Critical Conditions Regulating the Gelation in Macroionic Cluster Solutions.

The critical gelation conditions observed in dilute aqueous solutions of multiple nanoscale uranyl peroxide molecular clusters are reported, in the presence of multivalent cations. This gelation is dominantly driven by counterion-mediated attraction. The gelation areas in the corresponding phase diagrams all appear in similar locations, with a characteristic triangle shape outlining three critical boundary conditions, corresponding to the critical cluster concentration, cation/cluster ratio, and the degree of counterion association with increasing cluster concentration. These interesting phrasal observations reveal general conditions for gelation driven by electrostatic interactions in hydrophilic macroionic solutions.

Molecular assembly indices of mineral heteropolyanions: some abiotic molecules are as complex as large biomolecules.

Molecular assembly indices, which measure the number of unique sequential steps theoretically required to construct a three-dimensional molecule from its constituent atomic bonds, have been proposed as potential biosignatures. A central hypothesis of assembly theory is that any molecule with an assembly index ≥15 found in significant local concentrations represents an unambiguous sign of life. We show that abiotic molecule-like heteropolyanions, which assemble in aqueous solution as precursors to some mineral crystals, range in molecular assembly indices from 2 for H2CO3 or Si(OH)4 groups to as large as 21 for the most complex known molecule-like subunits in the rare minerals ewingite and ilmajokite. Therefore, values of molecular assembly indices ≥15 do not represent unambiguous biosignatures.

A New Molybdenum Blue Structure Type: How Uranium Expands this Family of Polyoxometalates.

The assembly of molybdenum polyoxometalates (POMs) has afforded large discrete nanoclusters with varied degrees of reduction such as the ~20 % reduced molybdenum blues. While many heterometals have been incorporated into these clusters to afford new properties, uranium has yet to be reported. Here we report the first uranium containing molybdenum blue clusters and the unique properties exhibited by this incorporation. The uranyl ion (UO2 2+) directs formation of Mo72U8, a square POM comprised of two faces connected by eight edge-sharing molybdenum dimers. Mo72U8, a chiral cluster, crystallizes as a racemic mixture and, in the solid state, has a 'negative' charge localized on one face of the cluster opposite the 'positively' charged face of another cluster. Using U(IV) as both heterometal and molybdenum reductant afforded crystals of Mo97U10, a wheel cluster with a heptamolybdate cap on one face. Mo97U10 dissociates in solution, losing the heptamolybdate, to form Mo90U10. Using more solvent during synthesis afforded crystals of Mo90U10S4 which, instead of heptamolybdate, contains four sulfate ions. Crystals of Mo90U10S4 undergo a dehydration induced phase change where clusters form a sheet through oxide bridges. Half of the bridges are cation-cation interactions between the uranyl oxygen atom and molybdenum, the first reported of this kind.

Pu(VI) Oxalate Crystal Structure and Evidence of Photoreduction to Pu(IV) Oxalate.

We report the first crystal structure of a Pu(VI)-oxalate compound. This compound, [PuO2(C2O4)(H2O)]·2(H2O) (1), crystallizes in space group P21/c with a = 5.5993(3) Å, b = 16.8797(12) Å, c = 9.3886(6) Å, and ß = 98.713(6)°. It is isostructural with the previously reported U(VI) compound, [UO2(C2O4)(H2O)]·2(H2O). Each plutonyl ion (PuO22+) is coordinated in the equatorial plane by two side-on bidentate oxalates, creating an infinite chain along [001]. A coordinated water molecule and twisting of the oxalates lead to a distorted pentagonal bipyramidal geometry of the Pu. A photochemical degradation was observed for 1, which resulted in the formation of a secondary crystalline phase. The absorption spectrum of this secondary phase confirmed the presence of Pu(IV), but it did not match the spectrum of Pu(C2O4)2·6H2O, which is considered to be the primary product of Pu-oxalate precipitation. While compound 1 has previously been proposed to exist in solution, this is the first time it has been isolated via crystallization. Although redox interactions between Pu and oxalate have been documented in the literature, the present study is the first observation of a photochemical reduction of Pu(VI)-oxalate. As a result, this study has expanded on the limited understanding of the Pu(VI)-oxalate system, which is important for nuclear fuel cycle applications.

Removal of Aqueous Uranyl and Arsenate Mixtures after Reaction with Limestone, PO43-, and Ca2.

