Journal ofRaddoanalytical andNuclear Chemistry, Articles, VoL 17~, No. I (1994) 19-26

DEVELOPMENT OF A SO~CALLED HOUR METHOD FOR ANALYSIS

OF PLUTONIUM IN SOIL SAMPLES

J. M. GODOY, M. L. GODOY, Z. L. CARVALHO, D. C.LAURIA

Comiss~o Nacional de Energia Nuclear, Insatuto de Radioprotefao e D o s ~ Caixa Postal 37750-CEP 22793, Barra da Tijuca, Rio de Janeiro, RI (Brazil)

(Received Oclober 1, 1993)

Following the definition from the International Atomic Energy Agency, a so-Called hour

method is that which needs six hours time interval from the sample solution preparation to the data

evaluation. Two alternative procedures to be applied to soil samples were studied, One based on

LaF 3 precipitation and ion-exchange mad another on extraction chromatography with TOPO/Silicagel.

Both methods gave good results ill terms of accuracy, chemical yield and lime requirement, but the

extraction chromatographic procedure is recommended because it involves, fewer steps.

During the first meeting of the Co-ordinated Research Programm (CRP) on Rapid

Instrumental and Separation Methods for Monitoring Radionuclides in Food and Environmental

Samples. which focuses on the development of rapid methods for the determination of

radionuclides in the intermediate and late post-accident phase in contaminated food and

environmental samples, some needs and requirements related with these so-called rapid methods

were identified. ~

In particular, related to the alpha emitters, the need was identified of a so-called "hour"

method to be applied to water and soil samples. An hour method was defined as a method with

a processing time of 6 hours or less and a desired accuracy between + 20% to a factor of 2.

Supposing 2 hours for counting and evaluation, one has 4 hours to perform the work from sample

dissolution to the counting source preparation. Since the counting time of around 100 minutes is

not so large, soil sample amounts of at least 10 grams were recommended. 1

As a consequence of this 4-hour period for the whole chemical procedure, it became clear

that time-consuming steps such as evaporation should be avoided and., when possible, one step

should be used with two simultaneous purposes, e.g. micro-precipitation with LaF 3 for Pu/U

separation and source preparation.

We decided to try two different procedures for the separation of plutonium from the matrix

and from the natural-occurring alpha emitters. One was based on the co-precipitation with LaF 3

and ion-exchange, and the other one was based on the extraction chromatography with Tri-n:octil

phosphine oxide (TOPO). The present work shows the results obtained with both procedures.

Elsevier Sequoia S. A., Lausanne Akadtmiai Kiadt, Budapest

J. M. GODOY ct al.: DEVELOPMENT OF A SO-CAI 1,ED HOUR METHOD

Experimental

Tracers

z~SPu, ~ u , ~ r h , 2z~ and ~3U solutions were obtained by gravimetric dilution from

Amersham standard solutions. ~ solution was prepared as described by Abr~o)

TOPO/Silicagel

The impregnation of silanised silicage170-230 mesh was performed as described by Lauria

and Godoy)

.Columns

Columns used for the ion-exchange separations are shown in Figure 1. Columns used for

the chromatographic separations are shown in Figure 2.

( ) ' \~ . 1 . /

7.5 cm

DI=0.5 [

Figure 1. Glass column for the ion-exchange

separation.

- I cm

5.0 cm ~ TOPO/Silicagel

Figure 2. Glass column for the extraction

chromatography.

.Alpha spectrometer

The alpha spectrometer was ORTEC 576 surface harder detectors coupled to one ORTEC

ADCAM 918 and a personal computer.

20

I. M. GODOY et aL: DEVELOPMENT OF A SO-CALLED HOUR METHOD

Ion-exchan ger

The ion-exchanger was Dowex 1XS, 50-100 mesh, C1 form.

Chemicals

All reagents used were of analytical grade.

Results and Discussion

Ion-exchange method

The sample solution was produced from a 10grn soil sample, leached twice with boiling

7.2 M HNO3 + 0.1 M HF, 15 minutes each? The next step is a LaF 3 precipitation which separates

the bulk matrix and allows for further adjustment of the solution volume and medium in

preparation for the anionic ion-exchange step. This precipitation also separates uranium and radium

from plutonium. The anionic ion-exchange in 7.2 M HNO3 washing with 7.2 M HNO3 and 10 M

HC1, separates plutonium from thorium, polonium and the remaining matrix. 4 Plutonium is eluted

with 0.36 M HC1 + 0.01 M HF. To increase the plutonium decontamination factor from uranium

and also to avoid evaporation, the micro-precipitation with LaF 3 was the choice as source

preparation method, s

Adjusting the I-IF concentration to I M and using two additions of 20 mg La for the

plutonium co-precipitation from the sample solution, and applying the ion-exchange conditions

described by Schuettelkopf, 4 a decontamination factor ~ of only 40 was obtained. As shown

in Figure 3, it was necessary to increase the 10 M HC1 volume from I0 ml to 50 ml in order to

separate Pu from Th. Under these conditions the chemical yield was 95% and the decontamination

factors from uranium, polonium and thorium were >103, >103, 2 x 102, respectively.

