Commercial Titanium Dioxide Nanoparticles in Both Natural and Synthetic Water: Comprehensive Multidimensional Testing and Prediction of Aggregation Behavior

Engineered nanoparticles (ENPs) from industrial applications and consumer products are already being released into the environment. Their distribution within the environment is, among other factors, determined by the dispersion state and aggregation behavior of the nanoparticles and, in turn, directly affects the exposure of aquatic organisms to EPNs. The aggregation behavior (or colloidal stability) of these particles is controlled by the water chemistry and, to a large extent, by the surface chemistry of the particles. This paper presents results from extensive colloidal stability tests on commercially relevant titanium dioxide nanoparticles (Evonik P25) in well-controlled synthetic waters covering a wide range of pH values and water chemistries, and also in standard synthetic (EPA) waters and natural waters. The results demonstrate in detail the dependency of TiO2 aggregation on the ionic strength of the solution, the presence of relevant monovalent and divalent ions, the presence and copresence of natural organic matter (NOM), and of course the pH of the solution. Specific interactions of both NOM and divalent ions with the TiO2 surfaces modify the chemistry of these surfaces resulting in unexpected behavior. Results from matrix testing in well-controlled batch systems allow predictions to be made on the behavior in the broader natural environment. Our study provides the basis for a testing scheme and data treatment technique to extrapolate and eventually predict nanoparticle behavior in a wide variety of natural waters.