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Dual-Bioinspired Design for Constructing Membranes with Superhydrophobicity for Direct Contact Membrane Distillation Zhigao Zhu, Yuanren Liu, Haoqing Hou, Wenxin Shi, Fangshu Qu, Fuyi Cui, and Wei Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06227 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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Environmental Science & Technology

1

Dual-Bioinspired Design for Constructing Membranes with

2

Superhydrophobicity for Direct Contact Membrane Distillation

3 4

Zhigao Zhua, Yuanren Liua, Haoqing Houb, Wenxin Shia, Fangshu Qua, Fuyi Cuia, Wei

5

Wanga,*

6 7

a

8

School of Environment, Harbin Institute of Technology, Harbin 150090, P. R. China

9

b

10

State Key Laboratory of Urban Water Resource and Environment (SKLUWRE),

Department of Chemistry and Chemical Engineering, Jiangxi Normal University,

Nanchang 330022, P. R. China

11 12

*Corresponding

13

Email address: [email protected] (W. Wang)

Author

14 15

1

ACS Paragon Plus Environment


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ABSTRACT: Water flux and durability are the two critical parameters that closely

17

associated with the practical application of membrane distillation (MD). Herein, we

18

report a facile approach to fabricate superhydrophobic polyimide nanofibrous

19

membranes (PI NFMs) with hierarchical structures, interconnected pores and high

20

porosity, which was derived from the electrospinning, dual-bioinspired design and

21

fluorination

22

polydopamine/polyethyleneimine (PDA/PEI) composite was firstly linked onto

23

membrane substrates, and then assembled lotus leaf hierarchical structure by binding

24

the negatively charged silica nanoparticles (SiO2 NPs) via electrostatic attraction. The

25

resultant superhydrophobic PI NFMs exhibits a water contact angle of 152o, robust

26

hot water resistance of 85 oC and high water entry pressure of 42 kPa. Moreover, the

27

membrane with omniphobicity presents high water flux over 31 L m-2 h-1 and high

28

salts rejection of ~100% as well as robust durability for treating high salinity

29

wastewater containing typical low surface tension and dissolved contaminants

30

(∆T=40 oC). Significantly, the novel dual-bioinspired method can be used as a

31

universal tool to modify various materials with hierarchical structures, which is

32

expected to provide more effective alternative membranes for MD and even for other

33

selective wetting separation fields.

processes.

Bio-inspired

adhesive

34

2


based

on

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INTRODUCTION

36

The ever-increasing geographical inequalities and deterioration of water quality

37

impose an urgent task to develop appropriate technologies for wastewater treatment

38

and make good utilization of abundant seawater resources.1-4 Among various

39

technologies, membrane distillation (MD) using a hydrophobic microporous

40

membrane has emerged as a promising method because it can utilize low-grade or

41

waste heat to generate high-quality water with high recovery and high rejection of

42

salts (100% in theory).5-7 MD is driven by a vapor pressure gradient induced by the

43

different temperatures between a hot feed stream and a cold collected stream. The

44

hydrophobic membrane aims at resisting the permeation of water droplets and

45

allowing the transmission of water vapor, thus achieving the purpose of purification of

46

seawater or treatment of wastewater.8

47

Water flux and durability are the two critical parameters that closely associated

48

with the practical application of MD for seawater desalination and wastewater

49

treatment.9 Water flux is highly determined by the porous structure of the hydrophobic

50

membrane.10-12 Various methods including mechanical fibrillation, template and melt

51

blown, phase inversion as well as electrospinning methods have been employed to

52

fabricate microporous membranes.13-16 It has been well-recognized that high porosity,

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narrow pore size distribution and low tortuosity structure can maximum increase the

54

water vapor transmission.17-18 Durability of the MD membrane is another crucial issue,

55

which is highly related to the hydrophobicity of membranes.19 An increase in

56

hydrophobicity would slow the rate of crack formation and the capillary attraction of 3



57

the water into the pore, thus reducing the surface pore wetting and wicking and

58

maintaining the MD membrane stability.19 Some commercial hydrophobic

59

microfiltration membranes were firstly used for MD, but most of them are generally

60

suboptimal and suffer from progressive membrane wetting in the MD process.20 In

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recent years, two other methods have been developed for the design of

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superhydrophobic membranes. Both of them are based on the roughness construction

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and grafting or mixing with low surface energy materials. The first one is that the

64

nanoparticles and hydrophobic agent was directly mixed with the as-prepared

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solutions for membrane preparation, but most nanoparticles embedded in the

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membrane matrix greatly reduced the utilization of nanoparticles for roughness

67

construction.21-22 Moreover, the existence of wrinkles, nano-protrusions and

68

nanoparticles may become the flaws of microporous membranes, thus deteriorating

69

their physical and chemical properties.23-24 The second one is post treatment strategy

70

for superhydrophobic surface modification of membrane.25 For example, polymer

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porous membrane was firstly etched by chemicals to form reactive functional groups,

72

then served as substrates to construct membrane roughness.26-27 During the above

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processes, the structure of polymeric substrates, to some extent, were destroyed, and

74

then affect the physical robustness of the MD membrane. Thus, it still requires a green

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method that can construct MD membrane with robust interconnected porous structure

76

and excellent durability for MD performance.

