- Email: [email protected]
polyethylene glycol-400 composite membranes for reverse osmosis
Accepted Manuscript Title: Fabrication of tethered carbon nanotubes in cellulose acetate/polyethylene glycol-400 composite membranes for reverse osmosis Author: Aneela Sabir Muhammad Shafiq Atif Islam Afsheen Sarwar Muhammad Rizwan Dilshad Amir Shafeeq Muhammad Taqi Zahid Butt Tahir Jamil PII: DOI: Reference:
S0144-8617(15)00542-1 http://dx.doi.org/doi:10.1016/j.carbpol.2015.06.035 CARP 10024
To appear in: Received date: Revised date: Accepted date:
7-4-2015 10-6-2015 11-6-2015
Please cite this article as:http://dx.doi.org/10.1016/j.carbpol.2015.06.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Highlights Surface Engineered-Multiwall CarbonNanoTubes (SE-MWCNT) made by dissolution casting
4 5
SEM micrographs of PM/SE-MWCNTs showed uniform dispersed dense structured membranes
6
PM/SE-MWCNTs composite membranes improved salt rejection properties up to 99.8%
7 8
Thermal properties augmented PM/SE-MWCNTs composite membrane compared to PM membrane
us
cr
ip t
2 3
9
Ac ce p
te
d
M
an
10
1
Page 1 of 41
Fabrication of tethered carbon nanotubes in cellulose acetate/polyethylene
11
glycol-400 composite membranes for reverse osmosis
12
Aneela Sabir1, Muhammad Shafiq1, Atif Islam1, Afsheen Sarwar1, Muhammad Rizwan
13
Dilshad2, Amir Shafeeq2, Muhammad Taqi Zahid Butt3, Tahir Jamil 1
2
us
Pakistan.
15
Institute of Chemical Engineering and Technology (ICET), University of the Punjab, Lahore, 54590 Pakistan.
17 18
3
19
Abstract
an
16
Department of Polymer Engineering and Technology, University of the Punjab, Lahore, 54590
cr
1
Faculty of Engineering and Technology, University of the Punjab, Lahore, 54590 Pakistan.
M
14
ip t
10
In this study pristine multi-walled carbon nanotubes (MWCNTs) were surface engineered (SE)
21
in strong acidic medium by oxidation purification method to form SE-MWCNT. Five different
22
amount of SE-MWCNT ranging from 0.1- 0.5wt% were thoroughly and uniformly dispersed in
23
cellulose acetate/polyethylene glycol (CA/PEG400) polymer matrix during synthesis of
24
membrane by dissolution casting method. The structural analysis, surface morphology and
25
roughness was carried out by Fourier transform infrared spectroscopy (FTIR), scanning electron
26
microscopy (SEM), and atomic force microscopy (AFM) respectively which showed that the
27
dispersed SE-MWCNT was substantially tethered in CA/PEG400 polymer matrix membrane.
28
The thermogravimetric analysis (TGA) of membranes also suggested some improvement in
29
thermal properties with the addition of SE-MWCNT. Finally, the performance of these
30
membranes was assessed for suitability in drinking water treatment. The permeation flux and
Ac ce p
te
d
20
Corresponding author: Aneela Sabir; Email: [email protected], Phone: +92 322 4569 950 2
Page 2 of 41
salt rejection were determined by using indigenously fabricated reverse osmosis pilot plant with
32
1000ppm NaCl feed solution. The results showed that the tethered SE-MWCNT/CA/PEG400
33
polymer matrix membrane, with strong SE-MWCNTs /polymer matrix interaction, improved
34
the salt rejection performance of the membrane with the salt rejection of 99.8% for the highest
35
content of SE-MWCNT.
ip t
31
Keywords
37
Polymer matrix membrane, Surface Engineered Multiwalled carbon nanotubes, Tethering,
38
Water purification.
39
1. Introduction
an
us
cr
36
The availability of portable and hygienic drinking water is continuously depleting throughout the
41
world. Major initiatives are underway to find viable scientific and engineering solutions to
42
enhance supply of portable water via recycling, conservation and desalination processes (Glater
43
1998).These processes are currently in use to mitigate the risk of water shortage. In past,
44
desalination has been pursued by converting non-portable high saline sources (brackish and
45
seawater) to fresh drinkable water. Nowadays, state-of-the-art systems for desalination are in
46
practice. Most recently, reverse osmosis (RO) membranes are grabbing attention in scientific
47
community (Xu, Chang et al. 2010).
48
The renewable, biodegradable and eco-friendly natural polymers have gathered much attention in
49
recent years regarding membrane technology. Cellulose acetate (CA) is a kind of natural
50
thermoplastic polymer produced via esterification of wood, cotton, recycled paper and bagasse
51
(Sassi and Chanzy 1995; Chandure, Bhusari et al. 2014). CA has the potential tendency for
52
water permeation and salt rejection properties (He, Zhang et al. 2009). CA improved the
Ac ce p
te
d
M
40
3
Page 3 of 41
membrane performance, permeability and permselectivity when incorporated in other polymers
54
like poly (vinyl pyrolidone), poly (vinyl alcohol) poly (ethylene glycol), poly (amide) etc., as
55
compared to pristine polymer (Qin, Li et al. 2003; Saljoughi, Sadrzadeh et al. 2009).
56
Polyethylene glycol (PEG) can render the hydrophilicity and thermal stability in the membranes
57
(Ahumada, Delgado et al. 2012; Zhou, Fu et al. 2014). PEG 400 has low toxicity, which makes it
58
an ideal additive for use in water purification industry and is being used in a variety of
59
biotechnological and biochemical processes (Zavastin, Cretescu et al. 2010). Many studies have
60
shown that the incorporation of additives like silica (SiO2), titania (TiO2), alumina (Al2O3),
61
zirconia (ZrO2) and carbon nanotubes (CNTs) to polymer matrix can effectively improve the
62
thermal stability and mechanical strength of the membranes (Wara, Francis et al. 1995; Castro,
63
Cohen et al. 1996; Kalra, Garde et al. 2003; Yang, Zhang et al. 2007). The chemical modification
64
of polymers with various additives can improve the diffusive, hydrophilic and the salt water
65
purification capacity of the membranes (Haddada, Ferjani et al. 2004; Sivakumar, Mohan et al.
66
2006).
67
Among different additives, CNTs have gathered a considerable attention as nanofiller in
68
desalination. It has also an ideal reinforcement capabilities due to its exceptionally unique
69
thermal, mechanical and permeation properties. An exclusively dispersed CNTs provide
70
enhanced interfacial adhesion between polymer chains (Hummer, Rasaiah et al. 2001; Joseph
71
and Aluru 2008 ; El Badawi, Ramadan et al. 2014). Many studies have been reported where
72
CNTs were blended with other polymers like aromatic polyamides (PA), chitosan, polysulfone
73
(PS), and polyacrylonitrile (PAN) for water filtration applications (Choi, Jegal et al. 2006; Tang,
74
Zhang et al. 2009; Shawky, Chae et al. 2011).
Ac ce p
te
d
M
an
us
cr
ip t
53
4
Page 4 of 41
75
Shawky et al. (2011) synthesized aromatic polyamide/MWCNT nanocomposite membranes with
76
different concentration of MWCNTs by a polymer grafting process. The data showed a decrease
77
in the membrane permeability with the increase in CNTs concentration whereas the salt rejection
78
increased to 76 %.
79
MWCNTs and PA network structure, due to strong interaction between MWCNTs and polymer
80
matrix (Shawky, Chae et al. 2011). Tang et al. (2009) blended chitosan with different loadings
81
of CNT for membrane synthesis and measured pure water flux. The performance of membranes
82
showed that the water flux was 4.6 times more than the pristine chitosan membrane (Tang,
83
Zhang et al. 2009). Choi et al. (2006) prepared PS/CNT membranes by phase inversion method
84
and showed that the final water flux was directly proportional to the increase in CNTs weight
85
percentage. The reason behind this behavior was the increase in the hydrophilicity of the
86
membrane which increased up to the threshold level by loading1.5wt% of CNTs. However, with
87
more CNT concentration, the pore size decreased but the permeation flux increased. (Choi, Jegal
88
et al. 2006). The hydrophilic nature of the membrane due to the presence of CNTs, which
89
increases the water flux, has also been studied by Majeed et al. (2012). In the synthesized
90
PAN/CNTs ultrafiltration membrane (UF), pure water flux increased by the addition of 0.5wt%
91
carbon nanotubes (Majeed, Fierro et al. 2012).
92
In this work, we delineated the synthesis of polymeric matrix based on CA/PEG400 which has
93
been tethered with surface engineered carbon nanotubes. Indigenously lab scale fabricated RO
94
plant was intended to assess the permeation flux and salt rejection. The structural analysis,
95
thermal stability, morphology and surface roughness of the synthesized membranes were
96
measured by Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning
97
electron microscopy and atomic force microscopy, respectively.
Ac ce p
te
d
M
an
us
cr
ip t
These results were explained in terms of structural compactness, creating
5
Page 5 of 41
98
2. Experimental
99
2.1 Materials Analytical grade cellulose acetate (CA) with 39.7% acetyl content was obtained from Fluka
101
(USA). Pristine Multi-walled carbon nanotubes (MWCNTs) of around 20-40 nm diameter, inner
102
diameter of 15 nm and >90 % purity were manufactured by CNME International Co. Ltd.
103
Sulphuric acid (H2SO4) (> 95%) and nitric acid (HNO3) (65-70%) were purchased from BDH
104
Chemicals Ltd. Polyethylene glycol (PEG 400) and N, N-dimethyl formamide (DMF) (99%)
105
were obtained from Fluka and Riedel-de Haȅn, respectively. All the chemicals were used without
106
further purification.
107
2.2 Synthesis of polymer matrix membranes tethered with SE- MWCNTs
108
2.2.1 Preparation of SE- MWCNTs by oxidation purification in strong acid medium
109
Pristine MWCNTs were first surface engineered by oxidation purification in a concentrated
110
H2SO4/HNO3 (3:1 v/v) acidic medium as the oxidant to enhance their dispersion to prepare
111
MWNT-COOH within the polymer matrix solution. In a 1L flask equipped with a condenser,
112
pristine MWNTs (3.0 g) with H2SO4/HNO3 were added and stirred vigorously. The flask was
113
then subjected to ultrasonic bath under vibrations (40 kHz) for 12 h at 140 ̊C. After cooling to
114
room temperature, the reaction mixture was diluted with 500 mL of deionized water and then
115
filtered in vacuum through a filter paper (Fisher). The solid was dispersed in 500 mL of water
116
and filtered again, and then 300 mL of water was used to wash the filter cake several times.
117
The dispersion, filtering, and washing steps were repeated until the pH of the filtrate reached 7.0
118
(at least five cycles were required). The filtrate as then washed with 250 mL of acetone five
Ac ce p
te
d
M
an
us
cr
ip t
100
6
Page 6 of 41
times to remove most of the water from the sample and dried under vacuum for 24 h at 60 ̊C,
120
giving 2.1 g (~70% yield) of MWNT-COOH. The prepared SE-MWCNTs are shown in Scheme
121
I.
an
us
cr
ip t
119
M
122
2.2.2 Synthesis of CA/PEG400 polymer matrix membrane
124
Six different concentrations of polymer matrix solutions of CA/PEG400 were synthesized using
125
dissolution casting method. 10 g of CA was dissolved in 100 mL of DMF solvent with
126
continuous stirring at 70 ̊C for 6 h. The varying blend composition (w/w) of CA/PEG400 (50/50,
127
60/40, 70/30 and 80/20) were prepared and labeled as shown in Table 1. The solutions were
128
cooled at room temperature for 1 h in an air tight condition. Step 1 in Scheme II shows possible
129
interaction between CA and PEG400.
Ac ce p
te
d
123
7
Page 7 of 41
130
Table 1
131
Effect of concentration CA/PEG400 at 4 bar on membrane hydraulic resistance (Rm),
132
permeation flux and salt rejection.
Rm
Salt Rejection
(L/m2.h)
(%)
0.42
76
0.51
68
0.83
62
1.21
54
0.0128
80/20
PM 1
0.0115
70/30
PM 2
0.0102
60/40
PM 3
0.0100
us
PM
M
(bar/ m2.h)
50/50
te
d
133
Permeation Flux
ip t
CA/PEG400
cr
Type
Hydraulic resistance
an
Membrane
2.2.3 Tethering SE-MWCNTs with polymer matrix
135
CA/PEG400 composition of 80/20 was selected, for tethering with SE-MWCNTs, on the basis of
136
maximum salt rejection (76%) compared to other polymer matrix membranes as shown in Table
137
1. Step 2 in Scheme II depicts the interaction between PM chains with modified SE-MWCNTs
138
(0.1-0.5 wt%). The samples were subjected under ultrasonic vibration for 12 h at 65 ̊C. The PM
139
tethered with SE-MWCNTs was labeled as PM-CNTs.
