Influence of Surface Oxidation of Multiwalled Carbon Nanotubes on

Apr 23, 2012 - or (π-EDA) bonds, and hydrogen bonds, were responsible for adsorption of organic compounds by CNTs.6,15,16,18−22 Polanyi theory has ...
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Influence of Surface Oxidation of Multiwalled Carbon Nanotubes on the Adsorption Affinity and Capacity of Polar and Nonpolar Organic Compounds in Aqueous Phase Wenhao Wu,†,‡ Wei Chen,†,‡ Daohui Lin,†,‡ and Kun Yang*,†,‡ †

Department of Environmental Science, Zhejiang University, Hangzhou 310058, China Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China

‡

S Supporting Information *

ABSTRACT: Adsorption of organic contaminants on carbon nanotubes (CNTs) is a critical behavior in the environmental application of CNTs as sorbents and in the environmental risk assessment of both organic contaminants and CNTs. Oxidation of CNTs may introduce oxygen-containing groups on CNTs’ surface and then alter the adsorption of organic contaminants. In this study, adsorption of polar and nonpolar organic compounds on four multiwalled carbon nanotubes (MWCNTs) containing varied amounts of surface oxygen-containing groups were investigated to examine the influence of CNTs’ surface oxidation on adsorption. We observed that surface oxidation of MWCNTs reduced the surface area-normalized adsorption capacity of organic compounds significantly because of the competition of water molecules but did not alter the adsorption affinity. The interactions (i.e., hydrophobic effect, π−π bonds, and hydrogen bonds) and the interaction strength for adsorption of organic molecules on MWCNTs could not be altered by the surface oxidation of MWCNTs and thus were responsible for the unaltered adsorption affinity. In addition, the decrease of surface area-normalized adsorption capacity of the organic compound with more polarity and higher adsorption affinity by surface oxidation was less because of the heterogeneous nature of hydrophilic sites of MWCNTs’ surface.

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INTRODUCTION Adsorption of organic compounds by carbon nanotubes (CNTs) in aqueous phase is of great importance for their applications in water treatment1−6 and solid-phase extraction7−10 as superior sorbent, and in the synthesis and surface modification of CNTs with desired physicochemical properties.11 This process is also essential for the assessment of the environmental and health risks of both CNTs and organic compounds once they are released into the environment.12−14 Adsorption of an organic compound by CNTs should be described in two aspects, i.e., adsorption capacity and adsorption affinity.15 Adsorption capacity is the maximum amount of the compound adsorbed on CNTs, while adsorption affinity is a parameter of the strength of all interactions between the compound and CNTs for adsorption.15 Three main interactions, i.e., hydrophobic effect, π-electron donor−acceptor (π-EDA) bonds, and hydrogen bonds, were responsible for adsorption of organic compounds by CNTs.6,15,16,18−22 Polanyi © 2012 American Chemical Society

theory has been regarded as the most powerful theory dealing with aqueous adsorption of organic compounds on CNTs.3,5,6,15,17,18,23,24 Adsorption capacity and affinity of organic compounds including nonionic compounds (e.g., naphthalene, phenanthrene, and pyrene) and ionizable compounds (e.g., aniline, phenol, and their substitutes) by CNTs can be identified by the fitted isotherm parameters of the Polanyi theory-based Dubinin−Ashtakhov (DA) model.15,18,23,24 The graphite surface of CNTs can be oxidized during the processes of synthesis, purification, and oxidation treatment of CNTs, and thus contain oxygen-containing functional groups such as −OH, −CO, and −COOH.19,22,24 These oxygenReceived: Revised: Accepted: Published: 5446

February April 19, April 23, April 23,

6, 2012 2012 2012 2012

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Table 1. Selected Properties of Organic Chemicals chemical naphthalene nitrobenzene 1,2-dinitrobenzene (1,2-DNB) 1,3-dinitrobenzene (1,3-DNB) 1,4-dinitrobenzene (1,4-DNB) 1,3,5-trinitrobenzene (TNB) 4-nitrophenol (4-NP) 4-nitroaniline (4-NA) 4-chlorophenol (4-CP) 4-chloroaniline (4-CA)

Csa 31.7 1936 133 574.9 69 278 16000 600 26300 2755

MWb 128.20 123.11 168.11 168.11 168.11 213.11 139.11 138.13 128.56 128.58

λmaxc

MPe d

280/328 269 255 242 266 228 317 380 225 238

80.2 5.7 117 89 174 121.5 113 146 42.8 72.5

pKaf

VI/100g

π*h

βmi

αmj

7.15 1.00 9.38 4.15

0.753 0.631 0.733 0.733 0.733 0.851 0.685 0.685 0.625 0.653

0.7 1.01 1.42 1.30 1.31 1.63 1.01 0.91 0.72 0.73

0.15 0.30 0.37 0.46 0.46 0.61 0.32 0.46 0.23 0.40

0.00 0.00 0.00 0.00 0.00 0.00 0.93 0.47 0.67 0.31

a Cs: water solubility (mg/L). bMW: molecular weight (g/mol). cλmax: UV-wavelength (nm). dNaphthalene was determined by a fluorospectrophotometer at the excitation and emission wavelengths of 280 and 328 nm, respectively. eMP: melting point (°C). fpKa: dissociated constant. gVI: intrinsic molar volume (mL/mol). hπ*: polarity/polarizability parameter. iβm: hydrogen-bonding acceptor parameter. jαm: hydrogenbonding donor parameter. Data of Cs, MP, and pKa were obtained from refs 6 and 18 and http://toxnet.nlm.nih.gov/cgi-bin/sis/htmlgen?HSDB. Data of VI, π*, βm, and αm of DNB and TNB were calculated from eq 8 in ref 31 and eq 6d in ref 33, while these data for other chemicals were obtained from refs 18 and 32.

