Versatile Surface Modification of TFC Membrane by Layer-by-Layer

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Energy and the Environment

A Versatile Surface Modification of TFC Membrane by Layer-by-Layer Assembly of Phytic Acid-Metal Complexes for Comprehensively Enhanced FO Performance Shu Xiong, Sheng Xu, Anny Phommachanh, Ming Yi, and Yan Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06628 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Environmental Science & Technology

1

A

Versatile

Surface

Modification

2

Membrane by Layer-by-Layer Assembly of Phytic

3

Acid-Metal

4

Enhanced FO Performance

5

Shu Xiong,† Sheng Xu,† Anny Phommachanh,† Ming Yi,† Yan Wang†,‡*

Complexes

for

of

TFC

Comprehensively

6 7

† Key Laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong

8

University of Science & Technology), Ministry of Education, Wuhan, 430074, China

9

‡Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and

10

Chemical Engineering, Huazhong University of Science & Technology, Wuhan, 430074,

11

China

12 13

*Corresponding author: Email: [email protected] (Y.W.)

14 15 16 17

Key words: phytic acid-metal complexes, layer-by-layer assembly, versatile modification,

18

TFC membrane, anti-fouling, forward osmosis

1

ACS Paragon Plus Environment


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Abstract: Polyamide TFC membranes are widely applied in the membrane-based water

21

treatment but generally suffer various fouling problems. In this work, the layer-by-layer

22

assembly of phytic acid (PA) and metal ions (M) is constructed on the surface TFC membrane

23

for the first time, to improve the bio/organic fouling resistances and separation performance of

24

TFC membranes simultaneously. PA molecule with six phosphonic acid groups of strong

25

chelation ability acts as the organic ligand, and the metal ion acts as the inorganic cross-linker,

26

inducing the assembly of hydrophilic and antibacterial PA-M (Ag or Cu) complexes on the

27

TFC membrane surface. Various characterizations including FTIR, XPS, SEM, AFM and EDX

28

are employed to confirm the successful and uniform modification of PA-M. FO performance

29

of the PA-M modified TFC membranes, i.e., TFC_PA-Ag and TFC_PA-Cu, is optimized by

30

varying PA concentration and assembly cycles, where the water flux can be improved by 157%

31

and 168% respectively without compromising the membrane selectivity. Additionally, the PA-

32

M modification improves the biofouling and organic fouling resistances of the TFC membrane

33

remarkably, owing to the enhanced antibacterial ability and hydrophilicity. The modified TFC

34

membranes are also proven to show the excellent stability by the quantitative release test.

2


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1. Introduction

37

Waste water reuse and saline water desalination by highly efficient membrane-based

38

technologies could be effective and sustainable ways to alleviate the worldwide clean water

39

shortage. Osmotically driven forward osmosis (FO) is potentially more energy-saving than

40

other pressure-driven membrane processes, and therefore has attracted tremendous attentions

41

in recent years.1-4 As a pivotal component in FO system, the membrane determines the

42

separation efficiency in the practical application. The thin film composite (TFC) membrane

43

with a polyamide selective layer formed by the interfacial polymerization of m-

44

pheylenediamine (MPD) and trimesoyl chloride (TMC) is one predominant type of FO

45

membranes owing to its facile fabrication and good selectivity.5-6 However, the highly cross-

46

linked, rough and relatively hydrophobic polyamide selective layer put FO membranes

47

confronted with the low water permeation and easy fouling tendency by organic or biologic

48

substances either.7

49

Enhancing the hydrophilicity of the polyamide selective layer, is therefore considered to be

50

a feasible and efficacious strategy to achieve an improved water flux and reduced foulant

51

adhesion, by employing hydrophilic monomers5, 8-10 or introducing hydrophilic nanomaterials

52

in the monomer solutions,11-12 and grafting hydrophilic compounds on the membrane surface.7

53

However, the enhancement in membrane hydrophilicity exhibits the limited efficacy on the

54

biofouling mitigation for its inefficiency dealing with the fast reproduction of the microbial

55

and the formation of biofilm on the membrane surface.12-14 Modifying the polyamide layer of

56

the TFC membrane with biocides, such as silver (Ag) or copper (Cu) nanoparticles,15-18

57

quaternary ammoniums19 and halamines,20-22 endows the membrane with the excellent

58

biofouling resistance, which however generally reduces the membrane water flux to a certain

59

degree due to the increased hydrophobicity and/or mass transport resistance with the introduced

60

biocides. 3



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Efficient modifications to enhance the organic fouling/biofouling resistance and separation

62

performance of TFC membranes simultaneously is therefore in high demand. So far, only a

63

few effective strategies have been reported to realize the above target. For example, hydrophilic,

64

antibacterial and compatible polyrhodanine nanoparticles (PRh-NPs) have been synthesized

65

and incorporated into the polyamide layer, resulting in the TFC membrane with simultaneously

66

improved flux, selectivity, organic fouling and biofouling resistance.23 In addition, hydrophilic

67

and antibacterial zwitterion-Ag nanocomposites have been constructed on the polyamide layer

