The zwitterionic homopolymer poly[2-(methacryloyloxy)ethyl-dimethyl-(3-sulfopropyl) ammonium hydroxide was coated onto the surface of commercial polyamide reverse osmosis (RO) membranes. Aqueous solutions of the polymer at different concentrations were applied to modify the polyamide membranes through an in situ surface coating procedure. After membrane modification, cross-flow filtration testing was used to test the antifouling potential of the modified membranes. The obtained data were compared with experimental data for unmodified membranes. Each test was done by cross-flow filtering tap water for 60 hours. Yeast extract was added as a nutrient source for the naturally occurring bacteria in tap water, to accelerate bacteria growth. Fourier transform infrared spectroscopy, contact angle, scanning electron microscopy, atomic force microscopy, and permeation tests were employed to characterize membrane properties. The results confirmed that modifying the membranes enhanced their antifouling properties and cleaning efficiency, the fouling resistance to bacteria improving due to the increased hydrophilicity of the membrane surface after coating. In addition, the water permeability and salt rejection improved. This in situ surface treatment approach for RO membranes could be very important for modifying membranes in their original module assemblies as it increases water production and reduces the salt content.

INTRODUCTION

Polymeric reverse osmosis (RO) membranes are widely used for water treatment, desalination, and downstream processing (Kang et al. 2011; Wagner et al. 2011). Two types of membranes are used for the RO process: asymmetric and thin-film composite (TFC) membranes (Lee et al. 2011). At present, TFC aromatic polyamide membranes are the most sold RO membranes on the market. They can be synthesized by interfacial polymerization between aromatic polyamine on a reinforced microporous support (Fritzmann et al. 2007). Polyamide membranes have several advantages, i.e., high water permeability and ion rejection, high thermal stability, stability over wide pH ranges, and high resistance to pressure compaction. However, fouling remains a serious problem (Cheng et al. 2013; Suwarno et al. 2014). Fouling refers to the deposition of inorganic materials, organic materials, and microorganisms on the membrane surface, increasing the hydraulic resistance of the membrane (Rana & Matsuura 2010). One serious problem during RO, observed in all desalination plants, is bacterial growth on membrane surfaces, i.e., biofouling (Khulbe et al. 2010). Biofouling is difficult to remove through either disinfection or chemical cleaning (Khulbe et al. 2010), so several methods have been developed to reduce it, for example, pretreatment processes, membrane modification, and the design of special modules (Ben-Sasson et al. 2014; Blok et al. 2014). The most effective and relatively economic choice is membrane surface modification. It is widely known that the hydrophilicity, charge, and roughness of membrane surfaces greatly influence membrane fouling (Wilbert et al. 1998). Several attempts have been made to modify membrane surfaces by the physical adsorption of hydrophilic polymers (Wilbert et al. 1998), surface coating (Louie et al. 2006; Matin et al. 2014), grafting polymerization (Freger et al. 2002; Wei et al. 2010), and plasma polymerization (Zou et al. 2011; Moghimifar et al. 2014). Of these methods, surface coating has been widely used to modify the surface properties of polyamide membranes. Several polymers have been tested to reduce biofilm resistance, polymers such as poly(ethylene glycol) (Belfer et al. 1998; Sagle et al. 2009), heparin (Riedl et al. 2002; Ayres et al. 2008; Liu et al. 2010), phospholipid polymers (Ishihara 2000; Kyomoto et al. 2010), sulfobetaine, and zwitterionic polymers (Yuan et al. 2008; Chiang et al. 2009; Xuan & Liu 2009; Carr et al. 2010). Zwitterionic polymers, in which anionic and cationic groups are arrayed on the pendant side chain of the molecular backbone, seem to be a promising choice (Carr et al. 2010). However, few papers have reported membrane surface modification using zwitterionic polymers to enhance anti-biofouling properties (Huang & Xu 2010; Azari & Zou 2012; Razia et al. 2012). Zwitterionic polymers strongly bind water molecules (Yang et al. 2011). Furthermore, modifying surfaces with zwitterionic polymers reduces bacterial adhesion and biofilm formation (Cheng et al. 2009).

