Surface complexation modelling of uranyl adsorption onto kaolinite based clay minerals using FITEQL 3.2

The aim of this study is to explain how the kaolinite-based clay minerals adsorb hexavalent uranium (uranyl ion), and to model uranyl adsorption based on inner-sphere surface complexation with the kaolinite edge hydroxyl sites and outer-sphere complexation with the permanent charge sites. The adsorption of UO22+ on kaolinite-based clay was modeled with the aid of FlTEQL 3.2 code using the single-site binding model of the Langmuir approach. Potentiometric titrations and adsorption capacity experiments were carried out in solutions containing different concentrations of the inert electrolyte NaClO4, however adsorption modeling was deliberately done at low ionic strength (0.01 M electrolyte) for enabling adsorption onto the permanent negatively charged sites of kaolinite. When a 'two-site' binding model was adopted. i.e., S1OH sites representing silanol and S2OH sites aluminol, a good fit was not obtained, and therefore the involvement of merely aluminol sites in surface complexation was assumed along with the ion exchange-held X-2(2-) - UO22+ species by the NaX permanent charge sites. The uranyl cation was assumed to bind to the clay surface as the sole (urthydrolyzed) UO22+ ion and form monodentate surface complexes. The modeling system comprised of surface complexation and ion exchange was resolved with respect to species distributions and relevant stability constants. Electrostatic effects were accounted for using a diffuse layer model (DLM) requiring the minimum number of adjustable parameters. Metal adsorption onto clay showed a steady increase with increasing pH up to pH 5.5, i.e., the edge pH for bulk precipitation at the studied concentration range.