Highly Permeable Thin-Film Composite Forward Osmosis Membrane

Subscriber access provided by UNIV OF TASMANIA

Article

Highly Permeable Thin-film Composite Forward Osmosis Membrane Based on Carbon Nanotube Hollow Fiber Scaffold with Electrically Enhanced Fouling Resistance Xinfei Fan, Yanming Liu, Xie Quan, and Shuo Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05341 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology 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.

Page 1 of 23

Environmental Science & Technology

1

Highly Permeable Thin-film Composite Forward Osmosis Membrane Based

2

on Carbon Nanotube Hollow Fiber Scaffold with Electrically Enhanced

3

Fouling Resistance

4

Xinfei Fan, Yanming Liu, Xie Quan,* Shuo Chen

5

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

6

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

7

116024, China

8

*Corresponding author e-mail: [email protected]

9

1

ACS Paragon Plus Environment


10

Abstract：：Forward osmosis (FO) is an emerging approach in water treatment, but its application is

11

restricted by severe internal concentration polarization (ICP) and low flux. In this work, a

12

self-sustained carbon nanotube hollow fiber scaffold supported polyamide thin film composite (CNT

13

TFC-FO) membrane was firstly proposed with high porosity, good hydrophilicity and excellent

14

electro-conductivity. It showed a specific structure parameter as low as 126 µm, suggesting its

15

weakened ICP. Against a pure water feed using 2.0 M NaCl draw solution, its fluxes were 4.7 and 3.6

16

times as high as those of the commercial cellulose triacetate TFC-FO membrane in the FO and

17

pressure retarded osmosis (PRO) modes, respectively. Meanwhile, the membrane showed excellent

18

electrically assisted resistance to organic and microbial fouling. Its flux was improved by about 50%

19

during oil-water simulation separation under 2.0 V voltage. These results indicate that the CNT

20

TFC-FO membrane opens up a frontier for stably and effectively recycling potable water from

21

electrochemical FO process.

22

Keywords: fouling mitigation, water treatment, forward osmosis, carbon nanotube, electrochemistry

23

TOC Art:

24

2


Page 2 of 23

Page 3 of 23

25


Introduction

26

Water scarcity is one of the global crises in the 21th century, and membrane separation provides

27

promising solutions for addressing this worldwide issue. Among various membrane technologies,

28

forward osmosis (FO) has attracted growing attention in recent years to providing clean water.1-3 FO

29

is an osmotically driven membrane process with no or low hydraulic pressure requirement.

30

Compared to pressure-driven membrane processes, FO delivers the advantages of low energy

31

consumption, efficient water recovery, low fouling propensity, and easy fouling removal.4 However,

32

despite the significant application advancements, FO has not been widely used in industries primarily

33

due to the absence of high-performance membranes.

34

The conventional FO membranes are typically asymmetric in a thin-film composite (TFC)

35

membrane structure.5-8 These TFC membranes are composed of a densely active layer on the top of a

36

porous sublayer. As a result, the design of TFC membranes offers the possibility to optimize their

37

active layer and sublayer separately, which favors improving the performance of the final TFC-FO

38

membranes.9 Specially, the FO sublayers should possess a low structural parameter (S) to reduce the

39

internal concentration polarization (ICP) phenomenon.4, 10 For the FO processes, ICP is recognized

40

as the predominantly responsible for the decline in both effective osmotic driving force and water

41

production. It can reduce more than 80% flux at higher draw solution concentration. However, the

42

conventional sublayers, which are fabricated via phase inversion method, usually present isolated

43

channels in a tortuous 1D architecture.11 Such structure is one of the bottlenecks for developing

44

high-flux FO membranes because it would form an un-turbulent barrier to the diffusions of draw

45

solute and water.

46

Recently, constructing 3D architecture with interconnected pores opens a fascinating insight into 3



Page 4 of 23

47

developing high-flux TFC-FO membrane. For example, the electrospun scaffold-like nanofibrous

48

matrixes usually present high porosity (σ), low tortuosity (τ) and easily controllable thickness (t).12-14

49

These unique structure features ensure the electrospun nanofibrous matrixes with a potentially low S

50

value (S=σ·τ/t). As the ICP degree is positively relates to S, constructing TFC-FO membranes on

51

electrospun scaffold-like sublayers provides an efficient approach for breaking the ICP bottleneck

52

and achieving high FO flux. However, despite low fouling tendency during the FO processes, a

53

higher flux enables a greater hydrodynamic force. The increased hydrodynamic force would drag the

54

foulants toward to the membrane surface and then cause more severe membrane fouling. Finally,

55

these fouling, particularly during wastewater treatment, in turn lead to an additional hydraulic

56

resistance which then lowers the effective osmotic pressure and water permeability. Developing

57

novel strategies to mitigate membrane fouling is a long-active area in the FO processes.4,

10

58

According to the electrically enhanced fouling resistance on pressure-driven membranes,15,

16

59

applying a small external potential might be an alternative and scatheless approach to significant

60

fouling mitigation on the FO membranes. However, one critical challenge is the electrospun

61

polymeric matrixes are intrinsically insulate.

62

In the past decades, carbon nanotubes (CNTs) with great aspect ratio have been widely employed

63

in preparing nanofibrous mats for pressure-driven membrane applications.17-19 In addition to the

64

competitive S value comparing to electrospun flat sheets, such membranes with randomly entangled

65

CNT meshes possess fascinating electrical/electrochemical properties. Moreover, the non-woven

66

networks of entangled CNTs has been recently fabricated into a hollow fiber architecture composed

67

of finger-like macrovoids and interconnected pores.20, 21 For FO processes, hollow fiber membranes

68

are desirable because of their high packing density for small footprint systems. Therefore, these 4


Page 5 of 23


69

material and structure uniqueness might ensure the CNT hollow fiber as an alternative sublayer for

70

developing FO membrane with high flux and good fouling resistance under electrochemical

71

assistance.

