Journal of Environmental Radioactivity 136 (2014) 121e126
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Evaluation of radiation hazard potential of TENORM waste from oil and natural gas production M.A. Hilal a, M.F. Attallah a, Gehan Y. Mohamed b, *, M. Fayez-Hassan b a
Analytical Chemistry and Control Department, Hot Laboratories and Waste Management Center (HLWMC), Atomic Energy Authority, Post Office No. 13759, Cairo, Egypt b Experimental Nuclear Physics Department, Nuclear Research Center (NRC), Atomic Energy Authority, Post Office No. 13759, Cairo 13759, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2014 Received in revised form 22 May 2014 Accepted 24 May 2014 Available online
In this study, a potential radiation hazard from TENORM sludge wastes generated during exploration and extraction processes of oil and gas was evaluated. The activity concentration of natural radionuclides 238 U, 226Ra and 232Th were determined in TENORM sludge waste. It was found that sludge waste from oil and gas industry is one of the major sources of 226Ra in the environment. Therefore, some preliminary chemical treatment of sludge waste using Triton X-100 was also investigated to reduce the radioactivity content as well as the risk of radiation hazard from TENORM wastes. The activity concentrations of 226Ra and 228Ra in petroleum sludge materials before and after chemical treatment were measured using gamma-ray spectrometry. The average values of the activity concentrations of 226Ra and 228Ra measured in the original samples were found as 8908 Bq kg1 and 933 Bq kg1, respectively. After chemical treatment of TENORM samples, the average values of the activity concentrations of 226Ra and 228Ra measured in the samples were found as 7835 Bq kg1 and 574 Bq kg1, respectively. Activity concentration index, internal index, absorbed gamma dose rate and the corresponding effective dose rate were estimated for untreated and treated samples. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Sludge wastes Oil and gas production Environmental radioactivity Radiation hazard indices
1. Introduction Wastes associated with various industrial activities were enhanced levels of natural radioactivity which called TENORM; this acronym means “Technological Enhanced Naturally Occurring Radioactive Materials”. The TENORM waste produced from several industrial sectors such as uranium mining overburden, phosphate ore processing, coal ash, water treatment, metal mining and processing, geothermal energy production wastes, and petroleum industry (Egidi and Hull, 1999). TENORM scales may build up inside oil field production tubing and may concentrate considerable quantities of radioactive material that has the potential to expose humans to relatively high dose of radioactivity (Attallah et al., 2012, 2013). The available data from the literature show that TENORM wastes contain activity concentrations of 226Ra ranged from undetectable levels more than 1000 kBq kg1 (Exploration and Production Forum
* Corresponding author. E-mail addresses: [email protected] (M.F. Attallah), gg_yousef@ yahoo.com (G.Y. Mohamed). http://dx.doi.org/10.1016/j.jenvrad.2014.05.016 0265-931X/© 2014 Elsevier Ltd. All rights reserved.
Report, 1987). The activity concentrations of 226Ra in TENORM can be much higher than the permissible values established by IAEA (IAEA, 1994). The production of large amounts of NORM and the potential radiological hazards has been extensively described in studies related to the measurement of the activity concentrations in generating wastes for the assessment of the radiological impact on workers and members of the public (Turhan et al., 2011; Ugur et al., 2013; Iwaoka and Yonehara, 2012; Hrichi et al., 2013; Landsberger et al., 2013). Radiochemical characterization and attempts for chemical treatment of TENORM waste that's produced from oil and gas production in Egypt has been investigated (El Afifi and Awwad, 2005; El Afifi et al., 2009b; Attallah et al., 2013). Recently, attention was focused on the environmental and health impacts from the uncontrolled release of TENORM wastes. From the natural risk point of view, it is necessary to know the dose limits of public exposure and to measure the natural environmental radiation level provided by TENORM to estimate human exposure to natural radiation sources (Papastefanou et al., 1988; Senthilkumar et al., 2013). The aim of this work is to determine the activity concentration and some radiation parameters to evaluate the risks of radiations from TENORM wastes that produced from oil and gas industry. A
122
M.A. Hilal et al. / Journal of Environmental Radioactivity 136 (2014) 121e126
preliminary attempt to decrease the activity contents from TENORM wastes by chemical treatment using surfactants was carried out. This would protect workers in oil and natural gas production industry as well as our environment by reducing its activity concentration consequently and the health hazard potential.
1991). Certified reference materials contain a known activity concentration for radionuclides, (238U, 232Th and 226Ra) covering the energy range of natural radioactivity (El Afifi et al., 2009a).
