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Equilibrium and kinetic studies on the removal of NaCl from aqueous solutions by electrosorption on carbon nanotube electrodes
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Separation and Purification Technology 58 (2007) 12–16
Equilibrium and kinetic studies on the removal of NaCl from aqueous solutions by electrosorption on carbon nanotube electrodes Shuo Wang ∗ , DaZhi Wang, LiJun Ji, Qianming Gong, YueFeng Zhu, Ji Liang Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
Abstract In this paper, we prepared carbon nanotube (CNT) electrodes with polytetrafluoroethylene (PTFE) as binders to adsorb NaCl from water. The electrodes were characterized using scanning electron micrograph (SEM), nitrogen adsorption and cyclic voltammetry. The adsorption/electrosorption rate and capacity of CNT electrodes with different bias potentials were measured, and the electrosorption isotherms and kinetics were also investigated. The results showed that electrosorption can effectively increase the adsorption capacity and rate of NaCl on CNT electrodes. The adsorption/electrosorption of NaCl on CNT electrodes followed Langmuir isotherms and the pseudo-first-order adsorption kinetics. The equilibrium adsorption capacity at 1.2 V was 9.35 mg/g, which was nearly eight times higher than that at 0 V. Compared with the adsorption rate at 0 V, the adsorption rate at 1.2 V was increased by 58.9%. The enhancement of capacity and rate may be due to the forming an electric double layer on the surface of CNT electrodes in the solution. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon nanotube; Electrosorption; Adsorption isotherms; Adsorption kinetics
1. Introduction With a continuing increase of the world population, water shortage has become one of the major problems in many countries worldwide. It is generally recognized that seawater desalination is a very attractive method for the solution of this problem. There are several traditional methods for desalination, such as multiple effect distillation, multi-stage flash, vapor compression distillation, electrodialysis, reverse osmosis, etc. [1–5]. Electrosorption is generally defined as adsorption on the surfaces of charged electrodes by applying potential or current. Since electrosorption can increase adsorption capacity and increase the removal efficiency, it exhibits promising applications in desalination and purification of wastewater [6–8]. Several porous carbon materials, such as activated carbon and carbon aerogel, have been employed as electrosorption electrodes because of their large surface areas and superb chemical stability [9–11]. But many defects such as high resistivity and low mechanical strength limit their application. Carbon nanotubes (CNTs) are new nanoscale materials. Their large surface area, high mechanical strength, remarkable electrical conductivities and high stability indicate ∗
Corresponding author. Tel.: +86 10 62773641; fax: +86 10 62782413. E-mail address: [email protected] (S. Wang).
1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.07.005
their tremendous potential for application as electrode material [12–14]. Adsorption kinetics and isotherms are essential to estimate the removal rate and capacity of CNT electrode. However, to the author’s knowledge, there is no report about the kinetics of electrosorption of CNT electrode in NaCl solution in the literature. In this paper, we prepared CNT electrodes with polytetrafluoroethylene (PTFE) emulsion as binders, and investigated the adsorption isotherms and kinetics of electrosorption on CNT electrodes. We studied the performance of different bias potentials on the electrosorption capacity and rate of NaCl on CNT electrode to obtain a solid basis for estimating this technical potential for desalination. The CNT electrodes were also studied by means of cyclic voltagram. 2. Materials and experiments 2.1. Materials CNTs, which are multi-walled carbon nanotubes (MWNTs) used in this work, were synthesized by chemical vapour deposition with Ni particles as the catalyst. Propylene-hydrogen (C3 H6 :H2 = 2:1) mixture gas was introduced to a vertical furnace at a flow rate of 15 L/min at 750 ◦ C for 30 min. CNTs were formed on these Ni particles with the sizes of
S. Wang et al. / Separation and Purification Technology 58 (2007) 12–16
13
The amount of NaCl adsorption per unit mass of CNT electrodes was calculated from the following equation: m=
Fig. 1. TEM image of CNTs.
