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

Abstract

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.

Bimolecular versus Trimolecular Reaction Pathways for H2O2 with Hypochlorous Species and Implications for Wastewater Reclamation

“…Second, we emphasize the novelty that 1 O 2 and 3 O 2 can be generated via both bimolecular and trimolecular pathways, and the continuous generation and accumulation of 1 O 2 is of practical benefit. The remarkable selective reactivity of 1 O 2 toward arenes with electron-rich functional groups (e.g., phenols and aromatic amino acids) renders it a highly reactive species for the further reduction of soluble microbial products (SMPs), as their major component consists of phenolic and amino moieties. , It also exhibits low reactivity toward background organic and inorganic water matrices, allowing for more efficient SMP reduction. In addition, 1 O 2 also kills cells through oxidatively generated damage to methionine, cysteine, DNA guanine nucleobases, lipids, and other biomolecules .…”