Catalyst-Free Extraction of U(VI) in Solution by Tribocatalysis

Extraction of U(VI) in water is of great significance in energy and environmental fields. However, the traditional methods usually fail due to the indispensable extra addition of catalyst, adsorbent, precipitant, or sacrificial agents, which may lead to enhanced extraction costs and secondary pollution. Here, a new efficient uranium extraction strategy is proposed based on triboelectricity without adding a catalyst or other additives. It is found only under the friction between the microbubbles (generated under ultrasonication) and the water flow, that reactive oxygen species (ROS) can largely be generated, which thus contributes to the solidification of U(VI) from water. In addition, the magnetic field can affect the phase of the product. Under mechanical stirring, the product contains (UO2)O2·2H2O, while which contains UO2(OH)2 and (UO2)O2·4H2O under the magnetic stirring. Quenching experiments are also carried out to explore the influence of environmental factors. Most importantly, it shows great potential in the extraction of U(VI) from seawater. This work proposes a catalyst-free and light-free strategy toward the solidification of U(VI) from water, which avoids the secondary pollution of the catalyst to the environment and is low-cost, and has great potential in the real application.

Polymorphism and phase transitions in Na2U2O7 from density functional perturbation theory

Polymorphism and phase transitions in sodium diuranate, Na₂U₂O₇, are investigated with density functional perturbation theory (DFPT). Thermal properties of crystalline α-, β- and γ-Na₂U₂O₇ polymorphs are predicted from DFPT phonon calculations, i.e., the first time for the high-temperature γ-Na₂U₂O₇ phase (R3̄m symmetry). The standard molar isochoric heat capacities predicted within the quasi-harmonic approximation are for P2₁/a α-Na₂U₂O₇ and C2/m β-Na₂U₂O₇, respectively. Gibbs free energy calculations reveal that α-Na₂U₂O₇ (P2₁/a) and β-Na₂U₂O₇ (C2/m) are almost energetically degenerate at low temperature, with β-Na₂U₂O₇ becoming slightly more stable than α-Na₂U₂O₇ as temperature increases. These findings are consistent with XRD data showing a mixture of α and β phases after cooling of γ-Na₂U₂O₇ to room temperature and the observation of a sluggish α → β phase transition above ca. 600 K. A recently observed α-Na₂U₂O₇ structure with P2₁ symmetry is also shown to be metastable at low temperature. Based on Gibbs free energy, no direct β → γ solid-solid phase transition is predicted at high temperature, although some experiments reported the existence of such phase transition around 1348 K. This, along with recent experiments, suggests the occurrence of a multi-step process consisting of initial β-phase decomposition, followed by recrystallization into γ-phase as temperature increases.

Incorporation of iodine into uranium oxyhydroxide phases

Herein, we have synthesised a novel uranium oxyhydroxide (UOH) phase, Rb₂K₂[(UO₂)₆O₄(OH)₆]·(IO₃)₂, under hydrothermal conditions which intercalates IO₃^-via a hybrid salt-inclusion and host-guest mechanism. The mechanism is based on favorable intermolecular bonding between disordered Rb⁺/K⁺ and IO₃^- ions and hydroxyl and layer void positions respectively. To examine whether the intercalation may occur ubiquitously for UOH phases, the known UOH mineral phases metaschoepite ([(UO₂)₈O₂(OH)₁₂]·12H₂O), compreignacite (K₂[(UO₂)₆O₄(OH)₆]·7H₂O) and also related β-UO₂(OH)₂ were synthesised and exposed to aqueous I^- and IO₃^- for 1 month statically at RT and 60 °C in air and the solid analysed using laser ablation inductively coupled plasma mass spectroscopy. Measurements indicate intercalation can occur homogeneously, but the affinity is dependent upon the structure of the UOH phases and temperature, where higher temperatures and when the interlayer space is free of initial moieties are favoured. It was also found that after repeated washing of the UOH samples with DI water the intercalated iodine was retained. UOH phases are known to form during the oxidative corrosion of spent nuclear fuel during an accident scenario in the near field, this work suggests they may help retard the transport of radiolytic iodine into the environment during a long-term release event.

