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.     

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 Å. 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 Å) and longer (2.2–2.3 Å) 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.     

Microanalysis (micro-XRF, micro-XANES, and micro-XRD) of a tertiary sediment using microfocused synchrotron radiation.

Micro-focused synchrotron radiation techniques to investigate actinide elements in geological samples are becoming an increasingly used tool in nuclear waste disposal research. In this article, results using mu-focus techniques are presented from a bore core section of a U-rich tertiary sediment collected from Ruprechtov, Czech Republic, a natural analog to nuclear waste repository scenarios in deep geological formations. Different methods are applied to obtain various, complementary information. Elemental and element chemical state distributions are obtained from micro-XRF measurements, oxidation states of As determined from micro-XANES, and the crystalline structure of selected regions are studied by means of micro-XRD. We find that preparation of the thin section created an As oxidation state artifact; it apparently changed the As valence in some regions of the sample. Results support our previously proposed hypothesis of the mechanism for U-enrichment in the sediment. AsFeS coating on framboid Fe nodules in the sediment reduced mobile groundwater-dissolved U(VI) to less-soluble U(IV), thereby immobilizing the uranium in the sediment.     

CARBEX process,a new technology of reprocessing of spent nuclear fuel

Advances in the CARBEX process, a new aqueous chemical method for reprocessing of spent nuclear fuel (SNF) in carbonate media, are considered. A review of carbonate methods for SNF reprocessing is given. The CARBEX process concept is presented and experimental data for every stage of the CARBEX process: high-temperature oxidation of spent fuel composition, its oxidative dissolution in carbonate aqueous solutions, extraction refining of U(VI) and Pu(VI), solid-phase re-extraction of carbonate complexes of U(VI) and Pu(VI), and obtaining of uranium and plutonium dioxide powders for fabrication of ceramic nuclear fuel, are discussed. It was shown that the CARBEX process can be more effective and safe than the well-known industrial PUREX process.     

Direct determination of the intracellular oxidation state of plutonium

Microprobe X-ray absorption near edge structure (μ-XANES) measurements were used to determine directly, for the first time, the oxidation state of intracellular plutonium in individual 0.1-μm(2) areas within single rat pheochromocytoma cells (PC12). The living cells were incubated in vitro for 3 h in the presence of Pu added to the media in different oxidation states (Pu(III), Pu(IV), and Pu(VI)) and in different chemical forms. Regardless of the initial oxidation state or chemical form of Pu presented to the cells, the XANES spectra of the intracellular Pu deposits were always consistent with tetravalent Pu even though the intracellular milieu is generally reducing.     

Extraction of uranyl ions from aqueous solutions using silica-gel-bound macrocycles for alpha contaminated waste water treatment

The efficiency for the extraction of U(VI) of new modified silica gels, namely N-tripropionate (or N-triacetate)-substituted tetraazamacrocycles-bound silica gels, has been studied. The effect of the nature of the ligand, the pH and the temperature was studied both in batch experiments as well as in continuous extraction. These silica gels are good candidates for the extraction of U(VI) when compared to a commercially available acid-type chelating resin. The breakthrough and regeneration tests showed that the total removal of U(VI) from a contaminated solution can be achieved by using a column packed with such tetraazamacrocycles-bound silica gels. Finally, the use of a modified silica gel in a pilot device allowed the total decontamination of 50 m³ of real effluents containing traces of uranium, plutonium, and americium.     

Controlled potential coulometry: the application of a secondary reaction to the determination of plutonium and uranium at a solid electrode

A method is described for the simultaneous determination of plutonium and uranium in mixed oxides by controlled potential coulometry at a gold working electrode in two stages: first a coulometric oxidation, at 0.73 V vs. a silver/silver chloride electrode, of Pu(III) and U(IV) to Pu(IV) and U(VI) by a combination of a direct electrode reaction and a secondary chemical reaction proceeding concurrently, and secondly, a coulometric reduction at 0.33 V of Pu(IV) to Pu(III), leaving uranium as U(VI). The determination is carried out in a mixture of sulphuric and nitric acids, and Ti(III) is used to reduce plutonium and uranium to Pu(III) and U(IV) before electrolysis. The precision (3sigma) of Pu:U ratio results obtained from mixtures containing about 30% and 2% plutonium was 0.5% and 1-5% respectively. The effect of experimental variables on the time taken to complete the coulometric determination is discussed.     

Solvent extraction of metals with hydroxamic acids

Solvent extraction with hydroxamic acids has been investigated. with comparison of aliphatic and aromatic reagents for the extraction of iron, copper, cobalt and nickel. Caprylohydroxamic acid has been evaluated for use in extraction systems for titanium, vanadium, chromium, molybdenum and uranium, both in terms of acidity of aqueous phase and oxidation state of the metal. It has been established that caprylohydroxamic acid in 1-hexanol is a suitable extractant for the removal of titanium(IV), vanadium(V), chromium(VI), molybdenum(VI) and uranium(VI) from 6M hydrochloric acid.     

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.     

