Journal of Colloid and Interface Science 311 (2007) 338–346
www.elsevier.com/locate/jcis
Studies of the equilibrium and thermodynamics of the adsorption of Cu
2
+
onto as-produced and modified carbon nanotubes
Chung-Hsin Wu
∗Department of Environmental Engineering, Da-Yeh University, 112, Shan-Jiau Road, Da-Tsuen, Chang-Hua, Taiwan Received 7 December 2006; accepted 24 February 2007
Available online 17 April 2007
Abstract
This study evaluates the Cu2+adsorption efficiency of as-produced carbon nanotubes (CNTs) and those modified by HNO3and NaOCl. The surface area, pHpzc, pore volume, FTIR analyses, and average pore size of CNTs were determined to compare the differences between nanotubes
before and after HNO3and NaOCl modification. The HNO3and NaOCl modifications increased the pore volume and the average pore size of
CNTs; in contrast, the pHpzcwas decreased. The modification processes produced some functional groups. The adsorption capacity of Cu2+on
as-produced and modified CNTs increased with the pH and temperature; however, the effects of the ionic strength on the adsorption of Cu2+on as-produced and modified CNTs were negligible. The linear correlation coefficients of Langmuir and Freundlich isotherms were obtained and the results revealed that the Langmuir isotherm fitted the experimental results better than did the Freundlich isotherm. The adsorption capacity of Cu2+followed the order NaOCl-modified CNTs > HNO3-modified CNTs > as-produced CNTs. Changes in the free energy of adsorption
(Go), enthalpy (Ho), and entropy (So)were determined. All Govalues were negative; the Hovalues of as-produced, HNO3-modified,
and NaOCl-modified CNTs were 10.84, 17.08, and 67.77 kJ/mol and the Sovalues were 96.89, 122.88, and 319.76 J/mol K, respectively.
©2007 Elsevier Inc. All rights reserved.
Keywords: Adsorption; Carbon nanotubes; Isotherm; Copper; Thermodynamics
1. Introduction
The disposal of industrial effluents that contain heavy metals into natural water systems is a serious environmental concern. Heavy metals are nondegradable and can accumulate in ani-mals and plants, so they must be removed from wastewater. In the field of wastewater treatment, various approaches have been used for removing heavy metals, such as precipitation, electro-chemical treatment, electro-chemical oxidation or reduction, solvent extraction, ion exchange, and adsorption. One of these meth-ods, adsorption, is a cost-effective, simple, and widely used method. Copper is one of the most widespread heavy metal contaminants of the environment. Copper is essential to human life and health but, like all heavy metals, is potentially toxic. The major sources of copper in Taiwan’s industrial effluents are metal cleaning and electroplating. Numerous adsorbents,
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E-mail address:chunghsinwu@yahoo.com.tw.
such as rubber-wood-sawdust activated carbon[1], manganese-oxide-coated sand[2], water-treatment sludge[3], γ -Al2O3[4],
grafted silica [5], kaolinite [6], montmorillonite [7], electric furnace slag[8], zeolite[9,10], granular activated carbon and powder activated carbon[9], tree fern [11], vanillin–chitosan membrane [12], Aspergillus niger [13], Tectona granges L.f.
[14], dehydrated wheat bran[15], Capsicum annuum[16], and
Sporopollenin[17], have been examined for their potential to remove copper from wastewater.
