RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Surface complexation models developed to simulate U(VI) adsorption by a weathered schist material in an earlier publication are extended by considering additional U(VI) surface species and a modified modelling approach. It is illustrated that the surface speciation of Component Additivity models can be different from Generalized Composite (GC) models, because of the different approaches to fitting the pH dependence of adsorption data. Collecting experimental adsorption data at more than one partial pressure of carbon dioxide was important in achieving excellent fits between U(VI) adsorption data and model simulations. Calibration of GC models across a wide pH range (4-10) with FITEQL 4.0 yields good predictions of percent U(VI) adsorbed, but appears to result in a significant overestimation of K_d values in the neutral pH range. Better agreement between the model and experimental K_d values was obtained by considering data across a narrower pH range (6-8.6), which is important for application of the GC modelling approach to field data. It is recommended that a modified optimization method be developed for calibration of surface complexation models with experimental K_d values.