Article pubs.acs.org/IC
Uranyl Peroxide Cage Cluster Solubility in Water and the Role of the Electrical Double Layer Kathryn M. Peruski,† Varinia Bernales,‡ Mateusz Dembowski,§ Haylie L. Lobeck,† Kristi L. Pellegrini,† Ginger E. Sigmon,† Sarah Hickam,† Christine M. Wallace,† Jennifer E. S. Szymanowski,† Enrica Balboni,† Laura Gagliardi,‡ and Peter C. Burns*,†,§ †
Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Department of Chemistry, Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455, United States § Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *
ABSTRACT: Uranium concentrations as high as 2.94 × 105 parts per million (1.82 mol of U/1 kg of H2O) occur in water containing nanoscale uranyl cage clusters. The anionic cage clusters, with diameters of 1.5−2.5 nm, are charge-balanced by encapsulated cations, as well as cations within their electrical double layer in solution. The concentration of uranium in these systems is impacted by the countercations (K, Li, Na), and molecular dynamics simulations have predicted their distributions in selected cases. Formation of uranyl cages prevents hydrolysis reactions that would result in formation of insoluble uranyl solids under alkaline conditions, and these spherical clusters reach concentrations that require close packing in solution.
clusters as approximately linear (UO2)2+ uranyl ions, and their O atoms truncate the inner and outer surfaces of the cages. Some uranyl peroxide cage clusters are soluble and stable in water,14−18 but the extent to which these could impact the concentration of uranium is solution is largely unexplored. Uranyl compounds are more soluble in water than U(IV) oxides, and in oxidizing conditions, minerals such as compreignacite, K2(UO2)6O4(OH)6(H2O)7, and meta-ankoleite, KUO2PO4(H2O)3, form by hydrolysis reactions and limit the U concentration in water to generally less than 100 ppm under most pH conditions. Studtite, [(UO2)(O2)(H2O)2](H2O)2, can form where peroxide is present. The log Ksp for its dissolution in water is −2.7 ± 0.2, and at pH = 8, a 0.01 mol/L NaClO4 aqueous solution in contact with studtite contains only 0.2 ppm of U.19,20 Industrial scale dissolution of uranium compounds in the nuclear fuel cycle relies upon nitric acid because uranium oxides have low aqueous solubility under less harsh conditions. Nanoscale uranyl peroxide clusters appear to provide a potential avenue for high loading of U in water, as shown by studies focused on the purification of uranium.21 Uranyl ions assemble in aqueous solution containing peroxide to produce anionic cage clusters with diameters in the range of 1.5−4 nm and as many as 124 uranyl ions.14,22
1. INTRODUCTION The aqueous solubility of metals is central to geochemical cycling and the distribution of the elements, transport of environmental contaminants, and many industrial processes. Dissolution of metals in simple ionic solutions is consistent with the Debye−Hückel theory,1 and Derjaguin−Landau− Verwey−Overbeek (DLVO) theory2 captures the essence of suspension of colloidal material stabilized by van der Waals forces balanced by electrostatic repulsions. Neither of these theories appears to apply to soluble nanoscale metal oxide macro-ions with sizes extending to a few nanometers. These cannot be treated as point charges and, unlike colloids, are rigorously dissolved species.3−6 They interact with smaller charge-balancing ions in solution, which can alter their aggregation behavior7−9 as well as their aqueous solubility. In solution, charge-balancing ions exist within the electrical double layer about the nanoscale macro-ions, and these ions also provide for crystallization of the charged macro-ions. Polyoxometalates are a major family of water-soluble macroions that are metal oxide clusters;10−12 their aqueous solubility is impacted by counterions in solution.13 Over the past decade, uranyl peroxide cage clusters have emerged as an extensive family of polyoxometalates in which uranyl ions are bridged through peroxide groups, as well as a variety of other ligands that include hydroxyl, phosphate, nitrate, oxalate, pyrophosphate, and phosphite.14 The U(VI) cations are present in these © XXXX American Chemical Society
Received: October 10, 2016
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DOI: 10.1021/acs.inorgchem.6b02435 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. U60 (left) and U24Pp12 (right) cage clusters, showing uranium (yellow), oxygen (red), and phosphorus (purple).
