Journal of Membrane Science 486 (2015) 97–105
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Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
Surface modiﬁcation of SWRO membranes using hydroxyl poly
(oxyethylene) methacrylate and zwitterionic
carboxylated polyethyleneimine
Hyoungwoo Choi a, Yongdoo Jung b, Sungsoo Han c, Taemoon Tak a, Young-Nam Kwon d,n
a
Department of Biosystems and Biomaterials Science and Engineering, Seoul National University, Seoul 151-921, Republic of Korea
Toray Chemical Korea, R&D Center, Gumi 730-707, Republic of Korea
c
Samsung Advanced Institute of Technology, Suwon 443-803, Republic of Korea
d
School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea
b
art ic l e i nf o
a b s t r a c t
Article history:
Received 16 December 2014
Received in revised form
12 March 2015
Accepted 16 March 2015
Available online 24 March 2015
Comb-like hydroxyl poly(oxyethylene) methacrylate homopolymer (HPOEM) and zwitterionic carboxylated polyethyeleneimine (carboxylated PEI) were synthesized and applied to the surface of commercially available seawater reverse osmosis (SWRO) membranes to introduce fouling resistance. The
successful synthesis of the polymers and modiﬁcation of the membrane surface were conﬁrmed by X-ray
photoelectron spectroscopy, zeta potential and contact angle measurements. Fouling test of the surfacemodiﬁed membranes with alginate and bovine serum albumin showed opposite behavior at high salt
concentrations: a salting-out effect on the HPOEM-coated membrane and a salting-in effect on the
carboxylated PEI-coated membrane. HPOEM-coated SWRO membranes showed better fouling resistance
under brackish conditions. However, the zwitterionic carboxylated PEI-coated membrane showed higher
afﬁnity to sodium chloride solution than deionized water, and presented an inhibitory effect to foulant
adsorption under seawater conditions. This indicates that an effective fouling resistant layer specialized
for seawater ﬁltration could be prepared with zwitterionic materials.
& 2015 Elsevier B.V. All rights reserved.
Keywords:
Seawater reverse osmosis
Membrane fouling
Surface modiﬁcation
Amphiphilic polymer
Zwitterionic polymer
1. Introduction
Reverse osmosis (RO) membrane technology has been regarded
as one of the most economic processes to separate dissolved salts
from solvent, and thus has been applied to a wide range of processes
including the puriﬁcation of brackish water, reclamation of wastewater, and desalination of seawater. However, the wide application of
RO technology has sometimes been limited due to the problem of
membrane fouling. Fouling is the attachment or adsorption of rejected particles or molecules onto the surface or within the pores of
the membrane, causing decline of water ﬂux. In order to mitigate
membrane fouling, several approaches have been used, such as pretreatment of the feed before membrane ﬁltration, the development
of new types of membranes [1–3], and surface modiﬁcation of
membranes [4–7], as well as effective membrane cleaning [8,9].
Surface modiﬁcation methods to enhance the antifouling characteristics include changes to the surface roughness, hydrophilicity,
and charge, without worsening the perm-selective properties. Polyethylene glycol (PEG) has widely been used for the preparation of
n
Corresponding author. Tel.: þ 82 52 217 2810.
E-mail address: [email protected] (Y.-N. Kwon).
http://dx.doi.org/10.1016/j.memsci.2015.03.040
0376-7388/& 2015 Elsevier B.V. All rights reserved.
fouling-resistant membranes [10–12]. Previous studies showed
surface-bound long-chain hydrophilic PEG to be very effective in
preventing the adsorption of macromolecules such as protein onto
the membrane surface due to the steric repulsion mechanism. They
have inherent afﬁnity and a tendency to form a huge complex with
the surrounding water molecules, forming hydrated layers on the
membrane surface in aqueous solution [13]. Although PEG is known
as one of the most effective materials for the preparation of antifouling membrane surfaces to date, it is noteworthy that PEG moieties
have also been frequently used for protein crystallization in combination with some salts [14]. This technique, which is called the “saltingout method”, has been used to separate and concentrate proteins in
mixed solution of high salt concentration.
