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.

Kinetics of nitrobenzene and 4-nitrophenol degradation by UV irradiation in the presence of nitrate and nitrite ions.

In the present work we analyze the degradation rates of nitrobenzene (NBE) and 4-nitrophenol (PNP), by using UV irradiation in the presence of HNO(2)/NO(2)(-) or NO(3)(-) added as sources of hydroxyl radicals. With both nitroaromatic substrates pseudo-first-order kinetics were observed for conversion degrees of at least 45% within the analyzed experimental domain. The apparent rate constants (k(app)) were higher for UV/NO(3)(-) systems than for UV/HNO(2)/NO(2)(-) systems at high additive concentrations, whereas the opposite trend was observed for low additive concentrations. In addition, k(app) values were found to decrease with increasing substrate concentration. The analysis of the distribution of the most important products detected by HPLC suggests that reactions involving nitrogen species are likely to play a secondary role in the apparent rate constants measured. NBE and PNP degradation rates induced by HNO(2)/NO(2)(-) photolysis increase to a maximum value and then decrease with increasing additive concentration. On the other hand, substrate degradation rates increase with NO(3)(-) concentrations until a plateau is reached, but no decrease of k(app) is observed even at very high additive concentrations. The results may be quantitatively described by a simplified model that considers two opposite effects. On the one hand, the increase of additive concentrations increases the amount of photons absorbed by the photoactive species in both systems. On the other hand, high additive concentrations significantly decrease the degradation rates in UV/HNO(2)/NO(2)(-) systems where HNO(2) and NO(2)(-) are efficient hydroxyl radical scavengers, whereas this decrease is not observed in UV/NO(3)(-) systems since the scavenging ability of NO(3)(-) is much lower. Inner filter and scavenging effects are discussed and the results are compared with the ones previously reported for UV/H(2)O(2) systems.

Abatimento de arsénico en aguas de consumo mediante el empleo de la técnica de hierro cero valente

El objetivo del trabajo ha sido caracterizar las variables que afectan la producción sostenible de Fe (III), intimamente relacionada con el mecanismo de remoción de arsénico, basado en procesos de remoción adsorción y/o coprecipitación. De acuerdo con los estudios realizados, la superficie de contacto de Fe cero valente, la disponibilidad de oxígeno y la presencia de sales en la matriz acuosa, son las variables que mayor influencia presentan en el proceso heterogéneo inicial de corrosión electroquímica. La segunda parte de este artículo se publica en el No. 93

Abatimento de arsénico en aguas de consumo mediante el empleo de la técnica de hierro cero valente

El objetivo del trabajo ha sido caracterizar las variables que afectan la producción sostenible de Fe (III), intimamente relacionada con el mecanismo de remoción de arsénico, basado en procesos de remoción adsorción y/o coprecipitación. De acuerdo con los estudios realizados, la superficie de contacto de Fe cero valente, la disponibilidad de oxígeno y la presencia de sales en la matriz acuosa, son las variables que mayor influencia presentan en el proceso heterogéneo inicial de corrosión electroquímica. La segunda parte de este artículo se publica en el No. 93