Sulfonated multi-walled carbon nanotubes for the removal of copper (II) from aqueous solutions

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JIEC-1523; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx

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Sulfonated multi-walled carbon nanotubes for the removal of copper (II) from aqueous solutions Yuanyuan Ge a,b, Zhili Li a,b,*, Duo Xiao a, Piao Xiong a, Na Ye a a b

School of Chemistry & Chemical Engineering, Guangxi University, Nanning 530004, China Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China

A R T I C L E I N F O

Article history: Received 23 May 2013 Accepted 28 August 2013 Available online xxx Keywords: Heavy metal ion Adsorption Carbon nanotube Sulfonation Wastewater

A B S T R A C T

Sulfonated multi-walled carbon nanotubes (s-MWCNTs) was prepared from purified multi-walled carbon nanotubes (p-MWCNTs) by concentrated H2SO4 at elevated temperature. The structure was characterized by SEM, FTIR, Raman, XPS, and BET. It could be dispersed steadily in water at a dosage of 1.0 mg/mL for a week. The adsorption performance of s-MWCNTs toward Cu(II) was investigated including the effects of pH and ionic strength. Results indicated the adsorption was much dependent on pH but not on ionic strength. The adsorption capacity for Cu(II) was enhanced 58.9% via the sulfonation. Moreover, the adsorption mechanisms were carefully analyzed by Freundlich and D-R models. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Water pollution by heavy metal ions through various industrial manufacturing is a worldwide environmental problem that includes metal finishing, plating facilities, mining operations and battery manufacturing. Generally, heavy metal ions are not biodegradable and can be accumulated in living organisms which can cause various diseases and disorders [1]. Accordingly, the removal of heavy metal ions from wastewater by an effective way has become an important issue nowadays. The often used technologies for removal of heavy metal ions from wastewater include precipitation, ion-exchange, electro dialysis, membrane separation, and adsorption [2–6]. Among them, adsorptive removal is based on the ability of a porous adsorbent to selectively adsorb contaminants compounds from wastewater. Adsorption is one of the most effective methods for the removal of colors, odors, organic and inorganic pollutions from wastewater due to its costeffectiveness, simplicity and easily operated [7,8]. The increasingly stringent limitation of the concentration of heavy metal ions in water has stimulated a growing effort in the exploitation of new highly efficient adsorbents. Carbon nanotubes (CNTs) [9,10], a new member of carbon family, has novel physical and chemical properties such as

* Corresponding author at: School of Chemistry & Chemical Engineering, Guangxi University, Nanning 530004, China. Tel.: +86 155 7815 0040; fax: +86 771 3233718. E-mail address: [email protected] (Z. Li).

mechanical electrical properties, high chemical stability, high porous and hollow structure and large surface area, which have led to interests in the potential applications for sensors, catalyst supports, electronics, optics and wastewater treatment, energy storage etc. [11,12]. CNTs include single-walled (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) depending on the numbers of layers. Due to the high surface area and chemical stability of CNTs, it is used for the removal of heavy metal ions from wastewater, such as lead, copper, cadmium, silver, and nickel [13–18]. In order to improve the adsorption capacity of CNTs for metal ions, oxidation was commonly used to introduce oxygen-containing functional groups onto CNTs [19]. Such as Kandah and Meunier found that the adsorption of Ni (II) by oxidized CNTs was 1.24 times greater than commercial activated carbon [20]. Considering the self-aggregating tendency of CNTs, even the functionalized CNTs with oxygen-containing groups [21] in aqueous environment, which results in a decrease of adsorption sites of CNTs and therefore decreases the adsorption capacity for heavy metal ions, we are trying to explore a feasible modification process to improve the water-dispersibility of CNTs for removal of heavy metal ions from aqueous solutions. Thus, in this paper, we reported the functionalization of MWCNTs by concentrated H2SO4 at elevated temperatures which introduced the hydrophilic functional groups –HSO3 onto MWCNTs. And the effects of sulfonation on the adsorption capacity of MWCNTs toward Cu (II) from aqueous solutions were also investigated.

