Identification of the Reactive Oxygen Species Responsible for Carbon Tetrachloride Degradation in Modified Fenton's Systems

The reactive oxygen species responsible for the transformation of carbon tetrachloride (tetrachloromethane, CT) by modified Fenton's reagent using hydrogen peroxide (H2O2) concentrations >0.1 M was investigated. Addition of the hydroxyl radical scavenger 2-propanol to modified Fenton's reactions did not significantly lower CT transformation rates. Scavenging by 2-propanol not only confirmed that hydroxyl radicals are not responsible for CT destruction, but also suggested that a major product of an iron (III)-driven initiation reaction, superoxide radical anion (O2•-), is the species responsible for CT transformation. To investigate this hypothesis, CT degradation was studied in aqueous KO2 reactions. Minimal CT degradation was found in CT−KO2 reactions; however, when H2O2 was added to the KO2 reactions at concentrations similar to those in the modified Fenton's reactions (0.1, 0.5, and 1 M), CT degradation increased significantly. Similar results were obtained when 1 M concentrations of other solvents were added to aqueous KO2 reactions, and the observed first-order rate constant for CT degradation correlated strongly (R2 = 0.986) with the empirical solvent polarity (ETN) of the added solvents. The results indicate that even dilute concentrations of solvents, including H2O2, can increase the reactivity of O2•- in water, probably by changing its solvation sphere. The higher reactivity of O2•- generated in modified Fenton's reagent, which has a less polar nature due to the presence of H2O2, may result in a wider range of contaminant degradation than previously thought possible.