Constructing substrate of low structural parameter by salt induction for high-performance TFC-FO membranes

Journal Pre-proof Constructing substrate of low structural parameter by salt induction for highperformance TFC-FO membranes Liang Shen, Xuan Zhang, Lian Tian, Zhou Li, Chun Ding, Ming Yi, Chao Han, Xi Yu, Yan Wang PII:

S0376-7388(19)32363-4

DOI:

https://doi.org/10.1016/j.memsci.2020.117866

Reference:

MEMSCI 117866

To appear in:

Journal of Membrane Science

Received Date: 30 July 2019 Revised Date:

17 January 2020

Accepted Date: 19 January 2020

Please cite this article as: L. Shen, X. Zhang, L. Tian, Z. Li, C. Ding, M. Yi, C. Han, X. Yu, Y. Wang, Constructing substrate of low structural parameter by salt induction for high-performance TFC-FO membranes, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117866. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Author Statement

Liang Shen: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data Curation, Writing Original Draft, Writing-Review&Editing, Visualization Xuan Zhang: Writing Original Draft Lian Tian: Investigation Zhou Li: Investigation Chun Ding: Investigation Ming Yi: Investigation Chao Han: Investigation Xi Yu: Visualization Yan

Wang:

Conceptualization,

Methodology,

Validation,

Investigation,

Writing-Review&Editing, Supervision, Project administration, Funding acquisition

Graphic Abstract for Constructing substrate with low membrane structural parameter by salt induction for developing high-performance TFC-FO membranes Liang Shen, Xuan Zhang, Lian Tian, Zhou Li, Chun Ding, Ming Yi, Chao Han, Xi Yu and Yan Wang *

Water bath Rapid solvent out-diffusion

Rapid water in-diffusion

IP Selective layer Substrate

Salt bath Delayed water in-diffusion

Rapid solvent out-diffusion

IP

Constructing substrate of low structural parameter by salt induction for high-performance TFC-FO membranes

Liang Shen a,b, Xuan Zhang a,b, Lian Tian a,b, Zhou Li a, Chun Ding a,b, Ming Yi a,b, Chao Han a,b, Xi Yu a,b and Yan Wang a,b,*

a

Key Laboratory of Material Chemistry for Energy Conversion and Storage

(Huazhong University of Science and Technology), Ministry of Education, Wuhan, 430074, P.R. China b

Hubei Key Laboratory of Material Chemistry and Service Failure, School of

Chemistry and Chemical Engineering, Huazhong University of Science & Technology, Wuhan, 430074, P. R. China

* Corresponding author. Tel.: 86 027-87543032; fax: 86 027-87543632. E-mail address: [email protected] (Yan Wang)

1

ABSTRACT

Forward osmosis (FO) is a promising membrane-based technology for water treatment and desalination. But, internal concentration polarization (ICP), a unique phenomenon in FO, generally leads to the sharp performance decline. In this study, a novel polyacrylonitrile (PAN) substrate with low polymer concentration (4 wt.%) of the thin-film composite (TFC) membrane with a low membrane structural parameter (S) is developed with sodium chloride (NaCl) aqueous solution as the coagulation bath, and exhibits high performance for FO applications. The existence of NaCl in the coagulation bath not only affects the phase inversion process significantly, resulting in the formation of a thin substrate with finely tuned morphology structure, but also benefits the formation of a uniform and defect-free top polyamide (PA) layer. Effects of NaCl content in the coagulation bath on the morphology and intrinsic properties of resulting PAN substrates, the formed PA layer, as well as the morphology and FO performance of resulting TFC membranes, are investigated systematically. Moreover, an extended study on cellulose acetate (CA) substrate is also conducted to study the universality of this salt induction method to fabricate high-performance TFC membranes. Compared to the control TFC membrane, modified TFC membranes show much lower S parameters and superior separation performance with the higher water flux (324% increment) and lower reverse salt flux (58% reduction).

Keywords: Forward osmosis; Thin-film composite membrane; Salt-containing coagulation bath; Substrate; Internal concentration polarization

2

1. Introduction

With potential advantages of relatively low energy consumption, low fouling tendency and high water recovery, the forward osmosis (FO) process has been widely applied in various fields, including desalination [1-4], wastewater treatment [5], power generation [6] and liquid food processing [7]. One prerequisite for the success of the FO technique is the availability of high-performance membranes. Thin-film composite (TFC) membrane, consisted of a porous support layer (substrate) and a dense active layer [8-10], is the most popular membrane type for the FO process, for its excellent separation performance and superior chemical stability [11, 12]. The dense selective layer acts as the barrier for feed solutes or contaminants while allows the permeation of water molecules [13, 14]. And the support layer, as the foundation of the composite membrane, not only provides mechanical strength and flow pathways, but also lays the platform for the growth of the top PA layer. The substrate properties, including its pore size, porosity, hydrophilicity, and surface roughness, exert great impacts on the microstructure of the formed top PA layer [15-18]. However, internal concentration polarization (ICP), an inevitable key issue in the support layer, influences the osmotic efficiency and thus the final water flux in the FO process. Generally, the ICP effect can be reflected by the membrane structural parameter (S), i.e., a lower S value indicates a less severe ICP phenomenon, and vice versa. It is positively related to the membrane thickness and pore tortuosity, and negatively relevant to the porosity of the substrate. Over past decades, tremendous efforts have been devoted to this end. The incorporation of inorganic nanomaterial fillers, such as graphene oxide [19], carbon nanotube [20] and etc. [21], is a common and effective approach to combine both 3

