Oxidation State Of Uranium Research Articles

238U-235U-234U fractionation between tetravalent and hexavalent uranium in seafloor phosphorites

Variations in the 238U/235U ratio are mostly observed in association with changes in the uranium oxidation state and therefore controlled by changes in the redox conditions, although evidence for this process has so far been indirect. Here, the δ238U and δ234U isotope composition of different redox species is studied for the first time within the same geological samples: the bulk, reduced (U(IV)) and oxidized (U(VI)) uranium species in seafloor phosphorites. In all cases, δ238U(IV) is higher (−0.27 to −0.81) than corresponding δ238U(VI) (−0.64 to −1.07), with δ234U(VI) displaying extremely high values (~500–2000) relative to δ234U(IV) (−240 to −100). By comparison, the bulk δ238U, δ234U and U concentrations are −0.42 to −0.85, −10 to +20, and 63–328ppm, respectively. These values are mostly in the range of natural variations in previously reported samples, with the bulk δ238U and δ234U values corresponding with seawater values except for a tail in δ238U toward lower values. The main exception is displayed by the composition of the U(VI) fraction, which ranges toward relatively low δ238U values but has a very strong positive δ234U excursion relative to comparable samples elsewhere.Given that the studied phosphorites formed in high productivity environments, where oxygen consumption was high and hence anoxic conditions could have been favored, it is assumed that most of the initial uranium in the samples was in reduced form. Indeed, even if some of the U oxidized over time, the studied samples still consist of approximately 60–80% U(IV). The process of 238U radioactive decay resulted in the oxidation of the decay product, 234U, and consequently, the δ234U(VI) within the samples has very high values. The evolution of δ238U is related to the effect of the ‘nuclear field shift’, which predicts that nuclides with higher atomic masses will be reduced preferentially over those with lower atomic masses. Accordingly, it is easier to reduce 238U than 235U in the same environment, which would result in higher δ238U of U(IV). This is indeed the case observed here, although in addition, the evidence here shows that the δ238U fractionation occurred during U oxidation and that the final differences of δ238U between both oxidation states reflect the combined effect of the depositional nuclear field shift and the in-situ recoil-related oxidation.A set of quantitative models are used to evaluate the role and rate of the different processes and suggest that because of the independent evolution of U(IV) and U(VI) over time, their relative fraction and isotopic compositions can be used to evaluate the formation ages of seafloor phosphorites, which has so far not been possible using U-decay series.

Direct Determination of Oxidation States of Uranium in Mixed-Valent Uranium Oxides Using Total Reflection X-ray Fluorescence X-ray Absorption Near-Edge Spectroscopy.

Total reflection X-ray fluorescence (TXRF)-based X-ray absorption near-edge spectroscopy has been used to determine the oxidation state of uranium in mixed-valent U3O8 and U3O7 uranium oxides. The TXRF spectra of the compounds were measured using variable X-ray energies in the vicinity of the U L3 edge in the TXRF excitation mode at the microfocus beamline of the Indus-2 synchrotron facility. The TXRF-based X-ray absorption near-edge spectroscopy (TXRF-XANES) spectra were deduced from the emission spectra measured using the energies below and above the U L3 edge in the XANES region. The data processing using TXRF-XANES spectra of U(IV), U(V), and U(VI) standard compounds revealed that U present in U3O8 is a mixture of U(V) and U(VI), whereas U in U3O7 is mixture of U(IV) and U(VI). The results obtained in this study are similar to that reported in literature using the U M edge. The present study has demonstrated the possibility of application of TXRF for the oxidation state determination and elemental speciation of radioactive substances in a nondestructive manner with very small amount of sample requirement.

