Availability of radiocaesium in soils: a new methodology

the Science of the Total Environment ELSEVIER

The Scienceof the Total Environment 157 (1994) 239-248

Availability of radiocaesium in soils: a new methodology J. Wauters*, L. Sweeck, E. Valcke, A. Elsen, A. Cremers Laboratoryfor Colloid Chemistry, Faculty of Agronomy, K.U. Leuven, Kardinaa/ Mercierlaan, 92, B-3001 Heuerlee, Belgium

Abstract Two issues are addressed in this paper: (a) the partitioning of radiocaesium between the micaceous specific site pool and the reversible ion exchange pool in mineral soils characterized by relatively low contents of organic matter; (b) the presentation of a new methodology for measuring radiocaesium availability in soils. The partitioning of radiocaesium between specific sites and reversible ion exchange sites is predicted on the basis of soil characterization: specific sites and overall ion exchange capacity. It is predicted that, in mineral soils, only very small fractions of radiocaesium can be expected to be present in readily reversible ion exchange sites. Such predictions are confirmed by an experimental screening study on radiocaesium desorption in a sandy, loamy sand, loam and clay soil, using a variety of desorption agents. A new methodology is presented for measuring radiocaesium availability, using an infinite bath scenario. The method is illustrated by a series of radiocaesium desorption protocols on humic acid, a reference illite clay, a sand loam, loam and clay soil and a set of podzolic soils, including samples from the Chernobyl 30-km zone. It is demonstrated that the (Ca + Mg)/K ratio in soils may play a key role in accelerating the radiocaesium fixation process in the specific sites. The implications of the positive effect of a high Ca-Mg status in the soil on its fixation potential are discussed in terms of the long-term effects of possible countermeasures. Keywords: Radiocesium; Soils; Sorption reversibility

I. Introduction There is now a general consensus that the selective sorption of radiocaesium in soils and sediments is directly related to the action of micaceous clays [1-5]. In particular, it is well established that the low hydration energy of ions such as Cs +, Rb ÷, K ÷ and NH~- is the key factor in this so-called fixation process. Moreover, these specific effects are thought to take place at the edges of the clay particles, characterized by par-

*Corresponding author. Elsevier Science BV. SSDI 0048-9697(94)04287-W

tially expanded layers, generated by the effect of weathering and possibly by the action of large hydrated cations such as Ca 2+ and Mg 2+ [6]. These specific sorption sites are often referred to as Frayed Edge Sites (FES). Fig. 1 shows a schematic view of the FES. A new procedure was recently developed for measuring the overall capacity of and the caesium selectivity pattern in the FES [7-9]. The technique is based on the use of the silverthiourea complex (AgTU) as a masking agent for the Regular Exchange Sites (RES) i.e. the humic acid and planar exchange sites. The extremely high ion selectivity of this bulky ion for the RES [10-12]

J. Wauters et al. / ScL Total Environ. 157 (1994) 239-248

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limits the caesium interception to the FES, allowing the generation of well-defined sorption plateaus. In addition, it could be established that the FES group is rather heterogeneous and its Cs-sorption behaviour can be described in terms of a model comprising three kinds of sites one of which is extremely selective for caesium (intrinsic K c = e ]e for the Cs-K pair). Evidently, such sites are responsible for the trace sorption of radiocaesium. This procedure is however inadequate for systems with very low FES capacity such as podzol or peaty soils. Such systems can however be characterized on the basis of an improved methodology allowing the measurement of the product of the [FES] and the trace selectivity coefficient K*(Cs/K) in the FES [8,9]. Such a product, represented by the symbol [KD.m K] has been defined as the radiocaesium interception potential for a K-scenario (FES:K saturated). Similarly, for a NH 4- scenario, the quantity [FES].K* (NH4/K), represented by the symbol [K o .m N] can be measured. It has been demonstrated that for soils and reference mica clays, the ratio [KD.mK] / [KD" m N] generally varies in the range 4-7, showing in fact that the NH 4 ion is more sorption competitive. NH 4 fertilization can therefore be expected to lead to a direct enhancement of radiocaesium availability, as demonstrated by field observations [13-14]. Although such characterization is important in rationalizing the short-term effects, it is of limited relevance in understanding the long-term availability and the potential of replenishment of a soil upon depletion of caesium in the soil solu-

