Article pubs.acs.org/ac
Pore-Filled Scintillating Membrane as Sensing Matrix for α‑Emitting Actinides Vivek Chavan, Chhavi Agarwal,* and A. K. Pandey Radiochemistry Division, Bhabha Atomic Research Centre (BARC), Trombay, Mumbai 400085, India ABSTRACT: Pore-filled membranes with scintillating properties have been synthesized for sensing α-emitting radionuclides. The membranes have been prepared by in situ UV-initiator-induced polymerization of monomer bis[2(methacryloxy)ethyl] phosphate in pores of the host membranes, poly(propylene) and poly(ethersulfone). The polymerization has been carried out in the presence of scintillating molecules, 2,5-diphenyloxazole. These scintillating molecules are physically trapped in the thus formed microgel in the membrane. Much higher αscintillation efficiency has been obtained for the 241Am-loaded poly(ethersulfone)based grafted membrane compared to poly(propylene)-based membrane. This was attributed to the aromatic backbone of the poly(ethersulfone) membrane. The scintillation response of poly(ethersulfone)-based membranes has been found to be linear over the range of 241Am activity studied. The pore-filled scintillating membranes have been found to be selective toward Pu4+ ions at higher HNO3 concentration compared to Am3+. The analytical performance of the pore-filled scintillating membranes has been evaluated. The membranes have been found to be stable and reusable. The scintillating membrane with optimized composition has been applied for quantification of Pu in a soil sample.
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separation and preconcentration. The scintillating molecules convert the kinetic energy of radiation emitted by the analyte to give an optical signal. This optical signal is then used to detect and quantify the analyte. An ideal α-/β-scintillating radionuclide sensor should have the following features: (i) It should be able to have high levels of selective preconcentration of radionuclides; (ii) α-/β-emitting radionuclides must spatially localize in close proximity to a scintillating material; (iii) the response of the sensor must be linear with a linear increase in the analyte amount and reproducible; and (iv) the sensor should be reusable for multiple measurements. Various forms of the extractive scintillating sensors have been developed such as resins,5−12 fibers,13 beads,14,15 and membranes.16−19 In these materials, the organic phase containing the extractant and scintillator is impregnated in polymer matrices. Roane and DeVol prepared transuranic selective scintillating resin by impregnating the TRU resin with organic fluors (2,5diphenyloxazole (PPO) and 1,4-bis(4-methyl-5-phenyl-2oxazolyl)benzene (POPOP)) with a combination of extractants (octyl(phenyl)-N,N-diisobutylcarbamoyl methylphosphine oxide (CMPO) in tributyl phosphate).10 Scintillating resins have also been adapted to be used as a flow detector when combined with a flow cell and photomultiplier tube.20 Park et al. have used the scintillating gels as sensing probes for optical fibers by impregnating the organic fluors in epoxy base.21 In a similar way, scintillating fibers have been explored to create dual functionality sensors for 137Cs by Headrick et al.13 Scintillating polymer inclusion membranes have been prepared for the
uman activities related to nuclear energy results in introduction of radionuclides in the soil and natural resources of water. Continuous monitoring of radionuclides especially actinides in the environment is important for the health and safety aspects. In the environment, wherever an ultratrace level of these radionuclides is present, direct determination with routinely used instruments is difficult. Hence the detection and quantification step needs to be preceded by separation and preconcentration steps, where the radionuclide is extracted from other elements present in bulk and is then preconcentrated to give a measurable analytical signal. Various methods such as ion exchange,1 solvent extraction,2 and chromatography3 are available for the separation and preconcentration of actinides. Subsequently, the detection of actinides is usually done using their nuclear properties. The detection of α-/β-emitting radionuclides is mostly done by liquid scintillation counting (LSC) technique due to its simple methodology and high sensitivity. In this, the scintillation cocktails absorb energy emitted by radioisotopes and re-emit it as UV−visible light, which is then detected. Here, the solvent acts as an efficient collector as well as a transmitter of energy and the presence of any chemical or color quencher in the scintillation medium would decrease the light output and hence affect sensitivity. This method of detection generates radioactive organic waste, the disposal of which needs special attention. A better option is the simultaneous extraction, preconcentration, and detection of the analyte which can be carried out using the extractive scintillating radionuclide sensors.4 An extractive scintillating radionuclide sensor is comprised of an extractant and a scintillator. The extractant is capable of selectively extracting the analyte and, therefore, results in its © XXXX American Chemical Society
Received: December 21, 2015 Accepted: March 2, 2016
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DOI: 10.1021/acs.analchem.5b04827 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
pore size = 0.1 μm). The thicknesses and porosities of these host membranes have been given in our earlier publication.26 The monomer MEP, UV-initiator 2,2′-dimethoxy-2-phenyl acetophenone (DMPA), and organic fluors PPO and MSB were obtained from Sigma-Aldrich (Steinheim, Switzerland). The chemical structures of the monomer and organic fluors used are shown in Figure 1a−c, respectively. N,N′-dimethyl-
