- Email: [email protected]
Distribution coefficients (Kd) and desorption rates of 137Cs and 241Am in Black Sea sediments
Chemosphere 49 (2002) 1367–1373 www.elsevier.com/locate/chemosphere
Distribution coefficients (Kd) and desorption rates of 137Cs and 241Am in Black Sea sediments S. Topcuo glu *, N. G€ ung€ or, C glu ß . Kırbasßo Radiobiology Department, C ß ekmece Nuclear Research and Training Center, P.O. Box 1, Atat€urk Airport, Istanbul, Turkey Received 21 August 2001; received in revised form 3 May 2002; accepted 3 June 2002
Abstract The distribution coefficients (Kd ) and desorption rates of 137 Cs and 241 Am radionuclides in bottom sediments at different locations in the Black Sea were studied under laboratory conditions. The Kd values were found to be 500 for 137 Cs and 3800 for 241 Am at the steady state and described exponential curves. Rapid uptake of the radionuclides occurred during the initial period and little accumulation happened after four days. The desorption rates for 137 Cs in different bottom sediments were best described by a three-component exponential model. The desorption half-times of 137 Cs ranged from 26 to 50 d at the slow components. However, the desorption rate of 241 Am described one component for all sediment samples and desorption half-time was found to be 75 d. In general, the results showed that the 241 Am radionuclide is more effectively transferred to bottom sediment and has longer turnover time than 137 Cs under Black Sea conditions. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords:
137
Cs;
241
Am; Kd ; Desorption; Black Sea sediment
1. Introduction The concentration of radionuclides in the dissolved phase of sea water will be decreased due to their adsorption onto suspended sediment particles or directly on bottom sediments. The distribution coefficient (Kd ) is used to express the ratio between adsorbed radionuclide concentration per unit weight of sediment and dissolved radionuclide concentration per unit volume of sea water. Generic Kd values associated with suspended sediments for selected elements in the marine environment are synthesized in IAEA (1985). Recently, the Kd values for suspended sediment samples have been determined in different marine ecosystems by various authors
* Corresponding author. Tel.: +90-212-548-4050; fax: +90212-548-2230. E-mail address: [email protected] (S. Topcuo glu).
(Martin and Thomas, 1990; Mitchell et al., 1991; Molero et al., 1995). It is well known that the estimation of suspended sediment–water distribution coefficients (Kd ) is important for explaining differences observed between the behavior of radionuclides in the marine environment. Bottom sediment can accumulate radionuclides due to deposition of suspended sediment and by direct adsorption from overlying water (Onishi, 1994). The association of radionuclides with bottom sediments in the marine environment is important. Sediments at the bottom are often the largest reservoir for most particle reactive radionuclides (137 Cs is exception in remaining mostly in sea water) (Carpenter, 1997). Transfer of radionuclides from contaminated bottom sediments to benthic organisms and overlying sea water has been determined to be very low (Beasley and Fowler, 1976; Hamilton et al., 1991). The sediment–radionuclide interactions have also been used to determine sediment accumulation and mixing rates for dating applications
0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 2 ) 0 0 2 9 0 - 4
1368
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373
(137 Cs activity profiles were not used to determine sedimentation rates) (Beasley et al., 1982; Carpenter et al., 1987). Moreover, the Kd value in bottom sediment can be used in pessimistic dose estimation for the control of radioactive discharges (IAEA, 2000). Radionuclides deposited in the bottom sediments can desorb and pass into the dissolved phase. The adsorption–desorption process in sediment–marine water systems will also be affected by pH, salinity and the chemical form of the radionuclide besides the exchange capacity, mineral composition and particle size of the sediments (Murray and Fukai, 1975; Stanners and € nl€ Aston, 1982; U u and Birol, 1986). This paper reports the results of experimental studies on the adsorption–desorption rates of 137 Cs and 241 Am radionuclides in bottom sediments collected from four stations of the Turkish Black Sea coast.
2. Materials and methods Surface sediment samples (the upper 4 cm) were collected in the near shore (35 m) zone of Amasra, Sinop, Persßembe and Rize along the Black Sea coast (Fig. 1) using a Lenz Bottom Sampler. The collected sediments were composed and sieved in the field and the <125 l size fraction was kept for the experiment. The samples were freeze-dried for 7 d to constant weight. The organic matter and carbonate fractions as percent of the sediment samples were determined using the methods of Puskaric et al. (1992). Determination of Fe concentrations in the sediment samples was similar to that previously described by Topcuoglu et al. (2002).
