Fabrication of hyperbranched polyether demulsifier modified PVDF membrane for demulsification and separation of oil-in-water emulsion

Journal Pre-proof Fabrication of hyperbranched polyether demulsifier modified PVDF membrane for demulsification and separation of oil-in-water emulsion Can Xu, Feng Yan, Mingxia Wang, Hao Yan, Zhenyu Cui, Jianxin Li, Benqiao He PII:

S0376-7388(19)33449-0

DOI:

https://doi.org/10.1016/j.memsci.2020.117974

Reference:

MEMSCI 117974

To appear in:

Journal of Membrane Science

Received Date: 9 November 2019 Revised Date:

8 February 2020

Accepted Date: 15 February 2020

Please cite this article as: C. Xu, F. Yan, M. Wang, H. Yan, Z. Cui, J. Li, B. He, Fabrication of hyperbranched polyether demulsifier modified PVDF membrane for demulsification and separation of oil-in-water emulsion, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/ j.memsci.2020.117974. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Graphical abstract

Manuscript submission to Journal of Membrane Science

Fabrication of hyperbranched polyether demulsifier modified PVDF membrane for demulsification and separation of oil-in-water emulsion Can Xu1,2, Feng Yan1,3*, Mingxia Wang1,4, Hao Yan2, Zhenyu Cui1,4, Jianxin Li1,4, Benqiao He1,4

1

State Key Laboratory of Separation Membranes and Membrane Processes,

Tiangong University, Tianjin 300387, P. R. China 2

School of Environmental Science and Engineering, Tiangong University, Tianjin

300387, P. R. China 3

School of Chemistry and Chemical Engineering, Tiangong University, Tianjin

300387, P. R. China 4

School of Material Science and Engineering, Tiangong University, Tianjin 300387,

P. R. China

Corresponding Author. Tel: +86-22-8395-5115; E-mail: [email protected] (Feng Yan); [email protected] (Feng Yan)

1

Abstract

2

Membrane-based treatment for oil-in-water emulsion remains a significant

3

challenge. The main difficulties are to solve membrane fouling and to destroy the

4

emulsion system to achieve high-efficiency emulsion separation. Herein, chemical

5

demulsification combined with membrane separation technology was proposed for

6

oil/water separation from the emulsion. A hyperbranched phenol-amine resin block

7

polyether demulsifier (AE2311) was grafted onto surface of styrene-co-maleic

8

anhydride (SMA) blend polyvinylidene fluoride (PVDF) membrane by alcoholysis

9

reaction. The successful preparation of the demulsifier modified PVDF membrane

10

was charaterized by X - rayphotoelectron spectroscopy (XPS), attenuated total

11

reflectance-fourier transform infrared spectroscopy (ATR-FTIR), scanning electron

12

microscopy

13

AE2311@SMA/PVDF

14

superhydrophilic (water contact angle of 0°) and underwater superoleophobic

15

properties (underwater oil contact angle over 150°). The modified membrane can

16

break the O/W emulsion and allow water to pass through. The separation efficiency

17

for dichloroethane-in-water, kerosene-in-water, toluene-in-water and petroleum

18

ether-in-water emulsions were all recorded over 99.0%, which indicates that the

19

modified membrane has excellent capability for oil-water separation. Moreover, the

20

modified membrane can be reused and exhibited long-term operation stability owing

21

to the excellent underwater anti-fouling performance. The intrinsic mechanism for the

(SEM)

and

atomic

membrane

force

microscope

with

grafting

(AFM).

time

of

9

The

obtained

h

exhibited

22

oil/water separation is the synergistic effect of the chemical demulsification of

23

AE2311, the hydrophilic and underwater super hydrophobic properties of the

24

membrane surface, and the sieving effect of the ultrafiltration membrane. The

25

demulsifier functionalized membrane provides a new idea for the fabrication

26

membrane of oil-water emulsion separation in the future.

27 28 29

Keywords: SMA/PVDF blend membrane; membrane demulsification; O/W emulsion

30

hyperbranched branched phenol-amine PPO-b-PEO polyether

31

32

1. Introduction

33

With the rapid growth of economy and industry, oil-in-water emulsions have

34

received great attention due to their increasing applications in medicine,

35

petrochemical engineering and cosmetics [1-4]. However, oil spill accidents happened

36

occasionally resulted from human activities during manufacturing, drilling,

37

transporting, storing, and waste management, which is now causing serious threat to

38

ecological environment and human health [5, 6]. Therefore, it is urgent need of

39

developing efficient separation methods for oil-water mixture, especially from

40

oil-in-water emulsions [7, 8].

41

Many

strategies,

such

as

sedimentation[9],

physical

absorption[10-14],

42

electroflotation [15] and centrifugation [16, 17] have been employed for oil/water

43

separation, and they are very effective in the treatment of oil-slick wastewater.

44

However, these strategies can only be used to separate oil from O/W emulsion, in

45

which the sizes of oil droplets are usually less than 20 µm [18-20].

46

Recently, continuous oil-water separation membrane has attracted extensive

47

attention due to its high water flux, robust removal capacity and low energy

48

consumption. The scientific back-ground is based on the surface wettability difference

49

and “sieving effect” [21-24]. It has been acknowledged universally that

50

superhydrophilic and underwater superoleophobic membrane can separate oil

51

contaminations from water with high efficiency. The hydrophilic property allows

52

water to pass-through the membrane continuous, whereas the underwater oleophobic

53

property retains the oil by hydrophilic-hydrophobic repulsion. The materials that

54

commonly used for membranes, including polypropylene, poly(vinylidene fluoride)

55

polysulfone and polytetrafluoroethylene, are hydrophobic. Thus, hydrophilic

56

modification strategies include blending and grafting hydrophilic components [25].