72

To verify the feasibility of this hypothesis, a TFC-FO membrane was fabricated by coating a

73

polyamide (PA) active layer onto a CNT hollow fiber scaffold (CNT TFC-FO) through wet-spinning

74

and interfacial polymerization methods. Its FO performance was investigated in comparison with a

75

standard commercial cellulose triacetate (CTA) TFC-FO membrane and a polyether sulfone (PES)

76

TFC-FO membranes. The effects of electrochemical assistance on the organic fouling, biofouling and

77

gypsum scaling were also studied under a low DC voltage.

78

Materials and Methods

79

Materials and Chemicals. CNTs (multi-walled carbon nanotubes, diameter: 60~100 nm; length:

80

5~15 µm) were supplied by Shenzhen Nanotech Port Co. Ltd. 1,3-phenylenediamine (MPD, >99%),

81

triethylamine and 1,3,5-benzenetricarbonyl trichloride (TMC, 98%) were purchased from

82

Sigma-Aldrich. N,N-dimethylacetamide (DMAc, anhydrous, 99.8%), sodium chloride (NaCl),

83

sulfuric acid (H2SO4, 98%), nitric acid (HNO3, 65%), and polyvinyl butyral (PVB) were obtained

84

from Sinopharm Chemical Reagent Co., Ltd. Unless otherwise specified, all chemicals with

85

analytical grade were used as received without further purification. Additionally, the commercially

86

asymmetric CTA FTC-FO membrane with 50 µm thickness was acquired from Hydration

87

Technologies Inc. (Albany, Oregon), as it has received popular recognition as the benchmark

88

membrane during the FO studies.

89

Preparation of CNT Hollow Fiber Sublayer. The CNT hollow fiber sublayer was prepared by

90

the wet-spinning method (Figure S1).21 Briefly, the original CNTs were oxidized by H2SO4/HNO3 5



91

solution. Then, 1.0 g oxidized CNTs were dispersed homogeneously in 8.5 g DMAc solution with

92

0.5 g PVB to yield dope suspension (Figure S2). After degasification, the dope was dispensed

93

through a home-made spinneret by an injection pump, and directly immersed into a water

94

coagulation bath. The bore-fluid was DMAc/water solution with volume ratio of 75/25. Finally, the

95

obtained hollow fiber sublayer was dried and calcinated at 1000 °C for 2 h in N2 atmosphere.

96

Preparation of CNT TFC-FO Hollow Fiber Membranes by Interfacial Polymerization.

97

Interfacial polymerization was performed on the outer surface of the CNT hollow fiber sublayer. In

98

brief, the CNT sublayer was immersed into isopropyl alcohol for better wetting. After rinsed with DI

99

water, the sublayer was immersed in 2.5 w/v% MPD aqueous solution for 120 s. Then, the excess

100

MPD solution was removed from the saturated sublayers by an air knife. After sealed both ends by

101

resin, a 0.15 w/v% TMC solution with n-hexane as solvent was brought into contact with the outer

102

surfaces of the MPD saturated sublayer. After reaction for 60 s to form an ultrathin PA layer, the

103

nascent membrane was taken out and cured in DI water at 90 °C for 120 s, then rinsed with a 200

104

ppm NaOCl aqueous solutions for 120 s, followed by rinsing with a 1000 ppm NaHSO3 aqueous

105

solutions for 30 s. Finally, the prepared CNT TFC-FO hollow fiber membranes were cured at 90 °C

106

for 300 s and then stored in DI water at 4 °C.

107

Characterization of CNT Sublayer and CNT TFC-FO Membrane. The microstructure and

108

surface morphology of both CNT sublayer and CNT TFC-FO membrane were characterized on a

109

scanning electron microscopy (SEM, Hitachi S4800). Fourier transform infrared spectrum (FT-IR,

110

Bruker Vertex 70) and X-ray photoelectron spectroscopy (XPS, EscaLab 250i) were used to analyze

111

the component of the sublayer and TFC-FO membrane. For the CNT sublayer, its measurements of

112

pure water flux, pore size distribution, porosity, water contact angle and electric conductivity were 6


Page 6 of 23

Page 7 of 23

113


described in Supporting Information.

114

According to the reported standard protocols,22-26 water permeability and salt permeability were

115

evaluated by testing the membrane under the pressurized cross-flow RO modes. Briefly, the water

116

permeability coefficient (A) was determined from pure water flux (25 °C) under the applied

117

trans-membrane pressure (∆P) of 0~1.0 bar for hollow fiber membranes and 0~8.0 bar for plate sheet

118

membrane, respectively. The salt rejection (Rs) was obtained from conductivity measurements of the

119

feed and permeate water by filtering 500 mg/L NaCl solution at 1.0 bar for hollow fiber membranes,

120

while 2000 mg/L NaCl solution at 8.0 bar for plate sheet. The salt permeability coefficients (B),

121

which is an intrinsic property of FO membranes, were calculated based on the average rejection

122

value (3 replicates) at a given pressure using the solution-diffusion theory:6

123

124

1− R   J  B = Jw ⋅   ⋅ exp − w   R   k  where ∆π denotes the osmotic pressure difference across the membrane.

125

Lab-Scale Forward Osmosis and Anti-fouling Tests under Electrochemical Assistance. The

126

forward osmosis tests were carried out in a lab-scale cross-flow filtration module. The feed and draw

127

solutions were kept at 25 °C and fed concurrently into the membrane module at flow rate of 25 cm/s.

128

Each membrane was tested under both PRO mode (active layer facing draw solution) and FO mode

129

(active layer facing feed solution). The change in feed solution weight was monitored by a balance.

130 131

The water flux (Jw, L/(m2·h), abbreviated LMH) was calculated as follows: Jw =

∆m ρ ⋅ M ⋅ ∆t

132

where ∆m (kg) is the weight change of feed solution over a predetermined time ∆t (h) in the FO

133

process; M is the effective membrane surface area (m2) and ρ is the water density of 1.0 kg/L.