2. Experimental
The radioactive decay series (238U and 232Th) are important series in nature since it provides, the more important isotopes of elements Ra, Rn and Po, which can be isolated in large amounts in the environment. When dealing with natural radioactive decay series of 238U and 232Th, secular equilibrium may exist between the parent radionuclide, and their respective decay products (Wilson, 1994). In the equilibrium exists, the concentration of the parent radionuclide will equal to that of each of the corresponding daughters, considering the respective decay constant in each case with the following relation:
2.3. Calculation of activity concentration
2.1. Preparation of TENORM samples The TENORM sludge wastes were precipitated on the bottom and around the walls of the storage tanks, due to the depression in temperature and pressure. These materials are precipitates of metal and radionuclides salts as well as organic residues. Since the TENORM sludge reduces the efficiency of the tanks storage, it is removed periodically during the maintenance process. The sludge represents wastes accumulated in large amounts (several hundred tonnes) surrounding workers in sites of the petroleum exploration. Therefore, 30 samples of the waste sludge are taken and collected in several plastic bags to prevent cross-contamination for radiometric investigation. The TENORM waste samples were obtained from the Abu Rudies onshore oil field at the east of the Suez Gulf, South Sinai Governorate, Egypt. The sludge samples were dried to eliminate the moisture content. Then, the samples were pulverized, to attain homogenized particle sizes. An accurately 50 g of the sludge samples was packed in a bottle (250 cm3), closed tightly and stored for 30 days.
l1N1 ¼ l2N2 ¼ l3N3/etc: where: l is the decay constant of the radionuclide and equal to (ln 2)/t1/2in seconds (s1), and N is the number of respective nuclei. The activity concentrations of the parent radionuclide can be determined in test samples through the quantitative analysis of any daughter radionuclide under secular equilibrium condition. 238U can be determined based on gamma-energy lines of its direct daughter 234Th at 63.3 and 92.6 keV, assuming secular equilibrium exists. 226Ra belongs to the 238U-series has decayed to a radioactive noble gas of 222Rn (3.82 d). The decay of radon produces airborne radioactive isotopes of Po, Bi and Pb. 226Ra reaches radioactive equilibrium with all its daughter products to 210Pb in several weeks i.e., seven half-life of 222Rn (3.82 d). Therefore, each packed sample was sealed and stored for 30 days to reach secular equilibrium between 226Ra and its daughters. The activity concentration of 226 Ra was determined from the average concentrations of these decay products 214Pb (351.9 keV) and 214Bi (609.3 and 1764.5 keV) (El-Taher et al., 2010) at the secular equilibrium of TENORM sludge samples (Wilson, 1994). Also, the activity concentration of 232Th was determined from the average concentrations of its daughters 212 Pb (238.6 keV), 228Ac (911.1 keV) and in 208Tl (583.1 and 2614.7). The activity concentration in Bq kg1 (A) in the sludge samples was calculated from following equation:
2.2. Instrumentation and calibration Measurement of activity concentration of 238U, 226Ra and 232Th was performed by an HPGe detector with relative efficiency 70 percent which coupled to a PC-MCA. Each sample was measured three times to get good statistical values and the time for measurement of activity concentration was 14,000 s. The data acquisition was collected by an 8 K multichannel analyzer (MCA) using PCA-III software program. Energy calibration was performed using certified several sealed point sources (Amersham, England) of known gamma-energy lines, namely, 137Cs (661.9 keV), 60Co (1173.2 and 1332.5 keV), 133Ba (81.9, 282.9, 302.9 and 356.1 keV). Detector efficiency calibration was done using certified reference materials IAEA-314 from the International Atomic Energy Agency, Vienna, Austria (Stracchnov et al.,
Untreated Samples
12000
Ra -2 2 6 Co ncentra tio n (Bq kg -1 )
A ¼ Np aεm
Treated Samples
10000
8000
6000
4000
2000
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Sample number Fig. 1. Average activity concentrations (Bq kg1) of
226
Ra in untreated and treated sludge samples.
26
27
28
29
30
M.A. Hilal et al. / Journal of Environmental Radioactivity 136 (2014) 121e126
1400
Untreated Samples
123
Treated Samples
Th-232 Concentration (Bq kg-1)
1200 1000 800 600 400 200 0 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Sample number
Fig. 2. Average activity concentration (Bq kg1) of
where Np, a, ε and m are the net counts per second under specified energy, the abundance of the each spec,ic gamma-ray line, the photo peak detection efficiency for the same gamma-ray line and the sample mass in Kg.
232
Th in treated and untreated sludge samples.
absorbed rate to the human effective dose equivalent with an outdoor occupancy of 20 and 80 percent for indoors. The annual effective dose in units of mSv y1 was calculated by the following formula (El-Taher et al., 2010):
Dannu ¼ Dabs T F
2.4. Estimation of dose rate Conversion factors to transform specific activities AK, ARa and ATh of K, Ra and Th respectively, in absorbed dose rate at 1 m above the ground (in nGy h1) by Bq kg1) are calculated from (Beretka and Mathew, 1985; UNSCEAR, 2000),
Dabs ¼ 0:0417Ak þ 0:462ARa þ 0:604ATh 2.5. The annual effective dose rate The annual estimated average effective dose equivalent received by a member of the public was calculated using a conversion factor of 0.7 Sv Gy1 (Singh et al., 2009), which was used to convert the
where Dannu is the calculated annual effective dose rate (nGy h1), T is the outdoor occupancy time (0.2 24 h 365.25 d z 1753 hy1), and F is the conversion factor (0.7 106 Sv Gy1). 2.6. Radiation hazard indices The natural activity concentration in petroleum materials is usually determined from 226Ra, 232Th and 40K contents. As 98.5 percent of the radiological effects of the 238U series were produced by 226Ra and its daughter products, the contribution from the 238U has been replaced with the decay product 226Ra. The gamma-ray radiation hazards due to the specified radionuclides were assessed by three different indices (NEA-OECD, 1979). Radium
Untreated Samples
420
Treated Samples
U-238 Concentration (Bq kg-1)
350
280
210
140
70
0 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 S ample number
Fig. 3. Average activity concentration (Bq kg1) of
238
U in untreated and treated sludge samples.
