IONIC ADSORPTION AND DIFFUSION IN POROUS METAL OXIDES (ION EXCHANGE, THERMODYNAMICS)

Adsorption and diffusion of ionic species in porous metal oxide is important in areas as diverse as soil science, pollution abatement, ion exchange, froth flotation, and catalyst impregnation. This thesis discusses the thermodynamics of ionic adsorption, consistency of adsorption models, determination of adsorption equilibria, and formulation of conservation equations for ionic diffusion. Major contributions of this work are: (1) The thermodynamics for ionic adsorption on metal oxides is developed to allow correct interpretation of adsorption data. We establish a general method for defining the adsorption invariants in terms of the proposed solution and surface species for any adsorption model. For the first time, rigorous material balances are derived to give two independent methods for determining ionic adsorption from surface titration experiments. Accepted procedures for analyzing ionic adsorption data, such as the common-point-of-intersection method for determining the absolute value of the adsorption, are shown to be in error except under very restricted conditions. This analysis demonstrates the procedures required to measure, interpret, and model ionic adsorption on metal oxides. (2) Thermodynamic consistency is exploited as a novel criterion for discrimination among models of ionic adsorption on metal oxides. Consistency tests show that adsorption equilibria models must allow all solution ions to compete for surface sites. This view of the surface equilibria contrasts with the thermodynamically inconsistent concept of H('+) and OH('-) as the sole potential determining ions. Background electrolyte ions must be specifically adsorbed. (3) For the first time, adsorption equilibria of (gamma)-alumina in contact with aqueous solutions of NaCl are completely characterized. We report 600 data for acid/base adsorption, and 200 data each for Na('+) and Cl('-) adsorption. Solution NaCl concentration and pH ranging from 0.0010 to 1.0 M and 2 to 12, respectively. In addition, the surface hydroxyl site density is measured at 3 x 10('-9) mol/cm('2) using tritium uptake. These data are the first to be proven thermodynamically consistent. Typical investigations of adsorption on oxides attempt to quantify all equilibria from only acid and base adsorption data. We demonstrate that this method gives only the ratios of the equilibrium constants, and illustrate the techniques required for rigorous characterization of the surface. (Abstract shortened with permission of author.)...