Anal. Chem. 2002, 74, 5629-5634
Simultaneous Separation and Detection of Actinides in Acidic Solutions Using an Extractive Scintillating Resin J. E. Roane and T. A. DeVol*
Department of Environmental Engineering and Science, Clemson University, Clemson, South Carolina 29634-0919
An extractive scintillating resin was evaluated for the simultaneous separation and detection of actinides in acidic solutions. The transuranic extractive scintillating (TRU-ES) resin is composed of an inert macroporous polystyrene core impregnated with organic fluors (diphenyloxazole and 1,4-bis-(4-methyl-5-phenyl-2-oxazolyl)benzene) and an extractant (octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide in tributyl phosphate). The TRU-ES resin was packed into FEP Teflon tubing to produce a flow cell (0.2-mL free column volume), which is placed into a scintillation detection system to obtain pulse height spectra and time series data during loading and elution of actinides onto/from the resin. The r-particle absolute detection efficiencies ranged from 77% to 96.5%, depending on the r energy and quench. In addition to the on-line analyses, off-line analyses of the effluent can be conducted using conventional detection methods. The TRU-ES resin was applied to the quantification of a mixed radionuclide solution and two actual waste samples. The on-line characterization of the mixed radionuclide solution was within 10% of the reported activities whereas the agreement with the waste samples was not as good due to sorption onto the sample container walls and the oxidation state of plutonium. Agreement between the on-line and off-line analyses was within 35% of one another for both waste samples. Radiochromatography involves the sequential or simultaneous separation of radioactive elements and radiation detection. Sequential actinide separation and detection has been achieved with either a heterogeneous or homogeneous flow cell scintillation detector coupled to an ion-exchange system1 or to a sequential injection extraction chromatography system.2-4 In sequential radiochromatographic applications, the ideal heterogeneous scintillation material is inert with no solvent interaction or analyte sorption to affect the quantitative analysis; however, this condition is rarely met in all cases. Conventional flow cell detection systems tend to have high minimum detectable activities due to the small detector volumes and short counting times required to maintain * Corresponding author. E-mail:
[email protected]. Fax: 864-6560672. (1) Reboul, S. H.; Fjeld, R. A. Radioact. Radiochem. 1994, 5 (3), 42-49. (2) Grate, J. W.; Egorov, O. B.; Fiskum, S. K. Analyst 1999, 124, 1143-1150. (3) Grate, J. W.; Egorov, O. E. Anal. Chem. 1998, 70, 3920-3929. (4) Grate, J. W.; Egorov, O. E. Anal. Chem. 1998, 779A-788A. 10.1021/ac026050z CCC: $22.00 Published on Web 09/25/2002
© 2002 American Chemical Society
analyte separation. The application of stop-flow data collection increases the counting time by retaining the activity within the detector for an extended period, thus resulting in increased sensitivity. The stop-flow method can be problematic due to the ability to stop the flow when the activity in the flow cell is at a maximum. While typical radiochromatographic applications involve separation and subsequent detection, whole-column chromatography incorporates the entire chromatography column into the radiation detector active volume for simultaneous separation and detection. Link and Synovec described a whole-column chromatography system for the analysis of 11C-labeled analogues of epinephrine using a Dionex C14 media phase placed within a cavity milled from two pieces of plastic scintillator to detect the penetrating annihilation radiation.5 The whole-column detection system produces an integral chromatogram since it measures the accumulated activity on the column. The output consists of a series of plateaus as all the radionuclides are on the column and are detected until each radionuclide (or radiolabeled component) is eluted