Influence of Surface Oxygen on the Interactions of Carbon Nanotubes with Natural Organic Matter Billy Smith, † Jin Yang, ‡ Julie L. Bitter, † William P. Ball, ‡ and D. Howard Fairbrother* ,†,§ † Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States ‡ Department of Geography and Environmental Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States § Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States * S Supporting Information ABSTRACT: The sorption properties of natural organic matter (NOM) with oxidized multiwalled carbon nanotubes (O-MWCNTs) in simple electrolytes has been studied, as well as the effect that NOM concentration, pH, and O-MWCNT surface chemistry have on CNT stability under environmentally relevant conditions. As O-MWCNT oxygen content increased, NOM sorption decreased in simple electro- lytes for a common set of solution conditions. For each O-MWCNT, NOM sorption increased with increasing ionic strength and decreasing pH, although the sensitivity of NOM sorption to these water quality parameters increased as the O-MWCNT oxygen content increased. Collectively, these observations indicate that NOM sorption by O- MWCNTs is determined by favorable hydrophobic π-π interactions that are moderated by repulsive electrostatic forces between negatively charged carboxylic acid functional groups on the O-MWCNTs and NOM. Stability studies conducted in artificial groundwater revealed that CNT stability is influenced by both the NOM concentration and pH, but stability was largely independent of the O- MWCNT oxygen concentration. These findings contrast with the marked effect that surface oxygen has on CNT stability in simple electrolytes. Electrophoretic mobility measurements revealed that the stabilizing effects of adsorbed NOM are due to the introduction of steric repulsion between NOM-coated CNTs, rather than from changes to surface charge. ■ INTRODUCTION Natural organic matter (NOM) consists of negatively charged macromolecules and is a ubiquitous component of all natural aquatic environments. 1,2 NOM molecules contain hydrophobic segments in addition to hydrophilic carboxylic acid and phenolic functionalities and readily adsorb onto most particles and surfaces encountered in natural waters. 3 Consequently, NOM plays an important role in determining the colloidal stability of suspended particles. In part, this is a consequence of the fact that adsorbed NOM alters a particle’s surface charge 4,5 and thereby influences electrostatic repulsive forces. The presence of adsorbed NOM also generates a macromolecular coating on the particle’s surface. This introduces repulsive steric particle-particle interactions that pose significant barriers to aggregation and deposition. 6-8 Indeed, numerous studies on different inorganic and organic particles have confirmed that adsorbed NOM strongly influences, and in many cases, controls particle stability in water. 4,5,9-12 Motivated by the need to understand the fate and transport of engineered nanomaterials in natural waters, several studies have examined the interactions of NOM with carbon nanotubes (CNTs). For example, Kim et al. have shown that pristine multiwalled carbon nanotubes (MWCNTs), which are extremely hydrophobic and do not form stable suspensions in polar solvents, are stabilized in water after NOM adsorption. 13 It has also been shown that NOM adsorption increases the colloidal stability of oxidized single-walled CNTs. 14 Other studies have focused on measuring the extent of NOM sorption onto CNTs, motivated by the possibility that CNTs could be used as fixed bed sorbents or active components in new membrane technologies designed to remove NOM from drinking water. 15,16 Indeed, comparative adsorption studies using different carbonaceous sorbents suggest that CNTs may be more efficient NOM sorbents than granular activated carbon. 15 CNT-NOM adsorption studies have also found that NOM adsorption is influenced by CNT properties (e.g., diameter) and water quality parameters (e.g., pH and ionic strength). 13 In other related studies designed to investigate how NOM composition influences the sorption properties of NOM with pristine CNTs, the extent of NOM adsorption and the mass of CNTs that could be stabilized in solution at a given concentration of NOM were both found to be proportional to the NOM’s aromatic content. 13 These results were taken to suggest a sorption mechanism that is regulated by the strength of π-π interactions. 13 In support of this basic sorption Received: August 4, 2012 Revised: November 5, 2012 Accepted: November 12, 2012 Published: November 12, 2012 Article pubs.acs.org/est © 2012 American Chemical Society 12839 dx.doi.org/10.1021/es303157r | Environ. Sci. Technol. 2012, 46, 12839-12847