Showing papers on "Uranyl published in 2006"
TL;DR: Based on the formation constants obtained in this study, speciation calculations indicate that at low Ca2+ concentration (e.g., <2.2 mmol/L), CaUO2(CO3)3(2-) is more important than Ca2UO 2(CO 3)3 and that the Ca2 UO2-CO3-3 distribution increased with increasing Ca2- concentration.
Abstract: The formation constants of ternary complexes (MUO2(CO3)32- and M2UO2(CO3)30) of uranyl and carbonate with alkaline earth metals (M2+ denotes Mg2+, Ca2+, Sr2+, and Ba2+) were determined with an anion exchange method by varying the metal concentrations (01−5 mmol/L) at pH 81 and a constant ionic strength (01 mol/L NaNO3) under equilibrium with atmospheric CO2 The results indicate that the complexes of MUO2(CO3)32- and M2UO2(CO3)3 are simultaneously formed for Ca2+ and Ba2+, while Mg2+ and Sr2+ form only the MUO2(CO3)32- complex under our experimental conditions The cumulative stability constants for the MUO2(CO3)32- complex obtained at I = 0 are as follows: logβ113 = 2611 ± 004, 2718 ± 006, 2686 ± 004, and 2668 ± 004 for Mg2+, Ca2+, Sr2+, and Ba2+, respectively For M2UO2(CO3)30, the value of logβ213 at I = 0 was measured to be 3070 ± 005 and 2975 ± 007 for Ca2+ and Ba2+, respectively Based on the formation constants obtained in this study, speciation calculations indicate that at low Ca2
442 citations
TL;DR: The intent of this Dalton Perspective is to highlight the recent advances in uranium molecular chemistry, with the results reported during the 2000-2006 period, with a number of tetravalent uranium complexes, which exhibit exciting structures and physicochemical properties.
Abstract: The intent of this Dalton Perspective is to highlight the recent advances in uranium molecular chemistry, with the results reported during the 2000–2006 period. This discipline is currently witnessing an impressive development, together with the theoretical chemistry and solid-state chemistry of the f-elements, and its face has profoundly changed, revealing unsuspected structural and reactivity features. This progress required and was facilitated by the use of new precursors. Studies of low-valent compounds gave a better insight into lanthanide(III)/actinide(III) differentiation and led to the discovery of unusual reactions, including activation of small molecules. A number of tetravalent uranium complexes, in particular polynuclear compounds, have been synthesized, which exhibit exciting structures and physicochemical properties. The potential of uranium(III) and uranium(IV) complexes in catalysis has been confirmed. The uranyl complexes, from mononuclear species to supramolecular assemblies, reveal a variety of novel structures, changing the generally accepted ideas on the coordination geometry and the stability of the UO22+ ion.
277 citations
TL;DR: In this paper, a review focusing on uranium cation coordination complexes with multidentate nitrogen-containing ligands appeared in the literature from 2000 to mid-2005, with reports prior to 2000 included where pertinent.
246 citations
TL;DR: The biocoordination chemistry of the uranyl cation (uranium(VI), [UO2]2+) is assessed utilizing data from the Protein Databank, solution thermodynamic data and biochemical experiments.
190 citations
TL;DR: In this article, 11 novel U(VI)-containing coordination polymers have been synthesized under hydrothermal conditions and characterized via single-crystal X-ray diffraction and fluorescence spectroscopy.
Abstract: Eleven novel U(VI)-containing coordination polymers have been synthesized under hydrothermal conditions and characterized via single-crystal X-ray diffraction and fluorescence spectroscopy. These inorganic/organic hybrid materials represent an important advance in the synthesis of polymeric materials in that multiple organic species have been employed in an effort to influence topology and properties. This family of materials is thus the result of a systematic pairing of the uranyl cation, UO22+, with aliphatic dicarboxylates (see part I, this issue) and the dipyridyl species, 4,4‘-dipyridyl and 1,2-bis(4-pyridyl)ethane. This second organic component assumes a number of roles in these structures, including direct coordination, charge balance, and structure direction. Distinction between which role(s) the dipyridyl species will assume appears to be correlated to the size (length) matching between it and the dicarboxylate linker used. Further, polymerization of primary building units (i.e., monomeric uranyl...