68

of the TFC membrane via a secondary interfacial polymerization using a synthetic zwitterionic

69

monomer followed by the in-situ binding of Ag NPs,24 resulting in both the superior separation

70

performance and higher anti-fouling properties. Similarly, the co-deposition of hydrophilic and

71

antibacterial tannic acid-ferric ion-polyethylenimine/Ag (TA-Fe-PEI/Ag) complexes on the

72

TFC membrane surface can also improve the membrane performance comprehensively.25

73

Herein, we report a novel versatile modification for polyamide TFC FO membrane by the

74

controllable layer-by-layer assembly of hydrophilic phytic acid (PA) and antibacterial metal

75

ions for the first time. PA is a nontoxic, biocompatible and natural electrolyte with phosphate

76

acid groups attached symmetrically to a cyclohexamehexol ring, as shown in Fig. S1,3 which

77

exhibits high affinity towards water molecules and strong chelation capacity with various metal

78

ions. Insoluble superhydrophilic PA-metal (PA-M) complexes can therefore be formed via the

79

spontaneous assembly of PA and metal ions.26-31 To our best knowledge, no study has been

80

reported on the modification with the assembly of metal ions and polyelectrolyte multilayer to

81

construct superhydrophilic surfaces on FO membranes or TFC membranes yet. In this study,

82

silver and copper ions (Ag+ and Cu2+) are chosen to assemble with PA to perform a hydrophilic

83

and antibacterial surface modification on the polyamide TFC membrane. The successful

84

modification of the TFC membrane was confirmed by various techniques including FTIR, XPS,

85

SEM, AFM and EDX. The variations in the surface properties and FO performance of the 4


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modified TFC membrane with various PA concentrations and LBL cycles are investigated

87

systematically. The organic fouling and biofouling resistance of the modified TFC_PA-Ag and

88

TFC_PA-Cu membranes are studied comprehensively. This work is believed to provide a

89

feasible and universal approach to simultaneously improve the separation performance and

90

organic fouling/biofouling resistances of TFC membranes.

91 92

2. Experimental

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2.1 Materials. Polyethersulfone (PES, Mw = 200 kDa) was supplied by Hubei

94

Chushengwei Corporation. Trimesoyl chloride (TMC, purity ≥ 98%), m-phenylenediamine

95

(MPD, purity ≥ 99.5%), and phytic acid (PA, 70% aqueous solution) were purchased from

96

Aladdin Chemical Reagent Co. Ltd. Sodium chloride (NaCl, purity ≥ 99.5%), sodium

97

hydroxide (NaOH, purity ≥ 99.5%), cupper chloride (CuCl2, purity ≥ 99.5%), n-hexane (purity

98

≥ 99.5%), N-methyl pyrrolidone (NMP, purity ≥ 99.5%), polyethylene glycol (PEG 400, purity

99

≥ 99.5%), glutaraldehyde (50% aqueous solution) and nitric acid (HNO3, 65% aqueous solution)

100

were all provided by Sinophatm Chemical Reagent Co. Ltd. Silver nitrate (AgNO3, purity ≥

101

99.5%) was obtained from Shanghai Lingfeng Chemical Reagent Co. Ltd. Escherichia coli (E.

102

coli) was supplied by Beijing ComWin Biotech Co., Ltd. Luria-Bertani (LB) power was

103

purchased from Thermo Fisher Scientific. Deionized (DI) water was produced by a lab-scale

104

ultrapure water system (Wuhan Pin Guan).

105

2.2 Membrane fabrication. 2.2.1 Preparation of TFC Membrane. Porous PES

106

substrate was prepared by non-solvent induced phase inversion by casting a PES dope solution

107

(PES 18 wt%, PEG400 10 wt%, NMP 72 wt%) on a glass plate with a cast knife of a 100 μm

108

thickness. TFC membrane was fabricated with a typical IP process as described in our previous

109

works.5-6 In brief, PES substrate was immersed in 2 w/v% MPD aqueous solution for 2 min,

110

wiped dry with a rubber roller, and then brought to contact with 0.1 w/v% TMC/n-hexane 5



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solution for 1 min. The as-prepared TFC membrane was stored in DI water at 4 oC for 12 h

112

before further modification.

113

2.2.2 Modification of TFC Membrane. The TFC membrane was immersed in an inorganic

114

salt aqueous solution (0.12 M AgNO3 or 0.06 M CuCl2) for 10 min and then rinsed with DI

115

water, resulting in a TFC membrane anchored with metal ions. Later, the treated TFC

116

membrane was soaked in a PA solution (pH=7, adjusted by NaOH) and the inorganic salt

117

(AgNO3 or CuCl2) solution alternatively, which is denoted as one cycle. After each soaking

118

step, a thorough water rinsing was performed to remove the weakly bonded PA or metal ions

119

on the membrane surface. The modified TFC membranes were marked as TFC_PA-Ag and

120

TFC_PA-Cu according to the metal ions incorporated. The concentration of PA solution (0.01,

121

0.02, 0.04, 0.06 M) and the number of modification cycles (2, 4, 6 cycles) were varied to

122

prepare different TFC_PA-Ag and TFC_PA-Cu membranes.

