A Surface Complexation Model of Selenium Sorption Through Illite and Montmorillonite in PHREEQC
The Nuclear Waste Management Organization is developing a database to predict sorption for elements of interest on crystalline and sedimentary rocks for Deep Geological Repository (DGR) site selection, monitoring, and safety. Se(-II), a uranium fission product, is a key radionuclide of interest due to its long half-life and potential for migration through groundwater solutions. To predict interactions of selenium with groundwater environments, this thesis examines selenium speciation, solubility, and sorption. First, selenium speciation is analyzed in PhreePlot to determine relevant ions of importance with respect to two groundwater reference solutions: SR-270-PW (sedimentary) and CR-10 (crystalline). Then, the solubilities of these ions are modelled in PHREEQC with respect to redox potential, pH, and ionic strength to determine which ions have the greatest potential to migrate through a wide variety of groundwater environments. Finally, sorption is modelled in PHREEQC on illite and montmorillonite, key sedimentary components used in the construction in the DGR and found in the surrounding areas. This model is used to calculate sorption constants (KD) that describe sorption reactions and allow for predictive capacity for a wide range of soils. Overall, this thesis serves as an analysis of the interaction of selenium ions in sedimentary and crystalline environments to help better understand concerns with spent nuclear fuel management.