Adsorption of Nonionic Aromatic Compounds to Single-Walled Carbon

Aug 27, 2008 - We systematically studied effects of pH, ionic strength, and presence of Cu2+ (50 mg/L) or a dissolved humic acid (HA, Fluka). (50 mg/L...
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Environ. Sci. Technol. 2008, 42, 7225–7230

Adsorption of Nonionic Aromatic Compounds to Single-Walled Carbon Nanotubes: Effects of Aqueous Solution Chemistry JUNYI CHEN,† WEI CHEN,‡ AND D O N G Q I A N G Z H U * ,† State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Jiangsu 210093, China, and Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China

Received May 21, 2008. Revised manuscript received July 7, 2008. Accepted July 8, 2008.

We systematically studied effects of pH, ionic strength, and presence of Cu2+ (50 mg/L) or a dissolved humic acid (HA, Fluka) (50 mg/L) on adsorption of three nonionic aromatic compounds, naphthalene, 1,3-dinitrobenzene, and 1,3,5-trinitrobenzene to single-walled carbon nanotubes. Presence of Cu2+ or variance in the ionic strength between 0.02 and 0.1 M (NaNO3) only slightly affected adsorption affinities. Presence of HA reduced adsorption of the three compounds by 29-57% for CNTs, as measured by change in distribution coefficient (Kd), and by 80-95% for graphite. In contrast to nonporous graphite, whose surface area was completely accessible in adsorption, CNTs formed aggregates with microporous interstices in aqueous solution, which blocked large HA molecules from competing with the surface area. Changing the pH from 2 to 11 did not affect adsorption of naphthalene, while it increased adsorption of 1,3-dinitrobenzene and 1,3,5-trinitrobenzene by 2-3 times. Increasing pH apparently facilitated deprotonation of the acidic functional groups (-COOH, -OH) of CNTs, which promoted the π-electron-donor ability of the graphene surface, therefore enhancing π-π electron-donor-acceptor (EDA) interactions of the two nitroaromatics (π-electron acceptors).

Introduction Recent studies have demonstrated that synthesized carbon nanotubes (CNTs) have very strong adsorption affinity toward many organic contaminants such as polycyclic aromatic hydrocarbons (PAHs), chlorobenzenes, and dioxin (1-6). In addition to the promising application as special adsorbents in water treatment and environmental remediation (7, 8), the carbon nanomaterials, once released to the environment, might also play an important role in the fate of common organic contaminants because of the strong adsorption affinity (6, 9). Therefore, elucidation of the molecular mechanisms and factors controlling adsorption of organic contaminants to CNTs is of great environmental importance. A number of studies have been conducted to characterize the mechanistic aspects of organic-CNT interactions * Corresponding author phone/fax: +86 025-8359-6496; e-mail: [email protected]. † Nanjing University. ‡ Nankai University. 10.1021/es801412j CCC: $40.75