10 nm and were cooled down to room temperature in argon atmosphere. Fig. 1 shows the transmission electron microscopy image (TEM) of CNTs. As is shown, CNTs have an average diameter of about 30 nm and a length of several micrometers. The as-prepared CNTs were then immersed in concentrated nitric acid and hydrofluoric acid to dissolve the catalyst particles and catalyst supports and then washed several times with deionized water until the washings show no acidity. Finally, CNTs were obtained after drying in a oven at 100 ◦ for 24 h. All other chemicals were analytical grade and used as received without further purification. 2.2. Preparation of electrodes Electrodes were prepared using the CNTs with PTFE as binders and the weight ratio is 95:5. Current collectors were made of titanium mesh with 0.8 mm thickness. Then electrodes were pressed on titanium mesh to keep them in contact with the current collectors. The assembled electrodes were packed in holders with an area of 4 cm × 3 cm and the distance between these two electrodes was 40 mm.
(C0 − Ci )V M
where C0 and Ci are the initial and equilibrium concentration of NaCl, respectively, V the volume of the solution and M is the mass of the CNT electrodes. The electrosorption capacity is similarly defined as the amount of NaCl adsorbed on CNT electrodes under polarization, which can also be calculated by the equation above, but C0 and Ci here are the concentration at the initial and electrosorption equilibrium state. The cyclic voltagrams of the CNT electrodes were obtained in two electrode systems on the same CNT electrodes and were recorded on an automatic cycle tester in a voltage range of 0.0–1.2 V with a scan rate of 50 mV/s at room temperature. During a cyclic voltammetric measurement, the solution was 0.5 M NaCl. 3. Results and discussion 3.1. Characteristics of CNT electrodes Fig. 2 shows the SEM image of CNTs electrode. It could be seen that CNTs were fixed by titanium mesh together to form the electrode. This may reduce the resistivity and increase efficiency during electrosorption. The dependence of adsorption and electrosorption capacities on characteristics of the CNT electrodes such as pore size distribution and specific area is of interest. Therefore, the CNT electrodes were characterized to obtain their BET specific surface area and pore size distribution using N2 as the adsorbate at 77 K. The BET specific surface area was 153 m2 /g and the pore size distribution was shown in Fig. 3. It was suggested that the pore size were mainly micropores and mesopores. The micropores were beneficial to the adsorption/desorption of ions, and the mesopores could been seen as the ions transportation route [15].
2.3. Test and measurement CNT electrodes were observed by SEM, and the BET specific surface area of CNT electrodes was determined by nitrogen (99.99% purity) adsorption/desorption at 77 K using a Micomeritics ASAP 2010 surface analyzer. The adsorbents were degassed at 423.15 K under vacuum. Porosity distribution was deduced from the experimental data using the BET method. Physically adsorbed amounts of NaCl were measured without applying electric potential. Electrical adsorption was measured under constant potential conditions. A dc power supply maintained a constant voltage between two electrodes during electrosorption, and the salt concentration was measured by a conductivity meter of DDS-307 in situ. The detectors were put in the solution and the data were got directly.
(1)
Fig. 2. SEM image of CNTs electrode.
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S. Wang et al. / Separation and Purification Technology 58 (2007) 12–16
Fig. 3. Pore size distribution of CNTs electrode.
Fig. 4. The adsorption/electrosorption isotherms of NaCl on CNT electrodes at different bias potentials, 20 ◦ C.
