Introduction

Naturally occurring radionuclides may be concentrated in the residues, waste materials and end products according to the non-nuclear industrial activities such as oil and gas production. Especially, waste that was produced by burning of coal, fossil fuel, petroleum products, or other related materials in thermal power plants, phosphates and fertilizers production, metals and rare earth elements mining may lead to a significant environmental hazards and human risk [1].Oil ashes are mostly the waste products of non-nuclear industries, such as oil ashes from electricity production, metrology, and oil and gas sludge [2]. Oil ash is the major industrial waste. This was produced from the burning of the heavy fuel oil (HFO). Heavy oil fuel was commonly used for production of electricity in thermal power stations, and to operate the boilers in power stations and water desalination facilities. Oil fly ash (OFA) and bottom ash (BA) are the major industrial waste that formed with the production of electricity in thermal power plants that burn HFO. Two kinds of oil ash are produced by power plants; Boiler ash is one type of ash fraction that is too heavy to be entrained in the flue gas and was deposited to the bottom of the furnace and is called bottom ash (BA). It is often categorized as high-grade ash and has no unburned carbon, nickel (2.7–8.5), and vanadium (4.4–19.2%) [3].Oil fly ash (OFA) is a fine combustion residue that collected from the stack or electrostatic precipitators. It is often classified as low-grade ash and comprises 30–80% unburned carbon, nickel (0.2–0.5%), and vanadium (0.3–1.5%) [4]. The characteristics and amount of the two types of oil ash produced depends on the type of fuel oil and boiler. The result of combustion fuel oil results in concentration of most trace elements in oil ash by approximately 10 times the concentration in the original fuel oil [5].The burns of heavy fuel oil result in concentrates the radioactive materials in the oil ash, which enters the environment by different pathways that can cause significant environmental concerns when stored as waste and released into the atmosphere [6]. The concentrations of radioelements varies greatly between oil fields, and then still requiring local survey studies in this area [7].Thermal power plants exist in several countries in the world and use a variety of fuels, including oil, coal, diesel, lignite, and natural gas Those plants produced significant quantities of fly ash [8]. In Saudi Arabia to operate the thermal power plant and sea water desalination the Saudi Arabia consumed annually about 40 million metric tons of crude oil and heavy fuel oil (HFO) [9]. Whereas electric power plants in Egypt burn around 7 million metric tons of HFO each yearly, the burning of HFO producing more than 4000 metric tons of oil ash [10, 11]. The combustion of heavy fuel in power stations generates huge amounts of oil ash. Oil fly Ashes (OFA), were collected by using electrostatic precipitators (ESPs) that had built-in at facilities and then the oil ash is disposed of in landfills. It was noted that the Egypt's power facilities, which were located in a heavily populated region, didn’t equipped with ESPs. Typically, about 80% of oil ash was gotten disposed of in Landfills however about 65% of oil ash which generated in power plants was disposed of in landfills and ponds that may cause a significant environmental hazard [12, 13]. When oil ash dispersion and accumulation in a large heap big around the power plants and when it was used in building materials. A high background indoors dose rates may be arisen from radioactive materials that found in oil ash [14].Trace metals were released from oil ash into soil and water through numerous methods, including volatilization, melting, decomposition, and oxidation [15]. Furthermore, these metals' reachability posed an additional environmental concern. Oil ashes disposal can pollute neighboring soil, rendering it unsuitable [16]. Most of the generated oil ashes were landfill and had an environmental hazard so it should be determining the activity concentration of the naturally occurring radionuclides of 238U, 235U, and 232Th and their respective decay products, as well as 40K that included in oil fly ashes. Therefore, it is more important to reuse FA for beneficial uses instead of disposal into the environment [17].

This study aims to investigate the specific activity concentrations of naturally occurring radioactivity in present in the oil fly ash and bottom ash by using gamma spectrometry. in addition, establishing these measurements and the distribution of these radionuclides in these materials were provided the main information for estimating radiation hazards and doses with comparing this estimated data with the global safety limits by using UNSCEAR additionally, detailed characterization studies of the oil ashes were conducted to determine the presence of toxic metals, analyze particle size distribution, and explore the practical applications of oil ashes. The samples of oil ashes were collected from the El Kriymat steam electric Power plants located in southern Cairo, Egypt.

Methods and materials

Oil ash samples were collected from the El Kriymat steam electric station, located south of Cairo, Egypt. Nine representative samples of oil ash were investigated; Four samples of oil fly ash were collected from a stack constructed for releasing the combustion gases and five samples of bottom fly ash were collected from a boiler unit that was shutdown in preparation for system cleaning.

Collection and preparation of samples

Nine samples of oil ash were homogenized and ground to fine particles less than 63 µm. 300 g of each waste sample were placed in a covered plastic cup. The samples were kept for three weeks before measurement to reach secular equilibrium between uranium, radium, and their decay products. The specific activity in (Bq/kg) in the oil ash studied samples were calculated according to the flowing relationship 1 ppm 226Ra = 12.35 Bq/kg, 1 ppm 232Th = 4.06 Bq/kg and 1 ppm 40K = 313 Bq/kg [19].