Zwitterionic polymers (polyampholytes) are polymers possessing
both positively and negatively charged functional groups, such as
polyphosphobetaine, polysulfonbetaine, and polycarboxybetaine
[15]. Ji et al. investigated composite nanoﬁltration membranes containing zwitterions [16]. The results showed that the extent of fouling
was signiﬁcantly reduced, and most of the fouling was reversible during the MgCl2 and protein ﬁltration test. Zhang et al. prepared ultraﬁltration membranes using cardo poly(arylene ether sulfone)s, one
bearing pendant zwitterionic carboxybetaine groups (PES-CB) and
the other bearing pendant sulfobetaine groups. The carboxybetaine
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H. Choi et al. / Journal of Membrane Science 486 (2015) 97–105
poly(arylene ether sulfone) (PES-CB) membrane showed signiﬁcant
resistance to protein adsorption [17]. Most previous studies on
zwitterionic polymers have primarily focused on improvement of
their biocompatibility [18,16,19], while little research has investigated the antifouling properties of zwitterionic materials [17,20].
Earlier researchers discovered that polyzwitterions show unique
behavior in aqueous solution. The solution behavior of polyampholytes is often opposite to that of polyelectrolytes at high salt
concentration, exhibiting the so called antipolyelectrolyte effect
(salting-in behavior). Polyzwitterions swell signiﬁcantly more in
saline solutions than in pure water due to chain expansion in the
presence of low molecular weight electrolytes [21]. Considering the
results of these studies, zwitterionic polymers appeared to be worth
of exploration as antifouling coating material for seawater reverse
osmosis (SWRO) membranes. Such an expanded polyzwitterionic
layer was expected to form an effective steric barrier against membrane fouling in seawater conditions.
In the present study, the antifouling properties of the PEG moiety
(hydroxyl poly(oxythylene) methacrylate homo polymer, HPOEM) and
zwitterionic carboxylated polyethyleneimine (carboxylated PEI) were
systematically investigated. Comb-like HPOEM homopolymer was synthesized via free radical polymerization and applied on a commercial SWRO membrane. A zwitterionic carboxylated PEI coating layer was
introduced on the SWRO membrane using the dip-coating process
followed by cross-linking and N-carboxylation steps. The physicochemical properties of the neat and modiﬁed membranes were characterized using various analytical tools, and the antifouling properties of the
membranes were evaluated by cross-ﬂow ﬁltration tests under treatment conditions of brackish water and seawater.
2. Experimental
2.1. Materials
Seawater reverse osmosis membranes (denoted as SHN) from Toray
Chemical Korea were used as the SWRO control membranes. The SHN
membrane is a thin ﬁlm composite (TFC) SWRO membrane with a
selective polyamide skin-layer polymerized on the interface of a
polysulfone sub-layer supported by non-woven fabric. Hydroxy poly
(oxyethylene) methacrylate (HPOEM, number average molecular
weight (Mn) 360), Azobisbutyronitrile (AIBN), tetrahydrofuran
(THF), petroleum ether, methanol, ethanol, branched polyethyleneimine (PEI, Mw 800), glutaraldehyde solution (GA, grade II,
25%), bromoacetic acid, potassium hydroxide, bovine serum
albumin (BSA), and alginic acid from brown algae were purchased from Aldrich Chemical (Korea). HCl and NaOH were purchased from Daejung Chemicals & Metals Co., Ltd. (Korea). All
chemicals used in this study were of reagent grade and used without further puriﬁcation.
