Photocatalytic nanofiber-coated alumina hollow fiber membranes for highly efficient oilfield produced water treatment

Accepted Manuscript Photocatalytic nanofiber-coated alumina hollow fiber membranes for highly efficient oilfield produced water treatment Nur Hashimah Alias, Juhana Jaafssar, Sadaki Samitsu, T. Matsuura, A.F. Ismail, M.H.D. Othman, Mukhlis A. Rahman, N.H. Othman, N. Abdullah, S.H. Paiman, N. Yusof, F. Aziz PII: DOI: Reference:

S1385-8947(18)32179-X https://doi.org/10.1016/j.cej.2018.10.217 CEJ 20280

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

13 August 2018 21 October 2018 28 October 2018

Please cite this article as: N.H. Alias, J. Jaafssar, S. Samitsu, T. Matsuura, A.F. Ismail, M.H.D. Othman, M.A. Rahman, N.H. Othman, N. Abdullah, S.H. Paiman, N. Yusof, F. Aziz, Photocatalytic nanofiber-coated alumina hollow fiber membranes for highly efficient oilfield produced water treatment, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.10.217

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1

Photocatalytic nanofiber-coated alumina hollow fiber membranes for highly efficient oilfield

2

produced water treatment

3 4

Nur Hashimah Aliasa,b, Juhana Jaafara*, Sadaki Samitsuc, T. Matsuurad, A. F. Ismaila, M. H. D.

5

Othmana, Mukhlis A Rahmana, N. H. Othmanb, N. Abdullaha, S. H. Paimana, N. Yusofa, F. Aziza

6 7

a

8

81310 Skudai, Johor, Malaysia.

9

b

Advanced Membrane Technology Research Center (AMTEC), Universiti Teknologi Malaysia,

Department of Oil and Gas Engineering, Faculty of Chemical Engineering, Universiti Teknologi

10

MARA, 40450 Shah Alam, Selangor, Malaysia.

11

c

12

Integrated System (MaDIS), National Institute for Materials Science (NIMS), 1-2-1 Sengen,

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Tsukuba, Ibaraki 305-0047, Japan.

14

d

15

University of Ottawa, 161 Louis Pasteur St, Ottawa, ON K1N 6N5, Canada.

Data-driven Polymer Design Group, Research and Services Division of Materials Data and

Industrial Membrane Research Laboratory, Department of Chemical and Biological Engineering,

16 17

Corresponding author: [email protected]

18 19

Abstract

20 21

Cost-effective purification technology of oilfield produced water (OPW) is becoming a

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global challenge for future petroleum exploration and production industry. Energy-efficient

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operation of membrane separation is potentially promising. However, severe fouling problem of

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oil droplets demands new robust and fouling-resistant membranes with high permeability and

25

rejection efficiency. Here, we propose a photocatalytic nanofiber-coated inorganic hollow fiber

26

membrane suitable for OPW treatment. The membrane was fabricated by coating polyacrylonitrile

27

(PAN) nanofiber incorporated with photocatalytic graphitic carbon nitride (GCN) on an alumina

28

(Al2O3) hollow fiber membrane. While the highly porous coating made of smooth hydrophilic

29

nanofibers facilitated water permeation, the coating effectively captured oil droplets in its opening,

30

resulting in a better rejection efficiency of oil contaminants. Its sparse mesh morphology prevented

31

oil contaminants to form dense fouling film on the membrane surface and maintained high

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permeate flux even after 180 min filtration. The best permeate flux of 640 L∙m−2∙h−1 and oil

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rejection percentage of 99% were recorded for 180 min crossflow filtration of OPW at 2 bar along

34

with the highest pure water flux of 816 L∙m−2∙h−1. The photocatalytic activity of GCN enabled the

35

coating to degrade the captured oil contaminants under UV irradiation, demonstrating permeate

36

flux of 577 L∙m−2∙h−1 and oil rejection of 97% after three cycles of 180 min filtration. The excellent

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fouling resistance and cleaning performances of the membrane are considerably beneficial for a

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long-term repeated filtration operation. This work will motivate researchers to develop nanofiber-

39

coated hollow fiber membranes for future membrane separation technology.

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Keywords: Graphitic carbon nitride; PAN nanofiber; Alumina hollow fiber membrane; Coating;

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Filtration; Oilfield produced water

43 44

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1.

Introduction

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A massive amount of oilfield produced water (OPW) is co-produced during petroleum

48

production and exploration, which accounts for 80%–95% of the production volume collected

49

from the production wells. Much attention has been given to reduce the pollution of OPW such as

50

aliphatic hydrocarbons, heavy aromatic compounds, alkylated phenols, and added production

51

chemicals because the contaminants are potentially hazardous to the environment [1,2]. Various

52

physical treatments such as hydrocyclone, floatation, centrifugation, evaporation, and extraction

53

have been traditionally used as a primary treatment to minimize the concentration of oil in OPW

54

[3]. Despite high efficiency of primary treatments, the resultant OPW still contains trace oil

55

contaminants, which is not allowed to be discharged according to stringent environmental

56

regulations to date [4].

57

Membrane separation is another technique that can be applied to remove trace

58

contaminants in water. Although current membrane processes generally consume more energy and

59

are more expensive compared with conventional biological and physicochemical treatment

60

processes, they have great potentials on improving their performance by developing advanced

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membranes and filtration processes [5,6]. In principle, membrane separation systems have many

62

attractive features such as energy-efficient separation without phase change, small footprint, and

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easy installation and maintenance [7,8]. Membrane separation for OPW treatment, however, needs

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more improvement on robust membranes due to the complex sticky composition and high

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temperature of OPW, which frequently degrade common polymeric membranes. Inorganic

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membranes are the promising candidates for OPW treatment due to the excellent chemical and

67

thermal stabilities. However, the development of inorganic membranes is still challenging in terms

68

of high permeability and rejection, fouling resistance, and easy cleaning of membrane fouling.

