Improving Ion Rejection of Conductive Nanofiltration Membrane

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Remediation and Control Technologies

Improving Ion Rejection of Conductive Nanofiltration Membrane through Electrically Enhanced Surface Charge Density Haiguang Zhang, Xie Quan, Xinfei Fan, Gang Yi, Shuo Chen, Hongtao Yu, and Yongsheng Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04268 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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Environmental Science & Technology

1

Improving Ion Rejection of Conductive Nanofiltration Membrane

2

through Electrically Enhanced Surface Charge Density

3 4

Haiguang Zhang†, Xie Quan*,†, Xinfei Fan†, Gang Yi†, Shuo Chen†, Hongtao Yu†, and

5

Yongsheng Chen‡

6 7

†

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of

8

Education, China), School of Environmental Science and Technology, Dalian

9

University of Technology, Dalian 116024, China

10 11

‡

School of Civil and Environmental Engineering, Georgia Institute of Technology,

Atlanta, Georgia 30332, United States

12 13

*Corresponding author: Xie Quan; School of Environmental Science and Technology,

14

Dalian University of Technology, Dalian, China; Phone: +86-411-84706140. Fax:

15

+86-411-84706263. E-mail: [email protected].

16 17 18 19 20 21 22 23 24 25 26 27 28 29 1

ACS Paragon Plus Environment


30

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ABSTRACT

31

Nanofiltration (NF) is considered a promising candidate for brackish and sea

32

water desalination. NF exhibits high multivalent ion rejection, but the rejection rate

33

for monovalent ions is relatively low. Besides, great challenges remain for

34

conventional NF membranes to achieve high ion rejection without sacrificing water

35

flux. This work presents an effective strategy for improving the ion rejection of

36

conductive

37

electrically-assisted enhancement of surface charge density. With increasing the

38

external voltage from 0 to 2.5 V, the surface charge density of the membrane

39

increases from 11.9 to 73.0 mC m−2, which is 6.1 times higher than that without

40

external voltage. Correspondingly, the rejection rate for Na2SO4 increases from 81.6

41

to 93.0% and that for NaCl improves from 53.9 to 82.4%; meanwhile, the membrane

42

retains high permeabilities of 14.0 L m−2 h−1 bar−1 for Na2SO4 filtration and 14.5 L

43

m−2 h−1 bar−1 for NaCl filtration. The Donnan-steric-pore model analysis suggests that

44

the Donnan potential difference between the membrane and bulk solution is increased

45

under electrical assistance, leading to increased ion transfer resistance for improved

46

ion rejection. This work provides new insight into the development of advanced NF

47

technologies for desalination and water treatment.

NF

membrane

without

decreasing

the

permeability

through

48 49

Keywords: Nanofiltration membrane, electrical assistance, surface charge density,

50

carbon nanotube, polyaniline

51 52

TOC Art

Feed

SO3

SO3 SO3

SO3

SO3

SO3

Permeate

SO3

SO3

SO3

SO3 SO3

SO3

SO3

SO3 SO3

SO3

SO3

SO3

SO3

Counter-ion concentration

53 Bulk solution

c1II

SO3

Co-ion (anion) Counter-ion (cation)

54

Δc2

c1I

c2I Membrane

Permeate

SO3

SO3 SO3

Potential

SO3

c2II

Δc1

Bulk solution SO3

Permeate

Membrane

Donnan equilibrium

ΔΦD1

Adding voltage

2


ΔΦD2

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55

1. INTRODUCTION

56

With overpopulation, climate change and water pollution, the shortage of

57

available freshwater has become more and more serious.1, 2 Almost 97% of the total

58

water on earth is brackish water and seawater. Thus, desalination of salty water could

59

provide an alternative way to expand the world’s clean water supply.3-5

60

Recently, reverse osmosis (RO) membrane desalination has become one of the

61

most important desalination technologies, thereby producing approximately 65% of

62

the total global desalination capacity.6, 7 Nevertheless, RO still suffers from several

63

limitations such as high operating pressure, low water permeability and serious

64

membrane fouling.6,

65

alternative to RO.9-11 Compared with RO, NF advantages include low energy

66

consumption and high water flux.12 With regard to salt rejection, NF is an effective

67

process for rejecting multivalent salt ions (e.g. Ca2+, Mg2+, SO42−).13 However, the

68

rejection rate for monovalent salt ions (e.g., Cl−, Na+) is relatively low, generally

69

between 10 and 60%.14 Such a low rejection rate makes it difficult for NF membranes

70

to meet the demand of desalination in spite of their high water flux. Therefore, it is

71

necessary to improve the ion rejection performance of NF membranes, especially for

72

monovalent ions, while retaining high permeability.

