Comparative EXAFS investigation of uranium(VI) and -(IV) aquo chloro complexes in solution using a newly developed spectroelectrochemical cell

The coordination of the U(IV) and U(VI) ions as a function of the chloride concentration in aqueous solution has been studied by U L(III)-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The oxidation state of uranium was changed in situ using a gastight spectroelectrochemical cell, specifically designed for the safe use with radioactive solutions. For U(VI) we observed the complexes UO2(H2O)5(2+), UO2(H2O)4Cl+, UO2(H2O)3Cl2(0), and UO2(H2O)2Cl3- with [Cl-] increasing from 0 to 9 M, and for U(IV) we observed the complexes U(H2O)9(4+), U(H2O)8Cl3+, U(H2O)(6-7)Cl2(2+), and U(H2O)5Cl3+. The distances in the U(VI) coordination sphere are U-Oax = 1.76+/-0.02 A, Oeq = 2.41 +/- 0.02 A, and U-Cl = 2.71 +/- 0.02 A; the distances in the U(IV) coordination sphere are U-O = 2.41 +/- 0.02 A and U-Cl = 2.71 +/- 0.02 A.     

Mixed-valent uranium(IV,VI) diphosphonate: synthesis, structure, and spectroscopy

A mixed-valent uranium(IV,VI) diphosphonate, (H(3)O)(2)(UO(2))(3)U(H(2)O)(2)[CH(2)(PO(3))(2)](3)·6H(2)O (UC1P2S), has been synthesized under hydrothermal conditions. S-2-butanol was used to reduce uranium VI to IV. The tetravalent uranium centers adopt eight-coordinate geometries, while hexavalent uranyl units are all tetragonal bipyramids. The UV-vis-NIR spectrum of UC1P2S shows absorption features for both U(VI) and U(IV).     

Reactions of laser-ablated uranium atoms with H2O in excess argon: a matrix infrared and relativistic DFT investigation of uranium oxyhydrides

Laser-ablated U atoms react with H2O during condensation in excess argon. Infrared absorptions at 1416.3, 1377.1, and 859.4 cm(-1) are assigned to symmetric H-U-H, antisymmetric H-U-H, and U=O stretching vibrations of the primary reaction product H(2)UO. Uranium monoxide, UO, also formed in the reaction, inserts into H2O to produce HUO(OH), which absorbs at 1370.5, 834.3, and 575.7 cm(-1). The HUO(OH) uranium(IV) product undergoes ultraviolet photoisomerization to a more stable H2UO2 uranium(VI) molecule, which absorbs at 1406.4 and 885.9 cm(-1). Several of these species, particularly H2UO2, appear to form weak Ar-coordinated complexes. The predicted vibrational frequencies, relative absorption intensities, and isotopic shifts from relativistic DFT calculations are in good agreement with observed spectra, which further supports the identification of novel uranium oxyhydrides from matrix infrared spectra.     

Volumetric determination of the U/O ratio in uranium oxides

A method is given for determining the general formula UO(2+x), of sintered uranium dioxide pellets and uranium dioxide powders. Uranium(VI) is reduced by titration with iron(II) ammonium sulphate, after which the total amount of uranium is oxidized by titration with potassium dichromate. The end-points of both titrations are detected electrometrically. Determination of x in the range 0.0001-1.00 is possible.     

Progress in neutron activation analysis for uranium

A new type of extractant, sym-dibenzo-16-crown-5-oxyhydroxamic acid (HL) is introduced. The extractions of UO22+, Na+, K+, Sr2+, Ba2+ and Br- were studied with HL in chloroform. The results obtained show that UO22+ can be quantitatively extracted at pH values above 5, whereas the extractions of K+, Na+, Sr2+, Ba2+ and Br- are negligible in the pH range of 2 - 7. The dependence of the distribution ratio of U(VI) on both the concentration of the HL and pH are linear, and they have the same slope of 2. This suggests that U(VI) appears to form a 1:2 complex with ligand. Uranium(VI) can be selectively separated and concentrated from interfering elements such as Na, K, Sr and Br by solvent extraction with HL under specific conditions. The recovery of uranium is nearly 100% and the radionudear purity of uranium is greater than 99.99%. Therefore, neutron activation analysis has greatly improved the sensitivity and accuracy for the detection of trace uranium from seawater.     

