Environ. Sci. Technol. 2007, 41, 8295–8300
Adsorption of Polar and Nonpolar Organic Chemicals to Carbon Nanotubes WEI CHEN,† LIN DUAN,† AND D O N G Q I A N G Z H U * ,‡ Tianjin Key Laboratory of Environmental Remediation and Pollution Control/College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China, State Key Laboratory of Pollution Control and Resource Reuse/ School of the Environment, Nanjing University, Jiangsu 210093, China
Received May 24, 2007. Revised manuscript received September 24, 2007. Accepted October 10, 2007.
Understanding adsorptive interactions between organic contaminants and carbon nanotubes is critical to both the environmental application of carbon nanotubes as special adsorbents and the assessment of the potential impact of carbon nanotubes on the fate and transport of organic contaminants in the environment. The adsorption of organic compounds with varied physical-chemical properties (hydrophobicity, polarity, electron polarizability, and size) to one single-walled carbon nanotube (SWNT) and two multiwalled carbon nanotubes (MWNTs) was evaluated. For a given carbon nanotube, the adsorption affinity correlated poorly with hydrophobicity but increased in the order of nonpolar aliphatic < nonpolar aromatics < nitroaromatics, and within the group of nitroaromatics, the adsorption affinity increased with the number of nitrofunctional groups. We propose that the strong adsorptive interaction between carbon nanotubes and nitroaromatics was due to the π-π electron-donor–acceptor (EDA) interaction between nitroaromatic molecules (electron acceptors) and the highly polarizable graphene sheets (electron donors) of carbon nanotubes. Additionally, we attribute the stronger adsorption of nonpolar aromatics compared to that of nonpolar aliphatics to the π-electron coupling between the flat surfaces of both aromatic molecules and carbon nanotubes. For tetrachlorobenzene, the bulkiest adsorbate, adsorption affinity (on a unit surface area basis) to the SWNT was much stronger than to the two MWNTs, indicating a probable molecular sieving effect.
Introduction Carbon nanotubes are a relatively new class of synthesized carbonaceous materials and are considered to be promising candidates for many areas of applications including special adsorbents in water treatment and environmental remediation (1–3). The rapid growth in production and industrial applications of carbon nanotubes, however, has also raised serious concerns over the potential environmental impact of these materials (4–7). One of the concerns is that the fate, transport, and environmental exposure of common organic * To whom correspondence may be addressed: Phone/fax: 8625-8359-6496; e-mail:
[email protected]. † Nankai University. ‡ Nanjing University. 10.1021/es071230h CCC: $37.00
Published on Web 11/17/2007
2007 American Chemical Society
contaminants could be largely changed due to their strong adsorptive interactions with carbon nanotubes released into the aquatic or soil environments. For example, carbon nanotubes might function similarly to residues of incomplete combustion in soils and sediments (such as soot, coal, and black carbon) in sequestrating organic contaminants. In a recent study, Hyung et al. (8) reported that natural organic matters in aquatic systems can interact strongly with carbon nanotubes and thus greatly enhance their stability and transport in natural aquatic systems. In such cases, the mobility and transport of contaminants adsorbed to carbon nanotubes might be enhanced. Therefore, characterizing the adsorption of organic contaminants to carbon nanotubes may shed light on both potential environmental applications and understanding of the environmental impact of these materials. Limited numbers of studies have been conducted to understand the adsorption of organic contaminants by carbon nanotubes (9–18). A few of these studies focused on the potential applications of carbon nanotubes for the removal of recalcitrant compounds such as dioxin and undesirable byproducts in water treatment (9–12). Several other studies were conducted primarily to understand the mechanistic aspects of organic chemical-carbon nanotube interactions, but only a few nonpolar chemicals have been studied (i.e., butane, 1,2-dichlorobenzene, naphthalene, phenanthrene, and pyrene) (13–18). A common observation from these studies was that carbon nanotubes are very strong adsorbents for hydrophobic organic compounds. This is understandable considering the strong hydrophobicity and high surface area of carbon nanotubes. Additionally, an important implication from several of the studies is that electronic polarizability of the aromatic rings on the surface of carbon nanotubes might considerably enhance adsorption of the organic compounds to carbon nanotubes (9, 13, 14, 18). For example, Long and Yang (9) found that, in the lowconcentration regime (the Henry’s Law region), the amount of dioxin adsorbed to carbon nanotubes was much higher than that adsorbed