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Development of a new electrodeposition method for Pu-determination in environmental samples
Applied Radiation and Isotopes PERGAMON
Applied Radiation and Isotopes 50 (1999) 851±857
Development of a new electrodeposition method for Pu-determination in environmental samples M.H. Lee a, *, M. Pimpl b a
Environmental Radiation Assessment Division, Korea Atomic Energy Research Institute, Taejon 305-353, South Korea b Central Safety Department, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany Received 5 February 1998; received in revised form 21 May 1998; accepted 4 August 1998
Abstract Using an ammonium oxalate-ammonium sulfate electrolyte containing diethyl triamino pentaacetic acid (DTPA), we developed a simple and quantitative technique for preparing sources for analytical alpha spectrometry. To determine the optimum conditions for plating plutonium, parameters aecting the electrodeposition of plutonium such as current and pH of electrolyte have been investigated. An optimized electrodeposition procedure for the determination of plutonium has been validated by application to IAEA-Reference soils. The developed method has been applied to determine fallout Pu in environmental samples such as soil, sediment and moss samples in Korea. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Plutonium; Electrodeposition; Environmental samples
1. Introduction Sources for alpha spectrometric measurements have usually been prepared by vacuum sublimation, direct evaporation, electrodeposition, or micro-coprecipitation. Among source preparations, electrodeposition is a commonly used method in the preparation of sources for alpha spectrometry, because the technique is simple and gives a very thin deposit, which is essential for a high resolution of the peak. Plutonium, americium and higher actinides are frequently plated from the solution of ammonium sulfate (Dutton and Lovett, 1977; Hallstadius, 1984; Liu et al., 1988; Tome and Sanchez, 1991), ammonium chloride (Mitchell, 1960; Veselsky, 1977), ammonium chloride-ammonium oxalate (Puphal and Olsen, 1972) or dimethylsulfoxide (Handley and Cooper, 1969). The most widespread method for electroplating actinides is that described by Talvitie (1972), using ammoniumsulfate-sulfuric acid as electrolyte in the plating solution. This method can be
* Corresponding author.
applied to analyze samples with a relatively simple matrix composition, like tracer solutions or water samples, but diculties occur when environmental samples with a complex matrix composition have to be analyzed. The ®nal step of the separation procedure, the electroplating from ammoniumsulfate-sulfuric acid solution, is easily disturbed by traces of matrix elements which have not been completely removed by the separation procedure. Even a small amount of organic carbon will reduce the recovery dramatically. To remove organic carbon quantitatively before electroplating by wet ashing with nitric, acid-sulfuric acid is therefore strictly recommended. By this, the time needed for analyzing environmental samples is lengthened at least for some hours. An additional disadvantage of the method is the necessity of a precise pH adjustment inside the electroplating solution. The use of ammonium hydroxide for pH adjustment may cause losses of Pu by polymerization, as hydrolysis of Pu(IV) can occur in the immediate vicinity of each drop of concentrated NH4OH used in the initial pH adjustment. Recently, in view of the diculties associated with electrodeposition, alternative techniques making
0969-8043/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 9 8 ) 0 0 1 5 5 - 9
852
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
use of micro-coprecipitation with rare earths have been developed to yield sources for a-spectrometry with a resolution comparable to electroplated sources and quantitative recovery of the actinides (Joshi, 1985; Hindman, 1986). In this study, to overcome the disadvantages of Talvitie's method, a simple and quantitative technique was developed for the electrodeposition of plutonium after radiochemical separation from environmental materials. The optimized method was validated by application to IAEA-reference soils. The developed method has been applied to determine fallout Pu in environmental samples from Korea, such as soil, sediment and moss samples.
2. Methods 2.1. Preparation of electrodeposition solution and electrodeposition cell The 242 Pu tracer of high alpha purity was obtained from Isotope Products Laboratories, USA. Radioisotope dilution was made by weight. Electroplating solution used in this study was prepared as follows: 30 g ammonium oxalate, 50 g ammonium sulfate, 18 g hydroxylammonium sulfate and 2 g diethyl triamino pentaacetic acid (DTPA) are dissolved in one litre of H2O and then the pH is adjusted to exactly 1.8 with H2SO4.
