Uranyl interaction with the hydrated (0001) basal face of gibbsite: A combined theoretical and spectroscopic study

International audienceThe sorption of uranyl cations and water molecules on the basal (001) face of gibbsite was studied by combining vibrational and fluorescence spectroscopies together with density functional theory ͑DFT͒ computations. Both the calculated and experimental values of O–H bond lengths for the gibbsite bulk are in good agreement. In the second part, water sorption with this surface was studied to take into account the influence of hydration with respect to the uranyl adsorption. The computed water configurations agreed with previously published molecular dynamics studies. The uranyl adsorption in acidic media was followed by time-resolved laser-induced fluorescence spectroscopy and Raman spectrometry measurements. The existence of only one kind of adsorption site for the uranyl cation was then indicated in good agreement with the DFT calculations. The computation of the uranyl adsorption has been performed by means of a bidentate interaction with two surface oxygen atoms. The optimized structures displayed strong hydrogen bonds between the surface and the-yl oxygen of uranyl. The uranium-surface bond strength depends on the protonation state of the surface oxygen atoms. The calculated U – O surface bond lengths range between 2.1–2.2 and 2.6– 2.7 Å for the nonprotonated and protonated surface O atoms, respectively

Effect of a thermal gradient on iron-clay interactions

Disposal facilities in deep geological formations are considered to be a possible solution for long-term management of high-level nuclear waste (HLW). The design of the repository generally consists of a multiple-barrier system including Fe-based canisters and a clay backfill material. The Fe-clay system will undergo a thermal gradient in time and space, thehe at source being the HLW insidetheca nisters. In the present paper, the effect of a thermal gradient in space on Fe-smectite interactions was investigated. For this purpose, a tube-in-tube experimental device was developed and an 80-300°C thermal gradient was applied to a mixture of MX80 bentonite, metallic Fe (powder and plate), magnetite, and fluid over periods of 1 to 10 months. Transformed and newly formed clay minerals were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Mössbauer spectroscopy. The main mineralogical transformations were similar to those described for batch experiments: smectite was destabilized into an Fe-enriched trioctahedral smectite and Fe-serpentine or chlorite as a function of the experimental conditions. Newly formed clay was observed all along the walls of the gold tube. Their crystal chemistry was clearly different from the clays observed in the hot and cold part of the tubes. The thermal diffusion of elements was also observed, especially that of Mg, which migrated toward the hottest parts of the tubes. In the end, the thermal gradient affected the redox equilibria; more reduced conditions were observed in the hotter parts of the tubes

Mineralogical evolution of a claystone after reaction with iron under thermal gradient

The design of the repository for high-level nuclear waste (HLW) in France consists of a multiple-barrier system including steel canisters in a clay host rock. The system will undergo temperature variations in time and space, the heat source being the HLW within the canisters. The effect of a thermal gradient in space on the Fe-claystone interaction was investigated here by applying a thermal gradient (150-300°C and 80-150°C) to a mix of claystone, Fe, and an aqueous chloride solution over periods of 3 and 6 months. Following the reaction, the starting clay minerals (mostly illite and mixed-layer illite smectite) evolved toward chlorite, Fe-serpentine, Fe-saponite, mixed-layer chlorite-smectite, or mixed-layer serpentine-smectite as a function of temperature. Iron corrosion made the medium basic and reductive. Magnesium enrichment of clay minerals was observed in the hottest part of the experiment due to Mg migration under the thermal gradient. Reaction progress was enhanced at the lowest temperatures, compared to batch experiments

Sulphide mineral reactions in clay-rich rock induced by high hydrogen pressure. Application to disturbed or natural settings up to 250 degrees C and 30 bar

International audienceAs abiotic hydrogen redox reactivity is kinetically limited, most of the possible redox reactions induced by hydrogen remain insignificant at low temperature, even at a geologic time scale. This is the case for sulphate and carbonate reduction, but some H-2 induced redox reactions may be significant at low temperature conditions, in particular the pyrite reduction into pyrrhotite. In this experimental study, the geochemical impact of hydrogen in a clay-rich rock containing 1-2 wt.% framboidal pyrite is evaluated under mid-hydrothermal condition (90 to 250 degrees C) and with hydrogen partial pressure ranging from 3 to 30 bar. This study demonstrates that the main geochemical perturbation induced by hydrogen in a claystone host-rock formation is the destabilisation of pyrite, which leads to the production of sulphide. Two different reaction mechanisms can be distinguished as a function of temperature and hydrogen pressure. When temperature is lower than 150 degrees C and the hydrogen partial pressure is below 6 bar, pyrite solubility controls the sulphide concentration at low values. By contrast, at higher temperature or at higher hydrogen partial pressure, the rate and extent of the reaction are driven by pyrrhotite precipitation. A complete replacement of pyrite into pyrrhotite occurs within a short interval of time at T > 90 degrees C and P(H-2) > 10 bar. The pH of the media is also a critical parameter controlling the extent of the reaction as alkaline conditions may promote pyrrhotite precipitation at lower temperature and hydrogen pressure. These conclusions have a direct application in the safety assessment of nuclear waste repositories, but may also be extended to other contexts such as the underground storage of hydrogen, and, given the wide range of temperatures used in the experiments, can even be applied to hydrothermal-hosted submarine systems where hydrogen is naturally produced by olivine serpentinization