Microwave-Assisted Sample Treatment in a Fully Automated Flow

Microwave-Assisted Sample Treatment in a Fully Automated Flow-Based Instrument: ... The effectiveness of the automated sample treatment procedures has...
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Microwave-Assisted Sample Treatment in a Fully Automated Flow-Based Instrument: Oxidation of Reduced Technetium Species in the Analysis of Total Technetium-99 in Caustic Aged Nuclear Waste Samples Oleg B. Egorov,* Matthew J. O’Hara, and Jay W. Grate

Microsensors and Microfluidics Group, Fundamental Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352

An automated flow-based instrument for microwaveassisted treatment of liquid samples has been developed and characterized. The instrument utilizes a flow-through reaction vessel design that facilitates the addition of multiple reagents during sample treatment and removal of the gaseous reaction products and enables quantitative removal of liquids from the reaction vessel for carryoverfree operations. Matrix modification and speciation control chemistries that are required for the radiochemical determination of total 99Tc in caustic aged nuclear waste samples have been investigated. A rapid and quantitative oxidation procedure using peroxydisulfate in acidic solution was developed to convert reduced technetium species to pertechnetate in samples with high content of reducing organics. The effectiveness of the automated sample treatment procedures has been validated in the radiochemical analysis of total 99Tc in caustic aged nuclear waste matrixes from the Hanford site. In the practice of elemental and radiochemical analysis, sample pretreatment and matrix modification procedures represent an important and often crucial element of the overall analysis scheme. Sample digestion and matrix modification steps are commonly employed to improve reliability of the analysis by reducing matrix effects and to ensure compatibility of the sample matrix with subsequent separation or detection methodologies.1-4 In the case of analytes with varying chemical and oxidation state speciation * To whom correspondence should be addressed. E-mail: oleg.egorov@ pnl.gov. Fax: 509-373-1457. (1) Gedye, R. J. Am. Chem. Soc. 1999, 121, 4729. 10.1021/ac0497196 CCC: $27.50 Published on Web 05/28/2004

© 2004 American Chemical Society

(e.g., As(III)/As(V); Se(IV)/Se(VI); Cr(III)/Cr(VI); Tc(IV)/Tc(VII)), chemical reactions are typically required to convert the analyte to a desired chemical form or fixed oxidation state prior to total analyte measurements.5-16 To enhance the effectiveness and speed of the sample treatment reactions, heating using conventional conductive and, more recently, microwave methods is often used. We are interested in the development of real-time or near-realtime monitors to analyze the radiochemical content of samples from the large-scale processing of aged nuclear waste. Due to the complex and variable matrixes involved, matrix modification and sample digestion procedures are required, and these must be rapid and fully automated. In this paper, we describe a flow-based system for sample treatment in the analysis of total technetium in aged nuclear waste, where technetium exists in a mixture of valence states that must all be converted to pertechnetate for subsequent radiochemical separation and analysis. Because of the rapid, uniform, and well-controlled heat delivery, microwave-assisted sample preparation offers significant advantages in effectiveness, speed, and reproducibility relative to conventional sample treatment techniques based on conductive heating.1,2,4 Various microwave instrumentation designs and procedures have been reported for sample digestions and other treatments using both closed- and open-vessel reactor formats.2 (2) Kingston, H. M.; Haswell, S. J. Microwave-Enhanced Chemistry: Fundamentals, Sample Preparation, and Applications; American Chemical Society: Washington, DC, 1997. (3) Mermet, J. M. In Microwave-Enhanced Chemistry: Fundamentals, Sample Preparation, and Applications; Kingston, H. M., Haswell, S. J., Eds.; American Chemical Society: Washington, DC, 1997; pp 371-400. (4) Richter, R. C.; Link, D.; Kingston, H. M. Anal. Chem. 2001, 73, 31A-37A.

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Closed-vessel microwave-assisted digestions can be carried out at elevated pressures of up to 80 bar and temperatures of up to 250 °C, using modern commercially available microwave instrumentation.2 In general, closed-vessel digestion is well suited for a single-step processing of small quantities of refractory sample matrixes. However, because of the need for cooling/depressurization periods and disassembly of the reaction vessels prior to reagent addition the closed-vessel format is not ideally suited for sample treatment procedures that require sequential addition of multiple reagents.3 In addition, digestion chemistries that result in generation of significant quantities of gaseous species are problematic in closed-vessel operation. Closed-vessel procedures executed at high pressures and temperatures present significant challenges for reliable implementation in an automated flow format.17 However, numerous applications of microwave-assisted sample treatment at low temperatures and pressures have been reported in a continuous, flow-through regime using coiled tube flow reactors, which are functionally similar to closed-vessel reactors.6,10,18-22 Nevertheless, flow-through tubing reactors do not allow addition of multiple reagents during sample treatment, and procedures that generate gaseous species result in the formation of gas segments in the tubing, which leads to uncontrolled and irreproducible flow behavior.21,22 In contrast, the open-vessel digestion format offers flexibility in the design of sample treatment procedures that require addition of multiple reagents and venting of gaseous products.3,23 However, the open-vessel format is more challenging for implementation in an automated flow format. Automation generally entails extensive fluid-handling operations required for the delivery of sample and reagents to the digestion vessel, retrieval of the processed sample for subsequent analysis, and washing of the reaction vessel for subsequent measurements. Commercial open-vessel microwave instrumentation allows automated addition of several re(5) Mena, M. L.; Gomez, M. M.; Palacios, M. A.; Camara, C. Lab. Autom. Inf. Manage. 1999, 34, 159-165. (6) Caballo-Lopez, A.; Luque de Castro, M. D. Anal. Chem. 2003, 75, 20112017. (7) Gurleyuk, H.; Tyson, J. F.; Uden, P. C. Spectrochim. Acta, Part B 2000, 55B, 935-942. (8) Thomas, P.; Finnie, J. K.; Williams, J. G. J. Anal. At. Spectrom. 1997, 12, 1367-1372. (9) Arruda, M. A. Z.; Gallego, M.; Valcarcel, M. J. Anal. At. Spectrom. 1996, 11, 169-173. (10) Burguera, J. L.; Carrero, P.; Burguera, M.; Rondon, C.; Brunetto, M. R.; Galignani, M. Spectrochim. Acta, Part B 1996, 51B, 1837-1847. (11) Eva Moreno, M.; Perez-Conde, C.; Camara, C. J. Anal. At. Spectrom. 2000, 15, 681-686. (12) He, Y.; El Azouzi, H.; Luisa Cervera, M.; de la Guardia, M. J. Anal. At. Spectrom. 1998, 13, 1291-1296. (13) de Andrade, J. C.; Rocha, J. C.; Baccan, N. Analyst 1984, 109, 645-647. (14) de Andrade, J. C.; Rocha, J. C.; Baccan, N. Analyst 1985, 110, 197-199. (15) Egorov, O. B.; O’Hara, M. J.; Ruzicka, J.; Grate, J. W. Anal. Chem. 1998, 70, 977-984. (16) Egorov, O. B.; Kurath, D. E. Automated 99Tc Analysis in AW-101 and AN107 “Diluted Feed ”Matrixes, Battelle NW, PNWD-3014, Richland, WA, 2000. (17) Pichler, U.; Haase, A.; Knapp, G.; Michaelis, M. Anal. Chem. 1998, 71, 4050-4055. (18) Burguera, J. L.; Burguera, M.; Rondon, C. Talanta 2002, 58, 1167-1175. (19) Burguera, M.; Burguera, J. L. Anal. Chim. Acta 1998, 366, 63-79. (20) Galignani, M.; Bahsas, H.; Brunetto, M. R.; Burguera, M.; Burguera, J. L.; Petit de Pena, Y. Anal. Chim. Acta 1998, 369, 57-67. (21) Huang, C.-C.; Yang, M.-H.; Shih, T.-S. Anal. Chem. 1997, 69, 3930-3939. (22) Stewart, L. J. M.; Barnes, R. M. Analyst 1994, 119, 1003-1010. (23) Flock, J.; Michael, F.; Ohls, K. D. Fresenius J. Anal. Chem. 1999, 363, 306310.

