Nanofiltration Membrane with a Mussel-Inspired Interlayer for

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Nanofiltration Membrane with A Mussel-Inspired Interlayer for Improved Permeation Performance Yang Xi, Yong Du, Xi Zhang, Ai He, and Zhi-Kang Xu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04465 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 12, 2017

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[Revised as an article for publication in Langmuir]

Nanofiltration Membrane with A Mussel-Inspired Interlayer for Improved Permeation Performance Xi Yanga,b, Yong Dua,b, Xi Zhanga,b, Ai Hea,b, Zhi-Kang Xua,b,*

a

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, and bKey Laboratory of

Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China

*Corresponding author. E-mail address: [email protected]; fax: + 86 571 8795 1773.

ACS Paragon Plus Environment

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ABSTRACT A mussel-inspired interlayer of polydopamine (PDA)/poly(ethyleneimine) (PEI) is co-deposited on the ultrafiltration substrate to tune the interfacial polymerization of piperazine and trimesoyl chloride for the preparation of thin-film composite (TFC) nanofiltration membranes (NFMs). This hydrophilic interlayer results in an efficient adsorption of piperazine solution in the substrate pores. The solution height increases with the PDA/PEI co-deposition time from 45 min to 135 min due to the capillary effect of the substrate pores. The prepared TFC NFMs are characterized with thin and smooth polyamide selective layers by ATR/IR, XPS, FESEM, AFM, zeta potential and water contact angle measurements. Their water permeation flux measured in a cross-flow process increases to two times as compared with those TFC NFMs without the mussel-inspired interlayer. These TFC NFMs also show a high rejection of 97% to Na2SO4 and an salt rejection order of Na2SO4 ≈ MgSO4 > MgCl2 > NaCl. Keywords: mussel-inspired coating, interlayer, interfacial polymerization, membrane surface, nanofiltration.

Introduction In recent years, tremendous developments have been taken in the field of nanofiltration technology1-3 for seawater desalination4 and wastewater treatment.5 Thin-film composite (TFC) membranes are a kind of very important category of nanofiltration membranes (NFMs) due to their commendable salt rejection and high water permeation flux at the same time, which have been widely used in practical processes.6,7 These membranes are usually 2


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fabricated by the interfacial polymerization of diamine with acyl chloride to form a polyamide selective layer on one side surface of an ultrafiltration substrate.8,9 Typically, the porous substrate is first impregnated by an aqueous solution of diamine monomer, and then contact with an organic solution containing acyl chloride. Therefore, it is a challenge to regulate the surface properties and the porous structures of the ultrafiltration substrate. These two factors dominate the adsorption/diffusion of diamine monomer and the followed reaction with acyl chloride during the interfacial polymerization, which have significant influences on the resulted polyamide selective layer of TFC NFMs.10,11 A large number of efforts have been made to develop desirable substrates and/or optimize the substrate surfaces for the interfacial polymerization. For example, water permeation flux was increased for TFC NFMs both by hydrophilizing the substrate surfaces12,13 and by using those substrates with high porosity14. It can be ascribed to the thin and smooth selective layer resulted from the uniform distribution of aqueous solution on the substrate surfaces and the relatively slow diffusion of diamine monomer during the interfacial polymerization process.15-21 Recently, Livingston and his co-workers proposed that a hydrophilic and uniform nanostrand coating can be introduced on the substrate to optimize the surface properties and then to control the interfacial polymerization to a certain extent.22,23 They fabricated polyamide nanofilms with ultrathin and uniform structure using hydrophilic cadmium hydroxide nanostrand as a sacrificial coating that can be removed after the interfacial polymerization. The as-prepared nanofilms have ultra-high solvent permeation flux without reducing solute rejection. However, the cadmium hydroxide nanostrand coating needs to be removed, which may lead to poor integrity and 3


