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Prince JA, Bhuvana S, Anbharasi V, Ayyanar N, Boodhoo KVK, Singh G.
Ultra-wetting graphene-based PES ultrafiltration membrane – A novel
approach for successful oil-water separation.
Water Research 2016, 130, 311-318.
Copyright:
© 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license
DOI link to article:
http://dx.doi.org/10.1016/j.watres.2016.07.042
Date deposited:
29/07/2016
Embargo release date:
19 July 2017
Accepted Manuscript
Ultra-wetting graphene-based PES ultrafiltration membrane – A novel approach forsuccessful oil-water separation
J.A. Prince, S. Bhuvana, V. Anbharasi, N. Ayyanar, K.V.K. Boodhoo, G. Singh
PII: S0043-1354(16)30554-1
DOI: 10.1016/j.watres.2016.07.042
Reference: WR 12237
To appear in: Water Research
Received Date: 14 March 2016
Revised Date: 14 July 2016
Accepted Date: 18 July 2016
Please cite this article as: Prince, J.A., Bhuvana, S., Anbharasi, V., Ayyanar, N., Boodhoo, K.V.K.,Singh, G., Ultra-wetting graphene-based PES ultrafiltration membrane – A novel approach for successfuloil-water separation, Water Research (2016), doi: 10.1016/j.watres.2016.07.042.
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Ultra-wetting graphene-based PES ultrafiltration membrane - A novel approach 3
for successful oil-water separation 4
5 6 7 J. A. Prince a,b,*, S. Bhuvana a, V. Anbharasi a, N. Ayyanar a, K.V.K. Boodhoob and G. Singh a, 8
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12 a Environmental & Water Technology – Centre of Innovation, Ngee Ann Polytechnic, Singapore 13 599489 14 15 16 17 b School of Chemical Engineering and Advanced Materials, Faculty of Science, Agriculture and 18 Engineering, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom 19 20 21 *Email: [email protected] 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 Keywords: (Ultra-wetting graphene, hydrophilicity, ultrafiltration, oil-water separation) 37 38 39
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ABSTRACT: 47
Oil pollution in water and separation of oil from water are receiving much attention in recent years 48
due to the growing environmental concerns. Membrane technology is one of the emerging solutions 49
for oil-water separation. However, there is a limitation in using polymeric membrane for oil water 50
separation due to its surface properties (wetting behaviour), thermal and mechanical properties. 51
Here, we have shown a simple method to increase the hydrophilicity of the polyethersulfone (PES) 52
hollow fiber ultrafiltration (UF) membrane by using carboxyl, hydroxyl and amine modified 53
graphene attached poly acrylonitrile-co-maleimide (G-PANCMI). The prepared membranes were 54
characterized for its morphology, water and oil contact angle, liquid entry pressure of oil (LEPoil), 55
water permeability and finally subjected to a continuous 8 hrs filtration test of oil emulsion in water. 56
The experimental data indicates that the G-PANCMI play an important role in enhancing the 57
hydrophilicity, permeability and selectivity of the PES membrane. The water contact angle (CAw) 58
of the PES membrane is reduced from 63.7± 3.8o to 22.6 ± 2.5o which is 64.5% reduction while, the 59
oil contact angle was increased from 43.6 ± 3.5o to 112.5 ± 3.2o which is 158% higher compared to 60
that of the PES membrane. Similarly, the LEPoil increased 350% from 50±10 kPa of the control PES 61
membrane to 175± 25 kPa of PES-G-PANCMI membrane. More importantly, the water 62
permeability increased by 43% with >99% selectivity. Based on our findings we believe that the 63
development of PES-G-PANCMI membrane will open up a solution for successful oil-water 64
separation. 65
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77 78 79
1.Introduction 80
In recent years, oil-water separation is receiving much attention due to the growing environmental 81
concerns related to oil pollution in water (Shannon, 2008). Large volumes of oil polluted 82
wastewater are produced in various industries such as oil fields, metallurgical, petrochemical, 83
pharmaceutical etc., in the form of oil water emulsion (Sirivedhin and Dallbauman, 2004). The 84
untreated oil polluted wastewater contains harmful chemicals and dissolved minerals which are 85
classified as hazardous waste and these will bring a negative impact on people’s health and even 86
will have damaging impact on the ecosystem and hence, governmental regulation are increasingly 87
more stringent to remove the hazardous waste before discharge (Reilly et al., 1991; Group 1998). 88