Black Sea water (salinity, 18.75‰ and pH 7.9) used in the experiments was filtered through 0.22 lm Millipore filters and stored until use at 4 °C in the dark. The experiments were performed in a temperature-controlled room (14 1 °C) under dark regime. Hundred ml of water were added to each of five plastic bottles of 250 ml capacity. 137 Cs and 241 Am were added into the bottles to provide concentrations of 100 kBq l1 each. Approximately 0.2 g dry sediment were added to the four bottles after the addition of the radionuclides. One bottle was used as control without addition of sediment. The bottles were then shaken mechanically during the adsorption and desorption periods. The pH of each sample was measured and adjusted to 7.9 during the adsorption period. At the same time, the radioactivity concentrations in the bottles were counted every 2 d in order to maintain the radionuclide concentrations at relatively constant levels. The desired activity concentrations were adjusted using a previously described method (Pentreath, 1975). Five ml of the samples from four bottles and the control bottle were taken and filtered through 0.45 lm filters. The filter and filtrate were counted to obtain a distribution coefficient which is defined cpm per g wet sediment divided by cpm of the radionuclide per ml sea water. Data of the adsorption experiments were fitted to the equation Kdt ¼ Kdss ð1 ekt Þ where Kdt is distribution coefficient at time t, Kdss is distribution coefficient at steady state and k is the desorption rate constant.
Fig. 1. Location of stations along the Turkish Black Sea coast.
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373
1369
At the end of the adsorption period the sediment samples were transferred to uncontaminated bottles after filtration on 0.22 lm filters. The desorption experiment was followed over 28 d and during that time the sea water was changed every 1 or 2 d. The results for radionuclide desorption from sediment samples are expressed as percentage of the initial activity concentrations at time zero. The desorption kinetics was described by the equation
7:6 7:6 cm well-type or 3 3 in. flat NaI (TI) crystals. An internal reference standard of the radionuclides was used to correct for different counting geometries and quantification of the radioactivity. The overall propagated counting error was generally less than 5% at the 1r level.
Cont ¼ Con1 ek1 t þ Con2 ek2 t ; . . . ; Conn ekn t
The results of the adsorption experiments are shown in Figs. 2 and 3. As can be seen, the differences of absorption rates of the two radionuclides in sediments from the four stations were not found to be statistically significant. For this reason, distribution coefficients were described by their exponential curves. Rapid sorption occurred during initial period with little increase in accumulation occurring after four days. The adsorption kinetics of 137 Cs and 241 Am were described by a nonlinear model over the experimental time considered and
3. Results
where Con1 ; Con2 ; . . . ; Conn are fractions of activity in components 1; 2; . . . ; n at time zero, respectively, and k1 , k2 , kn are desorption rate constants of components 1; 2; . . . ; n, respectively. The gamma emitting radionuclides 137 Cs (T1=2 ¼ 30:17 years) and 241 Am (T1=2 ¼ 423:7 years) were obtained from Amersham International plc. Radioactivities were measured using a multi-channel analyzer coupled to
Fig. 2. Distribution coefficient of
137
Cs in sediment samples over time.
1370
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373
Fig. 3. Distribution coefficient of
Table 1 137 Cs and
211
241
Am in sediment samples over time.
Am Kd s and other parameters determined experimentally in the Black Sea sediments
Radionuclide
Kdss
134
Cs
500
241
Am
3800
k (d1 ) 1.7748 Kdt ¼ 500ð1 e1:7748t Þ 0.0585 Kdt ¼ 3800ð1 e0:0585t Þ
s (d)
I (Bq g1 d1 )
Td1=2 (d)
0.5634
887.47
0.39
17.0940
222.30
11.85
s ðturnover timeÞ ¼ 1k 1 , I ðfluxÞ ¼ Kdss s1 , Kdss value is also equal to concentration of the activity at equilibrium as Bq g1 , Td1=2 ðdesorption half-timeÞ ¼ 0:693k 1 .
the parameters obtained are given in Table 1. The results showed that the bottom sediments did not accumulate 137 Cs any great degree with a Kd value of 500 at the steady state. On the other hand, the Kd value was calculated to be 3800 for 241 Am radionuclide. However, the flux of 137 Cs was significantly higher than that for 241 Am. In contrast, the turnover time and desorption half-time of 241 Am in the adsorption experiment were significantly greater. The desorption kinetics for 137 Cs in different bottom sediment samples were best described by three-component exponential models and are given separately for each station in Fig. 4 and Table 2. The desorption rate of 241 Am was described by only a single component for all stations (Fig. 5). The fast components of the 137 Cs desorption curves were characterized by very short de-
sorption half-times and very high flux values. The long desorption half-time for 137 Cs in the slow components were identical 49.5 d at Sinop and Persßembe stations, however, the half-life of the slow desorption rate was 25.7 d in the Rize sediment. The desorption rate of 241 Am was markedly different from those for 137 Cs; e.g. the desorption half-time of 241 Am was 75 d (Table 2). The organic matter, carbonate and Fe contents in Amasra, Sinop, Persßembe and Rize sediment samples are given in Table 3. The adsorption results showed that no significant effects were observed with the increasing concentrations of the substances in the sediments. However, the desorption half-time of 137 Cs in the Rize sediment was significantly lower than those for the other stations. The desorption gradient tends to decrease in the Rize sediment with the increasing Fe concentration.
