Investigation into Adsorption Performance and Mechanism for Copper on Red Mud Modified Biochar

Peanut shells from Shanxi Province were used as the biomass feedstock to prepare three red-mud-modified biochars at red mud-to-peanut-shell mass ratios of 1:1 (RMBC1), 1:5 (RMBC2), and 1:10 (RMBC3) through suspension loading followed by pyrolysis. Their adsorption behavior and mechanisms for removing Cu2+ from aqueous solution were systematically investigated. The optimal initial pH for Cu2+ adsorption was 6. The adsorption capacity increased with the initial Cu2+ concentration, and adsorption approached equilibrium within 240 min. Adsorption performance improved with increasing red-mud content. RMBC1 exhibited the highest experimental adsorption capacity (36.23 mg/g), which was 1.67 times that of pristine biochar. The equilibrium data were described more closely by the Langmuir model than by the Freundlich model, whereas the kinetic data were described most closely by the pseudo-second-order model. SEM, XRD, and FTIR analyses indicated that Cu2+ immobilization was governed primarily by surface complexation and may also involve interface-mediated mineral precipitation, together with ion exchange and other mineral-associated reactions.