Introduction

Phosphate rocks are used as sources of phosphorus for fertilizers and to produce phosphoric acid and gypsum. These rocks' mineral composition varies depending on their origin. In general, sedimentary phosphate rocks contain larger amounts of uranium decay products; whereas magmatic minerals have higher concentrations of thorium decay products and a lower concentration of nuclides of uranium decay products, even though the total activity of magmatic minerals is less than that of the sedimentary minerals [1]. Uranium and radium concentrations in phosphate rock are normally around 1500 Bq/kg, while some phosphate rocks can contain up to 20,000 Bq/kg of U3O8 [2]. The fertilizers are enriched in uranium, up to 150% relative to the ore, while phosphogypsum contains about 80% of Ra-226, 30% of Th-232, and 5% of U-238 [3]. In most cases, the phosphate rocks are treated with sulfuric acid, and as a result, it is expected that phosphate rocks may behave differently according to the acidulation processes, which is the most significant phase of all phosphate fertilizer industries [4].The chemical processing of phosphate rocks over a period of whole processing may anhance the uranium and radium content and then increase the activity concentrations which correspond to the increase in radiation doses and cause hazards for the human body. Occupational exposures occur during the mining and milling of phosphate rocks, processing and transportation and utilization of phosphate fertilizers [5]. The workers can receive additional external dose rates exposure up to 0.8 (nGy/h) [6]. Therefore, it is important to measure their radionuclides inventories, which is the main cause of high radioactive content and might lead to a significant exposure for miners, manufacturers, and end-users. The processing of phosphate rocks may generate particulate and gaseous emissions in the ambient air that contain U-238 and Ra-226. In a poorly ventilated environment, these radionuclides might lead to radiation exposure for workers and the population around the factory of phosphate production. When these radionuclides are discharged into the environment, higher levels of exposure to radon for the public may be expected at sites being developed for housing [7]. As an inert gas, radon gas is itself not harmful; rather, it enhances the dispersion of other daughter radionuclides throughout the environment, the field of work and contributes to the incorporation of lead and polonium isotopes into lung membranes. Except for Th-234, Pa-234, and Pb-210, which are beta emitters, all U-238 daughters are alpha emitters [8]. Because alpha and beta particle radiation are highly attenuated in air over short distances, ingestion of these radionuclides through food or water or inhalation through aerosols or dust may be highly hazardous. Therefore, the external dose rate of these radionuclides is lower than their internal dose rate. All these radionuclides may cause cancer, genetic disruptions, and a wide range of diseases when exceeding the limit.

This study's main goal was to determine the activity concentrations of naturally occurring radioactivity in phosphate rocks, phosphoric acid, phosphogypsum, and fertilizers, by using gamma spectrometry. Additionally, establishing these measurements and the distributions of these radionuclides in these materials were provided the fundamental information for estimating radiation hazards and doses, and comparing this information with a global safety limits using UNSCEAR equations.

Experimental methods and techniques

Chemical processing of phosphate rock

In the present study thirty-one phosphate samples from milling stages of chemical processing were collected which included nine samples of phosphate ore; five samples (PO1-PO5) from phosphate ore of El-Sebayia these samples were used to produce phosphoric acid and phosphogypsum, and four samples (PO6-PO9) were a mixture of western El-Sebayia and phosphate Abu-Tarture ore in Egypt that were used in the production of single superphosphate and triple superphosphate. Those samples were prepared for chemical processing. Also, ten samples from the chemical processing of pohosphoric acid production; five samples (PA1–PA5) from phosphoric acid, and five samples from phosphogypsum (PG1–PG5). Seven samples from single superphosphate (SSP1–SSP7), and five samples of triple superphosphate (TSP1–TSP5). All of these samples were represent a part of this work and were chosen to estimate the effective doses received by the workers at the fertilizer factory due to ore chemical processing. The major raw material of phosphate rock was used in the production of phosphoric acid and fertilizers. The mixture of phosphate ore attacks sulfuric acid in a wet process, resulting in two products: phosphoric acid and a calcium sulfate by-product known as phosphogypsum [9, 10]. This was shown in the following chemical reactions.

$$ {\text{Ca}}_{{{1}0}} \,\left( {{\text{PO}}_{{4}} } \right)_{{6}} \,{\text{F}}_{{2}} + { 1}0\,{\text{H}}_{{2}} {\text{SO}}_{{4}} + { 2}0\,{\text{H}}_{{2}} {\text{O }} \to { 1}0\,{\text{CaSO}}_{{4}} .{2}\,{\text{H}}_{{2}} {\text{O}} + { 6}\,{\text{H}}_{{3}} {\text{PO}}_{{4}} + { 2}\,{\text{HF}} $$

