sulfonated polysulfone

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Mesh-embedded polysulfone (PSU)/sulfonated polysulfone (sPSU) supported thin film composite membranes for forward osmosis Yuntao Zhao, Xiao Wang, Yiwei Ren, and Desheng Pei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15309 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Materials & Interfaces

Submitted to ACS Applied Materials & Interfaces Date: 2017.12.21

Mesh-embedded polysulfone (PSU)/sulfonated polysulfone (sPSU) supported thin film composite membranes for forward osmosis

Yuntao Zhao a b 1, Xiao Wang a 1, Yiwei Ren a*, Desheng Pei a*

a

Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences,

Chongqing, China, 400714 b

University of Chinese Academy of Sciences, Beijing, China, 100049

*Corresponding Author: Yiwei Ren Email: [email protected]

*Corresponding Author: Desheng Pei Email: [email protected]

1

These authors contributed equally to this work.

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ACS Applied Materials & Interfaces 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

Keywords: Sulfonated polysulfone; Forward osmosis; Mesh-embedded substrate; Thin film composite membrane; Structural parameter;

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Abstract In this work, mesh-embedded polysulfone (PSU)/sulfonated polysulfone (sPSU) supported thin film composite (TFC) membranes were developed for forward osmosis (FO). The robust mesh integrated in PSU/sPSU sublayer imparts impressive mechanical durability. The blending of hydrophilic sPSU in PSU sublayer affects the hydrophilicity, porosity, pore structure and pore size of mesh-embedded PSU/sPSU substrates, and the total thickness, crosslinking degree and roughness of the corresponding TFC-FO membrane active layers. An appropriate incorporation of sPSU not only significantly decreases the structural parameter, S of the mesh-embedded substrate to 220µm which is the lowest reported value for fabric backed FO membrane, but also optimizes the permselectivity of the formed active layer. Regarding the osmosis performance, TFC membranes with sPSU modified substrates gain a higher water flux (Jw) while keeping the specific reverse salt flux (Js/Jw) low. The optimal TFC-FO membrane has a Jw of 31.76LMH with Js/Jw of 0.19g/L in FO mode when using deionized water feed and 1M NaCl draw solution. This paper is practical for developing TFC-FO membrane on hydrophilic support membrane materials.

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1. Introduction Recently, FO technology research and development have received increased attention. FO processes utilize the osmotic pressure gradient between draw solution (DS) and feed solution (DS) instead of hydraulic pressure or heat for the spontaneous water transport through a semipermeable membrane which is typically impermeable to almost all dissolved and suspended constituents. This unique nature makes FO hold several advantages over the conventional pressure-driven (e.g., reverse osmosis) and thermally-driven (e.g., membrane distillation) membrane separation technologies: (1) low energy input and maintenance cost, (2) reduced membrane fouling propensity and good fouling reversibility, (3) high water recovery, (4) efficient contaminant rejection, and (5) extensive applications especially for the treatment of high salinity and high fouling potential feed waters1-3. Like other membrane separation processes, the membrane lies at the heart of FO process. A desirable FO membrane should achieve high osmotic water flux, efficiently reject the draw and feed solutes, and be chemically and mechanically robust. In past years, various integral asymmetric, layer-by-layer assembled and TFC membranes have been specially developed for FO process4-7. Among them, the aromatic TFC polyamide (PA) membrane serves as the benchmark for FO membrane because of its easy fabrication, good pH stability and excellent permselectivity8. The TFC-FO membrane typically consists of two chemically different layers: (1) a thin nonporous active layer prepared by interfacial polymerization (IP); (2) a thick porous support layer prepared via phase inversion method. The former contributes permselectivity, and the later contributes essential mechanical support and also acts as an unstirred diffusive boundary layer9. They can be separately tailored to maximize the osmosis performance. Recent studies point out that the water permeation resistance across FO membrane mainly results from the internal concentration polarization (ICP) in support layer3, 10. The level of ICP is determined by the structural and chemical properties of support membrane, which can be described quantitatively by structural parameter, S. Several studies have suggested that an ideal PSU or polyethersulfone (PES) support -4-