Further examples of the failure of surrogates to properly model the structural and hydrothermal chemistry of transuranium elements: insights provided by uranium and neptunium diphosphonates

In situ hydrothermal reduction of Np(VI) to Np(IV) in the presence of methylenediphosphonic acid (C1P2) results in the crystallization of Np[CH2(PO3)2](H2O)2 (NpC1P2-1). Similar reactions have been explored with U(VI) resulting in the isolation of the U(IV) diphosphonate U[CH2(PO3)2](H2O) (UC1P2-1), and the two U(VI) diphosphonates (UO2)2[CH2(PO3)2](H2O)3.H2O (UC1P2-2) and UO2[CH2(PO3H)2](H2O) (UC1P2-3). Single crystal diffraction studies of NpC1P2-1 reveal that it consists of eight-coordinate Np(IV) bound by diphosphonate anions and two coordinating water molecules to create a polar three-dimensional framework structure wherein the water molecules reside in channels. The structure of UC1P2-1 is similar to that of NpC1P2-1 in that it also adopts a three-dimensional structure. However, the U(IV) centers are seven-coordinate with only a single bound water molecule. UC1P2-2 and UC1P2-3 both contain U(VI). Nevertheless, their structures are quite distinct with UC1P2-2 being composed of corrugated layers containing UO 6 and UO 7 units bridged by C1P2; whereas, UC1P2-3 is found as a polar three-dimensional network structure containing only pentagonal bipyramidal U(VI). Fluorescence measurements on UC1P2-2 and UC1P2-3 exhibit emission from the uranyl moieties with classical vibronic fine-structure.     

Aqueous coordination chemistry and photochemistry of uranyl(VI) oxalate revisited: a density functional theory study

Using density functional theory (DFT) calculations, we revisited a classical problem of uranyl(VI) oxalate photochemical decomposition. Photoreactivities of uranyl(VI) oxalate complexes are found to correlate largely with ligand-structural arrangements. Importantly, the intramolecular photochemical reaction is inhibited when oxalate is bound to uranium exclusively in chelate binding mode. Previously proposed mechanisms involving a UO(2)(C(2)O(4))(2)(2-) (1:2) complex as the main photoreactive species are thus unlikely to apply, because the two oxalic acids are bound to uranium in a chelating binding mode. Our DFT results suggest that the relevant photoreactive species are UO(2)(C(2)O(4))(3)(4-) (1:3) and (UO(2))(2)(C(2)O(4))(5)(6-) (2:5) complexes binding uranium in an unidentate fashion. These species go through decarboxylation upon excitation to the triplet state, which ensues the release of CO(2) and reduction of U(vi) to U(v). The calculations also suggest an alternative intermolecular pathway at low pH via an electron transfer between the excited state *UO(2)(2+) and hydrogen oxalate (HC(2)O(4)(-)) which eventually leads to the production of CO and OH(-) with no net reduction of U(VI). The calculated results are consistent with previous experimental findings that CO is only detected at low pH while U(IV) is detected only at high pH.     

Dissolution Extraction of Uranium Oxides and Cerium Oxide with TBP-HNO3 Complex in Supercritical CO2

Dissolution extraction of uranium oxides, CeO2 and (U, Ce)O2 solid solution with TBP-HNO3 complex in supercritical CO2 (SC-CO2) was investigated. It is difficult to dissolve and extract directly UO2 pellets and CeO2 with TBP-HNO3 complex in SC-CO2. After UO2 pellets spontaneously turns into U3O8 powders under O2 flow and 600 °C, the extraction efficiency can reach more than 98%. For dissolution extraction of (U, Ce)O2 solid solution with TBP-HNO3 complex in SC-CO2 under 60 and 20 MPa, the extraction efficiency of U and Ce is 98.61% and 98.1% respectively.     

