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. Chlorometh anes degrade through combined reductive and oxidative processes, the i mportance of which varies on going from CCl4, to CHCl3 and CH2Cl2. Und er aerated conditions at pH 5, almost complete conversion into CO2 and HCl has been observed. Although chloride ions were evolved stoichiome trically, at pH 11 persistent formation of formaldehyde and formic aci d was observed. Stable intermediates, either chlorinated or dechlorina ted (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 ea rly stages of degradation, whereas the roles of the oxidative and redu ctive pathways were comparable for CHCl3. The effect of hole and elect ron scavengers highlights the importance of reductive and oxidative pa thways. Reductants such as alcohols (methanol, propan-2-ol, tert-butan ol) enhance CCl4 degradation remarkably, have a limited effect on CHCl 3 and strongly decrease the degradation rate of CH2Cl2. Electron scave ngers like periodate and peroxydisulfate decrease the degradation rate of CCl4 and have limited effects on CHCl3. Halides (chloride and brom ide) show a peculiar behaviour. Acting as hole scavengers, they produc e active species (Cl-. and Br-.) that participate in reactions with tr ansient 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 de tected at trace level.