Abstract
Levels of thirteen polycyclic aromatic hydrocarbons (PAHs) were studied in 24 soil samples collected from the Ahdab oil field in Waset region, Iraq. The whole analysis was carried out using the gas chromatography with mass spectrometry (GC/MS) in the single ion monitoring (SIM)-mode. The total PAHs concentrations were ranged between 19 μg/kg (Site 15) and 855 μg/kg (Site of 16). The results of these samples were compared among each other according to their toxicity equivalence in μg TEQ/g concentrations and with the results of other studies. The total estimated cancer risks of exposure to PAHs in soil samples were ranged from 1.02 × 10−7 to 4.59 × 10−6. By multiplying the estimated cancer risk values by 106, then the estimated theoretical cancer cases per million is between 0.0 and 5. The total estimated cancer risks in this study were within the acceptable range of excess cancer risk specified by the Environmental Protection Agency in the United States.
Keywords:
Introduction
Persistent organic pollutants (POPs) are organic compounds that include synthesized substances. Pesticides, polycyclic aromatic hydrocarbons, polychlorinated biphenyls and other by-products generated as a result of human and natural activity as dioxins and furans (Motelay-Massei et al., Citation2004). Extensive scientific studies have shown that POPs are some of the most dangerous pollutants released into the environment by humans. Great efforts have been made since the early 1960 to enhance chemical management and safety issues. Various conventions have been adopted for this purpose, the Stockholm conventions (May 2001) is one of the well-known meeting in this context, which focuses on reducing and eliminating the release of twelve POPs by the United Nations Environment Program (UNEP) (Liu et al., Citation2001).
Polycyclic aromatic hydrocarbons (PAHs) comprise a large group of organic contaminants that are formed as a result of incomplete combustion of organic material. They enter the environment through natural sources such as forest fires and volcanic eruptions and through a variety of anthropogenic activities. These include e.g. combustion of fossil fuels and wood, wood treatment processes, coke production, coal-tar production, metal smelting and asphalt production (Nestler, Citation1974). PAHs are composed of two or more fused benzene rings and contain only carbon and hydrogen atoms. Related heterocyclic aromatic rings often co-exist with PAHs in the environment, in which carbon atoms are substituted by nitrogen, sulfur or oxygen atoms. PAHs substituted with alkyl groups are also common co-pollutants. The whole group of PAHs and related compounds are referred to as polycyclic aromatic hydrocarbons compounds (PAHs).
PAHs have also been found in facilities where petroleum, petroleum products, or coal are used or where wood, cellulose, corn, or oil is burned. People living near waste sites containing PAHs may be exposed through contact with contaminated air, water, and soil.
Polycyclic aromatic hydrocarbons have shown a wide range of toxicological effects but the primary focus has been on their mutagenic and carcinogenic capacity. U.S. Environmental Protection Agency (USEPA, Citation2004) has classified seven of the 16 priority PAHs as probably carcinogenic to humans. Long-term exposure to mixtures containing PAHs and other chemicals via inhalation and skin contact has been shown to cause cancer in some individuals (Chen & Liao, Citation2006; USEPA, Citation1991; Wang, Citation2007). PAHs are acutely toxic to aquatic organisms. Generally, the toxicity increases with increased molecular weight and log Kow (Aislabie et al., Citation2006).
To compare our results with those of other researchers, Zhang et al., (Citation2006) have analyzed 53 soil rural and urban samples from Hong Kong and found the sum concentration of 16 EPA priority PAHs ranged from 7.0 to 410 μg/kg dry wt.
Honda et al. (Citation2007) have analyzed 21 PAHs in soil samples collected from particular sites in Japan from 1959–2005 and found the total concentration in the range of 53–2180 μg/kg.
Lin et al. (Citation2013) have analyzed forty surface soil samples (0–5 cm depth) from Kunming/China and found the total concentration of 15 PAHs was between 102 and 6208 μg/kg.
Man et al. (Citation2013) have evaluated 55 soil samples from different land use types in Hong Kong/China on human cancer risk related to PAHs. The incremental life cancer risks for car dismantling workshop samples were found to be of moderate potential for cancer development (244 × 10−6 – 209 × 10–6).
Kumar et al. (Citation2015) have studied sixteen PAHs in residential soils from industrial region, Ghaziabad/India. The incremental life time cancer risk (ILCR) was found within the acceptable distribution range (10−6–10−4) for human adults and children.
The objectives of this study are to analyze twenty four soil samples gathered from Al-Ahdab oil field in Iraq for their PAHs-content in order to estimate the health hazard on the people living in this area by comparing the results among each other according to their μg TEQ/g concentrations, through comparison with results of researches from other countries and through the estimation of the total cancer risks of exposure.
