Adsorption and Desorption of Chlorinated Compounds from Pristine

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Adsorption and Desorption of Chlorinated Compounds from Pristine and Thermally Treated Multiwalled Carbon Nanotubes Xingmao Ma,*,† Deepti Anand,† Xiangfeng Zhang,‡ and Saikat Talapatra*,‡ †

Department of Civil and Environmental Engineering and ‡Department of Physics, Southern Illinois University Carbondale, Carbondale, Illinois, United States ABSTRACT: We report on the adsorption and desorption behavior of chlorinated compounds on as-produced as well as heat-treated multiwalled carbon nanotubes (MWCNTs). Adsorption and desorption studies of trichloroethylene (TCE), 1,1,1-trichloroethane (1,1,1-TCA), and 1,3,5-trichlorobenzene (1,3,5-TCB), three chlorinated compounds with different molecular structures, on MWCNT bundles were performed. Adsorption measurements indicated that adsorption capacities of these chlorinated compounds to MWCNTs are greatly affected by the molecular structures and follow an order of 1,1,1-TCA < TCE < 1,3,5-TCB. A pronounced desorption hysteresis was observed for all the adsorbate compounds that were experimented on pristine MWCNTs. Similar adsorption and desorption investigations on thermally treated MWCNTs showed an increased sorption capacity of these compounds (39-97%). However, in the case of the heat-treated MWCNTs, reduced desorption hysteresis was noticed, possibly due to the removal of disorderly amorphous carbon and certain surface modification. These results indicate that simple heat treatment of MWCNTs could increase the adsorption capacity of these chlorinated compounds and drastically change the adsorption characteristics of MWCNTs.

’ INTRODUCTION Chlorinated compounds have become prevalent environmental pollutants due to their widespread use in various chemical processes, leading to diverse applications and unregulated disposal practices in the past.1 These compounds are highly detrimental to the health of human beings and aquatic life, and as a result, many of them are on the U.S. Environmental Protection Agency’s (USEPA’s) priority list to be removed from the environment. Carbon nanotubes (CNTs) have displayed many surface properties that are suitable for applications where surface adsorptive properties can be utilized to filter/remove a wide variety of environmental contaminants (especially from water).2-4 For instance, CNTs were found to be a stronger adsorbent than activated carbon for 1,2-dichlorobenzene in a broad pH range (3-10).5 The high adsorptive potential of CNTs derives from many appealing features of CNTs, such as their well-defined structure and uniform surface,6 multilayer adsorption characteristics,7,8 additional sorption sites, such as the interstitial regions between the CNTs bundles, and the defects on the tubes.9,10 The controllable pore size distributions and manipulative surface chemistry of CNTs provide additional advantages for CNTs as superior adsorbent.11 Even though various investigations on the adsorption of organic compounds to CNTs are available in the literature, systematic studies on the adsorption and desorption mechanisms of chlorinated compounds on CNTs are still lacking. In addition, most of the previous studies focused on the adsorption of contaminants and only a few studies addressed the desorption mechanisms, which comprise important implications on the fate and transport of environmental pollutants.12,13 r 2011 American Chemical Society

Chen et al.12 observed significant desorption hysteresis of naphthalene and 1,2-dichlorobenzene from fullerene (C60), and their result was in agreement with a later study on the desorption of polycyclic aromatic hydrocarbons (PAHs) from carbonaceous nanomaterials.13 Deformation and rearrangement of the interstitial spaces in spherical fullerene nanoparticles were ascribed as the main mechanism for the desorption hysteresis in fullerenes.13 The authors further concluded that cylindrical CNTs could not form such structures during the adsorption and desorption process and, therefore, carbon nanotubes did not show desorption hysteresis. The hypothesis was supported by a recent study showing no significant desorption hysteresis of atrazine from MWCNTs.14 However, a later study on the adsorption and desorption of two pharmaceuticals, bisphenol A (BPA) and 17Rethinyl estradiol (EE2), on CNTs showed significant desorption hysteresis.15 From these studies, it is clear that factors other than the CNT bundle restructuring also play a crucial role in dictating the mechanism of surface interactions (the adsorption and desorption process) of organic compounds and nanostructured carbon surfaces. Previous research has indicated that amorphous carbon present in as-grown CNTs affects the surface properties of CNTs and hinders the efficiency of the adsorption and desorption process in general by occupying the adsorption site of CNTs.8 Thus, several studies have employed various purification processes of the CNTs in order to modify/increase their adsorption Received: December 9, 2010 Revised: February 1, 2011 Published: March 03, 2011 4552

