Enhancing permeability of thin film nanocomposite (TFN) membranes

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Enhancing permeability of thin film nanocomposite (TFN) membranes via covalent linking of polyamide with the incorporated metal-organic frameworks (MOFs) Hengrao Liu, Jing Gao, Guanhua Liu, Miyu Zhang, and Yanjun Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00772 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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Industrial & Engineering Chemistry Research

Enhancing permeability of thin film nanocomposite (TFN) membranes via covalent linking of polyamide with the incorporated metal-organic frameworks (MOFs)

Hengrao Liu a, Jing Gao a, Guanhua Liu a, b, Miyu Zhang a, Yanjun Jiang a, c, *

a

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

b

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

c

Engineering Laboratory for Energy Conservation in Chemical Process Integration and Resources Utilization，Hebei University of Technology, Tianjin 300130, China

* Corresponding

author: Y. Jiang ([email protected])

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Abstract ： Thin film nanocomposite (TFN) membranes were invented in 2007 to enhance the permeability of thin film composite (TFC) membranes. Surface modification of nanofillers was a common method to improve the interaction and compatibility

at

amino-functionalized

polymer/nanofiller

interfaces.

Zirconium-based

metal-organic

Accordingly, framework

as

an

(MOF),

UiO-66-NH2 was synthesized and introduced into the preparation of TFN membranes via interfacial polymerization in this study. The super-hydrophilic characteristic of UiO-66-NH2 made it possible to be well dispersed in aqueous solution and the amino groups on particle surfaces could react with 1,3,5-benzenetricarboxylic acid chloride (TMC) to form covalent interaction with polymer thus inhibiting the formation of non-selective defects at PA/nanofiller interfaces. The morphology images and FT-IR spectra revealed PA selective layer successfully formed on the top of hydrolyzed polyacrylonitrile (HPAN) supports. The EDX characterization demonstrated UiO-66-NH2 nanoparticles were successfully introduced into the TFN membranes. The UiO-66-NH2 nanoparticles increased the surface hydrophilicity and roughness of the membranes, and provided additional passageways for mass transfer. Pure water permeability increased from 6.89 LMH/bar for TFC membrane to 14.55 LMH/bar for TFN-0.10 membrane indicating the distinct permeability elevation after the

2


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incorporation of UiO-66-NH2 nanoparticles. And the Na2SO4 rejection of TFN-0.10 membrane was up to about 99.0% and NaCl rejection was 38.1% at 4 bar, which was higher than that of TFN membrane incorporated with pristine UiO-66.

Key Words: Membrane separation; TFN membranes; UiO-66-NH2; interfacial polymerization; interfacial interaction; desalination/nanofiltration

1. Introduction Separation process is one of the largest energy consumers in chemical industry due to its thermally-driven characteristic which also brings negative effect on environment 1. For instance, distillation and the related processes consume about 40% of total energy for a chemical plant 2. So it is of utmost importance to exploit an energy-efficient and environmental friendly separation technology. Compared with the conventional separation technologies, membrane separation is considered as a promising and alternative technology thanks to its simple operation, no phase transition 3，no chemical reactions involved and no secondary waste products 4, 5. Polyamide (PA) thin film composite (TFC) membranes prepared by interfacial polymerization are widely used in nanofiltration and reverse osmosis for water treatment. In order to enhance the permeability of TFC membrane, thin film nanocomposite (TFN) membrane, which could overcome the trade-off relationship

3



between permeability and selectivity, was firstly reported by Hoek's research group in 2007 6. Whereafter, various kinds of nanoparticles, such as zeolite, SiO2, TiO2, etc., were used as fillers for the preparation of TFN membranes 7. But one of the greatest challenge during TFN membrane preparation is the generation of non-selective defect between inorganic nanoparticles and polymer matrix 8. This phenomenon is due to particle aggregation and poor compatibility between nanoparticles and polymers. Generally, hydrophilic nanoparticles can be better dispersed in aqueous solution. Zhu et al. synthesized ZIF-8, a kind of metal-organic frameworks (MOFs), and modified it with poly(sodium 4-styrenesulfonate) to improve the surface hydrophilicity followed by dispersion into aqueous phase to form PA membrane during the subsequent interfacial polymerization 9. In their research, the permeability of the TFN membrane doubled than that of TFC membrane, while the rejection was obviously decreased especially at high pressure (8 bar). For example, the Na2SO4 rejection declined from above 95% for TFC to less than 75% for TFN-mZIF3. The reason of this phenomenon, as described by the author, was the existence of non-selective interfacial defects between mZIF nanoparticles and PA caused by particle aggregation. To achieve better compatibility between nanofillers and polymer, Huiqing Wu et al. modified mesoporous silica nanoparticles with amino groups (-NH2) and introduced this materials into interfacial polymerization

10.

In their research, the -NH2 on the

mesoporous silica surface could react with 1,3,5-benzenetricarboxylic acid chloride (TMC) during interfacial polymerization process, which led to covalent bonding between nanofillers and polymer matrix to improve the compatibility at interfaces.

4


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Metal-organic frameworks, on account of the nano-sized porous structures made up of metal clusters and various organic ligands via strong coordinative bonds

11,

possessed inter-connective pores thus providing additional passageways for mass transfer

12.

