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Thermally stable core-shell star-shaped block copolymers for antifouling enhancement of water purification membranes
Journal Pre-proof Thermally stable core-shell star-shaped block copolymers for antifouling enhancement of water purification membranes Babak Soltannia, Muhammad Amirul Islam, Jae-Young Cho, Farshad Mohammadtabar, Ran Wang, Victoria A. Piunova, Zayed Almansoori, Masoud Rastgar, Andrew J. Myles, Young-Hye La, Mohtada Sadrzadeh PII:
S0376-7388(19)31732-6
DOI:
https://doi.org/10.1016/j.memsci.2019.117686
Reference:
MEMSCI 117686
To appear in:
Journal of Membrane Science
Received Date: 10 June 2019 Revised Date:
5 October 2019
Accepted Date: 20 November 2019
Please cite this article as: B. Soltannia, M.A. Islam, J.-Y. Cho, F. Mohammadtabar, R. Wang, V.A. Piunova, Z. Almansoori, M. Rastgar, A.J. Myles, Y.-H. La, M. Sadrzadeh, Thermally stable core-shell star-shaped block copolymers for antifouling enhancement of water purification membranes, Journal of Membrane Science (2019), doi: https://doi.org/10.1016/j.memsci.2019.117686. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Thermally Stable Core-shell Star-shaped Block Copolymers for Antifouling Enhancement of Water Purification Membranes
4 5
Babak Soltanniaa , Muhammad Amirul Islama*, Jae-Young Choc, Farshad Mohammadtabara, Ran
6
Wangb, Victoria A. Piunovab, Zayed Almansooria, Masoud Rastgara, Andrew J. Mylesc*, Young-
7
Hye Lab*, Mohtada Sadrzadeha*
8 9 10 11 12 13 14 15
a
16
Abstract
17
Star-shaped block copolymers (SPs), consisting of a hydrophobic core and hydrophilic arms, are
18
a new material architecture introduced to the area of antifouling coatings for water filtration
19
membranes. The unique self-assembly behavior of these SPs on hydrophobic membranes surface
20
allows the formation of an ultrathin coating (less than 15 nm), rendering a highly hydrophilic
21
antifouling membrane surface. Here, we have judiciously examined several important structural
22
and process parameters, such as the chemical composition of SPs and surface properties (e.g.,
23
surface potential, coverage, and wettability) driven by a mono- or bi-layered assembly of SPs, to
24
acquire a highly hydrophilic and thermally stable antifouling coating on a hydrophobic
25
ultrafiltration (polysulfone, PSF) membrane for oil sands produced water treatment. Among
26
three types of SPs (e.g., SP1 with 100 % amine, SP2 with 55 % amine/45% PEG, and SP3 with
27
35 % carboxylic acid/65% PEG moieties in the arms) and their variable coating conditions, the
28
bilayer coating comprised of SP3 top-layer and SP1 bottom-layer exhibited the best
29
hydrophilicity and anti-oil fouling efficiency showing 2 ~ 4 times higher water flux compared to
30
an unmodified PSF membrane during the filtration of synthetic oil-water emulsions conducted in
31
both constant flux mode and constant pressure mode. Outstanding antifouling efficiency of the
Department of Mechanical Engineering, 10-367 Donadeo Innovation Centre for Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada. b
IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120, United States
c
National Research Council Canada, Nanotechnology Research Centre, 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada * Corresponding Authors: [email protected], +1(587)710-3281; [email protected], +1(780)492-8745; [email protected], +1(408) 927-1256, [email protected], +1(780) 641-1758
1
1
bilayer coating toward natural organic matters (NOMs) was also verified by using bovine serum
2
albumin (BSA) and humic acid (HA) as the model foulants. The bilayer-coated membrane
3
retained more than 80 % and 90 % (for nano and micron size oil emulsion, respectively) of its
4
initial antifouling efficiency even after rigorous thermal treatment for an extended period (80 °C
5
for 1 week), which demonstrates its feasibility for high-temperature membrane operation.
6 7
Keywords: Star-shaped Block Copolymers; Membrane; Oily Wastewater Treatment;
8
Antifouling Coating; Oil Sands
9
1. Introduction
10
Imminent water scarcity as a result of global warming, water pollution and overconsumption of
11
freshwater has urged immense interest among researchers to develop emerging technologies for
12
water reclamation, recycling, and reuse. Membrane technology is a feasible and energy-efficient
13
solution to seawater desalination and wastewater treatment [1–3]. Membrane separation
14
technologies have secured a crucial role in industrial water purification due to minimal operating
15
costs, compact design, as well as high product quality. Given the fact that inorganic membranes
16
are so expensive and could be irreversibly fouled, research has been primarily focused on
17
modification of polymer materials to fabricate high-performance polymeric membranes [4–8].
18
Fouling phenomenon, mainly caused by the adsorption of particles, organic matter, and
19
microorganisms over the membrane surface or within its pores, is perhaps the main obstacle
20
against practical application of membrane technologies [9]. Accumulation of water contaminants
21
on the membrane surface leads to a severe flux decline due to the formation of a cake layer of
22
foulants and membrane pores blocking that consequently shortens membrane’s lifespan [10–12].
23
Particularly, the oily wastewater, such as oil sands produced water in Alberta (Canada), contains
24
highly stable emulsified oil which is one of the most notorious foulant encountered in membrane
25
purification processes. Fouling of membranes by emulsified oil necessitates periodic cleaning
26
using various chemical processes, demanding cleaning cost and generating further waste [13–16].
27
The oily wastewater is indeed the largest source of wastewater generated from industrial sectors
28
[14,15,17,18]. Therefore, enormous effort has been devoted to the membrane surface
29
modification techniques to mitigate the oil fouling propensity of the membranes [19–25].
2
1
The majority of recent studies for surface modification has been devoted to the chemical grafting
2
of functional materials to the membrane surface [26–29]. The objective was to tune the
3
hydrophilicity and surface charge of membranes to mitigate fouling caused by the hydrophobic
4
and electrostatic interactions between the water constituents and the membrane surface.
5
Although chemical grafting is found to be effective in inducing the desired characteristics to the
6
membrane, the major drawback is the separation of functional materials from the surface after a
7
long-time operation. Moreover, the chemical grafting does not ensure complete surface coverage,
8
requires chemical reaction and can result in increased costs. On the other hand, incorporation of
9
a very thick layer of these materials - by membrane surface activation and applying very reactive
10
grafting methods, such as radical polymerization - to the surface introduces resistance against
11
water transport through the membrane and adversely affects the membrane permeation properties.
12
Recently, a versatile and effective technique has been developed where modifying agents are
13
deposited on a substrate via the layer-by-layer (LBL) deposition technique to form a thin and
14
defect-free separation layer [30–33]. This technique relies on the sequential deposition of
15
alternating layers of polycations and polyanions on a porous substrate to make a multi-layer film.
16
The number of deposited layers can be arbitrarily determined so that the desired membrane
17
permeability and selectivity are achieved [31,33–36]. This technique also enables control over
18
surface potential and morphology [37–39]. So far, most of the studies on LBL deposition
19
technique utilized linear polyelectrolytes that require a considerable number of layers to achieve
20
desirable separation and/or antifouling properties [35,36,40–42]. This can severely reduce the
21
water flux though the membrane and commonly used to prepare nanofiltration (NF) membranes
22
[33,42–44]. These linear polyelectrolyte-based coatings also pose stability concern under
23
filtration conditions and often requires postdeposition treatment (for example cross-linking)
24
which is not feasible for large scale membrane modification and for commercial membrane
25
modules, such as spiral wound modules [42,43,45,46].
26
Charged nanomaterials or hyperbranched polyelectrolytes of globular shaped nanoarchitecture
27
have been demonstrated to reduce the number of deposition layers to achieve desirable
28
antifouling and/or separation properties since these materials bear high-charge density per unit
29
volume compared to linear polyelectrolytes [9,23,46–59]. However, bare inorganic nanoparticles
30
(NPs) are in general not suitable for direct deposition on hydrophobic membranes such as
31
polysulfone (PSF), and polyethersulfone (PES) due to the lack of enough interaction energy. The 3
1
NPs are either modified with charged polyelectrolyte or they are self-assembled within
2
alternative layers of cationic and anionic polyelectrolyte. This assembly process is mainly based
3
on electrostatic interaction and largely depends on the nature of the base membrane such as its
4
surface charge and the presence of surface ionic functional groups, and therefore, often requires
5
membrane surface activation and/or postdeposition treatment [47,54,59–61]. Titania NPs
6
possessing Ti4+ coordinative sites are directly deposited on PSF and PES surface since these NPs
7
can form coordinative bond with the sulfone groups of the membrane. However, bare NPs are
8
not well dispersed in solvent; therefore, the agglomeration of NPs results in uncontrolled surface
9
roughness, which is prone to fouling [62,63]. Long-term stability of such coordinatively
10
deposited films is also questionable [62]. Titania is commonly used to coat membrane with
11
abundant anionic surface groups, including polyamide, and sulfonated PES where it can form
12
stronger coordinative bonds [9,50,58,64]. Carbon based nanomaterials, for example carbon
13
nanotubes and graphene oxide, also require multistep surface activation [53,65]. Hyperbranched
14
polyelectrolyte is also deposited mainly based on electrostatic interaction and require negatively
15
charged hydrolyzed polyacrylonitrile support [49].
16
It is evident from a recent study that the application of multi-charged star-shaped block
17
copolymers (SPs) results in the superior performance of the coated membrane mainly due to the
18
high charge density and well-packed structure [66]. These SPs comprise of hydrophilic arms
19
(shell part) anchored to the hydrophobic core. While the hydrophilic arms help to mitigate the
20
membrane fouling, the hydrophobic core prevents undesirable swelling of the selective layer of
21
the membrane. The chemical composition of hydrophilic arms can be tuned to introduce different
22
types of functional groups, including amine groups representing the cationic segment, carboxylic
23
acid groups representing the anionic segment, or polyethylene glycol representing charge-neutral
24
hydrophilic moieties [67,68]. The functionalized SPs are water soluble due to the polarity of
25
hydrophilic arms and can be easily deposited on the hydrophobic PSF via dual interactions, that
26
is hydrophobic interaction with PS core and electrostatic interaction with cationic tertiary amine
27
containing arms. This collective interaction is expected to provide superior stability of the
28
monolayer coating over other types of nanomaterials used so far. The successive layers are also
29
presumed to be stable due to multitude of inter SP’s chain entanglement where the chains consist
30
of numerous oppositely charged functional groups. The water solubility of the SPs also enables
31
the surface-coating from a green, abundant, and membrane compatible aqueous medium. In 4
1
addition, the extra interstitial volume between adjacent SPs and the resultant interconnected
2
water channels significantly improves the water permeation [69–73]. It is noteworthy that the
3
high charge density per polymeric arm, significant local density of polymeric arms, and surface
4
functionality of the SPs remarkably lower the required number of stacking layers to achieve
5
desirable separation and antifouling properties [66,74–80]. Our previouse study with structurally
6
similar but compositionally different SPs was focused on fabicating NF membrane that required
7
10 alternative cationinc and anionic SPs layers to from a dense barrier film on a porous substrate
8
[66]. For ultrafiltration (UF) or microfiltration (MF) membranes that have been widely used for
9
waste water treatment and/or pre-treatment process for RO/NF, however, needs a cost-efficient,
10
surface modification technique and ultrathin coating layer to provide good antifouling properties
11
with minimal loss of intrinsic water permeability of the base membranes. In this context, fine-
12
tuning of the composition of hydrophilic arms on the SPs holds the potential for fabricating only
13
a monolayer or a bilayer coated membrane with higher permeability while maintaining the
14
desired antifouling properties for hydrophobic, porous UF or MF membranes.
15
Another technological challenge is the development of thermally stable membrane since the
16
temperature of produced water from thermally enhanced heavy oil recovery method for the
17
extraction of bitumen in Alberta, Canada, e.g., steam-assisted gravity drainage (SAGD) process,
18
is usually 70-95 °C [81]. In the SAGD process steam is injected underground to dilute the
19
bitumen and thus facilitate its extraction. The diluted bitumen is first separated from steam
20
condensate and the hot produced water, containing oil, organic matter and inorganic materials
21
(mainly silica), is first cooled down, then treated by a variety of conventional methods [82–85].
22
The treated water is finally reused in steam generators. Cooling and reheating cycles for
23
membrane filtration demand capital and operating expenses [86]. Therefore, it is economical to
24
perform the oil filtration at high temperatures. Low thermal stability of the functional materials
25
used for the surface modification of the membrane can hinder the application of the modified
26
membrane for such high temperature produced water. Degradation of the membrane surface
27
functional materials at higher temperatures can lead to the loss of their functionality together
28
with accelerated leaching out into the wastewater solution during crossflow filtration at elevated
29
temperatures. Therefore, one of the prime objectives of the membrane research has always been
30
subjected to the development of surface modification methodologies that provide excellent
31
antifouling properties, permeation properties, and thermal stability. Given that, it is essential to 5
1
assess the thermal stability of the SPs-modified membranes (e.g., dettachments of SPs, and their
2
structural and/or functinal change) to test the feasibility for the treatmnet of high-temperature
3
streams of oil sands industry in Alberta, Canada.
4
In order to address the issues associated with water permeability, antifouling property, and
5
thermal stability of water filtration membrane, we have investigated the effect of the chemical
6
composition of hydrophilic arms on the SPs to deposit a highly hydrophilic and thermally stable
7
monolayer and bilayer antifouling coating on a commercial polysulfone ultrafiltration (PSF UF)
8
membrane. The size, hydrophilicity, surface charge, and thermal stability of the SPs were
9
investigated prior to their coating on the porous PSF UF membranes. Following the detailed
10
investigation of the properties of the SPs, monolayers of each SP and bilayer of alternating
11
polycations and polyanions SPs were deposited on the PSF UF membranes. The SP-coated
12
membranes were thoroughly characterized (e.g., self-assembly behaviors including packing
13
density, surface charge, and wettability) and their separation performance and antifouling
14
property were also studied. Finally, the bilayer coated PSF UF membrane exhibiting the best
15
anti-oil fouling property was subjected to thermal stability test underwater at 80 °C and 250 rpm
16
stirring for 7 days. This membrane was thoroughly characterized to determine any dettachments
17
of SPs, their structural change and/or functinal group degradation, loss in hydrophilicity and
18
antifouling property.
