Showing papers on "Uranyl published in 2008"

Highly Sensitive and Selective Colorimetric Sensors for Uranyl (UO22+): Development and Comparison of Labeled and Label-Free DNAzyme-Gold Nanoparticle Systems

Abstract: Colorimetric uranium sensors based on uranyl (UO2(2+)) specific DNAzyme and gold nanoparticles (AuNP) have been developed and demonstrated using both labeled and label-free methods. In the labeled method, a uranyl-specific DNAzyme was attached to AuNP, forming purple aggregates. The presence of uranyl induced disassembly of the DNAzyme functionalized AuNP aggregates, resulting in red individual AuNPs. Once assembled, such a "turn-on" sensor is highly stable, works in a single step at room temperature, and has a detection limit of 50 nM after 30 min of reaction time. The label-free method, on the other hand, utilizes the different adsorption properties of single-stranded and double-stranded DNA on AuNPs, which affects the stability of AuNPs in the presence of NaCl. The presence of uranyl resulted in cleavage of substrate by DNAzyme, releasing a single stranded DNA that can be adsorbed on AuNPs and protect them from aggregation. Taking advantage of this phenomenon, a "turn-off" sensor was developed, which is easy to control through reaction quenching and has 1 nM detection limit after 6 min of reaction at room temperature. Both sensors have excellent selectivity over other metal ions and have detection limits below the maximum contamination level of 130 nM for UO2(2+) in drinking water defined by the U.S. Environmental Protection Agency (EPA). This study represents the first direct systematic comparison of these two types of sensor methods using the same DNAzyme and AuNPs, making it possible to reveal advantages, disadvantages, versatility, limitations, and potential applications of each method. The results obtained not only allow practical sensing application for uranyl but also serve as a guide for choosing different methods for designing colorimetric sensors for other targets.

Reduction and selective oxo group silylation of the uranyl dication

Abstract: Uranium occurs in the environment predominantly as the uranyl dication [UO2]2+. Its solubility renders this species a problematic contaminant which is, moreover, chemically extraordinarily robust owing to strongly covalent U-O bonds. This feature manifests itself in the uranyl dication showing little propensity to partake in the many oxo group functionalizations and redox reactions typically seen with [CrO2]2+, [MoO2]2+ and other transition metal analogues. As a result, only a few examples of [UO2]2+ with functionalized oxo groups are known. Similarly, it is only very recently that the isolation and characterization of the singly reduced, pentavalent uranyl cation [UO2]+ has been reported. Here we show that placing the uranyl dication within a rigid and well-defined molecular framework while keeping the environment anaerobic allows simultaneous single-electron reduction and selective covalent bond formation at one of the two uranyl oxo groups. The product of this reaction is a pentavalent and monofunctionalized [O = U...OR]+ cation that can be isolated in the presence of transition metal cations. This finding demonstrates that under appropriate reaction conditions, the uranyl oxo group will readily undergo radical reactions commonly associated only with transition metal oxo groups. We expect that this work might also prove useful in probing the chemistry of the related but highly radioactive plutonyl and neptunyl analogues found in nuclear waste.

Preparation, structures, and photocatalytic properties of three new uranyl-organic assembly compounds.

Abstract: Three new uranyl−organic coordination polymers (UO2)8(NDC)12(4,4′-bipyH2)3(4,4′-bipyH)3 (1), (UO2)3O[Ag(2,2′-bipy)2]2(NDC)3 (2), and (UO2)2(NDC)2(2,2′-bipy)2 (3), where NDC = 1,4-naphthalenedicarboxylate and bipy = bipyridine, have been prepared in hydrothermal conditions. Both 1 and 2 possess a 2D structure while 3 is composed of 1D zigzag chains. In 1 there are mononuclear pentagonal-bipyramidal U−O polyhedra and 1D channels filled with 4,4′-bipy molecules, whereas in 2 there are mononuclear hexagonal-bipyramidal U−O polyhedra and tetranuclear pentagonal-bipyramidal U−O clusters which form 2D channels with occluded [Ag(2,2′-bipy)2]+ counterions. In 3 both the NDC and the bipy ligands coordinate to uranyl centers, leading to hexagonal-bipyramidal polyhedra which are connected to form 1D zigzag chains. Under UV or visible irradiation, 1 and 2 degrade rhodamine B with similar efficiency. The correlation between photocatalytic reaction rate and oxygen concentration for 1 and 2 has also been elucidated. Crys...

