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Association of Chernobyl-derived 239 + 240Pu, 241Am, 90Sr and 137Cs with organic matter in the soil solution
J. Environ Radioactivity, Vol. 29 No. 3, pp. 257-269, 1995 Copyright 0 1995 Elswier Science Limited Printed m Ireland. All rights reserved 0265-931X/95 $9.50 + 0 00 0265-931X(95)00023-2
ELSEVIER
Association of Chernobyl-derived 239+240Pu,241Am,?3r and 13’Cs with Organic Matter in the Soil Solution
G. I. Agapkina, F. A. Tikhomirov,
A. I. Shcheglov
Moscow State University, Soil Science Faculty, Leninskie gory, 119899 Moscow, Russian Federation az
W. Kracke & K. Bunzl GSF-Forschungszentrum
fiir Umwelt und Gesundheit, Institut fiir Strahlenschutz, Neuherberg, Germany
85764
(Received 22 November 1994; accepted 14 March 1995)
ABSTRACT To investigate the extent of association of fallout radionuclides with soil organic matter, gel filtration was applied to the soil solution obtained from the three top horizons AOf, AOh and AOh f Al/A2 of a forest soil within the 10 km zone of the nuclear reactor at Chernobyl/Ukraine. In the five fractions isolated (fraction 1: nominal molecular weight Mw < 2000, fraction 2: Mw = 130&1000, fraction 3: Mw = 800, fraction 4: Mw = 400 daltons, fraction 5: inorganic compounds), 239f240P~, “8Pu, 24’Am, 90Sr and 137Cs were determined. For that purpose, an efficient method for the simultaneous determination of the actinides and 90Sr was developed. The data show that plutonium and americium are associated mainly with the high molecular fraction 1 and to a much smaller percentage also with the fraction 2. While the differences between plutonium and americium were rather small in the top two horizons, americium in the third soil layer is present to some extent also in the fractions 3,4, and 5. Strontium-90 from the AOf horizon is associated almost exclusively with fraction 4. In the other two soil layers, however, this radionuclide is present essentially only in fraction 5 (inorganic compounds). Caesium-I37 from the soil solution of 257
258
G. I. Agupkina et al
the AOf horizon is associated essentially only with the fraction 3, but in the deeper layers progressively also with all other fractions. Thus, in the third Iayer, 13’Cs is distributed almost uniformly between all five fractions. Because the mobility and biological availability of these radionuclides will depend on their association with soil organic matter, the present data suggest that the determination of only the total concentration of a radionuclide in the soil solution might not be sufficient to interpret or predict adequately the fate of radionuclides in the soil.
INTRODUCTION Most long-lived radionuclides deposited on the soil by fallout are sorbed rather efficiently by the various soil components (soil organic matter, clay minerals, sesquioxides). As a consequence, the concentrations of these radionuclides in the soil solution are usually quite low. Nevertheless, this quantity is of fundamental interest for a given radionuclide and for a given soil, because it is this fraction which is most readily available for ecological processes, such as plant uptake or vertical migration in the soil. The soil solution, however, contains not only dissolved inorganic ions but also (depending on the soil type, the soil horizon and the pH) various amounts of dissolved organic matter (e.g. fulvic or humic acids). As a result, one has to expect that radionuclides which are known to associate strongly with soil organic matter, as e.g. actinides (Choppin, 1988; Bertha & Choppin, 1984; Kim, 1986; Kim et al., 1989) and divalent metal ions, will be present in the soil solution not only as inorganic ions but also attached more or less firmly to dissolved organic molecules of various sizes. There is little doubt that these associations will have a significant effect on the biological availability (Agapkina et al., 1988; Tikhomirov et al., 1980) and mobility of these radionuclides in the soil. Recently, Livens et al. (1987), and Livens and Singleton (1991) showed that very interesting information on the association of plutonium and americium by soil organic matter can be obtained by using gel filtration. For that purpose they first dissolved all organic matter of a soil sample with 0.5 M NaOH, subsequently precipitated humic acid at pH 1, filtered and dissolved the precipitate again in 0.1 M NaOH for gel filtration. Finally, plutonium and americium (derived from nuclear fuel reprocessing plants) were determined in the separated fractions. Because the dissolution of the soil organic matter with NaOH may produce size fractions which are not actually present in a native soil solution, these experiments yield only general information on the association of actinides with humic and fulvic substances. For transport and biological availability, however,
Chernobyl-derived 239+z40Pu, 24’Am, 90Sr and 13’Cs
259
direct fractionation of dissolved soil organic matter in an in-situ soil solution, followed by quantitative determination of the radionuclides in these fractions, should be more relevant. Such investigations seem, as yet, to be unavailable. The purpose of the present investigation was to determine, in the soil solution of a contaminated soil, the association of plutonium, and americium, radiostrontium and radiocesium with dissolved organic molecules, as a function of their size. Gel filtration was used to separate the various fractions of the organic matter in the soil solution. Because the total activities of actinides, radiostrontium and radiocesium in the soil solution are usually very low, a heavily contaminated soil has to be used to determine these radionuclides with sufficient accuracy in the various fractions from the gel filtration. In order to obtain realistic results however, we did not contaminate the soil artificially in the laboratory, but selected a soil which was contaminated in 1986 by the fallout from the reactor accident at Chernobyl, and which is situated within the 1Okm zone around the reactor. Soil organic matter includes plant residues at various stages of decomposition. To investigate whether the stage of decomposition has an effect on the association of the radionuclides with soil organic matter, a forest soil was selected, consisting of a well defined Of-horizon (partially fermented litter), an Oh-horizon (well humified organic matter). and an underlaying mineral horizon. Gel filtration was applied to the soil solution from all three horizons. For the determination of 239f’40P~, 238Pu, 241Am and 90Sr in the various fractions from the gel filtration, a very efficient sequential method was developed, using supported highly specific extractants for these elements.