57

For instance, Liu et al. prepared a superhydrophilic polylactide membrane inlayed

58

with TiO2 nano-particles, which exhibited good separation performance for oil/water

59

mixture. The flux was up to 950 L·m-2·h-1, and the oil rejection ratio reached more

60

than 99% [26]. In pursuit of excellent performance for separation of oil and water,

61

similar strategies were further developed by coating SiO2 [27], graphene [28, 29] and

62

nanotube[30] on membrane surface. Grafting hydrophilic components onto membrane

63

surfaces is another way for fabricating super wettability membrane. Jin’s group

64

prepared superhydrophilic PVDF membranes via grafting of ionized poly(acrylic acid)

65

[31, 32]. The PVDF membrane showed underwater ultralow-oil-adhesive to various

66

oils because of the surface hierarchical structure and the great hydration capability of

67

the membrane. Unfortunately, the aforementioned modified membranes were mainly

68

effective for oil/water mixture rather than oil/water emulsion. To solve this problem,

69

Janus membranes with asymmetric configuration have been fabricated to separate

70

oil/water emulsion based on “sieving effect” [33-37]. Nevertheless, the separation

71

mechanism of oil/water emulsion by the aforementioned Janus membrane is still

72

unclear, and there are short of controllable ways to tailor the synergistic effect of

73

asymmetric configuration for performance of separation. Therefore, further research is

74

still needed to improve the separation efficiency of oil/water emulsion.

75

As two immiscible liquids, oil and water form a suspension of one liquid in another

76

with the help of emulsifying agent (usually surfactant), which is known as emulsion.

77

Water-in-oil (W/O) and oil-in-water (O/W) are two main emulsions based on the

78

nature of the dispersed phase. In either case, the emulsifying agents form a robust

79

interfacial film around water (for W/O emulsion) or oil (for O/W emulsion) droplets,

80

with their polar groups oriented toward the water phase and their nonpolar groups

81

toward the oil phase. As known, the physical nature of the oil-water interfacial film

82

and the electrostatic repulsion of the droplets are the two main mechanisms for

83

emulsion stability [38]. To break these emulsions, various techniques have been used.

84

Among them, chemical demulsification is recognized as the most effective method for

85

demulsification and is widely used [39]. The main mechanism of chemical

86

demulsification is that the demulsifiers adsorbed at the oil-water interface reduce the

87

intensity of the interface, which facilitates the coalescence of dispersed droplets to the

88

liquid beads. Therefore, the key for oil-water separation from emulsion is to change

89

the interfacial properties or the strength of the interfacial film by adding high

90

efficiency demulsifiers. The most commonly used demulsifiers are the poly(propylene

91

oxide-b-ethylene oxide) (PPO-b-PEO) copolymer [40-42], such as polyoxyalkylated

92

glycols, polyoxyalkylated epoxy resins, polyoxyalkylated alkyl-phenol-formaldehyde

93

resins, and polyoxyalkylated amines etc. Though chemical demulsification is of high

94

efficiency in oily water separation, it’s difficult to remove trace emulsified oil by

95

demulsification, and the demulsifier remaining in water leads to further water

96

pollution.

97

In our previous works, a type of hyperbranched phenol-amine resin block polyether

98

demulsifier with PPO-b-PEO copolymers [43, 44] was prepared for break of crude oil

99

emulsion by chemical demulsification. As far as we know, few works focused on

100

chemical demulsification coupling with membrane filtration for oil/water emulsion.

101

Herein, the hyperbranched phenol-amine resin block polyether demulsifier (AE2311)

102

was grafted onto surface of SMA blend PVDF membrane via the alcoholysis reaction

103

between maleic anhydride on membrane surface and terminal hydroxyl group on

104

AE2311 as shown in Scheme 1. The obtained AE2311@SMA/PVDF membrane

105

showed superhydrophilicity and underwater superoleophobicity. The O/W emulsions

106

can be separated effectively by the as-prepared demulsifier functionalized membrane,

107

which opens up a new way for practical surfactants-stabilize oil/water separation.

108 109

Scheme 1. Schematic diagram of the fabrication of AE2311@SMA/PVDF membrane.

110 111

2. Experimental

112

2.1 Materials

113

PVDF powder (FR904, average molecular weight MW≈2×106 g·mol‒1) was

114

purchased from 3F New Materials Co., Ltd. (Shanghai, China) Styrene-co-maleic

115

anhydride (SMA, MW≈1.0×105 g·mol‒1) was provided by Jiaxing Howin Chemical

116

Co., Ltd. Albumin Bovine Serum (BSA) was supplied by Solarbio Science and

117

Technology Co., Ltd. (Beijing, China)

118

The hyperbranched phenol-amine resin polyoxypropene (PPO) polyoxyethylene

119

(PEO) polyether (AE2311, MW≈1.3×104 g·mol‒1) was synthesized according to the

120

previous works [44] and the structure was shown in Scheme 1. All the other reagents

121

were obtained from Tianjin Guangfu Fine Chemical Reagent Co., Ltd.

122

2.2 Preparation of SMA blend PVDF membrane

123

PVDF is a commonly used membrane materials for oil-water separation due to its

124

chemical resistance, high mechanical strength, oxidation resistance and thermal

125

stability [45, 46]. However, the natural hydrophobicity makes it prone to foul in water

126

treatment [47]. SMA is a hydrophobic copolymer with alternating maleic anhydride

127

and styrene units. Therefore, PVDF has a good compatibility with SMA [48, 49].

128

Herein, the SMA/PVDF membranes were fabricated via the phase inversion method.

129

To begin with, 15 wt.% PVDF, 7 wt.% SMA and 5 wt.% LiCl were dissolved in

130

DMAc under stirring for 15 h at 70 ◦C to make a homogeneous solution. Following,

131

the mixture was kept in water bath at 70 ◦C for another 12 hours to ensure that the

132

bubbles were completely removed. Then the homogeneous solution was cast on a

133

cleaned glass plate at a thickness of 300 µm, and the liquid film was immediately

134

immersed into water coagulation bath at 40 ◦C. The prepared membranes (named as

135

M-0) were soaked in DI water for 48 h to remove the residual solvent.

