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

Abstract

The present work evaluates ozone driven processes (O3, O3/UVC, O3/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\u202fm2\u202fm-3. Ozone and n-decane streams were fed to alternate chambers of the mili-photoreactor, promoting a good contact between O3/n-decane/catalyst. Initially, direct reaction between n-decane and ozone (ozonation) was assessed for different O3/n-decane (O3/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 (rdec) of 6.8\u202fμmol\u202fmin-1 or 6.7\u202fmmol\u202fm-3reactor s-1. However, the low reactivity of ozone with the degradation by-products resulted in a quite poor mineralization (~10%). For the O3/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 from the reaction of active oxygen radicals (O) and water molecules. A strong synergistic effect was observed when coupling TiO2/UVA photocatalysis with ozonation (O3/TiO2/UVA), enhancing substantially the mineralization of n-decane molecules up to 100% under O3/dec feed molar ratio of 15, photonic flux of 2.67\u202f±\u202f0.03\u202fJ\u202fs-1 and a residence time of 2.0\u202fs. Different reaction intermediates were detected for O3, TiO2/UVA and O3/TiO2/UVA oxidative systems, indicating the participation of different oxidant species (O3, HO, O, etc.).