Adsorption of Natural Organic Matter Surrogates from Aqueous

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Adsorption of Natural Organic Matter Surrogates from Aqueous Solution by Multiwalled Carbon Nanotubes Fei-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 and Engineering, 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) with different diameters was investigated in aqueous solution. The tannic acid (TA) adsorption isotherm fit the Langmuir model best, while adsorption of gallic acid (GA) fit the Freundlich model. PAC has a higher adsorption capacity for GA due to the pore-filling effect, while TA adsorption on PAC was lower than that on MWCNTs because of the molecule sieving effect. Adsorption of GA and TA on MWCNTs decreased with increasing MWCNT diameter, but neither surface area nor total pore volume alone can fully explain adsorption of the two NOM surrogates on MWCNTs. Adsorption mechanisms including electrostatic interaction, hydrophobic interaction, hydrogen bond, and π−π interaction play roles in the adsorption process. This study implies that NOM properties have a great effect on their adsorption on MWCNTs, and these properties should be further considered when evaluating the risks of NOM-coated nanomaterials.

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INTRODUCTION Since their discovery by Iijima in 1991,1 carbon nanotubes (CNTs) have been at the center of nanoscience and nanotechnology research for a variety of applications such as adsorbents, composite filters, high-flux membranes, antimicrobial agents, environmental sensors, energy storage devices, and pollution prevention reagents due to their unique and outstanding chemical, electronic, and mechanical properties.2−6 Production and commercial use of CNTs are now of significant quantities, and hundreds of tons of CNTs, especially multiwalled carbon nanotubes (MWCNTs), are produced each year.7 As CNTs will inevitably be released into the environment accidentally or intentionally in the process of manufacturing and applications,8 there is serious concern over their possible toxicity and the associated risk to the environment.9−13 Natural organic matter (NOM), a complex and heterogeneous mixture of polyelectrolyte with diverse molecular weights, derives mainly from the decay of plant and animal residues.14 NOM is ubiquitously present in surface waters at dissolved organic carbon (DOC) concentrations typically ranging from 1 to 20 mg/L.15 Therefore, it is unavoidable that CNTs will contact with NOM once released into an aquatic environment. Thus far, several impacts of NOM on CNTs have been reported in previous research articles. First, NOM enhanced the stability of CNTs suspension by producing thermodynamically favorable surfaces and inducing electrostatic and steric repulsion between individual CNTs to overcome their strong hydrophobicity16,17 and further affect the © 2012 American Chemical Society

persistence behavior of CNTs in water. Second, NOM coating altered the adsorption capacity of CNTs toward other contaminants. Zhang et al. reported that the presence of NOM suppressed the adsorption of synthetic organic chemicals on CNTs resulting from the competition for limited adsorption sites and physical pore blockage effect of NOM.18 However, adsorption of heavy metals on NOM-coated CNTs could be enhanced because of the complexation between metal ions and functional groups on NOM.19 Third, NOM can influence the toxicity and bioavailability of CNTs and other pollutants. Kim et al. found that NOM-stabilized CNTs have a low acute ecotoxicity to Daphnia magna, while Cu toxicity increased with increasing concentration of NOM-associated CNTs.11 Thus, understanding the interaction between CNTs and NOM, especially adsorption of NOM on CNTs, is the first and essential 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 studies was extracted from river sediment or peat soil.21−28 However, NOM has an indeterminate molecular structure and molecular weight and the composition of NOM from different regions may differ, which is not favorable for determining the adsorption mechanism accurately. Research with appropriate small organic molecules as surrogates has provided valuable Received: July 17, 2012 Revised: November 6, 2012 Published: November 15, 2012 25783

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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

2.5 × 105

30.32

272

−1.19

gallic acid

C7H6O5

4.9 7.4 4.3 8.7 11.4

1.2 × 104

39.94

256

0.70

170.12

insight into the behavior of NOM in adsorption systems.19,29 As a substitute for NOM, tannic acid (TA) is an anionic and hydrophilic organic substance with relatively high molecular weight, which approximates to the mean value of NOM in surface waters.30 Another simple possible model for NOM is gallic acid (GA).31 The molecular weight of TA is exactly 10 times that of GA, and the solubility of the former in water is 20 times higher than the latter, so these two chemicals could represent 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 on different MWCNTs and determine the adsorption mechanisms. In addition, adsorption of NOM on MWCNTs was also compared with that on powdered activated carbon used in most commercial water treatment plants. We believe the results will shed light on the adsorption applications and environmental fate assessment of carbon nanomaterials.

