Analysis of Natural Radioactivity in Phosphogypsum by Gamma Spectrometry and Evaluation of Radiological Risk Parameters Associated with Phosphogypsum Storage in Senegal

Moussa Hamady Sy^1*, Ndèye Fatou Fall¹, Djicknack Dione¹, Papa Macoumba Faye¹, Nogaye Ndiaye¹, Oumar Ndiaye¹, Alassane Traoré^1,2, Jean Paul Latyr Faye^1,2, Ndèye Arame Boye Faye^1,2, Ababacar Sadikhe Ndao^1,2

¹Institute of Applied Nuclear Technology, University of Cheikh Anta Diop, Dakar, Senegal.
²Department of Physics, Faculty of Sciences and Technology, University of Cheikh Anta Diop, Dakar, Senegal.

DOI: 10.4236/wjnst.2025.153007                

Abstract

The HPGe gamma spectrometer was used to measure the concentrations of the natural radionuclides ²²⁶Ra, ²³²Th, and ⁴⁰K in eight samples of phosphogypsum, in order to assess the health risk indices associated with radiation and the excess lifetime cancer risk (ELCR). The ²²⁶Ra, ²³²Th, and ⁴⁰K were discovered with activity concentrations of 586.26 ± 129.91, 4.05 ± 1.47, and 12.22 ± 6.75, respectively. The average values of the estimated radiological risk parameters were 592.99 ± 141.06 Bq·kg⁻¹ for radium equivalent (Raeq), 273.81 nGy·h⁻¹ for the external absorbed dose rate (D_ext), and 544.81 nGy·h⁻¹ for the internal absorbed dose rate (D_int). The average annual effective dose (AEDE) was 335799.36 nSv·y⁻¹ for the external effective dose (AEDE_ext) and 2672601.09 nSv·y⁻¹ for the internal effective dose (AEDE_int). The annual dose equivalent to the gonads (AGED) was 1832.32 μSv. The average cancer risk rate (ELCR) was 10.22 × 10⁻³. With the External Risk Index (Hext), the Internal Risk Index (Hint), the Representative Gamma Index (I_γ), and the Alpha Index I_α, the average risk indices were 1.60; 3.19; 1.98; and 2.93. According to this study, the average values of radiological risk parameters for all examined samples exceed the internationally recommended thresholds.

Keywords

Natural Radioactivity, Gamma Spectrometry, Phosphogypsum, Activity Concentrations, Dose Parameters, Radiological Risks

1. Introduction

Several so-called primordial radioactive materials that were created during the planet’s formation are still present on Earth and continue to create others through nuclear disintegration processes. Potassium-40 and the radioisotopes from the natural series of uranium, actinium, and thorium, with the parent nuclei ²³⁵U, ²³⁸U, and ²³²Th, as well as the decay products resulting from successive alpha or beta decays, are the most abundant primordial radionuclides [1]. Phosphate rock, a sedimentary mineral mainly composed of apatite (Ca₅(PO₄)₃(OH, F, Cl)), contains radionuclides originating from the natural decay series of uranium and thorium, as demonstrated by research [1]-[3]. The phosphate rock is widely exploited to produce phosphoric acid and various brands of chemical fertilizers around the world. When phosphate rock is extracted and processed during mining activities, the radionuclides it contains are released into the environment and end up in finished products and by-products.

Phosphate rocks are treated with concentrated sulfuric acid (H₂SO₄) to produce phosphoric acid in a wet process (see “Equation (1)”), which accounts for about 90% of the world’s phosphoric acid production [4].

$C{a}_{5}F{\left(P{O}_4\right)}_3+5H_{2}S{O}_4+10H_{2}O\to 3H_{3}P{O}_4+5CaS{O}_4\cdot 2H_{2}O+HF$ (1)

The by-products, referred to as Phosphogypsum, are the result of this wet chemical treatment of phosphate, a technique known for its importance in enhancing natural radioactivity. It is primarily composed of calcium sulfate dihydrate and contains impurities such as radionuclides and heavy metals (Cd, Pb, As, Cr, etc.). Due to its radioactivity and potential environmental pollution, phosphogypsum is typically stored in large man-made piles.

