Influence of zeolite on the availability of radiocaesium in soil to plants

The Science of the Total Environment, 113 (1992) 287-295 Elsevier Science Publishers B.V., Amsterdam

287

Influence of zeolite on the availability of radiocaesium in soil to plants M . A . S h e n b e r a n d K.J. J o h a n s o n

The Swedish University of Agricultural Sciences, Department of Radioecology, P.O. Box 7031, S-75007 Uppsala, Sweden (Received October 20th, 1990; accepted December 3rd, 1990)

ABSTRACT Plant availability of radiocaesium is usually high immediately after incorporation in a soil. However, for some Swedish soils, e.g, peat soils, a high radiocaesium uptake in plants has also been observed during a second growth period after contamination. For these soils the reduction in the plant availability of the nuclide seems to be a slow process. In the last two years the mineral zeolite has become of interest and has been tested as a caesium binding agent in both animal and in soil-plant systems. The aim of this study was to evaluate the time dependency and the effect of zeolite on the plant availability of radiocaesium in a peat soil-plant system. The pot experiments designed for this purpose were carried out in a climate chamber, using winter wheat as the test crop. A significant reduction of the uptake of 134Cswas obtained in wheat when increasing amounts of zeolite were added. This reduction in plant uptake, up to a factor of 8, might depend on two factors. One is that zeolite has reduced the activity concentration of radiocaesium in the soil solution available to the plant roots. The other is that the potassium added with the natural zeolite increased the degree of dilution of caesium in the soil solution. Also, increasing equilibration time for caesium in soil before sowing brought about an effective reduction in the caesium uptake.

Key words: peat; uptake; caesium; zeolite

INTRODUCTION R a d i o c a e s i u m migrates in biological systems in a w a y r a t h e r similar to that o f potassium. It can be t a k e n u p directly by plants after wet or dry d e p o s i t i o n o n plants or indirectly by r o o t u p t a k e after g r o u n d deposition. In the soil, r a d i o c a e s i u m is strongly b o u n d to the soil material, s t r o n g e r to the mineral parts t h a n to o r g a n i c matter. T h e aim o f this w o r k was to study the plant availability o f r a d i o c a e s i u m in p e a t y soil a n d to determine the influence o f a caesium binder, zeolite, o n the p l a n t availability o f r a d i o c a e s i u m in soil. P o t experiments were p e r f o r m e d using p e a t y soil t a k e n f r o m fields where the 0048-9697/92/$05.00

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M.A. S H E N B E R A N D K.J. J O H A N S O N

root uptake of radiocaesium had been found to be extremely high. This high availability for plant uptake of radiocaesium is a serious set back in animal husbandry and remedial measures for reduction of the uptake are being investigated. This reduction can be achieved in at least two ways. Either potassium can be added to compete with radiocaesium at the root uptake, or a caesium binding agent can be added to bind radiocaesium and withdraw it from the plant available fraction. Such agents are minerals like zeolites and bentonite, or other clay minerals. MATERIALS AND METHODS

For the experiments, one organic soil was selected from a farm site with very high radiocaesium transfer to the grass and a natural zeolite material. The physical and chemical characteristics of the soil and zeolite used are shown in Table 1. The content of K was determined according to Egner et al. (1960). The potassium content of the zeolite was much higher than that of the soil and therefore the sorption of caesium were investigated in a more detailed study (Shenber and Eriksson, 1989). The design of the experiment considered different levels of the zeolite mixed with the soil before contamination with 134Cs, as well as various equilibration periods after contamination of the soil and before the crop was sown.

