Preparation and characterization of TiO2-sulfonated polymer embedded polyetherimide membranes for effective desalination application

Desalination 365 (2015) 355–364

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Desalination journal homepage: www.elsevier.com/locate/desal

Preparation and characterization of TiO2-sulfonated polymer embedded polyetherimide membranes for effective desalination application Y. Lukka Thuyavan a, N. Anantharaman a, G. Arthanareeswaran a,⁎, A.F. Ismail b,⁎, R.V. Mangalaraja c a b c

Membrane Research Laboratory, Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620 015, India Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia Advanced Ceramics and Nanotechnology Laboratory, Department of Materials Engineering, Faculty of Engineering, University of Concepcion, Concepcion 407-0409, Chile

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Sulfonated polymer and TiO2 are used as additive in PEI membrane preparation. • Hydrophilicity and permeability increased with addition of modifiers. • Electrolyte rejection improved with increase of membrane dielectric constant. • PEI/sulfonated polymer–TiO2 MMMs showed lesser flux reduction ratio.

a r t i c l e

i n f o

Article history: Received 3 February 2015 Accepted 3 March 2015 Available online xxxx Keywords: Polyetherimide Sulfonated polymer TiO2 Electrolytes Low transmembrane pressure Ground water

a b s t r a c t Nanocomposite polyetherimide/TiO2-sulfonated polymer membranes were fabricated by phase inversion process, using Dimethylacetamide as solvent. The sulfonated polymers such as sulfonated poly(ether ether ketone) (SPEEK) and sulfonated polyethersulfone (SPES) self-assemble with TiO2 to form resultant hybrid PEI membranes. Such hybrid membranes were characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, contact angle, scanning electron microscopy (SEM) and LCR meter. Among these membranes, TiO2–SPEEK incorporated PEI membrane shows a higher flux of 44.75 L/m2 h and lesser contact angle value of 61.38°. It is observed that such combination improves the membrane surface functionality. In addition, the filtration of three different electrolytes (NaCl, Na2SO4 and MgSO4) with varying concentrations from 50 to 500 mg/L on resulted membranes was analyzed. It is inferred that an increasing dielectric constant value increased the salt rejection in such membranes. Also, a higher retention coefficient of 0.72 was observed during filtration of Na2SO4 using PEI/SPEEK/TiO2 membrane. Effects of ground water on flux profile and flux reduction ratio in the fabricated membrane were also analyzed and discussions are provided. © 2015 Elsevier B.V. All rights reserved.

⁎ Corresponding authors. E-mail addresses: [email protected] (G. Arthanareeswaran), [email protected] (A.F. Ismail).

http://dx.doi.org/10.1016/j.desal.2015.03.004 0011-9164/© 2015 Elsevier B.V. All rights reserved.

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1. Introduction

2.2. Preparation and characterization of TiO2 nanoparticle

Ground water studies show a sharp decline in ground water levels in recent years and are experiencing an increase in salinity [1]. Membrane separation technology has been suggested to convert brackish water to potable standards [2,3]. The choice of membrane for desalination application should possess better stability, antifouling propensity higher salt rejection as well as with better flux rate [4]. Membrane modification is an effective technique to acquire the aforementioned characteristics for enhancing the consistent membrane performance [5]. Sulfonation is a method of incorporation of sulfonic group on polymers through electrophilic substitution reaction. Such sulfonated polymers have the property of miscibility, hydrophilicity, antifouling propensity and ion exchangeable mechanism, which are the major requirements for rejection of salts [6,7]. Hence, in this study sulfonated polymers of SPES and SPEEK were employed as modifier for the desalination purpose. PEI is a resilient material with good thermodynamic compatibility for the formation of membranes in gas separation and membrane distillation [8,9]. However, PEI has limited applications in desalination processes owing to its hydrophobicity and lower flux performance. In order to improve salt rejection, Bowen et al. [10] developed a charged PEI membrane by using sulfonated poly(ether ether ketone) (SPEEK) as a modifier. It was observed that a charged and hydrophilic characteristic of SPEEK has a significant role in enhancing the NaCl rejection and flux rate. Similarly, Lau and Ismail [11] observed that the surface charge density increased with PES/SPEEK nanofiltration membranes and this ultimately resulted in the salt rejection. Another charged polymer such as sulfonated polyethersulfone (SPES) was also widely used as a modifier to alleviate fouling and to improve the hydrophilicity [12,13]. Moreover, SPES membrane also showed excellent property in terms of retaining salts [14,15]. Recently, nanomaterials were incorporated extensively in polymer matrix to achieve the following characteristics; (i) alteration of required structure for the desired product separation [16], (ii) enhancement of hydrophilicity on membrane surface for fouling control [17,18] and (iii) provide better thermal and mechanical strength [19]. As compared to some of the nanomaterial like Al2O3, ZrO2, ZnO and Fe2O3, titania (TiO2) has numerous advantages such as super hydrophilic, higher specific surface area, photocatalytic nature, bactericidal, self-cleaning and resilient material in both thermal as well as chemical environment [20]. TiO2 was utilized effectively for the improvement of fouling mitigation properties in Psf membrane for humic acid filtration [21]. Moreover, Psf/PANI-TiO2 MMMs showed better performance for the filtration of BSA solution. It was also inferred that there is lesser binding affinity between BSA and membranes due to enhancement in hydrophilicity [22]. Thus, TiO2 was chosen as a modifier for the development of hybrid nanocomposite membrane for the current application. A review of literature indicated that studies on the fabrication of PEI/ sulfonated polymers with TiO2 ultrafiltration membrane are limited. In this study, charged polymers such as SPES and SPEEK were used as modifiers along with TiO2 nanoparticles in an attempt to modify the polyetherimide membranes and study their effectiveness in desalination.

Sol–gel method was followed in the synthesis of nano TiO2 particles as prescribed by Ramakrishnan et al. [23]. The precursor material used was titanium tetra-isopropoxide and it was mixed with isopropanol at a ratio of 17:40 to form titania sols. The titania sols obtained were added drop wise to the solution containing of acetic acid, iso-propanol and water with a proportion of 15:60:5. The addition of titania sols forms hydroxide precipitates with water molecule and further reacts with acetic acid to cleave the hydroxide peptides. Then, prepared nanoparticles were dried in hot air oven at a temperature of 110 °C for 24 h and further calcined at 500 °C. The crystalline structure and size of nano-TiO2 were studied using X-ray diffractometer (Model Rigaku Ultima III) equipped with monochromator Cu Kα radiation (λ = 1.541 Å) for 2θ value ranging from 10° to 80° under 40 kV. The average particle size (D) of titania was estimated using Debye Scherrer equation as given in Eq. (1) as

2. Materials and methods 2.1. Chemicals and materials Base polymer polyetherimide was purchased from M/s. Sigma Aldrich. Poly(ethylene glycols) constituting of different molecular weights, used for molecular weight cutoff determination and it was obtained from M/s. Alfa Aesar. The salts of sodium sulfate, sodium chloride and magnesium sulfate were procured from M/s. Merck chemical, India limited. The material used for the synthesis of TiO2 was titanium tetraisopropoxide which was purchased from M/s. Spectrochem Pvt. Ltd and acetic acid, iso-propanol, sulfuric acid and sodium hydroxide were purchased from M/s. Merck Chemical, India.

D¼

Kλ β Cosθ

ð1Þ

where, K represents the dimensionless shape factor and its value is 0.9, λ is the X-ray wavelength and β is the full width at half maximum intensity of peak corresponding to 2 θ. Moreover, the nano-titania morphology was also observed using Transmission electron microscopy (TEM, JEOL JEM 2000 EX). 2.3. Synthesis of sulfonated polymers Sulfuric acid was used as sulfonating reagent for the synthesis of sulfonated polyethersulfone (SPES) and sulfonated polyetherketone (SPEEK). SPES was synthesized as follows: Polyethersulfone (PES) was mixed with sulfuric acid in a double neck round bottomed flask at a ratio of 1:6 (w/w). It was then allowed to react for 6 h at a temperature of 50 °C. Subsequently, homogeneous sulfonated polymer solution was dropped slowly for precipitation into ice cold deionized water bath equipped with mechanical agitator. Then, the precipitated SPES was filtered and washed with deionized water until the pH settles at 7. Finally, it was dried for 24 h in a hot air oven maintained at a temperature of 80 °C. The protocol for the synthesis of sulfonated polyetherketone and its characterization is well discussed by both Pagidi et al. [24] and Jaafar et al. [25]. SPES was characterized using hydrogen-nuclear magnetic resonance (1HNMR) spectroscopy and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. 2.4. Membrane fabrication The pristine and modified PEI membranes were fabricated using phase inversion process by immersion precipitation method. All polymers and TiO2 were dried in oven at 60 °C for 8 h before preparing the dope solution. The various combinations of dope solution with PEI, SPES, SPEEK and TiO2 in the presence of Dimethylacetamide (DMAc) are listed in Table 1. Based on previous analysis [26], 10 wt.% SPEEK was selected as maximum composition in PEI. 10 wt.% of SPES and SPEEK was initially dissolved in DMAc solvent individually to attain clear homogeneous solution using a mechanical stirrer rotating at 300 rpm and maintained at 50 °C. Following this, a 90 wt.% of PEI was added to the SPES and SPEEK mixture separately and stirred for 18 h at a temperature of 50 °C. Subsequently, homogeneous dope solution was undisturbed in order to remove air bubbles. Finally, dope solution was poured on to the glass plate and casting was executed using a self-made doctor blade at a thickness of 250 μm. Then, the glass plate was set aside for the partial evaporation of solvent up to 30 s. This was followed by the immersion of glass plate in the non-solvent (water) coagulant bath at a temperature of 10 °C for 24 h. The resultant film was then washed with double distilled water and stored in 0.1% sodium

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357

Table 1 Composition of casting solution for fabricated membranes and its hydrophilicity along with permeability. Membrane code

Polymer and additive composition (17.5% polymer)/(82.5%DMAc)

Z1 Z2 Z3 Z4 Z5

PEI

SPES

SPEEK

TiO2

100 90 90 90 90

10 9.5 – –

– – – 10 9.5

– – 0.5 – 0.5

azide solution to prevent microbial attack. In the case of mixed matrix membranes (MMMs) fabrication, TiO2 was initially dissolved in DMAc under stirring for 2 h and then using ultrasonication homogeneity was obtained. Following this, sulfonated polymer was mixed with TiO2 containing dope solution and subsequently PEI was added. Thus, from the aforementioned procedure the membranes were cast and stored. The current study focuses to enhance the membrane surface charge properties for the better electrolyte rejection.

Contact angle (θ)

Surface free energy (mJ m−2)

rm (nm)

Permeability (×10−9 m/s·kPa)

77.40 ± (1.2) 69.43 ± (0.8) 67.5 ± (2.3) 66.38 ± (2.5) 61.38 ± (1.06)

37.06 42.05 43.22 43.91 46.70

0.69 1.45 2.12 2.32 2.50

0.315 4.04 9.79 10.91 25.22

out a a transmembrane pressure of 500 kPa and feed concentration of 0.1 mg/L.The average pore radius (rm) of the membranes is calculated using Eq. (4) given below [28]. The concentration of neutral solutes was estimated using spectrophotometer (Spectroquant® Pharo 300, Merck India Limited) by Dragendorff reagent method [29]. −10

rm ¼ 16:73  10

0:557

ðMWCOÞ

ð4Þ

2.5. Membrane characterization 2.6. Salt rejection The surface functional group analysis of membrane was performed using attenuated total-reflection Fourier transform infrared (ATRFTIR) spectroscopy (Thermo Scientific Nicolet iS5 FT-IR spectrometer). The spectra were collected in the range of wavelengths from 4000 to 550 cm−1. The membrane morphologies of both cross section and top surface were examined using scanning electron microscopy (SEM, Philips XL30). Prior to the sample analysis, membrane preparation involved drying, flashing with liquid N2 and finally gold coating. The dielectric spectrum of each fabricated membrane was measured by LCR meter (HIOKI 3532-50, Japan). The membrane was coated with the silver before the measurement of capacitance. Contact angle for the membrane was evaluated by sessile drop method using goniometer (DSA20B, Kruss, Germany). Wettability (hydrophilicity) of the membrane surface was analyzed by injecting 5 μl of water in drops on five different regions. The obtained values were averaged and taken into account for the determination of surface free energy. Surface free energy was calculated using Young Eq. (2) [27].

γlv cosθ ¼ γsv −γsl

ð2Þ

where γlv, γsv, and γsl are the interfacial tension of liquid–vapor, solid– vapor and solid–liquid interfacial energies respectively. θ represents the contact angle corresponding to membrane surface. The interfacial tension of liquid–vapor is 72.8 mJ/m2. The interfacial surface free energy of membrane cannot be calculated from the above Eq. (2). Hence, surface free energy of membrane was calculated by the procedure of Kwok and Neumann method using the below Eq. (3) [27].

