Download - Analysis of plutonium in soil samples

Analysis of plutonium in soil samples

M.P. Rubio Monteroa,*, A. MartõÂ n SaÂ ncheza, M.T. Crespo VaÂ zquezb,J.L. GascoÂ n Murillob

aDepartamento de FõÂsica, Universidad de Extremadura, 06071 Badajoz, SpainbUnidad de MetrologõÂa de Radiaciones Ionizantes, CIEMAT, 28040 Madrid, Spain

Received 22 October 1999; accepted 31 December 1999

Abstract

Procedures for analysis of plutonium in soil samples were developed using anion exchange as a puri®cationtechnique. Special attention was paid to removing impurities of 228Th which interferes in 238Pu determination by

alpha spectrometry. Two anion-exchange methods were compared. The determination of plutonium in soil involvesthe conversion of soil samples to acid-soluble form. Two methods for the extraction of plutonium from a naturalreference soil were compared. The ®rst method (a direct digestion in nitric acid) is suitable for the determination ofplutonium in large amounts of sample. The second method involves microwave digestion of soil (5 g) with a mixture

of HNO3, HCl and HF, and is suitable for saving time in routine determinations. Activities calculated with areference soil matrix were in good agreement with the reference value. The microwave digestion method was appliedin a study of di�erent soil samples, and recoveries ranged between 20% and 50%. 7 2000 Elsevier Science Ltd. All

rights reserved.

Keywords: Plutonium; Soils; Radiochemistry; Alpha spectrometry

1. Introduction

Determination of plutonium in environmentalsamples by alpha spectrometry involves tedious radio-

chemical procedures to separate this radionuclide fromthe matrix (Yu-Fu et al., 1991). In the case of soilsamples, several steps must be performed: conversionof the plutonium associated with the matrix into acid-

soluble form, radiochemical separation of the pluto-nium from the components of the sample and otherradionuclides, puri®cation of the plutonium isotopes,

and preparation of the source for measurement. The

aim of this work was to develop a procedure for rou-

tine determination of typical fallout levels of pluto-nium in soil samples using anion exchange puri®cation.Several factors must be taken into account, such as the

low activity concentrations of plutonium in thesamples and the high degree of puri®cation required toavoid possible interferences in the spectra from otheractinides such as thorium. Anion exchange procedures

have been extensively used for this purpose and havebeen reviewed in several works (Talvitie, 1971; Wong,1971; Thein et al., 1980; Yamato, 1982; Jiang et al.,

1986; HoÈ lgye, 1994).The methodological work presented in this paper

has two principal parts: extraction of plutonium from

the bulk material, and its puri®cation using anionexchange methods. For the extraction, dissolution of

Applied Radiation and Isotopes 53 (2000) 259±264

0969-8043/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S0969-8043(00 )00141-X

www.elsevier.com/locate/apradiso

* Corresponding author. Fax: +34-924-275-428.

E-mail address: [email protected] (M.P. Rubio Montero).

the leachable plutonium associated with the matrixwas carried out by strong acid leaching or by acid

digestion in microwave oven. The ®rst method (acidleaching) is extensively used in the analysis of largeamounts of sample (Krey and Bogen, 1987; HoÈ lgye,

1991), but the microwave acid digestion allows a majorsaving in time and is cleaner (Alexander and Shimield,1990; Smith and Yaeger, 1996). Also, two methods of

puri®cation through anion exchange were used.Establishment of the accuracy and reliability of the

results obtained is a delicate task, especially for soil

samples in which traceability is more di�cult toensure. Thus, there is still some controversy about thefull recovery of the plutonium associated with a soilsample when strong acid leaching is performed, if very

refractory compounds are present (Krey and Bogen,1987; Sill and Sill, 1995). The methods described belowhave been tested by participation in an interlaboratory

exercise promoted by the `Consejo de SeguridadNuclear'', the Spanish Nuclear and Radioactive Regu-latory Organization (C.S.N., 1998). The reference ma-

terial analyzed was a standard soil matrix provided byIAEA (International Atomic Energy Agency) withlevels of activity similar to those found in environmen-

tal samples. The procedure involving microwave diges-tion was applied to the analysis of environmentalsamples from the south-eastern Spanish zone of Palo-mares, accidentally contaminated with plutonium as a

result of an aircraft accident.

