Predominances and source implications of even n-alkenes in surface
sediments from coastal areas of Niger Delta, Nigeria
Ebirien P. Fubara 1,Bassey O. Ekpo2, Okon D. Ekpa2 And Hab L. Marynowski 3
1Department of Chemistry, Rivers State University of Education, Port Harcourt, Nigeria. 2Environmental and Petroleum Geochemistry Research Group (EPGRG), Department of Pure and Applied Chemistry, University of Calabar, C/R State, Nigeria. 3Faculty of Earth Science, University of Silesia, 41-200 Sosnowiec, Poland. Corresponding Author: Phone: +234 08037073954; e-mail: [email protected]
Abstract
Surface sediments from coastal areas of Bonny, Imo, Qua Iboe and Warri rivers in the Niger Delta region of
Southeastern Nigeria were characterized for n-alkene composition using gas chromatography-mass
spectrometry (GC-MS) technique. The total organic carbon (TOC) contents for the sediments ranged from
0.44 to 4.26% with an average value of 2.36 ± 1.03. The concentrations of n-alkenes C18:1 – C24:1 for the
sediments in the upper, middle and lower zones of the study area ranged from 0.13-1.33mg/kg, 0.26-
2.52mg/kg and 0.17-31.86mg/kg, respectively with a Carbon Preference Index (CPI) of 0.31 – 13.32. The
dominant carbon maximum (Cmax) of n-alkenes were C18:1, C22:1 for Warri, C22:1 for Qua Iboe and C20:1, C24:1
for Bonny coastal sediments. Factor analyses confirmed biogenic, anthropogenic (petroleum activities) and
microbial sources of sedimentary n-alkenes C18:1 – C24:1 in the study area. An unresolved complex mixture
(UCM) occurring in the range n-C18 – n-C35 is an indicator of petroleum contamination.
Hydrocarbons found in surface sediments reflect both natural and anthropogenic inputs as well as diagenetic processes taking place within the water column and during transport and sedimentation [1]. The distribution of alkanes and alkenes in surface sediments is characterized by distinct carbon number ranges and has been used for the diagnosis of paleoenvironments [2] and organic source input [3-8]. Hydrocarbons of coastal sediments are produced by various sources (marine organisms, terrestrial plants, and anthropogenic activities) leading to complex mixture of these compounds [9].
Even-numbered n-alkanes/alkenes predominance in the geosphere is uncommon. Slight even carbon number predominances or smooth distributions in the n-C20-C30 range have been ascribed to reductive processes or bacterial inputs [10]. The predominances of even numbered n-alkenes (terminal alkenes) in the range n-C12:1 - C18:1 [11], n-C14:1 – C26:1 [12], n-C22:1 – C32:1 [13, 14] in recent sediments had been reported. Formation of a series of isomeric n-nonadecadienes and n-heneicosadienes through either diagenetic modification of algal organic matter or de novo synthesis by microorganisms active in anaerobic decomposition of marsh sediments had been reported [15]. Series of n-alkanes and n-alk-1-enes in the carbon number range of nC16–nC26, the latter with an even predominance (Cmax 16 and 18) were observed.
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Such alkanes, previously and exclusively assigned a biogenic origin in sediments from the Calabar and Cross River Estuary, were considered to be distillation residues (n-alkanes) and derived from leaching of by-products from the production of polyethylene/polyvinyl chloride plastic containers (n-alkenes) where these solvents were stored [16]. In comparison to the large published works regarding the evaluation of potential sources of hydrocarbons in marine sediments, n-alkane distributions with odd number preferences (C27, C29, C31) and/or (C15, C17, C19) or with no carbon preferences and relating to vascular plants, marine animals and anthropogenic origin, have been reported [8, 14, 17-19]. The present study is part of a frame project titled Organic Tracers of Pollution in the Environments of the Niger Delta of Nigeria (OTPEND) designed for characterization and source evaluation of hydrocarbon pollutants in surface sediments from the Niger Delta coastal areas based on Advanced Chemical Fingerprinting (ACF). It is a systematic investigation of an uncommon group of organic constituents in the geosphere, namely n-alkenes in sediments from coastal areas of Bonny, Imo, Qua Iboe and Warri rivers in the South-western and eastern parts of the Niger Delta region of Nigeria. The objectives are to: (a) quantitatively characterise n-alkenes in the solvent extractable lipids of sediments from the Niger Delta coastal areas, (b) use the multivariate statistical approach to assess the different n-alkene sources, and (c) provide additional insight into the prospective sources and processes that modulate the flux of anthropogenic versus biogenic hydrocarbons ultimately accumulating in the Niger Delta coastal environment.