1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.08.030

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2

2. Experimental

adsorbed on the MWCNTs was calculated by subtracting the copper content in the filtrate from the initial content:

2.1. Materials qe ¼ MWCNTs were prepared by using chemical vapor deposition of methane in nitrogen flow at temperature 900 8C using Mo/MgO as catalyst [22]. The produced CNTs were purified by 2 M HCl under room temperature to remove the residual catalyst particles. Then it was filtered and washed for several times, followed by drying at 110 8C overnight. The sulfonation of MWCNTs was carried out according to the method described in our previous report [23]. In brief, 50 mL concentrated H2SO4 (98%) and 50 mg purified MWCNTs were mixed in a flask and sonicated for 30 min, followed by heating to 180 8C for 24 h with a reflux condensation. After the functionalization process, the suspension was cooled to environmental temperature and diluted by distilled water and filtered. The residue was washed with distilled water to remove excess acid until pH = 7.0 and then dried at 110 8C overnight to obtain the sulfonated MWCNTs (s-MWCNTs). 2.2. Characterizations The samples were characterized by microscopic and spectroscopic methods, including Scanning electron microscopy (SEM), Energy dispersive spectrometer (EDS), Fourier transform-infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Brunauer, Emmett, and Teller (BET) analyzer. SEM images were taken in a Hitachi SU8020 Microscope equipped with an INCA300 EDS by immobilizing the sample with conductive glue. FT-IR spectra were recorded with a Bruker 550 spectrometer. The scan region was between 4000 and 400 cm1. Each sample was prepared according to the potassium bromide slice method with a proportion of 1% to KBr. The Raman spectra were recorded with a Renishaw RM2000 spectrometer excited at 633 nm. XPS measurements were carried out on an ESCALAB 250 XPS spectrometer (Thermo-VG Scientific Co., USA). A monochromatic Al Ka X-ray source (1486.6 eV of photons) was used, with a spot area of 200 mm in diameter. The base pressure in the working chamber was 109 Torr. All binding energies were referenced to the neutral C1s peak at 284.6 eV to compensate for the surfacecharging effects. The data from XPS were analyzed by the XPSpeak 4.1 software. A Shirley baseline was used for the subtraction of the background, and Gaussian/Lorentzian (80/20) peaks were used for spectral decomposition. The specific surface areas and pore diameters of samples were calculated from nitrogen adsorption isotherm data (Micrometrics ASAP 2010, USA) at 77 K using the BET method.

Vðc0  ce Þ m

(1)

where qe is the adsorption amount (mg/g); V (L) is the volume of the aqueous solutions; c0 (mg/L) is the initial concentration of Cu (II); ce (mg/L) is the equilibrium concentration of Cu (II); m (g) is the amounts of the adsorbent. 3. Results and discussion 3.1. Characterization of MWCNTs Fig. 1(a) and (b) shows that MWCNTs are curved with length ranging from hundreds of nanometers to micrometers. The structure of the MWCNTs did not change significantly after sulfonation by concentrated sulfuric acid. EDS result demonstrates the existence of elements S and O by the distinct signal of sulfur at 2.2 keV and the signal of oxygen at 1.0 keV. Fig. 2(a) and (b) displays the Raman spectra of purified and sulfonated MWCNTs, respectively. It can be seen that there were two peaks located at about 1320 and 1575 cm1. The peak near 1320 cm1 is the so-called D band which is related to disordered sp2-hybridized carbon atoms of nanotubes. The peak near 1575 cm1 is the so-called G band which is related to the graphite E2g symmetry of the interlayer mode. This mode reflects the structural integrity of sp2-hybridized carbon atoms of the

2.3. Adsorption experiment For the adsorption experiment of carbon nanotubes toward Cu (II) from aqueous solutions, analytical grade copper sulfate was used to prepare a stock solution of 1000 mg/L of Cu (II), which was further diluted to the required concentrations before use. The pH values of the solution were maintained at 5.0 with the exception of pH effect study, in which the pH range of 2–6 were chosen. The pH was adjusted using 0.1 M HNO3 or 0.1 M NaOH. The effect of ionic strength on Cu (II) adsorption was studied for 0.001 M, 0.01 M, and 0.1 M NaCl solution. Batch adsorption behavior were studied by adding 0.025 g of the purified or sulfonated MWCNTs into 50 mL solutions with initial concentrations of Cu (II) from 10 to 100 mg/L with step values of 10 or 20 mg/L. After the suspensions were shaken for 6 h under 25 8C, they were filtered through 0.3 mm membrane filter. The filtrate was immediately measured by an atomic adsorption spectrometer (AA-6800, Shimadzu, Japan). The amount of copper

Fig. 1. SEM images of p-MWCNTs (a), s-MWCNTs (b), and EDS of s-MWCNTs (c).