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M.A. Hilal et al. / Journal of Environmental Radioactivity 136 (2014) 121e126
Table 1 Average radioactivity concentration (Bq kg1) of 238U, 232Th and 226Ra in treated and untreated sludge samples. Samplea
Untreated Treated Removal% a
238
U-series
238
U
240 ± 19.5 92.5 ± 8.6 61.5
226
Ra
8908 ± 585 7835 ± 467 12.1
232
Th-series
228
Ra
Table 3 Activity concentration of Country
where: ARa, ATh and AK are the activity concentration of 226Ra, 232Th and 40K in Bq kg1, respectively. The Raeq is related to the external gamma-dose and internal dose due to radon and its daughters. The maximum value of Raeq in natural radioactivity must be less than 370 Bq kg1 for safety use. Beretka and Mathew, 1985 defined two other indices that represent the external and internal radiation hazards. The external hazard index is obtained from Ra expression. This index value must be less than unity in order to keep the radiation hazard insignificant; i.e. the radiation exposure due to the radioactivity from construction materials is limited to 1.0 mSv y1. Then, the external hazard index can be defined as:
)
Ra and 228
228
Ra for sludge wastes in different countries.
Ra (Bq kg1) Ref.
Guidelines for naturally occurring radioactive materials, 2002 Brazil 50.000e168.000 49.000e52.000 Godoy and da Cruz, 2003 413.000 117.900 Gazineu et al., 2005 Malaysia 6e560 4.520 Omar et al., 2004 Norway 100e4700 100e4600 Lysebo et al., 1996 Tunisia 66e453 NR Testa et al., 1994 Egypt 11,950 1750 El Afifi et al., 2009 11,963 1747 Attallah et al., 2013 8908 933 This study
Average of 30 sludge samples.
Raeq ¼ ARa þ 1:43ATh þ 0:077AK
Ra (Bq kg
226
1
Australia 25.000
933 ± 122 574 ± 153 38.5
equivalent radioactivity (Raeq) is the first index that has been introduced to represent the specific activities of 226Ra, 232Th and 40 K by a single quantity, which takes into account the radiation hazards associated with them. This index can be calculated according to (UNSCEAR, 2000):
226
30.000
2.7. Chemical treatment of the sludge wastes Removal of the natural radioactivity was investigated based on suspending the fine particles containing radium using surfactant of Triton X-100 as extracting agent. The experimental procedure was carried out by shaking a known amount of the sludge waste with mixed ratio of liquid to solid of 5 ± 0.5 ml/g, for one hour at ambient room temperature (25 ± 1 C). The upper turbid aqueous phase was separated rapidly by decantation. The solid residue phase was left for drying and recounted. Weighted samples were placed in a glass bottle, of 250 cm3 volume. The bottles were completely sealed for 30 days to reach secular equilibrium where the rate of decay of the radon daughters becomes equal to that of the parent. This step is necessary to ensure that radon gas is confined within the volume and the daughters will also remain in the sample. 3. Results and discussion 3.1. Evaluation of activity concentration of TENORM sludge
Hex ¼ ðARa =370 þ ATh =259 þ AK =4810Þ 1
The gamma index (Ig) is calculated using the following equation as proposed by the European commission (EC, 1999). It is suggested by the European commission for the materials used in bulk amounts, the exemption dose criterion (0.3 mSv y1) corresponds to Ig 0.5, whereas the dose criterion 1 mSv y1 corresponds to Ig 1 (EC, 1999). On the other hand, for superficial and other materials, the corresponding values of Ig should be between 2 and 6.
The average values of activity concentration of 226Ra, 232Th and U for all tested samples from oil and gas productions before and after treatment were calculated as illustrated in Figs. (1e3) and Table 1. The average activity concentration of untreated sludge samples is found 240 Bq kg1, 8908 Bq kg1 and 933 Bq kg1 for 238 U, 226Ra and 228Ra, respectively. After chemical treatment, the activity concentration is found 92 Bq kg1, 7835 Bq kg1 and 574 Bq kg1 for 238U, 226Ra, and 228Ra, respectively. These values are high and more than the permissible values in both untreated and treated sludge samples. Although the results showed that the removal percent of radionuclides are 61.5 percent, 12.1 percent and 38.4 percent for 238U, 226Ra and 232Th, respectively has been obtained by chemical treatment. It might be the development of chemical treatment can be reached the permissible value of activity concentration of sludge waste.
Ig ¼ ðAU =300 þ ATh =200 þ AK =3000Þ
3.2. Evaluation of some radiation hazard indices of TENORM sludge
On the other hand, the internal hazard index (Hin) gives the internal exposure to carcinogenic radon and its short-lived daughter and is given by the following formula (El-Taher et al., 2010).