2. Study area
The fan-shaped Niger Delta, which is the third largest in the world after the Mississippi (USA) and Pantanal (South-West Brazil), lies between latitudes (4 and 6)0 north of the equator and longitudes (5 and 9)0 east of the Greenwich Meridian. The North-South extension, north of the equator, is expressly defined by the Great Atlantic Ocean in the South to Aboh (Delta State) in the North where River Niger forks in Rivers Nun and Forcados at a village called Obotor [20]. The East-West extension is from the boundary of the Bonny River to River Sapele, Delta State. According to [20], the geographical Niger Delta is just about 25,640km2 in size; made up of 7,400km2 low land area, 11,700km2 fresh water swamp, and 1,140km2 salt barrier islands as ecological zones.
A simplified subdivision of the Niger Delta as fresh water zone, the mangrove and the coastal sand ridges was given by [21]. The Northern part of the fresh water zone is often regarded as an extension of the lower Niger flood plain [22], and it is very susceptible to the annual floods during the rainy season. The southern part of the fresh water zone and most of the mangrove are swampy and hardly rise above 10m above mean sea level. This sub-zone covers a greater percentage of the Niger Delta. The strip of sand ridges and beaches lies close to the open sea and is bordered landwards by swamp areas with many
creeks [20]. The Niger Delta region displays a typical dendritic drainage pattern. This is because most of the tributaries join their main rivers at oblique angles. For example, Taylor Creek joins the main River Nun at Polaku, and the Epie Creek joins the River Nun/Ekole at Yenagoa. This also implies that there are many confluence towns in the Niger Delta. There is also a near parallel type of drainage, for example, New Calabar/Bonny River, Brass/Nun River, Forcados/Escravos River, Taylor Creek/Epie Creek and Orashi River.
The vegetation of the Niger Delta coastal areas is characterized by extensive freshwater and
mangrove swamps. The fresh water swamps consist of stilt rooted trees and shrubs. The main vegetation
of the mangrove swamps of the Niger Delta according to [23] is dominated by the red mangrove which
forms more than nineteen percent (19%) of the saline swamps. The white mangroves occur scattered
among the red mangroves and thrive in less water-logged places. Ferns, Nipa palms and herbs are found
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in areas where their salt content is not too high. The Niger Delta has provided the best conditions for the
thriving of vegetation on the Nigerian coast. By some estimates, over sixty percent (60%) of fishes caught
between the Gulf of Guinea and Angola breed in the mangrove belt of the Niger Delta. According to [20],
the vegetation of the Niger Delta region is arranged in the form of storey/strata or layers. There is the upper
stratum occupied by very tall trees of heights 30m and above; followed by the middle tier trees with heights
of 15-20m, while the lower layer has trees and shrubs with heights below 12m. The upper layer is normally
occupied by tall trees like the Iroko (Chlorophora excelsa), Mahogany (Khaya ivorensis), and Abura
(Mitragyra macrophylla) according to [24]. Palm trees (Elaeis guineensis) and others occupy the middle
layer. The availability of sunlight is also partly responsible for the growth of the tall plants as it provides
them with the necessary solar energy for the photosynthetic process. The interest in an accurate evaluation of
the sources of hydrocarbons in Niger Delta coastal sediments is becoming broader now than ever before due to the
increasing rate of environmental degradation and its social, economic and health impacts as a result of oil pollution
[25 - 34].
