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 NO_x 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 NO_x reduction.

Graphical abstract

Keywords

DMPO / Microbubbles / Millibubbles / Modelling / •OH radicals / Ozone

Highlight

	● Generation of •OH radicals is demonstrated using microbubble technology.
	● Radical formation is confirmed via ESR with DMPO as the spin-trapping agent.
	● MB system enhances gas dissolution in water and improves gas–liquid mass transfer.
	● A model describing mass transfer and reaction kinetics is developed.
	● A correlation for the mass transfer coefficient is established.

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