Showing papers on "Uranyl published in 1984"

Ion exchange/complexation of the uranyl ion by Rhizopus biosorbent.

Abstract: Nonliving biomass of nine Rhizopus species effectively sequestered the uranyl ion from solution, taking up 150-250 mg U/g dry cells at 300 ppm U equilibrium concentration in solution, and 100-160 mg U/g dry cells with 100 ppm U in solution. The affinity of this biosorbent for the uranyl ion was found to be affected by timing of harvesting and medium composition. Uptake of the uranyl ion by nonliving biomass of Rhizopus oligosporus was due to ion exchange or complexation, since binding was reversed by the addition of complexing ligands or the reduction of pH to a value less than 2. Uptake isotherms were interpreted in terms of a model of multiple equilibria. At pH

Photoassisted catalytic oxidation of isopropyl alcohol by uranyl-exchanged zeolites

Abstract: Suspensions of uranyl-exchanged zeolites mixed with solutions of isopropyl alcohol and acetonitrile undergo selective photocatalytic conversion of the alcohol to acetone. The photoassisted catalytic oxidation is sustained for over 300 h. Bulk photolysis experiments show that aging does occur. X-ray photoelectron spectroscopy experiments indicate the uranyl ions on the surface are slightly reduced with respect to the bulk. Luminescence lifetime measurements yield a range of long-lived components between 10 and 700 ..mu..s for the different zeolites. Quenching experiments show that the shortest lived components of crystalline uranyl-exchanged zeolites are responsible for this conversion to acetone. Liquid nitrogen and liquid helium temperature luminescence emission spectra show splitting of the vibrational rotational fine structure as the temperature is diminished. These splittings are indicative of site symmetry, and in the case of the uranyl-exchanged HZSM-5 with the isopropyl alcohol mixture, evidence is presented that the isopropyl alcohol binds to the excited uranyl ion. The nature of the active site and factors that are important in the mechanism of this reaction are described.

Relativistically parameterized extended Hueckel calculations. Part 8. Double-zeta parameters for the actinoids Th, Pa, U, Np, Pu, and Am and an application on uranyl

Abstract: Double-zeta Slater type fits to the radial, relativistic and nonrelativistic Hartree-Fock functions are given for the neutral atoms Th, Pa, U, Np, Pu, and Am. An application on uranyl indicates increased U(5f..pi..)-O(2p..pi..) bonding, compared to earlier relativistic single-zeta or nonrelativistic multiple-zeta calculations. The current understanding of bonding of uranyl is discussed. A possible CI mechanism for obtaining an eventual t/sub lg/ highest occupied molecular orbital (HOMO) for UF/sub 6/, instead of a t/sub lu/ one, is pointed out.

Photophysics of the excited uranyl ion in aqueous solutions. Part 1.—Reversible crossing

Abstract: Upon excitation at 337 nm by a nitrogen laser the uranyl ion undergoes biexponential decay from pH 1 to pH 4, this being more evident at the higher pH values. At pH 3 the biexponential is evident for [UO2+2] from 2 × 10–3 to 0.1 mol dm–3. The decay rates depend on [UO2+2], the laser intensity and the temperature. Temperature also affects the characteristics of the decay, giving single-exponential decay temperatures < 6 °C. Under steady-state conditions comparison of the fluorescence spectra at low and high temperatures reveals another emitting state of uranyl that has a spectrum which is similar to but weaker than the normal uranyl luminescence and shows a red shift of ca. 300 cm–1. Several possible kinetic models for analysing the experimental data are discussed and it is concluded that the data can be best interpreted in terms of a reversible crossing between two states of uranyl, U*ca. 300 cm–1 higher than X*, both of which decay by unimolecular kinetic processes. The oscillator strengths of the two states determined from the fluorescence decays and spectra are in good agreement with absorption spectral data. The model allows the estimation of the rate constants of the different processes at different temperatures, uranyl concentrations and excitation intensities. At pH 3, [UO2+2] affects only the rate of decay of the state U*, whilst an increase in the excitation intensity decreases the rate of the reversible crossing. The activation energies of all the processes suggest that they are of a chemical nature. It is suggested that the reversible decays are due to a solvent exchange process and the irreversible decays are due to hydrogen abstraction from coordinated water molecules.

