BIOTRANSFORMATION OF ENGINEERED NANOPARTICLES IN THE ENVIRONMENT

A major concern is that engineered nanomaterials will be released into the natural environment; therefore an understanding of the behavior of these materials in soil and groundwater is essential. Biogeochemical reactions (interaction with bacterial communities, mineral surfaces, and within pH/Eh gradients) will determine the ultimate fate of water-soluble nanoparticles (transport and persistence) and their ecotoxicity. This research aims to develop a molecular-level chemical understanding of the biogeochemical transformation in the natural environment of nanocrystals and nanoparticles functionalized with organic molecules to enhance their solubility in water. The hypothesis that guides this research is that nanoparticles interact with microorganisms present in soil and groundwater through passive and active mechanisms that alter the chemical form and hence the groundwater transport and soil retention characteristics of the nanoparticles, and ultimately the human exposure route and toxicity. Gold nanoparticles functionalized with citrate were studied for their mechanism of interaction with the soil microorganisms Pseudomonas fluorescens (aerobic) and Clostridium sp. (anaerobic). Changes in the nanoparticle surface chemical functionality and aggregation behavior in water were studied after exposure to growing and resting bacterial cells. Gold nanoparticle surface plasmon resonance, as measured by spectrophotometry, indicated modification of the citrate functionality and aggregation. Soft x-ray scanning transmission spectromicroscopy revealed that nanoparticles were attracted to the bacterial cell surface; nanoparticle-cell interactions were also studied by transmission electron microscopy. These batch studies were conducted in advance of column experiments to study advective-flow conditions and interactions with biofilms. The goal of these studies is to understand the predominant mechanisms of biotransformation in order to predict how such transformations will affect nanoparticle fate and transport in the environment.