Thermodynamics of adsorption of malachite green hydrochloride on treated and untreated corncob charcoal

Abstract

The need to explore an environmentally friendly and cheaper ads­orbent for removal of dye from polluted water has imposed a requirement for thorough thermodynamic studies of dye removal from synthetically polluted water. The sorption capacities of untreated corncob charcoal (UCC), treated corncob charcoal (TCC) and commercially available activated carbon (ACC) for malachite green (MG) were determined at pH 3.10 and 5.10, in the tempe­rature range 5–30 °C using the batch adsorption method. The adsorption cap­acity of each adsorbent for MG decreased with the increasing temperature in a manner suggestive of exothermic process. The equilibrium adsorption data were well fitted with Freundlich isotherm. The enthalpy of adsorption is higher at pH 5.10 for any particular adsorbent type compared to data collected at pH 3.10. The adsorption of MG is greatest in activated charcoal at pH 5.10. At pH 3.10 the adsorption process results in reduction in entropy changes in each adsorbent such that ∆S^o_TCC > ∆S^o_UCC > ∆S^o_ACC following the same trend as the change in enthalpy. At pH 5.10, however, ∆S^o_TCC > ∆S^o_ACC > ∆S^o_UCC and the change of enthalpy of adsorption of MG to the adsorbent follows the same order, ∆H^o_TCC > ∆H^o_ACC > ∆H^o_UCC. The values of the enthalpy changes suggest essen­tially physisorption in all cases.