69

Surface modification of membranes has attracted much attention for reducing membrane fouling,

70

which involves chemical modification to change surface hydrophilicity or hydrophobicity [9],

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control surface zeta potential [10], plasma treatment [11], UV irradiation [12], surface grafting

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[13], and surface coating [14]. Interestingly, a recent study demonstrated that electrospun

73

nanofiber coating enhanced fouling resistance and permeability for a flat sheet polymer membrane

74

[15]. The coating seems to be applicable to an inorganic hollow fiber membrane as actually

75

demonstrated in this study [16]. Hollow fiber membranes are probably more attractive for the

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practical treatment of OPW because they are suitable for constructing a microfiltration membrane

77

module which is easily up-scaled compared with that of flat-sheet membranes [17]. Surface

78

modification of membranes has attracted much attention for reducing membrane fouling. In OPW

79

purification process using microfiltration membranes, a considerable amount of oil droplets

80

adheres on the membrane surface while water molecules pass through the membrane.

81

Conventional membranes, which have smooth surface morphology and hydrophobic property (i.e.,

82

oleophilic), suffer from severe fouling because a dense oily layer usually covers the whole

83

membrane surface. The fouling layer seriously degrades permeate flux in addition to the reduction

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of separation efficiency, which is challenging for a long-term operation of microfiltration process.

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Membrane cleaning performance is another important issue in membrane development for

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OPW treatment. In addition to conventional physical and chemical washing processes,

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photodegradation of fouling contaminants has been considered as a promising technique in

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membrane cleaning process. Various polymeric and inorganic membranes incorporating

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photocatalyst into their matrices have demonstrated good membrane cleaning performance using

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photodegradation under light irradiation. Unfortunately, a low amount of photocatalyst was

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dispersed on the membrane top surface, leading to low photodegradation properties [18,19]. A

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higher dosage of photocatalyst had caused membrane pore blockage and deterioration of

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continuous porous structure due to the formation of agglomerates, which had seriously declined

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the membrane permeability [20,21].

95

To address several issues on robust membranes for OPW purification by integrating these

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preferential membrane properties, we developed a photocatalytic nanofiber-coated inorganic

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hollow fiber membrane. Recent emerging rediscovery of graphitic carbon nitride (GCN) has

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attracted a considerable attention for potential applications in photocatalysis [22–25]. Furthermore,

99

our previous study demonstrated an enhanced photodegradation of OPW using GCN-embedded

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polyacrylonitrile (PAN) nanofiber mesh [26]. Therefore, in this study, we electrospun GCN-

101

embedded PAN nanofiber on top of asymmetric alumina (Al2O3) hollow fiber membrane surface

102

using direct electrospinning technique. The thermal treatment after deposition significantly

103

enhanced the adhesion of nanofiber coating on the Al2O3 membrane and successfully offered a

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GCN-embedded nanofiber-coated Al2O3 hollow fiber membranes. The membrane demonstrated

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high permeability and rejection, fouling resistance, and easy cleaning of membrane fouling for a

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model example of OPW treatment.

107 108

2.

Experimental

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2.1. Materials

111 112

Urea from QReC Malaysia was used as the raw material to synthesize bulk graphitic carbon

113

nitride (bGCN). The bGCN was further exfoliated to form nanosheet graphitic carbon nitride

114

(nsGCN) using isopropyl alcohol (IPA) from QReC Malaysia. Polyacrylonitrile (PAN) from

115

Sigma Aldrich and dimethylformamide (DMF) from RCI Labscan were used as polymer binder

116

and solvent, respectively, for the electrospinning suspension to fabricate nanofibers.

117

Three different sizes of alumina powder (Al2 O3) were purchased from Alfa Aesar and used

118

to fabricate hollow fiber membranes: (1) α-Al2O3 (99% metal basis, average size of 1 µm and

119

surface area of 6–8 m2/g), (2) α- and γ-Al2O3 (99.5% metal basis, average size of 0.5 µm and

120

surface area of 32–40 m2/g), and (3) α- and γ-Al2 O3 (99.8% metal basis, average size of 0.01 µm

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and surface area of 100 m2/g). Polyethersulfone (PESf, Radal A300, Ameco Performance, USA),

122

N-methyl-2-pyrrolidone (NMP, AR grade, QRëCTM), and poly(ethylene glycol) 30-

123

dipolyhydroxystrearate (Arlacel P135 from Uniqema) were added into the spinning suspension to

124

fabricate Al2O3 hollow fiber membranes. All materials purchased in this work were used without

125

any further purification.

126

The crude oil sample used for membrane performance evaluation was supplied by Petronas

127

Refinery Malacca, Malaysia (API Grade 66). Sodium dodecyl sulfate (SDS) purchased from

128

Merck was used as surfactant during the preparation of the oilfield produced water (OPW)

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solution. Reverse osmosis (RO) water (Millipore: ASTM Type III) was used throughout the

130

experiments and membrane filtration tests.

131 132

2.2. Preparation of asymmetric alumina (Al2O3) hollow fiber membrane

133 134

The asymmetric alumina (Al2O3) hollow fiber membrane was fabricated according to the

135

scheme established in a previous report [27]. There are mainly three steps involved in the

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membrane preparation, which are preparation of ceramic suspension, spinning of hollow fiber

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precursor, and sintering of the precursor. In the first step, three Al2O3 powders with different

138

average sizes of 0.01, 0.05, and 1 µm were mixed at the ratio of 1:2:7, respectively, and 106 g of

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the powder mixture was added into 73.74 g of NMP solution containing 2.6 g of Arlacel P135.

140

The mixture was ball-milled in planetary ball machine (Magna NQM-2 Planetary Ball Mill) before

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17.66 g of PESf was added into the Al2O3 suspension. In the second step, the prepared Al2O3

142

suspension was extruded using stainless steel spinneret to form the hollow fiber membranes. The

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collected spun Al2O3 hollow fiber membranes were drenched into tap water to complete the phase

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inversion process. In the third step, the collected Al2O3 hollow fiber membranes were dried at

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room temperature before sintered at 1400 °C in a tubular furnace (Magna, XL-1700).