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agents during digestion operation but does not provide for automated delivery of the sample and subsequent recovery of the digested sample for analysis or for vessel cleanup.24 There have been few reports on open-vessel microwave-assisted sample pretreatment with automated or semiautomated fluid handling, and these have been concerned primarily with digestion of biological samples for metal determinations.21,22 Sample matrixes associated with the aged nuclear waste process streams are composed of caustic brine solutions with complex and varying chemical and radiological composition. In the case of 99Tc, the analyte speciation is varying and dependent on the source of the waste material.25,26 In certain types of the Hanford aged nuclear wastes with high organic content, up to 60-70% of total 99Tc may be present as reduced non-pertechnetate species of unknown chemical composition (to be referred to as “non-pertechnetate”).25,27-29 Recent studies indicate that nonpertechnetate may be associated with organic complexants such as gluconate.27 Sample treatment for automated total Tc determination must convert all the Tc to pertechnetate in acidic solution. This speciation and matrix composition is required in order to be compatible with the separation chemistries used to isolate Tc from other radioactive species before radiometric determination by counting. Non-pertechnetate species are not retained by anion exchange, extraction chromatographic, or sorbent extraction materials useful in the analytical separation of 99Tc.15,16,25,28-32 Our prior work in automated radiochemical analysis has focused on flow-based approaches for analytical separations and detection.15,33-41 In the present paper, we turn to the automated (24) http://www.cem.com. (25) Blanchard, D. L.; Brown, G. N.; Conradson, S. D.; Fadeff, S. K.; Golcar, G. R.; Hess, N. J.; Klinger, G. S.; Kurath, D. E. Technetium in Alkaline, HighSalt, Radioactive Tank Waste Supernate: Preliminary Characterization and Removal, Pacific Northwest National Laboratory, PNNL-11386, Richland, WA, 1997. (26) Blanchard, D. L.; Kurath, D. E.; Rapko, B. M. Small Column Testing of Superlig-639 for Removing 99Tc from Hanford Tank Waste Envelope C (Tank 241-AN-107), Pacific Northwest National Laboratory, PNWD-3028, Richland, WA, 2000. (27) Lukens, W. W.; Shuh, D. K.; Schroeder, N. C.; Ashley, K. R. Environ. Sci. Technol. 2004, 38, 229-233. (28) Schroeder, N. C.; Radzinski, S. D.; Ashley, K. R.; Truong, A. P.; Whitener, G. D. J. Radioanal. Nucl. Chem. 2001, 250, 271-284. (29) Schroeder, N. C.; Radzinski, S. D.; Ashley, K. R.; Truong, A. P.; Szczepaniak, P. A. Proc. Sci. Technol. Disposal Radioact. Tank Wastes 1998, 301-320. (30) Kurath, D. E.; Blanchard, D. L.; Bontha, J. R. Ion Exchange Distribution Coefficients for 137Cs and 99Tc removal from Hanford Wank Supernatants AW101 (Envelope A) and AN-107 (Envelope C,) Pacific Northwest National Laboratory, BNFL-RPT-009, Richland, WA, 1999. (31) Chamberlin, R. M.; Ashley, K. R.; Ball, J. R.; Bauer, E.; Bernard, J. G.; Berning, D. E.; Schroeder, N. C.; Sylvester, P. ACS Symp. Ser. 2004, No. 868, 177-192. (32) Nevissi, A. E.; Silverston, M.; R. S., S.; Kaye, J. H. J. Radioanal. Nucl. Chem. 1994, 177, 91-99. (33) Grate, J. W.; Strebin, R. S.; Janata, J.; Egorov, O.; Ruzicka, J. Anal. Chem. 1996, 68, 333-340. (34) Egorov, O.; Ruzicka, J.; Grate, J. W.; Janata, J., Egorov, O.; Ruzicka, J.; Grate, J. W.; Janata, J. Spectrum 96: Nuclear and Hazardous Waste Management International Topic Meeting, 1996; pp 189-195. (35) Egorov, O.; Grate, J. W.; Ruzicka, J. J. Radioanal. Nucl. Chem. 1998, 234, 231-235. (36) Grate, J. W.; Fadeff, S. K.; Egorov, O. Analyst 1999, 124, 203-210. (37) Grate, J. W.; Egorov, O. Anal. Chem. 1998, 70, 3920-3929. (38) Egorov, O.; O’Hara, M. J.; Grate, J. W.; Ruzicka, J. Anal. Chem. 1999, 71, 345-352. (39) Grate, J. W.; Egorov, O. B.; Fiskum, S. K. Analyst 1999, 124, 1143-1150. (40) Grate, J. W.; Egorov, O. B. Anal. Chem. 1998, 70, 779A-788A.

