Covalent organic framework modified polyamide nanofiltration membrane with enhanced performance for desalination

Author’s Accepted Manuscript Covalent organic framework modified polyamide nanofiltration membrane with enhanced performance for desalination Chongbin Wang, Zhiyuan Li, Jianxin Chen, Zhen Li, Yongheng Yin, Li Cao, Yunlong Zhong, Hong Wu www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)31077-8 http://dx.doi.org/10.1016/j.memsci.2016.09.055 MEMSCI14776

To appear in: Journal of Membrane Science Received date: 23 July 2016 Revised date: 14 September 2016 Accepted date: 14 September 2016 Cite this article as: Chongbin Wang, Zhiyuan Li, Jianxin Chen, Zhen Li, Yongheng Yin, Li Cao, Yunlong Zhong and Hong Wu, Covalent organic framework modified polyamide nanofiltration membrane with enhanced performance for desalination, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.09.055 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Covalent organic framework modified polyamide nanofiltration membrane with enhanced performance for desalination Chongbin Wanga,b, Zhiyuan Lic, Jianxin Chenc*, Zhen Lid, Yongheng Yina,b, Li Caoa,b, Yunlong Zhongc, Hong Wua,b a

Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

c

School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China d

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China.

*

Corresponding author: School of Marine Science & Engineering, Hebei University of Technology, Tianjin 300130, P R China. Tel: +86-22-60202759. [email protected] (J.X. Chen)

Abstract A novel thin film nanocomposite (TFN) membrane was prepared by incorporating covalent organic frameworks (COFs) into polyamide (PA) layer on a polyether sulfone (PES) substrate, through interfacial polymerization method. The porous structure of COFs (SNW-1) provided more passageway for water transport. Meanwhile, the reaction between SNW-1 and trimesoyl chloride (TMC) during interfacial polymerization formed strong covalent bonds for better interface compatibility. The surface hydrophilicity of the hybrid membrane was improved due

to the existence of amine-rich SNW-1. The influence of the interfacial polymerization conditions and piperazine (PIP) and the loading of SNW-1 on the membrane performance were investigated. The resultant PA-SNW-1/PES membrane with a SNW-1 loading of 1 g/m2 exhibited an increased pure water flux from 100 L m-2 h-1 MPa-1 to 192.5 L m-2 h-1 MPa-1 compared to the pristine PA membrane while the rejection to Na2SO4 maintained above 80%. Moreover, the membrane also showed long-term running stability.

Keywords: Thin film; nanocomposite; nanofiltration membrane; Covalent organic frameworks; Interfacial polymerization. 1. Introduction Membrane separation technique has gained tremendous attention due to its great potential applications in desalination, water softening and wastewater recycling, etc [1-4]. Nanofiltration (NF) is considered as an emerging pressure-driven separation process between reverse osmosis and ultrafiltration. Nanofiltration membranes with a pore size of 0.5-2 nm have great advantages in water treatment such as high retention to organic compounds with molecular weight above 200 Da or multivalent ions and low operating pressure [5]. Membrane flux and selectivity are the most important parameters in evaluating nanofiltration membranes [6]. Up to now, most of the NF membranes possess high selectivity but relatively low water flux. The performance of conventional polymer membrane is limited by the trade-off effect between permeability and selectivity. Nanocomposite membranes fabricated by incorporating nanoparticles into polymer matrix are considered the most desirable candidates for the

preparation of membranes with both improved permeability and selectivity. Several kinds of inorganic materials such as zeolite, silica particles, graphite, carbon nanotubes and titanium dioxide have been embedded into membrane structure in the preparation of NF membrane [7-10]. However, the inorganic materials are found to be likely to agglomerate in polymeric matrices, which present a great challenge in the preparation of hybrid membranes. Besides, it is noteworthy that the applications of inorganic materials in membrane are largely restricted as a result of the poor compatibility between inorganic particles and polymer phase which may lead to separation performance decline [11]. The above problems are expected to be solved by introducing alternative organic porous materials with not only intrinsic polymer characteristics, but also numerous pores and considerable hydrophilicity.

Covalent organic frameworks (COFs) is a burgeoning type of nanoporous materials synthesized by assembling organic building blocks linked solely via strong covalent bonds [12-15]. COFs have gained considerable interest in the field of gas separation, adsorption, catalysis and photoelectricity due to their specific characteristics such as permanent porosity, relatively low density and desirable thermal stability [16-20]. The entire organic-organic covalent bonds endow COFs better affinity with organic polymers, showing promising advantages over classical inorganic particles for the fabrication of composite membranes [21]. The pores from COFs may provide a passageway to allow water molecules through. Moreover, the synthesized COFs possess great potential for preparing highly stable membranes owing to their high thermal stability and water stability. Hence, the fabrication of COFs-based hybrid

membranes has been expected as to be an effective approach for liquid separation.

