| Silver nanoparticles (AgNPs) have received an increased attention in the past decade. This is a result of their unique size-dependent physical and chemical properties and their broad-spectrum toxicity to organisms. The AgNPs have been incorporated in a range of consumer products and have various medical, scientific and industrial applications. Because the hypothesized mechanisms that govern the fate, transport and toxicity of bulk materials may not directly apply to materials at the nanoscale, there are great concerns in the regulatory and research communities about potential environmental impacts associated with the release of AgNPs into the environment. Therefore, the current study aimed at conducting fundamental research to characterize the surface charging and aggregation properties of coated and uncoated AgNPs under various environmental conditions, examine their aggregation kinetics, investigate their mobility in reactive and nor-reactive porous media and assess their toxicity to organisms. The selected AgNPs represent the various surface charging scenarios and the common stabilization mechanisms. The AgNPs were, (1) electrostatically stabilized (uncoated H2-AgNPs, Citrate-AgNPs and NaBH4-AgNPs), (2) sterically stabilized (Polyvinylpyrrolidone coated (PVP-AgNPs)) and (3) electrosterically stabilized (branched polyethyleneimine coated (BPEI-AgNPs)). The type of capping agent and environmental conditions (pH, ionic strength and background electrolytes) had a great impact on the surface charge and aggregation potential of AgNPs. The electrostatically stabilized AgNPs aggregated at high ionic strength, acidic pH conditions and in the presence of divalent cations regardless of the pH. The ionic strength, pH and electrolyte valence had no impact on the surface charge and aggregation of the sterically stabilized AgNPs. This was not the case for the electrosterically stabilized AgNPs which exhibited major changes in surface charge and particle size as a result of the variation in solution chemistry. The examination of the electrolyte-induced aggregation kinetics of the selected AgNPs showed that the aggregation behavior of the electrostatically stabilized AgNPs was in agreement with the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (the AgNPs exhibited both reaction-limited and diffusion-limited regimes) and deviated from the DLVO theory in the case of the sterically and electrosterically stabilized AgNPs. The mobility of the selected AgNPs in quartz sand (QS), ferrihdrite coated sand (FcS) and kaolin coated sand (KcS) media was investigated. The impact of natural organic matter (NOM) on the mobility of the selected AgNPs was also examined. The electrostatically stabilized AgNPs were readily mobile in QS but significantly retained in FcS and KcS with more deposition achieved in the KcS media. The sterically stabilized AgNPs followed the same deposition order as the electrostatically stabilized AgNPs while the electrosterically stabilized BPEI-AgNPs was readily mobile regardless of the porous media investigated. Physicochemical interactions were the dominant filtration mechanism in the majority of the investigated cases but straining played the major role in the deposition of the electrostatically stabilized AgNPs in the KcS medium. The presence of humic acid enhanced the mobility of all investigated AgNPs regardless of the stabilization mechanism or soil reactivity. Finally, rapid screening toxicity tests showed that, under aerobic conditions, the AgNPs exhibited surface charge-dependent toxicity on bacillus species. |
