Abstract We describe the synthesis of cerium, thorium and uranium oxide nanoparticles embedded in a mesoporous matrix as template in a kind of nanocasting technique. The solid matrix is used as a template to obtain and stabilize the actinide oxide nanoparticles. We apply high resolution transmission electron microscopy (HR-TEM) to show evidence of metal oxide incorporation into the matrix pores and analyze their structure. Measured interplanar distances and calculated lattice parameters for synthesized nanosized CeO2−x and ThO2 samples differ from their bulk crystalline counterparts. We obtain with our synthesis CeO2−x particles containing both Ce4+ and larger sized Ce3+. The lattice parameter for these ceria nanoparticles is found to be larger than the bulk value due to the presence of Ce3+ with its larger ionic radius. The presence of Ce3+ was established by means of high resolution X-ray emission spectroscopy (HRXES), applied to the investigation of nanoparticles for the first time. The ThO2 nanoparticles exhibit a decrease in interplanar distances, as one might generally expected for these nanoclusters. However, the lattice distance decrease for our particles is remarkable, up to 5%, indicating that contact with the surrounding silica matrix may exert a bond distance shortening effect such as through significant external pressure on the particle surface.

Interaction of U(VI), Np(V), and Pu(IV, V) ions with colloidal particles of amorphous SiO2 under the conditions simulating disposal sites of radioactive wastes and spent nuclear fuel was studied. Uranium and plutonium are quantitatively sorbed on the colloidal particles, which creates prerequisites for the colloidal transport of actinides.

High resolution X-ray emission spectroscopy (HRXES) is becoming increasingly important for our understanding of electronic and coordination structures. The combination of such information with development of quantum theoretical tools will advance our capability for predicting reactivity and physical behavior especially of 5f elements. HRXES can be used to remove lifetime broadening by registering the partial fluorescence yield emitted by the sample (i.e., recording a windowed signal from the energy dispersed fluorescence emission while varying incident photon energy), thereby yielding highly resolved X-ray absorption fine structure (XAFS) spectra. Such spectra often display resonant features not observed in conventional XAFS. The spectrometer set-up can also be used for a wide range of other experiments, for example, resonant inelastic X-ray scattering (RIXS), where bulk electron configuration information in solids, liquids and gases is obtained. Valence-selective XAFS studies, where the local structure of a selected element's valence state present in a mixture of valence states can be obtained, as well as site-selective XAFS studies, where the coordination structure of a metal bound to selected elements can be differentiated from that of all the other ligating atoms. A HRXES spectrometer has been constructed and is presently being commissioned for use at the INE-Beamline for actinide research at the synchrotron source ANKA at FZK. We present the spectrometer's compact, modular design, optimized for attaining a wide range of energies, and first test measurement results. Examples from HRXES studies of lanthanides, actinides counter parts, are also shown.

The elemental and ionic composition of uranium oxides UO2+x was determined using a procedure based on characteristics of the structure of the X-ray photoelectron spectra of outer and inner electrons of various uranium oxides. Although the bulk composition of the initial sample was close to the stoichiometry of UO2, the surface had the composition UO2.26 (6% U4+, 68% U5+, and 26% U6+, XPS). Samples kept at 70 and 150°C for a long time in simulated natural water (pH 7.0 ± 0.1) containing Ca2+, Na+, K+, and HCO 3 − ions were also examined. In the course of 6-month leaching, the fluorite-like structure of UO2+x is preserved, but secondary U6+ phases, including schoepite, are formed on the sample surface. The oxygen coefficient k O = 2 + x in UO2+x on the surface of the examined samples was determined by XPS in relation to the temperature and time of keeping in water.

The study of a group of minerals including schoepite (UO2)(8)O-2(OH)(12)(H2O)(12) and metaschoepite (UO2)(4)O(OH)(6)(H2O)(5) on the basis of theoretical analysis of U L-3-edge XANES spectra was carried out. The experimental U L-3-edge X-ray absorption spectra of schoepite were measured in the synchrotron centre "ANKA" (of Karlsruhe, Germany).The theoretical analysis of the experimental data was made by full multiple-scattering method and finite differences method. (FEFF8.4 and FDMNES 2007 codes). The influence of non-muffin-tin effects on the shape of U L-3-XANES spectra of the compounds described above has been investigated. And it was shown that non-muffin-tin effects influenced on the energy position of the U L-3-edge XANES. (C) 2009 Elsevier B.V. All rights reserved.

Neptunium, Np(V) and Np(IV), sorption onto uranium dioxide surface was studied at various values of pH. Sorption was studied in two sets of experiments with different redox conditions that correspond to either Np(V) (Set 1) or Np(IV) (Set 2) in solution. In Set 1 the reduction of Np(V) was established when low pH solution covered a UO2 surface. When the pH increased, the sorption of neptunium is decreased. At pH>5.5 neptunium sequestration from solution is governed by Np(V) sorption onto UO2.25. In Set 2 (the more anoxic conditions) complete neptunium sorption is established at pH>2: it is present in the tetravalent form over the whole pH range. The proposed mechanisms of neptunium sorption was suggested by using pH sorption edges of Th(IV) as an analog to Np(IV) onto UO2 and Np(V) onto ThO2. The UO2 surface was characterized by X-ray photoelectron spectroscopy (XPS) after equilibration with aqueous solutions at different pH values.

Interaction of uranium dioxide with highly mobile radionuclides 237Np and 99Tc was studied under oxidative conditions. Sorption of these radionuclides at different pH was measured, and the mechanism of redox reaction occurring in the course of their sorption were determined. In alkaline solution, Np(V) is reduced on the UO2+x surface and is sorbed in the form of tetravalent species. In neutral solutions, Np is sorbed in the form of Np(V). This is due to the fact that the stoichiometry of the UO2+x surface corresponds to U4O9. In acid solution, U(VI) is leached to form surface UO2. Although the free surface area of a UO2+x sample is low, the Np distribution coefficients K d at pH > 6 are relatively high: log K d > 2. Unlike Np, Tc(VII) is not reduced on the UO2+x surface. However, the sorption capacity of uranium dioxide for Tc(IV) is high.

The solubility of three UO2 samples that were synthesized at different temperatures in reducing atmosphere and having different composition of surface oxidized layer was studied in distilled water and tuff groundwater (J-13). The solubility of the samples was governed by the presence of U(VI) in the surface oxidized layer. The sorption of Np(V) on those samples was accompanied by its partial reduction upon contact with UO2 (samples). For Np(IV) a partial oxidation was observed at pH values between 4.5 and 6.5. The most oxidized sample, with the highest U(VI) content in the surface layer, despite the highest solubility, was more efficient in the sorption of Np(V).

Possibilities of atomic tritium application as surface nanoprobe for structural investigations of adsorption layers on the liquid–air interface have been demonstrated. Frozen aqueous solutions of a series of amino acids and their mixtures and one well-known surface-active substance (cetyltrimethylammonium bromide, CTAB) were exposed to bombardment by tritium atoms generated on hot tungsten wire in a special vacuum device. This procedure resulted in substitution of hydrogen atoms by radioactive tritium in the thin surface layer of investigated samples. Curves of radioactivity changes depending on bombardment time and solution concentration for applied compounds were obtained and analyzed.