Complexity of Uranyl Peroxide Cluster Speciation from Alkali-Directed

Publication Date (Web): July 13, 2018 ... of UO2, with X:U molar ratios of 1.0, showing that alkali availability determines the U concentrations in so...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Complexity of Uranyl Peroxide Cluster Speciation from AlkaliDirected Oxidative Dissolution of Uranium Dioxide Sarah Hickam,† Sergey M. Aksenov,† Mateusz Dembowski,‡ Samuel N. Perry,† Hrafn Traustason,‡ Meghan Russell,† and Peter C. Burns*,†,‡ †

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Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Solid UO2 dissolution and uranium speciation in aqueous solutions that promote formation of uranyl peroxide macroanions was examined, with a focus on the role of alkali metals. UO2 powders were dissolved in solutions containing XOH (X = Li, Na, K) and 30% H2O2. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements of solutions revealed linear trends of uranium versus alkali concentration in solutions resulting from oxidative dissolution of UO2, with X:U molar ratios of 1.0, showing that alkali availability determines the U concentrations in solution. The maximum U concentration in solution was 4.20 × 105 parts per million (ppm), which is comparable to concentrations attained by dissolving UO2 in boiling nitric acid, and was achieved by lithium hydroxide promoted dissolution. Raman spectroscopy and electrospray ionization mass spectrometry (ESI-MS) of solutions indicate that dissolution is accompanied by the formation of various uranyl peroxide cluster species, the identity of which is alkali concentration dependent, revealing remarkably complex speciation at high concentrations of base.

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intermediates.1,4,10−12 Recently, it has been demonstrated that some salts of uranyl peroxide cage clusters are highly soluble in water, but the extent to which they may encourage dissolution of normally insoluble uranium oxides has not been systematically explored.13,14 Several countries currently reprocess spent (irradiated) nuclear fuel to recover fissionable isotopes and other useful radionuclides. Aqueous dissolution of UO2 spent nuclear fuel is needed prior to chemical separations of its components. The current plant-scale process, known as PUREX,15 involves dissolving UO2 in hot 50% HNO3, which results in a solution with about 400 g of uranium per liter. Spent fuel reprocessing strategies that emphasize alkaline conditions have also been studied in the USA, Japan, Korea, and Russia as an alternative to acid-based dissolution.16−23 Aqueous solutions rich in carbonate and peroxide are used to dissolve UO2 fuel that yields solutions with about 100 g of uranium per liter, which is generally lower than ideal.24 Studies focused on dissolution of UO2 in alkaline, peroxide-rich solutions have not reported the formation of uranyl peroxide cage clusters, although these studies do not report characterization methods that would have revealed the presence of clusters in solution.25

INTRODUCTION Factors affecting the dissolution of metal oxides under conditions conducive to formation of nanoscale metal oxide clusters in solution have not been extensively studied. Typically, transition-metal and actinide polyoxometalates (POMs) are synthesized in one-pot reactions using soluble precursors, with an emphasis on crystallizing clusters for structure characterization.1−4 Rarely have studies directly synthesized POMs by starting with solid materials,5,6 but one recent example provides initial evidence that formation of uranyl peroxide clusters encourages dissolution of UO2 in alkaline solutions.7 There have been no detailed studies of the factors encouraging dissolution of uranium solids to form POMs. A dozen years ago the first uranyl peroxide cage clusters were described, and this family has subsequently grown to more than 60 reported varieties.2,8,9 Uranyl peroxide cage clusters readily self-assemble in alkaline aqueous solutions at room temperature, with widely varied topologies and sizes resulting from variation of peroxide and pH, as well as the choice of alkali countercation. Whereas it was initially assumed that alkali cations played templating roles and therefore affected the resulting cluster topologies, more recent evidence suggests that alkali cations adventitiously select encapsulated locations, although alkali cations are important during the formation of clusters because they stabilize peroxide-bridged © XXXX American Chemical Society

Received: May 12, 2018

A

DOI: 10.1021/acs.inorgchem.8b01299 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Herein we provide the first systematic study of UO2 dissolution in alkaline aqueous peroxide solutions at various concentrations of alkali cations. Electrospray ionization mass spectrometry (ESI-MS), Raman spectroscopy, and singlecrystal X-ray diffraction (SC-XRD) have been used to provide insight into uranyl peroxide speciation. ESI-MS is particularly useful as a probe of the uranium speciation in the polydisperse cluster-bearing systems encountered here.