MATERIAL
AND METHODS
Site and soil
The soil type was a soddy-podzolic soil under a mixed forest (80% pine, 20% birch, age ~60 years), situated 6km west of the nuclear reactor near Chernobyl (Novoshepelitchi). The following soil horizons were identified and sampled in 1994: AOf (slightly decomposed litter): 1.0-2.0 cm; AOh (well humitied soil organic matter): 2.54.5 cm; AOh + Al/A2: 4-4-55 cm (mixture of the AOh horizon and the underlying mineral horizon). Due to the very high activity of the radionuclides in the AOf and AOh horizons (see Table l), the physico+hemical properties of these soil layers could not be
0.49 Zt 0.07 1.74 k 0.24 170 f 21 380 ZL58 20 Zt 3
608 Lt 2 4800 I/I 44 465 f 27
4330 i 490 2360 l 220 158 & 58
0.078 f 0.007 0.22 k 0.08
0.043 * 0.005
0.012 It 0.001 0.014 f 0.001 0.0019 f 0.0003
13* 1 6.3 f 0.7 I~OJrO~l 1.1 f 0.14
33 Zk2 0.3 f 0.1
52 Zt 4
2370 f 250 20 f 2
2210 f 220
3250 k 460 2840 k 330 250 f 67
0.065 & 0.007 0.14 f 0.04
0.057 f OXI
0.0077 f 0.0009 0.0067 + 0.0007 0.00080 f 0.0002
19It 1 0.2 f 0.02
25It2
1
and
4500 Lt 410 4160f460 8Ok 17
0.046 f 0.005 044 f 0.10
0.042 f 0.004
0.0080 f 0000.7 0.0077 f 0.0007 0.00 10 f 0.0002
32 f 2 0.08 k 0.007
36f
TABLE 1 in the Soil Solution, Percentage Amounts in the Extracted Soil Solution, Three Horizons (AOf, AOh, AOh + Al/A2) of the Forest Soil
0.039 Zt 0.001 0.075 f 0.004
0.31 f 0.001
Percentage amount in soil solution: From AOf ..
From AOh From AOh + A 1/A2 Distribution coefficient (& (ml g-‘): AOf AOh AOh+Al/A2
10 f- 0.03 1.1 f0.01 0.17 + 0.01
5290 f 6 79 f 0.3
AOh (2-54.5 cm) AOh + Al/A2 (4.5-5.5 cm) Activity in soil solution (Bq ml-‘): From AOf From AOh From AOh + Al/A2
6080 & 8
of the Radionuclides in the Soil, Activities Distribution Coeffkients 1yd for the Upper
Activity iu the soil (Bq g- ‘): AOf (l&2.0 cm)
Total Activities
k @ ??Y g a w r
a 5
Chernobyl-derived
23g+240Pu, “‘Am,
“Sr and 13’Cs
261
determined in our laboratories. In the AOh+ Al/A2 layer, the following values were found: organic matter: 5.3%; pH (H*O): 4.4; exchangeable Ca: 2.25, Mg: 0.55, Al: O.g9meq/lOOg; K (as K*O): 7.2mg/lOO g; size fraction < 0.01 mm: 66%; < 0.001 mm: 2.9%; bulk density: AOfi ~0.3, AOh: ~0.6, AOh + Al/A2: l.0gcm3. Isolation of the soil solution
Samples of 2&120 g (depending on soil horizon) of air dry soil from each of the above three layers were sieved to 2 mm and incubated for one week at a water content equal to 60% of the corresponding maximum water capacity. The samples were then centrifuged (4 h, 6000rpm) using a double cup, consisting of an inner cup, which held the soil sample and had a porous bottom covered with a coarse filter paper (pore size of 3.5 pm), and an outer cup for the collection of the soil solution. With respect to the total water content, the percentage of soil solution obtained in this way for the three soil layers was l-O-2.0 cm, 2.5-4.5 cm and 4.5-5.5 cm: 53%, 59% and 70%, respectively. Gel filtration
Acrylex P-2 (Reanal, Hungary), a polyacrylamide pearl polymer (bead size 120-320pm), produced by co-polymerisation of acrylamide and N,N’-methylenebis-acrylamide was used. Its special advantages over traditional gels (such as Sephadex) include high separation efficiency at elevated flow rate and mechanical strength at reasonable pressure, which allows flow rates up to 0.9 ml min-‘. This gel was found suitable for high resolution fractionation of organic molecules with a nominal molecular weight (Mw) of 250-2000 daltons. In this range, a linear selectivity curve was obtained. Standards for the determination of the selectivity curve of the Acrylex P-2 column (1.5 x 60 cm; flow rate O-50.9 ml mini, eluent O-05M Tris (trishydroxymethyl methylaminekHC1 buffer, pH 7-4, sample volume 1 ml) were: vitamin B-12 (1355 daltons), raffinose (532 daltons), and sugar (342 daltons). The exclusion limit, obtained by extrapolating the selectivity curve, was 2000 daltons. The void volume was 27.4ml (determined by using dextrane Blue, Mw = 2000000). For the separation of the soil solutions either 5 ml from the AOf horizon or 3 ml from the AOH or AOh +Al/A2 were applied to the column and eluted with the buffer and flow rate given above. After 20ml had passed through the column, the effluent was collected for analysis in the following five portions: O-20, 2-20, 3-15, 4-20. and 540ml. Ultraviolet absorbance measurements of the eluted
262
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et al.
solution at 254nm revealed that after the fourth portion had passed through the column, no organic material was detectable in the effluent. Radionuclides observed in the fifth fraction were, therefore, considered as ‘inorganic’ components. With the selectivity curve mentioned above, the first four fractions were attributed to the following (nominal) molecular weights (in dalton): 22000, 130&1000, 800, and 400, respectively. For further details see Agapkina and Tikhomirov (199 1) and Agapkina et al. (1994). Determination of 239t240P~, 238P~, 241Am, ?3r and 13’Cs
For the determination of the above actinides and of 90Sr, a sequential method was developed using commercially available, highly specific supported extractants. First, 20ml of an eluted fraction were made up to 35 ml with HNO3 (cont.) for the determination of i3’Cs by direct gamma spectrometry. Subsequently, after addition of the yield tracers and carriers (242Pu, 243Am, 20mg Sr and 20 mg Y) the solution was evaporated to dryness and fumed repeatedly with HN03 (cont.) until a white ash was obtained. After dissolving the ash in 50 ml 0.5 M HCl + 1 ml HI (57%), 100 ~1 of ammonium sulfite are added. After gently heating for 10 min, 5 g ammonium chloride are added and the hydroxides are precipitated by ammonia. In this way yttrium (but not strontium) is precipitated together with plutonium and americium. (Because from now on the isolated 9oY decays, this moment has to be recorded for the subsequent evaluation of the 90Sr via 9oY). After cooling, the suspension is centrifuged, washed with distilled water, and the precipitate dissolved in 20ml 2M HN03. The concentration of the nitric acid is then adjusted again to 2 M. The resulting solution (circa 20 ml) is fed via a peristaltic pump to a column (18 cm long, inner diameter 3.76mm) containing TEVAoSpecTM (particle size 50-100 pm, bed volume 2 ml, available from EICHroM Industries, Darien, IL, USA), at a rate of < 1 mlmin-’ (Bunzl & Kracke, 1994). Subsequently, at the same rate, the column is washed with 15ml nitric acid (2M) and the combined eluates (sample solution and washing) are saved for the determination of 24’Am and “Sr via 9oY (solution A). Next, the column is washed with 10ml hydrochloric acid (8N) to remove traces of iron and thorium. Plutonium is stripped first at a feed rate of 0.5 mlmin-’ with 25 ml hydrochloric acid (0.5 M) and then, at the same rate, with 25 ml of a mixture consisting of HCl (0.5 M) and HI (0.1 M), which contained 0.1 ml of an ammonium sulfite solution (35%). Traces of organic material are destroyed subsequently with HN03, HC104 and KN03. After fuming
Chernobyl-derived 239+240Pu, 241Am, wSr and 137C.~
263