Radioactivity measurements

The gamma ray spectrometer device NaI(Tl), was used to measure for the activity concentrations of 238U, 226Ra, 232Th, and 40K in the studied samples this was consisting of a Bicron scintillation detector, crystal, 76 × 76 mm, hermetically sealed with a photomultiplier tube in an aluminum housing,. After that, the detector was shielded from induced X-rays and a chamber of lead bricks against environmental radiation by copper cylindrical shielding (0.6 cm thickness) and then the detector was sealed by a lead sheet (5 cm thicken).The detector was linked with the main shaping amplifier of Nuclear Enterprises, model NE-4658, and the high voltage power supply of Tennelec, model TC 952 with HV digital display. The detector was also linked to the computer-based Nuclease PCA8000, 8192 multichannel analyzer with color graphical spectral display and high-level technical operation features. Each sample and background data were counted for 86,400 s. Gamma spectroscopy was used to determine the activities of 238U, 232Th, 226Ra and 40K. To ensure that the instrument accurately records the gamma radiation energy of the radioactive elements, permanent calibration was carreadout by using radioactive calibration sources; 137Cs (661.6 keV) and 57Co (122.1 keV). The 226Ra concentration was estimated from the 214Pb activity concentration (measured from its 351.9 keV γ-peak), 232Th concentration was estimated from the activity of 228Ac (obtained from its 911.1 keV γ-peak) and 40K concentration was evaluated from the 1460 keV γ-rays emitted during the decay of 40K itself. The radioactivity of natural radionuclides, of 238U series, 232Th series, and 40K, were investigated in the tested samples. The following formula has been used to determine these radionuclides' radioactivity concentrations equation [19] (1).

$${\text{A}}\left( {{\text{Bq}}/{\text{kg }}} \right) = \frac{{\text{C}}}{{{\text{PTW}}\varepsilon }}$$
(1)

where A, is the activity in (Bq/ kg), C is the net count above the background, P is the absolute emission probability of gamma ray decay, w, is the weight of the net dry sample (kg), t, is the time of measurement, and ε, is the detector’s absolute efficiency. The obtained results have been statically analyzed and the uncertainty (σ), standard deviation (SD), and standard error (SM), was evaluated at confidence intervals of 95% [18]. These analyses were conducted in the laboratory of chemical prospection at the Nuclear Materials Authority in Egypt.

Analytical procedures and instrumentation

Chemical compositions of oil ash

X- Ray diffraction (XRD)

The mineral phase analysis of the oil ash sample was identified by The X-ray diffraction (XRD) patterns which were obtained by using PANalytical X’Pert PRO diffractometer. With a Cu Ka radiation with a wavelength of 1.5418 A˚, and the angular range scanned was from 4° to 70° at a scanning rate of 4° per minute [20].

X-Ray fluorescence (XRF)

The chemical composition of two samples of oil ash; one samples from oil fly ash and the second samples from bottom ash; were characterized by using the portable X-ray fluorescence spectrometer (P-XRF). By using the quantitative analysis Standard FP method, the test device was a ThermoFisher NITON XL3t955-HE energy dispersive X-ray fluorescence spectrometer, which was used in the present study. For every sample, two parallel samples were tested, and the average findings were used to determine the heavy metal concentrations in the samples. For the test, the spot diameter was 8 mm, the measuring duration time for each measurement was more than 60 s and the test angle was 0° [21].

Particle size distribution

The measurement of oil ash particle size distribution participate on surface by the dynamic light scattering analyses was performed at 20 °C in 4 consequent measurements using a Zetasizer Nano ZS analyzer (Malvern Instruments Ltd., Malvern, UK) equipped with a 633 nm He–Ne laser [22].

Determination of radiation hazards generated from oil ash waste

The absorbed dose rate in air (ADR)

The absorbed dose rate outdoor (ADRout) (nGy/h) according to the external gamma radiation in the air at 1 m overhead the ground surface, and indoor air absorbed gamma dose rate (ADRin) were assessed from the gamma radiation of 226Ra, 232Th, and 40K natural radionuclides. The conversion factors that used for ADRout calculations for 226Ra, 232Th, and 40K are 0.462 (nGy/h), 0.604 (nGy/h), and 0.0417 (nGy/h) respectively [23, 24].

$${\text{ADR}}_{{{\text{out}}}} = 0.462{\text{A}}_{{{\text{Ra}}}} + 0.604{\text{A}}_{{{\text{Th}}}} + 0.0417{\text{A}}_{{\text{K}}} \left( {{\text{nGy}}/{\text{h}}} \right)$$
(2)

The conversion factors that used for ADRin calculations for 226Ra, 232Th, and 40K are 0.92 (nGy/h), 1.1 (nGy/h), and 0.08 (nGy/h) respectively Accordingly ADRout, ADRin (nGy/h) can be calculated as follow [25].

$${\text{ADR}}_{{{\text{in}}}} = 0.{\text{92A}}_{{{\text{Ra}}}} + 1.{\text{01A}}_{{{\text{Th}}}} + 0.{\text{08A}}_{{\text{K}}} \left( {{\text{nGy}}/{\text{ h}}} \right)$$
(3)

Finally, the total value of the external dose ADRtot (nGy/h) is then given by

$${\text{ADR}}_{{{\text{tot }}}} {\text{ = ADR}}_{{{\text{out}}}} {\text{ + ADR}}_{{{\text{in}}}} \left( {{\text{nGy/h}}} \right)$$
(4)

where the specific activities concentrations of 226Ra, 232Th and 40K are represented by ARa, ATh and AK are, respectively. The average worldwide values for the outdoor and indoor external absorbed dose rates are 58 nGy/h and 84 nGy/h [26].