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Synthesis of Li-U28. The Li salt of U28 was prepared from a modified method by combining 30 mL of 0.5 M aqueous uranyl nitrate, 30 mL of 30% H2O2, and 18 mL of 2.38 M aqueous LiOH.10 The resulting solution was divided into six parts, and each was centrifuged for 5 min, yielding yellow solutions and orange precipitates. Aliquots of solution were separated into 5 mL vials for slow diffusion with methanol. Bright yellow crystals of Li-U28 were harvested after 3 days. Synthesis of Li-U24. The synthetic route was derived from a previously published method.8 A 1.0 mL portion of 0.5 M aqueous uranyl nitrate and 1.0 mL of 30% H2O2 were combined in a scintillation vial. The reaction yielded a pale yellow precipitate that was dissolved by adding a 2.38 M LiOH solution with stirring. The vial was covered with Parafilm with small holes for slow evaporation. Yellow crystals formed in 7 weeks. Inductively Coupled Plasma Optical Emission Spectrometry. Samples were prepared for inductively coupled plasma optical emission spectrometry (ICP-OES) elemental analyses by diluting sample aliquots with water, followed by addition of 70% nitric acid to yield a concentration of 5% HNO3, as well as Y as an internal standard. Samples were analyzed using a PerkinElmer Optima 8000, and the concentration of each element was determined from a calibration curve developed using 10 standard solutions. Powder X-ray Diffraction. Solids were ground with an agate mortar and pestle and pressed onto a glass slide. Powder patterns were collected with a Bruker D8 Advance diffractometer with Cu Kα radiation using 0.01° steps over a range of 5−55° 2θ, counting for 1 s per step. Electrospray Ionization Mass Spectrometry. Electrospray ionization mass spectra (ESI-MS) were collected using a Bruker micrOTOF-Q II high-resolution quadrupole time-of-flight spectrometer in negative ion mode with a 2900 V capillary voltage, 0.85 bar of nebulizer, and 4 L/min of dry gas. Samples were diluted to less than 2000 ppm of U in water immediately prior to introduction into the system by direct injection. Data were collected over 3 min with an injection rate of 600 μL/h and scanned over the m/z 500−5000 region. Charge states were assigned to the resulting peaks using the Bruker software DataCompass. Raman Spectroscopy. Raman spectra were acquired using a Bruker Sentinel system with a fiber optic probe, thermoelectrically cooled CCD detector, and 785 nm excitation source. Spectra were acquired from solutions over 80 to 3200 cm−1 using three 15 s exposures at 400 mW laser power for most samples and 200 mW laser power for U samples with the highest concentrations. Raman spectra for crystals of K-U28, K-U28-U, Li-U28, and Na-U20 were collected using a Nikon optical microscope connected to the Raman probe with three 15 s exposures at 180 mW laser power. Single-Crystal X-ray Diffraction. Single-crystal X-ray diffraction data for crystals of K-U28-U, K-U28, and Na-U32-U28 were collected using a Bruker APEX II Quazar single-crystal diffractometer with graphite-monochromated Mo Kα X-ray radiation using a ω−θ scanning mode. Crystals were mounted on a cryoloop and cooled by flowing nitrogen gas at 120 K. Data were integrated using the program SAINT and were then scaled, merged, and corrected for Lorentz, polarization, and absorption effects using the SADABS package. The structure determination and refinement were carried out using the JANA2006 program package.26 Atomic scattering factors for neutral atoms together with anomalous dispersion correction were taken from ref 27. The final refinement included positional parameters for all atoms and anisotropic displacement parameters for U atoms. Illustrations were produced using JANA2006 in combination with DIAMOND.28 The experimental details of the data collection and refinement results are given in Table S4. Tables S5−S10 give the fractional atomic coordinates, site occupancies, and equivalent/ isotropic atomic displacement parameters.