Polynuclear cation-cation complexes of pentavalent uranyl: relating stability and magnetic properties to structure.

Abstract: Reaction of {[UO2Pys][mu-KI2Py2]}n (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl ([UO2(dbm),2]2[mu-K(Py)2]2[mu8-K(Py)]}2I2 x Py2 (2) (in 70% yield) and {[UO2(dbm)2]2[mu-K(MeCN)2][mu8-K]}2 (3) (in 40% yield) in which four UO2+ are mutually coordinated (T-shaped "cation-cation" interaction) The X-ray structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in UO2+K+ interactions Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex [UO2(dbm)2K(18C6)]2 (4) presenting a diamond-shaped interaction between two UO2+ groups, in 45% yield 1H and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of UO2+UO2+ and UO2+K+ interactions (tetrametallic species) in solution, which leads to a rapid (7 days) disproportionation of pentavalent uranyl to afford [U(dbm)4] and [UO2(dbm)2] species The UO2+K+ interaction plays an important synergistic role in the stabilization of the UO2+UO2+ interactions Accordingly, the lower affinity of (K(18C6))+ for the uranyl(V) oxygen in complex 4 results in a lower number of coordinated K+ and therefore in a weakened UP2+UO2+ interaction The UO24+UO2+ interactions is completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent uranyl Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the appearance of a maximum in chi versus T at approximately 5 K The different behavior of the tetrameric complex 3, which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different geometric arrangement of the interacting uranyl(V) groups

Uranyl Ion Complexes with Cucurbit[n]urils (n = 6, 7, and 8): A New Family of Uranyl-Organic Frameworks

Abstract: Uranyl ion complexation by cucurbit[n]urils (CBn, n = 6, 7, 8) was investigated, mostly under hydrothermal conditions, and the crystal structure of six novel complexes was determined. In compounds [UO2(H2O)5](NO3)2·CB6·4H2O (1) and [UO2(NO3)2(H2O)2]·CB6·7H2O (2), CB6 acts as a second-sphere ligand through hydrogen bonding to uranyl aquo ligands, while in all other complexes, [(UO2)4(CB6)O2(OH)2(H2O)6](NO3)2·CB6·8H2O (3), [(UO2)2(CB6)3(H2O)6][(UO2)2(NO3)4(C10H16O4)]2·CB6·8H2O (4), [(UO2)2(CB7)(SO4)2(H2O)2]·4H2O (5), and [(UO2)6(CB8)(SO4)6(H2O)10]·18H2O (6), uranyl coordination to one or two CB carbonyl groups is present. A one-dimensional polymer with tetranuclear, μ3-oxo-bridged subunits and a three-decker dinuclear complex are formed in 3 and 4, respectively, with in both cases numerous CH···O interactions between CB6 molecules, either within the complex or involving other CB6 molecules present as guests. The CB7 and CB8 complexes 5 and 6, both obtained in the presence of sulfuric acid, involve either di...

Use of bifunctional phosphonates for the preparation of heterobimetallic 5f-3d systems.

