Asymmetric Flow Field Flow Fractionation of Manufactured Silver Nanoparticles in Soil Solution.

Manufactured metallic silver nanoparticles (AgNP) are intensively utilized in a wide array of consumer products and this will inevitably lead to their release to soils. To assess the environmental risks of AgNP in soils, quantification of both the concentration and size of these nanoparticles in soil solution is essential. We developed a methodology consisting of asymmetric flow field flow fractionation (AF4) in combination with on-line detection by UV-Vis spectroscopy and off-line HR-ICP-MS measurements to quantify the concentration and size of AgNP coated with either citrate or polyvinylpyrrolidone (PVP) in water extracts from soils with varying contents of soil organic carbon and clay. The AgNP in the soil water extracts could be fractionated by AF4 using sodium dodecyl sulfate (SDS) as a mobile phase. The AF4 size measured with this mobile phase is close to the size as determined with dynamic light scattering, both representing the hydrodynamic diameter of the AgNP. The coating of the AgNP played a decisive role in their AF4 fractionation behaviour. With citrate as a coating, no or little interaction was found between the AgNP and natural clay colloids present in the 0.45 µm-filtered soil water extract of the clay soil. However, the PVP-coated AgNP formed colloidal aggregates with clay minerals, leading to a dramatic increase in their elution time. The primary size of AgNP in soil solution can be determined by AF4, except for PVP-coated AgNP when clay colloids are present. Similar interactions between PVP-coated AgNP and clay colloids can take place in the environment and as such facilitate the transport of PVP-coated AgNP in soils. AF4 in combination with UV-Vis spectroscopy and HR-ICP-MS measurements is a powerful tool to characterize AgNP in soil solution.