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2.3. Fabrication of a GCN nanofiber-coated Al2O3 hollow fiber membrane

148 149

Electrospinning technique has been successfully utilized to fabricate a nanofiber mesh on

150

a metallic foil placed on a flat plate or a cylindrical drum [28]. An electrically-conductive substrate

151

with small curvature generates uniform electric field between a needle and substrate, which is

152

readily applicable for electrospinning. There are, in contrast, limited reports on nanofiber

153

electrospinning on an insulating porous substrate having large curvature like a hollow fiber

154

membrane, and therefore, electrospinning technique on such substrate has not completely

155

established yet.

156

Photocatalysts of bGCN and nsGCN were synthesized from urea using green and facile

157

template-free method according to our previous work [26]. Figure 1 illustrates a schematic design

158

of electrospinning setup for the fabrication of GCN nanofiber-coated Al2O3 membrane.

159

Commercially available nanofiber electrospinning unit (NF-1000, Progene Link Sdn. Bhd.,

160

Malaysia) and a blunt metallic needle (21G × 1", Terumo Corporation) connected to a 10 mL

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syringe were employed in the nanofiber coating. An 8 wt% dope solution containing 7.2 wt% of

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PAN and 0.8 wt% of GCN was electrospun on an Al2O3 hollow fiber membrane at a solution feed

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rate of 1 mL/h and an acceleration voltage of 15 kV. The membrane of 7 cm in length was

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immersed in advanced in IPA for 15 min and fixed on a holder directed perpendicular to the needle

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while wrapped in cotton wadding. The holder was placed at 18 cm apart from the needle while

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rotating at 6 rpm to homogeneously coat the nanofibers on the membrane. Electrospinning process

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continued for nearly 3 h for each of the membrane. Al2O3 hollow fiber membranes coated with

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PAN nanofibers only, PAN nanofibers containing bGCN, and PAN nanofiber containing nsGCN

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were denoted as NF/Al2O3, NF-bGCN/Al2O3, NF-nsGCN/Al2O3 membranes, respectively. The

170

membranes coated with nanofibers were heated at 120 °C for 15 min, which enhanced both

171

mechanical stability of nanofiber mesh and adhesion between nanofiber layer and membrane as

172

demonstrated in the following section.

173 (d)

(a)

(b)

(e)

(c)

(f)

174

High voltage supply

175

Fig. 1. A schematic design of electrospinning setup for the fabrication of GCN nanofiber-coated

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Al2O3 membrane: (a) syringe containing dope solution, (b) blunt metallic needle, (c) syringe pump,

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(d) wood-based support, (e) grounded metal rotating holder, and (f) nanofiber-coated Al2O3 hollow

178

fiber membrane.

179 180

2.4. Characterization methods

181 182

A field emission scanning electron microscope (S-4800, Hitachi High-Tech. Co.) was

183

employed to examine the as-prepared membranes morphological structures. The samples were

184

fixed with double sided carbon tape and coated with a thin platinum layer under argon pressure of

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7 Pa for 50 s using an ion sputter (E-1030, Hitachi High-Tech. Co.). Thermal stability of the

186

nanofiber samples was determined using a thermal gravimetric analyzer (TGA4000, PerkinElmer

187

Inc.) under air flow at a heating rate of 10 °C/min. The nanofiber samples were cut into small

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pieces and placed in platinum pans.

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2.5. Filtration experiments

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Cross-flow microfiltration is known as a suitable way to remove colloidal particles and

194

reduce fouling effect [29]. In the crossflow filtration of colloidal droplets, droplets in a stream are

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subjected to two different forces in the direction perpendicular to the membrane surface. The

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balance between the forces determines the formation of a filter cake on the membrane surface. Due

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to the filtrate flow passing through the membrane surface, a drag force, Fy, moves droplets to the

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membrane surface. Fy can be calculated using Stokes equation as follows [29]:

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𝐹𝑦 = 3π 𝜂 𝐷 𝑣𝐹 ,

200

where η is the viscosity of the liquid medium, D is the droplet size, and vF is the permeate rate.

201

According to the equation, Fy is proportional to D. The other force acting on droplets is a lift force,

202

FL, which moves droplets away from the membrane surface. FL originates from shear flow and

203

estimated using an equation [29],

204

𝐹𝐿 = 0.761

𝜏𝑤 1.5 𝐷3 𝜌0.5 𝜂

205

where τw is the shear stress and ρ is the density of the liquid medium. According to the equation,

206

the lift force is proportional to the cube of D. As D increases, both Fy and FL increase. However,

207

due to different dependences on D, FL overcomes Fy when D becomes sufficiently large.

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Synthetic OPW at 1000 ppm was prepared according to a previously reported method [30].

209

The total organic carbon (TOC) measurement carried out using TOC analyzer (Shimadzu Co.,

210

TOC-LPCN) on 1000 ppm of OPW solution reveals the TOC value of 531.2 ppm. Oil particle

211

sizes in feed and permeate were measured using a particle analyzer (Zetasizer Ver. 7.11, Malvern

212

Instruments Ltd.). A benchtop crossflow membrane filtration setup was used to evaluate the

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separation performance of the synthesized membranes (Fig. 2). The membranes were wetted via 1

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h immersion in reverse osmosis (RO) water before installation. Prior to collecting the filtration

215

results, the membranes were compacted at a pressure of 3 bar for 10 min to ensure a steady state

216

condition of permeation experiment. The filtration setup was operated using a transmembrane

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pressure of 2 bar and a feed flow rate of 72 L/h. Pure water flux (PWF) and permeate flux, J

218

(L∙m−2∙h−1), are calculated using Eq. (1):

219

𝐽=𝐴

∆𝑉

𝑚 ∆𝑡

(1)

220

where Am is the effective membrane area (m²), Δt is the time used to collect permeate (h), and ΔV

