Thin-film composite forward osmosis membranes with novel hydrophilic supports for desalination

Journal of Membrane Science 423–424 (2012) 543–555

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Thin-film composite forward osmosis membranes with novel hydrophilic supports for desalination Gang Han a, Tai-Shung Chung a,n, Masahiro Toriida b, Shoji Tamai b a b

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117602, Singapore New Materials Development Center, Mitsui Chemicals, Inc., 580-32 Nagaura Sodegaura-City, Chiba 299-0265, Japan

a r t i c l e i n f o

abstract

Article history: Received 5 June 2012 Received in revised form 20 August 2012 Accepted 2 September 2012 Available online 10 September 2012

In this work, a novel sulphonated poly(ether ketone) (SPEK) polymer with super-hydrophilic nature was designed as the substrate material to fabricate high performance thin-film composite (TFC) membranes for desalination via forward osmosis (FO). m-Phenylenediamine (MPD) and 1,3,5-trimesoylchloride (TMC) were employed as the monomers for the interfacial polymerization reaction to form a thin aromatic polyamide selective layer. It has been demonstrated that blending a certain SPEK material into the polysulfone (PSU) substrate of TFC-FO membranes not only plays the key role to form a fully sponge-like structure, but also enhances membrane hydrophilicity and reduces structure parameter. The TFC-FO membrane comprising 50 wt% SPEK in the substrate shows the highest water flux of 50 LMH against deionized water and 22 LMH against the 3.5 wt% NaCl model solution, respectively, when using 2 M NaCl as the draw solution tested under the pressure retarded osmosis (PRO) mode (draw solution flows against the selective layer). It is found that the hydrophilicity and thickness of the substrates for TFC-FO membranes play much stronger roles in facilitating high water flux in FO for desalination compared to those made from hydrophobic substrates full of finger-like structures. Moreover, the reduced membrane structural parameter indicates that the internal concentration polarization (ICP) can be remarkably reduced via blending a hydrophilic material into the membrane substrates. Thermal treatment of TFC-FO membranes with optimized conditions can also improve the membrane performance and mechanical strength. & 2012 Elsevier B.V. All rights reserved.

Keywords: Composite membrane Interfacial polymerization Sulphonated poly(ether ketone) Hydrophilic substrates Forward osmosis Desalination

1. Introduction Global water scarcity is a significant problem that continues to grow worse with the rapid economic development and population growth. Development of alternative water sources by applying innovative membrane technologies with low costs, such as seawater desalination and wastewater reclamation, has become one of the most promising approaches to meet this critical challenge [1–5]. Over the past few decades, reverse osmosis (RO) technology has established as the industry benchmark for membrane-based water reuse and desalination because of its superior product water quality and competitive cost. However, its efficiency and sustainable operation are hampered by considerable energy consumption and membrane fouling [1,2,4]. Recently, forward (direct) osmosis (FO) has gained much attention as an emerging alternative technology to conventional pressure-driven membrane processes for seawater desalination [5–9], wastewater

n

Corresponding author. Tel.: þ65 65166645; fax: þ 65 67791936. E-mail address: [email protected] (T.-S. Chung).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.09.005

treatment [10], food processing [11], protein, pharmaceutical enrichment [12,13], and power generation [14–17]. In lieu of the applied hydraulic pressure, FO employs the osmotic pressure gradient as the driving force to induce osmotic flow from a lower solute/particle (correspondingly higher solvent) concentration feed solution to a higher solute/particle (correspondingly lower solvent) concentration draw solution [5,18]. The absence of external hydraulic pressure in FO is expected to reduce system energy consumption, equipment costs and the associated deleterious effects to the quality of the product water [1,2,19]. Moreover, FO may offer the advantages of higher rejections to a wide range of contaminants and lower membrane fouling propensities and higher water recovery compared to pressure-driven processes [2,20–22]. However, the inadequate number of commercially available high performance FO membranes [5,18,22–24] and the lack of draw solutes with friendly characteristics of low cost, non-toxicity, minimal reverse salt flux and easy of recycle [25–27] deter the full implementation of FO technology. In order to develop polymeric FO membranes with appropriate separation performance, two fabrication techniques have been often adopted: (1) asymmetric membranes made by non-solvent induced phase inversion [9,28–32], and (2) thin-film-composite (TFC) membranes made by

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interfacial polymerization on porous support layers [6,23,24,33–39]. Compared to the conventional asymmetric membranes (i.e., cellulose acetate-based membranes), TFC membranes have advantages such as a higher water permeability, greater solute rejection, and nonbiodegradability [23,33,40,41]. Aromatic polyamide TFC membranes were invented in 1970s. Since then, they have dominated the production of modern RO membranes [36–39,42–44]. Generally, the polyamide TFC membranes have a highly anisotropic structure consisting of a porous support layer and a thin rejection layer prepared via interfacial polymerization between an aqueous polyfunctional amine solution and a polyfunctional acyl chloride dissolved in an apolar organic solvent [6,7,17,23,33–39]. The support layer and active rejection layer can be independently tailored. Hence it permits easy optimization of the overall composite membrane with desirable separation performance (i.e., permeability, selectivity, and stability). These advantages inspire membrane scientists to explore polyamide TFC membranes for FO applications [6,7,23,24,33–35]. The design scheme for TFC-FO membranes is different from that for RO membranes. One major difference between RO and FO TFC membranes is their requirements for the membrane support. Usually, in order to withstand the high trans-membrane pressure of up to 60–100 bar, a thick and mechanically strong hydrophobic support should be required for TFC-RO membranes. However, FO is generally conducted under low or no applied pressures, a thin support is desired. In addition, a thin support layer can effectively minimize the effect of internal concentration polarization (ICP) in the FO process. This ICP generally cannot be eliminated by increasing the cross-flow velocity and turbulence along the surface because it happens inside the porous support layer which is different from the external concentration polarization (ECP) encountered in pressure driven membrane processes. Moreover, in FO processes both the top and bottom membrane surfaces have to simultaneously contact with two solutions, and water transport through the semi-permeable membrane is based on the solution-diffusion mechanism, thus the substrate physicochemical properties (hydrophilicity, porosity, pore size, pore-size distribution, and substructure resistance) also play very important roles in the overall FO performance [23]. The desired TFC-FO membranes should be: (1) an ultrathin semi-permeable active layer with a high solute rejection and water permeation flux; (2) a thin supporting layer with low ICP; (3) hydrophilic nature to enhance FO performance and reduce membrane fouling; and (4) sufficient mechanical strength and robust to sustain backwash, cleaning and vibration in industrial operations [5,6,23,45]. Studies have shown that the physicochemical properties of the substrates play key roles in determining the formation and morphology of the polyamide layer as well as the separation performance of the resultant TFC-FO membranes. For example, the rate and amount of m-phenylenediamine (MPD) diffusion into the reaction interface may alter the breadth of the reaction zone, the extent of interfacial polymerization reaction, and the polyamide layer thickness [23,46]. Kim and Kim [47] reported that the modification of the polysulfone support by plasma with hydrophilic materials prior to interfacial polymerization led to an increase in not only the water flux and rejection but also chlorine resistance. Singh et al. [48] showed that polysulfone support with a larger pore size resulted in the TFC membrane with a high water flux but a low rejection. Ghosh and Hoek [49] reported that the hydrophobicity of the support layer influenced the meniscus shape that formed between the organic and aqueous phases during interfacial polymerization and therefore determined the properties of the polyamide layer and the overall separation performance. Recently, high performance TFC-FO membranes have been fabricated by introducing a certain amount of hydrophilic sulphonated polymers, like sulphonated polysulfone [23] and PESU-co-sPPSU 11