The mixture was treated chemically with phosphoric acid in two different ratios to get the single superphosphate (SSP) fertilizer grains and the triple superphosphate (TSP) fertilizer grains [11]. Single superphosphate (SSP) was formed by reacting sulfuric acid with phosphate rock according to the following reactions equation.

$$ 0.{7}\,{\text{Ca}}_{{{1}0}} \,\left( {{\text{PO}}_{{4}} } \right)_{{6}} \,{\text{F}}_{{2}} + { 7}\,{\text{H}}_{{2}} {\text{SO}}_{{4}} + { 3}.{5}\,{\text{H}}_{{2}} {\text{O }} \to { 4}.{2}\,{\text{H}}_{{3}} {\text{PO}}_{{4}} + { 7}\,{\text{CaSO}}_{{4}} . \, \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} \,{\text{H}}_{{2}} {\text{O}} + { 1}.{4}\,{\text{HF}} $$

The phosphoric acid produced was reacted with rock phosphate in excess to give a final product of mono-calcium phosphate according to the following equation.

$$ 0.{3}\,{\text{Ca}}_{{{1}0}} \,\left( {{\text{PO}}_{{4}} } \right)_{{6}} \,{\text{F}}_{{2}} + { 4}.{2}\,{\text{H}}_{{3}} {\text{PO}}_{{4}} + { 3}\,{\text{H}}_{{2}} {\text{O }} \to { 3}\,{\text{Ca}}\,\left( {{\text{H}}_{{2}} {\text{PO}}_{{4}} } \right)_{{2}} .{\text{H}}_{{2}} {\text{O}} + \, 0.{6}\,{\text{HF}} $$

In the chemical process of triple superphosphate (TSP) production, the phosphoric acid reacts with apatite according to the reactions in the following equation.

$$ {\text{Ca}}_{{{1}0}} \,\left( {{\text{PO}}_{{4}} } \right)_{{6}} \,{\text{F}}_{{2}} + { 14}\,{\text{H}}_{{3}} {\text{PO}}_{{4}} + { 1}0\,{\text{H}}_{{2}} {\text{O }} \to { 1}0\,{\text{Ca}}\,\left( {{\text{H}}_{{2}} {\text{PO}}_{{4}} } \right)_{{2}} .{\text{H}}_{{2}} {\text{O}} + { 2}\,{\text{HF}} $$

Several samples from phosphate rocks, phosphoric acid, phosphogypsum, single superphosphate, and triple superphosphate are investigated radiometrically.

The measurements of radioactivity

Preparation of samples

Thirty one phosphate samples were crushed mechanically into a powdered form, then dried and ground to a 100-mish size. These homogenized samples, each weighing 200 g were packed into a polyethylene circular container (Merinelli beakers) of 10 cm diameter and 3 cm height. Each sample was pressed manually in its container until it was filled and then it was tightly closed and stored. At least it sealed for about one month to reach the state of secular equilibrium between radium and its decay products, which comprise 98.5% of the radiological impact of the uranium series. The contributions of uranium and its decay products from radium were ignored. Therefore, the reference to uranium-series radionuclides is usually written as radium instead of uranium [12]. A multi-channel gamma-ray spectrometer NaI (Tl) detector that had a high efficiency that was used for measuring the concentrations of uranium, thorium, radium, and potassium. These analyses were conducted in the laboratory of chemical prospection at the Egyptian Nuclear Materials Authority. The specific activity concentrations (Bq/kg) of each sample was computed in (Table 1) with accordance to the following relations: 1% K-40 = 313 (Bq/kg); 1 (ppm) U-238 = 12.35 Bq/ kg; 1 (ppm) Th-232 = 4.06 Bq/kg; and 1 (ppm) Ra-226 = 12.35 Bq/kg [13].

