Adsorption of NOM onto activated carbon: Electrostatic and non-electrostatic effects

Adsorption of natural organic material (NOM) onto two activated carbons was studied at high and low ionic strength over a range of pH values. It was found that at high surface concentrations an increase in ionic strength increased the adsorption onto both activated carbons at all pH. The increase was more pronounced at high pH as the electrostatic repulsions between segments of the NOM are higher in that pH region and are effectively screened by added salt. At low surface concentrations the direct surface-NOM interactions are predominant and electrostatic attraction, apparent on one activated carbon, was screened by added salt, thereby decreasing adsorption. The conversion from the ''screening reduced'' to ''screening enhanced'' adsorption regimes took place where the negative charge from the adsorbed NOM balanced the positive charge on the activated carbon surface. (C) 1997 Elsevier Science Ltd.