- Email: [email protected]
Source analysis for polycyclic aromatic hydrocarbon in road dust and urban runoff using marker compounds
Desalination 226 (2008) 151–159
Source analysis for polycyclic aromatic hydrocarbon in road dust and urban runoff using marker compounds T. Kose*, T. Yamamoto, A. Anegawa, S. Mohri, Y. Ono Graduate School of Environmental Studies, Okayama University, 3-1-1, Tsushima-naka, Okayama City, Okayama, Japan Tel./Fax +81 86 251 8149; email: [email protected] Received 17 January 2007; revised accepted 30 January 2007
Abstract Automobile traffic, although extremely important to human society has imparted a growing environmental impact through automobile emissions. For that reason, some studies have investigated origin specification methods for polycyclic aromatic hydrocarbons (PAHs) in road dust and sediments. Typically, PAHs biodegrade and photodegrade easily in environmental conditions. As marker compounds to specify their origin, biologically and photochemically stable compounds are desirable. In this study, we attempt to examine whether tri-terpanes are useful for origin specification of PAH pollution in road dust using an estimation method to determine the pollution source of PAH. This method might clarify pollution sources that cannot be clarified using conventional methods based on the PAH profiles. These results shown that the most important source of PAH in road dust seems to be tires while road pavement asphalt was a second pollution source. Furthermore, results clarified that the source of PAHs in runoff water is road dust. Keywords: Polycyclic aromatic hydrocarbon; Tri-terpane; Road dust; Runoff water; Pollution source
1. Introduction Automobile traffic is an extremely important system in modern human society. The environmental impact of automobile emissions is also increased. Particularly, road dust and aerosols might be injected into surface water during a rainy period [1,2]. The major contaminants within road *Corresponding author.
dust are polycyclic aromatic hydrocarbons (PAHs) and heavy metals. Some PAHs are known to be carcinogens and mutagens [3]. Furthermore, some PAHs have been identified in recent studies as endocrine disruptors. Reportedly, the half-life of PAHs in a water environment is several days to several months [4,5]. The persistence of PAHs is lower than that of heavy metals and dioxin. However, it is known that the degradation rate of PAHs adsorbed into sediments or soil will be
Presented at the 10 th IWA International Specialized Conference on Diffuse Pollution and Sustainable Basin Management, Istanbul, Turkey, 18–22 September 2006. 0011-9164/06/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.01.239
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slowed considerably [6]. Therefore, if PAHs in road dust cause sedimentary contamination, their environmental impact might persist for a long time. For that reason, some studies have reported an origin specification method for PAHs in road dust and sediments. Some studies have reported the major origin of PAHs in road dust as exhaust gases from automobiles [7]. Other studies have emphasized the contribution of dust from wear of road pavement material or asphalt [8]. Moreover, reports indicate that dust from tire rubber or lubricant oil is a major source of PAHs in road dust [9]. In those studies, the major source of PAHs in road dust is not necessarily consistent because the composition of the PAHs themselves was used for origin specification. As mentioned above, PAHs easily biodegrade or photodegrade in the environment [10]. The degradability of each PAH is quite different. Consequently, the PAH profiles used for specification of the origin might not be stable under environmental conditions. As marker compounds used for origin specification, biologically and photochemically stable compounds are desirable. Tri-terpanes are marker compounds used for origin specification of petroleum and its products [11]. Tri-terpanes are substances contained in crude oil; they have high stability to biodegradation and photochemical reactions. Some studies have used tri-terpanes as marker compounds to identify origins of marine and sediment contamination by oil spills [8,12]. Moreover, some studies have reported that hopanes (five-ring tri-terpanes) are useful to identify the origin of contamination by automobile traffic emissions [13,14]. However, the pollution origins of road dust are different. It remains unclear whether all sources of pollution contain tri-terpanes; few research examples of origin specification use this method. For those reasons, objectives of this study are to examine whether tri-terpanes might be used for origin specification of PAHs. Furthermore, specification of the main sources of PAHs in road dust in Okayama city was attempted.
2. Materials and methods 2.1. Sample collection procedure Five pollution sample types were collected and analyzed: road dust, runoff water, atmospheric deposit dust, atmospheric particle phase, and atmospheric gas. Road dust samples were collected at the sampling site shown in Fig. 1 (circles). Three sampling sites were selected along route 2 (the most heavily traveled route in Okayama city, around 100,000 vehicles per day). Two sampling sites were selected along route 53 (next heaviest traveled to route 2, around 30,000 vehicles per day). One sampling site was selected along prefect route 72 (8000 vehicles per day) [15]. Road dust samples were collected using a vacuum cleaner within a 1 m × 1 m frame. Sampling procedures were repeated three times per point and the position of the collection frame was kept from overlapping. Sampling was performed in June and November 2005 a day of sunny weather that was preceded by one week of sunny weather. Collected road dust was sieved using a 2.0 mm stainless sieve to remove large materials and to homogenize the sample. Runoff water was collected at the sampling site shown in Fig. 1 (square symbols) using an automatic water sampler (LYSAM-P; NKS Co. Ltd., Japan). Sampling was performed during a rain of more than 10 mm after sunny weather had continued for one week or more. Suspended solids of high concentration were contained in the sample collected within 10 min after a rain started. Therefore, water samples collected within 30 min after rain started were homogenized and analyzed. Atmospheric samples (deposit dust, particle phase and gaseous phase) were collected at the sampling site shown in Fig. 1 (diamonds). Deposit dust samples were collected using a pail can with milli-Q water to capture deposited dust. The pail can was filled with 5 L of milli-Q water and placed at a sampling site for five days. Subsequently,
T. Kose et al. / Desalination 226 (2008) 151–159
Okayama city
Road dust sampling point
53 0 km
153
3 km
Atmospheric sampling point Runoff water sampling point
2
Fig. 1. Sampling sites in Okayama city.
