Cation-Dependent Hierarchical Assembly of U60 Nanoclusters into Macro-Ion Assemblies Imaged via Cryogenic Transmission Electron Microscopy

Coacervate Formation in Dilute Aqueous Solutions of Inorganic Molecular Clusters with Simple Divalent Countercations

We report a complex coacervate formed by a 2.5 nm-diameter, rigid uranyl peroxide molecular cluster (Li68K12(OH)20)[UO2(O2)OH]60, U6060-) and SrCl2 salt in dilute aqueous solutions, including its location in the phase diagram, composition, rheological features, and critical conditions for phase transitions. In this coacervate, the Sr2+ cations are a major building component, and the coacervate phase covers a substantial region of the phase diagram. This coacervate demonstrates features that differ from traditional coacervates formed by oppositely charged long-chain polyelectrolytes, especially in its formation mechanism, dehydration, enhancement of mechanical strength with increasing ionic strength, and the change of salt partition preference into the coacervate and supernatant phases with ionic strength.

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.

Observing ion diffusion and reciprocating hopping motion in water

When an ionic crystal dissolves in solvent, the positive and negative ions associated with solvent molecules release from the crystal. However, the existing form, interaction, and dynamics of ions in real solution are poorly understood because of the substantial experimental challenge. We observed the diffusion and aggregation of polyoxometalate (POM) ions in water by using liquid phase transmission electron microscopy. Real-time observation reveals an unexpected local reciprocating hopping motion of the ions in water, which may be caused by the short-range polymerized bridge of water molecules. We find that ion oligomers, existing as highly active clusters, undergo frequent splitting, aggregation, and rearrangement in dilute solution. The formation and dissociation of ion oligomers indicate a weak counterion-mediated interaction. Furthermore, POM ions with tetrahedral geometry show directional interaction compared with spherical ions, which presents structure-dependent dynamics.

Crystallization of a Neptunyl Oxalate Hydrate from Solutions Containing NpV and the Uranyl Peroxide Nanocluster U60 Ox30

Uranyl peroxide nanoclusters are an evolving family of materials with potential applications throughout the nuclear fuel cycle. While several studies have investigated their interactions with alkali and alkaline earth metals, no studies have probed their interactions with the actinide elements. This work describes a system containing U₆₀ Ox₃₀ , [((UO₂ )(O₂ ))₆₀ (C₂ O₄ )₃₀ ]^60- , and neptunium(V) as a function of neptunium concentration. Ultra-small and small angle X-ray scattering were used to observe these interactions in the aqueous phase, and X-ray diffraction was used to observe solid products. The results show that neptunium induces aggregation of U₆₀ Ox₃₀ when the neptunium concentration is≤10 mM, whereas (NpO₂ )₂ C₂ O₄  ⋅ 6H₂ O(cr) and studtite ultimately form at 15-25 mM neptunium. The latter result suggests that neptunium coordinates with the bridging oxalate ligands in U₆₀ Ox₃₀ , leaving metastable uranyl peroxide species in solution. This is an important finding given the potential application of uranyl peroxide nanoclusters in the recycling of used nuclear fuel.

Self-Assembly and Ionic-Lattice-like Secondary Structure of a Flexible Linear Polymer of Highly Charged Inorganic Building Blocks

Among molecular building blocks, metal oxide cluster anions and their countercations provide multiple options for the self-assembly of functional materials. Currently, however, rational design concepts are limited to electrostatic interactions with metal or organic countercations or to the attachment and subsequent reactions of functionalized organic ligands. We now demonstrate that bridging μ-oxo linkages can be used to string together a bifunctional Keggin anion building block, [PNb₂Mo₁₀O₄₀]^5- (1), the diniobium(V) analogue of [PV₂Mo₁₀O₄₀]^5- (2). Induction of μ-oxo ligation between the Nb^V═O moieties of 1 in acetonitrile via step-growth polymerization gives linear polymers with entirely inorganic backbones, some comprising over 140 000 repeating units, each with a 3- charge, exceeding that of previously reported organic or inorganic polyelectrolytes. As the chain grows, its flexible μ-oxo-linked backbone, with associated countercations, coils into a compact 270 nm diameter spherical secondary structure as a result of electrostatic interactions not unlike those within ionic lattices. More generally, the findings point to new options for the rational design of multidimensional structures based on μ-oxo linkages between Nb^V═O-functionalized building blocks.

