Abstract
Triplet-state chromophoric dissolved organic matter (3CDOM*) plays an important role in aquatic photochemistry, yet much remains unknown about the reactivity of these intermediates. To better understand the kinetic behavior and reactivity of 3CDOM*, we have developed an indirect observation method based on monitoring time-resolved singlet oxygen (1O2) phosphorescence kinetics. The underpinning principle of our approach relies on the fact that O2 quenches almost all triplets with near diffusion limited rate constants, resulting in the formation of 1O2, which is kinetically linked to the precursors. A kinetic model relating 1O2 phosphorescence kinetics to triplet excited states produced from isolated humic substances and in whole natural-water samples (hereafter referred to as 3CDOM*) was developed and used to determine rate constants governing 3CDOM* natural lifetimes and quenching by oxygen and 2,4,6-trimethylphenol (TMP), a common triplet probe molecule. 3CDOM* was found to exhibit smaller O2 and TMP quenching rate constants, ∼9 × 108 and ∼8 × 108 M-1 s-1, respectively, compared with model sensitizers, such as aromatic ketones. Findings from this report shed light on the fundamental photochemical properties of CDOM in organic matter isolates and whole waters and will help refine photochemical models to more accurately predict pollutant fate in the environment.
Highlights
An important driver of indirect photochemistry in natural waters is chromophoric dissolved organic matter (CDOM), which is the light-absorbing fraction of the complex mixture of biologically derived organic molecules present in all aquatic systems
Light absorption by CDOM initiates the formation of several reactive intermediates, including hydroxyl radical (OH), hydrogen peroxide (H2O2), singlet oxygen (1O2), and others, collectively known as photochemically produced reactive intermediates (PPRIs).[1]. Many of these PPRIs are generated from triplet-state CDOM (3CDOM*), which are CDOM molecules in their electronically excited triplet state. 3CDOM* is both a producer of PPRIs and an important oxidant itself, playing a role in the transformation of aquatic contaminants, biomolecules, and the cycling of carbon and other elements.[2]
The processes considered in the kinetic analysis for CDOM and the nomenclature used for each respective rate constant are listed in Scheme 1
Summary
An important driver of indirect photochemistry in natural waters is chromophoric dissolved organic matter (CDOM), which is the light-absorbing fraction of the complex mixture of biologically derived organic molecules present in all aquatic systems. One common method used to assess 3CDOM* reactivity is to follow the loss of 2,4,6-trimethylphenol (TMP), which is known to be oxidized by 3CDOM*.4 This method is generally effective but only serves as a qualitative probe because the bimolecular reaction rate constant between 3CDOM* and TMP, kTMP, is not known and has only been estimated and may vary as a function of dissolved organic matter (DOM) structure and properties. If kTMP was known and consistent across DOM samples, TMP could be used as a quantitative probe to determine 3CDOM* steady-state concentrations ([3CDOM*]SS) and intersystem crossing quantum yields (ΦISC).[5,6] Typically, triplet reactivity is assessed using transient absorption (TA) spectroscopy, wherein the triplet excited-state intermediates are directly observed, allowing the bimolecular reaction rate constant to be determined. A kinetic model is proposed and used to determine average rate constants governing 3CDOM* quenching by oxygen and TMP as well as the natural triplet lifetimes for a number of DOM isolates and natural water samples. A portion of 1CDOM* forms 3CDOM* with efficiency of ΦISC. 3CDOM* can undergo quenching with substrate C or O2, with a fraction of O2 quenching events ( fΔ) forming 1O2, which typically ranges from ∼0.1−1 depending on the
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