Introduction

Natural and artificial radionuclides are transferred to plants through uptake from the soil via roots and absorption directly through leaves (James et al., 2011; Vandenhove et al., 2009). While some radionuclides are taken up as homologues of primary elements, others are taken up regardless of their biological emergency. For the growth and reproduction of vegetation, there are sixteen essential elements: hydrogen, carbon, nitrogen, oxygen, sulfur, phosphorus, calcium, potassium, iron, magnesium, zinc, manganese, molybdenum, copper, chlorine, and boron (Karunakara et al., 2013; Linsalata, 1994). However, a number of natural radioactive elements like 238U, 232Th, and 40K, cosmogenic radionuclides such as 7Be and artificial radionuclides such as 137Cs and 90Sr are present in plants in assorted concentrations (Karunakara et al., 2013). According to UNSCEAR, individual plants experience an 83% annual effective dose from natural radionuclides, 16% is contributed by primordial 40K, and the remaining 1% is due to artificial radionuclides (UNSCEAR, 2008).

The soil-to-plant transfer factor (TF), or the ratio of the concentration of radioactivity in the crop-to-soil radioactivity per unit mass (Bq kg−1 dry mass), is used to study the impact of radionuclides on the environment. TFs are convenient indices for establishing the degree of uptake of radionuclides from soil to plants. TFs are the most important parameters for modeling and simulating impact assessments of contamination in the surrounding environment.

Since TFS strongly depend on the soil and vary from site to site, site-specific data is recommended (James et al., 2011). In most countries in Europe and the USA, the TFs for most important agricultural products are known. In the rest of the world, especially developing countries, TFs are not so easily available. Therefore, the estimation of TFs in a country like Iraq is vital (IAEA, 2006).

Radionuclides present in the soil and not used in plant metabolism are absorbed regardless of their radioactive characteristics (Asaduzzaman et al., 2015). Soil flow by natural and fallout radionuclides has a nonstop radiological effect, since these radionuclides are transferred to the human body through the food chain and drinking water. Plant uptake is the major cause for the relocation of radionuclides from the soil into human foodstuffs (Shanthi et al., 2012; Shanthi et al., 2012). Radionuclides in the edible portions of plants may be a source of exposure (Shanthi et al., 2012). Nevertheless, radionuclide distribution and uptake in plants depend on various factors such as the kind and amount of clays, soil pH, exchangeable calcium and potassium, the physicochemical properties of the radionuclide, the kind of crop (species, variety, and cultivation practices), fertilizer application, irrigation, plowing, liming, climate conditions, organic matter content, etc. (Pulhani et al., 2005). Diet is the main cause of internal human exposure to radioactive elements (Saeed et al., 2012). After absorption by the root, radionuclides are transported into the plant along with other nutrients or minerals needed for their growth and reproduction (James et al., 2011). These radionuclides translocate toward various portions of the plant through the vascular system, including the xylem and phloem. They accumulate in various edible portions and lead to a continuous radiation dose once consumed (Pulhani et al., 2005).

The soil-to-plant transfer factor is one of the significant parameters widely used in the evaluation of internal radiation dose from food consumption (Tsukada et al., 2002). The transfer factor depends on soil properties, vegetation type, the type of radionuclides, and the climatic conditions (Asaduzzaman et al., 2015). Various studies on the transfer of natural radionuclides from soil to plant have been carried out in several regions around the world and have observed a notable difference in values (Alharbi & El-Taher, 2013; Currie, 1968; Mheemeed et al., 2014; Ononugbo et al., 2019; Pulhani et al., 2005; Shanthi et al., 2012; Shayeb et al., 2017; Velasco et al., 2012; Wang et al., 2015).

However, there seems to be little data on the transfer of natural radionuclides from soil to plant in the environment. Therefore, the current study aims to determine the natural radionuclide TF in some agricultural crops under natural field conditions. It will consider the concentration of the radioactive isotopes 40K, 226Ra, and 232Th in soil and plants. Finally, it will calculate the absorbed dose rate (Dr) due to gamma radiation in outdoor air 1 m above the soil surface, the radium equivalent activity (Raeq), the gamma index (Iγ), the external hazard index (Hex), and the internal hazard index (Hin).