the pail can was brought back to the laboratory and sample water was collected with deposited dust. Particle phase and gaseous phase samples were collected using a handy air pump (MP-Σ500; Shibata, Japan) equipped with a glass fiber filter (GC50; Advantec Toyo Kaisha Ltd., Japan) and a soil phase extraction disk (Empore disk SDB-XD; GL Sciences Inc., Japan). Particle phase materials were collected onto a glass fiber filter; gaseous phase components were collected onto a solid phase extraction disk. Five types of pollution source samples were analyzed: tires, road pavement asphalt, automotive fuel oil, automotive lubricant oil, and exhaust gases. Four types of new tires and four types of used tires were selected for analyses. Tire rubber was sliced from the tread surface and cut fine to 2 mm or smaller pieces. Three types of straight asphalt (paving materials) and two types of asphalt pavement (collected directly from the
road) were selected for analyses. Asphalt pavement samples were crushed finely to 2 mm or smaller pieces for extraction. Straight asphalt samples were dissolved directly into toluene. Eight types of automobile fuel were selected for analyses: two types of high octane gasoline, three types of regular gasoline, and three types of diesel. Fuel oils were concentrated using a rotary evaporator. However, diesel samples were difficult to concentrate because of diesel oil’s high vapor temperature. Therefore, results of five samples, without diesel, are presented in this study. We selected 14 automotive lubricant oils for analyses: 10 types of four-cycle engine oil, 3 types of diesel engine oil, and 1 type of two-cycle engine oil. 2.2. Sample pretreatment procedure Solid phase samples were extracted using an accelerated solvent extractor (ASE-200; Dionex Corp., California, USA): road dust, suspended
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solid from runoff water, deposit dust, atmospheric particle, tire rubber, and road pavement asphalt. Toluene/methanol: (50/50) mixed solvent was used for extraction. Extraction was carried out three times at 150°C, 2000 psi; the amount of solvent exchange for every cycle was 70%. Extract was concentrated using a rotary evaporator and separated for hydrocarbon (PAHs and tri-terpanes) analyses. The extract for hydrocarbon analysis was cleaned (Silica Sep-Pak Plus; Waters, USA). Solvents of the extract were exchanged to 1 mL of n-hexane and loaded onto a conditioned Sep-Pak cartridge. The saturated hydrocarbon fraction involving tri-terpanes was eluted with 5 mL of n-hexane. Aromatic hydrocarbon fractions involving PAHs were eluted with 5 mL of n-hexane/ dichloromethane: (70/30). Both elutes were dried under a nitrogen gas stream and re-dissolved into 1 mL of n-hexane involving internal injection standard compounds. Water phase samples, which comprised the dissolved phase of runoff water and milli-Q water for deposit dust collection, were separated by filtration using a GC50 glass fiber filter. Suspended solids were analyzed according to the procedure mentioned above. Water phase was concentrated using a conditioned Empore disk C18 for hydrocarbon analyses. After concentration, the disk was dried and an extract was eluted using 5 mL of dichloromethane. The eluate was concentrated to less than 1 mL under a nitrogen gas stream. The PAHs and triterpanes in the sample solution were separated and cleaned using the same procedure as that used for a solid sample using Sep-Pak Plus silica. To another portion of the water sample, 1 M NaOH aq. was added and pH was controlled to more than 10. A water sample was extracted using a conditioned Empore disk C18. The extract was eluted using 5 mL of dichloromethane. The eluate was concentrated to less than 1 mL under a nitrogen gas stream and cleaned using the same procedure as that used for the solid sample.
2.3. GC–MS analysis procedure The concentrations of PAHs, tri-terpanes and NCBA were determined using a quadropole mass spectrometer (QP-2000; Shimadzu Corp.) with an integrated GC-20A gas chromatograph equipped with a DB-5 capillary column (30 m fused silica 0.25 mm i.d. and 0.25 μm film thickness; J&W Scientific Inc.). The GC–MS operating conditions are summarized below. The ion source was maintained at 1.3 kV ionization potential at 280°C. For PAH analysis, the injection port was maintained at 280°C and the sample was injected with a splitless mode followed by a 1-min purge after the injection. The column temperature was maintained at 50°C for 2 min, then programmed to increase by 7°C min−1 to 310°C, and held for 10 min. Selected ion monitoring (SIM) was employed after a delay of 4 min. The PAHs were determined at a mass per charge ratio (m/z) of their molecular weight. Response factors of PAHs were defined using standard compounds (Aldrich Chemical Co., USA). For tri-terpane analyses, the injection port was maintained at 300°C and the sample was injected using splitless mode followed by a 1-min purge after injection. The column temperature was maintained at 50°C for 2 min and increased at 6°C min−1 to 300°C, then maintained for 15 min. Tri-terpanes were determined by m/z of 191. 3. Results and discussion Fig. 2 shows profiles of PAHs in runoff water and road dust. Horizontal axes denote ratios of concentrations of fluoranthene/pyrene. The vertical axis denotes that of phenanthrene/anthracene. Recent studies have shown that profiles of PAHs shown in Fig. 2 indicate the pollution sources of PAHs. The upper left area (Phen/Anth>10, Fluo/ Pyr<1) represents petrogenic pollution and the lower right area (Phen/Anth<10, Fluo/Pyr>1) includes pyrogenic pollution [16,17]. Profiles of runoff water were almost identical to those of road dust. The same correspondence
T. Kose et al. / Desalination 226 (2008) 151–159 50
road dust Road dust
Road road dust Runoff runoff water water
40
20 10 0
0
1
2
3 4 Fluo/Pyr (–)
5
tire Tire
Road pavement asphalt
lubricantoiloil Lubricant
fuel Fuel oil oil
atmospheric Atmospheric atmospheric Atmospheric gaseous phase gaseous phase particlephase phase particle automobile Automobile exhaust exhaust automobile exhaust Automobile exhaust particle particlephase phase gaseous phase gaseous phase atmospheric Atmospheric deposit deposit dustdust
40 30
Phen/Anth (–)
Phen/Anth (–)
50
155
30
20
6
10
Fig. 2. Profiles of PAHs in road dust and runoff water. 0 0
was apparent in the profiles of tri-terpanes in road dust and runoff water (Fig. 3). These results show clearly that road dust is the major source of the PAHs in runoff water. Profiles of road dust and runoff water were distributed at the halfway point of the petrogenic area and pyrogenic area. This result shows that the road dust was probably a mixture of pollution from petrogenic and pyrogenic sources. We next try to investigate the source of PAHs in road dust. Fig. 4 shows profiles of PAHs in road dust, various pollution source samples, and reference samples. Five source samples were selected: tire, road pavement asphalt, automobile fuel, lubricant oil, and exhaust gas. Data for two
3.5 Road dust Runoff water
Σ CC31~35/C30 (–)
3.0 2.5 2.0 1.5 1.0 0.5 0
0
0.5
1.0
1.5 2.0 C29/C30 (–)
2.5
3.0
Fig. 3. Profiles of tri-terpanes in road dust and runoff water.
