Showing papers on "Uranyl published in 2002"

Uranium speciation and bioavailability in aquatic systems: an overview.

Abstract: The speciation of uranium (U) in relation to its bioavailability is reviewed for surface waters (fresh- and seawater) and their sediments. A summary of available analytical and modeling techniques for determining U speciation is also presented. U(VI) is the major form of U in oxic surface waters, while U(IV) is the major form in anoxic waters. The bioavailability of U (i.e., its ability to bind to or traverse the cell surface of an organism) is dependent on its speciation, or physicochemical form. U occurs in surface waters in a variety of physicochemical forms, including the free metal ion (U4+ or UO2(2+)) and complexes with inorganic ligands (e.g., uranyl carbonate or uranyl phosphate), and humic substances (HS) (e.g., uranyl fulvate) in dissolved, colloidal, and/or particulate forms. Although the relationship between U speciation and bioavailability is complex, there is reasonable evidence to indicate that UO2(2+) and UO2OH+ are the major forms of U(VI) available to organisms, rather than U in strong complexes (e.g., uranyl fulvate) or adsorbed to colloidal and/or particulate matter. U(VI) complexes with inorganic ligands (e.g., carbonate or phosphate) and HS apparently reduce the bioavailability of U by reducing the activity of UO2(2+) and UO2OH+. The majority of studies have used the results from thermodynamic speciation modeling to support these conclusions. Time-resolved laser-induced fluorescence spectroscopy is the only analytical technique able to directly determine specific U species, but is limited in use to freshwaters of low pH and ionic strength. Nearly all of the available information relating the speciation of U to its bioavailability has been derived using simple, chemically defined experimental freshwaters, rather than natural waters. No data are available for estuarine or seawater. Furthermore, there are no available data on the relationship between U speciation and bioavailability in sediments. An understanding of this relationship has been hindered due to the lack of direct quantitative U speciation techniques for particulate phases. More robust analytical techniques for determining the speciation of U in natural surface waters are needed before the relationship between U speciation and bioavailability can be clarified.

Association of uranium with iron oxides typically formed on corroding steel surfaces

Abstract: Decontamination of metal surfaces contaminated with low levels of radionuclides is a major concern at Department of Energy facilities. The development of an environmentally friendly and cost-effective decontamination process requires an understanding of their association with the corroding surfaces. We investigated the association of uranium with the amorphous and crystalline forms of iron oxides commonly formed on corroding steel surfaces. Uranium was incorporated with the oxide by addition during the formation of ferrihydrite, goethite, green rust II, lepidocrocite, maghemite, and magnetite. X-ray diffraction confirmed the mineralogical form of the oxide. EXAFS analysis at the U L(III) edge showed that uranium was present in hexavalent form as a uranyl oxyhydroxide species with goethite, maghemite, and magnetite and as a bidentate inner-sphere complex with ferrihydrite and lepidocrocite. Iron was present in the ferric form with ferrihydrite, goethite, lepidocrocite, and maghemite; whereas with magnetite and green rust II, both ferrous and ferric forms were present with characteristic ferrous:total iron ratios of 0.65 and 0.73, respectively. In the presence of the uranyl ion, green rust II was converted to magnetite with concomitantreduction of uranium to its tetravalent form. The rate and extent of uranium dissolution in dilute HCl depended on its association with the oxide: uranium present as oxyhydroxide species underwent rapid dissolution followed by a slow dissolution of iron; whereas uranium present as an inner-sphere complex with iron resulted in concomitant dissolution of the uranium and iron.