136

2.3 Fabrication of AE2311@SMA/PVDF membrane

137

The anhydride ring of SMA on the blend membrane surface can be opened by

138

aminolysis or alcoholysis with functional amines or alcohols, which improves the

139

hydrophilicity

140

AE2311@SMA/PVDF membrane, a grafting solution containing 2 g·L‒1 phenolamine

141

resin

142

p-toluenesulfonic acid (PTSA) in 1 L deionized water was prepared. After oscillating

143

the solution in an ultrasonic oscillator for 10 min, SMA/PVDF membranes (4 cm × 4

144

cm) were soaked in the solution at 60 ◦C with constant stirring at a rate of 600 rpm.

145

The modified PVDF membranes were kept in water to remove the residual additives

146

and solvent. The membranes were named as M-3, M-6, M-9, M-12 and M-15

147

according to the grafting time.

148

2.4 Characterization of membranes

of

the

polyoxypropene

SMA/PVDF

polyoxyethylene

membrane

polyether

[50,

51].

(AE2311)

To

and

prepare

4

g·L‒1

149

Chemical structure and composition of membrane surface were characterized by

150

ATR-FTIR (Nicolet Nexus-670, USA) and X-ray photoelectron (XPS, Thermofisher,

151

USA). Morphologies of membranes were observed with a field emission scanning

152

electron microscopy (FE-SEM, Hitachi S-4800, Japan) and an atomic force

153

microscope (AFM, Agilent model 5400).

154 155

The overall porosity (ε, %) of membrane was measured by a dry-wet weight method [52] and was calculated by Eq. (1):

ε (%) =

156

mw-md ρAδ0

100%

(1)

157

where mw and md are the weights of membrane in wet and dry condition (g); ρ is the

158

density of deionized water (g·cm‒3); A and δ0 are the effective area (cm2) and the

159

thickness (cm) of membrane.

160

The average pore size was observed using Gas-liquid interface aperture tester

161

(Porolux 1000, Porometer, USA). The membrane samples were cut into a circular

162

sample with a diameter of 13 mm and then were infiltrated with the wetting liquid

163

(Porewick) for 6 h before measurement.

164

The graft of AE2311 on SMA/PVDF membrane was monitored by the variation of

165

membrane weight with grafting time. The grafted degree (GD, mg·g-1) was calculated

166

by Eq. (2):

167 168

GD =

mg-mo mo

(2)

where mo and mg are the weights of membrane before and after graft (g).

169

The pure water flux (J) and BSA rejection (R) of the AE2311@SMA/PVDF

170

membrane were evaluated by a cross-flow filtration at 0.1 MPa trans-membrane

171

pressure, and they were calculated by Eqs. (2) and (3), respectively:

V A∆t

J=

172

R = (1 ‒

173

Cp Cf

)

(3) 100%

(4)

174

where V is the volume of solution or pure water; ∆t is the permeation time (h); A is the

175

effective area of the membrane (m2); Cp is the BSA concentration of permeate; Cf is

176

the BSA concentration of feed.

177

The anti-fouling performance was evaluated by a three cycle filtration experiments

178

using BSA as the model of pollutant. The membrane was firstly compacted with DI

179

water for 30 min. The overall fouling procedure contained three cycles and each cycle

180

was operated as follows. The initial water flux was calculated through water filtration

181

for 60 min; Afterward, the DI water was replaced by BSA solution and the BSA

182

solution fouling stage was operated for 60 min. Subsequently, the fouled membrane

183

was thoroughly flushed with DI water for 30 min, then the water flux was measured

184

again [52, 53]. To assess the anti-fouling effect for protein fouling of membrane, the

185

flux recovery ratio (FRR) was calculated by Eq. (5):

FRR=

186

Jw3 Jw0

100%

(5)

187

where JW0 is the initial water flux before fouling; JW3 is the water flux after the third

188

cleaning (L·m‒2·h‒1).

189

The contact angle (UOCA) as well as underwater-oil-adhesion was determined by a

190

contact angle goniometer (DSA30E Krüss GmbH, Hamburg, Germany) equipped

191

with video capture at room temperature. The contact angle of each sample was

192

measured five times and was averaged in order to reduce the experimental error.

193

2.5 O/W emulsion separation experiments

194

The O/W emulsions were prepared by mixing 1.0 g oil (kerosene, dichloroethane,

195

toluene or petroleum ether) and 1.0 L DI water with the addition of 0.2 g SDS under

196

magnetic stirring at 2000 rpm for 1 h. The separation performance of O/W emulsion

197

by the membrane was also evaluated by the cross-flow filtration as shown in Scheme

198

2 at 0.1 MPa trans-membrane pressure. The permeate solution was collected and then

199

observed by an optical microscope (LEICA DM 2500 P, Germany) to obtain the

200

optical photograph of emulsion. The particle size of the prepared emulsion and

201

permeate was observed by a Malvern Mastersizer particle size analyzer (Zetasizer

202

Nano ZS90, UK). The total organic carbon (TOC) value of the feed and the permeate

203

solution was measured by a TOC analysis (GE Innovox, USA). The retention rate

204

(R, %) was calculated by Eq (6):

R = (1 ‒

205

206

where

207

value of the feed (ppm).

208

Cp Cf

)

100%

Cp is the TOC value of the permeate solution (ppm) and

(6) Cf is the TOC

209

Scheme 2 Schematic diagram of cross-flow filtration for O/W emulsion separation

210

3. Results and discussion

211

3.1 Characterization of AE2311@SMA/ PVDF membrane

212

The modification is realized by in-situ surface graft of demulsifier (AE2311) with

213

terminal hydroxyl group onto the SMA/PVDF membrane. In this work, the grafting

214

time is various from 0 to 15 h. As shown in Fig. 1(a), the grafted degree increased

215

linearly from 134.6 mg·g-1 to 683.2 mg·g-1 with increasing grafting time.

