Photochemical generation and decay kinetics of superoxide and hydrogen peroxide in the presence of standard humic and fulvic acids
2017
Fujii, Manabu | Otani, Erika
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.
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