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Materials Letters 59 (2

NaCl adsorption in multi-walled carbon nanotubes

Kai Daia,b,T, Liyi Shia,b,c, Jianhui Fangb, Dengsong Zhangb, Bingkun Yub

aSchool of Material Science and Engineering, Shanghai University, 99 Shangda Road, Shanghai 200436, PR ChinabCollege of Science, Shanghai University, 99 Shangda Road, Shanghai 200436, PR China

cNano-Science & Technology Center, Shanghai University, 99 Shangda Road, Shanghai 200436, PR China

Received 2 November 2004; accepted 7 January 2005

Available online 10 February 2005

Abstract

Three kinds of multi-walled carbon nanotube electrodes were fabricated in electrochemical double layer capacitors to adsorb Na+ and Cl�

from NaCl solution. The amount of ions adsorbed by electrodes depends on the specific surface area, pore specific volume. The specific NaCl

adsorption was investigated, it was found that purified carbon nanotube electrode after carbonization is the best NaCl adsorption electrode

with the largest specific surface area and pore specific volume, the percentage of NaCl desorption is nearly 90%, and the regeneration

property of the electrode was also studied in this paper.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Carbon nanotubes; NaCl; Adsorption

1. Introduction

In recent years, a great interest has been focused on

electrochemical double layer capacitors because of their

high energy density and long cycle life [1–4].

Since carbon nanotubes were discovered by Iijima in

1991 [5], they have good chemical stability, high electrical

conductivity, high inertia and significant mechanical behav-

ior [6–8], this suggested that they could be suitable for

electrochemical double layer capacitors used in batteries,

storage of hydrogen, flat panel display, chemical sensor and

so on [9–12]. Furthermore, their high accessible surface

area, low resistance, high adsorption and high pore specific

volume suggested that carbon nanotubes were suitable

materials to adsorb NaCl from brackish water.

The purpose of this study was to desalt NaCl solution

using multi-walled carbon nanotubes and to find a new way

to regenerate the carbon nanotubes. The carbon nanotube

electrodes were tested by passing through NaCl solution, the

0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.matlet.2005.01.042

T Corresponding author. School of Material Science and Engineering,

Shanghai University, 99 Shangda Road, Shanghai 200436, PR China. Fax:

+86 21 66134852.

E-mail address: daikai940@163.com (K. Dai).

influence on ion adsorption of the properties of the carbon

nanotubes, such as their specific surface area and pore

specific volume was investigated, it was found purified

carbon nanotube electrode after carbonization is the best

NaCl adsorption electrode. And the regeneration of the

electrode was also studied.

2. Principle

The key part of the desalinator was the electrochemical

double layer capacitor, ideally, the electrochemical double

layer formed at the electrode and NaCl solution interfaces in

the drive of direct current because the chemical potential of

positive ions (Na+) and negative ions (Cl�) was different in

polarized electrodes and electrolyte, the ions moved to the

electrode which has reverse polarity by electric adsorption.

The pores of electrodes were utilized to store ions, and fresh

water was obtained when voltage was applied, as Fig. 1(a)

shows. When ions were saturated in the electrodes and they

could not enter into the electrodes any more, we changed the

polarity of electrodes [13], as Fig. 1(b) shows, the ions can

flee from the electrodes by repulsive force, and the

electrodes regenerated.

005) 1989–1992

Na+ Cl - Na+

Cl -

(a) The process of NaCl adsorption (b) The process of regeneration

Na+ Cl -

NaCl solution

Fresh water

Na+

NaCl solution

Cl -

High concentration water

Fig. 1. Assembly drawing of desalination theory.

0 100 200 300 400 500 600 700 8000

20

40

60

80

100

120

140

160

180

Vo

lum

e ad

sorb

ed (

cm3 /

g S

TP

)

Pressure (mmHg)

Sample A

Sample B

Sample C

Fig. 3. Isotherms for N2 adsorption.

K. Dai et al. / Materials Letters 59 (2005) 1989–19921990

The capacitance of electrochemical double layer was

calculated according to [14]:

C ¼Z

e0edS4pd

ð1Þ

Where: C is the capacitance in Faraday (F), e is the dielec-

tric constant of solution, e0 is the dielectric constant of

vacuum, d is the distance between the surface of electrode

and the center of ion, and S is the surface area of electrode.

3. Experimental

Carbon nanotubes were produced by chemical vapor

deposition method using CH4 and La2NiO4 as the carbon

source and catalyst [15], the aperture of the carbon

nanotubes is 40–60 nm and the length is about several

micrometers. The raw materials dispersed by using ultra-

sound for 4 h in 40 wt.% nitric acids and then immersed in

20 wt.% nitric acids for 48 h, and then the mixture was

washed several times with double distilled water on a

sintered glass filter until the washings showed no acidity.

0 5 10 15 20 25 30 35 40 45 501000

1200

1400

1600

1800

2000

2200

2400

2600

2800

3000

Sal

t co

nce

ntr

atio

n (

mg

/L)

Time (min)

Sample A

Sample B

Sample C

Fig. 2. Comparison of removal characteristics for three samples.

Finally, purified carbon nanotubes were obtained after

drying in an oven at 100 8C for 2 h.

