J Radioanal Nucl Chem (2015) 305:199–205 DOI 10.1007/s10967-015-4123-4
Development of field-based separations for the rapid identification of uranium and plutonium Carol J. Mertz1 • Michael D. Kaminski1 • Ilya A. Shkrob1 • Michael Kalensky1 Vivian S. Sullivan1 • Yifen Tsai1
•
Received: 10 October 2014 / Published online: 8 May 2015 Ó Akade´miai Kiado´, Budapest, Hungary 2015
Abstract Method development for rapid processing and purification of uranium and plutonium was performed on a gas pressurized extraction chromatographic system using a single column of Eichrom’s DGA extraction resin. The demonstration of the U and Pu purification scheme provided in-line flowsheet processing in under 2 h with low reagent volumes (100–240 lL) for flowsheet processing stages. Quantitative recovery for U and Pu (95 and 98 %, respectively) with high selectivity between the target actinide analytes (99 % purity) in three bed volumes was achieved in the presence of a potential, environmental, interfering contaminant (iron). Keywords Uranium Plutonium Extraction chromatography Separations Flowsheet
Introduction The development of rapid, radioanalytical techniques to separate uranium and plutonium from complex, field samples are needed for the timely and accurate determination of nuclear material origin and processing activities. Widespread use of nuclear power and technology in the world has increased demands on analytical laboratories from the monitoring of numerous low-level, environmental samples with variable compositions. Environmental sampling has proven to be one of the strongest technical
& Carol J. Mertz
[email protected]; http://orcid.org/0000-0002-8431-3914 1
Argonne National Laboratory, 9700 S. Cass Ave, Argonne, IL 60439, USA
measures for detecting nuclear material and activities [1]. With the increase in sampling demands, new technologies must offer improvements such as automation, high throughput, reproducible chemical separations, short analysis times, and reduced costs to be effective. We have been developing a simplified flowsheet for the separation of uranium and plutonium separations based upon a single, extraction chromatographic resin. The flowsheet developed for recovery of U and Pu purified streams exploited the inline processing capability of extraction chromatographic resins as the primary means of concentrating the radionuclides from the raw acidic feed and separating the elements into purified streams. Demonstration of the flowsheet was performed using a gas pressurized extraction chromatography (GPEC) system with internal system volumes of less than 500 lL. Numerous procedures for radionuclide separations and purifications have been developed and explored for environmental samples [2–6]. Most of the methods for radionuclide analysis work have focused on a combination of multi-step processes involving co-precipitation, anion exchange and extraction chromatography separations. Varga and co-workers examined a sequential separation method for Pu and Am from soil samples which employed a coprecipitation step followed by extraction chromatographic separations in traditional resin columns of UTEVA and TRU resins [7]. Eikenberg et al. evaluated a number of separation methods for actinides in soils which employed a combination of multi-step extraction chromatographic and anion exchange separations [8]. Recent separations developed for U from sample fusion of environmental matrices of soil and organic-based samples employed the UTEVA resin [9]. Efforts to automate and reduce the time of actinide separations as a result of extensive chemical separations for complex sample matrices have explored
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extraction chromatography for bioassay matrices using the TRU resin [10]. Other researchers have explored a simple, rapid field approach for actinide separations based upon ligand polymer films [11, 12]. However, these films lack specificity and alpha spectrometry analyses will be complicated by complex field samples. The work presented here focuses on the development of a simplified flowsheet for the recovery of U and Pu which demonstrates in-line processing for rapid column separations. The flowsheet removes potential interferences from the target elements and provides a purified, and concentrated analyte stream that relaxes subsequent detection analyses for field-based applications. The U and Pu (U/Pu) separations in this work employed a diglycolamide extractant, N,N,N’,N’-tetrakis-2-ethylhexyldiglycolamide, as the stationary phase on a commercially available resin (Eichrom’s DGA Resin, Branched). Diglycolamides were developed as a result of studies on the actinide extraction properties of substituted diamides [13]. The tetraalkyldiglycolamides show unusually effective extraction of actinides and lanthanides from nitric acid media and their properties allow for ease in recovery of U and Pu by modifying the chemical solutions added to the column system [14]. The work in this study demonstrates flowsheets for the processing