Direct Determination of Oxidation States of Uranium in Mixed-Valent

Article pubs.acs.org/ac

Direct Determination of Oxidation States of Uranium in Mixed-Valent Uranium Oxides Using Total Reflection X‑ray Fluorescence X‑ray Absorption Near-Edge Spectroscopy Kaushik Sanyal,†,§ Ajay Khooha,‡ Gangadhar Das,‡,§ M. K. Tiwari,‡,§ and N. L. Misra*,†,§ †

Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Synchrotrons Utilisation Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India § Homi Bhabha National Institute, Mumbai 400094, India ‡

ABSTRACT: Total reflection X-ray fluorescence (TXRF)-based X-ray absorption near-edge spectroscopy has been used to determine the oxidation state of uranium in mixed-valent U3O8 and U3O7 uranium oxides. The TXRF spectra of the compounds were measured using variable X-ray energies in the vicinity of the U L3 edge in the TXRF excitation mode at the microfocus beamline of the Indus-2 synchrotron facility. The TXRF-based X-ray absorption near-edge spectroscopy (TXRF-XANES) spectra were deduced from the emission spectra measured using the energies below and above the U L3 edge in the XANES region. The data processing using TXRF-XANES spectra of U(IV), U(V), and U(VI) standard compounds revealed that U present in U3O8 is a mixture of U(V) and U(VI), whereas U in U3O7 is mixture of U(IV) and U(VI). The results obtained in this study are similar to that reported in literature using the U M edge. The present study has demonstrated the possibility of application of TXRF for the oxidation state determination and elemental speciation of radioactive substances in a nondestructive manner with very small amount of sample requirement.

U

catalytic and magnetic properties because of the presence of mixed valency of uranium in these compounds.12−14 There are conflicting reports on the oxidation states of uranium in U3O8.15,16 The oxidation state of uranium in U3O8 can be represented in two ways keeping the charge balance neutrality as shown below:

ranium oxides are important nuclear and technological materials.1 UO2 has a fluorite structure.2 After oxidation under different conditions it gets converted to different oxides, e.g., U4O9, U3O7, U3O8, etc., under different oxidation potentials.3 Knowledge about the oxidation states of uranium in theses oxides and other compounds of uranium is very important to understand the properties of these compounds, essential to use them for different applications, e.g., fuel production, spent fuel reprocessing, magnetic and electrical properties, etc. It is also helpful in understanding its behavior during reactor operating conditions as well as during spent fuel reprocessing.4−7 In addition, it is helpful to understand the role of such uranium oxides as catalyst and in environmental studies.8 U3O8 is a stable oxide of uranium and exists mainly in three phases. The room-temperature phase has orthorhombic structure. It has two more phases at high temperature having hexagonal and tetragonal lattice.9−11 U3O7 is another oxide of uranium which is metastable. Both of these oxides, U3O8 and U3O7, have uranium in mixed-valent states. Oxides of uranium having uranium in mixed-valent states are reported to have © XXXX American Chemical Society

U3O8 = UVIU2 VO8

or

U3O8 = U2 VIUIVO8

Similarly for U3O7 also there are two probable combinations of oxidation states: U3O7 = U2 IVUVIO7

or

U3O7 = U2 VUIVO7

X-ray photoelectron spectroscopy (XPS) is a well-known frequently used technique for the determination of elemental oxidation states. However, for actinides, especially when these Received: October 7, 2016 Accepted: November 21, 2016 Published: November 21, 2016 A

DOI: 10.1021/acs.analchem.6b03945 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

