Investigating the mechanism of hydrogen peroxide photoproduction by humic substances.

The mechanism(s) by which hydrogen peroxide (H(2)O(2)) is photoproduced by humic substances and chromophoric dissolved organic matter was probed by examining the dependence of the initial H(2)O(2) photoproduction rate (R(H(2)O(2))) and apparent H(2)O(2) quantum yields on dioxygen concentration for both untreated and borohydride-reduced samples. Although borohydride reduction substantially reduced light absorption, the R(H(2)O(2)) 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 H(2)O(2) 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(H(2)O(2)), indicating that neither (1)O(2) nor excited triplet states of quinones contribute significantly to H(2)O(2) photoproduction. The dependence of R(H(2)O(2)) 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.