1
2
3 4 Fluo/Pyr (–)
5
6
Fig. 4. Profiles of PAHs in road dust, pollution sources and reference samples.
reference samples, an atmospheric sample (separated into particle phase and gaseous phase) and atmospheric deposit dust, are also depicted in Fig. 4. The ratios of Phen/Anth and Fluo/Pyr in road dust (star symbol) were 6–14 and from 0.6–0.9, respectively. On the other hand, distribution of PAHs profiles in source samples was greater than that in road dust. For example, the respective values of Phen/Anth and Fluo/Pyr in asphalt for road pavement (open triangles) were 3–18 and 0.3–1.0. This wide range of values is probably attributable to high degradability of PAHs. Junfeng et al. [10] reported degradability of PAHs under natural sunlight by photo-oxidation. The half life of phenanthrene was reported as 72 h, but that of anthracene was only 26 h. The respective half-lives of fluoranthene and pyrene were reported as 26 h and 37 h. This result implies that the ratio of Phen/Anth and Fluo/Pyre might change according to the progress of photooxidation. The total concentration of PAHs in pavement samples collected from the road (21 ng/g, N = 3) was lower than that in fresh asphalt samples (5900 ng/g, N = 3). The total concentration of PAHs in used tires (128 ± 57 μg/g, N = 4) was
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also lower than that in new tires (58 ± 35 μg/g, N = 4). Lower total concentration of PAHs in used tires and pavement samples collected from the road also suggested the influence of photodegradation. Furthermore, profiles of PAHs in tires (closed triangles), asphalt for road pavement and lubricant oil (closed diamonds) showed a distribution that resembled that of road dust. Therefore, major pollution sources of PAHs in road dust might be difficult to investigate from these results. Stability of marker compounds is important for pollution source analysis. Fig. 5 shows profiles of tri-terpanes in road dust, various pollution source samples, and reference samples. Horizontal axes denote the ratio of the concentration of 29-carbon-number norhopanes per 30-carbon-number hopanes (C29/C30). The vertical axis shows ratios of the concentration of homohopanes with 31–35 carbon number per hopane with 30 carbon number (ΣC31–35/C30). Profiles of tri-terpanes in road dust resembled those in tires, implying that tires are a major pollution source of road dust. Other pollution source samples were apparent: profiles of fuel oil were shifted to the lower right side, but there was also a portion that overlapped slightly with that of road dust, which showed the tendency for 3.5
Road dust Tire Road pavement asphalt
3.0
Lubricant oil
2.5 ΣC31–C35/C30
Fuel oil Atmospheric particle phase Automobile exhaust particle phase Atmospheric deposit dust
2.0 1.5 1.0 0.5 0
0
0.5
1.0
1.5 2.0 C29/C30
2.5
3.0
Fig. 5. Profiles of tri-terpanes in road dust pollution sources and reference.
wide range profiles. Profiles of fuel oil also agreed to some degree with profiles of automobile exhaust emission. However, profiles of automobile exhaust emissions were shifted further to the lower left side, and the influence of engine oil was apparent from the profile. This result implies that fuel oil and lubricant oil were emitted into automobile exhaust gas. However, contributions of automobile exhaust gases to road dust were not great. Zakaria et al. [12] reported that lubricant oil is an important pollution source of contamination of sediments in Malaysia. This result is contradictory to the result of this paper. It might be attributable to differences of habits and materials used in that country. Zakaria et al. also reported that lubricant oil sometimes leaks from automobiles and that large amounts of waste oil are typically abandoned in Malaysia [12]. Such leakage of lubricant oil is not so common in Japan. Although the profiles of tri-terpanes in asphalt for road pavement differed from that in road dust, the possibility that not only tires, but asphalt for road pavement was major source of road dust was suggested from this result. The profile of tri-terpanes in atmospheric dust deposits was almost identical to that in road dust because road dust might spread to the atmosphere and be re-deposited near the road. These results indicate that tires, fuel oil and asphalt for road pavement are major sources of road dust. It is difficult to evaluate the contribution of the three sources only from the result shown in Fig. 5, which shows the ratios of concentration of several tri-terpanes. Therefore, we attempted to determine the importance of pollution sources based on the composition of all analyzed tri-terpanes using cluster analysis. The analysis was performed with Ward’s method using Euclidean distance, as calculated by computer software (STATISTICA6.0; StatSoft Inc., USA). Tree diagrams from analytical results are shown in Fig. 6. The letters denote the type of sample and the numbers denote the sample lot: R denotes a road dust sample; T denotes a tire
T. Kose et al. / Desalination 226 (2008) 151–159 20 At3 R7,R8,R9,R10 R11,R12 Pv1,Pv2
II
R1,R2,R3,R4,R5,R6 T1,T2,T3,T4,T5,T6 T7,T8 E3
III
At1,At2 Pt5,Pt6,Pt7,Pt13 Pt14,Pt15,Pt16 Pt17,Pt18,Pt20 E2 Pt1,Pt2,Pt3,Pt4 Pt8,Pt9,Pt10,Pt11 Pt12,Pt19 E1,E5 E4,E6
40
60
I
G1
IV V
G2 VI
VII
Fig. 6. Tree diagrams based on cluster analysis.
