The model structure of kaolinite in relation to surface complexation

Surface complexation modelling using the FITEQL software was applied to explain the surface acidity of kaolinite. In this context, a number of different models and modelling approaches were tested. Further, a density functional method, CASTEP (Accelrys, 2001), was used to investigate the crystal and surface structures of kaolinite, and to calculate surface charge densities. To enable the interpretation of adsorpti-on results with surface complexation modelling, an adequate characterization of the surface charging of the adsorbent is needed. In general, titration experiments are capable of providing this information for ionisable interfaces. However, results based on experimental data give only average values of surface charge densities. Therefore, in this study, molecular modelling was used to calculate surface charge den-sities of likely the most reactive kaolinite crystal faces separately. Based on this information, an attempt will be made to constrain the number of surface sites and magnitude of surface hydrolysis constants, of-ten considered as adjustable parameters devoid of deeper understanding, in surface complexation models. The molecular-level modelling produced invaluable background information of the kaolinite surface for further development of surface complexation models in terms of the surface charging and the adsorption of metal cations.