Polyetheramide organic solvent nanofiltration membrane prepared via an interfacial assembly and polymerization procedure

Separation and Purification Technology 234 (2020) 116033

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Polyetheramide organic solvent nanofiltration membrane prepared via an interfacial assembly and polymerization procedure

T

Muntadher M. Alwan Almijbileea, Xitian Wua, Ayang Zhoua, Xiaokuo Zhenga, Xingzhong Caob, ⁎ Wei Lia, a

Collaborative Innovation Center of Chemical Science and Chemical Engineering (Tianjin), School of Chemical Engineering & Technology, Tianjin University, Tianjin 300350, People’s Republic of China b Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese Academy of Science, Beijing 100049, People’s Republic of China

A R T I C LE I N FO

A B S T R A C T

Keywords: Organic solvent nanofiltration 4,4′-oxydianiline Interfacial assembly, Polyetheramide Interfacial polymerization Solvent activation

An interfacial assembly and polymerization procedure was adopted to synthesized organic solvent polyetheramide thin film composite (TFC) nanofiltration membrane using 4,4′-oxydianiline (ODA) and trimesoyl chloride (TMC) monomers upon the ethylenediamine-crosslinked polyetherimide (C-PEI) membrane substrate. DFT calculations indicate that the unreacted amine terminals located on the C-PEI membrane surface can form intermolecular hydrogen bonding with the ether group of ODA molecule meanwhile π-π stacking interaction could be formed among the aromatic rings of ODA and the aromatic section in the backbone of C-PEI. The separation efficiency of the synthesized polyetheramide TFC membranes was assessed with some organic solvents and characterized through ATR-FTIR, 13C NMR, XPS, SEM, DBES, etc. S parameters of DBES measurement indicate that the organic solvent activation indeed influences the arrangement of the polymer chain in the synthesized membrane layers, and consequently makes the diffusion faster through the top layer of the membrane. Through activating the TFC membrane by THF solvent, the optimum TFC membrane showed a rejection of 96.3% toward Crystal Violet (CV) (Mw 407.98 g mol−1) with a THF permeance around 4.5 L m−2 h−1 bar−1.

1. Introduction Organic solvent nanofiltration (OSN) membrane, also known as solvent resistance nanofiltration SRNF membrane [1], presently a growing technique compared to conventional separation techniques for recovering solvent [2–4], the separation and purification of compounds [5,6], and in a hybrid process that gathers nanofiltration with homogeneous catalysis [7–11] with the ability to reject the compounds which have molecular weight below 1000 g mol−1 in the non-aqueous solvents. Thin film composite (TFC) membranes [12–16] comprising of a thin polymeric top-layer on a support which is typically prepared from another type of polymer [17]. The thin layer (skin layer) often synthesized by the interfacial polymerization (IP) among a diamine and an acid chloride monomers have attracted more attention owing to the facile synthesis procedure and the superior resistance toward organic solvents, besides those integrally skinned asymmetric (ISA) membranes, which is asymmetric membrane synthesized by the phase inversion process [18,19] So far, the main difficulty for OSN polymeric membranes is to improve the resistance of polymeric materials toward a broad range of organic solvents [20,21], meanwhile maintaining the ⁎

comparable permeance and rejection. Amine compounds have been used as a crosslinking agent to intensify the solvent resistance property of OSN polymeric membranes [22,23]. For instance, Peinemann and co-workers used toluene diisocyanate (TDI) as a crosslinking reagent to prepare crosslinked Pebax ® membranes and the membrane capable to reject 95% of Brilliant Blue dye (Mw 792.85 g mol−1) with 10 L m−2 h−1 MPa−1 permeance of dimethylformamide (DMF) [2]. Vankelecom and co-workers used different diamine involving ethylene diamine (EDA), hexylenediamine (HDA), aliphatic compounds, and p-xylylenediamine (XDA), an aromatic compound, as crosslinker agents to improve the solvent resistance of polyimide (P84) membranes, which prepared through the phase inversion method using deionized water coagulation bath [24–26]. Chung and coworkers adopted the hydrazine monohydrate crosslinking to modulate the ultrafiltration polyacrylonitrile (PAN) hollow fiber membranes by minimizing the pore size of the membrane and modulate to nanofiltration separation with the permeability of the ethanol 23.2 L m−2 h−1 MPa−1 and the ability to reject 99.9% Remazol Brilliant Blue R (Mw 626.54 g mol−1) [27]. Recently, Zhou et al. noted the EDAcrosslinked polyetherimide (PEI) substrate has unreacted amine groups