Published on Web 08/27/2008

 2008 American Chemical Society

(1-6, 9). One common finding from these studies is that adsorption of aromatic compounds is greatly facilitated by the high electronic polarizability of graphene surfaces on CNT sidewalls. For example, Long and Yang (1) attributed the high adsorption affinity of dioxin on CNTs to the strong π-π stacking between the two benzene rings of dioxin and the carbon surface. In a previous study, we proposed a mechanism of π-π electron-donor-acceptor (EDA) interactions to explain the stronger adsorption of nitroaromatic compounds (nitrobenzene, 1,3-dinitrobenzene) relative to other nonpolar aromatic compounds (chlorinated and methylated benzenes) by CNTs (9). The strong electronwithdrawing ability of the nitro group causes the nitrosubstituted benzene rings to be electron-depleted and hence function as π-electron-acceptors, which interact strongly with the π-electron-rich sites (π-electron-donors) of the graphene surface of CNTs. At many contaminated sites, organic contaminants are present together with a complex suit of natural organic matter (e.g., dissolved humic substance), heavy metals, alkali and alkaline cations, and anions. The coexisting solutes can be adsorbed/complexed to the surface of CNTs and thus impact adsorption of organic contaminants. In a recent study, Hyung et al. (10) reported that natural organic matter interacts strongly with carbon nanotubes and forms stable complexes in aqueous solution. Moreover, changes in pH affect the protonation-deprotonation transition of surface functional groups (e.g., -COOH and -OH), which might modify the hydrophobicity of these groups and the net charge on the carbon surface. Recently Back and Shim (11) showed that the charge-transfer properties of transistors prepared from single-walled CNTs was pH dependent, which was likely the result of the specific electronic properties of the surface functional groups in response to the varied pH. Previous studies have demonstrated that presence of humic substance could significantly impact adsorption of organic contaminants to two other graphitized carbonaceous materials, activated carbon and black carbon (collectively termed for soot, char/charcoal, and other condensed environmental carbonaceous materials), respectively (12-14). Kilduff and Wigton (12) studied adsorption of trichloroethylene to activated carbon preloaded with humic substance and attributed the significantly reduced adsorption of trichloroethylene to a combination of molecular sieving, pore blockage, and competitive adsorption by the humic substance. Pignatello and coinvestigators (13, 14) also reported that presence of humic substance in adsorbed and coflocculated states significantly suppressed adsorption of organic compounds to wood-made charcoals because of similar mechanisms. In addition, changes in pH were found to influence the adsorption of organic compounds to activated carbon and black carbon by modification of the surface characteristics of the adsorbents and the electronic properties of the adsorbate molecules (15-17). More recently, Chen et al. (18) showed that complexing heavy metals (Cu2+, Ag+) with O functionalities on the charcoal surface could prominently affect adsorption of both polar and nonpolar aromatic compounds through modifying structures of the surrounding hydration shells of metal ions. The effects of aqueous solution chemistry on adsorption of organic compounds to activated carbons have been well documented in the literature (ref 19 and references therein). However, thus far adequate attention has not been paid to the effects of aqueous solution chemistry on adsorption of organic compounds by CNTs. Compared to activated carbons that normally do not self-assemble, CNTs suspended in VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Sorbate Water Solubility (Csat) and n-Octanol-Water Partition Coefficient (KOW), n-Hexadecane-Water Partition Coefficient (KHD), Freundlich Model Parameters KF, and n ± Standard Deviation for Sorption to Carbon Nanotubes (CNTs) and Graphite with and without Presence of Cu2+ or Humic Acid (HA) compound (abbreviation) naphthalene (NAPH)

Csata (mmol L-1) 0.25

KOWa (L L-1)

KHD (L L-1)

2140

b

2570

1,3-dinitrobenzene(DNB)

2.88

30.9

4.3c

1,3,5-trinitrobenzene (TNB)

1.55

15.1

1.00c

a

From Schwarzenbach et al. (32).

b

Experimental Section Materials. Sorbate compounds included naphthalene (NAPH, Aldrich), 1,3-dinitrobenzene (DNB, Aldrich), and 1,3,5trinitrobenzene (1000 mg/L in acetonitrile) (TNB, Supelco). All compounds were chemical graded or higher and were used as received. Selected physical-chemical properties of the compounds are listed in Table 1. In solution-phase experiments, naphthalene, phenanthrene (PHEN, Fluka), pyrene (PYR, Aldrich), and 1,4,5,8-naphthalene-tetracarboxylic acid (NTA, Fluka) prepared in the deprotonated form were used as model π-donors. A commercial humic acid (HA) (Fluka) extracted from natural coal was used as received. Results of elemental analysis and spectroscopic characterization of the HA samples were presented in a previous study (20). Single-walled CNTs were purchased from Nanotech Port Co. (Shenzhen, Guangdong Province, China). The samples of CNTs were treated to remove amorphous carbon and metal residues using a method reported earlier (21). The treated CNT samples were characterized in our previous study with respect of Fouriertransform infrared (FTIR) spectroscopy, elemental composition (78.0% C and 17.2% O), BET surface area (370 m2/g), and pore size distribution (average pore size of 37 Å) (9). Nonporous graphite powder (99.99% C, 325 mesh) (Aldrich) with a BET surface area of 4.5 m2/g (22) and polyethylene beads (medium density) (Aldrich) were used as two model sorbents. Batch Sorption. To prepare the bulk solution of HA, 50 mg of HA was dissolved in 5 mL of 0.1 M NaOH and mixed with distilled water to reach an apparent HA concentration of 50 mg/L. The HA solution was adjusted to pH 6.0 by 0.1 M HCl, followed by filtration through a 0.45 µm membrane. The bulk solution of HA had a total carbon concentration 9

