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All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Calorimetric Study of Functionalized Uranyl Peroxide Nanoclusters and Their Monomeric Building Block

The description of the energy landscape of polyoxometalates (POMs) enriches our understanding of their solution behavior through quantitative information that can be used to predict their existence, stability, and properties. Herein, we report thermodynamic values that describe the energy landscape of four uranyl peroxide nanoclusters (UPCs) (LiRb–U@U 24 , LiNa–U 24 Pp 12 , NaK–U 24 Pp 12 , and U 60 Ox 30 ) and a building block of UPCs, lithium uranyl triperoxide monomer (Li–UT). Furthermore, the results reveal relationships between the affinity of counter cations and the anionic uranyl peroxide units, enthalpy of dissolution, as well as enthalpy of crystallization and cluster solubility. Additionally, the calorimetric measurements of Li–UT allowed us to calculate a favorable enthalpy of formation of Li–U 24 and Li–U 28 from their monomeric units.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Oxidation of Dimethylsulfoxide in the Presence of Uranyl Salts

Here, we report here the oxidation of dimethyl sulfoxide in the presence of uranyl salts at temperatures typically considered ‘safe’ for its handling. Uranyl salts, even in the absence of light, appear to accelerate the well-known oxidative decomposition of DMSO in air. Sulfate, a product of the decomposition, was captured in the crystalline state as the uranyl sulfate phase, UO 2 (DMSO) 2 (SO 4 ).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Calcium-Uranyl-Carbonato Species Kinetically Limit U(VI) Reduction by Fe(II) and Lead to U(V)-Bearing Ferrihydrite

Reaction conditions and mechanisms promoting or inhibiting U reduction exert a central control on U solubility and, therefore, U transport and its associated risks. Here, we vary and track common aqueous uranium species to show that a kinetic restriction inhibits homogeneous reduction of the calcium-uranyl-carbonato species (CaUO$_2$(CO$_3$)$_3$$^{2–}$ and Ca$_2$UO$_2$(CO$_3$)$_3$) by Fe(II)$_{(aq)}$, while ferrihydrite surface-catalyzed reduction of all aqueous uranyl by Fe(II) proceeds. Using U L$_3$ high energy resolution fluorescence detection (HERFD) X-ray absorption near edge structure (XANES) spectroscopy, U L$_3$ extended X-ray absorption fine structure (EXAFS) spectroscopy, and transmission electron microscopy (TEM), we also show that U(V) is generated and incorporated into ferrihydrite formed from homogeneous U(VI) reduction by Fe(II)(aq). Through elucidation of the mechanisms that inhibit reduction of the calcium-uranyl-carbonato species and promote stabilization of U(V), we advance our understanding of the controls on U solubility and thus improve prediction of U transport in surface and subsurface systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Recovery of Oxidized Actinides, Np(VI), Pu(VI), and Am(VI), from Cocrystallized Uranyl Nitrate Hexahydrate: A Single Technology Approach to Used Nuclear Fuel Recycling

In this work, the utility of recovering cocrystallized Np, Pu, and Am from a uranyl nitrate hexahydrate crystalline phase has been examined. Progressive dissolution studies were carried out on samples of either Np(VI), Pu(VI), or Am(VI) cocrystallized with uranyl nitrate hexahydrate, resulting in a linear relationship between the amount of actinides dissolved as a function of volume of HNO 3 added, which indicates homogeneous distribution of the transuranic species throughout the crystalline solid. The ratio of the concentration of U to the heavier actinides was maintained over the entire dissolution and mimicked that of the starting cocrystallization solution. A series of five crystallization–dissolution–recrystallization cycles of uranyl nitrate hexahydrate from nitric acid have been achieved in the presence of Am(VI), where the behavior of Am(VI) followed that of U(VI) for each step of all five cycles. Negligible reduction of Am(VI) to Am(III) was observed between each recrystallization cycle. These results indicate the utility of a group hexavalent actinide cocrystallization approach toward used nuclear fuel recycling.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Two Ligands of Interest in Recovering Uranium from the Oceans: The Correct Formation Constants of the Uranyl(VI) Cation with 2,2'-Bipyridyl-6,6'-dicarboxylic Acid and 1,10-Phenanthroline-2,9-dicarboxylic Acid

