Study of ceramsite-supported iron and manganese oxides for enhancing soil immobilization and reducing rice plants uptake of cadmium

The use of metal oxides functionalized clay ceramsite as a soil amendment for immobilizing trace metals in the soil gives some novel insights and has gained extensive attention. The clay ceramsite functionalized with Fe/Mn oxides was controllably synthesized, including clay ceramsite (NCs), iron oxide ceramsite (FeOx-NCs), manganese oxide ceramsite (MnOx-NCs), and iron-manganese oxide ceramsite (FeOx/MnOx-NCs). The underlying mechanisms of these materials on Cd migration and transformation in soil, and bioaccumulation in rice plants were studied. The results revealed that compared to the control (without adding any materials), all the amendments increased soil pH, cation exchange capacity (CEC), and dissolved organic carbon (DOC). DTPA-Cd contents decreased by 7.15%, 4.14%, 34.21%, and 30.45% in the treatments of NCs, FeOx-NCs, MnOx-NCs, and FeOx/MnOx-NCs after 48 days, respectively. Exchangeable Cd fraction was mainly transformed into carbonate-bound and Fe/Mn-bound fractions. The concentrations of DCB-Fe, DCB-Mn, and DCB-Cd on the root surface of rice increased by 2.9%-79.6%, 5.1%-116.9%, and 56.7%-84.4%, respectively. Obviously, the contents of root-Cd, stem-Cd, and leaf-Cd decreased by 26.37%-49.22%, 45.39%-85.73%, and 43.41%-73.26%, respectively. Principal components analysis (PCA) and correlation analysis explained the migration pathways of Cd in the soil-rice-material system, which helped us to further explore the remediation mechanism of soil. All the amendments improved soil physicochemical properties, and decreased the Cd accumulation in rice plants. Meanwhile, the materials structural characterization pointed out that the chemical precipitation and surface complexation of oxygen-containing functional groups were main Cd immobilization mechanisms. Our results provided valuable references for Cd immobilization in the soil by using Fe/Mn oxides functionalized clay ceramsite.