Corresponding author. E-mail address: [email protected] (W. Li).

https://doi.org/10.1016/j.seppur.2019.116033 Received 18 May 2019; Received in revised form 30 August 2019; Accepted 6 September 2019 Available online 07 September 2019 1383-5866/ © 2019 Published by Elsevier B.V.

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which are capable to polymerize with the hydroxyl group of vanillic alcohol (VA) and the acid chloride groups of TMC, obtaining the polyarylester OSN membrane which showed 21 L m−2 h−1 MPa−1 dimethylsulfoxide (DMSO) permeance and rejection about 89% of the Brilliant Blue R-250 BB dye (Mw 825.97 g mol−1) [13]. It is suggested that on the crosslinked polymer surface there are a certain amount of amine terminals that can proceed further polymerization. However, in the view of synthesizing TFC membrane, it is desirable to generate a uniform distribution of amine terminals on the substrate surface, which would be beneficial to prepare a relative defect-free top layer through the polymerization with acyl chloride monomers. Taking into account the amine groups have the tendency to form weak interactions including hydrogen bonding and Van der Waals force, it is intriguing to construct a self-assembling layer of certain amine monomer molecules on the crosslinked substrate through the weak interactions, and then synthesize TFC membranes via the polymerization with acyl chloride monomers. In this article, the amine monomer of 4,4′-oxydianiline (ODA) and first studied the possible interactions between ODA and the EDA-crosslinked PEI (C-PEI) was adopted using the molecular calculations. DFT calculations indicate that the ODA molecule can form hydrogen bonding between the ether group of ODA and the terminal amine groups on the C-PEI. Meanwhile, the aromatic rings of ODA can form π-π stacking interaction with the aromatic section in the backbone of C-PEI (Fig. 1). As revealed by the top chart (Scheme 1), on the C-PEI surface there forms an ODA-assembling layer enriched with amine groups via the weak interactions of hydrogen bonding and π-π stacking interaction. Then the acyl chloride monomer, trimesoyl chloride (TMC), can react with the amine-enriched layer to create the TFC membrane with the chemical structure of polyetheramide. Next, we assessed the separation performance of the synthesized polyetheramide membranes with some organic solvents, and analyzed the organic solvent resistance of the membranes, in amalgamation with characterizations of ATR-FTIR, SEM, XPS, AFM, and DBES, etc. Finally, through the organic solvent activation, the optimal OSN membrane exhibits the THF permeance about 45 L m−2 h−1 MPa−1 with a 96.3% Crystal Violet (CV, 407.98 g mol−1) rejection. 2. Experimental 2.1. Materials The nonwoven polypropylene was obtained from Tianjin TEDA Filters Company. Ultem® 1000 unfilled Polyetherimide round bar (PEI) was provided by SABIC Innovative Plastics before each experiment the PEI was dried at 110 °C overnight. Trimesoyl chloride (TMC) and 4,4′oxydianiline (ODA) were procured from Tianjin Heowns Biochemical Technology Co., Ltd. Crystal Violet (CV, Mw 407.98 g mol−1), for rejection estimation, was taken from Aladdin Industrial Corporation. Ethylenediamine (EDA) and all the used organic solvents were procured from Tianjin Kemiou Chemical Reagent Co., Ltd. All the used chemicals were not purified further.

