A combined chemical and biological approach to transforming and mineralizing PAHs in runoff water

The water quality of lakes, rivers and streams associated with metropolitan areas is declining from increased inputs of urban runoff that contain polycyclic aromatic hydrocarbons (PAHs). Our objective was to transform and mineralize PAHs in runoff using a combined chemical and biological approach. Using ¹⁴C-labeled phenanthrene, ¹⁴C-benzo(a)pyrene and a mixture of 16 PAHs, we found that ozone transformed all PAHs in a H₂O matrix within minutes but complete mineralization to CO₂ took several weeks. When urban runoff water (7.6mgCL^-1) replaced H₂O as the background matrix, some delays in degradation rates were observed but transforming a mixture of PAHs was still complete within 10min. Comparing the biodegradability of the ozonated products to the parent structures in unsaturated soil microcosms showed that the 3-ring phenanthrene was more biodegradable (as evidence by ¹⁴CO₂ released) than its ozonated products but for the 5-ring benzo(a)pyrene, the products produced by ozone were much more biodegradable (22% vs. 3% mineralized). For phenanthrene, we identified diphenaldehyde as the initial degradation product produced from ozonation. By continuing to pump the ozonated products (¹⁴C-labeled diphenaldehyde or ozone-treated benzo(a)pyrene) onto glass beads coated with microorganisms, we verified that biological mineralization could be achieved in a flow-through system and mineralization rates improved with acclimation of the microbial population (i.e., time and exposure to the substrate). These results support a combined ozone and biological approach to treating PAHs in urban runoff water.