CNTs/Na+ CNTs/Cu2+ CNTs/HA graphite/Na+ graphite/HA CNTs/Na+ CNTs/Cu2+ CNTs/HA graphite/Na+ graphite/HA CNTs/Na+ CNTs/Cu2+ CNTs/HA graphite/Na+ graphite/HA

KF (mmol1-n L/kg)

n

R2

1420 ( 40 1330 ( 30 1220 ( 30 50 ( 5 15 ( 3 710 ( 40 780 ( 40 580 ( 30 64 ( 6 1 ( 0.4 650 ( 30 620 ( 30 500 ( 20 90 ( 7 1 ( 0.6

0.377 ( 0.005 0.373 ( 0.004 0.377 ( 0.005 0.59 ( 0.02 0.68 ( 0.05 0.37 ( 0.01 0.39 ( 0.01 0.37 ( 0.01 0.71 ( 0.02 0.42 ( 0.06 0.268 ( 0.008 0.271 ( 0.008 0.262 ( 0.007 0.67 ( 0.01 0.38 ( 0.07

0.999 0.999 0.999 0.991 0.963 0.992 0.994 0.993 0.995 0.835 0.991 0.993 0.993 0.998 0.753

From Zhu and Pignatello (29). c From Liu et al. (33).

aqueous solution could result in coagulation effects and hence different adsorption properties of organic pollutants. In the present study, we systematically evaluated adsorption of three nonionic compounds, naphthalene, 1,3-dinitrobenzene and 1,3,5-trinitrobenzene, from aqueous solution to single-walled CNTs and graphite served as a model for graphene sheets. The selected organic compounds differ in polarity and electron-donor/acceptor ability and represent two common types of organic pollutants, PAHs and nitroaromatic compounds, respectively. The objective of the study was to investigate the organic-CNT interactions under varied solution chemistry conditions of pH, ionic strength, and presence of humic acids or heavy metal ions.

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(TOC) of 19 ( 1 mg/L (TOC 5000A, Shimadzu, Japan), with the standard deviation calculated from three measurements in different experiments. The bulk solution of HA was used to prepare the operational background solution containing 0.02 M NaNO3 and 200 mg/L NaN3 (as bioinhibitor) in the experiments for testing the effect of dissolved HA on adsorption. Moreover, copper was selected as a representative heavy metal because of its toxicity and wide distribution in the environment for testing the effect of heavy metal ions on adsorption. A weighed amount of CuSO4 was added to 0.02 M NaNO3 solution to prepare a background solution containing 50 mg/L Cu2+ (with pH preadjusted to 6.0). Background solutions of NaNO3 over a concentration range of 0.02-0.1 M (with pH preadjusted to 6.3 for CNTs and 6.6 for graphite) were used in experiments to test the ionic strength effect on adsorption. In a separate set of experiments for testing the pH effect, background solutions of 0.02 M NaNO3 with preadjusted pH were used in consideration of the acid/ base-buffering ability of the adsorbent to ensure that the final pH at sorption equilibrium was close to the desired value. Sorption was carried out in polytetrafluoroethylene-lined screw cap glass vials of 8 mL for polyethylene, 22 mL for graphite, and 40 mL for CNTs. Vials received weighed amount of sorbent (15-20 mg of CNTs, 40 mg of graphite, and 2 g of polyethylene) and sufficient volume of operational background solution. The mass ratios of sorbent to solution were carefully selected to ensure data compatibility for different sorbate-sorbent combinations while maintaining analytical accuracy. After wetting the sorbent for at least 24 h, the test organic solute was spiked with a methanol carrier kept below 0.1% (v/v) to minimize cosolvent effects. Sorption data were collected in triplicate for ionic-strength experiments of CNTs, in single point for sorption experiments of HA to CNTs and graphite, and in duplicate for all the other experiments. The samples were mixed end-over-end at room temperature. Two weeks were used to reach apparent sorption equilibrium (no further uptake) for CNTs and 5 days for graphite and polyethylene on the basis of earlier kinetic studies (data not shown). After reaching sorption equilibrium, the samples were centrifuged at 4000 rpm for 30 min. The CNT samples were further left undisturbed on a flat surface for more than 24 h to completely settle down CNT samples. Concentration of NAPH, DNB, or TNB in an aliquot was analyzed by high-performance liquid chromatography with a UV detector using a 4.6 × 150 mm HC-C18 column (Agilent, USA). Isocratic elution was performed under the following