Carbon nanotubes (CNTs) are relatively new adsorbents for trace pollutants from water, because they have a large specific surface area and small, hollow, and layered structures. Much attention has been paid to adsorption by CNTs of such ions as Cd2+ [18], Zn2+ [19], Pb2+ [20–23], Cu2+ [23,24], and Cr6+ [25]. Earlier studies have suggested that CNTs may be a promising adsorbent for treating wastewater. An understand-ing of adsorption equilibrium and thermodynamics is required to design and operate adsorption equipment. Earlier works have presented equilibrium and kinetic adsorption data and a
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C.-H. Wu / Journal of Colloid and Interface Science 311 (2007) 338–346 339
few studies have measured the thermodynamic parameters of adsorption on CNTs: Peng et al. [26] measured the thermo-dynamic parameters of the adsorption of 1,2-dichlorobenzene. Li et al. [21]investigated the thermodynamics of the adsorp-tion of Pb2+. Wu [27]elucidated the thermodynamics of the adsorption of Procion Red MX-5B. Lu et al. [28] examined the thermodynamics of the adsorption of trihalomethanes on CNTs. Few studies have focused on the adsorption of heavy metals on CNTs and simultaneously determined the equilib-rium and thermodynamic parameters, and none have focused on the parameters that govern the adsorption of copper. This study elucidates the equilibrium and thermodynamics of the adsorp-tion of Cu2+ onto CNTs. Furthermore, the copper adsorption capacity is increased herein by modifying the surface of as-produced CNTs by HNO3 and NaOCl. The objectives of this
investigation were (i) to identify and compare the surfaces of as-produced, HNO3-modified, and NaOCl-modified CNTs; (ii) to
determine the effects of pH, ionic strength, and temperature on the adsorption of Cu2+ by as-produced and modified CNTs; (iii) to determine the coefficients of Langmuir and Freundlich isotherms; and (iv) to derive changes in the thermodynamic parameters—free energy (Go), enthalpy (Ho), and entropy
(So)—during adsorption.
2. Materials and methods 2.1. Materials
The as-produced CNTs utilized herein were multiwall nan-otubes (CBT, MWNTs-2040). As-produced CNTs were gener-ated by the pyrolysis of methane gas on particles of Ni in a chemical vapor deposition. The length of as-produced CNTs was 5–15 µm. All solutions were prepared using deionized wa-ter (Milli-Q) and reagent-grade chemicals. The Cu2+stock so-lution was prepared from Cu(NO3)2·3H2O and Milli-Q
deion-ized water. The background electrolyte concentrations were ad-justed by adding NaNO3to 0.01, 0.03, and 0.05 M.
2.2. Characterization of CNTs
The size and morphology of CNTs were observed by trans-mission electron microscopy (TEM) using a Model JEM-2010 (JEOL, Japan). The specific surface area, the average pore di-ameter, and the pore volume of CNTs were measured by the BET method, using a Model ASAP 2010 surface area analyzer (Micromeritics, USA). The functional groups of CNTs were identified by Fourier transform infrared spectroscopy (FTIR) analysis using a Spectrum One and Autoimagic system (Perkin Elmer, USA) via the KBr pressed-disc method. The pH of the point of zero charge of the CNTs was measured at pH values of 2–9 using a Zeta-Meter 3.0 (Zeta-Meter Inc., USA). Elec-trodes placed at each end of the chamber are connected to the Zeta-Meter 3.0 unit, creating an electric field across the cham-ber. Charged colloids move in the field and their velocity and direction are related to their zeta potential. Ten measurements were made of each sample at each pH and the mean was deter-mined as the zeta potential.
2.3. Experiments
The surfaces of as-produced CNTs were modified by HNO3
and NaOCl to remove the catalyst (Ni particles) and amorphous carbon. Before the modification experiments, as-produced CNTs (1 g) were heated at 623 K for 30 min. In the modifi-cation of HNO3, as-produced CNTs (1 g) were immersed in
HNO3(65%) and shaken in an ultrasonic bath for 30 min; they
were then stirred continuously for 36 h at 298 K. In the modi-fication of NaOCl, the as-produced CNTs (1 g) were immersed in NaOCl (60%) and shaken in an ultrasonic bath for 30 min; they were then continuously stirred for 3 h at 358 K. The mod-ified CNTs were washed repeatedly using Milli-Q deionized water until the solution reached pH 7. Finally, the modified CNTs were dried at 343 K for 12 h. All adsorption experiments were performed in a closed 250-ml glass pyramid bottle, con-taining 0.05 g of as-produced or modified CNTs; 100 ml of Cu2+ solution was placed in a water bath, which was shaken at 125 rpm for 24 h. In experiments on the effect of pH (ini-tial Cu2+ = 43.1 mg/L, ionic strength = 0.01 M, CNTs = 0.5 g/L, and temperature = 300 K), the pH of the solution was adjusted between 2 and 9 using 0.1 M HNO3and 0.1 M
NaOH. In the experiments on the effect of temperature (ini-tial Cu2+ = 43.1 mg/L, ionic strength = 0.01 M, and CNTs = 0.5 g/L), the temperature was held at 280, 290, 300, 310, and 320 K and the pH was fixed at 6.0. At the end of the equi-librium period, the suspensions were centrifuged at 4000 rpm for 10 min, and the supernatant was then filtered through 0.2-µm filter paper (Gelman Sciences) for subsequent analysis of the Cu2+concentration. The adsorption of Cu2+was detected using an inductively coupled plasma (ICP) spectrometer.