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EXPERIMENTAL METHODS Materials. TA and GA obtained from Tianjin Kemel Chemical Reagent Co., Ltd. (Tianjin, China) were of analytical grade and used without further purification. The physical and chemical properties of the two compounds are summarized in Table 1 and their chemical structures are presented in Figure 1. The carbonaceous adsorbents used in this study were multiwalled carbon nanotubes (MWCNTs) and powdered activated carbon (PAC). MWCNTs were purchased from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China) with different outer diameters. All MWCNTs were synthesized by chemical vapor deposition using the mixtures of CH4 and H2 at 700 °C with Ni as a catalyst. PAC was obtained from Tianjin Guangcheng Chemical Reagent Co., Ltd. (Tianjin, China). MWCNTs and PAC were used without any treatment to accurately replicate their occurrence in a commercial water treatment system due to their application or in natural waters resulting 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 a Quadrasorb SI-MP system (Quantachrome, U.S.). All samples were degassed at 373 K for 8 h in a vacuum before measurements. The Brunauer−Emmett−Teller (BET) equation and density functional theory (DFT) were used to calculate their BET surface area and pore size distribution. Transmission electron microscopy (TEM) images of MWCNTs and PAC were observed with a microscope (JEM100CXII, Japan). Samples were prepared by dispersing MWCNTs or PAC (95 >95 >95 >95

1−2 1−2 1−2 1−2

10−20 20−40 40−60 60−100

79.73 70.91 83.34 64.14 162.24

0.153 0.195 0.169 0.143 0.058

0.003 0.008 0.003 0.008 0.049

0.156 0.203 0.172 0.151 0.107

2.897 1.140 3.169 3.794 1.178

approximate energy distribution functions.32 For Freundlich isotherm, the approximate site energy distribution is described with the following function

are summarized in Table 1. Calibration curves were obtained using the standard solution with known concentrations. Blank samples were carried out for each experiment and did not indicate 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. Langmuir model (LM): qe =

F(E*) =

Freundlich model (FM): qe = KFCe

n

(4)

For the Langmuir isotherm, the site energy distribution is calculated through the following equation

qmKLCe 1 + KLCe

⎛ nE* ⎞ KFn(Cs)n ⎟ × exp⎜ − ⎝ RT ⎠ RT

(1)

F(E*) = (2)

qmKLCs RT

⎛ nE* ⎞⎡ ⎛ −nE* ⎞⎤ ⎟ 1 + K C exp⎜ ⎟ exp⎜ − L s ⎝ RT ⎠⎢⎣ ⎝ RT ⎠⎥⎦

−2

(5)

Polanyi−Manes model (PMM): qe d⎤ ⎡ ⎛ Cs ⎞ ⎥ 0 ⎢ = q exp Z ⎜RT ln ⎟ ⎢⎣ ⎝ Ce ⎠ ⎥⎦

33

According to the Polanyi adsorption potential theory, the energy of adsorption is related to the equilibrium liquid-phase concentration by ⎛ E* ⎞ ⎟ Ce = Cs exp⎜ − ⎝ RT ⎠

(3)

−1

where qe (mg g ) is the solid-phase concentration, Ce (mg L−1) is the solution concentration, qm and q0 are the adsorption capacity for LM and PMM, KL (L mg−1) is the Langmuir adsorption affinity parameter. KF and n are the Freundlich adsorption constants. Cs (mg L−1) represents the water solubility of adsorbate at 20 °C, Z and d are the PMM fitting parameters. R is the universal gas constant (8.314 × 10−3 kJ mol−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

(6)

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

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RESULTS AND DISCUSSION Characterization of Adsorbents. TEM images of the adsorbents are shown in Figure 2. As can be seen, a handful of metal catalyst and amorphous carbon existed within the pristine MWCNTs and the outer diameters were almost consistent with the data provided by the supplier. PAC was composed by several graphite sheets.

