Adsorption thermodynamics, kinetics and isosteric heat of adsorption of malachite green onto chemically modified rice husk

Rice husk treated with NaOH was tested as a low cost adsorbent for the removal of malachite green from aqueous solution in batch adsorption procedure. The adsorption experiments were carried out as a function of solution pH, initial dye concentration, contact time and temperature. The adsorption was found to be strongly dependent on pH of the medium. The Freundlich isotherm model showed good fit to the equilibrium adsorption data. The mean free energy (E) estimated from the Dubinin–Radushkevich model indicated that the main mechanism governing the sorption process was chemical ion-exchange. The kinetics of adsorption followed the pseudo-second-order model and the rate constant increased with increase in temperature indicating endothermic nature of adsorption. The Arrhenius and Eyring equations were used to obtain the activation parameters such as activation energy (Ea), and enthalpy (ΔH#), entropy (ΔS#) and free energy (ΔG#) of activation for the adsorption system. Thermodynamic studies suggested the spontaneous and endothermic nature of adsorption of malachite green by treated rice husk. The isosteric heat of adsorption (ΔHX) was also determined from the equilibrium information using the Clausius–Clapeyron equation. ΔHX increased with increase in surface loading indicating some lateral interactions between the adsorbed dye molecules.