determination of thorium, uranium and plutonium isotopes in atmospheric samples

Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 100, No. l (1986)65-72

DETERMINATION OF THORIUM, URANIUM AND PLUTONIUM ISOTOPES IN ATMOSPHERIC SAMPLES

F. S. JIANG, S. C. LEE, S. N. BAKHTIAR, P. K. KURODA

Department o f Chemistry, University of A rkansas, Fayetteville, AR 72 701 (USA)

(Received August 30, 1985)

A new procedure for the radiochemical measurements of thorium, uranium and plutonium in atmospheric samples is described. Analysis involves copreeipitation of these actirtides with iron hydroxide from a 40-to 50-din ~ sample of rainwater, followed by radioehernical separation and purification procedures by the use of ion exchange chromatography (Dowex AGlx8) and solvent extraction. The new procedure enables one to determine the isotopes of thorium, uranium and plutonium, which are found in rainwater at extremely low concentrations, with a chemical yield ranging from 60 to 80%.

Introduction

Several investigators ~ - 7 have reported on the method of analysis for thorium, uranium and plutonium isotopes in environmental samples. ANDERSON and FLEER 7 have recently carded out a detailed study on the determination of 227Ac, 228Th, 23~ 2a2Th, 234Th, 2a 1Pa, 234U and 2aSU in samples of

suspended marine particulate material and sediments. Their method involves

total dissolution of the samples, to allow equilibration of the natural isotopes,

with added isotope yield monitors, followed by coprecipitation of hydrolyzable metals at pH 7 with natural Fe and A1 acting as carriers to separate them from alkali and alkaline earth metals. Final purification is by ion exchange chromatography (Dowex Agl X 8) and solvent extraction for Pa. This method is not directly applicable, however, to the analysis of environmental samples such as rainwater, for the following reasons: (a) the concentrations of Fe and Al in rainwater are highly variable and usually not high enough to assure a complete

recovery of the actinides from a large volume of the sample solution; and (b) the concentrations of radon daughters such as 210po in rainwater are sometimes as

much as 100 to 1000 times those of the actinides, in terms of their activities and hence a serious contamination problem is encountered unless it is removed

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Articles 100/1

Elsevier Sequoia S. A., Lausanne Akad~miai Kiad6, Budapest

F. S. JIANG et al.: DETERMINATION OF THORIUM, URANIUM

radiochemically prior to the time the samples are counted. A modified procedure, which is presented here, enables one to carry out a simultaneous determination

of the concentrations of the isotopes of thorium, uranium and plutonium in

rainwater with a chemical yield of about 60 to 80%.

Experimental

Apparatus and material

The radioactivity measurements were carried out by the use of a silicon surface barrier detector (Ortec BR-21-300-100) with a resolution of 21 keV FWHM at the 5.486 MeV 24~Am peak. The detector was housed in a dual (Ortec-576) alpha spectrometer connected to a (Ortec-476) multiplexer/router and a (Ino-Tech 5300) multichannel analyzer.

Two kinds of ion exchange columns were used. The first column was made of

glass, which was an integral unit consisting of a 250 nun by 13 mm i.d. tube, a

stopcock with a Teflon plug, and a 150-milliliter reservoir. A plug of glass wool supported the resin bed. The second column, the i.d. of which was 10 nun, was made of polyethylene. The resin used was Agl X 8, chloride form, 100-200 mesh,

an analytical grade of Dowex 1 X 8 obtained from Bio-Rad Laboratories.

Reagent-grade acids and distilled or deionized water were used throughout

this work. Uranium-232 and 229Th tracers made by ORNL were calibrated by standard

solutions prepared from weighed amounts of 2 a s U and ~ a 2 Th. Plutonium-242 tracer was calibrated by the standard solution of z a 9pu" When 2 z S Th need

to be measured, the z a : U tracer must be purified, in order to remove 22 s Th, which is the daughter of 2 a 2 U. Ten days after the purification of 2 a 2 U solution, the radioactivity of 22 s Th in the 2 a 2 U tracer solution will become about one percent of that of the parent 2 a 2 U. The 2 a ~ Pa tracer used in this study was

prepared by the diisobutyl ketone extraction method, from a sample of natural uranium ore, according to the method described by KLUGE. s

