A Self-Standing, Support-Free Membrane for Forward Osmosis with

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Environmental Processes

A Self-standing, Support-Free Membrane for Forward Osmosis with No Internal Concentration Polarization Meng Li, Vasiliki Karanikola, Xuan Zhang, Lianjun Wang, and Menachem Elimelech Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00117 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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Environmental Science & Technology Letters

A Self-standing, Support-Free Membrane for Forward Osmosis with No Internal Concentration Polarization

Meng Li,1 Vasiliki Karanikola,2,3 Xuan Zhang,1,2* Lianjun Wang,1 and Menachem Elimelech 2*

1) Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science & Technology, Nanjing 210094, China 2) Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520-8286, USA 3) Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ 85721, USA

*Corresponding Author: Xuan Zhang: [email protected]; Menachem Elimelech: [email protected].

1

ACS Paragon Plus Environment


ABSTRACT 1

Conventional asymmetric or thin-film composite forward osmosis (FO) membranes suffer

2

from severe internal concentration polarization, which significantly hinders process

3

performance and practical applications. Here we report the synthesis of COOH-derived

4

polyoxadiazole copolymer for the fabrication of a self-standing selective thin film without a

5

support layer. The thickness of the membrane was controlled at merely a few micrometers

6

to achieve high rejection of the Na2SO4 draw solution, while maintaining acceptable water

7

permeability. Due to the symmetric architecture, the membrane exhibited excellent and

8

identical FO performance at both of its sides. The structural parameter of the fabricated

9

membranes was zero due to the absence of internal concentration polarization in the

10

symmetric FO membranes. Our results highlight the potential of support-free membranes

11

for the further development of FO technology.

12

2


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INTRODUCTION 13

Over the past decade, forward osmosis (FO) has attracted heightened attention as an

14

emerging technology for various separation/desalination applications.1,2 The spontaneous

15

process driven by osmotic pressure and the inherently low fouling propensity of the process

16

render FO an attractive platform for a wide range of applications, particularly those that

17

cannot be performed by reverse osmosis (RO).1,3-5 Nevertheless, the relatively low

18

module-averaged water flux6 and high energy consumption associated with the effective

19

regeneration and reuse of draw solutes1 remain as major challenges for successful

20

implementation of FO.

21

Membranes for FO are commonly derived from asymmetric cellulose triacetate6-8 or

22

polyamide thin-film composite (TFC) architectures9-14 and generally exhibit good water

23

permeability and rejection of draw solutes. However, as water permeates and dilutes the

24

draw solution at the interface between the porous substrate and active layer, the driving

25

force for water permeation, and hence water flux, are significantly reduced due to internal

26

concentration polarization (ICP).14 ICP is greatly influenced by the membrane support layer

27

structure (thickness, porosity, and tortuousity), which is characterized by the so-called

28

structural parameter,1,15 as well as the hydrophilicity of the support layer.16-18 Consequently,

29

extensive efforts have been devoted in recent years to fabricate membranes with reduced

30

ICP by designing membranes with thin and porous support layers as well as by enhancing

31

the hydrophilicity of the support layer.19-23 However, significant reduction of the structural

32

parameter of FO membranes is challenging because membranes with thin and highly porous

33

support layers lack the necessary mechanical strength for FO applications.

34

To address this problem, the idea of support-layer-free thin films with a symmetric and

35

homogenous structure merits attention. Although the concept of an FO membrane with no

36

ICP was proposed in a recent modeling study of a monolayer graphene24, the exploration of

37

mechanically robust polymeric materials for scalable fabrication of symmetric FO

38

membranes has yet to be systematically studied.

39

In this work, we fabricated, for the first time, a self-standing, support-free membrane for

40

forward osmosis. The polymeric membrane was synthesized by an initial polycondensation 3



41

reaction between 4,4’-oxybis (benzoic acid) and hydrazine sulfate salt and subsequent

42

grafting of carboxyl acid group to the side chains. The physicochemical and mechanical

43

properties of the membrane were thoroughly characterized and the water flux and reverse

44

draw solute flux of membranes of various thickness were determined as a function of draw

45

solution concentration. Our study provides a new platform for the fabrication of symmetric,

46

support-free FO membranes with no internal concentration polarization.

