Determination of recovery efficiency of ¹³⁷Cs in seawaterusing co-precipitation method

Nuclear Science and Technology, Vol.10, No. 1 (2020), pp. 40-46

Determination of recovery efficiency of ¹³⁷Cs in seawaterusing

co-precipitation method

Duong Duc Thang*, Vuong Thu Bac, Bui Dac Dung, Duong Van Thang, Doan Thuy Hau,

Le Dinh Cuong, Nguyen Van Khanh, Nguyen Thi Thu Ha, Cao Duc Viet,

Nguyen Huyen Trang, Nguyen Thi Oanh, Le Thị Hoa

Institute for Nuclear Science and Technology, 179 Hoang Quoc Viet – Cau Giay – Hanoi

* Email: [email protected], [email protected].vn

(Received 16 December 2019, accepted 04 March 2020)

Abstract: Recovery efficiency of ¹³⁷Cs in seawater samples with different volumes of 40, 50, 60, 80

and 100 liters using co-precipitation method by ferrocyanide compounds has been determined. ¹³⁴Cs

nuclide was used as a tracer to determine recovery efficiency. The results showed that the recovery

efficiency of ¹³⁴Cs ranged from 92.62% to 99.26% with mean value of (95.22 ± 2.61)% for different

sample volumes. Average recovery efficiency for samples with a volume of 50 liters was (95.70 ±

2.50)% and uncertainty when determining ¹³⁷Cs in seawater samples were still less than 20%.

Therefore, reducing the volume of sample to 50 liters still ensures reliability when determining ¹³⁷Cs

in seawater samples by co-precipitation method, thereby reducing the chemical and time when analyze

a large number of samples.

Keywords: Recovery efficiency of ¹³⁷Cs, co-precipitation method, seawater, HPGe.

The

determination

of

activity

I. INTRODUCTION

concentration of radionuclides in seawater

samples is a complicated issue, because

their activity in seawater samples is very

low, a large number of samples are required

(about 200 - 400 liters) [9]. Currently, there

are various processes in the world for

preliminary enrichment of seawater samples

on the field before laboratory analysis. A

Huge amounts of anthropogenic

radionuclides have been introduced into marine

environment as global fallout from large-scale

atmospheric nuclear-weapon testing, discharge

from nuclear facilities and ocean dumping of

nuclear wastes [1]. Their radiological and

ecological effects are still of world concern [2].

¹³⁷Cs is one of the most important

anthropogenic radionuclides in the field of

number

of

have

successfully reduced seawater sample

volume to 20 liters [10, 11] or 10 liters [12]

to reduce chemical and sample processing

time while still ensuring accuracy.

environmental

radioactivity

monitoring

because of its long physical half-life of 30.02

years. It is a major fission product (fission

yield is about 6–7%) from both plutonium and

uranium [1]. ¹³⁷Cs in the ocean has been

mainly derived from global fallout [1, 2, 3, 4,

5], together with close-in fallout from the

Pacific Proving Ground nuclear explosions [3,

4], discharge of radioactive wastes from

nuclear facilities and others [6, 7, 8].

This paper presents the survey results

on the recovery efficiency of ¹³⁷C in seawater

samples by different volumes, determining

the activity of ¹³⁷C and uncertainty by co-

precipitation method in the field then

analysis in the laboratory of Center for

Environmental Radiation Monitoring and

DUONG DUC THANG et al.

Impact Assessment (CERMIA), Institute for

the enrichment method must be applied prior to

analysis in the laboratory.

Nuclear Science and Technology (INST).

Based on the results obtained, it is

recommended to analyze ¹³⁷C in seawater

samples with optimal volumes.

Seawater samples were taken on Co To

island, Co To District, Quang Ninh province,

the sampling locations are shown in Figure 1.

Samples were taken at the time of high tide to

avoid the influence from the mainland. In this

study, we took samples of surface seawater

with different volumes of 40, 50, 60, 80 and

100 liters, pretreated by co-precipitation

method in the field (Figure 2) according to the

procedure presented as shown below.

II. MATERIALS AND METHODS

Sampling and sample processing

The activity concentration of ¹³⁷Cs

currently in the East Sea is quite small. It

ranges from (1.16 ± 0.06) Bq/m³to (1.62 ±

0.15) Bq/m³[9], therefore, to determine ¹³⁷Cs,

Fig. 1. Location of seawater sampling on Co To island.

Fig. 2. Sample treatment.

41

DETERMINATION OF RECOVERY EFFICIENCY OF ¹³⁷Cs IN SEAWATERUSING …

Result of analyzing seawater samples

on Co To island in 2018 (State-level

Scientific and Technological Program

(KC.05.07/16-20)) shows that there is not

¹³⁴Cs in seawater on Co To island. Because

¹³⁴Cs and ¹³⁷Cs have same chemical

properties. So ¹³⁴Cs is often used to

determine the recovery efficiency for Cs in

seawater samples. In this study, the ¹³⁴Cs

radioactive solution was used as a tracer to

determine the recovery efficiency for ¹³⁷Cs.

