High-performance thin-film composite polyamide membranes developed with green ultrasound-assisted interfacial polymerization

Author’s Accepted Manuscript High-performance thin-film composite polyamide membranes developed with green ultrasoundassisted interfacial polymerization Liang Shen, Wei-song Hung, Jian Zuo, Xuan Zhang, Juin-Yih Lai, Yan Wang www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(18)31814-3 https://doi.org/10.1016/j.memsci.2018.10.014 MEMSCI16530

To appear in: Journal of Membrane Science Received date: 2 July 2018 Revised date: 17 September 2018 Accepted date: 2 October 2018 Cite this article as: Liang Shen, Wei-song Hung, Jian Zuo, Xuan Zhang, Juin-Yih Lai and Yan Wang, High-performance thin-film composite polyamide membranes developed with green ultrasound-assisted interfacial polymerization, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.10.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

High-performance thin-film composite polyamide membranes developed with green ultrasound-assisted interfacial polymerization Liang Shena,b, Wei-song Hungc, Jian Zuod, Xuan Zhanga,b, Juin-Yih Laic, Yan Wanga,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 c

Center for Membrane Technology, Chung Yuan Christian University, Chung-Li, 32023, Taiwan,

d

Singapore Institute of technology, 10 Dover Drive, Singapore 138683, Singapore

*

Corresponding author. Tel.: 86 13871464406; fax: 86 027-87543632, [email protected]

1

Abstract Thin-film composite polyamide (TFC-PA) membranes prepared via interfacial polymerization (IP) are widely reported in water treatment applications, but the inefficient mixing of reactive monomers in the traditional IP process may cause the diffusion-limited growth of PA layer and the incomplete IP reaction, resulting in limited control over the morphology and microstructure of PA layer, and thus the membrane performance. Various strategies to address above issues have been explored via different

chemical

modifications.

In this

work,

a

"green"

ultrasound-assisted interfacial polymerization approach is employed for the first time to fabricate TFC membranes for forward osmosis and nanofiltration applications. Ultrasound in IP process enlarges the mixing area of reactive monomers, facilitates the mass transport of the amine monomer, therefore contributing to an efficient monomer mixing and the resultant higher IP reaction degree. Additionally, the disrupted PA chain packing, more penetrated amine monomers and generated nanovoids contribute to a relative loose PA layer. Effects of ultrasound power on the microstructure (crosslinking degree and free volume) and morphology (roughness and thickness) of the resultant TFC membranes are also investigated systematically. In comparison

with

the

control

membrane,

TFC

membranes

formed

via

ultrasound-assisted IP exhibit much superior separation performance.

2

Graphical Abstract:

Ultrasound-assisted interfacial polymerization

Keywords: ultrasound-assisted interfacial polymerization; thin-film composite polyamide

membrane;

forward

osmosis;

nanofiltration;

microstructure

and

morphology

3

1. Introduction

With the fossil fuel depletion and population growth, the scarcities of water and energy impel the human beings to hunt for alternative supplies. Accordingly, water treatment technologies are in the urgent demand to ensure the availability of clean water and sustainable energy [1-4]. Among them, membrane-based separation techniques with high efficiency for water treatment have attracted more and more attentions from the academia and industries [5-7]. It is worth noting that, the thin-film composite polyamide (TFC-PA) membrane with the desirable separation performance under wide operational conditions [8] has been widely applied in fields of seawater or brackish water desalination and wastewater treatment, including nanofiltration (NF) [5, 9, 10], reverse osmosis (RO) [11-13] and forward osmosis (FO) [14-16] processes. The TFC-PA membrane is generally prepared by the formation of the PA active layer on a porous support layer via the interfacial polymerization [17-19]. As well known, the reaction occurs in the organic phase by the diffusion of diamine molecules from the aqueous phase, since the highly unfavorable partition coefficient of the acyl chloride molecules limits their availability in the aqueous phase [20]. Therefore, the mixing rate of reactants, particularly the mass transfer rate of amine monomers into the organic phase, will determine the IP reaction rate, and exert significant impacts on the morphology and molecular microstructure of the formed PA layer and the resultant membrane performance [20]. Various approaches have been explored to optimize the monomer diffusion rates and facilitate PA layer formation, by incorporating additives [21, 22], phase-transfer catalysts [23, 24] or co-solvents [20, 25] into one phase, or using molecular layer-by-layer method [26]. However, these approaches may incur the additional cost with the incorporation of some environmentally-hazardous 4

