Natural Radioactivity Assessment of El- Sebeaya Low Grade Phosphate Rock and Hydrochloric Acid Leaching Residue

Transcription

1 doi: /ijnese Natural Radioactivity Assessment of El- Sebeaya Low Grade Phosphate Rock and Hydrochloric Acid Leaching Residue 1 Abdel-Hakim T. Kandil, 2 Hamed I. Mira, 2 Mohamed H. Taha, 2 Mohamed F. Kamel *dr.mohamedp@gmail.com Abstract Radionuclides concentrations of ( 238 U, 226 Ra, 232 Th and 40 K) maintained in El-Sebeaya low grade phosphate rock and the leaching residue samples were measured using Gamma-ray spectrometer. The radium equivalent activity, the external-internal hazard index, the absorbed dose, the exposure rate, the outdoor and indoor effective dose for El-Sebeaya low grade phosphate rock sample were 20.5 Bq kg -1, 0.06, 0.089, 9.5 ngy h -1, µr h and 0.01 msv y 1 respectively, and for leaching residue sample were Bq kg -1, 3.43, 6.83, 585.8nGy h -1, µr h and 0.72 msv y 1 respectively. These results indicated that, the El-Sebeaya low grade phosphate rock is acceptable for use as agricultural fertilizers material. On the other hand, the leaching residue sample shows a normal level for all indices factor's except Radium Equivalent Activity and the external and internal hazard exceeds the permissible limits. Keywords Radionuclide; Radiological Hazard; El-Sebeaya; Phosphate Rock; Hydrochloric Acid; Leaching Introduction Phosphoric acid, H3PO4, is second only to sulfuric acid in volume produced in the world. About 80 % of the rock phosphate is being used as raw material for the manufacturing of phosphate fertilizers (Becker, 1989). Phosphate deposits are classified into two main categories: igneous phosphate rocks (13%) as found in Russia, South Africa, Brazil and sedimentary phosphate rocks (87%) as found in Morocco, Algeria, Jordan, Egypt, and U.S.A (European Fertilizer Manufacturers Association, 2000). FIG. 1 LOCATION OF PHOSPHATE DEPOSITS IN EL-SEBEAYA IN EGYPT Phosphate ores occur in Egypt in three main provinces, Western desert, Nile valley, and Red sea. In addition, there are some phosphate- bearing sediments are present in WadiQena, WadiAraba, Esh El-Mallana range and Sinai (Nuclear Materials Authority, 1992; Zidan, 1998). El-Sebeaya plateau is located between latitudes ` ` 26

2 International Journal of Nuclear Energy Science and Engineering, Volume 6, and longitudes ` ` on both sides of the Nile Valley. There are many phosphate bearing localities in this region. El-Mahamid and El-Sebeaya are considered the most important areas in this location Figure 1 (Elmaadawy, Ezz El Din, Khalid, &Abouzeid, 2015). Several studies have investigated that phosphate rocks maintain various amount of natural occurring radioactive materials (NORM); uranium, thorium, theirdecay products and potassium (Olszewska-Wasiolek, 1995). The radioactivity of phosphate ores (due to uranium, thorium and radium) can be as high as 10,000 Bq/kg. In sedimentary phosphate ores, the concentrations of individual 238 U series radionuclides generally range from about 0.5 to 3 Bq/kg whereas the concentrations of 232 Th series radionuclides are generally not significantly elevated above values found in normal rocks and soils. However, in igneous phosphate ores, the concentrations of both the 238 U and the 232 Th series radionuclides are elevated (to about Bq/kg and Bq/kg, respectively). When compared with sedimentary phosphate deposits, the 238 U series radionuclide concentrations are lower but the 232 Th series radionuclide concentrations are higher (Korkmaz, &Turgut, 2005; IAEA, 2013). The most important sources of external and internal exposure are the gamma radiation and alpha particles emitted from the radionuclides of the uranium ( 238 U) series, thorium ( 232 Th) series and 40 K present within phosphate rocks. External exposure occurs directly by gamma-rays, whereas internal exposure to α-particles results from the inhalation of radon and its progenies. Mining and processing of phosphate ores redistribute those radionuclides among the various products, byproducts and wastes ofthephosphateindustry.effluentdischargesintotheenvironmentaswellastheuseof phosphate fertilizers in agriculture and by-products, such as gypsum (phosphogypsum) in the building industry, are