Evaluating Best Practices in Raman Spectral Analysis for Uranium

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Evaluating Best Practices in Raman Spectral Analysis for Uranium Speciation and Relative Abundance in Aqueous Solutions Grace Lu, Tori Z. Forbes, and Amanda J Haes Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03038 • Publication Date (Web): 25 Nov 2015 Downloaded from http://pubs.acs.org on November 26, 2015

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Analytical Chemistry

Evaluating Best Practices in Raman Spectral Analysis for Uranium Speciation and Relative Abundance in Aqueous Solutions Grace Lu, Tori Z. Forbes*, and Amanda J. Haes* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242 Email: [email protected], [email protected]; Tel: 319-384-1320, 319-384-3695

ABSTRACT

Raman spectroscopy is emerging as a powerful tool for identifying hexavalent uranium speciation in situ; however, there is no straight forward protocol for identifying uranyl species in solution. Herein, uranyl samples are evaluated using Raman spectroscopy, and speciation is monitored at various solution pH values and anion compositions. Spectral quality is evaluated using two Raman excitation wavelengths (532 and 785 nm) as these are critical for maximizing signal to noise and minimizing background from fluorescent uranyl species. The Raman vibrational frequency of uranyl shifts according to the identity of the coordinating ions within the equatorial plane and/or solution pH; therefore, spectral barcode analysis and rigorous peak fitting methods are developed that allow accurate and routine uranium species identification. All in all,

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this user’s guide is expected to provide a user-friendly, straight-forward approach for uranium species identification using Raman spectroscopy.

INTRODUCTION A wide range of spectroscopic techniques are employed for the identification of hexavalent uranium (U(VI)) in solution1-4 and solid phase,5,6 but Raman spectroscopy is increasingly being utilized for both species identification7 and quantitative analysis.8,9 Alpha spectrometry and mass spectrometry10 are considered the gold standard for quantitative analysis of U11 and provide information on isotopic signatures for nuclear forensics applications12-15. Pretreatment steps for both methods can be time consuming as the typical sample preparation includes dissolution of the solid phase in acid, followed by ion exchange,16 liquid-liquid extraction,17 or extraction chromatography.18,19 Optical spectroscopy methods including fluorescence, FTIR, and UV, do provide some additional chemical information;20-23 however, the resulting spectra contain broad overlapping features from various sample components. This limitation prevents positive identification of U species using UV spectroscopy and results in species validation if species also exhibit unique fluorescence spectra.20 In the hexavalent state, uranium forms two strong bonds with oxygen atoms resulting in the formation of the uranyl cation (UO2)2+. Uranyl cations exhibit strong, Raman-active symmetric stretches (ν1) centered between 750 and 900 cm-1 and are sensitive to the coordinating ligands. Location of this vibrational frequency provides a basis for positive identification of uranyl species, and proper spectral analysis can lead to quantitative assessment of the sample. The earliest characterization of a uranyl salt by Raman spectroscopy was performed by Conn and Wu24 in 1938, and this method continues to be used for investigations of both solid materials and solutions. Novel U(VI) materials are now routinely characterized by Raman spectroscopy25-


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through the position of the ν1 band, and several studies utilized this vibrational frequency to

calculate the force constants of a series of uranyl chloride complexes.32,33 This technique is also used in nuclear forensic applications to determine the chemical signature of uranium ore concentrates.34 Previously reported solution studies mainly focus on spectral assignments for identification of major species in solution35-41 with a few also focusing on the thermodynamics of uranyl complex formation.8,9,42 While the use of Raman spectroscopy to investigate uranyl materials and solutions is increasing substantially, there is limited information on using this technique beyond extracting speciation assignments through simple spectral feature analysis. Basic information, such as the importance of excitation wavelength selection in Raman spectroscopy was reported,7,43,44 but the impact of fluorescence interference on uranyl speciation identification using Raman spectroscopy was not demonstrated. In addition, specific guidance for Raman spectral analysis is significantly lacking in terms of identification of appropriate spectral ranges, peak positions, and peak widths for reproducible spectral analysis and uranyl speciation assignments.8,44,45 A more detailed description of Raman spectral analysis was recently reported, but the chosen boundaries for vibrational mode peak widths are debatable;9 therefore, practical guidelines for Raman spectral analysis for identifying uranyl species, particularly in aqueous solutions, are warranted. In this paper, a rigorous approach to uranyl species identification and abundance using Raman spectroscopy for samples containing the uranyl moiety is presented by considering Raman excitation wavelength, impact of coordinating ligands, and peak fitting for Raman spectral analysis that includes the use of spectral second derivative analysis to identify where significant U vibrational features are centered. The goal of this study is to provide a practical guide for uranyl species identification using Raman spectroscopy. First, by comparing and contrasting


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Raman spectra of uranyl samples in acidic, basic, and neutral solutions collected using excitation wavelengths at 532 and 785 nm, spectral analysis reveals that fluorescent U species can interfere with species identification. Next, uranyl samples containing dissolved carbonate are used to develop Raman spectral analysis guidelines and demonstrate how these can be used for userindependent species identification and relative abundance determination. The developed protocol provides guidelines for applying Raman spectroscopy for samples at different pH values and containing NO3-, SO42-, or Cl- ions. Finally, the spectral analysis results are validated using theoretical models and previously reported vibrational band assignments. By monitoring these samples over time, uranyl species are successfully identified in samples not yet at equilibrium. In the future, this protocol could be used and expanded for additional uranyl species of interest and in complex sample matrices for near real-time detection and monitoring in contaminated systems.

