UO2 Ions Research Articles

First Structured Uranium‐Based Monoliths Produced via Vat Photopolymerization for Nuclear Applications

AbstractUranium plays an unquestionable role in the framework of nuclear physics, biology, and radiopharmacy. Moreover, uranyl ion UO22+ offers an immense variety of applications due to the unique photosensitivity of its complexes. The excited state of uranyl cation is indeed accessible under ultraviolet‐visible (UV–vis) light, readily producing radical species UO22+* upon light irradiation. Herein, an innovative synthesis protocol is presented to explore the use of uranyl cations as photocatalyst systems for photocurable sol–gel‐based formulations, coupling the photochemical reactions of uranyl cations with photopolymerization‐based additive manufacturing processes. Additive manufacturing has nowadays revolutionized the production of complex structures with arbitrary geometries and has opened up enticing opportunities for innovative technological breakthroughs and highly tailorable systems. The fabrication of micro‐architected components is shown via vat photopolymerization, namely, the Digital Light Processing technique, and ‐3D) printed parts are converted into uranium dicarbide (UC2)/carbon nanocomposite upon carbothermal reduction. This uranyl‐mediated additive manufacturing process constitutes the first application of the synergistic role of uranyl motifs in a photopolymer platform, demonstrating for the first time the possibility to directly pattern uranium‐based materials in complex structures.

Grain subdivision and structural modifications by high-energy heavy ions in UO2 with different initial grain size

Irradiation with 84 MeV Xe ions at peak fluence above 7 × 1017 ions/cm2 has been carried out on two UO2 samples with different initial grain sizes at 300 °C and 500 °C. Characterization of both grain structure and bubble morphology has been performed using transmission electron microscopy, image analysis and precession electron diffraction. The bubble size in the large-grained sample was smaller than that in the standard-grain sample, confirming fission gas precipitation resistance in the former. The ion irradiation in the high electronic energy deposition range has caused formation of a large density of dislocations which resulted in grain polygonization for the sample with standard grain size, which was observed in multiple lamellas extracted next to each other at peak dose. The sample with large grains showed inconsistent behavior. In one case, dislocation pile-up occurred, but no sub-grain formation was observed. An adjacent location showed sub-grain formation, even if to less extent compared to the standard grain-size sample. These findings suggest a limited polygonization resistance in large-grained UO2 compared to standard-grain UO2. Recrystallization of both sample surfaces was present, leaving unclear if this is a surface effect or a consequence of the highest energy deposition rate in this area.

A study of non-iterative triples contributions in relativistic equation-of-motion coupled-cluster calculations using an exact two-component Hamiltonian with atomic mean-field spin-orbit integrals: Application to uranyl and other heavy-element compounds.

The implementation of an equation-of-motion coupled-cluster singles and doubles augmented with a noniterative triples correction [EOM-CCSD(T)(a)*] method [D. A. Matthews and J. F. Stanton, J. Chem. Phys. 145, 124102 (2016)] with an exact two-component Hamiltonian using atomic mean-field spin-orbit integrals (X2CAMF) is reported. Benchmark calculations show that both the correction from triple excitations in the cluster operator to the similarity-transformed Hamiltonian and the contribution from triple excitations in excited-state eigenvalue equations make important contributions to computed excitation energies of molecules containing heavy elements. X2CAMF-EOM-CCSD(T)(a)* excitation energies and excited-state properties for low-lying excited states of As2, Sb2, and Bi2 are in significantly better agreement with corresponding experimental values than EOM-CCSD ones. X2CAMF-EOM-CCSD(T)(a)* calculations have also been demonstrated to provide accurate results for excitation energies of low-lying excited states of bare uranyl ion UO2 2+, hereby resolving a long-standing debate on this topic.

