Environ. Sci. Technol. 2002, 36, 3530-3535
Carbon Isotope Signature of Polycyclic Aromatic Hydrocarbons (PAHs): Evidence for Different Sources in Tropical and Temperate Environments? WOLFGANG WILCKE,* MARTIN KRAUSS, AND WULF AMELUNG Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany
In tropical soils, naphthalene and, partly also, perylene occur at elevated concentrations while pyrolytic higher molecular weight PAHs are almost absent. We hypothesize that there are recent biological PAH sources in the tropical environment related with woody plants and termites. We used the C isotope signature of individual PAHs in temperate and tropical soils and in tropical wood and termite nests to distinguish different PAH sources. The mean δ13C values of the benzo[b+j+k]fluoranthenes and of benzo[a+e]pyrenes in temperate soils ranged between -24.6‰ and -25.3‰, being similar to values reported in the literature for PAHs with pyrolitic origin. The mean δ13C values of perylene decreased in the order temperate soils (-27.0‰) > termite nests (-31.4‰) > tropical soil (-32.4‰), while those of naphthalene (-24.6‰ to -26.2‰) were similar among the tropical and temperate soils, tropical wood, and termite nests. Our results support the assumption that perylene in the tropical environment is recently biologically produced, as indicated by the depletion in 13C. The C isotope composition of naphthalene, however, cannot be used to distinguish different sources.
Introduction Polycyclic aromatic hydrocarbons (PAHs) are produced during pyrolysis of fossil fuels or modern plant material and ubiquitously distributed in the environment (1, 2). In the temperate zone, the largest part of the PAHs results from anthropogenic activity and is mainly stored in soils (3). The little published work on PAHs in tropical soils indicates that the concentrations of most PAHs are lower than in the temperate zone and that the composition of the PAH mixture is markedly different. While in the temperate zone benzofluoranthenes, chrysene, and fluoranthene often dominate the PAH mixture, in the tropics, naphthalene, phenanthrene, and perylene are most abundant in many soils, particularly in rural areas (2). The works of Chen et al. (4, 5) and Wilcke et al. (6) have shown that the sources of naphthalene, phenanthrene, and perylene in tropical soils may be related to the activity of termites and woody plants because termite nests and wood * Corresponding author phone: ++49 30 314 73538; fax: ++49 30 314 73548; e-mail:
[email protected]. Present address of all authors: Department of Soil Science, Institute of Ecology, Technical University of Berlin, Salzufer 11-12, D-10587 Berlin, Germany. 3530
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from tropical regions contain high naphthalene and, partly also, high phenanthrene and perylene concentrations, while the concentrations of PAHs that are typically assigned to the pyrolysis of fossil fuels or modern organic matter and that dominate the PAH pattern in temperate soils are low (2). Considerable evidence has accumulated that perylene is produced biologically under anaerobic conditions, because of its presence in groundwater-influenced soils and in sediments where combustion-derived PAHs are almost absent (7-10). While Venkatesan (8) stated that perylene is microbially produced in anaerobic environments by incomplete degradation of pigments from insects, fungi, and marine organisms containing perylene quinones, other authors assume a microbial production independent of specific precursors (11). As in termite guts anaerobic conditions occur (12), it seems possible that perylene is produced there by intestinal microorganisms. In the past decade, the C isotope signature of individual PAHs has been used to determine the contribution from different sources to the PAH burden of soils and sediments (13-16). Enzymatically catalyzed biological processes frequently discriminate 13C, resulting in a shift of the δ13C value (i.e., the 13C/12C ratio relative to a standard) from that of the atmospheric CO2 to more negative 13C-depleted values. For plants following the C3 metabolic pathway, the main sources of fossil fuels, δ13C values of -32‰ to -22‰ are common in bulk biomass with a mean of -27‰ (17). The C isotope signature of pyrolysis-derived PAHs resembles the δ13C signal of the fuel, depending on combustion conditions. For coal combustion and gasification processes, high combustion temperatures lead to PAHs depleted in 13C as compared to parent materials, while for low temperatures the δ13C values of PAHs are close to those of the parent coals (18). Furthermore, it may not be ruled out that transport and postdepositional processes change the δ13C values of pyrolysis-derived PAHs after their emission to the environment. The objective of our study was to test the hypothesis that the compound-specific δ13C value allows for perylene and naphthalene produced by recent biological processes to be distinguished from those that were produced by pyrolysis of fossil fuels. For this purpose, we determined δ13C values of combustion-derived PAHs in anthropogenically contaminated temperate soils and compared these results with the C isotope signature of presumably biological naphthalene and perylene in tropical wood, termite nest, and soil samples.
Materials and Methods Samples. As temperate reference samples in which the PAHs were predominantly derived from the pyrolysis of fossil fuels, we chose three urban topsoils (0-5 cm) comprising a house garden, a former gas work site, and a roadside soil. The house garden and gas work site were in the city of Bayreuth and the roadside soil near a heavily frequented road in the village of Stephanskirchen, Germany. The soils were anthropogenically strongly modified Cambisols (house garden and gas work site) and a Leptosol (roadside, 19). In the Brazilian Amazon region, we collected in each of the Terra firme (never flooded), Va´rzea (seasonally flooded by white water), and Igapo´ (seasonally flooded by black water) regions near Manaus one composite sample of each of topsoil (0-10 cm), dead wood, and the central part of the nests of the woodfeeding termite genus Nasutitermes. The soil types in Brazil were Ferralsols (Terra firme) and Fluvisols (Va´rzea and Igapo´), and organic C and PAH concentrations are shown in Table 1. All samples were air-dried at ambient conditions near the location where they were collected and sieved to 0.95 for all PAHs. The replicate IRMS measurements of the samples showed a good reproduction of the results. For naphthalene, the standard error was
