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

Serpentinization and H2 production during an iron-clay interaction experiment at 90C under low CO2 pressure

International audienceInteractions between steel canisters and clay-rich material in deep geological repositories for high-level nuclear waste were studied by reacting metallic iron and a claystone from the Callovo-Oxfordian formation of the Paris Basin (COx). The experiment ran at 90 degrees C for 14 weeks in the presence of water and under 20 mbar of initial CO2 partial pressure (p(CO2)). At the end of the experiment, the pressure in the autoclave reached 50 bar due to H-2 production. Water was almost entirely consumed and poorly crystallized 7 A-greenalite-type clays were formed at the expense of illite and mixed-layered illite-smectite minerals (I/S). The dissolution of quartz and pyrite and the crystallization of pyrrhotite were also observed. Considering the results of Mossbauer and Fourier Transformed Infrared (FTIR) spectroscopies, iron carbonate and and/or ferrous hydroxycarbonate were suspected to precipitate but in very small amounts. A follow-up of the mineralogical evolution by XRD (X-ray diffraction), SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) shows that serpentinization of clays (formation of greenalite) is accompanied by a significant consumption of water and the production of H-2 gas due to the oxidation and dissolution of metallic iron. Experimental results are in good agreement with geochemical modelling with the PHREEQC code. The kinetics of the iron corrosion was evaluated from H-2 production and shows that diffusion processes are the rate limiting step

Raman spectra of Ni–Mg kerolite: effect of Ni–Mg substitution on O–H stretching vibrations

International audienceThe Ni-richmineral phases constituting the Ni-ores in New Caledonia are dominated inmost cases by kerolite–pimelite series, alsocalled talc-like minerals. Despite their economic and geologic interests, the crystallographic structures of these minerals are notfully understood. In order to improve the knowledge of their crystallographic structure, a set of natural talc-like minerals of varyingNi/Mg ratio was selected. A combination of SEM, EMPA, TEM, and Raman analysis shows that the studiedmineral contains onlyone mineral phase identified as the kerolite–pimelite series. Although kerolite and pimelite are not currently recognized as distinctminerals by the IMA, this study shows that these minerals exist as distinct minerals without any mixed layering with othermineral phases such as serpentine or other sheet silicates. The solid solution between the Ni and Mg endmembers was completewithout any gap, corresponding to a Ni–Mgsubstitution in the octahedral sheet. The Raman spectra in the OH stretching vibrationregion of representative samples covering the whole range of Ni–Mg substitution show a continuous and significant evolutionfrom the Mg to the Ni endmember. The signal was decomposed into nine Gaussian–Lorentzian functions, but the large overlappingof the peaks and the complexity of the band structure prevent any interpretation of the spectra from a structural point ofview. Finally, the problemis formalized as amultivariate curve resolution (MCR)which is solved using the Bayesian Positive SourceSeparation (BPSS) algorithm. This reveals that four possible arrangements of Ni and Mg in the octahedral sheet are encountered,and these arrangements are dependent on the Ni–Mg substitution rate. It also confirms that Raman microspectroscopy can bevery efficient in quick evaluation of the Ni content of the mineral