strengths of the polymer and inorganic nanomaterials. Meanwhile, it also tunes the micro-structure of the substrate with the altered phase inversion process, resulting in the improved permeation properties. Additionally, hydrophilic additives (such as polyvinylpyrrolidone and polyethylene glycol) in the dope solution can also affect the substrate structure significantly and improve the separation performance of TFC-FO membranes, resulted from the changed viscosity of the dope solution and the exchange rate between the solvent and non-solvent during the phase inversion process [22, 23]. Moreover, the employment of electro-spun nanofiber membrane as the substrate is also proven to be an effective method to alleviate the ICP effect, due to its low tortuosity, high porosity and inter-connected pores [24]. Other approaches such as adding pore forming agent [25] or non-solvent [26] into the dope solution, hydrophilization of substrate [27, 28], co-casting technique [29], are also reported to fabricate favorable substrates with low S parameters. Among them, the optimization of the coagulation bath condition is proven to be an efficient way to optimize the structures and properties of the substrate. Some researchers added solvents into the coagulation bath to delay the liquid-liquid demixing rate, resulting in the denser asymmetric membranes [30-36], or adjusted the coagulation bath temperature to tune the pore structure and morphology of resulting substrates [34, 37-39]. The employment of dual-coagulation bath was also reported to depress the formation of macrovoid structures significantly and translate the cross-sectional morphology from asymmetric structure to symmetric structure [40, 41]. Researchers also found that, the addition of inorganic salt into the coagulation bath could decrease the interaction of the polymer and water molecules, and therefore increase the out-diffusion rate of the solvent during phase inversion, resulting in the instant demixing and thus the formation of dense membrane structure with a low 4

thickness [42, 43]. It is therefore expected to result in a membrane substrate with a low structural parameter, which contributes to the low ICP effect of the composite FO membrane. However, no study has been reported on the salt-induced substrate of TFC membranes for FO applications yet. Therefore, in this work, the inorganic salt NaCl was added into the coagulation bath for the fabrication of polyacrylonitrile (PAN, 4 wt.%) and cellulose acetate (CA, 5 wt.%) substrates with low dope concentrations, in order to achieve the expected low S parameter of resulting TFC-FO membrane. Effects of NaCl concentration in the coagulation bath on the structures and separation properties of resulting substrates, as-formed PA layer and the corresponding separation performance of TFC membranes are systematically studied. An extended study on cellulose acetate (CA) substrate fabricated with the salt induction method and the resulted TFC-FO membranes are also conducted to explore the universality of this proposed method in this work.

2. Materials and methods

2.1 Materials

Polyacrylonitrile (PAN, Mw = 250 kDa) and cellulose acetate (CA, Mw = 100 kDa) powders were bought from Hubei Chushengwei Corporation and Acros Organics respectively. M-phenylenediamine (MPD, 99.5%) and trimesoyl chloride (TMC, 98%) as IP reaction monomers were bought from Aladdin. Solvents of N-methyl pyrrolidone (NMP, anhydrous, ≥99.5%) and hexane (anhydrous, AR), inorganic reagents of sodium chloride (NaCl, ≥99.5%) and sodium hydroxide (NaOH, 98%), and polyvinyl pyrrolidone (PVP, 99%) and polyethylene oxide (PEO, 98%) were 5

purchased from China National Medicine Corporation. The polyester nonwoven (Crane, CU632-UF) as the membrane support was provided by Crane (Hangzhou) Environmental Technology Co. Ltd, and its mechanical properties are shown in Fig. S1 of the Supporting Information.

2.2 Preparation of PAN and CA substrates

A prepared dope solution (PAN/NMP 4/96 wt.%, PAN/NMP 16/84 wt.% or CA/NMP 5/95 wt.%) after degassing was cast on the nonwoven pre-wetted with NMP by a casting knife (250 µm). Then the nonwoven with a casting solution on a glass plate was immersed in a water bath with NaCl (0-35 g NaCl / 100 g water) immediately. After that, the membranes phase inversed completely were stored in water. As for the PAN-4 (4 wt.% PAN) and PAN-16 (16 wt.% PAN) substrates, a hydrolyzation treatment (1.5 M NaOH, 45 ℃, 1 h) was also conducted to increase the hydrophilicity, with the details elaborated in our previous work [44]. The as-fabricated substrates were donated as SI-X (with PAN substrate) and SI-CA-X (with CA substrate) respectively, where X presents NaCl concentration in the coagulation bath.

2.3 Preparation of TFC membranes

The PA selective layer was prepared via interfacial polymerization (IP) process according to our previous studies [45-47]. Briefly, the obtained substrate was initially immersed into an aqueous solution (2 and 4 wt.% MPD solution for the fabrication of PAN and CA substrates respectively) for 5 min. Then the excessive amine solution was disposed by a rubber roller. Afterwards, an organic solution (0.1 wt.% TMC in 6

hexane) was brought into contact with the MPD-saturated substrate membrane for 1 min. Ultimately, after being drained off the TMC solution, the nascently-formed TFC membranes were left in air for 1 min and then stored in DI water before use. The as-fabricated TFC membranes were donated as SI-X-PA and SI-CA-X-PA.