The conventional oil-water separation techniques such as gravity separation, skimming and flotation 89
are useful for free oil/water mixtures (Oil droplet >150µm and dispersed oil size range of 20-150 90
µm), but are not applicable to small size (<20µm) oil/water emulsions (Cheryan et al., 1998; 91
Nordvik et al., 1996). Low efficiency and high operation cost are the other limiting factors of the 92
conventional oil-water separation techniques. Therefore, advanced techniques are urgently needed 93
to effectively separate various oil/water mixtures. 94
There is a growing tendency to use membrane technology for oil/water separation. Currently, there 95
are two different types of membrane are in use for oil-water separation based on their surface 96
properties. The first type is super hydrophobic-superoleophilic (Deng et al.,, 2013), these 97
membranes are favourable for the oil transportation while repel the liquid water entering the pores. 98
For example, silicon oxycarbide fibres (Lu et al.,, 2009), PTFE coated mesh (Feng et al., 2004), and 99
modified polyester textile (Zhang and Seeger 2011). Membranes with rationally controlled pore size 100
(to be smaller than the emulsified water droplets) are also suitable for effective oil-water separation 101
(Zhang et al., 2013; Shi et al., 2013). 102
The second type is super hydrophilic-super-oleophobic (Xu et al., 2013). These membranes are 103
favourable for the water transportation while repel the oil entering the pores. For example, aligned 104
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ZnO nanorod array coated mesh (Tian et al., 2012), Zeolite-coated mesh (Wen et al., 2013), 105
Alumina nanoparticles coated fabric (Samuel et al., 2011) and hydrogel-coated mesh are super 106
hydrophilic in nature (Xue et al., 2011). The first type super hydrophobic-super oleophilic 107
membrane has several drawbacks such as the adherence of high viscous oil to the membrane surface 108
which is generally difficult to be removed and requires more chemical usage to remove it (Chen et 109
al., 2013). The second type super hydrophilic-super oleophobic membranes are advantageous over 110
the first type super hydrophobic-super oleophilic membranes. Because, these membranes allow only 111
water to pass through, which reduces the possibility of membrane clogging. Similarly, they prevent 112
the formation of water barrier between the membrane surface and the oil phase due to the fact that 113
water is heavier than oil phase (Zhang et al., 2013). 114
Fig.1. shows how the water barrier affects the permeate flux in the first type super hydrophobic-115
super oleophilic membranes. For the first type super hydrophobic-super oleophilic membranes, the 116
system has to operate in very high turbulent flow to push the oil emulsion towards the membrane. 117
But, this process will increase the overall energy consumption of the system. 118
119 120
Fig. 1. a) Water in direct contact with membrane surface in the first type super hydrophobic-super 121 oleophilic membranes, b) water barrier between the membrane surface and the oil emulsion in the 122 second type super hydrophilic-super oleophobic membranes. 123
124 Generally, hydrophilic membrane exhibits an affinity for water. It possess a high surface energy 125
value and has the ability to form hydrogen-bonds with water. Hydrophilic surface will repel the 126
hydrophobic oily particles such as hydrocarbons, surfactants, grease etc. Recently, considerable 127
attention has been focused to improve the surface hydrophilicity of the membranes along with 128
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generation of surface micro-nano structures for oil-water separation, which results in super 129
oleophobic surfaces with low oil-adhesion (Zhu et al., 2013; Zhang et al., 2012; Kota et al., 2012). 130
Recently, carbon-based nanomaterials such as graphene (Gai et al., 2014), graphene oxide (Zinadini 131
et al., 2014), carbon nanotube (Duan, 2014) and fullerene (Tasaki et al., 2007) have gained much 132
attention in the field of membrane science and engineering due to its high surface area, high 133
mechanical strength and chemical stability. Graphene is a sp2-hybridized two-dimensional carbon 134
sheet (Novoselov, 2004). Incorporating graphene and its derivative graphene oxide in a polymer 135
matrix have shown improved membrane performance (Jin, 2013; Akin et al., 2014; Heo et al., 2013; 136
Han et al., 2013; Sun, 2013; Zhao, 2013) . However, graphite and graphene are generally 137
