Uranyl Concentration Research Articles

Oxygen-17 NMR Study of the Uranyl Ion. III. Factors Influencing 17O Chemical Shifts of Uranyl Oxygens in Aqueous Perchlorate Media

Abstract Effects of concentrations of perchlorate ion, nitrate ion, uranyl ion, and acid, isotopic substitutions, and temperature on 17O chemical shifts of oxygen in the uranyl ion (uranyl oxygens hereafter) have been studied in aqueous perchlorate media. Insensitivity of the 17O chemical shifts to [ClO4−] leads to the conclusion that the perchlorate ion is not coordinated to the uranyl ion. The stability constant (0.10±0.04 m−1 at 25 °C and ionic strength 5.0 m; m=mol kg−1) of a UO2NO3+ complex, has been obtained from the dependence of the 17O chemical shifts on [NO3−]. The 17O resonances shift toward the lower field with increasing concentrations of uranyl ion and acid, indicating the possibilities of the dimer formation of the uranyl ion and the protonation of uranyl oxygen atoms in concentrated solution, respectively. Isotope shifts in 17O nuclear magnetic shielding in the uranyl ion are interpreted in terms of the change of mean separation between the ground and lowest excited electronic states upon oxygen and uranium isotopic substitutions. Temperature dependence of 17O chemical shifts is discussed in connection with the isotope effects.

Ground state association and fluorescence quenching of aromatic molecules by uranyl ion in alcohols

The apparent rates of the fluorescence quenching of aromatic molecules by the uranyl ion in ethanol solutions are considerably higher than the rate of diffusion. At low uranyl ion concentrations this is attributed to the existence of ground state 1:1 (UO 2 2+ · aromatic) complexes which obtain their stability almost entirely from highly positive entropy changes; such associations are very sensitive to the solvent polarity and are not observed in less polar media. The existence of these entropically favourable complexes in ethanol could be confirmed neither by absorption spectroscopy nor by nuclear magnetic resonance spectroscopy, but it is supported by measurements of the viscosity of the solutions and by the effect of temperature on the quenching processes. Negative curvatures of the Stern-Volmer plots at high uranyl ion concentrations in ethanol solutions are attributed to the presence of ion pairs and/or polynuclear uranyl complexes.

An emission spectral study of uranyl ion binding to α-alumina

The effect of binding by α-alumina is examined on the luminescence spectrum of the uranyl ion. At relatively low concentrations, luminescence studies are complicated by the overlap of the uranyl luminescence and the relatively intense scattering bands of the α-alumina. The presence of uranyl ion is shown to lead to a considerable reduction in the intensity of this scattering, and comparison of emission spectra of this sample with those of alumina, uranyl nitrate and an alumina/uranyl nitrate suspension, followed by computer analysis of the common bands allows an idea of the probable uranyl emission spectrum under these conditions. At high uranyl concentrations less interference from the scattering bands is observed. X-ray diffraction of this sample shows uranyl binding without changing the alumina structure, but with modifications in the lattice dimensions. The room temperature emission spectrum of this sample is very similar to that of uranyl nitrate hexahydrate. In addition, certain other bands are observed which are probably due to Raman scattering. Isopropanol is shown to quench this emission, indicating that solvent access is possible to the uranyl binding site. Luminescence lifetime studies indicate a non-exponential decay, and suggest the possibility of uranyl ion being bound to more than one site. Support for this comes from emission spectra obtained at 77K.

Radical polymerization of N,N-diallyl quaternary salts in the presence of uranyl ions

1. The presence of uranyl ions in an aqueous solution of N,N-diallyl-N-isopropoxycarbonylmethyl-N-methylammonium chloride (DIMC) markedly accelerates the photoinitiated radical polymerization of this salt. 2. The initial polymerization rate is increased by a factor of up to 20 upon increasing the initial uranyl ion concentration in aqueous solutions of the monomer containing ammonium peroxydisulfate. The dependence of the initial polymerization rate and of the specific viscosity of the polymer formed on the UO22+/DIMC mole ratio passes through a maximum.