of simple, raw dissolved field samples using an in-line column processing system which employs a single chromatographic resin for the separation scheme. A summary of the extraction behavior of other elements besides U and Pu is useful for development of flowsheets for more complex dissolved field samples. However, our approach will focus on a simple simulant feed for the optimization of the separation. Flowsheets for the U/Pu separation were developed based upon retention profiles for the branched DGA resin [14]. There are several advantages for using the DGA resin to provide purified U and Pu streams from the processing of dissolved field samples. First, the extraction behavior of the DGA resin allows us to concentrate actinides and lanthanides while eluting interferences, especially Fe(III) through the column. Another advantage is that Pu(IV) is retained on the resin over a wide 0 range of HNO3 concentrations with retention factors, k , for Pu(IV) which remain nearly constant at 1000–2000 from 0.01 to 1 M HNO3 [14]. Reduction of Pu(IV) to Pu(III) allows elution of Pu as a concentrated stream at \0.1 M HNO3. Maintaining the Pu in the ?IV oxidation state during column washes provides a way to exploit separation schemes for the other actinides. Elution of concentrated U streams can be achieved with sequential elutions at 0.1 and 0.01 M HNO3, respectively. The wash steps are designed to retain the target actinides (U and Pu) while rinsing entrained interferents from the column such as transition metals, reducing agents, or residual actinides. One such reducing agent, acetohydroxamic
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acid (AHA), forms strong complexes with Pu(IV) in high HNO3 solutions, however the kinetics for reduction can be slow and hydrolysis of the AHA occurs [15]. Reduction of the Pu(IV) in 1 M HNO3 was attempted in this study to allow removal of the reducing agent while simultaneously reducing Pu. Once Pu(IV) is reduced, additional 1 M HNO3 can be passed through the column to remove the reducing agent while retaining the Pu(III). Once the Pu(III) is cleaned of the reducing agent, the acidity can be lowered to\0.1 M HNO3 to elute the Pu(III) from the column. Elimination of the reducing agent is assumed to be important for downstream analytical procedures as many reducing agents are also good complexing agents for ?3 cations. If the reducing agent is complexed with the Pu(III), it may hinder our ability to further concentrate the eluted plutonium onto magnetic capture beads for concentration and analysis. This concentration can be accomplished by sorbing the eluted plutonium onto magnetic beads containing a suitable extractant. However, if the eluted plutonium is complexed, its sorption onto the magnetic beads will be reduced.
Experimental All chemicals were ACS analytical grade reagents and high purity standards. The DGA-branched resin was purchased from Eichrom (Darien, IL). Reducing agent solutions were freshly prepared prior to use. Solutions were prepared from distilled, deionized 18 MX water obtained from a Barnstead water purification system and Optima Grade nitric acid (Fisher Scientific). Purified stock solutions of 232U, 238 U, and 242Pu were characterized by alpha spectrometry, liquid scintillation counting (LSC) and inductively coupled plasma mass spectrometry (ICP-MS) measurements. A 4 M HNO3 feed solution was prepared by dilution of stock solutions of 232U, 238U, 242Pu and stable iron. The concentrations of the uranium and plutonium isotopes used in the separation were characterized by ICP-MS and alpha spectrometry (Table 1). Corrections for isobaric interferences in ICP-MS data were performed for m/z (mass-tocharge ratio) 238 from 238U and 238Pu contributions in both the feed and stage fractions.
Table 1 Composition of feed solution for target isotopes m/z
Isotope
Run #1 mass (ng)
Run #2 mass (ng)
56 232
56
4460 2.03
4520 1.45
238
238
276
299
238
238
0.103
0.134
242
242
2580
3352
Fe U
232
U Pu Pu
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The in-line processing column system used for the separations of U and Pu is a GPEC system with an internal system volume of less than 450 lL (and less than 200 lL without the sample injection loop) designed by Idaho National Laboratory and is described elsewhere [16]. The GPEC system specifics for this study include: Eichrom’s DGA branched resin loaded in a 25 cm column, a 75 cm loading loop and a flow rate of 100 lL/min. Post-separation solution was collected from the DGA resin column output into pre-weighed vials. Various aliquots were taken from the fractions and analyzed separately. An aliquot (20-200 lL) of each fraction collected was analyzed by LSC during the separation to provide total alpha activity and to monitor the progress of the separation. For LSC, each aliquot was added to 1 mL of deionized water and 10 mL Packard Ultima Gold XR in a scintillation vial. Additional aliquots were analyzed by alpha spectrometry and ICP-MS to derive isotope ratios and stage purity.
