and polycyclic aromatic hydrocarbons (PAHs)

INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 3, No 1, 2012 © Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article

ISSN 0976 – 4402

Distribution and sources of aliphatic hydrocarbons (AHCs) and polycyclic aromatic hydrocarbons (PAHs) within the vicinity of a hot mix asphalt (HMA) plant in Port Harcourt, Nigeria Osuji L.C, Ilechukwu I.P, Onyema M.O Department of Pure and Industrial Chemistry, University of Port Harcourt, PMB 5323, Choba, Port Harcourt, Nigeria [email protected] doi:10.6088/ijes.2012030131068 ABSTRACT This study investigates the distribution and sources of aliphatic hydrocarbons (AHCs) and polycyclic aromatic hydrocarbons (PAHs) within the vicinity of a hot mix asphalt (HMA) plant in Port Harcourt, Nigeria. Five samples were collected at both surface (0-15cm) and subsurface (15-30cm) depths at an increasing distance from the HMA plant and the control samples for both levels were collected at 500m away from the HMA plant. The concentration of the aliphatic hydrocarbons varied from 16.74 to 2025.20mg/kg and 25.69 to 818.82mg/kg for surface and subsurface levels respectively while the polycyclic aromatic hydrocarbons ranged from 117.67 to 2226.44mg/kg and 414.40 to 1588.89mg/kg for surface and subsurface samples respectively. The diagnostic isomer ratios showed that the petroleum hydrocarbons were mainly of pyrogenic origin. Keywords: Hot mix asphalt, Polycyclic Aromatic Hydrocarbons, Diagnostic isomer ratio, Soil. 1. Introduction The growing demand for road construction and expansion in Nigeria has led to a huge increase in asphalt production. Asphalt is a primary road building material produced as hot mix asphalt (HMA) in a facility known as hot mix asphalt plant. This is an assemblage of mechanical equipment where aggregates (inert mineral materials such as sand, gravel, crushed stones, slag, rock dust or powder) are blended, heated, dried and mixed with bitumen. Hot mix Asphalt (HMA) plants are numerous in Nigeria since every major road project has a HMA plant installed for asphalt production or centrally located to feed more than one road project or as a commercial asphalt production plant (Rilwani and Agbanure, 2010). Bitumen, a dark brown to black cement-like semisolid or solid or viscous liquid, plays a significant role in asphalt production as the cement that binds the aggregates. It is produced by the nondestructive distillation of crude oil during petroleum refining and consists of aliphatic and cyclic alkanes, aromatic hydrocarbons, heterocyclic compounds containing oxygen, nitrogen and sulphur as well as metals like nickel, vanadium and iron (NIOSH, 2001). It liquefies when heated and may occur naturally in solid or semi-solid form or be obtained by petroleum refining. In HMA plants, petroleum hydrocarbons ranging from light products like diesel to heavy residues such as bitumen are used in production process. These petroleum products have variable composition and distribution of hydrocarbons that differ from each other (Wang et al., 2004). Ziegler et al (2008) had opined that source determination of these hydrocarbons makes it possible to identify, differentiate, determine risk to health and environment, direct remedial efforts and assign blame where necessary. Aliphatic Received on June 2012 Published on July 2012

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Distribution and sources of aliphatic hydrocarbons (AHC) and polycyclic aromatic hydrocarbons (PAH) within the vicinity of a hot mix asphalt (HMA) plant in Port Harcourt, Nigeria