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N2 adsorption isotherms of MWCNTs were of identical shape and can be divided into four sections.34 The first section appeared at ultralow pressures, and adsorption increased rapidly due to the micropore filling. Adsorption of the next section increased slowly and linearly due to surface monolayer formation. In addition, the last two sections of N2 adsorption were the hysteresis loops resulting from capillary condensation in the medium and high pressures, respectively. However, the N2 adsorption isotherm for PAC exhibited a different pattern (Figure S1, Supporting Information). From BET calculation of N2 isotherms, MWCNTs surface area was in the range of 64.14−83.34 m2 g−1, which was lower than the previously reported data due to the presence of amorphous carbon and metal catalyst.35,36 Among the four types of MWCNTs, the surface area decreased with increasing tube diameter except MWCNTs 40−60 (Table 2). The surface area of PAC was 162.24 m2 g−1, which was higher than any of the MWCNTs used 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 possess four possible adsorption sites including the external surface of the outmost MWCNTs, grooves of the MWCNTs bundles, interstitial spaces between tubes, and inner pores of the opened tubes.37 Because of the presence of amorphous carbon and metal catalyst in pristine MWCNTs, inner pores of MWCNTs are not available for adsorption. Therefore, the surface area, flexible interconnected grooves region, and interstitial space are the main adsorption sites in this study (Figure S2, Supporting Information). Different from MWCNTs, the pore structure of PAC was always fixed and closed. Clearly, MWCNTs are mesopores adsorbent, and the fraction of mesopores was about 94−98%, whereas PAC possessed both mesopores and micropores with an approximate ratio of 1.2:1 (Table 2). For MWCNTs, the pore volumes decreased with increasing outer diameter; however, the pore volume of MWCNTs 10−20 was the least. Figure S3, Supporting Information, shows the XRD patterns of MWCNTs and PAC. All of the adsorbents had a sharp diffraction peak at approximate 2θ = 26° corresponding to the (002) reflection planes, which indicated that both MWCNTs and PAC were composed by graphite sheets. It should be noted that 2θ increased but d002 (the interlayer spacing between the adjacent graphite layers) decreased with increasing tube diameter, which may result primarily from the low curvature and associated strain in the higher diameter nanotubes.38,39 Higher curvature could increase attraction energies between surfaces and adsorbed molecules40 and thus might increase NOM adsorption on lower diameter MWCNTs. FTIR measurements were performed to verify the functional groups on 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 PAC were fundamentally different. MWCNTs are composed of globally conjugated unsaturated carbons in three-dimensional arrays, while PAC contains carbons of varying saturation degree and oxidation state as well as functional groups formed during the activation process.41 Adsorption Isotherms. Isotherm data for TA and GA adsorption on the five adsorbents are shown in Figure 3. It is apparent that all adsorption isotherms were nonlinear when the qe vs Ce were plotted on linear coordinates. Therefore, three commonly used nonlinear adsorption isotherm models, LM,

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

FM, and PMM, were employed to fit the experimental data with Sigmaplot 12.0. Adsorption parameters, their probabilities (p), and adjusted square of correction coefficients (R2adj) for GA and TA adsorption are given in Tables S1 and S2, Supporting Information. Statistical significance was accepted when p was less than 0.01. For GA adsorption isotherms, it appears that the twoparameter FM had better fit to the experimental data than LM and PMM (Figure S5, Supporting Information) with high R2adj and low p (0.99), the PMM fitting parameter Z for MWCNTs 60−100 was significantly unreliable with p ≫ 0.01. LM fitted TA adsorption on the five adsorbents quite well with high R2adj and p < 0.01. Hence, the following discussion is based on the adsorption parameters calculated from FM and LM fitting results for GA and TA adsorption, respectively. Adsorption Sites Analysis. Figure 3a displays FM-fitted GA adsorption isotherms onto the five adsorbents on the unit mass basis. GA adsorption was markedly stronger on PAC than that on MWCNTs, and its surface area normalized adsorption capacity on PAC was also larger than that on MWCNTs at a given equilibrium concentration (Figure S7, Supporting Information). The adsorption distribution coefficients (Kd, L 25786

