Enhanced environmental mobility of carbon nanotubes in the presence of humic acid and their removal from aqueous solution.

Carbon nanotubes (CNTs) are important structural blocks for the preparation of composites with unique optical, electrical, and mechanical properties, and their production is expected to increase drastically in the years to come. This may increase the risk of human and environmental exposure to CNTs. CNTs are extremely hydrophobic and prone to aggregation, as they are subject to higher van der Waals forces along their length axis, and therefore are not readily dispersed in aqueous or non-aqueous solutions; this has been a major obstacle for the application of CNTs in industry. As a result, significant attention has been directed towards methods of CNT dispersion in aqueous solution. Two methods of exohedral functionalization of CNTs have been developed to disperse them; covalent and non-covalent methods. Non-covalent methods are more desirable since they incur little damage to the CNTs’ intrinsic structures and properties. Dispersants tested in the laboratory for non-covalent functionalization of CNTs include surfactants, synthetic polymers, and biopolymers. Therefore, even though some studies have shown that CNTs are biologically active and cause toxic responses in some cell cultures, CNTs are not usually considered as potential environmental toxins in the aqueous and soil environment because of their strong hydrophobicity and propensity to form insoluble aggregates in aqueous solution. Given the increase in production of CNTs, their potential release to the environment, and possible toxicity, of particular concern are two recent separate studies reporting that natural organic matter (NOM), in particular its major component, humic acid (HA), can disperse CNTs in the aqueous phase. HA constitutes a major fraction of soil organic matter, and of surface water organic matter, and is the most abundant naturally occurring organic macromolecule on Earth. The ubiquitous presence of HA will facilitate the dispersion of CNTs in the environment. Estimating the risk, both to humans and to the environment, of CNTs requires an understanding of their mobility in the environment, their bioavailability, and their toxicity. For this reason, an investigation on the transport of solubilized CNTs in the presence of HA within environmental media, such as soils, is necessary and important. In addition, given the possibility that CNTs will be dispersed by HA in ambient waters, a removal strategy for dispersed HA-stabilized CNTs is needed to reduce their potential environmental hazard. Thus, the objective of this study is to investigate the environmental behavior of CNT released in powder form into aqueous media and to develop a method for removing HAstabilized CNTs from these waters. Single-walled CNTs (SWNTs) and multi-walled CNTs (MWNTs) were considered. Although CNTs dispersed using synthetic surfactants or polymers can also be released, they are outside the scope of this study. The SWNTs, with a diameter of 1.4 nm, were synthesized by the high-pressure decomposition of CO (HiPco process purchased from Tubes@Rice) and the MWNTs, with a diameter of 35 nm, were produced by chemical vapor deposition (CVD). Dispersions of either SWNTs or MWNTs were prepared using HA at environmentally relevant concentrations ( 25mg L ). Although higher HA concentrations could be considered to disperse more CNTs, the objective was to study conditions that would lead to environmental mobility. UV/Vis absorption spectroscopy was used to determine the dispersion of CNTs in aqueous solution in the presence of HA. The concentration of HAcoated CNTs was determined by measuring the absorbance of the solution at 600 nm and by using an extinction coefficient of 2.23 10 cm g 1 for SWNTs and of 2.88 10 cm g 1 for MWNTs (see Supporting Information). Aromaticity, and the resulting p–p interactions, has been identified as an important parameter in CNT dispersion by various surfactants and polymers. The structure of HA is usually described as assemblies of covalently linked aromatic and aliphatic residues, in which the aromatic fraction ranges from 10–40%. Presumably, the aromatic fractions of HA preferentially associate with CNTs via p–p interactions. HA was found to be very effective in promoting the dispersion of both SWNTs and MWNTs in aqueous solution. The strong interaction between CNTs and HA molecules was demonstrated by Liu et al. through Raman spectroscopy. Sonicating a mixture of CNTs in an aqueous HA solution, followed by centrifugation to remove undispersed CNTs, resulted in dark-colored CNT suspensions (Figure S1 in the communications