Accumulation and release of 241Am by a macrophyte of the Yenisei River (Elodea canadensis)

Journal of Environmental Radioactivity 81 (2005) 33–46 www.elsevier.com/locate/jenvrad

Accumulation and release of 241Am by a macrophyte of the Yenisei River (Elodea canadensis) A. Bolsunovsky*, T. Zotina, L. Bondareva Institute of Biophysics, Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk 660036, Russian Federation Received 22 April 2004; received in revised form 14 October 2004; accepted 19 October 2004

Abstract The source of radioactive contamination of the Yenisei River floodplain, including contamination with transuranic elements, is the Mining-and-Chemical Combine of the Russian Ministry of Atomic Energy, which has for many years been producing weapons-grade plutonium. Transuranic elements have been detected not only in the soil and sediment of the river but also in the biomass of aquatic plants. This work is an investigation of accumulation and release of 241Am by a submerged macrophyte of the Yenisei River (Elodea canadensis) in laboratory experiments. In 2000–2003, laboratory experiments were carried out with biomass of E. canadensis Mich. and filtered river water. The samples were collected from the Yenisei River upstream of the discharge of the Combine’s radioactive effluent. The experiments showed that 241Am is accumulated by Elodea biomass: the activity concentration of 241Am can reach 3280 G 240 Bq/g, with the concentration factor for 241Am 16 600 G 2200 l/kg. Results of chemical fractionation have proved that in the course of 241Am accumulation by Elodea biomass, 241Am tightly bound to biomass increases from 11% to 27% of the total 241Am in the biomass. Release of 241Am from the decaying Elodea biomass has been evaluated experimentally. By the end of the experiment (lasting up to 127 days), the Elodea plants had

* Corresponding author. Fax: C7 3912 433400. E-mail address: [email protected] (A. Bolsunovsky). 0265-931X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2004.10.012

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lost up to 65% of their initial 241Am activity and the rate of environment reached 23 Bq/day. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Submerged plant Elodea canadensis; Yenisei River

241

Am release into the water

241

Am; Accumulation; Release; Laboratory experiments;

1. Introduction As a result of nuclear weapons tests, the Chernobyl accident, and long-term operation of radiochemical and nuclear facilities, very high levels of artificial radionuclides have been accumulated in the environment. In aquatic environments, organisms inhabiting them take up radionuclides and transfer them through the food chains; after their death, radionuclides concentrate in the sediment. When physicochemical conditions change, radionuclides can be transferred from the sediment back to the water and migrate via the food chains of the aquatic ecosystem. Aquatic plants, which have accumulated radionuclides from the water and sediment, can drift downstream and, when dead, form new sites of radioactive contamination. In the long-range estimate of consequences ensuing from the activities of nuclear facilities, including the Chernobyl accident, transuranic radionuclides, which have a long half-life, present potential danger. Earlier investigations of the level of radioactive contamination in the Yenisei River floodplain revealed transuranic radionuclides in the soil and sediment (Kuznetsov et al., 1994; Aturova et al., 1996, Bolsunovsky et al., 2002a). For many years, the Mining-and-Chemical Combine of the Russian Ministry of Atomic Energy (at Zheleznogorsk) has been producing weapons-grade plutonium and contaminating the floodplain with transuranic elements. Transuranic elements were detected not only in the soil and sediment but also in biomass of aquatic plants (Bolsunovsky, 2004). Plutonium isotopes were detected in biomass of aquatic plants both near the discharge point of the Mining-and-Chemical Combine (MCC) and at a distance of 200 km downstream (Bolsunovsky, 2004). The previously conducted laboratory experiments revealed a great capacity of aquatic plants and microalgae to accumulate transuranic elements (Gromow, 1976; Marchyulenene et al., 1988). Some authors report that aquatic plants accumulate americium better than plutonium (Eyman and Trabalka, 1980). However, mechanisms of intensive accumulation of transuranic elements by living organisms still remain unknown. Plants are an intermediate in the migration of 241Am in an aquatic ecosystem because the half-life of 241Am is much longer than the life time of the plants. Thus, it is necessary to estimate the retention of radionuclides by plant biomass and the release of 241Am from the biomass back into the water, where it can become biologically available again. The purpose of this work was to investigate accumulation and release of 241Am by a submerged plant of the Yenisei River (Elodea canadensis) in laboratory experiments.

