Superoxide is likely generated in natural waters when photo-produced one-electron reductants within chromophoric dissolved organic matter (CDOM) react with dissolved oxygen. One of the main decay pathways for superoxide is self-dismutation to produce hydrogen peroxide. Comparisons of the production rates of one-electron reductants, as a poxy for superoxide production, to that of hydrogen peroxide have been conducted previously, and a value of two was expected for this ratio due to the stoichiometry of the reactions. However, ratios ranging from five to sixteen were obtained for a wide variety of samples. Values this high suggest the presence of an additional sink for superoxide that does not produce hydrogen peroxide. The possibility of a light-dependent pathway for superoxide decay has been proposed but has yet to be explicitly demonstrated. Here we provide evidence for this light-dependent sink through spiking experiments in which superoxide is spiked into a sample that is being irradiated and the decay of the spike is observed. Rapid consumption of the superoxide spike occurs during irradiation, as compared to a spike introduced into the sample in the dark, suggesting the presence of a light-dependent sink. Additionally, superoxide spikes conducted under irradiation with longer wavelength cut-off filters have slower consumption, indicating a wavelength dependence of this sink. The existence of this sink could have significant impacts on the modeling of production and decay rates of superoxide in natural waters. Further experiments will be conducted to try to determine what the sink is and what it is producing, along with detailed kinetic analyses to improve existing superoxide models.

Investigation of the existence of a light-dependent decay pathway for superoxide within chromophoric dissolved organic matter

ACS Spring 2022

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