Procedures

Fig. 1 shows a flow diagram for the separation and purification of Th, U,

and Pu. Sample preparation. A 40-to 50-liter sample of rainwater was used for the

analysis. The coprecipitation procedure used was similar to that described by BISHOP et al.a The supematant was decanted carefully and about 500 cm a of

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(I) I 8M HNO3

Evaporation

Aqueous I 4M HCI solution discard ~ v 0 C q l - -

Back- .2M Hz 204 extraction Evaporation

I Conc. HCI J Anion exchange J

I [4M HC, I0.5M HCI

Discard U fraction

Rain sample ] Tracer I 23ZU, 242pu,229Th pH 9-10

Supernatant J Coprecipitation I "q discard

Conc. HNO3 Precipitation Conc. HCIO4 HF

Evaporation J

8M HNO3 I Boric acid, HzOz, NaNOz

Anion J

(11) I Conc. HCI (111) [Conc. HCI,NH4I

Evaporation ~ Evaporation 1

I 8MHNO, J 8M HNO3 J Anion Anion I

exchange ] exchange I

' I Discard [Th fraction I

8M HNO 3 ConcHCI 1 I ' Discard Discard HCI'NH4I l

[ Pu fraction I

Fig. 1. Flow diagram for the separation and purification of Th, U and Pu

the solution was retained. The solution containing the precipitate was evaporated to near dryness. A 25-cm 3 portion of 16M HNO3 was used to dissolve the precipitate. The solution and some insoluble residue were then transferred to a Teflon beaker. A 10-cm 3 portion of concentrated HF and l0 cm 3 of concentrated HC104 were then added to the beaker. The covered beaker was heated and the solution was digested gently at the boiling point for about 4 hours. The cover was then removed and the solution was evaporated until the appearance of perchlodc acid fumes. This solution was then transferred to a glass beaker and was further evaporated until perchloric acid fumes no longer appeared. ANDERSON and FLEER 7 have pointed out that simply heating the sample does not completely

s* 67


remove HF and cannot release Pa and Pu from their fluoride complexes. We

found that the fluoride can interfere with the adsorption of Th on anion

exchange resin. We have therefore decided to treat the solution with boric acid

or aluminum nitrate. The cake was dissolved in 30 cm 3 of concentrated HCI and

0.5 g of boric acid was added to the solution. When the solution was evaporated to dryness, a 60 cm 3 portion of 8M HNOs was added to dissolve the cake. A

3 cm 3 portion of 30% H2 02 was added to the solution and it was allowed to

stand for 10 minutes. The solution was then heated until bubbles of H2Oz were

no longer generated. After the solution became cold, 0.3 g of sodium nitrite and 0.5 g of boric acid were added and after 10 minutes the solution was heated

to boil for 2 minutes. Separation o f U, Th, and Pu. The glass column of the anion exchange resin

was selected to separate these elements. The volume of resin was 5 cm 3 . Before

the adsorption, 50 em 3 of 8M HNO3 was passed through the column. A 60 cm 3

portion of 8M HNO3 was then used as eluent for uranium after adsorption. The

thorium fraction was obtained by adding 60 cm 3 of concentrated HCI. The

plutonium fraction was eluted by 40 cm 3 of a solution containing 12M HCI and

0.09M NH4I and followed by 10 cm 3 concentrated HC1. The flow rate was kept

to 1.5 cm 3 per cm 2 per minute.

Purification of Pu and Th. After 10 cm 3 of concentrated HNO3 was poured

into the Pu fraction, the solution was evaporated to dryness. A few cm 3 of

concentrated HNO3 was then added and again evaporated to dryness. A 3 cm 3

portion of 8M HNO3 was used to dissolve this cake and 0.3 cm 3 of 30% H2 O2

and one tenth g of NaNO2 were added. The solution was then placed on top

of a polyethylene column, which contained 2 cm 3 of resin bed and through

which had been passed 20 cm 3 of 8M HNO3. A 40 cm 3 portion of 8M HNO3

was used to remove the remaining U and Po and a 24 cm 3 portion of

concentrated HC1 was also used to remove Th. A 16 cm 3 portion of a solution

which was 12M in HC1 and 0.09M in NH4I and a 4 cm 3 portion of concentrated HC1 were used for the elution of Pu. The purification of Th was similar to the

procedure for Pu, except that H2 O2 and NaNO2 were not needed. The fraction of 24 cm 3 of concentrated HC1 solution was collected for the determination of Th. The mean flow rate was 1.5 cm 3 per cm 2 per minute in

