High flux positively charged nanofiltration membranes prepared by UV-initiated graft polymerization of methacrylatoethyl trimethyl ammonium chloride (DMC) onto polysulfone membranes

Journal of Membrane Science 366 (2011) 363–372

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High flux positively charged nanofiltration membranes prepared by UV-initiated graft polymerization of methacrylatoethyl trimethyl ammonium chloride (DMC) onto polysulfone membranes Huiyu Deng a,b,∗ , Youyi Xu b , Qingchun Chen a , Xiuzhen Wei b , Baoku Zhu b a b

Department of Materials Science and Engineering, East China Institute of Technology, Fuzhou 344000, China Institute of Polymer Science, Key Laboratory of Macromolecule Synthesis and Functionalization, EMC-ZJU, Zhejiang University, Hangzhou 310027, China

a r t i c l e

i n f o

Article history: Received 3 August 2010 Received in revised form 9 October 2010 Accepted 12 October 2010 Available online 16 October 2010 Keywords: Positively charged Nanofiltration membrane UV-assisted grafting DMC

a b s t r a c t To achieve high water flux, the majority of positively charged NF membranes consist of a thin active skin deposited on a thick, permeable support, while the connection force between the active layer and the support is a physical force which may cause the membrane unstable in the long running. In general, chemical bond connection may show superior stability. Based on this consideration, a series of positively charged nanofiltration (NF) membranes were prepared by UV-initiated graft polymerization of methacrylatoethyl trimethyl ammonium chloride (DMC) onto polysulfone ultrafiltration membranes in this work. The salts rejection order of these membranes is MgCl2 > NaCl > MgSO4 ≥ Na2 SO4 . Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR-ATR), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and water contact angle were employed to characterize the resulting membranes. The results indicated that the grafting degree (DG) increased with increasing the monomer concentration, prolonging the irradiation time and reducing the irradiation distance. However, the filtration performance was not well correlated with the increasing DG. The NF membrane prepared by photografting in a 1.5 M DMC solution for 5 min demonstrated high MgCl2 rejection (94.8%) accompanied with high flux (20.3 L/m2 h) at 0.2 MPa. When operation pressure increased to 0.8 MPa, the solution flux increased to 60 L/m2 h, while MgCl2 rejection nearly maintained stable about 92.4%. An interesting phenomenon was also observed in this experiment that the flux of pure water was less than that of salt solution for some NF membranes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Nanofiltration (NF) is an attractive membrane technique similar to reverse osmosis (RO) in that pressure drives a solvent across a membrane against a concentration gradient [1]. However, nanofiltration of aqueous electrolyte solutions combines higher fluxes with acceptable selective rejection at low pressure. It can also reject molecules with molecular weight above 300. Hence, it has been applied for water softening, removal of micro-pollutants, as well as for treatment of waste and process water [2–6]. The selectively separation behavior is due to the fact that NF does not only rely on size exclusion, but is mainly governed by electrostatic effect, i.e., repulsion of co-ions by the surface charge of the membrane [1]. Thus, a positively charged membrane with a

∗ Corresponding author at: Department of Materials Science and Engineering, East China Institute of Technology, Fuzhou 344000, China. Tel.: +86 794 8258352; fax: +86 794 8258352. E-mail address: [email protected] (H. Deng). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.10.029

relatively small effective pore size is more suitable for cations separations than a negatively charged nanofiltration membrane, such as Mg2+ , Ca2+ removal (water softening) or heavy metal cations removal. However, most of commercial products are negatively charged [7–9]. Only in recent years, many people devoted to the preparation, characterization and application of positively charged NF membranes [10–16]. Su et al. [10] prepared asymmetric quaternized poly(phthalazinone ether sulfone ketone) NF membranes by phase inversion method and then introducing quaternary nitrogen groups into the membranes. The membrane prepared with polyethyleneglycol 400 (PEG 400) as additives showed perfect performance for the separation between dye and NaCl. However, in order to achieve high water flux, the majority of positively charged NF membranes are composite membranes which consist of a thin active skin deposited on a thick, permeable support. For instance, Huang et al. [11] prepared a positively charged NF membrane by coating quaternized chitosan (2-hydroxypropyltrimethyl ammonium chloride chitosan) on polyacrylonitrile (PAN) ultrafiltration (UF) membrane, then cross-linking by toluene diisocyanate. The permeability of pure water through this membrane was

