Waste Valorization: Effective Removal of Methylene Blue by Magnetic Composite Biochar Prepared from Red Mud and Medulla Tetrapanacis

Removing hazardous dyes from wastewater is essential for protecting the environment and reducing health risks. In this study, a magnetic composite biochar was successfully prepared via a pyrolysis method using industrial solid waste red mud and agricultural by-product Medulla tetrapanacis as raw materials, and its adsorption performance toward methylene blue (MB) was systematically evaluated. Among the prepared composites, MRMT exhibited superior removal capacity, with a Langmuir maximum adsorption capacity of 285.2 mg/g. The adsorption process followed the pseudo-second-order kinetic model and the Langmuir isotherm, indicating monolayer adsorption. Thermodynamic analysis gave ΔH0 = 19.691 kJ/mol and ΔS0 = 214.335 J/(mol·K), suggesting that the adsorption process was spontaneous and endothermic. Comprehensive characterization by XRD, FTIR, and XPS revealed that MB removal involved electrostatic attraction, π–π stacking, n–π interactions, and hydrogen bonding. MRMT exhibited a significant magnetic response (13.66 emu/g) and maintained an adsorption capacity of over 57% after five cycling experiments, demonstrating good recyclability. Even in the presence of coexisting ions (Pb2+, Cr3+, Ni2+, Cu2+, Zn2+), it still retained a relatively high adsorption efficiency. The research findings indicate that MRMT, prepared from solid waste red mud and the agricultural by-product Tongcao, is an adsorbent that is easily recovered magnetically; both types of raw materials have the potential for low-cost utilization and can provide valuable technical insights for dye wastewater treatment.