Adsorption of Natural Organic Matter Surrogates from AqueousSolution by Multiwalled Carbon NanotubesFei-fei Liu,† Shu-guang Wang,*,† Jin-lin Fan,† and Guang-hui Ma‡

†Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science andEngineering, Shandong University, Jinan 250100, P. R. China‡The Agricultural Extension Station of Agriculture Bureau in Anqiu, 262100, P. R. China

*S Supporting Information

ABSTRACT: Adsorption of two natural organic matter (NOM)surrogates (tannic acid and gallic acid) by powdered activated carbon(PAC) and four kinds of multiwalled carbon nanotubes (MWCNTs) withdifferent diameters was investigated in aqueous solution. The tannic acid(TA) adsorption isotherm fit the Langmuir model best, while adsorptionof gallic acid (GA) fit the Freundlich model. PAC has a higher adsorptioncapacity for GA due to the pore-filling effect, while TA adsorption on PACwas lower than that on MWCNTs because of the molecule sieving effect.Adsorption of GA and TA on MWCNTs decreased with increasingMWCNT diameter, but neither surface area nor total pore volume alonecan fully explain adsorption of the two NOM surrogates on MWCNTs. Adsorption mechanisms including electrostaticinteraction, hydrophobic interaction, hydrogen bond, and π−π interaction play roles in the adsorption process. This studyimplies that NOM properties have a great effect on their adsorption on MWCNTs, and these properties should be furtherconsidered when evaluating the risks of NOM-coated nanomaterials.

■ INTRODUCTION

Since their discovery by Iijima in 1991,1 carbon nanotubes(CNTs) have been at the center of nanoscience andnanotechnology research for a variety of applications such asadsorbents, composite filters, high-flux membranes, antimicro-bial agents, environmental sensors, energy storage devices, andpollution prevention reagents due to their unique andoutstanding chemical , electronic, and mechanicalproperties.2−6 Production and commercial use of CNTs arenow of significant quantities, and hundreds of tons of CNTs,especially multiwalled carbon nanotubes (MWCNTs), areproduced each year.7 As CNTs will inevitably be releasedinto the environment accidentally or intentionally in theprocess of manufacturing and applications,8 there is seriousconcern over their possible toxicity and the associated risk tothe environment.9−13

Natural organic matter (NOM), a complex and heteroge-neous mixture of polyelectrolyte with diverse molecularweights, derives mainly from the decay of plant and animalresidues.14 NOM is ubiquitously present in surface waters atdissolved organic carbon (DOC) concentrations typicallyranging from 1 to 20 mg/L.15 Therefore, it is unavoidablethat CNTs will contact with NOM once released into anaquatic environment. Thus far, several impacts of NOM onCNTs have been reported in previous research articles. First,NOM enhanced the stability of CNTs suspension by producingthermodynamically favorable surfaces and inducing electrostaticand steric repulsion between individual CNTs to overcometheir strong hydrophobicity16,17 and further affect the

persistence behavior of CNTs in water. Second, NOM coatingaltered the adsorption capacity of CNTs toward othercontaminants. Zhang et al. reported that the presence ofNOM suppressed the adsorption of synthetic organic chemicalson CNTs resulting from the competition for limited adsorptionsites and physical pore blockage effect of NOM.18 However,adsorption of heavy metals on NOM-coated CNTs could beenhanced because of the complexation between metal ions andfunctional groups on NOM.19 Third, NOM can influence thetoxicity and bioavailability of CNTs and other pollutants. Kimet al. found that NOM-stabilized CNTs have a low acuteecotoxicity to Daphnia magna, while Cu toxicity increased withincreasing concentration of NOM-associated CNTs.11 Thus,understanding the interaction between CNTs and NOM,especially adsorption of NOM on CNTs, is the first andessential step for further assessing CNTs transport behavior,bioavailability, and toxicity impacts.To date, in the investigations focused on CNTs−NOM

interactions,16,17,20,21 most of the NOM used in these studieswas extracted from river sediment or peat soil.21−28 However,NOM has an indeterminate molecular structure and molecularweight and the composition of NOM from different regionsmay differ, which is not favorable for determining theadsorption mechanism accurately. Research with appropriatesmall organic molecules as surrogates has provided valuable

