Showing papers on "Uranyl published in 1995"

The Influence of Uranyl Hydrolysis and Multiple Site-Binding Reactions on Adsorption of U(VI) to Montmorillonite

Abstract: Adsorption of uranyl to SWy- 1 montmorillonite was evaluated experimentally and results were modeled to identify likely surface complexation reactions responsible for removal ofuranyl from solution. Uranyl was contacted with SWy- 1 montmorillonite in a NaC104 electrolyte solution at three ionic strengths (I = 0.001, 0.01, 0.1), at pH 4 to 8.5, in a N2~) atmosphere. At low ionic strength, adsorption decreased from 95% at pH 4 to 75% at pH 6.8. At higher ionic strength, adsorption increased with pH from initial values less than 75%; adsorption edges for all ionic strengths coalesced above a pH of 7. A site-binding model was applied that treated SWy-1 as an aggregate of fixed-charge sites and edge sites analogous to gibbsite and silica. The concentration of fixed-charge sites was estimated as the cation exchange capacity, and non-preference exchange was assumed in calculating the contribution of fixed-charge sites to total uranyl adsorption. The concentration of edge sites was estimated by image analysis of transmission electron photomicrographs. Adsorption constants for uranyl binding to gibbsite and silica were determined by fitting to experimental data, and these adsorption constants were then used to simulate SWy-1 adsorption results. The best simulations were obtained with an ionization model in which A1OH2 + was the dominant aluminol surface species throughout the experimental range in pH. The pH-dependent aqueous speciation of uranyl was an important factor determining the magnitude of uranyl adsorption. At low ionic strength and low pH, adsorption by fixed-charge sites was predominant. The decrease in adsorption with increasing pH was caused by the formation of monovalent aqueous uranyl species, which were weakly bound to fixed-charge sites. At higher ionic strengths, competition with Na + decreased the adsorption of UOz 2+ to fixed-charge sites. At higher pH, the most significant adsorption reactions were the binding of UP22+ to A1OH and of(UO2)3(OH)5 + tO SiOH edge sites. Near-saturation of A1OH sites by UOz 1+ allowed significant contributions of SiOH sites to uranyl adsorption.

Multinuclear nmr, raman, exafs, and x-ray-diffraction studies of uranyl carbonate complexes in near-neutral aqueous-solution - x-ray structure of c(nh2)(3) (6) (uo2)(3)(co3)(6) center-dot-6.5h(2)o