Adsorption of uranyl onto ferric oxyhydroxides: Application of the surface complexation site-binding model

Uranyl adsorption was measured from aqueous electrolyte solutions onto well-characterized goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C. Adsorption was studied at a total uranyl concentration of 10 −5 M, (dissolved uranyl 10 −5 to 10 −8 M) as a function of solution pH, ionic strength and electrolyte concentrations, and of competing cations and carbonate complexing. Solution pHs ranged from 3 to 10 in 0.1 M NaNO 3 solutions containing up to 0.01 M NaHCO 3. All the iron oxide materials strongly adsorbed dissolved uranyl species at pHs above 5 to 6 with adsorption greatest onto amorphous ferric oxyhydroxide and least onto well crystallized specular hematite. The presence of Ca or Mg at the 10 −3 M level did not significantly affect uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate complexing severely inhibited adsorption. The uranyl adsorption data measured in carbonate-free solutions was accurately modeled with the surface complexation-site binding model of Davis et al. (1978), assuming adsorption was chiefly of the UO 2OH + and (UO 2) 3(OH) + 5, aqueous complexes. In modeling it was assumed that these complexes formed a monodentate UO 2OH + surface complex, and a monodentate, bidentate or tridentate (UO 2) 3(OH) + 5surface complex. Of the latter, the bidentate surface complex is the most likely, based on crystallographic arguments. Modeling was less successful predicting uranyl adsorption in the presence of significant uranyl carbonate and hydroxy-carbonate complexing. It was necessary to slightly vary the intrinsic constants for adsorption of the di- and tricarbonate complexes in order to fit the uranyl adsorption data at total carbonate concentrations of 10 −2 and 10 −3 M.

Microcomputer-Based Voltammetric Determination of Trace Uranium in the Presence of Dissolved Oxygen and Its Application to Sea Water

By eliminating the interfering effect of the oxygen reduction with the aid of a microcomputer, the voltammetric determination of trace uranyl ions after preconcentration on the TOPO-coated glassy carbon electrode was possible in the presence of dissolved oxygen. A voltammogram for the reduction of dissolved oxygen was obtained just before the preconcentration of uranyl ions. It was subtracted from the voltammogram measured after the preconcentration, in which the reduction wave for the concentrated uranyl ions and that for dissolved oxygen were overlapped. The resulting voltammogram was then differentiated, and the peak current for the derivative curve was measured. For the determination of uranium, a standard addition method was employed. The method could be applied to sea water samples by only adjusting their pH to 4.0.

The role of sulfate‐reducing bacteria in the deposition of sedimentary uranium ores

Abstract The formation of many important sediment‐hosted uranium ore deposits is thought to have resulted from the reduction of relatively soluble uranyl ion—U(VI)—to insoluble uranium (IV) oxides and silicates by aqueous sulfide species. This study focused on the influence that the sulfate‐reducing bacteria Desulfovibrio desulfuricans (ATCC 7757) has on this process. Preliminary studies showed that bacterial growth was not inhibited by concentrations of uranyl ion up to 100 mg U per liter. More detailed studies showed that sulfate‐reducing bacteria have an influence on uranyl ion removal beyond the simple production of the aqueous sulfide reductant. Comparative studies of bacterial cultures containing high densities of the sulfate reducers with bacterial cell‐free but otherwise identical media showed that the bacteria themselves enhance uranium removal from solution. At pH 8.0, no reaction was observed in H2S‐bearing cell‐free media, whereas at the same H2S concentration, the uranyl ion decreased markedly in the presence of the bacteria. At pH 7.0, some uranium removal occurred in the absence of bacteria, but it was much more rapid in their presence. We postulate that these effects are due to the ability of bacterial cell walls to adsorb uranium. Adsorption to surfaces is known from independent studies to enhance uranium reduction, and evidently this two‐step adsorption‐reduction mechanism is occurring in our experiments. We conclude that sulfate‐reducing and other bacteria may play a significant role in the geochemical cycling of uranium.

Luminescent solar concentrators using uranyl-doped silicate glasses

We describe solar concentrators containing uranyl. A fluorescence quantum yield of 0.7 is measured at low uranyl concentration (less than 1%) and decreases when the concentration increases. This variation is correlated with the lifetime of the excited state and is due to uranium-uranium interactions. These quantitative determinations can be used to optimize the thickness of the concentrator sheet.

Initial Process of Photo-oxidation of Ethanol by Uranyl Ions in the Rigid Matrix at 77 K

Abstract ESR spectra of radicals produced in the uranyl ion-photosensitized decomposition of ethanol have been measured in the rigid matrix at 77 K. At high concentrations of uranyl ions (>1 mol dm−3), methyl radicals were observed. With decreasing concentrations of uranyl ions, CH3\dotCHOH radicals became important, as expected from the previous studies. This result suggests that the charge transfer mechanism is important in the ethanol rigid matrix, although the hydrogen atom abstraction mechanism is accepted as the main initial process.

