ABSTRACT
Salts are essential for life which primarily composed of sodium chloride (NaCl). It has been widely used in various ways such as food seasoning, food additives and even in beauty line for cosmetic products. However, through industrialization and human activities, the concentration of radioactive materials and toxic elements in salt might exhibit above the legal limit amount, resulting hazardous to the public health for instance carcinogenic risk. As such, there are still insignificant data with regard to the latter of salt particularly Dead Sea salt (Jordan) , rock salt (Himalaya) and table salt (Malaysia), comprising natural and commercialised samples. Thus, from the viewpoint of quality control and food safety, present work investigates the values of naturally occurring radioactive material (e.g. 238U, 232Th and 40K) and toxic elements in the proposed sample materials which unreported elsewhere; use being made of High Purity Germanium (HPGE) gamma ray spectrometry system. Following to the radioactivity characterisation, elemental concentration analysis has been carried out using Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) and Energy-dispersive X-ray spectroscopy (EDX). The data obtained in this research are then compared with the limit given by the World Health Organization (WHO) and United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). As long as they values obtained are below the limit, they are safe to be consumed.
INTRODUCTION
Radiation is a spontaneous process of the decay for an unstable nuclei in order for it to become more stable. This process emits energy in the form of electromagnetic radiation. Radioactivity is an integral part of our environment. Apparently, they are unavoidable phenomenon that occurs in the world. Furthermore, radiation can either come from natural or artificial sources. Natural sources of radiation are usually called as naturally occurring radioactive material (NORM) while artificial sources as technology enhancing radioactive material (TENORM). The principle sources of NORM are cosmic rays, terrestrial, internal radiation and radon gas (UNSCEAR). The radiation which originates from outer space are called cosmic rays such as the sun and stars. The examples of cosmogenic radionuclides which are produced by the bombardment of stable nuclides by cosmic rays are Carbon-14, Tritium-3 and Beryllium-7 (NCRP, 1987; UNSCEAR, 1993; Bennett, 1997). For terrestrial radiation, the most important examples of radioactive atoms are such as Uranium-235, Uranium-238, Thorium-232 and Potassium-40. These radioactive materials have a very long half-lives. While for TENORM, it may occur due to human activities such as mining and nuclear power plants.
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Since salts are obtained from these natural sources and our body also seems already adapt to radiation as we exposed to it every day, it is very crucial to continuously asses the level of radiation contained in the salts consumed by the consumers. The main pathways for the radionuclides to enter human body is by internal exposure which can be consider as ingestion (IAEA, 1982). However, either internal or external exposure of radionuclides, both may be dangerous to our body. Somewhat unknowingly, we ingest radionuclides via daily food intake since in Malaysia, salt is the most essential for cooking. In fact, once radionuclides enters the human body system and if an individual consume the radioactive materials (specifically 238U, 232Th and 40K) in a large amount, they may get a higher risk of hazardous health issues . Under normal environmental conditions, some 90% of 226Ra enters the human body via the consumption of salts (Tettey-Larbi et al., 2013).
In addition, it is important to calculate the radiological human dose via the ingestion pathways as it is regarded as one of the most important parameters use in the evaluation studies on the impact of the radionuclides to human body. (IAEA, 1994) In Malaysia, the data of this kind of studies are not so readily available. (IAEA, 1994, 2000) Thus, a study for this local database is suggested to be useful and required for validation of existing data in order to predict impact of deposited radionuclides to human. The results also expected to give an awareness to the public consumers especially in Malaysia.
Moreover, apart from the contained of radionuclides in the salts, it also contains various type of heavy metals and trace elements. Salts plays a significant function in the process of digestion and essential element in the diet of living things. Different type of salts have different minerals since minerals are one of the natural components in Earth. Some of it may be toxic metals, for example, As, Cd, Cr, and Pb. Therefore, they might be harmful to human body if we take it in excess amount because heavy metals itself are poisonous. Food grade salts account for only a small part of salt production in industrialized countries. In spite of considerable variation, the daily intake for consumers is substantial (Usman and Filli, 2011). Thus, even a small contamination in the salt productions could create health risk to the consumers. Heavy metals contribute significantly to the pollution of the environment. In line with this concern, the concentration of daily intake of heavy metals are being investigate in this present research.
