ZnCl2-activated biochar derived from Zanthoxylum bungeanum for efficient adsorption of anionic and cationic dye pollutants

This study explored the production of biochar from zanthoxylum bungeanum branches using a low-oxygen heating technique and identified 550 °C as the optimal pyrolysis temperature. The biochar was modified with zinc chloride. FTIR, SEM, EDS, and XPS analyses collectively confirmed successful Zn²⁺ doping, enrichment of surface-active functional groups, and a 2629% increase in specific surface area relative to the pristine biochar. The highest removal efficiencies for Acid Red 73 and Methylene Blue dyes in aqueous solution using the modified biochar (ZBC-550) were 99.25% and 99.04%, and the adsorption capacity is 115.18 mg/g and 108.94 mg/g respectively. The adsorption mechanisms involved in the process included hydrogen bonding, electrostatic attraction, ion exchange, complexation reactions, and π-π interactions, consistent with Langmuir and Temkin isotherms. Kinetic assessments indicated that Methylene Blue adsorption was predominantly chemical, while Acid Red 73 adsorption was primarily physical. Density functional theory (DFT) calculations further supported these findings, demonstrating adsorption energies of -0.9599 eV for Acid Red 73 and -2.0386 eV for Methylene Blue.