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3000 2500 2000

Intensity

(b) 1500 1000

(a)

500 0 1000

1200

1400

1600

Raman Shift /cm

1800

2000

-1

3

of peaks around 1720 cm1 indicates both p-MWCNTs and sMWCNTs were not oxidized during the treatment processes. XPS was employed to investigate the chemical valences of the surface groups. The high resolution spectra of O1s, S2p and C1s are shown in Fig. 4. As expected, the binding energies of S2p1/2 at 169.1 eV and S2p3/2 at 168.7 eV can be assigned to S5 5O and S–OH, respectively [29]. The O1s spectra can be deconvoluted into three peaks. The three peaks in O1s curve at 533.5, 532.6 and 531.5 eV can be assigned as C–OH, S5 5O and S–OH, respectively. The C1s spectrum can be deconvoluted into two peaks, centered at 284.6 and 285.1 eV, respectively. The main peak centered at 284.6 eV originates in sp2-hybridized graphite carbon. The peak at 285.1 eV is contributed from sp3-hybridized carbon atoms [30] induced by the strong oxidation of concentrated H2SO4. The results strongly confirmed the sulfonic groups being successfully introduced onto MWCNTs by the treatment of concentrated H2SO4.

Fig. 2. Raman spectra of p-MWCNTs (a), and s-MWCNTs (b). 220

Intensity

200

180

160

140 175

174

173

172

171

170

169

168

167

166

165

Binding energy /eV

(a)

4100

4000

3900

Intensity

nanotubes [24–26]. Together, these bands can be used to evaluate the extent of carbon-containing defects. The intensity ratios of D band to G band (ID/IG) of p-MWCNTs and s-MWCNTs are 1.93 and 2.27, respectively. The ID/IG ratios of s-MWCNTs are higher than those of p-MWCNTs indicating that the surface properties of pMWCNTs were damaged by the sulfonation process for introducing sulfonic groups. FT-IR spectra of p-MWCNTs and s-MWCNTs were obtained to determine the structure of chemical groups formed during the treated process. Fig. 3(a) shows the FT-IR spectra of p-MWCNTs: the peak at 1635 cm1 is assigned to the C5 5C graphic stretching mode, the broad peak centered at 3415 cm1 is assigned to the – OH stretching mode which originated in the moisture containing in the sample. Fig. 2(b) shows the FTIP spectra of s-MWCNTs [27,28]: the peak at 1646 cm1 is attributed to the C5 5C stretching vibration. The broad band in the regions 3200–3500 cm1 is attributed to –OH groups. The strong band at 1185 cm1, was assign to the SO2 symmetric stretching mode, the strong band at 1061 cm1 is assigned to the asymmetric stretching mode of SO3 and the weak band at 997 cm1 are attributed to the corresponding symmetric stretching modes, the band at 850 cm1 is attributed to S5 5O bonds. In addition, the peak at 575 cm1 is assigned to the C–S stretching mode implying the covalent functionalization of sulfonic groups. These signals strongly confirms that the –HSO3 groups were successfully introduced onto MWCNTs. The absence

3800

3700

3600 528

530

532

534

536

Binding energy /eV 100

(a)

(b)

70000 60000

80

T /%

50000

Intensity

(b) 60

40000 30000 20000 10000

40 0 290

3500

3000

2500

2000

1500

Wavenumbers /cm

1000

-1

Fig. 3. FT-IR spectra of p-MWCNTs (a), and s-MWCNTs (b).

500

288

286

284

282

Binding energy /eV

(c) Fig. 4. High resolution XPS spectra of s-MWCNTs: S2p (a); O1s (b); C1s (c).

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0.030 0.025 3

Pore volume /cm g

-1

(a)

0.020 0.015 0.010

(b) 0.005 0.000 1

10

100

Pore diameter /nm Fig. 5. Pore diameter distribution of p-MWCNTs (a), and s-MWCNTs (b).

3.2. Effect of pH The solution pH is one of the dominant parameters controlling adsorption. At pH > 6, insoluble copper hydroxide begins to precipitate from aqueous solution, making true adsorption studies impossible [33]. Hence, Cu (II) adsorption onto MWCNTs as a function of pH ranging from 2 to 6 was carried out in this work, the results were given in Fig. 7. The removal of Cu (II) by s-MWCNTs Table 1 Specific surface area, pore diameter and pore volume of p-MWCNTs and sMWCNTs. Sample

SBET (m2 g1)

dp (nm)