cosθ ¼ −1 þ 2

rffiffiffiffiffiffiffi γsv −βðγlv −γsv Þ 2 e γlv

ð3Þ

where the constant value of β is 0.0001024 (mJ/m2)2. Now since θ and γlv are known, γsv can be calculated by using Eq. (3). This will enable one to estimate the membrane–solute interaction. The MWCO of membranes is calculated by solute rejection method using different molecular weight neutral solutes (Polyethylene glycol 1000 kDa, Polyethylene glycol 6000 kDa, Polyethylene glycol 10,000 kDa, Polyethylene glycol 20,000 kDa and Polyethylene glycol 35,000 kDa) in a dead end stirred cell module (Ultrafiltration cellS76-400-Model, Spectrum, USA). All the experiments were carried

The salt ion rejection and flux performance were compared for the nascent PEI and modified PEI membranes in dead end stirred cell module. The salts such as Na2SO4, NaCl and MgSO4 were used to evaluate the membrane efficiency and the performance was compared using the irreversible thermodynamic model. The transmembrane pressure was varied from 100 to 500 kPa for various salt concentrations in the range of 50 to 500 mg/L. The salt rejection was measured using the multiparameter analysis instrument CyberScan PCD650. Besides, the phenomenon of solute ions transport through the pressure driven membrane separation process can be explained by irreversible thermodynamic model. Kedem–Katchalsky [30] derived the following phenomenological transport flux relating equation of both volumetric flux (Jv) and solute (Js) flux which is represented as Eqs. (5) and (6) respectively. JV ¼ LP ðΔ P−σΔΠÞ

ð5Þ

The solute flux is expressed as a sum of both diffusive and convective transports through the membrane. The diffusive transport is based on the concentration gradient across the membrane on both permeate and feed side. Convective transport depends on applied transmembrane pressure. JS ¼ PS ðCm −CP Þ þ ð1−σ ÞC Jv

ð6Þ

where Lp is the pure water permeability (m/s·kPa), ΔP is the transmembrane pressure, σ is the retention coefficient, Δπ is the osmotic pressure of the solute, Ps is the solute permeability (m/s), Cm is the solute concentration on the membrane interface, Cp is the solute concentration in permeate side and C is the average molar solute concentration across the membrane. This above phenomenological transport model clearly indicates that the flux is a function of both retention and concentration of solute. In order to obtain the relationship between the rejection (R) and permeate flux as well as solute concentrations (permeate and retentate), Speigler and Kedem [31] derived from the following equations using Eqs. (5) and (6).

R ¼ 1−

Cp σ ð1−FÞ ¼ Cm m ð1−σ FÞ

ð7Þ

358

F¼

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expð−ð1−FÞ JV PS

ð8Þ

a.

240

SPES

230 220

3. Results and discussion

210

1017

200 190

Transmittance(%)

where, Ps and σ can be calculated using best-fit method from the experimental data on rejection (R) and the corresponding volumetric flux (Js). It determines the membrane sieving characteristics. In this study, the abovementioned parameters are determined and compared for the PEI and modified PEI membranes.

3.1. Morphology, crystallinity and functional group characterization of nano-titania

1233

1099

180

S=O

170 160

Z3

150 140 130

Z2

120 110 100 90

The XRD pattern of as-prepared nanoparticles has shown anatase phase with highly crystalline feature (Fig. 1a). All the peaks can be indexed with pure anatase TiO2 (tetragonal) accordingly in JCPDS no: 21–1272 and space group I41/amd (141) with lattice parameters and found to be a = 3.7899, c = 9.51645 (by using least squares regression). In addition, the crystalline size of as-prepared TiO2 nanoparticles was found to be 18 nm using Debye Scherrer equation (Eq. (1)) from the most intense plane (101). Further, TEM image of the TiO2 nanoparticles

Z1

80 70 60

500

1000

1500

2000

2500

3000

Wavenumber ( cm ) 220

180

a. Transmittance (%)

101

7000 6000

4000

b. SPEEK

200

8000

3500

-1

1023 1082 S=O

3434 O-H

1235

160

Z5

140

Z4

120

5000

Z1

204

1000

60 500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber ( cm )

116 220

2000

105 211

200

80

3000

004

Itensity

100

4000

Fig. 2. a. FTIR spectrum of the SPES and PEI/SPES membranes b. FTIR spectrum of the SPEEK and PEI/SPEEK membranes.

