Evaluation of Excess Lifetime Cancer Risk Due to Gamma Rays Exposure from Phosphate Fertilisers Used in Saudi Arabia

Activity concentrations were measured in phosphate fertilisers widely used in Saudi Arabia employing gamma ray spectrometry. The activity concentrations of 238U, 232Th and 40K were used to estimate the excess lifetime cancer risk (ELCR). The obtained data show that 238U and 40K concentrations are higher than the recommended values (35 Bq kg–1 and 400 Bq kg–1 for 238U and 40K, respectively) in most of phosphate fertilisers. The highest value of 238U was 4122 ± 82 Bq kg–1 for diammonium phosphate (DAP) fertiliser. Absorbed dose rate, annual effective dose rate and the ELCR were calculated. The results revealed that the values of outdoor, indoor and total ELCR values were higher than the world averages in all samples except one sample, sample of MAP fertiliser (monoammonium phosphate). The highest value of ELCRtot was for DAP fertiliser which is 50 times higher than the world average value (1.45 × 10–3). Moreover, the results indicated a strong correlation between uranium in phosphate fertilisers and ELCRtot. Based on the obtained results, the direct gamma radiation exposure from phosphate fertilisers is a serious radiological threat to the farmers.


INTRODUCTION
Natural environmental radioactivity and the associated external exposure due to gamma radiation depend primarily on the geological and geographical conditions, and appear at different levels in the soils of each region in the world. 1 One of the sources of radioactivity in soils other than those of natural origin is the extensive use of fertilisers rich in phosphates for agricultural purposes. 2,3 Phosphate fertilisers are used widely in the reclamation of farmland and increasing agricultural crops. Moreover, phosphate fertilisers are by far the most widely used for agriculture soil due to its cost-effectiveness and its availability in markets.
Sedimentary phosphate ores can be significantly enriched with naturally occurring radionuclides, uranium ( 238 U) and the daughter radionuclides that come from the radioactive decay of 238 U. 4 Phosphoric anhydride (P 2 O 5 ) and potassium oxide (K 2 O) are important materials used for phosphate fertilisers production. Previous studies have shown that the concentration of uranium and potassium is related to the concentration of P 2 O 5 and K 2 O in various fertilisers. 3,5 Phosphate ore is typically extracted from mineral deposits in phosphate rock. The phosphate rocks are used as raw materials in phosphate fertiliser industries, which contain radionuclides of the uranium and thorium natural series. Also, one of the important sources of radioactivity is potassium 40 K which exists in phosphate rocks. 6 Kingdom of Saudi Arabia is one of the largest producers and exporter of diammonium phosphate (DAP) and compound phosphate fertilisers worldwide. The phosphate fertiliser factories have been provided with phosphate ore from phosphate mine in northern Saudi Arabia. 3 Before being transported to the factories, the phosphate undergoes the beneficiation process in the plant in order to produce concentrated phosphate ore. Furthermore, several types of imported phosphate fertilisers have been used in agricultural activities. Therefore, concentrations of naturally occurring radionuclides ( 238 U, 232 Th and 40 K) in phosphate fertilisers in Saudi Arabia were measured, to evaluate the excess lifetime cancer risk (ELCR) for the population due to gamma rays exposure from phosphate fertilisers.

Samples Collection and Preparation
Seventeen samples of widely used phosphate fertilisers were collected from local markets, Saudi Arabia (Table 1). Four types of phosphate fertilisers were studied: DAP, monoammonium phosphate (MAP), single super phosphate (SSP) and NPK (nitrogen, phosphor and potassium). Concentrations of P 2 O 5 in fertilisers ranging at 5%-61% is shown in Table 1. Samples were grinded, dried and packed in the standard size can and tightly sealed and stored for 28 days to acquire secular equilibrium. Reference material was also packed in the same standard size can for efficiency calibration. 3

Experimental Setup
A hyper-pure Germanium detector (HPGe), coaxial type, p-type with relative efficiency of 20% was used. The resolution (full width at half maximum, FWHM) at the peak 122 keV, 57 Co, is 0.934 keV and at the peak 1332 keV, 60 Co, is 1.79 keV. The detector was shielded with lead shield to reduce the background radioactivity. The software Quantum Gold, Version 4.04.00, was used for analysis and evaluation of the gamma-ray spectra. The HPGe was calibrated for efficiency using the reference material RGU-1 from IAEA. The certified activity of uranium is 400 ppm which is equivalent to 4960 Bq kg -1 . The energy transitions of the 226 Ra daughters ( 214 Pb and 214 Bi) were used to develop the efficiency calibration curve. A fourth-degree polynomial fitting was performed to reach the best R 2 value (≈ 0.983). 3

Calculations of Activity Concentrations
After achieving secular equilibrium and subtraction of the background, the activity concentrations of 238 U, 232 Th and 40 K were calculated. 238 U was measured at the gamma lines 63.29 keV and 92.38 keV for 234 Th. For 232 Th, the activity concentration was assessed by using energy transition of 228 Ac at 93.4 keV and the energy transitions of 208 Tl at 583.19 keV and 228 Ac at 911.2. The average values were calculated. In the case of 40 K, the specific activity was estimated directly by its gamma line of 1460.75 keV.

