CHLORINATED BY-PRODUCTS FROM THE TIO2-MEDIATED PHOTODEGRADATION OF TRICHLOROETHYLENE AND TETRACHLOROETHYLENE IN WATER

The degradation mechanisms of aqueous solutions of tetrachloroethylene (PCE) and trichloroethylene (TCE) have been studied in a 5.6-L recirculating photoreactor utilizing Pyrex-filtered xenon radiation (lambda > 290 nm) and a photocatalytic semiconductor (Degussa P25 TiO2). Previously unreported byproducts from the photocatalytic degradation of each chloroalkene (CA) were observed. For PCE, at pH 8 and under an atmosphere of air, the major chlorinated byproducts were dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) (<15% based upon PCE degraded). For TCE, at neutral pH and under an atmosphere of air, low yields (<4% based upon TCE degraded) of dichloroacetaldehyde (DCAAD) and DCAA were formed. Also, there were traces of trichloroacetaldehyde (TCAAD) and TCAA. The effects of various additives to the reaction mixture suggest the existence of two mechanisms for substrate removal: a reductive pathway involving conduction band electrons which produces dichlorinated byproducts and an oxidative pathway leading to mineralization and trichlorinated byproducts.