Photooxidative and Photoreductive Degradation of Chlorinated Hydrocarbons on Aqueous Titanium Dioxide Colloids

Abstract

Studies on photocatalytic degradation reactions of chlorinated hydrocarbons on TiO2 colloids are presented in this thesis. Photoreactivities of metal-ion doped quantum-sized TiO2 colloids and photochemical mechanisms of CHCl3 and CCl4 degradation are investigated in detail. A systematic study of 21 metal-ion doped quantum-sized (2-4 nm) TiO2 colloids is performed by measuring their photoreactivities and the transient charge-carrier recombination dynamics. Doping with Fe3+, Mo5+, Ru3+, Os3+, Re5+, V4+, and Rh3+ at 0.1-0.5 atom% significantly increases the photoreactivity for both CHCl3 oxidation and CCl4 reduction while Co3+ and Al3+ doping decreases the photoreactivity. The quantum yields obtained during CW photolyses are quantitatively correlated with the measured transient absorption signals of the charge-carriers. 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. It is demonstrated that CCl4 can be fully degraded under both oxic and anoxic conditions. A photodegradation mechanism of CCl4 that includes both one-electron and two-electron transfer is proposed. The mechanism of photoreduction of CCl4 on illuminated TiO2 surfaces is investigated by selectively trapping transient free radical intermediates. Dichlorocarbene and trichloromethyl radical are trapped with 2,3-dimethyl-2-butene during the photocatalytic degradation of CCl4. The rate of formation of trapped :CCl2 and •CCl3 is found to be a function of [H2O], pH, [CCl4], the nature of the dissolved gas, and light intensity. A two-electron photoreductive pathway (via dichlorocarbene formation) is suggested to be the dominant mechanism leading to the full degradation of CCl4. The photocatalytic degradation reactions of CHCl3, CHBr3, CCl4, and CCl3CO2- are investigated in aqueous TiO2 suspensions. CHCl3 and CHBr3 are degraded into carbon monoxide and halide ion in the absence of dissolved oxygen. The anoxic degradation proceeds through a dihalocarbene intermediate which is produced by sequential reactions of the haloform molecule with a valence band hole and a conduction band electron. Degradation of haloform is enhanced dramatically at pH >11. This enhancement is ascribed to photoenhanced hydrolysis.