Materials and methods
EPA 525 stock standard 1 mL (500 μg/mL in methylene chloride) mixture of the following 13 priority pollutants: Fluorene, Phenanthrene, Anthracene, Fluoranthene, Pyrene, Benzo(a)anthracene, Chrysene, Benzo(b)fluoranthene, Benzo(k)fluoranthene, Benzo(a)pyrene, Indeno(1,2,3 cd) pyrene, Dibenzo(a,h)anthracene and Benzo(g,h,i)perylene was purchased from Supelco (Bellefonte, USA). 1-fluoronaphthalene which was used as internal standard for the PAHs analysis was purchased from Aldrich (USA). All used solvents were of GC-grade and purchased from Riedel de-Haën (Hannover, Germany).
Sampling and sample locations
Waset region is located in the south-east of Baghdad which contains huge amounts of Iraqi oil reserves, and presents an important industrial area in Iraq. Twenty-four samples were collected around the Al-Ahdab oil field as shown in . Each sampling site was 100 m apart from the next one. Each soil sample was collected from a depth of about 10 cm from the surface, consisted of 4–6 portions and within an area of 1 m2. All portions were pooled together, crushed, sieved using a sieve of 1.18 mm pores, dried at 110 °C for 3–4 h and then homogenized.
Figure 1. Locations of soil samples from Al-Ahdab oil field.
Sample extraction and cleanup
Ten grams of each homogenized soil sample were extracted in a Soxhlet apparatus, for 16 h using 150 ml toluene. The extracts were then evaporated using the rotary evaporator at 40 °C and 78 mbar to about 3 ml. The clean-up was done on a solid phase extraction (SPE) cartridges. C18, 1 g/6 ml SPE cartridges (TELOS, the Kinesis Group/UK) was used.
Each SPE cartridge was conditioned with 1 mL de-ionized water and 5 mL of a mixture of (1:1) dichloromethane:n-hexane. The residues from the extraction step (3 mL) were added to the cartridge, and passed through at a flow rate of 4–5 mL/min. Finally, the cartridge was eluted with 3 × 4 mL mixture of dichloromethane and n-hexane (1:1) at a flow rate of 1 mL/min. The eluates were evaporated at 40 °C and 335 mbar to 1 mL, then to dryness using a gentle stream of nitrogen. The residues were dissolved in 1 mL n-hexane containing 1 μg/mL internal standard (1-Fluoronaphthalene), and 2 μL were injected onto the GC/MS column.
Chromatographic conditions and MS-detection
The GC-MS analysis was carried out using an HP 6890 gas chromatograph with an autosampler series 7683 and an HP 5973 quadrupole mass spectrometer from Agilent Technologies (Waldbronn, Germany) under the following conditions:
Injected volume 2 μL/splitless, Column: DB5-MS, 30 m, 0.25 mm I.D; 0.25 μm film thickness, and stationary phase: 95% dimethyl – 5% phenyl polysiloixan from Agilent Technologies; Carrier gas: Helium (99.999%), (1 mL/min); Temperature program: 100° C (10 min), 100–160° C (25 °C/min), 160 °C – 265 °C (5 °C/min), 265 °C (17 min); Detector: mass selective quadrupole – detector, Agilent 5973 N (MSD); Auxiliary (transfer line): 300 °C, Electron Impact Ionization; Ionization Energy: 70 eV; Calibration substance: Perflourotributylamine (PFTBA); Tuning masses: 69/219/502.
Aquistion mode: Selective ion monitoring (SIM) - mode, quantitation masses, see .
Table 1. PAHs quantitation masses, according to Peters and Harlin (Citation1995).
Instrument calibration
Linear range
For the calculation of the performance data, a calibration was carried out with five concentration levels of PAHs in the range of 100–1000 μg/L. From the resulting calibration curves, the regression coefficients were calculated, characterizing the linearity of the calibration function. Regression coefficients were all > 0.99, indicating a good linearity of the calibration function in these concentration ranges.
Detection limits and limits of quantitation
The instrument limits of detection (LOD) were obtained by diluting the standard mixture solution until the ratio of signal to noise (S/N) is equal to 3, while the lower limits of quantitation (LLOQ) were calculated as (S/N) ratio equal to 10. shows the LOD and LLOQ values for PAHs. The method limits of detection (LOD) were obtained by spiking standard mixture solution into a blank sample until the ratio of signal to noise (S/N) is equal to 3 while the method lower limit of quantitation (LLOQ) was calculated as (S/N) ratio equal to 10.
Table 2. Instrument and method limits of detection (LOD) and lower limit of quantitation for PAHs.