Table 1 Activity concentrations of U-238, Ra-226, Th-232, and K-40 (Bq/kg) in phosphate samples
Full size table

Gamma spectrometry

Instrumentation and calibration

The NaI (Tl) gamma ray spectrometer was used to detect the activity concentrations of Ra-226, Th-232, U-238 and K-40.This was composed of a 76 mm by 76 mm NaI (Tl) crystal Bicron scintillation detector, that was hermetically sealed with the photomultiplier tube of aluminum chassis, that was determined for the sample material. Following that, the detector was shielded from induced X-rays and a chamber of lead bricks against environmental radiation by using shielding of copper cylindrical (thickness,0.6 cm) and then the lead sheet is used to seal the detector (thicken of 5 cm). Then the detector was coupled to a main shaping amplifier of Nuclear Enterprises, a model NE-4658, and a high voltage Tennelec of power supply, model TC 952 with a digital display HV. The detector was coupled to a computer-based Nuclease PCA-8000, 8192 multichannel analyzer with color graphical spectrum display and advanced operational capabilities. Each sample and background data were counted for 86,400 s. Then to determine an activity of U-238, Th-232, Ra-226 and K-40, gamma spectrometry was performed. To ensure that the instrument accurately records the gamma radiation energy of the radioactive elements, periodic calibration was performed by using radioactive calibration sources; Cs-137 (661.6 keV) and Co-57 (122.1 keV). By measuring the activity concentration of Pb-214, the Ra-226 concentration was calculated that measured from its 351.9 keV γ-peak, The activity of Ac-228, as measured by its 911.1 keV peak, that was used to determine the concentration of Th-232, and the 1460 keV rays, that was released by the decay of K-40 itself, that was used to assess the concentration of K-40.In the analyzed samples, the radioactivity of natural radionuclides such as U-238 and Th-232 series, as the same as K-40, that were examined. The following formula was used to determine these radionuclides' radioactivity concentrations [12].

$$ A\,{(}{\mathbf{Bq}}{/}{\mathbf{Kg}}{)} = {\varvec{C}}{/}{\varvec{Pwt}}\varepsilon $$
(1)

where: (A) is the concentration of activity in Bq/ kg. (C) is a net count, that more than a background. (P) is an absolute emission probability of gamma ray emission. (w); is a net weight of dry samples. (t); is the of time measurement. (ε); is an absolute efficiency of the detector. The obtained results is statically analyzed and an uncertainty (σ). The standard deviation is (SD). A standard error (SM) is evaluated at a confidence interval of 95% [14].

Special case study of assessment of the radiation risk generating from phosphate ore processing

The occupational workers were exposed to radiation hazards through the chemical processing of phosphate rocks under specific conditions to which the facility was subjected. The engineering design of the facility was the most important to reduce radon emissions which depend on the natural ventilation and uranium concentrations in dust [15]. The occupational workers were working hours per year, the International Commission on Radiological Protection was recommended a dose limit for the annual effective dose (AED) for the occupational workers by assuming 2000 working hours yearly. That maintains the radiation risk and annual effective doses below the international limits of 20 (mSv/y) [16].

Radiological hazard assessment

Absorbed dose rate in air (D air)

The absorbed dose rate in air (Dair) was estimated at 1 m over the ground surface. This was estimated from gamma radiation originating from Ra-226, Th-232, and K-40. Accordingly, all three constituents of radionuclides were contributing to the fact that, the individuals should be exposed to the same amount of externally incident gamma radiation. The dose conversion factors for Ra-226, Th-232 and K-40 were 0.462 (nGy/h), 0.604 (nGy/h) and 0.0417 (nGy/h) respectively [3]. The absorbed dose rate was computed by using the following equation:

$$ D_{{{\text{air}}}} \,({\text{nGy}}/{\text{h}}) = 0.462\,A_{{{\text{Ra}}}} + 0.604 \,A_{{{\text{Th}}}} + 0.0417 \,A_{{\text{K}}} $$
(2)

To comply with the safety standards, the worldwide average of the absorbed dose rate was 60 (nGy/h). According to estimate by spectrometry methods.

External annual effective dose (E ex)

The external annual effective dose Eex (mSv/y), that was received by occupational workers through chemical processing of fertilizer according to the exposure to external gamma radiation, and assuming the exposure duration time of 2000 working hours per year[17] was calculated as the following equation.

$$ E_{{{\text{ex}}}} \,\left( {{\text{mSv}}/{\text{ y}}} \right) \, = \, D_{{{\text{air}}}} \,\left( {{\text{nGy}}/{\text{ h}}} \right) \, \times { 1 }\left( {{\text{Sv}}/{\text{Gy}}} \right) \, \times { 2}000 \, \left( {{\text{h}}/{\text{y}}} \right) \, \times {1}0^{{ - {6}}} $$
(3)

The external annual effective dose was computed from the absorbed dose rate (nGy/h) according to ICRP [16] applying a factor of 1 Sv/Gy for the conversion factor coefficient for the workers from an absorbed dose rate in air to effective dose receive by the occupational workers at the fertilizer factory that [18].

