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Radiological impact of natural radionuclides from soils of Salamanca, Mexico
Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Radiological impact of natural radionuclides from soils of Salamanca, Mexico C.D. Mandujano-García a,b,n, M. Sosa a, J. Mantero b,d, R. Costilla c, G. Manjón b, R. García-Tenorio b,e a
Department of Physical Engineering, DCI, University of Guanajuato, Guanajuato, Mexico Department of Applied Physics II, ETSA, University of Seville, Seville, Spain c Department of Environmental Science, DCV, University of Guanajuato, Guanajuato, Mexico d Department of Radiation Physics, Institute of Clinical Sciences, University of Gothenburg, Sweden e Centro Nacional de Aceleradores (CNA), Seville, Spain b
H I G H L I G H T S
Radiometric characterization of soils around NORM industries in Mexico, is performed. Use of alpha-particle and gamma-ray spectrometry for measurements of NORM. Radiological impact is assessed by determining the external absorbed gamma dose rate.
art ic l e i nf o
a b s t r a c t
Article history: Received 31 October 2015 Received in revised form 19 January 2016 Accepted 29 January 2016
Salamanca is the centre of a large industrial complex associated with the production and refining of oilderived products in the state of Guanajuato, Mexico. The city also hosts a large chemical industry, and in past years a major fertilizer industry. All of them followed NORM (naturally occurring radioactive materials) industrial activities, where either raw materials or residues enriched in natural radionuclides are handled or generated, which can have an environmental radiological impact on their environmental compartments (e.g. soils and aquatic systems). In this study, activity concentrations of radionuclides from the 238U and 232Th natural series present in superficial urban soils surrounding an industrial complex in Salamanca, México, have been determined to analyse the possible environmental radiological impact of some of the industrial activities. The alpha-particle and gamma-ray spectrometry is used for the radiometric characterization. The results revealed the presence of 10–42, 11–51 and 178–811 Bq/kg of 238 U, 232Th and 40K, respectively, without any clear anthropogenic increment in relation to the values normally found in unaffected soils. Thus, the radioactive impact of the industrial activities on the surrounding soils can be evaluated as very low, representing no radiological risk for the health of the population. & 2016 Elsevier Ltd. All rights reserved.
Keywords: NORM industries Environmental radioactivity Alpha spectrometry Gamma spectrometry
1. Introduction Concentration of naturally occurring radionuclides in the environment may be affected by human activities. In particular, some industrial activities handle raw materials and/or may generate large amounts of products, by-products, residues and wastes enriched with natural radionuclides, which, associated with their management policy, can have a radiological impact on the
n Corresponding author at: Department of Physical Engineering, DCI, University of Guanajuato, Guanajuato, Mexico. E-mail address: cmandujano@fisica.ugto.mx (C.D. Mandujano-García).
environmental. These activities are referred to as naturally occurring radioactive materials (NORM) activities. Industrial activities involving NORM include processes such as exploration and extraction of oil and gas, combustion of fossil fuels, mining and processing of industrial minerals, production of phosphate fertilizers and treatment of drinking water (IAEA, 2003), increasing awareness on their possible occupational and public radiological impact. This study aims at evaluating the levels of natural radionuclides present in superficial soils collected near an important industrial complex from Salamanca, Mexico, where some NORM industrial activities occur, and where no information on the possible environmental/public impact due to natural radionuclides from NORM materials is available.
http://dx.doi.org/10.1016/j.apradiso.2016.01.031 0969-8043/& 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Mandujano-García, C.D., et al., Radiological impact of natural radionuclides from soils of Salamanca, Mexico. Appl. Radiat. Isotopes (2016), http://dx.doi.org/10.1016/j.apradiso.2016.01.031i
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C.D. Mandujano-García et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
In all NORM activities, the raw materials and/or products, byproducts, residues and wastes are enriched with natural radionuclides. For example, in the oil and gas industry, the produced water, scales and sludge may have elevated levels of 226Ra, 228Ra and 210Pb. Furthermore, in the mining industry associated with the extraction and processing of rare earth metals, niobium, tin, copper, gold, and other metal ores can be found in tailings and radionuclides in the order of thousands of Bq kg 1 from 238U and 232 Th can be found in water. In addition, significant amounts of natural radionuclides are present in the phosphate rock, the main raw material for the production of phosphate fertilizers. During the processing of phosphate rock, U and Th isotopes are concentrated in the final products (phosphoric acid and phosphate fertilizers), while Ra isotopes are present in the by-product phosphogypsum and wastewater (Casacuberta et al., 2009). Another important NORM industrial sector is associated with the production of building materials. This industry can use large amounts of rocks with nonnegligible concentrations of natural radionuclides, for instance, volcanic rocks rich in long-half-life radionuclides (Rizzo et al., 2001; Diano and Bellecci, 1998), and rocks containing a variety of minerals with some natural radioactivity (Righi et al., 2009). In addition, the last years have witnessed significant increase of valorization of some residues generated in NORM industries for the production of cement and some building materials, whose possible radiological impact has to be necessarily evaluated (Garcia-Tenorio et