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 Table of Contents 
ORIGINAL ARTICLE
Year : 2014  |  Volume : 37  |  Issue : 1  |  Page : 2-5  

Radiological impact of phosphate fertilizers on the agricultural areas in Iran


1 Radiation Applications Research School, Nuclear Science and Technology Research Institute, Tehran, Iran
2 National Radiation Protection Department, Iranian Nuclear Regulatory Authority, Tehran, Iran

Date of Web Publication8-Dec-2014

Correspondence Address:
A A Fathivand
Radiation Applications Research School, Nuclear Science and Technology Research Institute, Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.146449

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  Abstract 

It is common practice to use phosphate fertilizers in the soil to raise crop yield. Natural Radionuclides present in fertilizers are sources of external and internal radiation exposure. External radiation exposure is caused by gamma radiation originating from radionuclides in situ. Internal radiation exposure, mainly affecting the respiratory tract, is due to short-lived daughter products of radon which are exhaled from fertilizers. This paper describes the results of gamma spectrometric measurement of the concentration of the natural radionuclides namely 226 Ra, thorium-232 and potassium-40 in the soil samples from the fields using phosphate fertilizers. The radon concentration and exhalation rate have also been discussed. The values of outdoor annual effective dose are found to vary from 0.07 to 0.09 mSv/year in soil samples containing fertilizers, whereas the outdoor annual effective dose is 0.06 mSv/year in barren soil samples.

Keywords: Exhalation rate, fertilizer, gamma spectrometry, radiation exposure, radon


How to cite this article:
Fathivand A A, Moradi M, Kashian S. Radiological impact of phosphate fertilizers on the agricultural areas in Iran. Radiat Prot Environ 2014;37:2-5

How to cite this URL:
Fathivand A A, Moradi M, Kashian S. Radiological impact of phosphate fertilizers on the agricultural areas in Iran. Radiat Prot Environ [serial online] 2014 [cited 2020 Apr 5];37:2-5. Available from: http://www.rpe.org.in/text.asp?2014/37/1/2/146449


  Introduction Top


Most people tend to be concerned about man-made radiation sources. In fact, exposures predominantly occur from natural sources of radiation, to which all living organism are unavoidably exposed. [1] In general, the earth's crust contains natural radionuclides, most of which are members of radioactive decay chains beginning with uranium-238 ( 238 U) and thorium-232 ( 232 Th). In addition, there are many human activities which can enhance naturally occurring radioactive materials (NORMs) levels, which include mining, milling, phosphate and fertilizer manufacture and use, burning fossil fuels, metal refining, etc. [2] NORMs became focus of regulatory interest with the publication of International atomic Energy Agency (IAEA) BSS-No. GSR Part 3. [3] It has increased the regulatory awareness of natural radiation and the NORM industries. The natural radioactivity in soil comes mainly from 238 U and 232 Th series and the radioactive isotope of potassium-40 ( 40 K). It is common practice to use phosphate fertilizers in the soil to increase crop yield. Phosphate fertilizers are used in huge amounts around the world and are essential for food production. [4] The radiological impact of the use of phosphate fertilizers in land is due to both external and internal radiation exposure to the NORM. Internal exposure occurs through inhalation of radon gas, and external exposure occurs through the emission of penetrating gamma-rays. In the light of the above-mentioned facts, it is essential to assess the radiological hazards of use of the fertilizers in fields to man and the environment. [5] This paper outlines the methodology used for determining the type and specific activity of naturally occurring radionuclides found in soil with fertilizers using gamma spectrometry and radon exhalation rate using a radon gas analyzer. The results obtained, and the possible radiological health significance of the results is discussed.