Study of uranyl(VI) malonate complexation by time-resolved laser-induced fluorescence spectroscopy (TRLFS)

Abstract: The uranyl(VI) malonate complex formation was studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pH 4 and an ionic strength of 0.1 M NaClO 4 . The uranium concentration was 5 x 10 - 6 M at ligand concentrations from 1 x 10 - 5 to 1 × 10 - 2 M. The measured fluorescence lifetimes of the 1: 1 and 1: 2 uranyl(VI) malonate complexes are 1.24 ′ 0.02 μs and 6.48 ′ 0.02 μs, respectively. The fluorescence lifetime of the uranyl(VI) ion is 1.57 ′ 0.06 μs in 0.1 M perchloric media. The main fluorescence bands of the malonate complexes show a bathochromic shift compared to the uranyl(VI) ion and are centered at 494 nm, 515 nm and 540 nm for the 1: 1 complexes and at 496nm, 517 nm and 542nm for the 1: 2 complex. The spectra of the individual uranyl(VI) malonate complexes were calculated using a multi exponential fluorescence decay function for each intensity value at each wavelength, covering the entire wavelength range. Stability constants were determined for the complexes UO 2 C 3 H 2 O 4 ° ( a q ) and UO 2 (C 3 H 2 O 4 ) 2 2 from results of spectra deconvolution using a least square fit algorithm (log β 1 ° = 4.48 ′ 0.06, log β 2 ° = 7.42 ′ 0.06 or log K 2 ° = 2.94 ′ 0.04). The results are compared with literature values obtained by potentiometric measurements.

Uranyl Surface Complexes Formed on Subsurface Media from DOE Facilities

Abstract: A mechanistic understanding of U sorption in natural soils and sediments is useful for determining its transport and bioavailability in the environment. X-ray absorption spectroscopy (XAS) was used to determine the mechanisms by which U(VI) sorbs to three heterogeneous subsurface media reacted under static and dynamic flow conditions. Regardless of the media chosen, ternary surface complexes were the dominant type of sorption complex. Uranyl phosphate complexes were formed in subsurface media from more acidic environments. In contrast, uranyl carbonate ternary surface complexes formed in media from more neutral conditions. The complexes are predominantly inner-sphere, although some outer-sphere complexes may also be present, and appear to be on iron (hydr)oxides and possibly aluminosilicates. Additionally, the uranyl phosphate and carbonate complexes are highly disordered, which contributes to their reversible sorption properties.

Molecular Dynamics Study of Aqueous Uranyl Interactions with Quartz (010)

Abstract: Molecular dynamics simulations were used to study the structure and dynamics of the uranyl ion and its aquo, hydroxy, and carbonato complexes in bulk water and near the hydrated quartz (010) surface. All simulations were performed in the constant (NVT) ensemble with three-dimensional periodic boundary conditions, and a slab technique was used to model the quartz−water interface. The uranyl coordination shell exhibits pentagonal bipyramidal symmetry, with carbonate and hydroxide ions readily replacing water molecules in the first shell. Radial distribution functions of the hydroxy and carbonato complexes are characterized by a consistent splitting in the equatorial shell, caused by the close proximity of hydroxide and carbonate oxygen atoms. Average U−O distances are 2.31−2.35 A for hydroxide ions, 2.35−2.39 A for carbonate ions, and 2.49−2.55 A for water molecules. Two protonation states of the quartz surface were considered for adsorption simulations: singly protonated and partially deprotonated. Surfac...

Redox Energetics and Kinetics of Uranyl Coordination Complexes in Aqueous Solution

Abstract: Detailed voltammetric results for five uranyl coordination complexes are presented and analyzed using digital simulations of the voltammetric data to extract thermodynamic (E1/2) and heterogeneous electron-transfer kinetic (k0 and α) parameters for the one-electron reduction of UO22+ to UO2+. The complexes and their corresponding electrochemical parameters are the following: [UO2(OH2)5]2+ (E1/2 = −0.169 V vs Ag/AgCl, k0 = 9.0 × 10-3 cm/s, and α = 0.50); [UO2(OH)5]3- (−0.927 V, 2.8 × 10-3 cm/s, 0.46); [UO2(C2H3O2)3]- (−0.396 V, ∼0.1 cm/s, ∼0.5); [UO2(CO3)3]4- (−0.820 V, 2.6 × 10-5 cm/s, 0.41); [UO2Cl4]2- (−0.065 V, 9.2 × 10-3 cm/s, 0.30). Differences in the E1/2 values are attributable principally to differences in the basicity of the equatorial ligands. Differences in rate constants are considered within the context of Marcus theory of electron transfer, but no specific structural change(s) in the complexes between the two oxidation states can be uniquely identified with the underlying variability in the...

