Pedogenic Formation of Perylene in a Terrestrial Soil Profile: Evidence From Carbon Isotopic Ratios

Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants (POP), which are introduced into the environment mainly due to combustion of fossil fuel. Perylene is one compound of the PAHs that consists of 5 condensed rings like the well known carcinogenic benzo(a)pyrene. Apart from the pyrogenic formation, there are strong indications that it is produced biologically and/or diagenetically under anaerobic conditions. This conclusion was derived from the presence of perylene in deeper parts of marine and lacustrine sediment profiles, where the combustion-derived PAHs are almost absent ( Lima et al., 2003). 13C/12C compound-specific stable isotopic ratios were successfully applied for the differentiation of probably biologically generated perylene in tropical termite nests and pyrolytic perylene from surface soils of temperate regions ( Wilcke et al., 2002). Our study is the first aiming on the determination of the different processes of perylene formation at one location using carbon isotopic ratios such as 13C/12C. We determined PAHs in natural soils of southern Germany. At one location in the Black Forest we found for the first time evidence for natural perylene production in the subsoil of terrestrial environments. Apart from the combustion derived PAHs that accumulate at the top of all soil profiles, the depth distribution of perylene shows the highest peak in the subsoil about 1 m below the surface. Due to its very low solubility (0.4 μg l-1 at 25 °C) vertical transport of perylene with seepage water is very unlikely. Thus, we suggest atmospheric deposition of pyrogenic perylene at the top of the profile and in-situ generation in the subsoil, probably due to microbial activities. In order to distinguish between the pyrogenic and natural generation we employed 13C/12C compound-specific stable isotope analysis of perylene in soil samples from the top of the profile as well as from the subsoil. Preliminary measurements with soil extracts show strong indications for a difference in isotopic composition of perylene in the different horizons. High background of unresolved complex matrix (UCM) and co-eluating compounds require improved purification steps by flash-chromatography with silica to ensure baseline separation. The presentation will show first results from the ongoing study. In future we plan to extent the method to 14C/12C ratios. Additional soil samples from the surrounding area will be taken for the determination of the spatial distribution of perylene in the subsoils. Furthermore we want to perform studies about the microbial community that lives in the subsoil in order to find out which kind of gradients control the natural formation of perylene in the environment. References: Lima, A. L. C., Eglington, T. I. & Reddy, C. M. (2003): High-resolution record of pyrogenic polycyclic aromatic hydrocarbon deposition during the 20th century.- Environ. Sci. Technol., 37: 53-61. Wilcke, W., Krauss, M. & Amelung, W. (2002): Carbon isotope signature of polycyclic aromatic hydrocarbons (PAHs): Evidence for different sources in tropical and temperate environments?- Environ. Sci. Technol., 36: 3530-3535.