2.2. Preparation of HPOEM coated SWRO membrane
HPOEM homopolymer was synthesized via free radical polymerization, and the HPOEM homopolymer-coated SWRO was prepared
using the previously reported method [22]. Predetermined amounts of
HPOEM (80 g) and AIBN (2 g) were dissolved in THF (20 g) in a
dropping funnel. A 1000 mL 4-neck round bottom ﬂask containing
THF (300 g) was placed in a water bath and preheated to 6671 1C
under nitrogen atmosphere and stirred gently for 30 min. The monomer solution in the dropping funnel was added drop-wise to the reaction mixture for 3 h. Polymerization was allowed to proceed for 12 h
in reﬂuxing conditions. The resulting homopolymer was precipitated
in a mixture solution of methanol/petroleum ether (2:8 v/v), and the
precipitate was dissolved again in ethanol at least three times for
puriﬁcation. After puriﬁcation, the polymer was dried under vacuum
at room temperature for 24 h. The molecular weight of the synthesized HPOEM homopolymer was determined using GPC analysis. The
polymer had Mn (number average molecular weight) of 15,200, an
average molecular weight (Mw) of 38,600, and a polydispersity index
(PDI) of 2.54.
Neat SWRO membranes were rinsed with fresh deionized (DI)
water overnight to remove the moisturizer applied to protect the pores
of the support layer by the manufacturer before modiﬁcation. 0.1 wt%
of HPOEM homopolymer dissolved in DI water was prepared, and the
fresh SWRO membranes were dipped in the homogeneous HPOEM
solution for 1 min. Excess amounts of the solution were gently
removed with a rubber roller. The HPOEM homopolymer-coated SWRO
membranes were then exposed to aqueous GA solution (0.01 wt%) for
Fig. 1. Schematic diagram of the chemical reaction involved in preparation of the PEI-COOH coating layer.
H. Choi et al. / Journal of Membrane Science 486 (2015) 97–105
99
30 s to fasten the coating layer on the membrane surface [4,23]. After
modiﬁcation, the membranes were fully rinsed with DI water to
remove residual chemicals and kept in DI water before experiments.
2.3. Preparation of carboxylted PEI-coated SWRO membrane
The synthetic route for preparation of the zwitterionic PEICOOH coating layer is shown in Fig. 1. Surface modiﬁcation was
conducted in three consecutive steps: PEI coating, PEI networking,
and PEI carboxylation. The SWRO membranes were ﬁrst dipped
into aqueous PEI solution (0.1 wt%). The immersed membranes
were taken out of the solution after 1 min, and excess solution was
removed using a rubber roller. The membranes were then exposed
to aqueous GA solution (0.01 wt%) at pH 9 for 30 s to form a
network structure of the coating material on the membrane
surface. The membranes were taken out of the GA solution and
excess GA solution was removed. Finally, the GA-treated membranes were dipped in aqueous bromoacetic acid (0.1 wt%) in the
presence of potassium hydroxide (0.08 wt%), where they remained
for 10 h at 30 1C. The bromoacetic acid solution was used for the
N-carboxylation of PEI, and the KOH added to the solution was to
remove HBr formed from the reaction of amine and bromoacetic
acid [24]. After treatment, the membranes were fully rinsed with
DI water to remove residual chemicals and kept in DI water before
experiments.
2.4. Membrane surface characterization
The chemical compositions of the membrane surfaces were characterized using X-ray photoelectron spectroscopy (XPS) (SIGMA PROBE,
Thermo VG, U.K). The atomic percent and the atomic percent ratio of
carbon binding energies were calculated by the deconvolution of high
resolution C1s core level spectra. The streaming potentials were measured using a BI-EKA streaming potential analyzer (Brookhaven, NY)
with 1 mM KCl electrolyte solution. Contact angle measurements
were carried out with an FTÅ 200 contact angle analyzer (First Ten
Ångstroms, VA) using the sessile drop technique. A dried membrane
sample was mounted on a glass support on the specimen stage, and
5 μl droplets of DI and NaCl solution (32,000 ppm) were placed on the
membrane surface using a syringe driven by a syringe pump. At least
10 measurements were taken at different locations on the membrane
surface within 30 s after delivering the liquid drop onto the membrane
surface in order to determine the average contact angle value. Atomic
force microscopy (AFM) (XE-150, PSIA, CO) was used to quantify the
roughness of the SWRO membranes. AFM was operated in tapping
mode in air using a silicon nitride cantilever. AFM images of the dried
membranes were recorded over an area of 10 μm 10 μm.
