(688f) Role of Aggregation in the Adsorption Behaviors of Carbon Nanotubes

INTRODUCTION Carbon nanotubes (CNTs), as newly emerging nanomaterials, have been examined for various applications in many fields due to their unique structures and outstanding mechanical, optical, and electronic properties. However, due to their nanosize, CNTs can enter into cells and cause damage to plants, animals and humans (Oberdorster et al. 2005). In addition, owing to their strong adsorption affinities for organic chemicals, the fate and transport of organic contaminants can be significantly changed by the release of CNTs into the environment (Colvin 2003). Due to the strong van der Waals forces along the length axis, CNTs are prone to aggregate. Consequently, the availability of adsorption sites for organic chemicals is highly dependent on the aggregation of CNTs. Therefore, a fundamental understanding of the aggregation and adsorption properties of CNTs is critical for assessing the environmental risk of these nanomaterials and for evaluating their potential use in water/wastewater treatment and environmental remediation applications. METHODS One set of single-walled carbon nanotubes (SWNTs) and one set of multi-walled carbon nanotubes (MWNTs) in both pristine (SWNT, MWNT) and surface functionlized forms (SWNT-OH, SWNT-COOH, MWNT-OH, MWNT-COOH) were used as received. Three synthetic organic chemicals (SOCs), phenanthrene (PNT), biphenyl (BP), and 2-phenylphenol (2PP), were employed as adsorbates. Nitrogen adsorption at 77 K and water vapor adsorption at 273 K were performed with a physisorption analyzer (Micromeritics ASAP 2010) to characterize the surface areas, pore size distributions, and surface polarity of the CNTs. X-ray photoelectron spectroscopy (XPS) analysis was performed with an XPS-KRATOS AXIS 165 spectrometer (Kratos Analytical) to determine the elemental components. Constant carbon dose aqueous phase isotherm experiments were conducted using the completely mixed batch reactors (255 mL glass bottles with Teflon-lined screw caps). The isothermal data were analyzed with the Freundlich equation. RESULTS Theoretical calculation and nitrogen adsorption analysis demonstrate that the nanopores trapped in the aggregates of CNTs played a crucial role in their adsorption. Theoretically, the specific surface areas of the SWNTs and MWNTs used in this work are 1420 and 214 cm2/g, respectively. Due to the aggregation, a large portion of surface areas was reduced in the SWNTs. The remained surface area in the SWNTs was only about 30% to the theoretical value. The aggregation of the MWNTs generates pores 12 times to their inherent inner cavity volumes (0.056 cm3/g), whereas the ratio for the SWNTs was only 2.5, demonstrating that the aggregation structures of the MWNTs were much looser than those of the SWNTs. The aggregation of CNTs has caused attention from the practical point of view. In order to increase the water dispersibility of CNTs, surface functionalization was performed by CNT manufacturers. Water vapor adsorption isotherms indicate that the surface functionalization of the SWNTs provided by the manufacturer exerted little effect on their surface polarities, whereas significant effects were observed on the MWNTs. The lack of effect of surface functionalization on the SWNTs might be attributed to the limited functional groups formed from surface functionalization, which are proved by the oxygen contents (Table 1). For the liquid phase adsorption, as shown in Table 1, the adsorption capacities obtained from solubility normalized Freundlich constant (KFS) indicate that the adsorption affinities of the SWNTs to PNT and BP were about 5 and 3 times to those of the MWNTs. The Q values indicate that even normalized with surface area, the adsorption affinities of the SWNTs to PNT were still much higher than those of MWNTs. However, the differences between the SWNTs and the MWNTs to BP became insignificant on the surface area base, suggesting that the molecular configuration of SOCs played a significant role in their adsorption onto CNTs and the effects of molecular configuration of SOCs depended on the structure of CNTs. This is further supported by the Freundlich n values. The larger n values of the nonplanar BP and 2PP on the MWNTs indicate that the adsorption of nonplanar SOCs on the MWNTs were less site-selective than the planar PNT. The Q values in Table 1 also demonstrate that the surface functionalization with oxygen-containing groups had a slightly negative effect on their adsorption. CONCLUSIONS From this study, we got the following conclusions: 1) aggregation of CNTs played an important role in their adsorption to SOCs, 2) adsorption properties of CNTs depended on both the structure characteristics of CNTs and the molecular configuration of SOCs, 3) although the surface functionalization of CNTs is aimed to increase their water dispersibilities, the contribution of increased surface area due to better dispersion of CNTs in water was not large enough to compensate the negative effect caused by water cluster formation due to the presence of oxygen-containing functional groups, 4) the presence of NOM significantly reduced the adsorption capacities of CNTs to AOCs. ACKNOWLEDGEMENTS This work was partly supported by a research grant from National Science Foundation (CBET 0730694). However, the manuscript has not been subjected to the peer and policy review of the agency and therefore does not necessarily reflect its views. REFERENCES: Oberdorster, G., E. Oberdorster & J. Oberdorster (2005) Nanotoxicology: An emerging discipline evolving from studies of ultrafine particles. Environmental Health Perspectives, 113 (7), 823-839. Colvin, V. L. (2003) The potential environmental impact of engineered nanomaterials. Nature Biotechnology, 21 (10), 1166-1170.