3. Results and discussion
3.1. Surface characteristics of as-produced and modified CNTs
The TEM images demonstrated that the as-produced and modified CNTs were cylindrical and that the main external di-ameters of as-produced, HNO3-modified, and NaOCl-modified
CNTs were 50–60, 30–40, and 20–30 nm, respectively (Fig. 1). The decline in the diameter of the modified CNTs may be due to the removal of amorphous carbon from the as-produced CNTs by the oxidation of HNO3 and NaOCl. The good
dis-persibility of NaOCl-modified CNTs suggests that hydrophilic groups, such as carboxyl and hydroxyl, were generated on the surface of the NaOCl-modified CNTs. This suggestion will be verified in the FTIR analyses. Li et al. [22] established that the chemical oxidation of CNTs can introduce various acidic functional groups onto their surfaces and thereby improve their hydrophilicity. After they were modified by NaOCl, the dis-cretization of CNTs was increased, and the external surface of the CNTs was then enlarged. The modification by HNO3and
NaOCl can be expressed roughly as the following reactions: 10Cn+ 6H++ 6NO−3 + 2H2O
(1) → 10Cn−1− COOH + 3N2,
340 C.-H. Wu / Journal of Colloid and Interface Science 311 (2007) 338–346
Table 1
Surface characteristics of as-produced and modified CNTs
Sample Surface area (m2/g) Average pore diameter (nm) Pore volume (cm3/g) pHpzc
As-produced CNTs 82.2 2.5 1.07 4.86
HNO3-modified CNTs 64.3 4.2 1.32 2.85
NaOCl-modified CNTs 94.9 7.8 1.32 –
Fig. 1. TEM images of CNTs (a) as-produced CNTs, (b) HNO3-modified CNTs, and (c) NaOCl-modified CNTs (magnification: 30,000).
(2) 2Cn+ 3OCl−+ H2O→ 2Cn−1− COOH + 3Cl−.
Table 1presents the surface characteristics of as-produced and modified CNTs. The specific surface area of HNO3-modified
CNTs was less than that of as-produced CNTs. One possi-ble reason is that the CNTs became short and that the space among the isolated CNTs was reduced by modification[19]. In contrast, the specific surface area of NaOCl-modified CNTs ex-ceeded that of as-produced CNTs, suggesting that the oxidation of CNTs with NaOCl effectively removes amorphous carbons and can be used to probe the inner cavities of the CNTs, expos-ing their internal surfaces[18]. HNO3 and NaOCl
modifica-tion significantly increased the average pore diameter and pore volume of CNTs. The experimental results suggest that these treatments may damage the original structures of the CNTs and create some defects on the surface. Therefore, the average pore diameter and pore volume of modified CNTs increased. In par-ticular, the specific surface area and average pore diameter of NaOCl-modified CNTs both exceeded those of HNO3-modified
CNTs, implying that the oxidizing ability of NaOCl exceeded that of HNO3. The experimental results reveal that oxidation
treatments might not have only removed Ni particles and amor-phous carbons but also produced some structural defects. This observation was consistent with those of Monthioux et al.[29].
Fig. 2displays the FTIR spectra of as-produced and mod-ified CNTs. The as-produced CNTs exhibited only one peak near 1450–1600 cm−1, which could be assigned to the aro-matic –C=C groups; moreover, NaOCl oxidation significantly strengthened this peak. The NaOCl-modified CNTs yielded four major peaks in the ranges 1450–1600, 1700–1740, 2800– 3100, and 3200–3400 cm−1, which were associated with aro-matic –C=C groups [30], carbonyl groups (–C=O) from
car-boxylic acids (–COOH) [30,31], –CH groups [30,32], and hydroxyl groups (–OH) from carboxylic acids or alcoholic groups (–COH) [19,28,30–32], respectively. The aromatic –C=C groups, –CH groups, and hydroxyl groups of the HNO3
-modified CNTs were detected herein. In earlier research, aro-matic –C=C groups and hydroxyl groups were observed in HNO3-modified CNTs [28,31,32] and phenolic groups (O–
H), and carbonyl groups and hydroxyl groups were identified in NaOCl-modified CNTs [19]. Evidently, several functional groups were generated on the surface of modified CNTs, pro-viding various adsorption sites, increasing the adsorption ca-pacity[28].
