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Masoud Rastgar, Ali Chemical by the American Society. 1155 Sixteenth Bozorg, and Alireza Shakeri Street N.W., Washington, DC 20036


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ACS Paragon Plus Environment

Environmental Science & Technology

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A novel dimensionally-controlled nano-pore forming template in forward osmosis membranes

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Masoud Rastgara, Ali Bozorgb*, Alireza Shakeria**

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7 8 9 10

a

School of Chemistry, College of Science, University of Tehran, P.O. Box 14155-6619, Tehran, Iran b

Department of Biotechnology, College of Science, University of Tehran, P.O Box: 1417614411 Tehran, Iran

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12

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14 15 16 17 18

19 20 21 22 23

*

Corresponding author Tel: +98 (21) 66403672 Fax: +98 (21) 66405141 E-mail address: [email protected]

**

Corresponding author Tel: +98 (21) 61113812 Fax: +98 (21) 66972047 E-mail address: [email protected]

1 ACS Paragon Plus Environment

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Abstract

25

To lower the unfavorable internal concentration polarization effect in forward osmosis (FO)

26

membranes, support layers of highly porous interconnected structures with specifically large

27

surface-to-volume ratios are indispensable. Herein, zinc oxide (ZnO) has been introduced as a

28

new template to manipulate the porous structure of polyethersulfone (PES) support layer. The

29

ZnO can be readily synthesized as desired in different dimensionally controlled nanostructures.

30

The performance of the FO membrane was initially ameliorated in terms of permeability and

31

selectivity through simple incorporation of ZnO nanostructures in the PES support layer. The

32

PES support layer was blended with appropriate amounts of ZnO nanostructures, casted on a

33

glass plate, and subsequently acid washed to leach out the embedded ZnO nanostructures.

34

Different nano-porous structures were achieved when ZnO of different nanostructures were

35

used to modify the PES support layer. The experimental results indicated that the permeability

36

of FO membranes could be simply improved by incorporation of ZnO nanostructures in PES

37

support layer. Higher hydrophilicity and formation of suitable internal pores were mainly

38

responsible for such observation. Although surface hydrophilicity of the support layers was

39

reduced after being acid washed, water permeation through the membrane was intensified due

40

to the formation of interconnected porous structure.

41

42 43

Keywords: Forward Osmosis; Zinc Oxide; Nano Pore Forming Template; Structure Parameter; Water Flux



44

1.

Introduction

45

As one of the highest global systemic risks, increasing freshwater scarcity is a growing concern

46

threatening sustainable development of civil society 1. Today, improved water-use efficiency

47

besides enhanced freshwater production through the advanced desalination and water

48

reclamation processes are believed to be feasible strategies addressing increasing water

49

demand

50

technologies have gained widespread attention 2,3.

51

More than half of desalination plants installed worldwide are based on reverse osmosis (RO)

52

process, in which an external hydraulic pressure is applied to brackish feedwater to drive

53

freshwater through a semi-permeable membrane 3. In contrast to the pressure-driven RO

54

processes, in forward osmosis (FO), osmotic pressure difference across a semi-permeable

55

membrane drags water molecules from the lower osmotic pressure feed solution (FS) into the

56

draw solution (DS) of higher osmotic pressure 4. Compared to all the pressure-driven

57

membrane-based processes, FO profits from minimal fouling propensity due to the absence of

58

any external pressure gradient across the membrane. The loosely foulants can also be easily

59

removed upon application of minimal shear stress through physical washing 5. Besides such

60

minimal fouling propensity, relatively high water recovery, low equipment cost, and moderate

61

operating expenses are other advantages that have encouraged the application of FO and have

62

made it a promising technique of desalination 6. However, lack of an easily regenerable DS as

63

well as a highly efficient membrane with strong compatibility to conventional solutions and

64

long-term performance stability are the main obstacles hindering the widespread

65

commercialization of FO technology. As an appropriate DS that could be regenerated by

2

. Among the several proposed methods, membrane-based water treatment


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simple distillation at 60 °C, ammonium bicarbonate solution has been used extensively in FO

67

processes 7.

68

Owing to different water permeation mechanisms in RO and FO, the conventional RO

69

membranes do not possess appropriate fluxes in FO mode. Accordingly, in the fabrication of

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FO membranes, evolving osmotic pressure gradient across the membrane and the reverse

71

solute flux have to be carefully considered. A FO membrane generally consists of a thin active

72

layer, which is commonly made of polyamide (PA) thin film, deposited on a porous substrate

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(referred to as thin film composite (TFC) membranes) 8. The underlying thick porous substrate

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has a mechanically robust structure and provides the FO membrane with adequate physical

75

stability. Despite many advances in membrane fabrication, FO membranes still suffer from

76

internal concentration polarization (ICP) effect, which severely declines effective osmotic

77

pressure gradient across the membrane and thus, lower the overall water permeability 9. When

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water permeation dilutes the DS, simultaneous reverse solute diffusion naturally occurs from

79

high concentrated DS to FS to retain the solute concentration constant

80

processes drops the draw solute concentration at the active layer interface and consequently,

81

attenuates the osmotic pressure difference and lowers the overall membrane performance. ICP

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mainly occurs within the support layer due to the impediment of solutes diffusion through

83

active layer 12. Therefore, any dense, thick and tortuous structure with nano-sized thin layer on

84

top can impede facile solute transport through the membrane and exacerbate the ICP problem.

85

To better assess the ICP effect on FO membrane performance, structure parameter (S) has been

86

defined as:


10,11

. Such competing


=

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×

(1)

87

where l is the support layer thickness, τ is tortuosity, and ε is the membrane porosity 13,14. The

88

smaller the S parameter, the lower the ICP effect and thus, the better the overall FO membrane

89

performance would be

90

layer should be made as thin as possible with high porosity and minimal tortuosity. So far,

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ample efforts have been devoted to lower ICP and its negative impacts on the FO process. In

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the pressure-driven membrane processes, as prior to pass the support layer, water should

93

permeate through an active layer of high resistance via a solution–diffusion mechanism, the

94

porous support layer does not need to be fully wetted to achieve adequate water flux.