3. MATERIALS AND METHODS
Sampling
Samples of surface sediments were collected at different stations each along the coastal areas of Bonny, Qua Iboe, Warri and Imo rivers as depicted in the map of the study area (Fig 1) using a modified grab sampler (0.1m2). Samples were removed from the middle of the grab to avoid contact with the inner metallic surface of the grab sampler, wrapped in aluminium foil and stored frozen at - 4 0C. The sampling was carried out based on the nature of potential anthropogenic inputs. Prior to extraction, the samples were
freeze-dried, crushed and sieved through a 230 mesh (< 63µm) sieve.
Extraction and fractionation
To minimize contamination, all glassware was cleaned with detergent solution, rinsed with distilled water, heated in an oven at 5500C for eight hours to combust traces of surface organic matter, and finally rinsed with ANALAR grade dichloromethane. The total organic carbon (TOC) contents were determined
using a LECO CNS analyzer. Extraction of the crushed and sieved (< 63µm) sediment samples for extractable organic matter (EOM) was carried out using a soxhlet apparatus [35]. The thimbles and the glass wool used in the extraction were soxhlet-extracted with dichloromethane for 20 minutes on a water bath. Powdered sediment sample (50g) was then placed in the extracted thimble. The thimble with glass wool was filled with dichloromethane and extracted for 18hrs. Extracts were desulphurized by addition of 30g activated copper (copper immersed in 20ml of 0.1M concentrated hydrochloric acid for ten minutes) into the round-bottom flask during extraction. Extracts obtained were evaporated to near dryness using a vacuum evaporator. The weight of extracts was determined as a measure of the amount of extractable organic matter (EOM), made up of asphaltenes and maltenes. Precipitation of asphaltenes from the extractable organic matter (EOM) was carried out following the procedure described by [36] with a mixture (1:30) of dichloromethane /petroleum ether (b.p. 40-600C), and centrifuged at 3,000 rpm for about 20min. The asphaltenes precipitated from EOM were discarded after weighing. The separation of maltenes obtained from the extracts into aliphatic, aromatic and hetero-fractions was carried out by column chromatography (column 30 x1.2cm) using activated silica gel (20g activated by heating in an oven for two hours at 4000C) and alumina (neutral, 10g activated by heating in an oven for two hours at 5000C) on top of
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the silica gel. The concentrated extract (2ml) was carefully added to the top of the column already clamped to position. The saturated (aliphatic) fraction was eluted with 50ml hexane, while 200ml of 1:1 dichloromethane/hexane mixture was used for the elution of the aromatic fraction [18]. Finally, a mixture (1: 2, 60ml) of methanol /dichloromethane was used to remove the heterofractions.
Gas chromatography-mass spectrometry (GC-MS) analysis
The gas chromatography-mass spectrometry (GC-MS) analyses of the aliphatic and aromatic fractions from the sediment extracts were performed using an Agilent 6890 Series gas chromatograph (GC) interfaced to an Agilent 5973 Network Mass Selective Detector (MSD) and Agilent 7683 Series Injector. The GC Separation was achieved on a fused silica capillary column coated with DB 35 (60m x 0.25mm i.d., 0.25µm film thickness). The GC oven temperature was programmed from 500C (isothermal for 1 minute) to 1200C at a rate of 200C/ min, then to 3000C at a rate of 30C/ min. The final temperature was held for 45 minutes. Helium was used as the carrier gas. Samples were introduced into the cool on-column injector under electronic pressure control. The GC column outlet was connected directly to the ion source of the mass spectrometer. The GC-MS interface was kept at 280 0C, while the ion source and quadrupole analyzer were at 230 and 1500C, respectively. The mass spectrometer was operated in the electron impact (EI) mode at 70eV ionization energy. Mass spectra were recorded from 45- 550 dalton (0 - 40min) and 50 – 700 dalton (above 40 min). Quantification was obtained by acquiring mass chromatograms at selected m/z value for n-alkenes using the HP-MSD Chemstation Integrator. Individual compounds were identified by comparison of mass spectra with literature and library data.
