Showing papers on "Uranyl published in 1992"

Direct detection of trace levels of uranium by laser-induced kinetic phosphorimetry

Abstract: The kinetic phosphorimetric determination of uranyl ion in aqueous solutions at room temperature yielded a detection limit for UO 2 2+ of 1 ng/L. The response to uranium is linear from the detection limit up to 5 mg/L. The method is fast and accurate, with no separative pretreatment needed for most of the real samples investigated. In the analysis of biological samples, wet-ashing with HNO 3 /H 2 O 2 is required for measurements near the detection limit

Synthesis and structure of the first uranium(VI) organometallic complex

Abstract: The chemistry of complexes of the d-transition elements containing organoimido functional groups constitutes a rapidly expanding area of research. Examples of molecules containing multiple terminal imido functional groups now exist for representative elements from groups 5-8. In contrast, f-metal terminal organoimido complexes are still relatively rare, and examples of single actinide metal sites coordinated by greater than one imido ligand are unknown. On the basis of the stability of uranyl ion (UO[sub 2])[sup 2+], there appears to be no reason why the isoelectronic bis(organoimido) species should not be accessible. Development of this chemistry has been hampered by the lack of synthetic routes to high-valent (V,VI) uranium complexes and by the relatively small number of ligands which have been demonstrated to stabilize these oxidation states. Here we describe the synthesis and structural characterization of the first bis(organoimido) complex of uranium(VI). This species also represents the first complex of uranium(VI) supporting metal-carbon bonding in either a [sigma]- or [pi]-fashion. As part of our studies of preparative routes to uranium organoimido complexes, we are examining generation of uranium imido functionalities by abstraction of an amido hydrogen by a leaving hydrocarbyl or amido ligand. 4 refs., 1 fig.

Stereognostic coordination chemistry. 1. The design and synthesis of chelators for the uranyl ion

Abstract: A new approach to the molecular recognition of metal oxo cations is introduced based on a ligand design strategy that provides at least one hydrogen bond donor for interaction with oxo group(s) as well as conventional electron pair donor ligands for coordination to the metal center. This concept of stereognostic coordination of oxo metal ions is exemplified in the design of four tripodal ligands-tris[2-(2-carboxyphenoxy)ethyl]amine[NEB],tris[3-(2-carboxyphenoxy)propyl]amine[NPB], tris[3-(2-carboxynaphthyl-3-oxy)propyl]amine [NPN], and tris[3-(2-carboxy=4octadecylphenoxy)propyl]amine[NPodB] - for sequestration of the uranyl ion. The ligands NEB, NPB, and NPN form 1:1 complexes with UO{sub 2}{sup 2+}. The bidentate coordination of carboxyl groups of these compounds is indicated by the infrared spectra, which offer some support for the presence of a hydrogen bond to the uranyl group. Mass spectral data corroborate CPK model predictions that more than five intervening atoms between the tertiary nitrogen atom and the carboxylate groups are required for metal ion incorporation and monomeric complex formation. Solvent extractions of aqueous UO{sub 2}{sup 2+} into chloroform solutions of the ligands have shown them to be powerful extractants. In the case of the very hydrophobic ligand NPodB the stoichiometry of the complexation reaction is shown to be 1:1 UO{sub 2}/ligand complex formed by the release of 3 protons.more » The extraction is quantitative at pH 2.5, and an effective extraction coefficient of about 10{sup 11} is estimated for neutral aqueous solutions of UO{sub 2}{sup 2+}. 81 refs., 13 figs., 2 tabs.« less

DNA binding and photocleavage by uranyl(VI)(UO22+) salts

Abstract: The interaction of the uranyl(VI) ion (UO22+) with DNA and its light-induced cleavage of DNA has been studied using flow-linear dichroism and P-32-endlabeled oligonucleotides. It was found that binding of uranyl ion to DNA is a prerequisite for photocleavage; from run-off experiments the binding constant was estimated to be of the order of 10(10) M-1 at pH 4. The angular orientation of the [O=U=O]2+ chromophore is consistent with binding by bridging phosphate groups on opposite strands of the minor groove of DNA; at higher DNA concentration aggregation indicates intermolecular bridging as well. The uranyl-mediated photocleavage of DNA is not influenced by the presence of O2, is more efficient at low pH (<7), and is virtually absent at pH 8.5. Subsequent treatment of uranyl photocleaved DNA with hot piperidine does not significantly increase the cleavage. The free nucleobases (adenine, cytosine, guanine, and thymine) were the major ethanol-soluble products to be observed after uranyl photocleavage of calf thymus DNA. From experiments using P-32-endlabeled and methyl-phosphonate-containing oligonucleotides, it was concluded that upon irradiation attack by the uranyl ion occurs next to the phosphate to which it is bound, with equal preference for the 3'- and 5'-deoxyribose.