8

Nanofiltration (NF) has been suggested as an attractive

73

The separation mechanism of NF membranes for charged molecules and ions is

74

based on the size sieving effect and the electrostatic interaction (between the

75

membrane and charged target species) as well as the dielectric exclusion.15-18

76

Accordingly, reducing membrane pore size or enhancing electrostatic interaction may

77

be the effective and available ways to improve the rejection performance of the NF

78

membrane. Owing to the trade-off between permeability and rejection,19 narrowing

79

the membrane pore size leads to the decline of permeability even though the rejection

80

is improved, which is unfavorable for the overall separation performance.20 As an

81

alternative, the enhancement of electrostatic interaction may achieve high rejection

82

without decreasing the permeability.21 Previous researches have indicated that the

83

high surface charge density of NF membranes can lead to enhanced electrostatic

84

interaction through the surface modification.22, 23 Meanwhile, the introduced charged 3



Page 4 of 22

85

functional groups can also improve the hydrophilicity of a membrane for higher water

86

permeability.24 However, the increase of the surface charge density can be restricted

87

by the thin separation layer and the limited effective specific surface area of the

88

membrane.25-27 For further improving the electrostatic interaction, other strategies

89

should be sought to enhance the membrane surface charge density. Many previous

90

studies have shown that under electrical assistance the adsorption abilities of the

91

membrane for charged molecules or ions could be improved significantly.28, 29 Besides,

92

Hu et al. reported that the capacitance of the rGO-CNT NF membrane could also be

93

increased, which enhances the Donnan effect.30 All of these imply that electrical

94

assistance may be an effective approach to enhance membrane surface charge density

95

for stronger electrostatic interaction. Therefore, the ion separation performance could

96

be improved by electrically assistant enhancement of the surface charge density of NF

97

membrane.

98

Carbon nanotubes (CNTs) have been widely used as promising materials for

99

fabricating advanced NF membranes due to their excellent mechanical strength,

100

chemical stability and outstanding water-transport properties.31-33 Previous studies

101

have demonstrated that the performance of polymeric NF membranes can be

102

improved by incorporating CNTs into the membranes.26, 34-36 In addition, the excellent

103

electrical conductivity of CNTs provides an opportunity to fabricate conductive

104

membranes with good electrical properties.28,

105

CNT-polymer

106

polyethersulfone,26 polyethyleneimine35 and polydopamine39 are non-conductive,

107

which has a negative impact on the electron conduction between CNTs. Polyaniline

108

(PANi) is a promising conducting polymer and has both superior conductivity and

109

good CNT compatibility.40 Moreover, the rigid structure and redox reversibility of

110

PANi can endow a PANi-CNT composite with good structural and chemical

111

stability.41, 42 To obtain electrically-active and stable PANi, polystyrene sulfonate (PSS)

112

is used as the only dopant during the polymerization process of PANi.43, 44 Besides,

113

PSS can also improve the separation performance of the NF membrane as a modifier

114

through increasing the fixed charge of the membrane surface.45

composite

NF

membranes,

37

However, in the reported

polymers

4


such

as

polyamide,38

Page 5 of 22


115

Herein, a conductive high-permeable PANi-PSS/CNT NF membrane was

116

constructed and used as a cathode to investigate the ion separation under electrical

117

assistance. The effect of preparation conditions on the membrane performance was

118

investigated. And the ion separation performance of the membrane with electrically

119

assistance was evaluated under different external negative voltages. In addition, the

120

underlying electrical enhancement mechanism for ion separation was also elucidated.

121

2. MATERIALS AND METHODS

122

2.1 Chemicals and Materials

123

Pristine multiwalled CNTs (outer diameter: 10−20 nm) were provided by

124

Shenzhen Nanotech Port Co. Ltd., China. PVDF membranes (100 nm pore size, 47

125

mm diameter) were purchased from Merck Millipore Co. Ltd., Shanghai, China.

126

Polystyrene sulfonate (PSS, Mw=70,000 g mol−1) were supplied by Shanghai Macklin

127

Biochemical Co. Ltd., China. Other chemicals and reagents were purchased from

128

Sinopharm Chemical Reagent Co. Ltd., Shanghai, China.

129

2.2 Fabrication of PANi-PSS/CNT Membrane

130

To obtain the PANi-PSS/CNT composite membrane, the CNT membrane was first

131

prepared by vacuum-filtering CNTs onto PVDF membrane substrates.46 Then, the

132

coating of PANi-PSS was polymerized into the CNT membrane and in-situ

133

cross-linking with glutaraldehyde (GA) under acidic condition (Figure S1). Typically,

134

aniline (0.1 M) and PSS (1.0 wt%) were added to 1 M HCl solution and mixed for 15

135

min. The as-prepared CNT membranes were soaked in the aforementioned solution

136

for 20 min at 4 °C before draining off the excess solution. Afterwards, 0.1 M

137

ammonium persulfate (APS) solution was poured slowly onto the membrane at 4 °C.

