Inhomogeneous generation of hydroxyl radicals in hydrogen peroxide solution induced by ultraviolet irradiation and in a Fenton reaction system.

The density of hydroxyl radical (•OH) generation by degeneration of hydrogen peroxide (H2O2) during UVB irradiation and in a Fenton reaction system was estimated. The purpose of this study was to evaluate whether these reaction systems generate spatially uniform or inhomogeneous •OH from H2O2 in the reaction mixture. A series of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) solutions of several concentrations (0.13‒1661 mM) were prepared. For UVB irradiation, 1 μl of 98 mM, 980 mM, or 9.8 M H2O2 solution was added to a 100-μl aliquot of DMPO solution, and the reaction mixture was irradiated with UVB. For the Fenton reaction, 1 μl of 98 mM H2O2 and 1 μl of 100 mM FeSO4 were added to a 100-μl aliquot of DMPO solution. After UVB irradiation or adding FeSO4, the entire volume of the reaction mixture was drawn into PTFE tubing and measured by X-band EPR. The DMPO-OH concentration in the reaction mixture was plotted versus the molecular density of DMPO, and the density of •OH generation was estimated from an inflection point on the plotted profile. The local densities of the UV-induced •OH in the H2O2 water solutions depended on the concentration of H2O2 in the solution, and were likely localized. The energy absorption process of photons was suspected to occur in a step-wise manner in a limited volume. •OH generation in the Fenton reaction system was expected to be uniformly distributed, but inhomogeneous •OH generation was observed at the molecular level.