Polyphenol Coating as an Interlayer for Thin-Film Composite

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Polyphenol Coating as an Interlayer for Thin-Film Composite Membranes with Enhanced Nanofiltration Performance Xi Zhang, Yan Lv, Hao-Cheng Yang, Yong Du, and Zhi-Kang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10693 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 13, 2016

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ACS Applied Materials & Interfaces

Polyphenol Coating as an Interlayer for Thin-Film Composite Membranes with Enhanced Nanofiltration Performance Xi Zhanga,b, Yan Lva,b, Hao-Cheng Yanga,b, Yong Dua,b, Zhi-Kang Xua,b* a

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, and

b

Key

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

*Corresponding author. Fax: + 86 571 8795 1773; e-mail: [email protected].

ACS Paragon Plus Environment


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Abstract Thin-film composite (TFC) nanofiltration membranes are prepared via interfacial polymerization with polyphenol coating as an interlayer for thin and smooth polyamide selective layer. The polyphenol interlayer is simply fabricated by the co-deposition of tannic acid and diethlyenetriamine without changing the surface morphology of polysulfone ultrafiltration substrate. An interfacial polymerization is conducted from piperazidine and trimesoyl chloride on the polyphenol interlayer to construct the polyamide selective layer. The as-prepared TFC nanofiltration membranes show nearly tripled fold of water permeation flux as compared with those prepared at the same condition without an interlayer. They also exhibit a high rejection to Na2SO4 (>98%) because thin and defect-free polyamide selective layer is formed on the polyphenol interlayer. These nanofiltration properties have high reproducibility, which means the TFC nanofiltration membranes are suitable for scale-up industrial applications. Kerwords: polyphenol coating; interlayer; thin-film composite membrane; interfacial polymerization; nanofiltration.

1. Introduction Nanofiltration membranes have been widely used in various fields, including sea water desalination, waste water treatment and saline water softening.1-3 These membranes have merits of lower operation pressure and higher water permeation flux as compared with reverse osmosis ones, and they also show high rejection for most multivalent ions and organic 2


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molecules with low molecular weight.4-7 In general, nanofiltration membranes are usually fabricated by interfacial polymerization on ultrafiltration substrates to form thin-film composite (TFC) structures.8-11 In these cases, both the top selective layer and the bottom ultrafiltration substrate need to be optimized for desirable properties.12,13 Specifically, the selective layer dominates the service performance of TFC nanofiltration membranes, including water permeation flux, salt rejection, antifouling, and oxidative resistance.14 Therefore, it is essential to fabricate thin but defect-free selective layers for TFC nanofiltration membranes with high performance as their low thickness will greatly promote water permeation flux while their uniform structure can ensure high rejection.15 However, there is usually a “trade-off” effect between the thickness and the uniformity for the selective layers by common interfacial polymerization.16 It is reasonable to expect that the characteristics of ultrafiltration substrates, mainly including their porous structure and surface wettability,17-22 will significantly influence the selective layers formed in the interfacial polymerization procedure. This procedure is usually carried out on the porous substrate surface after absorbing an aqueous solution of diamine monomer, partially evaporating water with a drying process and then contacting with an organic solution of trimesoyl chloride to polymerize into polyamide thin film. Aim of the drying process is to make sure the polyamide thin film can be tightly adhered on the substrate surface.17 However, most ultrafiltration substrates are not hydrophilic enough to be homogenously wetted by the diamine aqueous solution,18,19 leading to a non-uniform monomer distribution on the substrate surface during the drying process and then forming defects in the 3



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polyamide selective layer. Researchers have made a series of efforts to hydrophilize the substrate surfaces by blending hydrophilic additives,20,21 partially hydrolyzing the polymer matrices,22 and grafting water-soluble polymers.23 Nevertheless, these methods all have impacts both on the porous structure and the surface wettability of the ultrafiltration substrates, which will influence the interfacial polymerization in a much complex way.24 For example, some hydrophilic additives can strongly suppress the diffusion of diamine monomer for the interfacial polymerization due to the strong hydrogen bonding among the monomers and the hydrophilized substrate surfaces, resulting in a relatively thick polyamide film.21 Livingston and coworkers fabricated TFC nanofiltration membranes with defect-free polyamide layers of sub 10 nm by introducing a sacrificing layer of cadmium hydroxide nanostrands for controlling the interfacial polymerization process. They have indicated that the introduction of a hydrophilic sacrificing layer on the substrate surface is a promising way to address the above-mentioned problems.25 It may be a drawback that the sacrificing layer needs to be removed after the interfacial polymerization and thus the as-formed polyamide layers will have relatively low adhesion on the substrate surfaces. In most recently, carbon nanotubes were also deposited on microfiltration substrates to form interlayers for fabricating TFC nanofiltration membranes with thin polyamide layers and high water permeation flux.26,27 However, the interactions seem to be not strong enough between the interlayer and the substrate surface and thus the TFC structures may collapse for corresponding nanofiltration membranes during practical applications. Therefore, it still remains a challenge to designing suitable interlayers for controlling the interfacial polymerization and fabricating TFC nanofiltration membranes 4


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with enhanced performance.

Figure 1. Schematic diagram for the fabrication process and the structures of TFC nanofiltration membranes with a polyphenol interlayer.

Herein, we propose to simply co-deposit a stable and hydrophilic interlayer on polysulfone ultrafiltration substrate to fabricate TFC nanofiltration membranes with thin, smooth and defect-free polyamide selective layers. It has been widely reported that mussel-inspired polydopamine coatings can strongly adhere on almost any substrate surfaces by multiple covalent and non-covalent interaction.28-31 In our previous work, we have demonstrated that polyphenols are able to form mussel-inspired coatings on various substrates via co-depositing with polymers or organics containing amino groups.32-34 These coatings are very uniform without large particles, which will block the pores of ultrafiltration substrate as in the cases of polydopamine ones.31,35 Therefore, we fabricated the polyphenol coatings via the co-deposition of tannic acid (TA) and diethlyenetriamine (DETA) as interlayers on polysulfone ultrafiltration substrates, followed by forming polyamide selective layers on the interlayers by the interfacial

5



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polymerization of piperazidine (PIP) and trimesoyl chloride (TMC). The polyphenol interlayers make the substrates easier to be wetted by the aqueous solution of diamine and can help to control the diffusion of PIP during the interfacial polymerization process. Thus, we constructed TFC nanofiltration membranes with thin, smooth and defect-free polyamide selective layers (schematically shown in Figure 1) for enhanced nanofiltration performance. Moreover, the TFC nanofiltration membranes prepared with the polyphenol interlayers exhibit improved operation reproducibility, which is great promising for practical applications.

2. Experimental 2.1 Materials N-(2-aminoethyl)-1,2-ethylenediamine (diethylenetriamine, DETA, chemically pure), tannic acid (TA), N,N-bis(2-hydroxyethyl) glycine (bicine, analytical reagent) and piperazidine (99%) were bought from Aladdin Chemistry Co. Ltd. (China). Trimesoyl chloride (99%) was purchased from Qingdao Sanlibennuo Chemistry Co. Ltd. (China). Other reagents, including hexane, ethanol, sodium hydroxide (NaOH), sodium chloride (NaCl), magnesium chloride (MgCl2), sodium sulfate (Na2SO4) and magnesium sulfate (MgSO4), were analytical reagents and obtained from Sinopharm Chemical Reagent Co. Ltd. (China). These chemicals were used without further purification. Bicine buffer with pH 7.8 was prepared as we previously reported35 with ultrapure water (18.2 MΩ, produced from an ELGA Lab Water system, France). Polysulfone (PSF) ultrafiltration membranes (pore size ranging from 40-80 nm, MWCO = 10-30 kDa) were obtained from Shanghai Megavision Membrane Engineering and Technology 6