77

Electrospun nanofibrous membranes constructing from randomly accumulation

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of nanofibers leaded to an interconnected porous structure.28-30 Therefore, the 4


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electrospun products with high porosity have potential application in MD process due

80

to ease transmission of water vapor across the hydrophobic membranes.18,31 Various

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chemical/coating methods have been used to construct nanofibrous membranes with

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roughness, but most of them are subjected to the damage of membrane as mentioned

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above. Recently, inspired by the unique adhesion and wettability of mussels in nature,

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polydopamine (PDA) containing catechol can serve as either the starting points or

85

covalent modification with desired molecules during the in situ polymerization

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process.32-34 The coating layer can inspire the membrane surface with functional

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groups without destroying the substrate. In our design, dopamine plays

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dual-functional roles as both the adhesive and the starting points to graft positively

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charged PEI.35,36 After that, the positively charged nanofibrous membrane can

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uniformly and compactly bind the negatively charged SiO2 NPs with excellent

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stability via electrostatic attraction. However, the studies on fabrication of such

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dual-bioinspired membranes with hierarchical roughness for MD or selective wetting

93

membrane are rather scarce.

94

Inspired by the mussel adhesive protein chemistry and lotus leaf hierarchical

95

structure (Figure 1 (a & b)), we presented a universal method to modify materials

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with hierarchical structures without destroying the properties of the parent material

97

structure. PI (polyimide), a traditional special engineering plastic, exhibits superior

98

thermal stability, high mechanical property as well as good hydrophobic

99

performance.37 Significantly, the excellent chemical resistance of PI plays key roles in

100

desalination or wastewater treatment.38 As a result, we rationally designed a 5



101

superhydrophobic PI nanofibrous membrane (PI NFMs) via electrospinning technique,

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electrostatic attraction and low surface energy modification. The detailed procedures

103

are shown in Figure 1c. The resultant interconnected porous structured membrane

104

presents superhydrophobic surface with a water contact angle (WCA) of 152o and

105

robust hot water resistance by simply turning the concentrations of colloidal SiO2. In

106

addition, the superhydrophobic nanofibrous membrane exhibited a high water entry

107

pressure of 42 kPa, which is finely fitted with the theoretical value calculating from

108

Young-Laplace equation. Ultimately, the superhydrophobic membranes demonstrated

109

a stable performance for treating high salinity wastewater containing typical low

110

surface tension and dissolved contaminants, which would be an appropriate candidate

111

for MD membrane especially for multi-component salinity wastewater purification.

112

Most importantly, the green dual-bioinspired method can be used as a universal

113

method for designing materials with roughness construction, demonstrating the

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potential applications in various selective separations.

115 116

Figure 1 (a) The in situ polymerization of DA and PEI. (b) Photograph and FE-SEM 6


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images of Lotus leaf. (c) Schematic illustrating the procedures for preparation of

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superhydrophobic PI NFMs with a lotus-leaf-like structure; the photographs showing

119

the large-scale PI nanofibrous membrane.

120

MATERIALS AND METHODS

121

Materials. 4,4'-oxydianiline (ODA), pyromellitic dianhydride (PMDA), trimethoxy

122

(heptadecafluorotetrahydrodecyl)-triethoxysilane

123

(DMAc) were purchased from Aladdin Chemical Regent Company. Dopamine (DA),

124

polyethyleneimine (PEI, Mw=600), methylene blue (MeB), ethanol (99 wt%), and

125

sodium dodecylbenzene sulfonate (SDBS) were purchased from Shanghai Runjie

126

Chemical Reagent Company, China. Silica nanoparticles with an effective surface

127

area of 198-250 m2/g (Ludox SM, 30 wt%) was purchased from Sigma-Aldrich

128

Company. Sodium hydroxide (NaOH) and hydrochloric (HCl) were purchased from

129

Xilong Chemical Company, China. Sodium chloride (NaCl), potassium chloride

130

(KCl), magnesium chloride (MgCl2), magnesium sulfate (MgSO4), calcium sulfate

131

(CaSO4) were purchased from Tianjin Benchmark Chemical Reagent Company, China.

132

The crude oil and the soya-bean oil were obtained from the 5th Daqing oil Production

133

Factory and Jiusan Oils & Grains Industries Group Co., Ltd, respectively. The

134

lubricating oil (L-DVB) was purchased from Japan Mitsubishi Heavy Industries Co.,

135

Ltd. The commercial PTFE and PVDF membrane were purchased from Haining

136

Lianzhong Filter Equipment Technology Co., Ltd, and Merck Millipore, respectively.

137

All chemicals were used as received without further purification.