3.2. Adsorption/electrosorption of NaCl 3.2.1. Adsorption/electrosorption isotherms Since CNTs have polar functional groups such as C O, –OH and –COOH on their surface [16], ions can be adsorbed on the surface of the CNTs without applying any electric field. The adsorbed amount and rate of ions on the CNTs, however, increased by applying an electric field because of the strengthened interaction between the electrodes surface and adsorbed ions, which formed an electric double layer. To investigate electrosorption behavior, electrosorption experiments at different bias potentials with different initial concentration were carried out, and obtained the electrosorption isotherm. The electrosorption of NaCl on CNT electrodes was evaluated at constant temperatures of 20 ◦ C for the adsorption isotherms as well as the kinetic models. The initial concentration of NaCl solutions were 500 mg/L, 1000 mg/L, 1500 mg/L, 2000 mg/L, 2500 mg/L, 3000 mg/L and 3500 mg/L and the bias potentials were 0 V (without connecting circuit), 0.4 V, 0.8 V and 1.2 V. Langmuir isotherm (Eq. (2)) and Freundlich isotherm (Eq. (3)) were used to simulate the experimental data for NaCl electrosorption on CNT electrodes. 3.2.1.1. Langmuir. qe =
qm bCe 1 + bCe
(2)
where Ce is the equilibrium concentration of adsorbate in solution, qm the largest mass of NaCl adsorbed per unit weight of adsorbent on the adsorbent surface and b is the Langmuir constant related to binding energy.
3.2.2. Adsorption/electrosorption kinetics Voltage is a key factor influencing electrosorption rate, as it plays an important role in driving ions onto the CNT electrode. Fig. 5 shows adsorption of ions on the CNT electrodes with application of electric field of various voltages. Four levels of bias potentials, 0 V, 0.4 V, 0.8 V, and 1.2 V were investigated. The adsorption rate constants of NaCl were determined by the Lagergren equation, which is often called the pseudo-first-order
Table 1 The isotherms parameters of NaCl adsorption/electrosorption on CNTs at different bias potentials Bias potential (V)
3.2.1.2. Freundlich. q = KF Ce1/n
Fig. 4 shows the adsorption isotherms, and the isotherm parameters obtained were summarized in Table 1. It was found that the Langmuir isotherm correlated better with the experimental data according to the correlation coefficient (R2 ). This phenomenon suggested that the monolayer adsorption was primary during the electrosorption process. The parameter qm in the Langmuir isotherm model was considered as the maximum adsorption capacity, so it suggest that the maximum adsorption capacity was improved as the bias potential rose. The equilibrium adsorption capacity at polarization of 1.2 V was 9.35 mg/g, which was nearly eight times higher than that at 0 V. The results suggested that the maximum adsorption capacity for NaCl on the CNT electrodes were improved as the bias potential and the concentration rose.
(3)
where KF and 1/n are Freundlich constants related to the capacity of adsorbent to adsorb and the tendency of the adsorbate to be adsorbed, respectively.
0 0.4 0.8 1.2
Langmuir
Freundlich (10−3 )
qm
b
1.04 3.09 6.01 8.91
1.45 0.86 0.89 0.83
R2
KF
n
R2
0.9948 0.9949 0.9968 0.9918
0.048 0.062 0.13 0.17
2.90 2.24 2.27 2.21
0.9840 0.9895 0.9861 0.9930
S. Wang et al. / Separation and Purification Technology 58 (2007) 12–16
15
Fig. 6. Cyclic voltammograms of CNTs electrode in 0.5 M NaCl aqueous solution at scan rate 50 mV s−1 . Fig. 5. The adsorption/electrosorption kinetics of NaCl on CNT electrodes at different bias potentials, 20 ◦ C.
adsorption kinetics (Eq. (4)): kt (4) 2.303 where k is the adsorption rate constant, qe and q the amounts of NaCl adsorbed at equilibrium and time t(min), respectively. The adsorption kinetics was fitted to the Lagergren law by nonlinear regression using the method of least squares. The results shown in Fig. 5 and Table 2 suggested that the adsorption capacity and rate constants of NaCl increased as voltage supplied rose from 0 V to 1.2 V. The adsorption/electrosorption were found to be in agreement with the Lagergren adsorption rate law. From Fig. 5 and Table 2, it could be seen that the adsorption capacity and rate were increased greatly by applying an electric field. Compared with the adsorption rate at 0 V, the adsorption rate at 1.2 V was increased by 58.9%. Therefore, it was concluded that electrosorption could effectively enhance the adsorption rate of ions in the solution on CNT electrodes.
