Published online 1 March 2006
Published in J Environ Qual 35:568-574 (2006)
DOI: 10.2134/jeq2005.0223
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Heavy Metals in the Environment
Performance Characteristics of Sequential Separation and Quantification of Lead-210 and Polonium-210 by Ion Exchange Chromatography and Nuclear Spectrometric Measurements
E. M. El Afifi and
E. H. Borai*
Hot Laboratories and Waste Management Center (HLWMC), Atomic Energy Authority, Post Office No. 13759, Cairo, Egypt
* Corresponding author (emadborai{at}yahoo.com)
Received for publication May 28, 2005.
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ABSTRACT
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A selective separation and quantitative determination procedure for 210Pb and 210Po in various environmental matrices from different sources such as IAEA-326 soil, phosphate rocks (PR), and phosphogypsum (PG) was developed. The tested samples were digested sequentially using concentrated mineral acids (HF, HNO3) by a programmable high-pressure microwave digestion system. The sample solution was loaded onto a preconditioned ion exchange column (Sr-resin) for chromatographic separation. Polonium-210 was eluted by 6 M HNO3 then spontaneously deposited onto polished silver discs to be measured using low-background alpha spectrometry. Lead-210 was sequentially eluted using 6 M HCl solution, precipitated as lead oxalate, dissolved in HNO3 solution, and mixed with scintillation cocktail to be measured by liquid scintillation counting (LSC). Performance of the developed procedure was tested using a reference soil (IAEA-326), with recommended isotope values, that was used as a quality control to assess separation and quantification efficiency (recovery %). The minimum detectable activities of 210Pb and 210Po were found to be 24 and 0.28 Bq kg–1 for the measurements using LSC and alpha spectrometry, respectively. The recoveries (%) of 210Pb and 210Po were found to be 80 and 60%, respectively. To test the validity of the proposed LSC method, a comparative study was performed by measuring 210Pb activity concentration in test samples by nondestructive gamma-ray spectrometry.
Abbreviations: LSC, liquid scintillation counting • MDA, minimum detectable activity • PG, phosphogypsum • PR, phosphate rock
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INTRODUCTION
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LEAD-210 and polonium-210 are naturally occurring radionuclides of the 238U decay chain and originate in the earth's crust (Ivanovich and Harmon, 1992; Ugur et al., 2002; Vrecek et al., 2004). Lead-210 was separated from its ancestor 222Rn through five short-lived alpha and beta emitters, while 210Po was largely produced from decay of 210Pb via the intermediate 210Bi. Lead-210 emits low energy beta particles (Eßmax = 20 keV [81%], 61 keV [19%]) and gamma rays (E
= 46.5 keV [4.06%]) and has a half-life of 22.3 yr, whereas 210Po emits alpha particles (E
= 5.3 MeV [100%]) and has a half-life of 138.5 d. Because 210Pb has high specific activity of 2.82 x 106 Bq µg–1, it is considered as one of the most toxic natural nuclides, remaining in the skeleton long enough to produce the highest skeletal dose of any natural nuclide under average conditions of background exposure (Swift, 1998). Polonium-210 has also a considerable radiological importance due to its high specific activity (1.66 x 108 Bq µg–1), high radiotoxicity, and high energy of its alpha emissions, and also due to its accumulation by many terrestrial plants and animals (Kelecom et al., 2002). Therefore, these nuclides are of environmental importance because of their contributions to the natural radiation dose and technologically enhanced releases from various sources of natural activity. Furthermore, they are of a great importance due to their major contribution to the internal dose in humans (Mihai et al., 1996).
Some radiochemical methods of 210Po determinations for environmental surveys are not accompanied by that of its grand parent 210Pb because of the complexity of the methods; however, the importance of measuring 210Pb in addition to 210Po must not be ignored. Lead-210 is a continuous source of 210Po, therefore knowledge of its content is necessary for an accurate determination of the 210Po content at the sampling time. It is necessary to calculate the 210Po content at the equilibrium that is obtained when 210Po input has ceased and unsupported 210Po has decayed. In this concern, 210Po was determined based on a destructive radiochemical procedure, which includes self-deposition onto a polished metallic disc followed by measurements using alpha spectrometry (Vrecek et al., 2004; Kelecom et al., 2002; Flynn, 1968; Gafvert et al., 2002).