hydrocarbons (AHCs) and polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants found in air, water and soil especially in Niger Delta area of Nigeria due to oil exploration and production activities (Anyakora et al., 2005). Some PAHs especially Benzo(a)pyrene has been identified as having high carcinogenic and mutagenic potential for human health (CCME, 2008). These hydrocarbons originate mainly from incomplete combustion or thermal treatment of biomass and fossil fuels (pyrogenic sources). They also enter the environment as a result of oil spills, refineries and offshore drilling sites otherwise known as petrogenic sources. Hot mix asphalt (HMA) plants are comprised of some basic environmental pollution sources. These are dryer, burner blower, exhaust fan, dust collection system, asphalt cement heating (bitumen heating), storage and reclaimed asphalt paving components (Rilwani and Agbanure, 2010). Other activities like asphalt loading, use and disposal of used engine oil as well as vehicle and plant maintenance may also contribute to environmental pollution sources within the HMA plant vicinity. Because of asphalt production activities in HMA plants and the nature of materials used in asphalt production especially bitumen, issues such as water quality degradation, eco-toxicity and occupational health are considered during asphalt production. As part of a study to determine the nature and contribution of hydrocarbons to the soil by HMA plants, this study seeks to characterise and determine the sources of hydrocarbons found in soil within the vicinity of an asphalt production plant in Port Harcourt. It is expected that distinguishing the different hydrocarbon sources will assist in implementing actions for pollution prevention and abatement. 2. Materials and method 2.1 Study site This study was conducted around the premises of an asphalt production plant located at Elelenwo area of Port Harcourt, Rivers state, Nigeria. The asphalt plant is between latitude (4051’9.32N and 4051’8.99) N and longitudes (70.05’7.7 and 70.05’2.69) E. It is located in a sparsely residential area is surrounded by farmlands. 2.2 Sampling Soil samples were collected at surface (0-15cm) and subsurface (15-30cm) depths from the vicinity of the HMA plant. Control samples were collected at 500m away from the plant at a site unlikely to be affected by the activities of the HMA plant. Five samples were collected cross sectionally within the asphalt plant vicinity and thoroughly mixed to form a homogenous composite sample. This was repeated at five successive points of increasing distance (10m, 20m, 30m, 40m and 50m) from the HMA plant for both surface and subsurface samples respectively. The soil samples were transferred into an aluminum foil bags, labeled accordingly and then taken to the laboratory. 2.3 Sample extraction and chromatographic analysis Five grams (5g) of homogenized soil samples were accurately weighed into clean dry volumetric flask and extracted with 10ml of dichloromethane after vigorous shaking. This was further repeated twice for each sample and the combined extract was filtered to remove impurities. The sample extract was concentrated to 1ml using a rotary evaporator. The extracts were further purified in a silica gel column topped with anhydrous sodium sulphate Osuji L.C, Ilechukwu I.P, Onyema M.O International Journal of Environmental Sciences Volume 3 No.1, 2012

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for water removal and eluted with 20ml hexane to obtain the aliphatic fraction and 30ml of hexane/dichloromethane (1:1 v/v) to obtain the PAHs fraction. Both fractions were concentrated to 1ml with a rotary evaporator. 1µl of the sample extract was analysed with Agilent 6890 gas chromatography equipped with a flame ionization detector (FID) with the following operational conditions; flow rate (H2 40ml/min, air 450ml/min, N2 30ml/min); injection temperature (initial 600C, final 3250C); detector temperature (3500C).The chromatograms were quantified with respect to the internal standards. 3. Result and discussion 3.1 Aliphatic hydrocarbons (AHCs) The gas chromatographic analysis of the aliphatic fractions of surface and subsurface samples from the HMA plant vicinity is presented in table 1 and 2 respectively. Aliphatic hydrocarbons ranging from C11-C38 were detected in the samples and the total concentration ranged between 16.74 to 2025.20mg/kg and 25.69 to 818.82mg/kg for surface and subsurface levels respectively while concentration of the control samples were 24.59mg/kg and 14.84mg/kg for both surface and subsurface levels respectively. Table 1: Aliphatics fractions in surface soil samples from the asphalt plant Aliphatics(mg/kg)

