Concomitant inactivation of Acanthamoeba spp. and Escherichia coli using suspended and immobilized TiO₂

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 (TiO₂); and, ii) a fixed-bed reactor using immobilized TiO₂ 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 TiO₂ 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.