Abstract: C-13 and O-17 NMR and Raman spectroscopies were used to monitor the fractions of UO2(CO3)(3)(4-) (1) and (UO2)(3)(CO3)(6)(6-) (2) in aqueous carbonate solutions as a function of pH, ionic strength, carbonate concentration, uranium concentration, and temperature. The multinuclear NMR and Raman data are consistent with the formation of (UO2)(3)(CO3)(6)(6-). The pH dependence of the C-13 NMR spectra was used to determine the equilibrium constant for the reaction 3UO(2)(CO3)(3)(4-) + 3H(+) reversible arrow (UO2)(3)(CO3)(6)(6-) + 3HCO(3)(-), log K = 18.1(+/- 0.5) at I-m = 2.5 m and 25 degrees C, and corresponds to log beta(36) = 55.6(+/- 0.5) for the reaction 3UO(2)(2+) + 6CO(3)(2-) reversible arrow (UO2)(3)(CO3)(6)(6-) under the same conditions. Raman spectra showed the uranyl nu(1) stretching band at 831.6 cm(-1) for monomeric 1 and at 812.5 cm(-1) for trimeric (UO2)(3)(CO3)(6)(6-) (2). EXAFS data from solid [C(NH2)(3)](6)[(UO2)(3)(CO3)(6)] and a solution of (UO2)(3)(CO3)(6)(6-) suggest that the same uranium species is present in both the solid and solution states. Fourier transforms of the EXAFS spectra of both solid and solution samples revealed five well-resolved peaks corresponding to nearly identical near-neighbor distances for solid and solution- samples of 2. Fitting of these peaks yields U-O(uranyl) = 1.79, U-O(carbonate) = 2.45, U- -C = 2.90, U- -O(terminal carbonate) = 4.16, and U- -U = 4.91 Angstrom for the solid and similar distances for the solution sample. A peak at 4.75 Angstrom in both Fourier transforms (uncorrected for phase shift) corresponds to a U- -U interaction at 4.91 Angstrom, a conclusion which is supported by the absence of this peak in the Fourier transform of the crystalline monomeric K-4[UO2(CO3)(3)] Multiple scattering along the uranyl vector is believed to play a significant role in the EXAFS of all three systems. The EXAFS data are consistent with the trimeric uranyl carbonate species indicated by NMR spectroscopy. Single crystals of [C(NH2)(3)](6)[(UO2)(3)(CO3)(6)]. 6.5H(2)O were obtained from a solution that contained stoichiometric amounts of uranyl nitrate and guanidinium carbonate and an excess of guanidinium nitrate at pH 6.5 under a CO2 atmosphere. The solid state molecular structure of [C(NH2)(3)](6)[(UO2)(3)(CO3)6]. 6.5H(2)O contains a planar D-3h trimetallic (UO2)(3)(CO3)(6)(6-) anion, the structure that Aberg and co-workers originally proposed for the trimeric solution species. The trimetallic anion contains three uranium atoms and all six carbonate ligands in the molecular plane with three uranyl oxygen atoms above and three below the plane. Uranyl U=O distances average 1.78(1) Angstrom, while U-O distances to the carbonate oxygen atoms average 2.41(1) Angstrom for terminal and 2.48(1) Angstrom for bridging ligands. Particularly significant is the average nonbonding U- -U distance of 4.97 Angstrom which compares favorably to the 4.91 Angstrom distance seen in the EXAFS analysis. The molecule crystallizes in the triclinic space group P ($) over bar 1, with a = 6.941(2) Angstrom, b = 14.488(2) Angstrom, c = 22.374(2) Angstrom, alpha = 95.63(2)degrees, beta = 98.47(2)degrees, gamma = 101.88(2)degrees, R = 0.0555, R(w), = 0.0607, V = 2158.5 Angstrom(3),d(cal), = 2.551 g cm(-3), and Z = 2.

Neutral carrier and organic resin based membranes as sensors for uranyl ions

Abstract: Uranyl ion selective electrodes have been constructed from poly(vinyl chloride) matrix membranes containing neutral carrier, 2,3,11,12-dicyclohexano-1,4,7,10,13,16-hexaoxacyclooctadecane and organic resin, 2,4-dihydroxypropiophenoneoxime–formaldehyde using dibutyl phthalate and dioctyl phthalate as plasticizing solvent mediators These electrodes demonstrate linear response to UO2+2 ions in the concentration range 10–5–10–1 mol l–1 with a Nernstian or near Nernstian slope at pH 3 The electrodes are inert towards acids, salt solutions and non-aqueous media for a period of 3 months and exhibit a response time as fast as 30 s The electrodes with plasticizers have been found to be better compared with electrodes without plasticizers The electrodes are sufficiently selective over a large number of bivalent and trivalent cations including calcium, iron(III) and copper(II), with selectivity coefficient values of the order of 001 for bivalent and 0001 for trivalent cations Monovalent ions cause interference at higher concentrations with selectivity coefficient values of the order of 01 The electrodes work satisfactorily in a partially non-aqueous medium

Uranium speciation in solution by time-resolved laser-induced fluorescence

Abstract: Complexation studies of radionuclides (such as uranium) are important to perform in order to predict their migration behavior in natural systems. Time-resolved laser-induced fluorescence, which is a very selective and sensitive method for actinide and lanthanide analysis, is used to study the interactions between uranyl cation and hydroxide ions at low uranium concentration (0.4 [mu]M). The principle of the study is based on time resolution and spectral convolution since free uranyl ion (UO[sub 2][sup 2+]) and first hydroxide complex give specific fluorescence spectra characterized by their spectral shift and fluorescence lifetime. Spectroscopic data (main fluorescence wavelengths, half-bandwidth, lifetime) for the first hydrolyzed complex UO[sub 2]OH[sup +] are obtained. From experimental spectra and time-resolved spectral convolution, the proportion of each species (free uranyl, complexed uranyl) can be estimated at different pH. 33 refs., 5 figs., 3 tabs.

