Journal of Radioanalyfical and Nuclear Chemistry, Voi. 220, No. 1 (1997) 15-19

Determination of plutonium and americium in moss and lichen samples

G. Ji&* D. Desideri,** F. Guerra,** M. A. Meli,** C. Testa** *China Institute of Atomic Energy, Beijing, P. R. China

**instituto di Chimica Generale, Universita di Urbino, Piazza Rinascimento 6, Urbino, Italy

(Received September 3, 1996)

A simple, sensitive and selective method is described for the simultaneous determination of plutonium and americium in lichen and moss samples which can be used as the atmospheric radioactivity b'mindicators. Plutonium is separated from a HCI leaching solution by a Micro~hene-TNOA colunug americimn is separated by a KL-HDEHP colonm and purified by PMBP-TOPO extraction. A special attention has been paid to the decontamination of plutonium and americium from 210po. Ten lichen and 12 moss samples from tree minks have been analyzed: staring from 2 g sample, the average yields and the detection limits were 70.2 + 12.5% and 28 mBq/kg for plutonium and 70.0 + 15.1% and 34 mBq/kg for americium. The concentrations (mBq/kg) ranged from < 28 to 4960 for 2393A0pu, from < 28 to 171 for 238pu and from < 34 to 1930 for 241Am, respectively.

Introduction

Radioactive fallout from nuclear weapon tests and accidental releases of radionuclides from nuclear power plants led to a worldwide contamination, especially on the areas of northern hemisphere. Over the past decades, since there was a dramatic increase in public concern regarding environmental issues, studies of the radioactive contamination of the environment have become the subject of many scientific researches. Among the studied pollutants, some attention has been given to the transuranic alpha emitters (mainly 239.24~ due to their long physical half-lives and high biological toxicity. From radiation protection point of view, ~lAm can be also significant, but it has not always been included in the most current environmental radioactive monitoring programs. The reason for such an absence is probably due to the difficulty of its analysis; as a matter of fact 241Am concentration in most environmental matrixes is not high enough to be detected directly by T-ray spectrometry and the radioanalytical procedures for ~lAm are complex and time-consuming if a-spectrometry is used.

The present study is devoted to develop a simple, sensitive and selective radioanalytical method for the simultaneous determination of plutonium and americium in mosses and lichens which are extremely efficient accumulators of radionuclides and non-radioactive heavy metals) -~

Experimental

Equipment

The a-spectrometry counting system was composed of a multichannel analyzer (Varro 8K), a digital plotter (Facit 4550) and a 450 mm 2 solid silicon detector (Mod. PP. 300-100-21) with a counting efficiency of 31% and a background < 5 . 1 0 --5 .s -1 over the interested energy region. The electroplating apparatus (Model IT-63, Carlo

Erba, Italy) consisted of 25 mm i.d. cells and 20 mm diameter stainless-steel disks. The chromatographic columns had 10 mm internal diameter.

Reagents

The ~2Pu and 243Am standard solutions, Microthene-710 (microporous polyethylene, 50-100 mesh) and the KL-I-IDEHP resin (50% Di(2-ethyl- hexyl)phosphofic acid, 60-100 mesh) were supplied by Amersham (G. B.), S. I. C. (Italy) and C. N. I. C. (P. R. China), respectively. 1 -phenyl-3-methyl-4-benzoyl-5- -pyrazoloue (PMBP), tri-octyl-phosphine oxide (TOPO), tri-n-octyl-amine (TNOA) and the other reagents were analytical grade (Carlo Erba, Italy).

Column preparation

Column A: 20 ml 20% TNOA in toluene (v/v) were added to 16 g Microtbene; the mixture was stirred for several minutes to get a homogeneous product, which was then placed in an oven at 40-50 ~ to evaporate toluene and obtain to a porous powder, containing about 20% TNOA w/w. Four grams of the Microthene-TNOA powder was mixed a small quantity of water and transferred into a column. After conditioning with 20 ml 6M HCI the column was ready for plutonium extraction.

Column B: 2.5 g KL-HDEHP resin were put into a beaker and immersed into 0.01M I-LNO 3 for 24 hours, then the resin was transferred into a column (the height of the resin was 10 cm).

Sampling

10 lichen and 12 moss samples were collected from tree trunks in a central region of Italy (Urbino, Marche). The samples were dried to constant weight for 24 hours at 105 ~ and then powdered.

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G . J I A e t ~1.: D E T E R M I N A T I O N O F P L U T O N I U M A N D A M E R I C I U M IN M O S S A N D U C H E N S A M P L E S

Preliminary tests

Lichens and mosses have been reported to accumulate a great variety of both man-made (SSFe, 9~ 137Cs, ]~ 144Ce, 125Sb and Pu etc.) and naturally occurring (TBe, 4~ Th, U and their decay products) radionuclides. Due to their high radioactive concentration a small sample quantity could be analyzed, with the possibility to use a simple procedure. Several analytical procedures for the determination of plutonium and americium in the environmental samples have been reported, s9 but most of them are tedious and time-consuming. In the preliminary tests of this study the basic procedure reported in reference 9 was chosen, but further studies have been made subsequently.

