Abstract
The present study provides a comprehensive evaluation of polycyclic aromatic hydrocarbons (PAHs) contamination in soil along Tehran-Semnan road, Iran, a densely populated road with a heavy load of vehicular traffics and several industrial complexes. Soil samples were collected from four different sites and then were analyzed for 16 PAHs by High Performance Liquid Chromatography (HPLC). There was variability in the total PAHs concentrations which ranged from 148.4 ng g-1 to 721 ng g-1. The Siman e Tehran (S1) site has the highest average total PAHs concentrations (654.55 ng g-1) and Dehenamak (S4) has the lowest average total PAHs concentrations (168.7 ng g-1) among studied sites. The total PAH concentrations in studied soil samples are less than those reported in a number of previous investigations from different regions and countries. The ratios of phenanthrene to anthracene (Phe/Ant) and fluoranthene to pyrene (Flu/Pyr) were used for identification of pyrogenic and petrogenic sources of PAHs, respectively. The derived results indicated that soil samples in studied sampling sites were contaminated by both pyrogenic and petrogenic sources of PAHs.
Keywords: Polycyclic Aromatic Hydrocarbons (PAHs), Soil, Environmental Health risk, Sources
Introduction
Polycyclic Aromatic Hydrocarbons (PAHs) are a group of organic chemical compounds with two or more fused aromatic rings (Nadal et al. 2004). These compounds are ubiquitous mutagenic, carcinogenic, and toxic environmental contaminants (Menzie et al. 1992; Yu 2002). PAHs have uniquely stable structures and present in the environment for long period of time (Dai et al. 2008;Sun et al. 2009; Brindha and Elango 2014). The US Environmental Protection Agency (US EPA) listed sixteen compounds of PAHs as priority pollutants and also classified seven of these compounds as probable human carcinogens (ATSDR 2005; NTP 2005) (Table 1).
PAHs can be released into the environment by both natural and anthropogenic sources (Raza et al. 2013; Hu et al. 2014). Natural sources of PAHs are mainly forest fires and volcanoes (Kim et al. 2003). Anthropogenic sources can be categorized into pyrogenic and petrogenic sources (Boonyatumanond et al. 2007). PAHs which generated through in incomplete combustion of organic materials during moderate to high temperature processes (300 to 700˚C) are pyrogenic (Feng et al. 2009) while PAHs which formed through moderate temperatures (100 to 300˚C) and associated with fossil fuels (Wang 2013; Johnston et al. 2015) are petrogenic. Anthropogenic activities are considered as a main source of PAHs and quantities and proportion of PAHs which can be emitted due to these activities depend on manufacturing process (Tang et al. 2005).
PAHs with three and more fused aromatic rings are generally stable and high hydrophobic compounds (low aqueous solubility) with high molecular weight and low vapor pressure (Means et al. 1980). These compounds are adsorbed rapidly on to the soil and sediment particles in the environment (Means et al. 1980). Consequently, PAHs are mainly accumulated in surface layer of soil and sediment in environment (approximately 95%) and are often suspended in water and air (Dong and Lee 2009; Mirza et al. 2014). PAHs in soil and sediment can also be dispersed by surface runoff and wind. Therefore soil and sediment can be considered as primary reservoir and sink for PAHs (Maltby et al. 1995a,b; Mai et al. 2003; Tang et al. 2005).
Soil is one of the most important natural resources and accumulation of PAHs in soils can have several direct and indirect negative effects on the environment and public health (Shi et al. 2015). Therefore, soil PAHs concentrations can be indicators of local pollution (Yu et al. 2014; Wild and Jones 1995). Several studies investigated concentrations, sources, and risk assessment of PAHs in soils in different regions (Essumang et al. 2011; Arienzo et al. 2015; Shi et al. 2015; Hussain et al. 2016).
The road from Tehran to Semnan is located in central northern part of Iran and has length of about 216 km. This road is a densely populated road with a heavy load of vehicular traffics which influences enormously the environment of this area. There are also several industrial complexes and companies along Tehran-Semnan road which are important sources of environmental pollutions. Public concern over possible adverse health effects for the population living along Tehran-Semnan road significantly has increased in recent years.
Data concerning concentrations of PAHs in soils along Tehran-Semnan road are of prime importance for assessment of environmental quality of this area. But only limited studies have been conducted to investigate PAHs pollution in this area. The main focus of present study is to determine the concentrations and possible sources of EPA priority PAHs in soils from four key sampling sites along Tehran-Semnan road in order to establish an environmental evaluation and health risk assessment. A comparison of derived results with data corresponding to similar studied areas in the literature has also been performed. The presented results in this paper will serve as essential reference information for PAHs in soil along Tehran-Semnan road in order to compare the current results with data obtained in future.
Materials and methods
Sampling procedure and description of sampling sites
Soil samples were collected during summer 2015 from four sampling sites (S1, S2, S3, and S4) along Tehran-Semnan road for evaluating EPA priority PAHs contamination levels. The locations of sampling sites were selected in order to reflect diverse exposure of soils to potential pollution sources along Tehran-Semnan road. Geographical location of each sampling site was determined by a handheld GPS. The sampling sites locations and specifications are presented in Table 2.
