ROLE OF CHLORINE AND OXYGEN IN THE PHOTOCATALYTIC DEGRADATION OF TRICHLOROETHYLENE VAPOR ON TIO2 FILMS

The photocatalytic degradation of trichloroethylene (TCE) was investig ated with ring-roughened annular reactors. Anatase titanium dioxide fi lms were immobilized by chemical vapor deposition using titanium tetra isopropoxide as the source of titanium. Experimental variables include d TCE concentration (0.1-10 parts per million by volume or ppmv), oxyg en content (20-200 000 ppmv), residence time (2.46-9.57 s), and reacto r length. In general, TCE conversion increased with increasing values of all of these variables up to a maximum of 99.4% within the range in vestigated. Ten chlorinated organic intermediates/products were observ ed primarily at low oxygen concentrations. Among these compounds, carb on tetrachloride, chloroform, hexachloroethane, pentachloroethane, and tetrachloroethylene were present at quantifiable levels. Additional e xperiments revealed that pentachloroethane and tetrachloroethylene als o decomposed with degradation efficiencies (up to 98 and 94%, respecti vely) being somewhat lower than that for TCE. Chloroform conversions w ere low (up to 30%), and no appreciable degradation of carbon tetrachl oride was observed for the range of operating conditions investigated. Chlorinated organic intermediate/product formation was the result of various reactions involving chlorine and hydrogen atom extractions and additions. Chlorine atom attack reactions were inhibited in the prese nce of relatively high oxygen concentrations including that in ambient air due to the predominance of competing oxidation reactions involvin g unidentified reactive oxygen species.