The cell used for these experiments was made of te¯on, it is shown in Fig. 1. The eective area of electrodeposition was 3.14 cm2. The anode was a polished platinum spiral wire. The upper lid, from which a small segment was cut out, was used not only to prevent releasing of the electrolyte outward but also to check visually the volume of the electrolyte inside the cell during the electrodeposition procedure.
2.2. Electrodeposition procedure 1. Add the Pu fraction obtained after the separation and cleaning procedure, which contains about 500 dpm 242 Pu tracer, into a 50 ml crystallizing dish, evaporate on a sand bath slowly to near dryness; 2. Rinse the crystallizing dish with 3 ml electroplating solution and transfer the rinsing solution into an electrodeposition cell; 3. Wash the crystallizing dish three times with 3 ml electroplating solution and transfer it to the cell; 4. Adjust the distance between the two electrodes to about 5 mm; 5. Turn on a current, adjusted to 950 mA, for 2 h; 6. Add about 1 ml conc. NH4OH into the cell before the end of electrodeposition; 7. Rinse the stainless steel plate with water and alcohol and hold it for 30 s in the ¯ame of a Bunsen burner.
Fig. 1. Schematic diagram of electrodeposition cell.
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
853
Fig. 2. Alpha counting eciency versus the source-to-detector distance.
2.3. Alpha spectrometry system The alpha spectrometer (EG&G ORTEC, Model 676A) was composed of an ion-implanted silicon detector (ORTEC, size: 450 mm2; alpha resolution: 25 keV FWHM at 5.486 MeV of 241 Am) in a vacuum chamber (Edwards Model E2M8), a detector bias supplier, a preampli®er, a linear ampli®er and a multichannel pulse-height analyzer. During the measurement inside the chamber a vacuum below 10 ÿ 2 Torr was maintained with a vacuum pump. As shown in Fig. 2, an increase of the sample-detector distance produces a decrease in the counting yield. In this study, the sample-detector distance was ®xed to 10 mm, because the detector is apt to be contaminated from recoil eects in the case of a shorter distance between sample and detector. 2.4. Radiochemical analyses of environmental samples
239,240
Pu and
238
with TOPO (trioctyl phosphineoxide) in cyclohexane. After back-extraction of the Pu fraction with ascorbic acid/hydrochloric acid, trace elements and disturbing alpha-emitting radionuclides of natural or man-made origin are separated radiochemically by coprecipitation of the Pu with lanthanium ¯uoride and an anion-
Pu in
To perform an accurate measurement of 239,240 Pu and 238 Pu by alpha-particle spectrometry requires ®rst a plutonium fraction free from major matrix components such as silica, iron, or aluminum, and from other alpha-emitters and second, a suitable method to prepare the source for alpha spectrometry. For the separation of a clean plutonium fraction from environmental samples a procedure was used which is schematically shown in Fig. 3 (SchuÈttelkopf, 1981). Pu is leached by aid of mineral acids from the ashed sample material (100 g of soil; 20 g of moss) and separated from most matrix components by extraction
Fig. 3. Analytical procedure for separation of fallout plutonium.
854
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
Fig. 4. Variation of the deposition yield of
exchange step. The obtained puri®ed Pu fraction is then electroplated on stainless steel platelets. The chemical yields attained with this analytical procedure were in the range of 70 to 80%. The detection limit was calculated from reagent blanks data using the following Eq. (1) (Curie 1968): LD Bq=kg
k1ÿa k1ÿb 2C 1=2 , TEYM
1
242
Pu with cell current.
where k 1 ÿ a, k 1 ÿ b are degrees of con®dence from Gaussian equation (at the 95% con®dence level: k 1 ÿ a = k 1 ÿ b = 1.645), C are background counts in the region of interest (ROI), T is background counting time, E is the counting eciency, Y is the chemical yield and the M is mass (kg). The detection limit was 0.0063 Bq/kg-dry for soil and sediment and 0.031 Bq/kg-dry for moss in 86,000 s of counting time in the alpha spectrometry.
