Natural radiation of chemical fertilisers and radiological impact on agriculture soil

To investigate the radiological hazards indices of chemical fertilisers commonly used in agriculture soils of Arasbaran, Iran, the activity concentration of 226Ra, 232Th, and 40K in four types of chemical fertilisers were measured by gamma spectrometry system. The concentration range for 226Ra, 232Th, and 40K was obtained from 54 ± 2.2 to 1585 ± 29.4, 6 ± 0.1 to 157 ± 9.1, and 31 ± 1.5 to 1496 ± 32.8 Bq/kg, respectively. The obtained results showed that the fertiliser's average radiation hazard parameters were higher than the reference level. The outcomes give rise to more parameter values over the permissible limit, increasing the likelihood of adverse health consequences.


Introduction
Natural radiation sources of terrestrial origin can be found in varying degrees in all media of the environment, including soil [1][2][3], sediment [4][5][6][7], water [8][9][10][11], air [12], and even the human body [13]. One of the radioactivity causes in soils other than those of natural origin is the widespread use of phosphate-rich fertilisers for agricultural reasons [14]. There is a direct relationship between the 238 U series and fertiliser phosphorus pentoxide (P2O5) content [15].
Phosphate rock is used as a raw material for fertilisers, and it contains radionuclides from the natural 238 U ( 226 Ra) and 232 Th series and 40 K. Chemical fertilisers, mostly compound commercially known as NPK (nitrogen (N), phosphorus (P), and potassium (K)) and NPKs, are currently used to replace nutrients in soils and thus supply substances to achieve high agricultural productivity (sulfate-based fertiliser). Their formulae differ significantly, and the concentrations are selected based on each soil and cultivation [16].
Excessive fertiliser usage may lead to increased radionuclides in soil and groundwater, which are exposure sources for food crops and drinking water. Gamma radiation from natural radioactivity accounts for 85% of the total global annual average ionising radiation [17]. Higher exposure to such radionuclides is hazardous to human health. It may result in various health issues, including lung, pancreatic, liver, hepatic, bone, and kidney cancer [4].
In general, fertiliser use has grown globally over the last decade, with demand for all fertilisers rising from 161 million tons in 2008 to more than 200 million tons in 2018, with nitrogen fertilisers being the most widely used [18][19][20]. As a result, during the last ten years, chemical fertilisers, particularly nitrogen, have grown, which has typically been to enhance crop production, the most significant of which was wheat and corn [21]. As a result of the increased harvesting due to the use of artificial fertilisers, environmental expenses and soil and ecosystem degradation are incurred.
Recently, research has been carried out and reported on natural radiation and radiological hazards from chemical fertilisers [22][23][24][25]. This study's objectives were to evaluate natural radioactivity concentration in chemical fertilisers used in the Ahar area, Iran, and to calculate the radiological risk due to chemical fertilisers used for agriculture.

Study area
The area under study covers the fertile regions of district Arasbaran. Arasbaran, located at 38.45138° N, 47.0676° E, is situated on the northern slopes of the Buzghush mountain. The chief river flowing from the west to east of Ahar is Aharchayi. The area is noted for its Moderate mountains. The area's topography involves rugged terrains, ranging from 800 to 1800 m a.s.l. The study area is shown in Fig. 1.

Sample collection and preparation
Four different main types of chemical fertilisers were gathered from farmers and markets. The investigated sample types are ammonium nitrate (AN), ammonium sulfate (AS), ammonium phosphate (AP), and potassium nitrate (PN). For each type of chemical fertiliser, different brands were sampled. Finally, 21 samples were packed in a polyethene container and transferred to the laboratory for measuring. The collected samples were dried in an oven at around 75 °C) for 24 h to eliminate all moisture; the samples were crushed, homogenised, and sieved through a 200 μm sieve. Twentyone samples were weighed and put in a 0.5 kg Marinelli beaker. The beakers were sealed entirely for four weeks to achieve secular equilibrium between daughter radionuclide 222 Rn and parent 226 Ra, which occurs when the daughters' decay rate equals that of the parent. This step is required to guarantee that 222 Rn gas is restricted inside the volume and that the daughters remain in the sample [26]. Table 1 shows several physical and chemical features of chemical fertilizers. Fig. 1 The geographical location of study area (Arasbaran, East Azerbaijan)

Measurement process
A gamma-ray spectrometer with a high-resolution coaxial p-type vertical HPGe detector was used to measure the radioactivity. The detector has a relative efficiency of 80% when compared to the NaI (Tl) (3 × 3) detector and has a resolution of 1.8 keV at full-width half-maximum FWHM of the 1332.5 keV gamma-ray photopeak from 60 Co. The system was coupled to a high count-rate Multi-Task 16 k MCA card. Commercial software Gamma 2000 from Silena-Italy was used for data analysis. The gamma-ray spectrometer's energy calibration was performed with point sources. The International Atomic Energy Authority supplied reference materials RGU-1 (U-ore), RGTh-1 (Th-ore), RGK-1 (K2SO4), and IAEA-375 were used for full energy peak (FEP) efficiency calibration of the gamma-ray spectrometer. The samples and standard materials geometry were the same geometry. Each of these reference materials was counted in the same geometry with samples until a good counting statistic was achieved. The FEP efficiency (ε γ ) for each gammaray energy of interest was then computed using the following equation [1]: where Cr is the net count rate of the region of interest's gamma-ray photopeak, P is the probability of the gammaray of interest, and A is the radioactivity of the reference material in Bq. Each soil sample was placed on the detector's top and counted for about 80,000 s. To acquire net counts for the sample, background radiations were measured under the same circumstances as sample measurements and removed. The activity concentrations in the 226 Ra and 232 Th decay series were averaged from gamma-ray photopeaks at various energies, assuming secular equilibrium. The activity concentration of 226 Ra was determined using the gamma-ray lines of 351.9 keV from 214 Pb and 609.3 keV from 214 Bi. The activity concentration of 232 Th was calculated using the gamma-ray photopeaks of 911.2 keV from 228 Ac and 583.2 keV from 208 Tl. The gamma-ray lines at 1460.8 keV were used to quantify the activity concentration of 40 K [27].

