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 oth er reactions that produce only hydroxyl radicals (OH.) in order to probe fo r additional or alternative intermediates that may contribute to the recogn ized potency of photo-fenton as an oxidant of organic compounds. Distinct d ifferences were found between photo-fenton and genuine OH. reactions. The k inetic deuterium isotope effect (KDIE) for cyclohexane in the photo-Fenton reaction increases from 1.2 to 1.4 with increasing concentration of OH. sca venger, 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 ter t-butyl alcohol. Photo-Fenton. catalyzed the epoxidation of cyclohexene at a much greater rate than H2O2/UV. The relative yields of chlorinated organi c acids from 1,1,2-trichloroethane, trichloroethene, and tetrachloroethene oxidation were markedly affected by the presence of iron. Time-resolved las er flash photolysis spectroscopy in the absence of organics revealed a tran sient, seen only in Fe3+ + H2O2 solutions, With broad absorbance in the vis ible and a lifetime of similar to 100 ns. The results suggest the participa tion of a high-valent oxoiron complex (ferryl) in addition to OH. in organi c compound oxidations. Hydrogen peroxide forms a complex with iron, Fe(O2H) (2+) (K-15 = 1.15 x 10(-2)), that absorbs in the visible region and could b e the precursor of the ferryl complex.