Journal of Membrane Science 407–408 (2012) 164–175
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
CF4 plasma surface modification of asymmetric hydrophilic polyethersulfone
membranes for direct contact membrane distillation
Xing Wei a,b , Baolong Zhao a , Xue-Mei Li a,∗ , Zhouwei Wang a , Ben-Qiao He c , Tao He a,b,∗ , Biao Jiang a
a
b
c
Shanghai Advanced Research Institute, CAS, Shanghai 201203, China
Nanjing University of Technology, Nanjing 210009, China
Tianjin University of Technology, Tianjin 300160, China
a r t i c l e
i n f o
Article history:
Received 22 December 2011
Received in revised form 5 March 2012
Accepted 12 March 2012
Available online 20 March 2012
Keywords:
Polyethersulfone
Membrane distillation
CF4 plasma
Surface modification
a b s t r a c t
This paper describes the use of CF4 plasma modification of a hydrophilic membrane into a hydrophobic
one for membrane distillation. Plasma surface modification conditions were optimized with respect to
plasma glow discharge power and treatment duration using a flat sheet PES membrane. The modified
membranes were characterized by X-ray photoelectron spectroscopy (XPS), SEM, contact angle measurements, pore size distribution, liquid entry pressure and atomic force microscopy. Results revealed that the
plasma modification converted hydrophilic membranes of a contact angle 0◦ into hydrophobic ones with
water contact angle above 120◦ . Fluorination was ascribed to the wettability change of the membrane
from hydrophilic to hydrophobic via insertion and possibly deposition. Direct contact membrane distillation of the hollow fibers using 4% NaCl solution yielded a water flux of 45.4 kg/m2 h at a feed temperature
of 63.3 ◦ C. A rather high evaporation efficiency of the membrane distillation process was estimated in
comparison with literature results. Direct contact membrane distillation (DCMD) stability test showed a
water flux of 42.1 kg/m2 h using 4 wt% NaCl as feed (at the temperature of 60.5 ± 0.2 ◦ C). No leakage was
observed for 54 h indicating a stable membrane performance. The high evaporation efficiency and water
flux were ascribed most probably to the high porosity of the base membrane.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Membrane distillation (MD) is a thermally driven process that
depends on the difference of the partial water vapor pressure across
a non-wetting, hydrophobic, porous membrane [1,2]. Emerged
nearly 50 years ago [3–5], no large scale MD plants have been
implemented yet for desalination. There are several scientific and
technological challenges that hamper its industrial applications
[6,7]. The major barriers include MD membrane and module design,
membrane pore wetting, low permeate flow rate, and flux decay as
well as uncertain energy and economic costs. These challenges have
attracted scientists and engineers striving for the best membrane
performance, module and process design [8–11], among which the
selection of membrane materials was the most important.
A key requirement for distillation membrane is that the membrane should not be wetted by water. Therefore, in early works,
commercial hydrophobic membranes were used because of their
intrinsically hydrophobic characteristics that resist the pore wetting of the membranes [12]. But these hydrophobic membranes
∗ Corresponding authors at: Shanghai Advanced Research Institute, Chinese
Academy of Sciences, China. Tel.: +86 21 20325162; fax: +86 21 20325034.
E-mail address: [email protected] (T. He).
0376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.memsci.2012.03.031
usually give low permeability in MD process. It is believed that
hydrophobic materials may cause severe temperature polarization
and thereby lower the evaporation efficiency in the membrane
distillation process due to their good thermal conductivity. On
the other hand hydrophilic membrane may suffer less temperature polarization and demonstrate higher evaporation efficiency
because of their high thermal resistance. In recent years, a
series of works have reported the development of dual layer
membrane consists of a hydrophobic layer and a hydrophilic
support.
Khayet et al. [13–15] prepared a series of hydrophobic/hydrophilic flat-sheet membrane for direct contact membrane
distillation (DCMD) by phase inversion. These membranes had a
composite structure with a thin hydrophobic layer (thus low resistance to water vapor diffusion) and a thick hydrophilic sublayer (a
low conductive heat loss). Although the concept was theoretically
attractive, results did not reflect significant advantages. Qtaishat
et al. [16] continued their work on the double layer membrane by
mixing fluorinated macromolecules (SMMs) with polyetherimide
(PEI). The membranes showed a contact angle of 100◦ at the top
surface with a LEPw value of 4.7 bars. Unfortunately, the water flux
in DCMD was fairly low, about 18 L/m2 h at a feed temperature of
around 65 ◦ C and a distillate temperature of 15 ◦ C. A separation
factor above 99% was observed. The reasons may lie in that the
X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
structure of the hydrophobic layer was not optimized and thereby
may impart higher mass transfer resistance.
Surface modification by plasma polymerization for the formation of a hydrophobic layer on a hydrophilic base membrane
was conducted as well. For example, Kong et al. [17,18] modified a hydrophilic microporous cellulose nitrate membrane surface
via plasma polymerization of octafluorocyclobutane (OFCB) and
vinyltrimethylsilicon/carbon tetrafluoride (VTMS/CF4 ). The membrane was tested in a DCMD system using a 0.3–0.5 M NaCl solution
as feed. A water flux of 32.0 kg/m2 h was observed at a feed and
cold distillate temperature of 70/25 ◦ C respectively. Unfortunately,
the membrane showed a salt rejection of only 92.1%, indicating
the salt transfer across the membrane. They studied the plasma
glow discharge time on the membrane performance and found
that longer polymerization time yielded membrane with higher
salt rejection up to 99% but the flux decreased significantly possibly due to that longer reaction time led to thicker coating layer
and consequently a higher mass transport resistance and eventually a higher salt rejection and a lower flux. The researchers from
Sirkar’s group [8,19–21] have reported in a number of publications
on hydrophobic/hydrophilic hollow fiber membranes having a thin
layer of microporous coating of silicon fluoropolymer plasma polymerized on the fiber outer surface. The modified hydrophobic PP
membranes showed significantly high water flux 79 kg/(m2 h) at
90 ◦ C in a cross flow module [21]. It should be noted that all these
surface modification has been mainly focused on the one side of
membrane surface, not in the membrane matrix and the other side
of the membrane.
