Nanocomposite Membrane with Polyethylenimine-Grafted Graphene

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Remediation and Control Technologies

Nanocomposite Membrane with Polyethylenimine-grafted Graphene Oxide as a Novel Additive to Enhance Pollutant Filtration Performance Lina Zhang, Baoliang Chen, Abdul Ghaffar, and Xiaoying Zhu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00524 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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Environmental Science & Technology

1

Nanocomposite Membrane with Polyethylenimine-grafted

2

Graphene Oxide as a Novel Additive to Enhance Pollutant

3

Filtration Performance

4 5

Lina Zhang1,2, Baoliang Chen1,2, Abdul Ghaffar1,2 and Xiaoying Zhu*1,2

6 7 8 9 10

1. Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310058, China. 2. Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China.

11 12 13

*Corresponding author:

14

Dr. Xiaoying Zhu, [email protected]

15 16

Tel.: +86-571-88982651

17

Fax: +86-571-88982651

18 19 20

Co-authors:

21

Lina Zhang, [email protected]

22

Abdul Ghaffar, [email protected]

23

Prof. Baoliang Chen, [email protected]

ACS Paragon Plus Environment


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Abstract

25

Synthetic membranes often suffer ubiquitous fouling as well as a trade-off

26

between permeability and selectivity. However, emerging materials which are able to

27

mitigate membrane fouling and break the permeability and selectivity trade-off are

28

urgently needed. A novel additive, GO-PEI, bearing a positive charge and hydrophilic

29

nature was prepared by the covalent grafting of polyethylenimine (PEI) molecules

30

with graphene oxide (GO) nanosheets, which later was blended with bulk

31

polyethersulfone (PES) to fabricate the graphene containing nanocomposite

32

membranes (NCMs). Strong π-π interactions contributed to the uniform dispersion of

33

GO-PEI nanosheets in bulk PES to form the asymmetric structure of NCM without

34

leaching. The ratio of the GO-PEI additive regulated the surface charge and

35

hydrophilicity of NCMs. To filter charged proteins, the designed NCM exhibited a

36

high permeability (flux) and high selectivity (retention) while showing resistance to

37

fouling by the charged proteins, which could be attributed to the asymmetric structure

38

and composition of the NCM that the porous internal and surface composited with the

39

GO-PEI additive was responsible for the NCM’s high flux; thereafter, the electrostatic

40

attraction of the NCM surface to the charged pollutant enhanced the solute/water

41

selectivity; finally, the synergistic effect of the hydrophilic and charged functional

42

groups of the GO-PEI contributed to the formation of a dense hydration layer on the

43

membrane surface thereby reducing membrane fouling. The NCM functionalized with

44

the GO-PEI additive demonstrated potential for high-performance pollutant removal

45

in water and wastewater treatments.

46 47 48

Keywords:

49

Graphene

50

permeability/selectivity trade-off, membrane fouling

oxide,

nanocomposite

membrane,

hydrophilicity,

51


surface

charge,

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TOC

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1. Introduction

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The development of membrane technologies has revolutionized the field of water and

57

wastewater treatment during the last few decades.1-4 Lower chemical dosage, higher separation

58

efficiency and smaller footprint are the main advantages of membrane separation technologies in the

59

fields of environmental and industrial processing.5, 6 However, current membrane technologies are

60

suffering from unavoidable issues, such as membrane fouling7, 8 and the permeability/selectivity

61

trade-off.9-11 Therefore, it is of great importance to solve or mitigate the abovementioned problems in

62

the development of high performance membrane. Nevertheless, both membrane fouling and the

63

permeability/selectivity trade-off are related to the interactions between the retained components and

64

the membrane surface. Furthermore, antifouling membranes have been developed based on

65

weakening the surface interactions between the foulants and the membrane surface.3, 8 However, a

66

high permeability and selectivity of the membrane could be achieved by introducing additional

67

features, such as the adsorption capacity, with minimal reduction in the membrane permeability.

68

The conventional methods for surface modification to improve the performance of a membrane,

69

such as high energy irradiation, polymer brush grafting, layer-by-layer (LbL) assembly, etc., have

70

been adopted extensively.3,

71

modification during commercial membrane production involves blending an additive with the bulk

72

polymer to fabricate membranes, thereby allowing surface modification.3,

73

during the membrane fabrication could be either natural or synthetic. However, natural additives,

74

such as aquaporin, would be easily denatured, losing their function during membrane fabrication and

75

field applications.5, 13 Moreover, organic molecules without stable anchoring, such as polyetherimide

76

as an additive, can easily leach from the membrane during filtration.14 In addition to natural additives,

77

synthetic additives such as functional nanomaterials, e.g., carbon nanotubes (CNTs)15,

78

graphene oxide (GO),17,

79

nanocomposite membranes.19 Nevertheless, it is difficult to accomplish uniform dispersions of

80

conventional functional nanomaterials within a membrane matrix due to the aggregation tendency

81

and low affinity between the inorganic nanomaterial and the bulk polymer. In addition, the stability

18

8

One of the most reliable and scalable approaches for surface

12

The additives used

16

and

have frequently been incorporated into a polymer matrix to form


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of the integrated nanomaterial within the membrane matrix is another important concern in the

83

fabrication of nanocomposite membranes since the leaching of the integrated nanomaterial may not

84

only affect the membrane structure, proper functioning and efficiency, but also lead to potential

85

environmental risks.

