Charge vs. porosity - Some influences on the adsorption of natural organic matter (NOM) by activated carbon

The adsorption of NOM ultrafiltration fractions onto ten activated carbons was studied. The aim of the research was the clarification of the effects of carbon charge and pore volume distribution and NOM charge and molecular weight on adsorption. The effect of pH and ionic strength on adsorption mechanisms was determined, and it was found that, in the absence of strong electrostatic effects ts, adsorption occurred by a pore filling mechanism in which the available pore volume was filled. At neutral pH the NOM has a significant negative charge which can affect the adsorption in several ways. At low surface concentrations on activated carbon with positive surface groups there is direct surface-NOM electrostatic attraction (screening reduced adsorption). At higher surface concentrations the adsorption becomes predominantly a physical surface-NOM attraction, possibly due to hydrophobic or aromatic ring interactions. The electrostatic effects are mainly caused by the lateral repulsion of the NOM that prevents the close-packed arrangement apparent at pH 3 (screening enhanced adsorption). For one activated carbon it was shown that the degree of ionisation of the adsorbed NOM is strongly dependent on the surface concentration. (C) 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved.