SPEEK blend nanofiltration membranes

Desalination 249 (2009) 996–1005

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Effect of SPEEK content on the morphological and electrical properties of PES/SPEEK blend nanofiltration membranes Woei-Jye Lau, A.F. Ismail ⁎ Advanced Membrane Technology Research Centre (AMTEC), Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

a r t i c l e

i n f o

Article history: Received 27 March 2008 Accepted 28 June 2009 Available online 8 October 2009 Keywords: Nanofiltration Steric-hindrance pore model Teorell–Meyer–Sievers model Sulfonated poly (ether ether ketone) Membrane separation

a b s t r a c t Polyethersulfone (PES)/poly (ether ether ketone) (SPEEK) blends nanofiltration membrane at different SPEEK contents were prepared using a simple dry-jet wet spinning technique. The SPEEK polymer with fixed sulfonation degree was used for membrane preparation and characterized using FTIR and nuclear magnetic resonance (NMR) spectrometer. The morphological and electrical properties of the blends membrane were deduced based on the combination of irreversible thermodynamic model, steric-hindrance pore model (SHP) and Teorell–Meyer–Sievers model (TMS). The modeling results have been analyzed and discussed. The effect of SPEEK content on the blend properties was further studied in detail by FTIR, DSC and TGA and the results were discussed. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, nanofiltration (NF) is becoming more and more important in environmental technology due to its capabilities of removing all pathogens, multivalent ions and small organic molecules in the molecular weight range of approximately 200–1000. Because of these properties, NF is widely applied in the treatment of industrial wastewater and in the production of drinking water from non-brackish, ground or surface water [1–5]. In order to improve NF membrane performances such as water flux and rejection rate, there are a number of studies reported on the membrane preparation in the literature. Interfacial polymerization (IP) technique is one of the most wellknown processes for the synthesis of thin film composite NF membranes. A cross-linking on a microporous membrane surface would give the composite membrane more chemical-resistant [6,7] and also better separation [1,2,5]. However, IP technique is quite sophisticated and laborious since the process can only be carried out in two different bath solutions (aqueous and organic solution) which in general involve various preparation conditions. Due to this reason, there is a growing interest in the development of NF membranes in a relatively simple way. Over the past few years, there is a number of studies reported the fabrication of charged asymmetric NF membranes through the single step process with using charged polymeric material as the main membrane forming or as an additive [8–13]. In general, NF membranes have unique characteristics which differ from ultrafiltration (UF) and reverse osmosis (RO) membranes since they are designed to selectively

⁎ Corresponding author. Tel.: +60 7 553 5592; fax: +60 7 558 1463. E-mail address: [email protected] (A.F. Ismail). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.09.016

remove compounds based on the combination effect of steric exclusion and electrostatic repulsion. A high retention for multivalent ions is frequently combined with a moderate retention for monovalent ions in NF membrane process due to the membrane surface charge properties. Wang and Chung [8] made an attempt to develop hollow fiber NF membranes directly from polybenzimidazole (PBI) without any postpolymerization modifications for chromate ions removal. It was reported that PBI is able to become self-charged in the aqueous environment and hence making it possesses charging characteristic [14]. The authors also employed PBI NF membrane as a forward osmosis membrane and found that both high water permeation flux and excellent salt selectivity were achieved by using this NF membrane [15]. Previously, it was also reported that cellulose acetate (CA) [11] and polysulfone (PSf) [12] could also be used for asymmetric NF membranes fabrication by varying the preparation conditions. The NaCl rejection rate is found to be affected by the structural and electrical properties of membranes prepared at different fabrication conditions. Apart from using one kind of polymer for the charged NF fabrication, the introduction of ionic groups into polymeric solution is also attracting considerable attention for the charged NF membrane fabrication. The blending of charged polymer into dope solution during membrane preparation is considered as a convenient and effective way to control the membrane structure besides combining the positive features of each component. It has been well known that the membrane properties can be controlled by the addition of a small amount of sulfonated poly (ether ether ketone) (SPEEK). As reported in literature, SPEEK has always been a good candidate for direct methanol fuel cell applications mainly due to its highly hydrophilic characteristics coupled with fairly good conductivity [16,17]. Since SPEEK demonstrates the ability to provide sulfonic acid groups (–SO3H) for membranes which is expected to be of benefits in separation of charge solutes, it has become the key component of NF membrane

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Nomenclature Ak/Δx C Cb Cm Cp dp Ds HD HF Js Jv k P Ps rp rs R SD SF T Δx X ϕX

ratio of porosity to thickness of a membrane (m− 1) mean concentration over the thickness of membrane (mol m− 3) concentration of solute in the bulk solution (mol m− 3) concentration of solute in the fluid at the feed/ membrane interface (mol m− 3) concentration of solute in the permeate solution (mol m− 3) the pore diameter (nm) the diffusivity of solute molecule in a dilute solution (m2 s− 1) the steric parameter related to wall correction factors in diffusion coefficient (–) the steric parameter related to wall correction factors in convection coefficient (–) flux of solute (mol m− 2 s− 1) volume flux (m s− 1) Boltzmann's constant (1.38 × 10− 23 J K− 1) solute permeability (m s− 1) solute permeability of salt (m s− 1) pore radius (nm) Stokes radius (nm) rejection (%) the distribution coefficient of solute in diffusion condition (–) the distribution coefficient of solute in convection condition (–) absolute temperature (K) membrane thickness (m) effective volume charge density (mol m− 3) effective fixed charge density (mol m− 3)

