TiO2 photocatalysis for destruction of trace CCl3-CH3, CFCl2-CH3 and CF3-CH2F in water

The title compounds have been chosen to further assess the degrading capabilities of TiO2 photocatalysis in water and to determine whether reductive degradation pathways exist along with the dominant oxidative pathways. These haloalkanes were found to be removed with rate constants in the order CCl3-CH3>CFCl2-CH3 approximate to CF3-CH2F. The lower removal rates for the Freons are consistent with the rate constants previously reported for the reactions between these ethane derivatives and OH degrees radicals in the gas phase. The CCl3-CH3 removal rate was only ca. 1.2 times lower than that of nitrobenzene taken as a reference. The efficient dechlorination of CCl3-CH3 was illustrated by the relationship d[Cl-]/dt approximate to -3 x d[CCl3-CH3]/dt and by the very low concentrations of CH2Cl-COO- (or -CHO) the only chlorinated intermediate products detected. Most carboxylates, including cis-butanedioate, intermediately formed from CFCl2-CH3 were the same as from CCl3-CH3. Trifluoroacetate was the preponderant organic final product of CF3-CH2F, which showed the stability of the CF3 group. The formations of CH3-CHO and CH2Cl-CHO from CCl3-CH3 proved tile existence of minor reductive pathways. Reaction mechanisms are presented.