Plasma surface modification has shown advantages in changing the surface wettability of the materials in the nanometer scale,
without affecting the bulk properties, and has been widely used in
membrane surface modification [22–25]. However, so far, plasma
treatment in membranes has been mostly focused on improving
the hydrophilicity of the membrane for better fouling resistance
[26,27]. Plasma polymerization has been widely used for the surface modification of membranes as mentioned above. CF4 plasma
has been used to improve the membrane hydrophobicity and fluorinated membrane has been tested for gas permeation [28–32] and
for blood compatibility [33], but using CF4 plasma without other
monomers for membrane surface modification has not yet been
widely applied for membrane distillation.
CF4 plasma treatment showed a moderate etching and a strong
fluorination effect which introduced fluorine functional groups in
the material. Therefore CF4 plasma surface modification can be used
to reduce the surface energy, enhance the material surface roughness and make the material surface more hydrophobic. We have
recently conducted CF4 plasma enhanced chemical vapor deposition (CVD) to make superhydrophobic composite resins [34]. It was
found that the process depends heavily on the process parameters. At an optimal condition, a suitable etching and fluorination
yields a superhydrophobic surface. It is believed that fluorination
and deposition of fluorocarbon materials was the main reason
for the wettability change of the surface but was not elucidated
clearly. Based on our previous work on surface modification using
CF4 plasma for polyester resins, we are going to investigate the
use of CF4 plasma modification of hydrophilic base membranes for
membrane distillation. The surface modification process was investigated in order to optimize the treatment conditions by changing
the glow discharge power and treatment time. The surface treatment effects were characterized by contact angle measurements,
liquid entry pressure and X-ray photo electron spectroscopy. The
membrane performance was evaluated in a direct contact membrane distillation of 4% NaCl solutions. The evaporation efficiency
of the membrane distillation process was estimated and compared with literature reports in order to assess the performance
of the membrane and to shed light on the new directions for
165
the preparation of high performance of membrane distillation
membrane.
2. Experimental
2.1. Chemicals
Sodium chloride (NaCl, AG grade) was supplied by Nanjing Ningshi Chemical Reagent Co., Ltd. All chemicals were used as received.
Deionized water was used in the direct contact membrane distillation (DCMD) processes.
2.2. Membrane
Permanently hydrophilic polyethersulfone (PES) flat sheet and
hollow fiber membranes were provided by Nanjing Altrateck Co.
Ltd. The thickness of the PES flat membrane was measured with a
micrometer and the average of 8 points is reported. The inner and
outer diameters of PES hollow fiber membrane were determined
via a zoom biological microscope (Nikon YS1000) equipped with
a CCD camera monitored by a computer. The accuracy was within
0.02 mm.
2.3. CF4 plasma surface modification
Plasma treatment was performed on a M4L plasma system provided by PVA TePla Co. Ltd. The membranes, either flat-sheet or
hollow fibers, were placed on the plate of the plasma chamber. The
chamber was evacuated to 50 mTorr at a speed of 40 L/min. Then
argon gas was injected at a rate of 100 standard cubic centimeters
(SCCM) with a glow discharge at 45 W for 30 s. The system was
evacuated to 100 mTorr. Then CF4 was injected at a speed of 18
SCCM till the pressure of 200 mTorr.
Radio frequency (RF) glow discharge power was set in the range
of 50–400 W and the treatment time was 5–40 min. Thereafter, the
system was evacuated again to a pressure below 100 mTorr and
N2 was introduced into the chamber to atmosphere pressure for
15 min. The membranes were taken out and preserved for further
investigation.
2.4. Instrumentation
X-ray photoelectron spectroscopy (K-alpha X-ray photoelectron spectroscopy, Thermo Fisher). Surface survey data was
collected followed by high resolution scans over C1s (278–298 eV),
O1s (525–545 eV), S2s (222–242 eV), N1s (392–402 eV) and F1s
(675–695 eV). Peak areas were calculated using Gaussian fit program. Relative peak area ratios were calculated according to
published results.
Liquid entry pressure (LEPw) membrane porosity was measured
by weight difference at wet and dry state. After the membrane sample wetted by ethanol, samples were immersed in water to replace
the ethanol within the membrane pores. Wet weight was measured
by wiping off surface water on the membrane and the dry weight
was measured after membrane was completely dried in an oven at
65 ◦ C. The membrane porosity was obtained by:
ε=
mwet − mdry
H2 O Vmem
× 100
(1)
in which ε is the membrane porosity, mwet and mdry are the wet and
dry weight of the sample, H2 O is the density of water and Vmem is
the total membrane volume.
Water contact angles (CA) were evaluated by means of a contact
angle goniometer (Maist DropMeter A-100P) at room temperature
by deposition of a 5 ␮L water droplet on the membrane surface.
Each reported value was calculated by averaging six measurements
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X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
at different points on the sample. The CA of hollow fiber membrane
inner surface were measured according to literature [35]. Part of
the hollow fiber was encased in a clear cylinder, and high vacuum
grease was used to seal the clearance between the fiber and the
cylinder. The fiber was slowly submerged into the water and the
height difference (h) between the top of the fiber and water surface was recorded upon water protrusion from the top of the fiber.
The experiments were repeated four times. The contact angles are
determined from:
hgd i
= cos−1 −
4l
(2)
where is water density, h is the height between the water protrusion and water surface in the beaker and g is acceleration due
to gravity, di is the inner diameter of the fiber, and l is the surface
tension of water.
Scanning electron microscopy samples were prepared by cryogenic breaking. The samples were allowed to dry under vacuum at
30 ◦ C overnight and then coated with a thin layer of gold. For low
magnifications, HITACH TM100 was used for analysis. For photos at
high magnifications, field emission scanning electron microscopy
(FESEM HITACHI JapanS-4800) was utilized for analysis.