The co-occurrence of uranyl and arsenate in contaminated water caused by natural processes and mining is a concern for impacted communities, including in Native American lands in the U.S. Southwest. We investigated the simultaneous removal of aqueous uranyl and arsenate after the reaction with limestone and precipitated hydroxyapatite (HAp, Ca10(PO4)6(OH)2). In benchtop experiments with an initial pH of 3.0 and initial concentrations of 1 mM U and As, uranyl and arsenate coprecipitated in the presence of 1 g L-1 limestone. However, related experiments initiated under circumneutral pH conditions showed that uranyl and arsenate remained soluble. Upon addition of 1 mM PO43- and 3 mM Ca2+ in solution (initial concentration of 0.05 mM U and As) resulted in the rapid removal of over 97% of U via Ca-U-P precipitation. In experiments with 2 mM PO43- and 10 mM Ca2+ at pH rising from 7.0 to 11.0, aqueous concentrations of As decreased (between 30 and 98%) circa pH 9. HAp precipitation in solids was confirmed by powder X-ray diffraction and scanning electron microscopy/energy dispersive X-ray. Electron microprobe analysis indicated U was coprecipitated with Ca and P, while As was mainly immobilized through HAp adsorption. The results indicate that natural materials, such as HAp and limestone, can effectively remove uranyl and arsenate mixtures.

Kinetics of Na- and K- uranyl arsenate dissolution.

We integrated aqueous chemistry analyses with geochemical modeling to determine the kinetics of the dissolution of Na and K uranyl arsenate solids (UAs(s)) at acidic pH. Improving our understanding of how UAs(s) dissolve is essential to predict transport of U and As, such as in acid mine drainage. At pH 2, Na0.48H0.52(UO2)(AsO4)(H2O)2.5(s) (NaUAs(s)) and K0.9H0.1(UO2)(AsO4)(H2O)2.5(s) (KUAs(s)) both dissolve with a rate constant of 3.2 × 10-7 mol m-2 s-1, which is faster than analogous uranyl phosphate solids. At pH 3, NaUAs(s) (6.3 × 10-8 mol m-2 s-1) and KUAs(s) (2.0 × 10-8 mol m-2 s-1) have smaller rate constants. Steady-state aqueous concentrations of U and As are similarly reached within the first several hours of reaction progress. This study provides dissolution rate constants for UAs(s), which may be integrated into reactive transport models for risk assessment and remediation of U and As contaminated waters.

Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries.

Invited for the cover of this issue is the group of Amy Hixon at the University of Notre Dame. The image depicts the newly identified structure of a PuIV oxalate sheet compared to the historically assumed structure. Read the full text of the article at 10.1002/chem.202301164.

Investigation of Radiation Effects in the Uranyl Mineral Metaschoepite.

The effects of water vapor and He ion irradiation on the alteration of particles of the uranyl hydroxide phase metaschoepite, [(UO2)8O2(OH)12](H2O)10, are determined. Raman spectra collected immediately postirradiation revealed the presence of a uranyl oxide phase structurally similar to γ-UO3 or U2O7. Short-term storage postirradiation at elevated relative humidity accelerated formation of the uranyl peroxide phase studtite, [(UO2)(O2)(H2O)2](H2O)2. Experiments examining the degradation of metaschoepite and the hydration of UO3 enabled spectral assignments and identification of reaction pathways. The results provide insights into thermal and radiolytic degradation products in both irradiated uranyl hydroxide phases and uranyl peroxide phases, which follow similar degradation pathways.

Predicting new mineral occurrences and planetary analog environments via mineral association analysis.

The locations of minerals and mineral-forming environments, despite being of great scientific importance and economic interest, are often difficult to predict due to the complex nature of natural systems. In this work, we embrace the complexity and inherent "messiness" of our planet's intertwined geological, chemical, and biological systems by employing machine learning to characterize patterns embedded in the multidimensionality of mineral occurrence and associations. These patterns are a product of, and therefore offer insight into, the Earth's dynamic evolutionary history. Mineral association analysis quantifies high-dimensional multicorrelations in mineral localities across the globe, enabling the identification of previously unknown mineral occurrences, as well as mineral assemblages and their associated paragenetic modes. In this study, we have predicted (i) the previously unknown mineral inventory of the Mars analogue site, Tecopa Basin, (ii) new locations of uranium minerals, particularly those important to understanding the oxidation-hydration history of uraninite, (iii) new deposits of critical minerals, specifically rare earth element (REE)- and Li-bearing phases, and (iv) changes in mineralization and mineral associations through deep time, including a discussion of possible biases in mineralogical data and sampling; furthermore, we have (v) tested and confirmed several of these mineral occurrence predictions in nature, thereby providing ground truth of the predictive method. Mineral association analysis is a predictive method that will enhance our understanding of mineralization and mineralizing environments on Earth, across our solar system, and through deep time.

Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries.

Plutonium(IV) oxalate hexahydrate (Pu(C2 O4 )2 ⋅ 6 H2 O; PuOx) is an important intermediate in the recovery of plutonium from used nuclear fuel. Its formation by precipitation is well studied, yet its crystal structure remains unknown. Instead, the crystal structure of PuOx is assumed to be isostructural with neptunium(IV) oxalate hexahydrate (Np(C2 O4 )2 ⋅ 6 H2 O; NpOx) and uranium(IV) oxalate hexahydrate (U(C2 O4 )2 ⋅ 6 H2 O; UOx) despite the high degree of unresolved disorder that exists when determining water positions in the crystal structures of the latter two compounds. Such assumptions regarding the isostructural behavior of the actinide elements have been used to predict the structure of PuOx for use in a wide range of studies. Herein, we report the first crystal structures for PuOx and Th(C2 O4 )2 ⋅ 6 H2 O (ThOx). These data, along with new characterization of UOx and NpOx, have resulted in the full determination of the structures and resolution of the disorder around the water molecules. Specifically, we have identified the coordination of two water molecules with each metal center, which necessitates a change in oxalate coordination mode from axial to equatorial that has not been reported in the literature. The results of this work exemplify the need to revisit previous assumptions regarding fundamental actinide chemistry, which are heavily relied upon within the current nuclear field.

Electrospray Ionization Tandem Mass Spectrometry With Collision Induced Dissociation of Uranyl Peroxide Nanoclusters Containing Various Uranyl Bridging Ligands.

Electrospray ionization tandem mass spectrometry with collision-induced dissociation (ESI-MS/MS) was utilized to study the gas phase fragmentation of uranyl peroxide nanoclusters with hydroxo, peroxo, oxalate, and pyrophosphate bridging ligands. These nanoclusters fragment into uranium monomers and dimers with mass-to-charge (m/z) ratios in the 280-380 region. The gas phase fragmentation of each cluster studied yields a distinct UO6 - anion attributed to the cleavage of a uranyl ion bound to 2 peroxide groups, along with other anions that can be attributed to the initial composition of the nanoclusters.

Electrochemistry of Uranyl Peroxide Solutions during Electrospray Ionization.

The ionization of uranyl triperoxide monomer, [(UO2)(O2)3]4- (UT), and uranyl peroxide cage cluster, [(UO2)28(O2)42 - x(OH)2x]28- (U28), was studied with electrospray ionization mass spectrometry (ESI-MS). Experiments including tandem mass spectrometry with collision-induced dissociation (MS/CID/MS), use of natural water and D2O as solvent, and use of N2 and SF6 as nebulizer gases, provide insight into the mechanisms of ionization. The U28 nanocluster under MS/CID/MS with collision energies ranging from 0 to 25 eV produced the monomeric units UOx- (x = 3-8) and UOxHy- (x = 4-8, y = 1, 2). UT under ESI conditions yielded the gas-phase ions UOx- (x = 4-6) and UOxHy- (x = 4-8, y = 1-3). Mechanisms that produce the observed anions in the UT and U28 systems are: (a) gas-phase combinations of uranyl monomers in the collision cell upon fragmentation of U28, (b) reduction-oxidation resulting from the electrospray process, and (c) ionization of surrounding analytes, creating reactive oxygen species that then coordinate to uranyl ions. The electronic structures of anions UOx- (x = 6-8) were investigated using density functional theory (DFT).

Solubility and Thermodynamic Investigation of Meta-Autunite Group Uranyl Arsenate Solids with Monovalent Cations Na and K.

We investigated the aqueous solubility and thermodynamic properties of two meta-autunite group uranyl arsenate solids (UAs). The measured solubility products (log Ksp) obtained in dissolution and precipitation experiments at equilibrium pH 2 and 3 for NaUAs and KUAs ranged from -23.50 to -22.96 and -23.87 to -23.38, respectively. The secondary phases (UO2)(H2AsO4)2(H2O)(s) and trögerite, (UO2)3(AsO4)2·12H2O(s), were identified by powder X-ray diffraction in the reacted solids of KUA precipitation experiments (pH 2) and NaUAs dissolution and precipitation experiments (pH 3), respectively. The identification of these secondary phases in reacted solids suggest that H3O+ co-occurring with Na or K in the interlayer region can influence the solubilities of uranyl arsenate solids. The standard-state enthalpy of formation from the elements (ΔHf-el) of NaUAs is -3025 ± 22 kJ mol-1 and for KUAs is -3000 ± 28 kJ mol-1 derived from measurements by drop solution calorimetry, consistent with values reported in other studies for uranyl phosphate solids. This work provides novel thermodynamic information for reactive transport models to interpret and predict the influence of uranyl arsenate solids on soluble concentrations of U and As in contaminated waters affected by mining legacy and other anthropogenic activities.

