determination of plutonium in soil

Journal of Radioanalytical and Nuelear Chemistry, Articles, Vol. 149, No. 2 [1991) 275-280

D E T E R M I N A T I O N OF P L U T O N I U M IN S O I L

z. HOLGYE

Institute of Hygiene and Epidem!ology, Center of Radiation Hygiene, 1 O0 42 Prague (Czechoslovakia)

(Received July 3, 1990)

A procedure for the determination of plutonium isotopes in soil is described. The method involves conversion of plutonium to acid-soluble form, dissolution, purification, electrodeposition and alpha-spectrometric determination. Final recovery is in the range from 27 to 51%. In this work increased attention was paid to the problem of equilibration of the chemical yield monitor with plutonium present in the soil sample.

Introduction

The radiochemical methods generally used to determine the concentration of plutonium isotopes in soil and sediment, as well as in other environmental and

biological materials, consist of several stages, such as plutonium extraction from the matrix, separation of plutonium from inorganic salts and from other alpha emitters, preparation of the counting source and alpha-spectrometric measurement of plutonium isotopes. Each stage poses specific problems and in our opinion, most problems are still connected with extraction of plutonium from the sample.

Two methods of extraction of plutonium from soil or sediment samples were used so far. The first method consists in direct dissolution of plutonium in nitric or hydrochloric acid or in a mixture of these acids. This method is suitable for determination of plutonium originating from atmospheric nuclear tests in soil. ~ ,2 The second method is used for the determination of plutonium oxide virtually insoluble in nitric

and hydrochloric acids in soil samples. In this case plutonium must be converted into acid-soluble form by fuming with hydrofluoric acid or by various fusion procedures, sw4 It can be seen from literature data that solubilization of plutonium oxide is mostly achieved by fuming with hydrofluoric acid rather than by fusion. However, some in- vestigators have reported erroneous results s with its use probably due to incomplete

dissolution of plutonium oxide and incomplete destruction of siliceous matrix, which subsequently causes incomplete isotopic exchange between plutonium present in the sample and that used as chemical yield monitor.

Elsevter ~equoia $. A., Lausanne Akad~mlai Kkwl6, Budapest

Z. HOLGYE: DETERMINATION OF PLUTONIUM IN SOIL

Within the project of monitoring the content of plutonium isotopes in the soil in various localities of Czechoslovakia, a radiochemical method for determination of both acid-soluble and acid-insoluble form of plutonium was tested in our laboratory. Repeated fuming with hydrofluoric and nitric acid was used in order to achieve solubilization of plutonium oxide. A number of experiments have proved that after such treatment complete solubilization of plutonium oxide occurs and during this treatment and leaching procedure complete isotope exchange between plutonium present in the sample and that used as chemical yield monitor occurs.

Experimental

Reagents and apparatus

Ostion AT 0806, 50-100 mesh (diameter of exchange column 1 cm and length 25 cm) was used as a strongly basic anion exchanger. Standard solution of 239pu was supplied by the Institute for Research, Development and Use of Radioisotopes, Prague and standard solution of 2aaPu was supplied by Amersham Radiochemical Centre (UK). All the reagents used were of analytical grade.

Agricultural soil, forest soil and pure sand were used for the experiments. Two former ones were collected up to 10 cm depth from a locality near Prague. Forest soil was taken also from 30 em below the surface. Soil samples were dried at 110 ~ and crushed. The fraction passing the 0.5 mm sieve was collected.

Plutonium oxide was prepared 6 by adding a solution containing 459.6 mBq of 2agPu (in nitrate form) to 5 g of the respective soil (in platinum dish) and heated for 4 hours at 1000 ~ The sample was then transferred to a porcelain crucible containing 45 g of the identical soil.

Activity of 2agPu and 23SPu on the disc after electrodeposition was measured by alpha-spectrometer consisting of a passivated implanted planar silicon detector (active area of 600 nun 2) and an 800 channel analyzer.

Preparation of samples for determination of plutonium oxide in soil

50 g of soil in porcelain crucible were heated in a muffle furnace at 550 ~ overnight. The sample was transferred with dilute nitric acid into a teflon beaker. Yield monitor was added to the sample and the solution was evaporated to dryness. 50 ml of water and 25 ml of cone. HF were added to the residue and the solution was evaporated to dryness under occasional stirring. This process was repeated. 50 ml of cone. HNO3 was added to the r~sidue and the solution was under occasional stirring evaporated to dryness. This process was two times repeated. 75 ml of 9M HNOa and

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5 g of H3 BO3 were added to the residue and the sample was occasionally stirred

for 3 hours. Insoluble residue was centrifuged. Supernatant was transferred to a beaker and leaching procedure was repeated with another 75 ml of 9M HNO3 and 5 g of n 3 B O 3 . The combined supernatant solution was filtered.

