Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Evidence of a Nonphotochemical Mechanism for the Solid-State Formation of Uranyl Peroxide Marie C. Kirkegaard,†,‡ Andrew Miskowiec,† Michael W. Ambrogio,† and Brian B. Anderson*,†,‡ †
Nuclear Security and Isotope Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, Tennessee 37996, United States
‡
S Supporting Information *
spectrometer (Renishaw) with an excitation wavelength of 785 nm. Multiple particles on each sample were tracked throughout the study, and the same region of each particle was analyzed each time (Figure S1). The initial Raman spectra of all three samples were characteristic of hydrated uranyl fluoride, with a uranyl stretching peak at 868 cm−1 and a shoulder at around 862 cm−1.4 No significant changes were noted throughout the course of the experiment in the control sample left at ambient RH. For the particles hydrated at 75% RH, the 868 cm−1 peak decreased in intensity, while additional peaks appeared at 845 and 820 cm−1 (Figure 1). After 190 days of equilibration at 75% RH, the NaCl-
ABSTRACT: We have demonstrated the solid-state formation of a uranyl peroxide (UP) species from hydrated uranyl fluoride via a uranyl hydroxide intermediate, the first observation of a UP species formed in a solid-state reaction. Water vapor pressure is shown to be a driving factor of both the loss of fluorine and the subsequent formation of peroxo units. We have ruled out a photochemical mechanism for formation of the UP species by demonstrating that the same reaction occurs in the dark. A radiolytic mechanism is unlikely because of the low radioactivity of the sample material, suggesting the existence of a novel UP formation mechanism. ranyl fluoride, the hydrolysis product of uranium hexafluoride (UF6), is a byproduct of the nuclear fuel cycle and of interest to the nuclear forensics community. At ambient conditions, uranyl fluoride exists as a hydrate of the form [(UO2F2)(H2O)]7·4H2O.1−4 It has been previously shown that mobile water exists within the pores of this structure and that the mobile water content increases upon exposure to elevated water vapor pressure.2,3,5 Thermal and spectroscopic studies have suggested that increased water vapor pressure may drive the formation of additional uranyl fluoride hydrate structures,6−13 whereas elemental analysis of aged samples has indicated a decrease in the F/U ratio over time.14 In the absence of conclusive structural data, however, there is significant uncertainty about what hydration products may exist. This uncertainty motivated a rigorous study of the chemical behavior of uranyl fluoride at elevated water vapor pressure. Uranyl fluoride powder used in this study was produced as described previously.2 Powder X-ray diffraction of the starting material was consistent with the uranyl fluoride hydrate, [(UO2F2)(H2O)]7·4H2O.5 Samples were prepared by depositing a small number of uranyl fluoride particles (10−100 μm in diameter) onto three adhesive carbon tabs on scanning electron microscopy (SEM) mounts. Two of these tabs were then suspended in capped glass vials above a NaCl-saturated salt solution in deionized water to achieve an environment of approximately 75% relative humidity (RH) at 20−23 °C (corresponding to a water vapor pressure of 1.75−2.11 kPa15). The third tab was suspended in an empty vial and thus exposed to ambient RH (approximately 45%−55% RH) as a control. Over the course of the experiment, each carbon tab was periodically removed from its vial and characterized using an inVia Raman
U
© XXXX American Chemical Society
Figure 1. Representative micro-Raman spectra of a uranyl fluoride particle ([(UO2F2)(H2O)]7·4H2O at t = 0) after increasing amounts of time spent equilibrating in a 75% RH environment and then a 100% RH environment after 190 days (20−23 °C). The lower-wavenumber region of each spectrum is scaled by a factor of 3 to show weaker intensity peaks more clearly. Vertical lines are at 136, 151, 188, 236, 257, 295, 348, 406, 460, 552, 820, 845, and 868 cm−1. All spectra are normalized to the Rayleigh line intensity (not shown) to allow for a better comparison of the peak intensities between different days. Received: March 2, 2018
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DOI: 10.1021/acs.inorgchem.8b00512 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
