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Adsorption of trihalomethanes from water with carbon nanotubes
ARTICLE IN PRESS
Water Research 39 (2005) 1183–1189 www.elsevier.com/locate/watres
Adsorption of trihalomethanes from water with carbon nanotubes Chungsying Lu, Yao-Lei Chung, Kuan-Foo Chang Department of Environmental Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung, Taiwan Received 23 June 2004; received in revised form 18 November 2004; accepted 12 December 2004
Abstract Commercial carbon nanotubes (CNTs) were purified by acid solution and were employed as adsorbents to study adsorption of trihalomethanes (THMs) from water. The properties of CNTs such as purity, structure and nature of the surface were greatly improved after acid treatment which made CNTs become more hydrophilic and suitable for adsorption of low molecular weight and relatively polar THM molecules. The adsorption of THMs onto CNTs fluctuates very little in the pH range 3–7, but decreases with pH value as pH exceeds 7. A comparative study between CNTs and powdered activated carbon (PAC) for adsorption of THMs from water was also conducted. The short time needed to reach equilibrium as well as the high adsorption capacity of CHCl3, which accounts for a significant portion of THMs in the chlorinated drinking water, suggests that CNTs possess highly potential applications for THMs removal from water. r 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Adsorption; Trihalomethanes; pH effect; Powdered activated carbon
1. Introduction Disinfection is routinely carried out in water treatment process or before finished water leaves the treatment plant to prevent microbiological degradation of drinking water quality. Until recently, chlorine was the most commonly employed disinfectant. To ensure the disinfection capacity of drinking water minimum chlorine residues must be maintained (Biswas et al., 1993). However, trihalomethanes (THMs; CHCl3, CHBrCl2, CHBr2Cl and CHBr3) were found to be formed during the chlorination of drinking water (Rook, 1974). THMs are recognized as potentially
Corresponding author. Tel.: +886 4 22840441; fax: +886 4 22862587. E-mail address: [email protected] (C. Lu).
hazardous and carcinogenic substances (Bull et al., 1995). Therefore, more stringent requirements for the removal of THMs from drinking water in recent years have necessitated the development of innovative, costeffective treatment alternatives. Carbon nanotubes (CNTs) are relatively new adsorbents for adsorption of trace pollutant from water or air. Long and Yang (2001) reported that a significantly higher dioxin removal efficiency is found with CNTs than that with activated carbon. Li et al. (2002b) found that CNTs have high lead adsorption capacity and can be used as an adsorbent for lead removal from water. Li et al. (2003) showed that CNTs are good fluoride adsorbents and their fluoride removal capability is superior to activated carbon. Peng et al. (2003) indicated that CNTs are good adsorbents to remove 1,2-dichlorobenzene from water and can be used in a wide pH range of 3–10.
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.12.033
ARTICLE IN PRESS 1184
C. Lu et al. / Water Research 39 (2005) 1183–1189
CNTs have been proved to possess great potential applications in environmental protection by the foregoing investigators. However, the studies on the adsorption of trace pollutants with CNTs are still very limited in the literature. This paper aims at investigating adsorption of THMs from water with CNTs. A comparative study between CNTs and powdered activated carbon (PAC) is also conducted.
THMs adsorbed by CNTs were calculated as follows: q ¼ ðC 0 C t ÞV =m;
(1)
where q is the amount of THMs adsorbed by CNTs (mg/g), C0 the initial THMs concentration (mg/l), Ct the THMs concentration after a certain period of time (mg/l), V the solution volume (l) and m the CNTs dosage (g). 2.4. Analytical methods
2. Materials and methods 2.1. Preparation of purified CNTs Multi-walled CNTs (Model CN3016, Nanotech Port Co., Shenzhen, China), with outer diameter range 10–30 nm and inner diameter range 5–10 nm, were fabricated by catalytic decomposition of the CH4/H2 mixture at 700 1C using Ni particles as catalyst. The length of as-prepared CNTs was in the range of 500 nm–500 mm. The as-prepared CNTs were dispersed into a 150 ml flask containing 40 ml concentrated acid solutions (30 ml HNO3+10 ml H2SO4) for 24 h to remove metal catalysts and then washed by deionized water. After cleaning, the CNTs were again dispersed into a 150 ml flask containing 40 ml concentrated acid solutions and refluxed using an ultrasonic cleaning bath (BRANSON 3510 Sonic cleaner, CT, USA) at 80 1C for 2 h to remove amorphous carbon. Finally, the CNTs-containing solution was filtered by 0.45 mm glass-fiber filter to obtain purified CNTs. 2.2. Preparation of THMs solution THMs solution was prepared by diluting a 2000 mg/l analytical grade THMs solution (Supelco Inc. Bellefonte, PA, USA), which contains equivalent concentration of CHCl3, CHCl2Br, CHClBr2 and CHBr3, into deionized water to obtain the desired THMs concentration. 2.3. Batch adsorption experiment Batch adsorption experiments were performed using 150 ml glass bottles with addition of 50 mg purified CNTs and 125 ml of THMs solution of increased initial concentrations (C0) from 0.2 to 12 mg/l. The glass bottles were sealed with Teflon and then mounted on a shaker. The shaker was placed within a temperature control box (Model CH-502, Chin Hsin, Taipei, Taiwan) to maintain water temperature at 25 1C. The pH value of the solution ranging from 3 to 11 was chosen to study the pH effect on THMs adsorption. The pH was adjusted using 1 M H2SO4 or 1 M NaOH.
THMs concentration was determined using a gas chromatograph (Hewlett Packard Model 5890 II gas chromatograph, MD, USA) equipped with an electron capture detector (ECD). A 30 m HP-5 fused silica capillary column (0.25 mm inside diameter, 1.5 mm film thickness) was used for THMs analysis. The GC-ECD was operated at injection temperature of 177 1C, detector temperature of 272 1C and oven temperature of 110 1C. The total surface area, mean pore size, pore size distribution and pore volume of purified CNTs were determined by a BET sorptometer (Model BET-202A, Porous Materials Inc., NY, USA).
