Environ. Sci. Technol. 2008, 42, 6862–6868
Adsorption of Hydroxyl- and Amino-Substituted Aromatics to Carbon Nanotubes WEI CHEN,† LIN DUAN,† LILIN WANG,† A N D D O N G Q I A N G Z H U * ,‡ College of Environmental Science and Engineering/Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin 300071, China, and State Key Laboratory of Pollution Control and Resource Reuse/ School of the Environment, Nanjing University, Jiangsu 210093, China
Received May 16, 2008. Revised manuscript received July 20, 2008. Accepted July 22, 2008.
The combined effects of hydroxyl/amino functional groups of aromatics and surface O-containing groups of carbon nanotubes on adsorption were evaluated. When normalized for hydrophobicity, 2,4-dichlorophenol and 2-naphthol exhibited a greater adsorptive affinity to carbon nanotubes and graphite (a model adsorbent without the surface O functionality) than structurally similar 1,3-dichlorobenzene and naphthalene, respectively, and 1-naphthylamine exhibited a much greater adsorptive affinity than all other compounds. Results of the pHdependency experiments further indicated that the hydroxyl/ amino functional groups of the adsorbates and the surface properties of the adsorbents played a combinational role in determining the significance of the nonhydrophobic adsorptive interactions. We propose that the strong adsorptive interaction between hydroxyl-substituted aromatics and carbon nanotubes/ graphite was mainly due to the electron-donating effect of the hydroxyl group, which caused a strong electrondonor-acceptor (EDA) interaction between the adsorbates and the π-electron-depleted regions on the graphene surfaces of carbon nanotubes or graphite. In addition to the EDA interaction, Lewis acid-base interaction was likely an extra important mechanism contributing to the strong adsorption of 1-naphthylamine, especially on the O-functionality-abundant carbon nanotubes. The findings of the present study might have significant implications for selective removal of environmental contaminants with carbon nanotubes.
Introduction Carbon nanotubes are considered promising candidates for many areas of nanotechnological applications, including special sorbents in water treatment to remove recalcitrant contaminants (1, 2). However, the rapid growth in production and industrial applications of carbon nanotubes also raised serious concerns on their potential environmental impacts (3-5). One of the concerns is that when released to the aquatic environment, carbon nanotubes might significantly affect the fate of common organic contaminants because of their strong affinity to carbon nanotubes (6). * Corresponding author phone/fax: +86-25-8359-6496; e-mail:
[email protected]. † Nankai University. ‡ Nanjing University. 6862
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 18, 2008
Limited numbers of studies have been conducted to understand the adsorption of organic contaminants to carbon nanotubes, using mainly nonpolar, nonionizable compounds (7-15). In addition to the strong hydrophobic effect often associated with the adsorption to carbonaceous materials, the high electronic polarizability of the graphene surfaces has also been found to be a significant factor affecting the adsorption to carbon nanotubes (7, 8, 10, 12, 15). Long and Yang (7) attributed the much higher adsorption of dioxin to carbon nanotubes than activated carbon to the strong interaction between the two benzene rings of dioxin and the surface of the carbon nanotubes. Gotovac et al. (12) and Zhao and Lu (15) both proposed π-π electron coupling between adsorbate molecules and carbon nanotube surfaces, via experimental study and density functional theory calculation, respectively. In our previous study, we found that adsorption affinity to carbon nanotubes followed the order of nitroaromatics . nonpolar aromatics (chlorinated and methylated benzenes) . cyclohexane. We proposed that the greater adsorption of nitroaromatics than nonpolar aromatics was due to the strong electron-withdrawing effect of the nitro group, which caused the aromatic rings to be electrondepleted, thus resulting in strong π-π electron donoracceptor (EDA) interaction with the electron-rich sites of the graphene surfaces of the carbon nanotubes (16). Thus far, the effects of several important substituent groups (e.g., hydroxyl and amino) of aromatic compounds on adsorption to carbon nanotubes have not been evaluated systematically. Many of these compounds, such as phenols and aromatic amines, are environmentally relevant. Additionally, the surface chemistry of carbon nanotubes (including O-containing groups such as -OH and -COOH) might have considerable effects on the adsorption of organic molecules. The overall objective of the present study is to investigate further the nonhydrophobic interactions between highly polar aromatic compounds and carbon nanotubes and, in particular, to investigate how the combined effect of adsorbate substituent groups (hydroxyl and amino) and the O-containing groups of carbon nanotube surfaces might affect adsorption. The chemicals selected were 2,4-dichlorophenol, 2-naphthol, 1-naphthylamine, 1,3-dichlorobenzene, and naphthalene. 1,3-Dichlorobenzene and naphthalene are structurally similar to 2,4-dichlorophenol and 2-naphthol/1-naphthylamine, respectively, but do not contain the polar functional group; these two compounds were included as a comparison to better understand the adsorption mechanisms. The adsorbents included three single-walled carbon nanotubes containing different amounts of surface O functionality, and graphite was also included as a model adsorbent for graphene surfaces. Adsorption isotherms were obtained to compare adsorption affinity of different adsorbate-adsorbent combinations. The effect of pH on adsorption was also evaluated to understand further the mechanisms controlling adsorptive interactions because the pH can significantly affect the electronic properties of the polar functional groups of both adsorbate molecules and carbon nanotubes.