Unpurified and purified carbon nanotube, respectively,

mixed with binder powders in a weight ratio of 80:20 was

mounded under 20 MPa pressure at 150 8C for 15 min,

where the binders made up of phenolic resin (95%) and a

minute of urotropine (5%), and they were signed as sample

A and sample B. Sample C was obtained by carbonization

of the sample B at 850 8C for 2 h under nitrogen

environment. All of the carbon nanotube electrodes were

115�75 mm2 in area, 1.2 mm in thickness and about 15 g in

weight.

The salt concentration was measured by a conductivity

meter of Cyerscan CON200 at the outlet of the apparatus,

Scanning electron microscopy (SEM) of the carbon nano-

tube electrodes was performed using JSM-6700F (JEOL),

Transmission Electron Microscope (TEM) of the carbon

nanotubes was measured by JEM 200CX, the Brunauer–

Emmett–Teller (BET) specific surface area values were

performed by Micromeritics ASAP2100, voltage was

measured by DF1730SB5A.

4. Results and discussion

Fig. 2 shows adsorption characteristics of sample A, B

and C, where the starting concentration is 3000 mg/l,

voltage is 1.2 V, water current is 10 ml/min and the number

of electrode piece is 10. Sample C was the best to adsorb

ions, and sample B was better than sample A. From Eq. (1),

Table 1

Data of BET surface area, pore specific volume and specific desalination

Sample BET (m2/g) Pore specific

volume (cm3/g)

Specific

desalination (mg/g)

A 47.015 0.175 0.535

B 93.107 0.204 0.907

C 129.368 0.383 1.734

Fig. 4. SEM images of samples A (a), B (b) and C (c).

100nm 100nm

(a) (b)

Fig. 5. TEM images of unpurified (a) and purified (b) carbon nanotubes.

0 50 100 150 200 250 3000

2000

4000

6000

8000

Sal

t con

cent

ratio

n (m

g/L)

Time (min)

Fig. 6. Removal and regeneration of sample C.

K. Dai et al. / Materials Letters 59 (2005) 1989–1992 1991

we can found that the NaCl adsorption capacity depends on

the specific surface area. Fig. 3 shows the N2 adsorption

isotherms for samples A, B and C at 77 K. The data of BET

surface area, pore specific volume and specific desalination

of three kinds of samples were listed in Table 1.

Fig. 4 shows the SEM images of samples A, B and C, for

sample A, the impurities clang to the outer surface of carbon

nanotubes, which will prevent outer wall to adsorb ions. Fig.

5 shows the TEM images of unpurified and purified carbon

nanotubes, untreated carbon nanotubes have many catalyst

particles and other carbonaceous phases such as amorphous

carbons and graphite particles, which will obstruct the

utilization of the carbon nanotubes because the tips of

unpurified carbon nanotubes were almost closed, the inner

surface of carbon nanotubes could not be used in adsorption.

After ultrasound and acid treatment, the surface area of

carbon nanotubes was increased with their tips opened and

surface cleared, and thus, the ions adsorption capacity was

increased.

From the nanostructural and microtextural characteriza-

tions, carbon nanotubes appear as a web of curved nanotubes

forming often intertwined entanglements, the porous struc-

ture of the electrodes is free of dead-end pores for purified

carbon nanotube electrodes [16]. Thus the pores of carbon

nanotube electrodes can be completely used theoretically. But

in this study, the electrode contained 20% binders, which will

worsen the structural performance of electrode and hinder the

diffusion of solvated ions towards the active surface, so it is

necessary to eliminate the bad influence of the binders. As a

kind of organic material, phenolic resin composed of carbon,

hydrogen and oxygen. Many oxygen functional groups were

found to act as cross-linking sites during the carbonization

and part of them were left as ether groups [17], carbon

powders were left, they cannot block the pores of carbon

nanotube electrodes and have high adhesive intensity and fine

conductivity, we can see sample C in Fig. 4, and a large

quantity of holes were engendered, so the values of specific

surface area and specific desalination increase dramatically.

The values of NaCl adsorption depended heavily on the

surface of the electrode–electrolyte interface, thus the more

developed the specific surface area of the electrode, the

higher the NaCl adsorption. However, this surface must be

electrochemically accessible for the ions, so the presence of

oxygen functional groups and holes is important for electro-

K. Dai et al. / Materials Letters 59 (2005) 1989–19921992

des to transport ions. These are the reasons why sample C is

the best to adsorb NaCl.

In this study, regeneration of the electrodes was very

important, we use the method by reverse voltage of each

electrode, the adsorbed ions could come out from electrodes

by electrostatic forces, Fig. 5 shows the characteristics of

adsorption and regeneration of sample C, where the starting

concentration is 5000 mg/l (Fig. 6), voltage is 1.2 V, water

current is 10 ml/min and the number of electrode piece is

40. The process of regeneration could be carried out easily

in a short time. And the percentage of NaCl adsorption

calculated nearly 90%.

5. Conclusion

It is possible to efficiently remove Na+ and Cl� from

dilute NaCl solution using carbon nanotube by electric

adsorption. It was confirmed that the amount of removal is

generally dependent on the surface area and pore volume of

the electrode. The purified carbon nanotube electrode after

carbonization was the best NaCl adsorption electrode. And

the regeneration of the electrode was very easy with high

efficiency. Therefore it is available to adsorb NaCl by this

method.

Acknowledgements

The authors acknowledge the support of the National

High Technology Research and Development Program (863

Program) of China (2002AA302302) and special nanometer

fund of Shanghai science and technology committee

(0215nm001).

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