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373
Fig. 4. Desorption rates of Table 2 Parameters for
137
Cs and
241
137
1371
Cs radionuclide in samples.
Am computed from the desorption experiments Pt¼0 (Bq g1 )
k (d1 )
Fast 63.16 Middle 20.79 Slow 19.67 Ct ¼ 63:16e0:5801t þ 20:79e0:2560t
153.98 50.69 47.96 þ 19:67e0:0150t
0.5801 0.2560 0.0150
1.7238 3.9063 66.6666
89.31 12.98 0.72
1.19 2.71 46.20
Sinop
Fast 47.96 Middle 37.84 Slow 21.54 Ct ¼ 47:96e1:0810t þ 37:84e0:1621t
216.76 171.42 97.58 þ 21:54e0:140t
1.0810 0.1621 0.0140
0.9251 6.1690 71.4286
234.32 27.77 1.37
0.64 4.28 49.50
Persßembe
Fast 54.12 Middle 27.63 Slow 27.13 Ct ¼ 54:12e0:6790t þ 27:63e0:0290t
305.24 155.83 153.01 þ 27:13e0:0140t
0.6790 0.0290 0.0140
1.4728 34.4828 71.4286
207.26 4.52 2.14
1.02 23.90 49.50
Rize
Fast 34.76 Middle 20.83 Slow 49.31 Ct ¼ 34:76e0:8181t þ 20:83e0:1190t
156.42 93.74 221.90 þ 49:31e0:0270t
0.8181 0.1190 0.0270
1.2223 8.4034 37.0370
127.95 11.15 5.99
0.85 5.82 25.67
Single 94.83 Ct ¼ 94:83e0:0092t
3609.5
0.0092
108.6957
33.21
75.33
Station
Component
Con (%)
s (d)
I (Bq g1 d1 )
Td1=2 (d)
134
Cs Amasra
241 Am Together
P (concentration of the radionuclides at time zero), s ðturnover timeÞ ¼ 1k 1 , I ðfluxÞ ¼ P s1 , Td1=2 ðdesorption half-timeÞ ¼ 0:693k 1 .
4. Discussion The Kd value of 137 Cs derived from our experiments is lower than many reported experimental values in liter-
ature (Duursma and Eisma, 1973; Duursma, 1973; Nyffeler et al., 1984). The recommended Kd values for 137 Cs in the pelagic ocean are given as follows: mean, 2 103 , maximum, 2 104 and minimum, 5 102 (IAEA, 1985).
1372
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373
Fig. 5. Desorption rates of Table 3 Organic matter, carbonate and Fe contents measured in the sediments Station
Organic matter (%)
Carbonate (%)
Fe (%)
Amasra Sinop Persßembe Rize
4.3 9.8 7.5 7.6
31.43 20.04 25.46 29.17
2.7 3.5 4.4 5.4
Nevertheless, this minimum value similar to that found in our study. In general, the Kd value for bottom sediment is assumed to be about 1/10th of the Kd values in suspended sediments. This situation partly depends on the particle size. The Kd value in fine (<125 l) €kcßekmece Lagoon sediment particles was found to K€ ußcu be significantly higher than that for coarse particles (250–500 l) in the same sediment sample under similar € nl€ conditions (U u and Birol, 1986). In the same way, the variations between Kd coefficients for suspended particles in aquatic environments can be explained by differences in the particle size fraction (Hetherington and Harvey, 1978). The minimum concentration factor for 241 Am in coastal sediments is given by IAEA (1985) as 1 104 . Our 241 Am Kd value was very low and out of the range of those reported in the literature. In most cases, variations likely result from differences in the sediment particle sizes. If a finer sediment particle size (<63 l) was used in our experiments we would most likely have observed a
241
Am in sediment samples.
higher Kd value for 241 Am. In fact, the mean percent particulate fraction of the fine sediment particles (<125 l) in the tested samples, 30 5%, was significantly higher than the value observed in the very fine sediment particle (<63 l), namely 8 3%. For this, we used only the particle size (<125 l) fraction in the present experiments. The higher Kd value and long desorption half-time observed in the case of 241 Am show that this radionuclide is more efficiently transferred to the bottom sediment and has a longer turnover time than 137 Cs. However, from the data obtained with 241 Am, it is impossible to compare the results in suspended particles under field conditions. On the other hand, the Kd value for 137 Cs in the bottom Black Sea sediments is generally assumed to be identical to Kd values in suspended sediments.
Acknowledgements Thanks are due to IAEA for supporting part of this study under Technical Co-operation contract no. RER/ 2/003 for the Marine Environment Assessment of the Black Sea Region.
References Beasley, T.M., Fowler, S.W., 1976. Plutonium and americium: uptake from contaminated sediments by the polychate Nereis diversicolor. Mar. Biol. 38, 95–100.