sample treatment necessary to convert nuclear waste process samples in the as-received matrix to samples containing the analyte in a suitable speciation and matrix for the subsequent automated separation and analysis. Specifically, we describe a novel flow-based sample treatment system designed to enable the determination of total 99Tc in caustic aged nuclear waste samples using radiochemical separation and analysis methodology. Automated microwave-assisted sample acidification and oxidation procedures using peroxydisulfate were developed and investigated that enable rapid and reliable conversion of the reduced form of 99Tc to 99Tc(VII). This sample treatment system is part of an integrated 99Tc automated radiochemical analyzer system that processes and analyzes nuclear waste samples in 15 min or less, using anion exchange chemistry to separate Tc from other radioactive species and a flow-through solid scintillator detector. The development, testing, and implementation of the fully integrated system will be described in more detail in a subsequent paper, while this paper focuses on automated microwave-assisted sample treatment in a flow format. The effectiveness of the automated instrumentation and sample treatment procedures was demonstrated using actual aged nuclear waste solutions with high content of organic and non-pertechnetate species. Technetium-99 is a radionuclide of significant concern in nuclear waste processing. It is important to immobilize it in stable waste forms because of its high abundance in aged nuclear waste from weapons production, its long half-life, and its mobility (as pertechnetate, 99Tc(VII)) in the environment if not contained.25,28,29,32,42 EXPERIMENTAL SECTION Microwave Instrument and Flow-Through Reaction Vessel. The automated instrument was configured using the STAR 2 open-vessel microwave digestion system (CEM Corp., Matthews, NC). The flow reaction cell was constructed using a 7-mL PTFE Teflon concave-bottom digestion vessel (CEM), originally designed for use as a replaceable inner vessel in MARS 5 closed digestion systems (CEM). To adapt this configuration to flowbased automated operation, the screw cap of the digestion vessel was modified by drilling two 1.5-mm-i.d. holes to enable an airtight tight, slip-fit insertion of two lines (Figure 1). A line made of a 0.8-mm-i.d. FEP tubing(Upchurch Scientific, Oak Harbor, WA) (line B in Figure 1) was used for delivery of sample and various reagent solutions to the reaction vessel and subsequent removal of the processed sample solution from the vessel. The second line (line C in Figure 1) was constructed of 0.5-mm-i.d. FEP tubing and used for venting of the gaseous reaction products during sample treatment to a waste receptacle (Figure 2). The opened end of the sample/reagent delivery line was cut to ∼30° angle and the length of the line was adjusted to enable the placement of the tubing end in contact with the center of the concave-shaped bottom of the reaction vessel. These measures were found critical in enabling reliable and reproducible quantitative removal of the liquids from the reaction vessel as required for automated operation. The flow-through reaction vessel was positioned in the center of the vessel holder machined from PTFE (Figure 1). The (41) Grate, J. W.; Egorov, O. B. In Handbook of Radioactivity Analysis, 2nd ed.; L’Annunziata, M. F., Ed.; Academic Press: Boston, 2003; pp 1129-1164. (42) Anders, E. The Radiochemistry of Technetium; National Academy of Science: Springfield, VA, 1960.

Figure 1. Schematic diagram of the flow-through reaction vessel design for microwave-assisted sample treatment: A, concave-bottom reaction vessel; B, sample/reagent delivery line; C, vent line; D, microwave cavity; E, reaction vessel holder.

Figure 2. Schematic diagram of the automated flow-based microwave-assisted sample treatment instrument.

vessel holder was designed to replace the standard commercial digestion vessel in the microwave instrument and to position a flow-through reaction vessel in the center of a microwave cavity. Automated Fluid-Handling System. The automated fluidhandling system shown schematically in Figure 2 was configured using two Kloehn 50300 (Kloehn Co., Las Vegas, NV) highprecision digital syringe pumps (resolution 48 000 steps). The pumps were equipped with the multiposition distribution valves and zero-dead volume syringes (syringe volume 5 mL). This design facilitates the use of a single pump for delivery of multiple reagents with minimal degree of carryover. Electronically actuated valves were of Valco Cheminert series (Valco Instruments Co., Houston, TX). A two-way six-port injection valve (valve 1 in Figure 2) equipped with an 8-mm-i.d. FEP tubing sample loop (volume 0.736 mL) was used to introduce sample into the digestion cell. A six-port three-position stream selection valves (model C5F) were used upstream (valve 2) and downstream (valve 3) from the digestion cell to facilitate delivery of sample/reagents and agitation gas stream to the reaction vessel. This valve configuration enables connection of the nonselected lines to the individual dedicated Analytical Chemistry, Vol. 76, No. 14, July 15, 2004

