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 SO₄^2-, 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.

Mechanism of nitrite-dependent NO synthesis by human sulfite oxidase

In addition to nitric oxide (NO) synthases, molybdenum-dependent enzymes have been reported to reduce nitrite to produce NO. Here, we report the stoichiometric reduction in nitrite to NO by human sulfite oxidase (SO), a mitochondrial intermembrane space enzyme primarily involved in cysteine catabolism. Kinetic and spectroscopic studies provide evidence for direct nitrite coordination at the molybdenum center followed by an inner shell electron transfer mechanism. In the presence of the physiological electron acceptor cytochrome c, we were able to close the catalytic cycle of sulfite-dependent nitrite reduction thus leading to steady-state NO synthesis, a finding that strongly supports a physiological relevance of SO-dependent NO formation. By engineering SO variants with reduced intramolecular electron transfer rate, we were able to increase NO generation efficacy by one order of magnitude, providing a mechanistic tool to tune NO synthesis by SO.

Phototransformation of p-arsanilic acid in aqueous media containing nitrogen species

The effects of co-existing nitrogen species in surface water on the phototransformation of organoarsenical p-arsanilic acid (p-ASA) have been investigated using a xenon lamp as a simulated solar light source. Significant enhancements of p-ASA phototransformation efficiency were observed in the presence of nitrate and nitrite, increasing with the concentration of these species and pH, whereas ammonia showed no obvious effect. The products, including inorganic arsenic species and organic derivatives, have been analyzed in order to reveal the phototransformation pathways. In the nitrate and nitrite systems, only small proportions of inorganic arsenic species were generated, with the majority of p-ASA being converted into other organoarsenical derivatives through hydroxylation, nitration, and nitrosation. Phototransformation of p-ASA in collected natural surface water was also observed. This work has implications for the phototransformation of p-ASA in nitrogen-contaminated surface water.

Photochemical transformation of dimethyl phthalate (DMP) with N(iii)(H2ONO+/HONO/NO2−) in the atmospheric aqueous environment

The primary step of photochemical transformation of dimethyl phthalate (DMP) with N(iii) is ˙OH-addition on the aromatic ring of DMP to form a DMP–OH adduct, followed by several decay channels and corresponding rate constants are determined.

Phototransformation of 5-nitro-2-furaldehyde in aqueous solution. A laser flash photolysis and product analysis study

Laser flash photolysis of 5-nitro-2-furaldehyde (NFA) in solution shows a short-lived transient absorption with λ_max = 475 ± 5 nm, which is relatively insensitive to solvent polarity and is assigned to the lowest triplet state of NFA (³NFA*). In water, the ³NFA* absorption decays to a long-lived absorption, the study of which, at different times after the end of the laser pulse, reveals it to be due to a furyloxyl radical (λ_max ≈ 375 nm) and to the radical anion NFA˙^- (λ_max ≈ 400 nm). These radicals were produced independently to confirm the assignment. The lifetime of ³NFA* depends both on the solvent and the ground-state concentration of NFA. An (n, π*) nature is attributed to ³NFA* on the basis of the propensity of ³NFA* to abstract a hydrogen-atom from the solvent. Kinetic evidence for triplet excimer formation was obtained from the self-quenching of ³NFA* in solvents where the triplet decay is slower. The effect of acidity on the triplet lifetime is discussed with respect to an electron-transfer self-quenching mechanism, assisted by the triplet excimer which is proposed to dissociate into radical ions. Chromatographic and spectroscopic analysis of the photolysed aqueous solution of NFA enabled the identification of 5-hydroxymethylene-2(5H)-furanone, nitrite ion and an unknown substance as the major photoproducts. Conclusive evidence is presented that the observed 5-hydroxymethylene-2(5H)-furanone is formed from the furyloxyl radical. It is shown that the unknown substance can also be obtained from both the photoreduction of NFA in propan-2-ol and chemical reduction of NFA by Fe(s) in water (along with 5-amino-2-furaldehyde). Based on ¹H- and ¹³C-NMR (with 2-D HMQC) and vibrational absorption spectroscopy, a tentative structure is proposed for the substance of t_R 3.69 minutes obtained as a photoreduction product of NFA in water. Inorganic anions are shown to be one-electron oxidised by ³NFA* (as indicated by the observation of both the radical anion of NFA and the inorganic radical) with second-order rate constants being dependent on E of the inorganic radical. The implications of the results from complete quenching of ³NFA* by inorganic anions, and subsequent product analysis, for the phototransformation mechanism of NFA in water are discussed.