138

After an oxidation time of 5−10 min, the excess solution was removed and the

139

membrane continued to react for 6 h under 4 °C. The membrane was then dried at

140

room temperature to remove the residual water. Finally, the composite membrane was

141

fully immersed into a crosslinker solution prepared by mixing 1 mL of glutaraldehyde

142

(GA, 50 wt% solution) and 1.2 mL of concentrated HCl (12 M) in 25 mL of ultrapure

143

water. After crosslinking for 30 min, the membrane was washed well with water and

144

dried at room temperature. 5



145

2.3 Membrane Characterizations

146

The membrane morphologies were observed by a field emission scanning

147

electron microscope (FE-SEM, Hitachi S-4800) equipped with an energy-dispersive

148

X-ray spectroscopy (EDS). A Fourier transform infrared spectrometer (FTIR, Bruker

149

Optics, VERTEX 70) was used to characterize the functional groups. The charging

150

properties of the membranes were studied by a SurPASS electrokinetic analyzer

151

(Anton Paar, Austria). The electro-conductivities of membranes were tested by a

152

four-probe measurement (Keithley 2400, United States). The membrane pore size

153

distribution was determined using a two-parameter log-normal distribution function

154

(Supporting Information 1).47

155

2.4 Membrane Performance Evaluations

156

The performance of PANi-PSS/CNT membranes were investigated by a lab-scale

157

electrical membrane filtration setup shown in Figure S2. A self-designed membrane

158

module was used to seal the membrane sample. The effective membrane area was

159

8.04 cm2. The membrane was pressurized with a peristaltic pump, and the

160

transmembrane pressure was maintained at 2.0 bar with a cross-ﬂow rate of 2.4 L h−1.

161

During the electrically-assisted filtration process, the PANi-PSS/CNT membrane was

162

served as a cathode and a titanium mesh was used as an anode. The distance between

163

the membrane and titanium mesh was 1 mm. Voltages were applied by a DC

164

stabilized power supply.

165

The pure water permeability was tested using ultrapure water as the feed and

166

weighed on an electronic scale balance (JJ 1000, Max.1000g, China). The ion

167

rejection rate was measured using 5 mM feed salt (Na2SO4 or NaCl) solution and

168

measured by a conductivity meter (Multi 3420, WTW, Germany). The solution was

169

filtered for 0.5 h at 2 bar before collecting the permeate sample. Each experiment was

170

repeated and analyzed at least three times. The Donnan-steric-pore model (DSPM)48-50

171

was used to analyze the separation performance of the PANi-PSS/CNT membrane

172

(Supporting Information 2).

173

3. RESULTS AND DISCUSSION

174

3.1 Membrane Characterization 6


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175 176 177 178 179 180

Figure 1 SEM images of (a) CNT membrane and (b) PANi-PSS/CNT membrane. The PANi-PSS/CNT membrane was prepared with CNT loading of 6.20 g m−2, ANi concentration of 0.1 M and PSS content of 1.0 wt%. Both (a) and (b) have insets showing high-resolution SEM images. (c) SEM image of the cross-section of the PANi-PSS/CNT membrane. (d) Amplified SEM image of the cross-sectional surface layer of the membrane (green box in (c)).

181

The surface morphologies of CNT and PANi-PSS/CNT membranes are presented

182

in Figures 1a and b, respectively. It can be observed that the CNT membrane has

183

abundant interconnected pores and the surface has no cracks. After the polymerization

184

of aniline (with PSS) in CNT membrane pores, the surface of the PANi-PSS/CNT

185

membrane becomes denser and rougher compared with the CNT membrane. As

186

shown in the high-resolution SEM image, many dispersed nodular structures are now

187

present on the surface of the composite membrane due to the coating of PANi and

188

PSS onto the CNTs. The photographic image shows that the membrane

189

macrostructure keeps intact and green PANi-PSS is coated on the membrane (Figure

190

S3). The cross section of the composite membrane in Figure 1c shows that the

191

membrane has a thickness of 2.8 μm. From the amplified SEM image of the

192

cross-sectional morphology (Figure 1d), a compact surface layer with the thickness of

193

286 nm can be observed, suggesting that the membrane presents an asymmetric

194

structure. This structure may be because that the polymerization reaction is initiated

195

from the membrane surface. The FTIR spectra of CNT, PANi/CNT and

196

PANi-PSS/CNT membranes are exhibited in Figure S4. Noteworthily, after the 7



Page 8 of 22

polymerization of PANi-PSS, the composite membrane shows four new peaks at

198

3,400, 1,300, 1030 and 704 cm−1, corresponding to N–H, C–N, S–O and C–S

199

stretching vibrations respectively (Table S1). This indicates the presence of PANi and

200

PSS in the synthesized PANi-PSS/CNT membrane. Moreover, EDS mappings (Figure

201

S5) further confirm the composite structure of the PANi-PSS/CNT membrane. The

202

ultrasonic shock measurement demonstrates that the membrane has excellent

203

mechanical stability (Figure S6).