with H2S04, plutonium is electrodeposited on a stainless steel disc at pH 2 in ammonium sulfate solution (Kracke & Bunzl, 1980). For the determination of 241Am and 9oY in solution A, its volume is made up to 50ml with distilled water, and 5 g ammonium chloride are added. Subsequently, while heating, yttrium hydroxide is precipitated by ammonia. The precipitate is washed with 50ml distilled water, centrifuged, dissolved in 100~1 HN03 (cont.), and the concentration of the nitric acid adjusted again to 2 M. The final volume (circa 1 ml) is fed to a column containing TRUoSpecTM (particle size 5&100pm) in the same way as described above. After washing the column with l-6ml 2 M HN03, yttrium is eluted with 20ml 1 M HN03 at a rate of 0.5 ml min-‘. To that fraction 2 ml oxalic acid (5 g 1-l) and ammonia are added, and yttrium is precipitated as oxalate at pH 2. The precipitate is filtered, dried at 12O”C, weighed, and its activity counted in a low-level beta counter. Americium is eluted from this column with 20 ml 4 M HCl and collected in a quartz crucible. Traces of organic material are destroyed with HN03, HC104 and KN03. After fuming with H$O4, americium is electrodeposited on a stainless steel disc at pH 2 in ammonium sulfate solution (Kracke & Bunzl, 1980). The chemical yields were for plutonium: 40-60%, americium: 60-80%, and strontium: 7%90%. The above radionuclides were not only determined in the eluted fractions from the gel filtration as described above, but also in the three extracted soil solutions and in the original three soil layers, from which the solution was obtained. In the latter case, 1 g soil was first completely digested by fuming with HN03, HF and HCL04. Alpha spectrometry
Alpha spectromet_ry of the Pu-isotopes was performed using a 300mm2 silicon surface barrier detector (alpha efficiency l&20%). The resolution was 4&50 keV FWHM at 4-6 MeV. The background counts were < 5 per 10 OOOmin. The detection limit for this counting time was about 0.1 mBq of 238Pu_239f240Pu and 24’Am. Estimation of errors and quality control
The errors (coefficients of variation) of the analytical determinations were generally less than 10%. Quality control over the accuracy of the data was assured by participating in intercomparison runs and by analysing standard reference samples for 239t240Pu and 24’Am. For 24’Am we used the ‘Fresh water lake sediment’, NBS standard Reference Material 4354. In
G. I. Agapkina et al.
264
the case of 239t240Pu, Soil-6 from the International Commission, Vienna, Austria, was used.
Atomic
Energy
RESULTS AND DISCUSSION Total amounts of radionuclides in the soil and the soil solution The total activities of the live radionuclides found in the three horizons of the forest soil as well as the corresponding activities found in the soil solution are given in Table 1. With these data and with the known quantities of soil solution extracted per gram from each horizon, the percentage amounts of activity extracted from the soil by the soil solution can be calculated. These values are also given in Table 1. They reveal that, with respect to the soil, the amounts extractable with the soil solution increase in the AOf horizon in the order Pu M Am < Cs < Sr; in the AOh horizon: Cs M Pu M Am < Sr, and in the AOh + Al/A2 layer: Cs < Pu < Am < Sr. This order results from the amount of soil solution present in each layer and from the sorption properties of the soil for the various radionuclides. Because the extent of this sorption is usually characterised by the distribution factor &, defined as the ratio between the amount of a radionuclide in the solid phase (in Bq per g air dry soil) and the corresponding amount in the soil solution (in Bq per ml), this quantity can be obtained from the data of the first two rows of Table 1, and is given in the fourth row of this table. These values show increased sorption in the AOh horizon in the order Sr < Cs < PUZ Am; in the AOh horizon Sr < Pu < Am z Cs, and in the AOh + Al/A2 layer Sr < Am < Pu < Cs. &-values, such as these, are frequently used to predict or to interpret
F&ion Molec.
1
weight > 2MlO
Fr&
2
1300 - 1ooo
Fraction 4
800
Fig. 1.
400
Fraction S
rnorganic
Chernobyl-derived 23g+240Pu,“‘Am, 90Sr and 13’Cs
265
the mobility and biological availability of radionuclides in the soil. In the following, however, it will become evident that the situation is more complicated, because these radionuclides are attached quite differently to dissolved soil organic matter. Gel filtration
The observed distributions of plutonium, americium, strontium and cesium between the five fractions isolated by gel filtration of the soil solutions from the three soil horizons are illustrated in Fig. 1. The activity 100
0 FWLtiOlll M&c.
weight >2UB
Frdm
2
Frdbm3
1300-loo0
100
Fraction 4
Fraction 5
400
inorganic
800
I
Leaend
I
2
F&l M&c.
weight >2Ml
Fr&ion2 1300-lcm
Fmeiim3
Fmcibn 4
800
4ocl
F&on
S
inorganic
Fig. I-contd. Distribution of 239+2‘?u, 238Pu, 24’Am 90Sr and ‘37Cs between five fractions isolated by gel filtration of the soil solutions from a forest soil. For the four organic fractions the corresponding nominal molecular weight (in daltons) is also given; the soil solutions were isolated from the AOf horizon, 1&2.0cm depth (top); the AOh horizon, 2.545cm (middle), and the AOh + Al/A2 horizon, 4.5-5.5 cm (bottom).
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of a given radionuclide in each fraction is shown there with respect to the corresponding activity found in all five fractions. The corresponding nominal molecular weight of each fraction, obtained as described above, is also given in this figure. Similar to Livens et al. (1987) and Livens and Singleton (1991) who used Sephadex for the gel filtration of humic acid and fulvic acid isolated from the soil by alkali extraction, we also observed sorption losses on the column material. They were only present however, for the soil solution from the organic horizons: plutonium and americium 645% in the AOf layer, and d 30% in the AOh layer. In the case of cesium, losses were only observed in the AOf horizon (~65%). For strontium, losses did not occur. Figure 1 reveals that in the solution of all three soil layers both plutonium isotopes and americium are associated mainly with the high molecular fraction (fraction 1, nominal Mw 2 2000 daltons) and to a much smaller percentage (-20%) also in fraction 2 (Mw = 130&1000 daltons). While the differences between plutonium and americium are rather small in the soil solution of the two top organic layers AOf and AOh, americium in the soil solution of the third layer (4.5-5.5 cm) is present to some extent also in the lower molecular fractions 3 and 4 and even in fraction 5 (inorganic compounds). For plutonium this is not the case. The strong association of the actinides with soil organic matter has, of course, been well known for many years (e.g. Bertha & Choppin, 1984; Choppin, 1988; Coughtrey et al., 1984; Kim, 1986; Kim et al., 1989; Livens et al., 1987; Livens & Singleton, 1991; Ramsay, 1988). As a result, these elements are attached in the soil mainly to undissolved humics and their mobility in the soil is very low (Bunzl et al., 1992, 1994; Coughtrey et al., 1984; Frissel et al., 1981). The present data demonstrate, however, that even the small fraction of plutonium present in the soil solution is also extensively associated with soil organic matter of a nominal molecular weight > 1000 daltons. This is essentially also the case for americium, with the exception of the soil solution from the AOH + A l/A2 horizon. There, americium seems to be present in a sizeable quantity (-25%) in the fraction of inorganic compounds. Strontium-90 from the soil solution of the AOf horizon is associated essentially (86%) with the low molecular fraction 4 (Mw = 400 daltons). The remainder (14%) is present in fraction 5 (inorganic compounds). In the soil solution of the two other soil layers this radionuclide is present essentially (90-95%) only in fraction 5 (inorganic compounds). The very small association of strontium with organic matter in the soil solution is probably also the reason why sorption losses on the column were not observed for this element.
