Equilibrium, Thermodynamic and Mechanism Studies of Malachite Green Adsorption on Used Black Tea Leaves from Acidic Solution

This study reports the equilibrium adsorption mechanism and thermodynamics for the removal of Malachite Green (MG) from acidic solution using Used Black Tea Leaves (UBTL) as a low cost adsorbent. The effect of initial dye concentration, processing temperature and the presence of electrolytes on the adsorption from acidic solutions were investigated in batch process. Adsorption process was attained to the equilibrium at about 24 hours. Adsorption isotherms of MG on UBTL for different temperatures were constructed at pH 2.0 and the equilibrium adsorption data were analyzed using different model equations such as Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (D-R), Harkin-Jura, Halsey, Elovich-Larionov and Flory-Huggens isotherms. The experimental results were reasonably correlated by Langmuir, Flory-Huggins and D-R models than other isotherm models. The maximum adsorption capacity ( q m ), intensity of adsorption ( b ) and separation factor ( R b ) were calculated from Langmuir plot and activation energy of adsorption ( E ad ) was determined from D-R isotherm. At pH 2.0, the equilibrium adsorption capacity of UBTL to MG is 110 mg∙g -1 at 30°C, which was increased with increasing temperature suggested endothermic nature of adsorption. Thermodynamics parameters: Δ G ads , Δ H ads and Δ S ads were calculated from Langmuir constant ( b ). The positive values of enthalpy and free energy, and negative value of entropy suggested that the adsorption is less spontaneous and surface migration or fragmentation of MG molecules on UBTL surface might be occur. Again, the low value of activation energy of adsorption ( E ad = 0.3 kJ∙mol -1