Simultaneous removal of Cd(II) and Sb(V) by MnFe₂O₄-biochar composite: Performance and mechanisms

The coexistence of cadmium (Cd) and antimony (Sb) in soils severely threatens environmental safety and human health. While biochar is widely used for soil remediation, its effectiveness in removing multiple metals, especially in the presence of anions, lacks dynamic quantification and mechanistic understanding. This study synthesized a MnFe₂O₄-biochar composite (MF-RBC) using the coprecipitation method, and explored its adsorption performance and mechanisms for coexisting Cd(II) and Sb(V). The maximum adsorption capacities of MF-RBC for Cd(II) and Sb(V) were 11.24 and 57.33 mg g^-1, respectively, higher than those of pure RBC (6.27 mg g^-1 for Cd(II), and 19.43 mg g^-1 for Sb(V)) and MF (9.25 mg g^-1 for Cd(II), and 48.01 mg g^-1 for Sb(V). The enhanced performance is attributed to the large specific surface area of MF-RBC (318.86 m² g^-1), improved dispersion of MF particles by biochar, and the attachment of oxygen-containing groups. Additionally, Cd(II) exhibited a synergistic effect on Sb(V) removal, likely due to reduced negative charge repulsion between MF-RBC and Sb(V), and the formation of MF-Cd(II)-Sb(V) ternary complexes. MF-RBC also decreased the availability of Cd and Sb by transforming them into more stable forms. Microscopic and mechanistic analysis revealed that Cd(II) forms complexes with both the CO/CO groups of biochar and the MnO bonds of MnFe₂O₄, while Sb(V) is primarily complexed with FeO bonds of MnFe₂O₄ in the Cd&Sb coexistence system. These findings facilitate a better understanding of Cd(II) and Sb(V) behavior in natural environments and offer valuable insights for improving soil remediation strategies.