These macro-ions can persist in solution for months15 and can aggregate into stable structures with diameters of a few tens of nanometers.16 Uranyl cage clusters are generally soluble in water, including under alkaline conditions, which may support high concentrations of uranyl in water. The aqueous solubility of two uranyl cage clusters, U 60 and U 24 Pp 12 (Pp: pyrophosphate), is studied here (Figure 1). These specific clusters were selected because it is relatively straightforward to prepare them in high purity. U60 contains 60 uranyl peroxide hydroxide polyhedra arranged in an anionic fullerene-topology cage with composition [(UO2)(O2)(OH)]6060−, with the charge balanced by Li and K.23 In U60, uranyl ions are bridged through both peroxide and hydroxyl groups, with the former being in a bidentate configuration to both uranyl ions. U24Pp12 is a cage with composition [(UO2)24(O2)24(P2O7)12]48− that consists of six four-membered rings of uranyl peroxide polyhedra in which the bridges are bidentate peroxide groups, and these rings are in turn bridged by pyrophosphate.24,25 The counterions are either K+ and Na+ or Na+ and Li+. The windows in uranyl peroxide cage clusters that are defined by four-, five-, or six-membered rings of uranyl polyhedra are large enough to permit the passage of cations, providing for overall charge variability in solution.7 Here, we report the measured aqueous solubility of two uranyl peroxide cage clusters and probe the impact of the countercations on their solubility, as well as a computational study of the distribution of countercations inside and about the U60 cage cluster in water. Such studies may be important for the eventual application of uranyl peroxide cage clusters in the nuclear fuel cycle, as, for example, in novel uranium purification approaches that take advantage of the substantial size and mass of the clusters.21 They also provide a valuable model for probing the relationship between nanoscale metal oxide clusters and countercations in solution.
combination of the other reactants, the LiOH solution was added in aliquots of 50 μL until a clear yellow-orange solution was obtained. Prior to the addition of LiOH, the solution was cloudy due to the precipitation of studtite. Na/K-U24Pp12 was synthesized by combining aqueous solutions of UO2(NO3)2 (0.5 M, 0.5 mL), H2O2 (30%, 0.5 mL), and tetraethyl ammonium hydroxide (40%, 0.5 mL). Subsequent to reactant dissolution and cessation of off-gassing of oxygen, Na4P2O7 (0.1 M, 1.5 mL) and HIO3 (0.5 M, 1.5 mL) were added. Na/LiU24Pp12 was produced by combining aqueous solutions of UO2(NO3)2 (0.5 M, 1.5 mL), H2O2 (30%, 1.5 mL), H2C2O4 (0.5 M, 0.75 mL), Na4P2O7 (0.2 M, 0.55 mL), and LiOH (2.4 M, 1.05 mL). Single-crystal X-ray diffraction was used to confirm the identity of selected crystals from each synthesis experiment. 2.2. Chemical Analyses. A PerkinElmer Optima 8000 DV inductively coupled plasma optical emission spectrometer (ICPOES) was used to quantify the concentration of elements in selected solutions. Standards and samples were diluted in 5% (v/v) nitric acid to produce 10 mL samples containing 0.25−10 ppm U, K, Li, and Na. All standards and samples were matrix-matched and internally spiked with ∼0.5 ppm Y to allow for normalization relative to instrumental drift. 2.3. Small-Angle X-ray Scattering (SAXS). SAXS data were collected using a Bruker Nanostar instrument equipped with a Cu microfocus source, Montel multilayer optics, and a HiSTAR multiwire detector. Data were collected with a sample-to-detector distance of 26.3 cm and the sample chamber under vacuum. During the course of study, multiple SAXS patterns were collected from both diluted and concentrated experimental solutions. For the diluted samples, SAXS patterns were collected at the end of each solubility experiment, to quantify the size of nanoclusters persisting in solution. The concentrated experimental solutions were diluted to ∼25 g/L nanocluster, and data were collected for 1 h using a 2 mm diameter capillary flow cell to introduce the sample into the sample chamber which was evacuated (Figure S2). A separate measurement was conducted for water in the same flow cell for background subtraction. The data were fitted using the Irena SAS package for Igor Pro26 or Bruker Nanofit software. Rg was derived via Guinier analysis from the slope of the experimental SAXS curve. For concentrated solutions, the high concentration of uranium precluded collection of SAXS data using the standard flow cell. We, therefore, prepared thin films of these solutions by placing a small drop of solution on a thin glass plate and covering it with an additional glass plate. Wax was used to provide a seal around the edges of the glass plates. A separate measurement was conducted for water using the capillaries and then glass plates for background subtraction. 2.4. Electrospray Ionization Mass Spectrometry (ESI-MS). ESI-MS was used to confirm the presence and persistence of uranyl peroxide nanoclusters in solution. Previous work has shown that uranyl peroxide nanoclusters display patterns with characteristic m/z (mass/charge) peaks in the 1200−2500 range.27 The spectra were