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2. Materials and methods From 2000 to 2003, experiments were conducted in laboratory, using biomass of E. canadensis Mich. (waterweed) – a submerged plant occurring in the Yenisei River (Bolsunovsky et al., 2002b). This is a cosmopolitan species, which is widely used in toxicological experiments (Kahkonen et al., 1997; Malet et al., 2002; SameckaCymerman and Kempers, 2003). Plant and water samples were collected from the Yenisei River upstream of the MCC discharge point in September–November. Plant samples were taken from the population growing in one of the river inlets. In our experiments we used 15–18-cm (long) apical shoots or 3–4-cm (short) apical shoots. The plants were pre-washed with the river and tap water. The Yenisei River water was aseptically filtered through 0.2–mm-pore-size cellulose nitrate membranes (d Z 47 cm, Shleicher&Shuell, Germany) to remove suspended particles and microflora. The plants were maintained in water, in 0.5–1.0–l cylindrical glass vessels, at a temperature of 15–20  C. The vessels were illuminated by luminescent lamps during 14–16 h a day and the side irradiance of a vessel was 4.5 klx. Fresh weight was determined on plants blotted with water absorbing paper. Dry weight was determined on dried plants. In some cases, the dry weight of the plants was calculated from the previously obtained calibration equations. In the experiments, the initial dry weight of the plants in one vessel could be different, varying from 0.23 to 0.54 g. The parameters of radionuclide accumulation by plants were calculated per unit of dry weight. During 2000–2003, we conducted different variants of the experiment with Elodea. Their objectives were as follows: In 2000: To estimate 241Am accumulation by long Elodea shoots in 8-day experiments. To estimate 241Am release by long Elodea shoots in 44-day experiments. In 2001: To estimate 241Am accumulation by long Elodea shoots in 11-day experiments. To estimate 241Am release by long Elodea shoots in 127-day experiments. In 2002: To estimate 241Am accumulation by long and short Elodea shoots in 20-day experiments. (In experiments on 241Am accumulation, new long and short shoots were added to the vessels to decrease 241Am activity of the medium.) To estimate 241Am release from biomass of long Elodea shoots in 110-day experiments. To estimate the binding of 241Am to the living and the decaying biomass of Elodea. In 2003: To estimate 241Am accumulation by long and short Elodea shoots in 28-day experiments. To estimate the binding of 241Am to the biomass of Elodea. Americium was added to water as a solution of 241Am(NO3)3. An equimolar amount of nitric acid was added to control vessels. The added solution of 241 Am(NO3)3 was neutralized with an NaOH solution (0.1 M) to pH 6.5–7.0. Then, plants were placed into the water. In the 2000–2002 experiments with Elodea, the initial activity of 241Am in the water was 1000 G 100 Bq/l, and in the 2003 experiments it was 1800 G 110 Bq/l. To calculate parameters of 241Am accumulation

