change with pH of the solution, we believe that the change The relationship between saturated adsorption amount ( A s ) and of z potential of the activated carbon with pH is the reason pH and the relationship between saturated adsorption amount for the effect of pH on the adsorption amounts. In order to (A s ) and z potentials for two cationic dyes and three anionic dyes prove this theory, zΓpH and pHΓA s curves were measured. on activated carbon have been studied. It has been found that A s We studied the effect of pH on the adsorption behavior of the dyes on activated carbon is correlated with the electrostatic of five different dyes: methyl green (MG), methyl violet forces between charges on the carbon surface and ionic dyes. The (MV), phenol red (PR), carmine (CM), and titan yellow experimental results showed three distinct states regarding the (TY). These dyes are not only different in molecular weight effect of z potential on carbon surface. In the neutral potential
and chemical structure but also different in the region of state where z potential is nearly zero, the electrostatic interactions between the carbon surface and the dyes are negligible and the color change with pH (4-7). The investigation was deadsorption forces are mainly dispersion forces. Therefore, pH has signed to study the adsorption of different dyes on activated no effect on A s . In the positive or negative potential states, there carbon at various conditions in order to gain an insight on exists the electrostatic attractive or repulsive forces between actithe adsorption mechanism for the ionic dyes on activated vated carbon and the ionic dyes. The adsorption forces are the sum carbon. or difference of dispersion and electrostatic forces. The adsorption amount, A s , changes with z potential. The different types of ad-EXPERIMENTAL sorption forces in the three potential states reveal the mechanism of adsorption for the dyes on activated carbon. The conclusion is The pH zpc of the zero-point charge of the powdered actifurther supported by the kinetic and thermodynamic parameters that were calculated from the experimental data. α§ 1998 Academic vated carbon (781-A type