Source identification of hydrocarbons in surface sediments of mangrove forests of Khoran Strait-Persian Gulf

 
Introduction
Persian Gulf is one of the most important water pathways in the world that has been heavily developed for crude oil production, transportation and exportation. It is well established that such activities result in contamination of the marine environment by petroleum and petroleum productions. The Persian Gulf represents a highly stressful environment due to prevailing natural conditions and development pressures along its coastline. It has approximately two-thirds of the worlds proven oil reserves and currently account for approximately one-fourth of the world oil production. The Persian Gulf is greatly intensified by oil spills and accidental discharges of various magnitudes have been reported in the region.
 
One of the most valuable of Persian Gulf ecosystems are mangrove forests. Mangroves are perhaps the dominant and most important intertidal habitat along tropical and subtropical coastlines and estuaries. These rich ecosystems are located in areas of high risk of acute or chronic petroleum pollution. Further, high levels of hydrocarbons may be expected to remain in mangrove sediments after occurrence petroleum contamination because these environments are not favorable for hydrocarbon depletion by sediment transport or degradation by bacteria. These zones are critical areas where valuable biological resources and rich biodiversity are threatened. These regions are continually exposed to anthropogenic contamination of hydrocarbons from different sources. Mangrove’s unique features such as high productivity, abundant detritus and rich organic carbon may make it an advantageous site for uptake and preservation of hydrocarbons.
 
The studies of distribution and concentration of hydrocarbons were mainly performed in marine water and sediments. Studies that have been carried out to assess the distribution and source of hydrocarbons in mangrove ecosystems are scarce. Straight chain alkanes (n-alkanes) and Polycyclic aromatic hydrocarbons (PAHs) are common biomarkers that have been applied to assess the petroleum pollution in the marine environment. Therefore the primary objective of this study is to investigate hydrocarbons in the mangrove forests of Khoran Strait (Persian Gulf) as well as  identify possible hydrocarbon sources in area.
Materials & Methods
In order to determine the source and concentration identification of hydrocarbons in the Khoran Strait, 17 surface sediments collected from mangrove Forest of this strait were analyzed for n-alkane and PAH compounds.
Sampling was conducted during low tide and any water was not overlying the sediments. Surface sediment samples were collected by using stainless spoon and then transferred into a stainless steel container to reduce any contamination. The containers were labeled and placed in icebox at 4°C then transportation to the laboratory for further analysis. The samples was stored in the Cold Room (-20°C) until further analysis.
The collected sediment samples were freeze dried for 3 days to get rid of any water contained in the samples. After the samples were freeze dried they were then stored frozen before proceeding with the analysis of the hydrocarbons.
About 10 g of each sample (dry weight) was used for extraction of hydrocarbons. The samples were extracted with a soxhlet extractor using 320ml dichloromethane (DCM) for 8 hours. In order to eliminate any sulfur in the samples, a few of copper chips were added into the samples and left overnight.
Volume of the solvents was reduced using rotary-evaporator to approximately 1 ml. The extracts were transferred onto the top of a 5% H₂O-deactivated silica gel column. The column was rinsed with 20ml dichloromethane/hexane (1:3, v/v). In this step most of the organic pollutants including n-alkanes, PCBs, LABs and PAHs were separated from other polar compounds. The eluted sample was reduced in volume by rotary evaporator for the second step of column chromatography. Fully activated silica gel was used In the 2^nd step column chromatography. The concentrated sample from first column was added to the second column and charged with 4ml of hexane to get alkane fraction. To get PAHs fraction 14ml dichloromethane/hexane (1:3, v/v) was used. All fractions were evaporated to approximately 1ml then transferred to glass ampoule and evaporated to dryness under gentle stream of nitrogen then alkane samples and re-dissolved into 100µl iso-octane and PAHs samples are re-dissolved into 100µl p-terphenyl-d₁₄ as an IIS (Internal Injection Standard) for PAHs.
Samples were analyzed by Gas Chromatography – Mass Spectrometry (GC – MS) using an Agilent Technologies 5975C quadrupole mass spectrometer coupled with an Agilent 7890A gas chromatoghraph. A 30m fused silica capillary column with 0.25 internal diameter and 0.25 µm film thickness, helium was used as carrier gas in the analysis.
 