-6 i - e -

I 0 e " 0

,3

6 0 -

50-

40-

30-

20-

10-

12

~13Am 21Opo

a)

,I �9 J , . , n . I , . , , , n ~ U , ~ l ~ l ~ I I I I

=1Am

3.95 4.23 4.50 4.78 5.05 5.33 5.60 5.88 Energy, MeV

Polonium elimination

Figures la and 2a and 2b show that 21~ (Tlr 2 = = 138 d, Ea=5.31 MeV) seems always to interfere

f - OB

: 3 0

0

6 0 -

5 0 -

4 0 -

3 0 -

2 0 -

10-- a)

0 3.95

~2pu

2SO, 240pu

lj- o I i ~ 2Upu

=J~ll..lali//lt.,ia . . . . . . . I " ; . . . . . . I I I I I : "

4.23 4.50 4.78 5.05 5.33 5.60 5.88

Energy, MeV

t-- t -

t - o

Q.

i - -'I

8

~2pu

60-

50--

40--

30--

20 - ~ 239, 24Opu

i ~ 21~176 10- b) I i 2 "pu

0 . . . . . . . . . ml ,. = l ....... j . . . . 4123 4'50 4178 5'.05 5'.33 5'.50

Energy, MeV

Fig. 1. Alpha-spectra of the same moss sample obtained at different washing conditions: a - washing with 100 ml 10M HCI and 80 ml

10M HNO3; b - washing with 120 ml 10M HCI and 150 ml

-6 e - t - - ca

=o

-- I O o

30-~

._,.,~ ~lAm mOpo

10- b) ~ .

12 , * * , , , , = l , , l IL

4'23 4'50 4:78 5'05 5'33 18o 588 Energy, MeV

-6 30- r e.-

20-

10- 0 o

~Am

0 ' ' " I ' ' 1 ' ' I ' '1 I I ~ l ' I ~'-

3.95 4.23 4.50 4.78 5.05 5.33 5.60 5.88 Energy, MeV

Fig. 2. Alpha-spectra of the same moss sample by analyzing at different conditions: a - with 243Am tracer and feeding solution in 6-7M HNO3;

b - without tracer and feeding solution in 6-7M HNO3; c - without tracer and feeding solution in 5-6M HCI

when simple procedures for plutonium and americium analyses are used. Several techniques for polonium elimination are available, 9-11 such as: (1) volatilization at high temperature (especially in the chloride chemical form), (2) washing out from TNOA column with high concentration HNO3, (3) adsorbtion on a TNOA column in a HCI medium and (4) polonium self-plating on nickel or copper powder using diluted HCI solution. In Reference 10, the first two techniques were used to eliminate 21~ for plutonium analysis. Due to the relatively low ashing temperature used in the present recommended procedure (450 ~ a great washing volume of 10M HNO 3 (from 80 ml to 150 ml) was tried in order to get a better decontamination from 21~ however this change caused a 10% to 30% plutonium loss. Later on, 150 ml 7.5M HNO 3 were chosen as a washing solution and the results show that the decontamination from 21~ is satisfactory (Fig. lb) and

16

G. Jt~ et al.: DlgrI~MINATION OF I'LUTONIUM AND AMERICIUM m MOSS AND LICHEN SAMPLI~

Table 1. 210po and 238U activity distributions (in mBq) after column A at two different feeding solution conditions (6M He1 and 7M HNO3)

Feeding 7.5M HNO 3 washing volmne, ml solution Nuclide Pu disk Am disk

0-40 41--80 81-120 121-160

6M HCI 21~ 0.42 + 0.13 8.35 + 0.56 0.38 + 0.12 0.15 + 0.08 0.19 + 0.07 0.09 + 0.04 238U 2.47:1:0.31 N.D. N.D. N.D. N.D. N.D.

7MHNO 3 21~ 0.33+0.19 0.42+0.11 0.29:1:0.11 0.16+0.08 0.20+0.08 1.85+0.30 238U N.D. N.D. N.D. N.D. N.D. N.D.

N.D.: not detectable.

that plutonium yields still remain high. Only in the case of large amounts of Fe, a washing with 20 ml 10MHNO 3 was necessary before the 7.5M HNO 3 washing.