Therefore, ten polar and nonpolar organic compounds with different functional groups and varied group numbers were selected to examine their adsorption capacity and affinity on oxidized CNTs in this study. The examined compounds included naphthalene, nitrobenzene, 1,2-dinitrobenzene, 1,3dinitrobenzene, 1,4-dinitrobenzene, 1,3,5-trinitrobenzene, 4chlorophenol, 4-nitrophenol, 4-chloroaniline, and 4-nitroaniline. These organic compounds were chosen also due to their notable concentrations and high toxicity in the aqueous environment.27 The examined CNTs were multiwalled CNTs (MWCNTs) including graphitized MWCNTs (G-MWCNTs), purified MWCNTs (P-MWCNTs), carboxylated MWCNTs (COOH-MWCNTs), and hydroxylated MWCNTs (HOMWCNTs). These MWCNTs were examined because of the vareid amounts of oxygen-containing groups on their surface. The parameters of DA model fitted isotherms were employed in this study to identify the adsorption capacity and affinity of organic compounds by MWCNTs, as has succeeded in our previous studies.6,15,18

containing functional groups altered the adsorption of organic compounds on CNTs.15 It was reported that adsorption of naphthalene,24 chlorophenol,25 and resorcinol26 on CNTs significantly decreased after oxidation treatment of CNTs, while that of 1,2-dichlorobenzene1 and trihalomethanes2 significantly increased. Competition of water molecules with organic molecules on the oxidized sites of CNTs’ surface may be responsible for the adsorption decrease of naphthalene, chlorophenol, and resorcinol, because oxygen-containing groups on CNTs’ surface are hydrophilic and can form strong H-bonds with water molecules to suppress adsorption of organic compounds, especially hydrophobic organic compounds.15,22,24−26 Conformation of CNTs’ aggregates resulting from surface oxidation treatment may enhance the exposed surface area of CNTs15 and thus could be employed to interpret the adsorption increase of 1,2-dichlorobenzene1 and trihalomethanes.2 Adsorption increase of aromatic compounds on CNTs after oxidation treatment was also partly attributed to the enhanced π-EDA interactions between the CNTs and aromatic compounds because (i) the surface functional groups such as carboxylic groups could act directly as the electron donors to form n−π bonds with aromatic molecules and (ii) the surface functional groups could promote the π-polarity of surface aromatic rings of CNTs to form stronger π−π bonds with aromatic molecules.19 At the present, it is still difficult to separately identify the significance of CNTs’ surface oxidation-induced competition of water molecules, enhancement of CNTs’ exposed surface area, or enhancement of π−EDA interactions in the adsorption capacity and affinity of aromatic compounds. Normalization of adsorbed amounts with surface area may be a useful way to rule out the influence of exposed surface area of CNTs on adsorption,15 and thus was employed in this study to explore the influence of oxygen-containing functional groups of CNTs on adsorption affinity and capacity. In addition, the influence of surface oxidation of CNTs on adsorption affinity and capacity could be different for organic compounds because the available surface area of a given CNTs (i.e., the ratios of available surface area to total measured surface area of CNTs) for adsorption is commonly varied with organic compounds due to the heterogeneous properties of CNTs’ surface,6,15,18 as is also not yet addressed.

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MATERIALS AND METHODS Chemicals. Naphthalene (>99%), nitrobenzene (>99%), 1,4-dinitrobenzene (1,4-DNB, >99%) and 4-chloroaniline (4CA, >98%) were purchased from Acros Organics Co., Ltd. 1,2Dinitrobenzene (1,2-DNB, >99%), 1,3-dinitrobenzene (1,3DNB, >99%), and 1,3,5-trinitrobenzene (TNB, wetted with 40% water) were purchased from Tokyo Chemical Industry Co., Ltd. 4-Nitrophenol (4-NP, >99.5%) and 4-nitroaniline (4NA, >99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. 4-Chlorophenol (4-CP, >99%) was purchased from Aladdin Reagent Co., Ltd. These chemicals were used as received. Selected properties of these chemicals are listed in Table 1. Carbon Nanotubes. Four multiwalled CNTs, i.e., GMWCNTs, P-MWCNTs, COOH-MWCNTs, and HOMWCNTs were purchased from Chengdu Organic Chemistry Co., Ltd. (China) and used as-received. P-MWCNT sample was synthesized by the chemical vapor deposition method and purified using HCl solution with the following treatments: 10 g of as-grown MWCNTs was added into 200 mL of the concentrated HCl solution and reacted at 80 °C for 12 h to remove the impurity such as Ni catalyst, and then, filtered, washed with deionized distilled water until a pH of 7.0 in