86

GO as a typical graphene derivative has attracted widespread attention because of its fascinating

87

properties,20 such as adsorption capability, its atomic thickness as well as abundant

88

oxygen-containing functional groups for easy dispersion in water21 and further chemical

89

modification.22-25 GO nanosheets used directly as sorbents in water reclamation and purification are

90

difficult to recover and recycle, introducing new risks to aquatic environments, as reported

91

earlier.26-28 Nonetheless, despite their easy loss or release into the natural environment, GO could be

92

used to construct membranes which were much easier to be recovered and showed high performance

93

in water purification.29 Moreover, GO can be utilized as a functional additive in bulk polymers via

94

blending to fabricate hydrophilic membranes. As reported earlier, GO functionalized with sulfonic

95

acid groups and subsequently blended with PVDF has been used to prepare antifouling membranes

96

with improved permeability.18 In another study, GO was incorporated with polyethersulfone (PES) to

97

prepare organic-inorganic blended membranes with a lower fouling tendency.17 The benzene ring of

98

PES may form a π-π interaction with the aromatic structure of the GO nanosheet, resulting in the

99

more stable incorporation of GO in the membrane matrix.17, 30 In addition to the hydrophilicity, the

100

surface charge is another vital surface property that determines the nature of the interactions between

101

the retained components and the membrane surface.31-34 However, the effects contributed by the

102

surface charge of the GO composite membranes have not been revealed in the literature. Furthermore,

103

the reported graphene-based additives were usually able to adjust only one surface property in one

104

direction. Thus, manipulating the interactions between the retained components and the membrane

105

surface simultaneously by tuning the surface charge and hydrophilicity using graphene-based

106

additives has not been reported yet.

107

The objective of this study is to develop a novel nanocomposite membrane with a synthetic

108

graphene derivative to overcome membrane fouling and the permeability/selectivity trade-off. A

109

functional additive (GO-PEI) was synthesized by covalent bonding between polyetherimide (PEI)

110

and GO nanosheets. Polyethersulfone (PES) was chosen as a bulk polymer to mix with the

111

graphene-based additive in different ratios for the fabrication of NCMs. Both the GO-PEI additive ACS Paragon Plus Environment


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and the prepared NCMs were well characterized. Subsequently, typical pollutants including

113

phenanthrene (PHE), proteins and bacteria were selected to evaluate the performance of the NCMs

114

through dynamic filtration and static adsorption tests. The NCM performance was evaluated based

115

on the contribution of different surface properties by different filtration processes. This work

116

proposed a facile and scalable method to prepare graphene-based nanocomposite membranes,

117

thereby regulating the surface properties for the enhanced removal of pollutants from aquatic

118

environments.

119

2. Materials and methods

120

2.1. Materials

121

Natural graphite flakes (325 meshes, 99.8%) were purchased from Alfa-Aesar. Polyethersulfone

122

(PES, Mw ~48 kDa) was provided by Sigma-Aldrich. Polyethylenimine (PEI, Mw ~1800 Da),

123

polyvinylpyrrolidone (PVP) (K29-32), N-N-dimethylacetamide (DMAc >99%), bovine serum

124

albumin (BSA, MW. 66 kDa) and lysozyme (LYZ, MW. 14.3 kDa) were obtained from Aladdin

125

Reagent Company (Shanghai, China) and phenanthrene (PHE) was provided by J&K Chemical. All

126

chemicals were used without further purification.

127

2.2. Synthesis and characterization of GO-PEI

128

GO was synthesized from natural graphite flakes by a modified Hummers’ method and detailed

129

earlier.26, 27 To prepare the GO-PEI additive, 200 mg GO was added to 20 mL of ultrapure water and

130

sonicated for 30 min. The pH of the solution was adjusted to 10 by subsequently adding a NaOH

131

solution (0.1 M). PEI (600 mg dissolved in 60 mL of DI water) was added to the GO solution, and

132

the mixture was sonicated for 1 h. Then, the mixture was heated in an oven at 100 °C for 24 h to

133

obtain the additive and was denoted as GO-PEI.

134

The surface functional groups of GO-PEI were characterized by Fourier transform infrared

135

spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The surface charge of the GO and

136

GO-PEI were evaluated by zeta (ζ) potential measurements, which were conducted at different

137

equilibrium pH values using a Nano-ZS90 Zetasizer (Malvern Instruments, Ltd.). The Zetasizer was ACS Paragon Plus Environment

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also used to investigate the size of the GO and GO-PEI.

139

2.3. Fabrication and characterization of NCM

140

The NCMs containing GO-PEI were fabricated by a non-solvent-induced phase inversion. The

141

casting solution compositions of all membranes are listed in Table 1. A certain amount of GO-PEI

142

was sonicated for 30 min to obtain a homogeneous dispersion in DMAc. Thereafter, PES and PVP

143

were added and dissolved in the dope solution at 70 °C under continuous stirring in sequence to

144

achieve a homogeneous solution; the dope solution was further stirred for 24 hours at 70 °C followed

145

by degassing under vacuum to release the bubbles. Finally, the degassed solutions were uniformly

146

casted on glass plates with a thickness of 200 µm by using an automatic film applicator (BEVS1811)

147

and a doctor blade (BEVS 1806/150). Subsequently, the glass plates were immediately immersed in a

148

coagulation bath containing DI water. The solidified membranes were stored in a fresh pure water

149

bath for 24 h to ensure a complete phase inversion. Eventually, the prepared membranes were stored

150

in pure water before further use.