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Table 1. Selected Physiochemical Properties of Three Chlorinated Compounds compounds

molecular weight (g/mol)

solubility (mg/L)

octanol-water partitioning coefficient (log Kow)

Henry’s law constant (atm-m3/mol)

TCEa

131.39

1280

2.42

0.00985

1,1,1-TCA

133.41

1290

2.49

0.0172

1,3,5-TCB

181.45

4.19

0.00189

6.01

a

Abbreviations: TCE, trichloroethylene; 1,1,1-TCA, 1,1,1-trichloroethane; 1,3,5-TCB, 1,3,5-trichlorobenzene. The physical properties of the compounds were obtained from the SRC PhysProp Database unless otherwise noticed http://www.syrres.com/esc/physdemo.htm.

efficiencies. One of the common purification procedures is to heat CNTs at 350-500 °C (in air) and then wash the products with strong acids.16 It is generally believed that the thermal treatment removes at least part of the amorphous carbon and that the acid wash removes most of the metal catalyst particles (that are used for growing CNTs) present in the CNTs. Following the purification process, surface properties of CNTs are generally changed. The most significant change is the addition of surface oxides on the sidewalls, and presumably at the defect sites and at any open ends of CNTs as well.17,18 The impact of surface oxides on sorption capacity has been shown to be significant.19 As stated earlier, the presence of amorphous carbon also affects the surface properties; thus, it is important to evaluate how the heating process (which removes the amorphous carbon) affects the sorption and desorption of organic compounds on CNTs. In this study, we have performed detailed experimental measurements of the adsorption behavior of three chlorinated compounds (trichloroethylene (TCE), 1,1,1-trichloroethane (1,1,1TCA), and 1,3,5-trichlorobenzene (1,3,5-TCB)) on as-produced and heat-treated MWCNTs. The objectives of this study were (1) to investigate the correlation between the molecular structure of three chlorinated compounds and their adsorption and desorption mechanisms and (2) to assess the effect of heat treatment on the adsorption-desorption properties of MWCNTs.

’ MATERIALS AND METHODS Chemicals. Trichloroethylene (TCE), 1,1,1-trichloroethane (1,1,1-TCA), and 1,3,5-trichlorobenzene (1,3,5-TCB) were purchased from Sigma Aldrich. All compounds have at least 99% purity. Selected physicochemical properties of these three compounds are listed in Table 1. TCE and 1,1,1-TCA were dissolved in water and 1,3,5-TCB was dissolved in hexane (>99%) to prepare stock solutions. The stock solutions were serially diluted with DI water to obtain different concentrations of these compounds. Synthesis and Treatment of Multiwalled Carbon Nanotubes. Multiwalled carbon nanotubes (MWCNTs) were grown using an air-assisted chemical vapor deposition (CVD) method (vapor-phase catalyst delivery) in a tube furnace.20 A silica substrate was placed inside the tube at an optimized position to yield a maximum growth of nanotubes. Argon gas was first flowed through the tube furnace at 500 sccm to clear the air, and then the temperature of the furnace was raised to 790 °C. Once the furnace was heated to 790 °C, the argon flow was stopped and a mixture of hydrogen and argon at 2.5 and 25 sccm, respectively, was flowed through the tube furnace. Meanwhile, a solution containing xylene and ferrocene was pumped into the tube furnace using a syringe pump (pumping speed ∼ 12 mL/h). A typical reaction time for CNT growth for these experiments was about 30 min. A small flow of air was also used in order to enhance the growth rate of the MWCNTs. After the growth aligned MWCNTS were peeled off from the substrates, they