204

3.2 Effect of Preparation Conditions on Membrane Performance

205

20

60

15

50

10

Permeability Na2SO4 rejection rate

5

NaCl rejection rate

0

40 30 20

1.55

3.1 4.65 6.2 9.3 -2 CNT loading (g m ) (ANi concentration: 0.15 M, PSS content: 1.0 wt%)

-1

70

-2

80

25

Rejection rate (%)

30

-1

90

Permeability (L m h bar )

(b) 45

100 35

-2

-1

-1


(a)

100 90 80 70 60 50 40 30 20 10 0

40 35 30 25 20 15

Permeability Na2SO4 rejection rate

10 5

NaCl rejection rate

0 0.025

0.05

0.1

0.15

Rejection rate (%)

197

0.2

ANi concentration (M) -2 (CNT loading: 6.20 g m , PSS content: 1.0 wt%)

80 70 20 60 15 50 10 40 Permeability Na2SO4 rejection rate

5 0

NaCl rejection rate

Rejection rate (%)

25

-2

-1

-1


(c) 30

30

0

0.5 1.0 1.5 PSS content (wt%) -2 (CNT loading: 6.20 g m , ANi concentration: 0.1 M)

206 207 208

Figure 2 Effects of (a) CNT loading, (b) ANi concentration and (c) PSS content on the membrane separation performances (pure water permeability and ion rejection rate).

209

To further study the effect of preparation conditions on membrane performance,

210

the membranes with different CNT loadings, ANi concentrations and PSS contents

211

were fabricated, and their filtration performances (pure water permeability, Na2SO4

212

and NaCl rejection rates) were evaluated. With increasing the CNT loading from 1.55

213

to 9.30 g m−2, the membrane surface looks flatter and the thickness increases from

214

0.73 to 4.05 μm (Figure S7). The increased thickness leads to decreased pure water

215

permeability and improved salt rejection rates (Figure 2a). Similarly, the permeability

216

decreases continually while the rejection rates increase gradually as the ANi 8


Page 9 of 22


217

concentration increases (Figure 2b), which may be because the membrane becomes

218

denser and smoother (Figure S8). With the increase of the PSS content from 0 to 1.5%,

219

the membrane has a more compact structure (Figure S9), and the water permeability

220

decreases, whereas the salt rejection rates first increase and then tend to become

221

constant (Figure 2c). Overall, comprehensively considering the water permeability

222

and rejection rate, the membrane prepared with CNT loading of 6.20 g m−2, ANi

223

concentration of 0.1 M and PSS content of 1.0 wt% was chosen for further

224

investigation. This membrane exhibits a water permeability of 16.1 L m−2 h−1 bar−1

225

and a Na2SO4 rejection rate of 81.6% as well as a NaCl rejection rate of 53.9%.

226

Furthermore, it shows a conductivity of 824 S m−1 (Figure S10). Such a good

227

conductivity is favorable for investigating the ion separation performance under

228

external voltage assistance.37, 51

229

3.3 Filtration Performances of the PANi-PSS/CNT Membrane under Electrical

230

Assistance

9



(b)

20

-1


(a)

Page 10 of 22

-2

Na+

Na+

Na+

Na+

Na+

Na+

Cl-

(e)

-1

20

80

15 10

70

5 0

60 0.0

0.5

1.0 1.5 2.0 Voltage (V)

2.5

3.0

8 4 0 0.0

0.5

2.5

3.0

90 Permeability Rejection rate

35 30 25

80 70

20 60

15 10

50

5 0

40 0.0

0.5

(f)

100

1.0 1.5 2.0 Voltage (V)

40

-1

25

90

12

-2

-1

30

(d)

100 Permeability Rejection rate

35

-2

-1


40

Rejection rate (%)

(c)

Cl-

16

Rejection rate (%)

SO42-

ClCl-

Cl-

Cl-

SO42-

-1

Na+


SO42-

1.0 1.5 2.0 Voltage (V)

2.5

3.0

Rejection rate (%)

Rejection rate (%)

80 80

2.5 V

60 40

Without voltage 2.5 V

20 0 1

2 3 Time (h)

4

Without voltage 2.5 V 0

(h) 100 Rejection rate (%)

Rejection rate (%)

2.5 V

20

5

90

231 232 233 234 235 236 237

40

0 0

(g) 100

60

80 70 60 50

1

2 3 Time (h)

4

5

80 60 40 20 0

40

0V 2.5V 0V 2.5V 0V 2.5V 0V 2.5V 0V 2.5V 0V

0V 2.5V 0V 2.5V 0V 2.5V 0V 2.5V 0V 2.5V 0V

Figure 3 (a) Enhanced view (artist’s illustration) of membrane filtration with electrical assistance. (b) pure water permeabilities of PANi-PSS/CNT membrane under different voltages. (c) Na2SO4, and (d) NaCl filtration performances of PANi-PSS/CNT membrane under different voltages. (e) Na2SO4, and (f) NaCl rejection rates of the membrane with and without external voltages over filtration time. The variations of (g) Na2SO4 and (h) NaCl rejection rates of the membrane with and without external voltages (2.5 V).