216

The ATR-FTIR spectra was applied to estimate the chemical structure of the

217

membrane surface before and after grafting, and the chemical structure of membranes

218

of PVDF, SMA/PVDF (M-0) and AE2311@SMA/PVDF with various grafting time

219

was shown in Fig. 1(b). It was observed that all the samples exhibited typical

220

characteristic peaks of PVDF. These peaks at 874, 1175 and 1402 cm-1 were assigned

221

to the skeletal vibration of C-C bond, -CF2- stretching, and -CH2- in-plane blending,

222

respectively [54]. In the spectrum of SMA/PVDF, three new peaks appeared at 703,

223

1779 and 1858 cm-1. They were assigned to the characteristic peaks of styrene,

224

anti-symmetric and symmetric vibrations of anhydride, which indicated that SMA

225

was blended with PVDF, and a considerable number of anhydride groups existed in

226

the surface of SMA/PVDF blend membrane (M-0). Comparing to the spectrum of

227

(M-0),

228

AE2311@SMA/PVDF membranes, accompanying with the appearance of a new

229

characteristic peak of ester group at 1720 cm‒1 for membranes with grafting time

the

peaks

at

1779

cm‒1

and

1858

cm‒1

disappeared

for

the

230

more than 3 h. These results indicated that the AE2311 had been grafted successfully

231

onto the SMA/PVDF matrix.

232

The chemical compositions of the surface of pristine and modified membranes

233

were further determined by XPS, and the XPS wide scan spectra were shown in Fig. 2.

234

The binding energies at 285.1, 533.1 and 688.1 eV were ascribed to C1s, O1s and F1s,

235

respectively. For M-0, the O1s peak was resulted from the anhydride group of SMA in

236

the blend membrane. While for AE2311@SMA/PVDF membrane, a new peak with

237

binding energies of 401.1 eV was observed, which was attributed to N1s. The

238

elemental

239

membranes and AE2311 were listed in Table 1. It can be found that the content of F

240

element decreased from 25.30% to 10.44% from M-0 to M-15, whereas the content of

241

O and N elements increased from 11.02% to 18.45 % and from 0.00% to 1.05%. The

242

increase of O/F ratio and N/F ratio suggested that: (1) AE2311 was grafted

243

successfully on SMA/PVDF membrane, and (2) the grafted degree rose with

244

prolonging the grafting time, which was consistent with the results monitored by the

245

weight method as mentioned previously.

246

compositions

of

SMA/PVDF

membrane,

AE2311@SMA/PVDF

247 248 249

250

(a)

251

(b)

252

Fig. 1. (a) Grafted degree as a function of grafting time. (b) ART-FTIR spectra of

253

membranes.

254

255 Fig. 2. XPS spectra of membranes.

256 257

Table 1 Elemental composition of membranes determined by XPS.

258

Composition (at %)

Atomic ratio

Samples C

F

O

N

O/F

N/F

M-0

63.68

25.30

11.02

0

0.44

0

M-3

64.75

21.97

13.12

0.16

0.60

0.0073

M-6

66.85

17.13

15.58

0.44

0.91

0.0257

M-9

68.40

13.12

17.75

0.73

1.31

0.0556

M-12

68.43

12.72

17.87

0.98

1.40

0.0770

M-15

70.06

10.44

18.45

1.05

1.77

0.1006

259 260 261

3.2 Membrane morphology and porosity

262

The SEM micrographs of SMA/PVDF and AE2311@SMA/PVDF membranes were

263

depicted in Fig. 3. As shown in Fig. 3(a), the SMA/PVDF membrane (M-0) exhibits a

264

finger-like structure on the top layer, a macroporous structure on the supporting layer,

265

and a much thinner spongy-like bottom. It is clearly observed from Fig. 3(b) to Fig.

266

3(f) that there were no obvious changes on the cross-section structures after AE2311

267

immobilization. In terms of the surface morphology, it can be found that the pore size

268

of the top surface of membrane decreased with the increase of grafting time. These

269

phenomena were resulted from the grafting of AE2311 with high molecular weight

270

onto the surface of base membrane. The pore size distribution of these membranes

271

was determined by Gas-liquid interface aperture tester, and is to be discussed later.

272 273

(a)-top

(a)-bottom

(a)-cross section

274 275

(b)-top

(b)-bottom

(b)-cross section

(c)-top

(c)-bottom

(c)-cross section

(d)-top

(d)-bottom

(d)-cross section

(e)-top

(e)-bottom

(e)-cross section

276 277

278 279

280 281

282

(f)-top

283 284

(f)-bottom

(f)-cross section

Fig. 3. FESEM photographs of membranes: (a) M-0; (b) M-3; (c) M-6; (d) M-9; (e) M-12; (f) M-15.

285 286 287

The membrane surface roughness was explored by AFM, and the results were

288

shown in Fig. 4(a). It can be seen that a smooth surface for M-0 and M-3 was

289

obtained, while abundant protuberances were found on the surface of M-6 to M-15. It

290

indicated that plenty of AE2311 was grafted on the membrane surface, which was in

291

consistence with the results from SEM. The results of mean surface roughness (Ra)

292

and root mean square roughness (Rq) obtained from AFM were shown in Fig. 4(b). It

293

can be observed that both Ra and Rq increased with increase of grafting time. The

294

mean surface roughness of M-0 was 21.9 nm, and it increased gradually to 49.1 nm

295

after grafting with AE2311 for 9 h. However, the mean surface roughness was up to

296

171 nm and 173 nm when the grafting time was 12 h and 15 h. Similar results were

297

observed for the root mean square roughness. The sudden increase of membrane

298

surface roughness was believed that abundant demulsifier was immobilized on the

299

SMA/PVDF membrane, and these results were also concordant with the observation

300

from SEM.

301 302

(a)

303 304

(b)

305

Fig. 4. (a) AFM images and (b) surface roughness values of membranes.