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373 Beasley, T.M., Carpenter, R., Jennings, C.D., 1982. Plutonium, 241 Am and 137 Cs ratios, inventories and vertical profiles in Washington and Oregon continental shelf sediments. Geochim. Cosmochim. Acta 46, 1931–1946. Carpenter, R., Beasley, T.M., Zahnle, D., Somayajulu, B.L.K., 1987. Cycling of fallout (Pu, 241 Am, 137 Cs) and natural (U, Th, 210 Pb) radionuclides in Washington continental slope sediments. Geochim. Cosmochim. Acta 51, 1897–1921. Carpenter, R., 1997. Interactions of radionuclides with sediments and suspended particles. Training Course Series No. 7, IAEA, Vienna, pp. 83–103. Duursma, E.K., 1973. Specific activity of radionuclides sorbed by marine sediments in relation to the stable element composition. In: Radioactive Contamination of the Marine Environment. Proc. Symp. Seatle July 10–14, 1972, pp. 57– 73. Duursma, E.K., Eisma, D., 1973. Theoretical, experimental and field studies concerning reactions of radioisotopes with sediments and suspended particles of the sea, Part C: applications to field studies. Neth. J. Sea Res. 6, 265–324. Hamilton, T.F., Fowler, S.W., La Rosa, J., Holm, E., Smith, J.D., Aarkrog, A., Dahlgaard, J., 1991. Comparative study of plutonium and americium bioaccumulation from two marine sediments contaminated in the natural environment. J. Environ. Radioact. 14, 211–223. Hetherington, J.A., Harvey, B.R., 1978. Uptake of radioactivity by marine sediments and implications for monitoring metal pollutants. Mar. Pollut. Bull. 9, 102–106. International Atomic Energy Agency, 1985. Sediments Kd s and concentrations factors for radionuclides in the marine environment. Technical Reports Series No. 247, IAEA, Vienna, p. 73. International Atomic Energy Agency, 2000. Generic models for use in the control of radioactive discharges into the environment. Revised Safety Series No. 57, IAEA, Vienna, p. 217. Martin, J.M., Thomas, A.J., 1990. Origins, concentrations and distributions of artificial radionuclides discharged by the Rhone River to Mediterranean Sea. J. Environ. Radioact. 11, 105–139. Mitchell, P.I., Vives Batlle, J., Ryan, T.P., McEnri, C., Long, S., OÕColmain, M., Cunningham, J.D., Caulfield, J.J.,
1373
Larmour, R.A., Ledgerwood, F.K., 1991. Plutonium, americium and radiocesium in sea water, sediments and coastal soil in Carlingford Lough. In: Kershaw, P.J., Woodhead, D.S. (Eds.), Radionuclides in the Study of Marine Processes. Elsevier Appl. Science, London, pp. 265– 275. Molero, J., Sanchez-Cabeza, J.A., Merino, J., Vives Batlle, J., Mitchell, P.I., Vidal-Quadras, A., 1995. Particulate distribution of plutonium and americium in surface waters from the Spanish Mediterranean coast. J. Environ. Radioact. 28, 271–283. Murray, C.N., Fukai, R., 1975. Adsorption–desorption characteristics of plutonium and americium with sediment particles in the estuarine environment. In: Impact of Nuclear Releases into the Aquatic Environment. International Atomic Energy Agency, Vienna, pp. 179–192. Onishi, Y., 1994. Contaminant transport modeling in surface waters. In: Chaudhry, M.H., Mays, L.W. (Eds.), Computer modeling of free-surface and pressurized flows. Kluwer Academic Publishers, The Netherlands, pp. 313– 341. Nyffeler, U., Li, H.Y., Santschi, P.H., 1984. A kinetic approach to describe trace element distribution between particles and solution in natural aquatic system. Geochim. Cosmochim. Acta 48, 1513–1522. Pentreath, R.J., 1975. Radiobiological studies with marine fish. In: Design of Radiotracer Experiments in Marine Biological Systems, Technical Report Series no. 167, IAEA, Vienna, pp. 137–170. Puskaric, S., Fowler, S.W., Miquel, J.C., 1992. Temporal changes in particulate flux in the northern Adriatic Sea. Estuar. Coast. Shelf Sci. 35, 267–288. Stanners, D.A., Aston, S.R., 1982. Desorption of 106 Ru, 134 Cs, 137 Cs, 144 Ce and 241 Am from intertidal sediment contaminated by nuclear fuel reprocessing effluents. Estuar. Coast. Shelf Sci. 14, 687–691. Topcuoglu, S., Kirbasßoglu, C ung€ or, N., 2002. Heavy metals ß ., G€ in organisms and sediments from Turkish Coast of the Black Sea, 1997–1998. Environ. Int. 27, 521–526. € nl€ U u, M.Y., Birol, E., 1986. Adsorption and desorption of 110 € kcßekmece Lagoon sediments. Doga Tr. Fiz. Ag in K€ ußcu Astrofiz 10, 171–179 (in Turkish).
