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ORIGINAL ARTICLE
Year : 2017  |  Volume : 40  |  Issue : 3  |  Page : 126-132  

Distribution and disequilibrium of natural radionuclides in the black shale from Wadi Naseib area, Southwest Sinai, and their environmental hazard indices


1 Department of Geochemical Exploration, Nuclear Materials Authority (NMA), El-Maadi, Cairo, Egypt; Department of Chemistry, Laboratory of Radiochemistry, University of Helsinki, Helsinki, Finland
2 Department of Geochemical Exploration, Nuclear Materials Authority (NMA), El-Maadi, Cairo, Egypt
3 Department of Nuclear Engineering, Military Technical College, Kobry El-Kobba, Cairo, Egypt

Date of Submission15-Jun-2017
Date of Decision26-Jul-2017
Date of Acceptance22-Aug-2017
Date of Web Publication16-Feb-2018

Correspondence Address:
Mahmoud R Khattab
Nuclear Materials Authority (NMA), P.o. Box 530, El-Maadi, Cairo; Department of Chemistry, Laboratory of Radiochemistry, University of Helsinki, Helsinki

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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_23_17

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  Abstract 


Several radionuclides of the 238U (or its decay daughters such as 234Pa, 234U, 230Th, 226Ra, 214Pb, and 214Bi), 232Th (228Ac, 208Tl, and 212Bi), and 235U decay series have been measured. The 234U/238U activity ratios (ARs) are >1 for most rock samples indicating preferential abundance of 234U relative to 238U from the rock due to prevailing reducing conditions. The 226Ra/238U ratios are ranging between 0.04 and 0.6 and averaging 0.4 which show disequilibrium in the direction of uranium concentrations. 230Th/238U and 230Th/234U ARs for all studied samples are less than unity, suggesting that all 230Th are produced by in situ decay of 234U and there is no allogeneic or initial 230Th. Exposure rate, dose rate, radium equivalent activity, external hazard index, internal hazard index, and radioactivity level index are determined.

Keywords: Gamma spectrometry, radiometric indices, radionuclides concentrations, shale rocks


How to cite this article:
Khattab MR, Ebyan OA, Abdel-rahman MA. Distribution and disequilibrium of natural radionuclides in the black shale from Wadi Naseib area, Southwest Sinai, and their environmental hazard indices. Radiat Prot Environ 2017;40:126-32

How to cite this URL:
Khattab MR, Ebyan OA, Abdel-rahman MA. Distribution and disequilibrium of natural radionuclides in the black shale from Wadi Naseib area, Southwest Sinai, and their environmental hazard indices. Radiat Prot Environ [serial online] 2017 [cited 2022 Jan 19];40:126-32. Available from: https://www.rpe.org.in/text.asp?2017/40/3/126/225583




  Introduction Top


Radiation is present in all environment of the Earth's surface, beneath the Earth and in the atmosphere. Human beings are exposed to external radiation and radiation from the naturally occurring radionuclides in their immediate surroundings, and also to internal radiation from food, water, and air they consume. Knowing the radiation levels and radionuclide distributions in the environment is important for assessing the effects of radiation exposure due to both terrestrial and extraterrestrial sources.

The natural environmental radioactivity depends mainly on local geological and geographical conditions and it is especially related to the rock types. Higher radiation levels are associated with igneous rocks, such as granite, and lower levels with sedimentary rocks, although some shale and phosphate rocks have a high content of radionuclides.[1] Natural radionuclides are mainly derived from three separate decay chains (235 U,238 U, and 232 Th) and 40 K. The longest-lived member is 232 Th, which has a half-life of 1.405 × 1010 years. The second is 238 U with 4.47 × 109 years. Naturally occurring uranium contains 99.2745% by weight 238 U, 0.7200%235 U, and 0.0055%234 U.[2]

Therefore, U and Th are present in minerals such as allanite, monazite, zircon, apatite, sphene, and thorite, and K occurs in major minerals such as feldspar and micas.[3] All of these accessory minerals and feldspars also enhance in silica-saturated acidic magmatic rocks such as granite, rhyolite, syenite, and pegmatite compared with intermediate, basic, and ultrabasic rocks.[4],[5] Thus, higher radiation levels are associated with such rocks. Most of the radioactivity associated with uranium in nature is due to its progenies that are left behind in mining and milling. The gamma-radiation detected by exploration geologists looking for uranium actually comes from associated elements such as radium (226 Ra) and bismuth (214 Bi) which over time have resulted from the radioactive decay of uranium. Hence, these two daughters are important to follow during hydrometallurgical processes.