Insight into the effect of A-site cations on structural and optical properties of RE2Hf2O7:U nanoparticles

Abstract A2B2O7 type pyrochlores have been recently proposed as a potential nuclear waste host due to their many interesting properties. To assess and understand the performance of these compounds as nuclear waste hosts, the speciation and structural investigations on actinide-doped RE2Hf2O7 are needed since both are imperative from their application perspective. In this work, we investigated the effect of uranium doping at different concentrations in the range of 0–10% on the structural and optical properties of RE2Hf2O7:U (RE = Y, Gd, Nd, and Lu) nanoparticles (NPs). The Y2Hf2O7 NPs exist in slightly disordered pyrochlore structure and the extent of disordering increases as a function of uranium doping while the structure reaches a cotunnite phase at 10.0% doping level. The Nd2Hf2O7 NPs also exist in a distorted pyrochlore structure and their distortion increases with increasing uranium doping inducing a phase transition into a disordered fluorite structure at 10.0% uranium doping. Both Gd2Hf2O7 and Lu2Hf2O7 NPs exist in a disordered fluorite structure and transforms into cotunnite structure at higher U concentrations (≥5.0%). Photoluminescence spectroscopy showed that uranium ions are stabilized in +6 oxidation state in all samples: in the form of uranate ion UO66− in the Y2Hf2O7, Nd2Hf2O7 and Lu2Hf2O7 NPs while in the form of uranyl ion UO22+ in the Gd2Hf2O7 NPs. Therefore, this work deepens the understanding of the behavior of uranium ions doped in different RE2Hf2O7 host matrices in the perspective of their application as nuclear waste hosts.

Selective removal of uranium from wastewater using sludge collected from refinery wastewater treatment: Equilibrium, thermodynamic and kinetics studies

Adsorption of uranium as uranyl ion UO22+ was investigated on the surface of pretreated refinery wastewater sludge. The sludge was chemically activated using different acids (HCl, HNO3, H2SO4, H3PO4), and its uranium uptake capacity from aqueous solution was checked after each activation process. The highest uptake capacity was obtained for sludge treated by phosphoric acid followed by washing with Na3PO4 (0.2M). The effect of pH, point of zero charge pH pzc, uranyl ion concentration and adsorbent dose on adsorption processes were studied. The adsorption is high in the pH range of 3.5–7.2 with optimum value at pH 5.3. The point of zero charge was found to be at pH 3.95. The adsorption capacity increases with increasing uranyl ion concentration but decreases with increasing sludge dose. The equilibrium studies showed that the adsorption fits Langmuir adsorption isotherm. Also, the kinetic studies showed that the adsorption is pseudo-second order with activation energy 31.15kJ/mol, indicating that the process is chemical in nature. The activated sludge showed high selectivity for UO22+ among Zr(IV), Fe(III), Cr(III), Gd(II), Sr(II), Co(II), Ca(II), Cs(I) and Na(I) ions.

Mechanism of chemiluminescent reaction of U4+ with oxygen in weakly acidic aqueous solutions

The mechanism of the chemiluminescent reaction of U(IV) oxidation with atmospheric oxygen is substantiated using published data. The suggested mechanism includes the initiating step of successive oxidation of U(IV) and U(V) with the formation of the excited ion [UO22+·aq]*, which forms with an unexcited ion UO22+·aq an excimer [UO22+·aq⋯UO22+·aq]*. The decay of the excimer yields H2O2 and two U(V) ions, causing the development of the chain process of the U(IV) oxidation.

Calculation of diffusion coefficients of defects and ions in UO2

This paper has presented molecular dynamics calculations of the diffusion coefficients of interstitials, vacancies, and vacancy complexes of oxygen and uranium in UO2, as well as the coefficients of ion diffusion provided by these defects. The interatomic potentials have been chosen by comparing the defect formation energies with data of the DFT + U calculations. The results of the calculations have been compared with experimental data on the annealing of defects and the measurements of self-diffusion coefficients of ions. The limitations of the model of point defects for the description of the self-diffusion in nominally stoichiometric UO2 have been discussed.