Received: July 17, 2012Revised: November 6, 2012Published: November 15, 2012

Article

pubs.acs.org/JPCC

© 2012 American Chemical Society 25783 dx.doi.org/10.1021/jp307065e | J. Phys. Chem. C 2012, 116, 25783−25789

insight into the behavior of NOM in adsorption systems.19,29

As a substitute for NOM, tannic acid (TA) is an anionic andhydrophilic organic substance with relatively high molecularweight, which approximates to the mean value of NOM insurface waters.30 Another simple possible model for NOM isgallic acid (GA).31 The molecular weight of TA is exactly 10times that of GA, and the solubility of the former in water is 20times higher than the latter, so these two chemicals couldrepresent NOM sources with a wide range of properties.Therefore, in the present work, we used TA and GA as NOM

surrogates to systematically examine their adsorption ondifferent MWCNTs and determine the adsorption mechanisms.In addition, adsorption of NOM on MWCNTs was alsocompared with that on powdered activated carbon used in mostcommercial water treatment plants. We believe the results willshed light on the adsorption applications and environmentalfate assessment of carbon nanomaterials.

■ EXPERIMENTAL METHODS

Materials. TA and GA obtained from Tianjin KemelChemical Reagent Co., Ltd. (Tianjin, China) were of analyticalgrade and used without further purification. The physical andchemical properties of the two compounds are summarized inTable 1 and their chemical structures are presented in Figure 1.The carbonaceous adsorbents used in this study were

multiwalled carbon nanotubes (MWCNTs) and powderedactivated carbon (PAC). MWCNTs were purchased fromShenzhen Nanotech Port Co., Ltd. (Shenzhen, China) withdifferent outer diameters. All MWCNTs were synthesized bychemical vapor deposition using the mixtures of CH4 and H2 at700 °C with Ni as a catalyst. PAC was obtained from TianjinGuangcheng Chemical Reagent Co., Ltd. (Tianjin, China).MWCNTs and PAC were used without any treatment toaccurately replicate their occurrence in a commercial watertreatment system due to their application or in natural watersresulting from released accidentally or intentionally.Characterization of Adsorbents. A series of techniques

was employed to characterize the adsorbents. N2 adsorption−desorption isotherms were performed at 77 K with aQuadrasorb SI-MP system (Quantachrome, U.S.). All sampleswere degassed at 373 K for 8 h in a vacuum beforemeasurements. The Brunauer−Emmett−Teller (BET) equa-tion and density functional theory (DFT) were used tocalculate their BET surface area and pore size distribution.Transmission electron microscopy (TEM) images ofMWCNTs and PAC were observed with a microscope (JEM-100CXII, Japan). Samples were prepared by dispersingMWCNTs or PAC (<1 mg) in ethanol (50 mL) byultrasonication (80 kHz, 30 min), and a drop of suspensionwas placed on a 200-mesh copper TEM grid. Fourier transforminfrared spectroscopy (FTIR) spectra were recorded in Avatar370 spectrometer (Thermo Nicolet, U.S.) within the range of400−4000 cm−1 with the samples prepared as KBr discs. X-raydiffraction (XRD) patterns of MWCNTs and PAC were

obtained from a Rigaku D/max-γA powder diffractionmeter(Rigaku, Japan) using Cu Kα radiation at a scanning ratio of8°/min ranging from 5° to 70°.

Batch Adsorption Experiment. All adsorption experi-ments were conducted using a batch equilibration technique at25 ± 1 °C. Briefly, certain amounts of adsorbent and adsorbatesolution with a series of initial concentrations (5−50 mg L−1)were added into 40 mL glass tubes. Background solutioncontained 10 mM NaClO4 to maintain constant ionic strengthand was purged with N2 for 15 min to prevent any degradation.Tubes were sealed with Teflon caps and placed on a rotaryshaker (200 rpm) in the dark for 48 h. Preliminary studyindicated that apparent equilibrium was reached in 24 h. Afterequilibrium, the tubes were left undisturbed in the shakerovernight to allow settling of the adsorbents. Then thesupernatants were taken out to separate the solid and solutionusing 0.22 μm cellulose acetate membrane filter. Finally, theconcentrations of TA or GA in the supernatants weredetermined by a UV−vis method. Duplicate samples wererun for all of the experiments.

