Fate of engineered nanomaterials in surface water: Factors affecting interactions of Ag and CeO2 nanoparticles with (re)suspended sediments

Engineered nanoparticles (NPs) are increasingly being utilized in numerous commercial applications, thereby augmenting the likelihood that they will find their way into aquatic or terrestrial environments, where their fate and behaviour still are not yet fully understood. The main objective of this study was to examine the fate of Ag and CeO2 NPs in sediment suspensions. Particularly, the association of these nanoparticles and their corresponding ions with sediment constituents was studied. Four different intertidal sediments were sampled along the river Scheldt (Belgium), dried, ground, and thoroughly characterized, before being re-suspended with Milli-Q water. The suspensions were spiked with nanoparticles or ions, continuously shaken for 24h, and subjected to gravitational settling or centrifugation, where after the remaining silver or cerium concentration in the supernatants were determined via ICP-OES. It was shown that centrifugation speed significantly affects the amount of suspended material and the TOC content in the liquid phase. In the absence of sediment, Ag and CeO2 NPs as well as their corresponding ions were unaffected by centrifugation speed over a 24-h time period, whereas significant differences were observed in the presence of sediment, indicating that sediment properties influence the partitioning behaviour of the nanoparticles. The amount of silver or cerium remaining in suspension was positively correlated with the TOC and suspended matter content, suggesting interactions of the nanoparticles with sediment colloidal matter. Remarkably, Ag and CeO2 NPs appear to bind less strongly to sediment constituents in comparison to their ionic counterparts, rendering the nanoparticles more mobile.