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

Abstract

The abatement of aromatic pollutants in water requires their oxidation to nontoxic products by resource-intensive reactions with hydroxyl radicals (•OH). We elucidate the mechanisms of •OH-induced aromatic ring degradation by combining kinetic measurements, electron paramagnetic resonance spectroscopy, density functional theory 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-) forms by •OOH deprotonation, which is generated by ultrasound (alongside •OH) and as a byproduct of •OH-benzyl alcohol reactions. •O2- acts as a 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 number of chemical initiators needed to degrade aromatic compounds, thus advancing •OH-based oxidation processes in water treatment.