al., 2015). The presence of higher amounts of natural radionuclides in building materials can lead to the generation and presence of radon in homes, which should be monitored. Many studies on indoor radon measurements have been published (Ahmad and Khatibeh, 1997; Çelik et al., 2008). Although combustion of fossil fuels generates energy, there have been some reports indicating elevated levels of natural radionuclides in the ashes produced. Coal-burning plants produce airborne effluents with high levels of 210Po and 210Pb released into the atmosphere, and generates bottom and fly ash with elevated natural radionuclide contents (Eisenbud and Petrow, 1964). Although, for over two decades (Heaton and Lambley, 1995), there have been concerns on the radiological evaluation of ashes from fuel oil plants, in some cases, ashes with very low levels of radionuclides have also been detected (Alonso-Hernández et al., 2014). However, measurements of radionuclides in products and wastes from oil-refining process have been reported (Al-Saleh and Al-Harsham, 2008). Although no radiological risk has been found to be associated with these industrial activities, more studies on NORM in this context are still necessary. Although Mexico is one of the major producers of oil, gas and several important industrial minerals in the world, there have been few reports of NORM associated with industrial activities in the country. Moreover, an important industrial complex including an oil refinery, fuel oil thermoelectric plant and agrochemical manufacturing company is located in Salamanca. The industrial complex was constructed over an active subsidence fault, which damaged the refinery pipelines, provoking hydrocarbon spills. An accidental explosion occurred in the agrochemical zone, releasing pollutants into the atmosphere and surroundings. Some studies conducted in the zone have reported high levels of pollutants such as As, Pb, V and hydrocarbons in soil and groundwater around the industrial complex (Rodríguez and Armienta, 2002; Mejia et al., 2007; Hernandez and Rodriguez, 2012); however, no NORM contamination study has been conducted yet. The objective of this study is to determine the activity and concentration levels of representative naturally occurring radionuclides in soils of the mentioned area based on the obtained data and assess the radiological risk for the population living near the
industrial complex.
2. Materials and methods A total of 19 soil samples were collected around the industrial complex based on their distance from the complex and the zones with open access to the public (Fig. 1). Approximately 1 kg of each sample was collected from a square of sides and depth 30 and 5 cm, respectively, dug in the study area. Universal transverse mercator (UTM) coordinates were registered from each sample point using a commercial global positioning system (GPS). The soil samples were sealed in plastic bags and transported to the laboratory, where they were dried at room temperature and sieved to a particle size of o250 mm. After homogenization of the samples, aliquots of each sample were separated for radiometric analysis. The radiometric characterization of the soil samples was performed via alpha-particle and gamma-ray spectrometry. 2.1. Alpha-particle spectrometry A well-defined radiochemical process was used to determine the activity concentration of U and Th isotopes (Lehritani et al., 2012). Aliquots of 1 g of each homogenized sample were first burnt to ashes in an oven at 600 °C for 24 h. An alkali fusion method was then used to dissolve 200 mg of each burnt sample with KHSO4, following the process described by Mantero et al. (2010) at 800 °C for 20 min. After natural cooling, the digested samples were put into a solution of 8 M HNO3. Actinides were then precipitated carefully by adding concentrated ammonia until pH of the solution becomes 9. The precipitate had high concentrations of U and Th radionuclides, which was separated from the remaining liquid by centrifugation. The precipitate was then diluted in 8 M HNO3 and isolated. Liquid–liquid solvent extraction with tri-n-butyl phosphate (TBP) and xylene was used to isolate uranium and thorium radionuclides. Thorium-isolated fraction was additionally purified using AG1-X8 ionic exchange resin to remove impurities remaining in the fraction. Finally, U and Th fractions were electrodeposited in stainless steel discs to prepare alpha-particle measurement sources. Electrodeposition was performed in polytetrafluoroethylene cells with a platinum anode. The electrodeposition parameters were 1 h and 1.2 A for uranium fraction and 1.5 h and 1.5 A for thorium fraction. U and Th sources were measured in a Canberra Alpha Analyst system equipped with semiconductor passivated implanted planar silicon (PIPS) detectors (active surface, 450 mm2). Minimum detectable activities (MDAs) for alpha spectrometry with 95% confidence level and 4250,000 s of measurement time were o0.01 Bq/kg. 2.2. Gamma-ray spectrometry Activity concentrations of 226Ra, 228Ra, 210Pb and 40K were determined by gamma ray spectrometry, for which aliquots of soil samples were packed and vacuum-sealed into cylindrical polypropylene containers, whose height and diameter were 1.29 and 5.26 cm, respectively. The hermetically sealed soil samples were stored for 430 days before measurement. This was done to avoid radon escape and ensure secular equilibrium between 222Rn and their short-half-life daughters. The samples were measured in a spectrometric system consisting of a Canberra HPGe (hyperpure germanium) detector of type XtRa (extended range), with a relative efficiency of 37% and resolution of 1.77 keV for the 1.33-MeV photopeak of 60Co. The
Please cite this article as: Mandujano-García, C.D., et al., Radiological impact of natural radionuclides from soils of Salamanca, Mexico. Appl. Radiat. Isotopes (2016), http://dx.doi.org/10.1016/j.apradiso.2016.01.031i
C.D. Mandujano-García et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
Fig. 1. Location of the study area and sampling points in Salamanca, Mexico.
