Quantitative Analysis of Uranium in Aqueous Solutions Using a

Technical Note pubs.acs.org/ac

Quantitative Analysis of Uranium in Aqueous Solutions Using a Semiconductor Laser-Based Spectroscopic Method Hye-Ryun Cho, Euo Chang Jung,* Wansik Cha, and Kyuseok Song Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong-gu, Daejeon 305-600, Republic of Korea ABSTRACT: A simple analytical method based on the simultaneous measurement of the luminescence of hexavalent uranium ions (U(VI)) and the Raman scattering of water, was investigated for determining the concentration of U(VI) in aqueous solutions. Both spectra were measured using a cw semiconductor laser beam at a center wavelength of 405 nm. The empirical calibration curve for the quantitative analysis of U(VI) was obtained by measuring the ratio of the luminescence intensity of U(VI) at 519 nm to the Raman scattering intensity of water at 469 nm. The limit of detection (LOD) in the parts per billion range and a dynamic range from the LOD up to several hundred parts per million were achieved. The concentration of uranium in groundwater determined by this method is in good agreement with the results determined by kinetic phosphorescence analysis and inductively coupled plasma mass spectrometry.

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medium. In this method, the Raman scattering intensity of water acts as an internal standard signal for the quantitative analysis of U(VI). One advantage is that the calibration curve can be used continuously, because the slope of the calibration curve remains unchanged despite fluctuations in the power of the light source and the sensitivity of the detector. Moreover, a calibration procedure can be performed using only one cell, because the U(VI) concentrations are determined by measuring the ratio of the luminescence intensity of U(VI) to the Raman scattering intensity of water, both of which are measured simultaneously in the same cell. A compact spectroscopic system was constructed with a cw semiconductor laser as the light source. A solution of 0.05 M Na4P2O7 in 0.4 M H3PO4 was added to the U(VI) samples as a luminescence enhancing reagent. A quantitative analysis of U in groundwater was performed using the proposed method, and the results are compared with the U concentrations determined by KPA and inductively coupled plasma mass spectrometry (ICPMS).

he selective and sensitive detection of uranium (U) is required in various research fields of the nuclear fuel cycle including the preparation of fuel, waste management, and environmental monitoring.1−6 Particular interest has been focused on a quantitative analysis of trace amounts of U in an aqueous medium for the safety assessment of nuclear waste disposal. Spectroscopic methods adopting time-resolved laserinduced fluorescence and laser-induced kinetic phosphorescence measurements have been widely used for the quantitative analysis of hexavalent uranium ions (U(VI)).1−14 These methods are based on the excitation of the U(VI) species by an appropriate laser pulse and the measurement of luminescence intensity with temporal resolution. In particular, a commercially available instrument called a kinetic phosphorescence analyzer (KPA) yields a limit of detection (LOD) of approximately 0.01 parts per billion (ppb) for U(VI) with the aid of a luminescent enhancing reagent such as Uraplex (phosphate-based complex). KPA uses two cells, one for a reference solution and the other for a sample. The phosphorescence signal measured from the reference solution containing the known U(VI) concentration is used to correct the signal obtained from the U(VI) in the sample cell. The correction procedure used in KPA is a type of external standardization that can eliminate analytical errors generated by variations in laser pulse energy and drifts of the high voltage supplied to the photomultiplier tube (PMT).11,12 With the aid of a kinetic analysis of luminescence, most geochemical samples can be directly analyzed by KPA. However, a calibration procedure should be performed for each day of operation, and thus, repeating the procedure used to prepare calibration curves can be time-consuming. In the present work, a simple analytical method is reported for determining the U(VI) concentration in an aqueous © 2013 American Chemical Society

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EXPERIMENTAL SECTION Instrumentation. Figure 1 shows a schematic diagram of the experimental setup. A cw diode laser at a center wavelength of 405 nm (model no. LQC405-85E, Newport Corporation) radiated an elliptical beam. An anamorphic prism pair (model no. APP 405J, Toptica Photonics) was used to make a circular laser beam. The power of the incident laser beam was measured with an optical power meter (model no. PD300-3W, Ophir Optronics), and the average power was approximately 50 mW. Received: December 24, 2012 Accepted: March 28, 2013 Published: March 28, 2013 4279

dx.doi.org/10.1021/ac303752t | Anal. Chem. 2013, 85, 4279−4283

Analytical Chemistry

Technical Note

The groundwater was filtered using a cellulose membrane filter with a pore size of 100 nm (mixed cellulose ester, Advantec MFS, Inc.) to eliminate particulates. A portion of the filtered groundwater was boiled to dryness, and 1 mL of concentrated nitric acid (Suprapur, Merck) and several drops of 30% H2O2 were added. After several hours, the solution was boiled to dryness and reconstituted with 0.5 M HClO4. The uranium concentrations in the filtered and the wet-ashed groundwater samples were analyzed by KPA, ICPMS, and the method proposed in this work.