3. Results and discussion 3.1. Characterization of purified CNTs Shape and size are essential as identifying characteristics of adsorbents since these parameters determine the specific surface area and thus influence adsorption rate and sensitivity to environmental conditions. The scanning electron microscope (SEM) image indicates that the isolated CNTs are in cylindrical shapes with an average external diameter of around 25 nm. The transmission electron microscope (TEM) image shows that the purified CNTs possess multi-wall with the hollow inner tube diameter of around 8 nm. Fig. 1 exhibits the pore size distribution of purified CNTs. It is obvious that the pore size is a bimodal distribution. The major peak is located in the size range of 1–4 nm while the secondary peak is located in the size range of 20–40 nm. The 1–4 nm pores are the CNT inner cavities and are responsible for about 50% of the total pore volume. The 20–40 nm pores are likely to be contributed by aggregated pores which are formed by the confined space among the isolated CNTs (Yang et al., 2001) and are responsible for around 20% of the total pore volume. This is because the inner diameter of CNTs is only around 8 nm. The BET surface area of CNTs increases from 225 to 295 m2/g after acid treatment. A possible reason may be the fact that the acid treatment can untie entwined CNTs and thus increase the surface area.
ARTICLE IN PRESS C. Lu et al. / Water Research 39 (2005) 1183–1189
0.25
30
Aggregated pores
CNT 0.20
D band
28
G band
26 24
Intensity (a.u.)
(volume fraction / nm)
1185
0.15 0.10
22 20 18 16 14
0.05
12 10
0.00 1
10 Pore size (nm)
100
0
(a)
Fig. 1. Pore size distributions of CNTs after acid treatment.
200 400 600 800 1000120014001600180020002200
Raman shift (cm-1) 30 G band
28
Weight remained (%)
100
Intensity (a.u.)
26
a
80 b
60
D band
24 22 20 18 16 14
40
12 10
4.95%
20
1.13% 0 0
100 200 300 400 500 600 700 800 900 Temperature (° C)
0
(b)
200 400 600 800 1000120014001600180020002200
Raman shift (cm-1)
Fig. 3. Raman spectra of CNTs: (a) before acid treatment, and (b) after acid treatment.
Fig. 2. Thermogravimetric analyses of CNTs: (a) before acid treatment, and (b) after acid treatment.
Thermogravimetric analysis (Fig. 2) and Raman spectra (Fig. 3) are employed to investigate the changes of purity and structure of CNTs after acid treatment. Thermogravimetric analysis of CNTs indicates that gasification ends at around 740 1C for as-prepared CNTs and around 700 1C for purified CNTs, in which 4.95% and 1.13% remaining weight was observed, respectively. This implies that the purity of CNTs increases from 95.05% to 98.87% after acid treatment. Raman spectrum of CNTs shows that there are two sharp peaks. The peak near 1334 cm1 is the so-called D-band which is related to disordered sp2-hybridized carbon atoms of CNTs (Tsai and Chen, 2003). The peak near 1582 cm1 is the so-called G-band which is related to the structure integrity of sp2-hybridized carbon atoms of the CNTs. The ID/IG ratio decreases from 1.047 to 0.845 after acid treatment, implying that the purified
CNTs possess more graphitized structures, thereby, improving their properties. Fig. 4 shows the Fourier transformed infrared spectra of CNTs before and after acid treatment. It is seen that the as-prepared CNTs exhibit no peaks. In contrast, the purified CNTs exhibit three major peaks at wavenumber of 1400, 1700 and 3500 cm1, which are associated with carboxylic acids and phenolic groups (O–H), carbonyl groups ð4C QOÞ and hydroxyl groups (–OH) (Li et al., 2002c). It is evident that there are many other functional groups attached on the surface of purified CNTs. 3.2. Effect of contact time Figs. 5a and b show the effect of contact time on the adsorption of THMs onto CNTs with the initial THMs concentrations (C0) of 0.2 and 3.2 mg/l. The concentration of each THM molecule is identical, while the pH of water was 6.87. It is noted that the adsorption of THMs
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50
40 C=O O-H b
-OH 30
20 4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1) Fig. 4. Fourier transformed infrared spectra of CNTs: (a) after acid treatment, and (b) before acid treatment.
0.12 0.10
3.3. Adsorption isotherms
q (mg/g)
0.08 0.06 CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.04 0.02 0.00 0
100
200
(a)
300 t (min)
400
500
1.6 1.4
q (mg/g)
1.2
Fig. 6 shows the adsorption isotherms of THMs with CNTs. As can be seen, the adsorption capacity of CHCl3 is the highest, followed by CHBrCl2, CHBr2Cl and then CHBr3. The adsorbed amounts of CHCl3, CHCl2Br, CHClBr2 and CHBr3 are equal to 2.41, 1.23, 1.08 and 0.92 mg/g, respectively, for equilibrium concentration of 1 mg/l. There are two possible reasons to explain the high capacity for adsorption of CHCl3 onto CNTs. First, previous study indicated that all wetting and filling results of CNTs are related to the surface tension of the liquid. Only low surface tension liquid will wet the CNT surface and fill open CNTs by capillarity (Ebbesen, 1996). The wettability with water certainly influences adsorption capacity of THM molecules from
1.0
3.0
0.8 2.5
0.6 CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.4 0.2
2.0
0.0 0 (b)
C 0 ¼ 3:2 mg=l: The final adsorption capacities of CHCl3, CHBrCl2, CHBr2Cl and CHBr3 reach 0.11, 0.06, 0.05 and 0.05 mg/g for C 0 ¼ 0:2 mg=l and achieve 1.51, 0.78, 0.72 and 0.64 mg/g for C 0 ¼ 3:2 mg=l: The longer contact time to reach equilibrium for lower initial THMs concentration may be explained by the fact that diffusion mechanisms control the adsorption of THMs onto CNTs. Reid et al. (1988) indicated that the mass diffusivity decreases with decreasing concentration under very dilute solution and causes the decrease in diffusion flux of adsorbate onto the surface of the adsorbent. It is also noted that the smallest molecule, CHCl3, is the most preferentially adsorbed onto CNTs, followed by CHBrCl2, CHBr2Cl and then CHBr3. This may be attributed to the fact that adsorption occurs with molecules that are small enough in size to enter the inner cavities through the pores. Therefore, the adsorption rate increases with decreasing size of the adsorbed molecules.