2.4 Characterizations of substrate membranes

2.4.1 Characterizations

Surface hydrophilicity of the substrate membrane was evaluated by the water contact angle (WCA) measurement using a Contact Angle Goniometer (DSA 25, KRÜSS, Germany) at ambient temperature. The mechanical strength of PAN-4 substrate membranes without nonwoven was measured by the tensile test (MTS, CMT 800, USA). Surface and cross-sectional morphologies of substrate membranes were observed by Scanning Electron Microscopy (SEM, VEGA3, TESCAN, Czech). The surface topology of as-fabricated substrates was observed by Atomic Force Microscope (AFM, SPM9700, Shimadzu, Japan) to obtain the average roughness (Ra). The porosity (ε) of the substrates was measured by the gravimetric method, as calculated by Eq. (1). Briefly, a dry membrane sample was immersed into water for overnight. Then the saturated membrane sample was weighted after the water droplets on the membrane surface being removed gently by a tissue. ε = (

(



where respectively,



)/

)/

(

and

/

(1)

)

are the weights of the wet membrane and dry membrane and

are the densities of water and the polymer (1.184 and 1.3

g/mL for PAN and CA respectively) respectively. 7

2.4.2 Measurements of pure water permeability, pore size and pore size distribution

The pure water permeability (PWP, LMH/bar) of as-fabricated substrates was determined by a cross-flow nanofiltration (NF) apparatus (Suzhou Faith & Hope membrane technology Co., Ltd) equipped with a stainless membrane cell (17.35 cm2) and calculated by Eq. (2). The fresh membrane sample tested at 0.5 bar using deionized (DI) water as the feed. PWP =

,

∆

(2)

×∆ ×∆

The pore size and pore size distribution of PAN and CA substrates were determined by solute rejection (R) results of PVP or PEO (1000 ppm), which could be obtained from the feed and permeate concentrations measured by a Total Organic Carbon (TOC, Vario, Elementar, Germany). Stokes radii of PEO and PVP were calculated from Eqs. (3) and (4) based on the molecular weight, PVP: r = 8.4 × 10

'(

PEO: r = 10.44 × 10

× )*.+,-

(3)

'(

(4)

× )*.+./

where r presents the Stokes radius of the chosen solute, M refers to the corresponding molecular weight. The pore size and pore size distribution could be determined by the log-normal probability density function with PVP/PEO rejection curves fitted. The geometric mean pore size of solutes (Rs = 50%) was considered as the mean pore size (dP). The geometric standard deviation 01 was defined as the ratio of the solute diameters at R of 84.13% and 50%. The detailed testing procedures and theoretical descriptions can be found in our previous studies [48-50].

8

2.5 Characterizations of TFC membranes

2.5.1 Characterizations

Chemical properties of the PA layer were examined by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000, Thermo VG Scientific, UK). Surface hydrophilicity, morphologies and topologies of the PA layer were also characterized by Contact Angle Goniometer, SEM and AFM respectively.

2.5.2 RO and FO tests

Intrinsic transport properties of the as-fabricated TFC membranes were evaluated by RO tests, in terms of water permeance (A, LMH/Bar), salt permeability (B, LMH) and salt rejection (Rs, %). The testing parameters were the same as those for the substrate characterization, i.e., the testing pressure was 0.5 bar. DI water or 250 ppm NaCl aqueous solution was used as the feed to measure the pure water flux (J) and the salt rejection respectively. The permeate weight and salt concentrations in the permeate (Cp) and the feed (Cf) were monitored by a digital balance (FX3000-GD, AND, Japan) and a conductivity meter (FE30, Mettler Toledo, Switzerland) respectively. The pure water flux (J), water permeance (A), salt permeability (B) and salt rejection (Rs) can be calculated by Eqs. (5) - (8). 2=4 A=

∆3

5,67 ×∆8

:

(5) (6)

∆

?@

;< = =1 − ? B × 100% A

(7)

9

' DE DE

=

4(∆

F

(8)

∆G)

A lab-scale FO equipment was employed to conduct FO tests at 22 ± 0.5 °C, using DI water and NaCl aqueous solution (0.5 or 2.0 M) as the feed and draw solutions, respectively. The fresh membrane with an effective area of 3.87 cm2 was stabilized for 30 min before the data collection under FO and PRO modes. The variations of NaCl concentration in the feed solution and weight in the draw solution were monitored by the conductivity meter (FE30, Mettler Toledo, Switzerland), and balance (FX3000-GD, AND, Japan), respectively. The water flux (Jv, LMH) and the reverse salt flux (Js, gMH) were employed for the FO performance evaluation of the TFC membrane, as calculated by Eqs. (9) and (10). ∆3

(9)

∆(?K 3K )

(10)

2H = 4 2J = 4

5,I7 ∆8

5,I7 ∆8

S parameter is an important index of the ICP effect in the FO membrane, determined by Eqs. (11) and (12). FO mode: PRO mode:

L

4 × GO F

2H = < MN 4 × G L

2H = < MN

I

:P F

4 × GO :P F 4 × GI F

(11) (12)

3. Results and discussion

3.1 Characterization of PAN substrates fabricated with NaCl-containing coagulation bath

Since NaCl salts are poorly miscible with the polymer solvent, its addition in the coagulation bath is expected to decrease the solvent/coagulant (water) miscibility, 10

