Transport and blocking behavior of engineered silver nanoparticles in aquifer materials

Engineered silver nanoparticles (AgNPs) are widely used in consumer products, textiles, medical products, and electronics because of their antimicrobial and physical-chemical properties. Through the use of such products, AgNPs are released into the environment, so that information about the factors influencing their transport and fate are essential, e.g. for risk assessment. The aim of this thesis is to investigate the transport and blocking behavior of two commercially available AgNPs with different capping agents in laboratory conditions in natural unconsolidated aquifer material, relevant for the drinking water production. Column experiments were conducted with two common types of aquifer material, namely a silicate-dominated and a carbonate-dominated material. The AgNPs were stabilized with non-ionic surfactants or the polymer polyvinylpyrrolidone (PVP), respectively. The transport experiments covered variations in flow velocity, ionic strength (IS) and cation (calcium and sodium), and anion (nitrate and phosphorous) composition of the background solution as well as the presence and absence of silt and clay-sized particles, and varying surface properties of the porous medium. Total silver concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS). Breakthrough curves (BTCs) were plotted with their normalized effluent concentration against the exchanged pore volumes. BTCs were modeled using the numerical finite element code HYDRUS-1D with one or two kinetic retention sites with Langmuirian blocking. The results of the transport experiments showed that the stabilizing agent of the AgNPs had a distinct effect on the transport and blocking behavior in the saturated column experiments. With increasing IS and ion valence, a higher retention of both AgNP products was observed in silicate-dominated aquifer material whereas higher retention under all tested conditions was observed for PVP-AgNPs. A decrease in flow velocity led to a higher retention of AgNPs in the silicate-dominated porous medium as did the presence of fine-grained materials. Increased AgNP retention with increased IS and ion valence occurred because of a compression of the double layer and lower magnitudes in the zeta potential. Furthermore, cation bridging in the presence of calcium was related to the presence of clays. A reduction in flow velocities increased the residence time and decreased the hydrodynamic forces. Time dependent blocking behavior was more pronounced for the PVP-AgNPs, which was reflected in increased values for the simulated values for the maximum retained Ag concentration on the solid phase, and which increased for the first order AgNP retention coefficient for the solid-water interface. The retention of the PVP-AgNP decreased in the presence of phosphorous (P) in the background solution because of competitive blocking. Phosphorous blocks the retention sites of iron oxides, which leads to enhanced transport of AgNPs. The results indicated that the difference in AgNP transport was attributed to the P species. Bound inorganic P, such as sodium dihydrogen phosphate, leads to a faster breakthrough of PVP-AgNPs compared to bound organic P, such as myo-inositol hexakisphosphate. Experiments with carbonate-dominated porous media showed similar results in the transport behavior of surfactant and polymer-stabilized AgNPs. With an increasing calcium carbonate sand content in an artificially mixed porous medium with quartz sand, PVP-AgNPs show higher retention. This behavior was attributed to the surface roughness of the calcium carbonate in contrast to the quartz sand. Furthermore, the zeta potential of the calcium carbonate sand was weakly positive compared to the quartz under the conditions tested which provided favorable attachment conditions. In natural carbonate-dominated porous media, PVP-AgNPs exhibited a higher time-dependent blocking behavior compared to surfactant-stabilized AgNPs. While the effect of the removal of organic impurities of the natural porous medium had only a minor effect on the blocking behavior of PVP-AgNPs, the surfactant-stabilized AgNPs showed a dramatic increase in their mobility. Analyses carried out with asymmetric flow field-flow fractionation (AF4) coupled with UV/Vis- detection and dynamic light scattering (DLS) (AF4-UV/Vis-DLS) revealed that the size of the effluent PVP-AgNPs did not change while effluent surfactant-stabilized AgNPs show a slight increase in their particle size. To conclude, the results from the laboratory experiments and from the mathematical modeling provided important insights into the transport and fate of AgNPs in unconsolidated aquifer material. This knowledge can be used later in field scale studies and risk assessment