Removing PAHs from urban runoff water by combining ozonation, adsorption, and biodegradation

Abstract

The water quality of lakes and rivers associated with metropolitan areas is declining from increased inputs of urban runoff that contain polycyclic aromatic hydrocarbons (PAHs). Our objective was to develop a treatment technology that removes PAHs from urban runoff. We accomplished this by developing a flow-through system that uses ozone (O₃) to quickly transform PAHs in a runoff stream and then removes the O₃-transformed PAHs via adsorption to either activated carbon or carbon nano-onions (CNOs); adsorbed PAH products are then further biodegraded. To quantify the efficacy of this approach, ¹⁴C-labeled phenanthrene and benzo(a)pyrene, as well as a mixture of 16 PAHs were used as test compounds. These PAHs were pumped from a reservoir into a flow-through reactor that continuously ozonated the solution. Outflow from the reactor then went to a chamber that contained either activated carbon or CNOs that adsorbed the O₃-treated PAHs and allow clean water to pass. By adding a microbial consortium to the CNOs following adsorption, we observed that bacteria were able to degrade the adsorbed products and release more soluble, transformed products back into solution. Control treatments confirmed that parent PAH structures were not biologically degraded following CNO adsorption and that O₃-treated PAHs were not released from the CNO in the absence of bacteria. For phenanthrene, we identified diphenaldehyde as the product of ozonation and diphenaldehydehdric acid as the biological product released from the CNOs. We then compared the biodegradability of these products to the parent structures in unsaturated soil microcosms. Results showed that the parent phenanthrene structure was more biodegradable (Σ ¹⁴CO₂ released = 51%) than the transformed products (34.5 - 36.7%) but for the 5-ring benzo(a)pyrene, the products produced by ozone (22.3%) or released from the CNO following biological treatment (35.2%) were significantly more biodegradable than the parent compound (2.7%). As an alternative to using activated carbon or CNOs, we also verified that the ozonated product (diphenyldehyde) could be biologically mineralized in a bioreactor and that mineralization rates improved with acclimation of the microbial population. These results support the combined use of ozone and biological degradation as a means of removing PAHs from urban runoff.