The photochemical generation of hydroxyl radicals in the UV-vis/ferrioxalate/H2O2 system

A reaction mechanism has been validated for the photochemical generation of hydroxyl radicals by ultraviolet or visible irradiation of oxalato iron(II I) complexes (ferrioxalate) in the presence of hydrogen peroxide and 2-prop anol as a model substrate. A kinetic simulation program incorporating the s et of reactions was written to predict the behavior of this photochemical s ystem. The program calculates the quantum yield of oxidation of 2-propanol used as a hydroxyl radical scavenger. The scavenger's oxidation product, 2- propanone, was analyzed by gas chromatography after controlled exposure of the solution to a calibrated light source. The theoretical quantum yields o f 2-propanol oxidation, Phi(RH), agreed reasonably well with experimentally determined Phi(RH) values under a variety of initial reaction conditions. The value of Phi(RH), which under appropriate conditions is directly propor tional to the quantum yield for the generation of hydroxyl radicals, Phi(OH ), was considerably greater than unity in most cases (often Phi(RH) = 3.0-4 .0), indicative of a chain mechanism involving iron cycling between the II and III oxidation states.