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for TFC-FO membrane should have finger-like pore morphology underneath a thin sponge-like structure to decrease the ICP5, 11-12. These developed TFC-FO membranes with tailored support structure showed higher water flux than commercial asymmetric cellulose triacetate membranes. Recently, the TFC-FO membrane performance was found to be significantly enhanced if the support membrane material was intrinsically hydrophilic13-15. These FO membranes were developed using freestanding polymeric substrates without supporting fabric. The fabric-free FO membranes show remarkably low mechanical strength and are not practical for designing commercially available membranes. The osmosis performance of the FO membranes with a typical fabric is severely compromised11-12. Hence, there is a need to develop high performance and mechanically robust FO membrane. Until just recently, Han et al.16 combined the strengths of directly sulfonated material and woven fabric for the first time to fabricate high performance mesh-reinforced TFC-FO membranes. Ren et al.17 first integrated PSU/post-sulfonated PSU blend at a ratio of 3:1 with suitable polyester (PET) nonwoven to fabricate low structural parameter TFC-FO membrane. However, the effects of sulfonic acid (-SO3H) group content in polymer system on the properties of support layer and active layer of the resultant TFC-FO membrane were not investigated thoroughly. To date, there are very limited studies on high performance fabric-reinforced FO membranes. Nguyen et al.8 indicated that the mesh, or woven fabric, is more suitable than nonwoven fabric to reinforce the FO membrane substrate compared to nonwoven fabric. In this work, PSU was blended with a certain amount of sPSU and cast onto a PET mesh to prepare mesh integrated support layer. The hydrophobic PSU is a commonly commercial membrane material with good chemical, thermal and mechanical stabilities. The sPSU with hydrophilic -SO3H groups is synthesized via directly copolymerized sulfonation method, which is good for the repeatability and scale-up in membrane fabrication. The sPSU has a good compatibility with PSU due to their similar molecular chains18. This facilitates the incorporation of sPSU into the PSU membrane via polymer blending. Also, the sPSU characterizes by low cost and -5-



easy processability when compared with sPPSU. The sPSU content in polymer blend system was changed from 0 to 66.7wt% for optimizing the structural and chemical properties of support membrane. The PA selective layer on the substrate surface was formed via IP. The effects of sPSU content on the thickness, crosslinking degree and roughness of the formed active layer were investigated and the related mechanisms were discussed. Finally, the resultant TFC membranes on different PSU/sPSU substrates were evaluated in FO. This is the first time that mesh-embedded PSU/sPSU supported TFC membrane for FO is studied comprehensively. This paper provides useful guidelines for TFC-FO membrane development using hydrophilic support membrane materials. 2. Materials and methods 2.1. Materials Two polymer materials, i.e., PSU (Udel® P-3500, 77,000~83,000g/mol, Solvay) and sPSU (15% of sulfonation degree, 90,000~95,000g/mol, Trumpchemicals), were used for fabricating the membrane substrates. The sPSU was directly synthesized with a certain amount of sulfonated monomer in the copolymerization reaction. Prior to use, they were vacuum dried overnight at 80°C. N-methyl-2-pyrrolidone (NMP, >99.5%, Merck) was employed as the solvent. A commercial PET mesh was purchased from Shanghai Shangshai Bolting Cloth Manufacturing Co., Ltd., China. Trimesoyl chloride (TMC, 98%, Sigma-Aldrich) and M-phenylenediamine (MPD, >98%, TCI) were used during IP. Hexane (>99.0%, Merck) was utilized as the solvent for TMC. Deionized (DI) water (18.25MΩ·cm, MolecularΣH2O®) and Sodium Chloride (NaCl, 99.5%, KeLong Chemical Reagent) were used in osmosis performance tests. 2.2. Preparation of mesh-embedded PSU/sPSU substrates The mesh-embedded PSU/sPSU substrate was prepared by non-solvent induced phase separation (NIPS). 15wt% polymers with different sPSU contents were completely dissolved into NMP under continuous mechanical stirring for 24h. Before casting, the dope solution was rested and degased overnight at room temperature. All -6-


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the prepared casting solutions were homogeneous and transparent. The series names and corresponding dope compositions for each substrate were summarized in Table 1. It is noted that when the sPSU content in total polymer is higher than 66.7wt%, the formation of PSU/sPSU blend membrane is difficult due to slow precipitation19. The PET mesh was taped smoothly to a clean plate glass. A 100µm casting knife was used for spreading the dope solution uniformly onto the mesh. The nascent film was immersed quickly in a coagulation bath (DI water) to initiate NIPS. Afterwards, the as-prepared membrane substrate was rinsed and preserved in pure water until further use. Table.1 The composition of PSU and PSU/sPSU casting solutions Casting solution composition (wt%)

sPSU content

Membrane

PSU/sPSU in the total

code

PSU

sPSU

mass ratio

NMP polymer (%)

sPSU-0

15

0

85

0

sPSU-1

13.5

1.5

85

10

9:1

sPSU-2

12

3

85

20

4:1

sPSU-3

10

5

85

33.3

2:1

sPSU-4

7.5

7.5

85

50

1:1

sPSU-5

5

10

85

66.7

1:2

2.3. Fabrication of PA active layer By IP of MPD and TMC, the PA active layer on top surface of the prepared PSU and PSU/sPSU substrates was formed. The process involves the following steps: immersing the membrane support in 2.0wt% MPD/water solution for 2min; removing the excess amine solution residual from the support surface; placing 0.15wt% TMC/hexane solution on the MPD-saturated support for 1min; rinsing the PA skin layer with hexane to remove the unreacted TMC; curing the membrane at 60°C for 3min. The resultant composite membranes were placed in pure water at 4°C before osmosis performance tests. The prepared TFC-FO membranes were denoted as TFC/sPSU-0,