In situ spectroscopy and spectroelectrochemistry of uranium in high-temperature alkali chloride molten salts

Soluble uranium chloride species, in the oxidation states of III+, IV+, V+, and VI+, have been chemically generated in high-temperature alkali chloride melts. These reactions were monitored by in situ electronic absorption spectroscopy. In situ X-ray absorption spectroscopy of uranium(VI) in a molten LiCl-KCl eutectic was used to determine the immediate coordination environment about the uranium. The dominant species in the melt was [UO 2Cl 4] (2-). Further analysis of the extended X-ray absorption fine structure data and Raman spectroscopy of the melts quenched back to room temperature indicated the possibility of ordering beyond the first coordination sphere of [UO 2Cl 4] (2-). The electrolytic generation of uranium(III) in a molten LiCl-KCl eutectic was also investigated. Anodic dissolution of uranium metal was found to be more efficient at producing uranium(III) in high-temperature melts than the cathodic reduction of uranium(IV). These high-temperature electrolytic processes were studied by in situ electronic absorption spectroelectrochemistry, and we have also developed in situ X-ray absorption spectroelectrochemistry techniques to probe both the uranium oxidation state and the uranium coordination environment in these melts.     

Pentavalent uranium oxide via reduction of [UO2]2+ under hydrothermal reaction conditions

The synthesis, crystal structure, and spectroscopic characterization of [U(V)(H2O)2(U(VI)O2)2O4(OH)](H2O)4 (1), a mixed-valent U(V)/U(VI) oxide material, are reported. The hydrothermal reaction of UO2(2+) with Zn and hydrazine at 120 degrees C for three days yields 1 in the form of a dark red crystalline solid. Compound 1 has been characterized by a combination of single-crystal and powder X-ray diffraction (XRD), elemental analysis, thermogravimetric analysis, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The structure consists of an extended sheet of edge and corner shared U(VI) pentagonal bipyramids that are further connected by edge sharing to square bipyramidal U(V) units. The overall topology is similar to the mineral ianthinite. The uranium L(III)-edge XAS revealed features consistent with those observed by single-crystal X-ray diffraction. High resolution XPS data analysis of the U4f region confirmed the oxidation states of U as originally assigned from XRD analysis and bond valence summations.     

N(CH2CH2NH3)3][U3O2F13].2H2O: a novel organically templated mixed-valent uranium oxyfluoride with a hybrid network structure

The synthesis and characterization of a novel mixed-valent uranium oxyfluoride is described; the inorganic network consists of 2-D [U(2)F(10)](2)(-) sheets constructed from corner- and edge-sharing U(IV)F(9) tricapped trigonal prisms and 1-D [UO(2)F(3)](-) chains constructed from edge-sharing U(VI)O(2)F(5) pentagonal bipyramids with the organic cations and water molecules between the sheets. This is the first example with a hybrid network structure in the system of uranium fluoride or oxyfluoride. The variable-temperature magnetic susceptibility confirms the oxidation state of the uranium ions. Crystal data follow: C(6)H(25)N(4)O(4)F(13)U(3), monoclinic, space group P2(1) (No. 4); a = 8.6876(4) A, b = 7.3158(4) A, c = 16.3376(8) A, beta = 93.7285(9) degrees , V = 1036.2(2) A(3), and Z = 2.     

Complexes formed in the chloroform extraction of uranium(VI) with oxine

Equilibrium distribution ratios have been determined for uranium(VI) with oxine between chloroform and 0.1M perchlorate as a function of pH and reagent concentration at 20 degrees . It is concluded that the extractable complex is UO(2)Ox(2)HOx. The equilibrium constants for the extraction of uranium have been determined as K(u,1) = [UO(2)Ox(2)HOx](0)/[UO(2)(2+)][Ox(-)](3)[H(+)] = 10(36.18) at low pH and K(U,2) = [UO(2)Ox(2)HOx](0)/[UO(2)Ox(2)OH(-)][Ox(-)][H(+)](2) = 10(25.40) at high pH.     