Hydrothermal syntheses, structures, and properties of the new uranyl selenites Ag(2)(UO(2))(SeO(3))(2), M[(UO(2))(HSeO(3))(SeO(3))] (M = K, Rb, Cs, Tl), and Pb(UO(2))(SeO(3))(2).

Abstract: The transition metal, alkali metal, and main group uranyl selenites, Ag(2)(UO(2))(SeO(3))(2) (1), K[(UO(2))(HSeO(3))(SeO(3))] (2), Rb[(UO(2))(HSeO(3))(SeO(3))] (3), Cs[(UO(2))(HSeO(3))(SeO(3))] (4), Tl[(UO(2))(HSeO(3))(SeO(3))] (5), and Pb(UO(2))(SeO(3))(2) (6), have been prepared from the hydrothermal reactions of AgNO(3), KCl, RbCl, CsCl, TlCl, or Pb(NO(3))(2) with UO(3) and SeO(2) at 180 degrees C for 3 d. The structures of 1-5 contain similar [(UO(2))(SeO(3))(2)](2-) sheets constructed from pentagonal bipyramidal UO(7) units that are joined by bridging SeO(3)(2-) anions. In 1, the selenite oxo ligands that are not utilized within the layers coordinate the Ag(+) cations to create a three-dimensional network structure. In 2-5, half of the selenite ligands are monoprotonated to yield a layer composition of [(UO(2))(HSeO(3))(SeO(3))](1-), and coordination of the K(+), Rb(+), Cs(+), and Tl(+) cations occurs through long ionic contacts. The structure of 6 contains a uranyl selenite layered substructure that differs substantially from those in 1-5 because the selenite anions adopt both bridging and chelating binding modes to the uranyl centers. Furthermore, the Pb(2+) cations form strong covalent bonds with these anions creating a three-dimensional framework. These cations occur as distorted square pyramidal PbO(5) units with stereochemically active lone pairs of electrons. These polyhedra align along the c-axis to create a polar structure. Second-harmonic generation (SHG) measurements revealed a response of 5x alpha-quartz for 6. The diffuse reflectance spectrum of 6 shows optical transitions at 330 and 440 nm. The trailing off of the 440 nm transition to longer wavelengths is responsible for the orange coloration of 6.

Synthesis and characterization of uranyl compounds with iminodiacetate and oxydiacetate displaying variable denticity