100
200
300 t (min)
400
500
600
Fig. 5. Effect of contact time on the adsorption of THMs with CNTs: (a) C 0 ¼ 0:2 mg=l; and (b) C 0 ¼ 3:2 mg=l:
increases quickly with time and then reaches equilibrium. The contact time to reach equilibrium is equal to 180 min for C 0 ¼ 0:2 mg=l and equal to 150 min for
q (mg/g)
Transmission (%)
a
1.5 1.0 0.5
CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Ce (mg/l) Fig. 6. Adsorption isotherms for THMs with CNTs.
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of one component was changed, the adsorption of other three components will change even if their concentrations are kept at the same value of this study.
their aqueous solution. The surface tension of CHCl3 (27.14 dyn/cm) (Holbrook, 1993) is much lower than that of CHBr3 (46.2 dyn/cm) (Jasper, 1972), which made adsorption of CHCl3 onto CNTs much easier. Second, there are many oxygen-containing groups attached on the surface of purified CNTs, which made CNTs become more hydrophilic and suitable for adsorption of relatively polar molecules. In THM molecules, the dipole moments of C–Cl bond and C–Br bond are equal to 1.56 and 1.48 Debye, respectively. This causes the polarity of covalent to become less as the number of bromine ion in THM molecule increases (Morris and Boyd, 1987). The polarity of CHCl3 is thus the highest, followed by CHBrCl2, CHBr2Cl and then CHBr3, which made adsorption of CHCl3 onto CNTs much easier. The experimental data for THMs adsorption onto CNTs could be approximated by the isotherm models of Langmuir (2) and Freundlich (3) q¼
abC e ; 1 þ bC e
3.4. Effect of pH value Fig. 7 shows the effect of pH on adsorption of THMs onto CNTs with the initial THMs concentration of 2.0 mg/l. It is obvious that the adsorption of THMs onto CNTs fluctuates very little in the pH range of 3–7. This may be due to the fact that the employed CNTs have been purified by acid solution to improve their properties which may enhance the resistance of CNTs to acid environment. However, the adsorption of THMs decreases as the pH exceeds 7. This is due to the fact that more oxygen-containing groups on the CNTs surface are ionized at higher pH values and thus they adsorb more water (Peng et al., 2003). The formation of water cluster on these groups blocks the access of THM molecules to adsorption sites and results in less adsorption of THMs.
(2)
q ¼ K f C ne ;
1187
(3) 2.6
where q is the mass of THMs adsorbed by CNTs, Ce the equilibrium THMs concentration, a and b Langmuir constants and Kf and n are Freundlich constants. The isotherm constants are obtained from fitting the adsorption equilibrium data and are listed in Table 1. As can be seen, both Langmuir and Freundlich isotherm models match the experimental data very well, with the correlation coefficient values of 0.9710.996 and 0.9530.980, respectively. The constants a and Kf, which are related to adsorption capacity, are the highest for adsorption of CHCl3, followed by CHBrCl2, CHBr2Cl and then CHBr3, which is consistent with the experimental observation. It should be mentioned that these experiments were carried out in the system of competitive adsorption; the parameters of individual adsorption listed in Table 1 correlated with each other. That is, if the concentration
2.4 2.2 q (mg/g)
2.0 CHCl3 CHBrCl2 CHBr2Cl CHBr3
1.8 1.6 1.4 1.2 1.0 0.8 3
4
5
6
7 pH
8
9
10
11
Fig. 7. Effect of pH on the adsorption of THMs with CNTs (C 0 ¼ 2 mg=l).
Table 1 Constant of Langmuir and Freundlich models for adsorption of THMs onto CNTs THMs
CHCl3 CHBrCl2 CHBr2Cl CHBr3
Langmuir model
Freundlich model 2
a
b
R
4.730 2.925 2.763 2.695
0.00233 0.00087 0.00048 0.00067
0.979 0.971 0.996 0.986
Kf
n
R2
2.721 1.087 1.022 0.901
0.583 0.699 0.741 0.755
0.953 0.968 0.980 0.977
Note: The starting solution contains equivalent concentration of four THM molecules. Unit: a ¼ mg=g; b ¼ l=mg; K f ¼ 1n mg1n =g; n ¼ dimensionless; R ¼ dimensionless:
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3.5. Comparative experiment
3.2
In the comparative study, a test for adsorption of THMs with PAC (Model 18001, Ridel deHae¨n Co., Seelze, Germany) was conducted. The surface area of PAC is equal to 900 m2/g. Figs. 8a and b show the effect of contact time on the adsorption of THMs onto PAC with the initial THMs concentrations (C0) of 0.2 and 3.2 mg/l, respectively. It is noted that the adsorption of THMs onto PAC also increases quickly with time and then reaches equilibrium. The contact time to reach equilibrium is equal to 450 min for C 0 ¼ 0:2 mg=l and equal to 360 min for C 0 ¼ 3:2 mg=l: The final adsorption capacities of CHCl3, CHCl2Br, CHClBr2 and CHBr3 reach 0.063, 0.089, 0.108 and 0.119 mg/g for C 0 ¼ 0:2 mg=l and achieve 0.98, 1.2, 1.29 and 1.52 mg/g for C 0 ¼ 3:2 mg=l: It is also noted that the largest molecule, CHBr3, is most preferentially adsorbed onto PAC, followed by CHBr2Cl, CHBrCl2 and then CHCl3. This is because the adsorption capacity of activated carbon generally increases with the compound’s molecular weight or boiling point. Furthermore, activated carbon has greater
2.8
0.14 0.12
q (mg/g)
0.10 0.08 0.06 CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.04 0.02 0.00 0
200
400
(a)
600 t (min)
800
1000
1.6 1.4
q (mg/g)
1.2 1.0 0.8 CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.6 0.4 0.2
(b)
100
200
300
400 500 t (min)
600
700
q (mg/g)
2.0 1.6 1.2
CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.8 0.4 0.0 0.0
0.5
1.0 Ce (mg/l)
1.5
2.0
Fig. 9. Adsorption isotherms for THMs with PAC.