123

2.3 Membrane Characterization. The surface chemistry of TFC membrane was

124

characterized by Fourier Transform Infrared Spectroscopy (FTIR, Brucker VERTEX-70) and

125

X-ray Photoelectron Spectroscopy (XPS, AXIS-ULTRA DLD-600W). The water contact

126

angle on the membrane surface was tested with a Geniometer (Kruss ZSA25) at ambient

127

conditions. The membrane surface morphologies were observed with a Scanning Electron

128

Microscope (SEM, Tescan VEGA 3 SBH). The element distribution on the membrane surface

129

was characterized by Energy Dispersive X-Ray Spectroscopy (EDX) (Inca X-max 50).

130

2.4 FO Performance Evaluation. The FO performance of TFC membranes was

131

evaluated with a cross-flow filtration setup, as demonstrated in our previous works.4 The test

132

was conducted at room temperature (25 ± 1C) with the fixed liquid flow rate of 300 mL/min

133

for both feed (DI water) and draw solution (2 M NaCl) sides in FO mode (active layer face the

134

feed solution). The calculation of water flux (Jw, LMH) was based on Eq. (1),

6


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135 136 137 138 139

JW =

∆m A × ∆t × ρ0

(1)

where Δm (g) is the weight change of the draw solution within a time interval Δt (h), A (m2) is the effective membrane area (3.96 cm-1), and ρ0 is the water density. The reverse solute flux (Js, gMH) was calculated with Eq. (2), JS =

(CtVt) - (C0V0) A × ∆t

(2)

140

where C0 and V0 are the initial concentration and volume of the feed solution, while Ct and

141

Vt are its concentration and volume at time t, respectively. The feed concentration was

142

determined using a calibrated conductivity meter (Mettler toledo, FE30).

143

2.5 Stability of PA-M Modified TFC Membranes. The stability of the modified

144

TFC membranes was assessed by determining the content of Ag or Cu released from TFC_PA-

145

M membranes. A circular membrane sample (diameter of 1 cm) was immersed in 10 mL DI

146

water with continuous powerful shaking for 5 days and water changed daily. The content of

147

Ag and Cu released into the DI water was determined by Atomic Absorption Spectroscopy

148

(AAS, AA300 Agilent Technologies). The total amount of Ag and Cu bonded on the TFC_PA-

149

M membrane was also determined by immersing a membrane sample in 10 mL 10 wt% HNO3

150

solution with continuous shaking for 1 week, followed by the determination of Ag and Cu

151

concentrations by AAS.

152

2.6 Anti-fouling Properties of TFC Membranes. 2.6.1 Biofouling Resistance of

153

TFC Membranes. E.coli, a representative microbe in the general water source, and with a rod-

154

like shape for easy experimental observation, was chosen as the model microbial foulant to

155

evaluate the anti-biofouling property of TFC membranes.

156

The inhibition zone test12 and the bacteria suspension test were conducted to evaluate the

157

anti-biofouling properties qualitatively with the methods reported in our previous works12-13.

158

The experimental details can be found in the Supporting information (S1.1 and S1.2). 7



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A dynamic biofouling test was also carried out according to previously reported methods.32-

159 160

33

161

NaCl draw solution (1~2 M) were employed to achieve an initial flux of approximate 15 LMH.

162

Afterwards, E. coli suspension was added into the feed solution to initiate the biofouling test,

163

where the bacteria concentration is about 7.5 × 107 CFU/L. During the test, the volume of feed

164

and draw solutions were 1.5 L, and other conditions were the same with FO performance test

165

mentioned in section 2.4. The weight change of the draw solution during the fouling test was

166

recorded with a balance connected to a computer, to obtain the real-time water flux of the TFC

167

membrane. More details can be found in the Supporting Information (S1.3).

Briefly, in a sterilized FO system, the synthetic wastewater feed solution (Table S1) and

168

2.6.2 Organic Fouling Resistance of TFC Membranes. The dynamic organic fouling test

169

of TFC membrane was evaluated under FO mode with sodium alginate synthetic wastewater

170

(200 ppm sodium alginate, 7 mM NaCl and 1 mM CaCl2) as the feed solution5, 34. After 18-h

171

fouling test, the synthetic waste water feed solution was replaced by DI water for a 2-h water

172

recovery test. The weight change of the draw solution during the recovery test was also

173

recorded to calculate the recovered water flux.

174 175 176

3. Results and Discussion 3.1 Preparation and Characterization of TFC_PA-M Membranes

177

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178 179

Fig. 1. Preparation of TFC_PA-M membranes.

