Research Use of Compound-Specific Stable Isotope Analysis to Source Anthropogenic Natural Gas-Derived Polycyclic Aromatic Hydrocarbons in a Lagoon Sediment C A R O L E M C R A E , † C O L I N E . S N A P E , * ,‡ CHENG-GONG SUN,† DANIELE FABBRI,§ DANIELE TARTARI,§ CLAUDIO TROMBINI,§ AND ANTHONY E. FALLICK| Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, U.K., School of Chemical, Mining and Environmental Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., Laboratorio di Chimica Ambientale, Universita` di Bologna, via Marconi 2, 48100 Ravenna, Italy, and Scottish Universities Environmental Research Centre (SUERC), East Kilbride, Glasgow, G75 0QF, U.K.
This paper reports the first identification of polycyclic aromatic hydrocarbons (PAHs) in the environment with extremely low carbon stable isotopic ratios. For sediments collected from a coastal lagoon in northern Italy, these low 13C/12C isotopic ratios for many of the PAHs together with the presence of cylcopentafused PAHs (CP-PAH) indicate that the PAHs have predominantly been derived from a hightemperature industrial process utilizing the local biogenic natural gas. The process used the biogenic gas as the feedstock for the production of acetylene. From the evidence available, the large variability in the PAH isotopic δ13C values (from -31 to -62 ‰) is ascribed mainly to biodegradation rather than to major inputs from sources of isotopically heavy PAH, such as the neighboring modernday carbon black plants. Polycyclic aromatic hydrocarbons (PAHs) in the environment arise from a number of natural and anthropogenic sources, with major contributions from coal utilization (1), vehicle exhaust emissions (2), and biomass combustionsboth accidental and intentional (3). Although conventional analytical techniques are useful for the identification and quantification of PAH in environmental samples, they provide little information on the source of these pollutants. The potential of using compound-specific stable carbon isotope ratios determined by gas chromatography-isotope ratio mass spectrometry (δ13C GC-IRMS) for the source apportionment of environmental PAH has been demonstrated by O’Malley et al. (4, 5), Lichtfouse et al. (6), and four of the authors (7-10). Although coal, transport fuel, and biomass contributions have been resolved successfully in a number of cases, * Corresponding author telephone: +44 115 951 4166; fax: +44 115 951 4115; e-mail:
[email protected]. † University of Strathclyde. ‡ University of Nottingham. § Universita ` di Bologna. | Scottish Universities Environmental Research Centre. 4684
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the possibility of anthropogenic natural gas-derived PAHs in the environment has not been addressed thus far. This paper reports the first identification of environmental PAHs with extremely low stable carbon isotopic values from a suite of lagoon sediments that are characteristic of a natural gas source. Full details on the lagoonal site in northern Italy, sampling and analytical procedures, and the variations in PAH concentrations throughout the sediments will be given in a subsequent paper (11). Sediment cores were collected from four different areas of the lagoon Pialassa Baiona (Ravenna, Italy) in September 1998 (sites B and C) and in February 1999 (sites A and D). In relation to the discharge channel into the lagoon, sample A is the most distant and sample B is close to the point of discharge while sample C is from the channel itself, close to the point of exit. Cores B and D are two sites located in a pond where the water flow is lowest. Cores were extruded and segmented at 6- (cores A and D) or at 10-cm (cores B and C) depth intervals, homogenized, and extracted with dichloromethane (DCM). After solvent removal, the extracts were subjected to open column chromatography on silica gel, the PAHs being eluted with dichloromethane/hexane (40:60 v/v). The solvent was eliminated by rotary evaporation, and the aromatics were dissolved in dichloromethane for GC-IRMS analyses, O’Malley et al. (4) demonstrated that evaporative losses have no significant effect on the measured δ13C values of PAHs. This paper reports the isotopic data for the extracts containing the highest total PAH concentration found in each core. For purposes of comparison, PAHs were also extracted from carbon black currently produced in the Ravenna area where