promoting the instant demixing and resulting in the formation of a thinner membrane structure [43]. It is also expected to result in the larger out-flux of the solvent than that of the non-solvent (water) during the phase inversion process, resulting in the instant demixing [42]. Additionally, a low polymer concentration (4 wt.% and 5 wt.% for PAN and CA dope solutions respectively) is employed in this study to obtain a relatively loose substrate, which also could magnify the modification efficiency to a greater extent, because of the faster diffusion of the solvent and nonsolvent through the dope solution of the lower viscosity. The morphological change is testified by SEM characterization as displayed in Fig. 1 (a). It shows that all PAN substrates exhibit the asymmetric structure with a smooth top surface and subjacent porous sublayer. For the pristine membrane (SI-0) with pure water as the coagulation bath, the relatively dense top surface without any visible pores is observed because of the instantaneous phase separation [51, 52]. Alternatively, with the increase of NaCl content in the coagulation bath, the top surface of modified substrates becomes rougher and much more porous with more and bigger visible pores, ascribed to the reduced driving force for the membrane precipitation and the delayed phase separation process, as a result of the reduced chemical potential of saline water (salt effect) [53]. In principle, the delayed phase separation should favor the formation of the sponge-like structure instead of the finger-like or macrovoid structure. However, no obvious increase of the sponge-like structure portion in the cross-sectional of modified substrates can be observed, probably resulted from the following two factors: (1) the relatively larger surface pores of modified substrates facilitate the exchange between the solvent and non-solvent; and (2) the addition of NaCl weakens the interaction between PAN and water molecules, promoting the instant demixing [42]. These two factors offset the 11

delayed phase separation in the initial stage. Therefore, the regular finger-like structure of the modified PAN substrate of modified PAN substrates can be obtained. Furthermore, with the increase of NaCl content in the coagulation bath, the top surface becomes thinner with the better interconnectivity between the top surface and the sublayer, which is beneficial to the lower permeation resistance and ICP alleviation.

(a)

(b)

Fig. 1. (a) SEM and (b) AFM images of the top-surface and cross-sectional morphology of PAN substrate membranes

The porosity (Q) of the fabricated substrates is measured and listed in Table 1. The 12

data reveal that the overall porosity increases from 91.5% (SI-0 membrane) to 96.0% (SI-35 membrane) with the higher NaCl content in the coagulation bath. The increased porosity is consistent to the morphology change of the corresponding substrates. In addition, SEM results also show that with the higher NaCl content in the coagulation bath, the thinner PAN substrate is obtained (60% reduction). As discussed above, the relatively larger surface pores of modified substrates, the declined solvent/coagulant miscibility, the increased out-flux of solvent and the weakened PAN/water interaction, all favor the accelerated phase separation, thus leading to the membrane shrinkage and therefore the thinner membrane substrate [49, 54]. Correspondingly, the decreasing trend of the membrane structural parameter S is expected, as determined by Eq. (13), R =

S × T

(13)

U

where M, V and Q are the membrane thickness, tortuosity and porosity respectively. Table 1 Intrinsic properties of PAN substrate membranes

Thickness Porosity Code (l, µm) (Q, %)

WCA (°)

PWP (LMH/Bar)

Mean pore size (dP, nm)

σg

MWCO (kDa)

SI-0

131.8±3.5

91.5±0.2 57.3±2.6 1303.3±31.9

22.6

2.06

630.1

SI-5

122.6±1.5

92.4±0.1 48.0±1.4 1481.8±60.7

24.2

1.98

653.7

SI-15

113.3±3.7

93.0±0.3 43.5±1.1 1592.6±27.3

26.3

1.88

674.0

SI-25

75.1±2.4

94.1±0.4 40.5±2.1 1726.3±57.8

28.2

1.82

708.9

SI-30

54.4±0.6

95.0±0.3 36.4±2.3 1863.6±35.2

29.1

1.77

719.2

SI-35

53.0±0.7

96.2±0.2 31.3±2.2 2043.2±70.6

30.6

1.73

736.5

The surface topologies of the as-fabricated PAN substrates were further investigated by AFM. Fig. 1 (b) shows that the top surface of the PAN substrates 13

becomes much rougher (86% increment in Ra) with the increase of NaCl content in the coagulation bath, possibly resulting from the severer membrane shrinkage during the accelerated phase separation process. And this affect is more obvious for the membrane prepared with the relatively low polymer dope concentrations employed in this work, since the accelerated flowing of solvent/non-solvent exerts greater effects on the microstructure of the membrane with a higher porosity. In addition, the accelerated phase separation also reduces the residual time for the PAN chains to stretch back to the original state, which is another factor leading to the rougher surface of the formed PAN substrates [49]. Accordingly, there is a negative correlation between WCA of modified PAN substrates and NaCl content in the coagulation bath, as summarized in Table 1, which should be ascribed to the larger pore size and rougher surface of the formed PAN substrate. The pore size distribution of the fabricated PAN substrates was determined by solute rejection tests. The good linearity shown in Fig. 2 (a) confirms the uniform pore size of membrane substrates, even though the PAN concentration in casting solutions is relatively low. In addition, the values of geometric standard deviations (σ) of modified substrates (listed in Table 1) are smaller than 2.0, also indicating the good uniformity of pore size distribution. Curves in Fig. 2 (b) and (c) reveals that the pore size of PAN substrates increases gradually with the higher NaCl content in the coagulation bath (from 22.6 nm of the SI-0 membrane to 30.6 nm of the SI-35 membrane), which is in accordance with the SEM morphologies. Above behaviors are ascribed to the altered phase separation process as discussed previously. MWCOs of the PAN substrates also exhibit the increasing trend, which is consistent with the changes in the membrane pore size.

14

(a)

(b)

(c)

Fig. 2. (a) Solute rejection curves, (b) probability density function curves and (c) cumulative pore size distribution curves of PAN substrate membranes

The pure water permeability of as-fabricated substrates is also studied. The results listed in Table 1 reveal that, with the higher NaCl content in the coagulation bath, the PWP gradually increases from 1350 LMH/bar (SI-0 membrane) to 2050 LMH/bar (SI-35 membrane), which should be attributed to the higher porosity, larger pore size, better surface hydrophilicity, and much lower thickness of modified substrate as aforementioned. The mechanical strength of those PAN-4 substrate membranes without nonwoven is also measured, as displayed in Fig. 3 and Table 2. It can be seen that the modified substrate membranes exhibit the higher Young’s modulus and tensile strength resulted 15

from the shrunken structure with smaller finger-like pores, indicating the improved mechanical property, which benefits the stability of resultant TFC membranes.