221

is the respective collected permeate volume (L). The oil rejection (%) was assessed according to

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our previous study [26] using Eq. (2): 𝑂𝑖𝑙 𝑟𝑒𝑗𝑒𝑐𝑡𝑖𝑜𝑛 (%) = (

223

𝐶𝑖 −𝐶𝑜 𝐶𝑖

) × 100 (2)

224

where Ci and Co are the absorbance values of feed and permeate solutions, respectively, and were

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measured using a UV–visible spectrophotometer (DR5000, Hach, U.S.A) at the wavelength of 238

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nm. The crude oil used in this study has large UV absorbance at the 238 nm wavelength. The OPW

227

filtration experiment was conducted for 180 min by collecting permeate solutions at every 10 min

228

to examine antifouling property. To examine the cleaning property after the 180 min filtration, the

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contaminated membrane was taken out from the membrane module, immersed in RO water, and

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subjected to UV irradiation (30 W UV lamp, peak wavelength of 312 nm) for 180 min. The GCNs

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photocatalyst has strong UV absorbance in this wavelength range, which was demonstrated in our

232

previous paper [26]. The membrane was then reinstalled into the module and the OPW filtration

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was performed for another 180 min. The filtration–cleaning cycle was repeated for three times

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using fresh feed of OPW solution in each cycle.

Retentate

OPW feed tank Valve

Pump

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Membrane module

Permeate Pressure gauge

Nanofiber coated Al2O3 hollow fiber membrane

Fig. 2. Schematic diagram of crossflow membrane filtration setup.

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3.

Results and discussion

239 240

3.1. Fabrication of nanofiber-coated on alumina (Al2O3) hollow fiber membrane surface

241 242

As described in Section 2.3, the electrospinning technique was able to coat PAN

243

nanofibers homogeneously on a thin Al2O3 hollow fiber membrane by selecting appropriate

244

electrospinning parameters such as acceleration voltage, solution flow rate, and tip-to-membrane

245

distance based on our previous study [26]. The whole membrane was uniformly covered by

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nanofiber mesh, which was at least confirmed by visual inspection of the membrane appearance.

247

In spite of the uniform nanofiber coating on the membrane surface, when the membrane

248

was used in the filtration experiment, some of the nanofibers were left in the filtrate and the coating

249

sometimes peeled off from the membrane surface due to insufficient adhesion. The as-prepared

250

coating of electrospun nanofibers suffered from low mechanical properties and weak adhesion on

251

substrates due to low electrical conductivity of Al2O3 hollow fiber membrane, which made the

252

coating impossible to be applicable to a filtration membrane. To ensure good membrane stability

253

in a prolonged filtration process, the as-prepared nanofiber-coated membranes were further heated

254

at a high temperature. When the membrane was heated at 150 °C for 15 min, the membrane was

255

lightly browned. The result indicates that the PAN nanofibers partially decomposed although the

256

thermal gravimetric analysis (TGA) exhibited no detectable nanofibers degradation at a

257

temperature below 280 °C [31] (Fig. S1) (Details in Supplementary Information). On the other

258

hand, the 15 min heat treatment at 120 °C did not cause detectable change of nanofiber coating by

259

visual inspection. In addition to no color change, the mechanical properties of the nanofiber mesh

260

were also significantly improved by the heat treatment as demonstrated in Fig. 3. When the as-

261

prepared and heat-treated NF-nsGCN meshes were immersed in RO water for 60 min, the as-

262

prepared mesh was so soft that tweezers could not hold it firmly while the heat-treated mesh

263

offered a mechanical strength enough for handling. Figure 3(a) shows the photographs of the as-

264

prepared and heat-treated meshes containing RO water. The as-prepared mesh showed many

265

wrinkles with a deformed shape while the heat-treated mesh exhibited a flat rectangular shape.

266

Furthermore, when they were subjected to 60 min ultrasonication in RO water, the heat-treated

267

mesh maintained its shape (Fig. 3c) while the as-prepared mesh lost its original shape and exhibited

268

rough appearance accompanying partial fragmentation (Fig. 3b). The results directly indicate a

269

significant improvement on the mechanical strength through the heat treatment process. Since the

270

temperature of 120 °C is higher than the glass transition temperature of PAN (97 °C) [32], the

271

high-temperature short-time treatment allows PAN polymers to plasticize and bound strongly at

272

nanofiber intersections while keeping the nanofiber shape as confirmed by SEM. The high

273

mechanical strength of nanofiber coating plays a significant role to determine the stability of the

274

membrane during prolonged filtration operation.

(a) (i)

(b)

(ii)

(c)

275 276

Fig. 3. Photographs of NF-nsGCN meshes (a) immersed in RO water and ultrasonicated for 60

277

min: (b) as-prepared and (c) heat-treated. The samples initially had a rectangular shape of 12

278

(width) × 17 (length) × 0.05 (thickness) mm.

279 280

3.2. Characterization of membranes

281 282

The SEM images show the surface morphologies of bare Al2O3 membrane (Fig. 4a) and

283

NF-coating layers (Fig. 4b–d). The Al2O3 membrane exhibited smooth surface having many small

284

pores with a diameter range of 175–375 nm. All the NF-coating layers formed sparse mesh of

285

straight nanofibers with an average diameter of 250 nm. The opening sizes of all NF layers are

286

similar and approximately several micrometers, which are almost ten times larger than the pore

287

size of the Al2O3 membrane. In contrast to smooth straight shape of NF, NF-bGCN had several

288

bulges on smooth nanofibers. The spherical bulges with a few micrometers in size correspond to

289

the bGCN particles embedded on the nanofibers. Due to the successful exfoliation of nsGCN, NF-

290

nsGCN had a less numbers of thin bulges compared with those in NF-bGCN. It is difficult to

291

accurately measure the thickness of the coating layer because the coating layer did not result in a

292

clear cross section on an SEM image even using a freeze fracture technique. In our previous work,

293

the obtained FTIR spectra confirmed that GCNs were successfully embedded into the PAN

294

polymer matrix. The well dispersion of GCN into PAN polymer matrix promotes a vast number

295

of active sites for effective interactions between the reactant and photocatalyst, thus enhancing the

296

photocatalytic activity [26]. Meanwhile, in comparison with NF-bGCN (mean nanofiber diameter

297

of 207 ± 2 nm), the mean nanofibers diameter of NF-nsGCN is 262 ± 6 nm with a smooth and

298

straight infinite length structure [26].