19
2. Material and Methods
20
2.1. Material
21
The SPs used in this study comprised of a hydrophobic polystyrene (PS) core and different types
22
of
23
polydimethylaminoethyl methacrylate (PDMAEMA), and polymethacrylic acid (PMAA). The
24
cationic and anionic SPs (PS-PDMAEMA, Mw = 1,174 kDa and PS-PMAA, Mw = 560 kDa)
25
contain about 30 independent PDMAEMA and PMAA arms covalently linked to a cross-linked
26
PS core, respectively [66–68,80]. The PS cores of the SPs are similar while the arms are of a
27
variable combination of PEGMA, PDMAEMA, and PMAA polymers (SP1: 100% PDMAEMA,
28
SP2: 55% PDMAEMA-45% PEGMA, and SP3: 35% PMAA-65% PEGMA). The chemical
29
structures of these SPs are shown in Figure 1. The synthesis method of these SPs is provided
hydrophilic
arms
including
polyethylene
6
glycol
methacrylate
(PEGMA),
1
elsewhere [68,87]. PSF UF membranes (PSF-20, MWCO 20 kDa) were purchased from Sepro
2
Membranes Co. and used as substrate. Tetrahydrofuran (THF) and isopropyl alcohol (IPA) were
3
purchased from Sigma-Aldrich and used for preparing SP solutions and treating the surface of
4
the PSF membranes, respectively. Synthetic oily wastewater solutions were prepared using
5
vegetable oil (Crisco® Pure Vegetable Oil from Safeway) with Tween 20 surfactant (Bio-Rad)
6
and Hexadecane (n-Hexadecane, 99%, pure, Fisher Scientific, Canada) with Tween 80 surfactant
7
(Bio-Red). n-Decane was purchased from Sigma-Aldrich and used for the captive bubble contact
8
angle measurements. Bovine Serum Albumin (BSA) was purchased from ChemCruz and was
9
dissolved in 1× Phosphate buffer saline (PBS) of pH 7.4 (kindly provided by Biochemistry lab of
10
National Research Council Canada) followed by adjustment of pH to 7.0 with 0.1 M
11
hydrochloric acid (HCl). Humic acid (HA) purchased from Sigma Aldrich was dissolved in 0.1
12
M sodium hydroxide solution and pH was adjusted to 7.0 by adding 4 M HCl.
13
2.2. Solution preparation
14
To prepare 0.1 wt% solution, 100 mg of the SPs were dissolved in 800 µl THF in a 15 mL glass
15
vial using an ultrasonic bath. Then, 10 ml DI water was added slowly in 100 µL portions every
16
30 seconds while stirring at 1000 rpm. The solution was sonicated for 10 minutes in an ultrasonic
17
bath. Finally, the solution was diluted to 100 mL with DI water and sparged with nitrogen for 3
18
hours to remove the THF. The pH of the DI water was about 6.5 ± 0.2, which fluctuated slightly
19
by adding different SPs to the aqueous solutions. Variation of the pH can be attributed to
20
deprotonation of carboxylic acid groups and consequently acquiring negative charge in the case
21
of SP3. Partial protonation of tertiary amine groups in SP1 and SP2 could also increase the
22
alkalinity of water [66]. Three different types of solutions were prepared, following the
23
procedure mentioned above, containing 0.1% wt% of SP1, SP2, and SP3, respectively.
7
1 2 3 4 5
Figure 1. (a) Scheme and examples of the chemical formula of the SPs with PS core and arms of PEGMA and DMAEMA. SP1: 100 mol % DMAEMA (x = 1), SP2: 55 mol % DMAEMA and 45 mol % PEGMA (x = 0.55), (b) Scheme and example of chemical formula of SPs with PS core and arms of PEGMA and PMAA. SP3: 65 mol % PEGMA and 35 mol % PMAA (x = 0.35).
6
2.3. Characterization of SPs
7
Hydrodynamic diameter and surface charge density of the SPs (0.1 wt % aqueous solution) were
8
measured by using a Malvern Zetasizer Nano-ZS (Malvern Instruments Ltd., Worcestershire,
9
UK). The average hydrodynamic diameter was calculated against polystyrene latex standard by
10
performing three series of 11 measurements. The zeta potential was calculated from
11
electrophoretic mobility measurements in three series of 20 measurements. All measurements
12
were done at 25 oC.
13
The size distribution of SPs was evaluated using transmission electron microscopy (TEM: JEOL
14
2200 FS TEM – 200kV Schottky field emission instrument equipped with an in-column omega
15
filter). A TEM sample was prepared by depositing a droplet of SP solution (0.01%) on a carbon-
16
coated 400-mesh copper grid (Electron Microscopy Sciences) and blotting using filter paper after
17
10 s. The sample was then stained by applying one droplet of uranyl acetate (2 % aqueous
18
solution) for 120 s, followed by blotting and air-drying. The hydrophilic arms on the SPs stained
19
with uranyl acetate were imaged via bright field-scanning transmission electron microscopy (BF8
1
STEM) operated at 30 kV accelerating voltage and 30 µA emission current on an ultra-high-
2
resolution Hitachi S–5500 cold field emission SEM.
3 4
2.4. Thermal stability of SPs
5
Thermal stability of SPs was evaluated under high-temperature TEM (HT-TEM) on a Hitachi H-
6
9500 environmental TEM at 100kV acceleration voltage with 5°C/min elevation of Temperature
7
until 80°C and structural changes were monitored (Please refer to section 2.3 for TEM sample
8
preparation).
9
2.5. Self-assembly of SPs on Commercial UF Membranes
10
The PSF UF membrane was pretreated to remove the protective coating layer by soaking into
11
IPA for 30 minutes, followed by rinsing with DI water. The membrane was finally kept in DI
12
water overnight prior to the coating to ensure complete removal of IPA. To coat the pretreated
13
PSF membrane with SP through the self-assembly process, the substrate was first placed between
14
two frames with sealing rubbers. Frames were held tightly with paper clips, and an SP solution
15
was poured onto the active surface of the membrane. After 15 min, the solution was removed,
16
and the membrane was immersed in DI water for 20 minutes to remove the loosely bound SPs.
17
For the bilayer coating, the same process was repeated with carboxylated-SP (SP3) solution on
18
top of the first layer prepared with the amine-functional SP (SP1) solution. Then, the membrane
19
was removed from the frame and was stored in DI water overnight. The first and the second
20
membrane samples denoted as PSF-SP1 and PSF-SP2 comprise a monolayer of SP1 with amine-
21
functional groups, and a monolayer of SP2 with amine and PEG functional groups, respectively.
22
The third membrane sample (PSF-SP1-SP3) consists of a bilayer featuring SP3 with carboxylic
23
acids deposited on the monolayer of amine-functional SP1. The compositions of the membrane
24
samples prepared in this work are presented in Table 1.
25 26
Table 1. Specification of different types of SP coated PSF UF membranes used in this work. Sample
Active layer
PSF-SP1
Monolayer of SP1
PSF-SP2
Monolayer of SP2
PSF-SP1-SP3
Bilayer of SP1 and SP3
27 9
1
2.6. Characterization of SPs coated PSF UF membranes
2
XPS analysis was done on a Physical Electronics Quantum 2000 ESCA, using a monochromated
3
Al Kα X-ray source at 1496 eV, 200 µm spot size throughout and charges were neutralized. 1000
4
eV survey spectra (187 eV pass energy, 1.6 eV/step) were taken. Self-assembly and
5
hydrophilicity of SPs on PSF membranes were evaluated by AFM. Specifically, the sample
6
surface was observed using a Digital Instruments/Veeco Instruments MultiMode Nanoscope IV
7
AFM equipped with an E scanner. In order to have information about local hydrophilicity &
8
hydrophobicity from the sample surface, high resolution, soft tapping mode AFM Probes
9
(MikroMasch USA, Inc.) with low spring constants of 5.0 N/m and 1 nm radius were used. To
10
obtain a clear phase mode image from the surface, low scan rate (0.3-0.5 Hz) and amplitude
11
setpoint were choosen during measurement. Contact angle measurements were performed using
12
a contact angle analyzer (Data Physics Instruments GmbH) with a 2 µL DI water drop under air
13
or 6 µL captive n-decane bubble in water. For captive n-decane bubble in water contact angle
14
measurement, a 2 cm wide membrane strip was first placed face down in a custom-made holder.
15
The membrane holder assembly was then accommodated in a small, clear cuvette so that the
16
membrane was fully immersed in water. A high-speed computer-controlled camera was focused
17
on the membrane surface and at least three n-decane bubbles were placed on the membrane
18
surface using a syringe with an inverted needle. Contact angle samples were kept under
19
deionized water and rinsed before analysis. The measurements of membrane zeta potential were
20
made using an Electrokinetic Analyzer (SurPASS 3, Anton Paar) equipped with a gap adjustable
21
cell (20 mm × 10 mm). An automatic pH titration was performed at room temperature within a
22
pH range from 3.0 to 8.0. Streaming potential measurements were done using different salt
23
solutions with a target ramp pressure of 0.5 bar. Before measurements, membranes were
24
thoroughly rinsed in MilliQ water, and the pH of the solutions was adjusted by the addition of
25
0.05 M NaOH or 0.05 M HCl. A KCl solution of 0.001M was used as an electrolyte solution.
26
2.7. Pure-water flux test of SPs coated membranes
27
A dead-end filtration module (Millipore) equipped with nitrogen pressure control was used to
28
measure the pure water flux. The dead-end filtration module has an active membrane area of
29
14.6 cm2. A pressure of 50 psi was applied to the system at room temperature. A stirring speed of
30
700 rpm was used to ensure the feed solution was well mixed inside the cell during the
10
1
experiment. The weight of permeate was recorded by a digital balance connected to a computer.
2
Pure water flux of membrane was calculated using the following equation [88]: =
∆
3
where Jw is pure water flux (LMH), m is the mass of the permeate water (kg), ρ is water density
4
(kg/m3), Am is the effective area of membrane (14.6 cm2), and ∆t is the time of permeation (hr).
5
2.8. Antifouling tests of SPs coated membranes
6
Fouling behavior of UF membranes, before and after SPs coating, was evaluated by filtering
7
synthetic oily wastewater containing 1000 ppm of hexadecane or 1500 ppm vegetable oil
8
emulsion. To prepare the hexadecane oil emulsion 0.5 g of hexadecane and 0.75 mg of Tween 80
9
was blended in 500 mL DI water and the mixture was homogenized with a homogenizer
10
(Homogenizer 150, Fisherbrand, Canada) at the highest speed for 4 minutes. To prepare the
11
vegetable oil emulsion, 4.5 g of vegetable oil was mixed with 0.32 g of Tween 20 via mechanical
12
stirring. Then 900 mL of DI water was added slowly dropwise via addition funnel while stirring
13
at 1000 rpm. The resultant emulsion was diluted to 3 L by adding DI water and stirred for 1 hour
14
followed by homogenization with the same homogenizer. Oil emulsion droplet size was
15
determined by dynamic light scattering (DLS) using using Malvern Zetasizer Nano series (Nano-
16
ZS). In case of constant-flux mode filtration with hexadecane oil emulsion, a 400 ml dead-end
17
filtration cell (Amicon, UFSC40001) was used. Nitrogen gas was used to pressurize water
18
through the membrane. The bare PSF, PSF-SP1, and PSF-SP2 were pressurized with nitrogen at
19
3.5-5.0 psi and PSF-SP1-SP3 was perssurized at 10.0-15.0 psi to obtain a stable initial flux of ca.
20
185 LMH. The mass of permeate water was monitored and recorded over time using a digital
21
weighing balance (ME4002, Mettler Toledo, USA). After 30 minutes of constant pure water flux,
22
400 mL of hexdecane oil emulsion was added to the cell and the filtration was performed for 2
23
hours at 450 rpm while the cell was refilled with the permeate on demand. At the end of oil
24
filtration, the oil emulsion was discarded and the membrane was rinsed with 400 mL DI water
25
for 20 minutes at 450 rpm. Then, pure water flux was recorded for 30 minutes. The antifouling
26
properties against BSA and HA solutions were evaluated in the smilar processes. To study the
27
stability of membrane coating for longtime operation under rigorous operating condition a satble
28
nanosized oil emulsion prepared with a very viscouse vegetable was used as the foulant and the
29
filtration was conducted with a crossflow filtration setup (Water Planet). All the membranes 11
1
were compacted by running DI water at 80 PSI for 90 minutes, and water flux was stabilized at
2
70 PSI for 90 minutes. Then, the vegetable synthetic oil emulsion was added as the feed solution
3
and filtration was run for 15 hours. The oil or BSA concentrations in the feed and permeate
4
solutions were measured using a total organic carbon (TOC) analyzer (Shimadzu TOC-L with
5
ASI-L and TNM-L). The concentration of HA in the feed and permeate solution was calculated
6
by measuring their absorbance at 254 nm using a UV-Vis spectrophotometer (Thermo Electron
7
Corporation, model: Genesys 10-S) and dividing the absorbance by their absorptivity obtained
8
from the calibration curve. Feed HA solution was diluted 20 times before absorbance
9
measurement in order to comply with Beer-Lambert law. The retention ratio (or rejection) of the
10
oil, BSA, and HA was calculated by the following equation:
11
R(%) = 1 −
12
where Cf and Cp represent the oil, BSA, or HA concentration in the feed and permeate solution,
13
respectively.
14
The flux recovery ratio after simple wash was calculated by the following equation:
15
FRR (%) =
16
2.9 Thermal stability analysis of bilayer SP coated PSF UF membrane (PSF-SP1-SP3)
17
For the evaluation of thermal stability, the PSF-SP1-SP3 membrane was clamped to a glass slide
18
with stainless steel paper clips and immersed in 2L DI water in a clean glass beaker and stirred at
19
250 rpm at 80 °C for 1 week. The beaker was covered and wrapped with a plastic bag to slow
20
down the water evaporation. The thermal stability of the coating was evaluated by measuring the
21
elemental composition, contact angle, pure-water flux, streaming surface zeta potential, and anti-
22
oil fouling property.
23
3. Results and discussion
24
3.1. Characterization of SPs
25
Table 2 shows the chemical structures, hydrodynamic diameter, and Zeta potentials of the core-
26
shell SPs used in this study. Each SP has a cross-linked, hydrophobic polystyrene (PS) core,
27
which is made of approximately 30 individual PS linear polymers via crosslinking in the
28
presence of para-divinylbenzene. The hydrophilic methacrylate polymer arms contain PEGMA,
× 100
× 100
12
1
PDMAEMA, and/or PMAA with various compositions, leading to different hydrodynamic
2
diameters as measured by DLS. SP1showed positive surface potential (ζ = +28 mV), indicating
3
the pendant amine functional groups in the outer shell are partially protonated in water under
4
neutral pH conditions, while SP3 is negatively charged (ζ =-17 mV) due to carboxylic acid
5
functional groups in the arms. Meanwhile, SP2 exhibited less positive charge compared to SP1
6
due to the presence 45% charge neutral PEGMA in the hydrophilic arms. The charge
7
characteristics of SPs will influence the surface potential of membranes after coating.
8 9
Table 2. List of SPs with different compositions
Sample Code.