In Senegal, phosphoric acid production began in 1960 and reached 0.65 million tons in 2010 [5]. The phosphogypsum (PG), produced during this chemical reaction is stored in extensive piles near the plant. Furthermore, the fact that arable land remains occupied by the vast piles of phosphogypsum, natural radioactivity, the presence of heavy metals, and the impurities found in this material can have a toxic and radioactive impact on the local environment.

Soil contamination by heavy metals and radionuclides, following their dispersion or leaching by rain, can alter its structure and fertility. Water, whether groundwater or surface water, can be polluted by toxic leachate from this waste, with consequences for drinking water quality and aquatic ecosystems. The air can be laden with fine dust and radon, a radioactive gas naturally emitted by these residues, which represents a health hazard if inhaled. Living beings, fauna, and flora, can suffer the effects of the bioaccumulation of these toxic substances, causing ecological disruption. Finally, humans are indirectly exposed through contaminated water, food, or air, which can increase the risk of respiratory, kidney, or cancerous diseases.

Today, the search for a limited impact on the environment, driven by the identification of issues related to inadequate management, pressure from the population, and a better scientific understanding of the risks associated with radiological elements, has led governments and industries to take a series of regulatory measures to reduce the impact of this type of management. As part of a sustainable approach to this by-product, its enhancement has been encouraged worldwide. Various uses of phosphogypsum have been studied, particularly in the civil engineering sector to produce plaster [6]. In agriculture, it is used to fertilize and improve the soil [7] as well as in construction materials to replace natural gypsum in the production of Portland cement in order to regulate the hydration reaction rate of the cement and aggregates [8] [9]. Natural radioactivity must therefore be assessed not only in phosphate rock but also in the by-products. High radioactivity can lead to significant exposure for miners, manufacturers, and end users. The objective of this study is to analyze the levels of natural radionuclides such as ²²⁶Ra, ²³²Th, and ⁴⁰K in phosphogypsum samples using high-resolution gamma spectrometry. Furthermore, this study will also examine the radiological risk factors associated with the outdoor storage of phosphogypsum.

2. Materials and Methods

2.1. Sample Collection and Preparation

The sampling was carried out at the storage piles of the industrial facilities for phosphoric acid production, in ICS (Chemical Industries of Senegal). The map below of the area, which is the focal point of the study, shows the sampling sites as well as the main communes located in this region (Figure 1).

Figure 1. Location of the main cities and study area: Darou Khoudoss, Mboro, and Taiba Ndiaye, sample collection points.

Eight samples of phosphogypsum were collected from stockpiles and stored in polyethylene bags. After being collected in polyethylene bags, the samples were placed in appropriate measuring containers and sealed to prevent any cross-contamination. The samples were prepared according to standard packaging norms to ensure their suitability for spectrometric analysis. In the laboratory, the samples were first dried in an oven at 105˚C for 24 hours to remove moisture until a constant weight was reached. Then, they were crushed and passed through a series of sieves to check their homogeneity [10]. A very fine powder was obtained from the pulverized samples, but to ensure homogeneity, the fractions passing through the 250-micrometer sieves or smaller were used, and a mass of the sieved sample was weighed. The homogeneous samples were transferred into Marinelli Beaker-type containers up to the gauge mark, and the mass that will be used for the measurement was recorded. The sealed samples were left to rest for 4 weeks to allow for the establishment of secular equilibrium between radon-222 (²²²Rn) and its descendants, which is essential for determining the activity of radium-226 (²²⁶Ra). To monitor this balance, additional measurements of 86,400 seconds were taken for each sample, with a one-week interval between each series of measurements, repeated four times.

