Abstract
Due to increased global demand for clay, the present work involves the use of INAA for elemental analysis and pollutants concentration in clay. The samples were collected from Aswan in South Egypt. The samples were irradiated using the thermal neutrons “at the TRIGA Mainz research reactor” and at a neutron flux “of 7 × 10 n/cm s”. Twenty-six elements quantitatively and qualitatively were specified for the first time upon studying the samples. The elements determined are U, Th, Ta, Hf, Lu, Eu, Ce, Ba, Sn, Nb, Rb, Zn, Co, Fe, Cr, Sc, Sm, La, Yb, As, Ga, K, Mn, Na, Ti, and Mg. The concentrations of natural radionuclides 232Th, 226Ra, and 40K were also calculated. Based on these concentrations, to estimate the exposure risk for using clay as raw materials in building materials, the radiation hazard indices such as radium equivalent activities, effective doses rate, and the external hazard indices have been computed. The obtained results were compared with analogous studies carried out in other countries and with the UNSCEAR reports.
1. Introduction
Due to increased global demand for clay and its industrial importance, for diversity of uses, it is considered as one of the leading minerals worldwide [1]. Instrumental neutron activation analysis INAA using HPGe detector coupled with a multichannel pulse height analyzer and optimum choice of irradiation and delay times may yield promising results and have been employed in a number of methods for the analysis of geochemical materials. The elemental content in different environmental media has been widely employed by INAA technique [2, 3]. Until now, there is no database for the constituent elements for clay. Therefore, our results can be considered as reference data for the Egyptian clay. Natural radionuclides research has contributed greatly to developing a quantitative understanding of the environmental performance. In order to attain an improved understanding of the environmental destiny of contaminant radionuclides, it is necessary to characterize not only the biogeochemical properties of the radionuclides but also the expected biogeochemical processes occurring in the receiving environment [4, 5].
The present work dealt with measuring the elemental content of clay samples collected from Aswan, South Egypt, and shed more light on the activity concentrations of the naturally occurring radioactive materials (NORM) to assess the radiation hazard parameters due to using clay as building materials.
2. Experimental Technique
2.1. Samples Preparation
Clay samples were collected from Aswan, South Egypt. Each of the samples weighs about 1 kg and is then dried in an oven at about 105° to make sure that all the moisture has been removed. For elemental analysis using instrumental neutron activation analysis, the powder samples were sieved through a set of standard sieves with diameters ranging within 63–125 mm, and an electric shaker was used to obtain homogeneous samples; the samples were irradiated using thermal neutrons. With regard to measurement of the natural radioactivity, each sample was grinded and homogenized and thereafter the powder clay samples were sieved through a 200-mesh sieves, “which is the optimum size when enriched in heavy minerals” to become homogenized powder [6]. The samples were weighed, Packed, and sealed in polyethylene Marinelli beakers, of 350 cm3 volume each, and then stored for 4 weeks to attain secular equilibrium with the short-lived daughters of 232Th and 226Ra and their long-lived parent radionuclides [7].
The activity concentrations of radionuclides were determined from the significant average energy lines of 609.3, 1120.3 and 1764.5 keV (214Bi), 352.9 keV (214Pb) for. 226Ra, 1460.7 keV for 40K and 968.9, 338.4 and 911.1 keV (228Ac) for 232Th series [8].
2.2. Instrumentation and Irradiations
Polyethylene capsules were filled with a hundred mg from powder clay samples and then irradiated with a Dolerite WSE and Microgabro PMS standard reference material with thermal neutrons “at the University of Mainz Triga research reactor (100 kWth) with a flux of 7 × 1011 n/cm2 s”. The concentration of the elements determined in the irradiated samples was quantitatively specified by comparison with the activities of the reference materials [9, 10]. After appropriate cooling times, the data were collected to conduct different measurements [11]. The irradiation conditions for the elements determined were shown in Table 1.
The measuring of activity concentration for radionuclide in studied samples was defined using gamma ray spectrometer system by HPGe detector with an electronic circuit. The HPGe detector has specifications as a follows: energy resolution (FWHM) is 1.70 keV at 1.33 MeV 60Co, Peak-to-Compton ratio 60Co is 65.2, and relative efficiency is 29.2 at 1.33 MeV 60Co. The analysis of results was accomplished by the Inter-Gamma Software that was generated by Intertechnique “Deutschland GmbH, Mainz, Germany” [12–17]. In all measurements, the electronic dead time is less than 10% and the Inter-Gamma Software has performed the correction automatically [2].