Fig. 1. Molecular interactions between one ODA molecule (a, b) or two ODA molecules (c, d) and the amine-terminals above the C-PEI surface.

eliminate the unreacted EDA, the EDA-crosslinked PEI membrane was washed and drenched in methanol for 3 h, which was denoted as C-PEI. Next, the C-PEI was fixed using a polytetrafluoroethylene frame and soaked in the ODA solution with the mixed solvent of water and DMF (1:1 v/v) at 25 °C for 2 min, in order to form the assembled layer of ODA on the C-PEI surface by the weak interactions of hydrogen bonding and π-π conjugation. The ODA concentration was adopted as 0.5, 1 or 1.5% (w/v). Following this interfacial assembly process, a TMC nhexane solution was decanted upon the membrane, which was fixed by Teflon frame, for the IP of 1 min after removing the surplus ODA solution. Finally, the attained TFC membrane was treated at 70 °C for 6 min, followed by washing using deionized water and stored at 4 °C. The TMC concentration was used as 0.1, 0.15, 0.2, or 0.25% (w/v). To make the discussion clear, the prepared TFC membrane was named in terms of ODA concentration when the TMC concentration was 0.2%.

2.2. Synthesis of polyetheramide TFC membrane TFC membrane was synthesized by means of interfacial assembly tandem polymerization method. First, the phase inversion method was used for synthesizing the C-PEI membrane on the non-woven polypropylene fabric. The casting solution was prepared by dissolution 23 g of PEI in 77 g dimethylacetamide (DMAc) and agitated for 6 h at 60 °C and then was put in a desiccator for 24 h, to remove air blebs. After degassing, the solution was cast on the polypropylene fabric fixed to a glass lamina. A casting knife with 200 µm knife gap was used at 25 ± 3 °C and 30 ± 5% relative humidity, followed by water coagulation bath at 25 °C for 60 min to remove DMAc and form microporous PEI membrane. Thereafter, the membrane was drowned in a methanol crosslinking solution contains 6% EDA for 1 h at room temperature. To 2

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Scheme 1. The schematic to prepare OSN membrane via the interfacial assembling of ODA on the C-PEI substrate and the following polymerization between amine terminals and acyl chloride of TMC. The top panel displays the hydrogen bonding (with green data indicating the bond length) and π-π stacking (in the view of the blue arrow) interactions between ODA and the EDA terminals on the substrate (with the gray column shadow indicating the cross-linked substrate).

Fig. 2. SEM images of TFC membranes prepared with various ODA concentrations.

2.3. Membrane characterization

For example, 0.5-TFC and 1-TFC indicate the TFC membranes prepared respectively with the ODA concentration of 0.5% and 1% and the TMC concentration of 0.2%.

Attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy (Bio-Rad FTS-6000, USA) was used to distinguish changing the functional groups of the chemical composition of the membranes through crosslinking and IP processes. Elements percentage 3

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399.4 N-C=O

399.3 C-NH2

400.8 -NH+3

404

402

400 398 B.E. (eV)

400.8 -NH+3

404

396

N 1s

399 C-NH2

402

400 398 B.E. (eV)

396

404

(b)

-NH+3

400

398

396

1-TFC

100

404

398.8 C-NH2 400.8 -NH+3

402

400 398 B.E. (eV)

396

1.5-TFC

O=C-N NH2 -NH+3

80

Integrated areas (%)

400.8

402

399.3 N-C=O

399.1 C-NH2

B.E. (eV)

0.5-TFC

C-PEI

N 1s

399.7 N-C=O

Intensity

N 1s

399.9 N-C=O

Intensity

Intensity

N 1s

Intensity

(a)

60

40

20

0 C-PEI

0.5-TFC

1-TFC

1.5-TFC

Fig. 3. (a) The deconvoluted N1s spectra of TFC membranes. (b) The percentage of individual N species in the TFC surfaces.