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waste ports. Reagent and transport lines were constructed from FEP Teflon tubing of 0.8-mm i.d. (Upchucrh Scientific). A 50-cmlong transfer line (Figure 2) constructed from 0.5-mm-i.d. PEEK tubing (Upchurch Scientific) was used for the delivery of the digested sample to the distribution valve (valve 3 in Figure 2) and provided sufficient degree of cooling. The Gilson 223 robotic sample changer (Gilson Inc., Middleton, WI) was connected to a syringe pump for delivery of the sample solution to the injection valve and enabled remote, automated selection of the sample solutions. Fluid handling and microwave instrumentation were controlled via RS-232 serial communication protocol using a laptop PC computer running under Windows 2000 Professional operating system (Microsoft, Redmond, WA). A four-port serial PCMCIA card (B&B Electronics, Ottawa, IL) was used in the laptop PC to provide additional communication ports. Two serial ports were dedicated to controlling the pumps and valves, which were connected to the serial ports via two five-port programmable serial switches (B&B Electronics). The sample changer and microwave digestion system were controlled via dedicated serial ports. The automation and control software was developed in ANSI C programming language using LabWindows version 5.5 Integrated Development Environment (National Instruments, Austin, TX). The software enabled comprehensive control of all individual instrument devices and operation sequences using multithreaded logic for asynchronous control of the individual devices. Reagents, Standards, and Nuclear Waste Samples. Deionized water (MilliQ-Plus, 18.1MΩ) was used for the preparation of reagent solutions. Unless otherwise stated, all chemicals used were of analytical grade. Nitric acid solutions of the 99Tc(VII) standards were prepared by dilution of the NIST-traceable standard stock solutions. Activity of the prepared standards was verified by liquid scintillation counting and inductively coupled plasma mass spectrometry (ICPMS). Peroxydisulfate reagent was prepared by dissolution of the Na2S2O8 salt (98%+ purity) (Aldrich) in DI water. Nitrogen gas was used for sample agitation during microwave-assisted sample treatments. Nuclear waste simulant solutions (∼5 M Na+) were obtained as ready to use solutions and were formulated to emulate nonradioactive composition of the processed aged waste samples.43 Nuclear waste samples were processed solutions derived from aged nuclear wastes stored at the U.S. DOE Hanford site. Processed liquid samples used in this study were composed of the blend of supernatants and caustic sludge washes adjusted by dilution to ∼5 M Na+ concentration. To enable safe handling outside the shielded radiological facility, the samples were first placed in contact with the crystalline silicotitanate inorganic anion exchanger for removal of bulk 137Cs activity.26Removal of the bulk 90Sr and transuranic elements was carried by coprecipitation treatment. Coprecipitation was carried out by adding Sr(NO3)2 and NaMnO4 salts to the sample and removing precipitates after several hours of stirring. Described sample treatments were noted not to significantly affect total 99Tc concentration and speciation.26 Resulting sample solutions were filtered using 0.45-µm filters prior to experimentation. To determine the content of non-pertechnetate species, the nuclear waste samples were batch contacted with (43) Hassan, N. M.; Nash, C. A.; Saito, H. H.; Marra, J. C. SuperLig- 644 Resin Accelerated Aging Study, Westinghouse Savannah River Co., WSRC-MS2002-00872, Aiken, SC, 2002.

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excess Superlig 639 solid-phase extraction material (IBC Technologies, Inc., American Fork, UT). This sorbent material selectively removes pertechnetate from caustic samples but does not exhibit affinity toward non-pertechnetate species.26 The content of non-pertechnetate was determined by subtracting the total 99Tc concentration in the samples with removed pertechnetate from the total 99Tc concentration of the untreated samples. Specific samples selected for this study were derived from tanks with documented high content of organic complexants and documented high fraction of the total 99Tc present in nonpertechnetate form.25,28,29 These samples present the most challenging scenario with regard to the feasibility of reliable and quantitative control of the 99Tc speciation. Studies directed at the optimization of the microwave-assisted oxidation procedure studies were carried out using an aged nuclear waste sample with ∼57% content of the total 99Tc present in the non-pertechnetate form and total organic carbon (TOC) content of 1.5 M or 18.2 g/L. Analytical Methods. Baseline total 99Tc levels in nuclear waste samples were determined by ICPMS analysis in the in-house analytical laboratory using standard procedures.44 Following automated microwave-assisted sample treatment, total 99Tc determination was carried out via a radiochemical analysis method,41 where the separation of 99Tc(VII) from interfering radionuclides was accomplished using a 0.83-mL bed column (Upchurch) packed with MP-1 M (Bio-Rad Laboratories, Hercules, CA) strongly basic anion exchanger, particle size 38-75 µm.32 The separation was carried out by loading a 1.6-mL aliquot of the acidified digestion sample on the column preconditioned with 0.2 M nitric acid. Next, a sequence of column wash steps using 5 mL of 0.2 M HNO3, 6 mL of 1 M NaOH, 6 mL of 0.2 M HNO3-0.5 M H2C2O4, and 5 mL of 2 M HNO3 was used to remove interfering radionuclide species. Following the column wash sequence, the 99Tc(VII) was eluted using 5.6 mL of 8 M HNO Flow-through 3. scintillation counting using the Beta-Ram 2B (IN/US Systems, Inc. Tampa, FL) detector equipped with a 1-mL glass scintillator flow cell was employed for the detection of 99Tc(VII) activity eluted from the separation column.15,16,33,40 Standard addition methodology was used to correct for detection efficiency and analyte recovery during sample treatment and separation.38 During method development experiments, standard addition was performed for each measurement in order to correct for varying digestive treatment parameters (i.e., digestion times and amount of oxidizing reagent). In the analysis of nuclear waste samples, where the sample digestion conditions were constant, standard addition measurement was performed for each sample matrix in order to account for detection efficiency and recovery variations in the analysis of samples with varying matrix composition (see Analysis of the Aged Nuclear Waste Samples section). The quantitative analytical results were calculated using the standard addition formula, A ) (NAs)/(Ns - N), where A is the sample activity (Bq/mL), As is the added 99Tc(VII) standard activity (Bq/ mL), N is the detector response for the unspiked sample, and Ns is the detector response for the spiked sample. The effectiveness of the automated microwave-assisted nonpertechnetate oxidation was determined by comparing total 99Tc (44) Urie, M. W.; Wagner, J. J.; Greenwood, L. R.; Farmer, O. T., III; Fiskum, S. K.; Ratner, R. T.; Soderquist, C. Z. Inorganic and Radiochemical Analysis of AW-101 and AN-107 “Diluted Feed” Materials, Battelle, Pacific Northwest Division, PNWD-2463, Richland, WA, 1999.

