Effects of Dissolved Oxygen and Light Exposure on Determination of KOC Values for PAHs Using Fluorescence Quenching

The fluorescence of some polycyclic aromatic hydrocarbons (PAHs) has been observed to decline over time, potentially leading to overestimation of PAH−humic binding coefficients ( K OC ) determined by fluorescence quenching. This phenomenon was investigated using phenanthrene, anthracene, pyrene, triphenylene, and a soil humic acid. Fluorescence quenching experiments were conducted under ambient conditions and under conditions that minimized exposure of the samples to UV radiation and dissolved oxygen. All four compounds experienced decays in fluorescence intensity when exposed to UV radiation and dissolved oxygen, but anthracene and pyrene were affected to a much larger extent than phenanthrene and triphenylene. Baseline decays in fluorescence intensity were reflected in inflated apparent K OC values, particularly for anthracene and pyrene. For example, the apparent K OC for anthracene when no attempt was made to limit dissolved oxygen or exposure to UV radiation was 10.9 × 10 4 mL/g of C. When exposure to UV radiation alone was minimized, the apparent K OC decreased by a factor of 2. A similar result was found when exposure to dissolved oxygen alone was minimized. When exposure to both UV radiation and dissolved oxygen was minimized, K OC was 3.69 × 10 4 mL/g of C. Photooxidation processes may explain these observations; however, more work is needed to confirm this.