tion. Evidence has been presented regarding partial irreversibility of sorption as discussed by Comans et al. [6] recently. However, no clear-cut interpretations have been proposed for the seemingly large differences in desorption reversibility in systems showing otherwise quite similar sorption properties. This paper addresses two issues directly related to the question of partial irreversibility. In the first part of the paper, the question is raised regarding the partitioning of radiocaesium between the FES and RES. In the second part, a new methodology is presented for measuring radiocaesium availability and the technique is illustrated for a variety of substrates (humics, reference clay, sand, loam and clay soils). On the basis of the results shown, some tentative interpretation is presented for the difference in the extent of irreversibility. 2. The partitioning of radiocaesium between FES and RES

It is often thought that the fraction of radiocaesium displaced by a reagent such as 1 M NH 4acetate is the one associated with the RES. The question whether some fraction of radiocaesium intercepted in the FES is also displaced by 1 M NH 4 may seem to be an acedemic one. The answer to such a question however is quite important as will be demonstrated below. If in fact, significant fractions of radiocaesium are displaced from the FES pool, then perhaps some (counter)measures can be taken to influence such a process. If on the other hand, all of the radiocaesium displaced originates from the RES, thus remaining permanently available, then possibly little can be done to alter the transfer from the soil to the plant. It is therefore of interest to attempt some estimate of radiocaesium partitioning between FES and RES. Such an estimate can be based on the overall radiocaesium interception potential of the two pools. In fact, such an exercise was made recently for peaty soils [15]. Let us take a podzol as an example. The specific radiocaesium interception potential is typically in the range of 0.2 mequiv./g for [K D.mK] as shown in Table 6. If

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we take a capacity of 0.1 mequiv, for the RES and a K saturation of 5% in the RES, then the interception potential of the RES amounts to 0.005 mequiv./g. In this calculation, we have taken a unit value for K*(Cs/K) in the RES which are mostly humic acid sites (evidence presented in section 4 below). Consequently, the ratio of radiocaesium levels in the F E S / R E S would amount to 40, i.e. radiocaesium can be expected to be nearly quantitatively present in the FES. Of course, the situation can be expected to be even more favourable in loamy and clay soils on account of much higher FES capacities. The condition is quite different for a peaty soil. Taking a [KD-m K] value of 0.1 mequiv./g and a capacity of 1 mequiv./g for the RES, then the ratio of radiocaesium in F E S / R E S would drop to a value of 2 (taking a 5% K saturation in the RES). In other words, very significant fractions of radiocaesium may be expected to be present in readily reversible ion exchange sites. The question however is whether such estimates can be backed up experimentally. Evidence was obtained through an extensive screening study on radiocaesium desorption on four soils (sandy, sandy loam, loam, clay). Evidence pertaining to peaty soils is presented in another paper at this workshop [16]. Radiocaesium desorption tests were carried out using a broad range of displacing agents which can be divided in two categories. The first one is a set of bulky ions characterized by very pronounced ion selectivity for the RES [17-19] and which should displace radiocaesium quantitatively from the RES, even at moderate concentration. The second group concerns concentrated solutions (1 M) of calcium, potassium and ammonium. The reagents used and concentrations are listed in Table 1. The relevant characteristics of the soil are listed in Table 2. The procedure is as follows. Soil samples (5 g) were dispersed in 25 ml 10 - 4 M KC1, labelled with caesium-137. After overnight equilibration (end-over-end shaking), the samples were centrifuged at high speed and supernatants assayed for caesium-137 activity, discarded and replaced by 25 ml of the displacement reagent. Samples were shaken for 24 h, centrifuged and supernatants assayed radiometrically. This procedure

Table 1 List of reagents used in radiocaesium desorption tests Reagent

Concentration

Bistrimethylammoniumhexane (BTM-6) Silver-ethylene-thiourea (AgENTU) Silver-methyl-thiourea (AgMTU) Silver-thiourea (AgTU) Trimethyl ammonium (TMA) Copper-ethylene-diamine (CuENDA) Calcium chloride Potassium chloride Ammonium acetate