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Table 2. Selected Properties of Carbon Nanotubes surface atomic percent (XPS) CNTs G-MWCNTs P-MWCNTs COOHMWCNTs HO-MWCNTs

surface acidic group content (mmol/g) carboxyl groups

lactonic groups

total acidic groups

pHzpca

Asurfb (m2/ g)

Vmesob (cm3/ g)

Vmicrob (cm3/ g)

C%

O%

hydroxyl groups

100 98.1 94.0

NDc 1.9 6.0

0.009 ND 0.079

0.012 0.018 0.138

ND 0.010 0.038

0.021 0.028 0.255

6.2 5.7 3.0

114 127 161

0.475 0.588 0.665

0.046 0.050 0.064

93.6

6.4

0.139

0.158

0.085

0.382

3.0

230

0.850

0.088

a

pHzpc values were from the Zeta potential curves of MWCNTs at varied solution pH (Figure S1). bAsurf (surface area), Vmeso (mesopore pore volume), and Vmicro (micropore volume) were calculated from the adsorption−desorption isotherms of N2 at 77 K by multipoint BET, BJH, and DR methods, respectively. cND: Not Detectable.

filtrate was reached, and dried at 150 °C. G-MWCNTs were produced by treating P-MWCNTs under inert gas at 2800 °C for 10 days. COOH-MWCNTs were produced by treating PMWCNTs with KMnO4 oxidation in NaOH solution with the following treatments: 2.0 g of P-MWCNTs was added into 200 mL of the mixed solution containing 0.5 mol/L KMnO4 and 0.5 mol/L NaOH and reacted at 100 °C for 1 h; after that, 100 mL of 1.5 mol/L H2SO4 and 20.8 g of NaHSO3 were added into the mixture, followed by a filtering to remove the produced MnO2; and then, the mixture was washed with deionized distilled water until a pH of 7.0 in filtrate was reached, and it was dried at 150 °C. HO-MWCNTs were produced by treating P-MWCNTs with KMnO4 oxidation in H2SO4 solution with the following treatments: 2.0 g of P-MWCNTs was added into 200 mL of the mixed solution containing 0.75 mol/L KMnO4 and 1.5 mol/L H2SO4 and reacted at 100 °C for 1 h; after that, 26 g of NaHSO3 was added into the mixture, followed by a filtering to remove the produced MnO2, and then the mixture was washed with deionized distilled water until a pH of 7.0 in filtrate was reached, and it was dried at 150 °C. The purity, length, outer diameter, and inner diameter of these MWCNTs are >95%, 10−50 μm, 8−15, nm and 3−5 nm, respectively. Surface acidic group (i.e., carboxyl, hydroxyl, and lactonic groups) contents, surface oxygen and carbon atomic percents, specific surface area, and pore volume values of these MWCNTs listed in Table 2 were characterized by Boehm titration method,28 X-ray photoelectron spectroscopy (XPS) technique,24 and multipoint Brunauer−Emmett−Teller (BET) method of nitrogen adsorption isotherm,29 respectively. The total oxygen-containing acidic group contents, the surface oxygen atomic contents, and the measured surface area of four MWCNTs increased with an order of G-MWCNTs < PMWCNTs < COOH-MWCNTs < HO-MWCNTs (Table 2). Zeta potential values of the four MWCNTs (Figure S1) were measured with a Zetasizer (Nano-ZS90, Malvern Instruments, UK) to get the pHzpc values in Table 2. Experiment of the measurement of Zeta potential values is described in detail in the Supporting Information. Adsorption Experiments. Adsorption experiments were conducted at 25 ± 1 °C using a batch equilibration technique.6,17,18,23 Briefly, chemicals, except for naphthalene, 1,2-DNB, and 1,4-DNB, were dissolved in background solution containing 0.01 mol/L CaCl2 and 200 mg/L NaN3 (as a biocide) in deionized distilled water. Naphthalene, 1,2-DNB, and 1,4-DNB in methanol were added to the background solution for adsorption. Eight or 22 mL of aqueous solutions of chemicals were mixed with MWCNTs in 8 or 22 mL screw cap vials, respectively. The added amounts of MWCNTs were adjusted to achieve that more than 20% of the added organic