Calculation of radon gas concentration

Calculation of the radon emanation coefficient f of phosphate samples

Abdel-Razek [19], proposed a radon emanation model to obtain an equation:

$$ f \, = \, \left( {{4 }/{\text{A}}_{{{\text{Ra}}}} } \right) \, + \, 0.00{5} $$
(4)

f: is emanation coefficient of the studied samples and ARa is the radium (Ra-226) activity concentration in phosphate samples (Bq/kg).

Calculation of the radon flux density from phosphate piles into the working atmosphere at fertilizer factory.

Phosphate piles at fertilizer factories were considered as spherical caps of radius r = 5 m and height H = 1 m.These dimensions fit with the semi-infinite model reported in the UNSCEAR [20]. Accordingly, radon flux density from such piles was calculated as follows:

$$ J_{{\text{D}}} \,\left( {{\text{Bq}}/{\text{m}}^{{2}} {\text{.s}}} \right) = \, A_{{{\text{Ra}}}} \,\lambda_{{{\text{Rn}}}} \,f\,\rho_{{\text{s}}} \left( {{1} - \varepsilon } \right)\, \, L $$
(5)

λRn: is the decay constant of Rn-222 (2.1 × 10–6) s−1, ARa: is the activity concentration of Ra-226; f: is the emanation fraction for earth material Eq. (8), ρs: is the soil grain density (1600), ɛ: is the porosity of dry earth material (0.15) and L: is the diffusion length, (1 m).

The piles at phosphate mills were considered as cap-shaped. The area of the cap or the emanating area SA of the phosphate piles at a hanger was calculated as following equation.

$$ S_{{\text{A}}} \,\left( {{\text{m}}^{{2}} } \right) = {2}\,\pi \, \, r \, \,H $$
(6)

where; SA = 31.42 (m2).

Calculation of radon gas concentrations (CRn)

Radon gas concentrations in the atmosphere of phosphate mills were calculated as follows [20].

$$ C_{{{\text{Rn}}}} \,\left( {{\text{Bq}}/{\text{m}}^{{3}} } \right) = \, \left( {\left( {J_{{\text{D}}} \times \, S_{{\text{A}}} } \right) \, / \, V} \right) \, /\lambda {\text{v }}) \, + \,C_{{{\text{Rn}}}} \,\left( {{\text{out}}} \right) $$
(7)
$$ V\,\left( {{\text{m}}^{{3}} } \right) = \, H \, \times \, w \, \times l $$
(8)

CRn, is the activity concentrations of radon gas (Bq/m3). JD; is radon flux density from phosphate piles (Bq/m2 s), SA: is emanating area of the piles (m2). V; Volume of the hanger (m3), H: is the height of the hanger, 5 m, w is the width of hanger, 10 m, l: is the long of hanger 20 m, λv: ventilation rate, 1 (h−1), and CRn(out); average worldwide radon concentration outdoors, 10 (Bq/m3).

Annual internal effective doses from radon gas (E Rn)

The internal annual effective doses due to radon gas, that were received by the workers at a phosphate mill due to the inhalation of radon gas and its decay products are calculated as follows [17, 20].

$$ E_{{{\text{Rn}}}} \,\left( {{\text{mSv}}/{\text{y}}} \right) \, = C_{{{\text{Rn}}}} \times \, 0.{4 } \times { 2}000 \, \times { 9} \times {1}0^{{ - {6}}} $$
(9)

Annual internal effective dose from airborne dust (EU)

In the environment of the operational areas at phosphate mills, the average annual effective dose due to the inhalation of airborne dust was 0.19 mSv [21, 22]. The author chooses this value as the contribution of the airborne dust to the total effective dose at the mills studied.

Estimation of the total annual effective dose ( E tot)

The total annual effective doses to individual workers were the summation annual effective doses due to external gamma irradiation and internal exposure due to inhalation of radon and uranium in dust was given by the following formula.

$$ E_{{{\text{tot}}}} \,\left( {{\text{mSv}}/{\text{y}}} \right) = E_{{{\text{ex}}}} + E_{{{\text{Rn}}}} + E_{{\text{U}}} $$
(10)

where Eex; is the external annual effective dose due to gamma radiation, and ERn; is the internal annual effective dose due to radon inhalation. It was assumed that the actual exposure period for occupational was taken to be 2000 h per year [23].

