Sumana Paul*a,
Ashok K. Pandeyb,
Raju V. Shaha,
D. Alamelua and
Suresh K. Aggarwal*a
aFuel Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India. E-mail: sumana@barc.gov.in; skaggr2002@gmail.com; Fax: +91 22 25505150, +91 22 25505151; Tel: +91 22 25593740
bRadiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India. E-mail: ashokk@barc.gov.in
First published on 14th December 2015
Single resin bead-based thermal ionization mass spectrometry (TIMS) offers numerous advantages for Pu(IV) determinations in complex aqueous samples. These include removal of the matrix and interfering ions by a one-step process, selective preconcentration of Pu(IV) ions with a high chemical recovery, transportation without heavy shielding, avoiding the possibility of cross-contamination, and acting as a point source. The single-bead TIMS method reported in literature is based on an anion-exchange resin that lacks sorption-selectivity toward Pu(IV) ions, and where the beads are not easily retrievable from a large volume sample. Therefore, silica-coated superparamagnetic Fe3O4 embedded functionalized porous poly(ethersulfone) (PES) beads were developed for Pu(IV) preconcentration and analysis by TIMS. The beads were functionalized with a phosphate bearing monomer along with or without a quaternary ammonium bearing monomer by UV-induced surface grafting. Since the beads were used in a highly acidic solution, the Fe3O4 nanoparticles were protected with a silica coating formed by the hydrolysis and condensation of tetraethoxysilane. The PES porous beads were prepared by a phase inversion method. The monomers used for UV-grafting were 2-hydroxyethyl methacrylate ester and (3-acrylamidopropyl) trimethylammonium chloride. The functionalized beads were characterized by scanning electron microscopy, energy dispersive analysis, and vibrating sample magnetometry. The sorption studies indicated that the bi-functionalized PES beads consisting of phosphate and quaternary ammonium groups not only have a higher distribution coefficient (Kd) for Pu(IV) but also high selectivity toward Pu(IV) ions in the presence of a large excess of U(VI) ions (Kd(Pu(VI))/Kd(U(IV)) = 11.5). The phosphate-functionalized PES beads showed comparable selectivity (Kd(Pu(VI))/Kd(U(IV)) = 9.1), but a lower Kd value for Pu(IV). The quaternary ammonium-functionalized PES beads were found to have lower selectivity and Kd values toward Pu(IV) ions. The analytical performance of single bi-functionalized bead-based TIMS for the determination of Pu(IV) using isotope dilution was compared with the solution-based TIMS, validated using the Pu isotopic standard reference material NIST SRM-947 and applied to real samples such as dissolver solutions and soil leach liquors.
A combination of anion-exchange and extraction chromatography is preferable for achieving a higher Pu chemical recovery and U decontamination factor.6 An automated sequential injection separation system in conjunction with MC-ICP-MS has been developed for the simultaneous analysis of 237Np and Pu isotopes in environmental samples.7 The high chemical recovery (>90%) and low detection limits, 2.5, 2.1, and 0.42 fg mL−1 obtained for 237Np, 239Pu, and 240Pu, respectively, reduces the soil and sediment samples requirement to as low as 1 g.7 The chemical purification along with isotope dilution technique makes MC-ICP-MS highly reliable for the analysis of ultratrace concentrations of Pu in environmental samples.8
However, TIMS in conjunction with isotope dilution is also commonly used for the quantification of Pu in the ng range in nuclear facilities.1,2 TIMS analysis gives a reproducibility of 0.1% (2 s) for ≈1 ng Pu, 1.5% (2 s) for 200–500 fg Pu, and 10% for <50 fg Pu.3b In general, Pu analysis by TIMS requires an appropriate sample preparation procedure involving dissolution/leaching to bring Pu into solution, matrix elimination and the selective preconcentration of Pu, and finally, manual deposition of a small volume (≈10 μL) of solution onto a filament surface.9 The general comparison of TIMS and ICP-MS based analytical methods is given in Table 1.
