1 Introduction

Studies have been conducted in many regions and countries to determine the concentration of naturally occurring radionuclides and the effective dose from ingestion in several foodstuffs [1,2,3,4,5]. Nevertheless, there is a paucity of data in South Africa, especially with the estimation that a large portion, at least one-eighth of the mean annual effective dose internally due to natural sources, is by food intake [5, 6].

The Maize (Zea mays) remains South Africa’s most dominant grain crop grown under diverse environmental conditions [7]. It is a principal staple food of the population’s daily diet, consumed in a variety of ways and accounting for around 70% of caloric human intake [7,8,9]. According to reports [9, 10], the total production of maize (white and yellow) in South Africa increased by 6% to approximately 15.8 million metric tons during the 2020/2021 cropping season, with about 5.2 million metric tons for human consumption. This is a vegetable in corn or when processed as maize meal, polenta, grits, and derivatives as corn starch, syrup, dextrose, or corn oil. Conversely, the second-cycle consumption by man is found in the increased demand for animal protein, eggs, and dairy products [9].

Radionuclides transmission to foodstuffs is through direct deposition from the atmosphere or root uptake with other nutrients or mineral elements necessary for plant growth and reproduction in the soil [11, 12]. These radionuclides accumulate in several parts of the plant, including the edible portions, through the xylem and phloem’s vascular system [13].

Once ingested, the contaminated food results in internal radiation exposure, posing a health effect depending on the radionuclides concentration and the duration of exposure [14,15,16].

Although the KwaZulu-Natal (KZN) province ranked after the Free State, Mpumalanga, and Northwest in the major maize-producing areas of South Africa, the region has good environmental conditions with agricultural land devoted to the production of grain and seed crops [9, 17]. It is grown widely by large-scale commercial and emerging farmers with smaller plots of land and limited access to external inputs for household consumption [17]. Moreover, the uMzinyathi and eThekwini districts of the province are important role players in maize exportation due to the availability of suitable exporting facilities for agricultural commodities, such as the Durban harbor [19].

The quest for “growing more from less” consequently demands an appraisal of human radiation exposure owing to the ingestion of radionuclides via maize consumption, specifically in rural subsistence areas. A realistic and credible assessment of the radiation dose and potential risks should be site-specific, considering the food consumption and occupancy rates [20].

The present effort therefore aimed at evaluating the exposure dose of 238U, 232Th, and 40K radionuclides concentrations in samples of maize representing the daily diet of the KwaZulu-Natal residents.

A detailed understanding of the internal exposure and risk to individuals will aid adequate regulation measures in the foreseen likelihood the growing population becomes increasingly dependent on maize directly and indirectly for feed.

2 Methodology

2.1 Sample collection and preparations

The maize crops (Yellow variety) were obtained from farms and food markets in major towns of the KwaZulu-Natal province. The sampling method is such that the crops were representatives of six distinct regions of the area (Table 1). Analyses were carried out in triplicate for each maize species from the study location in a region. The edible parts retained for analysis were air-dried before oven drying at 80ºC for constant weight. After drying, the samples were pulverized, homogenized, and packed into pre-weighed airtight 1 L Marinelli beakers. This was stored for 6 weeks before gamma spectroscopy counting to ensure that radioactive equilibrium was reached between parent radionuclides and their progeny [2, 20].

Table 1 Spatial coordinates and locations of the sampling sites
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2.2 Source of Investigated Maize Samples

Information relating to the source of the analyzed yellow maize crops, including the characteristics of the farmlands and basic growing practices employed by the farmers, were obtained through a survey and from available literature [18, 22]. The crops come largely from the Northern region of the province and the Midlands areas between the low-lying coastal strip of the Indian Ocean and the Drakensberg mountains.

The area is primarily distributed by sedimentary rocks with highly weathered acidic soils [18, 23]. The climate is typically oceanic, with annual rainfall ranging from 521 to 1120 mm, average maximum summer temperatures of 28.2 °C, and average minimum winter temperature of 3.2 °C [23]. Besides, the northern region also boasts minerals (aluminium, calcitic marbles) and sand mining operations [24].

Maize planting is from late October to mid-December, depending on the expected rainfall, while harvesting usually takes place from April till the end of August [25]. Agricultural practices such as ploughing, crop rotation, liming, and application of herbicides and fertilizers after soil analysis before planting are carried out mechanically through driven tractors and by hand.