Abstract: The hydrothermal reaction of phosphonoacetic acid (H2PO3CH2C(O)OH, PAA) with UO3 and Cu(C2H3O2)2·H2O results in the formation of the crystalline heterobimetallic uranium(VI)/copper(II) phosphonates UO2Cu(PO3CH2CO2)(OH)(H2O)2 (UCuPAA-1), (UO2)2Cu(PO3CH2CO2)2(H2O)3 (UCuPAA-2), and [H3O][(UO2)2Cu2(PO3CH2CO2)3(H2O)2 (UCuPAA-3) The addition of sodium hydroxide to the aforementioned reactions results in the formation of Na[UO2(PO3CH2CO2)]·2H2O (NaUPAA-1) These compounds display 1D (UCuPAA-1), 2D (UCuPAA-2, NaUPAA-1), and 3D (UCuPAA-3) architectures wherein the phosphonate portion of the ligand primarily coordinates the uranium(VI) centers; whereas the carboxylate moiety preferentially, but not exclusively, binds to the copper(II) ions Fluorescence measurements on all four compounds demonstrate that the presence of copper(II) mostly quenches the emission from the uranyl moieties

Affinity of the highly preorganized ligand PDA (1,10-phenanthroline-2,9-dicarboxylic acid) for large metal ions of higher charge. A crystallographic and thermodynamic study of PDA complexes of thorium(IV) and the uranyl(VI) ion.

Abstract: The hydrothermal synthesis and structures of [UO2(PDA)] (1) and [Th(PDA)2(H2O)2].H2O (2) (PDA = 1,10-phenanthroline-2,9-dicarboxylic acid) are reported. 1 is orthorhombic, Pnma, a = 11.1318(7) A, b = 6.6926(4) A, c = 17.3114(12) A, V = 1289.71(14), Z = 4, R = 0.0313; 2 is triclinic, P1, a = 7.6190(15) A, b = 10.423(2) A, c = 17.367(4) A, alpha = 94.93(3) degrees , beta = 97.57(3) degrees , gamma = 109.26(3) degrees , V = 1278.3(4) A (3), Z = 2, R = 0.0654. The local geometry around the U in 1 is a pentagonal bipyramid with the two uranyl oxygens occupying the apical positions. The donor atoms in the plane comprise the four donor atoms from the PDA ligand (average U-N = 2.558 and U-O = 2.351 A) with the fifth site occupied by a bridging carboxylate oxygen from a neighboring UO2/PDA individual. The PDA ligand in 1 is exactly planar, with the U lying in the plane of the ligand. The latter planarity, as well as the near-ideal U-O and U-N bond lengths, and O-U-N and N-U-N bond angles within the chelate rings of 1 suggest that PDA binds to the uranyl cation in a low-strain manner. In 2, there are two PDA ligands bound to the Th (average Th-N = 2.694 and Th-O = 2.430 A) as well as two water molecules (Th-O = 2.473 and 2.532 A) to give the Th a coordination number of 10. The PDA ligands in 2 are bowed, with the Th lying out of the plane of the ligand. Molecular mechanics calculations suggest that the distortion of the PDA ligands in 2 arises because of steric crowding. UV spectroscopic studies of solutions containing 1:1 ratios of PDA and Th(4+) in 0.1 M NaClO4 at 25 degrees C indicate that log K1 for the Th(4+)/PDA complex is 25.7(9). The latter result confirms the previous prediction that complexes of PDA with metal ions of higher charge and an ionic radius of about 1.0 A such as Th(IV) would have remarkably high log K1 values with PDA. The origins of this very high stability are discussed in terms of a synergy between the pyridyl and the carboxylate donor groups of PDA. Metal ions of high charge normally bond poorly with pyridyl donors in aqueous solution because such metal ions require donor groups that are able to disperse charge to the solvent via hydrogen-bonding, which pyridyl groups are unable to do. In PDA, the carboxylates fulfill this need and so enable the high donor strength of the pyridyl groups of PDA to become apparent in the high log K1 for Th(IV) with PDA.

Density functional model studies of uranyl adsorption on (001) surfaces of kaolinite.

Abstract: The adsorption of uranyl on two types of neutral (001) surfaces of kaolinite, tetrahedral Si(t) and octahedral Al(o), was studied by means of density functional periodic slab model calculations. Various types of model surface complexes, adsorbed at different sites, were optimized and adsorption energies were estimated. As expected, the Si(t) surface was found to be less reactive than the Al(o) surface. At the neutral Al(o) surface, only adsorption at protonated sites is calculated to be exothermic for inner- as well as outer-sphere adsorption complexes, with monodentate coordination being preferred. Adsorption energies as well as structural features of the adsorption complexes are mainly determined by the number of deprotonated surface hydroxyl groups involved. Outer-sphere complexes on both surfaces exhibit a shorter U-O bond to the aqua ligand of uranyl that is in direct contact with the surface than to the other aqua ligands. This splitting of the shell of equatorial U-O bonds is at variance with common expectations for outer-sphere surface complexes of uranyl.

