Novel Photocatalytic Mechanisms for CHCl_3, CHBr_3, and CCl_3CO_2^-Degradation and the Fate of Photogenerated Trihalomethyl Radicals on TiO_2

Abstract

The photocatalytic degradation of CHCl_3, CHBr_3, CCl_4, and CCl_3CO_2^- is investigated in aqueous TiO_2 suspensions. A common intermediate, the trihalomethyl radical, is involved in the degradation of each substrate except for CCl_3CO_2^-. CHCl_3 and CHBr_3 are degraded into carbon monoxide and halide ions in the absence of dissolved oxygen. The anoxic degradation proceeds through a dihalocarbene intermedi ate, which is produced by sequential reactions of the haloform molecule with a valence band hole and a conduction band electron. Carbon dioxide and halide ion are formed as the primary products during CHCl_3 degradation in the presence of oxygen. Under these conditions, the trihalomethyl radicals react rapidly with dioxygen. At pH > 11, degrada tion of the haloforms is enhanced dramatically. This enhancement is ascribed to photoenhanced hydrolysis. The secondary reactions of the trichloromethyl radical generated during CCl_4 photolysis is strongly influenced by the nature of the electron donors. Both •CCl_3 and Cl^- production increase substantially when 2-propanol is present as an electron donor. A new photocatalytic mechanism for CCl_3CO_2^- degradation, which involves the formation of a dichlorocarbene intermediate, is proposed.

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