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Co. Ltd. (China). PSF substrates were washed with ethanol for 30 min to remove impurities and dried in a vacuum oven overnight to a constant weight before used. 2.2 Preparation of the TFC nanofiltration membranes TA was dissolved in bicine buffer to form solution with a concentration of 2 g/L. Then DETA was added into the solution with TA/DETA mass ratio of 1/20. PSF substrates were cut into circle pieces with a diameter of 4 cm and pre-wetted in ethanol for 10 min. These ultrafiltration membranes were then immersed in the freshly prepared TA/DETA solution and shaken for certain time at room temperature (25 oC). After that, the TA/DETA co-deposited ultrafiltration substrates, designated as TA/DETA-PSF, were washed with deionized water for several times and dried in a vacuum oven for 24 h at 40 oC. The as-prepared TA/DETA-PSF substrates were used for the following interfacial polymerization. Polyamide selective layers were fabricated on the TA/DETA-PSF substrates by the interfacial polymerization of piperazidine and trimesoyl chloride. The TA/DETA-PSF substrates were firstly immersed in 3 g/L PIP aqueous solution for 10 min and then taken out. The excess piperazidine solution on the substrate surfaces was drained off in air. Then the piperazidine saturated substrates were immersed into 3 g/L trimesoyl chloride in hexane solution immediately. After reacting for 120 s, the membranes were taken out from the organic solution. The as-prepared TFC nanofiltration membranes were air-dried to evaporate the hexane and then post-treated at 60 °C for 30 min to stabilize the structures. As control, TFC nanofiltration membranes were also fabricated using the pristine PSF membranes as substrates. Afterwards, the two kinds of TFC nanofiltration membranes, signed as PA-PSF and PA-TA/DETA-PSF membranes were washed with deionized water for several times and stored in it for further study. 7



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2.3 Membrane characterization Deposition Degree (DD) of TA/DETA on the pristine PSF substrate was measured by weighting method and calculated by equation (1): ‫= ܦܦ‬

ௐ೎ ିௐ೚ ௐ೚

× 100%

(1)

where Wc and Wo represent the weight of the TA/DETA-PSF substrate and the pristine PSF one, respectively. The surface wettability of the substrates was characterized by a DropMeter A-200 contact angle system (MAIST VisionInspection & Measurement Co. Ltd., China) at room temperature. The surface and cross sectional morphologies of the membranes were observed by field emission scanning electron microscopy (FESEM, Hitachi, SU-8010, JAPAN). The membranes were frozen and fractured in liquid nitrogen for cross sectional observation and all the samples were sputtered with gold before FESEM observation. The chemical structures of the membrane surfaces were characterized via Fourier transform infrared spectrometer equipped with an attenuated total reflectance accessory (FT-IR/ATR, Nicolet 6700). Elemental compositions of the membrane surfaces were analyzed by X-ray photoelectron spectroscopy (XPS, PerkinElmer, USA) using Al Kα excitation radiation (1486.6 eV). The surface roughness of the TFC nanofiltration membranes was analyzed using an atomic force microscopy (AFM, multimode, Vecco, USA) in the tapping mode. An electro kinetic analyzer (SurPASS Anton Paar, GmbH, Austria) was adopted to measure the charging properties of the membrane surfaces using stream potential method with KCl (1 mmol/L) as electrolyte solution. The water uptake (WU) was measured by immersing the substrates into water for 10 min and weighing them, calculated by equation (2):36 8


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ܹܷ =

ௐೠ ିௐೞ ௐೞ

× 100%

(2)

where Wu represents the substrate weight saturated with water and Ws represents the weight of dry substrate. 2.4 Evaluation of nanofiltration performance A laboratory scale cross-flow module was used to evaluate the separation performance of TFC nanofiltration membranes under 0.6 MPa at room temperature (25 °C) with an effective diameter of 3 cm for each membrane sample. The PA layer was faced to the feed solutions and each sample was pre-compacted at 0.8 MPa for 2 h before testing. Na2SO4, MgSO4, MgCl2 and NaCl aqueous solutions with concentration of 2 g/L were used as feed solutions and the flow rate was fixed at 30 L/h. Water flux (Fw, L/m2 h) was calculated by equation (3): ொ