Excess lifetime cancer risk (ELCR)

The excess lifetime cancer risk was known as the likelihood of developing cancer throughout a lifetime at a specific exposure level. It was estimated according to the total annual effective dose by using the following equation [24].

$$ {\text{ELCR}} = E_{{{\text{tot}}}} \,\left( {{\text{mSv}}/{\text{ y}}} \right) \, \times T_{{\text{D}}} \times \, \left( {{2}000/{ 876}0} \right) \, \times {\text{ RF }} \times {1}0^{{ - {3}}} $$
(11)

TD; is a work duration time (40 year) for occupational, the multiplier (2000/8760) is introduced to correct for the actual working time, and RF (Sv−1); is the coefficient of the risk factor. For stochastic effects following low dose rate radiation exposure uses values of 0.041 for the adult working individual [16].

Annual effective dose for different tissues and organs (ET)

The total annual effective dose for different tissues and organs ET, that was delivered to a particular tissue and organs was calculated by using the following equation,

$$ E_{T} \left( {{\text{ mSv}}/{\text{y}}} \right) = E_{tot} \times w_{T} $$
(12)

where ET; is the annual effective dose for different tissues and organs, Etot: is the total annual effective dose internal and external and wT,: is the tissue weighting factors as reported ICRP [16].

Results and discussion

The occupational workers at the phosphate fertilizer factory were classified into three groups. These groups represented the full process of line production. Group (I), the preparation group that included crushing, sieving, and mixture of ore samples. Group (II), treating the ore mixture with sulfuric acid, which ended with phosphoric acid (PA) and phosphogypsum (PG). Group (III), chemical treatment of a mixture of ore with phosphoric acid, which ended with the products, single superphosphate (SSP) and triple superphosphate (TSP).

The activity concentrations for phosphate ore of Group (I) for U-238, Ra-226, Th-232, and K-40 were ranged from 494 ± 25 to 1186 ± 59 Bq/kg, 55.5 ± 2.8 to 386 ± 19 Bq/kg, 2.4 ± 0.1 to 28.4 ± 1.4 Bq/kg, and 175 ± 16 Bq/kg to 8896 ± 801 Bq/kg, respectively. With average values 893 ± 45 Bq/kg, 235 ± 12 Bq/kg, 13 ± 0.6 Bq/kg and 3513 ± 316 Bq/kg, respectively (Table 1). The activity concentrations of uranium and radium in all samples were higher than the worldwide average of 33 and 32 Bq/kg, respectively [3]. The activity concentrations of thorium in all samples were less than the worldwide average of 45 Bq/kg. The activity concentrations of K-40 in some ore samples (PO1:PO5) were less than the worldwide average 412 Bq/kg [3]. While the samples (PO6:PO9) were higher than the worldwide average. The radioactivity of the uranium series (U-238, U-234, Th-230, Ra-226, and Pb-210) and thorium series (Th-232, Ra-228, and Th-228) are in equilibrium in the rock samples. The main radioactivity content of phosphate rock was due to the accumulation of dissolved uranium and its products in the form of uranyl complex, in the seawater during the geological formation of the phosphate rocks and their decay products [25].

Group-II, the activity concentrations of U-238, Ra-226, Th-232, and K-40 for phosphoric acid samples (PA) were ranged from 865 ± 43 to 1136 ± 57 Bq/kg, 6.7 ± 0.3 to 77.7 ± 4 Bq/kg, 4.1 ± 0.2 to 24.4 ± 1.2 Bq/kg, and 542 ± 27 to 942 ± 47 Bq/kg respectively. The average values were 973 ± 49 Bq/kg, 34.6 ± 1.7 Bq/kg, 16.2 ± 0.8 Bq/kg, and 718 ± 36 Bq/kg, respectively. The activity concentrations for phosphogypsum samples (PG) of U-238, Ra-226, and K-40 were ranged from 111 ± 5.6 to 235 ± 12 Bq/kg, 187.4 ± 9.2 to 199 ± 9.9 Bq/kg, and 222 ± 11 to 466 ± 23 Bq/kg respectively. The average values of U-238, Ra-226, Th-232, and K-40 were 175 ± 8.8 Bq/kg, 194.1 ± 9.7 Bq/kg, 2.4 ± 0.1 Bq/kg, and 392 ± 20 Bq/kg respectively (Table 1).