EXPERIMENTAL METHODS Sample Preparation. Uranium was purchased from Flinn Scientific, and all other chemicals were purchased from Sigma-Aldrich (St. Louis, MO). A stock solution of 0.1 M UO22+(aq) was prepared

from

UO2(NO3)2·6H2O(s) and

diluted

to

0.03

M

in

water.

CAUTION:

(UO2)(NO3)2·6H2O contains radioactive 238U, which is an alpha emitter, and like all radioactive materials, must be handled with care. These experiments were conducted by trained personnel in a licensed research facility with special precautions taken towards the handling, monitoring, and disposal of radioactive materials. Unless otherwise noted, samples were prepared and allowed to equilibrate for 30 minutes prior to analysis. Samples containing additional ions were prepared in two steps by first mixing 0.1 M UO22+(aq) with 0.5 M Na2CO3(aq) so that the final [CO32-]/[UO22+]


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was 3.5. Second, HNO3(l), HCl(l), or H2SO4(l) was added to achieve a pH of 3 and diluted so that final concentrations of 0.03 M UO22+(aq) and 0.105 M Na2CO3(aq) were achieved. Samples were also prepared at various carbonate concentrations with Na2CO3(aq) solutions at 7.5, 15, 30, and 300 mM mixed with 0.03 M UO22+(aq). Concentrations of sulfuric acid equal to 9.5, 17.5, 34, and 279 mM, respectively, are used to acidify these solutions to a final pH of 3. The impact of pH on speciation was evaluated by preparing samples with 0.03 M UO22+(aq) and 0.105 M Na2CO3(aq) and adjusting the final solution pH with HNO3. In all cases, pH verification was performed using pH paper (Whatman, Maidstone, England), which was pH meter validated, for repeated and small-volume measurements. Chemical equilibrium diagrams were constructed using Hydra and Medusa software46 using the solution composition for speciation prediction and validation. All stability constants are the defaults with the exceptions of UO2(SO4)34- (3.02).47 Newly reported stability constants for the hydrolysis products48 were considered but did not significantly impact the results in this study. Ionic strengths for solutions were considered by performing iterative calculations (assuming U speciation) until ionic strength converged. Raman Spectroscopy and Spectral Analysis. Raman spectra were collected using either an ExamineR532 (DeltaNu) module mounted on an Olympus IX71 inverted microscope or an iRaman BWTEK system (excitation wavelengths (λex) = 532 and 785 nm, respectively). Samples were prepared and evaluated in either glass vials or quartz cuvettes. Integration times (tint) for spectra collected using λex = 532 and 785 nm were 5 and 20 sec, respectively. Laser powers (P) were 10-11 and 80 mW, respectively. All reported Raman spectra represent an average of 10 spectra, which were background corrected using a blank (water) Raman spectrum, and intensities were corrected by dividing spectral counts by laser power and integration time. To compare


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spectral intensities collected on the various Raman detectors, an ethanol standard was used so that instrument correction factors (from detectors and excitation wavelengths) were estimated. Detailed step by step instructions for spectral analysis are included in Supplemental Information. Briefly, Raman bands from the symmetric stretching mode of uranyl (UO2)2+ were analyzed using a peak analysis protocol (Origin 9.1) using data between 770 – 910 cm-1. A baseline was established between these two vibrational frequencies, and a background slope was determined. Next, the second derivative of this spectral window were smoothed using the Savitsky-Golay filter (8 points) and an 8% threshold using MATLAB (R2015a) were used to identify possible vibrational modes as shown in Figure S1.49 Because detector broadening50 limits vibrational frequency band widths in these experimental setups, Gaussian functions were used for spectral analysis.42,51 Assuming all U species exhibit similar Raman cross sections,8,42 the relative abundance of each species can be estimated using the peak height (i.e., Raman intensity) of the Gaussian functions. Spectral noise was estimated as the standard deviation of the baseline from 1150-1250 cm-1. Vibrational features were only considered significant and reported as a signal (and species) if a signal to noise ratio of 3+ was observed. Species were determined by setting vibrational frequency windows expected from the literature (Table 1) and FWHM (Γ) values of 13-15 cm-1 for all species except UO2(SO4)34-, UO2(CO3)22-, and UO2(CO3)34- in which a Γ ranging from 16-20 cm-1 was used. RESULTS AND DISCUSSION Simplifying Raman Spectra Complexity of U Samples for Speciation Determination. Spectroscopic identification of U(VI) species in aqueous samples depends on many factors including fluorescence, which is a common spectroscopic signal that arises from select U(VI) complexes20-22,52-54 and can interfere with Raman spectral analysis. Polynuclear hydrolysis


