Analysis of plutonium in soil samples

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<ul><li><p>Analysis of plutonium in soil samples</p><p>M.P. Rubio Monteroa,*, A. Martn Sancheza, M.T. Crespo Vazquezb,J.L. Gascon Murillob</p><p>aDepartamento de Fsica, Universidad de Extremadura, 06071 Badajoz, SpainbUnidad de Metrologa de Radiaciones Ionizantes, CIEMAT, 28040 Madrid, Spain</p><p>Received 22 October 1999; accepted 31 December 1999</p><p>Abstract</p><p>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</p><p>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</p><p>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.</p><p>Keywords: Plutonium; Soils; Radiochemistry; Alpha spectrometry</p><p>1. Introduction</p><p>Determination of plutonium in environmentalsamples by alpha spectrometry involves tedious radio-</p><p>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-</p><p>soluble form, radiochemical separation of the pluto-nium from the components of the sample and otherradionuclides, purification of the plutonium isotopes,</p><p>and preparation of the source for measurement. The</p><p>aim of this work was to develop a procedure for rou-</p><p>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</p><p>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</p><p>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.,</p><p>1986; Holgye, 1994).The methodological work presented in this paper</p><p>has two principal parts: extraction of plutonium from</p><p>the bulk material, and its purification using anionexchange methods. For the extraction, dissolution of</p><p>Applied Radiation and Isotopes 53 (2000) 259264</p><p>0969-8043/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.PII: S0969-8043(00 )00141-X</p><p>www.elsevier.com/locate/apradiso</p><p>* Corresponding author. Fax: +34-924-275-428.</p><p>E-mail address: pilar@unex.es (M.P. Rubio Montero).</p></li><li><p>the leachable plutonium associated with the matrixwas carried out by strong acid leaching or by acid</p><p>digestion in microwave oven. The first method (acidleaching) is extensively used in the analysis of largeamounts of sample (Krey and Bogen, 1987; Holgye,</p><p>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</p><p>purification through anion exchange were used.Establishment of the accuracy and reliability of the</p><p>results obtained is a delicate task, especially for soil</p><p>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</p><p>refractory compounds are present (Krey and Bogen,1987; Sill and Sill, 1995). The methods described belowhave been tested by participation in an interlaboratory</p><p>exercise promoted by the Consejo de SeguridadNuclear, the Spanish Nuclear and Radioactive Regu-latory Organization (C.S.N., 1998). The reference ma-</p><p>terial analyzed was a standard soil matrix provided byIAEA (International Atomic Energy Agency) withlevels of activity similar to those found in environmen-</p><p>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</p><p>result of an aircraft accident.</p><p>2. Experimental</p><p>Radiotracer 242Pu provided by NIST (Ref. SRM-4334F) was used to determine the chemical recovery.Environmental samples of soil were dried in a mue</p><p>furnace (1108C), sieved and homogeneized before thechemical treatment.</p><p>2.1. Hot-plate acid leaching</p><p>This is one of the two methods used in this work toextract the plutonium from the original matrix.</p><p>Samples of about 50 g of soils were conditioned with200 ml of 8 M HNO3. The</p><p>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-</p><p>plate for several hours, with sporadic addition ofH2O2. Before the acid was consumed, the residue wascentrifuged o and the supernatant was stored; fresh</p><p>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</p><p>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-</p><p>ing leachate was reduced by evaporation to approxi-mately 200 ml, or until the appearance of salts. After</p><p>dilution with deionized water, actinides were coprecipi-tated at pH 9 as iron hydroxides. Iron was eliminatedwith diisopropylether (DIE) in a conventional decanta-</p><p>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.</p><p>2.2. Microwave acid digestion</p><p>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-</p><p>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</p><p>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</p><p>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</p><p>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</p><p>added to the beaker containing the sample. At thisstage, the 242Pu radiotracer was added because sampledigestion process was completed assuming no losses in</p><p>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</p><p>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</p><p>Fig. 1. Procedure for soil sample analysis involving acid leach-</p><p>ing.</p><p>M.P. Rubio Montero et al. / Applied Radiation and Isotopes 53 (2000) 259264260</p></li><li><p>was eliminated by filtration. The sample was evapor-ated and the process of addition of 9 M HNO3 and</p><p>H3BO3 was repeated until the sample appeared clear.Finally, the sample was evaporated until dryness andthe Pu purification was carried out using the anion</p><p>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-</p><p>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-</p><p>longed time consume for the calcination of the sampleswas saved.