Wadi Naseib area is located about 40 Km east Abu Zenima town in the southwestern part of Sinai Peninsula. It is bounded by longitude 33° 22' and 330 25' E and latitude 29° 01' 30” and 29°05'N [Figure 1]. The area is covered by different rocks of Precambrian, Paleozoic, and Mesozoic ages. The Precambrian rocks have limited exposure and are represented by diorite–granodiorite complex and granite. The diorite–granodiorite complex is exposed in the southern and central parts of Wadi Naseib and along some parts of Wadi El Lehian, to the east of the study area. The granite is less common and exposed at the southern part of the map area, especially along Wadi Abu Hamata. The Paleozoic rocks are unconformably overlying the crystalline basement complex and overlain by a thick basaltic sheet of Jurassic (178 ± 25 million years) age.[6] The Paleozoic rocks of southern Sinai were studied by many authors, especially from the geological, mineralogical, and geochemical radioactivity points of view.[7] According to these studies, the Paleozoic section at Wadi Naseib area can be subdivided into two main types of sediments: early Paleozoic and late Paleozoic sediments.
Figure 1: Geological map of Wadi Naseib area with the location of the collected samples

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The aim of this contribution is to study the distribution of radionuclide and evaluation of radioactive disequilibrium condition in 238 U decay series of the black shales from Wadi Naseib Area, Southwest Sinai, as well as their environmental impacts.


  Materials and Methods Top


Sample preparation

Twenty shale rock samples were collected from the top surface layers of the studied area and then crushed using a mixer mill type MM-2 into fine powder, and then sieved using a 2 mm mesh to obtain a uniform particle size. The collected samples were dried in a drying oven at 110°C for 24 h to remove the moisture from the samples and also until its weight remains constant.

Analytical method

5 g of rock samples were sealed with Polyvinyl chloride (PVC) insulation tape in 20 ml plastic vials and stored for 4–8 weeks before analysis to prevent the escape of the radiogenic gases 222 Rn and 220 Rn, and to allow the attainment of radioactive secular equilibrium in the decay chains. After the attainment of secular equilibrium between 238 U,232 Th, and their daughters (progenies), the samples were subjected to gamma-ray spectrometric analysis using hyperpure germanium (HpGe) detector. The samples were measured for 24 h with a Canberra GX8021 gamma spectrometer with a thin (625 um) carbon epoxy window, and a relative efficiency of 95.8%. It was connected to a Canberra DSA-1000 digital signal analyzer, and the spectra were collected and analyzed with Genie 2000 software. The primordial 238 U is the most abundant isotope of U (99.27%) and the initial member of the 238 U-decay chain [8] with a long half-life time (4.4683 G year). It decays to 234 Th with the emission of the α-particle through two consecutive β-transitions,234 Th decays to 234 Pa (half-lives of 24.10 days and 6.69 h, respectively) and to 234 U, with the half-life time of 245,250 years, which decays to 230Th.[9]238 U activity was measured from the gamma ray emitted by its daughter product (234m Pa) whose activity is determined from the 1001 keV photo peak.[10]

Before the analysis, the energy calibration of the measuring system was performed using a multinuclide (137 Cs and 60 Co) standard source (Eckert and Ziegler isotopes product GMBH). The activity is deposited on 9 mg/cm 2 aluminized Mylar (polyester) disk, and covered with 0.9 mg/cm 2 Kapton (polyamide). The source is supplied in a removable aluminum holder. In the holder, the source has an overall diameter of 1” (25.4 mm) and a thickness of 0.125” (3.18 mm). Out of the holder, the source is 0.937” (23.8 mm) in diameter with a thickness of approximately 0.030” (0.76 mm). The active diameter is 0.12 (3 mm).