Sorption of Uranyl Ions on Hydrous Silicon Dioxide

Abstract: Sorption of uranyl ions on SiO[sub 2].[chi]H[sub 2]O (silica gel) is investigated in absence and in presence of carbonate as function of pH. The curves obtained are very similar to those observed for sorption of uranyl ion on TiO[sub 2].[chi]H[sub 2]O, indicating the dominating influence of the uranium species in solution. Between pH 2 and 5 the sorption ratio R[sub s] increases with hydrolysis of uranyl ions (formation of UO[sub 2]OH[sup +]), around pH 7 it is nearly independent of pH, and at higher pH it decreases again. The equilibrium constants are calculated for these ranges. In presence of carbonate R[sub s] decreases drastically above pH 6, due to the formation of carbonato complexes in solution. Sorption of uranyl ions on SiO[sub 2].[chi]H[sub 2]O, on TiO[sub 2].[chi]H[sub 2]O, and on cryst. SiO[sub 2] and Al[sub 2]O[sub 3] is compared. The problems of 'surface complexation' modelling are discussed. (orig.).

Sorption of uranyl ions by a modified chitosan: Kinetic and equilibrium studies

Abstract: The uranium sorption of an ascorbic chitosan derivative polymer (N.D.T.C. N‐[2‐(1,2‐dihydroxyethyl) tetra‐hydrofuryl] chitosan) was shown to be three to four times greater than chitosan under the same conditions, reaching 600 mgU.g−1 polymer (i.e. 2.5 mmolU.g−1 polymer). The effect of variation of the classic parameters was studied: pH, metal concentration and particle size. pH and metal concentration, which induced a change in metal solution chemistry, were found to be the controlling parameters, mainly affecting intraparticle diffusion; the diffusion rate is strongly affected by ionic size of solute, as opposed to a limited effect on external mass transfer. pH plays an important part in equilibrium studies, and sorption isotherms are mainly a function of pH. The Freundlich model shows a better correlation coefficient than the Langmuir model when fitted to experimental results; the uptake mechanism involves a monolayer sorption with molecular interactions between molecules sorbed on surface.

Monomeric uranyl alkoxide and amide complexes

Abstract: Reaction of UO 2 Cl 2 (Ph 3 PO) 2 with 2 equiv of KO-t-Bu or NaN(SiMe 3 ) 2 in THF results in the formation of UO 2 (O-t-Bu) 2 (Ph 3 PO) 2 or UO 2 [N(SiMe 3 ) 2 ] 2 (Ph 3 PO) 2 .

Synthesis of cyclic and acyclic oligocytidylates by uranyl ion catalyst in aqueous solution

Abstract: Uranyl ion catalyses the oligomerization of cytidine-5′-phosphoroimidazolide in aqueous solution, yielding 3′–5′-linked cyclic di- and tri-cytidylates preferentially at high catalyst concentration, or 2′–5′-linked linear oligocytidylates at lower catalyst concentration. Addition of Ag+ affects the uranyl ion-catalysed oligocytidylate formation and alters the product distribution.

Interaction of Uranyl(VI) with Silicic Acid

Abstract: The interaction of uranyl(VI) with silicic acid was investigated by a solvent extraction method. The equilibrium constant of the reaction υ Ο Γ + Si(OH)4 = U020Si(0H)î + H + was determined to be (3.6 + 0.5) χ I O 3 in 6.7 χ Ι Ο 2 M silicic acid and pH 3 — 4.5. The extraction behavior of uranyl(VI) was also studied from silicic acid-acetate medium. The results are discussed in relation to metal-silicate interaction and silica polymerization.

Infrared vibrational characterization and synthesis of a family of hydrous alkali uranyl silicates and hydrous uranyl silicate minerals

Abstract: Synthesis studies in the system K 2 O-UO 3 -SiO 2 -H 2 O are presented as an isothermal isobaric projection through the system at 300°C and 30MPa. In additionto the compatibility relationships for weeksite, boltwoodite and soddyite, a new previously unreported hydrous alkali uranyl silicate similar in composition to weeksite was identified. Infrared spectroscopy supplemented X-ray diffraction methods to characterize the structures of the quaternary phases

Effect of Solvent Type on Neutral Macrocycle-Facilitated Transport of Uranyl Ions across Supported Liquid Membrane using Dicyclohexano-18-crown-6 as Carrier

Abstract: The diffusion-Iimited transport of neutral macrocycle-facilitated uranyl ion permeation across thin sheet supported liquid membrane (SLM) has been investigated to quantify the membrane solvent effects on the important parameters such as cation flux and diffusion coefTicients. Also, results obtained with the aim of optimizing the permeation fluxes, physical stability (loss of carrier) and chemical stabihty of SLM's using various aromatic and aliphatic diluents are evaluated in detail. The uranyl ion transport is tested across SLM using dicyclohexano-18-crown-6 as membrane carrier. The diluents studied include Chloroform, 1,2-dichloroethane, dichloromethane, o-dichlorobenzene, toluene, benzene, xylene, and a mixture containing varying proportions of toluene and o-dichlorobenzene. Among the various aliphatic and aromatic diluents tested, the chlorinated hydrocarbons such as Chloroform, 1,2-dichloroethane, dichloromethane, afford much poor permeability compared to aromatic diluents like toluene and o-dichlorobenzene. Among the various mixtures tested for membrane impregnation, a good stability and efficient uranyl ion transport are achieved by using toluene + o-dichlorobenzene admixtures.