Greek letters α the transport number of cation in free solution (–) σ reflection coefficient (–) reflection coefficient of salt (–) σs λ ratio of solute radius to pore radius (–) ξ the ratio of effective volume charge density of membrane to the electrolyte concentration of feed solution (–) μ solvent viscosity (water viscosity at 25 °C, 0.8937 × 10− 3 kg m− 1 s− 1)

development [9,10,18]. Bowen et al. [9] investigated the effect of SPEEK content on PSf membrane formation and found that the charged UF/NF membranes with higher flux and higher salt rejection could be obtained compared to the PSf base membrane. In comparison with two commercial membranes, Bowen et al. [10] experienced that PSf/SPEEK membranes showed high effective removal of humic substances from surface water with low fouling properties and attributed this phenomena to high porosity and high charge of the blends. On the other hand, it was reported in the work of Arthanareeswaran et al. [19] that both water flux and pore radius of blend PSf UF membrane were increased with the increasing SPEEK content in the casting solution. In addition to this, the authors also investigated the performances of the CA UF membrane blended with SPEEK elsewhere [20,21]. However, a study on the salt removal can hardly be found in these works as the authors did not take into consideration the effect of ionic charges introduced by SPEEK in the rejection of solutes. Since there have several works reported the effect of SPEEK on the membrane performance, therefore, we have made first attempt to

997

study systematically the relationships between SPEEK content and membrane properties based on the theoretical model. The NF membranes were prepared with the addition of SPEEK concentration ranging from zero to 4 wt.% and the fine morphological details and electrical properties of the NF membranes prepared were studied by employing the combination of irreversible thermodynamic model, steric-hindrance pore model (SHP) and Teorell–Meyer–Sievers model (TMS). Based on the quantitative analysis of the experimental results and modeling data, it is expected that the study can provide an improved basis for understanding the effect of SPEEK concentration on membrane properties and separation performance. 2. Theoretical background Many methods have been used for pore size determination on membrane porous structure. Among the measurement methods, SEM and FESEM are the simplest and direct methods. The microscopic techniques however is not suitable on the pore size measurement in NF membrane which typically in the near or sub-nanometer range, ≤1 nm [5]. Furthermore, the heavy metal that is coated on the sample during coating process might give some artifacts and hence affect the accuracy of measurement. High beam energy as required in microscopes for high resolution also tends to damage polymeric membranes. Due to this reason, the measurement of the membrane pore size is generally conducted by means of solute transport method. 2.1. Irreversible thermodynamic model In pressure-driven NF processes, the transport phenomena in the process can be described by irreversible thermodynamics model. The solute flux, Js through NF membrane was originally proposed by Kedem and Katchalsky [22] and can be expressed with the following equation: Js = PðCm −Cp Þ + ð1−σÞJv C

ð1Þ

where P is solute permeability, Cp concentration of solute in the permeate solution, Cm concentration of solute in the fluid at the feed/ membrane interface, σ reflection coefficient, Jv the volume flux and C is the mean concentration over the thickness of membrane. Generally, the transport equation for the components through a NF membrane consists of two components: a diffusion component (first term in Eq. (1)) and a convection component (second term in Eq. (1)). The diffusion component is dependent on solute concentration while the convection component is proportional to the applied pressure. From the simple phenomenological transport model, it should be realized that retention is not only dependent on the flux but also on the solute concentration. Spiegler and Kedem [23] expressed the flux of solute, Js in a differential form if the concentration difference between retentate and permeate is very high: Js = −P

′




 dc + ð1−σÞJv C dx

ð2Þ

where P′ is the local solute permeability, defined as P′ = P · Δx. Spiegler–Kedem equation (SK) can be expressed as follows by integrating Eq. (2) across the membrane thickness: R = 1−

Cp σð1−FÞ = Cm ð1−σFÞ

ð3Þ

where
 1−σ Jv F = exp − P

ð4Þ

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In this study, it is assumed that the real rejection is the same as the observed rejection due to the low concentration of solute used. It is because the effect of concentration polarization which decreases the driving force is not obvious at a very low concentration of aqueous solute solution [24,25]. Therefore, the solute concentration at the membrane surface, Cm is considered to be similar as in the bulk solution, Cb. By substituting Eq. (4) into Eq. (3), a well-known SK equation can now be rearranged and described by R=

   σ 1− exp − 1−σ P Jv   1−σ 1−σ exp − P Jv

ð5Þ

Referring to the above equation, the rejection, R increases with an increase of the solvent flux, Jv. Therefore, one can assume that R becomes equal to the reflection coefficient, σ (maximum solute rejection) when the volume flux is infinite as the filtration flow overtakes solution diffusion. The membrane parameters, σ and P therefore can be determined directly from the experimental data of R as a function of 1/Jv using best-fitting method. According to the irreversible thermodynamic model, membrane is treated as a black box because no information about the membrane structural and electrical properties can be obtained from it. Thus, to further evaluate these membrane properties, theoretical models such as steric-hindrance pore model and Teorell–Meyer–Sievers model are required.