138

Preparation of PI Nanofibrous Membranes. The pristine poly (amic acid) (PAA)

(FAS),

7


N,N-dimethylacetamide


139

solution was synthesized using PMDA and ODA through polycondensation

140

reaction.39 Generally, 2.7 g of PMDA and 2.7g of ODA were dissolved in 24.6 g of

141

DMAc solution with continuously stirring at 0 oC for 12 h. The PAA NFMs was

142

fabricated via electrospinning using the pristine PAA solution with the solid content

143

of 18 wt%. Typically, 10 mL of PAA solution was loaded in a syringe at a fixed

144

voltage of 15 kV and a feed rate of 0.25 mL h-1 with a work distance of 20 cm

145

between tip of the needle and the collector. The fabrication chamber temperature and

146

humidity were fixed at 23±1 oC and 45±3%, respectively. All the collected

147

electrospun nanofibrous membranes were dried at 60 oC in a vacuum for 3 h to

148

remove the residual solvent and then thermally imidized to obtain PI nanofibers

149

heating up at a rate of 5 oC min-1 to 100, 200 and 300 oC at each temperature stage for

150

30 min.

151

Surface Modification of PI NFMs. The PI NFMs was modified with aqueous

152

solution of DA (2 mg mL-1) and PEI (6 mg mL-1) in l0 mM Tris-buffer.36 The DA/PEI

153

solution was stirred at room temperature for 24 h in order to perfectly cover the PI

154

nanofiber with core-shell structure. The PI NFMs after PDA/PEI modification were

155

rinsed thoroughly with deionized water at least three times, following dried in a

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vacuum oven at 50 °C for 6 h. The positively charged PI NFMs was then immersed in

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various concentrations of Ludox SM for 3 h in acetate buffer with an ionic strength of

158

~1 mM. The pH of the suspension was adjusted to 4.5 to accelerate the electrostatic

159

attraction between the positively charged PI nanofibers and the negatively charged

160

SiO2 NPs. After the SiO2 NPs were successfully anchored on the PI nanofibers 8


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surface, the composite membrane was then immersed in 1 wt% of FAS in ethanol

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solution for 12 h to low the membrane surface energy. After that, the fluorinated

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SiO2-PDA/PEI@PI NFMs were subjected to heat treatment at 120 oC for 2 h.

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Characterization. The morphologies of fibers were observed by a field emission

165

scanning electron microscope (FE-SEM) (Zeiss SUPRA 55 SAPPHIRE) and

166

transmission electron microscopy (TEM) (JEM-2100F, JEOL Ltd). The elemental

167

compositions of the membranes were investigated by Fourier transform infrared

168

(FT-IR) spectra with attenuated total reflectance (ATR) (PerkinElmer Spectrum). Zeta

169

potential of membranes surface were evaluated by a streaming potential analyzer

170

using streaming current measurements (SurPASS, Anton Paar GmbH, Australia).

171

Measurements were performed with a solution containing 1 mmol/L KCl. The

172

near-surface chemical information of the materials was analyzed by X-ray

173

photoelectron spectroscopy (XPS, K-Alpha, Al Kα radiation). The surface area of the

174

samples was derived from N2 sorption measurements using an automatic micropore

175

physisorption analyzer (Tristar 3020, USA). The pore size distribution was

176

characterized via a bubble point method using a capillary flow porometer

177

(CFP-1100ai, Porous Materials InC., USA). The mechanical property of relevant

178

samples was performed on a tensile tester (XQ-1C, Shanghai New Fiber Instrument

179

Co., Ltd., China). Water contact angles (WCAs) of 3 µL were performed using a

180

contact angle goniometer (Kino SL200B). The liquid entry pressure was measured

181

using a capillary flow porometer (POROLUX 1000, Germany). The surface

182

roughness of single fibers were measured by atomic force microscope with a scan 9



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area of 1 µm ×1 µm (AFM, Nanoscope Ⅳ, Digital instruments).

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MD Performance Tests. The membrane performances were tested using a lab-scale

185

direct contact membrane distillation (DCMD) system with an effective membrane

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area of 16 cm2 as shown in Figure S1. The feed and permeate solution were cycled

187

through the flat-sheet membrane cell. The feed solutions were prepared with various

188

concentrations of salinity water and operated at desired temperatures. The simulated

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reverse osmosis concentrated water was composed of 32.61 g/L of NaCl, 1.03 g/L of

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KCl, 4.52 g/L of MgCl, 2.91 g/L of MgSO4 and 1.80 g/L of CaSO4. Meanwhile, the

191

temperature of cold permeate side was always fixed at 20 oC with a conductivity

192

below 5 µS/cm circling by a peristaltic pump.

193

RESULTS AND DISCUSSION

194 195

Figure 2 (a) Zeta potential as a function of pH of colloidal silica and PI NFMs

196

modified by PDA and PDA/PEI, respectively. (b) ATR-FTIR spectra and (c-f) TEM

197

images

198

F-SiO2-PDA/PEI@PI NFMs. (h) FE-SEM and (i) AFM images of single nanofiber of

of

PI

based

nanofibrous

membranes.

10


(g)

FE-SEM

image

of

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F-SiO2-PDA/PEI@PI, respectively. (j) Cross-section image of F-SiO2-PDA/PEI@PI

200

NFMs.