2. Experimental

Radiotracer 242Pu provided by NIST (Ref. SRM-4334F) was used to determine the chemical recovery.Environmental samples of soil were dried in a mu�e

furnace (1108C), sieved and homogeneized before thechemical treatment.

2.1. Hot-plate acid leaching

This is one of the two methods used in this work toextract the plutonium from the original matrix.

Samples of about 50 g of soils were conditioned with200 ml of 8 M HNO3. The

242Pu spike was added andthe mixture was kept for one day in a beaker coveredwith a watch glass. This mixture was heated on a hot-

plate for several hours, with sporadic addition ofH2O2. Before the acid was consumed, the residue wascentrifuged o� and the supernatant was stored; fresh

acid was added to complete the washing of the matrix.The process was repeated until the leachate was clear,and all the resulting liquids were combined. The total

volume of acid employed depended on the sampleweight: for 50 g of soil, the total quantity necessarywas about 1 l of 8 M HNO3. The volume of the result-

ing leachate was reduced by evaporation to approxi-mately 200 ml, or until the appearance of salts. After

dilution with deionized water, actinides were coprecipi-tated at pH 9 as iron hydroxides. Iron was eliminatedwith diisopropylether (DIE) in a conventional decanta-

tion funnel, and the sample was dried. The puri®cationprocedure used will be described in Section 2.3. Ascheme of the procedure is shown in Fig. 1.

2.2. Microwave acid digestion

The other method used to extract the plutoniumfrom the soil matrix was the microwave acid digestionof the sample. In this work, a Millestone Ethos-900Microwave labstation for digestion with 900 W deliv-

ered microwave power was used. This device is pro-vided with a rotor with capacity for ten mediumpressure PFA type vessels of 100 ml each. Dried

samples of about 0.5 g of soils were placed into eachvessel jointly with an acid mixture of 65% HNO3 (2ml), 35% HCl (5 ml), and 48% HF (3 ml). The vessels

were covered and installed in the rotor. The additionof HCl is essential for a complete digestion of the soilsamples analyzed. When the digestion was ®nished and

vessels had cooled, 2 ml of 70% HClO4 were added toeach vessel. All the aliquots were passed to a glass bea-ker. The PFA vessels were washed with concentratednitric acid and deionized water, and the washings were

added to the beaker containing the sample. At thisstage, the 242Pu radiotracer was added because sampledigestion process was completed assuming no losses in

the process. Finally, the sample was evaporated to dry-ness over a hot-plate. The residue was treated with60% HNO3 (50 ml) and evaporated again. The sample

was dissolved in 50±75 ml 9 M HNO3, and about 3 gof H3BO3 were added. After several hours of stirringthe mixture with a magnetic stirrer, the excess H3BO3

Fig. 1. Procedure for soil sample analysis involving acid leach-

ing.

M.P. Rubio Montero et al. / Applied Radiation and Isotopes 53 (2000) 259±264260

was eliminated by ®ltration. The sample was evapor-ated and the process of addition of 9 M HNO3 and

H3BO3 was repeated until the sample appeared clear.Finally, the sample was evaporated until dryness andthe Pu puri®cation was carried out using the anion

exchange method described in Section 2.3. A schemeof the procedure is shown in Fig. 2, together with themicrowave digestion protocol followed in the prep-

aration of the samples. The ®rst and second stages(bold in Fig. 2) were included to destroy any residualorganic matter from the non calcined soil, and the pro-

longed time consume for the calcination of the sampleswas saved.

2.3. Anion exchange

Dried residues from either of the previous treatmentsfor the soil samples were dissolved in 1 N HCl; a few

milligrams of NH2OH�HCl (hydroxylamine hydrochlo-ride) were added. The mixture was evaporated in acovered beaker by heating on a hot plate (Yamato,

1982). The new residue was dissolved in 8 M HNO3,and a few milligrams of NaNO2 were added to the sol-ution. The sample was passed through an anion

exchange column containing the resin Dowex 1 � 8,where Pu(IV) was retained. The adsorbed plutonium

was puri®ed from interference elements by washing thecolumn with 150±200 ml 8 M HNO3, and 200±250 ml

10 N HCl. The washing with HCl is necessary toremove thorium isotope impurities that would interferein the determination of plutonium isotopes, principally

with 238Pu: Then, plutonium was eluted with 150±200ml of a mixture of 0.1 N HI and 9 N HCl. Themethod is shown schematically in Fig. 3.