log(qe − q) = log qe −
3.3. Cyclic voltagram of CNT electrodes Fig. 6 shows cyclic voltagrams obtained in the electrosorption and desorption of NaCl on the CNT electrodes. The variation of potential from 0 V to 1.2 V between electrodes indicated the adsorption behavior of ions on an electrode surface. A steady increase with current in cyclic voltagrams is caused by faster response of ions with increasing potential, while a peak appearing in this process shows the emergency of redox reaction. Table 2 The adsorption/electrosorption parameters of first-order adsorption kinetics for NaCl at different potentials Bias potentials (V)
qe (mg/g)
K (×10−2 min−1 )
R2
0 0.4 0.8 1.2
1.01 2.51 4.91 7.74
2.19 3.37 3.41 3.48
0.9977 0.9792 0.9773 0.9753
As shown in Fig. 6, there was no evident redox pseudocapacitance peaks that can confirm strong oxidation or reduction occurred in the potential range. This revealed the enhancement of adsorption capacity and rate at bias potential supplied were mainly not due to direct electrochemical reaction caused by active oxidizing, but due to the increase in electrosorption by forming an electric double layer between the surface of electrodes and ions. Hence, the enhancement of NaCl adsorption capacity and rate were mainly due to the increase in electrosorption. 4. Conclusion CNT electrodes with PTFE as binders are prepared and the adsorption/electrosorption ability are also measured. It is clear that applying an electric field on CNT electrodes has an accelerating effect on the adsorption of NaCl, and the electrosorption capacity and rate are dependent on bias potential, increased with increasing bias potential. The electrosorption is in agreement with Langmuir adsorption isotherms and the pseudo-first-order adsorption kinetics. The equilibrium adsorption capacity at 1.2 V was 9.35 mg/g, which was nearly eight times higher than that at 0 V. Compared with the adsorption rate at 0 V, the adsorption rate at 1.2 V was increased by 58.9%. The cyclic voltagrams result reveals the enhancement of adsorption capacity and rate at bias potential supplied were mainly due to the increase in electrosorption by forming an electric double layer. Therefore, the electrosorption may show a promising application for desalination and purification of drinking water. References [1] [2] [3] [4] [5] [6] [7]
M.S. Mohsen, J.O. Jaber, M.D. Afonso, Desalination 157 (2003) 167. M. Al-Shammiri, M. Safar, Desalination 126 (1999) 45–59. Y.M. ElSayed, Desalination 107 (1996) 13–27. H. Tanaka, T. Nosoko, T. Nagata, Desalination 130 (2000) 295–304. B. Pilat, Desalination 139 (2001) 385–392. E. Ayranci, B.E. Conway, J. Appl. Electrochem. 31 (2001) 257–266. E. Ayranci, N. Hoda, J. Hazard. Mater. 112 (2004) 163–168.
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[8] Y.H. Han, X. Quan, S. Chen, H.M. Zhao, C.Y. Cui, Y.Z. Zhao, Sep. Purif. Technol. 50 (2006) 365–372. [9] J.C. Farmer, D.V. Fix, G.V. Mack, R.W. Pekala, J.F. Poco, J. Electrochem. Soc. 143 (1996) 159–169. [10] T.Y. Ying, K.L. Yang, S. Yiacoumi, C. Tsouris, J. Colloid Interf. Sci. 250 (2002) 18–27. [11] H. Oda, Y. Nakagawa, Carbon 41 (2003) 1037–1047. [12] D.S. Zhang, L.Y. Shi, J.H. Fang, K. Dai, L.Y. Zhang, J. Inorg. Mater. 21 (2006) 87–93.