On the photophysics and speciation of actinide ion in MgAl2O4 spinel using photoluminescence spectroscopy and first principle calculation: A case study with uranium

Actinide chemistry is very interesting not from scientific perspective but also from technological importance. Elucidating the valence state and coordinating environment of actinide ion like uranium in technologically important magnesium aluminate spinel (MAS) is important to fully understand its hazardous and other harmful effect in human as well as environment. Magnesium aluminate spinel doped with 1.0 mol % of Uranium ion has been synthesized using citric acid assisted gel-combustion route at 800 °C. The as prepared powder is characterised using X-ray diffraction (XRD), time resolved photoluminescence spectroscopy (TRPLS) and density functional theory (DFT) calculations. Uranium is an interesting element because it exhibits multiple oxidation state and each one of them is having characteristics fluorescence behavior. TRPLS is used to investigate the oxidation state and coordination behavior of uranium in MgAl2O4. Indeed in our earlier work on undoped and lanthanide ion doped MAS; it was oberved that in undoped sample itself defect induced emission could be seen in visible region which was probed using DFT. Here on doping uranium in MAS; complete energy of host is transferred to uranium ion which is explained using DFT. From excitation and emission spectroscopy it was observed that uranium stabilizes in +6 oxidation state in the form of UO22+ ion. Based on luminescence lifetime and its comparison with the emission profile of uranyl fluoride crystal it was inferred that majority of uranium is occupying relatively asymmetric MgO4 polyhedra and minority substitutes AlO6. The site stability of the uranyl ion in MAS was also validated using DFT based first principle calculations. Time resolved emission shows the uranyl at Mg2+ site differs from the one at Al3+ site in terms of peak position and intensity.

Thorium effect on the oxidation of uranium: Photoelectron spectroscopy (XPS/UPS) and cyclic voltammetry (CV) investigation on (U1 − xThx)O2 (x = 0 to 1) thin films

Thin films of U1−xThxO2 (x=0 to 1) have been deposited via reactive DC sputter technique and characterized by X-ray/Ultra-violet Photoelectron Spectroscopy (XPS/UPS), X-ray Powder Diffractometer (XRD) and Cyclic Voltammetry (CV) in order to understand the effect of Thorium on the oxidation mechanism. During the deposition, the competition between uranium and thorium for oxidation showed that thorium has a much higher affinity for oxygen. Deposition conditions, time and temperature were also the subject of this study, to look at the homogeneity and the stability of the films. While core level and valence band spectra were not altered by the time of deposition, temperature was affecting the oxidation state of uranium and the valence band due to the mobility increase of oxygen through the film. X-ray diffraction patterns, core level spectra obtained for U1−xThxO2 versus the composition showed that lattice parameters follow the Vegard's law and together with the binding energies of U-4f and Th-4f are in good agreement with literature data obtained on bulk compounds. To study the effect of thorium on the oxidation of U1−xThxO2 films, we used CV experiments at neutral pH of a NaCl solution in contact with air. The results indicated that thorium has an effect on the uranium oxidation as demonstrated by the decrease of the current of the oxidation peak of uranium. XPS measurements made before and after the CV, showed a relative enrichment of thorium at the extent of uranium at the surface supporting the formation at a longer term of a thorium protective layer at the surface of uranium-thorium mixed oxide.

Effect of Ce doping on UO2 structure and its oxidation behavior

Abstracts The structure of U1-yCeyO2 was studied in terms of added Ce cation. The oxygen vacancy, the oxidation state of cerium and uranium, and changes in the lattice parameter were investigated. Most Ce ions exist as tetravalent ions, which cause a lattice contraction due to the ionic radius. However, some Ce ions exist as trivalent Ce3+ ions, causing the oxidation of U4+ into higher oxidation states as well as a formation of oxygen vacancy for a charge balance. In addition, the effect of Ce on the oxidation of uranium dioxide was also studied by thermo-gravimetric analysis. The oxidation of Ce doped UO2 proceeds through dominant path given as two step reaction U1-yCeyO2-δ → ( U 1 − y Ce y 4 + )4O9 → ( U 1 − y Ce y 4 + )3O8. However, for higher Ce content (y = 0.136 to 0.332), an alternative pathway is proposed as ( U 1 − y Ce y 4 + )4O9 → ( U 1 − y Ce y 3 + )4O9. The increasing Ce content stabilizes the cubic (U1-yCey)4O9 phase, prohibits the formation of a metastable tetragonal (U1-yCey)3O7 phase, and inhibits the formation of (U1-yCey)3O8 proportional to the Ce content.

