New processes in the environmental chemistry of nitrite: nitration of phenol upon nitrite photoinduced oxidation

The role of nitrite as an environmental factor has been widely recognized. Nitrite is a relevant source of *OH in the atmosphere, both in the gas phase via photolysis of gaseous HNO2 and in atmospheric hydrometeors by photolysis of NO2-. In aqueous systems, *OH production through nitrite photolysis can be negligible due to the competition for light absorption by dissolved Fe(III), colloidal iron oxides, and nitrate. These photoexcited oxidants interact with NO2- and HNO2 to form *NO2, either directly or via formation of *OH. As a consequence, nitrite and nitrous acid may act as *NO2 rather than *OH sources. The radical *NO2 is involved in the nitration of many aromatic compounds, of which phenol is a model in this work. Kinetic measurements using 2-propanol as *OH scavenger show that the direct production of *OH by aqueous Fe(III) species decreases as pH increases. At slightly acidic and neutral pH values, oxidation of nitrite occurs by direct electron transfer to photoexcited Fe(III)aq species or colloidal iron oxides, in addition to the *OH-mediated oxidation of NO2-. The reported findings suggest a completely new role of nitrite in aquatic environments.