Silver nanoparticle (Ag-NP) retention and release in partially saturated soil: Column experiments and modelling

The need for better understanding of the environmental fate and transport of engineered nanoparticles (ENPs) is now a scientific consensus. However, the partially saturated zone, a critical region that links the earth's surface to aquifers, has to date received only minor attention in the context of ENP mobility. We investigate the transport and fate of a representative ENP, silver nanoparticles (Ag-NPs), in partially saturated soil. Here we present a set of column experiments and modelling simulations to examine breakthrough curves (BTCs), retention profiles, and mass balances that characterize Ag-NP transport, and gain insights into retardation mechanisms. Unlike Ag-NP transport in sand columns, where the BTC pattern often resembles that of a conservative tracer, Ag-NP transport in soil columns shows moderate mobility and more complex BTC patterns; these results also emphasize the importance of employing realistic porous media in environmental studies. Overall, Ag-NP mobility decreases in the presence of Ca(NO₃)₂, and increases when the solution contains humic acid, at higher water saturation levels, or at higher input concentrations of Ag-NPs. In addition, a different pattern was observed for Ag-NP aggregates, indicating nanospecific behaviour. Modelling analysis of Ag-NP transport in partially saturated soil suggests that a two-site kinetic model with a time-dependent retention function quantifies the transport behaviour of Ag-NPs.