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Section 16. Chemistry
Section 16. Chemistry
Sadikhova Leyla Rzaqulu, Baku State University, Doctoral student, the Faculty of Ecology and Soil Science E-mail: [email protected] Aminbekhov Aliqismat Feyzulla, Baku State University, PhD in Chemistry, the Faculty of Ecology and Soil Science Shamilov Nazim Telman, Baku State University, Doctor of chemical sciences, the Faculty of Ecology and Soil Science
Distribution and sources of polycyclic aromatic hydrocarbons in surficial sediments of the Caspian Sea in the vicinity of the Absheron peninsula
Abstract: The study explores the distribution of 16 polycyclic aromatic hydrocarbons (PAHs) in surficial sediments of the Caspian Sea in the vicinity of the Absheron peninsula. PAHs were extracted with solvent and analyzed by gas chromatography with mass selective detector. The calculations were carried out using deuterated PAH standards. Total concentration of 16 PAHs in the samples varied from 2.1 to 26,6433 ng/g dry weight. In order to determine the source of contamination, the ratios of various individual PAH were calculated. The results showed that PAHs in the studied area were presented by both pyrogenic and petrogenic sources of pollution.
Keywords: Caspian Sea, sediment, polycyclic aromatic hydrocarbons, chromatography.
1. Introduction
In recent years, the focus of many environmentalists has been the deterioration of the marine environment of the Caspian Sea as a result of pollution from various sources due to human activities. Environmental problems of the Caspian Sea have emerged due to extensive economic development in the countries of the region.
One of the sources of pollutants in the sea is the industrial and domestic effluents from the settlements located along the shoreline. The Caspian Sea shores are home to large cities with more than 220 sources of water pollution. Every year 39 km 3 of wastewater, of which 8 km 3 is contaminated, is dumped into the sea. About 30 tons of petroleum hydrocarbons are discharged into the sea along with wastewater [1]. The nature and amount of organic substances in sediments depend on deposition conditions.
The content of persistent organic pollutants, including polycyclic aromatic hydrocarbons, is affected by metabolism between sea water and sediments. According to available research, organic substances, including oil, accumulates in sediments and remains there for many years [2]. Anthropogenic sources dominate natural resources in many areas affected by human activities [3]. The study of the hydrocarbon composition of sediments may reflect the degree of anthropogenic pressure, as well as provide information about the sources of their entry into the environment [4; 5]. Offshore oil and oil
spills, industrial waste, sewage discharges flowing from the river water are considered the main sources of anthropogenic hydrocarbons in the marine environment [6]. There are two types of anthropogenic sources of hydrocarbons: pyrogenic and petrogenic sources [7]. Pyrogenic sources of hydrocarbon compounds form by incomplete combustion of organic substances such as oil, wood, coal, stone, etc. [3; 8; 9]. Crude oil and hydrocarbons of petroleum are the petrogenic sources of pollution [10; 11]. Oftentimes, to assess oil pollution of the marine environment, polycyclic aromatic hydrocarbons (PAHs) are used [12; 13]. Polycyclic aromatic hydrocarbons of pyrogenic origin are mainly represented by high molecular weight compounds with a high number of cycles. Pyrogenic PAHs are relatively stable to weathering. This allows using them as markers for identification of oil sources [14]. High concentrations of low molecular weight PAHs in the samples indicate the petrogenic origin of major pollution [15].
Many international conventions and environmental legislations, including Environmental Protection Agency of the United States, the Helsinki Convention (HELCOM 2008), and the World Health Organization recognize some PAHs with unsubstituted rings as priority pollutants. These compounds are known for their carcinogenic and mutagenic properties [16]. The carcinogenic properties of PAHs were found in 1933 with the release of hydrocarbons from coal tar [17].
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Distribution and sources of polycyclic aromatic hydrocarbons in surficial sediments of the Caspian Sea...
Solubility of PAHs in water medium decreases with an increase of the number of aromatic rings. During the sedimentation process in marine environment, selective extraction of high molecular weight hydrocarbons from dissolved forms occurs with suspension by means of adsorption. Due to their hydrophobic properties, the majority of PAHs get adsorbed by the particles, transported from the surface of the water into the water column, then settle on the seabed [18]. Being a source of a large amount of particles, phytoplankton also plays an important role in the transference of PAHs from water column to sediments [19]. Due to there ability to retain pollutants, sediment serve as a reservoir for PAHs [20]. Settled PAHs affect the tissue of benthic organisms [21]. PAHs detected in sediment are considered to be resistant to bacterial biodegradation. The accumulation of PAHs in sediments or soil leads to the decrease of their bioavailability for bacteria that slows down the process of biodegradation [22; 23]. However, it is worth noting that in comparison with high molecular weight PAHs, low molecular weight PAHs, such as naphthalene and phenanthrene, can degrade in marine sediment more rapidly [24]. An important factor influencing the behavior of PAHs (migration, deposition, transformation) when released into different natural environment, is the particle size distribution of precipitation [25]. The reasons for the accumulation of PAHs in sediments may be either anthropogenic or natural. Being an integral part of higher plants, polycyclic aromatic hydrocarbons of biogenic origin
are formed during the stages of sedimentation and early diagenesis at biochemical bacterial transformation of the original organic matter [26].