2. EXPERIMENTAL SECTION 2.1. Material Preparation. Crystals containing nanoclusters (designated K/Li-U60, Na/K-U24Pp12, and Na/Li-U24Pp12, wherein the countercations are indicated in each case) were synthesized in aqueous solution at room temperature in 20 mL scintillation vials left open to ambient conditions until crystals formed. Crystals were recovered using disposable pipettes, were lightly washed with 18 MΩ water, and vacuum filtered. K/Li-U60 was synthesized by combining solutions of UO2(NO3)2 (0.5 M, 1 mL), KCl (0.4 M, 0.25 mL), H2O2 (30%, 1 mL), and LiOH (2.4 M, 0.75 mL). Subsequent to the B
DOI: 10.1021/acs.inorgchem.6b02435 Inorg. Chem. XXXX, XXX, XXX−XXX
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pH values of 9.23 for the Na/Li salt, and 9.13 for the Na/K salt. ICPOES analyses of the resulting solutions indicated steady states occurred from 1 day through 18 days (Figure S1, Tables S2, S3). Single-crystal X-ray diffraction was used to obtain unit-cell parameters for the remaining crystals. 2.8. Ultrafiltration of Solutions. Ultrafiltration of solutions of K/ Li-U60 and Na/K-U24Pp12 was done using established methods.38
collected using a Bruker microTOF-Q II high-resolution quadrupole time-of-flight mass spectrometer in negative ion mode (3600 V capillary voltage, 0.8 bar nebulizer gas, 4 L/min dry gas, and 180 °C dry gas temperature). Samples were directly infused at a rate of 10 μL/ min and scanned over an m/z range of 1000−5000. MaxEnt was employed for deconvolution of data where possible, although the prevalence of multiple species in solution prevented exact mass measurements in some cases. Samples were diluted to a final U concentration of approximately 100 ppm in ultrapure water for analysis. 2.5. Molecular Dynamics Simulations. Molecular dynamics (MD) simulations were performed to study the distribution of cations in the electrical double layer (EDL) around the U60 cages and the number and identity of cations inside the cages for different concentrations of clusters. Calculations were done using the DL_POLY 1.9 classic package.28,29 The U60 cluster was simulated using the universal force field,30 combined with density functional theory derived atomic partial charges,29,31 while the solvent interactions were described using the SPC/E water model.32 Two different sets of Lennard-Jones parameters were applied for the cation−cation and water−cation interactions (Joung and Cheatham33 and OPLS-AA34) using the corresponding mixing rules. Details are provided in the Supporting Information. To explore solutions with different concentrations of U60, three different periodic boundary constraints were employed in which the center-to-center distance between adjacent K/Li-U60 clusters was 6.1, 5.5, and 3.6 nm, respectively, which bracket the predicted distance from the solubility measurements (see below), and correspond to uranium concentrations ranging from 9.39 × 104 to 3.39 × 105 ppm. Simulations were equilibrated for 800 ps in the isobaric−isothermal ensemble and consecutively for 1.0 ns in the canonical ensemble, followed by 1.0 ns in the isobaric−isothermal ensemble. Finally, a 1.0 ns simulation was executed using the latest ensemble. A cubic box containing one U60 and 40 Li+ and 20 K+ cations was used for the MD simulations. Electrostatic interactions were accounted for using the Ewald summation technique35 with a convergence threshold of 0.210 Å−1 and considering a maximum of six wave vectors in each direction. The Verlet leapfrog algorithm36 was used to integrate the equations of motion with a time step of 1 fs and a cutoff of 10 Å. In order to reduce the computational cost of the simulations, we used a Verlet neighbor list35 with a cutoff radius of 10 Å. Rigid body rotational motion was handled under the leapfrog scheme with Fincham’s implicit quaternion algorithm37 with a tolerance of 10−6 Å. The nonbonded interaction energy (uNB ij ) considers the dispersion term as the Lennard-Jonnes (uLJ ij ) equation plus the Coulombic interaction (uCoul ij ). The nonelectrostatic parameters for the oxygen centers in the studied species and the counterions were considered by using the UFF/OPLS-AA parameters, since the nonbonded term in the uranium−counterion interactions is considered to be negligible as the interaction is so dominated by the electrostatic term (Table S6). In the electrostatic term, atomic partial charges derived from DFT calculations were used. In particular, the CM5 charge scheme was used to obtain partial atomic charges for [(UVIO2)2(O2)]2+,41 [(UVIO2)2(OH)2]2+, [(UVIO2)20(μ2-O2)28(OH)16]32− (U20R), [(UVIO2)24(μ2O2)36(OH)12]36− (U24R), and [(UVIO2)20(O2)30]20−27,42 (U20) (see Table S6). We averaged the partial charges of uranium, oxygen, and hydrogen atoms in the above clusters and used these average values in the U60 simulation. 