The outdoor and indoor annual effective doses (AED)

The external outdoor and indoor annual effective doses AEDout AEDin due to gamma rays were estimated by using the conversion coefficient from the absorbed dose rate in the air to the effective dose that reported as 0.7 Sv/Gy for adult, with used the occupancy factor for outdoor and indoor occupancy factor of 0.2, and 0.8 respectively, during a year of 8760 h [25]. Therefore, the annual effective doses in mSv/y were calculated by the following Eq. (3), (4).

$${\text{AED}}_{{{\text{out}}}} {\text{ADR}}_{{{\text{out}}}} \left( {{\text{nGy}}/{\text{h}}} \right) \times 0.7\left( {{\text{Sv}}/{\text{Gy}}} \right) \times 0.2 \times 8760\left( {{\text{h}}/{\text{y}}} \right) \times 10^{{ - 6}} \left( {{\text{mSv}}/{\text{y}}} \right)$$
(5)
$${\text{AED}}_{{{\text{in}}}} = {\text{ADR}}_{{{\text{in}}}} \left( {{\text{nGy}}/{\text{h}}} \right) \, \times \, 0.7 \, \left( {{\text{Sv}}/{\text{Gy}}} \right) \, \times \, 0.8 \, \times \, 8760 \, \left( {{\text{h}}/{\text{y}}} \right){\text{ x }}10^{ - 6 } \left( {{\text{mSv}}/{\text{y}}} \right)$$
(6)

Finally, the total value of the annual effective dose is then given

$${\text{AED}}_{{{\text{tot}}}} = {\text{ AED}}_{{{\text{out}}}} + {\text{ AED}}_{{{\text{in}}}} \left( {{\text{mSv}}/{\text{y}}} \right)$$
(7)

The annual effective dose measures the risk of stochastic and deterministic effects in the irradiated individuals. the terrestrial gamma annual outdoor AEDout (mSv/y) and AEDin (mSv/y) indoor effective doses and the average total annual effective dose AEDtot (mSv/y) for adults are 0.07 mSv/y, 0.41 mSv/y, and 0.48 mSv/y, respectively, as reported in UNSCEAR [23, 26].

Excess lifetime cancer risk (ELCR)

The excess lifetime cancer risk is used in radiation protection assessment to predict the probability of an individual developing cancer over his lifetime due to low radiation dose exposure, if it will occur at all. According to the evaluation of the annual effective doses (AEDout), (AEDin), (AEDtot) mSv/y, the excess lifetime cancer risk indoor (ELCRin), outdoor (ELCRout) and total (ELCRtot) were calculated using the following equation [27, 28].

$$\left( {{\text{ELCR}}_{{{\text{out}}}} } \right)\, = \,\left( {{\text{AED}}_{{{\text{out}}}} } \right){\text{ mSv}}/{\text{y}}\, \times \,\left( {{\text{DL}}} \right){\text{ y}}\, \times \,\left( {{\text{RF}}} \right){\text{Sv}}^{{ - {1}}} \, \times \,{1}0^{{{-}{3}}}$$
(8)
$$\left( {{\text{ELCR}}_{{{\text{in}}}} } \right){ } = \, \left( {{\text{AED}}_{{{\text{in}}}} } \right){\text{ mSv}}/{\text{y }} \times \, \left( {{\text{DL}}} \right){\text{ y }} \times \, \left( {{\text{RF}}} \right){\text{ Sv}}^{ - 1} \times {10}^{ - 3}$$
(9)
$$\left( {{\text{ELCR}}_{{{\text{tot}}}} } \right) = \left( {{\text{AED}}_{{{\text{tot}}}} } \right){\text{ mSv}}/{\text{y}} \times \left( {{\text{DL}}} \right){\text{ y}} \times \left( {{\text{RF}}} \right){\text{ Sv}}^{ - 1} \times 10^{ - 3}$$
(10)

(AEDout), (AEDin), (AEDtot) the annual effective doses (mSv/y). (DL), represented the life of duration (70 yr), and RF, represented the risk factor (0.05 Sv−1) respectively [27].