Fig. 5. Variation of the deposition yield of
242
Pu with cell pH.
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
855
Table 1 Optimum conditions of electrodeposition Anode Cathode Voltage Volume Duration of electrodeposition Electrolyte solution
Platinum wire, 1 mm dia. stainless steel disk 7.5 V 12 ml 2h 0.2 M ammonium oxalate, 0.4 M ammonium sulfate, 0.1 M hydroxyl ammonium sulfate and 0.005 M DTPA 1.8
pH of electrolyte solution
Table 2 Plutonium results for soil reference samples obtained by the `optimized method' Concentration of
239,240
Pu (Bq kg ÿ 1)
Concentration of
238
Pu (Bq kg ÿ 1)
Sample
ref. value
this methoda
ref. value
this method
IAEA-326 Soil IAEA-327 Soil
0.4952 0.025 0.5842 0.018
0.48420.031b 0.58720.012
0.01920.002 0.02020.005
0.02220.003 0.02320.006
a
Number of aliquots analyzed is 3.bError is 1s.
3. Results and discussions 3.1. Modi®cation of the electrodeposition step To determine the optimum conditions for plating plutonium, it is necessary to determine the eects of such parameters as time, current, electrolyte concentration, chelating reagent, pH and volume of electrolyte on the electrodeposition for plutonium. In this study, the eects of chelating reagent, current and pH of electrolyte were investigated on the base of the ammonium oxalate-ammonium sulfate electrolyte. In general, a chelating agent such as NTA (Nitrilo triacetic acid), EDTA (ethylene diamine tetraacetic acid) and DTPA has good acid solubility and prevents polymerization and hydrolysis during the electrodeposition. Puphal and Olsen (1972) used DTPA to improve the
yield of several nuclides on the electrodeposition. However, if one uses Puphal and Olsen's method, the electrodeposition must be performed in a fume hood because of evolution of chlorine during the electrodeposition time. To overcome this shortcoming of Puphal and Olsen's method, in our study 0.005 M DTPA chelating agent was added to ammonium oxalate-ammonium sulfate electrolyte. The eects of varying the current at ®xed electrodepostion time (2 h), volume (12 ml) and pH (1.8) are shown in Fig. 4. The maximum yields were obtained with a current of 950 mA. Fig. 5 shows how plating yields vary with pH at constant time (2 h), current (950 mA) and volume (12 ml) of electrodeposition. The pH value was controlled with 0.1 M H2SO4 and NH4OH. The results shown in Fig. 5 indicate that the optimum pH is 1.8.
Table 3 Comparison of chemical yields between the optimized method and Talvitie's method Chemical yield in % Sample material
This methoda
Talvitie's methoda
Soil (100 kg-dry) Sediment (100 kg-dry) Moss (20 g-dry)
75.32 14.5b 70.62 12.7 74.62 18.1
65.4212.7 65.1212.1 70.4213.4
a
Number of aliquots analyzed is 4.bError is 1s.