detector was shielded by 10 cm of lead and lined with a 5-mm layer of Cu. Efficiency was calibrated by measuring two reference standard materials packed in the same type of containers; IAEA-RGU1 and IAEA-RGTH1. These reference materials are silicon minerals enriched with U and Th with well-known activity concentration and apparent densities similar to the ones found for the soil samples analysed in this study. In order to determine the activity concentration of 226Ra, a weighted mean value was obtained from the prompt γ-ray emissions of their short-half-life decay products; 214Pb (295 and 352 keV) and 214Bi (609, 1120 and 1764 keV). On the contrary, activity concentration of 228Ra was determined by mean of the main gamma emissions of 208Tl (583 and 2614 keV), 212Pb (238.6 keV), 212 Bi (727.3 keV) and 228Ac (338 and 911 keV) considering the corresponding branching ratios for some of them. The activity concentrations of the analysed decay products of 226Ra and 228Ra were consistent in all gamma emissions considering 1 s uncertainties. In efficiency calibration, the effects of coincidence summing for efficiency determination can be neglected because of the use of reference materials with the same radionuclides of interest. For low-energy gamma rays of 210Pb (46.5 keV), self-absorption corrections were applied by using the so-called transmission method following the methodology described by Cutshall et al. (1983). Activity concentration of 40K was obtained by fitting the experimental efficiency values obtained for the reference materials in the energy range of 295–2204 keV. For gamma ray spectrometry, the measurement times were 4 150,000 s and with 95% confidence, MDAs cover the range of 2–7 Bq/kg. Finally, in order to assess the radiological impact of the soil samples on the public, the external absorbed gamma dose rate (Dγr) in nGy/h was determined. The total absorbed gamma dose rate was estimated by following the guidance reported in UNSCEAR (1993). Eq. (1) describes the parameters involved in the calculation of the gamma dose rate absorbed by the public exposed to radionuclides 226Ra, 232Th and 40K in outdoor air at a height of 1 m above the ground:
Dγ r =0. 462CRa −226+0. 662CRa −228+0. 043CK −40,
(1)
where CRa-226, CRa-228 and CK-40 are the activity concentrations of 226Ra, 228Ra and 40K, respectively, in Bq/kg, determined via gamma-ray spectrometry. An average total absorbed gamma dose rate of 55 nGy/h is acceptable (Mirjana et al., 2009).
3. Results and discussion The alpha spectrometry measurements of U and Th isotopes in soil samples from Salamanca, Mexico, are presented in Table 1. After alpha spectrometry measurements, radionuclides from the upper part of the 238U natural series presented values in the range of 10–42, 12–60 and 12–82 Bq/kg for 238U, 234U and 230Th, respectively. Activity concentrations of 238U in soil samples from Salamanca are of the order of values found in unaltered soils for North America. The mean activity concentration of 238U in soils from the United States and Costa Rica are reported for comparison as 35 and 46 Bq/kg, respectively (UNSCEAR, 2000). Table 1 Activity concentration (Bq/kg) of U and Th isotopes from Salamanca, Mexico. Uncertainties are expressed in brackets as one sigma. Sample
238
U
234
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19
18 31 42 10 24 28 31 31 38 31 17 17 17 19 22 28 19 20 24
(2) (3) (2) (1) (1) (2) (3) (3) (3) (2) (2) (2) (1) (2) (2) (3) (2) (2) (2)
17 25 60 12 37 28 26 28 40 28 29 18 14 22 21 18 21 17 20
U (2) (3) (3) (1) (2) (2) (3) (3) (3) (2) (3) (2) (2) (2) (2) (2) (2) (2) (2)
230
Th
16 36 23 15 12 25 21 52 51 23 38 25 21 50 82 34 32 36 26
(1) (3) (2) (1) (1) (2) (3) (4) (4) (2) (4) (2) (2) (4) (5) (3) (3) (3) (3)
234
U/238U
0.9 0.8 1.4 1.2 1.5 1.0 0.9 0.9 1.1 0.9 1.7 1.0 0.8 1.1 0.9 0.6 1.1 0.9 0.8
(0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1)
232
Th
16 27 15 18 11 34 25 51 31 37 37 18 26 40 29 34 23 20 34
(1) (3) (1) (1) (1) (2) (2) (4) (2) (2) (3) (2) (2) (3) (2) (3) (2) (2) (3)
Please cite this article as: Mandujano-García, C.D., et al., Radiological impact of natural radionuclides from soils of Salamanca, Mexico. Appl. Radiat. Isotopes (2016), http://dx.doi.org/10.1016/j.apradiso.2016.01.031i
C.D. Mandujano-García et al. / Applied Radiation and Isotopes ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
Experimental Data Linear fit of LnEff
1.6
Equation 1.4
y = a + b*x
Adj. R-Square0.99582 Value Standard Error 5.96956 0.14763
1.2 1.0
B
Intercept
B
Slope
-0.80946
0.0214
0.8
LnEff
0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 5.5
6.0
6.5
7.0
7.5
8.0
LnEnergy Fig. 2. Linear fit in ln–ln scale of experimental efficiency versus energy. Table 2 Activity concentrations (Bq/kg) of natural radionuclides in soils from Salamanca, Mexico, determined by γ-ray spectrometry. The corresponding values of absorbed gamma dose rate (nGy/h) are also presented. Uncertainties are expressed in brackets as one sigma. Sample