2X~ interference is also a critical problem for 24xAm analysis. In the procedure reported in Reference 9, a special TNOA column was prepared for 21~ elimination, as its or-particle energy (5.31 MeV) is near to that (5.27 MeV) of 243Am used as a yield tracer. Therefore high activities of 2t~ in the final americium source could give an 243Am overestimation and consequently an 241Am underestimation. In order to prepare a short procedure, it was tried to adsorb and eliminate 2Wpo completely by column A. Experiments with and without tracer were carried out at two different feeding solution conditions (6M HCI and 7M HNO3) by analyzing the same moss sample containing high concentration of Pu, ~4tAm, 21Opo and 23sU. Moreover, all the 7.5M HNO 3 washing solutions were collected and analyzed in the different sections for 21~ and 238U by eleclroplating directly the solution residue at pH 1.5-2.0 with 10 ml 0.025M H2C20~0.1M HNO 3 solution, in order to better know where the majority of 21~ and 238U Was. The results reported in Table 1 show clearly that first condition had to be preferred for a good decontamination of Pu and Am from 23sU and 21~ The americium alpha spectra obtained in other moss samples by using the different feeding solution conditions are shown in Fig. 2.

Americum decontamination from rare earth elements

In the preliminary Am determination procedure, no separation step from rare earth elements was included and Am was eluted from column B with 3M HNO 3 as recommended by other authors. 12 However in this investigation, it was found that some rare earth elements were present in moss and lichen samples. Although they do not interfere with the Am electroplating, the Am obtained disks did not result clean, showing a resolution deterioration in the alpha-spectra. In order to improve the method, it was tried to elute Am by 1M HNO 3. The elution curve (Fig. 3) indicates that 20 ml 1M HNO 3 are sufficient to strip most of Am. In this case, some of the rare earth elements (Y, Eu etc.) remain on the column and the Am sources are clean, showing good resolution features.

0.3'

0.2-

0.1

0.0

- L i i I I i

0 8 16 24 32 40

Elution volume, ml

Fig. 3. Elution diagram of americium from KL-DEHP column. Eluent: 1M HNO3; flow rate: 0.15 ml/min

"ID

1 0 0 ~

8 0 . - -

6 0 -

4 0 ~

20.

|

l I ) I i ! 3O 6O 9O

Time, rain

0 I I ; I 0 120 150

Fig. 4. Americimn yield as a function of the electroplating time. Current density: 550 mA/cm2; medimn: 10 ml 0.025M H2C204/0.1M HNO 3

adjusted to pH 2 with ammonia

17

G . J I A e t a l . : D E T E R M I N A T I O N O F P L U T O N I U M A N D A M E R I C I U M IN M O S S AND U C l i i E N S A M P L E S

Americum electroplating

It was reported |2 that the optimum pH value for the Am electroplating in the oxalic acid and nitric acid medium is -~ 3o- r -

2. Actually, Am electroplating is more difficult when compared with Pu for which a relatively wider pH range ~. 20 - (fi'om 1 to 2) can be used in the same medium. Am yield fluctuations can occur when the electroplating time and the ~ 1 o - current density are not accurately controlled. In the present study a 0.025M I-l,zC20,~0.1 HNO 3 solution was used and o the Am yields as a function of electroplating time at a 3.95 current density of 550 mA/cm 2 were studied. As Fig. 4 shows a quantitative recovery of Am was obtained after 90 minute electroplating.

[i ii i I i i , t �9 ,

423 450 2.r8 s'.05

=r

~4~Am

I ! I =~ 5.33 5.60 5.88

Energy, MeV

Fig. 6. Americium o~-spectmm of a moss sample obtained by suggested procedure

Recommended procedure

Plutonium determination

2 g of dried sample are put into a ceramic crucible. Known activities of z42pu and z43Am (0.01-0.03 Bq) are added as the chemical yield tracers. The sample is placed on a heater for carbonization and then transferred to a muffle for ashing at 450 ~ for 20 hours. After cooling, two leachings are carried out with 15 ml 6M HC1 each. The leacning solutions are centrifuged, combined and mineralized. 1 ml of 1M NH,zOH �9 HCI is added to obtain Pu(III), which is then oxidized to Pu(IV) by adding I ml of 8M NaNO 2. The 5-6M HCI leaching solution is passed through column A at 0.8 ml/min for the plutonium retention, then the column is washed with 30 ml 6M HCI and the two effluents are kept for americium analysis. After a further washing with 120 ml 10M HCI and 150 ml 7.5MHNO 3, plutonium is eluled with 18 ml 0.025MH2C20~0.15MHNO 3 at 0.1 ml/min and it is

6 0 -

50 - -

"e 40- - t- i- ra 5 ~. : ~ -

0 o 20--

1 0 -

0 3.95

242

2311, 24Op u 23epu

4123 4:50 . . . . . . ' ' 1 I1"11~1 11'1 '1 17- 4.'78 5.05 5.33 5.60 5.88

Energy, MeV

Fig. 5. Plutonimn a-spectmrn of a moss sample obtained by the suggested procedure

electroplated at pH 1.5-2.0 for one hour at 550 mA/cm 2 current density. Finally plutonium activity in the disk is measured by (x-spectrometry.