The efficiency calibration was performed using three well-known reference materials obtained from the International Atomic Energy Agancy for U, Th, and K activity measurements: RGU-1, RGTh-1, and RGK-1. The efficiency-calibrated standard has similar geometry, density, dimensions, and chemical composition to the samples that will be analyzed to reduce the effect of solid angle and sample material variation correction. The absolute full-energy peak efficiency of HPGe detector was calculated using the following equation:



Where Ct is the total number of count recorded per unit time. Nγ is the number of gamma quanta emitted by the source per unit time and can be calculated by:

Nγ= DsIγ(Eγ)(2)

Where Ds is the activity or the disintegration rate of the source. Iγ (Eγ) is the branching ratio or the emission probability per disintegration. This method has been accredited by UKAS in accordance with the International Standard ISO/IEC 17025:2005 General Requirements for the competence of testing and calibration laboratories.

The activity concentration measurements of 238 U, A238 in Bq/kg, were calculated based on the count rate from the gamma lines of its decay chain (i.e. 1001.3 KeV gamma line from 234m Pa, daughter of 238 U) using the following equation:



Where Cnet is the net number of counts in a given peak area corrected from background peaks. m is the mass of the measured sample (kg). εabs is the absolute detector efficiency at specific energy (Eγ). T is the counting time.

The specific activity of 40 K was measured by its own gamma-ray at 1460.8 keV, while activities of 226 Ra and 232 Th were calculated based on the weighed mean value of their respective decay products in secular equilibrium. The specific activity of 226 Ra was measured using the 186.1 keV from its own gamma-ray (after the subtraction of the 185.7 keV of 235 U). The specific activity of 214 Pb was measured using the 241.9 keV, 295.2 keV, and 351.9 keV while the specific activity of 214 Bi was measured using the 609.3 keV. The specific activity of 232 Th was measured using the 338.4 keV and 911.2 keV from 228 Ac and 583 keV and 2614.4 keV from 208 Tl.[11],[12]

The 234 U activity was determined directly from the gamma rays emitted from this nuclide at energies of 53.2 and 120.9 keV while the 210 Pb was measured using 46.5 keV.


  Results and Discussion Top


The activity concentrations of 238 U,232 Th,226 Ra, and 40 K (Bq/kg) for black shale samples are listed in [Table 1]. The activity concentration of 238 U is ranging between 214.9 and 2845.1 Bq/kg with average of 1505.9 Bq/kg.226 Ra activity concentrations are ranging between 104.1 and 852.4 Bq/kg with average of 400.1 Bq/kg.232 Th is ranging between 48.50 and 162.20 Bq/kg with average of 108.24 Bq/kg, while 40 K is ranging between 123.30 and 981.50 Bq/kg with average of 550.15 Bq/kg. The 235 U concentrations of the analyzed samples vary from 17.90 to 3360.20 Bq/kg with 728.85 Bq/kg as an average. These results are higher than the average of UNSCEAR, 2000[13] and the average of argillaceous sediments of IAEA, 1979,[14] while 40 K has lower averages.
Table 1: Specific activity (Bq/kg) of radionuclides and different activity ratios for studied samples

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The observation of a 234 U/238 U AR that is less than or greater than 1 indicates that an isotope of uranium has migrated within the rock in the last 1–2 million years; the time required for 234 U to reach its equilibrium activity.[15] Other daughter/parent ARs can be used to detect radionuclide migration over shorter time-scales, such as 230 Th/238 U (30, 0000 years; the time required for 230 Th to reach its equilibrium activity if 234 U/238 U activity ratio (AR) is 1) and 226 Ra/230 Th (8,000 years; the time required for 226 Ra to reach its equilibrium activity with 230 Th). The 234 U/238 U activity ratios were higher than 1 for most rock samples with the exception of (A 2, B 1, and B 2), indicate preferential enrichment of 234 U on 238 U from the rock, due to prevailing reducing conditions. For shale samples (A-2, B-1 and B-2),234 U/238 U AR <1 may be related to its presence within oxidizing conditions. Another possible explanation of low 234 U/238 U AR is that 234 Th, daughter of 238 U, is adsorbed on to clay and iron minerals produced by weathering of host rock, and 234 Th and its daughter nuclide,234 U, are subsequently fixed in the minerals.[16] The 226 Ra/238 U ratios are ranging between 0.04 and 0.6 and averaging 0.4 which show disequilibrium in the direction of uranium concentrations indicating U-migration in these sedimentary rocks where radium appeared to be relatively mobile in the more reducing condition.[17],[18] Radium generally has an intermediate mobility between U(IV) and U(VI), but, in contrast to uranium, is less mobile in oxidizing conditions because it is strongly adsorbed by clay and iron minerals.[14],[19]

