Photocatalytically Assisted Hydrolysis of Chlorinated Methanes under Anaerobic Conditions

The photocatalytic degradation of CCl4, CHCl3, and CH2Cl2 over irradiated TiO2 has been investigated at pH 5 and pH 11 under anaerobic conditions. Chloromethanes degrade through combined reductive and oxidative processes. Photocatalytically assisted hydrolysis, given by sequential reactions involving ¥OH/e-/H+, is 106-108 times faster than the corresponding thermal process. Dechlorination of chloromethanes is achieved with degradation rates in the order CCl4>CHCl3>CH2Cl2. Stable intermediates, either chlorinated (chloromethanes, C2Cl6, and C2Cl4) or dechlorinated (formic acid, formaldehyde, and methanol) have been quantified. The average carbon oxidation number remains almost unchanged at the end of the degradation process, although for CCl4 in the early stages it is markedly reduced with slow regrowth of nC toward the initial value. Reaction pathways accounting for the observed results are presented based on literature data concerning transient intermediates.