2.2. Determination of Phosphogypsum Activity Concentration by Gamma Spectrometry

The activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K were measured on various samples using a high-resolution gamma spectrometry system. A high-purity germanium (HPGe) N-type detector from the brand CAMBERRA is used in this spectrometry system, with a relative efficiency of 15%, for an energy range of 10 keV to 10 MeV. A multi-nuclide reference standard was used to calibrate the energy and relative efficiency of the detector, emitting gamma radiation in an energy range from 5.9 to 1332 keV. The typical energy resolutions of the detector used in this study range from 0.45 keV to 5.9 keV and from 1.9 keV to 1332.5 keV, respectively, with 55Fe and 60Co. The adjustment and verification of the geometry were carried out using the mathematical software Labsocs. All measurements were taken for at least 86,000 seconds to ensure accurate detection of radionuclides and minimize statistical errors. The peak total energy of its descendant ²²⁸Ac at 911.6 keV was used to assess the activity concentration of thorium ²³²Th. In gamma spectrometry, the direct quantification of radium ²²⁶Ra is challenging due to the overlap of its peak at 186.2 keV with that of uranium ²³⁵U at 185.77 keV. The problem is all the more complex because the limited activity of uranium-235 makes its precise measurement difficult [11]. In this study, the activity concentration of radium ²²⁶Ra was determined from its short-lived decay products once secular equilibrium was reached. The activity concentrations of radium ²²⁶Ra were calculated by averaging the measured activities for two distinct photoelectric peaks of its daughter nuclides: ²¹⁴Pb at 351.92 keV and ²¹⁴Bi at 609.31 keV. For potassium ⁴⁰K, the activity concentration was determined by using the gamma line at 1460.81 keV as illustrated in Table 1.

Table 1. Major gamma lines used for natural radionuclides.

Radionuclides	E, keV	$P_{\gamma }$ %
226Ra
214Bi	609.31	46.30
214Pb	351.92	37.20
232Th
228Ac	911.60	27.70
40K	1460.81	10.67

The following equation was used to calculate the activity concentration of each radionuclide (Bq·kg⁻¹).

$A=\frac{1000\cdot N}{m\cdot \epsilon \left(E\right)\cdot P_{\gamma }\left(E\right)\cdot t_{\text{mesured}}}$ (2)

where:

• A represents the activity concentration at the time of sampling (in Bq/kg);

• N is the net number of counts of the peak (cps);

• $t_{\text{mesured}}$ is the live measurement time in seconds (s);

• m is the mass of the sample used in grams (g);

• $\epsilon \left(E\right)$ is the detector efficiency for gamma radiation (dimensionless); and

• $P_{\gamma }\left(E\right)$ is the probability of gamma emission by the radionuclide (dimensionless).

2.3. Evaluation of Dose and Radiological Risk Parameters

The following equations have been used to calculate the radiological hazard indices related to exposure to excessive gamma radiation from phosphogypsum resulting from phosphoric acid production.

2.3.1. Radium Equivalent Activity

In general, the concentration and distribution of ⁴⁰K, ²²⁶Ra, and ²³²Th in the soil vary by country [12]. Thus, in order to represent the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K with a single measure, taking into account the radiological hazard associated with them, it is common to calculate a widely used common radiological hazard index: the radium equivalent activity (R_aeq) [13]. The radium equivalent activity can be expressed as follows:

$R_{\text{aeq}}\left(\text{Bq}\cdot {\text{kg}}^{-1}\right)=A_{\text{Ra}}+A_{\text{Th}}\times 1.43+A_{\text{K}}\times 0.077$ (3)

where A_Ra, A_Th, and A_K representing the specific activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in Bq·kg⁻¹, respectively.

2.3.2. Absorbed Dose Rates

An assessment of the external dose rate of radiation (D_ext) at 1 meter above the ground surface is conducted using the gamma radiation emitted by ²²⁶Ra, ²³²Th, and ⁴⁰K, which are assumed to be evenly distributed in the soil. The following equations have been used to calculate the doses of external and internal radiation. (D_ext et D_int) [3]:

$D_{\text{ext}}\left(\text{nGy}\cdot {\text{h}}^{-1}\right)=0.462A_{\text{Ra}}+0.604A_{\text{Th}}+0.0417A_{\text{K}}$ (4)

$D_{\text{int}}\left(\text{nGy}\cdot {\text{h}}^{-1}\right)=0.92A_{\text{Ra}}+1.1A_{\text{Th}}+0.081A_{\text{K}}$ (5)