Synthesis and study Co, Mn, Cu, Ni and UO 2-ion coordination complexes with aliphatic dinitrones

A series of new 3-d metal and UO 2-ion complexes based on glyoxal ( L 1 ) and glutaric (L 2) dinitrones were synthesized. Compounds of the general formula MLCl 2· xSolv (M = Co 2+, Mn 2+, Cu 2+, Ni 2+) were prepared by the reaction of the above mentioned ligands with the corresponding chloride salts in isopropanol or in an isopropanol–water solution. Each complex was studied by spectroscopic methods. The molecular structure of L 1 , CoL 1Cl 2·2H 2O, and UO 2 L 2 1 ( NO 3 ) 2 have been characterized by X-ray diffraction. In cobalt case the coordination polyhedron of the cobalt atom is formed by two oxygen atoms from two ligands, two oxygen atoms from water, and two chloride ions. The geometry of the coordination sphere is a significantly distorted octahedron. L 1 coordinates in a bridging, bidentate manner, forming a 1D coordination polymer. While in the uranyl-complex, one nitrone group of L 1 is coordinated to the uranyl atom; the other remains free.

Influence of temperature on the lifetime of electronically excited uranyl ion: IV. Effect of isotope enrichment in low-temperature crystallization of partially deuterated aqueous solutions of sulfuric acid

The isotope effect manifested in phase transitions in deuterium-substituted aqueous solutions of sulfuric acid was found. The phase transitions occurring with variation of temperature of 0.5 M H2SO4 solution in H2O, 0.5 M D2SO4 solution in D2O, and their mixtures were studied by differential thermal analysis. Additional data on the phase transitions in these samples were obtained using chemiluminescence [low-temperature reaction of U(IV) with XeF2] and τ-metry (measurement of the lifetime τ of uranyl ion UO22+ in the electronically excited state). The uranyl ion was used as a luminescence probe. The whole set of data obtained can be interpreted on the basis of the concept that a mixture of 0.5 M H2SO4 and 0.5 M D2SO4 solutions (H/D = 1), subjected to fast (10–15 K s−1) cooling to 77 K followed by warming, contains a liquid phase in which the deuterium content in the temperature range 202–210 K is higher than that in the initial solution.

Determination of alkali, alkaline earth and transition metal ions in UO2, ThO2 powders and sintered (Th,U)O2 pellets by ion chromatography

An ion chromatographic method has been developed for the determination of traces of Li+, Na+, K+, Ca2+, Mg2+, Sr2+, Fe3+, Cu2+, Ni2+, Co2+, Zn2+, Cd2+, Mn2+ in UO2, ThO2 powders and sintered (Th,U)O2 pellets. This new method utilizes poly-(butadiene-maleic acid) (PBDMA) coated silica cation exchange column and mixed functionality column of anion and cation exchange to achieve the separation of alkali, alkaline earths and transition metal ions, respectively. It involves matrix separation after sample dissolution by solvent extraction with TBP (tri butyl phosphate)-TOPO (tri octyl phosphine oxide)/CCl4. Interference of transition metal ions in the determination of alkali, alkaline earth metal ions are removed by using pyridine 2,6-dicarboxylic acid (PDCA) in the tartaric acid mobile phase. Mobile phase composition is optimized for the base line separation of alkali, alkaline earth and transition metal ions. Linear calibration graphs in the range 0.01–20 μg mL−1 were obtained with regression coefficients better than 0.999. The respective relative standard deviations were also determined. Recoveries of the spiked samples are within ±10% of the expected value. The developed method is authenticated by comparison with certified standards of UO2 and ThO2 powders.

Probing the oxygen environment in UO22+ by solid-state O17 nuclear magnetic resonance spectroscopy and relativistic density functional calculations

A combined theoretical and solid-state (17)O nuclear magnetic resonance (NMR) study of the electronic structure of the uranyl ion UO(2)(2+) in (NH(4))(4)UO(2)(CO(3))(3) and rutherfordine (UO(2)CO(3)) is presented, the former representing a system with a hydrogen-bonding environment around the uranyl oxygens and the latter exemplifying a uranyl environment without hydrogens. Relativistic density functional calculations reveal unique features of the U-O covalent bond, including the finding of (17)O chemical shift anisotropies that are among the largest for oxygen ever reported (>1200 ppm). Computational results for the oxygen electric field gradient tensor are found to be consistently larger in magnitude than experimental solid-state (17)O NMR measurements in a 7.05 T magnetic field indicate. A modified version of the Solomon theory of the two-spin echo amplitude for a spin-5/2 nucleus is developed and applied to the analysis of the (17)O echo signal of U (17)O(2)(2+).