230 Th/238 U and 230 Th/234 U ARs for all studied samples are less than unity, suggesting that all 230 Th are produced by in situ decay of 234 U and there is no allogeneic or initial 230 Th. The 226 Ra/230 Th AR is higher than unity in samples A-2, B-1, B-2, B-3, C-1, GAD-2, and N-1 and lesser in samples D-2, B-4, and H-3 while sample D-1 in equilibrium. This could be attributed to the promotion of 230 Th solubility by forming complexes with Cl − or SO42− ions (e.g., [ThCl3] + and [ThSO4]2+), indicating that the system was not closed to surface and groundwater movement for a maximum time period of 8 ky.226 Ra/210 Pb ARs are greater than unity for five (D-2, A-2, B-3, H-3, and GAD-2) out of twelve samples, indicating loss of radon from these samples or a preferential abundance of 226 Ra in these samples.234 U/235 U ARs range between 0.8 and 68.9 which means a U-234 leaching out and in due to alteration processes.[20]

Environmental hazard impacts of the radionuclides

Calculation of the dose rate

The γ radiation dose rates (DR) (D in nGy/h) are due to the sample content of radionuclides which can be estimated by employing.[13]

D = 0.462A (Ra)+ 0.604 A(Th)+ 0.0417 A(K)(4)

Where Au, ATh, and Ak are the activity concentrations of uranium, thorium, and potassium in Bq/kg, respectively, in the measured samples. From [Table 2], it is clear that all DR values are higher than the worldwide average levels corresponding to 55 nGy/h.[21]
Table 2: The dose rate and external hazard indices in rock samples

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External and internal hazard indices

The external hazard index (Hex) is used to measure the external hazard due to the emitted gamma radiation. It is calculated by the following equation:[22]



Where Hex is the external hazard index and ARa, ATh, and AK are the specific activities of Ra, Th, and K in Bg/kg, respectively. The upper limit of this index is 1. The obtained results indicate high values (>1) of Hex with the exception of one sample (GAD2) that has lower value than 1 [Table 2].

The internal hazard index (Hin) is used to control the internal exposure to 222 Rn and its radioactive progeny. It is given by the following equation:[22]



The calculated values confirm the above result where the Hin values exceed the permissible value with the exception of one sample (GAD2) [Table 1]. This means that 222 Rn and its radioactive progeny play a significant role in the expected internal hazard due to radiation in the studied samples.

Radioactivity level index

This index is used to estimate the level of radiation risk, especially γ-rays, associated with natural radionuclides in specific materials. It is defined as:



Where Iγ is the radioactivity level index and ARa, ATh, and AK are the specific activities of Ra, Th, and K in Bg/Kg, respectively. The safety value for this index is ≤1. The calculated values indicate very high radioactivity index for the studied samples.


  Conclusions Top


The investigated mineralized sediments have high uranium and radium contents due to the presence of some pockets and lenses of secondary U-mineralization. The values of exposure and DRs as well as radium equivalent activity, external and internal hazardous indices, and radioactivity level index of the mineralized sediments exceed the international permissible values. The potential for environmental exposure is high in the study area. This is attributed to the occurrences and areas of U-mineralization and then limited areas for radiation sources and radiation exposure.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

  [Figure 1]
 
 
    Tables

  [Table 1], [Table 2]


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Radiochemistry. 2021; 63(5): 627
[Pubmed] | [DOI]



 

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