Analysis of plutonium in soil samples

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  • Analysis of plutonium in soil samples

    M.P. Rubio Monteroa,*, A. Martn Sancheza, M.T. Crespo Vazquezb,J.L. Gascon Murillob

    aDepartamento de Fsica, Universidad de Extremadura, 06071 Badajoz, SpainbUnidad de Metrologa de Radiaciones Ionizantes, CIEMAT, 28040 Madrid, Spain

    Received 22 October 1999; accepted 31 December 1999


    Procedures for analysis of plutonium in soil samples were developed using anion exchange as a purificationtechnique. 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 first 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 dierent soil samples, and recoveries ranged between 20% and 50%. 7 2000 Elsevier Science Ltd. Allrights 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, purification 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 purification.Several factors must be taken into account, such as the

    low activity concentrations of plutonium in thesamples and the high degree of purification 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; Holgye, 1994).The methodological work presented in this paper

    has two principal parts: extraction of plutonium from

    the bulk material, and its purification using anionexchange methods. For the extraction, dissolution of

    Applied Radiation and Isotopes 53 (2000) 259264

    0969-8043/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.PII: S0969-8043(00 )00141-X

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

    E-mail address: (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 first method (acidleaching) is extensively used in the analysis of largeamounts of sample (Krey and Bogen, 1987; Holgye,

    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

    purification 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 dicult 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 mue

    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 purificationprocedure 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 finished 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 5075 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-


    M.P. Rubio Montero et al. / Applied Radiation and Isotopes 53 (2000) 259264260

  • was eliminated by filtration. 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 purification 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 first 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 NH2OHHCl (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 purified from interference elements by washing thecolumn with 150200 ml 8 M HNO3, and 200250 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 150200ml 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 Holgye (1994). In the presentcase, a dierent method to fix the oxidation state of

    plutonium was used. The dry residue from the firststep 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


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