PHOTONIT · Phototransformation and photonitration processes of aromatic compounds in surface waters: environmental significance and impacts on living organisms

Phototransformation and photonitration processes of aromatic compounds in surface waters: environmental significance and impacts on living organisms

During the two years of the project, the research activity was focused into the understanding of photochemical processes that can occur in surface waters and would influence the fate of dissolved organic pollutants such as pesticides and their transformation intermediates. A particular interest was addressed to the photoinduced nitration reactions of aromatic compounds, which produce harmful nitroderivatives. The toxicity of a chloronitroderivative (2,4-dichloro-6-nitrophenol) toward the brackish-water crustacean Artemia salina was also assessed. Before addressing the actual environmental problem, fundamental research work was carried out to gain better understanding of the processes that may be involved in aromatic photonitration and in the phototransformation of dissolved pollutants. The photonitration reactions of aromatic compounds in surface waters involve nitrogen dioxide (•NO2), which is generated upon nitrate photolysis and nitrite photoxidation. The first part of the work was devoted to the assessment of the effects that the bicarbonate anion, the most widespread ionic species in freshwater, has on the photolysis of nitrate. It has been found that bicarbonate and/or carbonate can significantly influence nitrate photochemistry by reacting with transient species generated by nitrate irradiation, such as the radicals •OH + •NO2 within the cage of the water molecules and the unstable anion ONOO (peroxynitrite). Such processes could enhance the photochemical production of •NO2 by photolysis of nitrate. As far as nitrite is concerned, while its reaction with •OH is rather well known, very little information is available concerning oxidation to •NO2 in the presence of photoactive dissolved organic compounds under irradiation. To this purpose, nitrite oxidation to •NO2 was studied in the presence of anthraquinone-2-sulphonate (AQ2S) and 1-nitronaphthalene as proxies of dissolved organic matter. The choice of the former was motivated by the fact that it enables the study of the reactivity of an excited triplet state without the additional complication caused by the contemporary formation of other reactive transients such as singlet oxygen and the •OH radical (vide infra). In contrast, 1-nitronaphthalene is of interest because it yields all the cited transient species, in a similar way as chromophoric dissolved organic matter (CDOM) does in surface waters. The nitration of phenol into nitrophenols was used in both cases as a probe reaction for •NO2. In the case of AQ2S it was possible to use the experimental data to model the formation of •NO2 upon nitrite oxidation by the excited triplet states of CDOM. The model results indicated that the process can be a major source of •NO2 in organic-rich waters.

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