Investigating the mechanism of the hydrogen peroxide photoproduction from chromophoric dissolved organic matter

Abstract

The photochemical pathways for H2O2 production from chromophore dissolved organic matter (CDOM) were investigated extensively, employing in part a selective sodium borohydride reduction method to examine the structural basis of the H2O2 formation. Estimates of the lifetime of possible H2O2 precursor(s) have been acquired by examining the dependence of H2O2 production rates on dioxygen concentration. The results indicate that H2O2 arises from intramolecular electron transfer from an excited singlet donor to a ground-state acceptor. Possible donors include substituted phenols, while possible acceptors included quinones, which unlike ketones/aldehydes are not irreversibly reduced by borohydride.

The relationship between the rate of H2O2 formation and the rate of reducing intermediate(s) formation upon irradiation of CDOM was thoroughly investigated employing radical molecular probe. The results obtained from the dependence of dioxygen concentrations and wavelengths indicate that approximate 90% of produced one-electron reducing intermediates is converted to superoxide. The stoichiometric ratio between the H2O2 and total reducing radicals suggests that 67% of the photochemically produced superoxide decays through oxidant sinks other than dismutation to H2O2.

The effect of adding external phenol electron donors on the H2O2 production rate was also examined. Substantially enhanced rates of H2O2 production are observed, which is substantially inhibited by borohydride reduction (40-50%), similar to the loss of TMP previously reported. In addition, H2O2 production rate increases as the dioxygen concentration decreases, consistent with reaction with a triplet state intermediate. The results all indicate that an additional pathway of H2O2 formation is introduced in the presence of sufficient high concentration electron donors. Reaction between the phenol electron donors and the excited triplet state of aromatic ketones/aldehydes yields a ketyl radical that subsequently react with O2 to form O2- and then H2O2. The enhancement generally follows the reduction potential of the added phenols, as DMOP> MOP> PHE, with the exception of TMP. In the case of TMP, secondary radical reactions could be the cause of this difference.