2.5. Fouling experiments
V
A Ut
C permeate
R ð%Þ ¼ 1 100
C f eed
ð1Þ
ð2Þ
where Jw is the pure water ﬂux (L/m2 h), V is the permeate volume (L),
A is the membrane area (m2), t is the time (h), and Cpermeate and Cfeed
are the conductivities of the permeate and feed solutions, respectively. Conductivities were measured using a calibrated conductance
meter (Orion model 115). Normalized ﬂux was deﬁned as the ﬂux of
membranes divided by the initial ﬂux.
BSA and alginate were used as a model protein and a polysaccharide foulant, respectively, to evaluate the anti-fouling properties of the
neat and modiﬁed SWRO membranes. After measuring the initial
water ﬂux and rejection, alginate or BSA stock solution was added to
NaCl solution to reach the total concentration of 30 ppm of alginate or
10 ppm of BSA. The ﬂux decline was then measured for 20 h.
To evaluate the cleaning efﬁciency and fouling resistance of the
membranes, pure water ﬂux of the fouled membranes was measured
again after cleaning. The membrane system was ﬁrst rinsed with DI
water for 30 min, after which the cleaning solution (32,000 ppm NaCl
solution at pH 11) was added. Cleaning of the fouled membrane was
carried out for 30 min with the ﬂow rate of 6 L/min and no applied
pressure (no permeate) [25]. After cleaning, the reservoir and membrane cell were rinsed with DI water to ﬂush out the residual cleaning
solution. The ﬂux recovery test was then performed to investigate the
cleaning efﬁciency of the membranes. Water ﬂux was measured at the
same conditions as the initial ﬁltration test, and the ﬂux recovery (FR)
was calculated according to the following equation:
FR ð%Þ ¼
All ﬁltration experiments were conducted using a cross-ﬂow
ﬁltration system (Taerang Engineering Co., Ltd., Korea) equipped with
membrane cells with the effective area of 30 cm2 (4.0 cm 7.5 cm)
(Fig. 2). As a compaction step, the SWRO membranes were ﬁltrated
using DI water at 900 psi (6.21 Mpa) until the permeate ﬂux was
stabilized. Stock NaCl solution and CaCl2 were added to the feed
solution (2571 1C) and the concentrations were controlled to be
32,000 ppm NaCl and 70 ppm CaCl2 at pH 8.0 in 20 L Milli-Q water.
The performances of the membranes were characterized in terms of
water ﬂux and salt rejection at the operating pressure of 800 psi
(5.52 Mpa) and the ﬂow rate of 4 L/min. Water ﬂux and salt rejection
were obtained using the following equations:
Jw ¼
Fig. 2. Schematic of the reverse osmosis ﬁltration system.
Jw
100
Ji
ð3Þ
where Ji is the initial water ﬂux and Jw is the water ﬂux after membrane
cleaning.
Fouling experiments were performed for another 20 h after the
membrane cleaning step following the same procedure as above.
The antifouling properties of the HPOEM and PEI-COOH-coated
SWRO membranes were evaluated at the treatment conditions of
both brackish water (BW) (2,000 ppm NaCl and 70 ppm CaCl2) and
seawater (SW) (32,000 ppm NaCl and 70 ppm CaCl2). Fouling
experiments in the brackish water condition were conducted with
100 ppm alginate solution (2,000 ppm NaCl and 70 ppm CaCl2, pH
8, 400 psi (2.76 Mpa)). All other conditions and the overall
procedure of the tests were identical to the case of seawater
conditions.