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RESULTS AND DISCUSSION Figure 2A,B shows the Raman and luminescence spectra measured using a spectrofluorometer for pure water and a

Figure 1. Schematic diagram of the experimental setup for simultaneous measurements of Raman scattering spectra of water and luminescence spectra of U(VI). APP: anamorphic prism pair.

The laser beam passed through the sample cell (model no. 111QS, Hellma Analytics), which was located inside a dark box to avoid ambient light. The luminescence and Raman scattered light perpendicular to the laser beam propagating direction were delivered to the entrance slit of the monochromator using an optical fiber bundle. The inset of Figure 1, enclosed by the dotted line, represents a cross-section of the fiber bundle (2 × 8 mm2, 345 fibers). The luminescence and Raman spectra were recorded using a PMT (R928, Hamamatsu Photonics) attached to the Czerny-Turner type monochromator with a 320 mm focal length (iHR 320, Horiba Jobin Yvon) in which a holographic grating (1200 grooves/mm) was installed. The output signal of the PMT was saved on a PC using the data acquisition module (SpectrACQ1, Horiba Jobin Yvon). A conventional spectrofluorometer (AMINCO Bowman Series 2, Spectronic Instruments, Inc.) was used as an auxiliary spectroscopic system to measure the luminescence and Raman spectra induced at different excitation wavelengths. To confirm the U concentration in groundwater determined by the present method, KPA (KPA-11, Chemcheck Instruments, Inc.) and ICPMS (RedTop, Varian, Inc.) were used for quantitative analyses of U contained in the same groundwater sample. Sample Preparation. A quantity of 2.7 g of natural uranium oxide (UO2) powder was dissolved and oxidized in concentrated HClO4 (TraceSELECT Ultra, Fluka) with drops of 30% H2O2 (reagent grade, Sigma-Aldrich). Excess H2O2 and HClO4 were removed by evaporating to dryness. The residue was dissolved in 0.1 M HClO4 with slight heating. The concentration of U in the stock solution was 89.5 ± 0.15 mM as determined by potentiometric titration with a standard dichromate solution based on Davies and Gray’s method.15 U(VI) standard solutions in a concentration range of 1 ppb to 240 ppm were prepared through dilution of the stock solution using 0.5 M HClO4 as the diluent. Two solutions were prepared as luminescence enhancing reagents using H3PO4 (reagent grade, Sigma-Aldrich) and Na4P2O7·10H2O (reagent grade, Sigma-Aldrich). One was a solution of 0.76 M H3PO47 and the other a solution of 0.05 M Na4P2O7 in 0.4 M H3PO4.9 A volume of 1 mL of the U(VI) sample was mixed with 1 mL of a luminescence enhancing reagent. Because the pyrophosphate ions may decompose into phosphate ions, all measurements were performed within 1 day after the sample preparation to avoid the hydrolysis effect of pyrophosphate ions.16,17 All solutions in this work were prepared with deionized water purified by a Milli-Q system (Element, Millipore Co.).

Figure 2. (A) Raman scattering spectra of water and (B) luminescence and Raman scattering spectra of a uranium solution in 0.5 M HClO4 measured using a conventional spectrofluorometer. Dashed and solid lines represent the spectra measured with the excitation wavelengths at 355 and 414 nm, respectively.

U(VI) standard solution ([U(VI)] = 24 ppm), respectively. Raman peaks appeared at 403 and 481 nm at excitation wavelengths of 355 and 414 nm, as shown in Figure 2A. These excitation wavelengths correspond to the wavelengths of the third harmonic beam of the Nd:YAG laser (355 nm) and the highest absorbance of U(VI) in perchloric acid (414 nm, molar absorption coefficient of 8 M−1cm−1).18 The wavelength difference between the relevant Raman peaks and the excitation wavelengths represents a wavenumber shift of 3375 ± 42 cm−1, which is known to be an overlapped intramolecular Raman peak of water.19 Because the Raman scattering of water is intrinsic in aqueous solutions, these Raman peaks also appeared at the same positions with the luminescence peaks of U(VI) in the U(VI) standard solution, as shown in Figure 2B. Luminescence peaks of U(VI) in perchloric acid appeared at wavelengths of 470, 488, 510, 533, and 560 nm, and these wavelengths agree with the reported values.14 In Figure 2B, the ratio of the luminescence intensity of U(VI) to the Raman scattering intensity of water is much higher at the excitation wavelength of 414 nm than that at 355 nm. A laser beam at a wavelength of 414 nm can be easily adopted for this experiment using a compact semiconductor laser. InGaN-based semiconductor lasers are promising candidates for short wavelengths ranging from UV to visible wavelengths. The wavelengths of these lasers reported to date have ranged between 390 and 440 4280

