XPS determination of uranium oxidation states

This contribution is both a review of different aspects of X‐ray photoelectron spectroscopy that can help one determine U oxidation states and a personal perspective on how to effectively model the X‐ray photoelectron spectroscopy of complicated mixed‐valence U phases. After a discussion of the valence band, the focus lingers on the U4f region, where the use of binding energies, satellite structures, and peak shapes is discussed in some detail. Binding energies were shown to be very dependent on composition/structure and consequently unreliable guides to oxidation state, particularly where assignment of composition is difficult. Likewise, the spin orbit split 4f_7/2 and 4f_5/2 peak shapes do not carry significant information on oxidation states. In contrast, both satellite‐primary peak binding energy separations, as well as intensities to a lesser extent, are relatively insensitive to composition/structure within the oxide–hydroxide–hydrate system and can be used to both identify and help quantify U oxidation states in mixed valence phases. An example of the usefulness of the satellite structure in constraining the interpretation of a complex multivalence U compound is given. Copyright © 2011 John Wiley & Sons, Ltd.     

Positronium in solid phases of phenanthrene

Temperature dependence of positronium formation in phenanthrene was investigated. On crossing the solid-solid phase transition point in both directions a sharp decrease of positronium creation probability was observed. The effect is discussed in terms of formation of an unstable phase, and of possible pyroelectricity of phenanthrene.     

Kinetic study of uranium residue dissolution in ammonium carbonate media

The purpose of this study was to determine the kinetics of the dissolution of a uranium residue in ammonium carbonate media. The residue is generated in the production of medical isotopes. The effects of parameters, such as varying peroxide and carbonate concentrations, dissolution time as well as temperature on the extraction rate have been separately studied. Results indicate complete dissolution of the residue at 60 °C, after 30 min, in ammonium carbonate solution enriched with hydrogen peroxide. The yield and rate of uranium extraction were found to increase as a function of both temperature, in the range of 25–60 °C, and hydrogen peroxide concentration. The extraction process was governed by chemical reaction as the activation energy was found to be 45.5 kJ/mol. The order of reaction with respect to uranium concentration was found to be approximately first order.     

Positronium in solid phases of long-chain paraffins

The study of positronium intensity rise in long-chain alkanes shows two kinds of electron traps: the first, which are discharged at low temperature (≈200 K); the other, which remain populated up to the transition point to the rotator phase; e.g. in C₃₀H₆₂ they are still observed at 328 K.In the rigid phase of even-numbered alkanes o-Ps lifetime is shorter than that of odd-numbered ones, due to the difference in the width of gap between the molecular lamellae.     

Spectrophotometric determination of the oxygen to uranium ratio in uranium oxides based on dissolution in sulphuric acid

The oxygen to uranium ratio in uranium oxides such as U(3)O(8), UO(2+x) powders and UO(2) fuel pellets has been determined by a new spectrophotometric method. The method can be used for determination of O/U ratio in UO(2) pellets and powders on a routine basis. In the described method, uranium oxides in the powder form are dissolved in 2M sulphuric acid containing a few drops of HF. The concentrations of U(IV) and U(VI) are directly determined by means of the absorbances of these species at different wavelengths. For determination of the O/U ratio in U(3)O(8) powder samples, 630 and 310 nm are the wavelengths chosen for U(IV) and U(VI), respectively. For UO(2+x) powder, where the O/U ratio lies between 2.04 to 2.15, U(IV) and U(VI) are determined at 630 and 300 nm respectively, whereas for UO(2) fuel pellets, where the O/U ratio is less than 2.01, 535 and 285 nm are used. The molar absorptivity of U(IV) at 630 and 535 nm is 21.4 and 6.8 l.mole(-1).cm(-1) and that of U(VI) at 310, 300 and 285 nm is 178.1, 278.6 and 585 l.mole(-1).cm(-1), respectively. Standard deviations of +/-0.002 O/U ratio units for pellets and +/-0.004 O/U ratio units for powders have been achieved.     