Results and discussion First U/Pu separation Two separation schemes were performed on a gas pressurized chromatographic column system to demonstrate the separation and purification of uranium and plutonium from a simple solution containing iron, uranium, and plutonium in nitric acid media. The first column separation of uranium and plutonium using the branched DGA resin was performed using the separation scheme described next and summarized in Table 2. After pre-equilibrating the column by passing several bed volumes of 4 M HNO3, we loaded the feed solution onto the column and collected the eluate in separate fractions. The first wash was performed to scrub interferents such as Fe(III) from the column. Next, the U(VI) was stripped and rinsed from the column using dilute nitric acid (0.1 M HNO3). The next step of the flowsheet was to simultaneously reduce the Pu(IV) to Pu(III) Table 2 Flowsheet run #1
with the reductant (0.5 M AHA) and retain the Pu(III) on the column in high acid (1 M HNO3). This would eliminate AHA from downstream operations, but not strip it from the column. The behavior of Pu(III) is based upon the k’ for Am(III) which acts as a surrogate for Pu(III) at 1 M HNO3 whereby k’ [ 100 [14]. Finally, recovery of the Pu(III) was to be eluted using dilute nitric acid (0.1 M HNO3). Figure 1 shows the recovery of U and Pu as a percentage of the initial feed content for each fraction collected from the column (the eluate). Quantification of U and Pu was determined by ICP-MS and alpha spectrometry. During the separation, the progress of the separation was monitored by liquid scintillation using alpha/beta mode discrimination. Sample loading did not produce breakthrough, and the first rinse was also successful in keeping uranium and plutonium on the column. The stripping of uranium from the column with 0.1 M HNO3 was nearly quantitative. Detailed analysis showed that we recovered 95 % of uranium with 99 % purity from Pu. The subsequent rinse of the column with additional 0.1 M HNO3 produced a cumulative uranium recovery of 97 %. The Pu reduction with 0.5 M AHA in 1.0 M HNO3 followed by rinsing with additional 1.0 M HNO3 to remove residual AHA and its breakdown products appeared to be usuccessful. Subsequent stripping of plutonium with 0.1 M HNO3 failed to elute plutonium and analyses of the reducing agent was not performed. In an attempt to recover the Pu, the column was allowed to soak in 0.1 M HNO3 overnight. However, when the column was drained the following morning, little alpha activity was detected. This overnight soak produced an aliquot that was \2 % of the total Pu feed with 95 % Pu purity. An additional attempt to elute the Pu from the column was performed with 0.5 M AHA in 0.1 M HNO3. The reducing agent in dilute nitric acid recovered 91 % of the Pu from the feed with 99 % purity from U. An additional wash of the column was performed to remove the remaining Pu in 0.1 M HNO3. Excellent agreement was observed between LSC (data not shown) and ICP-MS results for the U and Pu separations.