2.6. K/Li-U60 Solubility in Water. An excess of crystallized K/LiU60 was placed in ultrapure water in contact with air at room temperature. The resulting pH was 9.0 and the U concentration, measured by ICP-OES, was stable from 12 h to 14 days (Figure S1, Table S1). Mass spectrometry (ESI-MS, Figure S3, Table S4) was collected for aliquots of the resulting solution. Single-crystal X-ray diffraction using a Bruker APEX II diffractometer equipped with Mo Kα was used to determine unit-cell parameters for the remaining crystals. 2.7. U24Pp12 Solubility in Water. An excess of crystals of Na/KU24Pp12 with Na:K = 42:8 or Na/Li-U24Pp12 with Li:Na = 36:12 was placed in ultrapure water, as for K/Li-U60. The resulting solutions had
3. RESULTS AND DISCUSSION Crystals of K/Li-U60 readily and rapidly dissolved in ultrapure water, although an excess of crystals remained subsequent to achieving steady state. The uranium concentration in the resulting solution in contact with crystals reached a steady state within 1 day (Figure S1). ESI-MS data collected for aliquots of the solution demonstrated that K/Li-U60 was the dominant aqueous species (Figure S3). Single-crystal X-ray diffraction confirmed that the remaining crystals were identical to the starting K/Li-U60 material. Equilibrium was demonstrated by time-resolved chemical analyses and by a reversal that produced the crystalline form of K/Li-U60. The reversal consisted of evaporation of solution that resulted in the crystallization of K/ Li-U60, as confirmed by single-crystal X-ray diffraction. The measured uranium concentration in solution in contact with K/Li-U60 crystals at steady state was 1.77 ± 0.11 × 105 ppm (0.92 mol of U/1 kg of H2O). Congruent dissolution of the K/Li-U60 crystals was confirmed by the U:Li:K ratio of 60:44:16 in solution (Figure S1). Ultrafiltration experiments showed that 98% of the uranium in solution was rejected by the membrane, indicating that most of the uranium was contained in clusters. Crystals of Na/K-U24Pp12 and Na/Li-U24Pp12 also rapidly dissolved in water, and in each case, the remaining crystals had unit-cell dimensions that are identical to those of the starting materials, as determined by single-crystal X-ray diffraction. For Na/K-U24Pp12 with Na:K = 42:8, the steady state aqueous solution at pH = 8.2 contained 3.78 ± 0.19 × 104 ppm U (0.17 mol of U/1 kg of H2O) (Figure S1, Table S2). For Na/LiU24Pp12 with Li:Na = 36:12, the steady state solution contained 2.94 ± 0.17 × 105 ppm U (1.82 mol of U/1 kg of H2O) (Figure S1, Table S3). Ultrafiltration experiments for concentrated solutions of Na/K-U24Pp12 resulted in the rejection of 99% of the uranium in solution, indicating that almost all of the uranium was contained in clusters. ESI-MS and SAXS data demonstrated that the highly concentrated solutions of Na/KU24Pp12 and Na/Li-U24Pp12 contained the expected clusters (Figures S2, S3). The uranium concentrations that result upon dissolution of U60 and U24Pp12 in mildly alkaline water under ambient conditions are 3 or more orders of magnitude higher than would be expected for simple inorganic species under similar conditions. Uranium loading levels achieved by dissolving uranyl peroxide cage clusters in water approach the levels attainable when uranium is dissolved in strong nitric acid, as is typically done in fuel-cycle applications. Furthermore, the data presented here demonstrate that not only is the solubility of uranyl peroxide cage clusters high in water but also it exhibits a strong dependence on the identity of the countercations. As shown for the case of the U24Pp12 cluster, a higher cluster solubility occurs when Li is used as a countercation. This is compatible with results for transition metal polyoxometalates, which are more soluble in the presence of lithium chloride.13 In solution, an electrical double layer (EDL) consisting of bound cations, as well as a diffuse distribution of cations and C
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Figure 2. Schematic showing the U60 cages separated by 3.0 nm and their corresponding EDLs.
