Photocatalytic inactivation of airborne microorganisms. Performance of different TiO2 coatings

Abstract

The quality of indoor air has a fundamental importance for human health. Indoor air pollutants include, among others, biological contaminants present as bioaerosols. The latter are mostly composed of bacterial cells, fungal spores and bacterial spores (Stetzenbach et al., 2004). In particular, bacterial endospores are characterized by their extreme latency, resistance to adverse environmental conditions and lack of metabolism. It is the resistance to treatments, including heat, desiccation, UV light and the action of harmful chemicals, the most important feature of spores (Atrih and Foster, 1999). This characteristic makes them a useful reference tool for decontamination tests. Moreover, bacterial endospores are frequently used to verify the efficiency of sterilization devices (Josset et al., 2008). Heterogeneous photocatalysis, an advanced oxidation technology that uses UV-A radiation (315-400 nm) with a catalyst (mainly TiO2), is a potential alternative to address the problem of biological contamination indoors. This technology has emerged as an effective treatment method for air containing chemical and microbial contaminants due to its numerous advantages. When TiO2 is photoexcited by UV-A, formation of electron-hole pairs occurs, with the consequent generation of oxidative radical species. These radical species may cause fatal damage to the microorganisms in contact with the catalyst surface. The process does not involve any expensive chemical oxidant, and uses atmospheric oxygen in the presence of water vapor. The effectiveness of photocatalysis for the elimination of organic pollutants in air has been proven for a wide variety of chemicals, as it is the case of alcohols, ketones, aromatics, nitrogen and halogenated compounds (Hoffman et al., 1995; Peral et al., 1997; Blake, 2001; Zhao and Yang, 2003;Vohra et al., 2009). However, the applicability of photocatalysis for the inactivation of airborne microbial contaminants remains scarce.The pioneeringwork in the field of photocatalytic gas phase disinfection was done by Goswami et al. (1997), reporting the total inactivation of Serratia marcescens in a photocatalytic reactor with recirculation using TiO2 Degussa P-25. More recently different studies on the application of photocatalysis to inactivate spore-forming bacteria over TiO2-coated surfaces have been published (Wolfrum et al., 2002; Lin and Li, 2003; Pal et al., 2005; Vohra et al., 2005; Zhao et al., 2009; Chuaybamroong et al., 2010; Prasad et al., 2011). Arelevant issue concerning photocatalytic disinfection is kinetic modeling. Kinetic expressions governing the process are of utmost importance to identify the variables that influence the reaction rate and to optimize the inactivation of microorganisms Additionally, modeling is essential for the design of photocatalytic devices to solve real scale problems. In this chapter, we present a comprehensive study on the photocatalytic inactivation of Bacillus subtilis spores over UV-irradiated TiO2 films. In the first part of the chapter, a kinetic model of the process is proposed (Zacarias et al., 2010). TiO2 was immobilized by the sol-gel method employing borosilicate glass plates as support. A reaction rate expression was derived from an inactivation scheme which, although simple, still retains the essentials of the intervening events and the influence of the most relevant operating variables upon the whole process. The rate of photon absorption by the TiO2 catalyst, which represents one of the most important parameters affecting the photocatalytic reaction rate, was predicted by using CFD software (Fluent 6.3). The kinetic expression considers the effect of the photon absorption, the concentration of viable spores, and the irradiation time on the spore inactivation rate. In the second part of the chapter three different coating techniques with increasing number of layers were assayed to improve the photocatalytic inactivation rate: TiO2 sol-gel, TiO2 Degussa P-25, and composite TiO2 sol-gel/Degussa P-25. The amount of photocatalyst deposited on the borosilicate glass plates and the fraction of energy effectively absorbed by the catalytic films were determined. Diffuse transmittance and reflectance measurements of the coated and uncoated glass plates and the subsequent application of the Net-radiation Method (Siegel and Howell, 2002) were used to compute the fraction of energy absorbed by theTiO2 films. Finally, to objectively compare the performance of the coatings, the photonic efficiency and the quantum efficiency of inactivation were calculated and discussed.