sample; E denotes an automobile exhaust sample; Pt denotes an atmospheric particle sample; and Pv and At respectively denote pavement samples collected from the road and fresh asphalt samples. All road dust samples are distributed in clusters II and III. Cluster III also contained all tire samples. This result shows clearly that tires are a major source of road dust pollution. Asphalt for road pavement samples (Pv and At) is distributed in clusters I, II and IV. Only cluster II was in agreement with road dust. However, clusters I and IV belong to group 1 (G1) and might be related closely with clusters II and III. Therefore, asphalt for road pavement is a major pollution source of road dust that ranks second to tires. Clusters V–VII include automobile exhaust and atmospheric particles belonging to group 2, which
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has low relevance to road dust. Therefore, the importance of automobile exhaust and atmospheric particles as sources of road dust were lower than that of tire and asphalt for road pavement. It was inferred from the similarity of profiles of tri-terpanes and results of cluster analyses that tires were the largest pollution source of road dust, followed by asphalt for road pavement. However, evaluation of this contribution is based on tri-terpane composition, which might not agree with the contribution of PAHs. That is, if the concentration of tri-terpanes in pollution source samples were high, the contribution in road dust would be large, but if the concentration of PAHs in the pollution source were low, the contribution as a pollution source of PAHs would become necessarily less. To clarify this point, the total concentrations of PAHs and tri-terpanes in road dust and pollution source samples and their ratios are shown in Table 1. The total concentration of PAHs in tires and asphalt for road pavement was higher than that of other pollution source samples. Therefore, even if based on the ratio of concentration shown in Table 1, the result that the contribution of tires and asphalt for road pavement was large did not change. 4. Conclusions An estimation method to determine the pollution source of PAHs in road dust based on the profiles of tri-terpanes was used in this study.
Table 1 Total concentration of PAHs and tri-terpanes in road dust and pollution source samples
Total PAHs (ng/g) Total tri-terpanes (ng/g) Number of samples PAHs/tri-terpanes (−)
Tire
Road pavement asphalt
Fuel oil
Lubricant oil
Exhaust particle phase
Road dust
93,139 9201 8 10.1
3562 1400 5 2.5
89 41 5 2.2
784 527 14 1.5
1493 402 6 3.7
45,809 21,831 8 2.1
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This method can clarify pollution sources that cannot be clarified using conventional methods based on the PAH profiles. Simultaneously, this study specified the main sources of PAHs in road dust in Okayama city using this technique. Results revealed that the most important source of PAHs in road dust in Okayama city is automobile tires; asphalt for road pavement was the next greatest pollution source. Results also clarified that the source of PAHs in runoff water was road dust. Acknowledgements A part of this study was supported by the Core Research for Environmental Science and Technology (CREST) by Japan Science and Technology Agency. We also thank Fukutaro Uchida and Masatomo Nishihara for their help in sample collection and analyses. References [1]
[2]
[3]
[4]
[5]
J.V. Hunter, T. Sabatino, T. Gompers and M. Mackenzie, Contribution of urban runoff to hydrocarbon pollution, J. Water Pollut. Cont. Fed., 51 (1979) 2129–2138. H. Kumata, Y. Sanada, H. Takada and T. Ueno, Historical Trends of n-cyclohexyl-2-benzothiazolamine, 2-(4-morpholinyl)benzothiazole, and other anthropogenic contaminants in the urban reservoir sediment core, Environ. Sci. Technol., 34 (2000) 246–253. A.H. Neilson, The Handbook of Environmental Chemistry 3 J: PAHs and Related Compounds Biology, Springer-Verlag, Berlin, 1998, pp. 312–350. L.M. Carmichael, R.F. Christman and F.K. Pfaender, Desorption and mineralization kinetics of phenanthrene and chrysene in contaminated soils, Environ. Sci. Technol., 31 (1997) 126–132. J.C. White, J.W. Kelsey, P.B. Hatzinger and M. Alexander, Reduced biodegradability of desorption-resistant factions of polycyclic aromatic hydrocarbons in soil and aquifer solids, Environ. Toxicol. Chem., 15 (1996) 1973–1978.