201

Surface Properties and Morphologies of Modified Nanofibrous Membrane. To

202

obtain a superhydrophobic membrane surface for durable MD with high water flux

203

and salt rejection, hierarchical roughness and surface chemistry are the two crucial

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factors in determining the anti-wetting characteristic.40,41 In the present work, SiO2

205

NPs were used to decorate PI nanofibers with uniform roughness because the SiO2

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NPs with abundance hydroxyl functional groups allow surface fluorination via

207

well-established silane chemistry.6,42 Electrostatic attraction has been considered as

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one of the best ways to uniform disperses SiO2 NPs onto the nanofiber surface owing

209

to its ease of operation and perfect uniform dispersion. However, both the PI NFMs

210

and SiO2 NPs possess negative surface charge in the pH values ranging from 3 to 11,

211

which were monitored by zeta potential as shown in Figure 2a. In order to obtain the

212

positively charged membrane, the PI membrane was firstly modified by in situ

213

polymerization of DA/PEI to generate positive amine functional groups. The

214

PDA/PEI@PI NFMs exhibited a superhydrophilic surface with a water contact angle

215

of 0o, and increased the positive zeta potential of the PI NFMs with an extrapolated

216

isoelectric points of～6.5 mV. Mechanism for the in situ polymerization of DA/PEI is

217

well clarified in our previous study as shown in Figure 1a.36 For comparison, the PDA

218

coated PI NFMs was found still to be negatively charged in the entire pH range

219

because the as-prepared PI NFMs prepared from electrospinning technique was more

220

negatively charged than PDA, which was not beneficial for electrostatic attraction. 11



221

Fortunately, during the polymerization of DA, the catechol of DA can serve as the

222

starting points to crosslink with PEI. Thus, the modified PI NFMs with positive

223

charge coating (PDA/PEI) can bind the negatively charged SiO2 NPs via electrostatic

224

attraction, and then graft FAS to low the surface energy of the nanofibrous

225

membranes.6,42

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The surface modification of PI NFMs with PDA/PEI, SiO2 NPs and FAS were

227

measured by attenuated total reflectance Fourier transform infrared (ATR-FTIR)

228

spectroscopy measurement and X-ray photoelectron spectroscopy (XPS) as shown in

229

Figure 2b and Figure S2. The formation of covalent bonds between catechol moieties

230

of PDA and amines of PEI through Michael addition reaction can also remarkably

231

enhance the acid resistance in the following roughness construction in acid condition

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(pH=4). FT-IR and XPS analyses for the polymerization of DA/PEI are shown in

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Figure 2b and Figure S2. After the electrostatic attraction with SiO2 NPs, a new peak

234

rose at 1110 cm-1 was assigned to silanol groups of SiO2 NPs.27 In addition, the

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surface fluorination of SiO2-PDA/PEI@PI NFMs could also be confirmed by the

236

enhanced absorbance intensity at 1140 cm-1, corresponding to -CF2 symmetric

237

stretching mode.27

238

The morphologies of single PI nanofiber and nanofibrous membrane treated at

239

different stages were monitored by TEM and FE-SEM images as shown in Figure

240

2(c-g) and Figure S(3 & 4). All the nanofibers oriented randomly with high aspect

241

ratio, which constructed the 3-dimensional network structure. The macropores could

242

be served as interconnected passageways for vapor transfer and limit water from 12


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permeation. After PDA/PEI coating, the average diameters of PDA/PEI@PI nanofiber

244

was increased to 375±25 nm as shown in Figure S5, a thickness of～38 nm of

245

PDA/PEI layer could be observed (Figure 2d) compared with single PI nanofiber

246

(Figure 2c).

247

NFMs because the interaction between catechol groups of PEI and the amino group of

248

DA can effectively suppress the PDA aggregation.36 After electrostatic attraction, the

249

dense SiO2 NPs are uniformly and compactly anchored on the fiber surface without

250

flaws, as shown in Figure 2(e & f), which is a key factor in determining the membrane

251

durability because the flaws resulting from the uneven nanoparticles on membrane

252

substrate give opportunities for water permeation after long-term operation.19

253

Meanwhile, the fiber diameter was sharply increased to 470±35 nm (Figure S3 & 5),

254

demonstrating the SiO2 NPs with multilevel structure successfully decorated onto the

255

PI fiber surface. In addition, the roughness of fiber surface was greatly increased from

256

3.56 to 31.75 nm as shown in Figure 2(h & i) and Figure S6. Significantly, the

257

interaction between SiO2 NPs and the fiber surface was very strong and no visible

258

changes were observed regarding the fiber morphology after 1 h sonication treatments

259

with a power of 150 W (Figure S7). This demonstrates the electrostatic attraction

260

method between positive PDA/PEI and negative SiO2 for roughness construction

261

shows robust stability. The cross-section images of PI nanofibrous membrane and

262

relevant single nanofiber treated at different stages are shown in Figure 2j and Figure

263

S8, revealing the nanofibers accumulated layer by layer constructed the

264

interconnected porous structures. The treated membrane also exhibited denser than the

Most importantly, the PDA/PEI layer was uniformly coated on the PI

13



265

as-prepared PI nanofibrous membrane due to the crosslinking of fibers as shown in

266

Figure S3b. The membrane surface with and without SiO2 NPs after fluorination did

267

not affect the surface morphologies and fiber diameters as shown in Figure S(3d & 4).

