RADIOACTIVE RESIDUE IN HONEY

Th e concentration of radioactive isotopes in honey is an important bioindicator of environmental contamination. For that purpose, a total of 66 samples of diff erent types of honey (acacia, meadow, linden, sunfl ower, fl ower, oilseed rape, chestnut) were examined. Th e samples were collected during 2020 and 2021 at diff erent localities in the Republic of Serbia (Vojvodina, Central Serbia, Kosovo and Metohija). Gamma spectrometric analysis was used to determine natural radionuclides potassium-40 (40K), thorium-232 (232Th ), radium-226 (226Ra), uranium-238 (238U), uranium-235 (235U) and sodium-22 (22Na) and anthropogenic radionuclide caesium-137 (137Cs). Th e obtained results indicate that the predominant radionuclide in all the analyzed honey samples is natural K-40, whose average activity was 74 Bq/kg. Th e activity of other tested radionuclides ranged as follows: Th 232: < 1 2.0; Ra-226: 1.9 15.6; U-238: < 1 31.4; U-235: < 0.2 1.61 and Na-22: < 0.2 2.4 Bq/kg. Th e activity of the artifi cial radionuclide Cs-137 was measured in 53% of the tested samples from the territory of Kosovo with the maximum value of 3.63 Bq/kg. Regarding the determined level of radioactive residues, it can be concluded that the honey produced in the Republic of Serbia is healthy and environmentally safe food.


INTRODUCTION
Rapid development of industry and technology worldwide is leading to an increase in global environmental pollution. Th is can result in the presence of a large number of contaminants of various origins in foods for human consumption, with radionuclides being the most signifi cant ones. Th ey are primarily the result of the application of nuclear energy for peacetime use. Past accidents at nuclear power plants (Chernobyl, 1986;Fukushima, 2011) have caused high levels of radioactive contamination of the biosphere. Increased content of radionuclides in agricultural land as a result of the use of artifi cial phosphate fertilizers and coal combustion in thermal power plants with their solid waste (ash, slag) are the most important source of "technologically increased natural radioactivity" because they contain natural radionuclides (uranium, thorium) and their derivatives. Also, during the bombing in 1999, the territory of the Republic of Serbia was contaminated with depleted uranium and the consequences of this are long-term and unforeseeable. All these factors indicate that it is signifi cant to measure the level of radioactivity in the environment and predict the eff ects it might have for the fl ora and fauna, humans and ecosystems in diff erent ways in the years to come (Blagojević and Simić, 2012). Th ese sources of radioactive contamination provide ample opportunities for disturbing the ecological balance because they are by far the largest anthropogenic source of increasing the content of radionuclides in the soil, and therefore also in plants and other links in the food chain. Honey bee (Apis mellifera) has an extremely important and complex role in nature because it aff ects the production of human and animal food, various industrial raw materials of agricultural origin as well as human health. However, the conditions for beekeeping have been deteriorating for a long time because plants absorb various harmful substances like radionuclides, through roots, leaves and fl owers -and the fl owers of plants are the main sources of bee food, nectar and pollen. It has also been documented (Mihaljev et al., 1992) that honey contains chemical components of plants. Honey bees collect pollen and nectar from the environment, so the content of elements and their amount in honey depends on environmental conditions. Honey is a food that is exclusively a product of honey bees. Its quality depends on the type and geographical origin, environmental conditions, the origin of bees, the method of processing and storage. It is not allowed to change the composition of honey by adding some substances.
Bees are a part of nature and represent one of the most nutritious foods, which is rich in many nutrients of great physiological and preventive value for the human body. However, bees are also exposed to radioactive substances, i.e., ionizing radiation that is in the environment. Some observations aft er the Chernobyl nuclear disaster showed that in some areas bees had very intense reaction to increased radioactivity like anxiety, disorientation and mass abandonment of hives (Vorgić, 2004). Increased content of radionuclides on bee pastures can aff ect general health of bees, and the presence of radionuclides in honey can signifi cantly change the quality of this high-quality food. Given the characteristics of honey (Stanimirović et al., 2000), which imply a relatively wide area where bees collect nectar, honey is certainly one of the potential bioindicators of radioactive contamination.

