Investigating the Mechanism of Hydrogen Peroxide Photoproduction by Humic Substances

The mechanism(s) by which hydrogen peroxide (H2O2) is photoproduced by humic substances and chromophoric dissolved organic matter was probed by examining the dependence of the initial H2O2 photoproduction rate (R-H2O2) and apparent H2O2 quantum yields on dioxygen concentration for both untreated and borohydride-reduced samples. Although borohydride reduction substantially reduced light absorption, the R-H2O2 values were largely unaffected. Apparent monochromatic and polychromatic quantum yields thus increased following reduction. The results indicate that light absorption by charge-transfer states or by (aromatic) ketone/aldehydes does not lead to significant H2O2 photoproduction. High concentrations of triplet quenchers relative to that of dioxygen produced only small decreases (sorbic acid) or small increases (Cl- and Br-) in R-H2O2, indicating that neither O-1(2) nor excited triplet states of quinones contribute significantly to H2O2 photoproduction. The dependence of R-H2O2 on O-2 concentration provides evidence that the intermediate(s) reacting with O-2 to produce superoxide are relatively long-lived (approximately tens of microseconds or more). Evidence of the photochemical formation of O-2-reducing intermediates under anaerobic conditions was also obtained; these reducing intermediates appeared to be relatively stable in the absence of O-2. Our data suggest that these O-2-reducing intermediates are generated by intramolecular electron transfer from short-lived excited states of electron donors to ground-state acceptors.