Parameter | TIMS | ICP-MS |
---|---|---|
Ionization | Thermal ionization (produces ion beam with very high ionization yield and selectivity) | Inductively coupled plasma source at normal pressure |
Sample | Amenable to solid samples (≈10 μL of an aqueous sample solution is deposited on a high-purity Re filament surface or resin/bead mounted on Re filament) | More suited for liquid sample injection |
Interference | Formation of plutonium oxide can be prevented by filament carburization | Isobaric (e.g. 238U and 238Pu, 241Am, and 241Pu) and polyatomic (e.g. 238UH and 239Pu) interferences |
Sample purification methods | Ion-exchange and extraction chromatography | Ion-exchange and extraction chromatography |
Analytical performance | Advantage: excellent sensitivity, precise isotopic ratio determination, small amount of analyte needed | Advantage: excellent sensitivity, low detection limit, small amount of analyte, excellent simultaneous multi-element analysis ability, on-line separation and analysis |
Disadvantage: lacks multi-element capability, complicated sample preparation, restricted to elements with ionization potential <6 eV | Disadvantage: solid samples cannot be used, matrix effect | |
Major applications | Isotopic composition determination of actinides | Quantification of isotopes in various environmental, biological, and geological samples |
There is a possibility to use solid samples in TIMS instead of liquid samples. A single anion-exchange resin-bead-based TIMS method has been developed for the isotopic analysis of U and Pu in safeguarded nuclear materials.10 In this method, Pu(IV) ions are loaded in to a bead either from the original sample or after purification using an anion-exchange or extraction chromatography, and the Pu-loaded-bead is then fixed on a Re filament and pyrolyzed during TIMS. Thus, the bead acts as the matrix for chemical separation, helps to secure physical transportation, hence reducing the possibility of instrument contamination or cross-contamination of the sample, and it is a point source for thermal ionization. Sequential mass spectrometric analysis of uranium and plutonium was carried out in our laboratory employing a single Dowex 1X8 resin bead loaded with about 10 ng of plutonium.10a To enhance the Pu(IV) sorption kinetics in a single bead for TIMS, the acoustic streaming was used for small sample volumes.11
In the present study, the Fe3O4 nanoparticles (NPs) embedded bi-functional poly(ethersulfone) (PES) beads were developed for the single-bead-based TIMS analysis of Pu in aqueous samples. The choice of quaternary ammonium and phosphate functional groups bearing monomers for UV-grafting on the PES beads was based on the high affinity of the phosphate group toward Pu(IV) ions,12 and the quaternary ammonium group facilitates the sorption of hexanitrato anionic complex, Pu(NO3)62−, existing in a solution with a high concentration of 7–8 mol L−1 HNO3. The PES polymer was used as the base matrix due to its amenability for UV grafting13 and easy formation of the porous beads by the phase inversion method. The superparamagnetic Fe3O4 NPs were immobilized in the bi-functional PES beads for their easy retrieval from large volume samples using an external magnet. This concept is similar to magnetically assisted chemical separation for remediation and analytical applications.14 For the single-bead TIMS analysis of Pu, the isotope dilution method was employed for the quantification to make the results independent of chemical recovery and unknown variations in the experimental parameters.15
Poly(ethersulfone) was obtained from Goodfellow Cambridge Ltd. (England). Tetraethoxysilane, phosphoric acid 2-hydroxyethyl methacrylate ester (HEMP) containing 25% diester, 3-(acrylamido propyl) trimethylammonium chloride (APTAC), and N-N′-dimethylformamide (DMF) were obtained from Sigma-Aldrich (Steinem, Switzerland). The chemical structures of the monomers are given in Scheme S1 (ESI†). The surface grafting of monomers on the PES beads was carried out in a photo-reactor (Heber Scientific, Chennai, India) consisting of eight 8 Watt 365 nm UV lamps (Sankyo Denki, Japan) fitted in a circular geometry.