TFC/sPSU-1,

TFC/sPSU-2,

TFC/sPSU-3,

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TFC/sPSU-4

and


TFC/sPSU-5, corresponding to the membrane substrates sPSU-0, sPSU-1, sPSU-2, sPSU-3, sPSU-4 and sPSU-5. 2.4. Characterization of membranes The structure and morphology of mesh-embedded PSU/sPSU substrates and TFC-FO membranes were viewed with field emission scanning electron microscope (FESEM, JSM-7800F, JEOL). Before SEM imaging, all the samples were vacuum dried at 30°C for 12h and coated by gold using a sputtering coater (108auto, Cressington Scientific Instruments). For cross-sectional view, the sample was placed in liquid nitrogen and carefully cut with a sharp razor knife. X-ray photoelectron spectroscopy (XPS, Thermo Scientific) was conducted for measuring the atomic percentage of PA skin layer of TFC-FO membrane. The water contact angles of PSU and PSU/sPSU substrates in dry state were acquired through an optical goniometer (DSA100, KRÜSS). The contact angle value for each designated membrane support was taken as an average of three samples (seven measurement points per sample). The membrane surface chemistry was characterized via attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR, Thermo Scientific) with 2cm-1 resolution. The porosity of membrane substrate was obtained with dry-wet weight method. The wet circular membrane stored in DI water was firstly weighed (mw, g) after using filter paper to absorb the water droplets from its surface. The dried membrane disc was then re-weighed (md, g). According to Eq. (1), the porosity, ε (%) was calculated. − ε= × 100% (1) × ×

where t (cm) and A (cm2) represent respectively the thickness and area of the test sample in wet state. A bench thickness gauge (CH-12.7-STSX, Liuling Instrument Factory) was applied for measuring the membrane thickness. The measurement on the

pure water permeability (PWP, Lm-2h-1bar-1) of mesh-embedded substrate was carried out at 25±1°C and a hydraulic pressure of 100kPa using a customized cross-flow filtration system as depicted elsewhere20. The mean pore radius, rm (µm) of substrate -8-


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membranes was determined by the Guerout-Elford-Ferry equation: =

(2.9 − 1.75 ) × 8 (2) × × ∆

where Q (m3s-1) is the volumetric flow rate of permeate water under an operational pressure ∆P (100kPa) and µ is the viscosity of water (8.9×10-4Pa·s). Membrane mechanical properties including tensile strength (MPa), elongation at break (%) and Young's modulus (MPa) were characterized via a universal electronic tensile tester (Haida HD-609A) following Han et al.16. 2.5. Osmosis performance evaluation In terms of Jw, reverse salt flux (Js) and Js/Jw, the osmosis performance of the as-prepared TFC-FO membrane was obtained through a custom cross-flow FO system as presented in our previous work20. During tests, FS and DS flowed co-currently at 6.4cm/s along membrane surface. All the tests were performed under both FO mode (FS against active layer) and PRO mode (DS against active layer) with DI water feed. In FO mode, NaCl solutions of four concentrations (0.5M, 1 M, 1.5 M and 2 M) were prepared and used as draw solutions. In PRO mode, 1.0M NaCl solution was applied as DS. The changes in FS conductivity and DS weight were recorded automatically through a conductivity meter (DDSJ-308A, INESA Scientific Instrument) and digital balance (BSA6202S-CW, Sartorius), respectively. Each test was conducted for about 30 min and repeated three times with different membrane coupons. The change of DS concentration during osmosis tests was negligible because the osmotic water flow was small in relation to DS volume. The Jw (Lm-2h-1 or LMH) was calculated via Eq. (3): =

∆ (3) ∆ ×

where ∆V (L) represents the permeate water volume over a set time period ∆t (h) and Am (m2) is the test area of membrane. The water density is supposed to be 1000g/L. The Js (gm-2h-1 or gMH) was calculated via Eq. (4): " =

#$ × $ (4)

× ∆

where Ct (g/L) and Vt (L) are the draw solute (herein NaCl) concentration and volume -9-



of FS over ∆t, respectively. The Ct was determined based on a standard curve of draw solute concentration-conductivity. 2.6. Determination of membrane intrinsic properties The intrinsic properties (i.e., water permeability (A), salt permeability (B) and S) of the developed TFC-FO membranes were obtained by following a standard testing procedure introduced by Tiraferri et al21. 3. Results and discussion 3.1. Characteristics of mesh-embedded membrane substrates Incorporating hydrophilic materials into membrane substrate is able to enhance osmosis performance for the resultant TFC-FO membrane14, 22. Herein, PSU/sPSU blends at varied mass ratios were used as support materials for FO membrane. The used sPSU with a degree of sulfonation of 15% is water-insoluble and will not leak out from the blend membranes, which maintains the long-term stability of membrane. The FTIR spectra of PSU and PSU/sPSU support membranes are displayed in Fig.1. A small absorption peak at ~1028cm-1 for PSU/sPSU blend membranes is ascribed to symmetrical stretching vibration of -SO3H groups, which indicates the successful sPSU incorporation in support membranes. Another characteristic peak at ~1180cm-1 that corresponds to asymmetrical stretching vibration of -SO3H groups is not readily observed due to near overlapping absence23.