268

Furthermore, the average thickness of PI, PDA/PEI@PI, SiO2-PDA/PEI@PI and

269

F-SiO2-PDA/PEI@PI NFMs are 81±3, 91±2, 102±3 and 105±4 µm, respectively,

270

which was due to the increased thickness of the bottom and top layer after PDA/PEI

271

coating and SiO2 NPs decoration. The increased thickness was also in accordance

272

with the diameter changes.

273 274

Figure 3 (a) N2 ad/desorption, pore size distribution and (c) stress-strain curves of PI,

275

PDA/PEI@PI, SiO2-PDA/PEI@PI and F-SiO2-PDA/PEI@PI NFMs, respectively.

276

Membrane Specific Surface Area, Pore Size Distribution and Mechanical

277

Strength Analyses. The unique introduction of SiO2 NPs created the pristine PI fiber

278

surface with hierarchical roughness, thus dramatically increased the effective surface

279

area. Therefore, N2 ad/desorption isotherms as shown in Figure 3a reveal a typical-IV

280

isotherms, indicating the presence of open mesopores and macrospores.43 The

281

mesopores and macrospores are mainly coming from the SiO2 NPs with mesopores in

282

nature and the stacking of PI nanofibers. The surface area of the as-prepared PI,

283

PDA/PEI@PI, SiO2-PDA/PEI@PI and F-SiO2-PDA/PEI@PI NFMs are 7.32, 6.18,

284

27.30 and 25.22 m2 g-1, respectively, demonstrating the major contribution role of 14


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SiO2 NPs in determing the specific surface area and roughness structure, which was in

286

aggrement with the FE-SEM image results (Figure S3). Figure 3b gives typical pore

287

size distributions of PI, PDA/PEI@PI, SiO2-PDA/PEI@PI and F-SiO2-PDA/PEI@PI

288

NFMs measured through a bubble point method using a capillary flow porometer. It is

289

worth noting that the pore size distribution of all the relevant membranes was shown

290

in the range from 1.5 to 4 µm, and the average pore size gradually decreased from

291

2.65 to 2.23 µm. The decrease in pore size distribution of PI NFMs could be attributed

292

to that the layer by layer modification gradually narrowed the average pore size

293

distributions. The mechanical strength of relevant nanofibrous membranes was also

294

measured as shown in Figure 3c. Before broken, all the samples exhibited a nonlinear

295

elastic deformation in the first region (0 to 25%) under a stress load, and then the

296

stress-strain curves displayed a quasi linear plastic behavior until breakage.

297

Interestingly, the mehanical strength and the tensile strain were increased to 321±22

298

cN and 50.48 %, respectively. The PDA/PEI used as the positively charged adhesive

299

have the functionality to adhere the fiber points, which slightly enhanced the

300

mechanical strength and tensile strain. After the roughness construction and

301

fluorination, the stress was slightly decreasd to 290±16 and 277±21 cN, which could

302

be attributed to that the hydrophilic SiO2-PDA/PEI@PI NFMs displays slight

303

swelling.36

15



304 305

Figure 4 (a) WCAs of modified F-PDA/PEI@PI NFMs with various concentrations

306

of SiO2 NPs. (b) WCAs of the water droplet with increasing time on PI based

307

nanofibrous membrane surface, respectively; the inserts showing the optical profiles

308

of the evaporation of water droplets. (c) WCAs of different water quality on relevant

309

PI based nanofibrous membrane surface. (d) In-air sessile drop CAs for two different

310

surfaces with five liquids. (e) photographic images of different liquid droplets on the

311

commercial PVDF microporous membrane and F-SiO2-PDA/PEI@PI NFMs. (f) The

312

proposed mechanism of waterproof property based on Yong-Laplace equation and a

313

plausible mechanism of vapor permeation across the porous membrane.

314

Wetting Resistance of Superhydrophobic Membrane Surface. According to the

315

Wenzel and Cassie models, a rough surface is essential for enhancing the surface from

316

hydrophobic to superhydrophobic and hydrophilic to superhydrophlic depending on

317

the nature of the corresponding flat surface.44,45 The roughness can be improved by

318

texturing with multiple scaled roughnesses to improve the wettability. Colloidal

319

assemblies of inorganic nanoparticles via van der Waals interactions have been widely

320

used to create hierarchical rough structures owing to its low cost and no expensive

321

lithographic technique.6,27 PDA/PEI@PI NFMs modified with various concentrations 16


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of colloidal SiO2 NPs through elecronic attraction are presented in Figure S9. It is

323

clearly shown that the morphology of the resultant membranes was significantly

324

changed after being created with nano-scaled roughness structures on the nanofiber

325

surface. All the SiO2 NPs are facilely positioned without obvious agglomeration, and

326

a negligible amount of SiO2 NPs is present among the voids of nanofibers as the

327

concentration of colloidal SiO2 NPs was increased from 0.02 to 0.2 wt% (Figure

328

S9(a-d)). However, an obvious adhesion structure was appeared and the nanoparticles

329

can large area fill the pores when the concentration of colloidal SiO2 NPs was

330

increased over 0.5 wt% as shown the inserts of Figure S9(e & f). It is reasoned that

331

the increased concentration of colloidal SiO2 NPs resulted in higher viscosity can

332

block the membrane pores and greatly affect the roughness of nanofibers.