the two plants use petroleum residue as the feedstock. The compound-specific δ13C measurements were carried out using a VG Isochrom II GC-IRMS instrument as previously described (7-10). The reported carbon isotopic data, expressed in terms of the conventional δ-notation giving the per mill deviation of the isotope ratio from the standard Peedee belemnite (PDB), represent the arithmetic means of at least two duplicate analyses where variations were generally less than 0.5‰. Normal GC-FID and GC-MS analyses were conducted on the aromatics prior to carrying out the compound-specific δ13C measurements. Conventional sealed tube combustion was used to determine the isotopic composition of the bulk carbon black sample. The GC traces for the neutral aromatics from the four sediment samples investigated are shown in Figure 1. Together with phenanthrene, fluoranthene, and pyrene, cyclopentafused PAHs (CP-PAH), namely, acenaphthylene, one with m/z 202 eluting between fluoranthene and pyrene (tentatively identified as acephenanthrylene; 11), and one with m/z 226 (cyclopenta[cd]pyrene; 11) are also major components, the other m/z 226 compound present being benzo[ghi]fluoranthene (elutes first). The assignments of the CP-PAHs will be described fully in our subsequent paper (11), but suffice it to say here that their presence is indicative of a high-temperature pyrolysis/combustion process (12) being the source of a significant fraction of the PAHs. Some differences are evident in the distributions of unsubstituted PAHs including the more prominent underlying broad shoulder from alkylated aromatics in the GC trace for sample D (Figure 1). Total PAH concentrations in the samples investigated were in the range of 3-110 ppm, and the δ13C values for the major parent PAHs are listed in Table 1. The major three- and fourring PAHs plus the CP-PAHs present in the sediment extracts 10.1021/es0010002 CCC: $19.00
2000 American Chemical Society Published on Web 09/28/2000
FIGURE 2. GC trace of neutral aromatics from DCM extract of carbon black sample: 1, phenanthrene; 2, fluoranthene; 3, m/z 202; 4, pyrene; 5, m/z 226, benzo[ghi]fluoranthene; 6, m/z 226, cyclopenta[cd]pyrene.
FIGURE 1. GC traces of neutral aromatics from Ravenna sediment samples: 1, acenaphthylene; 2, phenanthrene; 3, anthracene; 4, fluoranthene; 5, m/z 202; 6, pyrene; 7, m/z 226, benzo[ghi]fluoranthene; 8, m/z 226, cyclopenta[cd]pyrene; 9, benzo[b]fluoranthene; 10, benzo[k]fluoranthene; 11, benzo[ghi]perylene.
TABLE 1. Stable Carbon Isotopic Ratios for the PAHs Separated from the Lagoon Sediments Collected near Ravenna sample designation/ δ13C value, ‰ PDB PAH acenaphthylene phenanthrene anthracene fluoranthene m/z 202 pyrene m/z 226 benzo[ghi]fluoranthene m/z 226 cyclopenta[cd]pyrene benzo[ghi]perylene
A
B
C
D
-30.8 -30.8 -43.6
-53.2 -61.7 -63.5 -63.3 -62.3 -59.9 -57.4 -48.3
-60.5 -41.7 -47.3 -33.4 -44.4 -50.4 -39.4 -36.0 -30.7
-61.8 -59.5 -59.5 -61.7 -61.1 -62.0 -57.9 -56.4
-47.7 -39.2 -40.4
are exceedingly isotopically light with their δ13C values varying between -31 and -62‰. Samples B and D exhibit the most isotopically light values. These low isotopic values can only be explained by the PAHs being sourced principally from natural gas (the range of values for biomass, coal, and petroleum is ca. -15 to -34‰ (4-10)) of a biogenic rather than a petrogenic (isotopically heavier) origin. Indeed, the δ13C range is -69 to - 73 for the biogenic methane in the reservoirs around Ravenna that has been utilized by the local chemical industry since the late 1950s (13). The process most likely responsible for releasing the isotopically light PAH into the lagoon system is one that used the local biogenic gas (because of its extremely high methane content, 99.8% v/v) as the feedstock for the production of acetylene, which was a low yield process (about 6%) generating a huge amount of soot as a coproduct. This process operated at close to 1400 °C but ceased operation in the early 1990s. Since PAHs are essentially an intermediate in soot formation, it is probable that, if they were obtained in relatively low yield, they and the acetylene produced might
