Adsorptive Removal of Methylene Blue (MB) Dye from Synthetic Wastewater by Biochar Prepared from Olive Mill Solid Wastes: Process Performance, Kinetics, Equilibrium, and Thermodynamics

This study evaluated the potential of biochar (BC) produced from olive mill solid waste (OMSW) for methylene blue (MB) removal from synthetic solutions. BC was prepared at pyrolysis temperatures of 400 °C, 500 °C, and 600 °C for residence times of 1, 3, and 5 h. The produced BCs were characterized using BET, SEM, FTIR, XRD, zeta potential, and elemental analysis, while adsorption performance was assessed through equilibrium, kinetic, and thermodynamic studies. The results demonstrated that increasing pyrolysis temperature improved the physicochemical properties of BC. The BET surface area increased from 1.34–1.9 m2/g at 400 °C to 135.23 m2/g at 600 °C, accompanied by enhanced pore volume and the development of a highly porous structure observed by SEM. FTIR analysis revealed progressive transformation of oxygen-containing functional groups, while XRD indicated increased structural ordering and carbonization at higher pyrolysis temperatures. BC produced at 600 °C exhibited the highest adsorption performance, achieving a maximum MB removal efficiency of 99.57% and an adsorption capacity of 17 mg/g. Adsorption kinetics were best described by the pseudo-second-order model (R2 > 0.99), indicating strong adsorbate–adsorbent interactions. Equilibrium results were best fitted by the Langmuir isotherm with a maximum R2 of 0.9963, suggesting predominantly monolayer adsorption, whereas the Freundlich model showed a maximum R2 of 0.9788 under elevated temperature and prolonged contact conditions, indicating surface heterogeneity and multilayer adsorption. Thermodynamic analysis confirmed that MB adsorption was spontaneous (ΔG° = −38.61 to −47.57 kJ/mol), predominantly endothermic (ΔH° up to 4.11 kJ/mol), and associated with increased randomness at solid–solution interface (ΔS° > 0). Overall, BC produced at 600 °C for 3–5 h exhibited the most favorable structural characteristics and adsorption performance, highlighting its potential as a sustainable and low-cost adsorbent for wastewater treatment applications.