134

Salt concentration in feed was determined by conductivity measurement using a NaCl calibration 7



135

curve. Salt reverse-diffusion, Js (g/(m2·h), gMH) was calculated from the increase of feed

136

conductivity: Js =

137

( C t ⋅ mt − C 0 ⋅ m0 ) ρ ⋅ M ⋅ ∆t

138

where Ct and C0 denote the salt concentration (g/L) after and before FO tests, mt and m0 are the

139

weights (kg) of the feed after and before FO test, respectively.

140

The organic fouling test was performed by treating feed water containing 100 mg/L humic acid

141

(HA), 1.0 mM CaCl2 and 10 mM NaCl. The biofouling test was carried out by using LB solution

142

containing 3× 107 cfu/mL. Gypsum scaling was tested by feed water containing 35 mM CaCl2, 20

143

mM Na2SO4, and 19 mM NaCl (gypsum saturation index (SI) of 1.3). The adsorbed HA, E. coli and

144

gypsum on the fouled membrane surface were observed by SEM. The oil/water separation was

145

performed by treating feed water containing 50000 mg/L emulsified oil with 2.0 wt% surfactant.

146

After operating for 6 h, the oil was desorbed from membrane and re-suspended in water, which was

147

then measured by a total organic matter (TOC) analyzer (TOC-VCPH, Shimadzu). The zeta potential

148

of emulated oil droplets was measured by Malvem nano-ZS90. During the electrically assisted FO

149

processes, the membrane worked as cathode and a Ti mesh as anode (distance of 3 mm). The voltage

150

was supplied by an outer DC power.

151

Results and Discussion

152

Properties of CNT Sublayer. According to the SEM images in Figure 1, a hollow fiber structure

153

with macro-voids sandwiching from inner to outer layers can be observed. This structure arises from

154

the instantaneous polymer precipitation of PVB contacting with coagulation bath. It keeps consistent

155

with those of conventional polymeric membranes from wetting spinning.25, 26 After PVB pyrolysis at

156

high temperature, structure collapses did not occur. Moreover, the entangled CNTs constructs the 8


Page 8 of 23

Page 9 of 23


157

hollow fiber architecture with a feature of interconnected pores like electrospun fibrious matrixes

158

(Figure 1c~e). Attributed to the co-presence of macro-voids and interconnected scaffold, the CNT

159

sublayer presents a high porosity of 93%. This value is much higher than 60~80% of the polymeric

160

hollow fibers. In addition, the pore size of 194 nm suggests that the CNT sublayer is a microfiltration

161

membrane. A water contact angle (WCA) of 37° indicates that it possesses a good hydrophilic

162

property. A high pure water flux of about 5000 LMH at 0.6 bar implies its low membrane resistance

163

for water transport. The result of mechanical strength reveals the CNT sublayer has a measured

164

tensile strength of 4.4 MPa, higher than that of the electrospun polymeric sublayer.14 Therefore, the

165

prepared CNT sublayer possesses desirable properties (e.g. high porosity, good hydrophilicity and

166

low water transport resistance) as potential sublayer for developing high-flux TFC-FO membrane.9, 27

167 168 169

Figure 1. SEM images of CNT hollow fiber: cross-section (a~c), inner surface (d) and outer surface (e).

Properties of TFC-FO Hollow Fiber Membrane. The complete TFC-FO hollow fiber membrane 9



170

is comprised of a salt-rejecting PA active layer interfacially polymerized on the outer surface of the

171

CNT hollow fiber. As presented in Figure 2a, the outer-view SEM image of the TFC-FO membrane

172

shows a typical ridge-and-valley morphology like the conventional PA layer.5, 8 Moreover, the PA

173

active layer (thickness: about 210 nm) is integrally bonded with CNTs (Figure 2b), indicating strong

174

bonding between PA active layer and its sublayer. In addition, the cross-section and inner surface of

175

the final TFC-FO membrane display a similar interconnected porous structure to the original CNT

176

sublayer (Figure S6). These results indicate that the sublayer was not affected during interfacial

177

polymerization except for a thin PA layer deposited on its outer surface.

178 179

Figure 2. SEM images of outer-view (a) and cross-section (b) of TFC-FO membrane based on CNT hollow fiber

180

sublayer, FT-IR (c) and XPS (d) spectrum of CNT sublayer and final CNT TFC-FO membrane.

181

To further verify PA active layer formation, both the final TFC-FO membrane and original CNT

182

sublayer were characterized by FT-IR and XPS. Compared with the original CNT sublayer, the

183

formation of PA active layer is evidenced by the appearance of some additional absorption peaks in

184

the FT-IR spectrum of the TFC-FO membrane (Figure 2c). The new peaks at 1541 and 1672 cm-1

185

correspond to the bending vibration of –N-H (amide II peak) and stretching vibration of –C=O

186

(amide I peak), respectively. The absorption peak at 1613 cm-1 is ascribed to the aromatic ring 10


Page 10 of 23

Page 11 of 23


187

breathing vibration in the PA molecule. All these results confirm the success of PA layer formation in

188

the TFC-FO membrane, which was further evidenced by XPS spectrum. As presented in Figure 2d,

189

an additional N1s peak, arising from the nitrogen element of the amide group in the PA layer, appears

190

at the binding energy of 400 eV after the interfacial polymerization.

