Insights into Remediation of Cd Contamination in Vegetable Soils by Red Mud-Modified Biochar

Recently, heavymetal contamination in soil has been widely concerned. Herein, three types of biochar-based materials, including biochar (BC), Fe-modified biochar (Fe-BC), and red mud-modified biochar (RM-BC), were prepared and characterized, and the effects of BC, Fe-BC, and RM-BC on the bioavailability of Cd in vegetable soils were studied by pot experiments. Results showed that the RM-BC treatment exhibited better immobilization effect on soil Cd than BC and Fe-BC, and reduced the bioavailability of soil Cd and effectively inhibited the migration of Cd from soil to the edible parts of onion plants. After RM-BC treatment, the acid-soluble state of soil Cd decreased by 23%, while the residual state of soil Cd increased by 21%, promoting the form transformation of soil Cd from the acid-soluble state to the residual state. Furthermore, the RM-BC treatment raised the soil pH, soil enzyme (β-glucosidase, alkaline phosphatase, and urease) activities, soil organic carbon, and the biomass of onion plants. The immobilization mechanism of soil Cd following RM-BC treatment was explored, involving in surface adsorption, ion exchange, and complexation reactions. Importantly, the Cd concentration of onion stem is about 0.13 mg/kg following RM-BC treatment, which did not exceed China's food safety standard of 0.20 mg/kg. Therefore, the RM-BC treatment shows potential for the remediation of Cd-contaminated soils.