[6] G. Cornelissen and P.C.M. Van Noort, Rapidly desorbing fraction of PAHs in contaminated sediments as a predictor of the extent bioremediation, Environ. Sci. Technol., 32 (1998) 966–970. [7] H. Takada, T. Onda and T. Ogura, Determination of polycyclic aromatic hydrocarbons in urban street dusts and their source materials by gas chromatography, Environ. Sci. Technol., 24 (1990) 1179–1186. [8] P. Faure, P. Landais, L. Schlepp and R. Michels, Evidence for diffuse contamination of river sediments by road asphalt particles, Environ. Sci. Technol., 34 (2000) 1174–1181. [9] P. Pengchai, N. Nakajima and H. Furumai, PAH profiles of vehicle exhaust, tires and road materials and their contributions to PAHs in road dusts, Environ. Sci., 15 (2002) 433–442. [10] J. Niu, J. Chen, D. Martens, X. Quan, F. Yang, A. Kettrup and K. Schramm, Photolysis of polycyclic aromatic hydrocarbons adsorbed on spruce Picea abies (L.) Karst. needles under sunlight irradiation, Environ. Pollut., 123 (2003) 39–45. [11] A.D. Venosa, M.T. Suidan, K.L. Strohmeier, J.R. Haines, B.L. Eberhart, D. King and E. Holder, Bioremediation of an experimental oil spill on shoreline of Delaware Bay, Environ. Sci. Technol., 30 (1996) 1764–1775. [12] M.P. Zakaria, T. Okuda and H. Takada, Polycyclic aromatic hydrocarbon (PAHs) and hopanes in stranded tar-balls on the coasts of Peninsular Malaysia: applications of biomarkers for identifying sources of oil pollution, Mar. Pollut. Bull., 42 (2001) 1357–1366. [13] M.P. Zakaria, H. Takada, S. Tsutsumi, K. Ohno, J. Yamada, E. Kouno and H. Kumata, Distribution of polycyclic aromatic hydrocarbons in Malaysia: a widespread input of petrogenic PAHs, Environ. Sci. Technol., 36 (2002) 1907–1918. [14] M.P. Zakaria, A. Horinouchi, S. Tsutsumi, H. Takada, S. Tanabe and A. Ismail, Oil pollution in the Straits of Malacca Markers for source identification, Environ. Sci. Technol., 34 (2000) 1189–1196. [15] Road Bureau, Japan Ministry of Land Infrastructure and Transport (1999) Census of Road Transport 1999: General traffic census CD-ROM, Japan Society of Traffic Engineers.
T. Kose et al. / Desalination 226 (2008) 151–159 [16] H.H. Soclo, P.H. Garrigues and M. Ewald, Origin of polycyclic aromatic hydrocarbons (PAHs) in coastal marine sediments: case studies in Cotonou (Benin) and Aquitain (France) areas, Mar. Pollut. Bull., 40 (2000) 387–396.
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[17] M.A. Sicre, J.C. Marty, A. Saliot, X. Aparicio, L. Grimalt and J. Albaiges, Aliphatic and aromatic hydrocarbons in different sized aerosols over the Mediterranean Sea, Atmos. Environ., 21 (1987) 2247–2259.
Source analysis for polycyclic aromatic hydrocarbon in road dust and urban runoff using marker compounds T. Kose*, T. Yamamoto, A. Anegawa, S. Mohri, Y. Ono Graduate School of Environmental Studies, Okayama University, 3-1-1, Tsushima-naka, Okayama City, Okayama, Japan Tel./Fax +81 86 251 8149; email: [email protected] Received 17 January 2007; revised accepted 30 January 2007
Abstract Automobile traffic, although extremely important to human society has imparted a growing environmental impact through automobile emissions. For that reason, some studies have investigated origin specification methods for polycyclic aromatic hydrocarbons (PAHs) in road dust and sediments. Typically, PAHs biodegrade and photodegrade easily in environmental conditions. As marker compounds to specify their origin, biologically and photochemically stable compounds are desirable. In this study, we attempt to examine whether tri-terpanes are useful for origin specification of PAH pollution in road dust using an estimation method to determine the pollution source of PAH. This method might clarify pollution sources that cannot be clarified using conventional methods based on the PAH profiles. These results shown that the most important source of PAH in road dust seems to be tires while road pavement asphalt was a second pollution source. Furthermore, results clarified that the source of PAHs in runoff water is road dust. Keywords: Polycyclic aromatic hydrocarbon; Tri-terpane; Road dust; Runoff water; Pollution source
1. Introduction Automobile traffic is an extremely important system in modern human society. The environmental impact of automobile emissions is also increased. Particularly, road dust and aerosols might be injected into surface water during a rainy period [1,2]. The major contaminants within road *Corresponding author.
dust are polycyclic aromatic hydrocarbons (PAHs) and heavy metals. Some PAHs are known to be carcinogens and mutagens [3]. Furthermore, some PAHs have been identified in recent studies as endocrine disruptors. Reportedly, the half-life of PAHs in a water environment is several days to several months [4,5]. The persistence of PAHs is lower than that of heavy metals and dioxin. However, it is known that the degradation rate of PAHs adsorbed into sediments or soil will be
Presented at the 10 th IWA International Specialized Conference on Diffuse Pollution and Sustainable Basin Management, Istanbul, Turkey, 18–22 September 2006. 0011-9164/06/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.01.239
152
T. Kose et al. / Desalination 226 (2008) 151–159
slowed considerably [6]. Therefore, if PAHs in road dust cause sedimentary contamination, their environmental impact might persist for a long time. For that reason, some studies have reported an origin specification method for PAHs in road dust and sediments. Some studies have reported the major origin of PAHs in road dust as exhaust gases from automobiles [7]. Other studies have emphasized the contribution of dust from wear of road pavement material or asphalt [8]. Moreover, reports indicate that dust from tire rubber or lubricant oil is a major source of PAHs in road dust [9]. In those studies, the major source of PAHs in road dust is not necessarily consistent because the composition of the PAHs themselves was used for origin specification. As mentioned above, PAHs easily biodegrade or photodegrade in the environment [10]. The degradability of each PAH is quite different. Consequently, the PAH profiles used for specification of the origin might not be stable under environmental conditions. As marker compounds used for origin specification, biologically and photochemically stable compounds are desirable. Tri-terpanes are marker compounds used for origin specification of petroleum and its products [11]. Tri-terpanes are substances contained in crude oil; they have high stability to biodegradation and photochemical reactions. Some studies have used tri-terpanes as marker compounds to identify origins of marine and sediment contamination by oil spills [8,12]. Moreover, some studies have reported that hopanes (five-ring tri-terpanes) are useful to identify the origin of contamination by automobile traffic emissions [13,14]. However, the pollution origins of road dust are different. It remains unclear whether all sources of pollution contain tri-terpanes; few research examples of origin specification use this method. For those reasons, objectives of this study are to examine whether tri-terpanes might be used for origin specification of PAHs. Furthermore, specification of the main sources of PAHs in road dust in Okayama city was attempted.