Experimental Section Materials. Single-walled carbon nanotubes were purchased from Nanotech Port Co. (Shenzhen, Guangdong Province, China). According to the manufacturer, the carbon nanotubes contained less than 5% amorphous carbon. A portion of the carbon nanotubes were treated with two HNO3 and H2SO4 mixtures (1:9 and 1:3 by volume, respectively) to obtain carbon nanotubes with different degrees of surface oxidation, 10.1021/es8013612 CCC: $40.75
2008 American Chemical Society
Published on Web 08/20/2008
TABLE 1. Summary of Adsorbate Properties [Water Solubility (Csat), n-Octanol-Water Partition Coefficient (KOW), n-HexadecaneWater Partition Coefficient (KHW), and Acid Dissociation Constant (pKa)] and Freundlich Model Coefficients (KF and n) Obtained from Adsorption Results adsorbate
Csat (mmol/L)
1,3-dichlorobenzene
8.16 ×
2,4-dichlorophenol
10-1a
log KOW a
log KHW
pKa
c
3.47
3.69
2.70 × 101a
3.09a
1.48c
naphthalene
2.50 × 10-1a
3.33a
3.41c
2-naphthol
6.93 × 100 b
2.70b
0.255 d
9.51b
1-naphthylamine
1.19 × 101a
2.25a
1.20d
3.92a
7.85a
adsorbente
KF (mmol1-n Ln/kg)
n
R2
graphite r-SWNT l-SWNT d-SWNT graphite r-SWNT l-SWNT d-SWNT graphite r-SWNT l-SWNT d-SWNT graphite r-SWNT l-SWNT d-SWNT graphite r-SWNT l-SWNT d-SWNT
190 ( 30 2980 ( 90 1510 ( 30 1320 ( 40 51 ( 5 1050 ( 20 680 ( 10 580 ( 10 30 ( 2 2150 ( 50 1270 ( 30 1040 ( 30 30 ( 3 2280 ( 80 1600 ( 100 1450 ( 90 12.0 ( 0.4 3900 ( 30 4040 ( 80 4180 ( 50
1.01 ( 0.04 0.53 ( 0.01 0.50 ( 0.01 0.49 ( 0.01 0.64 ( 0.02 0.25 ( 0.01 0.25 ( 0.01 0.25 ( 0.01 0.45 ( 0.01 0.380 ( 0.005 0.380 ( 0.006 0.370 ( 0.007 0.38 ( 0.02 0.210 ( 0.006 0.23 ( 0.02 0.23 ( 0.02 0.220 ( 0.007 0.084 ( 0.002 0.052 ( 0.005 0.061 ( 0.003
0.993 0.999 0.999 0.997 0.992 0.991 0.987 0.989 0.997 0.999 0.999 0.999 0.989 0.997 0.974 0.981 0.996 0.996 0.980 0.994
a From Schwarzenbach et al. (18). b From the U.S. National Library of Medicine Hazardous Substance Data Bank (http:// toxnet.nlm.nih.gov/cgi-bin/sis/htmlgen?HSDB). c From Zhu and Pignatello (19). d Measured in the present study; values are the average of six replicates. e r-SWNT, l-SWNT, and d-SWNT represent untreated, lightly oxidized, and deeply oxidized carbon nanotubes, respectively.