Therefore, MOFs are considered as ideal fillers to fabricate TFN

membranes. Zirconium-based MOFs have attracted great attention of researchers because of their high physical and chemical stability characteristic

15

13, 14,

super-hydrophilic surface

， as well as changeable organic ligands with various functional

groups such as hydroxy (-OH), amine (-NH2), carboxyl (-COOH), etc 16, 17. Ma et al. explored UiO-66 in the fabrication of forward osmosis membranes, and the optimum performance of UiO-66 incorporated membrane was 50% increase in water flux over the pristine membrane with good salt rejection

18.

Gao et al. prepared mixed matrix

membranes containing UiO-66-NH2 by vapor-phase crosslinked method for the first time. The optimal composite membrane showed excellent dye rejection properties and good stability in organic solvent nanofiltration applications

19.

He et al. fabricated

nanofiltration membranes incorporated with UiO-66 to obtain increased permeability and rejection property for selenium and arsenic removal from water 20. As a type of Zr-based MOF, UiO-66-NH2 consists of Zr6O4(OH)4 clusters and 2-amino-1,4-benzenedi-carboxalate (NH2-BDC) ligands. As mentioned above, the super-hydrophilic surface characteristic make it possible to be well dispersed in aqueous solution and the intrinsic existence of -NH2 on the outside surfaces of UiO-66-NH2 nanoparticles can react with TMC during interfacial polymerization to generate covalent amide bonds, which can enhance the interaction and compatibility

5



between UiO-66-NH2 nanoparticles and PA polymer thus eliminating or inhibiting the formation of non-selective defects at PA/UiO-66-NH2 interfaces so as to keep up rejection property of TFN membranes in the case of increased permeability. The illustration of covalent linking in the membrane is shown in Fig. 1. In this study, UiO-66-NH2 nanoparticles were synthesized and homogeneously dispersed into aqueous piperazine (PIP) solution with the assistance of ultrasonic. The nanofiltration membranes were prepared through interfacial polymerization between aqueous PIP solution and TMC/n-hexane solution on the surface of hydrolyzed polyacrylonitrile (HPAN) supports. The sketchy preparation process is shown in Fig. 2. The physicochemical properties of UiO-66-NH2 nanoparticles and the membranes were optimized and characterized. The salts rejection performance of the TFC and TFN membranes was extensively evaluated.

Fig. 1. Illustration of covalent linking between UiO-66-NH2 and PA

6


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Fig. 2. The preparation process of TFN membranes

2. Experimental section 2.1. Materials and Chemicals Polyacrylonitrile (PAN) ultrafiltration membrane was purchased from Beijing Separate Equipment Co.Ltd (Beijing, China). NaOH, Na2SO4, NaCl, piperazine, n-hexane, and 1,3,5-benzenetricarboxylic acid chloride (TMC) of analytical grade were purchased from Macklin Biochemical Co.Ltd (Shanghai, China). Zirconium tetrachloride (ZrCl4), N,N-dimethylformamide (DMF) and acetic acid glacial were purchased from Aladdin Biochemical Technology Co. Ltd (Shanghai, China).

7



2-aminoterephthalic acid (NH2-BDC) was purchased from Dibai Chemical Technology Co.Ltd (Shanghai, China). Ultrapure water was used throughout the experiments. 2.2. Synthesis and concentration calibration of UiO-66-NH2 nanoparticles UiO-66-NH2 nanoparticles were synthesized via a solvothermal method similar to UiO-66

16.

In a typical synthesis procedure, ZrCl4 (0.686 mmol, 0.1598 g) and

NH2-BDC (0.686 mmol, 0.1242 g) were dissolved in 40 mL DMF via alternate ultrasonication and magnetic stirring to obtain clear and pale yellow solution. 1.2 mL acetic acid glacial was added into the solution as modulator to improve crystallization during reaction. Then 20 μL water was added, which was essential to obtain well-ordered regular octahedron UiO-66-NH2 morphology and increase yield of product. The mixed solution was sufficiently stirred and heated at 100 ℃ for 24 h. The resulting product was centrifuged at 10000 r/min for 15 min, and then washed with DMF for three times and water for two times to thoroughly remove unreacted reagents. The purified UiO-66-NH2 nanoparticles were re-dispersed in 10 mL water by stirring and ultrasonication to achieve a homogeneous suspension. 1 mL of the suspension was dried at 100 ℃ overnight, and then weighed to obtain the concentration of UiO-66-NH2 in suspension. 2.3. Modification of PAN ultrafiltration membrane PAN ultrafiltration membranes were used as supports to prepare TFC and TFN 8


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membranes. Prior to interfacial polymerization, the PAN supports were immersed into 1 mol/L NaOH solution at 50 ℃ for 2 h to hydrolyze cyano groups into carboxyl. The carboxyl could adsorb PIP and enhance the interfacial compatibility between PA layer and PAN support. 2.4. preparation of TFC and TFN membranes TFC and TFN membranes were prepared by interfacial polymerization according to the following procedures shown in Fig. 2. For TFC, the previous hydrolyzed PAN support was firstly immersed into aqueous PIP solution (0.15% w/v) for 20 min to adsorb PIP sufficiently. Afterwards, the HPAN support was taken from aqueous solution and the excess solution on PAN support surface was removed by absorbent paper. Then the hydrolyzed PAN support with PIP inside was immersed into TMC solution (0.15% w/v) for 2 min to obtain a PA layer on the surface of HPAN support via interfacial polymerization. Soon after that, the membrane was washed with n-hexane 3 times to remove the unreacted TMC solution. Then, the as-prepared membrane was dried at 60 ℃ for 20 min to further polymerize between PIP and TMC. The preparation of TFN membranes was similar to that of TFC except adding UiO-66-NH2 into the aqueous solution. The TFN membranes, named as TFN-0.05, TFN-0.10 and TFN-0.15, were prepared with UiO-66-NH2 concentration of 0.05% w/v, 0.10% w/v and 0.15% w/v, respectively. The concentration of UiO-66-NH2 nanoparticles in aqueous solution can be identified by adding different volume of the as concentration calibrated suspension.