Determination of Adsorbates. Concentrations of TA andGA were quantified by an UV−vis spectrophotometer (UnicoUV-2000, China) at their optimum wavelengths. Detailed λmax

Table 1. TA and GA Properties

full name chemical formula molecular weight (g/mol) pKa solubility (mg/L) TOC (mg/L) λmax log Kow

tannic acid C76H52O42 1701.2 4.9 2.5 × 105 30.32 272 −1.197.44.3

gallic acid C7H6O5 170.12 8.7 1.2 × 104 39.94 256 0.7011.4

Figure 1. Molecular structures of TA (a) and GA (b).

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are summarized in Table 1. Calibration curves were obtainedusing the standard solution with known concentrations. Blanksamples were carried out for each experiment and did notindicate any degradation or loss during the experiment process,which was confirmed by UV−vis spectra analysis.Data Analysis. Three different nonlinear isotherm models

were applied to fit the adsorption data.

=+

qq K C

K CLangmuir model (LM):

1em L e

L e (1)

=q K CFreundlich model (FM): ne F e (2)

−

=⎡⎣⎢⎢

⎛⎝⎜

⎞⎠⎟

⎤⎦⎥⎥

q

q Z RTCC

Polanyi Manes model (PMM):

exp lnd

e

0 s

e (3)

where qe (mg g−1) is the solid-phase concentration, Ce (mgL−1) is the solution concentration, qm and q0 are the adsorptioncapacity for LM and PMM, KL (L mg−1) is the Langmuiradsorption affinity parameter. KF and n are the Freundlichadsorption constants. Cs (mg L−1) represents the watersolubility of adsorbate at 20 °C, Z and d are the PMM fittingparameters. R is the universal gas constant (8.314 × 10−3 kJmol−1K−1), and T is the absolute temperature (K).Site Energy Distribution. To analyze the energetic

characteristics of interactions between adsorbate and adsorbent,the condensation approximation was used to produce

approximate energy distribution functions.32 For Freundlichisotherm, the approximate site energy distribution is describedwith the following function

* = × − *⎜ ⎟⎛⎝

⎞⎠F E

K n CRT

nERT

( )( )

expn

F s

(4)

For the Langmuir isotherm, the site energy distribution iscalculated through the following equation

* = − * + − * −⎜ ⎟ ⎜ ⎟⎛⎝

⎞⎠⎡⎣⎢

⎛⎝

⎞⎠⎤⎦⎥F E

q K C

RTnERT

K CnE

RT( ) exp 1 expm L s

L s

2

(5)

According to the Polanyi adsorption potential theory,33 theenergy of adsorption is related to the equilibrium liquid-phaseconcentration by

= − *⎜ ⎟⎛⎝

⎞⎠C C

ERT

expe s (6)

where E* is the difference of adsorption energy at Ce and Cs.

■ RESULTS AND DISCUSSIONCharacterization of Adsorbents. TEM images of the

adsorbents are shown in Figure 2. As can be seen, a handful ofmetal catalyst and amorphous carbon existed within the pristineMWCNTs and the outer diameters were almost consistent withthe data provided by the supplier. PAC was composed byseveral graphite sheets.

Figure 2. Transmission electron microscopy (TEM) images of MWCNTs 10−20 (a), MWCNTs 20−40 (b), MWCNTs 40−60 (c), MWCNTs 60−100 (d), and PAC (e). Scale bars in all images are 100 nm.

Table 2. Selected Properties of the Multiwalled Carbon Nanotubes (MWCNTs) and Powdered Activated Carbon (PAC)

adsorbent purity (%) length (μm) outer diameter (nm) SBET (m2/g) Vmeso (cm3/g) Vmicro (cm

3/g) Vtotal (cm3/g) pore width (nm)

MWCNTs 10−20 >95 1−2 10−20 79.73 0.153 0.003 0.156 2.897MWCNTs 20−40 >95 1−2 20−40 70.91 0.195 0.008 0.203 1.140MWCNTs 40−60 >95 1−2 40−60 83.34 0.169 0.003 0.172 3.169MWCNTs 60−100 >95 1−2 60−100 64.14 0.143 0.008 0.151 3.794PAC 162.24 0.058 0.049 0.107 1.178

