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-<i>Lactobacillus plantarum</i> composite biochar (RM)-by immobilizing red mud and <i>Lactobacillus plantarum</i> 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.