2. Materials and methods 2.1. Sample collection procedure Five pollution sample types were collected and analyzed: road dust, runoff water, atmospheric deposit dust, atmospheric particle phase, and atmospheric gas. Road dust samples were collected at the sampling site shown in Fig. 1 (circles). Three sampling sites were selected along route 2 (the most heavily traveled route in Okayama city, around 100,000 vehicles per day). Two sampling sites were selected along route 53 (next heaviest traveled to route 2, around 30,000 vehicles per day). One sampling site was selected along prefect route 72 (8000 vehicles per day) [15]. Road dust samples were collected using a vacuum cleaner within a 1 m × 1 m frame. Sampling procedures were repeated three times per point and the position of the collection frame was kept from overlapping. Sampling was performed in June and November 2005 a day of sunny weather that was preceded by one week of sunny weather. Collected road dust was sieved using a 2.0 mm stainless sieve to remove large materials and to homogenize the sample. Runoff water was collected at the sampling site shown in Fig. 1 (square symbols) using an automatic water sampler (LYSAM-P; NKS Co. Ltd., Japan). Sampling was performed during a rain of more than 10 mm after sunny weather had continued for one week or more. Suspended solids of high concentration were contained in the sample collected within 10 min after a rain started. Therefore, water samples collected within 30 min after rain started were homogenized and analyzed. Atmospheric samples (deposit dust, particle phase and gaseous phase) were collected at the sampling site shown in Fig. 1 (diamonds). Deposit dust samples were collected using a pail can with milli-Q water to capture deposited dust. The pail can was filled with 5 L of milli-Q water and placed at a sampling site for five days. Subsequently,
T. Kose et al. / Desalination 226 (2008) 151–159
Okayama city
Road dust sampling point
53 0 km
153
3 km
Atmospheric sampling point Runoff water sampling point
2
Fig. 1. Sampling sites in Okayama city.
the pail can was brought back to the laboratory and sample water was collected with deposited dust. Particle phase and gaseous phase samples were collected using a handy air pump (MP-Σ500; Shibata, Japan) equipped with a glass fiber filter (GC50; Advantec Toyo Kaisha Ltd., Japan) and a soil phase extraction disk (Empore disk SDB-XD; GL Sciences Inc., Japan). Particle phase materials were collected onto a glass fiber filter; gaseous phase components were collected onto a solid phase extraction disk. Five types of pollution source samples were analyzed: tires, road pavement asphalt, automotive fuel oil, automotive lubricant oil, and exhaust gases. Four types of new tires and four types of used tires were selected for analyses. Tire rubber was sliced from the tread surface and cut fine to 2 mm or smaller pieces. Three types of straight asphalt (paving materials) and two types of asphalt pavement (collected directly from the
road) were selected for analyses. Asphalt pavement samples were crushed finely to 2 mm or smaller pieces for extraction. Straight asphalt samples were dissolved directly into toluene. Eight types of automobile fuel were selected for analyses: two types of high octane gasoline, three types of regular gasoline, and three types of diesel. Fuel oils were concentrated using a rotary evaporator. However, diesel samples were difficult to concentrate because of diesel oil’s high vapor temperature. Therefore, results of five samples, without diesel, are presented in this study. We selected 14 automotive lubricant oils for analyses: 10 types of four-cycle engine oil, 3 types of diesel engine oil, and 1 type of two-cycle engine oil. 2.2. Sample pretreatment procedure Solid phase samples were extracted using an accelerated solvent extractor (ASE-200; Dionex Corp., California, USA): road dust, suspended
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solid from runoff water, deposit dust, atmospheric particle, tire rubber, and road pavement asphalt. Toluene/methanol: (50/50) mixed solvent was used for extraction. Extraction was carried out three times at 150°C, 2000 psi; the amount of solvent exchange for every cycle was 70%. Extract was concentrated using a rotary evaporator and separated for hydrocarbon (PAHs and tri-terpanes) analyses. The extract for hydrocarbon analysis was cleaned (Silica Sep-Pak Plus; Waters, USA). Solvents of the extract were exchanged to 1 mL of n-hexane and loaded onto a conditioned Sep-Pak cartridge. The saturated hydrocarbon fraction involving tri-terpanes was eluted with 5 mL of n-hexane. Aromatic hydrocarbon fractions involving PAHs were eluted with 5 mL of n-hexane/ dichloromethane: (70/30). Both elutes were dried under a nitrogen gas stream and re-dissolved into 1 mL of n-hexane involving internal injection standard compounds. Water phase samples, which comprised the dissolved phase of runoff water and milli-Q water for deposit dust collection, were separated by filtration using a GC50 glass fiber filter. Suspended solids were analyzed according to the procedure mentioned above. Water phase was concentrated using a conditioned Empore disk C18 for hydrocarbon analyses. After concentration, the disk was dried and an extract was eluted using 5 mL of dichloromethane. The eluate was concentrated to less than 1 mL under a nitrogen gas stream. The PAHs and triterpanes in the sample solution were separated and cleaned using the same procedure as that used for a solid sample using Sep-Pak Plus silica. To another portion of the water sample, 1 M NaOH aq. was added and pH was controlled to more than 10. A water sample was extracted using a conditioned Empore disk C18. The extract was eluted using 5 mL of dichloromethane. The eluate was concentrated to less than 1 mL under a nitrogen gas stream and cleaned using the same procedure as that used for the solid sample.
