Disinfection of Bacillus spp. spores in drinking water by TiO2 photocatalysis as a model for Bacillus anthracis

Abstract

During the last decade the use of titanium dioxide has been the focus of water purification studies for photocatalytic degradation of organic compounds. Various studies have shown that TiO2 photocatalysis is a very efficient process for removal (by mineralization) of a large variety of hazardous chemicals. However, the potential use of this technology for water disinfection has been essentially unexplored. Only a few papers have described the photocatalytic destruction of microbial cells, such as E. coli bacteria, MS2 bacteriophages and recently B. fragilis phages, D. radiophilus. The mechanism of photocatalysis in the presence of TiO2 is the enhanced formation of hydroxyl (HO·) radicals, active in oxidation processes. The HO· radicals have a significant effect on chemical oxidation of anthropogenic organic compounds in the environment. Complete mineralization of many organic substances is possible in aqueous systems, when sufficient HO· radical flux can be generated in situ. Various water treatment technologies inherently produce HO· radicals in relatively minuscule quantities (i.e., < 10−12 M). Examples of such processes include ozonation, direct photolysis of hydrogen peroxide, and radiolysis. In contrast to the above system, steady-state HO· concentrations of the order of 10−9 M can be generated in UV-irradiated aqueous suspensions of immobilised particles of titanium dioxide. The use of TiO2 for microbial inactivation and disinfection of potable water is suggested, since free radicals such as HO· might act as a strong biocide, because of its high oxidation potential and nonselective reactivity. In the present study, two bacilli strain spores (B. subtilis and B. cereus) were tested for photocatalytic inactivation in water as simulators of B. anthracis spores. B. subtilis was selected for its high resistance to disinfection and B. cereus for its phylogenetic proximity to B. anthracis. Two UV sources were used: 1) monochromatic UV lamp with irradiation intensity of 7mW/cm2 at 365nm; and 2) Natural sunlight (irradiation intensity at 365nm of ∼4 mW/cm2 between 12:00 and 14:00 hours). TiO2 at 0.25g/L was found to be the optimal concentration needed for the reduction of four orders of magnitude in B. subtilis spores viability after irradiation for 300 minutes. B. cereus subjected to similar photocatalysis conditions was reduced by five orders of magnitude revealing lower endurance to this process. Comparison of artificial and natural (sunlight) UV irradiation source on B. subtilis resulted in increased inactivation of 5 orders of magnitude in favour of sunlight. Combined inactivation by photocatalytic process (UV 365nm) and detrimental activity of UV at 265nm can explain this result. There was no difference between the two irradiation sources when B. cereus was tested. Under both irradiation types, B. cereus was reduced by four orders of magnitude during 300 minutes time interval. Additional experiments including TiO2 concentration, irradiation intensity, water depth, initial spore number, etc. were performed. Taking into account that B. anthracis spores have hydrophobic properties, the photocatalytic process seems to be the method of choice in water disinfection eliminating the possibility of by products formation such as halogens.