The main purpose of our work is a study of the distribution of PAHs in surficial sediments of the Caspian Sea by methods of gas chromatography-mass spectrometry, as well as identifying the source of their income in the studied area.
2. Materials and methods
2.1. Study area and Sampling
As part of a research program to study the patterns of the quantitative distribution of PAHs, as well as to establish the level of contamination in the period from 2012 to 2013, a monitoring have been organised in the coastal territory of the Absheron Peninsula. Research materials were sediment samples collected from west to east in the Baku Bay area toward the western shore of Absheron State Reserve. The sampling points were selected on the basis of environmental condition in the study area.
Baku Bay and Absheron State Reserve are located along the Apsheron Peninsula, oppossite to each other. They differ from each other both on economic, social, as well as hydrochemical parameters. Baku Bay is located in the south of the Absheron Peninsula. It is the dirtiest area of the entire Caspian region. This is facilitated by a limited water exchange with the open sea, as well as a release of a huge amount of municipal waste for many years, resulting in the formation of a thickness of anthropogenic soils [27].
Fig. 1. Map of sudy area and sampling locations in the Caspian Sea
The location of sampling points is shown in Figure 1. A total of 19 samples were collected and analyzed. Three of these were collected in the vicinity of Baku Bay from points lying far from each other, in order to obtain the average PAHs content in this coastal part of the Caspian Sea (Fig. 1). After Baku Bay, 16 samples were collected in the area all the way to the western shore of the Absheron State Reserve. In this area, sampling points were positioned on four sections. The sec-
tions extended from the sea shore, with depths of 0.5 meters towards the central part of the sea, with depths up to 14 meters. Four sampling points were located along each section (Fig. 1). Sediment samples were collected by means of Van Ween grab from the surface of the bottom at a depth of about 10 centimeters into pre-cleaned aluminum cans. The samples were delivered to the laboratory, frozen at -20 °C and kept frozen for subsequent analyses.
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2.2. Sample extraction and clean up
Analyses were performed by gas-chromatography methods. Sediments were homogenised and shells were rej ected prior to analysis. As internal standard, five deuteriated PAH compounds were added to the sediment samples prior to the extraction. The samples were extracted by 50 ml. portions of methylene chloride using an ultrasonic bath three times for 30 min. each time. 50 ml. of methanol was added into extracts before starting the first extraction. After each extraction and sample settle down the solvent was decanted through a glass fiber filter into 1-liter separating funnel containing solvent extracted deionized water. The separating funnel was shook for 1 min.; organic layer was run off into a flask. All three extracts were combined and concentrated on a rotary evaporator at temperatures below 35 °C reducing the volume to about 2-2.5 ml. Concentrated extracts purified by column chromatography using a pre-activated silica gel (Silica gel 60, 0.063-0.200 mm. for column chromatography, Merck). Silica was activated overnight in a 200 °C oven. For extract clean up a glass column was filled with 5 g. of silica. The extracts were transferred into the column and elution was performed using 35 ml. of eluent. A mixture of pentane: dichloromethane with the ratio of 2:1 was used as an eluent. Cleaned extracts were concentrated first on a rotary evaporator and then under a gentle stream of nitrogen, and transferred into GC vials (1 ml.). Activated elemental copper was used for the desulfurization of the extracts.
Chromatographic purity grade solvents methylene chloride (Rathburn, Scotland), methanol (Promochem, LGC standards GmbH, Germany) and n-pentane (HiPerSolv for HPLC, BDH, England) were used for the analysis.
