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 x 106 and 2.5 x 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 x 10(7) and 3.95 +/- 0.01 x 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.

Automated summary

Photochemical transformation is an important process for trace organic contaminants (TrOCs) in sunlit surface waters. This study focuses on the self-photosensitization pathway of 1-nitronaphthalene (1NN), a representative nitrated polycyclic aromatic hydrocarbon. The results provide quantitative evidence for the environmental relevance of the excited triplet state (31NN*) in waters and indicate the formation of hydroxyl and sulfate radicals through the oxidation of inorganic ions.