chemical agents in the interfacial polymerization, such as acetone [20], toluene [26], 1-butyl-3-methylimidazolium chloride [24]. To address aforementioned challenges, here we report a “green” ultrasound-assisted interfacial polymerization (UAIP) approach for the first time, to prepare high-performance TFC membranes. The critical step is to introduce ultrasound into the IP process, in order to contribute to an efficient mixing between two monomers and complete IP reaction. Like all sound energy, the ultrasound propagates via a series of compression and rarefaction waves induced in the medium [27]. When the ultrasound power is sufficiently high, the force in the rarefaction cycle may exceed the inter-molecular interaction of the medium, creating some cavitation bubbles, which may grow to the equilibrium size rapidly till the resonance frequency matches the ultrasound frequency [27]. However, the bubbles, which are unstable due to their mutual disturbance, may inflate suddenly to an unstable size and collapse violently [27], and act as the localized microreactor in the aqueous system, generating a lot of heat (causing the temperature to go up to several thousand degrees) and high pressure (resulting in more than one thousand atmospheres) [28]. The energy generated by the acoustic cavitation has been reported in various sonochemistry fields for the accompanying chemical and mechanical effects (enhanced mass transport, facilitated emulsification effect, bulk thermal heating, etc.) [27, 28]. In this work, the energy generated by the acoustic cavitation may facilitate the mass transport of amine monomers, enlarge the interface area, and accelerate the IP reaction, therefore contributing to the formation of rougher PA layer with a higher crosslinking degree [29, 30]. A schematic diagram of the traditional IP and UAIP process is proposed in Fig. 1. As illustrated in Fig. 1 (a), the mixing interface in traditional IP process is relatively narrow, resulting in an insufficient monomer mixing. Additionally, 5

the quick IP reaction coupled with the diffusion-limited growth of PA layer may lead to little control over the microstructure and morphology of resultant PA layer. While in UAIP process (Fig. 1 (b)), the pristine interface may be disrupted by the collapse of cavitation bubbles at or near the interface under the ultrasound assistance, resulting in an enlarged mixing area [27, 28], the facilitated mass transfer of amine monomers [27, 28], and an efficient monomer mixing. As well known, IP reaction rate is determined by the concentrations of two reactive monomers, that is, the higher reactive monomer concentration, the faster the reaction rate. Therefore, the introduction of ultrasound could enhance IP reaction by increasing MPD concentration in the organic phase (where IP reaction takes place). Additionally, nanobubbles generated by ultrasonication can remain stable for a long time (103-104 s) at the solid-liquid interface [31, 32] or in the aqueous solution [33, 34], therefore result in nanovoids and increase the free volume in the PA layer of resultant TFC membranes [35]. Moreover, the packing density of PA chains may be disrupted by the ultrasound, and the penetration of more amine monomers in the organic phase may increase the free volume hole size of resultant PA layers [30], resulting in a larger free volume. By adjusting the ultrasound parameters (including time, power or frequency), the microstructure of PA layer can be optimized to obtain a desirable TFC membrane with a high separation performance.