where rs and MW are Stokes radius (nm) and molecular weight (g/mol), respectively. 2.3. Teorell–Meyer–Sievers (TMS) model For systems consisting of 1–1 type electrolyte (NaCl) and NF membrane with negative charge polarity, σsalt and Psalt can be expressed as follows based on the TMS model. σsalt = 1−

2 ð2α−1Þξ +

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðξ2 + 4Þ

Psalt = Ds ð1−σs ÞðAk = ΔxÞ

ð13Þ

ð14Þ

where α is the transport number of cation in free solution defined as Dcation / (Dcation + Danion) and ξ is the effective volume charge density of membrane to the electrolyte concentration of feed solution defined as X/C. In order to determine the diffusion coefficient of cation and anion of NaCl as defined in parameter α, the Stokes–Einstein equation as described in the following equation is required.

Di =

kT 6π μri

ð15Þ

2.2. Steric-hindrance pore (SHP) model Nakao and Kimura [26] proposed that SHP model can be used to determine the pore radius rp and the ratio of porosity to thickness of a membrane Ak/Δx if the system only consists of single neutral solute. These parameters can be written as follows: σs = 1−HF SF

ð6Þ

Ps = HD SD Ds ðAk = Δ xÞ

ð7Þ

where HF and HD are the steric parameter related to wall correction factors in convection coefficient and diffusion coefficient, respectively and SF and SD are the distribution coefficient of solute in convection condition and diffusion condition, respectively. Variables that are involving the steric parameters related to the wall correction factors of solute in convection and diffusion conditions (HF and HD) are expressed by the following equations: HF = 1 +

16 2 λ 9

ð8Þ ð9Þ

HD = 1

where λ is ratio of solute radius (rs) to pore radius (rp). Meanwhile, SF and SD which are the steric hindrance factors for convection flow and diffusion flow can be defined by: 2

2

SF = ð1−λ Þ½2−ð1−λÞ 

ð10Þ

2

ð11Þ

where k, T, μ and ri are Boltzmann's constant, temperature, solvent viscosity and ion radius, respectively. Prior to the determination of membrane electrical properties, the reflection coefficient σsalt and solute permeability Psalt of salt must be first obtained in order to use the TMS model. The parameter σsalt and Psalt can be obtained by plotting the NaCl rejection as a function of flow rate and applied pressure which is similar as described in Section 2.2. Based on the TMS model from Eq. (13), the membrane surface charge properties ξ and X can be obtained from σsalt at different NaCl concentrations. 3. Experimental 3.1. Materials Radel A300 polyethersulfone (PES) was supplied by Amoco Chemicals and poly(ether ether ketone) (PEEK) from Victrex US Inc. Ltd were used for the preparation of the membranes. Poly(vinyl) pyrrolidone K15 (PVP K15) and N-methyl-2-pyrrolidone (NMP) were purchased from Fluka and were used as blend polymer and solvent, respectively. Uncharged neutral solutes, poly (ethylene glycol)s (PEGs) from Aldrich and inorganic salt, sodium chloride (NaCl) from Merck, were used to characterize membrane structural and electrical properties. All the chemicals used in the experiments were analytical grade and were used as received without further purification. All the solutions used in the experiments were prepared using distilled water. 3.2. Sulfonation of PEEK

SD = ð1−λÞ

Based on Eqs. (6)–(11), the membrane structural properties, rp and Ak /Δx can be obtained using the values of reflection coefficient σs and solute permeability Ps from the irreversible thermodynamic model for each of the neutral solutes. In this work, the Stokes radius of the neutral solutes used in the membrane characterization was determined using the equation as proposed by Bowen and Wahab [27]. The relationship between Stokes radius and known solute molecular weight can be fitted by: log rs = −1:4854 + 0:461 log MW

ð12Þ

The sulfonation procedures of PEEK were similar as reported previously [28]. PEEK is sulfonated in solution using a sulfonating acid solvent to produce ion-containing polymer containing –SO− 3 groups. In this study, the SPEEK solution was prepared by dissolving 45 g of PEEK in 1 l of concentrated H2SO4. After completing the dissolution of PEEK, the temperature was controlled at 55 °C (±2 °C) and stirring thoroughly for 3 h before the acid solution was poured into large excess of ice water where the SPEEK would be precipitated. The precipitate was then washed with distilled water until the pH was nearly 7. To remove residual water completely, SPEEK was then dried in an oven at 70 °C for at least 48 h before it was ready to be used.