selective extraction and detection of lanthanides and actinides.17−19 These membranes are prepared by immobilizing the extractant (bis(2-ethylhexyl)phosphoric acid (HDEHP)) or the ion exchanger (Aliquat-336) along with scintillator 2,5diphenyloxazole (PPO) and wavelength shifter 1,4-bis(2methylstyryl)benzene (MSB) in a plasticized matrix such as cellulose triacetate (CTA) or poly(styrene) (PS). Since these scintillating matrices have the liquid phase extractant physically adsorbed onto the substrate, the leaching of the liquid phase from the substrate during column loading and elution have been observed.5,6 This leads to decreased detection efficiency, thereby limiting it to a single use. Also, the regeneration of these materials is difficult, again hampering their multiple usage.7 For resins, the problem of leaching has been dealt with by coating ion-exchange resins with polymer.22 Recently, the synthesis of polymerizable scintillator which has been copolymerized with styrene/4-methylstyrene to form beads has been reported.23−25 However, until now, the problem of leaching of scintillating polymer inclusion membranes has not been dealt with. Also, the poor mechanical stability of these membranes limits their application for practical purposes. The problem of leaching can be circumvented by replacing the mobile carrier in a liquid membrane with a covalently bonded carrier. The latter class of membranes is termed as a fixed-site/ pore-filled membrane. In this, the desired functional group bearing polymer is covalently anchored on the polymer base matrix and imparts separation characteristics to the membrane. The host matrix provides mechanical containment to the functionalized polymer. Along with good chemical and mechanical stability, the advantage of the pore-filled membranes is their high flux and hence fast sorption kinetics. This is an important parameter in designing any separation process for generating analytical response in a reasonably short time. This is due to the close proximity of the binding sites in pores of the host membrane which facilitates the fast transfer of ions to the interior matrix. Also, the microgels are hydrophilic which would ensure that there would not be a dielectric barrier in the transfer of ions from the aqueous solution to the membrane matrix. In the present work, the possibility of using pore-filled scintillating membranes as sensors for α-emitting actinides has been explored. The phosphate bearing monomer, i.e.,MEP, which is capable of preferentially extracting actinide ions, has been chosen as the monomer. Organic fluor PPO has been introduced in the polymerizing solution, leading to its physical entrapment in the highly cross-linked network of the MEP polymer. Simultaneous preconcentration and detection of αemitting radionuclides has been studied. The effect of the host microporous membrane, its pore size, and the organic fluor concentration on the scintillation response has been studied. The membrane composition has been optimized, and its selectivity toward actinides as a function of acidity, its stability, and its reusability have been evaluated. The scintillating membrane with optimized composition has been applied for quantification of Pu in a soil sample.
Figure 1. Chemical structures of (a) monomer bis[2(methacryloyloxy)ethyl] phosphate (MEP), organic fluors; (b) 2,5diphenyloxazole (PPO); (c) 1,4-bis(2-methylstyryl)benzene (MSB) used for preparing pore-filled membranes.
formamide (DMF), methanol (MeOH), and xylene were obtained from Merck (Mumbai, India). Deionized water (18 MΩ·cm; Gradient A-10 model, Milli-Q, Billerica, MA, USA) were used in the present experiments. 241Am activity obtained from the Board of Radiation and Isotope Technology, Mumbai was used for uptake experiments. The Pu activity used for the present study is basically a research reactor grade plutonium with its isotopic composition (at. %) as 238Pu (0.16 ± 0.006), 239 Pu (68.79 ± 0.03), 240Pu (26.94 ± 0.03), 241Pu (2.09 ± 0.005), and 242Pu (2.02 ± 0.006).27 Preparation of Scintillating Pore-Filled Grafted Membrane. Scintillating pore-filled membranes were prepared by UV-initiator-induced in situ polymerization. The method was chosen based on the fact that UV-initiator-induced grafting leads to uniform polymerization throughout the polymer matrix unlike the surface grafting with plasma radiation, and requires less stringent conditions than heat-initiator-induced polymerization. Also, UV radiation does not damage the polymer matrix significantly like γ and electron beam radiation, thereby preserving the original morphology of the polymer matrix. The host PP (4 × 4 cm2 size) and PES membranes (5 cm diameter) were presoaked in methanol for a half-hour. The polymerizing solution was prepared by dissolving the monomer MEP, organic fluor PPO, and UV-initiator DMPA in appropriate solvent. For PP membranes, DMF was used, and for PES membranes, due to the substrate solubility in pure DMF, DMF:methanol = 1:1 mixture was used as solvent. A minimum amount of UV initiator required for initiating polymerization, i.e., 1 wt %, was used. The solution was homogenized further by ultrasonicating it for 10 min. Then the membranes were soaked for 3 h in this polymerizing solution. For some pore-filled membranes, MSB was also added in the polymerizing solution as a wavelength shifter. For these
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EXPERIMENTAL SECTION Materials. The microporous host membranes used for the preparation of the pore-filled membranes were PES membranes from Sartorius Stedim India Private Ltd. (pore size = 0.1 and 0.2 μm) and poly(propylene) (PP) membranes from Membrana GmbH, Wuppertal, Germany (AccurelR PP 1E; B
DOI: 10.1021/acs.analchem.5b04827 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry membranes, 0.5 mL of xylene was additionally used to dissolve MSB. After soaking, excess polymerizing solution adhering to the surface was removed. These solution-filled membranes were sandwiched between two transparent polyester sheets to prevent any possible loss of solution filled in the pores. The sandwiched membranes were UV-irradiated at 365 nm for a period of 15 min in a multilamps photoreactor (Heber Scientific Model No. HML-SW-MW-LW-888) having eight 8 W UV lamps arranged in a circle. The unpolymerized components were then removed by washing membranes thoroughly with DMF:methanol mixture. The samples were washed until a constant weight was obtained and were then vacuum-dried at 50 °C. The amount of cross-linked microgel anchored in the polymer matrix (%) was obtained gravimetrically as % mass gain = (Wf − Wi ) × 100/Wi
Figure 2. Scintillation counting arrangement used for monitoring α radioactivity in the membrane samples.