using a method developed by Yang et al. (17). Elemental analysis, Fourier transform infrared (FTIR) transmission spectra, and surface area and pore size distribution were determined using previously reported methods (16). 2,4Dichlorophenol, 2-naphthol, 1-naphthylamine, 1,3-dichlorobenzene, and naphthalene were purchased from Sigma Aldrich (St. Louis, MO). Selected physical-chemical properties of the adsorbates are listed in Table 1. Adsorption Experiments. Adsorption experiments were conducted using a batch adsorption approach developed in our previous study (16). Duplicate samples were done for each adsorption isotherm data point, and triplicate samples were done for each pH effect data point. For adsorption isotherm experiments, the pH for 2,4-dichlorophenol, 2-naphthol, 1,3-dichlorobenzene, and naphthalene was between 5.5 and 5.8 (measured at the end of batch sorption) and the pH for 1-naphthylamine was approximately neutral. Within these pH ranges, the hydroxyl- and amino-substituted solutes existed predominantly in the neutral form (see pKa values in Table 1). For the pH effect experiments, the equilibrium pH was set over a range of 3-11. Prior to the initiation of an adsorption experiment, a certain amount of carbon nanotubes or graphite was transferred to a 40-mL amber EPA vial and was prewetted for 24 h with approximately 40 mL of an electrolyte solution containing 0.01 mol/L NaCl and 200 mg/L NaN3 (as the bioinhibitor). Afterward, a stock solution of an adsorbate (in methanol) was added to the vial using a microsyringe, and the volume percentage of methanol was kept below 0.1% to minimize cosolvent effects. The vial was then filled with an electrolyte solution to leave minimal head space and was tumbled at 3 rpm at room temperature for more than 15 days. The time required to reach adsorption equilibrium was predetermined. Then, the vials were removed from the tumbler and were left undisturbed on a flat surface for more than 24 h to allow complete settling of the carbon nanotubes/ graphite. Aliquots of the aqueous solution were then withdrawn from the vials and were extracted with hexanes (for 1,3-dichlorobenzene and naphthalene) or dichloromethane (for all other adsorbates).
For the pH effect experiments of 2,4-dichlorophenol, 2-naphthol, and 1-naphthylamine, the equilibrium pH of supernatant was adjusted with a weighed amount of a HCl or NaOH solution to ensure that solutes were in the nonionized forms before extraction with dichloromethane. The pH was unadjusted for 1,3-dichlorobenzene and naphthalene solutions in extractions. Recoveries of all adsorbates were found to be near complete. Calibration curves were obtained from control samples receiving the same treatment as the adsorption samples but without the adsorbent. Each calibration curve included at least 12 data points covering the concentration range of the isotherm data. The organic extracts were analyzed by gas chromatography (GC) with an electron capture detector for 1,3-dichlorobenzene and with a flame ionization detector for the other four compounds. No peaks were detected in the GC spectra for potential degraded/transformed products of the target compounds. The adsorbed mass at each equilibrium concentration was calculated as the difference between the total and solutionphase mass.
Results and Discussion Characterization of Carbon Nanotubes. The nitrogen adsorption and desorption isotherms and the pore size distribution data are given in Figure S1 and Table S1, Supporting Information. The elemental composition and Brunauer-Emmett-Teller (BET) surface areas of the untreated, lightly oxidized, and deeply oxidized carbon nanotubes are given in Table S2, Supporting Information. Results of the elemental composition analysis show that a considerable amount of oxygen was introduced to the carbon nanotubes after the treatment: 12.1% and 14.9% for the two respective treated carbon nanotubes compared with 4.77% of the untreated one. The FTIR transmission spectra (Figure S2, Supporting Information) confirm the existence of the CdO and C-O functional groups, and the intensities of the peaks correlate qualitatively with the oxygen contents listed in Table S2, Supporting Information. Adsorption Isotherms. The adsorption results of the five chemicals on carbon nanotubes and graphite are shown in VOL. 42, NO. 18, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6863
FIGURE 1. Comparison of the adsorption isotherms of five adsorbates on a given adsorbent. 1,3-DCB, 2,4-DCP, NAPH, 2-NATH, and 1-NALA are acronyms for 1,3-dichlorobenzene, 2,4-dichlorophenol, naphthalene, 2-naphthol, and 1-naphthylamine, respectively; r-SWNT, l-SWNT, and d-SWNT represent untreated, lightly oxidized, and deeply oxidized carbon nanotubes, respectively. Error bars, in most cases smaller than the symbols, represent standard deviations of duplicate samples. Table 1 and Figures 1 and 2. The adsorption data were fitted with the Freundlich isotherm: q ) KFCwn, where q (mmol/kg) and Cw (mmol/L) are equilibrium concentrations of an adsorbate on the adsorbent and in the aqueous solution, respectively; KF (mmol1-n Ln/kg) is the Freundlich affinity coefficient; and n (unitless) is the Freundlich linearity index. In general, adsorption was highly nonlinear (except adsorption of 1,3-dichlorobenzene to graphite) and the Freundlich model provided reasonably good fits to the data. Figure 1 compares the adsorption affinities of different chemicals to a given adsorbent. Adsorption affinities to graphite increased as 1,3-dichlorobenzene < 2,4-dichlorophenol < naphthalene < 2-naphthol/1-naphthylamine. Adsorption affinities to the three different carbon nanotubes followed the order of 1,3-dichlorobenzene/2,4-dichlorophenol/naphthalene < 2-naphthol , 1-naphthylamine. These trends correlated poorly with the