Adsorption Characterization and Mechanism of a Red Mud–Lactobacillus plantarum Composite Biochar for Cd2+ and Pb2+ Removal

Pb2+ and Cd2+ represent common heavy metal contaminants in aquatic environments, posing significant risks to ecosystem stability and human health. To develop efficient adsorbents for removing Cd2+ and Pb2+ while achieving resource utilization of industrial by-products (red mud and distiller’s grains), this study synthesized a novel composite biochar—red mud–Lactobacillus plantarum composite biochar (RM)—by immobilizing red mud and Lactobacillus plantarum onto biochar derived from distiller’s grains. The structural and chemical properties of RM were characterized using SEM-EDS, XRD, and FTIR. Batch adsorption experiments were conducted to evaluate the effects of various experimental factors on Cd2+ and Pb2+ adsorption. The adsorption process was further elucidated through kinetic and isothermal models, revealing that it follows the pseudo-second-order kinetic model. Equilibrium data were best described by the Langmuir model for Cd2+ and the Freundlich model for Pb2+. The maximum adsorption capacities reached 12.13 mg/g for Cd2+ and 130.10 mg/g for Pb2+. The primary mechanisms involved in Cd2+ and Pb2+ adsorption by RM include surface complexation, cation–π interactions, ion exchange, and coprecipitation. These findings demonstrate that RM represents a promising and effective adsorbent for the remediation of heavy metal-contaminated water.