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Hydrated uranyl fluoride, [(UO2F2)(H2O)]7·4H2O, is not stable at elevated water vapor pressure, undergoing a complete loss of fluorine to form a uranyl hydroxide hydrate. Powder X-ray diffraction data of the resultant uranyl hydroxide species is presented for the first time, along with Raman and infrared (IR) spectra. The new uranyl hydroxide species is structurally similar to the layered uranyl hydroxide hydrate minerals schoepite and metaschoepite, but has a significantly expanded interlayer spacing (c = 15.12 vs. 14.73 A), suggesting that additional H2O molecules may be present between the uranyl layers. Comparison of the Raman and IR spectra of this new uranyl hydroxide hydrate and synthetic metaschoepite ([(UO2)4O(OH)6]·5H2O) suggests that the equatorial environment of the uranyl ion may differ and that H2O molecules in the new species participate in stronger hydrogen bonds. In addition, the interlayer spacing of both this new uranyl hydroxide species and synthetic metaschoepite is shown to be sensitive to the environmental humidity, contracting and re-expanding with desiccation and rehydration. Structural distinction between the new uranyl hydroxide species and synthetic metaschoepite is confirmed by a comparison of the thermal behavior; unlike metaschoepite, the new hydrate does not form α-UO2(OH)2 upon dehydration.

Uranium trioxide, UO3, has a T-shaped structure with bent uranyl, UO22+, coordinated by an equatorial oxo, O2-. The structure of cation UO3+ is similar but with an equatorial oxyl, O center dot-. Neutral and cationic uranium trioxide coordinated by nitrates were characterized by collision induced dissociation (CID), infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory. CID of uranyl nitrate, [UO2 (NO3)3]- (complex A1), eliminates NO2 to produce nitrate-coordinated UO3+, [UO2 (O. )(NO3)2]-(B1), which ejects NO3 to yield UO3 in [UO2 (O)(NO3)]- (C1). Finally, C1 associates with H2O to afford uranyl hydroxide in [UO2(OH)2 (NO3)]- (D1). IRMPD of B1, C1, and D1 confirms uranyl equatorially coordinated by nitrate(s) along with the following ligands: (B1) radical oxyl O.-; (C1) oxo O2-; and (D1) two hydroxyls, OH- . As the nitrates are bidentate, the equatorial coordination is six in A1, five in B1, four in D1, and three in C1. Ligand congestion in low-coordinate C1 suggests orbital-directed bonding. Hydrolysis of the equatorial oxo in C1 epitomizes the inverse trans influence in UO3, which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl v3 IR frequencies indicate the following donor ordering: O2- [best donor] >> O.- > OH-> NO3-.

Nuclear energy generated through fission of uranium (U) is a re-producible, cleaner, and more sustainable energy source. While the terrestrial U reserve can just supply nuclear power industry for ~100 years, enormous U reserve in ocean can supply the nuclear power industry for ~72,000 years at the current capacity. However, developing viable and cost-competitive technologies for U mining from ocean is an extraordinary challenge. In this work, we developed magnetic mesoporous silica nanoparticles (MMSNs) that were functionalized with several organic ligands to improve their selectivity and capacity of uranium extraction from seawater simulant. The functionalized MMSNs were demonstrated to be very effective in U extraction from seawater with a capacity up to 54 mg/g. The functionalized MMSNs after exposed to U in seawater were characterized by a host of spectroscopic methods including FTIR, EPR, and synchrotron XANES and EXAFS spectroscopies. The dominant U species on the functionalized MMSNs is uranyl or uranyl hydroxide, rather than uranyl carbonates as expected in seawater. The uranyl-like species are bound with nitrogen binding ligand as η2 bound-like motifs or with phosphonate binding ligand as a monodentate, as well as on tetrahedral Si sites as an edge-sharing bidentate in the mesopore structure.

Carboxycellulose nanofibers (NOCNF) were extracted from untreated jute fibers using a simple nitro-oxidation method, employing nitric acid and sodium nitrite. The resulting NOCNF possessed high surface charge (−70 mV) and large carboxylate content (1.15 mmol/g), allowing them to be used as an effective medium to remove UO22+ ions from water. The UO22+ (or U(VI)) removal mechanism was found to include two stages: the initial stage of ionic adsorption on the NOCNF surface following by the later stage of uranyl hydroxide mineralization, as evidenced by the Fourier transform infrared, scanning electron microscopy with energy dispersive spectroscopy capabilities, transmission electron miscroscopy, and wide-angle X-ray diffraction results. Using the Langmuir isotherm model, the extracted NOCNF exhibited a very high maximum adsorption capacity (1470 mg/g), about several times higher than the most efficient adsorbent reported (poly(acrylic acid) hydrogel). It was also found that the remediation of UO22+ ions by N...

It has been proven, by the analysis of macroscopic amount of materiel pressed into pellets, that Raman spectrometry is a well suited technique to determine precisely the chemical compositions of uranium ore concentrates (UOCs), which is useful information for nuclear forensics and safeguards. However, as in some cases, only low amounts of UOC are available or the sample is a mixture of different UOCs, there is a need to carry out the identification at the individual grain or particle’s level. To evaluate the capability of micro-Raman spectrometry (MRS) for this purpose, 15–30 particles with typical sizes of a few micrometers from five UOCs of known bulk chemical compositions (uranyl peroxide – UP, sodium di-uranate – SDU, ammonium di-uranate – ADU, tri-uranium octo-oxide – TUO, and uranyl hydroxide – UH), production process and origins, were analyzed. Spectra were also compared with the ones obtained with macroscopic amounts of material pressed into pellets. Results show that for some UOCs (SDU, ADU, UH), micro-Raman spectra are reproducible from one particle to another and in good agreement on one side with available bibliographic data and on the other side with Raman spectra performed on macroscopic amounts of UOC. However, spectra of particles from the UP and TUO UOCs show that these compounds are mixtures of three species which were identified. In these cases, an acceptable agreement is obtained between the average Raman spectrum on compressed pellets and the one of most abundant species in analyzed particles. Consequently, an UOC compound or components of a mixture of UOCs can be reliably identified by the analyses of a limited number of isolated particles of a few μm in size by means of MRS.