6

Fig. 1 Schematic illustration of (a) traditional IP and (b) UAIP processes

2. Materials and methods

2.1 Materials

Polysulfone (PSf) (Mw: 800,000 Da) for fabricating substrate membranes was bought from Beijing HWRK Chem co. Ltd. (China). M-phenylenediamine (MPD, 99.5%) and 1, 3, 5-trimesoyl chloride (TMC, 98%) for preparing the PA layer by interfacial polymerization were obtained from Aladdin. N-methyl pyrrolidone (NMP, anhydrous, ≥99.5%), polyethylene glycol 400 (PEG 400, CP), hexane (anhydrous, AR) and sodium chloride (NaCl, ≥99.5%) were all bought from China National Medicine Corporation.

7

2.2 Fabricating TFC membranes for FO and NF processes

PSf substrate membranes were prepared via non-solvent induced phase separation. The detailed process can be referred to our previous studies [15, 36, 37] and also elaborated in the Supporting Information. The dense PA layer of TFC membranes was prepared via IP process on the PSf membrane. As for the fabrication of TFC-FO membranes, the substrate membrane was soaked in 2.0 wt% MPD solution for 2 min firstly. Next, a rubber roller was used to wipe away the superabundant MPD solution. Subsequently, the MPD-saturated PSf membrane was contact with a 0.1 wt% TMC solution (dissolved in hexane) for 1-min contact. After the excess TMC solution was drained off, the as-prepared TFC membrane was stored in DI water before use. As to the modified TFC-FO membranes, the IP process was conducted in an ultrasonication bath with a fixed ultrasound frequency of 40 Hz and variable ultrasound powers (120 to 600 w). The as-fabricated TFC membranes were denoted as PA-0 (control membrane), PA-120, PA-240, PA-360, PA-480 and PA-600, where the number refers to the ultrasound power. As for the fabrication of TFC-NF membranes, reactive monomer solutions employed for IP process were 0.35 wt% PIP/water and 0.15% w/v TMC/hexane solutions. The soaking time of PSf substrate membrane in PIP solution was 5 min, at the IP reaction time was fixed at 2 min. The as-fabricated TFC-NF membranes were denoted as PA-X-NF, where X presents the ultrasound power applied.

2.3 Characterizing TFC membranes

Fourier Transform Infrared (FTIR, Brucker, VERTEX-70) and X-ray Photoelectron 8

Spectroscopy (XPS, VG Multilab 2000, Thermo VG Scientific, UK) were applied to characterize the chemical changes of TFC membranes. Water contact angles (WCAs) of as-fabricated membranes were measured by a Contact Angle Goniometer (DSA 25, KRÜSS, Germany) at room temperature. The morphology and topology of TFC membranes were examined by Scanning Electron Microscopy (SEM, VEGA3, TESCAN, Czech) and Atomic Force Microscopy (AFM, SPM9700, Shimadzu, Japan), respectively. The microstructure change of the PA layer in as-fabricated TFC membranes was detected by Position Annihilation Lifetime Spectroscopy (PALS). The details were described in the Supporting Information.

2.4 Membrane performance tests

The water permeance (A), salt permeability (B) and salt rejection (Rs) of as-fabricated TFC membranes were evaluated using a RO setup (Suzhou Faith Hope Membrane Technology) with the cross-flow filtration mode and an applied transmembrane pressure of 2 bar. The water permeance (A) was obtained from the pure water flux (J) according to Eqs. (1) and (2) using DI water as the feed, while the salt rejection (Rs) was determined by Eq. (3) using 1000 ppm NaCl aqueous solution as the feed. The salt permeability (B) was then calculated based on Eq. (4) accordingly. The salt concentrations of the feed and the permeation were monitored by a conductivity meter (FE30, Mettler Toledo, Switzerland). Each membrane sample was repeated at least three times and stabilized under an applied transmembrane pressure of 3 bar for 30 min before the test, and at least three parallel tests were repeated to get the average data. J=𝐴

∆𝑉

𝑚,𝑅𝑂 ×∆𝑡

(1) 9

𝐽

A = ∆𝑃

(2) 𝐶𝑝

𝑅𝑆 = (1 − 𝐶 ) × 100 𝑓

1−𝑅𝑠 𝑅𝑠

𝐵

= 𝐴(∆𝑃−∆𝜋)