2.3 Gamma Spectroscopy

The gamma spectroscopic measurements of the samples were carried out at the Environmental Radiation Laboratory (ERL) of iThemba LABS Gauteng, South Africa. A Canberra brand, lead-shielded broad energy High-Purity Germanium (HPGe) detector with an electronic data acquisition system (Canberra DSA-1000 digital signal processing (DSP) system) interfaced to a multichannel analyzer (MCA) was employed (Fig. 1). The energy (FWHM) and efficiency calibration of the detector system was determined using a standard multi-isotope source from the International Atomic Energy Agency (IAEA) in the same geometry as the experimental samples for the energies of interest. All measurements were made with the samples in contact with the detector housing for 28,800 s, and the spectral analysis was performed with a Canberra-developed GENIE2000 software. The activity concentration of 238U was determined using peaks at 234 Pa (1001 keV), 214Pb (351.7 keV), and 214Bi (1764.5 keV). Similarly, 228Ac (911.2 keV) and 208TI (583.19 keV) were used to identify 232Th. The activity concentration of 40K was evaluated directly from its 1460.5 keV gamma-ray peak.

Fig. 1
figure 1

photos adapted from [26]

The HPGe measurements set-up. a Schematic of the counting geometry with the sample in a Marinelli beaker. b A top-view photo of the HPGe crystal inside the lead castle lined with copper. c Overall photography of the HPGe set-up with (left) the lead castle housing the detector supported on a mechanically rigid cryostat connected to a liquid nitrogen Dewar. On the right is a block diagram of the various units used with the HPGe detector to measure and process the detector’s signal;

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3 Results and Discussion

3.1 Radioactivity Calculations

The activity concentration of radionuclides in the samples was calculated from the net activity after the deduction of the background counts using [27]:

$$A=\frac{{N}_{c}}{{P}_{\gamma } \varepsilon { m}_{s }t}$$
(1)

where \(A\) is the radionuclide-specific activity (Bq/kg), \({N}_{c}\) is the background corrected net peak area for a count time \(t\), \(\varepsilon\) is the detection efficiency at a given energy, \({m}_{s}\) is the mass of the measured sample in kg, and \({P}_{\gamma }\) is the probability of the gamma-ray emission for selected energy.

Table 2 presents the radioactivity concentrations of natural radionuclides 238U and 40K in the maize crop samples from this study. No detectable activity concentrations were observed for 232Th. The mean values of the radionuclides are considered representative, as the standard deviation is relatively small in all cases. The mean concentrations were 21.3, 26.0, 21.0, 52.0, 53.3, and 80.7 Bq/kg for 238U, and 203.3, 386.7, 183.3, 336.7, 320.0 and 526.7 Bq/kg for 40K in the maize samples from Mtubatuba, Richards Bay, Ladysmith, Winterton, Imbali and Ballito respectively. 238U and 40K showed the highest concentration (80.7 and 526.7 Bq/kg) in samples from Ballito and the lowest concentration (21.0 and 183.3 Bq/kg) from Ladysmith. Furthermore, the activity concentrations of 40K were about 6–9 times higher than values obtained for 238U in all the samples, which can be a good sign from the health point of view since potassium, as an essential nutrient of human health, is homeostatically regulated [13, 28].

Table 2 Radioactivity concentrations of natural radionuclides 238U, 232Th, and 40K in the maize samples
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The ranking of radionuclides activity (40K > 238U) agrees with previous studies [28,29,30,31], with variations due in part to the characteristics of the soil where the maize is planted and conditions under which it is grown, such as the extra use of phosphate fertilizers and their by-products. The application of fertilizers on farmlands generally redistributes and increases the amount of potassium in the soils and its subsequent transfer to the human food chain [13, 28, 29].

3.2 Annual Effective Ingestion Dose \(\left(\mathrm{D}\right)\)

The annual effective ingestion dose enables the determination of radiation-induced health risks to the population with intakes of radionuclides in the maize samples. The effective dose over a year for these radionuclides following consumption of the investigated maize samples was calculated using the relation (Eq. 2) adapted from an ICRP report [32].

$$D\left(mSv/y\right)=\sum \left({A}_{r}{U}_{m}\right){H}_{r}$$
(2)

where \({A}_{r}\) and \({U}_{m}\) denotes the radionuclide activity measured in the maize (Bq/kg) and the annual consumption rate (kg). \({H}_{r}\) is the dose conversion factor for radionuclide \(r\)(\(m\) Sv/Bq), given as 2.8 × 10–4 and 6.2 × 10–6 Sv \(m\)/Bq for 238U and 40 , respectively [33]. Based on the annual consumption rate of 81.03 (kg/year) in South Africa [34], the annual effective doses to an individual in all study locations are presented in Table 3. For comparison in the same Table, the effective dose values reported for maize in other locations across the globe were also given.