We confirm that synthetic uranyl hydroxide hydrate metaschoepite [(UO)24O(OH)6]·5H2O is unstable against dehydration under dry conditions, and we present a structural and vibrational spectroscopic study of synthetic metaschoepite and its ambient temperature dehydration product. Complementary structural (X-ray diffraction and neutron diffraction) and vibrational spectroscopic techniques (Raman spectroscopy, infrared spectroscopy, and inelastic neutron scattering) are used to probe different components of these species. Analysis of the dehydration product suggests that it contains both pentagonally coordinated and hexagonally coordinated uranyl ions, necessitating that some uranyl ions undergo a coordination change during the dehydration of pentagonally coordinated metaschoepite. Vibrational spectra of metaschoepite and its dehydration product are interpreted with power spectra generated from ab initio molecular dynamics trajectories, allowing assignment of all major features. We identify the uranyl symmetric stretching modes of the four distinct uranyl ions in synthetic metaschoepite and clarify the assignment of lower energy Raman modes in both structures. The coanalysis of experimental and computational data reveals a strong coupling between the uranyl stretching modes and hydroxide bending modes in the anhydrous structure, leading to the presence of several high-energy combination bands in the inelastic neutron scattering data.

International audience; The purpose of this work is to determine how long UF$_4$ in micro-particles can be detected after release in the environment, what the degradation products are and what are the parameters which mostly lead to degradation of UF$_4$. For this, Raman spectra of UF$_4$ μm-sized particles (typical diameters ∼5 μm) stored in various conditions (humid or dry air/argon, without or with UV light, 20 °C, 40 °C, 80 °C) were monitored by micro-Raman spectrometry over three months. The characteristic spectral signature of UF$_4$ followed here is the very intense and broad fluorescence peak which covers the 700—1100 cm-1 region when a visible laser at 514 nm is used. This study shows that persistence of the UF$_4$ signature in μm-sized particles released in the environment is strongly dependent on ambient conditions: typically a few days when submitted to both humid air or argon atmosphere and only intense UV light, a few weeks when exposed to a humid air or argon atmosphere in the dark, till at least several months and most probably several years, when kept in a dry atmosphere. Reaction of UF4 with water vapor ultimately leads to the formation uranyl hydroxide (schoepite) whereas exposure to both water vapor and UV light produces mainly uranyl peroxide (studtite).

Immobilization of U(VI) by naturally ubiquitous ferrous ions (Fe(II)) has been considered as an efficient and ecofriendly method to retard the migration of aqueous U(VI) at many nuclear sites and surface environments. In this study, we conducted Fe-U coprecipitation experiments to investigate the mechanism and stability of uranium (U) precipitation induced by a small quantity of Fe(II) under oxygen-rich conditions. The experimental results suggest that the sedimentation rates of U(VI) by Fe(II) under neutral oxygen-rich conditions are more than 96%, which are about 36% higher than those without Fe(II) and 16% higher than those under oxygen-free conditions. The Fe-U coprecipitates were observed to remain stable under slightly acidic to neutral and oxygen-rich conditions. Fe(II) primarily settles down as low-crystalline iron oxide hydroxide. U(VI) mainly precipitates as three forms: 16–20% of U forms uranyl hydroxide and metaschoepite, which is absorbed on the surface of the solids; 52–56% of U is absorbed as discrete uranyl phases at the internal pores of iron oxide hydroxide; and 27–29% of U is probably incorporated into the FeO(OH) structure as U(V) and U(VI). The U(V) generated via one-electron reduction is somewhat resistant to the oxidation of O2 and the acid dissolution. In addition, nearly 70% of U and only about 15% of Fe could be extracted in 24 h by a hydrochloric acid solution with the H+ concentration ([H+]) of 0.01 M, revealing that U(VI) immobilization by low concentration of Fe(II) combined with O2 has potential applications in the separation and recycling of aqueous uranium.

Under humid conditions, uranyl fluoride ([(UO2F2)(H2O)]7·4H2O) undergoes a loss of fluorine to form a uranyl hydroxide species, which can be further hydrated to form a uranyl peroxide species. X-ray diffraction data of the uranyl peroxide product is presented for the first time. In addition, the temperature and humidity conditions under which these reactions occur have been clarified by a 220-day experiment using microRaman spectroscopy to track chemical changes in individual particles of uranyl fluoride. At 25 and 35∘C, uranyl fluoride is found to be stable at 32% relative humidity but not stable at and above 59% relative humidity. We show that water vapor pressure is the driving factor in formation of both the hydroxide and peroxide products. The kinetics of the transformation from uranyl fluoride into uranyl hydroxide is consistent with a denucleation reaction following the absorption of water molecules.

We have demonstrated the solid-state formation of a uranyl peroxide (UP) species from hydrated uranyl fluoride via a uranyl hydroxide intermediate, the first observation of a UP species formed in a solid-state reaction. Water vapor pressure is shown to be a driving factor of both the loss of fluorine and the subsequent formation of peroxo units. We have ruled out a photochemical mechanism for formation of the UP species by demonstrating that the same reaction occurs in the dark. A radiolytic mechanism is unlikely because of the low radioactivity of the sample material, suggesting the existence of a novel UP formation mechanism.

Raman spectroscopy was used on 95 samples comprising mainly of uranium ore concentrates as well as some UF4 and UO2 samples, in order to classify uranium compounds for nuclear forensic purposes, for the first time. This technique was selected as it is non-destructive and rapid. The spectra obtained from 9 different classes of chemical compounds were subjected to multivariate data analysis such as principal component analysis (PCA), partial least square-discriminant analysis (PLS-DA) and Fisher Discriminant Analysis (FDA). These classes were ammonium diuranate (ADU), sodium diuranate (SDU), ammonium uranyl carbonate (AUC), uranyl hydroxide (UH), UO2, UO3, UO4, U3O8 and UF4. Unsupervised PCA of full spectra shows fairly good distinction among the classes with some overlaps observed with ADU and UH. These overlaps are also reflected in the poorer specificities determined by PLS-DA. Higher values of sensitivities and specificities of remaining compounds were obtained. Supervised FDA based on reduced dataset of only 40 variables shows similar results to that of PCA but with closer clustering of ADU, UH, SDU, AUC. As a rapid and non-destructive technique, Raman spectroscopy is useful and complements existing techniques in multi-faceted nuclear forensics.