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sPSU-5

-1

1028cm

sPSU-4

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


sPSU-3

sPSU-2

sPSU-1

sPSU-0

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Fig.1 FTIR spectra of PSU and PSU/sPSU membrane substrates. For clarification, the spectra were shifted parallelly. Blending hydrophilic polymer into dope solution can affect its phase inversion behavior and thus change the fabricated membrane properties17, 24. The effect of blend ratio of PSU/sPSU on the mesh-reinforced substrate formation was investigated. Fig.2 illustrates the surface and cross-section FESEM images for mesh-embedded supports. The top surfaces of all six mesh-embedded supports have a relatively dense and nano-porous morphology. This is because an instantaneous demixing occurs for the top surface layer of polymer solution film during NIPS. The observed membrane surfaces without any visible defects are suitable to deposite qualified PA layers subsequently formed via IP. Interestingly, at a low magnification, the pattern of the intersections of mesh lines is visibly seen on the top surfaces of sPSU-3, sPSU-4 and sPSU-5 substrates, indicating their ultra-thin structures in dry state. The other three supports are relatively smooth on the surface. This phenomenon has not been clearly indicated before. It is known that water uptake of membranes, physically reflected by the swelling ratio, is closely related with the affinity between water and membrane materials25. Hence, with an increase in sulfonated material content, the PSU/sPSU matrix tends to have a high water-induced swelling because of its enhanced hydrophilicity. After being dry, the highly sulfonated membranes may be subject to -11-



high degree of shrinkage. Membrane dehydration can result in a significant decrease of osmotic water flux in FO26. Moreover, a serious drying shrinkage may give rise to irreversible membrane damage. The above findings implicate that precaution must be taken when preparing and using PSU/sPSU supported TFC-FO membrane to prevent potential membrane dehydration.

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Cross-section

sPSU-4

sPSU-3

sPSU-2

sPSU-1

sPSU-0

Top surface

sPSU-5

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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Bottom surface


Fig.2 FESEM images of mesh-embedded membrane substrates For practical applications, the developed FO membrane is required to possess reasonable mechanical properties to withstand the nominal hydraulic pressure in FO membrane module. For effective control of ICP, embedding a nonwoven/woven fabric within the sublayer of FO membrane as mechanical support is the state-of-the-art method8, 16-17, 27. The specially selected open mesh enables dope solution penetration before solidification in a coagulation bath, thus pushing the fabric upward into the polymer layer. As displayed in Fig.2 (the second column), the PET mesh is integrated within the PSU and PSU/sPSU matrixes instead of being a separate backing layer on the bottom, which reduces the membrane overall thickness. The presence of such backing mesh can influence the support membrane pore structure5, 27. Regarding the cross-sectional images of Fig.2, some finger-like pores and sponge-like regions are terminated by the mesh lines before they reach the bottom surface of substrates. This will likely result in a high membrane tortuosity and hence compromise the structure parameter12. Also, the impermeable mesh filaments may impart an additional resistance to water transport28. Therefore, the performance improvement for the fabric backed FO membranes is more challenging. The pore formation and properties of sPSU hydrophilized PSU supports are mainly associated with the polymer blend ratios29. It is observed that the mesh-embedded substrate cast from pure PSU exhibits a large number of finger-like macrovoids initiating just beneath the top surface. The sPSU-4 and sPSU-5 substrates show a fully sponge-like or cellular morphology. In between, the cross-section morphology of PSU/sPSU substrates gradually changes with the increase of sPSU content: (1) the width of macrovoids decreases firstly and increases afterwards; (2) the number of macrovoids is reduced; (3) the position of macrovoids becomes deeper away from top surface. These are due to the fact that the incorporation of sulfonated material in dope solution tends to cause a less rapid (delayed) demixing and phase inversion in comparison with conventional hydrophobic materials24,

29

. The delayed demixing is expected to suppress the

formation of finger-like macrovoid and facilitate the growth of sponge-like structure. -14-