Vp (cm3 g1)

p-MWCNTs s-MWCNTs

169.7 28.7

15.9 50.9

0.675 0.366

Fig. 6. Photographs of s-MWCNTs: (a) 0.5, (b) 1.0 mg/mL, and p-MWCNTs: (c) 0.5 mg/mL dispersions in distilled water.

was highly dependent on pH. The adsorption of Cu (II) increases from 0% (pH 2–3) to 95.2% (pH 6) for s-MWCNTs. The solution pH affects the surface charge of s-MWCNTs, the degree of ionization. It is known that copper species can be present in deionized water in the forms of Cu2+, Cu(OH)+1, Cu(OH)2, Cu(OH)3 and Cu(OH)42 [34]. At pH < 6, the predominant copper species is always Cu2+ and the removal of Cu2+ is mainly accomplished by adsorption reaction. The low Cu2+ adsorption that takes place at low pH can be attributed mainly to competition between H3O+ and Cu2+ ions on the same sites. Therefore, the increase in Cu (II) adsorption as the pH increases could be explained on the basis of a decrease in competition between H3O+ and Cu (II) ions for the same adsorption sites. 3.3. Effect of ionic strength The effect of ionic strength on Cu (II) adsorption by s-MWCNTs at pH = 5 is fairly negligible, as also shown in Fig. 7. The adsorption percentage of Cu (II) by s-MWCNTs increases slightly from 21.9% to 28.3% as the background electrolyte concentrations increase from

pH 1

2

3

4

5

6

7

100

Adsorption percentage /%

Fig. 5(a) and (b) exhibits the pore diameter distribution of pMWCNTs and s-MWCNTs, respectively. The peaks of p-MWCNTs are located mainly in the size range of 1.5–7 nm, and with a small peak at >100 nm. The pore diameter of s-MWCNTs is a typical bimodal distribution. The first peak of s-MWCNTs is located in the size range of 2–6 nm while another peak is located in the size range of 60–200 nm. It can be seen that the change in the pore size range of the first peak of MWCNTs after sulfonation is insignificant. But the pore size range of 60–200 nm of MWCNTs increased after sulfonation, which indicated the confined space of MWCNTs decreased by sulfonation [31]. The physical properties of pMWCNTs and s-MWCNTs measured by a BET analyzer were listed in Table 1. As can be seen, the specific surface area (SBET) and pore volume (Vp) of p-MWCNTs are higher than those of s-MWCNTs but the average pore diameter (dp) of p-MWCNTs is lower than that of s-MWCNTs. Possible reasons might be attributed to the fact that the confined spaces between different graphitic walls decreased due to the strong corrosion of concentrated H2SO4 to graphitic walls at elevated temperature [31,32]. Additionally, the ends and defective sites of MWCNTs were occupied by the introduced –HSO3 groups which make N2 molecules being hard to adsorb onto the sites of carbon nanotubes surface. Therefore, the BET surface area of MWCNTs was significantly decreased after sulfonation. Photographs of aqueous dispersions of the MWCNTs are shown in Fig. 6. As can be seen, the s-MWCNTs are highly dispersible in distilled water under ambient conditions. With concentration of 0.5 and 1.0 mg/mL in distilled water, the dispersions showed no indication of s-MWCNTs precipitation after a week, but the pMWCNTs immediately precipitated in distilled water even at 0.5 mg/mL.

80

60

40

20

0 0.00

0.02

0.04

0.06

Ionic strength /mol L

0.08

0.10

-1

Fig. 7. Effect of pH (Top X) and ionic strength (Bottom X) on the adsorption of Cu (II) by s-MWCNTs.