Ozone-Facilitated Formation of Uranyl Peroxide in Humid Conditions.

Metaschoepite, [(UO2)8O2(OH)12](H2O)10, maintained in a high relative humidity (RH) environment with air initially transformed into an intermediate phase that subsequently was replaced by the peroxide phase studtite, [(UO2)(O2)(H2O)2](H2O)2, over the course of 42 days, as observed using Raman and infrared spectroscopy and powder X-ray diffraction. Addition of atmospheric ozone vastly increased the rate and extent of the transformation to studtite but only in a high-RH atmosphere. Owing to its strong affinity for peroxide, uranyl reacted with hydrogen peroxide as it formed and precipitated stable studtite. In this work, we provide a previously unidentified source of hydrogen peroxide and make a case for the re-examination of storage systems where the consequences of atmospheric ozone are not considered.

One of Nature's Puzzles Is Assembled: Analog of the Earth's Most Complex Mineral, Ewingite, Synthesized in a Laboratory.

Through the combination of low-temperature hydrothermal synthesis and room-temperature evaporation, a synthetic phase similar in composition and crystal structure to the Earth's most complex mineral, ewingite, was obtained. The crystal structures of both natural and synthetic compounds are based on supertetrahedral uranyl-carbonate nanoclusters that are arranged according to the cubic body-centered lattice principle. The structure and composition of the uranyl carbonate nanocluster were refined using the data on synthetic material. Although the stability of natural ewingite is higher (according to visual observation and experimental studies), the synthetic phase can be regarded as a primary and/or metastable reaction product which further re-crystallizes into a more stable form under environmental conditions.

Targeting Diverse Bridging Motifs within Actinide Borosulfates and Establishing an Unconventional Structural Hierarchy.

The first actinide borosulfates, (UO2)[B(SO4)2(SO3OH)] (TSUBOS-1) and (UO2)2[B2O(SO4)3(SO3OH)2] (TSUBOB-1), were synthesized solvothermally in oleum using UO3. The classical borosulfate crystal structure of TSUBOS-1 is partially consistent with an established conventional hierarchy. Uranyl pentagonal bipyramids limit the anionic network linkages and isolate sulfate tetrahedra within the anionic network. Therefore, the classical one-dimensional chain established in the hierarchy does not fully describe the structure. The structure of TSUBOB-1 is the first actinide borosulfate that contains an unconventional borate-to-borate bridging mode (denoted B-O-B) and a zero-dimensional oxoanionic unit consisting of one sulfate tetrahedron that shares vertices with two B-O-B bridged borate tetrahedra that each share a vertex with two sulfate tetrahedra. As this structure departs from the existing structural hierarchy, a modified approach for understanding the unconventional borosulfate substructure and dimensionality is proposed, together with a new graphical notation. In the course of our synthesis experiments, a novel uranyl disulfate compound (UO2)2[(S2O7)(SO3OH)2] (TSUDS) was isolated and characterized. The structure of TSUDS is a framework consisting of uranyl pentagonal bipyramids and sulfate tetrahedra. Each uranyl pentagonal bipyramid is surrounded by five sulfate tetrahedra, two of which share a vertex creating a disulfate with a S-O-S bridging mode. The uranyl bipyramids are linked to one another via the singular sulfate or disulfate groups.

Gamma-Ray-Induced Formation of Uranyl Peroxide Cage Clusters.

Aqueous solutions of lithium uranyl triperoxide, Li4[UO2(O2)3] (LiUT), were irradiated with gamma rays at room temperature and found to form the uranyl peroxide cage cluster, Li24[(UO2)(O2)(OH)]24 (Li-U24). Raman spectroscopy and 18O 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. This differs from the 18O-labeled copper-catalyzed formation of U24, which progresses at a steady rate with no observation of intermediates. Li-U24 in solution decomposes at high radiation doses resulting in a solid insoluble product similar to Na-compreignacite, Na2(UO2)6O4(OH)6·7H2O, which contains uranyl oxyhydroxy sheets.