were used for further experimentation. For purification, the pristine MWCNTs were baked in an oven at 360 °C for more than 8 h in the presence of oxygen. Thermally treated CNTs were cooled at room temperature and were directly used for adsorption and desorption studies. Adsorption and Desorption Studies. Adsorption and desorption measurements were carried out with 15 or 40 mL centrifuge tubes. The molar concentration of compounds to be tested was from 0.08 to 15 μM for TCE, 0.08 to 15.5 μM for 1,1,1-TCA, and 5 to 137 μM for 1,3,5-TCB. A 15-50 mg portion of MWCNTs was weighed into each centrifuge tube, and the exact weight was recorded. Solutions containing various desired concentrations of TCE, 1,1,1-TCA, or 1,3,5-TCB were then filled into the tubes to their capacity. DI water used to prepare the targeted concentrations of the three compounds was first mixed with 0.5 mM sodium azide (NaN3) and 5 mM calcium chloride (CaCl2). NaN3 was added to the solution to inhibit bacterial activities and CaCl2 to facilitate coagulation. Coagulation is necessary to achieve good solid/water separation in sorption and desorption studies with CNTs. The tubes were then quickly sealed with aluminum foil-lined Teflon screw caps and were tumbled for 7 days in a tumbler at room temperature (2023 °C). Previous studies have shown that 4-6 days are adequate for the system to reach equilibrium.21 Two or three replicates were prepared for each concentration. At the termination, the tubes were centrifuged at 2000 rpm for 30 min and 10 mL of the supernatant containing TCE and 1,1,1-TCA was taken and transferred to a clean 20 mL vial. The vials were sealed immediately with a Teflon-lined screw cap, and the compounds were analyzed with a PerkinElmer Autosystem gas chromatograph (GC), affiliated with an electronic capture detector (ECD). The supernatant containing 1,3,5-TCB was transferred to a 2 mL vial and analyzed with an Ultra GC also affiliated with an ECD. Immediately after the collection of the supernatant for analysis, the rest of the supernatants were discarded and replaced with DI water containing 0.5 mM sodium azide and 5 mM calcium chloride. The solutions were filled to the top of the centrifuge tubes and closed immediately with aluminum foillined Teflon caps. The tubes were put back to the tumbler and were mixed for another 7-10 days at room temperature. At the end of the mixing, the centrifuge tubes were centrifuged at 2000 rpm for 30 min and 10 mL of supernatant was collected for pollutant analysis with GC. A separated study showed that about 6-8% of CNTs were lost during the preparation process for the desorption study (primarily due to the attachment of the CNTs to the cap foil), and the loss was adjusted in the calculation of desorption coefficients. Adsorption Data Fitting. Various models have been applied to fit adsorption isotherms to carbonaceous nanomaterials. The Freundlich model and Polanyi-Manes model were shown by numerous researchers to be the most effective models to fit sorption isotherms to CNTs.13,14 Considering that the Freundlich model can be viewed as a special form of the Polayni-Manes 4553

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Figure 1. Adsorption and desorption isotherms of (A) TCE, (B) 1,1,1TCA, and (C) 1,3,5-TCB on pristine MWCNTs. Reported values are the average of two or three replicates (n = 2, 3). Error bars represent standard deviation.

model (b = 1), the latter was adopted in this study to fit the adsorption isotherms. The Polanyi-Manes model is described as log qe ¼ log Q o þ aðεsw =Vs Þb where qe (μg/g) is the solid concentration at equilibrium, Qo (μg/g) is the sorption capacity, and εsw (J/mol) is the adsorption potential, which is defined as εsw ¼ RT lnðCs =Ce Þ where R (8.314 J/mol 3 K) is the universal gas constant, T (K) is the absolute temperature, Cs (μg/L) is the aqueous water solubility, and Ce (μg/L) is the liquid concentration at equilibrium. a and b are fitting parameters. Vs is the molecular volume and was calculated based on the atomic volumes and molecular structure of a molecule, following an established method.22