The ligands BDA (2,2'-bipyridyl-6,6'-dicarboxylic acid) and PDA (1,10-phenanthroline-2,9-dicarboxylic acid) are of interest as functional group types for ion-exchange materials for extracting uranium from the oceans, reported in a previous paper for PDA Lashley, M. A. ( Inorg. Chem. 2016 55 10818 10829). Yang, Y. ( Inorg. Chem. 2019, 58, 6064 6074) have published what they claim to be a more accurate result for the formation of the UO 2 2+ /PDA complex of log K 1 = 22.84 compared with our reported value of log K 1 = 16.5, as well as log K 1 = 21.52 for the BDA complex. The determination of log K 1 for the PDA and BDA complexes with the UO 2 2+ cation was carried out by Yang et al. using a competition reaction between DTPA (diethylenetriamine pentaacetic acid) and BDA or PDA, monitoring the absorbance due to the BDA and PDA ligands. This competition method using absorbance versus pH titrations was developed for determining the formation constants of the complexes of several polypyridyl ligands plus PDA complexes of metal ions, which were too stable for log K determination by competition with protons. A key feature of such titrations is that in the competition reaction, the displacement of the pyridyl donor ligand (e.g., PDA) by the competing ligand (e.g., DTPA), the absorbance spectrum of the displaced pyridyl donor ligand should be observed. Competing ligands used to date have been EDTA (ethylenediamine tetraacetic acid), DTPA, or the hydroxide ion. In the study of Yang et al., no such displaced PDA or BDA was apparent in the absorbance spectra in their titrations so that their reported log K 1 values have no validity. Their log K 1 values are so much higher than log K 1 for the uranyl DTPA complex (~13.6) that DTPA could not possibly displace BDA or PDA from the uranyl cation, and a competition reaction could not possibly occur. Here, we report the correct value of log K 1 = 15.4 (ionic strength = zero) for the uranyl BDA complex, to illustrate the correct determination of such a constant by a competition reaction between BDA and hydroxide, showing how the characteristic absorbance spectrum for a BDA complex, here the UO 2 2+ complex, disappears, and the distinctive absorbance spectrum of the free nonprotonated BDA ligand appears as the pH is increased, and BDA is displaced by the hydroxide ion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Gamma-Ray-Induced Formation of Uranyl Peroxide Cage Clusters

Aqueous solutions of lithium uranyl triperoxide, Li 4 [UO 2 (O 2 ) 3 ] (LiUT), were irradiated with gamma rays at room temperature and found to form the uranyl peroxide cage cluster, Li 24 [(UO 2 )(O 2 )(OH)] 24 (Li–U 24 ). Raman spectroscopy and 18 O labeling were used to identify the Raman-active vibrations of LiUT. With these assignments, the concentration of LiUT was tracked as a function of radiation dose. A discrepancy between monomer removal and cluster formation suggests that the reaction proceeds by the assembly of an intermediate. Non-negative matrix factorization was used to separate Raman spectra into components and resulted in the identification of a unique intermediate species. Much of the conversion appears to be driven by water radiolysis products, particularly the hydroxyl radical. Furthermore, this differs from the 18 O-labeled copper-catalyzed formation of U 24 , which progresses at a steady rate with no observation of intermediates. Li–U 24 in solution decomposes at high radiation doses resulting in a solid insoluble product similar to Na-compreignacite, Na 2 (UO 2 ) 6 O 4 (OH) 6 ·7H 2 O, which contains uranyl oxyhydroxy sheets.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Systematic Study of Solid-State U(VI) Photoreactivity: Long-Lived Radicalization and Electron Transfer in Uranyl Tetrachloride

Reported are the syntheses, structural characterizations and luminescence properties of three novel [UO 2 Cl 4 ] 2- bearing compounds containing substituted 1,1’-dialkyl-4,4’-bipyridinum dications (i.e. viologens). These compounds undergo photoinduced luminescence quenching upon exposure to UV radiation. Further, this reactivity is concurrent with two phenomena: radicalization of the uranyl tetrachloride anion and photoelectron transfer to the viologen which constitutes the formal transfer of one electron from the [UO 2 Cl 4 ] 2- to the viologen species. This behavior is elucidated using electron paramagnetic resonance (EPR) spectroscopy and further probed through a series of characterization and computational techniques including Rehm-Weller analysis, time-dependent density functional theory (TD-DFT), and density of states (DOS). This work provides a systematic study of the photoreactivity of the uranyl unit in the solid state, an under-described aspect of fundamental uranyl chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Distinguishing Desirable and Undesirable Reactions in Multicomponent Systems for Redox Activation of the Uranyl Ion