2.4. Alternative anion exchange procedure

Other methods have been described for the prep-aration of plutonium samples, such as those proposedby Jiang et al. (1986) or HoÈ lgye (1994). In the presentcase, a di�erent method to ®x the oxidation state of

plutonium was used. The dry residue from the ®rststep in extracting the plutonium from the sample wasdissolved in 10 ml of 1 M HNO3, and quantity of

NaNO2 was added to the solution. When the salt haddissolved, the sample was evaporated, conditioned in 9N HCl and added to the anion exchange column con-

taining the Dowex resin 1 � 4, where Pu(IV) wasretained. The adsorbed plutonium was puri®ed frominterferent elements by washing the column with about

100 ml 10 N HCl and 150 ml 8 M HNO3; the pluto-nium fraction was eluted with 100 ml of 9 N HCl and2 ml of H2O2. This method has the advantage thatthorium is not adsorbed in the resin, so that interfer-

ence with the plutonium fraction is less probable (thisis the reason why the resin was washed with a lowervolume of acid than in the procedure described in Sec-

tion 2.3).

2.5. Electrodeposition of actinides and counting details

The resulting elutions for each procedure were elec-trodeposited onto a stainless steel disc from a sulphuric

Fig. 3. Anion exchange procedure used to purify the pluto-

nium fraction coming from either of the chemical treatments

resumed in Fig. 1 or Fig. 2.

Fig. 2. Procedure for the analysis of soil samples using micro-

wave acid digestion. In the inner box the microwave protocol

used with 10 PFA vessels in the rotor. Steps indicated in bold

are necessary when the samples have not been previously cal-

cined.

M.P. Rubio Montero et al. / Applied Radiation and Isotopes 53 (2000) 259±264 261

acid electrolyte at pH 2.1±2.4 (Hallstadius, 1984) for

50 min.Alpha-particle spectra were obtained by measuring

the plutonium planchets with passivated ion-implanted

silicon detectors of 450 mm2 active area. Sources weresituated as near to the detector as possible. The count-ing e�ciency was 33% measured with a standard

sample of 164.5 Bq of 241Am in the same geometricalarrangement as that of the unknown samples.

3. Results and discussion

3.1. Hot-plate leaching versus microwave digestion

One of the principal questions to be answered iswhether it is preferable to use acid leaching or micro-

wave digestion for soil samples in the determination ofplutonium. In this section, a discussion of the resultsobtained with the two methods is presented.In the acid leaching method, there was observed to

be an in¯uence of the amount of sample analyzed onthe recovery. This e�ect is minimized when a coprecipi-tation step using Fe+3 in basic media is added to the

procedure after the leaching. Some experiments wereperformed to determine the in¯uence of this coprecipi-tation step on the ®nal recovery, using di�erents

amounts of sample. Firstly, experiments not includingthe coprecipitation step were performed. The resultsshowed that, when the sample size was less than 10 g

of dry soil, the recovery was 45±72%, but for 50 g ofsoil, the recovery fell to values lower than 10%, a

value which is insu�cient in the analysis of plutonium

due to the low activity concentrations in environmentalsamples. However, in the analysis of 50 g of soilincluding the coprecipitation step, the recoveries

attained were greater than 50% for all the cases. Wetherefore concluded that, in the analysis of largeamounts of sample, the coprecipitation step is necess-

ary.In the microwave acid digestion of soils, HF was

used to destroy silicate compounds and eliminate silicathat would interfere in the subsequent separation step.

The drawback is that HF is a strong complexing agent(Sill and Sill, 1995) of tri- and tetravalent ions (includ-ing Pu). Recoveries in experiments using only HClO4

to eliminate the silica for di�erent amounts of sampleanalyzed (0.3, 3, 5 and 10 g), without subsequent ad-dition of H3BO3 were less than 10%. When ¯uorides

were dissolved in nitric acid media and complexed withboric acid, the recoveries obtained were greater than10% in all cases. The amount of boric acid used was 9g, divided into three fractions, for 5 g of soil, and

using 30 ml 48% HF in the attack. Addition of furtheramounts of boric acid did not yield higher values forthe chemical recovery in the same matrix.