Table 3 Annual effective dose due to 238U and 40K in analyzed maize samples
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The total effective dose from intake of detected radionuclides (238U and 40K) from maize consumption in all study locations was below the 1.0 mSv/year acceptable level for the public by the International Commission on Radiological Protection [32] except for Winterton, Imbali, and Ballito respectively. However, the doses were all lower than the average worldwide annual effective dose (2.4 mSv) for an individual from natural radiation sources [5]. As seen in Table 3, the total effective doses were much higher than values previously reported globally for other locations but can be justified by the lower consumption rates of maize in those countries and the activity of the measured radionuclide. For instance, the consumption rates of 43.8 kg/year for Ghana and 20.67 kg/year for Nigeria were lower than 81.03 kg/year for South Africa in this study. Conversely, Mlwilo et al. [35] and Martinez et al. [36] reported a lower annual effective dose from Tanzanian and Mexico despite the higher intake rate of 140 and 109.5 kg/year than in the current study. The measured radionuclide activity concentrations were relatively lower than our obtained values.

Percentage contributions of dose from 238U and 40K in the maize samples are depicted in Figs. 2 and 3, respectively. From the figures, it is noteworthy to observe that the radionuclide’s contributions vary from location, which depends largely on the cultivation sites and practices, including the individual chemical properties of the radionuclides [13, 31, 37]. Maize sampling for this study was done from different markets in major cities of the province, grown and supplied within the precinct of the KwaZulu-Natal region. Therefore, the observed variation in the dose commitment could be attributable to the physio-chemical properties of the farmland soil and varieties of maize since the growth patterns are more or less the same in the province [7]. It is reported that acidic soils increase the availability and uptake of radionuclides in the upper soil surface more than in alkaline soil [38, 39]. Besides, the region is noted for its harbor facilities, hilly topography, and several industrial activities associated with mined minerals such as rock phosphate, zircon, and hematite, considered naturally occurring Radioactive Materials (NORMs) due to their presence in mineral grains [40].

Fig. 2
figure 2

Percentage contributions of 238U to the annual effective dose

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Fig. 3
figure 3

Percentage contributions of 40K to the annual effective dose

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Generally, the main contribution to the effective ingestion dose in this work is 238U, with the highest observed in Ballito and the lowest in Ladysmith.

3.3 Radiological Risk Analysis

The procedure proposed by the United States Environmental Protection Agency, US EPA [42] was used to quantify the lifetime cancer risk associated with exposure to 238U and 40K through maize consumption.

The lifetime risk of cancer \(\left(R\right)\) was determined according to Eq. (3) [13, 43].

$$R={A}_{I}\times {R}_{c}\times {E}_{D}$$
(3)

where \({A}_{I},{R}_{c}\) and \({E}_{D}\) are the annual intake of radionuclides (Bq), cancer risk coefficients (Bq−1), and the exposure duration (average lifetime expectancy, 64 years for South Africa). The mortality and morbidity risk coefficients for food ingestion are 1.51 × 10–9, 2.34 × 10–9 for 238U, and 5.89 × 10–10, 9.26 × 10–10 for 40K, respectively [43].

Table 4 presents the estimated cancer risk values, largely within the acceptable limit of 10–3 for radiological risk [44].

Table 4 Lifetime risk of cancer mortality and morbidity values due to 238U and 40K intake
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4 Conclusion

Radionuclides concentrations in samples of maize consumed within KwaZulu-Natal, South Africa, were determined to ascertain their variations and assess the radiological health hazards to the public from consumption. The trend of activity concentrations 40K > 238U from all study locations agrees with previous results obtained in other countries. This poses no health concern given that the potassium body content is maintained regardless of intake quantities. 232Th was not detected in any of the analyzed samples. The estimated annual effective doses are about one order of magnitude higher than the ICRP recommended permissible limits in some study locations for public exposure but generally within the world average annual effective dose to an individual from naturally existing radioactivity in the environment.

The dose values in the present study may still be considered high, given its due to the consumption of a single diet. However, the values could be refined with specific data on individuals’ regularity of maize consumption.