- Add 6 g of NiCl₂, 40 g of CaCl₂, 25 g

of K₄Fe(CN)₆, mix well, leave for 3 hours.

- Add to the sample 800 g NH₄Cl, 400 g

Na₂CO₃, mix well, leave for 2 hours.

- Add to the sample 40 ml of FeCl₃15%,

mix well, leave for 12 hours.

- Discard the sample supernatant by

siphon, pour the precipitated substance into

5 L beaker.

For the process of precipitation of seawater

samples with a volume of 40, 50, 60 and 80 liters,

the amount of chemicals as well as ¹³⁴Cs to be

used will increase or decrease proportionally to the

volume of seawater samples.

For a 100-liter seawater sample, the

following procedure has been applied [9]:

- Pour the 100 L of seawater sample

into the water tank, add (3.33 ± 0.05) Bq of

¹³⁴Cs tracer.

After pretreatment on the site, the

precipitate is transferred to the laboratory. The

precipitate was filtered thought paper filter,

dried to dryness at 105⁰C and put into the

measuring box. Typical gamma spectra of

seawater samples are shown in Figures 3 and 4.

- Add 100ml of HCl (1:1) solution to

adjust the pH to 2 ÷ 3.

- Add 10ml of CsCl solution (100

mg/ml), 15ml BaCl₂(100mg/ml), mix well.

Fig. 3. The gamma spectrum represents the peak of 604.7 keV of ¹³⁴Cs.

42

DUONG DUC THANG et al.

Fig. 4. The gamma spectrum represents the peak of 661.7 keV of ¹³⁷Cs.

The samples were measured on a low

The recovery efficiency of ¹³⁴Cs was

used to determine ¹³⁷Cs activity in seawater

samples. The radioactivity concentration of

¹³⁷Cs was calculated as follows (IAEA,

International Atomic Energy Agency, 1989):

background gamma spectrometer with

CANBERRA's HPGe GC5019 detector with

energy resolution and relative efficiency at the

peak of 1332.5 keV of ⁶⁰Co of 1.8 keV and

50%, respectively. The spectrometer is

calibrated using the IAEA RGU-1, IAEA-

RGTh-1 and IAEA-soil 6 reference samples of

comparable geometry.

=

(2)

Where

is the activity

The activity of ¹³⁴Cs and ¹³⁷Cs in the

precipitation samples were determined,

respectively, through gamma rays of energy

604.7 keV and 661.7 keV with a time of

240000 seconds to ensure the counting

statistical error below 10%.

concentration of ¹³⁷Cs in seawater sample

(Bq/m³), N is the net peak area in the sample

spectrum, t is the live time of the sample

spectrum collection in seconds, ε is the

detector efficiency of the gamma-ray, γ is the

emission probability of the gamma line

corresponding to the peak energy, V is the

volume of the measured sample (m³), HS is the

recovery efficiency of ¹³⁴Cs (%).

The recovery efficiency of ¹³⁴Cs is

determined by the formula 1:

(1)

III. RESULTS AND DISCUSSION

Where HS is recovery efficiency (%),

is the initial activity of ¹³⁴Cs used as

10 water samples with the different

volumes have been analyzed. The recovery

efficiency of ¹³⁷Cs in those samples were

calculated and presented in the Table I.

the tracer (Bq),

is measured activity

of ¹³⁴Cs in precipitation (Bq).

43

DETERMINATION OF RECOVERY EFFICIENCY OF ¹³⁷Cs IN SEAWATERUSING …

Table I. Recovery efficiency of ¹³⁷Cs in seawater samples by different volumes.

Sample

M1

Volume (L)

Recovery efficiency (%)

2.29

2.47

2.24

2.17

2.27

2.25

2.26

2.03

1.93

3.25

40

50

60

80

100

92.62

99.26

98.50

94.54

93.15

93.41

98.92

93.93

93.39

94.48

±

M2

M3

M4

M5

M6

M7

M8

M9

M10

Table I shows that the recovery

efficiency varies from (92.62 to 98.92)%

with an average of (95.22 ± 2.61)%. In

which, the recovery efficiency of ¹³⁷Cs in

different volumes of 40, 50, 60, 80 and 100

liters are (95.94 ± 3.32)%, (95.70 ± 2.50)%,

(93.93 ± 2.03)%, (93.39 ± 1.93 )%, (94.48 ±

The average recovery efficiency of the

sample volumes of 40 and 50 liters are quite

similar, but for our method, the volume of 50

liters is optimal. Because the precipitates

obtained

from

40-liter

samples

are

approximately the same as the amount to be

measured, while the precipitation volume of

50-liter samples is always greater than the

amount to be measured. This makes the

recovery efficiency of the sample volume of 50

liters stable and has less uncertainty.

