Assessing Limitations for PAH Biodegradation in Long-Term Contaminated Soils Using Bioaccessibility Assays

Polycyclic aromatic hydrocarbons (PAHs) are generated by a range of industrial processes including petroleum and gas production and are often found in high concentrations at industrial sites. Once PAHs enter the environment, the predominant mechanisms for removal are biological via microbial activity. However, PAHs have the potential to partition onto soil organic matter thereby decreasing their bioavailability to microorganisms and limiting their degradation. This explanation was felt to be the reason for a lack of evidence of PAH biodegradation in a study of long-term contaminated soils. To test the hypothesis that bioavailability was a limiting factor for biodegradation in theses soils, PAH bioavailability was determined using nonexhaustive extraction (propanol, butanol, hydrooxypropyl-β-cyclodextrin) and oxidation (persulfate) methodologies designed to determine the fraction of contaminants within soil which are available for biological uptake. The assays gave varying results for each soil, and no specific trends across all soils were observed. PAH bioaccessibility, derived from the HP-β-CD assay which has been the most extensively tested in the literature, was estimated to be between 0 and 10 % for most soils, with the exception of pyrene, indicating that a large fraction of the soil-borne PAHs at the site are not available to microorganisms and that bioavailability limitations may be a primary cause for the lack of observed biodegradation at this site. These results highlight the importance of bioavailability to PAH degradation as well as the relevance of utilizing an assay that has been evaluated across many soil conditions and parameters.