Journal of Membrane Science 483 (2015) 25–33
Contents lists available at ScienceDirect
Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
Impact of support layer pore size on performance of thin film
composite membranes for forward osmosis
Liwei Huang, Jeffrey R. McCutcheon n
University of Connecticut, Department of Chemical and Biomolecular Engineering, Center for Environmental Sciences and Engineering,
191 Auditorium Rd. Unit 3222, Storrs, CT 06269-3222
art ic l e i nf o
a b s t r a c t
Article history:
Received 29 August 2014
Received in revised form
7 January 2015
Accepted 11 January 2015
Available online 19 January 2015
Previous investigations of forward osmosis (FO) concluded that thin film composite (TFC) membranes
should be designed with hydrophilic supports to help mitigate internal concentration polarization and
improve water flux. A number of research groups and companies around the world have responded to
those findings by developing TFC membranes with hydrophilic supporting materials. However, there has
been few fundamental studies on how hydrophilic support structure affects selective layer formation
and hence membrane performance. Here, a systematic investigation on the influence of support layer
pore size on the osmotic performance of thin film composite membranes is conducted for the first time.
Specifically, TFC membranes were made by interfacial polymerization to form a polyamide selective
layer on top of a series of commercially available nylon 6,6 microfiltration membranes with similar
physical and chemical properties but different pore sizes. The interfacial polymerization process is
affected by the support pore dimensions and the resulting polyamide composite membranes exhibited
varying film morphology, cross-linking degree, mechanical integrity, and permselectivity. Osmotic flux
tests show that the osmotic flux performances (water flux, salt flux and specific salt flux) are dependent
on a permeability-selectivity trade-off which is in part impacted by the pore size of the support layer.
& 2015 Elsevier B.V. All rights reserved.
Keywords:
Forward osmosis
Pressure-retarded osmosis
Thin film composite
Interfacial polymerization
Nylon 6,6
1. Introduction
Forward osmosis (FO) is an emerging platform technology that
that exploits the natural phenomenon of osmosis to address water
and energy scarcity [1–3]. When two solutions of differing concentration are placed on opposite sides of a semi-permeable membrane, an osmotic pressure differential is generated to drive the
permeation of water across the membrane from the dilute solution
(the feed solution) to the concentrated solution (the draw solution).
The immense promise of this technology has been demonstrated in
various applications such as concentrating high-value dissolved
solids [4–6], seawater and brine desalination [7–9], and electric
power generation [10–12]. However, the further advancement and
ultimate commercialization of FO processes has been hindered
by the lack of an appropriately designed membrane. Until very
recently, the only commercially available FO membrane has been
the asymmetric cellulose triacetate (CTA) membrane from Hydration Technology Innovations (HTI, Albany, OR). The hydrophilic
nature of CTA favors osmotic transport, but its susceptibility to
hydrolysis [13], its relatively low water permeance, and low salt
rejection have limited its use to niche applications.
n
Corresponding author. Tel.: þ 1 860 486 4601.
E-mail address: [email protected] (J.R. McCutcheon).
http://dx.doi.org/10.1016/j.memsci.2015.01.025
0376-7388/& 2015 Elsevier B.V. All rights reserved.
Another potential FO membrane platform is the thin film
composite (TFC) membrane. Commonly used in reverse osmosis
(RO) and nanofiltration (NF), conventional TFC membranes are
comprised of an aromatic polyamide thin film formed in-situ on
top of an asymmetric polysulfone (PSu) mid-layer which is itself
cast by phase inversion over a polyester (PET) nonwoven backing
layer. The ultra-thin polyamide selective layer gives superior
permeance and selectivity over conventional asymmetric integrated membranes, such as CTA [14]. Furthermore, TFC membranes are more flexible in their design as both the selective and
support layers can be tailored for specific needs. These membranes, while accelerating the adoption of RO and NF, were never
tailored for use in any FO process and thus have performed poorly
when tested under relevant conditions [15]. Poor performance has
been attributed to thick, dense and hydrophobic support layers
that, while necessary in RO and NF to provide mechanical integrity
under hydraulic pressure, create severe mass transfer resistance
near the interface of the selective thin film layer in FO. This
phenomenon, widely described as internal concentration polarization (ICP) [8,15,16], reduces effective osmotic driving force and
results in poor water flux performance.
