Photoproduction Rates of One-Electron Reductants by Chromophoric Dissolved Organic Matter via Fluorescence Spectroscopy: Comparison with Superoxide and Hydrogen Peroxide Rates

One-electron reductants (OER) photoproduced by chromophoric dissolved organic matter (CDOM) have been shown to be likely precursors for the formation of superoxide and subsequently hydrogen peroxide. An improved method that employs a nitroxide radical probe (3AP) has been developed and utilized to d...

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Bibliographic Details
Published in:Environmental science & technology 2021-09, Vol.55 (17), p.12095-12105
Main Authors: Le Roux, Danielle M, Powers, Leanne C, Blough, Neil V
Format: Article
Language:English
Subjects:
Biogeochemical Cycling
Dissolved organic matter
Flow injection analysis
Fluorescence
Fluorescence spectroscopy
Hydrogen
Hydrogen peroxide
Hydroxylamine
Measurement methods
Nitroxide
Oxidation
Oxygen
Photoproduction
Reducing agents
Superoxide
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Summary:One-electron reductants (OER) photoproduced by chromophoric dissolved organic matter (CDOM) have been shown to be likely precursors for the formation of superoxide and subsequently hydrogen peroxide. An improved method that employs a nitroxide radical probe (3AP) has been developed and utilized to determine the photoproduction rates of OER from a diverse set of CDOM samples. 3AP reacts with OER to produce the hydroxylamine, which is then derivatized with fluorescamine and quantified spectrofluorometrically. Although less sensitive than traditional methods for measuring R O2•–, measuring R H provides a simpler and faster method of estimating R O2•– and is amenable to continuous measurement via flow injection analysis. Production rates of OER (R H), superoxide (R O2•–), and hydrogen peroxide (RH2O2) have a similar wavelength dependence, indicating a common origin. If all the OER react with molecular oxygen to produce superoxide, then the simplest mechanism predicts that R H/R H2O2 and RO2•–/R H2O2 should be equal to 2. However, our measurements reveal R H/R H2O2 values as high as 16 (5.7–16), consistent with prior results, and R O2•–/R H2O2 values as high as 8 (5.4–8.2). These results indicate that a substantial fraction of superoxide (65–88%) is not undergoing dismutation. A reasonable oxidative sink for superoxide is reaction with photoproduced phenoxy radicals within CDOM.
ISSN:0013-936X
1520-5851
DOI:10.1021/acs.est.1c04043