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

• Published in: Environmental science & technology 2023-12, Vol.57 (49), p.20627-20635
• Main Authors: Le Roux, Danielle M., Powers, Leanne C., Blough, Neil V.
• Format: Article
• Language: English
• Subjects: Biogeochemical Cycling; Dissolved Organic Matter; Heavy metals; Hydrogen peroxide; Hydrogen Peroxide - chemistry; Irradiation; Natural waters; Oxygen; Post-irradiation; Reducing agents; Singlet Oxygen - chemistry; Superoxide; Superoxides
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.3c08254

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.