Sources Of Polycylic Aromatic Hydrocarbons In Mangrove Sediments Biology Essay

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In this study, the distributions of PAHs in surface sediments of fifteen stations from a small mangrove island located within the estuary of Kemaman River were determined. Sediments were soxhlet extracted using a 1:1 mixture of dicholoromethane and hexane as solvent. Desulphurization was done using mercury treatment while fractionation was done using silica-alumina column chromatography technique. The hydrocarbons were detected using Gas Chromatography-Mass Spectrometer (GCMS). Results showed total identified PAHs (TIP) ranged between 0.12 - 1.42 g g-1. Regardless of site, the most common compounds detected were those of higher molecular weight largely known to be associated with combustion of fuel materials. A strong correlation exists between benzo[ghi] perylene and TIP suggested that combustion of fuel materials (vehicular emissions) could be a major contributor of PAHs to these sediments. Based on the strategic location of the island within the Kemaman estuary, it is postulated that the most likely sources of the PAHs compounds are runoffs and atmospheric deposition from various related activities from Chukai Township and from maritime sources such as discharges from ships and fishing boats.

Keywords: South China Sea, river estuary, PAHs, fuel combustion, vehicular emission

INTRODUCTION

Occurrence of hydrocarbons in the marine environment comes from many different sources and can generally be classified into two major sources viz. biogenic and anthropogenic sources. Hydrocarbons of biogenic origin naturally occur at low concentrations in different substrates, such as water and sediments, and are part of the natural hydrocarbon baseline of an ecosystem. These hydrocarbons originate from terrestrial plant waxes, biosynthesis by marine organisms and diagenesis transformation of non-hydrocarbon biosynthetic natural products such as sterols and hopanols. Hydrocarbons of anthropogenic origin, on the other hand, are introduced into the environment as a result of activities related to oil exploration, maritime transport (operational discharges and tanker accidents), harbour activities and domestic and industrial effluents. Due to their hydrophobic nature, hydrocarbons especially polycyclic aromatic hydrocarbons (PAHs) in aquatic environment rapidly tend to be adsorbed onto particulate materials and then deposited in sediment as a final sink. Thus, it not surprising that sediment is generally recognized as the most important reservoir of these contaminants [1, 2]. Background values for PAHs in sediments reported to be in the range of 0.01 to approximately 1 mgkg-1 dry weight. However there is a high probability of finding them higher in the river mouth, estuaries and bays, as well as in areas associated with ship traffic, oil production and transportation [1]. It is generally accepted that the Malaysian marine environment is increasingly being threatened by oil pollution, namely oil and grease pollutants [3-5].

Consequently, mangrove forests, an important nurseries and breeding grounds for a variety of living resources, being a coastal ecosystem are therefore extremely vulnerable to the threats of oil pollution. Few studies have been carried out to assess the distribution and accumulation of PAHs in mangrove forests and estuaries in certain regions. Tam et al. [6] have reviewed the levels of PAHs observed in mangrove and marine sediments of Hong Kong, China and other areas in the tropical region; generally the values (Table 1) showed significant variation with locations and these differences could be related to variations in and the extent of activities in the areas of study.

Table1: Concentration of total polycyclic aromatic hydrocarbons (PAH; ngg-1 d.w.) in mangrove sediments and marine sediments in Hong Kong and other cities in China.

Sites

Mean

S.E.

Range

Sample Stations

Reference

Mangrove sediments in Puerto Rico

1820

2349

500-6000

14

Klekowski et al. (1994)

Mangrove sediments in Carribean Island

502

554

103-1657

13

Bernard et al. (1996)

Mai Po mudflat in Hong Kong

547

142

400-831

3

Zheng (unpublished)

Marine coastal sediments in Hong Kong

553

253

7-4420

20

Zheng and Richardson (1999)

Marine bottom sediments in Hong Kong

Connell et al. (1998)

Background

40-60

66

Eastern buffer

94

93

40-230

4

Deep bay

99

89

47-231

4

Tolo Harbour

42

4

39-50

4

Victoria harbour

66-116

6

Typhoon shelter

42-1159

9

Marine sediments in Pearl River Estuary

2196

3087

408-10 811

10

Mai et al. (2000)

Marine sediments in South China Sea

146

25-275

16

Yang et al. (2000)

Marine sediments in Xiamen Harbour

367

87

247-480

9

Zhou et al. (2000)

Source: Tam et al. 2001 [6];