To the infrared spectroscopy of natural uranyl phosphates

Abstract: An interpretation based on site and factor symmetry analysis has been given of infrared spectra of the natural layered uranyl phosphates metaautunite, metatorbernite, metauranocircite I and II, saleeite and sabugalite, of the deuteroanalogues of metaautunite, metatorbernite and metauranocircite II and of the Raman spectrum of metatorbernite. The principal correlation relationships for the analysis of virbrational spectra G M (symmetry of free ion) ⊒ G S (site symmetry) ⊑ G F (factor symmetry) have been described. The results have been correlated with data from X-ray structure analysis.

Catalysts for the hydrotreatment of hydrocarbons and their preparation

Abstract: The present invention relates to new catalysts for the hydrotreatment of hydrocarbons, their preparation and their application. The catalyst for the hydrotreatment of hydrocarbons, according to the present invention, incorporates a refractory inorganic support combined with an active phase comprising a layer of uranium oxide and at least one oxide of a Group VIII metal. The active phase comprises a layer of uranium oxide fixed with a homogenous distribution on the said support through the intermediacy of direct --O-- bonds resulting from an impregnation of the support with the aid of an ethanolic solution of uranyl acetylacetonate.

Photochemistry of the uranyl ion in colloidal silica solution

Abstract: Uranyl acetate, once adsorbed onto a silica particle, exhibits an extremely long excited-state lifetime with t/sub 1/2/ approx. 440 ..mu..s as compared to 11 ..mu..s in water. The fluorescence spectrum is slightly blue shifted, and the transient absorption spectrum is quite different, consisting of only one peak on the silica but two peaks in water. The rate constants for several quenchers are determined and used to probe the nature of the silica surface. The interactions of the uranyl ion with various surfactants are also discussed, which further illustrate the uniqueness of the uranyl-silica system. A revised energy diagram is presented to account for the various spectral features encountered.

Oxygen Isotope Shifts in 17O NMR Spectra of the Uranyl Ion

Abstract: The 17O NMR spectrum of uranyl oxygens enriched with 17O gave three peaks. These peaks are assigned to the signals of three isotopomers, [16O=U=17O]2+, [17O=U=17O]2+, and [18O=U=17O]2+. Chemical shifts for the signals of [17O=U=17O]2+ and [18O=U=17O]2+ were −0.054±0.002 and −0.108±0.002 ppm relative to [16O=U=17O]2+, respectively.

Photophysics of the excited uranyl ion in aqueous solutions. Part 2.—Acidity effects between pH 0.5 and 4.0

Abstract: Fluorescence quantum yields and decays of excited uranyl ion in aqueous solutions have been studied over the pH range from 0.5 to 4.0. The fluorescence yield decreases between pH 0.5 and 2, is constant up to pH 2.5 and then varies again with a maximum at pH 3.5. The pH dependence of the emission decay is complex but when analysed in terms of a reversible-crossing mechanism reveals that the rate of reversible crossing has a maximum at pH 3 whereas the rate of irreversible decay has a maximum at pH 2 and a minimum at pH 3. All these effects have been interpreted in terms of several aquo, hydroxo–aquo and polynuclear uranyl cations (UO2+2)*, [UO2(OH)+]*, [UO2(OH)2]* and [(UO2)2(OH)–5]*, with a pH-dependent distribution curve and with different rates of fluorescence decay. Good agreement between the decay and the stationary-fluorescence data is found within a reversible-crossing kinetic scheme between two uranyl states that are energetically very close, a higher U* state and a lower X* state. Increasing uranyl concentration increases only the rate of decay of state U*, except at pH 1 where some effect is detected in the X* decay. The acidity effect on the autoquenching rate constant is not very pronounced. The laser intensity does not affect the rates of irreversible decay of both states, but affects the rate for the reversible transition between U* and X*. This effect depends on the pH.

Uranium isotope effects in some ion exchange systems involving uranyl–carboxylate complexes

Abstract: Isotope fractionation between uranyl complexes in aqueous solution and uranyl ions in cation exchange resin was experimentally studied. The isotope separation coefficients e of the uranyl complex systems determined by a chromatographic method were (0.25±0.1)×10−4 (uranyl–chloride system), (0.88±0.1)×10−4 (acetate), (1.03±0.1)×10−4 (glycolate), (1.42±0.15)×10−4 (lactate), and (2.18±0.2)×10−4 (malate) at 25 °C. IR spectroscopic measurements were conducted on the uranyl complexes of which isotope effects were determined. It was found that the separation coefficients are linearly proportional to the shift of the asymmetric stretching vibration ν3 of O=U=O in uranyl complexes. The observed relation is e=4.62×10−6 (960−ν3/cm−1). A theoretical treatment indicated that approximately one third of the observed isotope effects could be attributed to the asymmetric stretching vibration.