hydrophobic in nature which limits their application in water filtration (Li et al., 2008). 138
Here we report a novel method to produce ultra-wetting graphene based membrane for successful 139
oil water separation. Initially, the wettability of graphene was increased by amine and carboxyl 140
functionalisation. Graphene was first carboxylated using highly concentrated acid mixture 141
(hydrochloric acid and sulphuric acids). The carboxylic group was further modified to acid chloride. 142
Finally the acid chloride modified graphene oxide was amine functionalised by using ethylene 143
diamine. The functionalized graphene oxide was then attached to a highly hydrophilic water 144
insoluble polymer (poly acrylonitrile co maleic anhydride). The graphene oxide grafted poly 145
acrylonitrile co maleimide (G-PANCMI) was used to prepare the dope solution. The hollow fibre 146
ultrafiltration membranes were prepared by dry wet spinning. 147
The prepared membranes were characterized using (FTIR) spectroscopy, Contact angle (CA), 148
Tensile testing, Zetapotential (surface charge analyser), scanning electron microscopy (SEM), and 149
the Porometer. Both control PES and modified G-PANCMI-PES membrane were tested for the oil 150
entry pressure and clean water flux. Finally, all the prepared membranes were tested for oil water 151
separation, permeability, selectivity and antifouling property in long term experiments at two 152
different temperatures. 153
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2. Experimental 155
2.1 Materials 156
Polyethersulfone (PES) k-3010 powder was purchased from Sumitomo chemicals pte ltd, Japan. 157
Acrylonitrile, Maleic anhydride, dichloroethane and azobisisobutyronitrile (AIBN) were purchased 158
from sigma Aldrich with 99% purity. High purity ethanol, Nitric acid (HNO3), Sulphuric acid 159
(H2SO4), thionyl chloride (SOCl2) and DMAc (N-N-Dimethyl acetamide), were also purchased 160
from Sigma Aldrich and used as received. Castrol brake fluid oil was purchased locally. The xGnP, 161
exfoliated graphite nano platelets were purchased from XG Sciences. The oily waste water (oil 162
emulsion) was prepared by constantly mixing 200ppm of the oil in DI water at 400rpm using a 163
multi blade mechanical stirrer. The water used for the reaction was distilled and de-ionized (DI) 164
with a Milli-Q plus system from Millipore, Bedford, MA, USA. 165
166 2.2. Synthesis of functionalised xGnP 167
About 1 gram of the pristine xGnP was initially treated with an excess of acid mixture 168
(H2SO4/HNO3, 3:1) to introduce the acid functionality on to the graphene surfaces. After successful 169
oxidation, the functionalised graphene was centrifuged, filtered and washed with excess water until 170
the pH of the wash water was neutral. After through drying, the acid functionalised xGnP was 171
further refluxed with 150ml of thionyl chloride at 80OC for 24 hours. The excess thionyl chloride 172
after the reaction was filtered off and then about 150ml of ethylene diamine was added to the 173
reaction vessel and continued to reflux for another 48hrs. The amine functionalised xGnP was 174
finally separated out by centrifugation and washed with excess ethanol to remove the unreacted 175
reagents and further with water (Fig.2). The detailed synthesis of ultra-wetting graphene has been 176
discussed in our recent publication (Prince et al., 2016). 177
2.3. Synthesis of xGnP grafted PANCMI 178
As shown in our recent study (Prince et al., 2016), PANCMA was synthesised as per our 179
previously reported procedure using azobisisobutyronitrile as an initiator. The synthesised 180
PANCMA was allowed to react overnight with the amine functionalised xGnP in 500ml of DMAc. 181
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Further, 100ml of toluene was added to the reaction mixture and refluxed at 110oC for about 5 hours 182
and the toluene was distilled off from the reaction vessel. The product in DMAc was poured into 183
methanol to separate the product in polyamic acid form. This intermediate product was further 184
subjected to thermal imidisation using a multistage heating of 200oC for 2hours and finally at 260oC 185
for another 30 mins to obtain the final xGnP grafted PANCMI (G-PANCMI). Fig. 3 shows the 186
schematic representation of the G-PANCMI synthesis.187
188
189 190 191
Fig. 2. Different steps involved in the amine functionalisation of xGnP 192 193
194 2.4 Fabrication of PES and PES-G-PANCMI hollow fibre membranes by dry wet spinning 195
The control Poly ether sulfone (PES) and xGnP grafted poly (acrylonitrile co maleimide) (G-196