Uncertainty calculation
Uncertainty assessment was carried out before activity computation, with the activity concentration uncertainty (U A ) computed using the equation [28]: where U N is the sample counting uncertainty; U B , background counting uncertainty; U ε , efficiency uncertainty, U M , mass measurements uncertainty and U P G amma line energy uncertainty [29].

Radiological risk assessment
Some parameters, such as absorbed dose rate, Ambient dose equivalent rate (H*(10)), gamma index, and alpha index, can be used to calculate exposure risk.

Absorbed dose rate
The absorbed dose rate (D, nGy/h) of gamma radiations in one meter above the ground surface by 226 Ra, 232 Th, and 40 K radionuclides is calculated using Eq. (3). The absorbed dosage rate was calculated using conversion factors based on the particular radioactivity levels, A K , A Ra , and A Th (Bq/ kg) of 40 K, 226 Ra, and 232 Th, respectively [30].

Ambient dose equivalent rate (H*(10))
H*(10) is a quantifiable quantity providing an effective dose assessment that measures the radiation exposure risk to human health. The ambient dose equivalent rate of 226 Ra, 232 Th, and 40 K is calculated 1 m above the ground surface. The computation of the ambient dose equivalent rate (H*(10)) is as follows [1,31]: where A Ra , A Th and A K (Bq/kg) are the activity concentration of 226 Ra, 232 Th and 40 K, respectively.

Gamma index
This factor calculates the degree of γ-radiation risk in connection with phosphate fertilisers. The hazard gamma index (Iγ) in Bq/kg is calculated using the following equation [32]: To be at the safe level, the value of Iγ must be less than one, which is related to an annual effective dose of ≤ 1.0 mSv/y [33].

Alpha index
Alpha index (Iα) is a significant hazard evaluation aspect used to detect excess alpha radiation from radioactive sources. This parameter is evaluated using the following formula [34]: According to reference [35], the radiative material activity level of 226 Ra above 200 Bq/kg is assumed to contribute to 222 Rn concentration > 200 Bq/m 3 of 222 Rn exhalation, which recommended activity level of 226 Ra is 200 Bq/kg, which gives Iα = 1 [35].

Radioactivity concentration
The activity concentrations of 226 Ra, 232 Th, and 40 K of different fertiliser types, together with the measurement uncertainty (1σ) and average, minimum, and maximum values, are presented in Table 2   3 Bq/kg. The recommended limit is more than the unit (< 1) for Iα that is according to 200 Bq kg −1 of 226 Ra [35]. The alpha index (Iα) in the sample Ammonium Sulfate was less than the given limit. In contrast, ammonium nitrate (AN) and potassium nitrate (PN) were equal to the limit range. Also, the alpha index value in ammonium phosphate (AP) was more than four times. These observed values are  more than unity and show that the ammonium phosphate fertilisers are not safe from environmental radiation hazards and need more investigation. Figure 3 shows the studied samples' alpha index's maximum, minimum, and average values. As shown in Table 3, the value of absorbed dose rate (D) in Ammonium Nitrate, Ammonium Sulfate, Ammonium Phosphate and Potassium Nitrate samples ranged from 65.9-250.7 nGy/h, 94.8-167.6 nGy/h, 212.2-817.2 nGy/h and 91.9-254.4 nGy/h, respectively. The maximum and minimum absorbed dose rate (D) was found in Ammonium Phosphate (AP) and Ammonium Nitrate (AN). The average value of absorbed dose rate (D) in all studied fertilisers is higher than the international limit of 59 nGy/h [13].

Radiation hazard indices
As illustrated in Table 3, the values of ambient dose equivalent rate (H*(10)) vary between 89.0-234. 6  The H*(10) average values in all fertilisers were higher than the average ambient dose equivalent rate value in soil samples(119.16 nGy/h) reported by the author [1]. Figure 4 shows the maximum and minimum with the main ambient dose equivalent rate (H*(10)) in four types of studied fertilisers. Table 3. The gamma index(Iγ), alpha index(Iα), absorbed dose rate (D), and ambient dose equivalent rate (H*(10)) with maximum-minimum and average values in fertiliser samples.

Conclusion
Low-level gamma spectrometry was used to determine the activity concentrations of natural radionuclides 226 Ra, 232 Th, and 40 K in 21 samples from 4 types of fertiliser in the Arasbaran area, Iran. The obtained results have shown that the radiation hazard parameter Iγ for all fertiliser type samples) is higher than the reference level (> 1), except for the AN-3 sample, which is equal to the limit value. Also, the radiation hazard parameter Iα average value for all fertiliser samples is higher than the reference level > 1, except for the ammonium sulfate (AS) samples. In all investigated samples, the average values of absorbed dose rate (D) due to the fertiliser samples are higher than the reference level of 59 nGy/h [13] and ranged from 137.2 to 454.3 nGy/h.
The computed average ambient dose equivalent rate (H*(10)) values for each group fertiliser were found to be 1.6 to 5.5-fold in typical soil samples (Fig. 4). The use of fertilisers has a significant impact on radionuclide concentrations in agriculture soils, especially phosphorus and potassium-containing fertilisers being one of the causes of the high activity of 226 Ra and 40 K in the soil. Using these fertilisers causes a buildup of radioactivity in soils, which can be detrimental to the health of farmers, employees, and product consumers.  Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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