(a)

(b)

10 µm

10 µm (c)

(d)

10 µm

10 µm

299 300

Fig. 4. SEM micrographs of (a) bare Al2O3, (b) NF/Al2O3, (c) NF-bGCN/Al2O3, and (d)

301

NF-nsGCN/Al2O3 membranes.

302 303

The water affinity of the nanofiber meshes was assessed on the basis of water absorption

304

capacity. The water absorption capacity for NF and NF-nsGCN meshes were measured by

305

immersing them into RO water for 60 min. The excess water on the surface was removed using a

306

filter paper and the wetted nanofiber meshes were weighed. The water absorption capacity is

307

calculated using Eq. (3):

308

𝐴𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 =

309

where ww and wd are the masses of wetted and dry nanofibers, respectively. The NF and NF-nsGCN

310

meshes both have high water absorption capacity of 34 ± 8 and 27 ± 4, respectively. The water

𝑤𝑤 −𝑤𝑑 𝑤𝑑

(3)

311

absorption capacities correspond to 96%–97% volume of water being retained in wetted nanofiber

312

meshes, demonstrating a high water affinity and large porosity of nanofiber meshes. The slightly

313

higher absorption of NF-nsGCN suggests that the incorporation of nsGCN enhanced water affinity

314

probably due to the high water affinity of GCN. In fact, Hansen solubility parameter (HSP)

315

expressing chemical affinity between molecules and colloids [33] suggested better water

316

miscibility of GCN: HSP of GCN (δD ~ 17.8 MPa1/2, δP ~ 10.8 MPa1/2, and δH ~ 15.4 MPa1/2)

317

[34] is much closer to that of water (δD ~ 15.5 MPa1/2, δP ~ 16.0 MPa1/2, and δH ~ 42.3 MPa1/2)

318

than that of PAN (δD ~ 22.4 MPa1/2, δP ~ 14.1 MPa1/2, and δH ~ 9.1 MPa1/2). Note that the contact

319

angle measurement is not adopted as an informative characterization in this study. Cassie effect

320

resulting from large air pocket and micron-range roughness in nanofiber has a significant

321

contribution on water contact angle of nanofiber meshes [35]. As a result, a contact angle of a

322

nanofiber mesh does not indicate hydrophilicity or hydrophobicity of the nanofibers. In fact, the

323

contact angles of the nanofiber-coated membranes were relatively high (135.1°, 121.5°, and 114.9°

324

for NF/Al2O3, NF-bGCN/Al2O3, and NF-nsGCN/Al2O3 membranes, respectively) in spite of the

325

high water absorption capacity.

326 327

3.3.

Pure water flux of membranes

328 329

The PWF of bare Al2O3, NF/Al2O3, NF-bGCN/Al2O3, and NF-nsGCN/Al2O3 membranes

330

were assessed in a crossflow filtration system (Fig. 5). In spite of additional coating on bare Al2 O3

331

membrane, the average PWF of the NF/Al2O3 membrane (514 L∙m−2∙h−1) is 20% higher than that

332

of the bare membrane (421 L∙m−2∙h−1). Both NF-bGCN/Al2O3 and NFnsGCN/Al2O3 membranes

333

exhibited a much higher PWF than the bare membrane, the deviations of which were clearly

334

beyond the experimental error range. This finding is consistent with several previous studies

335

addressing that a surface coating of electrospun nanofibers is capable of effectively improving flux

336

of polymeric flat sheet membranes [15,36,37]. Although a definite mechanism of the high PWF

337

has yet to be revealed, we speculate that a coating layer of a sparse nanofiber mesh on a hollow

338

fiber membrane disturbs the laminar flow of water along the membrane surface, which results in

339

the enhanced permeation of water in a transmembrane direction [38]. The sparse mesh structure

340

of nanofibers resemble the effect of a mesh spacer placed in a spiral module of a flat sheet

341

membrane [39].

342

The PWF of the NF-bGCN/Al2O3 and NF-nsGCN/Al2O3 membranes were 728 and 816

343

L∙m−2∙h−1, respectively, both of which are much higher than that of the NF/Al2O3 membrane

344

because of the enhanced water affinity by the incorporation of GCN into the PAN nanofiber. This

345

result is consistent with the larger water capacity for NF-nsGCN as described in the previous

346

section. Furthermore, previous studies have reported that electrospun membranes improve water

347

flux by introducing hydrophilic organic nanofillers due to the enhancement of water affinity

348

[40,41]. Compared with the NF-bGCN/Al2O3 membrane, the NF-nsGCN/Al2O3 membrane gave

349

10% higher PWF, which is possibly due to the smooth surface morphology of nanofibers as shown

350

in Fig. 4(d). Among all the membranes fabricated, the NF-nsGCN/Al2O3 membrane showed the

351

highest PWF, which is nearly twice the PWF of bare membrane without the NF coating. Since the

352

nanofibers-coated membrane were completely wetted, the high porosity of nanofiber meshes

353

contributes to the large flow rates [42]. Three key parameters on the enhanced PWF were assigned:

354

(1) sparse mesh structure disturbing laminar water flow, (2) better affinity to water, and (3) smooth

355

nanofiber morphology.

356

357 358

Fig. 5. Pure water flux for bare Al2O3, NF/Al2 O3, NF-bGCN/Al2O3, and NF-nsGCN/Al2O3

359

membranes for 90 min crossflow filtration at transmembrane pressure of 2 bar.

360 361

3.4.