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8.96 L/(m2 h1 MPa1 ). Using poly(allyamine, hydrochloride) (PAH) as separating active layer, Ouyang et al. [12] prepared positively charged NF membranes with high flux of 0.85 m3 /(m2 day) at 4.8 bar by layer by layer self-assembly process. A similar positively charged NF membrane was also prepared by dynamic self-assembly process in our previous work [13]. Alternately, in this work, UVassisted grafting polymerization of methacrylatoethyl trimethyl ammonium chloride (DMC) onto polysulfone ultrafiltration membrane was applied to prepare NF membranes. Compared with the works mentioned above [11–13], the connection force between the substrate membrane and the active layer in this work is chemical bond, hence, may increase the stability of the NF membranes. Due to the advantages of simple operation, low cost, selectively to absorb UV light without affecting the bulk polymer in dilute monomer solutions, UV-assisted graft polymerization has been applied for surface modification for years [17–19]. Recently, it was also successfully used to prepare negatively charged NF membranes [20–24], however, to our knowledge, few focus on preparing positively charged NF membranes by this method. Whether the positively charged NF membranes prepared by this method have similar filtration performance to other positively charged NF membranes prepared by different methods? Whether it is convenient to control the performance of these NF membranes by altering the grafting conditions? What is the dependence of filtration performance on the grafting conditions? To answer these questions and to further broaden our knowledge on characteristics of positively charged NF membranes, a series of experiments were carried out in this work. Substrate membrane used in this work is polysulfone ultrafiltration membrane. Polysulfone is a typical aromatic polymer which is widely used as membrane materials, owing to its good thermal stability, excellent mechanical properties, as well as its stability to water, acid and many chemicals [25]. It was also discovered that polysulfone is intrinsically photosensitive and can generate free radicals upon irradiation with UV light without photoinitiators [26–28]. In the presence of vinyl monomers, free radical grafting polymerizations occurs, forming polymer chains that covalently bonded to the surface. Grafting monomer applied in this experiment is methacrylatoethyl trimethyl ammonium chloride (DMC) which is a normal chemical resource widely used to produce flocculants for a long time [29,30]. The effect of monomer concentration, irradiation time and irradiation distance on the grafting degree and the filtration performance were investigated. The rejection coefficients of different salts of these NF membranes and the effect of operation conditions on the filtration performance were also studied. The FTIR-ATR, XPS, AFM were applied to characterize the chemical and physical changes among the substrate membrane and the NF membranes.

2. Experimental 2.1. Materials Commercially available methacrylatoethyl trimethyl ammonium chloride (DMC) was supplied by Wuxi Xinyu Chemical Engineering Corporation as a 74.86% (w/w) aqueous solution and used as received. Other chemicals used were all of AR grade. Sodium chloride (NaCl), sodium sulfate (Na2 SO4 ), magnesium chloride (MgCl2 ), magnesium sulfate (MgSO4 ) were purchased from shanghai guoyao factory and used as received. Polysulfone ultra-filtration membranes used as support membrane were kindly offered by the Development Center of Water Treatment Technology, State Oceanic Administration (Hangzhou, China), which were prepared by traditional phase-inversion method on polyester non-woven fabric. The molecular weight cut-

Table 1 Synthesis conditions of the NF membranes. Membranes

[Monomer] (mol/L)

Irradiation time (min)

Distancea (cm)

NF1 NF2 NF3 NF4 NF5 NF6 NF7 NF8 NF9 NF10 NF11

1.5 1.5 1.5 1.5 1.5 1.5 0.5 1 2 1.5 1.5

1 3 5 10 15 20 5 5 5 5 5

12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 17.2 25.7

a

Distance refers to the distance between the UV lamp and the membrane surface.

off of these membranes is 50,000. Water flux of these membranes is 442.6 L/m2 h at 0.2 MPa. 2.2. Membrane preparation The apparatus of UV-induced graft polymerization is similar to the Ref. [22]. The photoreactor used for grafting DMC onto the polysulfone ultrafiltration membrane was a square stainless-steel chamber (10 cm × 10 cm) in which the membrane was fixed on the bottom, while the UV lamp,  > 300 nm (Hg medium pressure with a glass filter) of low power (100 W), was placed at the ceiling center of the reactor. A 30 ml water solution containing different concentrations of monomer was added into the chamber. Then the graft polymerization of DMC onto the polysulfone (PSf ) membrane was carried out by UV irradiation while the membrane was immersed in the monomer solution. Due to the practical interest, we preferred to focus our study on the grafting in air. The irradiation intensity was controlled by varying the distance between the UV lamp and the membrane surface. The grafting degree was controlled by altering the monomer concentration, UV irradiation time and the irradiation distance etc. After photo-grafting, the membrane was sufficiently washed with deionized water in ultra-sonic bath to remove the residual monomer and the homopolymer. The membranes were stored in deionized water before analyses. The molecular structures of methacrylatoethyl trimethyl ammonium chloride (DMC) and polysulfone were shown in Fig. 1. The grafting mechanism was also shown in Fig. 1 [25,26]. Synthesis conditions of the NF membranes are shown in Table 1. 2.3. Membrane characterization 2.3.1. Measurement of grafting degree The grafting degree (DG) of the PSf membrane was determined gravimetrically according to Eq. (1): DG =

Wi − W0 × 100% A

(1)

where W0 represents the weight of original PSf membrane, Wi is the weight of grafted PSf membrane and A represents the grafting surface area. 2.3.2. FTIR-ATR measurements To confirm the grafting of DMC on the polysulfone ultrafiltration membrane, FTIR-ATR spectroscopy was carried out on a Bruker Vector 22 FT-IR spectrophotometer. The spectra were measured in the wave number range of 4000–700 cm−1 . The spectra were collected by cumulating 32 scans at a resolution of 2 cm−1 .

