An improved method for the determination of uranium ... - Springer Link

Journal of Radioanalytical and Nuclear Chemistry, Vol. 262, No. 2 (2004) 433–441

An improved method for the determination of uranium isotopes in environmental samples by alpha-spectrometry Guogang Jia,* G. Torri, P. Innocenzi Italian Environmental Protection Agency and Technical Services, Via V. Brancati 48, 00144 Rome, Italy (Received May 4, 2004)

In order to improve the selectivity of the uranium isotopes determination in environmental samples, further studies have been carried out, including (1) interference of 210Po with uranium isotope determination, (2) distribution coefficients of polonium between 5% TOPO in toluene and aqueous hydrochloric and nitric acids, (3) decontamination factor of uranium from polonium of the recommended procedure, and (4) leaching effect comparison of two different leaching procedures in a lichen sample. Based on the new findings, a more accurate extraction chromatographic/ -spectroscopy method has been developed. For the method’s validation, four kinds of reference materials supplied by the IAEA have been tested. It is observed that nearly all the 238U, 234U and 235U concentrations obtained are in good agreement with the recommended or information values, showing that the method can give reliable results. A comparison with existing uranium determination methods has also been made. It is concluded that due to involving preconcentration and chemical separation, the extraction chromatographic/ -spectroscopy method is a more selective, very sensitive and accurate, and low cost method.

Introduction The measurements of low-level concentration of uranium isotopes in various environmental, geological and biological samples are very important in many fields including nuclear industry, radioactive waste management and disposal, health physics, geology, geochronology and environmental science. The extraction chromatographic/ -spectroscopy method is one of the most common techniques. This has the advantage of low cost and high selectivity, sensitivity and accuracy, and can be widely used in routine uranium isotope analysis, but extensive sample preparation procedures are required to achieve accurate results. Uranium is an actinide and, because of its position in the periodic table and electronegativity, shows strongly lithophile characteristics. Most commonly it occurs as oxides, hydroxides, phosphates, carbonates, sulphates, arsenates, vanadates, molybdenates and silicates, and tends to have affinities to hydrocarbon complexes. The distinctive characteristics of uranium in nature create many problems for its analysis in environmental and biological samples. Losses in several stages of uranium analysis were one of the main problems affecting the accuracy of the final results. There are a number of possible loss mechanisms during sample decomposition, including gas evolution, absorption onto surfaces, precipitation and persistence of undissolved material.1 Therefore, for many analytical techniques, the first step in analyzing solid samples is the wet acid digestion and chemical dissolution or fusion. Various sample decomposition methods were investigated including: (1) open-vessel dissolution with nitric, perchloric, hydrochloric and hydrofluoric acids, (2) closed vessel microwave digestion with nitric and hydrofluoric acids

followed by open-vessel dissolution with perchloric acid, and (3) lithium metaborate fusion and Na2CO3, NaF and Na2O2 fusion in platinum crucible followed by mineralization with nitric, hydrochloric and hydrofluoric acids. It was originally expected that most, if not all, of the sample decomposition methods investigated would produce final solutions with negligible fractions of insoluble materials. As a matter of fact, fusion is the most efficient method for uranium dissolution in solid samples. An additional problem affecting the accuracy of the final results in uranium analysis for -spectroscopy was encountered the isolation of uranium from both the large quantities of inactive substances (iron, phosphates and silicates, etc.) present and potential -emitting interferents in some typical samples. Numerous procedures, based on precipitation,2 ion exchange,2,3 liquid-liquid extraction,4 extraction chromatography,5–7 and thereof,2 have been suggested for this preconcentration and separation. In most of the uranium determination procedures much attention has been given to the separation of uranium from Th, Pu, Am and Cm, but less care has been taken to the interference of 210Po. Based on uranium isotope determination in environmental samples by -spectrometry developed in this Laboratory,8 further studies for improving the method have been carried out, including (1) interference of 210Po with uranium isotope determination, (2) distribution coefficients of polonium between 5% TOPO in toluene and aqueous hydrochloric and nitric acids, (3) decontamination factor of uranium by polonium in the recommended procedure, (4) leaching effect comparison of two different leaching procedures in a lichen sample, and (5) quality control of the recommended procedure evaluated by analyzing four kinds of reference materials