2.3. Analytical methods
Quantitative analysis of PAHs was performed on GC Hewlett Packard 6890N with Hewlett Packard 5975 mass selective detector, GC-MS (Agilent, USA), equipped with
splitless injector and capilary column (fused capillary column) ZB-5 (Phenomenex, USA). Specifications of column ZB-5 are — 5 % diphenyl 95 % — dimethylsiloxane copolymer, length — 60 m., inner diameter — 0.32 mm., film thickness — 0.25 pm. Selected Ion Monitoring (SIM) mode was used for identification and calculation of16 individual PAHs. Helium was used as a carrier gas at a flow 1.5 ml/min. Temperatures were maintained at 270 °C for the injector and at 230 °C for the ion source. Oven temperature programmed as follow: 60 °C for 2 min., 60-120 °C at 15 °C/min, 120-320 °C at 6 °C /min. Detector temperature was 300 °C. Injection volume was 1 pl. Extracts were injected using an auto sampler.
Five deuterated PAHs Naphthalene-d8, Phenanthrene-d10, Pyrene-d10, Crysene-d12 and Perylene-d12 (Cambridge Isotope Laboratories, Inc., Andover, USA) were used for the quantification of individual PAHs. A mixture of 16 PAHs ( T CL PAH MIX, Supelco, USA), containing 2,000 mg/L of each component (Acenaphthene - Ace, Acenaphthylene - Acy, Anthracene - Ant, Benzo (a) anthracene - BaA, Benzo (a) pyrene - BaP, Benzo (b) fluoranthene - BbF, Benzo (k) fluoranthene - BkF, Benzo (g, h, i) perylene - B (ghi) P, Chrysene - Chr, Diben-zo (ah) anthracene - DBA, Fluoranthene - Flu, Fluorene - Fl, Indeno (123) pyrene - Inp, Napthalene - Nap, Phenanthrene -Phen, Pyrene - Pyr) was used as a standard solution. The calibration standard samples were prepared by dissolving a mixture of PAHs in dichloromethane. External calibration curve was used to determine the PAH quantification of extracts. Calibration curve was plotted on eight points. Continuing calibration was performed daily and relative percentage differences between the eight-point calibration and the daily calibrations were within allowable limit for all of the target compounds. Quantitative data were determined by comparing the peak area of the five internal deuterated standards (Fig. 2) with an area of compounds of interest
Fig. 2. GC-MS selected ion chromatogram of sixteen PAHs and five deuterated internal standards:
2 - Napthalene, 3 - Acenaphthylene, 4 - Acenaphthene, 5 - Fluorene, 7 - Phenanthrene, 8 - Anthracene, 9 - Fluoranthene, 11 - Pyrene, 12 - Benzo(a)anthracene, 14 - Chrysene, 15 - Benzo(b)fluoranthene, 16 - Benzo(k)fluoranthene, 17 - Benzo(a)pyrene, 19 - Indeno(123)pyrene, 21 - Benzo(g,h,i)perylene, 20 - Dibenzo(ah)anthracene; 1 - Naphthalene-d8, 6 - Phenanthrene-d10, 10 - Pyrene-d10,
13 - Crysene-d12, 18 - Perylene-d12. Detection limit for PAH analysis was 0.5 ng/g
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Table 1. - Concentration of PAHs in the Caspian Sea sediment samples, ng/g dw
Station Code 2-ring 3-ring 4-ring 5-ring 6-ring IEPA 16 PAHs
Nap Асу Асе Е Phen Ant б E Pyr BaA Chr В (b+k)F BaP DBA Inp В (ghi)P
S 1 32.6 8.1 16.8 26.7 101 66.6 209 633 237 864 217 132 28.8 66.7 81.3 2720
S 2 9.3 51.3 30.4 22.4 24.0 241 258 1304 846 683 794 605 143 225 385 5621
S3 1980 3135 2744 7483 40631 8148 7084 22107 35164 91158 21779 9188 6670 2737 6437 266443
A 1 1.8 5.4 2.3 10.2 20.8 13.5 16.6 44.2 24.7 97.2 31.5 12.8 9.8 10.4 22.7 323.8
A2 <0.5 1.6 <0.5 1.2 1.7 4.1 1.0 2.0 2.2 6.1 5.0 1.7 2.3 5.2 6.8 40.9
A3 1.6 6.2 1.3 5.1 11.4 28.0 9.4 40.2 28.3 20.8 73.1 36.8 28.3 60.0 76.2 426.7
A4 3.3 4.5 1.2 3.4 10.3 26.8 7.0 50.2 18.9 15.2 73.4 46.0 26.8 65.2 70.9 423.0
В 1 11.5 2.6 4.4 29.4 42.2 7.1 10.7 31.6 14.2 48.1 12.3 5.5 3.2 6.8 9.3 238.9
В 2 19.7 0.9 6.6 6.4 20.9 5.9 18.4 14.4 13.2 17.0 18.9 9.9 3.1 11.4 6.3 173.0
ВЗ 1.6 4.4 1.4 4.6 19.9 22.2 15.0 22.4 21.3 27.0 48.8 24.4 21.6 42.9 47.6 325.2
В 4 6.2 2.1 0.6 2.9 9.1 12.6 7.9 11.4 12.4 18.1 29.5 13.3 12.5 22.9 25.1 186.6