Constant composition dissolution of mixed phases. II. Selective dissolution of calcium phosphates

Characterization of the dissolution kinetics of individual synthetic and biological calcium phosphates is of considerable importance since these phases often coexist in biological minerals. The constant composition method has been used to study the dissolution kinetics of a series of synthetic calcium phosphates, brushite (DCPD), beta-tricalcium phosphate (TCP), octacalcium phosphate (OCP), hydroxyapatite (HAP), and carbonated apatite (CAP) in the presence and absence of citric acid, as a function of pH and thermodynamic driving force. While citric acid markedly accelerates the dissolution of TCP, HAP dissolution is significantly inhibited. Moreover, this additive has almost no influence on the dissolution of DCPD, OCP, and CAP. Dual constant composition dissolution studies of mixed calcium phosphates in the presence of citric acid have also been made. Another factor, pH, also plays an important role in the dissolution of these calcium phosphates. In suspensions of calcium phosphate mixtures, specific phases can be selectively dissolved by changing experimental parameters such as pH and the presence of rate modifiers. This result has important applications for the dissolution control of dental hard tissues such as dentin, enamel, and calculus.     

Solution states of cellulose in selected direct dissolution agents

Properties of cellulose solutions in different direct dissolving liquids such as N-methylmorpholine-N-oxide and ionic liquids with varied cations and anions were investigated. The effects of different cations and anions of the used ionic liquids on the solution state were studied on the basis of the rheological characteristics of the resulting polymer solutions. The influence of these components is discussed in terms of zero shear viscosities, master curves with storage and loss moduli as well as complex viscosities using comparable molar ratios between cellulose and solvent and comparable polymer concentrations. Furthermore anisotropic properties of highly concentrated cellulose solutions were determined by means of polarised light microscopy and rheological methods subjected to the used solvent and variation of the polymer concentration as well as the temperature.     

Determination of oxidation states of uranium and plutonium in cooling solution of spent fuel element assemblies from NPP A-1

The knowledge of oxidation states of uranium and plutonium is necessary from the point of view of the proposed vitrification technology for the final liquidation of the cooling solution as a high radioactive waste containing alpha activity. The valence states of uranium and plutonium have a major influence on the subsequent leaching rate of these elements from vitrification matrices. Using the various chemical behaviours of these elements, in accordance with their valence states, we made an attempt to establish their oxidation state in the original solution.     

Multiparametric dissolution of thorium-cerium dioxide solid solutions

The dissolution of Th(1-x)Ce(x)O(2) solid solutions samples prepared by thermal conversion of oxalate precursors was studied by varying independently several parameters (such as chemical composition, leachate acidity, leaching temperature, firing temperature, and crystallization state). The relative effects of these parameters on the normalized dissolution rate were examined. Either the obtained partial order related to the proton activity (n = 0.50 ± 0.01) or the activation energy (E(A) = 57 ± 6 kJ.mol(-1)) suggested that the dissolution was mainly driven by surface reactions occurring at the solid/liquid interface. Conversely to that observed for Th(1-x)U(x)O(2) and Ce(1-x)Nd(x)O(2-x/2) solid solutions, the chemical composition did not induce strong modifications of the chemical durability of the leached samples. While the initial normalized dissolution rate slightly depended on the elimination of crystal defects for firing temperatures below 800 °C, it was mainly independent of the crystallite size (T ≥ 900 °C). The role of crystal defects (then that of preparation conditions) appeared thus important to consider since they contributed to modifications of the normalized dissolution rates of the same order of magnitude than that of the leachate acidity.     

A new method for alkaline dissolution of uranium metal foil

In order to develop a production process of ⁹⁹Mo by fission of low-enriched uranium, the first purification step, which consists of dissolution of a uranium metal foil target, was studied. It was found that alkaline NaClO gave good results, reaching the dissolution of up to 300 m of uranium foil. The different conditions for the dissolution were studied and the optimum ones were found. The influence of NaClO and NaOH concentration, temperature, dissolving solution volume per unit of surface and dissolution time were investigated. During this step, a gas, identified as H₂, was generated, and a precipitate characterized as Na₂U₂O₇ was observed. A stoichiometric reaction for this uranium dissolution is proposed.     

Stabilization of oxidation states in transition metals

Stabilization of oxidation states (OSs) for transition elements is considered. Distinctions between methods for stabilizing OSs in compounds in solution and in a solid state are discussed. The factors influencing stabilization are elucidated. Generation of high OSs of metals in alkali solutions is noticed. The specifics of stabilization of high OSs in solid compounds, including those in which mixed-valence effects are observed, and in solid matrix systems, in particular, in perovskite-related ones, are discussed. New data are cited concerning the stabilization of metal OSs in trans-dioxo complexes [MO₂L₄] ₂ ^z , where M = Mo, Tc, Ru, W, Re, or Os; and in the paramagnetic clusters that were discovered in neutral tungstates and hydroxo complexes of Rh, Ir, Pt, Pu, or Fe, and other elements in unusual high OSs.     

