Investigation of the Iron–Peroxo Complex in the Fenton Reaction: Kinetic Indication, Decay Kinetics, and Hydroxyl Radical Yields

The Fenton reaction describes the reaction of Fe­(II) with hydrogen peroxide. Several researchers proposed the formation of an intermediate iron–peroxo complex but experimental evidence for its existence is still missing. The present study investigates formation and lifetime of this intermediate at various conditions such as different Fe­(II)-concentrations, absence vs presence of a hydroxyl radical scavenger (dimethyl sulfoxide, DMSO), and different pH values. Obtained results indicate that the iron–peroxo complex is formed under all experimental conditions. Based on these data, stability of the iron–peroxo complex could be examined. At pH 3 regardless of [Fe­(II)]0 decay rates for the iron–peroxo complex of about 50 s–1 were determined in absence and presence of DMSO. Without DMSO and [Fe­(II)]0 = 300 μM variation of pH yielded decay rates of about 70 s–1 for pH 1 and 2 and of about 50 s–1 at pH 3 and 4. Hence, the iron–peroxo complex becomes more stable with increasing pH. Furthermore, pH-dependent hydroxyl radical yields were determined to investigate whether the increasing stability of the intermediate complex may indicate a different reaction of the iron–peroxo complex which might yield Fe­(IV) instead of hydroxyl radical formation as suggested in literature. However, it was found that hydroxyl radicals were produced proportionally to the Fe­(II)-concentration.