New Processes in the Environmental Chemistry of Nitrite: Nitration of Phenol upon Nitrite Photoinduced Oxidation DAVIDE VIONE, VALTER MAURINO, CLAUDIO MINERO, AND EZIO PELIZZETTI* Dipartimento di Chimica Analitica, Universita ` di Torino, Via P. Giuria 5, 10125 Torino, Italy 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 HNO 2 and in atmospheric hydrometeors by photolysis of NO 2 - . 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 NO 2 - and HNO 2 to form • NO 2 , either directly or via formation of • OH. As a consequence, nitrite and nitrous acid may act as • NO 2 rather than • OH sources. The radical • NO 2 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 NO 2 - . The reported findings suggest a completely new role of nitrite in aquatic environments. Introduction Nitrite is a relevant environmental factor both in natural waters (1-4) and in the atmosphere (5). Up to now, the attention wasmainlyfocused on nitrite UVphotolysis,leading to hydroxyl radical and nitrogen monoxide (6): As far as nitrite is the only photoactive species, it can play a dual role. Besides being a source of • OH, nitrite can act as a sinkfor the same radicals,reactingwith them at a diffusion- controlled rate giving • NO2 as product (6): Nitrogen dioxide promotes the nitration of a variety of organic compounds. Phenol (7-11) and various azaarenes (3) can be nitrated directly by • NO2, whereas the nitration of benzene(12) and various PAHs (13) is assisted by • OH.Other aromatic molecules of environmental concern that can be nitrated are resorcinol, catechol, hydroquinone, biphenyl, and some ofits derivatives (2).However,reaction 2 becomes significant onlyif[NO2 - ]isconsiderablyhigherthan theusual nitrite concentration in the environment.Nitration ofphenol (14) and various azaarenes (3) upon nitrite photolysis is actually possible but only at high [NO2 - ] (up to 0.1 M). Environmentalfactorshave often been studied separately, although most environmentalprocessesare the result ofvery complex interactions among several species. As a conse- quence, the study of the interactions between the various environmental factors is of foremost importance to the understanding of real environmental processes. We recently demonstrated that phenol photonitration in the presence of nitrite is enhanced by TiO2 in aqueous suspension (15). Titanium dioxide is both a promising photocatalyst for water detoxification (16) and an environ- mentalfactor (1). The reactive species in TiO2 photocatalysis are surface adsorbed hydroxyl radicals, free or trapped valence band holes (17), and conduction band electrons, which promote the redox processes of many dissolved molecules. Thereaction causingtheenhancementofphenolnitration in the presence of TiO2 is very similar to reaction 2. Photogenerated species on TiO2 particles ( • OHads) perform like homogeneous • OH (15, 17). As a consequence, the rate oflight-induced • OH generation is no longer limited byNO2 - photolysis and thus by [NO2 - ] (see reaction 1). The con- centration of nitrite only influences the formation rate of • NO2. Thus, the generation rate of • NO2 can be relevant at nitrite concentrations lower than in the case of nitrite photolysis. This has a potential environmental significance. In this framework, an interesting point is whether titanium dioxide is a unique case or also other substances present in the environment can induce the same processes. In this paper, we report on the effects ofdissolved Fe(III), R-Fe 2O3, -FeOOH,NO3 - ,and humicacid on phenolnitration in irradiated systemscontainingNO2 - .Actually,the formation of toxic nitroderivatives, in particular in the atmosphere, is of concern for the consequences on human health (18)and theenvironment(19).Forinstance,3-nitrobenzanthrone and 1,8-dinitropyrene are the most powerful direct mutagens so far detected in atmospheric particulate (20). Phenolwaschosen asa modelaromaticmolecule.Phenol nitration takes place via reaction with • NO2 or N2O4 ([N2O4] ∝ [ • NO2] 2 (2, 7-10)) or the oxidation of nitrosoderivatives (14). The first pathway, which has more general environ- mental significance, involves the generation of nitrogen dioxide by nitrite oxidation. Accordingly, the significance of the second mechanism was assessed by evaluation of the oxidation rate of4-nitrosophenolto 4-nitrophenolunder the same experimental conditions used for phenol nitration. Experimental Section Reagents and Materials. Phenol (P), 2-nitrophenol (2-NP), 4-nitrophenol(4-NP),4-nitrosophenol(4-NOP)(puritygrade >98%), and humic acid sodium salt were purchased from Aldrich,NaNO2 (>97%) and FeSO4 × 7H2O (99.5%) from Carlo Erba, Fe(ClO4)3 × 9H2O(>97%) from Fluka, NaNO3 (>99%), HClO4 (70%), and HNO3 (65%)from Merck.Allreagents were used as received without further purification. Acetonitrile and 2-propanol were LiChrosolv gradient grade, purchased from Merck. R-Fe 2O3 and -FeOOH have been synthesized following the procedure given by Leland and Bard (21). *Correspondingauthorphone: +39-011-6707630; fax: +39-011- 6707615; e-mail: pelizzet@ch.unito.it. NO 2 - + h v f • NO + • O - [Φ 310 nm ≈ 0.06] (1a) • O - + H + / • OH [p K a,1b ) 11.9] (1b) • OH + NO 2 - f • NO 2 + OH - [k 2 ) 1.0 × 10 10 M -1 s -1 ] (2) Environ. Sci. Technol. 2002, 36, 669-676 10.1021/es010101c CCC: $22.00  2002 American Chemical Society VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 669 Published on Web 01/15/2002