To design TFC membranes specifically for FO processes, support
layers must be redesigned to incorporate a combination of
characteristics. Support layers need to be thin, highly porous and
have a low tortuosity. The support layer must also be hydrophilic,
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L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–33
allowing for complete wetting throughout the structure [17]. The
support layer must exhibit excellent chemical and thermal stability while retaining reasonable mechanical strength and be easy to
fabricate at full scale. After all of this, the support layer must
adequately support the selective layer during both formation and
operation and that layer must match the permselectivity of a
conventional RO TFC membrane.
Early efforts in developing high-flux TFC-FO membranes have
been reported with flat-sheet and hollow-fiber platforms. Yip et al.
invented one of the first TFC-FO flat sheet membranes with tailored
support layers consisting of both finger-like and sponge-like pores
[18]. Bui et al. employed a new support fabrication technique known
as electrospinning to develop nanofiber based TFC membranes with
intrinsically low resistance [19]. Wang et al. designed novel hollow
fiber TFC membranes with high water flux [20]. All these studies,
however, focus on support structures but not support chemistry. In
large part, conventional support materials, such as hydrophobic PSu
and polyethersulfone are still used.
Other groups have addressed this issue by considering more
hydrophilic polymers as support materials. For instance, some
have considered sulfonated polysulfone [21] and sulfonated poly
(ether ketone) blends for use in TFC membrane substrates [22].
These studies still require a hydrophobic polymer to be blended to
avoid severe swelling or even dissolution in water, ultimately
making the support somewhat hydrophobic. A chemical modification method has been reported by Arena describing the modification of the PSu support layers of commercial TFC RO membranes
using polydopamine, a novel bio-inspired hydrophilic polymer
[23]. These efforts resulted in dramatically improved water flux
compared to the native membranes, but complete modification of
the support is difficult with the polyamide layer already formed.
More recently, intrinsically hydrophilic polymers have been considered as alternatives to modified hydrophobic materials as new
TFC substrates. FO membranes supported by polyacrylonitrile
(PAN) [24], cellulose triacetate [25] and cellulose acetate propionate [26] phase-inversion films as well as PAN/cellulose acetate
blended nanofibers [27] have been previously reported. None of
these efforts considered support materials fabricated on an industrial scale. Our previous work considered a commercially-available
hydrophilic nylon 6,6 microfiltration (MF) membrane from 3M™
made on a continuous line as a support for a TFC membrane [28].
We found that we could make a high performance TFC with this
unconventional support, but we only considered a single membrane (a 0.1 μm pore size membrane). There are, in fact, several
other membrane pore sizes that are available as well. By using
these membranes, we can control the material and vary the pore
size of the support, giving us a tool for understanding how pore
size impacts polymide formation on a hydrophilic support.
There are only a few studies on the role of support membrane
properties in formation of polyamide composite membranes in
recent years. Singh et al. found that two PSu supports with different
pore dimensions result in TFC membranes with different salt
rejections [29]. Gosh et al. proposed a conceptual model describing
the role of support membrane pore structure and chemistry (i.e.
pore size, porosity, and hydrophilicity) on performance of polyamide
based composite membranes [30]. Tian et al. adopted this model to
explain the permselectivity difference of two TFC membranes
supported by two nanofibers with different fiber diameters [31].
Yet most of these studies are based on hydrophobic polymer
substrates. It is uncertain that the conclusions drawn from these
studies hold for intrinsically hydrophilic supports. In addition, the
supports used in these studies are either lab-scale hand-cast films or
electrospun nanofiber mats. It is difficult to keep consistency when
casting in a lab and a commercial platform is far more consistent.
In this study, we explored the use of a series of commercially
available nylon 6,6 MF membranes as supports for FO TFC
membranes. We found that support layer pore size played an
important role in polyamide properties and osmotic flux performance. These performance differences were not caused by the
support layer itself, but rather differences in polyamide selective
layer formation and mechanical support during operation.