Although it has been established that covalent functionalization of the U–O bonds in the uranyl dication (UO 2 2+ ) generally requires use of strong reductants and electrophiles, little work has examined how interactions between the individual reaction components could affect final outcomes in solution. Here, the patterns of such reactivity have been studied in a UO 2 2+ -containing model system supported by a workhorse pentadentate ligand, 2,2′-[(methylimino)bis(2,1-ethanediylnitrilomethylidyne)]bis-phenol. Oxo activation and functionalization have been tested with (i) electrochemical and chemical reduction, and (ii) coordinating and noncoordinating solvents. In acetonitrile, uranyl reduction was achieved cleanly, but treatment of the reduced species with tris(pentafluorophenyl)borane (BCF) resulted in a mixture of products arising from direct electron transfer to BCF. In dichloromethane (CH 2 Cl 2 ), electrochemical reduction of uranyl was achieved cleanly, but clean chemical reactivity was inaccessible. Despite these challenges, one trinuclear and oxo-deficient uranium-containing product was crystallized from CH 2 Cl 2 solution and characterized; thus, desirable electrophilic reactivity can proceed to some degree in CH 2 Cl 2 with BCF. Computational studies were used to investigate the properties of the trinuclear uranium product and the changes that could be inducible by further reduction. Here, taken together, the reactivity patterns identified here could inform design of improved systems for actinyl oxo functionalization.

cations↗
DOE OSTI2025

Harnessing Solvent Displacement Crystallization for Actinide Synthesis: Insights from Uranyl Oxalate

To address the challenge of actinide crystallization in systems with a low chemical potential, solvent displacement crystallization (SDC) techniques are applied to synthesize uranyl oxalate in a series of alcohols with varying solvent polarity. This work demonstrates the simplicity of applying SDC to actinides and indicates that solvent polarity affects crystallizations. Uranyl oxalate trihydrate was synthesized from methanol, ethanol, 1-propanol, and isopropanol as additive solvents, with characterization indicating an absence of solvent influence on the bulk structure. The choice of solvent did cause changes to the observed morphology and particle size. Additionally, the total yield of uranyl oxalate was found to decrease with increasing solvent polarity. These data support the use of SDC techniques for the crystallization of high-solubility actinide compounds.

Anions↗
DOE OSTI2023

Spectroscopic identification of Ca-bearing uranyl silicates formed in C–S–H systems

Portland cement-based grouts used for radioactive waste immobilisation contain a Ca- and Si-rich binder phase, known as calcium–silicate–hydrate (C–S–H). Depending on the blend of cement used, the Ca/Si ratio can vary considerably. A range of C–S–H minerals with Ca/Si ratios from 0.6 to 1.6 were synthesised and contacted with aqueous U(VI) at 0.5 mM and 10 mM concentrations. Solid-state 29 Si MAS-NMR spectroscopy was applied to probe the Si coordination environment in U(VI)-contacted C–S–H minerals and, in conjunction with U L III -edge X-ray absorption spectroscopy analysis, inferences of the fate of U(VI) in these systems were made. At moderate or high Ca/Si ratios, uranophane-type uranyl silicates or Ca-uranates dominated, while at the lowest Ca/Si ratios, the formation of a Ca-bearing uranyl silicate mineral, similar to haiweeite (Ca[(UO 2 ) 2 Si 5 O 12 (OH) 2 ]·3H 2 O) or Ca-bearing weeksite (Ca 2 (UO 2 ) 2 Si 6 O 15 ·10H 2 O) was identified. This study highlights the influence of Ca/Si ratio on uranyl sequestration, of interest in the development of post-closure safety models for U-bearing radioactive waste disposal.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2020

Conversion of Uranium Metal to Uranyl Sulfate Solution

Experiments involving conversion of uranium (U) metal to U 3 O 8 , followed by dissolution to form uranyl sulfate solution, were performed with batches of up to 300 g of U metal. The focus was to identify major U species formed during the oxidation process, determine the particle size distribution of the uranium oxide product, and identify the size of the dissolution vessel needed to accommodate the exothermic reaction during the conversion of U 3 O 8 to uranyl sulfate using 30% hydrogen peroxide and H 2 SO 4 . The information obtained in this study will be used for designing full-scale processes to convert kilogram quantities of U metal to uranyl sulfate solution with a concentration of 300 g U/L.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2022