METHODOLOGY
Sample collection
Five different type of salts were used as samples in this research. The samples were chosen based on their popularity of the usage by Malaysians. The five salts used were the Jordan Dead Sea salt (natural, cooking and beauty salt), Himalayan salt (commercialized rock salt) and table salt. The natural Jordan dead sea salt was provided by the University of Jordan while the other commercialized Dead Sea salt, Himalayan salt and table salt were purchased from the supermarket located in Kuala Lumpur.
Sample preparation
For HPGE:
First, the sample was annealed at 100C in the oven for 24 hours. This process was to ensure that the moistures contained in the sample were removed so that a constant dry weight of the sample can be attained. Then, the sample was sieved to ensure the portion of parent and daughter nuclei were evenly distributed. Incomplete homogenization process would cause the increase of the sampling error in the data. The sample was then stored in a clean Marinelli beaker, sealed and was left for about six weeks before it was analyzed. The reason of using the Marinelli beaker for the samples of HPGE detectors was because Marinelli beaker provides high counting efficiency and it can minimize gamma ray attenuation throughout the analyzing process. In addition, the step of sealing the beaker using electrical tape was to ensure that there will be less interaction of radon gas with sample. If the sample interacts with radon gas, the decay rates will be slow. Lastly, the sample must be left for 6 weeks so that secular equilibrium can be achieved which means that, the parent and daughter activities were in the same rate. Since 226 Ra (Parent: 238 U) and 228 Ra (Parent: 232 Th) have long half-lives which are 1600 years and 5.75 years respectively, their activities were measured from their daughters’ activities rate during this secular equilibrium process. Each of the samples will undergo the same preparation process as stated above.
For ICP-MS:
Firstly, 0.5g of sample was weight and diluted with 100ml of nitric acid, HNO3 and 2ml of hydrochloric acid, HCl. Since ICP-MS is very sensitive to large particles, the samples need to be digest before proceed to analyze it. All nitrate salts were soluble in water. Those acids were also useful for keeping the elements of interest in the solution until they the plasma of ICP-MS other than to breakdown the organic component of the sample. In fact, those acids were used because they do not generate interference or spectral difficulties on most inorganic analytical instrument and also it was compatible with all type of samples. Next, the solution of mixture was heated at 100C for 1 hour in order to speed up the digestion process. The sample was then filtered to ensure no large particles were included in the sample mixture. Furthermore, the sample was then diluted more with the ration of 1:100 ml of sample and deionized water respectively. In addition, the standard solutions were prepared in order to calibrate with the results obtained later.
For EDX:
The raw samples of each salts were place individually on the sample holder of the instrument and then it can straightly to be analyzed.
Data calculation
For HPGE:
Radioactivity concentration of 226Ra, 228Ra and 40K
226Ra, 228Ra and 40K are three important radionuclides which their activity concentrations can be determined from the -characteristics line of their short-lived daughter nuclides. Each of them does not interfere other different decay series since different decay series represented by different nuclides. In order to calculate the activity concentrations, the relevant decay data (Table 2) of the detected radionuclides are used by referring to the NUDAT-2.6 data base and since the samples are assumed to be in the secular equilibrium state, the activity concentrations can be calculated by the following equation (Khandaker et al., 2012; Asaduzzaman et al.; 2015a):
DISCUSSIONS
Radioactivity concentration of 226Ra, 228Ra and 40K
The radioactivity concentrations of natural radionuclides present in the salt samples are presented in Table 3. The results are reported in Bq kg-1 on the dry-weight basis, ranging from 1.070.1 to 2.560.4 Bq kg-1 for 226Ra, 0.50.1 to 1.510.3 Bq kg-1 for 228Ra and 6.961.1 to 3813175.2 Bq kg-1 for 40K. The result shows that the activity concentration of the investigate samples varies considerably with respect to type of salts. As an instance, Himalayan salts showed the highest activity concentration of 226Ra and 228Ra. This is because Himalayan salts were obtained in the mining area where that was the area which natural radionuclides were most abundant. In addition, the concentration of 226Ra is slightly higher than 228Ra can be explained since salt was obtained mainly by water, the transfer factor in water of uranium is higher than thorium (Dlamini, Mathuthu and Tshivhase, 2016). Dead Sea beauty salts showed the highest concentration of 40K. This may be because of the other mixture of the Dead Sea salts itself with the other beauty solutions. It can be seen from Table 3 that 40K was in relatively large amount compared to the other radionuclides because it is vital to all living things.