hydrophilic polymer [6], and sulphonated poly(arylene ether sulfone)s [50] into the substrate membrane matrix. The blended sulphonated materials not only adjust the hydrophilicity and the morphology of the membrane substrates but also significantly enhance the FO performance of the result TFC membranes. Sulphonated poly(ether ketone) (SPEK) polymers are one of the most important hydrophilic sulphonated aromatic polymers [51–53]. In addition to the unique chemical and physical properties such as high hydrolytic, thermal and oxidation stabilities, adjustable hydrophilicity and excellent mechanical properties, they are characteristic of low costs and easy processability [54]. According to the aforementioned requirements for TFC-FO membranes, we believe the addition of SPEK into the support substrate may significantly enhance the performance of the TFC-FO membranes. Therefore, the objectives of this study are (1) to fabricate high performance TFC-FO membranes for desalination using a Mitsui Chemicals’ hydrophilic SPEK as part of substrate materials; (2) to examine the relationship between substrate morphology and resultant TFC-FO membranes’ performance as a function of SPEK content; and (3) to reveal the fundamental science bridging SPEK chemistry, SPEK loadings, membrane substrates properties, TFC formation, thermal treatment and membrane separation performance. To our best knowledge, this is the first study reporting the development and application of the hydrophilic SPEK material for TFC-FO membranes.

2. Experimental 2.1. Materials Udels polysulfone (PSU, UDEL P-3500) was used as the polymer material for the fabrication of membrane substrates. m-Phenylenediamine (MPD) with 499% purity and trimesoyl chloride (TMC) with 98% purity supplied by Sigma-Aldrich were used as the monomers for the interfacial polymerization reaction. N-hexane from Merck with 499.0% purity was utilized as the solvent for the TMC monomer. Bis(4-hydroxy-3,5-dimethyl-phenyl) methane (TMBPF) and 4,40 -difluorobenzophenone (DFBP) from Tokyo Kasei Kogyo Co. Ltd. and the 50% fuming sulfuric acid, dimethylsulfoxide (DMSO) from Wako Junyaku Kogyo Co. Ltd. were used as received for the synthesis of SPEK polymer. N-methyl2-pyrrolidone (NMP) and diethylene glycol (DEG) purchased from Merck were employed as the solvent and pore former, respectively. For FO desalination tests, sodium chloride (NaCl) purchased from Merck was dissolved in deionized (DI) water and used as the draw solution. Polyethylene oxide (PEO) and polyethylene glycol (PEG) from Sigma-Aldrich with different molecular weights were used to measure the mean pore size and its distribution of membrane substrates through the solute transport method. The deionized water was produced by a Milli-Q unit (Millipore) with a resistivity of 15 MO cm. 2.2. Synthesis of SPEK polymer The sulphonated poly (ether ketone) (SPEK) polymer was synthesized by Mitsui Chemicals using a method similar to that described by Zhou and Xiao et al. [52,55]. Typically, a mixture containing 772 g of DMSO, 257 g of toluene, 16.47 g (0.039 mol) of 5,50 -carbonylbis(2-fluorobenzene sulfonic acid) sodium salt, 76.59 g (0.351 mol) of DFBP, 99.98 g (0.390 mol) of TMBPF and 67.38 g (0.488 mol) of potassium carbonate was charged into a five-necked reactor equipped with a nitrogen-introducing tube, a reflux condenser, a stirrer and a thermometer. The reaction system was then heated to, and maintained at 140 1C with stirring for 12 h under a nitrogen atmosphere to remove water generated

G. Han et al. / Journal of Membrane Science 423–424 (2012) 543–555

545

Fig. 1. The chemical structure of: (a) PSU and (b) SPEK with a DS ¼10 wt%.

by the system. The reaction mixture was then distilled for 2 h and diluted by 585 g of toluene at 100 1C. The polymer powder was precipitated in 2500 g of methanol, then filtered and washed with methanol and deionized water. 161.5 g (yield of 91%) of the polymer product was collected after drying at 50 1C for 8 h and 150 1C for 4 h under nitrogen atmosphere. Fig. 1 describes the chemical structures of commercial PSU and the synthesized SPEK polymer with a degree of sulphonation (DS) of 10 wt%, determined by titration results. The inherent viscosity of the polymer in a solvent (NMP/DMSO¼1/1 (wt/wt)) was 96 cm  3 g  1, measured at a concentration of 0.005 g cm  3 at 35 1C. 2.3. Preparation of membrane substrates The membrane substrates were prepared by the conventional Loeb–Sourirajan wet phase inversion method [23]. The polymer solutions with various SPEK content were prepared at room temperature (about 23 1C) and the solution compositions are summarized in Table 1. The solutions were degassed for more than 24 h after completely dissolution. Afterwards, the homogeneous solution was cast using a knife blade at a height of 100 mm over a clean glass plate at ambient temperature. Then the entire assembly was immediately immersed into a water bath at room temperature. The phase inversed substrate membranes were removed from the water bath and washed thoroughly with deionized water and then stored in deionized water at room temperature. 2.4. Fabrication of polyamide TFC-FO membranes The polyamide TFC-FO membranes were fabricated on the top surface of the cast membrane substrates via the interfacial polymerization reaction between MPD and TMC, as shown in Fig. 2. The membrane substrate was firstly immersed in a 2 wt% MPD aqueous solution for 120 s. Then, the excess water droplets on the membrane surface were removed by a filter paper. Subsequently, the top surface of the substrate membrane was brought into contact with a 0.2 wt% TMC/hexane solution for 120 s immediately. In the whole process, membranes were fixed in a frame so that only the top surface of the substrate was exposed to the reactants. The resultant TFC-FO membranes were then thermally treated in hot water and the treatment conditions will be discussed later. All the TFC-FO membranes were finally kept in deionized water at room temperature for further characterization. 2.5. Characterizations 2.5.1. Morphology, contact angle, and membrane porosity Membrane morphology was observed via a field-emission scanning electron microscope (FESEM, JEOL JSM-6700F). The membrane samples dried by a freeze dryer (ModulyoD, Thermo Electron Corporation, USA) were frozen and then fractured in liquid nitrogen

