[Preparation of Silicon-based Magnetic Biochar and Its Remediation Mechanism for Cd（Ⅱ） and As（Ⅲ） Co-contaminated...

Cadmium （Cd） and arsenic （As） often coexist in water and agricultural soils around mining areas, and it is difficult to remove them at the same time due to their opposite chemical behaviors. Therefore, this study employed a co-precipitation-pyrolysis method to synthesize silica-based magnetic biochar （SMB） materials for the remediation of water contaminated with both Cd and As. The optimization of preparation conditions involved introducing three different types of silicates （Na₂SiO₃, CaSiO₃,and SiO₂） into the biomass-magnetite mixture, followed by pyrolysis at various temperatures （300℃, 500℃, and 700℃）, and the optimal preparation conditions were determined based on the composite batch experiments. Finally, batch experiments in both single-element and composite systems were used to systematically investigate the impact of initial pH, adsorption time, and initial concentration on the adsorption behavior of SMB for Cd（Ⅱ） and As（Ⅲ）. The adsorption mechanism was elucidated by combining it with characterization analyses, such as SEM, FT-IR, and XRD. The composite adsorption test determined that the material obtained from a pyrolyzing biomass-magnetite mixture containing 5% CaSiO₃ at 700℃ exhibited the most effective adsorption, with removal rates reaching 92.04% for Cd（Ⅱ） and 60.59% for As（Ⅲ） in Cd（Ⅱ） and As（Ⅲ） solutions with initial concentrations of 30 and 10 mg·L^-1. Characterization including SEM, FT-IR, XRD, and BET showed that the material had a significant surface area, rich functional groups, and magnetic properties. In the single-element batch experiment, the optimal adsorption pH for Cd（Ⅱ） and As（Ⅲ） was 6, with equilibrium reached at 1 h and 8 h, respectively. The quasi-secondary kinetic model effectively described the adsorption process of Cd（Ⅱ） and As（Ⅲ） by SMB. In the composite system, the optimum adsorption pH was 7. The Freundlich model better fitted the isothermal adsorption processes of Cd（II） and As（III） by the materials. The results of the batch experiments in the composite system revealed that both synergistic and antagonistic effects existed between Cd（Ⅱ） and As（Ⅲ）. Synergistic effects were manifested through the formation of A-type and B-type ternary surface complexes during the adsorption of As（Ⅲ） and Cd（Ⅱ） by SMB. The formation of A-type ternary surface complexes significantly enhanced the material's adsorption capacity for As（Ⅲ）. However, in the coexistence system, the synergistic effect was primarily controlled by electrostatic interaction, co-precipitation, and the formation of B-type ternary surface complexes due to the preference for adsorbing As（Ⅲ）. Antagonistic effects resulted from the competition between the two heavy metal elements for binding sites with hydroxyl groups. In summary, the synthesized SMB material demonstrated effective adsorption of both Cd and As in water, presenting a promising approach for the efficient remediation of water bodies co-contaminated with Cd（Ⅱ） and As（Ⅲ）.