Hin ¼ ðARa =185 þ ATh =259 þ AK =4810Þ
where, AU, ATh and AK are the activity concentration of and 40K in Bq kg1, respectively.
238
U,
232
Th,
238
Different radiation indices were evaluated of untreated and treated sludge as illustrated in Table 2. In untreated sludge, the
Table 2 The values of radiation hazard parameters for untreated and treated TENORM sludge wastes. Valuesa Untreated sludge Range Average Treated sludge Range Average a
D (nGy h1)
Raeq (Bq kg1)
Hin
Hex
Ig (Bq kg1)
AEDR (mSv y-1)
5029 4230 4678 ± 436
11030 9251 10240 ± 978
55.1 47.0 51.8 ± 4.9
29.8 25.0 27.7 ± 2.6
37.0 31.0 34.4 ± 3.3
6172 5192 5741 ± 546
4455 3462 3965 ± 487
9722 7551 8654 ± 924
50.0 39.2 44.6 ± 5.0
26.3 20.4 23.4 ± 3.1
32.6 25.3 28.9 ± 3.2
5467 4249 4867 ± 614
Values of 30 TENORM sludge samples.
M.A. Hilal et al. / Journal of Environmental Radioactivity 136 (2014) 121e126 Table 4 The average worldwide activity levels of U, Th and K (UNSCEAR, 1994). Radionuclide
U
Th
K
Activity level (Bq Kg1)
50
50
500
results showed that the values of absorbed dose rates vary from 4230 to 5029 nGy h1 with an average value of 4678 ± 436 nGy h1 for untreated sludge samples. The values of the Raeq ranged from 9251 to 11030 Bq kg1 with an average value of 10,240 ± 978 Bq kg1. Also, the values of external and internal hazard indices were found in the range 25e29.8 and 47.0e55.1 with the mean values 27.7 ± 2.6 and 51.8 ± 4.9, respectively. The annual effective dose due to gamma radiations at one meter over the surface was determined and was found in the range 5192e6172 mSv y1 with mean values 5741 ± 546 mSv y1. The gamma index (Ig) values ranged from 31 to 37 Bq kg1 and the average value of this index is 34.4 ± 3.3 Bq kg1. In these regards, the radiation hazard indices for the 30 sludge samples after chemical treatment has been evaluated. The results showed that the absorbed dose rates vary from 3462 to 4455 nGy h1 with the average value of 3965 ± 487 nGy h1 for treated sludge samples. The values of Raeq ranged from 7551 to 9722 Bq kg1 with an average value of 8654 ± 924 Bq kg1. The values of external and internal hazard indices are found in the range 20.4e26.3 and 39.2e50.0 with the mean values 23.4 ± 3.2 and 44.6 ± 5.0, respectively. The annual effective doses are found in the range 4249e5467 mSv y1 with mean values 4867 ± 614 mSv y1. The gamma index (Ig) values ranged from 25.3 to 32.6 Bq kg1 and the average value of this index is 28.9 ± 3.2 Bq kg1. The results in Table 3 show that the different radiation hazard indices of the treated samples are less than the same samples untreated. This attributed to removal portion of natural radionuclides (U-series and Th-series) by a chemical treatment process. These results are considered valuable because of the chemical treatment process that could be reducing the risks arising from TENORM wastes in oil and gas production. 3.3. Comparison the results with the different countries In this part, comparison between our results in this study and the other investigation for sludge waste generated from oil and gas production as well as the permissible level of different radiation parameters are presented. The activity concentration of 226Ra (Useries) and 228Ra (Th-series) in the TENORM sludge waste from oil and gas production in Egypt and other countries has been reported in Table 3. The average worldwide activity levels of uranium (U), thorium (Th) and potassium (K) (UNSCEAR, 1994) and the exemption activity levels of NORM as recommended in the IAEA basic safety standards (IAEA, 2001), were given in Tables 4e5. The average worldwide level of the most common radiological indices (UNSCEAR, 1994) was given in Table 6. 4. Conclusions TENORM sludge waste from oil and natural gas industry has been generated radiation exposure levels, which require attention Table 5 The exemption activity levels of NORM as recommended in the IAEA basic safety standards (IAEA, 2001). 238
Radionuclide 1
Exemption level (Bq g
)
1
U
226
10
Ra
232
1
Th
228
Ra
10
125
Table 6 The average worldwide levels of the most common radiological indices (UNSCEAR, 1994). Radiological indices
Ra-eq (Bq Kg1)
Dgr (nGy h1)
EADR for worker (mSv yr1)
EADR for public (mSv yr1)
Activity level
370
55
20
1
and continuous monitoring during some routine operation in this industry. The activity concentration of 226Ra and 228Ra is 8908 Bq kg1 and 933 Bq kg1, respectively, in sludge wastes. Since the concentration of 226Ra is found in sludge waste exceeds the permissible level by the international regulations. It was found necessary to reduce the radiation risks due to indoor radon and direct gamma-radiation in sludge waste to protect the workers in this field and to decrease the environmental impact. Reduced the activity contents as well as some radiation hazardous indices such as Ra-eq, Dabs, Dann, Hin and Hex was performed using chemical treatment by Triton X-100. The chemical treatment process is removed