95

Conversely, in osmotically driven membrane processes, if not being sufficiently wetted, vapor

96

or air trapped within the pores of the support layer can block the water flow, and consequently

97

worsen the ICP effect. In such conditions, as just a fraction of pore volume would be accessible

98

to water flow, water molecules could not continually flow through all the void spaces and thus,

99

the effective porosity of the membrane would be reduced

9,15

. Accordingly, to lower the ICP effect, the FO membrane support

16

. This implies that, lowering the

100

resistance against water and draw solute diffusions through the support layer would be

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probably one of the best strategies to minimize ICP effect

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enhancement, in which both water and solute permeations would be improved, is one of the

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most popular methods used extensively in practice to mitigate ICP

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entrapment within the pores of a hydrophilic support layer can lead to the formation of porous

105

structure with minimal tortuosity 19. Grafting hydrophilic polymers 20, substitution of specific

106

functional groups

107

have been successfully implemented to modify hydrophilicity of the FO membranes. However,

21,22

17

. Support layer hydrophilicity

, and addition of hydrophilic nanostructures


16,18

9,14,23–25

. Reduction of air

are introduced and

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it should be noted that more swelling in aqueous environments that occurs in the hydrophilic

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support layers could deteriorate membrane structure in real applications 19. Therefore, although

110

such strategies have addressed the ICP problem to some extent, FO membranes need to be

111

studies in more details and attempts should be made to achieve membranes of advanced

112

mechanical stability with long-term performance 26.

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Method of pore-forming in soft materials (i.e. polymer, carbon nitrides, or carbonaceous

114

materials) using hard templates has been recently introduced and has shown promising

115

potentials in various research areas including sensors, catalysis, and membranes 27. To fabricate

116

porous networks, templating materials can be easily formed using a simple single molecule or

117

even an aggregated matrix of complex morphology. As a pioneering work in this field, acid-

118

soluble carbonate nanoparticles were incorporated in polyethersulfone (PES) and applied to

119

ultrafiltration process

120

fabricated via dissolvable limestone nanoparticles as pore templates 29. By doing so, it has been

121

shown that dialysis membranes of unique features such as narrow pore size distribution and

122

fast dialysis rate at low protein adsorption might be achieved. Likewise, in order to improve

123

water permeation, mixture of calcium compounds has been embedded within a nanofibrillated

124

cellulose membrane structure and then removed to generate an open pore network

125

impossible, it is practically difficult to manipulate dimensions and morphology of the

126

monovalent and divalent salts used as hard templating structures. To control the ICP effects in

127

FO membranes, silica nanoparticles have also been incorporated in PES polymer and

128

subsequently were removed by using hydrofluoric acid to make a template-assisted support

129

layer 31. Owing to their tailored porous structure and interconnected-pore network, significant

130

water permeation was achieved in the obtained FO membranes. The utilized hydrofluoric acid

28

. In another study, PES membranes modified by triethyl citrate were


30

. If not


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131

used to remove silica particles is toxic and thus, such process may not be readily scaled up

132

toward industrial applications. In this study, zinc oxide (ZnO) has been introduced as a novel

133

non-toxic and dimensionally controllable pore-forming template. As an unique semiconductor

134

with many distinguished applications in electronics, material science, and photonics

32

135

different methods have been proposed to engineer the morphology of ZnO nanostructures

33

136

Herein, to verify the impacts that morphology of nanostructures could have on modified

137

membrane performance, facile methods have been used to synthesize two different ZnO

138

nanostructures including nanoparticles (NPs) and nanorods (NRs). The obtained nanostructures

139

were embedded in PES support layer as hard templating materials and then the membrane

140

support layer was fabricated through a conventional phase inversion process. Subsequently, the

141

ZnO nanostructures were removed by an acidic solution to fabricate high permeable FO

142

membranes of highly interconnected porous structures.

143

2.

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2.1. Materials and reagents

145

Polyethersulfone (PES, molecular weight: 58,000, Ultrason® E 6020, BASF Co., Germany) as

146

main support layer polymer, polyethylene glycol 400 (PEG, Mn = 400 g/mol, Merck) as pore-

147

forming agent, and N-methyl-2- pyrrolidone (NMP, Merck) as solvent were used to fabricate

148

membrane substrates. The top active layer consisted of a thin polyamide (PA) film, synthesized

149

by using 1, 3-phenylenediamine (MPD, >99%, Merck) and 1, 3, 5-benzenetricarbonyl

150

trichloride (TMC, >98%, Merck) as initial monomers. Zinc(II) acetate dihydrate (Zn(Ac),

151

Merck), NaOH (Sigma-Aldrich, US), and ethanol (≥99.9%) were used to synthesize ZnO

152

nanoparticles (ZnO NPs) and ZnO nanorods (ZnO NRs). During the synthesis of ZnO NPs,

,

.

Experimental


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153

diethylene glycol (DEG, Merck) was used to prevent particle agglomeration. In both FO and

154

RO experiments, sodium chloride (NaCl, ≥99.8%, Iran Mineral Salts Company) was used to

155

prepare solutions of different salt concentrations. All solutions were prepared using deionized

156

(DI) water supplied by a Milli-Q system (Millipore).

157

2.2. Synthesis of ZnO NPs and NRs

158

Throughout the experiments, seeded alcoholysis of the Zn(II) precursor (Zn(Ac)) was used to

159

synthesize ZnO NPs

160

ratio) were mixed in 100 ml of DEG followed by 10 min sonication in an ultrasonic bath. After

161

being heated at 160 °C for 1 h, the mixture was centrifuged at 5000 RCF and the supernatant

162

was used as seed solution. Zn(Ac) (0.1 M), NaOH (0.1 M), and DI water (0.2 M) were all

163

dispersed in the obtained supernatant DEG solution, heated for 1 h at 180 °C, centrifuged for

164

10 min, and finally washed three times with DI water. It was then dried at room temperature

165

and used as a pore-forming template to fabricate FO membranes. Facile thermal decomposition

166

method was also carried out to synthesize ZnO NRs

167

was placed, covered by a piece of alumina lid, and then heated to 300 °C under static air

168

condition with a ramping rate of 10 °C/min for 1 h. The obtained white powder was directly

169

used as ZnO NRs pore-forming template in FO membrane structure without any further

170

modifications.

