Comparative extraction efficiencies of tri-<Emphasis Type="Italic">n</Emphasis>-butyl phosphate and <Emphasis Type="Italic">N,N</Emphasis>-dihexyloctanamide for uranium recovery using supercritical CO<Subscript>2</Subscript>

Extraction of uranium from tissue paper, synthetic soil, and from its oxides (UO₂, UO₃ and U₃O₈) was carried out using supercritical carbon dioxide modified with methanol solutions of extractants such as tri-n-butyl phosphate (TBP) or N,N-dihexyl octanamide (DHOA). The effects of temperature, pressure, extractant/nitric acid (nitrate) concentration, and of hydrogen peroxide on uranium extraction were investigated. The dissolution and extraction of uranium in supercritical CO₂ modified with TBP, from oxide samples followed the order: UO₃ ≫ UO₂ > U₃O₈. Addition of hydrogen peroxide in the modifier solution enhanced the dissolution/extraction of uranium in dynamic mode. DHOA appeared better than TBP for recovery of uranium from different oxide samples. Similar enhancement in uranium extraction was observed in static mode experiments in the presence of hydrogen peroxide. Uranium estimation in the extracted fraction was carried out by spectrophotometry employing 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) as the chromophore.     

Synthesis, Characterization and Thermal Decomposition of Hydroxylammonium Uranyl Acetate

The synthesis of hydroxylammonium uranyl acetate is described. The identity of the synthesized compound was confirmed by chemical and infrared analysis. The intermediates and final products of the thermal decomposition were identified by means of thermogravimetric analysis, differential thermal analysis and X-ray diffraction. The thermal decomposition of hydroxylammonium uranyl acetate involves several steps. Two of them are due to decomposition of this compound to UO₂ via UO₂(CH₃COO)₂, and the third to the partial oxidation of UO₂ to UO₃ and the formation of U₂O₈ in the solid state at higher temperature.This revised version was published online in November 2005 with corrections to the Cover Date.     

The gravimetric determination of uranium in uranyl nitrate

A study of the factors which affect the gravimetric determination of uranium in uranyl nitrate is described. In the gravimetry of uranium, the U₃O₈ (weighing form) produced by ignition is usually assumed to deviate <0.02% from theoretical composition; and elemental impurities are assumed to form common oxides within the U₃0₈ matrix. It is shown that these assumptions are incorrect. Ignition temperature and time affect U₃O₈ stoichiometry. Ignitions of uranyl nitrate for 1–3 h at 850° produce U₃O₈ that deviates as much as 0.15% from stoichiometric U₃O₈; deviations are negligible when uranyl nitrate is ignited at 1000° for 2 h. Elemental impurities, particularly calcium and phosphorus, affect the composition of U₃O₈ formed in the ignition of uranyl nitrate. A variety of impurity complexes such as uranates and phosphates are found within the U₃O₈ matrix. Formation of these impurity complexes depends on the elements present, their concentration, and ignition temperature. Therefore, in the gravimetric determination of uranium in uranyl nitrate, the effects of ignition parameters and nonvolatile impurities must be considered in order to obtain accurate uranium determinations.     

Template‐Free Synthesis and Mechanistic Study of Porous Three‐Dimensional Hierarchical Uranium‐Containing and Uranium Oxide Microspheres

A novel type of uranium‐containing microspheres with an urchin‐like hierarchical nano/microstructure has been successfully synthesized by a facile template‐free hydrothermal method with uranyl nitrate hexahydrate, urea, and glycerol as the uranium source, precipitating agent, and shape‐controlling agent, respectively. The as‐synthesized microspheres were usually a few micrometers in size and porous inside, and their shells were composed of nanoscale rod‐shaped crystals. The growth mechanism of the hydrothermal reaction was studied, revealing that temperature, ratios of reactants, solution pH, and reaction time were all critical for the growth. The mechanism study also revealed that an intermediate compound of 3 UO₃?NH₃?5 H₂O was first formed and then gradually converted into the final hydrothermal product. These uranium‐containing microspheres were excellent precursors to synthesize porous uranium oxide microspheres. With a suitable calcination temperature, very uniform microspheres of uranium oxides (UO_2+x, U₃O₈, and UO₃) were successfully synthesized.     