Synthesis, Characterization, and Reactivity of a Uranyl β-Diketiminate Complex

Abstract: Addition of 1 equiv of Li(Ar2nacnac) (Ar2nacnac = (2,6-iPr2C6H3)NC(Me)CHC(Me)N(2,6-iPr2C6H3)) to an Et2O suspension of UO2Cl2(THF)3 generates the uranyl dimer [UO2(Ar2nacnac)Cl]2 (1) in good yield....

Imidazo[4,5-f]-1,10-phenanthrolines: Versatile Ligands for the Design of Metallomesogens

Abstract: 2-Aryl-substituted imidazo[4,5-f]-1,10-phenanthrolines were used as building blocks for metal-containing liquid crystals (metallomesogens). Imidazo[4,5-f]-1,10-phenanthrolines are versatile ligands because they can form stable complexes with various d-block transition metals, including platinum(II) and rhenium(I), as well as with lanthanide(III) and uranyl ions and they can easily be structurally modified by a judicious choice of benzaldehyde precursor. None of the ligands designed for this study were liquid-crystalline. However, mesomorphism could be induced by their coordination to various metallic fragments. The thermal behavior of the metal complexes depended on the metal-to-ligand ratio and the substitution pattern of the coordinating ligands. Complexes with a metal-to-ligand ratio of 1:1 [ML, with M = Pt(II), Re(I)] were not liquid-crystalline. The lanthanide(III) complexes with a metal-to-ligand ratio of 1:2 [ML2, with M = Ln(III)] formed an enantiotropic cubic mesophase or were not liquid-crystall...

The stereochemistry and chemical composition of interstitial complexes in uranyl-oxysalt minerals

Abstract: The crystal structures and chemical compositions of uranyl-oxysalt minerals (primarily with sheet structural units) are interpreted in terms of the binary representation – bond-valence approach to the structure and chemistry of oxysalts. There is a coherent relation between the average basicity of the structural unit and [CN in ], the average number of bonds to oxygen atoms of the structural unit from the interstitial complex and adjacent structural units. This relation allows calculation of the range of Lewis basicity for specific structural units. The Lewis acidity of an interstitial complex is expressed graphically as a function of the amounts and coordination numbers of monovalent, divalent and trivalent interstitial cations and the amount of interstitial transformer (H 2 O) groups. The range in Lewis basicity for a specific structural unit may also be expressed graphically, and where there is overlap of the Lewis acidity and Lewis basicity, the valence-matching principle is satisfied, and the details of the possible interstitial complexes can be derived. There are three distinct types of interstitial (H 2 O) groups: transformer, non-transformer and inverse-transformer. Inverse-transformer (H 2 O) groups accept three bonds from cations, other (H 2 O) groups and (OH) groups of the structural unit. Their occurrence is rare and is limited to minerals with low bond-valence distribution factors. Detailed predictions of the number of transformer, non-transformer and inverse-transformer (H 2 O) groups in the minerals of the meta-autunite, uranophane, phosphuranylite, carnotite, zippeite and uranyl-hydroxy-hydrate groups (and synthetic analogues) are in good agreement with the observed chemical compositions.