[13] K. Dai, L.Y. Shi, D.S. Zhang, J.H. Fang, Chem. Eng. Sci. 61 (2006) 428–433. [14] X.Z. Wang, M.G. Li, Y.W. Chen, R.M. Cheng, S.M. Huang, L.K. Pan, Z. Sun, Electrochem. Solid State Lett. 9 (2006) E23–E26. [15] S.B. Wang, Z.H. Zhu, A. Coomes, F. Haghseresht, G.Q. Lu, J. Colloid Interf. Sci. 284 (2005) 440–446. [16] K. Niesz, A. Siska, I. Vesselenyi, K. Hernadi, D. Mehn, G. Galbacs, Z. Konya, I. Kiricsi, Catal. Today 76 (2002) 3–10.
Separation and Purification Technology 58 (2007) 12–16
Equilibrium and kinetic studies on the removal of NaCl from aqueous solutions by electrosorption on carbon nanotube electrodes Shuo Wang ∗ , DaZhi Wang, LiJun Ji, Qianming Gong, YueFeng Zhu, Ji Liang Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
Abstract In this paper, we prepared carbon nanotube (CNT) electrodes with polytetrafluoroethylene (PTFE) as binders to adsorb NaCl from water. The electrodes were characterized using scanning electron micrograph (SEM), nitrogen adsorption and cyclic voltammetry. The adsorption/electrosorption rate and capacity of CNT electrodes with different bias potentials were measured, and the electrosorption isotherms and kinetics were also investigated. The results showed that electrosorption can effectively increase the adsorption capacity and rate of NaCl on CNT electrodes. The adsorption/electrosorption of NaCl on CNT electrodes followed Langmuir isotherms and the pseudo-first-order adsorption kinetics. The equilibrium adsorption capacity at 1.2 V was 9.35 mg/g, which was nearly eight times higher than that at 0 V. Compared with the adsorption rate at 0 V, the adsorption rate at 1.2 V was increased by 58.9%. The enhancement of capacity and rate may be due to the forming an electric double layer on the surface of CNT electrodes in the solution. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon nanotube; Electrosorption; Adsorption isotherms; Adsorption kinetics
1. Introduction With a continuing increase of the world population, water shortage has become one of the major problems in many countries worldwide. It is generally recognized that seawater desalination is a very attractive method for the solution of this problem. There are several traditional methods for desalination, such as multiple effect distillation, multi-stage flash, vapor compression distillation, electrodialysis, reverse osmosis, etc. [1–5]. Electrosorption is generally defined as adsorption on the surfaces of charged electrodes by applying potential or current. Since electrosorption can increase adsorption capacity and increase the removal efficiency, it exhibits promising applications in desalination and purification of wastewater [6–8]. Several porous carbon materials, such as activated carbon and carbon aerogel, have been employed as electrosorption electrodes because of their large surface areas and superb chemical stability [9–11]. But many defects such as high resistivity and low mechanical strength limit their application. Carbon nanotubes (CNTs) are new nanoscale materials. Their large surface area, high mechanical strength, remarkable electrical conductivities and high stability indicate ∗
Corresponding author. Tel.: +86 10 62773641; fax: +86 10 62782413. E-mail address: [email protected] (S. Wang).
1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.07.005
their tremendous potential for application as electrode material [12–14]. Adsorption kinetics and isotherms are essential to estimate the removal rate and capacity of CNT electrode. However, to the author’s knowledge, there is no report about the kinetics of electrosorption of CNT electrode in NaCl solution in the literature. In this paper, we prepared CNT electrodes with polytetrafluoroethylene (PTFE) emulsion as binders, and investigated the adsorption isotherms and kinetics of electrosorption on CNT electrodes. We studied the performance of different bias potentials on the electrosorption capacity and rate of NaCl on CNT electrode to obtain a solid basis for estimating this technical potential for desalination. The CNT electrodes were also studied by means of cyclic voltagram. 2. Materials and experiments 2.1. Materials CNTs, which are multi-walled carbon nanotubes (MWNTs) used in this work, were synthesized by chemical vapour deposition with Ni particles as the catalyst. Propylene-hydrogen (C3 H6 :H2 = 2:1) mixture gas was introduced to a vertical furnace at a flow rate of 15 L/min at 750 ◦ C for 30 min. CNTs were formed on these Ni particles with the sizes of
S. Wang et al. / Separation and Purification Technology 58 (2007) 12–16
13
The amount of NaCl adsorption per unit mass of CNT electrodes was calculated from the following equation: m=
Fig. 1. TEM image of CNTs.
