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16Original Russian Text © The authors, 2020. Published in MORSKOY GIDROFIZICHESKIY ZHURNAL, Vol. 36, Iss. 2 (2020)
Distribution of 137Cs in the Surface Layer of the Black Sea in Summer, 2017
1.1. Dovhyi1, *, D. A. Kremenchutskii1, N. A. Bezhin1, O. N. Kozlovskaia1, V. V. Milyutin2, E. A. Kozlitin2
Marine Hydrophysical Institute of RAS, Sevastopol, Russian Federation 2Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Moscow, Russian Federation *dovhyi.illarion@yandex.ru
Purpose. The purpose of the study is to investigate distribution of 137Cs in the coastal and deep-water part of the Black Sea. To achieve this goal, the following scientific problems has to be solved: choosing a method for concentrating 137Cs from seawater samples, seawater sampling and 137Cs concentrating, measurements of the obtained samples, discussion of the obtained results and their comparison with the available scientific literature data.
Materials and Results. Distribution of 137Cs activity in the coastal and deep-water part of the Black Sea was studied. The in situ data on spatial-temporal variability of the 137Cs volumetric activity field in the Black Sea surface layer obtained in course of the 95th cruise of the R/V "Professor Vodyanitsky" in June, 14-July 7, 2017 were used. The data on the radionuclide vertical distribution in the sea active layer were obtained at a number of stations. 22 seawater samples were taken and processed at 11 stations. To separate 137Cs from seawater, a ferrocyanide sorbent of the FCC brand was first applied; it was intentionally developed for selective recovery and separation of 134Cs and 137Cs radionuclides from the technological processing solutions and radioactive waste water. Conclusions. According to the results, the volumetric activity of 137Cs varied spatially in the range 5.7-8.8 Bq/m3 and amounted on average 6.9 ± 0.2 Bq/m3. Within the active layer boundaries, vertical distribution of 137Cs was found to be uniform.
Keywords: 137Cs, volumetric activity analysis, Black Sea, active layer, spatial-temporal variability, sorption, selective sorbent.
Acknowledgments: the study was carried out within the framework of the state task No. 0827-20200003 "Oceanological processes", methodological aspects of sorption were studied at the RFBR financial support within the framework of scientific project No. 19-33-60007.
For citation: Dovhyi, I.I., Kremenchutskii, D.A., Bezhin, N.A., Kozlovskaia, O.N., Milyutin, V.V. and Kozlitin, E.A., 2020. Distribution of 137Cs in the Surface Layer of the Black Sea in summer, 2017. Physical Oceanography, [e-journal] 27(2), pp. 152-160. doi:10.22449/1573-160X-2020-2-152-160
DOI: 10.22449/1573-160X-2020-2-152-160
© I. I. Dovhyi, D. A. Kremenchutskii, N. A. Bezhin, O. N. Kozlovskaia, V. V. Milyutin, E. A. Kozlitin, 2020
© Physical Oceanography, 2020
Introduction
The distribution of 137Cs after the Chernobyl accident (Chernobyl nuclear power plant) has been studied in many marine expeditions. The main works in this area for 30 years and to date have been carried out at the Institute of Biology of the Southern Seas. The results of these works were published in a number of monographs [1-3] and a large number of scientific papers [4-6]. Significant contribution to the study of the issue was made by American researchers during the expeditions on R/V "K. Piri Reis" (September 1986, May 1987), R/V "Knorr" (June 1988), a number of joint expeditions on R/V "Professor Vodyanitsky" (April
152 ISSN 1573-160X HYSICAL OCEANOGRAPHY VOL. 27 ISS. 2 (2020)
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1989, June 1990, August 1992). The main results of these works were published in [7, 8]. A number of works were carried out at Marine Hydrophysical Institute [9].
The result of these studies was the calculation of 137Cs fluxes in the Black Sea [2, 10] and the development of mathematical models of 137Cs transport [11, 12]. In these studies, half-life periods of 137Cs activity, 137Cs activity in the Black Sea before and after the Chernobyl accident and subsequent secondary pollution of the Black Sea were determined.
This study examined the distribution of 137Cs in the coastal and deepwater parts of the Black Sea in the modern period.
Materials and methods Sampling technique. The arrangement of stations is shown in Fig. 1 The drawings were plotted in Surfer (Fig. 1, 3, 4) and MS Excel 10 (Fig. 2) application programs).