Annual gonadal dose equivalent (AGDE)

The three organs gonads, the bone marrow and the bone, had an interest of UNSCEAR [26] according to their sensitivity to the radiation. It was declared that an increase in AGDE (mSv/y) had a hazard impact on the bone marrow, and consequently causing red blood cells to be impaired by replacement of white blood cells. The bone marrow was more sensitive to radiation than the nerve tissue or muscle. This caused a blood cancer known as leukemia that is fatal. The AGDE was given following the equation [29].

$${\text{AGDE}} = \frac{{3.09 {\text{C }}_{{{\text{Ra}}}} + 4.19 {\text{C}}_{{{\text{Th}}}} + 0.314{\text{C}}_{{\text{K}}} }}{1000}\;\;\;\; \left( {\text{mSv/y}} \right)$$
(11)

Hence, the annual gonadal dose equivalent due to the activity concentrations of 226Ra, 232Th and.40K in the studied the oil ash radioactive waste samples [29]

Results and discussion

Radiological characterization

Gamma-ray spectrometry

Table 1. The data in the table was showed the specific activity concentrations of 226Ra, 232Th and 40K of examined samples of both oil fly ash (OFA) and bottom ash (BA) and the overall samples of oil ash (OFA, BA), and examined the some typical values given for comparison. Figure 1 representrd a box plot that showed the range of specific activity concentrations values for all radionuclides oil ash samples for 226Ra, 232Th, 40K, which were investigated, as well as the median value for each one of radionuclides.

Table 1 the specific activity concentrations of 226Ra, 232Th, and 40K (Bq/kg) of the oil ash
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Fig. 1
figure 1

Showed the box plot of 226Ra, 232Th, and 40K activity concentrations (Bq/kg) in oil ash

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The activity concentrations of oil fly ash (OFA) samples for 226Ra, 232Th, and40K were ranged from (12.3 ± 0.6 to 70.5 ± 3.5) Bq/kg, (20.3 ± 1.01 to 30.8 ± 1.5) Bq/kg, and (ND to 18.8 ± 0.9) Bq/kg respectively, with an average values of (45.3 ± 2.3) Bq/kg, (24.9 ± 1.3) Bq/kg and (18.8 ± 0.9) Bq/kg respectively Table 1. The average values of OFA samples activity concentration for 226Ra was higher than the worldwide averge 32 (Bq/kg) as reported in UNSCEAR [23, 30]. However the average values of OFA activity concentrations for 232Th and 40K were less than the worldwide average 45, 412 respectively [23, 30].

The activity concentrations of bottom ash (BA) for 226Ra, 232Th, and 40K were ranged from (2226.3 ± 111.3 to 4022.1 ± 201.1) Bq/kg, (101.5 ± 5.1 to 657.72 ± 32.9)Bq/kg, and (46.9 ± 2.3 to 528.9 ± 26.4) Bq/kg respectively with average values of (3055.3 ± 152.7) Bq/kg, (408.7 ± 24.1) Bq/kg and (353.7 ± 17.7) (Bq/kg) for 226Ra, 232Th, and40K respectively. It was concluded that the average activity concentrations samples of bottom ash (BA) samples for 226Ra and 232Th were greater than the worldwide average 32, 45 (Bq/kg) respectively. However the average the activity concentrations samples for 40K was lower than the worldwide average 412 (Bq/kg).

Overall, the highest activity concentrations of oil ash(OFA, BA) for 226Ra, 232Th, and 40K were found to be 4022.1 ± 201.1, 657.7 ± 32.9, and 160.6 ± 8.02, (Bq/kg), respectively, while the lowest values were observed to be 12.3 ± 0.6, 20.3 ± 1.01, and 123.5 ± 6.2 (Bq/kg), respectively. And the average values were found to be 1718 ± 85.9, 278.1 ± 13.9, and 136 ± 6.7 (Bq/kg), respectively. Then the average values of oil ash for 226Ra and 232Th other than were greater than the UNSCEAR reported reference values of 32, 45, and 412 (Bq/ kg) for radium, thorium, and potassium [23, 30]. Furthermore, from Table 1, it was declared that the average activity concentrations of 226Ra > 232Th > 40K. This indicated that 226Ra, as a decay product of the 238U series, was the major radionuclide that contributed to gamma radiation levels in the oil ash samples. Then to determine the activity concentrations of natural occurring radioactive materials (NORM), that were found in Egyptian oil ashes, the origin of NORM and secular equilibrium status of long-lived radionuclides in the crude oil and the HFO must been discussed, according to Hyden [31] the radioactive materials that were included in crude oil were less affected by oil type and are more likely to be comparable to that of reservoir rocks. The 238U and 232Th decay series is significant NORM in the oil sector. Under reducing factors in oil reservoirs, 238U and 232Th, isotopes were insoluble and immobile. During crude oil migration, only lighter hydrocarbons may move, leaving the heavy asphalting fraction with the majority of 238 U and 232Th. however the solubility of 226Ra isotope was more than 238U and 232Th isotopes [32].

The natural occurring radioactive materials(NORMs) in oil ash were different significantly that in crude oil this due to that In crude oil case, NORMs migration during oil formation and production may cause large fluctuations in the concentrations of NORMs overtime. Second were the conditions of combustion process used and the behavior of NORMs during fuel combustion.