The activity concentrations of uranium in all samples of group-II were higher than the worldwide average. The activity concentrations of radium in the samples of this group were higher than the worldwide average, except in samples PA1 and PA3. The activity concentrations of thorium in all samples were less than the worldwide average. The activity concentrations of K-40 in the samples of this group were higher than the worldwide average, except in samples PG1 and PG3. Through the acidulation process of phosphate rocks that disrupted the equilibrium between uranium and thorium during the separation, according to their chemical properties and solubility, uranium was fractionated between phosphoric acid and phosphogypsum. About 80–90% of naturally occurring radium (Ra-226) is in phosphogypsum, while 86% of U-238 and 70% of Th-232 end up in the phosphoric acid [10]. In this study, it was noticed that the variation in the present results indicated that higher concentrations of U-238 and Th-232 were remaining in the phosphoric acid, which explains the low level of these radionuclides presented in the phosphogypsum.

Group-III, The activity concentrations of single superphosphate samples (SSP) for U-238, Ra-226, Th-232, and K-40 were ranged from 568 ± 28 to 790 ± 40 Bq/kg, 161 ± 8 to 289 ± 14 Bq/kg, 2.4 ± 0.1 to12 ± 0.6 Bq/kg, and 43.8 ± 2 to 545 ± 27 Bq/kg respectively, with average values 676 ± 34 Bq/kg, 198 ± 10 Bq/kg, 4.9 ± 0.2 Bq/kg, and 350 ± 18 Bq/kg, respectively. The activity concentrations of triple superphosphate (TSP) samples for U-238, Ra-226, Th-232, and K-40 were ranged from 1322 ± 66 to 1556 ± 78 Bq/kg, 90 ± 4.5 to 105 ± 5.2 Bq/kg, 16.2 ± 0.8 to 36.5 ± 1.8 Bq/kg, and 254 ± 13 to 416 ± 21 Bq/kg, with average values 1410 ± 71 Bq/kg, 98.2 ± 5 Bq/kg, 25.2 ± 1.3 Bq/kg, and 343 ± 17 Bq/kg, respectively (Table 1). The activity concentrations of uranium and radium in all samples were higher than the worldwide average. However the activity concentrations of thorium in all samples were less than the worldwide average. The activity concentrations of K-40 in the samples of this group were less than the worldwide average, except in the samples SSP2, SSP3, SSP4, SSP5, and TSP2. The fertilizer samples of TSP that were obtained by mixing phosphoric acid with different amounts of phosphate rock present a significant amount of uranium isotopes. It was declared from the results that uranium was concentrated in the fertilizers, whereas thorium was distributed, following the same pattern as uranium. The difference in the activity concentrations of the radium that was found in the samples of fertilizer can be due to the different concentrations of uranium and thorium series radionuclides in the raw material phosphate rock and the different compositions of fertilizers SSP and TSP. The fertilizer TSP samples, which was derived from mixing different amounts of phosphate rocks with phosphoric acid, present higher concentrations of radium. These results were agreed with [11], which was found that radium concentrations were distributed in the fertilizer TSP and in phosphogypsum. Approximately 80% of the radium was included in bone, that corresponding to an average annual effective dose of about 70 µSv [26]. Figure 1 showed the comparison between the activity concentrations of U-238, Ra-226, Th-232, and K-40 for samples of phosphate rocks, phosphoric acid, and phosphogypsum, and fertilizers of SSP, TSP, and superphosphate, compared with the worldwide average.

Fig. 1
figure 1

the comparison between the average values of activity concentration of U-238, Ra-226, Th-232 and K-40 over the stages of chemical processing

Full size image

The absorbed dose rate in air Dair, According to the UNSCEAR[20], the worldwide average values of Dair range from 18 to 93 (nGy/h) and a typical variability range for measured absorbed dose rates in air outdoors were from 10 to 200 nGy/h. The population weighted value of the absorbed dose rate in air outdoors from terrestrial gamma radiation was estimated by the spectrometry analysis of soil and rock samples given an average of 60 nGy/h. The average values of the absorbed dose rate Dair was changed over the different stages due to the change in the activity concentration of Ra-226. It was 263 ± 13 (nGy/h) for the ore rocks declaring a respective high value compared to the worldwide population weighted value. Fortunately, no public activities were recognized at the locations from which these samples were collected. Over the chemical stages, the average values of Dair are within the typical range even its value at PA stage was 56.8 ± 3.8 (nGy/h) which was lower than the worldwide weighted value. This result was consistent with a previous conclusion by Abdel-Razek et al. [17] that the chemical treatment of the radioactive materials can be a good choice to minimize the radiation doses.