С 1 <0.5 <0.5 <0.5 <0.5 0.6 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 0.7 0.8 2.1
С 2 <0.5 <0.5 <0.5 <0.5 2.7 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 0.6 3.3
сз 1.4 5.9 1.0 4.2 12.2 24.6 10.1 15.8 16.3 17.7 34.7 18.9 17.7 44.6 55.2 280.3
С 4 0.7 1.1 <0.5 1.2 2.1 5.2 3.0 3.8 3.9 4.7 11.2 6.0 5.8 15.7 18.4 82.9
D 1 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5
D 2 5.2 2.3 <0.5 2.5 6.2 8.2 4.6 7.8 6.6 8.6 14.7 6.6 6.4 15.7 22.7 118.2
D 3 9.9 7.4 1.0 6.7 20.7 16.2 18.7 34.6 22.6 25.1 32.6 24.8 10.8 33.6 44.8 309.6
D 4 2.3 0.9 <0.5 1.9 3.9 2.7 3.1 4.0 3.8 5.1 6.5 2.9 2.6 3.9 6.0 49.7
Мах 1980 3135 2744 7483 40631 8148 7084 22107 35164 91158 21779 9188 6670 2737 6437 266433
Min 0.70 0.90 0.60 1.20 0.60 2.70 1.00 2.00 2.20 4.70 5.00 1.70 2.30 0.70 0.60 2.10
Note: Nap — Napthalene, Асу — Acenaphthylene, Ace — Acenaphthene, Fl — Fluorene, Phen — Phenanthrene, Ant — Anthracene, Flu — Fluoranthene, Pyr — Pyrene, BaA — Benzo (a)anthracene, Chr — Chrysene, В (b+k)F — Benzo (b+k)fluoranthene, BaP — Benzo (a)pyrene, Inp — Indem (l23)pyrene, В (ghi)P — Benzo (g, h, i)perylene, DBA — Dibenzo (ah) anthracene
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Distribution and sources of polycyclic aromatic hydrocarbons in surficial sediments of the Caspian Sea...
Section 16. Chemistry
2.4. Quality Control
The analyses included Laboratory quality control procedures. Appropriate method blank and duplicate samples were analyzed with each batch of sediment samples.
3. Results and discussions 3.1. PAHs in sediments
A total of16 PAHs compounds were used for identification and calculation. GC-MS of representative sample S1 is presented in Fig. 3. The distribution of individual and total ZPAHs in Caspian Sea sediments is presented in Table 1. Taking into account the fact that the Baku Bay has limited water exchange with the open sea, as expected, the distribution of PAH concentrations in the samples collected in this area are much higher than in the samples collected in the area after the bay toward the Absheron State Reserve. As seen from the Table 1, minimum and maximum concentrations in samples collected in the bay vary between 2,720 ng/g to 266,433 ng/g dry weight, whereas in other samples, the concentration ranges from 2.1 ng/g to 426 ng/g dry weight. The highest concentration was found in station S3.
In this sample, the highest concentrations of crysene (61,158 ng/g dry weight), phenanthrene (40,631 ng/g dry weight), benzo (a) antracene (35,164 ng/g dry weight), and pyrene (22,107 ng/g dry weight) were detected. This sample was collected in the area of the Baku Bay at a distance of approximately 1 km. from the coastline. Total concentrations of PAHs for stations S1 and S2 were 2,720 ng/g dry weight and 5,621 ng/g dry weight respectively. Such concentrations are typical for highly industrialized areas [28; 29]. The lowest concentrations were detected in stations C1 and C2. Total concentrations of 16PAHs were 2.1 ng/g and 3.3 ng/g dry weights respectively. PAHs were not detected in a sample D4, which has been collected at a distance of 10 km. from the shore.
Due to resistance to biodegradation, high molecular PAH compounds with a large number of rings were represented in the samples at higher concentrations than low and medium molecular PAHs. This factor may be indicative of a dominant presence of pyrogenic sources of contamination in the study area.
Fig. 3. GC-MS chromatogram of representative sample S1 in SIM mode:
1 - Naphthalene-d8; 2 - Phenanthrene-d10; 3 - Pyrene-d10; 4 - Crysene-d12; 5 - Perylene-d12
3.2. Sources of PAHs in sediment
Anthropogenic sources of PAHs can be oil spills, as well as the products of combustion of organic matter. According to various researches, in most industrialized countries PAHs in aquatic sediments are presented by pyrogenic sources [30; 31; 32].