7,12

5 9

10 134

3

200

180

160

recorded when the droplets sat on the surface. The images were analyzed to calculate the contact angle. For each membrane sample at least five different locations were measured so as to obtain the average of the contact angle with a small error. The variation of the pore size and dense layer thickness were tested by Doppler broadening energy spectroscopy (DBES) technique. DBES with a 22Na radioisotope source and an HP Germanium detector utilizing positron annihilation spectroscopy (PAS). The total peak energy range of 499.5–522.5 keV were implanted into the samples. The S parameter is described as the proportion of integrated counts among 510.2 and 511.8 keV to the total count's region The molecular interactions between ODA and the surface of C-PEI were performed by density functional theory (DFT) simulation [28,29]. All the calculations were carried out using the Materials Studio DMol3 program from Accelrys. The nonlocal exchange and correlation energies were estimated with PW91 functional of the generalized gradient approximation (GGA). The parameter standard for the tolerances of energy and self-consistent field convergence are 2.0 × 10−5 Ha and 1.0 × 10−6, respectively. In order to analyze the molecular interactions between ODA and the unreacted amine-terminal in the surface of C-PEI, the binding energy Ebinding was calculated by Eq. (1).

8,11

2

140

120

100

80

Chemical shift (ppm) Fig. 4. 13C NMR spectra of the TFC membrane top layer synthesized by the IP of ODA and TMC monomers.

and chemical composition of membrane surface were studied by X-ray photoelectron spectroscopy (XPS) with a (PHI-5000 Versa Probe III, ULVAC-PHI Inc., Japan). Scanning electron microscopy (SEM) was used for analyzing the morphology and estimating dense layer thickness TFC membrane, synthesized by different ODA concentrations, by using (LEO 1530VP, Germany). An Agilent 5500 atomic force microscopy (AFM) was used to measure the effect of solvent activation on the surface roughness of the TFC membrane and the images analyzed by NanoScope Analysis (version 1.8). Solid-state 13C NMR (300 MHz Varian Infinity Plus 300WB) was used for identification of polymerization product. The measurement of dynamic contact angles on the membrane surface using the contact angle meter (KINO SL200KB, Solon Tech (Shanghai) Co. Ltd). Using the sessile drop method, a membrane sample with the size of 1 cm × 4 cm was fixed carefully on a glass slide, then the images of individual liquid droplets (each about 5 μL) were

ΔEbinding = Etotal − Emembrane − EODA

(1)

where Etotal is the energy of the optimized structure of the adsorption complex, Emembrane and EODA are respectively the total energy of the CPEI and the monomer ODA after the geometry optimization. The more negative the value of ΔEbinding, the stronger interactions occur. 2.4. Nanofiltration experiments All membranes were pre-conditioned using a pure solvent (same as feeding solvent) to remove any leachable until a constant flux was obtained. Nanofiltration experiments were accomplished at room temperature using a dead end stirred cell with 38.5 cm2 active area (A). The diffused solvent was collected after 2 h (reload the permeated solvent 4

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a

60 50

Contact angle, (o)

100

Contact angle, (o)

b

0.5-Sec 1-Sec 1.5-Sec 2-Sec

120

80 60 40

0.5-Sec 1-Sec 1.5-Sec 2-Sec

40 30 20 10

20

0

0 0.5-TFC

1-TFC

0.5-TFC

1.5-TFC

1-TFC

1.5-TFC

Fig. 5. The dynamic contact angles of TFC membranes measured using the liquid of (a) water and (b) ethylene glycol.

(b) 96.8

100

Permeance (L m-2 h-1 bar-1)

Permeance (L.m-2.h-1.bar-1)

89.4

1.35 97 1.21

80 1.0 60 40

0.5

Rejection (%)

1.43

1.5

2.0

P R

20 0.0

0.5%

1%

98 89

1.587.1

1.35

60

1.0

40 0.5

0.0

0.1%

0.15%

Fig. 6. The separation performance for a THF solution with 0.01 g L 1% ODA and different TMC contents (b).

60 42.2 30

40

0

Rejection (%)

Permeance (L m-2 h-1 bar-1)

8

2

P=

80

Methanol

EA

(2)

(3)

3. Results and discussion 20

1.4

ACN

V t × A × ΔP

Cp ⎞ R (%) = ⎜⎛1 − ⎟ × 100% Cf ⎠ ⎝

2.85 1.52

0

UV–Vis spectrophotometer and the rejection (R%) was measured by Eq. (3).

