Deleterious effect of homogeneous and heterogeneous near-neutral photo-Fenton system on Escherichia coli. Comparison with photo-catalytic action of TiO2 during cell envelope disruption

The mechanisms underlying bacterial inactivation by solar photo-Fenton at near-neutral pH have not yet been investigated in detail. In particular, no consensus exists on the bacterial inactivation mechanism under solar light enhanced by the Fenton's reagents (Fe3+, H2O2). In this study, cell envelope damage during bacterial inactivation by near-neutral photo-Fenton and TiO2 photocatalysis were comparatively studied using lipid peroxidation and cell permeability change indicators. TiO2 photocatalysis was found to result in marked cell envelope damage, in contrast to the near-neutral photo-Fenton process. However, similar kinetics of inactivation were observed for both types of processes. This finding corroborated with the results of an electron spin resonance (ESR) study, which pointed to higher efficiency of photo-generation of reactive oxygen species (ROS) in the presence of TiO2 photocatalyst compared with the photo-Fenton system at near-neutral pH. In the context of the photo-Fenton processes, the bactericidal effect of Fe3+/hv was attributed to the adsorption of Fe3+ ions on the bacterial cell wall and the subsequent photosensitization of these iron-bacteria exciplexes, thus leading to the direct oxidation of the cell membrane. In contrast, the effect of Fe2+/hv was associated with diffusion into the cell by the FeO system and its participation in intracellular dark Fenton's reactions. Based on these experimental results and literature reports, a mechanistic interpretation of the photo-inactivation of Escherichia coli in the presence of Fe2+, Fe3+, and the Fenton's reagent is proposed. Moreover, we suggest that extensive cell envelope damage might not necessarily be a unique pathway in bacterial inactivation by near-neutral photo-Fenton treatment. In particular, the enhancement of an internal (photo)-Fenton process by the synergistic action of UVA and the external Fenton's reactants seems an important contribution to bacterial inactivation

Castles fall from inside: Evidence for dominant internal photo-catalytic mechanisms during treatment of Saccharomyces cerevisiae by photo-Fenton at neutral pH

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Iron oxides semiconductors are efficients for solar water disinfection: A comparison with photo-Fenton processes at neutral pH

The photocatalytic activities of four different commercially available iron (hydr)oxides semiconductors, i.e. hematite (alpha-Fe2O3), goethite (alpha-FeOOH), wustite (FeO) and magnetite (Fe3O4), were evaluated for bacteria inactivation at neutral pH in the absence or presence of H2O2. Our results showed that heterogeneous photocatalysis and/or photo-Fenton processes catalyzed by low concentrations of reagents (0.6 mg/L Fe3+ and 10 mg/L H2O2) under sunlight may serve as a disinfection method for waterborne bacterial pathogens. In particular, we found that, with the exception of magnetite which need H2O2 as electron acceptor, all the other semiconductor iron (hydr)oxides were photoactive under sunlight in absence of H2O2 (using only oxygen as electron acceptor). Furthermore, for all iron (hydr)oxide studied in this work, no bacterial reactivation and/or growth was observed after photo-Fenton treatment. The same antimicrobial activity was obtained for the photocatalytic semiconducting action of hematite and goethite. Additionally, a delayed disinfection effect was observed to continue in the dark for the photo-assisted wilstite-based treatment. Electron spin resonance (ESR) in combination with spin-trapping was employed to detect reactive oxygen species (ROS) involved in heterogeneous photocatalysis and/or photo-Fenton treatments mediated by iron (hydr)oxide particles. In particular, ESR confirmed that center dot OH and O-2(center dot-) radicals were the principal ROS produced under photo-assisted action of iron (hydr)oxide particles in the absence or presence of H2O2. We also found that the components of natural water (i.e. natural organic matter (NOM) and inorganic substances) did not interfere with the photocatalytic semiconducting action of hematite to bacterial inactivation. However, these components enhance the bacterial inactivation by heterogeneous photo-Fenton action of hematite. Overall our results demonstrated, for the first time, that low concentration of iron (hydr)oxides, acting both as photocatalytic semiconductors or catalysts of the heterogeneous photo-Fenton process at neutral pH, may provide a useful strategy for efficient bacterial disinfection. (C) 2014 Elsevier B.V. All rights reserved