’ RESULTS AND DISCUSSION Adsorption Isotherms on Pristine and Thermally Treated MWCNTs. All adsorption isotherms demonstrated strong

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Figure 2. Adsorption and desorption isotherms of (A) TCE, (B) 1,1,1TCA, and (C) 1,3,5-TCB on thermally treated MWCNTs. Reported values are the average of two or three replicates (n = 2, 3). Error bars represent standard deviation.

nonlinearity, as indicated in Figures 1 and 2. PMM fitting was performed according to the least-squares method, and the fitting parameters and correlation coefficients are listed in Table 2. At any given concentration at equilibrium, the sorption capacity increased in an order of 1,1,1-TCA < TCE < 1,3,5-TCB for both pristine and thermally treated MWCNTs. The sorption capacity of all three compounds was significantly higher for thermally treated MWCNTs than for pristine MWCNTs. The adsorption capacity on thermally treated MWCNTs ranged from 38.6% for TCE, 96.8% for 1,1,1-TCA, and 56.5% for 1,3,5-TCB higher than the adsorption capacity toward as-grown MWCNTs (Table 2). The presence of high-energy adsorption sites is typically ascribed as the reason for heterogeneous adsorption for different adsorbents.23 For example, high-surface-area carbonaceous materials (HSACM), such as charcoals, are often cited as a major source of adsorption nonlinearity for soils and sediments.24 With regard to MWCNTs, these high-energy adsorption sites could be the defect sites or the functional groups on the surface. Inner pores are generally not accessible to chemicals because the tube openings are relatively smaller than most molecules and they are 4554

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Table 2. Fitting Parameters of the Polanyi-Manes Model (PMM) for Sorption Isotherms of Three Chlorinated Compounds on Pristine and Thermally Treated MWCNTs compounds TCE 1,1,1-TCA 1,3,5-TCB

adsorbent

a

b

r2

Pa

552.1 ( 25.4

-3.15  10-5 ( 6.71  10-6

1.97 ( 0.052

0.9314

a

T P

765.5 ( 49.5 347.5 ( 74.9

-9.71  10-6 ( 1.13  10-6 -1.95  10-5 ( 6.36  10-7

2.18 ( 0.034 2.01 ( 0.014

0.9028 0.9556

T

683.3 ( 38.4

-2.35  10-5 ( 5.66  10-6

1.95 ( 0.084

0.9399

-5.88  10-5 ( 4.24  10-7

2.17 ( 0.014

0.9257

-3.25  10-5 ( 7.07  10-7

2.16 ( 0.015

0.9526

P T

a

Qo

1285 ( 14.4 2011 ( 9.19

Abbreviations: P, pristine MWCNTs; T, thermally treated MWCNTs.

Figure 3. Transmission electron microscope images of (a) as-grown CNTs and (b) thermally treated CNTs. The bar on the figure is 40 nm.