306

The average pore size and porosity were further investigated to assess the effect of

307

grafting time on performance of the modified membrane. As illustrated in Fig. 5, the

308

average pore size and porosity of pristine SMA/PVDF membrane were 65.26 nm and

309

70.30%, while both of them declined after AE2311 being grafted onto the membrane.

310

This is because that AE2311 is a phenolamine resin polyoxypropene polyoxyethylene

311

polyether with molecular weight about 13000. It can be also found in Fig. 5, that the

312

average pore size and porosity decreased continuous from 59.47 nm to 41.98 nm and

313

70.30% to 62.33% in the first 9 h. However, no obvious change was observed after 12

314

h, and the average pore size and porosity for 12 h and 15 h were 38.98 nm, 38.28 nm,

315

60.94% and 60.17%, respectively.

316 317

Fig. 5. Porosity and average pore size of various membranes.

318 319

3.3 Surface wettability

320

The surface wettability of SMA/PVDF membrane and AE2311@SMA/PVDF

321

membranes were characterized by contact angle test. Fig. 6 showed the changes in the

322

WCA on the surface of pristine SMA/PVDF membrane and AE2311@SMA/PVDF

323

membranes with various grafting time. It was found that the value of WCA of PVDF

324

membrane was around 118°, which suggested the strong hydrophobicity of the PVDF

325

membrane. In addition, the pristine value of WCA on the SMA/PVDF membrane

326

surface was 68.5°, and it changed slightly in 60 s, indicating a vast improvement in

327

hydrophilicity of PVDF by blending with SMA. The value of WCA further declined

328

after AE2311 was grafted on the SMA/PVDF membrane. The minimum pristine WCA

329

value was 18.5° with grafting time of 9 h, and it dropped to 0° in 30 s. These results

330

demonstrated that the introduction of AE2311 greatly improved the hydrophilicity of

331

the PVDF membrane, and a superhydrophilic membrane of M-9 was obtained. It

332

should be noted that the values of WCA was actually going up, rather than down for

333

the modified membrane with grafting time of 12 h and 15 h. This can be explained by

334

the topological structure theory [55]. Generally, the wettability of materials was

335

influenced by the topological structure and chemical composition. In another word,

336

the surface pore structure and surface chemical composition controlled the

337

hydrophilicity. The surface wettability was dominated by surface chemical

338

composition when the grafting time was less than 9 h. Thus the WCA decreased with

339

prolonging time. However, membranes with dense surface, lower porosity and smaller

340

pore size were obtained with increasing grafting time to 12 h and 15 h. Therefore, the

341

values of WCA increased rather than decreased when grafting time increased to 12 h

342

and 15 h.

343 344

Fig. 6.

Dynamic water contact angle of membranes.

345

In view of O/W emulsion separation, the underwater oil contact angles (UOCA)

346

was tested to characterize the surface oleophobicity in water for the modified PVDF

347

membrane. Fig. 7(a) showed the UOCA that were observed on the M-9 membrane

348

with different oil. It was found that the UOCA of M-9 with kerosene, dichloroethane,

349

toluene and petroleum ether were 168.5°, 150.5°, 158.5° and 152.5°, respectively. For

350

comparison, the UOCA for pristine SMA/PVDF and modified M-9 membranes with

351

kerosene at different time were also tested, and the results were exhibited in Fig. 7(b).

352

The UOCA was 93.5° at 0 s for SMA/PVDF membrane, and it decreased to 85.0° at

353

60 s, which was much less than that of M-9 membrane. These results demonstrated

354

that the surface hydrophilicity of the modified PVDF membrane improved the

355

interactions for water and the membrane surface, and a thick hydrated layer was

356

formed subsequently. Therefore, the AE2311@SMA/PVDF membrane became

357

underwater superoleophobic, which is critical for separation of O/W emulsion.

358

Dynamic underwater-oil-adhesion experiments were carried out to evaluate the

359

performance of underwater anti-oil-fouling. As shown in Fig. 7(c), the underwater

360

oil-adhesion forces were conducted on M-0 and M-9 membranes using kerosene

361

droplets as detecting probes. A drop of oil was forced to contact with the membrane

362

surface sufficiently, then it was continued to be brought into contact with the

363

membrane for 5s. Afterwards, the oil droplet was lifted down and left the membrane

364

surface. It was found that the oil droplet was adsorbed on the pristine SMA/PVDF

365

membrane (M-0) surface because of the adhesion-force between oil and M-0. On the

366

contrary, no residual droplets remained on M-9, and no visible deformation of oil

367

shape was observed when the oil was lifted down. Obviously, the AE2311 modified

368

PVDF membrane (M-9) had the potential to exhibit outstanding anti-oil-fouling

369

performance in oil/water separation.

370 371

(a)

372 373

(b)

374

375 376 (c)

377 378

Fig. 7.

(a) Underwater oil contact angles on the surface of M-9 membrane with

379

different oil. (b) Underwater oil contact angles on the surface of M-0 and M-9

380

membrane with kerosene as function of time. (c) Dynamic underwater-oil-adhesion

381

behavior of M-9 and M-0 with kerosene.

382 383

3.4 Water permeability and antifouling performance

384

The results of the pure water flux of M-0 and AE2311@SMA/PVDF membranes

385

were exhibited in Fig. 8. As shown, the pure water flux of M-0 was 561.54 L·m‒2·h‒1,

386

while a gradual decrease in water flux was observed after AE2311 was grafted on the

387

SMA/PVDF membrane. The pure water flux of M-12 was only 230.54 L·m‒2·h‒1,

388

which was much lower than the flux of M-0. This apparent decrease in water flux was

389

mainly resulted from the decrease of average pore size and porosity of membranes as

390

expound in Section 3.2. It can also be observed that the pure water flux of M-15 was

391

218.49 L·m‒2·h‒1, which was lower than that of M-12 with comparative average pore

392

size and porosity. This may be explained by the hydrophilicity of M12. As we all

393

know, hydrophilicity of the membrane usually favors higher permeability [56].

