Combined Effects of Surface Charge and Pore Size on Co-enhanced

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Combined Effects of Surface Charge and Pore Size on Co-enhanced Permeability and Ion Selectivity through RGO-OCNT Nanofiltration Membranes Haiguang Zhang, Xie Quan, Shuo Chen, Xinfei Fan, Gaoliang Wei, and Hongtao Yu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00515 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Environmental Science & Technology

1

Combined Effects of Surface Charge and Pore Size on Co-enhanced

2

Permeability

3

Membranes

and

Ion

Selectivity

through

RGO-OCNT

Nanofiltration

4 5

Haiguang Zhang, Xie Quan*, Shuo Chen, Xinfei Fan, Gaoliang Wei, and Hongtao Yu

6 7

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education,

8

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

9

Dalian 116024, China

10 11

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

12

University of Technology, Dalian, China; Phone: +86-411-84706140. Fax: +86-411-84706263.

13

E-mail: [email protected].

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1

ACS Paragon Plus Environment


31

Abstract

32

Nanofiltration (NF) has received much attention for wastewater treatment and

33

desalination. However, NF membranes generally suffer from the trade-off between

34

permeability and selectivity. In this work, the co-enhancement of permeability and ion

35

selectivity was achieved through tuning the surface charge and pore size of oxidized

36

carbon nanotube (OCNT) intercalated reduced graphene oxide (RGO) membranes.

37

With the increase of OCNT content from 0 to 83%, the surface charge and the pore

38

size are increased. The permeability increased to 10.6 L m−2 h−1 bar−1 and rejection

39

rate reached 78.1% for Na2SO4 filtration at a transmembrane pressure of 2 bar, which

40

were 11.8 and 1.3 times higher than those of pristine RGO membrane. The composite

41

membrane also showed 11.1 times higher permeability (11.1 L m−2 h−1 bar−1) and 2.9

42

times higher rejection rate (35.3%) for NaCl filtration. The analyses based on Donnan

43

steric pore model suggest that the increased permeability is attributed to the combined

44

effects of enlarged pore size and increased surface charge, while the enhanced ion

45

selectivity is mainly dependent on the electrostatic interaction between the membrane

46

and target ions. This finding provides a new insight for the development of

47

high-performance NF membranes in water treatment and desalination.

48 49

Keywords: Nanofiltration membrane, reduced graphene oxide, oxidized carbon

50

nanotube, surface charge, pore size

51 52

TOC Art

53

54 55 2


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1. INTRODUCTION

57

Water scarcity has emerged as a global concern of growing severity due to the

58

increasing demand, serious pollution and unbalanced distribution of water resources.1,

59

2

60

feasible and effective ways to alleviate the worldwide water crisis,3, 4 for which novel

61

water treatment technologies should be developed.

Currently, seawater desalination and wastewater treatment have been regarded as the

62

Membrane separation is one of the most favorable and efficient technologies for

63

water treatment due to its simple operational process, non-phase change, no chemical

64

addition and small footprint requirements.5 In particular, nanofiltration (NF), with the

65

membrane pore size range of 0.5~2 nm, can efficiently reject multivalent salt ions and

66

organic molecules above 300 Dalton.6-9 This technology has been rapidly developed

67

in the last few decades, which has great potential in desalination and removal of heavy

68

metal ions and organic pollutants.10 However, NF membranes are generally subject to

69

the trade-off between permeability and selectivity11, 12: increased permeability results

70

in a decreased selectivity, and vice versa.13 Previous studies14-16 have devoted to

71

preparing high-flux membranes by incorporation of nanomaterials, but the selectivity

72

is negatively affected by enlarged membrane pore size or loosened active layer. Some

73

researchers17-19 have focused on improving the rejection performance of membranes,

74

but still the permeability is decreased due to the thickened or compacted active layer

75

which leads to reduced membrane pore size. The trade-off is a ubiquitous and

76

pernicious problem, which can restrict the membrane separation performance.12

77

Therefore, it is of great significance to develop advanced NF membranes with both

78

high permeability and selectivity.

79

Recently, nanocarbon-based membranes such as graphene-based membranes and

80

carbon nanotube-based membranes are of great interest due to their extraordinary

81

physicochemical stability, high water permeability and high rejection rate.20-23 These

82

membranes are expected to become promising candidates for high-performance

83

membranes. Among various nanocarbon membranes, graphene-based laminar

84

membranes (graphene oxide (GO) and reduced GO (RGO) membranes) provide a

85

more practical approach for NF separation.24 Due to the superior hydrophilicity of GO 3



86

nanosheets, the interlayer spacing of GO membranes can dramatically increase and

87

even reach several nanometers.25-27 Compared with GO membranes, RGO membranes

88

possess more stable membrane structure and higher rejection performance.28, 29 But

89

the reduction of RGO causes decreased water permeability and low membrane surface

90

charge.30 Carbon nanotubes (CNTs) as an intercalated nanomaterial can tune the layer

91

spacing of graphene-based laminar membranes,14 which could increase the membrane

92

pore size for enhanced permeability.22 Besides, the chemical oxidation of CNTs can

93

graft abundant oxygen-containing functional groups (carboxyl, hydroxyl, and epoxide

94

groups) to the surface of CNTs.31 These functional groups could increase the surface

95

charge for enhanced electrostatic interaction between the membrane and charged

96

species.31, 32 In NF process, the electrostatic interaction can endow membranes with an

97

ability to reject ions or charged molecules that are smaller than the membrane pore

98

size.30, 33 Therefore, such a oxidized CNT (OCNT) intercalated RGO (RGO-OCNT)

99

laminar NF membrane may achieve co-enhanced permeability and selectivity through

100

controlling the membrane surface charge and pore size.