The objective of this work was to modify the surface of polyamide RO membranes to reduce membrane fouling and thereby ensure high water flux through the membrane. We attempted to improve the antifouling properties of commercial polyamide membranes by coating the membrane surface with a synthesized zwitterionic polymer named (poly[2-(methacryloyloxy)ethyl-dimethyl-(3-sulfopropyl) ammonium hydroxide], i.e., P(MEDSAH). The MEDSAH monomer has a cationic quaternary ammonium (QA) (N+) group and an anionic sulfonate (SO3) group on its backbone. The virgin and modified membranes were tested for flux permeability and salt rejection. Furthermore, the antibiofouling properties of virgin and modified membranes were evaluated by filtering tap water for 60 hours. To obtain realistic biofouling data, it was important that the bacteria attached to or growing on the membrane be naturally occurring, so no bacteria was added to the tap water before filtration. To enhance bacteria growth, yeast extract was added to the tap water. Yeast is often used as a non-specific nutrient source for bacteria.

EXPERIMENTAL

Materials and reagents

The commercial TFC polyamide RO membrane (RO98pHt) used in this study was supplied by Alfa Laval Company (Lund, Sweden). The salt rejection of this commercial membrane is approximately 93% tested using sodium chloride (NaCl) (2,000 mg L1) aqueous solution at 15.5 bar and 25°C. The membrane was manufactured through the interfacial polymerization of m-phenylenediamine with trimesoyl chloride on a reinforced polysulfone porous substrate. The monomer [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide, i.e., (MEDSAH), and the initiator 2,2′-Azobis(2-methylpropion-amidine) dihydrochloride were purchased from Sigma-Aldrich (St Louis, MO, USA) and used without further purification. Granulated yeast extract for microbiology (E. Merck, Darmstadt, Germany) was used as a nutrient source to enhance bacterial growth. The yeast extract was made from autoprotolyzed yeasts and contained a mixture of organic compounds. All other chemicals were of analytic purity grade and used as received. The chemical structure of the MEDSAH monomer is shown in Figure 1.

Figure 1

Chemical structure of the MEDSAH monomer.

Figure 1

Chemical structure of the MEDSAH monomer.

Synthesis of P(MEDSAH) polymer

Zwitterionic monomer (0.5 mol) was polymerized in DI water using 0.01 mol of 2,2′-Azobis(2-methylpropionamidine) dihydrochloride as a free radical initiator. The reactants were purged with nitrogen for 30 minutes and the reaction was started by heating the sample in an oil bath at 65°C. After 24 hours, the reaction mixture was transferred to a dialysis bag (Spectra/Por®; Spectrum Laboratories, Rancho Dominguez, CA, USA) with a molecular weight cut-off of approximately 3,500 Da and dialyzed with DI water for 3 days to remove the initiator, oligomers, and unreacted monomers. The water was changed twice every day. After dialysis, the mixture was freeze dried and characterized using infrared (IR) and gel permeation chromatography (GPC).

Surface modification of polyamide RO membrane

The zwitterionic polymer P(MEDSAH) was dissolved in feed-deionized (DI) water at predetermined concentrations ranging from 200 to 800 mg L1. The polyamide TFC membrane samples were soaked in DI water for 24 hours to remove all preservative compounds and then mounted in the flow cell of a specially constructed laboratory-scale cross-flow filtration unit to test the membrane permeability. The flow cell was 7.8 cm long, 2.7 cm wide, and 0.3 cm high. The test membrane was placed between the top and bottom plates of the cell and was held tightly by o-rings. The membrane was prepressurized with DI water at 15.5 bar for 30 minutes until the transmembrane pressure (TMP) was stable, after which the pure water permeability and salt rejection were recorded. The membrane surface was modified by replacing the feed with P(MEDSAH) polymer solution and alternately circulating and soaking the membrane at 5 bar for 2 hours and then at 10 bar for another 1 hour at 25°C and pH 6.8 so that the zwitterionic polymer could adhere to the polyamide membrane surface via hydrogen bonding (Wei et al. 2010). After membrane coating, the P(MEDSAH) solution was removed from the feed reservoir and the membranes were rinsed with DI water at 1 bar for 3 hours. The membranes were kept wet throughout the modification process. After modification, the membranes were left in the permeation cell for subsequent RO and fouling tests.