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products (UO2)2(OH)22+ and (UO2)3(OH)5+ are dominant species that form at pH ~6 and emit 10 to 20 times higher fluorescence emission between the wavelength range of 450-600 nm. This is observed in Figure 1A, which demonstrates that the hydrolyzed uranyl species present at pH 6 exhibit strong fluorescence emission versus uranyl (UO22+) species52 present at pH 3 and 11. If fluorescence emission overlaps with the wavelength range where the Stokes Raman signals are to be detected, the radiated fluorescence is collected along with Raman vibrational signatures thus reducing spectral quality. To evaluate Raman spectral interferences from fluorescent uranyl species, three samples at pH 3, 6, and 11 containing 30 mM UO22+(aq) and 105 mM Na2CO3 were prepared then evaluated using excitation wavelengths at 532 and 785 nm. Sample conditions mimicking naturally-occurring uranium samples which often contain carbonate are selected and often contain up to three times molar excess of carbonate vs. U concentration.53 These data

Figure 1. (A) Fluorescence emission spectra of 30 mM UO22+(aq) and 105 mM Na2CO3 at

are summarized in Figures 1B-1D, and pH (1) 3, (2) 6, and (3) 11. Representative several observations are noted. First, all Raman spectra of 30 mM UO22+(aq) and 105 spectra reveal characteristic Raman features mM Na2CO3 at pH (B) 3, (C) 6, and (D) 11 consistent with unique uranyl species with vibrational frequencies centered at 871, 837, and 814 cm-1 for samples prepared at a pH of

collected using λex = (1) 532 and (2) 785 nm. Each Raman spectrum was collected using 10 averages, tint = 5 and 20 sec, P = 10-11 and 80

3 (Figure 1B), 6 (Figure 1C), and 11 (Figure mW for λmax = 532 and 785 nm, respectively.


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1D), respectively (see Table 1 for vibrational frequency assignments). All of these vibrational modes arise from the symmetric UO22+ stretching mode that is sensitive to coordinating ligand identity. As expected, these vibrational frequencies are independent of excitation wavelength. Second, the Raman intensities from a particular U(VI) sample are larger when the 532 nm excitation wavelength is used. This is consistent with Raman scattering cross sections being inversely proportional to the fourth power of the excitation wavelength and also depends on the detector efficiency of the various Raman systems. Third, spectral background depends on excitation wavelength. For instance, the Raman spectra collected using 532 nm excitation exhibit larger backgrounds underneath the Raman signals, which arise from the fluorescence signal from the hydrolyzed species present in solution. This broad fluorescence emission is easily observed in the U sample prepared at pH 6 (Figure 1C-1). Because this background feature can interfere with Raman spectral analysis (i.e., identifying the vibrational frequencies and speciation assignments), choosing excitation wavelengths that do not produce fluorescence emission is desired. In contrast, Raman spectra collected using the 785 nm excitation wavelength are free of fluorescence, which produce quality Raman spectra for universal U species identification. As a result, Raman spectra collected using the 785 nm excitation wavelength is used for all subsequent studies. Although the vibrational bands of uranyl species are narrow, most Raman-active modes are centered in a small spectral window ranging from 750 – 900 cm-1 (Table 1), and many of these species co-exist in solution. As a result, speciation assignments using Raman spectroscopy are limited by spectral complexity. To maximize uranyl species identification using Raman spectroscopy, spectral deconvolution is achieved using barcode analysis and multi-peak spectral fitting. To extract unique vibrational modes from Raman spectra, spectra are evaluated in


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multiple steps. Briefly, identification of U species from Raman spectra were determined by first, calculating a second derivative of the Raman spectrum then applying a smoothing function and threshold value (8%) as previously described.49 Examples of this barcode analysis are shown in Figures S1 and 3. Barcodes are generated by converting signals above and below the threshold value in the second derivative spectra into zeros and ones, respectively. Features with vibrational modes assigned values of one are used for peak fitting. Previously, Raman spectral features were fit using Gaussian, Lorentzian, and mixed GaussianLorentzian functions.50 In general, Gaussian line shapes can be used to accurately model Ramanactive vibrational modes when instrumental

broadening and sample inhomogeneity limits

vibrational line widths.42,51 To apply spectral peak fitting analysis for U(VI) species identification, the Gaussian function, Iν=

1

e- ν-µ σ√2π

2

/(2σ2 )

(eq 1)

is used where I( ) is peak intensity, is the vibrational frequency, µ is maximum vibrational frequency for a given vibrational mode, and the vibrational mode full width at half maximum (Γ) is equal to 22 ∗ ln 2 where is the standard deviation. To develop a protocol for U species identification, known literature vibrational band assignments (Table 1) for relevant uranyl species were selected to guide our analysis. Next, the Γ for each band was estimated considering sources of instrumental broadening as well as the identity of the coordinating ligands. Finally, samples and spectra collected with well-known speciation compositions were used to determine the maximum number of U(VI) species and Gaussian functions that could be used during the spectral peak fitting protocol. Vibrational bands with reasonable widths for U species (> 13 cm-1) and intensities with signal to noise values greater than 3 were considered as possible U species.