voltage of 10 kV and a beam current of 1.1 nA at the conclusion of the experiment showed a lack of measurable fluorine in both hydrated samples, indicating that a loss of fluorine occurs in the transition between β and γ. The presence of Raman-active modes in the 750−900 cm−1 region suggests that the uranyl ion remains intact throughout hydration, identifying γ and δ as uranyl hydroxyl/peroxo species rather than uranium oxides. Although the exact crystal structures of γ and δ cannot be determined using Raman spectroscopy, the Raman spectra of these species are characteristic of uranyl hydroxides and uranyl peroxides, respectively.18−21 The 845 cm−1 uranyl stretching mode in γ, along with a peak at 550 cm−1 that we attribute to a U−OH stretching mode, matches that of uranyl hydroxide species like metaschoepite, [(UO2)4(O)(OH)6]·5H2O.20 The pair of peaks in the 700−1000 cm−1 region in δ is suggestive of uranyl peroxide (UP) species such as the mineral studtite, [(UO2)(O2)(H2O)2]· 2H2O, which have a Raman-active peroxo stretching mode in this region in addition to the uranyl stretching mode. In fact, excellent agreement is noted between the full Raman spectra of δ and that of synthetic studtite, which has peaks at 68, 106, 135, 151, 163, 187, 237, 263, 294, 348, 406, 819, and 864 cm−1 (Figure S7),21 confirming the formation of a UP species with a structure at least similar to that of studtite. Figure 3 suggests that the formation of this UP species (δ) from uranyl fluoride occurs in two steps. Hydrated uranyl fluoride first undergoes a loss of fluorine to form a uranyl hydroxide species, which can be subsequently converted to a UP species upon further hydration. Kips et al. previously noted a reduction in the F/U ratio in uranyl fluoride particles exposed to high humidity,14 as well as the growth of a peak at 843 cm−1 in the Raman spectrum,12 but these observations were not correlated. By systematically tracking the same particles throughout our study, we confirm a complete loss of fluorine over the course of 30 days at 75% RH. We propose that, at elevated water vapor pressure, water molecules interact with the bridging fluorine ligands in [UO2F2(H2O)]7·4H2O, forming hydroxyl bridges and releasing HF molecules, which leave the crystal lattice. Although UP species have been previously synthesized by exposing solid uranium dioxide or uranyl hydroxide to dilute solutions of hydrogen peroxide (H2O2; 5 × 10−4−5 × 10−1 M),18,22−28 H2O2 was not added as a reagent in our system. Uranyl peroxides have also been observed to form in systems where H2O2 is produced in situ via water radiolysis, such as on the surface of fresh UO2 fuel pellets irradiated in water,25,27,29,30 on the surface of spent fuel pellets exposed to deionized water,24 and in irradiated samples of hydrated uranyl hydroxide.31 The radiolytic production of H2O2 has also been suggested to be responsible for the formation of the UP mineral studtite in nature. Although 238U and 235U have very long half-lives (4.5 billion and 700 million years, respectively), uraninite minerals can be relatively radioactive because of a buildup of radioactive daughter products. Assuming an α activity of 35000 dpm/cm2 at the surface of uraninite, Kubatko et al. calculated that the H2O2 concentration could reach 3.5 × 10−3 M in the layer of water between uraninite crystals in just 4 years.32 Our material was made with depleted uranium, however, and without a buildup of radioactive daughter products, has an α activity several orders of magnitude below that of uraninite. In addition, we observe the formation of UP on a time scale of weeks and not years. Thus, it seems unlikely that a radiolytic mechanism is responsible for the formation of a UP species in our system. In the absence of added H2O2 or significant α activity, the formation of multiple uranyl peroxide species has been
saturated salt solution in one of the vials was replaced with deionized water to further increase the water vapor pressure. This led to the rapid growth of peaks at 820 and 866 cm−1 and complete disappearance of the 845 cm −1 peak. Some deliquesence around the edges of the particles was noted in particles exposed to 100% RH (Figure S2), but otherwise no changes to the particle morphology were observed under the Raman objective lenses. The correlated growth of the 820 and 866 cm−1 peaks at 100% RH suggests that they are related to the same species and thus that this 866 cm−1 peak is distinct from the initial 868 cm−1 peak that is characteristic of hydrated uranyl fluoride. Singular-value decomposition of the data sets supported the presence of three distinct species, and multivariate curve resolution (MCR-ALS GUI 2.0 in Matlab16,17) was used to deconvolute the data sets into these three components. Spectra from the particles hydrated at both 75% and 100% RH were used to determine the spectral components (Figure 2) because of the more limited variation in
Figure 2. Deconvoluted spectral components. The lower-wavenumber region of each spectrum is scaled by a factor of 3 to show weakerintensity peaks more clearly. Vertical lines show peaks at 136, 151, 188, 236, 257, 295, 348, 406, 460, 552, 819, 845, and 868 cm−1.