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low combination between the polyamide layer and the substrate surface. Therefore, we suggest it is better to use the hydrophilic coating as an interlayer for the TFC NFMs. The benefit of the hydrophilic interlayer between the ultrafiltration substrate and the polyamide layer is still lack of sufficient understanding. Our research has successfully verified that with this PDA/PEI hydrophilic interlayer, there are more amine monomers in aqueous phase drawn up to the substrate surface, distributed uniformly for further interfacial polymerization, and formed the polyamide layer possessing high water permeation performance. In addition, the construction of the novel PDA/PEI interlayer requires less reaction time and has enhanced stability and uniformity compared with the traditional interlayer constructed by polydopamine.24-29 Polydopamine (PDA) is known as “bio-glue” since it has been found to use as a universal coating agent on a wide range of material surfaces. Compared with the polydopamine coatings being used as the selective layer or for the surface modification of membranes,30,31 our PDA/PEI coatings can act as a uniform, stable, and hydrophilic interlayer binding the ultrafiltration substrate and the selective polyamide layer. The reaction of PDA with PEI through Michael addition or Schiff base reaction greatly reduce the deposition time to less than 2 h, compared with the oxidation and self-polymerization of PDA in air (over 24 h).32 Herein, we construct this kind of uniform and hydrophilic PDA/PEI coating as an interlayer on polysulfone ultrafiltration substrate (as shown in Figure 1), on which the interfacial polymerization is carried out from the polycondensation of piperazine and trimesoyl chloride. TFC NFMs are fabricated with smooth polyamide selective layers, high rejection to divalent ions and doubled water permeation flux compared with those membranes prepared without the 4


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interlayer. In addition, the capillary effect is used to explain the adsorption/diffusion of diamine solution in the substrate pores and the results are used to speculate about the formation of smooth and detect-free polyamide selective layers in the TFC NFMs.9,19,33-35 Furthermore, the as-prepared TFC NFMs also show high stability because the mussel-inspired interlayer acts as glue between the polyamide selective layer and the substrate surface. ---Figure 1---

Experimental Section Materials. Polysulfone (PSf) ultrafiltration substrate with pore sizes ranging from 40 to 80 nm (MWCO = 10~30 kDa) was obtained from Shanghai MegaVision Membrane Engineering

&

Technology

Co.

Ltd

(China).

Dopamine

hydrochloride

and

polyethyleneimine (PEI, Mw = 600 Da) were purchased from Sigma-Aldrich (USA) and Aladdin (China), respectively. Piperazine and trimesoyl chloride were obtained from Sigma-Aldrich (USA). Other chemicals, including tris(hydroxymethyl) aminomethane (Tris), hydrochloric acid solution (18 mol/L), n-hexane, ethanol and inorganic salts, were procured from Sinopharm Chemical Reagent Co. Ltd (China). All of the chemicals were received without further purification. Ultrapure water (18.2 MΩ) was produced from an ELGA Lab Water system (France). Preparation of the TFC NFMs. PSf substrate was rinsed in ethanol for 24 h and then in deionized water for 2 h. Dopamine hydrochloride and PEI were dissolved in Tris–HCl buffer solution (pH = 8.5, 50 mM) with a mass ratio of 1:1. The substrate sample was prewetted by ethanol for 30 min, and then transferred into a freshly prepared dopamine/PEI solution and shaken at 25 ºC for certain time from 45 min to 135 min. The as-prepared substrate (PDA/PEI coated) was then washed by ultrapure water for several times and dried 5


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at room temperature. Then, the PSf substrate with a PDA/PEI interlayer was immersed into an aqueous solution of piperazine (0.3 wt.%) for 10 min. This piperazine impregnated substrate was taken out and the excess water was drained off in air. The organic phase with trimesoyl chloride dissolved in n-hexane solution (0.3 wt.%) was then poured to cover the substrate surface for 2 min, thus forming the selective polyamide layer by interfacial polymerization. The as-formed TFC NFM was dried for 30 min in air to evaporate the excess n-heptane and subsequently post-treated at 60 ºC to activate the formed polyamide layer. Finally, the TFC NFM was rinsed with deionized water and conserved in ultrapure water before performance evaluation and characterization. Characterization of the TFC NFMs. The chemical structure of the PDA/PEI interlayer and the polyamide selective layer were evaluated by attenuated total reflectance Fourier transform infrared spectrometer (FT-IR/ATR, Nicolet 6700, USA). The surface composition was further analyzed by X-ray photoelectron spectrometer (XPS, PerkinElmer 5300, USA) using Al Kα excitation radiation (1486.6 eV). The XPS spectra can be used to calculate the cross-linking degree.36 O/N ratio can be obtained from the XPS analysis by the following equation:

O 3m + 4n = N 3m + 2n

(1)

And degree of cross-linking (%) can be calculated by:

D=

m ´100% m+n

(2)

where m and n are the cross-linked and linear proportion of the polyamide selective layer, respectively. And the value of m and n can be calculated from XPS analysis of O/N ratio as equation (1), which are used to evaluate the degree of cross-linking by equation (2). Field emission scanning electron microscopy (FESEM, Hitachi S4800, Japan) was used to 6


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observe the surface and cross sectional morphologies of the membranes. The membranes were dried and fractured in liquid nitrogen for cross-sectional samples before test. Atom force microscopy (AFM, Bruker MultiMode 8, USA) was used for examining the surface roughness of the membranes in the tapping mode. The static and dynamic water contact angles were measured by a surface tension system (Surface-Meter, OSA 200, China) at room temperature. A streaming potential method was used to detect the charge properties of the membrane surface by an electro-kinetic analyzer (SurPASS Zeta, Litesizer 500, Austria) with KCl (1 mmol/L) as the electrolyte solution. The pH dependence of surface zeta potential was measured by adjusting the pH value. The surface zeta potential ζ was determined by the Helmholtz–Smoluchowski equation: 37

z=

DE hk DR e

(3)

where ΔE is the streaming potential, ΔP is the pressure, ε is the dielectric constant, η and κ are the viscosity and conductivity of the solution, respectively. Nanofiltration performance of the TFC NFMs. The nanofiltration performance of the TFC NFMs was measured using a cross-flow module under 0.6 MPa at 30 ºC. The effective filtration area was 7.07 cm2. The inorganic salt solutions of Na2SO4, MgSO4, MgCl2 and NaCl at the concentration of 1000 mg/L were used as feed solutions with the fixed cross-flow rate of 30 L/h. The water flux (Fw, L/m2h) and rejection rate (R, %) were calculated by the following equations:

Fw =

Q t×A

(4)

where Q, t and A represent the volume of permeated water, the permeation time, and the effective filtration membrane area, respectively.

æ Cp ö ÷ ´100% R = ç1 ç C ÷ f è ø

(5) 7


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where Cp and Cf are the solute concentration in permeate and feed solution, which were

detected by an electrical conductivity meter (METTLER TOLEDO, FE30K, Switzerland). The structural stability of the TFC NFMs was examined by ethanol swollen treatment for 1~10 days. Then, the swollen membranes were taken out and fully washed with deionized water to remove the remained ethanol. The water flux and the rejection rate of Na2SO4 solution were examined again. All the results were the average value calculated from three measurements under the same condition.

Results and Discussion The adhesive and hydrophilic PDA/PEI interlayer is constructed according to our previous work, using the co-deposition method which the oxidized reaction can be induced merely by air (Figure S1 in Supporting Information). Its thickness mainly depends on the mass ratio of dopamine/PEI and the co-deposition time.28,29 In this work, the mass ratio of dopamine/PEI is kept as 1:1 and the PSf substrates are co-deposited with different times from 0 to 135 min. Figure 2 shows that the surface pores almost maintain the pristine morphology (Figure 2a-d) but the pore diameter declines from 43 ± 22 to 28 ± 10 nm (Figure S2 in Supporting Information). These results indicate the PDA/PEI coating partially covers both the top and the pore surfaces of the PSf substrate. On the other hand, the surface hydrophilicity also changes obviously with the co-deposition of PDA/PEI interlayer. The initial water contact angle decreases from 83° to 58°. However, it is very interesting that the water contact angle decreases quickly with time for those substrates with PDA/PEI interlayer and the water droplets can penetrate into the substrate in several seconds (Figure 2e). This means these substrates are easy to adsorb the aqueous solution of diamine monomer into their porous structure. ---Figure 2--8