chemical hydrophobicity, shown by the water solubility (Csat), n-octanol-water partition coefficient (KOW), and n-hexadecane-water partition coefficient (KHW) values in Table 1. KHW has been found to be a better indicator of the solute hydrophobicity than KOW (16). For example, 2,4-dichlorophenol is much less hydrophobic (with lower KOW and much higher Csat) than 1,3-dichlorobenzene, but the adsorption affinities of these two compounds to a given adsorbent did not differ very much. 2-Naphthanol is much less hydrophobic than naphthalene but adsorbed much more strongly. Moreover, 1-naphthylamine is one of the least hydrophobic compounds, but its adsorption was the strongest, especially on the three carbon nanotubes. The enhanced adsorption of hydroxyl- and amino-substituted compounds was apparent when adsorption data were normalized for the hydrophobic effect using KHW (Figure S3, Supporting Information). Thus, the results presented in Figures 1 and S3, indicate that the stronger adsorption of the hydroxyl- and amino-substituted com6864
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 18, 2008
pounds was likely related to the specific electronic properties stemming from the -OH and -NH2 functional groups. Figure 2 compares the adsorption of a given chemical to the four different adsorbents. The adsorbed concentrations were normalized for the BET surface area. The figure shows that, for 1,3-dichlorobenzene, naphthalene, and 2,4-dichlorophenol, the effect of the adsorbent properties on the adsorption affinity was insignificant. For 2-naphthol and 1-naphthylamine, however, adsorption was much stronger on carbon nanotubes than on graphite. Additionally, adsorption of 1-naphthylamine to the two treated carbon nanotubes was considerably stronger than adsorption to the untreated carbon nanotubes. An interesting observation was that, even though both 2,4-dichlorophenol and 2-naphthol contain an -OH group, only the adsorption of 2-naphthol responded noticeably to a change of the adsorbent surface functionality. These results seem to indicate that the surface functionality of the carbon nanotubes can play an important role in adsorption, but only when the adsorbate molecules possess certain electronic properties. Effects of pH on Adsorption. Figure 3 shows that the pH effect on adsorption varied for different chemicals. For 1,3dichlorobenzene and naphthalene, the pH effect on adsorption to all four adsorbents was minimal. Changing the pH over the range of 3-11 should have significantly affected the protonation-deprotonation transition of carbon nanotube surface groups such as -OH and -COOH. However, it appears that such a transition had little effect on the adsorptive affinity of nonpolar compounds. The effect of pH on the adsorption of 2,4-dichlorophenol was insignificant when the pH was below the compound’s pKa (7.85), but the adsorption was considerably hindered when the pH was above the pKa. Moreover, this pH effect was more significant for the adsorption to carbon nanotubes than to graphite. Surprisingly, when the pH was below its pKa (9.51), the
FIGURE 2. Comparison of the adsorption isotherms of a given adsorbate on four different adsorbents. Solid-phase concentrations are normalized with the BET surface area. r-SWNT, l-SWNT, and d-SWNT represent untreated, lightly oxidized and deeply oxidized carbon nanotubes, respectively. Error bars, in most cases smaller than the symbols, represent standard deviations of duplicate samples. adsorption of 2-naphthol increased with the pH, and the trend was also much more significant for the adsorption to carbon nanotubes than to graphite. However, when the pH was above the pKa, different trends were observed on the different adsorbents: adsorption to graphite was enhanced; adsorption to untreated carbon nanotubes was similar to that at pH 8 and 9 but stronger than that at the lower pH values; adsorption to treated carbon nanotubes was weaker at pH 11 than at pH 8 but still considerably stronger than that at the lower pH values (pH 7 or below). For 1-naphthylamine, changing the pH had little effect on the adsorption to graphite. However, the adsorption to treated carbon nanotubes increased by over 1 order of magnitude over the pH range of 3-5 but slightly decreased when the pH further increased. A similar trend of the pH effect was observed on untreated carbon nanotubes when the pH was less than 9, but the adsorption affinity decreased with the pH pronouncedly at higher pH values. A striking observation shown in Figure 3 is that 2,4dichlorophenol and 2-naphthol exhibited remarkably different trends with a change of the pH. While adsorption of
2,4-dichlorophenol was significantly impeded once the pH was raised above its pKa, adsorption of 2-naphthol was much less affected. In fact, the Kd values of 2-naphthol at pH 11 were considerably higher than the corresponding Kd values at pH 7 or below, especially for graphite and untreated carbon nanotubes. At pH 11, the predominant fraction of 2-naphthol is the ionized form, which is much less hydrophobic compared with the nonionized form (log KOW ) -0.26, calculated with the ChemOffice software). Thus, the seemingly counterintuitive pH dependence suggests that certain specific adsorption-enhancement interaction must have counterbalanced (and probably overwhelmed) the decrease in the hydrophobicity. Another interesting observation shown in Figure 3 was that the adsorption of 1-naphthylamine to carbon nanotubes significantly increased when the pH exceeded its pKa, but the adsorption to graphite was essentially pH-independent. The pH dependence results further suggest that both the adsorbent surface functionality and the specific electronic properties of the adsorbate functional groups play important roles in the adsorption to carbon nanotubes. VOL. 42, NO. 18, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6865