Thin film nanocomposite (TFN) membranes have attracted much interest in nanofiltration process. TFN membranes are usually fabricated via interfacial polymerization (IP) method [22-24]. For example, Wu et al. [8] prepared a mixed matrix membrane by embedding modified mesoporous silica nanoparticles into polyamide (PA) thin selective layer and the results showed that the water permeability was improved. Sorribas et al. [25] prepared a hybrid membrane with enhanced performance, using several kinds of metal-organic frameworks (MOFs) as fillers. These reports indicated that a higher loading of additives inside PA thin active layer could improve the flux significantly. However, this approach owing to incorporate fillers by adsorption, leads a generally low filler loading. To address this issue, an idea for fabricating membranes has been explored by a filtration-assisted assembly strategy instead of dipping substrates into aqueous phase.

In this study, the COFs modified thin film nanocomposite membranes were prepared by interfacial polymerization of trimesoyl chloride (TMC) and piperazine (PIP) monomers. The SNW-1, a kind of COFs with rich secondary amine groups was selected as the fillers, and the reaction between the -NH- groups of SNW-1 and the -COCl groups of TMC during interfacial polymerization may form strong covalent bonds to construct structurally stable membranes. To the best of our knowledge, the present work is the first attempt to utilize COFs as additives to prepared TFN nanofiltration membranes. The effect of SNW-1 on the morphology, hydrophilicity

and the thermal stability of the prepared membranes were analyzed. The influence of the amount of adsorption for piperazine, the interfacial polymerization reaction time and the loading of SNW-1 on the membrane desalination performance were investigated.

2. Experiment 2.1 Materials and chemicals Polyether sulfone (PES, E6020 P, flake form) from BASF Co. (Germany) was used to fabricate the microporous support layer and dried at 65 °C for 1 d before use. Poly (ethylene glycol) (PEG, MW=2000g/mol), terephthalaldehyde (TA) were obtained from Kermel Chemical Reagent Co. (Tianjin, China). Trimesoyl chloride (TMC) was purchased from Heowns Biochemical Technology Co. (Tianjin, China). Chloroform (CHCl3), n-heptane, N,N-dimethyl formamide (DMF), dimethylsulfoxide (DMSO) and tetrahydrofuran (THF) were purchased from Benchmark Chemical Reagent Co. (Tianjin, China). Ethanol, methanol, piperazine (PIP) and the inorganic salts including Na2SO4, MgSO4, NaCl were supplied by Guangfu Fine Chemical Research Institute (Tianjin, China). Deionized water was used in this work. 2.2 Synthesis of N-rich Schiff based COF (SNW-1) The synthesis of SNW-1 was described as follows. Briefly [26], 0.5 g melamine was dissolved in a solution containing 0.8 g terephthalaldehyde and 25 ml dimethylsulfoxide under a nitrogen atmosphere. Then, the mixture was heated to 180 °C for 3 days under vigorous stirring condition. The resulting yellow precipitate was separated by filtration with a Buchner funnel, washed with N,N-dimethyl

formamide, tetrahydrofuran and refined via Soxhlet extraction with methanol and tetrahydrofuran. Finally, the obtained SNW-1 was dried under vacuum at 120 °C overnight. The synthesis route and chemical structure of the SNW-1 was shown in Fig. 1. It's worth noting that the major pore size in SNW-1 was evaluated to be around 5 Å, allowing water molecular pass through. NH2 HN N CHO

NH2 N H2N

N N

N NH

HN N

N

NH2

NH N

HN N

NH2

N

5Å CHO

N N

HN HN N

NH2

:H,

NH N

N

+

HN

:C,

NH2

:O,

:N,

N N

N

NH HN N

N

NH2

NH N

NH2

:H2O

Fig. 1. The Synthesis route and chemical structure of the SNW-1

2.3 Fabrication of the TFN membrane PES microporous membrane was fabricated in laboratory as the supporting of TFN membrane. The fabrication of sheet support layer with a molecular weight retention above 60000 Da has been detailedly described in previous literature [27]. The SNW-1 was dispersed in 80 ml aqueous solution containing PIP (1 g/L) and sonicated for 30 min. The mixture was filtrated on the surface of PES support layer with a dead-end stirred cell filtration system which connected with a liquid receiver and an inert gases cylinder. An ultrafilter with an effective area 28.7 cm2 (model 8200, Millipore Co.)

was used as a filtration cell. The inner diameter of the filtration cell was 62mm. Afterwards, the excess solution on the surface of support layer was removed with filter papers. Then the PIP impregnated support layer was immersed into the organic phase for 2 min to form a polyamide layer. The organic phase was prepared by mixing TMC(1

g/L)

in

n-hexane.

