E. Simoni

Solubility of actinide surrogates in nuclear glasses

Abstract This paper discusses the results of a study of actinide surrogates in a nuclear borosilicate glass to understand the effect of processing conditions (temperature and oxidizing versus reducing conditions) on the solubility limits of these elements. The incorporation of cerium oxide, hafnium oxide, and neodymium oxide in this borosilicate glass was investigated. Cerium is a possible surrogate for tetravalent and trivalent actinides, hafnium for tetravalent actinides, and neodymium for trivalent actinides. The material homogeneity was studied by optical, scanning electron microscopy. Cerium LIII XANES spectroscopy showed that the Ce3+/Cetotal ratio increased from about 0.5 to 0.9 as the processing temperature increased from 1100 to 1400 °C. Cerium LIII XANES spectroscopy also confirmed that the increased Ce solubility in glasses melted under reducing conditions was due to complete reduction of all the cerium in the glass. The most significant results pointed out in the current study are that the solubility limits of the actinide surrogates increases with the processing temperature and that Ce3+ is shown to be more soluble than Ce4+ in this borosilicate glass.

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.

Osteopontin: A Uranium Phosphorylated Binding-Site Characterization

Herein, we describe the structural investigation of one possible uranyl binding site inside a nonstructured protein. This approach couples spectroscopy, thermodynamics, and theoretical calculations (DFT) and studies the interaction of uranyl ions with a phosphopeptide, thus mimicking a possible osteopontin (OPN) hydroxyapatite growth-inhibition site. Although thermodynamical aspects were investigated by using time-resolved laser fluorescence spectroscopy (TRLFS) and isothermal titration calorimetry (ITC), structural characterization was performed by extended X-ray absorption fine structure (EXAFS) at the U LIII -edge combined with attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. From the vibrational and fluorescence spectra, several structural models of a UO2 (2+) /peptide complex were developed and subsequently refined by using theoretical calculations to fit the experimental EXAFS obtained. The structural effect of the pH value was also considered under acidic to moderately acidic conditions (pH 1.5-5.5). Most importantly, the uranyl/peptide coordination environment was similar to that of the native protein.

Structure of early actinides(V) in acidic solutions

Abstract Protactinium occupies a key position in the actinide series between thorium and uranium. In aqueous acidic solution, it is stable at oxidation state (V), occurring either as an oxocation or as a naked ion, depending on the media. Very few structural information on the hydration sphere of Pa(V) in acidic medium is available, in particular in hydrofluoric acid. Combined EXAFS and theoretical calculations have been used in this work to characterize the protactinium coordination sphere at various HF concentrations. The correlation of the XAFS data with quantum chemical calculations provides complementary structural and electronic models from ab initio techniques. At HF concentrations from 0.5 to 0.05 M, both theoretical calculations and EXAFS data suggest that the protactinium coordination sphere is mainly composed of fluoride ions. At the lowest HF concentration, the occurrence of a monooxo bond is observed with EXAFS, in agreement with the literature. A comparison of these data with related neptunium(V) and plutonium(V) diooxocations in perchloric acid is also presented.

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.

Molecular Characterization of Actinide Oxocations From Protactinium to Plutonium

This presentation addresses the structural characterization by EXAFS of actinide cations at oxidation states (V) and (VI) as one walks across the periodic table from Z = 91 (protactinium) to Z = 94 (plutonium). A structural comparison between Pa, U, Np and Pu oxocations in aqueous solution at formal oxidation states (V) and (VI) is carried out. These results are corroborated by quantum chemical and molecular dynamics calculations.

Molecular solids of actinide hexacyanoferrate: Structure and bonding

The hexacyanometallate family is well known in transition metal chemistry because the remarkable electronic delocalization along the metal-cyano-metal bond can be tuned in order to design systems that undergo a reversible and controlled change of their physical properties. We have been working for few years on the description of the molecular and electronic structure of materials formed with [Fe(CN)6]n- building blocks and actinide ions (An = Th, U, Np, Pu, Am) and have compared these new materials to those obtained with lanthanide cations at oxidation state +III. In order to evaluate the influence of the actinide coordination polyhedron on the three-dimensional molecular structure, both atomic number and formal oxidation state have been varied : oxidation states +III, +IV. EXAFS at both iron K edge and actinide LIII edge is the dedicated structural probe to obtain structural information on these systems. Data at both edges have been combined to obtain a three-dimensional model. In addition, qualitative electronic information has been gathered with two spectroscopic tools : UV-Near IR spectrophotometry and low energy XANES data that can probe each atom of the structural unit : Fe, C, N and An. Coupling these spectroscopic tools to theoretical calculations will lead in the future to a better description of bonding in these molecular solids. Of primary interest is the actinide cation ability to form ionic ? covalent bonding as 5f orbitals are being filled by modification of oxidation state and/or atomic number.