101

In this work, a series of RGO-OCNT membranes are fabricated by a facile

102

vacuum filtration process. Their surface charge and pore size are controlled by tuning

103

the OCNT content in composite membrane. The permeability and selectivity are

104

tested by two target salt (Na2SO4 and NaCl) solutions. Donnan steric pore model

105

(DSPM) is used to analyze the experimental results and investigate the effects of

106

membrane pore size and surface charge on permeability and selectivity. Additionally,

107

a reasonable mechanism for ion separation of the prepared RGO-OCNT membrane is

108

proposed.

109

2. MATERIALS AND METHODS

110

2.1 Chemicals and Materials

111

Natural graphite powder (8000 mesh) was purchased from Aladdin Chemistry Co.,

112

Ltd. (Shanghai, China). Pristine CNTs (multi-walled, diameter: 10~20 nm) were

113

purchased from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China). PVDF

114

membranes (diameter: 47 mm, pore size: 0.1 µm) were provided by Merck Millipore

115

Co., Ltd. (Shanghai, China). Other chemicals and reagents used in all experiments 4


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were supplied by Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China).

117

2.2 Preparation of RGO and OCNT dispersions

118

RGO dispersion were prepared by chemical reduction of graphene oxide (GO) in

119

solution. GO was synthesized from natural graphite powder using a modified

120

Hummers’ method.34 After being purified by several runs of centrifugation/washing

121

and dried for 24 h at a temperature of 40ºC, the as-prepared GO was exfoliated into

122

ultrapure water by ultrasonication for 30 min. The obtained GO dispersion was diluted

123

to 0.05 mg mL−1. 30 mL of aforementioned GO dispersion was mixed with 80 µL of

124

ammonia solution (25% in water) and 12 µL of hydrazine solution (80 wt% in water)

125

in a 50 mL glass beaker. After being stirred for 10 min, the mixed dispersion was then

126

placed in a 50 mL Teflon-lined stainless steel autoclave and heated at 120ºC for 2 h.

127

OCNT was prepared by refluxing in HNO3/H2SO4 (1:3, v/v) at 120ºC (and 90, 60,

128

30ºC for different oxidation degrees of OCNTs) for 4 h. After being purified by

129

several runs of filtration/washing and dried at 80ºC for 24 h, 5 mg of OCNT was

130

dispersed into 100 mL of ultrapure water to form a uniform dispersion with the

131

assistance of ultrasonication.

132

2.3 Fabrication of RGO-OCNT membranes

133

The membrane preparation process is shown in Figure S1. Briefly, 4 mL of

134

as-prepared RGO dispersion (0.05 mg mL−1) was first mixed with different volumes

135

of OCNT dispersion (0.05 mg mL−1) to form a series of RGO-OCNT mixtures using

136

sonication for 10 min at 500 W. Then the uniform RGO-OCNT mixtures were

137

vacuum-filtrated on PVDF membrane substrates to yield composite membranes.

138

Finally, the resultant fresh RGO-OCNT membranes were dried in air at room

139

temperature for 12 h to remove the residual water. The membranes prepared with

140

OCNT contents of 0, 25, 50, 75 and 83 wt% are designated as RGO, RGO-OCNT25,

141

RGO-OCNT50, RGO-OCNT75 and RGO-OCNT83 membranes, respectively.

142

2.4. Membrane characterizations

143

The morphologies of as-prepared RGO-OCNT membranes were examined using

144

a field−emission scanning electron microscope (FESEM, Hitachi S−4800) and a

145

transmission electron microscopy (TEM, FEI Tecnai F30). The molecular and crystal 5



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146

structures of the samples were investigated by a Fourier transform infrared

147

spectrometer (FTIR, Bruker Optics, VERTEX 70), a Raman spectrometer (Raman,

148

DXR Microscope, Thermo Fisher), and an X−ray diffractometer (XRD, EMPYREAN,

149

PANalytical). The thickness of RGO sample was examined by an atomic force

150

microscopy (AFM, Pico Scan 2500, Molecular Imaging, US). The surface charge

151

properties of the membranes were analyzed by a SurPASS electrokinetic analyzer

152

(Anton Paar, Austria). The water contact angles were observed by an optical contact

153

angle & interface tension meter (KINO SL 200KB).

