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.