Source Apportionment of Petroleum Hydrocarbons Hopane and Sterane in Anzali Port Area- South of Caspian Sea

Introduction
This work is the first study where the contributions of major hydrocarbon sources are investigated in detail in south of Caspian Sea area. In other previous studies, the major source of polyaromatic hydrocarbons (PAHs) and other hydrocarbons in south Caspian Sea rivers and coasts was from petrogenic origin (Nemati Varnosfaderany et al., 2015; Shirneshan et al., 2016a; Azimi et al., 2017). However, the investigation of what are the main petroleum products entering into rivers and to southern coasts of Caspian Sea is still necessary. Hopanes and steranes are ubiquitous components of crude oil which are present in traffic-related sources (automobile exhaust, tires, asphalt, engine oil and fuels). They are present as homologs and stereoisomers of each other and their composition differ among crude oils depending on their source and maturation. Therefore, hopane and steranes profiles have been used to identify the sources of petroleum pollution (Zakaria et al., 2000, 2001; Shirneshan et al., 2016a, b; Volkman et al., 1997; Yunker and Macdonald., 2003; Maioli et al., 2010).
Anzali is the most important port town in the southern Caspian Sea located in the north of Iran. This is one of the densest populated cities in Iran. Abundant rainfalls (1892 mm average annual precipitation) and crossing rivers inside the city accelerate the transport of street dust particles containing petroleum hydrocarbons into Anzali international wetland and rivers by surface runoffs (Kumata et al., 2000). In the present work, identification and distribution of the main sources of petroleum hydrocarbons in the street dust, runoffs and urban river sediments of different locations of Anzali port are investigated and they are related to automobile exhaust, tires, pavement asphalt, engine oil, gasoline and diesel possible sources. In addition, the contribution of each proposed source had apportioned by PCA and MCR-ALS chemometrics assays and fingerprinting diagnostic ratios.
Materials and Methods
Sample collection procedures
Four type of receptor samples were collected and analyzed: street dust, runoff suspended sediment, runoff soluble water and river sediment samples. Street dust and Runoff samples were collected from the surface of all bridges on the urban rivers in Anzali city (8 bridges), and from the surface of the four streets with more traffic. Sampling was performed a day of sunny weather in September 2016 which was preceded by more than one week of sunny weather. Sampling of Runoff was performed in October 2016 during a rain of more than 10 mm after some days of sunny weather using a water sampler. Runoff samples were filtered by a vacuum pump which separates suspended solids from the water-soluble fraction. River sediment samples were collected from all urban rivers of Anzali city and from harbor (24 stations) utilizing Van Veen grab (50 cm×50 cm) in 3 replicates for every station.
Six types of specific known pollution source samples were also sampled: tires, street pavement asphalt, gasoline, diesel, engine lubricant oil, and exhaust soot.
Sample pretreatment procedure
All solid phase samples including sediment, street dust, runoff filtered sediment, exhaust soot, tire and asphalt samples were extracted by the Soxhlet method. The Extraction and fractionation procedure is based on the method described in Zakaria et al. (2000). Petroleum hydrocarbons extraction from runoff soluble water was performed using liquid-liquid extraction, LLE, (Titato and Lancas, 2005; Okoli et al., 2011). Twenty milligrams of gasoline, diesel and engine oil samples were accurately weighed and dissolved in 2 ml DCM/n-hexane (1:3, v/v). These sample extracts were purified and fractionated using the same procedure as for the solid phase samples.
Limit of detection (LOD) and limit of quantification (LOQ) of the analytical method were 0.13-23.06 and 0.41- 75.10 ng.g−1, respectively for all hydrocarbons.
GC–MS analysis procedure
GC/MS analyses were carried out by a gas chromatograph (GC, model 7890A, Agilent Technologies, PaloAlto, CA, USA) instrument coupled to a quadrupole mass spectrometer (MS, 5975C, Agilent Technologies, PaloAlto, CA, USA).
