22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma surface modification of hydrophilic polyethersulfone membranes for air
gap membrane distillation (AGMD)
S. Pedram1,2, F. Arefi-Khonsari1, H.R. Mortaheb2, H. Fakhouri1, S. Mehvari2 and J. Pulpytel1
1
Université Pierre et Marie Curie, Laboratoire Interfaces et Systèmes Electrochimiques, Paris, France
2
Chemistry and Chemical Engineering Research Centre of Iran, Tehran, Iran
Abstract: Present research work is devoted to develop hydrophobic polyethersulfone
(PES) membranes for wastewater treatment. Ultrathin polymer films were prepared in two
ways: plasma enhanced chemical vapor deposition with propane as a precursor and physical
vapor deposition with PTFE target. By combining the enhanced polymer hydrophobicity
and high porosity, static contact angles up to 124 were obtained. The plasma modified
membranes were examined for benzene removal using an air gap contact membrane
distillation process.
Keywords: hydrophobic membrane, air gap membrane distillation, low pressure plasma
enhance chemical vapour depositions, sputtering, PES
1. Introduction
Serious threatens by the presence of chemical
compounds released by industries in environment, makes
it inevitable to use efficient treatment methods for
controlling emissions by these polluting resources.
Among the environmental pollutants, volatile organic
compounds (VOCs) are especially important because of
their destructive effects and their extensive presence in
refinery wastewaters as well as petrochemical industries,
but also rubber, plastics, paper, leather and textiles [1].
Membrane Distillation (MD) is a recent thermallydriven separation process technology in water purification
processes, in which vapour molecules pass through a
porous hydrophobic membrane. The process driving
force is the difference between partial pressures at both
sides of the membrane. Separation of volatile pollutants
by this method is based on their evaporation and transfer
through the hydrophobic porous membrane [2-4].
The main requirements for the MD process are that the
membrane must not be wetted and only vapour and noncondensable gases would pass through its pores. Then, a
hydrophobic microporous membrane meets these
requirements [5]. Polyethersulfune (PES) is a noncrystalline polymer which is suitable for membrane
preparation in terms of cost and processability. However,
the main disadvantage of the polymer for this particular
application is its relative hydrophilicity. Therefore, some
corrective surface treatments are needed to make these
membranes applicable in membrane distillation process.
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 [6].
Surface modification by plasma
polymerization for the formation of a hydrophobic layer
on a hydrophilic base membrane was conducted
accordingly.
P-III-6-57
The aim of this work is to investigate the performance
of treated membranes by physical vapour deposition
(sputtering) as well as plasma assisted chemical vapour
deposition in MD system. In this regards, attempts were
made to deposit fluoropolymer films in an RF magnetron
system, using Ar, as the sputtering gas on a PTFE target.
In parallel, propane plasma has been used by low pressure
plasma to improve the membrane hydrophobicity.
2. Experimental procedure
PES membranes were prepared by phase inversion
technique (100 µm thick) according to reference 7 and in
were used as substrates.
The membranes were placed on central substrate
holder. The chamber was evacuated to 0.4 mbar. The
substrates were etched in a first step in air which was
introduced in the chamber with a radio frequency
13.56 MHz (RF) glow discharge for 2 minutes. Then
propane was injected by using a LF (40 KHz) power
supply generator. Pressure was set in the range of
0.7-1.5 mbar by changing the propane gas flow rate in
range of 9-20 sccm at a constant power of 3.6 W.
In the PVD process, fluorocarbon coatings were
deposited by rf magnetron sputtering at 13.56 MHz with
Teflon target fixed on a cooled magnetron cathode, fixed
at a distance of 120 mm from the substrate. The
sputtering chamber was evacuated to a background
pressure of 10-5 mtorr before admitting the argon gas.
The Teflon films were also deposited also on silicon
wafers for analytical purposes.
The AGMD experiments were carried out using the
experimental set-up presented schematically in Fig. 1.
The feed solution was supplied from the feed tank to the
feed chamber of the membrane module and the retentate
was turned back to the feed tank by a circulation pump.
A cooling liquid (water) was recycled from the cooling
tank to the cooling chamber of the membrane module.
1
190
170
150
CA (°)
The evaporated water molecules at the liquid/membrane
interface crossed the membrane pores and the air gap to
finally condense over the cooling stainless steel metallic
plate. The thickness of the air gap is 6 mm. The inlets
and outlets of the membrane module for both feed
solution and cooling liquid were read out by inserting
thermometers in the isolated openings in each side.
130
110
90
70
50
0.4
0.6
0.8
1
1.2
1.4
1.6
C3H8 Plasma pressure (mbar)
Fig. 2. Water contact angle of modified PES flat sheet
membranes as a function of the C 3 H 8 plasma pressure for
10 min.
R
Fig. 1. Schematic diagram of experimental AGMD setup;
1) feed compartment, 2) cold water compartment, 3)
permeate collection, 4) feed vessel, 5) permeate (cooling)
vessel, 6) paraffin bath, 7) heater, 9) peristaltic pumps,
10) thermometers, 11) sampling valves.
To characterize the surface morphology, scanning
electron microscopy (SEM), attenuated total reflection
infrared spectroscopy (ATR-IR) measurements were
taken from films deposited on silicon substrates. The
contact angles of prepared membranes were measured by
optical contact angle (OCA). An average of five
measurements on different points on the membranes was
reported. The performance tests were done on an air gap
membrane distillation setup, and the results were
compared with those of a commercial PTFE membrane.
3. Results and Discussion
3.1. Plasma polymerization
Plasma treatment effect is strongly dependent on the
glow discharge power and pressure. In order to optimize
the treatment condition, a series of experiments were
carried out to investigate the effect of the propane plasma
pressure on the membrane water contact angle with
treatment duration set for 10 min. As shown in Fig. 2, the
original membrane showed a water contact angle of 70°.