compounds were adsorbed by MWCNTs. The pH of the mixtures was adjusted by adding 0.1 mol/L HCl or 0.1 mol/L NaOH solution. After 5-day equilibration, the mixtures were separated by centrifugation at 3500g for 20 min. (Blank experiments of weighted MWCNTs in background solution without organic compounds were conducted. After centrifugation, the UV absorbance values of the supernatants in the blank experiments, determined at the maximum UV wavelength of organic compounds, were lower than the detectable limits, indicating that the suspended MWCNTs in the supernatants after centrifugation was insignificant and thus the centrifugation technique is rigorous enough to separate the MWNTs from the aqueous phase). The compound concentrations in the supernatants were determined by a UV-spectrophotometer (Shimadzu, UV-2450) at their maximum adsorption wavelength (λmax, listed in Table 1), except for naphthalene which was determined by a fluorospectrophotometer (Shimadzu, RF5301PC) at the excitation and emission wavelengths of 280 and 328 nm, respectively. Experimental uncertainties were evaluated in vials without carbon nanomaterials, which showed that total uncertainty was less than 4% of the initial concentrations. Therefore, the adsorbed amounts of organic compounds by MWCNTs were calculated directly by the mass difference of organic compounds in initial solution and equilibrium solution. Isotherms of 4-NP, 4-CP, 4-NA, and 4-CA were obtained at final solution pH of 3.0, 3.0, 6.5 and 6.5, respectively, to ensure their neutral forms in solutions. Isotherms of naphthalene and nitrobenzenes were all obtained at a final solution pH of 6.5. The experiments of pH-dependent adsorption of organic compounds at a give concentration, in the pH range from 1 to 12 and adjusted by adding HCl or NaOH solution, has also been conducted, following the same procedure as described above. Sorption Model and Regression Analysis. The Dubinin−Astakhov (DA) model30 (eq 1) used successfully in fitting adsorption isotherms of nonionic compounds (e.g., naphthalene, phenanthrene, and pyrene) and ionizable compounds (e.g., aniline, phenol, and their substitutes) by CNTs in previous studies,5,6,15,18,23,24 was employed here to fit the experimental data. log qe = log Q 0 − (ε /E)b

(1)

where qe [mg/g] is the adsorbed amount of organic compound in equilibrium; Q0 [mg/g] is the adsorption capacity of organic compound; ε [KJ/mol], ε = RTln(Cs/Ce), is the effective adsorption potential; Ce [mg/L] is the equilibrium concentration of organic compound in aqueous phase; Cs [mg/L] is the water solubility of organic compound; R [8.314 × 10−3 KJ/ 5448

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Figure 1. Surface area-normalized adsorption isotherms of naphthalene, nitrobenzene, 1,2-dinitrobenzene (1,2-DNB), 1,3-dinitrobenzene (1,3DNB), 1,4-dinitrobenzene (1,4-DNB), 1,3,5-trinitrobenzene (TNB), 4-nitrophenol (4-NP), 4-nitroaniline (4-NA), 4-chlorophenol (4-CP), and 4chloroaniline (4-CA) on four MWCNTs. Solid lines represent isotherms fitted by DA model.

(mol·K)] is the universal gas constant; T [K] is the absolute temperature; E [KJ/mol] is the “correlating divisor” of the effective adsorption potential ε, i.e., the characteristic energy of adsorption;30 and b is a fitting parameter.30 Both E and b are

also parameters that can be used to identify successfully the adsorption affinity of organic compounds (i.e., the strength of interactions for adsorption of organic compound) on CNTs because the E value has a multiple linear relationship with the 5449

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Table 3. Results of DA Model Fits to Adsorption Data of Selected Organic Chemicals on MWCNTsa chemical naphthalene

nitrobenzene

1,2-DNB

1,3-DNB

1,4-DNB

TNB

4-NP

4-NA

4-CP

4-CA

MWCNT

Q0 (mg/g)

G-MWCNTs P-MWCNTs COOH-MWCNTs HO-MWCNTs G-MWCNTs P-MWCNTs COOH-MWCNTs HO-MWCNTs G-MWCNTs P-MWCNTs COOH-MWCNTs HO-MWCNTs G-MWCNTs P-MWCNTs COOH-MWCNTs HO-MWCNTs G-MWCNTs P-MWCNTs COOH-MWCNTs HO-MWCNTs G-MWCNTs P-MWCNTs COOH-MWCNTs HO-MWCNTs G-MWCNTs P-MWCNTs COOH-MWCNTs HO-MWCNTs G-MWCNTs P-MWCNTs COOH-MWCNTs HO-MWCNTs G-MWCNTs P-MWCNTs COOH-MWCNTs HO-MWCNTs G-MWCNTs P-MWCNTs COOH-MWCNTs HO-MWCNTs

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

45.7 47.9 42.7 49.0 67.6 77.6 83.2 85.1 45.7 49.0 40.7 46.8 47.9 52.5 49.0 57.5 39.8 38.9 33.1 39.8 31.6 35.5 31.6 37.2 47.9 55.0 63.1 77.6 38.0 42.7 47.9 53.7 45.7 56.2 60.3 72.4 69.2 83.2 75.9 83.2

1.0 1.1 1.1 1.0 1.1 1.0 1.1 1.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

f

E (kJ/mol) 11.4 11.4 11.8 12.4 14.3 13.8 12.5 13.8 11.2 11.6 10.7 12.2 19.4 19.0 16.7 18.4 13.4 14.3 13.4 14.6 22.0 21.8 18.5 20.1 25.4 24.6 23.6 23.7 19.7 19.1 18.6 19.0 21.8 21.3 21.3 21.6 12.4 12.3 13.8 14.0