2. Experimental
2.1. Materials
Four types of multi-zoned nylon 6,6 MF membranes with different
pore sizes (STL01, BLA010, BLA020 and BLA045) were provided by 3M
Purification Inc. (Meriden, CT) as the support membranes for the TFC
membranes. The details of membrane structure (i.e. pore size and
porosity) and other characteristics will be discussed in Section 3.1. Mphenylenediamine (MPD) and trimesoyl chloride (TMC) were purchased from Sigma-Aldrich. Hexane, the solvent for TMC, was purchased from Fisher Scientific. Deionized water (DI) obtained from a
Milli-Q ultrapure water purification system (Millipore, Billerica, MA)
was used as the solvent for diamine monomers. Sodium chloride was
purchased from Fisher Scientific. A commercial asymmetric cellulose
triacetate (HTI-CTA) FO membrane (Hydration Technology Innovations
Inc., Albany, OR) was provided for comparison.
2.2. Interfacial polymerization of TFC membrane
Nylon 6,6 MF support membranes were first taped onto a glass
plate with the side supporting the polyamide film facing up. The
plates were then immersed into a 1.0% (w/v) aqueous MPD solution
for 120 s. Excess MPD solution was removed from the support
membrane surface using a rubber roller. The support membranes
were then dipped into a solution of 0.15% (w/v) TMC in hexane for
60 s to form an ultrathin polyamide film. The resulting composite
membranes were air dried for 120 s and subsequently cured in an
air-circulation oven at 80 1C for 5 min for attaining the desired
stability of the formed structure [32]. The TFC polyamide membranes were thoroughly washed and stored in deionized water at
4 1C before testing.
2.3. Support layer characterization
The thickness of the support membranes was measured using a
digital micrometer at 5 different locations for each membrane
sample. A CAM 101 series contact angle goniometer (KSV Company, Linthicum Heights, MD) was used to measure the contact
angle of the support surface. The values were taken as an average
of at least five points with a volume of 10 71 μL.
The support surface porosity was analyzed as a ratio of total pore
area vs. total image area using Image-J software based on top-surface
scanning electron microscopy (SEM) images) using a cold cathode
field emission scanning electron microscope JSM-6335F (JEOL Company, USA). This technique, though widely adopted in previous studies
to quantify surface pore size and porosity of phase-inversion films
[30,33], is challenging for our support membranes as there is no
distinct surface interface. Images therefore are thresholded to distinguish the surface from the bulk. Results might slightly differ if others
were to analyze the same image since the threshold value is user
biased. We do, however, keep the image contrast and threshold value
constant for all SEMs to ensure consistency between samples. Before
imaging, samples were kept overnight in a desiccator and then
sputter coated with a thin layer of platinum to obtain better contrast
and to avoid charge accumulation.
The average porosity (ε) of the substrate was determined by a
gravimetric method which measures the weight of DI water as the
wetting agent contained in membrane pores. The following equation
L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–33
was used to calculate the porosity of the membrane.
ðW wet W dry Þ=ρwater
ε¼
V
ð1Þ
where Wdry is the weight of dry membrane, Wwet is the total weight
of membrane after wetting with water, ρwater is the water density,
and V is the total volume of the sample that can be obtained by
measuring the dimensions. We assume that the hydrophilic structure
fully wets.
2.4. Selective layer characterization
Surface morphology and cross-sectional structure of the TFC
membranes were also imaged with SEM. Samples were prepared
for cross-sectional imaging using a freeze fracture technique
involving liquid nitrogen. A razor blade was submerged into liquid
nitrogen with the sample simultaneously and then used to quickly
cut the sample into half once removed from the liquid nitrogen.
The thickness of the selective layer was measured at 3 different
locations for each membrane and then averaged.
The cross-linking degree of formed polyamide film can be
examined based on the element ratio of O/N [31,34]. The chemical
formula of fully cross-linked polyamide is [C15H10O3N3] in which
the O/N ratio is 1; while the O/N ratio is 2 for the fully linear
structure of [C15H10O4N2]. The relative fractions of cross-linked
portion, m, and linear portion, n, were calculated based on the
following relations:
m þn ¼ 1
ð2Þ
O 3m þ 4n
¼
N 3m þ 2n
ð3Þ
The O/N ratio can be experimentally determined by performing
element composition analysis using X-ray photoelectron spectroscopy (XPS) (Kratos, AXIS 165, UK) with a monochomated Al K-α
source. A normal sample position of 0 degree to detector direction
was applied. A charge neutralization system was used to obtain
high-resolution spectra for the insulating polymers by reducing
the surface charge. Pass energy of 160 eV and 20 eV were used for
survey and high-resolution spectroscopy. No chemical degradation
of the surface membrane was found due to the exposure to X-rays.
