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A New Method of Polonium Source Preparation using Tellurium Micro-precipitation for Alpha Spectrometry Lijuan Song, Yan Ma, Yadong Wang, Yonggang Yang, Maoyi Luo, and Xiongxin Dai Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04422 • Publication Date (Web): 26 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017
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Analytical Chemistry
A New Method of Polonium Source Preparation using Tellurium Micro-precipitation for Alpha Spectrometry †
Lijuan Song, Yan Ma, † Yadong Wang, † Yonggang Yang, †, ‡ Maoyi Luo, †, ‡ Xiongxin Dai*,†, ‡ †China Institute for Radiation Protection, Taiyuan, China ‡Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Suzhou, P. R. China ABSTRACT: A thin-layer source for the counting of polonium isotopes by alpha spectrometry can be rapidly prepared using micro-precipitation with tellurium. Polonium was first co-precipitated with the reduction of tellurium by stannous chloride followed by micro-filtration onto a membrane filter for counting. This micro-precipitation method is faster, cheaper and more convenient than the traditional spontaneous deposition method, with an excellent Po recovery (>90%) under optimal conditions. The influences of several experimental parameters, including Te(IV) quantity, reaction time and HCl molarity, were examined to determine the optimal conditions for Te micro-precipitation. The decontamination factors of potential interferences from various radionuclides (Ra, Th, U, Pu, Am) for the counting of long-lived polonium isotopes (208Po, 209Po and 210Po) were also evaluated, and the results confirmed a good selectivity on polonium by this micro-precipitation method. Due to its strong resistance to high acidity up to 12M HCl, the method would be particularly suitable for rapid determination of 210Po in acid leaching solution of solid samples.
Polonium-210 is a naturally occurring radioisotope produced during radioactive decays through a series of intermediate radionuclides including 226Ra and 210Pb in the 238U decay chain.1 This alpha emitter is considered to be highly radioactive and toxic and is dangerous to handle: even 1 µg of 210Po is fatal to an adult.2, 3 Direct damage could occur from energy adsorption into tissues from alpha particles emitted by 210Po. Due to the importance of 210Po in biological effect and environmental tracing studies, numerous methods have been developed for the determination of 210Po in environmental and biological samples such as water, food, tobacco, cigarettes, urine, soil and sediment.4-11 The traditional preparation of polonium counting source for alpha spectrometry is the spontaneous deposition method, in which 210Po can be selectively deposited to some extent by displacement on metals (e.g., Ag, Ni, Fe, Au, Pt, and Cu) in acid solutions.12-13 Metallic discs, covered on one side, are fixed in a holder and immersed in diluted acid solution. Spontaneous deposition of polonium is usually carried out at an elevated temperature (80-95℃) for about 4 hours to obtain a satisfactory recovery.12-15 The discs are then rinsed with water and ethanol. After drying, the polonium isotopes on the disc are measured on alpha spectrometer. To obtain preferred results, this method may require preliminary separation of 210Po from a chemically complicated mixtures.16 Several chemical agents (e.g., organic matter, oxidants, and elements which also deposit on the metallic discs) tend to interfere. In addition, the spontaneous deposition procedure require Po to be deposited on the disc at a higher temperature for several hours, which is inconvenient and time-consuming.17 Recently, few microprecipitation techniques for the preparation of Po counting source have been developed for rapid determination of 210Po by alpha spectrometry. Guerin and Dai have demonstrated that alpha counting sources of Po can be rapidly prepared using the CuS micro-precipitation technique in HCl solution due to the very low solubility of PoS, but this micro-precipitation method has a reduced recovery when the molarities of HCl is higher than 1 M.18 A finely divided Te precipitate can be formed by reduction of either Te(IV) or Te(VI) in acid solution with a strong reducing agent, which could be utilized to co-precipitate Po for the
