UV irradiation mediated aggregation of TiO2 nanoparticles in simulated aquatic system

The environmental behavior of engineered nanomaterials is complex due to their photochemical transformation and interactions with the surrounding aquatic environment. In this report, the aggregation of UV-irradiated TiO2 nanoparticles under different aquatic conditions (ionic strength, ionic composition, and humic acids) was investigated. With increased UV irradiation time and elevated ionic strength, TiO2 aggregation was remarkably accelerated due to the suppression of the electrostatic repulsion between the nanoparticles. Humic acids normally stabilize nanoparticle suspensions in salt solutions by the steric hindrance effect. But the change of TiO2 surface chemistry induced by UV irradiation reduced their adsorption capacity toward humic acids and thus weakened their stabilizing effect. In a simulated aquatic solution with 450 mM NaCl and 10 mg L− 1 SRHA, aggregation rate a TiO2 nanoparticle suspension increased from 5.8 nm min− 1 before UV irradiation to 26.7 nm min− 1 after 40 h irradiation. These results demonstrate the critical role of light irradiation in mediating the environmental behaviors of nanomaterials in aquatic environments.