Effects of an additive (hydroxyapatite-biochar-zeolite) on the chemical speciation of Cd and As in paddy soils and their accumulation and translocation in rice plants

A pot experiment was carried out to investigate the remediation of paddy soils contaminated with both Cd and As. The effects of a soil additive (HZB: hydroxyapatite + zeolite + biochar) on the chemical speciation of Cd and As in the soil and on the accumulation and translocation of these two elements in rice plants were studied. The application of HZB decreased the concentration of acid-extractable Cd in the soil by 7.3-32.6% by promoting the transformation of soil Cd from soluble to insoluble species; as a result, the concentration of organically bound Cd in the soil increased by 6.8-49.5%. The application of HZB also promoted the transformation of soil As from soluble to insoluble species, thus increasing the concentration of Ca-bound As by 34.1-93.4% and reducing the concentration of soil-exchangeable As by 12.2-55.1%. However, when the application rate of HZB was greater than 4.0 g kg(-1), the concentration of soil-exchangeable As increased again. It was found that the application of HZB decreased the Cd and As bioaccumulation capacity of rice root, and among various rice organs, rice husk was the highest in Cd transferring capacity, whereas rice root was the highest in As transferring capacity. When the amount of HZB applied was 0.05-0.2%, the Cd and As concentrations in the various parts of the rice plants decreased significantly.