Extraction recoveries
Two samples, each of 10.0 g of pure blank sample (sand from the Jordanian desert) were spiked with the PAHs standard mixture to give the concentrations of 100 and 1000 μg/kg. These samples were mixed thoroughly and extracted, cleaned-up and analyzed according to the abovementioned method. The recovery tests were done in triplicate at different times. The average recoveries of PAHs were found between 81 and 100%. All recoveries are shown in and were found within the acceptable range for trace analysis (Gonzalez & Herrador, Citation2007) which is 80–120%.
Table 3. Results of recovery studies using blank sand samples spiked with standard solution of PAHs mixture at 1000 μg/kg and 100 μg/kg levels.
Instrument precision
The precision of the instrument was measured using standard solutions (100, 500 and 1000 ng/mL) prepared by dilution of the stock solution mentioned in the section “Materials and methods”. Each solution was injected three times. The peak area ratio for the three injections, the average, standard deviations and the coefficient of variation (CV) were calculated. The coefficients of variation (CV) were found to be less than the acceptable limit value for trace analysis (Gonzalez & Herrador, Citation2007) which is <15%. These results show that the used method have a good instrument precision.
Risk assessment
Toxicity equivalent (TEQ) method was used to assess the ecotoxicological risk at a specific site. The total BaP equivalent concentration (BaPeq) was calculated by the sum of BaPeq for each PAH using toxicity equivalent factors.
The incremental lifetime cancer risk (ILCR) was employed to evaluate the potential risk of PAH in the Iraqi soil samples to human health in this study. The ILCRs for adults terms of direct ingestion, dermal contact, and inhalation using the following EquationEquations (1(1) , 2 and 3).
(1)
where Cancer riskingest is the cancer risk via ingestion of soil particles; Csoil is the concentration of the pollutant in soil (mg/kg); IngR is the ingestion rate of soil (mg/day); ED is the exposure duration (years); BW is the average body weight (kg); AT is the averaging time (days); CF is the conversion factor (1 × 10−6 kg/mg); SFO is the oral slope factor (mg/kg/day)−1.
(2)
where Cancer riskdermal is the cancer risk via dermal contact of soil particles; SA is the surface area of the skin that contacts soil (cm2/day); AFsoil is the skin adherence factor for soil (mg/cm2); ABS is the dermal absorption factor (chemical specific); EF is the exposure frequency (days/year); GIABS is the gastrointestinal absorption factor (USEPA, Citation1989).
(3)
where Cancer riskinhale is the cancer risk via inhalation of soil particles; ET is the exposure time (h/day); IUR is the inhalation unit risk (mg/m3)−1; AT is the averaging time (h); PEF is the particle emission factor = 1.36 × 109 m3/kg. PEF is the inhalation of pollutants that are adsorbed to respirable particles (PM10) and relates the concentration of a pollutant in soil with the concentration of respirable particles in the air, due to fugitive dust emissions from contaminated soils (USEPA, Citation2009). shows parameters for estimating human cancer risk (USEPA, Citation2001).
Table 4. Parameters for estimating human cancer risk for adults.
Qualitative descriptions of lifetime cancer risks are as follows:
Very low when the estimated value is ≤ 10−6; from 10−6 < to < 10−4, moderate, from 10−4 ≤ to < 10−3, high, and from 10−3 ≤ to < 10−1 very high.
Analysis of PAHs in soil samples
Each sample was analyzed three times (weighing, Soxhlet extraction and SPE clean up) and each extract was injected three times onto the GC/MS column. shows the results, presented as the average ± standard deviation for each compound. From these results, the concentrations were converted to the μg TEQ/kg soil sample using the toxicity equivalence factors (TEFs) and then the cancer risk assessment was calculated ().
Table 5. Toxicity equivalent factors (TEFs), recovery-corrected concentrations (μg/kg) of PAH and μg TEQ/kg of the samples 1–12(A) and 13–24(B).
Results and discussion
Concentration of PAHs in the soil samples
and show the concentrations of the PAHs in the studied soil samples.
Figure 2. Concentrations of PAHs in soil samples for site 1–8 (A), 9–16 (B) and 17–24 (C).
The overall concentration of the 13 priority PAHs in the studied Al-Ahdab soil samples ranged from 19 to 855 μg/kg dry weight with a mean concentration of 320 μg/kg. The detection frequencies of Fluorene and Phenanthrene were the highest (100%) followed by Fluoranthene and Pyren (79%). The detection frequencies of Benzo(k)fluoranthene, Indeno(1,2,3-cd) pyrene and Dibenzo(a,h)anthracene were the lowest of all PAHs at a detection rate of 0%.