191

FO Water Flux and Salt Reverse Transport. To evaluate the performance of CNT TFC-FO

192

membrane, both osmotic water fluxes (Jw) and reverse salt fluxes (Js) were investigated under FO

193

and PRO modes by using 2.0 M NaCl as draw solution against a DI water feed. Meanwhile, a

194

commercial CTA TFC-FO planar membrane (without macrovoids and interconnected pore structure)

195

and PES TFC-FO hollow fiber membrane (with macrovoids but no scaffold structure) were taken as

196

the control membranes to reflect the sublayer structure effect on the FO performance (Figure 3 and

197

S7). According to the experimental results (Figure 3a and 3b), the CNT TFC-FO membrane displays

198

high Jw of 61.0 and 81.9 LMH with relatively low Js of 8.8 and 11.2 gMH for respective FO and

199

PRO mode. In the case of PES TFC-FO membrane, although it exhibits lower Js of 5.9 and 9.5 gMH,

200

its Jw is only 30.7 and 59.0 LMH for FO and PRO modes, respectively. Specific salt flux (Js/Jw) is

201

usually used to evaluate the FO performance by determining the amount of draw solute loss per unit

202

of water production.14, 28 It is noteworthy that the Js/Jw of CNT TFC-FO membrane (FO: 0.144 g/L,

203

PRO: 0.137 g/L) is lower than those of the PES TFC-FO membrane (FO: 0.192 g/L, PRO: 0.161

204

g/L). The lower Js/Jw ratio reflects a better FO efficiency on the CNT TFC-FO membrane. As their

205

active layers were prepared in the same method, the difference in FO performance between CNT and

206

PES TFC-FO membranes might arise from their sublayers. It can be found in Table S2 that the

207

porosity of CNT sublayer (93%) is much higher than that of the PES sublayer (79%). Such high

208

porosity is attributed to that the CNT sublayer possesses both macro-voids and interconnected mesh 11



209

structures, while PES sublayer only with macro-voids. On the other hand, the CNT sublayer presents

210

a WCA (37°) lower than the PES sublayer (62°), suggesting the CNT sublayer is more hydrophilic.

211

The high hydrophilicity is because the CNTs were oxidized by acid treatment before sublayer

212

preparation. Such treatment can introduce hydrophilic oxygen-containing groups (e.g. ‒COOH, ‒OH,

213

etc.) onto the CNT surface. According to the reported works,9, 13, 14, 29, 30 both high porosity and good

214

hydrophilicity are favorable to a lower water diffusion resistance and a higher concentration gradient

215

between the two sides of the active layer on the FO membranes. As a result, such fascinating

216

properties of CNT sublayer might be the primary reason that endows the CNT TFC-FO membrane

217

with enhanced mass transfer and improved water permeability (Figure 3c and 3d). To confirm this,

218

the structural parameter (S) is calculated to express the contribution of the sublayers on the ICP

219

based on the intrinsic properties (including A, Rs and B) of the two TFC-FO membranes (Table 1). It

220

is noteworthy that the CNT TFC-FO membrane possesses a significantly low S of 126 µm, which is

221

nearly 1/3 of that of the PES TFC-FO membrane. As S is an intrinsic membrane property positively

222

indicating the ICP degree, the result implies that a weaker ICP effect formed in the CNT hollow fiber

223

sublayer. Combined with the flux comparison, the weakening ICP is substantiated to contribute to

224

higher flux on the CNT TFC-FO membrane. Thus, the higher flux on the CNT TFC-FO membrane is

225

evident from its lower S.

12


Page 12 of 23

Page 13 of 23


226 227

Figure 3. Water flux (a) and reverse salt flux (b) of the TFC-FO membranes (Conditions: 2.0 M NaCl as the draw

228

solution against a DI water feed at the same crossflow velocity of 25 cm/s and temperature of 25 °C), schematic

229

images of ICP in the CNT TFC-FO (c) and PES TFC-FO (d) membranes.

230

Table 1. Transport properties and structural parameters of TFC-FO membranes Water permeabilitya

Salt rejectionb

Salt permeability

Structural parameterc

(A, L/(m2·h·bar))

(Rs, %)

(B, L/(m2·h))

(S, µm)

CNT

2.45 ± 0.10

92.6 ± 1.4

0.119 ± 0.041

125.57 ± 7.51

PES

1.93 ± 0.09

90.7 ± 0.9

0.573 ± 0.106

324.18 ± 28.87

CA

0.68 ± 0.01

88.4 ± 1.2

0.120 ± 0.026

634.26 ± 41.51

Sample

a

A was obtained at 0~1.5 bar for CNT (PES) samples, and 0~8.0 bar for CA sample (Figure S8). b Rs was measured

at a fixed crossflow velocity of 25 cm/s using pressure of 1.0 bar for CNT (PES) samples (500 mg/L NaCl feed), and 8.0 bar for CA sample (2000 mg/L NaCl feed). c S was calculated based on experiments in FO mode using 1.0 M NaCl as draw solution and deionized water as feed. All experiments were performed at 25 °C. 231

On the other hand, both TFC-FO hollow fiber membranes showed much better performance than

232

the commercial CTA TFC-FO planar membrane under both FO and PRO modes. Particularly, the 13



233

CNT TFC-FO membrane presents S of 6 times lower than the standard CTA TFC-FO membrane. Its

234

Jw values of 4.7 times higher than the commercial CTA TFC-FO membrane in the FO mode, and 3.6

235

times higher in the PRO mode. Meanwhile, the Js/Jw of CNT TFC-FO membrane is only about 1/4

236

and 1/5 of those of standard CTA TFC-FO membrane under FO and PRO modes, respectively. A

237

quick comparison to the reported TFC-FO membranes is also taken and presented in Table S3. It can

238

be found that the CNT TFC-FO hollow fiber membrane displays the FO performance better than or

239

comparable to those reported TFC-FO membranes. Therefore, this work establishes the potential

240

prospect of using CNT hollow fiber scaffold as a new sublayer for interficially polymerized TFC-FO

241

membranes with low ICP and high flux.