151

Table 1. Composition of casting solutions for NCM preparation Membrane name Component M0

M1

M2

M3

PES (wt. %)

20

19.5

19

18.5

GO-PEI (wt. %)

0

0.5

1

1.5

PVP (wt. %)

1

1

1

1

Solvent (wt. %)

79

79

79

79

152 153

The surface morphology and cross-section of the prepared membranes were observed using a

154

field emission scanning electron microscope (FESEM, Hitachi S-4800, Tokyo Japan). To prevent the

155

collapse of pores during drying, a multi-step solvent exchange method was used to dry the

156

membranes. The membrane samples were immersed in each water/ethanol mixture (80:20, 60:40,

157

40:60, 20:80 and 0:100) for 20 min in sequence followed by drying in air. The membrane samples

158

were freeze-fractured in liquid nitrogen at 77 K to obtain regular cross-sections. The membrane

159

samples and cross-sections were then coated with a thin layer of platinum prior to SEM observation.



160

ImageJ software was used to measure the pore size of the NCMs from SEM images. The surface

161

morphologies of these NCMs were also analyzed by atomic force microscopy (AFM (Bruker

162

Dimension Icon). The membrane surface scanning was performed both in air and water by using the

163

ScanAsyst mode developed by Bruker. The surface roughness of each membrane was measured from

164

the height profile of the three-dimensional AFM images. In addition, a Bruker Dimension Icon AFM

165

system was used to measure the adhesion force between the probe and the membrane surface. All

166

force curve measurements between the SiO2 probe (diameter of 2 nm; Bruker's SNL-10-A) and the

167

membrane surfaces were performed in liquid conditions (PBS, pH=7.2). All force measurements

168

were conducted using the peak force quantitative nanomechanics (PFQNM) mode at a vertical scan

169

rate of 1 Hz. The AFM data were analyzed by the NanoScope Analysis software from Bruker.

170

Nitrogen adsorption-desorption isotherms of the NCMs were measured by using a

171

NOVA-2000E surface area analyzer (Quantachrome Instruments) at -196 °C. The membrane samples

172

were dried in an oven overnight prior to the surface area (SA) analysis. The adsorption capacities of

173

the NCMs were calculated by the Brunauer-Emmett-Teller (BET) method. The pore size distribution

174

(PSD) was determined from the desorption branches of the isotherms by using the

175

Barrett-Joyner-Halenda (BJH) model.

176

The mechanical strength, including the tensile stress and strain parameters of the NCMs, was

177

measured using a Universal material experiment machine (Zwick/Roell Z020). The dried membrane

178

samples were cut into dumbbell shapes (6 cm in length, and 1 cm in width) and vertically clamped at

179

a gauge length of 5 cm. The dragging rate of the grip was 2 mm/min in the test. Five measurements

180

were taken for each membrane sample. The surface wetting properties of the prepared membranes

181

were evaluated by the water contact angle. A surface analyzer (OSA200 Optical) provided by Ningbo

182

NB Scientific Instruments Co., Ltd. was used for the contact angle measurements. A 10 µL water

183

droplet was dropped onto the dry sample surface through the microsyringe of the device. The water

184

droplet image was captured once it touched the membrane surface and was analyzed by the

185

instrument to obtain the contact angle (CA) value of the tested membranes. The ζ potentials of the

186

NCMs were measured by using a SurPASS 3 electro-kinetic analyzer from Anton Paar (GmbH,

187

Austria). The membranes were cut into 1×2 cm slides and attached to the sample holders. A 0.01 M

188

KCl aqueous background solution was used for the determination of the ζ potential of the membranes

189

at different pH values ranging from 2.0 to 10.0. ACS Paragon Plus Environment

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The stability of the additive in the membrane was evaluated through measuring the TOC of the

191

permeate during the filtration of DI water using the prepared membranes. The DI water was pumped

192

into a filtration cell equipped with a membrane (diameter: 2.2 cm) using a peristaltic pump. The flux

193

was set at 0.5 mL/min and the permeate was collected per hour over the 5-hour filtration. TOC of the

194

collected permeates were measured using a total organic carbon analyzer (TOC-Vcph, SHIMADZU).

195

2.4. Dynamic filtration tests

196

In the dynamic filtration tests, the feed concentration (100 mg/L) of the BSA or LYZ solution

197

was prepared by dissolving BSA or LYZ in PBS (pH=7.2). Escherichia coli (E. coli) were cultivated

198

in a LB broth (10 g of tryptone, 5 g of yeast extract and 10 g of NaCl) for 24 h at 37 °C. Then, the

199

bacterial suspension was centrifuged at 4000 rpm for 10 min, and the supernatant was discarded. The

200

bacteria were washed three times using PBS and then re-suspended with PBS. The final bacterial

201

suspensions were prepared by diluting the original bacterial suspension with PBS for a final

202

concentration at ~105 CFU/mL.

203

The filtration experiments were carried out by using a dead-end filtration system, as shown in

204

Figure S1. The filtration was driving by compressed nitrogen gas from a cylinder, while the

205

transmembrane pressure was controlled by a pressure controller (Alicat PC series); once the feed

206

solution was filtrated through the membrane (diameter: 47 mm) installed in a stirred cell (Millipore

207

XFUF04701), the permeate was collected and weighted by a digital balance (METTLER TOLEDO

208

ME4002E); the weight of the permeate was transferred to a computer in time for flux calculation,

209

using a software programed by Labview (National Instruments). The dead-end filtration experiments

210

were conducted in 3 stages. First, the pure water flux was measured for 0.5 h to achieve a steady flux,

211

denoted as J0. Then, BSA, LYZ or the bacterial solution was filtered through a membrane for 2 h, and

212

the permeate flux at 2 h was denoted as Jp. Finally, the pure water flux was recorded again for 0.5 h

213

after immersing the membrane in 10 mL of DI water with continuous shaking at 250 rpm for 10 min,

214

and the recovered water flux was denoted as J1. The relative flux decay (RFD) was calculated by

215

RFD = [(J0-Jp)/J0] ×100%, and the relative flux recovery (RFR) was calculated by RFR = (J1/J0)

216

×100%.