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by Elodea biomass, we used the averaged data for three experimental vessels with long Elodea shoots and for three experimental vessels with short Elodea shoots. To calculate the binding of 241Am to the biomass, we used the data for three experimental vessels with long Elodea shoots and for five experimental vessels with short Elodea shoots. To estimate 241Am release from the biomass, we took the plants out of the experimental vessels at a stage of maximal 241Am concentration in the biomass and placed them into bags made of synthetic fabric (bolting cloth, mesh size 56–82 mm). Then, the bags with plants were placed into aquaria filled with the clean Yenisei water (0.8–1.2 l), which had been aseptically filtered through 0.2-mm-pore-size membranes (Shleicher&Shuell, Germany) to remove suspended particles and microflora. The aquaria were maintained at room temperature under natural lighting. Initial 241Am activity in the plant biomass in the aquaria was 1700–2700 Bq. Activity concentration of 241Am in the plant and water samples was measured on a g-spectrometer (Canberra, USA) coupled to a hyper-pure germanium detector. The g-spectra were processed using the software CANBERRA GENIE-PC and GENIE-2000 (USA). Activity of 241Am in plants and water was measured in the standard Russian geometry (a 120-ml ‘‘Denta’’ cylindrical vessel): first, we measured the activity of the plant immersed in a certain volume of water and then, we measured the activity of 241Am in the water. To determine the activity concentration of 241Am in the plants, we subtracted the activity of the water. The concentration factor (CF) of americium was calculated as the ratio of the radionuclide content of plant dry biomass (Bq/kg) to the radionuclide content of water (Bq/l). To estimate the mobility of 241Am contained in plant biomass, we used the method of chemical fractionation (Peters et al., 1974; Bondareva and Bolsunovsky, 2002). Americium of the exchangeable fraction was separated by exposing the plant biomass to the action of a CH3COONH4 solution (1 M) for 24 h. To separate 241Am of the adsorbed fraction, the plant biomass was treated with an H2SO4 solution (0.2 M) for 20 min. The americium that was still retained by the biomass was considered to be tightly bound to plant components. Americium bound by organic compounds and mineral residue of the plant biomass was separated by ‘‘wet combustion’’ (Siebert et al., 1996), using H2O2 (30%) and an HNO3 solution (0.1 M).

3. Results and discussion 3.1. Accumulation of

241

Am by biomass of E. canadensis

Typical dynamics of 241Am activity concentration in Elodea biomass and in the water environment is shown in Figs. 1 and 2. It is evident that during the first 4 h of the experiment some 241Am is taken up from the environment and is registered in the biomass of the long and short shoots of the plant. In our experiments the dynamics of 241Am activity concentration in the plant was shaped as a plateau curve (Figs. 1 and 2). When the curve reaches a plateau, activities of 241Am in the plant biomass

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Long shoots 2002 1

4000

1200

2 3

1000

4

3000

800

2500

600

2000 1500

400

1000

Am-241 in water, Bq/l

Am-241 in biomass, Bq/g

3500

200

500 0 0

100

200

300

400

500

0 600

Hours 241

Fig. 1. Dynamics of Am activity in the biomass of long Elodea shoots (1 and 2, Bq/g) and in the water of experimental vessels (3 and 4, Bq/l). 1 – in the initial plant biomass accumulating 241Am; 3 – in the water containing initial plant biomass accumulating 241Am; 2 – in the new plant biomass accumulating 241Am; 4 – in the water containing new plant biomass accumulating 241Am.

and in the water remain rather constant, so we term the activity in this phase equilibrium activity. The time of reaching equilibrium activity was approximately the same in all of our experiments (2000–2003). In 2002, in the experiment with long Elodea shoots, 241Am reached its equilibrium activity in the biomass on days 3–4 (Fig. 1), and the average activity concentration of the radionuclide, 3280 G 240 Bq/g dry weight, remained unchanged during the following 4–5 days, till the end of the experiment. The activity of 241Am in the water decreased from its initial value – 1070 Bq/l – to the equilibrium level – 200 G 20 Bq/l (Fig. 1). In 2002, in the experiment with short Elodea shoots, 241Am also reached its equilibrium activity in the biomass on day 4 (Fig. 2), and the average activity concentration of the radionuclide, 3010 G 250 Bq/g dry weight, remained unchanged during the following 4 days, till the end of the experiment. The activity of 241Am in the water decreased from its initial value – 1070 Bq/l – to the equilibrium level – 300 G 30 Bq/l (Fig. 2). Throughout the experiment, the plant remained viable, growing new leaves and rootlets, but the biomass did not increase significantly. There were no significant differences between 241Am activity concentrations of long shoots and short shoots of Elodea (3280 G 240 and 3010 G 250 Bq/g, respectively). The calculated concentration factors were 16 600 G 2200 and 10 300 G 1200 l/kg (Table 1). In our 2000 and 2001 experiments, we solely used long Elodea shoots. In 2002 and 2003, we experimented with both long and short shoots. It is well known that the top part of Elodea (a short shoot) is the most actively growing part of the plant and the amount