140
2.2.4 Membrane Casting
141
Finally, Step 3 in Scheme II showed the interaction of PM-CNTs. To maintain uniform thickness
142
of the membrane on a clean dried glass plate, a doctor’s blade was used. The solvent was
Ac ce p
134
8
Page 8 of 41
removed by placing PM-CNTs membrane in an oven at 60 ̊C. PM-CNTs were dried for 12 h in
144
vacuum oven and removed from the glass plates with membrane thickness ranging from 25-35
145
µm, as measured by screw gauge.
ip t
143
Ac ce p
te
d
M
an
us
cr
146
147
9
Page 9 of 41
3. Characterization of Membranes
149
3.1 Fourier transform infrared spectroscopy
150
FTIR spectra of PM-CNTs were logged by IR Prestige-21 (Shimadzu) using attenuated total
151
reflectance (ATR) with ZnSe crystal. The air background of the instrument was run before each
152
test of the membrane. The wavenumber range was persistent from 4000-600 cm-1 at 120 scans
153
per spectrum.
154
3.2 Thermogravimetric analysis
155
The thermogravimetric analysis (TGA) measurements of the PM-CNTs were conducted using
156
Mettler Toledo, TGA/SDTA851e instrument. It was carried out from ambient temperature to
157
1200 ̊C at a ramp rate of 20 ̊C/ min under nitrogen flow (15 mL/min).
158
3.3 Scanning electron microscopy
159
Scanning electron micrographs of PM-CNTs were obtained using JEOL (JSM-6480LV)
160
microscope to analyze the morphology of the membranes at different magnifications. The PM-
161
CNTs were examined on a carbon conductive tape as its more significant compared to many of
162
the other common adhesives that have been used in SEM mounting. The low vacuum mode was
163
used to operate PM-CNTs at 20 kV.
164
3.4 Atomic force microscope
165
The topographical images were obtained using scanning probe microscope (SPM 9500J3,
166
Shimadzu) with tapping mode at room temperature. The scanning area was 5 μm2. The values of
167
root mean square (RMS) roughness were derived from AFM images, which were obtained from
168
the average values measured in random areas. The topography of membrane expressed in terms
Ac ce p
te
d
M
an
us
cr
ip t
148
10
Page 10 of 41
of various roughness parameters like mean roughness (Ra) which represents the mean value of
170
the surface relative to the center plane. The volumes enclosed by the image above and below this
171
plane are equal. It is calculated by Equation 1.
ip t
169
(1)
172
Where
shows surface relative to the center of plane while Lx and Ly denotes the
174
dimensions of surface in x and y directions, respectively.
175
However, the root mean square average (RMS) of the measured height deviations from the mean
176
surface taken within the area evaluation and calculated by Equation 2.
M
1/2
177
an
us
cr
173
(2)
3.6 Water content
179
The membranes were kept for 48 h in oven under vacuum at 70 ºC for drying (Ahumada,
180
Delgado et al. 2012). Dried PM-CNTs (1 g) were placed in a vial filled with distilled water (100
181
mL) and the vials were set at room temperature. The water content (%) of the membranes was
182
attained after 24 h by Equation 3.
Ac ce p
te
d
178
183
(3)
184
Whereas, WC stands for water content, WS for wet sample weight and DS for dried sample
185
weight.
186
3.7 Membrane hydraulic resistance (Rm)
11
Page 11 of 41
187
It defines the membrane tolerance towards hydraulic pressure. It can be calculated using
188
Equation 4 [15].
(4)
ip t
189
3.8 Reverse osmosis performance test
191
The RO experiments were employed on the plate and frame membrane module as revealed in
192
Fig.1 (process flow diagram) [2]. NaCl (1000 ppm) solution in water was used as a feed of
193
known conductance. The effective area of membrane in contact with the continuous feed flow
194
was 154 cm2. Permeate was collected for 12 h in a continuous RO membrane unit operation. The
195
provided feed tank capacity was 25 L and feed inlet along with circulation pump of 1 KW
196
followed by rotameter.RO performance tests of the membranes were evaluated on the basis of
197
salt rejection SR (%) and permeation flux (L/m2.h) using Equations 5 and 6. Permeate and
198
retentate were collected during operation from their respective sampling points. The pressure
199
range during the process was varied from 0.5- 4 bar. The conductance of the feed and permeate
200
acquired by using Cyber Scan Waterproof PC 300 Series (EUTECH).
202
203
us
an
M
d
te
Ac ce p
201
cr
190
(5)
(6)
12
Page 12 of 41
ip t cr us an M d te Ac ce p
204 205
4. Results and Discussion
206
4.1 Fourier transform infrared spectroscopy (FTIR)
207
The FTIR spectrum of PM compared with that of PM-CNTs and emergence of functional groups
208
on acid treated MWCNTs which illustrated the functional group analysis of the membranes is
209
shown in Fig. 2. In case of PM membranes, the band at 3425-3464 cm-1 (–OH stretching
210
vibration), weak band at 2924 cm-1 (-C-H stretching), 1737 cm-1 (-C=O stretching), (Costa-
211
Júnior, Barbosa-Stancioli et al. 2009; Salihu, Goswami et al. 2012),1431 cm-1 (-C-H bending)
212
followed by bands at 1371 and 1224 cm-1 were described by the rocking and wagging mode of -
13
Page 13 of 41
C-H bond. The existence of adsorption band at 1036 cm-1 was attributed to -C-O (acyclic)
214
stretching vibrations (Xing and Ho 2009; Zavastin, Cretescu et al. 2010; Atif Islam 2012; Feng,
215
Ren et al. 2013; Worthley, Constantopoulos et al. 2013) while weak band at 906 and 1155cm-1
216
confirmed the presence of pyranose ring and saccharine structure (Atif Islam 2012, Atieh,
217
Bakather et al. 2011, Costa-Júnior, Barbosa-Stancioli et al. 2009).
218
Furthermore, FTIR spectra of PM-CNTs membranes with different concentrations of SE-
219
MWCNTs (0.1-0.5 wt %) are shown in Fig. 2. The specific bands owing to created functional
220
groups were detected on MWCNTs. The acid treated MWCNTs exhibits bands having hydroxyl
221
(-OH) and carbonyl (-C=O) groups. This confirmed that carbonyl and hydroxyl functional
222
groups were introduced on the CNTs. The observed feature at 3425-3464 cm-1 was associated
223
with the -OH stretching vibrations from the inter-molecular hydrogen bonds (Scheme II) (Costa-
224
Júnior, Barbosa-Stancioli et al. 2009) that tend to weaken the force constant and moving the
225
absorbance to lower energy (lower wave number). The carbonyl group was also shifted slightly
226
towards lower wavenumber (1738 to 1734 cm-1) due hydrogen bonding (Scheme II) which
227
proposes interactions between –COOH functional groups of MCWT-COOH and -C=O groups of
228
CA (Zeeshan and Gopiraman 2013).
cr
us
an
M
d
te
Ac ce p
229
ip t
213
14
Page 14 of 41
ip t cr us an M d te
230
4.2. Thermogravimetric analysis (TGA)
232
The thermal stability of PM and PM-CNTs (0.1-0.5 wt%) were accomplished in three steps
233
shown in Fig. 3. In the first step, for PM, the removal of moisture and also dehydration was
234
observed at about 30-280 ̊C up to weight loss of 4.63%. The second step ranges from 280-450 ̊C
235
was the major degradation of the polymer backbone with weight loss of about 90.78% which was
236
probably due to main thermal decomposition of CA chains. In the final stage, almost constant
237
thermal profile was observed from 450-1200 ̊C due to carbonization of the degraded product to
238
ash. This TGA profile indicated that PM membrane could highly be unstable at high temperature.
Ac ce p
231
15
Page 15 of 41
Thermograms of PM-CNTs (0.1-0.5wt %) showed similar steps which involves the
240
decomposition from 30-280 ̊C with weight loss of 3.3-4.4% which was attributed to moisture
241
removal, dehydration and loss of bound water. The second step showed onset of polymer
242
degradation from 250 ̊C (Shieh and Chung 1998; Lucena, V. de Alencar et al. 2003) with weight
243
loss of about 92.5-92.62% up to offset temperature of 450 ̊C. The final step indicated the
244
temperature range from 450-1200 ̊C showing carbonization of the thermally decomposed product
245
to ash (Chatterjee and Conrad 1968). The experimental data showed that 30 percent weight
246
losses for PM, PM-CNT1 and PM-CNT5 were occurred at 338, 353 and 372 ̊C, respectively.
247
Similarly, 80 percent weight losses for PM, PM-CNT1 and PM-CNT5 were observed at 383, 385
248
and 407 ̊C, respectively. This confirmed that the experiential TGA curve of PM-CNT5 was
249
thermally stable as compared to the other PM-CNTs exhibiting the improvement in the thermal
250
properties with the addition of CNTs. The percentage residue left for the PM and PM-CNTs are
251
also shown in Fig.3.
253 254 255 256 257
cr
us
an
M
d te
Ac ce p
252
ip t
239
16
Page 16 of 41
ip t cr us an 261 262 263 264
d
M 260
te
259
Ac ce p
258
265 266 267 268
17
Page 17 of 41
269
Table 2
270
Sample name hydraulic resistance (Rm) and roughness values of with and without SE-MWCNTs (0.1-0.5
271
wt%) tethered polymer matrix
272
Average
Square average roughness roughness
Root mean square roughness
Weight Fraction of SEMWCNTs loading (wt%)
(bar/m2.h)
(%)
Ra (nm)
Ry (nm)
PM
0
0.0128
78.1
18.17
115.60
76.64
22.27
PM-CNT 1
0.1
0.0123
80.8
19.21
240.47
115.90
24.913
PM-CNT 2
0.2
0.0127
83.2
30.83
310.21
146.75
38.801
PM-CNT 3
0.3
0.0134
88.4
34.16
332.06
161.88
43.673
PM-CNT 4
0.4
0.0145
90.1
17.61
192.80
93.98
22.045
PM-CNT 5
0.5
0.0156
92.2
21.80
210.07
102.08
27.071
te
d
M
an
us
Rm
Mean depth
ip t
Water Content
Rz (nm)
cr
Hydraulic resistance
Rms (nm)
Ac ce p
Membrane Sample
273
4.3 Scanning electron microscopy (SEM)
274
Scanning electron microscopy technique was used for the characterization of PM and PM-CNTs
275
as shown in Fig. 4. Membranes employed for examination provide the information of surface
276
morphology (Zavastin, Cretescu et al. 2010). The membrane is dense and it has even and smooth
277
surface. This indicated that PEG400 was homogeneously mixed with CA due to capability of
278
two polymers interacting mutually through hydrogen bonding between –OH and -C=O groups
279
(Yang, Zhang et al. 1999). By tethering different amount of SE-MWCNTs (0.1–0.5 wt%),
280
surface morphology were changed significantly and SE-MWCNTs were well and uniformly 18
Page 18 of 41
distributed in the PM. During high SE-MWCNTs tethering i.e. PM-CNT5 (Fig. 4 a,b) as shown
282
in Fig 4, were found to form their aggregates and agglomeration to some extent in the composite
283
membranes and were not dispersed well in the membrane because of the van der Waals
284
interaction between the neighboring SE-MWCNTs (Li, Kim et al. 2010).
Ac ce p
285
te
d
M
an
us
cr
ip t
281
286
4.4 Atomic force microscope (AFM)
287
The tethering of the SE-MWCNTs on the polymer matrix was confirmed from AFM
288
complementing SEM. Samples of membrane were employed for examination to provide the
289
information of surface topography and roughness. The bright regions signify the highest points
290
or nodules of membrane surface whereas the darker regions signify the depressions as shown in
291
Fig. 5. The descriptions showed that surface roughness was increased with an increase in the
292
concentrations of SE-MWCNTs upto 0.3 wt% in the membranes. Whereas, the decline in
293
roughness parameters were observed when the concentration of SE-MWCNTs was increased 19
Page 19 of 41
upto 0.4 wt% i.e. PM-CNT4 as shown in Table 2. This decrease owed to the low electrostatic
295
interactions between SE-MWCNTs and the polymer matrix (Vatanpour, Madaeni et al.
