Nitrification Inhibition by UVA Photocatalytic TiO₂ Nanoparticles: The Role of Reactive Oxygen Species on Nanotoxicity

We measured the toxicity of TiO₂ nanoparticles to nitrifying bacteria under ultraviolet conditions. Nitrifying bacteria play an important role of nitrification in wastewater treatment plants. Although there was no inhibitory effect of TiO₂ nanoparticles under ambient light or dark conditions, nitrification inhibition was significantly increased when TiO₂ nanoparticles were exposed to ultraviolet (UV) for 30 minutes. Under UV exposure, both TiO₂ nanoparticles and bulk TiO₂ generated the same amount of reactive oxygen species (ROS) in the cells although TiO₂ nanoparticles were more toxic than the bulk counterpart. While the inhibition was well correlated to intracellular ROS concentration, the ROS correlations were different for the different forms of TiO₂ or for the different nanoparticles (e.g., Ag vs. TiO₂), indicating that ROS is not a good chemical marker of the toxicity of nanoparticles.