Aggregation behaviour of TiO₂ nanoparticles in natural river water

The purpose of this study was to determine and understand the aggregation behaviour of industrial nanoparticulate TiO₂ (NPs) in the river water near a TiO₂ production plant. The aggregation was tested in near-reality conditions with industrial NPs and the filtered river water in which they are potentially released. The initial size of TiO₂ NPs is around 5 nm. The evolution of the hydrodynamic diameters of the TiO₂ aggregates in the presence of added Suwannee River fulvic acid (SRFA) and illite in the filtered river water was measured at pH 8 for at least 30 min with dynamic light scattering and laser diffraction. The experiments performed in the filtered river water allowed the determination of the attachment efficiency coefficients, and the experiments performed under conditions facilitating aggregation (with higher Ca²⁺ content) were used to understand the potential aggregation processes. When no Ca²⁺ was added into the river water, the initially aggregated TiO₂ did not develop a secondary aggregation in the presence of SRFA and illite. Upon the addition of 2.75 mM Ca²⁺, TiO₂ was shown to heteroaggregate with illite at all tested concentrations. Consequently, in the studied river, the fate of the TiO₂ NPs does not seem to be related to that of the clay suspended particles upstream of the plant. However, the behaviours of the TiO₂ NPs and the clays are closely linked in the water with higher salt content, which is the case downstream of one of the industrial effluent release points.