296
2011).Thus, the MWCNTs were frequently positioned, giving rise to smooth surface (Qiu, Wu et
297
al. 2009). However, with high SE-MWCNT concentrations PM-CNT5 (0.5 wt%), again an
298
increase in roughness parameters and non-uniform surface was observed due to agglomeration of
299
SE-MWCNT (Phao, Nxumalo et al. 2013).
us
cr
ip t
294
Ac ce p
te
d
M
an
300
301
4.5 Water content
302
The water content % of the membrane exhibited the hydrophilic nature of the membrane
303
(Sivakumar, Mohan et al. 2006). Equation 3 was used to calculate the WC for PM i.e. 78.1 % as
304
shown in Table 2 above. In case of PM-CNT1, the water content % showed the value of 80.8 %.
20
Page 20 of 41
When SE-MWCNTs contents reached to 0.5 wt% i.e. PM-CNT5, the water content % was
306
increased up to 92.2 %. Similar increase in water content % was shown in which Pluronic (F127)
307
was added in the CA membranes (Lv, Su et al. 2007). The reason acclaimed to the absorbance of
308
water in the membranes was the hydrophilic nature of SE-MWCNTs
309
4.6 Hydraulic resistance
310
Membrane hydraulic resistance (Rm) controls the resistance when pressure is applied to the
311
membrane. The linear proportionality of permeation flux to the applied pressure is directly
312
proportional to the transport resistance. Equation 4 was used for evaluating the hydraulic
313
membrane resistance. The permeation flux was calculated by varying the transmembrane
314
pressure from 0.5 to 4.0 bar. It is evident from Table 1 that Rm is directly proportional to the
315
extent of CA present in the membrane.
316
The polymer matrix membrane with highest content of cellulose acetate i.e. PM offers maximum
317
hydraulic resistance compared to those which have greater PEG. PM offers resistance of 0.0128
318
bar/m2.h while PM3 shows minimum resistance of 0.01 bar/m2.h. The PM with greater Rm
319
entails more amount of cellulose acetate and it comes up with stable dense membrane
320
(Arthanareeswaran, Sriyamuna Devi et al. 2008; Arthanareeswaran, Sriyamuna Devi et al. 2009).
321
The formation of dense membrane formation is also evident from the SEM images shown in PM
322
of Fig. 4. When the quantity of PEG400 was increased, the formation of porogen in membranes
323
was increased as depicted in cross sectional view of membranes. These pores gave extended free
324
volume and as a result Rm was reduced (Sivakumar, Mohan et al. 2006).
325
The Rm values for tethered PM-CNTs are given in Table 2 which shows a steady increase in
326
SE-MWCNTs. The improved interaction in the PM-CNTs was due to the segmental gaps in the
Ac ce p
te
d
M
an
us
cr
ip t
305
21
Page 21 of 41
membranes which exhibited a decline with increase in Rm. Values in Table 2 showed the
328
comparison in Rm values of PM with PM-CNTs. It was observed that PM-CNTs have enhanced
329
Rm. The reason attributed to this enhanced resistance was due to the development of electrostatic
330
interaction between the polymer chains.
331
4.7 Reverse osmosis performance test
332
4.7.1Transport properties of membranes
333
The salt water feed 1000 ppm NaCl in water with pH=7.4 i.e. slightly alkaline was pumped into a
334
vessel in indigenously fabricated RO pilot plant. The pressure was varied from 0.5- 4.0 bar when
335
25 L/h (feed flow rate) was applied.
336
Table 1 shows the permeation flux and salt rejection of polymer matrix membranes which
337
exhibited that PM3 membrane has maximum permeation flux of 1.21 (L/m2.h) while it has
338
minimum salt rejection capacity of 54%. As the CA content was increased, the flux slightly
339
decreased while capacity of salt rejection increased remarkably. PM membrane showed 76 % salt
340
rejection and its flux was 0.42 (L/m2.h) with maximum CA and minimum PEG400 content.
341
The transport through reverse osmosis membrane was described in terms of diffusive flow in
342
which separation occurs in membrane when both solute and solvent permeate by solution and
343
diffusion process. The transport mechanism was best elucidated on the basis of solution-
344
diffusion model. According to this model, three steps occur in transport process within
345
membrane i.e. sorption at the surface of membrane, diffusion into dense membrane under
346
pressure and then desorption. The hydrophilic nature of PEG400 acts as driving force for
347
sorption of water on membrane (Lonsdale, Merten et al. 1965; Burghoff, Lee et al. 1980; Mazid
Ac ce p
te
d
M
an
us
cr
ip t
327
22
Page 22 of 41
1984).Salt removal was increased due to the desalting nature of CA (Malaisamy, Mahendran et
349
al. 2002).
350
Furthermore, salt rejection occurred on the basis of electrostatic repulsion known as Donnon
351
Effect (Lonsdale, Merten et al. 1965).The ions excluded by the Columbic Forces which arise
352
from the charges residing on the surface of membrane. CA showed desalting property and
353
improves the salt rejection efficiency which was the reason behind the maximum salt rejection
354
ability of PM.
355
PEG400 depicted the hydrophilic nature and act as porogen (Burghoff, Lee et al. 1980). PM 3
356
(polymer matrix membrane) had maximum quantity of PEG400 giving more water flux but at the
357
same time showed salt rejection compromised. This may be attributed to the formation of
358
porogen (Malaisamy, Mahendran et al. 2002) on membrane which allows the passage of salt
359
along with water, resulting in the maximum flux and minimum salt rejection. Greater the
360
PEG400 content, higher will be the flux. The diffusion rate of water was accelerated by the
361
presence of PEG400 due to its hydrophilic nature. PEG400 proliferate the tendency of porogen
362
and as a consequence permeation flux was increased (Kurokawa and Ueno 1982). Whereas,
363
when salt rejection efficiency of membrane was optimum, the salt passage % reduced depending
364
upon the concentration of CA/PEG400.
365
On the basis of salt rejection, PM polymer matrix membrane was selected for tethering of SE-
366
MWCNTs. After tethering, these PM-CNTs were characterized and compared with PM polymer
367
matrix membrane. The permeation flux and salt rejection of PM-CNTs using various
368
concentrations of SE-MWCNTs are given in Table 2. The water salinity was decreased with the
369
increment of SE-MWCNT in the PM. The permeation flux, and salt rejection % were
Ac ce p
te
d
M
an
us
cr
ip t
348
23
Page 23 of 41
significantly affected by the integration of SE-MWCNTs. PM-CNT1 showed maximum
371
permeation flux of 0.84 L/m2.h but as tethering of SE-MWCNTs increased permeation flux
372
declined gradually. Fig. 6 shows the comparison between permeation flux of PM and PM-CNTs.
373
As evident from Fig. 6 that PM-CNT 5 showed reduced permeation flux and gave lowest flux of
374
0.61 L/m2.h.
375
The salt rejection capacity of PM-CNTs was enhanced when compared with PM membrane as
376
indicated in Fig. 6. PM-CNT 5 showed highest capacity of salt rejection of 99.8% attributed to
377
0.5 wt% SE-MWCNTs tethered on PM. The permeation flux effect was owed to tethering of SE-
378
MWCNTs. These PM-CNTs infatuated a hydrophilic nature as its driving force. Resultantly, the
379
salt rejected through the hydrophilic membranes because of its ability for hydrogen bonding. The
380
tethering of SE-MWCNTs changed the polymer chains segmental motion and also the mobility
381
of permeate which increased the free volume of PM-CNTs.
384 385
cr
us
an
M
d te
383
Ac ce p
382
ip t
370
24
Page 24 of 41
ip t cr us an
386
5. Conclusion
388
This study has examined the relation of permeability flux and salt rejection using RO membranes
389
in indigenously fabricated pilot plant. It showed that 0.1-0.5 wt% tethering of SE-MWCNTs into
390
PM membrane manifestly increased the selective salt rejection % and permeation flux at 4.0 bar.
391
Higher content of SE-MWCNTs (0.5 wt%) led to the noteworthy increase in salt rejection of
392
99.8%, whereas, permeation flux was decreased upto 0.61 L/m2.h as compared to the PM
393
membrane. Nevertheless, data presented in this study encompassing the most common
394
membrane materials (cellulose acetate, polyethylene glycol and multi-walled carbon nanotubes)
395
used in RO reconfirm a single general trend connecting intrinsic water permeability flux and
396
selectivity to salt rejection of these materials. The reported results can improve the understanding
397
of the state of the art function of RO membranes and can be valuable in design of new polymeric
398
materials with high salt rejection and good permeability. Moreover, the structural
399
characterization of PM-CNTs membrane is confirmed by ATR-FTIR. The SEM and AFM
Ac ce p
te
d
M
387
25
Page 25 of 41
micrographs showed uniform dispersion of SE-MWCNTs on PM-CNTs having influence on
401
morphology and topography of membrane structure. Generally, tethering of SE-MWCNTs led to
402
increase in the thermal stability of membranes analyzed by TGA thermograms. Also, the WC %
403
exhibited that PM-CNT 5 tethered membrane has the highest water content as compared to other
404
membranes.
405
Acknowledgements
406
The authors gratefully acknowledge University of the Punjab, Lahore Pakistan for providing
407
financial support of this work.
an
us
cr
ip t
400
M
408
410
Ac ce p
te
d
409
26
Page 26 of 41
ip t cr us an M d te Ac ce p
411 412 413 414
27
Page 27 of 41
416
Ac ce p
te
d
M
an
us
cr
ip t
415
28
Page 28 of 41
ip t cr us an M d te
418
Ac ce p
417
29
Page 29 of 41
ip t cr us an M d Ac ce p
te
419
420 30
Page 30 of 41
an
us
cr
ip t
421
422
Ac ce p
te
d
M
423
424 425
31
Page 31 of 41
426
Table 1
428
Effect of concentration CA/PEG400 at 4 bar on membrane hydraulic resistance (Rm),
429
permeation flux and salt rejection Hydraulic
CA/PEG400
Type
Resistance
Permeation
Salt
Flux
Rejection
us
(Rm)
432 433 434 435 436 437
70/30
PM 2
0.0102
60/40
PM 3
0.0100
50/50
0.51
68
0.83
62
1.21
54
M
0.0115
76
d
PM 1
0.42
te
431
80/20
(%)
Ac ce p
430
0.0128
(L/m2.h)
an
(bar/ m2.h) PM
cr
Membrane
ip t
427
438 439
32
Page 32 of 41
440
Table 2
441
Sample name hydraulic resistance (Rm) and roughness values of with and without SE-MWCNTs
442
(0.1-0.5 wt%) tethered polymer matrix
443 444 445 446 447 448
Root mean square roughness
0
0.0128
78.1
18.17
115.60
76.64
22.27
PM-CNT 1
0.1
0.0123
80.8
19.21
240.47
115.90
24.913
PM-CNT 2
0.2
0.0127
83.2
30.83
310.21
146.75
38.801
PM-CNT 3
0.3
0.0134
88.4
34.16
332.06
161.88
43.673
PM-CNT 4
0.4
0.0145
90.1
17.61
192.80
93.98
22.045
PM-CNT 5
0.5
0.0156
92.2
21.80
210.07
102.08
27.071
te
d
an
M
(%)
ip t
PM
cr
Mean depth Rz (nm)
Hydraulic Resistance (Rm) (bar/m2.h)
us
Water Content
Average Square roughness average roughness Ra (nm) Ry (nm)
Weight Fraction of SEMWCNTs loading (wt%)
Rms (nm)
Ac ce p
Membrane Sample
449 450 451 452 33
Page 33 of 41
References
454 455
Ahumada, E. A., D. R. Delgado, et al. (2012). "Solution thermodynamics of acetaminophen in some PEG 400 + water mixtures." Fluid Phase Equilibria 332(0): 120-127.
456 457 458 459
Arthanareeswaran, G., T. K. Sriyamuna Devi, et al. (2009). "Development, characterization and separation performance of organic–inorganic membranes: Part II. Effect of additives." Separation and Purification Technology 67(3): 271-281.
460 461 462 463
Arthanareeswaran, G., T. K. Sriyamuna Devi, et al. (2008). "Effect of silica particles on cellulose acetate blend ultrafiltration membranes: Part I." Separation and Purification Technology 64(1): 38-47.
464 465 466 467
Atieh, M. A., O. Y. Bakather, et al. (2011). "Effect of carboxylic functional group functionalized on carbon nanotubes surface on the removal of lead from water." Bioinorganic chemistry and applications 2010.
468 469 470
Atif Islam, T. Y. (2012). "Controlled delivery of drug from pH sensitive chitosan/poly (vinyl alcohol) blend." Carbohydrate Polymers 88(3): 1055-1060.
471 472 473 474
Burghoff, H. G., K. Lee, et al. (1980). "Characterization of transport across cellulose acetate membranes in the presence of strong solute–membrane interactions." Journal of applied polymer science 25(3): 323-347.