154 citations
TL;DR: Water structure appears to be governed by the presence of adsorbed ions and not by the location of layer charge associated with the substrate, as illustrated by atomistic detail provided to help explain experimental trends in uranyl adsorption onto natural media containing smectite clays.
Abstract: Molecular dynamics simulations were performed to provide a systematic study of aqueous uranyl adsorption onto the external surface of 2:1 dioctahedral clays. Our understanding of this key process is critical in predicting the fate of radioactive contaminants in natural groundwaters. These simulations provide atomistic detail to help explain experimental trends in uranyl adsorption onto natural media containing smectite clays. Aqueous uranyl concentrations ranged from 0.027 to 0.162 M. Sodium ions and carbonate ions (0.027-0.243 M) were also present in the aqueous regions to more faithfully model a stream of uranyl-containing groundwater contacting a mineral system comprised of Na-smectite. No adsorption occurred near the pyrophyllite surface, and there was little difference in uranyl adsorption onto the beidellite and montmorillonite, despite the difference in location of clay layer charge between the two. At low uranyl concentration, the pentaaquouranyl complex dominates in solution and readily adsorbs to the clay basal plane. At higher uranyl (and carbonate) concentrations, the mono(carbonato) complex forms in solution, and uranyl adsorption decreases. Sodium adsorption onto beidellite occurred both as inner- and outer-sphere surface complexes, again with little effect on uranyl adsorption. Uranyl surface complexes consisted primarily of the pentaaquo cation (85%) and to a lesser extent the mono(carbonato) species (15%). Speciation diagrams of the aqueous region indicate that the mono(carbonato)uranyl complex is abundant at high ionic strength. Oligomeric uranyl complexes are observed at high ionic strength, particularly near the pyrophyllite and montmorillonite surfaces. Atomic density profiles of water oxygen and hydrogen atoms are nearly identical near the beidellite and montmorillonite surfaces. Water structure therefore appears to be governed by the presence of adsorbed ions and not by the location of layer charge associated with the substrate. The water oxygen density near the pyrophyllite surface is similar to the other cases, but the hydrogen density profile indicates reduced hydrogen bonding between adsorbed water molecules and the surface.
141 citations
TL;DR: The present study successfully demonstrates the feasibility of removing uranyl ions selectively in the range 5 microg - 300 mg present in 500 mL of synthetic nuclear power reactor effluent containing a host of other inorganic species.
Abstract: Major quantities of uranium find use as nuclear fuel in nuclear power reactors. In view of the extreme toxicity of uranium and consequent stringent limits fixed by WHO and various national governments, it is essential to remove uranium from nuclear power reactor effluents before discharge into environment. Ion imprinted polymer (IIP) materials have traditionally been used for the recovery of uranium from dilute aqueous solutions prior to detection or from seawater. We now describe the use of IIP materials for selective removal of uranium from a typical synthetic nuclear power reactor effluent. The IIP materials were prepared for uranyl ion (imprint ion) by forming binary salicylaldoxime (SALO) or 4-vinylpyridine (VP) or ternary SALO-VP complexes in 2-methoxyethanol (porogen) and copolymerizing in the presence of styrene (monomer), divinylbenzene (cross-linking monomer), and 2,2'-azobisisobutyronitrile (initiator). The resulting materials were then ground and sieved to obtain unleached polymer particles. Leached IIP particles were obtained by leaching the imprint ions with 6.0 M HCl. Control polymer particles were also prepared analogously without the imprint ion. The IIP particles obtained with ternary complex alone gave quantitative removal of uranyl ion in the pH range 3.5-5.0 with as low as 0.08 g. The retention capacity of uranyl IIP particles was found to be 98.50 mg/g of polymer. The present study successfully demonstrates the feasibility of removing uranyl ions selectively in the range 5 microg - 300 mg present in 500 mL of synthetic nuclear power reactor effluent containing a host of other inorganic species.