Abstract: Eight uranyl compounds containing the dicarboxylate ligands iminodiacetate (IDA) or oxydiacetate (ODA) have been characterized in the solid state. The published polymeric structures for [UO2(C4H6NO4)2] and [UO2(C4H4O5)]n have been confirmed, while Ba[UO2(C4H5NO4)2] ·3H2O, [(CH3)2NH(CH2)2 NH(CH3)2][UO2(C4H4 O5)2] [orthorhombic space group Pnma, a = 10.996(5) A, b = 21.42(1) A, c = 8.700(3) A, Z = 4], and [C2H5NH2(CH2)2NH 2C2H5][UO2 (C4H4O5)2] [monoclinic space group P21/n, a = 6.857(3) A, b = 9.209(5) A, c = 16.410(7) A, β = 91.69(3), Z = 2] contain monomeric anions. The distance from the uranium atom to the central heteroatom (O or N) in the ligand varies. Crystallographic study shows that U-heteroatom (O/N) distances fall into two groups, one 2.6-2.7 A in length and one 3.1-3.2 A, the latter implying no bonding interaction. By contrast, EXAFS analysis of bulk samples suggests that either a long U-heteroatom (O/N) distance (2.9 A) or a range of distances may be present. Three possible structural types, two symmetric and one asymmetric, are identified on the basis of these results and on solid-state 13C NMR spectroscopy. The two ligands in the complex can be 1,4,7-tridentate, giving five-membered rings, or 1,7-bidentate, to form an eight-membered ring. (C4H12N2) [(UO2)2(C4H5 NO4)2(OH)2]·8H2O, [monoclinic space group P21/a, a = 7.955(9) A, b = 24.050(8) A, c = 8.223(6) A, β = 112.24(6), Z = 2], (C2H10N2) [(UO2)2(C4H5 NO4)2(OH)2]·4H2O, and (C6H13N4)2 [(UO2)2 (C4H4O5)2(OH)2] ·2H2O [monoclinic space group C2/m, a = 19.024-(9) A, b = 7.462(4) A, c = 2.467(6) A, β = 107.75(4), Z = 4] have a dimeric structure with two capping tridentate ligands and two μ2-hydroxo bridges, giving edge-sharing pentagonal bipyramids.

[(CH3)4N][(C5H5NH)0.8((CH3)3NH)0.2]U2Si9O23F4 (USH-8): an organically templated open-framework uranium silicate.

Abstract: The open-framework uranium fluorosilicate [(CH3)4N][(C5H5NH)0.8((CH3)3NH)0.2]U2Si9O23F4 (USH-8) has been synthesized hydrothermally by using tetramethylammonium hydroxide and pyridine−HF. The compound has a framework composition U2Si9O23F4 based on silicate double layers that are linked by chains of UO3F4 pentagonal bipyramids. The framework has 12-ring channels along [010] and 7-ring channels along [100]. The [010] 12-ring channels have a calabash-shape with the middle part partially blocked by the uranyl oxygen atoms. The narrow side of the 12-ring channels is occupied by well-ordered TMA cations while the wide side is occupied by disordered pyridinium and trimethylammonium cations.

New Heteropolytungstates Incorporating Dioxouranium(VI). Derivatives of α-[SiW 9 O 34 ] 10− , α-[AsW 9 O 33 ] 9− , γ-[SiW 10 O 36 ] 8− , and [As 4 W 40 O 140 ] 28−

Abstract: Reaction of uranium salts with several lacunary polyoxotungstate anions yields four new heteropolyanion assemblies in which the uranium atoms occupy pentagonal bipyramidal coordination polyhedra. Treatment of A,α-[SiW9O34]10− with UO2(NO3)2 leads to Na14[Na2(UO2)2(SiW9O34)2]⋅38H2O (1, Monoclinic, P21/c, a=16.5719(8) A, b=14.1689(7) A, c=21.2528(10) A, β=111.6670(10)°, V=4786.6(4) A3, Z=2) which proves to be isostructural with the analogous derivative of [PW9O34]9− reported previously. Solutions of 1 exhibit the 5-line W-NMR spectrum expected for the structure of C i point symmetry. The salt (NH4)17[(UO2)3(H2O)4As3W26O94]⋅16H2O (2, Orthorhombic, Pnma, a=40.1747(2) A, b=18.25840(10) A, c=18.0817(2) A, V=13263.4(2) A3, Z=4) was isolated in 64% yield from a reaction of UO2(NO3)2 with B,α-[AsW9O33]9−. The structure of the anion in 2 has C s symmetry and contains one α-AsW9O33 and two novel β-AsW8O30 units linked by the UO2+ 2 groups; an additional WO6 links the two AsW8 fragments. Spectrophotometric titration of UCl4 with the sodium salt of [As4W40O140]28− indicated the formation of a 4:1 U:As4W40 complex. During attempts to isolate a crystalline product from this reaction the uranium became oxidized and a guanidinium salt of [Na(UO2)3(OH)(H2O)6As4W40O140(WO)]18− (3, Orthorhombic, Fdd2, a=54.848(3) A, b=80.809(4) A, c=20.2874(2) A, V=89919(7), Z=16) was isolated. The partially disordered structure of 3 shows the S2 and adjacent sites of the lacunary As4W40 anion to be occupied by three UO5 and one WO5 polyhedra. A tetrameric assembly of γ-SiW10 units linked by UO2+ 2 groups occurs in [{M(OH2)}4(UO2)4(OH)2(SiW10O36)4]22− (lithium salt, M=Na, 4a, tetragonal, P42/nmc, a=b=26.5285(2) A, c=15.0463(2) A, V=10589.0(2) A3, Z=2; sodium-potassium salt, M=K, 4b, orthorhombic, Fddd, a=24.180(5) A, b=31.696(6) A, c=58.012(12) A, V=44460(15) A3, Z=8). Tungsten-183 NMR spectra show the slow transformation of the expected 5-line (1:1:1:1:1) spectrum of 4a to a new species giving a 6-line spectrum (2:2:2:1:2:1). The latter complex has not been successfully isolated.