(3) (4)

where ∆V is the permeate volume, 𝐴𝑚,𝑅𝑂 is the active membrane area (17.35 cm2), Cp and Cf are NaCl concentrations of the permeate and the feed, Δπ and ΔP were the transmembrane pressure and the osmotic pressure difference, respectively. FO performance of as-fabricated TFC membranes was measure at 22 ± 0.5 C by a self-made cross-flow filtration apparatus. The volumes of both feed (DI water) and draw (0.5 and 2 M NaCl) solutions were 1.0 L, which were co-currently circulated with the fixed velocity of 0.3 L min-1 (150 rpm). The changes of weight and NaCl concentration in the draw solution and feed solution were monitored by a digital weight balance (FX3000-GD, AND, Japan) connected to the computer and a conductivity meter (FE30, Mettler Toledo, Switzerland), respectively. The FO test was conducted under both FO mode (the active layer facing the feed solution) and PRO mode (the active layer facing the draw solution). Each membrane sample was pre-compacted for 0.5 h before data recording and at least three parallel tests were repeated. Water flux (Jv, LMH) and reverse salt flux (Js, gMH) of as-fabricated TFC membranes were obtained by Eqs. (5) and (6). 𝐽𝑣 = 𝐴

∆𝑉

𝑚,𝐹𝑂 ∆𝑡

∆(𝐶𝑡 𝑉𝑡 )

𝐽𝑠 = 𝐴

𝑚,𝐹𝑂 ∆𝑡

(5) (6)

where ∆V is the permeate volume of the feed solution, Ct and Vt are the NaCl concentration and the volume of the feed solution at the end of the FO test, Am,FO is the active membrane area (3.87 cm2). 10

3. Results and discussion

3.1 Mechanism of UAIP

The enhanced IP reaction with the ultrasound assistance is testified by FTIR and XPS characterizations. FTIR spectra in Fig. 2 show that, in the spectra of modified membranes, peak intensity ratios I1544/I1660 increases, and peak intensities at 3410 cm-1 decrease, demonstrating the more complete reaction between TMC and MPD in UAIP process [36]. The detailed description on FTIR spectra can be found in the Supporting Information. Similar conclusion can also be derived from XPS results of TFC membranes as displayed in Figs. S1 and S2. The surface elemental compositions of PA layers listed in Table 1 show that, O/N ratios of these modified ones are all lower than that of the control one, demonstrating the higher crosslinking degree again, resulted from the enhanced IP reaction with the assistance of ultrasound [36, 38]. The deconvolution of O 1S peak is further performed to study the chemical changes of the PA layer quantitatively and the results are displayed in Fig. S2 and Table S1. It can be observed that, IOI/IOII ratios of modified ones are higher than that of the control one, because of the more complete reaction between TMC and MPD [36]. The detailed elaboration can be found in the Supporting Information. Similar conclusion can be derived from TGA profiles of PA powders formed by traditional IP and UAIP (Fig. S3). It exhibits that, the profile of PA-360 powder displays a higher decomposition temperature and less weight loss than that of PA-0 powder, indicating the better stability of the formed PA network under UAIP with a 11

higher crosslinking degree.

3410

1660

1544

O-H

C=O

N-H/C-N

PA-480 PA-240 PA-0

3500

1700

1600

Wavenumber (cm

1500

1400

-1 )

Fig. 2 FTIR spectra of TFC membranes formed by traditional IP (PA-0) and UAIP (PA-240, PA-480)

Table 1 Surface elemental composition of PA membranes and powders formed by traditional IP and UAIP Code