Pore size analysis was conducted with capillary flow porometry
(Model CFP-1100-A, supplied by Porous Material Inc.). The membrane was pre-wetted with commercial low surface tension liquid
Porewick (surface tension of 16 dyn/cm based on the supplier’s
datasheet). After mounting the sample onto the test cell, the measurement was managed with a program consisting of wet-run and
dry-run. The wet-run was realized by replacing the wetting liquid
within certain pore size by compressed air at certain pressure till
the membrane was dried out (wet-run). Then the air flow rate of
the membrane was tested by decreasing the air pressure (dry-run).
Membrane pore size was determined by commercial software
from PMI based on Young–Laplace equation:
p =
2 cos r
(3)
where p, , , r are the pressure difference, surface tension, contact angle and membrane pore size, respectively.
In case that the membrane pore size was out of the measurement
range of PMI, bovine serum albumin (BSA) rejection measurement
was used for pore size characterization [36]. For BSA rejection measurement, a cross-flow setup was employed. The concentration of
BSA was monitored by UV photospectrometry (UV-2802, UNICO
(Shanghai) Instruments Co., Ltd.) and determined according to the
measured standard curve.
Fig. 1. Schematic of direct contact membrane distillation process: (1) test cell; (2)
digital thermometer; (3) flowmeter; (4) circulation pump; (5) distillate tank with
thermostatic jacket; (6) feed tank with thermostatic jacket; (7) overflow collector.
The salt rejection rate R was determined according to the following equation:
R=
Cf − Cp
× 100
Cf
(5)
where Cf , Cp representing the NaCl concentration of the feed and the
permeate, respectively. Mettler-Toledo FE30 conductivity meter
was used for monitoring the conductivity of the feed and distillate.
A conductivity and salt concentration standard curve was established by plotting the conductivities of a series of solutions against
their salt concentrations. The concentrations of the experimental
solutions were then determined according to the plot.
For the flat-sheet membrane, both feed and distillate flow rates
were kept constant at 0.36 m/s. For the hollow fiber membrane
module, feed flows inside of the fiber at a rate in the range of
0.5–3.0 m/s, and the cold water circulates outside of the hollow
fiber within the range of 0.17–1.36 m/s. The inlet temperature at
the cold side was kept at 20.0 ± 3 ◦ C and the hot water temperature was varied within 40.0–75.0 ◦ C. To keep the feed concentration
constant, additional deionized water was added to the feed side
according to the amount of water that has been transferred
across the membrane. Stability of the hollow fiber membranes
performance was tested for 6 h/day and for 9 days continuously.
2.5. DCMD experiments
Fig. 1 schematically illustrates the direct contact membrane
distillation process. The membrane was placed in the membrane
module as shown in Fig. 2. The feed solution and distillate (deionized water) were circulated concurrently. The inlet and outlet
temperatures of the feed and distillate were read out with digital
thermometer (calibrated with a standard mercury thermometer).
Water transferred across the membrane from feed to distillate was
collected as an overflow. Water flux was calculated according to
FMD =
V
A·t
(4)
With FMD , V, A, and t representing the water flux, L/m2 h;
amount of permeate (L or kg), effective membrane surface area
(m2 ), and the time duration (h), respectively.
Fig. 2. Schemes of the flat and hollow fiber membrane modules with the dimensions
of the test cell.
X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
100
100
80
80
CA(o)
120
CA(o)
120
60
Top
Bottom
40
167
60
Top
Bottom
40
20
20
0
0
0
50
100
150
200
250
300
350
400
450
0
CF4 plasma power(W)
Fig. 3. Water contact angles of top and bottom surface of modified PES flatsheet
membrane as a function of the CF4 treatment power. The pretreatment was using
argon plasma at a glow discharge power of 45 W with a duration of 30 s (Ar 45 W
30 s), and the CF4 treatment time was 10 min.
Between each operation, the membrane module was kept in the
setup.
3. Results and discussion
A flat sheet polyethersulfone (PES) membrane was used for optimization of the plasma treatment process. The PES membrane was
hydrophilic with a static water contact angle of 60.0 ± 2.0◦ and 0◦
at the top and bottom surface, respectively. It was mechanically
robust and its water flux was 1070 LMHBar. The membrane showed
no water transfer when applied directly in a DCMD process. In order
to use this membrane in MD, surface modification with CF4 plasma
treatment was carried out.
3.1. Plasma treatment
Plasma treatment effect is strongly dependent on the glow discharge power and treatment duration. In order to optimize the
treatment condition, a series of experiments were carried out with
respect to these two issues, respectively. The pretreatment was
realized with argon plasma. The purpose of pretreatment with
argon was to make the membrane free of dust particles and ready
for further treatment. The condition was optimized by varying the
treatment time and glow discharge power. It was found that at
45 W at 30 s the pretreatment was sufficient. Other plasma gases,
for example O2 , should not be used because oxygen atom can be
inserted in the surface, which is not favorable for the creation of
hydrophobic surface.
The surface after pretreatment was then exposed to CF4 plasma
and the influence of the glow discharge power of CF4 plasma on the
membrane water contact angle was investigated with treatment
duration set for 10 min. As shown in Fig. 3, the original membrane
showed a water contact angle of 60.0 ± 2.0/0◦ at the top/bottom
surfaces. After treated with the CF4 plasma, at 25 W for 10 min,
the top surface contact angle increased slightly to 71.0 ± 2.0◦ while
the CA of bottom surface remained unchanged. At 50 W, the CA
of both surfaces increased significantly to 115.0 ± 2.0◦ . At 100 W,
the CA of the surfaces increased slightly to 117.0 ± 2.0◦ /119.0 ± 2.0◦
for top/bottom surfaces. Further increment of the treatment power
resulted in very slight change in the contact angle indicating the
surface modification has reached saturation. We chose 200 W for
further surface modification.
The influence of the CF4 plasma treatment time at 200 W on the
membrane contact angle was shown in Fig. 4. Similar to the trend
in glow discharge power effect, the water contact angle change
10
20
30
40
CF4 plasma treat time(min)
Fig. 4. Water contact angles of top and bottom surfaces of modified PES flatsheet
membrane as a function of CF4 treatment. The pretreatment was argon 45 W 30 s,
and the CF4 treatment glow discharge power was 200 W.
showed initially significant increase within short time and thereafter a level-off. At 5 min of treatment, the top surface contact angle
increased from 62.0◦ to 113.0◦ and the bottom from 0◦ to 118.0◦ .
