Effective and sustainable methylene blue removal from wastewater using palm kernel cake

This research evaluates unmodified palm kernel cake (PKC), an agricultural by-product of palm oil processing as an inexpensive and sustainable adsorbent in the removal of methylene blue (MB) in aqueous solutions. Experiments on batch adsorption were carried out at 298-313 K, 5-20 mg/L of initial MB concentration, and 2-8 pH. The highest removal of 81% was obtained at pH 8 within 90-120 min. The Langmuir isotherm (R² = 0.982) provided the best fit, yielding a theoretical maximum monolayer capacity (q_m) of 151.5 mg/g; the experimentally measured adsorption capacity (q_e,exp) was 4.8 mg/g under the tested conditions. Pseudo-second-order kinetics (R² = 0.999) best described the adsorption rate, indicating intraparticle diffusion controlled physical adsorption confirmed by an activation energy of 37.8 kJ/mol. Thermodynamic analysis revealed a spontaneous (ΔG° =  - 3.98 to - 4.25 kJ/mol), endothermic (ΔH° = 1.47 kJ/mol) process. The adsorption process is mainly physical, which depends on the electrostatic attraction, hydrogen bonding, π-π stacking, and intraparticle diffusion as shown in the Weber-Morris, Boyd, FTIR, and SEM analyses. PKC maintained a high adsorption efficiency of 96.5% even after four regeneration cycles, which showed great reusability and high potential of sustainable treatment of textile wastewater.