Evaluation of RO membrane performance

Both virgin and modified RO membranes were tested by measuring the permeate flux (J) when filtering a 2,000-ppm NaCl aqueous solution, as follows: 
formula
where A is the effective permeation area of the membrane (m2) and Q is the volume of permeation (L) over time interval t (hours). The salt rejection (R) was evaluated using the following equation: 
formula
where Cp and Cf are the salt concentrations in the permeate and feed, respectively. Salt rejections for the coated and uncoated membranes were tested using 2,000 mg L1 NaCl aqueous solution at 15.5 bar at 25°C and pH 6.8. The membranes were operated for at least 1 hour before data were collected.

Membrane characterization

FTIR analysis

Fourier transform infrared (FTIR) spectroscopy (Varian 610 IR, Agilent Technologies, USA) was used to analyze the surface composition of the virgin and modified polyamide RO membranes. Membrane samples used for FTIR analysis were removed from the test cells and allowed to air dry for a day before the experiment. The IR scans were conducted on the active surface of the virgin and modified membranes.

Contact angle measurement

The membrane contact angle was measured using a camera connected to a computer. A drop of DI water (10 μL) was carefully placed on the membrane surface at 25°C and the angle between the membrane–water and water–air interfaces was measured. Images were captured 5 s after introducing the droplet and the contact angles were calculated by analyzing the image. At least 10 images were captured for each droplet and about five measurements acquired at different locations on the membrane sample were averaged to obtain the contact angle of the measured membrane sample. All the results presented are averages from five membrane samples with the standard deviations of the measured values.

Scanning electron microscope and atomic force microscopy analysis

Scanning electron microscope (SEM) analysis of the surface of the virgin and modified membranes was conducted using a Zeiss EVO 60 SEM (Carl Zeiss, Oberkochen, Germany). The surface roughness of the membranes was measured by means of atomic force microscopy (AFM) imaging and analysis in the tapping mode using an NTEGRA atomic force microscope (NT-MDT, Moscow, Russia).

Fouling experiments

Standardized biofouling procedure

Membrane fouling experiments were carried out using tap water to which 25 mg L1 of yeast extract was added as the nutrient source for the naturally occurring bacteria. The fouling experiments were conducted at 5 bar for 60 hours and 25°C employing the cross-flow permeation test unit previously mentioned. During the fouling experiments, the permeate flux was recorded every 12 hours by raising the pressure to 15.5 bar. The resulting flux profile was used to analyze the biofouling behavior of the virgin and modified TFC membranes. After the fouling experiments, the membranes were washed with DI water for approximately 4 hours; after membrane cleaning, the salt rejection and permeation flux were determined.

Staining and microscopy

The fouled membranes were stained by adding 25 μL of SYTO® 9 green fluorescent nucleic acid stain (Life Technologies, Carlsbad, CA, USA) to the surface of the membranes; a cover slide was then placed on top to distribute the stain. The slides were placed in darkness and incubated at room temperature for 20 minutes before analysis. As the DNA stain only emits light with an optimum at 498 nm if it binds to DNA, intracellular as well as extracellular cells can be monitored in a confocal microscope without removing the excess stain. Images were recorded using a Zeiss LSM510 confocal laser scanning microscope (Carl Zeiss, Jena, Germany) equipped with a meta-filter and a 63× oil objective. The microscope was adjusted to match the optimum excitation/emission wavelengths of SYTO 9 bound to DNA of 485/498 nm, respectively.

RESULTS AND DISCUSSION

Synthesis and characterization of P(MEDSAH)

Zwitterionic polymer was formed by the free radical polymerization of the monomer. Poly[2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide, i.e., P(MEDSAH), was synthesized and characterized using FTIR and GPC measurements. The weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity of the resulting polymer are 216,000, 61,000 Da, and 3.5, respectively.

Figure 2 shows the FTIR spectra of the virgin and modified membranes. Both the virgin and modified membranes have characteristic peaks at 1,660 and 1,540 cm1, attributable to the amide group and C=C aromatic of the polyamide thin film, respectively. The IR spectrum of the modified membrane also displayed amide group peaks as well as a new peak at 1,730 cm1 attributable to the carbonyl (C = O) of the ester group of the zwitterionic polymer. Furthermore, the modified membrane displays absorption peaks at 963 and 1,040 cm1, characteristic of the QA and sulfonate (SO3) groups of the P(MEDSAH) polymer, respectively (Razia et al. 2012). This indicates the successful deposition of the zwitterionic polymer onto the membrane surface.