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As an example, samples containing 30 mM UO22+(aq) and 105 mM Na2CO3 prepared at various pH values are evaluated using both Raman

spectroscopy

and

speciation

predictions. A chemical equilibrium diagram is initially constructed using Hydra and Medusa software for this purpose. An example of a speciation diagram for a sample

Figure 2. (A) Equilibrium diagram of U(VI)

prepared in a pH 6 solution is shown in

species for 30 mM UO22+(aq) with 105 mM

Figure 2A. These predictions were used to validate U(VI) speciation assignments from Raman spectra and the spectral peak fitting protocols. As expected, uranyl speciation is highly dependent on solution pH. The predicted

and

experimentally

observed

species present in solution at pH 3, 6, and 11 are summarized in Figure 2B (predictions shaded bars, experimental – solid bars). Important similarities and differences are noted and are understood through closer evaluation of the Raman spectra.

Na2CO3 at pH 6. Solid lines represent aqueous species while dash and dash dot lines represent precipitated species. (B) Relative abundance of U(VI) species at pH 3, 6, and 11. The shaded bars represent the theoretical results, and the solid bars represent speciation determination using Raman spectral analysis. Error bars represent the standard deviation from 10 trials. U vibrational frequencies: 871 (UO22+),

859

((UO2)2(OH)3+),

853

((UO2)2(OH)22+), 836 ((UO2)3(OH)5+ and UO2(CO3)22-), and 814 (UO2(CO3)34-) cm-1.

First, clear speciation differences are evident from Raman spectra collected at pH 3, 11, 6 (after 1 day), and 6 (after 12 days) and are


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shown in Figure 3. Final spectral barcodes and overlays of best-fit Gaussian functions representing the most probable U species are included (Figures 3B and 3C) and were used to report relative abundance of the species present in each sample (solid bars in Figure 2B). As noted previously, each sample exhibits unique vibrational frequencies in the 750-900 cm-1 spectral envelope. Of all three samples, spectra collected using the samples prepared at pH 3 are the narrowest (Figure 3A-1). From the speciation diagram, this sample should contain primarily uranyl (UO22+) and small amounts of UO2(NO3)+ and (UO2)2(OH)3+. Both UO22+ and UO2(NO3)+ should exhibit identical vibrational frequencies;7,43 additionally, the generated barcode (Figure 3B-1) shows only two Gaussian functions are used in spectral peak fitting for the sample prepared at pH 3. As shown in Figure 3C-1, the Raman spectrum for this sample can be fit using two Gaussian functions. The free uranyl species is centered at 871.2 ± 0.1 cm-1 and exhibits a Γ = 13 ± 1 cm-1 while the vibrational frequency for (UO2)2(OH)3+ is centered at 858.8 ± 1.0 cm-1 with a Γ = 15 ± 1 cm-1. Assuming the Raman scattering cross section for UO22+ is independent of coordinating ligand composition,8,42 the relative abundance of these species can be predicted. As shown in Figure 2B, relative abundance of the two species using Raman spectral peak fitting agree well with what is theoretically predicted at pH 3 in that ~95% of the U(VI) is present as UO22+ with the remainder of the sample being present as (UO2)2(OH)3+. A similar approach is used to evaluate the uranyl species present at pH 11; however, only species that do not precipitate are considered. At pH 11, uranyl tricarbonate (UO2(CO3)34-) is the only predicted water-soluble species. Because coordination between the multiple carbonate ions and metal center weakens the uranyl bond, the vibrational frequency for the symmetric stretch of UO22+ should be red-shifted and broader55 when compared to other weakly coordinated ligands to uranyl.7 As shown in Figures 3B-2 and 3C-2, the Raman spectrum for this sample is best fit


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with a single Gaussian function. This uranyl tricarbonate species exhibits a vibrational frequency centered at 813.4 ± 0.2 cm-1 and a Γ = 20 ± 1 cm-1. Figure 3. (A) Representative Raman spectra, In comparison to the Raman spectral (B) barcode, and (C) spectral peak fitting analysis and speciation assignments for analyses for samples containing 30 mM U(VI) in the pH 3 and 11 samples, Raman UO2(NO3)2 and 105 mM Na2CO3 at pH (1) spectra are much more complex at pH 6 3,(2) 11, (3) pH 6 (24 h), and (4) pH 6 (12 because of the multiple species present and potentially slow kinetics.56 As a result, the

days).