Table 1 Compositions of casting solutions for the fabrication of membrane substrates. Composition of Casting Casting Casting Casting casting solution 1 solution 2 solution 3 solution 4 solutions (wt%) (50 wt% SPEK) (50 wt% SPEK) (25 wt% SPEK) (0 wt% SPEK) PSU SPEK DEG NMP

7.5 7.5 – 85

7.5 7.5 17 68

11.25 3.75 17 68

15 – 17 68

before sputtering with platinum using a sputtering coater (JEOL LFC-1300). The surface hydrophilicity of membrane substrates was measured by a Contact Angle Geniometer (Rame Hart, USA) using Milli-Q deionized water as the probe liquid at room temperature (about 23 1C). To minimize the experimental error, the contact angle was randomly measured at more than 10 different locations for each sample and the average value was reported. To measure the membrane substrate porosity, wet membranes were carefully blotted using tissue paper to remove the excess water on the surface. Then the wet membranes were immediately weighed (m1, g), freeze dried overnight, and re-weighed (m2, g). The water content was calculated as m1  m2, and the dry weight of the membrane was m2. Since the densities of both water (rw, 1.00 g cm  3), PSU (rp1, 1.24 g cm  3) and SPEK (rp2, 1.18 g cm  3) are known, the overall porosity e (%) of the membrane was then obtained as follows [6,23]:

e¼

ðm1 m2 Þ=rw ðm1 m2 Þ=rw þ m2 =rp

ð1Þ

2.5.2. Pore size and pore-size distribution of membrane substrates The mean effective pore size and pore size distribution of each membrane substrate were examined by solute rejection experiments and the detailed procedures were reported elsewhere [6,23,56]. The membrane substrate was first tested to measure its pure water permeability (PWP) (in L/(m2 bar h)) by using an ultrafiltration membrane permeation cell with a sample diameter of 3.5 cm. Subsequently, the membrane was subjected to the neutral solutes (polyethylene glycol (PEG) or polyethylene oxide (PEO)) separation tests with a concentration of 200 ppm under a pressure of 1 bar on the liquid solution side. The concentrations of the neutral solutes in the feed and permeate were measured via a total organic carbon analyzer (TOC ASI-5000A, Shimadzu, Japan). The measured feed (Cf) and permeate (Cp) concentrations were used for the calculation of the effective solute rejection coefficient R (%):   Cp R ¼ 1  100% Cf

ð2Þ

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Fig. 2. Scheme of the interfacial polymerization reaction to form the polyamide layer.

The relationship between Stokes radius (rs, nm) and molecular weight (Mw, g mol  1) of the neutral solutes can be expressed as: for PEG : r ¼ 16:73  1012  M 0:557

ð3Þ

for PEO : r ¼ 10:44  1012  M 0:587

ð4Þ

The mean effective pore size and the pore size distribution were obtained based on the traditional solute transport approach by ignoring the influence of the steric and hydrodynamic interaction between solute and membrane pores, the mean effective pore radius (mp) and the geometric standard deviation (sp) can be assumed to be the same as ms (the geometric mean radius of solute at R¼50%) and sg (the geometric standard deviation defined as the ratio of the rs at R¼84.13% over that at R¼50%). Therefore, based on dp and sp, the pore size distribution of a membrane can be expressed as the following probability density function: " # ðlndp lnmp Þ2 dRðdp Þ 1 pffiffiffiffiffiffi exp  ¼ ð5Þ ddp dp lnsp 2p 2ðlnsp Þ2

2.5.3. Mass transport characteristics of TFC-FO membranes The water and salt permeability of the TFC-FO membranes were characterized by testing the membranes under the RO mode [6]. The RO filtration apparatus applied is a stainless steel cell with an effective membrane area of 19.5 cm2. The water permeability coefficient (A) was calculated from the pure water permeation fluxes under a trans-membrane pressure varying from 1 to 5 bar. The salt rejection (Rs) was tested by one feed water containing 200 ppm NaCl and determined according to the conductivity measurements of permeate and feed solutions. The salt permeability coefficient (B) was determined according to the solution-diffusion theory as follows [16,23,57]: 1Rs B ¼ AðDPDpÞ Rs

ð6Þ

where DP and Dp are the hydraulic pressure difference and osmotic pressure difference across the membrane, respectively. 2.5.4. FO performance of TFC-FO membranes FO performance of the fabricated TFC membranes was evaluated via a lab-scale cross-flow filtration unit as reported by Widjojo and Zhang et al. [6,9]. The cross-flow permeation cell was a plate-and-frame design with a spacer free rectangular channel (2.0 cm in length, 2.0 cm in width and 0.28 cm in height) on each side of the membrane. The solution flow velocities during the FO tests were kept at 0.2 L min  1 for both the feed and draw solutions which flowed counter-currently along the membranes. The temperatures of the feed and draw solutions were maintained at room temperature of about 23 1C. Each membrane was evaluated under two different modes: (1) pressure retarded osmosis

(PRO mode) where the draw solution flows against the polyamide selective layer; and (2) FO mode where the draw solution flows against the porous support layer. NaCl solutions with different concentrations (wt%) and deionized water were used as the draw solutions and feed solution, respectively. During FO tests, the dilution of the draw solution was ignored, because the ratio of water permeation flux to the volume of the draw solution was less than 2%. A balance (EK-4100i, A&D Company Ltd., Japan) connected to a computer recorded down the mass of water permeating into the draw solution over a selected period of time. The water permeation flux (Jv, L m  2 h  1, abbreviated as LMH) is calculated from the volume change of the feed or draw solution: Jv ¼

DV D t Sm

ð7Þ

where DV (L) is the permeation water collected over a predetermined time Dt (h) in the FO process; Sm is the effective membrane surface area (m2). The salt concentration in the feed water solution was determined from the conductivity measurement based on a standard concentration–conductivity curve. The salt leakage rate or salt reverse diffusion flux Js in g m  2 h  1 (abbreviated as gMH) from the draw solution to the feed side was determined from the increase of the feed conductivity: Js ¼

DðC t V t Þ Dt Sm

ð8Þ

where Ct and Vt are the salt concentration and the volume of the feed solution at the end of FO tests, respectively. To minimize errors, every experiment was carried out more than twice and the average value was reported. The water flux in FO processes was also modeled by the following equations [16,58]. For the PRO mode (selective layer against the draw solution): Jw ¼