EXPERIMENTAL SECTION

UO2 Dissolution Studies. Time-resolved dissolution of solid UO2 into solutions containing various concentrations of alkali hydroxide and hydrogen peroxide was studied, and uranium concentrations measured for these solutions were determined in parts per million (ppm). Often, dissolution reactions were exothermic and evolved gas over the first several minutes, which delayed initial sampling. The uranium concentration in solution for most experiments stabilized within hours and remained steady for at least 7 days. Comparisons here are made for different experiments, specifically for uranium concentrations measured on the seventh day after commencement of each dissolution experiment. UO2 powders (International Bio-Analytical Industries, Inc.) were used as received (characterization in the Supporting Information). For experiments involving LiOH, 10 mg to 1.5 g of UO2 powder was placed in Teflon vials. The initial mass of UO2 for each experiment was selected on the basis of results of preliminary experiments to avoid complete dissolution of UO2. Subsequently, 30% H2O2 (EMD Millipore Corporation) was added, followed within 1 min by a 2 M LiOH solution prepared using LiOH powder (ACROS) or, in some cases, solid LiOH powder. No efforts were made to eliminate carbonate in these systems. The quantity of LiOH added was predetermined to achieve the target Li concentrations, although the actual concentration of Li in solution was measured by chemical analyses. Target concentrations of Li were 200, 1400, 2500, 5000, 10000, 14000, 17500, 20000, and 22500 ppm. After the reactants were combined and the evolution of gas had visibly ceased, the vials were capped and covered by Parafilm to prevent evaporation. Experiments involving NaOH or KOH were conducted as for LiOH described above, except that the target concentrations of NaOH were 500, 2500, 7500, 15000, 20000, 25000, 35000, and 40000 ppm of Na, accomplished by adding 2, 4, or 8 M NaOH prepared from NaOH powders (BDH), and the target concentrations for KOH experiments were 200, 2500, 5000, 7500, 15000, 17500, and 19000, 20000, and 25000 ppm of K, produced by adding 2 or 4 M stock solutions prepared from KOH powders (Strem Chemicals). Time-resolved pH measurements, as well as aliquots of each solution, were taken at designated times, and samples were centrifuged prior to further analyses, which are described below. The remaining solid was collected on day 7 and was vacuum-filtered prior to further analysis (see below). Synthesis of K-U28. Two K salts containing the U28 cluster were synthesized and are designated K-U28 and K-U28-U. For K-U28, 5 mL of 0.5 M uranyl nitrate (International Bio-Analytical Industries, Inc.), 5 mL of 30% H2O2, and 3 mL of 2 M aqueous KOH were combined in the stated order, which produced a cloudy, pale green solution that was centrifuged for 5 min before being divided into small aliquots. One day of vapor diffusion with methanol yielded green, acicular crystals. K-U28-U was prepared by mixing 2 mL of 0.5 M uranyl nitrate, 2 mL of 30% H2O2, and 2 mL of 2 M aqueous KOH. The resulting redbrown solution was centrifuged for 5 min and divided into small aliquots for vapor diffusion with methanol. Orange, acicular crystals formed within 1 day. Synthesis of Na-U20. For the synthesis of Na-U20,10 approximately 0.04 g of 30% H2O2 was added to 1.5 mL of 0.026 M aqueous uranyl nitrate, followed by 0.08 g of 2 M aqueous NaOH. The solutions were briefly vortexed and then centrifuged. The resulting solution was pipetted into a 5 mL glass vial, and ethanol diffusion resulted in crystals after 2 days.

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RESULTS AND DISCUSSION Alkali-Concentration-Dependent Dissolution of UO2. Oxidative dissolution of UO2 powders in the presence of H2O2 B

DOI: 10.1021/acs.inorgchem.8b01299 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and aqueous XOH (X = Li, Na, K) occurs rapidly, is exothermic, and evolves O2 gas. In the systems under study, peroxide is responsible for the rapid oxidation of the U(IV) in UO2 to form (UO2)2+ uranyl in solution, and during this reaction peroxide is reduced. The evolution of O2 gas during dissolution of UO2 is associated with the assembly of uranyl ions into uranyl peroxide cages and likely is liberated when peroxide is oxidized during production of bridges between uranyl ions.12 Time-resolved chemical analyses of solutions in contact with UO2 show that concentrations stabilize within hours (Figure S1). Uranium and alkali concentrations measured for solutions that had been in contact with UO2 for 7 days are compared in Figure 1. There is a linear relationship between the quantity of

and KNa-U24Pp12) in water, in which the presence of heavier alkali countercations yields lower compound solubility.1,13 Lithium has a larger hydration radius in comparison to Na or K and is less likely to be strongly associated with or form bridges between cluster cages.13 The maximum concentration of uranium following dissolution of UO2 with H2O2 and LiOH is 4.20 × 105 ppm of U (4.14 mol of U/kg of H2O), which is the highest concentration of uranium reported to date for any uranyl peroxide cluster system, with the previous maximum being 2.94 × 105 ppm of U (calculated here as 2.61 mol of U/ kg of H2O) for LiNa-U24Pp12.13 Starting concentrations of alkali that are higher than those in the regions of linear alkali−uranium concentration relations in Figure 1 (e.g., 20000 ppm of K) exhibit decreased solution U concentrations and alkali:U ratios are greater than 1 (Table S2). This deviation from a 1:1 ratio corresponds with the observation of Raman signals in addition to those observed for uranyl peroxide clusters for LiOH and NaOH solutions, possibly corresponding to uranyl peroxide monomers, [(UO2)(O2)3]4− (Figure S2).29 Weak Raman signals in KOH solutions indicate that clusters are still the dominant solution species. Formation of Secondary Phases. Powder X-ray diffraction (PXRD) of solids after dissolution experiments indicate that secondary phases formed (Figure S3). Typical PXRD patterns of solids from low alkali concentration experiments contain the uranyl peroxide phase studtite, [(UO2)(O2)(H2O)2](H2O)2.30 At higher alkali concentrations, PXRD patterns include signals from up to four phases: UO2, [(UO2)(O2)(H2O)2](H2O)2, uranyl oxide hydrates, and an amorphous phase. The oxide hydrate phase in the Li system is schoepite, [(UO2)8O2(OH)12](H2O)12.31 Solids from Na experiments yield patterns with d spacings that are similar to those of uranyl oxide hydrates but that do not precisely correspond to existing synthetic compounds or mineral structures, including Na2U2O7, which has been observed in alkaline, peroxide-based UO2 dissolution experiments.25 Uranyl peroxide clusters have very large d spacings; thus, the precipitate is likely not a uranyl peroxide cluster phase. An X-ray amorphous phase is particularly evident in diffraction patterns collected for experiments with high concentrations of KOH. Visual inspection of the reactants in these experiments revealed that crystallization occurs at the highest concentrations of added base in the KOH system. Crystal structure determination identified the crystals as K-U28U (U28 encapsulating one U site);8 however, the PXRD patterns of these samples show UO2 and an amorphous phase. Amorphous phases have been found in previous studies as a coprecipitate with crystallized uranyl peroxide clusters.14,32 On the basis of this observation, the amorphous phase may contain uranyl peroxide clusters. Solution Speciation. Several techniques have been used to gain insight into uranium speciation in the solutions that resulted from dissolution of UO2. Whereas some studies have emphasized much smaller uranyl peroxide species in alkaline, peroxide-rich solutions, we found no evidence of their significant presence in the systems under study over solution concentrations that are within the linear trend.33,34 The determined solution characteristics generally group according to pH (Table 1), and they can generally be described as dominated by uranyl peroxide cluster species. Solutions that have pH values ranging from 6 to 8 yield discrete Raman vibrational modes and produce ESI-MS spectra that contain broad but well-resolved signals. Solutions generally with a pH