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H. Choi et al. / Journal of Membrane Science 486 (2015) 97–105
3. Results and discussion
3.1. XPS analysis
High resolution XPS spectra for carbon 1s are shown in Fig. 3
for neat, HPOEM-coated, and PEI-COOH-coated SWRO membranes. Peak deconvolution was carried out to investigate the
binding energy shift for carbon 1s and to provide chemical
bonding information. The spectra were deconvoluted into four
symmetric Gaussian peaks with binding energies (BEs) around
285.0 eV assigned to CH species, at 286.0 eV to CO and CN species,
at 288.0 eV to COO species, and at 291.0 eV representing the
aromatic carbon species (π–π* shake-up satellite) [26, 27]. Bonding energies of the peaks were observed at the same position for
each membrane; however, the peak intensity varied.
The ratios of atomic percent of the curve-ﬁtted carbon species
to the total carbon atomic percent were calculated and shown in
Table 1. When comparing the control and HPOEM-coated SWRO
membrane, the ratios of the peaks around 285.0 eV, 288.0 eV and
291.0 eV decreased, while the ratio of the peak at around 286.0 eV
increased from 18.3% to 25.3%. It is thought that introduction of
the PEG moiety in the HPOEM homopolymer to the membrane
surface caused the increase in the atomic percent of CO (at
291.0 eV) and the subsequent decrease of other types of carbon.
The carboxylated PEI-coated SWRO showed decreased peak
intensity at 285.0 eV and 291.0 eV compared with the control,
while showed enhancement of the peak intensities from 18.3% to
24.0% at 286.1 eV and from 12.9% to 14.5%, at 288.3 eV were
observed. The decrease in aromatic carbon content and increase
in C–N, C–O, CQO contents indicated building up of the PEI-COOH
coating layer on the membrane surface after coating.
3.2. AFM
Surface morphologies of the SWRO membranes were characterized using AFM. The three-dimensional AFM images
(10 μm 10 μm) of the membranes before and after modiﬁcation
are shown in Fig. 4. The peak-to-valley distance (Rpv), root mean
square roughness (Rms), and average roughness (Rav) of the
membrane surfaces are listed in Table 2. Typical ridges and valley
structure were observed in all the membranes, but the surface
roughness values decreased slightly after surface coating. The
images of the modiﬁed membranes were almost identical to that
of the unmodiﬁed control membrane. The average roughness
Table 1
Atomic percent and peak ratio of control, HPOEM, and PEI-COOH coated membranes determined from the curve-ﬁtted C1s core-level spectra.
Control
HPOEM
PEI-COOH
Fig. 3. C 1s core level spectra of (a) control SWRO, (b) HPOEM and (c) PEI-COOH
coated membranes.
Peak BE
Atomic percent
Peak ratio (%)
285.02
286.16
288.30
291.21
284.99
286.37
288.26
290.93
285.0
286.15
288.31
291.24
49.28
14.15
9.96
4.03
44.74
18.94
8.56
2.7
43.51
18.23
11.05
3.29
63.7
18.3
12.9
5.2
59.7
25.3
11.4
3.6
57.2
24.0
14.5
4.3
H. Choi et al. / Journal of Membrane Science 486 (2015) 97–105
101
in surface roughness after surface coating was considered to be
attributed to the coating conditions: the size of the coating
material, coating solution concentration, dipping time, reaction
time, etc. The surface roughness of the modiﬁed SWROs is
expected to decrease after further increase in the coating solution
concentration, dipping time and reaction time, as the result of
sufﬁcient coverage of the membrane surface with the coating
materials [28,29].
3.3. Zeta potential
Surface charge of the membranes was investigated using
streaming potential measurement in the pH range from 4 to 10
to conﬁrm successful formation of the coating layers after modiﬁcation (Fig. 5). The control SWRO membranes showed charges of
9.0 mV at pH 4.8 and 30 mV at pH 9.9; however, coating of the
surface with HPOEM made the membrane less negative due to
surface coverage of the negative charge with the neutral PEG
moiety of the HPOEM polymer [30].