134 citations
TL;DR: The described synthetic method allows us to isolate a stable derivative of the elusive pentavalent UO2+ species providing a potential starting material for the development of anhydrous UO 2+ coordination chemistry.
Abstract: The polymeric complex {[UO2Py5][KI2Py2]}n was isolated by controlled oxidation of uranium tris-iodide in pyridine and structurally characterized using X-ray diffraction. The described synthetic method allows us to isolate a stable derivative of the elusive pentavalent UO2+ species providing a potential starting material for the development of anhydrous UO2+ coordination chemistry.
127 citations
TL;DR: The linear uranyl dication [UO2]2+ can be bound in one of two coordination sites in the ditopic Pacman-shaped pyrrolic macrocyle H4L using Mn2+, Fe2+, or Co2+ cations in the second donor compartment.
Abstract: The linear uranyl dication [UO2]2+ can be bound in one of two coordination sites in the ditopic Pacman-shaped pyrrolic macrocyle H4L. Incorporation of Mn2+, Fe2+, or Co2+ cations in the second donor compartment affords the first uranyl complexes with a transition-metal-functionalized oxo group.
121 citations
TL;DR: In this paper, four novel coordination polymers, UO2(C7H10O4) (1), UO 2C8H12O4 (2) and UO 3C10H16O4(4), were synthesized from UO(NO3)2·6H2O and aliphatic dicarboxylic acids (pimelic, suberic, azelaic, and sebacic acids, respectively) using hydrothermal techniques and were characterized by single-crystal diffraction.
Abstract: Four novel U(VI) containing coordination polymers, UO2(C7H10O4) (1), UO2(C8H12O4) (2), UO2(C9H14O4) (3), and UO2(C10H16O4) (4), were synthesized from UO2(NO3)2·6H2O and aliphatic dicarboxylic acids (pimelic, suberic, azelaic, and sebacic acids, respectively) using hydrothermal techniques and were characterized by single-crystal diffraction. Compounds 1 and 4 consist of (UO2)2O8 dimers connected by the acid molecules to form layers, whereas 2 and 3 are constructed of chains of hexagonal bipyramids connected to form layers. These materials complete the series of aliphatic uranyl carboxylates to C10 (sebacic acid) and may be considered to be carboxylate “end-members” of a series of dipyridyl/carboxylate structures described in part II of this investigation (following in this issue).
117 citations
TL;DR: In this paper, U(VI) reactivity and surface speciation on a poorly crystalline aluminosilicate mineral, synthetic imogolite, were investigated using batch adsorption experiments, X-ray absorption spectroscopy (XAS), and surface complexation modeling.
TL;DR: The solid-state molecular structures of 5, 7, and 9 have been determined by X-ray crystallography, and these complexes, like their parent compounds, exhibit linear N-U-N angles and short U-N bonds.
Abstract: Addition of 1.5 equiv of I2 to a THF solution of UI3(THF)4, containing either 6 equiv of tBuNH2 or 2 equiv of RNH2 (R = Ph, 3,5-(CF3)2C6H3, 2,6-(iPr)2C6H3) and 4 equiv of NEt3, generates orange solutions containing U(NtBu)2I2(THF)2 (1) or U(NAr)2I2(THF)3 (Ar = Ph, 2; 3,5-(CF3)2C6H3, 3; 2,6-(iPr)2C6H3, 4), respectively, all of which can be isolated in good yields. Alternatively, 1 can be prepared by reaction of uranium metal with 3 equiv of I2 and 6 equiv of tBuNH2, also in good yield. Complexes 1−4 have been characterized by X-ray crystallography, and each of these complexes exhibits linear N−U−N linkages and short U−N bonds. Using density functional theory simulations of complexes 1 and 2, two triple bonds between the metal center and the nitrogen ligands were identified. Complexes 1 and 2 readily react with neutral Lewis bases such as pyridine or Ph3PO to form U(NR)2I2(L)2 (R = tBu, L = py, 5; Ph3PO, 7; R = Ph, L = py, 6; Ph3PO, 8), and with PMe3 to form U(NR)2I2(THF)(PMe3)2 (R = tBu, 9; Ph, 10). The so...
