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Assessing the Risk of Engineered Nanomaterials in the Environment: Modeling Fate, Exposure, and Bioaccumulation


Engineered nanomaterials (ENMs) are a relatively new class of material for which the risks of negative environmental impacts are still being determined. A comprehensive assessment of the environmental risks of ENMs entering the environment is essential, in part due to the continued increase in ENM production and release to the environment. The technical difficulty in measuring ENM fate and toxicity in complex and dynamic environmental media necessitates the use of mathematical models. In this research, the environmental risks of ENMs are assessed through: (i) the collection and analysis of emerging information on significant fate and transport processes; (ii) development of an ENM-specific fate and transport model to predict the accumulation of ENMs and their exposure to organisms in the environment; (iii) development of a statistical model to predict the distribution of species toxicity from specific ENMs in freshwater; and (iv) development of a bioaccumulation model to predict the long-term accumulation of ENMs through a food chain.

The NanoFate model, which was developed as part of the research described in this paper, is used to predict the temporal variability in fate across a broad range of complex environmental media at various spatial scales using both traditional fate and transport processes such as advection, deposition, and erosion, but also using ENM-specific processes and transformations such as heteroaggregation, sedimentation, and dissolution. A case study on San Francisco is then used to explore how fate and accumulation may vary among 4 different metallic ENMs, n-CeO2, n-CuO, n-TiO2, and n-ZnO, because the rates of fate processes and the toxicity are known to vary among these four ENMs.

Chapter 1 specifically explores how these processes and toxicities vary among different types of ENMs. Chapter 2 explores how species sensitivities vary between different ENMs within a freshwater ecosystem. A species sensitivity distribution (SSD) is a cumulative probability distribution of a chemical’s toxicity measurements obtained from single-species bioassays that can be used to estimate the ecotoxicological impacts of that ENM. The SSD results indicate that size, formulation, and the presence of a coating can alter toxicity, and therefore the corresponding range of toxic concentrations. Chapter 3 describes the development of the NanoFate model and explores the implications of the San Francisco case study. By investigating both the range in rate processes and release scenarios, ENM fate was found to vary by multiple orders of magnitude among different environmental media and that even with an improved understanding of ENM fate, predictions of environmental concentrations are still very uncertain. We compare the predicted environmental concentrations for San Francisco Bay across many different release scenarios with the results of the SSDs and found that while CuO, TiO2, and ZnO are likely to exceed No Observed Effect Concentrations (NOEC) in freshwater, this is not the case for soils. The worst-case scenario, where the predicted concentrations would exceed lethal concentrations (LC50), was not found in any scenario explored within the case study. Chapter 4 explores the range in bioaccumulation that could result from the NanoFate predictions for a freshwater ecosystem. A toxicokinetics model, using as much species-specific and ENM-specific uptake, biotransformation, and elimination rates as were available for CuO, TiO2, and ZnO is used to predict the likelihood of bioconcentration and biomagnification through a simple food chain. Though bioconcentration was found for most species, biomagnification was not predicted to be significant with increasing trophic levels. Uncertainty analysis indicates that these results may vary by as much as two orders of magnitude. A parameter sensitivity analysis highlights key biological and environmental parameters that can be used to focus future research. While further developments will improve these predictions as our understanding of ENM fate and toxicity progresses, current understanding indicates that risk is likely low for most ENMs at predicted environmental concentrations though there is some concern that under high and localized release scenarios, toxic impacts will occur.

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