The Roles of Hydroxyl Radicals and Superoxide in Oxidizing Aqueous Benzyl Alcohol under Ultrasound Irradiation.

The abatement of aromatic pollutants in water requires resource-intensive oxidation to nontoxic products by hydroxyl radicals (•OH). In this study, we elucidate the mechanisms of •OH-induced aromatic ring degradation by combining kinetic measurements, electron paramagnetic resonance spectroscopy, density functional theory (DFT) calculations, and kinetic modelling. We demonstrate that benzyl alcohol, a model aromatic compound, is oxidized by •OH radicals, generated by ultrasonic irradiation in an O2-rich environment, into aromatic compounds (benzaldehyde and phenol derivatives) and C1-C2 oxygenates (formic acid, glyoxal, and oxalic acid). Through pathways akin to atmospheric chemistry, these •OH radicals de-aromatize and fragment benzyl alcohol, producing 5-hydroxy-4-oxo-pentenal and other dicarbonyl products. Unique to the aqueous phase, however, superoxide (•O2–) is generated as a byproduct of •OH-benzyl alcohol reactions. •O2– acts as a potent nucleophile, oxidizing 5-hydroxy-4-oxo-pentenal into oxalic acid and C1 oxygenates via aldehyde and ketone intermediates. This process regenerates •O2– and does not consume •OH, thereby further degrading ring fragmentation products while preserving •OH to activate the refractory aromatic ring of benzyl alcohol. These nucleophilic •O2– reactions can therefore reduce the energy and chemical demands needed to degrade aromatic compounds, thus promoting the sustainable and scalable application of •OH-based oxidation processes in water treatment.