MATERIAL AND METHODS
A total of 66 honey samples (acacia, meadow, linden, sunfl ower, fl ower, oilseed rape, chestnut) were collected from 2020 to 2021. Th e specifi c activities of caesium-137 ( 137 Cs), potassium-40 ( 40 K), uranium-238 ( 238 U), uranium-235 ( 235 U), radium-226 ( 226 Ra), thorium-232 ( 232 Th ) and sodium-22 ( 22 Na) were determined using a coaxial HPGe detector (ORTEC-AMETEK, GEM25P4-70) with an energy resolution of 1.67 keV at the 1.33 MeV of 60 Co, Peak-to-Compton ratio 67:1 (Amp Shape Time 6μs) and the relative effi ciency of 28%. Th e detector was housed in a 10 cm thick lead shield lined by copper layer, cadmium layer and a layer of plexiglass (IAEA, 1989). Th e instrument was calibrated by the accredited laboratory for gamma-ray spectroscopy calibration and activity of radioactive sources of gamma emitters -Chair of Nuclear Physics, Department of Nuclear Physics, Faculty of Sciences, University of Novi Sad. Th e precision and accuracy of the instrument operation was controlled using standard reference material Source No. LR 320 certifi ed by the Calibration Laboratory for Measurements of Radioactivity (Deutscsher kalibrierdienst, Braunschweig, Germany).
Th e counting time for each sample was 100,000 s, and the analytical precision of the measurement was approximately ± 10%. Th e specifi c activity of 238 U was calculated from gamma-rays of 234 Th at 63.3 keV and gamma-rays Pa-234m (1001keV). Th e specifi c activity of 226 Ra was determined from the peak areas at 609.3, 1120.3 and 1764.5 keV of 214 Bi and 295.2 and 351.9 keV of 214 Pb. Gamma-ray peaks with energies 911.2 and 969.0 keV ( 228 Ac) and gamma-ray doublet 238.6 keV ( 212 Pb) and 241 KeV ( 214 Pb) as well as Tl-208 (583.2 KeV) were used for calculation of the specifi c activities of 232 Th . Specifi c activities of 40 K and 137 Cs were determined using their own 1460.8 keV and 661.7 keV peaks, respectively (Bikit et al., 1995).
Input signals to the detector through ORTEC type pre-amplifi ers and spectra amplifi ers were channelled to a multichannel analyser MCA with an analog-to-digital converter of 16384 channels total memory. MCA was directly connected with PC where the measured spectra were stored and analysed. Th e gamma spectra were acquired and analysed using the GammaVision ® software. Th is program calculates the activity concentration of an isotope from all prominent gamma lines aft er background subtraction. All the measurement of uncertainties are presented at 95% confi dence level. Th e measured specifi c activity of 137 Cs was decay corrected to the sampling date (Ortec, 2015).

Results
Th e content of radioactive substances -radionuclides on honey plants is the result of the transfer of radioactive fallout from the atmosphere through air currents, water (rain, snow, erosive processes) and soil, or their direct and indirect reach to crops of meadows, pastures and forests. Gamma-spectrometric analysis in the examined samples of honey determined the presence of natural radionuclides, namely: sodium-22, potassium-40, thorium-232, radium-226, uranium-238 and uranium-235. Th e results of the measured activities of these radionuclides are shown in Table 1 and Table 2. When it comes to artifi cial (anthropogenic) radionuclides, the activity of cesium-137 was recorded, but only in the honey samples originating from the area of Kosovo and Metohija (Table 2). A total of 15 samples from diff erent localities were tested, and the presence of biologically signifi cant 137 Cs radionuclide was determined at eight localities (which represents more than 50% of the total tested samples). It is important to note that all the measured activities of the isotope cesium-137 in the analysed honey samples are below the maximum allowable values (Offi cial Gazette of RS, No. 36/2018).