210
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19
35 33 63 51 46 33 35 28 31 40 35 55 20 34 24 34 36 15 37
Pb (3) (3) (4) (4) (4) (3) (2) (3) (3) (3) (2) (3) (2) (3) (3) (3) (3) (2) (3)
226
33 37 25 50 11 30 33 31 29 29 21 20 21 29 24 29 32 18 26
Ra (1) (2) (1) (2) (1) (2) (1) (1) (1) (2) (2) (1) (2) (3) (1) (1) (2) (1) (1)
228
32 44 27 50 11 28 39 27 29 34 22 17 29 43 39 27 31 23 38
Ra (4) (4) (4) (4) (3) (2) (2) (2) (2) (2) (2) (2) (2) (3) (3) (2) (2) (2) (4)
40
Dγr (nGy/h)
K
686 570 271 789 178 811 809 604 396 535 539 465 510 490 528 597 588 320 623
(25) (22) (12) (35) (12) (32) (24) (23) (19) (32) (27) (22) (25) (23) (25) (28) (31) (17) (30)
63(6) 68 (5) 39 (4) 86 (8) 19 (3) 65 (7) 72 (5) 56 (5) 48 (4) 56 (7) 46 (6) 39 (5) 49 (6) 60 (5) 57 (5) 55 (6) 58 (7) 36 (4) 61 (7)
However, several samples presented disequilibrium between radionuclides of the 238U natural series, with ratios ranging from 0.6 to 1.7 for 234U/238U and 0.3 to 3.9 for 234U/230Th. This could be attributed to the higher mobility of 234U than 238U. It can also be attributed to natural processes or agriculture (some of the soil samples were collected from nearby crops). On the contrary, the obtained activity concentrations of 232Th were in the range of 11–51 Bq/kg. Most of the samples presented exhibited 232Th activity concentrations of the order of unaltered soils from the United States and Costa Rica (35 and 11 Bq/kg, respectively). Regarding gamma-ray spectrometry measurements, the experimental efficiency curve obtained from the reference materials is presented in Fig. 2, whose energy range is 295–2447 keV. This curve was used to determine the photopeak efficiency of 40 K. For gamma-emitting radionuclides of 238U and 232Th natural series, the obtained experimental efficiency data were used directly. Activity concentration values of 210Pb, 226Ra, 228Ra and 40K found in soils from Salamanca are presented in Table 2. The results of gamma-ray spectrometry cover a range of 15–63,
11–50, 11–50 and 178–811 Bq/kg for 210Pb, 226Ra, 228Ra and 40K, respectively. The following conclusions can be derived from these results: First, the wide range found for the 40K results indicates the nonhomogeneous composition between the analysed soils. In addition, in several samples, the activity values found for 210 Pb are higher than those of 226Ra, indicating a possible atmospheric contribution of this radionuclide as a daughter of 222Rn. Unsupported 210Pb is deposited in the soil after radon decay. In most of the soil samples, the activity concentration values determined by gamma spectrometry for 226Ra and 228Ra are consistent with the values determined by alpha spectrometry for 238U and 232Th. Nevertheless, in samples collected from a zone very near to the agrochemical plant, disequilibrium between 226Ra and 238 U could be noted.. In addition, by comparing the activity ratios of 238U/232Th (obtained via alpha spectrometry) and their corresponding decay products 226Ra/228Ra (obtained via gamma spectrometry), it is possible to observe that in most of the samples, the ratio of the parent radionuclides is lower than that of the decay products. This fact ratifies the results of alpha spectrometry, indicating that part of uranium has been lixiviated from the soils. The absorbed gamma dose rate is found to be in the range of 19–86 nGy/h, whose average value is approximately 55 nGy/h. These values are typical for soils in unperturbed zones. Therefore, the estimated gamma dose rates indicate that there is no radiological hazard for the exposed public. The possible contamination of the analysed area does not include NORM contamination, and hence any radiation protection measure can be adopted.
4. Conclusions Alpha-particle and gamma spectrometry measurements of soils collected from an industrial area in Salamanca, Mexico, indicate that the 238U and 232Th natural series radionuclides presented activity concentration values very similar to mean values found in unaffected soils from North America. Although there is disequilibrium between radionuclides of the same series, the obtained results indicate that there is no clear anthropogenic increment in the levels of NORM. Consequently, the calculated absorbed gamma dose rates indicate that the radiological impact of the industrial activities in the surrounding soils is very low. Hence, from the viewpoint of radiological protection, the soils studied do not pose radiological risk to the health of the population.
Acknowledgements The participation of the Radioisotope Service (CITIUS) at the University of Seville and Dr Santiago Hurtado in this study is deeply acknowledged. Mandujano-García acknowledges the financial support of CONACyT (Grant: 328910) and University of Guanajuato (Grant: CBM-05-2015-BMS-16) for a research stay in the University of Seville.
