Investigation of UV-TiO2 photocatalysis and its mechanism in Bacillus subtilis spore inactivation

The inactivation levels of Bacillus subtilis spores for various disinfection processes (ultraviolet (UV), TiO₂ and UV-TiO₂) were compared. The results showed that the inactivation effect of B. subtilis spores by UV treatment alonewas far below that for bacteriawithout endospores. TiO₂ alone in the dark, as a control experiment, exhibited almost no inactivation effect. Compared with UV treatment alone, the inactivation effect increased significantly with the addition of TiO₂. Increases of the UV irradiance and TiO₂ concentration both contributed to the increase of the inactivation effect. Lipid peroxidation was found to be the underlying mechanism of inactivation. Malondialdehyde (MDA), the degradation product of lipid peroxidation,was used as an index to determine the extent of the reaction. TheMDA concentration surged surprisingly to 3.24 nmol/mg dry cellwith the combination disinfection for 600 sec (0.10 mW/cm² irradiance and 10 mg/L TiO₂). In contrast, for UV alone or TiO₂ in the dark, the MDA concentration was 0.38 and 0.25 nmol/mg dry cell, respectively, under the same conditions. This indicated that both UV and TiO₂ were essential for lipid peroxidation. Changes in cell ultrastructure were observed by transmission electron microscopy. The cell membrane was heavily damaged and cellular contents were completely lysed with the UV-TiO₂ process, suggesting that lipid peroxidation was the root of the enhancement in inactivation efficiency.