Kinetics and Thermodynamics of Acid Red 1 Adsorption on Used Black Tea Leaves from Aqueous Solution

This study presents the investigation of kinetics and thermodynamics of adsorption of Acid Red 1 (AR1) from aqueous solution on used black tea leaves (UBTL) as a biosorbent. The effects of initial dye concentration, solution pH and temperature on the adsorption kinetics were evaluated through batch adsorption experiments. Different kinetic model equations such as pseudo first order, pseudo second order, Elovic and Intra-particle diffusion were applied to the experimental data to evaluate their validity and determined their respective parameters. The results revealed that the adsorption kinetics is well described by pseudo second order kinetic equation. The equilibrium amounts adsorbed, calculated from the pseudo second order kinetic plots for different initial concentrations, were used to construct the adsorption isotherm which is well expressed by Langmuir equation compared with Freundlich one and the maximum adsorption capacity was 30.03 mg/g. Again, the equilibrium amounts adsorbed, calculated from the pseudo second order kinetic plots for different temperatures, were increased with the increase of adsorption temperature indicating the adsorption is endothermic. The calculated values of apparent activation energy (Ea = 3.35 kJ/mol), enthalpy change (∆H = +30.05 kJ/mol), entropy change (∆S = +0.119 kJ/mol) and free energy change (∆G) revealed that the adsorption of AR1 on UBTL at pH 2.0 was endothermic, spontaneous and physical in nature. The equilibrium amount adsorbed for different initial pH of solution, calculated from pseudo order kinetic plots, was found to be decreased with increase of solution pH from 2.0 to 8.0 suggesting the adsorption mechanism is dominated by electrostatic interaction between positive surface of UBTL at pH 2.0 and negative AR1 dye molecules.