Comparative analysis of H₃PO₄ and KOH activated African oil bean husk ( Pentaclethra macrophylla ) for adsorption of methylene blue from aqueous solutions: Isothermal, thermodynamic, and kinetic studies

This study shows that African oil bean husk was effectively converted to high-performance activated carbons (AC) to eliminate methylene blue (MB) in aqueous solution. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), SEM/ Energy-dispersive X-ray spectroscopy (EDS), X-ray fluorescence (XRF), Thermogravimetric Analysis (TGA), and N₂ adsorption Brunauer Emmett–Teller (BET) were used in an extensive study to confirm surface and structural changes after activation. PZC supported pH-dependent adsorption, shifting from 5.8 (RAOBH) to 5.4 (KOH) and 6.9 (H₃PO₄). KOH and H₃PO₄ chemical activation also remarkably improved the raw biomass, as demonstrated by the BET surface area of 216.011 m ² /g RAOBH, 582.102 m ² /g AOBH-KOHAC, and 492.968 m ² /g AOBH-H₃PO₄AC, and iodine number of 330 mg/g, 901.203 mg/g, and 939.282 mg/g, respectively. AOBH-KOHAC and AOBH-H₃PO₄AC removed 99.8–99.9% of the dye in 30 minutes after reaching equilibrium, indicating surface-controlled interactions. Kinetic and isotherm analysis revealed that a pseudo second order and Temkin isotherm (TI) model was associated with adsorption respectively. The highest adsorption capacities were 142.86 mg/g on AOBH-H₃PO₄AC, 109.53 mg/g on AOBH-KOHAC and 116 mg/g on RAOBH. Thermodynamic validation of the study indicates spontaneous and exothermic adsorption of negative values of Gibbs free energy (-7 kJ/mol) and enthalpy adjusted to maximum value of -60.64 kJ/mol. The study shows the potential of AOBH to be a sustainable source of ACs for dye wastewater treatment.