Composition (mol %) Diameter (nm)
Potential (mV)
PDMAEMA
PEGMA
PMAA
SP1
100
0
0
56.7±0.1
28.1±0.1
SP2
55
45
0
64.2±1.0
10.8±2.3
SP3
0
65
35
44.2±0.7
-17±0.2
10 11
Provided that DLS measures the hydrodynamic diameter of the SPs in an extended state of the
12
arms in water, which includes water corona in the size measurement, the real sizes were
13
determined by TEM imaging of uranyl acetate stained SPs (Figure 2). Note that the uranium
14
metal staining was done to enhance the Z-contrast of SPs against similar Z-contrast of
15
background carbon. The sizes of SP1, SP2, and SP3 are 21 ± 3, 17 ± 2, and 16 ± 2 nm,
16
respectively, which are very small and is beneficial for their high packing density on the
17
membrane. However, this would not be the real size in the water. The samples were dried, and
18
hydrophilic arms could be folded into the core to minimize surface energy in the TEM samples.
19
The existence of hydrophilic arms around the hydrophobic core of the SPs was visualized via
20
uranyl acetate staining followed by BF-STEM imaging (Figure 3). The BF-STEM images show a
21
darker contrast on the periphery of the SPs associated with the uranium metal bonded to the
22
amine or carboxylic acids groups of the hydrophilic polymeric arms.
13
1 2 3
Figure 2. TEM images demonstrating the size distribution of SPs on carbon coated copper grids: a) SP1, b) SP2 ),
and c) SP3.
4
5 6 7 8
Figure 3. BF-STEM images of uranium metal stained SPs demonstrating the presence of hydrophilic arms of SP1
9
3.3. Thermal stability of SPs
(Left), SP2 (Middle), and SP3 (Right), respectively. Corresponding insets clearly shows the hydrophilic arms stained with uranium metal.
10
Thermal stability of the SPs is pivotal for their application as coating materials under high-
11
temperature filtration condition for the treatment of oil-gas produced water. In this context, the
12
thermal stability of the SPs stained with uranium metal was evaluated under high-temperature
13
TEM (HT-TEM) with a temperature ramp of 5 °C/min up to 80 °C (Figure 4). No obvious
14
change in the shape or size of the SPs were observed during or after the heat treatment, which
15
implies their stability at an elevated temperature up to 80 °C.
14
1 2
Figure 4. HT-TEM images from three different SPs: room temperature (Top panel), and at 80°C (Bottom panel)
3 4
3.4. Evaluation of SP coatings on porous PSF UF membrane
5
It is extremely difficult to directly coat hydrophilic materials on top of a hydrophobic membrane
6
surface. Although hydrophobic PSF membrane surface is known to exhibit negative surface zeta
7
potential, the electrostatic interaction of this surface with a positively charged polymeric
8
hydrophilic material is not enough to obtain a stable coating. Therefore, a combination of highly
9
hydrophobic and positively charged segments on the coating material are essential for obtaining
10
a very stable coating on such membrane. As mentioned earlier, the SPs consist of hydrophobic
11
PS core and cationic or anionic functional groups containing hydrophilic arms. When cationic 15
1
arms containing SPs are used as the first deposition layer, the unique hydrophobic core of the
2
SPs promotes interaction with the hydrophobic membrane in addition to the electrostatic
3
interaction. The PS core is prepared by cross-linking PS arms, and hydrophilic arms are built on
4
these flexible PS arms. The flexibility of the arms allows the folding of the arms upwards in the
5
water phase and expose the hydrophobic core resembling the structure of a jellyfish; this
6
structural rearrangement during coating process facilitates the interaction of PS core with the
7
hydrophobic PSF membrane. Meanwhile, owing to the flexible nature, the cationic arms near the
8
membrane surface can align on the surface via electrostatic interaction. As a result, SPs attach to
9
the PSF membrane surface and form a uniform single layer by self-assembly and self-limiting
10
behaviors, while most of the hydrophilic arms will be exposed to the water solution. To assess
11
this unique interaction, SPs aqueous solutions with different compositions (0.1 wt%) were coated
12
on commercial PSF UF membranes (PSF-20, MWCO 20k). As shown in the AFM height images
13
(Figure 5), SP1 (left panel) and SP2 (middle panel) resulted in a uniform monolayer coating of
14
SPs on the membrane surface. This observation suggests that the unique interaction of the
15
hydrophobic PS cores of SPs with the hydrophobic membrane materials along with electrostatic
16
results in the monolayer coating excluding multilayer formation. However, there are interparticle
17
gaps for the monolayer coated membranes that is assumed to be accessible for foulants. The SP2
18
monolayer coating provided better packing and surface coverage than the SP1 monolayer and
19
has reduced interparticle gaps (10.8 ± 4.9 nm for PSF-SP1 verses 9.5 ± 5.5 nm for PSF-SP2). It
20
is worth noting that interparticle gaps are calculated from the phase images shown in Figure 6
21
since phage mode shows the structural details of SPs better than the height mode. The bilayer
22
coating of SP1 followed by SP3 is also uniform and further filled the interparticle gaps (8.1 ± 4.5
23
nm). Most importantly, bilayer provided obviously higher surface coverage than the monolayers.
24
The carboxylic acid arms containing SPs (SP3) of the second layer are assembled surrounding
25
the amine arms containing SPs of the first layer that provided the higher surface coverage.
26
Encouragingly, the unique characteristic of these SPs is that their hydrodynamic diameters are
27
larger (ca. 57, 64, and 44 nm, Table 2) since hydrophilic arms are extended under water. This
28
size extension of the SPs can fill the gaps and shield the access of foulants to membrane surface.
29
Overall, all three different coating process provided an ultra-thin SPs coating with less than 15
30
nm thickness. Such an ultra-thin coating will be beneficial to modify the surface property (e.g.,
31
hydrophilicity) of base membranes with minimal changes in their water permeation properties. 16
1
The coating of SPs on the PSF UF membranes was also evaluated by XPS elemental analysis
2
(Table 3). The XPS element analysis in Table 3 showed that SP1, SP2 and SP3 coating in mono
3
and bilayer increased the amount of nitrogen and oxygen and reduced the amount of carbon and
4
sulfur on the PSF UF membrane surface. These composition changes are due to the fact that SP1,
5
SP2, and SP3 have higher amounts of nitrogen and oxygen than PSF UF membrane. Among
6
them, SP1 has a higher amount of nitrogen, because it has 100% DMAEMA arms with more
7
amine groups. SP3 coating in the bilayer introduced more oxygen, because of the PMAA arms
8
consisting of pendant carboxylic functional groups.
9
10 11 12 13
Figure 5. AFM images of UF membranes coated with SPs: monolayer of SP1 (left panel), monolayer of-SP2
(middle panel), and bilayer of SP1 and SP3 (right panel). The corresponding insets show the sizes of the SPs.
XPS elemental compositions Samples
14
C (%)
N (%)
O (%)
S (%)
PSF
82.5
0
14.3
3.2
PSF-SP1
80.2
1.2
16.0
2.6
PSF-SP2
80.3
0.8
16.3
2.7
PSF-SP1- SP3 76.8 1.7 20.4 Table 3. XPS element analysis of PSF UF membranes before and after SP coating.
1.1
15 16
Given that the hydrophilic nature of the membrane surface is essential for antifouling property,
17
the local hydrophilicity of the SPs coated membranes was analyzed via AFM phase imaging 17
1
modes (Figure 6). Obvious brighter contrast at the periphery versus the darker contrast at the
2
center indicates the presence of two different phases originating from the hydrophilic arms and
3
hydrophobic core. The root-mean-square (RMS) values of phase images indicate the degree of
4
the phase difference between the hydrophilic arms and the hydrophobic core, which we define
5
here as the extent of hydrophilicity of SPs coated membranes [89]. However, the hydrophilic
6
arms are folded inward in dry state to minimize the surface energy and partly exposed the SPs
7
hydrophobic core. This collapsing of the SPs also increased the interparticle gaps exposing the
8
hydrophobic membrane surface to some extent. Therefore, this method cannot provide accurate
9
degree of surface hydrophilicity for such SPs coated membranes. However, it is very useful to
10
estimate and illustrate the difference in local hydrophilicity between different coatings. The RMS
11
value is the highest for bilayer (SP1 and SP3) coated membrane (5.33) followed by SP1 (4.96)
12
and SP2 (4.87) monolayer coated membranes. The highest local hydrophilicity of bilayer coated
13
membrane can be attributed to the highest packing density and consequently the highest surface
14
coverage via the self-assembly of multiple hydrophilic carboxylic acid and PEG functional group
15
containing SPs (SP3) surrounding an amine-containing SP (SP1) (Figure 6 right panel and its
16
inset).
17 18 19 20
Figure 6. AFM phase images of UF membranes coated with SPs: SP1 monolayer (Left panel), SP2 monolayer
(Middle panel) and bilayer of SP1 followed by SP3 (Right panel). The inset in the right panel clearly shows the selfassembly of several carboxylic acids containing SPs around an amine-containing SP on the bilayer coated membrane.
21 22
The hydrophilicity of the SPs coated membranes was also estimated from the under-air water
23
contact angle measurements. The water contact angles are ca. 76°, 66°, 62°, and 57° for bare
24
PSF, PSF-SP1, PSF-SP2, and PSF-SP1-SP3, respectively. This decreasing trend of contact 18
1
angles clearly demonstrates the increasing hydrophilicity of PSF membrane coated with SP1 and
2
SP2 monolayers as well as SP1-SP3 bilayer. However, in-air measurement of contact angle
3
might not be the accurate representation of the wettability of the membrane since the hydrophilic
4
arms of SPs are folded under dry state forming a globular shape and exposing the hydrophobic
5
pristine PSF surface between SPs (Figure 6). The hypothesis behind the design of such core-shell
6
SPs is that the hydrophilic arms are hydrated and extended outwards under water providing a
7
barrier for oil adsorption. Therefore, the oleophobicity of the membranes was evaluated from the
8
under-water captive n-decane bubble contact angle measurements (Figure 7). The water contact
9
angle (WCA), which is a complementary angle to n-decane contact angle, reduces from ca. 90°
10
for bare PSF membrane to ca. 71° for SP1 coated PSF membrane to ca. 52° for SP2 coated
11
membrane to ca. 35° for bilayer (SP1 and SP3) coated PSF membrane. This means that the
12
coating of SPs highly increased the hydrophilicity of the membrane surface, which implies the
13
modified membrane has an anti-oil fouling ability. The SP2 monolayer coated membrane has
14
lower WCA that is higher hydrophilicity than the SP1 monolayer coated membrane presumably
15
due to the presence of PEGMA arms of superior hydrophilicity on SP2 and higher packing
16
density SP2 over SP1. The bilayer coated membrane has the lowest water contact angle,
17
consequently the highest hydrophilicity, which is consistent with its highest local hydrophilicity
18
observed by AFM phase contrast imaging. As explained in the previous section, the highest
19
hydrophilicity of the bilayer coated membrane can be attributed to highest packing of SP3s of
20
highly hydrophilic arms composed of PEG and carboxylic acid groups around SP1. This highest
21
hydrophilicity of bilayer coated membrane makes it the best choice of all four types of
22
membranes for antifouling application.
23
19
1 2 3 4
Figure 7. Under-water captive n-decane bubble contact angles of PSF UF membranes a) Bare PSF, b) PSF-SP1 c)
PSF-SP2, and d) PSF-SP1-SP3.
5
The zeta potential changes of PSF membranes before and after SP coatings as a function of pH
6
are shown in Figure 8. In the streaming potential experiments, the original PSF membranes had
7
more negative zeta potential values under the studied conditions due to the preferential
8
adsorption of anions (e.g., OH−, and Cl−) over cations (e.g., H+, K+, and Na+) of the electrolytic
9
solution on hydrophobic PSF surface [90–92]. When the positively charged SPs were coated, the
10
potentials of the modified membranes shifted to a positive or less negative range. The isoelectric
11
points (IEP) are 3.15, 4.94, 5.00, and 4.60 for bare PSF, PSF-SP1, PSF-SP2, and PSF-SP1-SP3,
12
respectively. The monolayer coated membranes became less negatively charged, or more neutral,
13
while the bilayer coated membrane became more negatively charged around pH 7 solutions. The
14
more negative charge for bilayer coated membrane is arising from the deprotonated carboxylate
15
anions of the hydrophilic arms on SP3s. The excessive negative charges on the bilayer coated
16
membrane around pH 7 and above are suitable to repel commonly encountered negatively
17
charged foulants in wastewater.
18
20
1 2 3 4
Figure 8. The streaming zeta potentials of PSF UF membrane before and after SPs coating. IEP are 3.15, 4.94, 5.0,
4.6 for PSF blank, PSF-SP1, PSF-SP2, and PSF-SP1-SP3, respectively.
5
3.5. Evaluation of filtration performance of PSF UF membranes before and after SP
6
coating
7
Additional coating layers on porous MF and UF membranes always reduce the water flux due to
8
the added barrier to water transport. As a result, antifouling property and separation efficiency
9
can be achieved at the expense of a flux decline. In order to maintain high porosity and high flux,
10
a proper coating should not significantly block the pores or change the porous structure. Figure 9
11
shows the permeate flux through unmodified and SP coated membranes. As can be seen, water
12
flux increased slightly from 1505 LMH for unmodified membranes to 1547, and 1712 LMH for
13
PSF-SP1, and PSF-SP2, respectively. On the other hand, the flux declined to 926 LMH for
14
bilayer coated (PSF-SP1-SP3) membrane. Although the bilayer membrane experienced the
15
largest flux decline of all coated membranes, it still maintained a high pure water flux that falls is 21
1
in the range between MF and UF membranes. The slight increase in water flux for monolayer
2
coated membranes is attributed to the ultra-thin (less than 15 nm) self-assembled layer on the
3
membrane surface which increased the surface hydrophilicity and permeate diffusion rate
4
through the membrane. PSF-SP2 has higher hydrophilicity than PSF-SP1. Therefore, PSF-SP2
5
provides higher water flux than PSF-SP1. In contrast, the reason for water flux decline for
6
bilayer coated membrane is due to the higher packing density of the carboxylic acid containing
7
SPs (SP3) around amine containing SPs (SP1) and the resulting high surface coverage leading to
8
the reduction in access of through the membrane (Figure 6, rightmost panel).
9
10 11
Figure 9. Pure water flux of PSF UF membranes before and after coating by three types of SPs
12
Synthetic oily wastewater (average emulsion droplets of 1.5 ± 0.4 µm with only 1.8% of smaller
13
sizes of 164 ± 41 nm) was filtered to investigate the anti-oil fouling properties of membranes.
