Sustainable production of biochar from red kidney bean ( Phaseolus vulgaris L.) pod waste and its application for phenol removal

Abstract

Converting agricultural residues into functional carbon materials offers a sustainable route for wastewater remediation and biomass valorization. Biochar was synthesized from waste red kidney bean (Phaseolus vulgaris L.) pods via ZnCl2-assisted chemical activation and evaluated for phenol removal from aqueous solutions. Characterization by SEM, EDX, XRD, BET/BJH, FTIR, and zeta potential analysis revealed a fractured, porous carbon skeleton with residual Zn and Cl from activation and coexisting amorphous carbon and crystalline ZnO. The biochar exhibited a hierarchical micro-mesoporous structure (BET surface area 228.4 m2 g−1, pore volume 0.128 cm3 g−1) with a negatively charged surface above pH 3. Batch experiments (contact time, pH, temperature) showed equilibrium within 45–60 min and removal efficiencies approaching 98% under near-neutral pH. Kinetic uptake followed the pseudo-second-order model, while equilibrium data fitted the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherms. The non-linear Langmuir maximum adsorption capacity reached 18.27 mg g−1, and the D-R mean free energy (E = 8.61 kJ mol−1) indicated combined physisorption and chemisorption. The process was spontaneous and exothermic (ΔG° = −17.3 to −18.7 kJ mol−1), retaining over half its efficiency across five reuse cycles. This work highlights bean pod waste as a low-cost, reusable adsorbent for phenol removal from water.