Implication of volume changes in uranium oxides: A density functional study

In severe nuclear accident scenarios (in air environments and high temperatures) UO₂ fuel pellets oxidise to produce uranium oxides with higher oxygen content, e.g., U₄O₉ or U₃O₈. As a first step in investigating the microstructural changes following UO₂ oxidation to hexagonal high temperature phase of U₃O₈, density functional quantum mechanical calculations of the structure, elastic properties and electronic structure of U₃O₈ have been performed. The calculated properties of hexagonal phase of U₃O₈ are compared to those of the orthorhombic pseudo-hexagonal phase which is stable at room temperature. The total energy technique based on the local density approximation plus Hubbard U as implemented in the CASTEP code is used to investigate changes in the lattice constants. The first-principles calculations predict a 35–42% increase in volume per uranium atom as a result of the transformation from UO₂ to U₃O₈, in agreement with experimental data. The implications of this prediction on the linear expansion and fragmentation of fuel are discussed.     

The Energy Landscape of Uranyl–Peroxide Species

Nanoscale uranyl peroxide clusters containing UO₂²⁺ groups bonded through peroxide bridges to form polynuclear molecular species (polyoxometalates) exist both in solution and in the solid state. There is an extensive family of clusters containing 28 uranium atoms (U₂₈ clusters), with an encapsulated anion in the center, for example, [UO₂(O₂)_3?x(OH)_x^4?], [Nb(O₂)₄^3?], or [Ta(O₂)₄^3?]. The negative charge of these clusters is balanced by alkali ions, both encapsulated, and located exterior to the cluster. The present study reports measurement of enthalpy of formation for two such U₂₈ compounds, one of which is uranyl centered and the other is peroxotantalate centered. The [(Ta(O₂)₄]‐centered U₂₈ capsule is energetically more stable than the [(UO₂)(O₂)₃]‐centered capsule. These data, along with our prior studies on other uranyl–peroxide solids, are used to explore the energy landscape and define thermochemical trends in alkali–uranyl–peroxide systems. It was suggested that the energetic role of charge‐balancing alkali ions and their electrostatic interactions with the negatively charged uranyl–peroxide species is the dominant factor in defining energetic stability. These experimental data were supported by DFT calculations, which agree that the [(Ta(O₂)₄]‐centered U₂₈ capsule is more stable than the uranyl‐centered capsule. Moreover, the relative stability is controlled by the interactions of the encapsulated alkalis with the encapsulated anion. Thus, the role of alkali‐anion interactions was shown to be important at all length scales of uranyl–peroxide species: in both comparing clusters to clusters; and clusters to monomers or extended solids.     

Ex‐Situ Kinetic Investigations of the Formation of the Poly‐Oxo Cluster U38

The ex‐situ qualitative study of the kinetic formation of the poly‐oxo cluster U₃₈, has been investigated after the solvothermal reaction. The resulting products have been characterized by means of powder XRD and scanning electron microscopy (SEM) for the solid phase and UV/Vis, X‐ray absorption near edge structure (XANES), extended X‐ray absorption fine structure (EXAFS), and NMR spectroscopies for the supernatant liquid phase. The analysis of the different synthesis batches, stopped at different reaction times, revealed the formation of spherical crystallites of UO₂ from t=3 h, after the formation of unknown solid phases at an early stage. The crystallization of U₃₈ occurred from t=4 h at the expense of UO₂, and is completed after t=8 h. Starting from pure uranium(IV) species in solution (t=0–1 h), oxidation reactions are observed with a U^IV/U^VI ratio of 70:30 for t=1–3 h. Then, the ratio is inversed with a U^IV/U^VI ratio of 25/75, when the precipitation of UO₂ occurs. Thorough SEM observations of the U₃₈ crystallites showed that the UO₂ aggregates are embedded within. This may indicate that UO₂ acts as reservoir of uranium(IV), for the formation of U₃₈, stabilized by benzoate and THF ligands. During the early stages of the U₃₈ crystallization, a transient crystallized phase appeared at t=4 h. Its crystal structure revealed a new dodecanuclear moiety (U₁₂), based on the inner hexanuclear core of {U₆O₈} type, decorated by three additional pairs of dinuclear U₂ units. The U₁₂ motif is stabilized by benzoate, oxalates, and glycolate ligands.     

An overview of studies on the highly dispersed uranium oxide species occluded within mesoporous MCM-41 and MCM-48 host materials