Fig. 3. Mechanical properties of the pristine and modified PAN substrate membranes

Table 2 Mechanical properties of the pristine and modified PAN substrate membranes Membrane Young’s Code modulus (MPa)

Elongation at break (%)

Tensile strength (MPa)

SI-0

92.02±3.99

2.07±0.25

1.53±0.09

SI-5

153.39±8.18

2.85±0.24

2.18±0.04

SI-15

199.12±3.47

4.26±0.21

3.14±0.11

SI-25

331.91±17.50

2.27±0.23

5.14±0.08

SI-30

606.66±12.43

2.07±0.27

8.97±0.23

SI-35

557.89±7.29

1.77±0.11

7.26±0.18

3.2 Properties of PA/PAN-TFC membranes

Theoretically, a substrate with the large pore size, high porosity and better surface hydrophilicity could absorb more MPD monomers into the pores with uniform 16

distribution, resulting in a thicker and rougher PA layer with a higher crosslinking degree [16, 55], as confirmed by XPS and SEM results in Figs. S2, 4 and Table 3. Surface elemental compositions of the PAN-based TFC membranes in Table 3 reveal that, O/N ratios of modified membranes (1.54 and 1.43) decrease compared to that of the control membrane (1.72), and decrease with increasing NaCl content in the coagulation bath for the PAN substrate preparation, indicating the higher crosslinking degrees of the modified PA layers (calculated to be 35.80% and 46.81%) than that of the pristine PA layer (calculated to be 20.62%).

Table 3 Surface elemental compositions of PAN-based TFC membranes by XPS analysis Code

C

O

N

O/N

Crosslinking degree (%)

SI-0-PA

72.56

17.35

10.09

1.72

20.63

SI-25-PA

71.12

17.53

11.35

1.54

35.80

SI-30-PA

69.15

18.16

12.69

1.43

46.81

Fig. 4 (a) shows that modified TFC membranes exhibit more obvious and uniform ridge-and-valley structure. Since modified substrates with the higher surface hydrophilicity and larger pore size can absorb more MPD, more MPD molecules diffuse into the organic phase to participate in the interfacial polymerization, resulting in the formation of more obvious and uniform ridge-and-valley structure [55]. Besides, the rougher surface of the modified substrate also favors the formation of a rougher PA layer [16]. The SEM results also reveal that with the increase of NaCl content (within 30 %) in the coagulation bath, the ridge-and-valley structure of the PA layers formed with a modified substrate becomes more obvious. However, since the excess NaCl (> 30%) in the coagulation bath leads to the formation of large pores on the 17

substrate surface, the formed PA layer exhibits less obvious ridge-and-valley structure with defects, possibly due to the fact that the nascently-formed PA layer could not cover the large pores and grow into the pores of the substrate. Besides, as shown in cross-sectional morphology images, the thicknesses of the PA layers formed on modified substrates (93% increment) are all higher than that on the control substrate, and increase with the higher NaCl content in the coagulation bath for the fabrication of the PAN substrates, ascribed to the more complete IP reaction. However, with the pores on the substrate surface are getting bigger with a further higher NaCl content (SI-35 membrane), the growth of PA layer into the substrate pores may occur and lead to the thinner PA layer of the SI-35-PA membrane. AFM images of TFC membranes in Fig. 4 (b) further confirms the rougher surface of the PA layers formed on modified substrates (59% increment), consistent with their more obvious ridge-and-valley structures, particularly for the SI-30-PA membrane. Accordingly, the lower WCAs of corresponding TFC membranes are resulted (28% reduction), as exhibited in Fig. 5.

(a)

18

(b)

Fig. 4. (a) SEM and (b) AFM images of PAN-TFC membranes

90

WCA (°)

80

70

60

50

SI-0-PA SI-5-PA SI-15-PA SI-25-PA SI-30-PA SI-35-PA

Fig. 5. Water contact angles of PAN-TFC membranes

3.3 Separation performance of TFC membranes with modified substrates

FO performance of resulted TFC membranes is investigated using DI water as the feed solution and 0.5 M NaCl as the draw solution. As revealed in Fig. 6 that the water fluxes of the modified membranes are higher than that of the control one in both operation modes. It increases initially with a higher NaCl content (till 30 wt%), ascribed to the thinner substrate with reduced ICP effect, as well as the rougher and more hydrophilic PA layer. Instead, the water flux decline of the SI-35-PA membrane 19

is possibly due to the relative smoother PA layer with lower hydrophilicity. Contrary to the permeability-selectivity trade-off relationship, the reverse salt flux of modified membrane is lower than that of the control membrane, and decreases as a function NaCl content in the coagulation bath (if within 30 wt%), resulted from the thicker and denser PA layer with the higher crosslinking degree. But the higher reverse salt flux of the SI-35-PA membrane is due to the thinner and defective PA layer aforementioned. Moreover, a 48-h FO test was also conducted to evaluate the stability of the modified membrane (SI-30-PA). As shown in Fig. S3, in spite of some slight performance decline, the relatively stable performance demonstrates the stability of the as–fabricated TFC membranes in this work.

