Desorption, Persistence, and Leaching of Dibenzofuran in European Soils

Polycyclic aromatic hydrocarbons (PAHs) and their heterocyclic analogs (hetero-PAHs) represent the predominant chemical classes in many contaminated soils. Despite the fact that hetero-PAHs are more soluble and hence potentially more mobile and bioavailable than similar nonsubstituted compounds, the existing information about the fate of these compounds in the soil environment is limited. In this article, laboratory experiments were designed to assess the desorption, persistence, and leaching behavior of the hetero-PAH dibenzofuran in four European soils with different physicochemical characteristics. Previous research has shown that organic matter was important in the extent of sorption of dibenzofuran by the soils studied, although sorption was not predictable exclusively from the organic carbon content. Dibenzofuran desorption isotherms showed little hysteresis, in particular from higher equilibrium points of the sorption isotherms, indicating weak interaction mechanisms between dibenzofuran and the soils. The half-life of dibenzofuran in the soils during incubation experiments ranged between 5 and 55 d, depending on soil type and experimental conditions. In general, the persistence of dibenzofuran decreased with the organic carbon content of the soils (0.56–2.09%) and with temperature (4!C/20!C) and increased with the initial concentration of the chemical in soil (10 mg kg/100 mg kg). Leaching tests, performed in hand-packed and undisturbed soil columns, indicated that dibenzofuran is likely to move slowly through the soil profile, although dissolved organic matter and soil structure can influence the amount of dibenzofuran leached. The effect of dissolved organic matter on leaching of dibenzofuran was found to be soil dependent and less important than the effect of soil structure. SOIL and ground water contamination by polycyclic aromatic hydrocarbons (PAHs) and their heterocyclic analogs (hetero-PAHs) containing N, S, or O is a widespread problem in industrialized countries. These organic compounds represent the predominant chemical classes in soil of former coal gasification sites, tar oil distillation plants, and wood-preserving facilities (Meyer and Steinhart, 2001). Oil spills, leakages, and pyrolytic sources, such as incomplete combustion, significantly contribute to the input of these chemicals into the environment (Traulsen et al., 1999). Soils contaminated with creosote, a product of high-temperature treatment of hard coal, contain hundreds of different organic compounds where PAHs constitute about 85%, hetero-PAHs 5–15%, phenolic compounds 1–10%, and monoaromatic hydrocarbons, 1% (Broholm et al., 1999; Meyer et al., 1999). Because of the presence of the heteroatom, heteroPAHs have larger water solubilities than the corresponding PAHs (Meyer et al., 1999), which may result in greatermobility and bioavailability comparedwith similar nonsubstituted compounds. For instance, when water comes in contact with creosote, polar compounds are enriched in the aqueous phase, and the fraction of heteroPAHs increases to 35 to 40%, whereas the fraction of PAH compounds decreases to about 15% (Kiilerich and Arvin, 1996; Broholm et al., 1999). Therefore, regarding ground water contamination arising from contaminated soils, hetero-PAHs usually represent a larger problem thanPAHs (Johansen et al., 1997; Broholmet al., 1999). In fact, high concentrations of hetero-PAHs, such as thiophene, benzothiophene, dibenzothiophene, benzofuran, dibenzofuran, pyridines, quinolines, indole, and carbazole, have been detected in ground water at creosotecontaminated sites (Johansen et al., 1997). In addition, the larger presence of these compounds in the aqueous phase comparedwith nonsubstituted compounds enhances their bioavailability and consequently the risk of ecological and human effects. In this regard, a number of heteroPAHs have been reported to display toxic, mutagenic, and carcinogenic properties, even when present in low concentrations (Meyer et al., 1999). Although they can be more mobile and bioavailable than their nonsubstituted analogous, little is known about the environmental fate of hetero-PAHs (Traulsen et al., 1999; Meyer and Steinhart, 2000). In fact, the heteroaromatic compounds containing nitrogen, sulfur, or oxygen in the aromatic ring are not normally included in the chemical analysis of contaminated sites. The polar metabolites from the degradation of creosote are not usually analyzed (Kiilerich and Arvin, 1996). Extensive knowledge about the environmental fate of the more hydrophilic fraction of compounds present in contaminated soils has been stressed as important in improving risk assessments and success evaluation of remediation technologies (Langbehn and Steinhart, 1994; Meyer et al., 1999; Vila et al., 2003). The present study was designed to provide insight into the fate of an oxygen-substituted hetero-PAH (dibenzofuran) in four European soils differing in their physicochemical characteristics. The sorption behavior of dibenzofuran in a number of soils, including those selected for this study, was reported in a previous article (Celis et al., 2006). Here, batch, incubation, and column leaching tests were designed to assess the desorption, persistence, and leaching behavior of dibenzofuran in selected soils. Because dissolved organic matter can strongly influence the transport of hydrophobic organic compounds, such as PAHs and related compounds, by formation of dissolved organic matter (DOM)-solute complexes (Rav-Acha and Rebhun, 1992; Sabbah et al., Instituto de Recursos Naturales y Agrobiologı́a de Sevilla, CSIC, Avda Reina Mercedes 10, P.O. Box 1052, 41080 Sevilla, Spain. Received 14 Sept. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:1310–1317 (2006). Soil Chemistry doi:10.2136/sssaj2005.0304 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: BTC, breakthrough curve; DOM, dissolved organic matter; HPLC, high-performance liquid chromatography; PAHs, polycyclic aromatic hydrocarbons. R e p ro d u ce d fr o m S o il S ci e n ce S o ci e ty o f A m e ri ca Jo u rn a l. P u b lis h e d b y S o il S ci e n ce S o ci e ty o f A m e ri ca . A ll co p yr ig h ts re se rv e d . 1310 Published online June 21, 2006