3.32)%,

respectively.

The

recovery

efficiency of ¹³⁷Cs in different volume of

seawater samples tends to decrease when the

volume increases.

Fig. 5. The recovery efficiency of ¹³⁷Cs versus sample volume.

44

DUONG DUC THANG et al.

Figure 5 and Table I shows that the

Table II shows that the activity

concentration of ¹³⁷Cs are in the range of (1.12

± 0.23) Bq/m³to (1.69 ± 0.21) Bq/m³with an

average of (1.42 ± 0.22) Bq/m³, with

uncertainty less than 20%. The activity

concentration of ¹³⁷Cs in different seawater

volumes of this study is similar to previous

publication [9].

recovery efficiency of ¹³⁷Cs in seawater by

different sample volumes is quite similar

and greater than 90%. The recovery

efficiency of the smaller volume seawater

samples are greater a bit than those of the

larger volume samples.

Activity concentrations of ¹³⁷Cs in

different seawater samples with different

volumes calculated by formula 2 are shown in

Table II.

Table III presents a comparison of the

recovery efficiency of ¹³⁷Cs in seawater in this

study with some results published before.

Table II. Activity concentration of ¹³⁷Cs in seawater samples.

Sample

Volume, L

Activity concentration ¹³⁷Cs (Bq/m³)

M1

M2

M3

M4

M5

M6

M7

M8

M9

M10

40

50

60

80

100

1.12

1.19

1.68

1.42

1.46

1.69

1.14

1.56

1.61

1.29

±

0.23

0.19

0.17

0.28

0.20

0.21

0.23

0.21

0.19

Table III. The comparison of the recovery efficiency.

Volume, L

Recovery efficiency (%)

10

20

40

50

60

80

100

93

99

±

3

K. Hirose [10]

Nakano [11]

J. Kamenik [12]

This work

90

÷

±

95

92.62

93.15

93.93

93.39

94.48

99.26

98.92

2.03

This work

1.93

This work

3.25

This work

From Table III, we find that the recovery

efficiency of ¹³⁷Cs in seawater in this study is quite

good and as similar as results of other papers.

IV. CONCLUSIONS

In this study, we achieved the initial goal

of determining the recovery efficiency of ¹³⁷Cs

45

DETERMINATION OF RECOVERY EFFICIENCY OF ¹³⁷Cs IN SEAWATERUSING …

“Anthropogenic radionuclides in seawater in

the East Sea/Japan Sea: Results of the first-

stage Japanese–Korean–Russian expedition”.

J. Environ. Radioact. 43, 1–13, 1999.

in seawater with different volumes from 40, 50,

60, 80 and 100 liters. The recovery efficiency of

¹³⁷Cs in seawater samples varied from 92.62%

to 99.26% with an average of (95.22 ± 2.61)%

and relatively good repeatability.

[5]. Reiter, E.R. Atmospheric Transport Processes.

“Part 4: Radioactive Tracers. Technical

Information Center”, U.S. Department of

Energy, Washington, DC, pp. 1–605, 1978.

The average recovery efficiency for

samples with a volume of 50 liters is (95.70 ±

2.50)% and the uncertainty when determining

the activity concentration of ¹³⁷Cs is still less

than 20%. Therefore, it is possible to reduce

the sample volume to 50 liters while

maintaining reliability when determining the

activity concentration of ¹³⁷Cs in seawater

samples by the co-precipitation method,

thereby reducing chemicals and time when

analyzing many samples at the same time.

[6]. Livingston, H.D., Povinec, P.P. “A millennium

perspective on the contribution of global

fallout radionuclides to ocean science”. Health

Phys. 82 (5), 656–668, 2002.

[7]. Pentreath, R.J. “Sources of artificial

radionuclides in the marine environment”.

Radionuclides: A Tool for Oceanography.

Elsevier, London, pp. 12–34, 1988.

[8]. Sugiura, Y., Saruhashi, K., Miyake, Y..

“Evaluation on the disposal of radioactive

wastes into the North Pacific”. Pap. Meteorol.

Geophys. 27, 81–87, 1975.

ACKNOWLEDGMENTS

The authors would like to thank Vietnam

Atomic Energy Institute (VINATOM) under

grant CS/19/04-05 and the State-level

[9]. N. Q. Long, L. D. Cuong, T. V. Giap, D. V.

Thang, D. D Thang, N. V. Khanh, D. X. Anh.

“The dispersion of artificial radionuclides in

seawater from the Fukushima nuclear

accident to the Eastsea, Vietnam”. The 12^th

regional conference on nuclear science and

technology, 2017.

Scientific

and

Technological

Program

(KC.05.07/16-20) for funding this study.

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