Overlooked Transformation of Nitrated Polycyclic Aromatic Hydrocarbons in Natural Waters: Role of Self-Photosensitization

Photochemical transformation is an important process that involves trace organic contaminants (TrOCs) in sunlit surface waters. However, the environmental implications of their self-photosensitization pathway have been largely overlooked. Here, we selected 1-nitronaphthalene (1NN), a representative nitrated polycyclic aromatic hydrocarbon, to study the self-photosensitization process. We investigated the excited-state properties and relaxation kinetics of 1NN after sunlight absorption. The intrinsic decay rate constants of triplet (31NN*) and singlet (11NN*) excited states were estimated to be 1.5 × 106 and 2.5 × 108 s–1, respectively. Our results provided quantitative evidence for the environmental relevance of 31NN* in waters. Possible reactions of 31NN* with various water components were evaluated. With the reduction and oxidation potentials of −0.37 and 1.95 V, 31NN* can be either oxidized or reduced by dissolved organic matter isolates and surrogates. We also showed that hydroxyl (•OH) and sulfate (SO4•–) radicals can be generated via the 31NN*-induced oxidation of inorganic ions (OH– and SO42–, respectively). We further investigated the reaction kinetics of 31NN* and OH– forming •OH, an important photoinduced reactive intermediate, through complementary experimental and theoretical approaches. The rate constants for the reactions of 31NN* with OH– and 1NN with •OH were determined to be 4.22 × 107 and 3.95 ± 0.01 × 109 M–1 s–1, respectively. These findings yield new insights into self-photosensitization as a pathway for TrOC attenuation and provide more mechanistic details into their environmental fate.