HETEROAGGREGATION BETWEEN ENGINEERED NANOMATERIALS AND HEMATITE NANOPARTICLES IN AQUATIC ENVIRONMENTS

The rapid growth in the use of engineered nanomaterials for industrial and research applications, as well as in consumer products, will inevitably result in the release of these nanomaterials into the environment. In natural and subsurface waters, engineered nanomaterials can undergo aggregation with each other (homoaggregation) or with other types of nanoparticles such as naturally occurring colloids (NOCs) (heteroaggregation). Since the concentration of engineered nanomaterials in natural aquatic systems is likely to be much lower than that of NOCs, heteroaggregation is expected to play a more important role than homoaggregation in determining the environmental fate and transport of engineered nanomaterials. While homoaggregation of engineered nanomaterials has been extensively investigated, the heteroaggregation behavior of these nanomaterials has rarely been studied and therefore is not well understood. The objectives of this dissertation work were to investigate the heteroaggregation of engineered nanomaterials with hematite nanoparticles (HemNPs), a model NOC, and the effects of heteroaggregation on the antimicrobial activity of engineered nanomaterials. The engineered nanomaterials studied in this dissertation work were carbon nanotubes (CNTs) and silver nanoparticles (AgNPs). The first part of the dissertation effort focused on the heteroaggregation between CNTs and HemNPs. The rate and mechanism of CNT–HemNP heteroaggregation were demonstrated to depend on the concentration ratios of the two nanomaterials. Heteroaggregation rates were found to increase with increasing CNT/HemNP concentration ratio up to a point and to then decrease with further increase in the concentration ratio. In the presence of humic acid, the maximum heteroaggregation rate was observed to decrease when the humic acid concentration was increased. As the CNT–HemNP heteroaggregates were exposed to solution chemistries where there should be electrostatic and electrosteric repulsion between these nanoparticles, the strength of the heteroaggregates was presumably weakened, thus making the heteroaggregates more susceptible to disaggregation. The heteroaggregation behavior of two other carbon-based nanomaterials, namely, graphene oxide nanosheets and fullerene (C60) nanoparticles, with HemNPs was also investigated and compared with that of CNTs with HemNPs. The second part of the dissertation effort focused on the homoaggregation behavior of citrate- and polyvinylpyrrolidone (PVP)-coated AgNPs in different solution chemistries. The nature of homoaggregation of citrate-coated AgNPs NaCl solution was shown to be in excellent agreement with the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. PVP was found to be a more effective stabilizer than citrate in both monovalent (NaCl) and divalent (CaCl2 and MgCl2) electrolyte solutions. The adsorption of humic acid on citrate- and PVP-coated AgNPs was found to increase the colloidal stability of these nanoparticles in NaCl solutions and also at low CaCl2 concentrations. Conversely, humic acid was observed to enhance the homoaggregation kinetics of both AgNPs at high CaCl2 concentrations. In addition, the heteroaggregation between citrate-coated AgNPs and HemNPs was demonstrated to reduce the antimicrobial activity of AgNPs toward Escherichia coli bacteria. To our knowledge, this work is the first to describe this effect