856 Table 4 Activity concentration of
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
239,240
Pu and
238
Pu together with activity ratio of
238
Pu/239,240 Pu in the environmental samples 238
Concentration (Bq/kg-dry) Sample name
239,240
Gongju soil Uljin soil Sesan soil Jungsun soil Upo sediment Daeduk sediment East Sea sediment Yellow Sea sediment Uljin moss Kimje moss Muju moss Gongju moss
1.10 20.10 0.55 20.04 0.32 20.03 0.80 20.09 0.37 20.06 0.28 20.05 2.03 20.27 0.77 20.08 4.70 20.71 0.59 20.12 1.47 20.11 0.39 20.04
Pu
The distance between the electrodes is not critical as long as the current is maintained at 950 mA. Darkening of the cathode occurred at a spacing of 2 mm, and excessive voltage was required at a spacing of about 8 mm, resulting in boiling and electrolyte loss. A distance of about 5 mm gave the best plate. The optimum conditions of the electrodeposition are summarized in Table 1. 3.2. Validation of the electroplation procedure The optimized electrodeposition method for the determination of plutonium in environmental samples
238
Pu
0.04420.005 0.02520.003 0.01420.001 0.02920.004 0.01020.003 0.01320.003 0.08320.009 0.02520.013 0.24920.040 0.02320.002 0.04420.006 0.03020.004
Pu
239,240
Pu
0.04020.006 0.04520.006 0.04420.005 0.03620.006 0.02720.009 0.04620.013 0.04120.007 0.03220.017 0.05320.012 0.03920.009 0.03020.005 0.07720.013
was validated by its application to several IAEAReference soils. As shown in Table 2, the concentrations of plutonium using the optimized electrodeposition step are so consistent with reference values reported by IAEA that this method can be applied to dierent soils with reliable results. In Table 3, the chemical yields obtained for dierent environmental samples were compared between the Talvitie method and the modi®ed one. The chemical yield of the optimized method of electrodeposition step was about 7% higher than that of the Talvitie method. The reason for the high chemical yield in the optimized method may be the eect of the chelating agent.
Fig. 6. Alpha spectra of fallout Pu in the Gongju Soil.
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
DTPA used in this method perhaps suppresses polymerization of plutonium owing to local concentration of ammonium hydroxide during electrodeposition time. 3.3. Application to environmental samples The optimized method of fallout Pu determination described above has been applied to environmental samples such as soil, sediment and moss samples. As shown in Table 4, the 239,240 Pu and 238 Pu contamination level in the terrestrial environment due to atmospheric fallout varies in a broad range: for 238 Pu between 0.010 and 0.249 Bq/kg-dry and for 239,240 Pu between 0.28 and 4.70 Bq/kg-dry. However, the measured speci®c activities of 239,240 Pu and 238 Pu in environmental samples of Korea are in good agreement with data reported by UNSCEAR (1982) for the north temperate zone (40±308). The activity ratios of 238 Pu/ 239,240 Pu in the Korean samples are in the range of 0.027 to 0.077, with a mean value of 0.043. The mean activity ratio of 238 Pu/239,240 Pu agrees well with observations on the total deposition by the global fallout performed by Perkins and Thomas, 1980, who determined a 238 Pu/239,240 Pu ratio of 0.037. Fig. 6 shows the typical a-particle spectrum of plutonium fraction isolated from the Gongju soil using the method described above. The alpha peaks of 242 Pu, 239,240 Pu and 238 Pu are well resolved (FWHM; 26.6 keV) and the spectrum is free from contributions due to various thorium and uranium isotopes.
Acknowledgements This study has been carried out under the Nuclear R & D program by the Ministry of Science and Technology of Korea.
857
References Curie, L.A., 1968. Limits for qualitative and quantitative determination: application to radiochemistry. Anal. Chem. 40, 586. Dutton, J.W.R., Lovett, M.B., 1977. Supported 242 Cm in the marine environment. Nature 267, 37. Hallstadius, L., 1984. A method for the electrodeposition of actinides. Nucl. Instrum. Meth. Phys. Res. 223, 266. Handley, T.H., Cooper, J.H., 1969. Quantitative electrodeposition of actinides from dimethylsulfoxide. Anal. Chem. 41 (2), 381. Hindman, F.D., 1986. Actinide separations for a spectrometry using neodymium ¯uoride coprecipitation. Anal. Chem. 58, 1238. Joshi, S.R., 1985. Lanthanum ¯uoride coprecipitation technique for the preparation of actinides for alpha-particle spectrometry. J. Radioanal. Nucl. Chem. Article 90 (2), 409. Liu, S.T., Ban, Y., Sumiya, S., Ishida, J., Iwai, M., 1988. Simultaneous electrodeposition of actinides. J. Radioanal. Nucl. Chem. Article 121 (1), 167. Mitchell, R.F., 1960. Electrodeposition of actinide elements at tracer concentrations. Anal. Chem. 32 (3), 326. Perkins, R.W., Thomas, C.W., 1980. World wide fallout. In: Transuranic Elements in the Environment, Hanson, W.C. (Ed.), DOE/TIC-22800. National Technical Information Service, Spring®eld, VA, p. 53. Puphal, K.W., Olsen, D.R., 