Phase Analysis of Australian Uranium Ore Concentrates Determined by Variable Temperature Synchrotron Powder X-ray Diffraction

The chemical speciation of uranium oxides is sensitive to the provenance of the samples and their storage conditions. Here, we use diffraction methods to characterize the phases found in three aged (>10 years) uranium ore concentrates of different origins as well as in situ analysis of the thermally induced structural transitions of these materials. The structures of the crystalline phases found in the three samples have been refined, using high-resolution synchrotron X-ray diffraction data. Rietveld analysis of the samples from the Olympic Dam and Ranger uranium mines has revealed the presence of crystalline α-UO₂(OH)₂, together with metaschoepite (UO₂)₄O(OH)₆·5H₂O, in the aged U₃O₈ samples, and it is speculated that this forms as a consequence of the corrosion of U₃O₈ in the presence of metaschoepite. The third sample, from the Beverley uranium mine, contains the peroxide [UO₂(η²-O₂)(H₂O)₂] (metastudtite) together with α-UO₂(OH)₂ and metaschoepite. A core-shell model is proposed to account for the broadening of the diffraction peaks of the U₃O₈ evident in the samples.

Characterization of Uranyl Coordinated by Equatorial Oxygen: Oxo in UO3 versus Oxyl in UO3. J Phys Chem A 2021;125:5544-5555. [PMID: 34138571 DOI: 10.1021/acs.jpca.1c03818]

Uranium trioxide, UO₃, has a T-shaped structure with bent uranyl, UO₂²⁺, coordinated by an equatorial oxo, O^2-. The structure of cation UO₃⁺ is similar but with an equatorial oxyl, O^•-. 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 O^2-; 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 ν₃ IR frequencies indicate the following donor ordering: O^2-[best donor] ≫ O^•-> OH^-> NO₃^-.

Impact of Controlled Storage Conditions on the Hydrolysis and Surface Morphology of Amorphous-UO3

The hydration and morphological effects of amorphous (A)-UO₃ following storage under varying temperature and relative humidity have been investigated. This study provides valuable insight into U-oxide speciation following aging, the U-oxide quantitative morphological data set, and, overall, the characterization of nuclear material provenance. A-UO₃ was synthesized via the washed uranyl peroxide synthetic route and aged based on a 3-factor circumscribed central composite design of experiment. Target aging times include 2.57, 7.00, 14.0, 21.0, and 25.4 days, temperatures of 5.51, 15.0, 30.0, 45.0, and 54.5 °C, and relative humidities of 14.2, 30.0, 55.0, 80.0, and 95.8% were examined. Following aging, crystallographic changes were quantified via powder X-ray diffraction and an internal standard Rietveld refinement method was used to confirm the hydration of A-UO₃ to crystalline schoepite phases. The particle morphology from scanning electron microscopy images was quantified using both the Morphological Analysis of MAterials software and machine learning. Results from the machine learning were processed via agglomerative hierarchical clustering analysis to distinguish trends in morphological attributes from the aging study. Significantly hydrated samples were found to have a much larger, plate-like morphology in comparison to the unaged controls. Predictive modeling via a response surface methodology determined that while aging time, temperature, and relative humidity all have a quantifiable effect on A-UO₃ crystallographic and morphological changes, relative humidity has the most significant impact.

Thermodynamic properties of the uranyl carbonate minerals roubaultite, fontanite, widenmannite, grimselite, čejkaite and bayleyite

The thermodynamic properties of six important uranyl carbonate minerals, roubaultite, fontanite, widenmannite, grimselite, čejkaite and bayleyite, are determined as a function of temperature using first principles methods.

The crystal structures and mechanical properties of the uranyl carbonate minerals roubaultite, fontanite, sharpite, widenmannite, grimselite and čejkaite

The structure, hydrogen bonding, X-ray diffraction pattern and mechanical properties of six important uranyl carbonate minerals, roubaultite, fontanite, sharpite, widenmannite, grimselite and čejkaite, are determined using first principles methods.