Table (2), Showed the results of the present study Compared with relevant literature result for oil ash from many countries. Table 2 declared that the activity concentrations of the oil ash samples are considerably different. The observed variation of the activity concentrations for the radionuclides at variant sampling sites worldwide can be related to the geographical and geological differences among all mining areas and the variability of different fuel used (crude oil, heavy fuel oil, or natural gas HFO mixture), and the behavior of each metal during fuel combustion at different conditions, chemical additives, type and efficiency of the ash removal system. High contents of radioactive materials were found in the Egyptian oil ashes, with values significantly higher than those documented in published literature.

Table 2 the mean activity concentrations (Bq/kg) of the oil fly ash from electric power plant in the worldwide
Full size table

Figures (2), (3), (4), declared the correlation analysis between activity concentrations of oil ash radionuclides. A strong correlation was observed between the 232Th and 226Ra, R2 = 0.81 Fig. 2. On the other hand a weak correlation was declared between 232Th and 40K (R2 = 0.26) this due to the higher activity concentration of 232Th than 40K Fig. 3. While there was a median correlation between 226Ra and 40K, R2 = 0.36 Fig. 4.

Fig. 2
figure 2

The correlation between activity concentrations of 232Th, and 226Ra (Bq/kg) in oil ash samples

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Fig. 3
figure 3

The correlation between activity concentrations of 232 Th and 40K (Bq/kg) of oil ash samples

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Fig. 4
figure 4

Showed the correlation between activity concentrations of 226Ra and 40K of oil ash

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The absorbed dose rate in air ADR (nGy/h), with accordance to UNSCEAR [23] states that the average ADR in air values globally range from 18 to 93 (nGy/h), while the normal variable range for observed absorbed dose rates in outdoor air was found to be between 10 and 200 nGy/h [23, 26]. The spectrometry analysis of soil and rock samples with an average dose rate of 60 (nGy/h) was used to determine the population weighted value of the absorbed dose rate in air outdoors from terrestrial gamma radiation [24]. The values of the outdoor absorbed dose rate ADRout (nGy/h) were changed over both the oil fly ash samples and the bottom ash samples due to the variability in the gamma radiation activity concentration of 226Ra, 232Th and 40K. The values of ADRout (nGy/h) in oil fly ash (OFA) samples were ranged from (18.6 ± 0.9 to 51.7 ± 2.6) nGy/h, with an average of (36.5 ± 1.8) nGy/h Table 3. And the values of ADRout for bottom ash samples (BA) were ranged from (1147 ± 57to 2228 ± 111) nGy/h, with an average value of (1711 ± 85.9) nGy/h. The total ADRtot in oil ash samples were ranged from (1918.6 ± 0.9 to 2228 ± 111) nGy/h, with an average of (967 ± 48.8)nGy/h. For oil fly ash samples, the values of ADRout were lower than the world’s average value (58 nGy/h) [23, 26] according to UNSCEAR. However the values of ADRout for the bottom oil ash and overall samples of oil ash were higher than the worldwide limit.

Table 3 The external absorbed dose rates (ADRout, ADRin, and ADRtot) nGy/h, the annual effective doses (AEDout, AEDin, and AEDtot) mSv/y in the oil ash(OFA, BA)
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On the other hand, The values indoor absorbed dose rate (ADRin) nGy/h of in oil fly ash (OFA) samples were ranged from (35.1 ± 1.8 to 98.7 ± 4.9) nGy/h, with an average of (69.5 ± 3.5) nGy/h Table 3. And the values of ADRin for bottom ash samples (BA) were ranged from (2276.8 ± 114 to 4387 ± 219) nGy/h, with an average value of (3356.3 ± 167.8) nGy/h. The total ADRin in oil ash samples were ranged from (35.1 ± 1.8 to 4387 ± 219) nGy/h, with an average of (1896 ± 94.8) nGy/h. For the oil fly ash samples, the average values of ADRin were lower than the world’s average value (84 nGy/h) [23, 26]. However the values of ADRin for the bottom oil ash and overall samples of oil ash were higher than the worldwide limit (84 nGy /h). Furthermore, the ADRtot of oil fly ash (OFA) samples were ranged from (53.7 ± 2.7 to 150.4 ± 8) nGy/h, with an average of (106 ± 5.3) nGy/h. Similarly, the ADRtot of bottom ash (BA) ranged from 3424 ± 171 to 6615 ± 331 nGy/h, with an average of (5068 ± 253.4) nGy/h. The average value of total samples of oil ash was (2863 ± 143.2) (nGy/h).

Moreover, the average total absorbed dose rate ADRtot (nGy/h) was greater than the world’s average (143 nGy/h) [23, 26] for bottom oil ash, and lower than the world’s average in oil fly ash ( OFA), Then the total average for all samples for oil ashes were greater than the worldwide average (Table 3). The samples of higher values confirmed that the use of these materials might be obeyed to radiation protection regulation.