171

2.3. Membrane fabrication

172

2.3.1. Fabrication of support layers

34

. To do so, the precursor containing Zn(Ac) and DI water (1:2 molar

35

. In an alumina crucible, Zn(Ac) (2 g)



Page 10 of 39

173

Different support layers were prepared using non-solvent induced phase inversion through

174

immersion precipitation method. Dope solutions used for fabrication of support layers have

175

been summarized in Table 1. Different homogeneous polymer solutions were prepared and

176

cooled down to the room temperature and then degassed overnight prior to the casting

177

operation. The porous support layers were cast on a glass plate using a laboratory hand casting

178

knife of 150 µm gate height and subsequently immersed in a coagulation bath of DI water to

179

precipitate the polymers as a thin support layer. After being dried overnight, obtained support

180

layers were cut into circular pieces (36 mm in diameter). Half of the modified support layers

181

were then soaked for 1 h in a 1 M HCl solution to dissolve ZnO nanostructures, leaving void

182

spaces therein. Eventually, the support layers modified by ZnO nanostructures, the ones treated

183

with HCl solution, as well as the pristine PES were all characterized in details and employed in

184

FO experiments to verify their performances. The procedures used to fabricate the support

185

layers are schematically illustrated in Fig. 1.

186

Table 1. The compositions of dope casting solution used to fabricate different support layers. Support layer PES PES-HCl PES-ZNP-0.5 PES-HClZNP-0.5 PES-ZNP-1 PES-HClZNP-1 PES-ZNP-2 PES-HClZNP-2 PES-ZNR-0.5 PES-HClZNR-0.5 PES-ZNR-1 PES-HClZNR-1 PES-ZNR-2 PES-HClZNR-2

PES (wt%) 15 15 15 15 15 15 15 15 15 15 15 15 15 15

NMP (wt%) 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5

PEG-400 (wt%) 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5

Nanomaterial type ZnO NPs ZnO NPs ZnO NPs ZnO NPs ZnO NPs ZnO NPs ZnO NRs ZnO NRs ZnO NRs ZnO NRs ZnO NRs ZnO NRs


Weight content 0.5 0.5 1 1 2 2 0.5 0.5 1 1 2 2

Immersion in HCl solution No Yes No Yes No Yes No Yes No Yes No Yes No Yes

Page 11 of 39


187

2.3.2. Formation of PA active layer

188

To conclude fabrication process of the membranes, an active PA rejection layer was placed on

189

the support layers through interfacial polymerization between MPD and TMC monomers. Each

190

of the obtained support layers was immersed in an aqueous solution of 2 wt% MPD monomer

191

for 120 s and then dried by a piece of filter paper to just remove the excess water droplets from

192

its top surface. In order to form a thin selective PA layer, membranes were further soaked in a

193

0.1 wt% TMC/n-hexane solution for another 90 s, dried at room temperature for 1 h, washed

194

with DI water for three times to remove any residuals, and finally stored in DI water until use.

195

To be identified, the final membranes were denoted by their support layer, in which the starting

196

PES expression was replaced by TFC.

197 198 199

Fig. 1. Schematic illustration of the preparation methods used for fabrication of ZnO NPs and NRs modified support layers used in FO desalination process.

200

2.4. Characterizations



201

2.4.1. Characterization of synthesized ZnO NPs and NRs

202

Morphology of the synthesized ZnO NPs and NRs was observed by Scanning Electronic

203

Microscopy (SEM) (Tescan, VEGA). X-ray diffraction (XRD) patterns of ZnO NPs and NRs

204

were also provided at 2θ ranging from 20 ° to 70 ° using an X-ray diffractometer (Bruker, D8-

205

advance) with monochromatized CuKa radiation (λ=1.541874 Å) operated at 40 kV/30 mA.

206

Also, using the Brunauer–Emmett–Teller (BET) theory, surface area (SBET) and mean pore

207

diameter of the synthesized nanostructures were determined based on N2 adsorption–

208

desorption isotherms at 77 K (ASAP 2010, Micromeritics, USA). Particle size distribution of

209

the prepared ZnO NPs and NRs were also determined using a dynamic light scattering (DLS)

210

particle size analyzer (Malvern, Micro-P).

211

2.4.2. Membrane characterization

212

SEM images (Tescan, VEGA) were used to determine top, bottom, and cross-sectional

213

morphologies of the TFC membranes. Energy dispersive X-ray spectroscopy (EDS) was also

214

carried out to perform elemental analysis of the fabricated support layers. Using sessile drop

215

method, averages calculated for the contact angles measured at three points of each support

216

layer surface at room temperature (Dataphysics, OCA 15 plus) were used to evaluate the

217

hydrophilicity of the surfaces. The obtained contact angles were also compared using Student

218

t-test to verify the significance of the differences. The chemical structure of the obtained

219

membranes was also analyzed using an attenuated total reflection Fourier transform infrared

220

spectroscopy (ATR-FTIR, Bruker, Equinox 55). To evaluate the mechanical strength of the

221

prepared support layers, a rectangular stripe (10 mm × 40 mm) was carefully cut from each

222

casted film and stretched at ambient temperature by a tensile machine (CT3 Texture Analyzer 11 ACS Paragon Plus Environment

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tensile testing equipment, Brookfield engineering). In addition, porosity (ε) of each support

224

layer was determined gravimetrically 25,36. From each support layer, three fresh pieces were cut

225

and immersed in DI water overnight and then, using a digital balance (A&D FZ-5000i, Tokyo,

226

Japan), weights of the fully saturated membranes with no excess water on the surfaces (m1)

227

were measured. Subsequently, following overnight vacuum-drying at 60 °C, the dried support

228

layers were weighed again (m2) and using the densities of water (ρw) and substrate polymer

229

(ρp), following equation was used to calculate the porosity of each membrane substrate:

=

− / − / + /

(2)

230

Surface morphology of the support layers and corresponding final membranes were also

231

studied using atomic force microscope (AFM, Femto Scan, 2012) technique with scanning area

232

of 3 µm × 3 µm.