In vitro dissolution study of uranium dioxide and uranium ore with different particle sizes in simulated lung fluid

Inhalation is one of the most important routes for aerosol particles of uranium compounds to enter the body. The main step for uranium to be available for blood circulation and for interaction with bio-molecules is the dissolution of the particles. Particle size effects on dissolution of uranium dioxide and uranium ore were studied in simulated lung fluid using the “batch/filter” method. Samples were fractionated to ten size ranges from <0.43 μm to >10 μm by cascade impaction prior to dissolution experiments. Dependence of dissolution kinetics on particle size and on the amount of uranium trioxide contained in the particles was observed.     

Estimation of vanadium in different oxidation states

Summary In the present investigations, the different oxidation states of vanadium have been estimated volumetrically in mixtures with the help of visual indicators.

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Zusammenfassung Die verschiedenen Wertigkeitsstufen des Vanadiums wurden volumetrisch in Mischungen bestimmt. Zur Endpunktanzeige dienten visuelle Indicatoren.

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Thanks of the authors are due to the C.S.I.R. authorities for providing a Research Fellowship to one of them (R.K.M.).     

Neutron powder profile studies of the gamma uranium trioxide phases

Neutron powder profile studies show the existence of three phases in gamma uranium trioxide between 373°K and 77°K. The three phases are closely related and the transitions smooth and displacive. At 373°K, γ-UO₃ is tetragonal, with a = 6.9013 (5) and c = 19.9754 (18) Å, and space group $\text{I4}{}_1\text{amd(}\text{D}{}^{19}{}_{\mathrm{4h}}\text{)}$. At 323°K, γ-UO₃ becomes orthorhombic, space group F_ddd(D²⁴_2h), with the cell dimensions (293°K) a = 9.787 (3), b = 19.932 (4) and c = 9.705 (3) Å. There is a further transition between 293°K and 77°K, and, at 77°K, the orthorhombic dimensions of the pseudocell are a = 9.8225 (7), b = 19.8487 (15), and c = 9.6318 (7) Å. The neutron diffraction studies show that, in all three phases, the coordination polyhedra of the two crystallographically distinct uranium atoms are octahedral and (dodecahedral-2) respectively. At 293°K, the shortest UO distance is 1.796 (6) Å, and thus there are no pure uranyl bonds, in agreement with the infrared spectrum. The UO distances are precise to about ± 0.006 Å, about ten times the precision of an earlier X-ray single-crystal study, in which the conclusions were in conflict with the infrared spectrum. The structure is made up of parallel chains of edge-fused U(2) octahedra, cross-linked by U(1) dodecahedra. The atomic shifts are not great in going from 373°K to 77°K; at 293°K the data will refine in the pseudotetragonal cell as well as the true orthorhombic cell, and the 77°K data will refine in the F_ddd cell.     

Simultaneous determination of uranium carbide dissolution products by capillary zone electrophoresis

Separation and simultaneous determination of a number of organic acid anions (oxalate, mellitate, trimellitate and benzoate) and U(VI) with direct UV detection is developed for analysis of uranium carbide (UC) dissolution products by capillary zone electrophoresis (CZE). Reverse polarity mode is used. It is found that complex formation of U(VI) with carbonate, used as a carrier electrolyte, allows U(VI) to be determined, as negatively charged species, in a single run with organic acid anions. Some parameters such as pH value, composition of electrolyte and detection wavelength are optimized. Under the chosen conditions (carbonate buffer (ionic strength of 100 mM), pH 9.8, 0.15 mM tetradecyltrimethylammonium bromide (TTAB)) a complete separation is achieved. Calibration plots are linear in two ranges of concentration for U(VI) (∼1 × 10⁻⁵ to 1 × 10⁻³), mellitate and trimellitate (∼5 × 10⁻⁶ to 5 × 10⁻⁴), and about one range (∼1 × 10⁻⁴ to 5 × 10⁻³) for oxalate and benzoate. Accuracy of the procedure is checked by the “added-found” method in standard mixture solutions. Relative standard deviation is within the range of 2–10% and the recovery is in the range of 90–110%. This method is applied for the analysis of real UC dissolution samples.