TL;DR: Optimize in a polarizable continuum (specifically, the conductor-like screening model in conjunction with the zero-order regular approximation and triple-zeta Slater basis sets) can qualitatively reproduce the geometrical changes from explicit hydration.
Abstract: Results from Car−Parrinello molecular dynamics simulations are reported for [UO2(OH2)5]2+, UO2(NO3)2(OH2)2, and UO2(NO3)2(η2-tmma) (tmma = tetramethylmalonamide) in the gas phase and in aqueous solution. The distances between uranyl and neutral ligands such as water and tmma are decreased by up to 0.2 A upon hydration, whereas those between uranyl and the nitrate ion are increased by up to 0.08 A. According to pointwise thermodynamic integration involving constrained molecular dynamics simulations, solvation facilitates the transition of the chelating nitrate ligand to a η1-bonding mode: the free energy of UO2(η2-NO3)(η1-NO3)(OH2)2 relative to the bis-chelating minimum drops from 3.9 kcal/mol in vacuo to 1.4 kcal/mol in water. Optimizations in a polarizable continuum (specifically, the conductor-like screening model in conjunction with the zero-order regular approximation and triple-ζ Slater basis sets) can qualitatively reproduce the geometrical changes from explicit hydration.
TL;DR: In this paper, two new uranyl-organic compounds (UO2)3O(OH)3(NA)2]−H3O+ (1) (HNA =nicotinic acid) and UO22(phen)2(BTEC) have been prepared under hydrothermal conditions.
TL;DR: The gas-phase infrared spectra of discrete uranyl complexes ligated with acetone and/or acetonitrile were used to evaluate systematic trends of ligation on the position of the O=U=O stretch and to enable rigorous comparison with the results of computational studies.
Abstract: The gas-phase infrared spectra of discrete uranyl ([UO2]2+) complexes ligated with acetone and/or acetonitrile were used to evaluate systematic trends of ligation on the position of the OUO stretch and to enable rigorous comparison with the results of computational studies Ionic uranyl complexes isolated in a Fourier transform ion cyclotron resonance mass spectrometer were fragmented via infrared multiphoton dissociation using a free electron laser scanned over the mid-IR wavelengths The asymmetric OUO stretching frequency was measured at 1017 cm-1 for [UO2(CH3COCH3)2]2+ and was systematically red shifted to 1000 and 988 cm-1 by the addition of a third and fourth acetone ligand, respectively, which was consistent with increased donation of electron density to the uranium center in complexes with higher coordination number The values generated computationally using LDA, B3LYP, and ZORA-PW91 were in good agreement with experimental measurements In contrast to the uranyl frequency shifts, the carbonyl fr
TL;DR: In this paper, the authors examined samples from the Hanford Site at microscopic scale to determine the mode of uranium occurrence, and constructed a conceptual model for its emplacement, which was tested using a model of reactive diffusion at that scale.
TL;DR: NMR studies showed unambiguously the presence of the cation-cation complex in pyridine solution while more polar solvents lead to the disruption of the UO2/UO2+ interaction and increased stability.
Abstract: A polymetallic assembly containing mutually coordinated highly reactive UO2+ groups was isolated in the presence of dibenzoylmethanate. NMR studies showed unambiguously the presence of the cation−cation complex in pyridine solution while more polar solvents lead to the disruption of the UO2+/UO2+ interaction and increased stability.
TL;DR: Raman spectroscopy has been used to study the molecular structure of a series of selected uranyl silicate minerals including uranophane, sklodowskite, cuproskloda, boltwoodite and kasolite as mentioned in this paper.