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.3 0.4 0.4 0.4 0.3 0.3 0.4 0.4 0.1 0.1 0.0 0.1 0.6 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.6 0.2 0.2 0.1 0.5 0.2 0.2 0.2 0.3 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1

b 1.05 1.01 1.03 1.01 1.41 1.33 1.21 1.25 1.62 1.52 1.50 1.55 2.02 1.94 1.84 1.92 1.69 1.79 1.59 1.64 2.15 2.11 1.63 1.88 2.18 2.23 2.07 2.08 1.89 1.75 1.79 1.88 2.57 2.31 2.23 2.29 1.29 1.19 1.26 1.33

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.06 0.06 0.06 0.07 0.13 0.05 0.06 0.08 0.01 0.04 0.02 0.04 0.20 0.08 0.06 0.06 0.09 0.06 0.04 0.03 0.15 0.07 0.07 0.03 0.20 0.13 0.10 0.09 0.10 0.08 0.06 0.05 0.06 0.06 0.06 0.04 0.03 0.03 0.02 0.02

r2

MWSEb

Nc

Q0SAd (mg/m2)

Q0SA/Ce

0.996 0.994 0.993 0.994 0.989 0.997 0.996 0.994 0.998 0.999 1.000 0.999 0.989 0.997 0.998 0.999 0.996 0.998 0.999 0.999 0.996 0.998 0.997 0.999 0.992 0.996 0.997 0.998 0.996 0.997 0.999 0.999 0.999 0.999 0.999 1.000 0.999 0.999 0.999 1.000

0.00208 0.00171 0.00173 0.000859 0.00578 0.00434 0.00523 0.00505 0.0011 0.00098 0.000328 0.000906 0.00249 0.00164 0.00107 0.00119 0.00089 0.00087 0.000477 0.000336 0.000903 0.000843 0.00219 0.000448 0.00192 0.00262 0.00206 0.00186 0.000916 0.00154 0.000942 0.000902 0.000816 0.000888 0.00113 0.000644 0.00124 0.00107 0.000706 0.000613

17 17 17 16 16 18 18 17 19 19 19 19 13 16 17 16 16 19 19 19 12 14 16 15 14 16 16 16 14 16 16 16 17 17 17 17 17 17 17 17

0.401 0.377 0.265 0.213 0.593 0.611 0.517 0.370 0.401 0.386 0.253 0.203 0.420 0.413 0.304 0.250 0.349 0.306 0.206 0.173 0.277 0.279 0.196 0.162 0.420 0.433 0.392 0.337 0.333 0.336 0.297 0.233 0.401 0.443 0.374 0.315 0.607 0.655 0.471 0.362

0.98 0.92 0.65 0.52 0.92 0.95 0.81 0.58 0.99 0.95 0.62 0.50 0.98 0.97 0.71 0.59 1.04 0.91 0.61 0.51 0.97 0.98 0.69 0.57 0.97 1.00 0.90 0.77 0.97 0.98 0.86 0.68 0.93 1.03 0.87 0.73 0.95 1.03 0.74 0.57

a

All estimated parameter values and their standard errors were determined by commercial software (Origin 7.0) with nonlinear regression program. MWSE is mean weighted square error, equal to 1/v[(qmeasured − qmodel)2/q2measured], where v is the amount of freedom; v = N − 3 for DA model. cN is the number of observations. dQ0SA is the surface area-normalized adsorption capacity. eQ0SA/C is the ratio of Q0SA to the intercept of eq 4 (C values are listed in Table S1). fMean ± standard deviation. b

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solvatochromic parameters of organic compounds.6,15,18 All estimated model parameter values and their standard errors were determined by a commercial software program (Origin 7.0). Mean weighted square error (MWSE), equal to sum of 1/ v[(qmeasured − qmodel)2/q2measured] of different solute concentrations, and correlation coefficients (r2) were used to evaluate the goodness of fitting results; where v is the degree of freedom (v = N − 3 for DA model), N is the number of experimental data points, qmeasured is the measured equilibrium adsorbed concentration, and qmodel is the estimated equilibrium adsorbed concentration by the model. The MWSE calculation was a useful method which can avoid the good fits of isotherms resulted from the overparameterization of models. 17

RESULTS AND DISCUSSION Influence of Surface Oxidation of MWCNTs on Adsorption Affinity of Organic Compounds. Adsorption isotherms of all organic compounds by four MWCNTs and the Polanyi theory-based DA model fitting are presented in Figures 1 and S2. DA model fitted parameters (i.e., Q0, E and b) of these isotherms are listed in Table 3. The good fitting of isotherms by DA model was indicated by the low MWSE values, which avoided the effect of model overparameterization on isotherm fits,17 and high relative coefficients (r2) in Table 3. The E and b values of a given organic compound on four MWCNTs are almost invariable (Table 3), indicating they are independent of surface oxygen-containing functional group contents and surface area values of MWCNTs. This phenomenon was also observed by Cho et al.,24 i.e., the E 5450

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Figure 2. Relationship between DA model fitted E and b values (A), and correlation between calculated E (Ecal) values by eq 3 and DA model fitted E (Eexp) values from experiment (B) for organic chemicals on CNTs. Dashed line in Figure A derives from linear regression. Solid line in Figure B represents equation, y = x, and sample deviation (SDEV) = (Σ((Ecal − Eexp)/Eexp)/(n − 1))1/2 × 100.