Structure-thermodynamics relationship of schoepite from first-principles

The relationship between the structure and thermodynamic properties of schoepite, an important uranyl phase with formula [(UO2)8O2(OH)12]·12H2O formed upon corrosion of UO2, has been investigated within the framework of density functional perturbation theory (DFPT). Experimental crystallographic lattice parameters are well reproduced in this study using standard DFT. Phonon calculations within the quasi-harmonic approximation predict standard molar entropy and isobaric heat capacity of S0 = 179.60 J mol-1 K-1 and C0P = 157.4 J mol-1 K-1 at 298.15 K, i.e., ∼6% and ∼4% larger than existing DFPT-D2 calculations. The computed variation of the standard molar isobaric heat capacity with water content from schoepite (UO3·xH2O, x = 2.25) to dehydrated schoepite (x = 1) is predicted to be essentially linear along isotherms ranging from 100 to 500 K. These findings have important implications for the dehydration of layered uranyl corrosion phases and hygroscopic materials.

Negative linear compressibility in uranyl squarate monohydrate

The mechanical properties of the uranyl squarate monohydrate material, [Formula: see text], were studied using theoretical solid-state methods based in density functional theory employing plane waves and pseudopotentials. Very demanding calculation parameters were utilized in order to obtain a realistic description of the mechanical behavior of this material. Since the determination of the positions of the hydrogen atoms in the unit cell of uranyl squarate monohydrate was not possible from x-ray diffraction data by structure refinement, they were fully optimized theoretically. The computed lattice parameters, bond distances, angles, and x-ray powder diffraction patterns of this material were in very good agreement with the experimental data. This material was found to be mechanically and dynamically stable since the corresponding stability conditions were satisfied. The values of the bulk modulus and its pressure derivatives, shear and Young moduli, Poisson ratio, ductility, hardness, and mechanical anisotropy indices of this material were reported. Furthermore, this study showed that this material exhibits the important negative Poisson ratio (NPR) and negative linear compressibility (NLC) phenomena. Uranyl squarate monohydrate is a very anisotropic brittle material characterized by a bulk modulus of ~33 GPa, which shows a minimum value of the NPR of the order of  -0.5. Besides, this material displays NLC values for a limited range of positive pressures, from 0.025 GPa to 0.094 GPa, applied along the direction of minimum negative Poisson ratio. The analysis of the crystal structure as a function of pressure demonstrates that the mechanism of NLC of this material is associated to the change in shape of the uranyl pentagonal bipyramids and unrelated to the wine-rack structural mechanism commonly used to rationalize this phenomenon.

The layered uranyl silicate mineral uranophane-β: crystal structure, mechanical properties, Raman spectrum and comparison with the α-polymorph

The crystal structure, elastic properties and Raman spectrum of the calcium uranyl silicate pentahydrate mineral uranophane-β, are studied using first-principles solid-state methods and compared with the corresponding information for the α polymorph.

Structural, mechanical, spectroscopic and thermodynamic characterization of the copper-uranyl tetrahydroxide mineral vandenbrandeite

The full crystal structure of the copper-uranyl tetrahydroxide mineral (vandenbrandeite), including the positions of the hydrogen atoms, is established by the first time from X-ray diffraction data taken from a natural crystal sample from the Musonoi Mine, Katanga Province, Democratic Republic of Congo. The structure is verified using first-principles solid-state methods. From the optimized structure, the mechanical and dynamical stability of vandenbrandeite is studied and a rich set of mechanical properties are determined. The Raman spectrum is recorded from the natural sample and determined theoretically. Since both spectra have a high-degree of consistence, all spectral bands are rigorously assigned using a theoretical normal-coordinate analysis. Two bands in the Raman spectra, located at 2327 and 1604 cm⁻¹, are recognized as overtones and a band at 1554 cm⁻¹ is identified as a combination band. The fundamental thermodynamic functions of vandenbrandeite are computed as a function of temperature using phonon calculations. These properties, unknown so far, are key-parameters for the performance-assessment of geological repositories for storage of radioactive nuclear waste and for understanding the paragenetic sequence of minerals arising from the corrosion of uranium deposits. The thermodynamic functions are used here to determine the thermodynamic properties of formation of vandenbrandeite in terms of the elements and the Gibbs free-energies and reaction constants for a series of reactions involving vandenbrandeite and a representative subset of the most important secondary phases of spent nuclear fuel. Finally, from the thermodynamic data of these reactions, the relative stability of vandenbrandeite with respect to these phases as a function of temperature and in the presence of hydrogen peroxide is evaluated. Vandenbrandeite is shown to be highly stable under the simultaneous presence of water and hydrogen peroxide.