154

2.5. Membrane performance evaluations

155

All performance evaluations were carried out by using an in-house made

156

dead-end membrane filtration setup (Figure S2a). The membrane sample was sealed

157

in a self-designed membrane module for each experiment (Figure S2b). The effective

158

filtration area of the membrane was 8.04 cm2. In the filtration test, the permeability

159

was measured using ultrapure water or salt solution (Na2SO4, NaCl) as the feed, while

160

the rejection rate was determined by using a feed salt solution of 5 mM under the

161

transmembrane pressure of 2 and 5 bar. Prior to rejection tests, the membrane was

162

pressurized for 2 h. The permeability and rejection rate are calculated as follows:

163 164

J=

Q A ⋅ ∆P

 C  R = 1 − p  × 100%  C  f  

(1) (2)

165

where Q is the flow rate (L h−1) of water or salt solution at the permeate side, A is the

166

effective membrane area (m2), ∆P is the transmembrane pressure (bar), and Cp and Cf

167

are the concentrations of the salt solution in the permeate and feed, respectively,

168

which are determined by a conductivity meter (Multi 3420, WTW, Germany). All

169

experiments were performed independently at least three times. DSPM was used to

170

investigate the rejection performance of RGO-OCNT membranes (Supporting

171

Information, SI 1).35 This model has proved to be very successful in modeling various

172

solutes like Na2SO4 and NaCl.36, 37

173

3. RESULTS AND DISCUSSION

174

3.1. Characterizations of RGO-OCNT membrane 6


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175 176

Figure 1 (a) Low-resolution and (b) high-resolution SEM images of the RGO-OCNT75

177

membrane. (c) SEM images (the inset: high-resolution) of the cross-section of RGO-OCNT75

178

membrane. (d) TEM image of the RGO-OCNT75 membrane.

179

To obtain the RGO-OCNT membrane, RGO nanosheets with ultrathin structure

180

(1~2 layers) and OCNTs with rough surface were prepared, separately (Figure S4 and

181

S5). Their mixture was filtrated on a PVDF membrane substrate to form the

182

composite membrane. The morphology of the prepared composite membrane was

183

characterized by SEM and TEM. As shown in Figure 1a, RGO nanosheets and

184

OCNTs are stacked and interlaced tightly and uniformly. The membrane surface has

185

no obvious defects and aggregation due to the excellent dispersibility of RGO

186

nanosheets and OCNTs (Figure S6). Compared with pristine RGO membrane (Figure

187

S7a), the composite membrane possesses a rougher surface because of the

188

intercalation of OCNT. From the high-resolution SEM image (Figure 1b), it can be

189

obviously observed that the RGO laminates are intercalated by OCNTs, which could

190

create continuous 3D nanostructured channels in membrane. The composite

191

membrane has a thickness of ~351 nm (Figure 1c), which is much thicker than the

192

RGO membrane (~71 nm) (Figure S7b). This obvious difference in thickness

193

indicates that the intercalated OCNTs expand the interlayer spacing of the composite

194

membrane. As shown in TEM image of the composite membrane (Figure 1d), RGO 7



195

nanosheets possess many wrinkles and integrate closely with OCNTs to form strong

196

and stable network. Figure S8 shows that the composite membrane possesses superior

197

mechanical strength, which is undamaged after ultrasonication (60 kHz, 100 W) for

198

10 min.

199

XRD, Raman and FTIR patterns of RGO and RGO-OCNT membranes are shown

200

in Figure S9. In the XRD spectra of RGO membrane, a single peak at about 22.6° is

201

observed. For the composite membrane, two peaks located at 22.3 and 25.6°

202

demonstrate that the OCNT has been intercalated into the RGO laminates. From the

203

Raman spectra, typical D and G bands could be found at approximately 1350 and

204

1596 cm−1, respectively. After the intercalation of OCNT, a higher Raman D/G peak

205

height ratio can be observed for the composite membrane (ID/IG = 1.2), compared with

206

RGO membrane (ID/IG = 1.0). The FTIR patterns show that the relative intensity of

207

C=O (1653 cm−1), C−OH (1404 cm−1) and C−O (1221 cm−1) peaks are all increased,

208

meaning that the intercalated OCNT increases the oxygen-containing functional

209

groups of the composite membrane. All of the above results indicate that the

210

membrane structure and surface property of RGO-OCNT membrane could be greatly

211

influenced by the intercalated OCNT.

212

3.2. Effect of OCNT on surface charge and pore size of RGO-OCNT membrane

213

In order to investigate the effect of OCNT on membrane surface charge and pore

214

size, a series of RGO-OCNT membranes containing different OCNT contents (0, 25,

215

50, 75 and 83%) were fabricated. As shown in Figure S10, the color of the composite

216

membranes become darker as the OCNT content increases, and all the membrane

217

surfaces are defect-free and smooth, suggesting that the macrostructure of composite

218

membrane remains intact after the intercalation of OCNT. The surface and

219

cross-sectional morphologies of the membranes were characterized by SEM, as shown

220

in Figure 1, S7 and S11. With the increase of OCNT content, the distribution of

221

OCNT is denser and the membrane surface becomes rougher. Cross-section of the

222

membranes show an increased membrane thickness from ~71 nm of RGO membrane

223

to ~532 nm of RGO-OCNT83 membrane. Such a change in thickness implies that the

224

interlayer spacing between the RGO nanosheets could be tuned by controlling the 8


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225

OCNT content.

(a) 0

RGO

RGO-OCNT25 RGO-OCNT50 RGO-OCNT75 RGO-OCNT83

(b) RGO RGO-OCNT25 RGO-OCNT50 RGO-OCNT75 RGO-OCNT83

1.6

-10

1.2

-30 Surface charge density 4.6 mC m-2

-50

RGO

-60

RGO-OCNT25

5.8 mC m-2

-70

RGO-OCNT50

8.7 mC m-2

-80

RGO-OCNT75 RGO-OCNT83

Average pore size

-1

-40

f(d) (nm )

Zeta potential (mV)

-20

RGO

1.25 nm

RGO-OCNT25

1.30 nm

RGO-OCNT50

1.41 nm

RGO-OCNT75

1.51 nm

RGO-OCNT83

1.77 nm

0.8

0.4

11.8 mC m-2 12.9 mC m-2

0.0

-90

0

1

226

2

3

4

5

6

Diameter (nm)

227

Figure 2 (a) Zeta potentials and surface charge densities (the inset), (b) pore size

228

distributions and average pore sizes (the inset) of RGO-OCNT membranes with different OCNT

229

contents.