In this article we have consolidated our recent studies on anchoring of uranyl groups and encapsulation of highly dispersed nano-particles of -U₃O₈ in mesoporous MCM samples. The size of uranium oxide crystallites and the binding of uranyl groups at framework sites of host matrix depended on the preparation method, viz. wet impregnation, exchange of template cations, and the hydrothermal route. These uranium species contributed individually to the catalytic oxidation of organic molecules, such as methanol, toluene and benzyl alcohol; the uranyl groups playing a more important role at lower reaction temperatures. Also, the size and the lattice oxygen of uranium oxide crystallites played a vital role, not only in the lowering of reaction onset temperature but also in deciding the nature and the reactivity of the transient surface species formed during the oxidation of above mentioned organics. For instance, the results of in situ IR spectroscopy experiments have shown that while larger-size U₃O₈ crystallites help in the growth of certain oxymethylene (–OCH₂) and polymerized oxymethylene (–OCH₂)_n species, adsorption of methanol on smaller size particles helped in the additional formation of formate-type complexes. Thus, a relationship was found between the size of uranium oxide crystallites, the nature of the transient species formed and the catalytic conversion of methanol to form CO₂, CO and methane. In addition, the uranyl ions anchored within the pore system of host matrix are found to serve as efficient heterogeneous photocatalysts for the sunlight-assisted deep oxidation of organic molecules in the vapor phase and at room temperature. The reaction mechanisms, accounting for the catalytic properties of occluded UO_x species without and in the presence of radiation, are discussed in the light of the above mentioned results.     

Determination of oxygen to uranium ratio in irradiated uranium dioxide by controlled potential coulometry

This work reports the determination of oxygen to uranium (O/U) ratio in irradiated UO_2+x fuel pellet of burnup of ca. 34 GWd/t by controlled potential coulometry. The method is based on the dissolution of the nuclear fuel in strong phosphoric acid (SPA) at 180–190 °C under an inert atmosphere. After dissolution, 8% sulphuric acid is added in order to obtain a 20% SPA in 8% sulphuric acid. A controlled potential coulometric determination of uranium(VI) is carried out at ?0.60V vs. ferri-ferrocyanide. The uranium(IV) contained in an aliquot of the fuel solution is oxidised to uranium(VI) with cerium(IV) sulphate, and the total uranium content is then determined by coulometry. Optimum experimental conditions have been established using simulated irradiated fuel solution containing various fission products which include cerium, tellurium, palladium, ruthenium, molybdenum and zirconium. Interference of the fission products and the possible removal of their interferences by preelectrolysis at +0.5 V vs. saturated calomel electrode (SCE) have been investigated. The accuracy of the coulometric method is confimed by polarographic measurement using several unirradiated UO_2+x fuel of known stoichiometry.     

Development of a pH titration method for the simultaneous determination of uranium,nitrate and free-acid in the feed solution of the sol-gel process of nuclear fuel fabrication

pH titration curves generated by slow addition of alkali to solutions containing varying concentrations of uranyl nitrate and nitric acid were studied using an autotitrator linked to a personal computer. A procedure with multiple choice of equations has been developed for the estimation of free acid, nitrate and uranium in pure uranyl nitrate solution by a single titration. The technique provides a simple single-step method with required accuracy and precision for the simultaneous estimation of the three quantities in the uranyl nitrate feed solution of the sol-gel process for making UO₃ microspheres. The relative standard deviations in the determination of uranium and nitrate were ±0.82% and ±1.52%, respectively, in 15 determinations.     

Photoelectron Spectroscopy (XPS) and Cyclic Voltammetry (CV) Investigation of UxTh1-xO2 Thin Films

Thin film samples of UO₂ and U_0.55Th_0.45O₂ have been prepared by sputter co-deposition under argon atmosphere in presence of oxygen (reactive sputtering) onto gold foil. Films were characterized by X-Ray Photoelectron Spectroscopy (XPS). Cyclic Voltammetry (CV) studies have been made by using the thin films as electrodes in 0.01 M NaCl pH neutral non-purged electrolyte. The effect of thorium in the UO₂ lattice is observed by comparing the U_0.55Th_0.45O₂ electrode to the UO₂ electrode. The results indicate that uranium develops enhanced resistance to the oxidation when thorium is added to the lattice. After the CV measurements, the films were again characterized by XPS. The surface is enriched in thorium by 11%. Uranium is in a higher oxidation state; however, uranium is less oxidized in U_0.55Th_0.45O₂ than in UO₂.     

Oxidation state and coordination environment of uranium in sodium iron aluminophosphate glasses

An analysis of the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) of uranium determined the oxidation state and coordination environment of uranium atoms in glasses containing 40 mol % Na₂O, 10 mol % Al₂O₃, 10 mol % Fe₂O₃, and 40 mol % P₂O₅ to which uranium oxides were added to a concentration of 50 wt % (above 100%). If the added amount of UO₂ was small, uranium occurred as U(IV) in a near-octahedral oxygen environment with an average U–O distance in the first coordination sphere of 2.25 Å. At higher concentrations of uranium oxides introduced both as UO₂ and as UO₃, uranium occurred as U(V) and U(VI); the first coordination sphere is split; shorter (~1.7–1.8 Å) and longer (2.2–2.3 Å) distances were observed, which corresponded to the axial and equatorial U–O bonds in uranyl ions, respectively; and the redox equilibrium shifted toward U(VI). The glass with the maximal (~33 wt %) UO₃ concentration contained mainly U(VI). The existence of low-valence uranium species can be related to the presence of Fe(II) in glasses. The second coordination sphere of uranium manifests itself only at high concentrations of uranium oxides.     

