Effect of biochar derived from co-pyrolysis of sewage sludge and rice straw on cadmium immobilization in paddy soil

The remediation of cadmium (Cd) contaminated cropland has been related to food safety and public health. While biochar derived from sewage sludge (SS) has been widely used for soil remediation due to its high efficiency of Cd immobilization, it has a low specific surface area and the ecological risk of heavy metals. Co-pyrolysis of straws and SS could resolve these issues. To date, little is known about the effect of biochar from SS/rice straw (RS) on Cd immobilization in soils. Here, we explored the soil remediation efficiency and mechanism of biochar derived from different mixing ratios (1:0, 3:1, 2:1, 1:1, 1:2, 1:3 and 0:1) of RS and SS named as RBC, R3S1, R2S1, R1S1, R1S2, R1S3 and SBC. It was shown that R1S2 amendment had the most efficiency of Cd immobilization among all amendments, which decreased the bioavailable Cd by 85.61% and 66.89% compared with RBC and SBC amendments. Results of biochar after soil remediation revealed that cation-pi interaction, complexation, ion exchange and precipitation were the key mechanisms of Cd immobilization by biochar. All biochar amendments indirectly promoted Cd immobilization through increasing soil pH values, cation exchange capacity (CEC), soil organic carbon (SOC) and available phosphorous (AP). Compared with RBC, R1S2 reduced bioavailable Cd mainly through the increased soil pH, CEC and AP. However, the enhanced efficiency of Cd immobilization in R1S2 amendment than that in SBC amendment because of the more developed pore structure, functional groups and larger specific surface area of R1S2. Overall, our study showed a new type of biochar for the effective remediation of Cd-contaminated soil.