After treated with the C 3 H 8 plasma, at 0.7 mbar for
10 min, the surface contact angle increased slightly to
94°. At 1 mbar, the CA of the surfaces increased to 102,
and at 1.5 mbar, PES polymer became superhydrophobic
with a CA of 165°. In fact, under such conditions the
deposited hydrocarbon was quite thick and partially or
completely plugged the pores of the PES membrane. . The
consequence of the latter was a decrease of water flux in
the membrane distillation process. Therefore, these
superhydrophobic membranes could not be used for water
distillation and VOC removal.
R
2
R
R
R
R
R
Water contact angle is a surface property related to the
surface composition, roughness and surface porosity.
SEM images of the PES membrane structure are
illustrated in Fig. 3. The background pressure in the
chamber is 0.4 mbar, and the propane precursor flow rate
is increased to deposit different thin films. One can note
how the morphology of the deposited thin films from
propane varies, as a function of the pressure. The
increase of the latter, obtained by an increase of the
precursor flow rate gives rise to a higher deposition rate at
1.5 mbar and a rougher surface.
Fig. 3. SEM images of modified PES flat sheet
membranes: a) untreated membrane, b) at 0.7 mbar, c) at
1 mbar, c) at 1.5 mbar.
R
The coatings were also prepared by means of RF
magnetron sputtering of PTFE target using argon as the
working gas. The effect of the sputtering power was
studied on the contact angle of the deposited layer,.
Substrates were treated by changing the applied power
from 50 to 200 W at different deposition time (to reach
similar thicknesses which were determined in-situ by
means of a QCM).
The experimental results for
d = 12 cm, P = 10 mtorr and F = 30 sccm are presented in
the Table 1.
P-III-6-57
Table1. Water contact angle of fluorocarbon plasma
polymer films on the power during sputtering.
Time (min)
Power (W)
Thickness (nm)
CA (°)
120
50
250
110
85
75
300
105
50
100
300
107
30
125
270
108
20
150
290
106
15
200
290
98
FTIR spectroscopy of the PTFE film deposited on
silicon substrate shown in Fig. 4 indicates a fluorocarbon
plasma polymer film deposited on the substrate. It shows
a broad absorption peak of CF x (x = 1,2) group stretching
vibration, positioned between 1000 cm-1 and 1100 cm-1.
This is accompanied by an absorption peak positioned at
1210 cm-1 that has been assigned to CF 2 symmetric
stretching vibration for the film. Furthermore, a band at
about 1725 cm-1 of the C=C stretching vibrations and a
band at 740 cm-1 assigned to deformation vibrations of
CF 3 groups can be observed in the survey spectrum.
0.14
CFx,x=1,2
0.12
transmission (%)
0.10
CF2
0.08
0.06
0.04
0.02
CF3
-C=CF2
0.00
500
1000
1500
2000
2500
3000
3500
4000
4500
wavenumber (cm-1)
Fig. 4. FTIR spectra of the treated silicon substrate.
The separation performance of a membrane for a given
pollutant was defined as (C initial -C remaining )/C initial . The
separation performance of C 3 H 8 plasma treated PES flat
sheet membranes were tested in the AGMD process for
benzene removal from water, using 100 ppm benzene
solution. Feed inlet and condensation surface temperature
were adjusted at 45 °C and 10 °C, respectively. Table.2
shows separation factor through commercial (Sartorius
PTFE filter 0.2 mm, used as comparison) and plasma
modified membranes.
P-III-6-57
Table 2. Permeate flux and separation factor of treated
membranes for benzene removal by AGMD.
Membrane
Plasma
pressure
Separation
factor
Permeate
flux (kg/m2.h)
×102
P
P
P
PTFE
PES
PES
PES
-0.7
0.9
1.2
15
18
22
33
125
110
50
--
One can point out that all modified membranes showed
lower permeate fluxes and higher separation factors in
comparison to the PTFE industrial membrane. These
values were well correlated with the thickness of the
deposited polymer which resulted in a partial or complete
plugging of the pores observed at higher precursor flow
rates and higher pressures as shown by SEM images.
4. Conclusion
Hydrophilic PES flat sheet membranes were surface
modified by PECVD of C 3 H 8 precursor as well as
depositing of a fluorocarbon rich layer by sputtering of a
Teflon target. Both treatments lead to hydrophobic
surfaces with change in the roughness of the coatings.
Thin coatings were required in order to less affect the
permeability of the membranes. Membranes were tested
in AGMD process for benzene removal with a
concentration of 100 ppm in water. Results of the treated
PES membranes demonstrated better performance than
the commercial PTFE ones. However the simple design
of the MD module used which will be improved in our
future studies.
R
R
R
R
5. References
[1] A. Berenjian, N. Chan and H.J. Malmiri. Am. J.
Biochemi. Biotechnol., 8, 220 (2012)
[2] Z. Ding, L. Liu, Z. Li, R. Ma and Z. Yang.
J. Membrane Sci., 286, 93 (2006)
[3] M. Khayet. Desalination, 321, 60 (2013)
[4] I. Gancarz, M. Bryjak, J. Kujawski and J. Wolska.
Mat. Chem. Phys., 151, 233 (2015)
[5] S. Al-Obaidani and E. Curcio. J. Membrane Sci.,
323 85 (2008)
[6] X. Wei, B. Zhao, X. Li, Z. Wanga, B. He, T. He and
B. Jiang. J. Membrane Sci., 407, 164 (2012)
[7] S.S. Shahrabi, H.R. Mortaheb, J. Barzin and
M.R. Ehsani. Desalination, 287, 281 (2012)
3