The experimental full crystal structure of vandenbrandeite is stablished for the first time and verified using first-principles methods. A detailed mechanical, spectroscopic and thermodynamic characterization is obtained from the optimized structure.

First-Principles Structural, Mechanical, and Thermodynamic Calculations of the Negative Thermal Expansion Compound Zr2(WO4)(PO4)2

The negative thermal expansion (NTE) material Zr₂(WO₄)(PO₄)₂ has been investigated for the first time within the framework of the density functional perturbation theory (DFPT). The structural, mechanical, and thermodynamic properties of this material have been predicted using the Perdew, Burke and Ernzerhof for solid (PBEsol) exchange-correlation functional, which showed superior accuracy over standard functionals in previous computational studies of the NTE material α-ZrW₂O₈. The bulk modulus calculated for Zr₂(WO₄)(PO₄)₂ using the Vinet equation of state at room temperature is K ₀ = 63.6 GPa, which is in close agreement with the experimental estimate of 61.3(8) at T = 296 K. The computed mean linear coefficient of thermal expansion is -3.1 × 10^-6 K^-1 in the temperature range ∼0-70 K, in line with the X-ray diffraction measurements. The mean Grüneisen parameter controlling the thermal expansion of Zr₂(WO₄)(PO₄)₂ is negative below 205 K, with a minimum of -2.1 at 10 K. The calculated standard molar heat capacity and entropy are C _P ⁰ = 287.6 and S ⁰ = 321.9 J·mol^-1·K^-1, respectively. The results reported in this study demonstrate the accuracy of DFPT/PBEsol for assessing or predicting the relationship between structural and thermomechanical properties of NTE materials.

Becquerelite mineral phase: crystal structure and thermodynamic and mechanical stability by using periodic DFT

The structure, thermodynamic and mechanical properties of becquerelite mineral, Ca(UO₂)₆O₄(OH)₆·8H₂O, were studied by means of theoretical solid-state calculations based on density functional theory using plane waves and pseudopotentials. The positions of the hydrogen atoms in the unit cell of becquerelite mineral were optimized theoretically since it was not possible to determine them from X-ray diffraction data by structure refinement. The structural results, including the lattice parameters, bond lengths and X-ray powder pattern, were found to be in excellent agreement with their experimental counterparts. The fundamental thermodynamic properties of becquerelite mineral, including specific heat, entropy, enthalpy and Gibbs free energy, were then computed by performing phonon calculations at the computed optimized structure. Since the experimental values of these properties are unknown, their values were predicted. The values obtained for the isobaric specific heat and entropy of becquerelite at the temperature of 298.15 K were 148.4 and 172.3 J K⁻¹ mol⁻¹, respectively. The computed thermodynamic properties were combined with those of the corresponding elements in order to obtain the enthalpy and Gibbs free energy of formation as a function of temperature. The availability of these thermodynamic properties of formation allowed to determine the enthalpies and free energies and associated reaction constants of a series of reactions involving becquerelite and other uranyl containing materials. Futhermore, knowledge of these properties permitted the study of the thermodynamic stability of becquerelite with respect to a rich set of secondary phases of spent nuclear fuel, including dehydrated schoepite, schoepite, metaschoepite, studtite, metastudtite, rutherfordine and soddyite under different conditions of temperature. Becquerelite is shown to be highly stable in the presence of hydrogen peroxide. It is the second most stable phase under intermediate hydrogen peroxide concentrations (after schoepite), and the fourth most stable phase under high hydrogen peroxide concentrations (after studtite, schoepite and metaschoepite). Finally, the equation of state and elastic properties of this mineral, unknown to date, were determined. The crystal structure of becquerelite was found to be stable mechanically and dynamically. Becquerelite can be described as a brittle material exhibiting large anisotropy and large compressibility in the direction perpendicular to the sheets characterizing the structure of this layered uranyl containing material. The dependence of the elastic properties of becquerelite with respect to the strain orientation is shown to be analogous to that of schoepite mineral. The calculated bulk modulus is also very similar to that of schoepite, B ∼ 31 GPa.