possible sources of radiation exposure to the public (Saueia, &Mazzilli, 2006; Sabiha-Javied, Tufail, &Asghar, 2010). Therefore, radiation released from NORMs (phosphate ores and fertilizers) has a theoretical potential to cause cancers in individuals who are exposed at significant levels so that monitoring of natural radioactivity in human surroundings is primary importance from the viewpoint of radiation protection(nazaroff, 1992). During the last decade, there have been numerous published articles which indicate that phosphate deposits reserves are expected to be depleted in years and peak phosphate is expected to be reached in (Déry, & Anderson, 2007; De Bruijne, & Caldwell, 2009; Rockström, Steffen, Noone, Persson, Chapin, Lambin, & Foley, 2009; Vaccari, 2009). In this concern, it essentially means that low-grade resources, presently considered uneconomical have to be developed to meet future demands. Therefore, a sustainable use of these resources is of prime importance. Dissolution of phosphate rocks to obtain phosphate fertilizers is carried out by acidulation using mineral acids. These acids are sulfuric, nitric, or hydrochloric acid. While sulfuric acid acidulation is the most popular industrial process developed, yet it suffers from the disadvantage of producing enormous amount of phosphogypsum contaminated with radioactive radium (Becker, 1989). Hydrochloric acid can be used instead of sulfuric acid in phosphate ore decomposition process. The reaction between phosphate rock and hydrochloric acid yield acidulate solution containing phosphoric acid and calcium chloride according to equation (1.5): Ca 10 F 2 PO 20HCl 6H PO 0CaCl 2HF...(1.5 ) 4 6 Generally, decomposition of phosphate rock by using hydrochloric acid is followed by recovering of phosphoric acid (Takhim, 2005; Aly, Ali, &Taha, 2013). When phosphate rock is digested with sulfuric acid to produce phosphoric acid (the primary starting point for the manufacture of several major phosphate products), some of the radionuclides, particularly isotopes of radium, become concentrated in phosphogypsum (IAEA, 2013). While in the digestion of phosphate rock with hydrochloric acid, little is known about the fate of the radionuclides introduced via the phosphate rock feedstock. The chemical conditions suggest that all the radionuclides, including

3 uranium and radium, dissolve in the hydrochloric acid with only trace quantities remaining in the residue consisting of small amounts of silica, silicates and insoluble organic matter (IAEA, 2013). In this concern, the aim of the present work is to estimate the activity concentration of radionuclides 238 U, 226 Ra, 232 Th, and 40 K in the working sample collected from El-Sebeaya low grade phosphate rock and the leaching residue after dissolution by hydrochloric acid. In addition, to evaluate the radiological indices and their effects on the population living in this environent. Experimental Sample Preparation Representative sample of El-Sebeaya low grade phosphateore and phosphate leaching residue-after dissolution with hydrochloric acid (Aly, Ali, &Taha, 2013), were collected. The selected sample was crushed to a fine powder form and sieved through a1 mmmeshsizetoremovethelargergrainssizeandtherenderthemwillbemorehomogenous. Then, it was dried in a temperature controlled furnace (oven) at 110 o C for 24 hours to ensure that moisture is completely removed. After moisture removal, the sample was cooled in a desiccator to be ready for radionuclides and radon measurements. Energy and Efficiency Calibrations The calibration of the gamma ray detection system always includes two stages: the energy and the efficiency calibrations. The energy calibration converts channel numbers to γ -ray energy in MeV and the efficiency calibration aimed to determine the gamma ray counting efficiencies over the full energy range of measurement. The energy and efficiency calibrations of the detection system were performed using a set of high quality certified reference sources (IAEA, RG-set) with density similar to the densities of the measured samples. For calibrations, the reference source was placed in the same place as the samples when measuring their γ -ray spectra. For energy calibration, the