Materials and methods

Sample collection and processing

The transfer factors of 238U, 232Th, and 40K from the soil into cultivated plants were examined for ten crop samples in agricultural areas in the outskirts of Baghdad and one crop sample from the Al-Mishkab district in Al-Najaf. The region is known for cultivating the finest type of rice in the world (amber) (Fig. 1 and Table 1). The crops include cereals (rice and wheat), fruits (lemon and orange), podded vegetables (vigna and okra), fruity vegetables (chili pepper, Solanum melongena), and leafy vegetables (Apium graveolens, Raphanus sativus, and Ocimum basilicum). The crop samples were collected with cultivated soil. The samples were prepared by separating them from non-edible parts and drying, crushing, and sifting them with a sieve (630 µm mesh size). They were fully mobilized in sealed Marinelli beakers and stored for 30 days so that a secular equilibrium between 238U and 232Th with their decay products was reached.

Fig. 1
figure 1

Locations of the plants under study

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Table 1 Crop and soil information
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Soil samples were collected at a depth of 20 cm below the soil surface. These samples were prepared by removing unwanted materials such as roots, gravel, stone, and leaves. About 1 kg of soil was dried in an oven at 100 °C for 1 h to achieve a constant dry weight. The samples were crushed into a fine powder, homogenized and placed inside a Marinelli beaker to be examined 30 days later via gamma-ray spectrometry.

Gamma-ray spectrometry with a HPGe detector

The specific activity of 238U, 232Th, and 40K in the samples under study were measured via shielded γ-ray spectrometry with a HPGe detector (a cylindrical single crystal with a dimension of 3 × 3 inches) connected to a multi-channel analyzer (model: DSPEC-LF, ORTEC, USA). The HPGe detector was calibrated with a 152Eu source (activity = 1 µCi) with the following energy lines: 121.8, 244.7, 344.3, 411.1, 778.9, 964.0, 1085.8, 1112.0, 1299, and 1408.0 keV.

For the present work, calibration efficiency was achieved with a standard mixture source. The source contains ten mixed radionuclides: 241Am-109Cd-139Ce-57Co-60Co-137Cs-113Sn-88Sr-88Y-203Hg. This source is specialized for gamma spectroscopy calibration systems. The measuring time for the background and the samples was 24 h (Ammer et al., 2017). Figure 2 shows the radionuclides in the spectrum of the standard mixed source. Table 2 shows the information of isotopes in the mixed source. The uncertainty of the measured specific activity concentration of samples (UA) is estimated by Eq. 1 (Kadhim et al, 2021):

$$\frac{{U}_{A}}{A}=\sqrt{{\left(\frac{{U}_{N}}{N}\right)}^{2}{+\left(\frac{{U}_{B}}{B}\right)}^{2}{+\left(\frac{{U}_{\varepsilon }}{\varepsilon }\right)}^{2}{+\left(\frac{{U}_{M}}{M}\right)}^{2}+{\left(\frac{{U}_{{P}_{\gamma }}}{{P}_{\gamma }}\right)}^{2}}$$
(1)

where UN is the uncertainty of the sample count rate; UB is the uncertainty of background count rate; Uε is the efficiency uncertainty at choose energy; UM is the uncertainty of spices mass measurements; and UPγ is the uncertainty gamma line. The average uncertainty 6% is founded in the present measurements.

Fig. 2
figure 2

The detected radionuclides in the spectrum of the standard mixed source

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Fig. 3
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Spectrum of (S7) sample, Solanum melongena

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Table 2 Radionuclides with energy peak E (keV), half-life t1/2, decay constant λ, original activity Ao, activity at time of measurement At, intensity I (%), and efficiency ε (%) for each isotope in the mixed source
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Specific activity

The specific activities, Bq kg−1, for i radionuclide (Ai) at energy peak Eγ are calculated as follows (Kadhim & Ridha, 2019):

$${A}_{i}({E}_{\gamma })=\frac{N}{t\times {I}_{\gamma }({E}_{\gamma })\times \varepsilon ({E}_{\gamma })\times m}$$
(2)

where N is the net peak area, Iγ is the abundance of energy Eγ, t is the time of measurement, ε is the detection efficiency at photo peak energy, and m is the weight of the sample.

The lower limits of detection (LLD [Bq kg−1]) used to estimate the lowest activity of a specific radionuclide at the time of measurement are listed in Table 3 for 238U, 232Th, and 40K (Currie, 1968). Table 3 shows the radionuclides detected in the samples and some other important information. Figures 3 and 4 show the spectrum of the soil and crop samples (S7 and C7) of Solanum melongena.

