Mechanistic understanding of the behavior of diuron in the adsorption from water onto activated carbon

The adsorption of diuron from aqueous phase by activated carbon has been analyzed from a molecular point of view thanks to a computational approach based on COSMO-RS methodology, with the aim of providing a mechanistic explanation of the experimental results. The adsorption experiments were carried Out at different pH values (3-7) and temperatures (15-45 degrees C) using an activated carbon prepared by chemical activation of grape seeds. The most relevant characteristics of the adsorption of diuron were the increase of diuron uptake with temperature and the occurrence of multilayer adsorption at high equilibrium concentrations. Likewise, the contribution of cooperative adsorption was also found to increase with temperate, as shown by the change of the isotherm pattern from L-3 type (Giles classification) at 15 degrees C to S-3 at 45 degrees C. The formation of multilayer and the contribution of cooperative adsorption were not observed at the highest pH studied. The results obtained from the computational approach were consistent with the trends shown by the experimental data. The molecular and thermodynamic properties of the solvent-adsorbate-adsorbent system were estimated using the quantum-chemical COSMO-RS method. Thus, the increase in diuron uptake at increasing temperature was ascribed to a higher population of diuron planar conformers, whose affinity for activated carbon is higher as evaluated in terms of the activated carbon/water partition coefficient. COSMO-RS simulations predicted strong interaction among diuron molecules due to the amphoteric character of the molecule. Likewise, the formation of clusters was found to be especially favored from a thermodynamic point of view in the case of planar conformers adsorbed on activated carbon, which supports the occurrence of cooperative adsorption and the formation of a multilayer. (C) 2012 Elsevier B.V. All rights reserved.