FIGURE 1. Adsorbed concentration (q) vs aqueous phase concentration (CW) at adsorption equilibrium to carbon nanotubes (CNTs) in 0.02 M NaNO3 shown separately for naphthalene (NAPH), 1,3-dinitrobenzene (DNB), and 1,3,5-trinitrobenzene (TNB) with and without presence of Cu2+ (initially at 50 mg/L) or humic acid (HA) (initially at 50 mg/L). For a given adsorbate/adsorbent combination, samples were spiked with same series of initial concentrations. conditions: 75% methanol/25% water (v:v) with a wavelength of 254 nm for NAPH; 60% methanol/40% water with a wavelength of 238 nm for DNB; 55% methanol/45% water with a wavelength of 226 nm for TNB. The concentration of Cu2+ was determined by an atomic absorption spectrometer (Thermo Electro GF95Z). Concentration of HA was measured by the TOC analyzer. To account for solute loss from processes other than sorbent sorption (i.e., sorption to septum and glass wall), calibration curves were built separately from controls receiving the same treatment as the sorption samples but no sorbent. Calibration curves included at least 7 standards over the test concentration range. On the basis of the obtained calibration curves, the sorbed mass of organic solute, Cu2+, or HA was calculated by subtracting mass in aqueous solution from mass added. The equilibrium pH of samples, as measured at the end of sorption, was approximately 5.7 for CNTs, 4.0 for CNTs/Cu2+, 6.0 for CNTs/ HA, 6.8 for graphite, and 7.0 for graphite/HA. Spectroscopic Studies. Carbon nuclear magnetic resonance (13C NMR) spectra of per-C13 atom-labeled DNB (99% C13, Cambridge Isotope Laboratories, Inc.) in mixtures with model π-donor compounds (NAPH, PHEN, and PYR) in chloroform-d (>99.8% deuterium) were recorded at room temperature on a Bruker-DRX 500 MHz spectrometer (Silberstreifen, Germany). Ultraviolet-visible (UV-vis) absorbance spectra of TNB/NTA mixtures in aqueous solution at different ratios of NaOH to NTA were acquired on a UV-2450 spectrometer (Shimadzu) against a solvent blank. Zeta Potential (ζ) Measurement. The ζ values of CNT samples suspended in deionized water (500 mg CNTs per litter water) versus pH was measured from four replicate samples using a domestically made instrument (Zhongchen JS94H, Shanghai).

Results and Discussion Adsorption results of the three compounds to CNTs and graphite are shown in Table 1 and Figures 1 and 2. The adsorption data were fitted with the Freundlich model q ) KFCWn, where q (mmol/kg) and CW (mmol/L) are equilibrium concentrations of an adsorbate on the adsorbent and in the