Applying the procedure to a soil sample, the chemical yield decreases to 35%. All the steps

were verified and it was found that plutonium remained in the sample solution, probably due to

the presence of AI +3 complexing the fluoride ion. The LaF 3 co-precipitation, starting from a soil

sample solution, was studied and it was found that it was necessary to increase the HF

concentration to 5 M. Under this condition 92% of plutonium was co-precipitated.

The method (Figure 4) was tested applying it to the NBS 4353 Rocky Flats Soil Number I

reference sample which contains 8.0 mBq/g 239+2~pu and 0.4 mBq/g 23Spu, using 23SPu as a tracer.

The 239+24~ results (Table 1) are in agreement with the reference values. The chemical yields

were all around 50%

The time needed for the whole procedure between soil sample leaching and source

preparation is about 6 hours.

21

J. M. GODOY et aL: DEVELOPMENT OF A,SO-CALLED HOUR METHOD

8000 ~ , ,

B

0 0

6000

4000

2000

o i 0 t 0 20 50 40 50 60

Figure 3.

I I

I I I

10 M HCI (ml)

Thorium elution with 10 M HC1.

TABLE 1

Plutonium concentrations in NBS 4353 reference soil sample

Aliquot number

Mean value

2 3 ~ c o n c e n ~ a f i o n s

mB~

8.0

7.6

8.7

8.1 •

Extraction chromatography

The following procedure is based on the work of Testa, ~ who used a TOPO/Microthene

column to separate plutonium from urine samples; We introduced some modifications because

Testa had used a mix of batch and column procedure and, for the Pu elution, the reductant agent

was iodide.

22

J. M. GODOY ct al.: DEVELOPMENT OF A S O ~ E D HOUR METHOD

I 0 GRAMS SOIL SAMPLE + 74 ~Bq

PU TRACER.

TWICE LEACHING WITH 50 ~L

( 7 , 2 MHO 3 + O . i MHF), 15

MINUTES BOILING EACH ONE.

/

HF up ~ [ (SOLUTION) TO 5 M

COPRECIPITATION WITH 20 ~g

La . WASHING WITH iO ML IM HF.

TWO TIMES EACH.

(PRECIPITATE)

DISSOLUTION OF THE LaF~ WITH

iO ML SATURATED BORIC ACID

+ iO ~L HNO z CONC.

l ION-EXCHANGE SEPARATION

Z ~ E R - - - ~ ~ - I O 0 MESH

- FEED SOLUTION = 20 ~L 7 . 2 M HNO~ (AS ABOVE)

- WASHINGS = 50 ~L 7 . 2 M HNO~, 50 ~L i 0 M HC%

- ELUANTE = 20 ~L ( 0 . 3 6 MCL + 0 . 0 1 M HF)

I MICRO-PRECIPITATION WITH LaF 3

ALPHA SPECTROMETRY

Figure 4. LaF 3 - ion exchange separation schema.

Based on the previous experience 3 with Th and U, a 2.5 cm height TOPO/Silicagel column

was used at first. Since fluoride could disturb the ex~action of plutonium with TOPO due to

~,omplexing effects, the l0 gm soil sample lixiviation was changed to a first step with

23

J. M. GODOY ct al.: DEVELOPMENT OF A SO-CALLED HOUR METHOD

50 ml 7.2 M HNOa + 0.1 M I-IF and a second with 50 ml 7.2 M HNO 3 + 1 M AI(NO~)v 4 In order

to obtain the fight HNO 3 concentration, only dilution with water was necessary. Therefore, after

a soil sample lixiviation, one will obtain 200 ml or less of a 3.6 M HNO3 solution. After the

percolation at a flow rate of 2-3 ml/min 1, two washing steps were included, one with

20 ml 3.6 M HNO 3 and another with 10 ml 6 M HCI. The elution is performed with

6 M HCI + 0.2 M ascorbic acid.

Due to a possible interference of 21~ deposition of the plutonium onto a TOP(3 film was

the method of choice for the alpha spectrometry source preparation, since it is submitted to a

calcination after the deposition. 7 From 3 ml 4 M HNO 3, the deposition is completed after 30-

minutes stirring at 2000 rpr~ The first test with z39pu, 23313 and ZZSTh showed a Pu/U

decontamination factor higher than l0 s, but only 20 for Pu/rh.