(−COOR) are strong electron-donating groups.16,19−21 These electron-donating groups can directly form n−π EDA interactions with aromatic molecules.19 These electrondonating groups can also enhance the π-polarity of surface aromatic rings of CNTs to form stronger π−π EDA interactions with aromatic molecules.19 Both π−π and n−π EDA interactions will be stronger when surface acidic groups of CNTs are ionized to be their dissociated species with the increase of solution pH because of the fact that their dissociated species (i.e., −O− and −COO−) are even stronger electrondonating groups.16,19,21 Consequently, adsorption affinity and capacity of aromatic molecules on the oxidized surface of CNTs will increase as pH increased.19 Moreover, this pH-dependent adsorption increase of organic compounds on oxidized CNTs (e.g., COOH-MWCNTs and HO-MWCNTs) should be much more significant than that on nonoxidized CNTs (e.g., GMWCNTs and P-MWCNTs) because oxidized CNTs contain more acidic groups. In other words, adsorbed amounts of organic molecules on oxidized CNTs at a unit of surface area will be more than that on nonoxidized CNTs in the base solution. The E and b values of a given organic compound on four MWCNTs were almost invariable (Table 3), implying that the role of surface oxidation induced n−π and π−π EDA interactions of CNTs for the adsorption enhancement of aromatic compounds is insignificant. The insignificance of surface oxidation of CNTs on the n−π and π−π EDA interactions of CNTs with aromatic molecules was also confirmed by (i) the constant adsorbed amounts of nonionic organic compounds (e.g., naphthalene and nitrobenzenes) at a given concentration on MWCNTs in the pH range of 1 to 12 (Figure S3), and (ii) the lower surface area normalized adsorbed amounts of organic compounds on oxidized CNTs than that on nonoxidized CNTs in the base solution (Figures S3 and S4). The pH dependence of adsorbed amounts of the ionizable compounds (e.g., 4-NP, 4-NA, 4-CP, and 4-CA) at a given concentration on MWCNTs (Figures S4) is a result of the dissociation of these ionizable compounds with pH around their pKa values,6,15,18 and thus their adsorbed amounts remain invariable at solution pH away from their pKa values (Figures S4). The insignificance of n−π EDA interactions of surface acidic groups with aromatic molecules could be attributed to the competition of water molecules.15,22,24,36 Surface sites containing acidic groups of CNTs are more favorable to combine with water molecules than organic molecules by H-bond interactions and to form water molecule clusters.24,36 Thus, the formed

and b values of naphthalene on nine MWCNTs with surface oxygen contents ranging from 3.3% to 14.0% were almost the same. The significant linear intrinsic relationship between E and b values, established in our previous study,15,18 was also observed (eq 2 and Figure 2A). E = 8.018( ±0.793) × b + 3.133(± 1.406) r 2 = 0.637

(2)

The E and b values of naphthalene, phenanthrene, pyrene, aniline, phenol and their substitutes, reported in our previous studies17,18 were included in eq 2 and Figure 2A, too. The E values of examined organic compounds on four MWCNTs in Table 3 followed a multiple linear relationship (eq 3 and Figure 2B) with the solvatochromic parameters (i.e., the hydrogen bonding donor parameter αm and the π-electron polarizability parameter π*) of organic compounds (Table 1). E = 11.58(± 2.03) × αm + 8.15(± 3.64) × π * + 5.50(± 3.42)

(3)

Equation 3 was established in our previous study for aniline, phenol and their substitutes.18 The constant intercept of eq 3, i.e., 5.50 ± 3.42, is a parameter to identify the strength of hydrophobic effects induced by van der Waals force of organic compounds on adsorption.15,18 Figure 2B shows that the calculated E values of these organic compounds, calculated from the αm and π* values31−35 in Table 1 using eq 3, agreed well with the experimental E values. The calculated and experimental E values of naphthalene, phenanthrene, pyrene, aniline phenol and their substitutes, reported in our previous studies17,18 are also presented in Figure 2B. The multiple linear relationship (eq 3) indicated that6,15,18 (i) E is a parameter to identify the adsorption affinity of organic compounds (i.e., the strength of all interactions for adsorption of organic compound on CNTs), and (ii) the examined organic molecules adsorbed on MWCNTs’ surface by the interactions of hydrophobic effect, π−π bonds and hydrogen bonds. The invariable E and b values of a given organic compound on four MWCNTs (Table 3) implied that these interactions and their strength could not be altered by the surface oxidation of MWCNTs. Surface oxidation of CNTs was suggested to enhance the π− EDA interactions (i.e., enhance the adsorption affinity) between CNTs’ surface and aromatic compounds because surface acidic groups of CNTs including hydroxyl groups (−OH), carboxyl groups (−COOH), and lactonic groups 5451