238

Electrically-assisted filtration experiments were conducted using a two-electrode

239

test setup (Figure 3a). The PANi-PSS/CNT membrane served as a cathode and the

240

same-sized titanium mesh was used as an anode. Different voltages from 0 to 3.0 V

241

were applied to investigate the effect of electrical assistance on the filtration

242

performance. As shown in Figure 3b, the pure water permeability of the composite

243

membrane is almost unchanged with the increase of applied voltage, suggesting the 10


Page 11 of 22


244

electrical assistance has negligible influence on the pure water permeability. Figure 3c

245

and d show the filtration performances of the membrane for Na2SO4 and NaCl,

246

respectively, under different voltages. Obviously, similar variation tendencies in

247

permeability and salt rejection rate can be observed for Na2SO4 filtration and NaCl

248

filtration. With the external voltage ranging from 0 to 2.5 V, the permeabilities remain

249

at around 14.0 L m−2 h−1 bar−1 for Na2SO4 filtration and 14.5 L m−2 h−1 bar−1 for NaCl

250

filtration. Notably, the rejection rate for Na2SO4 increases from 81.6 to 93.0% and that

251

for NaCl improves from 53.9 to 82.4% as the voltage increases from 0 to 2.5 V. These

252

results demonstrate that electrical assistance can improve the ion rejection

253

performance of the membrane without decreasing the permeability. As shown in

254

Figure S11, compared with the reported NF membranes and commercial NF

255

membranes, the PANi-PSS/CNT membrane exhibits superior water permeability and

256

high ion rejection under electrical assistance. However, when the voltage is further

257

increased to 3.0 V, both of the permeability and rejection rate are decreased, either for

258

Na2SO4 or NaCl. To explore the reason why the permeability and the salt rejection

259

rate are declined at 3.0 V, the cathode potential and CV curve of the membrane were

260

tested (Figure S12). At the voltage of 3.0 V, the corresponding cathodic potential is

261

around −1.7 V (vs. SCE, Figure S12a). From the CV curve (Figure S12b), an obvious

262

current increase can be observed at the potential of −1.7 V (vs. SCE). This result

263

demonstrates that the electrochemical reaction occurs at the membrane cathode when

264

the applied voltage is 3.0 V. Such a reaction may increase water transport resistance

265

and disturb the ion rejection, thereby decreasing the permeability and the salt rejection

266

rate. To avoid the impact of the electrochemical reaction, the following studies were

267

carried out at an applied voltage between 0 and 2.5 V.

268

One of the concerns that has surfaced as a result of using the PANi-PSS/CNT

269

membrane for ion separation with external voltage is the electrochemical stability of

270

the membrane under cathodic conditions. Here, 20 cycles of CV scans of the

271

membrane were performed at cathodic potentials ranging between 0 and −1.5 V vs.

272

SCE (Figure S13). The result shows a good overlap of all CV curves, indicating that

273

the membrane has good electrochemical stability. Figure 3e and f display the change 11



Page 12 of 22

274

in the rejection rates of Na2SO4 and NaCl over filtration time. Without the voltage, the

275

Na2SO4 and NaCl rejection rates of the membrane are 81.9 and 52.6%, respectively.

276

After applying the voltage of 2.5 V at 0.5 h, the rejection rates increase to 93.3% for

277

Na2SO4 and 82.9 % for NaCl. With further filtrating for 4.5 h, the rejection rates

278

remain almost constant. Furthermore, the long-time (30 h) operation result shows

279

stable permeability and rejection (Figure S14). These studies suggest that the

280

electrically-assisted PANi-PSS/CNT membrane exhibits good operation stability. By

281

applying and removing the external voltage, continuous switches (improvement and

282

restoration) for Na2SO4 and NaCl rejection rates can be observed (Figure 3g and h),

283

demonstrating that the membrane possesses excellent recyclability.