sequentially. By utilizing stop-flow conditions, the width of the counting plateau can be manipulated to provide an adequate counting time to obtain the desired sensitivity. The application of whole-column chromatography for R- and β-emitting radionuclides utilizing simultaneous separation and detection has been demonstrated. For a radionuclide sorbed to the resin, there is a high probability of radiation interaction with the scintillating resin and thus a high probability of detection. The technique of integrating concentration, separation, and detection into a single analytical tool provides a unique approach to identifying and quantifying R- and β-emitting radionuclides, which are problematic to detect due to their short ranges in aqueous media. Schram and Lombaert,6 and later adapted in the work of Schutte,7 developed systems incorporating both absorption and radioactivity detection with granular anthracene. Ion-exchange resins developed for separating and detecting β-emitting radionuclides in aqueous solution came into being in the early 1960s with the introduction of anionic and cationic scintillating ionexchange resins (SIERs).8 The SIERs were tested successfully in a variety of environmental and experimental scenarios, whereby (5) Link, J. M.; Synovec, R. E. Anal. Chem. 1999, 71, 2700-2707. (6) Schram, E.; Lombaert, R. Anal. Biochem. 1962, 3, 68-74. (7) Schutte, F. J. Chromatogr. 1972, 72, 303-309. (8) Heimbuch, A. M.; Gee, H. Y.; DeHaan, A., Jr.; Leventhall, L. Radioisotope Sample Measurement Techniques in Medicine and Biology; Proceedings of the International Atomic Energy Agency Symposium: Vienna, 1965; pp 505519.
Analytical Chemistry, Vol. 74, No. 21, November 1, 2002 5629
solutions containing radioactivity were contacted with the resins and then counted off-line. Extractive scintillating resins, which are analyte selective, are produced by coating a thin, transparent extractant onto a scintillating support. The concept, feasibility, and development of the extractive scintillator resin was demonstrated using the extractants, octyl(phenyl)-N,N-diisobutyl-carbamoylmethylphosphine oxide (CMPO) in tri-butyl phosphate (TBP), Aliquat 336, and bis-4,4′(5′′)-tert-butylcyclohexano-18-crown-6 in 1-octanol impregnated on a macroporous scintillating support for the analyte selective concentration and detection of actinides, pertechnetate, and strontium, respectively.9 The application of the Aliquat 336 and 18-crown-6 in 1-octanol-based extractive scintillators has been demonstrated for the detection of 99Tc and 89/90Sr in aqueous solutions, respectively.10-12 An extractive scintillator utilizing the R-NPO fluor and Dipex extractant has been developed for gross R detection in groundwater down to ∼0.5 Bq/L.13 In an independent effort, Egorov et al. developed selective scintillating microspheres for the simultaneous separation and detection of 99Tc or 90Sr/90Y in aqueous solutions using a bead injection radionuclide sensor system.14 An alternative approach undertaken by Headrick et al. utilized an extraction resin epoxied to scintillating fibers.15 The scintillating fibers are coated with cesiumselective dual-mechanism bifunctional polymer (DBFP) beads, which have rapid transfer kinetics to bring the analyte of interest into the polymer matrix and close to the recognition sites that retain the analyte. The research described herein presents the simultaneous separation and detection capability of the CMPO/TBP-based transuranic extractive scintillating (TRU-ES) resin for quantification of the actinides via a sequential elution method. Sequential separation of trivalent, tetravalent, and hexavalent actinides (americium, plutonium, neptunium, thorium, and uranium) using the CMPO/TBP extractants with off-line detection has been demonstrated.16 Also, CMPO/TBP has been used in a sequential injection system for the separation of actinides with on-line detection.2-4 In this work, the TRU-ES resin was characterized for sequential elution of the actinides and then applied to the simultaneous separation and quantification of a mixture of radionuclides, an acid-digested high-level waste (HLW) tank sludge, and a high-activity drain (HAD) tank supernatant. EXPERIMENTAL SECTION The TRU-ES resin, which consist of 120-µm-diameter macroporous cross-linked polystyrene beads (Amberchrom CG-161c, TosoHaas) impregnated with the fluors 2,5-diphenyloxazole (PPO; Sigma Chemical) and 1,4-bis-(4-methyl-5-phenyl-2-oxazolyl)benzene (DM-POPOP; Sigma Chemical) and the extractant CMPO/ (9) DeVol, T. A.; Roane, J. E.; Williamson J. M.; Duffey, J. M.; Harvey, J. T. Radioact. Radiochem. 