H. Deng et al. / Journal of Membrane Science 366 (2011) 363–372

365

CH3 H2C

C

COOC2H4N(CH3)3Cl

(1) O

CH3 C

O

S

O

n

CH3

O

(2) O

CH3 C

O

hν

S

O

n

O

CH3 CH3 C

SO2

O

.+ .

O

CH3

CH3

CH3 C

SO2

O

H2C

.

C COOC2H4N(CH3)3Cl

CH3

C

CH3

O

CH3 O

CH3

S O

H2 C

*

C

n

COOC2H4N(CH3)3Cl

CH3

.

O

H2C

CH3

C COOC2H4N(CH3)3Cl

O

H2 C

C

n COOC2H4N(CH3)3Cl

Fig. 1. Chemical structure of (1) DMC, (2) PSf , and (3) proposed mechanism for the grafting of PSf with DMC [25,26].

2.3.3. X-ray photoelectron spectroscopy The surface compositions of membranes were characterized using an (XPS) (PHI 5000c, Peking-Elmer instruments). The sample was directly pressed to a self-supported disk (10 mm × 10 mm) and mounted on a sample holder then transferred into the analyzer chamber. The whole spectra (0 ∼ 1100 (1200) eV) and the narrow spectra of all the elements with much high resolution were both recorded by using RBD 147 interface (RBD Enterprises, USA) through the AugerScan 3.21 software. Binding energies were calibrated by using the containment carbon (C 1s = 284.6 eV). The data analysis was carried out using XPS Peak 4.1 provided by Raymund W.M. Kwok (The Chinese University of Hongkong, China). 2.3.4. Atomic force microscopy (AFM) AFM is a powerful asset in characterizing membranes as it allows determination of surface morphology and surface pore size distribution with no special sample preparation. An atomic force microscope (AFM, SPI3800N, Seiko Instrumental, Japan) was used to image the membranes. Membrane morphology was measured in air with a scan rate of 1.00 Hz at 256 × 256 resolution in tapping mode. The average surface roughness (the average deviation of the peaks and valleys height from the mean value) was determined on 5 ␮m × 5 ␮m areas of the membrane at three different spots on the surface using built-in software.

2.3.5. Scanning electron microscopy (SEM) A field emitting SEM (SIRION-100, FEI Co., Ltd., Netherlands) was used to image the cross-sectional morphologies of the membranes. To avoid destroying the cross-section structure, the membrane samples were fractured in liquid nitrogen and then sputtered with gold before observation.

2.3.6. Water contact angle measurements The hydrophilicity of the membrane top surface was characterized on the basis of water contact angle measurements (OCA20, Dataphysics, Germany) equipped with video capture at room temperature. The membrane was kept in vacuum at 35 ◦ C for 24 h to obtain the dry membrane sample for contact angle characterizing. A total of 1 ␮L of de-ionized water was dropped onto a dry dense membrane with a micro-syringe in an atmosphere of saturated water vapor and the size of the drip was captured. At least 10 contact angles were averaged to get a reliable value.

2.3.7. Characterization of membrane for salts rejection Nanofiltration experiments occurred at a pressure of 0.8 MPa, in a flat-sheet cross-flow NF test cell with an active membrane area of 23.75 cm2 (Fig. 2). The membranes were stabilized at 0.8 MPa for at least 30 min with de-ionized water before testing. NaCl, Na2 SO4 , MgCl2 and MgSO4 salt solution of 1000 mg/L were flowed across

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C1s O1s (1)

(2)

S2s

S2P 3

N1s Cl2p 3

1000

800

600

400

200

0

Binding energy,ev Fig. 2. Schematic representation of the nanofiltration evaluation cell. (1) Feed solution (initial volume: 5 L, temperature 25 ◦ C), (2) feed pump, (3 and 9) valve, (4) buffer tank, (5 and 8) pressure gauge, (6) membrane cell, (7) permeate end, (10) flow meter.

the membrane, respectively. NF rejection is defined by Eq. (2):



R=

1−

cp cf



× 100%,

(2)

Here R is the percent solute rejection, Cp and Cf are the concentrations of solute in permeate and feed, respectively. The salt concentration in the feed and permeate were measured with a conductor (DDS-11A, Shanghai Leici Instruments, China). To investigate the effect of operation pressure on the NF performance, the operation pressure changed from 0.2 MPa to 0.8 MPa.

Fig. 4. XPS spectra of (1) PSf and (2) NF5 membranes.

nal at 1724 cm−1 assigned to carbonyl in the DMC, while 955 cm−1 contributed to the characteristic peak of NH4 + . There were wide absorbing band in 3091–3629 cm−1 , which contributed to the vibrations of hydroxyl in water and NH4 + in DMC. Meanwhile, the absorption peaks at 1724 cm−1 and 955 cm−1 increased with prolonging irradiation time which we will discuss in detail later. These data indicated that the DMC had been successfully grafted onto the PSf membrane. However, one cannot infer from the IR spectra whether the grafted polymer as intended is deposited onto the PSf membrane surface or filled the pores of membrane or adjacent support layer. The permeate data will indicated that some grafting polymer filled the pores of the membrane which we will discuss later.

