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Preparation of Polymer-Coated, Scintillating Ion-Exchange Resins for Monitoring of 99Tc in Groundwater Ayman F. Seliman,† Azadeh Samadi,‡ Scott M. Husson,‡ Emad H. Borai,† and Timothy A. DeVol*,§ †
Department of Analytical Chemistry and Environmental Control, Atomic Energy Authority, Cairo, Egypt Department of Chemical and Biomolecular Engineering and §Department of Environmental Engineering and Earth Sciences, Clemson University, Clemson, South Carolina 29634, United States
‡
ABSTRACT: The present study was oriented to prepare new scintillating anion-exchange resins for measurement of 99 TcO4 in natural waters. The organic fluor 2-(1-naphthyl)5-phenyloxazole was diffused into (chloromethyl)polystyrene resin. Thereafter, a thin layer of poly[[2-(methacryloyloxy)ethyl]trimethylammonium chloride] was grafted from the resin surface by surface-initiated atom transfer radical polymerization as an attempt to overcome potential problems related to the leaching of fluor molecules during usage. The residual chloromethyl groups of the polymer-coated resin were aminated by reaction with two different tertiary amines, triethylamine (TEA) and methyldioctylamine (MDOA). Off- and on-line quantification of 99Tc was achieved with high detection efficiencies of 60.72 ( 1.93% and 72.83 ( 0.81% for resin with TEA and MDOA functional groups, respectively. The detection limit was determined to be less than the maximum contaminant level (33 Bq L1) established under the Safe Drinking Water Act. The two functionalized resins were demonstrated to be selective for pertechnetate from synthetic groundwater containing up to 1000 ppm Cl, SO42, and HCO3 and up to 1200 ppb Cr2O72 in an acidic medium.
ignificant 99Tc has been released to the environment, contaminating the soil, surface, and groundwater in several sites. The main sources of 99Tc to the biosphere are the nuclear fuel cycle and the fallout from nuclear weapons tests.1 However, trace quantities of this radionuclide occur naturally in the Earth’s crust as a spontaneous fission product in uranium ores. Insignificant levels of the isotope are released into the environment as a result of the use of 99 mTc in radiopharmaceuticals. 99Tc draws special concern from environmental engineers and scientists due to its long half-life (t1/2 = 213 000 years) and mobility in the environment. High levels of 99Tc caused a reduction in thyroid hormone (T4) levels, decreased iodine uptake, and a reduction in fertility. The chemical toxicity to the thyroid is not observed as most damage is due to the radiological effects of 99Tc.2 The measurement of the 99Tc concentration is important to assess its environmental impact, but the measurement process is complicated by several factors. These factors include (1) the lack of any γ-ray emission, (2) the fact that it is typically found at low concentration, (3) the volatility of pertechnetate, and (4) radiometric or isobaric interferences during β spectroscopic or ICP-MS measurement, respectively.2 To overcome these difficulties, a highly selective extractant or anion-exchange resin for 99 TcO4 separation is required, which concentrates Tc in the presence of interfering anions (e.g., Cl, SO42, NO3, HCO3, and Cr2O72) of the natural water samples. A variety of commercial products have been used for the removal of pertechnetate from groundwater. The quaternary ammonium chloride extractant Aliquat-336 is one of the most common pertechnetate
S
r 2011 American Chemical Society
extractants.3,4 Aqueous biphasic extraction chromatography resins have successfully removed TcO4 from high ionic strength solutions.5 Also, several strong-anion exchangers such as Purolite A-520E triethylamine resin6 and Reillex HPQ7 have been used for 99Tc removal from aqueous waste. Other bifunctional, pertechnetate-selective ion-exchange resins have been developed.8,9 After separation, 99Tc concentrations commonly are measured by liquid scintillation counting or ICP-MS; however, these two techniques are difficult to use in situ or in the field. Therefore, suitable in situ sensors for long-term environmental monitoring need to be developed. Improved sensors could reduce costs considerably and facilitate different monitoring applications such as those needed for performance assessment