Chernobyl-derived 239+240Pu, 24rAm. 90Sr and 13’Cs
267
Radiocesium from the soil solution of the AOf horizon is associated essentially (900/,) only with fraction 3 (Mw =800 daltons), but in the deeper layers, progressively also with the other fractions. Thus, in the third layer, 13’Cs is distributed almost uniformly between all five fractions.
CONCLUSIONS The different distributions of plutonium and americium, but especially of strontium and cesium between the five fractions obtained by gel filtration of the soil solution from the three soil layers demonstrate clearly that the nature of soil organic matter present in these three horizons must be different. For a forest soil, as used here, this is, of course, not surprising, because the chemical composition and structure of humic material from the Of horizon is necessarily different from that of the AOh horizon. It is interesting, however, to observe that these differences have quite different effects with respect to the association of the various radionuclides with this material. While the sorption of radiocesium is obviously most sensitive to the nature of the soil organic matter, plutonium seems to be least affected. This latter element is obviously in any case associated mainly with the high molecular weight fraction. Strontium seems to be affected differently only by dissolved organic material from the AOf horizon and the AOh horizon, while the elution curves for this radionuclide from the AOH and AOH + Al/A2 were alike. The association of the radionuclides in the soil solution with soil organic matter will undoubtedly have effects on their mobility and biological availability. As mentioned, distribution coefficients Kd are frequently determined to interpret or to predict the behaviour of radionuclides in the soil. However, even if the soil solution used to determine Kd values is separated from the solid phase by, e.g. a 0.45pm membrane filter, it will still contain organic material well above 50000 daltons. Kd values obtained in this way thus yield no information on the association of a radionuclide with soil organic matter in the solution phase. Additionally, because the extent of this association can be quite different for soil solutions from different horizons (see above), similar Kd values observed for a radionuclide in different soil layers do not necessarily imply a similar radioecological behaviour of this element. Thus, the rather similar Kd values found for 137Cs in the present soil in the AOf and AOh + Al/A2 horizon (608 and 465 ml g-i, respectively, see Table 1) do not necessarily indicate the same mobility of cesium in these soil layers, because the corresponding association with soil organic matter is quite different (see Fig. 1).
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ACKNOWLEDGEMENT This work was carried out in the frame of the ‘Agreement for International Collaboration on the Consequences of the Chernobyl Accident’ between the Commission of the European Communities and the states of Belarus, Russia and Ukraine.
REFERENCES Agapkina, G. I. & Tikhomirov, F. A. (1991). Organic compounds of radionuclides in soil solutions and their role in uptake of elements by plants. Ekologiya, 6, 22-28 (in Russian, translated in USA). Agapkina, G. I., Berketova, L. V. & Tikhomirov, F. A. (1988). Availability of calcium for plants in organic compounds of the soil solution. Agrokhimiya, 1, 61-6 (in Russian). Agapkina, G. I., Tikhomirov, F. A. dz Shcheglov, A. I. (1994). Dynamics and chemical forms of radionuclide compounds in the liquid phase of a forest soil at the Chernobyl accident zone. Ekologiya, 1,21-28 (in Russian, translated in USA). Bertha, E. L. & Choppin, G. R. (1984). Interactions of humic and fulvic acids with Eu(II1) and Am(II1). Radiochim. Acta., 35, 143-7. Bunzl, K. & Kracke, W. (1994). Efficient radiochemical separation for the determination of plutonium in environmental samples, using a supported, highly specific extractant. J. Radioanal. Nucl. Chem. Lett., 186,401-13. Bunzl, K., Fbrster, H., Kracke, W. & Schimmack, W. (1994). Residence times of fallout 239+240pu 238pu, 241~~ and 137Csin the upper horizons of an undisturbed grassland’soil. J. Environ. Radioactivity, 22, 1l-27. Bunzl, K., Kracke, W. & Schimmack, W. (1992). Vertical migration of plutonium-239+240, americium-241 and cesium-137 in a forest soil under spruce. Analyst, 117,469-74. Choppin, G. R. (1988). Humics and radionuclide migration. Radiochim. Acta., 44/45,23-g. Coughtrey, P. J., Jackson, D., Jones, C. H., Kane, P. & Thome, M. C. (1984). Radionuclide Dhtribution and Transport in Terrestrial and Aquatic Ecosystems, Volume IV, ed. A. A. Balkema. Plutonium, Rotterdam, chapter 29, pp. l-93. Frissel, M. J., Jakubik, A. T., van der Klugt, N., Pennders, R., Poehtra, P. & Zwemmer, E. (1981). Modeling of the transport and accumulation of strontium, cesium and plutonium. Experimental verification. Report 185-76-1 BIA N; Brussels, Commission of the European Communities. Kim, J. I. (1986). Chemical behaviour of transuranic elements in natural aquatic systems. In Handbook on the Physics and Chemistry of the Actinides, ed. A. J. Freeman & C. Keller. North Holland, Amsterdam, pp. 413-56. Kim, J. I., Buckau, G., Bryant, E. & Klenze, R. (1989). Complexation of Am(II1) with humic acid. Radiochim. Acta., 48, 13-3. Kracke, W. & Bunzl, K. (1980). Determination of 239+240Puand 137Csin livers of cattle. Radiochem. Radioanal. Lett., 42, 77-86.
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Livens, F. R. & Singleton, D. L. (1991). Plutonium and americium in soil organic matter. J. Environ. Radioactivity, 13, 323-39. Livens, F. R., Baxter, M. S. & Allen, S. E. (1987). Association of plutonium with soil organic matter. Soil Sci., 144, 24-8. Ramsay, J. D. F. (1988). The role of colloids in the release of radionuclides from nuclear waste. Radiochim. Acta., 44145, 165-70. Tikhomirov, F. A., Moiseev, I. T. & Rusina, T. V. (1980). Dynamics of biological availability of radioiodine for plants in different soils. Agrokhimiya, 11, 116 20 (in Russian).
