Kinetics and Thermodynamics of Methylene Blue Adsorption on Polyfurfuryl Alcohol-Derived Templated Carbon and Activated Carbon from H3PO4-Impregnated Moringa oleifera Seed Hull

Abstract

This work probes the kinetics and thermodynamics of adsorption of methylene blue (MB) from aqueous phase on a templated carbon (TC) synthesized from polyfurfuryl alcohol using kaolinite template at 773 K; and activated carbon produced by the pyrolysis of H3PO4-impregnated Moringa oleifera seed hull (MOSH) at 723 K. Factors such as initial concentration of MB (1-3 mg/50 mL), reaction time (0-60 min.) and temperature (302-328 K) for the process were investigated using isotherms, sorption kinetics and thermodynamics. From the results obtained, adsorption of MB on TC fits both Langmuir and Freundlich isotherms well. The Langmuir isotherm describes adsorption of MB on the activated carbon from MOSH (MOSHC) better than the Freundlich isotherm. The adsorption capacities of the active carbons observed for MB were 29.3 mg/g for TC and 29.8 mg/g for MOSHC. The thermodynamic values evaluated:  (14.15 kJ/mol. and 11.48 kJ/mol.),  (87.93 kJ/mol. K and 49.55 kJ/mol. K) and  (7.10 kJ/mol. and 14.26 kJ/mol.) for TC and MOSHC respectively, indicate endothermic and physical nature of adsorption, and enhanced randomness at the adsorbate-adsorbent interface. The sticking probability ( ) values (3.25 x 10–3 and 1.36 x 10–4 for TC and MOSHC respectively) show that the probability of MB molecules sticking on the surfaces of these active carbons is very high, with that for MOSHC being higher. Negative  values (– 12.28 to – 14.68 kJ/mol.K and – 3.56 to – 4.92 kJ/mol.K) for TC and MOSHC respectively, at 302–328 K confirms a spontaneous adsorption process dominated by physisorption. Adsorption of MB on TC supports multilayer formation and was dominated by pseudo-second order kinetics while its adsorption on MOSHC was dominated by pseudo-first order kinetics with a single MB molecule occupying more than one active site. MOSHC, the low-cost adsorbent prepared, is thus very assuring for the removal of organic pollutants like MB from aqueous systems.