Effects of two combined amendments on heavy metal bioaccumulation in paddy soil

Stabilization of heavy metals in situ was investigated. Two combined amendments (LS, limestone + sepiolite; HZ, hydroxyhistidine + zeolite) were applied to paddy soil with multi-metal (Pb, Cd, Cu, and Zn) contamination at ratios of 0.2%, 0.4%, and 0.8% (w/w). The effects of these two combined amendments on heavy metal bioavailability in soil, and on uptake and accumulation of heavy metals in rice plants were investigated. The experiment results indicated that application of LS and HZ significantly increased soil pH values and cation exchange capacity contents, while the organic matter contents did not change obviously. Higher soil pH values and CEC contents were observed with LS treatment soils than HZ. Application LS and HZ resulted in a reduction in toxicity characteristic leaching procedure (TCLP) extract metal contents of amended soils. Compared with the control soil, contents of Pb, Cd, Cu, and Zn in TCLP extracts were decreased by 25.7%~52.2%, 12.7%~25.7%, 6.4%~17.2%, and 8.6%~23.4%, respectively, as a result of 0.2% to 0.8% addition of LS and decreased by 57.6%~80.1%, 7.0%~40.9%, 2.3%~22.7%, and 4.5%~33.2%, respectively, as a result of 0.2% to 0.8% addition of HZ. LS and HZ significantly decreased the bioavailability of Pb, Cd, Cu, and Zn in the paddy soils, and inhibited the bioaccumulation of heavy metals in rice organs, resulting in decreasing the concentrations of Pb, Cd, and Cu of rice roots, shoots, husks, and brown rice. Significant positive linear correlations were found between heavy metal concentrations in TCLP extracts and those in rice tissues (roots and brown rice) except for Zn. Extracting heavy metals with TCLP was a suitable method for estimating heavy metal bioavailability concentrations of amended soil.