Positively charged capillary nanofiltration membrane with high rejection for Mg2 + and Ca2 + and good separation for Mg2 + and Li +

Desalination 420 (2017) 158–166

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Positively charged capillary nanofiltration membrane with high rejection for Mg2 + and Ca2 + and good separation for Mg2 + and Li+

MARK

Hai-Zhen Zhang, Zhen-Liang Xu⁎, Hao Ding, Yong-Jian Tang State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R & D Lab, Chemical Engineering Research Center, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

G RA P H I C A L AB S T R A C T

A R T I C L E I N F O

A B S T R A C T

Keywords: Polyethyleneimine Modified multiwall carbon nanotubes Positively charged Nanofiltration membrane Preparation

High water permeability and good separation property are greatly desired in water production due to energy concerns. To explore nanofiltration (NF) membrane with high permeability for cations separation, a positively charged NF membrane was fabricated via interfacial polymerization using polyethersulfone (PES) three-channel capillary ultrafiltration (UF) membrane as substrate, polyethyleneimine (PEI) as the aqueous precursor. The NF membrane preparation conditions were optimized. To enhance the permeability of the prepared NF membrane, modified hydroxyl contained multi-walled carbon nanotubes (MWCNTs-OH), grafting with piperazine (PIP), were utilized. The resultant NF membrane showed increased water flux from 20.8 L·m− 2·h− 1 to 56.1 L·m− 2·h− 1 at 4 bar after adding 0.01 wt% modified MWCNTs-OH in aqueous solution. Interestingly, MgCl2 rejection of the membrane also increased from 94.2% to 96.9%. The positively charged NF membrane exhibited above 97% rejection for divalent cations (Mg2 + and Ca2 +) and low rejection (< 70%) for monovalent cations (Na+ and Li+), and it also showed long durability and good separation for Mg2 + and Li+ when the membrane was used to separate mixed salts solution simulated the composition of salt lake brine. The fabricated membrane would have potential for effective water softening and for reclamation of lithium from brine or seawater with high Mg2 +/ Li+ ratio.

1. Introduction Freshwater and energy scarcity become worldwide problems due to

⁎

Corresponding author. E-mail address: [email protected] (Z.-L. Xu).

http://dx.doi.org/10.1016/j.desal.2017.07.011 Received 21 March 2017; Received in revised form 25 May 2017; Accepted 13 July 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.

the rapid development of global industry, the overgrowth of population and the pollution of available water resources. Membrane technology, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse

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vulnerable during module fabrication and nanofiltration process [31]. While the effective area is relatively small for NF membrane with inner selective layer. Capillary NF membrane with inner selective layer does not have the problems aforementioned [32]. In this work, a positively charged NF membrane was fabricated via IP using PES three-channel UF capillary membrane as substrate, PEI and TMC as aqueous precursor and organic monomer, respectively. The membrane preparation conditions have been systematically investigated. To improve the permeation of the prepared NF membrane, MWCNTs-OH were modified via grafting with PIP, the characteristics of the modified MWCNTs were analyzed by FTIR, TGA, TEM. Then the modified MWCNTs-OH were incorporated into the selective layer by adding them in the aqueous solution. Finally, the morphology and the permeation and rejection performance of the resultant membrane were observed and determined, respectively.