Daily intake of radioactive materials and committed effective dose per year of the consumption of salt by an average adult.
From Table 4, the daily intake of each radionuclide from different type of salts varies from 3.11 to 507.96 mBq d-1 for 226Ra, 1.46 to 183.39 mBq d- for 228Ra and 20.26 to 11099.78 mBq d- for 40K. Dead Sea beauty salt shows the highest value of daily intake followed by table salt, Himalayan salt, natural Jordan Dead Sea salt and Dead Sea cooking salt. On the other hand, for the committed effective dose per year of the consumption of salt by an average adult, which referred to Table 5, the values were found to be in the average of 13.45 µSv y-1 for 226Ra, 12.01 µSv y-1 for 228Ra and 9.90 µSv y-1 for 40K. According to the report from UNSCEAR (2000), the total of exposure per person resulting from the ingestion of radionuclides should be ≤290 µSv y-1.
Excess lifetime cancer risk
Based on the data obtained in Table 6, the average of ELCR varied from 8.87x10-7 to 1.31x10-3 for 226Ra, 1.77x10-6 to 1.36x10-4 for228Ra and 6.11x10-7 to 1.06x10-4 for 40K. These values shows a positive results since they were much lower than the acceptable ELCR limit of 10-3 for radiological risk in general (Patra et al, 2013). Table salt shows the highest contribution to the cancer risk. It may not contained highest content of radionuclides but due to greater consumption rate, the risk shown was high enough.
The comparison of the concentration of heavy metals in the samples.
It was clearly stated that different type of salts contained different concentration of heavy metals. Natural Jordan Dead Sea salt has the highest content of As. While for Himalayan salt, it has the highest content of Ca and Zn. In addition, Mn was seen to be highest in table salt. For Dead sea cooking salt, Ni shows the highest reading. Last but not least, Dead Sea beauty salt content Cr and Fe.
Estimated daily intake for heavy metals in samples
From the data shown in Table 7, all the elements found have lower value than the tolerable daily intake given by the World Health Organization (WHO). This is because, the consumption of salts in Malaysia is low enough. This makes the intake of heavy metals lower than the limit given. However, too much intake of those salts will give higher chances of carcinogenic risk to the consumers.
The other elements that contained in the samples detected by EDX.
Refer to the data obtained in Table 8, other than the heavy metal elements detected by ICP-MS there were other elements too. EDX seems that it can detect more specific percentage of some other elements such as Rh, Ge, Si, In and many others. These elements were all may cause hazardous health risk. However, as long as consumers took adequate amount of salts intake the health risk are low.
CONCLUSION
The activity concentrations of naturally occurring radionuclides (NORM) in the most common used salts in Malaysia were determined. This study shows that the radioactivity concentrations of 238U, 232Th, 40K for JDSS, HS, TS, CS and BS varies from (1.07-2.56) Bq kg-1, (0.50-1.51) Bq kg-1 and (6.96-3813) Bq kg-1 respectively. This means that the radioactivity concentration were not uniform and varied with respect to salt usage type, location and geological formation of the area of the salts obtained. Himalayan salts has the highest total value of the radionuclides shows that the mining area contained highest NORM concentrations compared to the open sea area. On the other hand, for the analysis of ICP-MS, Fe shows the highest concentration value followed by Ca, Zn, Ni, Cr, Mn, As, Cd and Cu. The EDI values were all below the TDI value given by the World Health Organization (WHO). The analysis of EDX shows that there were other elements contained in the salts. However, the elements contained not a threaten to consumers’ health since the percentage of it were all low. In the nutshell, present results shows no serious health burden to the consumers since all the values obtained were below the limit given by the responsible organizations. Thus, all salts are safe to be used but with the adequate amount of the intake or else, too much intake of it will give negative impact to the consumer health.
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