Subsequently

the

resulting

TFN

membrane

(PA-SNW-1/PES) was air dried at room temperature for 30 min. The obtained membrane was washed with deionized water thoroughly and stored in deionized water. The PA/PES membrane without COFs in PIP solution was also fabricated for comparison. 2.4 Characterizations The morphology of the membrane was analyzed by utilizing a field emission scanning electron microscope from FEI Co., Ltd. (FE-SEM, Nanosem 430). Before tests, the samples of the PES and TNF membrane frozen in liquid nitrogen were broken and gold-sputtered. The Fourier transform infrared spectroscopy spectra of the obtained SNW-1 and membranes were obtained with a FT-IR analyzer (Bruker Vertex 80 V). XPS (XPS, PHI-1600) measurement was used to determine the near-surface element composition of the resultant membrane. A contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China) was used to measure the surfaces hydrophilicity at ambient temperature. Six random locations on the samples surface were taken to acquire an average value.

The thermal stability of membranes were studied using a thermal gravimetric analyzer (TGA, NETZSCH, TG209 F3) under a N2 atmosphere from 20 °C to 800°C at a heating rate of 10 °C min-1. 2.5 Separation performance of the prepared TFN membrane Separation performance of the prepared TFN membrane was evaluated using water flux and salt rejection. The device was mentioned in Subsection 2.3. All the samples of nanofiltration membrane were initially pretreated under 0.25 MPa operating pressure for 30 min before tests. After that the experiments were carried out at operation pressure of 0.2 MPa and stirring speed of 200 rpm at room temperature. Several salt solutions (1 g/L) including Na2SO4, MgSO4 and NaCl were used to evaluate salt rejection. The pure water flux (Fw, L m-2 h-1 MPa-1) and salt rejection (R%) were evaluated using the following equations (1) and (2) respectively,

FW 

V At P

(1)

where V (L) represented the total volume of permeated water, A (m2) was the membrane effective area, Δt (h) was the operation time and ΔP (MPa) was the operation pressure,  Cp R  1   C f 

  100% 

(2)

where Cp and Cf were the salt concentration of permeate and feed solutions, respectively. To measure the concentration of the salt solution, an electrical conductivity meter (DDS-11A, Leichi Instrument Co., China) was used to detect the conductivity of solution. Each TFN membrane was tested at least three times to get a

reliable data. 3. Results and Discussion 3.1 Fabrication of the TFN membrane Fig. 2 displayed the fabrication process of the TFN membrane. A mixture including PIP and SNW-1 was firstly filtrated on the PES microporous membrane. Then, the PIP impregnated membrane was immersed into the organic phase to form a polyamide (PA) layer after removing the excess water on the membrane surface with filter papers. The SNW-1 could react with the -COCl of TMC during interfacial polymerization and formed strong covalent bonds. To investigate the chemical reaction between SNW-1 and TMC, SNW-1-TMC compound was prepared via adding SNW-1 into TMC/n-hexane organic phase. Fig. 3 presented the FTIR spectra of the SNW-1 and SNW-1-TMC. The FTIR spectra of SNW-1 showed two peaks located at 1550 cm-1 and 1648 cm-1 which were assigned to the triazine ring, indicating SNW-1 synthesized successfully. To verify the reaction between the -NH- groups of SNW-1 and -COCl of TMC during polymerization reaction, a certain amount of SNW-1 were immerged into a n-hexane solution of TMC (1 g/L). A new peak at 1614 cm-1 was observed from the FTIR spectra of SNW-1-TMC compared to SNW-1. The appearance of the new peak was due to the C=O stretching vibration of the formed amide group, indicating the reaction between SNW-1 and TMC occurred as expected. Hence, the SNW-1 was covalently linked to the PA active layer which was beneficial for stability of the hybrid membrane.

3.2 Characterization of SNW-1 and membranes

The micromorphology and size of the SNW-1 were detected by TEM as shown in Fig. 4. The obtained SNW-1 was uniform sphere-like nanoparticles with an average diameter about 30 nm.

