Modeling of Ion-Exchange Reactions on Metal Oxides with the Frumkin Isotherm. 1. Acid−Base and Charge Characteristics of MnO2, TiO2, Fe3O4, and Al2O3 Surfaces and Adsorption Affinity of Alkali Metal Ions

Abstract

Metal oxides abundant in natural environments affect the concentrations of ions in waters by adsorption. The ion adsorption ability of oxides arises from the acid−base nature of surface hydroxyl groups formed by dissociative chemisorption of water molecules. The protonation and deprotonation reactions of hydroxyl groups produce electric charges, resulting in ion adsorption to maintain electric neutrality (ion exchange). The amount of surface charge in alkali metal nitrate solutions 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. This model embodies not only electrical but also chemical, geometrical, and/or other lateral interactions and can be applied to “real”, not well-defined oxide/solution systems in natural environments. 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 of adjacent lattice oxide ions and, hence the acid−base nature of hydroxyl sites, changes. Also, the adsorption affinity of alkali metal ions was evaluated and discussed within the model parameters.