1972. Electrodeposition of alphaemitting nuclides from a mixed oxalate-chloride electrolyte. Anal. Chem. 44 (2), 284. SchuÈttelkopf, H., 1981. KfK-Report 3035. Talvitie, N.A., 1972. Electrodeposition of actinides for alpha spectrometric determination. Anal. Chem. 44 (2), 280. Tome, F.V., Sanchez, A.M., 1991. Optimizing the parameters aecting the yield and energy resolution in the electrodeposition of uranium. Appl. Radiat. Isot. 42 (2), 135. Veselsky, J.C., 1977. Problems in the determination of plutonium in bioassay and environmental analysis. Analytica Chimica Acta 90, 1. United Nations, 1982. Ionizing radiation: sources and biological eects, UNSCEAR (United Nations Scienti®c Committee on the Eects of Atomic Radiation), Report to the General Assembly, New York.
Applied Radiation and Isotopes 50 (1999) 851±857
Development of a new electrodeposition method for Pu-determination in environmental samples M.H. Lee a, *, M. Pimpl b a
Environmental Radiation Assessment Division, Korea Atomic Energy Research Institute, Taejon 305-353, South Korea b Central Safety Department, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany Received 5 February 1998; received in revised form 21 May 1998; accepted 4 August 1998
Abstract Using an ammonium oxalate-ammonium sulfate electrolyte containing diethyl triamino pentaacetic acid (DTPA), we developed a simple and quantitative technique for preparing sources for analytical alpha spectrometry. To determine the optimum conditions for plating plutonium, parameters aecting the electrodeposition of plutonium such as current and pH of electrolyte have been investigated. An optimized electrodeposition procedure for the determination of plutonium has been validated by application to IAEA-Reference soils. The developed method has been applied to determine fallout Pu in environmental samples such as soil, sediment and moss samples in Korea. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Plutonium; Electrodeposition; Environmental samples
1. Introduction Sources for alpha spectrometric measurements have usually been prepared by vacuum sublimation, direct evaporation, electrodeposition, or micro-coprecipitation. Among source preparations, electrodeposition is a commonly used method in the preparation of sources for alpha spectrometry, because the technique is simple and gives a very thin deposit, which is essential for a high resolution of the peak. Plutonium, americium and higher actinides are frequently plated from the solution of ammonium sulfate (Dutton and Lovett, 1977; Hallstadius, 1984; Liu et al., 1988; Tome and Sanchez, 1991), ammonium chloride (Mitchell, 1960; Veselsky, 1977), ammonium chloride-ammonium oxalate (Puphal and Olsen, 1972) or dimethylsulfoxide (Handley and Cooper, 1969). The most widespread method for electroplating actinides is that described by Talvitie (1972), using ammoniumsulfate-sulfuric acid as electrolyte in the plating solution. This method can be
* Corresponding author.
applied to analyze samples with a relatively simple matrix composition, like tracer solutions or water samples, but diculties occur when environmental samples with a complex matrix composition have to be analyzed. The ®nal step of the separation procedure, the electroplating from ammoniumsulfate-sulfuric acid solution, is easily disturbed by traces of matrix elements which have not been completely removed by the separation procedure. Even a small amount of organic carbon will reduce the recovery dramatically. To remove organic carbon quantitatively before electroplating by wet ashing with nitric, acid-sulfuric acid is therefore strictly recommended. By this, the time needed for analyzing environmental samples is lengthened at least for some hours. An additional disadvantage of the method is the necessity of a precise pH adjustment inside the electroplating solution. The use of ammonium hydroxide for pH adjustment may cause losses of Pu by polymerization, as hydrolysis of Pu(IV) can occur in the immediate vicinity of each drop of concentrated NH4OH used in the initial pH adjustment. Recently, in view of the diculties associated with electrodeposition, alternative techniques making
0969-8043/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 9 8 ) 0 0 1 5 5 - 9
852
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
use of micro-coprecipitation with rare earths have been developed to yield sources for a-spectrometry with a resolution comparable to electroplated sources and quantitative recovery of the actinides (Joshi, 1985; Hindman, 1986). In this study, to overcome the disadvantages of Talvitie's method, a simple and quantitative technique was developed for the electrodeposition of plutonium after radiochemical separation from environmental materials. The optimized method was validated by application to IAEA-reference soils. The developed method has been applied to determine fallout Pu in environmental samples from Korea, such as soil, sediment and moss samples.