Table 3 The lower detection limits for each radionuclide, their related series, half-lives, gamma energies and intensities (Currie, 1968)
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Fig. 4
figure 4

Spectrum of (C7) sample, Solanum melongena

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Fig. 5
figure 5

Specific activities (238U) of soil and plant samples with the transfer factor (TF) for each

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Soil-to-crop TF

The TF from soil to crop is calculated from the specific activity of the natural isotope in both crop and soil samples by Eq. 2 (Alharbi & El-Taher, 2013; Karunakara et al., 2013; Mheemeed et al., 2014):

$$\mathrm{TF }= \frac{\mathrm{The\;specific\; activity\; of\; an\; isotope\; in\; a\; crop }\left(\mathrm{in\; Bq }\;{\mathrm{kg}}^{-1}\;\mathrm{ dryweight}\right)}{\mathrm{The\; specific\; activity\; of\; an\; isotope\; in\; soil }\left(\mathrm{in\; Bq }\;{\mathrm{kg}}^{-1}\;\mathrm{ dryweight}\right)}$$
(3)

Radiation hazard parameters

The radiation hazard parameters for the soil and crop samples were calculated. The absorbed dose rate (Dγ) due to gamma radiation in outdoor air 1 m above the soil surface, the radium equivalent activity (Raeq), the gamma index (Iγ), the internal hazard index (Hin), the annual effective dose rate (Eff), and the annual gonadal dose equivalent (AGDE) were calculated via the following equations 3, 4, 5, 6, 7, 8 and 9 (UNSCEAR, 2000, 2008, 2010):

$$D\gamma (\mathrm{nGy h}-1)=0.462{\mathrm A}_{\mathrm {Ra}}+0.621{\mathrm A}_{\mathrm {Th}}+0.0417{\mathrm A}_{\mathrm K}$$
(4)
$${\mathrm{Ra}}({\mathrm{eq}})={\mathrm{A}}_{\mathrm{Ra}} + 1.43{\mathrm{A}}_{\mathrm{Th}}+0.077{\mathrm{A}}_{\mathrm{K}}$$
(5)
$${I}_{\gamma }=\frac{{A}_{Ra}}{300}+\frac{{A}_{Th}}{200}+\frac{{A}_{K}}{3000} \le 1$$
(6)
$${H}_{in}=\frac{{A}_{Ra}}{185}+\frac{{A}_{Th}}{259}+\frac{{A}_{K}}{4810} \le 1$$
(7)
$$\mathrm{Eff\; dose} (\mathrm{mSv y}-1)=\mathrm{D}\gamma (\mathrm{nGy h}-1)\times 8760\hspace{0.17em}\times 0.7\times(103 \mathrm{mSv}.10-9) \mathrm{nGy}-1\times 0.2$$
(8)
$$\mathrm{AGDE} \left(\mathrm{mSv y}-1\right)=\left(3.09\mathrm{A}_\mathrm{Ra}+4.18{\mathrm A}_{\mathrm {Th}}+0.314\mathrm{A}_\mathrm{K}\right)\times 10-3$$
(9)

where ARa, ATh, and AK are the activity concentrations of 238U, 232Th, and 40K, respectively (Kadhim & Ridha, 2019; UNSCEAR, 2008).

Results and discussion

Specific activity

238U, 232Th, and 40K activity are detected in the soil and crop samples with 214Pb and 214Bi from the uranium-238 series, 212Pb, 208Tl, and 228Ac from the thorium-232 series and the single radionuclide 40K. Cesium-137 was not detected in any of the samples, which indicates that these areas are not contaminated. The 238U, 232Th, and 40K specfic activity concentrations, Bq kg−1, are presented in Figs. 5, 6, and 7 as cumulative bars for soil and crop samples.

Fig. 6
figure 6

Specific activities (232Th) of soil and plant samples with the transfer factor (TF) for each

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Fig. 7
figure 7

Specific activities (40K) of soil and plant samples with the transfer factor (TF) for each

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The 238U specific activities of the soil varied from 16.66 ± 4.08 to 11.87 ± 3.53 Bq kg−1. The 232Th soil specific activities had a low value of 12.19 ± 3.49 Bq kg−1and a high value of 37.46 ± 6.12. The 40K specific activities of the soil samples ranged from 242.38 ± 15.57 Bq kg−1 to 308.67 ± 17.57 Bq kg−1.

