Kinetic, equilibrium, and thermodynamic study of Methylene Blue adsorption on orange peel biochar prepared by microwave-assisted pyrolysis

This study presents a sustainable approach for Methylene Blue (MB) dye removal using pristine, non-activated biochar derived from orange peel waste via Microwave-Assisted Pyrolysis (MAP). The key novelty lies in the systematic comparison of the biochar’s adsorption performance under both pH-controlled (constant pH 4) and unregulated pH conditions, demonstrating that pH regulation is essential for optimizing adsorption efficiency. The resulting biochar exhibited a high fixed carbon content (60.89%), an alkaline surface (Point of zero charge (pHpzc) = 11.20, ZPotential = 0.1 mV @ pH 9), and oxygenated functional groups. Best MB removal of 83% was achieved at pH 4, despite the expected electrostatic repulsion. Kinetic studies showed the best fit with the Elovich model, indicating a heterogeneous surface. The Langmuir isotherm accurately described the equilibrium data, revealing a maximum adsorption capacity (qmax) of 20.57 mg g⁻1 under pH-controlled conditions, representing an 83% increase over the 11.24 mg g⁻1 obtained in the unregulated scenario. Thermodynamic analysis confirmed the process is spontaneous (ΔG° < 0), endothermic (ΔH° = + 4.88 kJ mol⁻1 at constant pH), and governed by physisorption mechanisms, including hydrogen bonding and π-π interactions. This work demonstrates that pristine orange peel biochar generated via MAP is a highly effective adsorbent and highlights the critical impact of pH control on improving adsorption capacity and elucidating the dominant physisorption mechanisms.