242

Fouling Resistance of FO membranes. Membrane fouling is an inevitable problem in all

243

membrane processes, including FO. Thus, the fouling resistance to organic fouling, biofouling and

244

gypsum scaling has been investigated on the CNT TFC-FO membrane, and then taken in comparison

245

with the two control membranes. Figure 4 displays that the fluxes on the three membranes can reach

246

the steady state in 24 h operation for all fouling tests. After the fluxes reaching the steady state, the

247

flux loss on the three FO membranes are less than 20%, 31% and 18% for organic fouling,

248

biofouling and gypsum scaling, respectively. These results keep in consistent with the fact of FO

249

with low fouling tendency under FO mode.4,

250

membrane can reach steady state after 12, 20 and 16 h during the organic fouling, microbial fouling

251

and gypsum scaling tests, respectively. These values are lower than those on the two control

252

membranes. Moreover, the CNT TFC-FO membrane not only presents less flux loss, but also shows

253

better flux recovery than the PES TFC-FO membrane in all fouling tests. According to the reported

254

works,31-34 CNTs can be used to change the membrane wettability and surface charges for improving

10

Meanwhile, the fluxes on the CNT TFC-FO

14


Page 14 of 23

Page 15 of 23


255

fouling resistance. This might be the reason that results in the high antifouling performance on the

256

CNT TFC-FO membrane over PES TFC-FO membrane. In the case of CTA TFC-FO planar

257

membrane with the lowest flux loss and best flux recovery, the primary reason is that its initial water

258

flux is much less than those of the other two TFC-FO hollow fiber membranes. Its lowest initial flux

259

results in minimal fouling. In addition, compared to CTA, the PA active layer on the CNT TFC-FO

260

and PES TFC-FO membranes is more susceptible to foulant adsorption due to its higher surface

261

heterogeneity. However, the CNT TFC-FO membrane presents steady fluxes of 4.6, 4.3 and 4.6 times

262

higher than the CTA TFC-FO membrane in the organic fouling, biofouling, and gypsum scaling tests,

263

respectively (Figure S9). Therefore, the CNT TFC-FO membrane not only possesses high flux, but

264

also has relatively good antifouling ability. (c)100

90 80 70 CTA CNT PES

60

(e)100 Normalized Flux (%)

Normalized Flux (%)

90 80 70 CTA CNT PES

60

50

50 0

4

8

12

16

20

24

4

8

Time (h) After cleaning After fouling

100

90

80

265

12

(d)

16

20

CTA CNT PES

60

0

4

90

80

12

16

20

24

(f)

After cleaning After fouling

100

90

80

70 CTA

CNT

8

Time (h)

70 PES

70

24

After cleaning After fouling

100

70 CTA

80

Time (h) Normalized Flux (%)

Normalized Flux (%)

(b)

90

50 0

Normalized Flux (%)

Normalized Flux (%)

(a)100

PES

Membrane

CNT

Membrane

CTA

PES

CNT

Membrane

266

Figure 4. Comparison of organic fouling (a) and cleaning (b), biofouling (c) and cleaning (d), gypsum scaling (e)

267

and cleaning (f) in FO mode. (Organic foulant: 100 mg/L HA in 10 mM NaCl solution. Bio-foulant: 3×107 cfu/mL

268

E. coli LB suspension. Gypsum scaling solution: 35 mM CaCl2, 20 mM Na2SO4, and 19 mM NaCl, gypsum

269

saturation index (SI) of 1.3. FO conditions: crossflow velocity of 15 cm/s and the temperature of 25 °C for both

270

feed and 2 M NaCl drawn solution. Cleaning: DI water (25 °C) as feed to rinse membrane after 12 h fouling tests

271

for 30 min at a crossflow velocity of 25 cm/s.) 15



272

Antifouling Performance of CNT TFC-FO Membrane under Electrochemical Assistance.

273

Enhancing fouling resistance can decrease the frequency of membrane cleaning and replacement,

274

then lowering the economic consumption. As the newly developed CNT TFC-FO membrane

275

possesses a good electric conductivity of 1500 S/m, an integration of FO with electrochemistry was

276

performed to evaluate the electrochemical effects on its antifouling performance. Prior to the

277

antifouling tests, water splitting and flux change under electrochemical assistance were investigated

278

by treating DI water feed. The results suggested no gas evolution and flux changes occurring on the

279

membrane even under 2.0 V voltage (Figure S10 and S11). As expected, the flux loss declines with

280

increasing the voltages on the membrane for organic fouling under the FO mode (Figure 5a). When

281

the applied voltage is 2.0 V, the membrane presents an inapparent flux loss even after 12 h. By

282

contrast, the flux loss was 14.6% under the open circuit condition. These results suggest that organic

283

fouling can be significantly suppressed under the electrochemical assistance. To understand this

284

phenomenon, the fouled membranes were observed by SEM. It can be found that an obvious organic

285

fouling layer has formed on the membrane surface under the open circuit condition. In contrast, the

286

electrically assisted membrane presents similar architecture as the fresh membrane. These results

287

imply the organic foulant accumulation on the membrane surface is significantly mitigated by the

288

electrochemical assistance. It is well known that the HA molecule is negatively charged with

289

electrophoretic mobility of -3.3 × 10-8 m2/(V·s) (10 mM NaCl, pH 7).35 Thus, the electrostatic

290

repulsive force between the HA and cathodic membrane would be enhanced by increasing the

291

voltages. The enhanced repulsive force then prevents the HA attaching to the membrane surface.

292

Furthermore, the effect of electrical field force on the foulant was also considered in the experiment.

293

By calculation in SI, the critical field strength (Ecritical, V/cm), that can counterbalance the convective 16


Page 16 of 23

Page 17 of 23


294

migration of HA toward membrane, is about 5.1 V/cm. As the electric field strength is about 6.7

295

V/cm at 2.0 V voltage, electrical field force is evidenced to drive HA away from membrane surface.