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N2 adsorption isotherms of MWCNTs were of identicalshape and can be divided into four sections.34 The first sectionappeared at ultralow pressures, and adsorption increasedrapidly due to the micropore filling. Adsorption of the nextsection increased slowly and linearly due to surface monolayerformation. In addition, the last two sections of N2 adsorptionwere the hysteresis loops resulting from capillary condensationin the medium and high pressures, respectively. However, theN2 adsorption isotherm for PAC exhibited a different pattern(Figure S1, Supporting Information). From BET calculation ofN2 isotherms, MWCNTs surface area was in the range of64.14−83.34 m2 g−1, which was lower than the previouslyreported data due to the presence of amorphous carbon andmetal catalyst.35,36 Among the four types of MWCNTs, thesurface area decreased with increasing tube diameter exceptMWCNTs 40−60 (Table 2). The surface area of PAC was162.24 m2 g−1, which was higher than any of the MWCNTsused in this study.Information about pores of PAC and MWCNTs are listed in

Table 2. As van der Waals force exists between individual tubes,MWCNTs tend to aggregate to form bundles which possessfour possible adsorption sites including the external surface ofthe outmost MWCNTs, grooves of the MWCNTs bundles,interstitial spaces between tubes, and inner pores of the openedtubes.37 Because of the presence of amorphous carbon andmetal catalyst in pristine MWCNTs, inner pores of MWCNTsare not available for adsorption. Therefore, the surface area,flexible interconnected grooves region, and interstitial space arethe main adsorption sites in this study (Figure S2, SupportingInformation). Different from MWCNTs, the pore structure ofPAC was always fixed and closed. Clearly, MWCNTs aremesopores adsorbent, and the fraction of mesopores was about94−98%, whereas PAC possessed both mesopores andmicropores with an approximate ratio of 1.2:1 (Table 2). ForMWCNTs, the pore volumes decreased with increasing outerdiameter; however, the pore volume of MWCNTs 10−20 wasthe least.Figure S3, Supporting Information, shows the XRD patterns

of MWCNTs and PAC. All of the adsorbents had a sharpdiffraction peak at approximate 2θ = 26° corresponding to the(002) reflection planes, which indicated that both MWCNTsand PAC were composed by graphite sheets. It should be notedthat 2θ increased but d002 (the interlayer spacing between theadjacent graphite layers) decreased with increasing tubediameter, which may result primarily from the low curvatureand associated strain in the higher diameter nanotubes.38,39

Higher curvature could increase attraction energies betweensurfaces and adsorbed molecules40 and thus might increaseNOM adsorption on lower diameter MWCNTs. FTIRmeasurements were performed to verify the functional groupson MWCNTs and PAC (Figure S4, Supporting Information).It was clear that all MWCNTs displayed no significant bands,but PAC showed some apparent bands as MWCNTs and PACwere fundamentally different. MWCNTs are composed ofglobally conjugated unsaturated carbons in three-dimensionalarrays, while PAC contains carbons of varying saturation degreeand oxidation state as well as functional groups formed duringthe activation process.41

Adsorption Isotherms. Isotherm data for TA and GAadsorption on the five adsorbents are shown in Figure 3. It isapparent that all adsorption isotherms were nonlinear when theqe vs Ce were plotted on linear coordinates. Therefore, threecommonly used nonlinear adsorption isotherm models, LM,

FM, and PMM, were employed to fit the experimental datawith Sigmaplot 12.0. Adsorption parameters, their probabilities(p), and adjusted square of correction coefficients (R2

adj) forGA and TA adsorption are given in Tables S1 and S2,Supporting Information. Statistical significance was acceptedwhen p was less than 0.01.For GA adsorption isotherms, it appears that the two-

parameter FM had better fit to the experimental data than LMand PMM (Figure S5, Supporting Information) with high R2

adjand low p (<0.01). The lowest R2

adj and p of the fittingparameter b for MWCNTs 40−60 was larger than 0.01,suggesting that LM was not an appropriate model to describeGA adsorption. PMM also failed to fit the adsorption data withalmost all of the fitting parameters significantly unreliable (p ≫0.01).For TA adsorption, lowest R2

adj excluded FM as anappropriate model for the adsorption isotherm data (FigureS6, Supporting Information). Although PMM seemed to fit TAadsorption well with high R2

adj (>0.99), the PMM fittingparameter Z for MWCNTs 60−100 was significantly unreliablewith p ≫ 0.01. LM fitted TA adsorption on the five adsorbentsquite well with high R2

adj and p < 0.01. Hence, the followingdiscussion is based on the adsorption parameters calculatedfrom FM and LM fitting results for GA and TA adsorption,respectively.