0 0

10

20

30

40

50

60

70

80

90

Two theta (degree)

b.

is shown in Fig. 1b. Here, the TiO2 particles are well distinguished, spherical in shape and 15–25 nm of size between for each. It has been reported that anatase form of TiO2 was favored widely as modifier in PES membrane for use in desalination applications [32]. Moreover from the FTIR results, it is clear that the desired characteristic peak of hydroxyl group band was observed at 3384 cm−1 and the same is presented in the supporting information as Supplementary Fig. 1. It is well known that the occurrence of hydroxyl group can enhance the hydrophilicity of membrane surface and thereby it is expected to reduce the fouling under separation process [32]. 3.2. Membrane surface chemistry

Fig. 1. a. Characterization of nano-titania (a) X-ray diffraction (XRD) pattern of nano-titania. b. Transmission electron micrograph of nano-titania.

FTIR spectral bands for the prepared membranes and additives of SPPEEK and SPES are presented in Fig. 2a and b. From Fig. 2a and b, characteristic spectral band of PEI groups is clearly visible in the membranes (Z1 to Z5) at the following wavelengths (i) 1234 cm−1 of the aromatic ether C–O–C, (ii) 1780 and 1720 cm−1 of the imide carbonyl asymmetrical and symmetrical stretch group and (iii) 1355 and 743 cm−1 corresponds to C–N stretching and bending [33]. The specific sulfone groups of strong asymmetric and symmetric stretch spectral bands of 1020, 1080 and 1255 cm−1 were clearly identified in the membranes with additives SPEEK and SPES [34]. This confirms the occurrence of sulfonation

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359

a.

b.

Hb

He

Ha

Hc, Hd

ppm Fig. 3. a. Schematic representation of the interaction mechanism between TiO2 and sulfonated polymers. b. 1H NMR spectra of SPES poymer.

reaction on both PES and PEEK modifiers. It is interesting to note that a new peak corresponding to hydroxyl radicals was identified at the wavelength of 3434 cm−1 on both the sulfonated polymers modified PEI membranes. Among membranes, SPEEK incorporated PEI membranes (Z4 and Z5) showed better intensity of O–H moiety on SO3 group (Fig. 2b). The other observation showed that TiO2 embedded within both PEI/SPEEK and PEI/SPES membranes and have the higher intensity as compared to PEI/SPEEK and PEI/SPES membrane (Fig. 2a and b). It indicates that hydrophilicity improved with the TiO2 added MMMs. It is generally known that metal ion (Ti4+) has greater affinity towards the more electronegative oxygen atom of sulfone group [35]. This drives the self-assembly of the TiO2 on sulfonated polymeric matrices. The schematic depiction and mechanism of selfassembling of TiO2 in sulfonated polymeric (SPES and SPEEK) moiety has been clearly described in Fig. 3a. These interactions may aid in compatibility in the formation of hybrid PEI membrane resulting in the improvement of flux and reduction of fouling properties. Fig. 3b clearly illustrates the conformation data of SPES using 1H NMR spectra which is a versatile tool to identify the chemical structure of molecules. The changes in the chemical shift values and the observance of new peaks reveal the introduction of SO3H group in the phenyl ring of PES. Klaysom et al. [36] also observed similar pattern while using 1H NMR for the SPES.

3.3. Membrane surface morphology The cross section and top surface morphology of both nascent PEI and modified PEI membranes are presented in Figs. 4 and 5. Fig. 4Z1 displays the lesser pores and smaller pore size for nascent PEI membrane. It is clearly seen in Fig. 4Z4 and Z5 that large pore size formation was noticed for SPEEK modified PEI membranes on the surface. It is mainly because of hydrophilic SPEEK which has an affinity to move towards the water molecules in coagulation bath during phase inversion process. Thus it serves to promote the phase separation reaction and results in the formation of larger pores on the surface. As can be seen in Fig. 5, the cross section morphology of all the fabricated membranes resembles a classic asymmetric structure with thick skin followed by bottom layer. The pristine PEI membrane has the hydrophobic character which results in the formation of thick skin layer. Further, the thickness of skin layer is reduced in the case of sulfonated polymers modified PEI membranes. Such alteration in membrane morphology is mainly based on the thermodynamic properties of solvent–non-solvent exchange rate in coagulation bath under phase inversion process. These modifiers enhance the exchange rate and result in fast phase separation [37,38]. The distinct cross sectional morphology was seen in the SPES modified membrane (Fig. 5Z2 and Z3). As discussed in Section 3.1, TiO2 has a

360

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Z1

Z2

Z4

Z5

Z3

Fig. 4. Top surface morphology of (Z1) Neat PEI membrane, (Z2) PEI/SPES membrane, (Z3) PEI/SPES/TiO2 membrane, (Z4) PEI/SPEEK membrane and (Z5) PEI/SPEEK/TiO2 membrane.

