Preparation of thin-film composite nanofiltration membranes with improved antifouling property and flux using 2,2′-oxybis-ethylamine

Desalination 355 (2015) 141–146

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Preparation of thin-film composite nanofiltration membranes with improved antifouling property and flux using 2,2′-oxybis-ethylamine Jin-bo Jin, Dong-qing Liu ⁎, Dan-dan Zhang, Yu-hang Yin, Xin-yu Zhao, Yu-feng Zhang State Key Laboratory of Hollow Fiber Membrane Materials and Processes, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China

H I G H L I G H T S • • • •

2,2′-Oxybis-ethylamine was used to improve surface hydrophilicity of TFC membrane. Enhanced hydrophilicity of composite membranes was confirmed. Small amount of 2,2′-oxybis-ethylamine can keep high Na2SO4 rejection and water flux. Flux recovery ratio exceeds 98% for composite membrane.

a r t i c l e

i n f o

Article history: Received 9 June 2014 Received in revised form 22 October 2014 Accepted 24 October 2014 Available online xxxx Keywords: 2,2′-Oxybis-ethylamine Hydrophilicity Nanofiltration membrane Interfacial polymerization Antifouling

a b s t r a c t Thin-film composite (TFC) nanofiltration membranes were fabricated via interfacial polymerization using 2,2′oxybis-ethylamine (2,2′-OEL) as an aqueous monomer. The chemical composition and morphology of the membrane surface were confirmed by X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and scanning electronic microscopy (SEM). The results showed that interfacial polymerization successfully occurred and pore size of the resultant membranes was in the range of nanofiltration. Hydrophilicity of the membranes was investigated through water contact angle measurement. Water flux and salt rejection measurement showed that the prepared TFC membranes had a high flux at 35.6 L/m2h despite the salt rejection slightly decreased. The prepared TFC membranes showed good antifouling performance (best flux recovery ratio was 98.5%) and fouling resistant capacity increased with the concentration of 2,2′-OEL. Compared with the PIP-TMC membranes, low concentration of 2,2′-OEL could maintain as high Na2SO4 rejection of membrane as PIP-TMC membranes and show better fouling resistance performance. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Membrane separation technology has been widely used in wastewater treatment, water purification, seawater desalination, food, medicine and other fields due to advantages of simplify, high efficiency, low cost and eco-friendly. Especially, process based on nanofiltration (NF) technology has been extensively applied in wastewater treatment and seawater desalination [1]. The molecular weight cutoff (MWCO) of NF membrane generally ranges from 200 to 1000 Da, and the pore sizes are 1–3 nm [2]. The main driving force of NF membrane is pressure difference between the feed and the permeate sides. Thin-film composite (TFC) nanofiltration membrane, coated with a thin barrier layer on the surface of the porous support, has proven to be an effective separation technology for NF. The skin layer of TFC membrane is looser than that of reverse osmosis membrane, but denser than ultrafiltration membrane due to various preparing techniques. Currently, there are several ⁎ Corresponding author. E-mail address: [email protected] (D. Liu).

http://dx.doi.org/10.1016/j.desal.2014.10.036 0011-9164/© 2014 Elsevier B.V. All rights reserved.

methods to prepare TFC membranes like phase transformation, surface grafting, and interfacial polymerization [3–6]. Among these methods, interfacial polymerization is a common and effective approach to prepare TFC membrane, which is the maximum yield among commercial products currently. Nowadays, with the development of industrial activities and environmental pollution, seawater desalination has become the main trend to solve the scarcity of fresh water. However, NF technology is greatly limited by membrane contamination from solute, colloidal or organic particles by physical or chemical interactions with membranes [7]. Generally, pore size, the charge density, and the surface hydrophilicity of NF membranes are critical factors for rejection behavior of NF membrane and application fields [8–11]. However, the pores are small enough and prone to be polluted by microorganism or protein. Contaminated pores were blocked up and diminished by deposition and adsorption of pollutants [12]. Consequently, adsorption of particles will damage the membrane properties and significantly affect the membrane permeability [13,14]. It is difficult to fully restore to the original level once the membrane was fouled. So the researchers did lots of research in the direction of

