Diffusion of H+ and OH– in porous solids

Abstract

A novel formulation is presented to describe H⁺ and OH^– diffusion in adsorptive porous solids. As a result of solution and surface reaction equilibrium constraints, only the difference in H⁺ and OH^– concentrations appears in the convective-diffusion equation. For the supporting electrolyte case, diffusion of H⁺ and OH^– can be treated uniformly over the entire pH range. With linear sorption isotherms, analytic solutions illustrate the coupled behaviour typical of H⁺ and OH^– diffusion. Using the suggested formulation, the adsorption and diffusion of H⁺ and OH^– in ionogenic porous oxide solids has been quantified for the first time. Adsorption equilibria are described by the triple-layer, site-binding model of Davis, James and Leckie (J. Colloid Interface Sci., 1978, 63, 480). A priori numerical calculations are presented for the uptake of aqueous acid and base into γ-alumina pellets from an infinite solution bath. Agreement is found between the proposed diffusion theory and new experimental data for tank depletion of both acidic and basic solutions over a range of NaCl background electrolyte concentrations from 0.010 to 1.0 mol dm^–3.