FIGURE 2. Adsorbed concentration (q) vs aqueous phase concentration (CW) at adsorption equilibrium to graphite in 0.02 M NaNO3 shown together for naphthalene (NAPH), 1,3-dinitrobenzene (DNB), and 1,3,5-trinitrobenzene (TNB) with and without presence of humic acid (HA) (initially at 50 mg/L). aqueous solution, respectively, KF (mmol1-n Ln/kg) is the Freundlich affinity coefficient; and n (unitless) is the Freundlich linearity index. Adsorption was highly nonlinear, and in general, the Freundlich model fitted reasonably well with the isotherms except that of DNB with graphite/HA and TNB with graphite/HA because of the extremely low adsorption. For all the three compounds adsorption to CNTs was remarkably more nonlinear than adsorption to graphite, probably because of a more heterogeneous distribution of the adsorption sites on CNTs. It also seems that presence of Cu2+ or HA did not affect much the adsorption nonlinearity to CNTs, while the presence of HA significantly changed the adsorption nonlinearity to graphite. Effects of Cu2+ Cosolute. Figure 1 compares the adsorption affinities of different compounds to CNTs with and without presence Cu2+. Given the same initial concentrations of adsorbate, the presence of Cu2+ at 50 mg/L reduced most adsorption points by up to 20 ( 4%, as measured by decline in distribution coefficient (Kd). In a previous study of charcoal adsorption, we observed that presence of Cu2+ at 50 mg/L reduced Kd of 2,4-dichlorophenol, 1,2-dichlorobenzene, and naphthalene to a higher degree by 30-60% (18). The adsorption suppression of organic compound on charcoal VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Effects of ionic strength and pH on distribution coefficient (Kd) to carbon nanotubes (CNTs) shown together for naphthalene (NAPH), 1,3-dinitrobenzene (DNB), and 1,3,5-trinitrobenzene (TNB). (a) Effect of ionic strength. The spiked concentrations were 0.14 mmol/L for NAPH, 0.070 mmol/L for DNB, and 0.070 mmol/L for TNB. (b) Effect of pH. The spiked concentrations were 0.20 mmol/L for NAPH, 0.047 mmol/L for DNB, and 0.095 mmol/L for TNB. Error bars in figures represent standard deviations. was attributed to complexation of Cu2+ with surface O functionalities to form hydration shells of dense water that directly compete with organic compound for adsorption surface area. Fairly high contents of O-containing functional groups (-COOH, -OH, etc.) (17.2% O) were introduced to samples of CNTs during the course of pretreatment (9). These functional groups are likely located at sites of tube ends and defect sites on sidewalls (23). The acidic surface functionalities (-COOH, -OH) of CNTs could complex Cu2+ ions strongly via ligand-exchange reactions and release protons, which was evidenced by declines in pH when Cu2+ was present (approximately 1.7 pH units compared to the initial pH of Cu2+-containing background solution). The adsorption coefficient of Cu2+ to CNTs was measured to be 600 ( 100 (standard deviation measured from all Cu2+-containing samples in sorption). Although CNTs showed even higher binding ability of Cu2+ than certain wood-made charcoal (adsorption coefficients of Cu2+ on charcoals shown in ref 18), adsorption suppression of organic compound by Cu2+ was weaker on CNTs than on charcoal. Compared with the interstitial spaces formed by individual carbon nanotubes in aggregates, the micropores in charcoal are strictly rigid and averagely smaller in size (pore size distribution data reported earlier 9, 22). Accordingly, adsorption of organic compound on the charcoal surface becomes more sensitive to the competitive effect caused by Cu2+-complexed hydration shells. Effects of HA Cosolute. It is clear to see that presence of HA at 50 mg/L moderately reduced adsorption of all the three compounds to CNTs (Figure 1). The Kd was lowered by 29-35% for NPAH, 35-44% for DNB, and 47-57% for TNB by HA (errors are approximately 4%). It was shown in previous studies that coating humic substance on activated carbon surfaces significantly suppressed adsorption of organic compounds (12-14). Mechanisms of molecular sieving, pore blockage, and competitive adsorption by humic substance have been proposed to explain the observation. These mechanisms are likely responsible for the HA-suppressed adsorption of organic compounds to CNTs observed in the present study. Furthermore, it appears that the adsorption suppression of organic compound is proportional to molecular size (NAPH < DNB < TNB). This was likely because of a coupled effect of competitive adsorption by HA and molecular sieving in microporous interstices formed by individual carbon nanotubes. With the same loading of HA, the adsorption space would be more inaccessible to the organic compounds with larger molecular sizes. In a previous study Pignatello et al. reported that suppression on adsorption of nonpolar solutes to charcoal by humics increased with molecular size of solute (benzene < naphthalene < phenanthrene, 1,2,4-trichlorobenzene) (14). The effect of HA on organic compound adsorption to graphite is presented in Figure 2. One clear trend is that compared with CNTs adsorption suppression on graphite 7228