analysis results obtained using a radiochemical analysis approach, specific to 99Tc(VII), with the speciation-independent total 99Tc data obtained by ICPMS. TOC analysis of nuclear waste samples and digested sample fractions was performed in the in-house analytical laboratory using a UIC coulometric carbon analyzer (UIC Inc., Joliet, Ill) and the hot peroxydisulfate oxidation procedure described previously.45 The stability of peroxydisulfate reagent with respect to its decomposition in water was determined by indirect measurement of its decomposition product, sulfate. Sulfate determination in peroxydisulfate reagent was carried out gravimetrically as BaSO4 using 0.3 M Ba(NO3)2 reagent.46 Safety Considerations. Highly radioactive nuclear waste samples used in this work present severe radiological and chemical toxicity hazards and can be handled only in a specialized nuclear facility. Proper safety protocols for sample handling and waste disposal must be observed. RESULTS AND DISCUSSION Sample Oxidation and Matrix Modification Chemistries. Previous studies on the radiochemical determination of the total 99Tc in nuclear waste samples with high content of organic and non-pertechnetate species established that reliable and quantitative oxidation of non-pertechnetate species represents a significant challenge.25,28 The addition of the Cr(VI), H2O2, and Ce(IV) oxidizing reagents to acidified, heated samples was ineffective in enabling rapid and quantitative oxidation of the non-pertechnetate species.25 Quantitative oxidation of non-pertechnetate was possible only after vigorous digestions with the repeated additions of the Ce(NH4)2(NO3)2-16 M HNO3 reagent followed by evaporation of the digestate to incipient dryness.28 The reported time required to process a batch of samples was 8 h, which would be excessive for a rapid automated analysis. Further studies by Schroeder et al. of reagents for nonpertechnetate oxidation in caustic nuclear wastes have identified peroxydisulfate as a promising oxidizing reagent.28,29 Oxidation of non-pertechnetate was possible in matrixes with stoichiometric excess of nitrite and total organic carbon relative to peroxydisulfate. In the aged nuclear waste samples, nitrite and organic species represent major sources of reducing capacity. Peroxydisulfate is a strong oxidizing reagent, and both nitrite and organic compounds will be oxidized by peroxydisulfate (E° )2.01 V for the S2O82-/SO42- pair)).46,47 In these reports, the authors observed an appreciable degree of oxidation of non-pertechnetate in caustic samples after reaction time of 4-12 h at 60 °C. Our own preliminary manual experiments indicated the possibility of successful non-pertechnetate oxidation using peroxydisulfate in diluted acidified samples after ∼5-min reaction time at 100 °C, Although these experiments provided precipitate-free solutions suitable for subsequent radioanalytical separations,16 the oxidation of caustic samples was problematic and resulted in formation of precipitates, which precludes reliable automation in the flow regime. (45) Gillespie, B. M.; Stromatt, R. W.; Baldwin, D. L.; Hoopes, F. V. 101-SY Hydrogen Safety Project Chemical Analysis Support: Window “C” Total Organic Carbon Analysis, Pacific Northwest National Laboratory, Richland, WA, 1992. (46) Skoog, D. A.; West, D. M. Fundamentals of Analytical Chemistry; CBS College Publishing: Philadelphia, 1982. (47) Goulden, P. D.; Anthony, D. H. J. Anal. Chem. 1978, 50, 953-958.

Based on these considerations, we selected peroxydisulfate chemistry with prior sample acidification for investigation of the feasibility of rapid, quantitative 99Tc speciation control procedures in automated format. Sample acidification using nitric acid eliminates sample reduction capacity due to nitrite by converting it to HNO2, which is unstable in acid media and rapidly decomposes at elevated temperatures.48 Acidification provides sample matrix compatible with subsequent separation procedures. Furthermore, prior studies established that the kinetics of oxidation of organic compounds by peroxydisulfate are more rapid in acidic solutions.47 Instrument Design and Procedure for Automated Microwave-Assisted Sample Treatment. Automated microwave-assisted sample preparation steps were implemented in the fluidhandling apparatus shown schematically in Figure 2. The instrument was set up to execute the following operations: (1) introduction of the sample aliquot to the reaction vessel; (2) addition of reagents required for sample acidification and oxidation; (3) mixing of the sample solution; (4) delivery of microwave energy to the reaction mixture in a controlled, reproducible, and safe manner; (5) recovery of the digested sample for subsequent off-line or on-line processing; (6) wash of the transfer lines and reaction vessel to eliminate cross-contamination. In the current instrument design implementing zero dead volume syringe pumps, sequential addition of all reagent solutions to the reaction vessel, recovery of the processed sample for subsequent analysis, and wash of the sample preparation module components (Table 1) was accomplished using a single syringe pump (pump 2 in Figure 2). The flow cell for microwave-assisted digestion consisted of a small-volume, concave-bottom reaction vessel modified with one line extending to the bottom center of the vessel for solution deliveries and sample retrieval. A second line was added as a vent for outgassing, and a holder was designed to position the vessel in the microwave cavity. This flow cell is described in more detail in the Experimental Section and is shown in Figure 1. This flow cell enabled straightforward procedures for delivery of sample and reagent solutions to the reaction vessel, venting of the gases evolved during sample treatment, and consistently reliable quantitative retrieval of the digested sample from the reaction vessel for subsequent analysis. The small volume of the reaction cell without a condenser allowed implementation of a rapid, comprehensive wash cycle to eliminate cross-contamination without excessive generation of secondary waste. This flow cell design is well suited for automated processing of small samples and represents an improvement in design and ease of fabrication and operation relative to previously reported flow cells for automated open-vessel microwave digestions.21 The instrument design also provides a capability to deliver a gas stream to the sample/reagent delivery line of the reaction vessel for sample agitation. Gas delivery is accomplished by connecting a regulated gas source to the port of a multiposition valve 2. (Figure 2). Delivery of nitrogen stream at a flow rate of ∼10 mL/min enabled continuous sample mixing and agitation during digestion and facilitated removal of the gaseous reaction products. The sample agitation step was found to be critical in enabling steady and well-controlled microwave-assisted heating (48) Cotton, F. A. Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John Wiley & Sons: New York, 1988.

