Photocatalytic transformations of chlorinated methanes in the presence of electron and hole scavengers

The photocatalytic degradation in the aqueous phase of CCl4, CHCl3 and CH2Cl2 over irradiated TiO2 has been investigated in the presence of electron and hole scavengers, including atmospheric oxygen. Chloromethanes degrade through combined reductive and oxidative processes, the importance of which varies on going from CCl4, to CHCl3 and CH2Cl2. Under aerated conditions at pH 5, almost complete conversion into CO2 and HCl has been observed. Although chloride ions were evolved stoichiometrically, at pH 11 persistent formation of formaldehyde and formic acid was observed. Stable intermediates, either chlorinated or dechlorinated (formic acid, formaldehyde and methanol) have been quantified. An initial reductive dominance was observed for CCl4 even in the presence of oxygen. CH2Cl2 degradation was largely oxidative, even from the early stages of degradation, whereas the roles of the oxidative and reductive pathways were comparable for CHCl3. The effect of hole and electron scavengers highlights the importance of reductive and oxidative pathways. Reductants such as alcohols (methanol, propan-2-ol, tert-butanol) enhance CCl4 degradation remarkably, have a limited effect on CHCl3 and strongly decrease the degradation rate of CH2Cl2. Electron scavengers like periodate and peroxydisulfate decrease the degradation rate of CCl4 and have limited effects on CHCl3. Halides (chloride and bromide) show a peculiar behaviour. Acting as hole scavengers, they produce active species (Cl-. and Br-.) that participate in reactions with transient intermediates. During degradation of CHCl3 in the presence of chloride, formation of CCl4 was reported. Interestingly, for CCl4 the degradation rate decreases and in the presence of bromide CBrCl3 is detected at trace level.