amplifier gain has been adjusted to measure γ -rays in the energy range of 100 kev up to 2600 kev. Measurement of Radionuclide Concentrations (238U, 226Ra, 232Th and 40K) with Gamma-Ray Spectroscopy After the samples were prepared, they were packed in a plastic containers, normal cylindrical plastic container (6 cm diameter and 8 cm height) made from polyethylene. The containers were sealed using adhesive to avoid any possibility of out gassing of radon from them. Each sample was stored in a sealed container for 30 days to achieve radioactive secular equilibrium between 238 U and its daughters. An empty container with the same geometry was sealed and left for 30 days in order to measure background. Since radium ( 226 Ra) and its progeny produce 98.5 % of the radiological effects of uranium series, the contribution of 238 U and the precursors of 226 Ra are normally ignored. Therefore, the reference of 238 U series radionuclides is often 226 Ra instead of 238 U (Farai, &Ademola, 2005). 226 Ra concentration was measured from the kev γ-peaks of 214 Pb, kev and kev γ- peaks of 214 Bi. 232 Th concentration was estimated from kev γ-peak of 228 Ac and and kev of 208 Tl. 40 K concentration was estimated using 1460 kev γ-peak from 40 K itself. The specific activity concentration (Bq kg -1 ) of those radionuclides was calculated from the following Equation (Farai, &Ademola, 2005). where, C is the net count above the background, p is the absolute emission probability of the gamma ray decay, w is the net dry sample weight (kg), t is the measurement time, and ε is the absolute efficiency of the detector. Results and Discussion Phosphate deposits contain the naturally occurring radionuclides 238 U and 232 Th together with their decay progeny. The presence of these radionuclides of natural origin creates a potential need to control exposures of workers and members of the public in accordance with IAEA safety standards. The mining of phosphate ore, the processing of it 28

4 International Journal of Nuclear Energy Science and Engineering, Volume 6, into intermediate, end products, the handling and use of those products can all give rise to exposures of workers and members of the public. When phosphate rock is digested with acid to produce phosphoric acid, some of the radionuclides, particularly isotopes of radium, become concentrated in residues. The environmental radiation impact as well as radiation hazard indices factors (exposure rate and absorbed dose rate, effective dose rate, the radium equivalent activity (Raeq), external hazard, internal hazard and (level index) I- gamma) have been measured and calculated. Activity Concentrations of 238U, 232Th, 226Ra and 40K The measured radioelement concentration 238 U, 232 Th, 226 Ra and 40 K and specific activities in El-Sebeaya low grade phosphate rock and leaching residue samples were measured and presented in Tables 1 and 2. From the Tables, it is clear that, the concentration of natural radionuclides of concentration 238 U, 232 Th, 226 Ra and 40 K are 1.0, 23.0, 1.0, and 0.1 ppm for El-Sebeaya low grade phosphate rock sample and 102.0, 10.0, 1.0, and 0.1 ppm for leaching residue sample. The specific activities of 238U, 226Ra, 232Th, and 40K for El-Sebeaya low grade phosphate rock sample are 12.35, , 4.06 and 31.3 respectively. While the specific activities of 238U, 226Ra, 232Th, and 40K for leaching residue sample are , 123.5, 4.06 and 31.3 respectively. TABLE 1 THE MEASURED RADIOELEMENT CONCENTRATION 238U, 232TH, 226RA AND 40K Sample 226Ra, ppm 238U, ppm 232Th, ppm 40k, % El-Sebeaya Phosphate Leaching residue TABLE 2 THE SPECIFIC ACTIVITIES OF 238U, 232TH, 226RA AND 40K Sample 226Ra, Bq/Kg 238U, Bq/Kg 232Th, Bq/Kg 40k, Bq/Kg El-SebeayaPhosphate Leaching residue World limit (UNSCEAR, 2008) Committee on the Effects of AtomicRadiation, UNSCEAR, 2008 (UNSCEAR, 2008) reported atypical 238U and 226 Raconcentrations in phosphate ores of 1500Bqkg-1.This means that both of El-Sebeaya low grade phosphate rock and leaching residue samples show uranium and radium concentrations in the same range of UNSCEAR s typical values. Moreover, Radionuclides concentrations of 232Th,and 40K maintained in El-Sebeaya low grade phosphate rock and leaching residue