All spectroscopy were calibrated to hydrocarbon C 1 s peak at
284.6 eV. Sensitivity factors of C 1s, N 1s and O 1s from the
manufacture were used for quantitative calculation.
2.5. Determination of transport properties in cross-flow reverse
osmosis
A bench-scale cross-flow RO testing unit was used to evaluate
the pure water permeance, A, observed salt rejection, %R, and
solute permeability, B, of the TFC membranes and the HTI control
membrane (HTI-CTA) at 20 71 1C using a method described elsewhere [28]. The system was operated at 150 and 250 psi with a
fixed cross-flow velocity of 0.26 m/s (Re 1200) using DI or a
2000 ppm NaCl feed solution to determine A and %R, respectively.
The determined A and B were used to derive structural parameter
in FO tests.
27
mode (the membrane active layer faces the draw solution) and FO
mode (the membrane active layer faces the feed solution). The
cross-flow velocities were kept at 0.18 m/s for both the feed and
draw sides. Conductivity of the feed was measured to estimate the
reverse salt flux through the membrane.
The osmotic water flux, Jw, was calculated by dividing the
volumetric flux by the membrane area. By measuring the conductivity of the feed solutions at certain times during the tests, the
reverse salt flux, Js, was calculated by dividing the NaCl mass flow
rate by the membrane area. The specific salt flux [35,36], Js/Jw, was
determined as the ratio of the reverse salt flux and the water flux.
The structural parameter (S) was determined by using the following equation [18] where the membrane is orientated in FO mode:
B þ Aπ D;b
D
S¼
ln
ð4Þ
Jw
B þ J w þAπ F;m
In this equation, D is the diffusion coefficient of the draw solute,
Jw is the measured water flux, B is the solute permeability, A is the
pure water permeance, πD,b is the bulk osmotic pressure of the
draw, and πF,m is the osmotic pressure at the membrane surface on
the feed side (0 atm for DI feed).
The tortuosity of the support can be estimated using the
following Equation that defines structural parameter [15].
S¼
tτ
ε
ð5Þ
where t, τ, and ε are the thickness, tortuosity and porosity of the
support, respectively.
3. Results and discussion
3.1. Support layer characterization
All MF substrates share the similar surface morphology
(Fig. 1(a), (c), (e), and (f)) and cross-sectional structure(described
elsewhere [28]). Their other characteristics are listed in Table 1.
Except the STL01, the other MF membranes have an asymmetric
structure consisting of three regions: (1) a large-pore region at the
upstream side of the membrane; (2) a nonwoven scrim used as a
mechanical support to facilitate manufacturing; and (3) a smallpore region on the downstream side of the membrane. The pore
sizes of the upstream and downstream sides of STL01 are the
same. For interfacial polymerization, the membrane was oriented
to have the smallest pores supporting the selective layer to avoid
defects. These four supports are labeled based on their pore size
(Support-0.025, Support-0.1, Suppport-0.2, and Support-0.45). The
overall porosity of the support membranes ranges from approximately 55% to 70%, gradually increasing with the pore size. The
surface porosity, on the other hand, ranges from 46.8% to 54.7%,
lower than the overall porosity but substantially higher than
conventional RO support materials [30]. Note that a higher surface
porosity at the support-selective layer interface might help to
improve the osmotic water flux because the selective layer is not
shadowed by the support. The contact angle of the membrane
surface is measured to be approximately 41–421 for all four
supports, indicating their intrinsic hydrophilicity.