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g−1) were calculated from the equilibrium adsorption data and are shown in Figure S8, Supporting Information. It can be seen that the value of Kd followed the same order as the adsorption capacity within the tested adsorption concentrations range. Previous studies proposed that microporous adsorbents such as activated carbon and charcoal have higher adsorption affinity to low molecular compounds (naphthalene, benzene, and toluene) due to the micropore filling effect resulting from the closeness of the molecular size of the adsorbate and the pore size of adsorbent.42,43 In the current study, PAC has the highest micropores volume (0.049 cm3 g−1) and the almost lowest average pore diameter (1.178 nm). Moreover, the molecular diameter of GA was approximate 0.57 nm calculated by Gaussian software. Therefore, micropore filling was the main contributor for the largest GA adsorption affinity on PAC. However, due to large molecular weight and thus high steric hindrance of TA, it was very difficult for TA molecules to enter the 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 different adsorption patterns and the adsorption affinity followed an order of MWCNTs > PAC. Besides, the surface-normalized adsorption isotherms and the trends of Kd were all in the same order of MWCNTs > PAC (Figures S7 and S8, Supporting Information). The difference of TA adsorption on MWCNTs and 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 pores which were larger than 1.7 times of the adsorbate molecule’s second widest dimension.44 Therefore, TA cannot access pores smaller than 2.72 nm. Average pore width of MWCNTs except MWCNTs 20−40 exceeded 2.72 nm, and thus, TA adsorption on MWCNTs was not affected by the sieving effect. Unlike MWCNTs, PAC had rigid pore structures and a relatively smaller pore width. Accordingly, PAC exhibited a more severe molecule sieving effect than MWCNTs when encountering bulky adsorbates. It is worth noting that although the pore width of MWCNTs 20−40 was the lowest and closest to that of PAC, TA adsorption on MWCNTs 20−40 was significantly higher than that on PAC. MWCNTs existed as bundles due to aggregation of individual nanotubes and formed flexible pores; however, the aggregation behavior of MWCNTs may change when introduced in water due to the presence of a minor amount of functional groups. Surface functionalization with Ocontaining functional groups had a positive effect on the water dispersibility of MWCNTs,45 which likely loosened the MWCNTs aggregates and further enlarged the average pore width 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 GA adsorption 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 decreased with increasing MWCNT diameter. Su et al. studied the adsorption of NOM on MWCNTs and attributed the high adsorption capacity to the large pore volume of MWCNTs.46 Yang et al. used fulvic acid (FA) as NOM to study the effect of MWCNTs characteristics on FA adsorption and observed that FA adsorption capacity on MWCNTs depended greatly on the surface area of MWCNTs, thus concluding that surface area was 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 > MWCNTs 60−100 and BET surface area decreased with the increase of MWCNTs diameter except MWCNTs 40−60. Both of the above orders were different with the order of GA and TA adsorption on the four types of MWCNTs in the current study. Therefore, neither MWCNTs surface area nor the total pore volume alone can fully explain the GA and TA adsorption on MWCNTs. 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

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

increasing qe for both of GA and TA on the five adsorbents but exhibited different patterns. For GA adsorption, E* followed the order PAC > MWCNTs 10−20 > MWCNTs 20−40 > MWCNTs 40−60 > MWCNTs 60−100 at a certain qe (Figure 4a), which was in agreement with the order of GA adsorption capacity on PAC and MWCNTs. E* decreased sharply at first but trended slowly with increasing qe, which verified the heterogeneous sites for GA adsorption on PAC and MWCNTs.47 For TA, E* was also in accordance with the order 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 increasing E* (Figure 5a), and the values of F(E*) for PAC were above those for the four MWCNTs, reflecting more sites on PAC for GA adsorption. F(E*) curves for TA adsorption are illustrated in Figure 5b. It can be seen that MWCNTs and PAC had similar distribution shapes but different peaks. All of the F(E*) values for MWCNTs were higher than that for PAC, suggesting the presence of more sites on MWCNTs for TA adsorption. F(E*) values for the four types of MWCNTs decreased with increasing MWCNT diameter, which was consistent with the decrease of TA adsorption capacity. It is worth noting that 25787