XPS Study of Ion Irradiated and Unirradiated UO2 Thin Films.

XPS determination of the oxygen coefficient kO = 2 + x and ionic (U(4+), U(5+), and U(6+)) composition of oxides UO2+x formed on the surfaces of differently oriented (hkl) planes of thin UO2 films on LSAT (Al10La3O51Sr14Ta7) and YSZ (yttria-stabilized zirconia) substrates was performed. The U 4f and O 1s core-electron peak intensities as well as the U 5f relative intensity before and after the (129)Xe(23+) and (238)U(31+) irradiations were employed. It was found that the presence of uranium dioxide film in air results in formation of oxide UO2+x on the surface with mean oxygen coefficients kO in the range 2.07-2.11 on LSAT and 2.17-2.23 on YSZ substrates. These oxygen coefficients depend on the substrate and weakly on the crystallographic orientation. On the basis of the spectral parameters it was established that uranium dioxide films AP2,3 on the LSAT substrates have the smallest kO values, and from the XRD and EBSD results it follows that these samples have a regular monocrystalline structure. The XRD and EBSD results indicate that samples AP5-7 on the YSZ substrates have monocrystalline structure; however, they have the highest kO values. The observed difference in the kO values was probably caused by the different nature of the substrates: the YSZ substrates provide 6.4% compressive strain, whereas (001) LSAT substrates result only in 0.03% tensile strain in the UO2 films. (129)Xe(23+) irradiation (92 MeV, 4.8 × 10(15) ions/cm(2)) of uranium dioxide films on the LSAT substrates was shown to destroy both long-range ordering and uranium close environment, which results in an increase of uranium oxidation state and regrouping of oxygen ions in uranium close environment. (238)U(31+) (110 MeV, 5 × 10(10), 5 × 10(11), 5 × 10(12) ions/cm(2)) irradiations of uranium dioxide films on the YSZ substrates were shown to form the lattice damage only with partial destruction of the long-range ordering.

Chemical speciation of U, Fe, and Pu in melt glass from nuclear weapons testing

Nuclear weapons testing generates large volumes of glassy materials that influence the transport of dispersed actinides in the environment and may carry information on the composition of the detonated device. We determine the oxidation state of U and Fe (which is known to buffer the oxidation state of actinide elements and to affect the redox state of groundwater) in samples of melt glass collected from three U.S. nuclear weapons tests. For selected samples, we also determine the coordination geometry of U and Fe, and we report the oxidation state of Pu from one melt glass sample. We find significant variations among the melt glass samples and, in particular, find a clear deviation in one sample from the expected buffering effect of Fe(II)/Fe(III) on the oxidation state of uranium. In the first direct measurement of Pu oxidation state in a nuclear test melt glass, we obtain a result consistent with existing literature that proposes Pu is primarily present as Pu(IV) in post-detonation material. In addition, our measurements imply that highly mobile U(VI) may be produced in significant quantities when melt glass is quenched rapidly following a nuclear detonation, though these products may remain immobile in the vitrified matrices. The observed differences in chemical state among the three samples show that redox conditions can vary dramatically across different nuclear test conditions. The local soil composition, associated device materials, and the rate of quenching are all likely to affect the final redox state of the glass. The resulting variations in glass chemistry are significant for understanding and interpreting debris chemistry and the later environmental mobility of dispersed material.

Oxidation state and coordination environment of uranium in sodium iron aluminophosphate glasses

An analysis of the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) of uranium determined the oxidation state and coordination environment of uranium atoms in glasses containing 40 mol % Na2O, 10 mol % Al2O3, 10 mol % Fe2O3, and 40 mol % P2O5 to which uranium oxides were added to a concentration of 50 wt % (above 100%). If the added amount of UO2 was small, uranium occurred as U(IV) in a near-octahedral oxygen environment with an average U–O distance in the first coordination sphere of 2.25 A. At higher concentrations of uranium oxides introduced both as UO2 and as UO3, uranium occurred as U(V) and U(VI); the first coordination sphere is split; shorter (~1.7–1.8 A) and longer (2.2–2.3 A) distances were observed, which corresponded to the axial and equatorial U–O bonds in uranyl ions, respectively; and the redox equilibrium shifted toward U(VI). The glass with the maximal (~33 wt %) UO3 concentration contained mainly U(VI). The existence of low-valence uranium species can be related to the presence of Fe(II) in glasses. The second coordination sphere of uranium manifests itself only at high concentrations of uranium oxides.