1 ApD,m J w þ B ln Km ApF,b þ B

ð9Þ

For the FO mode (selective layer against the feed solution): Jw ¼

ApD,b þ B 1 ln ApF,m þJ w þ B Km

ð10Þ

where pD,b and pF,b refer to the osmotic pressures in the respective bulk draw solution and feed solution, pD,m and pF,m are the corresponding osmotic pressures on membrane surfaces in the draw and feed solutions. The relationship among solute diffusion resistivity within the porous layer Km, diffusivity Ds, membrane structural parameter S, membrane tortuosity t, membrane thickness l and membrane porosity e can be represented as follows:

Km ¼

S lt ¼ Ds e Ds

ð11Þ

G. Han et al. / Journal of Membrane Science 423–424 (2012) 543–555

overnight, they had to be exposed to air at atmospheric conditions for around 20 min when preparing for the TGA tests. It can be seen that for PSU polymer, there is only one major weight loss starting from around 500 1C which is ascribed to the decomposition of the polymer backbone. However, three weight-loss stages are observed for the synthesized SPEK polymer. The initial mass loss below 100 1C is due to the evaporation of water bounded to the sulphonic groups, indicating that the hydrophilic nature of the SPEK polymer which can easily adsorb certain amount of moisture during the short time when exposed to air. The mass loss centered at around 450 1C is attributed to the degradation of the sulphonic groups which can be observed from the FTIR spectrum (Fig. 3(b)). The final decomposition of the polymer is observed at above 530 1C, which is in agreement with typical fully aromatic polymers [54,55,60]. The FTIR spectra of the SPEK and PSU polymers are shown in Fig. 3(b). The peaks at 1651, 1500 and 1240 cm  1 are attributed to the carbonyl stretching of phenyl ketone group (–Ar–C(QO)–Ar–), CQC of benzene and phenyl ester group (–Ar–O–), respectively. The SQO stretching vibration of PhSO2Ph is present at 1165 cm  1. The characteristic absorption at 1192 cm  1 is due to the asymmetric stretching vibration of the aromatic SO3H [51]. Compared with PSU, the synthesized SPEK polymer is more hydrophilic because of the sulphonic groups and is expected to be a good substrate material for the fabrication of high performance TFC FO membranes [6,23].

2.5.5. Membrane mechanical strengths The mechanical properties of membrane substrates were measured by using an Instron 5542 tensile testing equipment. The flat sheet membranes were cut into stripes with 5 mm width and clamped at the both ends with an initial gauge length of 30 mm and a testing rate of 10 mm/min. At least five stripes were tested for each casting condition to obtain the average values of tensile stress, Young’s modulus and elongation at break of the membranes. 2.5.6. Other characterizations A FTIR (Fourier transform infrared spectroscopy) spectroscope (Bio-Rad FTS 135) over the range of 700–4000 cm  1 in the attenuated total reflectance (ATR) mode was used to characterize the membranes. Thermogravimetric analysis (TGA) data was collected on a TGA 2050 Thermo-gravimetric Analyzer (SDT 2960, TA Instruments, New Castle, DE) with a heating rate of 10 1C/min in a nitrogen flow of 100 mL/min. The ion exchange capacity (IEC) and the degree of sulphonation (DS) of the synthesized SPEK material were obtained from the titration results and the detail procedures were reported elsewhere [52]. The DS and IEC values were calculated with the following equations, modified from Guan et al. [59]: DSð%Þ ¼ IEC ¼

0:434MV  100 W0:08MV

ð12Þ

1000DS 434 þ 81DS

547

ð13Þ 3.2. Characteristics and performance of membrane substrates

where W, M and V are the mass of SPEK (g), concentration of NaOH (mol/l) and volume of NaOH (ml) reacted with the SPEK, respectively; the 434 and 81 are the molecular weights of the PEK repeat unit and the –SO3H, respectively.

The synthesized SPEK material (15 wt% in NMP) cannot form a free standing asymmetric membrane substrate when pure water is used as the non-solvent to induce the phase inversion maybe due to its highly hydrophilic nature and slow precipitation rate. Consequently, polymer blends consisting of PSU and different concentrations of SPEK hydrophilic polymer were used to prepare the substrates for TFC-FO membranes. Table 1 summarizes the polymer composition in casting solutions for substrate preparation.

3. Results and discussion 3.1. Characterization of the synthesized SPEK polymer The DS and IEC values of the synthesized SPEK polymer are calculated to be 10 wt% and 0.49 meq/g, respectively, suggesting the successful preparation of the sulphonated material, which is further proven by the TGA measurement as shown in Fig. 3(a). Although the PSU and SPEK polymer samples were dried at 80 1C

120

3.2.1. Role of the substrate membrane structures It is well accepted that membrane substrates for TFC-FO membranes should have a relatively high membrane porosity and proper pore size for the purpose of enhancing water permeability,

PSU

80

Transmittance, %

Weight (%)

100

SPEK

60 40 20 0 0

100

200

300

400

500

600

700

800

900

600

800

1000

1200

1400

1600

1800

2000

Wavenumber, cm-1

Temperature (°C)

Fig. 3. TGA curves (a), and FTIR spectra (b) of PSU and synthesized SPEK polymer.

Table 2 Summary of the mean effective pore size (mp), PWP and MWCO of the PSU/SPEK (50 wt% SPEK) membrane substrates cast from solutions with/without DEG additive. Membrane

mp (nm)

sP

PWP [L/(m2 bar h)]

Porosity (%)

MWCO (kDa)

Casting solution 1 (50 wt% SPEK without DEG) Casting solution 2 (50 wt% SPEK with DEG)

8.3 10.7

1.22 1.25

95.5 152.7

62.6 77.2

31.1 66

548

G. Han et al. / Journal of Membrane Science 423–424 (2012) 543–555

while maintaining good mechanical properties. This can be partially achieved by the addition of some pore forming agents in the polymer solution. Table 2 and Fig. 4 display the pore size characteristics and the PWP values of membrane substrates cast from solutions 1 and 2 as tabulated in Table 1. The PWP can be significantly increased from 95.5 L/(m2 bar h) to 152.7 L/(m2 bar h) after adding the pore former DEG. Meanwhile, the MWCO of the as-cast membranes also shows an increment form 31 kDa to 66 kDa. This is mainly because of the enlarged membrane mean pore size from 8.3 nm to 10.3 nm and membrane porosity from 62.6% to 77.2%. In addition, as shown in Fig. 4, the addition of DEG can slightly broaden the pore size distribution of the substrate membranes. Fig. 5 shows the morphology of aforementioned membrane substrates cast from solutions 1 and 2. Both substrates exhibit a

0.35

Casting solution 2 (50 wt% SPEK with DEG) Casting solution 1 (50 wt% SPEK without DEG)

Probability density function, (nm-1)

0.30

0.25

fully sponge-like structure with no observed macrovoids. This is due to the delayed demixing induced by the hydrophilic sulphonated material which can suppress the formation of marovoids [6] and reduced membrane thickness [61,62]. Under SEM observation at a high magnification of 50,000, the membrane substrate containing DEG exhibits bigger pore sizes at the top and bottom surface and more porous cross-section morphology than those cast from solution 2. These observations are consistent with the results shown in Table 2 and Fig. 4. Table 3 presents the FO performance of the TFC membranes formed on the substrates cast from solutions 1 and 2 in terms of water flux and salt reverse flux using 2 M NaCl as the draw solution and deionized water as the feed solution. The TFC membranes constructed on the substrates cast from solution 2 containing DEG exhibit a higher water flux and a comparable salt reverse flux. These indicate that the addition of the pore former DEG in the casting solution for substrates is favorable for the fabrication of TFC-FO membranes with better performance due to the reduced substrate resistance.