475 476 477
Castro, R. P., Y. Cohen, et al. (1996). "Silica-suported polyvinylpyrrolidone filtration membranes." Journal of Membrane Science 115(2): 179-190.
478 479 480 481
Chandure, A. S., G. S. Bhusari, et al. (2014). "Synthesis, Characterization, and Biodegradation Studies of Poly(1,4-cyclohexanedimethylene-adipate-carbonate)s." Journal of Polymers 2014: 11.
482 483
Chatterjee, P. K. and C. M. Conrad (1968). "Thermogravimetric analysis of cellulose." Journal of
484
Polymer Science Part A‐1: Polymer Chemistry 6(12): 3217-3233.
Ac ce p
te
d
M
an
us
cr
ip t
453
485
34
Page 34 of 41
Choi, J.-H., J. Jegal, et al. (2006). "Fabrication and characterization of multi-walled carbon nanotubes/polymer blend membranes." Journal of Membrane Science 284(1–2): 406-415.
488 489 490 491
Costa-Júnior, E. S., E. F. Barbosa-Stancioli, et al. (2009). "Preparation and characterization of chitosan/poly(vinyl alcohol) chemically crosslinked blends for biomedical applications." Carbohydrate Polymers 76(3): 472-481.
ip t
486 487
492
El Badawi, N., A. R. Ramadan, et al. (2014). "Novel carbon nanotube–cellulose acetate nanocomposite membranes for water filtration applications." Desalination 344(0): 79-85.
495 496 497
Feng, S., J. Ren, et al. (2013). "Poly (amide-12-b-ethylene oxide)/polyethylene glycol blend membranes for carbon dioxide separation." Separation and Purification Technology 116: 25-34.
498 499 500
Glater, J. (1998). "The early history of reverse osmosis membrane development." Desalination 117(1–3): 297-309.
501 502 503
Haddada, R., E. Ferjani, et al. (2004). "Properties of cellulose acetate nanofiltration membranes. Application to brackish water desalination." Desalination 167(0): 403-409.
us
an
M
d te
504
cr
493 494
He, J., M. Zhang, et al. (2009). "High‐quality cellulose triacetate prepared from bamboo
506
dissolving pulp." Journal of applied polymer science 113(1): 456-465.
507 508 509 510
Hilal, N., A. W. Mohammad, et al. (2003). "Using atomic force microscopy towards improvement in nanofiltration membranes properties for desalination pre-treatment: a review." Desalination 157(1–3): 137-144.
511 512 513
Hummer, G., J. C. Rasaiah, et al. (2001). "Water conduction through the hydrophobic channel of a carbon nanotube." Nature 414(6860): 188-190.
514 515 516
Joseph, S. and N. R. Aluru (2008). "Why Are Carbon Nanotubes Fast Transporters of Water?" Nano Letters 8(2): 452-458.
517 518 519
Kalra, A., S. Garde, et al. (2003). "Osmotic water transport through carbon nanotube membranes." Proceedings of the National Academy of Sciences 100(18): 10175-10180.
Ac ce p
505
35
Page 35 of 41
Kurokawa, Y. and K. Ueno (1982). "Reverse osmosis rejection by hydrous inorganic precipitate– cellulose composite membrane." Journal of applied polymer science 27(2): 621-630.
523 524 525 526
Li, M., I. H. Kim, et al. (2010). "Cellulose acetate/multiwalled carbon nanotube nanocomposites with improved mechanical, thermal, and electrical properties." Journal of applied polymer science 118(4): 2475-2481.
527 528 529
Lonsdale, H., U. Merten, et al. (1965). "Transport properties of cellulose acetate osmotic membranes." Journal of applied polymer science 9(4): 1341-1362.
530 531 532
Lucena, M. d. C. C., A. E. V. de Alencar, et al. (2003). "The effect of additives on the thermal degradation of cellulose acetate." Polymer Degradation and Stability 80(1): 149-155.
533 534 535 536
Lv, C., Y. Su, et al. (2007). "Enhanced permeation performance of cellulose acetate ultrafiltration membrane by incorporation of Pluronic F127." Journal of Membrane Science 294(1–2): 68-74.
537 538 539 540
Majeed, S., D. Fierro, et al. (2012). "Multi-walled carbon nanotubes (MWCNTs) mixed polyacrylonitrile (PAN) ultrafiltration membranes." Journal of Membrane Science 403–404(0): 101-109.
541 542 543 544
Malaisamy, R., R. Mahendran, et al. (2002). "Cellulose acetate and sulfonated polysulfone blend ultrafiltration membranes. I. Preparation and characterization." Journal of applied polymer science 86(7): 1749-1761.
545 546 547
Mazid, M. (1984). "Mechanisms of transport through reverse osmosis membranes." Separation Science and Technology 19(6-7): 357-373.
548 549 550
Meindersma, G. W., C. M. Guijt, et al. (2006). "Desalination and water recycling by air gap membrane distillation." Desalination 187(1–3): 291-301.
551 552 553 554
Phao, N., E. N. Nxumalo, et al. (2013). "A nitrogen-doped carbon nanotube enhanced polyethersulfone membrane system for water treatment." Physics and Chemistry of the Earth, Parts A/B/C 66(0): 148-156.
555 556 557
Qin, J.-J., Y. Li, et al. (2003). "Cellulose acetate hollow fiber ultrafiltration membranes made from CA/PVP 360 K/NMP/water." Journal of Membrane Science 218(1): 173-183.
Ac ce p
te
d
M
an
us
cr
ip t
520 521 522
36
Page 36 of 41
Qiu, S., L. Wu, et al. (2009). "Preparation and properties of functionalized carbon nanotube/PSF blend ultrafiltration membranes." Journal of Membrane Science 342(1–2): 165-172.
561 562 563
Salihu, G., P. Goswami, et al. (2012). "Hybrid electrospun nonwovens from chitosan/cellulose acetate." Cellulose 19(3): 739-749.
564 565 566 567
Saljoughi, E., M. Sadrzadeh, et al. (2009). "Effect of preparation variables on morphology and pure water permeation flux through asymmetric cellulose acetate membranes." Journal of Membrane Science 326(2): 627-634.
568 569 570
Sassi, J.-F. and H. Chanzy (1995). "Ultrastructural aspects of the acetylation of cellulose." Cellulose 2(2): 111-127.
571 572 573
Shawky, H. A., S.-R. Chae, et al. (2011). "Synthesis and characterization of a carbon nanotube/polymer nanocomposite membrane for water treatment." Desalination 272(1–3): 46-50.
574 575 576 577
Sheikholeslami, R. (1999). "Fouling mitigation in membrane processes: Report on a Workshop held January 26–29, 1999, Technion — Israel Institute of Technology, Haifa, Israel." Desalination 123(1): 45-53.
578 579 580 581
Shieh, J.-J. and T. S. Chung (1998). "Effect of liquid-liquid demixing on the membrane morphology, gas permeation, thermal and mechanical properties of cellulose acetate hollow fibers." Journal of Membrane Science 140(1): 67-79.
582 583 584 585
Sivakumar, M., D. R. Mohan, et al. (2006). "Studies on cellulose acetate-polysulfone ultrafiltration membranes: II. Effect of additive concentration." Journal of Membrane Science 268(2): 208-219.
586 587 588 589
Tang, C., Q. Zhang, et al. (2009). "Water transport behavior of chitosan porous membranes containing multi-walled carbon nanotubes (MWNTs)." Journal of Membrane Science 337(1–2): 240-247.
590 591 592 593
Vatanpour, V., S. S. Madaeni, et al. (2011). "Fabrication and characterization of novel antifouling nanofiltration membrane prepared from oxidized multiwalled carbon nanotube/polyethersulfone nanocomposite." Journal of Membrane Science 375(1–2): 284-294.
594 595 596
Wara, N. M., L. F. Francis, et al. (1995). "Addition of alumina to cellulose acetate membranes." Journal of Membrane Science 104(1): 43-49.
Ac ce p
te
d
M
an
us
cr
ip t
558 559 560
37
Page 37 of 41
Worthley, C. H., K. T. Constantopoulos, et al. (2013). "A study into the effect of POSS nanoparticles on cellulose acetate membranes." Journal of Membrane Science 431(0): 62-71.
600 601 602 603
Xing, R. and W. S. W. Ho (2009). "Synthesis and characterization of crosslinked polyvinylalcohol/polyethyleneglycol blend membranes for CO2/CH4 separation." Journal of the Taiwan Institute of Chemical Engineers 40(6): 654-662.
604 605 606
Xu, J., C.-Y. Chang, et al. (2010). "Performance of a ceramic ultrafiltration membrane system in pretreatment to seawater desalination." Separation and Purification Technology 75(2): 165-173.
607 608 609
Yang, G., L. Zhang, et al. (1999). "Role of polyethylene glycol in formation and structure of regenerated cellulose microporous membrane." Journal of Membrane Science 161(1–2): 31-40.
610 611 612 613
Yang, Y., H. Zhang, et al. (2007). "The influence of nano-sized TiO< sub> 2 fillers on the morphologies and properties of PSF UF membrane." Journal of Membrane Science 288(1): 231238.
614 615 616 617
Zavastin, D., I. Cretescu, et al. (2010). "Preparation, characterization and applicability of cellulose acetate–polyurethane blend membrane in separation techniques." Colloids and Surfaces A: Physicochemical and Engineering Aspects 370(1–3): 120-128.
618 619 620
Zeeshan, B. K. I. K. K. and K. F. M. Gopiraman (2013). "Structural and mechanical properties of cellulose acetate/graphene hybrid nanofibers: spectroscopic investigations." eXPRESS Polymer Letters 7(6).
621 622
Zhou, J., H. Fu, et al. (2014). "Solubility and solution thermodynamics of flofenicol in binary PEG 400 + water systems." Fluid Phase Equilibria 376(0): 159-164.
624 625 626
cr
us
an
M
d
te
Ac ce p
623
ip t
597 598 599
627 628 629 38
Page 38 of 41
630
Table 1
631
Effect of concentration CA/PEG400 at 4 bar on membrane hydraulic resistance (Rm),
632
permeation flux and salt rejection.
Rm
(%)
0.42
76
0.51
68
0.83
62
1.21
54
80/20
PM 1
0.0115
70/30
PM 2
0.0102
60/40
PM 3
0.0100
M
te
d
us
0.0128
Ac ce p
634
PM
50/50
Salt Rejection
(L/m2.h)
(bar/ m2.h)
633
Permeation Flux
ip t
CA/PEG400
cr
Type
Hydraulic resistance
an
Membrane
39
Page 39 of 41
634
Table 2
635
Sample name hydraulic resistance (Rm) and roughness values of with and without SE-MWCNTs (0.1-0.5
636
wt%) tethered polymer matrix
637 638 639
Average
Square average roughness roughness
Root mean square roughness
Weight Fraction of SEMWCNTs loading (wt%)
(bar/m2.h)
(%)
Ra (nm)
Ry (nm)
PM
0
0.0128
78.1
18.17
115.60
76.64
22.27
PM-CNT 1
0.1
0.0123
80.8
19.21
240.47
115.90
24.913
PM-CNT 2
0.2
0.0127
83.2
30.83
310.21
146.75
38.801
PM-CNT 3
0.3
0.0134
88.4
34.16
332.06
161.88
43.673
PM-CNT 4
0.4
0.0145
90.1
17.61
192.80
93.98
22.045
PM-CNT 5
0.5
0.0156
92.2
21.80
210.07
102.08
27.071
te
d
M
an
us
Rm
Mean depth
ip t
Water Content
Rz (nm)
cr
Hydraulic resistance
Rms (nm)
Ac ce p
Membrane Sample
40
Page 40 of 41
Figure, Scheme and Table Captions Scheme I: preparation of Surface Engineered-Multiwalled carbon nanotubes by oxidation purification in strong acid medium.
ip t
Scheme II: Tethering of Surface Engineered Multiwalled carbon nanotubes with cellulose acetate/polyethylene glycol polymer matrix
cr
Fig. 1. Process flow diagram of indigenously fabricated reverse osmosis pilot plant. Fig.2. FTIR spectra of PM and PM-CNTs membranes.
us
Fig.3.TGA of PM and PM-CNTs membranes at various percentage weight loss
an
Fig.4. Surface image of PM and PM-CNTs membranes, (a) top surface and (b) crosssection at different magnifications. Fig.5. AFM 3-dimensional image of PM and PM-CNTs membranes.
M
Fig.6 Comparison of permeation flux and salt rejection between PM and PM-CNTs membranes.
d
Table 1. Effect of concentration CA/PEG400 at 4 bar on membrane hydraulic resistance (Rm),permeation flux and salt rejection.