2. Methods 2.1. Preparation of electrodeposition solution and electrodeposition cell The 242 Pu tracer of high alpha purity was obtained from Isotope Products Laboratories, USA. Radioisotope dilution was made by weight. Electroplating solution used in this study was prepared as follows: 30 g ammonium oxalate, 50 g ammonium sulfate, 18 g hydroxylammonium sulfate and 2 g diethyl triamino pentaacetic acid (DTPA) are dissolved in one litre of H2O and then the pH is adjusted to exactly 1.8 with H2SO4.
The cell used for these experiments was made of te¯on, it is shown in Fig. 1. The eective area of electrodeposition was 3.14 cm2. The anode was a polished platinum spiral wire. The upper lid, from which a small segment was cut out, was used not only to prevent releasing of the electrolyte outward but also to check visually the volume of the electrolyte inside the cell during the electrodeposition procedure.
2.2. Electrodeposition procedure 1. Add the Pu fraction obtained after the separation and cleaning procedure, which contains about 500 dpm 242 Pu tracer, into a 50 ml crystallizing dish, evaporate on a sand bath slowly to near dryness; 2. Rinse the crystallizing dish with 3 ml electroplating solution and transfer the rinsing solution into an electrodeposition cell; 3. Wash the crystallizing dish three times with 3 ml electroplating solution and transfer it to the cell; 4. Adjust the distance between the two electrodes to about 5 mm; 5. Turn on a current, adjusted to 950 mA, for 2 h; 6. Add about 1 ml conc. NH4OH into the cell before the end of electrodeposition; 7. Rinse the stainless steel plate with water and alcohol and hold it for 30 s in the ¯ame of a Bunsen burner.
Fig. 1. Schematic diagram of electrodeposition cell.
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
853
Fig. 2. Alpha counting eciency versus the source-to-detector distance.
2.3. Alpha spectrometry system The alpha spectrometer (EG&G ORTEC, Model 676A) was composed of an ion-implanted silicon detector (ORTEC, size: 450 mm2; alpha resolution: 25 keV FWHM at 5.486 MeV of 241 Am) in a vacuum chamber (Edwards Model E2M8), a detector bias supplier, a preampli®er, a linear ampli®er and a multichannel pulse-height analyzer. During the measurement inside the chamber a vacuum below 10 ÿ 2 Torr was maintained with a vacuum pump. As shown in Fig. 2, an increase of the sample-detector distance produces a decrease in the counting yield. In this study, the sample-detector distance was ®xed to 10 mm, because the detector is apt to be contaminated from recoil eects in the case of a shorter distance between sample and detector. 2.4. Radiochemical analyses of environmental samples
239,240
Pu and
238
with TOPO (trioctyl phosphineoxide) in cyclohexane. After back-extraction of the Pu fraction with ascorbic acid/hydrochloric acid, trace elements and disturbing alpha-emitting radionuclides of natural or man-made origin are separated radiochemically by coprecipitation of the Pu with lanthanium ¯uoride and an anion-
Pu in
To perform an accurate measurement of 239,240 Pu and 238 Pu by alpha-particle spectrometry requires ®rst a plutonium fraction free from major matrix components such as silica, iron, or aluminum, and from other alpha-emitters and second, a suitable method to prepare the source for alpha spectrometry. For the separation of a clean plutonium fraction from environmental samples a procedure was used which is schematically shown in Fig. 3 (SchuÈttelkopf, 1981). Pu is leached by aid of mineral acids from the ashed sample material (100 g of soil; 20 g of moss) and separated from most matrix components by extraction
Fig. 3. Analytical procedure for separation of fallout plutonium.