Rapid and selective uranium extraction from aqueous solution under visible light in the absence of solid photocatalyst

Extraction of uranium from radioactive waste-water is of significant importance for environmental protection and the recovery of uranium resource. Different from the previous reports to use the solid absorbent/photocatalyst for U(VI) removal, herein, we proposed a new eco-friendly method for the rapid and selective extraction of uranium from aqueous solutions under visible light without solid materials. At optimal p H value and in the presence of organics like alcohols, the U(VI) could be extracted efficiently to form brown uranium solid over wide uranium concentrations under anaerobic condition and visible light, by utilizing the excitation of the given U(VI) species. With comprehensive modelling of the electronic ultraviolet-visible(UV-Vis)properties, it is proved that p H adjusting towards U(VI) could induce efficient ligand-to-metal-charge-transfer(LMCT) within the uranyl complex under visible light and the reduction of U(VI) to form U(V), which can be transformed into U(IV) via disproportionation reaction. The resulting U(IV) in solid phase makes the extraction much more convenient in operation. More importantly, the excellent selectivity for uranium extraction over interfering alkali metal ions, transition metal ions and the lanthanide metal ions shows a powerful application potential.     

Mechanism of U(VI) extraction by CYANEX 301 in kerosene from nitrate medium

The rate of extraction of U(VI) by CYANEX 301 in kerosene from nitrate medium of constant ionic strength (0.1M) was investigated using a stirred Lewis cell. The different parameters affecting the extraction rate of U(VI) as well as the temperature were studied. The effects of interfacial area and tension on the extraction rate showed that the extraction process is controlled by a chemical reaction taking place at the interface. The kinetic results indicated that the extraction of U(VI) is of first order, dependent on CYANEX 301 and nitrate concentrations while it is of inverse first order for uranium concentration. This revised version was published online in July 2006 with corrections to the Cover Date.     

Uranium(VI) speciation by spectroscopy

The application of UV-Vis and time-resolved laser-induced fluorescence (TRLF) spectroscopies to direct speciation of uranium(VI) in environmental samples offers various prospects that have, however, serious limitations. While UV-Vis spectroscopy is probably not sensitive enough to detect uranium(VI) species in the majority of environmental samples, TRLFS is principially able to speciate uranium(VI) at very low concentration levels in the nanomol range. Speciation by TRLFS can be based on three parameters: excitation spectrum, emission spectrum and lifetime of the fluorescence emission process. Due to quenching effects, the lifetime may not be expected to be as characteristic as, e.g., the emission spectrum. Quenching of U(VI) fluorescence by reaction with organic substances, inorganic ions and formation of carbonate radicals is one important limiting factor in the application of U(VI) fluorescence spectroscopy. Fundamental photophysical criteria are illustrated using UV-Vis and fluorescence spectra of U(VI) hydrolysis and carbonato species as examples.     

Preconcentration of trace uranium from seawater with solid phase extraction followed by differential pulse polarographic determination in chloroform eluate

In the present study, an effective method is presented for the separation and preconcentration of uranium (VI) by solid phase extraction (SPE). For this purpose, U(VI) oxinate is formed by the reaction of U(VI) with 8-hydroxyquinoline and adsorbed onto the octylsilane (C-8) SPE cartridge. The analyte is completely eluted with chloroform and determined by differential pulse polarography. The SPE conditions were optimized by evaluating the effective factors such as pH, oxine concentration, type and concentration of buffer and masking agent. By the proposed method a preconcentration factor more than 100 was achieved. The average recovery of uranium (VI) oxinate (0.1 mg l(-1)) was 99.8%. The relative standard deviation was 1.6% for seven replicate determinations of uranyl ion in the solution with a concentration 20 mug l(-1). Some concomitant ions such as Ca(+2), Mg(+2) and Fe(+3) which interfere in extraction or determination process of uranium were masked with EDTA in aqueous phase during the extraction process. The proposed method was successfully used for the determination of uranium in Caspian Sea and Persian Gulf water samples.     

Kinetic studies on the solvent extraction of uranium(VI) from phosphoric acid solution with HDEHP using laser-induced optical fiber fluorimetry

Laser-induced optical fiber fluorimetry has been first used to analyze uranium(VI) concentration in the kinetic studies on the extraction of uranium(VI) between 0.5 mol/l H₃PO₄ solution and HDEHP-cyclohexane system with a Lewis cell. The effects of stirring speed, temperature and concentrations of uranium(VI) and HDEHP on the rate of extraction were examined. These data show that the extraction rate of uranium(VI) in this system is controlled by the chemical reaction at the interface. The rate equations and the rate constants of forward and reverse extraction are obtained. The mechanism of the extraction is discussed.     