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The content of -SO3H groups has some influence on the bottom surface morphology of fabric-free sulfonated polymer modified membrane15, 24, 30. As shown in Fig.2 (the last column), the addition of sPSU in PSU substrate seems to facilitate the formation of large pore (pointed with the yellow arrow in Fig.2) at the bottom surface, while the pure PSU substrate possesses a bottom surface with relatively dense morphology. The formation of different bottom layers depends on two factors: (1) the time between the onset of precipitation and film floating; (2) the affinity between casting solution and glass plate30. A more open backside surface is expected to enhance the transport of solute and water in support layer. Table 2 and Fig.3 summarize the porosity, contact angle, PWP, mean pore size, thickness and mechanical properties of the developed mesh-embedded substrates. The results indicate that the porosities of PSU/sPSU blend supports are comparable but much higher than that of pure PSU support, which is consistent with previous findings. The enhancement in membrane porosity may result from that: (1) sPSU blending causes a delayed demixing; (2) the hydrophilic modified support material, fully wet in water, increases the effective porosity15, 30. A high-porosity support layer is desired to mitigate the severity of ICP. The contact angles for both support membrane surfaces are firstly decreased and then increased with an increase of sPSU concentration from 0 to 66.7wt%. This “down-up” trend could imply that the polymer blend system starts to phase separate when the mass ratio of sPSU/PSU exceeds a certain value29-30. However, it is noteworthy that the membrane hydrophilicity is inevitably improved upon incorporation of sPSU containing hydrophilic -SO3H groups. The PWP values of the substrates are varied with different sPSU contents. The difference in PWP and its order results from the combined effects: (1) PWP is positively correlated with the pore size and porosity; (2) finger-like macrovoids tend to increase the PWP; (3) the proper increase of hydrophilicity can enhance the PWP; (4) a high degree of water-induced swelling can lower the PWP. For example, the sPSU-1 shows the highest PWP due to its relatively more macrovoids and increased hydrophilicity. The sPSU-5 has the lowest PWP because of its fully sponge-like structure and high water-induced -15-



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swelling. The mean pore sizes of the supports are in the UF range, which is suitable for the subsequent IP to form a defect-free PA active layer27, 31. The total thicknesses of these supports under wet condition are similar (70.0±3.0µm). But it can be seen from Fig.2 that the PSU and PSU/sPSU thicknesses at different locations are highly non-uniform due to the embedment of mesh, and regions where mesh fibers are located are thinner than those away from the fibers. Fig.3 shows that the mesh-reinforced substrates developed in this work possess excellent mechanical robustness. Their tensile stress and Young’s modulus are almost 10 times higher than those of freestanding sulfonated polymer based membranes15, 32. On the whole, the sPSU content has little influence on mechanical properties of the mesh-embedded PSU/sPSU substrates because the mesh mainly contributes to the high mechanical robustness.

Table.2 The porosity, mean pore size (dp), contact angle, thickness and PWP of the developed membrane substrates Contact angle (º)

Thickness a

Membrane

Porosity

code

(%)

Top surface

Bottom surface

(Lm-2h-1bar-1)

sPSU-0

79.2±1.1

76.9±0.6

80.4±4.5

1579.4±98.7

63.7±2.0

68.3±2.8

sPSU-1

85.7±2.0

73.0±0.9

73.9±4.7

2193.3±152.6

71.2±2.5

71.7±4.0

sPSU-2

86.1±0.8

74.0±1.5

74.5±2.2

1425.4±128.9

57.1±2.6

70.9±3.9

sPSU-3

87.0±1.2

79.0±0.6

98.0±1.1

1239.2±101.2

52.7±2.2

68.3±3.7

sPSU-4

85.3±1.0

80.0±1.3

106.9±2.4

963.8±28.0

47.4±0.7

72.7±2.5

sPSU-5

85.0±1.8

79.0±2.5

101.0±4.2

299.6±78.7

26.4±3.5

67.9±2.7

a

PWP

The thickness was measured under wet condition.

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dp (nm) (µm)

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120 800

90

60

30

0 sPSU-0

sPSU-1

sPSU-2

sPSU-3

sPSU-4

90

60

Young's modulus (MPa)

Elongation at break (%)

120

Tensile strength (MPa)

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


600

400

30

200

0

0

sPSU-5

Fig.3 The mechanical properties of mesh-reinforced membrane substrate 3.2. Characteristics of TFC-FO membranes FTIR spectra for the resultant TFC-FO membranes are shown in Fig.4. The characteristic peaks at ~1545 and ~1660cm-1 correspond respectively to the amide II (C-N stretching and in-plane N-H bending vibration) and amide I (C=O stretching vibration)8, indicating the successful synthesis of PA active layer. The appearance of characteristic peaks at ~1151, ~1294 and ~1323cm-1 attributed to sulfone groups (O=S=O) is also observed, indicating that the penetration depth of the beam is more than the PA layer thickness in ATR-FTIR measurement. Herein, the absorption peak intensity ratio of amide and sulfone groups in FTIR spectrum of each TFC-FO membrane is employed as a quantitative indication for its PA layer thicknesses and the results are shown in Fig.4. The higher intensity ratio of (-CONH-)/(-SO2-) indicates a thicker PA layer8, 33. It can be seen in Fig.4 that when the sPSU content in support layer increases, thickness of resultant active layer first decreases to reach a minimum and then increases. A thicker PA layer may result in higher resistance to salt and water transport. It is noted that the thickness of PA active layer is obviously lower than that of support membrane and ICP is mostly in the support. Furthermore, only the dense inner layer of PA film, but not the total film thickness, is responsible for membrane separation properties34-35.