The purified stock solutions of mixedPu, 241Am, and 233U were obtained from Fuel Chemistry Division, B.A.R.C., Mumbai, India. Pu was generated in an Indian Pressurized Heavy Water Reactor (PHWR) with an average burn-up of ∼10000 megaWatt day per ton of U. The isotopic compositions of these tracers were determined by TIMS. The isotopic composition of Pu(IV) was 238Pu (0.16%), 239Pu (68.79%), 240Pu (26.94%), 241Pu (2.09%), and 242Pu (2.02%). The irradiated UO2 fuel dissolver solution was obtained from the Fuel Reprocessing Division, BARC, Mumbai, India, and contained the fission products, major actinides (U and Pu), minor actinides, and activation products. The alpha activity of actinides in an aqueous solution was measured by a home-built liquid scintillation counter with an EMI 9514 photomultiplier tube using an Ultima Gold AB scintillation cocktail (Perkin Elmer). The gamma activity measurement was carried out using a well-type 3′′ × 3′′ NaI(Tl) detector coupled to a single channel analyzer (NUCLEONIX). The TIMS analyses were carried out using a model MAT-261 (Finnigan, Germany) equipped with 9 Faraday cups, each cup connected to a resistor of 1011 ohm. The specifications of the TIMS instrument used in the present study are given in Table 2. The Pu+ ion currents were measured under a static mode of multi-collection, and a double Re filament assembly was used for the sample loading.
Parameter | MAT-261 TIMS instrument |
---|---|
Acceleration voltage | 10 kV |
Ion source & filament | Thermal ionization with a high-purity rhenium double-filament assembly |
Analyser | Nine variable Faraday cup detectors, designated as FAR2 to FAR10, each coupled to a 1011 ohms resistor |
Isotopic mass assigned to the Faraday cup | 239Pu to L1, 240Pu to axial/centre cup, 241Pu to H1 and 242Pu to H2 |
Mass resolution (ΔM/M) at 10% valley | 450 |
Abundance sensitivity | 10 ppm (at m/z = 237/238) |
Typically, PES granules were dissolved in an appropriate volume of DMF to obtain a saturated solution. To this, the TEOS-coated Fe3O4 NPs (Fe3O4@SiO2) were homogeneously dispersed by sonication for 30 min. The Fe3O4@SiO2 dispersed PES solution was added dropwise to water with continuous stirring to produce nearly spherical magnetic PES beads with diameters ranging from 0.5 to 1 mm, as shown in Scheme 1. For grafting functional groups, the Fe3O4@SiO2-loaded PES beads were equilibrated with an equimolar mixture of the two monomers HEMP and APTAC in a solution consisting of 1:
1 v/v water
:
ethanol. After equilibration, the polymerizing solution filled beads were irradiated for 15 min in a UV reactor (λmax = 365 nm). During irradiation, the poly(ethersulfone) polymer chains underwent photolysis to generate free radicals,13 which initiated graft polymerization of HEMP and/or APTAC monomers on the PES beads. The poly(HEMP), poly(APTAC), and poly(HEMP-co-APTAC) grafted magnetic PES beads obtained after irradiation were washed with ethanol to remove unreacted monomers, then air-dried and conditioned with 0.1 mol L−1 NaCl before use. A detailed schematic of the steps involved in formation of the bi-functional magnetic PES bead is given in Scheme 1.
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Scheme 1 Schematic of the synthesis of bi-functional magnetic PES beads by a phase inversion method and UV-grafting. |
![]() | (1) |
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Scheme 2 Depiction of different steps involved in the determination of Pu(IV) ions using single-bead-based isotope dilution thermal ionization mass spectrometry developed in the present study. |
Single-bead-based isotope dilution thermal ionization mass spectrometry (ID-SB-TIMS) technique was employed for the determination of Pu concentration in the dissolver solution of irradiated UO2 fuel and in the soil leach liquor samples spiked with a known amount of Pu(NO3)4. The Pu concentration was determined by an isotope dilution (ID) technique, which involved the addition of a known weight Wsp of a pre-calibrated spike solution, with a Pu concentration Csp, to a known weight Wsa of the sample solution. Each sample and its spiked mixture was equilibrated with a bi-functional magnetic PES bead and placed on a Re filament for isotopic composition analysis by TIMS. The Pu concentration Csa in the sample can be correlated with the change in either the 240Pu/239Pu or 242Pu/239Pu atom ratio in the spiked mixture (Rm) with respect to that in the sample (Rsa) and spike (Rsp) using the following equation:
![]() | (2) |
For determination of the Pu concentration, the single bi-functional magnetic PES bead was equilibrated with 3 mL of the dissolver solution (U:
Pu mole ratio ∼1000
:
1) and its spiked mixture, separately, for 1 h and with continuous stirring.
