Photolytic and photocatalytic degradation of surface oil from the Deepwater Horizon spill.

The photochemical behavior of Deepwater Horizon oil collected from the surface of the Gulf of Mexico was studied. Thin oil films on water were subjected to simulated sunlight, and the resulting chemical and optical changes were observed. Polycyclic aromatic hydrocarbons (PAHs) showed substantial photodegradation, with larger PAHs being more rapidly decomposed. About 60% of the fluorescence at the excitation and emission maxima was observed with 12h of simulated solar irradiation equivalent to approximately 3d of sunlight. Synchronous scan fluorescence measurements showed 80-90% loss of larger PAHs with 12h of simulated solar irradiation. Absorbance of the oil decreased by only 20% over the same time period. Alkanes showed no significant photochemical losses. After irradiation, the toxicity of water in contact with the oil significantly increased, presumably due to the release of water soluble photoproducts that were toxic. Photocatalyst addition resulted in enhanced degradation rate for PAHs, and toxicity of the aqueous layer was altered in the presence of photocatalysts added to the oil film. Photochemistry is an important pathway for degradation of large PAHs, which are typically resistant to biodegradation.