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Short shoots 2002

4000

1 2

1000

3

3000

4

2500 2000

800

600

1500

400

1000

Am-241 in water, Bq/l

Am-241 in biomass, Bq/g

3500

1200

200

500 0 0

100

200

300

0 400

Hours 241

Fig. 2. Dynamics of Am activity in the biomass of short Elodea shoots (1 and 2, Bq/g) and in the water of experimental vessels (3 and 4, Bq/l). 1 – in the initial plant biomass accumulating 241Am; 3 – in the water containing initial plant biomass accumulating 241Am; 2 – in the new plant biomass accumulating 241Am; 4 – in the water containing new plant biomass accumulating 241Am.

of periphyton attached to it is the smallest. Hence, 241Am concentration in the short shoots must be indicative of the radionuclide accumulation by the plant rather than by periphyton. However, our 2002 and 2003 experiments showed (Table 1) that there was no significant difference between 241Am activity concentrations in long and short Elodea shoots. This suggests a conclusion that the contribution of periphyton to accumulation of 241Am by the tested plant samples was not considerable. In all our experiments with Elodea, during the equilibrium phase the environment retained considerable 241Am activity – 20%–40% of the initially added activity (Table 1). That could indicate that the added 241Am activity was larger than the activity the plants were able to accumulate or that 241Am was present in the water in the form unavailable to plants (e.g. as radiocolloids). As mentioned above, in our 2002 experiments with Elodea, 200–300 Bq/l 241Am was registered in the environment at the end of the experiment. We made an attempt to decrease the residual activity of 241Am in the environment. After removing the Elodea biomass at the end of the experiment, we placed new short and long Elodea shoots, which did not contain 241Am, into the water. As a result, the 241Am activity in the environment decreased (Figs. 1 and 2). Immediately after the new biomass was added, the decrease was rapid, but then it slowed down and a new equilibrium was established in the system. The 241Am activity in the environment decreased to 82 Bq/l in the system with long shoots and to 150 Bq/l in the system with short shoots. Activity

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A. Bolsunovsky et al. / J. Environ. Radioactivity 81 (2005) 33–46 Table 1 Results of experiments on

241

Am accumulation by Elodea canadensis

Plant part, year of experiment

Initial dry mass, g

Added Am activity in water, Bq/l

Long shoots, 2000 Long shoots, 2001 Long shoots, 2001 Long shoots, 2002, 0–240 h Long shoots, 2002, 240–485 h Long shoots, 2003 Short shoots, 2002, 0–170 h Short shoots, 2002, 170–340 h Short shoots, 2003