Raman spectroscopy of uranopilite of different origins—implications for molecular structure

AbstractUranopilite, [(UO2)6(SO4)O2(OH)6(H2O)6](H2O)8, the composition of which may vary, can be understood as a complex hydrated uranyl oxyhydroxy sulfate. The structure of uranopilite from different locations has been studied by Raman spectroscopy at 298 and 77 K. A single intense band at 1009 cm−1 assigned to the ν1 (SO4)2− symmetric stretching mode shifts to higher wavenumbers at 77 K. Three low‐intensity bands are observed at 1143, 1117 and 1097 cm−1. These bands are attributed to the (SO4)2− ν3 anti‐symmetric stretching modes. Multiple bands provide evidence that the symmetry of the sulfate anion in the uranopilite structure is lowered. Three bands are observed in the region 843 to 816 cm−1 in both the 298 and 77 K spectra and are attributed to the ν1 symmetric stretching modes of the (UO2)2+ units. Multiple bands prove the symmetry reduction of the UO2 ion. Multiple OH stretching modes prove a complex arrangement of OH groupings and hydrogen bonding in the crystal structure. A series of infrared bands not observed in the Raman spectra are found at 1559, 1540, 1526 and 1511 cm−1 attributed to δ UOH bending modes. U‐O bond lengths in uranyl and OH/dotbondO bond lengths are calculated and compared with those from X‐ray single crystal structure analysis. The Raman spectra of uranopilites of different origins show subtle differences, proving that the spectra are origin‐ and sample‐dependent. Hydrogen‐bonding network and its arrangement in the crystal structure play an important role in the origin and stability of uranopilite. Copyright © 2006 John Wiley & Sons, Ltd.

Heterogeneous Photocatalytic Degradation of Methanol over Uranyl-Anchored Nanoporous MCM-41 and MCM-48

The vapor-phase photodegradation of methanol to carbon dioxide was carried out over uranyl-anchored nanoporous MCM-41 and MCM-48 hosts (designated as UO2(2+)/MCM-41 and UO2(2+)/MCM-48, respectively) under simulated light and ambient conditions. Preliminary results indicate that the photoactivity of the latter is considerably decreased as compared to the former due to the presence of a smaller fraction of photoactive uranyl (UO2(2+)) ions in UO2(2+)/MCM-48.

Selective binding of uranyl cation by a novel calmodulin peptide

A 33-amino acid peptide corresponding to the helix-loop-helix motif of the calcium binding site I of the protein calmodulin from Paramecium Tetraurelia has been synthesized its binding properties with heavy metal ions have been studied. Herein, we demonstrate that two mutations of two aspartic acid residues in the peptide sequence gave access to a new peptide, which was selective for the uranyl ion UO2 2+. This new peptide can be useful for the development of selective uranyl biosensors to monitor the presence of uranium in contaminated environments.

Behavior of radium, thorium and uranium in groundwater near the Buena Lagoon in the Coastal Zone of the State of Rio de Janeiro, Brazil

Groundwater found near the head of the Buena Lagoon in the State of Rio de Janeiro, Brazil, has high salinity and low pH values. There is a strong correlation between concentrations of radium and light rare earth elements (LREEs) that suggests the leaching of monazite as a common source. Radium is present predominantly as 228Ra. The factors responsible for high radium mobility in groundwater seem to be high levels of salinity resulting in the competition for adsorption sites, and low pH values resulting in the limited adsorption of Ra2+ on the positively-charged surface of adsorbents. The behavior of uranium and thorium is also influenced by their speciation and low pH conditions. Uranium is present as a positively charged uranyl ion UO22+ in low pH samples and is very mobile. Similarly, the presence of positively charged Th4+ and thorium complexes with sulfate result in relatively high thorium concentrations at a low pH range. On the other hand, the mobility of phosphate released by dissolving monazite is probably reduced due to its adsorption and precipitation close to its source.