9



2.5.

Characterization of UiO-66-NH2, TFC and TFN membranes Various kinds of characterization methods and analytic techniques were used to

research the synthesized UiO-66-NH2 nanoparticles, and the prepared TFC and TFN membranes. The morphology of UiO-66-NH2 nanoparticles was observed by field emission scanning electron microscopy (FE-SEM FEI Nova NanoSEM450). Prior to analysis, UiO-66-NH2 nanoparticles were dispersed in ethanol and dripped onto silicon wafers. The X-ray diffraction (XRD) patterns of UiO-66-NH2 nanoparticles were recorded on an X-ray diffractometer (BRUKER AXS D8 FOCUS) at room temperature, using Cu Kα as excitation radiation source. The 2θ degree was 5 to 60° with scanning speed of 5°/min. Fourier transform infrared (FTIR) spectra of UiO-66-NH2 and membranes were obtained by an ATR-FTIR spectrometer (BRUKER VERTEX 70). Size distribution and zeta potential of UiO-66-NH2 were measured by Malvern Zetasizer Nano ZS-90 particle size and potential analyzer. Prior to analysis, UiO-66-NH2 nanoparticles were dispersed in water with the assistance of ultrasonication for 1 h. Surface morphologies together with elemental analysis of TFC and TFN membranes were obtained by FE-SEM equipped with an energy-dispersive X-ray (EDX) spectrometer under 30 kV acceleration voltage. Before analysis, membrane samples were coated with gold particles to increase conductivity. The surface wettability of membranes was measured via optical contact angle measuring device (KRUSS DSA-100, Germany). The zeta potential of the membrane surfaces was obtained using SurPASS electrokinetic analyzer (Anton Paar GmbH, Austria), with 10


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KCl solution (0.2 mM) as background. The membrane roughness was investigated by FastScan Atomic Force Microscope (AFM, Bruker, America) with membrane scan area of 10 μm×10 μm. 2.6. Permeability and separation property The separation properties and permeability of TFC and TFN membranes were measured via a lab-made dead-end filtration cell with an effective membrane area of 20.4 cm2. The permeability was measured using ultrapure water. Na2SO4 solution (0.007 mol/L) and NaCl solution (0.017 mol/L) were used to evaluate membrane separation properties. Before measurement, the membranes were stabilized at 5 bar for 1 h, then varied pressure to 2, 4, 6 and 8 bar, respectively. The permeation flux (J, L m-2 h-1, LMH) is calculated via equation 1:

J

V A  t

(1)

where V (L) is the volume of liquid at permeate side, A (m2) is the effective area of membranes and Δt (h) is the operation time. And the rejection property (R) is calculated as follows:

R  (1 

Cp Cf

) 100%

(2)

CP and Cf are concentrations in permeate and feed solutions, respectively. The concentrations of inorganic salt are measured by a conductivity meter (Rex, DDSJ-308A). Prior to experiments, standard curves of salt conductivity for

11



concentration were plotted with solute concentrations at different levels. All membranes were tested at least three times to obtain an exact value. 3. Results and discussion 3.1. Characterization of UiO-66-NH2 nanoparticles The FE-SEM image of UiO-66-NH2 is shown in Fig. 3. The octahedron shape made up of eight regular triangles could be clearly observed. Inorganic fillers with smaller size could play a better role in improving permeation flux, because they could be more homogeneously dispersed in selective layer leading to stronger interaction with organic components

20.

As shown in the FE-SEM image, the synthesized

UiO-66-NH2 nanoparticles exhibited almost uniform size of about 100 nm or less, which was small enough to be applied in the preparation of TFN membranes. In order to further investigate the particle size, size distribution of the synthesized UiO-66-NH2 nanoparticles was shown in Fig. S1. The result demonstrated that the particle size spanned from 43.8 to 164.2 nm with an average size of 80.0 nm, which fitted well with the FE-SEM. The proportion of particle size from 50 to 120 nm accounted for above 90% with polymer dispersity index of 0.162. The narrow particle size distribution of UiO-66-NH2 nanoparticles could be beneficial for homogeneous dispersion in the PA selective layer. The regular surface morphology, relatively small particle size and narrow size distribution revealed a commendable method for the synthesis of nanosized UiO-66-NH2 nanoparticles. 12


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Zeta potential of UiO-66-NH2 nanoparticles was shown in Fig. S2. The zeta potential ranged from 37.8 to 97.0 mV with maximum intensity of 71.1. All of the UiO-66-NH2 nanoparticles displayed positive charge for the existence of -NH2 in organic ligands and Zr ions on particle surface. Generally, surface charge has significant influence on the dispersity of nanoparticles because the electrostatic repulsion between nanoparticles can prevent their aggregation

21.

In addition,

UiO-66-NH2 nanoparticles consisted of polar chemical contents such as -NH2, -COOH and Zr ions which were compatible well with water based on the principle of “like dissolves like” because water is a kind of strongly polar solvent.