2000, 11 (1), 34-46. (10) DeVol, T. A.; Egorov, O. B.; Roane, J. E.; Paulenova, A.; Grate J. W. Radioanal., Nucl. Chem. 2001, 249 (1), 181-189. (11) Ayaz, ?. ?.; DeVol, T. A. Nucl. Instrum. Methods, in press. (12) DeVol, T. A.; Duffey, J. M.; Paulenova, A. Radioanal., Nucl. Chem. 2001, 249 (2) 295-301. (13) Hughes, ?. ?.; DeVol, T. A. Nucl. Instrum,. Methods, in press. (14) Egorov, O. B.; Fiskum, S. K.; O’Hara, M. J.; Grate, J. W. Anal. Chem. 1999, 71, 5420-5429. (15) Headrick, J.; Seaniak, M.; Alexandratos, S.; Datskos, P. Anal. Chem. 2000, 72, 1994-2000. (16) Horwitz, E. P.; Chiarizia, R.; Dietz, M. L.; Diamond, H. Anal. Chim. Acta 1993, 281, 361-372.
5630
Analytical Chemistry, Vol. 74, No. 21, November 1, 2002
TBP. The fluors were impregnated into the beads using a modification of a previously reported method.17 The scintillating beads were subsequently impregnated with an actinide-selective extractant solution consisting of CMPO in TBP using established Eichrom Technologies, Inc. procedures.18 The metal ion uptake performance of the TRU-ES resin was comparable to the commercially available TRU resin when subjected to the quality control tests of Eichrom Technologies, Inc.18 The TRU-ES resin was dry packed into FEP Teflon tubing with a small amount of silanized glass wool packed at each end of the resin to prevent resin washout from the tubing. The tubing (0.3175-cm outer diameter × 0.1587-cm inner diameter) was filled with 0.103 ( 0.012 g of TRU-ES resin to a resin bed length of 14.9 ( 0.3 cm. The resulting pore volume of the flow cell was ∼0.2 mL (assuming a 0.65 void fraction16). The tubing was formed into a spiral coil of approximately three loops with the outer loop having a diameter of 2.86 cm. The flow cell scintillation detection system used in this study was similar to a previously published design.19 Figure 1 illustrates the experimental setup. A coiled flow cell is placed between two photomultiplier tubes (PMTs) that are shielded with 3.8-5.1 cm of lead to reduce the effects of background radiation, all of which is contained within a light-tight box. Two Aptec model 5008 multichannel analyzers (MCAs) are used to simultaneously acquire count data in pulse height and multichannel scaling modes (count rate time series data). Typical pulse height and multichannel scaling counting times were 10 and 1.67 min, respectively. The typical background count rate obtained after nitric acid conditioning of the flow cell was 0.2 counts/s. Solutions are pumped through the flow cell at a flow rate of 0.5 mL min-1 using a piston pump (Alltech model 426 HPLC pump). The loading effluent and eluted solutions were collected and retained for offline analysis by liquid scintillation counting (LSC) to determine the loading efficiency and percent total actinide recovery, respectively. The LSC analyses were performed using a Wallac 1415 R/β liquid scintillation counter (Perkin-Elmer Life Sciences) for a typical counting time of 20 min. The flow cell detection efficiencies were determined as a ratio of the net count rate and the loaded activity. Solutions of the actinides (233U, 239Pu, 241Am, 244Cm) as well as 90Sr/90Y were obtained from in-stock inventories and standardized as necessary in 2 M HNO3. Acid-digested sludge from a HLW tank and supernatant from a HAD tank were obtained from the Savannah River Technology Center (SRTC) at the Savannah River Site. Activity analyses of the samples were conducted at SRTC. The HLW sludge (pH
