Effect Of Environmental Factors On The Aggregation Behavior Of Silver Nanoparticles In The Aquatic Environment

Understanding the environmental factors that affect the fate and effects of engineered nanoparticles (ENPs) in the environment is crucial for NP risk assessment. Silver nanoparticles (Ag NPs) readily transform in the environment, which alters their properties and subsequently their transport, fate, and toxicity. The overall aim of this dissertation was to evaluate the effects of Ag NPs physicochemical properties (e.g., size, surface charge, surface coating) and water chemistry (e.g., buffer concentration, organic ligands, natural organic matter) in controlling Ag NPs colloidal stability. This aim was achieved by systematically 1) review and rationalize many studies that investigated the aggregation kinetics of Ag NPs; and 2) investigate the aggregation kinetics of Ag NPs under specific conditions that has not been studied in the literature, in particular presence of organic ligands with different structures, natural organic matter, and phosphate buffer concentration. The critical coagulation concentration (CCC) is independent of NP concentration for pure electrostatic interactions. However, in the presence of chemical constituents of high affinity to NPs such as cystine, carbonate, and phosphate anions, the CCC is NP concentration-dependent. Although sterically stabilized Ag NPs do not aggregate even at high ionic strength; they are prone to destabilization following surface coating replacement by molecules with higher affinity to Ag NPs that do not provide steric stabilization. For instance, sterically stabilized polyvinylpyrrolidone (PVP) Ag NPs remained colloidally stable at high Na+ and Ca2+ concentrations, but the presence of cystine destabilized PVP Ag NPs at similar Na+ and Ca2+ concentrations. The molecular structures of organic ligands play a significant role in determining the stability of Ag NPs. L-cysteine (L-cys) decreases the stability of Ag NPs, whereas N-acetyl-L-cysteine (NAL-cys) increases the stability of Ag NPs. Whereas Suwanee River fulvic acid (SRFA) increased the stability of Ag NPs, addition of a mixture of SRFA and L-cys decreased the stability of Ag NPs. Cystine significantly impacts the stability of citrate, PVP, and polyethylene glycol (PEG) coated Ag NPs, resulting in a concentration-dependent aggregation of Ag NPs, with a shift in the CCC toward lower concentrations of cystine at lower concentrations of Ag NPs. Furthermore, the CCC for Ag NPs decreased with the increase in phosphate buffer concentration in the presence and absence of SRFA. The majority of NP aggregation studies have focused on the effect of counter ions on NP aggregation kinetics, typically at environmentally irrelevant, high NP concentrations. This PhD focused on the impact of the less studied, yet equally important, environmental factors such as buffers, anions, organic ligands with different structures and properties (e.g., L-cys, NAL-cys and cystine) on Ag NPs stability, especially at environmentally relevant NP concentrations. The findings of this PhD are important to underpin NP risk assessment and environmental fate and behavior studies