Introduction

The typical classification of mineral fertilizers (so called N-P-K convention) is based on the concentration of the main nutrients: nitrogen (N), phosphorus (P), potassium (K). Among the fertilizers one can find single nutrient (or straight) containing one of the mentioned element, binary fertilizers containing two elements and the most commonly used group, so called NPK fertilizers, containing different amounts of all mentioned elements.

Poland has a long agricultural history, which along with the availability of domestic sources of sulphur and phosphate rocks created good conditions for chemical industry to develop. The increase in production capacities was necessary to meet the growing demand on fertilizers occurring in Polish agriculture after World War II. In 2017 Poland was in the second place among EU countries in consumption of P, K, and NPK fertilizers [1].

Originally Polish chemical plants that produce fertilizers were operating as independent entities, each specializing in one type of the fertilizer. Currently there are two major groups associating most of the biggest chemical plants. The industrial production profile of individual plants is primarily focused on nitrogen-based components, only one plant specializes in processing phosphate rocks. Additionally one plant operating independently of the mentioned groups, which is the successor of the former sulphur mine in the Tarnobrzeg area, specializes in sulphur-based components. This complicated situation caused most of the fertilizers producers to diversify their supply sources between domestic production and import.

Phosphate rock used in the production of fertilizers is a sedimentary rock that contains high levels of phosphate minerals. The concentration of phosphate rocks in Europe is relatively small, however, several European countries (e.g. Finland, Spain) exploit their phosphate deposits. On smaller scale phosphate rocks are also mined and processed in Poland, significant phosphate deposits can be found in several areas of the country, especially Świętokrzyskie Voivodeship in central Poland, Area of Gierczyn in western Poland and The Lublin Basin located in eastern Poland. It is worth mentioning that the phosphate components introduce considerable amounts of radioactive impurities to the fertilizers, as there is a relationship between concentration of uranium and thorium isotopes and the content of P2O5 in the fertilizer. For this reason, the phosphate components are often classified as NORM (naturally occurring radioactive material) [2,3,4].

As the use of fertilizer in Poland is relatively high it is important to investigate the concentration of radioactive isotopes in most popular brands and to asses their potential impact on the radiological safety of the population. The number of scientific papers about studies on radionuclides concentrations in fertilizers is constantly increasing, but most of the described papers focus on the fertilizers produced in geographic areas far from central Europe like Asia, Arabian Peninsula, Africa or South America [5, 6]. The reason for that is the poor quality of soil or small area suitable for agriculture, which in this areas results in increase in demand on high food productivity from unit area achieved mostly by fertilization [7,8,9]. There is a distinct lack of knowledge about radioactivity of Polish fertilizers. The importance of this problem was indicated in the research done by Olszewski et al. who confirmed negative radiological impact on local environment by phosphogypsum waste heap—the result of phosphorite fertilizers deposition produced by Phosphoric Fertilizers Industry in Gdańsk. [2].

In this work several commercially available fertilizers produced in Poland were subjected to gamma spectrometric measurement in order to calculate the concentration of natural radionuclides (235U, 232Th, 226Ra, 40K). The obtained results, along with the chemical compositions and dosing recommendations were used to calculate several factors important from the point of view of radiation protection.

Experimental

Samples

A total of 16 samples were examined, all of which were commercially available fertilizers produced by the Polish chemical industry. The studied fertilizers were primarily differentiated by their proposed use. Nine brands intended for home use were collected from local hardware or garden shops, while 7 brands intended for agriculture—typically sold in large quantities—were obtained directly from farmers. Further study on chemical composition was performed in order to subdivide the samples, however, differences in macronutrient part of the fertilizers were negligible, therefore the initial division based on the proposed use remained.

The composition of studied fertilizers was shown in Fig. 1. The concentrations shown in the figure describe the active (nutritious) part of the fertilizers, the inactive part (filler) is usually composed of insoluble compounds and was not taken into account.

Fig. 1
figure 1

Chemical composition of the studied fertilizers, the N, P, K, S stands for NO3−, P2O5, K2O, and SO3 respectively, micronutrients are marked as ‘other’

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As shown in the figure the most commonly represented group were NPK fertilizers, there were only 3 straight phosphate (P) fertilizers, 2 straight nitrogen (N) fertilizers and one straight potassium (K) fertilizer. Two of the studied fertilizers were special binary fertilizers, namely phosphorus—potassium (PK) fertilizer and phosphorus—nitrogen (PN) fertilizer. The concentrations of the main elements were based on the information provided by the manufacturers with the fertilizer.

Sample preparation

Most of studied fertilizers were in a form of granules formed from homogenous material, only several samples consisted of the mix of granules of different types. However, for all of the samples one analytical procedure was applied. Fertilizers were grinded in blade grinder and sieved through 0.5 mm sieve in order to obtain proper distribution of individual components.

After homogenization, material was transferred into 450 cm3 plastic Marinelli beakers. Beakers were air tightly closed and additionally sealed with rubber foam tape in order to prevent radon gas from escaping. Sealed samples were stored for at least 21 days, which is the time needed to achieve radiochemical equilibrium between 226Ra and its progenies (214Pb and 214Bi) [10].