Figure 3. X-ray diffraction data for saturated solutions of K/Li-U60, Na/K-U24Pp12, and Na/Li-U24Pp12. The spacing corresponding to the centroid of the peak is given in black, and compared to that calculated from the chemical data in blue.
Using the crystallographic radii 1.36 nm for U60 and 1.13 nm for U24Pp12, these center-to-center distances correspond to cluster−cluster separations of 3.0 nm for K/Li-U60, 4.8 nm for Na/K-U24Pp12, and 1.3 nm for Na/Li-U24Pp12. For comparison, in the crystal, the center-to-center distance between U60 clusters is 2.7 nm.23 Given the small separations between U60 and U24Pp12 clusters predicted on the basis of the chemical analyses, we hypothesize that, at maximum uranium concentration, aqueous solutions containing these clusters must contain a relatively ordered distribution of clusters that approaches a closestpacked arrangement. If such a state were to exist, it should be revealed by the diffraction of X-rays. X-ray diffraction patterns were, therefore, collected using our SAXS instrument for saturated aqueous solutions of U60 and U24Pp12 contained between two very thin plates of glass. The resulting patterns contain intense Bragg reflections that correspond to the average distances between the centers of mass of the clusters (Figure
H2O, must surround the anionic U60 and U24Pp12 clusters. The cage windows are large enough to allow passage of cations,39−42 and the clusters likely encapsulate some cations in solution, as they do in the crystalline state.14,43 The thickness of the corresponding EDL for a given cage cluster should depend on its specific cation constituents and their hydration behaviors (their hydrated radii), and the number of cations inside the cage as these lower its overall charge. The thickness of the EDL limits how close clusters can approach each other in solution under the most concentrated conditions (Figure 2). For a hypothetical homogeneous closestpacked distribution of identically sized spherical clusters and their EDLs in solution, their center-to-center distances can be calculated for the measured U concentrations. To do this, we noted that the theoretical maximum fraction of space that can be occupied by packed spheres of identical sizes is 74%. The calculated center-to-center distances are 5.7 nm for K/Li-U60, 7.1 nm for Na/K-U24Pp12, and 3.6 nm for Na/Li-U24Pp12. D
DOI: 10.1021/acs.inorgchem.6b02435 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. Calculated cation distribution relative to the center of mass (COM) of U60 cage clusters from molecular dynamics simulations. The cluster center-to-center distances were 6.1, 5.5, and 3.6 nm, and two sets of force field parameters were used. The red bars indicate the quantity of each cation predicted to be fully inside the U60 cage. The orange bars show the number of cations within the cage wall (in the pentagonal and hexagonal windows) and inside the cage. The yellow and green bars give the number of cations predicted to be inside U60 as well as outside, but near the cage (yellow), and all cations within 2.0 nm of the COM (green).
We hypothesize that, in an aqueous solution with high U60 concentration and a K/Li ratio of 16/44, as is the case in our experimental system, most of the K+ cations are inside the cage. This is supported by the results of the simulations reported here and is attributable to the good fit of K+ cations in the sites defined by five oxygen atoms of uranyl ions inside the pentagonal topological units of U60,40,42 and by the large hydrated radius of Li+, which reduces interactions with the anionic cage.17 The Li+ cations are not effective in bridging U60 clusters to form crystals, and the separation of U60 clusters at maximum concentration is, therefore, determined dominantly by the size of U60 and its associated EDL (Figure 2). For Na/KU24Pp12, both countercations can bridge clusters to form crystals, and the maximum concentration of the cluster in solution is accordingly lowered as the crystalline state is favored. The higher U concentration for a solution of Na/LiU24Pp12 as compared to K/Li-U60 indicates a thinner EDL about U24Pp12, as compared to U60, which is consistent with the higher charge-to-surface area ratio of the cluster. The high aqueous solubility of uranyl cage cluster materials is largely attributable to the disparity of sizes of the anionic clusters and charge balancing cations, the ability of the cations to enter the cages, the formation of electrical double layers about the clusters, and the resistance of the clusters to hydrolysis reactions that would lead to precipitation of uranyl oxyhydrate compounds. These results show that controlling the speciation of uranyl at the nanoscale can promote high uranium loading in water, which has potential applications in the nuclear fuel cycle as well as being important for understanding the geochemical mobility of uranium.
