A Critical Assessment of Surface Adsorption Models

The theory on variable ionization energy of surface sites is criticized based on inconsistencies of traditional potentiometric titration data (singular reference curve methods) with electrophoretic mobility (EM) data, and on the nature of dissolution of the solid phase. The mathematical consequences of including the Boltzmann distribution term in models using intrinsic equilibrium constants are most probably merely to track the solubility behavior of the solid phase. The EM of an Al oxide was studied with respect to pH. The isotherm showed clear maximum mobility at low and high pH values with the zero point of charge (ZPC) ranging from pH 9.5 to 9.8. The ZPC lowers with increasing electrolyte concentration due to the increased competitive behavior of the cations in solution for the surface sites. Assuming a constant ionization energy of the surface sites, a mass balanced model that predicted both anion-cation adsorption data and potentiometric titration data (backtitration technique) also correlated well with the EM data. The EM behavior was modeled based on the varying mobility of each type of surface species present on the oxide at various pH values. Whereas traditional models treat the oxide as being capable of forming a surface charge that induces ion adsorption, this model treats the oxide as being capable of undergoing ion exchange or substitution reactions resulting in surfaces that exhibit mobility in an applied electric field. The proposed model assumes that anion adsorption results in positive mobility, and cation adsorption results in negative mobility. Additional Index Words: point of zero salt effect (PZSE), zero point of charge (ZPC), surface charge, intrinsic equilibrium constants (A"), Boltzmann distribution term, solid phase dissolution,