Activity Concentrations in Phosphate Fertilisers
The activity concentrations of 238 U, 232 Th and 40 K in phosphate fertilisers were measured as shown in  Table 1. The value of 40 K ranges from 60.0 ± 3.0 Bq kg -1 to 6430 ± 62 Bq kg -1 . From these results, it can be observed that the activity concentrations of uranium in 15 phosphate fertiliser samples are higher than permissible limit in soil (35 Bq kg -1 ). 7 The highest value was 4122 ± 82 Bq kg -1 for DAP with 46% of P 2 O 5 which is 114 times higher than the permissible limit. Uranium content in DAP sample was higher than the values in the other phosphate fertiliser samples. High concentration of uranium in DAP fertiliser (KSA) may be due to the use of phosphate rocks as raw materials for manufacturing phosphate fertilisers. 8,9 Similarly, the activity concentrations of 40 K in all fertiliser samples were higher than the recommended value (400 Bq kg -1 ), except of DAP and MAP phosphate fertilisers. 7 The high concentration of potassium may be related to the concentration of K 2 O. 3 The result indicates that the large use of phosphate fertilisers leads to increase the radioactivity of soil, especially for DAP fertiliser. The elevated levels of 238 U may due to wet phosphate processing where phosphate ore is firstly attacked by sulfuric acid to produce the phosphoric acid where uranium will be mainly concentrated in the phosphoric acid, while radium, thorium and other insoluble radionuclides will be precipitated as sulphate salts and concentrated in the phosphogypsum by product. 10 In 2000, Mazzilli et al. found that 90% of 226 Ra and 80% of 232 Th fractionate into phosphogypsum, while 238 U was being predominantly incorporated in phosphoric acid. 11 Uranium and its progenies are usually in secular equilibrium in the different geological formation, while secular equilibrium could be disturbed during chemical treatment of phosphate ore to produce phosphoric acid. 10

