Reduction of perchloroalkanes by ferrioxalate-generated carboxylate radical preceding mineralization by the photo-fenton reaction

Perhalogenated aliphatic compounds in water are difficult to degrade by advanced oxidation processes that rely on hydroxyl radical as the reactive oxidant. We have successfully degraded carbon tetrachloride and hexachloroethane in UV-illuminated (300-400 nm) acidic oxic or anoxic solutions containing Fe(III) and oxalate. Kinetic and product studies were carried out to elucidate the mechanism and role of O-2. The initial step is a one-electron reduction of the perhaloalkane by CO2.-, which is generated by photolysis of iron(III) oxalate complexes. The products are lower-chlorinated compounds, several of which are unreactive toward CO2.-. In the presence of O-2, the reaction is catalytic in Fe(III) due to the re-oxidation of Fe(II) by ore-radical intermediates; however, O-2 also retards the rate by scavenging CO2.-. After the perhalogenated substrate has been transformed, the system can be made oxidizing by the addition of H2O2, resulting in mineralization of the remaining organochlorine intermediates by a photo-Fenton reaction.