and environmental remediation. Several promising approaches to 99Tc sensing have been developed based on preconcentration and scintillating flow cell or minicolumn sensors. The first trial to accomplish radionuclide separation by an ion-exchange resin with scintillating properties was reported over 40 years ago.10 After addition of the organic fluors during bead synthesis, the resin was aminated by trimethylamine. This scintillating anion exchanger was used “off-line” in several steps for measuring some individual radionuclides, such as 131I, 14C, 35S, and 36Cl. Other investigators used two different Received: December 21, 2010 Accepted: April 30, 2011 Published: May 24, 2011 4759
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Analytical Chemistry techniques based on the same principle to measure 99Tc in different matrixes. DeVol et al.3 and Egorov et al.4 coimmobilized organic scintillating fluors 2,5-diphenyloxazole and 1,4-bis(4methyl-5-phenyloxazol-2-yl)benzene or 1,4-bis(2-methylstyryl)benzene with the Aliquat-336 extractant within polymer support beads. DeVol et al.3 reported that the detection efficiency for 99 Tc decreased from 50% to 1% by the fifth loading/elution trial using 8 M HNO3 and attributed loss of efficiency to the removal and/or oxidation of the fluor during subsequent loading and elution cycles. Egorov et al.4 succeeded to partially overcome the fluor degradation or removal problem by using a minicolumn sensor, which renewed itself by fluidic replacement of the bifunctional resin. Despite the improved performance, this setup requires additional microfluidics to the system, thus increasing the complexity of the system. In another work, Aliquat-336 was used with 2-(1-naphthyl)-5phenyloxazole (R-NPO) as an organic fluor and the reported counting efficiency was 26.3%.11 Simultaneous separation and detection of 99Tc also was achieved by mixing extractant-loaded chromatography resin and inorganic scintillator beads.3,11 Generally, this technique has low counting efficiency except when the scintillating/extractant bead ratio is high. The low efficiency is attributed to the extracted radionuclides residing on the resin and the scintillator (radiation transducer) being an adjacent bead, which decreases the energy transfer to the scintillator. Egorov et al.12 reported a detection efficiency of 38 ( 1% by a mixture of a weak-anion exchanger with Bicron BC-400 scintillating beads. Although this sensor could be reused several times, its performance was sensitive to changes in groundwater properties, especially the concentration of chromate anion (Cr2O72) that accumulated in the column and had a strong quenching effect.13 Quenching is the decrease in detection efficiency due to the presence of certain chemicals or color in samples that reduce the number of photons detected for a given amount of energy deposited by each β-particle. Atom transfer radical polymerization (ATRP) is a recently developed,14 catalyst-activated reaction that allows for precise modification of the physical and chemical properties of surfaces.15 ATRP is a type of controlled polymerization. With proper ATRP formulation, termination reactions are minimized, yielding a narrow molecular weight distribution of polymer chains. When used for surface modification, surfaceinitiated ATRP produces highly uniform polymer nanolayers with thicknesses that are varied easily by adjusting the polymerization time.16 These features are important for concentration measurements of pure β emitters that can be affected easily by surface layer thickness and heterogeneity. Utilizing an ultrathin functional layer was expected to improve energy deposition in the scintillator and increase transmission of scintillation light relative to traditional, “thick” coatings.17 Thus, the fundamental objective of the current research is to prepare extractive scintillator materials that have physical and chemical stability for reliable, sustained performance in field measurements. These materials can be developed by utilizing surface-initiated ATRP to produce a stable, sensitive sensor that is selective for pertechnetate under a wide range of environmental conditions. Selectivity for 99Tc is achieved by aminating the coated scintillating resin beads with methyldioctylamine (MDOA) or triethylamine (TEA).