FIGURE 3. Changes of distribution coefficients (Kd) with the pH at single-point initial concentrations for adsorption of all adsorbates to graphite, untreated carbon nanotubes (r-SWNT), lightly oxidized carbon nanotubes (l-SWNT), and deeply oxidized carbon nanotubes (d-SWNT). The initial concentrations for 1,3-dichlorobenzene on graphite and carbon nanotubes are 0.014 and 0.27 mmol/L, those for 2,4-dichlorophenol are 0.023 and 0.28 mmol/L, those for naphthalene are 0.0058 and 0.10 mmol/L, those for 2-naphthol are 0.027 and 0.24 mmol/L, and those for 1-naphthylamine are 0.028 and 0.66 mmol/L. Error bars, in most cases smaller than the symbols, represent standard deviations of triplicate samples. Vertical dashed lines represent pKa values of the respective adsorbates. Mechanistic Aspects. As discussed earlier, 2,4-dichlorophenol and 2-naphthol exhibited stronger nonhydrophobic interaction(s) than the structurally similar 1,3-dichlorobenzene and naphthalene, respectively. The stronger nonhydrophobic interaction(s) appear to be related to the -OH functional group. Several possible mechanisms should be considered based on the literature: (1) hydrogen bonding between the adsorbate -OH group and the adsorbent O-containing groups; (2) π-hydrogen bonding between the adsorbate -OH group and the adsorbent aromatic surfaces; (3) molecular attraction between the adsorbed molecules and molecules in aqueous solution (20-24). Hydrogen bonding was likely not the primary cause of the enhanced adsorption for two main reasons. First, for 2-naphthol, when the pH exceeds its pKa (9.51), both the -OH group on 2-naphthol and the O-containing groups on carbon nanotubes would be ionized, and the hydrogen-bonding effect should have been significantly impeded. However, the adsorption affinity did not change accordingly. Second, 6866
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 18, 2008
graphite contains no O-containing groups, but the adsorption of 2-naphthol and 2,4-dichlorophenol to graphite was still stronger than that of naphthalene and 1,3-dichlorobenzene, respectively. Efremenko and Sheintuch (22) evaluated the interaction between phenol in the aqueous solution and different O-containing groups on activated carbon using density functional theory and molecular mechanics methods and concluded that the interaction was negligible because of the competitive and much stronger interactions of water molecules with the polar sites on activated carbon. Similarly, π-hydrogen bonding could not be the major cause of the enhanced adsorption because of the observed pH effects for both carbon nanotubes and graphite. For example, the adsorption of 2-naphthol to graphite was enhanced at a pH above the pKa, when -O- prevailed over -OH on 2-naphthol. Molecular attraction might have existed between the adsorbed molecules and the aqueous-phase molecules. For example, molecular attraction of chlorinated phenols has been reported (23, 24). However, this effect was also not
likely the cause of the enhanced adsorption. First, when the pH was above their respective pKa values, both 2-naphthol and 2,4-dichlorophenol were negatively charged, and the dominating electronic force between the molecules would have been electrostatic repulsion, but not attraction. Second, adsorption was single-layered for most adsorption data points. For example, the highest q value of naphthalene on graphite was 2.79 mmol/kg, which was equivalent to a 58% surface coverage using a surface area of naphthalene of 155.8 Å2 (25). On the basis of the analysis above and our previous work, we propose that EDA interactions were the primary mechanism for the enhanced adsorption of 2-naphthol and 2,4dichlorophenol. One type of EDA interaction is the π-π EDA interaction between the π-electron-rich aromatic ring(s) of the adsorbates and the π-electron-depleted regions on the graphene surfaces of carbon nanotubes or graphite. Both carbon nanotubes and graphite contain polarized electronrich and -depleted sites, caused primarily by the surface defects of carbon nanotubes or graphite (10, 26-30). For example, McDermott and McCreery (26) reported that the basal plane of graphite in the vicinity of the edges is often electron-rich, while the regions in the center of the graphene surface are typically electron-depleted. As a strong electrondonating group (31), -OH makes the benzene ring(s) of 2,4dichlorophenol and 2-naphthol electron-rich, thus allowing these two compounds to interact more strongly with the electron-depleted surfaces of carbon nanotubes and graphite. Several literature studies are consistent with the proposed mechanism. Zhu et al. (27) reported that π-π EDA interaction resulted in stronger adsorption of 2-methylphenol than nonpolar aromatics on wood charcoal. Fagan et al. (10, 28) concluded that the defected sites on carbon nanotubes exhibited much higher binding energy than the perfect surface. Chakrapani et al. (29) reported that the adsorption to defects such as structural vacancies was much stronger. Robinson et al. (30) also reported strong binding of acetone to defect sites of the carbon nanotubes. The fact that adsorption of 2-naphthol was slightly stronger on carbon nanotubes than on graphite was likely because of the difference in the type, geometry, and abundance of defected sites on carbon nanotubes and on graphite. Moreover, when -OH dissociates to -O- at high pH, the electron-donating strength would be further improved (31); this was probably why adsorption of 2-naphthol to graphite increased when the pH was above its pKa despite the fact that the hydrophobic effect decreased. The opposite trend of the pH effect on 2,4dichlorophenol might indicate that the EDA interaction was not strong enough (see the detailed