Multivariate statistics (Factor analysis)
For a better understanding of the principal sources and biogeochemical processes responsible for the sedimentary n-alkenes in coastal sediments from Niger Delta of Southeastern Nigeria, varimax rotated factor analyses using Statistica version 7.0 were carried out on the data set. The varimax rotated factor analyses were calculated using eigen values greater than 1.0 and sorted by results having values greater than 0.6 being considered significant influences towards the principal sources.
RESULTS AND DISCUSSION
The sampling stations, percentage total organic carbon (%TOC) contents and concentrations of n-alkenes C18:1 - C24:1 in the upper, middle and lower zones of the study area are presented in Table 1. The concentrations of n-alkenes in the range C18:1 - C24:1 for sediments in the upper, middle and lower zones of the study area ranged from 0.13 - 1.33mg/kg dry wt, 0.26 -2.52mg/kg and 0.17 - 31.86mg/kg dry wt, respectively. The total organic carbon (TOC) contents ranged from 0.44 to 4.26% with an overall average value of 2.36 ± 1.03.
The signatures of the homologous compound series present in the surface sediment extracts are characterised by unresolved complex mixture (UCM) and carbon number maximum (Cmax), which lend supportive evidence for the relative incorporation of recent biogenic organic material [12]. Typical distribution pattern for n-alkanes and n-alkenes in sediments from the study area are shown in Fig. 2. The n-alkanes range from C15 to C35 with a strong even carbon number predominances between C16 and C24 and a strong odd carbon number predominance above C25. All hydrocarbon fractions contain petroleum residues and this is reflected in the n-alkane distributions shown by the presence of significant amount of pristane and phytane and the envelope of petroleum-derived n-alkanes as reported by [37]. Most of the
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samples exhibit a characteristic bimodal n-alkane profile with a strong even carbon number predominances in the range n-C14 to n-C26 (CPI 0.31 – 13.35
Another salient feature displayed by the GC profile of the samples is the occurrence of even predominance of n-alk-1-enes in the range n-C18 - n-C24 (Fig. 2b) parallel to the alkanes which occurred in most of the samples. The most abundant compound is n-C20:1. The occurrence of this even predominance of n-alk-1-ene was however not observed in samples from Imo river. Higher homologues of the alkanes derived from higher plant waxes are abundant in most of the samples; and exhibit odd carbon predominance in the range n-C27 - n-C35, maximising at n-C29. This second mode is similar and typical for immature hydrocarbons with an origin from higher terrestrial plant wax inputs [10, 12, 38, 39]. There is also an occurrence of an unresolved complex mixture (UCM) of branched and cyclic compounds in the range of n-C18 – n-C35 n-alkanes (Fig. 2a) indicative of petroleum contamination. The predominance of even numbered n-alkenes (terminal alkenes) in the range n-C12:1 - C18:1 in recent sediments has been linked with the presence of unsaturated fatty acids, indicators of recent biogenesis [11] while the occurrence of even numbered n-alkenes in the range C14:1- C24:1 in surface sediments of Gabes Gulf of Tunisia had been reported to be linked with various sources (marine organisms, terrestrial plants, and anthropogenic activities) leading to complex mixture of these compounds [9]. In our samples, homologous even-numbered n-alkenes ranging from C18:1 to C24:1 were identified and quantified in stations from coastal areas of Bonny, Qua Iboe, and Warri rivers, but were not detected in stations from Imo river (Table 1). This observation calls for explanation. The results of varimax rotated factor analyses, showed that the even-numbered n-alkenes C18:1 - C24:1 in stations from Bonny river were positively loaded on terrestrial/biogenic and microbial sources, with greater contributions (more loadings) from microbial sources. For stations from Warri and Qua Iboe rivers, n-alkenes C18:1- C24:1 