Quinoline-8-ol-immobilized Amberlite XAD-4: synthesis, characterization, and uranyl ion uptake properties suitable for analytical applications.

Abstract: Amberlite XAD-4 has been functionalized by coupling it with 5-aminoquinoline-8-ol after acetylation. The resulting resin has been characterized by elemental analysis and IR spectra and has been used for preconcentrating uranyl ions prior to its determination by spectrophotometry. The optimum pH value for quantitative sorption is 4–6, and desorption can be achieved by using 5 mL of 1 mol L–1 HCl. The sorption capacity of the resin is 11.5 mmol g–1. The effect of various cations and anions on the preconcentration of uranium in conjunction with the determination procedure has been studied and we have found that none of the ions interfere except thorium. The enrichment factor for preconcentration of uranium was found to be 200. Ten replicate determinations of 40 µg of uranium present in 1 L of sample gave a mean absorbance of 0.185 with a relative standard deviation of 2.64%. The detection limit corresponding to three times the standard deviation of the bank was found to be 2 µg L–1. The validation of the developed preconcentration procedure was carried out by successfully analyzing standard marine sediment reference material. The uranyl content of sediment and soils is estimated by spectrophotometry after its preconcentration with the above chelating resin.

The first structural and spectroscopic characterization of a neptunyl polyoxometalate complex.

Abstract: The reaction between PW9O349- and NpO2+ has yielded the first structurally characterized neptunyl(V) polyoxometalate complex, [Na2(NpO2)2(A-PW9O34)2]14-. This complex is isostructural with the uranyl(VI) analogue, and there is also spectroscopic evidence for its existence in solution. The complex is readily extracted into toluene, and this may have significance in the sequestering and/or separation of the neptunyl ion in terms of nuclear waste management.

The hydration of the uranyl dication: Incorporation of solvent effects in parallel density functional calculations with the program PARAGAUSS

Abstract: The parallel density functional program PARAGAUSS has been extended by a tool for computing solvent effects based on the conductor-like screening model (COSMO). The molecular cavity in the solvent is constructed as a set of overlapping spheres according to the GEPOL algorithm. The cavity tessellation scheme and the resulting set of point charges on the cavity surface comply with the point group symmetry of the solute. Symmetry is exploited to reduce the computational effort of the solvent model. To allow an automatic geometry optimization including solvent effects, care has been taken to avoid discontinuities due to the discretization (weights of tesserae, number of spheres created by GEPOL). In this context, an alternative definition for the grid points representing the tesserae is introduced. In addition to the COSMO model, short-range solvent effects are taken into account via a force field. We apply the solvent module to all-electron scalar-relativistic density functional calculations on uranyl, UO22+, and its aquo complexes in aqueous solution. Solvent effects on the geometry are very small. Based on the model [UO2(H2O)5]2+, the solvation energy of uranyl is estimated to be about −400 kcal/mol, in agreement with the range of experimental data. The major part of the solvation energy, about −250 kcal/mol, is due to a donor–acceptor interaction associated with a coordination shell of five water ligands. One can interpret this large solvation energy also as a compounded effect of an effective reduction of the uranyl moiety plus a solvent polarization. The energetic effect of the structure relaxation in the solution is only about 8 kcal/mol. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Carbamoylphosphine oxide complexes of trivalent lanthanide cations: role of counterions, ligand binding mode, and protonation investigated by quantum mechanical calculations.