Impact of Irradiation and Decay Time on Chemical Speciation in Uranyl Sulfate Solution

A pH-1, 200-g U/L, highly enriched uranyl sulfate solution was neutron-irradiated using an accelerator-driven neutron source (the Universal Neutron Irradiator, UNI) to fission U-235 under conditions similar to those used by SHINE to produce Mo-99 in a subcritical assembly. We completed a series of five uranyl sulfate irradiations to assess the impact of irradiation and decay time on uranium precipitation and iodine speciation in solution. The duration of each irradiation was increased in order, with the shortest irradiation taking 2.9 hours and the longest, 64.5 hours. A total of 9.5E16 U-235 nuclei were fissioned in all irradiations, and a maximum fission power density of 0.26 W/mL was reached in the final irradiation. We observed no evidence of uranyl peroxide or other precipitates, demonstrating that the addition of Fe 2+ to a final Fe concentration of 200 ppm in the target solution was successful at preventing precipitation under representative irradiation conditions. Nonvolatile I - was found to be the dominant (>70%) iodine species in solution, both immediately after irradiation and after days to weeks of decay time.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2023

Electric field gradients at rubidium sites in rubidium uranyl nitrate and quadrupole moments of 85 Rb and 87 Rb

The electronic structure along the axial direction of the uranyl ion in rubidium uranyl nitrate (RbUO 2 (NO 3 ) 3 ) has been studied by nuclear quadrupole resonance and nuclear magnetic resonance measurements of 85 Rb and 87 Rb quadrupole couplings. Here, the parameter of interest in these experiments is the electric field gradient at the Rb sites, which may be compared with values com- puted by various theoretical approaches. From this analysis an accurate ratio of the quadrupole moments of 85 Rb and 87 Rb has also been obtained.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Anion photoelectron spectroscopic and relativistic coupled-cluster studies of uranyl dichloride anion, UO 2 Cl 2 –

A joint relativistic coupled-cluster and experimental photoelectron (PE) spectroscopic study of the uranyl dichloride anion, UO 2 Cl 2 – , is reported. Sophisticated electronic-structure calculations predict the photodetachment of UO 2 Cl 2 – to involve a U 5f electron and to be followed by significant geometry relaxation. Therefore, the adiabatic electron affinity (EA a ) of the uranyl dichloride neutral molecule, UO 2 Cl 2 , and the vertical detachment energy (VDE) of its anion, UO 2 Cl 2 – , provide valuable information about its uranium 5f orbital energies. The EA a value was computed to be 3.15 eV. The VDE value was calculated to be 3.55 eV by augmenting the computed EA a with a shift derived from a Franck–Condon simulation using coupled-cluster potential energy surfaces. Here, the VDE, which corresponds to the highest intensity peak in the PE spectrum, was measured to be 3.69 ± 0.20 eV, in good agreement with the computed value. The origin transition in the PE spectrum, whose electron binding energy corresponds to the EA a , was assigned to the feature at 3.2 ± 0.20 eV, consistent with the computed EA a .

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2025

To thread or not to thread: Reaction of uranyl (UO 2 2+ ) with two Aza-crown macrocycles

The complexation of the uranyl ion (UO 2 2+ ) by macrocyclic ligands is hindered by the rigidly enforced trans orientation of its oxo ligands. To further investigate this coordination chemistry challenge, here we investigate the reactivity of UO 2 2+ with two different 18-membered aza-crown macrocycles, diaza-dibenzo-18-crown-6 (DADBC) and 7,16-bis(N-methylacetamide)diaza-18-crown-6 (BAM). The reaction of uranyl triflate, [UO 2 (OTf) 2 (THF) 3 ], with DADBC in toluene afforded [UO 2 (DADBC)][OTf] 2 , in which the UO 2 2+ ion is fully encapsulated by the macrocycle. This conclusion was supported by NMR, UV–Vis, IR, and Raman spectroscopies, as well as single-crystal X-ray crystallography. Attempts to form the in-macrocycle complex of BAM, however, were unsuccessful. Here, the reaction of BAM with [UO 2 (OTf) 2 (THF) 3 ] in THF/CH 2 Cl 2 unexpectedly afforded crystals of [H 2 BAM][OTf] 2 , where the protons are derived from solvate water in the BAM crystal lattice.