were positively loaded on terrestrial/biogenic and anthropogenic sources; with greater contributions from anthropogenic sources for stations from Warri river. Stations from coastal areas of Qua Iboe river appeared to have almost equal loadings from both terrestrial/biogenic and anthropogenic sources. The results showed that the origin of even-numbered n-alkenes C18:1 - C24:1 in our study area could be linked to mangrove vegetation, especially around the coastal areas of Bonny and Warri rivers, in addition to synthesis by diverse microorganisms inhabiting petroleum contaminated environment [12]. Higher plant waxes have been reported to contain even carbon number n-alkenes in the range C22:1 - C32:1 [13, 14]. The possibility of synthesis of even-numbered n-alkenes from waxes of higher plants by defunctionalization of alcohols and fatty acids was unlikely as no sample in our study area exhibited even-numbered carbon above C24 assigned to waxes of higher plants. Variations in the concentrations of the n-alkenes (Fig.3) truly reflect variations in the relative source inputs at the different stations in the study area. Relatively high concentrations of the homologous even numbered n-alkenes were recorded for most stations at the lower zone (e.g. stations QIR25 and WR8) and station BN02 (middle zone) of the entire study area. This observation illustrates the influence of fluvial geochemistry (e.g. lateral transport and tidal currents) on distribution of sedimentary organic matter, with increasing sedimentation and accumulation at stations in the lower and middle course of a flowing river. The absence of n-alkenes C18:1- C24:1 in stations from coastal areas of Imo river could be attributed to strong terrestrial higher plants signal over microbial and anthropogenic source inputs. The dominant carbon maximum (Cmax) for n-alkenes were C18:1, C22:1 for Warri, C22:1 for Qua Iboe and C20:1, C24:1 for Bonny coastal sediments. These results are comparable with the Cmax of C18:1 and C20:1 reported for n-alkenes in surface sediments from Calabar River [12] and C16:1,
C18:1, C20:1 and C22:1 [9]. The identification of homologuous n-alkanes in the aliphatic fractions from GC traces has allowed
the determination of carbon number maximum (Cmax) and the Carbon Preference Index (CPI). This provides information on the relative incorporation of the different aliphatic hydrocarbon sources. A CPI >1 indicates the relative contribution of n-alkanes from natural (biogenic) sources and a CPI ~1 indicates
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anthropogenic (petroleum) pollution. The CPI characteristic of the bottom sediments ranged between 0.13 and 13.32. Carbon number maximum (Cmax) > C25 for n-alkanes of odd carbon number reflect the incorporation of high plant waxes, and Cmax of lower even carbon number indicates a major input from microbial sources [40]. In the study area, CPI for most stations is greater than 1, except for stations BN15, BN08, BN02 and WR08. This indicates the relative contribution of n-alkanes from natural (biogenic/terrestrial), while CPI < 1 for stations BN15, BN08, BN02 and WR 08 indicates anthropogenic (petroleum pollution)[41, 40,12].
CONCLUSION
Molecular biomarkers i.e. organic compounds detected in the geosphere with structures suggesting an unambiguous link with known contemporary natural product, which are specific indicator compounds (found in extracts from geological and environmental samples) were detected in samples from the study area. Even-numbered alk-1-enes are rare occurrences in the geosphere. The predominances of even numbered n-alkenes (terminal alkenes) in the range C18:1 – C24:1 in coastal sediments from the Niger Delta study area were ascribed to biogenic, anthropogenic (petroleum activities) and synthesis by microorganisms enhanced by petroleum contamination. Further studies should be carried out to determine the occurrences of even numbered n-alkenes in biota and suspended particulate matter (SPM) in the Niger Delta environmental ecosystem.
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