217

The concentrations of the BSA, LYZ and E. coli in the feed and permeate solutions were



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218

measured using a UV-Vis spectrophotometer at the wavelengths of 280, 280 and 600 nm,

219

respectively. Subsequently, the retention of the pollutant was calculated by dividing the concentration

220

difference between the feed and the permeate by the feed concentration.

221

2.5. Static adsorption tests

222

Phenanthrene (PHE), bovine serum albumin (BSA, 66KDa, IEP = 4.8), and lysozyme (LYZ,

223

14.3 KDa, IEP = 11.6) were selected as model pollutants to test the adsorption capacity of the NCMs.

224

The membranes were cut into 1 cm × 2 cm for the adsorption process. PHE was dissolved in

225

acetonitrile to prepare a stock solution with a concentration of 1 mg/mL. Subsequently, the stock

226

solution was diluted to 1 mg/L using pure water. BSA and LYZ were dissolved in PBS (pH = 7.2) to

227

prepare solutions with a concentration of 50 mg/L. Subsequently, the membrane samples were

228

immersed in the prepared solutions for 24 h with continuous shaking. After adsorption, the PHE

229

concentration was quantified by high-performance liquid chromatography (HPLC, Agilent 1200)

230

under certain conditions (column: XDB-C18, mobile phase: acetonitrile/water=90/10, flow rate: 1

231

mL/min, detector: G1321 fluorescence spectrophotometer with excitation at 244 nm and emission at

232

237 nm). A UV-Vis spectrometer was used for the quantification of proteins at a wavelength of 280

233

nm. The adsorption capacity of the membrane samples was calculated by dividing the concentration

234

difference (before and after adsorption) multiplied by the solution volume by the mass of the

235

membrane.

236

3．Results and discussion

237

3.1. Characterization of GO-PEI

238

A novel additive was synthesized by grafting PEI with GO nanosheets through amide bonds, as

239

shown in Figure 1. The chemical structure of the GO-PEI additive was characterized by FTIR and

240

XPS. The FTIR spectra of GO indicates the presence of alkoxy C-O (1052 cm-1), epoxy C-O-C

241

(1225 cm-1), hydroxyl C-OH (1402 cm-1) and carbonyl C=O (1725 cm-1) groups, as shown in Figure

242

2a.17 Two new peaks were observed at 1573 and 1467 cm-1 in GO-PEI, which could be assigned

243

simultaneously to the asymmetric stretching of amide I (HNC=O) and the stretching of C-N, ACS Paragon Plus Environment

32, 35

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and the C=O stretching vibration at 1725 cm-1 completely disappeared. These results verify the

245

formation of amide bond between the carboxyl group from GO and the amine group from PEI.

246 247

Figure 1. The schematic illustration of the interactions in the GO-PEI additive and the NCM.

248

The surface chemical composition of GO-PEI was further analyzed by XPS. Figure 2(b)

249

presents the XPS scans of GO and GO-PEI, in which the main difference is the presence of the N1s

250

peak (394-401eV) from GO-PEI.35, 36 This was contributed by the nitrogen from PEI. In addition, the

251

oxygen peak intensity of GO-PEI was lower than that of GO, which indicate the partial reduction of

252

GO by PEI and may restore a portion of the sp2 regions, resulting in an enhanced capability to adsorb

253

aromatic pollutants. The nitrogen and carbon elements in GO-PEI were further analyzed through

254

high-resolution N1s and C1s spectra, respectively. The peak at 397.8 eV could be assigned to the

255

nitrogen of the amide group, and the peak at 396.5 eV was initiated by the nitrogen of the amine

256

group in PEI after fitting, as shown in Figure 2(c).32, 35 The C1s spectrum of GO in Figure 2(e)

257

showed the characteristic peaks of the C-C, C-O, C=O, and COO- bands at 282.9, 284.6, 285.7 and

258

287.3 eV, respectively,26, 36 whereas the spectrum of GO-PEI in Figure 2(d) presented two additional

259

peaks at 282.7 eV and 286.15 eV, belonging to C-N and N-C=O, respectively.32, 35 The XPS results

260

further confirmed the formation of the amide bond between GO and PEI.

261

The formation of the amide bond proved the successful synthesis of the GO-PEI additive.

262

Moreover, the surface charge and size of GO-PEI were characterized by the particle Zetasizer. As

263

shown in Figure 2(f), GO was negatively charged throughout the pH scan from 2 to 12, while

264

GO-PEI indicated an isoelectric point (IEP) at a pH of approximately 10.5. The acidic groups, such

265

as carboxylic acid and phenolic hydroxyl groups, were identified on the GO nanosheet as observed in

266

the FTIR and XPS spectra, which could be ionized easily and possess a negative charge in aquatic

267

environments.21, 37 However, GO-PEI showed a positiveζpotential in solutions at pH values lower

268

than 10, indicating that the as-prepared GO-PEI nanosheets bear high density positive charges, which

269

is desirable for the subsequent surface charge control of the membrane.



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270

In addition, the size of GO-PEI was approximately 700 nm (Figure S2), which is smaller than

271

the GO nanosheets (approximately 1.7 µm) prior to modification. It is possible that the sonication

272

process during the GO-PEI synthesis caused the GO nanosheets to break into smaller pieces.38 The

273

Tyndall effects of the GO-PEI nanosheets suspended in water and DMAc (50 ppm) indicated well

274

dispersed nanosheets in the solvents (Figure S3). The well embedded GO-PEI additive was also

275

observed

276

S4).

through

SEM

from

the

cross-section

of

the

prepared

membrane

(Figure

277 278

Figure 2. The (a) FTIR, (b-e) XPS spectra and (f) Zeta (ζ) potential of the GO nanosheets and

279

GO-PEI additive.