preparation, a very small amount of sample may be rubbed on TXRF sample support,30 and this specimen may be probed for TXRF-EXAFS or XANES measurements. Recently, a study for finding out the uranium oxidation state in U4O9 and U3O8 has been reported using the U M5 edge.8 In this study about 10 mg of the sample was taken and pelletized with B4N for such measurements. The study has concluded that U is present in a mixed state of U(V) and U(VI) in U3O8 and U(IV) and U(V) in U4O9. The TXRF-EXAFS and TXRF-XANES measurements are feasible at the microfocus beamline BL-16 of the Indus-2 synchrotron light source Indore, India. It is reported that direct information about the sample can be obtained using the U M edge. However, the microfocus beamline (BL-16) at Indus-2 is not well-suited for low-energy X-rays. For speciation studies, it is very important to avoid any chemical transformation of the element being studied. Since this type of study does not require any elaborate sample preparation and requires rubbing the sample on TXRF supports, it will be very advantageous for such speciation studies also. Though such studies are reported in literature for the speciation of arsenic in cucumber xylem sap and speciation of copper and zinc in aerosol samples using TXRF-XANES,31,32 there is not any report in the literature on the speciation of nuclear materials using TXRF-XANES. Due to the above features of TXRF, a systematic study to explore the possibility of TXRF-XANES for the oxidation state determination of uranium in U3O8 and U3O7 using the U L3 edge (17.16 keV) was initiated. The results of this study are reported in this manuscript.

are in mixed-valent states, e.g., U in U3O8, U4O9, or U3O7, it is very difficult to determine the oxidation states in such compounds because the chemical shift of the main U 4f lines is very small for uranium.17 Similar is the case for other actinides. Ultraviolet and visible photoelectron spectroscopy (UPS) is another technique which can be used for such oxidation state determinations. However, still it is not so popular and can provide only 5f occupancies, and it is very hard to distinguish between the uranium present in different species by this technique.18 The synchrotron-based X-ray absorption near-edge spectroscopy (XANES) technique is a very reliable technique for such oxidation state determinations. The XANES spectrum is limited within 50 eV of the absorption edge of the element in the X-ray absorption fine structure (XAFS) spectrum. The XAFS spectrum arises upon the absorption of the X-ray photon, emission of the photoelectron, and thereby photoelectron scattering of this electron by the surrounding atoms in the sample. In the XANES region, the kinetic energy of the photoelectron is small (within 330 eV), and this makes it very sensitive to the surrounding potential and bonding effects. The details of the XANES spectra are reported in several references.19 Micro-XANES is another technique which can perform speciation of individual particles in a nondestructive manner. In literature several reports are available for the speciation of nuclear materials in soil samples, sea sediments, aerosol particles, and individual particles containing uranium and plutonium using micro-XANES.20−23 For preparation of the micro-XANES specimen the samples were dispersed in nhexane containing rubber cement.20 The oxidation states of U and Pu in environmental radioactive particles are reported using micro-XANES.24 In normal XANES measurements, the sample amount required is in the milligram level in the form of boric acid/ cellulose pellets or supported on membrane supports. Such an approach for nuclear materials, especially Pu, Am, etc. based materials, is not desirable as such a large sample amount poses a radiation hazard risk as well as produces more analytical waste, disposal of which is a difficult task. In recent years, several studies have been reported in literature on total reflection X-ray fluorescence (TXRF), and it has been found to be a favorable technique for the elemental determination in radioactive nuclear materials.25,26 TXRF is a variant of the energy-dispersive X-ray fluorescence (EDXRF) technique and is progressively finding its application in different scientific and technological areas.27 Different features of this technique, especially the requirement of only a few nanograms of analyte and multielemental analytical capability, are wellsuited for elemental characterization of radioactive nuclear materials.28 Due to the very small amount of sample required, the TXRF study of nuclear materials poses comparatively less radiation hazard to the operator and generates a small amount of radioactive analytical waste. Extended X-ray absorption fine structure (EXAFS) and XANES studies are possible in TXRF mode also using a variable energy excitation of the sample and measuring the intensity of the characteristic X-ray line of the analyte element below and above the absorption edge.29 This approach may reduce the sample amount drastically compared to that required in normal XANES and thus shall be very much suited for radioactive materials. In addition the sample can be studied as such after sticking a few particles on TXRF supports without any processing, e.g., pelletization with boric acid or cellulose or dispersion in hexane. For an effective sample