Figure 1. Concentration of U in solution following dissolution of UO2 versus concentration of alkali in solution, measured by ICP-OES. Each marker is an average of replicate experiments. The error, represented by red bars, is the standard deviation of the averages of replicates. The data were fitted by the following linear regressions: Li system, y = 33.769x − 4362.1 (R2 = 0.9982); Na system, y = 11.318x − 1423.3 (R2 = 0.9983); K system, y = 6.7378x − 966.51 (R2 = 0.9952).

alkali cation and uranium in solution for each of the three alkali cations studied, to a maximum uranium concentration that differs for the three alkali cations, with the highest corresponding to LiOH. Solutions studied here in contact with UO2 contain a 1:1 molar ratio of alkali to uranium (Li:U, 1.06 ± 0.07 ; Na:U, 1.00 ± 0.16 ; K:U, 0.99 ± 0.1). This indicates that uranium in solution is dominated by one or more species in which the overall charge per uranyl ion is 1−, as is the case for uranyl peroxide cage clusters but not for uranyl monomer species that could form in these systems. Characterization of the aqueous solutions confirms the dominance of uranyl peroxide cages (discussed below). We note that the solubility of UO2 in alkaline aqueous solutions containing H2O2 and alkalis is orders of magnitude higher than that in comparable solutions that do not contain peroxide, as also noted by several previous studies (Table S1).13,14 The maximum concentration of U in our experiments is strongly dependent on the alkali cation used, with the trend Li > Na > K. This is consistent with typical solubility trends of POMs as well as the measured aqueous solubility of the Li/Na and K/Na salts of [(UO2)24(O2)24(P2O7)12] (LiNa-U24Pp12 C

DOI: 10.1021/acs.inorgchem.8b01299 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Alkali Hydroxide Systems Grouped According to Solution Speciation, Discussed in Detail in the Text, and Corresponding Measured Uranium Concentrations and pH Values base

group

U concn (ppm)

LiOH

1 2 1 2 1 2 3

0 to 2.0 × 105 >2.0 × 105 0 to 6.0 × 104 >6.0 × 104 0 to 1.4 × 104 (1.4−2.0) × 104 >2.0 × 104

NaOH KOH

average pH, day 7 7.97 8.53 8.37 9.70 8.22 11.36 11.88

± ± ± ± ± ± ±

0.58 0.95 0.77 0.22 1.53 0.18 0.46

greater than 8.5 are characterized by more complex Raman and ESI-MS data and divergent behaviors of the Li, Na, and K systems. Some KOH solutions yield additional Raman signals and distinct ESI-MS peak envelopes. The following paragraphs discuss the solution speciation of these distinct groups. Solutions with pH from 6 to 8.5. Raman spectra for solutions containing Li, Na, and K with pH in the range of 6− 8.5 contain three characteristic peaks in the 800−900 cm−1 regions (Figure 2). The peak at 875 cm−1 is assigned to free

Figure 3. Representative ESI-MS spectra of group one samples (pH less than 8.5) for the LiOH (red), NaOH (green), and KOH (blue) systems.

