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.