* E-mail: [email protected] 0236–5731/2004/USD 20.00 © 2004 Akadémiai Kiadó, Budapest

Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht

GUOGANG JIA et al.: AN IMPROVED METHOD FOR THE DETERMINATION OF URANIUM ISOTOPES

supplied by the IAEA. These studies were very helpful to develop more accurate methods for uranium isotope determination by -spectrometry, to validate the method and the results obtained in routine analyses, and to improve other methods involving polonium separation as well. Experimental Apparatus and reagents The uranium and polonium sources were counted by -spectrometry (Canberra, U.S.A.) with a counting efficiency of 31.2% and a background of 2.10–6 s–1 in the interested energy region. The apparatus for uranium electrodeposition (model PL320QMD; Thurlby Thandar Instruments Ltd., England) was composed of a Perspex cell of 25 mm internal diameter and a stainless-steel disk of 20 mm diameter. A Perspex disk holder with a silver disk of 23 mm diameter and 0.15 mm thickness was used for 210Po spontaneous deposition. Chromatographic columns were 150 mm long and had 9 mm internal diameter. 232U, 236U and 209Po standard solutions, Microthene (microporous polyethylene, 60–140 mesh), tri-octylphosphine oxide (TOPO, 99%) and reference materials (IAEA-140/TM, IAEA-326, IAEA-327, IAEA-336, IAEA-381) were supplied by Amersham (G. B.), Ashland (Italy), Fluka (Switzerland) and IAEA (Vienna), respectively. FeCl3 was used to prepare the carrier solution for uranium separation in water and all other reagents were analytical grade (Merck, Germany). Column preparation A solution (50 ml) of 0.3M TOPO in cyclohexane was added to 50 g of Microthene; the mixture was stirred for several minutes until homogeneity and than evaporated at 50 °C to eliminate cyclohexane. The porous powder thus obtained contained about 10.4% TOPO. A portion (1.6 g) of the Microthene-TOPO powder, slurred with 3 ml concentrated HCl and some water, was transferred to a chromatographic column; after conditioning with 30 ml of 2M HNO3, the column was ready for use.

uranium isotopes (238U, 236U, 235U, 234U and 233U). The decontamination effects of the method from some important -emitters (Th, Pu, Am and Cm) have been studied in detail. However, during the application of the method for analyzing uranium activity in some typical lichen and leave samples, it was found that yields higher than 100%, and even 200% can be obtained. This arose a doubt of the possible interference of 210Po with uranium determination in the method. 210Po (E = 5.304 MeV), almost a pure -emitter with a halflife of 138.4 days, is the grand daughter of 210Pb, which is one of the intermediate products in the 238U decay chain. Interference of 210Po could be very critic in uranium determination in environmental samples by spectrometry due to (1) the widespread existence of 210Po as a natural radionuclide and (2) its E similarity to 232U [E = 5.2635 MeV (31%) and 5.3203 MeV (69%)]. It is owing to the interference of 210Po that yield correction from the 232U counts can lead to wrong results and can make the uranium concentration of the samples much lower than the real values. In order to verify the possible interference of 210Po, 4 lichen and 4 leave samples have been analyzed based on the method in Reference 8, but 236U was used as tracer instead of 232U. The -spectra obtained show that there exists a small peak in the energy region of 5.20– 5.32 MeV and the total counts are much higher than the reagent and instrument background. It is sure that there is interference from an -emitter. In order to identify the radionuclide, the sources obtained from the eight samples were counted by -spectrometry continually for a period of about 10 months to trace their radioactivity decay. After the data treatment, an average half-life of 138±11 days were obtained (Fig. 1), which is similar to the half-life of 210Po (138.4 days). Therefore, it was confirmed that there exists 210Po interference at the uranium isotope determination if 232U is used as a tracer.