296

To further evaluate the electrically assisted fouling resistance on CNT TFC-FO membrane, both

297

biofouling and gypsum scaling tests were performed. Interestingly, the flux loss also declines to 3.0 %

298

at 2.0 V from 12.9 % under open circuit conditions (Figure 5d) during the biofouling tests (Figure

299

5d). According to the SEM images of the fouled membrane surface, bacterial adhesion occurred on

300

the membrane under the open circuit conditions (Figure 5e), but did not happen on the electrically

301

assisted membrane (Figure 5f). Such results suggest that the electrochemical assistance can also

302

inhibit biofouling formation. As E. coli presents an electrophoretic mobility of -4.7 × 10-8

303

m2/(V·s)),36 both electric field force and electric repelling can prevent microbial movement toward to

304

the membrane surface, mitigating biofouling formation. Unfortunately, the gypsum scaling cannot be

305

inhibited by the electrochemical assistance. More flux loss and gypsum formation occur at higher

306

voltages (Figure 5g, 5h and 5i). According to Mi’s work,37 the PA active layer can form complexes

307

with Ca2+ ions, initiating the formation of gypsum prenucleation clusters and subsequently

308

amorphous gypsum particles on the PA membrane surface. When the membrane worked as the

309

cathode, more Ca2+ ions might be dragged toward to the negatively charged membrane surface,

310

resulting in more severe gypsum scaling (Figure 5h and 5f). Thus, the electrically assisted fouling

311

mitigation in FO was mainly dominated by electrostatic interaction and electric field force (Figure 6),

312

which was further confirmed by the flux changes on anodic membrane in the fouling tests (Figure

313

S12).

17



Page 18 of 23

314 315

Figure 5. Normalized flux and SEM images of electrically assisted fouling resistance to organical (a~c), microbial

316

(d~f) and scaling (g~i) fouling under FO mode. (Organic foulant: 100 mg/L HA in 10 mM NaCl solution.

317

Bio-foulant: 3×107 cfu/mL E. coli LB suspension. Gypsum scaling solution: 35 mM CaCl2, 20 mM Na2SO4, and 19

318

mM NaCl, gypsum saturation index (SI) of 1.3. FO conditions: crossflow velocity of 15 cm/s and the temperature

319

of 25 °C for both feed and 2 M NaCl drawn solution.) (+)

Feed

Draw

(–)

Feed

Feed (+)

(–)

(a) FO E. coli

320 321

Ca2+

Shearing force

(–)

Draw

(+)

Feed (–)

(+)

(b) Electrically assisted FO HA

Gypsum

Ti mesh

PA layer

Hydrodynamic force

CNT sublayer Electric field force

Figure 6. Mechanism of FO (a) and electrically assisted FO (b) processes.

322

Compared to FO mode, PRO mode usually presents much higher water flux at the same osmotic

323

pressure because of its low ICP.4, 10 However, the application of PRO mode is limited due to its

324

higher fouling tendency than FO mode. Therefore, it would be significant if the PRO mode presented 18


Page 19 of 23


325

good antifouling ability for more water production. Here, the antifouling performance in both

326

organic fouling and biofouling tests were investigated in the PRO mode under electrochemical

327

assistance. As expected, the flux loss in both fouling tests were significantly mitigated. These results

328

suggest that the electrically assisted fouling resistance can also achieve in the PRO mode (Figure 7a

329

and 7b). It is because the electric repelling and electric field force dominate the foulant movement

330

away from sublayer surface, like that occurred in FO mode. This would prevent the foulant

331

accumulation and pore blocking. Consequently, the electrochemical assistance also endows the PRO

332

mode with good antifouling ability that provides higher water production over FO mode in water

333

treatment processes. To extend this application, the electrically assisted FO process was applied to

334

separate oil-water mixture in the PRO mode. Figure 7c depicts that the water flux of the FO alone

335

quickly declines to only 36% of its initial value after 6 h. It is consistent with the fast fouling

336

propensity on FO membrane under the PRO mode during oily wastewater.38, 39 This phenomenon

337

might arise from that the oil droplets can easily deposit on the surface and/or entrance into the porous

338

substrate, resulting in pore blocking and water flux loss. Interestingly, the water flux is improved on

339

the cathodic CNT TFC-FO hollow fiber membrane at 2.0 V voltage. Its water flux is still 88% of its

340

initial value after 6 h, suggesting the oil fouling is significantly mitigated by the electrochemical

341

assistance. These results are confirmed by analyzing the amount of TOC on the fouled membrane

342

surfaces. As presented in Figure 7d, the TOC values of desorb and re-suspend oil from

343

electrochemically assisted FO membranes are much lower than that from FO membrane alone. These

344

results can be also attributed to the enhanced electric field force and repulsion interaction between

345

the negatively charged oil droplets (-27 mV) and membrane cathode. Moreover, the electrically

346

assisted FO process showed an acceptably energy consumption of 0.6 kWh/ton water under 19



Page 20 of 23

347

electrochemical assistance (Supporting Information). Therefore, this work provides an alternative

348

sublayer for interfacially polymerized TFC-FO membrane with low ICP and high flux. Meanwhile, it

349

gives a useful insight in applying such membrane with electrically assisted fouling resistance to

350

organic and microbial fouling. 100

(b) Normalized flux (%)

Normalized flux (%)

(a) 100 90

80

70

90 80 70 60 50

Cathode: 2.0 V Open circuit 60

Cathode: 2.0 V Open circuit

40

0

2

4

6

8

10

12

0

2

4

6

Time (h)

(d)

90 80 70 60 Cathode: 2.0 V Open circuit

40 30 0

351

2

4

6

8

10

12

Normalized TOC of fouled oil (%)

Normalized flux (%)

(c) 100

50

8

10

12

Time (h) 100 80 60 40 20 0 Open circuit

Time (h)

Cathode: 2.0 V

Condition

352

Figure 7. Normalized flux of electrically assisted antifouling tests under PRO mode: organic fouling (a), biofouling

353

(b), oil/water emulsion separation (c) and normalized TOC of fouled oil on membrane surface (d). (Organic foulant:

354

100 mg/L HA in 10 mM NaCl solution. Bio-foulant: 3×107 cfu/mL E. coli in LB suspension. Oil/water emulsion

355

solution: 50000 mg/L oil/water emulsion with 2.0 wt% surfactant and 10 mM NaCl. FO conditions: crossflow

356

velocity of 15 cm/s and the temperature of 25 °C for both feed and 1 M NaCl drawn solution.)