Results and Discussion
In this study n-alkanes in range (n-C₁₄-nC₃₃) and twenty-three compounds of PAHs were investigated in this study. Total concentration of n-alkanes (mean±SE) was 2802±334µg/g and for PAHs was 1.918±0.267µg/g (dry weight).
The diagnostic hydrocarbonic ratios were used for source identification of hydrocarbon in this regain. The sources of PAHs, whether from pyrolytic or from petrogenic origins, may be determined by ratios of individual PAH compounds based on properties in PAH composition and distribution pattern as a function of the emission source. The molecular patterns produced by each source, however, are like fingerprints, which make it possible to hypothesize which processes produce PAHs. In order to source identification of PAHs, ratio values of phenanthrene/anthracene (Phe/Ant) are widely employed. Usually pyrogenic source of PAHs characterized with low amount of this ratio. Generally Phe/Ant ratios >10 and Phe/Ant ratios Ratio of Fluoranthen/Pyren is one of the ratios that widely employed as characteristic tools. Similarly, Flr/Pyr ratios >1 and The benzo(a)anthracene/chrysene (BaA/Chr) ratio has also been suggested to identify PAH origins, this ratio tended to increase as petrogenic contribution decreased. In present study, the mean value of this ratio was 0.22. PAHs of molecular mass 178 and 202 are widely used to identify between petrogenic and pyrogenic sources. For mass178, ratio of anthracene to anthracene plus phenanthrene (An/ An+Ph) are employed. Ratio< 0.10 and >0.10 indicate petrogenic and pyrogenic sources, respectively. In present study, the value of this ratio was 0.05 that strongly indicated major source of PAHs in this regain is petrogenic. 
For mass 202, ratio of fluoranthene to fluoranthene plus pyrene (Fl/Fl+Py) has been suggested to characterize the source of PAHs. The value 0.40 for this ratio, specified as the petrogenic/pyrogenic transition point. Most petroleum samples have (Fl/Fl+Pyr) ratio below 0.40 while those of most combustion produced PAHs are above 0.40. In this investigation mean value of this ratio was 0.35 that indicated petrogenic source.
Among PAHs with molecular mass of 228, the ratio of benzo(a)anthracene to the sum of benzo(a)anthracene and chrysene, (BaA/BaA+Chr) is also declarative of the PAHs sources. Values lower than 0.20 for this ratio suggests a petrogenic source, whereas values from 0.20 to 0.35 indicates a petroleum or combustion source, and any values higher than 0.35 signify a combustion source. For present study the mean value of this ratio was 0.16. likewise, petrogenic sources may be idetified by a ratio of indeno(1,2,3-cd) pyrene to the sum of indeno(1,2,3-cd) pyrene and benzo[g,h,i] perylene, IP/(IP+Bghi), lower than 0.20. A ratio between 0.20 and 0.50 may suggest liquid fossil fuel combustion, and a ratio higher than 0.50 indicates biomass and coal combustion. In this study the mean value of this ratio was 0.17.
 The samples were also calculated using methylphenanthrene/phenanthrene (MP/P) ratio to determine the source of PAHs. The value of less than 1 is the combustion sources and more than 1 consists of petroleum sources. In present study the mean value of this ratio was 2.
Also ratio of LMW/HMW is employed for indentification of PAHs source. The high amount of this ratio strongly indicated petrogenic source. The mean value of this ratio was 19.78, that this amount is rather high.
 We use of some ratios to source identification of n-alkanes in Khuran strait, too. Wide range of CPI, TAR and U/R for sediment samples of Khuran strait have indicated that there are combined sources (biogenic and petrogenic sources) for organic matter of surface sediments. Therefore predominate petrogenic source in some of the Middle part stations and biogenic in some others could be explained for these reasons. Pr/Ph, Pr/n-C₁₇ and Ph/n-C₁₈ ratio is close to 1 indicating background petrogenic source in surface sediments of Persian Gulf mangrove forests.
In summary, results showed that the main source of hydrocarbons in this regain is mixed source of biogenic and petrogenic origin.