Assembly of Uranyl Peroxides from Ball Milled Solids.

Mechanochemistry enables transformations of highly insoluble materials such as uranium dioxide or the mineral studtite [(UO2)(O2)(H2O)2]·(H2O)2 into uranyl triperoxide compounds that can subsequently assemble into hydroxide-bridged uranyl peroxide dimers in the presence of lithium hydroxide. Dissolution of these solids in water yields uranyl peroxide nanoclusters including U24, Li24[(UO2)(O2)(OH)]24. Insoluble uranium solids can transform into highly soluble uranyl peroxide phases in the solid state with miniscule quantities of water. Such reactions are potentially applicable to uranium processing in the front and back end of the nuclear fuel cycle.

Radiation-Induced Solid-State Transformations of Uranyl Peroxides.

Single-crystal X-ray diffraction studies of pristine and γ-irradiated Ca2[UO2(O2)3]·9H2O reveal site-specific atomic-scale changes during the solid-state progression from a crystalline to X-ray amorphous state with increasing dose. Following γ-irradiation to 1, 1.5, and 2 MGy, the peroxide group not bonded to Ca2+ is progressively replaced by two hydroxyl groups separated by 2.7 Å (with minor changes in the unit cell), whereas the peroxide groups bonded to Ca2+ cations are largely unaffected by irradiation prior to amorphization, which occurs by a dose of 3 MGy. The conversion of peroxide to hydroxyl occurs through interaction of neighboring lattice H2O molecules and ionization of the peroxide O-O bond, which produces two hydroxyls, and allows isolation of the important monomer building block, UO2(O2)2(OH)24-, that is ubiquitous in uranyl capsule polyoxometalates. Steric crowding in the equatorial plane of the uranyl ion develops and promotes transformation to an amorphous phase. In contrast, γ-irradiation of solid Li4[(UO2)(O2)3]·10H2O results in a solid-state transformation to a well-crystallized peroxide-free uranyl oxyhydrate containing sheets of equatorial edge and vertex-sharing uranyl pentagonal bipyramids with likely Li and H2O in interlayer positions. The irradiation products of these two uranyl triperoxide monomers are compared via X-ray diffraction (single-crystal and powder) and Raman spectroscopy, with a focus on the influence of the Li+ and Ca2+ countercations. Highly hydratable and mobile Li+ yields to uranyl hydrolysis reactions, while Ca2+ provides lattice rigidity, allowing observation of the first steps of radiation-promoted transformation of uranyl triperoxide.

Hyperstoichiometric Uranium Dioxides: Rapid Synthesis and Irradiation-Induced Structural Changes.

Uranium dioxide (UO2), the primary fuel for commercial nuclear reactors, incorporates excess oxygen forming a series of hyperstoichiometric oxides. Thin layers of these oxides, such as UO2.12, form readily on the fuel surface and influence its properties, performance, and potentially geologic disposal. This work reports a rapid and straightforward combustion process in uranyl nitrate-glycine-water solutions to prepare UO2.12 nanomaterials and thin films. We also report on the investigation of the structural changes induced in the material by irradiation. Despite the simple processing aspects, the combustion synthesis of UO2.12 has a sophisticated chemical mechanism involving several exothermic steps. Raman spectroscopy and single-crystal X-ray diffraction (XRD) measurements reveal the formation of a complex compound containing the uranyl moiety, glycine, H2O, and NO3- groups in reactive solutions and dried combustion precursors. Combustion diagnostic methods, gas-phase mass spectroscopy, differential scanning calorimetry (DSC), and extracted activation energies from DSC measurements show that the rate-limiting step of the process is the reaction of ammonia with nitrogen oxides formed from the decomposition of glycine and uranyl nitrate, respectively. However, the exothermic decomposition of the complex compound determines the maximum temperature of the process. In situ transmission electron microscopy (TEM) imaging and electron diffraction measurements show that the decomposition of the complex compound directly produces UO2. The incorporation of oxygen at the cooling stage of the combustion process is responsible for the formation of UO2.12. Spin coating of the solutions and brief annealing at 670 K allow the deposition of uniform films of UO2.12 with thicknesses up to 300 nm on an aluminum substrate. Irradiation of films with Ar2+ ions (1.7 MeV energy, a fluence of up to 1 × 1017 ions/cm2) shows unusual defect-simulated grain growth and enhanced chemical mixing of UO2.12 with the substrate due to the high uranium ion diffusion in films. The method described in this work allows the preparation of actinide oxide targets for fundamental nuclear science research and studies associated with stockpile stewardship.