357

Associated Content

358

Supporting Information. Preparation of PES sublayer; schematic diagram of wetting spinning;

359

characterization and optimization of CNT sublayer; outer surface and cross-section SEM images of

360

CNT TFC-FO membrane; property comparisons for the three TFC-FO membranes; electrochemical

361

properties of CNT TFC-FO membrane; flux changes on anodic membrane; comparison between the

362

prepared and reported FO membranes; energy consumption calculation. This material is available

363

free of charge via the Internet at http://pubs.acs.org.

364

Author Information 20


Page 21 of 23


365

Corresponding Author E-mail: [email protected]

366

Notes

367

The authors declare no competing financial interests.

368

Acknowledgment

369

This work was supported by the National Natural Science Foundation of China (No. 21437001 and

370

51708085) and the China Postdoctoral Science Foundation (No. 2016M601314).

371

References

372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401

1.

Coday, B. D.; Yaffe, B. G. M.; Xu, P.; Cath, T. Y., Rejection of trace organic compounds by forward osmosis membranes: A literature review. Environ. Sci. Technol. 2014, 48, 3612-3624.

2.

Lutchmiah, K.; Verliefde, A. R. D.; Roest, K.; Rietveld, L. C.; Cornelissen, E. R., Forward osmosis for application in wastewater treatment: A review. Water Res. 2014, 58, 179-197.

3.

Valladares Linares, R.; Li, Z.; Sarp, S.; Bucs, S. S.; Amy, G.; Vrouwenvelder, J. S., Forward osmosis niches in

4.

Zhao, S.; Zou, L.; Tang, C. Y.; Mulcahy, D., Recent developments in forward osmosis: Opportunities and challenges.

seawater desalination and wastewater reuse. Water Res. 2014, 66, 122-139. J. Membr. Sci. 2012, 396, 1-21. 5.

Sukitpaneenit, P.; Chung, T. S., High performance thin-film composite forward osmosis hollow fiber membranes with macrovoid-free and highly porous structure for sustainable water production. Environ. Sci. Technol. 2012, 46, 7358-7365.

6.

Yip, N. Y.; Tiraferri, A.; Phillip, W. A.; Schiffman, J. D.; Elimelech, M., High performance thin-film composite forward osmosis membrane. Environ. Sci. Technol. 2010, 44, 3812-3818.

7.

You, S.; Tang, C.; Yu, C.; Wang, X.; Zhang, J.; Han, J.; Gan, Y.; Ren, N., Forward osmosis with a novel thin-film

8.

Zhong, P.; Fu, X.; Chung, T.S.; Weber, M.; Maletzko, C., Development of thin-film composite forward osmosis

inorganic membrane. Environ. Sci. Technol. 2013, 47, 8733-8742. hollow fiber membranes using direct sulfonated polyphenylenesulfone (sPPSU) as membrane substrates. Environ. Sci. Technol. 2013, 47, (13), 7430-7436. 9.

Lu, X. L.; Chavez, L. H. A.; Castrillon, S. R. V.; Ma, J.; Elimelech, M., Influence of active layer and support layer surface structures on organic fouling propensity of thin-film composite forward osmosis membranes. Environ. Sci. Technol. 2015, 49, (3), 1436-1444.

10. Klaysom, C.; Cath, T. Y.; Depuydt, T.; Vankelecom, I. F. J., Forward and pressure retarded osmosis: potential solutions for global challenges in energy and water supply. Chem. Soc. Rev. 2013, 42, 6959-6989. 11. Qin, D.; Liu, Z.; Bai, H.; Sun, D., Three-dimensional architecture constructed by graphene oxide nanosheets polymer composite for high-flux forward osmosis membranes. J. Mater. Chem. A 2017, 5, 12183-12192. 12. Bui, N. N.; Lind, M. L.; Hoek, E. M. V.; McCutcheon, J. R., Electrospun nanofiber supported thin film composite membranes for engineered osmosis. J. Membr. Sci. 2011, 385–386, 10-19. 13. Song, X.; Liu, Z.; Sun, D. D., Nano gives the answer: Breaking the bottleneck of internal concentration polarization with a nanofiber composite forward osmosis membrane for a high water production rate. Adv. Mater. 2011, 23, 3256-3260. 21