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’ EXPERIMENTAL SECTION Materials and Chemicals. (Chloromethyl)styrene divinylbenzene beads (100200 mesh and 0.9 mmol Cl/g) were obtained from TCI America (Tokyo, Japan). Organic fluor R-NPO and MDOA were from Alfa Aesar (Ward Hill, MA). Toluene and N,N0 -dimethylformamide (DMF) were from EMD Chemicals (Gibbstown, NJ). 2,20 -Bipyridyl (bipy; >99%), copper(I) chloride (>99.995%), copper(II) chloride (99.99%), HPLC water, methanol (99.8%), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (75 wt % solution in water), and TEA were from Aldrich (St. Louis, MO). All chemicals were used as received. Preparation of the Selective Scintillating Anion Exchangers. New solid-phase extractive scintillating materials were produced by a three-step procedure (Scheme 1). First, an organic fluor dissolved in toluene was diffused into a (chloromethyl)polystyrene resin for an 8 h period. The toluene solvent was separated from the scintillating beads by rotary evaporator under vacuum and at 60 °C. Thereafter, a thin layer of poly[[2-(methacryloyloxy)ethyl]trimethylammonium chloride] (poly[METAC]) chains was grown from benzyl chloride initiator sites of (chloromethyl)polystyrene resin beads by surface-initiated ATRP. For the ATRP reaction, a mixture of 2 parts by mass solvent (80:20 (v/v) methanol/water) and 1 part by mass monomer was used. CuICl, CuIICl2, and 2,20 -bipyridyl were added to the mixture in the following molar proportions: [METAC]:[CuICl]:[bipy]:[CuIICl2] = 100:2:5:0.1. The mixture was degassed using three freezepumpthaw cycles. The reaction mixture was then transferred into an oxygen-free glovebox, and 4 g of the scintillating (chloromethyl)polystyrene resin beads was put into 70 mL of the polymerization solution at room temperature for 5 h. These polymer-coated particles were then filtered on a filter paper, washed with large amounts of water until neutral and then with methanol, and dried overnight at 40 °C in a vacuum. After polymerization, residual chloromethyl groups of the polymer-coated resins were aminated by reaction with TEA or MDOA. This amination was conducted in a stirred suspension of the resin, 50 mL of DMF, and 10 mL of the amine at 60 °C for 4 h. The beads were washed with 1 M HCl and water until they became neutral and then finally with methanol and left to dry in air. The result is a chemically stable scintillating resin, with covalently bound ligands. Batch Uptake Experiments. The uptake properties were evaluated for the new tertiary amine MDOA by determining the distribution constant (Kd) using double distilled water (DDW) acidified with 0.05 M HNO3 and synthetic groundwater containing a 50 ppm concentration each of Cl, SO42, and HCO3 anions in 0.05 M HNO3. Approximately 15 mg of the prepared resin was weighed into a 6 mL glass vial, and a 4 mL volume of 99Tc-spiked DDW or groundwater was delivered to the sample vial. The sample was placed on a shaker and allowed to mix with the sorbent for 24 h. The experiment was repeated in triplicate. Each sample was then filtered into a clean vial to remove all sorbent from the liquid. Finally, a 1 mL volume of the filtrate was delivered to 10 mL of Ultima Gold AB scintillation cocktail for determination of the radioactivity remaining in the filtrate. Analysis was performed using a Tri-Carb 2700TR liquid scintillation counter (LSC). Sensor Stability. To investigate the effect of coating to stabilize the scintillator, two different columns were packed with the resin. One column contained uncoated resin aminated with MDOA, and the other had ATRP that was aminated with 4760
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Scheme 1. Synthetic Steps for Preparation of Scintillating Anion-Exchanger Resins