47 MATERIALS AND METHODS 48

Materials and Chemicals. 4,4’-Oxybis (benzoic acid) (OBBA, 98%) and polyphosphoric

49

acid (PPA, >85%) were purchased from J&K Chemical Reagent Co. Ltd. (Shanghai, China).

50

Hydrazine sulfate (HS, 99%) and 4-aminobenzoic acid (p-ABA, 98%) were supplied by

51

Energy Chemical (Shanghai, China). Anhydrous N-methyl-2-pyrrolidone (NMP, 99.5%),

52

N,N-dimethylformamide (DMF, 99.5%), and other reagents/solvents were used as received.

53

Deionized (DI) water was obtained from Millipore System (Millipore-Q, Ultrapure Water

54

System, resistance 18 MΩ-cm).

55

Synthesis of PODH and PTAODH. Polyoxadiazole-co-hydrazide (PODH) was

56

synthesized according to previous studies.25-28 In brief, a 500 mL fully-dried three-necked

57

flask was charged with OBBA (4.001 g, 15.5 mmol), HS (2.419 g, 18.6 mmol), and PPA (60.7

58

g). The solution with reactants was heated to 160oC and kept for 3 h with continuous stirring.

59

Next, the highly viscous solution was poured into water to isolate the fiber-like polymers.

60

After thoroughly washing with water, 5% NaOH solution, and DI water, successively, PODH

61

was obtained by drying under vacuum at 100oC for 24 h (yield: 92%). FTIR (υ, cm-1): 1670

62

(C=O), 1605, 1468 (C=C linkage of aromatic rings), 1237 (C-O-C from OBBA), 1085, 1030

63

(C-O-C from oxadiazole ring); 1H NMR (DMSO-d6, ppm): δ = 8.5 (-NH-NH-), 8.3–8.0

64

(Ar-H), 7.4-7.0 (Ar-H).

65

A 100 mL fully-dried three-necked flask, equipped with N2 inlet and outlet, was charged

66

with PODH (1.011 g, 4.282 mmol), p-ABA (0.588 g, 4.282 mmol), PPA (0.1 g), and NMP

67

(20.0 mL). Following complete dissolution of polymers, the mixture was heated for 12 h at

68

195oC. After cooling to room temperature, the brownish solution was poured into DI water. 4


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69

The fiber-like polymer, polytriazole-co-oxadiazole-co-hydrazide (PTAODH-1.0), was then

70

washed thoroughly with hot water to remove the unreacted precursor and was dried under

71

vacuum at 80oC for 24 h (yield: 79%). PTAODH-1.5 was synthesized by the same

72

procedure except the feed amount of p-ABA was increased to 0.881 g (6.423 mmol). FTIR

73

(υ, cm-1): 1670 (C=O), 1530, (C=N from triazole ring), 1605, 1468 (C=C), 1237 (C-O-C

74

from OBBA), 1085, 1030 (C-O-C from oxadiazole ring); 1H NMR (DMSO-d6, ppm): δ =

75

8.5 (-NH-NH-), 8.3–7.0 (Ar-H).

76

Preparation of Self-standing Symmetric FO Membranes. Self-standing

77

thin-films, PODH and PTAODH, were prepared via the solvent evaporation method. In brief,

78

homogeneous solutions of both copolymers were obtained by dissolving them into DMF at a

79

concentration of ~1 wt%. After being filtered and degassed, the solutions were poured into

80

ultraflat petri dishes, allowing them to dry at 80oC overnight. The membranes were detached

81

by first immersing them in water, followed by thorough washing with DI water and finally

82

storing in DI water.

83

Membrane Characterization. 1H NMR spectra of polymers were characterized by a

84

Bruker AVANCEIII spectrometer (500 MHz) using DMSO-d6 as the solvent. FTIR spectra

85

were obtained from a Perkin Elmer Spectrum FTIR spectrometer. Morphologies of the

86

self-standing thin-film samples were studied by field emission scanning electron

87

microscopy (FESEM, S-4800, Hitachi, Japan). All membrane samples were air-dried,

88

fractured in liquid nitrogen, and sputter coated with gold prior to the test. Membrane surface

89

charge was characterized by streaming potential using an electrokinetic analyzer with a set