Adsorption Sites Analysis. Figure 3a displays FM-fittedGA adsorption isotherms onto the five adsorbents on the unitmass basis. GA adsorption was markedly stronger on PAC thanthat on MWCNTs, and its surface area normalized adsorptioncapacity on PAC was also larger than that on MWCNTs at agiven equilibrium concentration (Figure S7, SupportingInformation). The adsorption distribution coefficients (Kd, L

Figure 3. Adsorption isotherms of GA (a) and TA (b) on PAC andMWCNTs. Dashed lines and solid lines are the fitting curves of FMand LM, respectively.

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g−1) were calculated from the equilibrium adsorption data andare shown in Figure S8, Supporting Information. It can be seenthat the value of Kd followed the same order as the adsorptioncapacity within the tested adsorption concentrations range.Previous studies proposed that microporous adsorbents such asactivated carbon and charcoal have higher adsorption affinity tolow molecular compounds (naphthalene, benzene, andtoluene) due to the micropore filling effect resulting from thecloseness of the molecular size of the adsorbate and the poresize of adsorbent.42,43 In the current study, PAC has the highestmicropores volume (0.049 cm3 g−1) and the almost lowestaverage pore diameter (1.178 nm). Moreover, the moleculardiameter of GA was approximate 0.57 nm calculated byGaussian software. Therefore, micropore filling was the maincontributor for the largest GA adsorption affinity on PAC.However, due to large molecular weight and thus high sterichindrance of TA, it was very difficult for TA molecules to enterthe micropores in PAC and MWCNTs.Figure 3b presents the adsorption isotherms of TA on PAC

and MWCNTs fitted by the LM on the unit mass basis.Compared with GA, TA exhibited apparently differentadsorption patterns and the adsorption affinity followed anorder of MWCNTs > PAC. Besides, the surface-normalizedadsorption isotherms and the trends of Kd were all in the sameorder of MWCNTs > PAC (Figures S7 and S8, SupportingInformation). The difference of TA adsorption on MWCNTsand PAC was probably caused by the molecular sieving effect.TA is a bulky molecule with an average diameter of 1.6 nm.Pelekani et al. reported that adsorption happened in poreswhich were larger than 1.7 times of the adsorbate molecule’ssecond widest dimension.44 Therefore, TA cannot access poressmaller than 2.72 nm. Average pore width of MWCNTs exceptMWCNTs 20−40 exceeded 2.72 nm, and thus, TA adsorptionon MWCNTs was not affected by the sieving effect. UnlikeMWCNTs, PAC had rigid pore structures and a relativelysmaller pore width. Accordingly, PAC exhibited a more severemolecule sieving effect than MWCNTs when encounteringbulky adsorbates. It is worth noting that although the porewidth of MWCNTs 20−40 was the lowest and closest to that ofPAC, TA adsorption on MWCNTs 20−40 was significantlyhigher than that on PAC. MWCNTs existed as bundles due toaggregation of individual nanotubes and formed flexible pores;however, the aggregation behavior of MWCNTs may changewhen introduced in water due to the presence of a minoramount of functional groups. Surface functionalization with O-containing functional groups had a positive effect on the waterdispersibility of MWCNTs,45 which likely loosened theMWCNTs aggregates and further enlarged the average porewidth accessible for TA.Adsorption isotherms of GA and TA on the four types of

MWCNTs are also shown in Figure 3. It was apparent that GAadsorption increased in the order of MWCNTs 10−20 >MWCNTs 20−40 > MWCNTs 40−60 > MWCNTs 60−100.In addition, adsorption of TA by MWCNTs also decreasedwith increasing MWCNT diameter. Su et al. studied theadsorption of NOM on MWCNTs and attributed the highadsorption capacity to the large pore volume of MWCNTs.46