affinity for adsorption of relatively nonpolar molecules (Li et al., 2002a). Fig. 9 shows the adsorption isotherms of THMs with PAC. The adsorbed amounts of CHCl3, CHCl2Br, CHClBr2 and CHBr3 are equal to 1.2, 1.68, 2.19 and 2.75 mg/g, respectively, for equilibrium concentration of 1 mg/l. The adsorbed amount of CHCl3 onto PAC is much lower than those of other THM molecules. By comparing the adsorption of THMs with CNTs and PAC, it is evident that it takes less contact time for CNTs to reach equilibrium. This may be explained by the fact that CNTs have no porous structure like PAC in which THMs have to move from the exterior surface to the inner surface of the pores on PAC to reach equilibrium (Peng et al., 2003). Although the surface area of purified CNTs (295 m2/g) is much lower than that of PAC (900 m2/g), the adsorption capacity of CHCl3 onto CNTs (2.72 mg/g) is approximately twice higher than onto PAC (1.32 mg/g). This can be attributed to functional groups attached on the CNTs surface, which made CNTs become more hydrophilic and suitable for the adsorption of low molecular weight and relatively polar THM molecules. The short time needed to reach equilibrium as well as the high adsorption capacity of CHCl3, which accounts for a significant portion (80–90%) of THMs found in the chlorinated drinking water (Hsu et al., 2001), suggest that CNTs have great potential applications for THMs removal from water.
4. Conclusions
0.0 0
2.4
800
Fig. 8. Effect of contact time on the adsorption of THMs with PAC: (a) C 0 ¼ 0:2 mg=l; and (b) C 0 ¼ 3:2 mg=l:
The following conclusions could be drawn from this study: 1. The properties of CNTs such as purity, structure and nature of the surface could be greatly improved after
ARTICLE IN PRESS C. Lu et al. / Water Research 39 (2005) 1183–1189
2. 3.
4.
5.
acid treatment which made CNTs become more hydrophilic and suitable for adsorption of low molecular weight and relatively polar THM molecules. The adsorption of THMs onto CNTs can be well described by both Langmuir and Freundlich models. Adsorption of THMs onto CNTs fluctuates very little in the pH range of 3–7 but decreases with pH value as pH exceeds 7. The smallest molecule, CHCl3, is the most preferentially adsorbed onto CNTs, followed by CHBrCl2, CHBr2Cl and then CHBr3. In contrast, the largest molecule, CHBr3, is the most preferentially adsorbed onto PAC, followed by CHBr2Cl, CHBrCl2 and then CHCl3. The short time needed to reach equilibrium as well as the high adsorption capacity of CHCl3 suggests that CNTs possess highly potential applications for THMs removal from water.
Acknowledgement Support from the National Science Council, Taiwan, under a contract no. NSC 93-2211-E-005-020 is gratefully acknowledged.
References Biswas, P., Lu, C., Clark, R.M., 1993. A model for chlorine concentration decay in pipes. Water Res. 27, 1715–1724. Bull, R.J., Brinbaum, L.S., Cantor, K.P., Rose, J.B., Butterworth, B.E., Pegram, R., Tuomisto, J., 1995. Water Chlorination: essential process and cancer hazard. Fundam. Appl. Toxicol. 28, 155–166. Ebbesen, T.W., 1996. Wetting, filling and decorating carbon nanotubes. J. Phys. Chem. Solids 57, 951–995. Holbrook, M.T., 1993. Chlorocarbons and chlorohydrocarbons: chloroform. In: Kirk–Othmer Encyclopedia of
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Chemical Technology, vol. 5, fourth ed. Wiley, New York, USA, pp. 1051–1062. Hsu, C.H., Jeng, W.L., Chang, R.M., Chien, L.C., Han, B.C., 2001. Estimation of potential lifetime cancer risks for trihalomethanes from consuming chlorinated drinking water in Taiwan. Environ. Res. 85, 77–82. Jasper, J.J., 1972. Surface tension of pure liquid compounds. In compilation of data of some 2200 pure liquid compounds. J. Phys. Chem. Reference Data 1, 859. Li, L., Quinlivan, P.A., Knappe, D.R.U., 2002a. Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution. Carbon 40, 2085–2100. Li, Y.H., Wang, S., Wei, J., Zhang, X., Xu, C., Luan, Z., Wu, D., Wei, B., 2002b. Lead adsorption on carbon nanotubes. Chem. Phys. Lett. 357, 263–266. Li, Y.H., Xu, C., Wei, B., Zhang, X., Zheng, M., Wu, D., Ajayan, P.M., 2002c. Self-organized ribbons of aligned carbon nanotubes. Chem. Mater. 14, 483–485. Li, Y.H., Wang, S., Wei, J., Zhang, X., Wei, J., Xu, C., Luan, Z., Wu, D., 2003. Adsorption of fluoride from water by aligned carbon nanotubes. Mater. Res. Bull. 38, 469–476. Long, R.Q., Yang, R.T., 2001. Carbon nanotubes as superior sorbent for dioxin removal. J. Am. Chem. Soc. 123, 2058–2059. Morris, R.B., Boyd, R.N., 1987. Organic Chemistry, fifth ed. Allyn & Bacon Inc., New York, USA. Peng, X., Li, Y., Luan, Z., Di, Z., Wang, H., Tian, B., Jia, Z., 2003. Adsorption of 1,2-dichlorobenzene from water to carbon nanotubes. Chem. Phys. Lett. 376, 154–158. Reid, R.C., Prausnitz, J.M., Poling, B.E., 1988. The Properties of Gases & Liquids, fourth ed. McGraw-Hill, New York. Rook, J.J., 1974. Formation of haloforms during chlorination of natural waters. Water Treatment Exam. 23, 234–243. Tsai, C.L., Chen, C.F., 2003. Characterization of biascontrolled carbon nanotubes. Diamond Relat. Mater. 12, 1615–1620. Yang, Q.H., Hou, P.X., Bai, S., Wang, M.Z., Cheng, H.M., 2001. Adsorption and capillarity of nitrogen in aggregated multi-walled carbon nanotubes. Chem. Phys. Lett. 345, 18–24.