Abstract: We present a quantum mechanical study of carbamoylphosphine oxide (CMPO) complexes of MX3 (M3+ = La3+, Eu3+, Yb3+; X- = Cl-, NO3-) with a systematic comparison of monodentate vs bidentate binding modes of CMPO. The per ligand interaction energies ΔE increase from La3+ to Yb3+ and are higher with Cl- than with NO3- as counterions, as a result of steric strain in the first coordination sphere with the bidentate anions. The energy difference between monodentate (via phosphoryl oxygen) and bidentate CMPO complexes is surprisingly small, compared to ΔE or to the binding energy of one solvent molecule. Protonation of uncomplexed CMPO takes place preferably at the phosphoryl oxygen OP, while in the Eu(NO3)3CMPOH+ complex carbonyl (OC) protonation is preferred and OP is bonded to the metal. A comparison of uranyl and lanthanide nitrate complexes of CMPO shows that the interaction energies ΔE of the former are lower. Finally, the effect of grafting CMPO arms at the wide rim of a calix[4]arene platform is described...

Steric Control of Substituted Phenoxide Ligands on Product Structures of Uranyl Aryloxide Complexes

Abstract: A series of uranyl aryloxide complexes has been prepared via metathesis reactions between [UO2Cl2(THF)2]2 and di-ortho-substituted phenoxides. Reaction of 4 equiv of KO-2,6-tBu2C6H3 with [UO2Cl2(THF)2]2 in THF produces the dark red uranyl compound, UO2(O-2,6-tBu2C6H3)2(THF)2·THF, 1. Single-crystal X-ray diffraction analysis of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo groups, two cis-aryloxides, and two THF ligands. A similar product is prepared by reaction of KO-2,6-Ph2C6H3 with [UO2Cl2(THF)2]2 in THF. Single-crystal X-ray diffraction analysis of this compound reveals it to be the trans-monomer UO2(O-2,6-Ph2C6H3)2(THF)2, 2. Dimeric structures result from the reactions of [UO2Cl2(THF)2]2 with less sterically imposing aryloxide salts, KO-2,6-Cl2C6H3 or KO-2,6-Me2C6H3. Single-crystal X-ray diffraction analyses of [UO2(O-2,6-Cl2C6H3)2(THF)2]2, 3, and [UO2Cl(O-2,6-Me2C6H3)(THF)2]2, 4, reveal similar structures in which each U atom is coordinated by...

CRYSTAL CHEMISTRY OF URANYL MOLYBDATES. V. TOPOLOGICALLY DISTINCT URANYL DIMOLYBDATE SHEETS IN THE STRUCTURES OF Na2((UO2)(MoO4)2) AND K2((UO2)(MoO4)2)(H2O)