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adsorption capacity of organic compounds is very complex and varied with organic compounds. Both competition of water molecules with organic molecules decreasing the adsorption15,22,24−26 and enhancement of CNTs’ exposed surface area increasing the adsorption1,2,15 of organic molecules on oxidized sites of CNTs’ surface were suggested to interpret the complex results. Moreover, the competition degree of water molecules with organic molecules was dependent on the polarity of organic compounds and hydrophilicity of CNTs’ surface.15,18 Because the influence of enhancement of exposed surface area of CNTs on adsorption can be ruled out by normalizing adsorbed amounts of organic compounds with CNTs’ surface area,15 normalization of adsorbed amounts with surface area was used here to explore the influence of surface oxidation-induced competition of water molecules and the properties of organic compounds on adsorption capacity. Surface area-normalized adsorption capacities Q0SA (mg/m2, Table 3) and even the normalized adsorption amounts of all examined compounds in the whole concentration range of isotherms with surface area (Figure 1) decreased with the increase of total oxygen-containing acidic group contents of examined MWCNTs (Table 2), following an order of GMWCNTs > P-MWCNTs > COOH-MWCNTs > HOMWCNTs (Table 3). A significant linear relationship (eq 4) between Q0SA values of a given organic compound (Table 3) and the total acidic group contents of four MWCNTs (Nacidic groups, mmol/g, Table 2) was observed (Figure S6).

water molecule clusters can suppress the direct interactions including n−π EDA interactions of surface acidic groups of CNTs with organic molecules and exclude the adsorption of organic molecules on these sites through steric hindrance.24,36 The increase of surface area-normalized adsorbed amounts of water-vapor on MWCNTs (Figure S5) with the increase of total oxygen-containing acidic group contents of the four MWCNTs (Table 2) was evidence for the possible adsorption suppression of organic molecules by water molecules on oxidized MWCNTs. The insignificance of surface oxidation on the π-polarity enhancement of surface aromatic rings of CNTs and the consequent enhancement of π−π EDA interactions between CNTs with aromatic molecules could be attributed to the high molar ratios of aromatic rings (i.e., benzene rings) to acidic groups of oxidized MWCNTs’ surface. The percent enhancement of the π-polarity of surface aromatic rings of CNTs by surface acidic groups decreased sharply with the increase of the molar ratios of surface aromatic rings to surface acidic groups of CNTs, as was presented in Figure 3.35 For example, the ratios

Q 0SA = S × Nacidicgroups + C

(4)

The linear regression parameters, i.e., S and C values, are listed in Table S1. The negative S values of all chemicals (Table S1) indicated that surface oxidation of CNTs will reduce adsorption capacity of organic compounds if the surface area values are not enhanced after oxidation treatment. Adsorption of water molecules on surface sites containing acidic groups of MWCNTs, indicated by the increase of surface area-normalized adsorbed amounts of water-vapor on MWCNTs (Figure S5) with the increase of total oxygen-containing acidic group contents of the four MWCNTs (Table 2), was responsible for the negative S values and the decrease of Q0SA.15,24,36 Thus, a fraction of adsorption sites on MWCNTs’ surface originally available for adsorption of organic molecules could be not available for adsorption after oxygen-containing groups were introduced into these surface sites. A significant linear relationship between the S values and the E values (average values of four MWCNTs in Table 3) of organic compounds (Figure 4 and eq 5) was observed.

Figure 3. Percent contributions of oxygen-containing groups including hydroxyl groups (−OH), carboxyl groups (−COOH), and lactonic groups (−COOR) on π* values of CNTs versus the molar ratios of benzene rings to oxygen-containing groups (i.e., [C6H6:OH], [C6H6:COOH], and [C6H6:COOR], respectively) on CNTs’ surface. [C6H6:OH], [C6H6:COOH], and [C6H6:COOR] are the molar ratios of benzene ring to hydroxyl group, carboxyl group, and lactonic group, respectively. The π* values of C6H6, −OH, −COOH, and −COOR, used for calculation of the percent contribution, are 0.59, 0.13, 0.15, and 0.17, respectively.35.

of surface benzene rings to hydroxyl groups, carboxyl groups, and lactonic groups of the most deeply oxidized HOMWCNTs, calculated from the surface oxygen and carbon atomic percents of HO-MWCNTs listed in Table 2, are 7.3:1, 14.6:1, and 14.6:1, respectively. These high ratios suggested that the π-polarity enhancement of HO-MWCNTs by its surface acidic groups can be negligible due to the relationships presented in Figure 3. Influence of Surface Oxidation of MWCNTs on Adsorption Capacity of Organic Compounds. The adsorption capacities (Q0, mg/g) of nitrobenzene, 4-CP, 4NP, and 4-CA on four MWCNTs increased with the increase of total oxygen-containing acidic group contents and surface oxygen atomic contents (Table 2), following an order of GMWCNTs < P-MWCNTs < COOH-MWCNTs < HOMWCNTs (Table 3). This order was not observed for other examined compounds, i.e., naphthalene, 1,2-DNB, 1,3-DNB, 1,4-DNB, TNB, and 4-NA (Table 3). This observation indicated that the influence of surface oxidation of CNTs on

