Natural Radioactivity In The Cultivated Land Around A Fertilizer Factory

The Second All African IRPA Regional Radiation Protection Congress 22-26 April 2007 Ismailia Egypt

EG0800200 Natural Radioactivity In The Cultivated Land Around A Fertilizer Factory A. Hamdy, H. M. Diab, S. A. El-Fiki and S. A. Nouh [email protected] ABSTRACT Natural radioactivity is a part of our natural surrounding. Concentrations of natural radionuclides in the environment increase with the development of technological activities such as mining operations, chemical and fertilizer factories. Phosphate materials used for production of phosphate fertilizers contain a minor quantity of radioactive material, mainly various members of the uranium and thorium series, and radiopotassium. The aim of the present work is to determine the levels of some natural radionuclides in soil samples collected from the cultivated area around to the phosphate fertilizer plant in Abu-Zabal area. The specific activities of radium (226Ra), thorium (232Th), and radiopotassium (40K) have been determined using gamma spectrometry. Also, the environmental impact of fertilizer industry on the surrounding site has been estimated and the estimated annual effective doses to the public living near the factory have been estimated. Key words: Radioactivity, pollution, monitoring, phosphate fertilizers INTRODUCTION Phosphate ores contain, due to geological reasons, an important amount of natural radioactivity, 238

232

40

mainly U and Th series and K, which is the major naturally occurring radionuclide [1]. The phosphate material is very insoluble, and therefore, in its original state is practically unavailable as a plant phosphorus source [2]. The operation of phosphate fertilizer factories clearly enhanced the natural radiation levels of its close environment. Exposure of workers and the public to radiation from phosphate rock and fertilizer is therefore not unlikely. The European Commission has issued a draft proposal for revision of the Basic Safety Standards for the protection of workers and the general public against the dangers of ionizing radiation [3]. Phosphate ores, especially sedimentary ores, can be significantly enriched with naturally occurring radionuclides; uranium (U-238) and the ‘daughter’ radionuclides that come from the radioactive decay of U-238. Processing of these ores into fertilizer products and phosphoric acid results in the contamination of the products and waste materials with many of these radionuclides [4]. This study was carried out around a fertilizer factory in Egypt, which may represent a site of significant environmental contamination due to fertilizer production and phosphogypsum deposition in the area. This study aims to assess the impact of phosphate industry on the radiation level in the environment of the region. MATERIALS AND METHODS 1 Sampling and sample preparation The phosphate fertilizer factory is located in Abou-Zabal area between latitudes 31.36903 & 31.38784 and between longitudes 30.26522 & 30.28175. The factory is around 30 km from Cairo. The factory is surrounded by farmlands, owned by farmers. Ismalia Lake is the main source of water for


the irregation. Most of the cultivated area considered the main source of food for the public leave in Cairo. Soil samples were collected from the area around the factory taking into account the wind direction. Over the selected area, some shore sediment and water samples were collected from the lake. Figure 1 represents the studied area and the samples locations. The samples were prepared for analysis by drying in an oven at 115oC. The samples were mechanically crushed and sieved through 0.8 mm mesh sieve. The sieved portion of the sample was transferred into a 100 ml Marinelli beaker for gamma spectrometry and sealed for four weeks to reach secular equilibrium between the thorium and radium contents of the sample and their daughters [5]. 2 Instruments A HpGe gamma-spectrometer with 40% efficiency and 2.0 KeV resolution at 1.33 Mev photons of Co-60, shielded by 4’’Pb 1 mm Cd and 1 mm Cu linked up to a multichannel analyzer was used for gamma measurements. The system was calibrated and the calibration quality control was carried out by using a standard reference materials soil (IAEA-226 and IAEA-375) whose concentration of natural radioactivity has been certified by the IAEA. 3 Radiation hazard indices To represent the activity levels of 226Ra, 232Th and 40K by a single quantity, which takes into account the radiation hazards associated with them, a common radiological index has been introduced. This index is called radium equivalent (Ra-eq) activity and is mathematically defined by [6] : Raeq(Bq kg−1) = ARa + 1.43 ATh + 0.077 AK , where ARa, ATh and AK are the activity concentrations of 226Ra, 232Th and 40K. In the above relation, it has been assumed that 10 Bq kg−1 of 226Ra, 7Bq kg−1 of 232Th and 130 Bq kg−1 of 40K produce equal gamma dose. The absorbed dose rates due to gamma radiations in air at 1m above the ground surface for the uniform distribution of the naturally occurring radionuclides (226Ra, 232Th and 40 K) were calculated based on guidelines provided by UNSCEAR 2000 [7]. We assumed that the contributions from other naturally occurring radionuclides were insignificant. Therefore, D can be calculated according to UNSCEAR 2000. D(nGy h−1) = 0.462 ARa + 0.621 ATh + 0.0417 AK To estimate the annual effective dose rates, the conversion coefficient from absorbed dose in air to effective dose (0.7SvGy−1) and outdoor occupancy factor (0.2) proposed by UNSCEAR 2000 are used. Therefore, the annual effective dose rate (mSv yr−1) was calculated by the following formula: Effective dose rate (mSv yr−1) = D(nGy h−1) × 8760 h yr−1 × 0.7 × (103 mSv /109) nGy × 0.2 = D (mSv yr−1) × 1.23 × 10−3 A widely used hazard index called the external hazard index Hex is defined as follows [8]: Hex= (ARa/370) + (ATh/259) + (AK/4810) In addition to external hazard index, radon and its short-lived products are also hazardous to the respiratory organs. The internal exposure to radon and its daughter products is quantified by the internal hazard index Hin, which is given by the equation Hin= ARa/185+ ATh/259+AK/4810 The values of the index (Hex, Hin) must be less than unity for the radiation hazard to be negligible. RESULTS AND DISCUSSION