3. Results and discussion 3.2. XPS 3.1. FTIR-ATR Fig. 3 shows the FTIR-ATR spectra of the substrate PSf membrane and NF membranes. In the IR spectra of the substrate PSf membrane, a characteristic peak at 1487 cm−1 due to the vibration of benzene ring. While the characteristic peaks at 1321 cm−1 and 1149 cm−1 , 1106 cm−1 attributed to the S O asymmetric and symmetric stretching, respectively. Compared with the PSf UF membrane, the IR spectrum of NF membranes exhibited evident absorbance signals at 1724 cm−1 and 955 cm−1 . The absorbance sig-

To further verify the chemical changes underwent on the PSf membrane, X-ray photoelectron spectra (XPS) of both NF5 membrane and original PSf substrate membrane were taken. As shown in Fig. 4, compared with the original PSf membrane, there were obviously two peaks at the NF5 membrane. A peak appeared at 404.1 eV assigned to N 1s, while the peak at 199.3 eV assigned to Cl 2p3 . Meanwhile, the intensity of S 2s and S 2p3 of PSf decreased, which indicated that the DMC was successfully grafted onto the PSf membrane. 3.3. AFM images

(1)

1321

(2)

1106 1149

1487

1724

955

(3)

3500

3000

2500

2000

1500

1000

Wavenumber/cm-1 Fig. 3. FTIR/ATR spectra of PSf and NF membranes. (1) PSf substrate membrane, (2) NF3 membrane (monomer concentration 1.5 M, irradiation time 5 min, irradiation distance 12.7 cm), and (3) NF6 membrane (monomer concentration 1.5 M, irradiation time 20 min, irradiation distance 12.7 cm).

AFM was chosen for morphological characterizing the surface of the nascent and NF membranes. Average roughness is defined as the average deviation of the peaks and valleys from the mean plane, and root mean squared (RMS) roughness is the RMS deviation of the peaks and valleys. The AFM images (Fig. 5) indicated that the average roughness of membranes changed with the grafting condition. When irradiation time was only 1 min, there was obvious nodular (hills and valleys) appeared on the substrate membrane, the average roughness increased from 5.635 nm (Fig. 5A) for original membrane to 14.16 nm (Fig. 5B). The membrane became smooth by prolonging the irradiation time to 5 min, the roughness decreased to 3.717 nm (Fig. 5C). However, further prolonging the irradiation time to 20 min, the roughness increased to 116.0 nm (Fig. 5D). Altering the monomer concentration can also change the roughness of NF membranes. For instance, the roughness of membrane changed from 5.635 nm for original substrate membrane (Fig. 5A) to 8.530 nm (not shown), 3.717 nm (Fig. 5C), and then 7.095 nm (Fig. 5E) when the monomer concentration were 0 M, 0.5 M, 1.5 M

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367

Fig. 5. AFM images of PSf substrate membrane and NF membranes. (A) PSf membrane, (B) NF1 ([DMC] 1.5 M, irradiation time 1 min, irradiation distance 12.7 cm), (C) NF3 ([DMC] 1.5 M, irradiation time 5 min, irradiation distance 12.7 cm), (D) NF6 ([DMC] 1.5 M, irradiation time 20 min, irradiation distance 12.7 cm), and (E) NF9 ([DMC] 2.0 M, irradiation time 5 min, irradiation distance 12.7 cm).

and 2 M, respectively. Hirose et al. [31,32] found a relationship between the flux of reverse osmosis membrane and their roughness parameters measured by AFM, the results indicated that an increase in surface roughness resulted in a higher water permeation flux. However, similar phenomena did not appear in this work, because both the surface morphology and the pore structure were changed after the photo-grafting.

membrane is, the better antifouling is. Some NF membranes, e.g., NF3 membrane not only showed good hydrophilic but also is very smooth which may render the membrane with good antifouling property. However, it is out of the scope discussed in this paper and needs further working. 3.5. Effect of grafting conditions on the grafting degree and filtration performance of NF membranes

3.4. Water contact angle The NF membranes prepared by photografting polymerization of DMC show better hydrophilic than PSf membrane (Table 2). The smaller the contact angle, the more hydrophilic the membrane is. The smallest water contact angle of NF membrane was 32.6◦ , while that of the original PSf membrane was 95.2◦ . Contrary to the irregular roughness changes appeared with changing grafting conditions. The water contact angle decreased regularly with prolonging the irradiation time, increasing the monomer concentration or reducing the irradiation distance. In general, the smoother a hydrophilic

In the grafting polymerization, two steps in series which play an important role in grafting degree must be considered: one is the molecular diffusion of monomers from the bulk solution to an area close to the membrane surface caused by the concentration gradient from the bulk phase to the surface; another is the transfer of molecules from this nearby position to the grafted state [33]. These can be controlled by monomer and substrate membrane nature, monomer concentration, solvent type, irradiation conditions, reaction time, temperature, etc. In this work, we only focus on the effects of monomer concentration, irradiation time, irradiating dis-

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Table 2 Water contact angle of PSf membrane and different NF membranes.

