Photocatalytic inactivation of Legionella pneumophila and an aerobic bacteria consortium in water over TiO 2 /SiO 2 ﬁbres in a continuous reactor Juan M. Coronado a , Javier Soria a , Jose´ Carlos Conesa a, *, Ricardo Bellod a , Cristina Ada´n a , Hiroyuki Yamaoka b , Vittorio Loddo c , and Vincenzo Augugliaro c a Instituto de Cata ´lisis y Petroleoquı´mica, CSIC, Marie Curie, s/n, Campus de Cantoblanco, 28049 Madrid, Spain b Inorganic Functional Materials Research Dpt., UBE Industries LTD., 1978-5 Kogushi, Yamaguchi, 755-8633 Japan c Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Viale delle Scienze, Universita ` di Palermo, 90128 Palermo, Italy A continuous photoreactor, working in a total recycle mode and irradiated by a low-pressure Hg lamp, has been used to study the bactericidal effect of a photocatalyst, formed by TiO 2 embedded in SiO 2 ﬁbres, on Legionella pneumophila and a consortium of common gram-negative aerobic bacteria (Escherichia coli, Klebsiella sp., Pseudomona sp. and Proteus sp.) in water. The kinetic modeling of the inactivation process, carried out with the measured values of viable bacteria concentration at the outlet of photoreactor, evidenced that for each pass inside the photoreactor the ratio between the outlet and inlet cell concentrations was of order of 0.01 for the inactivation of L. pneumophila. For the other aerobic bacteria, which are usually taken as reference in photocatalytic bacteria inactivation studies, this ratio was of order of 0.3 for the ﬁrst hour of illumination, while upon prolonged irradiation (up to 9 h) this ratio increased to 0.7. Several factors inducing this latter decrease of eﬃciency are possible, as e.g. competition for photocatalytic attack between microorganisms and organic compounds released by damaged bacteria or photoinduced alteration of a small fraction of still viable bacteria making them less interactive with the photocatalyst. The inactivation mechanism normally proposed for common bacteria involves an initial attack of the photogenerated radicals to the outer membrane; the consequent membrane dispersion allows the radicals to damage the cytoplasmic membrane. The higher lethality of the photocatalytic method observed towards Legionella (in comparison to the other aerobic bacteria) is explained considering that the radicals attack the Legionella secretion system, which is adapted for high virulence and would become activated for and through adhesion to the TiO 2 surface. This attack would then be able to inactivate L. pneumophila without dispersing the outer membrane. Apart from this, the water ﬂow through the catalyst ﬁbres can facilitate the bacteria transport towards the anatase surface, and additionally the generated shear stress may help adhesion, at least for some bacteria as E. coli, contributing further to improve the photokilling eﬃciency; both factors may contribute to the eﬃciency of this photoreactor conﬁguration. KEY WORDS: photocatalysis; disinfection; bacteria; Legionella; titania; silica ﬁbers; photoreactor. 1. Introduction Heterogeneous photocatalysis using UV radiation and a semiconductor oxide, which in most cases is the anatase form of TiO 2 , has emerged as an innovative method for water treatment [1–3]. By using aqueous oxygenated suspensions of polycrystalline TiO 2 and UV light it is possible to carry out the oxidation of organic species adsorbed at the solid surface. These oxidation processes, which are started by the photogeneration of active oxygenated radicals such as OH  or O 2 ) , can also be used to attack the cell membrane of microorganisms and to cause their inactivation. In this way, the antimicrobial activity of UV-irradiated TiO 2 photocat- alyst has been proved against several types of bacteria, yeasts, algae and viruses [4–12]. This system presents, over other disinfection technologies currently available, some important advantages such as its eﬃciency to oxidize organic compounds when they are at low concentration and without formation of dangerous or malodorous compounds. An important drawback for the application of photocatalytic methods has been the use, in most cases, of photoreactors containing the TiO 2 photocatalyst as suspensions of nanometric powders, so that a solid separation stage is required after the photoreactor to recover the photocatalyst. In this respect, the use of recently developed photocatalytic materials based on TiO 2 strongly bound to diﬀerent types of supports [10,12,13] can prevent catalyst loss from the photoreactor, and facilitate the use of a ﬁxed bed of photocatalyst conveniently illuminated. The photocatalyst formed by TiO 2 embedded in SiO 2 ﬁbres, prepared by Ishikawa et al. [13], appears as an attractive option for continuous operation photoreac- tors, required for most large-scale applications of photocatalysis as disinfection method. To assess the disinfection eﬃciency of those TiO 2 /SiO 2 photocatalytic ﬁbers, a study on the photocatalytic inactivation of Legionella pneumophila has been carried out here. In order to gain insight on the photocatalytic inactivation mechanism, this study has been extended to other less virulent aerobic bacteria, that are inactivated more eﬃciently than L. pneumophila by treatments with oxidant agents such as chlorine or ozone [14,15]. For this purpose, a commercial continuous plug ﬂow * To whom correspondence should be addressed. E-mail: jcconesa@icp.csic.es Topics in Catalysis Vol. 35, Nos. 3–4, July 2005 (Ó 2005) 279 DOI: 10.1007/s11244-005-3835-z 1022-5528/05/0700–0279/0 Ó 2005 Springer Science+Business Media, Inc.