These results showed that site 16 have the highest total concentration (855 μg/kg) of all studied samples and obviously because of its proximity to the Al-Ahdab Iraqi oil field, followed by site 12 (782 μg/kg) and site 19 (593 μg/kg), while the sites 1, 3 and 15 show very low total concentrations, namely 47, 72 and 19 μg/kg, respectively. The range of total PAHs concentrations found in this study is much lower than that found in soil samples taken from Shenyang/China, which was 950–2790 μg/kg (Song et al., Citation2006).
Risk assessment of PAHs in Al-Ahdab soil samples
Toxicity equivalent (TEQ) method was used to assess the ecotoxicological risk for each sample. BaPeq was calculated by the sum of BaPeq for each PAH using toxicity equivalent factors (USEPA, Citation1993). In this study, the total BaPeq concentration of all studied PAHs in soil samples were in the range of 0.10–473.08 μg/kg BaPeq with a mean value of 102.14 BaPeq (). An half of all the samples showed total BaPeq concentrations less than 3.96 μg/kg BaPeq and all of the samples had an exposure risk of less than 473.08 μg/kg BaPeq. According to the Canadian soil quality guidelines (CCME, Citation2008), soils containing <100 μg/kg BaPeq are considered uncontaminated, soils containing 100–1000 μg/kg BaPeq are considered slightly contaminated and soils containing 1000–10 000 μg/kg BaPeq are considered significantly contaminated. In the present study 70.8% of the sample sites contained less than 100 μg/kg BaPeq and 29.2% (7sites) had concentrations in the range 100–473.08 μg/kg BaPeq indicating most of soil samples in Al-Ahdab oil field could be considered uncontaminated to slightly contaminated.
Incremental lifetime cancer risk (ILCR) is a carcinogenic risk used to evaluate the human health risk. The calculated ILCR from exposure to PAH by direct ingestion, inhalation and dermal contact pathways, to an individual for 70 years, starting at birth using the USEPA (Citation1991,Citation2004) recommended standard default exposure parameters () for Reasonable Maximum Exposure (RME) resident. Generally an ILCR between 10−6–10−4 indicated a potential risk (USEPA Citation1993). and show the ILCRs levels calculated for the Al-Ahdab soil samples, indicating a low human health risk from the exposure of direct ingestion, inhalation and dermal contact.
Figure 3. The total estimated cancer risks of exposing to soil samples 1 to 24 PAHs through different exposure pathways.
Table 6. Total calculated incremental lifetime cancer risks (ILCRs) from exposure to PAHs in soil samples through all exposure pathways.
The highest values of ILCRs were found in sites 12, 16 and 18. This was accord with highest concentrations of PAHs were found in these sites.
By multiplying the estimated cancer risk of exposing to soil samples-PAHs by 106, it is possible to determine the theoretical number of potential cancer risk cases per million of people (USEPA, Citation1989). The total estimated cancer risk of exposing to PAHs in Al-Ahdab soil samples were ranged from 1.02 × 10−7 to 4.59 × 10−6 and the estimated cancer cases per million were between 0 and 5 (). The total number of cancer cases per million for the sample group 1–11 (far from Al-Ahdab oil field) was 8 cases, while those for the sample group 12–24 (in the proximity of Al-Ahdab oil field) was 31 cases. This shows the effect of Al-Ahdab oil field contamination on human health.
Concluding remarks
A comprehensive study was performed to monitor the concentrations of PAHs in soil samples collected from the vicinity of the Al-Ahdab oil field in Waset area/Iraq. It was possible to determine the concentrations of 13 PAHs in the studied samples. The studied PAHs compounds were found in almost all the samples. The total concentrations of the studied PAHs were ranged from 19 μg/kg (site 15) to 855 μg/kg (site 16). The main PAH pollutants in Al-Ahdab oil field were found to be Fluorene, Phenanthrene, Fluoranthene and Pyrene. Majority of the sampling sites (70.8%) exhibited PAH concentrations less than 100 μg/kg BaPeq which could be considered uncontaminated and the rest of sampling sites (29.2%, 7 sites) had concentrations in the range 100–417 μg/kg BaPeq which could be considered slightly contaminated.
The BaPeq concentrations of the soil samples group 1–11 are ranged between 0.103 and 3.960 μg TEQ/kg which are very low (only 8 total cancer cases per million) compared to the group 12–24 which are ranged between 19.000 and 417.173 μg TEQ/kg (31 total cancer cases per million).
The estimated total cancer risks associated with the exposure to the studied soil samples-PAHs in all sites were found to be acceptable. Even under the worst-case scenario, the estimated cancer risk for adults and children in all sites was ranged from 1.02 × 10−7– 4.59 × 10−6 which is lower than the acceptable range of excess cancer risk specified by the US-EPA. The environmental situation in the surrounding of the Al-Ahdab oil field area in Iraq concerning PAHs is acceptable.
Declaration of interest
The authors report no declaration of interest regarding the publication of this manuscript.
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