osmosis (RO) or forward osmosis (FO) and membrane distillation (MD), are extensively adopted for water treatment in the past decades [1–4]. In which, NF process has widespread applications in desalination, water softening and separation, and concentration of solutes, etc. [5–7]. Many researchers focus on developing and constructing composite NF membrane with high performance and long life-span. Nevertheless, most of the composite NF membranes are negatively charge currently, and NF membrane with negative charge has better rejection performance for anions according to Donnan effect [8]. However, the separation effect is unsatisfactory when these NF membranes are utilized in dealing with positively charged solutes such as cationic dyes and metal ions from electroplating wastewater. Thus, it is essential to fabricate NF membrane with positive charge. Various approaches have been used to fabricate the positively charged active layer on substrate surface, such as interfacial polymerization (IP) [9,10], chemical cross-linking [11,12], layer-by-layer assembly [13] and UV-induced photografting polymerization etc. [5]. IP has been a dominating method for fabricating composite NF membrane because the performance of the obtained membrane could be fine-tuned by varying the preparation conditions, e.g. the concentration of monomers, immersion time, reaction time and also the heat treatment temperature [14,15]. For the monomers, polyethyleneimine (PEI), a positively charged polymer, has been broadly employed to fabricate positively charged composite NF membranes [16–19]. However, the permeation of the composite NF membrane prepared using PEI should be improved considering energy efficiency [18]. Therefore, many efforts have been attempted to enhance the water permeability of this positively charge NF membrane through introducing a surfactant or coreactant in the aqueous phase. Fang et al. added sodium dodecyl sulfate (SDS) in PEI aqueous and investigated the additive amount on the permeation and salt rejection of NF membrane [20]. The membrane prepared under optimized conditions shows pure water permeability and MgCl2 rejection of 16.5 L·m− 2·h− 1·bar− 1 and 96.5%, respectively. Bera et al. used dextran (Dex) conjugate of PEI (PEI-Dex) as the comonomer in PEI aqueous solution to prepare composite NF membrane via IP [21], and the pure water permeate flux of membrane improved significantly without sacrifice of salt rejection. In addition, PIP was also employed as co-monomer with PEI to fabricate high-permeable NF membrane [22]. Carbon nanotubes (CNTs) are promising materials for the preparation of nanohybrids and nanocomposites with polymers [23]. At present, MWCNTs are widely used in improving the permeation performance of NF membrane for they can afford transport channels for water molecular [24]. However, the pristine MWCNTs cannot be used directly because of their poor dispersion. Many efforts have been tried to improve the dispersion of MWCNTs by introducing hydrophilic groups on the wall edge of MWCNTs. Liu et al. modified CNTs with hydroxyl and applied them in biomedical area [25]. Chan et al. synthesized zwitterionic CNTs and incorporated them into polyamide thin film composite membranes to improve the permselectivity [26]. The permeation flux of membrane increased by approximately three-fold after introducing zwitterionic CNTs. Zhao et al. adopted poly(dopamine) (PDA) to modified MWCNTs, and the PDA-MWCNTs show admirable dispersion in water [27]. Moreover, even a little amount of PDAMWCNTs can enhance membrane permeability greatly. In addition, sulfonated MWCNTs were synthesized from hydroxyl-functionalized multiwall carbon nanotube for the improvement of membrane water flux [28]. Although there are a number of publications using different CNTs to improve the permeability of membranes, to the best of our knowledge, there are no reports about the modification of hydroxyl contained MWCNTs with PIP. Recent years, more and more researchers focus on constructing composite NF membrane with hollow fibers as substrate [8,29,30]. The selective layer can be located at both the shell side and the lumen side of hollow fibers. The hollow fiber NF membrane with outer selective layer would have larger surface area, but the composite layer is

