Anal. Chem. 2003, 75, 6769-6774
Separation of Plutonium and Neptunium Species by Capillary Electrophoresis-Inductively Coupled Plasma-Mass Spectrometry and Application to Natural Groundwater Samples Bernhard Kuczewski,† Christian M. Marquardt,‡ Alice Seibert,‡ Horst Geckeis,‡ Jens Volker Kratz,*,† and Norbert Trautmann†
Institut fu¨r Kernchemie, Johannes Gutenberg-Universita¨t Mainz, 55099 Mainz, Germany, and Institut fu¨r Nukleare Entsorgung, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany
Capillary electrophoresis (CE) was coupled to ICPMS in order to combine the good performance of this separation technique with the high sensitivity of the ICPMS for the analysis of plutonium and neptunium oxidation states. The combination of a fused-silica capillary with a MicroMist AR 30-I-FM02 nebulizer and a Cinnabar smallvolume cyclonic spray chamber yielded the best separation results. With this setup, it was possible to separate a model element mixture containing neptunium (NpO2+), uranium (UO22+), lanthanum (La3+), and thorium (Th4+) in 1 M acetic acid. The same conditions were also suitable for the separation of various oxidation states of plutonium and neptunium in different aqueous samples. All separations were obtained within less than 15 min. A detection limit of 50 ppb ≡ 2 × 10-7 M (3-fold standard deviation of a blank) was achieved. To prove the negligible disturbance of the plutonium and neptunium redox equilibria during the CE separations, plutonium and neptunium speciation by CE-ICPMS in acidic solutions was compared with the results of UV/visible absorption spectroscopy and was found to be in good agreement. The CEICPMS system was also applied to study the reduction of Pu(VI) in a humic acid-containing groundwater at different pH values. High-level radioactive waste is planned to be disposed in deep geological formations. To perform a long-term safety assessment of a nuclear waste repository, it is necessary to understand the geochemical behavior of the radionuclides and their migration through the geological barrier to the biosphere. The long-lived actinides neptunium, plutonium, americium, and curium determine the radiotoxiticity of the waste for a long time period and are of special interest for long-term safety considerations.1 Plutonium and neptunium belong to the redox-sensitive actinides, which may * Corresponding author. E-mail:
[email protected]. † Johannes Gutenberg-Universita¨t Mainz. ‡ Institut fu ¨ r Nukleare Entsorgung. (1) Actinide and fission product partitioning and transmutation; OECD/NEA Report, Organisation for Economic Co-operation and Development, Nuclear Energy Agency: Paris, 1999 (PDF-file download: http://www.nea.fr/html/ trw/docs/neastatus99). 10.1021/ac0347213 CCC: $25.00 Published on Web 11/04/2003
© 2003 American Chemical Society
exist in different oxidation states depending on the geochemical conditions. In solution, up to four oxidation states of plutonium (Pu(III), Pu(IV), Pu(V), and Pu(VI)) can coexist. Under natural conditions, neptunium can only occur in the tetra- and pentavalent states. Depending on their oxidation state, the actinides show different geochemical and migration properties, which may lead to either strong retardation or enhanced mobilization of the species. Direct speciation based on spectroscopic methods allows the noninvasive identification of the different actinide species without influencing their chemical equilibrium. However, classical absorption spectroscopy provides only limited sensitivity, and its application in the case of plutonium is restricted to relatively high concentrations, g10-5 M, due to the small absorption coefficients of the oxidation states III-V.2 Additionally, the absorption bands of these species overlap partially when coexisting in solution. At unfavorable concentration ratios, it is difficult to deconvolute the different absorption bands of the species from the spectra. That is why mainly “chemical speciation methods” are applied at low actinide concentrations relevant to natural conditions. These methods combine chemical separation procedures such as solvent extraction,3,4 ion exchange,5-8 or coprecipitation9 with very sensitive detection techniques, such as nuclear spectroscopy or mass spectrometry. However, such methods suffer from the fact that oxidation states may be altered during chemical separation due to, for example, acidification of the sample, contacting the sample with strongly complexing agents, or chemical impurities. Within the present work, capillary electrophoresis (CE) was combined on-line with inductively coupled plasma-mass spectrometry (ICPMS) and studied as a potential alternative to existing actinide speciation methods at low metal ion concentrations. The main advantages of such a combination are the short separation times (2) Cohen, D. J. Inorg. Nucl. Chem. 1961, 18, 211-218. (3) Saito, A.; Choppin, G. R. Anal. Chem. 1983, 55, 2454-2457. (4) Bertrand, P. A.; Choppin, G. R. Radiochim. Acta 1982, 31, 135-137. (5) Gehmecker, H.; Trautmann, N.; Herrmann, G. Radiochim. Acta 1986, 40, 81-88. (6) Duff, M. C.; Amrhein, C. J. Chromatogr., A 1996, 743, 335-340. (7) Coates, J. T.; Fjeld, R. A.; Paulenova, A.; DeVol, T. J. Radioanal. Nucl. Chem. 2001, 248, 501-506. (8) Ro ¨llin, S.; Eklund, U.-B. J Chromatogr., A 2000, 884, 131-141. (9) Foti, S. C.; Freiling, E. C. Talanta 1964, 11, 385-392.