Preliminary tests The preliminary tests for determination of uranium isotopes in environmental samples by alphaspectrometry were mainly based on the procedure reported in Reference 8. Polonium interference: In Reference 8, TOPO was used as an extractent in the chromatographic column and 232U was selected being used as a uranium yield tracer with an advantage capable of determining nearly all 434

Fig. 1. Radionuclide identification test: radioactivity decay in the energy region of 5.20–5.32 MeV obtained from a source prepared from a lichen sample


Distribution coefficients of polonium between 5% TOPO in toluene and aqueous HCl and HNO3: To improve the decontamination effect of uranium from 210Po, it is certainly necessary and important to better understand the extraction behavior of polonium between TOPO in toluene and aqueous HCl and HNO3. For this purpose, 209Po with an activity of 0.0271 Bq in a beaker was transferred into a 10 ml graduated extraction tube with 3 ml HCl or HNO3 solution at different acidities (0.010, 0.10, 0.50, 1.0, 2.0, 3.0, 4.0, 6.0, 8.0, and 10.0M). Then 3.0 ml of 5% TOPO in toluene were added and shaking was carried out at room temperature (22±2 °C) for 5 minutes. After centrifugation for 5 minutes, the 209Po in the aqueous and organic phases was separated, deposited on a silver disk and measured by -spectrometry. The detailed extraction procedure of polonium and the calculation process of the distribution coefficients (D) were presented elsewhere.9 The distribution coefficients of polonium between 5% TOPO in toluene and aqueous hydrochloric acids at different acidity are given in Fig. 2. It is shown that: (1) about 6.4% of polonium is extracted by 5% TOPO in toluene from 0.010M HCl; (2) nearly all polonium (>98%) can be extracted by 5% TOPO in toluene at acidity ranges from 0.10 to 10.0M HCl, and (3) the D values of polonium are increasing with increasing HCl acidities. Therefore, it seems impossible to eliminate polonium from 5% TOPO in toluene in a wide range of HCl acidities from 0.10 to 10.0M. The distribution coefficients of polonium between 5% TOPO in toluene and aqueous nitric acid at different acidities are shown in Fig. 3. It is seen that: (1) there exists a peak of D values at the acidity of 1.0M HNO3; (2) when the acidity is higher than 2M HNO3 the D values are decreasing dramatically; and (3) at the acidities of 6M HNO3 and above, only 1.5% polonium is extracted by 5% TOPO in toluene. The polonium extraction rate of 1.5% can be considered a negligible amount. This gives an opportunity to eliminate and separate polonium from uranium effectively in the liquid-liquid extraction and column extraction chromatography of TOPO. In such condition nearly all polonium exists in the aqueous phase, but uranium still remains in the organic phase of TOPO with D values of 200.10 Decontamination factor of uranium from Po: The decontamination effect of uranium from polonium of the method in Reference 8 was verified by analyzing 0.5 g of IAEA-327 soil reference material where 0.0542 Bq of 209Po was added. In the method, 20 ml of 2M HNO 3 were used to eliminate polonium from the MicrotheneTOPO column. At that condition the obtained decontamination factor (DF) of uranium from polonium is 1657±437, which seems not sufficient to meet the need for polonium decontamination due to the fact that the D values of polonium (1.87±0.25) between 5%

TOPO in toluene and 2M HNO3 in liquid-liquid extraction are not low enough. For further improving the method, the Microthene-TOPO column was washed with 30 ml of 6M HNO3, and the corresponding DFs from polonium have increased to 8302±1478, showing a much better decontamination effect of uranium from polonium. Leaching of uranium from lichen samples: Loss of uranium in its refractory form in the step of sample decomposition is the main problem affecting the accuracy of the final results. The leachability of uranium in some IAEA reference materials (soil) has been studied in detail in Reference 8 using four different leaching techniques, including (1) wet digestion with HNO3, HCl and HF, (2) wet digestion with HNO3, HCl, HF and HClO4, (3) wet digestion with HNO3, HCl, HF and H3BO3, and (4) fusion with Na2CO3 and Na2O2 at 600 °C and further digestion with HNO3, HCl and HF. The results showed that the last technique is the best one for the complete dissolution of uranium in soil.