90

of AgCl electrodes (SurPASSIII, AntonPaar, Austria). For the streaming potential

91

measurements, an electrolyte solution of 0.01 M KCl was used to provide the background

92

ionic strength and was automatically titrated with 0.05 M HCl and 0.05 M NaOH to

93

investigate the effect of pH on the zeta potential. Membrane hydrophilicity was assessed

94

using a contact angle and drop shape analyzer (KRÜSS, DSA30, Germany). At least three

95

measurements were made at different locations for each membrane surface and their

96

averages were recorded. The number- and weight-average molecular weights (Mn and Mw)

97

of PODH and PTAODH polymers were determined by gel permeation chromatography 5



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98

(GPC) on a Waters 1515 HPLC system with a fixed 1.0 mL min-1 flow rate;

99

N,N-dimethylformamide (DMF) containing 0.05 M LiBr was used as eluate. Mechanical

100

properties of membranes were measured by a universal material tester (AGS-100NX,

101

Shimadzu) at room temperature (20oC) and 60% relative humidity. Membrane samples were

102

cut into rectangle shapes (2 cm × 0.5 cm) and fixed onto the cantilever, with the extension

103

speed set at 2 mm s-1.

104

Membrane Performance Testing. To evaluate the performance of the fabricated

105

membranes, a lab-scale cross-flow FO module was utilized, with an effective membrane

106

area of 10 cm2 as described elsewhere.5,21 DI water and Na2SO4 at different concentrations

107

(0.5, 0.75, 1, 1.25, and 1.5 M) were used for feed solution (FS) and draw solution (DS),

108

respectively. The flow rate for both solutions was fixed at 10.36 cm/s. The variation in

109

Na2SO4 concentration of the FS was calculated by measuring the electric conductivity

110

(Conductivity Meter DDS-307, Shanghai, China) and the rate of weight change of the FS

111

was recorded by a digital weight balance. All experiments were carried out at room

112

temperature (25oC).

113

The stability of the PTAODH-1.0 membrane (thickness of 8 µm) was evaluated by a

114

continuous 16 h test at 25oC. DI water and Na2SO4 solution were used as FS and DS,

115

respectively. The concentration of Na2SO4 solution was monitored every ~2 h, and

116

additional Na2SO4 was added to the draw solution to keep the salt concentration constant at

117

1.5 M. To ensure the symmetric character of the membrane, water fluxes (Jw) were

118

measured for both the top and bottom sides of the membrane.

119

Modeling. Water flux, Jw, and salt flux, Js, in FO can be determined by6

π exp − − π exp Jw Fb kF Db Jw =A πDm − πFm = A 1+ B exp Jw − exp − kF Jw

(1)

c exp − − c exp Jw Fb kF Db Js =B (cD − cFm ) = B m 1+ B exp Jw − exp − kF Jw

(2)

6


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120 121

where A and B are the membrane water and solute permeability coefficients, respectively;

122

πDm and πFm are the osmotic pressures of the draw and feed solutions at the

123

membrane/solution interface, respectively; cDm and cFm are the solute concentrations of the

124

draw and feed solutions at the membrane/solution interface, respectively; kF and kD are the

125

mass transfer coefficients of the solute at the feed boundary layer and of the feed at the draw

126

boundary layers; πDb and πFb are the osmotic pressures of the draw and feed bulk solutions,

127

respectively; and cDb and cFb are the draw and feed bulk solution concentrations,

128

respectively. Calculation of the mass transfer coefficient kF is described in Supporting

129

Information. Internal concentration polarization is neglected in this study as the dense

130

membranes synthesized are support-free and the structural parameter, S, is zero due to the

131

lack of a support layer.

132

The sum of squared errors method was used to fit the permeability coefficients A and B

133

by calculating the absolute difference between the predicted (eq 1 and 2) and the

134

experimental water and salt fluxes. With the aid of the Solver function in Excel, the total

135

sum of errors was then minimized by fitting the permeability coefficients A and B in the

136

water and salt flux equations. The new obtained values for A and B were then compared to

137

the obtained experimental water and solute permeability coefficients (the latter is described

138

in Supporting Information). The difference between the value fitted by the experimental

139

data and the value fitted based on the sum of squared error is always less than 10%.