PSf NF1(1 min) NF2 (3 min) NF3 (5 min) NF4 (10 min) NF5 (15 min)

95.2 70.4 65.2 40.9 38.5 32.6

± ± ± ± ± ±

2.0 0.7 2.3 1.6 1.3 0.9

Membranes

Contact angle (◦ )

NF6 (20 min) NF7 (0.5 M) NF8 (1 M) NF9 (2 M) NF10 (17.2 cm) NF11 (25.7 cm)

33.4 79.5 47.2 51.4 69.0 68.2

± ± ± ± ± ±

1.5 0.5 0.9 0.4 2.1 1.7

500 2

Contact angle (◦ )

DG (ug/cm )

Membranes

600

400 300 200

tance on the grafting degree and filtration performances which we discuss below. 3.5.1. Effect of grafting conditions on the grafting degree The dependence of grafting degree on the monomer concentration is shown in Fig. 6. It is obviously that grafting degree increased with increasing the monomer concentration from 0.5 M to 1.5 M and nearly level off when monomer concentration was further increasing to 2.0 M. In this experiment, DMC mainly takes part in two reactions. One is the grafting polymerization which caused by combination with the free radical generated by PSf upon UV irradiation, then prolonging by itself; the other is the homopolymerization by itself under UV irradiation. The former reaction increased DG while the latter reaction, i.e., homopolymerization increased the viscosity of reaction solution and limited monomer’s diffusion to the living sites of PSf membrane, thus retarding the grafting polymerization. Increasing the monomer concentration in a certain range can accelerate the two reactions mentioned above. However, the former one may play a more important role, so the grafting degree increased with increasing the monomer concentration from 0.5 M to 1.5 M. When the concentration was higher to 2.0 M, the positive effect for increasing DG may be balanced by negative effect generated by homopolymerization, so DG nearly leveled off. The results also indicated that prolonging the irradiation time is benefit for increasing grafting degree (Fig. 7). The increase rate from 10 min to 15 min is larger, while that from 15 min to 20 min was smaller. During the primary grafting polymerization, the monomer concentration is high and the solution viscosity is low. So the monomer can easily diffuse from the bulk phase to the membrane surface which causes an obvious increase in DG. The largest increase rate appeared in the range of 10–15 min may attribute to the gel effect which is the result of a sudden self acceleration of polymerization reaction due to an increased reaction solution viscosity. Further prolonging irradiation time, the increase rate slow down which may attribute to the decrease of the monomer diffusion.

100 0 0

5

10

15

20

Irradiation time (min) Fig. 7. Effect of irradiation time on the grafting degree (monomer concentration 1.5 M, irradiation distance 12.7 cm).

Irradiation intensity was controlled by varying the distance between the UV light and membrane in this experiment. The shorter the distance is, the more intense the irradiation. The primary radical generated by UV irradiation on PSf membrane increased with shorter irradiation distance, hence, the DG increased. When irradiation distance was 25.7 cm, DG was only 24 ␮g/cm2 , decreasing the distance to 12.7 cm, DG reached to 106 ␮g/cm2 . 3.5.2. Effect of grafting conditions on filtration performance Solution flux and salt rejection are the major parameters that characterize a membrane which mainly depends on the nature of the active layer and the membrane morphology [34,35]. As mentioned above, the chemical composition and surface morphology of PSf changed a lot after grafting, which successfully transfer the PSf ultrafiltration membrane without any salt rejection capacity to NF membrane. The dependence of membranes’ filtration performance on the monomer concentration is shown in Fig. 8. The results indicated the solution flux of NaCl and MgCl2 firstly decreased and then leveled off. While the salts rejection firstly increased and then leveled off with increasing the monomer concentration. When the monomer concentration was 0.5 M, the salt retention of NaCl and MgCl2 were 6.6% and 10.1%, respectively, while the permeation solution flux were 414.6 L/m2 h and 437.0 L/m2 h under 0.8 MPa. The NaCl and MgCl2 retention can reach 62.2% and 92.4%, respectively, meanwhile, permeation flux were 68.3 L/m2 h and 60.5 L/m2 h when the monomer concentration increased to 1.5 M. Further increasing the

120

100

500 NaCl MgCl2

400

R (%)

2

80

Flux (L/m .h)

80

2

DG (ug/cm )

100

60

40

60

300

40

200

20

100

0

20 0.5

1.0

1.5

2.0

monomer concentration (mol/L) Fig. 6. Effect of monomer concentration on the grafting degree (irradiation time 5 min, irradiation distance 12.7 cm).

0.5

1.0

1.5

2.0

0

monomer concentration (mol/L) Fig. 8. Effect of monomer concentration on the performance of the NF membranes (irradiation time 5 min, irradiation distance 12.7 cm).