284

3.4 Discussion of the Electrical Enhancement Mechanism

(a) - - ++ - - + + + + + + + + Membrane pore + + + + + + + + - + + - - - NF membrane

+

-

Titanium mesh

ΔΦD

0.6 0.4

25 20 15 10 5 0

0.0

0.5

1.0 1.5 2.0 Voltage (V)

(c) 100

0.0 V 0.5 V 1.0 V 1.5 V 2.0 V 2.5 V

2.5

0.2 0.0 100

200

300

400

500

2.5 V

80 60 40 20

1 mm

600

(e) 100

0

80

Rejection rate (%)

(d) 20 -20 -40 -60 -80 S i-PS PAN

2 mm

3 mm

Distance between the anode and membrane cathode

Time (s)

Zeta potential (mV)

Without votage

0

0

285 286 287 288

+

-- -- -- --+ + + -- - - ----NF membrane

3 Enhanced electrostatic interaction

2 External electric field (force)

Rejection rate (%)

Adsorption quantity (μg)

Current (mA)

0.8

+

-

+

NF membrane

1 Ion electro-adsorption

(b) 1.0

Donnan effect

-

+

--- -- -- --+

+

Without voltage 2.5 V

60 40 20 0

1 T3 T4 T2 /CNT S/CN S/CN S/CN i-PS i-PS i-PS PAN PAN PAN

4 1 2 3 /CNT /CNT /CNT /CNT i-PSS i-PSS i-PSS i-PSS PAN PAN PAN PAN

Figure 4 (a) Illustration of ion electro-adsorption, external electric field and enhanced electrostatic interaction. (b) I-t curves of PANi-PSS/CNT membrane under different voltages (the inset: ion adsorption quantities). (c) NaCl rejection rates of PANi-PSS/CNT membrane with and 12


Page 13 of 22


289 290 291 292 293 294 295

without external voltage under different distances between the anode and membrane cathode. (d) Zeta potentials and (e) NaCl rejection rates of four PANi-PSS/CNT membranes with similar pore sizes and different surface charges (PANi-PSS/CNT1, PANi-PSS/CNT2, PANi-PSS/CNT3, and PANi-PSS/CNT4). The zeta potentials were measured in a NaCl solution with the concentration of 5 mM and the pH of 7.0. The black bars in (e) represent the rejection rates of the membranes without voltage, and the red bars represent the rejection rates of the membranes with the voltage of 2.5 V.

296

The results of flirtation performance tests verify that the ion rejection performance

297

of PANi-PSS/CNT membrane can be improved by electrical assistance. To better

298

define the enhancement mechanism, NaCl was used here as the target salt to

299

investigate the effect of electrical assistance on enhanced ion rejection.

300

Previous studies have indicated that water desalination can be achieved by ion

301

adsorption of the membrane (Figure 4(a-1)).52 Therefore, the ion adsorption

302

performance of the membrane was evaluated under different applied voltages. Figure

303

4b shows the I–t curves of the PANi-PSS/CNT membrane under different voltages.

304

All of the current curves are first decreased due to the electrosorption of ions,53 and

305

then the curves tend to retain constant values which indicates that the adsorption of

306

the membrane has achieved saturation. Accordingly, the ion adsorption quantities can

307

be obtained through the I–t curves (the inset in Figure 4b). The calculated adsorption

308

quantity increases from 0.8 to 23.2 μg with increasing the voltage from 0 to 2.5 V.

309

Such low adsorption quantities demonstrate that the enhanced ion rejection is not a

310

result of ion electro-adsorption. It is well known that an electric field is existed

311

between the membrane cathode and the anode after applying the external voltage

312

(Figure 4(a-2)). With this electric field force, the ionic electro-migration can occur in

313

the electric field (caused by the electric field force), which could result in ion

314

rejection.3, 54 To explore the influence of external electric field force on ion rejection,

315

we adjusted the distance between membrane cathode and anode from 1 to 3 mm (to

316

tune the electric field force) and tested the NaCl rejection of the membrane

317

with/without external voltage. When the distance is 1 mm, the NaCl rejection rate

318

increases from 53.9 to 82.4% by applying a voltage of 2.5 V (Figure 4c). Besides, as

319

the distance increases to 2 and 3 mm, no obvious changes in the rejection rate at 2.5 V

320

are exhibited. The results suggest that the electric field force is not the main cause of 13



321

Page 14 of 22

the enhanced ion rejection.

322

During the NF process, the electrostatic interaction between the membrane and

323

ions (caused by the electrostatic force between the membrane surface and ions) is

324

mainly dependent on the fixed charge of the membrane, which derives from the

325

functional groups on the membrane surface.45, 55 Therefore, after applying the external

326

voltage, the electrostatic interaction could be enhanced (Figure 4(a-3)). In this case,

327

electrostatic interaction is contributed by external voltage and fixed charge. To

328

demonstrate the effect of these two factors on ion rejection, the separation

329

performance of the membranes with different fixed charges were evaluated in the

330

absence/presence of external voltage. Here, the membrane pore size and the fixed

331

charge were controlled by tuning the PSS content and the ANi concentration, and four

332

membranes with similar pore sizes and different surface charges were fabricated

333

(Table S2, Figure 4d and 4e). Figure 4d shows that the Zeta potential of the membrane

334

changes from 7.3 to −69.4 mV, indicating enhanced electrostatic interaction.