sampleshave specificactivities withintherange of the worldwide values of radionuclide concentrations (7-50 Bq/kg) for 232Th and ( Bq/kg) for 40K (UNSCEAR, 2000). Dose Assessment and Radiological Effects 1) Radium Equivalent Activity (Raeq) Radium equivalent concentration (Raeq) is a common index to compare specific activities in materials (Beretka, & Mathew, 1985). where, ARa, AThandAK arespecificactivitiesof226ra,232th,and40k,respectively,inbqkg-1. Radium equivalent concentration was calculated based on the estimation that 370Bqkg-1of226Ra, 259Bqkg-1of232Thand4810Bqkg- 1of40K produce the equivalent gamma dose. The value of RaeqinEl-Sebeaya low grade phosphate rock and leaching residue samplesmustbelessthan370bqkg-1tokeepγ-raydosebelow1.5msvy -1. The calculated date of radium equivalent concentration in El-Sebeaya low grade phosphate rock and leaching residue samples are presented in Table 3. 29

5 TABLE 3 THE RADIUM EQUIVALENT ACTIVITY INDEX, RAEQ Sample Raeq, Bqkg-1 El-Sebeaya Phosphate 20.5 Leaching residue World limit (UNSCEAR, 2008) 370 The value of Raeq for El-Sebeaya low grade phosphate rock and leaching residue samples were 20.5 and Bq kg-1 respectively. This means that El-Sebeaya low grade phosphate rock samples showed greater hazard index values than the recommended world values while the leaching residue sample showed higher value than there commended worldwide mean value of370bqkg-1 (Beretka, & Mathew, 1985). 2) The External and Internal Hazard Index (Hex&Hin) The external hazard index (Hex) was determined from the criterion formula as follow (Krieger, 1981) where, ARa, ATh and AK are specific activities of 226Ra, 232Th, and 40K, respectively. To limit the external gamma radiation dose to 1.5mSvy-1external hazard index must be less than unity in order to maintain the radiation hazard negligible. The calculated external hazard indexes for El-Sebeaya low grade phosphate rock and leaching residue samples were tabulated in Table 4. TABLE 4 THE EXTERNAL AND INTERNAL HAZARD INDEX FACTORS, (HEX&HIN) Sample Hex Hin El-SebeayaPhosphate Leaching residue World limit (UNSCEAR, 2008) 1 1 The results obtained from the Table show that, the external hazard index for El-Sebeaya low grade phosphate rock and leaching residue samples are 0.06 and 3.43 respectively. This means that El-Sebeaya low grade phosphate rock sample less than there commended values. On the other hand, the leaching residue sample was greater than there commended values. In addition to the external hazard, radon and its short-lived products are also hazardous to respiratory organs. The internal exposure to radon and its progeny produce disquantified by an internal hazard index (Hin) which can be defined as (Krieger, 1981); For the safe use of a material, Hin should be less than unity. The internal hazard index for El-Sebeaya low grade phosphate rock is and leaching residue samples is This means that El-Sebeaya low grade phosphate rock sample is within the world safe limit while the leaching residue sample was higher than the safety limit. 3) Radioactivity Level Index (Iγ) Theγ-ray radiation hazards associated with the natural radionuclides maintained in the selected materials can be assessed by means of radioactivity level index, Iγ. According to the European Commission guidelines, for safe use of phosphateore, radioactivity level index should be less than 6 (Iγ< 6) for radiation dose of 1 msvy - 1 (Rafique, Rehman, Matiullah, Malik, Rajput, Rahman, &Rathore, 2011). Radioactivity level index (Iγ) can be calculated from this equation (European Commission, EC). 30

6 International Journal of Nuclear Energy Science and Engineering, Volume 6, The selected samples have radioactivity level index (Iγ) less than six, as shown in Table 5. TABLE 5 THE RADIOACTIVITY LEVEL INDEX, IΓ Sample Iγ El-Sebeaya Phosphate 0.07 Leaching residue 4.23 World limit (UNSCEAR, 2008) 370 The workers in the phosphate mines and fertilizer factors are being exposed to radiation from two ways; one is due to the external γ-ray radiation (discussed previously) and the other is due to the inhalation of long-lived α- emitters from radon radioactive gas and its daughters associated with dust particles (UNSCEAR, 2008). Therefore, it is very important to