Dynamics of Cation-Induced Conformational Changes in Nanometer-Sized Uranyl Peroxide Clusters

Conformational changes of the pyrophosphate (Pp)-functionalized uranyl peroxide nanocluster [(UO₂)₂₄(O₂)₂₄(P₂O₇)₁₂]^48- ({U₂₄Pp₁₂}), dissolved as a Li/Na salt, can be induced by the titration of alkali cations into solution. The most symmetric conformer of the molecule has idealized octahedral (O_h) molecular symmetry. One-dimensional ³¹P NMR experiments provide direct evidence that both K⁺ and Rb⁺ ions trigger an O_h-to-D_4h conformational change within {U₂₄Pp₁₂}. Variable-temperature ³¹P NMR experiments conducted on partially titrated {U₂₄Pp₁₂} systems show an effect on the rates; increased activation enthalpy and entropy for the D_4h-to-O_h transition is observed in the presence of Rb⁺ compared to K⁺. Two-dimensional, exchange spectroscopy ³¹P NMR revealed that magnetization transfer links chemically unique Pp bridges that are present in the D_4h conformation and that this magnetization transfer occurs via a conformational rearrangement mechanism as the bridges interconvert between two symmetries. The interconversion is triggered by the departure and reentry of K (or Rb) cations out of and into the cavity of the cluster. This rearrangement allows Pp bridges to interconvert without the need to break bonds. Cs ions exhibit unique interactions with {U₂₄Pp₁₂} clusters and cause only minor changes in the solution ³¹P NMR signatures, suggesting that O_h symmetry is conserved. Single-crystal X-ray diffraction measurements reveal that the mixed Li/Na/Cs salt adopts D_2h molecular symmetry, implying that while solvated, this cluster is in equilibrium with a more symmetric form. These results highlight the unusually flexible nature of the actinide-based {U₂₄Pp₁₂} and its sensitivity to countercations in solution.

The role of cations in uranyl nanocluster association: a molecular dynamics study

Actinyl ions can self-assemble in aqueous solution to form closed cage clusters ranging from 1.5 to 4.0 nm in diameter. The self-assembly, stability, and behavior of the nanoclusters depend on the nature of the aqueous environment, such as the pH and cations present. In this work, a classical force field for [(UO2)20(O2)30]20- (U20) peroxide nanoclusters in aqueous solution was developed from quantum-mechanical calculations. Using molecular dynamics simulations, the preferred binding sites of six cations (Li+, Na+, K+, Rb+, Cs+, and Ca2+) to the nanocluster were determined. Replica exchange molecular dynamics was used to equilibrate the structure and determine the equilibrium distribution of cations and water with respect to the nanocluster cage. In addition, the free energy barriers associated with cations entering the cluster were computed. Finally, the association of two cages was investigated by computing the free energy as a function of intercage distance. The free energy profiles reveal that the nanoclusters prefer to be associated when neutralized with divalent cations, but do not associate when neutralized with monovalent cations. This could explain the formation of tertiary structures observed experimentally.

Beyond Charge Balance: Counter-Cations in Polyoxometalate Chemistry

Polyoxometalates (POMs) are molecular metal-oxide anions applied in energy conversion and storage, manipulation of biomolecules, catalysis, as well as materials design and assembly. Although often overlooked, the interplay of intrinsically anionic POMs with organic and inorganic cations is crucial to control POM self-assembly, stabilization, solubility, and function. Beyond simple alkali metals and ammonium, chemically diverse cations including dendrimers, polyvalent metals, metal complexes, amphiphiles, and alkaloids allow tailoring properties for known applications, and those yet to be discovered. This review provides an overview of fundamental POM-cation interactions in solution, the resulting solid-state compounds, and behavior and properties that emerge from these POM-cation interactions. We will explore how application-inspired research has exploited cation-controlled design to discover new POM materials, which in turn has led to the quest for fundamental understanding of POM-cation interactions.

Stability of Solid Uranyl Peroxides under Irradiation

The effects of radiation on a variety of uranyl peroxide compounds were examined using γ-rays and 5 MeV He ions, the latter to simulate α-particles. The studied materials were studtite, [(UO₂)(O₂)(H₂O)₂](H₂O)₂, the salt of the U₆₀ uranyl peroxide cage cluster, Li₄₄K₁₆[(UO₂)(O₂)(OH)]₆₀·255H₂O, the salt of U₆₀Ox₃₀ uranyl peroxide oxalate cage cluster, Li₁₂K₄₈[{(UO₂)(O₂)}₆₀(C₂O₄)₃₀]·nH₂O, and the salt of the U₂₄Pp₁₂ (Pp = pyrophosphate) uranyl peroxide pyrophosphate cage cluster, Li₂₄Na₂₄[(UO₂)₂₄(O₂)₂₄(P₂O₇)₁₂]·120H₂O. Irradiated powders were characterized using powder X-ray diffraction, Raman spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, and UV-vis spectroscopy. A weakening of the uranyl bonds of U₆₀ was found while studtite, U₆₀Ox₃₀, and U₂₄Pp₁₂ were relatively stable to γ-irradiation. Studtite and U₆₀ are the most affected by α-irradiation forming an amorphous uranyl peroxide as characterized by Raman spectroscopy and powder X-ray diffraction while U₆₀Ox₃₀ and U₂₄Pp₁₂ show minor signs of the formation of an amorphous uranyl peroxide.