333

The surface wettability of relevant SiO2-PDA/PEI@PI NFMs after fluorination

334

were measured by static water contact angels as shown in Figure 4a, the WCAs of

335

F-SiO2-PDA/[email protected],

336

F-SiO2-PDA/[email protected] are 133°, 138°, 145° and 152°, respectively, indicating a

337

noteworthy raising WCAs accompanied with increasing concentration of colloidal

338

SiO2 NPs. However, further increases the concentration of colloidal SiO2 NP to 0.5

339

and 1.0 wt% have decreased the WCAs to around 147° and 145°, respectively. This

340

phenomenon was due to the filling of voids among the nanofibers that reduced the

341

roughness of the membranes. As a result, F-SiO2-PDA/[email protected] NFMs was

342

chosen as the optimal candidate for further study. In order to better understand the

343

water dynamic behavior of the composite membrane, a high-speed camera system was

344

used to examine the water droplet adhesion ability. As shown in Figure S10, a water

345

droplet of 3 µL was forced to contact the membrane surface; the droplet can easily

346

leave the membrane surface and remained its spherical shape without obvious


17




347

deformation even at high load pressure, demonstrating the low water adhesion

348

surface.

349

Contact angles of evaporating water droplets (3 µL) on the as-prepared and

350

modified PI NFMs were measured to evaluate the water resistance for durability, as

351

shown in Figure 4b. In the case of PI and F-PDA/PEI membranes, the WCAs

352

decreased rapidly to 65o and 84o at the end of evaporation as time increased to 30 min,

353

illustrating the incursion of water into the voids of the nanofibrous membranes.

354

Alternatively, the WCAs of superhydrophobic F-SiO2-PDA/PEI@PI NFMs exhibited

355

few signs of sliding with evaporation, which was in support of the Cassie–Baxter

356

model and revealed the hierarchical roughness surface can low the pinning of the

357

water droplet.46 The detailed optical profiles of water droplets evaporation state on

358

different membranes are shown in the insert in Figure 4b. The WCAs can remain high

359

and the droplet maintain its spherical shape during evaporation. In addition, the

360

membranes were measured to evaluate their resistance to a wide range of surface

361

tension and water quality including NaCl, MeB, SDBS, HCl and NaOH solutions, the

362

as-prepared PI and F-PDA/PEI@PI membrane had water contact angles over 100o,

363

indicating the initially hydrophobicity of PI matrix. The resultant fluorinated

364

membrane coated with SiO2 NPs have high WCAs toward various liquids compared

365

to those with no SiO2 NPs decoration (Figure 4c), demonstrating the air-gaps in the

366

pores have strongly liquid repulsive according to the Cassie theory. Comparing the

367

wetting properties between the commercial PVDF microporous membrane with

368

hydrophobicity and the F-SiO2-PDA/PEI@PI NFMs with omniphobicity as shown in

369

Figure 4 (d & e), the commercial PVDF microporous membrane was resistance to

370

wetting by the high-surface-tension liquids but easily wetted by low-surface-tension

371

liquids. The F-SiO2-PDA/PEI@PI NFMs exhibiting robust resistance to all the liquids 18


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372

demonstrates the excellent antifouling performance.

373

The MD membrane also should possess robust waterproof performance to prevent

374

the membrane from wetting and permeation by salinity water. The mechanism of

375

waterproof membrane are based on two criteria as shown in Figure 4f: (1) the surface

376

of membrane should be hydrophobic; (2) the maximum pore size of membrane should

377

be less than the average pore size of water droplet (＜100 µm), which can effective

378

resist the water droplet permeation. Here, the waterproof performance of the

379

F-SiO2-PDA/PEI@PI NFMs was tested by hydrostatic pressure, which was associated

380

with the liquid surface tension (γliquid), liquid contact angle on the membrane surface

381

(θadv) and shape of membrane pores according to Laplace-Young equation as follows:

382

47

383

Hydrostatic pressure=−

384

Here, the γwater =72.58 mN/m, θadv =152o, dmax =2.3 µm, the calculated Hydrostatic

385

pressure was 45.3 kPa, which is finely matched well with the measured hydrostatic

386

pressure of 42 kPa. The results demonstrated that the hydrostatic pressure was

387

strongly related to the surface wettability and the maximum pore size of membrane.