2.3. GC–MS analysis procedure The concentrations of PAHs, tri-terpanes and NCBA were determined using a quadropole mass spectrometer (QP-2000; Shimadzu Corp.) with an integrated GC-20A gas chromatograph equipped with a DB-5 capillary column (30 m fused silica 0.25 mm i.d. and 0.25 μm film thickness; J&W Scientific Inc.). The GC–MS operating conditions are summarized below. The ion source was maintained at 1.3 kV ionization potential at 280°C. For PAH analysis, the injection port was maintained at 280°C and the sample was injected with a splitless mode followed by a 1-min purge after the injection. The column temperature was maintained at 50°C for 2 min, then programmed to increase by 7°C min−1 to 310°C, and held for 10 min. Selected ion monitoring (SIM) was employed after a delay of 4 min. The PAHs were determined at a mass per charge ratio (m/z) of their molecular weight. Response factors of PAHs were defined using standard compounds (Aldrich Chemical Co., USA). For tri-terpane analyses, the injection port was maintained at 300°C and the sample was injected using splitless mode followed by a 1-min purge after injection. The column temperature was maintained at 50°C for 2 min and increased at 6°C min−1 to 300°C, then maintained for 15 min. Tri-terpanes were determined by m/z of 191. 3. Results and discussion Fig. 2 shows profiles of PAHs in runoff water and road dust. Horizontal axes denote ratios of concentrations of fluoranthene/pyrene. The vertical axis denotes that of phenanthrene/anthracene. Recent studies have shown that profiles of PAHs shown in Fig. 2 indicate the pollution sources of PAHs. The upper left area (Phen/Anth>10, Fluo/ Pyr<1) represents petrogenic pollution and the lower right area (Phen/Anth<10, Fluo/Pyr>1) includes pyrogenic pollution [16,17]. Profiles of runoff water were almost identical to those of road dust. The same correspondence
T. Kose et al. / Desalination 226 (2008) 151–159 50
road dust Road dust
Road road dust Runoff runoff water water
40
20 10 0
0
1
2
3 4 Fluo/Pyr (–)
5
tire Tire
Road pavement asphalt
lubricantoiloil Lubricant
fuel Fuel oil oil
atmospheric Atmospheric atmospheric Atmospheric gaseous phase gaseous phase particlephase phase particle automobile Automobile exhaust exhaust automobile exhaust Automobile exhaust particle particlephase phase gaseous phase gaseous phase atmospheric Atmospheric deposit deposit dustdust
40 30
Phen/Anth (–)
Phen/Anth (–)
50
155
30
20
6
10
Fig. 2. Profiles of PAHs in road dust and runoff water. 0 0
was apparent in the profiles of tri-terpanes in road dust and runoff water (Fig. 3). These results show clearly that road dust is the major source of the PAHs in runoff water. Profiles of road dust and runoff water were distributed at the halfway point of the petrogenic area and pyrogenic area. This result shows that the road dust was probably a mixture of pollution from petrogenic and pyrogenic sources. We next try to investigate the source of PAHs in road dust. Fig. 4 shows profiles of PAHs in road dust, various pollution source samples, and reference samples. Five source samples were selected: tire, road pavement asphalt, automobile fuel, lubricant oil, and exhaust gas. Data for two
3.5 Road dust Runoff water
Σ CC31~35/C30 (–)
3.0 2.5 2.0 1.5 1.0 0.5 0
0
0.5
1.0
1.5 2.0 C29/C30 (–)
2.5
3.0
Fig. 3. Profiles of tri-terpanes in road dust and runoff water.
1
2
3 4 Fluo/Pyr (–)
5
6
Fig. 4. Profiles of PAHs in road dust, pollution sources and reference samples.
reference samples, an atmospheric sample (separated into particle phase and gaseous phase) and atmospheric deposit dust, are also depicted in Fig. 4. The ratios of Phen/Anth and Fluo/Pyr in road dust (star symbol) were 6–14 and from 0.6–0.9, respectively. On the other hand, distribution of PAHs profiles in source samples was greater than that in road dust. For example, the respective values of Phen/Anth and Fluo/Pyr in asphalt for road pavement (open triangles) were 3–18 and 0.3–1.0. This wide range of values is probably attributable to high degradability of PAHs. Junfeng et al. [10] reported degradability of PAHs under natural sunlight by photo-oxidation. The half life of phenanthrene was reported as 72 h, but that of anthracene was only 26 h. The respective half-lives of fluoranthene and pyrene were reported as 26 h and 37 h. This result implies that the ratio of Phen/Anth and Fluo/Pyre might change according to the progress of photooxidation. The total concentration of PAHs in pavement samples collected from the road (21 ng/g, N = 3) was lower than that in fresh asphalt samples (5900 ng/g, N = 3). The total concentration of PAHs in used tires (128 ± 57 μg/g, N = 4) was
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also lower than that in new tires (58 ± 35 μg/g, N = 4). Lower total concentration of PAHs in used tires and pavement samples collected from the road also suggested the influence of photodegradation. Furthermore, profiles of PAHs in tires (closed triangles), asphalt for road pavement and lubricant oil (closed diamonds) showed a distribution that resembled that of road dust. Therefore, major pollution sources of PAHs in road dust might be difficult to investigate from these results. Stability of marker compounds is important for pollution source analysis. Fig. 5 shows profiles of tri-terpanes in road dust, various pollution source samples, and reference samples. Horizontal axes denote the ratio of the concentration of 29-carbon-number norhopanes per 30-carbon-number hopanes (C29/C30). The vertical axis shows ratios of the concentration of homohopanes with 31–35 carbon number per hopane with 30 carbon number (ΣC31–35/C30). Profiles of tri-terpanes in road dust resembled those in tires, implying that tires are a major pollution source of road dust. Other pollution source samples were apparent: profiles of fuel oil were shifted to the lower right side, but there was also a portion that overlapped slightly with that of road dust, which showed the tendency for 3.5
Road dust Tire Road pavement asphalt
3.0
Lubricant oil
2.5 ΣC31–C35/C30
Fuel oil Atmospheric particle phase Automobile exhaust particle phase Atmospheric deposit dust
2.0 1.5 1.0 0.5 0
0
0.5
1.0
1.5 2.0 C29/C30
2.5
3.0
Fig. 5. Profiles of tri-terpanes in road dust pollution sources and reference.
