Thermodynamic and kinetics studies on adsorption of Indigo Carmine from aqueous solution by activated carbon

The effect of the initial dye concentration (10-60 mg/L), contact time (0-90 min), pH (1−12), agitation speed (0-600 rpm), adsorbent dose (1-10 g/L) and temperature (298-323 K) were determined to find the optimal conditions for a maximum adsorption. The adsorption mechanism of Indigo Carmine onto AC was studied using the first pseudo order, second pseudo order and Elovich kinetics models. The adsorptions kinetic were found to follow a pseudo second order kinetic model with a determination coefficient (R2) of 0.999. To get an idea on the adsorption mechanism, we applied the Webber-Morris diffusion model. The equilibrium adsorption data for Indigo Carmine on AC were analyzed by the Langmuir, Freundlich, Elovich, Dubinin and Temkin models. The results indicate that the Langmuir model provides the best correlation at 25 °C (qmax = 79.49 mg/g) and Dubinin at 40 °C (qmax = 298.34 mg/g). The adsorption isotherms at different temperatures have been used for the determination of thermodynamic parameters such as free energy (ΔG° = −0.071 to −1.050 kJ/mol), enthalpy (ΔH° = 28.11 kJ/mol), entropy (ΔS° = 0.093 kJ/mol·K) and activation energy (Ea) of 51.06 kJ/mol of adsorption. The negative ΔG° and positive ΔH° values indicate that the overall adsorption is spontaneous and endothermic.