180 181

In this study, the assembly of PA and antibacterial metal ions (Ag+ and Cu2+) was performed

182

for the surface modification of the polyamide TFC membrane, as illustrated in Fig. 1. The

183

abundant functional groups on the surface of the pristine TFC membrane, including amide

184

groups, carboxyl groups generated from acyl chloride hydrolysis, and terminal amino groups,

185

can work as electron pair donors and enable the embolization of Ag+ and Cu2+ onto the

186

polyamide layer by the strong chelation interaction and electronic attraction.35 Then the

187

immobilized metal ions on TFC membrane can provide the strong adhesion for the subsequent

188

self-assembly of PA-M complexes. PA molecule with six phosphonic acid groups of strong

189

chelation ability acts as the organic ligand, and the metal ion acts as the inorganic cross-linker,

190

inducing the assembly of hydrophilic and antibacterial PA-M (Ag or Cu) complexes on the

191

TFC membrane surface.28

192

To confirm the successful modification of the TFC membrane, FTIR spectra of the pristine

193

TFC membrane, PA-M modified TFC membranes and PA-M solids are investigated, as

194

exhibited in Fig. 2 (a). Compared with the pristine TFC membrane, a new peak at 965 cm-1 and 9



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195

the peak with enhanced intensity at 1058 cm-1, ascribed to the stretching vibrations of C-O-P

196

and P-OH groups in PA,36 can be observed in the spectra of TFC_PA-Ag and TFC_PA-Cu

197

membranes, as well as the corresponding PA-Ag and PA-Cu solids. In addition, the existence

198

of the characteristic peaks at 965 and 3300 cm-1 in the FTIR spectra of PA-M or TFC_PA-M

199

membranes indicates the presence of free hydroxyl groups.

200

(a)

(b)

TFC_PA-Ag

TFC_PA-Cu

(c) Pristine TFC

TFC_PA-Ag

TFC_PA-Cu

5um Ra=39.3 nm

(d)

Ra=49.3 nm

Ra=39.7 nm

Binding energy (eV)

P

Ag

P

Cu 18

201 202

Fig. 2. (a) FTIR spectra of pristine and PA-M modified TFC membranes, as well as PA-M

203

solids, (b) XPS spectra of TFC_PA-Ag and TFC_PA-Cu membranes, (c) Surface morphologies 10


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204

of different TFC membranes, and (d) EDX maps of P and M on TFC_PA-M membrane surface.

205

(TFC_PA-M membrane is prepared with 2 cycles of PA-M assembly and 0.02 M PA solution).

206 207

The surface chemical properties of TFC_PA-Ag and TFC_PA-Cu membranes are further

208

studied by XPS as demonstrated in Fig. 2 (b). The presences of P, Ag and Cu elements are all

209

detected, indicating the existence of PA-M complex on the surface of TFC_PA-M membranes.

210

In addition, the high resolution XPS spectra of different elements are further deconvoluted to

211

obtain the chemical states of above elements.37-38 For TFC_PA-Ag membrane, the peaks of O-

212

Ag bond (O 1s, 530.1 eV), P-O-Ag bond (P 2p, 135.0 eV) and Ag-O bond (Ag 3d3/2, 375.6 eV

213

and 3d2/5, 369.4 eV) verify the strong chelation interaction between Ag+ and PA, confirming

214

again the formation of PA-Ag complexes by the assembly of Ag+ and PA. Moreover, the

215

existence of N-Ag (N 1s, 401.2 eV), Ag-O=C (Ag 3d3/2, 375.6 eV and 3d5/2, 369.4 eV) and Ag-

216

N (Ag 3d3/2, 374.8 eV and 3d5/2, 368.5 eV) bonds suggests the interaction between Ag and

217

amide groups, carboxyl groups, and the residual terminal amine (from the pristine polyamide

218

selective layer), indicating the good adhesion of PA-Ag complexes on the TFC membrane

219

surface as proposed above. Similarly, for TFC_PA-Cu membrane, the successful deposition of

220

PA-Cu complexes is proved by the existence of O-Cu (O 1s, 530.0 eV), P-O-Cu (P 2p, 135.1

221

eV), Cu-O (Cu 2p, 933.9 eV) bonds; and the interaction between PA-Cu complexes and the

222

polyamide selective layer of TFC membrane can be ascertained by the characteristic peaks of

223

N-Cu (N 1s, 401.2 eV), Cu-O=C (Cu, 2p, 935 eV) and Cu-N (Cu 2p, 932.6 eV) bonds.

224

Meanwhile, the peak of P-OH (P 2p, 133.8 eV) bonds can be observed in the spectra of both

225

TFC_PA-Ag and TFC_PA-Cu membranes, implying the existence of free hydroxyl groups.

226

The SEM images in Fig. 2(c) demonstrate that all TFC membranes remain typical ridge-

227

and-valley structure, but additional granule-like structures on the surface of TFC_PA-Ag

228

membranes and amorphous floc-like structures on the surface of TFC_PA-Cu membrane can 11



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229

be observed, due to the aggregation of PA-M complexes.28,31 These morphologies are

230

consistent with the previous observation that the deposition of PA-M could form a thin film

231

with lots of microscale and nanoscale protuberances on a substrate 31. As for the morphology

232

difference between TFC_PA-Ag and TFC_PA-Cu membranes, it is probably ascribed to the

233

different structure feature of PA-M complex with different valence of metal ions.28 AFM

234

images also show that the surface roughness of TFC membranes increases after the deposition

235

of PA-M complexes. Furthermore, EDX element maps of the TFC_PA-M membrane surface

236

in Fig. 2 (d) demonstrate that both P and Ag/Cu elements distribute evenly on the membrane

237

surface, indicating the homogeneous coating of PA-M complex on the TFC membrane surface.