3). The corresponding measured spacings of 5.0 nm for K/LiU60, 6.5 nm for Na/K-U24Pp12, and 3.2 nm for Na/Li-U24Pp12, reveal a consistent trend with separations from the concentration-based calculations, although the measured values are about 0.5 nm shorter in each case, suggesting domains of closer-packed clusters exist in the bulk solution. However, the diffraction data demonstrate that the distribution of uranyl peroxide cage clusters in solution at maximum uranium concentration is approximated by a closest packing of spheres arrangement. Results of the molecular dynamics simulations for K+ and Li+ about U60 are provided in Figure 4. The simulations predict that K+ cations are much more likely to be inside the U60 cage, or within the pentagonal or hexagonal windows, than the Li+ cations, despite there being twice as many Li+ cations in the simulation. This prediction is not unexpected, as Li+ has a larger hydration sphere and tends not to form inner-sphere complexes as readily as K+. Furthermore, simulations by Miro have shown that K+ cations are more compatible with the binding sites provided by pentagonal rings of uranyl ions than Li + cations.40,42 Higher concentrations of U60 were simulated by equilibrating a computational box containing less water. Where the concentration of U60 is higher in the model (where there is less water), the simulations predict that there will be more cations encapsulated within the cage, and most of these cations will be K+ (Figure 4). Most of the Li+ cations are outside the cage even at high U60 concentrations, although more Li+ cations occur within 2 nm of the center of the cluster than predicted for the case of lower cluster concentration, indicating that increasing the cluster concentration causes a less diffuse EDL (Figure 4). Radial distribution functions calculated for the simulation results are provided in Figure S7. These show that the K+ cations preferentially interact with the O atoms of uranyl ions or peroxide groups, both of which are part of the cage, whereas the Li+ cations interact more strongly with water molecules. This is again consistent with the large hydration sphere of Li+, and with K+ associating with the U60 cages as inner-sphere complexes.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02435. Synthetic methods, ICP-OES analysis, small-angle X-ray scattering data for solutions, electrospray ionization mass spectrometery data for solutions, and details of molecular dynamics simulations (PDF) E
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(13) Nyman, M.; Burns, P. C. A comprehensive comparison of transition-metal and actinyl polyoxometalates. Chem. Soc. Rev. 2012, 41, 7354−7367. (14) Qiu, J.; Burns, P. C. Clusters of actinides with oxide, peroxide, or hydroxide bridges. Chem. Rev. 2013, 113, 1097−1120. (15) Armstrong, C. R.; Nyman, M.; Shvareva, T.; Sigmon, G. E.; Burns, P. C.; Navrotsky, A. Uranyl peroxide enhanced nuclear fuel corrosion in seawater. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 1874− 1877. (16) Li, D.; Simotwo, S.; Nyman, M.; Liu, T. Evolution of actinyl peroxide clusters U-28 in dilute electrolyte solution: exploring the transition from simple ions to macroionic assemblies. Chem. - Eur. J. 2014, 20, 1683−1690. (17) Nyman, M.; Alam, T. M. Dynamics of uranyl peroxide nanocapsules. J. Am. Chem. Soc. 2012, 134, 20131−20138. (18) Qiu, J.; Ling, J.; Jouffret, L.; Thomas, R.; Szymanowski, J. E. S.; Burns, P. C. Water-soluble multi-cage super tetrahedral uranyl peroxide phosphate clusters. Chem. Sci. 2014, 5, 303−310. (19) Gimenez, J.; de Pablo, J.; Casas, I.; Martinez-Llado, X.; Rovira, M.; Martinez Torrents, A. Solubility study and point of zero charge of studtite (UO2O2. 