discussion below) to compensate for the decrease in the hydrophobicity caused by the ionization of the adsorbate molecules at the pH above the pKa. Another possible EDA interaction was that the -OH group or -O- (n-electron donor) might have directly interacted with the electron-depleted sites (π-electron acceptor) of carbon nanotubes and graphite. The n-π EDA mechanism has been proposed for oxygen electron pairs (n-electron donor) of the siloxane surface of clays and nitroaromatic compounds (π-electron acceptor) (32, 33). Similar to the π-π EDA interaction, such n-π EDA interaction is also enhanced when -OH is ionized to -O-, which is an even stronger electron-donating group. However, further studies are needed to verify the n-π EDA mechanism for adsorption to carbon nanotubes. It is necessary to note that, even though both 2-naphthol and 2,4-dichlorophenol contain the -OH group, the adsorption-enhancement effect of the EDA interactions was more prominent for 2-naphthol. This is probably due to a combination of two factors. First, 2-naphthol has two benzene rings and, accordingly, has much stronger π-electron-
conjugating potential than the single-ringed 2,4-dichlorophenol. Second, the two chlorine atoms in 2,4-dichlorophenol have an electron induction effect (even though relatively weak) that 2-naphthol does not possess. This effect results in less electron density in the benzene ring, which compensates for the electron-donating effect of the oxygen atom, even after -OH dissociated into -O-. This is supported by the finding of Sulaymon and Ahmed (34) that the withdrawing inductive character of chlorine substituents decreases the electron density of the p-chlorophenol benzene ring compared with that of phenol benzene ring. The EDA interactions mentioned above can also be used to explain the high adsorption affinity of 1-naphthylamine. Amino group (-NH2) is a strong electron-donating group (stronger than -OH) (31), and the unshared pair of electrons of nitrogen can result in strong electron conjugation with the π electrons in the benzene rings, making the benzene rings electron-rich. Accordingly, the electron-rich benzene rings can interact strongly with polarized positively charged regions on carbon nanotubes and graphite via π-π EDA interaction, causing extremely strong adsorption of 1-naphthylamine. This might also explain why the adsorption of 1-naphthylamine was much stronger at low concentrations, because the amount of polarized positively charged regions on carbon nanotubes is likely limited. As in Figure 1, equilibrium aqueous-phase concentrations of 1-naphthylamine increased by nearly 4 orders of magnitude, but adsorbed-phase concentrations on the three carbon nanotubes only increased slightly. Additionally, the strong electrondonating -NH2 group might directly interact with the electron-depleted surfaces of the carbon nanotubes and graphite via n-π EDA interactions, in a fashion similar to that of the -OH group, as mentioned above. Another important mechanism controlling the strikingly strong adsorption of 1-naphthylamine might have been the Lewis acid-base interaction, wherein -NH2 serves as the Lewis base and the O-containing groups on carbon nanotubes serve as Lewis acids. Experimental results in this study indicate that the Lewis acid-base interaction was an important extra driving force for the adsorption of 1-naphthylamine. As shown in Figure 2, the adsorption of 1-naphthylamine to carbon nanotubes was much stronger than that to graphite, and the adsorption to treated carbon nanotubes was stronger than that to untreated carbon nanotubes. This can be explained by the fact that graphite does not possess any O-containing functionality and, thus, the Lewis acid-base interaction cannot be in effect. Moreover, the treated carbon nanotubes contained a higher abundance of O-containing groups as Lewis acid sites and therefore exhibited a stronger adsorption affinity than untreated carbon nanotubes. In addition, Figure 3 shows that when the Lewis acid-base interaction was impeded at a pH less than pKa (3.92) as most 1-naphthylamine molecules and surface O-containing groups were protonated, adsorption on carbon nanotubes was hindered much more greatly than was the adsorption on functionality-free graphite. The different trends observed in the higher pH range (9-11) between graphite and carbon nanotubes are also consistent with the mechanism of Lewis acid-base interaction. At high pH, some of the Lewis acid sites (e.g., -COOH, -OH) of carbon nanotubes were ionized and, accordingly, the Lewis acid-base interaction was weaker. Additionally, as the pH was increased, the adsorption to treated carbon nanotubes decreased less significantly compared to untreated carbon nanotubes. This was likely because the treatment introduced significant amounts of O-containing groups such as -O-, dO, -CHO, and the like, which cannot undergo the protonation-deprotonation transition over the pH range tested. VOL. 42, NO. 18, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6867
A promising environmental application of carbon nanotubes is special sorbents with high capacity and selectivity for recalcitrant organic contaminants. Findings in this study indicate that both electronic properties of organic molecules and surface properties of carbon nanotubes can play important roles in adsorption. Additionally, the pH has a significant effect on the adsorption of polar compounds and, particularly, the enhanced adsorption at high pH for hydroxylsubstituted aromatics has not been reported previously. Thus, it is possible that selective removal of organic pollutants can be achieved by controlling the solution chemistry and by designing specific surface functional groups for carbon nanotubes to enhance the adsorptive interactions with target contaminants.
