CURRENT CONSIDERATIONS FOR THE DISSOLUTION KINETICS OF SOLID OXIDES WITH SILICA

A general dissolution model valid for solid oxides has been developed. This kinetic model consists of reactions for the cleavage and formati on of element-oxygen bonds as the rate-limiting steps and protolysis r eactions for the surface groups of the oxide and the monomer in equili brium. The experimental data for silica show that the dissolution rate in the strong acid region can be compared with that in an alkaline me dium. Therefore all possible reactions must be taken into consideratio n so that a realistic description in a wide pH range can be achieved i n terms of kinetic and thermodynamic parameters of the dissolution beh avior. The dissolution kinetics of a porefree, spherical silica depend ent on pH at 0.1, 0.01 and 0.001 mol/L background electrolyte concentr ation (NaCl) at 40 degrees C is looked into to confirm the model. The protolysis reactions are described in three different ways: first with out formation of a double layer, second with the formation of a diffus e double layer, and third with the additional formation of a gel layer . The results of the three approaches are then compared. The character istic parameters such as the pristine point of zero charge, the protol ysis constants of the surface groups, the saturation concentration, an d the partial rate constants are found in acceptable orders of magnitu de. A further result is a detailed expression for the pH dependence of the overall rate constant of dissolution determined by three partial rate constants and the pH-dependent charge state of the surface.