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the surface while the surface of PAC was more hydrophilic with O-containing functional groups. Therefore, it is believed that hydrophobic interaction was more important for GA and TA adsorption on MWCNTs than that on PAC. Hydrophobic interaction can be evaluated by the octanol−water distribution coefficient (Kow) of organic chemicals, and stronger hydrophobic interaction results from higher Kow. GA is more hydrophobic with lower solubility and higher Kow (Table 1), but its adsorption affinity on MWCNTs was obviously lower than TA. From the above, hydrophobic interaction was not a key mechanism for adsorption of GA and TA on the five adsorbents. A hydrogen bond (H bond) has been proposed as a mechanism for interpreting adsorption of chemicals on carbonaceous materials.53,54 A H bond was formed between the adsorbate −OH groups and the adsorbent O-containing groups. Similarly, GA and TA have a large amount of −OH groups and thus can form H bonds with the O-containing functional groups of PAC. However, this mechanism might not be important in GA and TA adsorption on MWCNTs because the amount of oxygen on MWCNTs was very low. Though the contribution of surface groups on MWCNTs to the H bond was negligent, the aromatic rings on the MWCNT surface can act as a H-bond donor and form a H bond with −OH on GA and TA molecules.55 Therefore, the H bond was one of the mechanisms for GA and TA adsorption on the five adsorbents, although the source of H-bond donors was different for PAC and MWCNTs. π−π EDA interaction was a specific, noncovalent force of attraction between π-donor and π-acceptor molecules. This interaction was usually regarded as one of the most important driving forces for adsorption of chemicals with aromatic rings on graphene structures.43,56,57 PAC is an organic semiconductor with delocalized π electrons on its surfaces, thus showing electron−donor properties.58 As for MWCNTs, each carbon atom has a π-electron orbit perpendicular to their surface,59 so they can be viewed as either electrodonors or electroacceptors. Previous studies have revealed the role of π−π EDA interaction in the adsorption of phenols and some antibiotics on PAC and MWCNTs.42,53 GA and TA have a large amount of −OH, which makes aromatic rings on both of them electrondonors due to the electron-donor ability of −OH. Therefore, π−π EDA interaction can be responsible for the strong adsorption between GA/TA on PAC and MWCNTs.

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

F(E*) curves for MWCNTs 40−60 and MWCNTs 60−100 crossed with each other, which also can be seen from almost overlapping of TA adsorption on MWCNTs 40−60 and MWCNTs 60−100 (Figure 3b). Adsorption Mechanisms. The adsorption behaviors of GA and TA on the five adsorbents were different, suggesting various mechanisms occurred in the adsorption process. On the basis of the literature, several possible mechanisms should be considered to explain the adsorptive interactions between GA/ TA and MWCNTs/PAC including electrostatic interaction, hydrophobic interaction, hydrogen bond, and π−π electrondonor−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 and TA have more than one pKa (Table 1), and both of them can be positively charged, negatively charged, or zwitterionic due to the variation of solution pH. PAC used in this study has a large amount of various functional groups such as −OH (3400 cm−1), −COOH (1700 cm−1), and CO (1400 cm−1), and its surface can be protonated or deprotonated at different pH values. Therefore, electrostatic attraction or repulsion between GA/TA and PAC was likely to occur in the adsorption process. However, this mechanism was not responsible for adsorption of GA/TA on MWCNTs. As shown in Figure S4, Supporting Information, the as-grown MWCNTs were almost free of Ocontaining functional groups without any chemical modification, and hence, the surface of MWCNTs cannot be charged. Both MWCNTs and PAC are heterogeneous, being composed of hydrophobic regions due to their bare carbon backbone. Thus, hydrophobic interaction should be considered to understand the adsorption of some organic chemicals on carbon adsorbents.50−52 In the current investigation, MWCNTs were completely hydrophobic with minor functional groups on

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CONCLUSIONS It is very important to understand the interaction between NOM and CNTs as NOM has a severe effect on the fate and possible risks of CNTs to the environment. Evaluating the adsorption of NOM on CNTs was the first essential step for further assessing the potential environmental behavior of CNTs. In this study, TA and GA were employed to investigate the adsorption of NOM on MWCNTs. The results suggested that MWCNTs has a higher adsorption capacity for larger molecular weight NOM, and several mechanisms act simultaneously in the adsorption process. Further studies are needed to focus on the fate of NOM-coated CNTs to promote development and application of carbon nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

Tables of nonlinear fits of adsorption isotherms of GA on MWCNTs and PAC and nonlinear fits of adsorption isotherms 25788

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of TA on MWCNTs and PAC; figures of nitrogen adsorption isotherms of MWCNTs and PAC, distribution of adsorption sites on MWCNTs bundles, X-ray diffraction (XRD) patterns for MWCNTs and PAC, FTIR spectrum of MWCNTs and PAC, 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, and Kd curves of GA and TA adsorption on PAC and MWCNTs. This material is available free of charge via the Internet at http://pubs.acs.org.