82.8

10

4

0.25%

CV dye using TFC membranes synthesized with 0.2% TMC and different ODA contents (a) or

100

91.8 9.9

6

0.2%

20

TMC (%) −1

P R

100 80

1.2

ODA (%)

12

96.8

0.25

0

1.5%

1.5

P R

Rejection (%)

(a)

DMF

3.1. Molecular interactions between ODA and the C-PEI 0.04 DMSO

0

As displayed in Fig. 1a and b, if there is one ODA molecule attached to the C-PEI surface, the negative values of binding energy suggest that this ODA probably forms hydrogen bonding (H bonding) with the amine terminal of C-PEI (as shown by the green lines with the bond distance), or generates π-π stacking interactions with the aromatic rings of C-PEI (as shown by the inset). If there are two ODA molecules on the surface of C-PEI, more H bondings and π-π stacking interactions occur, as shown in Fig. 1c and 1d. It is illustrated that there exist interactions between ODA molecules and the amine-terminals to promote the formation of the assembled layer of ODA on the surface of C-PEI, which is beneficial to increase the content and the distribution of amine terminals on the surface of C-PEI (Scheme 1).

Fig. 7. Performance of 1-TFC membrane for filtration different organic solvent contain 0.01 g L−1 CV dye.

each 30 min) for permeance and rejection measurements. The permeance (P) was measured through Eq. (2) by gathering the permeated solvent (V) for a specific time (t) and pressure (ΔP), expressed as (L m−2 h−1 bar−1). The rejection was calculated using a 10 ppm CV dye in the feed solution. The content of the dye in the feed solution (Cf) and penetrate solution (Cp) were calculated by an (Agilent, Cary 300 Bio) 5

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Table 1 Physical properties of the used organic solvents and the calculated complex parameter [42,43]. Solvent

MW (g mol−1)

Viscosity (mPa) at 25 °C

Molar volume (ml.mol−1)

δd (MPa1/2)

δh (MPa1/2)

(Vm µ δd δh)−1 × 10−4

ACN EA Methanol DMF THF DMSO

41.0 88.1 32.0 73.1 72.1 78.1

0.38 0.46 0.6 0.82 0.55 2.00

52.6 97.7 40.7 77.0 81.7 71.3

15.3 15.8 15.1 17.4 16.8 18.4

6.1 7.2 22.3 11.3 8.0 10.2

5.36 1.19 1.21 0.80 1.65 0.37

aromatic carbon at 284.7 eV, CeN at 285.5 eV, CeO ether at 286.3 eV, and C]O amide at 288.1 eV, while two kinds of oxygen species attributed to the C]O amide at 531.4 eV and the CeO ether at 533.3 eV [33]. 13 C NMR spectra were used to characterize the chemical composition of the TFC membrane top layer synthesized via the IP between 1% ODA and 0.2% TMC solutions, corresponding to the conditions of 1-TFC membrane. Fig. 4 shows the peaks of carboxylic acid and carboxylic amide carbon atoms at 172 and 165 ppm (as labeled by the number of 3 and 5 in the inset formula), the carbon in phenyl rings attached with O atoms at 155, 153 ppm (as labeled by the number of 9, 10), and the carbon in aromatic ring attached to amine groups at 146.5 ppm (as labeled by the number of 13) [34–36]. In order to study the effect of ODA monomer concentration on the surface hydrophilicity of TFC membranes, the dynamic contact angles evaluation was used. As displayed in Fig. 5, the dynamic contact angles of TFC membranes measured using water and ethylene glycol, respectively. The contact angle measured using water is respectively about 84° on the membrane 0.5-TFC, 78° on 1-TFC, 71° on 1.5-TFC, in particular, the values of contact angles are little dependent on the dynamic process of droplet sitting. Whereas using the high viscous liquid of ethylene glycol (EG), the contact angle is associated with the droplet sitting status, with the value decreasing a little as the droplet stands onto the membrane surface. At the first touch of the EG droplet on the surface (the time is 0.5 s), the contact angle value is 53° on 1.5-TFC, 41° on 1TFC, and 35° on 0.5-TFC. As reflected by ATR-FTIR and XPS spectra, 1.5-TFC membrane has more amount of hydrophilic amide groups on the surface. It illustrates that the contact angle value of the nanofiltration membrane is associated with the amount of hydrophilic groups on the surface, the pore structure on the toplayer as well as the viscosity of the liquid in the measurement.

