A lot of human activities have negative impact on water quality and sometimes result in the biological water contamination. Currently used chemical (chlorine, ozone, and etc.) and physical (UV) water disinfection methods have strong environmental disadvantages or suffers from limited efficiency. To overcome these problems, scientists suggest to use photocatalyst activated advanced oxidation processes. One of the most studied photocatalysts which attracts a lot of research interest is titanium dioxide. TiO2 application for the disinfection of water, air or surfaces is increasingly encouraged by researchers. However, to unlock its full potential it is highly desirable to make it suitable for the visible light activation. In the current study the effect of visible light assisted photocatalytic treatment to the outer membrane permeability of Salmonella enterica bacteria and how it changes under different titanium dioxide concentrations was analysed. The results from the treatment of relatively complex Salmonella enterica bacteria organism were compared to the visible light activated TiO2 ability to oxidise considerably simpler objects like methylene blue molecules. The efficiency of TiO2 photocatalytic disinfection process was evaluated using spread plate technique. Membrane permeability of the treated Salmonella enterica bacteria was determined by NPN uptake factor assay. Generation of intracellular reactive oxygen species was evaluated by Dichlorodihydrofluorescein diacetate fluorescence measurements. The key finding of this study was that intense wide spectrum visible light irradiation and TiO2 powder synergistically inactivate S. enterica bacteria and halt its potential to form colonies. High amounts of intracellular reactive oxygen species could be seen as the main suspects for the observed inactivation of S. enterica.
The beneficial photocatalytic properties of UV light activated TiO2 powder are well-known and have been demonstrated with various pollutants and pathogens. However, traditionally observed photocatalytic activity of visible light activated pristine TiO2 is insignificant but there are a few studies which have reported that under some specific conditions commercially available TiO2 powder could at least partially disinfect microorganisms even under visible light. To better understand this phenomenon, in the current study we focused on bacteria response to the treatment by visible light and P25 TiO2 powder. More specifically, we analyzed the relationship between the bacteria viability, outer membrane permeability, metabolism, and its capacity to generate intracellular reactive oxygen species. During the study we assayed the viability of treated bacteria by the spread plate technique and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction method. Changes in bacterial outer membrane permeability were determined by measuring the fluorescence of N-phenyl-1-naphthylamine (NPN). To detect intracellular reactive oxygen species formation, the fluorescence of dichlorodihydrofluorescein diacetate (DCFH-DA) was assayed. Results of our study indicated that TiO2 and wide spectrum visible light irradiation damaged the integrity of the outer membrane and caused oxidative stress in the metabolizing bacteria. When favorable conditions were created, these effects added up and unexpectedly high bacterial inactivation was achieved.
Usually, the most efficient photocatalyst materials are synthesized as fine nanocrystalline powders and this rises significant handling and repetitive application issues. More recently, researchers started to immobilize photocatalyst (nano)materials on relatively large low density supports creating floating photocatalyst particles. Such approach allows to effectively retrieve and re-apply the used photocatalyst material. In current study we used reactive magnetron sputtering technique and deposited unconventional orange colour nanocrystalline ZnO based photocatalyst on floating high-density polyethylene (HDPE) grains. The structure of the synthesized photocatalyst was characterized by XRD, SEM, and XPS techniques. The repetitive measurements of Rhodamine B (RhB) dye bleaching by the ZnO based photocatalyst film under visible light irradiation showed high stability over ten cycles. Visible light induced photocatalytic efficiency of the floating photocatalyst grains (FPG) was also estimated by the repetitive treatment of water samples containing Salmonella typhimurium (strain SL1344) and Micrococcus luteus bacteria, as well as water samples containing PRD1 and T4 bacteriophages. These tests indicated complex interaction between the bacteria, viruses, photocatalyst and its HDPE support. For example, they revealed that FPGs lose most of its photocatalytic efficiency in just 3 cycles. To stabilize the ZnO based FPGs and enhance its photocatalytic efficiency under the visible light irradiation, before the depositing of ZnO films we pre-covered HDPE grains by Ni underlayer. The addition of Ni resulted in mixed results – Ni underlayer reduced the efficiency of S. typhimurium disinfection during the first cycle but increased the efficiency and detoxication stability over consecutive tests using the same set of FPGs.
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