Fig. 3. FE-SEM image of UiO-66-NH2 nanoparticles

13



1708

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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NH2-BDC UiO-66-NH2 UiO-66-NH2-TMC UiO-66-TMC

4000

3500

3000

2500 2000 1500 Wave number (cm-1)

1000

500

Fig. 4. FT-IR spectra of NH2-BDC, UiO-66-NH2 , UiO-66-NH2-TMC and UiO-66-TMC

The synthesized UiO-66-NH2 was washed with DMF for three times to remove the unreacted reagents such as ZrCl4 and NH2-BDC in the internal pore and on particle surface, then washed with water three times to remove DMF. The FT-IR spectra of UiO-66-NH2 and NH2-BDC were measured and shown in Fig. 4. The absorption peak at 1708 cm-1 was attributed to carboxylic (-COOH) stretching vibration

22,

and the absorption band at 3500~3300 cm-1 was partly attributed to

amino stretching vibration 23. Comparing UiO-66-NH2 with NH2-BDC, the absorption peak at 1708 cm-1 stood out observably in NH2-BDC but disappeared in UiO-66-NH2, which demonstrated that nearly all carboxyl groups in NH2-BDC were coordinated with Zr ions leading to the absence of free carboxyl groups in UiO-66-NH2. The

14


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results also revealed that all NH2-BDC inside the inner pore and outside the surface of UiO-66-NH2 nanoparticles were removed thoroughly. In order to verify the reaction between UiO-66-NH2 and TMC, the UiO-66-NH2 and UiO-66 nanoparticles were added to TMC/n-hexane solution and then washed by n-hexane to thoroughly remove unreacted TMC. Then the above materials were disposed in water. As shown in Fig. 4, compared with UiO-66-NH2 and UiO-66-TMC, the absorption peak at 1708 cm-1 appeared obviously in UiO-66-NH2-TMC, which was attributed to -COOH stretching vibration of hydrolyzed TMC indicating the reaction between TMC and UiO-66-NH2. It could be inferred that during the preparation process of TFN membranes, UiO-66-NH2 can take part in interfacial polymerization, which makes MOF nanoparticles link with PA via covalent amido bonds thus improving the interaction and compatibility at nanoparticle/polyamide interfaces and decreasing the formation of non-selective interfacial defects. For most unfunctionalized nanoparticles, they can only interconnect with PA through non-covalent weak interaction due to the absence of -NH2 groups. MOFs are a kind of coordination compound constructed by inorganic metal ions and organic ligands via coordination bonds. The strength of coordination bonds plays a predominant role in the stability of MOFs. Water solution or high temperature can destroy the stereochemical structure via hydrolysis or dissociation of chemical bonds 14, 24, 25.

For example, the structure of isoreticular metal-organic framework (IRMOF)

shows low stability in water because water molecules tend to substitute ligands to 15



coordinate with metal ions and consequently leading to dissociation of the IRMOF structure. The fillers used to prepare TFN membranes for water treatment need to be stable in water. In order to illustrate the long-term stability of UiO-66-NH2, XRD patterns of UiO-66-NH2 nanoparticles before and after exposed in water for 10 days were shown in Fig. 5. The XRD patterns matched well with the reported literatures 26-28.

And after exposed to water for 10 days, the XRD pattern remained almost the

same as before. Taking into account the above results, it could be concluded that UiO-66-NH2 nanoparticles with high crystalline degree were synthesized by the above method and these materials were highly stable in water and suitable for the preparation of TFN membranes for water treatment.

UiO-66-NH2 UiO-66-NH2 exposed to water for 10 days

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

20

30 2θ (°)

40

50

Fig. 5. XRD patterns of UiO-66-NH2 nanoparticles

16


60

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3.2. Characterization of the TFC and TFN membranes 3.2.1. Morphology of TFC and TFN membranes

17



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Fig. 6. Surface morphology images and elemental analysis of TFC (a,b), TFN-0.05 (c,d), TFN-0.10 (e,f) and TFN-0.15 membranes (g,h)

The FESEM image of HPAN support membrane was displayed in Fig. S3, the pore diameter of the support membrane was about 10 nm or smaller. In order to study the influence of UiO-66-NH2 nanoparticles on the morphology of the TFN membranes, surface FE-SEM images and elemental analysis

29

of the prepared

membranes were presented in Fig. 6. For the TFC membrane without adding UiO-66-NH2 nanoparticles into PIP solution during the preparation process, a typical “ridge and valley” network structure composed of striped Turing structures distributed on the HPAN support

30.

The white light stripes represented “ridges”, and the dark

regions corresponded to “valleys”. Generally, the reaction kinetic of the interfacial polymerization indicates that many parameters affect the property of PA layer: partition coefficient, reactivity ratio, diffusion rates of the reactants, etc. Due to the low partition coefficient of TMC in water, the polymerization takes place initially on the organic side of the water/n-hexane interface, and then the PA layer grows into the organic phase 30, 31. The morphology of PA layer depends on the diffusion coefficient of PIP molecules. Upon most occasions, the diffusion coefficient of PIP is a little less than that of TMC within an order of magnitude, hence the morphology of PA layer is smooth. However, when the system meets great difference in diffusion coefficients of PIP and TMC resulting in diffusion-driven instability, the striped Turing structure will appear. 18


