Correlations between Dissolved Organic Matter Optical Properties and Quantum Yields of Singlet Oxygen and Hydrogen Peroxide

Various aquatic dissolved organic matter (DOM) samples produce singlet oxygen (O-1(2)) and hydrogen peroxide (H2O2) with quantum yields of 0.59 to 4.5% (O-1(2) at 365 nm) and 0.017 to 0.053% (H2O2, 300-400 nm integrated). The two species' yields have opposite pH dependencies and strong, but opposite, correlations with the E-2/E-3 ratio (A(254) divided by A(365)). Linear regressions allow prediction of both quantum yields from E-2/E-3 in natural water samples with errors ranging from -3% to 60%. Experimental evidence and kinetic calculations indicate that less than six percent of the H2O2 is produced by reaction between O-1(2) and DOM. The inverse relationship between the O-1(2) and H2O2 yields is thus best explained by a model in which precursors to these species are populated competitively. A model is presented, which proposes that important precursors to H2O2 may be either charge-transfer or triplet states of DOM.