Infrared Spectroscopic Study of Uranyl Biosorption by Fungal Biomass and Materials of Biological Origin

Abstract: Metal ion biosorption has been well studied, but the nature of the interactions between metal ions and biomass is still controversial. This paper deals with a description of the influence of pH and sorbent on the removal of uranyl in solution. Infrared spectroscopy allows the influence of amino functions to be characterized for various fungal biomasses such as Aspergillus niger, Penicillium chrysogenum, or Mucor miehei. The binding of uranyl to amine sites confers that it has a structure similar to that of amide. Metal ion sorption is thus examined through its effects on the amide bands. Uranyl sorption changes the relative intensities of the amine or amide bands. Moreover, the UO vibration band appears at a wavenumber that varies according to the pH and the nature of the metal ion species in the solution. As the cell wall, which mainly consists of chitosan or glycoprotein fibers, has been identified as the major site of metal accumulation, the sorption of uranyl by chitosan is studied by spectroscopic studies. Observed differences are explained by the influence of several functional groups in the fungal biomass. The complexation of uranyl by amino ligands such as the monomeric units present in chitin or chitosan (i.e., acetylglucosamine or glucosamine) is governed by the pH.

Supercritical fluid extraction of uranium and thorium from nitric acid solutions with organophosphorus reagents.

Abstract: Extraction techniques for the recovery of uranium and transuranic elements from acid waste solutions are important in nuclear waste management. This paper examines the feasibility of extracting uranyl and thorium ions from nitric acid solutions with supercritical CO{sub 2} containing the different organophosphorus reagents. In this study, an organophosphorus reagent is dissolved in supercritical CO{sub 2} by passing the fluid through a reagent vessel placed upstream of the sample vessel in the extractor. Using TBPO or TOPO in supercritical CO{sub 2}, effective extraction of uranyl and thorium ions can be achieved even in dilute HNO{sub 3} solutions, thus yielding the possibility of reducing acidic waste volumes in nuclear waste treatment. The results may form the basis of a novel extraction process for the treatment of acidified nuclear wastes, while minimizing the production of secondary wastes. 12 refs., 2 figs., 2 tabs.

Characterisation of Hydroxide Complexes of Uranium(VI) by Time-Resolved Fluorescence Spectroscopy

Abstract: Deconvolution of composite fluorescence spectra and related decay curves for UVI in the pH ranges 0–5 and 9–11 at different total U concentrations, yielded decay times and individual fluorescence spectra for various hydroxo-complexes (UO2)m(OH)n(2m–n), where (m, n)=(1, 1); (1, 3); (2, 2) and (3, 5). Depending on the emission wavelength, the fluorescence efficiencies of (2, 2) and (3, 5) were found to be, respectively, 7 to 85, and 3 to 4 times higher than that of the uranyl ion, whereas the latter is ca. 3 times more fluorescent than the (1, 1) complex. Thus, relatively small concentrations of polynuclear species contribute appreciably to the overall fluorescence of aqueous UVI solutions. The fairly large difference between decay times and the high sensitivity of the equipment used made it possible to detect even small amounts of the hydroxo complexes. In the alkaline pH range 10–12, the (1, 3) hydroxo-complex was found to predominate. In the presence of 0.05 mol dm–3 phosphate and in the same pH range, there was a pronounced change in the fluorescence spectrum indicating that the chemical speciation in the system had changed. These observations make it necessary to revise the current equilibrium data for uranium (VI) phosphate complexes at high pH.