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Evaluation of radiation hazard potential of TENORM waste from oil and natural gas production M.A. Hilal a, M.F. Attallah a, Gehan Y. Mohamed b, *, M. Fayez-Hassan b a
Analytical Chemistry and Control Department, Hot Laboratories and Waste Management Center (HLWMC), Atomic Energy Authority, Post Office No. 13759, Cairo, Egypt b Experimental Nuclear Physics Department, Nuclear Research Center (NRC), Atomic Energy Authority, Post Office No. 13759, Cairo 13759, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2014 Received in revised form 22 May 2014 Accepted 24 May 2014 Available online
In this study, a potential radiation hazard from TENORM sludge wastes generated during exploration and extraction processes of oil and gas was evaluated. The activity concentration of natural radionuclides 238 U, 226Ra and 232Th were determined in TENORM sludge waste. It was found that sludge waste from oil and gas industry is one of the major sources of 226Ra in the environment. Therefore, some preliminary chemical treatment of sludge waste using Triton X-100 was also investigated to reduce the radioactivity content as well as the risk of radiation hazard from TENORM wastes. The activity concentrations of 226Ra and 228Ra in petroleum sludge materials before and after chemical treatment were measured using gamma-ray spectrometry. The average values of the activity concentrations of 226Ra and 228Ra measured in the original samples were found as 8908 Bq kg1 and 933 Bq kg1, respectively. After chemical treatment of TENORM samples, the average values of the activity concentrations of 226Ra and 228Ra measured in the samples were found as 7835 Bq kg1 and 574 Bq kg1, respectively. Activity concentration index, internal index, absorbed gamma dose rate and the corresponding effective dose rate were estimated for untreated and treated samples. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Sludge wastes Oil and gas production Environmental radioactivity Radiation hazard indices
1. Introduction Wastes associated with various industrial activities were enhanced levels of natural radioactivity which called TENORM; this acronym means “Technological Enhanced Naturally Occurring Radioactive Materials”. The TENORM waste produced from several industrial sectors such as uranium mining overburden, phosphate ore processing, coal ash, water treatment, metal mining and processing, geothermal energy production wastes, and petroleum industry (Egidi and Hull, 1999). TENORM scales may build up inside oil field production tubing and may concentrate considerable quantities of radioactive material that has the potential to expose humans to relatively high dose of radioactivity (Attallah et al., 2012, 2013). The available data from the literature show that TENORM wastes contain activity concentrations of 226Ra ranged from undetectable levels more than 1000 kBq kg1 (Exploration and Production Forum
* Corresponding author. E-mail addresses: [email protected] (M.F. Attallah), gg_yousef@ yahoo.com (G.Y. Mohamed). http://dx.doi.org/10.1016/j.jenvrad.2014.05.016 0265-931X/© 2014 Elsevier Ltd. All rights reserved.
Report, 1987). The activity concentrations of 226Ra in TENORM can be much higher than the permissible values established by IAEA (IAEA, 1994). The production of large amounts of NORM and the potential radiological hazards has been extensively described in studies related to the measurement of the activity concentrations in generating wastes for the assessment of the radiological impact on workers and members of the public (Turhan et al., 2011; Ugur et al., 2013; Iwaoka and Yonehara, 2012; Hrichi et al., 2013; Landsberger et al., 2013). Radiochemical characterization and attempts for chemical treatment of TENORM waste that's produced from oil and gas production in Egypt has been investigated (El Afifi and Awwad, 2005; El Afifi et al., 2009b; Attallah et al., 2013). Recently, attention was focused on the environmental and health impacts from the uncontrolled release of TENORM wastes. From the natural risk point of view, it is necessary to know the dose limits of public exposure and to measure the natural environmental radiation level provided by TENORM to estimate human exposure to natural radiation sources (Papastefanou et al., 1988; Senthilkumar et al., 2013). The aim of this work is to determine the activity concentration and some radiation parameters to evaluate the risks of radiations from TENORM wastes that produced from oil and gas industry. A
122
M.A. Hilal et al. / Journal of Environmental Radioactivity 136 (2014) 121e126
preliminary attempt to decrease the activity contents from TENORM wastes by chemical treatment using surfactants was carried out. This would protect workers in oil and natural gas production industry as well as our environment by reducing its activity concentration consequently and the health hazard potential.
1991). Certified reference materials contain a known activity concentration for radionuclides, (238U, 232Th and 226Ra) covering the energy range of natural radioactivity (El Afifi et al., 2009a).
