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 ( 3 1NN*) and singlet ( 1 1NN*) excited states were estimated to be 1.5 × 10 6 and 2.5 × 10 8 s -1 , respectively. Our results provided quantitative evidence for the environmental relevance of 3 1NN* in waters. Possible reactions of 3 1NN* with various water components were evaluated. With the reduction and oxidation potentials of -0.37 and 1.95 V, 3 1NN* can be either oxidized or reduced by dissolved organic matter isolates and surrogates. We also showed that hydroxyl ( • OH) and sulfate (SO 4 •- ) radicals can be generated via the 3 1NN*-induced oxidation of inorganic ions (OH - and SO 4 2- , respectively). We further investigated the reaction kinetics of 3 1NN* and OH - forming • OH, an important photoinduced reactive intermediate, through complementary experimental and theoretical approaches. The rate constants for the reactions of 3 1NN* with OH - and 1NN with • OH were determined to be 4.22 × 10 7 and 3.95 ± 0.01 × 10 9 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.