14
Figure 10 shows the change in water permeate flux over time at a constant-flux mode. The flux
15
decline is 71% for bare PSF membrane and only about 6-7% improvement was obtained for
16
monolayer coated membranes of PSF-SP1 and PSF-SP2. In contrast, bilayer coated membrane
17
(PSF-SP1-SP3) exhibited only 29% flux decline. Meanwhile, the bare PSF membrane did not
18
provide any flux recovery upon washing with DI water while the flux recovery ratio (FRR) for
19
PSF-SP1, PSF-SP2, and PSF-SP1-SP3 were ca. 41%, 44%, and 79%, respectively. The
20
insignificant improvement in anti-oil fouling property of PSF-SP1 compared to the bare PSF is 22
1
expected considering their almost similar contact angle and susceptibility of cationic arms of
2
these SPs to fouling by slightly negatively charged oil droplets prepared by using a nonionic
3
surfactant (surface zeta potential: -6mV). However, the similar anti-oil fouling property of PSF-
4
SP1and PSF-SP2 is unexpected since the contact angle of the latter was remarkably lower than
5
that of the former. Moreover, SP2 contain only 55% amine-containing hydrophilic arms while
6
the rest of the hydrophilic arms are PEG that should reduce the attachment of oil. Nevertheless,
7
this similar anti-oil fouling performance of PSF-SP1, and PSF-SP2 is presumably governed by
8
the inter SPs gaps on the coated membrane leaving the exposed parts of the PSF membrane
9
prone to fouling. Of important note here is that the applied normal pressure force in the dead-end
10
filtration module might have forced the hydrophilic arms of the SPs to fold inward during the
11
repulsive interaction with the oil droplets and created gaps for oil adsorption on exposed PSF
12
membrane. The remarkable observation is that the anti-oil fouling property of the membrane
13
improved substantially by only one additional layer. This improvement for bilayer coated
14
membrane (PSF-SP1-SP3) has arisen from the reduction of exposed membrane area via the self-
15
assembly of many SP3s around one SP1 (Figure 6 right panel and its inset). As mentioned earlier,
16
the hydrophilic arms of SP3s are composed of both negatively charged carboxylate and charge
17
neutral PEG functional groups, and these functional groups are well known for repelling oil
18
droplets. The rejection of oil emulsions for all membranes was in the range of 97.8-99.2%
19
resulting in clear oil free permeates (picture at the bottom of Figure 10).
23
1 2 3 4
Figure 10. Water flux vs. time for oil/water emulsion filtration tests of bare PSF, PSF-SP1, PSF-SP2, PSF-SP1-SP3
(bilayer), and PSF-SP1-SP3 (bilayer stability) membranes. The picture at the bottom represents the corresponding feed and permeate solutions.
5 6
In order to evaluate the longtime stability of membrane coatings and their respective antifouling
7
property under rigorous conditions, the anti-oil fouling experiments were conducted with a very
8
stable nano-emulsion (53% of 235± 50 nm and 47% of 64 ±11 nm, prepared from highly viscous
9
vegetable oil) under extreme transmembrane pressure (70 PSI) and high feed flow rate (150 L/hr)
10
using a crossflow filtration setup. The coatings of SPs are supposed to be removed from the PSF
11
membrane surface during the period of compaction and flux stabilization for a total of 3 hours
12
followed by oil filtration for 15 hours under high tangential flow rate and extreme pressure if
13
they are loosely bound to the surface. The high viscosity of vegetable oil and the corresponding
14
stable nano-emulsion is expected to increase the oil droplet adhesion to the membrane surface.
15
The extreme transmembrane pressure should intensify the oil adhesion. As anticipated from the
24
1
rigorous operating condition for anti-oil fouling experiment, a sharp flux decline was observed
2
for all membranes followed by a gradual leveling to a steady flux (Figure 11). The bilayer
3
modified membrane (PSF-SP1-SP3) exhibited the highest steady-state flux (4.1 times), followed
4
by SP2 monolayer coated membrane (3.4 times), and SP1 monolayer coated membranes (1.6
5
times). These results are consistent with the cumulative effect of the hydrophilicity, and the
6
nature of the charges of the SPs coated membranes (Figure 7, 8, Table1). The hydrophilicity of
7
the membranes determined by the water contact angle measurements (Figure 7) has the same
8
trend as the antifouling property. Bilayer consists of SP second layer with hydrophilic arms of 35%
9
negatively charged carboxylic acid and 65% PEG functional groups. In contrast, SP1 and SP2
10
have 100% and 55% positively charged PDMAEMA arms, respectively. Oil-in-water emulsion
11
droplets themselves are slightly negatively charged. Therefore, the SPs with more PEGMA
12
and/or PMAA will repel more oil emulsion droplets more efficiently but SPs with more
13
PDMAEMA will attract more oil emulsion droplets. This antifouling test indicates that the
14
numbers of hydrophilic arms of the SPs and their charges are critical to repel hydrophobic
15
foulants. However, this trend of anti-oil fouling property for PSF-SP1 and PSF-SP2 could not be
16
realized under a dead-end setup. As mentioned in the above section, the pressure normal to the
17
membrane surface and the repulsion from the oil droplet might have resulted in inward folding of
18
hydrophilic arms of the SPs under dead-end setup whereas tangential pressure in cross flow setup
19
alleviated the folding of the arms. The rejections of oil were again in the range of 98.1 to 98.5%
20
for all membranes, including the bare PSF.
25
1 2 3 4 5
Figure 11. Normalized water flux vs. time for oil/water emulsion filtration tests of bare PSF, PSF-SP1, PSF-SP2,
PSF-SP1-SP3 (bilayer), and PSF-SP1-SP3 (bilayer stability) membranes under longtime and rigorous operating conditions.
6
Since the bilayer coated membrane (PSF-SP1-SP3) exhibited the highest anti-oil fouling
7
property, the scope of this membrane was further examined for the filtration of a model bio-
8
foulant (BSA) and a model organic matter (HA) (Figure 12) under dead-end filtration setup. As
9
seen in Figure 12(a), the flux decline during BSA filtration was same (71 LMH, 50%) for bare
10
PSF and bilayer coated membrane (PSF-SP1-SP3). The instantaneous flux decline can be due to
11
the collective effect of concentration polarization and fouling where the former is presumably
12
playing the predominant role; particularly for the tighter bilayer coated membrane. Note that the
13
BSA solution was prepared in 1× phosphate buffer saline containing a large amount of many
14
types of salts, including NaCl, KCl, Na2HPO4, and KH2PO4. These salts ions in addition to BSA
15
can result in a large concentration polarization and increase the resistance for water diffusion.
16
However, no flux recovery was observed for bare PSF, indicating the complete irreversible
17
fouling (pore blocking) of this membrane. On the other hand, 60 LMH flux was recovered (FRR:
18
92.4%) for PSF-SP1-SP3 by a simple DI water washing. This high flux recovery for PSF-SP1-
19
SP3 demonstrates that the rapid flux decline for this membrane was mainly due to concentration
20
polarization besides some contribution from loosely and/or firmly bound BSA protein adsorption. 26
1
The rejections of BSA were 57.7% and 80% for bare PSF and bilayer coated membrane,
2
respectively.
3
The anti-HA fouling property of the bare PSF and bilayer coated PSF (PSF-SP1-SP3) was
4
examined by a single batch filtration of 400 mL of the HA solution for 30 minutes using the
5
dead-end filtration setup. Bare PSF exhibited a flux decline by 98 LMH (50%) whereas the flux
6
decline was 76.4 LMH (39.5%) for PSF-SP1-SP3. The flux decline here is also due to the mutual
7
effect of fouling and concentration polarization. The HA feed solution also contains large
8
amount of NaCl solution since the HA was initially dissolved in 0.1M NaOH solution and the pH
9
was adjusted to 7.0 by adding 4M hydrochloric acid. The presence of NaCl salt increased the
10
salinity of the feed solution, which contributed to the increase in concentration polarization
11
and/or increases in the resistance for water diffusion through the membrane. The FRR for bilayer
12
coated membrane is 91.6% which is only 18% higher than that for bare PSF membrane (73.4%).
13
Interestingly, a significantly high flux recovery was observed even for bare PSF in contrast to no
14
flux recovery for this membrane during oil and BSA filtration. The rejection of HA was 97.5%
15
for bare and 99.5% for bilayer coated membrane which is reflected in the color differences of the
16
feed and permeate solutions (Figure 12b inset).
17 18 19
Figure 12. Water flux vs. time for a) BSA, and b) HA filtration tests of bare PSF, and PSF-SP1-SP3 (bilayer),
membranes, respectively. The inset in figure b) shows the photographs of HA feed and permeate solutions.
20 21
3.6. Evaluation of thermal stability of bilayer coated PSF UF membrane
27
1
The thermal stability of the bilayer coated membrane was examined thoroughly since it exhibited
2
superior antifouling properties. The thermal stability of the bilayer coated membrane was studied
3
by immersing the membrane under water at 80 °C and agitating the water for 1 week at 250 rpm.
4
Then, the anti-oil fouling performance was evaluated. The flux decline under constant flux mode
5
oil emulsion filtration (Figure 10) was 37% which only 8% higher that the bilayer membrane
6
before stability test (29%). It also exhibited slightly lower FRR compared to bilayer coated
7
membrane (73.9% verses 78.6%) before thermal treatment under water at 250 rpm agitation.
8
During the oil filtration under rigorous conditions, the steady state flux after initial fouling was
9
20% lower than that for the membrane before stability test, which was still 3.2 times that of bare
10
PSF membrane (Figure 11). To reveal the reason behind the slight loss in the antifouling
11
property after under-water thermal treatment, a wide range of complementary characterizations
12
was carried out.
13
The XPS elemental composition was found to be 78.5% C, 0.8% N, 19.7% O, and 0.9% S versus
14
the initial composition of 76.8% C, 1.7% N, 20.4% O, and 1.1% S. The slight decrease in the N
15
and O content can be associated with detachment of SP particles from the membrane surface
16
and/or the thermal hydrolysis of ester bonds connecting the amine and PEG containing pendant
17
moieties with polymeric backbone.
18
The water contact angle was almost unchanged (Initial 36.7° ± 1.4° and Final 38.1° ± 0.3°),
19
indicating no noticeable loss in surface hydrophilicity. The pure water flux should increase if the
20
loss in hydrophilic components and corresponding anti-oil fouling property is due to the
21
detachment of SPs from the surface. In fact, the pure water flux declined by 110 LMH. Therefore,
22
the slight loss in hydrophilicity can be a consequence of hydrolysis of hydrophilic arms and/or
23
folding of the hydrophilic arms inwards due to the thermo-responsive property of PDMAEMA
24
arms. However, pure water flux of bare PSF membrane also decreased by 202 LMH pointing out
25
to the shrinkage of pores of PSF membrane itself at high temperature. This shrinkage of the base
26
membrane at high temperature suggest the folding of thermo-responsive PDMAEMA arms is the
27
reason for slightly losing antifouling property. Finally, AFM imaging technique was used to
28
assess the stability of the coating. The packing density of SPs did not change significantly,
29
although some extent of collapsed globule formation is observed. (Figure 13). Meanwhile, the
30
decrease in RMS value of phase contrast images from 5.33 to 4.72 again indicates the loss
31
hydrophilicity which can be attributed the folding of the hydrophilic arms inwards rather than 28
1
detachments of SPs from the membrane surface. Overall, the loss in hydrophilicity and anti-oil
2
fouling property were not dramatic.
3
4 5 6 7
Figure 13. AFM height images (Top panel) and phase image (Bottom panel) of bilayer coated membrane surface
before and after under-water thermal stability test for 1 week. The top and bottom insets in the top images are the height profiles of SP1 and SP3, respectively.
8
4 Conclusion
9
We have investigated anti-oil fouling efficiency and thermal stability of ultrathin SP coatings
10
(monolayers and bilayers) on a hydrophobic PSF UF membrane to validate their feasibility for 29
1
oil-gas produced water treatment. The SPs are made of hydrophobic PS cores and hydrophilic
2
arms of variable compositions of a tertiary amine, polyethylene glycol, and carboxylic acid
3
functional groups. The presence of the hydrophobic core, as well as the cationic tertiary amine
4
group in the arms, facilitated self-assembly of the SPs on a hydrophobic membrane surface,
5
forming an ultrathin coating layer (less than 15nm). The bilayer coated membranes with SP3-
6
comprising the most abundant hydrophilic and negatively charged arms- as the second layer
7
(PSF-SP1-SP3) showed the highest water flux during filtration of a synthetic oil-water emulsion.
8
All membrane samples showed more 98% of oil rejection. Although the anti-oil-fouling property
9
of the bilayer coated membrane was compromised to some extent after rigorous thermal
10
treatment, there was no evidence of SP loss on the membrane surface and the membrane still
11
exhibited more than 2 and 3 times higher water flux than unmodified PSF membrane under dead-
12
end and crossflow filtration, respectively. The results imply that the interactions between star
13
polymers and the hydrophobic membrane are considerably strong to endure high-temperature
14
membrane operation. The thermal and/or chemical stability of the coatings could be further
15
improved by substituting some vulnerable moieties in the hydrophilic arms (e.g., ester backbone).
16
The unique design of SPs that drives a self-assembly process on a hydrophobic membrane
17
surface could be advantageous in modifying a commercial-scale membrane module through a
18
simple post-treatment process (e.g., in-situ SP assembly by flowing a solution). The tunable
19
functionality of the SPs also opens more opportunities to target different types of foulants
20
associated with wastewater discharged from biochemical, pharmaceutical, and food processing
21
industries. For instance, the bilayer coated PSF membrane provided superior antifouling property
22
to unmodified PSF membrane against a model bio-foulant, bovine serum albumin, and a model
23
organic matter, humic acid.
24
Acknowledgments
25
The authors wish to acknowledge the financial support from the Natural Sciences and
26
Engineering Research Council of Canada (NSERC), the IBM Alberta Centre for Advanced
27
Studies (CAS), IBM Research-Almaden, University of Alberta, the National Research Council
28
and Alberta Innovates (AI). The authors would like to thank Mr. Amin Karkooti for training zeta
29
potential measurement, Mr. Farhad Ismail for training contact angle measurement, Mr. Asad
30
Asad for training the operation of membrane crossflow filtration set up, Ms. Laleh 30
1
Shamaeighahfarokhi for training on coating related facilities, and Mr. Bradley Smith for all
2
supports relevant to the National Research Council (NRC) – Nanotechnology Research Centre
3
lab supply, equipment, and facility. The authors would like to offer special thanks to Dr. Joseph
4
S. Sly (a talented researcher at IBM Almaden Research Center), who, although no longer with us,
5
provided inspiration for this study.
6 7
Declarations of interest: none
8
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treatment of oil sands produced water: Process integration using ion exchange
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(2016) 14869–14887.