determine the released radon from selected materials and its emanation coefficient and its exhalation rate as well. 4) Gamma-absorbed Dose Rate (D) The absorbed dose rates due to gamma radiations in air at 1 m above the ground surface for the uniform distribution of the naturally occurring radionuclides (226Ra,232Th and40k,) were calculated based on guidelines provided by UNSCEAR 2000 (UNSCEAR, 2000). The conversion factors used to compute absorbed gamma dose rate (D) in air per unit activity concentration in Bq kg -1 (dry weight) corresponds to ngy h -1 for 226Ra, ngy h -1 for 232Th and ngy h -1 for 40k. Therefore, D can calculate as follows (Ross, & Riaz, 2008): where, ARa, ATh and AK are the specific activities of226ra,232th and40k, respectively. The absorbed dose was calculated for the representatives amples as seen in Table 6. TABLE 6 THE GAMMA-ABSORBED DOSE RATE, D Sample Dose rate (ngy h -1 ) El-Sebeaya Phosphate 9.5 Leaching residue World limit (UNSCEAR, 2000) 70 From the Table it is clear that, the absorbed dose for El-Sebeaya low grade phosphate rock sample is 9.5 ngy h - 1 and585.8 ngy h -1 for the leaching residue sample. UNSCEAR, 2000 (UNSCEAR, 2000), indicated that the world average limit value is 70nGy h -1.Thus,the leaching residue sample is higher than the world dose rate limit while El-Sebeaya low grade phosphate rock sample is below that value. 5) Annual Effective Dose Equivalent, AEDE To estimate the AEDE to γ-ray emitted from radionuclides of 226Ra, 232Th and40k in the El-Sebeaya low grade phosphate rock sample and leaching residue samples, the conversion factor (0.7 Sv Gy -1 ) from absorbed dose rate in air in ngy h -1 to effective dose rate in msv y -1 is used with outdoor occupancy factor of 0.2 and indoor occupancy factor of 0.8. The AEDE was calculated using the following formulae (UNSCEAR, 2000): AEDE ( Indoor)( msvyr ) Dair ( ngyh ) 8766 h Sv Gy

7 AEDE ( outdoor )( msvyr ) Dair ( ngyh ) 8766h Sv Gy 0 6 These indices measure the risk of stochastic and deterministic effects in the irradiated individuals. The recommended value of the annual effective dose equivalent is 0.5 msv y -1 and the criterion of the total annual effective dose equivalent (indoors and outdoors) should be less than 1 msv y -1 (UNSCEAR, 2000). The calculated total Annual effective dose equivalent AEDE, indoor AEDE, and outdoor AEDE values are quoted in Table 7. TABLE 7 TOTAL ANNUAL EFFECTIVE DOSE EQUIVALENT, AEDE AND INDOOR AND OUTDOOR AEDE Sample Indoor AEDE, msvy -1 Outdoor AEDE, msvy -1 El-Sebeaya Phosphate Leaching residue World limit (UNSCEAR, 2000) The results of outdoor and indoor effective dose for El-Sebeaya low grade phosphate rock sample were 0.05 and 0.01 msv y 1 respectively, while the outdoor and indoor effective dose for leaching residue sample were 2.88 and 0.72 msv y 1, respectively. It can be seen that the above-mentioned values were within the corresponding worldwide values respectively (UNSCEAR, 2000). 6) Exposure Rate (ER) The exposure rate was calculated using the following relation (Akhtar, Tufail, Ashraf, Mohsin, &Iqbal, 2005): where, ARa, ATh and AK are the specific activities of226ra,232th and40k, respectively. The calculated exposure rate for both of El-Sebeaya low grade phosphate rock and leaching residue samples were calculated and presented in Table 8. TABLE 8 THE EXPOSURE RATE (µr H-1) Sample Exposure rate (µr h -1 ) El-Sebeaya Phosphate Leaching residue World limit (UNSCEAR, 2008) 50 From Table 8 it is clear that, the values of exposure rate for El-Sebeaya low grade phosphate rock and leaching residue samples are and µr h -1 respectively. In reference to the world exposure rate which was (50 µr h -1 ) (UNSCEAR, 2000), it is clear that, the exposure rate El-Sebeaya low grade phosphate rock samples are lower than the world exposure rates but the leaching residue samples are higher than the world exposure rates. Conclusion The environmental radiation impact as well as radiation hazard indices factors for El-Sebeaya low grade phosphate rock and leaching residue samples have been investigated. The obtained results clarified that, El-Sebeaya low grade phosphate rock and leaching residue samples have radioactivity level index (Iγ) less than six. The radium equivalent activity, the external-internal hazard index, the gamma absorbed dose rate, the exposure rate, the outdoor and indoor effective dose for El-Sebeaya low grade phosphate rock sample were 20.5 Bq kg -1, 0.06, 0.089, 9.5 ngy h -1, µr h and 0.01 msv y 1 respectively, and for leaching residue sample were Bq kg -1, 3.43, 6.83, ngy h -1, µr h and 0.72 msv y 1 respectively. 32

