Abstract
Metal oxide particles in aqueous solutions have electric charges due to the (de)protonation of acid-base surface hydroxyl groups formed by dissociative chemisorption of water molecules. The charges adsorb anions and cations from the solution to maintain electric neutrality, resulting in ion exchange. The amount of surface charge was measured as a function of pH by titration, and the ion-exchange reactions accompanying the charge formation were modeled by using the Frumkin isotherm, which assumes suppression of the reaction due to lateral interactions between the interphase species. From the model parameters, it was found that the intensity of cation exchange (deprotonation) increases in the order: Al2O3 Fe3O4 TiO2 MnO2, and the intensity of anion exchange (protonation) decreases in the same order. The electronegativity of the lattice metal ions of these oxides was estimated and found to increase in the order above. It is suggested that, with electronegativity of the lattice metal ions, the electron density and hence the acid-base nature of hydroxyl sites changes. Also, the adsorption affinity of alkali metal ions was evaluated and it is suggested that these ions are adsorbed in the hydrated form.
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