Abstract: Raman spectroscopy has been used to study the molecular structure of a series of selected uranyl silicate minerals including uranophane, sklodowskite, cuprosklodowskite, boltwoodite and kasolite. Raman spectra clearly show well resolved bands in the 750 to 800 cm-1 region and in the 950 to 1000 cm-1 region assigned to the ν1 modes of the (UO2)2+ units and to the (SiO4)4- tetrahedra. Sets of Raman bands in the 200 to 300 cm-1 region are assigned to ν2 δ (UO2)2+ and UO ligand vibrations. Multiple bands indicate the non-equivalence of the UO bonds and the lifting of the degeneracy of ν2 δ (UO2)2+ vibrations. The (SiO4)4- tetrahedral are characterized by bands in the 470 to 550 cm-1 and in the 390 to 420 cm-1 region. These bands are attributed to the ν4 and ν2 (SiO4)4- bending modes. The minerals show characteristic OH stretching bands in the 2900 to 3500 cm-1 and 3600 to 3700 cm-1 region ascribed to water stretching and SiOH stretching vibrations. The high wavenumber position of the δH2O bands indicate strong hydrogen bonding of water in these uranyl silicates. Bands in the 1400 to 1550 cm-1 region are attributed to δSiOH modes. The Raman spectroscopy of uranyl silicate minerals enabled separation of the bands attributed to distinct vibrational units. This enabled definitive assignment of the bands. The spectra are analysed in terms of the molecular structure of the minerals.
TL;DR: ExAFS modeling shows that uranyl is present in these oxides predominantly in the tunnels of the Mn oxide structure in a tridentate complex, implying that Mn-oxidizing bacteria may significantly influence dissolved U(VI) concentrations in impacted waters via sorption and incorporation into Mn oxide biominerals.
Abstract: Βiogenic manganese oxides are common and an important source of reactive mineral surfaces in the environment that may be potentially enhanced in bioremediation cases to improve natural attenuation. Experiments were performed in which the uranyl ion, UO22+ (U(VI)), at various concentrations was present during manganese oxide biogenesis. At all concentrations, there was strong uptake of U onto the oxides. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray diffraction (XRD) studies were carried out to determine the molecular-scale mechanism by which uranyl is incorporated into the oxide and how this incorporation affects the resulting manganese oxide structure and mineralogy. The EXAFS experiments show that at low concentrations ( 2 mol % U, >4 μM U(VI) in solution), the presence of U(VI) affects the stability and structure of the Mn oxide to form poor...
TL;DR: The UO2X(THF)n (X = I, OSO2CF3) complexes were obtained by reduction of their U(VI) parents with TlC5H5 or KC5R5 in THF.
TL;DR: In this article, the authors show that M2UOO2(dipic)2, M = H, Rb, Cs, dipic = dipicolinate = pyridine-2,6-dicarboxylate, all of which show strong solid-state luminescence from the uranyl centre, have as a common feature extended π-stacking arrays of the dipicolinates.
Abstract: Crystal structure determinations on the hydrated forms of the complexes M2UO2(dipic)2, M = H, Rb, Cs, dipic = dipicolinate = pyridine-2,6-dicarboxylate, all of which show strong solid-state luminescence from the uranyl centre, have as a common feature extended π-stacking arrays of the dipicolinate ligands. Comparisons with related but non-luminescent complexes indicate that these stacking arrays may be associated with efficient energy transfer from the dipicolinate “antennae” to the uranyl unit. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
TL;DR: The first crystal structure of uranyl citrate is reported, as well as that of Uranyl sodium tricarballylate, which are polymeric compounds, with all acid groups coordinated, but they differ strongly in their coordination modes.
TL;DR: It is shown that steric and/or fit/misfit requirements of actinyl cations to the ligand cavities, among the studied complexes, are the most favorable for the bi-pyen ligand 2, because its flexibility allows for optimal metal-to-donor-atom distances.
Abstract: Formation of complexes of alaskaphyrin 1, bi-pyen 2 and bi-tpmd 3 ligands with actinyl ions AnO2(n+), An = U, Np, Pu and n = 1, 2, was studied using density functional theory (DFT) within the scalar relativistic four-component approximation. The alaskaphyrin complexes of the uranyl are predicted to have a bent conformation, in contrast to the experimentally available X-ray data. This deviation is likely due to crystal packing effects. Apart from these conformational differences, calculated geometry parameters and vibrational frequencies are in agreement with the available experimental data. The character of bonding in the complexes is investigated using bond order analysis and extended transition states (ETS) energy decomposition. Metal-to-ligand bonds can be described as primarily ionic although substantial charge transfer is observed as well. Based on ETS analysis, it is shown that steric and/or fit/misfit requirements of actinyl cations to the ligand cavities, among the studied complexes, are the most favorable for the bi-pyen ligand 2, because its flexibility allows for optimal metal-to-donor-atom distances. Planarity of the equatorial coordination sphere of the actinide atom is found to be less important than the ability of a ligand to provide optimal uranium-to-nitrogen bond lengths. Experimental differences in demetalation rates between similar alaskaphyrin, bi-pyen and bi-tpmd uranyl complexes are explained as a result of easier protonation of the Schiff-base nitrogen of the latter. Reduction potentials calculated for the uranium complexes show a good agreement with the experiment, both in relative and in absolute terms.