The negatively charged control SWRO was more effectively
neutralized after coating with PEI. It was considered that the
positively charged PEI molecules were readily adsorbed on the
membrane surface through electrostatic attraction [31]. The zetapotential of the PEI-coated membrane was 13 mV at pH 4.8 and
18 mV at pH 9.9. After treatment with bromoacetic acid, the
zeta-potential of the carboxylated PEI-coated membrane became
more negative compared with the PEI-coated membrane. The shift
to the more negative zeta potential was thought to be attributed to
the introduction of COO– groups in acetic acid anchored at the
nitrogen molecules of PEI. The carboxylated PEI-coated membrane
showed various charge values depending on the pH of the
solution. This phenomenon was attributed to the protonation
and deprotonation of the amine and carboxylic groups on the
membrane surface, and implied that an amphoteric carboxylated
PEI layer had been successfully introduced after modiﬁcation.#
3.4. Contact angle
Fig. 4. AFM images of the membrane active layer. (a) Control SWRO, (b) HPOEM
and (c) PEI-COOH membranes.
Wettability of the membranes was investigated from measurement of the contact angle between the membrane surface and
liquid/vapor interface. The contact angle is a measure of the
tendency of a liquid to wet the solid surface. The higher the contact
angle, the lower the tendency of the liquid to wet the surface and
the lower the afﬁnity of the membrane surface to the liquid.
Table 2
Root mean square roughness (Rms) and peak to valley distance (Rp-v) of control,
HPOEM and PEI-COOH coated SWRO membranes.
Sample
Rp-v
(nm)
Rms
(nm)
Rav
Median
(nm) Ht (nm)
Control
HPOEM
PEI-COOH
742.3 74.42 58.68 209.0
619.6 68.54 55.19 265.6
669.9 73.31 58.14 211.4
Mean
Surface
Ht (nm) area
(μm2)
Prj
area
(μm2)
201.0
260.7
202.3
100.0
100.0
100.0
186.4
184.9
190.9
values of the control SWRO (a), HPOEM-coated SWRO (b) and
carboxylated PEI-coated SWRO (c) membranes were 58.68 nm,
55.19 nm and 58.14 nm, respectively, while the Rms roughness
values were 74.42 nm, 68.54 nm and 73.31 nm for each membrane, respectively. The roughness values of the membranes did
not show distinctive change after surface coating. The little change
Fig. 5. The zeta potentials of SWRO and HPOEM, PEI, and PEI-COOH coated SWRO
membranes.
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H. Choi et al. / Journal of Membrane Science 486 (2015) 97–105
Photographic images and contact angles of water drops in the
presence or absence of salts are presented in Fig. 6.
The control SWRO membrane had almost the same contact
angle when drops of DI and NaCl solution were delivered on the
surface of the membrane. The change of probe liquid from DI
water to NaCl solution increased the average contact angle of the
HPOEM-coated SWRO from 30.61 to 40.21. However, the carboxylated PEI-coated SWRO showed the opposite behavior, with the
average contact angle of 57.81 in the absence of salt declining to
43.01 in the presence of salt. This opposite wettability behavior of
the HPEOM and carboxylated PEI-coated membranes observed in
the presence of salty liquid is likely due to the salting-out and
salting-in effects of polymeric membranes exposed to a liquid.
The PEG moiety in the neutral HPOEM coating layer formed
hydrogen bonds with surrounding water molecules, allowing the
membrane surface to be hydrophilic and wettable [6,7,11,13].
When the salt concentration became high enough to attract the
water molecules interacting with the coating layer, the interactions between the coating material became stronger than the
coating surface–water interaction, causing the surface to be less
soluble at higher salt concentration (salting-out effect) (Fig. 7).
However, carboxylated PEI had both negatively charged carboxyl
groups and positively charged amine groups in its structure. These
oppositely charged groups had strong interactions between them,
exposing hydrophobic hydrocarbon groups on the surface [32].
Fig. 6. Drop contact angle of DI water and 32,000 ppm aq. NaCl solution on
membranes: SWRO membrane with DI water (a) and NaCl (b); HPOEM-coated
membrane with DI water (c) and NaCl (d); and PEI-COOH-coated membrane with
DI water (e) and NaCl (f).
