NANOFLOW – Mobility of synthetic nanoparticles in water-saturated and variably water-saturated subsoils
Within the NanoFlow project, laboratory and field tests were conducted investigating the transport behaviour of synthetic nanoparticles in solid rock and soils under water-saturated and water sub-saturated conditions. Special focus was on the behaviour of multi-walled carbon nanotubes (MWCNTs) and stabilised silver nanoparticles (Ag-NPs).
The laboratory tests revealed that the mobility of the respective carbon nanotubes depends on their initial concentration and the soil grain size. The field tests with lysimeters showed that the MWCNTs are subject to a high retention in the upper soil horizon. All in all, the results of the environmentally relevant experiments (unsaturated flow, low initial concentration, undisturbed soil) suggest that soils are most effective sinks for carbon nanotubes. In view of this fact, only low MWCNT transport in the vadose zone and, hence, a low risk of groundwater contamination is to be expected in the investigated soils.
The results of the laboratory tests with stabilised silver nanoparticles (Ag-NPs) revealed that, under certain conditions, the nanoparticles show a high mobility and can reach transport ranges of several meters. This has been demonstrated in a model soil, in pure sandstone or in a soil with high sand contents in the presence of monovalent ions in the surrounding solution. Under other conditions e.g., in sandstones of a more complex mineralogical composition and with smaller pore sizes, in fine-grained soils with higher clay content, and at higher ion strengths or in the presence of bivalent ions in the soil solution, the transport of the silver nanoparticles was found to become strongly limited. If the ion strength is being reduced, as may be the case e.g. during heavy rain, or if the bivalent ions are replaced by monovalent ones, the silver nanoparticles can be released again from the soil. Such remobilisation of the silver nanoparticles is also caused by the release of soil colloids that act as Ag-NPs carriers (co-transport). Also in fissured sandstone, the tests have revealed a relatively high mobility of silver nanoparticles which partly is enabled also by co-transport of Ag-NPs on sandstone fragments.