Pots with a surface area of 1.77 dm-2and containing 1 kg dry soil were used. The zeolite levels were 0, 50, 300 g per pot or 0, 5 and 30°/5 of the soil

TABLE 1 Characteristics of the soil and the zeolite Characteristic

Soil

Zeolite

pHaq" pH (KCJ,0.1M) Organic matter (%) Caexch (mg/100 g soil) Kexch (mg/100 g soil) CaHo a (mg/100 g soil) KHct a (mg/100 g soil) Kto t (mg/100 g soil) Ka q b (mg/100 g soil)

5.05 4.70 88 200 16 2600 30 ---

---900 940 1200 1190 5000 30

aExtractable with hot 2 M HC1, 4:100. bExtractable with aq. dest., 10:100.

ZEOLITE

A N D

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R A D I O C A E S I U M

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Fig. 1. Regression plot of equilibration time against zeolite amendments on the wheat uptake of I34Cs; (A) control, (B) 50 g zeolite per pot, (C) 300 g zeolite per pot.

290

M.A. SHENBER AND K.J. JOHANSON

weight. The fertilizer was 1.55 g N P K 2 0 - 5 - 9 per pot corresponding to 200 kg/ha. Zeolite and fertilizer were homogeneously mixed and ~34Cs (25 kBq/pot) was added. During the equilibration periods, 0, 15, 30, 60, and 120 days, the water content of the soil was kept on the same level. At the end of these periods the crop, winter wheat (Folke), was sown and cultivated for 30 days in climate chamber with a temperature of 22 4- I°C and a relative humidity of about 50%. The light period was 16 h per day given by General Electric F 96 PG 17 W white lamps. Water loss was replaced every third day. The plants were cut after four weeks at the soil surface level and shoots and roots were separated. The plant material was dried (12 h at 50°C) and ground into a homogeneous powder. The contents of potassium in the samples were determined after ashing with an atomic absorption spectrophotometer (Varian Techtron AA-6).

Ra~omet~ All 134Cs activity measurements were carried out by three hyper-pure Ge-detectors (ORTEC, PGT) in the low-background laboratory at the Department of Radioecology. The output signals of the detectors were fed into two 4096-channel analyzers (ORTEC Adcam) for gamma-spectrometry. The measuring time was chosen in order to reduce the standard deviation to < 5%. Detailed information on radiometric technique has previously been published (Mascanzoni, 1987). RESULTS

AND

DISCUSSION

A parameter commonly used for describing the radionuclide transport between the compartments 'soil' and 'plant' is the soil-to-plant transfer factor (TF,p), defined as (UIR, 1984):

TFsp

Activity in plant dry matter (Bq/kg) Activity in dry soil (Bq/kg)

As shown in Fig. 1, both the zeolite amendments to the soil as well as the extension of the equilibration times had very large effects on the ~34Cs uptake by wheat. The effect of the former was larger at short or no equilibration time before sowing the crop. For this particular soil, with its high content of organic matter the equilibration of ~34Cs between fractions sorbed at mineral surfaces and those sorbed by ionic attraction is a slow process, i.e. at least a period of three months. However, the process goes on under moist

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Fig. 2. Regression plot of equilibration time against zeolite amendments against the Cs/K ratio in wheat (shoot); (A) control, (B) 50 g zeolite per pot, (C) 300 g zeolite per pot.