Surface-functionalized magnetite nanoparticles have high capacity for U(VI) adsorption and can be easily separated from the aqueous phase by applying a magnetic field. A surface-engineered bilayer structure enables the stabilization of nanoparticles in aqueous solution. Functional groups in stearic acid (SA), oleic acid (OA), and octadecylphosphonic acid (ODP) coatings led to different adsorption extents (SA≈ OAODP) under the same conditions. The impact of water chemistry (initial loading of U(VI), pH, and the presence of carbonate) has been systematically examined for U(VI) adsorption to OA-coated nanoparticles. A diffuse double layer surface complexation model was developed for surface-functionalized magnetite nanoparticles that could simulate both the measured surface charge and the U(VI) adsorption behavior at the same time. With a small set of adsorption reactions for uranyl hydroxide and uranyl carbonate complexes to surface sites, the model can successfully simulate the entire adsorption data set over all uranium loadings, pH values, and dissolved inorganic carbon concentrations. The results show that the adsorption behavior was related to the changing U(VI) species and properties of surface coatings on nanoparticles. The model could also fit pH-dependent surface potential values that are consistent with measured zeta potentials.

Surface complexation models use experimental adsorption measurements to calculate stability constants that quantify the thermodynamic stability of adsorbed species. However, these constants are often poorly constrained due to nearly complete removal of the solute from solution and/or because the tested adsorbate:adsorbent ratios are not varied sufficiently. Using data sets that quantify the adsorption of U(VI) to multilayered graphene oxide (MLGO), we tested whether three different U(VI):MLGO ratios (3 ppm U; 20–210 mg L–1 MLGO) affect the ability of nonelectrostatic and diffuse layer models to predict U(VI) adsorption behaviors across a range of ionic strength (1–100 mM) and pH (2–9.5) conditions. Model formulations assumed interactions between discrete MLGO surfaces sites and the most abundant aqueous U(VI) complex(es) within a given pH range. We determined that the observed extents of U(VI) binding require adsorption of more than one U(VI) species (UO22+ and uranyl hydroxide(s) and/or carbonate(s)) and...

Actinide nanomaterials have great potential for application in the fabrication of nuclear fuels and spent fuel reprocessing in advanced nuclear energy systems. In this work, we used track-etched nanoporous membranes as hard templates to synthesize uranium-based nanomaterials with new structures by electrodeposition. Through electrochemical behavior investigations and subsequent product characterization, the chemical compositions of the deposition product has been confirmed to be uranyl hydroxide. More importantly, accurate control of the morphologies of the deposition product (i.e., nanowires and nanotubes) could be achieved by carefully adjusting the growth parameters such as deposition time and current density. The preferred morphology of the electrodeposition product was nanowires when a low current density was applied, whereas nanotubes could be formed only when a high current density and a short deposition time were both applied. The formation of nanotubes is attributed to the hydrogen bubbles generated by water electrolysis under the overpotential electroreduction conditions. Additionally, we transformed the main chemical composition of the deposition products from uranyl hydroxide to triuranium octoxide by calcination, and SEM results showed that the morphologies of the nanowires and nanotubes were very well maintained after the calcination. Our work provides a useful protocol for the synthesis of one-dimensional uranium-based nanomaterials.

Solid-state 1H magic-angle spinning (MAS) NMR was used to investigate local proton environments in anhydrous [UO2(OH)2] (α-UOH) and hydrated uranyl hydroxide [(UO2)4O(OH)6·5H2O (metaschoepite). For the metaschoepite material, proton resonances of the μ2-OH hydroxyl and interlayer waters were resolved, with two-dimensional (2D) double-quantum (DQ) 1H–1H NMR correlation experiments revealing strong dipolar interactions between these different proton species. The experimental NMR results were combined with first-principles CASTEP GIPAW (gauge including projector-augmented wave) chemical shift calculations to develop correlations between hydrogen-bond strength and observed 1H NMR chemical shifts. These NMR correlations allowed characterization of local hydrogen-bond environments in uranyl U24 capsules and of changes in hydrogen bonding that occurred during thermal dehydration of metaschoepite.

The kinetics of dissolution of stoichiometric uraninite (UO2) synthesized by electroreduction (electrolysis) of uranyl chloride in a melt of an eutectic mixture of alkali metal salts was studied. It follows from the results obtained that, in the initial step of uraninite dissolution under both dynamic and static conditions, schoepite is not an intermediate phase. It is formed, apparently, in the further steps by oxidation of uranium dioxide. The diffusion coefficients of uranium in solutions over uraninite and schoepite were determined. It was found that, at least in the initial step of the dissolution, the forming uranyl hydroxide occurs in the solution either in the dissociated state or in the form of mononuclear hydroxo complexes.

Paulscherrerite, UO2(OH)2, occurs as an abundant dehydration product of metaschoepite at the Number 2 Workings at Radium Ridge, Northern Flinders Ranges, South Australia. The mineral name honors the contribution of Swiss physicist Paul Scherrer (1890–1969) to mineralogy and nuclear physics. Individual paulscherrerite crystals are tabular, reaching a maximum of 500 nm in length. Paulscherrerite has a canary yellow color and displays no fluorescence under UV light. Chemically, paulscherrerite is a pure uranyl hydroxide/hydrate, containing only traces of other metals (

A recently proposed pathway for the scrambling of axial (uranyl) and equatorial 0 atoms in [UO2(OH)4]2- (1) is refined using Car-Parrinello molecular dynamics (CPMD) simulations in an explicit solvent (water) and with model counterions (NH4+). According to constrained CPMD/BLYP simulations and thermodynamic integration, 1 can be deprotonated to [UO3(OH)3]3- with a T-shaped UO3 group (Delta A = 7.1 kcal/mol), which in turn can undergo a solvent-assisted proton transfer via a cis-[UO2(OH)4]2-center dot OH- complex and a total overall barrier of Delta A(double dagger) = 12.5 kcal/mol. According to computed relative energies of trans- and cis-[UO2(OH)4]2- in the gas phase and in a polarizable continuum, "pure" functionals such as BLYP underestimate this overall barrier somewhat, and estimates of Delta A(double dagger) approximate to 16 and 17 kcal/mol are obtained at the B3LYP and CCSD(T) levels, respectively, in excellent agreement with the experiment. Postprint

