Combined factors influencing the surface charge and aggregation behaviors of TiO2 nanoparticles in the presence of humic acid and UV irradiation

The environmental behavior and ultimate fate of the engineered nanoparticles could inevitably be affected by the widespread dissolved organic matter and sunlight irradiation in the natural environment. In this study, an analysis combined with experimental investigations was used to explore the influence of humic acid (HA) and UV irradiation on the changes in surface charge and aggregation behavior of TiO 2 nanoparticles. A prolonged UV irradiation caused the point of zero charge (PZC) of TiO 2 nanoparticles shifted to lower values and the aggregate size reached to a maximum value at the PZC. TEM image and FT-IR spectroscopy analysis confirmed the formation of TiO 2 cluster aggregates and successive growth of the hydroxyl groups on different sites on TiO 2 surface, respectively, although no effect of irradiation on the phase transformation of TiO 2 nanoparticles was observed through the use of Raman spectroscopy. The fluorescence spectroscopy analysis illustrated the interactions between the surface hydroxyls formed during UV irradiation on TiO 2 surface and functional groups on HA molecules, which accelerated the aggregation process of TiO 2 nanoparticles. Widespread HA and UV irradiation in the natural environment significantly altered the surface properties and stability of the nanoparticles in aqueous environments. Graphical abstract