‫ܨ‬௪ = ஺×௧

(3)

where Q, A and t are the permeated volume of water, the area of membrane and the permeation time, respectively. The salt rejection (R) was calculated by equation (4): େ೛

ܴ = (1 − େ ) × 100%

(4)

೑

where Cp and Cf represent the concentration of feed solution and permeated solution, respectively. The concentration of all the salt solutions was determined by an electrical conductivity meter (METTLER TOLEDO, FE30, China). Each result was detected for three times to obtain an average value and the standard deviations were calculated simultaneously.

3. Results and discussion 3.1 Co-deposition of TA/DETA on PSF ultrafiltration substrates 9



TA and DETA can be co-deposited and form robust and hydrophilic cross-linked coatings on various substrates via Michael addition, as we reported previously.35 Figure 2 shows that the PSF substrate surfaces become yellow after co-deposition and the deposition degree increases with the co-deposition time. XPS spectra have a new peak assigned to N1s (Figure 3a), indicating the formation of TA/DETA coatings. Notably, these coatings should be quite thin (about several nanometers) because the XPS signal of S can be still detected from the PSF substrates after the co-deposition of TA/DETA. 0.8

Deposition Degree (%)

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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0.6

0.4

0.2

0.0 0

30

60

90

Co-deposition Time (min) Figure 2. Digital photos of TA/DETA-PSF substrates and deposition degree with different co-deposition times.

It has been suggested that the surface wettability of substrate should not only affect the interfacial polymerization process with the structures of the formed selective layer, but also influence the separation performance of the TFC nanofiltration membranes.19 In our cases, the surface wettability of TA/DETA-PSF substrates can be simply adjusted by controlling the co-deposition time. Figure 3b presents the dynamic water contact angles of the pristine PSF 10


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and the TA/DETA-PSF substrate surfaces. The pristine PSF surface shows a constant water contact angle around 60-70°, while the TA/DETA-PSF substrates exhibit a final apparent water contact angle of ~0° when the co-deposition time is above 30 min, Moreover, Figure 3b also shows that the initial water contact angle of the TA/DETA-PSF substrate decreases and the decline tendency is increased with the co-deposition time. These results suggest that the apparent surface wettability can be improved significantly for the PSF substrate by the TA/EDTA coatings, and water can spread homogeneously on the coated substrate surface. 90 C1s

(a)

80

(b)

0 min 10 min 30 min 50 min 90 min 120 min

70

O1s

N1s

TA/DETA-PSF S2s

S2p

PSF

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


60 50 40 30 20 10 0

600

500

400

300

200

100

0

1

2

Binding Energy (eV)

3

4

5

6

7

8

9

10 11

Time (s)

Figure 3. (a) XPS spectra of the pristine PSF substrate and the TA/DETA-PSF one co-deposited for 50 min; (b) dynamic water contact angles of the pristine PSF substrate and the TA/DETA-PSF ones with different co-deposition times.

Surface morphologies of the PSF and TA/DETA-PSF substrates were characterized by FESEM. Figure 4 indicates that no changes can be obviously seen after TA/DETA co-deposition, which further means thin and particle-less TA/EDTA coatings are fabricated on the PSF substrates. These coatings have almost no impact on the surface porosity and the pore size of the pristine substrate, thus avoid the effects of morphology variation on the interfacial polymerization process. Moreover, a particle-free coating is beneficial for water permeation as 11



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the deposited particles would block the ultrafiltration pores and cause decrease in the water permeation flux of TFC nanofiltration membranes.31

Figure 4. Surface morphologies of (a) PSF and TA/DETA-PSF substrates with co-deposition time of (b) 10 min, (c) 30 min and (d) 50 min.