Synthesis of a uranium(VI)-carbene: reductive formation of uranyl(V)-methanides, oxidative preparation of a [R2C═U═O]2+ analogue of the [O═U═O]2+ uranyl ion (R = Ph2PNSiMe3), and comparison of the nature of U(IV)═C, U(V)═C, and U(VI)═C double bonds

We report attempts to prepare uranyl(VI)- and uranium(VI) carbenes utilizing deprotonation and oxidation strategies. Treatment of the uranyl(VI)-methanide complex [(BIPMH)UO(2)Cl(THF)] [1, BIPMH = HC(PPh(2)NSiMe(3))(2)] with benzyl-sodium did not afford a uranyl(VI)-carbene via deprotonation. Instead, one-electron reduction and isolation of di- and trinuclear [UO(2)(BIPMH)(μ-Cl)UO(μ-O){BIPMH}] (2) and [UO(μ-O)(BIPMH)(μ(3)-Cl){UO(μ-O)(BIPMH)}(2)] (3), respectively, with concomitant elimination of dibenzyl, was observed. Complexes 2 and 3 represent the first examples of organometallic uranyl(V), and 3 is notable for exhibiting rare cation-cation interactions between uranyl(VI) and uranyl(V) groups. In contrast, two-electron oxidation of the uranium(IV)-carbene [(BIPM)UCl(3)Li(THF)(2)] (4) by 4-morpholine N-oxide afforded the first uranium(VI)-carbene [(BIPM)UOCl(2)] (6). Complex 6 exhibits a trans-CUO linkage that represents a [R(2)C═U═O](2+) analogue of the uranyl ion. Notably, treatment of 4 with other oxidants such as Me(3)NO, C(5)H(5)NO, and TEMPO afforded 1 as the only isolable product. Computational studies of 4, the uranium(V)-carbene [(BIPM)UCl(2)I] (5), and 6 reveal polarized covalent U═C double bonds in each case whose nature is significantly affected by the oxidation state of uranium. Natural Bond Order analyses indicate that upon oxidation from uranium(IV) to (V) to (VI) the uranium contribution to the U═C σ-bond can increase from ca. 18 to 32% and within this component the orbital composition is dominated by 5f character. For the corresponding U═C π-components, the uranium contribution increases from ca. 18 to 26% but then decreases to ca. 24% and is again dominated by 5f contributions. The calculations suggest that as a function of increasing oxidation state of uranium the radial contraction of the valence 5f and 6d orbitals of uranium may outweigh the increased polarizing power of uranium in 6 compared to 5.     

An EXAFS and TRLFS investigation on uranium(VI) sorption to pristine and leached albite surfaces

Uranium(VI) was sorbed to freshly ground and leached albite in batch and flow-through systems in the pH range 5.0-6.4. The uranium(VI) surface complexes were studied by extended X-ray absorption fine structure (EXAFS) spectroscopy and time-resolved laser-induced fluorescence spectroscopy (TRLFS). The EXAFS analysis of uranium(VI) sorbed on albite at pH 5.8 and 5 x 10(-6) M U(VI) showed one silicon atom at a USi distance of 3.09 A, which is indicative of the formation of an inner-sphere, mononuclear, bidentate uranium(VI) surface complex, Si(O)2UO2, on the silicate tetrahedra of albite. Two additional uranium(VI) sorption complexes were detected by TRLFS at higher initial aqueous U(VI) concentrations. However, the structure of these surface complexes could not be derived from EXAFS, since the measured EXAFS spectra represent the average of two surface complex structures. In order to simulate U(VI) sorption onto weathered feldspar surfaces, albite was leached with 0.01 M HClO4, resulting in surface material similar to amorphous silica gel. EXAFS showed that the equatorial oxygen shell of uranium(VI) sorbed on this material at pH 5.0 and 5.8 was split in two distances of 2.23 and 2.44 A. This indicates the formation of an inner-sphere surface complex.     

