Direct Evidence of a Light-Dependent Sink of Superoxide within Chromophoric Dissolved Organic Matter

Superoxide (O₂^•-) is produced photochemically in natural waters by chromophoric dissolved organic matter (CDOM) via the reaction of molecular oxygen with photoproduced one-electron reductants (OERs) within CDOM. In the absence of other sinks (metals or organic radicals), O₂^•- is believed to undergo primarily dismutation to produce hydrogen peroxide (H₂O₂). However, past studies have implicated the presence of an additional light-dependent sink of O₂^•- that does not lead to H₂O₂ production. Here, we provide direct evidence of this sink through O₂^•- injection experiments. During irradiations, spikes of O₂^•- are consumed to a greater extent (∼85-30% loss) and are lost much faster (up to ∼0.09 s^-1) than spikes introduced post-irradiation (∼50-0% loss and ∼0.03 s^-1 rate constant). The magnitude of the loss during irradiation and the rate constant are wavelength-dependent. Analysis of the H₂O₂ concentration post-spike indicates that this light-dependent sink does not produce H₂O₂ at low spike concentrations. This work further demonstrates that simply assuming that the O₂^•- production is twice the H₂O₂ production is not accurate, as previously believed.