It appears that from 5 min to 40 min, the contact angle at the top
surface increased slightly from 113.0◦ to 125.0◦ , and the contact
angle of the bottom surface increased from 118.0◦ to 124.0◦ .
Water contact angle is a surface property related to the surface composition, roughness and surface porosity [37]. In general, a
rough and highly porous surface normally shows a high water contact angle. Scanning electron microscopy (SEM) images of the PES
membrane structure are shown in Fig. 5. The membrane is asymmetric in the cross section (Fig. 5a) with a dense skin layer (Fig. 5b)
and a porous support (Fig. 5c). Small pores of 20–30 nm are observable from the top surface (Fig. 5d) while the bottom surface shows
much larger pores in the range of 1.0–4.0 ␮m, in agreement with
the cross section observation. The asymmetric structure might be
the reason for the difference in the water contact angles between
the top and bottom surface both before and after plasma modification [37]. To guarantee sufficient surface modification, we have
chosen 200 W 30 min for CF4 glow discharge and 45 W 30 s for argon
pretreatment.
The characteristics of PES membranes before and after plasma
treatment are listed in Table 1. The membrane thickness, porosity,
gas permeability remained unchanged. Pore size measurement by
PMI failed to give exact bubble point due to the low limit in the set
pressure, however, indicating that the bubble pore size is below
70 nm, which agrees to the SEM observations with the presence of
pores of 20–30 nm (Fig. 5d). The modification is more pronounced
in the change of liquid entry pressure of water (LEPw). The LEPw of
the PES before and after treatment were approximately 0.1 bar and
3.7 bar, respectively.
LEPw is an indication of the ability of a hydrophobic membrane against wetting in the MD process. If the LEPw is low, water
Table 1
Characteristics of PES membrane before and after surface treatment.
Properties
Units
Before
After
Membrane thickness
Porosity
Average pore size
Surface pore size
CA (t/b)
LEPw
Elongation at break
Mechanical strength
PWP
␮m
%
nm
nm
deg
bar
%
N
kg/m2 h bar
201 ± 14
79.2 ± 1.8
<70
40
58.0/0
0.1
19.7 ± 3.1
3.0 ± 0.8
1070
201 ± 14
79.2 ± 1.8
<70
40
124.0/125.0
3.7 ± 0.1
13.6 ± 2.3
2.8 ± 0.2
–
168
X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
Fig. 5. SEM images of PES flatsheet membranes. (a) Cross-section at 500×; (b) cross-section of the top layer at 5000×; (c) cross-section of the bottom layer at 5000×; (d) top
surface at 30K×; (e) bottom surface at 30K×.
could be pressed easily inside the pore of the membrane leading to
pore wetting and possibly solute leakage. Therefore, a membrane
with a higher LEPw is expected to perform better than that with a
lower one. Based on the LEPw and contact angle changes of the PES
membrane, one may conclude that the hydrophilic membrane has
indeed been transformed into a hydrophobic one.
It is noted that the mechanical property of the membrane
changed. In literature, when surface modification was carried out
on membrane, the mechanical properties for example, mechanical
strength is normally strengthened [17,18]. However, the mechanical strength decreased in our case. In order to elucidate the surface
modification mechanism, we have carried out X-ray photoelectron
spectroscopy (XPS).
3.1.1. Plasma modification mechanism
Plasma modification is a rather complicated process including
etching, atomic insertion, deposition and polymerization. From our
former research [34] and published literatures [25,38], the main
effects of CF4 plasma treatment are etching and F atom insertion,
and possibly deposition depending on the operation conditions.
XPS is always used to assess the composition of a surface and moreover can be used to analyze the bonding status of an atom. XPS
survey scans (Fig. 6a and b) revealed that before surface modification, C is the major component and after surface modification,
F becomes dominant. The atomic concentrations of different elements are listed in Table 2. After surface modification, the carbon
atomic concentration decreased drastically from 74.0% to 43.4% and
F becomes dominant with 50.8% abundance. The F/C ratio increased
from 0 to 1.2 indicating that CF4 plasma treatment has brought a
fluorinated layer to the surface, which is responsible for the wettability change of the membrane
Carbon XPS spectra (with simulation) before and after modification are shown in Fig. 6c and d, respectively. It can be seen that
the before modification, C atoms were mainly in the form of C–C,
C–H status. After surface modification, carbon atoms were present
as CF2 –CF2 , C–F, CF3 , among which CF2 –CF2 accounts for the highest percentage. Based on this analysis we envisioned the following
scenario for the process of the CF4 plasma modification as depicted
in Fig. 7.
A plasma gas is overall neutral but is in fact a mixture of cations,
anions and radicals. These particle species are highly active and
energetic and thus react drastically when collide into any substances on their way. As such, fluorine elements are introduced
to the membrane surface leading to a hydrophobic membrane.
Furthermore, the gaseous species penetrates into the membrane
pores or channels inside the porous structure of the membrane
(as depicted in Fig. 7), and thereby modifies the interior of the
membrane, which helps to avoid pore wetting during membrane
distillation. In some cases, when CF3 species is inserted in the
Table 2
PES membrane top surface composition before and after modification (at.%).
Sample
C1s
O1s
S2p
N1s
Cl2p
F1s
O/C
F/C
Original
Modified
74.0
43.4
17.5
4.8
7.0
1.0
1
0.0
0.9
0.00
0.0
50.8
0.2
0.1
0.0
1.2
X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
169
1.0M
C1s
(a)
250.0k
(b)
F1s
800.0k
Intensity (a.u.)
Intensity (a.u.)
200.0k
O1s
150.0k
100.0k
S2p
50.0k
600.0k
400.0k
N1s
200.0k
C1s
N1s O1s
S2p
0.0
0
200
400
600
800
1000
1200
0.0
1400
0
200
Bonding Engergy/eV
40k
(c)
C-C/C-H
400
600
800
18.0k
C1s
Intensity (a.u.)
Intensity (a.u.)