MDOA. The two columns were loaded separately into the radioHPLC detector where 5000 pore volumes of 25% methanol/ water (v/v) solution was pumped through the column at a flow rate of ∼1 mL/min. The activity on the column was monitored each minute using the multichannel scaling (MCS) mode. To quantify the leaching process, the effluent was collected and allowed to evaporate, and then the effluent was counted by the liquid scintillation technique. Sensor Setup. The prepared resins were evaluated using two different techniques, including off-line (minicolumnLSC) and on-line (flow cellflow scintillation analyzer (FSA)) detection modes. For off-line application (static measurement), scintillating ion exchanger was dry packed into an FEP Teflon column with a small amount of glass wool packed at each end to prevent resin washout from the tubing. The tubing (1/8 in. external and 1/16 in. internal dimensions) was filled with ∼0.05 g of resin beads to a resin bed length of 4.45 ( 0.13 cm. After passing aqueous solutions containing different concentrations of groundwater constituents and 99Tc, the whole column was fixed in the center of a 7 mL liquid scintillation plastic vial and quantified by the LSC without the introduction of cocktail. The effect of different interfering anions on the sorption behavior of pertechnetate in simulated groundwater on the two minicolumn sensors was quantified by the LSC as previously described. For on-line detection (dynamic measurement), a coil flow cell was packed in a manner similar to that of the off-line column and then placed in an FSA IN/US β-ram model 3 with the analog output connected to an Aptec model 5008 multichannel analyzer (MCA) set to acquire either the pulse height spectrum or multichannel scaling spectrum with a 100 s dwell time. Uncontaminated synthetic groundwater was counted as a background
solution, and all solutions were pumped through the flow cell at a flow rate of 1.0 mL min1 using a gradient pump (Dionex model AGP). The loading and effluent solutions were quantified using a Wallac 1415 R/β liquid scintillation counter (PerkinElmer, Inc. Life and Analytical Sciences) to determine the loading efficiency and 99Tc recovery, respectively. The effect of color quench from the presence of chromate anion was examined through pumping synthetic groundwater solutions (50 ppm mixed anions) with different Cr(VI) concentrations of 200, 500, 750, and 1200 ppb. The solutions were passed through the flow cell at a flow rate of 1 mL min1. The activity level was measured on-line by the FSA, while the effluents were collected and analyzed for Cr(VI) concentration using an X Series II ICP-mass spectrometer (Thermo Scientific). The same experiment was repeated using groundwater at elevated mixed anion concentrations.
’ RESULTS AND DISCUSSION Resin Characterization. The elemental analysis of the resin composition will not give sufficiently accurate results to quantify the ammonium groups because R-NPO also has a nitrogen atom in its structure that can interfere with the nitrogen content from the amination step. The ammonium group content is estimated to be in the range of 0.360.54 mmol g1 on the basis of the conversion (%) of CH2Cl groups that reported to be typically ∼4060% of the total CH2Cl (0.9 mmol g1) content during the amination process for more than eight different samples.18 Therefore, the prepared MDOA resin was characterized using an FT-IR spectrometer (PerkinElmer, Inc. Life and Analytical Sciences). The FT-IR spectra of the resin before and after amination show a significant fall in the intensity of the band at 1268 cm1 4761
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Figure 1. FT-IR spectra for the scintillating anion-exchange sensor before and after amination.
Table 1. Technetium-99 Distribution Coefficient (Kd) for MDOA Resin and Selected Commercial Strong-Base Resins anion-exchange resina resin þ ATRPb b
MDOA MDOAd
group METACc
mesh size
Kd (mL/g)
100200
8.83 ( 1.05 this work
reference
methyl(octyl)2N 100200 28954 ( 1132 this work methyl(octyl)2N 100200 6364 ( 243 this work
Purolite A-850
(methyl)3N
1650
Amberlite
(methyl)3N
1650
102 ( 5.10 7 2640 ( 132
7 7
IRA-900 Purolite A-520E (ethyl)3N
1650
12800 ( 640
(butyl)3N
1650
20700 ( 1035 7
Syborn Ionac SR-6 a
All resins are strong-base macroporous and have a polystyrene backbone except for Purolite A-850, which is a gel resin with a polyacrylic backbone and trimethylamino group attached to a propyl linker to the acrylic ester functionality. All test solutions were used without acidification and composed of 6 mM NH499TcO4 and the sodium salts of Cl, NO3, and SO42 (60 mM in each anion). b The background solution is 0.05 M HNO3. c [2-(Methacryloyloxy)ethyl]trimethylammonium chloride. d The background solution is 50 ppm from the anion mixture (Cl, HCO3, SO42) in 0.05 M HNO3.