238

3.2 Manipulation of Properties and Separation Performance of TFC

239

Membranes. Owing to the controllability of the layer-by-layer assembly technique,29, 39 the

240

efficacy of PA-M modification can be regulated by varying the PA concentration and PA-M

241

assembly cycle. In this study, the surface properties and separation performance of the modified

242

TFC membranes with various PA concentrations and PA-M assembly cycles are investigated.

243

3.2.1 Membrane Properties. As shown in Fig. S2, with the increase in both PA concentration

244

and the cycle number of PA-M assembly, the granule-like structure on TFC_PA-Ag membrane

245

surface and the amorphous floc-like structure on TFC_PA-Cu membrane surface both become

246

more visible, because of the higher deposition amount of PA-M complex.

247

The deposition amount of PA-M complexes on the TFC membrane surfaces are further

248

characterized quantitatively by determining Ag and Cu loadings. As shown in Fig. 3 (a), the

249

metal loading increases with the increase in both PA concentration and cycle number of PA-M

250

assembly, consistent with the membrane morphology changes shown in Fig. S2. And it can

251

also be found that Cu loading is higher than Ag loading owing to the stronger chelation ability

252

of Cu2+ ions, which possess the larger binding strength to the phosphate groups than Ag+ ions,

253

and therefore benefits to the assembly of PA and Cu2+.40-42 And the stronger chelation ability 12


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254

of Cu2+ also ensures a stronger adhesion of PA-Cu with polyamide layer, resulting in more PA-

255

Cu complexes deposition on the TFC membrane surface.

256

2

Ag/Cu loading (mol /cm )

(a)

0.6 0.5

0.4

0.3

0.3

0.2

0.2

0.1

0.1 0.01

Water contact angle (o)

0.06

TFC_PA-Ag TFC_PA-Cu

65

0.0

55

50

50

45

45

40

40

35

35

0.06

6

65 60

0.04 0.01 0.02 PA concentraion (M)

4 Number of PA-M cycles

TFC_PA-Ag TFC_PA-Cu

70

55

0

2

75

60

30

257

0.04 0.02 PA concentration (M)

75 70

Ag@TFC_PA-Ag Cu@TFC_PA-Cu

0.5

0.4

0.0

(b)

0.6 Ag@TFC_PA-Ag Cu@TFC_PA-Cu

30

0

4 2 Number of PA-M cycles

6

258

Fig. 3. Effects of PA concentration and of PA-M assembly cycles on (a) the metal loading and

259

(b) the surface hydrophilicity of TFC_PA-M membranes.

23

260 261

The water contact angles of the TFC membranes with PA-M modification at different

262

conditions are also determined to evaluate the membrane surface hydrophilicity. As presented

263

in Fig. 3 (b), all PA-M modified TFC membranes show lower water contact angle than that of

264

the pristine TFC membrane, owing to the excellent hydrohilicity of PA-M complexes.31

265

Additionally, the water contact angle decreases continuously to with the increase of PA 13



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266

concentration and PA-M assembly cycles, consistent with the increasing deposition amounts

267

of PA-M complexes on the membrane surface as exhibited in Fig. 3 (a).

268

Besides, the mechanical properties of TFC membranes with different PA-M cycles are

269

determined as summarized in Table S2. It can be seen that the Young’s modulus and tensile

270

strength of TFC_PA-M membranes are higher than the pristine TFC membrane, indicating the

271

enhanced mechanical properties of TFC membranes with the additional PA-M complexes

272

deposited on the surface.

273 274

3.2.2 FO Performance of TFC_PA-M Membranes. The separation performances of

275

different membranes are evaluated under FO process. As demonstrated in Fig. 4, all TFC_PA-

276

Ag and TFC_PA-Cu membranes show enhanced Jw compared to the pristine TFC membrane,

277

ascribed to the improved membrane hydrophilicity. More detailedly, Fig. 4 (a) illustrates that

278

Jw elevates with PA concentration arising from 0.01 to 0.04 M, and then declines with the

279

further increase in PA concentration. Similarly, Fig. 4 (b) demonstrates that Jw increases

280

initially by increasing the cycle number of PA-M assembly from 2 to 4, and then decreases

281

after 4 assembly cycles. The up-and-down trends of Jw are resulted from two opposite factors.

282

The enhancement of membrane hydrophilicity with the increasing PA concentration and PA-

283

M assembly cycles contributes to the higher Jw, while the more PA-M deposition increases the

284

selective layer thickness and the water diffusion resistance, offsetting the improvement of Jw.