2.6. Evaluation of osmotic water flux and reverse salt flux
3.2. Selective layer characterization
Osmotic water flux and reverse salt flux of polyamide TFC
membranes were evaluated using a custom lab-scale cross-flow
forward osmosis system described in details elsewhere [23]. A
1.5 M sodium chloride solution was used as the draw while DI
water was the feed at a temperature of 20 71 1C. Osmotic flux
tests were carried out with the membrane oriented in both PRO
3.2.1. Scanning electron micrographs
The top surface SEM images for polyamide selective layers built
on different supports are shown in Fig. 1. Defect free films with
typical ridge-and-valley structure were obtained for all TFC membranes. Smoother surfaces are seen in TFC membranes based on
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L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–33
Fig. 1. Top surface SEM images of different supports and corresponding TFC membranes (Magnification 5000 ): (a) Support-0.025; (b) TFC-0.025; (c) Support-0.1;
(d) TFC-0.1; (e) Support-0.2; (f) TFC-0.2; (g) Support-0.45; (h) TFC-0.45.
Table 1
Properties of nylon 6,6 MF supports with different pore sizes.
Average pore size (μm)
Contact angle (degree)b
Thickness (μm)
Surface porosity (%)b
Porosity (%)
Structural parameter (μm)c
Estimated tortuosityd
a
Top zone
Bottom zone
Support-0.025 (STL01)
Support-0.1 (BLA010)a
Support-0.2 (BLA020)
Support-0.45 (BLA045)
0.025
0.025
41.9 7 1.7
142 7 2
46.8 7 0.9
54.2 7 0.5
22107 170
8.4
0.1
0.45
40.5 7 3.1
181 74
51.17 1.9
57.7 70.1
1940 7 240
6.5
0.2
0.65
41.0 7 4.6
1767 2
53.2 7 1.5
66.87 0.1
1220 7 380
4.6
0.45
0.8
40.7 74.7
1817 4
54.7 73.1
70.57 1.0
14007 160
5.5
Support-0.1 data is from [28].
Measured on the small pores zone (i.e. the surface for interfacial polymerization).
c
Structural parameter is determined by fitting the experimental results into Eq. (4).
d
Tortuosity is estimated by dividing structural parameter by the thickness and porosity.
b
L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–33
larger pore size support (i.e. TFC-0.45) than those with smaller
pores (i.e.TFC-0.025).
The cross-sectional SEM images for polyamide TFC membranes
are shown in Fig. 2. The average thickness of the polyamide selective
layers was measured to be approximately 0.10 μm for all four
supports, indicating that the thickness of the selective layer is
independent of the support pore size. Different results have been
observed by Singh et al. that the polyamide coated on smaller pore
size (70 nm) PSu support was thicker than that coated on bigger pore
size (150 nm). This difference could be attributed to the hydrophobicity of the PSu supports. Singh believes the smaller pores of the
hydrophobic PSu support might be resistant to penetration of MPD
aqueous solution, limiting polyamide formation to the surface. Larger
pores favor the penetration of MPD into the pores, leading to
formation of polyamide film inside the pore. This is not the case
for our membranes, though, since MPD aqueous solution easily wet
out the entire nylon 6,6 support structure. In addition, the selective
layer is better supported by the smaller pores than by the larger
pores. The selective layer spanning over the large voids seen on
larger pore size support (i.e. TFC-0.45) is susceptible to failure at high
hydraulic pressure.
3.2.2. Cross-linking degree
The relative atomic concentrations and composition ratios of
the polyamide selective layer are presented in Table 2. All four
TFC membranes exhibited a cross-linking degree between 0 and 1,
implying the presence of a partially cross-linked and partially linear
polyamide structure. With increasing the support pore size from
0.025 μm to 0.45 μm, the resulting polyamide cross-linking degree
decreased from 0.69 to 0.37. In other words, under the same interfacial
polymerization condition, lower selectivity membranes results from
larger pores size supports. This finding indicates that the support
structure not only performs as a mechanical anchor for the selective
layer, but directly impacts the interfacial polymerization processes and
strongly affects the properties of the formed selective layer.
3.3. Polyamide formation mechanism
The difference in cross-linking degree can be explained by a
conceptual model to illustrate the polyamide formation mechanism
proposed by Ghosh [30], which is shown schematically in Fig. 3.