Gamma spectrometry

Samples were measured by means of gamma spectrometry. In this study a Canberra GX 3520 detector equipped with carbon composite window was used. The background radiation was reduced by the use of passive custom made shield consisted of steel, lead, cadmium and copper. The typical counting time was between 18 and 24 h. The minimum activity concentrations, determined using the Currie method, were 14.1 Bq kg−1, 2.7 Bq kg−1, 0.9 Bq kg−1, and 8.2 Bq kg−1 for 226Ra, 228Ac, 235U and 40K respectively [11].

The efficiency calibration was performed numerically by the use of Gennie 2000 software. Each sample was individually modeled in order to obtain efficiency curve.

For geometry modeling the chemical compositions provided by the producers were used. Additionally known volume of the beakers and mass of the material were used to calculate density.

The efficiency calibration was verified by the IAEA-447 certified reference material measurement.

Radium equivalent activity (Raeq)

Radium equivalent activity (Raeq) is a commonly used parameter in the field of environmental monitoring, occupational safety, and public health. It is used to assess the potential health risks associated with natural gamma emitters present in NORM materials, as it allows to unify the radiation exposure effect caused by different isotopes. The Raeq was calculated by the following equation:

$${\text{Ra}}_{{{\text{eq}}}} = {\text{C}}_{{{}_{ }^{226} {\text{Ra}}}} \left( {{\text{Bq kg}}_{ }^{ - 1} } \right) + 1.43 {\text{C}}_{{{}_{ }^{232} {\text{Th}}}} \left( {{\text{Bq kg}}_{ }^{ - 1} } \right) + 0.077 {\text{C}}_{{{}_{ }^{40} {\text{K}}}} \left( {{\text{Bq kg}}_{ }^{ - 1} } \right)$$
(1)

where \({\mathrm{C}}_{{}_{ }{}^{226}\mathrm{Ra}},{\mathrm{C}}_{{}_{ }{}^{232}\mathrm{Th}}, {\mathrm{C}}_{{}_{ }{}^{40}\mathrm{K}}\) are the activity concentrations of 226Ra, 232Th and 40K respectively. The factors in formula 1 were proposed by Mahamood [12] and represent the contribution of mentioned isotopes to the gamma dose rate.

Absorbed dose rate increase (\({ }\mathop {\text{D}}\limits^{ \cdot }_{{{\text{inc}}}}\))

Absorbed dose is an important factor in radiation protection, allowing to prospectively estimate the potential negative effect of radiation from soil on human. In order to calculate the external gamma dose rate 1 m above the ground level caused by the use of fertilizers the conversion factors from UNSCEAR report were used. The Ḋinc was calculated from the Eq. 2.

$${\dot {\text{D}}_{{\text{inc}}}} = {\text{k}}_{{{\text{Ra}}}} {\text{C}}_{{{}_{ }^{226} {\text{Ra}}}} + {\text{k}}_{{{\text{Th}}}} {\text{ C}}_{{{}_{ }^{232} {\text{Th}}}} + {\text{k}}_{{\text{K}}} {\text{ C}}_{{{}_{ }^{40} {\text{K}}}}$$
(2)

where \({\text{C}}_{{{}_{ }^{226} {\text{Ra}}}} ,\) \({\text{C}}_{{{}_{ }^{232} {\text{Th}}}} ,\) \({\text{C}}_{{{}_{ }^{40} {\text{K}}}}\) are the activity concentrations of 226Ra, 232Th and 40K in soil, kRa is the conversion factor for 226Ra equal to 0.462 (nGy kg h−1 Bq−1), kTh is the conversion factor for 232Th equal to 0.604 (nGy kg h−1 Bq−1), and kRa is the conversion factor for 40K equal to 0.0417 (nGy kg h−1 Bq−1) [13].

As the aim of this calculations was to asses the increase of absorbed dose from ground treated with the fertilizer, not from the fertilizer itself according to the methodology given by Ugolini et al. [14]. For this purpuse the following assupmtions were made.

The concentrations of elements introduced into the soil were calculated for a 5 cm thick soil layer of 1.3 g cm−3 density. This represents typical properties of cultivated soils in Poland [15]. The amount of fertilizer to be dispersed was taken from recommended dosing, provided with each fertilizer by the producers. As the dosing recommendations differ significantly between intended crop types or time of the year, for this study maximum values were taken into account.

Increase in annual effective dose equivalent (AEDE)

Results from annual effective dose rate increase were used for further calculations in order to provide information about the annual increase in effective dose. The EADE was calculated by the following equation:

$${\text{AEDE}} = {\dot {\text{D}}_{{\text{inc}}}} \left( {{\text{nGy h}}^{ - 1} } \right) \times 8760 \left( {{\text{h y}}^{ - 1} } \right) \times 0.2 \times 7 \times 10^{ - 7} \left( {{\text{mSv nGy}}^{ - 1} } \right)$$
(3)

The 0.2 factor in (3) represents the average amount of time spent outdoors [13]. The conversion factor from absorbed dose to effective dose was taken from Diwa et al. [16].