The Key Role of U28 in the Aqueous Self‐Assembly of Uranyl Peroxide Nanocages

For 11 years now, the structural diversity and aesthetic beauty of uranyl–peroxide capsules have fascinated researchers from the diverse fields of mineralogy, polyoxometalate chemistry, and nuclear fuel technologies. There is still much to be learned about the mechanisms of the self‐assembly process, and the role of solution parameters including pH, alkali template, temperature, time, and others. Here we have exploited the high solubility of the UO₂²⁺/H₂O₂/LiOH aqueous system to address the effect of the hydroxide concentration. Important techniques of this study are single‐crystal X‐ray diffraction, small‐angle X‐ray scattering, and Raman spectroscopy. Three key phases dominate the solution speciation as a function of time and the LiOH/UO₂²⁺ ratio: the uranyl–triperoxide monomer [UO₂(O₂)₃]^4?and the two capsules [(UO₂)(O₂)(OH)]₂₄^24?(U₂₄) and [(UO₂)(O₂)_1.5]₂₈^28?(U₂₈). When the LiOH/U ratio is around three, U₂₈ forms rapidly and this cluster can be isolated in high yield and purity. This result was most surprising and challenges the hypothesis that alkali templating is the most important determinant in the cluster geometry. Moreover, analogous experiments with KOH, NH₄OH, and TEAOH (TEA=tetraethylammonium) also rapidly yield U₂₈, which suggests that U₂₈ is the kinetically favored species. Complete mapping of the pH–time phase space reveals only a narrow window of the U₂₈ dominance, which is why it was previously overlooked as an important kinetic species in this chemical system, as well as others with different counterions.     

ChemInform Abstract: Density Functional Theory Calculations of UO2 Oxidation: Evolution of UO2‐x,U4O9‐y,U3O7, and U3O8.

DFT calculations of UO₂ oxidation indicate stable compounds U₄O_8.889, U₃O₇, and U₃O_7.333, which are based on ordering of split quad‐interstitial clusters.     

Binuclear Uranium(VI) Complexes with a “Pacman” Expanded Porphyrin: Computational Evidence for Highly Unusual Bis‐Actinyl Structures

On the basis of uranyl complexes reacting with a polypyrrolic ligand (H₄L), we explored structures and reaction energies of a series of new binuclear uranium(VI) complexes using relativistic density functional theory. Full geometry optimizations on [(UO₂)₂(L)], in which two uranyl groups were initially placed into the pacman ligand cavity, led to two minimum‐energy structures. These complexes with cation–cation interactions (CCI) exhibit unusual coordination modes of uranyls: one is a T‐shaped ( T ) skeleton formed by two linear uranyls {O_exo?U₂?O_endo→U₁(?O_exo)₂}, and another is a butterfly‐like ( B ) unit with one linear uranyl coordinating side‐by‐side to a second cis‐uranyl. The CCI in T was confirmed by the calculated longest distance and lowest stretching vibrational frequency of U₂?O_endo among the four U?O bonds. Isomer B is more stable than T , for which experimental tetrameric analogues are known. The formation of B and T complexes from the mononuclear [(UO₂)(H₂L)(thf)] ( M ) was found to be endothermic. The further protonation and dehydration of B and T are thermodynamically favorable. As a possible product, we have found a trianglelike binuclear uranium(VI) complex having a O?U?O?U?O unit.     

Mechanochemical effects in U3O8

The effect of the mechanical treatment of U₃O₈ in a planetary ball mill in air or as a suspension in benzene solution of thyolilthreefluoroacetone (TTA) on the nature of the oxide and on the leaching of U and ²³⁴Th into diluted aqueous solutions of HCl, Na₂EDTA and NaCl has been studied. Transformation of U₃O₈ to UO₂, is much stronger expressed when the mechanoactivation is performed in air is established. The leaching behavior of U and Th depends significantly on the activation mode and on the leaching reagent nature. The role of mechanochemically enhanced UO₂-ThO₂ solid solution formation for the observed effects is discussed.     

