Engineered nanomaterials (ENMs) are small anthropogenic colloids with at least one length dimension <100 nanometers. Due to the nature of their use, ENMs are being increasingly released to the environment. Yet the environmental risks posed by ENMs are unknown due to a variety challenges, including limitations with detecting and quantifying ENMs in environmental systems. To support ENM risk assessments, environmental fate models have been developed to estimate predicted environmental concentrations (PECs). The objective of this research was to (1) aid the refinement of these models by investigating certain processes that have been shown to influence ENM environmental fate and (2) identifying the dominant factors affecting those processes under highly realistic environmental conditions. In particular, this research explores the aggregation behavior of ENMs in freshwater systems and how surface coatings applied to ENMs during their manufacture and the transformation of ENMs during their life-cycle may alter this behavior.
To accomplish this, the aggregation of a suite of model ENMs was first investigated in simulated freshwater media under varying but well-controlled aquatic chemistry. This allowed mechanistic insights into how variations in the physiochemical properties of the surface coatings alter ENM aggregation. This was followed by examining the aggregation of the model ENMs under more realistic conditions using an actual freshwater media. Then, to better understand the form of ENMs being released to the environment, the transformations of the ENMs during conventional wastewater treatment processes was investigated. The impact of these transformations on the aggregation behavior of the ‘aged’ ENMs was then evaluated and compared against the behavior of their ‘pristine’ analogs in an actual freshwater media.
From this body of work, it was found that variations in the surface coating physiochemical properties can alter the aggregation behavior of ENMs in both simulated and actual freshwater media. Of particular importance was the role of surface coatings in mediating the adsorption of natural organic macromolecules that are ubiquitous in the environment. Likewise, we find that the transformations occurring to the ENMs during conventional wastewater treatment processes were influenced by the initial properties of the surface coatings. More importantly, however, was that the ‘aged’ ENMs no longer resembled their ‘pristine’ analogs, likely due to the formation of a corona layer comprised of organic macromolecules from the wastewater media. In an actual freshwater media, the aggregation behavior of the ‘aged’ ENMs was found to differ from that of the ‘pristine’ ENMs.
These findings have significant implications when trying to understand the processes and factors influencing the environmental fate of ENMs. First, surface coatings influence ENM aggregation behavior in complex aquatic media, which can be explained by studying their behavior in simulated media. However, under highly realistic conditions mimicking the nature of ENMs being released to the environment (i.e., ‘aged’ ENMs to actual freshwater environments), the environmental fate of ENMs is best explored using ‘aged’ ENMs. It is recommended that future studies examining the processes and factors impacting ENM environmental fate consider the life-cycle of ENMs in order to reflect the nature of ENMs released to the environment.