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resistance to pollution. Coating hydrophilic functional layer on the membrane surface is an effective method to reduce membrane fouling [15]. But post-surface treatment could increase the penetration resistance leading to flux decrease. Covalent bonding of hydrophilic groups is another attempt to improve fouling resistant performance [16–19]. Addition of monofunctional structure (o-Aminobenzoic acid-triethylamine salt) into the ingredient of interfacial polymerization, then coating of cross-linked PEG can improve the flux and antifouling performance of membranes according to Ho's work [20]. Ethyloxy groups have been widely applied to improve hydrophilicity of materials, such as nonion surfactant, owing to their amphiphilic property. Gol and Jewrajka [21] reported a novel approach for in-situ PEGylation on TFC membranes, which could effectively enhance antifouling capability of membranes. Meanwhile, salt rejection and water flux were almost unaffected. Hydrophilic surface modifying macromolecules, such as combination of active diisocyanate and PEG, were reported to enhance antifouling property of membranes when added into trimesoyl chloride (TMC) solution before interfacial polymerization [22]. Prince et al. [23] grafted PEG onto surface of PES membrane and then immobilized silver nanoparticles (Ag) through thermal grafting. As a result, the modified surface was more hydrophilic than the original PES membrane. Elimelech et al. [24] reported a forward osmosis (FO) TFC membrane functionalized using PEG derivatives which showed improved antifouling performance. Zhang et al. [25] introduced poly(ethylene oxide) segments into polyamide layer to enhance the hydrophilicity of composite film. An amino terminated poly(propylene oxide)/poly(ethylene oxide) block copolymer (PEGamine) was used to attach PEG chains onto the surface of an asymmetric poly(etherimide) (PEI) ultrafiltration membrane, which improved resistance to protein fouling for the modified PEI membrane [26]. In recent years, many diamines had been screened to obtain suitable monomers for interfacial polymerization to prepare pollution resistance membranes. In this paper, we use 2,2′-oxybis-ethylamine(2,2′-OEL) as a hydrophilic ingredient mixed with piperazine (PIP) in aqueous solution, and then polycondensed with TMC in n-hexane to prepare TFC nanofiltration membranes. This diamine possesses an ethyloxy group in the structure, which is more hydrophilic and flexible than hydrocarbon diamines, therefore is an important intermediate for organic synthesis, such as crown ethers. Owing to short molecular chain, swelling of polymer would be limited comparing with long-chain PEG derivatives, which could improve the combine strength of base membrane and skin layer. Nanofiltration performance and fouling resistance property of the membranes were investigated to evaluate the effect of 2,2′-OEL in fabrication of TFC nanofiltration films. Water flux and salt rejection of the membranes were investigated to test separation performance of the TFC membranes. SEM, FTIR-ATR and XPS were used to characterize surface chemical features of the resultant membranes. Hydrophilicity of the membranes was investigated through water contact angle (WCA) measurement. Protein fouling resistance of the membranes was evaluated by water recovery rate after treatment in the presence of bovine serum albumin. 2. Materials and methods 2.1. Materials Trimesoyl chloride (TMC, N99%), piperazine (PIP, N99.5%) and n-hexane were obtained from Sinopharm Chemical Reagent. 2,2′Oxybis-ethylamine(2,2′-OEL N 95.0%) was purchased from J&K Chemical. NaCl, MgSO 4 and Na 2 SO 4 were purchased from Kermel Chemical Reagent Co., Ltd. (Tianjin, China) and used without any further purification. Bovine serum albumin (BSA) was purchased from Shanghai Aladdin Reagent Company. Phosphate-buffered saline was purchased from Sigma. Polysulfone ultrafiltration (PSF-UF) sheet membrane (MWCO = 20,000) with nonwoven fabric supporting was supplied by GE company. Deionized water was used throughout the study.

2.2. Preparation of the TFC membranes Interfacial polymerization was operated on the surface of PSF-UF sheet membrane to fabricate nanofiltration composite membrane. Firstly, blend aqueous solution of PIP and 2,2′-OEL in different ratios was poured on the top of the PSF-UF sheet membrane for 10 min, then the membrane was taken out and excess solution was drained off with filter papers. Organic phase was prepared by dissolving TMC in n-hexane (0.1 wt.%). Then, the membrane was dipped in the organic phase for 30 s to obtain polyamide layer through interfacial polymerization between diamines or PIP and TMC. Afterward, the resultant composite membrane was air dried for 30 min to allow the n-hexane to evaporate and subsequently post-treated for attaining the desired stability of the formed structure. Finally, the membrane was washed with deionized water and stored in deionized water for further investigation. The total aqueous monomer concentration was set at 2 wt.% and the concentration of PIP was varied at 2, 1.33, 0.9 and 0 wt.% (Table 1) and the concentration of 2,2′-OEL at 0, 0.67, 1.1 and 2 wt.% correspondingly. The prepared TFC membranes were named as N1, N2, N3 and N4, respectively.