233

2.5. Evaluation of membranes intrinsic properties

234

Using the membranes with an effective surface area (Am) of 9.60 cm2, dead-end RO filtration

235

tests were carried out to determine salt rejection (Rs), water permeability (A), and salt

236

permeability (B) coefficient of the TFC membranes (Eqs 3 to 6). All RO tests were conducted

237

at room temperature under 5 bar of feed pressure. DI water was used as the feed to determine

238

water permeability coefficients (A) using Eqs. (3) and (4) 37:

=

∆ × ∆

(3)



=

∆

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(4)

239

where J, ∆V, ∆t, and ∆P are RO water flux, permeate volume change, time interval, and

240

applied hydraulic pressure, respectively. By using a feed solution of 1000 ppm NaCl, the salt

241

rejection rate (Rs) was also determined as follows 38:

= 1 −

× 100

(5)

242

where Cf and Cp represent NaCl concentrations at feed and permeate sides, respectively. In

243

addition, considering the solution-diffusion theory and by using the transmembrane hydraulic

244

pressure difference (∆P) and the osmotic pressure difference across each membrane (∆π), Eq. 6

245

was employed to evaluate salt permeability coefficients (B) 37:

1 − " = ∆ − ∆#

(6)

246

2.6. Forward osmosis performance tests

247

Water flux and salt rejection were used to evaluate the FO performance of the membranes. The

248

lab-scale cross-flow FO setup used in this study has been previously described in details

249

The experiments were conducted at room temperature. At a constant flow rate of 0.2 L/min (~

250

8 cm/s), DS and FS were counter currently circulated over either sides of the membrane. To

251

better assess the membrane performance, all FO tests were conducted under both PRO (PA

252

active layer facing DS) mode and FO (PA active layer facing FS) mode. By using a digital

253

weight balance (EK-4100i, A&D Company, Japan) connected to a computer data logging

254

system, the amount of permeate water collected at DS side was recorded. The water flux (jv, 13 ACS Paragon Plus Environment

14,22

.

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255

Lm2−h−1, denoted as LMH) and reverse salt flux (Js, gm−2 h−1, denoted as gMH) of the TFC

256

membranes were calculated as following 14,39:

$ =

∆ ∆

(7)

% =

& & − ( ( ∆

(8)

257

where ∆V (L) is the volume change of the DS over a predetermined time interval ∆t (h) in each

258

FO experiment. Also, C0 and V0 denote the initial salt concentration and FS volume, and Ct

259

and Vt are their corresponding values at time t. To measure salt concentration values during the

260

FO tests, a conductivity meter (WTW GmbH, Germany) was used to monitor the conductivity

261

of FS.

262

As described earlier, the membrane structural parameter (S) (Eq. 1) is an intrinsic membrane

263

property, indicating the degree of ICP effects on the membrane overall performance. The lower

264

the S value, the better the membrane performance would be with respect to the water

265

permeation. The S parameter could be attained by the recorded FO water flux values, when 1M

266

NaCl solution and DI water were respectively used as DS and FS in either FO mode (Eq. 9) or

267

PRO mode (Eq. 10) as following 40,41:

$ =

1 #+. − $ + " * ) #-.. + "

(9)

$ =

1 #+.. + " * ) #-. + $ + "

(10)



268

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where #+.. and #-.. refer respectively to the osmotic pressure of the bulk DS and FS, while

269

#+. and #-. represent the actual osmotic pressures applied to membrane surfaces at DS and

270

FS sides. Also, the S values were used to evaluate the solute resistivity (Km) within the porous

271

support layer as:

) =

= / /

(11)

272

where Ds denotes the diffusion coefficient of solute used in DS.

273

3.

274

3.1. Characterization of synthesized nanostructures

275

XRD characterization of the synthesized ZnO NPs and NRs was conducted to determine the

276

crystal structure and sizes of the synthesized nanostructures (Fig. 2a). All diffraction peaks can

277

be indexed as hexagonal wurtzite structure of ZnO, with cell constants of a = 0.324890 nm and

278

c = 0.520620 nm, without any impurities 42. The existence of sharp peaks in the XRD patterns

279

confirmed the well-crystallized structure of the synthesized nanostructures. Also, using the

280

isotherms achieved by the physisorption of nitrogen gas (Fig. 2b), specific surface area and

281

pore structure of the ZnO NPs and NRs were assessed to be 10.32 m2/g and 8.89 m2/g and the

282

mean pore diameter were determined to be 4.23 nm and 1.13 nm, respectively. Morphologies

283

of the nanostructures are also illustrated in Fig. 2c and d. The obtained SEM images revealed

284

that the ZnO NPs were uniformly synthesized. Due to the strong surface attractions, ZnO NPs

285

tended to aggregate and form clusters. In addition, typical SEM image of ZnO NRs (Fig. 2d)

286

clearly demonstrated successful formation of the desired nanostructures. By using the DLS

Results and discussion


Page 17 of 39


287

profiles shown in Fig. 2e, the average particle size of ZnO NPs was determined to be 35 nm

288

which was accordance with the obtained SEM images. However, two distinct peaks were

289

observed in the DLS curve of ZnO NRs (Fig. 2e). It was more likely that the first peak

290

represented the average diameter of the synthesized NRs (~85 nm), while their average length

291

was indicated by the second peak at almost 1.2 µm.

292 293 294 295 296 297 298

Fig. 2. (a) XRD patterns of the synthesized ZnO nanostructures with cell constants of a = 0.324890 nm and c = 0.520620 nm that could all be indexed as hexagonal wurtzite structure. (b) Nitrogen adsorption–desorption isotherms achieved by the physisorption and desorption of nitrogen gas revealed specific surface areas of 10.32 m2/g and 8.89 m2/g and mean pore diameter of 4.23 nm and 1.13 nm for ZnO NPs and NRs, respectively. (c) SEM images of ZnO NPs and (d) ZnO NRs, and (e) DLS size distributions of synthesized nanostructures.