Experiment

condition:

Excitation

wavelength: λex = 785 nm; tint = 20 s; P = 80 U(VI) spectral features observed at pH 6 mW, and averages = 10. Uranium vibrational occur over a broad range and vary with time frequencies: 871 (UO22+), 859 (UO2)2OH3+), (Figures 3A-3 and 3A-4). For instance, 853 ((UO2)2(OH)22+), 836 (UO2(CO3)22-), and Raman spectra collected for a sample 814 (UO2(CO3)34-) cm-1. prepared at pH 6 collected 1 and 12 days after preparation are shown in Figures 3A-3 and 3A-4, respectively. As predicted from the speciation diagram in Figure 2A, both UO2CO3(s) and Na2U2O7(s) should form a precipitate at equilibrium. This precipitate is not observed until ~12 days following sample preparation. Analysis of the solution as a function of time, however, can still be used to predict and validate the presence of soluble uranyl species. Further validation of uranyl speciation of the sample not at equilibrium can be strengthened by evaluating both fluorescence emission and Raman spectra. After 1 day, four uranyl species including (UO2)2(OH)22+, (UO2)3(OH)5+, UO2(CO3)22- and UO2(CO3)34- are likely present in


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solution. These speciation predictions are based on several factors. First, fluorescence emission spectra collected for this sample suggest that both (UO2)2(OH)22+ and (UO2)3(OH)5+ are present in solution (Figure 1A).20 Second, the vibrational frequency assignments from the literature for (UO2)3(OH)5+ and UO2(CO3)22- range from 834-840

42,43,57,58

and 829-835 cm-1,7 respectively;

therefore, these vibrational bands were considered both separately and collectively. The generated barcode (Figure 3B-3) and spectral peak fitting protocols that considered these two species as one vibrational mode provided better fits. Using this approach, three unique vibrational frequencies centered at 853.8 ± 1.1 cm-1 (Γ = 14 ± 1 cm-1), 836.8 ± 0.2 cm-1 (Γ = 19 ± 1 cm-1), and 813.4 ± 0.2 cm-1 (Γ = 18 ± 2 cm-1) are predicted in the Raman spectra and represent (UO2)2(OH)22+, (UO2)3(OH)5+ and UO2(CO3)22-, and UO2(CO3)34-, respectively. These results are summarized in Figures 2B and 3C-3. After aging the solution for 12 days, precipitate formed and the spectrum showed changes in the vibrational frequencies and reductions in fluorescence emission. Both generated barcodes (Figure 3B-4) and the presence of precipitate suggest that near equilibrium conditions are achieved and, as such, analyses of these spectra are best evaluated using speciation predicted at equilibrium. Thus, two Gaussian functions, which represent (1) (UO2)3(OH)5+ and UO2(CO3)22- and (2) UO2(CO3)34- are observed and centered at 835.7 ± 0.5 cm-1 (Γ = 19 ± 1 cm-1), and 813.6 ± 0.5 cm-1 (Γ = 20 ± 1 cm-1), respectively (Figure 3C-4). Comparison of the relative abundance of species in this sample is consistent in terms of speciation and approximate abundance with respect to what is predicted in the speciation diagram (Figure 2B). Slight deviations are hypothesized to arise from a small pH change in solution (pH increased from 6 to 8 over the course of 12 days) and small deviations from equilibrium conditions. All in all, these analyses provide a straight forward approach to U


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species identification and validation of their relative abundance using Raman spectroscopy and the spectral analysis protocol. Identification of U(VI) Species in the Presence of Multiple Coordinating Ligands. Our initial studies focused on uranyl speciation in the presence of carbonate because the anion forms strong complexes in solution59 and possesses relatively simple speciation at all pH values.60 Because sulfate forms stable complexes with uranyl, H2SO4 was selected so that the effectiveness of the Raman spectral analysis protocol described previously could be rigorously tested using a complex sample. To do this, four samples containing 30 mM uranyl with carbonate to uranyl ([CO32-]/[UO22+]) ratios ranging from 0.25, 0.5, 1, to 10 are prepared and then acidified to pH 3 using H2SO4. While the carbonate to uranyl ratio was intentionally varied, the uranyl species predicted to form at this pH are aqua and sulfate species. Upon acidification, carbonate is converted into CO2(g) and released into the environment61 and unavailable for uranyl coordination. As a result, the previously described [CO32-]/[UO22+] ratios also correspond to [SO42-]/[UO22+] ratios of 0.3, 0.6, 1.0, and 9.0, respectively. Figure 4 reveals clear Raman spectral differences as a function of sulfate to uranyl ratio. Identified species are extracted using the previously described spectral peak analysis protocol and validated using Hydra and Medusa software. As shown in Figure 4A, the uranyl symmetric stretch red-shifts with increasing [SO42]/[UO22+]. This spectral trend is consistent with UO2(H2O)52+ forming inner sphere complexes with sulfate.8,62 As a result, three sulfate species including UO2SO40, UO2(SO4)22-, and UO2(SO4)34- are identified in the Raman spectra and exhibit similar relative abundancies. Vibrational frequency assignments associated with UO2SO40 and (UO2)2(OH)3+ are indistinguishable and centered at 860 cm-1 as previously described.7,43 Additionally, the