Fig. 2. Distribution coefficients of polonium between 5% TOPO in toluene and aqueous hydrochloric acid

Fig. 3. Distribution coefficients of polonium between 5% TOPO in toluene and aqueous nitric acid

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It is known that botanical and/or biological materials (vegetation, lichen, moss leave and so on) may contain various soil and/or refractory mineral fractions,1 therefore, studies on the leachability of uranium in such matrices are also significant for the development of an accurate method. For this purpose, IAEA-336 reference material (Lichen) was selected for a test with an information value (95% confidence interval) of 0.492 (0.369–0.616) Bq.kg–1 for 238U. The sample was analyzed using two different leaching techniques, (1) fusion with Na2CO3 and Na2O2 at 600 °C and further digestion with HNO3, HCl and HF (method for soil samples suggested in Reference 8), (2) wet digestion with Na2CO3, and then HNO3 and HF (method for biological samples suggested in Reference 8). The uranium isotope concentrations are given in Table 1. The uncertainty given for individual analysis in the table is 1 , which is estimated from the uncertainties associated with the tracer (232U) activity, the addition of the tracer to the sample, counting statistics and the weighing of the sample (the same in Tables 2 to 5). From Table 1 it is seen that the average 238U concentrations of the sample are 0.595±0.028 Bq.kg–1 obtained from the soil method and 0.529±0.026 Bq.kg–1 obtained from the biological sample method. The 238U concentration obtained from the soil method is 12.5% higher than from the biological sample, perhaps due to existing of a refractory uranium fraction in the sample. Although both results are in good agreement with the information value within the 95% confidence interval from the statistics point of view, it is quite evident that the leaching efficiency of the soil method is better. Therefore, it is suggested that the soil leaching method should also be used to biological samples.

Recommended procedures Based on the new findings in the section above, the methods for determination of uranium isotopes in environmental samples by alpha-spectrometry is suggested as follows: Preconcentration of uranium in sea and fresh water: Forty mg of Fe3+ (40 mg Fe3+.ml–1) as carrier, 0.03 Bq of 232U as tracer and 20 ml of conc. HNO3 are added to one litre of filtered water sample. After boiling for 30 minutes, the solution is removed to an electric-magnetic stirrer and adjusted to pH 9.5–10 with concentrated ammonia solution to coprecipitate uranium with iron(III) hydroxide. The solution is stirred for another 30 minutes and the precipitate is allowed to settle down for at least 4–6 hours and preferable overnight. The supernatant is

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carefully siphoned off and the hydroxide slurry is centrifuged at 4000 rpm. The supernatant is discarded, the precipitate is dissolved in 5 ml of conc. HNO3 and a few drops of 40% HF and 30% H2O2, and transferred to a 150 ml beaker. The solution is evaporated to incipient dryness and the residual is dissolved in 2 ml of conc. HNO3, 13 ml of water and a few drops of 40% HF by heating. Further separation is carried out following the uranium determination procedure given below. Leaching of uranium from soil or sediment: Two grams of milled sodium carbonate powder are put in a 30-ml ceramic or platinum crucible and made into a crucible shape with a smaller crucible. Then, 0.5 g of soil or sediment sample ( 150 µm) and 2 g Na2O2 are added to the sodium carbonate crucible. After mixing well the sample with Na2O2, the crucible with a cover is put in a muffle at 600 °C for 15–20 minutes. The fused sample is taken out from the muffle, cooled at room temperature and 0.03 Bq of 232U are added. Leaching is carried out by adding some water and sufficient conc. HNO3, and the leachate is transferred into a 100 ml Teflon beaker. 5 ml of conc. HNO3, HCl and 40% HF are added to the leachate which is then evaporated to incipient dryness and this step is repeated another two or three times to eliminate most of the silicates. The residue is dissolved again with 5 ml of conc. HNO3 and some water and filtered through a 0.1 µm cellulose nitrate membrane filter (Whatman, England) together washing with 10 ml 2M HNO3 to a 150 ml beaker. Some concentrated ammonia solution is added to the beaker to adjust the solution pH to 9.5–10 to coprecipitate uranium with hydroxide, which is then centrifuged at 4000 rpm in a 100 ml glass tube. After discarding the supernatant, the precipitate is dissolved with 3 ml of concentrated HNO3, 12 ml of water and a few drops of 40% HF by heating and transferred to a 150 ml beaker and washed with 10 ml of 2M HNO3. Leaching of uranium from biological samples (vegetables, mosses and lichens, etc.): A 30 ml ceramic or platinum crucible containing 1–5 g dried biological sample is put in a muffle. To avoid burning the sample, the muffle is heated slowly (over 10 hours) starting from room temperature to 600 °C. After 14 hours at 600 °C, the temperature of the muffle slowly decreases to room temperature. Two grams of Na2O2 and Na2CO3 each are added in the crucible and mixed well with the ashed sample. The crucible is heated at 600 °C for 15–20 minutes. The fused sample is taken out from the muffle, cooled at room temperature and 0.03 Bq of 232U are added. The next treatments are the same as the corresponding steps for soil leaching described in the former section.