Results and discussion

Activity concentrations

The activity concentrations of 235U, 232Th, 226Ra, and 40K were presented in Table 1. The activities of 235U and 40K were calculated directly from 143.8 keV and 1460.8 keV gamma lines respectively. The 232Th thorium activity was calculated upon assumption of activity equilibrium with 228Ac and 212Pb. The activity of 226Ra was calculated also by assuming equilibrium with progenies (namely 214Pb and 214Bi) but also by direct counting at 186.2 keV. The second method required subtracting overlapping counts from 235U (line 185.7 keV) and was therefore described by higher uncertainty.

Table 1 Activity concentrations of gamma radionuclides in studied samples
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The N fertilizers were the only samples with almost all results below the minimum activity concentrations. It indicates that the introduction of gamma radionuclides to the environment (and consequently food chain) is in this case negligible.

Among the multinutrient fertilizers the highest activity concentrations were observed for 40K in NPK fertilizers (maximum value reached 6110 ± 600 Bq kg−1) however the highest overall activity of 40K was observed for single nutrient potassium fertilizer (13,800 ± 1400 Bq kg−1). These results are directly related to the concentration of 40K in natural potassium, present in samples in different amounts. The phosphate fertilizers were in general described by high activities of 235U and 226Ra, where the maximum uranium (69.3 ± 7.4 Bq kg−1) and radium (2120 ± 370 Bq kg−1) activity concentrations were found for P2 and P1 samples respectively.

As the chemical composition of fertilizers is widely variated (Fig. 1), the results were rearranged. The activity concentrations of 232Th and 226Ra were plotted versus the concentration of phosphorus in the nutrient part of fertilizers. The results are presented in Fig. 2.

Fig. 2
figure 2

Relationship between activity concentrations of 226Ra and 232Th and phosphorus concentration in fertilizers

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The increase of the activity can be observed in both plots, however the 226Ra results are better fitted with the linear trend (marked as the dashed line) than 232Th. This observation was verified by calculating the Pearson correlation coefficient, that measures linear correlation between two sets of data [17]. The calculated coefficients were equal to 0.95 for radium and 0.37 for thorium. This weak correlation for thorium may indicate some disturbance in equilibrium in 232Th → 228Ra → 228Ac part of the decay chain. This aspect requires further investigation involving direct determination of 232Th.

Radium equivalent and absorbed dose increase

The calculated results of radium equivalent and absorbed dose rate increase were presented in Table 2.

Table 2 Radiological parameters of the studied samples
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The potassium fertilizer along with phosphate fertilizers were described by the highest radium equivalent activity values. The highest activity measured in this study was found for 40K in potassium fertilizer however, the highest Raeq values characterize the phosphate fertilizers. This was reflected in the low conversion factor for potassium activity in Eq. 1, related to the low probability of gamma radiation emission for 40K of about 10.5%.

Two factors, the radium equivalent but also the recommended dosing of the fertilizer affected the absorbed dose rate increase along with the increase in annual effective dose equivalent. Despite the fact, that P fertilizers had the highest values of Raeq in the entire study, the maximum observed values of \(\mathop {\text{D}}\limits^{ \cdot }_{{{\mathbf{inc}}}}\) and AEDE were found for some of the NPK fertilizers (namely NPK2, NPK4, NPK6, and NPK8). This is mainly due to the different restrictions of the maximal recommended dosing of the fertilizer effecting in introducing more radionuclides with them.

In order to compare the obtained results with the fertilizers reported in the literature maximum values of AEDE for each group of fertilizers were taken as the representative values of Polish fertilizers. The results of the comparison are presented in Table 3.

Table 3 Radium equivalent activity and increase in annual effective dose quivalent of fertilizers from different countries
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Conclusions

The consumption of artificial fertilizers for agricultural production in Poland is significantly high compared to other European countries, however lack of information about radionuclides concentrations in those fertilizers was observed. In this work, 16 samples of commercially available fertilizers were studied.

The highest activities of uranium and radium isotopes were found for phosphate fertilizers. Positive correlation between phosphorus content and 226Ra activity was confirmed by calculating Pearson correlation factor. The relatively low value of correlation factor for 232Th activity and phosphorus content needs further investigation.

It was found that in 7 cases the Radium Equivalent (Raeq) was above the limit of 370 Bq kg−1 set by Nuclear Energy Agency, the highest result was almost 6 times higher than the limit [18]. On the other hand, absorbed dose increase and annual increase in effective dose calculated from the obtained results, allows to conclude that there is no significant radiological hazard posed by the studied fertilizers. The highest value of the AEDE was 0.67 mSv y−1 which is considerably low in comparison to the natural gamma radiation in Poland, which is 2.48 mSv y−1[19].

The comparison of the results with the available data allows to conclude, that despite having the highest values of radium equivalent activity, in terms of absorbed dose increase and annual increase in effective dose Polish fertilizers do not achieve any extreme values in comparison to other countries.