1200
1400
C1s
(d)
CF2-CF2
15.0k
30k
20k
10k
1000
Bonding Engergy/eV
C-N
12.0k
C-C/C-H
CH2-CF2
9.0k
6.0k
3.0k
C-F
CF3
N-C=O
pi-pi*
0
0.0
280 282 284 286 288 290 292 294 296 298
Bonding energy/eV
280 282 284 286 288 290 292 294 296 298
Bonding energy/eV
Fig. 6. XPS survey scans of the PES flatsheet membranes (a, b) and carbon spectra before and after surface modification: (a) original surface; (b) surface modified membrane
surface; (c) carbon spectra before surface modification; (d) carbon spectra after surface modification (with simulations).
polymer chain, the C–C bond is broken and the polymer chain
becomes shorter, which might explain the lower mechanical
strength after surface modification.
We used atomic force microscopy (AFM) to characterize the
PES membrane before and after surface modification as shown in
Fig. 8. Tapping mode AFM height images showed granular particulate structures for both before and after surface modification.
The membrane pores were assigned to the cavities among the
granular structure. The average grain size before surface modification was 51 ± 5 nm, and 65 ± 5 nm after surface modification.
The growth of the grain size was ascribed to the deposition of
the fluorinated layer. By the grain size analysis, we estimated
that a fluorinated layer of less than 10 nm was deposited on the
PES membrane, which helps to the clear the surface modification
mechanism.
3.1.2. DCMD properties of hydrophobic flat sheet PES membrane
Using a 4 wt% NaCl water solution as feed, the water flux and
separation performance of CF4 plasma treated PES flat sheet membranes were studied. Fig. 9 shows the water flux as a function of feed
water temperature from 47.4 to 74.5 ◦ C. At a feed inlet temperature
of 47.4 ◦ C, the water flux was 12.6 kg/m2 h, when the feed temperature increased to 74.5 ◦ C, the water flux increased to 40.9 kg/m2 h.
Since the membrane thickness was about 200 ␮m, it was expected
that if the membrane thickness was 100 ␮m, the membrane flux
would be much higher. A salt rejection of 99.97% was obtained. It
has been reported, for example, the surface plasma polymerization modified cellulose nitrate membrane showed a water flux of
32.0 kg/m2 h at a feed and cold distillate temperature of 70/25 ◦ C,
respectively with a salt rejection of 92.1% [17,18]. At similar operating condition, our membrane has shown a higher flux with much
higher salt rejection. Probably our membrane is more homogeneous in surface modification and thus less prone to pore wetting
and thereby higher salt rejection.
3.2. PES hollow fiber membrane
With the successful surface modification of flat sheet membrane, we continued surface modification of the PES hollow fiber
membranes due to advantages of higher packing density and wider
availability.
Fig. 7. Schematic illustration of CF4 plasma modification of the porous membranes.
3.2.1. Characteristics of PES hollow fiber membrane
Similar to the flat-sheet PES membrane, the hollow fiber PES
membrane is asymmetric in the cross section with a relatively
dense inner skin and a sponge-like porous support (Fig. 10a–c).
At 5000× magnification, the inner surface (Fig. 10d) appeared
relatively smooth and the outer surface (Fig. 10e) showed open
170
X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
Fig. 8. Tapping mode AFM height images of the PES membrane before (a) and after surface modification. Image size: 1 ␮m × 1 ␮m, Z range: 250 nm.
methods reported in literature [35] with CA of 120◦ /96◦ for
outer/inner surfaces.
2
Flux (kg/m h)
45
30
15
45
60
75
o
Tf ( C)
Fig. 9. Effect of the feed inlet temperature (Tf ) on the water flux in DCMD for surface
modified PES flat membrane. Feed was 4 wt% NaCl water solution, and distillate was
deionized water (18.3 ± 1.9 ◦ C). Flow rate at both feed and distillate was 0.36 m/s.
pores. The inner surface of the hollow fiber membranes shows
much smaller pore size than its flat sheet membrane counterparts.
Table 3 lists the characteristics of the hollow fiber membranes. The
BSA rejection of the membrane was about 40% indicating that the
membrane was a UF membrane with relatively large pores. The
inner and outer diameters of the hollow fiber were measured to be
0.78/1.28 mm, respectively, in agreement with the standard measure of ultrafiltration hollow fiber membranes. Water permeability
of the membrane was 1436 ± 260 kg/m2 h bar, higher than the
flat sheet membrane. After CF4 plasma treatment, the membrane
was changed from hydrophilic to hydrophobic. The water contact
angle of the hollow fiber membrane was monitored according to
Table 3
Characteristics of PES hollow fiber membranes.
Properties
Units
Original
Surface modified
ID/OD
PWP
BSA rejection
LEPw
Inner/outer CA
␮m
kg/m2 h bar
%
bar
degree
780/1280
1436 ± 260
40
NA
0/0
NA
NA
3.1 ± 0.3
96 ± 2/120 ±2
NA: not applicable.
3.2.2. Surface composition analysis
Fig. 11 shows the atomic composition before and after plasma
modification of the hollow fiber membranes. Similar to the flat
sheet membrane, the hollow fiber membrane does not contain any
fluorine atoms before surface modification as expected. At the inner
surface, after surface modification the F/C ratio increased slightly to
0.07 however with a contact angle of 96.4◦ . For the outer surface, the
F/C ratio was 1.09, similar to that in the flat PES membrane and the
CA was 120◦ (Table 4). From this comparison, it can be seen that the
degree of fluorination inside the hollow fiber was less compared to
outer surface possibly due to that the plasma had to diffuse through
the membrane wall to reach the inner surface leading to asymmetric modification of the hollow fiber membrane. It was nevertheless
very interesting to test the membrane distillation performance of
such a membrane.
3.2.3. DCMD properties of hydrophobic PES membrane
The MD performance of the PES hollow fiber was tested using
4% NaCl as a feed and the feed temperature varied from 45.3 to
73.8 ◦ C. The water flux plotted against the feed temperature showed
an exponential increase from 20.4 to 66.7 kg/m2 h against the feed
temperature (Fig. 12). Throughout the experiment, the permeate
conductivities were all below 10 ␮s/cm, and the salt rejections were
as high as 99.97%, indicating a nearly complete rejection of NaCl.