Fig. 8. The relationship among the permeance of 1-TFC membrane and the organic solvents properties.

3.2. Characterization of TFC membranes synthesized by ODA and TMC SEM images show the morphology of the TFC membranes prepared with various ODA monomer percentages. Fig. 2 shows an asymmetrical structure of the TFC membrane with a dense layer at the top of the finger-like structure. The dense layer thickness increases with increasing the ODA concentration, equal 651, 683, and 730 nm respectively for the membrane 0.5-TFC, 1-TFC, and 1.5-TFC. ATR-FTIR spectra were computed to analyze the chemical compositions of TFC membranes including 0.5-TFC, 1-TFC, and 1.5-TFC versus PEI and C-PEI. As viewed in Fig. S1, the signals at 1778, 1720 and 1363 cm−1 are attributed to C]O and CeN imide groups [30,31], while the peaks at 3260 and 1547 cm−1 are referred to the amine and amide groups, respectively [13]. For the prepared TFC membranes 0.5TFC, 1-TFC, and 1.5-TFC, the peak intensities increase at 3260 and 1547 cm−1as the ODA concentration rises from 0.5 to 1.5 (w/v). Further, the deconvoluted N1s XPS spectra of the TFC membranes were used to analyze the nitrogen species on the membrane surface. Fig. 3 indicates that three kinds of N species exist in the membrane surface, corresponding to the binding energy at 399.3 eV (CeNH2 of the amine group), 399.9 eV (NeC]O of the amide group), and 400.8 eV (eNH3+) [32]. Comparing with the C-PEI without IP, the prepared membranes (0.5-TFC, 1-TFC, and 1.5-TFC) have a higher content of NH2, which is due to the assembled layer of ODA on the surface. In addition, the content of NeC]O in these TFC membranes is lower than that in C-PEI. The eNH2 and eNH3+ groups over the surface of C-PEI represented the unreacted amine terminals during the crosslinking process. Moreover, the deconvoluted C1s and O1s XPS spectra of these membranes are displayed in Fig. S2 in the supporting information. It indicates the existence of four carbon species corresponding to the

3.3. Separation efficiency of TFC membranes Membrane separation efficiency was studied using a THF solution containing 0.01 g L−1 CV dye at room temperature under 6 bar pressure. Fig. 6a shows the permeance and rejection of the TFC membranes synthesized with different ODA monomer concentrations. The membrane 0.5-TFC, synthesized at 0.5% ODA and 0.2% TMC, shows the THF permeance of 1.43 L m−2 h−1 bar−1 and rejection 89.4% of CV. As the ODA content rises to 1% while maintaining the TMC content of 0.2%, the synthesized membrane 1-TFC exhibits 1.35 L m−2 h−1 bar−1 permeability and the rejection of 98.6%. With much higher ODA content (1.5%), the permeance decreases to 1.21 L m−2 h−1 bar−1. Fig. 6b displays the effectiveness of TFC membranes synthesized at 1% ODA but different TMC contents (0.1–0.25%). It indicates that at 0.1% TMC the permeance is higher than 0.2% TMC, but the rejection is 87%, lower than 98% for 0.2% TMC; thus the optimal TMC content is selected as 0.2% to synthesize the nanofiltration membrane. Fig. 7 shows the performance of the membrane 1-TFC in different organic solvents have 0.01 g L−1 CV dye. It indicates that 1-TFC shows the acetonitrile (ACN) permeance up to 9.9 L m−2 h−1 bar−1 with the possibility to reject 91.8% of the CV dye, however, the performance becomes deteriorated in other solvents including methanol, DMF, DMSO, and EA. Such great variation of the permeance in different 6

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(b) 40

0 -20 -40 -60 -80

3

4

5

6

7

8

9

10

P R

100

Permeance (L.m-2.h-1.bar-1)

20

Zeta potential (mV)

1.8

Original THF-treated

1.6

80 60

1.4

40

Rejection (%)

(a)

1.2 20 1.0

11

3

pH

4.5

0

10

pH

(c)

Fig. 9. (a)Zeta potential of the membrane surface at different pH; (b) The rejection 0.01 g L−1 CV in THF solution at different pH; (c)Molecular structure of CV dye [51].