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nm.20 The currently available InGaN lasers offer powers of up to 80 mW with guaranteed lifetimes of approximately 10 000 h. The results shown in Figures 3−5 were obtained using a cw diode laser at a center wavelength of 405 nm. The molar absorption coefficient of U(VI) at 405 nm is 7 M−1cm−1 in 1 M hydrochloric acid.18

bands were not changed with the addition of luminescence enhancing reagents. In contrast, the luminescence peaks were red-shifted in wavelength and the intensities dramatically increased when these reagents were added. The enhancement factors in the luminescence intensities were approximately 35 (dotted line) and 54 (dashed line) compared with the luminescence intensity of U(VI) in 0.5 M HClO4 (solid line). The most intense luminescence peak appeared at 519 nm with a solution of 0.05 M Na4P2O7 in 0.4 M H3PO4. Thus, the luminescence intensity at 519 nm was used to analyze the uranium concentration. Figure 4A shows the calibration curves measured with four different high voltages applied to the PMT for the determination of the uranium concentration using peak intensities at 519 nm. Although the luminescence intensities correlate very well with the uranium concentration at each voltage, the slopes of the calibration curves increase with the increase in the applied voltage. This finding means that drifts in the voltages of the PMT and the power of the laser beam may cause empirical errors when calculating the uranium concentration. In general, uranium at low concentration should be measured at an experimental condition with a high gain, whereas a relatively low gain is preferable for measuring uranium in high concentrations to prevent a saturation of the luminescence intensity. Therefore, the processes used to obtain the calibration curves for the routine analysis of samples with various concentrations must be repeatedly performed each time in daily operation. Figure 4B shows the ratios of the luminescence intensity at 519 nm to the Raman intensity at 469 nm for U samples measured at four different high voltages supplied to the PMT. The ratio for each U sample was constant with the change in voltage. The average values (solid lines) and standard deviations (dotted lines) of the measured ratios were 0.1936 ± 0.0034, 0.3817 ± 0.0059, and 0.7345 ± 0.0133 for the U samples at concentrations of 0.5 (triangles), 1.0 (circles), and 2.0 (rectangles) ppm, respectively. This result suggests that the ratio of the luminescence intensity of U(VI) to the Raman intensity of water is not affected by the drift of detector gain or the fluctuation of the laser beam power.

Figure 3. Luminescence and Raman scattering spectra for uranium ([U(VI)] = 5 ppm) in 0.5 M HClO4 induced by irradiation of a diode laser beam. Solid, dotted, and dashed lines represent the spectra measured without luminescence enhancing reagents, with a solution of 0.76 M H3PO4 and a solution of 0.05 M Na4P2O7 in 0.4 M H3PO4, respectively.

Figure 3 shows the measured spectra of a 5 ppm U(VI) standard solution through the irradiation of a laser beam with a power of 50 mW. The solid line in Figure 3 represents the spectrum of U(VI) in 0.5 M HClO4. The Raman peak of water appeared at 469 nm and the luminescence peaks of U(VI) appeared at the same wavelength positions as those shown in Figure 2B. The measured spectra for the uranium samples mixed with the luminescence enhancing reagents, a solution of 0.76 M H3PO4 and a solution of 0.05 M Na4P2O7 in 0.4 M H3PO4, were shown by the dotted and dashed lines in Figure 3, respectively. The peak position and intensity of the Raman

Figure 4. (A) Luminescence intensities at 519 nm measured at different voltages supplied to PMT as a function of uranium concentration and (B) ratios of the luminescence intensity at 519 nm to the Raman intensity at 469 nm measured for three different U(VI) samples as a function of the voltages. 4281

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Technical Note

band of the organic compounds strongly interfered with the neighboring luminescence band of U(VI). In the filtered groundwater sample, the luminescence lifetime measured by KPA was 122 ± 2 μs, as listed in Table 1. After the organic

Figure 5 shows the calibration curve obtained by plotting the ratios of the luminescence intensity of U(VI) at 519 nm to the

Table 1. U Concentrations in a Groundwater Sample Determined by ICPMS, KPA, and the Present Method groundwater sample