44-
42-
28 30 32 34 36 38 40 °E 42
F i g. 1. Location of the sampling stations in the cruise of R/V "Professor Vodyanitsky" (June, 14-July 7, 2017)
Sea water was pumped with a submersible pump into 8 plastic containers (the volume of each was 1m3) installed on the board of the vessel. From the containers, using the Unipump JS 60 on-board pump, the sea water was pumped at 6.7-9.0 l/min volume flow rate through a system of three cartridges connected in series and then dumped overboard. The first cartridge in the direction of water flow was a polypropylene filter with 1 ^m pore size (FCPS1M series, Aquafilter Europe Ltd). It served to remove suspended particles from the water. In the second and third cartridges of 700 ml volume, 450 g of ferrocyanide sorbent (of FSS brand) were loaded. On this sorbent the sorption of radiocesium occurred. FSS sorbent is an inorganic sorbent based on nickel-potassium ferrocyanide deposited on the silica gel surface. A pilot batch of sorbent was manufactured at Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences (Moscow) according to TU 2641-003-51255813-2007. PHYSICAL OCEANOGRAPHY VOL. 27 ISS. 2 (2020) 153
In appearance, FSS sorbent has the form of irregular-shaped green granules of 0.25-3.0 mm size. This sorbent is used for processing liquid radioactive waste containing 137Cs; it was first used to concentrate 137Cs from the seawater samples.
After filtration of the water batch, the cartridges with a polypropylene filter and sorbents were dried in air and packed in plastic bags.
Vertical temperature distribution and salinity data were obtained using Seabird CTD (SBE 911 Plus CTD, Sea-BirdElectronics).
Determination of 137Cs activity in samples of sorbents. Measurement of 137Cs activity in sorbent samples was carried out in Marinelli vessels on a low-background gamma spectrometer with a NaI(Tl) scintillation detector (diameter -63 mm, height - 63 mm, the resolution is 7 % based on 137Cs peak, detection efficiency of gamma quantum with an energy of 661.7 keV is 4.5 %). The detector was located in a shield consisting of cast iron and lead rings of 150 and 140 mm thickness, respectively. Registration and processing of spectrometric data was carried out by software on a personal computer. The time of recording the activity of a single sample was on average 16 hours. Calibration of the efficiency of recording 137Cs activity in the samples was performed using a certified soil source with known specific activity IAEA CU-2006-03. The error in measuring the activity of each sample (a) usually did not exceed 5 %.
The degree of 137Cs (S) recovery from the seawater was calculated according to the equation from [13]:
where A, B (Bq) are 137Cs activity in the second and third sorbent cartridges, respectively. A gamma spectrometric analysis of the polypropylene filter contents was not performed.
Sampling technique. A lot of techniques for the radioanalytical determination of 137Cs [14] were proposed. So, for 137Cs concentration a coprecipitation with ammonium phosphormolibdate (APM) is used, as well as sorption on selective sorbents based on APM and insoluble ferrocyanides of various metals.
In order to carry out the sorption concentration of 137Cs, fibrous sorbents obtained by impregnating polymer fibers with transition metal ferrocyanides, for example copper ferrocyanide, are widely used [15]. The disadvantages of this sorbent include the associated recovery of 40K natural radioactive isotope from the seawater. Fibrous sorbents based on mixed potassium-nickel ferrocyanide exhibit greater selectivity with respect to 137Cs and therefore are often used for radioanalytical determination of 137Cs [15]. Thus, a polyacrylate fiber coated with potassium nickel ferrocyanide - KNiFC-PAN - was used to determine 137Cs content in the seawater, and the fibrous sorbent consumption was 5 ml per 20 l of sample [16]. After concentration, the activity of the sorbent solid phase was determined by high-resolution y-spectrometry using a semiconductor detector from high-purity germanium. The advantage of this method is the use of relatively small volumes of seawater for analysis; the disadvantage is the need to use expensive semiconductor y-spectrometers. At the same time, in many radiochemical
Results and discussion
laboratories, cheaper and more affordable scintillation y-spectrometers are used to determine the activity of radionuclides.
During the tests the dependence of 137Cs recovery rate on the volumetric flow rate of seawater transmission through adsorbers with an FCC sorbent was determined (Fig. 2). The results showed that there is a slight decrease in 137Cs recovery rate, on average, from 70 to 60 % with a rate increase from 3 to 9 dm3/min.