C

O

N

O/N

Crosslinking%

PA-0

72.21

17.51

10.28

1.70

21.95

PA-120

72.7

16.65

10.65

1.56

34.07

PA-240

72.52

16.52

10.96

1.51

39.30

PA-360

77.31

13.36

9.33

1.43

46.72

PA-480

76.76

13.52

9.72

1.39

50.95

PA-600

75.73

13.83

10.44

1.32

58.10

12

3.2 Separation performance

Corresponding FO performance of as-fabricated TFC membranes prepared by traditional IP and UAIP processes are evaluated, and corresponding results are exhibited in Fig. 3. It shows that, compared with the control one, water fluxes of these modified ones significantly increase in both operation modes, and exhibit an up-and-down trend with a higher ultrasound power. Especially, the water flux of PA-360 membrane (106.4±3.3 LMH) more than doubles that of the control membrane (45.4±4.3 LMH) with 2 M NaCl solution as the draw solution under PRO mode. Moreover, the lower water flux in FO mode than that in PRO mode is due to the severer internal concentration polarization effect. Meanwhile, as compared to the control membrane, PA-120, PA-240 and PA-600 membranes exhibit higher reverse salt fluxes, while PA-360 and PA-480 membranes with a relatively modest ultrasound power display lower reverse salt fluxes.

(a) 12

50

10

10

8

8

6

6

4

4

40

40

30

30

20

20

10

10

0

0 0

120

240

360

480

0

600

12

FO PRO

120

240

360

480

Js (gMH)

Jv (LMH)

50

60

Js (gMH)

FO PRO

Jv (LMH)

60

600

Ultrasound power (W)

Ultrasound power (W)

(b)

13

90

75

75

60

60

45

45

30

30

15

15

0

0 0

120

240

360

480

Jv (LMH) Js (gMH)

Jv (LMH)

90

25

FO PRO

105

20

20

15

15

10

10

5

600

Js (gMH)

25 FO PRO

105

5 0

120

240

360

480

600

Ultrasound power (W)

Ultrasound power (W)

Fig. 3 FO performance of TFC membranes formed by traditional IP and UAIP with (a) 0.5 M NaCl solution and (b) 2 M NaCl solution as the draw solution (feed solution: DI water)

The improved water fluxes of modified membranes under UAIP are mainly ascribed to their larger free volume of resultant PA layer as confirmed by Positron Annihilation Lifetime Spectroscopy (PALS) characterization in Figs. 4-5 and Table 2. Basically, the large S parameter demonstrates the formation of loose PA layer [39, 40]. Fig. 4 shows that, S values of all modified membranes are higher relative to that of the control one, and exhibit the same up-and-down trend with the rising ultrasound power, suggesting the formation of a looser PA layer by UAIP process. This behavior possibly ascribe to the following factors: (1) the more penetrated MPD molecules into the organic phase favors the formation of a looser PA layer caused by the enlarged spatial distance between PA chains [30]; (2) the introduction of ultrasound in IP process may disrupt the packing density of nascently formed PA chains, resulting in enlarged aggregate pores; (3) the ultrasound introduced in IP process could also generate much more nanobubbles, which are further encapsulated into the crosslinked PA layer as nanovoids to form a looser PA layer [35].

14

0.49 0.48 0.47 0.485

0.46

S

0.480 0.475 0.470

S

0.45

0.465 0.460

0.44

0.455 0.450 1.0

1.5

2.0

2.5

3.0

3.5

Positron Incident Energy (keV)

0.43

PA-Control PA-360

0

5

10

PA-120 PA-480

15

PA-240 PA-600

20

Positron Incident Energy (keV) Fig. 4 S parameters of TFC membranes formed by traditional IP and UAIP

The change of free volume in the PA layer under UAIP is further investigated by the ortho-positronium (o-Ps) lifetime distribution with MELT analysis as shown in Fig. 5 and corresponding results by PATFIT analysis are displayed in Table 2. Fig. 5 shows that, all curves exhibit two distinct peaks, where the sharp peak with o-Ps lifetime at around 1-1.3 ns corresponds to the network pores (pore radius of 1.746-2.046 Å, R2), i.e. the small pores formed from the amide linkage and their crosslinking, while the wide peak at 3-4 ns is related to the aggregate pores (pore radius of 3.753-4.234 Å), i.e. the large pores formed among polymer aggregates in the aromatic PA layer [30, 39]. Additionally, it is worthwhile to note from Fig. 5 that, both two types of pores are larger than the water molecule (radius of 1.3 Å), and smaller than the hydrated NaCl molecule (radius of 4.75 Å), therefore ensuring a high separation performance. In comparison with the PA cluster formed by traditional IP, the aggregate pores in the PA cluster formed via UAIP is larger, due to the generated nanovoids [35], higher amine concentration [30] and the disrupted chain packing density; while the network pores are smaller than those of the PA network formed by 15