The form of solid phase U after Fe(II) induced anaerobic remineralization of ferrihydrite in the presence of aqueous and absorbed U(VI) was investigated under both abiotic batch and biotic flow conditions. Experiments were conducted with synthetic ground waters containing 0.168 mM U(VI), 3.8 mM carbonate, and 3.0 mM Ca2+. In spite of the high solubility of U(VI) under these conditions, appreciable removal of U(VI) from solution was observed in both the abiotic and biotic systems. The majority of the removed U was determined to be substituted as oxidized U (U(VI) or U(V)) into the octahedral position of the goethite and magnetite formed during ferrihydrite remineralization. It is estimated that between 3 and 6% of octahedral Fe(III) centers in the new Fe minerals were occupied by U. This site specific substitution is distinct from the nonspecific U coprecipitation processes in which uranyl compounds, e.g., uranyl hydroxide or carbonate, are entrapped within newly formed Fe oxides. The prevalence of site specific U incorporation under both abiotic and biotic conditions and the fact that the produced solids were shown to be resistant to both extraction (30 mM KHCO3) and oxidation (air for 5 days) suggest the potential importance of sequestration in Fe oxides as a stable and immobile form of U in the environment.

Extracellular polysaccharide (EPS) produced by a Pseudomonas aeruginosa strain BU2 was characterized for its ability to remove uranium from aqueous solution. The EPS was acidic in nature and found as a potent biosorbent for uranium (U), showing pH dependence and fast saturating metal sorption, being maximum (985 mg U g− 1 EPS) at pH 5.0. The polymer showed enhanced uranium sorption capacity and affinity with increasing solution pH, suggesting a preferential sorption of monovalent uranyl hydroxide ions over the nonhydroxylated divalent species. Pseudo-first-order and pseudo-second-order kinetic models were applied to the experimental data, assuming that the external mass transfer limitations in the system can be neglected and biosorption is sorption controlled. Equilibrium metal binding showing conformity to the Freundlich model suggested a multilayer sorption involving specific binding sites with affinity distribution. The presence of two types of metal binding sites corresponding to strong and w...

The surface contamination by uranium in the primary circuit of PWR type nuclear reactors is a fairly complex problem as (i) different chemical forms (molecular, colloidal and/or disperse) of the uranium atoms can be present in the boric acid coolant, and (ii) only limited pieces of information about the extent, kinetics and mechanism of uranium accumulation on constructional materials are available in the literature. A comprehensive program has been initiated in order to gain fundamental information about the uranium accumulation onto the main constituents of the primary cooling circuit (i.e., onto austenitic stainless steel type 08X18H10T (GOSZT 5632-61) and Zr(1%Nb) alloy). In this paper, some experimental findings on the time and pH dependences of U accumulation obtained in a pilot plant model system are presented and discussed. The surface excess, oxidation state and chemical forms of uranium species sorbed on the inner surfaces of the stainless steel tubes of steam generators have been detected by radiotracer (alpha spectrometric), ICP-OES and XPS methods. In addition, the passivity, morphology and chemical composition of the oxide-layers formed on the studied surfaces of steel specimens have been analyzed by voltammetry and SEM-EDX. The experimental data imply that the uranium sorption is significant in the pH range of 4–8 where the intense hydrolysis of uranyl cations in boric acid solution can be observed. Some specific adsorption and deposition of (mainly colloidal and disperse) uranyl hydroxide to be formed in the solution prevail over the accumulation of other U(VI) hydroxo complexes. The maximum surface excess of uranium species measured at pH 6 ( Γ sample = 1.22 μg cm −2 U ≅ 4 × 10 −9 mol cm −2 UO 2 (OH) 2 ) exceeds a monolayer coverage.

We systematically studied the adsorption of uranyl(vi) on two common edge surfaces, (010) and (110), of 2 : 1 smectite clay minerals, using standard periodic DFT models. To describe various types of permanently charged clay minerals, we introduced charged defects into the initially neutral layer of pyrophyllite, cation substitutions in tetrahedral (beidellitic) and octahedral (montmorillonitic) sheets. Comparing uranyl(vi) species at various sites of these two types of surfaces, we found that structural parameters of such adsorption complexes are essentially determined by the surface chemical groups forming the adsorption site, not by the type of the clay mineral. Even for sites involving a substituted cation we noticed only a weak effect of the substitution on the geometric parameters. Geometry optimization resulted in adsorbed uranyl or uranyl hydroxide, with coordination numbers of 4 or 5. However, in most cases the same species was determined on the same type of site, independent of the substitutions. Optimization of adsorbed uranyl leads to hydrolysis at sites close to a AlOH(-1/2) surface group, resulting in uranyl monohydroxide as adsorbate and protonation of the AlOH(-1/2) group. While most species are equatorially five-coordinated, coordination 4 is preferred when uranyl adsorbs on mixed AlO(H)-SiO(H) sites. Calculated formation energies of surface complexes do not single out a preferred species or site, but point to an equilibrium of several species. Comparison to experiment and consideration of pH conditions suggests AlOHOH and AlOH-SiO sites of (010) surfaces and AlOmOH, SiOOm, and AlOH-SiO sites of (110) surfaces as most probable for uranyl adsorption.

Four types of reaction mechanisms for the oxo ligand exchange of monomeric and dimeric neptunyl(VI) hydroxide in aqueous solution were explored computationally using density functional theory (DFT) and ab initio classical molecular dynamics. The obtained results were compared with previous studies on the oxo exchange of uranyl hydroxide, as well as with experiments. It is found that the stable T-shaped [NpO3(OH)(3)](3-) intermediate is a key species for oxo exchange in the proton transfer in mononuclear Path I and binuclear Path IV, similar to the case of uranyl(VI) hydroxide. Path I is thought to be the preferred oxo exchange mechanism for neptunyl(VI) hydroxide in our calculations, due to the lower activation energy (22.7 and 13.1 kcal mol(-1) for Delta H-double dagger and Delta H-double dagger, respectively) of the overall reaction. Path II via a cis-neptunyl structure assisted by a water molecule might be a competitive channel against Path I with a mononuclear mechanism, owing to a rapid dynamical process occurring in Path II. In Path IV with the binuclear mechanism, oxo exchange is accomplished via the interaction between [NpO2(OH)(4)](2-) and T-shaped [NpO3(OH)(3)](3-) with a low activation energy for the rate-determining step, however, the overall energy required to fulfill the reaction is slightly higher than that in mononuclear Path I, suggesting a possible binuclear process in the higher energy region. The chemical bonding evolution along the reaction pathways was discussed by using topological methodologies of the electron localization function (ELF).