TL;DR: Calcite can be effective at sequestering U in vadose zone sediments and validate U-related dating methods and show that the uranyl environment may evolve over long time scales, becoming more calcite-like and more stable for long-term sequestration of uranium.
Abstract: The mechanism of uranium (U) incorporation into calcite (calcium carbonate) is of fundamental importance to the fate and transport of U at the surface and in the shallow subsurface and has implications for (a) the accuracy of U−Pb and U-series isotope ratio methods used to determine the ages of ancient deposits and (b) potential remediation strategies based on sequestration of U in the subsurface. Extended X-ray absorption fine structure (EXAFS) spectroscopy is uniquely suited to the study of U−calcite systems. The sensitivity of the EXAFS spectrum to the local atomic Ca coordination about U(VI) in the calcite structure results in an increase in the number and amplitude of Ca signals as the U(VI) becomes more ordered within the crystal structure. Our X-ray microprobe (10-μm) measurements of an ancient 298 million-year-old organic-rich calcite (calcrete) clearly revealed three coordination shells of Ca atoms, defining a well-ordered calcite structure about uranyl to a distance of ∼6.5 A. These results indi...
TL;DR: In this article, a phosphate functionalized cysteamine self-assembled monolayer based on gold electrode is designed for uranyl ion (UO2 2+ ) detection by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry.
15 Apr 2006
TL;DR: Measurement of the stretching frequency of nitrate ions coming from electrolyte showed a pure electrostatic adsorption and exclude the formation of a ternary complex with uranyl, indicating that the same surface species is responsible for the sorption in the pH range 5-8.
Abstract: In this paper, we present results of ATR-IR spectroscopy of uranyl complexes adsorbed on hematite. This method allowed the in situ recording of infrared spectra of uranyl sorbed on hematite in presence of aqueous solution and to detect one peak at 906 cm(-1) attributed to the antisymmetric O=U=O stretching. The intensity of the peak increases with pH, but its shape does not evolve, indicating that the same surface species is responsible for the sorption in the pH range 5-8. The reversibility experiments confirm that the hematite deposit reacts in the same way as dispersed suspensions. Measurement of the stretching frequency of nitrate ions coming from electrolyte showed a pure electrostatic adsorption and exclude the formation of a ternary complex with uranyl.
TL;DR: In this paper, the position of the antisymmetric stretching vibration occurs at lower wavenumbers than the symmetric stretching mode of the (UO2)2+ and (SeO3)2-2-units respectively.
Abstract: The mineral haynesite, a uranyl selenite, has been characterised by Raman spectroscopy at 298 and 77 K. Two bands at 811.5 and 800.2 cm-1 are assigned to the symmetric stretching modes of the (UO2)2+ and (SeO3)2- units respectively. These values give calculated U-O bond lengths of 1.799 and/or 1.801 A. The broad band at 861.8 cm-1 is assigned to the ν3 antisymmetric stretching mode of the (UO2)2+ (calculated U-O bond length 1.813 A). Additional bands are observed in the 77 K spectrum. In the spectroscopy of selenite compounds the position of the antisymmetric stretching vibration occurs at lower wavenumbers than the symmetric stretching mode and thus the band at 746.6 cm-1 is attributed to the ν3 antisymmetric stretching vibration of the (SeO3)2- units. The ν4 and the ν2 vibrational modes of the (SeO3)2- units are observed at 418.5 cm-1 and 472.1 cm-1. Bands observed at 278.3, 257.3 and 218.8 cm-1 are assigned to OUO bending vibrations.