Crossflow filtration of oilfield produced water

362 363

In the PWF measurement, the fabricated membranes were subjected to the crossflow

364

filtration of OPW, which were evaluated in terms of permeate flux and oil rejection efficiency.

365

Fig. 6 shows the photographs of OPW feed and permeate solutions obtained by the crossflow

366

filtration. In contrast with the opaque appearance of the feed solution containing 1000 ppm oil

367

droplet, all the permeate solutions were transparent, directly indicating that most of the oil droplets

368

were successfully removed by the filtration operation.

OPW

369

(a)

(b)

(c)

(d)

370

Fig. 6. OPW feed and permeate solutions obtained by crossflow filtration using hollow fiber

371

membranes: (a) bare Al2O3, (b) NF/Al2O3, (c) NF-bGCN/Al2O3, and (d) NF-nsGCN/Al2 O3.

372 373

Figure 7(a) shows the permeate flux of bare Al2O3, NF/Al2O3, NF-bGCN/Al2O3, and

374

NF-nsGCN/Al2O3 membranes assessed in the crossflow filtration system. After 180 min, the bare

375

membrane showed a permeate flux of 236 L∙m−2∙h−1, which was only 56% of its PWF. After the

376

filtration, the SEM images demonstrated that the surface of the membrane was almost completely

377

covered by oily component (Fig. 8a). Such a dense fouling layer significantly reduced the permeate

378

flux of OPW due to the severe blockage of the open pores on the surface of the membrane. The

379

NF/Al2O3 membrane exhibited a permeate flux of 386 L∙m−2∙h−1, which was higher than that of

380

the bare membrane. The permeate flux of the NF/Al2O3 membrane corresponds to 75% of its PWF,

381

maintaining a higher permeate flux of OPW compared with that of the bare membrane. An SEM

382

image of the NF/Al2O3 membrane after filtration displayed some amount of oily component

383

captured on the opening of the nanofiber mesh (Fig. 8b). However, individual nanofibers and the

384

openings between nanofibers can be still observed because the fouling layer made of oily

385

component covered only a small portion of the nanofiber coating layer. The large opening size of

386

the nanofiber mesh prevented the oily component to form dense continuous fouling layer,

387

maintaining the permeation pathway of water to a large extent. As a result, the nanofiber coating

388

layer has the capability of preserving high permeate flux of OPW. In fact, the NF-bGCN/Al2O3

389

and NF-nsGCN/Al2O3 membranes gave permeate fluxes of 577 and 640 L∙m−2∙h−1, respectively,

390

which are much higher than that of the bare membrane. Compared with the NF/Al2O3 membrane,

391

the SEM images exhibited a less amount of oily component on the surface of the NF-bGCN/Al2 O3

392

and NF-nsGCN/Al2O3 membranes (Fig. 8c–d), which agrees with the higher permeate flux of

393

OPW for the membranes. As demonstrated in the PWF experiment, the NF–nsGCN/Al2O3

394

membrane gave a higher permeate flux of OPW than that of the NF–bGCN/Al2O3 membrane due

395

to the smooth surface morphology of the nanofibers resulting from the uniform distribution of thin

396

nsGCN. (a)

(b)

397 398

Fig. 7. (a) Permeate flux and (b) oil rejection percentage in 180 min crossflow filtration of OPW

399

using bare Al2O3, NF/Al2O3, NF-bGCN/Al2O3, and NF-nsGCN/Al2O3 membranes at a

400

transmembrane pressure of 2 bar.

401

(b)

(a)

10 µm

10 µm (d)

(c)

10 µm

10 µm

402 403

Fig. 8. Surface SEM micrographs of (a) bare Al2 O3, (b) NF/Al2 O3, (c) NF-bGCN/Al2O3, and (d)

404

NF-nsGCN/Al2O3 membranes after 180 min OPW filtration.

405 406

3.5.

Oil rejection of membranes

407 408

Oilfield produced water generally contains a complex mixture of organic and inorganic

409

materials similar to those found in crude oil, and the composition varies with the location and the

410

life of a producing field [43]. Therefore, we measured the organic species in the crude oil using

411

gas chromatography–mass spectroscopy (GC–MS) (Fig. S2) to identify the components in OPW

412

solution (Details in Supplementary Information).

413

To assess the trace amount of oil contaminants in the transparent permeates, we

414

quantitatively determined the rejection efficiency of OPW based on UV absorbance. All

415

membranes showed a high oil rejection above 94% even after 180 min filtration as shown in Fig.

416

7(b). Obviously, the nanofiber-coated membranes resulted in a better oil rejection than that of the

417

bare membrane. The NF-nsGCN/Al2O3 membrane exhibited an excellent oil rejection percentage

418

of 99% along with the highest permeate flux, demonstrating the best filtration performance among

419

the membranes examined in this study. Furthermore, oil rejection percentage of this permeate flux

420

was matched with the result obtained from TOC analysis at 8.601 ppm (98.3% oil rejection). To characterize the dispersion state of oil contaminants in the permeates, the size

422

distribution of oil droplets in the OPW feed and permeate solutions were measured using a particle

423

size analyzer, which is suitable for understanding the separation mechanism of the fabricated

424

membranes (Fig. 9).

Number weighted frequency (%)

421

425 426

Fig. 9. Particle size distribution of oil droplets in OPW feed and permeate solutions produced by

427

bare Al2O3 and NF-nsGCN/Al2O3 membranes after 180 min.

428

429

The feed solution contained a large number of oil droplets with the sizes between 0.57 and

430

1.3 μm. These size oil droplets strongly scatter light, which is consistent with the white color of

431

the feed solution. The permeate solution obtained by crossflow filtration using the bare Al2O3

432

membrane also contained oil droplets sizing from 0.24 to 0.37 µm. In addition to the low

433

concentration of oil droplets suggested from the oil rejection efficiency, the small droplet size

434

agrees with the transparent appearance of the permeate solution. The average size of oil droplets

435

significantly reduced compared with that in the feed solution.