often closed in as-grown MWCNTs.25 However, the architecture of the MWCNT bundles are highly porous (due to alignment and bundling), and hence, adsorption of all these three compounds is possible in the spaces between the aligned bundles of the MWCNTs. However, the majority of the adsorption process occurs on the outer surfaces of the MWCNTs. Such a conclusion is also arrived from previous investigations (for organic compounds).26 Adsorption on the surface is governed by multiple intermolecular forces, such as hydrophobic forces, electrostatic forces, and hydrogen bonding. All three compounds tested here have moderate hydrophobicity, and therefore, hydrophobic forces should contribute to the adsorption, but are unlikely to be dominant on the adsorption process. For instance, 1,1,1-TCA has a slightly higher octanol-water partitioning coefficient than TCE (Table 1), yet the adsorption capacity of 1,1,1-TCA is smaller than that of TCE for both pristine and thermally treated MWCNTs. Previous research has shown that πdπ interactions could occur between molecules with CdC double bonds or benzene rings and CNT surfaces. In this study, TCE contains a CdC double bond and 1,3,5-TCB contains a benzene ring, and therefore, electrostatic forces, such as πdπ interactions, would contribute to the adsorption of TCE and 1,3,5-TCB. Besides the πdπ interactions, 1,3,5-TCB has three chlorines attached to its benzene ring. The -Cl is a typical electron acceptor, and the CNT surface can serve as both an electron donor and electron acceptor. Therefore, an electron donor-acceptor (EDA) interaction is also likely to occur between 1,3,5-TCB and CNT surfaces. The stronger interactions from TCA to TCB, found in our studies, are in agreement with the general trends previously reported in the literature that the adsorption capacity on CNTs increased from aliphatic to aromatics.12 The sorption capacities to thermally treated MWCNTs were found to be higher than that of pristine MWCNTs for all three compounds. Because purification is not expected to considerably change the structures of nanotubes, as demonstrated by previous research,19 the substantial change of adsorption that occurs due

to the porous structure of the MWCNT assemblies are not expected. Examination of CNTs with a Hitachi 7650 transmission electron microscope (TEM) also indicates that the morphology of the MWCNTs is modified due to the heat treatment, and a cleaner MWCNT surface (perhaps due to the removal of the amorphous carbon) was observed in Figure 3. It has been reported earlier that thermal treatment at 300-450 °C could increase the effective surface area of CNTs.27 We believe that, in our case also, the surface cleaning of the MWCNTS, due to the mild thermal treatment, allows the molecules to access more adsorption sites on the surface and, consequently, results in a higher adsorption capacity for all three compounds. In addition to the adsorption capacity, we also examined the PMM fitting parameters to gain more insight on the adsorption and desorption mechanisms. The parameter “b” in the PMM modeling equation is associated with the adsorption potential energy of activated carbons and is primarily a function of adsorbent properties rather than chemical properties of adsorbates.28,29 In this study, the value of b was essentially the same for all three compounds for both pristine and thermally treatment MWCNTs, supporting the statement that b was primarily a function of the adsorbent nature. The similarity of b between pristine and thermally treated MWCNTs may be because the thermal treatment did not significantly change the structure of the CNTs. The excellent fit of the sorption isotherms with the PMM model further motivated us to examine the applicability of this model to capture the adsorption mechanisms of these three compounds on MWCNTs. One of the key assumptions of the PMM model is that all adsorption spaces of a given adsorbent are accessible for all adsorbates and molecular volume is the only property that affects the adsorption. In other words, if the sorption isotherm is normalized with molecular volume, the sorption isotherms of all compounds should coalesce. This was not what was observed in this study; see Figure 4. The correction curves of TCE and 1,1,1-TCA appeared to coalesce for both adsorbents, but 1,3,5-TCB deviated substantially from the other two curves. The inadequacy of molecular volume as a sole correcting factor of sorption isotherms of different compounds to CNTs has been reported before. The result may be because using molecular volume as a sole adjustor neglects the impact of specific intermolecular forces contributing to adsorption. Strong electron donor-acceptor (EDA) interactions may be a reason for the deviation of 1,3,5-TCB from the other two compounds studied in this work. A comprehensive normalizing factor, F, based on intermolecular forces, such as hydrogen bonding, EDA interaction, and polarizability, has been suggested, yet even a comprehensive normalizing factor as F is often inadequate to obtain a single correction curve for all compounds.30 A close 4555

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Figure 4. Correction curves of TCE, 1,1,1-TCA, and 1,3,5-TCB adsorption on (top) pristine and (bottom) thermally treated MWCNTs.