The radon emanation coefficient (f) for group-I was ranged from 0.02 ± 0.001 to 0.04 ± 0.002 with an average value of 0.08 ± 0.004 as shown in (Table 2). Group-II, Radon emanation coefficient; these values were computed only for phosphogypsum (PG) in Group II. All values were ranged from 0.03 ± 0.001 to 0.03 ± 0.001, with an average value of 0.03 ± 0.001 (Table 2). Group-III, the radon emanation coefficient (f) for SSP samples were ranged from 0.02 ± 0.001 to 0.03 ± 0.002 with an average value of 0.03 ± 0.001, and for TSP samples, it was ranged from 0.04 ± 0.002 to 0.05 ± 0.002 with an average value of 0.046 ± 0.002 (Table 2).

Table 2 Absorbed dose rate Dair (nGy/h), external annual effective dose Eex (mSv/y), the total annual effective dose Etot (mSv/y), radon emanation coefficient f, radon flux JD (Bq/m2.s), and the excess life time cancer (ELCR) due to different phosphate samples at the fertilizer factory
Full size table

Radon flux JD for group-I, the samples were ranged from 0.012 ± 0.001 to 0.02 ± 0.001 (Bq/m2.s), with an average value of 0.017 ± 0.001 (Bq/m2 s). All values of the studied samples in this group were less than the worldwide average of 0.016 (Bq/m2.s) [20], (Table 2).Group-II, radon flux these values were computed only for phosphogypsum (PG) samples. The average value of radon flux for PG production was 0.014 ± 0.001 (Bq/m2.s). All values of the studied samples in this group were less than the worldwide average. Group-III, radon flux, for (SSP) samples was ranged from 0.014 ± 0.001 to 0.02 ± 0.001 (Bq/m2.s) with an average value of 0.014 ± 0.001 (Bq/m2.s). And the average value of radon flux for (TSP) samples were 0.013 ± 0.003 (Bq/m2.s). All values of the studied samples in this group were less than the worldwide average (Table 2). At all the values of the radon flux density from the piles at phosphate mills are comparable of worldwide average.

The radon gas concentrations, CRn According to Eq. (7), it was clear that, the concentration of radon gas which was emanating from the treated piles were affected by the volume of the hangers. This minimized the value of radon concentrations into only a value of 0.001 (Bq/m3) which will neglect in the further calculations. Then, the outdoor radon concentrations only represented the radon concentrations at the studied area which is 10 Bq/m3.

The average of total annual internal effective doses Eint that was come from the radon gas and the uranium concentration in dust. According to Eq. (9) the average of three groups of annual internal effective doses due to radon; ERn was equals 0.07 (mSv/y). While the internal effective dose; EU due to uranium concentration in dust was equal 0.19 mSv/y according to [21, 22]. This was lead to a value of 0.26 (mSv/y) for Eint. It was noticed that the annual internal effective doses due to radon gas concentrations were extremely low due to the wide dimensions of the fertilizer factory and the good ventilation. The concentrations of radon gas were due to the outdoor radon gas concentrations only.

To evaluate the hazard risks from the exposures to the ionizing radiation the ICRP [23] relies on the operational quantity "Effective Dose E." At higher values of E the risks are classified as deterministic and the severity of a certain risk is proportional to the dose with a threshold of 2 Sv. At the range of mSv the risks are classified as probabilistic or stochastic. In this situation the probability of a risk is proportional to the dose with no threshold i.e. any value of the effective dose is associated with a probable radiation risk. The ICRP recommended certain dose limits. These limits are 20 mSv/y for occupational radiation workers with 100 mSv averaged over 5 years and for members of the public it is 1 mSv/y[23]. Also, ICRP stated that any exposure higher than the natural radiation background should be kept as low as reasonably achievable, ALARA principle.

