The photo-Fenton reaction (Fe 3+ + H 2 O 2 + UV) has potential applications in wastewater treatment. This reaction was compared to H 2 O 2 photolysis and other reactions that produce only hydroxyl radicals (OH • ) in order to probe for additional or alternative intermediates that may contribute to the recognized potency of photo-Fenton as an oxidant of organic compounds. Distinct differences were found between photo-Fenton and genuine OH • reactions. The kinetic deuterium isotope effect (KDIE) for cyclohexane in the photo-Fenton reaction increases from 1.2 to 1.4 with increasing concentration of OH • scavenger, tert-butyl alcohol; whereas the KDIE in genuine OH • reactions (H 2 O 2 / UV, Fe 3+ /UV, and Fe 2+ + H 2 O 2 ) is 1.1 and unchanged in the presence of tert-butyl alcohol. Photo-Fenton catalyzed the epoxidation of cyclohexene at a much greater rate than H 2 O 2 /UV. The relative yields of chlorinated organic acids from 1,1,2-trichloroethane, trichloroethene, and tetrachloroethene oxidation were markedly affected by the presence of iron. Time-resolved laser flash photolysis spectroscopy in the absence of organics revealed a transient, seen only in Fe 3+ + H 2 O 2 solutions, with broad absorbance in the visible and a lifetime of ∼100 ns. The results suggest the participation of a high-valent oxoiron complex (ferryl) in addition to OH • in organic compound oxidations. Hydrogen peroxide forms a complex with iron, Fe(O 2 H) 2+ (K 15 ) 1.15 × 10 -2 ), that absorbs in the visible region and could be the precursor of the ferryl complex.

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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 O2. 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 O2, the reaction is catalytic in Fe(III) due to the re-oxidation of Fe(II) by oxo-radical intermediates; however, O2 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 reac tion.