examination of the assumptions may be necessary, and the properties of adsorbents might need to be considered to obtain a universal correction curve if this could occur. Desorption Hysteresis. Desorption hysteresis, a phenomenon when part of the adsorbate is irreversibly bonded with the adsorbent, was observed for all three compounds from pristine MWCNTs, and interestingly, the desorption hysteresis was substantially reduced after a simple heat treatment of MWCNTs; see Figures 1 and 2. Desorption hysteresis can originate from artifacts, such as slow desorption kinetics and loss of adsorbates during the experimental period. The first artifact can be corrected by providing adequate desorption time, and the second artifact can be accounted by running a control group. A previous study showed that a sorption and desorption equilibrium of bisphenol A from MWCNTs could be reached in 5 days.15 As mentioned above, approximately 6-10% of compounds are lost in the preparatory stage of the desorption study, primarily due to volatilization or adsorption to the centrifuge tube walls, and have been taken into consideration in the computation of desorption coefficients. Degradation should be negligible because all three compounds are recalcitrant environmental pollutants and rapid aerobic degradation is unlikely. No degradation product was ever detected throughout the process. In addition, the desorption study for pristine and thermally treated MWCNTs is conducted in a near identical manner, and a substantially large hysteresis was observed for pristine MWCNTs than thermally treated MWCNTs. Therefore, it is unlikely that artifacts would be the principle cause of the desorption hysteresis observed for the asgrown MWCNTs in this study. Real desorption hysteresis may originate from two sources: the formation of metastable states of adsorbates in fixed pores, such as capillary condensation hysteresis,31 or irreversible deformation of the sorbent by the sorbate, a possible mechanism for

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desorption hysteresis from carbonaceous nanoparticles.32 As far as the desorption of organic contaminants from MWCNTs is concerned, desorption hysteresis of bisphenol A (BPA) and 17Rethinyl estradiol (EE2) from MWCNTs was observed,15 but desorption hysteresis of PAHs from the same adsorbate was negligible. The authors concluded that desorption hysteresis of BPA and EE2 was caused by the deformation or rearrangement of CNT bundles and this rearrangement is only significant for highly adsorptive compounds. BPA and EE2 demonstrated much stronger adsorption toward MWCNT bundles than PAHs, and therefore, adsorption hysteresis was observed for BPA and EE2, but not PAHs. Some compounds tested here, however, have a weaker adsorption affinity to MWCNTs than PAHs, and it is generally accepted that, for a given nanotube, the adsorption affinity increases in the order of nonpolar aliphatics < nonpolar aromatics < polar aromatics.12 In view of this, it is unlikely that desorption hysteresis of TCE or 1,1,1-TCA was caused by bundle rearrangement in this study. Because hysteresis is much more pronounced for pristine MWCNTs than for thermally treated MWCNTs and a major difference of these two adsorbents is the disorderly amorphous carbon on the as-grown MWCNTs, it is reasonable to conclude that amorphous carbon is intimately related to the desorption hysteresis observed in pristine MWCNTs. While it is generally believed that adsorption on amorphous carbon is linear and reversible, the observation is not necessarily in conflict with the general belief because the state and structure of amorphous carbon on the CNT surface can be highly different from those of typical amorphous carbon. Previous research indicated that amorphous carbon forms a microporous structure after high-temperature annealing due to outgassing.33 Herein, we propose a new mechanism for adsorption desorption hysteresis. We postulate that the desorption hysteresis observed on as-produced MWCNTs could be due to the penetration of adsorbates into the pore spaces of disorderly amorphous carbon in the adsorption process. Localized condensation and pore deformation could both occur in the adsorption process, resulting in irreversible desorption for pristine MWCNTs. After thermal treatment, disorderly amorphous carbon is removed and, consequently, their contribution to the desorption process was lost. The surface of thermally treated MWCNTs became much cleaner, as shown in Figure 3. As a result, desorption hysteresis was significantly reduced. This investigation revealed highly interesting phenomena and suggested important roles of amorphous carbon in the adsorption and desorption process. Indeed, it is very interesting to note that a simple heat treatment could drastically change the adsorption and desorption behavior of organic chemicals to CNTs, which has important environmental and health implications of CNTs. Environmental Implications. The rapid expansion of carbonbased nanotechnology and the potential release of carbonaceous nanomaterials into our ecosystem have important environmental consequences, which need to be understood. As a superior adsorbent for a wide array of environmental contaminants, CNTs strongly affect the fate and transport of coexisting environmental contaminants through adsorption and desorption processes. Depending upon the adsorption and desorption mechanisms, CNTs can serve either as a superior adsorbent for water purification or as a pollutant carrier and source. Pristine MWCNTs have a high adsorption capacity and pronounced desorption hysteresis, which means that part of the pollutants adsorbed onto the pristine MWCNTs will not be released to the environment at the same environmental condition and, 4556

