Adsorption of 2,4-dichlorophenoxyacetic acid on activated carbons from macadamia nut shells

This study reports on the application of activated carbons from macadamia nut shells as adsorbents for the removal of 2,4-dichlorophenoxyacetic acid, a commonly used pesticide, from water. Different activating agents (FeCl₃, ZnCl₂, KOH and H₃PO₄) were used to obtain adsorbents within a wide range of porous texture and surface properties. The characterization of the resulting activated carbons was performed by N₂ adsorption-desorption, elemental analysis, TG and pH_PZC. The adsorption experiments were conducted in batch at 25, 45 and 65 °C. The adsorption kinetics on activated carbons obtained with FeCl₃ H₃PO₄ or KOH was well described by the pseudo-second order model, whereas for the resulting from ZnCl₂ activation the experimental data fit better the pseudo-first order model. The equilibrium studies were performed with the KOH- and ZnCl₂-activated carbons, the two showing higher surface area values. In both cases, high adsorption capacities were obtained (c.a. 600 mg g^-1) and the experimental data were better described by the Langmuir and Toth models. The thermodynamic study allows concluding the spontaneous and endothermic character of the adsorption process, as well as an increase of randomness at the solid/liquid interface. Breakthrough curves were also obtained and fitted to the logistic model.