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EXPERIMENTAL SECTION Sample Preparation. The samples and standards prepared earlier and available in our laboratory were used for such studies. U3O8 and U3O7 were prepared from nuclear grade UO2. U3O8 (α phase) was prepared by heating UO2 in a furnace at 600 °C for about 24 h,33 and U3O7 (β phase) was prepared by careful heating of UO2 in air atmosphere at around 150 °C for 24 h.34 Similarly UO3, TlUO3, and UO2 available in our laboratory were used as standards for U(VI), U(V), and U(IV), respectively. For UO3 preparation, U3O8 available in our laboratory was dissolved in nitric acid and NH3 solution was added to this solution to precipitate (NH4)2U2O7 which was heated in air atmosphere at 300 °C to get UO3. TlUO3 was prepared following the procedure given in literature.35 TXRF-XANES Measurements. For TXRF-XANES measurements, a few tiny particles of the samples to be investigated were taken directly in powder form with the help of a micropipette tip on quartz sample supports. The samples were spread uniformly on the TXRF supports. During such sample preparation, a few tiny particles got stuck with the support. The specimen thus prepared was dabbed vertically on a clean glass support so that any loose particle comes out. This type of specimen preparation brings a fresh surface of the sample on the support and avoids any error due to surface oxidation. These specimens were presented for TXRF-XANES measurements. U L3 edge XANES measurements in fluorescence mode and total reflection geometry were carried out at the microfocus Xray Fluorescence beamline (BL-16) of Indus-2 synchrotron facility.36 A Si(111) double-crystal monochromator was used for selecting different X-ray energies of the exciting beam from the continuous synchrotron X-ray spectrum. The beam spot size is 10 mm (H) × 0.1 mm (V). The energy resolution (ΔE/ E) of the monochromator was ∼10−4, which yielded an overall B


Article


Figure 1. Totally reflected beam (top) and direct beam (bottom) shown by the CCD camera indicating that TXRF conditions are satisfied during the sample measurements.

fluorescent screen and charge-coupled device (CCD). The totally reflected and direct beams in TXRF conditions are shown along with the experimental hutch of the beamline in Figure 1. The TXRF measurements were made by utilizing the anisotropic nature of the scattered synchrotron radiation in the horizontal detection geometry. Though such arrangement increases the path length of emitted X-rays coming to the detector and thereby reduces the solid angle of the detector, as explained by Pepponi et. al.,37 it does not change the nature of the XANES spectrum. This geometry offers a significant advantage of reduced spectral background during the TXRF measurements and thus an improved signal-to-background ratio for a fluorescence peak.38 Moreover, as we have measured U Lα X-rays (13.61 keV), the absorption effect of the sample− detector air column path (∼20 mm) would not be significant. A TXRF spectrum before starting the XANES measurement of each sample was measured. Such TXRF spectrum of U3O8 for a spectra acquisition time of 10 s is shown in Figure 2. From Figure 2 we can see that only a few nanograms of sample is giving a TXRF spectrum of reasonable elemental X-ray line intensities. The region of interest (ROI) for the U Lα line for each spectrum was fixed and averaged to get the U Lα ROI counts at each energy. With the help of the initial X-ray intensity I0, the mass absorption coefficients at each energy were calculated to generate a mass absorption versus energy

energy resolution of ∼2 eV. All the measurements were carried out in air atmosphere at room-temperature conditions. The TXRF-XANES measurements on each sample were carried out in steps of 1 eV in the range of 17.027−17.260 keV around the U L3 edge (17.16 keV) with a spectrum acquisition time of 4 s. The intensity of exciting radiation (I0) was determined using an ionization chamber before the sample. The fluorescence X-rays emitted from the samples were detected using a Vortex energydispersive spectroscopy detector (SII NanoTechnology, U.S.A.). Three spectra were measured at each step of energy for each sample. These spectra were merged in data treatment procedure. For energy calibration, a standard Zr (Zr K edge 17.99 keV) foil was used. The XANES measurements of the standard Zr foil was carried out in EDXRF mode. A linear combination analysis of the XANES spectrum was carried out with the software ATHENA, which is included in the IFEFFIT program package. Fitting was performed over an energy range of −30 to +30 eV about the L3 edge energy of uranium.