PANCMI) modified PES-G-PANCMI hollow fibre ultrafiltration membranes were prepared by 197
dry wet spinning method. PES was used as the base polymer, NMP was the base solvent, DEG 198
was used as a non-solvent, PVP was used as an additive (pore forming agent) and G-PANCMI 199
was used as a hydrophilic additive. Based on the results of our previous studies, the weight 200
percentage of the polymeric additive to the PES dope was fixed as 5wt% (G-PANCMI)34. The 201
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composition of the casting solution consists of 21 wt% PES, 5 wt% PVP-K-30, 5 wt% DEG, 69 202
wt% NMP respectively 5% of G-PANCMI was added to the PES-G-PANCMI dope 203
composition by replacing 5% of NMP where the NMP concentration was 64%. The phase 204
diagram of the dope compositions are presented in Fig.4. PVP powder was first added into the 205
NMP /DEG mixture in a RB flask and the solution was stirred by a mechanical stirrer for at 206
least 1-1.5 hours. After complete dissolution of PVP, PES and G-PANCMI were added and 207
allowed to stir at a constant speed of 250~350 rpm for at least 24 h at 80o C, to obtain a 208
completely dissolved / dispersed homogeneous polymeric solution. The dope solution was 209
poured into the polymer tank and degassed at a negative pressure of -0.6 bar for 15-20 min. 210
Nitrogen gas was purged into the dope tank to create inert atmosphere and to push the polymer 211
towards the polymer pump. NMP and water were mixed in 80:20 volume ratio (NMP: Water 212
80:20) was used as a bore liquid. The polymer solution and the bore liquid were pumped to the 213
spinneret (OD 1.2 mm, ID 0.6 mm). The air gap was fixed at 50mm. The hollow fibre 214
membranes were fabricated at around 25o C and at around 65-70% relative humidity with a take 215
up speed of 0.21 m/s. The membranes were then collected from the winder and left inside a 216
water tank (post coagulation tank) for 24 hrs to washout the residual NMP, DEG and PVP that 217
was not removed from the solution at the point of fabrication process. The membranes were 218
immersed into a post treatment solution of 40% water and 60% glycerine before testing the 219
clean water flux. 220
221
222 Fig. 3. Synthesis of xGnP grafted PANCMI 223
224
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228 229
Fig.4. Phase diagram for the dope compositions 230 231 2.5 Characterization 232
A scanning electron microscope (SEM) Jeol Jsm-7600F coupled to a XmaxN detector for energy-233
dispersive X-ray (EDX) analysis was used to study the morphology and the overall chemical 234
composition and the distribution of the chemical elements of interest in the membrane. The water 235
contact angle (CAw) of the unmodified and modified hollow fibre membranes were determined 236
using the Sigma 701 Tensiometer. Five readings were measured for each sample and an average 237
was obtained from the results. The pore size of the membrane was measured using the Porometer 238
3G instruments (equipped with 3GWin control software) from Quanta chrome. Thermo gravimetric 239
(TG) analysis of the samples (10-15mg) was performed on a Mettler-Toledo thermo gravimetric 240
analyzer in temperature range of 30-500⁰C with a heat ramping rate of 15°C min−1 under nitrogen 241
atmosphere. The mechanical properties of the membranes were studied using an Instron universal 242
materials testing machine (Model 3366). The hollow fibre samples (5 numbers) of length 100 mm 243
were used for the test. 244
2.6 Liquid Entry Pressure experiment 245
The liquid entry pressure of oil (LEPoil) was measured for the PES and PES-G-PANCMI 246
membranes. 10 numbers of hollow fiber membranes of 30cm length were used to fabricate the 247
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membrane module. The membrane modules were potted using epoxy glue to seal one side of the 248
hollow fibers while keeping the other lumen side open to feed the liquid oil. The membrane module 249
was kept in a non-pressured transparent box and the open lumen side of the membrane module was 250
connected to the feed tank topped-up with oil (Castrol Brake Fluid Dot 4). Compressed nitrogen 251
was used to apply pressure in the tank. The pressure was increased to 25kPa at a time interval of 60 252
s to examine if any oil droplet appeared on the membrane surface. The pressure was noted when the 253
oil droplets appear on the membrane surface. The experiment was carried out three times using 254