  Materials and methods Top


Soil samples were collected from the fields in which variety of phosphate fertilizers were used. Samples were also collected from the land areas where no fertilizers were used. All samples were first ground to a coarse powder, weighed and poured into polyethylene container of 300 cm−3 volume. The containers were well-sealed with celluloid tape for at least 3 weeks. No radon escape during this period was allowed to ensure the secular equilibrium between 226 Ra and 232 Th and their respective radioactive progenies. All samples were analyzed using a low level hyper pure germanium gamma-ray detector coupled to a PC with special electronic card to make it equivalent to a 4096 CANBERRA multi-channel analyzer. The energy resolution was 2 keV for 1332 keV gamma-ray of 60 Co with relative efficiency of 40%. A cylindrical lead (CANBERRA 747) with a moveable cover shielded the detector to reduce gamma-ray background. The energy calibration of the spectrometer was performed using a mixed standard of multi-nuclide source. Reference materials (RGU-1), (RGTh-1), and (RGK-1) for uranium, thorium, and potassium were prepared from IAEA and used for precise efficiency calibration. An empty container was analyzed for subtraction of background. The quality assurance of the measurement was carried out by daily measurement of energy and efficiency and by repeating the background and sample measurement. The counting time was 60000 s to obtain spectra with good statistics. The gamma-ray transitions energies of 352 keV ( 214 Pb) and 609 keV ( 214 Bi) were used to determine the concentration of 226 Ra. The gamma-ray transition energies of 583 keV ( 208 Tl) and 911 keV ( 228 Ac) were used to determine the concentration of 232 Th. [6],[7] The 40 K activities were determined from the line at 1460 keV. The distribution of 232 Th, 226 Ra, and 40 K in different types of soil is not uniform, so that with respect to exposure to radiation, the radioactivity has been defined in terms of radium equivalent activity (R aeq ) in Bq/kg to compare the specific activity of materials containing different amount of 226 Ra, 232 Th, and 40 K. It is calculated through the following relation: [8]

Ra eq = A 226 + 1.43 A 232 + 0.077 A 40

Where A 226 , A 232 , and A 40 are activity concentrations in Bq/kg of 226 Ra, 232 Th, and 40 K, respectively. To calculate the total absorbed gamma dose rate in the air at one meter above the ground level, the following relation was used. [9]

D (nGy/h) = 0.427 C Ra + 0.662 C Th + 0.043 C K

Where C Ra , C Th , and C K are the activity concentration (Bq/kg) of 226 Ra, 232 Th, and 40 K, respectively in the samples. The annual effective dose equivalent (AEDE) in μSv was calculated as follows:

AEDE = D × DCF × OF × T

Where D, DCF, and OF are absorbed dose rate in air, dose conversion factor (DCF) and outdoor occupancy factor (OF), respectively, and T is the time (8760 h/year). A DCF of 0.7 Sv/Gy and outdoor OF of 0.2 were used as recommended by the UNSCEAR. [10]

A radon gas analyzer (ALPHA GUARD 2000) from Genitron Instrument was used to measure radon emanation from the samples. The radon analyzer and samples were placed one at a time in an emanation and calibration cylindrical container with a removable gas-tight lid. The container dimensions were 45.0 cm in diameter and 54.3 cm in height. The average dimension of samples was around 12 × 12 × 12 cm and their mass varied from 0.5 to 0.7 kg. The concentration of radon emanated from each soil sample inside the emanation container was allowed to build-up with time and was measured in 1 h cycles for an average time of 50 h. The build-up of radon activity inside the emanation container follows the well-known equation:

A t = A 0 (1 − e−λt )

Where λ is the decay constant of radon (7.567 × 10 − 3 /h) and A 0 is the final value of the radon activity at t ~ seven half-lives of radon, which is approximately 27 days. Using a computer program for curve fitting the radon build-up, the final activity A 0 in Bq/m 3 was calculated. The radon exhalation rate per unit area of soil, E, is defined as a flux of radon released at the surface of the material. This was calculated using the following formula: [11]

E = A 0 λ (V/F)

Where V is the volume of the emanation container (50.4 × 10 − 3 m 3 ) and F is the total surface area of the samples. The unit of E is Bq/m 2 /h. In order to compare emanated radon with radium contents, a final activity of emanated radon per unit mass was calculated using following formula: [12]

C Rn = A 0 λ (V/m)

Where m is the mass of the sample, A 0 is the final activity, and V is the volume of the emanation container.