Water Research 39 (2005) 1183–1189 www.elsevier.com/locate/watres
Adsorption of trihalomethanes from water with carbon nanotubes Chungsying Lu, Yao-Lei Chung, Kuan-Foo Chang Department of Environmental Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung, Taiwan Received 23 June 2004; received in revised form 18 November 2004; accepted 12 December 2004
Abstract Commercial carbon nanotubes (CNTs) were purified by acid solution and were employed as adsorbents to study adsorption of trihalomethanes (THMs) from water. The properties of CNTs such as purity, structure and nature of the surface were greatly improved after acid treatment which made CNTs become more hydrophilic and suitable for adsorption of low molecular weight and relatively polar THM molecules. The adsorption of THMs onto CNTs fluctuates very little in the pH range 3–7, but decreases with pH value as pH exceeds 7. A comparative study between CNTs and powdered activated carbon (PAC) for adsorption of THMs from water was also conducted. The short time needed to reach equilibrium as well as the high adsorption capacity of CHCl3, which accounts for a significant portion of THMs in the chlorinated drinking water, suggests that CNTs possess highly potential applications for THMs removal from water. r 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Adsorption; Trihalomethanes; pH effect; Powdered activated carbon
1. Introduction Disinfection is routinely carried out in water treatment process or before finished water leaves the treatment plant to prevent microbiological degradation of drinking water quality. Until recently, chlorine was the most commonly employed disinfectant. To ensure the disinfection capacity of drinking water minimum chlorine residues must be maintained (Biswas et al., 1993). However, trihalomethanes (THMs; CHCl3, CHBrCl2, CHBr2Cl and CHBr3) were found to be formed during the chlorination of drinking water (Rook, 1974). THMs are recognized as potentially
Corresponding author. Tel.: +886 4 22840441; fax: +886 4 22862587. E-mail address: [email protected] (C. Lu).
hazardous and carcinogenic substances (Bull et al., 1995). Therefore, more stringent requirements for the removal of THMs from drinking water in recent years have necessitated the development of innovative, costeffective treatment alternatives. Carbon nanotubes (CNTs) are relatively new adsorbents for adsorption of trace pollutant from water or air. Long and Yang (2001) reported that a significantly higher dioxin removal efficiency is found with CNTs than that with activated carbon. Li et al. (2002b) found that CNTs have high lead adsorption capacity and can be used as an adsorbent for lead removal from water. Li et al. (2003) showed that CNTs are good fluoride adsorbents and their fluoride removal capability is superior to activated carbon. Peng et al. (2003) indicated that CNTs are good adsorbents to remove 1,2-dichlorobenzene from water and can be used in a wide pH range of 3–10.
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.12.033
ARTICLE IN PRESS 1184
C. Lu et al. / Water Research 39 (2005) 1183–1189
CNTs have been proved to possess great potential applications in environmental protection by the foregoing investigators. However, the studies on the adsorption of trace pollutants with CNTs are still very limited in the literature. This paper aims at investigating adsorption of THMs from water with CNTs. A comparative study between CNTs and powdered activated carbon (PAC) is also conducted.
THMs adsorbed by CNTs were calculated as follows: q ¼ ðC 0 C t ÞV =m;
(1)
where q is the amount of THMs adsorbed by CNTs (mg/g), C0 the initial THMs concentration (mg/l), Ct the THMs concentration after a certain period of time (mg/l), V the solution volume (l) and m the CNTs dosage (g). 2.4. Analytical methods
2. Materials and methods 2.1. Preparation of purified CNTs Multi-walled CNTs (Model CN3016, Nanotech Port Co., Shenzhen, China), with outer diameter range 10–30 nm and inner diameter range 5–10 nm, were fabricated by catalytic decomposition of the CH4/H2 mixture at 700 1C using Ni particles as catalyst. The length of as-prepared CNTs was in the range of 500 nm–500 mm. The as-prepared CNTs were dispersed into a 150 ml flask containing 40 ml concentrated acid solutions (30 ml HNO3+10 ml H2SO4) for 24 h to remove metal catalysts and then washed by deionized water. After cleaning, the CNTs were again dispersed into a 150 ml flask containing 40 ml concentrated acid solutions and refluxed using an ultrasonic cleaning bath (BRANSON 3510 Sonic cleaner, CT, USA) at 80 1C for 2 h to remove amorphous carbon. Finally, the CNTs-containing solution was filtered by 0.45 mm glass-fiber filter to obtain purified CNTs. 2.2. Preparation of THMs solution THMs solution was prepared by diluting a 2000 mg/l analytical grade THMs solution (Supelco Inc. Bellefonte, PA, USA), which contains equivalent concentration of CHCl3, CHCl2Br, CHClBr2 and CHBr3, into deionized water to obtain the desired THMs concentration. 2.3. Batch adsorption experiment Batch adsorption experiments were performed using 150 ml glass bottles with addition of 50 mg purified CNTs and 125 ml of THMs solution of increased initial concentrations (C0) from 0.2 to 12 mg/l. The glass bottles were sealed with Teflon and then mounted on a shaker. The shaker was placed within a temperature control box (Model CH-502, Chin Hsin, Taipei, Taiwan) to maintain water temperature at 25 1C. The pH value of the solution ranging from 3 to 11 was chosen to study the pH effect on THMs adsorption. The pH was adjusted using 1 M H2SO4 or 1 M NaOH.
THMs concentration was determined using a gas chromatograph (Hewlett Packard Model 5890 II gas chromatograph, MD, USA) equipped with an electron capture detector (ECD). A 30 m HP-5 fused silica capillary column (0.25 mm inside diameter, 1.5 mm film thickness) was used for THMs analysis. The GC-ECD was operated at injection temperature of 177 1C, detector temperature of 272 1C and oven temperature of 110 1C. The total surface area, mean pore size, pore size distribution and pore volume of purified CNTs were determined by a BET sorptometer (Model BET-202A, Porous Materials Inc., NY, USA).