Preparation of sample for determination of acid-soluble forms of plutonium in soil

50 g of soil in the porcelain crucible were heated in a muffle furnace at 400 ~ overnight. After cooling, the sample was transferred with dilute nitric acid ( " 100 ml) into a glass beaker. 150 ml of conc. HNO3 and yield monitor were added to the sample. The beaker was covered with a watch glass and the sample was heated on a hot plate for 5 hours under occasional stirring and occasional addition of "" 5 ml of H2 02. After cooling, the insoluble residue was centrifuged and the leaching procedure was repeated for 2 hours (without addition of H202) with 150 rm of 8M HNO3. The supernatant solutions were evaporated to dryness, the residue was" dissolved in 100 ml o f 9M HNO3 and the solution was f'dtered.

Separation, purification and electrodeposition of plutonium

0.5 g of NaNO2 was added to the solution and after 15 min standing, the solution was passed through an anion-exchange column (preconditioned before with 50 ml of 8M HNO3). The column was subsequently washed with 25 ml of 8M HNO3 and then with 200 ml of 9M HC1. Plutonium was eluted with a mixture of 100 ml

1.2M HC1 and 2 ml 30% H2 02. The flow rate was 1.5 ml/min throughout. The solution was evaporated to dryness and the residue was fumed with 5 ml cone. HNO3

containing several drops of H2 02. The residue was dissolved in 50 ml of 1.SM HC1 and the solution was transferred

into a polyethylene centrifuge tube. 1 ml 0.08M La(NO3)3 and 150 mg hydroxylamine hydrochloride were added and after 20 minute standing 2 ml cone. HF was added to the solution. After 20 minute standing the precipitate formed was centrifuged and the precipitation of LaFa was repeated by addition of another 0.5 ml 0.08M La(NO3)3 to the supernatant. The supernatant was then discharged.

LaF3 was dissolved in 3 ml of conc. HC1 and 1 ml 1M AI(NO3)a. 20 ml of water, 50 mg of hydroxylamine hydrochloride and, after 20 minute standing, 2.5 ml of cone. HF was added to the solution. The precipitate was centrifuged after 20 minutes and the precipitation was repeated by addition of another 0.2 ml 0.08M La(NOa)3 to the supernatant. The supernatant was discharged.

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The precipitate was washed with water and then dissolved in 3.5 ml aluminium nitrate solution (prepared by dissolution of 12 g Al(NO3)3 �9 9H20 in 50 ml 1.2M HNO3) under slight heating on a water bath. The solution was transferred into a stoppered test tube (10 ml) together with 0.5 ml water from washing the centrifuga- tion tube. To the solution 0.3 ml of 2M NaNO2 was added and after 15 minutes plutonium was extracted (15 rain) by 4 ml of 0.25M TTA in benzene. The organic phase was washed (2 min) first with 4 ml 1M HNO3 and then with 4 ml 0.5M HNO3. Each washing solution contained one drop of 2M NaNO2. Plutonium was then twice reextracted (3 min) with 4 ml 9M HC1. The aqueous phase was evaporated to dryness. Organic substances in the residue were destroyed by repeated evaporation with cone. HNO3 ('~ 5 ml) containing several drops of H202.

The residue was transferred into a polyethylene tube with 2 ml 5M HCI and 2.8 ml 1M HC1. The solution was neutralized by 10M NH4OH to pH ~ 3. The solution was then transferred into an electrolytic cell and plutonium was electrodeposited for 50 minutes using a current of 1 A. Twenty seconds before the end of electrolysis, the solution was made alkaline with 2 ml cone. NH4 OH. The disc was washed thor- oughly with water (pH ~ 7) and dried.

Results and discussion

The radiochemical method studied for determination of plutonium isotopes in soil was verified by determination of recovery of 239pu and ~ 3 a pu (as chemical yield monitor) from 50 g of soil artificially contamined by 459.6 mBq 239pu in the form of insoluble plutonium oxide and in acid-soluble form. The 23apu activities added to the sample were in the range of 378.5-415.8 mBq. In the determination of recovery of insoluble plutonium oxide 4, and in the determination of recovery of acid- soluble form of plutonium 3 (besides pure sand) determinations were carried out for

each type of soil. Activities of plutonium isotopes were measured (by alpha spectrometer with detection efficiency 22.5%) for 86400 seconds. At the activity measurement the relative standard deviation was less then 1.5% for each sample. The results are presented in Table 1. It can be seen that when soil samples were contaminated with insoluble plutonium oxide, the recovery of 239pu and 238pu from pure sand

and from soil collected 30 em below surface were very close. For agricultural and especially for forest soil collected 0 - 1 0 cm below surface the recovery of 239pu is even higher than the recovery of 2aSPu. This increase is probably due to 2a9,24~

present in the surface soil mainly as the result of atmospheric nuclear tests. Similar results were obtained when soil samples were eontamined with acid-soluble form of plutonium. The results in Table 1 suggest that the types of soil studied show no