3.2. Selectiviness for plutonium in the anion exchangeprocedures

The anion exchange method for the determinationof plutonium in environmental samples must yield high

values for the chemical recovery, and must also beselective for this actinide due to its low occurrencecompared with other actinides such as uranium andthorium in environmental samples. Emissions of these

last nuclides interfere with those of plutonium in thealpha spectrum. For comparison between the methodsdescribed in Sections 2.3 and 2.4, parallel experiments

following two di�erent schemes were carried out using242Pu tracer, and adding some tracer activities of 232U

and 228Th to check the selectiviness of the methods. In

experiment 1, known activities of the mentioned stan-dards were added to 10 ml 1 N HCl, and the steps ofthe anion exchange procedure described in Section 2.3

Table 2

Activity concentrations of 239�240Pu obtained in a reference

soil material obtained with the procedures developed

Procedure Amount analyzed

(g)

[ 239�240Pu�(Bq/kg)

Acid leaching 50.053 1.0420.03

Microwave digestion 5.119 0.9120.06

IAEA reference value 1.0420.10

Table 1

Recoveries obtained in uranium, thorium and plutonium fractions arising from the application of the methods described in the

texta

Experiment Recovery Elution

Fraction of U (%) Fraction of Th (%) Fraction of Pu (%)

1 Pu ± ± 81.6

2 Pu 8.5 15.5 39.5

a `±' mean that no trace of the radionuclide indicated was found in the corresponding fraction.


were followed. In experiment 2, known activities of242Pu, 228Th and 232U tracers were added to 10 ml of 1M HNO3, and the rest of the anion exchange pro-cedure described in Section 2.4 was followed.

All the elutions resulting from each fraction in bothexperiments were electrodeposited (Section 2.5) andalpha emissions of 242Pu, 232U and 228Th were

measured in each spectrum. Recoveries for plutonium(including the electrodeposition step) were about 80%and 40% for experiments 1 and 2, respectively. In ex-

periment 2, not only was the Pu recovery less than inexperiment 1, but also a high percentage of Puappeared in the U and Th fractions (see Table 1), indi-cating a premature elution of this element. This fact

can be explained by an incomplete stabilization of thePu oxidation state when only NaNO2 is used for thispurpose (C.I.E.M.A.T., 1994).

3.3. Applications

The analysis of the IAEA reference material was

made by extracting the plutonium from the soilthrough strong acid leaching or using microwave aciddigestion, for comparison. The results obtained for the

activity concentration of 239�240Pu are given in Table 2(quoted total uncertainties are 1s). These values werein agreement with the reference value given by theIAEA. Although strong acid digestion seems to be

more appropriate for the characterization of a zone,due to the greater amounts of sample used, microwaveacid digestion is faster, cleaner and there are no acid

emissions during the process.In order to apply the method for analyzing pluto-

nium using alpha spectrometry, the microwave diges-

tion and the anion exchange (Section 2.3) procedureswere used for some soil samples from Palomares(AlmerõÂ a). Some examples of the results are listed in

Table 3.

4. Conclusions

Stabilization of the plutonium oxidation state withhydroxylamine hydrochloride and NaNO2 prior to thechemical separation seems to be necessary to avoid the

premature elution of plutonium in the puri®cation pro-cedure, and the subsequent poor recovery in this step.Strong acid leaching of the soil matrix and microwaveacid digestion yielded similar results in the study of a

reference material, showing that both methods can beused for the analysis of plutonium in soil samples. Anexplanation of this could be that contamination due to

plutonium is in the outer part of the grains formingthe initial matrix, so that leaching or microwave diges-tion of the sample give similar results. For natural

alpha-emitting nuclides, slightly di�erent results forleaching or microwave digestion could perhaps beexpected depending on the grain size, composition ofthe bulk matrix, etc. The methods described in this

work are valid for the determination of levels of en-vironmental contamination due to plutonium. How-ever, the microwave acid digestion procedure is faster

and cleaner than acid leaching. Minimum detectableactivities reached with the recoveries obtained with themicrowave acid digestion method permit the determi-

nation of the activity concentration of plutonium insamples of about 5 g.

Acknowledgements

Thanks are due to the Junta de Extremadura (Con-

sejerõÂ a de EducacioÂ n y Juventud, Project no.IPR98C021) and DGICYT (Project no. PB95-1139A)for ®nancial support.

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Soil 2 4.76 1,000,000 3.2920.07 0.0820.02 41.020.5

Soil 4 4.558 270,848 0.4720.06 < 0.16 33.120.4

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