S1

S2

S3

S4

S5

CS

C9

ND

ND

ND

ND

ND

ND

C10

ND

ND

ND

ND

ND

ND

C11

ND

ND

ND

ND

ND

ND

C12

6.82

ND

ND

ND

ND

ND

C13

28.21

ND

ND

ND

ND

ND

C14

57.23

7.20

ND

ND

ND

ND

C15

53.82

4.58

ND

ND

ND

ND

C16

169.41

13.24

ND

ND

ND

ND

C17

112.79

7.63

6.45

ND

ND

ND

Pristane

66.25

7.04

ND

ND

ND

ND

C18

108.72

30.90

6.97

ND

ND

ND

Phytane

71.54

12.18

ND

ND

ND

ND

C19

54.13

37.92

3.60

ND

ND

ND

C20

131.79

13.41

3.93

ND

2.79

ND

C21

94.29

8.02

2.00

2.13

2.78

ND

C22

22.98

9.6

2.61

ND

ND

ND

C23

16.83

4.54

3.26

ND

ND

ND

C24

49.26

3.92

3.87

ND

4.71

ND

C25

45.46

4.99

4.39

ND

ND

ND

C26

14.79

7.61

5.84

ND

ND

ND

C27

74.10

8.32

7.72

ND

6.46

ND

C28

79.02

9.87

10.12

6.43

ND

6.62

Osuji L.C, Ilechukwu I.P, Onyema M.O International Journal of Environmental Sciences Volume 3 No.1, 2012

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Distribution and sources of aliphatic hydrocarbons (AHC) and polycyclic aromatic hydrocarbons (PAH) within the vicinity of a hot mix asphalt (HMA) plant in Port Harcourt, Nigeria C29

64.20

9.53

8.56

6.85

ND

6.89

C30

153.98

31.93

21.84

7.17

ND

ND

C31

136.29

10.49

9.81

ND

ND

ND

C32

65.22

6.55

7.83

ND

ND

ND

C33

148.62

6.91

6.98

ND

ND

ND

C34

46.70

7.00

ND

ND

ND

ND

C35

26.30

9.83

15.94

9.76

ND

11.08

C36

26.27

8.33

ND

ND

ND

ND

C37

42.45

9.67

ND

ND

ND

ND

C38

57.73

11.42

ND

ND

ND

ND

TOTAL

2025.20

302.63

131.72

32.34

16.74

24.59

S; surface sample, CS; control surface, ND: Not detected Table 2: Aliphatics fractions in subsurface soil samples from the asphalt plant Aliphatics(mg/kg)

SS1

SS2

SS3

SS4

SS5

CSS

C9

ND

ND

ND

ND

ND

ND

C10

ND

ND

ND

ND

ND

ND

C11

8.67

ND

ND

ND

ND

ND

C12

7.99

ND

ND

ND

ND

ND

C13

20.40

ND

ND

ND

ND

ND

C14

51.86

ND

ND

ND

ND

ND

C15

39.59

ND

ND

ND

ND

ND

C16

121.46

ND

ND

ND

ND

ND

C17

76.73

8.99

ND

ND

ND

ND

Pristane

44.10

6.99

ND

ND

ND

ND

C18

40.37

8.72

ND

ND

ND

ND

Phytane

11.65

1.40

ND

ND

ND

ND

C19

46.81

6.29

ND

ND

ND

ND

C20

15.45

5.36

2.78

2.94

2.80

ND

C21

47.37

2.36

2.77

2.92

2.71

ND

C22

17.48

2.73

ND

ND

ND

ND

C23

7.50

3.34

ND

ND

ND

ND

C24

16.32

ND

4.66

4.73

ND

ND

C25

12.90

4.25

ND

ND

ND

ND

C26

21.49

4.71

ND

ND

ND

ND

C27

25.54

5.38

6.73

7.04

6.86

ND

C28

13.57

5.90

ND

ND

ND

ND

C29

19.84

7.49

ND

5.10

ND

ND

C30

42.63

12.03

ND

ND

ND

ND


700

Distribution and sources of aliphatic hydrocarbons (AHC) and polycyclic aromatic hydrocarbons (PAH) within the vicinity of a hot mix asphalt (HMA) plant in Port Harcourt, Nigeria C31