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sands produced water: systematic study of influential parameters, Desalin. Water Treat. 57
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4
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6
40
Highlights: 1. IBM-invented star block copolymers (SPs) were utilized to synthesize antifouling UF membranes. 2. Fine-tuning of the composition of SPs was done for making monolayer and bilayer coated membranes. 3. Hydrophilic arms of SPs have mitigated the membrane fouling. 4. For the first time, the thermal stability of SPs-modified membranes was investigated. 5. Bilayer coating of SPs exhibited the best hydrophilicity, anti-oil fouling properties, and thermal stability.
1
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0376-7388(19)31732-6
DOI:
https://doi.org/10.1016/j.memsci.2019.117686
Reference:
MEMSCI 117686
To appear in:
Journal of Membrane Science
Received Date: 10 June 2019 Revised Date:
5 October 2019
Accepted Date: 20 November 2019
Please cite this article as: B. Soltannia, M.A. Islam, J.-Y. Cho, F. Mohammadtabar, R. Wang, V.A. Piunova, Z. Almansoori, M. Rastgar, A.J. Myles, Y.-H. La, M. Sadrzadeh, Thermally stable core-shell star-shaped block copolymers for antifouling enhancement of water purification membranes, Journal of Membrane Science (2019), doi: https://doi.org/10.1016/j.memsci.2019.117686. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
1 2 3
Thermally Stable Core-shell Star-shaped Block Copolymers for Antifouling Enhancement of Water Purification Membranes
4 5
Babak Soltanniaa , Muhammad Amirul Islama*, Jae-Young Choc, Farshad Mohammadtabara, Ran
6
Wangb, Victoria A. Piunovab, Zayed Almansooria, Masoud Rastgara, Andrew J. Mylesc*, Young-
7
Hye Lab*, Mohtada Sadrzadeha*
8 9 10 11 12 13 14 15
a
16
Abstract
17
Star-shaped block copolymers (SPs), consisting of a hydrophobic core and hydrophilic arms, are
18
a new material architecture introduced to the area of antifouling coatings for water filtration
19
membranes. The unique self-assembly behavior of these SPs on hydrophobic membranes surface
20
allows the formation of an ultrathin coating (less than 15 nm), rendering a highly hydrophilic
21
antifouling membrane surface. Here, we have judiciously examined several important structural
22
and process parameters, such as the chemical composition of SPs and surface properties (e.g.,
23
surface potential, coverage, and wettability) driven by a mono- or bi-layered assembly of SPs, to
24
acquire a highly hydrophilic and thermally stable antifouling coating on a hydrophobic
25
ultrafiltration (polysulfone, PSF) membrane for oil sands produced water treatment. Among
26
three types of SPs (e.g., SP1 with 100 % amine, SP2 with 55 % amine/45% PEG, and SP3 with
27
35 % carboxylic acid/65% PEG moieties in the arms) and their variable coating conditions, the
28
bilayer coating comprised of SP3 top-layer and SP1 bottom-layer exhibited the best
29
hydrophilicity and anti-oil fouling efficiency showing 2 ~ 4 times higher water flux compared to
30
an unmodified PSF membrane during the filtration of synthetic oil-water emulsions conducted in
31
both constant flux mode and constant pressure mode. Outstanding antifouling efficiency of the
Department of Mechanical Engineering, 10-367 Donadeo Innovation Centre for Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada. b
IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120, United States
c
National Research Council Canada, Nanotechnology Research Centre, 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada * Corresponding Authors: [email protected], +1(587)710-3281; [email protected], +1(780)492-8745; [email protected], +1(408) 927-1256, [email protected], +1(780) 641-1758
1
1
bilayer coating toward natural organic matters (NOMs) was also verified by using bovine serum
2
albumin (BSA) and humic acid (HA) as the model foulants. The bilayer-coated membrane
3
retained more than 80 % and 90 % (for nano and micron size oil emulsion, respectively) of its
4
initial antifouling efficiency even after rigorous thermal treatment for an extended period (80 °C
5
for 1 week), which demonstrates its feasibility for high-temperature membrane operation.
6 7
Keywords: Star-shaped Block Copolymers; Membrane; Oily Wastewater Treatment;
8
Antifouling Coating; Oil Sands
9
1. Introduction
10
Imminent water scarcity as a result of global warming, water pollution and overconsumption of
11
freshwater has urged immense interest among researchers to develop emerging technologies for
12
water reclamation, recycling, and reuse. Membrane technology is a feasible and energy-efficient
13
solution to seawater desalination and wastewater treatment [1–3]. Membrane separation
14
technologies have secured a crucial role in industrial water purification due to minimal operating
15
costs, compact design, as well as high product quality. Given the fact that inorganic membranes
16
are so expensive and could be irreversibly fouled, research has been primarily focused on
17
modification of polymer materials to fabricate high-performance polymeric membranes [4–8].
18
Fouling phenomenon, mainly caused by the adsorption of particles, organic matter, and
19
microorganisms over the membrane surface or within its pores, is perhaps the main obstacle
20
against practical application of membrane technologies [9]. Accumulation of water contaminants
21
on the membrane surface leads to a severe flux decline due to the formation of a cake layer of
22
foulants and membrane pores blocking that consequently shortens membrane’s lifespan [10–12].
23
Particularly, the oily wastewater, such as oil sands produced water in Alberta (Canada), contains
24
highly stable emulsified oil which is one of the most notorious foulant encountered in membrane
25
purification processes. Fouling of membranes by emulsified oil necessitates periodic cleaning
26
using various chemical processes, demanding cleaning cost and generating further waste [13–16].
27
The oily wastewater is indeed the largest source of wastewater generated from industrial sectors
28
[14,15,17,18]. Therefore, enormous effort has been devoted to the membrane surface
29
modification techniques to mitigate the oil fouling propensity of the membranes [19–25].
2
1
The majority of recent studies for surface modification has been devoted to the chemical grafting
2
of functional materials to the membrane surface [26–29]. The objective was to tune the
3
hydrophilicity and surface charge of membranes to mitigate fouling caused by the hydrophobic
4
and electrostatic interactions between the water constituents and the membrane surface.
5
Although chemical grafting is found to be effective in inducing the desired characteristics to the
6
membrane, the major drawback is the separation of functional materials from the surface after a
7
long-time operation. Moreover, the chemical grafting does not ensure complete surface coverage,
8
requires chemical reaction and can result in increased costs. On the other hand, incorporation of
9
a very thick layer of these materials - by membrane surface activation and applying very reactive
10
grafting methods, such as radical polymerization - to the surface introduces resistance against
11
water transport through the membrane and adversely affects the membrane permeation properties.
12
Recently, a versatile and effective technique has been developed where modifying agents are
13
deposited on a substrate via the layer-by-layer (LBL) deposition technique to form a thin and
14
defect-free separation layer [30–33]. This technique relies on the sequential deposition of
15
alternating layers of polycations and polyanions on a porous substrate to make a multi-layer film.
16
The number of deposited layers can be arbitrarily determined so that the desired membrane
17
permeability and selectivity are achieved [31,33–36]. This technique also enables control over
18
surface potential and morphology [37–39]. So far, most of the studies on LBL deposition
19
technique utilized linear polyelectrolytes that require a considerable number of layers to achieve
20
desirable separation and/or antifouling properties [35,36,40–42]. This can severely reduce the
21
water flux though the membrane and commonly used to prepare nanofiltration (NF) membranes
22
[33,42–44]. These linear polyelectrolyte-based coatings also pose stability concern under
23
filtration conditions and often requires postdeposition treatment (for example cross-linking)
24
which is not feasible for large scale membrane modification and for commercial membrane
25
modules, such as spiral wound modules [42,43,45,46].
26
Charged nanomaterials or hyperbranched polyelectrolytes of globular shaped nanoarchitecture
27
have been demonstrated to reduce the number of deposition layers to achieve desirable
28
antifouling and/or separation properties since these materials bear high-charge density per unit
29
volume compared to linear polyelectrolytes [9,23,46–59]. However, bare inorganic nanoparticles
30
(NPs) are in general not suitable for direct deposition on hydrophobic membranes such as
31
polysulfone (PSF), and polyethersulfone (PES) due to the lack of enough interaction energy. The 3
1
NPs are either modified with charged polyelectrolyte or they are self-assembled within
2
alternative layers of cationic and anionic polyelectrolyte. This assembly process is mainly based
3
on electrostatic interaction and largely depends on the nature of the base membrane such as its
4
surface charge and the presence of surface ionic functional groups, and therefore, often requires
5
membrane surface activation and/or postdeposition treatment [47,54,59–61]. Titania NPs
6
possessing Ti4+ coordinative sites are directly deposited on PSF and PES surface since these NPs
7
can form coordinative bond with the sulfone groups of the membrane. However, bare NPs are
8
not well dispersed in solvent; therefore, the agglomeration of NPs results in uncontrolled surface
9
roughness, which is prone to fouling [62,63]. Long-term stability of such coordinatively
10
deposited films is also questionable [62]. Titania is commonly used to coat membrane with
11
abundant anionic surface groups, including polyamide, and sulfonated PES where it can form
12
stronger coordinative bonds [9,50,58,64]. Carbon based nanomaterials, for example carbon
13
nanotubes and graphene oxide, also require multistep surface activation [53,65]. Hyperbranched
14
polyelectrolyte is also deposited mainly based on electrostatic interaction and require negatively
15
charged hydrolyzed polyacrylonitrile support [49].
16
It is evident from a recent study that the application of multi-charged star-shaped block
17
copolymers (SPs) results in the superior performance of the coated membrane mainly due to the
18
high charge density and well-packed structure [66]. These SPs comprise of hydrophilic arms
19
(shell part) anchored to the hydrophobic core. While the hydrophilic arms help to mitigate the
20
membrane fouling, the hydrophobic core prevents undesirable swelling of the selective layer of
21
the membrane. The chemical composition of hydrophilic arms can be tuned to introduce different
22
types of functional groups, including amine groups representing the cationic segment, carboxylic
23
acid groups representing the anionic segment, or polyethylene glycol representing charge-neutral
24
hydrophilic moieties [67,68]. The functionalized SPs are water soluble due to the polarity of
25
hydrophilic arms and can be easily deposited on the hydrophobic PSF via dual interactions, that
26
is hydrophobic interaction with PS core and electrostatic interaction with cationic tertiary amine
27
containing arms. This collective interaction is expected to provide superior stability of the
28
monolayer coating over other types of nanomaterials used so far. The successive layers are also
29
presumed to be stable due to multitude of inter SP’s chain entanglement where the chains consist
30
of numerous oppositely charged functional groups. The water solubility of the SPs also enables
31
the surface-coating from a green, abundant, and membrane compatible aqueous medium. In 4
1
addition, the extra interstitial volume between adjacent SPs and the resultant interconnected
2
water channels significantly improves the water permeation [69–73]. It is noteworthy that the
3
high charge density per polymeric arm, significant local density of polymeric arms, and surface
4
functionality of the SPs remarkably lower the required number of stacking layers to achieve
5
desirable separation and antifouling properties [66,74–80]. Our previouse study with structurally
6
similar but compositionally different SPs was focused on fabicating NF membrane that required
7
10 alternative cationinc and anionic SPs layers to from a dense barrier film on a porous substrate
8
[66]. For ultrafiltration (UF) or microfiltration (MF) membranes that have been widely used for
9
waste water treatment and/or pre-treatment process for RO/NF, however, needs a cost-efficient,
10
surface modification technique and ultrathin coating layer to provide good antifouling properties
11
with minimal loss of intrinsic water permeability of the base membranes. In this context, fine-
12
tuning of the composition of hydrophilic arms on the SPs holds the potential for fabricating only
13
a monolayer or a bilayer coated membrane with higher permeability while maintaining the
14
desired antifouling properties for hydrophobic, porous UF or MF membranes.
15
Another technological challenge is the development of thermally stable membrane since the
16
temperature of produced water from thermally enhanced heavy oil recovery method for the
17
extraction of bitumen in Alberta, Canada, e.g., steam-assisted gravity drainage (SAGD) process,
18
is usually 70-95 °C [81]. In the SAGD process steam is injected underground to dilute the
19
bitumen and thus facilitate its extraction. The diluted bitumen is first separated from steam
20
condensate and the hot produced water, containing oil, organic matter and inorganic materials
21
(mainly silica), is first cooled down, then treated by a variety of conventional methods [82–85].
22
The treated water is finally reused in steam generators. Cooling and reheating cycles for
23
membrane filtration demand capital and operating expenses [86]. Therefore, it is economical to
24
perform the oil filtration at high temperatures. Low thermal stability of the functional materials
25
used for the surface modification of the membrane can hinder the application of the modified
26
membrane for such high temperature produced water. Degradation of the membrane surface
27
functional materials at higher temperatures can lead to the loss of their functionality together
28
with accelerated leaching out into the wastewater solution during crossflow filtration at elevated
29
temperatures. Therefore, one of the prime objectives of the membrane research has always been
30
subjected to the development of surface modification methodologies that provide excellent
31
antifouling properties, permeation properties, and thermal stability. Given that, it is essential to 5
1
assess the thermal stability of the SPs-modified membranes (e.g., dettachments of SPs, and their
2
structural and/or functinal change) to test the feasibility for the treatmnet of high-temperature
3
streams of oil sands industry in Alberta, Canada.
4
In order to address the issues associated with water permeability, antifouling property, and
5
thermal stability of water filtration membrane, we have investigated the effect of the chemical
6
composition of hydrophilic arms on the SPs to deposit a highly hydrophilic and thermally stable
7
monolayer and bilayer antifouling coating on a commercial polysulfone ultrafiltration (PSF UF)
8
membrane. The size, hydrophilicity, surface charge, and thermal stability of the SPs were
9
investigated prior to their coating on the porous PSF UF membranes. Following the detailed
10
investigation of the properties of the SPs, monolayers of each SP and bilayer of alternating
11
polycations and polyanions SPs were deposited on the PSF UF membranes. The SP-coated
12
membranes were thoroughly characterized (e.g., self-assembly behaviors including packing
13
density, surface charge, and wettability) and their separation performance and antifouling
14
property were also studied. Finally, the bilayer coated PSF UF membrane exhibiting the best
15
anti-oil fouling property was subjected to thermal stability test underwater at 80 °C and 250 rpm
16
stirring for 7 days. This membrane was thoroughly characterized to determine any dettachments
17
of SPs, their structural change and/or functinal group degradation, loss in hydrophilicity and
18
antifouling property.