394

Therefore, the higher flux for M-12 than that for M-15 is mainly resulted from the

395

better hydrophilicity of M-12.

396 397 398

Fig. 8. Pure water flux of SMA/PVDF membrane (M-0) and AE2311@SMA/PVDF composite membranes (M-3, M-6, M-9, M-12 and M-15).

399

To further evaluate the fouling resistance of the AE2311@SMA/PVDF membrane,

400

the BSA solution permeation was carried out after the initial pure water fluxes (Jw0)

401

were obtained. After being fouled by BSA, the membrane was rinsed and the pure

402

water fluxes (Jw3) was measured again. These results were depicted in Fig. 9 and

403

Table 3. As shown in Table 3, the BSA rejection ratio for M-0 was 97.0%, while it

404

was almost 100% for the modified membranes. The high rejection of BSA by the

405

AE2311@SMA/PVDF membrane was attributed to the superhydrophilicity and

406

nanopore structure as expounded in Section 3.2 and 3.3.

407

Comparing to the pure water fluxes, the BSA solution permeate fluxes declined

408

significantly due to BSA depositing on the surface of membranes. After being fouled

409

by protein, a simple hydraulic clean was carried out. The pure water fluxes of the

410

cleaned membranes were further measured, and various flux recovery ratios (FRR)

411

were obtained as shown in Fig. 9 and Table 3. It showed that a low FRR of 39.61%

412

was obtained for pristine SMA/PVDF membrane after 3 cycles of fouling, while the

413

FRR was improved after the membrane was modified by AE2311, which indicated

414

that the AE2311@SMA/PVDF membranes exhibited excellent antifouling ability.

415

Furthermore, it is also noticeable that the FRR (92.36%) of M-9 is much higher than

416

that of other membranes. This suggests that the modified membrane M-9 possesses

417

the best antifouling capability, which can be attributed to the property of

418

superhydrophilicity and underwater superoleophobicity of M-9 that prevent the oil

419

droplets from contacting the membrane surface [57].

420

In consideration of good antifouling performance, superhydrophilic and underwater

421

superoleophobic properties, and high water permeability, M-9 was chosen for further

422

O/W emulsion separation experiments.

423 424

Fig. 9. Three cyclic filtration experiments of SMA/PVDF and AE2311@SMA/PVDF

425

membranes contaminated by BSA.

426 427

Table 3 Anti-fouling properties of pristine blend membrane and modified membranes* Membrane

RBSA (%)

JW0

JW3

FRR

(L·m-2·h-1)

(L·m-2·h-1)

(%)

M-0

97.0

548.34

217.21

39.61

M-3

100

486.75

349.68

71.84

M-6

100

424.85

343.23

80.78

M-9

100

362.14

334.49

92.36

M-12

100

230.54

115.29

50.01

M-15

100

218.49

117.94

53.98

428 429 430

* Jw0, initial

pure water flux of membrane, Jw3, pure water flux of membrane being

fouled 3 cycles by BSA.

431 432

3.5 Separation of O/W emulsion

433

It is believed that the prominent anti-oil-fouling performance endowed the

434

AE2311@SMA/PVDF membrane with outstanding capability to separate O/W

435

emulsion. This was confirmed by a cross-flow filtration of emulsion using M-9

436

membrane. The O/W emulsion stabilized by SDS was stable for more than 72 h, and

437

was turbid resulting from existence of emulsified oil droplets as shown in Fig. 10(a).

438

It was found that the diameter of the oil droplets in the pristine O/W emulsion ranged

439

from 2-10 µm as shown in Fig. 10(c), which was consistent with the general

440

observations of the oil droplets diameters in O/W emulsion as reported in the

441

literatures [18-20]. The diameters of oil droplets in the permeate solution decreased

442

and were found to be in the range of 200 nm to 700 nm (Fig. 10d) after the emulsion

443

was filtrated by M-0 membrane. This result indicated that only large oil droplets were

444

retained, and the emulsion passed through the M-0 membrane without being

445

demulsified. However, the turbid O/W emulsion became clear (Fig. 10b) after

446

filtration through M-9. As suggested in the optical microscope images, almost no

447

visible oil droplets could be observed in the permeate solution. Moreover, no oil

448

droplets were found in the permeate after filtrated by M-9 membrane in Malvern

449

particle size test. This illustrated that the AE2311@SMA/PVDF membrane only

450

enabled water to permeate through whereas preventing the oil droplets after they were

451

separated from the O/W emulsion.

452

453 454

(a)

455 456 457 458

(b)

459 460

(c)

461 462 463

(d) Fig. 10 (a) Photographs of O/W emulsion before and after filtration with M-0 and

464

M-9, (b) Optical microscopy images of O/W emulsion before and after filtration

465

with M-0 and M-9, (c) Oil particle size distribution of the original O/W emulsion,

466

(d) Oil particle size distribution of the permeate through M-0 membrane.

467 468

In order to further evaluate the separation capability of the modified membrane on

469

various emulsions, the quantitative measurement of the separation efficiency was

470

carried out using the TOC analysis. The results were listed in Table 4. The TOC

471

content of kerosene-in-water emulsion was as high as 1059.5 ppm. After being

472

separated by M-0 and M-9 membranes, the TOC contents decreased to 573.0 ppm and

473

8.6 ppm, and the TOC removals for M-0 and M-9 membranes were 45.9% and 99.2%,

474

respectively. Similarly, all of the TOC values of emulsions with dichloroethane,

475

toluene and petroleum ether decreased dramatically from more than 400 ppm to less

476

than 10 ppm after filtration with M-9 membrane. The results proved that the AE2311

477

modified PVDF membrane possessed high separation efficiencies for various O/W

478

emulsions.