2. Experimental 2.1. Materials and chemicals Polyethersulfone (PES) capillary UF membrane with three-channel was prepared via non-solvent phase inversion method as reported in our previous work [32]. The inner diameter and outer diameter of this capillary membrane are about 1.39 ± 0.03 mm and 3.92 ± 0.08 mm, respectively, and the pure water permeability of the PES membrane is 161 L·m− 2·h− 1·bar− 1. The hydroxyl-functionalized MWCNTs (purity > 95 wt%, –OH content 5.58%, outer diameter < 8 nm) were purchased from Shenzhen Nanotech Port Co. Ltd. Polyethyleneimine (PEI) with a molecular weight of 70,000 Da was provided from Aladdin Reagent Co., LLC, Shanghai, China. Trimesoyl chloride (TMC, ≥ 98%), the active monomer in organic phase, was purchased from Qingdao Benzo Chemical Co., China. Piperazine (PIP, GR) was obtained from Sigma-Aldrich. Glutaraldehyde (GA), n-Hexane, PEG 400 (AR), PEG 600 (AR), sucrose (AR), raffinose (AR), glucose (AR), sodium sulfate (Na2SO4, AR), magnesium sulfate (MgSO4, AR), magnesium chloride (MgCl2, AR), calcium chloride (CaCl2, AR), sodium chloride (NaCl, AR), lithium chloride (LiCl, AR) and lithium sulfate (Li2SO4, AR) were provided from Sinopharm Chemical Reagent Co. Ltd. (China). 2.2. Modification of MWCNTs The hydroxyl-functionalized MWCNTs were modified via grafting with PIP using glutaraldehyde (GA) as the cross-linking agent. The modification process is described as follows: MWCNTs-OH powder (50 mg) was added into a conical flask with 100 mL pure water, and dispersed by 5 min sonication (KQ100DB, 100 W, Kunshan Ultrasonic Instruments). Subsequently, PIP (50 mg), 25% GA (0.5 g) and 0.1 mL hydrochloric acid (pH = 2) were added, then the mixture was strongly stirred in water bath at 50 °C for 12 h. Afterward, the mixture was centrifuged (10 min, 4000 rpm) to eliminate assembled residues. Finally, the modified MWCNTs powder was collected by filtration supernatant (washed with DI water), and dried at 40 °C under vacuum for 24 h. The hydroxy contained MWCNTs and the modified MWCNTs were denoted as CNTs and M-CNTs respectively in this paper. The diagram of the grafting reaction is shown in Fig. 1. 2.3. Membrane fabrication The NF membranes with inner selective layer were fabricated via IP, and the schematic preparation of the NF membranes is shown in Fig. 2. Prior to IP, the PES capillary membrane was first sealed in a module using epoxy resin, and each module contained one piece of membrane with an effective length of 50 cm. As described in our previous work [32], the fabrication procedure started with one membrane module that was held vertically. PEI contained aqueous solution was guided inside the PES substrate with a rotary pump at a flow rate of 0.3 L/min. Excess 159

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HO

OH

OH

CH

CH N

Fig. 1. Diagram of the grafting reaction.

NH

O NH +

OH

HN

O

O H C

C H +

heat, H

O

OH

OH

CH

CH N

NH

area of the membrane (m2), and t is the permeation time (h). The molecular weight cut-off and the pore size of the prepared NF membranes were determined using 300 ppm organics (neutral organics, e.g. glucose, sucrose, PEG 400, raffinose, PEG 600) aqueous solution as feed, and the separation performance of the NF membranes was measured using 2000 ppm inorganic salt solution as feed. The rejection can be calculated using the following equation:

solution was removed and the membrane were dried using clean compressed air. Similarly, TMC/n-hexane solution was extruded into the lumen side of PES substrate at 25.0 mL/min for certain time. Finally, the fabricated NF membranes were dried and then stored in deionized (DI) water. The substrate and the NF membranes prepared with PEI, PEI/CNTs and PEI/M-CNTs in aqueous were labeled as M0, MP, MPC and MPMC, respectively. The amount of CNTs or M-CNTs in aqueous solution was fixed at 0.01 wt%.

Cp ⎞ R = ⎛1 − × 100% Cf ⎠ ⎝ ⎜

2.4. Characterization

⎟

(2)

where Cp and Cf are the concentration of permeate and feed, respectively. The concentrations of organics were obtained by a TOC analyzer (Shimadzu, Model TOCVPN, Japan). The concentrations of mono-dispersed salt solution were measured with an electrical conductivity meter (DDS-11A, Shanghai Neici Instrument Company, China) and concentrations of cations in the mixed salts solution were determined by atomic absorption spectrometer. Each measurement was tested three times and the value averaged. All experiments were conducted at ambient temperature (20 ± 2 °C).

FTIR was utilized to characterize the chemical composition of CNTs and M-CNTs. Thermogravimetric analysis (TGA) (NETZSCH, STA409PC) was conducted to measure thermal stability over 20–600 °C (heating rate 10 °C min− 1 and in N2 atmosphere). Morphologies of CNTs and M-CNTs were visualized by transmission electron microscopy (TEM) (JEM-2100) with an accelerating voltage of 120 kV. The surface chemical composition of membranes was determined by ATR-FTIR. Morphology of the membranes was observed via SEM and AFM, and surface roughness was determined by AFM at tapping mode. Water contact angle was used to measure the hydrophilicity of membranes at ambient temperature (20 ± 2 °C).