The column height was increased to 5.0 cm and the experiment was carried out with ~gPu

and ~ tracers. From the eluate, 10 mi fractions were collected and Z39pu and r~Th determinated

by alpha/beta counting. With the first 40 ml, 70% of Pu is eluated but only 0.3% of Th and, with

70 nil, 92% of Pu was recovered and a decontamination factor Pu/Th of 3.7 x 102 was achieved.

The method was tested with two-10 gm aliquots of the same NBS reference soil sample.

The presence of ~2'I'h and ~ peaks and 22STh interfering on the 23spu were observed. Based on

239pu, the chemical yields were 65% and 50%. Using the 238pu area, the 239pu soil content was 4.3

and 4.5 mBq/g, much lower than the expected value, due to the 23Spu peak interference. After a

period to allow the 2URa ingrowth, the 22STh contribution to the 238Pu peak could be corrected.

Another tentative was done, reducing the eluam volume to 40 ml. The obtained 239pu

activity was in agreement with the reference value, but the chemical yield was of only 40%.

Since the eluate evaporation takes too much time, the micro-precipitation with LaF 3 from

50 ml 6 M HC1 + 0.2 M ascorbic acid was tested and the yield was 85%. To apply it to the

separation scheme above, it was necessary to verify the decontamination factor 239+u~176 and

to include a wash of the r with CC14, in order to remove traces of TOPO in the aqueous phase.

The obtained value of 5 x 102 allows the application of the LaF 3 micro-precipitation, and the

consequent time-shortening. The method (Figure 5) was applied twice to the NBS reference soil

sample; the results were once more in agreement with the reference value, (8.0 + 0.5) nd3q/g and

(8.4"• 0.6) mBq/g. The thorium peaks as well as the ~ R a peak were no longer observed. The

chemical yields were around 60%, close to the expected value of multiplying the recovery from

the eluate and the yield from the LaF 3 micro-precipitation. The results in terms of chemical yield

and decontamination factors are well in agreement with the values reported by Testa. 6

24

J. M. GODOY ctal.: DEVELOPMENT OF A SO-CAIJ.~D HOUR METHOD

i O GRAMS SOIL SAMPLE + 74 MRq

PU TRACER. L E A C H I N G WITH 50

~L ( ? . 2 M HN a + 0 . r M HF) AND

50 ~L ( 7 . 2 H HOI+ r A L ( N O a ) 3 ) .

r M INUTES B O I L I N G EACH ONE.

(SOLUTION)

D IL U T IO N i l l HITH H20 .

PERCOLATION INTO A 5 CH

HEIGHT T O P O / S I L I C A GEL

COLUMN.

FLOW R A T E = 2 - 3 ~ L . M I N - 1 .

I W A S H I N G W I T H 2 0 ~ L 3 . 6 M HNO 3

AND i O ~ L 6 M HCL. E L U T I O N

I,IITH 50 ~L (6 M HCL + 0 . 2 M

ASCORBIC A C I D ) .

WASHING OF THE ELUATE WITH

CCL 4 . M I C R O - P R E C I P I T A T I O N

H I T H L a F B ,

ALPHA SPECTROMETRY

Figure 5. Extraction chromatography separation schema.

With a mean flow rate of 2.5 ml/min ~, a 4-hour period between soil sample leaching and

the source preparation can bc achieved.

25

J. M. GODOY ct aL: DEVELOPMENT OF A SO-CAIJ~D HOUR METHOD

Condu~ons

Both studied procedures can fit the so-called hour method requirements for soil samples,

but that based on extraction chromatographic is easier to be applied to a larger number of samples

simultaneously and, therefore, is the recommended one.

References

Ime~'national Atomic Energy Agancy (IAEA) - Report of the First Research Co-ordination Meeting on Co-ordinated

rese~ch Programme: Rapid Instrumental and Separation Methods for Monitoring Radionuclides in Food and

Environmental Samples, IAEA/AL/043, 1989.

A. ABRAO - IEA Report 217, Comiss~ Nacional de Energia Nuclear, Instituto de Pesquisas Nucleates, Sao Paulo,

Brasil, 1980.

D.C. LAURIA, M.J. GODOY, The Science of the Total Environment, 70 (1980) 83-90.

H. SCHUETTELKOPF - KfK report 3035, Kernforschungszenmmt Karlsruhe GmbH, Posffach 3640, 75 Karlsrube

1, Federal Republic of Germany, September 1981.

J.M. GODOY, H. SCHI.TE'ITELKOPF - KfK report 3510, March 1983.

C. TESTA, Exu'action Chromatography cd. by T. Braun and G. Gbersini, Elsevier Scientific Publishing Company,

1975, p. 566.

I.A. SACHETT, A.W. N O B ~ A , D.C. LAURIA, Health Phys., 46 (1980) 133-139.

26