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sPSU-5

I(C=O)/I(S=O=S)=0.56

sPSU-4

I(C=O)/I(S=O=S)=0.45

sPSU-3

I(C=O)/I(S=O=S)=0.47

sPSU-2

I(C=O)/I(S=O=S)=0.51

sPSU-1

I(C=O)/I(S=O=S)=0.61

sPSU-0

I(C=O)/I(S=O=S)=0.77

3500

3000

2500

2000

-1

-1

1500

1323cm

1545cm

1660cm

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

-1


1000

500

-1

Wavenumber (cm )

Fig.4 FTIR spectra of mesh-embedded PSU/sPSU supported TFC-FO membranes by baseline correction. For clarification, the spectra were shifted parallelly. The FTIR absorbance ratio of amide I at 1660cm-1 to sulfone groups at 1323cm-1 was included in the left figure. PA active layer synthesized via IP of MPD and TMC possibly is consisted of: (1) linear portion containing free pendant -COOH hydrolyzed from the unreacted -COCl and (2) crosslinked portion where the available -COCl groups are all involved in crosslinking. The crosslinking degree of PA plays a key role in determining the TFC membrane performance, e.g., salt rejection and water flux8, 36. XPS surface analysis was employed for investigation of the crosslinking of PA formed upon different support membranes and the relevant results are summarized in Fig.5 and Table 3. In PA synthesized from TMC and MPD, the oxygen atoms are present in the -COOH and -CONH-, whereas nitrogen atoms are from -CONH- and the -NH2 end groups. The theoretical O/N ratios of fully crosslinked and fully linear PA are 1.0 and 2.0, respectively. The crosslinking for PA layer thus is estimated with the O/N ratio. As tabulated in Table 3, the measured O/N ratios are closer to 1.0, which indicates that all six TFC-FO membranes have a highly crosslinked active layer. The results are consistent with the FTIR characterization where the characteristic peak of -COOH group is not observed. The PA film with more crosslinked structures tends to be less -18-


Page 18 of 33

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permeable for water and salt. Moreover, Fig.4 and Table 3 indicate that the sPSU content in support layer seems to have some effect on active layer formation. This may be because that the sPSU content influences the MPD transfer to reaction zone, thereby affecting the extent and rate of IP. It is generally recognized that the reaction of MPD and TMC mainly occurs in organic phase because diffusion of acyl chloride to water is relatively low. In the early stage, amine diffusion from pores or surface of the support membrane to interface of the polymerization is the rate-controlling step37. Thereafter, the newly-formed PA film becomes a barrier to further diffusion of amine into reaction region38-39. The IP of MPD and TMC is thus self-limited and diffusion-controlled. The faster MPD diffuses, the more likely it is to form a dense and crosslinked PA film, and conversely, the slower MPD diffuses, the more chance there is of linear PA formation. Thickness of as-formed PA film also correlates closely with the MPD amount for IP. In the case of PSU/sPSU blend supports with extended -SO3H groups and improved hydrophilicity, the more MPD solution can be adsorbed over support surface for the following IP. This, to some extent, facilitates the MPD migration to reaction interface, and hence a more crosslinked PA skin layer is favored. The denser PA film blocks, to a larger extent, the MPD diffusion for IP, which favors to a thinner active layer. It is shown from Table 3 and Fig.4 that with the raise of sPSU loading from 0 to 20wt%, the crosslinking degree and thickness of the formed active layer are increased and decreased, respectively. The much more sPSU in support layer may hold back the MPD diffusion because of the attractive interactions between support material and MPD36-37. This causes to decline the crosslinking degree and thickness of PA in TFC/sPSU-3 and TFC/sPSU-4 owing to the insufficient MPD supply to reaction zone. With the further increment in sPSU content, an excess of MPD could remain over substrate surface, which allows more MPD diffusion for IP. Thus, the crosslinking degree and thickness of PA in TFC/sPSU-5 are increased. It is noteworthy that the excess MPD may adversely affect active layer formation39.

-19-



C1s

O1s N1s

sPSU-5

sPSU-4

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

Page 20 of 33

sPSU-3

sPSU-2

sPSU-1

sPSU-0

1000

800

600

400

200

0

Binding Energy (eV)

Fig.5 XPS spectra of TFC-FO membranes. For clarification, the spectra were shifted parallelly.

