The mechanisms of hydrogen peroxide (H2O2) photoproduction by humic substances (HS) were investigated, with H2O2 production rates (RH2O2) and apparent quantum yields acquired for both untreated and borohydride-reduced samples. While borohydride reduction substantially reduced light absorption, the RH2O2 were largely unaffected. Thus, quantum yields increased after reduction, indicating that light absorption by charge-transfer states or by (aromatic) ketone/aldehydes does not contribute significantly to H2O2 photoproduction. The dependence of RH2O2 on dioxygen concentration provided lifetime(s) for the O2-reducing intermediate(s) in the 10's of microseconds range. High concentrations of triplet quenchers provided evidence that neither 1O2 nor excited triplet states of quinones contribute significantly to H2O2 photo-production. Based on these results, the O2-reducing intermediate appears to be formed through intramolecular electron transfer from an excited state electron donor to a ground-state acceptor. Employing stable nitroxides as radical/redox traps, the nature and extent of the O2-reducing intermediate(s) generated under irradiation was further investigated.

Mechanisms of hydrogen peroxide photoproduction by humic substances

245th National Meeting (2013)

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