This result was very interesting in that it showed that the asymmetrically surface modified PES hollow fiber has shown a good
MD performance. Moreover, compared to flat sheet membrane, the
water flux of the hollow fiber is about 50% higher, which might
be due to less mass transfer resistance of the base membrane and
possibly improved hydrodynamic conditions.
Fig. 13 shows the effect of the feed NaCl concentration at
63.3 ◦ C on the water flux. Following the increase in the NaCl concentration from 4% to 16%, the water flux declined from 45.4
Table 4
Surface elements analysis of PES hollow fiber membranes.
Sample
C1s
N1s
O1s
F1s
S2p
O/C
F/C
Original-inner
Original-outer
Modified-inner
Modified-outer
80.67
75.94
72.31
44.37
1.64
2.27
2.16
0.00
12.49
16.51
14.95
5.94
0.00
0.00
4.91
48.46
5.19
5.28
5.68
1.23
0.15
0.22
0.21
0.13
0.00
0.00
0.07
1.09
X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
171
Fig. 10. SEM photos of PES hollow fiber membranes. (a) Cross-section at 500×; (b) inner skin layer at 5000×; (c) outer skin layer at 5000×; (d) inner surface at 5000×; (e)
outer surface at 5000×.
to 30.1 kg/m2 h. The reduction of water vapor pressure at high
solute concentrations was probably the main cause for the decrease
in permeation flux. However, with the increase of feed concentration, the concentration polarization might also become more
announced and thus impart the decrease of water flux in the MD
process.
3.2.4. Effect of the flow rates on the DCMD flux
Membrane distillation process combines the mass transfer
along with heat transfer. Both on the feed side and the distillate
side, there exists the temperature polarization and the concentration polarization. The flow rate may alter both thereby affecting the
water flux of the process. Fig. 14 shows the effects of the feed flow
rate in the bore of the membrane on the permeate flux. The cold
distillate was circulating at the shell side. The water flux increases
gradually with the increase of the flow rate of hot NaCl solution.
At higher flow rate temperature polarization could be significantly
suppressed because of the improvement in hydrodynamic conditions and the trans-membrane temperature difference is thus
increased. This result agrees well with literature [39].
Fig. 15 shows the effect of the flow rate of the distillate at the
shell side of the membrane on the permeate flux. The flux increased
slightly when the flow rate of the cold distillate increased from 0.17
to 0.68 m/s. Further increase in the flow rate of the cold distillate did
not give significant improvement in water flux. Estimation of the
Reynold number at the shell side of the membrane indicated that at
the flow rate of 0.34 m/s, the Reynolds was around 1900. At higher
flow rate, the turbulent flow may occur, thus further increase in
the hydrodynamics does not add much to the improvement in the
water flux.
3.2.5. Performance stability
In membrane distillation, it is well-known that the long term
stability of the membrane is a key issue for the practical application of MD. In order to evaluate the performance stability of our
PES hollow fiber membrane, we carried out an experiment using a
NaCl solution as feed at 60.5 ◦ C. The membrane module was tested
in total 54 h for 9 days, 6 h per day. During the test interval, the
membrane module was in contact with both feed and distillate.
Fig. 16 shows the DCMD water flux against time. The flux increased
slightly from 38.8 to 42.1 kg/m2 h in the first two days, and then
declined slowly to 36.4 kg/m2 h. The first increase was due to the
removal of air bubbles adsorbed onto the membrane walls. Later on,
the flux stayed stable around 39 kg/m2 h and after 40 h, a slight drop
was observed to 36.4 kg/m2 h. As for salt rejection, the cold distillate water’s conductivity decreased slightly from 7.81 to 7.45 ␮s/cm
throughout the experiments showing a nearly 100% salt rejection.
Pore wetting has been ascribed as the main reason for salt leakage
and low flux. Thus no salt leakage was observed indicating there
was no membrane pore wetting throughout the whole test indicating that our membrane holds potentials to compete with other
hydrophobic membranes.
172
X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
400.0k
400.0k
(a)
Intensity (a.u.)
Intensity (a.u.)
C1s
300.0k
250.0k
200.0k
O1s
150.0k
0.0
250.0k
O1s
200.0k
150.0k
S2p
0
C1s
300.0k
N1s
100.0k
N1s
100.0k
50.0k
(b)
350.0k
350.0k
S2p
50.0k
200
400
600
800
1000
1200
0.0
1400
0
200
400
600
800
1000
1200
1400
1200
1400
Bonding Engergy/eV
Bonding Engergy/eV
1.2M
160.0k
(c)
120.0k
O1s
100.0k
80.0k
F1s
60.0k
N1s
40.0k
S2p
800.0k
600.0k
400.0k
200.0k
C1s
20.0k
0.0
F1s
C1s
Intensity (a.u.)
Intensity (a.u.)
(d)
1.0M
140.0k
S2p
0.0
0
200
400
600
800
1000
1200
1400
0
200
N1s
400
O1s
600
800
1000
Bonding Engergy/eV
Bonding Engergy/eV
Fig. 11. XPS survey scans of PES hollow fiber membranes before and after CF4 plasma treatment. (a) Original inner surface; (b) original outer surface; (c) modified inner
surface; (d) modified outer surface. Modification parameters: Ar 45 W 30 s, CF4 200 W 30 min.
3.3. Evaporation efficiency of the hollow fiber membranes
Evaporation efficiency (EE) of the MD process is defined as the
ratio between the heat transfer contributing to water flux (latent
heat) and the total feed heat-loss in the module at different temperatures, which is an indication of the efficiency of energy used to
produce condensate [2]. It can be calculated according to [40]
EE =
FMD A Hv
(6)
mf C̄P (tf1 − tf2 )
where FMD , A, Hv , C̄p , mf , tf1 , and tf2 representing the MD flux,
membrane area, latent heat of vaporization, average heat capacity
of water, feed mass flow rate, and feed inlet and outlet temperatures, respectively.