three different set of membranes made from the same condition. The results were averaged to obtain 255
the final LEPoil. 256
2.7 Clean Water Permeability experiment 257
The clean water flux of the control PES and the PES-G-PANCMI membranes were measured using 258
similar setup used in our previous study (Prince et al., 2014). The fibers with the total effective 259
membrane area of 90 cm2 (10 fibers and 30 cm length (effective length 24cm)) were used to 260
fabricate the membrane module. The two edges of the membrane module were sealed by using 261
epoxy glue while keeping the lumen open on one side to collect the clean water. The developed 262
membrane module was mounted to the filtration system. Cross-flow ultrafiltration experiments (out 263
to in) were carried out by using a filtration system at a constant feed pressure of 1bar. To evaluate 264
the performance of the prepared membrane in oil water separation, a long time (8 hrs) filtration test 265
was carried out using 200ppm oil (oil emulsion) in DI water at the same condition for both control 266
PES membrane and the modified PES-G-PANCMI membrane individually. 267
268
3.Results and Discussion 269 270 3.1. Structural analysis 271
272 The structure of the PES, G-PANCMI and PES/G-PANCMI membranes were confirmed using 273
Fourier Transform Infra Red Spectroscopy (FTIR) as shown in Fig.5. The FTIR spectra of PES 274
membrane showed a peak for the C-H stretching peak of benzene ring at 2974 cm-1. Three peaks 275
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between 1600 cm-1and 1400 cm-1were attributed to aromatic ring vibration. The C-O-C stretching 276
peaks were located at 1320 cm-1and 1233 cm-1. The S=O stretching peaks were present at 1150 cm-277
1and 1102 cm-1. The FTIR spectrum of G-PANCMI showed a broad band at 3219cm-1 278
corresponding to the –NH stretching vibration of the diamine moiety, a small peak at 2931cm-1 for 279
the –CH stretching vibration, a sharp peak at 2245cm-1 corresponding to the –CN stretching 280
vibration of the nitrile group and two sharp peaks at 1770cm-1 and 1718cm-1correspondingto the 281
C=O stretching vibrations of the imide carbonyl groups and finally a peak at 1386cm-1 for –C-N-C 282
stretching vibration confirming the formation of imide functionality by the attachment of amine 283
modified xGnP to PANCMA. The FTIR spectra of PES/G-PANCMI membrane showed the 284
presence of both PES and G-PANCMI peaks confirming the successful incorporation of G-285
PANCMI in PES matrix. 286
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Fig. 5. FTIR spectra of the control PES, G-PANCMI and the modified membrane PES-G-PANCMI 288 289 290
291
3.2 Mechanical and Thermal analysis 292
Thermo gravimetric (TG) analysis was performed to investigate the effect of incorporation of the 293
novel ultra-wetting graphene (G-PANCMI) on the thermal property of the PES UF membranes. The 294
differences in thermal stability of PES and PES-G-PANCMI based membranes are highlighted in 295
Fig. 6 (a). Compared to PES membrane, PES-G-PANCMI membrane showed excellent thermal 296
stability. The drastic weight loss for PES started at about 180°C. Whereas, PES-G-PANCMI 297
showed greater thermal stability up to a temperature of 210oC, without much weight loss 298
confirming the improved thermal properties of the PES due to the presence of G-PANCMI in the 299
membrane matrix. 300
301 302
Fig. 6. (a) Thermo gravimetric (TG) analysis and (b) the mechanical property of the control PES 303 and the modified membrane PES-G-PANCMI 304
305 The mechanical property of the control PES and the modified PES-G-PANCMI membranes were 306
studied using an Instron universal testing machine and the results are presented in Fig.6 (b). 307
Compared to PES membrane, PES-G-PANCMI membrane showed excellent mechanical stability. 308
The maximum load achieved for the PES membrane was 2.69 N whereas, PES-G-PANCMI showed 309
greater mechanical stability of 3.84 N which is around 30% higher than the PES membrane. 310
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Similarly, the elongation (extension) of the PES-G-PANCMI membrane (25%) was also higher 311
compared to PES membrane (15%). The improved mechanical properties of the PES UF membrane 312
is due to the presence of G-PANCMI in the membrane matrix. 313
314
3.3 Morphological analysis 315
The surface morphology and cross section of the PES and ultra-wetting graphene modified PES-G-316
PANCMI based hollow fibre membranes were examined using SEM and the pictures are presented 317