With the addition of small ionic molecules (NaCl), the charged
property of each group was screened and volumetric transition
occurred as swelling of the coating layer (salting-in effect). Baker
and coworkers [33] investigated the swelling properties of ampholytic hydrogels. They observed increase in the swelling of amphorytic hydrogels at appreciable NaCl concentrations (4 0.1 M), and
assumed the swelling behavior to be attributed to the salting-in
effect on ampholytic polymer. The results were agreed well with
our contact angle study.
3.5. Fouling experiments
The fouling propensity of the control SWRO, HPOEM and
carboxylated PEI-coated membranes was investigated using alginate and BSA in both BW and SW conditions.
3.5.1. Membrane fouling by alginate in BW condition
The antifouling effects of the HPOEM and carboxylated PEImodiﬁed membranes were evaluated compared with the control
membrane in BW conditions (Fig. 8). The initial ﬂuxes of the control,
HPOEM, and carboxylated PEI-coated membranes were 32.0 L/m2 h,
28.2 L/m2 h, and 30.3 L/m2 h, respectively. The HPOEM-coated SWRO
membrane lost about 13% of the initial ﬂux after coating. Meanwhile,
the carboxylated PEI-coated membrane showed only a 5% decline of
the ﬂux after surface coating, implying that the additional coating
layer of carboxylated PEI showed lower resistance to water permeability compared to HPOEM. However, the surface-modiﬁed membranes did not outperform the control SWRO membrane during 20 h
of fouling test in the BW conditions. The ﬂux of the control SWRO,
HPOEM and carboxylated PEI-coated membranes declined after 20 h
by about 22.5%, 15.2%, and 20.1%, respectively. The carboxylated PEIcoated membrane showed an almost identical ﬂux decline rate as the
control SWRO. An effect of the carboxylated PEI coating layer on alginate fouling was not observed. However, the HPOEM-coated membrane showed a slower rate of ﬂux decline in normalized ﬂux, indicating that the adsorption of alginate on the membrane surface was
inhibited by the HPOEM layer. In BW conditions, it was concluded
that the HPOEM-coated SWRO membrane showed better fouling resistance.
3.5.2. Membrane fouling by alginate in SW condition
Fig. 9 shows the ﬂux and the normalized ﬂux proﬁles of control and
modiﬁed membranes operated using alginate solution in SW conditions. The initial ﬂuxes (salt rejection) of the control, HPOEM, and
carboxylated PEI-coated membranes were 32.4 L/m2 h (99.2%), 29.8 L/
m2 h (99.3%), and 32.9 L/m2 h (99.1%), respectively. The initial ﬂux of
the HPOEM-coated membrane decreased by 8% compared with the
control membrane, but the carboxylated PEI-coated membrane showed almost the same initial water ﬂux as the control. In addition, the
carboxylated PEI-coated SWRO membrane showed a much slower ﬂux
decline rate and outperformed the others during the test. The degree
of ﬂux decline after 20 h operation was 15.2%, 14.9%, and 9.1% for the
control, HPOEM and carboxylated PEI-coated membrane, respectively.
After 20 h of operation, the membranes were cleaned and a second
run of the fouling experiment was performed. Both of the surface
modiﬁed membranes showed higher cleaning efﬁciency than the
unmodiﬁed membrane. The ﬂux recovery of the control, HPOEM and
PEI-COOH membranes after cleaning was measured as 92.3%, 96.2%
and 98.4%, respectively. Another 20 h of operation after cleaning showed that fouling proceeded on each membrane in the same manner.
The carboxylated PEI-coated membrane showed the slowest ﬂux
decline rate and the highest cleaning efﬁciency.
H. Choi et al. / Journal of Membrane Science 486 (2015) 97–105
103
Fig. 7. Schematic diagram of salting-out effect on HPEOM-coated membrane (a) and salting-in effect on carboxylated PEI-coated membrane (b) in the presence of salts.
Fig. 8. Time-dependent ﬂux and normalized of control and modiﬁed SWRO
membranes during alginate ﬁltration in BW conditions.
