Date of Award
Master of Science (MS)
Environmental Engineering and Earth Science
Lee , Cindy
Freedman , David
Carbon nanotubes (CNTs) are allotropes of carbon consisting of sheets of carbon atoms covalently bonded in hexagonal arrays that are seamlessly rolled into a hollow, cy-lindrical shape with both ends rounded by fullerene-like caps. As large amounts of CNTs have been manufactured and significant growth is expected in commercial CNT produc-tion, there is a major concern over their health and environmental risks once they enter into the environment. In aquatic systems, CNTs are likely to adsorb organic chemicals and aggregate as bundles due to their extremely hydrophobic surfaces. The aggregation state of CNTs plays a significant role in their fate and transport in the environment, and their adsorption of organic chemicals. There is still much to learn about the mechanisms and factors controlling aggregation behavior of CNTs and adsorption of organic contaminants by CNTs. This knowledge is critical for the environmental risk assessment of both CNTs and toxic organic chemicals as well as for evaluating the potential applications of CNTs as adsorbents in water and wastewater treatment systems.
This study had four objectives. The first one was to investigate the impact of ag-gregation (e.g. bundle structure) on the CNT surface area and pore volume in the bulk phase, and subsequent adsorption of synthetic organic compounds (SOCs) in aqueous so-lutions. Then, the roles of CNT physicochemical properties (e.g., surface area, pore vo-lume, morphology and functional groups) on the adsorption of SOCs were examined. The third objective was to study the role of selected SOC properties (e.g., hydrophobicity, molecular size and configuration) on their adsorption by CNTs. Finally, the importance of selected background aquatic chemistry components (i.e., pH, ionic strength and presence of NOM) on adsorption of SOCs by CNTs was investigated.
Theoretical calculations and nitrogen adsorption analysis results demonstrated that aggregation of CNTs led to a significant reduction in surface area, but a significant increase of pore volume due to interstices trapped in the CNT aggregates. However, based on the liquid phase adsorption results which showed that the adsorption of SOCs by CNTs was controlled by available adsorption surface area rather than pore volume, aggregation of CNTs was an unfavorable factor for the SOC adsorption.
The liquid phase adsorption results indicated that molecular configurations of SOCs played a role in their adsorption by CNTs. For the planar phenanthrene (PNT), the single-walled carbon nanotubes (SWNTs) showed significantly higher adsorption capaci-ties and site energies than the multi-walled carbon nanotubes (MWNTs), whereas for the nonplanar SOCs the adsorption capacity and site energy differences between the SWNTs and the MWNTs became smaller with increasing concentration of SOCs. Surface func-tionalization of CNTs improved their dispersion in aqueous solutions, but decreased their adsorption capacities for the hydrophobic SOCs, which was attributed to the formation of water clusters around the oxygen-containing functional groups.
Among the three background water characteristics (pH, ionic strength and NOM) examined, NOM showed the most significant effect on SOC adsorption, while solution pH and ionic strength exhibited minimal or negligible impacts on SOC adsorption, de-pending on the physical structure and surface chemistry of CNTs as well as the physico-chemical properties of SOCs. The presence of NOM, either spiked simultaneously with SOC or one-week-preloaded prior to the spiking of SOC, greatly suppressed the SOC adsorption by CNTs and the impact on the SWNTs was greater than that on the MWNTs. The planarity and hydrophobicity of SOCs were two important factors that determined the effects of NOM, pH and ionic strength on SOC adsorption by the CNTs.
In summary, this study showed the importance of CNT aggregation in the SOC adsorption. The SOC sorption affinity on the CNTs depends on SOC molecular configu-ration (planarity) and CNT surface area. Also, the different impacts of NOM on adsorp-tion indicated that planarity and hydrophobicity of SOCs were two important factors de-termining the effects of NOM. Future studies need to consider the factors and characteristics of CNTs that impact their aggregation and thus available surface area for adsorption, which is important in the investigation of their adsorption properties. Furthermore, more laboratory studies are required to investigate the adsorption of SOCs with different properties (e.g., functional groups).
Shao, Ting, "Factors influencing the adsorption of synthetic organic compounds by carbon nanotubes in aquatic environments" (2010). All Theses. 875.