stronger affinity with sulfonated polymers and such interaction leads to connecting the top thin skin layer and bottom sub layer (Fig. 5Z3 and Z5). 3.4. Hydrophilicity and water permeability The effect of additives on permeability, water flux analysis of the pristine PEI and modified PEI membranes are shown in Fig. 6 and Table 1. Fig. 6 clearly indicates that the flux in fabricated membranes

Z1

Z2

Z4

Z5

varies linearly with respect to transmembrane pressure. From Table 1, it is clear that pristine PEI membrane (Z1) has a higher contact angle of value 77° and lower water permeability of 0.315 × 10−9 m/s·kPa. Permeability data clearly shows that both the sulfonated polymers have a significant role in enhancement of flux (Table 1). This may be attributed to the increase of wettability on the membrane surface and formation of bigger pore size of the hybrid membrane [38]. It is interesting to note that the permeability enhanced significantly up to

Z3

Fig. 5. Cross sectional morphology of (Z1) Neat PEI membrane, (Z2) PEI/SPES membrane, (Z3) PEI/SPES/TiO2 membrane, (Z4) PEI/SPEEK membrane and (Z5) PEI/SPEEK/TiO2 membrane.

Y.L. Thuyavan et al. / Desalination 365 (2015) 355–364

50

100 Z1

Z5 40

Z2

30

Z4 20

Rejection (%)

2 Flux ( L/m h)

361

90

Z3 Z4 Z5

80

Z3

10

Z2

70 PEG1000

Z1

0 0

100

200

300

400

PEG6000

PEG 10000 PEG 20000 PEG 35000

Molecular weight (kDa)

500

Pressure (kPa) Fig. 6. Water flux analysis of pristine PEI and modified PEI membranes.

25.22 × 10−9 m/s·kPa for PEI/SPEEK/TiO2 membrane (Z5). Moreover, with the addition of TiO2, water permeability is enhanced compared to the PEI/sulfonated polymer blend analogues. It shows that the aforesaid two factors have enhanced the water penetration rate on the polymer matrix (MMMs). It is also clear that TiO2 has good affinity with sulfone (SO− 3 ) groups and thus mutual functionalities (hydrophilicity) aid in improving the water permeability. 3.5. Molecular weight cutoff studies Fig. 7 indicates multiple neutral solute rejection results of both nascent PEI and modified membranes. The results show that using lower molecular weight solutes (PEG 1000 kDa) resulted in higher rejection. While comparing both the sulfonated polymers, higher MWCO of 8 kDa was observed for PEI/SPEEK/TiO2 membrane. In the case of SPES modified PEI membranes, the MWCO of Z2 and Z3 membrane is found to be 3 and 6 kDa respectively. The pore radius is lower for the pristine PEI membrane and its value is 0.69 nm and their MWCO is lesser than 1 kDa (Fig. 7). In the case of modified membranes, average pore radius increased and thus the solute flows easily across the membrane. This is due to the sieving mechanism of neutral solutes and it relies on both solute radii of PEG (molecular weight) and pore size of the membrane [39]. The membrane of higher MWCO has lower rejection due to the formation of larger pores by the addition of sulfonated polymers, which is evident from surface morphology and water flux of all modified membranes. The pore radius increased while with the incorporation of TiO2 in PEI/SPES and PEI/SPEEK membranes. A similar trend of increase in pore radius was observed for the TiO2 embedded PVDF/ SPES MMMs [35]. On the whole, this study indicates that the fabricated membranes are lower MWCO ultrafiltration membranes. 3.6. Salt rejection analysis The flux and rejection of the electrolytes (Na2SO4, MgSO4 and NaCl) corresponding to transmembrane pressure of fabricated membranes are presented in Table 2 and in the supporting information as Supplementary Figs. 2 to 7. Supplementary Figs. 2 to 4 show the effect of transmembrane pressure on permeate flux for all the fabricated membranes. It is observed that flux increased with an increase in transmembrane pressure from 100 to 500 kPa. At higher electrolyte concentrations, ion diffusion displays dominance and hence it results in the reduction of flow across the membranes [40]. A higher solute permeability of 66.48 × 10− 7 m/s was observed for the PEI/SPEEK/TiO2 membrane

Fig. 7. MWCO determination of the membranes using neutral solute rejection.