discriminator level was selected to reduce the background noise of the scintillation counter. Analytical Application. A local soil sample (2 g) was spiked with a trace level of Pu by soaking the soil sample in a dilute solution of Pu and then evaporating the supernatant to dryness. The Pu from the radioactive soil sample was then leached using the procedure given in detail in our earlier publication by using 8 mol L−1 HNO3 (100 mL) containing 2− 3 mL of 30% H2O2.27 The leaching was performed by heating the solution slightly under an IR lamp. The volume of the supernatant was then made up to 50 mL using 8 mol L−1 HNO3. A known volume (10 mL) of the supernatant (166 Bq mL−1) was then equilibrated with a membrane of 1 × 2 cm2 size for overnight. After equilibration, the membrane sample was washed thoroughly with 8 mol L−1 HNO3, dried, and counted in a scintillation counter. The solution activities before and after the equilibration with membrane were also monitored using liquid scintillation counting.
(1)
where Wi and Wf are the weights of the nascent and the grafted membranes, respectively. Characterization. The FEG-SEM images of the nascent and the grafted membranes were taken using FEG-SEM (Model No. JSM-7600F) with a resolution of 1.0 nm (15 kV) and 1.5 nm (1 kV). The instrument had an accelerating voltage range from 0.1 to 30 kV with a magnification from ×25 to 1,000,000. Sorption and Desorption of Radionuclides. To study the extraction property of the grafted membranes, 1 × 2 cm2 of these samples were equilibrated with 1 mL of solution containing tracer levels of α-emitting actinide, 241Am, and Pu in different known acidities (0.1, 2, and 3 mol L−1 HNO3) for a definite time interval. The uptake of radionuclides in the membranes from aqueous feed was monitored by taking the samples of feed solution before and after equilibration and counting it by liquid scintillation counting. The extraction efficiency of the membranes for the two actinides was obtained as extraction efficiency/% =
Ai − Af × 100 Ai
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RESULTS AND DISCUSSION The compositions of the polymerizing solutions used for porefilling PP and PES microporous substrates are given in Table 1. Table 1. Composition of the Monomer Solution Used for Preparing the Pore-Filled Membranes
(2)
where Ai and Af are the activity in solution before and after equilibration with the membrane sample, respectively. Scintillation Counting. The α-activity of the 241Am radioisotope in the solution was monitored by liquid scintillation counting using a home-built liquid scintillation counter with EMI 9514 photomultiplier tube (13 stages) coupled with a PC-based multichannel analyzer (MCA). For this, a 50 μL aliquot from the sample was added to a vial containing 5 mL of scintillation cocktail-W. The composition of the cocktail-W was as follows: 10 g of 2,5-diphenyloxazole, 0.25 g of 1,4-di-2-(5-phenyloxazolyl)benzene, and 100 g of naphthalene in 1000 mL of dioxane. The scintillation responses of the 241Am-loaded membranes were monitored by counting these samples in the same scintillation counter by mounting the radionuclide-loaded membrane samples (1 × 2 cm2) on the inside wall of a quartz cell (1 × 1 × 5 cm3). To achieve a constant counting efficiency, the quartz cell was fixed on an Al sheet and this Al sheet was kept at the center of a 5 cm diameter groove present in front of the photomultiplier. The detailed counting geometry is shown in Figure 2. In this configuration, the distance of the membrane sample and the PMT is ∼1.5 cm. The appropriate lower
a
ID
UV irrad time (min)
MEP (mg)
PP1 PP2 PES1 PES2 PES3 PES4 PES5 PES6 PES7a
15 15 15 15 30 15 15 15 15
500 500 500 500 500 500 500 500 500
PPO (mg)
MSB (mg)
15 15 15 5 75 15 15
1.5
mass gain (%)
scintillation counting efficiency (%)
± ± ± ± ± ± ± ± ±