in Fig.7, (a) cross section, (b) outer surface and (c) actual image. Both membranes had an average 318
inner diameter of 0.6mm and an outer diameter of 1.2mm. However, the hollow fibre membranes 319
exhibit different internal structures depending on their composition. The internal structure of PES 320
membrane has a large number of macro voids. Whereas, the ultra-wetting graphene modified PES-321
G-PANCMI membranes has a lower macro voids with more sponge like structures in the cross 322
section next to the internal surface. This is due to the increase in viscosity and the coagulation value 323
of the casting solution. Further, G-PANCMI contains highly hydrophilic amine and carboxylic 324
groups which slows down the non solvent/solvent exchange. As a result less water was drawn into 325
the membrane which lead to the sponge like structure. Sponge like structure helps to enhance the 326
water permeability and selectivity. In addition to that, the even distribution of ultra-wetting 327
graphene nano sheets can be identified in the cross section and on the outer surface of the PES-G-328
PANCMI modified membrane.329
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331 332
Fig. 7. SEM images of PES and PES-G-PANCMI membranes (a) Cross section (b) Outer surface 333 (c) Actual image of synthesized PES and PES-G-PANCMA membranes 334
335 336
337
3.4 Pore size analysis 338
Pore size analysis: The average pore size of the PES membrane and the ultra-wetting graphene 339
modified PES-G-PANCMI membranes were measured and the experimental data indicated that 340
there is no significant difference on the mean pore size of both PES and PES-G-PANCMI 341
membranes. The average pore sizes of PES membrane was 0.07 ± 0.02µm and 0.07 ± 0.03µm for 342
the PES-G-PANCMI membrane. 343
3.5 Performance analysis 344
Fig. 8 (a) shows average LEPoil of the membranes together with its error range. Comparing the 345
LEPoil, even though the membrane pore size was almost same for PES and G-PANCMI, LEPoil 346
increased from 50±10kPa of PES membrane to 175± 25kPa of PES-G-PANCMI membrane (with 347
ultra-wetting graphene), which is 350% (3.5 times) higher than the PES membrane. 348
349
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350 351
Fig.8. (a) Liquid entry pressure of oil (LEPoil) analysis, (b) Water and oil contact angle and (c) 352 Clean water flux of the PES and PES-G-PANCMI membrane samples 353
354 355
The increase in LEPoil thus parallels to the increase in oil (dichloroethane) contact angle 356
(oleophobicity) of the ultra-wetting graphene modified PES-G-PANCMI membrane. The 357
oleophobicity of the PES membrane and PES-G-PANCMI membranes were measured by their oil 358
contact angle by using dichloroethane and the results are presented in Fig. 8 (b). The PES 359
membrane sample showed an oil contact angle of 43.6 ± 3.5o. Ultra-wetting graphene modified 360
PES-G-PANCMI membrane sample showed an oil contact angle of 112.5 ± 3.2o which is 158% 361
higher compared to that of the PES membrane. Similarly, the hydrophilicity of the PES membrane 362
and PES-G-PANCMI membranes were measured by their water contact angle and the results are 363
presented in Fig.8 (b). The PES membrane sample showed a water contact angle of 63.7± 3.8o. 364
Ultra-wetting graphene modified PES-G-PANCMI based membrane sample showed a water contact 365
angle of 22.6± 2.5o which is 64.5% reduction compared to that of the PES membrane sample. The 366
effectiveness of the ultra-wetting graphene on the hydrophilicity is clearly demonstrated by these 367
tests. The increased hydrophilicity is attributed to the presence of the amine (-NH2) and acid (-368
COOH) groups attached to the nano graphene sheets in the G-PANCMI matrix of the PES-G-369
PANCMI membrane. 370
The prepared PES membrane and the ultra-wetting graphene modified PES-G-PANCMI 371
ultrafiltration membrane were tested to evaluate the clean water flux of the membrane using a cross 372
flow filtration method. Fig.8 (c) shows the clean water flux for both membranes at a constant feed 373
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water pressure of 100 kPa (1bar). The PES membrane gave a pure water flux of 437± 18 LMH. 374
Even though the pore size are similar for both membrane, the ultra-wetting graphene modified PES-375
G-PANCMI based membrane gave higher pure water flux of 767± 23 LMH which is around 43% 376
higher compared to the PES based membrane. This increase in pure water flux is due to the increase 377
in hydrophilicity / wettability of the membrane. 378