</p><p>2.3. Anion exchange</p><p>Dried residues from either of the previous treatmentsfor the soil samples were dissolved in 1 N HCl; a few</p><p>milligrams of NH2OHHCl (hydroxylamine hydrochlo-ride) were added. The mixture was evaporated in acovered beaker by heating on a hot plate (Yamato,</p><p>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</p><p>exchange column containing the resin Dowex 1 8,where Pu(IV) was retained. The adsorbed plutonium</p><p>was purified from interference elements by washing thecolumn with 150200 ml 8 M HNO3, and 200250 ml</p><p>10 N HCl. The washing with HCl is necessary toremove thorium isotope impurities that would interferein the determination of plutonium isotopes, principally</p><p>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.</p><p>2.4. Alternative anion exchange procedure</p><p>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</p><p>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</p><p>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-</p><p>taining the Dowex resin 1 4, where Pu(IV) wasretained. The adsorbed plutonium was purified frominterferent elements by washing the column with about</p><p>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-</p><p>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-</p><p>tion 2.3).</p><p>2.5. Electrodeposition of actinides and counting details</p><p>The resulting elutions for each procedure were elec-trodeposited onto a stainless steel disc from a sulphuric</p><p>Fig. 3. Anion exchange procedure used to purify the pluto-</p><p>nium fraction coming from either of the chemical treatments</p><p>resumed in Fig. 1 or Fig. 2.</p><p>Fig. 2. Procedure for the analysis of soil samples using micro-</p><p>wave acid digestion. In the inner box the microwave protocol</p><p>used with 10 PFA vessels in the rotor. Steps indicated in bold</p><p>are necessary when the samples have not been previously cal-</p><p>cined.</p><p>M.P. Rubio Montero et al. / Applied Radiation and Isotopes 53 (2000) 259264 261</p></li><li><p>acid electrolyte at pH 2.12.4 (Hallstadius, 1984) for</p><p>50 min.Alpha-particle spectra were obtained by measuring</p><p>the plutonium planchets with passivated ion-implanted</p><p>silicon detectors of 450 mm2 active area. Sources weresituated as near to the detector as possible. The count-ing eciency was 33% measured with a standard</p><p>sample of 164.5 Bq of 241Am in the same geometricalarrangement as that of the unknown samples.</p><p>3. Results and discussion</p><p>3.1. Hot-plate leaching versus microwave digestion</p><p>One of the principal questions to be answered iswhether it is preferable to use acid leaching or micro-</p><p>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</p><p>be an influence of the amount of sample analyzed onthe recovery. This eect is minimized when a coprecipi-tation step using Fe+3 in basic media is added to the</p><p>procedure after the leaching. Some experiments wereperformed to determine the influence of this coprecipi-tation step on the final recovery, using dierents</p><p>amounts of sample. Firstly, experiments not includingthe coprecipitation step were performed. The resultsshowed that, when the sample size was less than 10 g</p><p>of dry soil, the recovery was 4572%, but for 50 g ofsoil, the recovery fell to values lower than 10%, a</p><p>value which is insucient in the analysis of plutonium</p><p>due to the low activity concentrations in environmentalsamples. However, in the analysis of 50 g of soilincluding the coprecipitation step, the recoveries</p><p>attained were greater than 50% for all the cases. Wetherefore concluded that, in the analysis of largeamounts of sample, the coprecipitation step is necess-</p><p>ary.In the microwave acid digestion of soils, HF was</p><p>used to destroy silicate compounds and eliminate silicathat would interfere in the subsequent separation step.</p><p>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 HClO4to eliminate the silica for dierent amounts of sampleanalyzed (0.3, 3, 5 and 10 g), without subsequent ad-dition of H3BO3 were less than 10%. When fluorides</p><p>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</p><p>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.</p><p>3.2. Selectiviness for plutonium in the anion exchangeprocedures</p><p>The anion exchange method for the determinationof plutonium in environmental samples must yield high</p><p>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</p><p>last nuclides interfere with those of plutonium in thealpha spectrum. For comparison between the methodsdescribed in Sections 2.3 and 2.4, parallel experiments</p><p>following two dierent schemes were carried out using242Pu tracer, and adding some tracer activities of 232Uand 228Th to check the selectiviness of the methods. In</p><p>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</p><p>Table 2</p><p>Activity concentrations of 239240Pu obtained in a referencesoil material obtained with the procedures developed</p><p>Procedure Amount analyzed</p><p>(g)</p><p>[ 239240Pu(Bq/kg)</p><p>Acid leaching 50.053 1.0420.03Microwave digestion 5.119 0.9120.06IAEA reference value 1.0420.10</p><p>Table 1</p><p>Recoveries obtained in uranium, thorium and plutonium fractions arising from the application of the methods described in the</p><p>texta</p><p>Experiment Recovery Elution</p><p>Fraction of U (%) Fraction of Th (%) Fraction of Pu (%)</p><p>1 Pu 81.6</p><p>2 Pu 8.5 15.5 39.5</p><p>a mean that no trace of the radionuclide indicated was found in the corresponding fraction.</p><p>M.P. Rubio Montero et al. / Applied Radiation and Isotopes 53 (2000) 259264262</p></li><li><p>were followed. In experiment 2, known activities of242Pu, 228Th and 232U tracers...</p></li></ul>

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