0.50 G 0.04 0.54 G 0.05 0.31 G 0.04 0.24 G 0.01

Equilibrium activity of

241

Am

241

Concentration factor (!103), l/kg

In plants, Bq/g dry mass

In water, Bq/l

1010 G 70 1000 G 60 970 G 70 1070 G 50

1270 G 170 1190 G 110 1830 G 230 3280 G 240

440 G 40 180 G 30 260 G 40 200 G 20

3.0 G 0.5 6.9 G 1.7 7.1 G 1.3 16.6 G 2.2

0.09 G 0.01

200 G 20

1340 G 260

82 G 12

17.1 G 4.3

0.27 G 0.02 0.23 G 0.01

1850 G 110 1070 G 50

3000 G 330 3010 G 250

510 G 40 300 G 30

6.0 G 0.8 10.3 G 1.2

0.13 G 0.02

260 G 20

780 G 140

150 G 15

5.3 G 1.1

0.29 G 0.01

1750 G 110

2820 G 280

620 G 40

4.8 G 0.6

concentrations were 1340 Bq/g for biomass of long shoots and 780 Bq/g for biomass of short shoots. The calculated 241Am concentration factors were 17 100 G 4300 l/kg for long shoots and 5300 G 1100 l/kg for short shoots (Table 1). The filtering of the water containing 241Am through membranes of various pore sizes, including 0.2-mm-pore-size ones, confirmed the presence of radiocolloids in the water at the end of experiment. The portion of 241Am incorporated into radiocolloids or pseudo-colloids, which were retained by 0.2-mm-pore-size filters, amounted to 60% and more of 241Am content in the environment at the end of the experiments. It was reported by Cochran et al. (2000) and Degueldre et al. (2001) that 241Am really forms complexes with organic and mineral substances of the water. Our experiments showed that 241Am is really accumulated by long and short Elodea shoots. However, it was not clear how and where (on the surface of cells or inside them) it is accumulated. To estimate the mobility of 241Am contained in Elodea biomass, we used the method of chemical fractionation. At the end of the first part of the 2002 experiment (Fig. 1), long Elodea shoots were subjected to the conventional procedure of chemical fractionation. The data listed in Table 2 show that on day 11 of the 2002 experiment, 241Am contained in biomass was mainly concentrated in the exchangeable and the adsorbed fractions (36%–46% and 35%– 52%, respectively). 241Am tightly bound to Elodea biomass constituted just 12%– 19% of the total 241Am activity. In another, longer, experiment with long Elodea shoots (2003), on day 28, 241Am was also concentrated mainly in the exchangeable and the adsorbed fractions of the biomass (41% and 38%, respectively). The fraction of 241Am tightly bound to Elodea biomass was larger – 21% of the total 241Am activity (Table 2). These data suggest that 241Am is incorporated into Elodea cells and that with time 241Am can be redistributed and the fraction of the 241Am tightly bound to biomass can increase. In 2003 we conducted an experiment with short Elodea shoots to investigate the possible dynamics of 241Am redistribution while it was being accumulated by

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Table 2 Results of fractionation of Elodea canadensis biomass Plant part year, day

Long shoots 2002, day 11a Long shoots 2002, day 11a Long shoots 2002, day 110b Long shoots 2002, day 110b Long shoots 2003, day 28a Short shoots 2003, day 1a Short shoots 2003, day 2a Short shoots 2003, day 4a Short shoots 2003, day 11a a b

Measured activity of Total, Bq (%)

241

Am in fractions, meanGsd

Exchangeable, Bq (%)

Adsorbed, Bq (%)

Organic, Bq (%)

Mineral, Bq (%)

234 G 20 (100%) 107 G 8 (46%)

82 G 6 (35%)

45 G 4 (19%)

185 G 20 (100%) 66 G 5 (36%)

97 G 7 (52%)

22 G 2 (12%)

490 G 30 (100%) 33 G 2 (13%) 144 G 8 (55%)

82 G 5 (31%) 3.2 G 0.5 (1.2%)

570 G 40 (100%) 40 G 3 (17%)

87 G 5 (38%) 5.5 G 0.5 (2.4%)

96 G 6 (42%)

560 G 20 (100%) 227 G 13 (41%) 212 G 13 (38%) 113 G 8 (20%) 7.5 G 1.5 (1.3%) 313 G 14 (100%) 170 G 11 (54%) 108 G 8 (35%)

34 G 3 (11%) 0.5 G 0.2 (0.2%)

400 G 16 (100%) 190 G 12 (47%) 140 G 9 (35%)

69 G 5 (17%) 1.6 G 0.5 (0.4%)

550 G 20 (100%) 245 G 14 (44%) 230 G 13 (42%) 67 G 5 (12%) 9.5 G 1.4 (1.7%) 690 G 17 (100%) 275 G 11 (40%) 230 G 10 (33%) 164 G 8 (24%) 20.9 G 1.4 (3.0%)

Time of fractionation of living biomass from the start of the experiment. Time of fractionation of decaying biomass from the start of the experiment.

biomass. Similarly to the 2002 experiments, in 2003 241Am also reached its equilibrium activity in the biomass on day 4, and the average activity concentration of the radionuclide, 2820 G 280 Bq/g dry weight, remained unchanged during the following 7–8 days, till the end of the experiment (Table 1). In this experiment, fractionation of Elodea biomass was conducted on days 1, 2, 4, and 11 (each time a different replicate vessel was used). In 24 h after the beginning of the experiment, the exchangeable fraction contained 54% and the adsorbed fraction 35% of the total 241 Am (Table 2). At that time the fraction of biomass-bound 241Am was just 11.2%: 11% was in the organic fraction and 0.2% in the mineral fraction. As the biomass accumulated 241Am, the portions of 241Am in the exchangeable and the adsorbed fractions decreased while 241Am in the biomass fractions – organic and mineral – increased (Table 2). At the end of the experiment, on day 11, 241Am tightly bound to biomass constituted 27% of the total 241Am in the biomass. Compared with the primary distribution of 241Am, its portion in the organic fraction increased 2 times and in the mineral fraction 10 times. Thus, our experiments on accumulation of 241Am by Elodea biomass have shown that the activity concentration of 241Am can reach 3280 G 240 Bq/g, with the concentration factor for 241Am 16 600 G 2200 l/kg. Results of chemical fractionation have proved that 241Am can be tightly bound to biomass. As Elodea biomass accumulates 241Am, it decreases in the exchangeable and the adsorbed fractions and increases in the organic and the mineral ones.