45

FO

3.0

PRO 3.0

27

18

18

9

9

0

0

SI-0-PA SI-5-PA SI-15-PA SI-25-PA SI-30-PA SI-35-PA

2.4

2.4

1.8

1.8

1.2

1.2

0.6

0.6

Js (gMH)

27

Js (gMH)

36

Jv (LMH)

Jv (LMH)

36

45

FO PRO

SI-0-PA SI-5-PA SI-15-PA SI-25-PA SI-30-PA SI-35-PA

Fig. 6. FO performance of TFC membranes (0.5 M NaCl and DI water were used as the draw and feed solutions respectively)

The intrinsic transport properties of resulting TFC membranes are also investigated as displayed in Table 4. It shows that both water permeances (A) (1.22 2.12 LMH/bar) and salt rejections (Rs) (90.84 - 95.71%) of TFC membranes with modified PAN substrates are all higher than those of the control membrane (1.03 LMH/bar and 90.42 %), and both exhibit the up-and-down trend with the higher NaCl content in the coagulation bath, as the result of the higher surface roughness, more hydrophilic, and thicker PA layers with the higher crosslinking degrees, as analyzed 20

above. Therefore, the rejections of all TFC membranes with modified substrates are higher, and the trend is consistent with that of the water permeance. Besides, salt permeances (B) of all modified membranes (0.035 - 0.037 LMH) increase compared to that of the control membrane (0.031 LMH), excluding the SI-30-PA membrane with the highest rejection. Contrary to the salt rejection result, B/A ratios (an index of the membrane selectivity) of modified membranes decline compared to that of the control membrane, and present a down-and-up trend with the increasing NaCl content in the coagulation bath, indicating the higher selectivity of modified membranes. Moreover, the structural parameter (S) values of the modified membranes decrease sharply compared to that of the control membrane (from 560.46±62.65 to 84.35±6.43µm), and decrease with the rising NaCl content in the coagulation bath, indicating the significantly mitigated ICP effect in the much thinner substrate with a higher porosity.

Table 4 Intrinsic transport properties of the control and modified TFC membranes Membrane Aa , ID LMH/Bar

Bb, LMH

Rejection Rs, %

B/A, Bar

S, µm

SI-0-PA

1.03±0.04

0.031±0.002 90.42±0.68 0.031±0.002 560.46±62.65

SI-5-PA

1.22±0.06

0.035±0.001

90.84±0.50 0.029±0.002 216.11±20.21

SI-15-PA

1.38±0.06

0.037±0.001

91.69±0.47 0.026±0.002 114.36±12.38

SI-25-PA

1.86±0.09

0.036±0.001

93.70±0.40 0.019±0.001 100.62±27.19

SI-30-PA

2.12±0.10

0.027±0.002

95.71±0.51 0.013±0.002

SI-35-PA

1.33±0.04

0.035±0.001

91.45±0.45 0.027±0.002 417.32±44.79

*a

84.35±6.43

DI water was used as the feed solution in the RO test with an applied pressure of

0.5 bar; b

250 ppm NaCl solution was used as the feed solution in the RO test with an applied

pressure of 0.5 bar.

To further testify the amplified effects introduced by the salt bath in a low dope 21

concentration system, the PAN substrate membranes with a relatively higher polymer concentration of 16 wt.% and corresponding TFC membranes are fabricated for comparison. It can be seen from Fig. 7a, the modified PAN-16 substrates show lower thickness, smaller and more finger-liker pores than that of the pristine PAN-16 substrate, consistent to the modified PAN-4 substrate structure. But, these finger-like pores of modified substrate membranes don’t cross from the top to the bottom (especially for SI-PAN-16-30 membrane), probably resulted from the restricted solvent out-flow during the phase inversion with the high viscosity of the dope solution and the salt effect. Unlike the interconnected pore structure of the PAN-4 substrate, the shorter finger-like pores of the PAN-16 substrate leads to the lower porosity (as shown in Fig. 7b) and therefore the declined PWP (as shown in Fig. 7b). Additionally, the FO performance of these TFC membranes in Fig. 7d shows that, modified membranes, especially the SI-25-16-PA membrane, exhibit higher water fluxes (7.8-31.4% increase) and lower reverse salt fluxes (14.9-49.0% decrease) compared to those of the control membrane. Similarly, the RO performance of the TFC membranes listed in Table 5 reveals that, water permeances and salt rejections of modified membranes are all higher than those of the control membrane. Additionally, salt permeabilities, B/A ratios and S values of modified membranes are all lower than those of the control membrane. However, despite the separation performance improvement of the modified membranes with a modified PAN-16 substrate, the water flux improvement ratio (7.8-31.4%) and reverse salt flux decline ratio (14.9-49.0%) is less pronounced than those achieved with a modified PAN-4 substrate, as a result of the less improvement of in the PA layer roughness and thickness (Fig. 7c), less alleviated ICP effect of comparable S values, and the less permeable pore structure of the PAN-16 substrate. 22

(a)

(b)

(c)

23

(d)

Fig. 7 (a) SEM images, and (b) porosity and pure water permeation of PAN-16 substrate membranes, as well as (c) SEM images and (d) FO performance (2 M NaCl and DI water were used as the draw and feed solutions respectively) of corresponding TFC membranes with PAN-16 substrates

Table 5 Intrinsic transport properties of the control and modified TFC membranes with PAN-16 substrates Membrane ID

Aa , LMH/Bar

Bb, LMH

Rejection Rs, %

B/A, Bar

S, µm

SI-16-0-PA

0.76±0.05

0.27±0.03

85.64±0.32

0.35±0.01

225.78±11.82

SI-16-25-PA

0.92±0.03

0.17±0.03

91.43±0.22

0. 19±0.02

181.44±22.60

SI-16-30-PA

0.82±0.05

0.18±0.02

90.45±0.41

0.22±0.02

219.58±616.64

*a

DI water was used as the feed solution in the RO test with an applied pressure of

3.0 bar; b

1000 ppm NaCl solution was used as the feed solution in the RO test with an applied

pressure of 3.0 bar.