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The substrates are immersed in an aqueous solution of piperazine (0.3 wt.%) for 10 min and the solution uptake is measured by weight change. It can be seen from Table 1 that the solution uptake increases from 100% to 350% for the nascent substrate to the PDA/PEI-coated one co-deposited by 135 min. Capillary rise is calculated for further evaluating this phenomenon. The ultrafiltration substrate has a non-woven fabric support with an asymmetric PSf layer. Capillaries in this porous substrate can be viewed as a tube assay, with an average pore size acquired from FESEM images (Figure 3a and Figure S2 in Supporting Information). It is known that the capillary rise can be measured both by the height approach and the weight approach.33 When the solution density is fixed, the value of the increased weight can represent the capillary rise and is easy to measure.34 In our cases, the weight change was measured and used for the calculation of the capillary rise by equation (6):38-41

H(R) =

2gcosq ragR

(6)

where g is the interfacial tension of the water/air interface, q is the dynamic water contact angle of the substrate surface. Considering the θ changes with time, cosθ here in the q2

equation should be replaced with

ò cosθdq (θ1 is the original water contact angle and θ2 is

q1

the value of contact angle at 10 min). a is the corner opening angle (satisfy a/2 + q £ 90o) and R is the pore radius, r and g are the solution density and gravitational constant. These two values are replaced with water here, because the solution concentration is very low. It can be seen from Table 1 that the capillary rise increases from 10.2 to 36.7 mm, which shows a same tendency with the solution uptake. This result can be ascribed to the hydrophilic characteristics of the substrate pores after PDA/PEI co-deposition. In our cases, as schematically shown in Figure 3, the Laplace pressure produced by the capillary effect will gradually pull the aqueous solution up to the substrate surface, which results in the 9


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increases of the capillary rise and the amount of aqueous solution. Both of these increases will influence the interfacial polymerization. ---Table 1---, ---Figure 3--In addition, the surface hydrophilicity will also change the water equilibrium vapor pressure of the PSf substrates filled with the aqueous solution, according to the Kelvin effect equation (7):42

é 2Vg cosq ù 0 0 Pm = Pi expêRTr úû ë

(7)

where θ, γ, V, R, and T are the water contact angle of the substrate, water surface tension, permeant molar volume, universal gas constant, and temperature, respectively. Further, Pm0 and Pi0 are the equilibrium vapor pressure of permeant water on the substrate surface and the bulk vapor pressure of water, respectively. The pore radius r is the same as R in the eq 6. The Kelvin equation predicts a decrease in the equilibrium vapor pressure (compared to its bulk value) with the increase of the surface hydrophilicity (θ < π/2). As a result, the aqueous solution can be easily conserved in the substrate pores for the followed interfacial polymerization. Typical interfacial polymerization was carried out on the PSf substrates with PDA/PEI coating as an interlayer. TFC NFMs were prepared with polyamide as the selective layer. FESEM and AFM characterize that these TFC NFMs have relatively smooth surface and thin polyamide layer as compared with those prepared from the nascent PSf substrate (Figure 4 and Figure S3 in Supporting Information). The rough surfaces of typical TFC NFMs have been attributed to the rapid, uncontrolled reaction occurring at the water/oil interfaces, whereas the formation of smooth surfaces are caused by the reduced hydraulic resistance, short diffusion path length for diamine solution to pass through the substrate pores.43-46 Freger suggested the diffusion rate and the concentration of diamine monomer along the reaction zone co-contribute to the thickness and morphology of the polyamide 10