292

M.A. S H E N B E R A N D K.J. J O H A N S O N

conditions in the soil, and the transfer factors obtained give relative values for the distribution of radiocaesium on different fractions, and for the plant availability of radiocaesium in the soil. The kinetics of the soil-plant transfer of 137Cs are very complex; root uptake is largely dependent on clay or organic content of soil, exchangeable potassium and mineral fraction (Barber, 1964; Evans and Dekker, 1966; Fredriksson and Eriksson, 1958; Squire and Middleton, 1966; Sawhney, 1972; Eriksson, 1977). The uptake of radiocaesium in the control plant shoot was reduced by a factor of 2 when the sowing was delayed by 15-30 days. Further reduction required comparatively longer equilibration periods before sowing. This seems to indicate that the net rate of the equilibration process when radiocaesium migrates to less accessible sites should be dependent on the number of sites. Adding zeolite to the system increased the 134Cs sorbing capacity of the soils and reduced the time required for equilibration. However, at the same time the potassium level in soil increased, which might compete with the radiocaesium for plant uptake. It is therefore evident that the reduction in radiocaesium transfer observed in Fig. 1 depends on three factors, time for equilibration, increased sorption capacity from the zeolite and increased dilution of radiocaesium in the soil solution by the potassium added with the zeolite. The first two factors affect the concentration of exchangeable radiocaesium in the soil solution, the third factor only affects the plant uptake of radiocaesium. Plant roots take up ions from the soil solution, in equilibrium with the ions in the solid phase (Fried and Shapiro, 1961; Schulz, 1965). A specific radionuclide in the soil available for root uptake is distributed on soluble, exchangeable and complexed fractions (Verloo et al., 1980). The major parameter which controls the availability of free ions is pH, which influences the transfer from solid to liquid phase. In the case of radiocaesium also the contents of potassium, clay and organic matter in the soil play important roles for the sorption as well as for the plant uptake (Coughtrey and Thorne, 1983). While the total effect of the zeolite is easy to determine in Figs 1 and 2, the separate effects of the radiocaesium fixing capacity and that of the dilution by potassium are more difficult to estimate. The potassium content in plants was increased by the addition of zeolite to the system and hence the competing effect on radiocaesium available in soil solution cannot be neglected. In fact the amounts of exchangeable potassium in the soil after cropping were increased considerably by the zeolite treatments (Table 2). The potassium content of plants do not differ very much between treatments (Table 3). The regression analysis has shown the existence of the correlation

ZEOLITEAND SOILRADIOCAESIUMAVAILABILITYTO PLANTS

293

TABLE 2 Pot experimental contents o f K and Cs in soil, exchangeable with a m m o n i u m acetate. Zeolite amendments 0, 5 and 30% o f soil weight

134Cs, exch.

Equili- K (mg/100 g soil) libration time Control 5% Z. 0 15 30 60 120

16 18 15 22 19

± .4± ± ±

2 4 2 1 6

62 63 63 65 81

.4.4.44.4-

30% Z. 3 8 2 1 9

270 250 267 273 299

+ + 4.4±

( % o f added)

Control 2 8 2 1 9

15.9 9.5 8.9 9.4 4.7

.4± .4.4.4-

5% Z. 1.7 0.10 0.60 0.50 0.10

62.3 64.7 57.3 57.9 41.8

± .44.4.4-

30% Z. 0.1 0.7 5.4 0.6 15

37.1 36.5 35.8 35.4 49.8

.4- 0.3 ± 0.5 ± 1.0 ± 1.2 .4- 18.5

of various equilibration times and Cs/K ratios in shoots. A reciprocal equation of the form 1/Y = a + bx gave the best fit and the coefficient determination of r 2 was calculated to assess the degree of explanation given by the model used. The results are presented in graphic form in Figs 1 and 2, respectively. The amount of exchangeable radiocaesium in soil after the cropping period (Table 2) shows considerable differences between control and zeolite treatments. This occurs because the organic soil and the zeolite material behave so differently to radiocaesium, both with regard to sorption and to desorption by ammonium ions (Shenber and Eriksson, 1989). Zeolite has a much higher sorption capacity for radiocaesium than the organic soil (Shenber and Eriksson, 1989) but, on the other hand, the radiocaesium sorbed is much more easily desorbed by ammonium ions from the zeolite. This

