Reactive oxygen species (ROS) such as superoxide (O2-) and hydrogen peroxide (H2O2) can be photochemically generated in aerobic waters containing natural organic matters (NOM) such as humic substances (HS). To investigate the effect of NOM molecular composition on the kinetics and mechanism of ROS transformation, photochemical O2- generation and subsequent H2O2 production via catalyzed and uncatalyzed (bimolecular dismutation) O2- decay were examined in the presence of 14 types of HS (pH 8.0). By using chemiluminescence and colorimetric techniques, the photochemical O2- generation rate, quasi-steady-state O2- concentration, catalyzed and uncatalyzed O2- decay rates, and H2O2 production rate were found to vary significantly by factors of 72, 18, 14, 320, and 7.7, respectively, depending on the type of HS and degree of photolysis. For more than half of the HS samples, both uncatalyzed and catalyzed reductive decay of photogenerated O2- were significantly involved in H2O2 generation, and their rates were comparable to those for O2- oxidative decay in which H2O2 is not generated. These results suggest that the chemical quality of HS influenced the H2O2 generation pathway. Correlation analyses indicated that rate constants associated with HS-mediated photochemical O2- and H2O2 generation are significantly correlated with HS molecular composition including total and aromatic C contents. In particular, practical indices representing NOM aromaticity including specific ultraviolet absorbance (SUVA) can be useful for predicting NOM-mediated ROS generation and decay kinetics. Overall, the present work suggests that NOM concentration and its quality influence NOM-mediated ROS dynamics in aqueous systems.
Keywords: Aromatic moiety; Humic substance; Photochemistry; Reactive oxygen species.
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