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vibrational frequencies of (UO2)2(OH)22+ (854 ± 2 cm-1) and UO2(SO4)22- (852 ± 2 cm1

) could not be resolved. At the lowest

carbonate to uranyl ratio ([CO32-]/[UO22+] = 0.25; [SO42-]/[UO22+] = 0.3), the predicted relative abundance of UO22+ and UO2NO3+ to UO2SO40

and

(UO2)2(OH)3+

to

(UO2)2(OH)22+ is 67:30:3, as shown in Figure 4B (shaded bars). The Raman spectral analysis of this sample identifies three Figure 4. (A) Representative Raman spectra unique vibrational frequencies centered at -1

870.8 ± 0.4 (Γ = 14 ± 1) cm (UO2

2+

with spectral peak fitting for 30 mM UO22+(aq) and

UO2NO3+), 859.2 ± 1.0 (Γ = 15 ± 1) cm-1 (UO2SO40, (UO2)2(OH)3+), and 850 (Γ = 15 ± 1) cm-1 (UO2(SO4)22-, (UO2)2(OH)22+)). The

acidified to pH 3 using H2SO4 in the presence of (1) 7.5, (2) 15, (3) 30, and (4) 300 mM Na2CO3. Same Raman conditions as in Figure 3. (B) Relative abundance from theoretical

relative abundance of these three Raman predictions

(from

Medusa,

shaded)

and

peaks is 71(± 3):23(± 3):8(± 1) – values that experiment (from Raman spectral analysis, are similar to what is predicted (Figure 4B, solid). Error bars represent standard deviation solid vs. shaded bars, respectively). in data from at least 10 measurements. U Similar comparisons can be made for the other samples. When the ratio is 0.5

[CO32-]/[UO22+]

([SO42-]/[UO22+]

vibrational frequencies: 871 (UO22+), 858 ((UO2)2OH3+, UO2SO40), 851 (UO2(SO4)22-,

= 0.6), the 2

predicted relative abundance for UO2 to

(UO2)2(OH)22+), and 841 (UO2(SO4)34-) cm-1.


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UO2SO40 and (UO2)2(OH)3+ to (UO2)2(OH)22+ is 55:43:2. Vibrational bands consistent with these same species are extracted from the Raman spectra and are centered at 870.7 ± 0.4 (Γ = 14 ± 1), 860.8 ± 0.5 (Γ = 13), and 850 (Γ = 15) cm-1, respectively (Figure 4A-2). The experimentally observed relative abundance of these three bands and species is 55(± 3):33(± 3):12(± 2). This agreement with prediction is good with slight deviations expected from unregulated solution contributions. Both species determination from Raman spectra and prediction show that total uranyl sulfate species (UO2SO40 and UO2(SO4)22-) concentration increases and uranyl abundance decreases as the [SO42-]/[UO22+] ratio increases from 0.3 to 0.6. Increasing this ratio to 1 should cause the relative abundance of total sulfate species (UO2SO40 and UO2(SO4)22-) to exceed UO22+ (Figure 4B, shaded bars). This same trend is observed through analysis of the Raman spectra for these samples (Figure 4A-3). Three vibrational features centered at 870.6 ± 0.3 (Γ = 14 ± 1), 860.5 ± 0.4 (Γ = 13 ± 1), and 850 (Γ = 15) cm-1 and are assigned to (UO22+and UO2NO3+), (UO2SO40 and (UO2)2(OH)3+), and (UO2(SO4)22-and (UO2)2(OH)22+), respectively. The predicted and observed relative abundance of these same species are 40:52:8 and 39(± 2):45(± 3):17(± 1), respectively (Figure 4B, shaded and solid bars) and agree well. Once the [SO42-]/[UO22+] ratio equals 9, UO2(SO4)34- should begin to form and the disulfate species (UO2(SO4)22-) should be the most dominate species in solution according to predictions (Figure 4B, shaded bars). Upon Raman spectral analysis using only vibrational features with signal to noise ratios greater than 3, three vibrational frequencies centered at 862.0 ± 0.1 (Γ = 15 ± 1), 851.5 ± 0.1 (Γ = 15 ± 1), and 841.3 ± 0.7 (Γ = 20) cm-1 are identified and consistent with the presence of UO2SO40 and (UO2)2(OH)3+, UO2(SO4)22- and (UO2)2(OH)22+, and UO2(SO4)34-, respectively. The relative abundances of these species are 33(± 2):49(± 2):18(± 1), respectively. This agrees qualitatively