0.01 M 0.015 M 0.015 M 0.015 M 0.01 M 0.01 M 1M 1M 1M

was repeated two more times. Results are summarized in Table 3 in terms of cumulative desorption yields for the three extractions. In the case of the bulky ions and 1 M CaCI2, it was found that roughly equal amounts were displaced in each step (with a clear trend towards a lower desorption yield in the final washing). In the case of K + and NH~-, the major displacement yield (70-90%) was obtained in the first step. These results require various comments. It is apparent that in the case of BTM-6 (the most bulky of the ions studied), negligibly small amounts of radiocaesium are displaced from the four soils studied. It would therefore appear that, in the four soils, radiocaesium is localized in the FES from which it cannot be displaced for steric reasons. The same is true (although to a lesser extent) for the other bulky ions in the clay, loam and sandy loam. In the sandy soil however, rather high displacement yields are obtained by the set of bulky cations, The same considerations apply for the case of 1 M CaC12. Examination of the K and NH4-data shows that a wide range in displacement yields are obtained, indicating that both ions are in fact able to disTable 2 Characteristics of soils studied Soil

CEC [KD'mK] [KD'mN] Kc(N/K) mequiv/g mequiv/g mequiv/g

Sandy (podzol) Sandy loam Loam Clay

0.083 0.112 0.105 0.364

0.151 1.64 3.30 6.64

0.044 0.29 0.52 1.04

3.5 5.7 6.5 6.4

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Table 3 Cumulative desorption yields (%) of radiocaesium in the four soils studied Reagent

Podzol

Sandy loam

Loam

Clay

BTM-6 AgENTU AgMTU AgTU TMA CuENDA CaCI e KCI

1.4 10.8 21.8 40.9 10.6 33.9 56.4 75.5 62.5

1.9 1.3 2.0 6.8 1.4 11.8 3.8 88.7 56.3

0.5 0.5 1.0 2.5 0.4 7.4 1.8 48.5 35.9

0.3 0.3 0.5 1.1 0.4 2.8 0.7 13.4 12.4

NI-'I4Ac

place radiocaesium readily from the FES. Attention is further drawn to the fact that displacement yields in K-solutions exceed those obtained in NH 4 (in spite of lower competitive effects). At this stage, it would be tempting to postulate some differences in structural properties among the FES (with otherwise identical sorption properties) to explain the large differences in displacement yield. However, we shall have to postpone further discussion of these differences and first consider in detail some further desorption tests, described in section 4. 3. Radiocaesium sorption reversibility: a new methodology The 'classical' procedure for measuring the extent of irreversibility ('availability') of radiocaesium sorption is to disperse the soil in a concentrated (usually 1 M) solution of NH~. Such practice is of course related to the fact that (reversible) ion exchange processes are known to be quite rapid and implies that the radiocaesium desorbed in such tests is associated with the RES. This particular procedure is of course one of the steps in the sequential extraction protocols. Such protocols have some merit in setting up some operational scale of availability but can offer very little in terms of the mechanistics of sorptiondesorption processes. For a number of reasons, we were not satisfied with these procedures (kinetic effects, effect of solid/liquid ratios) and have chosen to develop a technique based upon an 'infinite bath scenario'. Such an option was motivated by experimental

findings that the radiocaesium displacement is rate-controlled by the build-up of the radiocaesium level in the liquid phase (a finding which indicates that we are not dealing with simple reversible ion exchange processes which are 'instantaneous'). The principle of the methodology is as follows. The radiocaesium-labelled soil is equilibrated dialytically with the dispersion of an ion exchanger which has caesium sorption properties strongly exceeding these of the soil studied. Usually such equilibration is carried out in a 10 -3 M solution of either K o r N H 4. As a result, the liquid phase radiocaesium is selectively trapped in the ion exchanger, leading to a break-up of the soil-solution equilibrium. This generates a radiocaesium desorption flux from the soil into the liquid phase characterized by 'near-zero' levels of radiocaesium (hence the term 'infinite bath'). Usually such equilibration is carried out for 24 h (end-over-end shaking) whereafter the ion exchanger is replaced by a fresh sample. The procedure can be repeated any number of times. The process is monitored by counting the activity collected in the ion exchanger. At the termination of the experiment, residual activity in the soil is monitored as well (flocculating the soil by BTM-6 treatment to ensure proper counting geometry). The overall effect of the treatment can thus be quantified in terms of cumulative desorption yields and the number of treatments required to arrive at a reasonably well-defined plateau value of desorption can empirically be established. In general, five treatments (covering 1 week) are adequate although in some cases, longer desorption times were used. In general, the ion exchanger is present in the dialysis bag although other versions are possible (discussed below). Various options are possible for the ion exchanger. In most cases, we now use the Giese-granulat (Fachgebiet Medizinische Phyzik, Hannover), i.e. ammoniumcopper-hexacyanoferrate. This material is characterized by exceedingly high KD(CS) values. For example, in a 10 -3 M K or NH4-solution, K D is about 5 × 105 ml/g. Other options are of course possible and have been used such as Dowex 50 (K D +5 X 10 3 ml/g in 10 -3 M K or NH 4) or illite clay ( K D = 10 4 ml/g in 10 - 3 M K).

