2. Sensitized degradation of chlorophenols on iron oxides induced by visible light - Comparison with titanium oxide

The sensitized photocatalytic degradation of mono-, di- and trichlorophenols on iron oxides aqueous suspensions of alpha -Fe2O3 and alpha -FeOOH is reported in detail. The degradation of these compounds followed pseudo-first-order kinetics when alpha -Fe2O3 was used as photocatalyst. a-FeOOH was found to be inactive for chlorophenols degradation with the exception of 2,4-dichlorophenol (2,4-DCP) where a modest effect was observed. The formation of a surface complex by the chlorophenols with the iron oxide and the solubility of the particular chlorophenol in aqueous solution were observed to be the controlling parameters during the photodegradation. The results obtained with the most active catalyst alpha -Fe2O3 are compared with TiO2. Total mineralization of chlorophenols was observed on TiO2 while on alpha -Fe2O3 only partial mineralization was observed. In either case, the intermediates produced in solution during the photodegradation were found to be significantly more biodegradable than the initial compound. For mono-, di- and trichlorophenols the overall photocatalytic degradation was observed to increase in the order: 2,4,6-trichlorophenol (2,4,6-TCP) < 2,3-dichlorophenol (2,3-DCP) < 2-chlorophenol (2-CP) < 2,4-DCP. The former sequence shows that the recalcitrant 2,4-DCP degrades more rapidly than other chlorophenols tested during this study. The photodegradation of chlorophenols on <alpha>-Fe2O3 and TiO2 proceeds mechanistically through para-hydroxylation of the initial compound as suggested by the intermediates found by high-pressure liquid chromatography HPLC during the course of the degradation. (C) 2001 Elsevier Science B.V. All rights reserved.