Photodegradation of 4-chloronitrobenzene in the presence of aqueous titania suspensions in different gas atmospheres

The photocatalytic degradation of 4-chloronitrobenzene (4-CNB) was carried out using Degussa P25 TiO₂ as photocatalyst in three different gas atmospheres: nitrogen, oxygen, and ozone. The total organic carbon (TOC) and inorganic anions including chloride, nitrite, and nitrate anions were measured to monitor the mineralization processes, while the degradation of 4-CNB and the formation of intermediates were followed by liquid chromatography–mass spectrometry (LC/MS). Results showed that the photocatalytic efficiency followed the order of TiO₂/UV/N₂ < TiO₂/UV/O₂ < TiO₂/UV/O₃, which was further proved by evaluating the reaction activities using electron paramagnetic resonance (EPR) spin trapping technique. Chlorine atom, nitro group and hydrogen atom of the benzene ring could be displaced by hydroxyl radical (•OH) leading to the formation of chloride, nitrite (III) anions and a variety of phenols, then the nitrite (III) anions were further oxidized to nitrate (V) anions. 4-Nitrophenol and 5-chloro-2-nitrophenol were identified and quantified in both of the TiO₂/UV/N₂ and TiO₂/UV/O₂ processes while no aromatic intermediates were monitored in the process of TiO₂/UV/O₃.