Behaviors of Acrylamide/Itaconic Acid Hydrogels in Uptake of Uranyl Ions from Aqueous Solutions

Abstract: In this study, adsorptions of uranyl ions from two different aqueous uranyl solutions by acrylamide-itaconic acid hydrogels were investigated by a spectroscopic method. The hydrogels were prepared by irradiating with γ-radiation. In the experiment of uranyl ions adsorption, Type II adsorption was found. One gram of acrylamide-itaconic acid hydrogels sorbed 178–219 mg uranyl ions from the solutions of uranyl acetate, 42–76 mg uranyl ions from the aqueous solutions of uranyl nitrate, while acrylamide hydrogel did not sorb any uranyl ion. For the hydrogel containing 40 mg of itaconic acid and irradiated to 3.73 kGy, swelling of the hydrogels was observed in water (1660%), in the aqueous solution of uranyl acetate (730%), and in the aqueous solution of uranyl nitrate (580%). Diffusions of water onto hydrogels were a non-Fickian type of diffusion, whereas diffusions of uranyl ions were a Fickian type of diffusion.

Multiple-scattering calculations of the uranium L 3 -edge x-ray-absorption near-edge structure

Abstract: A theoretical study of the uranium ${\mathit{L}}_{3}$-edge x-ray absorption near-edge structure (XANES) is presented for several uranium compounds, including oxides, intermetallics, uranyl fluoride, and \ensuremath{\alpha}-uranium Calculations were performed using feff6, an ab initio multiple-scattering (MS) code that includes the most important features of current theories The results, which account for both the fine structure \ensuremath{\chi} and the atomiclike background ${\mathrm{\ensuremath{\mu}}}_{0}$ of the absorption coefficient \ensuremath{\mu}, are compared to new and previously measured experimental spectra, reavealing very good agreement for most systems For several compounds, a more detailed theoretical analysis determined the influence of cluster size and scattering order upon the calculated spectra Results indicate that MS paths and scattering paths that include rather distant atoms make significant contributions for ${\mathrm{UO}}_{2}$, whereas XANES for crystals with lower symmetry and density can be modeled using only shorter single-scattering paths In most cases, assumption of a screened final state in the calculation gives better agreement with experiment than use of an unscreened final state The successful modeling of spectra for a variety of different uranium compounds, with differing spectral features, indicates that the semirelativistic treatment of XANES used here is adequate even for heavy elements The well-known resonance, observed experimentally for uranyl (${\mathrm{UO}}_{2}^{2+}$) compounds \ensuremath{\approxeq}15 eV above the white line, is successfully modeled here for the first time, using multiple-scattering paths within the O-U-O axial bonds Overlapping muffin-tin spheres were required in the calculation, probably as a result of the short uranyl axial bonds

Adsorptions of Some Heavy Metal Ions in Aqueous Solutions by Acrylamide/Maleic Acid Hydrogels

Abstract: In this study, acrylamide—maleic acid (AAm/MA) hydrogels in the form of rod have been prepared by γ-radiation. They have been used for adsorption of some heavy metal ions such as uranium, iron, and copper. For the hydrogel containing 40 mg of maleic acid and irradiated at 3.73 kGy, maximum and minimum swellings in the aqueous solutions of the heavy metal ions have been observed with water (1480%) and the aqueous solution of iron(III) nitrate (410%), respectively. Diffusions of water and heavy metal ions onto hydrogels have been found to be of the non-Fickian type of diffusion. In experiments of uranyl ions adsorption, Type II adsorption has been found. One gram of AAm/MA hydrogels sorbed 14–86 mg uranyl ions from solutions of uranyl acetate, 14–90 mg uranyl ions from solutions of uranyl nitrate, 16–39 mg iron ions from solutions of iron(IV) nitrate, and 28–81 mg copper ions from solutions of copper acetate, while acrylamide hydrogel did not sorb any heavy metals ions.

Carboxylato complexes of the uranyl ion: Effects of ligand size and coordination geometry upon molecular and crystal structure

Abstract: The molecular and crystal structures of complexes with uranyl ion of carboxylate ions of varying type (mono- and polycarboxylate, aliphatic, aromatic, heterocyclic) are surveyed. The tendencies to form monomers on the one hand, or chains or sheets on the other, can be related to ligand size and functionality.

