Heteroaggregation of titanium dioxide nanoparticles with model natural colloids under environmentally relevant conditions

The heteroaggregation of engineered nanoparticles (ENPs) with natural colloids (NCs), which are ubiquitous in natural surface waters, is a crucial process affecting the environmental transport and fate of ENPs. Attachment efficiencies for heteroaggregation, α_hetero, are required as input parameters in environmental fate models to predict ENP concentrations and contribute to ENP risk assessment. Here, we present a novel method for determining α_hetero values by using a combination of laser diffraction measurements and aggregation modeling based on the Smoluchowski equation. Titanium dioxide nanoparticles (TiO₂ NPs, 15 nm) were used to demonstrate this new approach together with larger silicon dioxide particles (SiO₂, 0.5 μm) representing NCs. Heteroaggregation experiments were performed at different environmentally relevant solution conditions. At pH 5 the TiO₂ NPs and the SiO₂ particles are of opposite charge, resulting in α_hetero values close to 1. At pH 8, where all particles are negatively charged, α_hetero was strongly affected by the solution conditions, with α_hetero ranging from <0.001 at low ionic strength to 1 at conditions with high NaCl or CaCl₂ concentrations. The presence of humic acid stabilized the system against heteroaggregation.