Yang et al. used fulvic acid (FA) as NOM to study the effect ofMWCNTs characteristics on FA adsorption and observed thatFA adsorption capacity on MWCNTs depended greatly on thesurface area of MWCNTs, thus concluding that surface areawas a major factor in the adsorption process.21 From Table 2,total pore volume of MWCNTs followed the order MWCNTs

20−40 > MWCNTs 40−60 > MWCNTs 10−20 > MWCNTs60−100 and BET surface area decreased with the increase ofMWCNTs diameter except MWCNTs 40−60. Both of theabove orders were different with the order of GA and TAadsorption on the four types of MWCNTs in the current study.Therefore, neither MWCNTs surface area nor the total porevolume alone can fully explain the GA and TA adsorption onMWCNTs.E* can be calculated according to eq 6, and variation of E* as

a function of qe is shown in Figure 4. E* decreased with

increasing qe for both of GA and TA on the five adsorbents butexhibited different patterns. For GA adsorption, E* followedthe order PAC > MWCNTs 10−20 > MWCNTs 20−40 >MWCNTs 40−60 > MWCNTs 60−100 at a certain qe (Figure4a), which was in agreement with the order of GA adsorptioncapacity on PAC and MWCNTs. E* decreased sharply at firstbut trended slowly with increasing qe, which verified theheterogeneous sites for GA adsorption on PAC andMWCNTs.47 For TA, E* was also in accordance with theorder of TA adsorption capacity. However, different from GA,E* decreased slowly at first but then sharply with increasing qe.Figure 5 displays the site energy distribution of GA and TA

on the five adsorbents. F(E*) decreased for GA with increasingE* (Figure 5a), and the values of F(E*) for PAC were abovethose for the four MWCNTs, reflecting more sites on PAC forGA adsorption. F(E*) curves for TA adsorption are illustratedin Figure 5b. It can be seen that MWCNTs and PAC hadsimilar distribution shapes but different peaks. All of the F(E*)values for MWCNTs were higher than that for PAC, suggestingthe presence of more sites on MWCNTs for TA adsorption.F(E*) values for the four types of MWCNTs decreased withincreasing MWCNT diameter, which was consistent with thedecrease of TA adsorption capacity. It is worth noting that

Figure 4. Variation of E* as a function of qe for GA (a) and TA (b).

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F(E*) curves for MWCNTs 40−60 and MWCNTs 60−100crossed with each other, which also can be seen from almostoverlapping of TA adsorption on MWCNTs 40−60 andMWCNTs 60−100 (Figure 3b).Adsorption Mechanisms. The adsorption behaviors of

GA and TA on the five adsorbents were different, suggestingvarious mechanisms occurred in the adsorption process. On thebasis of the literature, several possible mechanisms should beconsidered to explain the adsorptive interactions between GA/TA and MWCNTs/PAC including electrostatic interaction,hydrophobic interaction, hydrogen bond, and π−π electron-donor−acceptor (EDA) interaction.Electrostatic interaction has been widely considered one of

the major factors controlling adsorption of aromatic chemicals,especially ionic compounds on carbon adsorbents.48,49 GA andTA have more than one pKa (Table 1), and both of them canbe positively charged, negatively charged, or zwitterionic due tothe variation of solution pH. PAC used in this study has a largeamount of various functional groups such as −OH (3400cm−1), −COOH (1700 cm−1), and CO (1400 cm−1), and itssurface can be protonated or deprotonated at different pHvalues. Therefore, electrostatic attraction or repulsion betweenGA/TA and PAC was likely to occur in the adsorption process.However, this mechanism was not responsible for adsorption ofGA/TA on MWCNTs. As shown in Figure S4, SupportingInformation, the as-grown MWCNTs were almost free of O-containing functional groups without any chemical modifica-tion, and hence, the surface of MWCNTs cannot be charged.Both MWCNTs and PAC are heterogeneous, being

composed of hydrophobic regions due to their bare carbonbackbone. Thus, hydrophobic interaction should be consideredto understand the adsorption of some organic chemicals oncarbon adsorbents.50−52 In the current investigation, MWCNTswere completely hydrophobic with minor functional groups on