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and the high separating efficiency of CE combined with the elemental and isotopic selectivity and the sensitivity of the ICPMS detection.10-17 Separations of the oxidation states with CE-UV/ visible18-20 and with CE-ICPMS21-23 are described for several elements. With the developed conditions for CE-ICPMS, it was possible to separate plutonium ions in the oxidation states III-VI and neptunium ions in the oxidation states IV and V. Subsequently, the method was applied to study the redox behavior of plutonium in a natural groundwater rich in humic substances under anaerobic conditions. EXPERIMENTAL SECTION Reagents. All the chemicals were of p.a. quality or better and were obtained from Merck (Darmstadt, Germany) or Riedel de Haen (Seelze, Germany). Milli-Q deionized water (18 MΩ) was used to prepare all solutions. The CE electrolyte (1 M acetic acid) was filtered and degassed in a supersonic bath before use. Plutonium Solutions. A plutonium stock solution containing 99.4 wt % Pu-242, and small amounts of Pu-238, -239, -240, and -241, with known specific activity was used for the experiments. The solution was prepared by dissolving PuO2 in 8 M HNO3 with subsequent purification by anion exchange.24 The effluent from the ion exchanger was evaporated to dryness, and the residue was dissolved in 1 M HClO4. After several additional evaporation steps, the solid was finally dissolved in 1 M HClO4, resulting in a 2 × 10-2 M plutonium (Pu-242) stock solution. This solution contains the plutonium ions in different oxidation states, as demonstrated by their characteristic absorption bands in the spectral range from 400 to 870 nm.2 Solutions with defined concentrations of plutonium in a single oxidation state were prepared by potentiostatic electrolysis as described by Cohen.25 UV/visible absorption spectroscopy showed that solutions could be prepared with the individual plutonium oxidation states being present at a high degree of purity (>95%). The concentrations of actinide ions in these solutions generally were ∼10-3 M. For the (10) Huber, G.; Passler, G.; Wendt, K.; Kratz, J. V.; Trautmann, N. In Handbook of Radioactivity Analysis, 2nd ed.; L’Annunziata, M. F., Ed.; Academic Press: San Diego, 2003. (11) Prange, A.; Schaumlo¨ffel, D. Fresenius J. Anal. Chem. 1999, 364, 452456. (12) Day, J. A.; Caruso, J. A.; Becker, S. J.; Dietze, H.-J. J. Anal. At. Spectrom. 2000, 15, 1343-1348. (13) Li, J.; Umemura, T.; Odake, T.; Tsunoda, K.. Anal. Chem. 2001, 73, 59925999. (14) Michalke, B. J. Anal. At. Spectrom. 1999, 14, 1297-1302. (15) Barnes, R. M. Fresenius J. Anal. Chem. 1998, 361, 246-251. (16) Taylor, K. A.; Sharp, B. L.; Lewis, D. J.; Crews, H. M. J. Anal. At. Spectrom. 1998, 13, 1095-1100. (17) Lu, Q.; Bird, S. M.; Barnes, R. M. Anal. Chem. 1995, 67, 2949-2956. (18) Kannamkumarath, S. S.; Wrobel, K.; Wrobel, K.; B’Hymer, C.; Caruso, J. A. J. Chromatogr., A 2002, 975, 245-266. (19) Dabek-Zlotorzynska, E.; Lai, E. P. C.; Timerbaev, A. R. Anal. Chim. Acta 1998, 359, 1-26. (20) Costa-Ferna´ndez, J. M.; Bings, N. H.; Leach, A. M.; Hieftje, G. M. J. Anal. At. Spectrom. 2000, 15, 1063-1067. (21) Ashdown, Ross P.; Marriott, Philip J. J. High Resolut. Chromatogr. 2000, 23, 430-436. (22) Pozdniakova, S.; Padarauskas, A. Analyst 1998, 123, 1497-1500. (23) Kohlicˇkova´, M.; Jedina´kova´-Krˇizˇova´, V. J. Radioanal. Nucl. Chem. 2000, 246, 549-556. (24) Korkisch, J. Handbook of Ion Exchange Resins: Their Application to Inorganic Analytical Chemistry; CRC Press: Boca Raton, 1989; Vol. II. (25) Cohen, D. J. Inorg. Nucl. Chem. 1961, 18, 207-210.