479

Table 4 The TOC results for emulsions before and after filtration* Emulsions with

TOCf

TOCpM-0

TOCpM-9

RM-0

R M-9

different oil

(ppm)

(ppm)

(ppm)

(%)

(%)

Kerosene

1059.5

573.0

8.6

45.9

99.2

Dichloroethane

542.8

253.3

5.0

53.3

99.1

Toluene

427.6

153.5

4.1

64.1

99.0

Petroleum

452.2

183.8

3.2

59.3

99.3

480

* TOC pM-0 and TOCp M-9 are the concentrations of the permeate after filtration with

481

M-0 and M-9. RM-0 and RM-9 are the oil rejection ratios of M-0 and M-9.

482

As the membrane is susceptible to oil droplet pollution during separation

483

experiment,

three-cycle

filtration

experiments

were

carried

out

using

484

kerosene-in-water emulsion as a contamination model to further investigate the

485

antifouling performance of M-9 membrane. As indicated in Fig.11, the emulsion flux

486

of M-0 reduced from 518.12 L·m‒2·h‒1 to 238.29 L·m‒2·h‒1 and almost no recovery

487

was found after rinsed by deionized water. The FRR of M-0 was 45.99% owing to the

488

fact that many emulsified oil droplets adhered on the walls of pore during filtration

489

and were problematic to be rinsed out. On the contrary, the water flux of M-9 only

490

decreased by 10.64 L·m‒2·h‒1 after three cyclic filtrations and the FRR was achieved

491

at 96.56%. This is because that it could encounter difficulties for oil droplets to enter

492

the pores of M-9 and these oil droplets could be easily rinsed out by water to make the

493

membrane reusable. The oil droplets size and distribution of the permeate solution

494

after the second cycle were further conducted to evaluate the antifouling performance

495

of membrane M-0 and M-9. As shown in Fig. 12(a), a large number of oil droplets

496

were found in the permeate solution after the second filtration by membrane M-0, and

497

the diameters of oil droplets were found to be in the range of 400 nm to 1200 nm as

498

shown in Fig. 12(b). These results indicated that a fouling layer was formed on the

499

membrane surface and in the pores. Thus more and more oil pollutants permeated

500

through M-0 in the second and subsequent cycle filtrations. On the contrary, hardly

501

any oil droplets can be observed in the permeate solution after the second filtration by

502

membrane M-9 as shown in Fig. 12(a). This result implied that M-9 membrane has

503

good

504

superhydrophilicity properties of the modified membrane [58].

anti-pollution

performance

due

to

the

superoleophobicity

and

505 506

Fig. 11. Three cyclic filtration experiments of SMA/PVDF membrane (M-0) and

507

AE2311@SMA/PVDF composite membranes (M-9) contaminated by kerosene-water

508

emulsion.

509

510 511

(a)

512

513 514

(b)

515

Fig. 12 (a) Optical microscopy images of the permeate through M-0 membrane (left)

516

and M-9 membrane (right) after the second filtration. (b) The oil droplets distribution

517

of the permeate through M-0 membrane after the second filtration.

518 519

The long-term operation stability of the modified membrane is extremely important.

520

Therefore, ten cycles of O/W emulsion separation - pure water rinsing experiments

521

were carried out. The O/W emulsion separation for each cycle lasted for 120 min. A

522

stable water flux of 335 L·m-2·h-1 and ultrahigh oil rejection of more than 99.0% were

523

obtained as shown in Fig. 13(a). What’s more, the WCA and UOCA were not changed

524

after the M-9 membrane was used for ten cycles as shown in Fig. 13(b), which

525

indicated that the superhydrophilic and underwater superoleophobic properties were

526

not destroyed. These results implied the outstanding stability of the modified PVDF

527

membrane.

528 529

(a)

530 (b)

531 532

Fig. 13. The long-term operation stability of M-9 membrane for O/W emulsion

533

separation. (a) Water flux and oil rejection, (b) water contact angle and underwater oil

534

contact angle.

535 536

3.6 Verification of the demulsification by AE2311@SMA/PVDF membrane and

537

demulsification mechanism

538

To further verify the demulsification effect of the modified membrane, membrane

539

demulsification experiment without permeation was carried out by using the same

540

cross-flow filtration system (Scheme 2) with the valve-2 off, and kerosene was used

541

for preparing the O/W emulsion. In this process, the emulsion flowed the membrane

542

surface at a flow rate of 0.1 mL/min. The mixture from the outlet of membrane cell

543

(valve-1 in Scheme 2) was collected in a beaker, and samples for particle size

544

determination were taken from the near-bottom of the beaker. The oil droplet size

545

distribution is shown in Fig. 14 after the emulsion contacted with the surface of M-0

546

and M-9 membranes. It was illustrated in Fig. 10(c) that the diameters of the oil

547

droplets in the pristine O/W emulsion were in the range of 2-10 µm. While there is

548

little change in the size of the oil droplets (Fig. 14a) after the emulsion passed the M-0

549

membrane surface. This is because there are no groups with demulsification function

550

on surface of M-0 membrane. However, only small oil droplets with diameters less

551

than 500 nm was observed after the emulsion passed the M-9 membrane surface (Fig.

552

14b). Because of no transmembrane transport, the “size sieving” effect [33, 37] for

553

O/W emulsion separation could be excluded. Therefore, the demulsification is

554

believed to take place when the emulsion contacts the AE2311 on the modified PVDF

555

membrane. The surface of the AE2311 modified PVDF membrane is equipped with a

556

large number of free PPO-b-PEO copolymers, which can act as demulsifying agent.

557

Therefore, the emulsion that right closed to the membrane surface was broken (the

558

emulsion that is far away from the membrane surface may keep no change). Thus a

559

large number of oil slick on the surface of the mixture came out from the membrane

560

cell, and some small oil droplets was left in the solution. These results were further

561

confirmed by the TOC determination. The TOC content in the original emulsion was

562

1059.5 ppm, while it was 1008.0 ppm and 451.0 ppm after the emulsion passed across

563

M-0 membrane and M-9 membrane, respectively.

