(13a) Adsorption and Desorption Dynamics of Hazardous Air Pollutants and Water Vapor On Engineered Nanomaterials

Engineered nanomaterials (1-100 nm size range) have unique chemical and physical properties that make them desirable for widespread applications including environmental remediation, energy conversion, catalyst support and biomedical use. Although the enhanced properties of nanomaterials make them very useful in industry, nanomaterials may ultimately pose a threat to the human population and the environment. Adsorption interactions of nanomaterials with atmospheric contaminants may affect their fate, transport and transformations in the environment. Atmospheric transformations of engineered nanomaterials with air pollutants can pose a greater threat to human health. Therefore, it is important to assess the physical, chemical, and adsorption properties of nanomaterials. This study assesses the equilibrium adsorption and desorption capacities and dynamics of hazardous air pollutants (HAPs) and water vapor on model engineered nanomaterials including silica (SiO2) nanopowder, C60 fullerene, and single-wall carbon nanotubes (SWCNT). These nanomaterials are also physically characterized in terms of their morphology. A gravimetric method was used to determine the equilibrium adsorption and desorption capacities of HAPs and water vapor at ambient conditions of temperature and pressure. A symmetrical gravimetric vapor sorption analyzer was used to obtain adsorption and desorption isotherms for the SiO2, C60, and SWCNT nanomaterials at adsorbate's relative pressures between 0.05 and 0.95 and at 25°C. The morphology of the nanomaterial samples was assessed using a field-emission scanning electron microscope (FE-SEM). The adsorption isotherms of water vapor on the SiO2 nanopowder showed an increased adsorption capacity from 7% to 27% as the relative humidity increased from 10% to 90%, respectively. Water vapor adsorption on SiO2 showed a Type II isotherm according to the Brunauer classification. The water vapor desorption from SiO2 nanopowder showed a similar profile as compared to the adsorption step with minimal hysteresis. The C60 and SWCNT samples showed hydrophobic properties. The C60 fullerene sample did not adsorb water vapor at relative humidity ranging from 10% to 90%. SWCNT samples observed water vapor adsorption ranging from 0 to 2.5% for relative humidity values in the range of 10% to 90 %. The Langmuir and Freundlich adsorption models provided good fit to SiO2 adsorption data. A total relative error of 6.03% and 7.58% were calculated when using the Langmuir and Freundlich equations, respectively. Adsorption and desorption dynamics of hazardous air pollutants on the nanomaterials will also be presented. FE-SEM images show that the morphology of C60 is tabular and amorphous, that SiO2 particles agglomerate at ambient conditions, and that SWCNT are cylindrical in shape. Results from this study are important because they will provide a better understanding of the fate, transport and transformations of these nanomaterials in the atmosphere. There is an urgent need for risk assessment on nanomaterials, and research in this area is receiving world wide support.