HNO3, and 0.75 cm 3 in HCI. Purification of U. The elutant of adsorption and the rinse solution of 8M

HNO3 from the first column were evaporated to dryness. The cake was dissolved

in 50 cm 3 of 4M HNO3. This solution was placed in a separatory funnel, which

contained 25 cm 3 of tributyl phosphate (TBP). In advance, this TBP was

equilibrated with 0.2M HNOs and 0.2M (NH4)2 CO3 in succession and it was

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finally equilibrated with deionized water three times. After shaking for three minutes, it was allowed to stand and the aqueous phase was discarded. The organic phase was washed with 25 cm 3 of 4M HNO3 and 25 cm 3 of 2.5M

NI~ OH in succession, each time shaking for one minute. A 25 cm 3 portion of 0.2M H2 C2 O4 was used as back-extraction solution and the step was repeated.

The back-extracted solution was evaporated to dryness in order to destroy H2C204. A 2-or 3-era 3 portion of concentrated HC1 was used to dissolve the cake. The solution was passed through the polyethylene column which contained 2 ml of resin bed, treated with 20 cm 3 of concentrated HC1 in advance. The ion exchange column was then washed with 14 cm 3 of concentrated HCI and a 16-em 3 portion of 4M HC1, to remove the remaining Pa, etc. The U was eluted with 20 ml of 0.5M HCI.

Electrodeposition

The purified fractions of U, Pu, and Th were electrodeposited on steeb planchets, according to the method of TALVITIE. 9

Results and discussion

It is known that U (VI), Pu (IV), Pa (V), Po and Fe (III) are all adsorbed on the anion exchanger-HC1 system, while Th (IV) is not. s ' ' ~ In 8M HN03, however, Th (IV) and Pu (IV) have large distribution coefficients on the anion exchanger, while U, Po, and Pa have smaller distribution coefficients, and Fe (III) cannot be adsorbed. Preliminary experiments showed that 15 or 20 cm 3 of resin bed must be used at a flow rate of one ml per square centimeter per minute for 0.5 g of iron, if the anion exchanger-HCl system is selected. The disadvantages of this system are that a large amount of reagents will be needed, and for the separation of Pu, fluoride 7 or highly concentrated NH41 solution 12 must be introduced. In order to choose a satisfactory procedure, some elution patterns had to be investigated. Experiments with 242pu ' 229Th and 231Pa were

carried out separately, because their alpha peaks overlapped partially. It was possible, however, to carry out the experiments with 234U, 23 s U and 208po

simultaneously. Table 1 shows the elution patterns of U and Po with 8M HNO3 on 2 cm 3

of AG1 • 8 resin at a flow rate of 1.5 cm 3 per cm 2 per minute. These data show that U and Po can be eluted out by 15 column volumes of 8M HNO3. In the first ion exchange separation column approximately 95% of the U can be recovered by the use of 12 column volumes of the adsorption solution and

69


Table 1 Elution patterns of u~anium and polonium with 8M HNOa on 2 cm 3

ofAGl x 8 resin at aflow rate of 1.5 cm 3 percm 2 per minute

Eluate fraction in the unit of column

volume, V c

Elements in the eluate, %

U Po

0 - 2 2 - 3 4 - 6 7 - 9

10-12 13-].5

4.5 13.4 38.4 35.9

8.3 1.8

0 0 7.8

51.1 27.3 12.8

Table 2 Elution patterns during the purification steps of thorium or

plutonium on 2 ml of AGI • 8 resin at a flow rate of 0.75 cm a per cm 2 per minute

Volume of Eluate eluate solution, solution ml


U Po Pa Pu Th

8M HNO~ 40 105 95 - 5 0.7

cone. HC1 24 0.2 0.7 1.0 0.5 97

12M HC1 + 0.09M 20 0 1.6 0.3 93 1.2 NH 4 I; cone. HC1

12 column volumes of the 8M HNO3 elution solution. The elution pattern of

Pa is similar to those of U and Po shown in Table 1, except for the fact that

only 20 percent of the Pa was eluted out by 20 column volumes of 8M HNO3.

Table 2 compares the elution patterns of various isotopes during the

purification steps of Th and Pu on 2 r n l of AG1 • 8 resin at the flow rate of

0.75 cm 3 per cm 2 per minute. These results indicate that Th and Pu can be

purified from each other, and U, Po and Pa by the proposed methods,

satisfactorily and with high chemical yields. On the basis of the data on the

distribution ratio reported by KLUGE, 8 we decided to carry out the separation

of U and Pa with 4M HC1 on the anion exchange resin.