Table 2. Mass Assignments of Group One (Solutions with pH less than 8.5) ESI-MS Spectra alkali

m/z

charge

calcd mass (kDa)

cluster (mass, kDa)

Li

1294 1510 1812 1146 1375 1718 1383 1621 1953

7− 6− 5− 6− 5− 4− 7− 6− 5−

9.06

Li28[(UO2)28(O2)42] (9.10)

6.87

Na20[(UO2)20(O2)30] (6.82)

9.99

K28[(UO2)28(O2)42] (10.0)

Na

K

both indicate the presence of U28 clusters and compositions Xy[(UO2)28(O2)42](28−y)− (X = Li, K; y = number of alkali cations) (Figure 4). In contrast, Na-bearing solutions with pH in the range of 6−8.5 contain U20 clusters, with composition Na20[(UO2)20(O2)30] (Figure 4). Previous investigations with K countercations have shown that the U28 cage can encapsulate an additional uranyl cation that may be partially occupied, bringing the number of total U to near 29.8,10 In a departure from these earlier observations, ESI-MS mass assignments corresponding to solutions having lower concentrations of KOH indicate a cluster with composition K28[(UO2)28(O2)42] (with no encapsulated U), which is supported by good agreement with the simulated mass spectrum (Figure 5). Subsequently, crystals of U28 with composition K28[(UO2)28(O2)42](H2O)30 were synthesized with a K:U ratio lower than is typical (see the Experimental Section). Single-crystal X-ray diffraction confirmed that the U28 clusters in these crystals do not encapsulate U (see crystallographic descriptions below), and the ESI-MS spectra of solutions produced by dissolving these crystals correspond to the cluster mass of 10.00 kDa observed for lower concentration KOH experiments (Figure 5). The assignments for Na-U20 and Li-U28 were confirmed by synthesizing and dissolving their respective crystals in water for Raman and ESI-MS measurements, which show good

Figure 2. Representative Raman spectra of group one solutions containing LiOH (red), NaOH (green), and KOH (blue) with pH less than 8.5. The intensities were normalized to the largest peak.

peroxide (O2)2−.35 Two discrete signals between 805 and 838 cm−1 are typical of vibrational modes of uranyl peroxide clusters, with peaks between 805 and 810 cm−1 attributed to symmetric vibrations of the uranyl, v1(UO2)2+, and those in the range of 832−838 cm−1 arising from the symmetric vibrations of peroxide, v1(O2)2−.35 ESI-MS spectra for solutions with pH in the range of 6−8.5 (Figure 3) contain broad peak envelopes, as are typical for highly charged POMs such as uranyl peroxide clusters.35,36 Such signals are in the spectrum of each solution, including those with the lowest concentrations of alkali cations, showing that clusters are formed in the presence of alkali hydroxide and excess H2O2 in each case. These spectra contain well-resolved broad peaks, facilitating charge assignments and mass determination using the relationship 1/P = z/m, where P is the peak position, z is the peak charge, and m is mass (Table 2).36 For the K and Li solutions, expected and observed masses D

DOI: 10.1021/acs.inorgchem.8b01299 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

agreement with dissolution experiment characterization (Figure S4). Additionally, Li-U28 was crystallized from dissolution experiments by vapor diffusion with methanol. The presence of Na-U20, Li-U28, and K-U28 in our solutions with pH in the range of 6−8.5 show that UO2 dissolution at relatively low concentrations of alkali cations is analogous to the dissolution of studtite, [(UO2)(O2)(H2O)2](H2O)2, at low alkali:U ratios, with the added observation of K-U28 with no encapsulated U.10 Raman and mass spectra do not reveal additional uranyl species in solution, which indicates that the uranium concentration in solutions studied here produced by the oxidative dissolution of UO2 is limited by the availability of cations to form the aforementioned clusters when hydrogen peroxide is available in excess. The 1:1 alkali:uranium molar ratios indicate that uranyl peroxide clusters are dominant solution species, in which each U polyhedron bears a 1− charge that is balanced by a single cation. The presence of additional species such as monomers or oligomers would affect the 1:1 ratio because charges per uranium are higher (e.g., in Na4[(UO2)(O2)3] and in K6[(UO2)(O2)2(OH)]2).33,37 Observations of studtite as a secondary uranium phase at low alkali concentrations also indicate that uranium concentrations in solution are regulated by alkali concentrations, as this phase requires no charge-balancing cations and forms as a result of UO2 oxidation and release of uranyl ions that cannot form additional clusters due to the limited cation availability. Under these experimental conditions, uranyl peroxide clusters in solution appear to be favored over insoluble studtite, provided alkali cations are available. Solutions with pH Greater than 8.5. Dissolution of UO2 into alkali peroxide aqueous solutions with pH generally greater than 8.5 yielded solutions for which the Raman spectra contain numerous overlapping peaks in the 800−850 cm−1 region, as well as complex ESI-MS spectra of Li and Na bearing systems that prevent reliable mass assignments, although some spectra exhibit signals that are attributable to Li-U28 and NaU20 (Figure 6). The mass spectra for K-bearing solutions contain more discrete signals, which facilitate a detailed analysis. Solutions with pH Greater than 8.5 and High K. The mass spectra for K-bearing solutions with pH greater than 8.5 arise from two clusters (Figure 7). One, with major peaks at m/z

Figure 4. Cluster topologies observed by dissolving UO2 with LiOH, NaOH, and KOH in the presence of 30% H2O2, shown in a polyhedral representation of uranyl bipyramids (left) and arrangement of U atoms showing square, pentagonal, and hexagonal faces (right).