3.2 Preparation and structures of the TFC nanofiltration membranes The pristine PSF and TA/DETA-PSF substrates were subjected to an interfacial polymerization process of piperazidine and trimesoyl chloride for fabricating TFC nanofiltration membranes, which are signed as PA-PSF and PA-TA/DETA-PSF, respectively. FT-IR/ATR was used to analyze the chemical structures of the PSF, PA-PSF and PA-TA/DETA-PSF surfaces. Figure 5a indicates that, compared with the pristine PSF substrate, new peaks appear at 1620 cm-1 and 1442 cm-1 in the spectra of PA-PSF and PA-TA/DETA-PSF nanofiltration membranes. These peaks should be assigned to the C=O stretching vibration of 12


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amide groups and O-H stretching vibration of carboxylic groups, respectively, since the unreacted acryl chloride groups will be hydrolyzed into carboxylic groups in the interfacial polymerization. Figure 5b depicts the XPS spectra of the PSF, PA-PSF and PA-TA/DETA-PSF surfaces. A new peak of N1s arises on the spectra of PA-PSF and PA-TA/DETA-PSF nanofiltration membranes, ascribing to the formation of the polyamide selective layers. The peaks of S2s and S2p disappear on the spectra of PA-PSF and PA-TA/DETA-PSF nanofiltration membranes because the membrane surfaces are totally covered by the polyamide selective layers. (b)

(a)

C1s

O1s

(1)

(3) N1s

(2) (2)

(3) (1) O-H

S2s

C=O

1800 1700 1600 1500 1400 1300 1200 1100 1000 600 -1

Wavenumber (cm

)

500

400

300

200

S2p

100

Binding Energy (eV)

Figure 5. (a) FT-IR/ATR spectra and (b) XPS spectra of (1) the pristine PSF substrate, (2) the PA-PSF and (3) the PA-TA/DETA-PSF nanofiltration membranes. The TA/DETA-PSF substrates are co-deposited for 50 min.

We observed the surface morphologies of the TFC nanofiltration membranes by FESEM. Figure 6 demonstrates that the polyamide selective layer shows a folded and nodular structure on the pristine PSF substrate, while it becomes smooth on the TA/DETA-PSF substrates. Moreover, the nodule size decreases with the increase of the TA/DETA co-deposition time. The 13



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surface roughness was further detected by AFM. Table 1 shows the mean surface roughness of PA-PSF membrane is 25.3 nm, while for PA-TA/DETA-PSF nanofiltration membranes, it gradually decreases to 9.77 nm with increasing the TA/DETA co-deposition time. These phenomena can be ascribed to the change in the surface wettability. The pristine PSF substrate cannot be wetted homogenously by the aqueous solution of piperazidine because of the low surface wettability and causes the non-uniform diffusion of piperazidine during the interfacial polymerization process, thus leads to a rough polyamide layer. As the surface wettability of TA/DETA-PSF substrate increases with the co-deposition time, the polyamide layers become smooth, which is coordinate to the literature.37

Figure 6. Surface morphologies of the TFC nanofiltration membranes fabricated using TA/DETA-PSF substrates with co-deposition time of (a) 0 min, (b) 10 min, (c) 30 min and (d) 50 min, and (e), (f), (g), (h) are the cross-section structures of (a), (b), (c), (d), respectively. Table 1. Surface roughness of the TFC nanofiltration membranes fabricated using TA/DETA-PSF substrates with different co-deposition times.