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In this research, the striped netlike morphology of TFC membrane was a kind of Turing structure. The HPAN support possessed hydrophilic pores with plentiful -COOH that could form hydrogen bonds with PIP molecule thus increasing the interaction between PIP and HPAN support and reducing the diffusion rate of PIP to the water/oil interface. As a result, the parameters of preparing TFC membrane met the condition of forming Turing structure, and the striped Turing structure morphology was presented on the hydrophilic support. The formation of this typical morphology revealed that the PA active layer was successfully formed on the HPAN support. For TFN membranes, comparing with TFC, certain number of spots could be observed within the striped networks in TFN-0.05. With the increase of UiO-66-NH2 loading amount, the spots in TFN-0.10 came to be more numerous and denser than that in TFN-0.05. Further increasing the loading amount, the striped networks in TFN-0.15 were overwhelmed by spots, leading to the inconspicuousness of striped Turing structure morphology. The formation mechanism for morphology of TFN membranes could be explained as follows. The hydrophilic UiO-66-NH2 nanoparticles adsorbed a certain amount of PIP, therefore, when contacted with TMC, the polymerization reaction occurred on the surface of UiO-66-NH2 leading to the nucleation of PA and the formation of PA spots within the striped networks. With the increase of UiO-66-NH2 loading amount, excess spots appeared and the striped networks became unclear. EDX characterization was carried out to certify the existence of UiO-66-NH2 in 19



the prepared TFN membranes. Four element contents (C, O, N and Zr, excluding H) were measured. The existence of Zr element demonstrated UiO-66-NH2 nanofillers were successfully incorporated into the PA selective layer of TFN membranes. The Zr element content for TFN-0.05, TFN-0.10 and TFN-0.15 was 1.47%, 2.07% and 2.16% by weight percentage, respectively, indicating incremental loading amount of UiO-66-NH2. The EDX mapping of TFN-0.10 membrane was shown in Fig. S4, from the Zr distribution in Fig. S4(d) we could know UiO-66-NH2 homogeneously distributed in polyamide layer, which also indicated the good dispersibility of UiO-66-NH2 in water.

Fig. 7. Cross-section morphology images of TFC (a), TFN-0.05 (b),

TFN-0.10 (c) and TFN-0.15 membranes (d)

20


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Fig.7. presented the cross-section morphology of TFC and TFN membranes. As shown in the above images, each membrane contained a porous HPAN supporting layer and a dense polyamide skin layer tightly covered on the support membrane surface. The thickness of polyamide layers marked by red arrows continuously rose with the increase of UiO-66-NH2 loading, which could be seen from the arrow length. The thicker polyamide layer could be ascribed to the hydrophilic character of UiO-66-NH2 nanoparticles which facilitated the diffusion of PIP from support layer surface to the organic phase during the interfacial polymerization process. 3.2.2. Chemical structure of TFC and TFN membranes The chemical structure of TFC and TFN membranes was studied by ATR-FTIR spectra and the results were shown in Fig. 8. The absorption peaks at 2933 and 2243 cm-1 appearing in HPAN, TFC, and TFN membranes were corresponded to stretching vibration of -OH in carboxyl groups and -C≡N in HPAN, respectively 32. This was due to the partial transformation of -C ≡ N groups in PAN membrane to -COOH groups via hydrolysis reaction under alkaline environment. Due to ionization of – COOH, the negatively charged support membrane could absorb positively charged PIP molecules by electrostatic attraction, which increased compatibility between PA layer and HPAN support

33.

Compared with HPAN membrane, the new absorption

peaks at 3406 cm-1 corresponding to –NH 34 emerged in TFC and TFN membranes. In addition, the new absorption peak at 1635 and 1367 cm-1 in TFC and TFN membranes

21



was corresponded to –C=O and –C–N in amide groups

35, 36.

The above three

emerging peaks indicated that a PA layer successfully formed on the top of HPAN support.

TFN-0.15

TFN-0.05 TFN-0.10

HPAN TFC

Transmittance

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3406

2933 2243

1367 1635

3500

3000

2500 2000 1500 Wave number (cm-1)

1000

500

Fig. 8. ATR-FTIR spectrum of TFC, TFN-0.05, TFN-0.10 and TFN-0.15 membranes

3.2.3. Hydrophilicity of TFC and TFN membranes Surface hydrophilicity is a crucial property for nanofiltration membrane because high hydrophilicity of membrane can improve affinity to water and hence obtain higher permeability

37-39.

To explore the hydrophilicity of the prepared membranes,

water contact angles were measured and the results were presented in Fig. 9. Compared with pristine PA TFC membrane, the contact angles of TFN membranes decreased after incorporating UiO-66-NH2. With the increase of UiO-66-NH2 loading amount, the contact angles decreased from 53.0±2.0° for TFC to 26.1±0.8° for 22


Page 23 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TFN-0.15. The results demonstrated that the incorporation of UiO-66-NH2 nanoparticles could increase the hydrophilicity of the membranes. The phenomenon was ascribed to that the hydrophilic UiO-66-NH2 nanoparticles contained various polar functional groups such as -NH2, -COOH and Zr ions on the particle surfaces. In addition, surface roughness served as another factor influencing hydrophilicity. With regard to hydrophilic surface, the rougher surface leads to a smaller apparent contact angle

40, 41.

In the later AFM test, TFN membranes showed larger surface roughness

compared with TFC membrane, which resulted in smaller contact angles as well.