140 RESULTS AND DISCUSSION 141

Synthesis and Physicochemical Properties of Polymer Membranes. Scheme S1

142

shows the synthesis route of PDOH and PTAODH. The final polymer was synthesized by

143

an initial polycondensation reaction between OBBA and HS, followed by side chain

144

grafting to form a COOH-functionalized pendant. In contrast to a typical condensation

145

reaction, which generally requires an equivalent feed ratio of the reactants, the molar ratio

146

used here for OBBA and HS was 1:1.2. This ratio is consistent with a previous report

147

optimized by statistical experimental design and kinetic modeling.27 The subsequent step 7



148

was the nucleophilic attack of the amino group on the electron-deficient oxadiazole ring.29,30

149

It should be noted that although the conditions of the reaction were similar to the synthesis

150

of OH-modified polyoxadiazoles,26,31 the conversion ratios in our case were always found in

151

the low range of 10-20% (calculated from 1H NMR spectra). One possible explanation is

152

the existence of a strong electro-withdrawing group (-COOH) which may result in a

153

decrease in nucleophilicity of –NH2 group, thus hindering the substitution reaction.

154

The chemical structures of both copolymers were studied by 1H NMR and FTIR spectra.

155

In Figure 1(A)-(a), the sharp peaks around 8.5 ppm and 8.3-7.0 ppm are assigned to the

156

hydrazide and phenyl protons in PODH, respectively. However, the spectra turn into the

157

scrambled states for both PTAODH-1.0 and PTAODH-1.5 with the occurrence of several

158

new peaks which overlapped (Figure 1(A)-(b) and (c)), indicating the complexity of the

159

grafting reaction. It has been reported that PODH was chemically unstable in an acidic

160

medium, undergoing a ring-opening reaction of the oxadiazole ring and hydrolysis of

161

hydrazide group.27,32 However, since a dehydrating agent is critically needed for the

162

subsequent nucleophilic substitution reaction, PPA is typically used as described

163

elsewhere.25,27 Such a predicament leads to the unavoidable degradation of PODH during

164

the synthesis. As proposed in Figure S1, degradation occurs either by the scission of

165

oxadiazole or of hydrazide units. Such scission produces more terminated carboxylic acid

166

groups at the end of the polymer chain; these groups could further react with p-ABA in the

167

presence of acid at high temperature. Yet, for the PTAODH polymers, a clear absorption

168

band at 1530 cm-1 is observed, corresponding to the stretching vibration of C=N from the

169

triazole ring,26 (Figure 1(B)), confirming the successful formation of the triazole ring.

170 171

Figure 1

172 173

The surface zeta potential was determined for the three fabricated membranes as a

174

function of pH (Figure S2). PODH, without -COOH groups, exhibited an electroneutral

175

character with a constant zeta potential of near zero over the pH range investigated. On the

176

other hand, highly negative zeta potentials were observed for the PTAODH-1.0 and 8


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177

PTAODH-1.5 membranes, with the latter exhibiting the highest negative charge among the

178

three membranes. We attribute this observation to the more degraded polymeric skeletons in

179

the

180

COOH-terminated groups, as we previously hypothesized. These findings are in accordance

181

with the water contact angles shown in Figure S3. Both PTAODH membranes showed

182

much lower contact angles after the introduction of polar carboxyl acid groups. Specifically,

183

the contact angle changed from the original 78.6±1.3 degree of PODH to 63.7±1.1 and

184

59.4±1.0 degrees for the PTAODH-1.0 and PTAODH-1.5 membranes, respectively,

185

suggesting improved surface hydrophilicity. We note that no significant difference in

186

contact angles existed between the top and bottom surface for any of the three membranes,

187

which further demonstrates their symmetric nature.

case

of

PTAODH-1.5

rather

than

PTAODH-1.0,

which

generated

more

188

Morphological Properties of Fabricated Membranes. The surface and

189

cross-section morphologies of all membranes were observed by FE-SEM. As shown in

190

Figure 2, the surface morphologies for all membranes are almost identical, and no visible

191

pores were identified in all the images (both top and bottom), suggesting their rather dense

192

character. In addition, uniform structures were also found in the cross-section images of all

193

membranes, which again indicates their symmetric properties.