The full crystal structure of becquerelite mineral phase was successfully determined using theoretical solid-state methods for the first time. Additionally, a complete study of its thermodynamic and mechanical properties and stability is reported.

Model representations of kerogen structures: An insight from density functional theory calculations and spectroscopic measurements

Molecular structures of kerogen control hydrocarbon production in unconventional reservoirs. Significant progress has been made in developing model representations of various kerogen structures. These models have been widely used for the prediction of gas adsorption and migration in shale matrix. However, using density functional perturbation theory (DFPT) calculations and vibrational spectroscopic measurements, we here show that a large gap may still remain between the existing model representations and actual kerogen structures, therefore calling for new model development. Using DFPT, we calculated Fourier transform infrared (FTIR) spectra for six most widely used kerogen structure models. The computed spectra were then systematically compared to the FTIR absorption spectra collected for kerogen samples isolated from Mancos, Woodford and Marcellus formations representing a wide range of kerogen origin and maturation conditions. Limited agreement between the model predictions and the measurements highlights that the existing kerogen models may still miss some key features in structural representation. A combination of DFPT calculations with spectroscopic measurements may provide a useful diagnostic tool for assessing the adequacy of a proposed structural model as well as for future model development. This approach may eventually help develop comprehensive infrared (IR)-fingerprints for tracing kerogen evolution.

Energetics of Sn2+ isomorphic substitution into hydroxylapatite: first-principles predictions

The energetics of Sn²⁺ substitution into the Ca²⁺ sublattice of hydroxylapatite, Ca₁₀(PO₄)₆(OH)₂, has been investigated within the framework of DFT.

Spectroscopic Raman characterization of rutherfordine: a combined DFT and experimental study

Study of a natural rutherfordine mineral by means of Raman spectroscopy and DFT calculations.

Relationship between crystal structure and thermo-mechanical properties of kaolinite clay: beyond standard density functional theory

The structural, mechanical and thermodynamic properties of 1 : 1 layered dioctahedral kaolinite clay, with ideal Al2Si2O5(OH)4 stoichiometry, were investigated using density functional theory corrected for dispersion interactions (DFT-D2). The bulk moduli of 56.2 and 56.0 GPa predicted at 298 K using the Vinet and Birch-Murnaghan equations of state, respectively, are in good agreement with the recent experimental value of 59.7 GPa reported for well-crystallized samples. The isobaric heat capacity computed for uniaxial deformation of kaolinite along the stacking direction reproduces calorimetric data within 0.7-3.0% from room temperature up to its thermal stability limit.

Thermodynamics of technetium: reconciling theory and experiment using density functional perturbation analysis

The structure, lattice dynamics and thermodynamic properties of bulk technetium were investigated within the framework of density functional theory. The predicted thermal expansion and isobaric heat capacity are in excellent agreement with available experimental data.

On the mechanical stability of uranyl peroxide hydrates: implications for nuclear fuel degradation

The mechanical properties and stability of studtite, (UO₂)(O₂)(H₂O)₂·2H₂O, and metastudtite, (UO₂)(O₂)(H₂O)₂, were investigated using density functional perturbation theory.