Fig. 9. Contact angles of TFC, TFN-0.05, TFN-0.10 and TFN-0.15 membranes

3.2.4. Surface charge properties of TFC and TFN membranes

23



Page 24 of 43

According to the Donnan effect, surface charge has a significant influence on membrane separation performance especially for salt rejection

42.

Based on this

consideration, the surface charge properties of the prepared nanofiltration membranes were tested and the results were shown in Fig. 10. It could be found that all the membranes exhibited increasing negative charge with the pH value ranging from acidity to alkalinity. When pH﹤3, all the membrane surfaces presented positive charge, which was ascribed to the presence of amine groups in unreacted PIP molecules and amido bonds in PA layer. Because N atoms have lone pair electrons in 2s atomic orbits which could combine H+ ions by coordination at strong acid environment leading to positively charged membrane surfaces. However, when the pH value went up, the charge of membrane surface changed to negative, which was strongly linked to the deprotonation of carboxyl groups from the unreacted acyl chloride of TMC hydrolyzed in water (-COCl→ -COOH→ -COO- )

43, 44.

Compared

with TFC membrane, TFN membranes showed higher Zeta potential around pH 7 with the elevation of UiO-66-NH2 loading amount. That could be clarified as follows: as shown in Fig. S2, when the positively charged UiO-66-NH2 nanoparticles were introduced into PA layer during interfacial polymerization process, the statistic positive electricity on membrane surfaces would be enhanced leading to higher Zeta potential than TFC. With the sequential improvement of UiO-66-NH2 loading amount, Zeta potential grew up continually.

24


Page 25 of 43

10 TFC TFN-0.05 TFN-0.10 TFN-0.15

5 0

Zeta potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5 -10 -15 -20 -25 -30 2

3

4

5

6 pH

7

8

9

10

11

Fig. 10. Zeta potential of TFC, TFN-0.05, TFN-0.10 and TFN-0.15 membranes

3.2.5. Roughness of TFC and TFN membranes AFM test was adopted to analyze the surface roughness of the prepared nanofiltration membranes. The 3D images were shown in Fig. 11, and the root mean surface roughness (Rq) and root average arithmetic roughness (Ra) were listed in Table 1. The “ridge and valley” structure of membrane surface could be obviously observed in Fig. 11. The brighter and higher region in AFM 3D images represented ridge on membrane surface, while the darker and lower region represented valley. Compared with TFC, the TFN membranes showed rougher surface according to Rq and Ra values in Table.1, which was in consistent with the SEM images (Fig. 6). With

25



the increase of UiO-66-NH2 loading, the Rq value increased from 21.7 to 29.7 nm and the Ra value rose from 17.1 to 23.3 nm. The rougher surface of TFN membranes was mainly attributed to the incorporation of UiO-66-NH2 nanoparticles which had been described in the previous discussion about morphology of the prepared membranes. The rougher membrane surfaces could provide larger contact areas with water, which was beneficial to higher water permeability for TFN membranes.

26


Page 26 of 43

Page 27 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 11. AFM 3D images of TFC (a), TFN-0.05 (b), TFN-0.10 (c) and TFN-0.15 (d) membranes

Table 1. Rq and Ra of TFC, TFN-0.05, TFN-0.10 and TFN-0.15 membranes

Membrane ID

TFC

TFN-0.05

TFN-0.10

TFN-0.15

Rq (nm)

21.7

25.9

27.7

29.7

Ra (nm)

17.1

20.9

22.4

23.3

3.3. Nanofiltration performance of TFC and TFN membranes 3.3.1. Permeability of TFC and TFN membranes Pure water permeability (PWP) of the prepared membranes was displayed in Fig. 12,

and

a linear fitting method

of data processing was adopted

to

depict

the

relationship between the permeation flux and applied pressure. It could be found that the pure water permeation flux increased linearly as the pressure increasing from 2 bar to 8 bar for each membrane. On the basis of linear fitting, the permeability of TFC membrane was 6.89 LMH/bar, and all the TFN membranes displayed obviously higher permeability after loading UiO-66-NH2 nanoparticles. Specifically, the permeability ascended to 12.68 LMH/bar at just 0.05% w/v UiO-66-NH2 content, and to the highest of 14.55 LMH/bar at loading content of 0.10% w/v. Many factors contributed to the rising permeability of TFN membranes. First of all, as discussed above, compared with TFC membrane, TFN membranes displayed

27



higher hydrophilicity which was favorable to cover the membrane surfaces with a hydration layer thus facilitating water molecules to contact the membrane surfaces and pass into the selective layer of TFN membranes . Secondly, rougher surfaces of TFN membranes offered larger water contact area, leading to higher water permeability as well. More importantly, the aperture size of UiO-66-NH2 was about 0.52nm 26, which was larger than water molecules of 0.28 nm 15.The porous structure of UiO-66-NH2 in TFN membranes could provide additional passageways for water 45.

molecules leading to a higher water permeability

For all the three factors

mentioned above, the TFN membranes showed superior water permeability in contrast with TFC membrane.