280

281

3.2. Surface properties of NCM

282

PES was chosen intentionally as the bulk polymer in the membrane since the benzene rings of

283

PES may form π-π interactions with the aromatic structures of GO,17, 30 as shown in Figure 1. The

284

stability of the additive in the membrane matrix was evaluated by measuring the TOC of the

285

permeate during filtration with DI water. As shown in Figure S5, the TOC values of the permeate

286

after a 5 h filtration process using M3 were all below 1.5 ppm and were very close to that of M0.

287

Apparently, the GO-PEI additive was embedded firmly in the membrane, and thus did not leach out


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during the filtration process. The π-π interaction ensured the stability of the additive (GO-PEI) in

289

NCMs, which may not only maintain the induced surface functionality of the NCM for a longer time

290

but also reduce the possible environmental hazards caused by the nanomaterial leaching.

291

The graphene-containing NCM was prepared by blending the GO-PEI additive with PES in

292

different ratios, followed by solidification through a non-solvent-induced phase inversion. The

293

prepared NCMs, namely, M1, M2 and M3, contained 0.5, 1 and 1.5 wt.% of GO-PEI, respectively.

294

The chemical compositions of the constructed NCMs were revealed by XPS. As shown in Figure

295

3(a), the XPS spectra indicated the presence of four elements, namely, carbon, oxygen, nitrogen and

296

sulfur, in the prepared membranes. Nevertheless, the proportions of nitrogen in M0, M1, M2 and M3

297

were 0, 18.1%, 23.9% and 27.9%, respectively (Table 2). The nitrogen content of the membrane

298

surface evidently increased with the amount of GO-PEI additive in NCM, which confirmed the

299

successful immobilization of GO-PEI on NCMs.

300

Table 2. The surface properties and structural characteristics of NCMs.

301

Membrane name Parameter M0

M1

M2

M3

0

18.1

23.9

27.9

Membrane thickness (µm)

160.9±0.9

153.7±1.1

144.8±2.3

125.6±17.4

Surface pore size (nm) b

10.2±1.8

13.6±2.7

14.6±2.1

16.9±2.9

Surface area (m2/g) c

24.4

25.4

25.2

31.1

Pore volume (cc/g) d

0.081

0.083

0.089

0.093

Isoelectric point (IEP)

N.A.

6.5

7.2

8.5

Dry roughness (Ra, nm) e

3.9±0.5

1.9±0.2

2.2±0.7

3.1±1.5

Wet roughness (Ra, nm) e

3.1±0.3

2.3±0.4

2.3±0.3

2.5±0.4

39.7±12.0

38.6±11.8

27.0±3.8

22.1±4.3

7.4±1.5

7.5±3.8

5.9±0.6

6.2±2.4

197.2±23.2

254.4±17.9

283.0±39.6

322.8±21.1

N content (%) a

Tensile strength (MPa) Tensile strain (%) Water flux (L /(m2 h)) 302

a

Molar percent based on the XPS measurements; b measured from the SEM images, c based on the

303

Brunauer-Emmett-Teller (BET) method, d the Barrett-Joyner-Halenda (BJH) method, and e the AFM



304

images.

305 306

In addition to the chemical composition, the surface charge, hydrophilicity and roughness of the

307

NCMs were characterized thoroughly by using different tools. The surface ζ potential, which is

308

measurable and directly related to the surface charge of membrane, was measured through SurPASS

309

3. As shown in Figure 3(b), the IEP values of M1, M2 and M3 were observed at pH values of

310

approximately 6.5, 7.2 and 8.5, respectively, while M0 indicated a negative charge in a pH range

311

from 3 to 10. It is well known that the physiological pH of most physiological fluids is

312

approximately 7.2. Thus, three membranes were designed to bear negative, neutral and positive

313

charges at pH 7.2, which was controlled by adjusting the content of the GO-PEI additive during the

314

membrane fabrication, since the additive contained a high density of positive charges.

315

The hydrophilicity of the prepared NCMs was evaluated through the water CA measurement. As

316

shown in Figure 3(c), M0 was hydrophobic and showed the highest water CA at approximately 95.9°.

317

The water contact angles of M1, M2 and M3 were approximately 87.2°, 78.7°and 60.5°, respectively.

318

It seems the hydrophilicity of NCM increased with the content of GO-PEI additive. This could be

319

attributed to the high hydrophilicity of the additive components, e.g., GO and PEI.

320

321 322

Figure 3. The (a) XPS spectra, (b) ζ potential and (c) water contact angle of NCMs.

323 324

Roughness is another important surface property that determines the interaction between the

325

retained components and the membrane surface. AFM was used to measure the roughness of the

326

prepared membranes both in dry and wet conditions. As shown in Table 2, the Ra values of M0, M1,

327

M2 and M3 in wet conditions were 3.1, 2.3, 2.3 and 2.5 nm, respectively. A similar trend but slightly

328

different Ra values of M0 (3.9 nm), M1 (1.9 nm), M2 (2.2 nm) and M3 (3.1 nm) were observed ACS Paragon Plus Environment

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329

under dry conditions (3D images of NCMs in Figure S6). In general, the surface of the NCM was

330

relatively smooth in both dry and wet conditions. In addition, the surface roughness of the M0

331

membrane was higher than that of the NCM with the GO-PEI additive, while the membrane surface

332

roughness increased slightly with the GO-PEI content. This may be due to the fast exchange of the

333

solvent and non-solvent during the NCM phase inversion process.