Fig. 3plots the zeta potential of as-produced and modified CNTs. All of the zeta potentials of CNTs became more nega-tive as the pH increased. The pH of the point of zero charge (pHpzc) for as-produced and HNO3-modified CNTs was
de-termined to be 4.86 and 2.85, respectively (Table 1). These values indicate that the surfaces of as-produced and HNO3
-346 C.-H. Wu / Journal of Colloid and Interface Science 311 (2007) 338–346
Avogadro number (6.02× 1023); A is the cross-sectional area of Cu2+ (1.58× 10−20 m2)and Mw is the molecular weight
of Cu2+. The theoretical specific surface areas of as-produced,
HNO3-modified, and NaOCl-modified CNTs were calculated
as 1.24, 2.08, and 7.10 m2/g, respectively. The ratio of available adsorption sites of CNTs was determined by dividing the theo-retical specific surface area by the actual specific surface area of CNTs. The ratios of available adsorption sites of as-produced, HNO3-modified, and NaOCl-modified CNTs were 1.5, 3.2, and
7.5%, respectively.
4. Conclusions
This study investigated the equilibrium adsorption of Cu2+ onto as-produced and modified CNTs at various pH values, ionic strengths, and temperatures. The average pore diame-ter and pore volume of CNTs increased markedly afdiame-ter HNO3
and NaOCl modification. Aromatic –C=C groups, –CH groups, and hydroxyl groups were detected on the surfaces of HNO3
-modified and NaOCl--modified CNTs. The negatively charged surfaces of modified CNTs electrostatically favor the adsorp-tion of Cu2+—more in NaOCl-modified CNTs than in HNO3
-modified CNTs. The effect of background electrolyte concen-trations on copper adsorption at pH 6 is fairly negligible. At a given pH, the adsorption capacity of Cu2+followed the or-der NaOCl-modified CNTs > HNO3-modified CNTs >
as-produced CNTs. The results shown that the adsorption of Cu2+ on CNTs increased with temperature. Positive Hovalues re-vealed that the adsorption of Cu2+ onto CNTs was endother-mic, which was supported by the increase in the adsorption of Cu2+ with temperature. The negative values of Go sug-gested that the adsorption of Cu2+onto as-produced and mod-ified CNTs was spontaneous. This study suggests that HNO3
and NaOCl modification not only increased the area of ac-tive adsorption sites of CNTs but also increased the proportion of available adsorption sites; additionally, NaOCl modified the surface of as-produced CNTs more effectively than HNO3.
Acknowledgments
The authors thank the National Science Council of the Re-public of China, Taiwan, for financially supporting this re-search under Contract NSC 94-2211-E-212-012. Additionally, Yi-Ling Hsu of the National Yunlin University of Science and Technology is appreciated for assistance in conducting some of the experiments.
References
[1] M.H. Kalavathy, T. Karthikeyan, S. Rajgopal, L.R. Miranda, J. Colloid Interface Sci. 292 (2005) 354.
[2] R. Han, Z. Lu, W. Zou, W. Daotong, J. Shi, Y. Jiujun, J. Hazard Mater. 137 (2006) 480.
[3] C.H. Wu, C.F. Lin, P.Y. Horng, J. Environ. Sci. Health A 39 (2004) 237. [4] C.H. Wu, C.F. Lin, H.W. Ma, T.Q. His, Water Res. 37 (2003) 743. [5] N. Chiron, R. Guilet, E. Deydier, Water Res. 37 (2003) 3079. [6] O. Yavuz, Y. Altunkaynak, F. Guzel, Water Res. 37 (2003) 948. [7] S.H. Lin, R.S. Juang, J. Hazard Mater. 92 (2002) 315.
[8] L. Curkovic, S.C. Stefanovic, A.R. Mioe, Water Res. 35 (2001) 3436. [9] H.K. An, B.Y. Park, D.S. Kim, Water Res. 35 (2001) 3551.