assigned to the CH2Cl group; see Figure 1. The thickness of the polymer layer is estimated to be in the nanometer range on the basis of another work previously done by one of the coauthors who studied the growth of the polystyrene brush thickness from silicon surfaces as a function of the polymerization time.19 Distribution Coefficient (Kd) of the Resin. Kd was evaluated for scintillating ion-exchange resins and compared with that of other anion-exchange resins, Table 1. This experiment was conducted for two reasons, to determine the role of the trimethylammonium chloride functional group in the anion-exchange process of TcO4 and to evaluate the uptake properties and selectivity of the new MDOA sensor. Although the polymer brushes on the resin beads containing trimethylammonium ions are expected to serve as anion-exchange sites, the resin shows very limited uptake for the TcO4 anion, giving a Kd value of
8.83 ( 1.05 mL g1. Therefore, the uptake property of the anionexchange sensors comes mostly from amination by TEA or MDOA. This behavior was in confirmation of previous work which stated that resin selectivity for pertechnetate ion sorption increases with the radius of the immobilized alkyl chain length of the quaternary ammonium groups on resin beads.8 A higher distribution coefficient value of 28954 ( 1132 mL g1 (uptake 99.1%) was reported for MDOA resin using 0.05 M HNO3 as a background solution relative to 6364 ( 243 mL g1 (uptake 96.0%) in the same matrix amended with a 50 ppm concentration each of Cl, SO42, and HCO3 as mixed anions. Although there is a significant difference in the Kd values of MDOA in both matrixes, the selectivity of the sensors toward 99Tc is still promising taking into account the small difference in the uptake values and the small mass of 750 Bq L1 TcO4 (∼1.21 108 mol L1) compared to the total mass of the anion mixture (∼0.054 mol L1). This means that the prepared scintillating resin still provided a significantly high percentage uptake even in the presence of different interfering anions. A similar behavior was observed by O'Hara et al.,13 who reported that the Kd value of 99Tc on the commercial weak-anion exchanger AG4-X4 using unacidified groundwater decreased from 750 mL g1 with 50% uptake to 350 mL g1 in the absence and in the presence of 100 ppm SO4 ions, respectively. Resin Stability. Leaching experiments to test the stability of the coated resin were conducted with the details from Grogan et al. [personal communication]. Grogan et al. mentioned that, for a resin without ATRP, washing the column with ∼5000 pore volumes of 25% methanol/water resulted in a decrease in the count rate of 8.47 ( 0.06% attributed to the leaching of R-NPO. The leaching test for MDOA resin with ATRP under the same experimental conditions resulted in a decrease in the count rate of 6.39 ( 0.07% attributed to the leaching of R-NPO. The error values are based on propagated 1σ counting statistics. Although there is a small difference between the ATRP-treated and -untreated resins under these experimental conditions, the ATRP resin was concluded to be slightly more stable. This indicates that the polymer layer does not encapsulate the fluor inside the resin beads completely, which may be attributed to the low number of initiator sites shared in the ATRP process, short length of the polymer brushes, and absence of any cross-linkage on the surface. Detection Geometry. Scintillating anion-exchange resins with two different sensor geometries, including off-line static measurement (minicolumn þ LSC) and on-line dynamic measurement (coil cell þ FSA), were investigated systematically with regard to linearity, detection efficiency, and selectivity. Minicolumn Static Measurement. The off-line measurement of pertechnetate using preconcentration minicolumn sensors was used to evaluate the characteristic performance of new scintillating materials. Figure 2 shows a typical background pulse height spectrum as well as pulse height spectra corresponding to the TEA- and MDOA-functionalized resin loaded with 7.1 Bq of 99Tc. The spectral count times were 21 600 s each. The MDOA sensor had slightly higher luminosity relative to the TEA sensor as determined by the shift in the pulse height spectrum (peak width) to higher channels (from 300 channels for TEA to 350 channels for MDOA). This indicates that the MDOA scintillating anion exchanger is expected to have a higher detection efficiency than TEA. Linearity and Detection Efficiency. The linearity and efficiency of the off-line detection mode were investigated for both minicolumn sensors. The minicolumn sensor with TEA and MDOA functional groups were evaluated by loading 1050 mL of a synthetic “groundwater” containing a 100 ppm concentration of 4762
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Figure 2. Pulse height spectra (360 min) collected before and after loading TEA and MDOA sensors with 7.1 Bq of 99TcO4 in a synthetic groundwater matrix acidified with 0.01 M HNO3.