436

The theoretical analysis as discussed in Section 2.5 expresses that crossflow microfiltration

437

has a tendency to accumulate the small droplets near the membrane surface and keep the large

438

droplets away from the surface. In addition to the spontaneous size-selectivity crossflow

439

microfiltration, membrane pore size plays a dominant role in the rejection of oil droplets. In fact,

440

the maximum size of oil droplet agrees with the maximum pore size on the surface of the bare

441

Al2O3 membrane (Fig. 4a). This indicates that the size-selectivity permeation of oil droplet through

442

the bare membrane was governed by the surface pore size of the membrane.

443

The permeate collected by crossflow filtration using the NF-nsGCN/Al2O3 membrane

444

contained oil droplets sizing from 0.07 to 0.3 μm, the average size of which is much smaller than

445

that obtained using the bare membrane. It is interesting because the opening size of the nanofiber

446

coating is much larger than the surface pore size of the bare membrane. This indicates that the

447

droplet size distribution is not directly determined by the opening size of the nanofiber coating.

448

This is probably because the nanofiber coating does not separate oil droplets based on size-

449

selectivity of the openings but absorbs them in the openings due to a large adhesive energy of the

450

droplets as illustrated in Fig.10. Indeed, our previous paper has demonstrated that the electrospun

451

PAN nanofibers incorporating bGCN or nsGCN have a good ability to absorb oil droplets

452

dispersed in OPW in a floating manner [26]. According to the evidence, we can expect the

453

nanofiber coating absorbs most of the droplets during the permeation of the droplets through the

454

coating. The SEM images proved that the nanofiber diameter definitely increased when the

455

nanofiber coating was subjected to crossflow filtration of OPW (Fig. 8). The nanofiber coating

456

captured the larger droplets more effectively due to the larger collision cross section of droplets,

457

which resulted in the permeation of smaller droplets. In addition of the effective absorption in the

458

nanofiber coating, the small pores of the bare membrane can remove large oil droplets that pass

459

through the nanofiber coating. As shown in Fig. 7(b), the ranking of oil rejection performance

460

coincides with that of oil absorption performance previously determined [26]: NF-nsGCN > NF-

461

bGCN > NF. The agreement completely supports our expectation that the absorption performance

462

of the nanofiber coating gave a better oil rejection efficiency and smaller size of oil droplets

463

remaining in the permeates. The effective absorption of oil droplets in the nanofiber coating

464

prevented the formation of fouling layer on the surface of the bare membrane. The fouling

465

suppression preserves a high flux of OPW even after 180 min operation.

466

467 468

Fig. 10. Crossflow filtration of OPW using NF-nsGCN/Al2O3 membrane.

469 470

3.6

Cycle operation of crossflow filtration

471 472

As discussed in the previous section, the nanofiber coatings on the Al2O3 hollow fiber

473

membrane was able to prevent dense fouling layer from covering the whole membrane surface

474

because of the rough surface morphology and high water affinity. However, a relative amount of

475

oil adhered on a part of the membrane surface after 180 min filtration. Thus, cleaning process is

476

necessary to recover the filtration performance for a long-term operation. To introduce good

477

cleaning properties, we incorporate GCN, an efficient photocatalyst, into the electrospun PAN

478

nanofiber. The photocatalyst-embedded nanofiber captured the oil droplets in water and exhibited

479

an efficient photodegradation property against the oil contaminants under UV and visible light

480

irradiations, which was demonstrated in our previous study [26]. On the basis of the experimental

481

evidence, the photodegradation process is expected to be applicable as a cleaning process of oil

482

contaminants absorbed by the nanofiber.

483

Figure 11 shows the permeation flux of OPW for 180 min crossflow filtration using the

484

fabricated membranes after three cycles of operation. There is a slight difference between Fig. 7

485

and 11 because the former is based on an averaged value and the latter represents one of the

486

experiments. After each interval, the membrane was taken out from the membrane module,

487

immersed in pure water, and irradiated with UV light. In the case of all filtration operation

488

independent on membrane types, the permeate flux exponentially decreased with the elapsed time

489

and fell down to 40%–50% of the initial flux after 180 min. Such reduction in the permeate flux

490

was recovered by the cleaning process, which strongly depends on membrane types. The bare

491

Al2O3 and NF/Al2O3 membranes, both of which have no photodegradation capability, exhibited a

492

recovery ratio of less than 90% of the initial flux compared with the previous operation. In contrast,

493

both NF-bGCN/Al2O3 and NF-nsGCN/Al2O3 membranes with photodegradation ability recovered

494

more than 90% of the initial flux. The results demonstrate that the photodegradation ability of the

495

nanofiber was suitable for cleaning membranes after filtration and maintained a high permeate flux

496

for repeated filtration cycles.

497

Figure 12 shows the photographs of NF-nsGCN membrane coated with nanofiber mesh.

498

While the as-fabricated membrane looks white (Fig. 12a), the membrane used for crossflow

499

filtration of OPW turned to pale orange, indicating an amount of oil component adhered on the

500

surface (Fig. 12b). By irradiating UV light in pure water, the membrane color faded away

501

considerably, suggesting an effective photodegradation of the oil component. The NF-

502

nsGCN/Al2O3 membranes gave the highest permeate flux of 577 L∙m−2∙h−1 after three cycles of

503

180 min filtration. In the third filtration cycle, the membrane maintained 87% of the permeate flux

504

after 180 min filtration of the first cycle while the fluxes of the other membranes reduced to 58%–

505

75% of their first cycle. The results agree with the highest photodegradation performance of

506

nanofiber containing nsGCN as confirmed in our previous paper [26] and provide an evidence that

507

the tphotodegradation ability of the nanofibers offers a better cleaning performance of the

508

membrane.

509

510 511

Fig. 11. Permeation fluxes of OPW filtration for bare Al2O3 (grey), NF/Al2O3 (green),

512

NF-bGCN/Al2O3 (blue), and NF-nsGCN/Al2O3 (red) at a pressure of 2 bar for 180 min in three

513

cycles operation.