29
According to this model, after the excess MPD solution was removed,
the aqueous phase meniscus was concave in hydrophilic pores
(the better wetting forces the meniscus to drop below the support
interface). In addition, MPD is likely to diffuse out of the pore slowly
due to the favorable hydrogen bonding interactions between MPD
and polar functional groups in the nylon 6,6 membrane pore walls
[30]. This “drag” on the MPD slows the diffusion rate and allows the
polyamide formation to occur deeper inside the pore.
Given the fact that four supports exhibited a small deviation of
surface porosity (i.e. between 46.8% and 54.7%), it is expected that a
smaller pore size support has more pores and than the larger pores.
In addition, the degree of concavity on the surface of support with
smaller pore size is also higher. As a result, the support with smaller
pores has greater surface area at the polymerization interface. This
higher surface area provides more sites for MPD partitioning into
the TMC organic phase and results in a higher local MPD/TMC ratio
and cross-linking degree.
3.4. Reverse osmosis tests
Table 3 summarizes the performances of TFC membranes in
reverse osmosis. The rejection of the polyamide selective layers
slightly decreased with increasing the pore size of the support then
showed dramatic change for a support of 0.45 μm pore size. Pure
water permeance A, on the other hand, gradually increased with
increasing the pore size. One cause for the perm-selectivity variation
is the cross-linking degree difference. TFC membrane based on
smaller pores support exhibits a higher cross-linking degree, which
is responsible for its higher selectivity. TFC membranes with a lower
Table 2
Relative atomic concentrations determined by X-ray photoelectron spectroscopy
(XPS) and composition ratios of aromatic polyamide TFC Membranes.
Fully cross-linked
Fully linear
TFC-0.025
TFC-0.1
TFC-0.2
TFC-0.45
C (%)
O (%)
N (%)
O/N
m
75
71.4
73.4
72.8
69.9
73.1
12.5
19.1
14.7
15.5
18.1
16.3
12.5
9.5
11.9
11.7
12.0
10.6
1
2
1.23
1.33
1.50
1.53
1
0
0.69
0.58
0.40
0.37
Fig. 2. Cross-sectional SEM images of polyamide-nylon 6,6 composite membranes built on different supports: (a) TFC-0.025; (b) TFC-0.1; (c)TFC-0.2; and (d) TFC-0.45.
30
L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–33
Fig. 3. Schematic diagram of polyamide thin film formation onto hydrophilic supports with different pore sizes. Figure modified from [30].
Table 3
Summary of salt rejection (%R), pure water permeability coefficient (A) and solute
permeability (B) of HTI and TFC membranes built on different MF supports.
Experimental conditions: 2000 ppm NaCl feed solution, cross-flow velocity of
0.26 m/s, and temperature of 20 1C.
%R
HTI
TFC-0.025
TFC-0.1b
TFC-0.2
TFC-0.45
a
b
150 psi
250 psi
85.4
97.7
95.8
92.9
47.5
94.8
97.5
96.5
92.9
Fail
A (LMH/bar)
B (LMH)a
0.599
0.673
0.917
1.548
1.930
0.942
0.122
0.300
0.697
18.14
Determined at a hydraulic pressure of 150 psi.
TFC-0.1 data is from [28].
cross-linking degree give a higher water permeation rate due to more
carboxylic acid group present in the linear portion of polyamide.
Also note that the TFC-0.45 exhibited less than 50% NaCl
rejection at 150 psi and failed at 250 psi. It also exhibited higher
solute permeability than the TFC-0.2. The difference in crosslinking degree might not explain this dramatic difference since
the cross-linking degree of TFC-0.45 is only slightly lower than
TFC-0.2. We are confident that the large poor size does not
adequately support the polyamide layer under hydraulic pressure.
3.5. Effect of support pore size on osmotic flux
3.5.1. Structural parameter and tortuosity determination
The osmotic water flux of the four TFC membranes is presented
in Fig. 4. The structural parameter for the TFC and HTI membranes
can then be derived by fitting the water flux data obtained in FO
as well as A and B values obtained in RO (Table 3) into Eq. (2).