230

To investigate the membrane surface charge, Zeta potentials of RGO-OCNT

231

membranes were measured at pH 7.0. As shown in Figure 2a, all the membranes are

232

negatively charged and the Zeta potential is greatly influenced by the OCNT content.

233

As the OCNT content increases from 0 to 83%, the Zeta potential changes from −18.7

234

to −49.2 mV. Correspondingly, calculated surface charge density increases from 4.6 to

235

12.9 mC m−2 (inserted table in Figure 2a). These results indicate that the intercalated

236

OCNT can increase the surface charge of composite membrane. Meanwhile, water

237

contact angle of the membrane decreases from 90.1 to 47.4° (Figure S12). This

238

phenomenon demonstrates that the addition of OCNT results in an enhancement of

239

membrane surface hydrophilicity. Furthermore, XPS measurements were conducted to

240

analyze the oxygen-containing functional groups of the composite membranes. Direct

241

comparisons of O1s and C1s curve fitting for all the membranes are shown in Figure

242

S13 and S14. For all investigated samples, the contents of functional groups were

243

obtained from curve fitting of C1s and O1s spectra (Table S2 and S3). The analysis of

244

the C1s signal reveals that the contents of COOH and C-OH groups increase from 2.2

245

to 7.7 at% (~3.5 times) and 6.1 to 10.8 at% (~1.8 times), respectively, as the OCNT

246

content increases from 0 to 83%. From the analysis results of O1s signal

247

deconvolution, increasing contents of both C=O and COOH groups are recorded with

248

the increase of OCNT content. Furthermore, a progressively increasing content of 9



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249

phenolic C-OH group and a consequently declining content of aliphatic C-OH group

250

can also be observed. From the above analyses, it could be concluded that the

251

intercalated OCNT mainly increases the carboxyl and phenolic hydroxyl groups of the

252

composite membrane, suggesting that these two groups could be the significant

253

contributors to the membrane surface charge.

254

For two-dimensional laminar membranes, the interlayer channels between

255

nanosheets are employed as membrane pores for selective transport of molecules or

256

ions.24, 38 Because of changed interlayer spacing of the composite membrane, the

257

membrane pore size can also be influenced by the intercalation of OCNT. Here, a

258

two-parameter log-normal distribution function was used to investigate the effect of

259

OCNT content on membrane pore size (Supporting Information, SI 3).39 The results

260

are shown in Figure 2b. It can be observed that the pore size increases with the

261

increase of OCNT content and distributes from 1 to 3 nm approximately. The average

262

pore size of RGO membrane is 1.25 nm (inserted table in Figure 2b), which is

263

comparable to that reported in previous literatures.22,

264

increases from 0 to 83%, the average pore size gradually increase to 1.77 nm. These

265

results demonstrate that the pore size of the composite membrane can be tuned by

266

controlling the OCNT content.

267

3.3. Performances of RGO-OCNT membranes

(a)

(b)

12

Na2SO4

NaCl Rejection rate (%)

-1

8

-2

269 270

NaCl

80

-1

Permeability (L m h bar )

268

As the OCNT content

100

Na2SO4 10

38

6 4

60

40

20

2 0

0 RGO


RGO


Figure 3 (a) Permeabilities and (b) Na2SO4 and NaCl rejection rates of RGO-OCNT membranes with different OCNT contents (transmembrane pressure: 2 bar).

271

The pure water permeabilities of RGO-OCNT membranes are shown in Figure

272

S15. Obviously, the pure water permeability increases from 1.2 to 11.3 L m−2 h−1 bar−1

273

with the increase of OCNT content from 0 to 83%. Moreover, increasing trends in 10


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274

permeability are also observed during the filtrations of Na2SO4 and NaCl (Figure 3a).

275

Specifically, the permeability increases from 0.9 to 10.6 and 1.0 to 11.1 L m−2 h−1

276

bar−1 for Na2SO4 and NaCl filtrations, respectively. The significant improvement of

277

permeability could be attributed to the increase of membrane pore size and the

278

enhancement of hydrophilicity, which result from the intercalation of OCNT into the

279

RGO laminates.

