Ion Exchange Properties of Surface Hydroxyl Groups onλ-MnO₂ for Sodium Ion Adsorption

Abstract

Lambda-MnO₂ has the ability to incorporate Li⁺ ions in solution selectively into its lattice vacancies, and can be applied as a sensor material for Li⁺ ions and as an agent for recovering Li⁺ ions from seawater. The surface of λ-MnO₂ in water is hydroxylated, and ion exchange reactions at surface hydroxyl groups may bring other ions into the recovered Li⁺ ions. The amount of Na⁺ ions adsorbed by exchanges with surface hydroxyl protons was measured by titration in NaNO₃ solutions, and modeled by considering reaction stoichiometry and equilibrium conditions with the Frumkin equation. The model equations reproduce the measured data, and can be used to estimate the amount of Na⁺ ions contaminating the recovered Li⁺ ions. A method for reducing the contamination due to adsorbed Na⁺ is proposed.