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Please cite this article as: Mandujano-García, C.D., et al., Radiological impact of natural radionuclides from soils of Salamanca, Mexico. Appl. Radiat. Isotopes (2016), http://dx.doi.org/10.1016/j.apradiso.2016.01.031i
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radiological impact of natural radionuclides from soils of Salamanca, Mexico C.D. Mandujano-García a,b,n, M. Sosa a, J. Mantero b,d, R. Costilla c, G. Manjón b, R. García-Tenorio b,e a
Department of Physical Engineering, DCI, University of Guanajuato, Guanajuato, Mexico Department of Applied Physics II, ETSA, University of Seville, Seville, Spain c Department of Environmental Science, DCV, University of Guanajuato, Guanajuato, Mexico d Department of Radiation Physics, Institute of Clinical Sciences, University of Gothenburg, Sweden e Centro Nacional de Aceleradores (CNA), Seville, Spain b
H I G H L I G H T S
Radiometric characterization of soils around NORM industries in Mexico, is performed. Use of alpha-particle and gamma-ray spectrometry for measurements of NORM. Radiological impact is assessed by determining the external absorbed gamma dose rate.
art ic l e i nf o
a b s t r a c t
Article history: Received 31 October 2015 Received in revised form 19 January 2016 Accepted 29 January 2016
Salamanca is the centre of a large industrial complex associated with the production and refining of oilderived products in the state of Guanajuato, Mexico. The city also hosts a large chemical industry, and in past years a major fertilizer industry. All of them followed NORM (naturally occurring radioactive materials) industrial activities, where either raw materials or residues enriched in natural radionuclides are handled or generated, which can have an environmental radiological impact on their environmental compartments (e.g. soils and aquatic systems). In this study, activity concentrations of radionuclides from the 238U and 232Th natural series present in superficial urban soils surrounding an industrial complex in Salamanca, México, have been determined to analyse the possible environmental radiological impact of some of the industrial activities. The alpha-particle and gamma-ray spectrometry is used for the radiometric characterization. The results revealed the presence of 10–42, 11–51 and 178–811 Bq/kg of 238 U, 232Th and 40K, respectively, without any clear anthropogenic increment in relation to the values normally found in unaffected soils. Thus, the radioactive impact of the industrial activities on the surrounding soils can be evaluated as very low, representing no radiological risk for the health of the population. & 2016 Elsevier Ltd. All rights reserved.
Keywords: NORM industries Environmental radioactivity Alpha spectrometry Gamma spectrometry
1. Introduction Concentration of naturally occurring radionuclides in the environment may be affected by human activities. In particular, some industrial activities handle raw materials and/or may generate large amounts of products, by-products, residues and wastes enriched with natural radionuclides, which, associated with their management policy, can have a radiological impact on the
n Corresponding author at: Department of Physical Engineering, DCI, University of Guanajuato, Guanajuato, Mexico. E-mail address: cmandujano@fisica.ugto.mx (C.D. Mandujano-García).
environmental. These activities are referred to as naturally occurring radioactive materials (NORM) activities. Industrial activities involving NORM include processes such as exploration and extraction of oil and gas, combustion of fossil fuels, mining and processing of industrial minerals, production of phosphate fertilizers and treatment of drinking water (IAEA, 2003), increasing awareness on their possible occupational and public radiological impact. This study aims at evaluating the levels of natural radionuclides present in superficial soils collected near an important industrial complex from Salamanca, Mexico, where some NORM industrial activities occur, and where no information on the possible environmental/public impact due to natural radionuclides from NORM materials is available.
http://dx.doi.org/10.1016/j.apradiso.2016.01.031 0969-8043/& 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Mandujano-García, C.D., et al., Radiological impact of natural radionuclides from soils of Salamanca, Mexico. Appl. Radiat. Isotopes (2016), http://dx.doi.org/10.1016/j.apradiso.2016.01.031i
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In all NORM activities, the raw materials and/or products, byproducts, residues and wastes are enriched with natural radionuclides. For example, in the oil and gas industry, the produced water, scales and sludge may have elevated levels of 226Ra, 228Ra and 210Pb. Furthermore, in the mining industry associated with the extraction and processing of rare earth metals, niobium, tin, copper, gold, and other metal ores can be found in tailings and radionuclides in the order of thousands of Bq kg 1 from 238U and 232 Th can be found in water. In addition, significant amounts of natural radionuclides are present in the phosphate rock, the main raw material for the production of phosphate fertilizers. During the processing of phosphate rock, U and Th isotopes are concentrated in the final products (phosphoric acid and phosphate fertilizers), while Ra isotopes are present in the by-product phosphogypsum and wastewater (Casacuberta et al., 2009). Another important NORM industrial sector is associated with the production of building materials. This industry can use large amounts of rocks with nonnegligible concentrations of natural radionuclides, for instance, volcanic rocks rich in long-half-life radionuclides (Rizzo et al., 2001; Diano and Bellecci, 1998), and rocks containing a variety of minerals with some natural radioactivity (Righi et al., 2009). In addition, the last years have witnessed significant increase of valorization of some residues generated in NORM industries for the production of cement and some building materials, whose possible radiological impact has to