Waterborne Escherichia coli inactivation by TiO2 photoassisted processes: A brief overview

Abstract

The most common used techniques for water disinfection in use now are chlorination, heating and ozonation. However in the case of chlorination, there is a negative effect: the appearance of trihalomethanes (THMs) as byproducts of the reaction between chlorine with organic matter [1-3]. Other methods, e.g., ozonation, are either expensive or involve a high consumption of electric energy; moreover after ozonation, chlorination is required in order to avoid bacteria re-growing. Solar disinfection (SODIS) is a simple technology developed to inactivate bacteria on water; in this case, SODIS makes a good use of the synergistic effect of the UV-component of the sunlight irradiation (5-6%) and the ambient heat created by the incoming sunlight to inactivate pathogen microorganisms [4, 5]. However this process sometimes leads to microbial re-growth and is not effective for the case of bacterial strains such as Salmonella typhimurium [6]. In this respect, the use of photochemical processes using or not photocatalysts in solution such as advanced oxidation processes (AOPs), photo-Fenton and heterogeneous and other supported photocatalysis seem a promising alternative to chlorination and could also be used to enhance the efficiency of the SODIS technology now in use [7, 8]. In 1985 Matsunaga et al. [9] reported the first use of heterogeneous photocatalysis by TiO2 as a sterilization process to inactivate E. coli, Lactobacillus acididophilus, and Saccaromyces cerevisiae cells, and this approach generated a few thousand studies during the last three decades. TiO2 exists in three crystalline forms: anatase, rutile, and brookite. Indeed, the photocatalytic activities of various anatase and rutile samples overlap because other parameters influence these activities. The allegation about the superiority of anatase per se is based on the observation that, at least until now, the most active TiO2 samples are anatase specimens [10], but it is not demonstrated that anatase is intrinsically more active. When TiO2 nanoparticles are illuminated with UV light below 385 nm, an electron (e-) is promoted from the valence band (VB) to the conduction band (CB), leaving in the VB a hole (h+). These charge carriers (e-/h+) can migrate to the TiO2 surface being trapped on surface defects, dangling bonds or impurities. Once the charge carriers are trapped, they undergo electronic transfer; the surface trapped h+ and e- can react with either electron donors or acceptors. Water molecules or hydroxyl groups adsorbed on the on TiO2 surface behave as electron donors leading to surface adsorbed hydroxyl radicals (?OH) with high oxidative power (1.8 V vs NHE). Trapped electrons can react with oxygen molecules previously adsorbed on TiO2 surfaces producing superoxide radical (?O2-) [11-15]. Recently it has been reported that anatase particles produce singlet oxygen (1O2) under UV light, through the oxidation of ?O2- radicals by vb holes [16, 17] (see the chapter by Y. Nosaka). In summary, irradiation of TiO2 nanoparticles by UV light lead to the production of reactive oxygen species (ROS) such as: ?OH, ?O2-, 1O2, and H2O2, all of them highly oxidative species able to oxidize/abate microorganism. These species are able to oxidize the cell constituents: proteins, lipids and nucleic acids.