335

Accordingly, the rejection rate improves from 34.6 to 53.8% (Figure 4e), which

336

suggesting that the enhancement of electrostatic interaction can improve the rejection

337

performance of the membrane. When the voltage of 2.5 V is applied on the membrane

338

system, the rejection rate increases in all membranes. This result indicates that the

339

electrical assistance can improve the ion rejection performances of the membranes

340

with different surface charges. Thus, it can be inferred that the electrical assistance

341

enhances the electrostatic interaction for improving the rejection performance. Under

342

electrical assistance, the membrane can be polarized to generate polarization-induced

343

charges due to the negative bias.56-60 Meanwhile, the PANi can endow the membrane

344

with high charge density, which can be doped with counter-ions (cations).61,

345

Moreover, the adsorbed ions on the membrane surface is increased significantly with

346

increasing the voltage (inset in Figure 4b). Therefore, the electrical assistance is likely

347

to further enhance the surface charge density of the membrane for enhanced

348

electrostatic interaction.

62

349

Figure 5a shows the charge quantities on the PANi-PSS/CNT membrane surface

350

under different external voltages, which were obtained by measuring the amount of 14


Page 15 of 22


351

transferred charges during the ion adsorption process. A significant change in charge

352

quantities from 0 to –38.2 mC can be observed as the applied voltage increases from 0

353

to 2.5 V. Accordingly, the calculated surface charge density changes from 0 to –61.1

354

mC m−2 (inset in Figure 5a). This result demonstrates that the surface charge density

355

of the membrane could increase when applied voltage increases. This phenomenon

356

implies that the Donnan potential difference between the membrane and the bulk

357

solution could be enlarged after applying the external voltage.

358

To further investigate the effect of electrical assistance on membrane performance,

359

DSPM50 was used to analyze the retention properties of the PANi-PSS/CNT

360

membrane, which has been proved to be successful in modeling the transport behavior

361

of NaCl.48, 63 To obtain the rejection rates through DSPM, the charge densities at

362

different voltages were calculated (Table S3), and the average pore size of the

363

membrane were measured (1.98 nm, Figure S15). Noteworthily, the applied voltage

364

has ignorable influence on the membrane pore size (Figure S16). The calculated

365

rejection rates at different voltages are shown in Figure S17, and the comparison of

366

calculated and experimental results are exhibited in Figure 5b. It can be observed that

367

all deviations between the calculated and experimental rejection rates are smaller than

368

5%, suggesting that the rejection performance of the membrane with electrical

369

assistance can be well fitted by DSPM. Table S4 shows the calculated counter-ion

370

concentrations at the membrane–solution interfaces under different external voltages.

371

The counter-ion concentration in the membrane increases significantly from 232.8 to

372

936.5 mol m−3 with increasing the voltage from 0 to 2.5 V. Accordingly, the

373

calculated Donnan potential at the membrane–feed solution interface changes from

374

−107.4 to −142.5 mV, and the Donnan potential at the membrane–permeate solution

375

interface increases from 103.5 to 167.1 mV (Table S5). These results indicate that

376

electrical assistance can enhance the ion concentration difference and the Donnan

377

potential difference between the membrane and bulk solution.

15


-35 -30 -25 -20 -15 -10

-60 -50 -40 -30 -20 -10 0

0.0

0.5

1.0

1.5

2.0

2.5

Voltage (V)

-5 0 0.0

0.5

1.0

1.5

2.0

2.5

80 60 40 20 0 0

20

SO3

SO3 SO3

SO3 SO3

SO3

SO3

Bulk solution

Feed

60

SO3

SO3 SO3

SO3

SO3 SO 3 SO3 SO3

SO3

SO3

SO3 SO3 SO3

SO3

SO3 SO3

SO3

SO3

Co-ion (anion)

SO3

Permeate

Membrane

Δc1

Adding voltage

c2II Δc2

c1I

c2I Membrane

Donnan equilibrium

SO3 SO3

Counter-ion (cation)

c1II

Bulk solution

Potential Φ (V)

Permeate Adding voltage

Permeate

Bulk solution

c1II

80

ΔΦD2

ΔΦD1

SO3 SO 3 SO3 SO3

100

Permeate

SO3 SO 3 SO3 SO3

SO3

SO3 SO3 SO3

SO3

SO3 SO3

SO3

SO3

Permeate

Membrane

c2II

Δc1

Δc2

c1I

c2I Bulk solution

Potential Φ (V)

Counter-ion concentration ci (M)

SO3

SO3 SO 3 SO3 SO3

Counter-ion concentration ci (M)

Feed

40

Experimental rejection rate (%)

Voltage (V)

(c)

Page 16 of 22

(b)100 -70

Predicted rejection rate (%)

Charge quantity (mC)

-40

-2

(a) -45

Surface charge density (mC m )


Membrane

Donnan equilibrium

Permeate

ΔΦD2

ΔΦD1

378 379 380 381 382 383

Figure 5 (a) Charge quantities on the PANi-PSS/CNT membrane surface under different voltages. The inset shows the calculated membrane surface charge densities. (b) Comparison of calculated and experimental NaCl rejection rates of PANi-PSS/CNT membrane under different voltages (yellow zone: < 5% deviation). (c) Schematic diagram of the enhancement mechanism of ion rejection performance with electrical assistance.

