Kinetics and thermodynamics of the Cr(III) adsorption on the activated carbon from co-mingled wastes

The kinetics and the thermodynamics of the chromium(III) adsorption on novel activated carbons prepared from co-mingled natural organic wastes have been studied by the sets of equilibrium and time-based experiments at various conditions (temperature, initial chromium concentration and carbon loading). The results are compared to those for commercially available Norit activated carbon post-oxidized with 1 M HNO₃. Two classical empirical models of Freundlich and Langmuir have been applied to fit the experimental data. Total chromium uptake by the novel activated carbon from co-mingled wastes was found to be higher then that for commercial Norit activated carbon under similar conditions (1.09 and 1.01 mmol/g, respectively, at 30 °C). Relatively low activation energy of ∼60 kJ/mol suggests a diffusion-controlled adsorption on the carbons from co-mingled waste, whereas relatively high values for the oxidized Norit carbon of 92 kJ/mol indicates the process controlled by chemical reactions. The free energy of the adsorption at all temperatures was negative indicating a spontaneous process. The positive entropy values indicate the existence of ion-exchange and the substitution reactions resulted in creation of steric hindrances in the systems studied. The overall processes were found to be endothermic and the enthalpy changes of 3-11 kJ/mol indicate the complex character of the Cr(III) adsorption on the studied activated carbons. Thus, the results obtained suggest that Cr(III) sorption on activated carbons from co-mingled wastes is either a physisorption or simple ion-exchange process, however, diffusion-controlled reactions have been taken into account for the description of the adsorption kinetics under studied experimental conditions. On the other hand, the Cr(III) adsorption on the oxidized Norit activated carbon, which is more rich by surface oxygen functionalities, is more likely to be a combined ion-exchange-surface complexation. The results of the present study are towards application of novel porous materials resulting from thermal co-utilization of natural carbon wastes as selective adsorbents for heavy metal removal from industrial wastewater. A Spanish leather industry wastewater stock solution was studied as the source of trivalent chromium.