2.3. Characterizations Chemical structure of membranes was characterized with a Vector22 FTIR spectrometer (Bruker Company, Germany). All the samples were dried thoroughly in vacuum at 60 °C for 24 h prior to characterization. The XPS data were obtained on an AXIS-Ultra instrument Kratos Analytical (SHIMADZU, Japan). Field emission scanning electron microscopy (FE-SEM) (S-4800, Hitachi, Japan) was used to analyze the surface and cross-sectional morphologies of the composite nanofiltration membranes. The samples frozen in liquid nitrogen were broken and sputtered with gold before SEM observation. Water contact angle measurements were performed with the sessile drop method using a contact angle meter (Drop Shape Analysis 100, KRUSS GmbH Co., Germany). A syringe was used to place a water droplet of 3 μL on the membrane surface. Tangent lines to both sides of the droplet static image were generated and averaged by the software Drop Shape Analysis. At least three readings at different locations on one surface were measured to get a reliable value.

2.4. Nanofiltration performance tests Performance of the composite membranes was investigated using a cross-flow module at 25 °C (unless otherwise specified, the following chapter performance tests were carried out at 25 °C) and 0.7 MPa. The membranes were pre-filtrated with deionized water at 0.7 MPa to reach a steady state before testing. Water flux and salt rejection of the membranes were investigated by 2 g/L Na2SO4, MgSO4 and NaCl aqueous solution, respectively. The solution conductivity was tested at appropriate intervals with permeated liquid collected by a small beaker. The conductivity meter was DDS-11A (Shanghai Honggai Instrument Plant, China).

Table 1 Chemical composition of the PSF, N1, N2, N3 and N4 membrane surfaces. Membranes

PSF N1 N2 N3 N4

2,2′-OEL:PIP (m%:m%)

0:2 0.67:1.33 1.1:0.9 2:0

Atomic Conc. (mol%)

N/O

C1s

N1s

O1s

S2s

77.35 73.89 71.99 74.37 73.9

2.93 10.11 7.43 6.46 7.91

14.94 14.17 16.09 15.73 15.75

2.49 0 0 0 0

0.19 0.71 0.46 0.41 0.50

J. Jin et al. / Desalination 355 (2015) 141–146

143

Fig. 1. Chemical structures of surface membranes.

fixed volume water, and the flux (F) was calculated from Eq. (1). Cp and Cf are the salt concentration of permeate and feed solution, respectively, which were measured by a conductance meter (EL30, METTLER TOLEDO, Switzerland), and salt rejection (R) was calculated from Eq. (2). Each sheet of membrane was tested three times over an arbitrarily selected position, and the average was recorded.

The water flux (F) and salt rejection (Rs ) were calculated by Eqs. (1) and (2): F¼

JðLÞ
A m2  TðhÞ

RS ¼

ð1Þ

  Cp  100% 1− Cf

ð2Þ

2.5. Antifouling performance measurements

where J (L) is the volume of permeated water, A (m2) is the effective area of membrane (7.07 cm2), T (h) is the time used by permeating a

(a) wide scan

Antifouling performance experiments were carried out with BSA solutions. BSA was dissolved in phosphate-buffered saline solution (PBS,

(c)C1s

C1s

C-H/C-C

O1s

C-O/C-S

C-N

N1s S2p 700

600

500

400

300

200

100

0 296

294

292

(b) wide scan

290

288

286

284

282

280

Binding energy (eV)

Binding energy (eV)

C-H/C-C

(d) C1s

C1s

O1s C-O N1s

C-N COO/N-C=O

700

600

500

400

300

Binding enegy(eV)

200

100

0 296

294

292

290

288

286

Binding energy (eV)

Fig. 2. The XPS spectra of the PSF (a and c) and N4 (b and d) membrane surfaces.

284

282

280

144

J. Jin et al. / Desalination 355 (2015) 141–146

3. Results and discussions

N4 N3 N2 N1 PSF

4000

3.1. Chemical composition of the surface

C=O

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1) Fig. 3. ATR-FTIR spectra of membrane surfaces.

pH = 7.4) with a concentration of 1 mg/mL. Before testing, the film was cut into an appropriate size and put in test cell. Before recording data, the membrane had been preload using pure water as testing liquid under an initial pressure of 0.7 MPa. The initial pure water flux was recorded every 10 min in continuous 40 min running, and then the feed solution was replaced by BSA/PBS solution. The system was stabled for 5 min under 0.7 MPa, then flux was measured once every 10 min in continuous 40 min running. The membrane was washed with deionized water for 30 min, and then the operation was repeated once. The antifouling performance was evaluated after 2.5 cycles. The recovery rate (J r ) of the TFC membrane was calculated through the following equation: Jt ¼ Jt =J0  100%

ð3Þ

where J0 and Jt are the water flux of initial and time t in the filtration process, respectively.