To identify the source of contamination, the ratios of concentrations of various individual PAH are commonly used. The most common ratios used for this purpose are ratios of anthracene to anthracene + phenanthrene Ant/(Ant + Phen), fluarentene to fluarentene + pyrene Flu/(Flu + Pyr), benzo (a) anthracene to benzo (a) anthracene + chrysene BaA/(BaA + Chr) [33; 25; 34; 35; 36; 29; 5]. The values of the ratio of BaA/(BaA + Chr) > 0.35, Ant/(Ant + Phen) > 0.1,
Flu/ (Flu + Pyr) > 0.5 indicate a pyrogenic origin of PAHs in samples and are characteristic for the products of combustion of coal or wood. Contamination has a petrogenic origin in case if ratios are Ant/(Ant + Phen) < 0.10, Flu/ (Flu + Pyr) < 0.4, BaA/(BaA + Chr) < 0.2. It should be noted, that although the ratio of 0.5 for the Flu/(Flu + Pyr) is a border of major transition from a petrogenic to a pyrogenic source, this value is less accurate than the ratio of 0.1 for Ant/(Ant + Phen) and may vary. If a ratio Flu/(Flu + Pyr) is between 0.4 and 0.5, the pollution source probable combustion products of petroleum origin. The value of the ratio for BaA/(BaA + Chr) between 0.2 and 0.35 serves as the evidence of a mixed pyrogenic and petrogenic nature of the source [37].
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Distribution and sources of polycyclic aromatic hydrocarbons in surficial sediments of the Caspian Sea...
The ratio values of individual PAH concentrations for the Caspian Sea surface sediment are presented in Table 3.
As canbe seen from the Table 3,the ratio ofAnt/(Ant+Phen) changes in the range from 0.14 to 0.91. The lowest ratios of Ant/(Ant + Phen) were calculated for three samples: S3, B1 and B2, with values 0.17, 0.14 and 0.22 respectively. These values are very close to the transition border from petrogenic to pyrogenic source. For all other stations the values were higher than 0.39. All ratios of BaA/(BaA + Chr) were higher than 0.35 and varied in range of 0.42 to 0.80. These results suggest that contamination in the studied samples has mainly a pyrogenic nature, which is the result of the pyrolysis process. The ratio of Flu/(Flu + Pyr) for samples B2, B3, B4, C4, D4 varies in the range from 0.40 to 0.56, suggesting the presence of combustion residues of petroleum origin products in these samples. In other samples the ratio of Flu/(Flu + Pyr) was less than 0.4. From these results, it can be concluded that in the studied samples contamination presented by products of mixed, pyrogenic and petrogenic origin (Fig. 4).
Table 3. Values of PAH ratios in the Caspian Sea surficial sediment samples
Station Ant/ Flu/ BnA/
Code (Ant + Phen) (Flu + Pyr) (BnA + Chr)
S 1 0.40 0.25 0.78
S 2 0.91 0.17 0.45
S 3 0.17 0.24 0.72
A1 0.39 0.27 0.80
A 2 0.71 0.33 0.73
A 3 0.71 0.19 0.42
A 4 0.72 0.12 0.45
B 1 0.14 0.25 0.77
B 2 0.22 0.56 0.56
B 3 0.53 0.40 0.56
B 4 0.58 0.41 0.59
C 3 0.67 0.39 0.52
C 4 0.71 0.44 0.55
D 2 0.57 0.37 0.57
D 3 0.44 0.35 0.53
D 4 0.41 0.44 0.57
a)
Peboleum
Peboleum GlassMood/coal combustion combustion
Combustion
Prtrolmtm
b)
Fig. 4. Plots of PAH ratios for source identification in the sediment: a) Ant/(Ant + Phen) versus Flu/Flu + Pyr; b) BaA/(BnA + Chr) versus Flu/Flu + Pyr
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Section 16. Chemistry
4. Conclusion
The present comprehensive study outlines the distribution and origin of PAHs in the Caspian Sea surficial sediment along the Absheron Peninsula. The results of the studies showed that oil pollution is still a problem in some areas of the sea. PAH compounds with a large number of rings were represented in the samples at high concentrations. Concentration of total
PAHs is important and alarming in some parts of study area. The most polluted surface sediments were in the coastal areas of the Baku Bay. Samples collected in the area after the bay toward the Absheron State Reserve were relatively clean. Considering the concentration ratios of individual polycyclic aromatic hydrocarbons, we can conclude that the pollution in the study area has both a pyrogenic and petrogenic origin.
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