Ab initio quantum chemical calculations on uranyl uo22+, plutonyl puo22+, and their nitrates and sulfates

Abstract: Ab initio molecular orbital calculations on the structure and stability of the nitrate and sulfate complexes of uranyl (UO{sub 2}{sup 2+}) and plutonyl (PuO{sub 2}{sup 2+}) using effective core potentials are reported. It is found that the binding energy of sulfate is greater than that of nitrate to both uranyl and plutonyl, with a slight preference for plutonyl. A method of decomposing the binding energy into electrostatic, Pauli repulsion, polarization, and charge-transfer components is described which predicts that electrostatic forces are dominant. A simple molecular mechanics potential is developed by using this finding, which is successful in reproducing the ab initio results. 38 refs., 2 figs., 6 tabs.

NUO+, a New Species Isoelectronic to the Uranyl Dication UO

Abstract: The nitridooxouranium cation NUO+ has been prepared as a new isoelectronic homologue to the uranyl dication UO+ by a sequence of ionmolecule reactions starting from the atomic uranium cation U+. Collision-induced dissociation experiments and ligand-exchange reactions serve to unambiguously identify the ion as a species in which the uranium cation is formally inserted in the NO molecule. New thermochemical data for NUO+ and UN+ are given, and the stability of NUO+ with respect to UO+ is investigated by means of quasi-relativistic density-functional calculations.

Enhancement of uranium bioaccumulation by a Citrobacter sp. via enzymically-mediated growth of polycrystalline NH4UO2PO4

Abstract: The objective of this work was to study the effect of pH and ionic matrix on product release and on the removal of uranyl ion by a Citrobacter. sp phosphatase enzyme-catalysed reaction. An improvement in the efficiency of uranyl removal was obtained by incorporating ammonium acetate (NH 4 Ac) into the solution. It was confirmed using X-ray diffraction (XRD), and other analytical techniques, that the insoluble, cell-bound product was NH 4 UO 2 PO 4

Extraction of lanthanide(iii) and uranyl(vi) from nitric acid solution by n,n'-dimethyl-n,n'-dibutylmalonamide

Abstract: The extractive properties of N,N'-dimethyl-N,N'-dibutylmalon-amide ( DMDBMA ) and the loading capacity of extraction solvents containing DMDBMA for lanthanide(III) and uranyl(VI) ions in nitric acid solution were investigated. The dependence of the distribution ratio on the concentration of nitric acid shows that DMDBMA has an ability to extract uranyl(VI) ion, while the extracting capacity for lanthanide(III) ions is not enough for the application in practice. The slope analyses for La(III), Nd(III), Eu(III) and UO2(VI) ions in 3.0 M HNO3 solution exhibit a straight line with the slope of 2.9, 2.9, 2.1 and 2.3, respectively. It was found that these extractions are an exothermic reaction with the enthalpy change of approximate -20[kJ/mo1]. The solubilities of DMDBMA into various diluents were related with the Hildebrand's solubility parameter, δ the Kamlet-Taft's polarity parameter, π*, and the Gutmann's acceptor number, AN, of the diluent. The loading percentages of DMDBMA with Nd(III) and UO2(V...

Enzymatically‐mediated uranium accumulation and uranium recovery using a Citrobacter sp. Immobilised as a biofilm within a plug‐flow reactor

Abstract: Biofilm-immobilised Citrobacter sp removed uranyl ion from flows supplemented with glycerol 2-phosphate The metal uptake mechanism was mediated by the activity of a cell-surface bound phosphatase that precipitated liberated inorganic phosphate with uranyl ion as HUO 2 PO 4 4H 2 O at the bacterial surface A modified integrated form of the Michaelis-Menten equation is proposed to describe the removal of metal ion by a columnar bioreactor, where the efficiency of metal removal is semi-quantitatively related to the input flow rate, the total enzyme loading (E 0 ) and the bioreactor activity With biofilm immobilised bacteria, E 0 was further divisible (split) into subparameters of phosphatase titre per bacterium and total biomass surface area Varying the split E 0 and the reaction temperature modified the bioreactor performance The immobilised bacteria retained high metal loads without loss in steady-state activity Accumulated metal was recovered as a concentrated solution