19
2. Material and Methods
20
2.1. Material
21
The SPs used in this study comprised of a hydrophobic polystyrene (PS) core and different types
22
of
23
polydimethylaminoethyl methacrylate (PDMAEMA), and polymethacrylic acid (PMAA). The
24
cationic and anionic SPs (PS-PDMAEMA, Mw = 1,174 kDa and PS-PMAA, Mw = 560 kDa)
25
contain about 30 independent PDMAEMA and PMAA arms covalently linked to a cross-linked
26
PS core, respectively [66–68,80]. The PS cores of the SPs are similar while the arms are of a
27
variable combination of PEGMA, PDMAEMA, and PMAA polymers (SP1: 100% PDMAEMA,
28
SP2: 55% PDMAEMA-45% PEGMA, and SP3: 35% PMAA-65% PEGMA). The chemical
29
structures of these SPs are shown in Figure 1. The synthesis method of these SPs is provided
hydrophilic
arms
including
polyethylene
6
glycol
methacrylate
(PEGMA),
1
elsewhere [68,87]. PSF UF membranes (PSF-20, MWCO 20 kDa) were purchased from Sepro
2
Membranes Co. and used as substrate. Tetrahydrofuran (THF) and isopropyl alcohol (IPA) were
3
purchased from Sigma-Aldrich and used for preparing SP solutions and treating the surface of
4
the PSF membranes, respectively. Synthetic oily wastewater solutions were prepared using
5
vegetable oil (Crisco® Pure Vegetable Oil from Safeway) with Tween 20 surfactant (Bio-Rad)
6
and Hexadecane (n-Hexadecane, 99%, pure, Fisher Scientific, Canada) with Tween 80 surfactant
7
(Bio-Red). n-Decane was purchased from Sigma-Aldrich and used for the captive bubble contact
8
angle measurements. Bovine Serum Albumin (BSA) was purchased from ChemCruz and was
9
dissolved in 1× Phosphate buffer saline (PBS) of pH 7.4 (kindly provided by Biochemistry lab of
10
National Research Council Canada) followed by adjustment of pH to 7.0 with 0.1 M
11
hydrochloric acid (HCl). Humic acid (HA) purchased from Sigma Aldrich was dissolved in 0.1
12
M sodium hydroxide solution and pH was adjusted to 7.0 by adding 4 M HCl.
13
2.2. Solution preparation
14
To prepare 0.1 wt% solution, 100 mg of the SPs were dissolved in 800 µl THF in a 15 mL glass
15
vial using an ultrasonic bath. Then, 10 ml DI water was added slowly in 100 µL portions every
16
30 seconds while stirring at 1000 rpm. The solution was sonicated for 10 minutes in an ultrasonic
17
bath. Finally, the solution was diluted to 100 mL with DI water and sparged with nitrogen for 3
18
hours to remove the THF. The pH of the DI water was about 6.5 ± 0.2, which fluctuated slightly
19
by adding different SPs to the aqueous solutions. Variation of the pH can be attributed to
20
deprotonation of carboxylic acid groups and consequently acquiring negative charge in the case
21
of SP3. Partial protonation of tertiary amine groups in SP1 and SP2 could also increase the
22
alkalinity of water [66]. Three different types of solutions were prepared, following the
23
procedure mentioned above, containing 0.1% wt% of SP1, SP2, and SP3, respectively.
7
1 2 3 4 5
Figure 1. (a) Scheme and examples of the chemical formula of the SPs with PS core and arms of PEGMA and DMAEMA. SP1: 100 mol % DMAEMA (x = 1), SP2: 55 mol % DMAEMA and 45 mol % PEGMA (x = 0.55), (b) Scheme and example of chemical formula of SPs with PS core and arms of PEGMA and PMAA. SP3: 65 mol % PEGMA and 35 mol % PMAA (x = 0.35).
6
2.3. Characterization of SPs
7
Hydrodynamic diameter and surface charge density of the SPs (0.1 wt % aqueous solution) were
8
measured by using a Malvern Zetasizer Nano-ZS (Malvern Instruments Ltd., Worcestershire,
9
UK). The average hydrodynamic diameter was calculated against polystyrene latex standard by
10
performing three series of 11 measurements. The zeta potential was calculated from
11
electrophoretic mobility measurements in three series of 20 measurements. All measurements
12
were done at 25 oC.
13
The size distribution of SPs was evaluated using transmission electron microscopy (TEM: JEOL
14
2200 FS TEM – 200kV Schottky field emission instrument equipped with an in-column omega
15
filter). A TEM sample was prepared by depositing a droplet of SP solution (0.01%) on a carbon-
16
coated 400-mesh copper grid (Electron Microscopy Sciences) and blotting using filter paper after
17
10 s. The sample was then stained by applying one droplet of uranyl acetate (2 % aqueous
18
solution) for 120 s, followed by blotting and air-drying. The hydrophilic arms on the SPs stained
19
with uranyl acetate were imaged via bright field-scanning transmission electron microscopy (BF8
1
STEM) operated at 30 kV accelerating voltage and 30 µA emission current on an ultra-high-
2
resolution Hitachi S–5500 cold field emission SEM.
3 4
2.4. Thermal stability of SPs
5
Thermal stability of SPs was evaluated under high-temperature TEM (HT-TEM) on a Hitachi H-
6
9500 environmental TEM at 100kV acceleration voltage with 5°C/min elevation of Temperature
7
until 80°C and structural changes were monitored (Please refer to section 2.3 for TEM sample
8
preparation).
9
2.5. Self-assembly of SPs on Commercial UF Membranes
10
The PSF UF membrane was pretreated to remove the protective coating layer by soaking into
11
IPA for 30 minutes, followed by rinsing with DI water. The membrane was finally kept in DI
12
water overnight prior to the coating to ensure complete removal of IPA. To coat the pretreated
13
PSF membrane with SP through the self-assembly process, the substrate was first placed between
14
two frames with sealing rubbers. Frames were held tightly with paper clips, and an SP solution
15
was poured onto the active surface of the membrane. After 15 min, the solution was removed,
16
and the membrane was immersed in DI water for 20 minutes to remove the loosely bound SPs.
17
For the bilayer coating, the same process was repeated with carboxylated-SP (SP3) solution on
18
top of the first layer prepared with the amine-functional SP (SP1) solution. Then, the membrane
19
was removed from the frame and was stored in DI water overnight. The first and the second
20
membrane samples denoted as PSF-SP1 and PSF-SP2 comprise a monolayer of SP1 with amine-
21
functional groups, and a monolayer of SP2 with amine and PEG functional groups, respectively.
22
The third membrane sample (PSF-SP1-SP3) consists of a bilayer featuring SP3 with carboxylic
23
acids deposited on the monolayer of amine-functional SP1. The compositions of the membrane
24
samples prepared in this work are presented in Table 1.
25 26
Table 1. Specification of different types of SP coated PSF UF membranes used in this work. Sample
Active layer
PSF-SP1
Monolayer of SP1
PSF-SP2
Monolayer of SP2
PSF-SP1-SP3
Bilayer of SP1 and SP3
27 9
1
2.6. Characterization of SPs coated PSF UF membranes
2
XPS analysis was done on a Physical Electronics Quantum 2000 ESCA, using a monochromated
3
Al Kα X-ray source at 1496 eV, 200 µm spot size throughout and charges were neutralized. 1000
4
eV survey spectra (187 eV pass energy, 1.6 eV/step) were taken. Self-assembly and
5
hydrophilicity of SPs on PSF membranes were evaluated by AFM. Specifically, the sample
6
surface was observed using a Digital Instruments/Veeco Instruments MultiMode Nanoscope IV
7
AFM equipped with an E scanner. In order to have information about local hydrophilicity &
8
hydrophobicity from the sample surface, high resolution, soft tapping mode AFM Probes
9
(MikroMasch USA, Inc.) with low spring constants of 5.0 N/m and 1 nm radius were used. To
10
obtain a clear phase mode image from the surface, low scan rate (0.3-0.5 Hz) and amplitude
11
setpoint were choosen during measurement. Contact angle measurements were performed using
12
a contact angle analyzer (Data Physics Instruments GmbH) with a 2 µL DI water drop under air
13
or 6 µL captive n-decane bubble in water. For captive n-decane bubble in water contact angle
14
measurement, a 2 cm wide membrane strip was first placed face down in a custom-made holder.
15
The membrane holder assembly was then accommodated in a small, clear cuvette so that the
16
membrane was fully immersed in water. A high-speed computer-controlled camera was focused
17
on the membrane surface and at least three n-decane bubbles were placed on the membrane
18
surface using a syringe with an inverted needle. Contact angle samples were kept under
19
deionized water and rinsed before analysis. The measurements of membrane zeta potential were
20
made using an Electrokinetic Analyzer (SurPASS 3, Anton Paar) equipped with a gap adjustable
21
cell (20 mm × 10 mm). An automatic pH titration was performed at room temperature within a
22
pH range from 3.0 to 8.0. Streaming potential measurements were done using different salt
23
solutions with a target ramp pressure of 0.5 bar. Before measurements, membranes were
24
thoroughly rinsed in MilliQ water, and the pH of the solutions was adjusted by the addition of
25
0.05 M NaOH or 0.05 M HCl. A KCl solution of 0.001M was used as an electrolyte solution.
26
2.7. Pure-water flux test of SPs coated membranes
27
A dead-end filtration module (Millipore) equipped with nitrogen pressure control was used to
28
measure the pure water flux. The dead-end filtration module has an active membrane area of
29
14.6 cm2. A pressure of 50 psi was applied to the system at room temperature. A stirring speed of
30
700 rpm was used to ensure the feed solution was well mixed inside the cell during the
10
1
experiment. The weight of permeate was recorded by a digital balance connected to a computer.
2
Pure water flux of membrane was calculated using the following equation [88]: =
∆
3
where Jw is pure water flux (LMH), m is the mass of the permeate water (kg), ρ is water density
4
(kg/m3), Am is the effective area of membrane (14.6 cm2), and ∆t is the time of permeation (hr).
5
2.8. Antifouling tests of SPs coated membranes
6
Fouling behavior of UF membranes, before and after SPs coating, was evaluated by filtering
7
synthetic oily wastewater containing 1000 ppm of hexadecane or 1500 ppm vegetable oil
8
emulsion. To prepare the hexadecane oil emulsion 0.5 g of hexadecane and 0.75 mg of Tween 80
9
was blended in 500 mL DI water and the mixture was homogenized with a homogenizer
10
(Homogenizer 150, Fisherbrand, Canada) at the highest speed for 4 minutes. To prepare the
11
vegetable oil emulsion, 4.5 g of vegetable oil was mixed with 0.32 g of Tween 20 via mechanical
12
stirring. Then 900 mL of DI water was added slowly dropwise via addition funnel while stirring
13
at 1000 rpm. The resultant emulsion was diluted to 3 L by adding DI water and stirred for 1 hour
14
followed by homogenization with the same homogenizer. Oil emulsion droplet size was
15
determined by dynamic light scattering (DLS) using using Malvern Zetasizer Nano series (Nano-
16
ZS). In case of constant-flux mode filtration with hexadecane oil emulsion, a 400 ml dead-end
17
filtration cell (Amicon, UFSC40001) was used. Nitrogen gas was used to pressurize water
18
through the membrane. The bare PSF, PSF-SP1, and PSF-SP2 were pressurized with nitrogen at
19
3.5-5.0 psi and PSF-SP1-SP3 was perssurized at 10.0-15.0 psi to obtain a stable initial flux of ca.
20
185 LMH. The mass of permeate water was monitored and recorded over time using a digital
21
weighing balance (ME4002, Mettler Toledo, USA). After 30 minutes of constant pure water flux,
22
400 mL of hexdecane oil emulsion was added to the cell and the filtration was performed for 2
23
hours at 450 rpm while the cell was refilled with the permeate on demand. At the end of oil
24
filtration, the oil emulsion was discarded and the membrane was rinsed with 400 mL DI water
25
for 20 minutes at 450 rpm. Then, pure water flux was recorded for 30 minutes. The antifouling
26
properties against BSA and HA solutions were evaluated in the smilar processes. To study the
27
stability of membrane coating for longtime operation under rigorous operating condition a satble
28
nanosized oil emulsion prepared with a very viscouse vegetable was used as the foulant and the
29
filtration was conducted with a crossflow filtration setup (Water Planet). All the membranes 11
1
were compacted by running DI water at 80 PSI for 90 minutes, and water flux was stabilized at
2
70 PSI for 90 minutes. Then, the vegetable synthetic oil emulsion was added as the feed solution
3
and filtration was run for 15 hours. The oil or BSA concentrations in the feed and permeate
4
solutions were measured using a total organic carbon (TOC) analyzer (Shimadzu TOC-L with
5
ASI-L and TNM-L). The concentration of HA in the feed and permeate solution was calculated
6
by measuring their absorbance at 254 nm using a UV-Vis spectrophotometer (Thermo Electron
7
Corporation, model: Genesys 10-S) and dividing the absorbance by their absorptivity obtained
8
from the calibration curve. Feed HA solution was diluted 20 times before absorbance
9
measurement in order to comply with Beer-Lambert law. The retention ratio (or rejection) of the
10
oil, BSA, and HA was calculated by the following equation:
11
R(%) = 1 −
12
where Cf and Cp represent the oil, BSA, or HA concentration in the feed and permeate solution,
13
respectively.
14
The flux recovery ratio after simple wash was calculated by the following equation:
15
FRR (%) =
16
2.9 Thermal stability analysis of bilayer SP coated PSF UF membrane (PSF-SP1-SP3)
17
For the evaluation of thermal stability, the PSF-SP1-SP3 membrane was clamped to a glass slide
18
with stainless steel paper clips and immersed in 2L DI water in a clean glass beaker and stirred at
19
250 rpm at 80 °C for 1 week. The beaker was covered and wrapped with a plastic bag to slow
20
down the water evaporation. The thermal stability of the coating was evaluated by measuring the
21
elemental composition, contact angle, pure-water flux, streaming surface zeta potential, and anti-
22
oil fouling property.
23
3. Results and discussion
24
3.1. Characterization of SPs
25
Table 2 shows the chemical structures, hydrodynamic diameter, and Zeta potentials of the core-
26
shell SPs used in this study. Each SP has a cross-linked, hydrophobic polystyrene (PS) core,
27
which is made of approximately 30 individual PS linear polymers via crosslinking in the
28
presence of para-divinylbenzene. The hydrophilic methacrylate polymer arms contain PEGMA,
× 100
× 100
12
1
PDMAEMA, and/or PMAA with various compositions, leading to different hydrodynamic
2
diameters as measured by DLS. SP1showed positive surface potential (ζ = +28 mV), indicating
3
the pendant amine functional groups in the outer shell are partially protonated in water under
4
neutral pH conditions, while SP3 is negatively charged (ζ =-17 mV) due to carboxylic acid
5
functional groups in the arms. Meanwhile, SP2 exhibited less positive charge compared to SP1
6
due to the presence 45% charge neutral PEGMA in the hydrophilic arms. The charge
7
characteristics of SPs will influence the surface potential of membranes after coating.
8 9
Table 2. List of SPs with different compositions
Sample Code.