(Table 2). This could be due to the synergetic effect of SPEEK and TiO2 which enhances the hydrophilicity on the membrane surface leading to an increase in the solute permeability (Ps). On the other hand, membrane assessment was compared as a function of electrolyte (Na2SO4, NaCl and MgSO4) rejection and is shown in Supplementary Figs. 5 to 7. The permeation flux and rejection data obtained from the membranes have a good correlation with Spiegler– Kedem model (Table 2 and Supplementary Figs. 2 to 4). The effect of transmembrane pressure on rejection is very minimal and it is mainly due to the lower feed concentration. It is interesting to note that electrolyte rejection coefficient improved significantly with both the sulfonated polymer modified membranes. It is solely because of an increase in surface charge (SO− 3 ) on the membrane surface. It is evident in the dielectric spectrum of the fabricated membrane and is illustrated in Fig. 8. Among the sulfonated polymers, dielectric constant was higher for SPEEK modified PEI membranes and further improved slightly with the both SPEEK–TiO2 and SPES–TiO2 incorporated PEI membrane. Incorporation of TiO2–SPEEK nanocomposite in PEI matrices enhances the functionality of the membrane for the separation of electrolytes. However, the PEI membrane made using SPES membrane also has higher dielectric constant value than nascent PEI membrane. Among the electrolytes, a higher rejection is observed for the Na2SO4 followed by NaCl and then MgSO4. The higher Na2SO4 retention coefficient of 0.72 is observed for PEI/SPEEK/TiO2 membrane at the concentration of 50 mg/L. This mechanism is attributed to the improvement of negatively charged (SO-3) groups on the membrane surface for the PEI membrane modified using sulfonated polymers. Sulphate (SO24 ) ions have higher negative surface charge and hydration radii than Cl− ions. Therefore, modified membranes reject the divalent SO24 − ions. Also since the ions thereby monovalent co-ions (Na+) are retained along with SO2− 4 to maintaining the electroneutrality condition. However, with the rejection of MgSO4 rejection decreased in all the membranes due to the coions Mg 2+ as it binds with membrane and reduces the rejection. Similar trend was observed with salt rejection for the other membranes [41,42]. It is well known that membrane with higher dielectric constant shows an improved rejection of electrolytes [43]. Sulfonated polymer combined TiO2 nanocomposite in PEI matrices that leads to operate effectively at desired low transmembrane pressure. The membrane performance was also evaluated for the filtration of ground water and the flux profile obtained with respect to time is shown in Fig. 9a. The ground water was collected from the Pudukkottai, Tamil Nadu, India and its characteristics are presented in Table 3. The flux value was higher for the PEI/SPEEK/TiO2 membrane. However the flux (Fig. 9a) value reduced while comparing with the dilute feed concentration (Supplementary Figs. 2 to 4). It is due to the presence of

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Table 2 Electrolyte rejection coefficient (σ) and solute permeability for the membrane. Concentration (mg/L)

PEI

PEI/SPS

PEI/SPS/TiO2

PEI/SPEEK

PEI/SPEEK/TiO2

50 100 250 500 50 100 250 500 50 100 250 500 50 100 250 500 50 100 250 500

Na2SO4 Solution

NaCl Solution

Ps (×10−7 m/s)

σ

Ps (×10−7 m/s)

σ

Ps (×10−7 m/s)

0.30 0.24 0.19 0.12 0.54 0.49 0.36 0.25 0.65 0.62 0.43 0.39 0.59 0.56 0.43 0.38 0.72 0.68 0.54 0.48

1.33 1.17 0.98 0.77 9.95 7.95 6.30 3.53 33.05 29.61 25.72 21.33 46.5 41.46 37.21 30.60 66.48 60.71 57.91 54.38

0.26 0.19 0.15 0.11 0.40 0.33 0.28 0.22 0.50 0.41 0.37 0.32 0.42 0.37 0.32 0.23 0.69 0.54 0.42 0.32

0.81 0.77 0.65 0.57 8.36 7.79 7.18 6.40 33.21 31.19 29.09 26.10 41.61 37.94 32.50 28.39 59.61 56.21 51.14 46.50

0.20 0.18 0.16 0.09 0.36 0.33 0.26 0.13 0.43 0.38 0.32 0.21 0.39 0.35 0.31 0.20 0.65 0.52 0.38 0.25

0.715 0.662 0.555 0.465 7.13 6.64 5.22 4.56 30.98 26.27 21.66 16.66 31.74 31.39 26.83 21.29 59.44 51.97 44.32 39.89

various mixtures of electrolytes. These solutes can interact with the membrane and reduce the performance. Nevertheless, the membrane modified by both SPEEK/TiO2 showed higher flux. Moreover, filtration performance of membranes was also assessed by flux reduction percentage (FRP) using Eq. (6) given below [45].

a.