Synthesis of Neutral and Anionic Uranyl Aryloxide Complexes from Uranyl Amide Precursors: X-ray Crystal Structures of UO2(O-2,6-i-Pr2C6H3)2(py)3 and [Na(THF)3]2[UO2(O-2,6-Me2C6H3)4]

Abstract: Reaction of anhydrous UO{sub 2}Cl{sub 2} with 2 equiv of KN(SiMe{sub 3})2 in THF leads to formation of the neutral bis(amido) complex UO{sub 2}[N(SiMe{sub 3})2]2 (1), whereas the use of 2, 3, or 4 equiv of NaN(SiMe{sub 3})2 in an analogous reaction produces the tetrakis (amido) salt [Na(THF){sub 2}]2[UO{sub 2}[N(SiMe{sub 3})2]4] (2). Alcoholysis of 1 with 2 equiv of HO-2,6-i-Pr{sub 2}C{sub 6}H{sub 3} in the presence of pyridine gives the bis(aryloxide) complex UO{sub 2}(O-2,6-i-Pr{sub 2}C{sub 6}H{sub 3})2(py){sub 3} (3) (py=C{sub 5}H{sub 5}N), whereas alcoholysis of 2 with 4 equiv of HO-2,6-Me{sub 2}C{sub 6}H{sub 3} in THF produces the tetrakis (aryloxide)salt [Na(THF){sub 3}]2[UO{sub 2}(O-2,6-Me{sub 2}C{sub 6}H{sub 3})4] (4) The molecular structures of 3 and 4 have been determined by means of single-crystal X-ray diffraction studies. Compound 3 exhibits pentagonal bipyramidal geometry, with the uranyl oxygen atoms occupying the axial positions and the aryloxide atoms and 2.197 (9) {Angstrom} for the aryloxide ligands, while U-N distances to the pyridine ligands average 2.616 (7) {Angstrom}. Compound 4 is comprised of a uranium metal center coordinated in a pseudo-octahedral fashion by six oxygen atoms, with two sodium cations each coordinated to one uranyl oxygen atom and one oxygen atom of an aryloxide ligand. U-Omore » bond distances to the uranyl oxygens average 1.814(5) {Angstrom}, while distances to the aryloxide ligands average 2.204(8) {Angstrom} for the two aryloxide ligands not involved in coordination to the sodium cations and 2.288(5) {Angstrom} for the aryloxide ligands forming U-O-Na interactions.« less

Speciation of Uranyl Species in Nitric Acid Medium by Time-Resolved Laser-Induced Fluorescence:

Abstract: The aim of this work is the development of an on-line analytical procedure for uranyl trace determination in the nuclear fuel reprocessing process using time-resolved laser-induced fluorescence. Because the uranyl fluorescence spectrum is strongly affected by the nitrate concentration, knowledge of composition of the medium is necessary to normalize measurements. This paper reports the assumptions made on the spectral distortion, leading to a spectral deconvolution model. Uranyl complex formation constants are obtained from the spectral deconvolution and validate the method. In this way, spectral distortion allows the determination of the nitrate concentration with good accuracy.

Aqueous uranyl complexes. 3. Potentiometric measurements of the hydrolysis of uranyl(VI) ion at 25°C

Abstract: Potentiometric titrations of uranyl(VI) solutions were conducted using a standard glasslcalomel electrode combination over the pH range 3 to 12 at 0.1 molkg−1 ionic strength with tetramethylammonium trifluoromethanesulfonate as the supporting electrolyte. The electrodes were calibrated directly on the hydrogen ion concentration scale during the initial stage of each titration. The species, UO 2 2+ , (UO2)2(OH) 2 2+ , (UO2)3(OH) 5 + , (UO2)3(OH) 7 − , (UO2)3(OH) 8 2− , and (UO2)3(OH) 10 4− identified in an earlier Raman study were compatible with the analysis of the titration data. Based on this analysis and application of the extended Debye-Huckel treatment, the polynuclear species indicated above were assigned overall formation constants at 25°C and at infinite dilution of −5.51±0.04, −15.3±0.1, −27.77±0.09, −37.65±0.14, and −62.4±0.3, respectively. The results are discussed in reference to hydrolysis quotients reported in the literature for the first three species. Formation quotients for the last two species have not been reported previously.