be necessarily evaluated (Garcia-Tenorio et al., 2015). The presence of higher amounts of natural radionuclides in building materials can lead to the generation and presence of radon in homes, which should be monitored. Many studies on indoor radon measurements have been published (Ahmad and Khatibeh, 1997; Çelik et al., 2008). Although combustion of fossil fuels generates energy, there have been some reports indicating elevated levels of natural radionuclides in the ashes produced. Coal-burning plants produce airborne effluents with high levels of 210Po and 210Pb released into the atmosphere, and generates bottom and fly ash with elevated natural radionuclide contents (Eisenbud and Petrow, 1964). Although, for over two decades (Heaton and Lambley, 1995), there have been concerns on the radiological evaluation of ashes from fuel oil plants, in some cases, ashes with very low levels of radionuclides have also been detected (Alonso-Hernández et al., 2014). However, measurements of radionuclides in products and wastes from oil-refining process have been reported (Al-Saleh and Al-Harsham, 2008). Although no radiological risk has been found to be associated with these industrial activities, more studies on NORM in this context are still necessary. Although Mexico is one of the major producers of oil, gas and several important industrial minerals in the world, there have been few reports of NORM associated with industrial activities in the country. Moreover, an important industrial complex including an oil refinery, fuel oil thermoelectric plant and agrochemical manufacturing company is located in Salamanca. The industrial complex was constructed over an active subsidence fault, which damaged the refinery pipelines, provoking hydrocarbon spills. An accidental explosion occurred in the agrochemical zone, releasing pollutants into the atmosphere and surroundings. Some studies conducted in the zone have reported high levels of pollutants such as As, Pb, V and hydrocarbons in soil and groundwater around the industrial complex (Rodríguez and Armienta, 2002; Mejia et al., 2007; Hernandez and Rodriguez, 2012); however, no NORM contamination study has been conducted yet. The objective of this study is to determine the activity and concentration levels of representative naturally occurring radionuclides in soils of the mentioned area based on the obtained data and assess the radiological risk for the population living near the
industrial complex.
2. Materials and methods A total of 19 soil samples were collected around the industrial complex based on their distance from the complex and the zones with open access to the public (Fig. 1). Approximately 1 kg of each sample was collected from a square of sides and depth 30 and 5 cm, respectively, dug in the study area. Universal transverse mercator (UTM) coordinates were registered from each sample point using a commercial global positioning system (GPS). The soil samples were sealed in plastic bags and transported to the laboratory, where they were dried at room temperature and sieved to a particle size of o250 mm. After homogenization of the samples, aliquots of each sample were separated for radiometric analysis. The radiometric characterization of the soil samples was performed via alpha-particle and gamma-ray spectrometry. 2.1. Alpha-particle spectrometry A well-defined radiochemical process was used to determine the activity concentration of U and Th isotopes (Lehritani et al., 2012). Aliquots of 1 g of each homogenized sample were first burnt to ashes in an oven at 600 °C for 24 h. An alkali fusion method was then used to dissolve 200 mg of each burnt sample with KHSO4, following the process described by Mantero et al. (2010) at 800 °C for 20 min. After natural cooling, the digested samples were put into a solution of 8 M HNO3. Actinides were then precipitated carefully by adding concentrated ammonia until pH of the solution becomes 9. The precipitate had high concentrations of U and Th radionuclides, which was separated from the remaining liquid by centrifugation. The precipitate was then diluted in 8 M HNO3 and isolated. Liquid–liquid solvent extraction with tri-n-butyl phosphate (TBP) and xylene was used to isolate uranium and thorium radionuclides. Thorium-isolated fraction was additionally purified using AG1-X8 ionic exchange resin to remove impurities remaining in the fraction. Finally, U and Th fractions were electrodeposited in stainless steel discs to prepare alpha-particle measurement sources. Electrodeposition was performed in polytetrafluoroethylene cells with a platinum anode. The electrodeposition parameters were 1 h and 1.2 A for uranium fraction and 1.5 h and 1.5 A for thorium fraction. U and Th sources were measured in a Canberra Alpha Analyst system equipped with semiconductor passivated implanted planar silicon (PIPS) detectors (active surface, 450 mm2). Minimum detectable activities (MDAs) for alpha spectrometry with 95% confidence level and 4250,000 s of measurement time were o0.01 Bq/kg. 2.2. Gamma-ray spectrometry Activity concentrations of 226Ra, 228Ra, 210Pb and 40K were determined by gamma ray spectrometry, for which aliquots of soil samples were packed and vacuum-sealed into cylindrical polypropylene containers, whose height and diameter were 1.29 and 5.26 cm, respectively. The hermetically sealed soil samples were stored for 430 days before measurement. This was done to avoid radon escape and ensure secular equilibrium between 222Rn and their short-half-life daughters. The samples were measured in a spectrometric system consisting of a Canberra HPGe (hyperpure germanium) detector of type XtRa (extended range), with a relative efficiency of 37% and resolution of 1.77 keV for the 1.33-MeV photopeak of 60Co. The
Please cite this article as: Mandujano-García, C.D., et al., Radiological impact of natural radionuclides from soils of Salamanca, Mexico. Appl. Radiat. Isotopes (2016), http://dx.doi.org/10.1016/j.apradiso.2016.01.031i
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Fig. 1. Location of the study area and sampling points in Salamanca, Mexico.