Distribution coefficients (Kd) and desorption rates of 137Cs and 241Am in Black Sea sediments S. Topcuo glu *, N. G€ ung€ or, C glu ß . Kırbasßo Radiobiology Department, C ß ekmece Nuclear Research and Training Center, P.O. Box 1, Atat€urk Airport, Istanbul, Turkey Received 21 August 2001; received in revised form 3 May 2002; accepted 3 June 2002
Abstract The distribution coefficients (Kd ) and desorption rates of 137 Cs and 241 Am radionuclides in bottom sediments at different locations in the Black Sea were studied under laboratory conditions. The Kd values were found to be 500 for 137 Cs and 3800 for 241 Am at the steady state and described exponential curves. Rapid uptake of the radionuclides occurred during the initial period and little accumulation happened after four days. The desorption rates for 137 Cs in different bottom sediments were best described by a three-component exponential model. The desorption half-times of 137 Cs ranged from 26 to 50 d at the slow components. However, the desorption rate of 241 Am described one component for all sediment samples and desorption half-time was found to be 75 d. In general, the results showed that the 241 Am radionuclide is more effectively transferred to bottom sediment and has longer turnover time than 137 Cs under Black Sea conditions. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords:
137
Cs;
241
Am; Kd ; Desorption; Black Sea sediment
1. Introduction The concentration of radionuclides in the dissolved phase of sea water will be decreased due to their adsorption onto suspended sediment particles or directly on bottom sediments. The distribution coefficient (Kd ) is used to express the ratio between adsorbed radionuclide concentration per unit weight of sediment and dissolved radionuclide concentration per unit volume of sea water. Generic Kd values associated with suspended sediments for selected elements in the marine environment are synthesized in IAEA (1985). Recently, the Kd values for suspended sediment samples have been determined in different marine ecosystems by various authors
* Corresponding author. Tel.: +90-212-548-4050; fax: +90212-548-2230. E-mail address: [email protected] (S. Topcuo glu).
(Martin and Thomas, 1990; Mitchell et al., 1991; Molero et al., 1995). It is well known that the estimation of suspended sediment–water distribution coefficients (Kd ) is important for explaining differences observed between the behavior of radionuclides in the marine environment. Bottom sediment can accumulate radionuclides due to deposition of suspended sediment and by direct adsorption from overlying water (Onishi, 1994). The association of radionuclides with bottom sediments in the marine environment is important. Sediments at the bottom are often the largest reservoir for most particle reactive radionuclides (137 Cs is exception in remaining mostly in sea water) (Carpenter, 1997). Transfer of radionuclides from contaminated bottom sediments to benthic organisms and overlying sea water has been determined to be very low (Beasley and Fowler, 1976; Hamilton et al., 1991). The sediment–radionuclide interactions have also been used to determine sediment accumulation and mixing rates for dating applications
0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 2 ) 0 0 2 9 0 - 4
1368
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373
(137 Cs activity profiles were not used to determine sedimentation rates) (Beasley et al., 1982; Carpenter et al., 1987). Moreover, the Kd value in bottom sediment can be used in pessimistic dose estimation for the control of radioactive discharges (IAEA, 2000). Radionuclides deposited in the bottom sediments can desorb and pass into the dissolved phase. The adsorption–desorption process in sediment–marine water systems will also be affected by pH, salinity and the chemical form of the radionuclide besides the exchange capacity, mineral composition and particle size of the sediments (Murray and Fukai, 1975; Stanners and € nl€ Aston, 1982; U u and Birol, 1986). This paper reports the results of experimental studies on the adsorption–desorption rates of 137 Cs and 241 Am radionuclides in bottom sediments collected from four stations of the Turkish Black Sea coast.
2. Materials and methods Surface sediment samples (the upper 4 cm) were collected in the near shore (35 m) zone of Amasra, Sinop, Persßembe and Rize along the Black Sea coast (Fig. 1) using a Lenz Bottom Sampler. The collected sediments were composed and sieved in the field and the <125 l size fraction was kept for the experiment. The samples were freeze-dried for 7 d to constant weight. The organic matter and carbonate fractions as percent of the sediment samples were determined using the methods of Puskaric et al. (1992). Determination of Fe concentrations in the sediment samples was similar to that previously described by Topcuoglu et al. (2002).
