Effects of rice straw ash amendment on Cd solubility and distribution in a contaminated paddy soil under submergence

The mobility and bioavailability of cadmium (Cd) in soils are determined by physical and chemical properties and constituents of soils. This study investigated the effect of rice straw ash (RSA) on the Cd distribution in a contaminated rice paddy soil with submerging incubation. RSA was obtained from a simulated open-field burning. The adsorption of Cd by RSA at pH 4 was characterized by L-shaped nonlinear isotherm and was fully reversible, suggesting nonspecific surface adsorption. The adsorption increased with increasing pH due to increased negative charges of RSA and hydrolysis and precipitation of Cd ion. During submerging incubation, soluble Cd concentration in a Cd-contaminated rice paddy soil gradually increased with incubation time. The results of sequential extraction for the soil before and after submerging incubation showed that the portion of Cd associated with Fe- and Mn-oxides decreased and transferred to exchangeable and residual fractions. The presence of RSA in soils suppressed the release of Cd into the soil solutions. The results suggest that the occurrence of RSA in rice paddy soil can retard the movement of Cd in soil and thus lower the potential environmental risk of Cd in soil. © 2014, The International Society of Paddy and Water Environment Engineering and Springer Japan.