Copper adsorption by Acacia auriculiformis biochar evaluated through batch, column, kinetic, thermodynamic, and regeneration studies

This study evaluates copper(II) ion removal using biochar synthesized from Acacia auriculiformis wood waste, an unmodified and locally abundant biomass. Batch, column, kinetic, thermodynamic, and regeneration studies were integrated to provide a systematic assessment of adsorption behavior. Batch experiments achieved a maximum copper removal efficiency of 97.08% at a biochar dosage of 0.5 g, with equilibrium attained within 240 min and a Langmuir-modeled adsorption capacity of 98.15 mg/g (R2 = 0.9862). Kinetic analysis indicated that chemisorption played a dominant role, as supported by the pseudo-second-order model (R2 = 0.9993; rate constant = 0.0036 g·mg⁻1·min⁻1). Thermodynamic parameters revealed a spontaneous and endothermic adsorption process (ΔG° = − 3.74 kJ/mol; ΔH° = 25.76 kJ/mol; ΔS° = 94.31 J·mol⁻1·K⁻1). Column studies conducted under varying flow rates (0.5–1.5 mL/min), bed depths (5–15 cm), and influent copper concentrations (50–250 mg/L) exhibited consistent adsorption capacities (48–50 mg/g) and breakthrough times ranging from 300 to 1140 min. Regeneration experiments demonstrated initial desorption and reuse efficiencies of 81.82% and 67.94%, respectively; however, adsorption performance declined over three successive cycles due to pore structure degradation and depletion of surface functional groups. Structural and chemical characterization using SEM, BET, FTIR, and EDAX indicated surface deterioration and residual copper retention after regeneration. Overall, A. auriculiformis biochar exhibits competitive adsorption performance compared with other lignocellulosic biochars, suggesting its potential as a low-cost and eco-friendly option for copper removal in water treatment applications.