Upon absorption of light, chromophoric dissolved organic matter (CDOM) produces one-electron reductants which are likely precursors for formation of superoxide and therefore hydrogen peroxide. The proposed mechanism involves photochemically produced one-electron reductants within CDOM reacting with dissolved oxygen to form superoxide, which dismutates into hydrogen peroxide. One-electron reductants have been detected previously using nitroxide radical probes and liquid chromatography with fluorescence detection. Modifications to this method have allowed it to be conducted simply using a fluorometer. Production rates of one-electron reductants and hydrogen peroxide have been measured for a variety of samples from reference materials to natural waters and their extracts. Based on the stoichiometry of the proposed mechanism, the ratio of the production rate of one-electron reductants to that of hydrogen peroxide should be equal to two. However, ratios from five to sixteen have been obtained. A value greater than two suggests that superoxide undergoes other oxidative reactions that are competing with dismutation. Work has since turned to studying superoxide production. The first direct measurements of the polychromatic wavelength dependence for superoxide production have been obtained and the dependence is similar to that of the one-electron reductants and hydrogen peroxide. Additionally, superoxide production is close in magnitude to one-electron reductant production and is therefore also much higher than hydrogen peroxide production, further highlighting the presence of an additional sink. The possibility of a light-dependent, oxidative sink for superoxide within CDOM has begun to be investigated. Spiking experiments have been conducted where superoxide is spiked into a sample that is being irradiated. The spikes of superoxide are consumed rapidly during the irradiation, suggesting the presence of a light-dependent sink. Future work will involve developing mathematical models to further study superoxide production and decay and using model compounds to study possible superoxide reactions with organic radicals.

Comparing photo-production rates of one-electron reductants, superoxide, and hydrogen peroxide by chromophoric dissolved organic matter; Investigating the oxidative sink of superoxide

ACS Spring Meeting 2021

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