Table 3 PRO and FO performance of the TFC-FO membranes formed on the PSU/SPEK (50 wt% SPEK) membrane substrates cast from solutions with/without DEG additive.

0.20

0.15

Sample Substrate

PRO mode Water flux (LMH)

0.10

0.05

0.00 0

10

20

30

40

50

Pore radius, dp(nm)

Fig. 4. Pore-size distribution of the PSU/SPEK (50 wt% SPEK) membrane substrates. with/without DEG additive.

40 TFC-FO Casting solution 1 (50 wt% SPEK) without DEG TFC-FO Casting solution 2 50 (50 wt% SPEK) with DEG

FO mode Salt leakage (gMH)

Water flux (LMH)

Salt leakage (gMH)

8

30

7

9

35

7

All TFC-FO membranes are thermally treated in 95 1C water for 1 min. Draw solution: 2 M, NaCl; Feed solution: DI water.

Top X10,000

Top X50,000

1µm

100 nm

1µm

100 nm

Cross section X50,000

Cross section X3,000

Cross section X50,000

Cross section X3,000

1µm

100 nm

Bottom X10,000

Bottom X50,000

1µm

100 nm

Top X10,000

1µm

Bottom X10,000

1µm

Top X50,000

100 nm Bottom X50,000

100 nm

Fig. 5. Typical morphology of membrane substrates cast from solutions 1 and 2.

G. Han et al. / Journal of Membrane Science 423–424 (2012) 543–555

Top X10,000

1 µm

Top X10,000

Bottom X10,000

1 µm

Bottom X10,000

549

Cross section X50,000

Cross section X1,000

100 nm

10 µm

Cross section X2,500

Cross section X50,000

10 µm

Top X10,000

Bottom X10,000

Cross section X3,000

Cross section X50,000

10 µm

Fig. 6. Typical substrate morphology as a function of SPEK concentration in substrates for TFC membrane fabrication: (a) casting solution 4 (0 wt% SPEK); (b) casting solution 3 (25 wt% SPEK); and (c) casting solution 2 (50 wt% SPEK).

0.20 Casting solution 2 (50 wt% SPEK polymer) -1

Probability density function, (nm )

3.2.2. Role of the sulphonated material blending concentration It has been known that blending a hydrophilic material into a casting solution made of hydrophobic polymers can alter original membrane properties such as morphology, hydrophilicity, pore size and its distribution, as well as mechanical strength because of different phase inversion paths [6,23]. A similar phenomenon is observed in this study. Fig. 6 shows the SEM morphology of membrane substrates cast from solutions comprising different SPEK concentrations as tabulated in Table 1. The membrane cast from pure PSU (solution 4) has a cross-section morphology full of finger-like macrovoids. However, the cross-sections of the membranes cast from the solutions containing 25 wt% (solution 3) and 50 wt% (solution 2) SPEK exhibit a fully sponge-like structure without macrovoids. This is due to the fact that the sulphonated material induces the delayed demixing and suppresses the macrovoid formation. Traces of macrovoids can be observed at the bottom surface of the membranes (10,000  magnification) cast from pure PSU (solution 4), while no macrovoids can be found in those membranes containing 25 wt% and 50 wt% SPEK. In addition, the pore size at the bottom surface, the membrane thickness and the porous structure across the membrane significantly decrease with an increase in SPEK content. All the cast membrane substrates show a similar top surface morphology with no visual pores observed at a magnification of 10,000. Fig. 7 and Table 4 display and summarize the pore size characteristics and PWP values of the membrane substrates as a function of SPEK content, respectively. With increasing the SPEK content from 0 wt% to 25 wt%, the mean pore size slightly increases from 19.8 nm to 21.5 nm. However, a higher SPEK content of 50 wt% corresponds to a much smaller mean pore size of 10.7 nm. The pore size distribution also narrows down with an increase in SPEK content, as indicated by the decreasing trend of the geometric standard deviation from 1.54 nm, 1.45 nm to 1.25 nm for substrates containing 0 wt%, 25 wt% and 50 wt% SPEK, respectively. In addition, the PWP values follow the order of pore sizes. In other words, the PWP value of the substrate containing

0.16

0.12 Casting solution 4 (0 wt% SPEK polymer)

0.08 Casting solution 3 (25 wt% SPEK polymer)

0.04

0.00 0

10

20

30

40

50

60

70

80

Pore radius, dp(nm)

Fig. 7. Pore size distribution of membrane substrates with different SPEK concentrations.

25 wt% SPEK is the highest, followed by that containing 0 wt% SPEK and then 50 wt% SPEK. The highest PWP for the substrate blended with 25 wt% SPEK arises from the effects of increased hydrophilicity, pore size and porosity. However, a further increase in SPEK content to 50 wt% does not help PWP. On the contrary, its PWP value is even lower than that of the PSU substrate (i.e., comprising 0 wt% SPEK). This is due to the fact that the former has a much higher degree of water induced chain swelling than the latter, thus reduces water transport across the membrane [6]. Contact angle measurements indicate that the PSU membrane without blending any SPEK has a contact angle of 81.01, while the membrane substrates blended with 25 wt% and 50 wt% SPEK have relatively low contact angles of 76.11 and 59.41, respectively. The decreased water contact angles are mainly because of the addition of hydrophilic SPEK. Table 5 shows the mechanical strengths of these membrane substrates. With an increment in SPEK content,

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Table 4 Summary of mean effective pore size (mp), PWP and MWCO of membrane substrates cast from solutions with different SPEK concentrations. Membrane

mp (nm)

sP

PWP [L/(m bar h)]

MWCO (kDa)

Porosity (%)

Contact angle (deg.)