384

The electrical enhancement mechanism could be explained based on the DSPM

385

analysis. For the PANi-PSS/CNT membrane, the sulfonic groups of PSS on the

386

membrane surface can be dissociated to generate negative charges (Figure 5c). The

387

negatively charged surface adsorbs co-ions (cations) and repulse counter-ions (anions).

388

During the ion separation process, ion transfers can take place between bulk solution

389

and the membrane through a combination of convection, diffusion and migration. This

390

makes it possible for a distribution equilibrium to be established at the

391

membrane–solution interface. Consequently, the membrane phase exhibits higher 16


Page 17 of 22


392

co-ion concentration but lower counter-ion concentration than the bulk solution phase,

393

which will lead to the formation of ion concentration difference and Donnan potential

394

difference between the membrane and bulk solution. The potential difference can

395

prevent ions (co-ions and counter-ions) from transferring into the membrane. Under

396

the electrical assistance, the membrane cathode is polarized to generate

397

polarization-induced charges due to the negative bias. The electric potential of the

398

(negatively charged) membrane becomes more negative. And the polarized surface

399

can further improve the adsorption of counter-ions. Meanwhile, the PANi can also be

400

doped with counter-ions for electrochemical charging. The polarization phenomenon

401

enhances the partitioning of counter-ions while limiting the partitioning of co-ions.

402

More counter-ions transfer into the membrane and the co-ions in the membrane

403

become rarer. As a result, the ion concentration difference and Donnan potential

404

difference become greater. Owing to the enlarged concentration difference and

405

potential difference, the ions in bulk solution are less likely to transfer through the

406

membrane. Therefore, the ion rejection performance of the PANi-PSS/CNT

407

membrane is improved under electrical assistance.

408

3.5 Implications and challenges

409

In this study, a conductive PANi-PSS/CNT membrane with good electrochemical

410

stability has been successfully fabricated and employed for ion separation under

411

electrically assistance. The result reveals that electrically-assisted enhancement of

412

membrane surface charge density is an effective strategy to improve the ion rejection

413

performance without decreasing the permeability. This enhancement strategy has

414

important implications for overcoming the trade-off between permeability and

415

rejection (or selectivity). During the ion rejection process, the electrical assistance

416

only causes very little additional energy consumption. As shown in Figure S18, the

417

energy consumption increases from 0 to 0.0017 kWh m−3 with increasing the voltage

418

from 0 to 2.5 V, which is much smaller than the overall energy consumption of NF

419

membrane process (around 1 kWh m−3).64, 65 Furthermore, Figure S19 shows that the

420

electrical assistance can also improve the antifouling ability of the membrane. This

421

work offers a guideline for fabricating high-performance NF membranes, and could 17



422

promote the development of the next generation of NF technologies for desalination

423

and water treatment.

424

In NF separation processes for ions or charged molecules, the charge screening

425

effect is a considerable disadvantage which can weaken the electrostatic

426

interaction.66-69 The electrically-enhanced ion rejection also suffers from this problem.

427

Higher ion concentration leads to stronger charge screening effect, resulting in weaker

428

electrical enhancement of ion rejection (Figure S20). In order to further improve the

429

ion rejection at higher salt concentrations, stronger electrostatic interaction is expected.

430

Applying higher external voltage may be a feasible approach to enhance the

431

electrostatic interaction. Thus new membrane and new strategy should be developed.

432

Besides, the charge screening length (Debye length) is described as the characteristic

433

length for the electrostatic interaction.66, 70 Therefore, controlling the membrane pore

434

structure could be another approach to achieve strong electrostatic interaction for high

435

ion rejection.

436

4. ASSOCIATED CONTENT

437

Supporting Information

438

The Supporting Information is available free of charge on the ACS Publications

439

website.

440

Detailed calculation methods and other supplementary data are presented (PDF)

441

5. AUTHOR INFORMATION

442

Corresponding Author

443

* Tel: +86-411-84706140. Fax: +86-411-84706263. E-mail: [email protected].

444

Notes

445

The authors declare no competing ﬁnancial interest.

446

6. ACKNOWLEDGMENTS

447

This work was supported by the National Natural Science Foundation of China

448

(21437001 and 51478075), the Programme of Introducing Talents of Discipline to

449

Universities (B13012), the Programme for Changjiang Scholars and Innovative

450

Research Team in University (IRT_13R05), and the Fundamental Research Funds for

451

the Central Universities (DUT16TD02). 18


Page 18 of 22

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452

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Improving Ion Rejection of Conductive Nanofiltration Membrane

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