J. Wauters et al. / Sci. Total Environ. 157 (1994) 239-248

The amount of ion exchanger to be used is critical. In order to generate suitable boundary conditions, it is necessary that K ~ . m ~ > 10 K~. m s (e referring to ion exchanger and s to soil; m is the mass used in the desorption experiment). In general, we have conditions corresponding to s • m s = 50-100. K o • m ~/ g D

measured in the HA stock: KD(CS) = 1.74 ( + 0.24) × 103 m l / g (three measurements). Such a result corresponds to a condition of about 70% of the radiocaesium adsorbed into the H A phase. The rate of radiocaesium desorption was monitored by intermittent sampling of the HA and liquid scintillation counting. The results for five runs are shown in Fig. 2. Examination of these results show that the desorption process is reversible and quite rapid and is completed (98%) within 2-3 min (tl/2 = 30-40 s). Of course, this implies that the replenishment of radiocaesium in organic soils is a nearly instantaneous process. One additional comment is required regarding the K D value found (1740 _ 240). This result is consistent with a near-unit selectivity coefficient between Cs and K. On the basis of exchange capacity and K concentration, one would expect a value of 1920 ml/g.

4. Results and discussion 4.1. H u m i c

acid

Radiocaesium desorption was studied in a commercial humic acid (Fluka) using a slightly modified procedure. Humic acid was first conditioned in the homoionic K-form (dialytic equilibration with 1 M KCl) and exhaustively equilibrated with 10 -3 M KCI. The dry weight of the stock solution so prepared was 1.11 g / l and its cation exchange capacity was 1.92 mequiv./g. This stock solution (caesium-137 labelled) was mixed with solutions of 10 -3 M KC1 (ratios of 1/2, 1/5, 1/11) containing appropriate amounts of KDowex. Prior to desorption, KD(CS) values were 1.00

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Desorption studies were carried out on a reference illite clay mineral (Montana illite obtained

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J. Wauters et al. / Sci. Total En~ron. 157 (1994) 239-248

from the American Clay Mineral Society) using Giese Granulat. Its characteristics are: exchange capacity = 0.163 mequiv./g; FES = 0.01 mequiv./g; [K D .m K] = 12.6 mequiv./g. Prior to caesium-137 labelling and desorption, clay systems had been preconditioned with three different ionic scenarios (a) 1 mM KCI, homoionically; (b) 1 mM KCI + 100 mM CaC12; (c) 1 mM KC1 + 100 mM MgC12. The systems were aged for 6 months in these three scenarios. Systems were labelled with caesium-137 and allowed to age overnight (16 h) and 24 days. K o values (ml/g: duplicate measurements) as obtained from ultracentrifugation and monitoring of the supernatants are given in Table 4. It is apparent that differences in K D are quite small and are essentially dependent on the K concentration (predicted value on the basis of [KD'mK] and K concentration is 1.26X 104 ml/g). Results on desorption yields are shown in Fig. 3. Desorption scenarios cover a period of about 2

Table 4 K o values (ml/g: duplicate measurements) as obtained from ultracentrifugation and monitoring of supernatants Scenario

Aging: overnight

Aging: 24 days

a(K) b (Ca) c (Mg)

