Nitration of nitrobenzene in Fenton's processes.

Previous studies of nitrobenzene (NB) degradation by Fenton and photo-Fenton technologies have demonstrated the formation and accumulation of 1,3-dinitrobenzene (1,3-DNB) as a highly toxic reaction intermediate. In the present study, we analyze the conditions that favor 1,3-DNB formation during NB degradation by Fe(2+)/H(2)O(2), Fe(3+)/H(2)O(2), UV/Fe(3+)/H(2)O(2) or UV/H(2)O(2) processes. Nitration yields in Fenton, Fenton-like and photo-Fenton techniques were much higher than those observed in UV/H(2)O(2) systems. Besides, several tests showed that 1,3-DNB formation increases with the initial iron concentration and decreases as the initial H(2)O(2) concentration increases. In order to asses the key species involved in NB nitration mechanism, additional experiments were performed in the presence of NO(2)(-)or NO(3)(-). In dark systems, 1,3-DNB yield significantly increased with increasing [NO(2)(-)]_(0), while it was not affected by the presence of NO(3)(-). In contrast, 1,3-DNB yields were higher and more strongly affected by the additive concentration in UV/NO(3)(-) systems than in UV/HNO(2)/NO(2)(-) systems. Dark experiments performed at pH 1.5 in excess of HNO(2) along with UV/NO(3)(-) tests conducted in the presence of 2-propanol show that hydroxyl radicals play an important role in NB nitration since NB molecule does not react with the nitrating agents ONOOH, .NO or .NO(2). The results indicate that, in the experimental domain tested, the prevailing NB nitration pathway involves the reaction between the .OH-NB adduct and .NO(2) radicals.