Abstract: Two alkali uranyl dimolybdates, Na2[(UO2)(MoO4)2] and K2[(UO2)(MoO4)2](H2O), have been synthesized by hightemperature solid-state reactions and hydrothermal methods, respectively. The structures of both compounds were solved by direct methods and refined on the basis of F 2 for all unique data collected with monochromatic MoK X-radiation and a CCD (charge-coupled device) detector. The structure of Na 2[(UO2)(MoO4)2] was refined to an agreement factor ( R1) of 3.1%, calculated from 2089 unique reflections (|Fo| ≥ 4F); it is orthorhombic, space group P212121, a 7.2298(5), b 11.3240(8), c 12.0134(8) A, V 983.5(1) A 3 , Z = 4. The structure of K 2[(UO2)(MoO4)2](H2O) was refined to an R1 of 3.7%, calculated from 2181 unique reflections (Fo ≥ 4F); it is monoclinic, space group P21/c, a 7.893(2), b 10.907(2), c 13.558(3) A, 98.70(3)°, V 1153.8(4) A 3 , Z = 4. Both structures are based upon sheets of uranyl pentagonal bipyramids and molybdate tetrahedra. The structural sheets in the two compounds are isochemical but topologically distinct.

Aqueous reactions of U(VI) at high chloride concentrations: syntheses and structures of new uranyl chloride polymers.

Abstract: The reactions of UO(3) with acidic aqueous chloride solutions resulted in the formation of two new polymeric U(VI) compounds. Single crystals of Cs(2)[(UO(2))(3)Cl(2)(IO(3))(OH)O(2)].2H(2)O (1) were formed under hydrothermal conditions with HIO(3) and CsCl, and Li(H(2)O)(2)[(UO(2))(2)Cl(3)(O)(H(2)O)] (2) was obtained from acidic LiCl solutions under ambient temperature and pressure. Both compounds contain pentagonal bipyramidal coordination of the uranyl dication, UO(2)(2+). The structure of 1 consists of infinite [(UO(2))(3)Cl(2)(IO(3))(mu(3)-OH)(mu(3)-O)(2)](2-) ribbons that run down the b axis that are formed from edge-sharing pentagonal bipyramidal [UO(6)Cl] and [UO(5)Cl(2)] units. The Cs(+) cations separate the chains from one another and form long ionic contacts with terminal oxygen atoms from iodate ligands, uranyl oxygen atoms, water molecules, and chloride anions. In 2, edge-sharing [UO(3)Cl(4)] and [UO(5)Cl(2)] units build up tetranuclear [(UO(2))(4)(mu-Cl)(6)(mu(3)-O)(2)(H(2)O)(2)](2-) anions that are bridged by chloride to form one-dimensional chains. These chains are connected in a complex network of hydrogen bonds and interactions of uranyl oxygen atoms with Li(+) cations. Crystal data: 1, orthorhombic, space group Pnma, a = 8.2762(4) A, b = 12.4809(6) A, c = 17.1297(8) A, Z = 4; 2, triclinic, space group P1, a = 8.110(1) A, b = 8.621(1) A, c = 8.740(1) A, Z = 2.

Acid-amide calixarene ligands for uranyl and lanthanide ions: synthesis, structure, coordination and extraction studies

Abstract: The synthesis of a series of calix[4]arene ligands bearing lower rim 1,3-acid-amide functionality for the two-phase solvent extraction of lanthanide and uranyl ions from aqueous wastes is described. X-Ray crystal structure studies demonstrate binding of lanthanide ions at the calixarene lower rim through phenolate, carboxylate and carboxamide oxygen atoms. The crystal structures of a centrosymmetric, dimeric lanthanum(III) and non-centrosymmetric dimeric sodium(I) complexes of a 1,3-acid-dihexylamidecalix[4]arene ligand are detailed. Electrospray mass spectrometry confirms retention of this 2 ∶ 2 dimeric structure for large lanthanide ions and that the smaller lanthanide ions e.g. Lu3+ form monomeric 1 ∶ 1 calixarene ∶ lanthanide complexes in solution. Extraction studies demonstrate a pH dependent uptake of lanthanide and uranyl cations by 1,3-acid-amide ligands with the nature of the carboxamide substituent largely irrelevant to extraction efficiency. Upper rim modified 1,3-dinitro and de-tert-butylcalix[4]arene ligands were prepared and NMR and ES-MS studies confirm uranyl coordination by these novel acid-amide ligands. The 1,3-dinitro and de-tert-butyl ligands exhibit superior and more efficient extraction of uranyl cations at lower pH values.