1.74(5:0.05) X 104 1.24 ( ± 0.07) X 104 1.43 ( _+0.01) x 104

1.80(+0.12) X 104 1.93 (+ 0,03) × 104 1.32 ( + 0,01) x 104

weeks, and involve six (overnight aging) and 10 (24 days of aging) replacements of the ion exchanger. Examination of the data relating to overnight aging shows that near quantitative desorption is accomplished for the three scenarios (97% for scenario a; 93% for b; 95% for c). Examination of the data for the 24-day aging period reveals some rather important effects of the composition of the liquid phase. It appears that for the K-scenario, aging effects are marginal (from 97 to 84% desorption in scenario a). In contrast, desorption yields in scenarios b and c decrease dramatically (from 93 to 39% in scenario b, and from 95 to 44% for scenario c). This

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J. Wauters et al. / Sci. Total Environ. 157 (1994) 239-248

loam), 48% (loam) and 18% (clay). Attention is drawn to the fact that these levels are significantly higher than those obtained with three extractions of 1 M N H 4 (section 2) in spite of the very low NH4-concentration (thus demonstrating the effect of the boundary conditions). The effect of aging is rather small and it thus appears that in some cases, a dramatic fixation effect may manifest itself over a very short time scale. Most surprising however is the finding that such pronounced differences in desorption behaviour are found in soils with otherwise similar caesium sorption properties. In view of the findings reported for illite, it is of interest to correlate the fixation behaviour with the K-Ca-Mg status of the soils. The ionic status of a soil can be specified in two ways: the ratio of K / ( C a + Mg) in the exchange complex and the Potassium Adsorption Ratio (PAR) defined as the inK~ x/mca+Mg in the soil solution. These characteristics are given in Table 5. PAR values are the averages of seven measurements as obtained from the analysis of the liquid

finding demonstrates that the composition of the liquid phase may play a key role in promoting the 'fixation' of radiocaesium in the FES. In particular, it would appear that the presence of calcium and magnesium ions is liable to accelerate the process of radiocaesium fixation in micaceous clays. This hypothesis is submitted to some further tests below.

4.3. Sandy loam, loam and clay soil Desorption runs were carried out on the three soils discussed in section 2. Soil samples (1 g) were dispersed in 10 ml 10 -5 M KC1, labelled with caesium-137 and allowed to age under wet conditions (4 h, 53 days, 109 days). Suspensions were diluted in 140 ml of 10 -3 M NH4CI containing a dialysis bag with 1 g of Giese Granulate. Oesorption measurements were carried out as described in section 3. Results are shown in Fig. 4. It can be seen that the desorption levels are quite different for the three soils: for freshly contaminated samples, the results are 84% (sandy

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J. Wauterset al. / Sci. Total Environ. 157 (1994) 239-248

246 Table 5 The ionic status of various soils Soil

K / C a + Mg

PAR

Sandy loam Loam Clay

0.152 0.069 0.023

1.5 ( + 0.5) 0.22 ( + 0.06) 0.072 ( _+0.016)

Potassium Adsorption Ratio (PAR) is defined as m ~ / ~/mc~+ Mg in the soil solution.

phase obtained by dispersing the soil in demineralized water at liquid/solid ratios ranging from 20 down to field capacity. PAR values are of course directly related to the K-status of the soil [20,21] and remain constant, irrespective of liquid/solid ratios. Examination of the data confirms the effect of a high Ca-Mg status on fixation behaviour as already illustrated for illite clay. In the case of the clay soil, characterized by relatively high Ca + Mg levels, fixation is very pronounced, the opposite being true for the sandy loam. This is a rather important finding in that it opens up some new possibilities in terms of countermeasures. Specifically, it can be anticipated that Ca-Mg amendments to a soil may accelerate the fixation process. Mechanistically, the process can probably be rationalized in terms of a faster migration of caesium through the expanded edge interlayers, as already speculated on the basis of sorption kinetics by Comans et al. [6]. Final proof of the effect of calcium and magnesium on fixation behaviour should be based on systematic studies of varying Ca-Mg regimes in a range of soils of varying textural properties.