Step
Stage
Reagent
Purpose
1
Pre-condition
4 M HNO3
Pre-equilibrate column
2
Load
4 M HNO3
Load target elements
3
Rinse
4 M HNO3
Remove interferents (Fe)
4
U strip/rinse
0.1 M HNO3
Recover U
5
Pu reduction/AHA rinse*
0.5 M AHA/1 M HNO3
Reduce Pu/strip AHA*
6
Pu strip*
0.1 M HNO3
Recover Pu*
7
Overnight Pu strip*
0.1 M HNO3
Recover Pu*
8
Pu strip/rinse
0.5 M AHA/0.1 M HNO3
Recover Pu
* Unsuccessful flowsheet steps for the removal of Pu from the column (and corresponds to flowsheet stage ‘‘No Pu strip/rinse’’ in Fig. 1)
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Fig. 1 Elution profile for separation of U and Pu from first flowsheet demonstration
Optimizing Pu column stripping The reduction and stripping scheme for Pu failed in the first separation run as discussed in the previous section. The Pu strip stage using 0.1 M HNO3 following the AHA reduction should have removed a measurable quantity of Pu(III), if it had formed. Instead, no significant activity was detected. Moreover, AHA appears to be an inferior complexant for Pu(IV) at 1 M HNO3 when compared with the functional groups of the diglycolamide resin. It was not until we lowered the acidity to 0.1 M HNO3 in the presence of AHA that we could successfully elute the Pu. The lower HNO3 concentration suggests that AHA may only reduce Pu(IV) to Pu(III) in low acid, or an AHA-Pu(IV) complex competes favorably with diglycolamide at 0.1 M HNO3, or a combination of these two mechanisms. Additional studies to understand the stripping mechanism of Pu in the presence of AHA have been initiated, but will not be discussed in this study. In an effort to optimize Pu stripping during a column separation, studies with different reducing/complexing solutions were examined for column separations. The DGAbranched resin column was loaded using 1 bed volume (BV) of purified plutonium feed (242Pu stock) at 4 M HNO3. Next, the column was rinsed with 2 BVs of 1 M HNO3. To strip, freshly prepared reducing agent solutions were eluted through the column with 3 BVs. A range of AHA concentrations (0.01–0.5 M) in 0.01 M HNO3 were compared with 0.5 M AHA in 0.1 M HNO3. In addition, some common reductants, sodium formaldehyde sulfoxylate (SFS, under the trade name RongaliteÒ), ascorbic acid, and TiCl3 were used in column runs. The stripping eluate was collected into small fractions and analyzed for Pu.
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Recoveries of Pu as a function of the Pu strip solutions in 3 BVs are shown in Fig. 2. The most effective Pu stripping solution was 0.5 M AHA in 0.01 M HNO3 with 97.3 % recovery in 3 BVs, followed by 0.5 M AHA in 0.1 M HNO3 with 90.7 % recovery in 3 BVs. While the concentrated AHA solution (0.5 M) in dilute nitric acid is more effective at stripping the Pu from the resin, the recovery of Pu decreased significantly (\20 % recovery) as the concentration of the AHA reducing agent was reduced in dilute nitric acid solutions of 0.01 M. All other reducing agent solutions had poor Pu recoveries (Pu recoveries of 29.8 % for the TiCl3 and others with less than 10 %). Second U/Pu separation Based upon results from the first flowsheet U/Pu demonstration and studies on the optimization of the Pu stripping, we optimized the Pu strip/rinse stage for the second flowsheet demonstration as shown in Table 3. The modification of the flowsheet was performed to optimize Pu recovery without any changes to feed solution (HNO3 solution containing iron, uranium, and plutonium), extractant resin (branched DGA resin), or the rest of the flowsheet for U recovery. Thus, the first 4 stages of the first and second flowsheet demonstration are the same. In the second flowsheet, the Pu was stripped from the column using 0.5 M AHA in 0.01 M HNO3 followed by column rinsing with 0.01 M HNO3. The recovery results for U and Pu are shown in Fig. 3 as a percentage of the initial feed content for each fraction collected from the column (the eluate). Loading of the feed showed no retention of the Fe and did not produce breakthrough of U or Pu. The first rinse was also successful in
J Radioanal Nucl Chem (2015) 305:199–205 Fig. 2 Recovery of Pu in 3 bed volumes for stripping with different reducing agent solutions: a) 0.5 M AHA in 0.1 M HNO3, b) 0.5 M AHA in 0.01 M HNO3, c) 0.1 M AHA in 0.01 M HNO3, d) 0.05 M AHA in 0.01 M HNO3, e) 0.01 M AHA in 0.01 M HNO3, f) 0.2 M ascorbic acid, g) 0.04 M SFS in 0.01 M HNO3, and h) 0.02 M TiCl3
203
100
Pu Recovery (%)
80
60
40
20
0
a
b
c
d
e
f
g
h
Stripping Stage Solution
Table 3 Flowsheet run #2
Step
Stage
Reagent
Purpose
1
Pre-condition
4 M HNO3
Pre-equilibrate column
2
Load
4 M HNO3
Load target elements
3
Rinse
4 M HNO3