The annual effective dose, the outdoor annual effective dose (AEDout) mSv/y for oil fly ash (OFA) samples was ranged from (0.02–0.001 to 0.06 ± 0.003) mSv/y, with an average of 0.043 ± 0.002 mSv/y. For bottom ash (BA), the AEDout mSv/y ranged from (1.41 ± 0.07 to 2.73 ± 0.1) mSv/y, with an average of (2.1 ± 0.1) mSv/y. the (AEDout) mSv/y for overall oil ash samples were ranged from (0.02 ± 0.001 to 2.73 ± 0.1) mSv/y with average value of ( 1.19 ± 0.06) mSv/y (Table 3). These values for OFA samples were lower than the worldwide average (0.07 msv/y) [23, 26]. However the average values for bottom oil ash (BA) and overall the samples of oil ah were higher than the world wide average.

Additionally, the indoor annual effective dose (AEDin) mSv/y of oil fly ash (OFA) were ranged from 0.2 ± 0.01 to 0.5 ± 0.02) mSv/y, with an average value of (0.35 ± 0.02) mSv/y. Additionally, the indoor annual effective dose (AEDin) mSv/y of bottom oil ash (BA) were ranged from (11.2 ± 0.56 to 21.5 ± 1.08) mSv/y, with an average (16.46 ± 0.8) mSv/y. The (AEDin) mSv/y for overall samples of oil ash were ranged from (0.02 ± 0.001 to 21.5 ± 1.08) mSv/y with average value of (9.30 ± 0.47) mSv/y (Table 3).The indoor annual effective dose (AEDin) values for (OFA) samples were close to the worldwide average (0.41 msv/y) according to UNSCEAR [23, 26]. However the average values for bottom ash (BA) and the overall samples of oil ash were greater than the world’s average (0.41 msv/y).

Finally, the total annual effective doses (AEDtot) mSv/y of oil fly ash (OFA) samples were ranged from 0.22 ± 0.01 to 0.56 ± 0.02 mSv/y, with an average value of (0.4 ± 0.02) mSv/y. Additionally, the (AEDtot) mSv/y of bottom ash (BA) samples were ranged from 12.61 ± 0.63 to 24.23 ± 1.18 mSv/y, with an average of (18.6 ± 0.93) mSv/y. moreover the average value of total annual effective doses (AEDtot) for the overall samples of oil ash was (10.5 ± 0.5) mSv/y, which ranged from ( 0.22 ± 0.01 to 24.2 ± 1.2) mSv/y (Table 3). The total annual effective dose (AEDtot) values for (OFA) samples were close to the worldwide average (0.48 msv/y) due to UNSCEAR [23, 26]. However the average values for bottom oil ash (BA) and the overall samples of oil ash were greater than the world’s average (0.48 msv/y). The comparison between the AEDin and AEDout values for oil fly ash waste with the dose limit, it is found that AEDin and AEDout values higher than worldwide average 0.41 and 0.07 mSv/y.Then according to the higher activity concentrations of most radionuclides in most studied samples of oil ash, that is the main cause of height values of annual effective doses indoor and outdoor than the dose limit. Then mean values of the total annual effective dose should be less than the dose limit of 0.48 mSv/y. According to these results prolonged exposure to a high dose has harmful consequences on health such as the development of cancer or cardiovascular disease or the deterioration of tissue or the occurrence of DNA damage in genes or DNA in RNA [38].

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 risk outdoor, indoor and total (ELCRout ELCRin, ELCRtot) for oil fly ash (OFA), bottom oil ash (BA) and subsequently all samples of oil ash (OFA, BA) and a comparison with world average were shown in Table 4.The excess lifetime cancer risk outdoor for the oil fly ash (OFA) samples was ranged from (0.07 ± 0.004 to 0.21 ± 0.01) × 10–3. The average value was (0.15 ± 0.01) × 10–3 (Table 4). While for bottom oil ash (BA) samples the (ELCRout) were ranged from (4.94 ± 0.2 to 9.6 ± 0.5) × 10–3 with an average value was (7.3 ± 0.5) × 10–3. The ELCRout for overall samples of the oil ash (OFA, and BA) were ranged from (0.07 ± 0.004 9.6 ± 0.5) × 10–3 with average value of (4.2 ± 0.21) × 10–3. The outdoor (ELCR) for oil fly ash samples were lower than the worldwide average 0.29 × 10–3 according to UNSCEAR [23, 26]. However the calculated values for (ELCRout) bottom oil ash samples were higher than the worldwide limit0.29 × 10–3. Accordingly the ELCRout of overall average value of oil ash (OFA, and BA) were higher than the worldwide limit for outdoor ELCR 0.29 × 10–3

Table 4 The excess lifetime cancer risk ELCRout, ELCRin, and ELCRtot, the annual gonadal dose equivalent AGDE (mSv/y) for in the oil ash samples
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The indoor excess lifetime cancer risk (ELCRin) for the oil fly ash (OFA) samples were ranged from was ranged (0.7 ± 0.04 to 1.8 ± 0.1) × 10–3 with an average value (1.23 ± 0.1) × 10–3. The values of (ELCRin) for bottom ash samples were ranged from (0.039 ± 0.002 to 0.075 ± 0.004), with an average value of (0.058 ± 0.003). The overall average value (ELCRin) of oil ash samples was (0.033 ± 0.002).The indoor excess lifetime cancer risk (ELCRin) for oil fly ash samples were comparable to the worldwide average 1.16 × 10–3 according to UNSCEAR [23, 26]. However the calculated values for ELCRin bottom oil ash samples were higher than the worldwide average 1.16 × 10–3. Accordingly the ELCRin of overall average value of oil ash (OFA, and BA) were greater than the worldwide limit for outdoor ELCR.