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Even in the long-duration experiments (up to 28 days), we did not register any significant release of the 241Am activity from the biomasses of long and short shoots into the aqueous environment (Figs. 1 and 2). 3.2. Release of

241

Am by biomass of E. canadensis

As mentioned above, when the uptake of 241Am by Elodea and its accumulation in the biomass reached steady state, the plants were removed from the experimental vessels and placed into bags made of synthetic fabric (bolting cloth, mesh size 56– 82 mm). Then, the bags with plants were placed into aquaria filled with the river water, which had been aseptically filtered through 0.2-mm-pore-size membranes (Shleicher&Shuell, Germany) to remove suspended particles and microflora. Prior to being placed into the bags, the plants had been maintained in vessels containing the Yenisei River water spiked with 241Am for 8–11 days. Over that time, the accumulation of radioactivity by the plants reached steady state, but no release of 241 Am from the biomass into the water environment was registered. At the time when the long Elodea shoots were placed into the bags, they were viable and their morphological structure was unchanged. The initial 241Am activity in Elodea biomass in the aquaria was (1700–2700) G 100 Bq. The aquaria were maintained at room temperature under natural lighting. The irradiance was certainly lower than that of artificial light. The typical dynamics of 241Am activity in the water of the aquaria containing the plants is shown in Fig. 3. 2500 1 2000

Am-241, Bq/l

2 1500

1000

500

0

0

30

60

90

120

150

Days Fig. 3. Dynamics of 241Am release from the biomass of Elodea canadensis into the water: the unfiltered water (1) and the water filtered through a paper filter of pore size %10 mm (2).

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Almost immediately after Elodea biomass was placed into the aquaria with the filtered water, we registered some 241Am activity in the water environment (Fig. 3). In the experiments conducted in 2001 and 2002, 95 G 20 Bq (3.5% of the initial 241 Am activity in the plant) and 175 G 20 Bq (9.4% of the initial 241Am activity in the plant) was released during 29 days (Table 3). During the following 30–40 days of experiment, the 241Am activity in the water increased, too, but not so significantly. On day 70, plants began to decompose rapidly. As a result, organic particles (microparticles of plant biomass) emerged from the bags into the water and 241Am activity in the water increased (Fig. 3). The morphological structure of the plant was completely destroyed and its biomass had become homogenized by days 110–127. By that time, the Elodea plants in the bags had lost up to 65% of their initial activity (Table 3). Almost all the initial 241Am activity was retained by the organic microparticles of decomposed plants in the water. To measure the 241Am activity released by plant biomass into the aqueous solution, we filtered the aquarium water through an ashless paper filter of pore size %10 mm (d Z15 cm, Russia). The filtered water contained not more than 10% of the total 241Am activity of the aquarium water (Fig. 3). In 2000, the plants decomposed much quicker (within 38–44 days) and in the initial phase the release of 241Am was somewhat larger (Table 3). The reason can be that in 2000 the initial Elodea biomass was in a worse physiological condition than in the following years. The rate of 241Am release into the aqueous environment was determined by the plant biomass decomposition rate, reaching 9–23 Bq/day for Elodea biomass. Experiments conducted in 2000–2002 showed that under low irradiance Elodea biomass containing 241Am rapidly decomposed in water (within 1–4 months of staying in the aquarium); the reason for such rapid decomposition is that Elodea has reduced mechanical tissues (Lukina and Smirnova, 1988). To estimate the mobility of 241Am in plant biomass, we also used chemical fractionation. Physicochemical investigations of the Elodea biomass showed that in the first phase of the experiment (prior to placing the plants into the aquaria), the fraction of 241Am tightly bound to biomass was not more than 20%. The remaining activity was concentrated in the exchangeable and the adsorbed fractions (Table 2). Table 3 Results of experiments on estimating environment Year of the beginning of experiment