10 nm and were cooled down to room temperature in argon atmosphere. Fig. 1 shows the transmission electron microscopy image (TEM) of CNTs. As is shown, CNTs have an average diameter of about 30 nm and a length of several micrometers. The as-prepared CNTs were then immersed in concentrated nitric acid and hydrofluoric acid to dissolve the catalyst particles and catalyst supports and then washed several times with deionized water until the washings show no acidity. Finally, CNTs were obtained after drying in a oven at 100 ◦ for 24 h. All other chemicals were analytical grade and used as received without further purification. 2.2. Preparation of electrodes Electrodes were prepared using the CNTs with PTFE as binders and the weight ratio is 95:5. Current collectors were made of titanium mesh with 0.8 mm thickness. Then electrodes were pressed on titanium mesh to keep them in contact with the current collectors. The assembled electrodes were packed in holders with an area of 4 cm × 3 cm and the distance between these two electrodes was 40 mm.
(C0 − Ci )V M
where C0 and Ci are the initial and equilibrium concentration of NaCl, respectively, V the volume of the solution and M is the mass of the CNT electrodes. The electrosorption capacity is similarly defined as the amount of NaCl adsorbed on CNT electrodes under polarization, which can also be calculated by the equation above, but C0 and Ci here are the concentration at the initial and electrosorption equilibrium state. The cyclic voltagrams of the CNT electrodes were obtained in two electrode systems on the same CNT electrodes and were recorded on an automatic cycle tester in a voltage range of 0.0–1.2 V with a scan rate of 50 mV/s at room temperature. During a cyclic voltammetric measurement, the solution was 0.5 M NaCl. 3. Results and discussion 3.1. Characteristics of CNT electrodes Fig. 2 shows the SEM image of CNTs electrode. It could be seen that CNTs were fixed by titanium mesh together to form the electrode. This may reduce the resistivity and increase efficiency during electrosorption. The dependence of adsorption and electrosorption capacities on characteristics of the CNT electrodes such as pore size distribution and specific area is of interest. Therefore, the CNT electrodes were characterized to obtain their BET specific surface area and pore size distribution using N2 as the adsorbate at 77 K. The BET specific surface area was 153 m2 /g and the pore size distribution was shown in Fig. 3. It was suggested that the pore size were mainly micropores and mesopores. The micropores were beneficial to the adsorption/desorption of ions, and the mesopores could been seen as the ions transportation route [15].
2.3. Test and measurement CNT electrodes were observed by SEM, and the BET specific surface area of CNT electrodes was determined by nitrogen (99.99% purity) adsorption/desorption at 77 K using a Micomeritics ASAP 2010 surface analyzer. The adsorbents were degassed at 423.15 K under vacuum. Porosity distribution was deduced from the experimental data using the BET method. Physically adsorbed amounts of NaCl were measured without applying electric potential. Electrical adsorption was measured under constant potential conditions. A dc power supply maintained a constant voltage between two electrodes during electrosorption, and the salt concentration was measured by a conductivity meter of DDS-307 in situ. The detectors were put in the solution and the data were got directly.
(1)
Fig. 2. SEM image of CNTs electrode.
14
S. Wang et al. / Separation and Purification Technology 58 (2007) 12–16
Fig. 3. Pore size distribution of CNTs electrode.
Fig. 4. The adsorption/electrosorption isotherms of NaCl on CNT electrodes at different bias potentials, 20 ◦ C.