514

(a)

(b)

(c)

515 516

Fig. 12. Photographs of GCN nanofiber-coated Al2O3 hollow fiber membranes (a) before OPW

517

filtration, (b) after 180 min filtration at a pressure of 2 bar, and (c) after 180 min irradiation under

518

UV light.

519

520

Figure 13 shows the oil rejection percentage for 180 min cross-flow filtration of OPW

521

using the fabricated membranes under three cycles of operation. The bare Al2O3 membrane

522

showed a decreased oil rejection percentage of around 89% after three operation cycles, which

523

corresponds to 8% reduction from the initial value. The NF/Al2O3 membrane oil rejection

524

degraded by 5%, while both NF-bGCN/Al2O3 and NF-nsGCN/Al2O3 membranes demonstrated

525

only 2% reduction due to the photodegradation of oil component by UV irradiation. The NF-

526

nsGCN/Al2O3 membranes gave the highest rejection percentage of 97% after three operation

527

cycles. The results indicate that the nanofiber mesh with photodegradation ability is a promising

528

coating layer for crossflow microfiltration membrane that maintains the high rejection

529

performance.

530

531 532

Fig. 13. Oil rejection percentage for bare Al2O3 (grey), NF/Al2O3 (green), NF-bGCN/Al2O3 (blue),

533

and NF-nsGCN/Al2O3 (red) at a pressure of 2 bar for 180 min in three cycles operation.

534

535

In comparison to other advance membrane modifications used to treat oily wastewater,

536

Yang and co-workers have investigated the efficiency of ZrO2/α-Al2O3 microfiltration membrane

537

with average pore size of 0.2 µm showed 99.8% of oil rejection from 5000 ppm of vegetable and

538

mineral oils solutions [44]. Graphene modified Al2O3 ceramic microfiltration membrane

539

fabricated by Hu et al., [45], revealed that flux increased 27.8% increased as compared to

540

unmodified Al2O3 membrane with 98.7% oil rejection. Lastly, Zhou et al., [46] reported that

541

zirconia modified membrane that have been fabricated obtained 88% initial flux and 97.8% oil

542

rejection using 1000 ppm engine-oil water emulsion as a feed solution. Nevertheless, we believe

543

that NF-nsGCN/Al2O3 membrane is a promising candidate to treat OPW based on revealed

544

excellent membrane properties, separation performances and long-term stability in repeating cycle

545

filtration.

546 547

4.

Conclusion

548 549

Photocatalytic nanofiber-coated hollow fiber membranes were successfully fabricated

550

using a newly design electrospinning technique. Polyacrylonitrile nanofibers incorporating

551

graphitic carbon nitride (GCN) were coated on alumina (Al2O3) hollow fiber membranes.

552

Crossflow filtration using the fabricated membranes exhibited a significant improvement in pure

553

water flux, OPW permeate flux, and oil rejection percentage compared with those of the bare Al2O3

554

membrane. The NF-nsGCN/Al2O3 membrane showed the highest pure water flux, OPW permeate

555

flux, and oil rejection at 816 L∙m−2∙h−1, 640 L∙m−2∙h−1, and 99%, respectively. Sparse mesh

556

structure, high water affinity, and smooth nanofiber morphology were found as key parameters for

557

nanofiber coatings that significantly improved membrane performances. Unlike the conventional

558

ultrafiltration membranes, nanofiber coating was able to prevent uniform fouling layer covering

559

the whole hollow fiber membrane surface, resulting in a high OPW permeate flux and oil rejection.

560

Furthermore, the NF-nsGCN/Al2O3 membrane also maintained the highest permeate flux (577

561

L∙m−2∙h−1) and oil rejection (97%) after three operation cycles, demonstrated an excellent cleaning

562

performance for a long-term membrane operation. The photodegradation ability of the NF-nsGCN

563

nanofiber enabled the coating to degrade the captured oil contaminants under UV irradiation,

564

which is beneficial to maintain the high permeate flux and rejection in repeated filtration system.

565

These findings indicate the potential application of the NF-nsGCN-coated Al2O3 hollow fiber

566

membrane in the industrial OPW treatment. They also provide useful information for further

567

research to develop nanofiber-coated hollow fiber membranes for future membrane separation

568

technology.

569 570

Acknowledgments

571 572

The authors gratefully acknowledge PETRONAS Penapisan (Melaka) Sdn. Bhd. for the

573

supply of crude oil sample. N.H.A. and S.S. would like to thank Dr. Masanobu Naito at NIMS for

574

his kind support. N.H.A, J.J, and N.Y. would like to express their sincere gratitude towards the

575

Malaysia Ministry of Higher Education for the research funds provided under UTM–HiCOE

576

Research Grants (R.J090301.7846.4J184) and (R.J090301.7846.4J185), UTM for the financial

577

support under Research University Grant (GUP) Tier 1 (Q.J130000.254616H43) and Japan

578

government for the Kurita Water and Environmental Foundation (KWEF) research grant

579

(18P001). N.H.A. would like to thank Universiti Teknologi Malaysia (UTM)–National Institute

580

for Materials Science (NIMS) Cooperative Graduate School Program (ICGP) 2017/18 for the

581

graduate fellowship awarded. A part of this work was supported by "Nanotechnology Platform"

582

(project No. A-17-NM-0208) of the Ministry of Education, Culture, Sports, Science and

583

Technology (MEXT), Japan.

584 585

Appendix A. Supplementary data

586 587

Supplementary data associated with this article can be found, in the online version, at

588 589

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Highlights

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

Graphitic carbon nitride (GCN) photocatalyst was incorporated in nanofibers

721



Photocatalytic nanofibers were electrospun on alumina hollow fiber membranes

722



Cross-flow microfiltration demonstrated purification of oilfield produced water

723



Hydrophilic, highly-porous nanofiber coating exhibited excellent fouling resistance

724



Photodegradation ability of GCN offered cleaning performance of robust membranes

725