XPS Investigation on Changes in UO2 Speciation following Exposure to Humidity

ABSTRACT High purity UO2 powder samples were subjected to accelerated aging under controlled conditions with relative humidity ranging from 34% to 98%. Characterization of the chemical speciation of the products was accomplished using X-ray photoelectron spectroscopy (XPS). A shift to higher uranium oxidation states was found to be directly correlated to increased relative humidity exposure. Additionally, the relative abundance of O2-, OH-, and H2O was found to vary with exposure time. Thus, it is expected that uranium oxide materials exposed to high relative humidity conditions during processing and storage would display a similar increase in average uranium valence.

Revealing the oxidation number and local coordination of uranium in Nd2Zr2O7 pyrochlore: A photoluminescence study

Uranium doped Nd2Zr2O7 is synthesized at 800°C (NUZO-800) using gel-combustion method and characterized using X-ray diffraction, scanning electron microscopy (SEM), X-ray photoelectron microscopy (XPS) and time-resolved photoluminescence (TRPL). As prepared sample was annealed further at 900, 1000, 1100 and 1200°C leading to increase in the size of the particle and the effect of same has been investigated on the photophysical properties of NUZO. Based on excitation and emission spectroscopy it was inferred that uranium stabilizes as +6 oxidation state in the form of UO66−. XPS measurement also establishes +6 oxidation state of uranium in Nd2Zr2O7. Both emission intensity and photoluminescence (PL) decay time increases with increase in temperature till 1000°C beyond which lifetime saturates whereas emission intensity decreases. The decay curve shows two different chemical environments for uranate ion in Nd2Zr2O7 pyrochlore matrix.

Use of HERFD-XANES at the U L3- and M4-Edges To Determine the Uranium Valence State on [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3

We report and discuss here the unambiguous uranium valence state determination on the complex compound [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] by using high-energy-resolution fluorescence detection-X-ray absorption near-edge structure spectroscopy (HERFD-XANES). The spectra at both U L3- and M4-edges confirm that all five nonequivalent U atoms are solely in the hexavalent form in this compound, as previously suggested by bond-valence-sum analysis and X-ray diffraction pattern refinement. Moreover, the presence of the preedge feature, due to the 2p3/2-5f quadrupole transition, has been observed in the U L3-edge HERFD-XANES spectrum, in agreement with theoretical and experimental observations of other uranium-based compounds. Recently, this feature has been proposed as a possible tool to determine the uranium oxidation state in a manner similar to that of 3d and 4d metals. Nevertheless, this feature is also very sensitive to the uranium local environment, as revealed by our theoretical calculations, and consequently could not be used to attribute without ambiguity the uranium valence state. In contrast, U M4-edge HERFD-XANES appears to be the most straightforward and reliable way to assess the uranium valence state in very complex materials such as [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] or a mixture of compounds.

Actinide Oxidation State and O/M Ratio in Hypostoichiometric Uranium-Plutonium-Americium U0.750Pu0.246Am0.004O2-x Mixed Oxides.

Innovative americium-bearing uranium-plutonium mixed oxides U1-yPuyO2-x are envisioned as nuclear fuel for sodium-cooled fast neutron reactors (SFRs). The oxygen-to-metal (O/M) ratio, directly related to the oxidation state of cations, affects many of the fuel properties. Thus, a thorough knowledge of its variation with the sintering conditions is essential. The aim of this work is to follow the oxidation state of uranium, plutonium, and americium, and so the O/M ratio, in U0.750Pu0.246Am0.004O2-x samples sintered for 4 h at 2023 K in various Ar + 5% H2 + z vpm H2O (z = ∼ 15, ∼ 90, and ∼ 200) gas mixtures. The O/M ratios were determined by gravimetry, XAS, and XRD and evidenced a partial oxidation of the samples at room temperature. Finally, by comparing XANES and EXAFS results to that of a previous study, we demonstrate that the presence of uranium does not influence the interactions between americium and plutonium and that the differences in the O/M ratio between the investigated conditions is controlled by the reduction of plutonium. We also discuss the role of the homogeneity of cation distribution, as determined by EPMA, on the mechanisms involved in the reduction process.