The total annual effective doses Etot for group-I, were ranged from 0.62 ± 0.03 to 1.1 ± 0.05 (mSv/y), with an average value 0.78 ± 0.04 (mSv/y) (Table 2). The total annual effective doses in all samples were lower than the dose limit for occupational 20 (mSv/y) [3]. Group-II, The total annual effective doses for (PA) samples were ranged from 0.27 ± 0.01 to 0.32 ± 0.02 (mSv/y) with an average value 0.30 ± 0.02 (mSv/y). And Etot for (PG) samples, were ranged from 0.46 ± 0.02 to 0.48 ± 0.02 (mSv/y) with average value 0.47 ± 0.02 (mSv/y) (Table 2).The total annual effective dose in all samples were less than the dose limit for occupational. Group-III, The total annual effective doses for (SSP) samples were ranged from 0.45 ± 0.02 to 0.55 ± 0.03 (mSv/y) with an average value 0.48 ± 0.02 (mSv/y); these values for (SSP) were less than the dose limit. And Etot for (TSP) samples were ranged from 0.39 ± 0.029 to 0.43 ± 0.02(mSv/y) with an average value 0.41 ± 0.02 (mSv/y) (Table 2).The total annual effective doses in all samples of TSP were less than the dose limit. For a phosphate mining and beatification facility in South Africa, summing of the doses from external gamma exposure and inhalation of airborne dust gives total annual effective doses in the range 0.1 to 1 (mSv/y) [21, 22]. This study was obtained comparable values for the annual effective doses. With respect to ICRP recommendations, the external annual effective dose limit for radiation for workers should has a value between 5 and 10 (mSv/y), unless their dosages can be measured more easily by another method, such as by flight crew [27, 28].

The ELCR is an additional risk that workers may be getting cancer if they have been exposed to cancer-causing materials for a longer time. The excess lifetime cancer for group- I, ranged from (0.23 ± 0.01) × 10–3 to (0.42 ± 0.02) × 10–3, with an average value of (0.3 ± 0.02) × 10–3 (Table 2). The excess lifetime cancer in the samples of this group was lower than the worldwide average of 1.45 × 10–3[3]. Group-II, The excess lifetime cancer for (PA) samples were ranged from (0.10 ± 0.01) × 10–3 to (0.12 ± 0.01) × 10–3 with an average value (0.11 ± 0.01) × 10–3, and ELCR for (PG) samples were ranged from (0.17 ± 0.01) × 10–3 to (0.18 ± 0.01) × 10–3 with an average value (0.18 ± 0.01) × 10–3 (Table 2). The excess lifetime cancers in phosphoric acid and phosphogypsum samples were lower than the worldwide average. Group-III. The excess lifetime cancer for (SSP) samples were ranged from (0.17 ± 0.01) × 10–3 to (0.21 ± 0.01) × 10–3 with an average value (0.18 ± 0.02) × 10–3. ELCR for (TSP) samples were ranged from (0.15 ± 0.01) × 10–3 to (0.16 ± 0.01) × 10–3 with an average value of (0.15 ± 0.03) × 10–3 (Table 2). The excess lifetime cancers in single superphosphate samples (SSP) and triple superphosphate (TSP) are lower than the worldwide average.

The distribution of annual effective doses over the different human organs and tissues depend on the computed total annual effective dose. Then the annual effective doses over the organs and tissues were less than the worldwide average in the three groups (Table 3). The radiation amasses in human organs and tissues for a human can be detected by ET. It was noticeable that, the average annual effective doses over the organs and tissues for bone marrow, colon, lung, stomach, breast and remainder tissues were (0.14 ± 0.07) mSv/y, that were received the highest dose relative to other human organs in all stages in chemical processing of phosphate that had the same value of tissue weighting factor (wT = 0.12). Controversy, for bone surface, brain, salivary glands, skin that were received the average annual effective dose 0.01 ± 0.01 (mSv/y) in all chemical processing of phosphate. These group of organs and tissues that had the same value of tissue weighting factor (wT = 0.01). That were received the lowest annual effective dose value relative to other human organs. According to the total annual effective doses through the three groups, all organs and tissues were received annual effective dose less than the dose limit for occupational workers are 20 mSv/y [3].

Table 3 Distribution of the annual effective doses over the different human organs and tissues ET due to phosphate samples at the fertilizer factory (mSv/y)
Full size table

Conclusion

Elemental concentrations of U-238, Th-232, Ra-226 and K-40 were affected by the chemical processing over the different stages. Accordingly, the activity concentrations of the studied radionuclides were changed. Wide dimensions of the studied phosphate mills lowered the activity concentrations of radon gas at the different stages. The values of the absorbed dose rate Dair (nGy/h) of the ore rocks were higher than the worldwide average 60 (nGy/h). Fortunately, no public activities were recognized at the locations from which the studied samples were collected. All the occupational worker in the suggested groups were received effective doses below 1 mSv/y. Accordingly, the studied phosphate mille need no radiation monitoring [29]. These dose limits are established on the prudent approach by assuming that there is no threshold dose below which there would be no effect. This means that any additional dose will cause a proportional increase in the chance of a health effect. The values of ELCR over all the stages were below the reported value of value of 1.45 × 10–3. This conclusion was consistent with the safe values of the annual effective doses indicating safe treatment of the samples.