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Fig. 9. Cross-section SEM images of PSf substrate membrane and NF membranes. (A) PSf membrane; (B) NF1 ([DMC] 1.5 M, irradiation time 1 min, irradiation distance 12.7 cm); (C) NF3 ([DMC] 1.5 M, irradiation time 5 min, irradiation distance 12.7 cm); (D) NF5 ([DMC] 1.5 M, irradiation time 15 min, irradiation distance 12.7 cm).

small and the effective charge density was not so high to reject most salts, both NaCl and MgCl2 rejection was very low, only 4% (NaCl) and 13.5% (MgCl2 ), respectively. Due to the improvement of DG by prolonging irradiation time, the pore-size decrease progressed, hence, increasing the effective charge density, leading to higher salts retention. When irradiation time was 5 min, the rejection of NaCl and MgCl2 reached 62.2% and 92.4%, respectively. However, further prolonging the irradiation time did not obviously lead to a higher rejection, only caused the flux decreased little. It may attribute to the increase of DG did not improve the effective charge density sufficiently. But the thickness of membrane increased (Fig. 9C and D) from about 871 nm (5 min) to about 1000 nm (15 min), which reduced the permeation solution flux.

1000 800

60

600

40

400

20

200

0

0

5

10

15

20

2

NaCl MgCl2

80

Flux (L/m .h)

100

R (%)

monomer concentration has little impact on the filtration performance. The effect of irradiation time on the performance of NF membranes was also studied. During the first 1 min irradiation, the pure water flux decreased drastically from 1559.3 L/m2 h of initial membrane to 1102.2 L/m2 h. The cross-section images showed there was no obvious change between the original membrane (Fig. 9A) and NF1 membrane (prepared under irradiation of 1 min) (Fig. 9B), both membranes were of nearly 615 nm. So the decrease in permeation flux reflects a drastic reduction in PSf ultrafiltration membrane pore size. The permeation solution flux of NaCl and MgCl2 for NF 1 were 907.62 L/m2 h, 908.3 L/m2 h, respectively, accompanied with very low salts rejection coefficient. After 5 min irradiation, the solution flux of NaCl and MgCl2 decreased sharply to 68.32 L/m2 h and 60.5 L/m2 h, the salts retention increased obviously to 62.2% and 92.4%. However, further prolonging the irradiation, the membrane’s filtration performance changed little (Fig. 10). These phenomena suggested that: in the primary irradiation time (0 ∼ 5 min), the grafting of DMC formed on both the pore walls and the dense part of the substrate surface can quickly decrease the pore size which increase the trans-membrane hindrance, leading to a progressive reduction in solvent flux and a salt rejection improvement. It should be noted that further prolonging the irradiation time, monomer concentration in the pore decreased. Although monomer can diffuse from bulk phase to the substrate membrane pore, the monomer consumption rate in the pore was larger than the diffusion rate, so it can not compensate the monomer in the pore efficiently which decrease the grafting polymerization in the pore, leading flux decreased slowly. Rejection of electrolyte of NF membranes mainly depends on size screen and electric repulsion. Although 1 min grafting polymerization can reduce the pore-size, but the pore size was not so

0

Irradiation time (min) Fig. 10. Effect of irradiation time on the performance of the NF membranes (monomer concentration 1.5 M, irradiation distance 12.7 cm).

H. Deng et al. / Journal of Membrane Science 366 (2011) 363–372

100

water NaCl MgCl2

80 NF3 NF4 NF9

40

60

60

30

600

50

20 4

6

400

8

10

12

14

16

18

20

22

40

Irradiation time (min)

40 20

200

NaCl 0

Na2SO4

MgCl2

MgSO4

30

Rejection of different salts and solution flux 0

5

10

15

20

Irradiation time (min) Fig. 11. Effect of irradiation time on the flux of the NF membranes (monomer concentration 1.5 M, irradiation distance 12.7 cm).

There was also an interesting phenomenon observed in this experiment. The NF membranes prepared by grafting under irradiation for relatively shorter time (1 min, 3 min) or relatively longer time (20 min) showed lager pure water flux than permeation NaCl and MgCl2 solution flux. While the NF membrane prepared by mediate irradiation time (5 min, 10 min and 15 min), the pure water flux was between that of NaCl solution and MgCl2 solution (Fig. 11). In general, the salts solution permeation flux decreases with increasing the feed solution concentration due to the higher osmosis pressure, so pure water flux is larger than salts solution flux. In this experiment, another element we must take into account, the grafting polymerization chain was polyelectrolyte which can change conformation in certain environments. The grafting polymer chain has less stretched conformation in salt solution than in pure water, which leads to a more open pore structure and causes the salt solution larger than that of pure water. For the membranes prepared under irradiation for 1 min and 3 min, the grafting polymer chains were relatively short, so the increasing osmosis pressure plays a more important role than polymer chain’s deformation change on the permeation flux. The pure water flux was larger than salt solution. When irradiation time increased to 5 min, 10 min and 15 min, the grafting polymer chain became longer, which means the chain’s conformation change in salt solution will increase the solution flux. However, the higher salts rejection with increased the osmosis pressure leads the salts solution flux decrease in a certain extent. For MgCl2 , the rejection is higher, may cause higher osmosis pressure than that of NaCl, so the pure water flux was between that of NaCl solution and MgCl2 solution. It should be noted that further prolonging irradiation time to 20 min, some polymer chain maybe longer which enhance the entanglement between the molecules and the combination termination between the different chains. These resulted in preventing the shrinkage of the polyelectrolyte chain. Therefore, the solution flux improvement caused by the chain shrinkage reduced, leading the pure water flux larger than salt solution permeation flux. Table 3 Effect of irradiation distance on the performance of NF membranes. Membranes

NaCl

a

NF3 (12.7 cm) NF10 (17.2 cm) NF11 (25.7 cm) a

70

50

2

2

2

Flux (L/m .h)

800

80

60

Flux (L/m .h)

1000

70

R(%)

1200

Flux(L/m .h)

370

MgCl2

R (%)

F (L/m2 h)

R (%)

F (L/m2 h)

62.2 3.5 4.0

68.3 668.9 682.2

92.4 4.8 5.8

60.5 629.9 732.4

The number in the bracket is irradiation distance.