Calcium-Facilitated Aggregation and Precipitation of the Uranyl Peroxide Nanocluster U60 in the Presence of Na-Montmorillonite

The unique and diverse features of uranyl peroxide nanoclusters may contribute to the enhanced mobility of uranium in the environment. This study examines the sorption of the uranyl peroxide nanocluster [UO₂(O₂)(OH)]₆₀^60- (U₆₀) to Na-montmorillonite (SWy-2), plagioclase (anorthite), and quartz (SiO₂) as a function of time, U₆₀ concentration, and mineral concentration. SWy-2 was studied in both its untreated form as well as after two different pretreatments, denoted as partially treated SWy-2 and fully treated SWy-2. U₆₀ was removed (∼99%) from solution in the presence of untreated and partially treated SWy-2. However, U₆₀ was not removed from suspensions containing anorthite, quartz, or fully treated SWy-2, even after several months. The removal of U₆₀ from suspensions containing untreated SWy-2 is promoted in part by the exchange of Li⁺ counter-ions, normally weakly associated with U₆₀ in solution, for Ca²⁺ ions naturally present in the clay. In solution, Ca²⁺ ions induce the aggregation of nanoclusters, which precipitate on the surface of SWy-2. Ca-rich U₆₀ aggregates associated with SWy-2 were identified and characterized by scanning electron microscopy with energy dispersive spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. This research enhances our understanding of the molecular-scale processes controlling U₆₀ behavior at the mineral-water interface.

In situ Raman spectroscopy of uranyl peroxide nanoscale cage clusters under hydrothermal conditions

The behaviours of two uranyl peroxide nanoclusters in water heated to 180 °C were examined by in situ Raman spectroscopy.

In situ nitroso formation induced structural diversity of uranyl coordination polymers

This work presents three possible pathways that could exist in the in situ reaction system. Structural analysis of these compounds revealed that the introduction of nitroso group exerted significant influences on the conformations of ligands, skeletons and 3D structures.

Kinetics of Uranyl Peroxide Nanocluster (U60) Sorption to Goethite

The unique properties of uranium-based nanomaterials may significantly impact our current understanding of the fate and transport of U(VI) in environmental systems. Sorption of the uranyl peroxide nanocluster [(UO₂)(O₂)(OH)]₆₀^60- (U₆₀) to goethite (α-FeOOH) was studied using batch sorption experiments as a function of U₆₀ concentration (0.5-2 g·L^-1), mineral concentration (100-500 m²·L^-1), and pH (8-10). The resulting rate law describing U₆₀ interactions with goethite at pH 9 was R = - k_rxn[U₆₀]^0.29±0.02[goethite]^1.2±0.1 where k_rxn = (6.7 ± 2.0) × 10^-4 (g·L^-1)^0.71±0.02(m²·L^-1)^-1.2±0.1(day^-1). The largest fraction of U₆₀ removed from solution was at pH 8, which is below the isoelectric point of the goethite used in this study. Site density calculations suggest that U₆₀ may exist on the goethite surface at a center-to-center distance of 5.4-6.5 nm, depending upon pH, which mirrors the center-to-center distance observed in the aqueous phase near the U₆₀ solubility limit. At pH 10, approximately 20% uranium was desorbed within 3 days. Analysis of the reacted mineral surface using X-ray photoelectron spectroscopy confirmed the presence of a single U(VI) species on the mineral surface, and electrospray ionization mass spectrometry revealed that U₆₀ remains intact during the sorption and desorption processes. These results demonstrate that the behavior of U₆₀ at the goethite-water interface is similar to that of discrete U(VI) but is governed by different sorption mechanisms and reaction kinetics, which has the potential to alter our current understanding of the fate and transport of uranium species in the environment.