299

3.2. Membrane characterizations

300

Fig. 3 shows the ATR-FTIR spectrum of the membranes modified by 2% of either ZnO NPs or

301

NRs. The characteristic bands of the PES support layer at 1578 cm-1 (C=C aromatic ring

302

stretching) and 1240 cm-1 (Aryl–O–Aryl C–O stretch), as well as the main characteristic band



Page 18 of 39

303

of PES at 1486 cm-1, attributed to the benzene ring and C–C bond stretch, could be clearly

304

observed in all ATR-FTIR spectrums

305

over the support layers was inferred by the peaks at 1548 cm-1 (–N–H bending vibration of

306

amide II), 1660 cm-1 (–C=O stretching vibration of amide I bond), and 1608 cm-1 (aromatic

307

ring breathing of amide) 44. As seen, the peak intensity of aromatic ring breathing in the TFC-

308

HClZNP-2 and TFC-HClZNR-2 membranes were higher than the ones containing ZnO

309

nanostructures. Also, the corresponding amide II peak arises when the C–N stretching

310

vibration of the C–N–H group couples with the in-plane N–H bending

311

observed spectra indicated that the PA polymer chains on the surface of TFC-HClZNP-2 and

312

TFC-HClZNR-2 membranes were primarily in straight configuration, while the other

313

membranes had PA layer of coiled configuration, a property that might have drastic impact on

314

their water permeation and salt rejection. Also, the broad absorption peak centered at 480 cm-1

315

could be due to the vibrations of Zn–O band 46. Therefore, after incorporation of ZnO NPs and

316

NRs, the peak corresponding to stretching vibration of Zn–O was appeared in the FTIR

317

spectrums of TFC-ZNP-2 and TFC-ZNR-2 membranes and disappeared after being washed

318

with acidic solution in TFC-HClZNP-2 and TFC-HClZNR-2 membranes.

43

. Moreover, successful formation of PA active layer


45

. Therefore, the

Page 19 of 39


319 320 321 322

Fig. 3. ATR-FTIR spectra of different FO membranes. Important functional groups in FTIR spectrum are 1240 cm−1 (C–O stretch), 1486 cm−1 (aromatic C–C), 1578 cm−1 (C=C), 1682 cm−1 (C=O). Each sample was scanned twice with 1 cm−1 step size.

323

Hydrophilicity of the membranes was assessed by performing contact angle analysis on all the

324

support layers (Fig. 4). The average contact angle of the PES support layer was determined to

325

be 77.08° and after being washed by HCl solution, with negligible changes, it reduced to

326

76.07°. However, by incorporation of 0.5% weight ratio of ZnO NPs and NRs to PES, the

327

contact angles of the modified support layers were considerably declined to 71.07° and 70.13°,

328

respectively. The contact angle reduction trend was consistently observed using higher

329

concentrations of ZnO nanostructures in the PES polymer which could be resulted from the

330

presence of hydrophilic ZnO NPs and NRs in the structure of the membranes. Furthermore,

331

contact angle measurements indicated that, when compared to the ZnO NPs, embedding ZnO



332

NRs had slightly higher impacts on the obtained support layer hydrophilicity. As expected, by

333

immersing ZnO modified support layers in acidic solution, and thus removing the embedded

334

nanostructures, the average contact angles substantially increased and almost returned to the

335

original values of pristine PES. Considering the fact that both ATR-FTIR and contact angle

336

analysis methods could survey the surface characteristics of the support layers rather than the

337

bulk, the obtained results reasonably suggest that ZnO nanostructures could be completely

338

leached out from outer parts of the support layers via the simple acid washing. The obtained

339

contact angle values were then analyzed using Student’s t-test (Table. S1) and it was concluded

340

that the differences between mean contact angle values were meaningful.

341 342 343 344

Fig. 4. Water contact angle values of different support layers. The contact angle measurements were conducted for each support layer at three different points. The mean values are presented with their standard deviations.

345

Thicknesses of the prepared support layers were measured by a digital micrometer and have

346

been reported along with their calculated porosities (Eq. 1) in Fig. 5. According to the obtained


Page 20 of 39

Page 21 of 39


347

results, as all the nanostructure modified support layers had smaller thicknesses than either of

348

the PES or PES-HCl ones, it could be concluded that increasing the ZnO nanostructure content

349

could lead to the reduced thickness of the modified support layers. In addition, compared to the

350

PES substrate, higher porosity values were achieved in the support layers modified by different

351

content of ZnO nanostructures. Such observations were in line with previous reports in which

352

high hydrophilicity of the casting polymer dope solution was found to be responsible in rapid

353

solidification in coagulation bath, reduced overall thickness, and enhanced porosity of the

354

fabricated support layer 47,48.

355

To study any impacts that embedded ZnO nanostructures and acid treatment could have on

356

mechanical stability of final support layer, stress–strain profiles of newly synthesized

357

substrates were analyzed and compared with pristine PES and PES-HCl ones. Tensile strengths

358

of 0.236 MPa and 0.239 MPa and Young’s moduli of 4.19 MPa and 4.25 MPa were

359

experimentally determined for PES and PES-HCl substrates, respectively. Also, all the

360

modified support layers revealed to have lower mechanical strengths and it was experimentally

361

confirmed that the effect of ZnO NPs was more significant in such reductions. The relatively

362

improved mechanical stabilities observed in the substrates modified by ZnO NRs could be

363

explained in part by the nanofibrous structure of such nanostructures. Since no external

364

hydraulic pressure is involved in FO process, lower mechanical strengths of the modified

365

support layers could be compromised by their improved permeation capabilities.



366 367 368 369

Fig. 5. (a) Overall porosities and thicknesses of different support layers measured in triplicates and reported as mean values along with the standard deviations and (b) stress–strain profiles of different support layers.

370

Fig. 6 illustrates cross-sectional morphologies of the pristine PES and other modified support

371

layers with different magnifications. According to the obtained SEM images, PES and PES-

372

HCl support layers had narrow loose finger-like pores. Conversely, the PES-ZNP-2 and PES-

373

HClZNP-2 substrates possessed more straight pores that could facilitate water permeation.

374

Furthermore, as shown in Fig. 6, although improved structures were evidently demonstrated in

375

the support layers modified by ZNRs (i.e. PES-ZNR-2 and PES-HClZNR-2), better

376

improvements were achieved when the substrates were modified by ZNPs (i.e. PES-ZNP-2 and

377

PES-HClZNP-2), possibly due the superior dispersion of ZNPs throughout the PES polymer.

378

Such observations elucidated the impact that morphology of nanomaterials could have on pore

379

structure of the modified support layers. Embedding hydrophilic nanostructures within the PES

380

can intensify thermodynamic instability of the PES polymer solution, thereby leading to an

381

accelerated exchange rate between solvent and non-solvent phases during the phase inversion

382

step

383

minimal tortuosity, and consequently, support layers of lower structural parameter (Eq. 1) 50.