Fig. 12. Rejection coefficients of different salts and solution flux (operation pressure 0.8 MPa, 25 ◦ C, salts solution concentration 1000 mg/L).

In this experiment, altering the irradiation distance can efficiently control the filtration performance (Table 3). When irradiation distance was 25.7 cm, NaCl and MgCl2 rejection were only 4.0% and 5.8%, while NF3 prepared by irradiation at 12.7 cm showed 62.2% rejection coefficient for NaCl and 92.4% rejection for MgCl2 . It provided people a convenient approach to prepare NF membranes with different performance. 3.6. Rejection of different salts for DMC NF membranes The ionic transport and selectivity of NF membranes mainly depend on two effects: charge repulsion and size screening. The first effect is caused by the charge nature of the membrane and electrolytes. The second effect is caused by the relative size of the ions (or hydration ions) to the membrane pores. In general, charged NF showed higher rejection for multivalent ions than for monovalent ions. Fig. 12 indicates that the DMC NF membranes were positively charged. For the same anion, the rejection of MgCl2 was larger than that of NaCl, while the rejection MgSO4 was slightly larger than that of Na2 SO4 . For NF3, the rejection of MgCl2 was 92.4%, while that of NaCl was only 62.2%. The salt rejection order follows R(MgCl2 ) > R(NaCl) > R(MgSO4 ) ≥ R(Na2 SO4 ), which was in agreement with the sequence indicated by Johan Schaep [36,37] and Huang et al. [11]. But the MgSO4 was lower than that of Huang. It may contribute to the structure differences between the different NF membranes. The NF membranes prepared by Huang maybe more compact than the membranes prepared in this work. For the NF membrane Huang prepared, the solution flux of MgCl2 was 11.5 L/m2 h at 1.0 MPa combined with rejection of MgCl2 was 87.2%, while the rejection of MgSO4 was 58.3%. In this work, the rejection of MgCl2 for NF3 was 92.4%, and the solution flux was 60.5 L/m2 h at 0.8 MPa, while the rejection of MgSO4 was 22.1%. Therefore, the electrostatic effect plays more important roles for NF membranes prepared in this work were, while for Huang, the size sieving effect cannot be neglected. It also indicated that the DMC NF membranes were suitable for the hardness removal from sulfate-poor waters. 3.7. Effect of operation pressure on the performance of DMC NF membranes Effect of operation pressure on the performance of the NF membranes was also investigated (Figs. 13 and 14). Two types of pressure-separation relationships were found for the NF membranes. The rejection of MgCl2 decreased with increasing the operation pressure for NF2, NF7, NF8 and NF10 membranes while the rejection of MgCl2 nearly maintained still when the operating pressure increased from 0.2 MPa to 0.8 MPa for NF3, NF4 and NF9 membranes. During the NF process for electrolyte solutions, the

H. Deng et al. / Journal of Membrane Science 366 (2011) 363–372

100

The short-term stability-test was also carried out at 0.2 MPa with 1000 mg/L MgCl2 on NF3 for 6 days. There was no significant change in flux or rejection. These results demonstrated that the studied NF3 membrane shows good stability in this experiment.

NF9 NF4

80

NF3

4. Conclusions

NF8

R(%)

60

40 NF2

20

NF7 NF10

0 0.2

0.3

0.4

0.5

0.6

0.7

0.8

Operating pressure(MPa) Fig. 13. Effect of operating pressure on MgCl2 (1000 mg/L) rejection.

ions can partially retained by surface forces (i.e., electrostatic and friction forces). These forces have to be considered in the case of small pores because the ratio of the pore surface to the pore volume is high, and therefore the residence time close to the pore wall cannot be neglected. These forces are constant when increasing the pressure, while the drag force increases due to the flux in the pore. When operation pressure increased from 0.2 MPa to 0.8 MPa, the drag force becomes higher. The effect of surface forces becomes more and more negligible for NF2, NF7, NF8 and NF10 membranes, and the solute flux increases, therefore, the difference between the solute and solvent fluxes decreased which in turn reduces the rejection rate. However, for NF3, NF4 and NF9 membranes, they were more compact and having higher charge density, so the surface forces were still high enough to resist the drag force. Therefore, the rejection coefficient nearly maintained stable. The permeate solution flux confirmed that the NF membranes (NF3, NF4 and NF9) are more compact than the other NF membranes (NF2, NF7, NF8 and NF10) (Fig. 14). The permeate solution flux for the former membranes changed less than that of the latter membranes with the increase of operation pressure. For instance, the solution flux to operation pressure ratio of NF3 was about 67 L/(m2 h1 MPa1 ), while that of NF10 can reach about 717 L/(m2 h1 MPa1 ). It should be noted that NF5 and NF6 followed same pattern as shown for NF3, and NF1, NF11 followed same pattern as shown for NF10 which does not shown in Figs. 13 and 14.