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Table 1. Automated Protocol for Microwave-Assisted Sample Treatment operation

step no. and description

sample load

1. using pump 1, aspirate 100-µL air segment to sample changer needle to eliminate dispersion; move sample changer needle to the sample tube 2. using pump 1, aspirate 1.4 mL of sample through valve 1 to load sample loop 3. wait for the sample to be dispensed from the sample loop to the reaction vessel (step 5) 4. move sample changer needle to rinse station; wash needle and sample loop with 3 mL of DI water 5. using pump 2, aspirate and deliver 2.8 mL of 1.6 M HNO3 to the digestion cell via sample loop 6. using pump 2, aspirate and deliver 2.5 mL of air to reaction vessel to flush transfer lines 7. begin delivery of the agitation gas to reaction vessel via valve 2 8. first digestion cycle 9. stop delivery of the agitation gas 10. using pump 2, aspirate and deliver 0.7 or 1 mL of 2 M Na2S2O8 to reaction vessela 11. using pump 2, aspirate and deliver 2.5 mL of air to reaction vessel to flush transfer lines 12. begin delivery of the agitation gas to the digestion cell via valve 2 13. second digestion 14. stop delivery of the agitation gas 15. using pump 2, aspirate 2 mL of the digested sample from the reaction cell; dispense to waste 16. using pump 2, aspirate 1.6 mL of the digested sample from the reaction cell; dispense solution via valve 3 for subsequent analysis 17. using pump 2, aspirate 5 mL from reaction cell to recover residual sample; dispense to waste 18. using pump 2 deliver 7 mL of 1.6 M HNO3 to the reaction cell 19. using pump 2 aspirate 8 mL of 1.6 M HNO3 from the reaction cell to recover wash solution and dispense to waste 20. repeat wash steps 18 and 19 using 1 mL of 1.6 M HNO3

acidification

oxidation

wash

a A 0.7-mL aliquot of 2 M Na S O was used in nuclear waste sample analysis; 1-mL aliquot was used in studies involving varying amounts of 2 2 8 oxidant.

of samples with high content of dissolved solids. Excessive violent boiling due to local overheating and sporadic sparking were observed without sample agitation. The temperature feedback control option is provided by the STAR 2 microwave instrument and uses the temperature reading on the infrared temperature sensor located in the microwave cavity to adjust microwave power settings as necessary to maintain a fixed digestion temperature. Nevertheless, we found this approach to be ineffective in enabling precise control of the digestion process when using small PTFE reaction vessels and small sample volumes. Precise control of the microwave energy delivery during digestion process was enabled via control software by timing of the length of the microwave digestion and by continuous monitoring and adjustment of the magnetron power setting performed via control software at 1-s intervals. Several operational parameters including sample/reagent volumes, reagent concentrations, microwave power levels, and lengths of the digestion intervals were investigated in order to develop practical working sample treatment protocols. The stepwise operational procedure developed for automated sample delivery, acidification, oxidation of non-pertechnetate using peroxydisulfate, recovery of the processed sample, and wash of the flow system is detailed in Table 1. The microwave-assisted sample treatment procedure consists two digestion cycles. The first digestion is performed after nitric acid is added to the sample solution; it facilitates removal of the nitrites and dissolution of the precipitates prior to addition of the oxidizing reagent. The second digestion cycle is performed following the addition of the peroxydisulfate reagent to the acidified diluted sample. The length of the digestion intervals and microwave instrument power settings for two digestion cycles are listed in Table 2. Microwave-Assisted Sample Acidification. We used nitric acid to convert caustic nuclear waste samples to dilute acidic 3874 Analytical Chemistry, Vol. 76, No. 14, July 15, 2004

Table 2. Microwave-Assisted Digestions Parameters microwave instrument parameter

first digestiona

second digestionb

ramp powerc ramp length digestion powerc digestion length

20% 10 s 10% 30 s

no ramp used no ramp used 5% (10-120)d; 90 se

a Performed after sample acidification. b Performed after addition of oxidizing reagent. c Magnetron power in percent of the full power.e See text for details. d A 90-s digestion was used in nuclear waste analysis.

solutions prior to oxidative treatment with peroxydisulfate. Several issues must be considered in the acidification of caustic aged nuclear wastes. Specifically, these sample matrixes have a high content of dissolved Al (4-17 g/L)44 and the formation of significant amounts of Al(OH)3 precipitate was evident upon acidification. Excess acid is required to completely dissolve this precipitate. The rate of dissolution was found to be slow and to increase with increasing acid concentration and heating. However, the addition of relatively concentrated acid (e.g., 8 M HNO3) resulted in a substantial degree of foaming. Moreover, exceedingly high acid concentrations (e.g., >3 M) are undesirable, because 99Tc(VII) uptake by anion exchange or extraction chromatographic sorbents, to be used in subsequent radioanalytical separation of 99Tc(VII), rapidly decreases with increases in acid concentration.15,16 Consequently, the volume and concentration of acid must be selected to achieve reliable dissolution of precipitate during heating without excessive foaming, while maintaining acid concentration in the range compatible with subsequent separations. Previous analytical characterization data and our own titration results indicated that total base content (determined as the number of equivalents of acid necessary to make solutions neutral)