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Separation and determination of uranium: The leachate solution is passed through a preconditioned Microthene-TOPO column at a flow rate of 0.6–0.8 ml.min–1. After washing with 30 ml of 6M HNO3, 60 ml of 1M HCl and 5 ml of water at the same flow rate, uranium is eluted with 30 ml of 0.025M (NH4)2C2O4 at a flow rate of 0.1 ml.min–1. The first 3 ml of eluant are discarded and the remains are directly collected in an electrodepositing cell. 0.62 ml of 8M HNO3 is added to the cell and the solution is adjusted to pH 1.0–1.5 with 1 : 4 ammonia solutions. Uranium is electrodeposited on a stainless steel disk at a current density of 400 mA.cm–2 for 4 hours and counted by -spectrometry. Results and discussion Comparison with existing uranium determination methods Several techniques have been used to measure uranium in various environmental, geological and biological samples, including kinetic phosphorescence analysis (KPA),11 energy-dispersive X-ray fluorescence (EDXRF),12,13 delayed neutron activation analysis (DNA),14 inductively coupled plasma-mass spectrometry (ICP-MS),15,16 thermal ionization mass spectrometry (TIMS),17,18 and -spectrometry.2–7,19,20 Evaluation of the typical characteristics (selectivity, sensitivity, accuracy, throughput and cost) of these methods can provide basic information for selecting suitable techniques to measure uranium for different research purposes. Of these methods, KPA is sensitive with a lower limit of detection (LLD) of 5 ng/sample, but is an elemental analysis technique and so it is not isotope specific. Calibration of the instrument typically uses a set of external standards and/or standard addition techniques and can be susceptible to matrix effects. EDXRF analyses the total uranium concentration (primarily 238U), and the LLD (50 µg/sample) is typically near to the average background soil abundances of 3 ppm.13 Hence, this method is most useful for rapid characterization of environmentally contaminated sites. DNA is typically sensitive to only 235U (LLD: 50 ng/sample), and so the estimation of the concentration of the other more radioactively abundant uranium isotopes (238U and 234U) is obtained only through knowledge of process. Due to the relatively poor sensitivity to 234U and 238U, the interpretation of the 235U results is difficult. ICP-MS is a more sensitive technique (LLD: 0.5– 50 ng/sample) capable of rapidly analyzing solutions in the sub ppb range and can be routinely used for 238U analysis. This technique can rapidly analyze multiple