To testify the feasibility of this approach for developing high-performance PA-TFC FO membranes, CA substrate was also fabricated with a NaCl coagulation bath and employed for TFC membrane fabrication. SEM and AFM images of CA substrates in Fig. 8 (a) show similar results to those of the PAN-4 substrates, where the pristine CA substrate shows a smooth surface without visible pores, while 24

modified CA substrates exhibit rougher surface with visible pores. Additionally, both the pore size and pore amount increase with the increasing NaCl content in the coagulation bath. Similarly, as displayed in Fig. 8 (b) PWPs of modified CA substrates are all higher than that the pristine CA substrate, and increase with the rising NaCl content.

(a)

(b) 2800

PWP (LMH/Bar)

2400 2000 1600 1200 800 400 0

SI-CA-0

SI-CA-25

SI-CA-30

Fig. 8. (a) SEM and AFM images and (b) Pure water permeate of the control and modified CA substrates

25

SEM surface morphologies of corresponding TFC membranes with modified CA substrates in Fig. 9 (a) also exhibit more obvious ridge-and-valley structures with rougher surfaces. Accordingly, the resultant FO performance of CA-based TFC membranes in Fig. 9 (b) shows that, water fluxes of TFC membranes with modified CA substrates are much higher than that of the control membrane, and also increase with the increase of NaCl content in the coagulation bath, while the reverse salt fluxes of modified membranes are all lower than that of the control membrane, and show an opposite trend with the water flux result. The obtained results are consistent with the TFC membranes prepared with modified PAN-4 substrates, which demonstrates that the substrate fabrication employing NaCl-containing coagulation bath is a universal and effective strategy to prepare high-performance TFC-FO membranes.

(a)

26

(b) 1.2

30

30

1.0

1.0

25

25

0.8

0.8

20

20

0.6

0.6

15

15

10

10

0.4

0.4

5

5

0.2

0.2

0

0.0

0

SI-CA-0-PA

SI-CA-25-PA

SI-CA-30-PA

FO PRO

SI-CA-0-PA

SI-CA-25-PA

SI-CA-30-PA

1.2

Js (gMH)

Js (gMH)

35

FO PRO

Jv (LMH)

1.4

35

Jv (LMH)

1.4

40

40

0.0

Fig. 9 (a) SEM and AFM images, and (b) FO performance of CA-based TFC membranes (0.5 M NaCl and DI water were used as the draw and feed solutions, respectively.)

3.4 Benchmarking

Table 6 summarizes the FO performance of recently-reported TFC membranes with various substrates. It shows that the TFC membrane with a PAN-4 substrate fabricated with the NaCl-containing coagulation bath and 4 wt.% dope concentration exhibits a much lower S parameter, higher water flux, lower reverse salt flux, and lower specific reverse salt flux (Js/Jv), compared to the control TFC membrane. Additionally, compared with other reported TFC membranes, the performance enhancement of PAN-TFC membranes developed in this work is more significant, which should be ascribed to the morphology structure optimization by the salt induction during the phase inversion process.

27

Table 6 FO performance benchmarking of recently developed TFC membranes with various substrates

Substrate

Modification route

PAN

Adding NaCl in coagulation bath

PAN

PVDF

PSf

Adding NaCl in coagulation bath Incorporating SiO2@MWC NT into substrate Incorporating TiO2 into substrate

Substrate thickness (µm)

Substrate Porosity (%)

131.80

91.50

Modified

54.40

95.00

Control

91.8

82.10

49.1

71.90

111.10

82.00

Membrane Control

PAN/NMP = 4/96

PAN/NMP = 16/84 Modified Control Modified Control Modified Control

PPSU

Sulphonation Modified

PES

Adding NMP in coagulation

Dope composition (wt.%)

Control

PVDF/PVPNMP = 18/3/79 PVDF/PVP/SiO2@MW CNT/NMP = 18/3/0.75/68.25 PSf/PVP/NMP = 17.5/0.5/82 PSf/PVP/TiO2/NMP = 17.41/0.5/0.5/81.59 PPSU/EG/NMP = 13/16/71 sPPSU/EG/NMP = 13/16/71 PES/PEG/NMP/H2O = 20/36.9/36.9/6.2

88.90

87.60

70-90

71.00

70-90

75.00

30-50

65.00

30-50

83.41

100-120

80.00

S Jv (µm) (LMH)

Js (gMH)

Test conditions Js/Jv （Feed/draw solutions, (g/L) operation mode） ）

560. 46 84.3 5 225. 78 181. 44 729. 00 240. 00

22.10

4.10

0.19

16.10

4.15

0.26

31.20

6.66

0.21

10.00

2.10

0.21

980. 00 420. 00 2940 652. 00 219. 00

9.48

2.94

0.31

40.16

1.22

0.03

33.85

23.35

0.69

44.49

11.9

0.27

10.01

11.34

1.12

54.00

8.80

0.16

57.10

6.93

0.12

Ref.