be isotopically heavier than the feed methane. This is because fewer C-C bonds are formed in relation to the soot, and this should give rise to enrichments in 13C (isotopically heavier species) since 13C-12C bonds form slower than their 12C-12C counterparts. Indeed, the acetylene was used to manufacture poly(vinyl chloride) (PVC), and a sample of the polymer isolated from one of the sediments has a stable isotope ratio of -44‰ (11). The large range of δ13C values for the PAH could arise as result of either the different extents of biodegradation and leaching undergone by the PAHs at the four sites investigated or a large input of isotopically heavy PAHs. Both transportation (diffusion) and biodegradation will give rise to positive shifts in the δ13C values of the remaining PAHs (i.e., values become less negative). Interestingly, the heaviest PAH isotopic signature has been obtained for the site (A) most distant from the discharge point into the lagoon (Table 1). The lightest signatures have been obtained for the two sites located in a pond where the water flow is lowest (B and D) and which are probably the most anoxic, possibly giving the least alteration of the PAHs in terms of their oxidative conversion to other species. Carbon black production in the Ravenna area may represent an additional source of pyrogenic PAHs into the lagoon. The production is still carried out at two plants, but these use petroleum residue feedstocks operating with cocombustion of natural gas at ca. 1200 °C. To help ascertain whether the PAHs released from the plants still producing carbon black could have contributed to the sediments, a carbon black sample from one of the plants in Ravenna (Cabot) was analyzed. The PAHs in the dichloromethane extract were found to comprise principally fluoranthene, pyrene, and the m/z 202 CP-PAH together with considerably smaller quantities of the two m/z 226 PAHs (Figure 2). These PAH exhibited isotopic compositions in the region of -25 to -26‰, consistent with the petroleum residue used in the process. Furthermore, the stable isotope ratio of the bulk carbon black sample was -26.5‰. The low ratio of the two m/z 226 PAHs to pyrene in the carbon black extract (Figure 2) as compared to those found for the sediment extracts (Figure 1) would tend to suggest that inputs from the plants still producing carbon black into the sediments are relatively small. Nevertheless, biodegradation could modify this profile considerably, and an input from the modern day carbon black plants cannot be ruled out entirely at this stage of the investigation. On the other hand, geographically, if PAHs were entering the lagoon via the discharge channel from the VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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carbon black plants, they would be expected to make the greatest contribution to samples B and C. However, the data presented in Table 1, with sample B being the isotopically lightest, do not provide a strong case for this hypothesis. Therefore, on balance, we suggest that biodegradation of the isotopically light PAH released from the former acetylene plant is the main factor responsible for the large range of δ13C values for the PAHs found in the four sediments.
Acknowledgments We thank the British Council (Conferenza dei Rettori delle Universita` Italiane Award) and the British Coal Utilisation Research Association (BCURA) for financial support. We acknowledge helpful discussions with Dr. M. Mongardi of Cabot.
Literature Cited (1) (2) (3) (4)
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(5) O’Malley, V. P.; Burke, R. A.; Schlotzhauer, W. S. Org. Geochem. 1997, 27, 567-581. (6) Lichtfouse, E.; Budzinski, H.; Garringues, P.; Eglinton, T. I. Org. Geochem. 1997, 26, 353-359. (7) McRae, C.; Love, G. D.; Murray, I. P.; Snape, C. E.; Fallick, A. E. Anal. Commun. 1996, 33, 331-333. (8) McRae, C.; Snape, C. E.; Fallick, A. E. Analyst 1998, 123, 15191523. (9) McRae, C.; Sun, C.-G.; Snape, C. E.; Fallick, A. E.; Taylor, D. Org. Geochem. 1999, 30, 881-889. (10) McRae, C.; Sun, C.-G.; Snape, C. E.; Fallick, A. E. Am. Environ. Lab. 1999, 8, 1-4. (11) Fabbri, D.; Trombini, C.; McRae, C.; Sun, C.-G.; Snape, C. E.; Fallick, A. E. To be submitted for publication in Environ. Sci. Technol. (12) Lafleur, A. L.; Howard, J. B.; Plummer E. F.; Taghizadeh, K.; Necula, A.; Scott, L. T. Polycyclic Aromat. Compd. 1998, 12, 223237. (13) Mattavelli, L.; Ricchiuto, T.; Grignani, D.; Schoell, M. Am. Assoc. Pet. Geol. Bull. 1983, 67 (12), 2239-2254.
Received for review February 14, 2000. Revised manuscript received August 7, 2000. Accepted August 29, 2000. ES0010002