Figure 2

FTIR spectra of virgin and coated membranes.

Figure 2

FTIR spectra of virgin and coated membranes.

Effect of surface modification on membrane surface hydrophilicity

Figure 3 shows the measured contact angles of the virgin and modified TFC membranes coated with varying amounts of polymer solution. The water contact angle decreased from approximately 78° to 35° when the coating solution concentration increased from 0 to 800 mg L1, indicating that the surface hydrophilicity increased at higher concentrations. This is mainly due to the surface coverage with the hydrophilic zwitterionic polymer (Yang et al. 2011).

Figure 3

Contact angle of virgin membrane and of coated membranes treated with polymer solutions in concentrations of 200, 400, 600, and 800 mg L1.

Figure 3

Contact angle of virgin membrane and of coated membranes treated with polymer solutions in concentrations of 200, 400, 600, and 800 mg L1.

Effect of surface modification on membrane surface morphological structure

The morphological structure of the coated and uncoated TFC membranes was characterized using SEM and AFM (Figure 4). The virgin membrane has a ridge-and-valley surface structure which appears relatively open and coarse (Figure 4(a)), while the modified membrane surfaces appear smoother with a finer structure (Figures 4(b)(d)). The surface structure becomes finer and smoother with higher concentrations of the coating solution. In addition, the zwitterionic polymer coating layer made the surface denser as the coating solution concentration increased, i.e., the amount of zwitterionic polymer deposited on the membrane surface increased with coating solution concentration.

Figure 4

SEM images of the surface of (a) virgin membrane, and coated membranes treated with polymer solutions in concentrations of (b) 200 mg L1, (c) 400 mg L1, and (d) 800 mg L1 at two magnifications (i.e., left, ×20,000; right, ×50,000).

Figure 4

SEM images of the surface of (a) virgin membrane, and coated membranes treated with polymer solutions in concentrations of (b) 200 mg L1, (c) 400 mg L1, and (d) 800 mg L1 at two magnifications (i.e., left, ×20,000; right, ×50,000).

Figure 5 shows the AFM images of the TFC membranes before and after modification. The roughness measurements are summarized in Table 1. The virgin TFC polyamide membrane (Figure 5(a)) has a characteristic ridge-and-valley structure and a rough surface, while the coated membranes appear smooth in structure, becoming relatively smoother with increasing coating solution concentration. The AFM results also indicate that the P(MEDSAH) polymer was deposited on the surface of the TFC membranes, a dense surface coating layer being formed on membranes modified with a 800 mg L1 polymer solution.

Table 1

The roughness data for the virgin membranes and the membranes coated with P(MEDSAH)

Membrane samplesRMS* (nm)Average roughnessMax. value (nm)Average height (nm)
Uncoated membrane 111.7 91.5 747.6 219.9 
Membrane treated with 200 mg L1 coating solution 103.6 81.5 843.3 383 
Membrane treated with 400 mg L1 coating solution 54.6 43.9 496.9 153 
Membrane treated with 800 mg L1 coating solution 5.6 4.3 43.6 18.3 
Membrane samplesRMS* (nm)Average roughnessMax. value (nm)Average height (nm)
Uncoated membrane 111.7 91.5 747.6 219.9 
Membrane treated with 200 mg L1 coating solution 103.6 81.5 843.3 383 
Membrane treated with 400 mg L1 coating solution 54.6 43.9 496.9 153 
Membrane treated with 800 mg L1 coating solution 5.6 4.3 43.6 18.3 

*RMS = root mean square roughness.

Figure 5

AFM images of (a) virgin membrane, and coated membranes treated with polymer solutions in concentrations of (b) 200 mg L1, (c) 400 mg L1, and (d) 800 mg L1.

Figure 5

AFM images of (a) virgin membrane, and coated membranes treated with polymer solutions in concentrations of (b) 200 mg L1, (c) 400 mg L1, and (d) 800 mg L1.

Effect of surface coating on membrane performance

Figure 6 shows the effect of surface modification on pure water permeability. The permeability increased for membranes treated with a coating solution up to 400 mg L1, and then gradually decreased with increasing coating solution concentration. The increase in the pure water permeability of the modified membrane was ascribed to the increased surface hydrophilicity. However, as the coating solution concentration increased, the membrane surface became completely covered by a denser coating layer, resulting in higher hydraulic resistance and therefore reduced water permeability.

