Photoproduction Rates of One-Electron Reductants by Chromophoric Dissolved Organic Matter via Fluorescence...

Area of Science:

• Environmental Chemistry
• Photochemistry
• Organic Geochemistry

Background:

• Chromophoric dissolved organic matter (CDOM) is a significant photosensitizer in aquatic environments.
• Photoproduction of one-electron reductants (OER) by CDOM is a key pathway for generating reactive oxygen species (ROS).
• Understanding the formation rates of OER, superoxide (O2•−), and hydrogen peroxide (H2O2) is crucial for aquatic chemistry.

Purpose of the Study:

• To develop and apply an improved method for determining OER photoproduction rates from CDOM.
• To investigate the relationship between OER, superoxide, and hydrogen peroxide production rates.
• To elucidate the fate of photochemically produced superoxide in aquatic systems.

Main Methods:

• Utilized a nitroxide radical probe (3AP) to quantify OER photoproduction rates.
• 3AP reacts with OER to form hydroxylamine, which is derivatized with fluorescamine.
• Quantification was performed spectrofluorometrically, with potential for flow injection analysis.

Main Results:

• The 3AP method provides a simpler and faster estimation of superoxide production rates compared to traditional methods.
• Production rates of OER (RH), superoxide (RO2•−), and hydrogen peroxide (RH2O2) exhibit similar wavelength dependencies, suggesting a common origin.
• Measured RH/RH2O2 ratios (5.7–16) and RO2•−/RH2O2 ratios (5.4–8.2) indicate that 65–88% of superoxide does not undergo dismutation.

Conclusions:

• The study presents an improved method for measuring OER photoproduction by CDOM.
• A significant portion of photochemically produced superoxide reacts with other species rather than dismutating to hydrogen peroxide.
• Photoproduced phenoxy radicals within CDOM are a likely sink for un-dismutated superoxide.