preparation of thin-layer alpha counting source. The reduction of Te(IV) with SnCl2 has been used to form an elemental Te precipitate for facilitating a separation of Po from massive amounts of Bi.19 Based on tellurium reduction, we developed a new micro-precipitation method for the preparation of Po counting source by alpha spectrometry. Compared to the CuS micro-precipitation method, elemental Te formed by reduction with SnCl2 can efficiently co-precipitate Po at much higher acidities up to 12 M HCl. This Te micro-precipitation method is also much faster and more convenient for batch processing of samples, with no heating step required for a high recovery compared to the spontaneous deposition method. Using a vacuum box system, large batches of the Po samples can be rapidly processed to increase sample analysis throughput. In this paper, a rapid method for the preparation of polonium counting source by alpha spectrometry using tellurium micro-precipitation is described. The optimal conditions for the micro-precipitation of Te were examined, and potential radionuclide and chemical interferences were also evaluated. EXPERIMENTAL SECTION Reagents and standards. All solutions were prepared using ultra-pure water (UPW) with a resistivity of >18 M⋅cm, which was produced from a Milli-Q Reference system. For the tellurium micro-precipitation, sodium tellurite (Na2TeO3), telluric acid (H6TeO6), tin chloride dihydrate (SnCl2⋅2H2O) and titanium trichloride (TiCl3) were used. Hydrochloric acid and isopropyl alcohol employed in the study were analytical grade reagents. The 209Po and 232U radioisotope standards were obtained from the National Physical Laboratory (NPL, UK), and the 210 Pb, 243Am, 226Ra, 242Pu standards were purchased from the National Institute of Standards Technology (NIST Gaithersburg, MD, USA). Among them, the 209Po and 210Pb standards were prepared in 1 M HCl solution while the rest radioisotope standards were diluted in 1 M HNO3 of solution composition. The certified reference materials (RM) IAEA-384 (for radionuclides in sediment collected from Fangataufa Lagoon, French Polynesia) and IAEA-385 (for natural and artificial
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radionuclides in sediment from the Irish Sea) were obtained from the Department of Nuclear Sciences and Applications of the IAEA Environment Laboratories. Method optimization. In order to determine the optimal conditions for the micro-precipitation with tellurium by SnCl2, the influence of several parameters (Te quantity, reaction time, HCl molarity) were examined. For each sample, approximately 25 mBq of 209Po was added in a 50-mL centrifuge tube and the total volume of all the sample solutions kept constant at 10 mL. The optimal HCl molarity was first studied with the HCl concentrations ranging from 1 M to 12 M. Under the optimal HCl molarity (1 M) determined above, the effects of the Te(IV) quantity and the reaction time on the micro-precipitation yield of Po were also examined. Micro-precipitation procedure. In a 50-mL centrifuge tube, a known activity (~25 mBq) of the 209Po tracer was weighed and mixed into 10 mL of 1 M HCl. For each sample, 7.87×10-7 moles of Na2TeO3 or H6TeO6 (~100 µg Te) and 5 mL isopropanol were added, and the sample was vigorously shaken. Isopropanol was added to ensure the uniformity of the precipitation. Then, 1 mL of SnCl2 (10% of Sn2+ m/v in 1 M HCl) or TiCl3 (15% Ti3+ m/v in 4 M HCl) was added to the sample, and black precipitate formed. After sitting for 10 min, the sample was filtered through a 0.1-µm Resolve® filter membrane (supplied by the TrisKem International) using a vacuum box system. The filter was subsequently rinsed with UPW followed by anhydrous ethanol. The alpha counting source was prepared by carefully mounting the filter on a 25mm diameter plastic disc with a double-sided adhesive tape (see the photographs in Figure 1). After being dried in air, the samples were counted by alpha spectrometry. Interference Assessment. Decontamination factors were determined for Ra and actinide (Th, U, Pu, and Am) nuclides, which could potentially interfere with the 208Po, 209Po, or 210Po measurements. For each sample, approximately 25 mBq of Ra and actinide standards were added in 10 mL of 1 M HCl. The tellurium micro-precipitation procedure was followed, and the filtrate solution was collected. The filters were counted by alpha spectrometry, and the decontamination factor was calculated as the ratio of the Ra or actinide activity found on the filter to the Ra or actinide activity added. Method application. Certified reference materials IAEA384 and IAEA-385 were used in the method application tests. About 0.5 g of the IAEA reference material (RM) sample was added into a glass beaker, and a known activity (~20 mBq) of the 209Po tracer was added. The 210Po in the sediment was leached out by digesting the sample in 20 mL of concentrated HCl on a hot plate at 100 ℃ for about 1 h. The sample was cooled and centrifuged to separate the 1st leaching solution. Then, 10 mL 6 M HCl was used to leach the residue and centrifuged for separation of the 2nd leaching solution. The same leaching process was repeated to obtain the 3rd leaching solution. All the leaching solutions (a total volume of ~40 mL) were combined and filtered through a 0.1-µm membrane into a 50-mL centrifuge tube. Finally, the leaching sample was processed through the Te micro-precipitation procedure as previously described to prepare the counting source for alpha spectrometry, in which 100 µg of Te(IV), 1 mL of 20% SnCl2 and ~10 mL of isopropanol were added to facilitate the microprecipitation.