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3.3. Dope solution and hollow fiber membrane preparation The dope solutions were prepared from 20 wt.% of PES in 70 wt.% NMP which containing different concentration of PVP K15 and SPEEK. The concentrations of SPEEK were varied from zero to 4 wt.% in order to produce five different batches of dope formulations. For simplicity, abbreviated PES, PES/SPEEK 1, PES/SPEEK 2, PES/SPEEK 3 and PES/ SPEEK 4 were used for the following sections to identify the membranes prepared from different SPEEK concentrations. To prepare the solution, PES was first dried in an oven at 100 °C for 5 h to remove the moisture contents. Then, the dried SPEEK was added slowly into a mixture of NMP and PVP K15 in a glass bottle and mixed thoroughly under constant mechanical stirring at 60 °C. The dried PES was started to add once SPEEK was completely dissolved in the NMP. To form a homogenous solution, at least 4–5 h was needed to blend and mix the solution. At last, the formulated dope solution was degassed before spinning to remove micro-bubbles that might exist. Table 1 summarizes the experimental parameters of spinning hollow fiber membranes. During the spinning process, all of the spinning parameters were constant. The nascent fiber take-up speed was almost the same as its free-falling velocity in the coagulation bath in order to minimize the effect of spin line (drawing) stress on fiber formation. The detail description for hollow fiber spinning was given elsewhere [29,30]. The as-spun membranes were stored in water bath at room temperature for at least 24 h to remove the residual solvent and then stored in 10 wt.% glycerol aqueous solution for at least 1 day. The membranes were then dried naturally in air at ambient condition before used for making test module. 3.4. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) FTIR studies for the PEEK and SPEEK samples were conducted using Nicolet–Magna 560 IR spectrometer in the wave number between 500 cm− 1 and 4000 cm− 1. In order to determine the degree of sulfonation of SPEEK polymer, NMR was conducted using Bruker Avance 300 NMR spectrometer at a resonance frequency of 300 MHz for 1H. Prior to the test, approximately 3 wt.% SPEEK polymer was dissolved in the deuterated dimethyl sulfoxide (DMSO-d6) for the purpose of analysis. The chemical shift of tetramethylsilance was used as the internal standard reference.

nitrogen atmosphere. On the other hand, differential scanning calorimetry (DSC) measurement of prepared samples was carried out using Mettler Toledo DSC822e. Prior to scanning, the samples with weight of 4–4.5 mg were first hermetically sealed into aluminum pans. The thermal analysis was first performed at a heating rate of 10 °C/min from 30 to 300 °C to remove moisture. After the first heating cycle, the sample was cooled down to 30 °C at a cooling rate of 10 °C/min and subsequently started the second heating cycle from 30 to 300 °C at a heating rate of 10 °C/min. The glass transition temperature, Tg was then determined at the intersection of the tangents to the corresponding DSC curve. 3.6. FESEM analyses The outer surface and cross-section morphologies of the hollow fiber membrane were observed under field emission scanning electron microscope (FESEM, JEOL JSM-6701F). The fiber samples were prepared by cryogenically fractured after immersion in liquid nitrogen for few minutes to reduce damage on morphology. As the fibers were non-conductive, the prepared samples were sputter coated with platinum using auto fine coater (JEOL JFC-1600). 3.7. Membrane performance evaluation All the permeation tests were carried out using laboratory-scale cross-flow filtration unit. The hollow fibers were potted into bundles consisting of 50 fibers of approximately 20 cm long (with approximately 188.5 cm2 of total membrane area). The feed was permeated through the shell-side of the hollow fibers, and permeate was collected from the lumen side. In this study the concentration of neutral solute was kept at 200 ppm while the salt concentration was varied between 250 and 1000 ppm. The pressure was controlled to the desired trans-membrane pressure by adjusting the valve of back-pressure regulator. It is assumed that the concentration of the feed solution is always remained constant as only a small quantity of the sample volume taken for TOC analysis and the concentrate stream is re-circulated to the feed container (10 L). The neutral solute concentration in the feed or in permeate was determined using TOC-VCSH/CSN Analyzer (Shimadzu, Japan). To realize the separation efficiency of NaCl, the following equation was used to determine the percentage of rejection, R (%).


3.5. Thermal stability Rsalt = Investigations on the thermal decomposition behavior and glass transition temperature behavior of the resultant membranes were carried out under nitrogen atmosphere using different instruments. The thermo gravimetric analysis (TGA) was conducted using a Mettler Toledo TGA/SDTA851e instrument. A sample of 5–5.5 mg of membranes was dried in an oven overnight at 80 °C to remove moisture that may exist in samples. The scans were then programmed from room temperature to 800 °C at a heating rate of 10 °C/min under

999

1−

Cp Cf

 ð16Þ

× 100%

where Cp and Cf represent the concentration of permeate and feed solution, respectively. The conductivity of aqueous salts solutions was measured using conductivity meter (EC300, YSI Inc). 4. Results and discussion 4.1. Determination of the degree of sulfonation (DS)

Table 1 Spinning conditions of PES and PES/SPEEK blend membranes. Parameter

Value for spun fiber

Dope flow rate (cm3/min) Bore fluid composition Bore fluid rate (ml/min) Bore fluid temperature (°C) Air gap distance (cm) External coagulant Coagulant temperature (°C) Take-up speed (cm/s) Dimension of spinneret (μm) Humidity (%)

3.5 Water 1.167 27 10 Water 27 23.58 i.d./o.d. 290/600 60

The presence of the SO3H group resulted in a distinct signal of protons HE at 7.5 ppm as shown in Fig. 1. The intensity of HE signal may be used for determination of the HE content which is equivalent to the DS of SPEEK per repeat unit. The DS of SPEEK can be estimated from the ratio between the peak area of the distinct HE signal (AHE) and the integrated peak area of all the other aromatic hydrogens (AHA , A′, B , B′,C , D) as shown on the bottom of Fig. 1 using the following mathematical expression [17,31,32]. AHE DS = 12−2DS ∑AH ′

A;A ;B;B′ ;C;D

ð0 ≤ DS ≤ 1Þ

ð17Þ

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Fig. 1. Nomenclature of the aromatic protons for the SPEEK repeat unit and its 1H NMR spectrum.

