A. Koua

G. Ardisson

G. Barci-Funel

H. Michel

H.K. Aka

J. Alabdullah

V. Barci

Comptes Rendus Chimie

Available online at www.sciencedirect.comC. R. Chimie 12 (2009) 850e853http://france.elsevier.com/direct/CRAS2C/Full paper / MémoireFirst measurements of anthropogenic and natural radionuclidesin surface soils (10 cm) of Côte d’IvoireA. Koua a, H. Michel b,*, J. Alabdullah b, V. Barci b, H.K. Aka c,G. Barci-Funel b, G. Ardisson ba Laboratoire de physique nucléaire et radioprotection, UFR sciences des structures de la matière et de la technologie,Université de Cocody, 22 BP 582 Abidjan 22, Côte d’Ivoireb Laboratoire de radiochimie, sciences analytiques et environnement, Université de Nice Sophia-Antipolis, 06108 Nice cedex, Francec Laboratoire de chimie-physique, UFR sciences des structures de la matière et de la technologie, Université de Cocody,22 BP 582 Abidjan 22, Côte d’IvoireReceived 9 May 2008; accepted after revision 19 November 2008Available online 23 May 2009Abstract239e240Pu and 137Cs activities in soil samples were measured in two sites of the littoral part of Côte d’Ivoire: Abidjan and theclassified forest of ‘‘la Comoé’’. Two areas were chosen: a forest and a clearing. The aim of this work is to establish a starting point fornuclear contamination data of a non-nuclear country. Measured soil cores were of 10 cm depth. 137Cs and 239e240Pu activities, mainlydue to weapon nuclear tests in the 1960s, were in the range 0.37e2.3 and 0.023e0.125 Bq kg1 respectively. 239e240Pu/137Cs valueswere found to be about 0.024e0.19 in agreement with the expected value for nuclear tests. To cite this article: A. Koua et al., C. R.Chimie 12 (2009). 2009 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.Keywords: 239e240Pu; 137Cs; Côte d’Ivoire; Soil1. IntroductionThe study of radionuclide spreading into the envi-ronment is of great interest. The long and medium half-life radionuclides were deposited in the world owing tothe intensive testing of nuclear weapons in the atmo-sphere in the 1960s, adding their radioactivity to that ofnatural series of uranium, thorium and to the naturalradionuclide 40K [1,2]. Moreover, the major accidentof Chernobyl in 1986 spread many fission products,* Corresponding author.E-mail address: hmichel@unice.fr (H. Michel).1631-0748/$ - see front matter  2009 Académie des sciences. Publisheddoi:10.1016/j.crci.2008.11.012mainly in the northern hemisphere. Very fewmeasurements of fission products and transuraniumelement activity have been made in West Africa. Arecent accident (2006 Decembere2007 January) con-cerning the discharge of toxic wastes in the Abidjanlaguna, for which we have few elements to judge thepresence of radioactivity, but which points out the needfor radioecological survey in the country, reinforces theinterest to carry out the first measurements of anthro-pogenic radioactivity in soil samples.by Elsevier Masson SAS. All rights reserved.Table 1Sample collection information.Site Area Depth (cm)1 Abidjan city A Under the tree canopy 0e33e66e10B Clearing area 0e33e66e102 The classified forestof ‘‘la Comoé’’A Under the tree canopy 0e44e7.27.2e10B Clearing area 0e3.63.6e6.86.8e10Adding tracers andcarrier to 10g of ashLeaching in acidic and oxydant mediaγ counting of 137Cs851A. Koua et al. / C. R. Chimie 12 (2009) 850e8532. Experimental2.1. Sample collectionThe samples were soil cores collected in the Abid-jan city (site 1) and the classified forest of ‘‘la Comoé’’(site 2), which is a protected primary forest located atabout 100 km of Abidjan (the economic capital)between the Grand Bassam and Alépé departments.The sites are situated in the south-east of Côte d’Ivoire,the big rainfall region, at about 5 N latitude, 4 Wlongitude (Fig. 1). The sampling was carried out in twodifferent areas: under tree canopy (sample A), and ina clearing site (sample B). Core samples were extrac-ted with an iron cylinder of 7 cm diameter and 10 cmlength. Each core was sliced into three sections(Table 1).Soils are coming from parent rocks, which areessentially constituted of granites, migmatic andschist. The profile of these tropical soils is constitutedby 2 horizons: the first horizon is rich in litter, due tothe biogeochemical cycles and the second horizon iscalled the sand accumulation horizon. Its thickness isabout 1 m. These soils are considered as ferralliticforest soils [3,4].2.2. Radiochemical separationAll reagents were of ACS grade: 65% HNO3, 37%HCl, 30% NH3. Dilutions were made with deionisedwater.To determine the plutonium activity, a radiochem-ical separation is needed (Fig. 2). The samples wereFig. 