ELSEVIER
Association of Chernobyl-derived 239+240Pu,241Am,?3r and 13’Cs with Organic Matter in the Soil Solution
G. I. Agapkina, F. A. Tikhomirov,
A. I. Shcheglov
Moscow State University, Soil Science Faculty, Leninskie gory, 119899 Moscow, Russian Federation az
W. Kracke & K. Bunzl GSF-Forschungszentrum
fiir Umwelt und Gesundheit, Institut fiir Strahlenschutz, Neuherberg, Germany
85764
(Received 22 November 1994; accepted 14 March 1995)
ABSTRACT To investigate the extent of association of fallout radionuclides with soil organic matter, gel filtration was applied to the soil solution obtained from the three top horizons AOf, AOh and AOh f Al/A2 of a forest soil within the 10 km zone of the nuclear reactor at Chernobyl/Ukraine. In the five fractions isolated (fraction 1: nominal molecular weight Mw < 2000, fraction 2: Mw = 130&1000, fraction 3: Mw = 800, fraction 4: Mw = 400 daltons, fraction 5: inorganic compounds), 239f240P~, “8Pu, 24’Am, 90Sr and 137Cs were determined. For that purpose, an efficient method for the simultaneous determination of the actinides and 90Sr was developed. The data show that plutonium and americium are associated mainly with the high molecular fraction 1 and to a much smaller percentage also with the fraction 2. While the differences between plutonium and americium were rather small in the top two horizons, americium in the third soil layer is present to some extent also in the fractions 3,4, and 5. Strontium-90 from the AOf horizon is associated almost exclusively with fraction 4. In the other two soil layers, however, this radionuclide is present essentially only in fraction 5 (inorganic compounds). Caesium-I37 from the soil solution of 257
258
G. I. Agupkina et al
the AOf horizon is associated essentially only with the fraction 3, but in the deeper layers progressively also with all other fractions. Thus, in the third Iayer, 13’Cs is distributed almost uniformly between all five fractions. Because the mobility and biological availability of these radionuclides will depend on their association with soil organic matter, the present data suggest that the determination of only the total concentration of a radionuclide in the soil solution might not be sufficient to interpret or predict adequately the fate of radionuclides in the soil.
INTRODUCTION Most long-lived radionuclides deposited on the soil by fallout are sorbed rather efficiently by the various soil components (soil organic matter, clay minerals, sesquioxides). As a consequence, the concentrations of these radionuclides in the soil solution are usually quite low. Nevertheless, this quantity is of fundamental interest for a given radionuclide and for a given soil, because it is this fraction which is most readily available for ecological processes, such as plant uptake or vertical migration in the soil. The soil solution, however, contains not only dissolved inorganic ions but also (depending on the soil type, the soil horizon and the pH) various amounts of dissolved organic matter (e.g. fulvic or humic acids). As a result, one has to expect that radionuclides which are known to associate strongly with soil organic matter, as e.g. actinides (Choppin, 1988; Bertha & Choppin, 1984; Kim, 1986; Kim et al., 1989) and divalent metal ions, will be present in the soil solution not only as inorganic ions but also attached more or less firmly to dissolved organic molecules of various sizes. There is little doubt that these associations will have a significant effect on the biological availability (Agapkina et al., 1988; Tikhomirov et al., 1980) and mobility of these radionuclides in the soil. Recently, Livens et al. (1987), and Livens and Singleton (1991) showed that very interesting information on the association of plutonium and americium by soil organic matter can be obtained by using gel filtration. For that purpose they first dissolved all organic matter of a soil sample with 0.5 M NaOH, subsequently precipitated humic acid at pH 1, filtered and dissolved the precipitate again in 0.1 M NaOH for gel filtration. Finally, plutonium and americium (derived from nuclear fuel reprocessing plants) were determined in the separated fractions. Because the dissolution of the soil organic matter with NaOH may produce size fractions which are not actually present in a native soil solution, these experiments yield only general information on the association of actinides with humic and fulvic substances. For transport and biological availability, however,
Chernobyl-derived 239+z40Pu, 24’Am, 90Sr and 13’Cs
259
direct fractionation of dissolved soil organic matter in an in-situ soil solution, followed by quantitative determination of the radionuclides in these fractions, should be more relevant. Such investigations seem, as yet, to be unavailable. The purpose of the present investigation was to determine, in the soil solution of a contaminated soil, the association of plutonium, and americium, radiostrontium and radiocesium with dissolved organic molecules, as a function of their size. Gel filtration was used to separate the various fractions of the organic matter in the soil solution. Because the total activities of actinides, radiostrontium and radiocesium in the soil solution are usually very low, a heavily contaminated soil has to be used to determine these radionuclides with sufficient accuracy in the various fractions from the gel filtration. In order to obtain realistic results however, we did not contaminate the soil artificially in the laboratory, but selected a soil which was contaminated in 1986 by the fallout from the reactor accident at Chernobyl, and which is situated within the 1Okm zone around the reactor. Soil organic matter includes plant residues at various stages of decomposition. To investigate whether the stage of decomposition has an effect on the association of the radionuclides with soil organic matter, a forest soil was selected, consisting of a well defined Of-horizon (partially fermented litter), an Oh-horizon (well humified organic matter). and an underlaying mineral horizon. Gel filtration was applied to the soil solution from all three horizons. For the determination of 239f’40P~, 238Pu, 241Am and 90Sr in the various fractions from the gel filtration, a very efficient sequential method was developed, using supported highly specific extractants for these elements.
MATERIAL
AND METHODS
Site and soil
The soil type was a soddy-podzolic soil under a mixed forest (80% pine, 20% birch, age ~60 years), situated 6km west of the nuclear reactor near Chernobyl (Novoshepelitchi). The following soil horizons were identified and sampled in 1994: AOf (slightly decomposed litter): 1.0-2.0 cm; AOh (well humitied soil organic matter): 2.54.5 cm; AOh + Al/A2: 4-4-55 cm (mixture of the AOh horizon and the underlying mineral horizon). Due to the very high activity of the radionuclides in the AOf and AOh horizons (see Table l), the physico+hemical properties of these soil layers could not be
0.49 Zt 0.07 1.74 k 0.24 170 f 21 380 ZL58 20 Zt 3
608 Lt 2 4800 I/I 44 465 f 27
4330 i 490 2360 l 220 158 & 58
0.078 f 0.007 0.22 k 0.08
0.043 * 0.005
0.012 It 0.001 0.014 f 0.001 0.0019 f 0.0003
13* 1 6.3 f 0.7 I~OJrO~l 1.1 f 0.14
33 Zk2 0.3 f 0.1
52 Zt 4
2370 f 250 20 f 2
2210 f 220
3250 k 460 2840 k 330 250 f 67
0.065 & 0.007 0.14 f 0.04
0.057 f OXI
0.0077 f 0.0009 0.0067 + 0.0007 0.00080 f 0.0002
19It 1 0.2 f 0.02
25It2
1
and
4500 Lt 410 4160f460 8Ok 17
0.046 f 0.005 044 f 0.10
0.042 f 0.004
0.0080 f 0000.7 0.0077 f 0.0007 0.00 10 f 0.0002
32 f 2 0.08 k 0.007
36f
TABLE 1 in the Soil Solution, Percentage Amounts in the Extracted Soil Solution, Three Horizons (AOf, AOh, AOh + Al/A2) of the Forest Soil
0.039 Zt 0.001 0.075 f 0.004
0.31 f 0.001
Percentage amount in soil solution: From AOf ..