[10] M.I. Panayotova, Waste Manag. 21 (2001) 671. [11] Y.S. Ho, Water Res. 37 (2003) 2323.
[12] A.R. Cestari, E.F.S. Vieira, C.R.S. Mattos, J. Chem. Thermodynam. 38 (2006) 1092.
[13] A. Dursun, Biochem. Eng. J. 28 (2006) 187.
[14] Y.P. Kumar, P. King, V.S.R.K. Prasad, J. Hazard Mater. 137 (2006) 1211. [15] A.S. Ozcan, B. Erdem, A. Ozcan, J. Colloid Interface Sci. 280 (2004) 44. [16] A. Ozcan, A.S. Ozcan, S. Tunali, T. Akar, I. Kiran, J. Hazard Mater. 124
(2005) 200.
[17] N. Unlu, M. Ersoz, J. Hazard Mater. 136 (2006) 272.
[18] Y.H. Li, S. Wang, Z. Luan, J. Ding, C. Xu, D. Wu, Carbon 41 (2003) 1057. [19] C.S. Lu, H.S. Chiu, Chem. Eng. Sci. 61 (2006) 1138.
[20] Y.H. Li, S.G. Wang, J.Q. Wei, X.F. Zhang, C.L. Xu, Z.K. Luan, D.H. Wu, B.Q. Wei, Chem. Phys. Lett. 357 (2002) 263.
[21] Y.H. Li, Z. Di, J. Ding, D. Wu, Z. Luan, Y. Zhu, Water Res. 39 (2005) 605. [22] Y.H. Li, Y. Zhu, Y. Zhao, D. Wu, Z. Luan, Diamond Relat. Mater. 15
(2006) 90.
[23] X. Peng, Z. Luan, Z. Di, Z. Zhang, C. Zhu, Carbon 43 (2005) 880. [24] Y.H. Li, J. Ding, Z. Luan, Z. Di, Y. Zhu, C. Xu, D. Wu, B. Wei, Carbon 41
(2003) 2787.
[25] Z.C. Di, J. Ding, X.J. Peng, Y.H. Li, Z.K. Luan, J. Liang, Chemosphere 62 (2006) 861.
[26] X. Peng, Y. Li, Z. Luan, Z. Di, H. Wang, B. Tian, Z. Jia, Chem. Phys. Lett. 376 (2003) 154.
[27] C.H. Wu, J. Hazard Mater. (2006),doi:10.1016/j.jhazmat.2006.09.083. [28] C.S. Lu, Y.L. Chung, K.F. Chang, J. Hazard Mater. 138 (2006) 304. [29] M. Monthioux, B.W. Smith, B. Burteaux, A. Claye, J.E. Fischer, D.E.
Luzzi, Carbon 39 (2001) 1251.
[30] W.M. Davis, C.L. Erickson, C.T. Johnston, J.J. Defino, J.E. Porter, Chemosphere 38 (1999) 2913.
[31] C.S. Lu, Y.L. Chung, K.F. Chang, Water Res. 39 (2005) 1183.
[32] C. Yang, X. Hu, D. Wang, C. Dai, L. Zhang, H. Jin, S. Agathopoulos, J. Power Sources 160 (2006) 187.
[33] K.F. Hayes, J.O. Leckie, J. Colloid Interface Sci. 115 (1987) 564. [34] A.S. Ozcan, B. Erdem, A. Ozcan, Colloids Surf. A 266 (2005) 73. [35] A. Ozcan, E.M. Oncu, A.S. Ozcan, Colloids Surf. A 277 (2006) 90. [36] A. Ozcan, E.M. Oncu, A.S. Ozcan, J. Hazard Mater. 129 (2006) 244. [37] H. Nollet, M. Roels, P. Lutgen, P. Meeren, W. Verstraete, Chemosphere 53
(2003) 655.
[38] M. Kara, H. Yuzer, E. Sabah, M.S. Celik, Water Res. 37 (2003) 224. [39] A. Ozer, D. Ozer, A. Ozer, Process Biochem. 39 (2004) 2183. [40] Y.S. Ho, C.T. Huang, H.W. Huang, Process Biochem. 37 (2002) 1421.
