Adsorption of Malachite Green onto Walnut Shells: Kinetics, Thermodynamic, and Regeneration of the Adsorbent by Chemical Process

The textile industry produces huge amounts of wastewaters containing synthetic and toxic dyes. The aim of this study was to evaluate the adsorption of Malachite green (MG) onto Activated Carbon from Walnut Shells (ACWS) realized in a batch system. The effects of contact time, initial pH, stirring speed, particle size, temperature, adsorbent dose, and initial MG concentration on the adsorption capacity were investigated graphically for determining optimum conditions. The experimental isotherm data were analyzed by the Langmuir, Freundlich, Temkin, and Elovich models. The adsorption follows well the Langmuir equation, providing a better fit of the equilibrium adsorption data. Under optimized conditions, up to 154.56 mg/g at 25 °C and 370.37 mg/g at 45 °C were removed from the solution. The adsorption mechanism of MG onto ACWS was studied using the first-pseudo-order, second-pseudo-order, Elovich and Webber–Morris diffusion models. The adsorptions’ kinetic was found to follow rather a pseudo-second-order kinetic with a determination coefficient ( R 2 ) of 0.999. The adsorption isotherms at different temperatures have been used for the determination of thermodynamic parameters, i.e., the free energy ΔG o (0.802 to − 2.123 kJ/mol), positive enthalpy change ΔH o (18.547 kJ/mol), entropy ( ΔS o = 0.064 kJ/molK), and activation energy ( Ea = 14.813 kJ/mol). The negative ΔG o and positive ΔH o values indicate that the overall MG adsorption is spontaneous and endothermic.