8 International Journal of Nuclear Energy Science and Engineering, Volume 6, The calculated values for El-Sebeaya low grade phosphate rock sample have a normal level of natural background where these values are within the world permissible limits. This means that the El-Sebeaya low grade phosphate rock is acceptable for using as agriculture as materials fertilizers. On the other hand, the leaching residue sample shows a high values for Radium Equivalent Activity, the external-internal hazard index, the gamma absorbed dose rate and the exposure rate which needs specific measures to control radiological hazards. REFERENCES [1] Akhtar N., Tufail M., Ashraf M., Mohsin A. and Iqbal M., (2005)."Measurement of environmental radioactivity for estimation of radiation exposure from saline soil of Lahore", Pakistan. Radiation Measurements, 39(1), [2] Aly H. F., Ali M. M. and Taha M. H., (2013) "Dissolution Kinetics of Western Desert Phosphate Rocks, Abu Tartur, with Hydrochloric Acid", Arab Journal of Nuclear Science and Application, Vol. 28, No. 4, p [3] Becker P., (1989). "Phosphates and phosphoric acid: raw materials, technology, and economics of the wet process", 2nd Edition, Marcel Dekker, Inc., New York. [4] Beretka, J., & Mathew, P. J. (1985)."Natural radioactivity of Australian building materials, industrial wastes and byproducts". Health Physics, 48(1), [5] De Bruijne A. and Caldwell I., (2009). "The Next Inconvenient Truth - Peak Phosphorus", The Broker, Issue 15, pp [6] Déry P. and Anderson B., (2007)."Peak phosphorus Energy Bulletin Rosemarin". [7] Elmaadawy, Kh. G., Ezz El Din M., Khalid A. M., Abouzeid A. Z., (2015) Mineral Industry in Egypt Part II Non Metallic Commodities Phosphate Rocks, Journal of Mining World Express (MWE) 4. [8] European Fertilizer Manufacturers Association, (2000).Booklet No. 4 of 8 Production of phosphoric acid. Belgium. [9] Farai I.P., Ademola J.A., (2005). "Radium equivalent activity concentrations in concrete building blocks in eight cities in Southwestern Nigeria". J. Environ. Radioact. 79, [10] International Atomic Energy Agency, (2013). "Radiation Protection and Management of Norm Residues in The Phosphate Industry", Safety Reports Series No.78, Vienna. [11] Korkmaz, B., Turgut, S. A., (2005)." Research on the urea hydrolysis rate in the soil of Thrace region".journal of Central European Agriculture, 6(2), 107e114. [12] Krieger R., (1981). Radioactivity of Construction Materials, BetonwerkFertigteil Technology, Vol. 47, No. 8, [13] Nazaroff W.W., (1992)." Radon transport from soil to air". Rev. Geophys. 30, [14] Nuclear Materials Authority, (1992)."The recovery of uranium from phosphoric acid produced by Abu-Zaable Company". Cairo, Egypt, Abu-Zaable Company. [15] Olszewska-Wasiolek, M., (1995)."Estimates of the occupational radiological hazard in the phosphate fertilizers industry in Poland", Radiat. Prot. Dosim. 58 (4), [16] Rafique M., Rehman H., Matiullah, Malik F., Rajput M., Rahman S. and Rathore M. (2011)."Assessment of radiological hazards due to soil and building materials used in Mirpur Azad Kashmir", Pakistan Iranian Journal of Radiation Research, 9 (2), [17] Rockström, J., Steffen, W., Noone, K., Persson, A., Chapin, F.S., Lambin, E.F. and Foley, J.A. (2009)."A safe operating space for humanity".nature, 461(7263), [18] Ross K. and Riaz A. (2008)."Naturally occurring radionuclides in materials derived from urban water treatment plants in southeast Queensland", Australia. Journal of Environmental Radioactivity, 99(4), [19] Sabiha-Javied, M. Tufail, M. Asghar, (2010)."Hazard of NORM from phosphorite of Pakistan", J. Hazard. Mater, 176, [20] Saueia C.H.R., Mazzilli B.P., (2006)."Distribution of natural radionuclides in the production and use of phosphate fertilizers in Brazil", J. Environ.Radioact. 89,

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