As shown in Fig. 17, EE increased from 56% to 79% along with the
increase of the feed temperature from 47.4 ◦ C to 66.7 ◦ C in a nearly
exponential relationship. From 60 ◦ C on, further increase in the feed
temperature did not lead to increase in the EE. The initial increase in
EE with temperature was probably due to the exponential relationship between the partial vapor pressure and the temperature. The
60
70
55
50
Flux (kg/m .h )
45
2
2
Flux (kg/m .h )
60
50
40
30
40
35
30
25
20
20
45
50
55
60
65
70
75
o
Tf ( C)
Fig. 12. The DCMD flux of the CF4 plasma modified PES hollow fiber membrane
as an effect of the feed temperature. Salt solution: flow rate 2.0 m/s. Cold distillate
water: 17.5 ◦ C, flow rate 0.68 m/s.
0
4
8
12
16
NaCl concentration(wt%)
Fig. 13. The DCMD flux of the CF4 plasma modified PES hollow fiber membrane as
an effect of the feed NaCl concentration. Feed solution. 2.0 m/s, 63.3 ± 0.2 ◦ C. Cold
distillate water: 0.68 m/s, 20 ± 0.5 ◦ C.
X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
173
90
Evaporation Efficiency(%)
45
2
Flux (kg/m .h )
40
35
30
25
80
70
60
50
20
0.5
1.0
1.5
2.0
2.5
3.0
40
-1
Flow rate m.s
60
55
Flux ( kg/m2h )
50
45
40
35
30
25
20
0.4
0.6
0.8
1.0
1.2
1.4
-1
Flow rate m.s
Fig. 15. The DCMD flux of the CF4 plasma modified PES hollow fiber membrane as an
effect of the flow rate of the distillate, 20 ± 0.5 ◦ C at the shell side of the membrane
on the permeate flux. Feed solution NaCl 4%: 63.3 ± 0.2 ◦ C, 2.0 m/s.
50
45
40
Flux (kg/m2h)
50
55
60
65
o
70
75
Feed inlet temperature( C)
Fig. 14. The DCMD flux of the CF4 plasma modified PES hollow fiber membrane as
an effect of the flow rate of the feed solution. Feed NaCl 4% with a temperature of
63.3 ± 0.2 ◦ C at the bore side of the membrane and cold distillate at the shell side
with a temperature of 20 ± 0.5 ◦ C, 0.68 m/s.
0.2
45
35
30
25
20
Fig. 17. Evaporation efficiency of the hollow fiber PES membranes at different feed
inlet temperatures.
permeation flux is proportional to the vapor pressure difference, or
to the vapor pressure of the feed temperature since the distillate
vapor pressure is significantly lower than the feed vapor pressure.
Therefore, the permeation flux shows an exponential relation with
the feed temperature, thus EE against temperature as well. It was
observed that when the feed temperature reached 74.5 ◦ C, the EE
value declined slightly to 76%. At higher feed temperature, the heat
loss at the feed side to environment may become more significant
resulting in lower EE. Zhang et al. [41] prepared a non-woven supported PTFE flat microfiltration membrane with a highest EE of 50%
at the feed inlet temperature of 70 ◦ C. Bonyadi et al. [40] prepared
PVDF hollow fiber membrane with a EE of 58% at the feed inlet temperature of 72 ◦ C. Wang et al. [42] prepared mixed matrix PVDF
membrane by addition of organophilic clay into the membrane
dope solution with the highest EE of 50% at the feed inlet temperature of 80 ◦ C. In contrast, our membrane showed much higher
EE than literature reports. The difference might be related to that
the base membrane is hydrophilic instead of hydrophobic. It is well
known that hydrophilic materials have lower thermal conductivity
than their hydrophobic counterparts. When used in membrane distillation, the low thermal conductivity of the base membrane may
show higher evaporation efficiency due to less temperature polarization. However, the exact causes need further investigation and
confirmation.
Recently, very high thermal efficiency or evaporation efficiency
values in the range between 70 and 85% were reported by Sirkar’s
group [21] using a cascade of cross-flow hollow fiber membrane
distillation device integrated with a heat exchanger. The evaporation efficiency values reported in this paper are quite close to what
has been observed in literature. Thus, application of the membranes
in this work might achieve at least comparable thermal efficiency
or evaporation efficiency by adopt of similar cross-flow distillation
device with a heat exchanger.
15
3.4. Performance comparison with literature results
10
5
0
0
5
10
15
20
25
30
35
40
45
50
55
Time(hr)
Fig. 16. Performance stability of the surface modified PES hollow fiber membrane.
Hot NaCl solution: 4 wt% NaCl, 60.5 ± 0.2 ◦ C, 2.0 m/s. Cold distillate water: 20 ± 0.5 ◦ C,
0.68 m/s.
Hydrophilic asymmetric PES flat and hollow fiber membranes
were surface modified by CF4 plasma treatment. Although the
present PES membranes, both flat and hollow fiber membranes,
had pore size significantly smaller than those of conventional MD
membranes, our membranes showed a quite high water flux up
to 66.7 kg/m2 h at a feed temperature of 73.8 ◦ C as seen in Fig. 12.
Table 5 lists the performance of 9 different hollow fiber membranes
used in DCMD for various separation purposes. Scientifically, it
174
X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
Table 5
Comparison of the performance in DCMD of the hollow fiber membranes in the literatures and the present work.
