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This work addresses photocatalytic oxidation for indoor air purification and more especially the key and realistic issue of VOC mixture treatment. The VOC mixture effect needs to be investigated since indoor air contains several tens of VOCs which may impact photocatalytic oxidation performances. In order to be closer to realistic conditions, concentrations in the ppb range were used and toluene, decane and trichloroethylene (TCE) were treated first as single compounds, then as an equimolar ternary mixture under the same experimental conditions. In a 120 L batch reactor, VOC removal kinetics, reaction intermediates and CO 2 mineralization are addressed using dedicated analytical devices compliant with typical ppb level monitoring. Regarding removal kinetics, the mixture effect affects the three VOCs. Regarding toluene and decane removal kinetic, the mixture effect is evidenced as mostly equivalent to a concentration effect, however TCE kinetic is further impacted. From the reaction intermediate point of view, the accurate monitoring of transiently produced intermediates evidences for the first time cross-reactivity between reaction intermediates originating from different primary VOCs. This phenomenon leading to novel reaction intermediate is mostly induced by chlorinated species produced by TCE degradation but remains moderate. An increase in VOC initial concentration to upper ppb levels emphasized a sequential degradation of primary VOC which may be related to competitive adsorption even on such a low concentration range. Finally, even if the mixture effect delays the removal of the primary VOCs, mineralization is slightly modified and, unlike formerly reported experiments on ppm range, final mineralization rates are equivalent under single or mixture condition at ppb levels. This work highlights the fact that photocatalytic treatment of VOC mixtures cannot be directly extrapolated from single VOC behaviour even at ppb level.

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An investigation of the photocatalytic degradation of indoor air model pollutants: toluene, n-decane and trichloroethylene (TCE), as single contaminants and in a mixture is proposed. The degradation of these contaminants was performed in a continuous closed-loop reactor operating in recirculation mode. Degradations were conducted at ppb level concentrations and under humid conditions (RH = 50%) in order to be closer to real applications of photocatalytic oxidation (PCO) systems developed for indoor air quality improvement. Accurate analytical methods were developed to identify and quantify the majority of the potential formed intermediates. Kinetic constants and the time constant of degradation were obtained for the model pollutants. Under these conditions, the degradation of the three VOCs in a mixture did not really show a decrease in the kinetic rates. Toluene and decane degradations were slightly slowed down when they were in a mixture but no significant interference was demonstrated with TCE. The intermediates formed during the PCO of the mixture of VOCs were the same as those identified during single degradation, leading to the hypothesis of few interactions between them. Only the formation and degradation of secondary intermediates, mostly composed of aldehydes, were time delayed. The total VOC concentration may lead to a competitive adsorption that seemed more sensitive for the last formed oxygenated intermediates. This investigation points out the need to always monitor the last formed aldehyde intermediates. Monitoring acetaldehyde and formaldehyde enables an evaluation of the efficiency and a better design of future PCO systems for indoor applications.

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