DISCUSSION
As can be seen form the tables, the highest activity was determined for radioactive potassium-40. In the tested samples, its activity ranged from 32 Bq/ kg (Table 1) to 123 Bq/kg ( Table 2). Th e values obtained for the concentration of 40 K activity are in agreement with the results reported by other authors (Borawska et al., 2013; Pöschl et al., 2011). Th e 40 K activity increased according to the type of honey in the following order: sunfl ower, fl ower (polyfl oral), linden, meadow, acacia, rapeseed and chestnut. From a biological and ecological point of view, 40 K is one of the most important natural radioactive elements. Th is radionuclide easily moves from the soil to plants and animals, and through foods of plant and animal origin, like honey it can also reach the human body. Since it is in the soil and in the human body, potassium-40 causes external and internal irradiation of all tissues, and especially radiation of soft tissues in the human body. Potassium is under homeostatic control in the body and it is estimated that the annual eff ective dose received by the human body as the result of the presence of 40 K is 165 μSv (Pavlović and Nikezić, 1995). From the obtained results, it can also be concluded that radioactive potassium-40 mostly contributes to the natural radioactivity of honey.
Th e measured activity of the natural radioisotope in the tested honey samples ranged between < 0.2 and 2.40 Bq/kg. Th ese 22 Na activities are common for most of the samples from the nature. It is estimated that the annual level of 22 Na intake by ingestion is about 50 Bq which contributes to the annual eff ective doze of 0.15 μSv (Pavlović and Nikezić, 1995). As a cosmogenic radionuclide that is formed in the constant interaction of cosmic radiation with various atoms and molecules in the atmosphere, earth and water, from the radioecological point of view it is not of great importance for the radiation safety of the biosphere.
Th e measured activity of the natural radionuclide in honey ranged from 0.82 to 2.0 Bq/kg. Th e measured values of 232 Th activity were in agreement with the values reported by other authors (Dizman et al., 2020). Various accidents at nuclear power plants can lead to a signifi cant increase in the concentration of this radionuclide in food. If 232 Th released in this way reaches the food chain, it can be deposited in bone tissue, lungs and liver, and thus increase its radioecological signifi cance in the overall radiation of the population. Th e average daily intake of 232 Th through food in the Republic of Serbia is 1.76 mBq (Đujić, 1995).
When it comes to natural radionuclides in uranium samples, the presence of uranium-238 and radium-226 was registered. Th e activity of 238 U was in the range between < 1 and 31.4 Bq/kg, which is signifi cantly higher than the values reported by other authors (Đurić and Popović, 2000) while the activity of 226 Ra ranged between 1.9 and 15.6 Bq/kg. Th e presence of these isotopes in honey is of great concern because U-238 and its derivatives Ra-226 and radon-222 pose the greatest risk for human health (Dangić, 1995). Th e total daily intake of 238 U through food is 12.48 mBq and the average daily intake of 226 Ra in Serbia is 52.3 mBq which corresponds to the average intake of 226 Ra in other countries, and its estimated annual dose is between 20 and 30 μSv (Đujić, 1995). Due to the very long physical and biological half-life, its amount in the body increases over time. Th e measured U-235 activity was in the range of < 0.2 and 1.61 Bq/ kg. In the body, uranium acts as a toxicant because it is a source of ionizing radiation, and also a chemically toxic element. Th e kidneys are critical organs for natural uranium and its isotopes because if they reach the lungs, they quickly pass into the blood and are eliminated from the body through the kidneys (Mitrović, 2001).
Th e results of honey contamination measurements, which are shown in Table 1 and Table 2, indicate that the most intense contamination comes from the artifi cial radionuclide cesium-137. It ranged from < 0.2 to 3.63 Bq/kg. Similar results for 137 Cs activity in honey and detection limit for most of the examined samples are reported by Beňová et al. (2019). Th is activity is particularly important because 137 Cs is a biologically important radionuclide which, if it reaches the human body through the food chain, is distributed to all organs almost evenly. Th e analysis of the 137 Cs content in the soil shows that there is a stronger binding of this radionuclide to the surface layers of the soil (0 -40 cm), while its migration to deeper layers (40 -80 cm) is signifi cantly slowed down. Th ese results are in full agreement with previous research on the deep penetration of 137 Cs, which showed that about 80% of its activity is retained in the surface layer of the soil.
It is interesting to point out the fact related to the radio contamination of bees. Given that honey is a bee product, it would make sense to assume that the bee organism itself will be signifi cantly contaminated. However, since bees have a short lifespan, this contamination cannot in any way aff ect the bee as a living organism, especially because insects as a species are known to be very resistant to ionizing radiation even at doses of a few tens of Gy, which are otherwise supraleathal for humans (Hadžimuratović et al., 1987). Also, when exposed to microwave radiation, bees did not show any abnormalities related to the ability to fl y, orientation, memory and effi ciency in performing work (Terzin, 2010).
Many studies of the radiochemical composition of honey (Mihaljev et al., 2001) found multiple links between the mineral composition of honey (con-tent of natural radionuclides and other micro and macro elements) and its geographical and botanical origin. Th is means that by applying appropriate statistical methods, certain types of honey can be very successfully identifi ed as well as classifi ed according to their geographical origin. Th e results indicate that the activity of natural radionuclides in honey varies depending on the geochemical characteristics of the soil, the proximity of industrial plants and that the content of radionuclides depends on the type of vegetation -honey fl ora (Đurić et al., 1996).

CONCLUSION
Th e presence of biologically signifi cant radionuclides, both natural ( 238 U, 235 U) and artifi cial ( 137 Cs), was determined in some analyzed honey samples. Th erefore, it is necessary to systematically study radioactive substances in bee pastures -their identifi cation, distribution and quantity in the system of soilhoney plant species-honey. Numerous radionuclides found in the nature with the potential to concentrate locally in honey in the levels that are dangerous for the health of the population, call for the need to carry out appropriate systematic research in order to prevent harmful eff ects of ionizing radiation.
Increasing intake of radionuclides from anthropogenic sources like accidents at nuclear power plants, application of phosphate fertilizers, pollution from thermal power plants and the use of ammunition with "depleted uranium" pose signifi cant risks to public health and environmental protection. Honey bee and its products and specifi c properties can serve as some kind of "environmental guard" because many changes and pollutants in the nature are quickly repercussed on bees that are very sensitive to any changes in the environment and react immediately to pollution (pesticides, toxic elements, radionuclides) that we normally do not notice. Th erefore, honey, as the fi nal product of bees and pollen deserves special attention when considering radioactive contamination.
Th e obtained results are of great importance for the protection of the environment of the examined areas because these data can be a reference since they represent a "zero state" in the case of future anthropogenic activities that may cause additional contamination. Regarding the determined concentrations of the activity of the tested radionuclides, it can be concluded that honey produced in the Republic of Serbia is a healthy and environmentally safe food and does not pose a radiation risk to human health.