Combined use of lime, bentonite, and biochar for immobilization of Cd and mobilization of Se in paddy soil

Remediation of soil contaminated with cadmium (Cd) that can produce rice enriched with selenium (Se) is highly significant for improving the public health in China. A key issue needing resolution in this regard is the simultaneous immobilization of Cd and mobilization of Se. To explore a potential promising method to remediate median-high Se soil that is contaminated by Cd, a potted experiment was conducted, and seven combined amendments (0.03–0.12% lime, 0.03–0.18% bentonite, and 0.3–1.2% biochar of the dry soil weight) were used to immobilize Cd in three paddy soils, in which the concentrations of Cd and Se are 0.46 and 0.45 mg/kg, 1.12 and 0.33 mg/kg, and 2.96 and 0.31 mg/kg, respectively. The soil pH increased by 1.5–2 units after the application of the amendments, and the soil organic carbon (SOC) concentration increased notably with the addition of large quantities of biochar. As the pH and SOC concentration increased, the concentrations of the available Cd in the soil decreased by 35–50%, and the FTIR spectrum showed that O-containing groups and Si–O facilitated the Cd immobilization. The concentration of Cd in brown rice decreased with a decrease in the available Cd. There was no apparent correlation between the Se concentration in the brown rice and the concentration of the available Se in the soil, although the available Se increased by 40–80% after the application of amendments. The accumulation of Se in rice grains was regulated by interactions among the lime, bentonite, biochar, and the soil. An SEM–EDS analysis showed that the biochar particles were covered with bentonite and other soil minerals that could postpone biochar aging and contribute to the longevity of the combined amendments in the soil.