Iron redox cycling in hydroxyl radical generation during the photo-Fenton oxidative degradation: Dynamic change of hydroxyl radical concentration

Effects of Fenton reagents, iron and hydrogen peroxide, on the generation of hydroxyl radicals (OH) during photo-Fenton degradation of acetaldehyde dissolved in water were examined. The dynamic change in OH radical concentration during the acetaldehyde degradation by the photo-Fenton process was measured using a coumarin fluorescence probing technique. The OH radical concentrations increased with increasing initial Fe concentrations within the Fe dosage range examined in this study. With increasing acetaldehyde loading the OH radical concentration decreased. The OH radical concentration changed rather complicatedly as the acetaldehyde degradation by the photo-Fenton oxidation process proceeded. The dynamic change in OH radical concentration could be divided into four phases: the OH radical concentration instantaneously and significantly increased just after the start of photo-Fenton oxidation process (Phase I), subsequently decreased (Phase II), increased again after reaching its minimum value (Phase III), and gradually decreased after the OH radical concentration reached a maximum point (Phase IV). This complicated change in OH radical concentration during photo-Fenton degradation of acetaldehyde could be rationally described by the Fe-redox cycle via the Fe–organic complex formation and its oxidation by dissolved oxygen besides the Fenton and photo-Fenton reactions. It was found that the oxidation of iron–organic complexes formed by ferric iron with acetaldehyde and its degradation products accelerated the rate of Fe-redox cycle. On the other hand, the formation of Fe–organic complexes led the decrease in Fe ions available for the Fenton and photo-Fenton reactions.

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• The OH radical concentration during the photo-Fenton process was measured.
• The change in OH radical concentration could be described by the Fe-redox cycle.
• The oxidation of iron–organic complexes accelerated the rate of Fe-redox cycle.
• The Fe–organic complexes led to decrease in Fe ions available for the degradation.