232

From the gamma spectrometric analysis, three naturally occurring radionuclides were chosen: 40

Th and K. The activity concentrations of

226

Ra,

232

40

226

Ra,

Th and K in the raw material were found to be


226

232

40

1180.6 Bq/kg, 16 Bq/kg and 1582 Bq/kg. The average activity concentration of Ra, Th and K in the collected soil samples were 29.4 Bq/kg ranged from (6 - 87.5) Bq/kg, 10.3 Bq/kg ranged from (4 19.3) Bq/kg and 271.3 Bq/kg ranged from (71.8 - 781.8) Bq/kg. Figures 2a, 2b, and 2c show the activities in soil samples around the factory. These data show that, the activity concentration of naturally occurring radionuclides in soil samples were within the world average ranges in soils, which are 35(10-50), 35(7-50) and 370(100-700) Bqkg-1 for (226Ra), 232Th and 40K respectively [7], and the exceed of 226Ra in some samples may be due to their neighboring of the stack of phosphate fertilizers factory (450 m far from the stack). However, the samples that locate in the zone of 500 m from the phosphate fertilizers stack shows higher concentrations than those locate from 500 m to 1200 m (this is the longest sampling zone) as shown in Fig. 2. This indicates that the higher concentrations levels are mainly due to fallout of phosphate dust generated during loading and processing of phosphate ore inside the phosphate fertilizers plant, which obvious increases the atmospheric release of the particles into the area affected by this industry. The activity concentrations of the four shore sediments collected from Al-Ismailia canal were ranged from (12.6±1.33 to 31.7±3.2) Bq kg-1 for 226Ra, from (5.5±1.1 to 10±1.6) Bq kg-1 for 232Th and from (71.8±24 to 550±10.5) Bq kg-1 for 40K, and were found to be in good agreement with the average world level. The gamma dose rates due to naturally occuring terrestrial radionuclides (226Ra, 232Th and 40K) were calculated based on their activities in soil samples, determined by gamma-ray spectrometry. The total absorbed gamma dose rate due to these radionuclides varied from 9.8 to 58.9 nGy h–1 with mean of 31.1 nGy h–1. The corresponding annual effective dose varied from 0.05 to 0.36 mSv with mean of 0.19 mSv, i.e. the dose was lower than the world wide average value. The external hazard index Hex was calculated. It ranged from 0.055 to 0.33, with an average value of 0.17. The internal hazard index Hin was also calculated, the values of Hin ranged from 0.072 to 0.570, with an average value of 0.255. The total hazard index was ranged from 0.12 to 0.9 with average 0.8. The values of Hex and Hin of all samples studied in this work are less than unity. CONCLUSION The assessment of the radioactive releases into the environment caused by a production plant of complex fertilizers is very important. The authors determined the concentrations of natural radioactivity present in raw material and the surroinding area around the factory. Although it is not possible to draw direct links between radionuclide contamination of fertilizers and health effects resulting from the consumption of food grown on soil amended with the fertilizer, there is clearly cause for concern over the use of such fertilizers on agricultural land. Because the radionuclide activity concentrations in most of the process materials are only slightly above levels in soil, the need for specific measures to control adiological hazards to individuals and the environment is very limited. In most cases, normal ccupational health and environmental protection measures designed for non-radiological hazards will be sufficient to protect against radiological hazards as well. REFERENCES (1) UNSCEAR (1988), Sources effects and risks ionizing radiation, United Nation Scientific Committee on the effects of Atomic Radiation. United Nations, New York, USA. (2) A.V.Slack: Fertilizer Products. In: The Fertilizer Handbook 47-65 (1972).


(3) EC (European Commission), 1997. Radiation protection 88, “Recommendations for the implementation of Title VII of the European Basic Safety Standards Directive (BSS) concerning significant increase in exposure due to natural radiation sources. DirectorateGeneral Environment, Nuclear Safety and Civil protection. Official Publication of the European Communities. (4) L. C. Scholten and C. W. M. Timmermans, “Natural radioactivity in phosphate fertilizer”, Nutrient Cycling Agrecosystem, Vol. 43 No. 1-3, P 103-107, 2005 (5) Khater, A.J. (2004) Environ. Radioact., Vol. 71, pp.33–41. (6) Beretka, J.; Mathew, P. J., ”Natural Radioactivity of Australian Building Materials, Industrial Wastes and By-products”, Health Physics. 48(1):87-95, January 1985. (7) UNSCEAR (2000), Sources effects and risks ionizing radiation, United Nation Scientific Committee on the effects of Atomic Radiation. United Nations, New York, USA.

Fig. 1: The distribution of samples in the studied area


Fig. (2a) Distribution of 226Ra (Bq kg-1) in the collected soil samples

Fig. (2b) Distribution of 232Th (Bq kg-1) in the collected soil samples


Fig. (2C) Distribution of 40K(Bq kg-1) in the collected soil samples