Novel photocatalytic mechanisms for CHCl3, CHBr3, and CCl3CO2- degradation and the fate of photogenerated trihalomethyl radicals on TiO2

Abstract

The photocatalytic degradation of CHCl3, CHBr3, CCl4, and CCl3CO2- is investigated in aqueous TiO2 suspensions. A common intermediate,the trihalomethylradical, is invoked in the degradation of each substrate except for CCl3CO2-. CHCl3 and CHBr3 are degraded into carbon monoxide and halide ions in the absence of dissolved oxygen. The anoxic degradation proceeds through a dihalocarbene intermediate, which is produced by sequential reactions of the haloform molecule with a valence band hale and a conduction band electron. Carbon dioxide and halide ion are formed as the primary products during CHCl3 degradation in the presence of oxygen. Under these conditions,the trihalomethyl radicals react rapidly with dioxygen. At pH > 11, degradation of the haloforms is enhanced dramatically. This enhancement is ascribed to photoenhanced hydrolysis. The secondary reactions of the trichloromethyl radical generated during CCl4 photolysis is strongly influenced by the nature of the electron donors. Both (CCl3)-C-. and Cl- production increase substantially when 2-propanol is present as an electron donor. A new photocatalytic mechanism for CCl3CO2- degradation, which involves the formation of a dichlorocarbene intermediate, is proposed.