Transport and retention of engineered silver nanoparticles in the presence of phosphorus

Nowadays engineered silver nanoparticles (AgNP) are being widely used for a multitude of purposes. At certain times during their life-cycle they might enter soils and freshwater resources and thus recent research has focused on their transport and fate in soils and the vadose zone as well as the saturated zone. AgNP retention in the subsurface depends on a multitude of parameters including the type and shape of the sediments through the nanoparticles are exposed to, the chemical composition of pore and groundwater acting as background solution or the type and quantity of soil organic matter present. One aspect that has received little attention so far is their transport behaviour in the presence of nutrients.

Here we study PVP-AgNP transport and retention in saturated columns containing silicate-dominated aquifer material that is also exposed to orthophosphate (NaH₂PO₄) or myo-inositol hexakisphosphate (IP6) via the background solution. In particular, we compare PVP-AgNP transport behaviour for different pH (6 and 4.5) in the background solution, for different mass concentrations of sediments <63 µm in the columns (0 and 2%) and in the presence/absence of soil organic matter (SOM). Experimental data were modelled using HYDRUS 1D.

Results of our experiments show that PVP-AgNP exhibit a higher mobility through the columns in the presence of phosphate as the latter can block attachment sites otherwise available to the nanoparticles. In the presence of SOM this mobility is even higher than in the absence of SOM as SOM and phosphate anions are both negatively charged and potentially bound to the same attachment sites. PVP-AgNP mobility also increased for both P-species when an increase in pH occurred but this increase was more pronounced in columns with orthophosphate. Results further show that PVP-AgNP are more mobile in columns with IP6 than orthophosphate in the absence of sediments <63 µm at pH 4.5. However, while for columns with material < 63 µm the overall AgNP mobility is decreased due to an overall increase in sediment surface area, AgNP are more mobile in the presence of orthophosphate as IP6 is more strongly bound to iron and aluminium oxides found in higher abundance in the fine sediments.