2000 2001 2002

Experiment duration, days

44 127 110

241

Am release from Elodea canadensis biomass into the water

Initial 241Am activity of biomass, Bq

1700 G 70 2700 G 90 1800 G 100

241

Am content of water (Bq) and its proportion of the initial activity of Elodea biomass (%) At the beginning of experiment (days 1–3)

At the end of experiment

Rate of 241 Am release into water, Bq/day

370 G 50 (21.5%) 95 G 20 (3.5%) 175 G 20 (9.4%)

1015 G 45 (59.7%) 1730 G 90 (65.1%) 1160 G 70 (63%)

23.0 18.8 9.4

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It is evident that during the first days of the experiment, the water received 241Am of the exchangeable fraction, which was released as a result of diffusion processes. However, 241Am in the exchangeable fraction considerably exceeded the released activity registered in the water. Our investigations showed that by the end of the experiment 241Am of the exchangeable fraction decreased from 46% to 17%, while the fraction of 241Am tightly bound to biomass increased from 20% to 30%–40%. At the end of the experiment, the mineral residue of the plant biomass contained not more than 2% of 241Am (Table 2). Thus, during the experiment, 241Am was redistributed and its fraction tightly bound to biomass increased. These experiments have confirmed our earlier conclusion that biomass binds 241 Am and it remains biomass-bound even when Elodea dies and decays. Some researchers report that physicochemical properties of 241Am allow it to efficiently bind to dissolved organic carbon, including humic acids (Zhang et al., 1997; Cochran et al., 2000). In the case study of the Ob River it was shown that radionuclides, including 241Am, can be transported by organic particles. In our case, 241Am associated with the organic matter that formed after the death of plants can be transported down the river over large distances and become a source of secondary contamination of the sediment and floodplain soils of the Yenisei River.

4. Discussion E. canadensis, a submerged macrophyte, has been widely used in toxicological studies to evaluate accumulation of heavy metals and organic substances in biomass (Kahkonen et al., 1997; Mal et al., 2002; Samecka-Cymerman and Kempers, 2003). It has been shown that the plant biomass can accumulate considerable amounts of heavy metals from both sediment and water. In some aquatic environments there was no correlation between the concentrations of metals in the different parts of Elodea and the corresponding concentrations in the sediment, which could indicate that the plant received metals from water (Kahkonen et al., 1997). Some metals, which are mobile in plants, such as copper, were observed to be translocated from one part of the plant to another (leaves, stem, and root). However, in most cases, heavy metals were confined to a particular part of Elodea plants (Kahkonen et al., 1997; Mal et al., 2002). Elodea has also been used as a test object to evaluate accumulation of artificial radionuclides (Chebotina et al., 1986; Kulikov and Chebotina, 1988; Marchyulenene et al., 1988; Shokodko et al., 1992; Cecal et al., 2002). Similarly to heavy metals, different radionuclides had different concentration factors in Elodea biomass. The highest concentration factors were recorded for 59Fe, 60Co, 65Zn, and 144Ce – from 3000 to 7000 (Kulikov and Chebotina, 1988). Our results are in good agreement with the abovementioned data, and we also found high concentration factors of 51Cr and 54 Mn in Elodea – 3200 and 1400, respectively (Bolsunovsky et al., 2002b). Radionuclides that have stable counterparts or analogues accumulate in plant biomass following the pattern of behaviour of stable elements (Kulikov and Chebotina, 1988). As there are no stable counterparts for transuranic elements, it is