854
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
Fig. 4. Variation of the deposition yield of
exchange step. The obtained puri®ed Pu fraction is then electroplated on stainless steel platelets. The chemical yields attained with this analytical procedure were in the range of 70 to 80%. The detection limit was calculated from reagent blanks data using the following Eq. (1) (Curie 1968): LD Bq=kg
k1ÿa k1ÿb 2C 1=2 , TEYM
1
242
Pu with cell current.
where k 1 ÿ a, k 1 ÿ b are degrees of con®dence from Gaussian equation (at the 95% con®dence level: k 1 ÿ a = k 1 ÿ b = 1.645), C are background counts in the region of interest (ROI), T is background counting time, E is the counting eciency, Y is the chemical yield and the M is mass (kg). The detection limit was 0.0063 Bq/kg-dry for soil and sediment and 0.031 Bq/kg-dry for moss in 86,000 s of counting time in the alpha spectrometry.
Fig. 5. Variation of the deposition yield of
242
Pu with cell pH.
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
855
Table 1 Optimum conditions of electrodeposition Anode Cathode Voltage Volume Duration of electrodeposition Electrolyte solution
Platinum wire, 1 mm dia. stainless steel disk 7.5 V 12 ml 2h 0.2 M ammonium oxalate, 0.4 M ammonium sulfate, 0.1 M hydroxyl ammonium sulfate and 0.005 M DTPA 1.8
pH of electrolyte solution
Table 2 Plutonium results for soil reference samples obtained by the `optimized method' Concentration of
239,240
Pu (Bq kg ÿ 1)
Concentration of
238
Pu (Bq kg ÿ 1)
Sample
ref. value
this methoda
ref. value
this method
IAEA-326 Soil IAEA-327 Soil
0.4952 0.025 0.5842 0.018
0.48420.031b 0.58720.012
0.01920.002 0.02020.005
0.02220.003 0.02320.006
a
Number of aliquots analyzed is 3.bError is 1s.
3. Results and discussions 3.1. Modi®cation of the electrodeposition step To determine the optimum conditions for plating plutonium, it is necessary to determine the eects of such parameters as time, current, electrolyte concentration, chelating reagent, pH and volume of electrolyte on the electrodeposition for plutonium. In this study, the eects of chelating reagent, current and pH of electrolyte were investigated on the base of the ammonium oxalate-ammonium sulfate electrolyte. In general, a chelating agent such as NTA (Nitrilo triacetic acid), EDTA (ethylene diamine tetraacetic acid) and DTPA has good acid solubility and prevents polymerization and hydrolysis during the electrodeposition. Puphal and Olsen (1972) used DTPA to improve the
yield of several nuclides on the electrodeposition. However, if one uses Puphal and Olsen's method, the electrodeposition must be performed in a fume hood because of evolution of chlorine during the electrodeposition time. To overcome this shortcoming of Puphal and Olsen's method, in our study 0.005 M DTPA chelating agent was added to ammonium oxalate-ammonium sulfate electrolyte. The eects of varying the current at ®xed electrodepostion time (2 h), volume (12 ml) and pH (1.8) are shown in Fig. 4. The maximum yields were obtained with a current of 950 mA. Fig. 5 shows how plating yields vary with pH at constant time (2 h), current (950 mA) and volume (12 ml) of electrodeposition. The pH value was controlled with 0.1 M H2SO4 and NH4OH. The results shown in Fig. 5 indicate that the optimum pH is 1.8.