194 195

Figure 2

196 197

Forward Osmosis Membrane Performance. The performance of the novel thin

198

films was examined in a typical FO system. Since the membranes were negatively charged,

199

two typical inorganic salts, Na2SO4 and NaCl, were used as draw solutes to verify the

200

feasibility of the membranes in FO. As shown in Figure S4, PODH, the less-charged

201

membrane, exhibited the highest reverse salt flux among the three membranes (609±42.8

202

mmol m-2 h-1 and 110±10.6 mmol m-2 h-1 for NaCl and Na2SO4, respectively), indicating its

203

inappropriateness for FO applications. However, both PTAODH-1.0 and PTAODH-1.5

204

membranes, with carboxyl side chain incorporated, showed much reduced reverse salt

205

fluxes. The reverse salt flux for Na2SO4 was lower than that for NaCl for both membranes, 9



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206

which is attributed to the higher electrostatic repulsion of the divalent sulfate ions by the

207

negatively charged membranes.33,34 Both PTAODH-1.0 and PTAODH-1.5 membranes

208

exhibited low reverse salt (Na2SO4) fluxes around 20-30 mmol m-2 h-1, highlighting their

209

potential application in FO.

210

At this point we will discuss the mechanical robustness of the membranes, since all the

211

membranes in the current study were support-free and fabricated with a thickness of a few

212

micrometers. Although the FO performance for both PTAODH membranes was quite

213

comparable, the PTAODH-1.5 membrane was more susceptible to fracture under long-term

214

operation. The lower maximum yield stress and elongation for the PTAODH-1.5 membrane

215

are indicative of the mechanical strength of this membrane compared to the other

216

membranes (Table S2). This result highlights the critical need to select an appropriate

217

degradation ratio for PODH to ensure that while satisfactory mechanical robustness is

218

achieved, the surface charge is still negative enough to maintain high salt rejection by

219

electrostatic repulsion.

220

FO performance as a function of salt concentration was studied using the PTAODH-1.0

221

as the optimal membrane, balancing performance and mechanical strength. The influence of

222

membrane thickness on FO performance was also evaluated. As shown in Figure 3(A), for

223

each FO membrane, water flux exhibits a linear relationship with draw solution

224

concentration and increases substantially with increasing Na2SO4 concentration from 0.5 to

225

1.5 M. It should be noted that water fluxes through the PTAODH-1.0 membranes were

226

improved proportionally with the decrease of membrane thickness at all draw solution

227

concentrations. For instance, water flux of the 5-µm PTAODH-1.0 membrane was

228

approximately twice of that for the 8-µm membrane and three times of that of the 15-µm

229

membrane at a given draw solution concentration. Results of reverse fluxes of draw solute

230

of the PTAODH-1.0 membranes are shown in Figure 3(B) and, as expected, exhibit a

231

similar trend to the observed water flux.

232

An important parameter characterizing FO membrane performance is the reverse flux

233

selectivity, which is the ratio of water flux to reverse salt flux, Jw/Js.35 This ratio is

234

proportional to the membrane selectivity (A/B), and is given by , where n is the

10


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235

number of dissolved species created by the draw solute (3 for Na2SO4), R is the ideal gas

236

constant, T is the absolute temperature, and A and B are the water and salt permeability

237

coefficients, respectively.35 For example, the averaged reverse flux selectivity was

238

412.0±30.9 L mol-1 for the 8-µm PTAODH-1.0 membrane, which is comparable to a

239

commercial FO membrane, suggesting its high FO performance.35-37 Detailed comparison of

240

the selectivities and structural parameters of the fabricated membranes and the commercial

241

FO membrane is presented in Table S3 of Supporting Information.

242 243

Figure 3

244 245

By using the calculated pure water permeability (A) and solute permeability (B) values

246

(see detailed calculations in Supporting Information), the structural parameters (S) for the

247

three PTAODH-1.0 membranes were found to be 4.5±2.0, 3.5±5.7, and 1.0±5.5 µm for

248

membrane thicknesses of 5, 8, and 15 µm, respectively. Considering the inherent errors

249

during the experiments, the S values for all the above three membranes could be regarded as

250

zero, which is expected for support-free FO membranes with no ICP. In other words, the

251

utilization of the osmotic pressure driving force is maximized in the symmetric FO system.

