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Applied and Environmental Microbiology, January 2005, p. 270-275, Vol. 71, No. 1
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.1.270-275.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Different Inactivation Behaviors of MS-2 Phage and Escherichia coli in TiO₂ Photocatalytic Disinfection

School of Chemical Engineering, College of Engineering, Seoul National University, Sillim-dong, Gwanak-gu, Seoul,¹ Water Microbiology Division, National Institute of Environmental Research, Kyungseo-dong, Seo-gu, Inchon,² School of Environmental Science and Engineering, Pohang University of Science and Technology, Hyoja-dong, Nam-gu, Pohang, South Korea³

Received 17 March 2004/ Accepted 6 August 2004

Despite a wealth of experimental evidence concerning the efficacy of the biocidal action associated with the TiO₂ photocatalytic reaction, our understanding of the photochemical mechanism of this particular biocidal action remains largely unclear. It is generally accepted that the hydroxyl radical (·OH), which is generated on the surface of UV-illuminated TiO₂, plays the main role. However, our understanding of the exact mode of action of the hydroxyl radical in killing microorganisms is far from complete, and some studies report that other reactive oxygen species (ROS) (H₂O₂ and O₂·^–, etc.) also play significant roles. In particular, whether hydroxyl radicals remain bound to the surface or diffuse into the solution bulk is under active debate. In order to examine the exact mode of action of ROS in inactivating the microorganism, we tested and compared the levels of photocatalytic inactivation of MS-2 phage and Escherichia coli as representative species of viruses and bacteria, respectively. To compare photocatalytic microbial inactivation with the photocatalytic chemical degradation reaction, para-chlorobenzoic acid, which rapidly reacts with a hydroxyl radical with a diffusion-limited rate, was used as a probe compound. Two different hydroxyl radical scavengers, tert-butanol and methanol, and an activator of the bulk phase hydroxyl radical generation, Fe²⁺, were used to investigate their effects on the photocatalytic mode of action of the hydroxyl radical in inactivating the microorganism. The results show that the biocidal modes of action of ROS are very different depending on the specific microorganism involved, although the reason for this is not clear. It seems that MS-2 phage is inactivated mainly by the free hydroxyl radical in the solution bulk but that E. coli is inactivated by both the free and the surface-bound hydroxyl radicals. E. coli might also be inactivated by other ROS, such as O₂·^– and H₂O₂, according to the present results.

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