3. Results and discussion 3.1. Characterization of purified CNTs Shape and size are essential as identifying characteristics of adsorbents since these parameters determine the specific surface area and thus influence adsorption rate and sensitivity to environmental conditions. The scanning electron microscope (SEM) image indicates that the isolated CNTs are in cylindrical shapes with an average external diameter of around 25 nm. The transmission electron microscope (TEM) image shows that the purified CNTs possess multi-wall with the hollow inner tube diameter of around 8 nm. Fig. 1 exhibits the pore size distribution of purified CNTs. It is obvious that the pore size is a bimodal distribution. The major peak is located in the size range of 1–4 nm while the secondary peak is located in the size range of 20–40 nm. The 1–4 nm pores are the CNT inner cavities and are responsible for about 50% of the total pore volume. The 20–40 nm pores are likely to be contributed by aggregated pores which are formed by the confined space among the isolated CNTs (Yang et al., 2001) and are responsible for around 20% of the total pore volume. This is because the inner diameter of CNTs is only around 8 nm. The BET surface area of CNTs increases from 225 to 295 m2/g after acid treatment. A possible reason may be the fact that the acid treatment can untie entwined CNTs and thus increase the surface area.
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0.25
30
Aggregated pores
CNT 0.20
D band
28
G band
26 24
Intensity (a.u.)
(volume fraction / nm)
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0.15 0.10
22 20 18 16 14
0.05
12 10
0.00 1
10 Pore size (nm)
100
0
(a)
Fig. 1. Pore size distributions of CNTs after acid treatment.
200 400 600 800 1000120014001600180020002200
Raman shift (cm-1) 30 G band
28
Weight remained (%)
100
Intensity (a.u.)
26
a
80 b
60
D band
24 22 20 18 16 14
40
12 10
4.95%
20
1.13% 0 0
100 200 300 400 500 600 700 800 900 Temperature (° C)
0
(b)
200 400 600 800 1000120014001600180020002200
Raman shift (cm-1)
Fig. 3. Raman spectra of CNTs: (a) before acid treatment, and (b) after acid treatment.
Fig. 2. Thermogravimetric analyses of CNTs: (a) before acid treatment, and (b) after acid treatment.
Thermogravimetric analysis (Fig. 2) and Raman spectra (Fig. 3) are employed to investigate the changes of purity and structure of CNTs after acid treatment. Thermogravimetric analysis of CNTs indicates that gasification ends at around 740 1C for as-prepared CNTs and around 700 1C for purified CNTs, in which 4.95% and 1.13% remaining weight was observed, respectively. This implies that the purity of CNTs increases from 95.05% to 98.87% after acid treatment. Raman spectrum of CNTs shows that there are two sharp peaks. The peak near 1334 cm1 is the so-called D-band which is related to disordered sp2-hybridized carbon atoms of CNTs (Tsai and Chen, 2003). The peak near 1582 cm1 is the so-called G-band which is related to the structure integrity of sp2-hybridized carbon atoms of the CNTs. The ID/IG ratio decreases from 1.047 to 0.845 after acid treatment, implying that the purified
CNTs possess more graphitized structures, thereby, improving their properties. Fig. 4 shows the Fourier transformed infrared spectra of CNTs before and after acid treatment. It is seen that the as-prepared CNTs exhibit no peaks. In contrast, the purified CNTs exhibit three major peaks at wavenumber of 1400, 1700 and 3500 cm1, which are associated with carboxylic acids and phenolic groups (O–H), carbonyl groups ð4C QOÞ and hydroxyl groups (–OH) (Li et al., 2002c). It is evident that there are many other functional groups attached on the surface of purified CNTs. 3.2. Effect of contact time Figs. 5a and b show the effect of contact time on the adsorption of THMs onto CNTs with the initial THMs concentrations (C0) of 0.2 and 3.2 mg/l. The concentration of each THM molecule is identical, while the pH of water was 6.87. It is noted that the adsorption of THMs
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50
40 C=O O-H b
-OH 30
20 4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1) Fig. 4. Fourier transformed infrared spectra of CNTs: (a) after acid treatment, and (b) before acid treatment.
0.12 0.10
3.3. Adsorption isotherms
q (mg/g)
0.08 0.06 CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.04 0.02 0.00 0
100
200
(a)
300 t (min)
400
500
1.6 1.4
q (mg/g)
1.2
Fig. 6 shows the adsorption isotherms of THMs with CNTs. As can be seen, the adsorption capacity of CHCl3 is the highest, followed by CHBrCl2, CHBr2Cl and then CHBr3. The adsorbed amounts of CHCl3, CHCl2Br, CHClBr2 and CHBr3 are equal to 2.41, 1.23, 1.08 and 0.92 mg/g, respectively, for equilibrium concentration of 1 mg/l. There are two possible reasons to explain the high capacity for adsorption of CHCl3 onto CNTs. First, previous study indicated that all wetting and filling results of CNTs are related to the surface tension of the liquid. Only low surface tension liquid will wet the CNT surface and fill open CNTs by capillarity (Ebbesen, 1996). The wettability with water certainly influences adsorption capacity of THM molecules from
1.0
3.0
0.8 2.5
0.6 CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.4 0.2
2.0
0.0 0 (b)
C 0 ¼ 3:2 mg=l: The final adsorption capacities of CHCl3, CHBrCl2, CHBr2Cl and CHBr3 reach 0.11, 0.06, 0.05 and 0.05 mg/g for C 0 ¼ 0:2 mg=l and achieve 1.51, 0.78, 0.72 and 0.64 mg/g for C 0 ¼ 3:2 mg=l: The longer contact time to reach equilibrium for lower initial THMs concentration may be explained by the fact that diffusion mechanisms control the adsorption of THMs onto CNTs. Reid et al. (1988) indicated that the mass diffusivity decreases with decreasing concentration under very dilute solution and causes the decrease in diffusion flux of adsorbate onto the surface of the adsorbent. It is also noted that the smallest molecule, CHCl3, is the most preferentially adsorbed onto CNTs, followed by CHBrCl2, CHBr2Cl and then CHBr3. This may be attributed to the fact that adsorption occurs with molecules that are small enough in size to enter the inner cavities through the pores. Therefore, the adsorption rate increases with decreasing size of the adsorbed molecules.