4.4. Sandy soils (Podzolic) Three soils originating from the Belgian Campine region (Kalmthout) and two soils from within the 30-km zone of the Chernobyl reactor (Kopatchi, Chistogalovka: sampled in November 1991) have been included in this study. Relevant characteristics are summarized in Table 6. As expected the ratio [KD.m K] / [ K D ' m N ] varies in the range 4-7 reflecting the fingerprint action of the micaceous clay component. In the case of the Kalmthout podzolic soils, a thorough preconditioning was carried out with a solution of the following composition: K, 0.5 mM;

Ca, 0.67 mM; Mg, 0.33 mM) i.e. a relatively high PAR value (PAR, 0.5). Desorption runs were made using illite clay as a sink which had been preconditioned with the same K-Ca-Mg solution. The two soils from the zone were labelled with caesium-137 (high activity) without pretreatment using the technique described in section 3. Desorption runs were carried out with Dowex 50 as a sink in a 10 -3 M K and a 10 -3 M N H 4 scenario. Aging time was 34 days. All results are summarized in Fig. 5. It can be seen that for the Kalmthout soils, nearly quantitative desorption (> 95%) is accomplished, a finding which is in line with the high K status. In contrast, desorption levels in the soils from the zone are much lower. For the Kopatchi soil, desorption levels of about 65% were obtained. In the case of the Chistogalovka, desorption levels as low as 15% were obtained. We have as yet no data available on the ionic composition of these soils. It is of interest to compare this result with data obtained for the 'Chernobyl-caesium'. The Chistogalovka (surface sample, undisturbed) is highly contaminated (9400 Bq/g) and has been submitted to a desorption run using the Giese Granulate as caesium sink. The desorption yield amounted to 21% and in spite of an aging time of 5.5 years, the result was practically the same as for a short aging time. For the Kopatchi sample, a desorption level of about 20% was obtained for the 'Chernobyl' caesium as well. At this stage, it is not clear whether this result is connected with the presence of some residual particulate forms. In any case, these soils will be submitted to a study of desorption dynamics at varying K-Ca-Mg scenarios.

Table 6 Characteristics of podzolic soils Area Kalmthout l Kalmthout 2 Kalmthout 3 Kopatchi Chistogalovka

C.E.C. mequiv/100 g

[ K D . m K]

[K o "mN]

mequiv/g

mequiv/g

18.3 10.1 10.6 3.7 3.5

0.15 0.13 0.28 0.66 0.13

0.045 0.034 0.066 0.126 0.019

J. Wauters et aL / Sci. Total Environ. 157 (1994) 239-248

5. Concluding comments One of the most disturbing findings in our recent work on the desorption behaviour of radiocaesium, as exemplified in the data presented in this paper and in a large number of unpublished results, is the widely diverging Cs-fixation patterns in soils exhibiting otherwise entirely similar sorption characteristics. In particular, short-term radiocaesium sorption behaviour could be rationalized extremely well in terms of soil sorption behaviour and K + and NH~--concentrations, the key competitive ions in a soil-chemical scenario. Ca and Mg levels and pH had practically no effect on caesium sorption K D values. In the light of these findings, many literature data such as the I U R data-base, the results of Adriano et al. [22]; Kerpen [23] and most recently Prister et al. [14] on the favourable effect of liming and pH are difficult to rationalize. The results presented in this paper may throw

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some light on the mechanistics involved in these effects. It would appear that liming and pH (which is of course directly related to the Ca-Mg status of the soil) have no direct ion exchange effect upon short-term Cs-availability but are indirectly effective by way of (long-term) enhancement of the Cs-fixation potential of the soil. When analysing soil TF behaviour of radiocaesium, correlations are generally made with single soilchemical properties but rarely with combinations (pH being an exception) of them. The data presented here suggest that it may be worthwhile to attempt to look for a connection between TF and the (Ca + Mg)/K ratio in a soil or with PAR values. The findings presented in this paper may open some promising perspectives in the area of possible countermeasures. In particular, it may be worth examining the effect of liming (operating at marginal K-levels) on the long-term availability of radiocaesium. Of course, crop selection under

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J. Wauters et al. / Sci. Total Enciron. 157 (1994) 239-248

such conditions would impose some obvious choices and would be restricted to crops such as grass and wheat.

[12]

Acknowledgements This research was funded by tion Protection Programme). J. cke and L. Sweeck acknowledge from the N.F.W.O. (J.W.) and and L.S.)

the CEC (RadiaWauters, E. ValPh.D. fellowships I.W.O.N.L (E.V.

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