Heteroaggregation of Titanium Dioxide Nanoparticles with Model Natural Colloids under Environmentally Relevant Conditions

摘要

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 (TiO_2 NPs, 15 nm) were used to demonstrate this new approach together with larger silicon dioxide particles (SiO_2, 0.5 μm) representing NCs. Heteroaggregation experiments were performed at different environmentally relevant solution conditions. At pH 5 the TiO_2 NPs and the SiO_2 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 CaCl_2 concentrations. The presence of humic acid stabilized the system against heteroaggregation.