Casting solution 4 (0 wt% SPEK) Casting solution 3 (25 wt% SPEK) Casting solution 2 (50 wt% SPEK)

19.8 21.5 10.7

1.54 1.45 1.25

385.4 497.6 152.7

305.1 316.4 66.0

77.4 83.0 77.2

81.0 71.8 76.1 71.9 59.4 71.5

Table 5 Mechanical properties of membrane substrates cast from solutions with different SPEK concentrations and thermal treatments. Membrane ID Membrane Membrane Membrane Membrane

substrate substrate substrate substrate

from from from from

casting casting casting casting

solution solution solution solution

4 3 2 2

(0 wt% SPEK) (25 wt% SPEK) (50 wt% SPEK) (50 wt% SPEK)—95 1C water for 1 min

Young’s modulus (MPa)

Tensile strength (MPa)

Elongation at break (%)

276.8 7 35.4 50.5 7 5.8 47.0 7 1.9 69.1 7 4.8

7.3 7 0.8 3.8 7 0.4 3.4 7 0.2 4.6 7 0.1

20.3 75.2 32.7 74.8 35.0 72.1 50.3 75.8

Table 6 Effects of thermal treatment on FO performance of TFC membranes made from different substrates. Sample ID

Temperature (1C)

Time (min)

PRO mode Water flux (Jw, LMH)

– 1 1 2 1 1 1

FO mode Salt leakage (Js, gMH)

55 16 45 16 50 9 25 10 Membrane shrinkage (defects) 38 6 25 8

Jw/Js (L/g)

Water flux (Jw, LMH)

Salt leakage (Js, gMH)

Jw/Js (L/g)

3.4 2.8 5.5 2.5

37 30 35 18

11 12 7 8

3.4 2.5 5.0 2.3

6.3 3.12

23 17

4 6

5.8 2.8

1 2 3 4 5 6 7

– 80 95 95 100 95 100

Substrate

1–5: TFC-FO membranes with substrates cast from casting solution 2 (50 wt% SPEK); 6 and 7: TFC-FO membranes with substrates cast from casting solution 4 (0 wt% SPEK)

Experimental conditions: Feed: deionized water; draw solution: 2 M NaCl; crossflow velocity and temperature of both feed and draw solutions of 0.2 L min  1 and 23 1C, respectively.

3.3. Characteristics and performance of TFC-FO membranes 3.3.1. Effects of the thermal treatment The thin polyamide layer formed via interfacial polymerization between TMC and MPD (Fig. 2) is the selective layer which determines the separation performance of the TFC-FO membranes. Curing temperature, reaction duration and posttreatment are three important parameters controlling the quality and separation performance of the polyamide layer. The post thermal treatment in hot water helps to elute un-reacted monomers such as MPD out of the TFC membranes. Table 6 summaries the FO performance of the TFC membranes constructed on the PSU and PSU/SPEK (50 wt% SPEK) substrates after being treated with different temperatures and durations. For the TFC-FO membranes consisting of a hydrophilic PSU/SPEK (50 wt% SPEK) substrate, although a high water flux can be achieved without any thermal treatment, a high salt revere flux is accompanied. When the TFCFO membrane was treated at relative moderate conditions, such as at 80 1C for 1 min, a decreased water flux and a comparable high reverse salt flux were observed which may be due to the thermal induced reduction in membrane permeability. However, when the thermal treatment conditions became too strong, for example 100 1C for 1 min or 95 1C for 2 min, the TFC-FO membranes shrank and exhibited a significant reduction in performance because of the defect formation. Similar

0.25 Casting solution 2 (50 wt% SPEK with DEG) Casting solution 2 (50 wt% SPEK with DEG) (95 °C for 1 min)

Probability density function, (nm-1)

Young’s modulus decreases, while the elongation at break increases. However, a further increase in SPEK content beyond 50 wt%, the membranes become too weak to be characterized under FO processes.

0.20

0.15

0.10

0.05

0.00 0

10

20

30

40

50

60

70

80

Pore radius, dp(nm)

Fig. 8. Pore-size distribution of PSU/SPEK (50 wt% SPEK) membrane substrates with/without thermal treatment.

phenomenon was observed for TFC-FO membranes with a PSU substrate (i.e., sample ID 7 in Table 6). In comparison, the TFC-FO membranes treated with 95 1C water for 1 min show the best performance indicated by the highest Jw/Js values. In addition, different substrates show different responses to thermal treatment, indicating that the effects of thermal treatment on TFC-FO membrane are restrained by the properties of the substrate. In fact, thermal treatment will also simultaneously alter the substrate properties, particularly for the SPEK material used in this

G. Han et al. / Journal of Membrane Science 423–424 (2012) 543–555

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Table 7 Summary of mean effective pore size (mp), PWP and MWCO of the PSU/SPEK (50 wt% SPEK) membrane substrates with/without thermal treatment. Membrane

mp (nm)

sP

PWP [L/(m2 bar h)]

Porosity (%)

MWCO (kDa)

Contact angle (deg.)

Casting solution 2 (50 wt% SPEK with DEG) Casting solution 2 (50 wt% SPEK with DEG) (95 1C water for 1 min)

10.7 13.1

1.25 2.03

152.7 84.1

77.2 50.4

66 248.8

59.4 7 1.5 68 7 2.4

Fig. 9. Scheme of the possible pathways for the formation of a sulfone linkage.

130 PSU&SPEK(50 wt%)

PSU/SPEK(50 wt%)

Transmittance, %

1080

1500 1165

120

PSU&SPEK(50 wt%)-95 °C for 1min

110

1651

100

1240 Weight (%)

90

PSU/SPEK(50 wt%)-95 °C for 1min

80 70 60 50 40 30

600

800

1000

1200

1400

1600

1800

2000

-1

Wavenumber, cm

Fig. 10. FTIR spectra of the membrane substrates cast from solution 2 (50 wt% SPEK) with/without thermal treatment.

study. Fig. 8 and Table 7 present the pore size characteristics, PWP and MWCO of the membrane substrates cast from solution 2 (Table 1) with or without the thermal treatment in 95 1C water. It is interesting to take note that after thermal treatment the pore size distribution of the substrate membrane significantly becomes broader with a much increased MWCO, while keeping a relatively similar membrane mean pore size. Moreover, the porosity and PWP of the membrane substrates decrease remarkably after thermal treatment. These results suggest that the porous structure

20 0

100

200

300

400

500

600

700

800

900

Temperature (°C)

Fig. 11. TGA curves of the membrane substrates cast from solution 2 (50 wt% SPEK) with/without thermal treatment.

has been changed, which may be due to the pseudo-cross-linking reaction happened during the thermal treatment. Fig. 9 illustrates the possible pathways for the formation of a sulphone linkage between polymer chains. The formation of the bridges occurs via an electrophilic aromatic substitution (SEAr) with a Friedel–Crafts type acylation mechanism [54,63]. FTIR spectroscopy and TGA analysis were employed to further characterize the pseudo-cross-linking reaction during the thermal treatment. Fig. 10 shows the typical FTIR spectrum of a PSU/SPEK