The total excess lifetime cancer risk (ELCRtot) for the oil fly ash (OFA) samples were ranged from was ranged (0.77 ± 0.04 to 1.96 ± 0.1) × 10−3with an average value of (1.38 ± 0.1) × 10–3. The values of (ELCRtot) for bottom ash samples were ranged from (0.044.1 ± 0.002 to 0.085 ± 0.004).and the average value was (0.065 ± 0.003). The overall average value (ELCRtot) of oil ash samples was (0.037 ± 0.0018). The total excess lifetime cancer risk (ELCRtot) for oil fly ash samples were comparable to the worldwide average 1.45 × 10–3 according to UNSCEAR [23, 26]. However the calculated values for ELCRtot bottom oil ash samples were higher than the worldwide average 1.45 × 10–3. Accordingly the ELCRtot of overall average value of oil ash (OFA, and BA) were greater than the worldwide limit for ELCRtot.

The annual gonadal dose equivalent (AGDE) mSv/y, of oil fly ash (OFA) samples were ranged from(0.13 ± 0.01 to 0.35 ± 0.02) mSv/y with an average value of(0.24 ± 0.01) mSv/y. Additionally, the (AGDE) mSv/y of bottom ash (BA) samples were ranged from(7.7 ± 0.4 to 15.04 ± 0.8) mSv/y, with an average of (11.6 ± 0.6) mSv/y. Moreover the overall average value (AGDE) of oil ash samples was (6.5 ± 0.3) mSv/y. The average value of all samples of both oil fly ash and oil ash were lower than the dose limiting (300) mSv/y [39].

X- Ray diffraction (XRD) analysis of the oil ash

Figure 5 presented the X-ray diffraction (XRD) analysis of the oil ash waste. This analysis provided detailed information about the crystalline phases present in the sample, allowing us to identify the mineralogical composition and understand the structural characteristics of the oil ash waste. The main economic minerals constituents in the oil ash sample were as follows: A peak at 2θ = 42.6° indicated the presence of dolomite (CaMg(CO3)2), as referenced by card 00–036-0426. A peak at 2θ = 67.9° suggests the presence of quartz (SiO2), as per card 04–008-7653. A peak at 2θ = 27.1° indicated the presence of paramontroseite (VO2), according to card 04–005-7408. The presence of calcite (CaCO3) is indicated by a peak at 2θ = 18.01°, referencing card 00–001-0837. A peak at 2θ = 18.01° also suggested the presence of paralstonite (BaCa(CO3)2), based on card 00–001-0770. Lastly, a peak at 2θ = 28.01° was present in the pattern, suggesting another significant phase, although its identification was not specified. These findings provided a comprehensive understanding of the mineralogical composition of the oil ash waste, which was crucial for evaluating its potential applications and environmental impact. [40].

Fig. 5
figure 5

represented the XRD analysis of the oil fly ash waste

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The chemical analysis of the oil ash

The oil Ash produced was composed of an inorganic compound, which included oxides of nickel oxide, vanadium oxide, iron oxide, magnesium oxide, aluminum oxide, zinc oxide, and others [41].

In the present study Table 5 show the XRF (X-ray fluorescence) analysis provided the elemental composition two samples of oil fly ash (OFA) and bottom ash (BA). The concentrations of (SiO2) in OFA and BA were 37.17%, and 2.07% respectively. While the concentrations of (CaO) in OFA and BA samples were 18.82% and 8.16%. Furthermore, SO3 was 19.2%, and 39.65 were contained in oil fly ash and bottom ash, respectively. Moreover, the hematite (Fe2O3) was higher in BA than OFA, 20.51% and 5.03%, respectively. The V2O5 was higher than in bottom ash for comparing oil fly ash. The V2O5 was 13.92% in BA and 3.56% in OFA. In addition, the NiO was 8.49% for bottom ash and 1.45% for oil fly ash. Al2O3 concentrations in oil fly ash were 3.34% and 0.38 in bottom ash. The oil fly ash and bottom ash contained a tiny amount of some chemical compositions of oxidation, such as P2O5, PbO, K2O, TiO2, and BaO[42].The oil ash had pose environmental and health risks due to the potential release of toxic elements during oxidation and weathering processes. In general, vanadium and nickel are present in heavy oil fly ash (HOFA) in concentrations of 2–5 wt% according to numerous studies that have characterized this material [43, 44]. Nickel and vanadium and Iron had the most hazardous metallic species found in heavy oil ash [45, 46]. Most of these ashes were easily leachable by rainwater in disposal landfills, leading to the dispersion of heavy metals into the surrounding groundwater and soil, causing environmental hazards. If not disposed of safely manner, they could lead to serious diseases and negatively impact biological diversity in the environment [47, 48]. To reduce the environmental impact of land disposal of oil ash from power plants and to valorize this waste material, it was important to consider the industrial applications of vanadium, nickel, and iron. These metals were valuable as alloying elements in steel and other alloys. and recycling for construction materials to mitigate environmental impact.Therefore, removing these minerals from the environment by reusing industrial wastes to recover vanadium, nickel, and iron from oil ash was of great interest both economically and environmentally [49, 50]. This approach helps mitigate toxicity and environmental hazards.