Table.3 XPS surface analysis of TFC-FO membranes Membrane

C%

N%

O%

N/C

O/C

O/N

TFC/sPSU-0

75.64

10.92

13.44

0.144

0.178

1.231

TFC/sPSU-1

73.95

11.32

14.74

0.153

0.199

1.302

TFC/sPSU-2

76.12

10.91

12.96

0.143

0.170

1.188

TFC/sPSU-3

76.80

9.87

13.33

0.129

0.174

1.351

TFC/sPSU-4

76.65

10.17

13.19

0.133

0.172

1.297

TFC/sPSU-5

76.61

10.65

12.74

0.139

0.166

1.196

The SEM and AFM images for resultant TFC-FO membranes are displayed in Fig.6 and Fig.7. The TFC membranes all present the typical ridge-and-valley top surface appearance. As aforementioned, sPSU loading in support layer can impact the synthesis of active layer. This fact is further confirmed from the AFM characterization. As shown in Fig.7 and Table 4, the more crosslinked and thinner PA film tends to be smoother with less leaf-like structure, since roughness of TFC membrane surface originates from the growth and overlap-add of rigid PA chain38. A rougher active layer can provide a larger surface area for water transport but is more prone to fouling. In conclusion, although TFC membrane allows separate optimization on support -20-


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membrane and active layer, the modifications for support membrane inevitably affect the active layer properties.

-21-



Cross-section

TFC/sPSU-4

TFC/sPSU-3

TFC/sPSU-2

TFC/sPSU-1

TFC/sPSU-0

Top surface

TFC/sPSU-5

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

-22-


Page 22 of 33

Page 23 of 33 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.6 FESEM images of TFC-FO membranes with different substrates

TFC/sPSU-0

TFC/sPSU-1

TFC/sPSU-3

TFC/sPSU-4

TFC/sPSU-2

TFC/sPSU-5

Fig.7 AFM surface images for TFC-FO membranes Table.4 Roughness for TFC-FO membrane surface Membrane

Rq

Ra

TFC/sPSU-0

86.08±7.35

68.70±5.26

TFC/sPSU-1

89.65±11.67

72.05±10.39

TFC/sPSU-2

79.10±6.65

62.85±5.16

TFC/sPSU-3

91.15±2.76

71.50±1.41

TFC/sPSU-4

91.50±6.08

73.20±5.37

TFC/sPSU-5

112.00±2.83

87.80±3.68

Table 5 lists the transport properties (i.e., A and B) and S of as-prepared TFC-FO membranes. As described above, the sPSU content in support layer can affect the active layer properties and thus alter its transport properties. The A value for resultant TFC-FO membrane increases from 1.21 to 2.65Lm-2h-1bar-1 with the sPSU content increased from 0 to 50wt%. The slight decline in the A of TFC/sPSU-5 could be due to its thicker PA layer (Fig.4) The Dense polymeric membranes, including PA TFC membranes have been postulated to follow an inherent permeability-selectivity tradeoff, in which increasing A value comes with even greater B40. As expected, the B value of the resultant TFC-FO membrane changes following the same trend as that -23-



Page 24 of 33

seen for A. Maximizing the A/B ratio is the key required property for the active layer of FO membrane3. Table 5 shows that compared with TFC/sPSU-0, the PSU/sPSU supported TFC-FO membrane (except for TFC/sPSU-5) has a higher A/B. Higher A/B means better permselectivity of the formed active layer and less draw solute loss in FO. The S value that quantifies the ICP extent in support membrane provides a simple yardstick for designing and comparing FO membranes. The S for resultant TFC-FO membrane is decreased from 371µm to 220µm as the sPSU loading in the substrate membrane is increased up to 33.3wt%. This is resulted from the improvements in porosity, hydrophilicity and pore connectivity of support membrane. With further increase of sPSU loading to 66.7wt%, the S value levels off. The currently achieved S value of about 220µm is the lowest compared to those of previously reported fabric-reinforced TFC-FO membranes16-17,

21, 27, 41

. The detailed comparisons are

presented in Table 6. Besides, the S obtained here is low even in comparison with the freestanding sulfonated polymer modified support layer19, 32. Table.5 Intrinsic properties for TFC-FO membranes Membrane

A (Lm-2h-1bar-1)

B (Lm-2h-1)

A/B (bar-1)

S (×10-6m)

TFC/sPSU-0

1.21±0.01

0.22±0.05

5.67±1.15

371±5

TFC/sPSU-1

1.58±0.15

0.26±0.08

6.22±1.39

253±17

TFC/sPSU-2

1.83±0.35

0.30±0.04

6.12±0.29

244±20

TFC/sPSU-3

1.93±0.16

0.31±0.06

6.27±0.69

220±6

TFC/sPSU-4

2.65±0.08

0.44±0.07

6.04±0.77

230±9

TFC/sPSU-5

2.35±0.01

0.44±0.03

5.41±0.41

222±8

-24-


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Table.6 Performance comparisons with previously reported fabric-reinforced TFC-FO membranes FO mode Membrane

DS

FS

PRO mode S, µm

Reference

0.23

577

[16]

N/A

N/A

513

[21]

49.40

7.00

0.14

314

[27]

0.49

23.90

36.33

1.52

N/A

[41]

5.64

0.23

55.05

9.01

0.17

401

[17]

29.95

3.11

0.10

66.89

6.87

0.10

277

[17]

DI

30.14

5.73

0.19

47.67

8.10

0.17

256

[16]