(b)

Permeance (L m-2 h-1bar-1)

6 4.5 4

80 60

3.12 40

P R

6

100

Permeance (L.m-2.h-1.bar-1)

96

96.3

Rejection (%)

P R

2 20

100 90

5

80 70

4

60 3

50 40

2

Rejection (%)

(a)

30 20

1

10 0

Methanol

THF

0

0

0 0

5

10

15

20

25

30

35

Time (h) Fig. 10. a, Performance of the treated 1-TFC membrane (after 24 h-THF immersion) for methanol and THF solvents contain 0.01 g L−1 CV dye. b, The long term filtration stability of THF-treated membrane using 0.01 g.L−1 CLP THF solution under 6 bar operating pressure.

volume and viscosity) and the membrane surface properties (sorption and surface energy) [38]. Darvishmanesh et al. sophisticated a model involving the solvent viscosity, the surface tension and the dielectric constant to correlate the permeance of the MPF 50 and HITK 275 membranes [39]. Recently, Carlos et al. showed that the flux of OSN ceramic membrane depends on Hansen solubility parameters of different solvents [40].

organic solvents has been declared formerly [37]. It is considered that the major influence factors toward the permeance include the properties of organic solvents (the solvent viscosity, the molar volume, etc.) and the solvent-membrane interactions. For instance, Bhanushali et al. studied the transport model to clarify the permeation of non-aqueous solvents within hydrophobic and hydrophilic NF membranes and revealed that the flux is dependent on the solvent properties (molar 7

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Fig. 11. Three-dimensional AFM images of the 1-TFC membrane top surface before and after treated by THF solvent. 0.51

0.50

S parameter

0.49

Here, a relationship was established between the permeance and the solvent properties (the absolute viscosity (µ), the molar volume (Vm)) as well as the dispersion and hydrogen bonding solubility parameters (δd and δh). Due to the close similarity of δd and δp (polarity solubility parameter) [41] and for simplicity δp was neglected. Table 1 views the physical properties of the organic solvents and the parameter (Vm µ δd δh)−1 value. As shown in Fig. 8, the permeance of 1-TFC membrane shows a linear correspondence with the complex parameter (Vm µ δd δh)−1. The physicochemical properties of the solute (e.g., molecular size, hydrophobicity, and charge) is one of the factors that affect the retention of solute by NF membrane which affect by the solution pH [44,45]. There are three major rejection mechanisms by NF are reported, including size exclusion, electrostatic repulsion, and hydrophobic interactions between membrane and solute [46,47]. The Zeta potential of the membrane (1-TFC) was measured at different pH, and assessed the CV dye rejection in THF solutions containing 0.01 g.L−1 CV at different pH (3, 4.5, 10). As shown in Fig. 9a, the membrane surface is positively charged at low pH whereas negatively charged at high pH, with the isoelectric point of 4.5. Fig. 9b indicates that 1-TFC membrane has the CV rejection of 98% at pH 3.0, 86% at pH 4.5 while 35% at pH 10. At 4.5 pH, the membrane has neutral charge, resulting in weak electrostatic repulsion between the membrane surface and CV molecules and consequently the lower rejection. At pH 10 the CV dye (with the pKa of 9.4 [48]) can be non-ionized, as shown in the molecular structure of CV (Fig. 9c), thus the electrostatic repulsion declines, which makes the CV rejection decreases greatly. Previously, the literatures have reported the similar effect of pH on the dye rejections of membranes [49,50]. In addition, the membrane 1-TFC was soaked into THF at room temperature for 24 h before separation process, considering the effect of the interfacial interactions on the diffusion across the membrane. As shown in Fig. 10a, after 24 h-THF immersion, the treated membrane 1TFC shows the THF permeance increased to 4.5 L m−2 h−1 bar−1 while maintaining the rejection at 96%, and the methanol permeance increased to 3.12 L m−2 h−1 bar−1 with enhanced the rejection of 96.3%. The long-term stability of the THF-treated membrane in organic solutions of Clindamycin phosphate (CLP) was tested, according to the literature [52,53]. CLP is a topical (for the skin) antibiotic used to treat acne vulgaris [54], of which the traditional production process involves the energy-consuming evaporation of organic solution [55]. Fig. 10b shows the separation performance of the THF-treated membrane for 0.01 g L−1 CLP THF solution, indicating the CLP rejection above 95% for 30 h, which suggests that the synthesized membrane is stable with excellent solvent resistance. The membrane roughness plays an important role in the separation performance of nanofiltration. Previously, Ramon and Hoek [56] reported the dilemma effect of the membrane roughness: 1-increasing the