Table 5 Chemical compositions of the investigated oil fly ash (OFA) and bottom oil ash (BA) (wt %) by XRF
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Particle size distribution of the oil ash dust

The particle size distribution of the oil ash dust was determined using zeta potential analysis. A swab was taken from surfaces adjacent to the oil fly ash waste to determine the particle size of respirable and airborne dust deposited on these surfaces due to wind movement. Particle size analysis confirmed that the mean diameter of oil ash sample was 521.1 nm as shown in Fig. 6. According to [51], the chemical and physical properties, and subsequently the potential industrial utilization of oil ash, greatly depended on particle size distribution. Particle size was an important property of oil ash: smaller particles provide a greater surface area. Particle size affected the mobilization of any trace element through oil ash. The dispersion of fine particles of oil ash in an atmosphere may cause pollution of air, water, and soil with heavy metal like (V and Ni). The toxicity of these heavy metals was due to the increased bioavailability of the soluble metals in the extracts of the samples. The dispersion of these fine particles had health hazards to human when inhaled because they penetrate deeper into the lung that cause a disease of respiratory and cardio vascular and may be the main cause of the induction of allergic responses in the respiratory system [52]. According to Banerjee [53], Pollution of particulate matter (PM) or particle pollution which contains a mixture of small solid particles and liquid droplets found in the air can be inhaled and caused serious health problem. Some particles that had diameters (≤ 2.5 µm) and (2.5 µm–10 µm) have the most concern, this due to their significant effects on the human health and the environment. The particle size of the studied oil ash sample was 521.1 nm that oil ash with diameter less than < (2.5 µm–10 µm). That caused an elevated risk to environment and human health that may be inhaled, and may cause lung cancer.

Fig. 6
figure 6

Showed the particle size distribution of the oil ash waste

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Conclusion

The electric power plants in Egypt burn around 7 million metric tons of HFO each yearly that generated more than 4000 metric tons of oil ash waste as a residual of industries that had a significant radiation potential hazard for the environment. That required attention and continuous monitoring during some routine operation in this industry. The radiological measurements oil ash average activity concentrations for 226Ra, 232Th, and 40K were (1718 ± 85.9, 278.1 ± 13.9, and 136 ± 6.7) Bq/kg respectively. Since the average of activity concentrations exceeded the worldwide average by the international regulations.

All the radiological hazardous assessments declared that, the outdoor and indoor absorbed dose rate for bottom oil ash are higher than that in oil fly ash and was exceeded the worldwide average.

Accordingly the higher absorbed dose rate lead to the heigher AEDoutdoor and AEDindoor for bottom oil ash. The higher value of indoor annual effective dose for the oil ash put it on the high side of the dose limit of the radiation dose rate in building materials. And then, most of the samples had an AEDin greater than 0.41 mSv/y safety limit. These results declare that the oil ash didn’t recommended to be ideal for building human residences.

The average values of outdoor and indoor ELCR, were higher than the worldwide average in bottom oil ash and also the average value of oil ash samples was exceeded the worldwide average. It is concluded that the higher radioactive materials that concentrated on the bottom ash and consequently higher activity concentrations of the radionuclides was the main cause in the higher radiological parameters in oil ash. It is necessary to found a method to reduce the radiation hazards due oil ash to protect the public from further exposure and decrease the environmental impact.

Iron, vanadium, nickel and other heavy elements were included in the waste content. The chemical composition of oil ash samples was investigated by XRF to determine the major elements in oil ashes. That had many hazards to the respiratory tract causing oxidative stress that due to these elements are water soluble, fractions of these elements can dissolve in the respiratory tract.

Recommendation

  • This study declares that oil ash had an environmental radiation hazards due to its high radioactivity. So it is recommended that the use of chemical treatment process of the oil ash waste to reduce the radioactive materials and toxic minerals from waste and reuse this waste in building materials with safe content of radioactive materials as well as protect the environment and all the workers in the field of the electric power station.

  • A safety process and environmental protection towards this industry should apply and monitor regularly.

  • None of Egypt's thermal power stations are equipped with a oil fly ash removal system. So, electrostatic precipitators (ESPs) should be utilized to minimize particle emissions that include high concentrations of harmful heavy metals to reduce air pollution [54].

  • It was concluded that the recovery of V, Ni and Zn and Iron could had an attractive important for economic and an alternative method to the oil ashes to be landfill this lead to minimize the environmental risk that associated with oil fly ash.