Adsorption of methylene blue on corncob charcoal: Thermodynamic studies

Abstract

To obtain the thermodynamic properties of adsorption of methylene blue (MB) on corncob carbonaceous adsorbents - untreated (UCC) and acid treated (TCC) - their equilibrium adsorption was determined between 10 ^o– 40^o C at different pH conditions. The adsorption isotherms were fitted to Freundlich, Langmuir and Temkin isotherm models. The point of zero charge of each of the adsorbents was also determined. The point of zero charge was 10.58 ±0.09 for TCC, and 7.55 ± 0.10 for UCC. Only Freundlich model could account for the observed thermodynamic properties of MB adsorption by the adsorbents, though Temkin and Langmuir models have higher correlation coefficients. MB adsorption by TCC was an entropically driven process which depends on pH;ΔS^o at pH 10.5 < ΔS^o at pH 8.0 <ΔS^o at pH 12.0. The ΔH^o of the endothermic process at pH 12 is > ΔH^o at pH 8 > ΔH^o at pH 10.5. The results suggest that MB adsorption by the adsorbents occur by physisorption and is optimum when the pH is around the point of zero charge. It is important to ensure that in addition to fitting and equilibrium adsoption data by an isotherm model, the fit of the relevant equilibrium parameter should also be good and give thermodynamic quantities that could satisfactorily account for the observed adsorption properties of the system.  Deciding the suitability of an isotherm model for fitting adsorption equilibrium experiment based on compared error function of the fitted curves or lines through single temperature isotherm could lead to erroneous conclusion.

Keywords: adsorption, adsorbent; methylene blue; enthalpy; entropy; Freundlich