Acknowledgments This project was supported by the National Science Foundation of China (Grants 20637030 and 20577024), Ministry of Education of China (Grant 20060055035), Tianjin Municipal Science and Technology Commission (Grant 06TXTJJC14000), and China-U.S. Center for Environmental Remediation and Sustainable Development.
Supporting Information Available Figure S1 shows nitrogen sorption and desorption isotherms at 77 K on carbon nanotubes, Figure S2 shows the FTIR transmission spectra of treated carbon nanotubes, Figure S3 shows the adsorption isotherms on a given adsorbent with aqueous-phase concentrations normalized for the hydrophobic effect using n-hexadecane as the reference solvent, Table S1 shows the pore volume and average pore size distribution of carbon nanotubes, and Table S2 shows elemental compositions and nitrogen BET surface areas for the carbon nanotubes. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Masciangioli, T.; Zhang, W. X. Environmental technologies at the nanoscale. Environ. Sci. Technol. 2003, 37, 102A–108A. (2) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chemistry of carbon nanotubes. Chem. Rev. 2006, 106, 1105–1136. (3) Robichaud, C. O.; Tanzil, D.; Weilenmann, U.; Wiesner, M. R. Relative risk analysis of several manufactured nanomaterials: an insurance industry context. Environ. Sci. Technol. 2005, 39, 8985–8994. (4) Wiesner, M. R.; Lowry, G. V.; Alvarez, P.; Dionysiou, D.; Biswas, P. Assessing the risks of manufactured nanomaterials. Environ. Sci. Technol. 2006, 40, 4336–4345. (5) Guzma´n, K. A. D.; Taylor, M. R.; Banfield, J. F. Environmental risks of nanotechnology: national nanotechnology initiative funding, 2000-2004. Environ. Sci. Technol. 2006, 40, 1401–1407. (6) Hyung, H.; Fortner, J. D.; Hughes, J. B.; Kim, J.-H. Natural organic matter stabilizes carbon nanotubes in the aqueous phase. Environ. Sci. Technol. 2007, 41, 179–184. (7) Long, R. Q.; Yang, R. T. Carbon nanotubes as superior sorbent for dioxin removal. J. Am. Chem. Soc. 2001, 123, 2058–2059. (8) 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. (9) Lu, C.; Chung, Y.; Chang, K. Adsorption of trihalomethanes from water with carbon nanotubes. Water Res. 2005, 39, 1183–1189. (10) Fagan, S. B.; Filho, A. G. S.; Lima, J. O. G.; Filho, J. M.; Ferreira, O. P.; Mazali, I. O.; Alves, O. L.; Dresselhaus, M. S. 1,2Dichlorobenzene interacting with carbon nanotubes. Nano Lett. 2004, 4, 1285–1288. (11) Hilding, J.; Grulke, E. A.; Sinnott, S. B.; Qian, D.; Andrews, R.; Jagtoyen, M. Sorption of butane on carbon multiwall nanotubes at room temperature. Langmuir 2001, 17, 7540–7544. (12) 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.