2.3. Estimation of Radiological Dose and Hazardous Indices
2.3.1. Radium Equivalent Activity ()
The radium equivalent activity () values in Bq/kg were obtained by using the following equation: where , , and are the activity concentrations of 40K, 232Th, and 226Ra in samples, respectively. The definition is based on the supposition that 130 Bq/kg of 40K, 7 Bq/kg of 232Th, and 10 Bq/kg of 226Ra create the same gamma radiation exposure dose [18, 19].
2.3.2. Absorbed Dose Rate ()
The absorbed dose rate in air due to radionuclides at 1 m above the ground surface for the uniform distribution of 226Ra, 232Th, and 40K was calculated according to guidelines supplied from UNSCEAR:where 0.462, 0.0417, and 0.604 nGy h−1 per Bq/kg were the conversion factors that correspond to 226Ra (238U-series), 40K, and 232Th [20, 21].
2.3.3. External Hazard Index ()
The recommended value of absorbed dose rate is 1.5 mSv y−1 [22, 23]. To limit the radiation dose value to this rate, the conservative model suggestion is based on infinitely thick walls, without doors and windows [24] to serve as a standard for the computation of index defined as external hazard index from the following relation:
3. Results and Discussion
Twenty-six elements were identified in studied clay samples. The average concentration of the elements determined is listed in Table 2. The elements Ti, K, Cr, Ga, Na, Mg, Mn, Sm, As, Sc, La, Co, Rb, Nb, Sn, Ce, Fe, Ba, Eu, Lu, Yb, Hf, Ta, Zn, Th, and U were determined. The concentration for all elements was expressed in μg/g except for Mn, Mg, Na, Fe, K, and Ti which were given in g/kg. In all other situations, the elements were measured by their most distinctive peaks, with lowest statistical errors and free of interference. The measurements accuracy has been estimated using the PMS and WSE analysis, for the standard reference materials. From the obtained results, we can say that INAA is an effective and successful means to supply valuable data for clay samples with a satisfying precision. The accuracy for most elements in present results is in the range of 10% of the reference values, and a good precision has been shown in most results [25, 26]. The activation converts 238U and 232Th into 239Np and 233Pa, respectively, by neutron capture and successive β-decay: The feature -rays can be detected using -spectroscopy [27, 28].
3.1. Assessment of Natural Radioactivity and Exposure Risk
To estimate the exposure risk which naturally occurs as a result of the use of clay as raw materials in construction, the radiological indicators like the external hazard index , absorbed dose rate , and radium equivalent activity and the specific activity concentrations of radionuclides in Bq/kg have been computed according to [19]. The obtained results were listed in Table 3. Average activity concentration of 40K, 232Th, 226Ra, and in clay samples was 208, 28, 36, and 101 Bq/kg, respectively. These values are within the worldwide range of the activity concentration values of 232Th, 226Ra, and 40K in soil and 30, 35, and 400 Bq/kg, respectively [21]. Up to our knowledge, there is no published world average activity concentration of natural radioactivity in clay deposits. All over the world clays deposits are widely used as a building material (e.g., bricks and ceramic industries). Their radioactivity content could be a source of dwelling internal and external radiation exposure. values were less than 370 Bq/kg, the maximum permissible activity concentration limit for building material. This value, 370 Bq/kg, is equivalent to maximum permissible limit [8, 29, 30].
The average activity concentrations of 226Ra, 232Th, 40K, and in clay bricks and building materials, from twelve different countries, were given in Table 4 [31]. The average activity concentrations of natural radionuclides in Egyptian clay samples that could be used as a raw material for bricks industry were less than the average of all studies for 226Ra, 232Th, 40K, and [32].
4. Conclusions
Twenty-six elements were quantitatively determined in the first time for clay samples collected from Aswan, South Egypt. The elements Ti, K, Cr, Ga, Na, Mg, Mn, Sm, As, Sc, La, Co, Ce, Sn, Rb, Nb, Fe, Ba, Eu, Lu, Yb, Ta, Zn, Th, and U were determined. From the obtained results, we can say that INAA is an effective and useful tool to provide good data for clay samples with a precision and satisfying accuracy. The average activity concentrations of natural radionuclides in Egyptian clay samples that could be used as a raw material for bricks industry were less than the recommended levels by UNSCEAR data for soil. It is hoped that our clay data are useful to those dealing with clay applications.
Competing Interests
The authors declare that they have no competing interests.
Acknowledgments
The authors acknowledge with thanks the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, for technical and financial support under Grant no. 363-95-D 1435.