Retention of selenium by calcium aluminate hydrate (AFm) phases under strongly reducing radioactive waste repository conditions

Safety assessment studies of future nuclear waste repositories carried out in many countries predict selenium-79 to be a critical radionuclide due to its presence in the anionic form resulting in weak retardation by most common rock minerals. This assumption, however, ignores its potential uptake by AFm phases, positively charged anion exchangers which are present in significant quantities in the cementitious materials used in artificial barriers. Here we report for the first time wet chemistry and spectroscopic data on the interaction of the most reduced selenium anion species, i.e. HSe-, with two AFm phases commonly found in cement, monocarbonate (AFm-CO3) and hemicarbonate (AFm-OHCO3). Batch sorption experiments show that Se(-II) is retained much more strongly (Rd = 100±50 L kg-1) by the hemicarbonate than by the monocarbonate (Rd = 4±2 L kg-1). The cause of this different sorption behavior was elucidated by extended X-ray absorption fine-structure (EXAFS) spectroscopy, showing that Se(-II) is mainly intercalated in the larger and hence more accessible interlayer of the hemicarbonate (d-spacing = 0.82 nm), whereas most Se(-II) is sorbed by the anion exchange sites on the outer surfaces of the AFm platelets in the case of monocarbonate, where the interlayer space is less accessible due to the smaller d-spacing of 0.75 nm. EXAFS spectra of oxidation experiments further show that Se(-II) in the interlayers is better protected from oxidation than Se(-II) sorbed to the outer surfaces. The quantitative sorption data along with the molecular-scale processes obtained from this study provide crucial insight into the Se mobility in the cementitious near-field of a radioactive waste repository under reducing conditions.