The potential for incorporating rare earth elements (REE) into/onto crystalline compounds has been evaluated by precipitating uranyl phases from aqueous solutions containing either cerium or neodymium. These REEs serve both as monitors for evaluating the potential repository behavior of REE radionuclides, and as surrogate elements for actinides (e.g., Ce4 and Nd3 for Pu4 and Am3, respectively). The present experiments examined the behavior of REE in the presence of ianthinite Formula Not Shown, becquerelite (Ca(UO2)6O4(OH)6(H2O)8), and other uranyl hydroxide compounds commonly noted as alteration products during the corrosion of UO2, spent nuclear fuel, and naturally occurring uraninite. The results of these experiments demonstrate that significant quantities of both cerium (Kd=1020) and neodymium (Kd=840) are incorporated within the uranium alteration phases and suggest that ionic substitution and/or adsorption to the uranyl phases can play a key role in the limiting the mobility of REE (and by analogy, actinide elements) in a nuclear waste repository.

Electrospray ionization was used to generate doubly charged complex ions composed of the uranyl ion and nitrile ligands. The complexes, with general formula [UO2(RCN)n]2+, n = 0-5 (where R=CH3-, CH3CH2-, or C6H5-), were isolated in an ion-trap mass spectrometer to probe intrinsic reactions with H2O. For these complexes, two general reaction pathways were observed: (a) the direct addition of one or more H2O ligands to the doubly charged complexes and (b) charge-reduction reactions. For the latter, the reactions produced uranyl hydroxide, [UO2OH], complexes via collisions with gas-phase H2O molecules and the elimination of protonated nitrile ligands.

The surface of the depleted uranium (DU)-0.75 wt.% titanium alloy has been studied using scanning electron microscopy, energy dispersive spectroscopy and optical microscopy. The samples were examined after mechanical polishing and again after nitric acid cleaning. The acicular martensitic microstructure is revealed after chemical etching. Several of the impurities have been identified and their prevalence has been found to change depending on the surface treatments. The impurities have also been found to vary from sample to sample and within the same sample. The electrochemistry and corrosion characteristics of the alloy were studied using open circuit potential measurements and potentiodynamic polarization techniques. This study has been directed towards developing environmentally friendly protective coatings for this alloy. In this paper, we discuss our efforts in finding suitable chemical species to act as inhibitors and activators during coating formation. The effect of various oxyanions, MoO42−, PO43−, VO43−, MnO4−, SiO44− and WO42−, on the electrochemical behavior of the depleted uranium alloy in quiescent nitric acid has been explored including their ability to inhibit corrosion. Results indicate that chemical or electrochemical activation of the DU alloy in 0.1 M HNO3 + 0.025 M MoO42− can lead to the formation of a rudimentary coating. The effect of several fluorine compounds was also examined and their electrochemical response indicates that several of them may have a potential use as a surface activator.

We developed a simple, safe method to remove uranium from contaminated metallic surfaces so that the materials can be recycled or disposed of as low-level radioactive or nonradioactive waste. Surface analysis of rusted uranium-contaminated plain carbon-steel coupons by X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy showed that uranium was predominantly associated with ferrihydrite, lepidocrocite, and magnetite, or occluded in the matrix of the corrosion product as uranyl hydroxide and schoepite (UO 3 .2H 2 O). Citric acid formulations, consisting of oxalic acid-hydrogen peroxidecitric acid (OPC) or citric acid-hydrogen peroxidecitric acid (CPC), were used to remove uranium from the coupons. The efficiency of uranium removal varied from 68% to 94% depending on the extent of corrosion, the association of uranium with the iron oxide matrix, and the accessibility of the occluded contaminant. Decontaminated coupons clearly showed evidence of the extensive removal of rust and uranium. The waste solutions containing uranium and iron from decontamination by OPC and CPC were treated first by subjecting them to biodegradation followed by photodegradation. Biodegradation of a CPC solution by Pseudomonas fluorescens resulted in the degradation of the citric acid with concomitant precipitation of Fe (>96%), whereas U that remained in solution was recovered (>99%) by photodegradation as schoepite. In contrast, in an OPC solution citric acid was biodegraded but not oxalic acid, and both Fe and U remained in solution. Photodegradation of this OPC solution resulted in the precipitation of iron as ferrihydrite and uranium as uranyl hydroxide.

The polyphosphate kinase gene from Pseudomonas aeruginosa was overexpressed in its native host, resulting in the accumulation of 100 times the polyphosphate seen with control strains. Degradation of this polyphosphate was induced by carbon starvation conditions, resulting in phosphate release into the medium. The mechanism of polyphosphate degradation is not clearly understood, but it appears to be associated with glycogen degradation. Upon suspension of the cells in 1 mM uranyl nitrate, nearly all polyphosphate that had accumulated was degraded within 48 h, resulting in the removal of nearly 80% of the uranyl ion and >95% of lesser-concentrated solutions. Electron microscopy, energy-dispersive X-ray spectroscopy, and time-resolved laser-induced fluorescence spectroscopy (TRLFS) suggest that this removal was due to the precipitation of uranyl phosphate at the cell membrane. TRLFS also indicated that uranyl was initially sorbed to the cell as uranyl hydroxide and was then precipitated as uranyl phosphate as phosphate was released from the cell. Lethal doses of radiation did not halt phosphate secretion from polyphosphate-filled cells under carbon starvation conditions.

This study examined the ability of alkaline desert soils to sorb depleted uranium (DU) as a function of soil horizon and assessed the solubility of corrosion and migration products from two DU kinetic energy penetrators exposed on the desert surface for a 22-y period. Both uranium corrosion products on the surface, and subsurface uranium originating from the dissolution of surface corrosion products followed by reprecipitation or sorption, were examined. A four-step sequential extraction method was used to classify uranium solubility at each site. Results show that distribution coefficient for uranium is highly variable, but can be correlated with the clay content (r = 0.55) and soil pH (r = 0.73) of the soil horizon considered. Surface corrosion products and near-surface uranium easily dissolve in a weak acid solution (25% acetic acid for two hours), suggesting a uranyl hydroxide form. As uranium migrates beyond several centimeters in depth, it forms insoluble aggregates with silicate minerals and requir...