Malaysia is estimated to have approximately 612,580 ha of mangrove forests, the bulk of which are located in Sabah and Sarawak. Peninsular Malaysia has an estimated 103,203.11 ha of mangrove forests, out of which only ca. 1987 ha. are found in Terengganu [7]. Kemaman estuary is unique in that not only it has the largest extent of mangrove forest in Terengganu, it is also in the vicinity of Kemaman supply base, the main supply port that service the oil and gas operation in the South China Sea. The estuary is also just ca. 20 km south of the Kerteh crude oil terminal, which receives oil from the South China Sea oil wells. Studies [4, 8, 9] on the hydrocarbon distributions in this region suggested some indications of oil and grease contamination when compared to other areas in the east coast of the Peninsular [10, 11]. Kemaman estuary is well studied with respect to physical processes (e.g. sediment transport phenomenon) in the estuary [12] and also the extent of mangrove forests [7] but studies on chemical transport and/or distribution of pollutants, particularly on hydrocarbons, are still limited. Yet, the definition of a baseline level of hydrocarbons is essential when attempting to determine chemical changes introduced by anthropogenic effects. Consequently, this study is initiated to provide the relevant and much needed information on current levels of polycyclic aromatic hydrocarbons as well as fingerprinting sources of these compounds in the area.

STUDY METHODOLOGY

Sampling and sample pre-treatment

KSB

Jetty

LKIM

Jetty

Pulau Cik Wan Dagang

Pekan

Cukai

Sg. Limbong

Jetty

Legend: KSB - Kemaman Supply Base

LKIM - Jetty (Lembaga Kemajuan Ikan Malaysia)

Figure 1: Location of Pulau Cik Wan Dagang and possible sources of hydrocarbon inputs

Figure 1 shows the location of Pulau Cik Wan Dagang which is located in the Kemanan estuary. The island is not populated and is actually a mangrove island which is subjected to tidal influence. Also included in the diagram are potential sources of hydrocarbon input into the estuary; Sg. Limbong is actually a small river which is now turned into a concrete channel and domestic sewerage system (sistem peparitan) of Chukai town actually drains into this channel bringing possible wastes into the estuary; jetties and KSB are potential sources as fishing boats and supply ships uses these facilities, respectively. A total of 15 stations were selected for the collection of surface sediments on the island (Figure 2). Sediments were collected using a Ekman grab sampler, wrapped in pre-cleaned aluminum foil and transported to the laboratory in an ice chest packed with ice. Once in the laboratory, samples were frozen until further analysis. These samples were later freeze-dried followed by sieving using a 500µm sieve. Only the fraction < 500µm were used in the analysis.

Figure 2: Map showing the location of sampling stations.

Analytical methodology

The analytical procedure adopted for the extraction of hydrocarbon compounds was based on standard method proposed by UNEP [13]. Briefly, sediments were soxhlet extracted using DCM:hexane (1:1) mixture for eight hours. Prior to the extraction, an internal surrogate standard (9,10 - Dihydroanthracene) was spiked into the sediment for recovery assessment; this procedure typically yields recoveries of PAHs in the region of 55 - 80%. PAHs concentration was recovery-corrected using the spiked surrogates. Removal of sulphur from the extract was done by using mercury treatment. The sulphur free extracts were then concentrated to about 1 ml using a combination of rotary evaporator (<35°C) and nitrogen blow-down followed by fractionation on partially deactivated (5%) silica-alumina column. The PAHs fraction was eluted using a combination of 30 ml 10% DCM and 20 ml 50% DCM in hexane, and the extracts collected in one fraction. Procedural blanks were run throughout the study to ensure there was little or no detectable contamination of the samples.

GCMS analysis

Identification and quantification of PAH's fraction were carried out using GC-MS by comparing the retention times compared to that of external PAHs standards. Confirmation of peaks was also carried out using the MS library. Sixteen USEPA priority PAHs compounds were analysed in this study; these compounds were as follows: naphthalene (NAP), acenaphtylene (ACY), acenaphtene (ACE), fluorene (FLU), phenanthrene (PHEN), anthracene (ANT), fluoranthene (FTH), pyrene (PYR), benzo(a)anthracene (BaA), chrysene (CHR), benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(a)pyrene (BaP), dibenz(a,h)anthracene (DA), benzo(g,h,i)perylene (BgP) and indeno(1,2,3,cd)pyrene (IP). Sums of these 16 compounds were collectively known as total identified PAHs (TIP).

In the GC-MS analysis, the GC operating condition was as follows: HP-5 fused silica column (30 m x 0.25 mm i.d; 0.25 m filmed thickness); injection temperature was set at 290°C using a splitless mode; column temperature was programmed in the following manner: hold at 50°C for 1 min, first temperature ramp of 50 - 140°C at 5°C min-1 followed by the second temperature ramp of 140 - 290°C at 4°C min-1 and then maintained at 290°C for 13 min resulting in a total run time of 82 mins; helium was used as the carrier gas with a flow rate at 1.2ml min-1. The GCMS interface was set at 300°C.