Hydrothermal synthesis of (C6N2H14)2(UVI2UIVO4F12), a mixed-valent one-dimensional uranium oxyfluoride

A new hybrid organic-inorganic mixed-valent uranium oxyfluoride, (C6N2H14)2(U3O4F12), UFO-17, has been synthesized under hydrothermal conditions using uranium dioxide as the uranium source, hydrofluoric acid as mineralizer, and 1,4-diazabicyclo[2.2.2]octane as template. The single-crystal X-ray structure was determined. Crystals of UFO-17 belonged to the orthorhombic space group Cmcm (no. 63), with a = 14.2660(15) A, b = 24.5130(10) A, c = 7.201(2) A, and Z = 4. The structure reveals parallel uranium-containing chains of two types: one type is composed of edge-sharing UO2F5 units; the other has a backbone of edge-sharing UF8 units, each sharing an edge with a pendant UO2F5 unit. Bond-valence calculations suggest the UF8 groups contain UIV, while the UO2F5 groups contain UVI. EXAFS data give results consistent with the single-crystal X-ray structure determination, while comparison of the uranium LIII-edge XANES of UFO-17 with that of related UIV and UVI compounds supports the oxidation-state assignment. Variable-temperature magnetic susceptibility measurements on UFO-17 and a range of related hybrid organic-inorganic uranium(IV) and uranium(VI) fluorides and oxyfluorides further support the formulation of UFO-17 as a mixed-valent UIV/UVI compound.     

Oxidation state delineation via U L(III)-edge XANES in a series of isostructural uranium coordination complexes

We present an X-ray absorption near-edge structure (XANES) study of a series of uranium coordination complexes that possess nearly identical first coordination spheres and geometries in a range of oxidation states from U(III) to U(VI). These compounds were obtained through the activation of small molecules, such as ketones, azides, and carbon dioxide, and upon oxidation of a high-valent U(V)≡O to [U(VI)≡O](+). Most of the compounds have been reported previously. All of them are fully characterized and their oxidation states have been confirmed by various spectroscopic methods (SQUID, (1)H NMR, and UV/vis/near-IR). Each uranium complex consists of a triazacyclononane anchor bearing three aryloxide side arms with bulky tert-butyl (t-Bu) or adamantyl (Ad) ortho substituents. All complexes have approximate C(3) symmetry and possess an axial cavity that is either empty (U(III)) or occupied by a seventh ligand, namely, terminal oxygen (U(V) and U(VI)) or an oxygen-containing ligand (U(IV)). The only exception is [(((t-Bu)ArO)(3)tacnU(VI)(O)][SbF(6)], which is the rare case of a complex that shows a strong inverse trans influence. The determined correlation between the uranium oxidation state and the U L(III)-edge XANES absorption in this series includes a single terminal oxo ligand bonded uranium(V,VI), for which data are essentially nonexistent. The correct assignment of the uranium valence in a U(IV)-L(?-) compound (L(?-) = ketyl radical) is shown to be only possible by a comparison to structurally similar compounds.     

Spectrophotometric measurement of uranium(VI)-tributylphosphate complex in supercritical carbon dioxide

UV-visible absorption spectra of uranium(VI)-tributylphosphate (U(VI)-TBP) complex dissolved in supercritical CO(2) at 40-60 degrees C and 100-250 kg cm(-2) were recorded. Wavelengths and molar extinction coefficients for the absorption peaks of U(VI)-TBP were determined and confirmed to be in good agreement with those of UO(2)(NO(3))(2)(TBP)(2) complex dissolved in organic solvents such as n-hexane. The absorbance at a given wavelength was proportional to the concentration of U(VI) species in supercritical CO(2), indicating a feasibility of in-situ determination of U(VI) concentration in CO(2) phase. A lower detection limit of U(VI)-TBP complex was estimated to be ca. 1x10(-3)M. The molar extinction coefficient of U(VI)-TBP in supercritical CO(2) decreased slightly with an increase of the density of CO(2) medium, suggesting that the solute-solvent interaction of U(VI)-TBP complex with CO(2) was affected by the density. On the basis of the spectra obtained, phase behavior and solubility of UO(2)(NO(3))(2)(TBP)(2)+H(NO(3))(TBP)+TBP in supercritical CO(2) were elucidated.     