Generally, the support with larger pores has lower S. TFC-0.45 did
Fig. 4. Osmotic water flux of HTI and TFC membranes built on different nylon 6,6
MF supports. Experimental conditions: 1.5 M NaCl as the draw solution; DI water as
the feed; cross-flow velocity of 0.18 m/s; 20 1C. TFC-0.1 data is from [28].
not follow this trend because the A and B values measured during
RO are impacted by the damage to the polyamide layer. The
artificially high A and B values impact the model substantially,
producing strange results. This is one of the challenges of using RO
to characterize membranes for use in FO. RO places the polyamide
layer under stresses that are not present in FO, meaning that the
membrane properties may not be the same between the two
processes. This has been observed by Tiraferri et al. in their recent
study [37]. They developed a new methodology for simultaneous
determination of A, B and S only by means of a FO experiment and
found that those values were different than those obtained by the
standard approach using both RO and FO. While this method is
reliant on fitting data to a model that may not be entirely accurate,
it is among the first studies to consider an alternative approach to
using RO to characterize A and B.
L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–33
Based on our porosity measurements, the lower S values calculated
for larger pore size substrates were attributed to tortuosity differences.
Tortuosity of the supports is estimated by dividing structural parameter by the thickness and porosity of the support membranes. The
calculated tortuosities for our membranes seem to be far greater than
a typical tortuosity value of a phase-inversion film (i.e. 1o τ o2).
They are consistent, however, with other studies characterizing
tortuosity of phase-inversion supports or membranes using the same
method [38,39]. One possible cause for the seemingly abnormal
tortuosity value is that the model for S calculation does not consider
ECP on both the support side and selective layer side, which would
over-estimate S and hence τ. In addition, dead-end pores might be
present that contribute to porosity but not to mass transport. The
interface between the selective layer and the support layer is also not
incorporated into the structural parameter in any way. The selective
layer is partially screened by the substrate and acts as another
resistance. This effect however, is not taken into account in modeling
of bulk structural parameters. We do notice in Table 1 that the
tortuosity decreases (generally) with increasing surface porosity,
suggesting that less screening of the selective layer by the support
layer reduces mass transport resistance.
Regardless of the absolute value of τ, we still can observe a
trend that smaller pore support membranes with lower porosity
generally have higher tortuosity than the ones with large pores.
This finding is consistent with previous investigations on other
porous media. Mualem and Dagan [40] suggested a pore-level
representation of tortuosity of soil media that is related inversely
to a power function of the pore radius, r, taking the form τ ¼r–b,
where b is an empirical parameter. Armatas [41] developed a
computational model to study flow and transport process of small
molecules in porous media and also found that the modeled
tortuosity is related inversely to the pore size of the channel,
where the parameter decreased significantly as the size of pores
increased. This means that pore channels with small pore size
exhibit tighter restrictions through tortuous pathways.
3.5.2. Water flux performance analysis
Further analyzing the water flux data shown in Fig. 4, we found
that in both PRO and FO mode the water flux first gradually increased
with increasing support pore size until pore size reaches to 0.2 μm,
and then dramatically dropped when pore size is increased to
0.45 μm. In PRO mode, TFC-0.2 achieved approximately two-fold
higher water flux than the other three TFCs as well as the HTI. The
TFC-0.45 exhibited the worst performance among the four. Fluxes in
the FO mode were lower than the PRO, as is typical for many FO
membrane tests, because of the severe ICP that impacts osmotic
driving force. TFC-0.2 still performed the best among the four TFCs
but its performance only matches the HTI in this orientation.
TFC-0.2 outperforms the other three membranes for two
reasons. The TFC-0.2 exhibited relatively high water permeance
in RO tests, reducing resistance to water transport in osmotic tests.
Second, its open and highly porous structure with lower tortuosity
creates less resistance to water transport and solute diffusion, as
indicated by its lower structural parameter. TFC-0.025 and TFC-0.1,
while more selective than the TFC-0.2, suffer from their lower
water permeance and higher structural parameter. This strongly
suggests a “permeance-selectivity” type tradeoff for FO membranes. The performance of the TFC-0.45, on the other hand, is
in a different category since the polyamide layer is insufficiently
supported to know exactly what its permselective properties are
in FO. The lower flux in the PRO mode can be attributed to the
relatively high salt flux among 4 TFC membranes (approximately
20 times higher than TFC-0.025 and TFC-0.1, and 3 times higher
than TFC-0.2), which causes a lower effective osmotic pressure and
severe ICP in the support layer.
