Photo-produced one-electron reductants within chromophoric dissolved organic matter (CDOM) have been shown to be likely precursors for formation of superoxide and therefore hydrogen peroxide. The current proposed mechanism involves photochemically produced one-electron reductants within CDOM reacting with dissolved oxygen to form superoxide, which dismutates to form hydrogen peroxide. Nitroxide radical probes have been used to trap one-electron reductants and their detection was previously performed using liquid chromatography with fluorescence detection. This method was modified to be achieved using a fluorescence spectrophotometer allowing for faster, easier, and more flexible measurements. The production rates of one-electron reductants and hydrogen peroxide have been measured for a wide variety of samples from reference materials to natural waters and 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 obtained thus far range from six (natural waters) to fourteen (extracts). A value greater than two indicates that not all photo-produced superoxide is undergoing dismutation to form hydrogen peroxide, suggesting that other reactions are competing with dismutation. In order to investigate the discrepancies in the ratios, various experimental factors were changed to see the resulting effects on the production rates of one-electron reductants and hydrogen peroxide. Some of the factors tested so far include irradiance wavelengths, concentration of CDOM, salinity, and addition of metal chelators. Results obtained from these experiments have shown that none of these factors affect the ratios observed. Interestingly, mixing reference fulvic acids with natural waters dampened one-electron reductant production. For future work, superoxide production rates will be measured. This will provide more information on the relationship between the reactions in the mechanism discussed above. Measurements of all three species (one-electron reductant, superoxide, and hydrogen peroxide) have yet to be conducted collectively on the same sample.

Comparing photoproduction rates of one-electron reductants and hydrogen peroxide in extracts and natural waters

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.