RESULTS AND DISCUSSION

Polycyclic aromatic hydrocarbon distribution

Table 2 shows the distribution of TIP and selected fingerprinting parameters in surface sediments of Pulau Cik Wan Dagang. TIP found in these sediments ranged from 0.12 to 1.42 µg/g dry wt with mean of 0.60 + 0.45 µg/g dry wt. Regardless of sampling sites, it is interesting to note that most common PAHs detected were those of higher molecular weight largely known to be associated with combustion of fuel materials viz. DA, BgP and IP. CHR was another compound quite commonly detected in the samples. The combustion of fuels generates a compositionally characteristic series of primarily non-alkylated PAHs, regardless of the fuel type burned [14]. The following nine compounds are often the most dominant members of this series viz. FTH, PYR, BaA, CHR, BbF, BkF, BaP, DA, BgP and IP. This primarily combustion derived PAHs are referred quantitatively in the following section as a composite term ∑Comb (Table 2), defined as the sum of the concentrations of these nine compounds in each sample. The results (Table 2) show that ∑Comb represent almost 41 to 93% of the total PAHs identified in the samples indicating the important contribution of this source of input into the area under study. Another interesting feature is that for some stations (e.g. stations 1,4 and 5), despite the relatively low TIP, ∑Comb represent extremely high proportions (>80%) of the TIP.

An association of BgP with vehicular emission source has long been established [15]; in this study, almost all stations showed the presence of BgP in their sediments. A strong correlation between BgP compound with the TIP (Figure 3) (r=0.91) indicates that vehicular emission could be a major source of PAHs found in these sediments. Exhaust emissions from road transports could be deposited onto soils and enter the estuary through run-offs from nearby Chukai township via Sg Limbong or through direct atmospheric deposition into the estuarine and marine environment. A major road runs across Chukai and Kemaman rivers; emissions from vehicles plying the route could potential contribute to the PAHs content through atmospheric deposition into these river systems. Equally important, emissions from maritime activities such as from ships and fishing boats using the jetties located along the estuary and supply vessels anchoring and /or using the Kemaman Supply Base (KSB) facilities in the area could also deposited the PAHs into the marine environment.

Table 2: Distribution of total identified PAHs and selected fingerprinting parameters

in surface sediments of Pulau Cik Wan Dagang, Kemaman Estuary

Sampling station

TIP (µg/g)

∑Comb (µg/g)

% Comb

BgP (µg/g)

1

0.241

0.215

89.2

0.042

2

1.133

0.513

45.3

0.218

3

0.413

0.236

57.1

0.132

4

0.158

0.137

86.7

nd

5

0.124

0.104

83.9

nd

6

0.274

0.192

70.1

nd

7

0.141

0.081

57.4

nd

8

0.261

0.215

82.4

0.042

9

0.231

0.215

93.1

0.996

10

0.657

0.317

48.2

0.204

11

0.908

0.419

46.1

0.116

12

1.417

0.587

41.4

0.362

13

1.135

0.723

63.7

0.397

14

0.918

0.571

62.2

0.320

15

1.042

0.515

49.4

0.355

TIP: total identified PAHs; ∑ Comb: sum of total combustion derived PAHs;

nd: not detected

Figure 3. Correlation between benzo(g,h,i) perylene and total identified PAHs

Review of the distribution pattern of the individual PAHs compounds found in this study showed the depletion of low molecular weight PAHs compounds such as ACE, ACY, PHEN, ANT, PYR from majority of the sediment samples. It is uncertain at present whether these absences are actually inherent to these samples or because of a possible technical problem occurring in the analysis of PAHs compounds. Thus, it is conceded that further study is needed to verify these findings. Nonetheless, present results clearly show that PAHs compounds were present in these samples and based on the present observation, they were of anthropogenic source probably resulting from combustion of fuel materials.

Admittedly, it is difficult to accurately identify PAHs origins based on the fact that these analytes could be emitted by number of possible sources or undergo various processes before being deposited into the sediments. Nonetheless, previous researchers [e.g. 1, 10-11, 16-17] have evaluated possible sources of PAHs found in sediments by employing individual ratios of selected PAHs compounds molecular markers in their source apportionment (Table 3). The abundance ratio of two and three ring to four to six ring PAHs (LMW/HMW) can be used to evaluate the petrogenic and pyrolytic sources. Ratio between benzo(ghi)perylene and indeno(1,2,3,cd)pyrene suggesting combustion input sources either from gasoline or diesel exhaust while ratio between Benzo[a]anthracene and the sum of Benzo[a]anthracene and chrysene can potentially differentiate between general combustion process with grass,wood and coal combustion sources. However a degree of caution is needed when using the latter ratio, a value of > 0.5 generally favours grass,wood and coal combustion sources but Yunker et al. (2002) as cited by Mille et al. (2006) [17], has reported that combustion residues from airport and automobile traffics and nautical activities sometimes yield values in excess of this ratio thus cannot be totally be excluded. In addition to ratios listed in Table 3, methyl phenanthrene to phenanthrene (MP/P) ratio of < 1 is also taken as an indication of combustion sources while a value of > 1 is indicative of petrogenic origin (Zakaria et. al, 2002).