TFC TFN-0.05 TFN-0.10 TFN-0.15

120

Pure water permeation flux (LMH)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 43

100

14.55 LMH/bar 13.13 LMH/bar

80 12.68 LMH/bar

60 40

6.89 LMH/bar

20 0 2

4

Pressure (bar)

6

8

Fig. 12. Pure water permeability of TFC, TFN-0.05, TFN-0.10 and TFN-0.15 membranes

28


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Nevertheless, it should be noted that the permeability dropped to 13.13 LMH/bar with the UiO-66-NH2 loading content increasing up to 0.15% w/v. This phenomenon could be explained by the larger thickness of polyamide layer. 3.3.2. Separation properties of TFC and TFN membranes Na2SO4 (0.007 mol/L) and NaCl (0.017 mol/L) solutions were used to evaluate separation properties of TFC and TFN membranes with applied pressure range of 2 to 8 bar. Fig. 13 and Fig. 14 show the experimental results with different regularity of Na2SO4 rejection between TFC and TFN membranes. With regard to the TFC membrane, the Na2SO4 rejection presented a consistently increasing trend with the pressure rising. There were two main reasons for this phenomenon. Firstly, the PA selective layer became much denser under higher pressure, which increased the selectivity of TFC membrane 9. More importantly, the dilution effect played an important role in the increase of Na2SO4 rejection. Specifically speaking, water and salt flux can be described as follows by solution-diffusion model 44 :

J w  A(p   ) Js 

Dsm K (Cb  C p ) l

(3)

(4)

where Jw and Js represent water flux and salt flux respectively. A is permeability parameter of the solvent. Δp and Δπ refer to the operating pressure and osmotic pressure, respectively. Dsm refers to diffusion of salt inside the membrane. K refers to solubility of salt in the membrane, and l is the membrane thickness. Cb and

29



Cp are the concentration at bulk solution side and permeate side, respectively. From equation 3 and 4, the increased Δp could enlarge Jw dramatically but had less influence on Js (Js altered under different operation pressure/salt solutions due to the simultaneous change of l and Dsm in equation 4, and the increment of Js could be less than Jw when Δp increased). That was to say, dramatically increased water permeability at high pressure diluted the solute concentration of permeate side, leading to higher Na2SO4 rejection of TFC membrane. However, TFN membranes showed a different rejection trend with the pressure increasing. When the pressure increased from 2 to 4 bar, the rejection varied with a slight elevation. While with the pressure higher than 4 bar, the rejection went down especially at 8 bar. The decline of rejection at high pressure (above 4 bar) could be explained by the unavoidable defects at the PA/nanofiller interfaces 10. Although the nanofillers were fully covered with -NH2 which could react with TMC during interfacial polymerization to inhibit the interfacial defects, the mobility of polymer chains and inorganic nanoparticles were intrinsically different, which had a great influence on polymer/nanofiller interfaces at high pressure thus resulting in the interfacial defects. In spite of this, compared with other nanofillers, for example mZIF-8 without reactive functional groups on the surface, UiO-66-NH2 still contributed to the relatively better rejection property for the prepared TFN membranes, which could be supported by the increasing Na2SO4 rejection from 2 to 4 bar rather than consistent decrease within the whole pressure range of 2 to 8 bar 9. During the interfacial polymerization process, besides the PIP monomers, the -NH2 on

30


Page 30 of 43

Page 31 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the UiO-66-NH2 nanoparticle surfaces could also react with TMC to form covalent amide bonds. But it’s difficult to discriminate amide bonds formed by TMC with UiO-66-NH2 or PIP monomers because of the similar physicochemical character. To reveal the chemical reaction between UiO-66-NH2 and TMC, UiO-66-NH2-TMC was obtained by adding UiO-66-NH2 nanoparticles into TMC solution and then washed with n-hexane and water. The FT-IR spectra in Fig. 4 confirmed the reaction between UiO-66-NH2 with TMC. So it could be believed that the covalent interaction occurred, which improved the compatibility between nanoparticles and polyamide thus inhibiting the formation of non-selective defects at interfaces. The TFN membrane incorporated with pristine UiO-66 nanoparticles (the concentration in aqueous solution was 0.10% w/v) was also prepared and tested to make a comparison with that incorporated with UiO-66-NH2. As shown in Fig. S5 and Fig. S6, the TFN membrane incorporated with UiO-66 showed higher permeability but lower rejection properties. The difference between two membranes also indicated the better compatibility of UiO-66-NH2 and polymer matrix, which inhibited the formation of non-selective defects and resulted in relatively higher rejection properties. Notably, TFN membranes showed higher rejection than TFC membrane at low pressure (below 4 bar), which might be due to the dilution effect as discussed above. All the TFN membranes showed higher permeability than TFC membrane, which diluted the solute concentration at permeate side and resulted in slightly higher rejection property. In a word, the TFN membranes exhibited excellent Na2SO4 rejection property above 98.5% within 6 bar and reach to the maximum at 4 bar. The

31



Donnan effect was an important factor for high Na2SO4 rejection. Because all the prepared nanofiltration membranes were negatively charged at pH > 3, the electrostatic repulsion between membrane surface and negative ion could repel SO42passing through the selective layer of nanofiltration membrane. The permeation flux was also measured using feed solution containing 0.007 mol/L Na2SO4. As shown in Fig. 13(b), the variation tendency agreed well with pure water permeability. Compared with pure water, the permeation flux of Na2SO4 solution was much lower for each membrane. This phenomenon was due to the concentration polarization of Na2SO4 between bulk solution side and permeate side of the membrane, which increased the osmotic pressure (Δπ in equation 3) and hence decreased the effective driving force for mass transfer.

