A COMPARISON OF THE DEGRADATION OF 4-NITROPHENOL VIA DIRECT AND SENSITIZED PHOTOCATALYSIS IN TIO2 SLURRIES

Sensitized photocatalysis has been demonstrated with colored pollutant s such as nitrophenols. With visible light excitation of an adsorbed n itrophenol, charge injection into the conduction band of the semicondu ctor 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 a n 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, hydroqui none, and benzenetriol-but the reaction pathway and distribution of th ese intermediates are dependent on the photocatalytic mechanism. Nitri te, nitrate, and ammonium ions were analyzed to determine the particip ation of oxidation vs reduction in the degradation pathway. The system s were also analyzed under Nr atmosphere to determine the role of oxyg en in the degradation pathway of 4-nitrophenol. The results presented herein verify those observed in powder systems indicating that sensiti zation in aqueous systems with a low 4-nitrophenol:semiconductor conce ntration achieves partial destruction of the parent compound and denit ration. It is thought that these chemical transformations may enhance the biodegradability of nitroaromatic compounds, suggesting that sensi tized photocatalysis has the potential to be used as a pretreatment pr ocess in combination with biological treatment. Copyright (C) 1996 Els evier Science Ltd.