70


Table 3 Elution patterns of protactinium and uranium with 4M HC1 on

2 ml of AG1 • 8 resin at a flow rate of 0.75 cm a per cm 2 per minute

E~uate fraction in the unit of column

volume, V c


Pa U

0 - 5 90 0

6 -10 8.6 0

1 1 - 1 5 0.9 1.5

Table 4 Recoveries and impurities found in the separated U, Th and Pu fractions

Element U fraction, % Th fraction, % Pu fraction, %

U 93 0.2 0.05

Th 0.4 93 0.2

Pu 0.4 0.5 90

Po 0 0.5 0.2

Pa 0.8 0.1 0

Table 5 Duplicate analysis of a sample of rainwater collected at

Fayetteville, Arkansas, on June 11, 1985. The concentrations are expressed in units of fCi/dm 3

Analysis I II

Sample taken (1) 40 40

~3~ 36.6 .+ 1.4 34.1 "," 1.2 2S~Th 25.1 + 1.1 26.6 • 1.0 234'U 46.8 • 4.3 47.2 + 3.0 2ss U 1.0 • 0 .9 1.3 • 0.4 2ssU 37.3 • 3.6 34.8 • 2.4 2S9'240pu 4.3 • 0.4 4.4 • 0.5

71

F. S. J1ANG et al.: DETERMINATION OF THORIUM, URANIUM

Table 6 Chemical yields obtained during the analyses of rainwater

Rain sampte U, % Th, % Pu, %

(1) 69 81 75

(2) 71 68 68

(3) 55 61 60

(4) 78 78 75

Average: 68 72 70

Table 3 shows that Pa can be separated from U by the use of 4M HC1 on 2 ml of AGI • 8 resin at a flow rate o f 0.75 cm a per cm 2 per minute. Table 4

shows the results of analysis of 50 cm 3 of a solution which contained known

amounts of U, Th, Pu, Po, and Pa isotopes as well as Fe (0.5 g), NaNO~ (1 g),

and Ha BOa (I g). The chemical yields o f U, Th and Pu were all 90% or greater and the contaminations from each other were less than 1%.

Table 5 shows the results o f duplicate analyses of a sample o f rainwater

collected at Fayetteville, Arkansas, on June 11, 1985. Table 6 shows examples

of chemical yields obtained during the analyses of four samples of rainwater. These results indicate that the radioanalytical procedures described in this

report are applicable to studies on the distributions and geochemical behaviors of the actinide elements in the atmosphere and the hydrosphere.

This investigation was supported by the National Science Foundation under Grants ATM 82-06718 and 84-07618

References

1. N. A. TALVITIE, Anal. Chem., 43 (1971) 1827. 2. C. W. GILL, K. W, PUPHAL, F. D. HINDMAN, Anal. Chem., 46 (1974) 1725. 3. C. T. BISHOP, A. A. GLOSKY, R. BROWN, C. A. PHILLIPS, NTIS, PB-285,435 (1978) 55. 4. T. G. SCOTT, S. A. REYNOLDS, Radiochem. Radioanal. Lett., 23 (1975) 275. 5. S. KPdSHNASWAMI, M. M, SARIN, Anal. Chin. Acta, 83 (1976) 143. 6. J. THOMSON, Anal. Chim. Acta, 142 (1982) 259. 7. R. F. ANDERSON, .4. P. FLEER, Anal. Chem,, 54 (1982) 1142. 8. E. KLUGE, K. H. LIESER, Radiochim. Acta, 27 (1980) 161. 9. N. A. TALVITIE, Anal. Chem., 44 (1972) 280.

10. J. E. GRINDLER, The Radiochemistry of Uranium, The Office of Technical Service, Department of Commerce, Washington 25, D. C.

11. E.K. HYDE, The Radiochemistry of Thorium, NAS-NS-3004. 12. V. R. CASELLA, C. T. BISHOP, A. A. GLOSBY, M. H. HIATT, N. F. MATHEWS, L. A.

BUNCE, P. B. HAHN, Radiochem. Radioanal. Lett., 55 (1982) 279.

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determination of thorium, uranium and plutonium isotopes in atmospheric samples

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