At low DS of SPEEK (~0.4), the HC and HD of the unsubstituted hydroquinone ring of PEEK repeat units in the SPEEK polymer appeared as a characteristic singlet at 7.25 ppm and 7.11 ppm respectively [31]. The two HB′ protons were a doublet at 7.00 ppm while the remaining HB protons absorbed at 7.15 ppm. Based on Eq. (17), it was found that the DS of the SPEEK prepared under given experimental conditions was reported to be 0.47. 4.2. FTIR analysis FTIR spectra were performed on PEEK and sulfonated PEEK polymers to confirm the chemical structures of the SPEEK polymer. As shown in Fig. 2, the presence of two absorption peaks of SPEEK at 1025 cm− 1 and 1081 cm− 1 were assigned to the symmetric and asymmetric of O S O stretching vibration of sulfonic acid groups, respectively. These bands were in close agreement with the results reported by Jaafar et al. [28]. The peak identified at approximately 1490 cm− 1 confirmed the presence of C–C aromatic ring in the PEEK and SPEEK polymers, which originally corresponding to 1,2,4-substitution. Furthermore, the peak observed at 1224 cm− 1 is corresponding to the presence of aromatic C–O–C structure in the PEEK and sulfonated PEEK.

Fig. 2. FTIR spectra of (a) SPEEK and (b) PEEK.

Fig. 3 illustrates the FTIR spectra of PES membranes with and without addition of SPEEK between 1000 and 1600 cm− 1. The presence of the peaks at specific wave number of 1105 cm− 1 (aromatic ring), 1149 cm− 1 (ester sulphone), 1239 cm− 1 (ether), 1485 and 1577 cm− 1 (C–C aromatic ring) indicated the specific chemical bonds of PES for all the studied membranes. A difference in the spectra for PES and PES/SPEEK blend membranes would be the appearance of small peak at around 1023 cm− 1. The appearance of small peak is most likely due to the presence of O S O stretching vibration, which was reported to be appeared at around 1025 cm− 1 (see Fig. 2). However, the intensity of the peak for all the blends (Fig. 3b–e) was found to be very similar to each other, probably due to the very small amount of the SPEEK presented in the blends. 4.3. Effect of SPEEK content on membrane structural parameters (rp and Ak/Δx) To evaluate the effect of SPEEK content on the membrane performance, the pure water flux was first investigated. By varying the SPEEK concentration from zero to 4 wt.% in the dope solutions, it had

Fig. 3. FTIR spectra of (a) PES, (b) PES/SPEEK 1, (c) PES/SPEEK 2, (d) PES/SPEEK 3 and (d) PES/SPEEK 4 membrane.

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resulted in an increase in membrane water flux from 3.75 × 10− 7 m/s to 7.84 × 10− 7 m/s, at operating pressure of 6 bar. This indicates the pure water flux of PES/SPEEK 4 membrane was enhanced more than two times than that of the PES membrane owing to the change in the hydrophilic nature of the blend membrane upon the addition of higher concentration of SPEEK. A similar observation was also reported in the work of Bowen et al. [9] that the effect of SPEEK in membrane formation had systematically increased the water permeability of the PSf membranes. To determine the membrane pore radius, studies were conducted to obtain the membrane rejection rates and fluxes at various operating pressure using different MWs of PEG at a feed concentration of 200 ppm. The neutral solutes having different MWs can be separated based on the sieving effect. Fig. 4 shows the relationship between rejections of neutral solutes and volume fluxes of PES and PES/SPEEK 4 membranes. Higher rejection was obtained when higher MW of solute was used. The membrane parameters (P and σ) of other membranes which determined by SK Eq. (5) based on a best fit method are listed in Table 2. Based on the parameters obtained, the pore radius was estimated through the reverse calculating of Eq. (6). The details of change in the structure can be clearly viewed by plotting the membrane pore radius versus the SPEEK content as shown in Fig. 5. Previously, Bowen et al. [9] and Arthanareeswaran et al. [19] reported that the pore radius of membrane increased with the addition of very small amounts of SPEEK on PSf membrane. However, in this present study, it is found that the membrane pore radius increased with increasing SPEEK content up to 2 wt.% and then decreased with further increasing SPEEK concentration to 3 wt.% and 4 wt.%. The slightly decrease in the pore radius can be attributed to the increase in the dope solution viscosity by which hinders the diffusion exchange between solvent (NMP) and non-

1001

Table 2 σ and P of neutral solutes and rp and Ak/Δx of membranes determined from Spiegler Kedem equation and SHP model, respectively. Membrane

Neutral solute

σ

P

λ

Ak/Δx

rp

[–]

[10− 7 m s− 1]

[–]

[m− 1]

[nm]

1058 3209 5321 24,017a 3196(av.) 1378 2748 4891 14,786a 3006(av.) 1299 1778 2822 22,747a 1986(av.) 1765 3231 4890 11,953a 3295(av.) 2651 5302 7675 26,348a 5209(av.)

