Under more alkaline conditions (i.e., pH N PZC) and low ionic
strength, the net charge on the surface of the solid phase is negative,
but positive charges due to the presence of remaining protonated
basic groups may still be available. These groups account for the electrostatic
attraction of the negatively charged aqueous species, as schematically
represented by Eq. (12). Thus, the lower adsorption density of
copper under such conditions could be ascribed to the smaller net positive
charge on the activated carbon surface.