Relative importance of multiple mechanisms in sorption of organic compounds by multiwalled carbon nanotubes
Introduction
Since their discovery, carbon nanotubes (CNTs) have been proposed for numerous potential applications because of their outstanding physiochemical properties. With increasing industrial production and use of CNTs, a large quantity of them will inevitably be released into the environment. Due to the large surface area (SA) of CNTs, they have been proposed as a promising adsorbent to remove hydrophobic organic compounds (HOCs) from the environment [1]. Since sorption is a critical factor that affects transport, fate, bioavailability and persistence of HOCs in the environment, a better understanding of the underlying interaction mechanisms between HOCs and CNTs is of importance for probing environmental applications of CNTs and the environmental behaviors of both CNTs and HOCs.
It has been documented that hydrophobicity of HOCs is an important factor affecting their sorption by CNTs [2]. A recent study, however, reported that although nitrobenzene was much less hydrophobic than benzene, toluene, and chlorobenzene, it had much higher sorption by multiwalled CNTs (MWCNTs) [3]. Another review reports the inability to establish a unifying relationship between Kd values of HOCs by CNTs and their Kow[4]. Furthermore, the hexadecane-water distribution coefficient (KHW)-normalized Kd values of various HOCs by CNTs differed over three orders of magnitude [5]. Other studies indicate the important roles of π–π interactions between aromatics and graphene sheets of CNTs or black carbons in HOC sorption [6], [7], [8], [9]. The role of hydrogen bonding in sorption of polar compounds by CNTs and activated carbons was also emphasized in previous studies [10], [11]. However, up to now, direct evidence for the relative importance of various mechanisms in HOC sorption by CNTs is scarce. As reported, molecular size of HOCs greatly influenced their sorption by black and activated carbons primarily due to the molecular sieve effect [12], but the relationship between molecular size of HOCs and their sorption by CNTs is largely unclear.
Functionalization of CNTs can happen by incidental exposure to strong oxidizing agents (e.g., O3 or OH radicals) after they enter the environment [13], [14]. Also, introduction of hydrophilic moieties to surfaces of CNTs broadens their usefulness and number of potential practical applications due to dispersion enhancement. However, the introduced polar functionalities reduced accessibility of HOC molecules to CNT surfaces, thus decreasing their sorption [15]. In another study, the introduced O-containing moieties increased sorption of p-xylene by CNTs, but decreased o-xylene sorption by CNTs [16]. These inconsistent findings showed that impact of CNT functionalization on its sorption for HOCs remains unclear.
The key objectives of this work were to: (1) provide direct evidence for the relative importance of multiple mechanisms in HOC sorption by CNTs; (2) examine the influences of introduced hydrophilic moieties on CNT sorption of HOCs; and (3) probe the roles of molecular size and specific interactions (e.g., hydrogen bonding) of HOCs in their sorption by CNTs. To achieve our aims, sorption of HOCs with strikingly different physicochemical properties by original and F-MWCNTs was examined. Results of this work will help systematically understand the underlying mechanisms controlling sorption of HOCs by CNTs.
Section snippets
Sorbates and sorbents
The original and OH-functionalized MWCNTs with purity > 95% were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences. MWCNT10, MWCNT20 and MWCNT50 were MWCNTs with outer diameters of 10–20 nm, 20–30 nm and >50 nm, and their functionalized derivatives were labeled as F-MWCNT10 and F-MWCNT20 and F-MWCNT50, respectively. Based upon our previous works and findings [11], [17], [18], [19], three chemicals (i.e., phenanthrene, lindane and atrazine) with major differences in
Elemental composition
All original MWCNTs had high carbon contents (i.e., >96.8%) and very low oxygen contents (Table 2), reflecting their high purity and hydrophobicity. Functionalization introduced substantial hydroxyl groups to MWCNTs thus increasing their polarity. MWCNT10 had the largest SA and porosity, and the SA and porosity of MWCNTs decreased with an increase in their outer diameter, noting the reduction in number of sorption sites for HOCs.
Sorption isotherms
Sorption data of phenanthrene, lindane and atrazine by the
Conclusions
Functionalization of MWCNTs with hydrophilic moieties (e.g., hydroxyl groups) broadens their practical application, but reduces their sorption of HOCs regardless of the chemical structure, suggesting their reduced influence on environmental behaviors (e.g., transport and fate) of HOCs. Hydrophobic interactions dominantly controlled HOC sorption by both the original and F-MWCNTs, and π–π interactions enhanced sorption of aromatic compounds. Results of this work provided direct evidence for the
Acknowledgments
This study was supported by the National Natural Science Foundation of China (40971246, 40730737 and 40710019001), the Startup Fund for the Peking University 100-Talent Program, the National Basic Research Program (2007CB407301), China Educational Foundation for Undergraduate Students of Sciences (No. J0630531), and USDA Hatch Program (MAS 00978). We thank Ellen Russell for her editing on the manuscript.
References (35)
- et al.
Assembly structure control of single wall carbon nanotubes with liquid phase naphthalene adsorption
Colloid Surf A
(2007) - et al.
Effect of chemical surface heterogeneity on the adsorption mechanism of dissolved aromatics on activated carbon
Carbon
(2000) - et al.
Comparison of molecular sieve properties in microporous chars from low-rank bituminous coal activated by steam and carbon dioxide
Carbon
(2005) - et al.
Adsorption of o-xylene and p-xylene from water by SWCNTs
Carbon
(2007) - et al.
Interactions of sodium azide with triazine herbicides: effect on sorption to soils
Chemosphere
(2006) - et al.
Molecular simulation study of hydrophilic and hydrophobic behavior of activated carbon surface
Carbon
(1998) - et al.
Sorption of atrazine on conventional and surface modified activated carbons
J Colloid Interface Sci
(2006) - et al.
Removal of pesticides from water using hypercrosslinked polymer phases: part 2 – sorption studies
Process Safety Environ
(1998) - et al.
Environmental technologies at the nanoscale
Environ Sci Technol
(2003) - et al.
Adsorption of polar and nonpolar organic chemicals to carbon nanotubes
Environ Sci Technol
(2007)
Adsorption mechanisms of organic chemicals on carbon nanotubes
Environ Sci Technol
Adsorption and hysteresis of bisphenol A and 17α-ethinyl estradiol on carbon nanomaterials
Environ Sci Technol
Photoluminescene and electronic interaction of anthracene derivatives adsorbed on sidewalls of single-walled carbon nanotubes
Nano Lett
Characterization of aromatic compound sorptive interactions with black carbon (charcoal) assisted by graphite as a model
Environ Sci Technol
Sorption of polycyclic aromatic hydrocarbons and polychlorinated biphenyls to soot and soot-like materials in the aqueous environment: mechanistic considerations
Environ Sci Technol
Giant thermopower effects from molecular physisorption on carbon nanotubes
Phys Rev Lett
Adsorption of phenolic compounds by carbon nanotubes: role of aromaticity and substitution of hydroxyl groups
Environ Sci Technol
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