35 30 25

  ðLiw −Law Þ  100 FRPð%Þ ¼ Liw

2

ð9Þ

where Liw is the initial water permeability before passing the feed solution and Law is the final water permeability after passing the feed solution. Fig. 9b shows the flux reduction ratio value for the fabricated membranes. The higher FRP of 47% was noticed for the pristine PEI membrane. It is mainly due to the decrease in the surface free energy. The surface free energy of pristine PEI membrane is 37.06 mJ m−2 and further it increased up to 46.70 mJ m−2 for the TiO2–SPEEK modified PEI membrane. This indicates that solute particle has lesser binding affinity to the membrane surface for the higher free surface energy [46] and dielectric constant membranes. This in turn reduced the FRP ratio on the membranes. Whereas in the case of SPEEK/TiO2 and SPES/TiO2 modified

20 15 10

Z5 Z4

5

Z3 Z2 Z1

0 0

20

40

60

80

100

120

140

160

180

200

Time (min)

b.

100

80

6 5

Z5 Z4 Z3 Z2 Z1

4 3 2

Flux reduction ratio (%)

7

Dielectric constant

MgSO4 solution

σ

Flux (L/m h)

Membrane

60

40

20

1

0 Z1

0 0

200000

400000

600000

800000

1000000

Z2

Z3

Z4

Z5

Membranes

Frequency (Hz) Fig. 8. Dielectric spectrum of the PEI and modified PEI membrane.

Fig. 9. a. Flux pattern for filtration of ground water using PEI and modified PEI membrane. b. Comparison of flux reduction ratio for PEI and modified PEI membrane.

Y.L. Thuyavan et al. / Desalination 365 (2015) 355–364 Table 3 Characteristics of ground water. Parameter

Accepted limit (WHO) [44]

Upper and lower limit values of collected samples

pH Sulphate (mg/L) Magnesium (mg/L) Chloride (mg/L) TDS (mg/L)

6.5–8.5 400 150 250 1000

6.0–7.5 34–78 12–43 74–415 585–1476

PEI MMMs showed lesser FRP ratio. On the whole the membrane performance was evaluated as a function of flux and salt rejection which is in the order, PEI/SPEEK/TiO2 (Z5) N PEI/SPEEK (Z4) N PEI/SPES/TiO2 (Z3) N PEI/SPES (Z2) N PEI. These studies give insight to other researchers for the use of combined charged polymers with nanomaterials in membrane fabrication for their improved performance in desalination application. 4. Conclusion Sulfonated polymers such as SPES and SPEEK were used as modifier to form resultant PEI membrane using phase inversion method. Moreover TiO2 was also self-assembled with sulfonated polymers and blended with PEI membranes. The characteristic results of fabricated membrane and their effects on electrolyte rejection studies are summarized as follows. 1) Nano TiO2 was synthesized successfully and its crystalline size was calculated as 18 nm using XRD and then confirmed by TEM. Moreover TiO2 has a tendency to form self-assembly with sulfonated polymers. ATR-FTIR result showed that the hydrophilicity enhanced with sulfonated polymer modified membranes. 2) Among the sulfonated polymers, SPEEK–TiO2 modified PEI membrane showed higher hydrophilicity and water permeability. The membrane molecular weight cutoff of the membrane was also higher and is 8 kDa. 3) With regard to electrolyte rejection, the rejection of salts in decreasing order was Na2SO4 N NaCl N MgSO4. The rejection study on membranes indicated that the incorporation of SO− 3 groups enhanced the dielectric constant and hence resulted in improved rejection. 4) As far as ground water filtration was concerned, the flux improved for modified PEI membranes. This indicates that the fouling reduced due to increase in the free surface energy. Overall, the SPEEK–TiO2 modified PEI membrane showed better flux and lesser fouling characteristics. Acknowledgment Support received from the Director of National Institute of Technology, Tiruchirappalli India and the Technical Education Quality Improvement Programme (TEQIP) Phase II for this work is gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.desal.2015.03.004. References [1] M.A. Montgomery, M. Elimelech, Water and sanitation in developing countries: including health in the equation, Environ. Sci. Technol. 41 (2007) 17–24. [2] A.A. Abuhabib, A.W. Mohammad, N. Hilal, R.A. Rahman, A.H. Shafie, Nanofiltration membrane modification by UV grafting for salt rejection and fouling resistance improvement for brackish water desalination, Desalination 295 (2012) 16–25. [3] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301–310.

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