285

The highest Jw of 23.8 and 25.3 LMH can be achieved for TFC_PA-Ag and TFC_PA-Cu

286

membranes with 0.02 M PA and 4 PA-M assembly cycles, which are 57% and 68% higher than

287

that of the pristine TFC membrane, respectively. On the other side, Js of the TFC membrane

288

increases slightly after PA-M modification, which is probably due to the reduced negative

289

charge of the membrane surface by the chelation of metal ions with polyamide layer.7 The

290

Js/Jw ratio, the indicator of the membrane selectivity, remains in a low level for most modified 14


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291

TFC membranes, implying that PA-M modification improves the water flux of TFC membrane

292

without comprising the membrane selectivity, with suitable PA concentration and PA-M

293

assembly cycles.

(a) 35

TFC_PA-Ag TFC_PA-Cu

30 25

25

20

20

15 10 5 0

0

0.02 0.01 0.04 PA concentration (M)

20

Js (gMH)

Js (gMH)


6

TFC_PA-Ag TFC_PA-Cu

15

10

10

5

0

1.0

0.02 0.04 PA concentration (M) 0.01

0

0.06

0

1.0

TFC_PA-Ag TFC_PA-Cu

0.6 0.4


6

TFC_PA-Ag TFC_PA-Cu

0.8 Js/Jw (g/L)

0.8 Js/Jw (g/L)

0

TFC_PA-Ag TFC_PA-Cu

5

0.6 0.4 0.2

0.2

294

10

0

0.06

15

0.0

15

5

20

0

TFC_PA-Ag TFC_PA-Cu

30

Jw (LMH)

Jw (LMH)

(b) 35

0.0 0

0.04 0.01 0.02 PA concentration (M)

0.06

0


6

295

Fig. 4. Effects of (a) PA concentration (b) number of PA-M assembly cycles on the FO

296

performance of TFC_PA-M membranes (FO mode).

297 298

3.3 Anti-fouling Properties of TFC_PA-M Membrane. 3.3.1 Static fouling

299

Resistance. Since Ag+ and Cu2+ are broad-spectrum antibacterial agents, TFC_PA-Ag and

300

TFC_PA-Cu membranes are believed to possess superior anti-biofouling properties towards 15



Page 16 of 29

301

most common bacteria and biofilm formation.17, 29 Firstly, the antibacterial abilities of the TFC

302

membranes are evaluated by the inhibition zone test with E. coli as a model bacterium. As

303

shown in Fig. 5 (a), no inhibition zone appears around the pristine TFC membrane, while

304

obvious inhibition zones (marked with the yellow arrow) can be observed near all TFC_PA-

305

Ag and TFC_PA-Cu membranes, indicating that PA-Ag and PA-Cu modification endow the

306

TFC membrane with the antibacterial property successfully.14 Moreover, it can be found that

307

the inhibition zone around TFC_PA-Ag membrane is significantly larger than that around

308

TFC_PA-Cu membrane, implying the stronger antibacterial ability of TFC_PA-Ag membrane,

309

which is mainly attributed to the much lower minimum inhibition concentration of Ag+ than

310

that of Cu2+. 43-44 However, with the increase of PA concentration and PA-M assembly cycle,

311

although Cu and Ag loading on the membrane surface increase as shown in Fig. 3 (a), the size

312

of inhibition zones around TFC_PA-M membranes exhibit no significant difference, probably

313

because the amounts of Ag and Cu ions released from different membrane surfaces are similar

314

and have exceeded the inhibition threshold. Besides, the inhibition zone test is a qualitative

315

antibacterial characterization and cannot definitely quantify the antibacterial ability of the

316

membrane.

16


Page 17 of 29


(a)

a-1

a-3

a-2 TFC_PA-Cu

Pristine TFC

2 cycles

2 cycles

4 cycles 6 cycles

a-4 TFC_PA-Cu

a-5

0.01 M 0.02 M

(b)

TFC_PA-Cu

0.04 M 0.06 M

a-6

TFC_PA-Ag

4 cycles

TFC_PA-Ag

0.01 M 0.02 M

6 cycles

a-7

TFC_PA-Ag

0.04 M 0.06 M

(c)

Pristine TFCTFC_PA-Cu TFC_PA-Ag

317 318

Fig. 5. (a) Photographs of inhibition zone test against E. coli of (a-1) pristine TFC membrane,

319

(a-2, 3) TFC_PA-Cu and TFC_PA-Ag membranes with different assembly cycles, (a-4, 5)

320

TFC_PA-Cu membranes with different PA concentrations and (a-6, 7) TFC_PA-Ag

321

membranes with different PA concentrations; (b) Photographs of E. coli suspension incubated

322

with different TFC membranes (the marker “X” behind the centrifuge tube is used to indicate

323

the transparency); and (c) SEM micrographs of the TFC membrane surface after incubation in

324

E. coli suspension. (TFC_PA-M membrane is prepared with 4 cycles of PA-M assembly and

325

0.02 M PA solution in (b-c).)

27

326 327

Fig. 5 (b) shows the photos of E. coli suspensions incubated with different TFC membranes.