334

Surface property tuning is the main concern in this study because it determines the performance

335

of the membrane in proposed applications. The additive introduces its own properties into the

336

membrane composition after blending. The chemical composition of the NCMs revealed the

337

abundance of functional groups, which was responsible for the changes in the surface charge and

338

hydrophobicity. The AFM images revealed that the surface roughness of NCM was also slightly

339

affected by additive blending.

340

3.3. Morphology of NCM

341

The morphologies and cross-sections of the prepared NCMs were observed through SEM. The

342

membrane surfaces were homogeneous and with regularly distributed pores, as shown in Figure 4. It

343

was reported earlier that the introduction of inorganic nanomaterials, such as TiO2, into polymeric

344

membranes may generate agglomerations and result in heterogeneous membrane surfaces because of

345

the low affinity of inorganic nanomaterial to the bulk polymer.39, 40 By contrast, the GO-PEI additive

346

was well dispersed in the polymer matrix due to the π-π interactions between the additive (GO-PEI)

347

and the bulk polymer (PES).17 The pore sizes of the prepared membranes were measured using the

348

ImageJ software based on the SEM images. As listed in Table 2, even the M3 membrane showed

349

slightly larger surface pores, and all the prepared membranes indicated similar surface pore sizes at

350

approximately 15 nm, which suggests that the prepared NCM fell into the ultrafiltration membrane

351

category, and the pore forming agent (PVP) along with GO-PEI contributed to the pore formation in

352

this study.

353

To study the morphology of the membrane under operating conditions, the surface of a wet

354

membrane immersed in water was imaged through liquid AFM. The height images of the wet NCMs

355

are displayed in Figure 5 and Figure S7. The valleys representing the pores on the membrane could

356

be clearly observed from the images. Apparently, the surface structures of NCMs were stable under



357

the operating conditions.

358

To reveal the internal porous structure of NCM, the SA and PV of the membrane were

359

determined by the BET method and the BJH method, respectively. M0 had the lowest SA and PV at

360

24.4 m2/g and 0.081 cc/g, and the SAs of M1, M2 and M3 were 25.4, 25.2 and 31.1 m2/g, while the

361

PVs of M1, M2 and M3 were 0.083, 0.089 and 0.093 cc/g, respectively, as listed in Table 2 and

362

Figure S8. The increased content of the GO-PEI additive in the NCM consequently resulted in larger

363

SA and PV values, which are desirable because more trans-membrane paths would be available in a

364

more porous internal membrane structure.

365

The cross-sections of the NCMs were also observed with SEM and are shown in Figure 4. The

366

cross-sections exhibited typical asymmetric structures with a dense outer layer supported by

367

finger-like porous channels for all the NCMs. It was reported earlier that mutual diffusion between

368

the non-solvent (water) and solvent (DMAc) could promote the formation of an asymmetric structure

369

in the cross-section during the phase inversion process.40 Meanwhile, the embedded GO-PEI

370

nanoparticles were observed in the magnified cross-sectional images of the NCMs (Figure S3).

371

The thickness and mechanical strength of the NCMs were examined. The NCMs with different

372

ratios of the GO-PEI additive presented similar thicknesses, as listed in Table 2. It seems that a lower

373

weight content of the additive in the membrane did not apparently contribute to the membrane

374

thickness. However, the mechanical strength of NCM was affected by the additive. The tensile

375

strength and strain were reduced with the increased proportion of GO-PEI in the NCMs, which is

376

possibly due to the integration of the inorganic nanomaterial and may result in partial breakage of the

377

internal connections within the polymer matrix. However, the M3 membrane with lowest mechanical

378

strength was strong enough to withstand normal filtration.41

379 380


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381 382

Figure 4. The SEM images of the top surface and cross-sections of the NCMs (a) M0 (b) M1 (c) M2

383

and (d) M3. ACS Paragon Plus Environment


384

385 386

Figure 5. The AFM height images of the NCMs (a) M0, (b) M1, (c) M2 and (d) M3 in water. (Scan

387

size: 1 µm × 1 µm).

388

3.4 Performance of NCMs during filtration

389

The functional NCMs were designed considering the surface properties and filtration efficiency.

390

Thus, the performance of the NCMs during filtration is one of the main concerns in this work. Three

391

typical pollutants, namely, one negatively charged protein (BSA), one positively charged protein

392

(LYZ) and one bacterium (E. coli), were chosen as representative model pollutants for filtration

393

through NCMs.

394

Prior to the filtration process, the pure water flux of the as-prepared NCMs was measured. The

395

control membrane (M0) had the lowest initial pure water flux (197.2 L/(m2 h)), while the M1, M2


Page 18 of 29

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396

and M3 membranes exhibited pure water fluxes of approximately 254.4, 283.0 and 322.8 L/(m2 h),

397

respectively, which clearly indicates that the pure water flux of the NCMs was correlated with the

398

GO-PEI additive ratio. This could be attributed to the increase in the surface hydrophilicity as the

399

surface pores increased and the internal porosity of NCMs improved.