Ac ce p
te
Table 2: Sample name hydraulic resistance (Rm) and roughness values of with and without SE-MWCNTs (0.1-0.5 wt%) tethered polymer matrix.
Page 41 of 41
S0144-8617(15)00542-1 http://dx.doi.org/doi:10.1016/j.carbpol.2015.06.035 CARP 10024
To appear in: Received date: Revised date: Accepted date:
7-4-2015 10-6-2015 11-6-2015
Please cite this article as:
1
Highlights Surface Engineered-Multiwall CarbonNanoTubes (SE-MWCNT) made by dissolution casting
4 5
SEM micrographs of PM/SE-MWCNTs showed uniform dispersed dense structured membranes
6
PM/SE-MWCNTs composite membranes improved salt rejection properties up to 99.8%
7 8
Thermal properties augmented PM/SE-MWCNTs composite membrane compared to PM membrane
us
cr
ip t
2 3
9
Ac ce p
te
d
M
an
10
1
Page 1 of 41
Fabrication of tethered carbon nanotubes in cellulose acetate/polyethylene
11
glycol-400 composite membranes for reverse osmosis
12
Aneela Sabir1, Muhammad Shafiq1, Atif Islam1, Afsheen Sarwar1, Muhammad Rizwan
13
Dilshad2, Amir Shafeeq2, Muhammad Taqi Zahid Butt3, Tahir Jamil 1
2
us
Pakistan.
15
Institute of Chemical Engineering and Technology (ICET), University of the Punjab, Lahore, 54590 Pakistan.
17 18
3
19
Abstract
an
16
Department of Polymer Engineering and Technology, University of the Punjab, Lahore, 54590
cr
1
Faculty of Engineering and Technology, University of the Punjab, Lahore, 54590 Pakistan.
M
14
ip t
10
In this study pristine multi-walled carbon nanotubes (MWCNTs) were surface engineered (SE)
21
in strong acidic medium by oxidation purification method to form SE-MWCNT. Five different
22
amount of SE-MWCNT ranging from 0.1- 0.5wt% were thoroughly and uniformly dispersed in
23
cellulose acetate/polyethylene glycol (CA/PEG400) polymer matrix during synthesis of
24
membrane by dissolution casting method. The structural analysis, surface morphology and
25
roughness was carried out by Fourier transform infrared spectroscopy (FTIR), scanning electron
26
microscopy (SEM), and atomic force microscopy (AFM) respectively which showed that the
27
dispersed SE-MWCNT was substantially tethered in CA/PEG400 polymer matrix membrane.
28
The thermogravimetric analysis (TGA) of membranes also suggested some improvement in
29
thermal properties with the addition of SE-MWCNT. Finally, the performance of these
30
membranes was assessed for suitability in drinking water treatment. The permeation flux and
Ac ce p
te
d
20
Corresponding author: Aneela Sabir; Email: [email protected], Phone: +92 322 4569 950 2
Page 2 of 41
salt rejection were determined by using indigenously fabricated reverse osmosis pilot plant with
32
1000ppm NaCl feed solution. The results showed that the tethered SE-MWCNT/CA/PEG400
33
polymer matrix membrane, with strong SE-MWCNTs /polymer matrix interaction, improved
34
the salt rejection performance of the membrane with the salt rejection of 99.8% for the highest
35
content of SE-MWCNT.
ip t
31
Keywords
37
Polymer matrix membrane, Surface Engineered Multiwalled carbon nanotubes, Tethering,
38
Water purification.
39
1. Introduction
an
us
cr
36
The availability of portable and hygienic drinking water is continuously depleting throughout the
41
world. Major initiatives are underway to find viable scientific and engineering solutions to
42
enhance supply of portable water via recycling, conservation and desalination processes (Glater
43
1998).These processes are currently in use to mitigate the risk of water shortage. In past,
44
desalination has been pursued by converting non-portable high saline sources (brackish and
45
seawater) to fresh drinkable water. Nowadays, state-of-the-art systems for desalination are in
46
practice. Most recently, reverse osmosis (RO) membranes are grabbing attention in scientific
47
community (Xu, Chang et al. 2010).
48
The renewable, biodegradable and eco-friendly natural polymers have gathered much attention in
49
recent years regarding membrane technology. Cellulose acetate (CA) is a kind of natural
50
thermoplastic polymer produced via esterification of wood, cotton, recycled paper and bagasse
51
(Sassi and Chanzy 1995; Chandure, Bhusari et al. 2014). CA has the potential tendency for
52
water permeation and salt rejection properties (He, Zhang et al. 2009). CA improved the
Ac ce p
te
d
M
40
3
Page 3 of 41
membrane performance, permeability and permselectivity when incorporated in other polymers
54
like poly (vinyl pyrolidone), poly (vinyl alcohol) poly (ethylene glycol), poly (amide) etc., as
55
compared to pristine polymer (Qin, Li et al. 2003; Saljoughi, Sadrzadeh et al. 2009).
56
Polyethylene glycol (PEG) can render the hydrophilicity and thermal stability in the membranes
57
(Ahumada, Delgado et al. 2012; Zhou, Fu et al. 2014). PEG 400 has low toxicity, which makes it
58
an ideal additive for use in water purification industry and is being used in a variety of
59
biotechnological and biochemical processes (Zavastin, Cretescu et al. 2010). Many studies have
60
shown that the incorporation of additives like silica (SiO2), titania (TiO2), alumina (Al2O3),
61
zirconia (ZrO2) and carbon nanotubes (CNTs) to polymer matrix can effectively improve the
62
thermal stability and mechanical strength of the membranes (Wara, Francis et al. 1995; Castro,
63
Cohen et al. 1996; Kalra, Garde et al. 2003; Yang, Zhang et al. 2007). The chemical modification
64
of polymers with various additives can improve the diffusive, hydrophilic and the salt water
65
purification capacity of the membranes (Haddada, Ferjani et al. 2004; Sivakumar, Mohan et al.
66
2006).
67
Among different additives, CNTs have gathered a considerable attention as nanofiller in
68
desalination. It has also an ideal reinforcement capabilities due to its exceptionally unique
69
thermal, mechanical and permeation properties. An exclusively dispersed CNTs provide
70
enhanced interfacial adhesion between polymer chains (Hummer, Rasaiah et al. 2001; Joseph
71
and Aluru 2008 ; El Badawi, Ramadan et al. 2014). Many studies have been reported where
72
CNTs were blended with other polymers like aromatic polyamides (PA), chitosan, polysulfone
73
(PS), and polyacrylonitrile (PAN) for water filtration applications (Choi, Jegal et al. 2006; Tang,
74
Zhang et al. 2009; Shawky, Chae et al. 2011).
Ac ce p
te
d
M
an
us
cr
ip t
53
4
Page 4 of 41
75
Shawky et al. (2011) synthesized aromatic polyamide/MWCNT nanocomposite membranes with
76
different concentration of MWCNTs by a polymer grafting process. The data showed a decrease
77
in the membrane permeability with the increase in CNTs concentration whereas the salt rejection
78
increased to 76 %.
79
MWCNTs and PA network structure, due to strong interaction between MWCNTs and polymer
80
matrix (Shawky, Chae et al. 2011). Tang et al. (2009) blended chitosan with different loadings
81
of CNT for membrane synthesis and measured pure water flux. The performance of membranes
82
showed that the water flux was 4.6 times more than the pristine chitosan membrane (Tang,
83
Zhang et al. 2009). Choi et al. (2006) prepared PS/CNT membranes by phase inversion method
84
and showed that the final water flux was directly proportional to the increase in CNTs weight
85
percentage. The reason behind this behavior was the increase in the hydrophilicity of the
86
membrane which increased up to the threshold level by loading1.5wt% of CNTs. However, with
87
more CNT concentration, the pore size decreased but the permeation flux increased. (Choi, Jegal
88
et al. 2006). The hydrophilic nature of the membrane due to the presence of CNTs, which
89
increases the water flux, has also been studied by Majeed et al. (2012). In the synthesized
90
PAN/CNTs ultrafiltration membrane (UF), pure water flux increased by the addition of 0.5wt%
91
carbon nanotubes (Majeed, Fierro et al. 2012).
92
In this work, we delineated the synthesis of polymeric matrix based on CA/PEG400 which has
93
been tethered with surface engineered carbon nanotubes. Indigenously lab scale fabricated RO
94
plant was intended to assess the permeation flux and salt rejection. The structural analysis,
95
thermal stability, morphology and surface roughness of the synthesized membranes were
96
measured by Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning
97
electron microscopy and atomic force microscopy, respectively.
Ac ce p
te
d
M
an
us
cr
ip t
These results were explained in terms of structural compactness, creating
5
Page 5 of 41
98
2. Experimental
99
2.1 Materials Analytical grade cellulose acetate (CA) with 39.7% acetyl content was obtained from Fluka
101
(USA). Pristine Multi-walled carbon nanotubes (MWCNTs) of around 20-40 nm diameter, inner
102
diameter of 15 nm and >90 % purity were manufactured by CNME International Co. Ltd.
103
Sulphuric acid (H2SO4) (> 95%) and nitric acid (HNO3) (65-70%) were purchased from BDH
104
Chemicals Ltd. Polyethylene glycol (PEG 400) and N, N-dimethyl formamide (DMF) (99%)
105
were obtained from Fluka and Riedel-de Haȅn, respectively. All the chemicals were used without
106
further purification.
107
2.2 Synthesis of polymer matrix membranes tethered with SE- MWCNTs
108
2.2.1 Preparation of SE- MWCNTs by oxidation purification in strong acid medium
109
Pristine MWCNTs were first surface engineered by oxidation purification in a concentrated
110
H2SO4/HNO3 (3:1 v/v) acidic medium as the oxidant to enhance their dispersion to prepare
111
MWNT-COOH within the polymer matrix solution. In a 1L flask equipped with a condenser,
112
pristine MWNTs (3.0 g) with H2SO4/HNO3 were added and stirred vigorously. The flask was
113
then subjected to ultrasonic bath under vibrations (40 kHz) for 12 h at 140 ̊C. After cooling to
114
room temperature, the reaction mixture was diluted with 500 mL of deionized water and then
115
filtered in vacuum through a filter paper (Fisher). The solid was dispersed in 500 mL of water
116
and filtered again, and then 300 mL of water was used to wash the filter cake several times.
117
The dispersion, filtering, and washing steps were repeated until the pH of the filtrate reached 7.0
118
(at least five cycles were required). The filtrate as then washed with 250 mL of acetone five
Ac ce p
te
d
M
an
us
cr
ip t
100
6
Page 6 of 41
times to remove most of the water from the sample and dried under vacuum for 24 h at 60 ̊C,
120
giving 2.1 g (~70% yield) of MWNT-COOH. The prepared SE-MWCNTs are shown in Scheme
121
I.
an
us
cr
ip t
119
M
122
2.2.2 Synthesis of CA/PEG400 polymer matrix membrane
124
Six different concentrations of polymer matrix solutions of CA/PEG400 were synthesized using
125
dissolution casting method. 10 g of CA was dissolved in 100 mL of DMF solvent with
126
continuous stirring at 70 ̊C for 6 h. The varying blend composition (w/w) of CA/PEG400 (50/50,
127
60/40, 70/30 and 80/20) were prepared and labeled as shown in Table 1. The solutions were
128
cooled at room temperature for 1 h in an air tight condition. Step 1 in Scheme II shows possible
129
interaction between CA and PEG400.
Ac ce p
te
d
123
7
Page 7 of 41
130
Table 1
131
Effect of concentration CA/PEG400 at 4 bar on membrane hydraulic resistance (Rm),
132
permeation flux and salt rejection.
Rm
Salt Rejection
(L/m2.h)
(%)
0.42
76
0.51
68
0.83
62
1.21
54
0.0128
80/20
PM 1
0.0115
70/30
PM 2
0.0102
60/40
PM 3
0.0100
us
PM
M
(bar/ m2.h)
50/50
te
d
133
Permeation Flux
ip t
CA/PEG400
cr
Type
Hydraulic resistance
an
Membrane
2.2.3 Tethering SE-MWCNTs with polymer matrix
135
CA/PEG400 composition of 80/20 was selected, for tethering with SE-MWCNTs, on the basis of
136
maximum salt rejection (76%) compared to other polymer matrix membranes as shown in Table
137
1. Step 2 in Scheme II depicts the interaction between PM chains with modified SE-MWCNTs
138
(0.1-0.5 wt%). The samples were subjected under ultrasonic vibration for 12 h at 65 ̊C. The PM
139
tethered with SE-MWCNTs was labeled as PM-CNTs.