Effect of sulfate, carbonate, and phosphate on the uranium(VI) sorption behavior onto bentonite

Abstract: Batch experiments were conducted to study the uranium U(VI) sorption onto bentonite as a function of pH (3 to 8), and initial U(VI) concentrations (5 x10 -6 and 5 x 10 -5 M) in the presence and absence of sulfate, carbonate, and phosphate. Uranium sorption onto bentonite depended on the initial U(VI) concentration with a stronger sorption at lower concentrations and was high over a wide range of pH in the absence of complexing ligands. In the presence of 0.005 M sulfate, U(VI) sorption was reduced at low pH values due to the competition between SO 4 2- and the uranyl ion for sorption sites on the bentonite surface, or the formation of uranyl-sulfate complexes. In the presence of 0.003 M carbonate, U(VI) sorption decreased sharply at a pH above 7, because of the formation of negatively charged uranyl-carbonate complexes, which are weakly adsorbed onto the bentonite. Uranium sorption onto bentonite was greatly enhanced in the presence of 0.003 M phosphate. Kinetic batch experiments carried out for 5 x 10 -5 M U(VI) at pH values of 3, 5, and 8 revealed that the sorption rate was generally rapid over the first lOmin of the experiments, then slowed down appreciably after 1 to 24 h. Sulfate had little effect on the kinetics of U(VI) sorption; both in the absence and presence of sulfate, sorption equilibrium was attained after 4 h. In the presence of carbonate, attainment of sorption equilibrium required more time than in its absence. The presence of 0.003 M phosphate reduced the time required to reach sorption equilibrium across a wide range of pH compared to phosphate-free systems.

Lanthanum(III) and uranyl(VI) diglycolamide complexes: synthetic precursors and structural studies involving nitrate complexation.

Abstract: The synthesis and structural characterization of lanthanum(III) and uranyl(VI) complexes coordinated by tridentate diglycolamide (DGA) ligands O(CH2C(O)NR2)2[R=i-Pr (L1), i-Bu (L2)] are described. Reaction of L with UO2Cl2(H2O) n forms the uranyl(VI) cis-dichloride adducts UO2Cl2L [L=L1 (1a), L2 (1b)], while reaction of excess L with the corresponding metal nitrate hydrate produces [LaL3][La(NO3)6] [L=L1 (2a), L2 (2b)] for lanthanum and UO2(NO3)2L [L=L1 (3a), L2 (3b)] for uranium. Compounds 2b and 3a have been structurally characterized. The solid-state structure of the cation of 2b shows a triple-stranded helical arrangement of three tridentate DGA ligands with approximate D3 point-group symmetry, while the counteranion consists of six bidentate nitrate ligands coordinated around a second La center. The solid-state structure of 3a shows a tridentate DGA ligand coordinated along the equatorial plane perpendicular to the OUO unit as well as two nitrate ligands, one bidentate and oriented in the equatorial plane and the other monodentate and oriented parallel to the uranyl unit with the oxygen donor atom situated above the mean equatorial plane. Ambient-temperature NMR spectra for 3a and 3b indicated an averaged chemical environment of high symmetry consistent with fluxional nitrate hapticity, while spectroscopic data obtained at -30 degrees C revealed lower symmetry consistent with the slow-exchange limit for this process.

Pyrazinetetracarboxylic Acid as an Assembler Ligand in Uranyl−Organic Frameworks

Abstract: The reaction of uranyl nitrate with pyrazinetetracarboxylic acid (H4PZTC) has been investigated under different experimental conditions, and the crystal structures of the resulting complexes have been determined In all cases, the uranyl ion is chelated in the ONO tridentate site, as in the complexes with pyridine-2,6-dicarboxylic acid, but much variety arises from the increased number of potential donor atoms Hydrothermal synthesis in the presence of NEt4Br led to the complex [UO2(PZDC)(H2O)] (1), from in situ decarboxylation of H4PZTC into pyrazine-2,6-dicarboxylic acid (H2PZDC) upon prolonged heating Complex 1crystallizes as ribbons held by bridging carboxylate groups and hydrogen bonds In the presence of NaOH and under hydrothermal conditions, two species could be obtained: [(UO2)2(PZTC)(H2O)]·2H2O (2) and [UO2Na2(PZTC)(H2O)4] (3) In these compounds, [UO2(PZTC)]n2n− linear chains with bis-chelating PZTC ligands are further assembled either into a two-dimensional assemblage by other, carboxylate-bo

