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, O/UVC, O/TiO/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 TiO-P25 thin film, resulting in a catalyst coated surface per reactor volume of 990 m m. Ozone and n-decane streams were fed to alternate chambers of the mili-photoreactor, promoting a good contact between O/n-decane/catalyst. Initially, direct reaction between n-decane and ozone (ozonation) was assessed for different O/n-decane (O/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) of 6.8 μmol min or 6.7 mmol m s. However, the low reactivity of ozone with the degradation by-products resulted in a quite poor mineralization (~10%). For the O/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 TiO/UVA photocatalysis with ozonation (O/TiO/UVA), enhancing substantially the mineralization of n-decane molecules up to 100% under O/dec feed molar ratio of 15, photonic flux of 2.67 ± 0.03 J s and a residence time of 2.0 s. Different reaction intermediates were detected for O, TiO/UVA and O/TiO/UVA oxidative systems, indicating the participation of different oxidant species (O, HO, O, etc.).