100
200
300 t (min)
400
500
600
Fig. 5. Effect of contact time on the adsorption of THMs with CNTs: (a) C 0 ¼ 0:2 mg=l; and (b) C 0 ¼ 3:2 mg=l:
increases quickly with time and then reaches equilibrium. The contact time to reach equilibrium is equal to 180 min for C 0 ¼ 0:2 mg=l and equal to 150 min for
q (mg/g)
Transmission (%)
a
1.5 1.0 0.5
CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Ce (mg/l) Fig. 6. Adsorption isotherms for THMs with CNTs.
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of one component was changed, the adsorption of other three components will change even if their concentrations are kept at the same value of this study.
their aqueous solution. The surface tension of CHCl3 (27.14 dyn/cm) (Holbrook, 1993) is much lower than that of CHBr3 (46.2 dyn/cm) (Jasper, 1972), which made adsorption of CHCl3 onto CNTs much easier. Second, there are many oxygen-containing groups attached on the surface of purified CNTs, which made CNTs become more hydrophilic and suitable for adsorption of relatively polar molecules. In THM molecules, the dipole moments of C–Cl bond and C–Br bond are equal to 1.56 and 1.48 Debye, respectively. This causes the polarity of covalent to become less as the number of bromine ion in THM molecule increases (Morris and Boyd, 1987). The polarity of CHCl3 is thus the highest, followed by CHBrCl2, CHBr2Cl and then CHBr3, which made adsorption of CHCl3 onto CNTs much easier. The experimental data for THMs adsorption onto CNTs could be approximated by the isotherm models of Langmuir (2) and Freundlich (3) q¼
abC e ; 1 þ bC e
3.4. Effect of pH value Fig. 7 shows the effect of pH on adsorption of THMs onto CNTs with the initial THMs concentration of 2.0 mg/l. It is obvious that the adsorption of THMs onto CNTs fluctuates very little in the pH range of 3–7. This may be due to the fact that the employed CNTs have been purified by acid solution to improve their properties which may enhance the resistance of CNTs to acid environment. However, the adsorption of THMs decreases as the pH exceeds 7. This is due to the fact that more oxygen-containing groups on the CNTs surface are ionized at higher pH values and thus they adsorb more water (Peng et al., 2003). The formation of water cluster on these groups blocks the access of THM molecules to adsorption sites and results in less adsorption of THMs.
(2)
q ¼ K f C ne ;
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(3) 2.6
where q is the mass of THMs adsorbed by CNTs, Ce the equilibrium THMs concentration, a and b Langmuir constants and Kf and n are Freundlich constants. The isotherm constants are obtained from fitting the adsorption equilibrium data and are listed in Table 1. As can be seen, both Langmuir and Freundlich isotherm models match the experimental data very well, with the correlation coefficient values of 0.9710.996 and 0.9530.980, respectively. The constants a and Kf, which are related to adsorption capacity, are the highest for adsorption of CHCl3, followed by CHBrCl2, CHBr2Cl and then CHBr3, which is consistent with the experimental observation. It should be mentioned that these experiments were carried out in the system of competitive adsorption; the parameters of individual adsorption listed in Table 1 correlated with each other. That is, if the concentration
2.4 2.2 q (mg/g)
2.0 CHCl3 CHBrCl2 CHBr2Cl CHBr3
1.8 1.6 1.4 1.2 1.0 0.8 3
4
5
6
7 pH
8
9
10
11
Fig. 7. Effect of pH on the adsorption of THMs with CNTs (C 0 ¼ 2 mg=l).
Table 1 Constant of Langmuir and Freundlich models for adsorption of THMs onto CNTs THMs
CHCl3 CHBrCl2 CHBr2Cl CHBr3
Langmuir model
Freundlich model 2
a
b
R
4.730 2.925 2.763 2.695
0.00233 0.00087 0.00048 0.00067
0.979 0.971 0.996 0.986
Kf
n
R2
2.721 1.087 1.022 0.901
0.583 0.699 0.741 0.755
0.953 0.968 0.980 0.977
Note: The starting solution contains equivalent concentration of four THM molecules. Unit: a ¼ mg=g; b ¼ l=mg; K f ¼ 1n mg1n =g; n ¼ dimensionless; R ¼ dimensionless:
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3.5. Comparative experiment
3.2
In the comparative study, a test for adsorption of THMs with PAC (Model 18001, Ridel deHae¨n Co., Seelze, Germany) was conducted. The surface area of PAC is equal to 900 m2/g. Figs. 8a and b show the effect of contact time on the adsorption of THMs onto PAC with the initial THMs concentrations (C0) of 0.2 and 3.2 mg/l, respectively. It is noted that the adsorption of THMs onto PAC also increases quickly with time and then reaches equilibrium. The contact time to reach equilibrium is equal to 450 min for C 0 ¼ 0:2 mg=l and equal to 360 min for C 0 ¼ 3:2 mg=l: The final adsorption capacities of CHCl3, CHCl2Br, CHClBr2 and CHBr3 reach 0.063, 0.089, 0.108 and 0.119 mg/g for C 0 ¼ 0:2 mg=l and achieve 0.98, 1.2, 1.29 and 1.52 mg/g for C 0 ¼ 3:2 mg=l: It is also noted that the largest molecule, CHBr3, is most preferentially adsorbed onto PAC, followed by CHBr2Cl, CHBrCl2 and then CHCl3. This is because the adsorption capacity of activated carbon generally increases with the compound’s molecular weight or boiling point. Furthermore, activated carbon has greater
2.8
0.14 0.12
q (mg/g)
0.10 0.08 0.06 CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.04 0.02 0.00 0
200
400
(a)
600 t (min)
800
1000
1.6 1.4
q (mg/g)
1.2 1.0 0.8 CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.6 0.4 0.2
(b)
100
200
300
400 500 t (min)
600
700
q (mg/g)
2.0 1.6 1.2
CHCl3 CHBrCl2 CHBr2Cl CHBr3
0.8 0.4 0.0 0.0
0.5
1.0 Ce (mg/l)
1.5
2.0
Fig. 9. Adsorption isotherms for THMs with PAC.