After equilibration, the single bead was washed and loaded onto a Re filament for the isotopic composition analysis of Pu in the dissolver solution and its spiked mixture. For quantification of Pu(IV) in a large volume sample, a known Pu concentration and its spiked mixture were added to 50 mL of 3 mol L−1 HNO3 and then equilibrated with the single bi-functional magnetic PES bead for 24 h with continuous stirring. For determination of the Pu concentration in the soil samples, the solution containing a known Pu concentration was added to about 2 g of soil sample, collected from the B.A.R.C. premises after digging one foot, homogenized manually, and dried under an IR lamp for 2–3 h. The solution was sufficient to soak 2 g of soil. Then, the soil sample was treated with 50 mL of 8 mol L−1 HNO3 containing 2–3 mL of 30% H2O2, and heated (65–70 °C) under an IR lamp for 8 h. The obtained soil leach liquor was used for the Pu quantification.
The same procedure was repeated for a spiked mixture of Pu. The leach liquor (50 mL) was equilibrated with a single bifunctional magnetic PES bead for 24 h with continuous stirring. After preconcentration of Pu(IV) ions, the beads were collected using a magnet, washed, dried, and loaded onto a Re filament for Pu isotopic composition analysis by TIMS. The Pu concentrations in various samples were calculated from the change in isotope ratios in the spiked mixture with respect to that in the sample and the spike.
The physical and elemental characterization of the bi-functionalized superparamagnetic beads was carried out with a scanning electron microscope (SEM) with an energy dispersive spectrometer attached to it. As can be seen from the SEM images given in Fig. 1, the PES beads prepared in the present study had a dense surface and highly porous interior. The physical structure of the beads did not change during grafting of the HEMP monomer with or without the co-monomer APTAC. The silica-coated Fe3O4 NPs were not visible as these may be embedded in the PES matrix. The phosphorus elemental mappings at different locations across a bead showed that the grafting occurred uniformly.
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Fig. 1 SEM images of poly(HEMP-co-APTAC) grafted superparamagnetic bead under different magnifications. |
The presence of elements such as S from the base PES matrix, Fe and Si from the silica-coated Fe3O4, phosphorus and nitrogen from the grafted poly(HEMP-co-APTAC) was observed in the EDS spectra. As can be seen from Fig. S1 (ESI†), the nitrogen (two N atoms in APTAC) and phosphorous (one P in HEMP) contents are comparable, indicating that both the monomers polymerized with the same efficiency. As beads have to be equilibrated with a solution containing a high concentration of HNO3, the beads were immersed in 3 and 8 mol L−1 HNO3 for 24 h, and then subjected to elemental composition analyses. It is evident from Fig. 1S (ESI†) that the elemental composition of beads did not change during equilibration with a high concentration of acid. Thus, all the components, including silica-coated Fe3O4 in the functionalized beads, are highly stable against leaching in a solution with a high concentration of HNO3. The changes in the superparamagnetic properties of Fe3O4 after coating silica and immobilizing in the bifunctional beads were studied by vibrating sample magnetometry (VSM).