Composition (mol %) Diameter (nm)
Potential (mV)
PDMAEMA
PEGMA
PMAA
SP1
100
0
0
56.7±0.1
28.1±0.1
SP2
55
45
0
64.2±1.0
10.8±2.3
SP3
0
65
35
44.2±0.7
-17±0.2
10 11
Provided that DLS measures the hydrodynamic diameter of the SPs in an extended state of the
12
arms in water, which includes water corona in the size measurement, the real sizes were
13
determined by TEM imaging of uranyl acetate stained SPs (Figure 2). Note that the uranium
14
metal staining was done to enhance the Z-contrast of SPs against similar Z-contrast of
15
background carbon. The sizes of SP1, SP2, and SP3 are 21 ± 3, 17 ± 2, and 16 ± 2 nm,
16
respectively, which are very small and is beneficial for their high packing density on the
17
membrane. However, this would not be the real size in the water. The samples were dried, and
18
hydrophilic arms could be folded into the core to minimize surface energy in the TEM samples.
19
The existence of hydrophilic arms around the hydrophobic core of the SPs was visualized via
20
uranyl acetate staining followed by BF-STEM imaging (Figure 3). The BF-STEM images show a
21
darker contrast on the periphery of the SPs associated with the uranium metal bonded to the
22
amine or carboxylic acids groups of the hydrophilic polymeric arms.
13
1 2 3
Figure 2. TEM images demonstrating the size distribution of SPs on carbon coated copper grids: a) SP1, b) SP2 ),
and c) SP3.
4
5 6 7 8
Figure 3. BF-STEM images of uranium metal stained SPs demonstrating the presence of hydrophilic arms of SP1
9
3.3. Thermal stability of SPs
(Left), SP2 (Middle), and SP3 (Right), respectively. Corresponding insets clearly shows the hydrophilic arms stained with uranium metal.
10
Thermal stability of the SPs is pivotal for their application as coating materials under high-
11
temperature filtration condition for the treatment of oil-gas produced water. In this context, the
12
thermal stability of the SPs stained with uranium metal was evaluated under high-temperature
13
TEM (HT-TEM) with a temperature ramp of 5 °C/min up to 80 °C (Figure 4). No obvious
14
change in the shape or size of the SPs were observed during or after the heat treatment, which
15
implies their stability at an elevated temperature up to 80 °C.
14
1 2
Figure 4. HT-TEM images from three different SPs: room temperature (Top panel), and at 80°C (Bottom panel)
3 4
3.4. Evaluation of SP coatings on porous PSF UF membrane
5
It is extremely difficult to directly coat hydrophilic materials on top of a hydrophobic membrane
6
surface. Although hydrophobic PSF membrane surface is known to exhibit negative surface zeta
7
potential, the electrostatic interaction of this surface with a positively charged polymeric
8
hydrophilic material is not enough to obtain a stable coating. Therefore, a combination of highly
9
hydrophobic and positively charged segments on the coating material are essential for obtaining
10
a very stable coating on such membrane. As mentioned earlier, the SPs consist of hydrophobic
11
PS core and cationic or anionic functional groups containing hydrophilic arms. When cationic 15
1
arms containing SPs are used as the first deposition layer, the unique hydrophobic core of the
2
SPs promotes interaction with the hydrophobic membrane in addition to the electrostatic
3
interaction. The PS core is prepared by cross-linking PS arms, and hydrophilic arms are built on
4
these flexible PS arms. The flexibility of the arms allows the folding of the arms upwards in the
5
water phase and expose the hydrophobic core resembling the structure of a jellyfish; this
6
structural rearrangement during coating process facilitates the interaction of PS core with the
7
hydrophobic PSF membrane. Meanwhile, owing to the flexible nature, the cationic arms near the
8
membrane surface can align on the surface via electrostatic interaction. As a result, SPs attach to
9
the PSF membrane surface and form a uniform single layer by self-assembly and self-limiting
10
behaviors, while most of the hydrophilic arms will be exposed to the water solution. To assess
11
this unique interaction, SPs aqueous solutions with different compositions (0.1 wt%) were coated
12
on commercial PSF UF membranes (PSF-20, MWCO 20k). As shown in the AFM height images
13
(Figure 5), SP1 (left panel) and SP2 (middle panel) resulted in a uniform monolayer coating of
14
SPs on the membrane surface. This observation suggests that the unique interaction of the
15
hydrophobic PS cores of SPs with the hydrophobic membrane materials along with electrostatic
16
results in the monolayer coating excluding multilayer formation. However, there are interparticle
17
gaps for the monolayer coated membranes that is assumed to be accessible for foulants. The SP2
18
monolayer coating provided better packing and surface coverage than the SP1 monolayer and
19
has reduced interparticle gaps (10.8 ± 4.9 nm for PSF-SP1 verses 9.5 ± 5.5 nm for PSF-SP2). It
20
is worth noting that interparticle gaps are calculated from the phase images shown in Figure 6
21
since phage mode shows the structural details of SPs better than the height mode. The bilayer
22
coating of SP1 followed by SP3 is also uniform and further filled the interparticle gaps (8.1 ± 4.5
23
nm). Most importantly, bilayer provided obviously higher surface coverage than the monolayers.
24
The carboxylic acid arms containing SPs (SP3) of the second layer are assembled surrounding
25
the amine arms containing SPs of the first layer that provided the higher surface coverage.
26
Encouragingly, the unique characteristic of these SPs is that their hydrodynamic diameters are
27
larger (ca. 57, 64, and 44 nm, Table 2) since hydrophilic arms are extended under water. This
28
size extension of the SPs can fill the gaps and shield the access of foulants to membrane surface.
29
Overall, all three different coating process provided an ultra-thin SPs coating with less than 15
30
nm thickness. Such an ultra-thin coating will be beneficial to modify the surface property (e.g.,
31
hydrophilicity) of base membranes with minimal changes in their water permeation properties. 16
1
The coating of SPs on the PSF UF membranes was also evaluated by XPS elemental analysis
2
(Table 3). The XPS element analysis in Table 3 showed that SP1, SP2 and SP3 coating in mono
3
and bilayer increased the amount of nitrogen and oxygen and reduced the amount of carbon and
4
sulfur on the PSF UF membrane surface. These composition changes are due to the fact that SP1,
5
SP2, and SP3 have higher amounts of nitrogen and oxygen than PSF UF membrane. Among
6
them, SP1 has a higher amount of nitrogen, because it has 100% DMAEMA arms with more
7
amine groups. SP3 coating in the bilayer introduced more oxygen, because of the PMAA arms
8
consisting of pendant carboxylic functional groups.
9
10 11 12 13
Figure 5. AFM images of UF membranes coated with SPs: monolayer of SP1 (left panel), monolayer of-SP2
(middle panel), and bilayer of SP1 and SP3 (right panel). The corresponding insets show the sizes of the SPs.
XPS elemental compositions Samples
14
C (%)
N (%)
O (%)
S (%)
PSF
82.5
0
14.3
3.2
PSF-SP1
80.2
1.2
16.0
2.6
PSF-SP2
80.3
0.8
16.3
2.7
PSF-SP1- SP3 76.8 1.7 20.4 Table 3. XPS element analysis of PSF UF membranes before and after SP coating.
1.1
15 16
Given that the hydrophilic nature of the membrane surface is essential for antifouling property,
17
the local hydrophilicity of the SPs coated membranes was analyzed via AFM phase imaging 17
1
modes (Figure 6). Obvious brighter contrast at the periphery versus the darker contrast at the
2
center indicates the presence of two different phases originating from the hydrophilic arms and
3
hydrophobic core. The root-mean-square (RMS) values of phase images indicate the degree of
4
the phase difference between the hydrophilic arms and the hydrophobic core, which we define
5
here as the extent of hydrophilicity of SPs coated membranes [89]. However, the hydrophilic
6
arms are folded inward in dry state to minimize the surface energy and partly exposed the SPs
7
hydrophobic core. This collapsing of the SPs also increased the interparticle gaps exposing the
8
hydrophobic membrane surface to some extent. Therefore, this method cannot provide accurate
9
degree of surface hydrophilicity for such SPs coated membranes. However, it is very useful to
10
estimate and illustrate the difference in local hydrophilicity between different coatings. The RMS
11
value is the highest for bilayer (SP1 and SP3) coated membrane (5.33) followed by SP1 (4.96)
12
and SP2 (4.87) monolayer coated membranes. The highest local hydrophilicity of bilayer coated
13
membrane can be attributed to the highest packing density and consequently the highest surface
14
coverage via the self-assembly of multiple hydrophilic carboxylic acid and PEG functional group
15
containing SPs (SP3) surrounding an amine-containing SP (SP1) (Figure 6 right panel and its
16
inset).
17 18 19 20
Figure 6. AFM phase images of UF membranes coated with SPs: SP1 monolayer (Left panel), SP2 monolayer
(Middle panel) and bilayer of SP1 followed by SP3 (Right panel). The inset in the right panel clearly shows the selfassembly of several carboxylic acids containing SPs around an amine-containing SP on the bilayer coated membrane.
21 22
The hydrophilicity of the SPs coated membranes was also estimated from the under-air water
23
contact angle measurements. The water contact angles are ca. 76°, 66°, 62°, and 57° for bare
24
PSF, PSF-SP1, PSF-SP2, and PSF-SP1-SP3, respectively. This decreasing trend of contact 18
1
angles clearly demonstrates the increasing hydrophilicity of PSF membrane coated with SP1 and
2
SP2 monolayers as well as SP1-SP3 bilayer. However, in-air measurement of contact angle
3
might not be the accurate representation of the wettability of the membrane since the hydrophilic
4
arms of SPs are folded under dry state forming a globular shape and exposing the hydrophobic
5
pristine PSF surface between SPs (Figure 6). The hypothesis behind the design of such core-shell
6
SPs is that the hydrophilic arms are hydrated and extended outwards under water providing a
7
barrier for oil adsorption. Therefore, the oleophobicity of the membranes was evaluated from the
8
under-water captive n-decane bubble contact angle measurements (Figure 7). The water contact
9
angle (WCA), which is a complementary angle to n-decane contact angle, reduces from ca. 90°
10
for bare PSF membrane to ca. 71° for SP1 coated PSF membrane to ca. 52° for SP2 coated
11
membrane to ca. 35° for bilayer (SP1 and SP3) coated PSF membrane. This means that the
12
coating of SPs highly increased the hydrophilicity of the membrane surface, which implies the
13
modified membrane has an anti-oil fouling ability. The SP2 monolayer coated membrane has
14
lower WCA that is higher hydrophilicity than the SP1 monolayer coated membrane presumably
15
due to the presence of PEGMA arms of superior hydrophilicity on SP2 and higher packing
16
density SP2 over SP1. The bilayer coated membrane has the lowest water contact angle,
17
consequently the highest hydrophilicity, which is consistent with its highest local hydrophilicity
18
observed by AFM phase contrast imaging. As explained in the previous section, the highest
19
hydrophilicity of the bilayer coated membrane can be attributed to highest packing of SP3s of
20
highly hydrophilic arms composed of PEG and carboxylic acid groups around SP1. This highest
21
hydrophilicity of bilayer coated membrane makes it the best choice of all four types of
22
membranes for antifouling application.
23
19
1 2 3 4
Figure 7. Under-water captive n-decane bubble contact angles of PSF UF membranes a) Bare PSF, b) PSF-SP1 c)
PSF-SP2, and d) PSF-SP1-SP3.
5
The zeta potential changes of PSF membranes before and after SP coatings as a function of pH
6
are shown in Figure 8. In the streaming potential experiments, the original PSF membranes had
7
more negative zeta potential values under the studied conditions due to the preferential
8
adsorption of anions (e.g., OH−, and Cl−) over cations (e.g., H+, K+, and Na+) of the electrolytic
9
solution on hydrophobic PSF surface [90–92]. When the positively charged SPs were coated, the
10
potentials of the modified membranes shifted to a positive or less negative range. The isoelectric
11
points (IEP) are 3.15, 4.94, 5.00, and 4.60 for bare PSF, PSF-SP1, PSF-SP2, and PSF-SP1-SP3,
12
respectively. The monolayer coated membranes became less negatively charged, or more neutral,
13
while the bilayer coated membrane became more negatively charged around pH 7 solutions. The
14
more negative charge for bilayer coated membrane is arising from the deprotonated carboxylate
15
anions of the hydrophilic arms on SP3s. The excessive negative charges on the bilayer coated
16
membrane around pH 7 and above are suitable to repel commonly encountered negatively
17
charged foulants in wastewater.
18
20
1 2 3 4
Figure 8. The streaming zeta potentials of PSF UF membrane before and after SPs coating. IEP are 3.15, 4.94, 5.0,
4.6 for PSF blank, PSF-SP1, PSF-SP2, and PSF-SP1-SP3, respectively.
5
3.5. Evaluation of filtration performance of PSF UF membranes before and after SP
6
coating
7
Additional coating layers on porous MF and UF membranes always reduce the water flux due to
8
the added barrier to water transport. As a result, antifouling property and separation efficiency
9
can be achieved at the expense of a flux decline. In order to maintain high porosity and high flux,
10
a proper coating should not significantly block the pores or change the porous structure. Figure 9
11
shows the permeate flux through unmodified and SP coated membranes. As can be seen, water
12
flux increased slightly from 1505 LMH for unmodified membranes to 1547, and 1712 LMH for
13
PSF-SP1, and PSF-SP2, respectively. On the other hand, the flux declined to 926 LMH for
14
bilayer coated (PSF-SP1-SP3) membrane. Although the bilayer membrane experienced the
15
largest flux decline of all coated membranes, it still maintained a high pure water flux that falls is 21
1
in the range between MF and UF membranes. The slight increase in water flux for monolayer
2
coated membranes is attributed to the ultra-thin (less than 15 nm) self-assembled layer on the
3
membrane surface which increased the surface hydrophilicity and permeate diffusion rate
4
through the membrane. PSF-SP2 has higher hydrophilicity than PSF-SP1. Therefore, PSF-SP2
5
provides higher water flux than PSF-SP1. In contrast, the reason for water flux decline for
6
bilayer coated membrane is due to the higher packing density of the carboxylic acid containing
7
SPs (SP3) around amine containing SPs (SP1) and the resulting high surface coverage leading to
8
the reduction in access of through the membrane (Figure 6, rightmost panel).
9
10 11
Figure 9. Pure water flux of PSF UF membranes before and after coating by three types of SPs
12
Synthetic oily wastewater (average emulsion droplets of 1.5 ± 0.4 µm with only 1.8% of smaller
13
sizes of 164 ± 41 nm) was filtered to investigate the anti-oil fouling properties of membranes.
