Photocatalytic degradation of methylene blue and inactivation of Gram-negative bacteria by TiO2 nanoparticles in aqueous suspension

The photocatalytic degradation of methylene blue (MB) and inactivation of Gram-negative bacteria Escherichia coli (generic) and Pseudomonas aeruginosa by TiO₂ nanoparticles in aqueous suspension were studied. TiO₂ resulted in significant reduction in MB absorption and a shift of MB absorption peak from 664 nm to 658 nm after a short time of irradiation. The maximum degradation of MB was observed when the concentration of TiO₂ in the aqueous suspension was 0.5 g L^?1. TiO₂ was also very effective with inhibiting growth of both Gram-negative bacteria E. coli and P. aeruginosa, although it took more than 60 min to observe the inactivation effects. The photocatalytic inactivation toward E. coli and P. aeruginosa by TiO₂ showed a similar trend with much higher effectiveness toward E. coli under the same experimental conditions. The inactivation kinetic behaviors could be explained by the modified Langmuir–Hinshelwood model, and well fitted to a pseudo-first order kinetic equation. The reaction rate constant for E. coli and P. aeruginosa were 7.768 × 10⁶ cfu mL^?1 min^?1 and 5.655 × 10⁶ cfu mL^?1 min^?1, respectively. The adsorption equilibrium constant for E. coli was 1.053 × 10^?8 mL cfu^?1, while it was 1.438 × 10^?8 mL cfu^?1 for P. aeruginosa. These results further demonstrate that in an aqueous system, TiO₂ nanoparticles can effectively both degrade organic compounds and inhibit Gram-negative bacteria under UVA light. Compared with the degradation activity of TiO₂ toward organic compounds, its antimicrobial activity against Gram-negative bacteria would be delayed by 60 min. The antimicrobial activity of TiO₂ against Gram-negative bacteria could vary with bacterial species.