328

It can be found that E. coli suspension with the pristine TFC membrane exhibits the highest

329

turbidity due to the rapid reproduction of E. coli after incubation, while that with TFC_PA-Ag

330

and TFC_PA-Cu membranes are clearer, since the released Ag+ and Cu2+ ions from the

331

membranes inhibit the reproduction of E. coli bacteria. The surface of different TFC 17



Page 18 of 29

332

membranes after incubation in E. coli suspension is also observed under SEM. As shown in

333

Fig. 5 (c), lots of E. coli bacteria can be found on the surface of pristine TFC membrane, due

334

to E. coli reproduction in the suspension and the relative hydrophobic membrane surface which

335

is prone to absorb E. coli. By comparison, E. coli bacteria on TFC_PA-Ag and TFC_PA-Cu

336

membranes surface are found to be much fewer ascribed to the inhibited E. coli reproduction,

337

and the less adhesion of E. coli on the more hydrophilic membrane surface. Additionally, more

338

E. coli bacteria are observed on the surface of TFC_PA-Cu membrane than that of the

339

TFC_PA-Ag membrane in spite of the higher hydrophilicity of the former (Fig. 3 (b)), implying

340

that inhibiting the reproduction and growth of the bacteria is more effective to alleviate the

341

biofouling than simply reducing the bacteria adhesion.

342

3.3.2 Dynamic anti-fouling property. The dynamic fouling behavior of different TFC

343

membranes are further investigated by a long-term FO fouling test with E.coli and alginate as

344

model microbial and organic foulants, respectively. As demonstrated in Fig. 6 (a), with E. coli

345

suspension as the feed solution, the dynamic anti-biofouling property of different TFC

346

membranes follows an order of TFC_PA-Ag > TFC_PA-Cu > pristine TFC. While with the

347

organic foulant sodium alginate in the feed solution, as shown in Fig. 6 (b), the fouling

348

resistance of different TFC membranes follows an order of TFC_PA-Cu > TFC_PA-Ag >

349

pristine TFC. In both cases, TFC_PA-M membranes exhibit better fouling resistance than the

350

pristine TFC membrane. It is well-known that the fouling resistance is a combined result of

351

multiple factors. Although the deposition of PA-M complexes increases the surface roughness

352

(Fig. S3), the results above still reveal that the enhanced antibacterial ability and hydrophilicity

353

of the modified TFC membranes outweighs the unfavorably increased surface roughness.

354

Besides, it can also been found that TFC_PA-Ag and TFC_PA-Cu membranes behave

355

differently in the biofouling and organic fouling tests. TFC_PA-Ag membrane shows the

356

stronger biofouing resistance than TFC_PA-Cu membrane, indicating that the antibacterial 18


Page 19 of 29


357

capacity dominates the bio-fouling resistance. In contrary, TFC_PA-Cu membrane exhibits a

358

better organic fouling resistance than TFC_PA-Ag membrane, which reveals that the

359

hydrophilicity of membranes plays a more important role in mitigating organic fouling.

360

(a) 1.00

Pristine TFC TFC_PA-Ag TFC_PA-Cu

0.98

Normalized flux

Normalized flux

0.96 0.94 0.92 0.90 0.88 0.86 0

361

(b) 1.0

2

4

6

8 10 Time (h)

12

14

16

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

Pristine TFC TFC_PA-Ag TFC_PA-Cu 0

2

4

6

after cleaning

8 10 12 14 16 18 20 22 Time (h)

362

Fig. 6. Variation of the water flux of different TFC membranes with operation time during the

363

dynamic test with (a) E.coli and (b) sodium alginate as model foulants. (TFC_PA-M membrane

364

is prepared with 4 cycles of PA-M assembly and 0.02 M PA solution.)

365 366

3.4 Stability of PA-M Modification. A 24-h FO test is carried out to evaluate the

367

performance stability of TFC_PA-M membranes. As seen from Fig. S4, the water fluxes and

368

Js/Jw values of both TFC_PA-M membranes are quite stable during the 24-h test. In addition,

369

the concentrations of Ag and Cu accumulated in the feed solution after 24h are found to be

370

lower than the detection limitation of AAS, indicating the leach out of Cu2+ and Ag+ is

371

negligible. The durability of PA-M modification layer is further evaluated by immersing

372

TFC_PA-M membranes in DI water with continuous and powerful shaking.12, 14 The content

373

of Ag or Cu released to DI water is measured to indicate the amount of PA-M complexes

374

detached from TFC_PA-M membrane. As shown in Fig. 7, with the increase of the immersion

375

time, the amounts of Ag and Cu released from the membrane surface decrease, and are even 19


29


Page 20 of 29

lower than 0.12 and 0.07 ug/cm2 after 4 days. Accordingly, the remained percentages of Ag

377

and Cu on the membrane surface decrease slightly in the first 3 days and then reaches a

378

relatively steady state. It can be found that, the remained PA-Ag and PA-Cu on the membrane

379

surface are still higher than 90% after a 5-day violent shaking, indicating the good stability of