traditional IP, resulted from the formed denser PA network with more amide linkages. The schematic diagram in Fig. 6 shows the corresponding above changes. The fractional free volume (FFV) in the selective PA layer corresponding to the two types of pores are further calculated by the pore dimension (pore radius, R) and the pore density (related to the o-Ps density, I3) [40] and shown in Table 2. It can be seen that, the total fractional free volume (FFVtotal) of the PA layers in these modified ones are all larger relative to that of the control one, which benefits the improved water flux. Additionally, FFVtotal value of modified membranes also exhibits an up-and-down trend, which is in accordance with the results of water flux. Detailed elaboration about the variations of the pore radius, o-Ps density and FFV can be found in the Supporting Information.

Radius (A) 1.66

2.85

3.64

4.24

0.04 RH O= 2

1.3 A

4.75 PA-0 PA-240 PA-360 PA-480

PDF

0.03

RNaCl= 4.75 A

0.02

0.01

0.00 1

2

3

4

5

6

o-Ps Lifetime (ns) Fig. 5 o-Ps lifetime distributions of TFC membranes formed by traditional IP and UAIP by MELT analysis (PDF: Probability Density Function)

Table 2 Positron lifetime results of the control and modified membranes by PATFIT analysis 16

Aggregate pore Sample code

Network pore FFVtotal (%)

δcal. (nm)

I3,1 (%)

R1 (Å)

FFV1 (%)

I3,2 (%)

R2 (Å)

FFV2 (%)

PA-0

15.858

3.753

6.322

9.531

2.004

0.578

6.900

119.5

PA-240

17.116

3.785

7.001

9.003

2.046

0.581

7.582

160.0

PA-360

19.119

4.234

10.945

7.789

1.941

0.430

11.375

253.5

PA-480

17.893

4.130

9.501

6.881

1.716

0.262

9.763

362.3

*I3: o-Ps density; R: pore radius; FFV: fractional free volume; δcal: calculated thickness of PA layer

Fig. 6 Schematic diagrams of the aggregate pores and network pores in PA network, formed in (a) traditional IP and (b) UAIP processes

Additionally, the rougher surfaces of modified membranes may also be another possible reason for the improved water flux, since it can provide a larger transport area for the water molecules. Fig. 7 shows the typical ridge-and-valley surface structure of all TFC membranes. The control membrane (PA-0) exhibits both nodular-like and leaf-like structures, probably due to the insufficient IP reaction. In comparison, the features of leaf-like and flake-like structures are more obvious in the modified membrane, especially with the higher ultrasound power (except for PA-600 membrane), ascribed to the facilitated IP reaction. While the relatively smoother 17

surface morphology of PA-600 membrane than those of other modified membranes is probably due to the fact that, the nascently-formed PA layer might be damaged under a strong ultrasound power, leading to an nonuniformly distributed ridge-and-valley structure and more defects on the membrane surface. AFM images of TFC membranes also present the similar variation trend of the surface roughness as shown in Fig. 8. Additionally, the PA layer thickness also experiences an up-and-down trend with the maximum value at the ultrasound power of 480 W, which is consistent with the thickness calculated from PALS results as listed in Table 2. Moreover, as a result of the increased surface roughness of the TFC membrane formed by UAIP, the membrane also exhibits a higher surface hydrophilicity (as presented in Fig. S4), which further contribute to the water flux improvement. While the reductions in water fluxes of PA-480 and PA-600 are resulted from the thicker PA layer and denser PA layer (Fig. 4) with a smoother surface, respectively. Detailed elaboration of Fig. S4 can be found in the Supporting Information.

