Photoreductive Mechanism of CCl_4 Degradation on TiO_2 Particles and Effects of Electron Donors

Abstract

The photoreductive degradation of CCl_4 in TiO_2 particulate suspensions in the presence of a variety of organic electron donors (alcohols, carboxylic acids, and benzene derivatives) has been examined. The rate of CCl_4 dechlorination can be enhanced significantly when alcohols and organic acids are used as electron donors. Alcohols with α-hydrogens show complex behavior due to the formation of intermediate α-hydroxyalkyl radicals, which directly reduce CCl_4. Kinetic isotope effects and structurere-activity relationships show that hydrogen-abstraction by hydroxyl radicals plays an important role in the hole-scavenging mechanism. The pH of the TiO_2 suspension influences the rate of CCl_4 reduction either by altering the electrostatic interactions of electron donors on the TiO_2 surface or by changing the reduction potential of the conduction band electron in a Nernstian fashion. It is demonstrated that CCl_4 can be fully degraded under both oxic and anoxic conditions. CHCl_3, C_2Cl_4, and C_2Cl_6 are detected as intermediates during photolysis at pH 2.8 while no intermediates are formed at pH 12.4. A photodegradation mechanism of CCl_4 that includes both one-electron and two-electron transfer processes is proposed. Dichlorocarbene, which is formed through a two-electron reduction of CCl_4, is directly trapped during the photolysis.

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