25.08

7.75

ND

ND

ND

ND

C32

7.85

ND

ND

ND

ND

ND

C33

12.68

ND

ND

ND

ND

ND

C34

8.48

ND

ND

ND

ND

ND

C35

17.76

10.63

10.97

11.35

13.32

14.84

C36

8.52

ND

ND

ND

ND

ND

C37

17.73

ND

ND

ND

ND

ND

C38

11.00

ND

ND

ND

ND

ND

TOTAL

818.82

104.32

27.91

34.08

25.69

14.84

SS; subsurface sample, CSS: control subsurface. The n-alkane (aliphatics) distribution of petroleum hydrocarbons can be used to indicate the source using isoprenoid ratios, carbon preference index (CPI) and Cmax as shown in table 3. The pristane/phytane (pr/ph) ratios in the samples were within the range of 0.58-0.93 and 3.79-4.99 for surface and subsurface samples respectively. This ratio has been a useful source index of hydrocarbon’s depositional environment (Udoetok and Osuji, 2008). High pristane/phytane ratio obtained for subsurface (SS1 and SS2) samples indicates an oxic depositional environment while low pristane/phytane ratio obtained for the surface samples (S1 and S2) indicate an anoxic depositional environment. Wagener et al (2010) had also opined that ratios involving isoprenoids below or close to unit indicates oil contamination. The isoprenoid ratios for the surface samples (S1 and S2) are below unity which indicates oil input while the high pristane/phytane ratio obtained for the subsurface samples (SS1 and SS2) largely suggests a mixture of oil and terrestrial plant material. This may be attributed to the decayed vegetation on the subsurface since the asphalt plant is sited on a land previously used for farming. Isoprenoids (pristane and phytane) was not detected in samples from distances beyond 30m from the HMA plant. This may be due to weathering since the nalkanes and in some cases the isoprenoids may be completely lost in heavily weathered oils (Osuji et al., 2006). Carbon preference index (CPI) of n-alkanes is useful for determining the degree of biogenic versus petrogenic input. It expresses the ratio of odd-carbon-numbered to even-carbonnumbered n-alkanes in a given sample and has been used to differentiate biogenic n-alkanes from those of petrogenic or anthropogenic input. CPI values > 2 indicate predominant biogenic sources such as epicuticular waxes of terrestrial plants, and CPI values of near unity signify the source as fossil fuels or incomplete combustion of petroleum products (Simoneit, 1984). The n-alkane distribution of the samples between C24-C34 shows that the CPI values for surface and subsurface samples were mostly below or close to unity signifying fossil fuel input except for subsurface sample SS4 that has a value of 2.57 (>2) that indicates a terrestrial plant input (table 3). The obtained CPI values indicate that inputs from anthropogenic activities are more significant than the input from terrestrial plants (Kavouras et al, 1999; Didyk et al, 2000). The maximum n-alkane concentration (Cmax) varied from C16-C35 among the samples. This is due to variations in the relative contribution of anthropogenic and biogenic sources. While the samples closer to the asphalt plant peaked at C16 and C19, indicating petrogenic input, the farther samples peaked at C35 except for S5 that peaked at C27 (table 3). While this may be attributed to easy degradation of short chain aliphatics compared to long chains, it also Osuji L.C, Ilechukwu I.P, Onyema M.O International Journal of Environmental Sciences Volume 3 No.1, 2012

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suggests biological input because high concentration of n-alkanes with odd number peaks is an indication of a significant biological input (Young and Wang, 2002). The total n-alkanes as shown in table 3 for each sampling point at both surface and subsurface levels indicates that the n-alkanes concentration decreases as the distance from the asphalt plants increases. This probably indicates that anthropogenic sources more than any other sources contributed significantly to the hydrocarbons within the asphalt plant vicinity. Table 3: Variations in source diagnostic ratios of AHCs with distance from the HMA plant Stations S1 SS1 S2 SS2 S3 SS3 S4 SS4 S5 SS5 CS CSS

Pr/Phy 0.93 3.79 0.58 4.99 ND ND ND ND ND ND ND ND

CPI24-34 1.22 0.91 0.64 1.10 0.76 1.44 0.50 2.57 1.37 0.00 1.04 0.00

Cmax C16 C16 C19 C30 C30 C35 C35 C35 C27 C35 C35 C35

Distance(M) 10 10 20 20 30 30 40 40 50 50 500 500

∑n-alkanes 2025.20 818.82 302.63 104.32 131.72 27.91 32.34 34.08 16.74 25.69 24.59 14.84

