A comparison of the degradation of 4-nitrophenol via direct and sensitized photocatalysis in TiO2 slurries

Sensitized photocatalysis has been demonstrated with colored pollutants such as nitrophenols. With visible light excitation of an adsorbed nitrophenol, charge injection into the conduction band of the semiconductor occurs and subsequent transformation of nitrophenols is possible. This work reports the results of experiments of direct and sensitized photocatalysis of 4-nitrophenol using an annular reactor, TiO2, and an artificial light source. The extent of 4-nitrophenol degradation was determined in order to assess mechanistic differences between direct and sensitized photocatalysis. Three major aromatic intermediates were identified for both types of photocatalysis—4-nitrocatechol, hydroquinone, and benzenetriol—but the reaction pathway and distribution of these intermediates are dependent on the photocatalytic mechanism. Nitrite, nitrate, and ammonium ions were analyzed to determine the participation of oxidation vs reduction in the degradation pathway. The systems were also analyzed under N2 atmosphere to determine the role of oxygen in the degradation pathway of 4-nitrophenol. The results presented herein verify those observed in powder systems indicating that sensitization in aqueous systems with a low 4-nitrophenol:semiconductor concentration achieves partial destruction of the parent compound and denitration. It is thought that these chemical transformations may enhance the biodegradability of nitroaromatic compounds, suggesting that sensitized photocatalysis has the potential to be used as a pretreatment process in combination with biological treatment.