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(50 wt%) membrane treated at 95 1C for 1 min and compares with that of an untreated PSU/SPEK(50 wt%) membrane. The characteristic peaks of the SPEK and PSU materials can be observed in the SPEK/PSU hybrid membrane when comparing with their individual spectra shown in Fig. 3. However, the peaks at 1080 cm  1, 1165 cm  1, 1192 cm  1 and 1240 cm  1 due to the absorption of 1:2:4-substituted phenyl rings, the SQO stretching vibration of PhSO2Ph, the asymmetric stretching vibration of the aromatic SO3H, and the phenyl ester group (–Ar–O–), respectively, show significantly shifts toward low frequency. In addition, the characteristic absorption at 1500 cm  1 and 1651 cm  1, which are attributed to the CQC of benzene and carbonyl stretching of phenyl ketone group (–Ar–C(QO)–Ar–), respectively, also slightly move to the low frequency direction [51,64]. Fig. 11 compares the thermal decomposition behavior of the thermal treated PSU/SPEK (50 wt%) membrane substrate with an untreated PSU/SPEK (50 wt%) membrane. As aforementioned, the mass loss in Fig. 3 occurs at around 450 1C because of the decomposition of sulfonic groups. Interestingly, this mass loss is clearly reduced for the thermal-treated membrane, indicating a loss of some sulfonic groups during the post thermal treatment. This phenomenon is consistent with the mechanism proposed in Fig. 9 that the bonds formation between adjacent polymeric chains consumes some SO2 moieties. Furthermore, Table 7 shows that the contact angle of the membrane substrate decreases after thermal treatment. This reduction in membrane hydrophilicity also confirms the consumption of some sulphonate groups. On the other hand, if the above hypothesis is occurred, the mechanical strength of the membrane substrates will increase after thermal treatment due to the enhanced interaction between polymer backbones. As displayed in Table 5, Young’s modulus (MPa), tensile strength (MPa) and elongation at break (%) are all significantly increased. In summary, all the results suggest that the interaction between macromolecular chains during the thermal treatment happens through the proposed pathways.

a

Top X10,000

Top X50,000

3.3.2. Effects of SPEK concentration on the TFC layer and FO performance TFC-FO membranes were also fabricated on membrane substrates consisting of different SPEK concentrations. For easy comparison, DEG was used as the pore-former for the substrates and all the TFC-FO membranes were treated in 95 1C water for 1 min. Fig. 12 displays the SEM morphology of TFC-FO membranes on substrates cast from solutions 2, 3 and 4, respectively. A typical ‘‘ridge-and-valley’’ morphology is observed for the TFC polyamide layer. However, the average thickness of the polyamide layer varies with SPEK content in the substrates and follows the order of 25 wt% SPEK (solution 3) 40 wt% SPEK (solution 4) 450 wt% SPEK (solution 2). The thickness of the polyamide layer is an average of at least three separate samples. Since the conditions for interfacial polymerization and post-treatment are the same and these three substrates also show a similar top surface pore structure (Fig. 6), the difference in substrate hydrophilicity may be mainly responsible for the deviation in their TFC layer thicknesses. Since interfacial polymerization between MPD and TMC occurs predominantly in the organic phase due to the relatively low solubility of TMC in water. The MPD molecules must diffuse from the water phase into the organic phase and react with the TMC molecules during the initial stage of interfacial polymerization that results in the formation of the nascent polyamide film consisting of many pendant acid chlorides. The adsorbed MPD monomers within the porous substrate can further diffuse out and react with these acid chlorides [65,66]. Compared with the hydrophobic PSU substrate, the substrates containing 25 wt% SPEK may absorb more MPD molecules in water and react with TMC, thus produces a thicker polyamide layer. On the other hand, a further increase in SPEK content to 50 wt% may not only enhance water content in the porous substrate but also limit the diffusion of MPD into the reaction zone due to the hydrogen bonding between MPD and sulfonic groups. As a result, the thickness of the TFC layer on highly hydrophilic substrates becomes smaller.

Cross section X10,000

Cross section X50,000

0 wt% SPEK 247 167 270 1µm

b

100 nm

25 wt% SPEK 322

c

241

244

255

410

50 wt% SPEK 145 185

205

210 285

Fig. 12. Typical morphology of TFC-FO membranes with different SPEK concentrations in the substrates: (a) casting solution 4 (0 wt% SPEK); (b) casting solution 3 (25 wt% SPEK); and (c) casting solution 2 (50 wt% SPEK).

G. Han et al. / Journal of Membrane Science 423–424 (2012) 543–555

Table 8 compares the performance of the TFC-FO membranes under both PRO and FO modes using 2 M NaCl as the draw solution and deionized water as the feed. The TFC-FO membranes comprising a fully sponge-like structure substrate made of 50 wt% SPEK show the highest water flux and slightly increased salt leakages for both PRO and FO modes. Water fluxes of 50 and 35 LMH can be achieved for PRO and FO modes, respectively, when using a 2 M NaCl as the draw solution. A similar sponge-like structure and high performance FO membranes were observed by Widjojo et al. [6] using other type hydrophilic material. Therefore, a highly finger-like structure is not an essential requirement to form a high water flux TFC-FO membrane as claimed by others [33]. In fact, a sponge-like structure with relatively hydrophilic characteristics is preferable since it can provide better performance stability for the TFC-FO membranes with enhanced antifouling properties. In comparison of PRO and FO data in Table 8, the water fluxes under PRO are much higher than those under FO due to severe internal concentration polarization within the porous substrate under the FO mode. Table 9 summarizes the basic transport properties of the TFC-FO membranes. The water permeability coefficient rises with

553

an increase in SPEK content in the substrates, while, the salt permeability coefficient also slightly increases. Interestingly, the calculated structure parameter (S) significantly decreases with an increase in SPEK content in the membrane substrates. This implies that the ICP effect can be significantly mitigated when the TFC-FO membrane is fabricated on a membrane substrate with higher hydrophilic and porous nature. Furthermore, since the TFC-FO membrane constructed on the substrate containing 50 wt% SPEK (solution 2 as shown in Table 1) shows the best FO performance with the lowest S value, it was further tested under both PRO and FO modes using different NaCl concentrations as draw solutions and deionized water as the feed solution. As shown in Fig. 13, the water flux linearly increases in both PRO and FO modes at low draw solution concentrations, while it seems to be leveled off at higher NaCl concentrations. This is most likely owing to the dilutive external concentration polarization (ECP) near the membrane surface and ICP within the porous substrate. In addition, a higher draw solution concentration may induce a higher salt leakage and thus reduces the overall osmotic driving force across the membrane [6].