400 401

Figure 6. The normalized permeate flux of the NCMs during the filtration of (a) BSA (b) LYZ and (c)

402

E. coli

403 404

BSA is a negatively charged protein and widely used as a model foulant for membrane

405

performance testing.6, 32, 34 Since the prepared membranes fell into the ultrafiltration category, the

406

NCMs indicated high retentions towards BSA (>90%), as listed in Table S2. The filtration curves of

407

BSA are shown in Figure 6(a), which suggest that the BSA molecules accumulated on the surface of

408

M0 very quickly, thus causing a sharp decline in the flux after 2 h of filtration. The permeate flux of

409

M0 dropped to only approximately 30% of its initial flux, and most of the declined flux was unable

410

to be recovered by physical cleaning. Obviously, the M0 membrane was fouled severely during the

411

filtration of BSA. However, the presence of GO-PEI in the NCM prevented the fouling of BSA such

412

that the flux decline was more gradual, and the flux recovery rate increased with the content of

413

GO-PEI in the NCM. Notably, the positively charged hydrophilic M3 membrane indicated the lowest

414

fouling tendency during the filtration of BSA. It should be noted that the M3 membrane and BSA

415

bear opposite charges. These results indicated that a hydrophilic surface with an opposite charge to

416

the foulant presented better antifouling effects than a hydrophobic surface bearing the same charge

417

during filtration. Apparently, the hydrophilicity rather than the surface charge of the membrane

418

determined the BSA adhesion during filtration. Furthermore, the dipole array of the water molecules

419

in the hydration shell on the membrane surface formed because the synergistic effect between the



420

surface charge and hydrophilicity was close enough to that of free water and could not be easily

421

penetrated by the protein molecule.32, 42 In other words, the enhanced hydration shell effectively

422

reduced the non-specific attachment of foulants. The distance between the foulant and the membrane

423

surface is enlarged by the hydration layer intercalation, which in turn diminishes the electrostatic

424

interactions. Furthermore, it is also possible that the electrostatic interaction between the protein

425

molecule and the membrane surface be inhibited in turbulent environments because of the hydraulic

426

shear force generated by the permeate flux and stirring.

427

As a negatively charged foulant, the gram-negative bacterium E. coli was filtered through the

428

NCMs, as shown in Figure 6(c). Since the size of bacteria is in the micrometer scale, all of the

429

prepared membranes almost completely retained E. coli during filtration (Table S4). During the

430

filtration of E. coli, the flux of the M0 membrane decreased rapidly to a steady-state value after a

431

short time, while the NCMs with a higher content of GO-PEI exhibited slower declines in the flux

432

during the filtration process. After filtration, the flux recovery changed according to the ratio of the

433

GO-PEI additive in the prepared membrane. Similar fouling trends of the NCMs were observed for

434

the filtration of E. coli and BSA. These results also indicated that the hydrophilicity played a more

435

important role than the surface charge in controlling membrane fouling during filtration.

436

The positively charged LYZ molecules were also filtered to further evaluate the performance of

437

the prepared membranes. The molecular weight of LYZ is 14.3 kDa, which is much smaller than that

438

of BSA. Thus, the retention rates of the prepared membranes towards LYZ were all above 55% but

439

below 70%. During the filtration of LYZ, the permeate flux of M0 decreased rapidly in the beginning,

440

and a relatively steady but very low flux was quickly reached (Figure 6(b)). However, the NCMs

441

with the GO-PEI additive exhibited much slower declines in the flux in the filtration process, and the

442

higher the content of GO-PEI additive in membrane was, the more gradual the decline in the flux

443

decline was during the filtration of LYZ using the NCMs. Both the lowest flux decline and the lowest

444

flux recovery were observed from the M3 membrane. M3 indicated the best resistance to LYZ during

445

filtration. This could be attributed simultaneously to the hydrophilicity and the positive charge of M3.

446

Similarly, an enhanced hydration layer induced synergistically by the combination of the surface

447

charge and hydrophilicity may reduce the chance of foulant attachment.32 Although electrostatic

448

interactions between the protein molecules and the membrane surface do not obviously contribute to

449

adhesion during dynamic filtration, the same charge for M3 and LYZ may initiate an electrostatic ACS Paragon Plus Environment

Page 20 of 29

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450


repulsion effect which might reduce the adhesion to some extent.

451

To evaluate the contribution of the electrostatic attraction to membrane selectivity, the filtration

452

of BSA or LYZ using the prepared NCMs were compared. As shown in Table S3, high retention

453

values of LYZ were achieved by negatively charged membranes, e.g., M0 (68%) and M1 (69%),

454

while only 56.9% of LYZ was retained by the positively charged M3. For the BSA, all the prepared

455

membranes indicated high retentions of approximately 97% (Table S2). The surface pore sizes of M0,

456

M1 and M3 were approximately 10, 14 and 17 nm, while the pure water fluxes of M0, M1 and M3

457

were 197.2, 254.4 and 322.8 L/(m2 h), respectively. If the selectivity was only provided by the pore

458

sieving effect during the filtration of LYZ, the M0 membrane should have a higher retention than M1.

459

However, the M1 membrane with approximately 40% larger pores and approximately 30% higher

460

flux than M0 showed a similar retention of LYZ. Therefore, the negative charge of both M0 and M1

461

contributed to better retention results for LYZ, while only M1 achieved high permeability and

462

selectivity during the LYZ filtration.

463

The contribution of the electrostatic attraction to the membrane selectivity was more dominant

464

during the filtration of BSA. In the BSA filtration process, the M3 membrane with approximately

465

70% larger pores and 64% higher flux than those of M1 exhibited a similar retention of BSA as M1.

466

In addition, the dimensions of the BSA molecule are approximately 9×5.5×5.5 nm, as reported

467

earlier,43 which is smaller than the average pore size of M3. It is apparent that the electrostatic

468

attraction provided the additional selectivity seen in M3 allowing for the effective retention of BSA

469

during high flux filtration, as shown in Figure 7. It seems that the current protocol can be used to

470

design a membrane that does not present a trade-off between the permeability and selectivity,

471

however, only when the membrane is treating a specific pollutant with known properties, such as the

472

surface charge.



473 474

Figure 7. Schematic illustration of the BSA filtration through M0 and M3.