31
3.5.3. Reverse salt flux
The reverse salt flux performances of our TFC and HTI membranes are shown in Fig. 5. With increasing pore size of the
support, the salt flux increased in both PRO and FO modes, which
corresponds to the trend of decreasing cross-linking degree.
Compared to HTI, our TFC membranes showed lower or equal
reverse salt flux in both orientations. Among those, the TFC-0.025
and TFC-0.1 achieved approximately 15 times lower salt flux than
the HTI membrane largely due to their superior selectivity compared to integrated asymmetric membranes. The TFC-0.2, with
1 to 2 fold higher water flux, still exhibited lower salt flux than
HTI. Interestingly enough, TFC-0.45, our least selective TFC, still
showed approximately equal salt flux to HTI, even though it
possesses an order of magnitude higher B when tested in RO.
Again this is probably due to the poor stability of the PA layer
under hydraulic pressure. These results suggest that in RO, the
selectivity is determined by combined effects of selective layer
properties and mechanical support provided by the substrate. In
FO, selectivity is primarily determined by the intrinsic selective
properties of the polyamide layer, or cross-linking degree.
3.5.4. Specific salt flux
Specific salt flux represents the amount of draw solute loss per liter
of water produced [35,36]. Lower specific salt flux is desirable as it
indicates a higher “efficiency”, meaning that the membrane passes
more water per unit of salt lost to reverse salt flux. As shown in Fig.6,
Fig. 5. Reverse salt flux of HTI and TFC membranes built on different nylon 6,6 MF
supports. Experimental conditions: 1.5 M NaCl as the draw solution; DI water as the
feed; cross-flow velocity of 0.18 m/s; 20 1C. TFC-0.1data is from [28].
Fig. 6. Specific salt flux of HTI and TFC membranes built on different nylon 6,6 MF
supports. Experimental conditions: 1.5 M NaCl as the draw solution; DI water as the
feed; cross-flow velocity of 0.18 m/s; 20 1C. TFC-0.1 data is from [28].
32
L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–33
TFCs based on smaller pore size substrates (i.e. TFC-0.025 and TFC-0.1)
generally have lower specific salt flux than those with larger pores (i.e.
TFC-0.2 and TFC-0.45). Compared to the HTI membrane, our TFC
membranes exhibited 10–30 times lower specific salt flux in PRO
mode and 3–9 times lower in FO mode. The exception is the TFC-0.45,
which exhibited a matched or slightly higher specific salt flux than the
HTI membrane.
4. Conclusions
During the course of this work, we identified that the support
layer pore size does have a significant impact on TFC membrane
performance in both RO and FO. Using a unique series of MF
membranes available from 3M, we were able to focus on pore size
as a singular independent variable impacting water and salt flux
performance. Generally, we found that smaller pore supports
yielded denser and more selective polyamide layers with better
pressure tolerance. However, the osmotic flux performance was
limited by a more tortuous structure. We found that support pore
sizes of around 0.2 μm favors both high water flux and low salt flux,
but even this membrane did not exhibit the lowest specific salt flux
of our group (0.1 μm).
An important finding of this study is that pore size is not a
feature of the structural parameter, yet it can impact the flux
performance dramatically in FO and PRO. Membrane fabricators
must keep this in mind when designing new membranes. It is not
only the thickness, tortuosity and porosity that must be adjusted
to minimize structural parameter. The pore size and its impact on
both structure parameter and selective layer formation must be
understood if we are to design high performance membranes for
FO or PRO.
Acknowledgements
The authors gratefully acknowledge funding from the National
Science Foundation (CBET #1067564), the Environmental Protection Agency STAR Program (R834872), and the 3M™ Non tenured
Faculty Award. Furthermore, we thank 3M for providing microfiltration membranes and for assisting with some of the SEM
imaging. Hydration Technologies Innovations provided their forward osmosis membrane for this work. We would also like to
acknowledge Dr. Heng Zhang and graduate student Xiaoqiang
Jiang in University of Connecticut for their help in XPS characterization and analysis.
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