in aged nuclear waste can range from 0.7 to 3 mol/L16,44 The automated sample acidification procedure listed in Table 1 uses 2.8 mL of 1.6 M nitric acid per 0.736-mL sample aliquot and provides a 4.8-fold dilution of the original sample. Assuming total base content of nuclear waste sample solutions to be between 0.7 and 3 mol/L, the expected final concentration of the acidified sample is ∼0.6 and 1.1 M H+, respectively. Delivery of the sample solution from the sample loop and addition of the nitric acid reagent were accomplished simultaneously by dispensing nitric acid solution through the sample loop to the reaction vessel, followed by flushing the transfer lines with air. (steps 5-6 in Table 1). The sample acidification step was followed by the initial digestion cycle under continuous sample agitation. Microwave power and digestion length settings of the first digestion cycle are shown in Table 2. The digestion cycle incorporates a short power ramp period designed to enable prompt gentle boiling of the reaction mixture. Microwave power and digestion interval length settings shown in Table 2 were successful in obtaining complete dissolution of precipitates and removal of the NOx gaseous species in the shortest time possible, while avoiding excessive effervescence, foaming, and uncontrolled boiling of the sample solution. Microwave-Assisted Oxidation with Peroxydisulfate. Following sample acidification and the initial digestion cycle, oxidative treatment was carried out by delivering an aliquot of the peroxydisulfate reagent to the reaction vessel and applying microwave power under continuous sample agitation. Experimentation with nuclear waste simulants indicated that relatively low microwave power level of 5% was sufficient to provide virtually instantaneous, gentle boiling of the previously heated digestate. In addition, this power setting enabled steady boiling conditions for extended periods of time without excessive evaporative losses of the sample solution. Digestions using higher power levels resulted in sparking and sporadic intense boiling accompanied by excessive evaporative losses. The effectiveness of non-pertechnetate oxidation as a function of the moles of peroxydisulfate added was examined by delivering 1-mL aliquots of the peroxydisulfate reagent (concentration range 0-2 M Na2S2O8) to acidified samples. The length of the subsequent digestion interval was 60 s. The percentage of the total Tc in the sample that is found by radiochemical analysis of the digested samples, where the total Tc is determined by ICPMS, is plotted in Figure 3 (graph A) as a function of the quantity of oxidant added to the sample. The relative total 99Tc results (radiochemical analysis/ICPMS analysis), indicate that addition of peroxydisulfate reagent in excess of 0.5 mmol (0.7 mmol/mL of the nuclear waste sample) results in close agreement between the pertechnetate-specific radiochemical analysis procedure and speciation-independent ICPMS techniques. Close agreement between these methods is indicative of the quantitative (within analytical error) conversion of reduced 99Tc species to 99Tc(VII) using peroxydisulfate oxidizing reagent in samples with high content of organics and non-pertechnetate species. Note that in the absence of oxidizing reagent (first data point of Figure 3A), sample acidification and microwave digestion treatments alone did not result in significant extent of conversion of the reduced 99Tc to 99Tc(VII). In this manner, the use of oxidizing reagent is imperative in enabling complete oxidation of the non-pertechnetate

Figure 3. Relative total 99Tc measurement (radiochemical analysis/ ICPMS analysis) (graph A) and the relative TOC content in the digested sample (graph B) as a function of peroxydisulfate mole quantity added to the acidified nuclear waste sample. Digestion times are 60 and 90 s for the relative total 99Tc and relative TOC results, respectively.

species in aged nuclear waste samples. To provide a safety margin with regard to the excess of the oxidizing reagent, we used 0.7 mL of 2 M Na2S2O8 (1.9 mmol of peroxydisulfate/mL of the nuclear waste sample) for use in the automated sample treatment procedure. The relative total 99Tc, obtained using 0.7 mL of 2 M Na2S2O8 reagent (1.9 mmol of peroxydisulfate/mL of the nuclear waste sample), is plotted as a function of the length of the second digestion in Figure 4 (graph A). Microwave-assisted Tc oxidation in acidified samples using peroxydisulfate is quite rapid, providing quantitative results after ∼30 s of digestion under selected conditions (5% continuous magnetron power level; gentle boiling under continuous gas agitation) Visual observations indicated noticeable and continuous reduction of the total digested sample volume with the increase in the length of the digestion interval past the ∼60-s mark. The decrease of the digested sample volume was accompanied by the increase in the 99Tc(VII) spike recovery in the standard addition measurements, which corresponds to an increase in apparent analyte concentration in the aliquot used for subsequent radiochemical measurement. These observations point out that 99Tc(VII) concentration occurs as the sample volume is reduced due to liquid evaporation during digestion treatment. These results show that no significant analyte volatilization losses occur during microwave digestion treatments. Potential volatilization losses of the 99Tc(VII) from hot acidic solutions were of initial concern in the development of microwave-assisted sample treatment procedures due to the known volatility of the Tc(VII) compounds in acid media.42,49 In addition, if so desired, sample volume reduction and corresponding analyte preconcentration can (49) Colton, R. The Chemistry of Rhenium and Technetium; John Wiley & Sons: London, 1965.

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Figure 5. Decomposition of 1.5 M sodium peroxydisulfate solution in 0.01 M NaOH (plot A, squares) and DI water (plot B, cicrles) as a function of reagent storage time at 25 °C.

Figure 4. Relative total 99Tc measurement (graph A) and the relative TOC content in digested samples (graph B) as a function of digestion time. Amount of peroxydisulfate added to the sample is 1.4 mmol in both cases.

be attained during microwave hearing by employing longer digestion intervals. Oxidation of TOC. Sample acidification and the first microwaveassisted digestion step results in the removal of the nitrite (as NOx),37 which would otherwise consume peroxydisulfate in the second digestion. In this scenario, organic species remaining in the acidified sample determine its reducing capacity with respect to peroxydisulfate oxidant. Because of the high content of the organic species in samples with high non-pertechnetate ratio, we were interested in examining the fate of the TOC during microwave-assisted sample treatments. The extent of the TOC removal was determined by performing TOC analysis of the digested samples and calculating percent of the TOC in the digested sample relative to the original untreated nuclear waste sample. TOC analysis results for samples processed under varying digestion times were also corrected for relative concentration increase due to evaporative sample volume reduction using 99Tc(VII) spike recovery data obtained in standard addition measurements. The effect of the digestion time on the TOC content in the digested sample was examined using 0.7 mL of 2 M Na2S2O8. Experimental results showing the plot of the remaining percent TOC as the function of the digestion time are presented in Figure 4B. Results in Figure 4 indicate that oxidation of the TOC (graph B) occurs at a slower rate compared to the oxidation of nonpertechnetate species (graph A). Moreover, no further oxidation of TOC is evident after 90 s of the microwave-assisted digestion with ∼33% of the initial TOC remaining unoxidized when using 0.7 mL of 2 M peroxydisulfate reagent. No further reduction in TOC content after 90 s of digestion is likely indicative of complete consumption of the oxidizing reagent. Prior experiments (see above section) and parallel total 99Tc radiochemical measurements 3876 Analytical Chemistry, Vol. 76, No. 14, July 15, 2004