3.4. Osmotic seawater desalination Table 8 Performance of TFC-FO membranes with different SPEK content in the membrane substrates. Sample Substrate

TFC-FO 0 TFC-FO 25 TFC-FO 50

PRO mode

Casting solution 4 (0 wt% SPEK) Casting solution 3 (25 wt% SPEK) Casting solution 2 (50 wt% SPEK)

FO mode

Water flux (LMH)

Salt leakage (gMH)

Water flux (LMH)

Salt leakage (gMH)

38

6

23

4

42

7

29

5

50

9

35

7

All TFC-FO membranes are treated in 95 1C water for 1 min. Experimental conditions: Feed: deionized water; draw solution: 2 M NaCl; crossflow velocity and temperature of both feed and draw solutions of 0.2 L min  1 and 23 1C, respectively.

Actual membrane performance for seawater desalination is important for industrial applications. Fig. 14 shows the water transport performance as a function of NaCl concentration in draw solutions under both PRO and FO modes when using a model seawater solution (3.5 wt% NaCl) as the feed. Our best TFC-FO membrane shows a water flux of 22 LMH under the PRO mode or 17 LMH under the FO mode using 2 M NaCl as a draw solution. To the best of our knowledge, these water fluxes outperform other flat sheet TFC-FO membranes in the application of seawater desalination. A comparison of Figs. 13 and 14 also indicates that water fluxes decrease in both PRO and FO modes when seawater is used as the feed. This is due to a reduction in overall osmotic pressure difference between the feed and the draw solution. In addition, the difference in water flux between PRO and FO modes is decreased during seawater desalination. This is resulted from a smaller osmotic driving force during seawater desalination and a smaller

Table 9 Transport properties and structural parameters of TFC-FO membranes with different SPEK content in membrane substrates. Membranea

Water permeability, A (L/(m2 bar h))

Salt rejection (%)b

Salt permeability, B (L m  2 h  1)

Km (s m  1)

S (m)c

TFC-FO 0 TFC-FO 25 TFC-FO 50

0.50 0.67 0.75

91 90 89.5

0.041 0.062 0.068

0.12  105 0.10  105 0.071  105

1.82  10  4 1.56  10  4 1.07  10  4

a b c

TFC-FO 0 represents the TFC-FO membrane with a substrate cast from solution 4 (0 wt% SPEK). Tested at 1 bar (14.5 psi) with 200 ppm NaCl solution. Structural parameters were calculated based on experiments under the FO mode using 2 M NaCl as the draw solution and deionized water as the feed. 60

15 PRO FO

Reverse salt flux (gMH)

Water flux (LMH)

50 40 30 20 10

PRO FO

12 9 6 3 0

0 0.0

0.5

1.0

1.5

2.0

Draw solution concentration, NaCl (M)

0.0

2.5

0.5

1.0

1.5

2.0

2.5

Draw solution concentration, NaCl (M)

Fig. 13. The water fluxes and salt leakages of TFC-FO membranes (50 wt% SPEK polymer in the membrane substrates) in the PRO and FO tests with different draw solution concentrations using deionized water as feed.

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entitled, ‘‘Advanced FO Membranes and Membrane Systems for Wastewater Treatment, Water Reuse and Seawater Desalination’’ (Grant No. R-279-000-336-281). Special thanks are due to Dr. Jincai Su and Mitsui Chemicals for their valuable suggestions.

30 PRO FO

Feed: 3.5 wt% NaCl

Water flux (LMH)

25

20 Nomenclature

15

10

5 0.5

1.0 1.5 2.0 Draw solution concentration, M(NaCl)

2.5

Fig. 14. The water flux in the PRO and FO tests with varying draw solution concentrations (NaCl) using seawater (3.5 wt% NaCl) as feed.

water flux tends to reduce ICP and ECP effects. Therefore the difference in water flux between PRO and FO modes is diminished.

4. Conclusions In this work, we have demonstrated hydrophilic sulphonated poly(ether ketone) (SPEK) is a good substrate material for the fabrication of high performance flat sheet TFC-FO membranes. The following conclusions can be further drawn from the current work: 1. Introduction of SPEK into the membrane substrates increases membrane hydrophilicity, reduces membrane thickness and changes membrane morphology from a finger-like to a spongelike structure. Membrane ductility can also be enhanced via blending SPEK material. 2. The TFC-FO membranes with the most hydrophilic nature of substrates, i.e., 50 wt% SPEK exhibit the lowest membrane thickness, fully sponge-like structure morphology and the highest water flux of 50 LMH and 35 LMH tested under PRO and FO modes, respectively, when using deionized water as the feed and 2 M NaCl as the draw solution. These membranes also show the highest water flux of 22 LMH under the PRO mode when a model seawater solution (3.5 wt% NaCl) used as the feed and 2 M NaCl as the draw solution. To the best of our knowledge, this seawater desalination water flux outperforms other flat sheet TFC-FO membranes among available published literatures. 3. The degree of membrane hydrophilicity and thickness of the substrates for TFC-FO membranes may play much stronger roles in enhancing the water transport in FO processes compared to membrane morphology (sponge-like or finger-like). TFC-FO membranes derived from substrates of hydrophobic nature and fingerlike structure do not necessarily facilitate a higher water flux in FO processes than those of hydrophilic characteristics and fully sponge-like structures. Post-treatment procedures of TFC-FO membranes can significantly alter the membrane performance. 4. It has been demonstrated that the membrane structure parameter, an indicator of potential internal concentration polarization (ICP), can be significantly decreased by blending a certain amount of hydrophilic sulphonated materials into the membrane substrates. Acknowledgments This research was funded by the Singapore National Research Foundation under its Competitive Research Program for the project

A B Cf Cp Ct Ds Js Jw Km l m P R S Sm Dt DV Vt

e p r s t m dp ds

water permeability (L/m2 h bar) salt permeability (L m  2 h  1) feed concentration (mol L  1) permeate concentration (mol L  1) salt concentration (mol L  1) diffusion coefficient of salt in the membrane substrate (m2 s  1) reverse salt flux (g m  2 h  1) water flux (L m  2 h  1) solute diffusion resistivity within the porous layer (s m  1) thickness (m) membrane weight (g) pressure (bar) solute rejection membrane structural parameter (m) effective membrane surface area (m2) operation time interval (h) water permeation volume (L) volume of the feed at a time interval of Dt (L) porosity osmotic pressure (bar) material density (g cm  3) geometric standard deviation tortuosity geometric mean radius pore diameter (nm) solute diameter (nm)

Subscripts s m b D F

solute membrane bulk solution draw solution side feed solution side

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