S = 0.0301( ± 0.0063) × E − 0.943(± 0.109) r 2 = 0.741

(5)

This linear relationship (eq 5) described quantitatively the dependence of competition of water molecules with organic molecules on the properties of organic compounds. Increase of S values with E values suggested that an organic compound adsorbed on MWCNTs’ surface with stronger interactions has less decrease in its adsorption capacity once oxygen-containing groups were introduced into the MWCNTs’ surface. The heterogeneous sites on oxidized CNTs’ surface, classified simply into three types including the most hydrophilic adsorption sites, the less hydrophilic adsorption sites, and the hydrophobic adsorption sites due to the hydrophilicity of CNTs’ surface,6,15 could be employed to interpret the linear 5452

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with more polarity and higher adsorption affinity has less decrease in its Q 0 SA on oxidized CNTs due to the heterogeneous sites of CNTs’ surface. The adsorption capacities (Q0) of nitrobenzene, 4-CP, 4-NP, and 4-CA on CNTs increased with the increase of total oxygen-containing acidic group contents and surface oxygen atomic contents, while other compounds, i.e., naphthalene, 1,2-DNB, 1,3-DNB, 1,4-DNB, TNB, and 4-NA did not. These observations suggested that surface oxidation of CNTs is not a potential way to engineer enhanced adsorption of organic contaminants for the application of CNTs as adsorbents. However, surface oxidation of CNTs may alter the adsorption of organic compounds. Therefore, surface oxidation of CNTs should be addressed for the assessments of environmental and health risks of both organic contaminants and CNTs, because not only the oxidized CNTs would be directly released into the environment but also the released CNTs would be oxidized in the oxidative environment. The relationships established in this study (eqs 3, 4, and 5) could be employed in the prediction of the adsorption capacity as well as the adsorption affinity of organic contaminants on CNTs in the environment. In addition, efforts to enlarge surface area and reduce surface oxygencontaining groups of CNTs at the same time would be perfectly valuable for the application of CNTs as sorbents, since adsorption capacity of organic compounds increased with CNTs’ surface area but surface area-normalized adsorption capacity decreased with CNTs’ surface oxygen content.

Figure 4. S values as a function of average E values of organic compounds on four MWCNTs.

relationship (Figure 4 and eq 5). The most hydrophilic adsorption sites are almost occupied by the oxygen-containing functional groups and can not be available for adsorption of most organic compounds due to the competition of water molecules.6,15 The less hydrophilic adsorption sites are some polarized electron-rich or -depleted sites such as surface defects on CNTs and can combine with both polar organic molecules or water molecules.6,15 The hydrophobic adsorption sites can combine with most organic compounds due to the hydrophobic effect of organic molecules but would be not favorable to water molecules.6,15 Therefore, the organic compound with more polarity and bigger E value, compared to the nonpolar organic compounds, can occupy more adsorption sites especially for the less hydrophilic adsorption sites of oxidized CNTs’ surface, and thus result in the less decrease in its surface area-normalized adsorption capacity. The higher adsorption capacities (Q0) of organic compounds such as nitrobenzene, 4CP, 4-NP, and 4-CA on oxidized MWCNTs (i.e., COOHMWCNTs and HO-MWCNTs) than that on G-MWCNTs and P-MWCNTs (Table 3) may be mainly attributed to (i) the exposed surface area enhancement of MWCNTs by surface oxidation treatment (Table 2) because adsorption capacity of an organic compound is dependent on the surface area of CNTs3,15,17,36 and (ii) nitrobenzene, 4-CP, 4-NP, and 4-CA are polar compounds with bigger E values and less decrease in their surface area-normalized adsorption capacity on the oxidized CNTs. The parameter C is the ideal surface area normalized adsorption capacity of a given organic compound on CNTs without surface acidic groups. Therefore, the C value of a given organic compound was almost equal to the Q0SA value of the organic compound on G-MWCNTs (Table 3) with a deviation of less than 5%. In addition, the ratio of Q0SA to C could be used as a parameter to identify the ratio of the available surface area to the total measured surface area of CNTs for adsorption of an organic compound. The ratios of Q0SA to C in Table 3 indicated that (i) the available surface area of a given CNTs for adsorption is varied with organic compounds and (ii) surface acidic groups of CNTs will decrease the ratio for all examined organic compounds.

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ASSOCIATED CONTENT

S Supporting Information *

Two tables and six figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-571-88982589; fax: 86-571-88982590; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported partly by NSF of China (21137003, 40973065 and 40971181), the Zhejiang Provincial NSF of China (R5110004), the Program for New Century Excellent Talents in University of China (NCET-08-493), the Fundamental Research Funds for the Central Universities, and the Key Innovation Team for Science and Technology of Zhejiang Province (2009R50047).

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REFERENCES

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ENVIRONMENTAL IMPLICATIONS Surface oxidation of CNTs was observed to reduce the surface area-normalized adsorption capacity (Q0SA) of organic compounds significantly because of the competition of water molecules on hydrophilic sites of oxidized CNTs but did not alter their adsorption affinity. Moreover, the organic compound 5453

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