2. Experimental
The radioactive decay series (238U and 232Th) are important series in nature since it provides, the more important isotopes of elements Ra, Rn and Po, which can be isolated in large amounts in the environment. When dealing with natural radioactive decay series of 238U and 232Th, secular equilibrium may exist between the parent radionuclide, and their respective decay products (Wilson, 1994). In the equilibrium exists, the concentration of the parent radionuclide will equal to that of each of the corresponding daughters, considering the respective decay constant in each case with the following relation:
2.3. Calculation of activity concentration
2.1. Preparation of TENORM samples The TENORM sludge wastes were precipitated on the bottom and around the walls of the storage tanks, due to the depression in temperature and pressure. These materials are precipitates of metal and radionuclides salts as well as organic residues. Since the TENORM sludge reduces the efficiency of the tanks storage, it is removed periodically during the maintenance process. The sludge represents wastes accumulated in large amounts (several hundred tonnes) surrounding workers in sites of the petroleum exploration. Therefore, 30 samples of the waste sludge are taken and collected in several plastic bags to prevent cross-contamination for radiometric investigation. The TENORM waste samples were obtained from the Abu Rudies onshore oil field at the east of the Suez Gulf, South Sinai Governorate, Egypt. The sludge samples were dried to eliminate the moisture content. Then, the samples were pulverized, to attain homogenized particle sizes. An accurately 50 g of the sludge samples was packed in a bottle (250 cm3), closed tightly and stored for 30 days.
l1N1 ¼ l2N2 ¼ l3N3/etc: where: l is the decay constant of the radionuclide and equal to (ln 2)/t1/2in seconds (s1), and N is the number of respective nuclei. The activity concentrations of the parent radionuclide can be determined in test samples through the quantitative analysis of any daughter radionuclide under secular equilibrium condition. 238U can be determined based on gamma-energy lines of its direct daughter 234Th at 63.3 and 92.6 keV, assuming secular equilibrium exists. 226Ra belongs to the 238U-series has decayed to a radioactive noble gas of 222Rn (3.82 d). The decay of radon produces airborne radioactive isotopes of Po, Bi and Pb. 226Ra reaches radioactive equilibrium with all its daughter products to 210Pb in several weeks i.e., seven half-life of 222Rn (3.82 d). Therefore, each packed sample was sealed and stored for 30 days to reach secular equilibrium between 226Ra and its daughters. The activity concentration of 226 Ra was determined from the average concentrations of these decay products 214Pb (351.9 keV) and 214Bi (609.3 and 1764.5 keV) (El-Taher et al., 2010) at the secular equilibrium of TENORM sludge samples (Wilson, 1994). Also, the activity concentration of 232Th was determined from the average concentrations of its daughters 212 Pb (238.6 keV), 228Ac (911.1 keV) and in 208Tl (583.1 and 2614.7). The activity concentration in Bq kg1 (A) in the sludge samples was calculated from following equation:
2.2. Instrumentation and calibration Measurement of activity concentration of 238U, 226Ra and 232Th was performed by an HPGe detector with relative efficiency 70 percent which coupled to a PC-MCA. Each sample was measured three times to get good statistical values and the time for measurement of activity concentration was 14,000 s. The data acquisition was collected by an 8 K multichannel analyzer (MCA) using PCA-III software program. Energy calibration was performed using certified several sealed point sources (Amersham, England) of known gamma-energy lines, namely, 137Cs (661.9 keV), 60Co (1173.2 and 1332.5 keV), 133Ba (81.9, 282.9, 302.9 and 356.1 keV). Detector efficiency calibration was done using certified reference materials IAEA-314 from the International Atomic Energy Agency, Vienna, Austria (Stracchnov et al.,
Untreated Samples
12000
Ra -2 2 6 Co ncentra tio n (Bq kg -1 )
A ¼ Np aεm
Treated Samples
10000
8000
6000
4000
2000
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Sample number Fig. 1. Average activity concentrations (Bq kg1) of
226
Ra in untreated and treated sludge samples.
26
27
28
29
30
M.A. Hilal et al. / Journal of Environmental Radioactivity 136 (2014) 121e126
1400
Untreated Samples
123
Treated Samples
Th-232 Concentration (Bq kg-1)
1200 1000 800 600 400 200 0 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Sample number
Fig. 2. Average activity concentration (Bq kg1) of
where Np, a, ε and m are the net counts per second under specified energy, the abundance of the each spec,ic gamma-ray line, the photo peak detection efficiency for the same gamma-ray line and the sample mass in Kg.
232
Th in treated and untreated sludge samples.
absorbed rate to the human effective dose equivalent with an outdoor occupancy of 20 and 80 percent for indoors. The annual effective dose in units of mSv y1 was calculated by the following formula (El-Taher et al., 2010):
Dannu ¼ Dabs T F
2.4. Estimation of dose rate Conversion factors to transform specific activities AK, ARa and ATh of K, Ra and Th respectively, in absorbed dose rate at 1 m above the ground (in nGy h1) by Bq kg1) are calculated from (Beretka and Mathew, 1985; UNSCEAR, 2000),
Dabs ¼ 0:0417Ak þ 0:462ARa þ 0:604ATh 2.5. The annual effective dose rate The annual estimated average effective dose equivalent received by a member of the public was calculated using a conversion factor of 0.7 Sv Gy1 (Singh et al., 2009), which was used to convert the
where Dannu is the calculated annual effective dose rate (nGy h1), T is the outdoor occupancy time (0.2 24 h 365.25 d z 1753 hy1), and F is the conversion factor (0.7 106 Sv Gy1). 2.6. Radiation hazard indices The natural activity concentration in petroleum materials is usually determined from 226Ra, 232Th and 40K contents. As 98.5 percent of the radiological effects of the 238U series were produced by 226Ra and its daughter products, the contribution from the 238U has been replaced with the decay product 226Ra. The gamma-ray radiation hazards due to the specified radionuclides were assessed by three different indices (NEA-OECD, 1979). Radium
Untreated Samples
420
Treated Samples
U-238 Concentration (Bq kg-1)
350
280
210
140
70
0 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 S ample number
Fig. 3. Average activity concentration (Bq kg1) of
238
U in untreated and treated sludge samples.
