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

Abstract

In the present work we analyze the degradation rates of nitrobenzene (NBE) and 4-nitrophenol (PNP), by using UV irradiation in the presence of HNO₂/NO₂⁻ or NO₃⁻ 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/NO₃⁻ systems than for UV/HNO₂/NO₂⁻ 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 HNO₂/NO₂⁻ photolysis increase to a maximum value and then decrease with increasing additive concentration. On the other hand, substrate degradation rates increase with NO₃⁻ 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/HNO₂/NO₂⁻ systems where HNO₂ and NO₂⁻ are efficient hydroxyl radical scavengers, whereas this decrease is not observed in UV/NO₃⁻ systems since the scavenging ability of NO₃⁻ is much lower. Inner filter and scavenging effects are discussed and the results are compared with the ones previously reported for UV/H₂O₂ systems.