in the digested sample indicated quantitative oxidation of nonpertechnetate under these digestion conditions. In this manner, quantitative oxidation of the non-pertechnetate is possible with a significant fraction of the TOC still present in the sample. Apparently, microwave-assisted oxidation using peroxydisulfate enables selective preferential oxidation of the non-pertechnetate species relative to organics in the acidified aged nuclear waste samples. As a result, complete destruction of the TOC is not a prerequisite for reliable control of the 99Tc speciation in aged nuclear waste samples. Graph B in Figure 3, shows a plot of the remaining TOC as function of the amount of peroxydisulfate added to the sample during digestion (digestion length 90 s). The first data point, corresponding to microwave-assisted digestion with no peroxydisulfate added, indicates that sample acidification and microwave digestion treatments alone do not result in the significant destruction of TOC content. The extent of the TOC oxidation is determined by the amount of oxidant added to the sample and increases linearly with the increase of the amount of peroxydisulfate. The TOC oxidation data were used to calculate the stoichiometry of the peroxydisulfate reaction with the organic carbon in acidified nuclear waste samples. For the sample matrixes and digestion conditions used in this study, 1.4 mol of S2O82- was required to oxidize 1 mol of organic carbon to CO2 in acidified aged nuclear waste sample. In this manner, if so desired, complete destruction of the organics in the sample can be accomplished during digestion by providing excess peroxydisulfate reagent. Stability of Peroxydisulfate Solutions. Aqueous peroxydisulfate solutions are known to undergo spontaneous decomposition during reagent storage.46 It is also known that the rate of peroxydisulfate decomposition is slower in basic media compared to neutral or acidic solutions.47 We assessed the extent of peroxydisulfate decomposition by performing gravimetric measurements of the decomposition product, sulfate, performed on the aliquots of the reagent solutions. Figure 5 shows plots of the percent peroxydisulfate remaining in solution as a function of storage time at 25 °C for a 1.5 M Na2S2O8 reagent prepared in

Table 3. Comparative Results of the Nuclear Waste Sample Analysis

nuclear waste sample matrix

non-pertechnetate (%)

total 99Tc, radiochemical measurementa,b (µg/mL)

AN-102-A AN-107 AP-104 AN-107-Td

57 57 72 100

4.10 ( 0.08 2.75 ( 0.17 10.9 ( 0.54 1.80 ( 0.04

total 99Tc, ICPMS measurementc (µg/mL) 4.35 ( 0.43 2.91 ( 0.29 10.5 ( 0.10 1.66 ( 0.16

a Using automated microwave-assisted sample treatment. b Errors are given as RSD of the triplicate measurements. c Reported errors of (10%. d Sample matrix with all 99Tc(VII) removed.

0.01 M NaOH (plot A) and DI water (plot B). These results indicate that peroxydisulfate reagent solutions exhibit good shelf life at room temperature. After a storage period of 40 days, the extent of reagent decomposition was only ∼9% for the reagent solution prepared in DI water. The extent of peroxydisulfate decomposition in basic solution was slightly less relative to the DI water matrix with ∼6% of the reagent decomposing after a 40day storage period for the 0.01 M NaOH solution. Analysis of the Aged Nuclear Waste Samples. Automated microwave-assisted sample acidification and Tc speciation control procedures were validated in the analysis of three types of aged nuclear waste samples. Selected samples are representative of the Hanford tank waste matrixes with high content of organics and non-pertechnetate species. Sample acidification and non-pertechnetate oxidation was carried out using the automated operational sequence detailed in Table 1 and the microwave power and digestion length settings listed in Table 2. The sample preparation and radiochemical analysis sequences for each sample matrix were performed in triplicate. Additional measurement sequences were performed on each sample matrix spiked with a known amount of 99Tc(VII) standard in order to enable quantification via standard addition methodology. Standard addition measurements were followed by processing and analyzing a blank sample, using a 99Tcfree nuclear waste simulant solution in place of the actual sample. Total 99Tc results obtained by radiochemical measurement using the automated sample treatment protocol are compared with ICPMS results in Table 3 and are in good agreement. No 99Tc was detectable in the blank samples processed after samples with high 99Tc content (carryover less than 0.1%). These results indicate that fluidic procedures designed to wash sample introduction module components, reaction vessel, and associated transfer lines (steps 4 and 17-20 in Table 1) were efficient in eliminating crosscontamination between samples. The total time required to execute the automated sample treatment protocol was 7.5 min, including a 2.6-min instrument wash procedure.

CONCLUSIONS AND SUMMARY We have developed a fully automated flow-based instrument for conducting microwave-assisted digestions of liquid samples in an open-vessel format. The instrument design enables straightforward and reliable fluidic procedures for delivering sequences of sample and reagent solutions to the reaction vessel, solution mixing, recovering the processed sample, and performing cleanup of the flow path for carryover-free operation. Furthermore, it tolerates outgassing during sample treatment operations. Matrix modification and speciation control procedures were developed for the determination of total 99Tc in caustic aged nuclear waste samples from the Hanford site, providing rapid and quantitative conversion of reduced forms of 99Tc to 99Tc(VII) and a final sample matrix compatible with subsequent radiochemical separation and analysis procedures. The effectiveness of the automated sample treatment procedures was validated in the analysis of challenging nuclear waste matrixes with high content of dissolved organic and non-pertechnetate species. Microwaveassisted acidification and oxidation using peroxydisulfate successfully overcome previously documented challenges in controlling the 99Tc speciation in such samples.25,28,29 Development of reliable, rapid, automated sample treatment procedures for caustic aged nuclear waste samples is significant because it opens the possibility of developing integrated radionuclide analyzer instrumentation for monitoring nuclear waste processes in near real time, using previously described automated radiochemical separation and detection methods.40,41 Development of the integrated radionuclide analyzer for automated radiochemical analysis of total 99Tc in aged nuclear waste will be a subject of a subsequent publication. Furthermore, we believe that the general automated microwaveassisted sample treatment methodology and instrumentation described in this study may find more diverse applications in the development of integrated measurement techniques for analyses of various biological and environmental samples, which require digestion, matrix modification, and speciation control chemistries prior to analyte detection. ACKNOWLEDGMENT This work has been supported in part with funding from the U.S. DOE Office of Science Environmental Management Science Program. The Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute.

Received for review February 18, 2004. Accepted April 27, 2004. AC0497196

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