3. Results and discussion

2.5. Measurement of membrane separation properties

3.1. Characterization of modified CNTs

The permeation and separation performance of NF membranes was conducted in cross-flow apparatus. All membranes were pre-filtrated with pure water at 4 bar for 30 min before test. The pure water flux of membrane was calculated as following:

ATR-FTIR was adopted to characterize the chemical composition of CNTs and M-CNTs and the results are presented in Fig. 3 (a). For CNTs, the characteristic absorption of eOH can be found, and the peaks at 2923 cm− 1 and 2851 cm− 1, the stretching vibration of CeH, is derived from the defects on the ends and sidewalls of CNTs [33]. The absorption at 1640 cm− 1 (C]C stretching vibration) and 1530 cm− 1 (C]C skeletal vibration) appears in CNTs [34]. In addition, the characteristic absorption of CeC at 1215 cm− 1 appears in both CNTs and M-CNTs.

PWF =

Q A×t

(1)

where Q is the volume of permeate pure water (L), A is the effective

Fig. 2. Schematic fabrication process of NF membrane.

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Fig. 3. Characterization of the modified MWCNTs: (a) FTIR spectra, (b) TG analysis, (c) TEM images, and (d) dispersion of MWCNTs (1) MWCNTs and (2) modified MWCNTs

Compared with CNTs, the absorption band around 3300 cm− 1 in MCNTs is much stronger which due to more eOH and eNH introduced via grafting. The absorption at 1712 cm− 1 and 1613 cm− 1 can ascribes to C]O and C ] OeOH in GA that unreacted or oxidized [35]. The peak at 1558 cm− 1 after modification (shown in M-CNTs) may due to the blue shift of C]C skeletal vibration (1530 cm− 1) which may be caused by grafting. The absorption at 1393 cm− 1 and 1011 cm− 1 are the in-plane bending vibration of eOH and the stretching vibration of CeO, respectively [36]. The peak at 1011 cm− 1 may also ascribe to CeN stretching vibration from PIP [22]. FTIR confirms the grafting reaction between CNTs and PIP. Thermogravimetric analyzer was employed to examine the thermal decomposition characteristics of M-CNTs and the TGA curves are shown in Fig. 3 (b). The weight loss of CNTs and M-CNTs are 10.6% and 27.8%, respectively, at temperature up to 600 °C, illustrating there are grafted segments on M-CNTs. From TEM images (Fig. 3 (c)), it can be found that the outer wall of M-CNTs roughened after modification compared with that of CNTs, which may be caused by the chain segments on M-CNTs. In addition, as a result of grafting modification, the dispersion of M-CNTs improved compared with that of CNTs shown in Fig. 3 (d). CNTs aggregated completely in 12 h while M-CNTs remained stable during this period, the improved dispersion of M-CNTs in water is due to the successful grafting of hydrophilicity segments onto M-CNTs.

concentration 0.15% and reaction time 30 s. The NF membrane fabricated under these conditions has a pure water flux of 20.8 L·m− 2·h− 1 and MgCl2 rejection of 94.2%. All the prepared NF membranes in this work were fabricated under the optimized conditions. 3.3. CNTs adding on the structure and properties of NF membrane The polyamide layer on membrane surface was confirmed by ATRFTIR and the spectrum of substrate and NF membranes are presented in Fig. 5. Compared with substrate, the new peaks at 2841 and 2957 cm− 1 in NF membranes are ascribed to the stretching vibration of CeH in PEI, and the absorption band around 3385 cm− 1, which corresponds to the eOH and unreacted eNH, also appears in all NF membranes. For MPMC, the absorption peak at 3385 cm− 1 is stronger than that of MP and MPC which may contribute by the eOH and eNH in M-CNTs. In addition, the peak at 1638 cm− 1 attributes to the C]O vibration in amide and the peak at 954 corresponds to CeN stretching vibration [11,20]. FTIR results verify the polyamide layer on membrane surface. The surface and cross-section morphology of substrate and NF membranes observed by FE-SEM are presented in Fig. 6. SEM images demonstrate that there are nanoscale pores on the surface of substrate, and the cross-section of substrate shows spongy-like structure. This structure of substrate can afford appropriate pore diameter for effective interfacial polymerization and robustness for high pressure operations [1]. For NF membranes, the surface is dense and no visible pores can be found. In which, MP membrane shows smooth surface, and there are lumps on the surface of MPC that may be caused by the aggregation of CNTs. In addition, there are fine grains on the surface of MPMC and these grains may form from the well dispersion of the modified M-CNTs. What's more, it can be found from the cross-section images that all NF membranes comprise a skin layer. The thickness of the selective layer on MP surface is about 100 nm, and thickness reduces when CNTs or MCNTs were added in the aqueous solution. The reason may lie in that the CNTs or M-CNTs in aqueous solution hinders the mass transfer of