DI water/0.5 M NaCl, PRO mode

This work

DI water/2.0 M NaCl, PRO mode

This work

DI water/1.0 M NaCl, FO mode

[48]

10 mM NaCl /0.5 M NaCl, PRO mode

[21]

DI water/2.0 M NaCl, PRO mode

[27]

DI water/2.0 M NaCl, PRO mode

[22]

28

bath

PVDF

PSf

PSf

PSf

PSf

PSf

Incorporating p-TiO2 into substrate Incorporating HNT into substrate Incorporating GO into substrate Incorporating CaCO3 into substrate then etched Incorporating LDH/ GO hybrid into substrate Incorporating zeolite into substrate

Modified Control Modified Control Modified

PVDF/PVP/NMP = 16/3/71 PVDF/PVP/p-TiO2/NMP = 16/3/0.9/70.1 PSf/PVP/DMAc = 17.5/0.5/82 PSf//PVP/HNT/DMAc = 17.5/0.5/0.5/81.5

100-120

82.00

114.10

79.40

252. 00 892. 80 268. 60 950. 00 370. 00

18.70

4.50

0.24

15.12

5.90

0.39

26.91

8.50

0.32

6.08

1.82

0.30

77.00

~52.00

~71.30

1060

19.77

3.36

0.17

3.60

~ 11.00

~ 3.06

~50.00

~74.80

Control

PSf/NMP = 15/85

150-180

71.20

4834

Modified

0.84

70-90

Modified

Control

7.50

74.00

191. 00

Modified

8.90

70-90

PSf/GO/DMAc = 18/0.045/82

Control

0.16

86.10

PSf/DMAc = 18/82

Modified

10.30

86.90

Control

PSf/CaCO3/NMP = 15/10/75 PSf/NMP/DMF = 12/66/22 PSf/LDH/GO/NMP/DM F =12/2/66/22 PSf/LiCl/PVP/NMP = 15.5/3/0.5/81 PSf/zeolite/LiCl/PVP/N MP = 15.5/0.5/3/0.5/80.5

62.70

150-180

90.00

66.80

75.60

58.60

82.20

70.60

81.40

66.30

79.80

525. 00 287. 00 138. 00 960. 00 340. 00

25.40

~ 57.00 ~ 2.24

15.30

7.20

0.47

23.60

6.90

0.29

~ 35 ~ 85

~ 15.00 ~ 0.43 ~ 55.00 ~ 0.65

DI water/1.0 M NaCl, FO mode

[50]

10 mM NaCl /0.5 M NaCl, PRO mode

[56]

DI water/0.5 M NaCl, FO mode

[57]

DI water/2.0 M NaCl, FO mode

[58]

DI water/1.0 M NaCl, PRO mode

[59]

DI water/2.0 M NaCl, PRO mode

[60]

29

4. Conclusion

In the present study, PAN substrates with the low membrane structural parameter is developed by employing a low dope concentration (4 wt.%) and a NaCl-containing coagulation bath, for the preparation of high-performance TFC-FO membranes with alleviated ICP effect. With the addition of NaCl in the coagulation bath, modified substrates exhibit the higher surface roughness and hydrophilicity, much lower thickness, better mechanical strength, as well as more and larger pores. Consequently, modified PAN substrates show significantly enhanced pure water flux compared to that of the pristine substrate. Additionally, resultant TFC membranes with modified substrates exhibit thicker PA layers with the higher crosslinking degree, better hydrophilicity, and rougher surface compared to those of the control membrane, resulted from more adsorbed MPD molecules and therefore the more complete IP reaction. Accordingly, in comparison with the control PAN-TFC membrane, the water fluxes of the modified PAN-TFC membranes increase significantly, and increase initially then decrease with the increasing NaCl content in the coagulation bath. Besides, the reverse salt fluxes of the modified PAN-TFC membranes decrease, and exhibit an opposite trend to the water flux result, attributed to the formed thicker and denser PA layer. Moreover, much lower S parameters can be achieved for modified PAN-TFC membranes as compared with that of the control membrane (560.46 µm), especially for the SI-30-PA membrane (84.35 µm). And the effect brought by the salt bath is more pronounced for the PAN substrate fabricated with a low dope concentration (4 wt.%), since the performance improvement and S value decline ratio of modified membranes with a modified PAN-4 substrate is more significant than those with a modified PAN-16 substrate, as a result of the low dope viscosity and fast 30

solvent-nonsolvent exchange. A further study on modified CA substrates prepared using NaCl-containing coagulation bath and the corresponding TFC membranes also exhibit the similar morphological change and performance improvement, which testifies the feasibility of the salt modification successfully.

Acknowledgement

We thank the financial supports from National Key Technology Support Program (no. 2014BAD12B06) and National Natural Science Foundation of China (no. 21306058). Special thanks are also given to the Analysis and Testing Center, the Analysis and Testing Center of Chemistry and Chemical Engineering School, as well as the State Key Laboratory of Materials Processing and Die & Mould Technology, in Huazhong University of Science and Technology for their help with material characterizations.

31

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39

Research Highlights for the manuscript “Constructing substrate of low structural parameter by salt induction for high-performance TFC-FO membranes” by Liang Shen, Xuan Zhang, Tian Lian, Zhou Li, Chun Ding, Ming Yi, Chao Han, Xi Yu and Yan Wang *

•

Substrates with low ICP were fabricated using salt-containing coagulation bath.

•

Modified substrates exhibit higher porosity, smaller thickness and large pore size.

•

TFC membranes with a modified substrate have rougher and thicker PA layers.

•

Modified TFC membrane possesses much higher water flux and lower reverse salt flux.

•

Modified TFC membrane has significantly lower structural parameter.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

No financial interest/personal relationship is considered as potential competing interests.