3.2. Adsorption/electrosorption of NaCl 3.2.1. Adsorption/electrosorption isotherms Since CNTs have polar functional groups such as C O, –OH and –COOH on their surface [16], ions can be adsorbed on the surface of the CNTs without applying any electric field. The adsorbed amount and rate of ions on the CNTs, however, increased by applying an electric field because of the strengthened interaction between the electrodes surface and adsorbed ions, which formed an electric double layer. To investigate electrosorption behavior, electrosorption experiments at different bias potentials with different initial concentration were carried out, and obtained the electrosorption isotherm. The electrosorption of NaCl on CNT electrodes was evaluated at constant temperatures of 20 ◦ C for the adsorption isotherms as well as the kinetic models. The initial concentration of NaCl solutions were 500 mg/L, 1000 mg/L, 1500 mg/L, 2000 mg/L, 2500 mg/L, 3000 mg/L and 3500 mg/L and the bias potentials were 0 V (without connecting circuit), 0.4 V, 0.8 V and 1.2 V. Langmuir isotherm (Eq. (2)) and Freundlich isotherm (Eq. (3)) were used to simulate the experimental data for NaCl electrosorption on CNT electrodes. 3.2.1.1. Langmuir. qe =
qm bCe 1 + bCe
(2)
where Ce is the equilibrium concentration of adsorbate in solution, qm the largest mass of NaCl adsorbed per unit weight of adsorbent on the adsorbent surface and b is the Langmuir constant related to binding energy.
3.2.2. Adsorption/electrosorption kinetics Voltage is a key factor influencing electrosorption rate, as it plays an important role in driving ions onto the CNT electrode. Fig. 5 shows adsorption of ions on the CNT electrodes with application of electric field of various voltages. Four levels of bias potentials, 0 V, 0.4 V, 0.8 V, and 1.2 V were investigated. The adsorption rate constants of NaCl were determined by the Lagergren equation, which is often called the pseudo-first-order
Table 1 The isotherms parameters of NaCl adsorption/electrosorption on CNTs at different bias potentials Bias potential (V)
3.2.1.2. Freundlich. q = KF Ce1/n
Fig. 4 shows the adsorption isotherms, and the isotherm parameters obtained were summarized in Table 1. It was found that the Langmuir isotherm correlated better with the experimental data according to the correlation coefficient (R2 ). This phenomenon suggested that the monolayer adsorption was primary during the electrosorption process. The parameter qm in the Langmuir isotherm model was considered as the maximum adsorption capacity, so it suggest that the maximum adsorption capacity was improved as the bias potential rose. The equilibrium adsorption capacity at polarization of 1.2 V was 9.35 mg/g, which was nearly eight times higher than that at 0 V. The results suggested that the maximum adsorption capacity for NaCl on the CNT electrodes were improved as the bias potential and the concentration rose.
(3)
where KF and 1/n are Freundlich constants related to the capacity of adsorbent to adsorb and the tendency of the adsorbate to be adsorbed, respectively.
0 0.4 0.8 1.2
Langmuir
Freundlich (10−3 )
qm
b
1.04 3.09 6.01 8.91
1.45 0.86 0.89 0.83
R2
KF
n
R2
0.9948 0.9949 0.9968 0.9918
0.048 0.062 0.13 0.17
2.90 2.24 2.27 2.21
0.9840 0.9895 0.9861 0.9930
S. Wang et al. / Separation and Purification Technology 58 (2007) 12–16
15
Fig. 6. Cyclic voltammograms of CNTs electrode in 0.5 M NaCl aqueous solution at scan rate 50 mV s−1 . Fig. 5. The adsorption/electrosorption kinetics of NaCl on CNT electrodes at different bias potentials, 20 ◦ C.
adsorption kinetics (Eq. (4)): kt (4) 2.303 where k is the adsorption rate constant, qe and q the amounts of NaCl adsorbed at equilibrium and time t(min), respectively. The adsorption kinetics was fitted to the Lagergren law by nonlinear regression using the method of least squares. The results shown in Fig. 5 and Table 2 suggested that the adsorption capacity and rate constants of NaCl increased as voltage supplied rose from 0 V to 1.2 V. The adsorption/electrosorption were found to be in agreement with the Lagergren adsorption rate law. From Fig. 5 and Table 2, it could be seen that the adsorption capacity and rate were increased greatly by applying an electric field. Compared with the adsorption rate at 0 V, the adsorption rate at 1.2 V was increased by 58.9%. Therefore, it was concluded that electrosorption could effectively enhance the adsorption rate of ions in the solution on CNT electrodes.
