Use of Copper in Evaluating the Role of Phenolic Moieties in 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.