280

Na2SO4 and NaCl rejection rates of the composite membranes are shown in Figure

281

3b. With the increase of OCNT content from 0 to 83%, the Na2SO4 rejection rate

282

increases from 58.8 to 80.0% and then slightly declines to 78.1%, while the NaCl

283

rejection rate continuously increases from 12.1 to 35.3%. It is worth noting that

284

increasing trends in both Na2SO4 and NaCl rejection rates are exhibited with the

285

increase of OCNT content, in spite of a slight decline in Na2SO4 rejection rate of the

286

RGO-OCNT83 membrane. This phenomenon demonstrates that the composite

287

membrane with higher OCNT content has better ion separation performance besides

288

its higher permeability. The separation performance of OCNT membrane was also

289

evaluated. The OCNT membrane exhibits poor ion rejection performance, despite

290

having much higher permeability (Figure S16), suggesting that the OCNT membrane

291

is unfavorable for ion rejection. Thus it is conclusive that the intercalated OCNT into

292

the composite membrane has created nanochannels and provided surface charge for

293

improved ion selectivity.22 When the transmembrane pressure is increased from 2 to 5

294

bar, the salt rejection rate is improved in all membranes (Figure S17). Notably,

295

simultaneous enhancements of permeability and ion selectivity can also be observed

296

with the increase of OCNT content in composite membrane. The RGO-OCNT

297

membrane is competitive compared with some membranes reported in literatures

298

(Table S4). It exhibits good water permeability and high rejection performance, which

299

is attribute to the intercalation of OCNT. Owing to the fact that OCNT content tunes

300

the surface charge and pore size of composite membrane, the co-enhancement of

301

permeability and ion selectivity is likely to ascribe to the combined effects of surface

302

charge and pore size.

303

3.4. Analysis of structure-performance relationship of RGO-OCNT membrane 11



(b) 100

(a) 100

Na2SO4 rejection rate Predicted rejection rate (%)

Predicted rejection rate (%)

Na2SO4 rejection rate NaCl rejection rate

80 60 40 20

0

305 306

NaCl rejection rate

80 60 40 20 0

0

304

Page 12 of 18

20

40

60

80

0

100

20

40

60

80

100

Experimental rejection rate (%)

Experimental rejection rate (%)

Figure 4 Comparisons of predicted and experimental rejection rates of RGO-OCNT membranes with different OCNT contents ((a) 2 bar, (b) 5 bar; yellow zone: < 5% deviation).

307

Previous studies indicated that the transport of electrolytes through NF

308

membranes can be theoretically described through DSPM.40, 41 Therefore, this model

309

was used to analyze the experimental results and investigate the phenomenon of

310

co-enhanced permeability and ion selectivity. Figure S18 exhibits the predicted

311

rejection rates of RGO-OCNT membranes with different OCNT contents. The

312

predicted and experimental rejection results are compared and shown in Figure 4. It

313

can be observed that the simulated results are in good agreement with the

314

experimental rejection rates of Na2SO4 and NaCl, either at 2 or 5 bar. These suggest

315

that the ion rejection performance of RGO-OCNT membrane can be fitted well by the

316

proposed model. For the rejection rates of Na2SO4 and NaCl, the consistent trends in

317

predicted and experimental results demonstrate that membrane surface charge and

318

pore size are two key parameters for the co-enhancement of permeability and ion

319

selectivity. Moreover, the effect of each parameter on the ion rejection performance

320

was further evaluated (Figure S19). Here, the surface charge was tuned by the

321

oxidation degree of OCNT in the composite membrane containing 75% OCNT (Table

322

S5). The results exhibit increased rejection rates of Na2SO4 and NaCl with the

323

increase of the oxidation degree, suggesting that the increase of surface charge can

324

enhance the ion rejection. In contrast, at constant surface charge (Table S6), Na2SO4

325

and NaCl rejection rates decrease as the pore size increases, suggesting that the

326

enlargement of pore size can decline the ion rejection rate. The above analyses

327

indicate that the surface charge is of great importance in the co-enhancement of 12


Page 13 of 18


permeability and ion selectivity.

rate (%)

71.37 62.70

36.67

Zone I 20 18

210.0

0

Su rfa ce ch arg e

) m (n

) m (n

2. 6 2. 8

ze

ze si

si

2 .0 2. 2 2. 4

8

re po

re po

6

20 18

16 14

1 .0

1 .4 1 .6

1 .8

ge ra ve A

4

12 10

Zone I 0.8

ge ra ve A

16 14

1.2

6.600

20

m -2 )

40

16.57

4

6

8

12 10

en sit y( mC

28.00

30

26.55

40

ch arg ed

50

36.52

Su rfa ce

45.35

1. 8

60

46.50

2. 0

54.02

56.48

Zone II

60

1 .4

70

66.45

1. 6

80

76.42

80

de ns ity (m Cm 2 )

) tion rate (% Na 2SO 4 rejec

80.05

Zone II

90

86.40

(b)

.2

88.72

1

97.40

(a) 100

NaCl rejection

328

329 330

Figure 5 (a) Na2SO4 and (b) NaCl rejection rate as a function of the average pore size and

331

surface charge density (the points in the figures are corresponding to ion rejection rates of

332

RGO-OCNT membranes at 2 bar).

333

The structure-performance relationships between the ion rejection performance

334

and membrane pore size as well as surface charge are exhibited in Figure 5 and S20.

335

For convenience in understanding the different effect of pore size and surface charge

336

on ion rejection, each figure is divided into two parts: Zone I and Zone II. Zone I

337

denotes the charge-dominated zone (average pore size > 1.8 nm for Na2SO4 or > 1.2

338

nm for NaCl) that the ion rejection rate is improved with the increase of surface

339

charge density but is almost unchanged over the average pore size. Zone II denotes

340

the charge & pore size co-dominated zone (average pore size < 1.8 nm for Na2SO4 or

341

< 1.2 nm for NaCl) that the ion rejection rate is simultaneously influenced by the

342

average pore size and surface charge density. Obviously, the boundary between Zone I

343

and Zone II for NaCl is smaller than that for Na2SO4. It is attributed to the smaller ion

344

radius of Cl− (0.12 nm) than that of SO42− (0.23 nm).42

345

The experimental rejection results for RGO-OCNT membranes at 2 and 5 bar are

346

also displayed in Figure 5 and S20 (the cyan colored points), respectively. Figure 5a

347

and S20a shows that all the cyan colored points (the Na2SO4 rejection rates of

348

composite membranes) are in Zone II, indicating that Na2SO4 rejection is

349

co-dominated by pore size and surface charge. As the content of OCNT increases

350

from 0 to 75%, the average pore size and surface charge density are increased.