Uranyl−Organic Frameworks with 1,2,3,4-Butanetetracarboxylate and 1,2,3,4-Cyclobutanetetracarboxylate Ligands

Abstract: The reaction of uranyl nitrate with 1,2,3,4-butanetetracarboxylic acid (H4BTC) and 1,2,3,4-cyclobutanetetracarboxylic acid (H4CBTC) under hydrothermal conditions gives various two- and three-dimensional frameworks. The complex [(UO2)2(BTC)(H2O)4]·4H2O (1) is a (4,4) grid in which the BTC4− ligands act as rectangular nodes and the uranyl ions as divergent, side-defining nodes. Complexes [(UO2)2(CBTC)(H2O)2]·2H2O (2) and [(UO2)2(CBTC)(H2O)2]·H2O (3), with the ligand in the cis,trans,cis form, present two types of three-dimensional architectures, with narrow channels formed in complex 2 only. In complex 4, [H3O]2[(UO2)5(CBTC)3(H2O)6], the ligand is in the noncentrosymmetric trans,trans,trans form, which assumes a saddle shape. This peculiar geometry of the ligand results in the formation of two types of subunits: 4:4 (metal/ligand) metallacycles and 8:12 cubic boxes, which are connected to one another to form a cubic lattice containing large channels. This latter result shows that the same geometric consider...

Interaction of uranium(VI) with lipopolysaccharide.

Abstract: Bacteria have a great influence on the migration behaviour of heavy metals in the environment. Lipopolysaccharides form the main part of the outer membrane of Gram-negative bacteria. We investigated the interaction of the uranyl cation (UO22+) with lipopolysaccharide (LPS) from Pseudomonas aeruginosa by using potentiometric titration and time-resolved laser-induced fluorescence spectroscopy (TRLFS) over a wide pH and concentration range. Generally, LPS consists of a high density of different functionalities for metal binding such as carboxyl, phosphoryl, amino and hydroxyl groups. The dissociation constants and corresponding site densities of these functional groups were determined using potentiometric titration. The combination of both methods, potentiometry and TRLFS, show that at an excess of LPS uranyl phosphoryl coordination dominates, whereas at a slight deficit on LPS compared to uranyl, carboxyl groups also become important for uranyl coordination. The stability constants of one uranyl carboxyl complex and three different uranyl phosphoryl complexes and the luminescence properties of the phosphoryl complexes are reported.

Sterically congested uranyl complexes with seven-coordination of the UO2 unit: the peculiar ligation mode of nitrate in [UO2(NO3)2(Rbtp)] complexes.