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(23) Schwyzer, I.; Kaegi, R.; Sigg, L.; Magrez, A.; Nowack, B. Environ. Pollut. 2011, 159, 1641−1648. (24) Ghosh, S.; Mashayekhi, H.; Pan, B.; Bhowmik, P.; Xing, B. Langmuir 2008, 24, 12385−12391. (25) Yang, K.; Lin, D.; Xing, B. Langmuir 2009, 25, 3571−3576. (26) Yang, K.; Xing, B. Environ. Sci. Technol. 2009, 43, 1845−1851. (27) Saleh, N. B.; Pfefferle, L. D.; Elimelech, M. Environ. Sci. Technol. 2008, 42, 7963−7969. (28) Zhang, L.; Petersen, E. J.; Huang, Q. Environ. Sci. Technol. 2011, 45, 1356−1362. (29) Floroiu, R. M.; Davis, A. P.; Torrents, A. Environ. Sci. Technol. 2000, 35, 348−353. (30) Newcombe, G.; Drikas, M.; Assemi, S.; Beckett, R. Water. Res. 1997, 31, 965−972. (31) Araujo, P. Z.; Morando, P. J.; Blesa, M. A. Langmuir 2005, 21, 3470−3474. (32) Carter, M. C.; Kilduff, J. E.; Weber, W. J. Environ. Sci. Technol. 1995, 29, 1773−1780. (33) Manes, M.; Hofer, L. J. E. J. Phys. Chem. 1969, 73, 584−590. (34) Yang, Q.-H.; Hou, P.-X.; Bai, S.; Wang, M.-Z.; Cheng, H.-M. Chem. Phys. Lett. 2001, 345, 18−24. (35) Fang, Q.; Chen, B. Carbon 2012, 50, 2209−2219. (36) Chen, W.; Duan, L.; Zhu, D. Environ. Sci. Technol. 2007, 41, 8295−8300. (37) Zhao, J.; Buldum, A.; Han, J.; Lu, J. P. Nanotechnology 2002, 13, 195. (38) Singh, D. K.; Iyer, P. K.; Giri, P. K. Diamond Relat. Mater. 2010, 19, 1281−1288. (39) Li, Z. Q.; Lu, C. J.; Xia, Z. P.; Zhou, Y.; Luo, Z. Carbon 2007, 45, 1686−1695. (40) Hilding, J.; Grulke, E. A.; Sinnott, S. B.; Qian, D.; Andrews, R.; Jagtoyen, M. Langmuir 2001, 17, 7540−7544. (41) Sontheimer, H.; Crittenden, J. C.; Summers, R. S. Activated carbon for water treatment; DVGW-Forschungsstelle, Engler-BunteInstitut, Universitat Karlsruhe (TH): Karlsruhe, Germany, 1988 (42) Ji, L.; Chen, W.; Duan, L.; Zhu, D. Environ. Sci. Technol. 2009, 43, 2322−2327. (43) Zhu, D.; Pignatello, J. J. Environ. Sci. Technol. 2005, 39, 2033− 2041. (44) Kasaoka, S.; Sakata, Y.; Tanaka, E.; Naitoh, R. Int. Chem. Eng. 1989, 29, 734−742. (45) Zhang, S.; Shao, T.; Bekaroglu, S. S. K.; Karanfil, T. Environ. Sci. Technol. 2009, 43, 5719−5725. (46) Lu, C.; Su, F. Sep. Purif. Technol. 2007, 58, 113−121. (47) Yuan, G.; Xing, B. Soil. Sci. 1999, 164, 503−509. (48) Newcombe, G.; Drikas, M. Carbon 1997, 35, 1239−1250. (49) Machado, F. M.; Bergmann, C. P.; Fernandes, T. H. M.; Lima, E. C.; Royer, B.; Calvete, T.; Fagan, S. B. J. Hazard. Mater. 2011, 192, 1122−1131. (50) Méndez-Díaz, J. D.; Prados-Joya, G.; Rivera-Utrilla, J.; LeyvaRamos, R.; Sánchez-Polo, M.; Ferro-García, M. A.; Medellín-Castillo, N. A. J. Colloid Interface Sci. 2010, 345, 481−490. (51) Soria-Sánchez, M.; Maroto-Valiente, A.; Guerrero-Ruiz, A.; Nevskaia, D. M. J. Colloid Interface Sci. 2010, 343, 194−199. (52) Li, X.; Zhao, H.; Quan, X.; Chen, S.; Zhang, Y.; Yu, H. J. Hazard. Mater. 2011, 186, 407−415. (53) Lin, D.; Xing, B. Environ. Sci. Technol. 2008, 42, 7254−7259. (54) Ahnert, F.; Arafat, H. A.; Pinto, N. G. Adsorption 2003, 9, 311− 319. (55) Hickey, J. P.; Passino-Reader, D. R. Environ. Sci. Technol. 1991, 25, 1753−1760. (56) Koh, B.; Kim, G.; Yoon, H. K.; Park, J. B.; Kopelman, R.; Cheng, W. Langmuir 2012, 28, 11676−11686. (57) Wang, X.; Liu, Y.; Tao, S.; Xing, B. Carbon 2010, 48, 3721− 3728. (58) Lavrinenko-Ometsinskaya, E. D.; Kazdobin, K. A.; Strelko, V. V. Theor. Exp. Chem. 1989, 25, 666−670. (59) Zhou, G.; Duan, W.; Gu, B. Chem. Phys. Lett. 2001, 333, 344− 349.