0.81 0.70 0.73 0.84 0.77(± 0.07)b 0.84 0.87 0.84 0.91 0.86(± 0.03)b 0.97 1.05 1.03 0.91 0.99(± 0.06)b 0.82 0.92 0.88 0.95 0.89(± 0.06)b 0.70 0.75 0.81 0.90 0.79(± 0.08)b

PES

PEG PEG PEG PEG

200 400 600 1000

0.33 0.74 0.90 0.98

1.63 0.85 0.38 0.24

0.46 0.74 0.85 0.94

PES/SPEEK 1

PEG PEG PEG PEG

200 400 600 1000

0.30 0.51 0.75 0.93

2.25 1.75 1.03 0.65

0.45 0.60 0.74 0.87

PES/SPEEK 2

PEG PEG PEG PEG

200 400 600 1000

0.23 0.37 0.53 0.93

2.60 1.77 1.40 1.00

0.39 0.49 0.61 0.87

PES/SPEEK 3

PEG PEG PEG PEG

200 400 600 1000

0.31 0.46 0.70 0.87

2.80 2.40 1.30 0.91

0.46 0.56 0.71 0.83

PES/SPEEK 4

PEG PEG PEG PEG

200 400 600 1000

0.42 0.66 0.79 0.93

3.07 2.00 1.30 1.00

0.54 0.69 0.77 0.88

a b

These data were excluded for the average value of Ak/Δx. These data were the average value of rp together with their standard deviation.

solvent (water) from the coagulation bath and leads to a higher polymer concentration at the interface between polymer solution and nonsolvent. As a consequence, it leads to the formation of membrane with smaller pore radius on top surface. Though there was a decrease in the membrane pore radius when higher content of SPEEK was added, the membranes still showed systematic increase in the water fluxes. Therefore, this phenomenon is mainly due to the fact of hydrophilic nature of SPEEK, which enhances the hydrophilicity properties of the blends to a greater extent. On the other hand, another structural property that may influence the permeate flux is the membrane porosity. The structural parameter, Ak/Δx can also be determined by employing SHP model. Based on SHP model, when rp is known, HD and SD can be calculated as they are a function of the ratio of solute radius to pore radius (rs/rp). Ak/Δx was then reverse calculated using Eq. (7). It is found that the Ak/ Δx increased with increasing neutral solute size. This observation is similar to work carried out elsewhere [22,33,34]. Table 2 summarizes the average values of Ak/Δx obtained from the modeling results. Since

Fig. 4. The neutral solute rejection of membrane as a function of volume flux. (a) PES membrane (b) PES/SPEEK 4 blend membrane (▼ PEG 200, ○ PEG 400, □ PEG 600 and ◊ PEG 1000).

Fig. 5. Fluxes and pore size of membranes at different SPEEK concentrations.

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there was a great deviation of the value Ak/Δx between PEG 1000 with other low MWs of neutral solutes for all the membranes, therefore the data were excluded for the calculation of the average Ak/Δx value. According to Wang et al. [22], the HP model is not suitable to be applied for calculating Ak/Δx when the reflection coefficient, σ is more than 0.90 due to highly restricted permeation. In order to increase confidence in the veracity of the modeling results, it is worth noting that porosity of membranes can be predicted based on the membrane top active layer thickness determined from the FESEM micrographs [11,12]. Fig. 6 shows the pore size and porosity of membranes prepared at different SPEEK concentrations. The calculated porosity seems to be reasonably accurate for NF membranes which typically showing very low porosity value due to the formation of dense skin layer. The results were further confirmed by studying the surface skin of membrane using FESEM. It is clearly seen that no pores can be visually observed on the membrane surface even up to magnification of 50,000×, as shown in Fig. 7. The results seemed to be line with the performance of NF which exhibiting small pore size and low porosity. Fig. 7 also reveals the cross sectional view of the membrane prepared from different SPEEK contents. Notable differences occurred at the intermediate substructure of the membrane. As reported by Kesting [35], generally macrovoids and finger-like structure are formed when the coagulation process is fast, whereas the slow coagulation rate results in a sponge-like substructure. From the figure, it is evident that at the intermediate substructure of membrane sponge-like structure slowly changed to marcovoids and finger-like structures with an increase in the SPEEK content, indicating the increase in hydrophilic characteristic of the blending solution resulted in faster coagulation process. Therefore it can be concluded that the formation of the marcovoids and finger-like structures at the intermediate substructure of PES/SPEEK membrane is one of the factors contributing to the improvement in water flux. 4.4. Effect of SPEEK content on membrane electrical parameters (ξ and X) Fig. 8 shows the dependency of the salt rejection of PES/SPEEK 2 and PES/SPEEK 3 blend membranes on volume flux. It is clearly observed that the higher the NaCl concentration, the lower the salt rejection. It is because the Donnan exclusion effect becomes weaker with the increasing NaCl feed concentration due to a greater anion screening of the surface charge [34]. The salt rejection would reach the maximum value, which corresponding the reflection coefficient when the volume flux is infinite. Due to the limitation of SHP model in describing the effect of electrostatic properties on membrane, TMS model was applied to allow the evaluation of membrane electrostatic properties. Single electrolyte solution with a fixed NaCl concentration was used for evaluating the membrane surface charge as TMS model considers only

Fig. 6. Pore size and porosity of membranes at different SPEEK concentrations.

