Microheterogeneous Distribution of Hydroxyl Radicals in Illuminated Dissolved Organic Matter Solutions

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.