3.2. Optimization of the fabrication conditions for NF membrane Selective layer is important for the separation performance of TFC NF membrane. Therefore, the parameters which may affect the formation of selective layer were investigated. The performance of the composite NF membranes was evaluated in terms of pure water flux and MgCl2 rejection, and the results are shown in Fig. 4. Considering both the permeation and rejection performance of membrane, the optimized fabrication conditions for NF membrane is as following, PEI concentration 0.6%, aqueous phase immersion time 15 min, TMC 161

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H.-Z. Zhang et al. 100

PWF R

98

94 10

PWF (L m-2 h-1)

96 15

96

30

(a) Rejection (%)

(b)

94

25 92 20

92

5

90

90

0 0.2

0.4

0.6

0.8

88

15

1.0

5

PEI concentration (%)

10

15

20

Aqueous phase immersion time (min)

35

100

100 30

PWF R

30

PWF R 95

90

15

PWF (L m-2 h-1)

20

96

(d)

25

(c)

Rejection (%)

PWF (L m-2 h-1)

25

Fig. 4. Optimization of preparation conditions of the positively charged NF membrane: effect of (a) PEI concentration, (b) aqueous phase immersion time, (c) TMC concentration and (d) reaction time on the permeation and rejection performance of NF membrane.

20

92

15

88

Rejection (%)

PWF (L m-2 h-1)

25

20

98

PWF R

Rejection (%)

30

85 10

80

5 0.15

0.30

0.45

0.60

10

20

TMC concentration (%)

30

40

Transmittance (%)

MP MPC

3385

4000

3500

2957 2841

3000 1800

60

enough from Fig. 3 (d) and some of them may aggregate on the surface of substrate during the immersion process, thus the roughness of MPC is higher than that of MP. It should be noticed that the surface roughness of MPMC is lower even than that of MP, and this may attribute to the well dispersion of M-CNTs and the interaction between PEI and MCNTs. M-CNTs would not assemble during immersion process of NF membrane fabrication due to the good dispersion, so the surface roughness of MPMC is lower than that of MPC. In addition, the grafted chains can react with TMC which may affect the reaction between PEI and TMC, thus reducing the surface roughness of MPMC. Water contact angle is an important parameter which can indicate the hydrophilicity of membrane. Dynamic water contact angle of membranes was measured and the results are shown in Fig. 9. The approximate 80 °C water contact angle of substrate illustrates its poor hydrophilicity. While all NF membranes show improved hydrophilicity after IP, in which MPMC shows better and MP shows poor. It is well known that hydrophilicity of membrane is associated with the surface chemical composition and the surface roughness of membrane. The improved hydrophilicity of NF membranes can be ascribed to the hydrophilic groups, amide groups and amino that unreacted, introduced via IP. The hydrophilicity of NF membrane further improved after adding CNTs or M-CNTs in aqueous solution because there are lowresistance water channels at the interface between CNTs and polyamide matrixes [27]. The grafted chains is favorable for the dispersion of CNTs as mentioned above, and the grafted hydrophilic chains and the good dispersion of M-CNTs endow the better hydrophilicity of MPMC membrane. The permeation and separation performance of NF membranes for deionized water and 2000 ppm MgCl2 aqueous solution were measured, respectively. And the results are presented in Fig. 10. It can be found from Fig. 10 that MP membrane shows a pure water flux of 20.8 L·m− 2·h− 1, and the pure water flux of NF membranes increased distinctly after adding CNTs or M-CNTs in aqueous solution during membrane fabrication. The water flux of MPMC is 56.1 L·m− 2·h− 1 and it is 40.7 L·m− 2·h− 1 for MPC. The salt rejections of all NF membranes are above 90%, and MPMC shows higher rejection while MPC shows lower. CNTs are widely used to improve the permeation of NF membranes [27,39,40] and there is no doubt that the water flux of MPC and MPMC is higher than that of MP. For MPMC membrane, the M-CNTs dispersed evenly in the selective layer which is beneficial for the water