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RESULTS AND DISCUSSION Before making any XANES measurements for a sample, it was ascertained that the TXRF conditions were satisfied and X-rays are falling on the sample at an angle less than the critical angle. The totally reflected and direct beams were detected using a C


Article


derivative of the XANES spectrum is also shown in Figure 4. From the figure we can see that the edge position of U3O8 is

Figure 2. TXRF spectrum of U3O8 specimen prepared for TXRFXANES measurements.

Figure 4. First derivative of the TXRF-XANES spectra of different uranium oxides showing shift in the edge position.

situated in between the edge position of TlUO3 [U(V)] and UO3 [U(VI)], which gives an impression that U3O8 is a mixed oxide of U(V) and U(VI). The edge energy values for all these compounds are summarized in Table 1. In order to calculate

plot. Finally, the XANES spectra were normalized. In such TXRF measurements self-absorption by uranium and damping of white line may occur as reported for As by Meirer et al.39 Their investigations show that the damping of the white line is significant only when the As amount is close to/or above 100 ng. However, Tiwari et al. have reported that not only the sample amount but particle size also plays an important role during the consideration of sample absorption effect in the TXRF geometry.40,41 It was shown that particle size above 1 μm only severely affects the emitted TXRF fluoresce intensity. In one of our earlier studies30 on the determination of U and Th in their mixed oxide pellets using TXRF, we have shown that the particle size of the analytes that stick on the TXRF sample support during the rubbing process ranges between 200 and 500 nm. In this study also we have used similar sample preparation, and therefore, such small particles shall not pose much absorption effect. The normalized TXRF-XANES spectra of all the compounds UO2, TlUO3, U3O8, and UO3 are shown in Figure 3. From the figure we can see that, as the oxidation state of uranium increases from 4 to 6 from UO2 to UO3, the edge position of the U L3 edges in the XANES spectra are shifting toward the higher energy side in an obvious manner. The exact edge energy positions were obtained from the maxima of the first derivative of the XANES spectrum. The

Table 1. U L3 Edge Energy Values in Different Oxides of Uranium as Obtained from the Maxima of the Second Derivative of Their TXRF-XANES Spectra uranium oxides

edge position (eV)

UO2 TlUO3 U3O8 UO3

17161 17162 17163 17165

the percentage of different oxidation states of U present in U3O8, linear combination analysis of the XANES spectrum was carried out with the software ATHENA with UO2, TlUO3, and UO3 as reference compounds for U(IV), U(V), and U(VI), respectively. During spectra fitting we considered two combinations of oxidation states of U: (1) U(IV) and U(VI) and (2) U(V) and U(VI). The best linear fitting of these two combinations by using ATHENA is shown in Figure 5, top and bottom panels, respectively. From these two figures it is clear that the fitting results with the combinations of U(V) and U(VI) are much better than those obtained with the U(IV) and U(VI) combination. From the above fitting, we could get U(V) = 70%, U(VI) = 30%; compared to the expected values of 66% and 34%, respectively [when we consider U3O8 is composed of U(V) and U(VI)], these are in good agreement. These values are also in good agreement with those reported by Kvashnina et al.8 who concluded the presence of U(V) and U(VI) and absence of U(IV) in U3O8. These authors used the U M edge as a probe for U in their XANES study, as most of the properties of U compounds originate from the localized 5f states and can be probed using the U M (3d3 = 2; 5 = 2) absorption edges directly. However, probing these edges is difficult due to several reasons, e.g., significant X-ray absorption by air, low energy difference, between several U M edges, etc. Moreover, presently the microfocus beamline at Indus-2 is not equipped for measurement of low-energy X-rays in the region of U M edges. Due to these reasons we have performed the experiments at the U L3 edge in TXRF-XANES mode and compared our results

Figure 3. TXRF-XANES spectra of U3O8 along with standard compounds. D


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Figure 6. Linear combination fitting of TXRF-XANES spectra of U3O7 using two different oxidation state combinations: U(IV) + U(V) and U(IV) + U(VI).

Figure 5. Linear combination fitting of TXRF-XANES spectra of U3O8 using two different oxidation state combinations: U(V) + U(VI) and U(IV) + U(VI).

