Adsorption of Indigo Carmine from Aqueous Solution by Chitosan and Chitosan/Activated Carbon Composite: Kinetics, Isotherms and Thermodynamics Studies

Activated carbon (AC) obtained from peanut shell, chitosan (CH) obtained from crab shell, and prepared chitosan/ activated carbon (CH/AC) composite were studied in a batch system for the adsorption of indigo carmine (IC) from aqueous solution. Characterizations of AC, CH, and CH/AC were investigated by FTIR, SEM, XRD, zero-point charge pHpzc, thermal analysis, surface area BET, and pore-size distribution. Adsorbent weight (0.01–0.1 g), initial pH solution (2–10), initial indigo carmine concentration (10–50 mg/l) and contact time (0–60 min) were used as parameters in the adsorption equilibrium experiments. Pseudo-second-order kinetic model was found to describe the adsorption process better than pseudo-first-order kinetic model. Langmuir, Freundlich, and Temkin isotherms applied to the adsorption data reveal that AC and CH/AC best fitted Langmuir and Freundlich models when CH data fitted Temkin model with maximum adsorption capacities of 82.64 mg/g for AC, 96.15 mg/g for CH, and 208.33 mg/g for CH/AC at 30 °C. Thermodynamic parameters such as standard Gibbs free energy (ΔG°), standard enthalpy (ΔH°), and standard entropy (ΔS°) were -23.42 kJ/mol, 10.66 kJ/mol, and 112.40 J/K/mol, respectively for CH/AC. The negative value of ΔG° and a positive value of ΔH° indicate that the removal of indigo carmine by CH/AC is spontaneous and an endothermic process.