Photophysics of the excited uranyl ion in aqueous solutions. Part 3.—Effects of temperature and deuterated water: mechanisms of solvent exchange and hydrogen abstraction from water in excited states

The effects of temperature and D2O on excited uranyl decay in aqueous solution have been studied at several pH values and uranyl concentrations. The data are interpreted in terms of reversible crossing between two excited states of uranyl with different electronic distributions. The highest state, called U*, has a π3uϕ1u configuration whereas the lowest state, called X*, is π3uδ1u(Ω= 1). The reversible decay is interpreted in terms of a solvent-exchange process. The values of ΔH† and ΔS† for this process reveal that solvent exchange proceeds by different mechanisms for the U* and X* states. The activation parameters are pH dependent, but the U* state always has more dissociative character for solvent exchange, whereas the X* state has more associative character. A weak effect on the solvent-exchange rates was found in D2O. The rates of the irreversible decays are attributed mainly to a hydrogen-abstraction reaction from water molecules by excited uranyl ions. The activation energies and the frequency factors are pH dependent, revealing that hydrogen abstraction from the coordinated equatorial H2O and from the more loosely bound water in axial positions can occur. The temperature and the deuterium isotope effects on the decays were analysed in terms of the current theories of radiationless transitions for the involvement of the O—H and UO stretching modes in the abstraction process. The autoquenching processes and the effect of the exciting intensity on the decays are in agreement with the proposed mechanisms.

Interaction of uranyl ions in carbonate media

The interaction of uranyl cation, in dilute solutions and seawater, with carbonate mineral solids was investigated in order to establish the relation between uranyl ion speciation and sorption. In solutions where the uranyl concentration approaches the carbonate alkalinity, rapid sorption followed by slow release occurs. This is accompaned by a color change in solution, spectroscopically determined to be the result of a change in the dominant uranyl species from hydroxy to carbonato complexes. In a heterogeneous system (solution and solid carbonate), the rate of reaction is directly dependent on the ratio of the surface area to the solution volume. The maximum extent of sorption is directly dependent on uranyl concentration and is almost independent of the surface area to volume ratio, the type of solution or the calcium carbonate mineral. Release of Ca, as a result of uranyl-carbonato complex formation, allows an estimation of the average uranyl to carbonate ratio of the complexes formed, which is in excellent agreement with predicted values.

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

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.

Determination of low levels of uranium(VI) in water solutions by means of the laser-induced thermal lensing effect

Thermal lensing spectrometry based on a pulsed, excimer-pumped, tunable dye laser as heating beam and a He-Ne laser as probe is described for the determination of low concentrations of uranyl ions in water solution. The dye laser, tuned at 414 nm, has a peak power of about 200 kW and a bandwidth of 0.02 nm. The smallest detectable sorptivity was 3 × 10 −5 cm −1 and the limit of detection for uranium was 4 × 10 −5 M.

Spectral analysis and kinetics of uranyl glasses

The absorption spectra of uranyl doped soda–lime and lithium phosphate glasses at 77 K consisted of broad bands. The fluorescence due to various continuous monochromatic excitations showed fine structure and revealed two electronic levels within the full fluorescence band. The energy levels in the visible region were located by spectroscopic measurements and were used to calculate the intramolecular decay rates. There was agreement between the theory and experiment. Besides intramolecular relaxations, intermolecular relaxations were observed and they involved uranyl–acceptor interactions via dipole–quadrupole coupling. Analysis on the spectral kinetics yielded the acceptor concentration, which enhanced with increasing uranyl concentration in soda–lime glass. Such a relationship between the acceptor and uranyl concentrations implied that some acceptors were the by-products of thermal decomposition of uranyl ions during sample preparation. Fluorescence decay of uranyl doped lithium phosphate glass consisted of multiple fluorescence decay rates. The thermal dependence of polyphosphate metamorphosis was studied qualitatively by fluorescence decays of uranyl doped samples.

Determination of free acidity in uranyl nitrate solutions

Two modifications of the existing method of determining free acidity in highly concentrated uranyl nitrate solutions by alkalimetric titrations in neutral potassium oxalate medium have been carried out to improve the reliability of the method. Free acidities of several synthetic solutions containing uranium and nitric acid in a wide range of concentration ratios were determined by all the three methods and compared with the results obtained by a more accurate ion exchange method. Interference from other hydrolyzable ions and the precision and accuracy were studied and compared.