Abstract: Addition of 1 or 2 molar equiv of Rbtp [Rbtp = 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine; R = Me, Pr ( n )] to UO 2(OTf) 2 in anhydrous acetonitrile gave the neutral compounds [UO 2(OTf) 2(Rbtp)] [R = Me ( 1), ( n )Pr ( 2)] and the cationic complexes [UO 2(Rbtp) 2][OTf] 2 [R = Me ( 3), Pr ( n ) ( 4)], respectively. No equilibrium between the mono and bis(Rbtp) complexes or between [UO 2(Rbtp) 2][OTf] 2 and free Rbtp in acetonitrile was detected by NMR spectroscopy. The crystal structures of 1 and 3 resemble those of their terpyridine analogues, and 3 is another example of a uranyl complex with the uranium atom in the unusual rhombohedral environment. In the presence of 1 molar equiv of Rbtp in acetonitrile, UO 2(NO 3) 2 was in equilibrium with [UO 2(NO 3) 2(Rbtp)] and the formation of the bis adduct was not observed, even with an excess of Rbtp. The X-ray crystal structures of [UO 2(NO 3) 2(Rbtp)] [R = Me ( 5), Pr ( n ) ( 6)] reveal a particular coordination geometry with seven coordinating atoms around the UO 2 fragment. The large steric crowding in the equatorial girdle forces the bidentate nitrate ligands to be almost perpendicular to the mean equatorial plane, inducing bending of the UO 2 fragment. The dinuclear oxo compound [U(CyMe 4btbp) 2(mu-O)UO 2(NO 3) 3][OTf] ( 7), which was obtained fortuitously from a 1:2:1 mixture of U(OTf) 4, CyMe 4btbp, and UO 2(NO 3) 2 [CyMe 4btbp = 6,6'-bis-(3,3,6,6-tetramethyl-cyclohexane-1,2,4-triazin-3-yl)-2,2'-bipyridine] is a very rare example of a mixed valence complex involving covalently bound U (IV) and U (VI) ions; its crystal structure also exhibits a seven coordinate uranyl moiety, with one bidentate nitrate group almost parallel to the UO 2 fragment. The distinct structural features of [UO 2(kappa (2)-NO 3) 2(Mebtp)], with its high coordination number and a noticeable bending of the UO 2 fragment, and of [UO 2(kappa (2)-NO 3)(kappa (1)-NO 3)(terpy)], which displays a classical geometry, were analyzed by Density Functional Theory, considering the bonding energy components and the molecular orbitals involved in the interaction between the uranyl, nitrate, and Mebtp or terpy moieties. The unusual geometry of the Mebtp derivative with the seven coordinating atoms around the UO 2 fragment was found very stable. In both the Mebtp and terpy complexes, the origin of the interaction appears to be primarily steric (Pauli repulsion and electrostatic); this term represents 62-63% of the total bonding energy while the orbital term contributes to about 37-38%.

Raman and infrared spectroscopic study of the molybdate-containing uranyl mineral calcurmolite

Abstract: Raman and infrared spectra of calcurmolite were recorded and interpreted from the uranium and molybdenum polyhedra, water molecules and hydroxyls point of view. U-O bond lengths in uranyl and Mo-O bond lengths in MoO6 octahedra were calculated and O-H…O bond lengths were inferred from the spectra. The mineral calcurmolite is characterised by bands assigned to the vibrations of the UO2 units. These units provide intense Raman bands at 930, 900 and 868 and 823 cm-1. These bands are attributed to the antisymmetric and symmetric stretching modes of the UO2 units respectively. Raman bands at 794, 700, 644, 378 and 354 cm-1 are attributed to vibrations of the MoO4 units. The bands at 693 and 668 cm-1 are attributed to antisymmetric and symmetric Ag modes of terminal MO2 units. Similar bands are observed at 797 and 773 cm-1 for koechlinite and 798 and 775 cm-1 for lindgrenite. It is probable that some of the bands in the low wavenumber region are attributable to the bending modes of MO2 units.

A cryogenic fluorescence spectroscopic study of uranyl carbonate, phosphate and oxyhydroxide minerals

Abstract: In this work we applied time-resolved laser-induced fluorescence spectroscopy (TRLIF) at both room temperature (RT) and near liquid-helium temperature (6 K) to characterize a series of natural and synthetic minerals of uranium carbonate, phosphate and oxyhydroxides including rutherfordine, zellerite, liebigite, phosphuranylite, meta-autunite, meta-torbernite, uranyl phosphate, sodium-uranyl-phosphate, becquerelite, schoepite, meta-schoepite, dehydrated schoepite and compreignacite, and have compared the spectral characteristics among these minerals as well as our previously published data on uranyl silicates. For the carbonate minerals, the fluorescence spectra of rutherfordine showed significant difference from those of zellerite and liebigite. The fluorescence spectra of the phosphate minerals closely resemble each other despite the differences in their composition and structure. For all uranium oxyhydroxides, the fluorescence spectra are largely red-shifted as compared to those of the uranium carbonates and phosphates and their vibronic bands are broad and less resolved at RT. The enhanced spectra resolution at 6 K allows more accurate determination of the fluorescence band origin and offers a complemental method to measure the O=U=O symmetrical stretch frequency, v,, from the spacings of the vibronic bands of the fluorescence spectra. The average v 1 values appear to be inversely correlated with the average pK a values of the anions.