Figure 5. Representative ESI-MS spectra of KOH system with pH less than 8.5 are shown in blue, dissolved crystals of K-U28 in black, and simulated mass spectra of K-U28 in red. Figure 6. Representative Raman spectra (left) and corresponding ESI-MS spectra (right) for LiOH (red) and NaOH (green) dissolution experiments with pH greater than 8.5 (group two). E

DOI: 10.1021/acs.inorgchem.8b01299 Inorg. Chem. XXXX, XXX, XXX−XXX

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is U28 with encapsulated U, as the encapsulated uranyl ion would yield Raman signals typical of uranyl triperoxide monomers. Uranyl triperoxide monomers produce multiple Raman signals from symmetric and asymmetric vibrations of peroxide ligands between 810 and 850 cm−1, which contribute to the complexity of signals in that region for these solutions. Diffusion of methanol vapor into the high-K-concentration solutions described in the preceding paragraph resulted in crystallization. Single-crystal X-ray diffraction data indicated that these were a mixture of orthorhombic and cubic crystals, but both were found to contain the U28 cluster encapsulating U (K-U28-U) (Figure 9; see crystallographic section below). The structure refinements for these crystals indicated a range of site occupancies for the encapsulated U, including full occupancy.

Figure 7. Representative ESI-MS spectra of group two (top) and three (middle) KOH system dissolution experiments and ESI-MS spectra of K-U28 (bottom, black) and K-U28-U (bottom, red) crystals dissolved in water.

1370, 1620, 1950, and 2460 (M = 10.00 kDa), corresponds to K-U28 without encapsulated U (as found for lower pH solutions). The second cluster has peaks at m/z 1450, 1720, 2070, and 2600 with 7−, 6−, 5−, and 4− charge states (M = 10.55 kDa). The difference in mass between the two clusters in this solution is 550 Da, which is similar to the mass of [UO2(O2)3]4− plus charge-balancing cations, suggesting that this species is U28 that does encapsulate U (K-U28-U). A further increase in KOH simplifies the ESI-MS that now contains only peaks corresponding to the 10.55 kDa cluster (group three, Figure 7). The Raman spectra of these high-K solutions are distinguished by low-intensity peaks at 670 and 693 cm−1 (Figure 8). Uranyl Raman vibrational modes generally trend toward lower wavenumber with increasing UOyl bond length,38 and these signals are within the range of vibrational modes of uranyl triperoxide monomers, which have 2+ modes between 680 and 740 cm−1.29 This v1 (UO2 ) observation supports the conclusion that the cluster species

Figure 9. Partial, polyhedral representation of K-U28-U shown with the disordered, encapsulated [(UO2)(O2)3]4− species. Dashed lines and arrows indicate the disorder of the central species. Blue spheres are oxygen atoms of water molecules, and potassium atoms are shown in green.

Figure 8. Raman spectra of solutions from UO2 dissolution with KOH and H2O2 with pH greater than 8.5 (group three). Arrows indicate v1(UO2)2+ signals arising from encapsulated U in K-U28-U.

K-U28-U crystals were also prepared by increasing the K:U ratio relative to the synthesis described for K-U28 in the Experimental Section. Elemental analysis of dissolved K-U28-U and K-U28 crystals confirms that the K:U ratio is higher for KU28-U crystals (1.14 versus 0.94), due to the extra negative charges on the encapsulated U species and corresponding extra K cations for charge balance. ESI-MS spectra of dissolved crystals of K-U28 and K-U28-U confirm our assignments for solutions produced by dissolving UO2 (Figure 7), and peaks between 670 and 700 cm−1 in their Raman spectra confirm that these signals arise from an encapsulated U. The Raman peaks arising from encapsulated U are weak, likely because these U atoms only correspond to 3.4% of the total and thus are not observed for solutions having a lower concentration of K, which contain U28 with and without encapsulated U. Also, K-U28-U crystallized at very high KOH concentrations in UO2 dissolution experiments, as mentioned earlier, indicating the maximum solubility of this cluster was reached, thus limiting the U concentration in solution. F