1. Location of Abidjan city and of the classified ‘‘la Comoé’’Forest.weighted and dried in a furnace at 600 C during 24 hin order to destroy the organic matter. 242Pu (100 ml,0.07368 0.00066 Bq/g) yield tracer was added to thesamples; afterwards they were leached twice with 8 MFerric coprecipitationPu oxidation stateadjustement to IVAnion exchangecolumnPu electroplatedα countingFig. 2. Synoptic scheme of the radiochemical separation.852 A. Koua et al. / C. R. Chimie 12 (2009) 850e853HNO3 during 4 h each. A few 30% H2O2 drops werealso added each hour. The supernatant (z200 ml) wascentrifuged and evaporated to 25 ml. The solution wasdiluted to 150 ml with deionised water in order to geta higher pH.Transuranium elements were then co-precipitatedwith ferric hydroxide in ammonia medium untilpH 8e9. The precipitate was washed with 1% HNO3,centrifuged twice, dried at 50 C, and finally dissolvedwith 50 ml 8 M HNO3.The sample solution was filtered and percolatedthrough anionic exchange column Dowex 1X8 (100e200mesh; diameter¼ 10 mm, length¼ 100 mm) pre-condi-tioned with 100 ml 8 M HNO3 solution. The column wasthen washed with 100 ml 8 M HNO3 to remove mainlyAm, Cm and with 100 ml 10 M HCl to desorb Th.Finally, Pu was stripped off the resin with 100 ml 0.1 MNH4Ie10 M HCl [5,6]. The eluate was evaporated todryness; a few drops of H2O2 were added to eliminateiodide.Pu isotopes were then electroplated in sulphuricacideammonia medium at pH¼ 2.4 on a stainless steeldisk with a current of 1 A during 1 h [7].2.3. Alpha spectrometryAlpha-spectra were measured with a 450 mm2 activearea boron implanted silicon detector. The energyresolution (FWHM), measured on the 5.47 MeV alphagroup of 241Am, was 20.0 keV. The a-spectrometer wasa dual vacuum chamber EG&G Ortec 576A; the outputpulses were analyzed with a multichannel bufferanalyzer (spectrum master Ortec 919).2.4. Gamma spectrometry measurements137Cs activity measurements were carried outwithout radiochemical separations. Soil samples wereoven dried at 100 C, homogenised and packed in thinplastic containers in a well-defined geometry.The g-spectrometer consisted of a 17% relativeefficiency HPGe detector with an energy resolution(FWHM) of 1.90 keV on the 1.33 MeV photo peak of60Co. The relative photo peak efficiency was deter-mined using standard activity sources of 152Eu, 137Csand 133Ba. The background contribution was reducedby surrounding the detector and source assembly witha 5 cm thick lead castle internally covered by coppersheets of 2 mm thickness. Four counting runs of about100,000 s were carried out for each sample in order toobtain good statistics on the unique photo peak of661.6 keV of 137Cs.3. Results and discussionTable 2 presents the organic matter content of thesamples after calcinations at 600 C. For the forest andthe clearing area in the two sites, the values are respec-tively in the range of 8.25e4.21% and 6.61e3.21%. Thetrees’ impact on the soil composition is shown by anorganic matter content more important in the forest areathan in the clearing area. But globally the studied soilsare poor in organic substance.The activity concentrations (Bq kg1 of dried matter)of 137Cs and of the 239e240Pu, measured in soil (0e10 cm)are reported in Table 2. For comparison, natural uraniumand thorium series and 40K are also presented.The general trend for the anthropogenic radionu-clides 137Cs and 239e240Pu is: (i) specific activityincrease with depth due to the leaching and migration ofelements by the rainfalls; (ii) activity is more importantin area B than A due to the biomass transfer of elementsby the vegetation. The trees have the capacity by theroots to pump elements up to the surface layers. Theobserved trend suggests that soil layers deeper than10 cm most likely contain more plutonium and cesium,part of sixties’ plutonium and