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by HA was much stronger (the Kd was lowered by 80-95%). Detailed structural characterization using solid-state 13C NMR and FTIR revealed that the Fluka humic acid primarily consists of poly(methylene)-rich aliphatics with high aromatic content and some COO/N-CdO structures but low polarity (20). The HA components are expected to interact strongly with the adsorbent graphene surface via van der Waals forces and π-π stacking and with the surface functional groups, if provided, via polar interactions (mainly H-bonding) and electrostatic forces. Gorham et al. (24) studied the adsorption of dissolved natural organic matter on highly ordered pyrolytic graphite using atomic force microscopy and found that the natural organic matter formed a densely packed monolayer coverage on the carbon surface. The calculated overall adsorption coefficient of HA to CNTs varies from 1300 to 2200 L/kg (adsorption data shown in Figure S1b). However, the effective adsorption coefficient would likely be much higher for the HA components that have small molecular sizes and can easily enter the microporous interstices of CNTs to cause competitive adsorption on the surface. The discrepancies between graphite and CNTs in reducing adsorption of organic compound can be explained by the varied accessibility of adsorbent surface area. The surface area of nonporous graphite is completely available to all sizes of HA molecules, therefore, resulting in stronger suppressive adsorption of organic compound. However, the interstitial spaces formed by individual carbon nanotubes in aggregates are expected to exclude large HA molecules from competing with the surface area. The proposed protocol can be better viewed by comparison of adsorption isotherms of organic compounds and HA to CNTs and to graphite with solidphase concentration normalized with BET surface area (Figure S1). For a given organic compound, the normalized adsorption data are nearly overlapped between CNTs and graphite, indicating that the interstitial space in CNT aggregates is fully available to organic compound. Consistent results have been shown in our previous study for other monoaromatic compounds (benzene, chlorobenzene, and 1,2,4,5-tetrachlorobenzene) when comparing adsorption data between single-walled CNTs and graphite (9). However, the surface area normalized adsorption of HA is much lower on CNTs than on graphite (Figure S1) because of size exclusion of large HA molecules from accessing the microporous interstices in CNT aggregates. In accordance with the size exclusion effect, Kilduff et al. (25, 26) reported earlier that small molecular size components of dissolved humic substance were adsorbed preferentially to activated carbon. Effects of Ionic Strength. Figure 3a shows the effect of ionic strength on adsorption of the three organic compounds to CNTs. Variance in the ionic strength between 0.02 and 0.1 M by NaNO3 did not influence adsorption affinities. Previous studies have indicated that an increase in ionic strength could enhance the uptake of ionic compounds by carbonaceous adsorbents because of a screening effect of the surface charge

FIGURE 4. Spectroscopic evidence for π-π EDA interactions in solution. (a) 13C NMR chemical shifts (δ) of per-C13 atom-labeled 1,3-dinitrobenzene (DNB) (0.01 M, 1,3 C) vs concentrations of π-donors, naphthalene (NAPH), phenanthrene (PHEN), and pyrene (PYR). Lines are for visual clarity only. (b) UV-vis difference spectra in water showing the charge-transfer absorption between π-acceptor of 1,3,5-trinitrobenzene (TNB) (0.0028 mmol/L) and π-donor of deprotonated 1,4,5,8-naphthalene tetracarboxylic acid (NTA) (0.032 mmol/L) and its dependence on the ratio of [NaOH] to [NTA]. produced by the added salt (27, 28). In the present study, the three organic compounds are nonionic, and their adsorption to CNTs is driven by a combined mechanism of hydrophobic effect and π-π electronic coupling with the graphene surface of CNTs (see detailed discussion below). These factors should not be significantly modified by changing the ionic strength over a relatively small range (0.02-01 M) by species (Na+, NO3-) with low electronic coordination ability. Therefore, adsorption affinities of the three compounds were not affected by the ionic strength studied. It should be pointed out that changing the ionic strength within the tested range by NaNO3 would not much affect the activity coefficient of adsorbate compounds, as reflected by negligible decreases in aqueous solubility of solute compared to pure water solubility (also referred as “salting out” effect, data not shown). Effects of pH. The effect of pH on adsorption of the three organic compounds is presented in Figure 3b. Changing the pH from ∼2 to 11 did not affect adsorption of NAPH, while increased adsorption of DNB and TNB by nearly three times. We have shown in a previous study that the CNT sample contains a significant amount of acidic functional groups (-COOH, -OH) on the basis of analyses of FTIR spectroscopy and elemental composition (9). The pH changes over the covered range apparently affect the protonation-deprotonation transition of these functional groups and hence the hydrophobicity of carbon surfaces. However, the pH-mediated adsorbent hydrophobicity can be ruled out as an explanation for the pH dependence of DNB and TNB considering the fact that no significant pH effect was observed for NAPH, which is more hydrophobic than DNB and TNB (see KOW values in Table 1). It is worth noting that no pH effect was observed for NAPH and DNB sorption to polyethylene beads (Figure S2, Supporting Information), a model sorbent which is dictated by the inert methylene structure units and cannot invoke any specific sorptive interactions. It thus appears that the pH-dependence of DNB and TNB sorption on CNTs was caused by a combined effect of speciation of surface functional groups on adsorbent and specific electronic properties of adsorbate. In a previous study, we proposed a mechanism of π-π EDA interactions between nitroaromatic compounds (πelectron acceptors) and polarized, electron-rich regions (πelectron donors) on graphene surfaces of CNTs to account for the stronger nonhydrophobic adsorptive interactions relative to other nonpolar aromatic compounds (methylated and chlorinated benzenes) (9). The specific adsorptive interactions of the two nitroaromatic compounds can be better evaluated with normalization of solute hydrophobicity. Solute n-hexadecane-water partition coefficient (KHD) has been found to be a better indicator of hydrophobicity than n-octanol-water partition coefficient (KOW) (values listed in Table 1) (29). When solute hydrophobicity was normalized