351

Though the enlarged pore size has an adverse effect on ion rejection, the increased 13



352

surface charge density enhances the electrostatic interaction, which improves the

353

Na2SO4 rejection rate (Figure 6). However, when the OCNT content increases from

354

75 to 83%, the electrostatic interaction is weakened by the further enlarged membrane

355

pore size, resulting a slight decline in Na2SO4 rejection rate (Figure S21). Despite

356

such a decline, the rejection rate still displays an increasing trend with the increase of

357

OCNT content. As shown in Figure 5b and S20b, all the cyan colored points (the

358

NaCl rejection rates of composite membranes) are in Zone I. The effect of membrane

359

pore size on the NaCl rejection can be ignored because of the smaller ion radius of Cl−

360

(rCl− < rSO42− < rmembrane). This suggests that the NaCl rejection can be only dominated

361

by the electrostatic interaction. As the OCNT content increases, the enhanced

362

electrostatic interaction results in the continuous increase of NaCl rejection rate.

363

Therefore, the simultaneous enhancements of permeability and ion selectivity can be

364

achieved under the combined effects of membrane surface charge and pore size.

365 366

Figure 6 Schematic diagram of the effects of surface charge and pore size on the ion

367

separation performance of RGO-OCNT membrane.

368

3.5. Implications for constructing NF membranes

369

The results from the present study reveal that the separation performance of NF

370

membranes is greatly influenced by the membrane surface charge and pore size. This

371

study has significant implications for fabricating high-performance NF membranes.

372

For obtaining high permeability, enlarging pore size and improving hydrophilicity of

373

the membrane are feasible approaches. Reducing membrane thickness is an alternative 14


Page 14 of 18

Page 15 of 18


374

way, but the selectivity could be decreased.7 For achieving high selectivity, the

375

membrane should be endowed with abundant and strong charged functional groups, or

376

other strategies are sought to enhance electrostatic interaction between the membrane

377

and target ions.

378

In summary, an interesting phenomenon of co-enhanced permeability and ion

379

selectivity was presented through RGO-OCNT membranes. By tuning the OCNT

380

content in the composite membrane, the membrane surface charge and pore size can

381

be controlled easily. As the OCNT content increases from 0 to 83%, the permeability

382

and rejection rate of the composite membrane are 11.8 and 1.3 times higher than those

383

of pristine RGO membrane for Na2SO4 filtration respectively. Besides, the membrane

384

also exhibit 11.1 times higher permeability and 2.9 times higher rejection rate for

385

NaCl filtration. Such a finding was investigated through DSPM, indicating that the

386

surface charge and pore size are two key parameters for the co-enhancement of

387

permeability and ion selectivity. Furthermore, the surface charge and pore size play

388

different roles in filtration of different ions. The rejection of Na2SO4 is co-dominated

389

by the surface charge and pore size, but the rejection of NaCl is mainly dominated by

390

the surface charge due to the smaller ion radius of Cl−. This work could provide a new

391

insight in future for the design of NF membranes with both improved permeability

392

and selectivity, which may greatly facilitate their development in water treatment and

393

desalination.

394

4. ASSOCIATED CONTENT

395

Supporting Information

396

Detailed calculation methods; Schematic illustration of the RGO-OCNT membrane preparation

397

process; characterizations of RGO nanosheets and OCNTs; ultrasonic shock test of RGO-OCNT

398

membrane; XRD, Raman, and FTIR spectra of RGO and RGO-OCNT75 membrane; the

399

separation performance of pure OCNT membrane; SEM images of the RGO, RGO-OCNT25,

400

RGO-OCNT50, and RGO-OCNT83 membranes; photographs of RGO-OCNT membranes with

401

different OCNT contents; photographs, water contact angles, XPS spectra and pure water fluxes of

402

RGO-OCNT NFMs with different OCNT contents; the separation performance of RGO-OCNT

403

membranes at 5 bar; Na2SO4 and NaCl rejection rates of RGO-OCNT75/x NFMs and 15



404

RGO-OCNTy/30 NFMs; and additional figures and tables. This material is available free of charge

405

via the Internet at http://pubs.acs.org.

406

5. AUTHOR INFORMATION

407

Corresponding Author

408

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

409

Notes

410

The authors declare no competing financial interest.