M0

MPMC

50

Reaction time (s)

1638

1600

954

1400

1200

1000

800

-1

Wavenumber (cm ) Fig. 5. Optimization of the preparation conditions of the positively charged NF membrane.

monomers which reducing the thickness of the formed skin layer [37,38]. To verify the hypothesis that the grains on MPMC surface are caused by M-CNTs, TEM technology was conducted to observe the polyamide layer on MP and MPMC membrane and the TEM images are shown in Fig. 7. It can be found that there are carbon nanotubes embedded in polyamide layer on MPMC compared with that on MP, and some of these M-CNTs are not fully covered by the polyamide. Thus it can be concluded that the protuberant M-CNTs caused the texture structure on MPMC surface. The surface topology of membrane is important for membrane performance and the topography of the prepared membranes is shown in Fig. 8. The substrate shows smooth surface from AFM images, while the surface roughness increases after IP. The roughness of NF membranes follows the order, MPC > MP > MPMC. The reason lies in the polymerization reaction and the aggregation of CNTs. On account of the large molecular weight of PEI, the reaction between PEI and TMC is not so severe as that between PIP and TMC, so the formed selective layer is relatively smooth [22]. The dispersion of CNTs in water is not well 162

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Fig. 6. Surface and cross-section structure of the substrate and the NF membranes. (1- Top surface; 2- Cross-section). Fig. 7. TEM images of the selective layer on membrane surface.

Ra=3.01 nm M0

Rms=4.40 nm

Ra=13.1 nm MPC

Rms=16.8 nm

Ra=9.15 nm MP

Fig. 8. AFM images of the substrate and the NF membranes.

Rms=12.6 nm

Ra=5.78 nm MPMC

Rms=7.42 nm

flux of membrane. There are some hydroxyl groups on the wall of CNTs while the activity of hydroxyl is relatively low compared with that of amino in the reaction with TMC. Therefore, there would be neither polymers covered on the surface of CNTs nor chemical bonds between the polyamide formed from PEI, TMC and CNTs. Therefore, the salt

rejection of MPC decreased comparing with MP. It is encouraging that there are amino contained chains on M-CNTs which can react with TMC, so some formed polyamide would cover on the wall of M-CNTs, and also these chains can connect with PEI via TMC. Hence, the salt rejection of MPMC is higher than that of MP and MPC.

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Table 1 Rejections of MPMC membranes for different neutral solutions.

80

Water contact angle ( )

70

60

M MP MPC MPMC

50

Solution

Molecular weight (Da)

Stokes radius (nm)

Rejection (%)

Glucose Sucrose PEG 400 Raffinose PEG 600

180 342 400 504 600

0.365 0.471 0.471a 0.584 0.590a

77.1 94.2 95.9 96.8 97.5

a

± ± ± ± ±

1.7 2.1 1.3 0.8 1.1

From Ref. [43,44].

40 100

96.9

98

97.6

30 80

20 0

20

40

60

68.9

80

time (s)

60

PWF R

50

100

56.1

91.1 90

30

20

20.8

MgCl2

80

Fig. 10. Pure water flux and salt rejection of the prepared NF membranes tested with deionized water and aqueous solution of 2000 ppm MgCl2, respectively.

100

Rejection (%)