each of the anions Cl, SO42, and HCO3 in 0.01 M HNO3 with 99Tc concentrations ranging from 20.30 to 81.20 Bq L1. The results demonstrated in parts a (TEA) and b (MDOA) of Figure 3 were collected from counting each minicolumn sensor (without the addition of a liquid scintillation cocktail) with a liquid scintillation counter for 1200 s at each 99Tc concentration. As shown in Figure 3, the linearity as determined for five different volumes was quite good for three different concentrations with an accumulated error range of (1.18% to (5.27%. A total of 5068 pore volumes of solution was loaded under vacuum through each minicolumn (∼10 μL pore volume), which is a good indication of the stability of the scintillating ion-exchange resins. The 99TcO4 uptake efficiency is almost 100% for both sensors. The detection efficiency as a function of loaded and measured activity was calculated for the minicolumn geometry and is reported in Table 2. The mean detection efficiencies were found to be 60.72 ( 1.93% and 72.83 ( 0.81% for TEA and MDOA, respectively. The obtained detection efficiencies are higher than 99Tc detection efficiencies previously reported for both inorganic scintillator (22.3348.29%)11 and organic scintillator (26.3%, 38.0%, 57.0%) sensors.12,4,11 Effect of Interfering Anions. The uptake percentage and detection efficiencies of 99Tc were measured using the off-line detection mode on TEA and MDOA scintillating anion exchangers as a function of the composition of synthetic groundwater. The synthetic groundwater was composed of a mixture of three common anions, Cl, SO42, and HCO3, at four different concentrations (100, 250, 1000, and 2000 ppm of each ion, which are referred to as 100 ppm GW, 250 ppm GW, 1000 ppm GW, and 2000 ppm GW, respectively). The results in Table 2 show that both scintillating anion exchangers TEA and MDOA have a very high uptake percentage (100%) for 99Tc in the presence of elevated anion concentrations compared to the maximum range of 1000 ppm reported for natural fresh waters.20 Moreover, the detection efficiency was not affected by the presence of these high composite anion concentrations, except in the case of TEA, especially at very high anion concentration (2000 ppm). Even at this highest anion level, the detection efficiency remained high. These results reflect the promising removal capabilities and high tolerance of both scintillating resins for 99Tc in real groundwater containing high anion concentrations.
Figure 3. Minicolumn responses to 99Tc at activity concentrations of 20.2, 40.6, and 81.2 Bq L1 in acidified synthetic groundwater containing a mixture of common anions (Cl, NO3, SO42, HCO3) each at 100 ppm using the off-line measurement mode: (a) TEA, (b) MDOA.
Table 2. Uptake Percentage and Detection Efficiency of 99Tc in Synthetic Groundwater (GW) Solutions Using the Off-Line Detection Mode for the TEA and MDOA Scintillating Resins TEA samplea
MDOA
uptake, % detection eff, % uptake, % detection eff, %
100 ppm GW
100
61.52
100
72.10
250 ppm GW
100
60.42
100
73.02
1000 ppm GW
100
62.75
100
72.30
2000 ppm GW
100
58.20
98
73.89
a
Groundwater acidified with 0.01 M HNO3 and containing concentration from each of Cl, SO42, and HCO3.
