[en] There has been an enormous amount of investigation of the luminescence of the uranyl (UO₂²⁺) ion and its photochemical reactions. Considerable attention has been given to a study of the photochemical reduction of UO₂²⁺ by organic compounds. It was long considered that UV irradiation of aqueous solutions of uranyl not containing organic impurities did not lead to any significant change in the chemical composition of the solution. However, the high oxidation-reduction potential of UO₂²⁺ (2.7 V) permits its reaction with water. Burrows and Kemp have proposed that some portion of UO₂⁺ and the ·OH radicals, which do not enter a geminal reaction, leave the cage and small amounts of U(IV) and H₂O₂ then accumulate in the solution. And the mechanism for the photochemical reduction of uranyl by water through electron transfer has found only indirect evidence by τ-metry (relative to the quenching of the photoluminescence of UO₂²⁺ by a water molecule). For a study of the radiation chemical reduction of uranium(IV) in aqueous sulfuric acid in the absence of ·OH radical acceptors, the authors proposed a chemiluminescence (CL) method based on the possibility of detecting small amounts of uranium(IV) (down to 10^-9 mole/liter) on the background of a significant excess of uranium(VI)(up to 10^-1 mole/liter) using the chemiluminescence of the oxidation of tetravalent uranium compounds by xenon trioxide

No abstract available

No abstract available

[en] When Penicillium digitatum Saccardo cultures are exposed to aqueous solutions containing soluble uranium salts, considerable amounts of this element are accumulated in the fungal mycelium. The accumulated uranium is retained after thorough rinsing with distilled water but is removed by alkali carbonate solutions. Analysis of thick sections (0.5 μm) of the fungal hyphae with TEM, after incubation in UO₂Cl₂ solutions of varying concentrations under both light and dark conditions, revealed conspicuous crystal-like deposits in UO₂Cl₂-exposed hyphae, but none in the control hyphae. Thick sections were necessary for crystal visualization. Using energy-dispersive X-ray analysis, uranium was detected as the only heavy element in these crystals. Uranium crystal biosorption was localized on the outside surface of the hyphal cell wall (following short exposures to relatively low uranium concentrations) or inside the cell wall (following long exposure to relatively high uranium concentrations). In some cases, crystal-like deposits of uranium salts were located on the outside surface as well as inside the cell. (author)

[en] The possibility of using thermal lensing spectrophotometry (TLS) for the quantitative determination of chemical equilibria in very dilute (∼ 10^-5 M) aqueous solution has been explored. The hyrolysis of uranium(IV) has been used as a test case. TLS turned out to be a convenient, precise, and rapid method to obtain information about the mononuclear hydrolysis, information that is difficult to obtain with more traditional solution chemical methods. The TLS data were obtained at 25 degree C in the concentration range of 0 ≤ -log [H⁺] ≤ 2.8 by using a 3 M (Na,H)ClO₄ ionic medium. The data were described with the following chemical model: U⁴⁺ + H₂O right equilibrium UOH³⁺ + H⁺, log *β₁ = -1.65 ± 0.05; U⁴⁺ + 2H₂O right equilibrium U(OH)₂²⁺ + 2H⁺, log *β₂ < -4.5. Apart from its higher sensitivity, the TLS method has the same characteristics as ordinary spectrophotometric methods. The experimental results of this study and previous literature information were interpreted by using the specific ion interaction theory to give log *β degree ₁ = -0.51 ± 0.03 (log *β degree ₁ is the equilibrium constant at zero ionic strength) and Δε is the ion interaction term Δε = ε(UOH³⁺, ClO₄^-) + ε(H⁺, ClO₄^-) - ε(U⁴⁺, ClO₄^-). 16 refs., 5 figs

[en] When the energy of a monochromatic gamma-ray beam is tuned over the K-edge of an element, the absorption of the beam due to this element changes abruptly. Using the tunable monochromatic gamma-ray source in Ghent, energies just above and just below the edge of an element were produced to identify this particular element by means of the absorption characteristics. Identification and quantitative analysis were possible for several heavy elements in samples of known composition. Unknown targets (pieces cut from uranium ore) were analysed as well, and the results obtained with the absorption method were compared with X-ray photographs taken of the surface of these pieces. Finally, two-dimensional heavy element tomographies were performed. (orig.)

[en] Published in summary form only

[en] Published in summary form only

No abstract available

[en] Although the chemistry of uranium(IV) chloride complexes in molten chloride melts has been widely investigated, no studies of uranium(IV) oxychloride complexes in these melts have appeared, with the exception of a recent paper from this laboratory. In that paper, visible and near-IR absorption bands overlapping with those of the uranium(IV) chloride complex are attributed to oxychloride species. Attempts have been made to prepare solutions containing uranium oxychloride by a direct dissolution of UOCl₂ into solvents such as water, acids, and molten pyridinium chloride. However, only uranium(IV) chloride complex species are recovered. This led Ewing to conclude that the uranium-oxygen bond in UOCl₂ is stabilized only in the solid state probably by the lattice energy of the crystal. Here the authors want to report, for the first time, the visible and near-IR spectra of uranium oxychloride in molten MgCl₂-KCl without interference from those of uranium(IV) chloride complexes. In addition, experimental evidence is provided to confirm that the new uranium(IV) species is an oxygen-containing chloride complex