ChemInform Abstract: Uranium‐Mediated Oxidative Addition and Reductive Elimination

AbstractReview: 115 refs.

Probing local coordination and oxidation state of uranium in ThO2: U nanostructured

Uranium doped thorium oxide nanoparticle (UDT) was synthesized using citric acid assisted combustion method. The concentration of uranium was varied from 0.5 to 5.0 mol % to investigate the effect of doping concentration on its optical properties. The synthesised UDT powder were characterized systematically using X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) respectively for phase purity, morphology and crystallinity. Pertaining to nuclear industry, UDT is an important material and investigating the local structure of uranium in UDT is interesting as well as challenging because of complexity involved in synthesis of such ceramic powder. We have used time resolved photoluminescence spectroscopy (TRPLS) to probe the local coordination and oxidation state of uranium in UDT. Based on PL emission spectroscopy it was confirmed that uranium stabilizes as UO22+ ion in UDT. Lifetime spectroscopy shows that uranyl ion is not homogenously distributed in UDT lattice; rather it has two different chemical environments. Effect of concentration on PL behaviour shows that, concentration quenching takes place beyond 2.0 mol %; and based on critical distance calculation multipolar interaction was found to be responsible for such non-radiative quenching. As far as application in luminescence industry is concerned PL measurement shows that UDT gives intense green emission under UV excitation.

Thermochemistry of rare earth doped uranium oxides LnxU1−xO2−0.5x+y (Ln = La, Y, Nd)

Lanthanum, yttrium, and neodymium doped uranium dioxide samples in the fluorite structure have been synthesized, characterized in terms of metal ratio and oxygen content, and their enthalpies of formation measured by high temperature oxide melt solution calorimetry. For oxides doped with 10–50 mol % rare earth (Ln) cations, the formation enthalpies from constituent oxides (LnO1.5, UO2 and UO3 in a reaction not involving oxidation or reduction) become increasingly exothermic with increasing rare earth content, while showing no significant dependence on the varying uranium oxidation state. The oxidation enthalpy of LnxU1−xO2−0.5x+y is similar to that of UO2 to UO3 for all three rare earth doped systems. Though this may suggest that the oxidized uranium in these systems is energetically similar to that in the hexavalent state, thermochemical data alone can not constrain whether the uranium is present as U5+, U6+, or a mixture of oxidation states. The formation enthalpies from elements calculated from the calorimetric data are generally consistent with those from free energy measurements.

Speciation and site occupancy of uranium in strontium orthosilicate by photoluminescence and X-ray absorption spectroscopy: A combined experimental and theoretical approach

Identifying the oxidation state and coordination geometry of radioactive element like uranium is important to fully understand its toxicology and other harmful effect in environment. Strontium orthosilicate is taken as a model compound for that. Strontium silicate doped with 1.0mol% of U has been synthesized using sol–gel method and characterized using X-ray diffraction (XRD), time resolved fluorescence spectroscopy (TRFS) and extended X-ray absorption fine structure (EXAFS). Uranium exhibits multiple oxidation state and each one of them is having characteristics luminescence is an interesting dopant from structural point of view. TRFS is used to investigate the oxidation state and coordination behavior of uranium in Sr2SiO4. From TRFS measurement it was observed that uranium stabilizes in +6 oxidation state in the form of uranyl ion. Based on luminescence lifetime and EXAFS studies it was inferred that uranyl is stabilized on both 9- and 10-coordinated strontium polyhedra but majority occupies relatively asymmetric 9-coordinated Sr sites. This is further confirmed using theoretical measurement.

Applications of polarizable continuum models to determine accurate solution-phase thermochemical values across a broad range of cation charge - the case of U(III-VI).

