Evidence for an Additional Oxidant in the Photoassisted Fenton Reaction

The photo-Fenton reaction (Fe3+ + H2O2 + UV) has potential applications in wastewater treatment. This reaction was compared to H2O2 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 (H2O2/UV, Fe3+/UV, and Fe2+ + H2O2) 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 H2O2/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 Fe3+ + H2O2 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(O2H)2+ (K 15 = 1.15 × 10-2), that absorbs in the visible region and could be the precursor of the ferryl complex.