2.3.3. Annual Effective Dose Equivalent

The following relationships were used to calculate the expected annual effective dose equivalent (AEDE) for the general public due to the radioactivity present in phosphogypsum:

${\text{AEDE}}_{\text{ext}}\left(\text{nSv}\cdot {\text{y}}^{-1}\right)=D_{\text{ext}}\left(\text{nGy}\cdot {\text{h}}^{-1}\right)\times 8760\text{h}\cdot {\text{y}}^{-1}\times 0.2\times 0.7\text{Sv}\cdot {\text{Gy}}^{-1}$ (6)

${\text{AEDE}}_{\text{int}}\left(\text{nSv}\cdot {\text{y}}^{-1}\right)=D_{\mathrm{int}}\left(\text{nGy}\cdot {\text{h}}^{-1}\right)\times 8760\text{h}\cdot {\text{y}}^{-1}\times 0.8\times 0.7\text{Sv}\cdot {\text{Gy}}^{-1}$ (7)

The annual effectiveness of the equivalent dose was determined using a conversion factor of 0.7 Sv·Gy⁻¹, which converts the absorbed dose in the air into an effective dose for adults. The external occupancy was 0.2 and the internal occupancy was 0.8 [2] [14].

2.3.4. Annual Gonadal Dose Equivalent

According to UNSCEAR, the organs of interest are the gonads (ovaries or testes, which generate gametes), the cells of the bone surface, and the active bone marrow [15]. The annual equivalent dose to the gonads (AGED) is estimated by using the equation provided by [16]:

$\text{AGED}\left(\mu \text{Sv}\cdot {\text{y}}^{-1}\right)=3.09C_{\text{Ra}}+4.18C_{\text{Th}}+0.314C_{\text{K}}$ (8)

where AGED is the Annual Equivalent Dose to the Gonads (µSv·y⁻¹), and C_Ra, C_Th, and C_K (Bq·kg⁻¹) are the radioactivity levels of ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively.

2.3.5. Excess Lifetime Cancer Risk

The following equations have been used to calculate the lifetime excess cancer risk value (ELCR) [15]:

${\text{ELCR}}_{\text{ext}}={\text{AEDE}}_{\text{ext}}\times \text{LE}\times \text{RF}$ (9)

${\text{ELCR}}_{\mathrm{int}}={\text{AEDE}}_{\mathrm{int}}\times \text{LE}\times \text{RF}$ (10)

where (AEDE_ext) and (AEDE_int) are the annual effective doses, LE is the life expectancy of 68.9 years according to the report from the National Agency of Statistics and Demographics of Senegal (ANSD, 2023), and RF (Sv⁻¹) is the fatal risk factor per Sievert, which is 0.05 [15].

2.3.6. Internal Hazard Index and External Hazard Index

The internal and external hazard indices (H_int; H_ext) are calculated using the following expressions [17]:

$H_{\text{ext}}=\frac{A_{\text{Ra}}}{370}+\frac{A_{\text{Th}}}{259}+\frac{A_{\text{K}}}{4810}$ (11)

$H_{\mathrm{int}}=\frac{A_{\text{Ra}}}{185}+\frac{A_{\text{Th}}}{259}+\frac{A_{\text{K}}}{4810}$ (12)

where A_Ra, A_Th, and A_K representing the specific activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in Bq·kg⁻¹, respectively.

2.3.7. Gamma and Alpha Radioactivity Level Indices

The gamma radioactivity level index (I_γ) is used to estimate the risk rate associated with gamma radiation from natural radionuclides in the investigated samples [18]. It is defined by the following equation:

$I_{\gamma }=\frac{C_{\text{Ra}}}{300}+\frac{C_{\text{Th}}}{200}+\frac{C_{\text{K}}}{3000}$ (13)

where C_Ra, C_Th, and C_K are the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively.