using KHD, adsorption affinities of the three compounds followed an order of NAPH < DNB < TNB (Figure S3, Supporting Information), providing support for π-π EDA interactions of the two nitroaromatic compounds with the graphene surfaces of graphite and CNTs. We herein further proposed that increasing pH over the tested range facilitated deprotonation of the acidic functional groups (-COOH, -OH) of CNTs, which promoted π-electron-donor ability of the graphene surface (-COOH is a weak electron donor and acceptor, -COO- is a strong electron donor but not an acceptor), therefore enhancing π-π EDA interactions of the two nitroaromatics (π-electron acceptors). The hypothesized mechanism was supported by the fact that the surface charge density, measured as zeta potential (ζ), was negative within the entire pH range studied and increased as the pH increased from ∼2 to 6 (Figure S4, Supporting Information). A previous study showed similar results of pH-dependence charge on the surface of activated carbon (30). The alkalinity-facilitated charge-transfer ability has also been observed on transistors prepared from single-walled CNTs (11). Spectroscopic Studies of Complexation in Solution. The π-π EDA interaction between π-electron-rich regions (πelectron donors) on the CNT surface and nitroaromatic compounds (π-electron acceptors) is supported by results in the solution-phase 13C NMR experiments. Placing a nucleus above or below the plane of an aromatic structure causes electronic shielding because of the “ring current” effect (31). Thus, for the “face-to-face” geometry of a π-π EDA complex, upfield chemical shifts (δ) of carbons on one ring would be induced by the ring current of the opposing ring. This may serve as an evidence for formation of π-π EDA complex in solution. A clear trend of 13C NMR upfield shifts was observed for DNB in mixtures with model π-electron donors, and the shifts increased with increasing π-donor concentration, as well as π-donor strength of PAH (pyrene > phenanthrene > naphthalene) (Figure 4a). In a previous study, Zhu and Pignatello reported 1H NMR upfield shifts of several nitroaromatic compounds with presence of PAHs in organic solvents (22). Solution-phase π-π EDA complexes often show a chargetransfer band in the UV-vis region. Figure 4b shows the appearance of π-π charge-transfer absorbance bands in the UV-vis spectrum for mixtures of TNB (π-electron acceptor) with a model π-electron-donor, deprotonated NTA in water. With increasing deprotonation of NTA by NaOH and hence its π-electron donor ability, the intensity of the charge-transfer band gradually increased. The results further verify the hypothesis that the π-π EDA interaction between nitroaromatic compounds and CNTs is enhanced with dissociation of surface acidic functional groups (-COOH, -OH) attached to the graphene surface as pH increases. VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Acknowledgments This work was supported by the China National Science Foundation (Grants 20637030 and 20777031). We thank Xiaolei Qu and Huiyu Sun for assisting part of the experimental work.

Supporting Information Available Figure S1 shows adsorption isotherms of organic compounds and HA to CNTs and graphite with solid-phase concentration normalized with BET surface area, Figure S2 shows pH effect on sorption of NAPH and DNB to polyethylene beads, Figure S3 shows adsorption isotherms of organic compounds to graphite and CNTs with hydrophobicity normalized using n-hexadecane as a reference state, and Figure S4 shows zeta potential of CNTs vs pH. This material is available free of charge via the Internet at http://pubs.acs.org.

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