411

6. ACKNOWLEDGMENTS

412

This work was supported by the National Natural Science Foundation of China (21437001), the

413

Programme of Introducing Talents of Discipline to Universities (B13012), the Programme for

414

Changjiang Scholars and Innovative Research Team in University (IRT_13R05), and the

415

Fundamental Research Funds for the Central Universities (DUT16TD02)

416 417

REFERENCES

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(1) Vörösmarty, C. J.; Green, P.; Salisbury, J.; Lammers, R. B., Global water resources: vulnerability from climate change and population growth. Science 2000, 289, (5477), 284-288. (2) Oki, T.; Kanae, S., Global hydrological cycles and world water resources. Science 2006, 313, (5790), 1068-1072. (3) Gupta, V. K.; Ali, I.; Saleh, T. A.; Nayak, A.; Agarwal, S., Chemical treatment technologies for waste-water recycling-an overview. RSC Adv. 2012, 2, (16), 6380-6388. (4) Kurihara, M.; Hanakawa, M., Mega-ton water system: Japanese national research and development project on seawater desalination and wastewater reclamation. Desalination 2013, 308, 131-137. (5) Yin, J.; Deng, B., Polymer-matrix nanocomposite membranes for water treatment. J. Membr. Sci. 2015, 479, 256-275. (6) Jin, L. M.; Yu, S. L.; Shi, W. X.; Yi, X. S.; Sun, N.; Ge, Y. L.; Ma, C., Synthesis of a novel composite nanofiltration membrane incorporated SiO2 nanoparticles for oily wastewater desalination. Polymer 2012, 53, (23), 5295-5303. (7) Chen, X.; Qiu, M.; Ding, H.; Fu, K.; Fan, Y., A reduced graphene oxide nanofiltration membrane intercalated by well-dispersed carbon nanotubes for drinking water purification. Nanoscale 2016, 8, (10), 5696-5705. (8) Ong, Y. K.; Li, F. Y.; Sun, S.-P.; Zhao, B.-W.; Liang, C.-Z.; Chung, T.-S., Nanofiltration hollow fiber membranes for textile wastewater treatment: Lab-scale and pilot-scale studies. Chem. Eng. Sci. 2014, 114, 51-57. (9) Saleh, T. A.; Gupta, V. K., Synthesis and characterization of alumina nano-particles polyamide membrane with enhanced flux rejection performance. Sep. Purif. Technol. 2012, 89, 245-251. (10) Mohammad, A. W.; Teow, Y. H.; Ang, W. L.; Chung, Y. T.; Oatley-Radcliffe, D. L.; Hilal, N., Nanofiltration membranes review: Recent advances and future prospects. Desalination 2015, 356, 226-254. 16


Page 16 of 18

Page 17 of 18


442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486

(11) Wang, C.; Li, Z.; Chen, J.; Li, Z.; Yin, Y.; Cao, L.; Zhong, Y.; Wu, H., Covalent organic framework modified polyamide nanofiltration membrane with enhanced performance for desalination. J. Membr. Sci. 2017, 523, 273-281. (12) Wu, C.; Liu, S.; Wang, Z.; Zhang, J.; Wang, X.; Lu, X.; Jia, Y.; Hung, W.-S.; Lee, K.-R., Nanofiltration membranes with dually charged composite layer exhibiting super-high multivalent-salt rejection. J. Membr. Sci. 2016, 517, 64-72. (13) Park, H. B.; Kamcev, J.; Robeson, L. M.; Elimelech, M.; Freeman, B. D., Maximizing the right stuff: The trade-off between membrane permeability and selectivity. Science 2017, 356, eaab0530. (14) Han, Y.; Jiang, Y.; Gao, C., High-flux graphene oxide nanofiltration membrane intercalated by carbon nanotubes. ACS Appl. Mater. Interfaces 2015, 7, (15), 8147-8155. (15) Yu, L.; Zhang, Y.; Wang, Y.; Zhang, H.; Liu, J., High flux, positively charged loose nanofiltration membrane by blending with poly (ionic liquid) brushes grafted silica spheres. J. Hazard. Mater. 2015, 287, 373-383. (16) Hu, J.; Lv, Z.; Xu, Y.; Zhang, X.; Wang, L., Fabrication of a high-flux sulfonated polyamide nanofiltration membrane: Experimental and dissipative particle dynamics studies. J. Membr. Sci. 2016, 505, 119-129. (17) Yao, Y.; Ba, C.; Zhao, S.; Zheng, W.; Economy, J., Development of a positively charged nanofiltration membrane for use in organic solvents. J. Membr. Sci. 2016, 520, 832-839. (18) Zhang, Y.; Zhang, S.; Chung, T.-S., Nanometric graphene oxide framework membranes with enhanced heavy metal removal via nanofiltration. Environ. Sci. Technol. 2015, 49, (16), 10235-10242. (19) Zhao, S.; Yao, Y.; Ba, C.; Zheng, W.; Economy, J.; Wang, P., Enhancing the performance of polyethylenimine modified nanofiltration membrane by coating a layer of sulfonated poly(ether ether ketone) for removing sulfamerazine. J. Membr. Sci. 2015, 492, 620-629. (20) Sun, P.; Wang, K.; Zhu, H., Recent developments in graphene-based membranes: structure, mass-transport mechanism and potential applications. Adv. Mater. 2016, 28, (12), 2287-2310. (21) Mahmoud, K. A.; Mansoor, B.; Mansour, A.; Khraisheh, M., Functional graphene nanosheets: The next generation membranes for water desalination. Desalination 2015, 356, 208-225. (22) Goh, K.; Jiang, W.; Karahan, H. E.; Zhai, S.; Wei, L.; Yu, D.; Fane, A. G.; Wang, R.; Chen, Y., All-carbon nanoarchitectures as high-performance separation membranes with superior stability. Adv. Funct. Mater. 2015, 25, (47), 7348-7359. (23) Das, R.; Ali, M. E.; Hamid, S. B. A.; Ramakrishna, S.; Chowdhury, Z. Z., Carbon nanotube membranes for water purification: A bright future in water desalination. Desalination 2014, 336, 97-109. (24) Liu, G.; Jin, W.; Xu, N., Graphene-based membranes. Chem. Soc. Rev. 2015, 44, (15), 5016-5030. (25) Sun, S.; Wang, C.; Chen, M.; Li, M., The mechanism for the stability of graphene oxide membranes in a sodium sulfate solution. Chem. Phys. Lett. 2013, 561-562, 166-169. (26) Stankovich, S.; Dikin, D. A.; Compton, O. C.; Dommett, G. H. B.; Ruoff, R. S.; Nguyen, S. T., Systematic post-assembly modification of graphene oxide paper with primary alkylamines. Chem. Mater. 2010, 22, (14), 4153-4157. (27) Zheng, S.; Tu, Q.; Urban, J. J.; Li, S.; Mi, B., Swelling of graphene oxide membranes in aqueous solution: Characterization of interlayer spacing and insight into water transport mechanisms. ACS Nano 2017, 11, (6), 6440-6450. (28) Liu, H.; Wang, H.; Zhang, X., Facile fabrication of freestanding ultrathin reduced graphene oxide membranes for water purification. Adv. Mater. 2015, 27, (2), 249-254. (29) Yang, E.; Ham, M.-H.; Park, H. B.; Kim, C.-M.; Song, J.-h.; Kim, I. S., Tunable semi-permeability 17



