Continuous •OH radical generation in microbubble-assisted wet oxidation for flue gas treatment: insights from radical quantification and modelling

Abstract

This study systematically examined SO2 and NOx removal via wet oxidation using H2O2 and O3 augmented by ultraviolet (UV) irradiation and microbubbles (MBs), combining experimental, spectroscopic, and modelling approaches. The presence of UV irradiation and MBs enhanced •OH radical generation and facilitated the conversion of NO into water-soluble NO2 and HNO3. Using 5,5-dimethyl-1-pyrroline N-oxide as a spin-trapping agent, electron spin resonance spectroscopy was used to quantify the •OH radicals generated across the six experimental setups. In millibubble systems, the radicals produced by O3 + UV, H2O2 + UV, and O3 + H2O2+ UV were rapidly depleted within 2–5 min. In contrast, the MB systems maintained a steady supply of radicals. The O3 + H2O2 + MB combination produced the highest radical concentration, comparable to that of O3 alone, which allowed continuous oxidation. Two-film modelling, incorporating experimentally measured radical yields, accurately predicted gas-phase removal, whereas neglecting radicals led to an underestimation of NO and SO2 removal. A system-specific mass-transfer correlation for O3 + MB, which increases the interfacial area, provides a reliable basis for scale-up. MB-based advanced oxidation processes also exhibited low energy consumption (0.46 kWh/m3 with H2O2) while efficiently removing pollutants under optimal conditions (298 K, pH 6, 0.2 mol/L H2O2, and 0.06 L/min). These findings highlight the critical role of •OH radicals, the advantages of MB stability in radical-mediated reactions, and the potential of MB-assisted wet oxidation as an energy-efficient and high-performance method for simultaneous SO2 and NOx reduction. © Higher Education Press 2026.