To evaluate the performance of the prepared membrane in oil water separation, a long time (8 hrs) 379
filtration test was conducted using 200ppm oil emulsion (oil emulsion was kept at constant stirring 380
at 400rpm during filtration in order to have a homogeneous emulsion) in DI water for the control 381
PES membrane and the PES-G-PANCMI membrane individually, and the results are summarized in 382
Fig. 9. It is observed that the ultra-wetting graphene modified PES-G-PANCMI membrane gives 383
stable flux compared to PES based membrane. The flux drop for the PES-G-PANCMI membrane is 384
only 9.2% (<10) of the initial flux after 8 hrs oil emulsion in water separation whereas the PES 385
membrane’s flux drop is 65% for the same duration of operation. The obtained results highlight that 386
the presence of G-PANCMI helps to reduce fouling (oil deposition) on the membrane surface. The 387
reduced oil adhesion is mainly due to the presence highly hydrophilic amine (-NH2) and acid (-388
COOH) groups attached to the nano graphene sheets in the G-PANCMI matrix (Prince et al., 389
2016)of the PES-G-PANCMI membrane Previous literature studies also indicate similar effects on 390
the oil separation efficiency of the hydrophilic membranes (Xu et al., 2013, Tian et al., 2012,Wen et 391
al., 2013,Xue et al., 2011) 392
In order to evaluate the oil-emulsion selectivity of the membrane, the total organic carbon (TOC) 393
of the feed (oil-emulsified solution) and permeate were measured every hour. Percentage of oil 394
emulsion rejection (selectivity) was calculated and presented in Fig. 9 (b). From the data, it is found 395
that the selectivity for the PES-G-PANCMI membrane is higher and stable compared to the control 396
PES membrane. The selectivity for control PES membrane drops over time which may be due to 397
change in surface properties of the membrane over time. These result further confirms the increased 398
hydrophilicity of the PES membranes by G-PANCMI. Based on our findings, we conclude that the 399
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ultra-wetting graphene offers the distinct potential to be an ideal material with significantly 400
improved properties for new generation water filtration membranes. 401
402
Fig. 9: (a) Permeability (flux drop) and (b) oil removal efficiency of the membrane samples PES & 403 PES-G-PANCMI in a long time study of 8 hrs 404 405 406 4. Conclusions 407
408 In this work, a simple method to increase the hydrophilicity of the PES hollow fibre UF membrane 409
by using hydrophilic functionalised graphene grafted poly acrylonitrile-co-maleimide (G-PANCMI) 410
or ultra-wetting graphene for successful oil-water separation has been investigated. The prepared 411
membranes were characterized thoroughly and the experimental data indicates that the G-PANCMI 412
play an important role in enhancing the hydrophilicity/wettability, water permeability and 413
selectivity of the PES UF membrane. The water contact angle (CAw) of the PES membrane is 414
reduced from 63.7 ± 3.8o to 22.6 ± 2.5o which is 64.5% reduction while, the oil contact angle was 415
increased from 43.6 ± 3.5o to 112.5± 3.2o which is 158% higher compared to that of the PES 416
membrane. Similarly, the LEPoil increased 350% from 50±10 kPa of the control PES membrane to 417
175± 25 kPa of PES-G-PANCMI membrane. More importantly, the water permeability increased 418
by 43% with >99% selectivity. Based on our findings we conclude that the development of PES-G-419
PANCMI membrane will create a new avenue for successful oil-water separation 420
421 Acknowledgements 422
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423 This work was supported from funding provided by Ministry of Education, Singapore under the 424
Innovation Fund. The authors also gratefully acknowledge the support from Newcastle University, 425
United Kingdom and Environmental & Water Technology Centre of Innovation (EWTCOI), Ngee 426
Ann Polytechnic, Singapore for the NU-Poly-SIT scholarship to J.A.P. 427
428
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429
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Highlights
1. A new water insoluble highly hydrophilic copolymer PANCMACDAMN was developed.
2. Ultra-wetting graphene additive was used to fabricate PES hollow fiber membranes.
3. The new copolymer additive increases the hydrophilicity of the membrane by 64.5%
4. The addition of 5% ultra-wetting graphene increase the LEPoil of the PES membrane by 350% 5. The permeability of the membrane was increased by 43% with the new ultra-wetting graphene