The specific activities of all crop samples for 238U were below LLD except for C4 (wheat), which had an activity of 4.13 ± 2.07 Bq kg−1. The specific activities of 232Th range from LLD for C6 (chili pepper) to 8.61 ± 2.34 Bq kg−1 for C4 (wheat). However, the 40K specific activity ranges from 39.94 ± 6.31 for C1 (rice) to 972.19 ± 31.17 Bq kg−1 for C7 (Solanum melongena).

Table 4 presents the soil and crop mean specific activities of 238U and 232Th, which are below the worldwide average for most samples under consideration. For the Solanum and Raphanus sativus soils, the specific activities of 232Th are higher than the worldwide average. This is due to the erosion of 232Th, which was adsorbed in the soil. However, 238U is removed simply with irrigation water. Also, this variation in radioactivity could result from the type of soil deposit and the geotechnical characteristic of the area. This may cause a higher accumulation of 232Th than 238U (Asaduzzaman et al., 2015; Jilbert et al., 2016; Zubair & Shafiqullah, 2020).

Table 4 The specific activities and transfer factor of the radionuclides in soil samples
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The specific activities of 40K for soil samples fall within the worldwide range (400 Bq kg−1), but the crop samples of vigna, okra, Apium graveolens, Raphanus sativus, and Ocimum basilicum have higher specific activities of 40K than the world average. This is due to the cation exchange capacity (CEC) in the soil, the pH of the soil and the type of soil (Asaduzzaman et al., 2015).

Transfer factor

The soil-to-crop TF is calculated from Eq. 2 and listed in Table 4. The maximum TF value of 238U is 0.32 for wheat. The maximum value of TF for 232Th is 0.7 in wheat; for 40K, it is 3.44 in Solanum melongena.

The variations in TF for different soils may be due to soil features such as granulometric production, mineralogical/organic matter content, pH, and hydrological conditions within the soil (Asaduzzaman et al., 2015; Jilbert et al., 2016; Zubair & Shafiqullah, 2020). The biological variability inherent in plants and differences between types and species are likely sources of the variations in transfer factors. Soil control, crop-farming technologies, the growing period, and the properties of root distribution also have an effect. The above parameters may change soil properties or cause the redistribution of radionuclides in the root zone; consequently, they influence radionuclide uptake in crops.

The kinds of soil and farming data are significant factors because the behaviors of radionuclides depend on the sampling conditions and soil properties. The soil-to-plant transfer of natural radionuclides is heavily influenced by the soil’s physio-chemical properties, such as potassium (K) content, cation exchange capacity (CEC), organic matter content, calcium (Ca) content, etc. (Asaduzzaman et al., 2015).

Hazard parameters

Tables 5 and 6 show the radiation hazard parameters of the crop and soil samples. The results show that all these parameters are below the global limits and world averages.

Table 5 Radium equivalent (Raeq), Hin, Iγ, Dγ, Eff dose, and AGDE for soil
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Table 6 Radium equivalent (Raeq), Hin, Iα, Iγ, Dγ, Eff dose, and AGDE for crops
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For crops, the highest radiation parameters were reported for C7 (Solanum melongena), while the lowest were reported for C1 (rice). For soil, the highest values were reported for S7, the soil collected from Al-Tuwaitha used to cultivate Solanum melongena. The lowest values were reported for S4, the soil collected from Abu Ghraib in which wheat was cultivated.

Conclusions

Natural radionuclides were detected in crop and soil samples. The specific activities of all crop samples for 238U were below LLD except for C4 (wheat). The absence of cesium in all samples indicates that these areas are not contaminated. The average specific activities of radionuclides in uranium-238 and thorium-232 chains were below the worldwide average in crop samples. The specific activities of potassium-40 were above the worldwide average in most of the crop samples, although the activities in the soil samples were below the recommended value set by UNSCEAR (2000). The soil-to-crop transfer factor (TF) values were higher than the default values set by IAEA. The radiation hazard parameters are lower than the global limits. The highest radiation hazard parameters for soil were detected in the sample from Al-Tuwaitha in which Solanum melongena was cultivated; this is because the soil sample from this region has the highest value of 232Th, 37.46 Bq kg−1. This may be related to the proximity of this farm to the Iraqi Atomic Energy Commission. The highest radiation hazard parameters for crops were detected in Solanum melongena because it has the highest value of 40K, 972.19 Bq kg−1. This may be related to the nature of this crop, which has a high potassium content.