F i g. 2. Dependence of the 137Cs recovery rate on the average volumetric seawater flow rate
The radionuclide recovery rate from seawater depends on many factors: the flow rate of pumping the sample through the sorbent, the final volume of the sample and the radionuclide content. According to the measurement results, 137Cs recovery rate by the FCC sorbent was on average 62 ± 9 %.
The main contribution to the error in the technique used to determine the activity of cesium in sea water is made by the statistical error of the product activity calculation, which did not exceed 7 % for the selected duration of measurement of the obtained activities of sorbents' counted samples and the detector we used.
137Cs distribution and activity profiles. During the period under study, 22 seawater samples were taken and processed at 11 stations. The results are represented in the table.
Tabic
CTn
Parameters of samples and stations
Station number Coordinates of sampling point Depth, m Salinity, %o Sample volume, m3 S, % Asp ' 37Cs, Bq m3 Confidence interval
Northern Latitude Eastern Longitude
2 44,456° 33,225° 5 18.28 1.978 72.9 6.01 0.25
25 18.31 3.096 59.5 6.85 0.25
65 18.57 2.932 68.1 6.75 0.26
14 44,628° 34,663° 3 18.57 2.013 84.6 6.00 0.35
20 18.35 3.045 58.0 6,55 0.24
50 18.52 2.966 53.1 7.29 0.26
22 44,946° 35.764° 3 18.10 2.959 60.9 6,56 0.24
23 18.29 4.107 60.4 7.17 0.24
30 44,469° 34,353° 3 18.11 2.000 68.4 5.70 0.26
16 18.14 3.084 43.3 6.99 0.25
50 18.52 3.083 51.7 6.74 0.24
31 44,372° 33,105° 3 18.25 1.977 66,7 6,44 0.23
20 18.25 3.056 56.6 6.31 0.23
60 18.60 3.009 51.1 7.13 0.25
36 43,802° 32,312° 3 18,42 1.984 64,8 6.32 0.27
20 18.39 3.095 50.3 6.91 0.24
45 18.63 3.062 70,6 6.95 0.26
50 44,430° 31,764° 3 18.32 8.250 60.8 6.64 0.20
56 44,537° 32,278° 3 18.25 2.000 68.3 7.37 0.30
67 44,646° 32,902° 3 18.30 8.149 58.8 8.76 0.26
73 43,993° 33,088° 3 18.20 8.196 63.6 7.94 0.25
83 44,099° 33,629° 3 17.96 3.717 74.4 7.56 0.34
137Cs distribution in the Black Sea surface layer is shown in Fig. 3. 137Cs activity value varied in space within 5.7-8.8 Bq/m3 range and averaged 6.9 ± 0.2 Bq/m3. Increased 137Cs activity was observed in the western part of the studied area, lower - in the eastern one. Such spatial variability may be related to the difference in remoteness from the Dnieper mouth [2], which ensures the entry of 137Cs radionuclide, washed out by groundwater from soils, into the Black Sea.
In [10], it was noted that according to observations carried out in 2007, 137Cs activity in the Black Sea surface layer was 20 Bq/m3. Thus, for the 11-year period 2007-2017 a decrease in 137Cs activity average value to 7-8 Bq/m3 is observed. The obtained data are consistent with the literature. According to [2], the half-life of 137Cs is 8.6 years for 1987-2011. The decrease in 137Cs activity in the Black Sea surface layer is associated with its radioactive decay and penetration into the lower layers [2]. The results of numerical experiments aimed at studying the evolution of 137Cs concentration field in the Black and Mediterranean Seas obtained using the POSEIDON-R model are presented in [12]. For the modeling area No. 69, according to the mentioned work, the activity of ~ 8 Bq/m3 refers to 2017.
32 32,5 33 33.5 34 34,5 35 35.5 °E Specific activity, Bq/m3
6 6.5 7 7.5 8 8.5
F i g. 3. Activity of 137Cs in the surface layer (up to 3 m) in the Black Sea nearby the Crimea Peninsula
137Cs activity profiles were obtained at 6 stations (Fig. 4). In all cases, the variation of 137Cs activity with depth did not exceed the error in determining this parameter. The obtained data are consistent with the literature [10], according to which no changes in 137Cs activity were detected up to 50 m depth compared to the surface activity.