100

(a)

99

Na2SO4 rejection (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 43

98 TFC TFN-0.05 TFN-0.10 TFN-0.15

97

96

95 2

4

6 Pressure (bar)

32


8

Page 33 of 43

80

(b)

70 Permeation flux (LMH)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60


9.98 LMH/bar

8.99 LMH/bar

50

8.62 LMH/bar

40 30 4.52 LMH/bar

20 10 2

4

6

8

Pressure (bar) Fig. 13. Na2SO4 rejection (a) and permeation flux (b) of TFC,

TFN-0.05, TFN-0.10 and TFN-0.15 membranes

NaCl solution (0.017 mol/L) was used to further measure the membrane rejection and permeation flux for monovalention. As shown in Fig. 14(a), the NaCl rejection of TFC membrane consistently increased within the applied pressure range of 2 to 8 bar. While that of TFN membranes increased at low pressure and then decreased at high pressure. TFN membranes showed higher NaCl rejection than TFC membrane at 2 and 4 bar. To sum up, NaCl rejection of TFN membranes varied in about 30%-40% without consistent falling when the applied pressure increased. Permeation flux of TFN membranes was higher than TFC membrane, which was in consistent with the above discussion.

33



45

(a)

NaCl rejection (%)

40 35 30 TFC TFN-0.05 TFN-0.10 TFN-0.15

25 20 15 2

4

6

8

Pressure (bar)

90

(b)

70


60

10.06 LMH/bar

80

Permeation flux (LMH)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 43

10.83 LMH/bar

9.62 LMH/bar

50 40 30

6.42 LMH/bar

20 10 2

4

Pressure (bar)

6

8

Fig. 14. NaCl rejection (a) and permeation flux (b) of TFC,

TFN-0.05, TFN-0.10 and TFN-0.15 membranes

3.3.3. Long-term operation stability of TFN-0.10 membrane TFN-0.10 membrane was used to evaluate the long-term operation stability, which was examined with pure water permeability and Na2SO4 rejection within 72 h. As shown in Fig. 15, the pure water permeability decreased slightly along with a 34


Page 35 of 43

slight increment of Na2SO4 rejection at first stage and then reached a stable level. The result indicated that the prepared TFN membrane showed good long-term operation stability.

100

16.0 15.5

98

15.0 96 14.5 94 14.0

Na2SO4 rejection (%)

Pure water permeability (LMH/bar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


92

13.5

90

13.0 0

10

20

30 40 Time (h)

50

60

70

Fig.15. The long-term operation stability test of TFN-0.10 membrane

35



Page 36 of 43

3.3.4. Performance comparison with other reported TFN membranes Table 2. Comparison of nanofiltration performance with other reported literatures

PWP Membranes

Na2SO4 rejection

NaCl rejection

Nanofiller

reference (LMH/bar)

(%)

(%)

TFC

—

6.89

98.8

32.7

This work

TFN-0.10

UiO-66-NH2

14.55

99.0

38.1

This work

TFN-mZIF2

mZIF

14.90

92.5

12

9

M3

Graphene oxide

14.5

98

58

46

Silica/polyamide

silica

9.45

97.3

25.6

47

PMMA-MWNTs/ polyamide

PMMA-MWNTs

6.98

99

44.1

48

TFN-30

UiO-66

11.5

92

30

20

TFN-f0.04

Graphene oxide

4.8

82

26

49

TFN-0.10 membrane showed the highest permeability as well as good rejection property and thus was used to make a comparison with other reported TFN membranes in literatures. The data were shown in Table 2. It could be found that after the incorporation of UiO-66-NH2 nanoparticles the PWP of TFN-0.10 membrane increased dramatically, which was higher than most other TFN membranes except TFN-mZIF-2 membrane which, however, displayed lower rejection property. The Na2SO4 rejection of TFN-0.10 in this work came up to 99.0%, which was one of the highest in all the listed TFN membranes. NaCl rejection also reached an acceptable

36


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level. Overall, TFN-0.10 membrane in this work showed excellent performance with both good permeability and rejection property. 4. Conclusion In this study, TFN membranes incorporated with UiO-66-NH2 nanoparticles were prepared via interfacial polymerization. The EDX characterization demonstrated UiO-66-NH2 nanoparticles were successfully introduced into the TFN membranes. Striped Turing structure surface morphology was obtained in TFC membrane, and a certain number of spots dispersed within the striped networks in TFN membranes. After the incorporation of the hydrophilic UiO-66-NH2 nanoparticles, hydrophilicity and surface roughness of TFN membranes increased. Furthermore, UiO-66-NH2 nanoparticles provided additional passageways for water molecules. PWP increased from 6.89 LMH/bar for TFC to 14.55 LMH/bar for TFN-0.10 indicating the distinct permeability elevation after the incorporation of UiO-66-NH2 nanoparticles. The membranes also showed high rejection property with Na2SO4 rejection of 99.0% and NaCl rejection of 38.1% for TFN-0.10 membrane at 4 bar owing to the good compatibility between UiO-66-NH2 nanoparticles and polyamide matrix.

Supporting information Experimental results, including size distribution and zeta potential of UiO-66-NH2; FESEM image, EDX mapping and property comparison of membranes in this study; Fig. S1-S6. 37



Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21576068, 21276060, 21276062, and 21306039), the Natural Science Foundation of Tianjin (16JCYBJC19 800), the Natural Science Foundation of Hebei Province (B2015202082, B2016202027, and B2017202056), the Program for Top 100 Innovative Talents in Colleges and Universities of Hebei Province (SLRC2017029) and Hebei High level personnel of support program (A2016002027).

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Enhancing permeability of thin film nanocomposite (TFN) membranes

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