The surface structure of α-uranophane and its interaction with Eu(III) – An integrated computational and fluorescence spectroscopy study

Uranophane is a rare U(VI) secondary silicate mineral that is relevant to the long-term performance of high level nuclear waste repositories. It can be formed under oxidizing conditions, potentially acting as an additional barrier to radionuclide migration through the accessible environment via mineral sorption reactions. To help understand the mechanisms involved in such sorption, a combination of theoretical calculations (classical molecular dynamics and ab initio density functional theory), and experimental work (sorption and laser induced fluorescence spectroscopy studies), have been employed to investigate the uranophane|water interface as well as the interfacial reactivity of the U(VI) silicate toward acidic conditions and radionuclide ion sorption. The combination of theoretical and experimental sorption studies help identify the molecular structure of the surface-sorbed species. Interfacial water is found to orient primarily with the hydrogen-atoms directed towards the negatively charged surface, with sorption sites belonging to three different groups: (1) those involving uranyl oxygen, (2) involving uranyl and silica hydroxyl oxygen atoms, and (3) involving hydroxyl hydrogen. Under basic conditions, deprotonation of the Si–OH groups is predicted to be responsible for uranophane dissolution, while protonation of bridging oxygens is likely responsible for acidic dissolution. Stable inner-sphere sorbed Eu(III) species are observed both experimentally and computationally. Moreover, Eu(III) is found to react with both protonated and deprotonated surface sites, indicating that radionuclides may sorb to the surface of uranophane under a broad range of pH conditions that alter the relative concentrations of protonated and deprotonated surface sites. This is in stark contrast to prior observations regarding other silicate minerals such as quartz, where sorption under basic conditions is enhanced.