Determination of plutonium and uranium in mixed oxide fuels by sequential redox titration

The use of a sequential determination of uranium and plutonium in a single sample solution results in a saving in analysis time and apparatus requirements. The method starts with U(IV) and Pu(in) in a mixture of sulphuric and nitric adds. Titration with dichromate, using amperometry at a pair of polarizable electrodes, produces two well-defined end-points corresponding to the sequential oxidation of U(IV) to U(VI) and Pu(III) to Pu(IV). The quantitative oxidation of U(IV) to U(VI) is achieved via the action of Pu(IV) as intermediate, and is dependent upon establishing conditions which favour rapid reaction between U(IV) and Pu(IV). The method is precise and accurate. With Pu-U mixtures containing between 15 and 30% plutonium the precision (3sigma) of the Pu: U ratio results is +/-0.6% on samples containing 100-120 mg of plutonium plus uranium. Iron and vanadium interfere quantitatively with plutonium, copper interferes non-quantitatively with uranium, and gross amounts of molybdenum mask the uranium end-point.     

Chemical equilibria in the binary and ternary uranyl(VI)-hydroxide-peroxide systems

The composition and equilibrium constants of the complexes formed in the binary U(VI)-hydroxide and the ternary U(VI)-hydroxide-peroxide systems have been studied using potentiometric and spectrophotometric data at 25 °C in a 0.100 M tetramethylammonium nitrate medium. The data for the binary U(VI) hydroxide complexes were in good agreement with previous studies. In the ternary system two complexes were identified, [UO(2)(OH)(O(2))](-) and [(UO(2))(2)(OH)(O(2))(2)](-). Under our experimental conditions the former is predominant over a broad p[H(+)] region from 9.5 to 11.5, while the second is found in significant amounts at p[H(+)] < 10.5. The formation of the ternary peroxide complexes results in a strong increase in the molar absorptivity of the test solutions. The absorption spectrum for [(UO(2))(2)(OH)(O(2))(2)](-) was resolved into two components with peaks at 353 and 308 nm with molar absorptivity of 16200 and 20300 M(-1) cm(-1), respectively, suggesting that the electronic transitions are dipole allowed. The molar absorptivity of [(UO(2))(OH)(O(2))](-) at the same wave lengths are significantly lower, but still about one to two orders of magnitude larger than the values for UO(2)(2+)(aq) and the binary uranyl(VI) hydroxide complexes. It is of interest to note that [(UO(2))(OH)(O(2))](-) might be the building block in cluster compounds such as [UO(2)(OH)(O(2))](60)(60-) studied by Burns et al. (P. C. Burns, K. A. Kubatko, G. Sigmon, B. J. Fryer, J. E. Gagnon, M. R. Antonio and L. Soderholm, Angew. Chem. 2005, 117, 2173-2177). Speciation calculations using the known equilibrium constants for the U(vi) hydroxide and peroxide complexes show that the latter are important in alkaline solutions even at very low total concentrations of peroxide, suggesting that they may be involved when the uranium minerals Studtite and meta-Studtite are formed by α-radiolysis of water. Radiolysis will be much larger in repositories for spent nuclear fuel where hydrogen peroxide might contribute both to the corrosion of the fuel and to transport of uranium in a ground water system.     

Determination of oxygen to uranium ratio in irradiated uranium dioxide by controlled potential coulometry

This work reports the determination of oxygen to uranium (O/U) ratio in irradiated UO_2+x fuel pellet of burnup of ca. 34 GWd/t by controlled potential coulometry. The method is based on the dissolution of the nuclear fuel in strong phosphoric acid (SPA) at 180–190 °C under an inert atmosphere. After dissolution, 8% sulphuric acid is added in order to obtain a 20% SPA in 8% sulphuric acid. A controlled potential coulometric determination of uranium(VI) is carried out at ?0.60V vs. ferri-ferrocyanide. The uranium(IV) contained in an aliquot of the fuel solution is oxidised to uranium(VI) with cerium(IV) sulphate, and the total uranium content is then determined by coulometry. Optimum experimental conditions have been established using simulated irradiated fuel solution containing various fission products which include cerium, tellurium, palladium, ruthenium, molybdenum and zirconium. Interference of the fission products and the possible removal of their interferences by preelectrolysis at +0.5 V vs. saturated calomel electrode (SCE) have been investigated. The accuracy of the coulometric method is confimed by polarographic measurement using several unirradiated UO_2+x fuel of known stoichiometry.