From AOh From AOh + A 1/A2 Distribution coefficient (& (ml g-‘): AOf AOh AOh+Al/A2
10 f- 0.03 1.1 f0.01 0.17 + 0.01
5290 f 6 79 f 0.3
AOh (2-54.5 cm) AOh + Al/A2 (4.5-5.5 cm) Activity in soil solution (Bq ml-‘): From AOf From AOh From AOh + Al/A2
6080 & 8
of the Radionuclides in the Soil, Activities Distribution Coeffkients 1yd for the Upper
Activity iu the soil (Bq g- ‘): AOf (l&2.0 cm)
Total Activities
k @ ??Y g a w r
a 5
Chernobyl-derived
23g+240Pu, “‘Am,
“Sr and 13’Cs
261
determined in our laboratories. In the AOh+ Al/A2 layer, the following values were found: organic matter: 5.3%; pH (H*O): 4.4; exchangeable Ca: 2.25, Mg: 0.55, Al: O.g9meq/lOOg; K (as K*O): 7.2mg/lOO g; size fraction < 0.01 mm: 66%; < 0.001 mm: 2.9%; bulk density: AOfi ~0.3, AOh: ~0.6, AOh + Al/A2: l.0gcm3. Isolation of the soil solution
Samples of 2&120 g (depending on soil horizon) of air dry soil from each of the above three layers were sieved to 2 mm and incubated for one week at a water content equal to 60% of the corresponding maximum water capacity. The samples were then centrifuged (4 h, 6000rpm) using a double cup, consisting of an inner cup, which held the soil sample and had a porous bottom covered with a coarse filter paper (pore size of 3.5 pm), and an outer cup for the collection of the soil solution. With respect to the total water content, the percentage of soil solution obtained in this way for the three soil layers was l-O-2.0 cm, 2.5-4.5 cm and 4.5-5.5 cm: 53%, 59% and 70%, respectively. Gel filtration
Acrylex P-2 (Reanal, Hungary), a polyacrylamide pearl polymer (bead size 120-320pm), produced by co-polymerisation of acrylamide and N,N’-methylenebis-acrylamide was used. Its special advantages over traditional gels (such as Sephadex) include high separation efficiency at elevated flow rate and mechanical strength at reasonable pressure, which allows flow rates up to 0.9 ml min-‘. This gel was found suitable for high resolution fractionation of organic molecules with a nominal molecular weight (Mw) of 250-2000 daltons. In this range, a linear selectivity curve was obtained. Standards for the determination of the selectivity curve of the Acrylex P-2 column (1.5 x 60 cm; flow rate O-50.9 ml mini, eluent O-05M Tris (trishydroxymethyl methylaminekHC1 buffer, pH 7-4, sample volume 1 ml) were: vitamin B-12 (1355 daltons), raffinose (532 daltons), and sugar (342 daltons). The exclusion limit, obtained by extrapolating the selectivity curve, was 2000 daltons. The void volume was 27.4ml (determined by using dextrane Blue, Mw = 2000000). For the separation of the soil solutions either 5 ml from the AOf horizon or 3 ml from the AOH or AOh +Al/A2 were applied to the column and eluted with the buffer and flow rate given above. After 20ml had passed through the column, the effluent was collected for analysis in the following five portions: O-20, 2-20, 3-15, 4-20. and 540ml. Ultraviolet absorbance measurements of the eluted
262
G. I. Agapkina
et al.
solution at 254nm revealed that after the fourth portion had passed through the column, no organic material was detectable in the effluent. Radionuclides observed in the fifth fraction were, therefore, considered as ‘inorganic’ components. With the selectivity curve mentioned above, the first four fractions were attributed to the following (nominal) molecular weights (in dalton): 22000, 130&1000, 800, and 400, respectively. For further details see Agapkina and Tikhomirov (199 1) and Agapkina et al. (1994). Determination of 239t240P~, 238P~, 241Am, ?3r and 13’Cs
For the determination of the above actinides and of 90Sr, a sequential method was developed using commercially available, highly specific supported extractants. First, 20ml of an eluted fraction were made up to 35 ml with HNO3 (cont.) for the determination of i3’Cs by direct gamma spectrometry. Subsequently, after addition of the yield tracers and carriers (242Pu, 243Am, 20mg Sr and 20 mg Y) the solution was evaporated to dryness and fumed repeatedly with HN03 (cont.) until a white ash was obtained. After dissolving the ash in 50 ml 0.5 M HCl + 1 ml HI (57%), 100 ~1 of ammonium sulfite are added. After gently heating for 10 min, 5 g ammonium chloride are added and the hydroxides are precipitated by ammonia. In this way yttrium (but not strontium) is precipitated together with plutonium and americium. (Because from now on the isolated 9oY decays, this moment has to be recorded for the subsequent evaluation of the 90Sr via 9oY). After cooling, the suspension is centrifuged, washed with distilled water, and the precipitate dissolved in 20ml 2M HN03. The concentration of the nitric acid is then adjusted again to 2 M. The resulting solution (circa 20 ml) is fed via a peristaltic pump to a column (18 cm long, inner diameter 3.76mm) containing TEVAoSpecTM (particle size 50-100 pm, bed volume 2 ml, available from EICHroM Industries, Darien, IL, USA), at a rate of < 1 mlmin-’ (Bunzl & Kracke, 1994). Subsequently, at the same rate, the column is washed with 15ml nitric acid (2M) and the combined eluates (sample solution and washing) are saved for the determination of 24’Am and “Sr via 9oY (solution A). Next, the column is washed with 10ml hydrochloric acid (8N) to remove traces of iron and thorium. Plutonium is stripped first at a feed rate of 0.5 mlmin-’ with 25 ml hydrochloric acid (0.5 M) and then, at the same rate, with 25 ml of a mixture consisting of HCl (0.5 M) and HI (0.1 M), which contained 0.1 ml of an ammonium sulfite solution (35%). Traces of organic material are destroyed subsequently with HN03, HC104 and KN03. After fuming
Chernobyl-derived 239+240Pu, 241Am, wSr and 137C.~
263
with H2S04, plutonium is electrodeposited on a stainless steel disc at pH 2 in ammonium sulfate solution (Kracke & Bunzl, 1980). For the determination of 241Am and 9oY in solution A, its volume is made up to 50ml with distilled water, and 5 g ammonium chloride are added. Subsequently, while heating, yttrium hydroxide is precipitated by ammonia. The precipitate is washed with 50ml distilled water, centrifuged, dissolved in 100~1 HN03 (cont.), and the concentration of the nitric acid adjusted again to 2 M. The final volume (circa 1 ml) is fed to a column containing TRUoSpecTM (particle size 5&100pm) in the same way as described above. After washing the column with l-6ml 2 M HN03, yttrium is eluted with 20ml 1 M HN03 at a rate of 0.5 ml min-‘. To that fraction 2 ml oxalic acid (5 g 1-l) and ammonia are added, and yttrium is precipitated as oxalate at pH 2. The precipitate is filtered, dried at 12O”C, weighed, and its activity counted in a low-level beta counter. Americium is eluted from this column with 20 ml 4 M HCl and collected in a quartz crucible. Traces of organic material are destroyed with HN03, HC104 and KN03. After fuming with H$O4, americium is electrodeposited on a stainless steel disc at pH 2 in ammonium sulfate solution (Kracke & Bunzl, 1980). The chemical yields were for plutonium: 40-60%, americium: 60-80%, and strontium: 7%90%. The above radionuclides were not only determined in the eluted fractions from the gel filtration as described above, but also in the three extracted soil solutions and in the original three soil layers, from which the solution was obtained. In the latter case, 1 g soil was first completely digested by fuming with HN03, HF and HCL04. Alpha spectrometry