The alpha index (I_α) is used to estimate the excess alpha radiation caused by the inhalation of radon from building materials. The alpha index is determined using the following equation [17]:

$I_{\alpha }=\frac{C_{\text{Ra}}}{200}$ (14)

3. Results and Discussion

3.1. Activity Concentration of ²²⁶Ra, ²³²Th, and ⁴⁰K

The characterization of radioactive species is the first step in assessing the radiological impact associated with phosphogypsum. In order to verify if the radionuclides ²¹⁴Bi and ²¹⁴Pb are in secular equilibrium, the graphs (Figure 2) present the respective activity concentration ratios of the different samples (labeled EPG1, EPG2, EPG3, EPG4, EPG5, EPG6, EPG7, and EPG8). According to the results, it was found that the measured values of the descendants were in secular equilibrium with those of radium-226, as the observed discrepancy between the measurements of the descendant radionuclides is less than 2%. This slight variation confirms the secular balance, thereby validating the accuracy of the measurements taken.

Table 2 presents the activity concentrations of natural radionuclides (²²⁶Ra, ²³²Th, and ⁴⁰K) measured in phosphogypsum samples using gamma spectrometry.

The results indicate a variation in activity concentrations between 408.66 ± 27.57 and 809.83 ± 54.54 Bq·kg⁻¹ for ²²⁶Ra, from 2.64 ± 0.61 to 6.63 ± 0.67 Bq·kg⁻¹ for ²³²Th, and from 6.55 ± 1.83 to 24.59 ± 3.11 Bq·kg⁻¹ for ⁴⁰K. For ²²⁶Ra, ²³²Th, and ⁴⁰K, the average values have been determined to be 586.26 ± 129.91 Bq·kg⁻¹, 4.05 ± 1.47 Bq·kg⁻¹, and 12.22 ± 6.75 Bq·kg⁻¹, respectively. The first two samples analyzed, EPG1 and EPG2, show significantly higher radionuclide activities than those of the other samples. This disparity is reflected by the statistical standard deviation, which measures the dispersion of values around the mean, thus indicating a greater variability in these two samples compared to the others. This difference could be explained by the fact that these two samples are more recent. On the other hand, older samples may have undergone phenomena such as leaching, leading to a reduction in their activity. This variation highlights the potential impact of time and environmental conditions on the concentrations of radionuclides in phosphogypsum.

Figure 2. Secular equilibrium between ²¹⁴Bi and ²¹⁴Pb for phosphogypsum samples.

Table 2. Distribution of activity concentrations (Bq·kg⁻¹) of ²²⁶Ra, ²³²Th, ⁴⁰K, and radium equivalent in phosphogypsum samples.

Sample Codes	Natural Radionuclides (Bq·kg−1)				Radium equivalent
	226Ra	232Th	40K	Raeq
EPG1	809.83 ± 54.54	6.63 ± 0.67	24.59 ± 3.11	821.20
EPG2	714.77 ± 48.16	6.22 ± 0.71	20.78 ± 2.71	725.26
EPG3	408.66 ± 27, 57	3.13 ± 0.46	11.02 ± 1.98	413.98
EPG4	451.34 ± 30.42	2.64 ± 0.61	10.64 ± 1.52	455.93
EPG5	598.78 ± 40.35	4.50 ± 0.57	6.66 ± 1.49	605.72
EPG6	472.37 ± 31.83	3.21 ± 0.54	6.55 ± 1.83	477.47
EPG7	573.93 ± 38.66	3.27 ± 0.45	9.67 ± 1.72	579.34
EPG8	660.42 ± 44.50	2.81 ± 0.55	7.86 ± 1.61	665.04
Min	408.66 ± 27.57	2.64 ± 0.61	6.55 ± 1.83	413.98
Max	809.83 ± 54.54	6.63 ± 0.67	24.59 ± 3.11	821.20
Mean ± SD	586.26 ± 129.91	4.05 ± 1.47	12.22 ± 6.75	592.99 ± 141.06