Fig. 7. FESEM images of the outer surface and cross section of (a) PES, (b) PES/SPEEK 2 and (c) PES/SPEEK 4 membrane.

electrostatic effect during the permeation. NaCl aqueous solutions with concentrations of 250 ppm (4.27 mol/m3), 500 ppm (8.05 mol/m3) and 1000 ppm (17.09 mol/m3) were prepared for the evaluation. With the help of SK Eq. (5), the membrane parameter (Psalt and σsalt) of other studied membranes were also determined and these fitted results together with the membranes electrical properties are listed in Table 3. From the table, ξ of PES, PES/SPEEK 1, PES/SPEEK 2, PES/SPEEK 3 and PES/SPEEK 4 membranes were deduced to be −1.17, −1.68, −2.14, −2.14 and −2.29, respectively. This indicates that SPEEK content has significant effect on blend membrane surface charge. Besides ξ, another electrical parameter X is also shown in Table 3. The value of X is defined as the fixed charge density of the membrane and can be calculated based on the TMS model using Eq. (13). Several researchers proposed to use the effective fixed charge density (ϕ X) instead of X because the fixed charge density varies with the salt concentrations [22,33]. In this case, the values of X obtained from different NaCl concentrations were considered for calculating the average values of X for all the membranes. Results show that as the SPEEK content was increased, X values were also increased. Compared to PES membrane, X value of the blends increased significantly as SPEEK content was varied. This was because of the introduction of ionic groups to PES membrane. Increasing the SPEEK content increases the negative surface charges of membrane which results in an increase in electrostatic repulsion between a negatively charged solute and membrane. Based on the Donnan exclusion mechanism, the Cl− ions (which have the same charge of membrane) in the NaCl feed solution are greatly repulsed by the membrane surface and to satisfy the electroneutrality condition, an equivalent number of counter-ions (Na+) is

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1003

Fig. 9. Pore size and ratio of volume charge density to electrolyte concentration of membranes at different SPEEK concentrations.

Fig. 8. NaCl rejection of blend membrane as a function of volume flux. (a) PES/SPEEK 2 membrane (b) PES/SPEEK 3 blend membrane. (▼ 1000 ppm, ○ 500 ppm and □ 250 ppm).

retained from passing through the membrane and consequently resulting in higher salt rejection [36]. As mentioned earlier, increase in the concentration of SPEEK also resulted in a higher water flux.

Therefore, it is believed that the increase in both surface charges and water flux are due to the combination effects of hydrophilic nature and sulfonic acid groups introduced by SPEEK. It is worth mentioning that only sieving effect is considered for the single neutral solute permeation and only electrostatic effect is considered for the NaCl permeation through a NF membrane, therefore by combining both the steric and Donnan effect, one would understand the effect of SPEEK content on the morphological and electrical properties of the blends. Fig. 9 shows the rp and ξ of the membranes as a function of SPEEK concentration. By addition of SPEEK content up to 2 wt.%, both rp and ξ values have been increased. However, rp decreased while ξ increased with further increasing SPEEK content. Though rp is decreased, it still in the subnanometer ranges. To the best of our knowledge, decreased pore radius would not result in lower salt rejection as Donnan exclusion is the dominant factor in repulsing charge solutes instead of steric effect. However, pore size may be an important driving factor in the rejection of low MW neutral solutes by NF membrane. These data provide valuable information of the structural and electrical properties of NF membranes as the combination of steric and Donnan effects between membrane and external solution plays an important role in the separation of small dissolved organic molecules and electrolytes. 4.5. Effect of SPEEK content on membrane thermal stability

Table 3 σsalt and Psalt of neutral solutes and ξ and X of membranes determined from Spiegler– Kedem equation and TMS model, respectively. ξ

NaCl solution

σsalt

Psalt

(ppm)

(–)

(10− 7 m s− 1)

(–)

(–)

PES

250 500 1000

0.240 0.215 0.206

1.70 0.40 1.39

PES/SPEEK 1

250 500 1000

0.375 0.349 0.247

1.35 1.20 2.35

PES/SPEEK 2

250 500 1000

0.470 0.390 0.355

1.55 1.25 1.29

PES/SPEEK 3

250 500 1000

0.464 0.400 0.355

2.35 2.50 2.95

PES/SPEEK 4

250 500 1000

0.470 0.423 0.345

2.45 2.50 2.55

−5.40 −9.82 −18.93 −11.39(av.) − 8.32 − 15.42 − 22.17 − 15.30(av.) − 10.89 − 17.38 − 31.39 − 19.88(av.) − 10.70 − 17.88 − 31.39 − 19.99(av.) − 10.88 − 21.71 − 30.47 − 21.02(av.)