Summary We determined the association of uranium with bacteria isolated from the Waste Isolation Pilot Plant (WIPP), Carlsbad, New Mexico, and compared this with known strains of halophilic and non-halophilic bacteria and archaea. Examination of the cultures by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) showed uranium accumulation extracellularly and/or intracellularly to a varying degree. In Pseudomonas fluorescens and Bacillus subtilis uranium was associated with the cell surface and in the latter it was present as irregularly shaped grains. In Halobacterium halobium, the only archeon studied here, uranium was present as dense deposits and with Haloanaerobium praevalens as spikey deposits. Halomonas sp. isolated from the WIPP site accumulated uranium both extracellularly on the cell surface and intracellularly as electron-dense discrete granules. Extended X-ray absorption fine structure (EXAFS) analysis of uranium with the halophilic and non-halophilic bacteria and archaea showed that the uranium present in whole cells was bonded to an average of 2.4±0.7 phosphoryl groups at a distance of 3.65±0.03 Å. Comparison of whole cells of Halomonas sp. with the cell wall fragments of lysed cells showed the presence of a uranium bidentate complex at 2.91±0.03 Å with the carboxylate group on the cell wall, and uranyl hydroxide with U-U interaction at 3.71±0.03 Å due to adsorption or precipitation reactions; no U-P interaction was observed. Addition of uranium to the cell lysate of Halomonas sp. resulted in the precipitation of uranium due to the inorganic phosphate produced by the cells. These results show that the phosphates released from bacteria bind a significant amount of uranium. However, the bacterially immobilized uranium was readily solubilized by bicarbonate with concurrent release of phosphate into solution.

We investigated the association of uranium with clean and corroded surfaces of 1010 carbon steel. Studying steel contaminated by uranium species will have an important effect on the development of methods used to clean radioactively contaminated metal waste. X-ray photoelectron spectroscopy, synchrotron infrared microspectroscopy and laboratory-based Fourier transform infrared analysis of steel surfaces exposed to uranyl nitrate showed the presence of crystallized hydrated uranyl oxides, uranyl hydroxides, iron oxyhydroxides and iron oxides. In general, heavily corroded areas physically shield the uranium species, which tended to associate spatially with hydroxyl groups and lepidocrocite. Lightly corroded areas contained uranium species with stronger axial U–O bonding. Infrared spectroscopy, Rutherford backscattering spectroscopy and energy-dispersive spectroscopy mapping analysis revealed that the uranium species are well distributed within the upper micron of the thick corrosion layer and associated more with areas of high hydroxide content. Parameters such as the concentration of uranyl nitrate solution used to expose the carbon steel coupons, the method of contamination (dipped or sprayed with dilute uranyl nitrate solution) and the degree of corrosion (accelerated corrosion before and/or after contamination) played significant roles in the distribution and nature of the uranyl hydroxide/iron oxyhydroxide corrosion products found on the surface of all coupons. These factors must be considered in the development and optimization of decontamination processes for metal waste. Copyright © 2003 John Wiley & Sons, Ltd.

New bonds for the uranyl: The controlled conversion of an uranyl oxo group ([UO2]+) into covalently bonded UOH and UOSi groups is described for pentavalent uranyl Pacman complexes. The unusual oxo–hydroxy motif is achieved by a protonation reaction and retains the normally unstable UV uranyl oxidation state. This product is readily silylated by treatment with a chlorosilane resulting in UOSi bond formation (see scheme).

In a prior study [Van Stipdonk; et al. J. Phys. Chem. A 2006, 110, 959-970], electrospray ionization (ESI) was used to generate doubly charged complex ions composed of the uranyl ion and acetonitrile (acn) ligands. The complexes, general formula [UO2(acn)n](2+), n = 0-5, were isolated in an 3-D quadrupole ion-trap mass spectrometer to probe intrinsic reactions with H2O. Two general reaction pathways were observed: (a) the direct addition of one or more H2O ligands to the doubly charged complexes and (b) charge-exchange reactions. For the former, the intrinsic tendency to add H2O was dependent on the number and type of nitrile ligand. For the latter, charge exchange involved primarily the formation of uranyl hydroxide, [UO2OH](+), presumably via a collision with gas-phase H2O and the elimination of a protonated nitrile ligand. Examination of general ion fragmentation patterns by collision-induced dissociation, however, was hindered by the pronounced tendency to generate hydrated species. In an update to this story, we have revisited the fragmentation of uranyl-acetonitrile complexes in a linear ion-trap (LIT) mass spectrometer. Lower partial pressures of adventitious H2O in the LIT (compared to the 3-D ion trap used in our previous study) minimized adduct formation and allowed access to lower uranyl coordination numbers than previously possible. We have now been able to investigate the fragmentation behavior of these complex ions completely, with a focus on tendency to undergo ligand elimination versus charge reduction reactions. CID can be used to drive ligand elimination to completion to furnish the bare uranyl dication, UO2(2+). In addition, fragmentation of [UO2(acn)](2+) generated [UO2(NC)](+), which subsequently fragmented to furnish NUO(+). Formation of the nitrido by transfer of N from cyanide was confirmed using precursors labeled with (15)N. The observed formation of [UO2(NC)](+) and NUO(+) was modeled by density functional theory.

Uranium-containing aqueous wastes have been treated by electrosorption on a carbon electrode composed of vapor-grown fibers in a continuous flow-through cell. Effective uranium (VI) removal is accomplished when a negative potential in the range of −0.45 to −0.9 V (vs. Ag/AgCl) is applied to the carbon electrode. For a feed concentration of 100 mg/l, the concentration of U(VI) in the cell effluent is reduced to less than 100 μg/l. The adsorbed uranium is stripped from the carbon fiber by passing a 0.1 M KNO 3 solution through the cell and applying a positive potential on the electrode. Almost all of the stripped uranium is removed as a suspended precipitate and recovered in solid form by filtration. A sorption capacity over 1.20 g uranium /g carbon is reached. The electro-adsorbed uranium is mainly in the form of uranyl hydroxide (UO 3 ·H 2 O), indicating very limited reduction of U(VI) to U(IV) and precipitation of U(IV). It is proposed that ion exchange and double layer charging are the dominant mechanisms for electrosorption of uranium at potentials less negative than −0.3 V, whereas surface-induced precipitation of uranyl hydroxide (UO 3 ·H 2 O) occurs at more negative potentials, thereby greatly enhancing the sorption capacity.