DI

34.86

10.11

0.29

58.60

16.99

0.29

300

[16]

DI

29.02

5.28

0.18

49.92

8.80

0.18

220

This work

DI

31.76

6.11

0.19

54.55

9.88

0.18

230

This work

Jw,

Js,

Js/Jw,

Jw,

Js,

Js/Jw,

LMH

gMH

g/L

LMH

gMH

g/L

15.28

4.89

0.32

32.56

7.49

23.2

7.78

0.34

N/A

DI

17.10

6.05

0.35

DI

19.20

9.41

DI

24.17

DI

1M HTI-TFC

DI NaCl 1.084M

0.5mM

NaCl

NaCl

Oasys TFC 1M TFC/PSF9 NaCl 0.5M PAI 2# MgCl2 1M TFC/SPSU-17 NaCl 1M TFC/SPSU-45 NaCl 1M #2 sPPSU-TFC NaCl 1M #1 sPPSU-TFC NaCl 1M TFC/sPSU-3 NaCl 1M TFC/sPSU-4 NaCl

3.3. Osmosis performance for TFC-FO membranes Mesh-embedded PSU/sPSU supported TFC membranes with various contents of sPSU were further evaluated in FO, and the results including Jw, Js and Js/Jw are presented in Fig.8. The PSU/sPSU supported TFC membranes generally gain much -25-



higher Jw than the TFC/sPSU-0. And, the Jw increases gradually as the sPSU content increases up to 50wt%. The TFC/sPSU-4 achieves a Jw of 31.76LMH with Js of 6.11gMH in FO mode with 1M NaCl as DS. Under the PRO mode, the Jw for TFC/sPSU-4 can be up to 54.55LMH with Js of 9.88gMH when using 1M NaCl as DS. The lower ICP in support layer and the less water transport resistance in active layer contribute to such remarkable improvement. TFC/sPSU-5 shows a slight decline in water flux because of its thicker active layer. It is also observed that the Jw and Js in FO mode increase with increasing DS concentration and are lower than those in PRO mode. The Js/Jw, representing the solute mass loss of DS per unit volume of osmotic water, is determined by permselectivity of active layer42. Fig.8 shows that for each FO membrane, the Js/Jw is similar at different DS concentrations. The Js/Jw for each DS decreases first and then increases with the raise of sPSU loading, and TFC/sPSU-3 shows the smallest Js/Jw of 0.18g/L.

FO mode

TFC/sPSU-0 TFC/sPSU-1 TFC/sPSU-2

40


Water 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 26 of 33

(a)

30

20

10 0.5

1.0

1.5

DS concentration (M)

-26-


2.0

Page 27 of 33

60 PRO mode

Water flux (LMH)

(d)

50

40

30

s C/ TF

0 -1 -2 -3 -4 -5 USU SU SU SU SU PS sP sP sP sP sP C/ C/ C/ C/ C/ TF TF TF TF TF

FO mode

TFC/sPSU-0 TFC/sPSU-1

10


Reverse salt flux (gMH)

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


TFC/sPSU-5

8 (b)

6

4

2

0 0.5

1.0

1.5


-27-


2.0


15

Reverse salt flux (gMH)

(e)

PRO mode

10

5

0

s C/ TF


0.3

Specific reverse salt flux (g/L)

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 33

TFC/sPSU-0

TFC/sPSU-1

TFC/sPSU-2

TFC/sPSU-3

TFC/sPSU-4

TFC/sPSU-5

FO mode

(c)

0.2

0.1

0.0 0.5

1.0

1.5


-28-


2.0

Page 29 of 33

0.3 PRO mode

(f)

Specific reverse salt flux (g/L)

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


0.2

0.1

0.0

s C/ TF


Fig.8 Water flux (a, d), reverse salt flux (b, e) and specific salt flux (c, f) of TFC-FO membranes under FO and PRO modes. In PRO mode, 1M NaCl was used as DS. 4. Conclusion In this work, TFC-FO membrane with mesh-embedded PSU/sPSU substrate was fabricated. The embedment of mesh within the sublayer imparts excellent mechanical robustness. The sPSU incorporation affects the hydrophilicity, porosity, pore structure and pore size of the PSU/sPSU substrates, and the total thickness, crosslinking degree and roughness of the corresponding TFC-FO membrane active layers. Appropriate sPSU blending effectively decreases the ICP and optimizes the permselectivity of active layer. TFC membranes with sPSU modified substrates gain a higher Jw (up to 31.76LMH in FO mode) with consistently low Js/Jw and a smaller S (as low as 220µm). Acknowledgments This research was funded by the National Natural Science Foundation of China (Grant No. 51503205 and 51478452), Chinese Academy of Sciences “Light of West China” Program, Three Hundred Leading Talents in Scientific and Technological Innovation Program of Chongqing (No. CSTCCXLJRC201714), and Key Application and Development Program of Chongqing Science and Technology Commission (No. -29-



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sulfonated polysulfone

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