The comparison of magnetization curves is shown in Fig. 2. As can be seen from Fig. 2, the Fe3O4 NPs retained their superparamagnetic properties, although the saturation magnetization decreased due to magnetic shielding by the silica coating and PES matrix. For example, the saturation magnetization of Fe3O4 particles decreased from 50 emu g−1 to 26 emu g−1 after the silica coating, and further decreased to 9 emu g−1 after embedding them (∼2 wt%) in the matrix of bi-functionalized PES beads. However, the saturation magnetization of beads was found to be good enough for withdrawing the single bead from an aqueous sample using an external permanent magnet kept outside.
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Fig. 2 VSM magnetization curves showing the superparamagnetic properties, and change in saturation magnetization of the bi-functionalized PES bead with respect to Fe3O4 and Fe3O4@SiO2 particles. |
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Fig. 3 The uptake profiles of actinides ions as a function of HNO3 concentration in the poly(HEMP) functionalized (a) and poly(HEMP-co-APTAC) functionalized (b) magnetic PES beads. |
In the case of U(VI) ions, the sorption efficiency of poly(HEMP)- and poly(HEMP-co-APTAC)-grafted beads remained high (80–90%) and decreased slightly with an increase in HNO3 concentration.
Thus, both types of beads do not differentiate between U(VI) and Pu(IV) ions present in the aqueous sample. The selectivity of polymer sorbents toward Pu(IV) and U(VI) ions was studied in a competitive mode by measuring their Kd values in solutions containing a 100:
1 mole ratio of U(VI) and Pu(IV) at 4 mol L−1 HNO3. It can be seen from the data given in Table 3 that the KdPu(IV) values are higher in all sorbents, including commercially available anion-exchange resin Dowex 1X8 and home-made poly(APTAC) PES beads. The data given in Table 3 suggest that the Kd value for Pu(IV) and ratio of Kd(Pu(IV)) to Kd(U(VI)) is highest in the bi-functional PES magnetic beads.
Polymer sorbent | Kd (Pu(IV)) (mL g−1) | Kd (U(VI)) (mL g−1) | Kd(Pu(IV))/Kd(U(VI)) |
---|---|---|---|
Dowex 1X8 | 197 | 32 | 6.15 |
Poly(APTAC)-PES | 98 | 46 | 2.13 |
Poly(HEMP)-PES | 227 | 25 | 9.09 |
Poly(HEMP-co-APTAC)-PES | 462 | 40 | 11.53 |
The kinetics of Pu(IV) sorption by the bi-functional magnetic PES beads was studied to ensure optimum recovery of Pu(IV) from the sample during equilibration. The rate of Pu(IV) sorption by the beads from 3 M HNO3 as a function of equilibration time is shown in Fig. S2 (ESI†). As shown in Fig. S2 (ESI†), the Pu(IV)-sorption equilibrium was attained within 80 min. The Pu(IV)-sorption rate followed a pseudo-second order equation (see ESI†).12 The initial Pu(IV)-sorption rate was fast but reduced thereafter due to the slow transfer of Pu(IV) ions from the surface to the interior of the matrix. The sorption capacity of the bi-functional magnetic PES beads determined from the slope of the plot of t/[Pu(IV)] vs. t/[Pu(IV)]eq. was found to be 93 μg of Pu g−1. This value is in a good agreement with the experimentally measured Pu loading capacity 92 μg of Pu g−1.
The effect of volume of the equilibrating solution on the Pu(IV)-sorption efficiency of the bi-functional magnetic PES beads was also studied. The data presented in Fig. S3 (ESI†) indicate that the Pu(IV)-sorption efficiency was nearly constant up to 30 mL, but reduced thereafter because of the slow diffusion of Pu(IV) ions in the large volume of the equilibrating solution. Therefore, the preconcentration of Pu(IV) ions from a large volume sample, higher than 30 mL, would require a longer equilibration time (24 h for 50–100 mL) or a better equilibration method such as a flow cell.