14,49

. The fast exchange rates can in turn result in formation of straight open pores of


Page 22 of 39

Page 23 of 39

384 385 386 387 388 389


Fig. 6. Cross sectional SEM images of different support layers magnified from left to right by 0.5kx, 3kx, and 30kx. All 30kx SEM images were obtained in back-scattered electrons mode to better illustrate blended nanostructures. As can be seen in PES-HClZNP-2 and PES-HClZNR-2 support layers, void spaces have been generated based on the incorporated ZnO nanostructure morphologies throughout the PES support layers. 22 ACS Paragon Plus Environment


390

The magnified cross sectional SEM micrographs revealed a dense morphology for PES support

391

layer that did not alter significantly after being washed by HCl. However, in the magnified

392

SEM image of PES-ZNP-2, the ZNPs that were uniformly distributed throughout the support

393

layer (identified as larger and brighter spheres) could be effectively removed after acid

394

washing, leaving pore spaces with similar dimensions throughout the support layer. Also, as

395

shown in the magnified cross sectional SEM image of PES-ZNR-2 support layer, ZNRs were

396

disorderly placed within the PES substrate. Following acid washing, SEM images obtained

397

from the PES-HCLZNR-2 confirmed that the NRs were successfully leached out of the support

398

layer and owing to their larger sizes, bigger interconnecting pores were generated. Such

399

specific channels were able to improve membrane permeability by providing the permeate

400

water with more void spaces. According to the EDS analysis results (Fig. S1), Zn element was

401

appeared in the EDS spectra of PES-ZNP-2 and PES-ZNR-2. After leaching out the ZnO from

402

support layer structure, as expected, the intensities of corresponding Zn peaks were

403

significantly lowered, implying that the ZnO was appropriately dissolved in HCl solution. It

404

was also concluded that the dissolution of ZnO NRs in acidic solution was more than ZnO

405

NPs, possibly due to their bigger sizes that makes them more prone to be dissolved by the

406

diffused HCl solution.

407

Therefore, considering the obtained SEM images, it could be concluded that the introduced

408

templating method have considerable influences on thickness, porosity, and tortuosity of the

409

support layer. Presence of hydrophilic ZnO nanostructures within the casting solution could

410

lead to a quick solidification and thus, resulting in a thinner membrane. Furthermore, after

411

leaching out the ZnO nanostructures, the PES polymer chain might be drawn into the newly-

412

developed void spaces, making these membranes even thinner (please refer to Fig. 5a). New 23 ACS Paragon Plus Environment

Page 24 of 39

Page 25 of 39


413

open pores could also be created via dissolution of embedded ZnO NPs and NRs. Such newly

414

developed secondary pores could act as channels connecting most of the micro-voids, making

415

an advanced interconnected pore network structure. In such structure, when parts of the PES

416

channels were clogged (e.g. trapped air), water could still flow through the highly established

417

interconnected pores and bypass the clogged regions. Fabrication of such porous structure

418

would be vitally important in the real applications of FO membrane as it can keep the ICP

419

minimal for a long period of time. In addition, as mentioned earlier, leaching out the ZnO

420

nanostructures embedded within the PES polymer could increase the support layer tortuosity,

421

resulting in an elevated S parameter (Eq. 1). However, given the fact that the straight pores

422

were much bigger than the interconnected ones created by the nanostructures on the void

423

channels (Fig. 6), water would still tend to flow through the primary straight micro-channels

424

and thus, variations in support layer tortuosity would be minimal, if any.

425

Using AFM technique, surface roughness (Ra) of the pristine PES and the other modified

426

support layers were measured to compare their surface morphology (Fig. S2). The results

427

showed that the PES and PES-HCl support layers possessed the lowest surface roughnesses of

428

11.75 nm and 11.51 nm. Moreover, the obtained values indicate that the ZnO NPs has more

429

significant impacts on the Ra values rather than the ZnO NRs. However, in either cases, Ra

430

was increased after acid treatment. It is well known that the water contact angle of a surface

431

could be influenced by surface chemical composition (hydrophilic/hydrophobic functional

432

groups) as well as its physical characteristics such as roughness. It has been reported that

433

surfaces with higher roughness demonstrate higher contact angle values

434

considering the results obtained for hydrophilicity and roughness values of the prepared


51

. Therefore,


435

support layers, it was concluded that variations in chemical composition of the active layers

436

were mainly responsible for the observed differences in surface wettability.

437

In FO processes, in order to prevent solute permeation as much as possible, presence of a

438

robust and selective active layer on top of the membrane is essentially important. Support layer

439

modifications should thus be accomplished without compromising the rejection rate of solute

440

molecules. Accordingly, to investigate the impacts that the proposed templating method could

441

have on the top PA active layer morphology, SEM images were obtained from top and cross

442

section of the PA layers (Fig. 7). The SEM images captured from the top PA layer of the TFC

443

membrane revealed an uniform ridge-and-valley morphology, being formed by typical IP

444

reaction 52,53. Also, the corresponding cross-sectional SEM image showed that the thickness of

445

the formed PA layer was 595 nm. The top PA morphology and its thickness in TFC and acid

446

washed TFC-HCl membranes were almost identical. However, incorporation of 2 wt% ZNPs

447

and ZNRs in the PES polymer resulted in formation of a leaf-like structure, which was more

448

evident in the TFC-ZNR-2 membrane. The PA layer thicknesses of the TFC-ZNP-2 and TFC-

449

ZNR-2 membranes were 330 nm and 230 nm, respectively. After being treated by acidic

450

solution, the PA layer formed on the ZNRs modified support layer was mostly similar to the

451

PES one with a ridge-and-valley morphology and thickness of 460 nm. In contrast, with 245

452

nm thickness, a leaf-like structure was observed in the TFC-HClZNP-2 membrane.

453

The IP reaction takes place at the interface of the porous support and the organic TMC

454

solution. It is believed that diffusion rate of MPD molecules within the IP reaction zone

455

controls the formation of PA layer. Therefore, as the surface of the porous support layer


Page 26 of 39

Page 27 of 39


456

immersed in aqueous MPD solution gets into contact with organic TMC solution, the PA film

457

starts growing from the water/organic interface toward the bulk of the organic solution 54.