Mechanochemical effects in U3O8

The effect of the mechanical treatment of U₃O₈ in a planetary ball mill in air or as a suspension in benzene solution of thyolilthreefluoroacetone (TTA) on the nature of the oxide and on the leaching of U and ²³⁴Th into diluted aqueous solutions of HCl, Na₂EDTA and NaCl has been studied. Transformation of U₃O₈ to UO₂, is much stronger expressed when the mechanoactivation is performed in air is established. The leaching behavior of U and Th depends significantly on the activation mode and on the leaching reagent nature. The role of mechanochemically enhanced UO₂-ThO₂ solid solution formation for the observed effects is discussed.     

Synthesis,X-ray crystallography,spectroscopy, electrochemistry,thermal and kinetic study of uranyl Schiff base complexes

Uranyl(VI) complexes [UO₂(L)(solvent)], where L denotes an asymmetric N₂O₂ Schiff base (salpyr, 3-MeOsalpyr, 4-MeOsalpyr, 5-MeOsalpyr, 5-Clsalpyr or 5-Brsalpyr; salpyr is N,N′-bis(salicyliden)-2,3-diaminopyridine), were synthesized and characterized in solution (UV–vis, ¹H NMR, cyclic voltammetry) and in the solid-state (X-ray crystallography, IR, TGA, C H N.). X-ray crystallography of UO₂(3-MeOsalpyr) revealed coordination of the uranyl by the tetradentate Schiff base and one disordered solvent, resulting in seven-coordinate uranium. Another disordered solvent was not coordinated. Cyclic voltammetry of [U^VIO₂(L)(solvent)] in acetonitrile was used to investigate the effect of the substituents of the Schiff base ligands on oxidation and reduction potential. The quasi-reversible redox reaction without any successive reactions was accelerated by groups with lesser electron withdrawing. We also investigated the kinetics and mechanism of the exchange reaction of the coordinated solvent with tributylphosphine using spectrophotometric method. The second-order rate constants at four temperatures and activation parameters showed an associative mechanism for all corresponding complexes with the following trend: UO₂(5-Clsalpyr)?>?UO₂(5-Brsalpyr)?>?UO₂(3-MeOsalpyr)?>?UO₂(4-MeOsalpyr)?>?UO₂(salpyr)?>?UO₂(5-MeOsalpyr). It was concluded that the steric and electronic properties of the complexes were important for the reaction rate.     

Specific absorption parameters for uranium octoxide and dioxide applicable to the ICRP Publication 66 respiratory tract model

Literature data from in vivo chest measurements and urinary excretion rates of individuals exposed to U₃O₈ and UO₂ were used to compare the results predicted by different models with empirical observations in humans. As a result the use of the respiratory tract model proposed in ICRP Publication 66 with its default absorption parameters underestimates urinary excretion of inhaled U₃O₈ and UO₂. The new respiratory tract model also overpredicts the Fecal/Urine activity ratio, independently of the systemic model. For U₃O₈ and UO₂ the choice of systemic model has very little influence on the predicted urinary excretion of inhaled compounds. On the other way, the choice of the respiratory tract model does influence the predicted urinary excretion significantly. In this work specific absorption parameters for U₃O₈ and UO₂ were derived to be used in the respiratory tract model proposed in ICRP Publication 66. The predicted biokinetics of these compounds were compared with those derived for Type M and Type S compounds of uranium.     

Hexavalent uranium dicarboxylates with hydrazine. Preparation,characterization and thermal studies

The uranium complexes of composition,UO₂X⋅N₂H₄⋅H₂O, X=succinate or glutarate, UO₂X₂⋅N₂H₄⋅H₂O, X=Hadipate, Hpimelate, Hsuberate, Hazelate and Hsebacate and UO₂X⋅N₂H₄, where X=malate and oxydiacetate have been prepared and characterized by analytical, spectral (IR and electronic), thermal and X-ray powder diffraction studies. Hydrazine acts as a monodentate ligand in uranyl succinate, glutarate, malate and oxydiacetate hydrazinates and bidentate in uranyl adipate, pimelate, suberate, azelate and sebacate hydrazinate hydrate complexes. The dicarboxylate anions bind the uranium through uni- and bidentate fashion depending upon the coordination polyhedra. All the dicarboxylate hydrazinate complexes in this series decompose to give U₃O₈ as the end product through their respective uranyl dicarboxylate intermediates. Malate and oxydiacetate compounds decompose exothermically in a single step. The coordinated water is confirmed from thermal data. The complexes of succinate to sebacate seem to possess hexagonal bipyramidal geometry around uranium, whereas pentagonal bipyramidal geometry has been proposed for both malate and oxydiacetate complexes. This revised version was published online in August 2006 with corrections to the Cover Date.