140
2.2.4 Membrane Casting
141
Finally, Step 3 in Scheme II showed the interaction of PM-CNTs. To maintain uniform thickness
142
of the membrane on a clean dried glass plate, a doctor’s blade was used. The solvent was
Ac ce p
134
8
Page 8 of 41
removed by placing PM-CNTs membrane in an oven at 60 ̊C. PM-CNTs were dried for 12 h in
144
vacuum oven and removed from the glass plates with membrane thickness ranging from 25-35
145
µm, as measured by screw gauge.
ip t
143
Ac ce p
te
d
M
an
us
cr
146
147
9
Page 9 of 41
3. Characterization of Membranes
149
3.1 Fourier transform infrared spectroscopy
150
FTIR spectra of PM-CNTs were logged by IR Prestige-21 (Shimadzu) using attenuated total
151
reflectance (ATR) with ZnSe crystal. The air background of the instrument was run before each
152
test of the membrane. The wavenumber range was persistent from 4000-600 cm-1 at 120 scans
153
per spectrum.
154
3.2 Thermogravimetric analysis
155
The thermogravimetric analysis (TGA) measurements of the PM-CNTs were conducted using
156
Mettler Toledo, TGA/SDTA851e instrument. It was carried out from ambient temperature to
157
1200 ̊C at a ramp rate of 20 ̊C/ min under nitrogen flow (15 mL/min).
158
3.3 Scanning electron microscopy
159
Scanning electron micrographs of PM-CNTs were obtained using JEOL (JSM-6480LV)
160
microscope to analyze the morphology of the membranes at different magnifications. The PM-
161
CNTs were examined on a carbon conductive tape as its more significant compared to many of
162
the other common adhesives that have been used in SEM mounting. The low vacuum mode was
163
used to operate PM-CNTs at 20 kV.
164
3.4 Atomic force microscope
165
The topographical images were obtained using scanning probe microscope (SPM 9500J3,
166
Shimadzu) with tapping mode at room temperature. The scanning area was 5 μm2. The values of
167
root mean square (RMS) roughness were derived from AFM images, which were obtained from
168
the average values measured in random areas. The topography of membrane expressed in terms
Ac ce p
te
d
M
an
us
cr
ip t
148
10
Page 10 of 41
of various roughness parameters like mean roughness (Ra) which represents the mean value of
170
the surface relative to the center plane. The volumes enclosed by the image above and below this
171
plane are equal. It is calculated by Equation 1.
ip t
169
(1)
172
Where
shows surface relative to the center of plane while Lx and Ly denotes the
174
dimensions of surface in x and y directions, respectively.
175
However, the root mean square average (RMS) of the measured height deviations from the mean
176
surface taken within the area evaluation and calculated by Equation 2.
M
1/2
177
an
us
cr
173
(2)
3.6 Water content
179
The membranes were kept for 48 h in oven under vacuum at 70 ºC for drying (Ahumada,
180
Delgado et al. 2012). Dried PM-CNTs (1 g) were placed in a vial filled with distilled water (100
181
mL) and the vials were set at room temperature. The water content (%) of the membranes was
182
attained after 24 h by Equation 3.
Ac ce p
te
d
178
183
(3)
184
Whereas, WC stands for water content, WS for wet sample weight and DS for dried sample
185
weight.
186
3.7 Membrane hydraulic resistance (Rm)
11
Page 11 of 41
187
It defines the membrane tolerance towards hydraulic pressure. It can be calculated using
188
Equation 4 [15].
(4)
ip t
189
3.8 Reverse osmosis performance test
191
The RO experiments were employed on the plate and frame membrane module as revealed in
192
Fig.1 (process flow diagram) [2]. NaCl (1000 ppm) solution in water was used as a feed of
193
known conductance. The effective area of membrane in contact with the continuous feed flow
194
was 154 cm2. Permeate was collected for 12 h in a continuous RO membrane unit operation. The
195
provided feed tank capacity was 25 L and feed inlet along with circulation pump of 1 KW
196
followed by rotameter.RO performance tests of the membranes were evaluated on the basis of
197
salt rejection SR (%) and permeation flux (L/m2.h) using Equations 5 and 6. Permeate and
198
retentate were collected during operation from their respective sampling points. The pressure
199
range during the process was varied from 0.5- 4 bar. The conductance of the feed and permeate
200
acquired by using Cyber Scan Waterproof PC 300 Series (EUTECH).
202
203
us
an
M
d
te
Ac ce p
201
cr
190
(5)
(6)
12
Page 12 of 41
ip t cr us an M d te Ac ce p
204 205
4. Results and Discussion
206
4.1 Fourier transform infrared spectroscopy (FTIR)
207
The FTIR spectrum of PM compared with that of PM-CNTs and emergence of functional groups
208
on acid treated MWCNTs which illustrated the functional group analysis of the membranes is
209
shown in Fig. 2. In case of PM membranes, the band at 3425-3464 cm-1 (–OH stretching
210
vibration), weak band at 2924 cm-1 (-C-H stretching), 1737 cm-1 (-C=O stretching), (Costa-
211
Júnior, Barbosa-Stancioli et al. 2009; Salihu, Goswami et al. 2012),1431 cm-1 (-C-H bending)
212
followed by bands at 1371 and 1224 cm-1 were described by the rocking and wagging mode of -
13
Page 13 of 41
C-H bond. The existence of adsorption band at 1036 cm-1 was attributed to -C-O (acyclic)
214
stretching vibrations (Xing and Ho 2009; Zavastin, Cretescu et al. 2010; Atif Islam 2012; Feng,
215
Ren et al. 2013; Worthley, Constantopoulos et al. 2013) while weak band at 906 and 1155cm-1
216
confirmed the presence of pyranose ring and saccharine structure (Atif Islam 2012, Atieh,
217
Bakather et al. 2011, Costa-Júnior, Barbosa-Stancioli et al. 2009).
218
Furthermore, FTIR spectra of PM-CNTs membranes with different concentrations of SE-
219
MWCNTs (0.1-0.5 wt %) are shown in Fig. 2. The specific bands owing to created functional
220
groups were detected on MWCNTs. The acid treated MWCNTs exhibits bands having hydroxyl
221
(-OH) and carbonyl (-C=O) groups. This confirmed that carbonyl and hydroxyl functional
222
groups were introduced on the CNTs. The observed feature at 3425-3464 cm-1 was associated
223
with the -OH stretching vibrations from the inter-molecular hydrogen bonds (Scheme II) (Costa-
224
Júnior, Barbosa-Stancioli et al. 2009) that tend to weaken the force constant and moving the
225
absorbance to lower energy (lower wave number). The carbonyl group was also shifted slightly
226
towards lower wavenumber (1738 to 1734 cm-1) due hydrogen bonding (Scheme II) which
227
proposes interactions between –COOH functional groups of MCWT-COOH and -C=O groups of
228
CA (Zeeshan and Gopiraman 2013).
cr
us
an
M
d
te
Ac ce p
229
ip t
213
14
Page 14 of 41
ip t cr us an M d te
230
4.2. Thermogravimetric analysis (TGA)
232
The thermal stability of PM and PM-CNTs (0.1-0.5 wt%) were accomplished in three steps
233
shown in Fig. 3. In the first step, for PM, the removal of moisture and also dehydration was
234
observed at about 30-280 ̊C up to weight loss of 4.63%. The second step ranges from 280-450 ̊C
235
was the major degradation of the polymer backbone with weight loss of about 90.78% which was
236
probably due to main thermal decomposition of CA chains. In the final stage, almost constant
237
thermal profile was observed from 450-1200 ̊C due to carbonization of the degraded product to
238
ash. This TGA profile indicated that PM membrane could highly be unstable at high temperature.
Ac ce p
231
15
Page 15 of 41
Thermograms of PM-CNTs (0.1-0.5wt %) showed similar steps which involves the
240
decomposition from 30-280 ̊C with weight loss of 3.3-4.4% which was attributed to moisture
241
removal, dehydration and loss of bound water. The second step showed onset of polymer
242
degradation from 250 ̊C (Shieh and Chung 1998; Lucena, V. de Alencar et al. 2003) with weight
243
loss of about 92.5-92.62% up to offset temperature of 450 ̊C. The final step indicated the
244
temperature range from 450-1200 ̊C showing carbonization of the thermally decomposed product
245
to ash (Chatterjee and Conrad 1968). The experimental data showed that 30 percent weight
246
losses for PM, PM-CNT1 and PM-CNT5 were occurred at 338, 353 and 372 ̊C, respectively.
247
Similarly, 80 percent weight losses for PM, PM-CNT1 and PM-CNT5 were observed at 383, 385
248
and 407 ̊C, respectively. This confirmed that the experiential TGA curve of PM-CNT5 was
249
thermally stable as compared to the other PM-CNTs exhibiting the improvement in the thermal
250
properties with the addition of CNTs. The percentage residue left for the PM and PM-CNTs are
251
also shown in Fig.3.
253 254 255 256 257
cr
us
an
M
d te
Ac ce p
252
ip t
239
16
Page 16 of 41
ip t cr us an 261 262 263 264
d
M 260
te
259
Ac ce p
258
265 266 267 268
17
Page 17 of 41
269
Table 2
270
Sample name hydraulic resistance (Rm) and roughness values of with and without SE-MWCNTs (0.1-0.5
271
wt%) tethered polymer matrix
272
Average
Square average roughness roughness
Root mean square roughness
Weight Fraction of SEMWCNTs loading (wt%)
(bar/m2.h)
(%)
Ra (nm)
Ry (nm)
PM
0
0.0128
78.1
18.17
115.60
76.64
22.27
PM-CNT 1
0.1
0.0123
80.8
19.21
240.47
115.90
24.913
PM-CNT 2
0.2
0.0127
83.2
30.83
310.21
146.75
38.801
PM-CNT 3
0.3
0.0134
88.4
34.16
332.06
161.88
43.673
PM-CNT 4
0.4
0.0145
90.1
17.61
192.80
93.98
22.045
PM-CNT 5
0.5
0.0156
92.2
21.80
210.07
102.08
27.071
te
d
M
an
us
Rm
Mean depth
ip t
Water Content
Rz (nm)
cr
Hydraulic resistance
Rms (nm)
Ac ce p
Membrane Sample
273
4.3 Scanning electron microscopy (SEM)
274
Scanning electron microscopy technique was used for the characterization of PM and PM-CNTs
275
as shown in Fig. 4. Membranes employed for examination provide the information of surface
276
morphology (Zavastin, Cretescu et al. 2010). The membrane is dense and it has even and smooth
277
surface. This indicated that PEG400 was homogeneously mixed with CA due to capability of
278
two polymers interacting mutually through hydrogen bonding between –OH and -C=O groups
279
(Yang, Zhang et al. 1999). By tethering different amount of SE-MWCNTs (0.1–0.5 wt%),
280
surface morphology were changed significantly and SE-MWCNTs were well and uniformly 18
Page 18 of 41
distributed in the PM. During high SE-MWCNTs tethering i.e. PM-CNT5 (Fig. 4 a,b) as shown
282
in Fig 4, were found to form their aggregates and agglomeration to some extent in the composite
283
membranes and were not dispersed well in the membrane because of the van der Waals
284
interaction between the neighboring SE-MWCNTs (Li, Kim et al. 2010).
Ac ce p
285
te
d
M
an
us
cr
ip t
281
286
4.4 Atomic force microscope (AFM)
287
The tethering of the SE-MWCNTs on the polymer matrix was confirmed from AFM
288
complementing SEM. Samples of membrane were employed for examination to provide the
289
information of surface topography and roughness. The bright regions signify the highest points
290
or nodules of membrane surface whereas the darker regions signify the depressions as shown in
291
Fig. 5. The descriptions showed that surface roughness was increased with an increase in the
292
concentrations of SE-MWCNTs upto 0.3 wt% in the membranes. Whereas, the decline in
293
roughness parameters were observed when the concentration of SE-MWCNTs was increased 19
Page 19 of 41
upto 0.4 wt% i.e. PM-CNT4 as shown in Table 2. This decrease owed to the low electrostatic
295
interactions between SE-MWCNTs and the polymer matrix (Vatanpour, Madaeni et al.
296
2011).Thus, the MWCNTs were frequently positioned, giving rise to smooth surface (Qiu, Wu et
297
al. 2009). However, with high SE-MWCNT concentrations PM-CNT5 (0.5 wt%), again an
298
increase in roughness parameters and non-uniform surface was observed due to agglomeration of
299
SE-MWCNT (Phao, Nxumalo et al. 2013).
us
cr
ip t
294
Ac ce p
te
d
M
an
300
301
4.5 Water content
302
The water content % of the membrane exhibited the hydrophilic nature of the membrane
303
(Sivakumar, Mohan et al. 2006). Equation 3 was used to calculate the WC for PM i.e. 78.1 % as
304
shown in Table 2 above. In case of PM-CNT1, the water content % showed the value of 80.8 %.