Review of Uranyl Mineral Solubility Measurements

Abstract: The solubility of uranyl minerals controls the transport and distribution of uranium in many oxidizing environments. Uranyl minerals form as secondary phases within uranium deposits, and they also represent important sinks for uranium and other radionuclides in nuclear waste repository settings and at sites of uranium groundwater contamination. Standard state Gibbs free energies of formation can be used to describe the solubility of uranyl minerals; therefore, models of the distribution and mobility of uranium in the environment require accurate determination of the Gibbs free energies of formation for a wide range of relevant uranyl minerals. Despite decades of study, the thermodynamic properties for many environmentally-important uranyl minerals are still not well constrained. In this review, we describe the necessary elements for rigorous solubility experiments that can be used to define Gibbs free energies of formation; we summarize published solubility data, point out difficulties in conducting uranyl mineral solubility experiments, and identify areas of future research necessary to construct an internally-consistent thermodynamic database for uranyl minerals.

Biogenic nanoparticulate UO2: Synthesis, characterization, and factors affecting surface reactivity

Abstract: The surface reactivity of biogenic, nanoparticulate UO2 with respect to sorption of aqueous Zn(II) and particle annealing is different from that of bulk uraninite because of the presence of surface-associated organic matter on the biogenic UO2. Synthesis of biogenic UO2 was accomplished by reduction of aqueous uranyl ions, UO 2 2 + by Shewanella putrefaciens CN32, and the resulting nanoparticles were washed using one of two protocols: (1) to remove surface-associated organic matter and soluble uranyl species (NAUO2), or (2) to remove only soluble uranyl species (BIUO2). A suite of bulk and surface characterization techniques was used to examine bulk and biogenic, nanoparticulate UO2 as a function of particle size and surface-associated organic matter. The N2-BET surface areas of the two biogenic UO2 samples following the washing procedures are 128.63 m2 g−1 (NAUO2) and 92.56 m2 g−1 (BIUO2), and the average particle sizes range from 5–10 nm based on TEM imaging. Electrophoretic mobility measurements indicate that the surface charge behavior of biogenic, nanoparticulate UO2 (both NAUO2 and BIUO2) over the pH range 3–9 is the same as that of bulk. The U LIII-edge EXAFS spectra for biogenic UO2 (both NAUO2 and BIUO2) were best fit with half the number of second-shell uranium neighbors compared to bulk uraninite, and no oxygen neighbors were detected beyond the first shell around U(IV) in the biogenic UO2. At pH 7, sorption of Zn(II) onto both bulk uraninite and biogenic, nanoparticulate UO2 is independent of electrolyte concentration, suggesting that Zn(II) sorption complexes are dominantly inner-sphere. The maximum surface area-normalized Zn(II) sorption loadings for the three substrates were 3.00 ± 0.20 μmol m−2 UO2 (bulk uraninite), 2.34 ± 0.12 μmol m−2 UO2 (NAUO2), and 2.57 ± 0.10 μmol m−2 UO2 (BIUO2). Fits of Zn K-edge EXAFS spectra for biogenic, nanoparticulate UO2 indicate that Zn(II) sorption is dependent on the washing protocol. Zn–U pair correlations were observed at 2.8 ± 0.1 A for NAUO2 and bulk uraninite; however, they were not observed for sample BIUO2. The derived Zn–U distance, coupled with an average Zn–O distance of 2.09 ± 0.02 A, indicates that Zn(O,OH)6 sorbs as bidentate, edge-sharing complexes to UO8 polyhedra at the surface of NAUO2 nanoparticles and bulk uraninite, which is consistent with a Pauling bond-valence analysis. The absence of Zn–U pair correlations in sample BIUO2 suggests that Zn(II) binds preferentially to the organic matter coating rather than the UO2 surface. Surface-associated organic matter on the biogenic UO2 particles also inhibited particle annealing at 90 °C under anaerobic conditions. These results suggest that surface-associated organic matter decreases the reactivity of biogenic, nanoparticulate UO2 surfaces relative to aqueous Zn(II) and possibly other environmental contaminants.