564 565 566

(a)

567 568

(b)

569

Fig. 14 Oil droplet size distribution after emulsion contacted with the surface of M-0

570

membrane (a) and M-9 membrane with demulsifier modification (b).

571 572

The demulsification mechanism was proposed as shown in Fig. 15. When M-0

573

membrane was used for O/W emulsion separation, it was easy for oils to adhere on

574

the surface of membrane because of the weak hydration ability and high adhesion free

575

energy [59]. Whereas, M-9 membrane presented excellent hydration ability and low

576

adhesion free energy which enabled the membrane with great anti-adhesion

577

performance towards oils. As depicted in Fig. 1, the surface of the modified PVDF

578

membrane was filled with PPO-b-PEO copolymers due to the immobilization of

579

AE2311. These PPO-b-PEO copolymers are just like innumerable “arms of God” or

580

“arm-like knives”, which can penetrate into oil-water interfacial film of the emulsion

581

due to the hydrophilicity of PEO and oleophobicity of PPO. Furthermore, the

582

interfacial film was torn up by the “arms”, which induced the deformation of the

583

interfacial film and the non-equilibrium distribution of interfacial active substances

584

that stabilized the emulsion. As a result, the thermodynamic equilibrium was

585

destroyed due to the removal and re-arrangement of the interfacial active substances

586

on the interfacial film [59]. This provided the condition for the coalescence of oil

587

droplets. Thus, the stable emulsion was broken and oil droplets came out. The oil

588

droplets coagulated to form large oil droplets, only to accumulate into a large number

589

of oil slick on the surface of the O/W mixture. Finally, the water after being

590

demulsified passed through the M-9 membrane continuously. Meanwhile, the

591

superhydrophilicity and underwater superoleophobicity properties enabled the

592

modified membrane to form a hydration layer on the surface, which in turn formed a

593

barrier to keep the oil droplets from contacting the surface of the membrane. On the

594

other hand, the capillary action could be used to clarify the superhydrophilic and

595

underwater superoleophobic property of AE2311@SMA/PVDF membrane. The

596

average pore size of the AE2311@SMA/PVDF membrane was 41.98 nm, which was

597

correspond with the requirements of capillary mechanics, so that these pores can be

598

called rich capillaries [60]. Therefore, water molecules could permeate through the

599

capillaries and maintain continuous permeation. On the contrary, the oil droplets were

600

isolated outside the water layer of the membrane surface because of the

601

superoleophobicity properties of the modified membrane, and then went to the

602

concentration tank or return to the feed tank. The cross-flow filtration process avoids

603

the contact of oil droplets with the membrane surface, and prevents the accumulation

604

of oil droplets on the membrane surface. So as to avoid clogging and fouling the

605

membrane pore, thus obtaining the ideal demulsification effect and realizing the

606

separation of O/W emulsion with high efficiency.

607 608

Fig. 15 Schematic diagram of demulsification mechanism of SMA/PVDF

609

membrane (M-0) and AE2311@SMA/PVDF membrane (M-9).

610 611

Conclusion

612

In this work, a polyether demulsifier (AE2311) grafted styrene-co-maleic anhydride

613

(SMA) blend polyvinylidene fluoride (PVDF) membrane was prepared by the in-situ

614

alcoholysis reaction between maleic anhydride on membrane surface and terminal

615

hydroxyl group on AE2311. The demulsifier modified membranes were characterized

616

by ATR-FTIR, XPS, SEM and AFM characterization, and the surface wettability was

617

tested by contact angle measurement. It suggested that the grafted degree increased

618

linearly with increase of reaction time. Accordingly, the pore size and porosity

619

declined, which resulted in the decrease of pure water fluxes of the membranes. The

620

surface wettability measurements suggested that the M-9 membrane exhibited

621

superhydrophilicity (WCA=0°), underwater superoleophobicity (UOCA>150°) and

622

excellent anti-oil-fouling property. The M-9 membrane can break the O/W emulsion

623

even by membrane surface contacting with the emulsion, and the oil removal rate was

624

up to 57.4% without membrane filtration. Furthermore, the oil removal rate was more

625

than 99.0% by the cross-flow filtration system using M-9 membrane, indicating that

626

the modified membrane has outstanding oil-water separation capability. Besides, the

627

modified membrane can be repeated use and exhibited long-term operation stability.

628

The oil rejection ratio and the UCOA were 99.4% and 156° after the M-9 membrane

629

was used for ten cyclic filtrations. Therefore, this research offers an attractive strategy

630

to fabricate membranes with superhydrophilicity and underwater superoleophobicity

631

for separating oil from O/W emulsions.

632 633

Acknowledgments:

634

This work was financially supported by Tianjin Science and Technology Planning

635

Project (Grant No. 18JCYBJC89300 and 18PTZWHZ00210), the National Natural

636

Science Foundation of China (Grant Nos. 21808166, 21878230 and 21376176), the

637

Chang-jiang Scholars and Innovative Research Team in the University, Ministry of

638

Education, China (Grant No. IRT-17R80) and the Cultivating Program for Innovative

639

Research Team of Tianjin High College, China (No. TD13-5044). The Program of

640

Introducing Talents of Discipline to Universities of China (111 Program) (Grant No.

641

D18021) is also appreciated.

642

643

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Highlights A polyether demulsifier graft PVDF blend styrene-co-maleic anhydride membrane was prepared The membrane exhibited superhydrophilicity and underwater superoleophobicity performance The membrane showed excellent anti-oil-fouling performance and the FRR was about 90% The oil removal rate of O/W emulsion was more than 99.0% by filtration coupling with demulsification The membrane can break the O/W emulsion even by membrane surface only contacting with the emulsion The modified membrane could be repeated use and exhibited long-term operation stability

Conflict of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Statement The authors have made substantial contributions to the conception or design of the work; or the acquisition, analysis for the work; The authors have drafted the work or revised it critically for important intellectual content; The authors have approved the final version to be published; The authors agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.