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capillary electrophoresis experiments, the solutions were diluted with 1 M acetic acid to obtain concentrations of ∼5 × 10-5 M. Neptunium Solutions. A solution of Np-237 was prepared by dissolving NpO2 in 8 M HNO3. Purification was performed by applying a procedure similar to that described for plutonium. The stock solution contained 5 × 10-2 M Np(V) in 0.1 M HClO4. An aliquot of this solution was electrochemically reduced26 to Np(III) at a potential of -0.1 V (NHE) and stored in a glovebox under argon atmosphere ( Th(IV) ∼ Np(V) > Pu(VI) ∼ Pu(V) > U(VI) ∼ Np(IV) > Pu(IV) This order is very similar to the experimental data. The lower (30) Pettit, L. D.; Powell, H. K. J. Stability Constants Database, International Union of Pure and Applied Chemistry; Academic Software; 1993. (31) Burney, G. A.; Harbour, R. M. The Radiochemistry of Neptunium, Report NAS-NS-3060, United States Atomic Energy Commission, Technical Information Centre: Oak Ridge, TN, 1974. (32) Katz, J. J.; Seaborg, G. T.; Morss, L. R. The Chemistry of the Actinide Elements, 2nd ed.; Chapman and Hall: New York, 1986; Vol. 1.
Table 2. Metal Species Expected in 1 M Acetic Acid with pH 2.4a
a Species are only shown with fractions of >10% of the total metal concentration. The main species are in boldface type.
mobility of Np(IV) and Pu(IV) as compared with Th(IV) may be explained by stronger complexation with acetate due to the smaller ionic radii of these two cations. The dissimilarities in the electropherograms of the model system and for Pu(III-VI) reflect the limited use of Th(IV), Np(V), and U(VI) as oxidation-state analogues for the plutonium oxidation states IV-VI. Comparison with UV/Visible Absorption Spectroscopy. An important prerequisite for the further application of the method is that the plutonium and neptunium oxidation states are not modified in the course of the analytical procedure. Therefore, a number of experiments were carried out to check the stability of the plutonium oxidation states in the electrolyte and to compare the results obtained by CE with UV/visible absorption spectroscopy. Pu(VI), being very sensitive to reduction by organic substances, showed no change in the UV/visible spectrum (λmax ) 831 nm) over a period of 24 h, indicating that no reduction is induced by the acetic acid. The analysis of a Pu(III)/Pu(V) mixture dissolved in HClO4 (1.1 × 10-4 M plutonium with 1 × 10-3 M HClO4) by UV/visible absorption spectroscopy is shown in Figure 4 (top). It is obvious that Pu(V) is very difficult to quantify with this spectroscopic method when Pu(III) and Pu(IV) are coexisting. The main absorption band of Pu(V) at 569 nm is overlapped by the absorption bands of the other oxidation states. In the studied mixture, Pu(III) can be unambiguously determined via the 600and 665-nm absorption bands. Then, Pu(V) is quantified by mathematical spectra deconvolution. This method yields 51% Pu(III) and 49% Pu(V) with an uncertainty of (5%. Other species could not be observed in the spectra (Pu(IV) at 470 nm; Pu(VI) at 831 nm). The electropherogram obtained from the same sample solution diluted by a factor of 10 with 1 M acetic acid shows two well-separated peaks, corresponding to Pu(III) (47.5%) and Pu(V) (51.9%), and a very small peak for Pu(IV) (0.6%). This electropherogram is shown in Figure 4 (bottom). This example illustrates that 1 M acetic acid does not significantly disturb the Pu(III)/Pu(V) equilibrium of the original solution. Additionally, it can be seen that even very small amounts of one species
Figure 4. Analysis of a mixture of Pu(III) and Pu(V) by UV/visible absorption spectroscopy (top) and CE-ICPMS (bottom) after addition of 1 M acetic acid. The y-axis of the electropherogram is shown in a logarithmic scale for better illustration. The percentage designations in the plots denote the fractions of the species. Table 3. Reproducibility of the Separation of Plutonium Oxidation State Species by CE-ICPMS Pu oxidation species
relative amount (%) peak data (% RSDa) migration time peak area peak height a
Pu(III)
Pu(IV)
Pu(V)
31.3
60.3
8.4
0.2 4.1 4.9
0.6 4.5 2.3
0.5 4.7 8.9
% RSD (relative standard deviation) of peak data (n ) 5).
coexisting with other species present at higher concentrations can be detected with the CE-ICPMS method. In the case of neptunium, the stability of Np(IV) under aerobic conditions was poor and oxidation to Np(V) occurred rapidly. Keeping the sample solution under argon atmosphere prior to the injection into the capillary minimized the oxidation of Np(IV). However, even traces of oxygen dissolved in the electrolyte were sufficient to produce considerable amounts of Np(V). By sample preparation under argon atmosphere and rapid injection, Np(IV) can be determined with an uncertainty of