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difficult to explain and predict the behaviour of actinoids (their biosorption and bioaccumulation). So far as we know, only Marchyulenene et al. (1988) reported that Elodea, as well as other aquatic plants, accumulates 238Pu with the concentration factor in dry biomass 2900. The authors made a conclusion that Elodea biomass similarly accumulates 238Pu and 234Th. This conclusion was based on similarities in both concentration factors and physicochemical states of these radionuclides in aqueous solutions. In natural waters 238Pu and 234Th have a propensity to form hydrolytic colloids, which are highly adsorbable. Kulikov and Chebotina (1988) showed that Elodea is really a good accumulator of 232Th and 226Ra, with the concentration factors up to 5000, but a poor accumulator of 238U, with the concentration factor 200. There are no available literature data on accumulation of 241 Am by Elodea biomass. However, Eyman and Trabalka (1980) reported that aquatic plants and microalgae accumulated americium more intensively than plutonium and the concentration factor of americium in aquatic plants could be an order of magnitude higher. As the 238Pu concentration factor in Elodea dry mass is 2900 (Marchyulenene et al., 1988), we could expect that the 241Am concentration factor in Elodea biomass would be about an order of magnitude larger, amounting to 29 000. In the experiments that we conducted in 2002, the highest concentration factor of 241Am in biomass of long Elodea shoots was 16 600 (Table 1), which is considerably higher than the reported concentration factor of 238Pu (2900). In the water bodies of the near zone of the Chernobyl accident, the highest 241Am concentration factors in aquatic plants reached 7500 and were several times higher than concentration factors of plutonium isotopes (Gudkov et al., 2002). Mechanisms of intensive accumulation of 241Am by living organisms still remain to be investigated. Perhaps, similarly to plutonium isotopes, 241Am occurs in natural waters as hydrolytic colloids (Cochran et al., 2000; Degueldre et al., 2001), which are highly adsorbable on cell surface. It should be taken into account that the surface of Elodea plants can be covered with a layer of calcium carbonate, and radionuclide concentration in it can be an order of magnitude higher than in plant tissues (Kulikov and Chebotina, 1988). The presence of calcium carbonate on the surface of the plant can also be a contributing factor in americium adsorption. The 241Am that has concentrated on the plant surface can penetrate into the plant and accumulate in organic and mineral components of the cell. We have proved this in our experiments, using chemical fractionation (Table 2). The organic components that contain 241Am still remain to be specified. Recently, it has been shown that fractions of polysaccharides and lipids are actively involved in accumulation of such radionuclides as 137Cs, 60Co, and 51Cr by Elodea (Cecal et al., 2002). To investigate this in detail can be an objective of our further study.

5. Conclusion 1. The experiments on accumulation of 241Am by Elodea biomass showed that the activity concentration of 241Am can reach 3280 G 240 Bq/g, with the concentration factor for 241Am 16 600 G 2200 l/kg. Results of chemical fractionation

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proved that 241Am can be tightly bound to biomass. At the end of the experiment, on day 11, 241Am tightly bound to biomass constituted 27% of the total 241Am in the biomass. As Elodea biomass accumulated 241Am, it decreased in the exchangeable and the adsorbed fractions and increased in the organic and the mineral ones. Compared with the primary distribution of 241Am, by the end of the experiment its portion in the organic fraction increased 2 times and in the mineral fraction 10 times. 2. The laboratory experiments revealed two typical phases in the dynamics of 241 Am release from Elodea biomass: in the first phase, during 1–3 days, the plant biomass lost up to 9% of the total 241Am activity and then, for a long time, 241 Am concentration in the water varied very little. In the second phase of the experiment, the major part of 241Am activity was released into the water, since the plants were decaying and biomass microparticles emerged into the environment. In the second phase, the rate of 241Am release into the aqueous environment was determined by the plant biomass decomposition rate, reaching 23 Bq/day. By the end of the experiment (lasting up to 127 days), the Elodea plants had lost up to 65% of their initial 241Am activity. The physicochemical investigations of the plant biomass showed that by the end of the experiment 241 Am had been redistributed: 241Am of the exchangeable fraction decreased from 46% to 17%, while the fraction of 241Am tightly bound to biomass increased from 20% to 30%–40%.

Acknowledgement The work was partly supported by the Siberian Branch of the Russian Academy of Sciences within the framework of Integration Project No. 96.

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