A Recyclable Magnetic Biochar from Corn Cobs and Red Mud for Treating Complex Contaminants Containing Dyes and Heavy Metals

<title>Abstract</title> The treatment of wastewater containing coexisting organic dyes and heavy metals remains challenging due to the competitive adsorption effect, which typically leads to a significant reduction in the removal efficiency of target pollutants, especially heavy metal ions. In this study, a magnetic multifunctional biochar (MMBC-400) was successfully synthesized via the co-pyrolysis of corn cobs and red mud at 400 °C for 2 hours, and it was used for the simultaneous removal of malachite green (MG) and Pb²⁺. The adsorption performance of MMBC-400 for MG and Pb²⁺ in both single and binary systems was systematically investigated through experiments involving different initial conditions, kinetics, isotherms, and cyclic regeneration. MMBC-400 exhibited a high adsorption capacity of 794.72 mg/g for MG, with a removal efficiency of 99.33%. More importantly, in the binary system with a high MG concentration of 500 mg/L, MMBC 400 still maintained a considerable adsorption capacity for Pb²⁺ (129 mg/g) and a removal efficiency of 96.82%, demonstrating strong anti-interference capability. Characterization and model analysis revealed that the adsorption mechanisms of the two pollutants included pore filling, complexation, and ion exchange. Notably, the two pollutants were preferentially adsorbed onto different sites: MG was adsorbed on the carbon matrix of the biochar through π-π interactions, while Pb²⁺ was immobilized on the red mud components via surface complexation. This selective adsorption behavior resulted in limited competitive adsorption between the two pollutants. Furthermore, MMBC-400 showed excellent recyclability. After five consecutive cyclic uses, the removal efficiency of MMBC-400 for both pollutants remained above 85%. Although the single adsorption capacity of MMBC-400 for Pb²⁺ was moderate, it has been proven to be an efficient and practical adsorbent for the treatment of complex polluted wastewater, attributed to its synergistic removal capability, weak competitive effect, and good recyclability. This study provides a valuable strategy for the design of specific adsorbents for complex wastewater systems.