The effect of the zeta (zeta) potential of activated carbon on the adsorption of cationic dyes from aqueous solution has been studied. The pH(zpc) of the zero-point charge of activated carbon has been measured by microelectrophoresis. Plots of saturated amount adsorbed for methyl green and methyl violet on activated carbon versus zeta potential have also been determined. The results show that the electrostatic interaction between cationic dyes and the surface of activated carbon has a great effect on adsorption capacity. When the zeta potential of activated carbon is lower than -30 mV, the electrostatic attraction between cationic dyes and the carbon surface greatly increases the adsorption capacity. However, when the potential of activated carbon is between -30 and +60 mV the electrostatic force between dyes and the carbon surface is diminished and the adsorption capacity of activated carbon is approximately unchanged. The capacity is significantly reduced due to the electrostatic repulsion between cationic dyes and the carbon surface when the zeta potential is higher than +60 mV. The adsorption kinetic and thermodynamic parameters as functions of pH have been determined. The mechanism for adsorption of cationic dyes on activated carbon has also been studied. (C) 1994 Academic Press. Inc.