15.1
Salinity. %■>
16.5
18.1
-1-
15.2
Salinity, %o
1-^-r
18.3 18.4
Temperature, °C
i— 18.5
Specific activity, Bq/m'
Specific activity, Bq/m'
13.5
___Salinity, %a
I-'-1-^-1-1-1
18 16,2 18,4 18,6
Temperature, °C
Specific activity, Bq/m3
Salinity, %>
I—T—I—1—I—•—T^—I—'—I—1—I 18 18,1 18,2 18.3 18,4 13,5 18.6
Temperature, °C
I-1-1-1-1-'-1-1-1-1-1
Specific activity, Bq/tn
18,1
1— 18,2
Salinily,
1—
16,3 18 4 18.5 Temperature, °C
18,8
18,3
—T~ 18 4
Salinity, %o
~r
18,5
Temperature, °C
-1— 186
Specitlc activity, Bq/m1
5 6 7 8 Specific activity, Bq/m3
—I 18,7
F i g. 4. Activity of 137Cs in the upper mixed layer of the Black Sea at stations 2 (a), 14 (b), 22 (c), 30 (d), 31 (e), 36 (f
Conclusion
For the first time, ferrocyanide sorbent of FCC brand based on potassium-nickel ferrocyanide and silica gel was used for the separation of 137Cs from the seawater samples. High recovery rates (over 60 %) of 137Cs by this sorbent are shown.
137Cs activity values obtained for the studied region vary within the range of 5.7-8.8 Bq/m3 and, on average, were equal to 6.9 ± 0.2 Bq/m3. Increased values of 137Cs activity are observed in the western part of the studied region, lower ones - in the eastern part. According to observational data, 137Cs activity in the depth range of 3-65 m varies within the error range of the method. 137Cs activity values obtained in the work are consistent with the results of numerical experiments that were carried out using the POSEIDON-R model.
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About the authors:
Illarion I. Dovhyi, Senior Research Associate, Marine Hydrophysical Institute of RAS (2 Kapitanskaya St., Sevastopol, 299011, Russian Federation), Ph. D. (Chem.), ORCID ID: 0000-00018706-3810, dovhyi.illarion@yandex.ru
Dmitrii A. Kremenchutskii, Junior Research Associate, Marine Hydrophysical Institute of RAS (2 Kapitanskaya St., Sevastopol, 299011, Russian Federation), ORCID ID: 0000-0002-87476612, d.kremenchutsky@gmail.com
Nikolay A. Bezhin, Senior Research Associate, Marine Hydrophysical Institute of RAS (2 Kapitanskaya St., Sevastopol, 299011, Russian Federation), Ph. D. (Tech.), ORCID ID: 0000-00021670-4251, nickbezhin@yandex.ru
Ol'ga N. Kozlovskaya, Junior Research Associate, Marine Hydrophysical Institute of RAS (2 Kapitanskaya St., Sevastopol, 299011, Russian Federation), SPIN-code: 5386-4791, o.n.kozlovska@gmail.com
Vitaly V. Milyutin, Chief of Laboratory of Chromatography of Radioactive Elements, Frumkin Institute of Physical Chemistry and Electrochemistry of RAS (31, bldg. 4, Leninsky Prospekt, Moscow, 119071, Russian Federation), Dr. Sci. (Chem.), SPIN-code: 3106-6795, vmilyutin@mail.ru Evgeny A. Kozlitin, Senior Research Associate, Laboratory of Chromatography of Radioactive Elements, Frumkin Institute of Physical Chemistry and Electrochemistry of RAS (31, bldg. 4, Leninsky Prospekt, Moscow, 119071, Russian Federation), Ph. D. (Chem.), ORCID ID: 0000-00020381-8786, evgeny_kozlitin@mail.ru
Contribution of the co-authors:
Illarion I Dovhyi - sampling during the 95th cruise of R\V "Professor Vodyanitsky", discussion of the obtained results
Dmitrii A. Kremenchutskii - sampling during the 95th cruise of R\V "Professor Vodyanitsky", discussion of the obtained results
Nikolay A. Bezhin - methodological aspects of 137Cs sorbtion, discussion of the obtained results Ol'ga N. Kozlovskaya - measuring of the samples, discussion of the obtained results Vitaly V. Milyutin - obtaining a sorbent for 137Cs concentration, discussion of the obtained results
Evgeny A. Kozlitin - obtaining a sorbent for 137Cs concentration, discussion of the obtained results
The authors have read and approved the final manuscript. The authors declare that they have no conflict of interest.