Flow-Cell Dynamic Measurement. The on-line quantification of 99Tc in synthetic groundwater matrixes using a flow scintillation analyzer and U-shaped flow cell was tested as an alternative detection mode with a different geometry. Figure 4 shows the flow profiles for TEA and MDOA scintillating anion 4763
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Figure 4. On-line quantification with TEA and MDOA U-shaped flow cell responses using ∼125 mL of acidified synthetic groundwater containing 33 Bq L1 99Tc and a mixture of common anions (Cl, NO3, SO42, HCO3) each at 100 ppm. The green horizontal bar indicates the DL.
exchangers. As shown in the figure, ∼30 mL of the unspiked 100 ppm GW solution (no 99Tc) was pumped at the beginning to equilibrate the flow cell followed by the spiked synthetic groundwater. The test solutions were spiked with pertechnetate ions to yield a final concentration of 33 Bq L1 (the U.S. maximum contaminant level, MCL). Finally ∼40 mL of unspiked GW solution was passed following the spiked GW. After pumping only 150 mL that contains very low activity (5 Bq), both sensors give a good response in the presence of groundwater matrix anions. The detection limit (DL) is the count rate 3 standard deviations above the mean of the count rate of the background solution (∼30 mL of unspiked GW that passed through the sensor). The DL was calculated to be 0.393 for a groundwater sample containing 33 Bq/L or more of pertechnetate, the DL can be achieved by pumping less than 20 mL of the sample. The effect of the groundwater matrix on the on-line detection response was investigated at three elevated anion concentrations, including 100 and 250 ppm each of Cl, SO42, and HCO3, as well as 250 ppm synthetic groundwater with 1200 ppb Cr(VI). Figure 5 shows the MDOA sensor response for the 99Tc activity level of 720 Bq L1 in the three different groundwater compositions. The results indicate that the sensor gives almost the same response (detection efficiency) regardless of the variation of the groundwater matrix. The same experiments were conducted for the TEA sensor under the same conditions with results that showed a behavior similar to that reported for MDOA. The detection response as a function of the successive loading of different groundwater samples without regeneration was investigated and is represented in Figure 6a. This evaluation refers to the possibility of using the same MDOA sensor to quantify the 99Tc radioactivity level in groundwater samples that have significant variations in chemical composition. The results demonstrated in Figure 6b confirmed that the detection efficiency is almost constant (62.7 ( 1.0%) and was not affected by pumping large a groundwater volume with different 99Tc activities. This value is lower than that obtained for the same sensor using minicolumn static detection. This difference is attributed to
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Figure 5. U-shaped MDOA flow cell responses after pumping 25 mL of different acidified groundwater matrixes containing 720 Bq L1 99Tc using the on-line measurement mode.
the photomultiplier tube (PMT) efficiency of the LSC and FSA as well as differences in the detection geometry. Color Quench Effect of Chromate Anions. Hexavalent chromate ion is a significant quenching and interfering anion for pertechnetate ion. To quantify the impact of chromate on the quantification of 99Tc, chromate ion (Cr(VI)) was added to the synthetic groundwater to investigate its effects on the detection efficiency of the scintillating anion-exchange resin. Samples of 50 mL of synthetic groundwater with different chromate concentrations from 200 to 1200 ppb were pumped through the online flow cell. The effluents were analyzed for chromium by ICP-MS, and the data are reported in Table 3. The results revealed that the uptake values of Cr(VI) were found to be less than 2.96%, indicating that chromate ions have insignificant interfering and quenching effects on the MDOA scintillating sensor. This behavior of Cr(VI) on the prepared resins is not surprising for two reasons: (1) The prepared resins are highly selective for pertechnetate TcO4 anions, and this was confirmed experimentally. (2) The chromate uptake depends mainly on its species distribution in the aqueous solution that is primarily related to the pH value. Since the working solution was acidified to 0.01 M with HNO3, the pH value was calculated to be 2. The predominant species for chromate are calculated to be >99% HCrO4 and a limited quantity of Cr2O72. The first species has a 1 charge that has difficulty localizing on the amino group. In a similar application,13 Cr(VI) was shown to have a strong quenching effect at a concentration of 300 ppb. This may be due to (1) the low acidity of the groundwater, the Cr2O72- ions may be the predominant species in the aqueous phase relative to HCrO4 species, or (2) the fact that the equilibration-based sensing approach may lead to the stepwise accumulation of chromate ions through continuous pumping of a large volume of groundwater solution. Recovery of TcO4 from the Resins. Several experiments with different common elution systems at various concentrations, including NaCl, NaOH, HNO3, and FeCl4, were conducted to study the recovery of 99TcO4 from the prepared 4764
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Figure 7. Elution curve of flow cell using 4 M HNO3.