458 459 460 461 462 463

Fig. 7. Top and cross sectional SEM images illustrating different morphologies of the membranes top PA active layers. Uniform ridge-and-valley structure was observed in TFC, TFC-HCl, TFC-ZNP-2, and TFC-HClZNR-2 while the TFC-HCl-ZNP-2 and TFC-ZNR-2 membranes revealed a smooth leaf-like structure. The cross sectional images were used to assess the thickness of the active layer.

464

This implied that any variation in the surface structural characteristics of the support layer,

465

including pore size and hydrophilicity, can influence the diffusion path, IP reaction rate, final

466

PA layer morphology, and ultimately the perm-selectivity of the fabricated TFC membrane. In

467

general, small surface pores enable the substrate to better hold the absorbed MPD solution

468

within its pore spaces, resulting in formation of a smoother PA active layer on top of the TFC

469

membrane. Conversely, broader pores on the substrate surface facilitate rapid migration of

470

amine molecules toward the organic TMC solution, producing a rough PA layer

471

substrate hydrophilicity can also impede facile MPD diffusion, forming a smooth PA layer.

472

Here, due to high hydrophilicity of the PES-ZNP-2 support layer, rapid diffusion of MPD

473

molecules through the IP reaction zone could be prevented, leading to formation of a thin PA

474

layer with leaf-like structure. Despite the fact that after acid washing and removal of ZNPs, the 26 ACS Paragon Plus Environment

55

.Higher


475

surface hydrophilicity reduced and got back to the original PES value, the formed nanoscale

476

pores were still able to trap the MPD molecules and impede further growth of PA layer.

477

However, in the case of PES-HClZNR-2 support layer, after leaching out the ZNRs, the

478

relatively larger open pores formed on the surface, along with lower hydrophilicity, might help

479

the MPD to substantially diffuse into the IP reaction zone and create a rigid PA layer.

480

Water contact angels and surface roughnesses were also studied to verify the hydrophilicity

481

and surface morphology of the active layer of the membranes. Water wettability of the TFC,

482

TFC-HCl, TFC-ZNP-2, TFC-HClZNP-2, TFC-ZNR-2, and TFC-HClZNR-2 membranes were

483

evaluated to be 91.31°, 86.47°, 83.91°, 77.16°, 87.42°, and 92.37°, respectively. Also, their

484

surface roughnesses were found to be 63.23 nm, 61.19 nm, 58.74 nm, 51.22 nm, 55.59 nm, and

485

60.06 nm, respectively. Given the similar chemical composition that all the active layers

486

possessed, the observed variations could be attributed to their different surface morphologies

487

manifested in roughness values.

488

3.3. FO membranes intrinsic separation properties

489

Independent from solute concentration and applied pressure, water and salt permeability

490

coefficients were measured through a RO dead-end process and then used in Eqs. 2 to 5 to

491

evaluate the other membrane properties (Table 2). As reported, all modified membranes

492

demonstrated higher water permeability coefficients than the TFC membrane. In the case of all

493

the membranes modified by ZnO nanostructures, improved water permeabilities could be

494

explained by the higher hydrophilicities of such modified membranes (see Fig. 4). For

495

instance, compared to the TFC membrane, water permeabilities were improved by 57.6% and

496

151.0%, reaching 3.09 LMH/bar and 4.92 LMH/bar, when 2 wt% of ZNPs and ZNRs were 27 ACS Paragon Plus Environment

Page 28 of 39

Page 29 of 39


497

incorporated in the support layers. However, the salt rejection rate of the TFC-HClZNR

498

membranes were almost similar to that of TFC-ZNR ones which could be in part attributed to

499

the practically identical top PA layers formed on these modified membranes (see Fig. 7).

500

As shown in Table 2, compared to the pristine TFC membrane, all modified membranes

501

demonstrated beneficially lower S values and consequently, minimized unpleasant ICP effect

502

in FO process. According to the obtained results, by incorporation of only 0.5 wt% of ZNPs

503

and ZNRs, the S parameter decreased by 23.5% and 43.8%, respectively. It worth to note that,

504

in reducing the S parameter, incorporation of ZNRs was always more beneficial. Moreover, in

505

the aforementioned modified membranes, even lower S values were achieved after being

506

treated by HCl. Known as three important characteristics, tortuosity, thickness, and porosity

507

specify the S value of the membranes support layer (Eq. 1). Accordingly, lower thickness as

508

well as higher porosity of the modified membranes (reported in Fig. 5) achieved by

509

incorporation of nanostructures could be assumed to be responsible for such experimental

510

observation.

511 512 513

Table 2. The intrinsic properties including water permeability coefficient (A), salt permeability coefficient (B), and structural parameters (S) of different membranes. Results are mean values of three replicates ± standard deviation. Membrane TFC TFC-HCl TFC-ZNP-0.5 TFC-HClZNP-0.5 TFC-ZNR-0.5 TFC-HClZNR-0.5 TFC-ZNP-1 TFC-HClZNP-1 TFC-ZNR-1 TFC-HClZNR-1

A (LMH/bar) 1.947±0.015 1.983±0.030 2.197±0.032 2.550±0.078 2.780±0.026 2.860±0.056 2.706±0.011 3.103±0.045 3.237±0.023 4.880±0.075

B (LMH) 1.230±0.027 1.447±0.045 1.557±0.049 2.887±0.032 2.776±0.023 3.063±0.051 2.340±0.040 3.600±0.020 4.530±0.051 8.230±0.020

B/A (bar) 1.583±0.040 1.371±0.021 1.412±0.027 0.883±0.037 1.001±0.016 0.936±0.003 1.157±0.020 0.862±0.017 0.714±0.007 0.592±0.010


S (µm) 2058.715±12.392 8 1727.624±5.874 1580.025±6.469 1152.271±13.951 1157.192±5.135 969.960±1.565 1127.235±4.893 763.961±4.696 700.437±2.019 518.050±3.021


TFC-ZNP-2 TFC-HClZNP-2 TFC-ZNR-2 TFC-HClZNR-2

3.096±0.006 4.810±0.098 4.920±0.105 5.177±0.064

3.923±0.023 7.387±0.080 8.453±0.006 9.103±0.097

0.789±0.004 0.651±0.020 0.582±0.001 0.568±0.002

Page 30 of 39

881.976±1.259 544.669±5.257 542.092±0.309 451.426±0.722

514 515

Similar to gas separation membranes, a permeability-selectivity trade-off exists in FO

516

polymeric membranes

517

water permeability coefficient to salt permeability coefficient was employed to verify such

518

trade-off relationship in fabricated polymeric FO membranes 58:

56,57

. Here, an empirical equation that has been developed to correlate

01 4 5 18 18 "= 3 7 12 2 6 519

where λ and β are empirical fitting parameters and L, Rg, T, MW, and L are the gas constant,

520

absolute temperature, molar mass of water, and the thickness of active layer, respectively.