N

N N H

HN

N N

N

NH N

NH

N

H N

N N

NH

ClOC

+

COCl

+ N H

HN N

COCl

N N O N C

O N C

O C N

O C

X O C N

Y N

N N

N N

N

N

N

O C N

N

N N

N

N

NH

N

N N

N N

Fig. 2. The fabrication process of the TFN membrane

SNW-1

Transmittance

SNW-1-TMC

1550 cm-1

1480 cm-1

1614 cm-1 2400

2200

2000

1800

1600

1400

1200

1000

800

Wavenumber (cm-1 )

Fig. 3. FTIR spectra of the SNW-1 and SNW-1-TMC

Fig. 4. TEM images of the SNW-1. The surface and cross-section morphologies of the membranes were analyzed using SEM. Fig. 5a, 5b, 5c showed the surface morphologies of the PES membrane, PA/PES membrane and PA-SNW-1/PES membrane,respectively. Some visible pores with a mean diameter of 12-22 nm were observed on the surface of PES membrane (Fig.5a). After polymerization reaction between PIP and TMC, a polyamide layer was formed on the top surface of PES support layer (Fig. 5b). The coating layer led to an almost invisible pore compared with the PES membrane. The SEM image of the

PA-SNW-1/PES membrane showed that the SNW-1 were embeded into the polyamide layer upon the porous PES layer (Fig. 5c). Moreover, the surface of PA-SNW-1/PES membrane was rougher than the membrane of PA/PES. Fig. 5d and 5e presented the cross-sectional morphology of PA/PES and PA-SNW-1/PES, respectively. Both of them were composed of a porous supporting layer and a dense skin layer. The skin layer thickness of the formed layer was around 300 nm and the morphology was not considerably affected after incorporation of the SNW-1.

Fig.5. Surface SEM images of a) the PES membrane, b) the PA/PES membrane, c) the PA-SNW-1/PES membrane and cross-section SEM images of d) the PA/PES membrane, e) the PA-SNW-1/PES membrane. The changes in surface chemical composition of PA/PES membrane and PA-SNW-1/PES membrane were characterized using FT-IR spectrum and the results were presented in Fig.6. Compared with the PES membrane, the PA/PES membrane appeared two new absorption peaks at 1625 cm-1 and 1442 cm-1 on the FT-IR spectrum, which were characteristics of the C=O stretching vibration of the formed amide group. The results clearly demonstrated that the interfacial polymerization reaction between TMC and PIP occurred. Additionally, the adsorption peaks at 1550 cm-1 and 1480 cm-1 of PA-SNW-1/PES membrane were assigned to the triazine ring from SNW-1. These peaks verified that the fillers were incorporated into the NF membrane during the interfacial polymerization process.

Transmittance

PA-COF/PES

PA/PES

PES

1625cm

-1

1442cm

1550cm

-1

1480cm

1800

1700

1600

-1

1500

-1

1400

1300

1200

Wavenumber (cm-1 )

Fig.6. FT-IR spectra of the PES, PA/PES and PA-SNW-1/PES membrane

Furthermore, XPS measurement was used to determine the surface element composition of the resultant membrane. The spectra of PES membrane, PA/PES membrane and PA-SNW-1/PES membrane were shown in Fig.7 and the chemical composition values including carbon, oxygen, nitrogen and sulfur were exhibited in Table 1. Compared with the PES supporting layer, the PA/PES membrane presented a lower oxygen content which declined from 22.4 to 19.6%. The appearance of N1s peak at 400 eV was taken as one element from the polyamide layer. Furthermore, the sign of sulfur element which only existed in the PES membrane disappeared. The results indicated that a polyamide layer formed successfully during interfacial polymerization process, coating on the PES membrane. The nitrogen percentage further increased for the PA-SNW-1/PES membrane, confirming the incorporation of SNW-1 in polyamide layer. Fig. 7b presented the XPS spectra of N 1s for the PA-SNW-1/PES membrane which was resolved into three peaks at the binding energy of 397.0, 400.6 and 402.2 eV. These peaks were attributed to C–N–C, C=N–C and

C–N–C=O, respectively. The SNW-1 was the only origin of C=N–C, proving the SNW-1 incorporated successfully.

O1s

N1s

C1s

a)

PA-SNW-1/PES

b) C=N-C Intensity(a.u.)