20
Page 20 of 41
When SE-MWCNTs contents reached to 0.5 wt% i.e. PM-CNT5, the water content % was
306
increased up to 92.2 %. Similar increase in water content % was shown in which Pluronic (F127)
307
was added in the CA membranes (Lv, Su et al. 2007). The reason acclaimed to the absorbance of
308
water in the membranes was the hydrophilic nature of SE-MWCNTs
309
4.6 Hydraulic resistance
310
Membrane hydraulic resistance (Rm) controls the resistance when pressure is applied to the
311
membrane. The linear proportionality of permeation flux to the applied pressure is directly
312
proportional to the transport resistance. Equation 4 was used for evaluating the hydraulic
313
membrane resistance. The permeation flux was calculated by varying the transmembrane
314
pressure from 0.5 to 4.0 bar. It is evident from Table 1 that Rm is directly proportional to the
315
extent of CA present in the membrane.
316
The polymer matrix membrane with highest content of cellulose acetate i.e. PM offers maximum
317
hydraulic resistance compared to those which have greater PEG. PM offers resistance of 0.0128
318
bar/m2.h while PM3 shows minimum resistance of 0.01 bar/m2.h. The PM with greater Rm
319
entails more amount of cellulose acetate and it comes up with stable dense membrane
320
(Arthanareeswaran, Sriyamuna Devi et al. 2008; Arthanareeswaran, Sriyamuna Devi et al. 2009).
321
The formation of dense membrane formation is also evident from the SEM images shown in PM
322
of Fig. 4. When the quantity of PEG400 was increased, the formation of porogen in membranes
323
was increased as depicted in cross sectional view of membranes. These pores gave extended free
324
volume and as a result Rm was reduced (Sivakumar, Mohan et al. 2006).
325
The Rm values for tethered PM-CNTs are given in Table 2 which shows a steady increase in
326
SE-MWCNTs. The improved interaction in the PM-CNTs was due to the segmental gaps in the
Ac ce p
te
d
M
an
us
cr
ip t
305
21
Page 21 of 41
membranes which exhibited a decline with increase in Rm. Values in Table 2 showed the
328
comparison in Rm values of PM with PM-CNTs. It was observed that PM-CNTs have enhanced
329
Rm. The reason attributed to this enhanced resistance was due to the development of electrostatic
330
interaction between the polymer chains.
331
4.7 Reverse osmosis performance test
332
4.7.1Transport properties of membranes
333
The salt water feed 1000 ppm NaCl in water with pH=7.4 i.e. slightly alkaline was pumped into a
334
vessel in indigenously fabricated RO pilot plant. The pressure was varied from 0.5- 4.0 bar when
335
25 L/h (feed flow rate) was applied.
336
Table 1 shows the permeation flux and salt rejection of polymer matrix membranes which
337
exhibited that PM3 membrane has maximum permeation flux of 1.21 (L/m2.h) while it has
338
minimum salt rejection capacity of 54%. As the CA content was increased, the flux slightly
339
decreased while capacity of salt rejection increased remarkably. PM membrane showed 76 % salt
340
rejection and its flux was 0.42 (L/m2.h) with maximum CA and minimum PEG400 content.
341
The transport through reverse osmosis membrane was described in terms of diffusive flow in
342
which separation occurs in membrane when both solute and solvent permeate by solution and
343
diffusion process. The transport mechanism was best elucidated on the basis of solution-
344
diffusion model. According to this model, three steps occur in transport process within
345
membrane i.e. sorption at the surface of membrane, diffusion into dense membrane under
346
pressure and then desorption. The hydrophilic nature of PEG400 acts as driving force for
347
sorption of water on membrane (Lonsdale, Merten et al. 1965; Burghoff, Lee et al. 1980; Mazid
Ac ce p
te
d
M
an
us
cr
ip t
327
22
Page 22 of 41
1984).Salt removal was increased due to the desalting nature of CA (Malaisamy, Mahendran et
349
al. 2002).
350
Furthermore, salt rejection occurred on the basis of electrostatic repulsion known as Donnon
351
Effect (Lonsdale, Merten et al. 1965).The ions excluded by the Columbic Forces which arise
352
from the charges residing on the surface of membrane. CA showed desalting property and
353
improves the salt rejection efficiency which was the reason behind the maximum salt rejection
354
ability of PM.
355
PEG400 depicted the hydrophilic nature and act as porogen (Burghoff, Lee et al. 1980). PM 3
356
(polymer matrix membrane) had maximum quantity of PEG400 giving more water flux but at the
357
same time showed salt rejection compromised. This may be attributed to the formation of
358
porogen (Malaisamy, Mahendran et al. 2002) on membrane which allows the passage of salt
359
along with water, resulting in the maximum flux and minimum salt rejection. Greater the
360
PEG400 content, higher will be the flux. The diffusion rate of water was accelerated by the
361
presence of PEG400 due to its hydrophilic nature. PEG400 proliferate the tendency of porogen
362
and as a consequence permeation flux was increased (Kurokawa and Ueno 1982). Whereas,
363
when salt rejection efficiency of membrane was optimum, the salt passage % reduced depending
364
upon the concentration of CA/PEG400.
365
On the basis of salt rejection, PM polymer matrix membrane was selected for tethering of SE-
366
MWCNTs. After tethering, these PM-CNTs were characterized and compared with PM polymer
367
matrix membrane. The permeation flux and salt rejection of PM-CNTs using various
368
concentrations of SE-MWCNTs are given in Table 2. The water salinity was decreased with the
369
increment of SE-MWCNT in the PM. The permeation flux, and salt rejection % were
Ac ce p
te
d
M
an
us
cr
ip t
348
23
Page 23 of 41
significantly affected by the integration of SE-MWCNTs. PM-CNT1 showed maximum
371
permeation flux of 0.84 L/m2.h but as tethering of SE-MWCNTs increased permeation flux
372
declined gradually. Fig. 6 shows the comparison between permeation flux of PM and PM-CNTs.
373
As evident from Fig. 6 that PM-CNT 5 showed reduced permeation flux and gave lowest flux of
374
0.61 L/m2.h.
375
The salt rejection capacity of PM-CNTs was enhanced when compared with PM membrane as
376
indicated in Fig. 6. PM-CNT 5 showed highest capacity of salt rejection of 99.8% attributed to
377
0.5 wt% SE-MWCNTs tethered on PM. The permeation flux effect was owed to tethering of SE-
378
MWCNTs. These PM-CNTs infatuated a hydrophilic nature as its driving force. Resultantly, the
379
salt rejected through the hydrophilic membranes because of its ability for hydrogen bonding. The
380
tethering of SE-MWCNTs changed the polymer chains segmental motion and also the mobility
381
of permeate which increased the free volume of PM-CNTs.
384 385
cr
us
an
M
d te
383
Ac ce p
382
ip t
370
24
Page 24 of 41
ip t cr us an
386
5. Conclusion
388
This study has examined the relation of permeability flux and salt rejection using RO membranes
389
in indigenously fabricated pilot plant. It showed that 0.1-0.5 wt% tethering of SE-MWCNTs into
390
PM membrane manifestly increased the selective salt rejection % and permeation flux at 4.0 bar.
391
Higher content of SE-MWCNTs (0.5 wt%) led to the noteworthy increase in salt rejection of
392
99.8%, whereas, permeation flux was decreased upto 0.61 L/m2.h as compared to the PM
393
membrane. Nevertheless, data presented in this study encompassing the most common
394
membrane materials (cellulose acetate, polyethylene glycol and multi-walled carbon nanotubes)
395
used in RO reconfirm a single general trend connecting intrinsic water permeability flux and
396
selectivity to salt rejection of these materials. The reported results can improve the understanding
397
of the state of the art function of RO membranes and can be valuable in design of new polymeric
398
materials with high salt rejection and good permeability. Moreover, the structural
399
characterization of PM-CNTs membrane is confirmed by ATR-FTIR. The SEM and AFM
Ac ce p
te
d
M
387
25
Page 25 of 41
micrographs showed uniform dispersion of SE-MWCNTs on PM-CNTs having influence on
401
morphology and topography of membrane structure. Generally, tethering of SE-MWCNTs led to
402
increase in the thermal stability of membranes analyzed by TGA thermograms. Also, the WC %
403
exhibited that PM-CNT 5 tethered membrane has the highest water content as compared to other
404
membranes.
405
Acknowledgements
406
The authors gratefully acknowledge University of the Punjab, Lahore Pakistan for providing
407
financial support of this work.
an
us
cr
ip t
400
M
408
410
Ac ce p
te
d
409
26
Page 26 of 41
ip t cr us an M d te Ac ce p
411 412 413 414
27
Page 27 of 41
416
Ac ce p
te
d
M
an
us
cr
ip t
415
28
Page 28 of 41
ip t cr us an M d te
418
Ac ce p
417
29
Page 29 of 41
ip t cr us an M d Ac ce p
te
419
420 30
Page 30 of 41
an
us
cr
ip t
421
422
Ac ce p
te
d
M
423
424 425
31
Page 31 of 41
426
Table 1
428
Effect of concentration CA/PEG400 at 4 bar on membrane hydraulic resistance (Rm),
429
permeation flux and salt rejection Hydraulic
CA/PEG400
Type
Resistance
Permeation
Salt
Flux
Rejection
us
(Rm)
432 433 434 435 436 437
70/30
PM 2
0.0102
60/40
PM 3
0.0100
50/50
0.51
68
0.83
62
1.21
54
M
0.0115
76
d
PM 1
0.42
te
431
80/20
(%)
Ac ce p
430
0.0128
(L/m2.h)
an
(bar/ m2.h) PM
cr
Membrane
ip t
427
438 439
32
Page 32 of 41
440
Table 2
441
Sample name hydraulic resistance (Rm) and roughness values of with and without SE-MWCNTs
442
(0.1-0.5 wt%) tethered polymer matrix
443 444 445 446 447 448
Root mean square roughness
0
0.0128
78.1
18.17
115.60
76.64
22.27
PM-CNT 1
0.1
0.0123
80.8
19.21
240.47
115.90
24.913
PM-CNT 2
0.2
0.0127
83.2
30.83
310.21
146.75
38.801
PM-CNT 3
0.3
0.0134
88.4
34.16
332.06
161.88
43.673
PM-CNT 4
0.4
0.0145
90.1
17.61
192.80
93.98
22.045
PM-CNT 5
0.5
0.0156
92.2
21.80
210.07
102.08
27.071
te
d
an
M
(%)
ip t
PM
cr
Mean depth Rz (nm)
Hydraulic Resistance (Rm) (bar/m2.h)
us
Water Content
Average Square roughness average roughness Ra (nm) Ry (nm)
Weight Fraction of SEMWCNTs loading (wt%)
Rms (nm)
Ac ce p
Membrane Sample
449 450 451 452 33
Page 33 of 41
References
454 455
Ahumada, E. A., D. R. Delgado, et al. (2012). "Solution thermodynamics of acetaminophen in some PEG 400 + water mixtures." Fluid Phase Equilibria 332(0): 120-127.
456 457 458 459
Arthanareeswaran, G., T. K. Sriyamuna Devi, et al. (2009). "Development, characterization and separation performance of organic–inorganic membranes: Part II. Effect of additives." Separation and Purification Technology 67(3): 271-281.
460 461 462 463
Arthanareeswaran, G., T. K. Sriyamuna Devi, et al. (2008). "Effect of silica particles on cellulose acetate blend ultrafiltration membranes: Part I." Separation and Purification Technology 64(1): 38-47.
464 465 466 467
Atieh, M. A., O. Y. Bakather, et al. (2011). "Effect of carboxylic functional group functionalized on carbon nanotubes surface on the removal of lead from water." Bioinorganic chemistry and applications 2010.
468 469 470
Atif Islam, T. Y. (2012). "Controlled delivery of drug from pH sensitive chitosan/poly (vinyl alcohol) blend." Carbohydrate Polymers 88(3): 1055-1060.
471 472 473 474
Burghoff, H. G., K. Lee, et al. (1980). "Characterization of transport across cellulose acetate membranes in the presence of strong solute–membrane interactions." Journal of applied polymer science 25(3): 323-347.
475 476 477
Castro, R. P., Y. Cohen, et al. (1996). "Silica-suported polyvinylpyrrolidone filtration membranes." Journal of Membrane Science 115(2): 179-190.