Extraction of uranium(IV) from phosphoric acid solutions with 1-phenyl-3-methyl-4-benzoylpyrazolone-5 (PMBP)

The extraction of uranium(IV) from phosphoric acid solutions with PMBP and PMBP-TOPO mixtures has been studied. The synergic extraction with PMBP-TOPO is more effective than the simple chelate extraction with PMBP and both systems are more effective than the synergic extraction of uranium(VI) with DEHPA-TOPO. It is established that the complexes extracted are U(PMBP)(4) and U(PMBP)(4).TOPO for the chelate and synergic extraction respectively. The most probable uranium(VI) species in the aqueous phase (2.9-6.33M H(3)PO(4)) is the neutral complex U(H(5)P(2)O(8))(4). Analytical methods suitable for determination of uranium in phosphoric acid solutions have been developed. The highest sensitivity is achieved by combining the synergic extraction with the uranium(IV)-arsenazo III colour reaction.     

Kinetic studies on the solvent extraction of uranium(VI) from phosphoric acid solution with HDEHP using laser-induced optical fiber fluorimetry

The laser-induced optical fiber fluorimetry has been used for the first time to analyse the concentration of uranium(VI) in the kinetic studies on the extraction of uranium(VI) between 0.5 mol L H₃PO₄ solution and HDEHP-cyclohexane system with Lewis cell. The effects of stirring speed, temperature and concentration of uranium(VI) and HDEHP on the rate of extraction were examined. These data show that the extraction rate of uranium(VI) in this system is controlled by the chemical reaction process at the interface. The rate equations and the rate constants of the forward and reverse extractions were obtained. The mechanism of the extraction has been discussed.     

Polarographic behaviour of uranium beta-diketonates in chloroform Application of ac polarography to the analysis of uranium minerals

The polarographic behaviour of U(VI) β-diketonates has been studied in chloroform. The conditions for reversible electrochemical reduction of U(VI) acetylacetonate and benzoylacetonate at a dropping mercury electrode was optimized by using a suitable ratio of piperidinium perchlorate and piperidine mixture as supporting electrolyte in chloroform. The one-electron nature of the reduction wave of U(VI) complexes was confirmed by controlled potential coulometry. The ac method preceded by a solvent extraction of U(VI) benzoylacetonate in chloroform was used for the determination of uranium. The calibration curve was linear over the range 0.5–20 μ/ml. The correlation coefficient was 0.9998 and the detection limit was about 0.2 μ/ml. The interference of some concomitant ions were examined and EDTA was used as an effective masking agent to separate uranium from other metals. The proposed method has been applied to the determination of uranium in uranium minerals.     

Effects of phosphate and fulvic acid on the sorption and transport of uranium(VI) on silica column

Column experiments were performed, breakthrough curves (BTCs) and displacement curves (DPCs) were obtained of uranium(VI) in the absence and presence of phosphate or fulvic acid individually and simultaneously which demonstrated the effects of phosphate and fulvic acid on the sorption and transport of U(VI) in a silica column at pH 3.7 and U(VI) concentration of 5·10⁻⁶ mol/L. It was found that in the presence of phosphate or fulvic acid sorbed preliminarily on the silica column, the amount of U(VI) sorbed increased significantly and the transport of U(VI) delayed significantly relative to that in the absence of phosphate or fulvic acid. Moreover, in the presence of phosphate and fulvic acids sorbed preliminarily and simultaneously on the silica column, the amount of U(VI) sorbed on the silica column is significantly increased again relative to that in the presence of phosphate or fulvic acid individually. Transport studies of U(VI) are important, since all uranium isotopes are radioactive, and uranium contamination of soils and groundwaters occurs at mining and mill sites. A fundamental understanding of the transport behavior of U(VI) in the water-mineral systems is necessary for accurate risk assessments.     

Determination of aqueous plutonium oxidation states by solvent extraction

The reliability of two solvent extraction techniques for the determination of Pu oxidation states in solution was tested with low-ionic-strength solutions and with high-Na and high-Mg brines that contained Pu concentrations sufficient for spectrophotometric analysis. One procedure only differentiates between reduced Pu [Pu(III) and Pu(IV)] and oxidized Pu [Pu(V) and Pu(VI)], whereas the second procedure was designed to differentiate between Pu(IV), Pu(V), and Pu(VI) in solution. Both procedures successfully differentiated between oxidized and reduced Pu in both dilute solutions and brines when tested with samples that contained only the Pu(IV), Pu(V), or Pu(VI) oxidation states. However, when the second solvent extraction procedure, which differentiates between Pu(V) and Pu(VI), was employed for solutions that did not contain a strong oxidant to maintain the Pu(VI) oxidation state, significant quantities of Pu(VI) were reduced to Pu(V) during extraction, indicating that accurate quantification of Pu(V) and Pu(VI) is not possible with this procedure.Work supported by the U. S. Department of Energy under Contract DE-ACO6-76RLO 1830.     

Further development in the high-precision volumetric method for the determination of uranium in nuclear-grade uranium compounds

The high-precision uranium determination by reduction with ferrous sulfate in phosphoric acid and titration with dichromate, which is applicable to nuclear-grade uranium compounds in which the uranium exists nearly exclusively as U(IV), has been modified. The modification enlarges the range of applicability of the original method to include the analysis of uranium compounds in which the uranium exists as U(VI) or as a mixture of U(IV) and U(VI), such as U₃O₈. The modified method has the same precision, relative freedom from interferences and applicability for routine use as the original method.