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0.001 to 0.1 M NaCl. The experimental results are similar to those of Sheng [35]. The background electrolyte concentration affects the thickness and interface potential of the double layer, influencing the binding of the adsorbates. Outer-sphere complexes are expected to be more sensitive to ionic-strength variations than inner-sphere complexes [33], because the background electrolyte ions are located in the same plane as the outer-sphere complexes. Thus, the adsorption of Cu (II) might be assigned to the innersphere complexes being formed during the adsorption process. As proposed by Hayes and Leckie [36], the effect of the background electrolyte concentration on adsorption could be used to predict the adsorption process. b-plane adsorption could be assumed to proceed when the background electrolyte distinctly influenced the adsorption; otherwise, o-plane adsorption might be occurring. Therefore, this work suggested that Cu (II) participated in an oplane complex adsorption reaction, without being influenced by the b-plane complex reaction of the background electrolyte (Na+ and Cl). 3.4. Comparative adsorption of Cu (II) from aqueous solutions by pMWCNTs and s-MWCNTs In the batch adsorption study, a test for adsorption of Cu (II) from aqueous solutions with s-MWCNTs and p-MWCNTs was conducted. The results were depicted in Fig. 8. As can be seen, with increasing initial concentration of Cu (II), the adsorption amounts of Cu (II) by s-MWCNTs and p-MWCNTs increase both. Particularly, the maximum adsorption amount of s-MWCNTs for Cu (II) is 59.6 mg/g, but it is only 37.5 mg/g as to p-MWCNTs. Namely, the adsorption capacity of Cu (II) by MWCNTs from aqueous solutions was effectively enhanced 58.9% by the sulfonation process. Although the surface area of s-MWCNTs (28.7 m2 g1) is much lower than that of p-MWCNTs (169.7 m2 g1), the adsorption capacities of Cu (II) onto s-MWCNTs are much higher than that onto p-MWCNTs. This is due to the fact that sulfonation of carbon surface can provide not only a more hydrophilic surface structure but also many sulfonic groups, which made it being well dispersed in water (Fig. 6) and increase the number of adsorption sites of carbon nanotubes, therefore, lead to an increase in the adsorption of Cu (II) onto MWCNTs. On the other hand, a strong electrostatic attractive force between the negative charged s-MWCNTs and the cations (Cu2+) also contributed to the increase of adsorption capacity of Cu (II) by s-MWCNTs.

Adsorption amounts / mg g

-1

60

(b)

50 (a)

40 30

5

The adsorption capacity of s-MWCNTs in our study is comparable to or better than other MWCNTs-based adsorbents given in literatures, such as Sheng [35] reported a maximum adsorption capacity of 3.5 mg/g for Cu (II) by MWCNT at initial concentration of 20 mg/L, the s-MWCNTs prepared in this study represents a higher adsorption capacity of 7.9 mg/g at the same initial concentration of 20 mg/L. Although Shao et al. [37] reported a maximum adsorption capacity of 13.6 mg/g for Cu (II) by plasma induced grafting MWCNTs with chitosan that was mainly contributed by the functional groups of carboxyl groups, hydroxyl groups, amino groups and acetyl groups, the new plasma technology was much limited used for large scale production. However, the s-MWCNTs prepared by sulfonation are very feasible and suitable for the preconcentration and immobilization of heavy metal ions in the environmental pollution cleaning. 3.5. Adsorption models In order to further understand the adsorption mechanisms of copper ions with s-MWCNTs, in this work, Freundlich and DubininRadushkevich (D-R) adsorption models were chose to fit the data, which are well-known and widely used to analyze adsorption data. The Freundlich model [38] assumes a heterogeneous adsorption surface with sites having different adsorption energies. It can be described by the following equation: qe ¼ K

(2)

1=n

FCe

Or the linear form: logqe ¼ logK F þ

1 logce n

(3)

where KF is the Freundlich constant indicating the adsorption capacity, n is the heterogeneity factor representing the adsorption intensity. The D-R model [39] is more general used to determine the nature of adsorption process whether it is physical or chemical. The D-R equation is given with the following Eq.: qe ¼ qm expðke2 Þ

(4)

The linearized form of this equation was given as Eq. (5): lnðqe Þ ¼ lnðqm Þ  ke2

(5) 2

where k is a constant related to the adsorption energy (mol /kJ2), qm is the maximum adsorption capacity mg/m2, e is the Polanyi potential which can be calculated by Eq. (6):   1 (6) e ¼ RTln 1 þ ce where R is the gas constant (8.314 J/mol K), T is the absolute temperature (K). To evaluate the nature of adsorption between heavy metal ions and adsorption sites, the mean free energy of adsorption (E) can be calculated from the k values by the following Eq.: ð1=2Þ

E ¼ ð2kÞ

(7)

20 10 0 10

20

30

40

50

60

Equilibrium concentration /mg L

70

80

-1

Fig. 8. Adsorption of Cu (II) from aqueous solutions by p-MWCNTs (a), and sMWCNTs (b).

