Photodegradation of chlorinated hydrocarbons in the presence and absence of dissolved oxygen in water.

The photodegradation rates of seven chlorinated hydrocarbons; C2Cl4, C2HCl3, C2H4Cl2, 1,1,1-C2H3Cl3, 1,1,2-C2H3Cl3, CHCl3, CCl4 were investigated under the UV bandwidths of 185 and 254 nm in the presence and absence of dissolved oxygen (DO) in water. These hydrocarbons are possible contaminants of groundwater. This study confirms that the degradation rates of all chlorinated hydrocarbons are elevated in the absence of DO. This was especially apparent for chlorinated methane and ethane. Tetrachloroethylene's rate was the highest among the seven hydrocarbons regardless of the DO levels. It was clear that the concentration of intermediate trichloroethylene produced by photodegradation of tetrachloroethylene in the absence of DO was 1/50th of that in the presence of DO. Photodegradation in the presence of DO resulted in the formation of O3 and hydroxyl radicals. Alternatively, the photodegradation in the absence of DO resulted in the formation of organic radicals and a dissociation of the bond. It is discussed that DO acts as an "inner filter" or "scavenger" that reduces the UV light intensity in the photoreactor. Molecular O2 has absorption bands at 185 and 254 nm, the former being stronger. The processes of degradation depend on the degradation rate relative to the presence and absence of DO.