475

476

3.5. Adsorption capabilities of NCMs

477

The interactions between the retained components and the membrane surface determine the

478

performance of a membrane during dynamic filtration. To investigate the contributions of the

479

different membrane surface properties to the pollutant adhesion or accumulation in static adsorption,

480

the interactions between the typical pollutants and the NCM surfaces were studied from both

481

macroscopic (adsorption capacity) and microscopic (molecular adhesion force) perspectives. Three

482

typical pollutants, namely, one negatively charged protein (BSA), one positively charged protein

483

(LYZ) and one aromatic hydrocarbon (PHE), were selected to evaluate the adsorption capacity of

484

NCMs in static adsorption tests.


Page 22 of 29

Page 23 of 29


485 486

Figure 8. (a) The adsorption capacities of the NCMs, (b) the adhesion force between the simulated

487

negatively charged molecule (AFM probe) and the NCMs.

488 489

Two model proteins, namely, negatively charged BSA and positively charged LYZ, were chosen

490

to evaluate the adsorption capability of the NCMs at a pH of 7.2. As shown in Figure 8(a), the

491

negatively charged M1 membrane adsorbed the lowest amount of BSA (1.8 mg/g), while the neutral

492

M2 membrane adsorbed 2.7 mg/g of BSA and M3 with a positive charge indicated the highest

493

adsorption capacity to BSA at 4.1 mg/g. However, the reverse trend was observed for the LYZ

494

adsorption tests with the NCMs. The M1 membrane adsorbed the highest amount of LYZ (4.8 mg/g),

495

while a lower adsorption (3.8 mg/g) was recorded on M2 with zero net charge, followed by M3 (2.9

496

mg/g). The adsorption behaviors of the protein molecules correlated well to the surface charge of the

497

NCMs. However, the difference in the hydrophilicity of the NCMs did not change the amount of

498

protein statically adsorbed. These results indicate that the electrostatic interaction (attraction or

499

repulsion) was dominant rather than the hydrophobic/hydrophilic interaction during the static

500

adsorption of the charged proteins towards the NCMs. It has been reported that the electrostatic

501

interaction determines the adsorption capability of the materials during static tests.44 In a typical

502

static adsorption process, the electrostatic force could either attract or repel the charged protein

503

molecules at the membrane surface in a relatively stable environment without any significant

504

turbulence (i.e., coagulation/flocculation processes in water treatment45). However, the contribution

505

of electrostatic interaction would be weakened during dynamic filtration because of the hydraulic

506

shear force initiated by the turbulence.



507

To further study the adsorption mechanism of the charged molecule on the surface of the NCMs

508

microscopically, a sharp SiO2 AFM probe (diameter: 2 nm) was used to simulate one negatively

509

charged hydrophilic molecule. The adhesion force between the probe and the nanoscale zone of the

510

NCMs was measured by AFM. The measured adhesion force data were plotted in Figure 8(b) and

511

mapped in Figure S9. As shown in Figure 8(b), the probe displayed the lowest average adhesion

512

force on the negatively charged M1 surface. A higher average adhesion force was shown on the

513

neutral M2 surface. The highest average adhesion force was measured between the probe and the

514

positively charged M3 surface, while the force data were distributed in a wider range. A single force

515

curve recorded the whole process of probe (or molecule) attached to and detached from the

516

nanoscale zone (adsorption site) of surface, which might identify the possible active sites having

517

enough attractive force for adsorption. In other words, the M3 surface might have active sites which

518

may provide a strong interaction with the hydrophilic and negatively charged molecules after contact.

519

Interestingly, these results were highly consistent with the BSA adsorption data of the NCMs. It is

520

possible that the strong attraction sites may trigger a fast initial accumulation, which might

521

eventually result in a higher accumulation during static adsorption.

522

For the PHE adsorption tests, the M0 membrane had the lowest adsorption capacity (4.6 mg/g)

523

for PHE, see Figure 8(a). Notably, the adsorption capacity of the NCM to PHE increased as the ratio

524

of the GO-PEI additive increased in the NCMs from 5.3 mg/g for M1 to 6.5 mg/g for M3. The 1.5

525

wt.% addition of GO-PEI in M3 exhibited a 42.5% increase in the PHE adsorption capacity.

526

Obviously, the aromatic structure of GO indicated a strong adsorption capacity to the aromatic

527

hydrocarbons because of the π-π interaction. Thus, the NCMs with a high ratio of GO containing

528

nanomaterial have the potential to selectively remove aromatic hydrocarbons through adsorption

529

instead of pore sieving and preserve a higher permeability by using larger pores for sieving.

530

Nevertheless, the GO content was low in the as-prepared NCMs in this study since the additive

531

demonstrated a strong influence in altering the membrane surface properties. Apparently, the facile

532

protocol developed in this study highlights the fabrication of a highly permeable and solute/water

533

selective nanocomposite membrane with non-specific fouling resistance for high-performance water

534

and wastewater treatments.

535 536


Page 24 of 29

Page 25 of 29

537


Supporting Information

538

The element molar percent of the NCMs, the BSA, LYZ and E.coli filtration parameters of the

539

NCMs, the schematic of the filtration system, the sizes of the GO sheets and the GO-PEI additive,

540

the Tyndall effects of GO and GO-PEI in different solvents, the SEM image of the M1 cross-section,

541

TOC of DI water filtrated through M0 and M3, the AFM height images of NCMs in air and in liquid,

542

the N2 adsorption-desorption isotherms and pore size distributions of NCMs, the adhesion force

543

mapping of NCMs.

544 545

Acknowledgments

546

This project was supported by the National Natural Science Foundation of China (21425730,

547

21621005 and 21607124), the National Key Research and Development Program of China

548

(2017YFA0207000) and the Fundamental Research Funds for the Central Universities.

549

550

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551

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