Americium determination

The collected HCI effluent is evaporated to dryness and 3 ml of conc. HNO 3 and some water are added to reach a volume of 50 ml which are then adjusted to pH 2 with conc. NHaOH and passed through column B at a flow rate of 0.8 ml/min for americium retention. After washing with 60 ml 0.1M HNO 3, americium is eluted with 20 ml 1M HNO 3. The elution solution is evaporated to dryness and the residue is transferred to a 10 ml extraction tube by 3 ml of 0.1M HNO 3. Three ml 0.05M PMBP/0.025M TOPO in cyclohexane are added to the tube and americium is extracted by shaking the tube for 5 minutes. After centrifuging, the aqueous phase is discarded. The organic phase is washed with the same volume of 0.1M HNO 3 and then diluted with 1 ml benzene and stripped twice with 4 ml of 5M HNO 3. The stripping solution is evaporated to dryness and the organic matter is destroyed with a few drops of I~O~ to obtain a white residue. Finally, the residue is transferred into the electrolytic cell by using 10 ml 0.025M HzC204/0.1M HNO 3 solution and americium is electroplated at pH 2 for 90 minutes at 550 mA/cm 2 current density for (x-spectrometry.

Results

Detection limits

The reagent blanks determined on 10 analyses were (6.1 + 0.4). 10 -5. s -l for 239,24~ and 238pu and (6.3 + + 0.5). 10 -5. s -1 for 241Am in the interested energy areas. When the counting efficiency was 31%, the yields of Pu and Am for 2 g samples were 70.2% and 70.0%, the detection limits for 239'24~ and 2aSpu and 241Am were 28 mBq/kg and 34 mBq/kg respectively as far as the 3a of the blank count rate are concerned.

18

G. JIA et aL: DETI~.MINATION OF PLUTONIUM AND AIV~RICIUM IN MOSS AND LICHEN S A ~

The application of the method

Ten lichen and 12 moss samples from tree trunks have been analyzed by using the suggested method. The average yield was 70.2+ 12.5% for Pu and 70.0+ 15.1% for Am. The concentrations (Bq/kg) in lichens ranged from < 0.03 to 1.87 for 239'2~Dpu, from < 0.03 to 0.15 for 238pu and from < 0.03 to 0.77 for 241Am. The corresponding values in mosses ranged from 0.32 to 4.96, from 0.03 to 0.17 and from 0.20 to 1.93, respectively. The average ratios of 238pu/239'240pu and 241AIIl/239"240pu Wel'e 0.09-----0.06 and

0.47 + 0.16, respectively. As an example, Figs 5 and 6 show the Pu and Am a-spectra obtained from the same moss samples. From the results, it can be concluded that (1) the recommended procedure is simple, sensitive and selective and (2) both lichens and mosses are effective biological indicators of air pollution of Pu and Am which are undetectable in most of the plants at such a small sampling quantity (2 g). More detailed results on lichen and moss sample concentration will be reported elsewhere. |3

References

1. J. E. LARSSON, Svemk. Bat. Tidskr, 64 (1970) 173. 2. O. ERAMLrrSA" J. YLmUOKAmm, Suom, Kemistil, B44 (1971) 372. 3. J. GAgTY, M. GALUN, C. FUc~ts, N. ZaSm, EL, Water Air Soil PoUut. 8

(1977) 171. 4. C. PAPASTEFANON, M. MANOLOPOUI~U, T. SAWIDIS, J. Environ.

Radioactivity, 9 (1989) 199. 5. N. A. TALVrrm, AnaL Chem., 43 (1971) 1827. 6. E. HOLM, R. FUlcra, Talante, 23 (1976) 853. 7. K. BUNZL, W. KRACtm, J. RadioanaL Nucl. Chem., 115 (1987) 13. 8. K. BATAREK~ D. IC ~ R A m , J. Radioanal. Nucl. Chem., 118 (1987)

415. 9. G. JIA, C. TESTA, D. DESlD•RL M. A, MELI, J. Radioanal. Nucl. Chem.,

133 (2) (1989) 227. 10. G. JIA, C. T~STA" D. DESmEPa, M. A. MELI, AnaL Chim. Acta, 220

(1989) 103. 11. S. A. IBRAmM, S. B. WEBB, A. KATIEL, J. RadioanaL Nucl. Chem.,

194 (I) (1995) 213. 12. M. ZHAO, Y. LI, Nuclear Protection, 2 (1980) 1. 13. JIA GUOGANO et al., in press.

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