affinity for adsorption of relatively nonpolar molecules (Li et al., 2002a). Fig. 9 shows the adsorption isotherms of THMs with PAC. The adsorbed amounts of CHCl3, CHCl2Br, CHClBr2 and CHBr3 are equal to 1.2, 1.68, 2.19 and 2.75 mg/g, respectively, for equilibrium concentration of 1 mg/l. The adsorbed amount of CHCl3 onto PAC is much lower than those of other THM molecules. By comparing the adsorption of THMs with CNTs and PAC, it is evident that it takes less contact time for CNTs to reach equilibrium. This may be explained by the fact that CNTs have no porous structure like PAC in which THMs have to move from the exterior surface to the inner surface of the pores on PAC to reach equilibrium (Peng et al., 2003). Although the surface area of purified CNTs (295 m2/g) is much lower than that of PAC (900 m2/g), the adsorption capacity of CHCl3 onto CNTs (2.72 mg/g) is approximately twice higher than onto PAC (1.32 mg/g). This can be attributed to functional groups attached on the CNTs surface, which made CNTs become more hydrophilic and suitable for the adsorption of low molecular weight and relatively polar THM molecules. The short time needed to reach equilibrium as well as the high adsorption capacity of CHCl3, which accounts for a significant portion (80–90%) of THMs found in the chlorinated drinking water (Hsu et al., 2001), suggest that CNTs have great potential applications for THMs removal from water.
4. Conclusions
0.0 0
2.4
800
Fig. 8. Effect of contact time on the adsorption of THMs with PAC: (a) C 0 ¼ 0:2 mg=l; and (b) C 0 ¼ 3:2 mg=l:
The following conclusions could be drawn from this study: 1. The properties of CNTs such as purity, structure and nature of the surface could be greatly improved after
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2. 3.
4.
5.
acid treatment which made CNTs become more hydrophilic and suitable for adsorption of low molecular weight and relatively polar THM molecules. The adsorption of THMs onto CNTs can be well described by both Langmuir and Freundlich models. Adsorption of THMs onto CNTs fluctuates very little in the pH range of 3–7 but decreases with pH value as pH exceeds 7. The smallest molecule, CHCl3, is the most preferentially adsorbed onto CNTs, followed by CHBrCl2, CHBr2Cl and then CHBr3. In contrast, the largest molecule, CHBr3, is the most preferentially adsorbed onto PAC, followed by CHBr2Cl, CHBrCl2 and then CHCl3. The short time needed to reach equilibrium as well as the high adsorption capacity of CHCl3 suggests that CNTs possess highly potential applications for THMs removal from water.
Acknowledgement Support from the National Science Council, Taiwan, under a contract no. NSC 93-2211-E-005-020 is gratefully acknowledged.
References Biswas, P., Lu, C., Clark, R.M., 1993. A model for chlorine concentration decay in pipes. Water Res. 27, 1715–1724. Bull, R.J., Brinbaum, L.S., Cantor, K.P., Rose, J.B., Butterworth, B.E., Pegram, R., Tuomisto, J., 1995. Water Chlorination: essential process and cancer hazard. Fundam. Appl. Toxicol. 28, 155–166. Ebbesen, T.W., 1996. Wetting, filling and decorating carbon nanotubes. J. Phys. Chem. Solids 57, 951–995. Holbrook, M.T., 1993. Chlorocarbons and chlorohydrocarbons: chloroform. In: Kirk–Othmer Encyclopedia of
1189
Chemical Technology, vol. 5, fourth ed. Wiley, New York, USA, pp. 1051–1062. Hsu, C.H., Jeng, W.L., Chang, R.M., Chien, L.C., Han, B.C., 2001. Estimation of potential lifetime cancer risks for trihalomethanes from consuming chlorinated drinking water in Taiwan. Environ. Res. 85, 77–82. Jasper, J.J., 1972. Surface tension of pure liquid compounds. In compilation of data of some 2200 pure liquid compounds. J. Phys. Chem. Reference Data 1, 859. Li, L., Quinlivan, P.A., Knappe, D.R.U., 2002a. Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution. Carbon 40, 2085–2100. Li, Y.H., Wang, S., Wei, J., Zhang, X., Xu, C., Luan, Z., Wu, D., Wei, B., 2002b. Lead adsorption on carbon nanotubes. Chem. Phys. Lett. 357, 263–266. Li, Y.H., Xu, C., Wei, B., Zhang, X., Zheng, M., Wu, D., Ajayan, P.M., 2002c. Self-organized ribbons of aligned carbon nanotubes. Chem. Mater. 14, 483–485. Li, Y.H., Wang, S., Wei, J., Zhang, X., Wei, J., Xu, C., Luan, Z., Wu, D., 2003. Adsorption of fluoride from water by aligned carbon nanotubes. Mater. Res. Bull. 38, 469–476. Long, R.Q., Yang, R.T., 2001. Carbon nanotubes as superior sorbent for dioxin removal. J. Am. Chem. Soc. 123, 2058–2059. Morris, R.B., Boyd, R.N., 1987. Organic Chemistry, fifth ed. Allyn & Bacon Inc., New York, USA. Peng, X., Li, Y., Luan, Z., Di, Z., Wang, H., Tian, B., Jia, Z., 2003. Adsorption of 1,2-dichlorobenzene from water to carbon nanotubes. Chem. Phys. Lett. 376, 154–158. Reid, R.C., Prausnitz, J.M., Poling, B.E., 1988. The Properties of Gases & Liquids, fourth ed. McGraw-Hill, New York. Rook, J.J., 1974. Formation of haloforms during chlorination of natural waters. Water Treatment Exam. 23, 234–243. Tsai, C.L., Chen, C.F., 2003. Characterization of biascontrolled carbon nanotubes. Diamond Relat. Mater. 12, 1615–1620. Yang, Q.H., Hou, P.X., Bai, S., Wang, M.Z., Cheng, H.M., 2001. Adsorption and capillarity of nitrogen in aggregated multi-walled carbon nanotubes. Chem. Phys. Lett. 345, 18–24.