Evidence for an Additional Oxidant in the Photoassisted Fenton Reaction

Abstract

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.