Ozonation and ozone-enhanced photocatalysis for VOC removal from air streams: Process optimization, synergy and mechanism assessment

The present work evaluates ozone driven processes (O-3, O-3/UVC, O-3/TiO2/UVA) in the NETmix mili-photoreactor, as a cost-effective alternative for the removal of volatile organic compounds (VOCs) from air streams, using n-decane as a model pollutant. The network of channels and chambers of the mili-photoreactor was coated with a TiO2-P25 thin film, resulting in a catalyst coated surface per reactor volume of 990 m(2) m(-3). Ozone and n-decane streams were fed to alternate chambers of the mili-photoreactor, promoting a good contact between O-3/n-decane/catalyst. Initially, direct reaction between n-decane and ozone (ozonation) was assessed for different O-3/n-decane (O-3/dec) feed molar ratios and total feed flow rates. Under the best conditions, ozonation process achieved total n-decane conversion (below the limit of detection), yielding a reaction rate (r(dec)) of 6.8 mu mol min(-1) or 6.7 mmol m(reactor)(-3) s(-1). However, the low reactivity of ozone with the degradation by-products resulted in a quite poor mineralization (similar to 10%). For the O-3/UVC system, an increase on relative humidity from 7 to 40% slight improved the n-decane oxidation rate, mainly associated with the generation of HO center dot from the reaction of active oxygen radicals (O center dot) and water molecules. A strong synergistic effect was observed when coupling TiO2/UVA photocatalysis with ozonation (O-3/TiO2/UVA), enhancing substantially the mineralization of n-decane molecules up to 100% under O-3/dec feed molar ratio of 15, photonic flux of 2.67 +/- 0.03 J s(-1) and a residence time of 2.0 s. Different reaction intermediates were detected for O-3, TiO2/UVA and O-3/TiO2/UVA oxidative systems, indicating the participation of different oxidant species (O-3, HO center dot, O center dot, etc.). (C) 2019 Elsevier B.V. All rights reserved.