PA/PES

PES

800

S2s

600

400

200

C-N-C CO-NH

S2p

0

408

Bingding Energy(eV)

406

404

402

400

398

396

394

Binding energy (ev)

Fig.7. XPS spectra of a) the PES, PA/PES and PA-SNW-1/PES membrane b) XPS

N1s core level spectra resolving results of the PA-SNW-1/PES membrane. Table 1. Chemical composition of the PES, PA/PES and PA-SNW-1/PES membrane surfaces.

Samples

C(%)

O(%)

N(%)

S(%)

PES

73.2

22.4

0

4.4

PA

75.4

19.6

4.7

0.3

PA-SNW-1/PES

69.6

13.5

16.9

0

Enhanced surface hydrophilicity plays an important role in improving the water flux of the NF membrane [29, 30]. Here, the static water contact angle was measured to evaluate the hydrophilicity of the obtained membrane. Fig. 8 showed the static water contact angle of the PA/PES membrane and PA-SNW-1/PES membrane. The water contact angle of the PA-SNW-1/PES membrane was 26.7±1.3o, which was much lower than the PA/PES membrane due to the existence of rich secondary amine on the SNW-1. This result indicated that the hydrophilicity of PA-SNW-1/PES

membrane increased significantly with the incorporation of SNW-1 into the polyamide layer.

Fig.8. Contact angles of PA/PES membrane (a) and PA-SNW-1/PES (b)

TGA was performed to analyze the thermal stability of the as-prepared membranes. Fig. 9 presented TGA curves of the PA/PES membrane, PA-SNW-1/PES membrane and SNW-1. Both the PA/PES membrane and the PA-SNW-1/PES membrane possessed two major stages in the decomposition process. The first weight loss in the range of 430–600 °C was due to the degradation of functional group from PES, PA and SNW-1. The second weight loss stage between 600°C and 800°C was mainly attributed to the decomposition of the PES backbone. In addition, compared to the SNW-1, the obtained hybrid membrane exhibited very less weight loss below 300 °C, indicating that the fabricated membrane possessed promising thermal stability.

100

SNW-1 PA/PES PA-COFs/PES

Weight (%)

80

60

40

20

0 100

200

300

400

500

600

700

800

Temperature (°C)

Fig.9. TGA curves of the PA/PES membrane, PA-SNW-1/PES membrane and SNW-1

3.3 Effects of fabrication conditions on the membrane performances 3.3.1 Effects of filtration time Fig. 10 presented the performance of PA-SNW-1/PES membrane fabricated using different filtration time under the condition of reaction time for 2 min and loading of 1g/m2 SNW-1 on membrane surface. The filtration time ranged from 10 min to 25 min. The salt rejection of the membrane significantly increased while water flux decreased with the filtration time increased from 10 to 20 min, then the salt rejection reached a maximum and water flux was identical. The absorption of PIP in membranes needs sufficient time since the wetting process of the pores is fairly slow during the filtration. At short period of filtration time, the amount of PIP in membrane was not sufficient to fabricate an integrate thin active layer, leading to a rather low salt rejection. When the filtration time exceeded 20 min, the amount of PIP in membrane was saturated and further filtration had no significantly effect on the subsequent interfacial polymerization process.

300 80

250 70

200

60

Flux Na2SO4

150

100

Rejection(%)

(Pure water flux(L/m2hMPa))

90

50

40

10

15

20

25

The time of filtration (min)

Fig.10. Effects of filtration time on the membrane performances 3.3.2 Effects of interfacial polymerization time Fig. 11 presented the performance of PA-SNW-1/PES membrane fabricated using different polymerization time under the condition of filtration time for 20 min and loading of 1g/m2 SNW-1 on membrane surface. The polymerization time ranged from 1 min to 4 min. As a consequence, the pure water flux decreased rapidly with the increase of reaction time which was due to the PA selectivity layer became dense. The formed thin film was not integrated enough to reject Na2SO4 in the first minute, because polymerization reaction was not sufficient, leading to defect in the membrane. [31]. The PA layer would tend to be flawless with the increase of reaction time, salt rejection to Na2SO4 enhanced obviously as the polymerization time extended from 1 to 2 min. When the reaction time exceeded 2 min, the rejection properties of the obtained membrane changed slightly but the pure water flux decreased still. The rejection to Na2SO4 changed a little with further reaction time due to self-limiting phenomenon which occurred during interfacial polymerization. With the increase of

reaction time, the thickness of PA layer was enhanced, which extended water channel

300

90

250

80

200

70

Flux Na2SO4 150

Rejection(%)

(Pure water flux(L/m2hMPa))

resulting in the pure water flux decreased.

