Hydroxyl radicals in ozone-based advanced oxidation of organic contaminants: A review

Many organic pollutants are chemically stable and thus cannot be degraded by classical wastewater treatment techniques. To solve this issue, ozone-based advanced oxidation processes using hydroxyl radicals with strong oxidation ability have been recently developed. Here we review hydroxyl radicals in ozone-based advanced oxidation processes with focus on reaction characteristics, generation, detection, and quantitation of hydroxyl radicals. Hydroxyl radicals are generated using ozone micro/nano-bubbles, peroxymonosulfate-activated ozone, ozone coupled with Fenton oxidation, electro-peroxone, or catalytic ozonation. Hydroxyl radicals are detected by electron paramagnetic resonance and quenching experiments. We also present applications in wastewater treatment and reactor design. Ozone-based advanced oxidation combines direct oxidation by ozone molecules and indirect oxidation by reactive oxygen species; regulating these two pathways remains challenging. The generation of hydroxyl radicals depends on the environmental matrix and on the chemical structure, properties, and ozone reactivity of contaminants. Chain reactions among reactive oxygen species induce contradictions during the analysis of results obtained by electron paramagnetic resonance, quenching techniques, and probe methods.