Sulfate radical (SO 4•− )-mediated advanced oxidation processes via peroxymonosulfate (PMS) activation have been extensively investigated. However, the phototransformation of PMS in sunlit dissolved organic matter (DOM) solution has not been previously examined. For the first time, the photosensitized transformation of PMS in DOM-enriched solutions under simulated solar irradiation was observed. The generation of reactive species, including 1 O 2 , SO 4•− , and • OH, was confirmed by electron paramagnetic resonance and quantified by chemical probes. SO 4•− was the primary reactive species generated via the reaction of excited triplet DOM ( 3 DOM*) with PMS. 3 DOM* acted as a reactive reductant and was quickly oxidized by PMS, with an estimated reaction rate constant of (4.09 ± 0.21) × 10 8 M −1 s −1 . Compared to 3 DOM*, one-electron-reducing DOM (DOM •− ) was a minor contributor to the photosensitized transformation of PMS, and the contribution of DOM •− relied on the phenolic constituents. In addition, a series of different types of DOM, including terrestrial DOM, autochthonous DOM, and effluent organic matter and its fractions, were employed to examine the photosensitized transformation kinetics of PMS. Overall, the photosensitized transformation of PMS by irradiated DOM could be a useful and economical approach to generate SO 4•− under environmentally relevant conditions.
Hydroxyl radicals ( • OH) are important reactive species that are photochemically generated through solar irradiation of chromophoric dissolved organic matter (CDOM) in surface waters. However, the spatial distribution within the complex three-dimensional structure of CDOM has not been examined. In this study, we used a series of hydrophobic chlorinated paraffins as chemical probes to elucidate the microheterogeneous distribution of • OH in illuminated CDOM solutions. The steady-state concentration of • OH inside the CDOM microphase is 210 ± 31-fold higher than the concentration in the aqueous phase. Our results suggest that the most photochemically generated • OH are confined into the CDOM microphase. Thus, illuminated CDOM behaves as a natural microreactor for • OH-based oxidations. By including intra-CDOM • OH, the quantum yield of • OH for CDOM solutions was estimated to be 2.2 ± 0.5 × 10 −3 , which is 2 orders of magnitude greater than previously thought. The elevated concentrations of photogenerated • OH within the CDOM microphase may improve the understanding of hydrophobic pollutant degradation in aqueous environments. Moreover, our results also suggest that • OH oxidation may play more important roles in the phototransformation of CDOM than previously expected.
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