Photocatalytic degradation of chlorinated pyridines in titania aqueous suspensions

The photocatalytic degradation of 3-chloropyridine was studied using different titania-based catalysts, its disappearance followed a zero-order kinetics. In our conditions, complete mineralization of 100 ppm of 3-chloropyridine occurred within ca. 480 min of UV-radiation on bare-titania systems. Doping of titania with metals was detrimental for photocatalysis. Some reaction intermediates were identified by SPME–GC–MS and a tentative degradation pathway was proposed implying two parallel routes with hydroxylation and ring opening prior to or after the release of chloride ions. Unlike 3-chloropyridine, 2-chloropyridine degradation follows a first-order kinetics. Competitive reactions on solutions containing 3-chloropyridine and 2-chloropyridine evidenced the stronger adsorption of the latter compound. Finally, in a similar way as described for 3-chloropyridine, photocatalytic degradation of 2,3-dichloropyridine followed a zero-order kinetics. Moreover, detection of coupling products containing 3 chlorine atoms evidenced the existence of a radical mechanism.