Removal of the herbicide MCPA by commercial activated carbons: Equilibrium, kinetics, and reversibility

The adsorption of the herbicide 4-chloro-2-methylphenoxyacetic acid (CAS 94-74-6) has been studied using four commercial activated carbons (Norit 0.8, Aquacarb 207C, Aquacarb 208A, and Aquacarb 208EA). Adsorption equilibrium isotherms were obtained in the 293-358 K temperature range. The trend of adsorption for MCPA onto the four activated carbon was in the order Norit 0.8 > Aquacarb 207C > Aquacarb 208A > Aquacarb 208EA. Among various adsorption isotherm models, the Freundlich equation best fit the experimental data. Experiments conducted at different temperatures allowed for the calculation of the isosteric heat of adsorption, revealing that the adsorption process was exothermic for three of the four adsorbents studied. The distribution of the herbicide into the pores of activated carbon Norit 0.8 was studied by considering the Dubinin-Radushkevich isotherm. Also, the adsorption kinetics were assessed by means of a simplistic mechanism based on the shrinking-core mass-transfer model. Finally, as a preliminary step in the study of the potential use of wet air oxidation (WAO) for the regeneration of exhausted activated carbon, the reversibility of the adsorption process was tested. Only partial desorption of MCPA was achieved under WAO conditions, which involves a likely double route of activated carbon regeneration, i.e., liquid and surface contaminant oxidation.