60

100

50 1

2

3

4

Reaction time (min)

Fig.11. Effects of reaction time on the membrane performance 3.3.3 Effects of SNW-1 loading Fig. 12 presented the performance of PA-SNW-1/PES membrane fabricated using different SNW-1 loading under the condition of filtration time for 20 min and reaction time for 2 min. The loading of SNW-1 ranged from 0 to 1.3 g/m2. The pure water flux increased rapidly as the loading of SNW-1 in polyamide layer which could be explained by several factors. First of all, the hydrophilicity was improved by incorporating SNW-1, making water molecules easier to pass through the hybrid membrane. Second, the pores from SNW-1 could provide a thoroughfare to allow water molecules pass. In addition, the voids at the interface between SNW-1 and polymer could increase the pure water flux to some degree [32]. The influence of SNW-1 loading on the separation performance of the hybrid membrane was examined utilizing several salt solutions including NaCl, MgSO4 and Na2SO4. It could be seen

that the property of the NF membrane in terms of salt rejection decreased slightly with increasing SNW-1 loading. The results complied with the following order: Na2SO4>MgSO4>NaCl. The membrane with a PA layer is a type of negatively charged membrane which endows it a stronger repulsion to dianionic anions than monovalent anions. Combining with the size sieving effects, the rejection of Na2SO4 and MgSO4 was higher than NaCl [33]. Obviously, 1 g/m2 SNW-1 loading was likely considered the best COFs concentration because of the fabricated membrane possessed a pure water flux of 192.5 L/m2hMPa while the rejection to Na2SO4 at 83.5%. The water flux of fabricated membrane was greatly potential for industrial applications and simultaneously maintained an acceptable rejection to Na2SO4. Compare flux and rejection results with other thin film nanocomposite membranes containing different fillers, our membranes exhibited excellent properties (see Table S1, Supporting Information).

250

100

Mg2SO4

NaCl

200

80

150

60

100

40

50

20

0

0

0.7

1.3

1

Rejection(%)

Na2SO4

2

(Pure water flux(L/m hMPa))

Flux

0

2

SNW-1 loading (g/m )

Fig.12. Effects of SNW-1 loading on the membrane performances 3.4 Long-time operation stability of the PA-SNW-1/PES membrane The stability of membrane was an important parameter in practical application

which was examined with pure water and Na2SO4 solution. The membrane was carried out at a 0.2MPa operation pressure for 72 h to evaluate the stability of the membrane. As presented in Fig. 13, the pure water flux and salt rejection varied slightly during long-time running. The results clearly showed that the prepared membrane possessed great potential for long-time operation.

90

200

70

150

Flux Rejection

Rejection(%)

Pure water flux(L/m2hMPa)

80

60

50 100 0

10

20

30

40

50

60

70

Operating time/h

Fig.13. The long-time operation stability of the PA-SNW-1/PES nanofiltration membrane 4. Conclusions A novel COFs (SNW-1)/polyamide (PA) TNF membrane (PA-SNW-1/PES) was fabricated by an improved interfacial polymerization between PIP and TMC on the PES membrane, incorporating the SNW-1 into the PA layer. The SNW-1 was incorporated into membrane via a filtration-assisted assembly strategy. The hydrophility of modified membranes was improved obviously due to the existence of rich secondary amine in the structure of SNW-1. The addition of SNW-1 facilitated the water transfer across the TFN membrane without sacrificing the salt-rejecting ability owing to the suitable pore size and the high porosity of SNW-1. Besides, the

SNW-1 was covalently linked to the polyamide layer which was also beneficial for the stability of membranes. The pure water flux of the obtained TFN membrane reached 192.5 L m-2 h-1 MPa-1, while the rejection to Na2SO4 was maintained at a relatively high level of above 80%. In addition, the membrane also exhibited a promising long-time stability. The results suggest that COFs is a potential candidate in the fabrication of high performance TFN nanofiltration membranes.

Acknowledge The authors gratefully acknowledge financial support from the National Natural Science Fundation of China (21276063, 21476059 and 21576189), Hebei Science and Technology Support Program (16273101D) and the Key Project of Natural Science Foundation of Tianjin (16JCZDJC36500).

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Highlights ● The amine-rich COFs is incorporated into polyamide layer to prepare hybrid membranes. ● Efficient water channels are fabricated in membrane by incorporating the COFs.

● The covalent bonds between COFs and polymer is benefit for interface compatibility. ● The water flux increased distinctly while salt rejection changed slightly.