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selective layer during the interfacial polymerization.9,19 According to the kinetic model for thin film formation, the thickness, δ, can be estimated by the following equation (8):9

é ù LD d~ê ú ë k (Ca f a + Cb f b )û

1/ 3

(8)

where Ca is the equilibrium concentration of the diamine at the organic side of the interface, Cb is the acyl chloride concentration in the organic phase, L is the thickness of the diffusion boundary layer at the interface, D is the diffusivity of the diamine monomer in the organic phase, k is the rate constant of the interfacial reaction between the two monomers, and fi is the functionality of monomer i. The equation implies that, during the interfacial polymerization process, both the increased monomer concentration and the reduced diffusion rate passing through the reaction interface can decrease the thickness of the polyamide selective layer. Therefore, our results can be ascribed to the PDA/PEI interlayer, which hydrophilizes the substrate surfaces, increases the adsorption amount of diamine monomer, and raises the capillary height of aqueous solution as mentioned above. The increased hydrophilicity can uniformize the monomer distribution at the interface and also reduce the diffusivity of diamine to the organic phase for polymerization. It means the diamine concentration, Ca, increases and the monomer diffusivity, D, decreases in Freger equation. The thickness, δ, will reasonably decrease. ---Figure 4--FT-IR/ATR and XPS spectra were used to demonstrate the chemical structures of the PSf substrates and the as-prepared TFC NFMs (Figure 5a). The peaks at 1570 cm-1 and 1650 cm-1 refer to the N-H and C-N vibrations of PDA/PEI.28,29 And the peaks at 1484 cm-1 with 1700 cm-1 are identified to the typical O-C=O stretching vibrations of the polyamide layer. XPS spectra can be further used to calculate the elemental composition and the cross-linking degree of the polyamide layer (Figure 5b and Table S1 in Supporting 11


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Information). The cross-linking degree increases from 51% for the typical TFC NFM to 76% for that with a PDA/PEI interlayer. This high cross-linking degree should endow the the membrane with dense selective layer and high salt rejection.36 ---Figure 5-- We

measured the nanofiltration performance of the as-prepared membranes by using a

cross-flow equipment. It can be seen from Figure 6a that the Na2SO4 rejection gradually increases from 94% to 97% with the deposition of PDA/PEI as the interlayer. This is due to the increases of the cross-linking degree of the polyamide selective layer. At the same time, the water permeation flux first increases and then decreases slightly with the prolonged deposition of PDA/PEI interlayer. This can be ascribed to the opposite effect of the thickness decrease and the cross-linking degree increase of the polyamide layer. It is promising that, when the deposition time of PDA/PEI is 90 min, TFC NFMs can be fabricated with the water permeation flux of two times and 97% Na2SO4 rejection. The water permeation flux of our NFMs with PDA/PEI interlayer is relatively high compared with other nanofiltration membranes in the published literatures (Table S2 in Supporting Information). These TFC NFMs show a rejection sequence of Na2SO4 ≈ MgSO4 > MgCl2 > NaCl (Figure 6b). It is due to the synergistic effect of the size sieving and the Donnan exclusion. These TFC NFMs possess less negatively charges than those without PDA/PEI interlayer (Figure S4 in Supporting Information). It is possible that the high rejection of Na2SO4 and MgSO4 can be ascribed to the high cross-linking degree for small pore sizes with the sieving effect. The negative TFC NFMs surface charges attribute to the high rejection of Na2SO4 and MgSO4 with the Donnan exclusion. In addition, the small hydrated ionic radius of Cl- cause the decreased rejection of MgCl2 and NaCl. ---Figure 6--Furthermore, it is significant of TFC NFMs long-term stability and adhesive strength 12