THF-Treated Original

interfacial zone dense skin

the support

0.48

0.47

0.46

0.45 0

2

4

6

8

10

12

14

16

18

20

Positron Energy (keV) Fig. 12. S parameter of 1-TFC membrane before and after immersing in THF.as a function of position incident energy.

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Original THF-treated 4

Stress (MPa)

3

2

1

0 0

1

2

3

4

5

6

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Strain (%) Fig. 13. Stress–strain relationship of the 1-TFC membrane before and after treating by THF solvent.

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membrane surface roughness would reduce the permeance, without trade off the thickness of the dense layer; 2- increasing the roughness in combination with reforming thinner dense layer would enhance the permeance. AFM images display the top surface morphology of the original 1-TFC and the partner experienced 24 h THF treatment (named as THF-treated membrane). As shown in Fig. 11, the average surface roughness (Ra) is respectively 5.09 nm for the original 1-TFC and 4.82 nm for THF-treated membrane. After THF treatment the membrane surface roughness becomes a little lower, and the membrane shows higher permeance in methanol and THF, which is probably corresponding to the first situation of Ramon and Hoek’s explanation. S parameter curve reflects the free-volume change in the polymeric membrane structure, since the positrons implanted in the solid experience diffusion and trapping processes and the transport of positrons is approximated by diffusion theory [57]. Fig. 12 indicates that in the top layer and the intermediate C-PEI layer S parameters of the original 1-TFC are larger than those of THF-treated membrane, suggesting that the diffusion through the THF-treated membrane is faster than that in the original 1-TFC membrane. Therefore, the THF-treated membrane shows the enhanced rejection and permeance. The mechanical properties of 1-TFC membrane before and after THF treatment were investigated. As shown in Fig. 13, for the original 1-THF the strain at break and Young’s modulus are respectively 4.54 MPa and 1.00 MPa, while for the THF-treated membrane they are 4.70 MPa and 1.04 MPa. Thus, the THF treatment shows no great effect on the mechanical property of 1-TFC membrane.

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4. Conclusions [13]

A TFC OSN membrane was first prepared through self-assembly interfacial polymerization method utilization ODA and TMC as monomers. The possible interactions between ODA and the C-PEI were calculated by using the DFT calculations. The negative values of binding energy suggest that the ODA probably forms hydrogen bonding with the amine terminal of C-PEI, or generates π-π stacking interactions with the aromatic rings of C-PEI. The effectiveness of the membrane was estimated by permeance and rejection method. The membrane shows good permeance for various organic solvent but low rejection. To enhance the performance of the membrane, the optimum TFC membrane immersed in THF solvents. Treating the membrane by THF solvent enhances the permeance from 1.52 to 3.12 L m−2 h−1 bar−1 with rising the rejection toward CV dye from 30 to 96.2% at the same time the permeance of THF solvent rises from 1.35 to 4.5 L m−2 h−1 bar−1 without sacrificing the rejection. AFM and DBES test of the treated membrane show reduce the membrane surface roughness and reduce the pore volume of the top layer. All the above make a promising route to select the membrane for organic solvent nanofiltration.

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Acknowledgements [21]

This work was supported by national key research and development program (2016YFC1201503), NSFC (21576206, 21621004) and the program for Changjiang Scholars and Innovative Research Team in University (IRT_15R46).

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Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116033.

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