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Table 1 Recovery of 239pu in the form of insoluble plutonium oxide (using fuming with HF

and HNO 3 for solubftization of plutonium before dissolution with acid) and that of acid-soluble form of 239 Pu (with the use of direct dissolution of plutonium in acid)

and of the 2 a 8 Pu used as a chemical yield monitor from studied softs

Studied softs

239pu present in soil in form of plutonium oxide

239 Pu present in soil in acid-soluble form

Recovery, % Recovery, %

239pu 2aapu 2agpu 2aapu

Pure sand

Forest soft taken 30 cm below surface

Agricultural soil

32.2 33.8 27.6 26.8 27.7* 27.9 30.4* 28.8

42.2 42.4 41.8 42.8 46.1 45.6 24.3 22.8 46.3* 44.1 26.6 27.6 46.1" 46.5

40.2 38.3 51.0 48.7 48.7 46.4 53.0 51.0 43.2* 41.6 50.8 48.5 44.3* 40.8

Forest soft taken up to 39.5 33.0 27.7 24.6 10 cm from surface 57,6 46.1 66.3 55.7

41.7" 37.1 48.9 42.9 62,2" 51.0

*The soil samples were fumed three times with 25 ml of cone. HF.

difference between recovery of 2aaPu and 2agPu when 2agPu was added to the soil

sample in acid-insoluble form and converted to acid-soluble form by evaporation with

HF and HNOa before extraction, and when 2 a 9pu was added to the soil in acid-soluble

form and extraction was performed by direct dissolution with HNOa. The results

allow to assume that: (1) Evaporation with cone. HF and cone. HNOa, as described

above, results in solubitization of plutonium oxide in all types of soil tested; (2) the

solubilization and leaching procedures result in isotope exchange between plutonium

present i n the sample and plutonium used as chemical yield monitor.

To achieve solubilization of plutonium oxide by hydrofluoric acid treatment

individual authors useT, s various amounts of conc. HF ( 0 . 2 - 1 0 ml/g of soil).

Evidently, these amounts depend on soil composition and mode of solubilization.

As can be seen from Table 1, no differences were found in recovery of 2agPu and

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~3Sl~ for soil samples which were evaporated to dryness with 25 ml of conc. HF two, or three times. As evidenced, for our solubilization process 1 ml of conc. HF per 1 g of soil is sufficient.

It is necessary to remove the fluoride ions prior to the extraction of plutonium from matrices 15y leaching with acids. The most used method for removal of fluoride ions consists in evaporation with perchloric acid. However, hydrofluoric acid obviously cannot be removed completely with perchloric acid, moreover, this acid can convert plutonium to Volatile PuF6 and, besides, an explosion hazard exists, a In the procedure designed by us, fluoride ions are removed from the sample by means of boric acid. The amount of boric acid needed for this purpose was chosen according to SILL s Leaching with nitric acid and boric acids must be carried out at room temperature. We have found that leaching at increased temperatures may cause formation of Colloidal solution.

After separation of plutonium by means of ion exchange on a strongly basic ion exchanger, plutonium was further purified especially from polonium, thorium and iron by coprecipitation with LaF3 and extraction with TTA in benzene. This separation has a very satisfactory course and has been applied for years in our laboratory in determining plutonium in biological materials. 9 The separation resulted in the absence 210po from the alpha spectrum of plutonium isotopes when determining plutonium in 50 g of soil and better separation of plutonium from thorium (< 0.01% of 22STh present in 50 g of soil is transferred to the disc).

References

1. S. A. REYNOLDS, T. G. SCOTT, Radiochem. Radioanal. Letters, 23 (1975) 269. 2. S. R. JOSHI, J. Radioanal. Nucl. Chem., 102 (1986) 187. 3. J. C. VESELSKY, Anal. Chim. Acta, 90 (I977) 1. 4. F. I. PAVLOCKAYA, T. A. GORYATSENKOVA, Z. M. FEDOROVA, V. V. EMELY.~NOV,

B. F. MYASOEDOV, Radiokhimiya, 26 (1984) 460. 5. C. W. SILL, Health phys., 29 (1975) 619. 6. C. W. SILL, F. D. HINDMAN, Anal. Chem., 46 (1974) 113. 7. H. SCHI3TTELKOPF, Report KfK 3036 Kernforschungszentrum, Karlsruhe, 1981. 8. R. F. ANDERSON, A. P. FLEER, Anal. Chem., 54 (1982) 1142. 9. Z. HOLGYE, Appl. Radiat. IsoL, 37 (1986) 1015.

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determination of plutonium in soil

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