RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Uranium^VI sorption experiments involving quartz and clinoptilolite, important mineral phases at the proposed US nuclear waste repository at Yucca Mountain, NV, were conducted to evaluate the ability of surface complexation models to predict U^VI sorption onto mineral mixtures based on parameters derived from single-mineral experiments. The experiments were conducted at an initial U^VI aqueous concentration of ∼2.0×10⁻⁷ mol·l⁻¹ (0.1 mol·l⁻¹ NaNO₃ matrix) and over the pH range ∼2.5 to ∼9.5. The U^VI solutions were reacted with either quartz or clinoptilolite only, or with mixtures of the two minerals. The experiments were carried out under atmospheric pCO₂(g) conditions (in loosely capped containers) or under limited pCO₂(g) (in capped containers or in a glove box).

Data from sorption experiments on quartz at atmospheric pCO₂ conditions were used to derive U^VI binding constants for a diffuse-layer surface complexation model (DLM). The DLM was then used with surface area as a scaling factor to predict sorption of U^VI onto clinoptilolite and clinoptilolite/quartz mixtures under both atmospheric and low pCO₂ conditions. The calculations reproduced many aspects of the pH-dependent sorption behavior. If this approach can be demonstrated for natural mineral assemblages, it may be useful as a relatively simple method for improving radionuclide transport models in performance-assessment calculations.