Useof Copper in Evaluating the Role of Phenolic Moietiesin the Photooxidation of Dissolved Organic Matter

In a recent study, copper was shown to act as a novel quencher for investigating the mechanism of the photooxidation and photobleaching of dissolved organic matter (DOM) by selectively quenching the one-electron oxidizing intermediates of DOM (DOM^D•+). However, the capture of DOM^D•+ by Cu is possibly partially due to strong competition from phenolic antioxidant moieties intrinsically present in DOM for DOM^D•+ quenching. In this study, the extent of interaction between DOM^D•+ and phenolic antioxidant moieties is quantified by measuring the inhibitory effect of Cu on DOM photooxidation and photobleaching under varying pH (5.2–10.0) conditions. The increase in pH facilitates formation of deprotonated phenolic moieties (p<i>K</i>_a ∼ 9–10), increasing their quenching capacity of DOM^D•+. Accordingly, our results indicate that the inhibitory effect of Cu on the DOM photobleaching and the loss of electron-donating moieties of DOM significantly decreased with an increase in pH, suggesting more pronounced competition for DOM^D•+ from antioxidant phenolic moieties within DOM. Considering the precursors of DOM^D•+ also originate from phenolic moieties of DOM, the findings of this study provide important insights into the long-distance charge transfer reactions occurring at different phenolic moiety sites during DOM photooxidation.