Selenide [Se(-II)] Immobilization in Anoxic, Fe(II)-Rich Environments: Coprecipitation and Behavior during Phase Transformations.

The radionuclide selenium-79 (Se-79) is predicted to be a key contributor to the long-term radiologic hazards associated with geological high-level waste (HLW) repositories; hence its release is of pertinent concern in the safety assessment of repositories. However, interactions of reduced Se species with aqueous Fe(II) species and solid phases arising from the corrosion of a steel overpack could play a role in mitigating its migration to the surrounding environment. In this study, we examined the immobilization mechanisms of Se(-II) during its interaction with aqueous Fe(II) and freshly precipitated Fe(OH) at circumneutral and alkaline conditions, respectively, its response to changes in pH, and its behavior during aging at 90 °C.

Mechanisms of Se(IV) Co-precipitation with Ferrihydrite at Acidic and Alkaline Conditions and Its Behavior during Aging.

Understanding the form of Se(IV) co-precipitated with ferrihydrite and its subsequent behavior during phase transformation is critical to predicting its long-term fate in a range of natural and engineered settings. In this work, Se(IV)-ferrihydrite co-precipitates formed at different pH were characterized with chemical extraction, transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS) to determine how Se(IV) is associated with ferrihydrite. Results show that despite efficient removal, the mode and stability of Se(IV) retention in the co-precipitates varied with pH.