Romuald Drot

Structural identification of europium(III) adsorption complexes on montmorillonite

A study of trivalent europium retention onto Na-montmorillonite is presented, combining both macroscopic and microscopic points of view. In order to investigate the metal sorption mechanisms at a molecular level and therefore experimentally identify both clay reactive sites and sorption equilibria, laser-induced fluorescence spectroscopy (LIF) and X-ray photoelectron spectroscopy (XPS) on europium ion loaded montmorillonite have been performed. Moreover, since this clay is an alumino-silicated mineral, we have interpreted our experimental results in terms of interactions between a metal ion and a cation exchange site, and distinct “aluminol” and “silanol” edge sites. Therefore, identical structural investigations have been carried out on both Eu/alumina and Eu/silica systems. These comparisons have allowed us to determine the nature of the europium surface complexes and thus led to an experimental definition of the sorption equilibria involved in the retention process. The obtained lifetime values and the Eu 3d XPS spectra of europium sorbed on the three solids have shown that this metal is sorbed, on the montmorillonite clay, on exchange sites as an outer-sphere complex and onto both “aluminol” and “silanol” edge sites as inner-sphere surface complexes, depending on the pH value and the ionic strength of the suspension.

Sorption of uranium (VI) species on zircon: structural investigation of the solid/solution interface

This work is an investigation of the mechanisms of interaction between uranium (VI) ions and zirconium silicate. The speciation of uranium (VI) sorbed on zircon was studied using four complementary techniques as probes of the local structure around the uranium atom: laser spectrofluorimetry, X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier-transformed (DRIFT) spectroscopy, and EXAFS spectroscopy. The sorption of uranyl on zirconium oxide was also studied to allow structural comparisons. Spectrofluorimetry and XPS results allowed an identification of the silicate sorption sites on the solid. These methods associated with spectrofluorimetry and DRIFT led to a characterization of the sorbed surface complexes, taking into account the influence of the nature of the background salt and of the pH on the structure of the U(VI) surface species. EXAFS measurements, either on air-dried samples or in situ, were then carried out on well-characterized samples and allowed identification of the sorption mechanism on zircon as the formation of an inner-sphere polydentate surface complex.

Theoretical Investigation of the Uranyl Ion Sorption on the Rutile TiO2(110) Face

Canister integrity and radionuclide retention is of first importance for assessing the long-term safety of nuclear waste stored in engineered geologic depositories. Uranyl ion sorption on the TiO(2) rutile (110) face is investigated using periodic density functional theory (DFT) calculations. From experimental observations, only two uranyl surface complexes are observed and characterized. When the pH increases (from 1.5 to 4.5), the relative ratios of these two surface complexes are modified. From a crystallographic point of view, three sorption sites can be considered and have been studied with different protonation states of the surface to account for very acidic and low acidic conditions. The two surface complexes experimentally observed were calculated as the most stable ones, while the evolution of their sorption energies agrees with experimental data.

Theoretical first step towards an understanding of the uranyl ion sorption on the rutile TiO2(110) face: A DFT periodic and cluster study

First results of a periodic and cluster Density Functional Theory (DFT) study of the uranyl ion (UO22+) sorption onto the rutile TiO2(110) face, based on plane wave and localised basis sets, are presented. A five layers slab with its most internal layer frozen to bulk positions was found to be a good surface model. In a first step and as reference data for the sorption process, the [UO2(H2O)n]2+ systems, with n=4 to 6 were studied. Relative solvation energies confirmed that the uranyl ion adopt a pentacoordinated structure in aqueous solution. From localised approach, an overall 0.91 electron transfer from the first hydration shell to the uranyl ion was calculated. Then, a periodic study of the uranyl sorption on a simplified hydroxylated TiO2(110) surface model was investigated. The resulting optimised structural parameters, for the three possible adsorption sites, show that the sorbed uranyl ion first coordination shell (saturated by three water molecules) plays an important role to model the adsorption process. Both methodologies (plane waves and localised atomic orbitals) were also used with a cluster model and gave similar results in agreement with experimental data. This first step in the understanding of the uranyl ion sorption onto the simplified hydroxylated TiO2(110) surface shows that hydrogen bonds should be included in the model in order to perform a more accurate description of the uranyl ion sorption process. A study with this surface model is currently performed in order to calculate the relative stabilities between the different uranyl adsorption sites and to compare with the experimental data.

Structural environment of uranium (VI) species sorbed onto ZrP2O7: X-ray absorption spectroscopy study

Abstract The structure of the surface complex formed during the sorption of UO 2 2+ aq ion onto the selected phosphate solid has been investigated mainly using X-ray absorption spectroscopy. Samples were prepared by batch experiments. L III U edge measurements have shown that uranyl ions are sorbed on the phosphate surface as a mononuclear bidentate inner-sphere complex.