3.5.3. Membrane fouling by BSA in SW condition
The ﬂux and normalized ﬂux declines of the membranes during the
BSA fouling test are shown in Fig. 10. The initial ﬂuxes (salt rejection) of
the control, HPOEM and carboxylated PEI-coated membranes were
32.3 L/m2 h (99%), 29.9 L/m2 h (99.2%), and 31.5 L/m2 h (99.3%), respectively. Although the initial ﬂux of the carboxylated PEI-coated SWRO
Fig. 9. Time-dependent ﬂux and normalized ﬂux of control and modiﬁed SWRO
membranes during alginate ﬁltration in SW conditions.
membrane was slightly lower than the control, it was soon compensated for by the slower ﬂux decline rate. The fouling resistance of the
HPOEM-coated membranes was not clearly shown through this experiment. The ﬂux declines of the membranes after 20 h of operation
were 20.7%, 18.7%, and 15.4%, while the ﬂux recoveries after cleaning
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H. Choi et al. / Journal of Membrane Science 486 (2015) 97–105
4. Conclusion
Amphiphilic HPOEM and zwitterionic carboxylated PEI antifouling layers were introduced on a commercially available SWRO
membrane and their performances were investigated in brackish
water and seawater conditions. The successful introduction of each
coating layer was conﬁrmed by measurements of the zeta potential and contact angle. Due to the ultrathin coating layer prepared
under mild modiﬁcation conditions, the modiﬁed membranes
showed little morphological changes. The contact angle of the
modiﬁed membranes shifted differently due to the salting-out and
salting-in behaviors of the coating layers in DI and saline solution.
The zwitterionic carboxylated PEI-coated SWRO membrane, which
presented lower contact angle value with 32,000 ppm of sodium
chloride solution than that with DI water, showed improved
fouling resistance in SW conditions. The results suggested that
effective antifouling SWRO membranes could be prepared with
zwitterionic materials. Further studies are needed to investigate
the antifouling mechanisms of the zwitterionic layer depending on
the salt concentration of feed water and to test the long-term
stability of modiﬁed membranes.
Acknowledgment
This study had been carried by support of Center for Eco-Smart
Water Works System As part of the Eco-Smart Water System Development Project of the Ministry of Environment (No. 2014001080001).
Fig. 10. Time-dependent ﬂux and normalized of control and modiﬁed SWRO
membranes during BSA ﬁltration in SW conditions.
were 92.9%, 94.3% and 95.4% for the control, HPOEM and carboxylated
PEI-coated membranes, respectively.
3.5.4. Fouling resistance of membranes in BW and SW conditions
The HPOEM and carboxylated PEI-coated membranes showed
opposite fouling propensity in BW and SW conditions. In the SW
condition, the carboxylated PEI-coating layer clearly showed fouling
resistance by demonstrating a lower initial ﬂux decline and slower
ﬂux decline rate. Due to these properties along with lower resistance
to water permeation, the carboxylated PEI-coated SWRO membrane
was able to outperform the other membranes during the period of
the fouling experiments. However, the exceptional antifouling properties of the PEG moiety, which have been reported in other studies and
shown in experiments herein for the BW condition, were hardly observed in the SW conditions. The lack of distinguishable decline proﬁles between the control and PEG-containing HPOEM-coated membranes in SW conditions implies that the strategy for the development of fouling resistant SWRO membranes should be differentiated
from that of BWRO membranes.
It is well-known that the physicochemical properties of the membrane surface such as roughness, hydrophilicity, and electrostatic
charge are major factors determining the degree of membrane
fouling [34]. Considering that the most noticeable differences in
surface properties between the HPOEM and carboxylated PEI-coated
membranes were the contact angle and fouling behavior, the fouling
of membrane surfaces was dominantly affected by the salting-in and
salting-out effect of the membrane surface. This ﬁnding suggests that
amphoteric carboxylated PEI, which showed salting-in behavior, was
more suitable as a fouling-resistant material in SW conditions.
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