Investigating the Mechanism of Phenol Photooxidation by Humic Substances

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It is well established that organic pollutants such as phenols are degraded in the presence of chromophoric dissolved organic matter (CDOM) and sunlight in natural waters. Early work attributed the photochemical loss of phenols to the involvement of photoproduced reactive oxygen species (ROS) such as singlet oxygen (1O2), hydroxyl radical (OH) or peroxy radicals (RO2). However, evidence for the involvement of triplet excited states of aromatic ketones/aldehydes within CDOM has accumulated in the literature. 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 (RTMP) on dioxygen concentration and irradiation wavelength was examined both for a variety of untreated as well as borohydride-reduced HS and C18 extracts from the Delaware Bay and Mid-Atlantic Bight. The effect of [O2] and borohydride-reduction of SRFA was also examined for a series of substituted phenols of varying one-electron reduction potentials. We find that RTMP was inversely proportional to dioxygen concentration at [O2] > 50 μM, a dependence consistent with reaction with triplet excited states, but not with 1O2 or RO2. Modeling the dependence of RTMP on [O2] provided rate constants for TMP reaction, O2 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 RTMP on [O2] for these reduced samples, suggests that there remains another class of oxidizing triplet sensitizer, perhaps quinones. However, the results of the wavelength dependence reveal that the sensitization is driven primarily by shorter wavelength UV-B and UV-A absorbing moieties, consistent with the involvement of aromatic ketones and aldehydes but appearing to exclude the longer wavelength (visible) absorbing quinones as sensitizers. An inverse dependence of Φ on one-electron reduction potential was observed where DMOP ≈ TMP > 4-MOP > 4-MP > phenol. Similar dependencies were observed for TMP and 4-MOP in the dependence of Rprobe on [O2] whereas DMOP did not exhibit a substantially lower Rprobe at high [O2] as would be expected for a triplet sensitization mechanism. Moreover, that a significant amount of sensitization is observed following borohydride reduction of SRFA for DMOP under high [O2], as well as the very low sensitization observed at low [O2] indicates that a separate pathway, unrelated to triplets, may be important for the mechanism of DMOP photooxidation by chromophoric dissolved organic matter.