Figure 6

Changes in pure water permeability with coating solution concentration for the membranes coated with zwitterionic polymer.

Figure 6

Changes in pure water permeability with coating solution concentration for the membranes coated with zwitterionic polymer.

The effect of surface modification on salt rejection is shown in Figure 7. The salt rejection of the membrane increased when the membrane was modified, as the zwitterionic polymer layer reduced the passage of the salt. The figure also shows that the water permeability increased slightly with coating solution concentration to a certain point, after which it gradually decreased.

Figure 7

Effect of coating solution concentration on water flux and salt rejection of RO membranes; feed concentration is 2,000 mg L1 NaCl (aqueous solution), temperature is 25 °C, and pressure is 15.5 bar.

Figure 7

Effect of coating solution concentration on water flux and salt rejection of RO membranes; feed concentration is 2,000 mg L1 NaCl (aqueous solution), temperature is 25 °C, and pressure is 15.5 bar.

Figure 8 shows that the salt rejection of the divalent cations (Mg+2 and Ca+2) is much higher than that of the monovalent cation Na+, and that the salt rejection of the membrane treated with the less-concentrated feed solution (2 g L1 NaCl) is higher than that treated with the more concentrated feed solution (4 g L1 NaCl). The salt rejection of NaCl increased with pressure and slightly decreased for the divalent cations. Increased rejection often occurs because the convective transport of ions becomes more important than diffusion at high fluxes. At very high fluxes, the rejection decreased again due to the high concentration which built up at the membrane surface (i.e., concentration polarization).

Figure 8

Effect of pressure on the salt rejection of the coated membranes for different salts; feed concentration is 2,000 mg L1 of each salt (if not stated otherwise), temperature is 25°C, and pressure is 15.5 bar.

Figure 8

Effect of pressure on the salt rejection of the coated membranes for different salts; feed concentration is 2,000 mg L1 of each salt (if not stated otherwise), temperature is 25°C, and pressure is 15.5 bar.

Figure 9 shows that the salt rejection of the coated membrane decreased with the increasing pH of the feed solution, i.e., desalination was better in the acidic medium than the alkaline medium.

Figure 9

Effect of pH on the salt rejection of the coated membrane at different pressures; feed concentration is 2,000 mg L1 NaCl (aqueous solution) and temperature is 25 °C.

Figure 9

Effect of pH on the salt rejection of the coated membrane at different pressures; feed concentration is 2,000 mg L1 NaCl (aqueous solution) and temperature is 25 °C.

Effect of surface coating on membrane fouling and cleaning properties

Figure 10 shows the results of the 60-hour fouling experiments with tap water and 25 mg L1 yeast extract for virgin and coated membranes. Membrane biofouling is initiated by the adhesion of bacterial cells to the membrane surface, followed by cell growth and multiplication at the expense of soluble feed water nutrients (i.e., yeast extract). The figure shows that the flux decline begins after 15 hours of filtering the feed solution. The water flux of the uncoated membrane decreased sharply during the filtration experiment because the biofouling on the membrane surface hinders the passage of water. However, the reduction in water permeability was lower with the coated membranes than with the virgin membrane. The zwitterionic polymer resulted in the surface hydration of the membrane, which was generally considered the key to its resistance to non-specific protein and bacterial adhesion.

Figure 10

Effect of fouling time on the permeate flux of virgin and coated membranes at 25 °C and 15.5 bar.

Figure 10

Effect of fouling time on the permeate flux of virgin and coated membranes at 25 °C and 15.5 bar.

Figure 11 shows that the pure water permeability of the coated membranes, unlike that of the uncoated membrane, substantially recovered after washing the fouled membranes with DI water. The flux recovery after washing increased for membranes treated with the high-concentration coating, attributable to the increased concentration of the zwitterionic polymer deposited on the membrane surface. The positive and negative charges of the zwitterionic polymer may form a hydrated layer which repels biofoulant from the membrane surface. The presence of zwitterionic polymer having hydrophilic non-fouling groups on the membrane surface, in addition to these groups' steric repulsion, enhances the removal of foulant from the membrane surface. Conversely, the large reduction in the water permeability of untreated membranes is attributable to hydraulic resistance arising from the accumulation of biofoulant on the membrane surface. The flux recovery rate of the modified membrane after cleaning with DI water was 87%, much higher than the 59% recovery for the virgin membrane.