Analysis of 210Pb in environmental samples typically involves leaching or dissolving the samples (for solid samples) or preconcentration (for liquid samples), followed by a chemical purification process. Different methods have been developed for the chemical purification of 210Pb, including selective precipitation as sulfate (Lebecka et al., 1993; Nieri Neto and Mazzilli, 1998; Kim et al., 1999) or chromate (Peres and Hiromoto, 2002), and solvent extraction (Chen et al., 2001). Lead-210 activity can be inferred from measurement of alpha particles of 210Po after the growth of 210Po from 210Pb. This method is very sensitive but cannot be applied for rapid analysis because of the establishment time needed for the growth of 210Po (over 6 mo) (Kelecom et al., 2002; Chen et al., 2001; Suzuki et al., 1996). Other indirect methods described the determination of 210Pb via its beta emitting daughter 210Bi (Jia et al., 2000). Activity of 210Pb is measured based on the growth of its intermediate 210Bi by using either low-level background alpha/beta proportional counting (El-Afifi, 2005) or the liquid scintillation counting technique (Wallner and Irlweek, 1997). Radiochemical separation procedures are sensitive but they usually require more time for the chemical separation and for the ingrowths of 210Bi and 210Po (Yener and Uysal, 1996).
Determination of 210Pb can be performed directly using nondestructive gamma-ray spectrometry at 46.5 keV (Yener and Uysal, 1996; Zielinski and Budahn, 1998; Shenber, 2002). This method has relative advantages in terms of rapidity of measurement and that it does not require preliminary chemical separation in preparing the samples; however, it is limited by low emission probability of the gamma line and the often difficult corrections for self absorption in some sample matrices (Gogrewe et al., 1996; Banford et al., 1998; Pilvio et al., 1999).
The present work was to describe a rapid and accurate analytical procedure for selective separation and direct measurement of 210Pb and 210Po in various environmental matrices such as soil reference material, phosphogypsum (PG), and phosphate rock (PR) samples using liquid scintillation counter (LSC) and alpha spectrometry systems. Performance characteristics of the developed method were analyzed to assess the validity of the proposed LSC method. Several parameters affecting the method performance, including the minimum detectable activity (MDA), the radiochemical recovery, matrix effect, and the validity of the detection mode, were carefully investigated. A comparative study was performed by measuring 210Pb activity concentration in some tested samples by nondestructive gamma-ray spectrometry.
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MATERIALS AND METHODS
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Reagents
All the chemical reagents used throughout this work were of analytical grade and purchased from Merck (Darmstadt, Germany). Deionized water of high purity (10–18 
) was provided by Milli-Q reagent water system (Millipore, Billerica, MA). Standard solution of 208Po tracer was obtained from Amersham (Little Chalfont, UK).
Samples
Samples from different sources were used to examine the proposed radioanalytical procedure. These samples included reference soil (IAEA-326 soil), phosphate rock (PR), and phosphogypsum (PG). Reference soil is a black soil obtained from the Kursk region of Russia in 1990. The soil originated from the surface layer and was processed on behalf of the International Atomic Energy Agency by Khlopin Radium Institute, Petersburg, Russia in 1994. Activity levels of 210Pb and 210Po in this soil (40 ± 3.4 Bq kg–1) were recommended by the analytical quality control services, IAEA laboratories, Vienna, Austria (Bojanowski et al., 2001). The PR samples used were raw materials for phosphoric acid and phosphate fertilizer production at Abu Zabaal plant, Egypt. The PG samples are described as technically enhanced naturally occurring radioactive materials, namely PG waste associated with phosphoric acid and phosphate fertilizer production in Egypt and Poland. All samples were homogenized, with particle sizes less than 0.5 mm, and dried at 105°C for 12 h before analysis.
Dissolution of Samples
The procedure for complete dissolution and leaching of 210Pb and 210Po in the different reference soil, PR, and PG samples is described in Fig. 1. To each aliquot (300–700 mg), 160 mBq of 208Po tracer and 35 mg of Pb2+ carrier as lead nitrate solution were added. The samples were digested with concentrated nitric (HNO3), hydrofluoric (HF), and 2 M hydrochloric acid (HCl) solution using a programmable high-pressure microwave digestion (PHPMD) system (mLS-1200; Milestone, Sorisole, Italy), through three subsequent addition steps as shown in Fig. 1. The contents dissolved in 2 M HCl were transferred into 50-mL polyethylene tubes and centrifuged for 10 min (1700 rcf). The supernatant of each sample was cautiously evaporated to dryness and redissolved in 2 M HCl. In a few cases where solid residue was observed 2 M HCl was added to the residue then heated gently, to avoid volatilization of Po, then stirred well until complete dissolution. The solution was then evaporated to dryness and dissolved again in a small volume of 2 M HCl. For quality control purposes, reagent blank samples were also performed at each run.
