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This work reports the application of photocatalytic disinfection to the inactivation of Acanthamoeba trophozoites, a free-living pathogenic amoeba. Two types of photocatalytic reactors configurations have been used: i) a slurry reactor using suspended titanium dioxide (TiO 2 ); and, ii) a fixed-bed reactor using immobilized TiO 2 onto glass Raschig rings. The effect of the chemical composition of water has been analysed, comparing the efficiency of the process in deionized water (DW) and synthetic wastewater treatment plant effluent (SWTPE). The inactivation of Acanthamoeba spp. has been compared to that of Escherichia coli bacteria, being also analysed the concomitant inactivation of both microorganisms. Our results show that 99% of inactivation of E. coli and Acanthamoeba spp. can be achieved using photocatalysis in both reactor configurations, but interestingly, the kinetics of inactivation of both microorganisms together differs from that found with them separately. Particularly, E. coli seems to be more resistant to the inactivation in the presence of Acanthamoeba spp. which has been justified by the screen effect caused by the bigger size of Acanthamoeba spp. This observation is more pronounced in DW as the composition of the SWTPE prevent the microorganisms from suffering osmotic and/or mechanical stress and protect cellular structures to the attack of reactive oxygen species (ROS). On the other hand, the difference between the inactivation rate of E. coli and Acanthamoeba, points out the importance of the different inactivation mechanisms, suggesting that the entry of small TiO 2 particles into the cytoplasm of the Acanthamoeba cells provokes the attack of inner structures and as a consequence a faster inactivation. This mechanism is not possible when the catalyst is immobilized leading to a higher cell resistance to inactivation and consequently lower efficiency of the disinfection process., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

TiO₂ photocatalysis is considered as an alternative to conventional disinfection processes for the inactivation of waterborne microorganisms. The efficiency of photocatalysis is limited by charge carrier recombination rates. When the photocatalyst is immobilized on an electrically conducting support, one may assist charge separation by the application of an external electrical bias. The aim of this work was to study electrochemically assisted photocatalysis with nitrogen doped titania photoanodes under visible and UV-visible irradiation for the inactivation of Escherichia coli . Aligned TiO₂ nanotubes were synthesized (TiO₂-NT) by anodizing Ti foil. Nanoparticulate titania films were made on Ti foil by electrophoretic coating (P25 TiO₂). N-doped titania nanotubes and N,F co-doped titania films were also prepared with the aim of extending the active spectrum into the visible. Electrochemically assisted photocatalysis gave higher disinfection efficiency in comparison to photocatalysis (electrode at open circuit) for all materials tested. It is proposed that electrostatic attraction of negatively charged bacteria to the positively biased photoanodes leads to the enhancement observed. The N-doped TiO₂ nanotube electrode gave the most efficient electrochemically assisted photocatalytic inactivation of bacteria under UV-Vis irradiation but no inactivation of bacteria was observed under visible only irradiation. The visible light photocurrent was only a fraction (2%) of the UV response.

Simultaneous Escherichia coli inactivation and oxidation of pharmaceuticals in simulated wastewater treatment plant effluents has been investigated using a photocatalytic treatment with TiO(2) in suspension and immobilised onto a fixed-bed reactor. Non-photocatalytic reference experiments of dark adsorption and photolysis showed a higher sensitivity of E. coli towards the chemical composition of water in comparison with the concentration of pharmaceuticals that remains unaffected. Moreover, it must be underlined that the presence of pharmaceuticals (including antibiotics) did not seem to affect the bacterial viability at such low concentrations. Concerning photocatalytic experiments, both suspended and immobilised TiO(2) were able to simultaneously inactivate and oxidise both kinds of pollutants (bacteria and pharmaceuticals). The fixed-bed reactor showed similar activity to that of the slurry without deactivation after several cycles of reuse. That makes TiO(2) photocatalysis a quite interesting technology for the treatment of drinking water supplies or wastewater plant effluents, allowing the removal of emerging contaminants such as pharmaceuticals during the disinfection treatment.

This work is focused on the comparison between the photocatalytic inactivation of Escherichia coli and the photocatalytic oxidation of methylene blue, regarding the reaction kinetics and the influence of water composition. Disinfection profiles show an initial delay, in contrast with the exponential decay shown by the decolorization of methylene blue solutions. A serial-event mechanism is proposed for both disinfection and mineralization processes, the number of intermediate species being the main difference between them. Concerning the influence of water composition, inactivation of bacteria is more sensitive to the presence of inorganic ions and/or organic matter in the solution. In some cases opposite behaviours are observed, such as in the presence of chloride ions, which enhance the disinfection rate but decrease degradation activity for methylene blue. Consequently, the results obtained in the evaluation of photocatalytic processes for the degradation of chemical pollutants cannot be always extrapolated to the inactivation of microorganisms.

This study reports the analogies and differences found when comparing TiO(2) photocatalytic treatment for chemical oxidation and microorganisms inactivation, using methylene blue and Escherichia coli as references, respectively. In both processes the activation is based on the same physicochemical phenomena and consequently a good correlation between them is observed when analyzing the effect of operational variables such as catalyst concentration or incident radiation flux, both factors influencing common stages such radiation absorption and generation of reactive oxygen species. However, different microbiological aspects (osmotic stress, repairing mechanism, regrowth, bacterial adhesion to the titania surface, etc) makes disinfection kinetics significantly more complex than the first-order profiles usually observed for the oxidation of chemical pollutants. Moreover, bacterial inactivation reactions are found to be extremely sensitive to the composition of water and modifications of the catalysts in comparison with the decolorization of the dye solutions, showing opposite behaviors to the presence of chlorides, incorporation of silver to the catalysts or the use of different types of immobilized TiO(2) systems. Therefore, the activity observed for the photocatalytic oxidation of organics can not be always extrapolated to photocatalytic disinfection processes., ((c) 2009 Elsevier Ltd. All rights reserved.)