  Results and discussion Top


The activity concentration of natural radionuclides in samples with and without fertilizers and estimated Ra eq is shown in [Table 1]. The average activity concentrations of 232 Th, 226 Ra, and 40 K in the soil with fertilizers are 29.3, 33.4, and 714.4 Bq/kg, respectively, whereas in soil with no fertilizers are 23.1, 20.2 and 613.4 Bq/kg, respectively. For one standard deviation, the counting error varies between 2 and 6%. The concentrations of 232 Th, 226 Ra, and 40 K vary in different soil samples depending on the type and quantity of fertilizers used. As can be seen from [Table 1], the radium equivalent (Ra eq ) in the soil sample in which no fertilizers were used is less than the soil samples collected from lands where fertilizers were used. From all the soil samples analyzed, the R aeq value is well below the permissible limits of 370 Bq/kg which is recommended by OECD. [13] [Table 2] shows the calculated absorbed gamma dose rate in air and AEDE from soil samples. The AEDE (outdoor) is found to vary from 0.07 to 0.09 mSv/year in soil samples with fertilizers, whereas the values of AEDE (outdoor) is 0.06 mSv/year in soil samples with no fertilizers. The AEDE for soil samples with fertilizers is higher than the barren soil samples. The radon concentration and radon exhalation rates (surface and mass) from soil samples are presented in [Table 3]. As can be seen from [Table 3], the average radon concentration, mass exhalation, and surface exhalation rates in soil samples with fertilizers, are 224 Bq/m 3 , 140 mBq/kg/h, and 6.4 mBq/m 2 /h, respectively, whereas the value of radon concentration, mass, and surface exhalation rates are 90 Bq/m 3 , 57 mBq/kg/h, and 2.85 mBq/m 2 /h, respectively for soil samples with no fertilizers. The calculated values of radon concentration and exhalation rates for samples with fertilizers are higher than soil sample with no fertilizers.
Table 1: Activity concentration of radionuclides and radium equivalent in soil samples


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Table 2: Absorbed gamma dose rates and annual effective dose from soil sample


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Table 3: Radon concentrations and radon exhalation rates from soil samples


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  Conclusion Top


It was found that the contribution of radium and thorium to background radiation dose from agriculture fields is marginally higher owing to the application of phosphate fertilizers which results in increased levels of activity concentrations and radon exhalation rates. However, the levels of radium and radon contents of all samples are found to be well within the acceptable limits. The application of phosphate fertilizers, in general, does not significantly affect the dose received by the general public. The results indicate that consumers are likely to be exposed to marginally higher levels of radium when they use food stuffs grown on soils which use fertilizers. Effort may be made at national and international level to reduce radium activity in fertilizers.

 
  References Top

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Bennett BG. Exposures to natural radiation worldwide. Proceeding of Fourth International Conference on High Levels of Natural Radiation. Beijing, China: Elsevier Science; 1997. p. 15-23.  Back to cited text no. 1
    
2.
O'Brien RS, Cooper MB. Technologically enhanced naturally occurring radioactive material (NORM): Pathway analysis and radiological impact. Appl Radiat Isot 1998;49:227-39.  Back to cited text no. 2
    
3.
IAEA. International Atomic Energy Agency. International Basic Safety Standards for Radiation Protection and Safety of Radiation Sources, Safety Standards Series No. GSR Part 3. Vienna: IAEA; 2014.  Back to cited text no. 3
    
4.
Eisenbud M, Gessell T. Environmental Radioactivity. 4 th ed. USA: Academic Press; 1997. p. 172-4.  Back to cited text no. 4
    
5.
IAEA. International Atomic Energy Agency. Safety Report Series for Radiation Protection and Management of Norm Residues in the Phosphate Industry, Safety Report Series No. 78. Vienna: IAEA; 2013.  Back to cited text no. 5
    
6.
Savidou A, Raptis C, Kritidis P. Natural radioactivity and radon exhalation from building materials used in Attica region, Greece. Radiat Prot Dosimetry 1995;59:309-12.  Back to cited text no. 6
    
7.
Sorantin H, Steger F. Natural radioactivity of building materials in Austria. Radiat Prot Dosimetry 1984;7:59-61.  Back to cited text no. 7
    
8.
Beretka J, Matthew PJ. Natural radioactivity of Australian building materials, industrial wastes and by-products. Health Phys 1985;48:87-95.  Back to cited text no. 8
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9.
Beck HL. The physics of environmentalradiation field. Natural radiation environment II. CONF-72085 P2. Proceeding of the 2 th international symposium on the natural radiation environment; 1972.  Back to cited text no. 9
    
10.
UNSCEAR. United Nation Scientific Committee on the Effect of Atomic Radiation. Sources and Effect of Ionizing Radiation. United Nation, New york; 1988.  Back to cited text no. 10
    
11.
Barton TP, Ziemer PL. The effects of particle size and moisture content on the emanation of Rn from coal ash. Health Phys 1986;50:581-8.  Back to cited text no. 11
[PUBMED]    
12.
al-Jarallah M. Radon exhalation from granites used in Saudi Arabia. J Environ Radioact 2001;53:91-8.  Back to cited text no. 12
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OECD. Organization of Economic Cooperation and Development. Exposure to radiation from the natural radioactivity in building materials. Report by group of expert of the OECD nuclear energy agency, Paris, France; 1979.  Back to cited text no. 13
    



 
 
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  [Table 1], [Table 2], [Table 3]



 

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