Adsorption behavior of Cu2 and UO22 ions on crosslinked poly[2,2-bis(acrylamido)acetic acid]

Abstract: The synthesis of a water-insoluble metal ion complexing resin was carried out by radical polymerization of 2,2-bis(acrylamido) acetic acid. The resin was characterized by elemental analyses, FTIR spectroscopy, and thermal analyses. The ability to bind copper(II), iron(II), iron(III), and uranium(VI) as well as the elution of the metal ions from the loaded resins were studied. For uranyl ions, pH 5 was the optimum sorption pH value. Sorption selectivity from the binary mixture Cu(II) + U(VI) was studied at pH 2. The thermal stability is increased by adsorption of UO. According to these results a coordination mechanism is suggested for sorption of copper and uranium. © 1995 John Wiley & Sons, Inc.

Kinetic Behavior of Carbonate Ligands with Different Coordination Modes: Equilibrium Dynamics for Uranyl(2+) Carbonato Complexes in Aqueous Solution. A 13C and 17O NMR Study

Abstract: Exchange processes (proton transfer and carbonate transfer) between uranyl and carbonate in solutions of uranyl carbonate were studied over the ranges 6 < pH < 9, and -5{degrees}C < t < 85{degrees}C NMR methods were utilized in these studies including the 2D-EXSY technique Rate constants for various reaction pathways are presented

Role of citrate as a complexing ligand which permits enzymically-mediated uranyl ion bioaccumulation

Abstract: Microorganisms can be used to remove toxic heavy metals from liquid industrial wastes. One potentially useful system utilizes the enzymically-mediated biomineralization of heavy metals at the surface of bacterial cells. This well-documented system harnesses a metal-resistant phosphatase enzyme overproduced by a Citrobacter sp.; metal uptake is mediated by the activity of this enzyme, which persists in non-growing cells, to liberate HPO{sub 4}{sup 2-} from glycerol 2-phosphate with stoichiometric deposition of heavy metals(M) as cell-bound MHPO{sub 4}. Recent attention has focused on the treatment of wastes from nuclear power and nuclear fuel reprocessing activities, together with discharges of native uranium in mining wastes. Previous investigations using the Citrobacter sp. demonstrated removal of uranium and the transuranic elements, plutonium and americium. Although uranium is inhibitory to the growth of the Citrobacter strain, and the activity of the cellular phosphatase, uranyl phosphate accumulates as polycrystalline HUO{sub 2}PO{sub 4} at the cell surface. The rate of uranyl removal into the growing crystal is primarily dependent on the rate of phosphate release by the enzyme catalysed reaction. This is inconsistent with the reported toxicity of uranyl ion to the mediating phosphatase; however, in the presence of an excess of substrate, the rapid rate of phosphatemore » release facilitated metal precipitation without toxic effect. Under substrate-limiting conditions uranyl toxicity was seen. The present investigation shows the inhibition of Citrobacter sp. phosphatase is related to the concentration of uranyl ion, and that citrate buffer can protect against this toxicity, and permit metal bioaccumulation. The toxicity pattern is dependent upon the substrate used; possible reasons for these effects, and environmental implications are discussed. 18 refs., 5 figs.« less

Flow injection spectrophotometric determination of uranium with in-valve ion-exchange column preconcentration and separation

Abstract: An in-valve column containing Duolite C-225(H) resin for preconcentration and separation flow injection determination of uranium is described. Uranyl ions loaded on the column were eluted with 1 mol l–1 HCl and mixed with a stream of buffer and masking solution [triethanolamine (1 mol l–1) and 1,2-cyclohexanediaminetetraacetic acid (0.7% m/v)]. After passing through a glass bead column the complex product was continuously monitored at 530 nm. Calibration by time (or volume) using a single standard was performed up to 7µg of U. The detection limit (3σ) and relative standard deviation were 0.27 µg of U and 2.1%(for 2 µg; n= 12), respectively. Interferences of some ions were studied. Applications to the determination of uranium in rock and water samples are described.