Figure 11

Effect of cleaning on the pure water flux for virgin and coated membranes at 25 °C and 15.5 bar.

Figure 11

Effect of cleaning on the pure water flux for virgin and coated membranes at 25 °C and 15.5 bar.

Figure 12 shows that the salt rejection of the coated membranes did not increase during filtration. In contrast, the salt rejection of the virgin membrane increased during filtration, even after washing away the fouling materials. This is an interesting result because the changed salt rejection indicates that the membrane became fouled during filtration. Biofouling forms a secondary layer which blocks the passage of salts through the membrane. The data support the idea that the coated polymers reduce biofouling. Furthermore, the salt rejection was higher for the washed virgin membranes than the fresh membranes, indicating that the fouling layer was not completely removed during the cleaning procedure.

Figure 12

Effect of fouling on the salt rejection of virgin and coated membrane; feed concentration is 2,000 mg L1 NaCl (aqueous solution), temperature is 25°C, and pressure is 15.5 bar.

Figure 12

Effect of fouling on the salt rejection of virgin and coated membrane; feed concentration is 2,000 mg L1 NaCl (aqueous solution), temperature is 25°C, and pressure is 15.5 bar.

Figure 13 illustrates the amount of biofouling on the studied membrane surfaces using confocal laser scanning micrographs. Figure 13(a) shows that relatively more biofouling occurs on the uncoated membranes after membrane filtration. The presence of bacteria on the membrane was not uniformly distributed, as would be expected with bacteria merely being drawn towards the membrane due to hydraulic drag. Instead, it was clear that bacteria seemed to be present in higher local density in some areas compared to others (high green intensity in Figure 13). This pattern resembles what is often described for surface attached biofilms, where initial colonizing bacteria start to grow and divide on the local surface indicating where they attach. The membrane treated with the low-concentration (200 mg L1) polymer solution, i.e., Figure 13(b), moderate biofouling was observed. On membranes treated with higher coating solution concentrations, i.e., Figures 13(c) and (d), almost no bacteria were observed, especially in Figure 13(d), which shows membrane treated with a 800 g L1 zwitterionic polymer solution. The lack of bacteria is attributable to less attachment of bacteria or to a toxic surface. However, no data from the literature indicate that the surface is toxic, so poor attachment seems to explain the fewer bacteria on the modified membranes. The attachment and settlement of bacteria is the key step in biofouling. Hence, the present data support the results of other studies (Cheng et al. 2009; Jiang & Cao 2010; Azari & Zou 2012) indicating that the polyamide membranes modified with zwitterionic polymer exhibit high antifouling properties in RO filtration.

Figure 13

Confocal laser scanning micrographs of fouled membranes for (a) uncoated membrane, and coated membranes treated with polymer solutions in concentrations of (b) 200 mg L1, (c) 400 mg L1, and (d) 800 mg L1.

Figure 13

Confocal laser scanning micrographs of fouled membranes for (a) uncoated membrane, and coated membranes treated with polymer solutions in concentrations of (b) 200 mg L1, (c) 400 mg L1, and (d) 800 mg L1.

The data do not indicate any detachment of the coated polymers after the 60-hour membrane filtration. Nevertheless, additional testing for long-term stability, resistance to cleaning, and the possibility of redepositing zwitterionic homopolymers may be relevant subjects for future studies.

CONCLUSIONS

An antifouling membrane was produced by modifying the surface of RO polyamide membranes with zwitterionic homopolymers. The zwitterionic homopolymer was successfully coated onto the polyamide membrane, making the membrane surface more hydrophilic. The membrane modification significantly enhanced both the water permeability and salt rejection of the membrane. Fouling tests indicated that the coating layer of zwitterionic polymer reduced bacterial fouling on the membrane. The permeate flux declined much less in the coated membranes in comparison to the uncoated membrane when they were used to filter tap water to which yeast extract was added. The coating layer markedly improved the membrane performance, hindering the biofouling of the membrane surface. After cleaning the membrane with DI water, 87% of the flux was recovered for the modified membrane, compared to 59% for the virgin membrane. Surface modification is therefore useful in enhancing membrane antifouling properties, resulting in high water flux and high salt rejection. The method is of particular interest from a practical perspective, as the membranes can be treated in their original module assemblies.