388

The ideal MD membrane not only requires good waterproofness, but also equips with

389

fast vapor transmission rate. The electrospun nanofibrous membrane provides a

390

3-dimensional network with interconnected porous structure, which possesses a robust

391

property of water vapor permeation without any barriers. As shown in Figure 4f, the

392

breathable property of the superhydrophobic nanofibrous membrane was driven by

393

the temperatures gradient between the two sides. The water droplet vaporized into

394

water molecules, following permeate through the hydrophobic porous membranes and

ସఊ౭౗౪౛౨ .௖௢௦ఏ౗ౚ౬ ௗౣ౗౮

19



395

be collected or condensed by pure water in the cold side (Figure 4f). In addition, the

396

distillation membrane was often operated at hot salinity water (＞60 oC), thus the

397

membrane should have the ability to resistance the hot water. The hot water repellent

398

characteristic of the as-prepared PI, F-PDA/PEI@PI and F-SiO2-PDA/PEI@PI NMFs

399

are displayed in Figure S11, it is clearly shown that both the PI and F-PDA/PEI@PI

400

NFMs exhibited obvious wetting behavior as dotted in red, demonstrating the

401

destruction of the smooth fiber surface. Interestingly, the F-SiO2-PDA/PEI@PI NFMs

402

still exhibited robust hydrophobicity toward hot water with no obvious wetting and

403

the hot droplets can easily slip on the membrane surface, implying the roughness

404

constructed by SiO2 NPs significantly enhanced the thermal stability of

405

F-SiO2-PDA/PEI@PI NFMs.

406 407

Figure 5 (a) Operation temperatures, (b) concentrations of salts, (c) salinity water

408

with different kinds of contaminants on the effect of membrane distillation. (d) 20


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409

Durability of the superhydrophobic F-SiO2-PDA/PEI@PI NFMs and the commercial

410

hydrophobic PTFE membrane in a simulated sea water containing 3.5 wt% of NaCl.

411

Membrane Performances in DCMD. The as-prepared PI NFMs treated at different

412

stages and commercial hydrophobic PTFE with different pore sizes and PVDF

413

microporous membranes were systematically evaluated using a lab–scale DCMD

414

device as shown in Figure S1. The hot feed side and cold side temperatures were fixed

415

at 60oC and 20 oC, respectively. It is observed that the flux of as-prepared PI NFMs

416

was dramatically decreased from 27.02 to 8.12 L m-2 h-1 in 180 min and the

417

F-PDA/PEI@PI membrane was decreased from 30.15 to 6.91 L m-2 h-1 in 240 min

418

(Figure

419

comparatively stable performance than the as-prepared PI NFMs in short time.

420

Subsequently, the water flux was sharply decreased and the conductivity was

421

substantially increased, revealing the membranes without roughness construction

422

show no resistance to hot water and the feed side salinity water can easily permeate

423

across the membrane. The results are in accordance with the above experiment that

424

dumping hot water onto the membrane surface aiming at measuring the hot water

425

resistance (Figure S11). Significantly, the F-SiO2-PDA/PEI@PI NFMs exhibited a

426

stable MD performance with a high water flux of 31.29 L m-2 h-1 and salt rejection of

427

~100% as presented in Figure S12(a & b). Both the water contact angles and

428

membrane distillation performance of modified nanofibrous membrane was higher

429

than that of traditional commercial hydrophobic PTFE membranes with different pore

430

sizes as shown in Figure S(13 & 14).

S12a),

demonstrating

the

F-PDA/PEI@PI

21


membrane

exhibited

a


431

As the membrane distillation is a non-isothermal separation process, the

432

temperature gradient has tremendous effect on the permeate flux.48,49 All the

433

membranes were performed over a period of 8 h using 3 wt% of NaCl solution. The

434

feed temperature was increased ranging from 40 to 90 oC and the collected side water

435

temperature was always fixed at 20 oC. Figure 5a demonstrates the water flux was

436

significantly increased from 14.23 to 63.25 L m-2 h-1 as the feed temperature was

437

increased from 40 to 90 oC, illustrating the increasing feed temperature can effectively

438

improve the vapor driving force. Fortunately, the membrane operated at high

439

temperature (90 oC) also exhibited excellent salt rejection due to the durable hot water

440

repellent characteristic as demonstrated in Figure S11. The permeate flux of

441

F-SiO2-PDA/PEI@PI NFMs operated at 3.0 wt% of NaCl solution and the hot side

442

temperature fixed at 60 oC is about 31.02 L m-2 h-1, which is slightly lower than that of

443

deionized water of 34.49 L m-2 h-1 (Figure 5b). Moreover, it was found that the water

444

flux was regularly decreased as increasing the NaCl concentrations, suggesting that

445

the hydration of ion and ionic association in salt solution reduced the water activity

446

and resulted in the decline of vapor pressure.