elements and detect less abundant isotopes of uranium (234U and 235U) when chemical separations are used prior to analysis. Isotope dilution ICP-MS provides analytical accuracy comparable to extraction chromatography/ -spectroscopy for 238U and more sensitive and precise measurement of 235U. Isotope dilution analysis by TIMS is the most sensitive (LLD: 0.01 pg/sample) and accurate method for measurement of all of the naturally occurring uranium isotopes in environmental samples,17,18,21 but is typically not used for routine human or environmental monitoring studies due to its limited throughput and high cost. Isotope dilution extraction chromatographic/ -spectroscopy method (Alpha-ID) has a reputation of low cost to maintain the instrument. Owing to preconcentration, chemical separation and purification steps in the method, most of the radioactive and nonradioactive interferences can be eliminated. Therefore, the method is very selective, sensitive and accurate and can be used to measure nearly all the natural and artificial uranium isotopes including 238U, 234U, 235U, 236U, 233U and 232U simultaneously in all kinds of liquid and solid samples. The extraction chromatography/ -spectroscopy method has been developed and used in the Italian Environmental Protection Agency for many years, and the sensitivity (LLD: 15 ng/sample), accuracy and throughput (8 samples/day) of the method are high enough to meet all needs in routine environmental monitoring purposes for several research programs.22–25 A comparison of uranium isotope concentrations (Table 2) was made by analyzing twelve environmental water samples with both extraction chromatography/ -spectroscopy and ICP-MS methods.25 It was observed that, (1) 238U, 234U and 235U concentrations in most of the samples obtained by both methods are comparable, and (2) the sensitivity and accuracy of the extraction chromatography/ spectroscopy method are as good as the ICP-MS method for 238U, better than ICP-MS method for 234U and less good than ICP-MS method for 235U. In brief, direct analysis techniques for solids (DNA and EDXRF) and aqueous samples (KPA and ICP-MS) have the highest throughput and lowest cost for the externally calibrated methods, since they do not require chemical separation. However, accuracy is generally higher for the isotope dilution methods (Alpha-ID, ICPMS-ID and TIMS-ID) relative to the externally calibrated methods. Due to involving preconcentration and chemical separation steps, Alpha-ID is a more selective, very sensitive and accurate, suitable throughput and low cost method. ICP-MS-ID and TIMS-ID (LLD: 0.01–1 pg/sample) can be considered as ultra-sensitive methods, but have the lowest throughput and highest cost due to the need for clean laboratory for chemical separations. 439


Fig. 4. Correlation between experimental values of 238U concentration (y, Bq.kg–1) and the IAEA recommended or information values (x, Bq.kg–1) in four kinds of reference materials with different levels of uranium activity

Quality control of the method One of the basic principles of quality assurance in radioanalytical chemistry is that only validated methods can be used for routine analyses. Even in cases when the methods are available from the literature and peerverified methods (intercomparison organized with various laboratories) they must be validated in the particular laboratory that is going to apply them. Methods developed in-house should undergo a complete method validation, which includes: accuracy (trueness + precision), sensitivity, selectivity, linearity, limit of detection, possible interferences, recovery and ruggedness. The selective, accurate and precise determinations of uranium concentrations are very important aspects in the method validation process. The use of certified reference materials containing the analyte at a known concentration and composition is one of the most appropriate tests to validate methods and the performance of the analysts. Moreover, it has long been recognized that uranium measurements in samples of terrestrial and marine origin are often subject to large errors in terms of precision and accuracy. A major part of the analyst’s work is to reduce these errors to a degree that the data becomes useful for environmental quality assessment. For this purpose, it is desirable to have included the analysis of a quality control or certified reference material that matches the sample as closely as possible with respect to its matrix and the concentrations of the constituents of interest to demonstrate the reproducibility and/or accuracy of the method. Method validation should be a continuous and regular process in order to obtain maximum confidence in the data set. In fact, for validation of the uranium isotopes determination method in Reference 8 extensive studies have been done, and especially, five soil or sediment reference materials supplied by IAEA were analyzed to 440

check statistically whether it is sufficiently precise and unbiased. In order to assess the reliability of the improved uranium determination method developed in this paper, four kinds of IAEA reference materials with different uranium activity levels are tested, including seawater, seaweed (algae), lichen and soil. Four- to sixfold analyses are done for each reference material and all the results with mean activity concentration and standard deviation are given in Tables 1 and 3 to 5. Linearity between the experimental and recommended or information values is a characteristic of the selectivity of a method. As shown in Fig. 4, for example, the correlation between experimental 238U activity concentrations (y, Bq.kg–1) and recommended or information values (x, Bq.kg–1) can be expressed as: y = 0.931x+0.284, n = 28, r = 0.9992. Similar evidences for 234U and 235U have also found in these samples. The nearly unity of the correlation coefficient demonstrates that (1) the method has very good selectivity and could be applicable to samples with different levels of uranium activities, and (2) the interferences from natural (232 Th, 228 Th, 226Ra, 210Po and etc) and artificial (237Np, 239+240Pu, 238Pu and 241Am) -emitters seem negligible. The precision was evaluated by the relative standard deviations. The accuracy was assessed by the term of relative errors, which reflect the difference between the experimental means and recommended or information values of uranium concentrations. From Tables 1 and 3 to 5, it is observed that the relative standard deviations of the results for all the four reference materials are