wide range profiles. Profiles of fuel oil also agreed to some degree with profiles of automobile exhaust emission. However, profiles of automobile exhaust emissions were shifted further to the lower left side, and the influence of engine oil was apparent from the profile. This result implies that fuel oil and lubricant oil were emitted into automobile exhaust gas. However, contributions of automobile exhaust gases to road dust were not great. Zakaria et al. [12] reported that lubricant oil is an important pollution source of contamination of sediments in Malaysia. This result is contradictory to the result of this paper. It might be attributable to differences of habits and materials used in that country. Zakaria et al. also reported that lubricant oil sometimes leaks from automobiles and that large amounts of waste oil are typically abandoned in Malaysia [12]. Such leakage of lubricant oil is not so common in Japan. Although the profiles of tri-terpanes in asphalt for road pavement differed from that in road dust, the possibility that not only tires, but asphalt for road pavement was major source of road dust was suggested from this result. The profile of tri-terpanes in atmospheric dust deposits was almost identical to that in road dust because road dust might spread to the atmosphere and be re-deposited near the road. These results indicate that tires, fuel oil and asphalt for road pavement are major sources of road dust. It is difficult to evaluate the contribution of the three sources only from the result shown in Fig. 5, which shows the ratios of concentration of several tri-terpanes. Therefore, we attempted to determine the importance of pollution sources based on the composition of all analyzed tri-terpanes using cluster analysis. The analysis was performed with Ward’s method using Euclidean distance, as calculated by computer software (STATISTICA6.0; StatSoft Inc., USA). Tree diagrams from analytical results are shown in Fig. 6. The letters denote the type of sample and the numbers denote the sample lot: R denotes a road dust sample; T denotes a tire
T. Kose et al. / Desalination 226 (2008) 151–159 20 At3 R7,R8,R9,R10 R11,R12 Pv1,Pv2
II
R1,R2,R3,R4,R5,R6 T1,T2,T3,T4,T5,T6 T7,T8 E3
III
At1,At2 Pt5,Pt6,Pt7,Pt13 Pt14,Pt15,Pt16 Pt17,Pt18,Pt20 E2 Pt1,Pt2,Pt3,Pt4 Pt8,Pt9,Pt10,Pt11 Pt12,Pt19 E1,E5 E4,E6
40
60
I
G1
IV V
G2 VI
VII
Fig. 6. Tree diagrams based on cluster analysis.
sample; E denotes an automobile exhaust sample; Pt denotes an atmospheric particle sample; and Pv and At respectively denote pavement samples collected from the road and fresh asphalt samples. All road dust samples are distributed in clusters II and III. Cluster III also contained all tire samples. This result shows clearly that tires are a major source of road dust pollution. Asphalt for road pavement samples (Pv and At) is distributed in clusters I, II and IV. Only cluster II was in agreement with road dust. However, clusters I and IV belong to group 1 (G1) and might be related closely with clusters II and III. Therefore, asphalt for road pavement is a major pollution source of road dust that ranks second to tires. Clusters V–VII include automobile exhaust and atmospheric particles belonging to group 2, which
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has low relevance to road dust. Therefore, the importance of automobile exhaust and atmospheric particles as sources of road dust were lower than that of tire and asphalt for road pavement. It was inferred from the similarity of profiles of tri-terpanes and results of cluster analyses that tires were the largest pollution source of road dust, followed by asphalt for road pavement. However, evaluation of this contribution is based on tri-terpane composition, which might not agree with the contribution of PAHs. That is, if the concentration of tri-terpanes in pollution source samples were high, the contribution in road dust would be large, but if the concentration of PAHs in the pollution source were low, the contribution as a pollution source of PAHs would become necessarily less. To clarify this point, the total concentrations of PAHs and tri-terpanes in road dust and pollution source samples and their ratios are shown in Table 1. The total concentration of PAHs in tires and asphalt for road pavement was higher than that of other pollution source samples. Therefore, even if based on the ratio of concentration shown in Table 1, the result that the contribution of tires and asphalt for road pavement was large did not change. 4. Conclusions An estimation method to determine the pollution source of PAHs in road dust based on the profiles of tri-terpanes was used in this study.
Table 1 Total concentration of PAHs and tri-terpanes in road dust and pollution source samples
Total PAHs (ng/g) Total tri-terpanes (ng/g) Number of samples PAHs/tri-terpanes (−)
Tire
Road pavement asphalt
Fuel oil
Lubricant oil
Exhaust particle phase
Road dust
93,139 9201 8 10.1
3562 1400 5 2.5
89 41 5 2.2
784 527 14 1.5
1493 402 6 3.7
45,809 21,831 8 2.1
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This method can clarify pollution sources that cannot be clarified using conventional methods based on the PAH profiles. Simultaneously, this study specified the main sources of PAHs in road dust in Okayama city using this technique. Results revealed that the most important source of PAHs in road dust in Okayama city is automobile tires; asphalt for road pavement was the next greatest pollution source. Results also clarified that the source of PAHs in runoff water was road dust. Acknowledgements A part of this study was supported by the Core Research for Environmental Science and Technology (CREST) by Japan Science and Technology Agency. We also thank Fukutaro Uchida and Masatomo Nishihara for their help in sample collection and analyses. References [1]
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