Investigating the Mechanism of Phenol Photooxidation by Humic Substances

To probe the mechanism of the photosensitized loss of phenols by humic substances (HS), the dependence of the initial rate of 2,4,6-trimethylphenol (TMP) loss (R-TMP) on dioxygen concentration was examined both for a variety of untreated as well as borohydride-reduced HS and C-18 extracts from the Delaware Bay and Mid-Atlantic Bight. R-TMP was inversely proportional to dioxygen concentration at [O-2] > 50 mu M, a dependence consistent with reaction with triplet excited states, but not with O-1(2) or RO2. Modeling the dependence of R-TMP on [O-2] provided rate constants for TMP reaction, O-2 quenching, and lifetimes compatible with a triplet intermediate. Borohydride reduction significantly reduced TMP loss, supporting the role of aromatic ketone triplets in this process. However, for most samples, the incomplete loss of sensitization following borohydride reduction, as well as the inverse dependence of R-TMP on [O-2] for these samples, suggests that there remains another class of oxidizing triplet sensitizer, perhaps quinones.