487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524

of graphene-based membranes by adjusting reduction degree of laminar graphene oxide layer. J. Membr. Sci. 2018, 547, 73-79. (30) Jang, J.-H.; Woo, J. Y.; Lee, J.; Han, C.-S., Ambivalent effect of thermal reduction in mass rejection through graphene oxide membrane. Environ. Sci. Technol. 2016, 50, (18), 10024-10030. (31) Xue, S.-M.; Xu, Z.-L.; Tang, Y.-J.; Ji, C.-H., Polypiperazine-amide nanofiltration membrane modified by different functionalized multiwalled carbon nanotubes (MWCNTs). ACS Appl. Mater. Interfaces 2016, 8, (29), 19135-19144. (32) Zhang, J.; Xu, Z.; Shan, M.; Zhou, B.; Li, Y.; Li, B.; Niu, J.; Qian, X., Synergetic effects of oxidized carbon nanotubes and graphene oxide on fouling control and anti-fouling mechanism of polyvinylidene fluoride ultrafiltration membranes. J. Membr. Sci. 2013, 448, 81-92. (33) Kim, K.; Kim, H.; Lim, J. H.; Lee, S. J., Development of a desalination membrane bioinspired by mangrove roots for spontaneous filtration of sodium ions. ACS Nano 2016, 10, (12), 11428-11433. (34) Zhang, H.; Fan, X.; Quan, X.; Chen, S.; Yu, H., Graphene sheets grafted Ag@AgCl hybrid with enhanced plasmonic photocatalytic activity under visible light. Environ. Sci. Technol. 2011, 45, (13), 5731-5736. (35) Maiti, S. K.; Lukka Thuyavan, Y.; Singh, S.; Oberoi, H. S.; Agarwal, G. P., Modeling of the separation of inhibitory components from pretreated rice straw hydrolysate by nanofiltration membranes. Bioresour. Technol. 2012, 114, 419-427. (36) Hussain, A. A.; Nataraj, S. K.; Abashar, M. E. E.; Al-Mutaz, I. S.; Aminabhavi, T. M., Prediction of physical properties of nanofiltration membranes using experiment and theoretical models. J. Membr. Sci. 2008, 310, (1–2), 321-336. (37) Fadaei, F.; Hoshyargar, V.; Shirazian, S.; Ashrafizadeh, S. N., Mass transfer simulation of ion separation by nanofiltration considering electrical and dielectrical effects. Desalination 2012, 284, 316-323. (38) Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R., Precise and ultrafast molecular sieving through graphene oxide membranes. Science 2014, 343, (6172), 752-754. (39) Shao, L.; Cheng, X. Q.; Liu, Y.; Quan, S.; Ma, J.; Zhao, S. Z.; Wang, K. Y., Newly developed nanofiltration (NF) composite membranes by interfacial polymerization for Safranin O and Aniline blue removal. J. Membr. Sci. 2013, 430, 96-105. (40) Bowen, W. R.; Welfoot, J. S., Modelling the performance of membrane nanofiltration—critical assessment and model development. Chem. Eng. Sci. 2002, 57, (7), 1121-1137. (41) Schaep, J.; Vandecasteele, C.; Wahab Mohammad, A.; Richard Bowen, W., Modelling the retention of ionic components for different nanofiltration membranes. Sep. Purif. Technol. 2001, 22-23, 169-179. (42) Cheng, S.; Oatley, D. L.; Williams, P. M.; Wright, C. J., Characterisation and application of a novel positively charged nanofiltration membrane for the treatment of textile industry wastewaters. Water Res. 2012, 46, (1), 33-42.

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Combined Effects of Surface Charge and Pore Size on Co-enhanced

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