The decreased concentrations of radium-226 in older phosphogypsum samples compared to more recent samples could be related to the precipitation of radium-226 as radium sulfate (RaSO₄). Indeed, it is plausible that radium-226 gradually transforms into RaSO₄ over time, leading to a reduction in the levels of free radium-226 in solid samples. Several factors can explain the relatively high presence of ²²⁶Ra in phosphogypsum samples compared to other radionuclides. First of all, radium has a strong chemical affinity for sulfate, which facilitates its coprecipitation in the form of radium sulfate (RaSO₄) during the wet process of phosphoric acid production. Consequently, ²²⁶Ra tends to preferentially accumulate in the solid phase of phosphogypsum. Secondly, the geological properties of the phosphates deposits being exploited can affect the initial rate of uranium (²³⁸U) and, consequently, the amount of ²²⁶Ra that result from its decay. Finally, leaching process or differential movement of radionuclides during storage can also explain the greater retention of radium compared to thorium and potassium. The average activity concentrations in phosphogypsum samples compared to the global average values (50 Bq·kg⁻¹ for radium-226, 50 Bq·kg⁻¹ for thorium-232, and 500 Bq·kg⁻¹ for potassium-40) reported by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and referenced by Ndour et al. (2021), have shown that the activities of thorium-232 and potassium-40 are significantly lower than the global average, while the activity of radium-226 remains significantly higher than this average.

The comparison of the average values of activity concentrations measured in this study for the analyzed samples with those reported in other countries is presented in Table 3 and Figure 3.

Table 3. Comparison of ²²⁶Ra, ²³²Th, and ⁴⁰K activity concentrations with those obtained in other countries.

Countries	Natural Radionuclides (Bq·kg−1)			References
	226Ra	232Th	40K
Egypt	203 ± 5.3	43.2 ± 3.5	126 ± 15	[16]
Korea	618	8. 5	24.1	[19]
Tunisia	188.0 ± 9.5	12.4 ± 1.4	<13.5	[4]
Spain	647	8	33	[20]
Jordan	376	4	40	[21]
Hungary	337.0 ± 179.3	5.4 ± 5.8	210.0 ± 122.2	[22]
Morocco	1420 ± 330	-	-	[1]
Portugal	735 ± 70	-	-	[23]
Florida	1120	3. 7	-	[24]
Senegal	586.26 ± 129.91	4.05 ± 1.47	12.22 ± 6.75	This Study

Figure 3. Comparison of activity concentrations of natural radionuclides with other countries around the world.

These results indicate that the activity concentration of ²²⁶Ra in the phosphogypsum samples of this study is higher than that observed in Egypt, Tunisia, Jordan, and Hungary. However, the activity concentrations recorded in Korea, Spain, Morocco, Portugal, and Florida are higher than those observed in this study. Apart from the concentration of ²³²Th measured in Egypt and the concentrations of ⁴⁰K recorded in Egypt and Hungary, the activity concentrations of ²³²Th and ⁴⁰K in this study are consistent with those observed in other countries (see Figure 3). This trend can be attributed to variations in industrial processes, the nature of phosphate deposits, as well as the environmental conditions specific to each region.

3.2. Dose Parameters and Radiological Risks

According to “Equations (3) to (14)”, the radiological dose parameters of the phosphogypsum samples have been evaluated. Thanks to these equations, it is possible to calculate various parameters essential for assessing the radiological risks associated with these materials, such as the radiological hazard index (Raeq), the absorbed dose rate, the annual equivalent effective dose, as well as the internal and external risk indices. They also offer the possibility to measure danger levels for gamma and alpha rays. Using these equations, it was possible to measure the potential impact of phosphogypsum residues on human health and the environment. The calculations also include the assessment of lifetime cancer risk and annual equivalent gonadal doses, providing a comprehensive overview of the potential radiological hazards associated with these materials. For the various phosphogypsum samples, the Raeq values were calculated using “Equation (3)”, and the results are presented in Table 2. The Raeq obtained for the phosphogypsum samples ranges from 413.98 to 821.20 Bq·kg⁻¹, with an average of 592.99 ± 141.06 Bq·kg⁻¹. The Raeq index exceeds the authorized limit of equivalent radium activity, which should not exceed 370 Bq·kg⁻¹, in order to ensure safe use in various civil applications [13]. Table 4 and Figure 4 presents the values of the absorbed dose rate outdoors, D_ext, which fluctuate between 191.15 and 379.17 nGy·h⁻¹, with an average of 273.81 nGy·h⁻¹.