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Fig. 1. Schematic procedure for separation of 210Po and 210Pb in samples. LSC, liquid scintillation counting; PHPMD, programmable high-pressure microwave digestion.
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Chromatographic Separation of Lead-210 and Polonium-210
After complete dissolution of 210Pb and 210Po, a sequential separation of both radionuclides was achieved using Sr-resin columns. In this concern, three grams of the Sr-resin (Eichrom Technologies, Darien, IL) (mesh size of 0.1–0.15 mm) was soaked in distilled water before filling the chromatographic column (Bio-Rad Laboratories, Hercules, CA) (10-mm internal diameter x 300 mm long). The column was conditioned by sequentially passing 100 mL of distilled water and 100 mL of 2 M HCl with flow rate of 1 mL min–1. After conditioning of the column, the sample solution was loaded. The column was washed using 100 mL of 2 M HCl to remove impurities (e.g., radionuclides and inactive elements and all the nonretained ions). The Po fraction was eluted first by passing 60 mL 6 M HNO3 through the column (Fig. 1), while the Pb fraction then eluted using 60 mL 6 M HCl.
Preparation of the Lead-210 Counting Aliquot
The 210Pb effluent fraction was evaporated to dryness. The sample was redissolved using 1 mL of concentrated HNO3 and evaporated again to dryness. This step was repeated two times to destroy any organic contaminants from the resin. The residue was dissolved in 20 mL of 1 M nitric acid, stirred, warmed, and mixed with 400 mg of oxalic acid. Lead oxalate precipitate was obtained by adjusting the pH of the solution to approximately 3 using concentrated ammonia solution. After 30 min, the fine precipitate was collected on filter paper under vacuum. The precipitate was washed using deionized water and 80% ethanol and dried in an oven at 50°C for 30 min. The precipitate was weighed for the gravimetric calculations of the recovery. The Pb oxalate precipitate was dissolved in 1 mL of 6 M HNO3 and mixed with 15 mL Insta Gel liquid scintillation cocktail (PerkinElmer, Wellesley, MA). The sample activities were measured using a liquid scintillation counter.
The double energy method was used to determine the activity level of 210Pb. In this respect, contribution due to Bi ingrowth in 210Pb region was evaluated as follows: the first energy window (window A) contains the full beta spectrum of 210Pb and the low energy part of the 210Bi spectrum, while the second energy window (window B) contains the medium energy part released by beta emission from 210Bi with no significant contribution from the alpha spectrum of 210Po (Fig. 2). Therefore, the activity concentration of 210Pb was calculated with correction for 210Bi contribution as follows:
 | [1] |
where CA is the net count rate (cps) in the energy window A of 210Pb + 210Bi; CB is the net count rate (cps) in the energy window B of 210Bi; f is the ratio between net count rate (cps) of 210Bi contribution in the energy window A to net count rate (cps) of 210Bi in the energy window B (this allows eliminating the 210Bi contribution involved with 210Pb in window A); E is the detection efficiency (of counting) in energy window A for 210Pb; R is the chemical recovery factor of 210Pb; and W is the sample weight (kg).
The signal percentage due to contribution of the 210Bi window relative to the window of 210Pb + 210Bi was approximately 5.25%.
Preparation of Polonium-210 Source
A fraction of 210Po was evaporated to dryness at about 150°C to destroy any organic contaminants coming from the resin through the separation process. Polonium was dissolved in a small amount of 2 M HCl and transferred into a deposition cell. Acidity of solution was adjusted to a pH of approximately 1 using 6 M NaOH. Ascorbic acid (100 mg) was added to the Po solution to prevent co-plating of other potentially interfering ions (i.e., Fe3+, Mn6+, and Cr6+). All Po isotopes were allowed to spontaneously plate onto a fixed Ag disc (2.5-mm diameter) with solution stirring at a temperature of 80°C for 90 min. After complete deposition of polonium, the disc was rinsed and washed using deionized water and acetone and dried in an oven at 50°C for 15 min. The prepared polonium source was counted for 24 h using alpha spectrometry (Ortec, Oak Ridge, TN) equipped with Alpha Vision 2.0 software.