Actinide↗
DOE OSTI2023

Finchite, Sr(UO 2 ) 2 (V 2 O 8 )·5H 2 O, a new uranyl sorovanadate with the francevillite anion topology

Finchite (IMA2017-052), Sr(UO 2 ) 2 (V 2 O 8 )·5H 2 O, is the first uranium mineral known to contain essential Sr. The new mineral occurs as yellow-green blades up to ~10 µm in length in surface outcrops of the calcrete-type uranium deposit at Sulfur Springs Draw, Martin County, Texas, U.S.A. Crystals of finchite were subsequently discovered underground in the Pandora mine, La Sal, San Juan County, Utah, U.S.A., as diamond-shaped golden-yellow crystals reaching up to 1 mm. In this work, the crystal structure of finchite from both localities was determined using single-crystal X-ray diffraction and is orthorhombic, Pcan, with a = 10.363(6) Å, b = 8.498(5) Å, c = 16.250(9) Å, V = 1431.0(13) Å 3 , Z = 4 (R 1 = 0.0555) from Sulfur Springs Draw; and a = 10.3898(16), b = 8.5326(14), c = 16.3765(3) Å, V = 1451.8(4) Å 3 , Z = 4 (R 1 = 0.0600) from the Pandora mine. Electron-probe microanalysis provided the empirical formula (Sr 0.88 K 0.17 Ca 0.10 Mg 0.07 Al 0.03 Fe 0.02 ) Σ1.20 (UO 2 ) 2 (V 2.08 O 8 )·5H 2 O for crystals from Sulfur Springs Draw, and (Sr 0.50 Ca 0.28 Ba 0.22 K 0.05 ) Σ0.94 (U 0.99 O 2 ) 2 (V 2.01 O 8 )·5H 2 O for crystals from the Pandora mine, based on 17 O atoms per formula unit. The structure of finchite contains uranyl vanadate sheets based upon the francevillite topology. Finchite is a possible immobilization species for both uranium and the dangerous radionuclide 90 Sr because of the relative insolubility of uranyl vanadate minerals in water.

58 GEOSCIENCES↗
DOE OSTI2020

Cover Feature: Rational Design of a Uranyl Metal–Organic Framework for the Capture and Colorimetric Detection of Organic Dyes (Chem. Eur. J. 61/2020)

A new uranyl containing metal–organic framework was prepared and the diffraction data reveals the structure consists of interpenetrating 3D nets with a 4,3-connected tbo topology. Furthermore, each net contains large 22.8 Å pores and is constructed from monomeric, hexagonal bipyramidal uranyl nodes that are joined by 1,2,4,5-tetrakis(4-carboxyphenyl)benzene ligands. The crystalline material is stable in aqueous media and due to the large void space (constituting 76% of the unit cell by volume) can sequester organic dyes, the uptake of which induces a visible change to the color of the material.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Bonding and reactivity of isostructural uranyl and neptunyl peroxide phases

Understanding the reactivity of actinide peroxides is critical for predicting the behavior of spent nuclear fuel in radiolytic environments. Herein, we report the synthesis and characterization of a lithium neptunyl(VI) hydroxo peroxo compound (LiNp), which is isostructural to the uranyl analogue (LiU). Single-crystal X-ray diffraction reveals that LiNp contains both [NpO 2 (O 2 ) 3 ] 4- and [NpO 2 (OH) 4 ] 2- units stabilized by Li + and hydrogen bonding and Raman spectroscopy shows systematic redshifts in neptunyl vibrational modes relative to uranyl. DFT calculations highlight the importance of secondary coordination in reproducing vibrational and structural features, but challenges remain with correctly modeling strong sigma donors. Solid-state EPR spectroscopy and DFT confirm superoxide stabilization within LiU and calculations suggest favorability of the analogous radical species in LiNp. Solution state EPR spectroscopy with the radical spin trap 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO) reveal evidence of superoxide in the LiU and LiNp phases and suggests stabilization of superoxide within actinyl triperoxide complexes, forming [AnO 2 (O 2 ) 2 (O) 2 • ] 3- .

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