14
Figure 10 shows the change in water permeate flux over time at a constant-flux mode. The flux
15
decline is 71% for bare PSF membrane and only about 6-7% improvement was obtained for
16
monolayer coated membranes of PSF-SP1 and PSF-SP2. In contrast, bilayer coated membrane
17
(PSF-SP1-SP3) exhibited only 29% flux decline. Meanwhile, the bare PSF membrane did not
18
provide any flux recovery upon washing with DI water while the flux recovery ratio (FRR) for
19
PSF-SP1, PSF-SP2, and PSF-SP1-SP3 were ca. 41%, 44%, and 79%, respectively. The
20
insignificant improvement in anti-oil fouling property of PSF-SP1 compared to the bare PSF is 22
1
expected considering their almost similar contact angle and susceptibility of cationic arms of
2
these SPs to fouling by slightly negatively charged oil droplets prepared by using a nonionic
3
surfactant (surface zeta potential: -6mV). However, the similar anti-oil fouling property of PSF-
4
SP1and PSF-SP2 is unexpected since the contact angle of the latter was remarkably lower than
5
that of the former. Moreover, SP2 contain only 55% amine-containing hydrophilic arms while
6
the rest of the hydrophilic arms are PEG that should reduce the attachment of oil. Nevertheless,
7
this similar anti-oil fouling performance of PSF-SP1, and PSF-SP2 is presumably governed by
8
the inter SPs gaps on the coated membrane leaving the exposed parts of the PSF membrane
9
prone to fouling. Of important note here is that the applied normal pressure force in the dead-end
10
filtration module might have forced the hydrophilic arms of the SPs to fold inward during the
11
repulsive interaction with the oil droplets and created gaps for oil adsorption on exposed PSF
12
membrane. The remarkable observation is that the anti-oil fouling property of the membrane
13
improved substantially by only one additional layer. This improvement for bilayer coated
14
membrane (PSF-SP1-SP3) has arisen from the reduction of exposed membrane area via the self-
15
assembly of many SP3s around one SP1 (Figure 6 right panel and its inset). As mentioned earlier,
16
the hydrophilic arms of SP3s are composed of both negatively charged carboxylate and charge
17
neutral PEG functional groups, and these functional groups are well known for repelling oil
18
droplets. The rejection of oil emulsions for all membranes was in the range of 97.8-99.2%
19
resulting in clear oil free permeates (picture at the bottom of Figure 10).
23
1 2 3 4
Figure 10. Water flux vs. time for oil/water emulsion filtration tests of bare PSF, PSF-SP1, PSF-SP2, PSF-SP1-SP3
(bilayer), and PSF-SP1-SP3 (bilayer stability) membranes. The picture at the bottom represents the corresponding feed and permeate solutions.
5 6
In order to evaluate the longtime stability of membrane coatings and their respective antifouling
7
property under rigorous conditions, the anti-oil fouling experiments were conducted with a very
8
stable nano-emulsion (53% of 235± 50 nm and 47% of 64 ±11 nm, prepared from highly viscous
9
vegetable oil) under extreme transmembrane pressure (70 PSI) and high feed flow rate (150 L/hr)
10
using a crossflow filtration setup. The coatings of SPs are supposed to be removed from the PSF
11
membrane surface during the period of compaction and flux stabilization for a total of 3 hours
12
followed by oil filtration for 15 hours under high tangential flow rate and extreme pressure if
13
they are loosely bound to the surface. The high viscosity of vegetable oil and the corresponding
14
stable nano-emulsion is expected to increase the oil droplet adhesion to the membrane surface.
15
The extreme transmembrane pressure should intensify the oil adhesion. As anticipated from the
24
1
rigorous operating condition for anti-oil fouling experiment, a sharp flux decline was observed
2
for all membranes followed by a gradual leveling to a steady flux (Figure 11). The bilayer
3
modified membrane (PSF-SP1-SP3) exhibited the highest steady-state flux (4.1 times), followed
4
by SP2 monolayer coated membrane (3.4 times), and SP1 monolayer coated membranes (1.6
5
times). These results are consistent with the cumulative effect of the hydrophilicity, and the
6
nature of the charges of the SPs coated membranes (Figure 7, 8, Table1). The hydrophilicity of
7
the membranes determined by the water contact angle measurements (Figure 7) has the same
8
trend as the antifouling property. Bilayer consists of SP second layer with hydrophilic arms of 35%
9
negatively charged carboxylic acid and 65% PEG functional groups. In contrast, SP1 and SP2
10
have 100% and 55% positively charged PDMAEMA arms, respectively. Oil-in-water emulsion
11
droplets themselves are slightly negatively charged. Therefore, the SPs with more PEGMA
12
and/or PMAA will repel more oil emulsion droplets more efficiently but SPs with more
13
PDMAEMA will attract more oil emulsion droplets. This antifouling test indicates that the
14
numbers of hydrophilic arms of the SPs and their charges are critical to repel hydrophobic
15
foulants. However, this trend of anti-oil fouling property for PSF-SP1 and PSF-SP2 could not be
16
realized under a dead-end setup. As mentioned in the above section, the pressure normal to the
17
membrane surface and the repulsion from the oil droplet might have resulted in inward folding of
18
hydrophilic arms of the SPs under dead-end setup whereas tangential pressure in cross flow setup
19
alleviated the folding of the arms. The rejections of oil were again in the range of 98.1 to 98.5%
20
for all membranes, including the bare PSF.
25
1 2 3 4 5
Figure 11. Normalized water flux vs. time for oil/water emulsion filtration tests of bare PSF, PSF-SP1, PSF-SP2,
PSF-SP1-SP3 (bilayer), and PSF-SP1-SP3 (bilayer stability) membranes under longtime and rigorous operating conditions.
6
Since the bilayer coated membrane (PSF-SP1-SP3) exhibited the highest anti-oil fouling
7
property, the scope of this membrane was further examined for the filtration of a model bio-
8
foulant (BSA) and a model organic matter (HA) (Figure 12) under dead-end filtration setup. As
9
seen in Figure 12(a), the flux decline during BSA filtration was same (71 LMH, 50%) for bare
10
PSF and bilayer coated membrane (PSF-SP1-SP3). The instantaneous flux decline can be due to
11
the collective effect of concentration polarization and fouling where the former is presumably
12
playing the predominant role; particularly for the tighter bilayer coated membrane. Note that the
13
BSA solution was prepared in 1× phosphate buffer saline containing a large amount of many
14
types of salts, including NaCl, KCl, Na2HPO4, and KH2PO4. These salts ions in addition to BSA
15
can result in a large concentration polarization and increase the resistance for water diffusion.
16
However, no flux recovery was observed for bare PSF, indicating the complete irreversible
17
fouling (pore blocking) of this membrane. On the other hand, 60 LMH flux was recovered (FRR:
18
92.4%) for PSF-SP1-SP3 by a simple DI water washing. This high flux recovery for PSF-SP1-
19
SP3 demonstrates that the rapid flux decline for this membrane was mainly due to concentration
20
polarization besides some contribution from loosely and/or firmly bound BSA protein adsorption. 26
1
The rejections of BSA were 57.7% and 80% for bare PSF and bilayer coated membrane,
2
respectively.
3
The anti-HA fouling property of the bare PSF and bilayer coated PSF (PSF-SP1-SP3) was
4
examined by a single batch filtration of 400 mL of the HA solution for 30 minutes using the
5
dead-end filtration setup. Bare PSF exhibited a flux decline by 98 LMH (50%) whereas the flux
6
decline was 76.4 LMH (39.5%) for PSF-SP1-SP3. The flux decline here is also due to the mutual
7
effect of fouling and concentration polarization. The HA feed solution also contains large
8
amount of NaCl solution since the HA was initially dissolved in 0.1M NaOH solution and the pH
9
was adjusted to 7.0 by adding 4M hydrochloric acid. The presence of NaCl salt increased the
10
salinity of the feed solution, which contributed to the increase in concentration polarization
11
and/or increases in the resistance for water diffusion through the membrane. The FRR for bilayer
12
coated membrane is 91.6% which is only 18% higher than that for bare PSF membrane (73.4%).
13
Interestingly, a significantly high flux recovery was observed even for bare PSF in contrast to no
14
flux recovery for this membrane during oil and BSA filtration. The rejection of HA was 97.5%
15
for bare and 99.5% for bilayer coated membrane which is reflected in the color differences of the
16
feed and permeate solutions (Figure 12b inset).
17 18 19
Figure 12. Water flux vs. time for a) BSA, and b) HA filtration tests of bare PSF, and PSF-SP1-SP3 (bilayer),
membranes, respectively. The inset in figure b) shows the photographs of HA feed and permeate solutions.
20 21
3.6. Evaluation of thermal stability of bilayer coated PSF UF membrane
27
1
The thermal stability of the bilayer coated membrane was examined thoroughly since it exhibited
2
superior antifouling properties. The thermal stability of the bilayer coated membrane was studied
3
by immersing the membrane under water at 80 °C and agitating the water for 1 week at 250 rpm.
4
Then, the anti-oil fouling performance was evaluated. The flux decline under constant flux mode
5
oil emulsion filtration (Figure 10) was 37% which only 8% higher that the bilayer membrane
6
before stability test (29%). It also exhibited slightly lower FRR compared to bilayer coated
7
membrane (73.9% verses 78.6%) before thermal treatment under water at 250 rpm agitation.
8
During the oil filtration under rigorous conditions, the steady state flux after initial fouling was
9
20% lower than that for the membrane before stability test, which was still 3.2 times that of bare
10
PSF membrane (Figure 11). To reveal the reason behind the slight loss in the antifouling
11
property after under-water thermal treatment, a wide range of complementary characterizations
12
was carried out.
13
The XPS elemental composition was found to be 78.5% C, 0.8% N, 19.7% O, and 0.9% S versus
14
the initial composition of 76.8% C, 1.7% N, 20.4% O, and 1.1% S. The slight decrease in the N
15
and O content can be associated with detachment of SP particles from the membrane surface
16
and/or the thermal hydrolysis of ester bonds connecting the amine and PEG containing pendant
17
moieties with polymeric backbone.
18
The water contact angle was almost unchanged (Initial 36.7° ± 1.4° and Final 38.1° ± 0.3°),
19
indicating no noticeable loss in surface hydrophilicity. The pure water flux should increase if the
20
loss in hydrophilic components and corresponding anti-oil fouling property is due to the
21
detachment of SPs from the surface. In fact, the pure water flux declined by 110 LMH. Therefore,
22
the slight loss in hydrophilicity can be a consequence of hydrolysis of hydrophilic arms and/or
23
folding of the hydrophilic arms inwards due to the thermo-responsive property of PDMAEMA
24
arms. However, pure water flux of bare PSF membrane also decreased by 202 LMH pointing out
25
to the shrinkage of pores of PSF membrane itself at high temperature. This shrinkage of the base
26
membrane at high temperature suggest the folding of thermo-responsive PDMAEMA arms is the
27
reason for slightly losing antifouling property. Finally, AFM imaging technique was used to
28
assess the stability of the coating. The packing density of SPs did not change significantly,
29
although some extent of collapsed globule formation is observed. (Figure 13). Meanwhile, the
30
decrease in RMS value of phase contrast images from 5.33 to 4.72 again indicates the loss
31
hydrophilicity which can be attributed the folding of the hydrophilic arms inwards rather than 28
1
detachments of SPs from the membrane surface. Overall, the loss in hydrophilicity and anti-oil
2
fouling property were not dramatic.
3
4 5 6 7
Figure 13. AFM height images (Top panel) and phase image (Bottom panel) of bilayer coated membrane surface
before and after under-water thermal stability test for 1 week. The top and bottom insets in the top images are the height profiles of SP1 and SP3, respectively.
8
4 Conclusion
9
We have investigated anti-oil fouling efficiency and thermal stability of ultrathin SP coatings
10
(monolayers and bilayers) on a hydrophobic PSF UF membrane to validate their feasibility for 29
1
oil-gas produced water treatment. The SPs are made of hydrophobic PS cores and hydrophilic
2
arms of variable compositions of a tertiary amine, polyethylene glycol, and carboxylic acid
3
functional groups. The presence of the hydrophobic core, as well as the cationic tertiary amine
4
group in the arms, facilitated self-assembly of the SPs on a hydrophobic membrane surface,
5
forming an ultrathin coating layer (less than 15nm). The bilayer coated membranes with SP3-
6
comprising the most abundant hydrophilic and negatively charged arms- as the second layer
7
(PSF-SP1-SP3) showed the highest water flux during filtration of a synthetic oil-water emulsion.
8
All membrane samples showed more 98% of oil rejection. Although the anti-oil-fouling property
9
of the bilayer coated membrane was compromised to some extent after rigorous thermal
10
treatment, there was no evidence of SP loss on the membrane surface and the membrane still
11
exhibited more than 2 and 3 times higher water flux than unmodified PSF membrane under dead-
12
end and crossflow filtration, respectively. The results imply that the interactions between star
13
polymers and the hydrophobic membrane are considerably strong to endure high-temperature
14
membrane operation. The thermal and/or chemical stability of the coatings could be further
15
improved by substituting some vulnerable moieties in the hydrophilic arms (e.g., ester backbone).
16
The unique design of SPs that drives a self-assembly process on a hydrophobic membrane
17
surface could be advantageous in modifying a commercial-scale membrane module through a
18
simple post-treatment process (e.g., in-situ SP assembly by flowing a solution). The tunable
19
functionality of the SPs also opens more opportunities to target different types of foulants
20
associated with wastewater discharged from biochemical, pharmaceutical, and food processing
21
industries. For instance, the bilayer coated PSF membrane provided superior antifouling property
22
to unmodified PSF membrane against a model bio-foulant, bovine serum albumin, and a model
23
organic matter, humic acid.
24
Acknowledgments
25
The authors wish to acknowledge the financial support from the Natural Sciences and
26
Engineering Research Council of Canada (NSERC), the IBM Alberta Centre for Advanced
27
Studies (CAS), IBM Research-Almaden, University of Alberta, the National Research Council
28
and Alberta Innovates (AI). The authors would like to thank Mr. Amin Karkooti for training zeta
29
potential measurement, Mr. Farhad Ismail for training contact angle measurement, Mr. Asad
30
Asad for training the operation of membrane crossflow filtration set up, Ms. Laleh 30
1
Shamaeighahfarokhi for training on coating related facilities, and Mr. Bradley Smith for all
2
supports relevant to the National Research Council (NRC) – Nanotechnology Research Centre
3
lab supply, equipment, and facility. The authors would like to offer special thanks to Dr. Joseph
4
S. Sly (a talented researcher at IBM Almaden Research Center), who, although no longer with us,
5
provided inspiration for this study.
6 7
Declarations of interest: none
8
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Highlights: 1. IBM-invented star block copolymers (SPs) were utilized to synthesize antifouling UF membranes. 2. Fine-tuning of the composition of SPs was done for making monolayer and bilayer coated membranes. 3. Hydrophilic arms of SPs have mitigated the membrane fouling. 4. For the first time, the thermal stability of SPs-modified membranes was investigated. 5. Bilayer coating of SPs exhibited the best hydrophilicity, anti-oil fouling properties, and thermal stability.
1
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: