Hydrophobic Model Systems for Oil Film Photooxidation: Part II: Direct and Indirect Pathways Controlled by Oxygen and Molecular Structure

Abstract

Sunlight-driven oil photooxidation alters petroleum composition and influences the environmental fate of spilled oil; however, the mechanisms controlling hydrocarbon photolysis remain poorly understood. To address this, we investigated the direct and indirect photodegradation of saturated and aromatic hydrocarbons in a hydrophobic model petroleum system. Photosensitizers (phenalenone, tetracene, xanthone, naphthol, and anthraquinone) and quenchers (furfuryl alcohol, sorbic acid, and DABCO) were used to probe reactive pathways. The effects of solvent type and oxygen saturation were also examined. Results indicated that PAHs with fewer than three rings exhibited limited direct photoreactivity due to limited overlap with solar radiation, whereas three- ring or larger- PAHs were photoreactive. Alkylation substantially increased direct photolysis rates for anthracene, pyrene, and benzo[a]pyrene. For anthracene and 9-methylanthracene, oxygen availability did not affect direct photodegradation, suggesting oxygen plays a minor role under the tested conditions. Except for 9-methylanthracene, photosensitizers markedly enhanced transformation of most three-ring or larger PAHs, and this effect was suppressed by singlet oxygen quenchers, confirming the role of reactive oxygen species. Saturated and monoaromatic hydrocarbons showed no indirect photoreactivity. For indirect photolysis, reduced oxygen saturation decreased degradation for anthracene, methylpyrene, and methylbenzo[a]pyrene, while other compounds were unaffected. Methyl substitution produced position-specific effects across PAHs, leading to variable photoreactivity. The findings of this study highlight the complex photochemical behavior of PAHs in oil films, where multiple direct and indirect pathways operate simultaneously in a highly substrate- and oxygen-dependent manner. These results further emphasize the importance of reactive oxygen species, energy transfer, and quenching dynamics in controlling oil photooxidation.