Role of Reactive Oxygen Species in Determining Nitrification Inhibition by Metallic/Oxide Nanoparticles

The toxicity of several metallic/oxide nanoparticles (TiO2, CuO, ZnO, and Ag) to nitrifying bacteria was evaluated individually or in combination in batch studies. Except for the mixture of ZnO and Ag, the mixture inhibition of nanoparticles was roughly the sum of individual inhibition among the nanoparticles studied. 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 min. Under UV exposure, both TiO2 nanoparticles (anatase) and bulk amorphous TiO2 generated the same amount of reactive oxygen species (ROS) in the bacterial cell 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 versus TiO2). ROS is therefore not a good chemical marker to indicate the toxicity of common metallic/oxide nanoparticles.