124
M.A. Hilal et al. / Journal of Environmental Radioactivity 136 (2014) 121e126
Table 1 Average radioactivity concentration (Bq kg1) of 238U, 232Th and 226Ra in treated and untreated sludge samples. Samplea
Untreated Treated Removal% a
238
U-series
238
U
240 ± 19.5 92.5 ± 8.6 61.5
226
Ra
8908 ± 585 7835 ± 467 12.1
232
Th-series
228
Ra
Table 3 Activity concentration of Country
where: ARa, ATh and AK are the activity concentration of 226Ra, 232Th and 40K in Bq kg1, respectively. The Raeq is related to the external gamma-dose and internal dose due to radon and its daughters. The maximum value of Raeq in natural radioactivity must be less than 370 Bq kg1 for safety use. Beretka and Mathew, 1985 defined two other indices that represent the external and internal radiation hazards. The external hazard index is obtained from Ra expression. This index value must be less than unity in order to keep the radiation hazard insignificant; i.e. the radiation exposure due to the radioactivity from construction materials is limited to 1.0 mSv y1. Then, the external hazard index can be defined as:
)
Ra and 228
228
Ra for sludge wastes in different countries.
Ra (Bq kg1) Ref.
Guidelines for naturally occurring radioactive materials, 2002 Brazil 50.000e168.000 49.000e52.000 Godoy and da Cruz, 2003 413.000 117.900 Gazineu et al., 2005 Malaysia 6e560 4.520 Omar et al., 2004 Norway 100e4700 100e4600 Lysebo et al., 1996 Tunisia 66e453 NR Testa et al., 1994 Egypt 11,950 1750 El Afifi et al., 2009 11,963 1747 Attallah et al., 2013 8908 933 This study
Average of 30 sludge samples.
Raeq ¼ ARa þ 1:43ATh þ 0:077AK
Ra (Bq kg
226
1
Australia 25.000
933 ± 122 574 ± 153 38.5
equivalent radioactivity (Raeq) is the first index that has been introduced to represent the specific activities of 226Ra, 232Th and 40 K by a single quantity, which takes into account the radiation hazards associated with them. This index can be calculated according to (UNSCEAR, 2000):
226
30.000
2.7. Chemical treatment of the sludge wastes Removal of the natural radioactivity was investigated based on suspending the fine particles containing radium using surfactant of Triton X-100 as extracting agent. The experimental procedure was carried out by shaking a known amount of the sludge waste with mixed ratio of liquid to solid of 5 ± 0.5 ml/g, for one hour at ambient room temperature (25 ± 1 C). The upper turbid aqueous phase was separated rapidly by decantation. The solid residue phase was left for drying and recounted. Weighted samples were placed in a glass bottle, of 250 cm3 volume. The bottles were completely sealed for 30 days to reach secular equilibrium where the rate of decay of the radon daughters becomes equal to that of the parent. This step is necessary to ensure that radon gas is confined within the volume and the daughters will also remain in the sample. 3. Results and discussion 3.1. Evaluation of activity concentration of TENORM sludge
Hex ¼ ðARa =370 þ ATh =259 þ AK =4810Þ 1
The gamma index (Ig) is calculated using the following equation as proposed by the European commission (EC, 1999). It is suggested by the European commission for the materials used in bulk amounts, the exemption dose criterion (0.3 mSv y1) corresponds to Ig 0.5, whereas the dose criterion 1 mSv y1 corresponds to Ig 1 (EC, 1999). On the other hand, for superficial and other materials, the corresponding values of Ig should be between 2 and 6.
The average values of activity concentration of 226Ra, 232Th and U for all tested samples from oil and gas productions before and after treatment were calculated as illustrated in Figs. (1e3) and Table 1. The average activity concentration of untreated sludge samples is found 240 Bq kg1, 8908 Bq kg1 and 933 Bq kg1 for 238 U, 226Ra and 228Ra, respectively. After chemical treatment, the activity concentration is found 92 Bq kg1, 7835 Bq kg1 and 574 Bq kg1 for 238U, 226Ra, and 228Ra, respectively. These values are high and more than the permissible values in both untreated and treated sludge samples. Although the results showed that the removal percent of radionuclides are 61.5 percent, 12.1 percent and 38.4 percent for 238U, 226Ra and 232Th, respectively has been obtained by chemical treatment. It might be the development of chemical treatment can be reached the permissible value of activity concentration of sludge waste.
Ig ¼ ðAU =300 þ ATh =200 þ AK =3000Þ
3.2. Evaluation of some radiation hazard indices of TENORM sludge
On the other hand, the internal hazard index (Hin) gives the internal exposure to carcinogenic radon and its short-lived daughter and is given by the following formula (El-Taher et al., 2010).