The experimental data were fitted by the two-parameter models. The plots of qe and ce with certain forms were shown in Figs. 9 and 10, the correlation coefficients (R2) and model parameters were tabulated in Table 2. As can be seen from Table 2, the values of R2 for p-MWCNTs and s-MWCNTs were 0.9396 and 0.9777, respectively, which indicates that the Freundlich models give good fits to the adsorption data for Cu (II) onto p-MWCNTs and s-MWCNTs. But the KF value of

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Table 2 Fitting parameters by Freundlich and D-R models. Freundlich model parameters

Adsorbent

p-MWCNTs s-MWCNTs

D-R model parameters 2

KF (mg/g)

n

R

qm (mg/g)

k (mol2/kJ2)

E (J/mol)

R2

4.38 1.75

0.96 1.39

0.9396 0.9777

36.82 43.16

88.47 97.27

75.18 71.69

0.9060 0.9431

1.75 mg/g and 4.38 mg/g for s-MWCNTs and p-MWCNTs respectively are not in accordance with the experimental data. The n values are 1.39 for s-MWCNTs, and 0.96 for p-MWCNTs, which indicates the adsorption intensity on s-MWCNTs is stronger than that on p-MWCNTs. The stronger adsorption intensity (n) of sMWCNTs was due to the heterogeneousity of the adsorption surface increased after the treating process of introducing the functional groups of –HSO3 that was accordant with results of Raman spectra and FT-IR spectra. The fitting results from D-R models shows a higher qm value of 43.16 mg/g for s-MWCNTs than the qm value of 36.82 mg/g for p-MWCNTs that are close to the experimental data. Besides, the values of E, the mean free energy of adsorption, are found to be 71.69 J/mol and 75.18 J/mol for sMWCNTs and p-MWCNTs, respectively. The smaller E value for sMWCNTs, indicating much less energy is needed to transfer Cu (II) 1.8

1.6

1.4

log(qe)

1.2

(b) 1.0

0.8

(a) 0.6

0.4 0.8

1.0

1.2

1.4

1.6

1.8

2.0

log(ce) Fig. 9. Freundlich model plots for Cu (II) on p-MWCNTs (a), and s-MWCNTs (b).

4.0 (b)

3.5

ln(qe)

3.0

4. Conclusion In this work, we presented a route to obtain water-dispersible MWCNTs treated by concentrated H2SO4 at elevated temperature for introducing hydrophilic functional groups –HSO3. The materials were characterized by SEM, EDS, FT-IR, Raman, XPS and BET. SEM pictures indicated the morphology of MWCNTs did not changed by sulfonation. The EDS, FT-IR, and XPS results confirmed the –HSO3 groups were successfully introduced onto MWCNTs. The s-MWCNTs exhibited a good water-dispersibility, i.e., it could be dispersed steadily in water at a dosage of 1.0 mg/ mL for a week without aggregation tendency. BET analysis showed the specific surface area of s-MWCNTs decreased after sulfonation. The adsorption of Cu (II) on MWCNTs was strongly dependent on pH values that was due to the competition adsorption between H3O+ and Cu2+. The adsorption of Cu (II) on s-MWCNTs was independent of ionic strength that indicated the adsorption of Cu (II) was mainly dominated by inner-sphere surface complexation. Moreover, the copper (II) adsorption capacity of MWCNTs was enhanced distinctly by the sulfonation process due to its good water-dispersibility, which contributed to the increase of adsorption sites, and the strong electrostatic attractive forces. The experimental data were fitted by Freundlich and D-R models, the results confirmed the adsorption process belonged to endothermic and physical adsorption, and the stronger adsorption of s-MWCNTs toward Cu (II) was due to the increased heterogeneousity and strong electrostatic attractive forces. The lower value of mean free energy (E) for Cu (II) being adsorbed onto s-MWCNTs indicated much less energy was needed to transfer the Cu (II) onto s-MWCNTs from the bulk solutions than that onto p-MWCNTs. This study provided a possible way to enable MWCNTs being more feasibly handled during wastewater treatment, chemical reactions, physical blending, and composite formation. Acknowledgments

(a)

The authors gratefully acknowledge the Projects Sponsored by the National Natural Science Foundation of China (21264002), Scientific Research Foundation of Guangxi University (XJZ120279), and Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2012Z07).

2.5

2.0

References

1.5 0.000

onto the surface of s-MWCNTs than onto the surface of p-MWCNTs, and this also indicates that the adsorption mechanism of Cu (II) onto s-MWCNTs might be quite different from that onto pMWCNTs, i.e., the strong electrostatic attractive forces. Moreover, the magnitude of the E values indicates the adsorption process is endothermic and belongs to physical adsorption [38,39].

0.004

0.008

0.012

0.016

0.020

0.024

2

e

Fig. 10. D-R model plots for Cu (II) on p-MWCNTs (a), and s-MWCNTs (b).

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