ICPMS (ppb)

filtered sample

44.2 ± 0.7

wet-ashed sample

43.2 ± 0.7

KPA (ppb) − too short lifetime (τ1/2 = 122 ± 2 μs) 39.2 ± 2.4 (τ1/2 = 318 ± 18 μs)

present work (ppb) − 42.7 ± 1.7

compounds in the groundwater were ashed using concentrated HNO3 with drops of 30% H2O2, the luminescence lifetime measured by KPA increased up to 318 ± 18 μs. These lifetime values measured by KPA agreed well with those measured using a time-resolved laser-induced fluorescence spectroscopy (TRLFS) system built in this laboratory (data not shown).23 In general, the luminescence lifetimes of most organic compounds are shorter than those of inorganic compounds. Thus, the unwanted luminescence of the organic compounds can be discriminated from the luminescence of the uranium by using TRLFS, but the detailed explanation regarding the TRLFS is beyond the scope of this paper. The filled symbol in Figure 5 represents the measured value of 42.7 ± 1.7 ppb using the proposed method. The results determined by KPA and ICPMS are listed in Table 1. The result determined by the proposed method is in agreement with the results determined by other common methods. Thus, we believe that the simple analytical method proposed in this paper will be useful to personnel requiring a rapid determination of U(VI) concentration before the shipment of samples to an accredited laboratory for a traditional analysis.

Figure 5. Calibration curve for the determination of U(VI) concentration obtained using the ratios of the luminescence intensity of U(VI) to the Raman scattering intensity of water (open symbol). The determined U concentration in a groundwater sample was 42.7 ± 1.7 ppb (filled symbol). The inset shows the luminescence spectrum for a U(VI) standard solution of 1 ppb.

Raman intensity of water at 469 nm as a function of the uranium concentration. The Raman and luminescence spectra for 18 U(VI) samples with concentrations ranging from 1 ppb to 240 ppm were measured at five different PMT voltages. The open symbols indicate the mean values obtained from more than three measurements. The relative standard deviations (RSD) are less than 3% for most samples. The error bars corresponding to RSD values are smaller than the size of the symbols; thus, the error bars are not presented in Figure 5. The calibration curve shows a straight line with a correlation coefficient close to unity. The shift of luminescence peak positions with the luminescence enhancing reagent, as shown in Figure 3, represents the formation of U(VI)−phosphate complexes in the solutions. However, mole fractions of the U(VI) species were constant in the wide range of U(VI) concentration because of the relatively high concentration of phosphate or pyrophosphate ions.21 The inset of Figure 5 represents the luminescence spectrum for the sample with a concentration of 1 ppb. A comparison of the luminescence intensity at 519 nm for the U(VI) sample with 3σ, where σ is the standard deviation of the background intensity at 519 nm for the blank solution without U, resulted in an LOD of approximately 1 ppb. The LOD is higher than that of ICPMS at the present time; however, it can be improved by using the appropriate optical filters, so that the PMT will detect only the luminescence at the peak wavelengths of analytes with the best signal-to-noise ratio, in replacement of the monochromator. To demonstrate the effectiveness of the proposed analytical method, the U concentration in groundwater was determined and the result was compared with the U concentrations determined by KPA and ICPMS. Although the mineral concentrations in the groundwater (Na [22 ppm], Ca [26 ppm], Mg [5 ppm], and Si [17 ppm]) were lower than the reported values, causing luminescence quenching,22 it was impossible to measure the luminescence of U(VI) in the groundwater because the very intense and wide luminescence

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CONCLUSIONS A quantitative analysis of U(VI) in an aqueous solution was performed using diode laser-based spectroscopy. A simple analytical method that measures the ratio of the luminescence intensity of U(VI) to the Raman scattering intensity of water was adopted. Because the Raman scattering intensity of water acts as an internal standard signal for the quantitative analysis of U(VI), a calibration procedure was performed using only one cell. The calibration curve can be used continuously, because the slope of the calibration curve is unchanged under a variety of experimental conditions, such as power fluctuation of the light source and sensitivity of the detector. An LOD of 1 ppb was achieved with a wide dynamic range in U(VI) concentration, from the LOD up to several hundred ppm. This method can be used for trace analysis of U(VI) in environmental monitoring as well as for analysis of relatively high concentrations of U(VI) in nuclear materials. The quantitative analysis of U(VI) performed using the present method agrees with the results determined by traditional methods such as KPA and ICPMS.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone +82-42-868-4737. Fax +8242-868-8148. 4282

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Analytical Chemistry

Technical Note

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the nuclear research and development program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.

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