Alpha spectromet_ry of the Pu-isotopes was performed using a 300mm2 silicon surface barrier detector (alpha efficiency l&20%). The resolution was 4&50 keV FWHM at 4-6 MeV. The background counts were < 5 per 10 OOOmin. The detection limit for this counting time was about 0.1 mBq of 238Pu_239f240Pu and 24’Am. Estimation of errors and quality control
The errors (coefficients of variation) of the analytical determinations were generally less than 10%. Quality control over the accuracy of the data was assured by participating in intercomparison runs and by analysing standard reference samples for 239t240Pu and 24’Am. For 24’Am we used the ‘Fresh water lake sediment’, NBS standard Reference Material 4354. In
G. I. Agapkina et al.
264
the case of 239t240Pu, Soil-6 from the International Commission, Vienna, Austria, was used.
Atomic
Energy
RESULTS AND DISCUSSION Total amounts of radionuclides in the soil and the soil solution The total activities of the live radionuclides found in the three horizons of the forest soil as well as the corresponding activities found in the soil solution are given in Table 1. With these data and with the known quantities of soil solution extracted per gram from each horizon, the percentage amounts of activity extracted from the soil by the soil solution can be calculated. These values are also given in Table 1. They reveal that, with respect to the soil, the amounts extractable with the soil solution increase in the AOf horizon in the order Pu M Am < Cs < Sr; in the AOh horizon: Cs M Pu M Am < Sr, and in the AOh + Al/A2 layer: Cs < Pu < Am < Sr. This order results from the amount of soil solution present in each layer and from the sorption properties of the soil for the various radionuclides. Because the extent of this sorption is usually characterised by the distribution factor &, defined as the ratio between the amount of a radionuclide in the solid phase (in Bq per g air dry soil) and the corresponding amount in the soil solution (in Bq per ml), this quantity can be obtained from the data of the first two rows of Table 1, and is given in the fourth row of this table. These values show increased sorption in the AOh horizon in the order Sr < Cs < PUZ Am; in the AOh horizon Sr < Pu < Am z Cs, and in the AOh + Al/A2 layer Sr < Am < Pu < Cs. &-values, such as these, are frequently used to predict or to interpret
F&ion Molec.
1
weight > 2MlO
Fr&
2
1300 - 1ooo
Fraction 4
800
Fig. 1.
400
Fraction S
rnorganic
Chernobyl-derived 23g+240Pu,“‘Am, 90Sr and 13’Cs
265
the mobility and biological availability of radionuclides in the soil. In the following, however, it will become evident that the situation is more complicated, because these radionuclides are attached quite differently to dissolved soil organic matter. Gel filtration
The observed distributions of plutonium, americium, strontium and cesium between the five fractions isolated by gel filtration of the soil solutions from the three soil horizons are illustrated in Fig. 1. The activity 100
0 FWLtiOlll M&c.
weight >2UB
Frdm
2
Frdbm3
1300-loo0
100
Fraction 4
Fraction 5
400
inorganic
800
I
Leaend
I
2
F&l M&c.
weight >2Ml
Fr&ion2 1300-lcm
Fmeiim3
Fmcibn 4
800
4ocl
F&on
S
inorganic
Fig. I-contd. Distribution of 239+2‘?u, 238Pu, 24’Am 90Sr and ‘37Cs between five fractions isolated by gel filtration of the soil solutions from a forest soil. For the four organic fractions the corresponding nominal molecular weight (in daltons) is also given; the soil solutions were isolated from the AOf horizon, 1&2.0cm depth (top); the AOh horizon, 2.545cm (middle), and the AOh + Al/A2 horizon, 4.5-5.5 cm (bottom).
266
G. I. Agapkinaet al.
of a given radionuclide in each fraction is shown there with respect to the corresponding activity found in all five fractions. The corresponding nominal molecular weight of each fraction, obtained as described above, is also given in this figure. Similar to Livens et al. (1987) and Livens and Singleton (1991) who used Sephadex for the gel filtration of humic acid and fulvic acid isolated from the soil by alkali extraction, we also observed sorption losses on the column material. They were only present however, for the soil solution from the organic horizons: plutonium and americium 645% in the AOf layer, and d 30% in the AOh layer. In the case of cesium, losses were only observed in the AOf horizon (~65%). For strontium, losses did not occur. Figure 1 reveals that in the solution of all three soil layers both plutonium isotopes and americium are associated mainly with the high molecular fraction (fraction 1, nominal Mw 2 2000 daltons) and to a much smaller percentage (-20%) also in fraction 2 (Mw = 130&1000 daltons). While the differences between plutonium and americium are rather small in the soil solution of the two top organic layers AOf and AOh, americium in the soil solution of the third layer (4.5-5.5 cm) is present to some extent also in the lower molecular fractions 3 and 4 and even in fraction 5 (inorganic compounds). For plutonium this is not the case. The strong association of the actinides with soil organic matter has, of course, been well known for many years (e.g. Bertha & Choppin, 1984; Choppin, 1988; Coughtrey et al., 1984; Kim, 1986; Kim et al., 1989; Livens et al., 1987; Livens & Singleton, 1991; Ramsay, 1988). As a result, these elements are attached in the soil mainly to undissolved humics and their mobility in the soil is very low (Bunzl et al., 1992, 1994; Coughtrey et al., 1984; Frissel et al., 1981). The present data demonstrate, however, that even the small fraction of plutonium present in the soil solution is also extensively associated with soil organic matter of a nominal molecular weight > 1000 daltons. This is essentially also the case for americium, with the exception of the soil solution from the AOH + A l/A2 horizon. There, americium seems to be present in a sizeable quantity (-25%) in the fraction of inorganic compounds. Strontium-90 from the soil solution of the AOf horizon is associated essentially (86%) with the low molecular fraction 4 (Mw = 400 daltons). The remainder (14%) is present in fraction 5 (inorganic compounds). In the soil solution of the two other soil layers this radionuclide is present essentially (90-95%) only in fraction 5 (inorganic compounds). The very small association of strontium with organic matter in the soil solution is probably also the reason why sorption losses on the column were not observed for this element.