Co-deposition time (min)

Rq (nm)

Ra (nm)

0

25.3

17.8 14


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10

12.9

10.2

30

11.9

9.44

50

9.77

7.76

Thickness of the polyamide layer was measured from the cross-sectional FESEM images of the TFC nanofiltration membranes (Figure 6). It is about 113 nm on the pristine PSF substrate, while it decreases to 57 nm as the co-deposition time of TA/DETA extends from 0 to 50 min. It is well known that the diffusion rate of piperazidine to organic phase dominates the interfacial polymerization process and the polyamide film structure.38 The initially formed polyamide film will have a high cross-linking degree if there is a high migration rate of piperazidine during the interfacial polymerization process. Then the highly cross-linked polyamide film will block the further diffusion of piperazidine, resulting in a thinner polyamide selective layer.38,39,40 TA/DETA-PSF substrates are positively charged while the pristine PSF substrates are negatively charged (Figure S1). There will be less electrostatic attraction between piperazidine and TA/DETA-PSF substrates compared with the pristine one because piperazidine is positively charged in aqueous solution, leading to a high migration rate of piperazidine from TA/DETA-PSF substrates to the oil phase. On the other hand, we also measured the water reserving abilities of PSF and TA/DETA-PSF substrates and the results are shown in Table 2. The water uptakes of TA/DETA-PSF substrate increase greatly with the increase of co-deposition time, which is consistent with the increasing surface wettability. Therefore, the TA/DETA-PSF substrates are able to reserve more aqueous solution than the pristine one and 15



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also help to promote the diffusivity of piperazidine. Thus it will form thin polyamide layers on TA/DETA-PSF substrates during the interfacial polymerization process. To further prove our speculation, we also calculated the content of carboxylic group on the top surface of polyamide layer on substrates with different co-deposition times by deconvolution of C 1s and O 1s XPS spectra (Table S1 and S2). The content of carboxylic group increases with the increase of co-deposition time, which is cooperated with the co-deposition time of substrates. The highly cross-linked initial polyamide film can hinder the diffusion of piperazidine and there will be little piperazidine at the top surface in the interfacial polymerization process. Thus there will have a high hydrolysis degree of trimesoyl chloride, leading to a high content of carboxylic group on the top surface. Table 2. Water uptake of PSF and TA/DETA-PSF substrates with different co-deposition times.

Co-deposition time (min)

0

10

30

50

Weight gain (mg)

65.1

73.5

77.5

91.7

Water uptake (%)

129.0

144.9

153.2

182.0

3.3 Nanofiltration performance of the TFC nanofiltration membranes The water permeation flux of TFC nanofiltration membranes is dramatically influenced by the thickness of the polyamide selective layers because the main trans-membrane resistance is originated from them. A thin selective layer will obviously lead to a promoted water permeation flux. Figure 7 indicates that the water permeation flux of our TFC nanofiltration membranes increases with the co-deposition time of TA/DETA on the PSF substrate, which 16


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should be ascribed to the decreased thickness of the polyamide selective layer as mentioned above. Especially, the water permeation flux increases dramatically from 26 L/m2⋅h to 63 L/m2⋅h for the TFC nanofiltration membranes with a co-deposition time of 50 min, and the rejection of Na2SO4 remains above 98%. It can be owed to the defect-free and uniform polyamide layers formed on substrates. Notably, when we further extended the co-deposition time to 90 min, the as-prepared TFC nanofiltration membranes show significant decrease in Na2SO4 rejection (~60%). This phenomenon can be ascribed to the increased hydrophilicity of substrates. As shown in Figure 3b, the water contact angle decreases below 20° within 4 min when co-deposition time is above 90 min. Thus the aqueous phase may penetrate through the substrate rather than reserved on the surface, leading to a discontinuous polyamide selective layer and consequently the reduction of salt rejection. 180

100

160 140

80

120

Water Permeation Flux Rejection of Na2SO4

100

60

80

40

60

Rejection (%)

2 Water Permeation Flux (L/m •h)

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


20

40 20

0 0

10

30

50

90

Co-deposition Time (min)

Figure 7. Nanofiltration performance of the TFC nanofiltration membranes using TA/DETA-PSF substrates with different co-deposition times. The feed is Na2SO4 solution with a concentration of 2 g/L.