Figure 6. Detection response and linearity of the MDOA sensor as a function of the successive loading of 99Tc activity: (a) accumulated count rate for groundwater samples with significant variations in the chemical composition; (b) measured net count rate versus the total loaded activity. The red dot indicates the corresponding detection efficiency for each point.
Table 3. Uptake Percentage of Cr(VI) by MDOA Scintillating Resin samplea GW blank
add before, ppb 0.0
measured after, ppb
uptake, %
0.133 ( 0.001
effluent 1
200
198.9 ( 0.48
0.55
effluent 2 effluent 3
500 750
487.7 ( 2.60 749.8 ( 1.42
2.46 0.03
effluent 4
1200
1164.6 ( 2.56
2.96
a
Groundwater acidified with 0.01 M HNO3 and containing a 50 ppm concentration each of Cl, SO42, and HCO3 was used as a background solution for all samples.
99
Tc on MDOA resin as measured with a
strong anion-exchange resins.21 On the basis of this study, FeCl4 eluent was applied in the current system, but failed to elute TcO4 from scintillating anion-exchanger resins. The Fe discolored the resin, so at the conclusion of the “elution”, the count rate on the flow cell was reduced. This quenching effect is mainly due to the yellow color of the ferric chloride solution. Subsequently, 4 M HNO3 was used to remove some of the color, but there was little removal of TcO4 from the column as illustrated in Figure 7. Pumping a large volume of 4 M HNO3 did not show any degradation for material scintillating properties where the count rate recovered completely after pumping groundwater instead of nitric acid solution. These results confirmed that the MDOA scintillating sensor has a strong retention efficiency for TcO4. Thus, the regeneration of spent resin becomes a particularly challenging point.
’ CONCLUSIONS The newly prepared scintillating anion exchanger can be used to produce efficient and stable sensors for measuring 99Tc at concentrations less than 33 Bq L1. Both on-line and off-line detection modes can be used successfully to quantify pertechnetate in groundwater. The sensor materials achieved high selectivity in the presence of elevated groundwater matrix anions and up to 1200 ppb Cr(VI). The results indicated that the anion concentrations have almost no effect on the minicolumn uptake of ∼100% or detection efficiencies of 62% and 72% for TEA and MDOA sensors. For on-line quantification the average detection efficiency of the MDOA sensor is 62.7 ( 1.0% even in the presence of a very high concentration of common interfering anions. The results confirmed that the MDOA scintillating sensor has a very high removal capacity as well as a strong affinity for TcO4. ’ AUTHOR INFORMATION Corresponding Author
materials. The results demonstrated that all these solutions were not able to elute the TcO4 that was sorbed to the resin. Gu et al.21 reported that tetrachloroferrate (FeCl4) solution succeeded in complete recovery of perchlorate (ClO4), which has a behavior similar to that of pertechnetate ions, from
*Fax: (864) 656-0672. E-mail:
[email protected].
’ ACKNOWLEDGMENT Funding for this research was provided in part by the Ministry of Higher Education and Scientific Research of the Egyptian 4765
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Analytical Chemistry
ARTICLE
Government and by the DOE Environmental Management Science Program, Grant DE-FG02-07ER64411, entitled “Radionuclide Sensors for Water Monitoring”.
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dx.doi.org/10.1021/ac103311p |Anal. Chem. 2011, 83, 4759–4766