DOI: 10.1021/acs.inorgchem.8b01299 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Solutions with pH Greater than 8.5 and Na or Li. Solutions produced by dissolution of UO2 under Li- and Narich conditions exhibit similarities to those containing K, including corresponding Raman spectra that contain multiple signals. ESI-MS spectra of aqueous solutions with high concentrations of LiOH indicate that polydisperse clusters are present. Efforts to identify the changes in uranyl cluster species over time showed that U28 is initially formed upon dissolution of UO2 with high concentrations of LiOH. U24 is known to assemble over time in solutions initially containing U28;10 thus, Li-U24 crystals were synthesized and the mass spectrum of a solution produced by dissolving Li-U24 crystals is shown in Figure 10, together with the spectra corresponding to

Figure 11. ESI-MS spectra of an NaOH dissolution experiment from which Na-U24 crystals formed (top), spectra of dissolved Na-U20 crystals (middle), and simulated spectra for Na-U24 (bottom).

energetically favored crystalline product at high concentrations of NaOH. Efforts to promote crystallization of solutions produced by dissolution of UO2 into NaOH solutions with pH greater than 8.5 by slow evaporation yielded the novel compound Na60[(UO2)32(O2)40(OH)16][(UO2)28(O2)42](H2O)x (x ≈ 134) (Figure 12). A detailed description of the crystal

Figure 10. Li system, y = 34.103x−3896.1 (R2 = 0.9979); Na system, y = 10.826x+173.39 (R2 = 0.9941); K system, y = 6.5919x− 913.17 (R2 = 0.9944).

the current UO2 dissolution systems. The ESI-MS spectra do not correspond to U24 in solution on day 7 of the UO2 dissolution experiments. Previous studies showed that it may take greater than 15 days for U24 to assemble; thus, the mass spectra on day 7 may represent intermediate cluster species. Dissolution of UO2 into aqueous peroxide solutions with NaOH and pH greater than 8.5 produces solutions that yield Raman signals typical of uranyl peroxide clusters and the 2+ of triperoxide monomers, and these monomers v1(UO2) could be free in solution or encapsulated in uranyl peroxide cages. At high concentrations of NaOH, Na-U24 crystallized within days.39 The mass spectrum of a corresponding solution, collected on the same day crystals were harvested for singlecrystal X-ray diffraction, is shown in Figure 11. Simulated mass spectra and comparison to spectra produced by a solution into which Na-U20 crystals were dissolved demonstrate that the primary solution species is U20, not the expected Na-U24. Noncorrespondence of the dominant solution species and that crystallized from solution was previously found during the pHdriven disassembly and reassembly of uranyl peroxide pyrophosphate clusters. In that case, U32Pp16 was crystallized as Li32Na32[(UO2)32(O2)32(P2O7)16](H2O)275 from a solution containing only weak nuclear magnetic resonance chemical shifts corresponding to the U 32 Pp 16 cluster. Thus, Li32Na32[(UO2)32(O2)32(P2O7)16](H2O)275 is the thermodynamically favorable crystalline product but U32Pp16 is not a favorable solution species.40 Similarly, Na-U24 may occur as an

Figure 12. Polyhedral representation of the crystal structure of NaU28-U32 (cations and water molecules omitted for clarity) showing two layers. Layer A contains disordered U28 clusters, and layer B contains U32.

structure is given in the crystallographic section below. This is the second reported instance of cocrystallization of two uranyl peroxide clusters into a single compound and the first in which both clusters contain identical bridging ligands.40 It is particularly notable that dissolution of UO2 into aqueous peroxide solutions containing NaOH produced four different uranyl peroxide cages (U20, U24, U28, and U32) (Figure 4). None of these clusters encapsulate U, possibly indicating that smaller uranyl peroxide units are entirely consumed during reassembly and subsequent crystallization of clusters. In any case, it is clear that factors other than cation templating are affecting cluster formation and stability, and this G

