On the Equi-adsorption Point of Hydrogen and Hydroxyl Ions at the Alumina-Water Interface

Abstract

The equi-adsorption point defined here as a pH value of the solution where hydrogen and hydroxyl ions are equal in adsorption density at oxide mineral-water interface is presented. The equi-adsorption point of alumina was found at pH 7.4 from the adsorption measurement, while the isoelectric point of the same material was at pH 9.0 in the electrophoresis study. It is shown that the equi-adsorption point is not always in agreement with the isoelectric point for oxide minerals. The equiadsorption point of alumina is also determined to be at pH 7.5 by considering the equilibrium solubility diagram which is obtained from thermodynamical information for several reactions in the alumina-water system. It is concluded that the adsorption of hydrogen and hydroxyl ions on oxide minerals is closely related to the chemical affinity between the constituent atoms of the mineral and the hydrogen or hydroxyl ion.