Reaction between β-cyclodextrin (β-CD) and diaqua(benzoate)chlorodioxouranium in aqueous solution leads to the formation of the adduct diaqua(benzoate)hydroxydioxouranium (VI)/β-CD. The compound has been characterized from crystallographic studies of two crystal forms. Monometallic uranyl-hydroxide is included in the CD cavity and is stabilized by hydro-phobic forces and hydrogen bonds. EXAFS studies of diaqua(benzoate)chlorodioxouranium in aqueous solution with α and β-CD suggest that uranyl insertion compounds are present in both solutions.

We investigated the transformations of uranyl nitrate, uranyl citrate, uranyl ethylenediaminetetraacetate (U-EDTA), and uranyl carbonate by a denitrifying halophilic bacterium, Halomonassp. (WIPP1A), isolated from the Waste Isolation Pilot Plant (WIPP) repository. The addition of uranyl nitrate, uranyl citrate, or uranyl EDTA to the brine or bacterial growth medium resulted in the precipitation of uranium. Extended X-ray absorption fine structure (EXAFS) analysis of the precipitates formed in the brine and in the growth medium were identified as uranyl hydroxide [UO 2( OH) 2 ] and uranyl hydroxophosphato species [K(UO 2 ) 5 (PO 4 ) 3( OH) 2 .nH 2 O], respectively. Dissolution of the uranium precipitate was concomitant with the growth of the bacteria under anaerobic conditions. The UV-vis spectra of the culture medium during growth showed that a uranyl dicarbonate complex [UO 2 (CO 3 ) 2 ] 2- was formed due to CO 2 production from the metabolism of the carbon source succinate. The bacterium completely metabolized the citrate released from the uranyl citrate complex but not the EDTA released from the U-EDTA complex. Adding uranyl carbonate to the growth medium caused no changes in the uranium speciation due to bacterial growth. Uranyl carbonate was not biosorbed by the growing culture nor by washed resting cells suspended in 20% NaCl brine (3.4 M) because the complex was either neutral or anionic. Our results demonstrate that bacterial activity can enhance the dissolution of uranium phosphate by forming uranyl carbonate species.

International audience; Infrared-attenuated total reflectance (IR-ATR) and Raman spectroscopies are used to identify the complexed species of uranyl with hydroxide and acetate in aqueous solutions as a function of pH and metal-to-ligand ratio. Three stoichiometries (UO2CH3COO+, UO2(CH3COO)2 and UO2(CH3COO)3−) are observed via irregular shifts of the uranyl stretching signals. The acetate vibrational modes ν(CO2) and ν(CC), allow the identification of two different ligand structures as a function of the complex stoichiometries: UO2CH3COO+ and one ligand of UO2(CH3COO)2 are pseudobridging, the second acetate of UO2(CH3COO)2 being bidentate. There is still uncertainty on the presence of a pseudobridging acetate in UO2(CH3COO)3−.

Two complimentary spectroscopic techniques, X-ray absorption and fluorescence spectroscopy have been conducted at spatial scales of 1 to 25 μm on uranium contaminated soil sediments collected from two former nuclear materials processing facilities of the DOE: Fernald, OH and Savannah River Site, SC. A method of imbedding particles in a non-reactive Si polymer was developed such that individual particles could be examined before and after extraction with a wide range of chemicals typically used in sequential extraction techniques and others proposed forex situ chemical intervention technologies. Using both the micro-X-ray fluorescence (XRF) and micro-X-ray Absorption Near Edge Structure (XANES) techniques, both elemental and oxidation state distribution maps were generated on individual particles before and following chemical extraction. XANES can determine the relative proportion of U(VI) and U(IV) in phases comprising individual particles before and after extraction and showed that greater than 85% of the uranium existed as hexavalent U(VI). Fluorescence spectra of contaminated particles containing mainly U(VI) revealed populations of uranyl hydroxide phases and demonstrated the relative efficacy and specificity of each extraction method. Correlation of XAS and fluorescence data at micron scales provides information of U oxidation state as well as chemical form in heterogeneous samples.

Leaching experiments on a simulated borosilicate waste glass were performed using a static method in the presence of PbO and bentonite. Ions in leachates were analyzed by ICP, while leached samples were characterized by several surface analytical methods such as XPS, SEM, EPMA, XRD and topography. As the simulated waste glass was leached in the presence of PbO and bentonite, lead hydroxide and insoluble elements (Ti, Nd, Ru and Zr ) were detected on the surface, respectively. On the other hand, a large amount of uranyl hydroxide was found on the surface of it when the borosilicate glass with 25 % U 3 O 8 was leached in a soxhlet apparatus.

The synthesis of the mono-uranyl complex [UO2(THF)(H2LMe)] of a ditopic Schiff-base pyrrole macrocycle is described and is shown to adopt a Pacman wedge-shaped structure in which the uranyl dication is desymmetrised and sits solely in one N4-donor compartment to leave the other vacant. While investigating the mechanism of the previously reported, base-initiated, reductive silylation chemistry of [UO2(THF)(H2LMe)], we found that uranyl hydroxide complexes could be isolated. As such, the reaction between [UO2(THF)(H2LMe)] and KH in THF generated the dimeric cation-cation hydroxide [{UO2(OH)K(C6H6)(H2LMe)}2] when crystallised from C6H6, or alternatively, when crystallised from THF, the monomeric THF-adducted cation-cation complex [UO2(OH)K(THF)2(H2LMe)] was isolated. These compounds result formally from the substitution of the equatorial THF molecule by hydroxide, and it was also shown that the reaction between dry KOH and [UO2(THF)(H2LMe)] generated [{UO2(OH)K(C6H6)(H2LMe)}2].