4H2O). Appl. Geochem. 2014, 49, 42−45. (20) Kubatko, K. A. H.; Helean, K. B.; Navrotsky, A.; Burns, P. C. Stability of peroxide-containing uranyl minerals. Science 2003, 302, 1191−1193. (21) Wylie, E. M.; Peruski, K. M.; Prizio, S. E.; Bridges, A. N. A.; Rudisill, T. S.; Hobbs, D. T.; Phillip, W. A.; Burns, P. C. Processing used nuclear fuel with nanoscale control of uranium and ultrafiltration. J. Nucl. Mater. 2016, 473, 125−130. (22) Burns, P. C.; Kubatko, K. A.; Sigmon, G.; Fryer, B. J.; Gagnon, J. E.; Antonio, M. R.; Soderholm, L. Actinyl peroxide nanospheres. Angew. Chem., Int. Ed. 2005, 44, 2135−2139. (23) Sigmon, G. E.; Ling, J.; Unruh, D. K.; Moore-Shay, L.; Ward, M.; Weaver, B.; Burns, P. C. Uranyl-peroxide interactions favor nanocluster self-assembly. J. Am. Chem. Soc. 2009, 131, 16648−16649. (24) Johnson, R. L.; Ohlin, C. A.; Pellegrini, K.; Burns, P. C.; Casey, W. H. Dynamics of a nanometer-sized uranyl cluster in solution. Angew. Chem., Int. Ed. 2013, 52, 7464−7467. (25) Ling, J.; Qiu, J.; Sigmon, G. E.; Ward, M.; Szymanowski, J. E. S.; Burns, P. C. Uranium pyrophosphate/methylenediphosphonate polyoxometalate cage clusters. J. Am. Chem. Soc. 2010, 132, 13395− 13402. (26) Ilavsky, J.; Jemian, P. R. Irena: tool suite for modeling and analysis of small-angle scattering. J. Appl. Crystallogr. 2009, 42, 347− 353. (27) Qiu, J.; Ling, J.; Sui, A.; Szymanowski, J. E. S.; Simonetti, A.; Burns, P. C. Time-resolved self-assembly of a fullerene-topology coreshell cluster containing 68 uranyl polyhedra. J. Am. Chem. Soc. 2012, 134, 1810−1816. (28) Todorov, I. T.; Smith, W.; Trachenko, K.; Dove, M. T. DL_POLY_3: new dimensions in molecular dynamics simulations via massive parallelism. J. Mater. Chem. 2006, 16, 1911−1918. (29) Marenich, A. V.; Jerome, S. V.; Cramer, C. J.; Truhlar, D. G. Charge model 5: an extension of Hirshfeld population analysis for the accurate description of molecular interactions in gaseous and condensed phases. J. Chem. Theory Comput. 2012, 8, 527−541. (30) Rappé, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III; Skiff, W. M. UFF, a full periodic table force field for molecular mechanics and molecular dynamics simulations. J. Am. Chem. Soc. 1992, 114, 10024−10035. (31) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. CM5PAC; University of Minnesota: Minneapolis, 2011. (32) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The missing term in effective pair potentials. J. Phys. Chem. 1987, 91, 6269−6271. (33) Joung, I. S.; Cheatham, T. E., III Determination of alkali and halide monovalent ion parameters for use in explicitly solvated biomolecular simulations. J. Phys. Chem. B 2008, 112, 9020−9041. (34) Jensen, K. P.; Jorgensen, W. L. Halide, ammonium, and alkali metal ion parameters for modeling aqueous solutions. J. Chem. Theory Comput. 2006, 2, 1499−1509.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Laura Gagliardi: 0000-0001-5227-1396 Peter C. Burns: 0000-0002-2319-9628 Funding
This research is funded by the Office of Basic Energy Sciences of the U.S. Department of Energy as part of the Materials Science of Actinides Energy Frontier Research Center (DESC0001089). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Chemical analyses were conducted at the Center for Environmental Science and Technology at the University of Notre Dame. Spectra and diffraction data were collected at the Materials Characterization Facility of the Center for Sustainable Energy at the University of Notre Dame. We thank Tianbo Liu, William Casey, Albert Migliori, and Jeremy Fein for their comments on an earlier draft of our manuscript.
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DOI: 10.1021/acs.inorgchem.6b02435 Inorg. Chem. XXXX, XXX, XXX−XXX