Contributing factors to the solution-phase correction to the free energy of the molecular clusters U(H2O)n(3+/4+) and UO2(H2O)m(1+/2+) (n = 8, 9, 30, 41, 77; m = 4, 5, 30, 41, 77) have been examined as a function of cavity type in the integrated-equation-formalism-protocol (IEF) and SMD polarizable continuum models (PCMs). It is observed that the free energy correction, Gcorr, does not smoothly converge to zero as the number of explicitly solvating water molecules approaches the bulk limit, and the convergence behavior varies significantly with cavity and model. The rates of convergence of the gas-phase hydration energy, ΔGhyd, wherein the bare metal ion is inserted into a molecular water cluster and ΔGcorr for the reaction exhibit wide variations as a function of ion charge, cavity, and model. This is the likely source of previously reported discrepancies in predicted free energies of solvation for metal ions when using different PCM cavities and/or models. The cancellation of errors in ΔGhyd and ΔGcorr is optimal for clusters consisting of only a second solvation shell of explicit water molecules (n = m = 30). The UFF cavity within IEF, in particular, exhibits the most consistent cancellation of errors when using a molecular cluster consisting of a second shell of solvating water for all oxidation states of uranium, leading to accurate free energies of solvation ΔGsolv for these species.

Estimation of the oxidation state of uranium in microparticles by using scanning electron microscope in the backscattered electrons mode

This paper considers the results of applying of scanning electron microscopes in the backscattered electron mode for the specifying of the contents of multielemental samples and minerals, which are presented in publications. This approach is used for estimation of oxidation state of uranium in microparticles. The possibility of this goal achieving is demonstrated on the example of the analysis of the UO2 and U3O8 microparticles and the samples of In, Sn, Pt and Pb.

ChemInform Abstract: The Crystal Chemistry of Uranium Carboxylates

AbstractReview: 345 refs.

High-T, High-P Hydrothermal Synthesis of Uranium Silicates and Germanates

Most uranium minerals can be classified as oxidized species in which U is fully oxidized to U(VI), and reduced species, in which U occurs primarily as U(IV). Uranyl silicates are an important group of uranium(VI) minerals in the altered zones of many uranium deposits [1]. Uranyl silicates have also received attention because they form when spent nuclear fuel interacts with water containing silicon under oxidizing conditions. One naturally occurring uranium(IV) silicate exists, namely coffinite (USiO4), which is the most important ore mineral for uranium after uraninite. Numerous synthetic uranium(VI) silicates and germanates containing organic amines or alkali metals as countercations have also been reported [2]. In contrast to the uranium(VI) compounds, the chemistry of materials containing uranium(V) is considerably less developed owing to the tendency of U(V) to either oxidize to U(VI) or disproportionate to U(IV) and U(VI). We have synthesized a pentavalent-uranium silicate and a germanate by a high-temperature, high-pressure hydrothermal method in gold ampoules contained in a high-pressure reaction vessel at ca. 600 °C and 170 MPa [3a,b]. Following the synthesis of the U(V) compounds, a number of mixed-valence uranium silicates and germanates have been synthesized, for example, a mixed-valence uranium(IV,V) silicate, Cs2K(UO)2Si4O12 [3c], a uranium(IV,VI) germanate, Cs8U(UO2)3(Ge3O9)3·3H2O [3d], uranium(V,VI) silicates and germanates, A3(U2O4)Ge2O7 (A = Rb, Cs) and [Na9F2][(UO2)3(Si2O7)2] [3e,f], and a uranium(IV,V,VI) silicate, Na7UO2(UO)2(UO2)2Si4O16 [3g] in which three oxidation states of uranium co-exist in one compound. In addition, tetravalent-uranium compounds, Cs2USi6O15 and Cs4UGe8O20 [3h,i], were also synthesized. All members in the family of uranium silicates and germanates with the oxidation states of uranium from +4 to +6 have been observed. In this presentation the high-temperature, high-pressure hydrothermal synthesis, crystal structures, and XPS spectroscopy of these interesting compounds will be reported.