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between the polyamide layer and the substrate. The adhesive strength can be characterized by immersing these TFC NFMs in ethanol for several days (from 1 to 10 days) to observe the long-term tendency of water permeation flux and salt rejection.29 Compared with the traditional TFC NFMs, this novel TFC NFMs with the PDA/PEI interlayer have the enhanced stability of relatively steady water permeation flux and salt rejection in the ethanol for up to 10 days (Figure S5 in Supporting Information). The interlayer binds the ultrafiltration substrate and the polyamide layer tightly through multiple non-covalent and covalent interactions, for example, π-π interactions, hydrogen-bindings, ester bonds and amide bonds.26,27 In addition, the high cross-linking degree of the polyamide layer could also contribute to the stabilization in ethanol.47 Because the highly cross-linked parts in the structure can hinder the solvent molecules from entering into the polyamide layer, so as to reduce the swollen phenomenon. As a result, the polyamide layer could be stable even in polar solutions such as ethanol for long-time usage. It means that the TFC NFMs have tremendous potential for long-time practical usage with the stable operation performance. In addition, the NFMs with the PDA/PEI interlayer possess better stability compared with other NFMs of practical applications or in the published literatures.11-13 It is because of the strong binding strength of the covalent (including ester bonds and amide bonds) and non-covalent interactions (includingπ-π interactions and hydrogen-bonding), endowing the excellence of long time performance for the designed NFMs, even in organic solutions.

Conclusion In summary, we introduce the hydrophilic mussel-inspired PDA/PEI interlayer on the ultrafiltration substrate for the preparation of TFC NFMs. Our results show that this hydrophilic interlayer can regulate the adsorption/diffusion of diamine monomer for the interfacial polymerization, and the resulted membrane structures with the nanofiltration 13


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performances. Moreover, the TFC NFMs with the PDA/PEI hydrophilic interlayer achieve the thin and smooth polyamide selective layer. The versatility of this mussel-inspired interlayer could potentially offer the new pathway for a wide range of TFC NFMs preparation and applications.

Acknowledgements The authors thank the financial support to this work by the National Natural Science Foundation of China (Grant No. 21534009) and the Open Research Fund Program of Collaborative Innovation Center of Membrane Separation and Water Treatment of Zhejiang Province (Grant No. 2016ZD04).

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Table 1. Solution uptake of the PSf substrates co-deposited with PDA/PEI for different times and the theoretically calculated capillary rise.

Deposition time

Dm

Solution uptake

dp

Capillary rise H

(min)

(g)

(%)

(nm)

(mm)

0

0.0131

100

43 ± 22

10.2

45

0.0210

160

35 ± 16

15.4

90

0.0380

290

32 ± 12

28.6

135

0.0459

350

28 ± 10

36.7

(Dm is the mass increase of the substrates immersed in piperazine solution for 10 min. Solution uptake is the ratio of mass change for substrates with different deposition times compared to the original one. The pore radius were inferred from FESEM images using the statistical method (Figure S2 in Supporting Information), with the value of dp equals 2R, and the capillary rise H was calculated from the eq. 6).

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Figure Captions: Figure 1. Schematic illustration of the interfacial polymerization mediated by PDA/PEI mussel-inspired interlayer and the resulted TFC NFMs polyamide layer. Figure 2. (a-d) Surface FESEM images of PSf substrates before and after certain time of PDA/PEI co-deposition (0 min, 45 min, 90 min and 135 min, respectively). (e) Water contact angle of the PSf substrate surfaces after PDA/PEI co-deposition with different times. Figure 3. (a) Schematic illustration of the original and PDA/PEI modified PSf substrates of increasing capillary rise. (b) Solution uptake and theoretical calculated capillary rise of PSf substrates before and after certain time of PDA/PEI co-deposition (0 min, 45 min, 90 min and 135 min, respectively). Figure 4. (a-d) Surface FESEM images of TFC NFMs after interfacial polymerization with the deposition times of PDA/PEI for 0 min, 45 min, 90 min and 135 min, respectively, the AFM images of resulted morphology of TFC NFMs (e) without or (f) with the PDA/PEI mussel-inspired interlayer. Figure 5. (a) FT-IR/ATR and (b) XPS spectra of PSf substrates before and after PDA/PEI co-deposition for certain time, as well as the TFC NFMs with or without the PDA/PEI mussel-inspired interlayer, respectively. Figure 6. Performance of TFC NFMs with or without PDA/PEI mussel-inspired interlayer. (a) Effect of different PDA/PEI deposition times on the separation performance of Na2SO4 solution. (b) Water permeation flux and rejection rate of different inorganic salt solutions with TFC NFMs prepared under the optimized condition. (Test conditions: inorganic salt concentration = 1000 mg/L, pH = 6, 30 ºC , 0.6 MPa, cross-flow rate = 30 L/h.)

