant accident, widespread contaminated areas appeared where radioactive cesium in soil exceeds the provisional standard in Japan (5000 Bq/kg), and recovery from the contamination is an urgent issue. As radioactive cesium in undisturbed fields is initially accumulated in a very thin surface layer (roughly 0-2 cm) in soil, removal of surface thin soil is effective in eliminating radioactive cesium. The removal of surface thin soil, however, could not apply for cultivated farmland. Thus, we have developed a new, cleaning method toremediate Cs-contaminated paddy fields by on-site stirring and cleaning (drainage of dispersed soil micro particle) of the surface layer of soil using water. The removal rate of Cs-137 from soil was 61.7% and can extensively decrease the amount of removed soil compared to the method of removal of contaminated surface soil. Cs-137 concentration in brown rice cultivated in the decontaminated paddy field was reduced by 60%.