521

When plotted on logarithmic scale, strong relation (R2 > 0.97) between A and B values was

522

observed and the slope of the trade-off line was determined to be slightly less than 2. This

523

implied that increased water permeability would be achieved at the expense of higher salt

524

permeability.

525


Page 31 of 39


526 527 528 529

Fig. 8. Double logarithmic plot of water and salt permeability coefficients. Permeability-selectivity trade-off was confirmed by the strong relationship obtained between A and B parameters. Eq. 12 was used to correlate the experimental data with T = 298 K, MW = 18 g/mol, and Rg = 8.314 J/K.mol.

530

3.4. Performance of FO membranes

531

Using a cross-flow FO system in both FO mode and PRO mode, water fluxes passed through

532

the fabricated membranes were measured in triplicates (Fig. 9). The experimental results

533

indicated that, in both operation modes, significantly higher water fluxes could be achieved in

534

the modified membranes rather than the pristine TFC membrane. The mean water fluxes in

535

PRO mode were generally higher than the ones obtained in FO mode which could be resulted

536

predominantly from the detrimental ICP effect and thus reduced effective osmotic driving

537

force across the membrane active layer in FO mode

538

that, when an identical weight ratio of nanostructures were used in the FO membrane support

539

layer, ZnO NRs would be a better candidate to improve the water permeation. This observation

540

was consistent with our previous study where hydrophilicity of additives was found to be more

59,60

. The experimental results confirmed



541

important than their surface area 14. Therefore, the higher the concentration of nanostructures,

542

the higher the hydrophilicity of the membrane support layer, and thus, the better the

543

performance of the membrane would be in terms of water permeation. The FO membranes

544

containing 2 wt% of ZnO NPs and NRs had respectively water fluxes of 17.9 and 26.8 LMH,

545

that were 54.3% and 83.5% more than the values achieved in the membranes modified by 0.5

546

wt% of the same nanostructures.

547

548 549 550 551 552 553

Fig. 9. Water fluxes measured in triplicates for the TFC and other nanostructured modified membranes in (a) FO mode, and (b) PRO mode. Experiments were all conducted at room temperature with DI water as FS, 1 M NaCl solution as DS, cross-flow velocity of 8 cm/s, and membrane surface area of 9.60 cm2. Mean values along with their associated standard deviations have been reported based on the water fluxes measured in triplicates for each membrane under identical operating conditions.

554

After acid washing, minor changes were observed in the water permeability of the TFC

555

membrane, whereas water fluxes of the membranes modified by ZnO nanostructures were

556

evidently improved. For instance, the water fluxes of the TFC-HClZNP-2 and TFC-HClZNR-2

557

membranes were respectively 53.6% and 18.6% higher than the TFC-ZNP-2 and TFC-ZNR-2

558

membranes. As expected, by using higher weight ratios of ZnO nanostructures to modify the


Page 32 of 39

Page 33 of 39


559

PES polymer, the influence of HCl treatment on water permeabilities was intensified. Also,

560

results of the contact angle measurements (Fig. 4) showed that the hydrophilicity of ZnO

561

incorporated support layers were considerably higher than the HCl treated ones. It implied that,

562

to fabricate an excellent FO membrane of minimal structural parameter, interconnected pore

563

network would be more fundamental than hydrophilicity of the porous support layer.

564

Using an EC-meter, the reverse salt fluxes were also recorded in the FS line throughout the

565

experiments (Fig. 10). As seen, the progressive trends recorded in the reverse salt fluxes were

566

similar to the observed variations in water permeation fluxes, and thereby, consistent with

567

many other studies

568

selectivity. The obtained results confirmed that, in FO processes, higher water permeability can

569

be achieved in the expense of lower selectivity and higher reverse salt leakage that reduces

570

effective osmotic pressure gradient across the membrane.

571 572 573 574 575 576 577

Fig. 10. Reverse salt flux of the TFC and other ZnO nanostructures modified membranes in (a) FO mode, and (b) PRO mode, Experiments were all conducted with DI water as FS, 1 M NaCl solution as DS, cross-flow velocity of 8 cm/s, membrane surface area of 9.60 cm2, and room temperature. Mean values along with their associated standard deviations have been reported for the reverse salt fluxes measured in triplicates for each membrane under identical operating conditions.

52,61,62

, indicated a trade-off relationship between water permeability and



Page 34 of 39

578

The membrane selectivity defined as the ratio of water flux to reverse salt flux can be used as a

579

parameter independent of both draw solution concentration and S value

580

the overall performance of FO membranes. Among all the modified support layers, with the

581

highest value of 3.80 L/g in TFC-HClZNP-1 membrane, relatively high selectivity ratios of

582

2.16 L/g, 2.34 L/g, and 3.40 L/g were achieved respectively in TFC-ZNP-1, TFC-ZNR-0.5,

583

and TFC-HClZNR-1 membranes in FO mode. Accordingly, it could be inferred that

584

nanostructures with specific geometry might be fabricated and used in an appropriate amount

585

to rationally improve the performance of FO membranes.

586

Associated contents

587

Supporting Information.

588

Acknowledgements

589

The authors gratefully acknowledge financial and instrumental supports received by University

590

of Tehran.

591

References

592 593

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(3) Li, D.; Wang, H. Recent developments in reverse osmosis desalination membranes. J. Mater. Chem. 2010, 20 (22), 4551.

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