Modeling Complexes of the Uranyl Ion UO2L2n+: Binding Energies, Geometries, and Bonding Analysis

A systematic study has been carried out with density functional theory of UO2L2n+ (n = 0 or 2) complexes. Thirty three ligands have been considered, enabling a large database of binding energies and geometries to be established that can improve our knowledge and understanding of the bonding between the uranyl ion UO22+ and potential ligands or solvent. A statistical study has been performed using the database with the twin aims of predicting the coordination energies of new complexes and revealing the dominant bonding parameters. We have shown that it is possible to predict the coordination energy satisfactorily just from the properties of the isolated ligand. However, the statistical analysis is shown to be physically unsound. To develop our understanding of the nature of the uranyl−ligand bond, the coordination energy has been decomposed into several contributions: electrostatic, repulsion, ligand polarization, uranyl polarization, charge transfer from the ligand to the uranyl ion, and inverse charge transfer. It is clear that the electrostatic term cannot be modeled just by the ligand's dipole moment; a multipole development extending at least to the quadrupole moment is necessary. Our quantitative analysis also shows that the polarization and charge-transfer terms are important and must be included in any force field if numerically and physically reliable results are sought.

Application of Relativistic Quantum Chemistry to the Electronic Energy Levels of the Uranyl Ion

The ground and excited electronic states of the uranyl ion UO22+ are computed using relativistic core and spin−orbit potentials and multireference graphical unitary group approach configuration interaction as implemented in the COLUMBUS suite of programs. Excitation energies, symmetric stretch vibrational frequencies, and angular momentum coupling properties are compared to the extent possible with spectroscopic data from Cs2UO2Cl4 and CsUO2(NO3)3.

Stable Analogs of the Uranyl Ion Containing the −NUN− Group

A series of compounds containing the −NUN− group has been prepared. Their physical and chemical properties are reported. They adopt a trans configuration, and are thermally robust. The chemistry of some recently-prepared OUN− compounds is also described. These are the first examples of stable analogs of the uranyl ion UO22+. The factors that determine their stability are discussed.

EXAFS Determinations of Uranium Structures: The Uranyl Ion Complexed with Tartaric, Citric, and Malic Acids

U LIII-edge EXAFS spectra were measured for 1:1 mixtures of the uranyl ion with tartaric, malic, and citric acids. The room-temperature EXAFS spectra of these mixtures demonstrate the characteristic uranium−oxygen interactions of the uranyl ion UO22+ and also show a U−U interaction at 3.92 A, indicating the presence of a uranyl dimer. On the basis of models proposed by Martell et al. from earlier potentiometric titration experiments, it is concluded that the dimer is the ligand-bridged species (UO2)2(L)2, in which bridging occurs through the α-hydroxyl groups of the polycarboxylate ligands.

Luminescence Linewidth Broadening and Nonradiative Energy Transfer Studies of Solid UO2+2-Datura

The emission spectra and fluorescence decay curves of solid UO2+2- Datura at liquid nitrogen temperature have been measured. The linewidth of the emission peaks of UO2+2 ions in UO2+2- Datura decreases with the UO2+2 concentration. This linewidth broadening phenomenon can be explained by the existence of resonance interactions between adjacent UO2+2- Datura species. The analysis of the emission peak position of the bound ions has been used to provide a measure of the electronic factors contributing to the interaction between the uranyl ion and phosphoryl and dicarboxyl moieties on the cell wall material. An observed blue shift of the uranyl fluorescence spectrum as a function of solution pH has been ascribed to a distortion of the normally linear O-U-O bond. An inter- and intra-molecular nonradiative energy transfer model has been successfully used to interpret the measured lifetime data of UO2+2- Datura.