Table 3 Comparison of chemical yields between the optimized method and Talvitie's method Chemical yield in % Sample material
This methoda
Talvitie's methoda
Soil (100 kg-dry) Sediment (100 kg-dry) Moss (20 g-dry)
75.32 14.5b 70.62 12.7 74.62 18.1
65.4212.7 65.1212.1 70.4213.4
a
Number of aliquots analyzed is 4.bError is 1s.
856 Table 4 Activity concentration of
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
239,240
Pu and
238
Pu together with activity ratio of
238
Pu/239,240 Pu in the environmental samples 238
Concentration (Bq/kg-dry) Sample name
239,240
Gongju soil Uljin soil Sesan soil Jungsun soil Upo sediment Daeduk sediment East Sea sediment Yellow Sea sediment Uljin moss Kimje moss Muju moss Gongju moss
1.10 20.10 0.55 20.04 0.32 20.03 0.80 20.09 0.37 20.06 0.28 20.05 2.03 20.27 0.77 20.08 4.70 20.71 0.59 20.12 1.47 20.11 0.39 20.04
Pu
The distance between the electrodes is not critical as long as the current is maintained at 950 mA. Darkening of the cathode occurred at a spacing of 2 mm, and excessive voltage was required at a spacing of about 8 mm, resulting in boiling and electrolyte loss. A distance of about 5 mm gave the best plate. The optimum conditions of the electrodeposition are summarized in Table 1. 3.2. Validation of the electroplation procedure The optimized electrodeposition method for the determination of plutonium in environmental samples
238
Pu
0.04420.005 0.02520.003 0.01420.001 0.02920.004 0.01020.003 0.01320.003 0.08320.009 0.02520.013 0.24920.040 0.02320.002 0.04420.006 0.03020.004
Pu
239,240
Pu
0.04020.006 0.04520.006 0.04420.005 0.03620.006 0.02720.009 0.04620.013 0.04120.007 0.03220.017 0.05320.012 0.03920.009 0.03020.005 0.07720.013
was validated by its application to several IAEAReference soils. As shown in Table 2, the concentrations of plutonium using the optimized electrodeposition step are so consistent with reference values reported by IAEA that this method can be applied to dierent soils with reliable results. In Table 3, the chemical yields obtained for dierent environmental samples were compared between the Talvitie method and the modi®ed one. The chemical yield of the optimized method of electrodeposition step was about 7% higher than that of the Talvitie method. The reason for the high chemical yield in the optimized method may be the eect of the chelating agent.
Fig. 6. Alpha spectra of fallout Pu in the Gongju Soil.
M.H. Lee, M. Pimpl / Applied Radiation and Isotopes 50 (1999) 851±857
DTPA used in this method perhaps suppresses polymerization of plutonium owing to local concentration of ammonium hydroxide during electrodeposition time. 3.3. Application to environmental samples The optimized method of fallout Pu determination described above has been applied to environmental samples such as soil, sediment and moss samples. As shown in Table 4, the 239,240 Pu and 238 Pu contamination level in the terrestrial environment due to atmospheric fallout varies in a broad range: for 238 Pu between 0.010 and 0.249 Bq/kg-dry and for 239,240 Pu between 0.28 and 4.70 Bq/kg-dry. However, the measured speci®c activities of 239,240 Pu and 238 Pu in environmental samples of Korea are in good agreement with data reported by UNSCEAR (1982) for the north temperate zone (40±308). The activity ratios of 238 Pu/ 239,240 Pu in the Korean samples are in the range of 0.027 to 0.077, with a mean value of 0.043. The mean activity ratio of 238 Pu/239,240 Pu agrees well with observations on the total deposition by the global fallout performed by Perkins and Thomas, 1980, who determined a 238 Pu/239,240 Pu ratio of 0.037. Fig. 6 shows the typical a-particle spectrum of plutonium fraction isolated from the Gongju soil using the method described above. The alpha peaks of 242 Pu, 239,240 Pu and 238 Pu are well resolved (FWHM; 26.6 keV) and the spectrum is free from contributions due to various thorium and uranium isotopes.
Acknowledgements This study has been carried out under the Nuclear R & D program by the Ministry of Science and Technology of Korea.
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