Remove interferents (Fe)
4
U strip/rinse
0.1 M HNO3
Recover U
5
Pu strip/rinse
0.5 M AHA/0.01 M HNO3
Recover Pu
6
Rinse
0.01 M HNO3
Rinse column
Fig. 3 Elution profile for cumulative recovery of plutonium, uranium and iron from second flowsheet demonstration
keeping uranium and plutonium on the column. The stripping of uranium from the column with 0.1 M HNO3 was nearly quantitative. Detailed analysis showed that we recovered 95 % of uranium with 99 % purity from Pu. The
subsequent rinse of the column with additional 0.1 M HNO3 produced a cumulative uranium recovery of 97 %. The next stage in the separation was elution of Pu with 0.5 M AHA in 0.01 M HNO. Recovery of the Pu was
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Table 4 Summary of uranium and plutonium separation using GPEC column system Separation stage
Target/separation element
U Strip
U/Pu
Pu Strip
a b
Pu/U
Bed volume
Recovery (%)
Enrichment factora
Separation factorb
0.5
54.3
20.4
189
1.6
90.8
10.8
113
2.7
94.9
0.4
55.5
24.5
3.94 9 103
1.3
94.8
13.3
2.52 9 103
3.0
98.3
6.65
6.23
75.3
1.52 9 103
Ratio of analytical concentrations for stage fraction to feed measured by ICP-MS Ratio of analytical concentrations for target to separation element (238U or 242Pu) in stage fraction measured by ICP-MS
nearly quantitative 98 % of the Pu with 99 % purity from U. A final wash of the column was performed in 0.01 M HNO3 and contained very little U or Pu. The chromatographic column system using the DGA resin and the developed flowsheets provided high selectivity for U and Pu purified streams. The results summarized in Table 4 for the purified U and Pu streams from separation run #2 were performed in less than 2 h and concentrated the stage aliquot versus the feed by at least a factor of 6 (in B3 BVs) with recoveries of the element of interest of C95 %. These purified U and Pu streams have \0.1 % Pu and \0.5 % U in the respective streams. While limitations of the first separation were exhibited by the inferior stripping of Pu, the flowsheet was modified to quantitatively strip Pu using the second flowsheet. Modification of the first flowsheet was successful, in that the second separation successfully demonstrated the effective and quantitative stripping of Pu using the eluent, 0.5 M AHA in 0.01 M HNO3.
While limitations of the first separation were exhibited by the inferior stripping of Pu, the flowsheet was modified to quantitatively strip Pu using the second flowsheet. Modification of the first flowsheet was successful, in that the second separation successfully demonstrated the effective and quantitative stripping of Pu using the eluent, 0.5 M AHA in 0.01 M HNO3. The AHA acted as a reductant for Pu(IV) to Pu(III) and/or complexant for Pu(IV) which effectively strips Pu from the DGA column in 0.01 M HNO3. Additional schemes to produce a complexfree Pu stream may be required for downstream processing which may explore hydrolysis mechanisms of AHA. This separation flowsheet offers significant improvements to traditional separation methods which employ a sequence of tedious, time-consuming steps and generate large waste volumes. The demonstrated flowsheet provides the premise for future developments in simple and fast, field-based identification of U and Pu when coupled to automated separation system designs and the appropriate sample digester and detection systems.
Conclusions
Acknowledgments The manuscript was created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (‘‘Argonne’’). Argonne, a U.S. Department of Energy Office of Science laboratory and was supported by the Defense Threat Reduction Agency under interagency agreement, through U.S. Department of Energy contract DEAC02-06CH11357.
A separation scheme was developed and optimized for the separation of U and Pu from an iron, nitric acid solution (simulant for dissolved field sample) in under 2 h. The main advantage of the separation scheme is the rapid and efficient separation of the U and Pu using a single, extraction chromatographic column. The gas pressurized extraction chromatographic column system using the DGA resin and the developed flowsheet provided high selectivity for U and Pu purified streams. In the developed separation schemes, loading of the resin column with Pu and U is achieved with 4 M HNO3 and subsequent washing of the resin column of any transition metals (i.e., iron) requires C1 M HNO3. Uranium is eluted as a purified stream from the column at 0.1 M HNO3, while Pu(IV) is retained. A reduction in oxidation state from Pu(IV) to Pu(III) using an appropriate reducing agent is required to elute the purified Pu(III) stream from the column.
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