The fate of cerium oxide nanoparticles in sediments and their routes of uptake in a freshwater worm

The relative importance of ingestion and transdermal uptake of nanomaterials is poorly understood, particularly in sediment dwelling organisms, where diet has the potential to contribute significantly to particle accumulation. In aquatic sediments, nanoparticles may partition to bind with the solid fraction of sediment, be freely mobile in the pore water or, for certain metal/metal oxides, undergo dissolution, each of which could influence the route of nanoparticle uptake. Here, we used the freshwater worm Lumbriculus variegatus as a model species. We took advantage of its unique feeding and non-feeding life-stages to assess the contribution of dietary and transdermal uptake in the bioaccumulation of cerium oxide nanoparticles (CeO2 NP) and soluble Ce(III)NO3. Distribution of cerium between the solid, colloidal and soluble fractions in the sediments was determined through sediment separations using micro and ultrafiltration techniques. We assessed particles of differing sizes (10, 28 and 615 nm CeO2) and stabilizing surfactants (10 nm electrostatic Citrate-CeO2 and steric stabilized PEG-CeO2). Soluble Ce(III)NO3, was found to accumulate readily across the skin of the worms whilst nanoparticles were not. Sediments reduced the uptake of Ce-III by limiting the presence of dissolved species of cerium in the pore waters. Neither particle size nor the coatings studied altered the distribution of nanoparticles between solid and colloidal fractions of the sediment, with similar to 99% associated to the solid phase. Any uptake of CeO2 nanoparticles into worms was only through ingestion. Stabilized 10 nm particles were retained even after gut clearance, indicating that these particles may translocate across the gut wall.