Study on the adsorption of methylene blue solution by activated clay biochar composites

This study fabricated an activated bentonite–rapeseed straw biochar composite (AC-RSB120-4:3) via hydrothermal carbonization, followed by FeCl₃ impregnation and NaOH treatment to enhance methylene blue adsorption. Characterization by BET analysis, XRD, SEM, and FTIR revealed a porous structure and abundant functional groups. The composite showed superior adsorption capacity (425.65 mg/g at neutral pH) compared to pristine biochar. The optimal adsorption performance was achieved when the solution pH was around 10. Kinetics obeyed the pseudo-second-order model (chemisorption control), yet thermodynamics (Δ<i>H</i> = −15.856 kJ/mol) simultaneously indicate prominent physisorption. Multilayer adsorption (Freundlich isotherm) arises from combined pore filling, electrostatic attraction, ion exchange and surface complexation, with physical interactions dominating the initial stage and chemical bonds progressively taking over. Overall, the composite exhibits high efficiency and stability for dye wastewater treatment.