20


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Figure 1. Schematic illustration of the interfacial polymerization mediated by PDA/PEI mussel-inspired interlayer and the resulted TFC NFMs polyamide layer.

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(a)

(b)

1μm

1µm (d)

(c)

1µm

1µm

Water contact angle (°)

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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(e)

100 80 60 40 Nascent PSf 45 min 90 min 135 min

20 0 0

5

10

15

20

25

30

35

Time (s) Figure 2. (a-d) Surface FESEM images of PSf substrates before and after certain time of PDA/PEI co-deposition (0 min, 45 min, 90 min and 135 min, respectively). (e) Water contact angle of the PSf substrate surfaces after PDA/PEI co-deposition with different times.

22


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(a)

Solution uptake (%)

500

Solution uptake Capillary rise

400

Capillary rise H (mm)

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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300

200

100

0

0

45

(b)

90

135

Deposition time (min)

Figure 3. (a) Schematic illustration of the original and PDA/PEI modified PSf substrates of increasing capillary rise. (b) Solution uptake and theoretical calculated capillary rise of PSf substrates before and after certain time of PDA/PEI co-deposition (0 min, 45 min, 90 min and 135 min, respectively).

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(b)

(a)

1µm

1µm (d)

(c)

1µm

1µm (e)

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(f)

Figure 4. (a-d) Surface FESEM images of TFC NFMs after interfacial polymerization with the deposition times of PDA/PEI for 0 min, 45 min, 90 min and 135 min, respectively, the AFM images of resulted morphology of TFC NFMs (e) without or (f) with the PDA/PEI mussel-inspired interlayer.

24


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PSf substrate

PDA/PEI-modified PSf substrate TFC NFMs without PDA/PEI interlayer -1 1570 cm TFC NFMs with PDA/PEI interlayer

-1 1484 cm

-1 1650 cm

(a) 1800

1600

1400

Wavenumber O1s

TFC NFMs with PDA/PEI interlayer

1200

1000

-1 (cm )

C1s

N1s

TFC NFMs without PDA/PEI interlayer

PDA/PEI-modified PSf substrate PSf substrate

S2s

(b) 700

600

500

400

300

S2p

200

Binding Energy (eV)

Figure 5. (a) FT-IR/ATR and (b) XPS spectra of PSf substrates before and after PDA/PEI co-deposition for certain time, as well as the TFC NFMs with or without the PDA/PEI mussel-inspired interlayer, respectively.

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120

(a)

Water flux Rejection

100

2 Water flux (L/m ×h)

100

80

80

60

60

40

40

20

20

0

0 0

120

45

90

Deposition time (min)

135

120

(b)

Water flux Rejection

100

100

80

80

60

60

40

40

20

20

0

Rejection (%)

2 Water flux (L/m ×h)

120

Rejection (%)

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

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0

Na2SO4

MgSO4

MgCl2

NaCl

Figure 6. Performance of TFC NFMs with or without PDA/PEI mussel-inspired interlayer. (a) Effect of different PDA/PEI deposition times on the separation performance of Na2SO4 solution. (b) Water flux and rejection rate of different inorganic salt solution with TFC NFMs prepared under the optimized condition. (Test conditions: inorganic salt concentration = 1000 mg/L, pH = 6, 30 ºC, 0.6 MPa, cross-flow rate = 30 L/h.)

26


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Graphic for Table of Content

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Nanofiltration Membrane with a Mussel-Inspired Interlayer for

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