Table 3. Relative Amounts of U(IV), U(V) and U(VI) Obtained from the Linear Combination Fit of the TXRFXANES Spectra of U3O7

with those reported by Kvashnina et al. and theoretical values as well as other literature-reported studies.8,42 The relative amounts of U(V) and U(VI) present in U3O8 from the linear combination fit of the TXRF-XANES spectra of U3O8 are shown in Table 2. The quality of fitting can be indicated by the parameters like χ2, reduced χ2, and R-factors. The values of these parameters, shown in the table, suggest good fitting of the spectra. Similarly, a linear combination fit for U3O7 was also carried out. The linear fitting of these two combinations, (1) U(VI) and U(IV) and (2) U(V) and U(IV), by using ATHENA is shown in Figure 6, top and bottom panels, respectively. The best fit data could be obtained when we took the combination of U(IV) and U(VI). The weight fractions calculated from the fitting and the fitting parameters are shown in Table 3. From this table we can see that the weight fractions of U(IV) and U(VI) as 70% and 30%, obtained from the fitted spectra, are also very close to the theoretically predicted values (66% and 34%, respectively), when we consider U3O7 as composed of U(IV) and U(VI) These agreements in experimentally determined values, theoretical values, and literature-reported values suggest the reliability of TXRF-XANES study using the U L3 edge for oxidation state determinations in mixed-valent compounds. Though a similar study, with the U M edge and in normal XANES measurement conditions, is also reported in literature, it has certain difficulties as described earlier. Moreover, in this study as reported by us the sample requirement is nanogram level only, and thereby, it is well suited for radioactive materials. Since the results obtained in this study are similar to those of

U3O7 oxidation state combinations U(IV)−(VI)

percentage % U(IV) 70 ± 2 % U(IV) 98 ± 4

U(IV)−(V)

% U(VI) 30 % U(V) 2

R-factor

χ2

reduced χ2

0.0001

0.007

0.0001

0.0006

0.030

0.0004

XANES study with the U M edge as probe, which is supposed to give more direct information, the probe of mixed-valent compounds using TXRF-XANES for U using the U L3 edge is reliable with the additional advantage of ease of experiment and small sample amount.

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CONCLUSIONS The oxidation states of U in mixed-valent oxides U3O8 and U3O7 could be successfully determined in a simple and direct manner probing the U L3 edge by TXRF-XANES. The study concluded that U3O8 contains uranium in U(V) and U(VI) states. The relative amounts of these oxidation states of uranium in U3O8 are 70% and 30%, respectively. These values are in agreement with the theoretical values of 66% and 34%, respectively. These results are in also agreement with the literature-reported data using M edge measurements. Similarly for U3O7, uranium was found to be in mixed-valent states of U(IV) and U(VI) (70% and 30%, respectively). These values are also in agreement with the theoretically calculated values.

Table 2. Relative Amounts of U(IV), U(V) and U(VI) Obtained from the Linear Combination Fit of the TXRF-XANES Spectra of U3O8 U3O8 oxidation state combinations U(V)−(VI) U(IV)−(VI)

percentage % U(V) 70 ± 5 % U(IV) 98 ± 4

% U(VI) 30 % U(VI) 2 E

R-factor

χ2

reduced χ2

0.000245

0.0167

0.0003

0.001

0.090

0.001


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The present study has demonstrated that TXRF-XANES is a reliable technique giving similar results as XANES spectroscopy. The added advantages for radioactive materials are that it requires sample in nanogram levels only with almost no sample preparation. These features are well-suited for other materials also, especially surface-reactive materials, as the samples can be probed fast without much sample preparation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Phone: 0091-22-25594565. Fax: 0091-22-25505151. ORCID

N. L. Misra: 0000-0002-8483-4912 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Dr. B. S. Tomar, Director of the Radiochemistry and Isotope group, and Dr. S. Kannan for their keen interest in this work. They express their sincere thanks to Dr. S. K. Sali for providing the U3O7 sample for this work and Mr. Ajit Kumar Singh for helping in the TXRF-XANES measurements.

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REFERENCES

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Direct Determination of Oxidation States of Uranium in Mixed-Valent

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