Black Sea water (salinity, 18.75‰ and pH 7.9) used in the experiments was filtered through 0.22 lm Millipore filters and stored until use at 4 °C in the dark. The experiments were performed in a temperature-controlled room (14 1 °C) under dark regime. Hundred ml of water were added to each of five plastic bottles of 250 ml capacity. 137 Cs and 241 Am were added into the bottles to provide concentrations of 100 kBq l1 each. Approximately 0.2 g dry sediment were added to the four bottles after the addition of the radionuclides. One bottle was used as control without addition of sediment. The bottles were then shaken mechanically during the adsorption and desorption periods. The pH of each sample was measured and adjusted to 7.9 during the adsorption period. At the same time, the radioactivity concentrations in the bottles were counted every 2 d in order to maintain the radionuclide concentrations at relatively constant levels. The desired activity concentrations were adjusted using a previously described method (Pentreath, 1975). Five ml of the samples from four bottles and the control bottle were taken and filtered through 0.45 lm filters. The filter and filtrate were counted to obtain a distribution coefficient which is defined cpm per g wet sediment divided by cpm of the radionuclide per ml sea water. Data of the adsorption experiments were fitted to the equation Kdt ¼ Kdss ð1 ekt Þ where Kdt is distribution coefficient at time t, Kdss is distribution coefficient at steady state and k is the desorption rate constant.
Fig. 1. Location of stations along the Turkish Black Sea coast.
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373
1369
At the end of the adsorption period the sediment samples were transferred to uncontaminated bottles after filtration on 0.22 lm filters. The desorption experiment was followed over 28 d and during that time the sea water was changed every 1 or 2 d. The results for radionuclide desorption from sediment samples are expressed as percentage of the initial activity concentrations at time zero. The desorption kinetics was described by the equation
7:6 7:6 cm well-type or 3 3 in. flat NaI (TI) crystals. An internal reference standard of the radionuclides was used to correct for different counting geometries and quantification of the radioactivity. The overall propagated counting error was generally less than 5% at the 1r level.
Cont ¼ Con1 ek1 t þ Con2 ek2 t ; . . . ; Conn ekn t
The results of the adsorption experiments are shown in Figs. 2 and 3. As can be seen, the differences of absorption rates of the two radionuclides in sediments from the four stations were not found to be statistically significant. For this reason, distribution coefficients were described by their exponential curves. Rapid sorption occurred during initial period with little increase in accumulation occurring after four days. The adsorption kinetics of 137 Cs and 241 Am were described by a nonlinear model over the experimental time considered and
3. Results
where Con1 ; Con2 ; . . . ; Conn are fractions of activity in components 1; 2; . . . ; n at time zero, respectively, and k1 , k2 , kn are desorption rate constants of components 1; 2; . . . ; n, respectively. The gamma emitting radionuclides 137 Cs (T1=2 ¼ 30:17 years) and 241 Am (T1=2 ¼ 423:7 years) were obtained from Amersham International plc. Radioactivities were measured using a multi-channel analyzer coupled to
Fig. 2. Distribution coefficient of
137
Cs in sediment samples over time.
1370
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373
Fig. 3. Distribution coefficient of
Table 1 137 Cs and
211
241
Am in sediment samples over time.
Am Kd s and other parameters determined experimentally in the Black Sea sediments
Radionuclide
Kdss
134
Cs
500
241
Am
3800
k (d1 ) 1.7748 Kdt ¼ 500ð1 e1:7748t Þ 0.0585 Kdt ¼ 3800ð1 e0:0585t Þ
s (d)
I (Bq g1 d1 )
Td1=2 (d)
0.5634
887.47
0.39
17.0940
222.30
11.85
s ðturnover timeÞ ¼ 1k 1 , I ðfluxÞ ¼ Kdss s1 , Kdss value is also equal to concentration of the activity at equilibrium as Bq g1 , Td1=2 ðdesorption half-timeÞ ¼ 0:693k 1 .
the parameters obtained are given in Table 1. The results showed that the bottom sediments did not accumulate 137 Cs any great degree with a Kd value of 500 at the steady state. On the other hand, the Kd value was calculated to be 3800 for 241 Am radionuclide. However, the flux of 137 Cs was significantly higher than that for 241 Am. In contrast, the turnover time and desorption half-time of 241 Am in the adsorption experiment were significantly greater. The desorption kinetics for 137 Cs in different bottom sediment samples were best described by three-component exponential models and are given separately for each station in Fig. 4 and Table 2. The desorption rate of 241 Am was described by only a single component for all stations (Fig. 5). The fast components of the 137 Cs desorption curves were characterized by very short de-
sorption half-times and very high flux values. The long desorption half-time for 137 Cs in the slow components were identical 49.5 d at Sinop and Persßembe stations, however, the half-life of the slow desorption rate was 25.7 d in the Rize sediment. The desorption rate of 241 Am was markedly different from those for 137 Cs; e.g. the desorption half-time of 241 Am was 75 d (Table 2). The organic matter, carbonate and Fe contents in Amasra, Sinop, Persßembe and Rize sediment samples are given in Table 3. The adsorption results showed that no significant effects were observed with the increasing concentrations of the substances in the sediments. However, the desorption half-time of 137 Cs in the Rize sediment was significantly lower than those for the other stations. The desorption gradient tends to decrease in the Rize sediment with the increasing Fe concentration.
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373
Fig. 4. Desorption rates of Table 2 Parameters for
137
Cs and
241
137
1371
Cs radionuclide in samples.