380

PA-M modification. In addition, the release rate of Cu from the TFC_PA-Cu membrane is

381

found to be lower than that of Ag from the corresponding TFC_PA-Ag membrane, indicating

382

the better stability of PA-Cu modification, which is because that PA-Cu complexes possess

383

stronger adhesion to the polyamide layer as aforementioned.40-42 Similarly, the remained Cu

384

percentage on the membrane surface is also higher than Ag percentage as shown in Fig. 7. To

385

quantify the duration of TFC_PA-M membranes, the lasting time of different TFC_PA-M

386

membrane is estimated as presented in Tables S3 and S4 and can be found to be longer than

387

300 days. 1.0

PA 0.02M 2 cycles PA 0.06M 2 cycles PA 0.02M 6 cycles

0.8 0.6 0.4 0.2 0.0

1

388

3 Time (day)

4

5

96 92 88 PA 0.02M 2 cycles PA 0.06M 2 cycles PA 0.02M 6 cycles

84 80

1

2

3 4 Time (day)

5

(a) PA_TFC-Ag

(a) TFC_PA-Ag 100

1.0


0.8 0.6 0.4 0.2 0.0

1

2 3 Time (day)

4

5

Percentage of remained Cu (%)

Content of released Cu (ug/cm2)

389

2

100

Percentage of remained Ag (%)

Content of released Ag (ug/cm2)

376


84 80

1

(b) PA_TFC-Cu


2

3 4 Time (day)

5

20

0.0

1

2

PA 0.02M 6 cycles

Perc

Con

Page 21 of 29

Environmental Science & 80Technology

3 Time (day)

4

5

1

2

3 4 Time (day)

5

390

1.0

100


0.8 0.6 0.4 0.2 0.0

1

2 3 Time (day)

4


84 80

1

2

3 4 Time (day)

5

(b) PA_TFC-Cu

(b) TFC_PA-Cu

391 392

5

Percentage of remained Cu (%)

Content of released Cu (ug/cm2)

(a) PA_TFC-Ag

Fig. 7. Results of the release test for (a) TFC_PA-Ag (b) TFC_PA-Cu membrane.

393 394

Besides, the release of Ag and Cu from the membrane surface may take the risk of water

395

contamination, since heavy metals are harmful to the environment and human being. WHO

396

guideline suggests that the concentrations of Ag and Cu in the drinking water should not exceed

397

0.1 and 2 mg/L, respectively.12,

398

membrane surface and Jw of corresponding TFC membranes, Ag and Cu concentrations in the

399

final product can be estimated, as listed in Table S5. It can be seen that released Ag and Cu

400

concentrations in the water product are in ranges of 0.001-0.013 and 0.001-0.010 mg/L

401

receptively, which are far lower than those suggested by WHO guideline, indicating the safety

402

of both TFC_PA-Ag TFC_PA-Cu membranes for water treatment applications.

45

According to the release rates of Ag and Cu from the

21



403

ASSOCIATED CONTENT

404

Supporting Information.

405

Page 22 of 29

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

406

http://pubs.acs.org.

407

Inhibition zone test, bacteria suspension test, dynamic biofouling test, chemical structure of

408

PA, EDX map of TFC_PA-M membranes, SEM morphologies of TFC_PA-M membranes,

409

AFM images of TFC membranes, long-term FO performance of TFC_PA-M membranes,

410

chemical composition of synthetic wastewater for the long-term biofouling test, mechanical

411

properties of TFC membranes with different PA-M cycles, estimated lasting time of the

412

antibacterial ability of TFC_PA-Ag membranes and the estimated concentration of Ag or Cu

413

in the water product.

414

Author Information

415

Corresponding Author

416

417

E-mail: Email address: [email protected] (Y. W.) Author Contributions

418

S. X. and Y. W. designed the experiments. S. X. and A. P. performed the experiments. S. X.,

419

S. X. and Y. W. wrote the paper. M. Y. conducted the biofouling characterization. All authors

420

discussed the results and commented on the manuscript. All authors have approved this

421

manuscript.

422

Notes

423

424

The authors declare no competing financial interest. Acknowledgements 22


Page 23 of 29


425

We thank the financial supports from National Key Technology Support Program (Grant

426

No. 2014BAD12B06), National Natural Science Foundation of China (Grant No. 21306058)

427

and Natural Science Foundation of Hubei Scientific Committee (2016CFA001). We would

428

also like to thank the Analysis and Testing Center, the Analysis and Testing Center of

429

Chemistry and Chemical Engineering School in Huazhong University of Science &

430

Technology for their help with material characterizations.

431

Abbreviations

432

TFC, thin film composite membrane; PA, phytic acid; LBL, layer-by-layer; PA-M, phytic

433

acid-metal complex; Jw, FO water flux; Js, reverse solute flux; Δm, mass change; A, effective

434

membrane area; C, concentration of feed solution; V, volume of feed solution; Δt, time interval;

435

ΔV, volume change.

436

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437

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Versatile Surface Modification of TFC Membrane by Layer-by-Layer

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