Membrane
PP Accurel® S6/2*
Modified PP MXFR 3*
PVDF
PVDF-2
PVDF/PAN dual-layer
PVDF HF with nanoscale pores
Clay particle PVDF hollow fiber
PVDF*
Surface modified PES hollow fibera
ID/OD
(mm)
1.8/2.6
0.63/0.33
0.9/1.2
0.97/1.4
0.52/1.2
1.0/1.38
0.86/1.1
0.92/1.47
0.78/1.2
Pore size
(␮m)
0.22
0.2
0.25
NA
0.41
NA
0.44
0.19–0.42
<0.070
Porosity
Feed
0.73
0.65
0.75
–
0.8
0.90
0.80
0.80–0.85
0.79
Distillate
Salt conc.
tf (◦ C)
Shell /Lumen
tp (◦ C)
Tap water (∼650 ␮s/cm)
1 wt%NaCl
5 mg/L fluoride
1000 ppm NaCl
3.5 wt% NaCl
3.5 wt% NaCl
3.5 wt% NaCl
3.5 wt% NaCl
4 wt% NaCl
90
90
80
85
90
81.3
86
70
73.8
Shell
Lumen
Shell
Shell
Lumen
Lumen
Lumen
Lumen
Shell
20
15–17
20
20
16.5
17.5
20.5
25
20
Flux
(kg/m2 h)
Ref
33.3
78.8
35.6
37.3
55.2
79.2 ± 1.2
70.1
67
66.7 ± 4.9
[44]
[8]
[45]
[46]
[47]
[42]
[40]
[48]
This study
Note: NA, not available.
a
Maximal pore size, others are mean pore size.
is difficult to make a solid conclusion due to variation in membrane dimension, feed temperature/feed concentration, flow rate
in the bore side and the shell side and most notably the membrane module design. Nevertheless, the present plasma modified
PES membrane showed relatively high water flux. It should be
note worthy that most of the existing surface modified membranes
focused on mainly the top surface or surfaces. In our case, the
whole membranes were attempted to be modified, which made
it less prone to pore wetting and may be more fouling, scaling
resistant.
Moreover, according to Li and Sirkar [8], the surface modified
PP membrane showed a flux of 78.8 kg/m2 h at a feed temperature of 90 ◦ C in a cross flow configuration, which helped to reduce
the temperature polarization. Our membranes were operated in a
concurrent flow fashion. At optimized conditions, our membrane
should give higher water flux, which is currently under investigation.
In addition, both our membrane and the PP membranes are
based on commercial products. The systematic research work on
the PP membranes at pilot scale indicates that the present membranes are promising in future desalination applications. Most
of the PVDF membranes did not show high flux. Yet, very high
water flux was obtained by blending PVDF with clay and utilization of delamination technology to form a relatively thin-wall
hollow fiber membrane [42]. The water flux of our PES membrane is comparable to the PVDF membrane at a same feed
temperature.
The water vapor flux in DCMD depends on membrane porosity, and water vapor pressure at the feed-side pore mouth
which is strongly affected by temperature polarization in the
feed side thermal boundary layer, membrane thickness, pore
connectivity and tortuosity, thermal conductivity of the membrane material and the distillate side water vapor pressure.
Under similar working conditions, high porosity, pore connectivity and low thermal conductivity may contribute most to the good
performance.
The membrane thermal conductivity km is generally used to
evaluate the membrane conductivity and is given by [43].
km = (1 − ε)ks + εkg
(7)
where ε is the membrane void fraction (in this case using porosity),
and ks and kg are the thermal conductivities of the solid membrane
material and of the vapor/air within the pores, respectively. For
instance, the thermal conductivity of Millipore PVDF (GVHP) membrane was 0.0858 W m−1 K−1 . Wang et al. [42] reported that the
thermal conductivity of a mixed matrix hollow fiber PVDF membrane with a porosity as high as 0.9 was about 0.04–0.05 W m−1 K−1 .
Calculation of thermal conductivity of the PES membrane is
0.052 W m−1 K−1 (the porosity = 0.79; thermal conductivity of PES,
Ks (PES) = 0.16 W m−1 K−1 ).
Surprisingly, the estimated thermal conductivity of present PES
membrane was in line with that of the earlier reported PP or PVDF
membranes. This comparison indicates that the thermal conductivity of PES membrane may not be the main cause for the high water
flux in MD process. The membrane porosity of the PES membrane
is relatively higher than some of the membranes, however, is still
at the same level as most of the membranes (as listed in Table 5).
Other parameters, such as pore connectivity, may contribute significantly to the performance of the PES membranes. However,
at this stage, we have no solid scientific proof to confirm this. A
more quantitative measure is worthy of investigation in future
research.
4. Conclusions
Hydrophilic asymmetric PES flat and hollow fiber membranes
were surface modified by CF4 plasma treatment. Results showed
that after plasma modification the membranes become hydrophobic. The plasma modification mechanism was explored and
fluorination was ascribed to the main cause for the converting the
hydrophilic membranes into the hydrophobic ones. DCMD tests of
the membrane demonstrated that both flat and hollow fiber membranes were good membranes materials in membrane distillation
with high water flux and salt rejection. The high salt rejection was
ascribed to the homogeneous surface modification by plasma treatment and thus was less prone to pore wetting and eventually less
salt leakage in the MD processes. The evaporation efficiency of
present MD processes were calculated and compared with literature results. The use of hydrophilic membrane was ascribed to the
high evaporation efficiency in the MD process. Finally a long-term
stability evaluation of the hollow fiber membrane showed a quite
stable water flux and 100% salt rejection. Overall, we have demonstrated a novel approach for converting a hydrophilic ultrafiltration
membrane to a hydrophobic one that has shown satisfactory performance in membrane distillation. Further work is expected to
investigate and the applicability of this process to other materials
and its effect on membrane performance and the scale up processes
for large scale applications.
Acknowledgements
The authors would like to thank the partial financial support from National Natural Science Fund China (Project nos.
20976083, 21176119), the National Key Basic Research Program of China (973 Program) (Project nos. 2012CB932800(TH),
2012CB720903(XML), 2009CB623402). China-Israel Joint Research
Program from MOST.
X. Wei et al. / Journal of Membrane Science 407–408 (2012) 164–175
List of symbols
A
Cp
Cf
EE
FMD
Hv
mdry
mf
mwet
p
r
t
tf1
tf2
V
C̄p
ε
H2 O
effective membrane surface area (m2 )
salt concentration in distillate (g/L)
salt concentration in the feed (g/L)
evaporation efficiency (%)
water flux (L/m2 h)
latent heat of vaporization (kJ/kg)
dry weight (kg)
feed mass flow rate (kg/h)
wet weight (kg)
pressure difference (Pa)
membrane pore size (m)
time of the experiment (h)
feed inlet temperatures (K)
feed outlet temperatures (K)
amount of water permeate through the membrane
during experiment (L or kg)
average heat capacity of water (J/kg K)
membrane porosity
density of water (kg/m3 )
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