Gamma-Ray Measurements
To validate the proposed destructive method for 210Pb determination using the LSC technique, the activity concentration of 210Pb was also measured and determined nondestructively using gamma-ray spectrometry. In this concern, 50 g of the 210Pb test samples was quantified nondestructively at 46.5 keV using a HPGe detector (Model 2201; Tennelec, Oak Ridge, TN) (resolution: 2.0 keV; efficiency: 20%) equipped with Genie 2000 software. A sealed point source containing 137Cs, 133Ba, and 60Co was used for energy calibration, while the efficiency calibration is performed using certified reference material (IAEA-314) provided by IAEA, Vienna, Austria.
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RESULTS AND DISCUSSION
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Chromatographic Separation of Lead-210 and Polonium-210
Figure 2 represents the spectrum obtained for 210Pb measured by LSC. The spectra showed two main windows. Window A is mainly related to 210Pb with small contributions from the low energy range of 210Bi, while window B is related to the high energy range of 210Bi emission. The spectrum does not show any other beta interferences, which confirmed the high purity of the 210Pb source.
After separation of 210Pb, 210Po was spontaneously deposited onto polished silver discs. Figure 3 shows typical alpha spectra of 210Po content for the reference soil (IAEA-326). It shows three peaks: the first, weak peak at 4.88 Mev is due to the contribution of 209Po (1.98%) present in the 208Po (98.02%) tracer solution. The second peak at 5.10 Mev is mainly for 208Po, while the third peak is due to 210Po (5.3 Mev). The spectra indicated that there were no other peaks related to alpha emitters. Resolution (FWHM: full width height maximum) of the spectra obtained for all samples does not exceed 26 keV, indicating the procedure was highly efficient in separating 210Po from 210Pb and other beta emitters.
Performance Characteristics of the Developed Analytical Method
The purpose of the present study was to elucidate the applicability (performance characteristics) of the proposed method for analysis of both 210Pb and its progeny (210Po) in various environmental samples. To fulfill this purpose, several factors have been investigated including the effect of sample matrices, radiochemical recovery, minimum detection activity (MDA), and validity of the detection modes.
Chemical Recovery
The chemical recovery of the separation processes of 210Pb is calculated from the following relation (Pimple, 1998):
 | [2] |
where W is the net weight of lead oxalate precipitate (mg); FG is the gravimetric factor of lead relative to lead oxalate (0.701); C is the Pb2+ concentration in the added carrier solution (mg mL–1); and V is the solution volume added (mL).
The radiochemical recovery of 210Po determination is calculated from the following relation (Pimple, 1998):
 | [3] |
where CPo208 is the net count rate of 208Po (5.1 Mev); APo208 is the standard activity of 208Po added; and 
is the detector efficiency.
Measurements of 210Pb yielded recovery values for reference soil, PR, PG (Egypt), and PG (Poland) of 72.2, 76.5, 80.5, and 75.3% with coefficients of variation of 6.9, 2.4, 3.8, and 3.9, respectively, while measurements of 210Po by alpha spectrometry yielded recovery values of 40.0, 67.3, 49.8, and 43.0% with coefficients of variation of 21.3, 14.4, 23.7, and 41.8, respectively.
Considering the fact that Pb recoveries were based on chemical/gravimetric measurements while Po recoveries were based on measurements of actual decay events, it was found that (Tables 1 and 2) the recoveries of 210Pb by chemical measurement are considerably high comparing with that of 210Po. The low recoveries of Po may be attributed to the partial volatilization of Po during the microwave digestion of sample matrices and/or during destruction of the organic residue eluted with Po fraction from the Sr resin. Although the recovery of Po was relatively low the final values determined for Po activity concentrations were adjusted by the radiochemical recovery of 208Po tracer.
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Table 1. Chemical recovery of 210Pb in various environmental samples using the liquid scintillation counting (LSC) technique.
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Matrix Effect
To investigate the performance of the separation process and both radiometric measuring systems, four samples with different matrices were tested: reference soil, PR, PG (Egypt), and PG (Poland). The reference soil sample was chosen as a quality control for digestion and separation efficiency and for accuracy in measuring the activity concentrations of 210Pb and 210Po by the two techniques, assuming the radionuclides were in radioactive equilibrium. As demonstrated in Tables 3 and 4, it was found that the mean activity concentrations of 210Pb in reference soil, PR, PG (Egypt), and PG (Poland) were 43.6, 1434, 993, and 971 Bq kg–1 with coefficients of variation of 8.5, 3.2, 3.7, and 0.4%, respectively. For 210Po the corresponding mean activities were 35.1, 1003, 716, and 815 Bq kg–1 with coefficients of variation of 0.9, 3.8, 3.7, and 1.4%, respectively.
