NUREG/CR-6708 "Surface Complexation Modeling of Uranium (VI) Adsorption on Natural Mineral Assemblages"

The value of surface complexation modeling of radionuclide adsorption in supporting the selection of KI values should be given wider recognition within the performance assessment modeling process. Studies of uranium(Vl) adsorption by a weathered schist and specimen mineral phases that comprise the schist were conducted as a function of aqueous chemical conditions in laboratory experiments. The schist was collected from weathered, subsurface soil in the vicinity of the Koongarra uranium deposit (Northern Territory, Australia). The variable aqueous chemical conditions in the experiments caused significant variations in the speciation of dissolved uranium(VI) and the distribution coefficient, Kd, that describes the partitioning of uranium(VVI) between the aqueous and solid phases. Kd values determined in the laboratory experiments compared favorably with in-situ partitioning constants derived from analyses of dissolved uranium(VI) in groundwater and in the subsurface soils. Mineral coatings were more important than bulk mineralogy in controlling U(VI) adsorption by the schist. Various surface complexation modeling approaches were developed to describe adsorption of uranium(VI) on the schist and its reference mineral phases. One of the modeling approaches, the Generalized Composite approach, can reduce the uncertainty in Kd values chosen for performance assessment modeling. Surface complexation modeling offers a scientifically defensible means of linking the selection of Kd values for performance assessment modeling to existing knowledge of thermodynamic data for radionuclides and radionuclide speciation in aqueous systems.