Photoreductive Mechanism of CCl4 Degradation on TiO2 Particles and Effects of Electron Donors

Abstract

The photoreductive degradation of CCl4 in TiO2 particulate suspensions in the presence of a variety of organic electron donors (alcohols, carboxylic acids, and benzene derivatives) has been examined. The rate of CCl4 dechlorination can be enhanced significantly when alcohols and organic acids are used as electron donors. Alcohols with alpha-hydrogens show complex behavior due to the formation of intermediate alpha-hydroxyalkyl radicals, which directly reduce CCl4. Kinetic isotope effects and structure-reactivity relationships show that hydrogen-abstraction by hydroxyl radicals plays an important role in the hole-scavenging mechanism. The pH of the TiO2 suspension influences the rate of CCl4 reduction either by altering the electrostatic interactions of electron donors on the TiO2 surface or by changing the reduction potential of the conduction band electron in a Nernstian fashion. It is demonstrated that CCl4 can be fully degraded under both oxic and anoxic conditions. CHCl3, C2Cl4, and C2Cl6 are detected as intermediates during photolysis at pH 2.8 while no intermediates are formed at pH 12.4. A photodegradation mechanism of CCl4 that includes both one-electron and two-electron transfer processes is proposed. Dichlorocarbene, which is formed through a two-electron reduction of CCl4, is directly trapped during the photolysis.