REFERENCES

REFERENCES
Ayres
N.
Holt
D. J.
Jones
C. F.
Corum
L. E.
Grainger
D. W.
2008
Polymer brushes containing sulfonated sugar repeat units: synthesis, characterization, and in vitro testing of blood coagulation activation
.
Journal of Polymer Science. Part A: Polymer Chemistry
46
,
7713
7724
.
Belfer
S.
Purinson
Y.
Fainshtein
R.
Radchenko
Y.
Kedem
O.
1998
Surface modification of commercial composite polyamide reverse osmosis membranes
.
Journal of Membrane Science
139
,
175
181
.
Ben-Sasson
M.
Lu
X.
Bar-Zeev
E.
Zodrow
K. R.
Nejati
S.
Qi
G.
Giannelis
E. P.
Elimelech
M.
2014
In situ formation of silver nanoparticles on thin-film composite reverse osmosis membranes for biofouling mitigation
.
Water Research
62
,
260
270
.
Chiang
Y. C.
Chang
Y.
Higuchi
A.
Chen
W. Y.
Ruaan
R. C.
2009
Sulfobetaine-grafted poly(vinylidene fluoride) ultrafiltration membranes exhibit excellent antifouling property
.
Journal of Membrane Science
339
,
151
159
.
Fritzmann
C.
Löwenberg
J.
Wintgens
T.
Melin
T.
2007
State-of-the-art of reverse osmosis desalination
.
Desalination
216
,
1
76
.
Khulbe
K. C.
Feng
C.
Matsuura
T.
2010
The art of surface modification of synthetic polymeric membranes
.
Journal of Applied Polymer Science
115
,
855
895
.
Kyomoto
M.
Moro
T.
Takatori
Y.
Kawaguchi
H.
Nakamura
K.
Ishihara
K.
2010
Self-initiated surface grafting with poly(2-methacryloyloxyethyl phosphorylcholine) on poly(ether-ether-ketone)
.
Biomaterials
31
,
1017
1024
.
Louie
J. S.
Pinnau
I.
Ciobanu
I.
Ishida
K. P.
Ng
A.
Reinhard
M.
2006
Effects of polyether–polyamide block copolymer coating on performance and fouling of reverse osmosis membranes
.
Journal of Membrane Science
280
,
762
770
.
Rana
D.
Matsuura
T.
2010
Surface modification for antifouling membranes
.
Chemical Reviews
110
,
2448
2471
.
Razia
F.
Sawada
I.
Ohmukai
Y.
Maruyama
T.
Matsuyama
H.
2012
The improvement of antibiofouling efficiency of polyethersulfone membrane by functionalization with zwitterionic monomers
.
Journal of Membrane Science
401–402
,
292
299
.
Riedl
C. R.
Witkowski
M.
Plas
E.
Pflueger
H.
2002
Heparin coating reduces encrustation of ureteral stents: A preliminary report
.
International Journal of Antimicrobial Agents
19
,
507
512
.
Sagle
A. C.
Van Wagner
E. M.
Ju
H.
McCloskey
B. D.
Freeman
B. D.
Sharma
M. M.
2009
PEG-coated reverse osmosis membranes: Desalination properties and fouling resistance
.
Journal of Membrane Science
340
,
92
108
.
Suwarno
S. R.
Chen
X.
Chong
T. H.
McDougald
D.
Cohen
Y.
Rice
S. A.
Fane
A. G.
2014
Biofouling in reverse osmosis processes: the roles of flux, cross flow velocity and concentration polarization in biofilm development
.
Journal of Membrane Science
467
,
116
125
.
Yuan
J.
Lin
S. C.
Shen
J.
2008
Enhanced blood compatibility of polyurethane functionalized with sulfobetaine
.
Colloids and Surfaces B: Biointerfaces
66
,
90
95
.
Zou
L.
Vidalis
I.
Steele
D.
Michelmore
A.
Low
S. P.
Verberk
J. Q. J. C.
2011
Surface hydrophilic modification of RO membranes by plasma polymerization for low organic fouling
.
Journal of Membrane Science
369
,
420
428
.