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with predictions of 27:40:27 for the same species. In addition, predictions indicate ~5% of UO22+ as well. While Raman spectral analysis indicated the possible presence of this species, the signal intensity was insignificant versus noise and not included. Given uncontrolled experimental conditions and possible uncertainties in equilibrium constants for the various U species, the agreement of uranyl species identification and relative abundancies vs. predictions is excellent for this complex sample. First, the relative abundance of each species determined using Raman spectral analysis is within 10 % of the predicted values for major U species. Second, the error between the Raman experiments and predictions increased for minor species in solution. We hypothesize that the samples analyzed using Raman spectroscopy are likely not in equilibrium. This discrepancy between equilibrium predictions and Raman spectra analysis are real. As such, the Raman spectral analysis protocol used is likely providing nearreal-time detection and with the added capability of in situ determination of water soluble U(VI) species. Raman Analysis for U Quantification. As already demonstrated, Raman spectroscopy can be used to quantify a complex matrix containing a mixture of coordinating ligands. Because the symmetric uranyl stretch observed with Raman spectroscopy is sensitive to coordinating anions, ion composition may also complicate Raman spectral analysis. After developing a protocol for determining and relating the relative abundance of water soluble U species using Raman spectroscopy, implications of ion composition are evaluated. Samples containing 30 mM UO22+(aq) and 105 mM Na2CO3 were prepared and acidified with the HCl, HNO3, and H2SO4. After the samples reached equilibrium, Raman spectra were collected. Figure 5 shows these spectra as well as vibrational band features and speciation assignments (frequency and Γ). For instance, the sample with HNO3 contains two unique vibrational frequencies centered at 870.6 ±


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Page 18 of 29

0.1 (Γ = 15 ± 1) and 859.8 ± 1.0 (15 ± 1) cm-1.

Similar

vibrational

features

are

observed when HCl is used and centered at 870.8 ± 0.2 (14 ± 1) and 858 (15 ± 1) cm-1 are determined. Samples acidified using H2SO4 suggest the presence of four unique vibrational frequencies centered at 871.7 ± 0.8 (15 ± 1), 860.0 ± 0.3 (13 ± 1), 851.5 ± 0.3 (13 ± 1), and 841.0 ± 0.1 (20) cm-1. As a reference, a Raman spectrum containing 30

Figure 5. Representative Raman spectra of (A)

mM UO22+(aq) (at pH 3 without Na2CO3) is

30 mM UO22+(aq) at a pH 3 (no pH adjustment)

also included and shows three vibrational

or 30 mM UO22+(aq) and 105 mM Na2CO3

features centered at 871.0 ± 0.1 (13 ± 1),

adjusted to pH 3 with (B) HNO3, (C) HCl, and

860.8 ± 1.0 (13), and 852.4 ± 0.8 (15 ± 1)

(D) H2SO4. Same Raman conditions as in

cm-1.

Figure 3. U vibrational frequencies: 871

Several trends are noted. First, the Raman

(UO22+), 858 ((UO2)2OH3+ for (A), (B), and

spectra for samples acidified using either

(C); UO2SO40 for (D)), 852 (UO2(SO4)22-,

HNO3 or HCl are simplest and reach

(UO2)2(OH)22+), and 841 (UO2(SO4)34-) cm-1.

equilibrium quickly (i.e., within 30 minutes). These acids promote straight-forward speciation distributions, which facilitate speciation identification as well as possible quantification because Raman cross sections could be established with these samples. Second, because coordination between uranyl and ligands increases the U-O axial bond length, the vibration frequency of uranyl should systematically decrease for UO2NO3+, UO2Cl+, and UO2SO42-.55 Furthermore, the


18

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formation constant of these species follow this same trend,47 which promotes the presence of a higher fraction of uranyl sulfate species in solution. Although the formation of uranyl chloride is possible, the impact of Cl- coordination on the U-O axial bond length is minimal55 and only induces a small 4 cm-1 red shift in the uranyl symmetric stretch vs. uncoordinated uranyl (UO2Cl+ at 866 ± 2 cm-1 vs. UO22+, UO2NO3+ at 870 ± 2 cm-1). Depending on detector resolution, these may not be distinguishable and are not with our detector (Figures 5A-C). In contrast, a significant red-shift in the uranyl symmetric stretch is observed in Figure 5D when the sample is acidified using H2SO4. This large change in vibrational frequency indicates the formation of new U(VI) species in solution and are assigned as UO2(SO4)22- (860 cm-1) and UO2(SO4)34- (841 cm-1). Additional speciation assignments are summarized in Table 1. All in all, speciation identification is easily determined using Raman spectroscopy. Because ion composition can impact speciation assignments as well as vibrational frequencies and band widths, care should be taken to ensure proper band assignments and spectral analysis. For practical applications of environmental U(VI) sample analysis, if an acidification reagent is used, nitric acid should be selected as this acid provides the simplest U species and as a result, Raman spectra for quantitative analysis. CONCLUSIONS In summary, this study systematically reveals how choice of excitation wavelength and Raman spectral analysis can be successfully used for the identification and relative abundance evaluation of U(VI) species in solution using the UO22+ fingerprint region from 750-900 cm-1 without extensive sample preparation or separations. Several guiding principles were demonstrated. First, while blue and green excitation wavelengths provide the largest Raman intensities, these wavelengths also promote fluorescence in some U(VI) species and thus should be avoided during