Synthesis of Ethane-1,2-bis(thioglyoxime) and Its Complexes with Nickel(II), Copper(II), Cobalt(III), Cadmium(II), and Uranyl(VI)

Abstract: A new tetra-oxime, ethane-1, 2-bis (thioglyoxime) (ETH4), has been prepared from 1,2-ethanedithiol and anti-chloroglyoxime. ETH4 has been polymerized by coordination with Ni(II), Cu(II), and Co(III). The Cd(II) complex is tetranuclear [(ET)Cl4(H2O)4Cd4] and the Zn(II) complex is binuclear [(ETH2)Cl2(H2O)2Zn2]. The uranyl complex is formed by coordination of O and S atoms of the ligand. Structures of the ligand and the complexes are proposed according to elemental analysis, magnetic measurements, 1H-n.m.r., i.r., and u.v.-visible spectral data.

Charge-transfer processes in the quenching of excited uranyl ion by organosulphur, organohalogen and organometal species

Abstract: Strong quenching of the emission of excited uranyl ion, [UO22+]*, is found on addition of the molecules RHal (Hal = Br, I), R2S and R4M (M = Si, Ge, Sn, Pb), both in terms of the intensity and lifetime of the emitting species as determined by 347 nm laser flash photolysis in acetone solution. Kinetic studies have been supplemented by quantum-yield determinations (for UIV), product analysis and, in certain cases, by e.s.r. investigation of the irradiated system at 77 K.Second-order rate constants k2 for the quenching process fall in the region 107–6 × 109 dm3 mol–1 s–1 and, in general, correlations can be found between log k2 and the ionisation potential of the donor for a particular series. Plots for the series R4Sn and RHal yield gradients of –1.12 ± 0.10 and –1.61 ± 0.07 eV–1, respectively, while that for the series cyclo-R2S is markedly different (–0.69 ± 0.07 eV–1). (None of these gradients conforms to the figure of –16.91 eV–1 expected from the linear sloping section of a so-called Weller plot.) ϕ(UIV) is generally rather low (<0.07) and in the case of RHal extremely so, although UIV yields could not always be determined because of precipitation of photochemical products. Organic products were identified by gas-chromatography–mass-spectrometry. E.s.r. examination indicated that one-electron transfer from R2S to [UO22+]* takes place to yield the species (R2S)2˙+ as the identifiable radical product, although a monomeric radical is found under certain conditions. (R2S)2˙+ was also observed as a transient species during laser flash photolysis of tetrahydrothiophene in the presence of UO22+ ion, while Me2S2 yielded Me2S2˙+.Mechanisms of the quenching processes are discussed in terms of fast, largely reversible electron transfer and the intermediacy of exciplexes.

Raman study of uranyl ion attachment to thorium(IV) hydrous polymer

Abstract: The association of uranyl ions to Th(IV) hydrous polymer in dilute aqueous nitric acid solutions has been studied by Raman spectroscopy over the pH range of 1.5-4.0. By monitoring changes in the frequency of the uranyl symmetric stretching vibration we have identified the attachment of UO/sub 2//sup 2 +/ to Th(IV) hydrous polymer through hydroxyl bridges. Unassociated UO/sub 2//sup 2 +/ has a symmetric stretching vibration giving rise to a Raman band at 869 cm/sup -1/ whereas the band appears at 851 cm/sup -1/ when UO/sub 2//sup 2 +/ is attached to Th(IV) polymer. Aging of the Th(IV) hydrous polymer to which uranyl ion is bridged causes the frequency of the UO/sub 2//sup 2 +/ symmetric stretching vibration to appear at 665 cm/sup -1/-indicative of a change in the bridging bond from hydroxyl to oxygen. Comparison of the Raman spectra of these aged polymers containing UO/sub 2//sup 2 +/ has been made with those of the crystalline uranates such as ..cap alpha..-Na/sub 2/UO/sub 4/. 14 references, 3 figures.

Separation of uranium from technetium in recovery of spent nuclear fuel

Abstract: A method for decontaminating uranium product from the Purex process comprises addition of hydrazine to the product uranyl nitrate stream from the Purex process, which contains hexavalent (UO2 2+) uranium and heptavalent technetium (TcO4 -). Technetium in the product stream is reduced and then complexed by the addition of oxalic acid (H2 C2 O4), and the Tc-oxalate complex is readily separated from the uranium by solvent extraction with 30 vol. % tributyl phosphate in n-dodecane.