Hin ¼ ðARa =185 þ ATh =259 þ AK =4810Þ
where, AU, ATh and AK are the activity concentration of and 40K in Bq kg1, respectively.
238
U,
232
Th,
238
Different radiation indices were evaluated of untreated and treated sludge as illustrated in Table 2. In untreated sludge, the
Table 2 The values of radiation hazard parameters for untreated and treated TENORM sludge wastes. Valuesa Untreated sludge Range Average Treated sludge Range Average a
D (nGy h1)
Raeq (Bq kg1)
Hin
Hex
Ig (Bq kg1)
AEDR (mSv y-1)
5029 4230 4678 ± 436
11030 9251 10240 ± 978
55.1 47.0 51.8 ± 4.9
29.8 25.0 27.7 ± 2.6
37.0 31.0 34.4 ± 3.3
6172 5192 5741 ± 546
4455 3462 3965 ± 487
9722 7551 8654 ± 924
50.0 39.2 44.6 ± 5.0
26.3 20.4 23.4 ± 3.1
32.6 25.3 28.9 ± 3.2
5467 4249 4867 ± 614
Values of 30 TENORM sludge samples.
M.A. Hilal et al. / Journal of Environmental Radioactivity 136 (2014) 121e126 Table 4 The average worldwide activity levels of U, Th and K (UNSCEAR, 1994). Radionuclide
U
Th
K
Activity level (Bq Kg1)
50
50
500
results showed that the values of absorbed dose rates vary from 4230 to 5029 nGy h1 with an average value of 4678 ± 436 nGy h1 for untreated sludge samples. The values of the Raeq ranged from 9251 to 11030 Bq kg1 with an average value of 10,240 ± 978 Bq kg1. Also, the values of external and internal hazard indices were found in the range 25e29.8 and 47.0e55.1 with the mean values 27.7 ± 2.6 and 51.8 ± 4.9, respectively. The annual effective dose due to gamma radiations at one meter over the surface was determined and was found in the range 5192e6172 mSv y1 with mean values 5741 ± 546 mSv y1. The gamma index (Ig) values ranged from 31 to 37 Bq kg1 and the average value of this index is 34.4 ± 3.3 Bq kg1. In these regards, the radiation hazard indices for the 30 sludge samples after chemical treatment has been evaluated. The results showed that the absorbed dose rates vary from 3462 to 4455 nGy h1 with the average value of 3965 ± 487 nGy h1 for treated sludge samples. The values of Raeq ranged from 7551 to 9722 Bq kg1 with an average value of 8654 ± 924 Bq kg1. The values of external and internal hazard indices are found in the range 20.4e26.3 and 39.2e50.0 with the mean values 23.4 ± 3.2 and 44.6 ± 5.0, respectively. The annual effective doses are found in the range 4249e5467 mSv y1 with mean values 4867 ± 614 mSv y1. The gamma index (Ig) values ranged from 25.3 to 32.6 Bq kg1 and the average value of this index is 28.9 ± 3.2 Bq kg1. The results in Table 3 show that the different radiation hazard indices of the treated samples are less than the same samples untreated. This attributed to removal portion of natural radionuclides (U-series and Th-series) by a chemical treatment process. These results are considered valuable because of the chemical treatment process that could be reducing the risks arising from TENORM wastes in oil and gas production. 3.3. Comparison the results with the different countries In this part, comparison between our results in this study and the other investigation for sludge waste generated from oil and gas production as well as the permissible level of different radiation parameters are presented. The activity concentration of 226Ra (Useries) and 228Ra (Th-series) in the TENORM sludge waste from oil and gas production in Egypt and other countries has been reported in Table 3. The average worldwide activity levels of uranium (U), thorium (Th) and potassium (K) (UNSCEAR, 1994) and the exemption activity levels of NORM as recommended in the IAEA basic safety standards (IAEA, 2001), were given in Tables 4e5. The average worldwide level of the most common radiological indices (UNSCEAR, 1994) was given in Table 6. 4. Conclusions TENORM sludge waste from oil and natural gas industry has been generated radiation exposure levels, which require attention Table 5 The exemption activity levels of NORM as recommended in the IAEA basic safety standards (IAEA, 2001). 238
Radionuclide 1
Exemption level (Bq g
)
1
U
226
10
Ra
232
1
Th
228
Ra
10
125
Table 6 The average worldwide levels of the most common radiological indices (UNSCEAR, 1994). Radiological indices
Ra-eq (Bq Kg1)
Dgr (nGy h1)
EADR for worker (mSv yr1)
EADR for public (mSv yr1)
Activity level
370
55
20
1
and continuous monitoring during some routine operation in this industry. The activity concentration of 226Ra and 228Ra is 8908 Bq kg1 and 933 Bq kg1, respectively, in sludge wastes. Since the concentration of 226Ra is found in sludge waste exceeds the permissible level by the international regulations. It was found necessary to reduce the radiation risks due to indoor radon and direct gamma-radiation in sludge waste to protect the workers in this field and to decrease the environmental impact. Reduced the activity contents as well as some radiation hazardous indices such as Ra-eq, Dabs, Dann, Hin and Hex was performed using chemical treatment by Triton X-100. The chemical treatment process is removed