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 531 88362802. Fax: +86 531 88364513. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Natural Science Foundation of Shandong Province (2009ZRB01618).

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REFERENCES

(1) Iijima, S. Nature 1991, 354, 56−58. (2) Long, R. Q.; Yang, R. T. Ind. Eng. Chem. Res. 2001, 40, 4288− 4291. (3) Mauter, M. S.; Elimelech, M. Environ. Sci. Technol. 2008, 42, 5843−5859. (4) Yin, Y. F.; Mays, T.; McEnaney, B. Langmuir 1999, 15, 8714− 8718. (5) Khoerunnisa, F.; Fujimori, T.; Itoh, T.; Urita, K.; Hayashi, T.; Kanoh, H.; Ohba, T.; Hong, S. Y.; Choi, Y. C.; Santosa, S. J.; Endo, M.; Kaneko, K. J. Phys. Chem. C 2012, 116, 11216−11222. (6) Singh, D. K.; Giri, P. K.; Iyer, P. K. J. Phys. Chem. C 2011, 115, 24067−24072. (7) Lehman, J. H.; Terrones, M.; Mansfield, E.; Hurst, K. E.; Meunier, V. Carbon 2011, 49, 2581−2602. (8) Nowack, B.; Bucheli, T. D. Environ. Pollut. 2007, 150, 5−22. (9) Jia, G.; Wang, H.; Yan, L.; Wang, X.; Pei, R.; Yan, T.; Zhao, Y.; Guo, X. Environ. Sci. Technol. 2005, 39, 1378−1383. (10) Petersen, E. J.; Akkanen, J.; Kukkonen, J. V. K.; Weber, W. J. Environ. Sci. Technol. 2009, 43, 2969−2975. (11) Kim, K.-T.; Edgington, A. J.; Klaine, S. J.; Cho, J.-W.; Kim, S. D. Environ. Sci. Technol. 2009, 43, 8979−8984. (12) Kang, S.; Mauter, M. S.; Elimelech, M. Environ. Sci. Technol. 2008, 42, 7528−7534. (13) Kang, S.; Mauter, M. S.; Elimelech, M. Environ. Sci. Technol. 2009, 43, 2648−2653. (14) Lam, B.; Baer, A.; Alaee, M.; Lefebvre, B.; Moser, A.; Williams, A.; Simpson, A. J. Environ. Sci. Technol. 2007, 41, 8240−8247. (15) Zhang, Y.; Chen, Y.; Westerhoff, P.; Crittenden, J. Water. Res. 2009, 43, 4249−4257. (16) Jiang, L.; Gao, L.; Sun, J. J. Colloid Interface Sci. 2003, 260, 89− 94. (17) Hyung, H.; Fortner, J. D.; Hughes, J. B.; Kim, J.-H. Environ. Sci. Technol. 2006, 41, 179−184. (18) Zhang, S.; Shao, T.; Bekaroglu, S. S. K.; Karanfil, T. Water. Res. 2010, 44, 2067−2074. (19) Chen, C. L.; Wang, X. K.; Nagatsu, M. Environ. Sci. Technol. 2009, 43, 2362−2367. (20) Hyung, H.; Kim, J.-H. Environ. Sci. Technol. 2008, 42, 4416− 4421. (21) Yang, K.; Xing, B. Environ. Pollut. 2009, 157, 1095−1100. (22) Wang, X.; Shu, L.; Wang, Y.; Xu, B.; Bai, Y.; Tao, S.; Xing, B. Environ. Sci. Technol. 2011, 45, 9276−9283. 25789

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