Raman spectra of uranyl ion and its hydrolysis products in aqueous nitric acid

Raman spectra of aqueous uranyl ion have been neasured as a function of pH and uranyl concentration. The ..nu../sub 1/ symmetrical stretching vibration has been found to consist of three bands at 869, 851, and 836 cm/sup -1/ which are identified as belonging to the UO/sub 2//sup 2 +/, (UO/sub 2/)/sub 2/(OH)/sub 2//sup 2 +/, and (UO/sub 2/)/sub 3/(OH)/sub 5//sup +/ ions, respectively. The Raman spectral measurements correlate with equilibrium thermodynamic measurements.

Sorption properties of oxides VIII: sorption of uranium on hydrous oxides

Abstract Sorption of unranium from nitrate solution on four hydrous oxides, namely, hydrous titanium, zirconium, cerium and thorium oxides (TiO 2 , ZrO 2 , CeO 2 , ThO 2 ) has been studied at pH 3.5 as a function of uranyl concentration and temperature. The sorbed species was uranyl ion in the case of hydrous TiO 2 and CeO 2 , while in the case of hydrous ZrO 2 and ThO 2 , electrolyte sorption involving uranyl nitrate predominated. Sorption site densities calculated from saturation capacities, evaluated from Langmuir adsorption isotherm and surface areas, bear a direct relationship with the heat of sorption values. Results indicate that, in terms of sorption site density, hydrous CeO 2 seems to be a better sorbent than hydrous TiO 2 for the sorption of uranium.

Preparation of Uranium Kernels by an External Gelation Process

A process has been developed for the production of dense fissile fuel kernels for the high-temperature reactor. A method is used whereby a concentrated uranyl nitrate solution with urea and ammonium nitrate as additives is dispersed horizontally into droplets that are hardened by external gelation. The process is very simple and consists of only a few steps, since no organic thickener is necessary and only aqueous solutions are used. The equipment, which has been developed for the special needs of the refabrication of 233U, has a throughput of ~0.7 kgU/h.

Study of the transport process of uranyl ions in an ion-exchange membrane by solid-state track detector fissionography

A study was made of the diffusion of uranyl ions in an ion-exchange membrane of polystyrenesulphonic acid type. The solid-state track detector technique was used to prepare radiograms on the diffusion concentration profiles in the interior of the membrane at various diffusion times and temperatures. Fission fragments from 238U were detected in mica plates, and the local variations in the uranyl ion concentration were employed to determine the characteristic parameters of the transport process: the diffusion coefficients, the frequency factor and the activation energy.

Surface Charge Density Determination of Micaceous Minerals by 235U Fission Particle Track Method

AbstractThe surface charge density of mica (001) cleavages was determined by counting the number of fission particle tracks in a given area of a 6-mm muscovite disc replica with optical and scanning electron microscopy after saturation of the layer charge by washing with 0.5 M UO2(NO3)2 solution, dilution of the excess salt by washing with 0.01 M UO2(NO3)2 in 0.005 M HNO3 (pH 2.4), blotting off the excess liquid, thermal neutron activation in contact with the muscovite disc, etching the muscovite, and counting the 235U fission tracks/cm2. In initial studies, the uranyl cations were found to hydrolyze from the cleavage surface continuously during the washings with water, ethanol or acetone to remove excess salts, but the uranyl cations in the interlayers near broken edges and crystallographical steps were strongly retained even against washings with 0.5 M CaCl2 solution. The hydrolysis of UO22 + from the smooth portions of the flake surfaces was avoided by the use of three 1-hr final washings with the 0.01 M UO2(NO3)2 in 0.005 M HNO3 solution. Each flake was pressed between filter papers three times to remove the excess solution. A negligible amount of excess salt remained on the cover glass controls. The UO22 + cations retained (mean, 3.6 ± 0.2 × 10−7 mequiv./cm2) on the cleavage surfaces of various micas were nearly equivalent to the theoretical surface charge (cation exchange capacity, 3.5 × 10−7 mequiv./cm2), showing that hydrolysis was prevented. The uranium on the unblemished mica planar surfaces increased with increasing uranyl concentrations in the final washing solution, indicating that the excess salt remaining on the surfaces had become significant. With a given UO22 + salt concentration, the uranium on the surface increased on increasing the solution pH from 2.5 to 3.5, attributable to the formation of polymeric ions such as U2O52 + and U3O82 + with higher uranium retention per unit positive charge equivalent to the fixed negative charge of the mineral surface. Uranyl cations replaced much of the interlayer cations from vermiculites even after K, Rb and Cs presaturation and drying from 110°C were employed. Strong adsorption of uranyl cations (in a form not replaced by washings with a neutral salt solution), which occurred in the defects of micaceous minerals, is important in the interpretation of actinide element retention in soils and sediments wherein these minerals are abundant.