Investigating the Mechanism of Phenol Photooxidation by Humic Substances

Abstract

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 μM, a dependence consistent with reaction with triplet excited states, but not with (1)O(2) or RO(2). 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.

Marine Photochemistry of Organic Matter

“…An alternative explanation for the increased DIC (and singlet oxygen) AQYs in the above studies is that photodegradation (bleaching) selectively removed less (and non-) photoproductive chromophores while not significantly decreasing overall photoproduction rates of DIC (and singlet oxygen), which would result in higher AQYs. This explanation is supported by a number of recent studies (Mostafa and Rosario-Ortiz, 2013; and may be in part due to LMW CDOM having higher photoproduction efficiencies than HMW CDOM (Dalrymple et al, 2010;Golanoski et al, 2012;Halladja et al, 2007;Richard et al, 2004;Sharpless, 2012), the latter being selectively photodegraded.…”

“…This variability appears to be due to differences in DOM/CDOM type, e.g., terrestrial vs. microbial, prior photobleaching history and irradiation wavelength (Boyle et al, 2009;Del Vecchio and Blough, 2004;Golanoski et al, 2012;Ma et al, 2010;Reader and Miller, 2012;Stubbins et al, 2011;White et al, 2010; see discussion of CDOM sources in Chapter 10). Similar variability was also observed for DIC photoproduction by broadband irradiations (Reader and Miller, 2012;White et al, 2010).…”

“…Thus, the role of superoxide in directly oxidizing DOM oxidation and producing DIC is unknown, but likely minor. Highly energetic, excited state ketone triplets 3 (R 1 R 2 -CO)* are also known to oxidize DOM (Canonica et al, 2000;Cottrell et al, 2013;Golanoski et al, 2012), but like superoxide, their role in oxidizing DOM and producing DIC in natural waters is not known.…”

Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter

“…The quenching rate constants reported here (2.2−7.3 × 10 6 s −1 ) are similar to those determined in SRNOM by a similar method (2.5−3 × 10 6 s −1 ) 66 but higher than quenching rates determined by measurements of 1 O 2 formation rates and TMP loss rates (5−50 × 10 4 s −1 ). 8,93 While inaccuracies in the estimated value of k p introduce some uncertainty, these results suggest shorter 3 DOM lifetimes for higher energy 3 DOM that is capable of reaction with t,t -HDA.…”

Photogeneration of reactive transient species upon irradiation of natural water samples: Formation quantum yields in different spectral intervals, and implications for the photochemistry of surface waters

“…The rationale was to use an elevated TMP concentration (still allowing an easy assessment of its degradation), to minimise the impact of uncertainties associated with the reaction rate constant between TMP and 3 CDOM* and with the quenching rate constant of 3 CDOM* (vide infra) (Halladja et al, 2007;Al Housari et al, 2010;Golanoski et al, 2012). Uncertainties are understandably important given the poorly known nature of CDOM, and of 3 CDOM* as a consequence.…”