Table 4. Radiological dose parameters, dose rate, AEDE, and AGED.

Sample Codes	Radiological Dose Parameters
	Dext (nGy·h−1)	Dint (nGy·h−1)	AEDEext (nSv·y−1)	AEDEint (nSv·y−1)	AGED (µSv·y−1)
EPG1	379.17	754.32	465013.36	3700415.57	2537.80
EPG2	334.85	666.11	410655.21	3267677.18	2241.16
EPG3	191.15	380.30	234427.66	1865614.31	1279.30
EPG4	210.56	419.00	258226.26	2055428.75	1409.01
EPG5	279.63	556.36	342938.74	2729298.94	1871.12
EPG6	220.45	438.65	270359.28	2151826.28	1475.12
EPG7	267.53	532.39	328099.99	2611693.89	1790.13
EPG8	307.14	611.31	376674.38	2998853.80	2054.91
Mean	273.81	544.81	335799.36	2672601.09	1832.32

Figure 4. The different dose parameters.

Conversely, the absorbed dose rate indoors ranges from 380.30 to 754.32 nGy·h⁻¹, with an average of 544.81 nGy·h⁻¹. These values exceed the global average of 58 nGy·h⁻¹ for D_ext and 84 nGy·h⁻¹ for D_int, respectively [25]. The results indicate that the values of the external effective dose (AEDE_ext) for the various samples range from 0.23 to 0.46 mSv·y⁻¹, with an average of 0.33 mSv·y⁻¹, which exceeds the permitted limit of 0.07 mSv·y⁻¹ (UNSCEAR, 2008). In the same way, the values of the effective internal dose (AEDE_int) for the samples range from 1.86 to 3.70 mSv·y⁻¹, with an average of 2.67 mSv·y⁻¹, thus exceeding the recommended value of 0.41 mSv·y⁻¹ (UNSCEAR, 2008), as shown in Table 4. The values obtained for the Annual Equivalent Dose to the Gonads (AGED), which fluctuate between 1279.30 and 2537.80 μSv·y⁻¹, with an average of 1832.32 μSv·y⁻¹. This average value is higher than the globally recommended limit, which is 1000 μSv·y⁻¹ for the general public [16]. These values indicate that gonad exposure levels may have significant radiological effects on the bone marrow and superficial bone cells of miners, workers and inhabitants of the area studied. In the long term, the combination of doses at this level of exposure could endanger the reproductive organs (ovaries and testicles) of individuals living near these mining sites. “Equations (9) and (10)” are used to calculate the values of the lifetime cancer risk excess (ELCR) for outdoor and indoor exposures, as presented in Table 5 and Figure 5.

Table 5. Radiological risk indices from different phosphogypsum samples.

Sample Codes	Radiological Risk Indices
	Hint	Hext	Iγ	Iα	ELCRint	ECLRext	ELCR total (10−3)
EPG1	4.41	2.22	2.74	4.05	12.75	1.6	14.35
EPG2	3.89	1.96	2.42	3.57	11.26	1.41	12.67
EPG3	2.22	1.12	1.38	2.04	6.43	0.80	7.23
EPG4	2.45	1.23	1.52	2.26	7.08	0.89	7.97
EPG5	3.26	1.64	2.02	2.99	9.40	1.18	10.57
EPG6	2.57	1.29	1.59	2.36	7.41	0.93	8.43
EPG7	3.12	1.57	1.93	2.87	8.98	1.13	10.11
EPG8	3.58	1.81	2.22	3.30	10.33	1.30	11.46
Mean	3.19	1.60	1.98	2.93	9.21	1.16	10.22

Figure 5. Risk indices compared to their limit values.