ELCR
There is always a chance of 33% (or 0.33% per person) that an individual will be diagnosed with cancer at some stage of life due to normal exposure to natural radioactivity and or a toxic substance. 12 The Department of Environmental Quality (DEQ) considers a chance of 1 in 100,000 persons. That means 1 × 10 −5 is allowable limit. ELCR is an additional possibility that a person may develop cancer if that person is exposed to additional cancer-causing materials for a longer time. In order to evaluate the ELCR of phosphate fertilisers, some radiological parameters were calculated ( Table 2). The outdoor absorbed dose rate (D out ) in air 1 m above the ground surface from the gamma radiation from 232 Th, 238 U and 40 K was calculated. 7 The conversion factors used to compute the outdoor external dose rate (D out ) in air per unit activity concentration in 1 Bq kg -1 correspond to 0.604 nGy h -1 for 232 Th, 0.462 nGy h -1 for 238 U and 0.0417 nGy h -1 for 40 K. Therefore, D out could be calculated from the following equation: 6,13 D out (nGy h -1 ) = 0.462 A U + 0.604A Th + 0.0417 A K The indoor gamma ray dose (D in ) from the above radionuclides was calculated using where A U , A Th and A K are the activity concentrations of 238 U, 232 Th and 40 K (Bq kg -1 ), respectively.
As shown in Table 2, The outdoor and indoor absorbed dose rates (D out and D in ) in air 1 m above the ground surface in phosphate fertilisers ranged from 22.1-1910 nGy h -1 and 43.7-3803 nGy h -1 , respectively. The outdoor and indoor absorbed dose rates in 16 phosphate samples, except MAP, are higher than the world averages for outdoor and indoor exposure due to terrestrial gamma radiation (51 nGy h -1 and 84 nGy h -1 , respectively). 7 The results indicate the direct connection between the outdoor and indoor absorbed dose rates in air and the specific activity of uranium and potassium, where the greatest part of gamma radiation comes from them.
Outdoor annual effective dose rate, E out (mSv y -1 ), received by the population is calculated using the following equation: 3,7 E out (mSv y -1 ) = D out (nGy h -1 ) × 8760 (h y -1 ) × 0.2 × 0.7 (Sv Gy -1 ) × 10 -6 (3) where, D out = the outdoor absorbed dose rate in air (nGy h -1 ); 8760 = the time for one year (h); 0.7 = the conversion factor (Sv Gy -1 ), which converts the absorbed dose rate in air to human effective dose; and 0.2 = the outdoor occupancy factor. 7 Outdoor annual effective dose rate, E out , ranged at 0.03-2.34 mSv y -1 . It can be observed that the outdoor annual effective dose rate for all samples is higher than the world average value from the outdoor exposure (0.07 mSv y -1 ). 3,7 Similarly, the indoor annual effective dose rate in air was calculated using the formula as follows: 3,7 E in (mSv y -1 ) = D in (nGy h -1 ) × 8760 ( h y -1 ) × 0.7 (Sv Gy -1 )×10 -6 × 0.8 (4) where, D in = the indoor gamma ray dose; 0.8 = the indoor occupancy factor; 0.7 × 10 -6 = the conversion factor from absorbed dose rate in air to effective dose received by the person; and 8760 = the time per year (h).
The annual effective dose rate for indoor ranged from 0.21 mSv y -1 to 18.7 mSv y -1 . The result indicates that the values of indoor annual effective dose rate for all samples, except MAP fertiliser, were higher than the world average of 0.41 mSv y -1 from indoor exposure. 7 The highest values of E out and E in are 1910 mSv y -1 and 18.7 mSv y -1 , respectively, in Sample 1 for DAP phosphate fertiliser. Therefore, the agricultural uses of DAP fertilisers could lead to significant radiation exposure. The annual effective dose rate for outdoor and indoor exposure must be less than 0,07 mSv y -1 and 0.41 mSv y -1 , respectively, to keep the radiation hazard insignificant.
The ELCR was calculated to estimate the probability of cancer incidence in a population of individuals. The indoor and outdoor ELCR were calculated using the following equations: 12,14 ELCR out = E out (mSv y -1 ) × DL (y) × RF (Sv -1 ) ELCR in = E in (mSv y -1 ) × DL (y) × RF (Sv -1 ) where E in , E out , DL and RF are indoor and outdoor annual effective dose, duration of life (70 y) and risk factor, 0.05, for the public, respectively. 15 From Table 2, the calculated ELCR out ranged from 0.09 × 10 -3 to 8.20 × 10 -3 for outdoor exposure. These values, except the value for MAP, were higher than the worldwide recommended value of 0.29 × 10 -3 . 7 The highest value is 8.20 × 10 -3 for DAP fertiliser which is greater than the world average value. The indoor ELCR in was calculated to estimate the total ELCR. The indoor ELCR in varied from 0.75 × 10 -3 to 65.3 × 10 -3 . The ELCR tot total for all samples was computed as shown in Table 2. The values of ELCR tot ranged at 0.84 × 10 -3 -73.5 × 10 -3 . The ELCR tot total for all samples, expect MAP, are greater than the word average value 1.45 × 10 -3 . 16 The highest value was in DAP sample which is 50 times higher than the world average value, 1.45 × 10 -3 . The variations of ELCR can be seen clearly as shown in Figure 1, where the highest value of ELCR was in DAP fertiliser and the lowest value was in MAP fertiliser.  The results indicate that the excessive use of phosphate fertilisers in agriculture soil can lead to increase the radiation risk for public, especially for DAP. Further, the obtained values of absorbed dose rate, annual effective dose rate and the excess lifetime cancer risk in phosphate fertilisers show that the agricultural uses of phosphate fertilisers could lead to increase the probability of cancer incidence in a population of individuals, with the exception of MAP fertiliser.

CONCLUSION
The activity concentrations of 238 U, 232 Th and 40 K in local and imported phosphate fertiliser samples were measured using gamma ray spectrometry technique. The data show that the activity concentrations of 238 U in 15 phosphate samples were higher than the world average value (35 Bq kg -1 ), while 40 K activity was higher than the world average value (400 Bq kg -1 ) in all samples except two samples (DAP and MAP fertilisers). However, the activity concentrations of 232 Th were found to be lower than the recommended value (30 Bq kg -1 ) in all phosphate samples. The absorbed dose rate and annual effective dose rate were calculated to evaluate the ELCR for the analysed phosphate fertiliser samples. The outdoor, indoor and ELCR values were higher than the world average values. This result reveals that the direct exposure to the phosphate fertilisers may increase the probability of developing cancer for farmers, especially for DAP fertilisers due to the elevated levels of uranium. Moreover, a strong correlation has been observed between ELCR and uranium. Based on this study, the current data indicate to the need of new rules to improve and develop the regulations that can control the import and production of phosphate fertilisers in Saudi Arabia.