detector was shielded by 10 cm of lead and lined with a 5-mm layer of Cu. Efficiency was calibrated by measuring two reference standard materials packed in the same type of containers; IAEA-RGU1 and IAEA-RGTH1. These reference materials are silicon minerals enriched with U and Th with well-known activity concentration and apparent densities similar to the ones found for the soil samples analysed in this study. In order to determine the activity concentration of 226Ra, a weighted mean value was obtained from the prompt γ-ray emissions of their short-half-life decay products; 214Pb (295 and 352 keV) and 214Bi (609, 1120 and 1764 keV). On the contrary, activity concentration of 228Ra was determined by mean of the main gamma emissions of 208Tl (583 and 2614 keV), 212Pb (238.6 keV), 212 Bi (727.3 keV) and 228Ac (338 and 911 keV) considering the corresponding branching ratios for some of them. The activity concentrations of the analysed decay products of 226Ra and 228Ra were consistent in all gamma emissions considering 1 s uncertainties. In efficiency calibration, the effects of coincidence summing for efficiency determination can be neglected because of the use of reference materials with the same radionuclides of interest. For low-energy gamma rays of 210Pb (46.5 keV), self-absorption corrections were applied by using the so-called transmission method following the methodology described by Cutshall et al. (1983). Activity concentration of 40K was obtained by fitting the experimental efficiency values obtained for the reference materials in the energy range of 295–2204 keV. For gamma ray spectrometry, the measurement times were 4 150,000 s and with 95% confidence, MDAs cover the range of 2–7 Bq/kg. Finally, in order to assess the radiological impact of the soil samples on the public, the external absorbed gamma dose rate (Dγr) in nGy/h was determined. The total absorbed gamma dose rate was estimated by following the guidance reported in UNSCEAR (1993). Eq. (1) describes the parameters involved in the calculation of the gamma dose rate absorbed by the public exposed to radionuclides 226Ra, 232Th and 40K in outdoor air at a height of 1 m above the ground:
Dγ r =0. 462CRa −226+0. 662CRa −228+0. 043CK −40,
(1)
where CRa-226, CRa-228 and CK-40 are the activity concentrations of 226Ra, 228Ra and 40K, respectively, in Bq/kg, determined via gamma-ray spectrometry. An average total absorbed gamma dose rate of 55 nGy/h is acceptable (Mirjana et al., 2009).
3. Results and discussion The alpha spectrometry measurements of U and Th isotopes in soil samples from Salamanca, Mexico, are presented in Table 1. After alpha spectrometry measurements, radionuclides from the upper part of the 238U natural series presented values in the range of 10–42, 12–60 and 12–82 Bq/kg for 238U, 234U and 230Th, respectively. Activity concentrations of 238U in soil samples from Salamanca are of the order of values found in unaltered soils for North America. The mean activity concentration of 238U in soils from the United States and Costa Rica are reported for comparison as 35 and 46 Bq/kg, respectively (UNSCEAR, 2000). Table 1 Activity concentration (Bq/kg) of U and Th isotopes from Salamanca, Mexico. Uncertainties are expressed in brackets as one sigma. Sample
238
U
234
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19
18 31 42 10 24 28 31 31 38 31 17 17 17 19 22 28 19 20 24
(2) (3) (2) (1) (1) (2) (3) (3) (3) (2) (2) (2) (1) (2) (2) (3) (2) (2) (2)
17 25 60 12 37 28 26 28 40 28 29 18 14 22 21 18 21 17 20
U (2) (3) (3) (1) (2) (2) (3) (3) (3) (2) (3) (2) (2) (2) (2) (2) (2) (2) (2)
230
Th
16 36 23 15 12 25 21 52 51 23 38 25 21 50 82 34 32 36 26
(1) (3) (2) (1) (1) (2) (3) (4) (4) (2) (4) (2) (2) (4) (5) (3) (3) (3) (3)
234
U/238U
0.9 0.8 1.4 1.2 1.5 1.0 0.9 0.9 1.1 0.9 1.7 1.0 0.8 1.1 0.9 0.6 1.1 0.9 0.8
(0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1)
232
Th
16 27 15 18 11 34 25 51 31 37 37 18 26 40 29 34 23 20 34
(1) (3) (1) (1) (1) (2) (2) (4) (2) (2) (3) (2) (2) (3) (2) (3) (2) (2) (3)
Please cite this article as: Mandujano-García, C.D., et al., Radiological impact of natural radionuclides from soils of Salamanca, Mexico. Appl. Radiat. Isotopes (2016), http://dx.doi.org/10.1016/j.apradiso.2016.01.031i
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4
Experimental Data Linear fit of LnEff
1.6
Equation 1.4
y = a + b*x
Adj. R-Square0.99582 Value Standard Error 5.96956 0.14763
1.2 1.0
B
Intercept
B
Slope
-0.80946
0.0214
0.8
LnEff
0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 5.5
6.0
6.5
7.0
7.5
8.0
LnEnergy Fig. 2. Linear fit in ln–ln scale of experimental efficiency versus energy. Table 2 Activity concentrations (Bq/kg) of natural radionuclides in soils from Salamanca, Mexico, determined by γ-ray spectrometry. The corresponding values of absorbed gamma dose rate (nGy/h) are also presented. Uncertainties are expressed in brackets as one sigma. Sample
210
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19
35 33 63 51 46 33 35 28 31 40 35 55 20 34 24 34 36 15 37