402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445

14. Bui, N. N.; McCutcheon, J. R., Hydrophilic nanofibers as new supports for thin film composite membranes for engineered osmosis. Environ. Sci. Technol. 2013, 47, 1761-1769. 15. de Lannoy, C. F.; Jassby, D.; Gloe, K.; Gordon, A. D.; Wiesner, M. R., Aquatic biofouling prevention by electrically charged nanocomposite polymer thin film membranes. Environ. Sci. Technol. 2013, 47, 2760-2768. 16. Sun, X.; Wu, J.; Chen, Z.; Su, X.; Hinds, B. J., Fouling characteristics and electrochemical recovery of carbon nanotube membranes. Adv. Funct. Mater. 2013, 23, 1500-1506. 17. Rahaman, M. S.; Vecitis, C. D.; Elimelech, M., Electrochemical carbon-nanotube filter performance toward virus removal and inactivation in the presence of natural organic matter. Environ. Sci. Technol. 2012, 46, 1556-1564. 18. Gao, G.; Vecitis, C. D., Electrochemical carbon nanotube filter oxidative performance as a function of surface chemistry. Environ. Sci. Technol. 2011, 45, 9726-9734. 19. Li, H.; Gui, X.; Zhang, L.; Wang, S.; Ji, C.; Wei, J.; Wang, K.; Zhu, H.; Wu, D.; Cao, A., Carbon nanotube sponge filters for trapping nanoparticles and dye molecules from water. Chem. Commun. 2010, 46, 7966-7968. 20. Wei, G.; Yu, H.; Quan, X.; Chen, S.; Zhao, H.; Fan, X., Constructing all carbon nanotube hollow fiber membranes with improved performance in separation and antifouling for water treatment. Environ. Sci. Technol. 2014, 48, 8062-8068. 21. Fan, X.; Liu, Y.; Quan, X.; Zhao, H.; Chen, S.; Yi, G.; Du, L., High desalination permeability, wetting and fouling resistance on superhydrophobic carbon nanotube hollow fiber membrane under self-powered electrochemical assistance. J. Membr. Sci. 2016, 514, 501-509. 22. Cath, T. Y.; Elimelech, M.; McCutcheon, J. R.; McGinnis, R. L.; Achilli, A.; Anastasio, D.; Brady, A. R.; Childress, A. E.; Farr, I. V.; Hancock, N. T.; Lampi, J.; Nghiem, L. D.; Xie, M.; Yip, N. Y., Standard methodology for evaluating membrane performance in osmotically driven membrane processes. Desalination 2013, 312, 31-38. 23. Ren, J.; McCutcheon, J. R., A new commercial thin film composite membrane for forward osmosis. Desalination 2014, 343, 187-193. 24. Setiawan, L.; Wang, R.; Li, K.; Fane, A. G., Fabrication of novel poly(amide–imide) forward osmosis hollow fiber membranes with a positively charged nanofiltration-like selective layer. J. Membr. Sci. 2011, 369, 196-205. 25. Chou, S.; Shi, L.; Wang, R.; Tang, C. Y.; Qiu, C.; Fane, A. G., Characteristics and potential applications of a novel forward osmosis hollow fiber membrane. Desalination 2010, 261, 365-372. 26. Rong, W.; Lei, S.; Tang, C. Y.; Shuren, C.; Changquan, Q.; Fane, A. G., Characterization of novel forward osmosis hollow fiber membranes. J. Membr. Sci. 2010, 355, (1-2), 158-167. 27. Huang, L. W.; McCutcheon, J. R., Impact of support layer pore size on performance of thin film composite membranes for forward osmosis. J. Membr. Sci. 2015, 483, 25-33. 28. Yasukawa, M.; Mishima, S.; Shibuya, M.; Saeki, D.; Takahashi, T.; Miyoshi, T.; Matsuyama, H., Preparation of a forward osmosis membrane using a highly porous polyketone microfiltration membrane as a novel support. J. Membr. Sci. 2015, 487, 51-59. 29. Han, G.; Chung, T. S.; Toriida, M.; Tamai, S., Thin-film composite forward osmosis membranes with novel hydrophilic supports for desalination. J. Membr. Sci. 2012, 423, 543-555. 30. Huang, L.; Nhu-Ngoc, B.; Meyering, M. T.; Hamlin, T. J.; McCutcheon, J. R., Novel hydrophilic nylon 6,6 microfiltration membrane supported thin film composite membranes for engineered osmosis. J. Membr. Sci. 2013, 437, 141-149. 31. Goh, K.; Setiawan, L.; Wei, L.; Jiang, W.; Wang, R.; Chen, Y., Fabrication of novel functionalized multi-walled carbon nanotube immobilized hollow fiber membranes for enhanced performance in forward osmosis process. J. Membr. Sci. 2013, 446, 244-254. 32. Zheng, J.; Li, M.; Yu, K.; Hu, J.; Zhang, X.; Wang, L., Sulfonated multiwall carbon nanotubes assisted thin-film nanocomposite membrane with enhanced water flux and anti-fouling property. J. Membr. Sci. 2017, 524, 344-353. 22


Page 22 of 23

Page 23 of 23

446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462


33. Dumée, L.; Lee, J.; Sears, K.; Tardy, B.; Duke, M.; Gray, S., Fabrication of thin film composite poly(amide)-carbon-nanotube supported membranes for enhanced performance in osmotically driven desalination systems. J. Membr. Sci. 2013, 427, 422-430. 34. Davood Abadi Farahani, M. H.; Hua, D.; Chung, T. S., Cross-linked mixed matrix membranes consisting of carboxyl-functionalized multi-walled carbon nanotubes and P84 polyimide for organic solvent nanofiltration (OSN). Sep. Purif. Technol. 2017, 186, 243-254. 35. Tsai, Y. T.; Yu C. L., A.; Weng, Y. H.; Li, K. C., Treatment of perfluorinated chemicals by electro-microfiltration. Environ. Sci. Technol. 2010, 44, 7914-7920. 36. Sonohara, R.; Muramatsu, N.; Ohshima, H.; Kondo, T., Difference in surface-properties between escherichia-coli and staphylococcus-aureus as revealed by electrophoretic mobility measurements. Biophys. Chem. 1995, 55, 273-277. 37. Mi, B. X.; Elimelech, M., Gypsum scaling and cleaning in forward osmosis: Measurements and mechanisms. Environ. Sci. Technol. 2010, 44, (6), 2022-2028. 38. Duong, P. H. H.; Chung, T. S.; Wei, S.; Irish, L., Highly permeable double-skinned forward osmosis membranes for anti-fouling in the emulsified oil-water separation process. Environ. Sci. Technol. 2014, 48, 4537-4545. 39. Han, G.; de Wit, J. S.; Chung, T. S., Water reclamation from emulsified oily wastewater via effective forward osmosis hollow fiber membranes under the PRO mode. Water Res. 2015, 81, 54-63.

463

23


Highly Permeable Thin-Film Composite Forward Osmosis Membrane

Recommend Documents