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Table 3. Effect of various matrices on the activity of 210Pb measured by the liquid scintillation counting (LSC) technique.
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The mean activity concentrations obtained by LSC and alpha spectrometry for the reference soil, after decay correction, were 43.6 and 35.1 Bq kg–1, respectively. Since the recommended value for this soil is 40.0 ± 3.4 Bq kg–1, it can be stated that both alternative analytical methods were highly accurate and precise. The overall measuring efficiencies including separation and detection steps were found to be 91.9 and 87.6% for LSC and alpha spectrometry, respectively.
Minimum Detectable Activity Background Estimation
To determine the counting rate corresponding to the background, a reagent blank sample was prepared in an identical way to the real samples. The MDA value should be taken as a reference index only. When the procedure is applied to environmental samples, the MDA should be evaluated individually for each sample taking this factor into account. The minimum detectable activities for 210Pb and 210Po measured using LSC, gamma-ray, and alpha spectrometry were determined using the formulas reported by Currie (1968) and Seymour et al. (1992). It was found that the MDA values for 210Pb using LSC and gamma-ray spectrometry were 24 and 30 Bq kg–1, respectively, whereas the MDA values for 210Po using alpha spectrometry was 0.28 Bq kg–1.
Validity of the Detection Mode
Tables 3 and 5 contain the results obtained for 210Pb from LSC and gamma-ray measurements. For the reference soil sample, the activity concentration of 210Pb had mean values of 43.6 and 41.4 Bq kg–1 with standard deviations of 3.7 and 5.1 Bq kg–1 and coefficient of variations of 8.5 and 12.3%, respectively. The values determined using both techniques are close and in agreement with the reference value (40.0 ± 3.4 Bq kg–1, after decay correction) which is recommended by IAEA (Bojanowski et al., 2001). It was observed that the LSC measurements had relatively low coefficient of variation and low MDA (24 Bq kg–1), therefore the LSC technique is favored over gamma-ray measurements with an MDA of 30 Bq kg–1.
In cases of PR and PG (from Egypt) samples, the activity concentrations of 210Pb measured by gamma-ray spectroscopy had mean values of 1440 and 985 Bq kg–1, standard deviation of 103 and 56 Bq kg–1, and coefficient of variation of 7.2 and 5.7%, respectively. Although the obtained values of 210Pb using LSC measurements were close to that measured using gamma-ray spectrometry, the latter had slightly higher coefficient of variation compared with LSC measurements. It is obvious that the developed procedure for determination of 210Pb by ion exchange separation followed by LSC measurement has many advantages compared to the other methods: (i) it is based on the direct measurement of 210Pb itself; (ii) it is relatively rapid because it does not require the establishment of radioactive equilibrium between 210Pb and its progeny (i.e., 210Bi), as reported for other methods (Peres and Hiromoto, 2002; El Afifi, 2005; Nevissis, 1991; Jia et al., 2003; Johansson and Skarnemark, 2001); (iii) the procedure is simple since 210Pb is separated and purified without complicated radiochemical processes using extractants such as tributyl phosphate or heavy amines (Peres and Hiromoto, 2002; Chen et al., 2001; El Afifi, 2005); (iv) it is relatively high in its radiochemical recovery compared to that obtained by others (Peres and Hiromoto, 2002; Kim et al., 2001); and (v) it has a low minimum detectable activity and coefficient of variation compared with nondestructive analysis by gamma-ray spectrometry.
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CONCLUSIONS
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A selective chromatographic procedure using Sr ion exchanger was used for simultaneous separation of 210Pb and 210Po from different samples. Lead-210 was measured directly using LSC, while 210Po was measured using alpha spectrometry. The separation chromatogram of both radionuclides confirmed that no interferences existed in their spectra. The procedure was examined using recommended reference soil and applied to environmental materials and waste samples of different origins. Activity concentrations for 210Pb determined using this procedure (LSC) were in agreement with that obtained using gamma-ray spectrometry; however, LSC is favored over gamma-ray spectrometry not only due to lower minimum detectable activity but also due to its relatively high recovery. The main disadvantage of LSC over nondestructive gamma-ray spectrometry is attributed to consumption of aggressive chemical reagent through the sample digestion, separation, and measuring process.
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ACKNOWLEDGMENTS
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Deep thanks to Mr. G. Kis-Benedek and Ms. R. Schorn on their help during carrying out part of this work in the IAEA Laboratories, Seibersdorf, Vienna, Austria.
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REFERENCES
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ABSTRACT
INTRODUCTION