Pb (3) (3) (4) (4) (4) (3) (2) (3) (3) (3) (2) (3) (2) (3) (3) (3) (3) (2) (3)
226
33 37 25 50 11 30 33 31 29 29 21 20 21 29 24 29 32 18 26
Ra (1) (2) (1) (2) (1) (2) (1) (1) (1) (2) (2) (1) (2) (3) (1) (1) (2) (1) (1)
228
32 44 27 50 11 28 39 27 29 34 22 17 29 43 39 27 31 23 38
Ra (4) (4) (4) (4) (3) (2) (2) (2) (2) (2) (2) (2) (2) (3) (3) (2) (2) (2) (4)
40
Dγr (nGy/h)
K
686 570 271 789 178 811 809 604 396 535 539 465 510 490 528 597 588 320 623
(25) (22) (12) (35) (12) (32) (24) (23) (19) (32) (27) (22) (25) (23) (25) (28) (31) (17) (30)
63(6) 68 (5) 39 (4) 86 (8) 19 (3) 65 (7) 72 (5) 56 (5) 48 (4) 56 (7) 46 (6) 39 (5) 49 (6) 60 (5) 57 (5) 55 (6) 58 (7) 36 (4) 61 (7)
However, several samples presented disequilibrium between radionuclides of the 238U natural series, with ratios ranging from 0.6 to 1.7 for 234U/238U and 0.3 to 3.9 for 234U/230Th. This could be attributed to the higher mobility of 234U than 238U. It can also be attributed to natural processes or agriculture (some of the soil samples were collected from nearby crops). On the contrary, the obtained activity concentrations of 232Th were in the range of 11–51 Bq/kg. Most of the samples presented exhibited 232Th activity concentrations of the order of unaltered soils from the United States and Costa Rica (35 and 11 Bq/kg, respectively). Regarding gamma-ray spectrometry measurements, the experimental efficiency curve obtained from the reference materials is presented in Fig. 2, whose energy range is 295–2447 keV. This curve was used to determine the photopeak efficiency of 40 K. For gamma-emitting radionuclides of 238U and 232Th natural series, the obtained experimental efficiency data were used directly. Activity concentration values of 210Pb, 226Ra, 228Ra and 40K found in soils from Salamanca are presented in Table 2. The results of gamma-ray spectrometry cover a range of 15–63,
11–50, 11–50 and 178–811 Bq/kg for 210Pb, 226Ra, 228Ra and 40K, respectively. The following conclusions can be derived from these results: First, the wide range found for the 40K results indicates the nonhomogeneous composition between the analysed soils. In addition, in several samples, the activity values found for 210 Pb are higher than those of 226Ra, indicating a possible atmospheric contribution of this radionuclide as a daughter of 222Rn. Unsupported 210Pb is deposited in the soil after radon decay. In most of the soil samples, the activity concentration values determined by gamma spectrometry for 226Ra and 228Ra are consistent with the values determined by alpha spectrometry for 238U and 232Th. Nevertheless, in samples collected from a zone very near to the agrochemical plant, disequilibrium between 226Ra and 238 U could be noted.. In addition, by comparing the activity ratios of 238U/232Th (obtained via alpha spectrometry) and their corresponding decay products 226Ra/228Ra (obtained via gamma spectrometry), it is possible to observe that in most of the samples, the ratio of the parent radionuclides is lower than that of the decay products. This fact ratifies the results of alpha spectrometry, indicating that part of uranium has been lixiviated from the soils. The absorbed gamma dose rate is found to be in the range of 19–86 nGy/h, whose average value is approximately 55 nGy/h. These values are typical for soils in unperturbed zones. Therefore, the estimated gamma dose rates indicate that there is no radiological hazard for the exposed public. The possible contamination of the analysed area does not include NORM contamination, and hence any radiation protection measure can be adopted.
4. Conclusions Alpha-particle and gamma spectrometry measurements of soils collected from an industrial area in Salamanca, Mexico, indicate that the 238U and 232Th natural series radionuclides presented activity concentration values very similar to mean values found in unaffected soils from North America. Although there is disequilibrium between radionuclides of the same series, the obtained results indicate that there is no clear anthropogenic increment in the levels of NORM. Consequently, the calculated absorbed gamma dose rates indicate that the radiological impact of the industrial activities in the surrounding soils is very low. Hence, from the viewpoint of radiological protection, the soils studied do not pose radiological risk to the health of the population.
Acknowledgements The participation of the Radioisotope Service (CITIUS) at the University of Seville and Dr Santiago Hurtado in this study is deeply acknowledged. Mandujano-García acknowledges the financial support of CONACyT (Grant: 328910) and University of Guanajuato (Grant: CBM-05-2015-BMS-16) for a research stay in the University of Seville.
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Please cite this article as: Mandujano-García, C.D., et al., Radiological impact of natural radionuclides from soils of Salamanca, Mexico. Appl. Radiat. Isotopes (2016), http://dx.doi.org/10.1016/j.apradiso.2016.01.031i