New insights into the environmental photochemistry of 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan): reconsidering the importance of indirect photoreactions

Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) is a widely used antimicrobial agent that undergoes fairly slow biodegradation. It is often found in surface waters in both the acidic (HTric) and basic (Tric(-)) forms (pKa ∼8), and it can undergo direct photodegradation to produce several intermediates including a dioxin congener (2,8-dichlorodibenzodioxin, hereafter 28DCDD). The latter is formed from Tric(-) and causes non-negligible environmental concern. Differently from current literature reports, in this paper we show that the direct photolysis would not be the only important transformation pathway of triclosan in surface waters. This is particularly true for HTric, which could undergo very significant reactions with (•)OH and, if the laser-derived quenching rate constants of this work are comparable to the actual reaction rate constants, with the triplet states of chromophoric dissolved organic matter ((3)CDOM*). Model calculations suggest that reaction with (3)CDOM* could be the main HTric phototransformation pathway in deep waters with high dissolved organic carbon (DOC), while reaction with (•)OH could prevail in low-DOC waters. In the case of Tric(-) the direct photolysis is much more important than for HTric, but triplet-sensitised transformation could produce 28DCDD + 27DCDD with higher yield compared to the direct photolysis, and it could play some role as dioxin source in deep waters with elevated DOC.

Theoretical and experimental evidence of the photonitration pathway of phenol and 4-chlorophenol: a mechanistic study of environmental significance

Light-induced nitration pathways of phenols are important processes for the transformation of pesticide-derived secondary pollutants into toxic derivatives in surface waters and for the formation of phytotoxic compounds in the atmosphere. Moreover, phenols can be used as ˙NO(2) probes in irradiated aqueous solutions. This paper shows that the nitration of 4-chlorophenol (4CP) into 2-nitro-4-chlorophenol (NCP) in the presence of irradiated nitrate and nitrite in aqueous solution involves the radical ˙NO(2). The experimental data allow exclusion of an alternative nitration pathway by ˙OH + ˙NO(2). Quantum mechanical calculations suggest that the nitration of both phenol and 4CP involves, as a first pathway, the abstraction of the phenolic hydrogen by ˙NO(2), which yields HNO(2) and the corresponding phenoxy radical. Reaction of phenoxyl with another ˙NO(2) follows to finally produce the corresponding nitrated phenol. Such a pathway also correctly predicts that 4CP undergoes nitration more easily than phenol, because the ring Cl atom increases the acidity of the phenolic hydrogen of 4CP. This favours the H-abstraction process to give the corresponding phenoxy radical. In contrast, an alternative nitration pathway that involves ˙NO(2) addition to the ring followed by H-abstraction by oxygen (or by ˙NO(2) or ˙OH) is energetically unfavoured and erroneously predicts faster nitration for phenol than for 4CP.

Formation of hydroxyl radicals by irradiated 1-nitronaphthalene (1NN): oxidation of hydroxyl ions and water by the 1NN triplet state

The excited triplet state of 1-nitronaphthalene ((3)1NN*) reacts with OH(-) with a second-order reaction rate constant of (1.66 ± 0.08)×10(7) M(-1) s(-1) (μ±σ). The reaction yields the ˙OH radical and the radical anion 1NN(-)˙. In aerated solution, the radical 1NN(-)˙ would react with O(2) to finally produce H(2)O(2) upon hydroperoxide/superoxide disproportionation. The photolysis of H(2)O(2) is another potential source of ˙OH, but such a pathway would be a minor one in circumneutral (pH 6.5) or in basic solution ([OH(-)] = 0.3-0.5 M). The oxidation of H(2)O by (3)1NN*, with rate constant 3.8 ± 0.3 M(-1) s(-1), could be the main ˙OH source at pH 6.5.