Table 3: Suggested PAHs sources based on the ratios of some PAH cmpounds

Ratio PAHs source

LMW/HMW

BgP/ InP

BaA/Σ228

Pyrolytic

<1a

Petrogenic

>1a

Gasoline exhaust

3.5a

Diesel exhaust

1.1a

Combustion

>0.35b

Grass/ wood/ coal combustion

>0.5b

LMW/HMW: ratio between the sum of two and three ring PAHs and the sum of four to six ring PAHs. BgP/InP: ratio between benzo(g,h,i)perylene and indeno(1,2,3,cd)pyrene. BaA/Σ228: ratio between benz[a]anthracene and the sum of benz[a]anthracene and chrysene.

aZhang et al. (2004) [16]; bMille et al. (2006) [17]

Table 4 shows the ratios of selected PAHs compound in mangrove sediment of Pulau Cik Siti Wan Dagang. All the values of LMW/HMW were lower than 1, indicating significant PAHs input from pyrolytic sources which is a common cause of contamination, such as atmospheric deposition that might have introduced PAHs into water bodies. The ratios of BgP/IP for stations 3, 13, 14 and 15 were approximately 3.5, suggesting combustion input of gasoline, while stations 2, 10, 11 and 12 were approximately 1.1, showing that these PAHs were derived from diesel combustion (Figure 4). BaA/Σ228 ratios of stations 7 and 11 were over 0.35, suggesting that they were from combustion sources. The ratios in excess of 0.5 as exhibited by stations 9, 13 and 14 are indicative of combustion of grass, wood or coal.

The concentration of PAHs analysed in this study were compared with the effects-based guideline values such as the effects range low (ERL) and effects range-median (ERM) values provided by the US National Oceanic and Atmospheric Administration (Long et al., 1995 as cited by Zhang et al. 2004 [16). This comparison shows that PAHs level from these stations (0.124-1.396 μg g-1) does not exceed the level of the ERL value (4.022 μg g-1), suggesting the PAHs found in the study area are not likely to cause any biological effect. However, it must be noted that the levels of PAHs in mangrove sediments of some stations on Pulau Cik Wan Dagang have exceeded the background level of PAHs found in uncontaminated sediments (0.01-1 μg g-1) indicating that some parts on the island are showing sign of contamination.

Table 4: Ratios of some PAH compounds in the mangrove sediments

Station

LMW/HMW

BgP/InP

BaA/Σ228

1

0.12

na

na

2

0.03

1.5

0.32

3

0.18

4.8

na

4

0.15

na

0.28

5

0.2

na

na

6

na

na

na

7

0.74

na

0.48

8

0.21

na

na

9

0.07

na

0.5

10

na

2.31

na

11

0.03

1.13

0.39

12

na

2.82

0.31

13

0.02

4.35

0.73

14

0.07

4.71

0.6

15

0.02

3.46

na

na: not available

Gasoline exhaust

Diesel exhaust

3

14

15

13

11

10

12

2

Figure 4: Plot of BgP/IP versus ΣPAHs for the sediments collected from Pulau Cik Wan

Dagang Mangrove forest.

CONCLUSION AND FURTHER RESEARCH

Data obtained from the PAHs analyses revealed the presence of PAHs compounds generally associated with the combustion of fuels materials i.e. pyrolytic sources. Analysis of diagnostic molecular markers suggest that the most likely sources of these compounds were of mixed origin, namely runoffs and atmospheric deposition from Chukai Township and also maritime sources such as discharges from ships and fishing boats. From the toxicity point of view, present levels of PAHs in these sediments are not likely to cause any serious biological effects. However, it must be conceded that a follow up study is timely and very needed since the results discussed in the present article is based on sampling carried out in year 2001. In future, organic carbon analysis should be conducted for the sediments. Normalization of PAHs to organic carbon is necessary since it is an important controlling factor of sorption of PAHs on sediment [18].

ACKNOWLEDGEMENT

The authors would like to acknowledge the support of the Department of Chemical Sciences, Universiti Malaysia Terengganu for the grant to final year undergraduate student (HM) and Malaysian Nuclear Agency-Reactor Interest Group (MNA-RIG) for partial funding of this project. Technical support from the analytical laboratory of FST and help in sampling from the Mangrove Research Unit (MARU) of Institute of Oceanography (INOS) UMT are also acknowledged.

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