Equilibrium, kinetic and thermodynamic studies of methyl orange removal by adsorption onto granular activated carbon

The adsorption of methyl orange (MO) onto granular-activated carbon (AC) is studied. The effects of initial dye concentration, AC dose, initial solution pH, stirring rate, and temperature on MO adsorption were investigated in a batch system. The experimental data were analyzed using the Langmuir, Freundlich, Elovich, Temkin, and Dubinin-Radushkevich adsorption isotherms, and the characteristic parameters and related correlation coefficients were determined for each isotherm at different temperatures. The best fit was achieved by the Langmuir isotherm equation, by increasing the maximum monolayer adsorption capacity from 20.7 to 46.1 mg/g, when the temperature increases from 283 to 313 K. Pseudo-first-order, pseudo-second-order, Elovich, Weber-Morris intraparticle diffusion and Boyd models were used to analyze the kinetic data obtained at different initial MO concentrations. The adsorption kinetic data were well described by the pseudo-second-order model, while the adsorption process was controlled by film diffusion. The Gibbs free energy, enthalpy, and entropy of the adsorption process were also evaluated, the results showing that the adsorption was spontaneous and endothermic under the examined conditions, and was presumably of a physical nature and non-reversible.