the spectra of the particles only hydrated at 75% RH. Three species (identified as β, γ, and δ) are fairly well resolved, with some artifacts noted because of the limited number of spectra in the data set. Normalized concentration profiles (Figure 3) were generated for each particle using these identified components. It is clear that β corresponds to the initial uranyl fluoride hydrate, [UO2F2(H2O)]7·4H2O, whereas γ and δ are distinct species. Energy-dispersive X-ray (EDS) spectra (Figure S3) collected with a Carl Zeiss MERLIN VP-SEM (Carl Zeiss) and XFlash detector 5030 (Bruker Nano GmbH) using an accelerating
Figure 3. Normalized concentration gradients of the three component species as determined by MCR-ALS analysis for two representative particles. Left: Particle exposed to 75% RH throughout the experiment. Right: Particle exposed to 75% RH for 190 days and then 100% RH for the remainder of the experiment (shaded portion of the graph). B
DOI: 10.1021/acs.inorgchem.8b00512 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
nonphotochemical, nonradiolytic mechanism responsible for the formation of UP in our system explains the presence of UP in this case. In summary, we have demonstrated the formation of a UP species from hydrated uranyl fluoride through a uranyl hydroxide intermediate. This is the first demonstration of the formation of UP via a nonphotochemical mechanism that does not appear to be radiolytic, as well as the first direct observation of the formation of UP via a solid-state reaction. It is evident that water molecules play a mechanistic role in the formation of the UP species because the rate of formation increases with water vapor pressure. The mechanistic details remain unclear and deserve further study, especially because this mechanism could explain the unexpected formation of UP in other environments as well.
demonstrated via photochemical mechanisms induced by sunlight.33−44 Two different photochemical mechanisms have been presented. In the first, the uranyl ion is excited by incident light (*UO22+), reduced to UO2+ via H-abstraction or electron transfer, and then reoxidized by atmospheric oxygen to regenerate UO22+, producing H2O2 in the process (eq 1).42 Alternatively, uranyl peroxide species have been produced in the absence of atmospheric oxygen, with μ-peroxo bridges forming via the photochemical oxidative coupling of water or hydroxyl bridges.43,44 2UO2+ + O2 + 2H+ → 2UO2 2 + + H 2O2
(1)
To determine whether or not a photochemical reaction is responsible for the formation of UP in this system, the hydration experiment was replicated in the dark. Particles of uranyl fluoride ([(UO2F2)(H2O)]7·4H2O) were again deposited onto an adhesive carbon tab and exposed to the headspace of a NaClsaturated salt solution. The vial was wrapped in aluminum foil to prevent light exposure to the particles during the experiment. To accelerate the previously observed reaction, the sample was stored in an incubator at 35 °C. The initial Raman spectrum is compared to the spectra collected after 57 and 81 days of hydration in Figure 4. Peaks at 845 and 820 cm−1 once again
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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.8b00512. Microscopic images, energy dispersive x-ray spectroscopy data, additional Raman spectra and curve fits, and multivariate curve resolution data for additional particles (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Brian B. Anderson: 0000-0002-0675-9750 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Lee Trowbridge, Darrell Simmons, Hal Jennings, Leigh Martin, and Ashley Shields in the Nuclear Security and Isotope Technology Division at Oak Ridge National Laboratory for source material production and helpful discussions. The SEM and EDS analyses were conducted at the Center for Nanophase Materials Sciences, which is a Department of Energy Office of Science User Facility. This manuscript has been authored by UT-Battelle, LLC, under Contract DE-AC0500OR22725 with the U.S. Department of Energy (DOE). The U.S. government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. government purposes. The DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http:// energy.gov/downloads/doe-public-access-plan). This material is based upon work supported by the U.S. Department of Homeland Security under Grant 2012-DN-130-NF0001. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Department of Homeland Security.
Figure 4. Representative micro-Raman spectra of a uranyl fluoride particle ([(UO2F2)(H2O)]7·4H2O at t = 0) after increasing amounts of time spent equilibrating in a 75% RH environment at 35 °C in the dark. Spectra are presented in the same manner as that in Figure 1. After 57 and 81 days of hydration, it is clear that both of the hydration products observed previously (uranyl hydroxide and UP) are present.
appear in the uranyl stretching region upon hydration, and the peaks at 820 and 868 cm−1 increase in intensity relative to the peak at 845 cm−1 between 57 and 81 days. It is thus clear that both of the hydration products identified previously are present in the particles hydrated in the dark, ruling out a photochemical reaction mechanism for formation of the UP species. The formation of UP via a nonphotochemical, nonradiolytic mechanism has not been demonstrated previously. The existence of such a mechanism could explain previous unexpected observations of UP species, however. We note the recent identification by Wang et al. of studtite in addition to metaschoepite as a corrosion product found on depleted uranium ammunitions retrieved from soil in Bosnia and Herzegovina.45 The authors attributed the formation of studtite to the same radiolytic reaction mechanism responsible for the formation of studtite from natural uraninite. However, as noted previously, the α activity of depleted uranium is several orders of magnitude less than that of uraninite in secular equilibrium with its radioactive daughter products. It is possible instead that the same
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