−1.264 −1.149 −1.108 −1.174(± 0.081)a −1.947 −1.804 −1.297 −1.683(± 0.342)a −2.547 −2.033 −1.836 −2.139(± 0.367)a −2.505 −2.092 −1.836 −2.144(± 0.338)a −2.547 −2.540 −1.783 −2.290(± 0.439)a

Membrane

a

X

These data were the average value of ξ together with their standard deviation.

In this study, the miscibility behavior and thermal properties of the polymer blends were carried out by DSC. It is generally agreed that the thermal stability of the blend membranes may be influenced by the presence of SPEEK in the PES membrane due to the difference in Tg between SPEEK and PES polymer [28,37]. Fig. 10 shows the

Fig. 10. DSC curves of the PES and PES/SPEEK blend membranes.

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representative DSC thermograms of PES and PES/SPEEK blend membranes. The existence of a single well-defined Tg over the entire composition examined, indicating that miscibility existed for these blend systems. Pure PES membrane showed the Tg at around 214 °C, which was slightly lower than that of the value reported [37]. This is mainly due to the use of PVP in the dope solution as an additive. The PVP however would not have much influence on the Tg value since it tends to be easily leached out from membranes during the membranes formation process [38]. As shown, low percentage of SPEEK in the blends led to small shift in the Tg to lower temperatures. The PES/SPEEK 1 and PES/SPEEK 2 membranes displayed the Tg at 211 and 202 °C, respectively. The decrease in Tg indicated lower thermal energy is required to overcome the rotation interactions between the polymer chains owing to increased chain mobility. The Tg however was found to be increased with increase in SPEEK content to 3 and 4 wt.%. The increase of Tg values in PES/SPEEK 3 (~216 °C) and PES/SPEEK 4 membranes (~217 °C) was mainly due to the stiffening of segmental motion of polymer chain [39]. The stronger intermolecular orientation between the polymer chains may be caused by the physical crosslinking arising from specific attractive interactions between PES and high content of SPEEK used due to their chemical structure similarity [40]. Due to this reason, the DSC results thus provided supporting evidence to the modeling results that PES/SPEEK 3 and PES/SPEEK 4 membranes possessed smaller pore radius compared to other blend membranes. On the other hand, the thermal properties of the membranes were further investigated by thermo gravimetric. Fig. 11 shows the weight loss versus temperature curves of the studied membranes. The results show that in the temperature range of 50–100 °C, the blends (particularly PES membrane blended with lower SPEEK content) displayed significant weight loss compared to that of PES membrane. The first weight loss was closely associated with the loss of water from the blends due to the presence of highly hydrophilic SPEEK polymer in the membrane. The second weight loss of blends at around 250–370 °C was attributed to the weight loss of the desulfonation of the sulfonic acid group, –SO3H in the parent PEEK [39,41]. According to Zhang et al. [42], the weight loss of SPEEK at 250–350 °C was owed to the splitting-off of sulfonic acid groups. Furthermore, the drastic weight loss of all the membranes at 450–550 °C was due to the degradation of main chain polymer. As can be seen, the thermal stability of PES membrane decreased slightly with the addition of SPEEK content up to 2 wt.% but was improved with further increasing the SPEEK content. Compared to

other membranes, PES/SPEEK 3 and PES/SPEEK 4 membranes displayed higher mass remaining at 800 °C, indicating specific interaction may be occurred between PES and SPEEK polymer as reported previously [40]. The results were in accord with the results of DSC where higher Tg was obtained by incorporation higher content of SPEEK into the PES dope solution. 5. Conclusions The present study has shown that the fine morphological details and electrical properties of the blend membranes deduced by theoretical models were in good agreement with the experimental results. Based on these results obtained, it offers a better understanding on the effect of SPEEK concentration on the membrane separation performance and draws the conclusions as follows. (a) Increasing SPEEK content into PES dope resulted in an increase in the membrane water flux. With the addition of SPEEK content up to 4 wt%, the water flux was enhanced more than two times than that of the membrane without presence of SPEEK. (b) The blend membrane possessed the largest pore radius when 2 wt.% SPEEK was added. With further increasing SPEEK content, it resulted in decrease in pore size without compromising membrane water flux. It is because the water flux increased with increasing SPEEK content due to the change in hydrophilic nature of blend membrane to a greater extent. (c) The SPEEK content showed a significant effect on membrane surface charge in which both X and ξ increased with increasing SPEEK content. This resulted in higher repulsion force between membrane and electrolyte solution and consequently causing higher salt rejection. (d) DSC results revealed that the two polymers showed good miscibility, which resulted in homogeneity of the blend membranes. For TGA analysis, the blends with high content of SPEEK exhibited higher thermal stability due to occurrence of specific attractive interactions between PES and SPEEK. Acknowledgement The authors would like to express gratitude to Ministry of Science, Technology and Innovation Malaysia for the financial support on this work. Thanks are also due to Dr. Fadhil B. Md. Din from Faculty of Civil

Fig. 11. The TGA curves of the PES and PES/SPEEK blend membranes.

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