95

90

85

80

285 Da 75

200

300

400

500

MgSO4

Na2SO4

Li2SO4

NaCl

LiCl

weight cut-off (MWCO) and the pore size of MPMC are about 285 Da and 0.47 nm, respectively, and the MWCO and the pore size of MPMC is lower than that of NF membranes with parallel permeation performance [22,41]. In fact, the skin layer of MPMC contained M-CNTs. On one hand, the M-CNTs could provide the channels for water to pass through [24]. On the other hand, the high stiffness of M-CNTs could insure that the skin layer of NF membrane would not be compacted in some extent during nanofiltration [42], and this is helpful for the permeation of membrane. Therefore, MPMC has high water flux though the pore size of it is small. The rejections of MPMC for different salts were measured and the results are shown in Fig. 12. The rejections for MgCl2, CaCl2 and MgSO4 are all above 96%, and the rejections for Na2SO4, Li2SO4, NaCl and LiCl are relatively low. The salts rejection is mainly affected by Donnan exclusion and steric hindrance [45]. Since MPMC membrane is positively charged, there is no doubt that it has high rejections for MgCl2 and CaCl2 and low rejections for sulfates according to Donnan effect. While it should be noticed that the rejection of MgSO4 is higher even than that of MgCl2 which is not consistent with other reported results [9,10]. The reason may lies in the small pore size of MPMC membrane and the PIP contained chains on M-CNTs. The hydrated radius of Mg2 + (0.428 nm) and SO42 − (0.379 nm) is similar with or a little smaller than the pore radius of MPMC membrane (< 0.47 nm), respectively. Thus, steric hinerance may dominate during nanofiltration process of MgSO4 aqueous solution, also PIP contained chains on M-CNTs after IP is beneficial for the rejection improve of sulfates [22]. Therefore the rejection of MgSO4 is high. Monovalent cations (Na+, Li+) have weaker electrostatic repulsive interaction to the positively charged membrane compared with multivalent ones [16], in addition, the small hydrated radius and the high diffusion coefficient of Na+, Li+, Cl− can also contribute to the low rejection of NaCl and LiCl. And the higher rejection to NaCl than LiCl of MPMC membrane is consistent with that of other positively charged membrane [43]. The performance comparison of the fabricated membrane with other NF membranes prepared by adding inorganic materials is listed in

MPMC

MPC

CaCl2

Fig. 12. Rejection performance of MPMC membrane to different salts (2000 ppm).

10

0

20.3

0

85

MP

40

20

95

40.7

40

49.3

96.9

Rejection (%)

PWF (L m-2 h-1)

94.2

R (%)

Fig. 9. The hydrophilicity of the substrate and the NF membranes.

60

68.9

600

Molecular weight (Da) Fig. 11. Rejections of MPMC membrane to glucose, sucrose, PEG 400, raffinose and PEG 600 tested with 300 mg/L feed aqueous solution at 4.0 bar and 20.0 °C.

3.4. Separation performance of the MPMC membrane The rejection performance of MPMC membrane for organics with low molecular weight is shown in Fig. 11, and the Stokes radius of these organics is given in Table 1. It can be concluded that the molecular 164

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Table 2 Comparison of MPMC membrane to some reported NF membranes prepared by adding inorganic materials. Membranes materials

PWP (L·m− 2·h− 1·bar− 1)

Salt rejection (%)

Operation pressure (bar)

Solute concentration (ppm)

Ref.

PIP-MWCNTs/PEI/PES PDA-MWCNTs/PEI/PSf mHT/PES NH2-MWCNT/PIP/PSf SMWCNT/PIP/PES MWCNTs/PIP/PES ZnO/MWCNTs/PES MWCNTs/PEI/PSf PMMA–MWNTs/PIP/PSf CNTs/PES NH2-MWCNTs/PES

14.0 15.32 6.3 6.17a 13.2 17.6 4.18 4.5b 6.98 9.7 5.9

96.9 to MgCl2 91.5 to MgCl2 ~8% to NaCl 95.7% to Na2SO4 96.8 to Na2SO4 ~95% to Na2SO4 / 93.5 to MgCl2 99.0% to Na2SO4 87.2 to Na2SO4 65% to Na2SO4

4 6 4 10 6 6 4 6 10 4 4

2000 1000 500 2000 1000 1000 / 1000 2000 2000 200

This work [27] [46] [47] [28] [40] [48] [49] [37] [50] [51]

Flux for NaCl solution (2000 ppm). Flux for MgCl2 solution (1000 ppm). 100

60

Permeate flux (L m-2 h-1)

100

80

80

LiCl

40

60

50

Flux MgCl2

50

(b)

60 30 40 20 20

10

40

Flux Ca2+ Mg2+ Na+ Li+

30

20

60

40

0

1

2

3

4

5

time (day)

Fig. 13. Permeation and rejection performance of MPMC for simulated aqueous solution (a) The simulated solution was prepared using MgCl2 and LiCl, the salts total concentration is 2000 ppm (MgCl2 1866 ppm, LiCl 134 ppm) and Mg/Li mass ratio is 21.4; (b) The salts solution was prepared using CaCl2, MgCl2, NaCl and LiCl, and the concentration of Ca2 +, Mg2 +, Na+ and Li+ are 0.082 g/L, 30.3 g/L, 0.60 g/L and 1.42 g/L, respectively.