A total of 85 soil samples were analyzed in current study. Nineteen soil samples were collected from area within the direct influence (distance less than 5 km) of Siman e Tehran’s plant (S1). Also, twenty eight soil samples derived from area with distance less than 5 km from Nirogah e Damavand’s emissions. Twenty one soil samples were collected from vicinity of Istgah e Sorkheh (S3) which is a gas station and mainly used by motor vehicles for gasoline and diesel recharging. Finally, seventeen soil samples were derived from Dehenamak (S4) which is proximity (15km) far away from area of influence of all the suspected sources of contamination.
Previous studies on soil profile data indicated that PAHs may extend into deep layers of soil but it is expected that more than 90% of the total burden of PAHs restricted to the surface layer of soil (Wild and Jones 1995; Maliszewska-Kordybach et al. 2008). All soil samples (about 500g) in current study were collected from surface layer of soil (0-20 cm) with a stainless steel scoop which was prewashed with methylene chloride and hexane. Each sample was a composite of five subsamples which collected from an area of 25 m2 (5m by 5m) and bulked together to form one sample.
All soil samples were transported to the laboratory and were moved in cleaned glass flasks which sealed with aluminum foil lids (in order to prevent sorption by plastic). All soil samples were also air dried for 48 hours at a temperature of 20°C to reach to constant weight and then sieved through a 2 mm mesh screen and stored in the dark at room temperature for a period not more than six months before further characterization.
Chemical Analysis
The reference standard mixture of 16 PAHs (naphthalene, acenaphthene, acenaphthylene, anthracene, phenanthrene, fluorene, fluoranthene, benzo (a) anthracene, chrysene, pyrene, benzo (a) pyrene, benzo (b) fluoranthene, benzo (k) fluoranthene, dibenzo (a,h) anthracene, Benzo (g,h,i) perylene, and indeno (1,2,3-cd) pyrene) was purchased from Macherey-Nagel (Düren, Germany) and diluted in acetonitril to make desired concentrations. Neutral silica gel (100-200 mesh) and alumina (70-230 mesh) which were obtained from Sigma-Aldrich (St. Louis, MO, USA) activated respectively at 130°C and 250°C for 10 hours and then kept in desiccators before using. Anhydrous sodium sulfate (Na2SO4) was also heated at 450°C for 4 hours and then stored in sealed container.
The details of the experimental procedures that include extraction, fractionation and PAHs analysis are described comprehensively in Samimi et al. (2009). Briefly, 2 g of each soil sample was weighed precisely and extracted with dichloromethane (20 mL) in an ultrasonic water bath for 30 minutes. The dichloromethane extract was then filtered over 0.5 μm filter paper and concentrated by a rotary evaporation in 35˚C before the cleanup procedure.
The cleanup procedure was performed by a glass chromatographic column (30 cm (L) by 1 cm (ID)) that was packed with anhydrous sodium sulfate in order to absorb any water in the sample extracts, 10 g of silica gel, and then 4 g Alumina. The column was also eluted with hexane prior to use. After introduction of concentrated extract, the aliphatic fraction was collected by elution with 20 mL hexane and then aromatic one was eluted with a 40 mL mixture of hexane / dichloromethane (50:50 v/v). Subsequently, all fractions were evaporated and concentrated with a gentle gas stream of purified nitrogen.
The final concentrated samples were analyzed for PAHs by High Performance Liquid Chromatography (HPLC) system which was equipped with 410 binary pumps, a 470 scanning fluorescence detector, and a Reodyne 7725i injection loop. The identification of PAHs were performed by comparison of individual PAH retention times with those of external reference standards. Quantification was based on the peak areas against an external calibration curves for each individual compound.
The pH value of each soil samples was determined. Ten grams of dried soil sample was weighed and dissolved in 20 ml of deionized water. After continues stirring for 30 minutes, suspension was stand for 1 hour to allow suspended particles to settle out from the suspension and then, pH value was measured by a pH-meter. Organic content of samples was also determined by drying the samples in oven at a temperature of 440°C according to ASTM D2974 (ASTM 2007).
Result and Discussion
PAHs contamination levels in soil samples
The mean pH value and organic content of studied samples was 7.96 (from 7.17 to 8.59), and 6.7% (from 3.1 to 11.2%), respectively. The derived results indicated that there is no significant statistically correlation between these variables and total concentrations of 16 PAHs in soils in current study. The average concentration of the 16 individual PAH analyzed in soils at studied sampling sites were presented in Fig. 1 to 4.
It could be seen from Fig. 1 to 4 that 16 EPA priority PAHs were present in majority of soil samples. The average concentrations of total PAHs in soil ranged from 168.7 ng g-1 (S4) to 654.55 ng g-1 (S1) with the mean value of 396.33 ng g-1.
The obtained results indicated that PAHs concentrations are strongly linked to the land use of each specific site. The S1 has highest total PAHs concentrations (721 ng g-1) and S4 has the lowest total PAHs concentrations (148.4ng g-1) among studied sites. The average concentrations of total PAHs in soil corresponding to S1 (654.55 ng g-1) and S2 (437.68 ng g-1) were relatively close together while that found for S3 (324.39 ng g-1) was notably lower than those of S1 and S2 sites and close to total PAHs concentration of S4 (168.7 ng g-1).