Fig. 7 SEM images of TFC membranes formed by traditional IP and UAIP

18

Fig. 8 AFM images of TFC membranes fabricated by traditional IP and UAIP

On the other hand, the variation in the reverse salt flux of TFC membranes could be ascribed to the changes in the surface negative charge, thickness and free volume of the PA layer. With a relative low ultrasound power (<360 w in this work), the higher reverse salt flux of PA-120 and PA-240 membranes are mainly caused by the less surface negative charges (as shown in Fig. S5), since the amount of their large aggregate pore (R>4.75Å) is nearly equal to that of the control membrane (Fig. 5). As to the abnormally-high reverse salt flux of PA-600 membrane, it is possibly caused by the existence of defects on the PA selective layer. While for membranes prepared under a relatively higher ultrasound power (360-480 W), the lower reverse salt flux should be resulted by the thicker PA layer. Detailed elaboration of Fig. S5 can be found in the Supporting Information. RO tests results of as-fabricated TFC membranes are summarized in Table 3. It displays that, water permeances (A) of modified TFC membranes (2.56±0.11, 3.14±0.07, 3.44±0.09, 3.21±0.09, 2.32±0.10 LMH/Bar) are all higher than that of the control TFC membranes (1.99±0.10 LMH/Bar), which should be ascribed to the 19

higher surface hydrophilicity, more larger aggregate pores, and higher surface roughness with larger surface area for the water transport. Similarly, the water permeance also exhibits an up-and-down trend with the higher ultrasound power (with the highest water permeability achieved at 360 w), as the result of variations in the free volume, thickness, roughness and hydrophilicity of resultant PA layers. Table 3 also shows that, salt rejections (Rs) of these modified membranes (94.30±0.82 and 93.96±1.18%) slightly decrease compared to that of the control TFC membrane (94.72±1.05%) with a relative low ultrasound power (<360 W), but higher with the higher ultrasound powers (95.92±1.05, 96.67±1.43% for PA-360 and PA-480 membranes). With a relative low ultrasound power (<360 W in this work), the less surface negative charges of PA-120 and PA-240 membranes should be the main reason for the lower salt rejections. While for membranes prepared under higher ultrasound powers (360-480 W), the higher salt rejection could be resulted by the thicker PA layer. Moreover, it is also found that the salt rejection of PA-600 membrane is the lowest, probably due to the defect formation on the selective layer with the strong ultrasound power. Accordingly, compared to the control one, these modified membranes (excluding PA-480 membrane) all possess the higher salt permeabilities (B), which are positively related to the water permeance and negatively related to the salt rejection of modified membrane. While the results of membrane selectivity (B/A ratio) exhibit the opposite trend to the salt rejection results with the higher ultrasound power.

Table 3 Intrinsic transport properties of the control and modified TFC membranes Code

Aa, LMH/Bar

Bb, LMH

Rejection Rs, %

B/A, Bar

PA-0

1.99±0.10

0.18±0.05

94.72±1.05

0.09

20

PA-120

2.26±0.11

0.22±0.04

93.96±1.18

0.11

PA-240

2.67±0.10

0.26±0.05

94.30±0.82

0.10

PA-360

3.44±0.09

0.23±0.07

95.92±1.05

0.07

PA-480

3.21±0.09

0.18±0.08

96.67±1.43

0.05

PA-600

2.32±0.10

0.35±0.02

91.13±0.87

0.15

a

DI water is used as the feed solution in RO test with an applied pressure of 2 bar (2.5 rpm); b 1000 ppm NaCl solution is used as the feed solution in RO test with an applied pressure of 2 bar (2.5 rpm);