447

In addition, we also evaluated the MD performance using feed salinity water

448

solutions containing typical organic pollutant (MeB, lubricating oil, and

449

representative surfactant SDBS). It is well known that organic pollutants are often

450

presented in high salinity wastewater; therefore, the distillation membrane also should

451

have the ability to isolate the organic pollutants preventing the clean water from

452

pollution. Unfortunately, after the oil was added in the saline water, the water flux of 22


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453

commercial macroporous PVDF membrane (Millipore, 0.45 µm) was continuously

454

decreased and salt rejection was substantially increased, suggesting that the

455

commercial PVDF membrane is prone to wetting by the low surface tension feed

456

waters. Our resultant F-SiO2-PDA/PEI@PI NFMs exhibited a stable separation and

457

purification performance for treatment of polluted salinity wastewater containing

458

typical low surface tension and dissolved contaminants as displayed in Figure 5c. We

459

attribute this fascinating result to the multilayer structured membrane and low surface

460

energy of F-SiO2-PDA/PEI@PI NFMs with omniphobicity. Meanwhile, the simulated

461

RO concentrated water was also performed under the same condition, the membrane

462

also showed a stable water flux and high salt rejection. Significantly, the

463

superhydrophobic F-SiO2-PDA/PEI@PI NFMs shows a water contact angle of 152o,

464

which exhibited disadvantage compared to previous electrospun nanofibrous

465

membranes employed in MD.50-52 However, it is worth noting that the above

466

mentioned electrospun MD membranes show comparatively high contact angles, but

467

the running time of membrane distillation is generally less than 12 h. We inferred that

468

traditional coating or co-electrospinning methods are inevitably faced the

469

inhomogeneity of roughness on membrane substrate because the coated inorganic

470

nanoparticles are not uniformly and compactly covered the membrane surface. The

471

flaws may become the beginning of the membrane wetting after long-time operation.

472

The superhydrophobic F-SiO2-PDA/PEI@PI NFMs membrane can maintain robust

473

durability and stability after 48 h operation, which have obvious advantages compared

474

with the above mentioned electrospun nanofibrous membranes and commercial PTFE 23



475

microporous membrane (Figure 5d). Therefore, the compactness and uniformity of

476

inorganic particles on the membrane substrate play key roles in determining the

477

durability of MD membrane, reducing the formation of defects and the wetting of

478

membrane.

479 480

Figure 6 (a-e) FE-SEM images of porous materials after hierarchical structure

481

construction. (f) The water contact angles of hierarchical structured porous materials

482

before and after fluorination.

483

A Simple and Universal Procedure for Fabricating Hierarchical Structured

484

Materials with Superhydrophobicity or Superhydrophilicity. To prove the

485

universality of this method, various materials including copper mesh, steel mesh,

486

silica fiber, hydrate membrane and cellulose triacetate membrane were used to

487

construct the raw materials with hierarchical structures. The raw materials before and

488

after roughness construction are shown in Figure S15 and Figure 6, all the materials 24


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489

whether the inorganic, metal or polymer materials can be successfully constructed

490

with roughness. Significantly, all the materials after SiO2 NPs construction exhibited

491

robust superhydrophlic surface with water contact angles of 0o. After fluorination, the

492

surface properties of materials were greatly turned from superhydrophlicity to

493

superhydrophobicity as shown in Figure 6f, demonstrating the dual-bioinspired design

494

can be used as a universal method and paved a new method for constructing

495

membranes surface with superhydrophobicity or superhydrophilicity for various

496

applications.

497

In summary, we have successfully demonstrated a facile and scalable method for

498

fabrication of superhydrophobic nanofibrous membrane with enhanced hot water

499

resistance via electrospinning, dual-bioinspired design and fluorination. The in situ

500

polymerized PDA/PEI layer endowed the as-prepared PI NFMs with strong positively

501

charged coating, which enable irreversible bind the negatively charged SiO2 NPs onto

502

the nanofiber surface through electrostatic attraction. After fluorination, the resultant

503

nanofibrous membrane displayed promising superhydrophobicity with a WCA of 152o

504

and displayed robust resistance to hot water (85 oC). In addition, the relationship

505

among hydrophobicity, hydrostatic pressure and maximum pore size were proved to

506

be finely accordance with the Young-Laplace equation. Significantly, the

507

superhydrophobic nanofibrous membrane exhibited a stable performance for treating

508

high salinity wastewater, in which contained typical low surface tension and dissolved

509

contaminants.

510

environmental benign synthesis and would pave the way for new types of hierarchical

The

dual-bioinspired

construction

method

25


may

provide

an


511

structured materials for various applications such as waterproof and breathable fabrics,

512

oil water separation and anti-pollution paint etc.

513 514

ASSOCIATED CONTENT

515

Supporting information

516

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

517

website at DOI: 10.1021/acs.est.XXX.

518

AUTHOR INFORMATION

519 520 521 522

Corresponding Author E-mail: [email protected]

523

The authors declare no competing financial interest.

Notes

524 525

ACKNOWLEDGEMENTS

526

The authors gratefully acknowledge National Natural Science Foundation of China

527

(Grant no. 51573034), State Key Laboratory of Urban Water Resource and

528

Environment in HIT of China (No. 2016DX02), HIT Environment and Ecology

529

Innovation Special Funds (No. HSCJ201606), Postdoctoral Science Foundation of

530

Heilongjiang Prov. (LBH-TZ0606 and LBH-Q16012). Scientific Research Foundation

531

for Returned Scholars, Heilongjiang of China (LC2017023). The authors also

532

gratefully acknowledge acknowledge Dr. Zheyu Li for useful discussions and

533

technical support.

534

REFERENCES

535

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