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Raman analysis. Of note, fluorescence interference from hydrolyzed species including (UO2)2(OH)22+ and (UO2)3(OH)5+, which form in solution at pH ≥ 4, interferes with the detection and speciation assignments of all U(VI) species using Raman spectroscopy. Second, the developed Raman spectral analysis protocol, which considered approximate vibrational band locations and band widths approximated using second derivative spectral analysis, allowed for the successful identification of U(VI) species at acidic, neutral, and basic pH values in the presence of the common anion, carbonate. By assuming that each U(VI) species exhibited similar Raman cross sections, the relative abundance of each U species was estimated under equilibrium and non-equilibrium conditions. Speciation in samples that were at equilibrium agreed well with theoretical predictions. This model and spectral analysis were challenged using a complex, sulfate containing sample, and successful speciation assignments were found. Notably, this was achieved without additional sample treatment or separation steps. Finally, the change of the relative abundance for U(VI) species was observed to change with time. As expected, because U(VI) species composition can be dynamic, the presented Raman spectral analysis procedure was used to describe the abundance and composition of the U(VI) species. All in all, this study demonstrates how Raman spectroscopy can be used for the qualitative, in situ, and on-site detection of U without the need for separations or additional sample treatment. Although quantification of U(VI) using Raman intensity can be improved by increasing laser power or integration times, signal intensity is limited by intrinsically low Raman scattering cross sections. Alternatively, signal enhancements are possible using techniques such as surface enhanced Raman63,64 and/or infrared spectroscopy.65 While fluorescence provides superior sensitivity and low detection limits if fluorescence lifetimes are measured, Raman spectroscopy remains a powerful approach to analyze U samples from the environment and lab. Clearly,


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normal Raman spectroscopy when combined with spectral barcode analysis and peak fitting can be used to extract U species information more accurately than previously demonstrated for aqueous phase U species. Moving forward, these considerations and spectral analysis approaches could be extended to other U(VI) samples for rigorous and real-time evaluation of these and other water soluble U(VI) species.

TABLES Table 1. Vibrational frequency assignments for possible species present in these samples. Speciation

Vibration Mode

Vibrational Frequency (cm-1) Literature

Fitting Range

Ref.

This Work (Γ)

UO2(CO3)34-

bound CO32- (ν4)

1378

66

CO32-

free CO32- (ν1)

1064.5

66

1048

67

1047

68,69

982

67

SO42NO3SO42UO22+, UO2(HSO4)+, UO2NO3+

free NO3- (ν1)

UO22+ symmetric 870±1 stretching (ν1)

870±2

UO2SO4

860±2

860±2

(UO2)2(OH)3+

860±1

(UO2)2(OH)22+

854±2

870.3±0.8

7,43

(14±1)

854±2

859.3±1.5

7

(15±1)

43

853.8±1.1

43

(14±1) UO2(SO4)22-

852±1

(UO2)2(OH)22+, UO2(SO4)22-

853±3

7

853±3

851.5±0.4

7,43

(15±1)


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UO2(SO4)34-

843±2 843±1

(UO2)3(OH)5+

837±3

UO2(CO3)22-

832±3

UO2(CO3)34-

812±3

Page 22 of 29

841.3±0.7

7

(20) 835±5

812±2

836.2±0.7

42,43,57,70

(19±1)

aq7; solid71

813.7±0.4

7

(19±1) UO2(NO3)20

Bound NO3- (ν4)

UO2(CO3)34-

Bidentate CO32- (ν4)

751

bound 735

66

ASSOCIATED CONTENT Supporting Information Additional information regarding spectral analysis including barcode analysis and peak fitting. The Supporting Information is available free of change on the ACS Publications website. AUTHOR INFORMATION Corresponding Authors *University of Iowa, Department of Chemistry, Iowa City, Iowa 52242; TZF: Phone: (319) 3841320, [email protected]; AJH: Phone: (319) 384-3695, Email: [email protected], Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. We thank Hoa Phan for providing Matlab code that was used for barcode generation.


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Funding Sources This work was funded through a University of Iowa Internal Funding Initiative major project award and the National Science Foundation, (CHE-1150135). The authors declare no competing financial interest.

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For Table of Contents Only

Synopsis Hexavalent uranium species identification and relative abundance in aqueous solutions are achieved for in situ analysis using Raman spectroscopy and peak fitting.


29

Evaluating Best Practices in Raman Spectral Analysis for Uranium

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