Am computed from the desorption experiments Pt¼0 (Bq g1 )
k (d1 )
Fast 63.16 Middle 20.79 Slow 19.67 Ct ¼ 63:16e0:5801t þ 20:79e0:2560t
153.98 50.69 47.96 þ 19:67e0:0150t
0.5801 0.2560 0.0150
1.7238 3.9063 66.6666
89.31 12.98 0.72
1.19 2.71 46.20
Sinop
Fast 47.96 Middle 37.84 Slow 21.54 Ct ¼ 47:96e1:0810t þ 37:84e0:1621t
216.76 171.42 97.58 þ 21:54e0:140t
1.0810 0.1621 0.0140
0.9251 6.1690 71.4286
234.32 27.77 1.37
0.64 4.28 49.50
Persßembe
Fast 54.12 Middle 27.63 Slow 27.13 Ct ¼ 54:12e0:6790t þ 27:63e0:0290t
305.24 155.83 153.01 þ 27:13e0:0140t
0.6790 0.0290 0.0140
1.4728 34.4828 71.4286
207.26 4.52 2.14
1.02 23.90 49.50
Rize
Fast 34.76 Middle 20.83 Slow 49.31 Ct ¼ 34:76e0:8181t þ 20:83e0:1190t
156.42 93.74 221.90 þ 49:31e0:0270t
0.8181 0.1190 0.0270
1.2223 8.4034 37.0370
127.95 11.15 5.99
0.85 5.82 25.67
Single 94.83 Ct ¼ 94:83e0:0092t
3609.5
0.0092
108.6957
33.21
75.33
Station
Component
Con (%)
s (d)
I (Bq g1 d1 )
Td1=2 (d)
134
Cs Amasra
241 Am Together
P (concentration of the radionuclides at time zero), s ðturnover timeÞ ¼ 1k 1 , I ðfluxÞ ¼ P s1 , Td1=2 ðdesorption half-timeÞ ¼ 0:693k 1 .
4. Discussion The Kd value of 137 Cs derived from our experiments is lower than many reported experimental values in liter-
ature (Duursma and Eisma, 1973; Duursma, 1973; Nyffeler et al., 1984). The recommended Kd values for 137 Cs in the pelagic ocean are given as follows: mean, 2 103 , maximum, 2 104 and minimum, 5 102 (IAEA, 1985).
1372
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373
Fig. 5. Desorption rates of Table 3 Organic matter, carbonate and Fe contents measured in the sediments Station
Organic matter (%)
Carbonate (%)
Fe (%)
Amasra Sinop Persßembe Rize
4.3 9.8 7.5 7.6
31.43 20.04 25.46 29.17
2.7 3.5 4.4 5.4
Nevertheless, this minimum value similar to that found in our study. In general, the Kd value for bottom sediment is assumed to be about 1/10th of the Kd values in suspended sediments. This situation partly depends on the particle size. The Kd value in fine (<125 l) €kcßekmece Lagoon sediment particles was found to K€ ußcu be significantly higher than that for coarse particles (250–500 l) in the same sediment sample under similar € nl€ conditions (U u and Birol, 1986). In the same way, the variations between Kd coefficients for suspended particles in aquatic environments can be explained by differences in the particle size fraction (Hetherington and Harvey, 1978). The minimum concentration factor for 241 Am in coastal sediments is given by IAEA (1985) as 1 104 . Our 241 Am Kd value was very low and out of the range of those reported in the literature. In most cases, variations likely result from differences in the sediment particle sizes. If a finer sediment particle size (<63 l) was used in our experiments we would most likely have observed a
241
Am in sediment samples.
higher Kd value for 241 Am. In fact, the mean percent particulate fraction of the fine sediment particles (<125 l) in the tested samples, 30 5%, was significantly higher than the value observed in the very fine sediment particle (<63 l), namely 8 3%. For this, we used only the particle size (<125 l) fraction in the present experiments. The higher Kd value and long desorption half-time observed in the case of 241 Am show that this radionuclide is more efficiently transferred to the bottom sediment and has a longer turnover time than 137 Cs. However, from the data obtained with 241 Am, it is impossible to compare the results in suspended particles under field conditions. On the other hand, the Kd value for 137 Cs in the bottom Black Sea sediments is generally assumed to be identical to Kd values in suspended sediments.
Acknowledgements Thanks are due to IAEA for supporting part of this study under Technical Co-operation contract no. RER/ 2/003 for the Marine Environment Assessment of the Black Sea Region.
References Beasley, T.M., Fowler, S.W., 1976. Plutonium and americium: uptake from contaminated sediments by the polychate Nereis diversicolor. Mar. Biol. 38, 95–100.