The inclusion of NH4+ as counterions in Car-Parrinello molecular dynamics (CPMD) simulations of anionic uranyl(VI) complexes is proposed as a viable approach to modeling "real" aqueous solutions. For [UO2F4(H2O)](2-) in water, it is shown that the inclusion of two NH4+ ions strengthens the bond between uranyl and the water ligand by ca. 2 kcal/mol, improving the accordance with experiment. According to CPMD simulations for [UO2X5]-[NH4](3) (X = F, OH) in water, the fifth fluoride is bound much stronger than the fifth OH-. Implications for a recently proposed model for oxygen exchange in uranyl hydroxide are discussed. Postprint

We present the complex formation of the uranyl ion ( UO 2 2 + ) in the aqueous system with phosphocholine, O -phosphoethanolamine and O -phosphoserine. These phosphonates ( R–O–PO 3 2 - ) represent the hydrophilic head groups of phospholipids. The complexation was investigated by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pH = 2–6. An increase of the fluorescence intensity, connected with a strong red-shift of about 8 nm compared to the free uranyl ion, indicates a complex formation between UO 2 2 + and the phosphonates already at pH = 2. Even at pH = 6 these complexes prevail over the uranyl hydroxide and carbonate species, which are generated naturally at this pH. At pH = 4 and higher a 1:2 complex between uranyl and O -phosphoserine was found. Complexes with a metal-to-ligand ratio of 1:1 were observed for all other ligands. Fluorescence lifetimes, emission maxima and complex stability constants at T = 22 ± 1 °C are reported. The TRLFS spectra of uranyl complexes with two phosphatidic acids (1,2-dimyristoyl-sn-glycero-3-phosphate and 1,2-dipalmitoyl-sn-glycero-3-phosphate), which represent the apolaric site of phospholipids, show in each case two different species.

Uranium concentrates obtained from leach solutions, known as uranium yellow cakes, represent an intermediate step in the processing of uranium ores. Yellow cake concentrates are prepared by various metallurgical methods, depending on the types of ores. Samples of yellow cakes prepared under various methods were analyzed; examined in detail by means of x-ray diffraction, infrared spectra, and wet chemical methods; and classified by mineralogic methods. The cakes were classified as uranyl hydroxide hydrate, basic uranyl sulfate hydrate, sodium para-uranate, and uranyl peroxide hydrate. The experimental preparation methods and characterization methodology used are described, and the significance of structural types to the physical and chemical properties of yellow cake production, as well as the pyrolytic transformations at high temperatures, are discussed.

Forty years of uranium mining in the German state of Saxony have left a legacy of uranium-contaminated pits, waste piles, mine tailings and surrounding soils. Since 1963, and more extensively since 1989, contaminated sites were covered in order to protect people and environment. Little is known on the further fate of uranium at these buried sites. Therefore, we investigated two mine tailing samples from hydrochloric-acid ore-extraction, which were buried for 30 years under several meters of mine and communal waste. The two samples were collected at depths of 5 m (sample 1) and 12 m (sample 2) below the surface. Due to the neutralizing influence of the waste cover, the upper sample 1 had a pH of 8, while the lower sample 2 had a pH of 4. Both samples were retrieved from oxic redox conditions. Chemical extractions showed that U is predominantly water soluble and/or ion exchangeable in sample 1, while U is predominantly bound in weakly soluble solid phases in sample 2. To further identify the uranium species, we applied a combination of Synchrotron-based methods, namely Extended X-ray Absorption Fine-Structure (EXAFS) spectroscopy of bulk samples, micro X-Ray Diffraction (µ-XRD) and micro X-Ray Fluorescence (µ-XRF) spectroscopy. In sample 1, uranium predominates homogeneously distributed at concentrations in the mg/kg range in aggregates with a diameter of tens to hundreds of µm. The aggregates consist of layer silicates (muscovite, kaolinite, illite), jarosite and gypsum. Chemical extractability, EXAFS, µ-XRD, and µ-XRF strongly suggest that U(VI) is adsorbed on edge sites of the layer silicates. In sample 2, U is heterogeneously distributed among single crystals and small aggregates with very high U concentrations (g/kg) and variable elemental composition. Besides the matrix minerals muscovite, kaolinite, illite and quartz, we identified pitchblende and coffinite, and found evidence for other uranyl hydroxide and vanadate solids. In addition, a smaller amount of uranium seems to be adsorbed to mineral surfaces as in sample 1. The results suggest that a substantial amount of uranium remained in the buried tailings as relatively mobile, adsorbed U(VI) species. No clear evidence for secondary uranium mineral precipitates was found.

Electrospray ionization was used to generate mono-positive gas-phase complexes of the general formula [UO2A(S)n]+ where A = OH, Cl, Br, I or ClO4, S = H2O, CH3OH or CH3CH2OH, and n = 1-3. The multiple-stage dissociation pathways of the complexes were then studied using ion-trap mass spectrometry. For H2O-coordinated cations, the dissociation reactions observed included the elimination of H2O ligands and the loss of HA (where A = Cl, Br or I). Only for the Br and ClO4 versions did collision-induced dissociation (CID) of the hydrated species generate the bare, uranyl-anion complexes. CID of the chloride and iodide versions led instead to the production of uranyl hydroxide and hydrated UO2+. Replacement of H2O ligands by alcohol increased the tendency to eliminate HA, consistent with the higher intrinsic acidity of the alcohols compared to water and potentially stronger UO2-O interactions within the alkoxide complexes compared to the hydroxide version.

The intrinsic hydration of three monopositive uranyl-anion complexes (UO(2)A)(+) (where A = acetate, nitrate, or hydroxide) was investigated using ion-trap mass spectrometry (IT-MS). The relative rates for the formation of the monohydrates [(UO(2)A)(H(2)O)](+), with respect to the anion, followed the trend: Acetateor = nitratehydroxide. This finding was rationalized in terms of the donation of electron density by the strongly basic OH(-) to the uranyl metal center, thereby reducing the Lewis acidity of U and its propensity to react with incoming nucleophiles, viz., H(2)O. An alternative explanation is that the more complex acetate and nitrate anions provide increased degrees of freedom that could accommodate excess energy from the hydration reaction. The monohydrates also reacted with water, forming dihydrates and then trihydrates. The rates for formation of the nitrate and acetate dihydrates [(UO(2)A)(H(2)O)(2)](+) were very similar to the rates for formation of the monohydrates; the presence of the first H(2)O ligand had no influence on the addition of the second. In contrast, formation of the [(UO(2)OH)(H(2)O)(2)](+) was nearly three times faster than the formation of the monohydrate.