CRYSTAL CHEMISTRY OF URANYL MOLYBDATES. VI. NEW URANYL MOLYBDATE UNITS IN THE STRUCTURES OF Cs4[(UO2)3O(MoO4)2(MoO5)] AND Cs6[(UO2)(MoO4)4]

Abstract: Two Cs uranyl molybdates, Cs4[(UO2)3O(MoO4)2(MoO5)] and Cs6[(UO2)(MoO4)4], have been synthesized by hightemperature solid-state reactions. The structures of these compounds were solved by direct methods and refined on the basis of F 2 for all unique data collected with monochromatic MoK X-radiation and a CCD (charge-coupled device) detector. The structure of Cs4[(UO2)3O(MoO4)2(MoO5)] was refined to an agreement factor ( R1) of 4.4%, calculated using the 4873 unique observed reflections (Fo ≥ 4F). It is triclinic, space group P1, a 7.510(2), b 7.897(2), c 9.774(2) A, 79.279(5), 81.269(5), 87.251(5)°, V 562.8(2) A 3 , Z = 1. The structure of Cs 6[(UO2)(MoO4)4] was refined to an R1 of 4.9%, calculated using the 4275 unique

300 Area Uranium Leach and Adsorption Project

Abstract: The objective of this study was to measure the leaching and adsorption characteristics of uranium in six near-surface sediment samples collected from the 300 Area of the Hanford Site. Scanning electron micrographs of the samples showed that the uranium contamination in the sediments is most likely present as co-precipitates and/or discrete uranium particles. Molecular probe techniques also confirm the presence of crystalline discrete uranium bearing phases. In all cases, the uranium is present as oxidized uranium (uranyl [U(VI)]). Results from the column leach tests showed that uranium leaching did not follow a constant solubility paradigm. Four of the five contaminated sediments showed a large near instantaneous release of a few percent of the total uranium followed by a slower continual release. Steady-state uranium leachate concentrations were never measured and leaching characteristics and trends were not consistent among the samples. Dissolution kinetics were slow, and the measured leach curves most likely represent a slow kinetically controlled desorption or dissolution paradigm. Batch adsorption experiments were performed to investigate the effect of pH and uranium and carbonate solution concentrations on uranium adsorption onto the uncontaminated sediment. Uranium adsorption Kd values ranged from 0 to > 100 ml/g depending on which solution parameter wasmore » being adjusted. Results of the experiments showed that carbonate solution concentration has the greatest impact on uranium adsorption in the 300 Area. Solution pH was shown to be important in laboratory tests; however, the sediment will dominate the field pH and minimize its overall effect in the 300 Area sediments. Results also showed that uranium sorption onto the background sediment is linear up to uranium concentrations of 3 mg/L, well above the values found in the upper unconfined aquifer. Therefore, the linear Kd model is defensible in predicting the fate of uranium in the 300 Area aquifer.« less

Detection of uranyl ion via fluorescence quenching and photochemical oxidation of calcein

Abstract: This paper reports the detection of uranyl ion via fluorescence quenching of a dye molecule followed by the selective photocatalysis of the dye molecule by excited state uranyl. We have found that selectivity can be obtained when calcein, a highly fluorescent fluorescein-type dye, is quenched by complex formation with uranyl in solution at low concentrations. Following the quenching by uranyl, the entire sample is excited at a wavelength that is strongly absorbed by uranyl at 425 nm. This produces the excited state uranyl ion that has a large oxidation potential and photocatalytically decarboxylates the dye, breaking the dye/metal bond. The photocatalysis product is highly fluorescent and produces an increase in the fluorescence signal. The detection limit for uranyl via quenching is 60 nM and via photocatalysis is 40 nM. Interfering metal ions such as iron and chromate have little effect on the amount of fluorescence regenerated since their absorbance bands, modes of quenching, and photochemistry are different from uranyl.