In summary, the water flux of TFC membranes in this work is affected by various factors, including the free volume, the surface roughness, the surface hydrophilicity and the thickness of resultant PA layers. While the reverse salt flux is impacted by the free volume, surface negative charge and thickness of resultant PA layers. When a relatively low ultrasound power (120-240 W) is applied during IP process, the impact is not very high, resulting in comparable total free volume, roughness and hydrophilicity to those of the control membrane, and thus the limited improvement in the water flux. While in comparison with the control membrane, the reduced surface negative charge as the predominant factor leads to the higher reverse salt flux of resultant membranes, with their comparable thickness and free volume distribution of the PA layer. When a strong ultrasound power (600 W) is employed, the resultant membrane has non-uniform PA layer with less obvious “valley-and-ridge” structure feature and more defects caused by destroyed ultrathin PA layer, leading to the sharply reduced water flux and increased reverse salt flux. Only with an intermediary ultrasound power (360-480 W) is employed, the resultant membrane can exhibit obviously larger total free volume, higher surface hydrophilicity and roughness, contributing to the significantly improved water flux. Additionally, the significantly 21

thicker PA layer becomes the predominant factor, resulting in the lower reverse salt flux as compared that of the control membrane. Therefore, the medium ultrasound power is preferred for the fabrication of PA-based TFC membranes. In order to testify the feasibility of UAIP method, TFC-NF membranes are also prepared and tested. Corresponding results in Fig. 9 shows that, modified TFC-NF membranes exhibit both higher water permeances and NaCl rejections. The improved water permeance of modified membranes is probably ascribed to the larger fractional free volume, rougher surface of resultant PA layers with improved hydrophilicity as analyzed above. And the thicker PA layer should be responsible for the improved NaCl rejection. Additionally, the water permeance of PA-360-NF membrane is larger than that of the PA-480-NF membrane, while NaCl rejection is lower, which is consistent with FO results as discussed above, The superior NF performance of TFC membranes demonstrates again that UAIP is a convenient and efficient method to

40

40

35

35

30

30

25

25

20

20

15

15

10

10

5

5

0

PA-0-NF

PA-360-NF

Rejection of NaCl (%)

Water permeance (LMH/Barr)

prepare high-performance PA-based TFC membranes.

0

PA-480-NF

Fig. 9 Nanofiltration performance of TFC membranes prepared by traditional IP and UAIP

22

4. Conclusion In summary, this study presents a novel UAIP strategy to prepare TFC membranes with promising FO and NF separation performance. The ultrasound assistance in IP process not only enlarges the mixing interface, but also enhances the diffusion rate of the amine monomer into the organic phase and therefore promotes the IP reaction. As a result, the PA layer formed by UAIP is thicker and exhibits a rougher surface morphology and a larger free volume, contributing to a higher water flux and a lower reverse salt flux in FO tests. Moreover, by adjusting the ultrasound parameters (such as power, time or frequency), the microstructure and morphology of the resultant PA layer can be optimized, with an improved membrane separation performance. As a facial approach for the IP modification, UAIP is believed to provide a new prospect to fabricate high-performance TFC membranes for various membrane-based separation applications.

Acknowledgement We thank the financial support from National Key Technology Support Program (no. 2014BAD12B06), National Natural Science Foundation of China (no. 21306058), Natural Science Foundation of Hubei Scientific Committee (no. 2016CFA001), and the Free Exploring Fundamental Research Project from Shenzhen Research Council, China (no. JCYJ20160408173516757). Special thanks are also given to the Analysis and Testing Center, the Analysis and Testing Center of Chemistry and Chemical Engineering School, and the State Key Laboratory of Materials Processing and Die & Mould Technology, in Huazhong University of Science and Technology for their help with material characterizations.

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Highlights 

Ultrasound-assisted interfacial polymerization is put forward for the first time.



Enlarged mixing area for reactive monomers and enhanced IP reaction are achieved.



Rougher, thicker and looser PA layer is obtained.



Resultant membranes possess superior FO and NF separation performance.

29