Uranium Adsorption by Chelating Resins Containing Amino Groups

Abstract: A series of porous polymers containing primary amino groups has been tested for their ability to complex the uranyl ion in aqueous solutions. The maximum capacity is obtained around pH 6, but that value is much lower than the theoretical value calculated on the basis of the loading in amino groups and the assumed stoichiometry of the metal ligand complex. Electron probe x-ray microanalysis demonstrated that the uranyl ion is chelated in a narrow surface layer, the thickness of which never exceed 250 μm, i.e., about 15% of the total diameter. On the other hand, copper can access all the chelating sites which are uniformly distributed inside the resins. This was explained by the formation of a precipitate of polyuranates in the first layer of the beads, preventing further penetration of the uranyl ion. This process is reversible, as shown by successive cycles of fixation and elution of the metal ion. The capacity of the resins toward uranium is not affected by the presence of an excess of calcium o...

Bacterial biosorption and retention of thorium and uranyl cations by mycobacterium smegmatis

Abstract: Biosorption of Th4+ and UO 2 2+ ions, both separately and in mixed equimolar ratio, was carried out using nitrate-buffered solutions of the cations at pH 1 in the presence of 5%w/w non-proliferative cell suspensions of Mycobacterium smegmatis. At equilibrium following a 3 h treatment, specific adsorption for 2 mM Th and U was, respectively, 102 and 115 μmol g−1 dry biomass for individual solutions and 102 and 42 μmol g−1 for the mixed 2/2 mM solution. Desorption studies of the cation-loaded biomass preparations in aqueous media and in soilbacterial suspensions within the pH range <1 to 11 showed that leaching of throium was generally less than 1% at pH 1–11 after 7 d, whereas uranium was leached to the extent of 2% at pH 1 and up to 10% under the same conditions in Th−U mixtures.

Experimental Study of Uranium(6+) Sorption on the Zeolite Mineral Clinoptilolite

Abstract: Experiments on the sorption of uranium(6+) on clinoptilolite from solutions in equilibrium with atmospheric CO{sub 2}(g) were conducted to understand the fundamental controls on uranium sorption on zeolite minerals, including the effects of pH, aqueous uranium speciation, and uranium concentration in solution. The results indicate that uranium(6+) species are strongly sorbed on the zeolite mineral clinoptilolite at near-neutral pH. The amount of uranium sorbed is strongly dependent on pH and, to some extent, on the total concentration of uranium. Uranium sorption on clinoptilolite is important in the pH range where UO{sub 2}(OH){sub 2}{degrees}(aq) is the predominant uranium aqueous species, whereas sorption is inhibited at pH`s where carbonate- and hydroxy-carbonate-complexes are the primary uranyl species. Surface adsorption appears to be the main sorption mechanism, although at pH<4 the results suggest ion exchange may occur between the UO{sub 2}{sup 2+} ions in solution and the cations in the intracrystalline cation exchange sites of clinoptilolite. The effectiveness of zeolite-rich horizons underneath Yucca Mountain, Nevada, as barriers to actinide transport through sorption processes will depend strongly on groundwater chemistry. Reliable predictions of radionuclide transport through these horizons will need to properly account for changes in solution chemistry.

SOLVENT EXTRACTION OF NITRIC, HYDROCHLORIC AND SULFURIC ACID AND THEIR URANYL SALTS WITH TRI-n-OCTYLPHOSPHINE OXIDE

Abstract: The equilibrium constants of the extraction of nitric, hydrochloric and sulfuric acid and their uranyl salts with tri-n-octylphosphine oxide (TOPO), dissolved in different diluents, have been determined. The extraction constants obtained have been correlated with the corresponding dissociation constants of the mineral acids used. The more mineral acid is dissociated, the fewer undissociated molecules of acid are available and, as a consequence, the extraction process is less pronounced. 12 refs., 1 fig., 5 tabs.

Photochemistry of the uranyl ion with alkenes in acetone—water mixtures

Abstract: Quenching reactions of the excited uranyl ion with various alkenes (1-heptene, 1-hexene, 1-octene, cycloheptene, cyclohexene, 1,4-cyclohexadiene, 2,4-hexadiene and 1,3-cyclohexadiene) were studied in aqueous 50% acetone mixtures. The quenching constants kq of various alkenes were determined by measuring the emission intensity and by the laser flash photolysis method. A plot of log kq vs. the free-enthalpy change ΔG2 for the photoinduced electron transfer showed a linear relationship, but the slope (1.0 eV−1) was much smaller than that predicted from the Rehm—Weller correlation. This indicates that direct electron transfer from alkene to excited uranyl ion was not the rate-determining step. These quenching reactions resulted in the formation of uranium(IV), except for the reactions with alkenes of high vertical ionization potentials. The quantum yields for the formation of uranium(IV) were measured. A plot of the reciprocal quantum yield for the reaction with 1,4-cyclohexadiene vs. its reciprocal concentration gave a straight line.