478 479 480 481
Chandure, A. S., G. S. Bhusari, et al. (2014). "Synthesis, Characterization, and Biodegradation Studies of Poly(1,4-cyclohexanedimethylene-adipate-carbonate)s." Journal of Polymers 2014: 11.
482 483
Chatterjee, P. K. and C. M. Conrad (1968). "Thermogravimetric analysis of cellulose." Journal of
484
Polymer Science Part A‐1: Polymer Chemistry 6(12): 3217-3233.
Ac ce p
te
d
M
an
us
cr
ip t
453
485
34
Page 34 of 41
Choi, J.-H., J. Jegal, et al. (2006). "Fabrication and characterization of multi-walled carbon nanotubes/polymer blend membranes." Journal of Membrane Science 284(1–2): 406-415.
488 489 490 491
Costa-Júnior, E. S., E. F. Barbosa-Stancioli, et al. (2009). "Preparation and characterization of chitosan/poly(vinyl alcohol) chemically crosslinked blends for biomedical applications." Carbohydrate Polymers 76(3): 472-481.
ip t
486 487
492
El Badawi, N., A. R. Ramadan, et al. (2014). "Novel carbon nanotube–cellulose acetate nanocomposite membranes for water filtration applications." Desalination 344(0): 79-85.
495 496 497
Feng, S., J. Ren, et al. (2013). "Poly (amide-12-b-ethylene oxide)/polyethylene glycol blend membranes for carbon dioxide separation." Separation and Purification Technology 116: 25-34.
498 499 500
Glater, J. (1998). "The early history of reverse osmosis membrane development." Desalination 117(1–3): 297-309.
501 502 503
Haddada, R., E. Ferjani, et al. (2004). "Properties of cellulose acetate nanofiltration membranes. Application to brackish water desalination." Desalination 167(0): 403-409.
us
an
M
d te
504
cr
493 494
He, J., M. Zhang, et al. (2009). "High‐quality cellulose triacetate prepared from bamboo
506
dissolving pulp." Journal of applied polymer science 113(1): 456-465.
507 508 509 510
Hilal, N., A. W. Mohammad, et al. (2003). "Using atomic force microscopy towards improvement in nanofiltration membranes properties for desalination pre-treatment: a review." Desalination 157(1–3): 137-144.
511 512 513
Hummer, G., J. C. Rasaiah, et al. (2001). "Water conduction through the hydrophobic channel of a carbon nanotube." Nature 414(6860): 188-190.
514 515 516
Joseph, S. and N. R. Aluru (2008). "Why Are Carbon Nanotubes Fast Transporters of Water?" Nano Letters 8(2): 452-458.
517 518 519
Kalra, A., S. Garde, et al. (2003). "Osmotic water transport through carbon nanotube membranes." Proceedings of the National Academy of Sciences 100(18): 10175-10180.
Ac ce p
505
35
Page 35 of 41
Kurokawa, Y. and K. Ueno (1982). "Reverse osmosis rejection by hydrous inorganic precipitate– cellulose composite membrane." Journal of applied polymer science 27(2): 621-630.
523 524 525 526
Li, M., I. H. Kim, et al. (2010). "Cellulose acetate/multiwalled carbon nanotube nanocomposites with improved mechanical, thermal, and electrical properties." Journal of applied polymer science 118(4): 2475-2481.
527 528 529
Lonsdale, H., U. Merten, et al. (1965). "Transport properties of cellulose acetate osmotic membranes." Journal of applied polymer science 9(4): 1341-1362.
530 531 532
Lucena, M. d. C. C., A. E. V. de Alencar, et al. (2003). "The effect of additives on the thermal degradation of cellulose acetate." Polymer Degradation and Stability 80(1): 149-155.
533 534 535 536
Lv, C., Y. Su, et al. (2007). "Enhanced permeation performance of cellulose acetate ultrafiltration membrane by incorporation of Pluronic F127." Journal of Membrane Science 294(1–2): 68-74.
537 538 539 540
Majeed, S., D. Fierro, et al. (2012). "Multi-walled carbon nanotubes (MWCNTs) mixed polyacrylonitrile (PAN) ultrafiltration membranes." Journal of Membrane Science 403–404(0): 101-109.
541 542 543 544
Malaisamy, R., R. Mahendran, et al. (2002). "Cellulose acetate and sulfonated polysulfone blend ultrafiltration membranes. I. Preparation and characterization." Journal of applied polymer science 86(7): 1749-1761.
545 546 547
Mazid, M. (1984). "Mechanisms of transport through reverse osmosis membranes." Separation Science and Technology 19(6-7): 357-373.
548 549 550
Meindersma, G. W., C. M. Guijt, et al. (2006). "Desalination and water recycling by air gap membrane distillation." Desalination 187(1–3): 291-301.
551 552 553 554
Phao, N., E. N. Nxumalo, et al. (2013). "A nitrogen-doped carbon nanotube enhanced polyethersulfone membrane system for water treatment." Physics and Chemistry of the Earth, Parts A/B/C 66(0): 148-156.
555 556 557
Qin, J.-J., Y. Li, et al. (2003). "Cellulose acetate hollow fiber ultrafiltration membranes made from CA/PVP 360 K/NMP/water." Journal of Membrane Science 218(1): 173-183.
Ac ce p
te
d
M
an
us
cr
ip t
520 521 522
36
Page 36 of 41
Qiu, S., L. Wu, et al. (2009). "Preparation and properties of functionalized carbon nanotube/PSF blend ultrafiltration membranes." Journal of Membrane Science 342(1–2): 165-172.
561 562 563
Salihu, G., P. Goswami, et al. (2012). "Hybrid electrospun nonwovens from chitosan/cellulose acetate." Cellulose 19(3): 739-749.
564 565 566 567
Saljoughi, E., M. Sadrzadeh, et al. (2009). "Effect of preparation variables on morphology and pure water permeation flux through asymmetric cellulose acetate membranes." Journal of Membrane Science 326(2): 627-634.
568 569 570
Sassi, J.-F. and H. Chanzy (1995). "Ultrastructural aspects of the acetylation of cellulose." Cellulose 2(2): 111-127.
571 572 573
Shawky, H. A., S.-R. Chae, et al. (2011). "Synthesis and characterization of a carbon nanotube/polymer nanocomposite membrane for water treatment." Desalination 272(1–3): 46-50.
574 575 576 577
Sheikholeslami, R. (1999). "Fouling mitigation in membrane processes: Report on a Workshop held January 26–29, 1999, Technion — Israel Institute of Technology, Haifa, Israel." Desalination 123(1): 45-53.
578 579 580 581
Shieh, J.-J. and T. S. Chung (1998). "Effect of liquid-liquid demixing on the membrane morphology, gas permeation, thermal and mechanical properties of cellulose acetate hollow fibers." Journal of Membrane Science 140(1): 67-79.
582 583 584 585
Sivakumar, M., D. R. Mohan, et al. (2006). "Studies on cellulose acetate-polysulfone ultrafiltration membranes: II. Effect of additive concentration." Journal of Membrane Science 268(2): 208-219.
586 587 588 589
Tang, C., Q. Zhang, et al. (2009). "Water transport behavior of chitosan porous membranes containing multi-walled carbon nanotubes (MWNTs)." Journal of Membrane Science 337(1–2): 240-247.
590 591 592 593
Vatanpour, V., S. S. Madaeni, et al. (2011). "Fabrication and characterization of novel antifouling nanofiltration membrane prepared from oxidized multiwalled carbon nanotube/polyethersulfone nanocomposite." Journal of Membrane Science 375(1–2): 284-294.
594 595 596
Wara, N. M., L. F. Francis, et al. (1995). "Addition of alumina to cellulose acetate membranes." Journal of Membrane Science 104(1): 43-49.
Ac ce p
te
d
M
an
us
cr
ip t
558 559 560
37
Page 37 of 41
Worthley, C. H., K. T. Constantopoulos, et al. (2013). "A study into the effect of POSS nanoparticles on cellulose acetate membranes." Journal of Membrane Science 431(0): 62-71.
600 601 602 603
Xing, R. and W. S. W. Ho (2009). "Synthesis and characterization of crosslinked polyvinylalcohol/polyethyleneglycol blend membranes for CO2/CH4 separation." Journal of the Taiwan Institute of Chemical Engineers 40(6): 654-662.
604 605 606
Xu, J., C.-Y. Chang, et al. (2010). "Performance of a ceramic ultrafiltration membrane system in pretreatment to seawater desalination." Separation and Purification Technology 75(2): 165-173.
607 608 609
Yang, G., L. Zhang, et al. (1999). "Role of polyethylene glycol in formation and structure of regenerated cellulose microporous membrane." Journal of Membrane Science 161(1–2): 31-40.
610 611 612 613
Yang, Y., H. Zhang, et al. (2007). "The influence of nano-sized TiO< sub> 2 fillers on the morphologies and properties of PSF UF membrane." Journal of Membrane Science 288(1): 231238.
614 615 616 617
Zavastin, D., I. Cretescu, et al. (2010). "Preparation, characterization and applicability of cellulose acetate–polyurethane blend membrane in separation techniques." Colloids and Surfaces A: Physicochemical and Engineering Aspects 370(1–3): 120-128.
618 619 620
Zeeshan, B. K. I. K. K. and K. F. M. Gopiraman (2013). "Structural and mechanical properties of cellulose acetate/graphene hybrid nanofibers: spectroscopic investigations." eXPRESS Polymer Letters 7(6).
621 622
Zhou, J., H. Fu, et al. (2014). "Solubility and solution thermodynamics of flofenicol in binary PEG 400 + water systems." Fluid Phase Equilibria 376(0): 159-164.
624 625 626
cr
us
an
M
d
te
Ac ce p
623
ip t
597 598 599
627 628 629 38
Page 38 of 41
630
Table 1
631
Effect of concentration CA/PEG400 at 4 bar on membrane hydraulic resistance (Rm),
632
permeation flux and salt rejection.
Rm
(%)
0.42
76
0.51
68
0.83
62
1.21
54
80/20
PM 1
0.0115
70/30
PM 2
0.0102
60/40
PM 3
0.0100
M
te
d
us
0.0128
Ac ce p
634
PM
50/50
Salt Rejection
(L/m2.h)
(bar/ m2.h)
633
Permeation Flux
ip t
CA/PEG400
cr
Type
Hydraulic resistance
an
Membrane
39
Page 39 of 41
634
Table 2
635
Sample name hydraulic resistance (Rm) and roughness values of with and without SE-MWCNTs (0.1-0.5
636
wt%) tethered polymer matrix
637 638 639
Average
Square average roughness roughness
Root mean square roughness
Weight Fraction of SEMWCNTs loading (wt%)
(bar/m2.h)
(%)
Ra (nm)
Ry (nm)
PM
0
0.0128
78.1
18.17
115.60
76.64
22.27
PM-CNT 1
0.1
0.0123
80.8
19.21
240.47
115.90
24.913
PM-CNT 2
0.2
0.0127
83.2
30.83
310.21
146.75
38.801
PM-CNT 3
0.3
0.0134
88.4
34.16
332.06
161.88
43.673
PM-CNT 4
0.4
0.0145
90.1
17.61
192.80
93.98
22.045
PM-CNT 5
0.5
0.0156
92.2
21.80
210.07
102.08
27.071
te
d
M
an
us
Rm
Mean depth
ip t
Water Content
Rz (nm)
cr
Hydraulic resistance
Rms (nm)
Ac ce p
Membrane Sample
40
Page 40 of 41
Figure, Scheme and Table Captions Scheme I: preparation of Surface Engineered-Multiwalled carbon nanotubes by oxidation purification in strong acid medium.
ip t
Scheme II: Tethering of Surface Engineered Multiwalled carbon nanotubes with cellulose acetate/polyethylene glycol polymer matrix
cr
Fig. 1. Process flow diagram of indigenously fabricated reverse osmosis pilot plant. Fig.2. FTIR spectra of PM and PM-CNTs membranes.
us
Fig.3.TGA of PM and PM-CNTs membranes at various percentage weight loss
an
Fig.4. Surface image of PM and PM-CNTs membranes, (a) top surface and (b) crosssection at different magnifications. Fig.5. AFM 3-dimensional image of PM and PM-CNTs membranes.
M
Fig.6 Comparison of permeation flux and salt rejection between PM and PM-CNTs membranes.
d
Table 1. Effect of concentration CA/PEG400 at 4 bar on membrane hydraulic resistance (Rm),permeation flux and salt rejection.
Ac ce p
te
Table 2: Sample name hydraulic resistance (Rm) and roughness values of with and without SE-MWCNTs (0.1-0.5 wt%) tethered polymer matrix.
Page 41 of 41