cesium having migrated tothe depth.The 2007 isotopic ratio 239e240Pu/137Cs (Table 2)ranges from 0.024 to 0.186. The ratio stays, after thedecay correction, in agreement with the weapon testingfallout ratio [8] for the 1960s.For the two areas the ratio increases with depth.Forest trees are particularly effective in enhancingradiocaesium recycling [9,10]. An explanation for thecurrent relative enrichment of forest surface soils inradiocaesium compared to plutonium may be thecapacity of trees with high fine root biomass inthe lower soil profiles to pump radiocaesium up to thesurface layers through uptake and further litterfall andto promote overall radiocaesium circulation in thesystem.The depositions of 137Cs and 239e240Pu fromChernobyl fallout in 1986 in general Côte d’Ivoire andWest Africa were in very small amounts.For the natural radionuclides, 40K soil activity isvery low and perturbed. It could be due to the highsolubility of this element and to the washing by theabundant rains in this region of the country. It can bestated that the quantity of potassium recycled annu-ally is high in comparison with the stocks in thevegetation and in the soil. The ecosystem of theforests is efficient for potassium retention. The siteaffects greatly the importance of the quantities putinto circulation. The natural uranium and thoriumTable 2Organic matter content, radionuclide activity concentration and Pu/Cs ratio results for studied soil samples.Sample Depth (cm) % Organic matter Activity concentration Bq kg1 with (error) in 2007 2007 ratio Pu/Cs137Cs 239e240Pu 226Ra (238U) 228Ac (232Th) 40K1A 0e3 6.94 0.75 0.20 0.023 0.006 16.7 0.4 15.0 0.3 Nd 0.031 0.0123e6 6.11 Nd 0.030 0.006 19.2 0.5 16.0 0.3 Nd6e10 4.21 0.37 0.20 0.069 0.007 14.7 0.4 18.7 0.4 Nd 0.19 0.111B 0e3 3.21 1.30 0.15 0.031 0.006 16.5 0.5 14.6 0.4 Nd 0.024 0.0063e6 5.30 1.55 0.14 0.047 0.007 12.0 0.4 15.6 0.3 14.4 0.4 0.030 0.0066e10 6.61 2.24 0.13 0.080 0.008 18.9 0.3 12.5 0.3 2.69 0.23 0.036 0.0052A 0e4 8.25 1.2 0.5 0.035 0.007 7.68 0.21 16.5 0.3 78.6 0.6 0.030 0.0144e7.2 7.31 2.0 0.4 0.079 0.010 8.5 0.3 14.5 0.3 46.2 0.4 0.039 0.0107.2e10.1 6.97 1.7 0.3 0.094 0.008 9.08 0.22 14.2 0.5 Nd 0.054 0.0112B 0e3.6 4.44 1.6 0.4 0.062 0.006 7.20 0.24 12.19 0.23 Nd 0.040 0.0113.6e6.8 3.55 2.1 0.4 Nm 8.7 0.5 14.6 0.4 Nd6.8e10.3 4.11 1.5 0.3 0.125 0.006 10.4 0.4 15.3 0.3 Nd 0.086 0.018Nd: not detected (<detection limit); Nm: not measured.853A. Koua et al. / C. R. Chimie 12 (2009) 850e853activities are similar in the soils and in the worldaverage.4. ConclusionThe first measurements of plutonium and cesiumradionuclides in surface soils (10 cm) of Côte d’Ivoire fora forest area and clearing area show very low radioactivecontaminations from global nuclear fallout. The Cher-nobyl nuclear power plant accident in 1986 had no impacton this region. Vegetation and precipitations seem toaffect the radionuclide vertical migration and redistribu-tion to the soil. The maximum of the artificial radioac-tivity (plutonium and cesium from nuclear atmospherictest fallout) should be in layers deeper than 10 cm.References[1] M.B. Gavini, Earth Planet. Sci. Lett. 41 (1978) 228.[2] R.J. Pentreath, Appl. Radiat. Isot. 46 (1995) 1279.[3] P. De Boissezon, Cath ORSTOM, ser. Pédol. 8 (1970).[4] B. Dadin, Cath ORSTOM, ser. Pédol. 21 (1984e1985).[5] H. Michel, G. Barci-Funel, J. Dalmasso, G. Ardisson,J. Radioanal. Nucl. Chem. 240 (1999) 467.[6] H. Michel, J. Gasparro, G. Barci-Funel, J. Dalmasso,G. Ardisson, G. Sharovarov, Talanta 48 (1999) 821.[7] N.A. Talvitie, Anal. Chem. 44 (1972) 280.[8] J. Cizdziel, V. Hadge, S. Faller, Health Phys. 77 (1999)67.[9] F. Goor, Y. Thiry, Sci. Total Environ. 325 (2004) 163.[10] B. Delvaux, N. Kruyts, E. Maes, E. Smolders, in: G.R. Gobran,W.W. Wenzel, E. Lombi (Eds.), Trace Elements in the Rhizo-sphere, CRC Press LLC, UK, 2001, p. 61.

First measurements of anthropogenic and natural radionuclides in surface soils (10 cm) of Côte d'Ivoire

https://comptes-rendus.academie-sciences.fr/chimie/item/10.1016/j.crci.2008.11.012.pdf

First measurements of anthropogenic and natural radionuclides in surface soils (10 cm) of Côte d'Ivoire

Abstract

Similar works

Full text

Available Versions

Comptes Rendus Chimie