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Fig. 4 Variations of observed 239Pu+ ion current from the solution and bi-functionalized magnetic-based Pu loadings as a function of vaporization filament temperature. |
With increasing temperature of the VF, the Pu+ ion current was found to increase according to the Saha–Langmuir equation. In the present study, no PuO+ ion current was observed during analysis due to the reducing atmosphere provided by the PES polymer present on the filament surface. The optimum VF heating current for the isotopic analysis of Pu was found to be about 4 amp, at which about 1 V signal corresponding to 239Pu+ ion current was obtained. The ionization efficiency of the SB-TIMS method was lower than that of the conventional solution-based loading method, but the ion collection efficiency was comparable for both the techniques. The magnetic PES bead on the Re filament did not decompose completely. Therefore, it was important to carry out degassing of the bead loaded onto the filament prior to TIMS analysis to remove volatile impurities and to obtain a stable Pu+ ion current. The effect of degassing on the precision of Pu isotopic atom ratio obtained from the single magnetic PES bead fixed on the Re filament was studied and compared with the precision obtained from the conventional TIMS method employing solution loading and 600 s degassing time.
As shown in Fig. S4 (ESI†), the best precision was obtained when degassing of the bead was carried out for a longer period of time. Therefore, the degassing was carried out for 1800 s prior to the analysis of each sample for obtaining a stable current. The comparison of Pu isotopic compositions in the standard NIST SRM-947 sample obtained by SB-TIMS and conventional TIMS is given in Table 4. The atom% of the two major isotopes, namely, 239Pu and 240Pu, determined by the two methods were found to agree within 0.2%, whereas the atom% of 241Pu and 242Pu, which were less abundant isotopes, was found to agree within 0.5%.
Isotope | Certified value (atom%) | Bead loading (a) (atom%) | Solution loading (b) (atom%) |
---|---|---|---|
239Pu | 79.03 ± 0.02 | 78.96 ± 0.04 | 78.96 ± 0.03 |
240Pu | 19.02 ± 0.02 | 19.04 ± 0.012 | 18.99 ± 0.03 |
241Pu | 0.808 ± 0.006 | 0.735 ± 0.006 | 0.723 ± 0.006 |
242Pu | 1.238 ± 0.004 | 1.243 ± 0.008 | 1.225 ± 0.006 |
Isotope | Dissolver solution (atom%) | 240Pu spike used for dissolver solution (atom%) | (U, Pu)C (atom%) | 242Pu spike used for (U, Pu)C (atom%) |
---|---|---|---|---|
238Pu | 0.022 ± 0.001 | 0.192 ± 0.005 | 0.150 ± 0.005 | 0.02 ± 0.001 |
239Pu | 89.92 ± 0.04 | 67.73 ± 0.03 | 69.20 ± 0.03 | 1.54 ± 0.006 |
240Pu | 9.119 ± 0.009 | 27.35 ± 0.03 | 26.23 ± 0.03 | 2.97 ± 0.005 |
241Pu | 0.861 ± 0.003 | 2.574 ± 0.005 | 2.568 ± 0.005 | 0.66 ± 0.003 |
242Pu | 0.070 ± 0.001 | 2.15 ± 0.006 | 1.846 ± 0.006 | 94.81 ± 0.04 |
The Pu spiked in aqueous samples and soil leach liquors contained ∼26 atom% of 240Pu, therefore a 242Pu-enriched spike was used and the concentration was determined from the change in 242Pu/239Pu atom ratio in the mixture with respect to that in the sample. The Pu concentrations in these samples determined by both SB-ID-TIMS and conventional ID-TIMS methods are given in Table 6. It is observed from this table that the results agree within 1%. This indicates that SB-ID-TIMS has a reasonably good analytical performance.
Sample | Pu concentration (μg g−1) | |
---|---|---|
SB-ID-TIMS | ID-TIMS | |
Dissolver solution | 11.8 ± 0.3 | 11.9 ± 0.2 |
4 mol L−1 HNO3 | 3.37 ± 0.02 | 3.39 ± 0.02 |
Soil leach liquor | 2.31 ± 0.02 | 2.32 ± 0.02 |
Footnote |
† Electronic supplementary information (ESI) available: Kinetics experiments. See DOI: 10.1039/c5ra18419c |
This journal is © The Royal Society of Chemistry 2016 |