It is well known that the surface charges play a crucial role for the rejection performance in nanofiltration process because of the Donnan effect. Figure 8a shows the streaming potential 17



of PA-PSF and PA-TA/DETA-PSF nanofiltration membranes under various pH values. The prepared TFC nanofiltration membranes exhibit negatively charges during the nanofiltration tests as they were conducted at pH ≈ 6.0. We evaluated the rejection performance of the TFC nanofiltration membranes with different co-deposition times for different salts. It can be seen in Figure 8b that all of the TFC nanofiltration membranes show very high rejection to Na2SO4 (>98%) and follow a rejection order of Na2SO4 > MgSO4 > MgCl2 > NaCl. The high rejection for divalent anions and low rejection for divalent cations can be rationalized to the Donnan effect and size exclusion effect as the membrane surfaces are negatively charged and the hydration radius of Na+ and Cl- are too small to be rejected. 20 PA-PSF PA-TA/DETA-PSF

10

Na2SO4

100

MgSO4

MgCl2

NaCl

80

Rejection (%)

Zeta Potential (mV)

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 -10 -20

60 40 20

-30

(b)

(a)

-40 2

0 3

4

5

6

pH

7

8

9

0

10

30

50

Co-deposition Time (min)

Figure 8. (a) Zeta potentials of PA-PSF and PA-TA/DETA-PSF nanofiltration membranes under various pH values. The TA/DETA-PSF substrates are co-deposited for 50 min; (b) Rejection performance of the TFC nanofiltration membranes fabricated using TA/DETA-PSF substrates with different co-deposition times for different salts.

Reproducibility of the TFC nanofiltration membranes is crucial for scale-up industrial application. We evaluated the water flux and rejection for Na2SO4 of PA-PSF and 18


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PA-TA/DETA-PSF nanofiltration membranes prepared under the same conditions and plotted the rejection versus the water permeation flux in Figure 9. The PA-PSF nanofiltration membranes exhibit low reproducibility because the rejection-flux data are randomly dispersed. (We only collected the data of PA-PSF nanofiltration membranes which have comparable salt rejection with PA-TA/DETA-PSF in Figure 7.) On the other hand, the PA-TA/DETA-PSF nanofiltration membranes show high reproducibility with similar nanofiltration performance of the water permeation flux above 60 L/m2⋅h and Na2SO4 rejection above 98%. This disparity can be ascribed to the different surface wettability of the PSF and TA/DETA-PSF substrates. The as-prepared polyamide layer is prone to be defective on the pristine PSF substrate ascribed to its unsatisfactory wettability and the resultant non-uniform distribution of aqueous solution on the substrate. Thus the interfacial polymerization process and the resultant structure of polyamide layer are more easily to be influenced by the operation condition. On the contrary, the TA/DETA-PSF substrates exhibit improved surface wettability, resulting in a uniform and defect-free polyamide layer with high reproducibility during the fabrication process independent to minor turbulence.

19



100 90 80

Rejection (%)

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70 60 50 40 30

PA-PSF PA-TA/DETA-PSF

20 10 0 30

40

50

60

70

200250300 2

Water Permeation Flux (L/m •h) Fig. 9. Reproducibility of the PA-PSF and PA-TA/DETA-PSF nanofiltration membranes fabrication. The co-deposition time is 50 min.

4. Conclusion We report a facile method to fabricate TFC nanofiltration membranes with thin, smooth and defect-free polyamide selective layers by introducing polyphenol coating as an interlayer. The interlayer does not change the surface morphology of the pristine ultrafiltration substrate but improve its surface wettability significantly, which makes it easier to be wetted thoroughly and homogeneously by the aqueous solution during the interfacial polymerization. Polyamide selective layer formed on the interlayer has a smooth, thin and defect-free structure. The as prepared TFC nanofiltration membranes exhibit unequivocally enhanced nanofiltration performance with water permeation flux of 63 L/m2⋅h and Na2SO4 rejection of 98%. The reproducibility of these TFC nanofiltration membranes is greatly improved as compared with the typical ones, which makes them highly suitable in large-scale practical applications. 20


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Acknowledgement This work is financially supported 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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Graphic for Table of Content

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Polyphenol Coating as an Interlayer for Thin-Film Composite

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