Interfacial polymerization was performed between TMC in n-hexane and diamines or PIP in aqueous solution, as showed in Fig. 1. Four composite membranes were prepared using 2 wt.% mixed amine by different ratios of 2,2′-OEL. Surface chemistry composition of the membranes was analyzed by XPS. Table 1 showed the element content of the PSF substrate and the TFC surface. It can be seen that the content of C and O is increased in TFC membranes compared with PSF. As can be seen in Fig. 2, the peak intensity of N and O of N4 (b) was higher than PSF (a) at 399.0 eV and 532.0 eV, respectively, which was the evidence of ethoxy group in surface chemistry. S2p peak at 169 eV (a) disappeared due to the formation of polyamide layer on the surface of PSF support. The peak at 285.08 eV was assigned to C1s in both spectra (a) and (b). The high-resolution XPS spectra for C1s of PSF and N4 membrane were illustrated in Fig. 2(c) and (d), respectively. As can be seen, peaks at 285.3 eV and 287.4 eV were C\N and O\C_O/N\C_O, respectively. Compared with spectra (c) and (d), the peak intensity of C\N in spectrum (d) was obviously higher than spectrum (c). These further demonstrated the formation of polyamide functional layer on the surface of PSF membrane. ATR-FTIR was also used to characterize the surface chemical composition of PSF membrane and the modified membranes (Fig. 3). Comparing with PSF, there appeared a new peak at 1628 cm− 1 on the TFC membrane surface, which is attributed to C_O of the amide characteristic [29]. This is the verification of interfacial polymerization that successfully occurred. While the adsorption peak of C\O\C of 2,2′oxybis-amine at 1100 cm−1 was not obvious due to overlapping with the adsorption of C\C/C\H.

3.2. Morphology of the membrane surface The surface and cross morphology of PSF membrane, N1 membrane and N3 membrane are shown in Fig. 4. Nodular structures could be observed on the surface of membranes in N1 (b) and N3 (c). The results showed that the interfacial polymerization successfully occurred and pore sizes of the resultant membranes were in the range of nanofiltration. These were typical for a nanofiltration surface made from TMC and PIP by polycondensation. Interfacial polymerization takes place in the organic

Fig. 4. FESEM images of surface and cross-section morphologies, (a/a′) PSF, (b/b′) N1 and (c/c′) N3.

J. Jin et al. / Desalination 355 (2015) 141–146

145

Table 2 Molecular structures of the monomers involved. Monomer parameters (J/cm )

Molecular structure

Piperazidine

18.07

2,2′-Oxybis-amine

21.09

n-Hexane

13.47

Received and predicted by the group contribution method.

phase, so the property of resultant membrane would be affected by the solubility of amines in an organic solvent. The solubility parameters of the three compounds were calculated and listed in Table 2. The difference of solubility parameters between PIP and n-hexane is smaller than the difference between 2,2′-OEL and n-hexane. 2,2′-OEL was less soluble than PIP in n-hexane, so when 2,2′-OEL reacted with TMC at a slower rate, which finally resulted to smaller granular morphology. According to Saha and Joshi [28], the rough surface morphology may arise from the stress during interfacial polymerization and swelling phenomena in the drying process [27]. The faster the monomer migrates, the rougher the surface is. Cross-section structures could be observed in N1 (b′) and N3 (c′), which show that a 200 nm functional layer was obtained.

3.3. Nanofiltration performance of the TFC membranes The parameters to evaluate nanofiltration performance included permeate flux and rejection. Pure water flux was determined by surface hydrophilicity and porosity of active layer. Rejection of neutral solutes by NF membranes was controlled by size-exclusion. Typically, the average pore size of nanofiltration membranes is around 1 nm, and the diameter of water molecule is 0.27 nm. Water molecule could permeate the membrane easily. Fig. 5 showed the performance of membranes tested by 2 g/L Na2SO4, MgSO4 and NaCl solution at 25 °C and 0.7 MPa. We can see that the flux of TFC membranes increased with the introduction of 2,2′-OEL. The flux of N4 for Na2SO4, MgSO4 and NaCl is 33.9, 35.6 and 41 L/m2h, respectively; while the flux of N1 is 22.1, 21.2 and 25.5 L/m2h, respectively. Fig. 6 shows the salt rejection of TFC membranes. It can be seen that salt rejection is almost unaffected by the appropriate incorporation of 2,2′-OEL. In detail, salt rejection for Na2SO4, MgSO4 and NaCl is around 97%, 90%, and 30%, respectively. So we can conclude that Donnan exclusion and size exclusion mechanism are critical factors for the studied

80 60

Na2SO4

100

Solubility

40 20 0

Rejection(%)

3 1/2

100

NaCl MgSO4

N1

N2

N3

N4

75

50

25

0

N1

N2

N3

N4

Fig. 6. Salt rejection performance of TFC membranes tested with 2 g/L different inorganic salt aqueous solutions at 25 °C and 0.7 MPa.