TL;DR: In this article, the UO bond lengths in uranyl were calculated from the Raman and infrared spectra, which were in agreement with those from the available X-ray single crystal structure analysis of parsonsite.
Abstract: The mineral parsonsite, with samples from The Ranger Uranium Mine, Australia, and the La Faye Mine, Grury, Saone-et-Loire, Burgundy, France, has been characterised by Raman spectroscopy at 298 and 77 K and complemented with infrared spectroscopy. Two Raman bands close to 807 and 796 cm−1 are attributed to the ν1(UO2)2+ symmetric stretching modes, while two bands close to 953 or 945 cm−1 and 863–873 cm−1 are assigned to the ν3(UO2)2+ anti-symmetric stretching vibrations. Four or five bands (953, 926, 910, 883 cm−1) are observed in the infrared spectrum in this region. Bands at 965–967 and 972 cm−1 are assigned to the ν1(PO4)3− symmetric stretching modes and bands that are observed in the 987 to 1078 cm−1region to the ν3(PO4)3− anti-symmetric stretching modes. Bands at 465, 439, 406, 394 cm−1 (298 K) and 466, 442, 405, 395 cm−1(77 K) are assigned to the split, doubly degenerate ν2(PO4)−3bending vibrations. Bands of very low intensity at 609, 595, 591, 582, 560 and 540 cm−1are attributed to the split, triply degenerate ν4(PO4)−3bending modes. Bands observed at wavenumbers lower than 300 cm−1are connected with the split ν2(δ) (UO2)2+ bending, ν(UOligand), δ(UOligand) and lattice vibrations. UO bond lengths in uranyl were calculated from the Raman and infrared spectra, which are in agreement with those from the available X-ray single crystal structure analysis of parsonsite. A short comment is given on the water content and the possibility of a hydrogen-bonding network in the parsonsite crystal structure. Copyright © 2006 John Wiley & Sons, Ltd.
TL;DR: McKinley et al. as mentioned in this paper presented a coupled microscopic reactive diffusion model by independently characterizing the intragrain diffusion and dissolution kinetics of Na-boltwoodite, which was used to infer the geochemical conditions that led to the uranyl precipitation when the sediment was contaminated by U-containing wastes at the site.
Abstract: Microscopic and spectroscopic analysis of uranium-contaminated sediment cores beneath the BX waste tank farm at the US Department of Energy (DOE) Hanford site revealed that uranium (U) existed as uranyl precipitates primarily associated with the intragrain fractures of granitic clasts in the sediment (McKinley et al. 2005). The dissolution of the precipitates appeared to be controlled by intragrain ion diffusion coupled with the dissolution kinetics of the uranyl precipitates most likely as Na-boltwoodite. Here we presented a coupled microscopic reactive diffusion model by independently characterizing the intragrain diffusion and dissolution kinetics of Na-boltwoodite. Diffusion characterization with a nuclear magnetic resonance (NMR) pulse gradient spin-echo (PGSE) technique showed that the intragrain fractures of the granitic clasts in the Hanford sediment contain two domans with distinct diffusivities. The fast diffusion domain has an apparent tortuosity of about 1.5, while the slow region has a tortuosity of two orders of magnitude larger. A two-domain diffusion model was assembled and used to infer the geochemical conditions that led to intragrain uranyl precipitation when the sediment was contaminated by U-containing wastes at the site. Rapid precipitation of Na-boltwoodite was simulated when a U-containing, alkaline caustic, and high carbonate tank waste solution diffused into intragrain fracturesmore » originally containing Si-rich solutions. The model was also used to simulate uranyl dissolution and release from the contaminant sediment to aqueous solutions. With independently characterized parameters for Na-boltwoodite dissolution, the model simulations demonstrated that diffusion could significantly decrease the rates of intragrain uranyl mineral dissolution due to diffusion-induced local solubility limitation, and the intragrain uranyl precipitates could serve as a long-term uranyl source for the vadose porewater and underlying groundwater at this site.« less