6868
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 18, 2008
(13) Yang, K.; Zhu, L.; Xing, B. Adsorption of polycyclic aromatic hydrocarbons by carbon nanomaterials. Environ. Sci. Technol. 2006, 40, 1855–1861. (14) Yang, K.; Wang, X.; Zhu, L.; Xing, B. Competitive sorption of pyrene, phenanthrene, and naphthalene on multiwalled carbon nanotubes. Environ. Sci. Technol. 2006, 40, 5804–5810. (15) Zhao, J.; Lu, J. Noncovalent functionalization of carbon nanotubes by aromatic organic molecules. Appl. Phys. Lett. 2003, 82, 3746–3748. (16) Chen, W.; Duan, L.; Zhu, D. Adsorption of polar and nonpolar organic chemicals to carbon nanotubes. Environ. Sci. Technol. 2007, 41, 8295–8300. (17) Yang, C.-M.; Park, J. S.; An, K. H.; Lim, S. C.; Seo, K.; Kim, B.; Park, K. A.; Han, S.; Park, C. Y.; Lee, Y. H. Selective removal of metallic single-walled carbon nanotubes with small diameters by using nitric and sulfuric acids. J. Phys. Chem. B 2005, 109, 19242–19248. (18) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry, 2nd ed.; Wiley-Interscience: New York, 2003. (19) Zhu, D.; Pignatello, J. J. A concentration-dependent multi-term linear free energy relationship for sorption of organic compounds to soils based on the hexadecane dilute-solution reference state. Environ. Sci. Technol. 2005, 39, 8817–8828. (20) Goss, K.-U.; Schwarzenbach, R. P. Linear free energy relationships used to evaluate equilibrium partitioning of organic compounds. Environ. Sci. Technol. 2001, 35, 1–9. (21) Suzuki, S.; Green, P. G.; Bumgarner, R. E.; Dasgupta, S.; Goddard, W. A.; Blake, G. A. Benzene forms hydrogen bonds with water. Science 1992, 257, 942–945. (22) Efremenko, I.; Sheintuch, M. Predicting solute adsorption on activated carbon: phenol. Langmuir 2006, 22, 3614–3621. (23) Gattrell, M.; MacDougall, B. The anodic eletrochemistry of pentachlorophenol. J. Electrochem. Soc. 1999, 146, 3335–3348. (24) Muna, G. W.; Tasheva, N.; Swain, G. M. Electro-oxidation and amperometric detection of chlorinated phenols at boron-doped diamond electrodes: a comparison of microcrystalline and nanocrystalline thin films. Environ. Sci. Technol. 2004, 38, 3647– 3682. (25) Yalkowsky, S. H.; Valvani, S. C. Solubilities and partitioning. 2. Relationship between aqueous solubilities, partition coefficients, and molecular surface areas of rigid aromatic hydrocarbons. J. Chem. Eng. Data 1979, 24, 127–129. (26) McDermott, M. T.; McCreery, R. L. Scanning tunneling microscopy of ordered graphite and glassy carbon surfaces: electronic control of quinone adsorption. Langmuir 1994, 10, 4307–4314. (27) Zhu, D.; Kwon, S.; Pignatello, J. J. Adsorption of single-ring organic compounds to wood charcoals prepared under different thermochemical conditions. Environ. Sci. Technol. 2005, 39, 3990–3998. (28) Fagan, S. B.; Santos, E. J. G.; Filho, A.G. S.; Filho, J. M.; Fazzio, A. Ab initio study of 2,3,7,8-tetrachlorinated dibenzo-p-dioxin adsorption on single wall carbon nanotubes. Chem. Phys. Lett. 2007, 437, 79–82. (29) Chakrapani, N.; Zhang, Y. M.; Nayak, S. K.; Moore, J. A.; Carroll, D. L.; Choi, Y. Y.; Ajayan, P. M. Chemisorption of acetone on carbon nanotubes. J. Phys. Chem. B 2003, 107, 9308–9311. (30) Robinson, J. A.; Snow, E. S.; Ba˘descu, S. C.; Reinecke, T. L.; Perkins, F. K. Role of defects in single-walled carbon nanotube chemical sensors. Nano Lett. 2006, 6, 1747–1751. (31) Carey, F. A. Organic Chemistry, 6th ed.; McGraw-Hill: New York, 2005. (32) Weissmahr, K. W.; Haderlein, S. B.; Schwarzenbach, R. P.; Hany, R.; Nuesch, R. In-situ spectroscopic investigations of adsorption mechanisms of nitroaromatic compounds at clay minerals. Environ. Sci. Technol. 1997, 31, 240–247. (33) Weissmahr, K. W.; Hildenbrand, M.; Schwarzenbach, R. P.; Haderlein, S. B. Laboratory and field scale evaluation of geochemical controls on groundwater transport of nitroaromatic ammunition residues. Environ. Sci. Technol. 1999, 33, 2593– 2600. (34) Sulaymon, A. H.; Ahmed, K. W. Competitive adsorption of furfural and phenolic compounds onto activated carbon in fixed bed column. Environ. Sci. Technol. 2008, 42, 392–397.
ES8013612
