Thermal Decomposition of Nano-Enabled Products: Potential Environmental Health and Safety Implications

Abstract

The widespread use of engineered nanomaterials (ENMs) in consumer products inevitably raises concern for potential exposure to ENMs released from such nano-enabled products (NEPs). It is therefore important to understand the scenarios and mechanisms by which ENMs are released over the NEP lifecycle causing exposure and potential environmental health implications. In this dissertation, I focus on understanding the fundamentals related to the incineration/thermal decomposition of industrially relevant NEPs, such as thermoplastics and coatings, widely used in many applications. The potential release of ENMs (used as nanofillers in NEPs) into the environment and the physicochemical, morphological (PCM) and toxicological properties of the released byproducts (released aerosol, residual ash and off-gases) are assessed. The role of NEP matrix composition, nanofiller type/size/mass-loading and thermal process conditions on the thermal decomposition behavior, release dynamics and byproduct PCM properties is investigated in detail. The potential cellular toxicological implications of the released aerosol are assessed and linked to PCM properties of the NEPs. Key findings suggest that inorganic (and catalytic) metal oxide nanofillers such as TiO2, Fe2O3 and CuO are more likely to be released into the aerosol compared to carbonaceous nanofillers (e.g., CNTs (carbon nanotubes)) and inert metalloid oxide nanofillers (e.g., SiO2). Moreover, majority of inorganic nanofillers are retained loosely in the residual ash as flaky clusters, prompting the concern for their subsequent release into the environment (due to weathering, leaching, etc.), whereas CNTs are completely combusted at the high temperatures (>800 °C). More importantly, in the case of released catalytic metal oxide nanofillers, due to their synergistic interactions with gaseous semi-volatile organic byproducts, carcinogenic polycyclic aromatic hydrocarbons (PAHs) could be formed and thus present a significant inhalation exposure hazard for incineration facility operators, firefighters and surrounding community. Furthermore, increased bioactivity and toxicity of the released aerosol compared to the control (no nanofiller) are also confirmed in cellular studies. Future studies should investigate effects of sequential lifecycle stresses such as weathering on thermal decomposition of NEPs. Additionally, environmental fate and transport of ENMs from residual ash should be performed to assess potential environmental health risks and develop mitigation strategies associated with incineration/thermal decomposition of NEPs.