Uranyl Peroxide Nanocluster (U60) Persistence and Sorption in the Presence of Hematite

The presence of uranium-based nanomaterials in environmental systems may significantly impact our current understanding of the fate and transport of U(VI). Sorption of the uranyl peroxide nanocluster [(UO₂)(O₂)(OH)]₆₀^60- (U₆₀) to hematite (α-Fe₂O₃) was studied using batch sorption experiments with varying U₆₀, hematite, and alkali electrolyte (i.e., NaCl, KCl, and CsCl) concentrations. Data from electrospray ionization mass spectrometry and centrifugal microfiltration revealed that U₆₀ persisted in the presence of hematite and the background electrolyte for at least 120 days. K⁺ ions were removed from solution with uranium whereas Li⁺ ions remained in solution, indicating that the U₆₀ cluster behaved like an anion and that the Li⁺ ions did not play a significant role in the sorption mechanism. Analysis of the reacted mineral surface using X-ray photoelectron and Raman spectroscopies confirmed the presence of U(VI) and uranyl species with bridged peroxo groups associated with the mineral surface. These results indicate that uranyl peroxide nanoclusters may persist in the aqueous phase under environmentally relevant conditions for reasonably long periods of time, as compared to that of the uranyl cation.

Review of the Scientific Understanding of Radioactive Waste at the U.S. DOE Hanford Site

This Critical Review reviews the origin and chemical and rheological complexity of radioactive waste at the U.S. Department of Energy Hanford Site. The waste, stored in underground tanks, was generated via three distinct processes over decades of plutonium extraction operations. Although close records were kept of original waste disposition, tank-to-tank transfers and conditions that impede equilibrium complicate our understanding of the chemistry, phase composition, and rheology of the waste. Tank waste slurries comprise particles and aggregates from nano to micro scales, with varying densities, morphologies, heterogeneous compositions, and complicated responses to flow regimes and process conditions. Further, remnant or changing radiation fields may affect the stability and rheology of the waste. These conditions pose challenges for transport through conduits or pipes to treatment plants for vitrification. Additionally, recalcitrant boehmite degrades glass quality and the high aluminum content must be reduced prior to vitrification for the manufacture of waste glass of acceptable durability. However, caustic leaching indicates that boehmite dissolves much more slowly than predicted given surface normalized rates. Existing empirical models based on ex situ experiments and observations generally only describe material balances and have not effectively predicted process performance. Recent advances in the areas of in situ microscopy, aberration-corrected transmission electron microscopy, theoretical modeling across scales, and experimental methods for probing the physics and chemistry at mineral-fluid and mineral-mineral interfaces are being implemented to build robustly predictive physics-based models.

Captivation with encapsulation: a dozen years of exploring uranyl peroxide capsules

Uranyl peroxide cages are an extensive family of topologically varied self-assembling nanoscale clusters with fascinating properties and applications.

Enzymatic reduction of U60 nanoclusters by Shewanella oneidensis MR-1

In this study, a series of reduction experiments were conducted using a representative uranyl peroxide nanocluster, U60 (K16Li44[UO2(O2)OH]60) and a bacterial species, Shewanella oneidensis MR-1, that is capable of enzymatic U(VI) reduction. U60 was reduced by S. oneidensis in the absence of O2, but the reduction kinetics for U60 were significantly slower than was observed in this study for aqueous uranyl acetate, and were faster than was reported in previous studies for solid phase U(VI). Our results indicate that U60 aggregates bigger than 0.2 μm formed immediately upon mixing with the bacterial growth medium, and that these aggregates were gradually broken down during the process of reduction. Neither reduction nor dissolution of U60 was observed during 72 h of control experiments open to the atmosphere, indicating that the breakdown and dissolution of U60 aggregates is caused by the reduction of U60, and that S. oneidensis is capable of direct reduction of the U(VI) within the U60 nanoclusters, likely due to the adsorption of U60 aggregates onto bacterial cells. This study is first to show the reduction capacity of bacteria for uranyl peroxide nanoclusters, and the results yield a better understanding of the long term fate of uranium in environmental systems in which uranyl peroxide nanoclusters are present.

Thermal Responsive Ion Selectivity of Uranyl Peroxide Nanocages: An Inorganic Mimic of K(+) Ion Channels

An actinyl peroxide cage cluster, Li48+m K12 (OH)m [UO2 (O2 )(OH)]60 (H2 O)n (m≈20 and n≈310; U60 ), discriminates precisely between Na(+) and K(+) ions when heated to certain temperatures, a most essential feature for K(+) selective filters. The U60 clusters demonstrate several other features in common with K(+) ion channels, including passive transport of K(+) ions, a high flux rate, and the dehydration of U60 and K(+) ions. These qualities make U60 (a pure inorganic cluster) a promising ion channel mimic in an aqueous environment. Laser light scattering (LLS) and isothermal titration calorimetry (ITC) studies revealed that the tailorable ion selectivity of U60 clusters is a result of the thermal responsiveness of the U60 hydration shells.