252

Modeling results for water flux and reverse mass flux of draw solution are in excellent

253

agreement with the experimental data (Figure 3). The fitted intrinsic membrane

254

permeabilities (i.e., material properties independent of membrane thickness) for water and

255

salt are 8.37 × 10-7 L m-1 h-1 bar-1 and 1.93 × 10-7 L m-1 h-1, respectively. Using these values,

256

the water and salt permeability coefficients (i.e., A and B) for the membranes with various

257

thicknesses can be calculated. For example, for the 5-µm PTAODH-1.0 membrane, the

258

calculated A and B are 0.16 L m-2 h-1 bar-1 and 0.039 L m-1 h-1, respectively, which are in

259

close agreement with the values determined from RO experiments (Table S3). Through this

260

validated model, we can predict the performance of the support-free FO membrane for any

261

thickness, operational variables, and draw solution concentration.

262 ASSOCIATED CONTENT 11



263

Supporting Information Available: Membrane performance testing methodology for

264

evaluation of structural parameter (S) and mass transfer coefficients. Supporting Scheme

265

includes the synthetic route of PODH and PTAODH. Supporting figures include any

266

possible degradation mechanisms during the synthesis of PTAODH (Figure S1), zeta

267

potential of the PODH, PTAODH-1.0, and PTAODH-1.5 FO membranes (Figure S2), static

268

water contact angles (Figure S3), FO performance of PODH, PTAODH-1.0 and

269

PTAODH-1.5 membranes as a function of different type of draw solutes (Figure S4), and

270

stability testing of water flux for representative PTAODH-1.0 membrane (Figure S5).

271

Supporting tables include GPC data of all polymers (Table S1), mechanical properties

272

(Table S2), and RO/FO performance of PTAODH-1.0 membrane with different thicknesses

273

(Table S3). This material is available free of charge via the Internet at https://pubs.acs.org/. AUTHOR INFORMATION

274

Corresponding Author

275

* Xuan Zhang. E-mail: [email protected], Tel./fax: +86-25-84315916.

276

* Menachem Elimelech: [email protected]. Notes

277

The authors declare no competing financial interest. ACKNOWLEDGMENTS

278

This work was supported by the National Natural Science Foundation of China (21774058,

279

51778292), Priority Academic Program Development of Jiangsu Higher Education

280

Institutions (PAPD), State Key Laboratory of Separation Membranes and Membrane

281

Processes (Tianjin Polytechnic University, M2-201604), and the Agnese Nelms Haury

282

Program in Environment and Justice at the University of Arizona.

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381

382 383 384 385

Figure 1. (A) 1H NMR spectra of (a) PODH, (b) PTAODH-1.0, and (c) PTAODH-1.5. In (b) and (c), the signals circled by the dashed ovals were attributed to the degradation of the

386

polymer chains. (B) FTIR spectra of PODH, PTAODH-1.0, and PTAODH-1.5.

16


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387 388 389

Figure 2. FESEM images of all three types of membranes (PODH, PTAODH-1.0, and

390 391

PTAODH-1.5 membrane, respectively): (a) (d) (g) top surface; (b) (e) (h) bottom surface; (c) (f) (i) cross-section.

17



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(A) 20 Experimental Simulated

5 µm

12

-2

-1

Jw (L m h )

16

8 µm

8 4 0 0.0

15 µm

0.4

0.8

1.2

1.6

-1

Na2SO4 Concentration (mol L )

392

(B)10

-2

-1

5 µm

60

6 40

4

-2

8 µm 15 µm

0 0.0

0.4

0.8

1.2

1.6

-1

20

2

JS (mmol m h )

8

JS (g m h )

80

Experimental Simulated

0

-1

393

Na2SO4 Concentration (mol L )

394 395 396

Figure 3. (A) Water flux (Jw) and (B) reverse draw salt flux (Js) for the PTAODH-1.0 membranes with different thickness. Experiments were carried out in an FO test cell with an

397 398

effective membrane area of 10 cm2 and a crossflow velocity of 10.36 cm/s using different concentrations of Na2SO4 as draw solution and DI water as feed solution. At least two

399 400

parallel tests were conducted at 25oC for 2 h, and the average values and error bars are presented.

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TOC Art

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A Self-Standing, Support-Free Membrane for Forward Osmosis with

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