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’ AUTHOR INFORMATION Corresponding Author

*Phone: 618-453-7774 (X.M.), 618-453-2270 (S.T.). Fax: 618453-3044 (X.M.), 618-453-1056 (S.T.). E-mail: [email protected]. edu (X.M), [email protected] (S.T.).

’ ACKNOWLEDGMENT Xingmao Ma and Saikat Talapatra acknowledge the Startup Funding they received from the Office of Research Development and Administration at Southern Illinois University Carbondale. ’ REFERENCES (1) Newman, L. A.; Strand, S. E.; Choe, N.; Duffy, J.; Ekuan, G.; Ruszaj, M.; Shurtleff, B. B.; Wilmoth, J.; Heilman, P.; Gordon, M. P. Uptake and biotransformation of brichloroethylene by bybrid poplars. Environ. Sci. Technol. 1997, 31, 1062–1067. (2) Lu, C. S.; Chung, Y. L.; Chang, K. F. Adsorption of trihalomethanes from water with carbon nanotubes. Water Res. 2005, 39, 1183–1189. (3) Hilding, J.; Grulke, E. A.; Sinnott, S. B.; Qian, D. L.; Andrew, R.; Jagtoyen, M. Sorption of butane on carbon multiwall nanotubes at room temperature. Langmuir 2001, 17, 7540–7544. (4) Liao, Q.; Sun, J.; Li; Gao, L. Adsorption of chlorophenols by multi-walled carbon nanotubes treated with HNO3 and NH3. Carbon 2008, 46, 553–555. (5) Peng, X.; Li, Y.; Luan, Z.; Di, Z.; Wang, H.; Tian, B.; Jia, Z. Adsorption of 1,2-dichlorobenzene from water to carbon nanotubes. Chem. Phys. Lett. 2003, 376, 154–158. (6) Pan, B.; Xing, B. Adsorption mechanisms of organic chemicals on carbon nanotubes. Environ. Sci. Technol. 2008, 42, 9005–9013. (7) Gotovac, S.; Hattori, Y.; Noguchi, D.; Miyamoto, J.; Kanamaru, M.; Utsumi, S.; Kanoh, H.; Kaneko, K. Phenanthrene adsorption from solution on single wall carbon nanotubes. J. Phys. Chem. B 2006, 110, 16219–16224. (8) Gotovac, S.; Song, L.; Kanoh, H.; Kaneko, K. Assembly structure control of single wall carbon nanotubes with liquid phase naphthalene adsorption. Colloids Surf., A 2007, 300, 117–121. (9) Shih, Y. H.; Li, M. S. Adsorption of selected volatile organic vapors on multiwall carbon nanotubes. J. Hazard. Mater. 2008, 154, 21–28. (10) Fagan, S. B.; Souza, A. G.; Lima, J. O. G.; Mendes, J.; Ferreira, O. P.; Mazali, I. O.; Alves, O. L.; Dresselhaus, M. S. 1,2-Dichlorobenzene interacting with carbon nanotubes. Nano Lett. 2004, 4, 1285–1288. (11) Mauter, M. S.; Elimelech, M. Environmental applications of carbon-based nanomaterials. Environ. Sci. Technol. 2008, 42, 5843–5859.

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dx.doi.org/10.1021/jp1117272 |J. Phys. Chem. C 2011, 115, 4552–4557