Analytical standards were: 17a(H)-22,29,30-trisnorhopane, 17β(H), 21β(H)-hopane, diploptene (17β(H), 21β(H)-hop- 22(29)-ene), and 17α(H), 21β(H) hopanes. The sterane standard mixtures include 5α (H)-cholestane, 24-methyl- 5α (H)-cholestane, and 24-ethyl-5α (H)-cholestane. Perdeuterated n-tetracosane-d50 (m/z 66; relative concentration values) was used as standard (Yunker and Macdonald 2003; Harris et al. 2011).
Recoveries were computed by spiking a known concentration of the SIS mixture (surrogate internal standard) to the sample followed by performing the whole analytical method. Recoveries of individual constituents of the spiked SIS were more than 85% for hopanes and steranes.
Software
All data analysis was performed under MATLAB (The Mathworks, MA, USA, 2014) numerical computing, visualization and programming environment. PLS Toolbox 7.0 (Eigenvector Research Ltd., Manson, WA, USA) and MCR-ALS Toolbox (www.ub.edu/mcr) were used for chemometric data analysis.
Results and discussion
Catagenetic hopanes composition is usually characteristic of petroleum sources, making them useful as possible molecular markers of petroleum pollution (Peters and Moldowan, 1993). The spatial distribution of catagenetic hopanes in the four type of samples, indicates the total amount of catagenic hopanes were higher in street dust and runoff (S and W) stations located in urban and populated areas with more traffic than in stations outside the city with lower traffic. Street dust particles in urban runoff act as a transport medium for pollutants such as PAH and petroleum markers (Brown and Peake, 2006; Herngren et al., 2006). Thus, these compounds arrive to the Anzali rivers and consequently to the coast of Caspian Sea and they are accumulated in river bottom sediments for a long time. In the case of river sediments also, the stations in runoff areas containing street dust and petroleum products, especially in the harbor area stations, had higher concentrations of catagenetic hopanes. Due to the closure of harbor by the artificial pier, pollutants accumulate in this area and settle in bottom sediments. In addition, in the harbor area, there are many ships and floats that release large amounts of petroleum hydrocarbons via their fuel and oil.
Chemical fingerprinting
To further investigate the distribution patterns of source-specific hydrocarbon markers, C23/C30 (ratio of C23 tricyclic terpane relative to 17α,21β(H)-hopane), Ts/Ts + Tm (ratio of 17α-22,29,30- trisnorhopane relative to 17α-22,29,30-trisnorhopane + 18α-22,29,30-trisnorhopane), C29/C30 (ratio of 17α,21β(H)-30 norhopane to 17α,21β(H)-hopane), C31HS/C31H(S + R), C32HS/C32H(S + R), ΣC31_C35/C30 (ratio of sum 17α,21β(H)–C31 homohopane to17α,21β(H)–C35 homohopane relative to 17α,21β(H)-hopane), C28 αββ/(C27 αββ + C29 αββ) and C29 αββ/(C27 αββ + C28 αββ) ratios were calculated for all the receptor samples and also for the proposed known source samples. Results show that especially in street dust and runoff samples, the relative amounts and concentration patterns of various terpanes and steranes in the street dust and runoff samples were rather similar, showing that they may have been originated from the same common sources. On the contrary, river sediment samples were confirmed to receive inputs from other unknown independent sources.
Source apportionment
According to MCR-ALS analysis, asphalt, and tire had the highest contributions (40% and 23%) to petroleum hydrocarbons in the analyzed receptor samples, and therefore, from a quantitative point of view, these two sources should be considered to be the main sources of petroleum hydrocarbons in the receptor samples analyzed in this study. There was only a 7% of the total mass analyzed that could not be explained by the proposed receptor model.
Conclusion
Results of this work demonstrated that street dust particles are identified as a major transport medium of petroleum hydrocarbons pollution in urban runoffs. Hydrocarbons from petroleum products stick to street dust and discharge with runoffs to Anzali rivers, Anzali international wetland and then accumulate consequently at the bottom of sediments of the Caspian sea coast for a long time. The contributions of asphalt and tire known sources to petroleum hydrocarbon contamination sources were larger than from other possible investigated sources in the studied samples. Automobiles exhaust soot had also some more specific contribution. Engine oil had only a minor contribution among the studied known sources to the petroleum hydrocarbons.