log(qe − q) = log qe −
3.3. Cyclic voltagram of CNT electrodes Fig. 6 shows cyclic voltagrams obtained in the electrosorption and desorption of NaCl on the CNT electrodes. The variation of potential from 0 V to 1.2 V between electrodes indicated the adsorption behavior of ions on an electrode surface. A steady increase with current in cyclic voltagrams is caused by faster response of ions with increasing potential, while a peak appearing in this process shows the emergency of redox reaction. Table 2 The adsorption/electrosorption parameters of first-order adsorption kinetics for NaCl at different potentials Bias potentials (V)
qe (mg/g)
K (×10−2 min−1 )
R2
0 0.4 0.8 1.2
1.01 2.51 4.91 7.74
2.19 3.37 3.41 3.48
0.9977 0.9792 0.9773 0.9753
As shown in Fig. 6, there was no evident redox pseudocapacitance peaks that can confirm strong oxidation or reduction occurred in the potential range. This revealed the enhancement of adsorption capacity and rate at bias potential supplied were mainly not due to direct electrochemical reaction caused by active oxidizing, but due to the increase in electrosorption by forming an electric double layer between the surface of electrodes and ions. Hence, the enhancement of NaCl adsorption capacity and rate were mainly due to the increase in electrosorption. 4. Conclusion CNT electrodes with PTFE as binders are prepared and the adsorption/electrosorption ability are also measured. It is clear that applying an electric field on CNT electrodes has an accelerating effect on the adsorption of NaCl, and the electrosorption capacity and rate are dependent on bias potential, increased with increasing bias potential. The electrosorption is in agreement with Langmuir adsorption isotherms and the pseudo-first-order adsorption kinetics. The equilibrium adsorption capacity at 1.2 V was 9.35 mg/g, which was nearly eight times higher than that at 0 V. Compared with the adsorption rate at 0 V, the adsorption rate at 1.2 V was increased by 58.9%. The cyclic voltagrams result reveals the enhancement of adsorption capacity and rate at bias potential supplied were mainly due to the increase in electrosorption by forming an electric double layer. Therefore, the electrosorption may show a promising application for desalination and purification of drinking water. References [1] [2] [3] [4] [5] [6] [7]
M.S. Mohsen, J.O. Jaber, M.D. Afonso, Desalination 157 (2003) 167. M. Al-Shammiri, M. Safar, Desalination 126 (1999) 45–59. Y.M. ElSayed, Desalination 107 (1996) 13–27. H. Tanaka, T. Nosoko, T. Nagata, Desalination 130 (2000) 295–304. B. Pilat, Desalination 139 (2001) 385–392. E. Ayranci, B.E. Conway, J. Appl. Electrochem. 31 (2001) 257–266. E. Ayranci, N. Hoda, J. Hazard. Mater. 112 (2004) 163–168.
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S. Wang et al. / Separation and Purification Technology 58 (2007) 12–16
[8] Y.H. Han, X. Quan, S. Chen, H.M. Zhao, C.Y. Cui, Y.Z. Zhao, Sep. Purif. Technol. 50 (2006) 365–372. [9] J.C. Farmer, D.V. Fix, G.V. Mack, R.W. Pekala, J.F. Poco, J. Electrochem. Soc. 143 (1996) 159–169. [10] T.Y. Ying, K.L. Yang, S. Yiacoumi, C. Tsouris, J. Colloid Interf. Sci. 250 (2002) 18–27. [11] H. Oda, Y. Nakagawa, Carbon 41 (2003) 1037–1047. [12] D.S. Zhang, L.Y. Shi, J.H. Fang, K. Dai, L.Y. Zhang, J. Inorg. Mater. 21 (2006) 87–93.
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