the surface while the surface of PAC was more hydrophilic withO-containing functional groups. Therefore, it is believed thathydrophobic interaction was more important for GA and TAadsorption on MWCNTs than that on PAC. Hydrophobicinteraction can be evaluated by the octanol−water distributioncoefficient (Kow) of organic chemicals, and stronger hydro-phobic interaction results from higher Kow. GA is morehydrophobic with lower solubility and higher Kow (Table 1),but its adsorption affinity on MWCNTs was obviously lowerthan TA. From the above, hydrophobic interaction was not akey mechanism for adsorption of GA and TA on the fiveadsorbents.A hydrogen bond (H bond) has been proposed as a

mechanism for interpreting adsorption of chemicals oncarbonaceous materials.53,54 A H bond was formed betweenthe adsorbate −OH groups and the adsorbent O-containinggroups. Similarly, GA and TA have a large amount of −OHgroups and thus can form H bonds with the O-containingfunctional groups of PAC. However, this mechanism might notbe important in GA and TA adsorption on MWCNTs becausethe amount of oxygen on MWCNTs was very low. Though thecontribution of surface groups on MWCNTs to the H bondwas negligent, the aromatic rings on the MWCNT surface canact as a H-bond donor and form a H bond with −OH on GAand TA molecules.55 Therefore, the H bond was one of themechanisms for GA and TA adsorption on the five adsorbents,although the source of H-bond donors was different for PACand MWCNTs.π−π EDA interaction was a specific, noncovalent force of

attraction between π-donor and π-acceptor molecules. Thisinteraction was usually regarded as one of the most importantdriving forces for adsorption of chemicals with aromatic ringson graphene structures.43,56,57 PAC is an organic semi-conductor with delocalized π electrons on its surfaces, thusshowing electron−donor properties.58 As for MWCNTs, eachcarbon atom has a π-electron orbit perpendicular to theirsurface,59 so they can be viewed as either electrodonors orelectroacceptors. Previous studies have revealed the role of π−πEDA interaction in the adsorption of phenols and someantibiotics on PAC and MWCNTs.42,53 GA and TA have alarge amount of −OH, which makes aromatic rings on both ofthem electrondonors due to the electron-donor ability of −OH.Therefore, π−π EDA interaction can be responsible for thestrong adsorption between GA/TA on PAC and MWCNTs.

■ CONCLUSIONSIt is very important to understand the interaction betweenNOM and CNTs as NOM has a severe effect on the fate andpossible risks of CNTs to the environment. Evaluating theadsorption of NOM on CNTs was the first essential step forfurther assessing the potential environmental behavior ofCNTs. In this study, TA and GA were employed to investigatethe adsorption of NOM on MWCNTs. The results suggestedthat MWCNTs has a higher adsorption capacity for largermolecular weight NOM, and several mechanisms actsimultaneously in the adsorption process. Further studies areneeded to focus on the fate of NOM-coated CNTs to promotedevelopment and application of carbon nanomaterials.

■ ASSOCIATED CONTENT*S Supporting InformationTables of nonlinear fits of adsorption isotherms of GA onMWCNTs and PAC and nonlinear fits of adsorption isotherms

Figure 5. Adsorption site energy distribution curves of GA (a) and TA(b) on the five adsorbents.

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of TA on MWCNTs and PAC; figures of nitrogen adsorptionisotherms of MWCNTs and PAC, distribution of adsorptionsites on MWCNTs bundles, X-ray diffraction (XRD) patternsfor MWCNTs and PAC, FTIR spectrum of MWCNTs andPAC, LM and PMM isotherms of GA on MWCNTs and PAC,FM and PMM isotherms of TA on MWCNTs and PAC,adsorption isotherms of GA and TA on unit surface basis, andKd curves of GA and TA adsorption on PAC and MWCNTs.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Phone: +86 531 88362802. Fax: +86 531 88364513. E-mail:wsg@sdu.edu.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was supported by the Natural ScienceFoundation of Shandong Province (2009ZRB01618).

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The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp307065e | J. Phys. Chem. C 2012, 116, 25783−2578925789