TFC membranes. There is a strong electrostatic repulsion between the multivalent anions (SO2− 4 ) and the membrane surfaces, so that the rejection for both Na2SO4 and MgSO4 is higher than that for NaCl. The hydrophilic TFC membranes can maintain a good salt separation capability with high water flux [30]. In addition, PIP is a small molecule which encourages a close-pack configuration of the molecules in the IP network, while 2,2′-OEL is a chain molecule, which could cause greater void space between TMC. It is a factor that the salt rejection was decreased and flux was increased by the incorporation of 2,2′-OEL.

3.4. Water contact angle measurement Contact angle measurements were used to characterize the surface hydrophilicity of the membranes. The water contact angles of TFC membranes were all smaller than PSF support (Fig. 7). It could be observed that the water contact angles decreased rapidly with the 2,2′-OEL concentration and meanwhile the developing trend was gradually obvious with the time, which indicated that the hydrophilicity increases with the concentration of 2,2′-OEL. The addition of 2,2′-OEL could increase the hydrophilicity of skin layer, which was a crucial insurance of high fouling resistance.

60 40

50

Na2SO4

75 PSF N1 N2 N3 N4

N1

N2

N3

Contact Angle( °)

10 0

-2

-1

Flux (L m h )

20

40

90

NaCl MgSO4

30

N4

30

20

60

45

30

10

0

N1

N2

N3

N4

15 0

20

40

60 Time (s)

Fig. 5. Water flux performance of membranes tested with 2 g/L different inorganic salt aqueous solutions at 25 °C and 0.7 MPa.

Fig. 7. Water contact angles of the membranes.

80

100

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J. Jin et al. / Desalination 355 (2015) 141–146

Flux Recovery Radio(%)

100

References

80

N4 N3 N2 N1

60

40

0

40

80

Water

BSA

Water

BSA

Water

120

160

200

Time(min) Fig. 8. Time dependent water flux recovery ratio of TFC membranes tested with deionized water and BSA/PBS aqueous solution (pH = 7.4) at 25 °C and 0.7 MPa.

3.5. Antifouling performance of the TFC NFMs Antifouling properties of all the TFC membranes were evaluated in cross-flow module employing deionized water and bovine serum albumin/phosphate-buffered saline solution (pH = 7.4) as model feed solution at 25 °C and 0.7 MPa. Fig. 8 had showed that four composite membranes had lower protein fouling than base membrane. It could be observed that N4 had shown the best antifouling property among all the samples. In detail, the Jr of N4 is 98.5% while Jr of N1 is 88%. The water flux recovery rate of N2 and N3 is 95% and 96%, respectively. With the increase of 2,2′-OEL, fouling resistance had been enhanced gradually. It has been demonstrated that proteins would be aggregated and adsorbed severely onto the membrane surface at their isoelectric point. Hydrophilic surface could strengthen its binding to water and reduce protein adsorption, thus make it easy to clean; hence pure water flux recovery rater had been increased correspondingly.

4. Conclusions Anti-pollution thin-film composite nanofiltration membranes were prepared using 2,2′-oxybis-amine. The resultant membranes showed stability against fouling and increased hydrophilicity. IR and XPS demonstrated the formation of polyamide functional layer on the surface of PSF membrane. The surface hydrophilicity of composite membranes was improved by introducing 2,2′-OEL. Hydrophilicity of membranes increased with the ratio of 2,2′-OEL in aqueous solution. 2 g/L Na2SO4, MgSO4 and NaCl were employed to evaluate nanofiltration performance of the membranes at 25 °C and 0.7 MPa. The water flux of the film increased and salt rejection of the film decreased with the increase of 2,2′-OEL in aqueous phase. The membranes showed better performance in Na2SO4 aqueous solution than in MgSO4 and NaCl solution. Na2SO4 rejection of N2 was 96% at the flux of 28 L/m2h, and water recovery rate was 95%.

Acknowledgments The authors gratefully acknowledge the National High Technology Research and Development Program of China (863 Program of China, 2012AA03A602) and the financial support by the National Nature Science Foundation of China (NFSC, 21206122). Dr. LDQ also thanks Prof. LZH (Tianjin Polytechnic University, China) and Prof. MJQ (Tianjin Polytechnic University, China) for their helpful discussions.

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