700 NF2 NF3 NF4 NF7 NF8 NF9 NF10

500

2

Flux (L/m .h)

600

400

NF10

NF7

NF2

300

NF8

200

NF9

100

NF3 NF4

0 0.2

0.3

0.4

0.5

0.6

371

0.7

0.8

Operating pressure (MPa) Fig. 14. Effect of operating pressure on the solution flux of MgCl2 (1000 mg/L).

Positively charged nanofiltration membranes with high flux were conveniently prepared by UV-initiated graft polymerization of methacrylatoethyl trimethyl ammonium chloride (DMC) onto polysulfone ultrafiltration membranes in this work. Filtration performance of NF membranes can be controlled by adjusting the monomer concentration, irradiation time and irradiation distance. The membrane prepared by photografting in 1.5 M DMC solution for 5 min demonstrated high MgCl2 rejection (94.8%) accompanied with high flux (20.3 L/m2 h) at 0.2 MPa. The MgCl2 rejection of this membrane nearly maintained stable when the operation pressure increased to 0.8 MPa, meanwhile, the flux increased to 60.5 L/m2 h. It maybe a very excellent NF membrane for the hardness removal from sulfate-poor water and heavy metal cations removal, etc. The stability of these NF membranes in different environments and the application of these NF membranes are under working. Acknowledgements The authors greatly acknowledge the National Nature Foundation of China (No. 50433010), National 973 Foundation of China (No. 2003CB615705) and the Foundation of Jiangxi Educational Committee (No. GJJ09524). References [1] M. Mulder, Basic Principles of Membrane Technology, 2nd ed., Kluwer Academic Publishers, Dordrecht, 1997. [2] S. Gomes, S.A. Cavaco, M.J. Quina, L.M.G. Ferreira, Nanofiltration process for separating Cr(III) from acid solutions: experimental and modelling analysis, Desalination 254 (2010) 80–89. [3] E.S. Tarleton, J.P. Robinson, J.S. Low, Nanofiltration: a technology for selective solute removal from fuels and solvents, Chem. Eng. Res. Des. 87 (2009) 271– 279. [4] M. Mika, P. Teuvo, N. Marianne, NF270, a new membrane having promising characteristics and being suitable for treatment of dilute effluents from the paper industry, J. Membr. Sci. 242 (2004) 107–116. [5] J.W. Lv, K.Y. Wang, T.S. Chung, Investigation of amphoteric olybenzimidazole (PBI) nanofiltration hollow fiber membrane for both cation and anions removal, J. Membr. Sci. 310 (2008) 557–566. [6] H.A. Kim, J.H. Choi, S.S. Takizawa, Comparison of initial filtration resistance by pretreatment processes in the nanofiltration for drinking water treatment, Sep. Purif. Technol. 56 (2007) 354–362. [7] R.M. Chennamsetty, I. Escobar, Evolution of a polysulfone nanofiltration membrane following ion beam irradiation, Langmuir 24 (2008) 5569–5579. [8] K. Hochoo, D. Joongkwon, K. Wonlee, A. Junechoi, Selective removal of Cobalt species using nanofiltration membranes, Environ. Sci. Technol. 36 (2002) 1330–1336. [9] B. Balannec, M. Yourch, M.R. Baudry, B. Chayfer, Comparative study of different nanofiltration and reverse osmosis membranes for dairy effluent treatment by dead-end filtration, Sep. Purif. Technol. 42 (2005) 195–200. [10] Y. Su, X. Jian, S.H. Zhang, G.Q. Wang, Preparation and characterization of quaternized poly(phthalazizone ether sulfone ketone) NF membranes, J. Membr. Sci. 241 (2004) 225–233. [11] R.H. Huang, G.H. Chen, B.C. Yang, C.J. Gao, Positively charged composite nanofiltration membrane from quaternized chitosan by toluene diisocyanate cross-linking, Sep. Purif. Technol. 61 (2008) 424–429. [12] L. Ouyang, R. Malaisamy, M.L. Bruening, Multilayer polyelectrolyte films as nanofiltration membranes for separating monovalent and divalent cations, J. Membr. Sci. 310 (2008) 76–84. [13] H.Y. Deng, B.K. Zhu, X.Z. Wei, Y.Y. Xu, A positively charged nanofiltration embrane prepared by dynamic self-assembly of PSS, PSS-co-MA and PAH, Gongneng Cailiao (China) 39 (2008) 1313–1317. [14] R.H. Du, J.S. Zhao, Properties of poly (N, N-dimethylaminoethyl methacrylate)/polysulfone positively charged composite nanofiltration membrane, J. Membr. Sci. 239 (2004) 183–188. [15] C.Y. Ba, D.A. Ladner, J. Economy, Using polyelectrolyte coatings to improve fouling resistance of a positively charged nanofiltration membrane, J. Membr. Sci. 347 (2010) 250–259.

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