ND: cannot be detected because of zero value of denominator and/or nominator Polyaromatic Hydrocarbons (PAHs) The concentration of each polyaromatic hydrocarbon at both surface and subsurface depth for each sampling station is shown in figure 1 and 2 respectively while the total concentration of PAH within the asphalt plant vicinity is shown in table 4. The total concentration expressed as ∑16PAH ranged from 117.67-2226.44mg/kg and 414.40-1588.89mg/kg for surface and subsurface samples respectively. There is also a decrease in ∑PAH as the distance from the asphalt plant increases giving an indication that the asphalt plant may be the primary source of hydrocarbons within the vicinity.

Figure 1: Polyaromatic hydrocarbons (PAHs) concentration of the surface samples


702


Figure 2: Polyaromatic hydrocarbons (PAHs) concentration of the subsurface samples The ratio of the low molecular weight PAHs (3-2 rings PAHs) to high molecular weight PAHs (4-6rings) PAHs (LMW/HMW) shows predominance of high molecular weight PAHs (4-6 rings PAHs) for both surface and subsurface samples suggesting that pyrogenic origin may be the most significant source of PAHs within the hot mix asphalt (HMA) plant vicinity. Petrogenic PAHs are related to non-combusted petroleum products with higher 2-3 rings PAHs concentration while higher molecular weight PAHs (4-6 rings) are typical pyrogenic products derived mainly from fossil fuel combustion (Yang et al: 2011). Table 4: Total concentration and PAHs diagnostic ratios for source determination Stations

∑PAHs (mg/kg)

S1

2226.44

0.47

0.42

0.51

0.43

SS1

1588.89

0.43

0.67

0.57

0.43

S2

1377.05

0.65

0.49

0.56

0.21

SS2

414.40

1.00

1.00

ND

0.17

S3

117.67

ND

ND

ND

ND

SS3

ND

ND

ND

ND

ND

S4

ND

ND

ND

ND

ND

SS4

ND

ND

ND

ND

ND

S5

ND

ND

ND

ND

ND

An/(An+Ph)

Fl/(Fl+Py)

B[a]An/(B[a]An+Chry)


LMW/HMW

703


SS5

ND

ND

ND

ND

ND

CS

ND

ND

ND

ND

ND

CSS

ND

ND

ND

ND

ND

0.35

>1.00 0.5 is for biomass and coal combustion (Yunker et al, 2002). These diagnostic ratios indicate that much of the petroleum hydrocarbons found within the vicinity of the HMA may have been from pyrogenic sources. This may be as a result of pyrolysis, a major process in hot mix asphalt production where aggregates and bitumen are heated at 1500C (Mamlouk and Zaniewski, 2011). 4. Conclusion This study shows that soil within the vicinity of the HMA was contaminated by petroleum hydrocarbons as compared with the control stations. The impact of this contamination was not significant beyond the immediate vicinity of the HMA. The diagnostic isomer ratios show that the petroleum hydrocarbons were mostly of pyrogenic origin. There is the need to closely monitor the HMA production process as well as the surrounding environment for possible clean-up and to prevent further contamination. 5. References 1. Anyakora C, Ogbeche A, Palmer P, Coker H, Ukpo G, Ogah C (2005), GC/MS analysis of polycyclic aromatic hydrocarbons in sediment samples from the Niger Delta Region, Chemosphere, 60, pp 990-997. 2. Didyk B.M, Simmoneit B.R.T, Pezoa L.A, Riveros M.L, Flores A.A (2000), Urban aerosol particles of Santiago, Chile: content and molecular characterization, Atmospheric Environment, 34, pp 1167-1179. 3. Canadian Council of Ministers of the Environment (CCME), (2008), Canadian Soil Quality Guidelines for Carcinogenic and Other Polycyclic Aromatic Hydrocarbons (Environmental and Human Health Effects), Scientific Supporting Document. pp 218.


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