to activated carbon; they attributed it to the strong interaction between the two benzene rings of dioxin and the surface of the carbon nanotubes. Thus far, no studies have been conducted to systematically compare adsorptive interactions between carbon nanotubes and organic compounds with significantly different physicalchemical properties (e.g., polarity, functional groups, etc.). In addition, engineered carbon nanomaterials can vary significantly in shape, size and morphology, and impurity (e.g., metal, amorphous carbon and O-containing groups), which can further complicate the adsorptive properties of these materials to organic contaminants. Therefore, much more research is still needed for a better understanding of the molecular interactions of carbon nanotubes and organic contaminants with different properties. In this study, we evaluated the adsorption of contaminants with varied physical-chemical properties (hydrophobicity, electron polarizability, polarity, size, etc.) to three types of carbon nanotubes, including one single-walled carbon nanotube (SWNT) and two multiwalled carbon nanotubes (MWNTs). The magnitude of nonhydrophobic interactions between chemicals and carbon nanotubes was determined by “normalizing” adsorption isotherms using an inert solvent, n-hexadecane, as the reference state. Mechanisms controlling the nonhydrophobic interactions between organic chemicals and carbon nanotubes are discussed on the basis of the experimental observations. VOL. 41, NO. 24, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Adsorbate Properties (Water Solubility, Csat; n-Octanol–Water Partition Coefficient, KOW; and nHexadecane-Water Partition Coefficient, KHW) and Freundlich Model Coefficients (KF and n) Obtained from Adsorption Results adsorbate
Csata (mmol/L)
log KOWa
log KHWb
adsorbent
KF (mmol1-n Ln/kg)
n
R2
SWNT MWNT1030 MWNT4060 SWNT SWNT MWNT1030 MWNT4060 SWNT SWNT MWNT1030 SWNT MWNT1030 MWNT4060 SWNT MWNT1030 SWNT SWNT MWNT1030 SWNTc
430 ( 20 290 ( 20 130 ( 10 720 ( 30 1000 ( 30 400 ( 10 410 ( 20 2200 ( 500 4500 ( 600 1700 ( 100 400000 ( 70000 16000 ( 15000 11000 ( 2000 520 ( 30 310 ( 30 2600 ( 300 1400 ( 200 150 ( 10 1230 ( 90
0.62 ( 0.04 0.95 ( 0.09 0.58 ( 0.08 0.57 ( 0.03 0.59 ( 0.02 0.94 ( 0.03 0.88 ( 0.06 0.45 ( 0.04 0.48 ( 0.02 0.64 ( 0.02 0.79 ( 0.04 0.75 ( 0.13 0.75 ( 0.03 0.41 ( 0.03 0.60 ( 0.04 0.43 ( 0.02 0.35 ( 0.03 0.25 ( 0.03 0.87 ( 0.05
0.990 0.980 0.965 0.995 0.997 0.998 0.989 0.979 0.993 0.998 0.994 0.940 0.997 0.989 0.993 0.992 0.982 0.985 0.994
benzene
2.24E+01
2.17
2.15
toluene chlorobenzene
6.03E+00 4.07E+00
2.69 2.78
2.74 2.81
1,2-dichlorobenzene 1,2,4-trichlorobenzene
8.91E-01 1.66E-01
3.4 4.06
3.52 4.43
1,2,4,5-tetrachlorobenzene
5.89E-03
4.72
4.95
nitrobenzene
1.62E+01
1.85
1.47
4-nitrotoluene 2,4-dinitrotoluene
3.72E+00 1.38E+00
2.37 2.0
2 1.41
cyclohexane
6.76E-01
3.44
3.91
a
From Schwarzenbach et al. (20). cyclohexane with five.
b
From Zhu et al. (21, 22).
Experimental Section Adsorbents. SWNT, MWNT1030, and MWNT4060 were purchased from Nanotech Port Co. (Shenzhen, Guangdong Province, China). On the basis of the information provided by the manufacturer, the carbon nanotubes were synthesized by a chemical vapor deposition method using cobalt, manganese, molybdenum, or nickel as catalysts. The SWNT contained more than 90% (by volume) carbon nanotubes (including both SWNT and MWNT) and less than 10% impurities, mainly amorphous carbon ( benzene). Nonporous graphite consists of stacked graphene surfaces and contains no functionality. The interlayer spacing between the coaxial tubes of MWNTs approximates to that of the bulk graphite (0.335 nm) (28) and is impenetrable to N2 molecules and organic chemicals. Therefore, for MWNTs, only the innermost and outermost surfaces of the tubes are available for adsorption. Figure 3a and b show that the three carbon nanotubes and nonporous graphite show very similar adsorption affinities toward benzene and chlorobenzene, indicating a predominant
Acknowledgments This project was supported by National Science Foundation of China (Grants 20637030, 20407013, and 20577024), Ministry of Education of China (Grant 105044), Tianjin Municipal Science and Technology Commission (Grant 06TXTJJC14000), Fok Ying Tung Education Foundation (Grant 101081), and China-US Center for Environmental Remediation and Sustainable Development.
Supporting Information Available Figure S1 shows transmission electron microscopy images of treated carbon nanotubes. Figure S2 shows nitrogen sorption and desorption isotherms at 77 K on treated carbon nanotubes. Figure S3 shows the Fourier transform infrared transmission spectra of treated carbon nanotubes. Figure S4 shows pH effect on adsorption of 1,2,4-trichlorbenzene and 2,4-dinitrotoluene to SWNT. Table S1 shows pore volume and average pore size distribution of treated carbon nanotubes. This material is available free of charge via the Internet at http://pubs.acs.org.
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