Chernobyl-derived 239+240Pu, 24rAm. 90Sr and 13’Cs
267
Radiocesium from the soil solution of the AOf horizon is associated essentially (900/,) only with fraction 3 (Mw =800 daltons), but in the deeper layers, progressively also with the other fractions. Thus, in the third layer, 13’Cs is distributed almost uniformly between all five fractions.
CONCLUSIONS The different distributions of plutonium and americium, but especially of strontium and cesium between the five fractions obtained by gel filtration of the soil solution from the three soil layers demonstrate clearly that the nature of soil organic matter present in these three horizons must be different. For a forest soil, as used here, this is, of course, not surprising, because the chemical composition and structure of humic material from the Of horizon is necessarily different from that of the AOh horizon. It is interesting, however, to observe that these differences have quite different effects with respect to the association of the various radionuclides with this material. While the sorption of radiocesium is obviously most sensitive to the nature of the soil organic matter, plutonium seems to be least affected. This latter element is obviously in any case associated mainly with the high molecular weight fraction. Strontium seems to be affected differently only by dissolved organic material from the AOf horizon and the AOh horizon, while the elution curves for this radionuclide from the AOH and AOH + Al/A2 were alike. The association of the radionuclides in the soil solution with soil organic matter will undoubtedly have effects on their mobility and biological availability. As mentioned, distribution coefficients Kd are frequently determined to interpret or to predict the behaviour of radionuclides in the soil. However, even if the soil solution used to determine Kd values is separated from the solid phase by, e.g. a 0.45pm membrane filter, it will still contain organic material well above 50000 daltons. Kd values obtained in this way thus yield no information on the association of a radionuclide with soil organic matter in the solution phase. Additionally, because the extent of this association can be quite different for soil solutions from different horizons (see above), similar Kd values observed for a radionuclide in different soil layers do not necessarily imply a similar radioecological behaviour of this element. Thus, the rather similar Kd values found for 137Cs in the present soil in the AOf and AOh + Al/A2 horizon (608 and 465 ml g-i, respectively, see Table 1) do not necessarily indicate the same mobility of cesium in these soil layers, because the corresponding association with soil organic matter is quite different (see Fig. 1).
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ACKNOWLEDGEMENT This work was carried out in the frame of the ‘Agreement for International Collaboration on the Consequences of the Chernobyl Accident’ between the Commission of the European Communities and the states of Belarus, Russia and Ukraine.
REFERENCES Agapkina, G. I. & Tikhomirov, F. A. (1991). Organic compounds of radionuclides in soil solutions and their role in uptake of elements by plants. Ekologiya, 6, 22-28 (in Russian, translated in USA). Agapkina, G. I., Berketova, L. V. & Tikhomirov, F. A. (1988). Availability of calcium for plants in organic compounds of the soil solution. Agrokhimiya, 1, 61-6 (in Russian). Agapkina, G. I., Tikhomirov, F. A. dz Shcheglov, A. I. (1994). Dynamics and chemical forms of radionuclide compounds in the liquid phase of a forest soil at the Chernobyl accident zone. Ekologiya, 1,21-28 (in Russian, translated in USA). Bertha, E. L. & Choppin, G. R. (1984). Interactions of humic and fulvic acids with Eu(II1) and Am(II1). Radiochim. Acta., 35, 143-7. Bunzl, K. & Kracke, W. (1994). Efficient radiochemical separation for the determination of plutonium in environmental samples, using a supported, highly specific extractant. J. Radioanal. Nucl. Chem. Lett., 186,401-13. Bunzl, K., Fbrster, H., Kracke, W. & Schimmack, W. (1994). Residence times of fallout 239+240pu 238pu, 241~~ and 137Csin the upper horizons of an undisturbed grassland’soil. J. Environ. Radioactivity, 22, 1l-27. Bunzl, K., Kracke, W. & Schimmack, W. (1992). Vertical migration of plutonium-239+240, americium-241 and cesium-137 in a forest soil under spruce. Analyst, 117,469-74. Choppin, G. R. (1988). Humics and radionuclide migration. Radiochim. Acta., 44/45,23-g. Coughtrey, P. J., Jackson, D., Jones, C. H., Kane, P. & Thome, M. C. (1984). Radionuclide Dhtribution and Transport in Terrestrial and Aquatic Ecosystems, Volume IV, ed. A. A. Balkema. Plutonium, Rotterdam, chapter 29, pp. l-93. Frissel, M. J., Jakubik, A. T., van der Klugt, N., Pennders, R., Poehtra, P. & Zwemmer, E. (1981). Modeling of the transport and accumulation of strontium, cesium and plutonium. Experimental verification. Report 185-76-1 BIA N; Brussels, Commission of the European Communities. Kim, J. I. (1986). Chemical behaviour of transuranic elements in natural aquatic systems. In Handbook on the Physics and Chemistry of the Actinides, ed. A. J. Freeman & C. Keller. North Holland, Amsterdam, pp. 413-56. Kim, J. I., Buckau, G., Bryant, E. & Klenze, R. (1989). Complexation of Am(II1) with humic acid. Radiochim. Acta., 48, 13-3. Kracke, W. & Bunzl, K. (1980). Determination of 239+240Puand 137Csin livers of cattle. Radiochem. Radioanal. Lett., 42, 77-86.
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Livens, F. R. & Singleton, D. L. (1991). Plutonium and americium in soil organic matter. J. Environ. Radioactivity, 13, 323-39. Livens, F. R., Baxter, M. S. & Allen, S. E. (1987). Association of plutonium with soil organic matter. Soil Sci., 144, 24-8. Ramsay, J. D. F. (1988). The role of colloids in the release of radionuclides from nuclear waste. Radiochim. Acta., 44145, 165-70. Tikhomirov, F. A., Moiseev, I. T. & Rusina, T. V. (1980). Dynamics of biological availability of radioiodine for plants in different soils. Agrokhimiya, 11, 116 20 (in Russian).