Kinetics and thermodynamics studies of chromium(VI) ions adsorption onto activated carbon from aqueous solutions

The removal of chromium(VI) from aqueous solutions by activated carbon has been investigated as a function of solution pH, initial chromium concentration C, solid/liquid ratio R and temperature T. The Freundlich and the Langmuir models have been applied and the equilibrium adsorption was found to best fit the Langmuir adsorption isotherm, where good correlation between theoretical and experimental equilibrium concentration of chromium(VI) ion was observed in most cases. The uptake distribution coefficient, KD indicated that the chromium(VI) removal was the highest at minimum solid–liquid ratio. An empirical modelling was performed by using a 24 full factorial design, and regression equation for adsorption chromium(VI) was determined from the data. The pH and the temperature are the most significant parameter affecting chromium(VI) adsorption, followed by the solid/liquid ratio whereas the initial chromium(VI) concentration has the most negative effect on the process. However, the interaction pH—solid–liquid ratio—temperature has a positive effect. The optimal parameters obtained, pH 3.62; C = 77.35 mg/L; R = 10 and T = 45 °C, have been applied to wastewater from the tannery plant of Rouiba (East Algiers, Algeria) in order to remove the contained chromium. The adsorption rate has been found to be 65.70%. A comparison of kinetic models applied to the adsorption of chromium(VI) ions on the activated carbon was evaluated for the pseudo first-order, the pseudo second-order, and intraparticle diffusion kinetic models, respectively. Results show that the pseudo first-order kinetic model was found to correlate the experimental data well. In other hand, the sorption rates at different temperatures were found to increase with increasing temperature and an activation energy of approximately 9.16 × 10−3 kJ/mol was determined. The findings of this investigation suggested that the physical sorption was the mode controlling the sorption rate. Thermodynamic parameters were calculated. The positive value of standard enthalpy of adsorption, ΔHads°=14.51×10−3 kJ/mol revealed the endothermic nature of the adsorption process, the positive entropy of adsorption, ΔSads°=104.12 J/mol K reflected the affinity of the adsorbent material toward chromium(VI) and the negative values of Gibbs free energy, ΔGads° were indicative of the spontaneity of the adsorption process.