The external and internal ELCRs have been estimated at 1.16 × 10⁻³ and 9.21 × 10⁻³, respectively, while the total average value of internal and external exposure is estimated at 10.22 × 10⁻³. These are values exceeding the recommended limits, which are 0.3 × 10⁻³ for external ELCR and 1.2 × 10⁻³ for internal ELCR. Furthermore, the global ELCR for all samples exceeds the world average of 1.45 × 10⁻³ [3]. It was found that all the samples showed an ELCR higher than the established standards. Mohammed and Ahmed [26] indicated that ELCR values of 1, 10, 100, and 1000 mSv·y⁻¹ would increase the risk of developing fatal cancer by 0.004%, 0.04%, 0.4%, and 4% respectively. Although the lifetime excess lifetime cancer risk (ELCR) observed in this study is above the global average, the increased risk of cancer over a lifetime remains negligible, even for indoor ELCR. The analysis reveals that the annual effective doses (AEDE) and excess lifetime cancer risks (ELCR) in certain cases surpass the limits recommended by the International Commission on Radiological Protection (ICRP), which stipulates a maximum public exposure of 1 mSv·y⁻¹. Sustained exposure to elevated levels of radium-226 may substantially heighten the incidence of bone cancer and leukemia within the exposed populations. Although all individuals are at risks, children represent a particularly sensitive subgroup due to higher cellular proliferation rates and longer latency periods for radiation-induced pathologies. These results highlight the necessity for rigorous radiological surveillance and the implementation of protective strategies aimed at minimizing exposure for the entire population residing near phosphogypsum storage areas. The external risk index (H_ext), the internal risk index, the representative gamma index, and the representative alpha index, calculated respectively from “Equations (11) to (14)”, are also shown in Table 5. It is essential that the results obtained for the H_ext and the H_int are both below 1 in order to keep the radiological risk (annual effective dose) within safe limits [27]. The averages of H_ext, H_int, I_γ, and I_α obtained are respectively 1.60; 3.19; 1.98; and 2.93. All these average values of radiological risk, namely H_ext, H_int, I_γ, and I_α, exceed the recommended limit of 1.0 for the public [28]. The internal risk index is three times higher than the allowed limit, while the gamma and alpha radiation level indices are nearly twice as high. According to the results, it has been demonstrated that phosphogypsum waste poses a radiological threat to the population, which represents a danger to human health and the environment. However, it is necessary to acknowledge certain limitation of this research, especially the limited sample, which may not represent all the existing phosphogypsum storage sites, as well as the lack of study on the migration of radionuclides in the surrounding soils and groundwater. It would be necessary to conduct additional research, with a broader spatial and temporal coverage, to validate these conclusions and more precisely assess the long-term risks.

4. Conclusions

Gamma spectroscopy was used to analyze the concentrations of natural radionuclides in eight samples of phosphogypsum. The measurements taken on ²²⁶Ra, ²³²Th, and ⁴⁰K have allowed for the evaluation of various radiological risk factors, such as the Absorbed Dose Rate (D), the Annual Effective Dose Equivalent (AEDE), the Radium Equivalent (R_aeq), the Annual Equivalent Dose to the Gonads (AGED), the Excess Lifetime Cancer Risk (ELCR), the External Risk Index (H_ext), the Internal Risk Index (H_int), the Representative Gamma Index (I_γ), and the Representative Alpha Index. Unlike the global average values for thorium-232 and potassium-40 activity (50 Bq·kg⁻¹ for thorium-232 and 500 Bq·kg⁻¹ for potassium), the activity of radium exceeds the recommended average value of 50 Bq·kg⁻¹. The average radium equivalent obtained in this study exceeds the recommended limit of 370 Bq·kg⁻¹. Furthermore, the other radiological risk parameters for all the samples studied are above the globally recommended thresholds.

These results highlight that the materials studied pose a significant radiological risk to the population, which requires appropriate monitoring and management to ensure safety. This assessment provides a comprehensive overview of the radiological hazards associated with the examined phosphogypsum samples, allowing for a better understanding of the potential consequences on health and the environment. It is important to monitor and manage the levels of radioactivity in construction materials to ensure public safety, as evidenced by the results obtained. It light of the high levels of radioactivity detected, the establishment of a regular monitoring program for phosphogypsum storage sites is recommended, accompanied by periodic examinations of soils, groundwater, and surrounding crops. Moreover, the development of strategies aimed at confining or controlled reuse of phosphogypsum could help minimize population exposure. Future studies should focus on analyzing the migration of radionuclides in the environment and assessing the transfer of pollutants in the food chain.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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