TABLE 3 K content in plant material (shoot) Equilibration time

0 15 30 60 120

K (mg/g) Control

50 g

300 g

51.26 46.61 52.67 57.45 42.56

65.19 65.02 60.89 73.10 61.47

70.17 70.72 69.32 80.84 70.37

294

M.A. S H E N B E R A N D K,J. J O H A N S O N

sensitivity makes it possible to consider the distribution of radiocaesium on the two fractions: either sorbed by the soil or sorbed by the zeolite. In this context it should be noted that radiocaesium sorbed by the organic soil is much more readily plant available than that sorbed by the zeolite or other mineral soils. The only possible explanation for this is that most of the ~34Cs originally applied to the soil has been sorbed by the zeolite during the equilibration time period and during the growth period. This movement of radiocaesium over the water bridges of the soil solution from and between particles may thus be very effective. The data on the exchangeability of ~34Cs in Table 2 shows 4-10-times higher exchangeability to ammonium ions for the treatment with 5% of zeolite material mixed initially into the soil than for the control. The K content in the soil was significantly correlated with the zeolite-level P < 0.0001 for 50 g zeolite and P < 0.001 for 300 g zeolite, respectively. CONCLUSION Experiments were conducted to study the influence of zeolite amendment on the plant availability of radiocaesium by wheat. The addition of zeolite to the soil caused a considerable reduction in the ~34Cs uptake by plants. The zeolite influenced the radiocaesium uptake in two ways: one by sorbing and reducing the caesium available in soil solution, and the other by diluting the radiocaesium available through the potassium content of the zeolite. There were also a significant reductions of the ~34Cs uptake by factors of 3 - 8 with an increasing length of time for radiocaesium in the soil before sowing. REFERENCES Barber, D.A., 1964. Influence of soil organic matter on the entry of Caesium-137 into plants. Nature, 204: 1326-7. Coughtrey, P.J. and M.C. Thorne, 1983. Radionuclide Distribution and Transport in Terrestrial and Aquatic Ecosystems, Vol. 1. A. A. Balkema, Rotterdam, Ch. 7. Egner, H., H. Riehm and W.R. Domingo, 1960. Untersuchungen uber die chemische Bodenanalyse als Grundlage fur die Beurteilung des Nfihrstoffzustandes der B6den. II, Chemische Extraktionsmethoden zur Phosphor-und Kaliumbestimmung. Lantbruksh6gsk Ann., 26: 199-215. Epstein, E. and C.E. Hagen, 1952, A kinetic study of the absorption of alkali cations by barley roots. Plant Physiol., 27: 457-474. Evans, E.J. and A.J. Dekker, 1966. Plant uptake of Cs-137 from nine Canadian soils. Can. J. Soil Sci., 46: 167-176. Eriksson, ,~., 1977. Fissionsprodukter i svensk milj6. Department of Radioecology, Swedish University of Agricultural Sciences, Uppsala, Report SLU-IRB-40.

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Fredriksson, L. and B. Eriksson, 1958. Studies on soil-plant-animal interrelationship with respect to fission products. Second United Nations International Conference on the Peaceful Uses of Atomic Energy. A/CONF. 15/P/177. Fried, M. and R.E. Shapiro, 1961. Soil-plant relationships in ion uptake. Annu. Rev. Plant Physiol., 12: 91-112. Mascanzoni, D., 1987. Chernobyl's challenge to the environment: report from Sweden. Sci. Total Environ., 67: 133-148. Sawhney, B.L., 1972. Selective sorption and fixation of cations by clay minerals: a review. Clays Clay Minorg., 20: 93-100. Schulz, R.K., 1965. Soil chemistry of radionuclides. Health Phys., 11: 1317-1324. Squire, H.M. and L.J. Middleton, 1966. Behaviour of Cs-137 in soils and pasture a long term experiment. Radiat. Bot., 6: 413-423.423. Shenber, A.M. and ,~,. Eriksson, 1989. Caesium sorption capacity of zeolite and soil. In: W. Feldt (Ed.), The Radioecology of Natural and Artificial Radionuclides. Verlag TUV Rheinland GmbH, K/51n, pp. 250-255. UIR, 1984. III Report of the workgroup on soil-to-plant transfer factors. UIR, RIMV, Bilthoven, The Netherlands. Verloo, M., L. Kiekens and A. Cottenie, 1980. Distribution of essential and non-essential trace elements in the soil-soil solution system. Pedologie, 30: 163-175.