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Inorganic Chemistry is consistent with the emerging conclusion that, although alkali cations are important in stabilizing intermediate species that ultimately form uranyl peroxide cage clusters, they are binding to the clusters in an adventitious fashion, rather than templating particular structural elements.4 The U20 and U28 cages have fullerene topologies consisting of pentagons and hexagons, whereas the U24 and U32 cages have topologies consisting of squares and hexagons, and U32 also contains pentagons. Each of these clusters form in solutions with high concentrations of NaOH and alkaline pH into which UO2 is dissolved, and free peroxide is consumed quickly, as indicated by Raman spectra acquired within minutes to hours after combining UO2 and the solutions. Under these conditions, hydroxyl bridges are likely to form.10 Likewise, in the KOH system, clusters encapsulate U in solutions depleted of peroxide. This suggests the importance of H2O2 availability and lack thereof in cluster speciation, as previously observed for Li-U28 decomposition and implied in the mechanism of decomposition via hydroxylation of peroxide bridges,10 and evidently has a critical role in the formation of a wide range of uranyl peroxide clusters. Crystal Structures of K-U28-U, K-U28, and Na-U28-U32 Compounds. Crystal structures of K-U28 and K-U28-U compounds contain U 2 8 clusters [(UO 2 ) 2 8 {(O 2 ), (OH)2}42]28− (Figures 4 and 9), formed by edge-shared UrO6 bipyramids (Ur = uranyl ion), which are topologically identical with those which were previously reported.8,10,41 The U28 clusters in both K-U28 and K-U28-U compounds are filled by disordered potassium atoms and water molecules. Adjacent U28 clusters are linked by extracluster potassium atoms and hydrogen bonds of water molecules. The main structural feature of K-U28-U is the presence of an additional fully occupied U site in the center of the U28 cluster (in the special position with local symmetry 4̅ 3m) with a disordered coordination environment. We propose that this is an orientationally disordered [Ur(O2)3]4− hexagonal bipyramid (Figure 9). The encapsulation of the U monomer in U28 increases the total negative charge, and as a result, there are 32 potassium atoms per formula unit. The corresponding formulas are K28[(UO2)28(O2)42](H2O)30 (Z = 4) and K32{[(UO2)(O2)3]@[(UO2)28(O2)30(OH)24]}(H2O)42 (Z = 2) for K-U28 and K-U28-U, respectively. The crystal structure of the Na-U28-U32 salt contains U32, [(UO2)32(O2)40(OH)16]32−, and U28, [(UO2)28(O2)42]28− (Figure 12), which form two types of tetragonal layers (A and B, Figure 12). These layers alternate along [001], with the cages in pseudo close packing (Figure 12). Layer A contains disordered U28 clusters corresponding to two orientations related by a noncrystallographic pseudotwin m* plane (Figure 13). Adjacent layers are linked via sodium atoms and water molecules. The refined crystal chemical formula of the Na-U28U32 compound is (Z = 2) Na60[(UO2)32(O2)40(OH)16][(UO2)28(O2)42](H2O)133.8.

Figure 13. Connectivity of U centers (spheres) and relationship between the two orientations of the disordered U28 clusters in NaU28-U32.

sufficient alkali cations, peroxide interaction with UO2 can result in the formation of studtite. The speciation of uranyl peroxide clusters in U-XOH-H2O2 systems is rather more complex than has been previously recognized. Here ESI-MS was shown to be powerful for observing and differentiating uranyl peroxide clusters in polydisperse solutions. Applying ESI-MS to the systems under study demonstrated that there is a compositiondependent transition from U28 clusters that do not and that do encapsulate uranyl monomers, which complicates the relations of uranyl peroxide cages and monomers and the distinction of encapsulation versus free uranyl peroxide monomers in solutions that contain clusters. The cluster polydispersity indicated by ESI-MS in all three alkali hydroxide systems correlates with the availability of peroxide in the system, suggesting that the formation of hydroxyl bridges is integral to the multiplicity of cluster topologies. Future studies should consider peroxide concentrations and limitations on dissolution of uranium phases. The prevalence of U28 in the KOH system, under conditions which are comparable to those that promote multiplicity of topologies in LiOH and NaOH systems, indicates that K favors and stabilizes this cluster, presumably because the K cation is an excellent bonding fit with uranyl ions that correspond to topological pentagons in its fullerene topology.11 On the other hand, the Na system shows that at least four cluster topologies containing four-, five-, and six-membered rings of polyhedra can form for systems containing the same alkali cation. The polydispersity of cluster solutions in the NaOH and LiOH systems indicates energetically complex cluster speciation and crystallization in simple U-XOH-H2O2 systems (X = alkali metal), which would benefit from thermodynamic and computational studies.

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CONCLUSIONS Owing to the formation of uranyl peroxide cage clusters upon dissolution of UO2 in alkali-rich peroxide aqueous solution, there is a 1:1 molar ratio of uranium to alkali cation in solution. The extent of cluster formation is governed by the quantity of alkali cation present, rather than the solution pH, which provides a novel way to control dissolution of uranium phases in solution to form uranyl peroxide clusters. In the absence of

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01299. Dissolution data, Raman spectra, and crystallographic data (PDF) H

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Inorganic Chemistry Accession Codes

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CCDC 1842556, 1842562, and 1842661 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.C.B.: [email protected]. ORCID

Sergey M. Aksenov: 0000-0003-1709-4798 Peter C. Burns: 0000-0002-2319-9628 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The initial stages of this research and the crystallographic analyses were 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). Later stages, including much of the synthesis and ESI-MS work, were supported by the Department of Energy, National Nuclear Security Administration, under Award Number DE-NA0003763. We thank the Center for Environmental Science and Technology, Materials Characterization Facility, and Mass Spectrometry and Proteomics Facility at the University of Notre Dame for the instrumentation used in this work.

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