S. Topcuo glu et al. / Chemosphere 49 (2002) 1367–1373 Beasley, T.M., Carpenter, R., Jennings, C.D., 1982. Plutonium, 241 Am and 137 Cs ratios, inventories and vertical profiles in Washington and Oregon continental shelf sediments. Geochim. Cosmochim. Acta 46, 1931–1946. Carpenter, R., Beasley, T.M., Zahnle, D., Somayajulu, B.L.K., 1987. Cycling of fallout (Pu, 241 Am, 137 Cs) and natural (U, Th, 210 Pb) radionuclides in Washington continental slope sediments. Geochim. Cosmochim. Acta 51, 1897–1921. Carpenter, R., 1997. Interactions of radionuclides with sediments and suspended particles. Training Course Series No. 7, IAEA, Vienna, pp. 83–103. Duursma, E.K., 1973. Specific activity of radionuclides sorbed by marine sediments in relation to the stable element composition. In: Radioactive Contamination of the Marine Environment. Proc. Symp. Seatle July 10–14, 1972, pp. 57– 73. Duursma, E.K., Eisma, D., 1973. Theoretical, experimental and field studies concerning reactions of radioisotopes with sediments and suspended particles of the sea, Part C: applications to field studies. Neth. J. Sea Res. 6, 265–324. Hamilton, T.F., Fowler, S.W., La Rosa, J., Holm, E., Smith, J.D., Aarkrog, A., Dahlgaard, J., 1991. Comparative study of plutonium and americium bioaccumulation from two marine sediments contaminated in the natural environment. J. Environ. Radioact. 14, 211–223. Hetherington, J.A., Harvey, B.R., 1978. Uptake of radioactivity by marine sediments and implications for monitoring metal pollutants. Mar. Pollut. Bull. 9, 102–106. International Atomic Energy Agency, 1985. Sediments Kd s and concentrations factors for radionuclides in the marine environment. Technical Reports Series No. 247, IAEA, Vienna, p. 73. International Atomic Energy Agency, 2000. Generic models for use in the control of radioactive discharges into the environment. Revised Safety Series No. 57, IAEA, Vienna, p. 217. Martin, J.M., Thomas, A.J., 1990. Origins, concentrations and distributions of artificial radionuclides discharged by the Rhone River to Mediterranean Sea. J. Environ. Radioact. 11, 105–139. Mitchell, P.I., Vives Batlle, J., Ryan, T.P., McEnri, C., Long, S., OÕColmain, M., Cunningham, J.D., Caulfield, J.J.,
1373
Larmour, R.A., Ledgerwood, F.K., 1991. Plutonium, americium and radiocesium in sea water, sediments and coastal soil in Carlingford Lough. In: Kershaw, P.J., Woodhead, D.S. (Eds.), Radionuclides in the Study of Marine Processes. Elsevier Appl. Science, London, pp. 265– 275. Molero, J., Sanchez-Cabeza, J.A., Merino, J., Vives Batlle, J., Mitchell, P.I., Vidal-Quadras, A., 1995. Particulate distribution of plutonium and americium in surface waters from the Spanish Mediterranean coast. J. Environ. Radioact. 28, 271–283. Murray, C.N., Fukai, R., 1975. Adsorption–desorption characteristics of plutonium and americium with sediment particles in the estuarine environment. In: Impact of Nuclear Releases into the Aquatic Environment. International Atomic Energy Agency, Vienna, pp. 179–192. Onishi, Y., 1994. Contaminant transport modeling in surface waters. In: Chaudhry, M.H., Mays, L.W. (Eds.), Computer modeling of free-surface and pressurized flows. Kluwer Academic Publishers, The Netherlands, pp. 313– 341. Nyffeler, U., Li, H.Y., Santschi, P.H., 1984. A kinetic approach to describe trace element distribution between particles and solution in natural aquatic system. Geochim. Cosmochim. Acta 48, 1513–1522. Pentreath, R.J., 1975. Radiobiological studies with marine fish. In: Design of Radiotracer Experiments in Marine Biological Systems, Technical Report Series no. 167, IAEA, Vienna, pp. 137–170. Puskaric, S., Fowler, S.W., Miquel, J.C., 1992. Temporal changes in particulate flux in the northern Adriatic Sea. Estuar. Coast. Shelf Sci. 35, 267–288. Stanners, D.A., Aston, S.R., 1982. Desorption of 106 Ru, 134 Cs, 137 Cs, 144 Ce and 241 Am from intertidal sediment contaminated by nuclear fuel reprocessing effluents. Estuar. Coast. Shelf Sci. 14, 687–691. Topcuoglu, S., Kirbasßoglu, C ung€ or, N., 2002. Heavy metals ß ., G€ in organisms and sediments from Turkish Coast of the Black Sea, 1997–1998. Environ. Int. 27, 521–526. € nl€ U u, M.Y., Birol, E., 1986. Adsorption and desorption of 110 € kcßekmece Lagoon sediments. Doga Tr. Fiz. Ag in K€ ußcu Astrofiz 10, 171–179 (in Turkish).