20

10

0

0

0

Rejection (%)

(a)

Permeate flux (L m-2 h-1)

b

Rejection (%)

a

0 0

4

8

12

16

20

time (h)

Table 2. The permeability of MPMC is better than most of the NF membranes prepared using MWCNTs, and also the salt rejection is outstanding. Therefore, MPMC membrane, with good permeability and rejection performance, would have potential in water treatment.

nanofiltration process of simulated brine.

3.5. Application of the MPMC membrane

MWCNTs-OH were modified by grafting PIP to improve its dispersion in water, FTIR, TGA and TEM techniques confirmed the existence of PIP grafted on MWCNTs-OH, and the dispersion of the modified CNTs was improved distinctly. A positively charged NF membrane was prepared via IP using PEI as aqueous precursor and TMC as organic monomer, and the M-CNTs were added in aqueous solution to enhance the permeability of the membrane. The existence of M-CNTs in the selective layer is very helpful for the improvement of membrane permeability and the pure water flux of membrane increases from 20.8 L·m− 2·h− 1 to 56.1 L·m− 2·h− 1. The salt rejection also increases from 94.2% to 96.9% which attributes to the PIP contained chains on M-CNTs. The positively charged NF membrane shows relatively small pore size, and both Donnan effect and size exclusion effect the separation of salts solution. The rejections of membrane for MgCl2, CaCl2 and MgSO4 are all above 97%, and the rejections for monovalent cations are below 70%. Also the membrane fabricated in this paper has good separation performance for Mg2 + and Li+ and the durability of the membrane is prominent. The high-flux positively charged NF membrane showing great potential for reclaiming lithium resource from brine or seawater with high Mg2 +/Li+ ratio.

4. Conclusions

The simulated aqueous solution based on the composition (Mg/Li mass ratio is 21.4) of East Taijiner brine in China was used to appraise the actual application performance of MPMC [52], and the results are shown in Fig. 13. When the salts concentration of the simulated solution is 2000 ppm (MgCl2 1866 ppm, LiCl 134 ppm), the solution flux of MPMC membrane is round 34 L·m− 2·h− 1 as shown in Fig. 13 (a). And the rejections of MgCl2 and LiCl are approximately 95% and 18% respectively during the long-time run, illustrating a 1.3 of Mg/Li ratio in the penetrating solution. The decreased flux of MPMC lies in the decline of the effective driving force of NF membrane. When 2000 ppm salts solution was used as feed, the osmotic pressure of the feed solution would increase, thus the transmembrane pressure reduced resulting in the decrease of the solution flux [27,53]. The slightly declined rejection of LiCl is caused by the low concentration in simulated solution. Fig. 13 (b) shows that the permeate flux of the salts solution decreases to 0.5 L·m− 2·h− 1 and the salt rejection also decreases especially for MgCl2 and NaCl, when the salt concentration is 130.1 g/L and there are Ca2 + and Na+ in the simulated solution. MgCl2 rejection is above 78%and LiCl rejection is 15% revealing a 3.0 of Mg/Li ratio in the penetrating solution. The apparently declined flux of MPMC may lie in two reasons. One is that the high concentration of the salts solution reduced the effective driving force significantly, as mentioned above. The other is that concentration polarization phenomenon and membrane fouling were unavoidable during nanofiltration process when using high concentration salts solution as feed, and the concentration polarization and fouling would lead to a decay in membrane permeate flux [20,54]. The declined rejections of cations are due to shield effect of high concentrations of Cl− [13,20]. In general, the MPMC membrane shows good separation ability for Mg2 + and Li+ and great stability during the

Acknowledgments The authors are thankful for the financial support received from the National Science and Technology Support Project of China (2014BAB07B01 and 2015BAB09B01), Project of National Energy Administration of China (2011-1635 and 2013-117) and the Key Technology R & D Program of Jiangsu Committee of Science and Technology in China (BE2013031).

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