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RESULTS AND DISCUSSION Method optimization. Tellurium precipitate can be formed by the reduction of either Te(IV) or Te(VI) species with a strong reducing agents such as SnCl2 or TiCl3. The standard reduction potentials of polonium, tellurium, tin, titanium in aqueous solution are given in Figure 2, and the overall oxidation-reduction reactions between Te(IV)/Te(VI) and Sn(II) are illustrated in Equations 1, 2a and 2b, respectively. Due to similar oxidation-reduction reactions, Po could also be effectively co-precipitated by the formation of either elemental Te or TeO2 precipitation using TiCl3.
TeO32 − + 2 Sn 2 + + 6 H + → Te ↓ ( Black ) + 2 Sn 4 + + 3H 2 O (1)
TeO42 − + Sn 2 + + 4 H + → TeO2 ↓ (White) + Sn 4+ + 2H 2O (2a)
TeO42 − + 3Sn 2 + + 8H + → Te ↓ ( Black ) + 3Sn 4 + + 4 H 2 O (2b) In aqueous solutions, the chemical thermodynamics and kinetics of the Te reduction to different valence states largely depend upon the nature and concentrations of the reactants (oxidant/reductant) as well as the acidity of the reaction medium (i.e., concentration of hydrogen ion). Thus, the influence of HCl molarities on the micro-precipitation yield of Po by Te(IV) and Te(VI) reduction with different reductants (e.g., SnCl2 and TiCl3) was investigated. Using SnCl2 as the reductant, the Po yield remained high (75%-100%) at all the HCl molarities ranged from 0.6 M to 12 M for different sample batches with the addition of either Te(IV) or Te(VI) (see Figure 3). While all the counting sources prepared by the Te(IV) reduction were black (see Figure 1a), a gradual color change of the Te precipitates on the filters (from light greyish to black) with increasing acidities have been observed for the samples prepared by the Te(VI) reduction (see Figure 1b). According to the standard reduction potentials in Figure 2, Sn(II) could reduce both Te(IV) and Te(VI) to elemental Te (a black precipitate). However, Te(IV) would only be reduced to the elemental Te (see Eq. 1), while the Te(VI) reduction could form either white TeO2 precipitate (Eq.2a) or black elemental Te (Eq.2b). Based on the Nernst Equation, the reduction from Te(VI) to TeO2 would be more favorable at the acidity of 1 M HCl as the standard reduction potential for TeO42-/TeO2 (1.00 V) is higher than that for TeO42-/Te4+ (0.93 V). Therefore, the counting sources prepared at lower acidities (6 M turned black (see Fig .1b), confirming that elemental Te has become the predominant product on the filter. When TiCl3 was used as the reductant, large variations in the Po yield (from 10% to 95%) for different sample batches have been observed at the lower HCl molarities of
