Fabrication of high-performance composite nanofiltration membranes for dye wastewater treatment: mussel-inspired layer-by-layer self-assembly

Journal Pre-proofs Fabrication of high-performance composite nanofiltration membranes for dye wastewater treatment: mussel-inspired layer-by-layer self-assembly Dongxue Guo, Yirong Xiao, Tong Li, Qingfeng Zhou, Liguo Shen, Renjie Li, Yanchao Xu, Hongjun Lin PII: DOI: Reference:

S0021-9797(19)31266-4 https://doi.org/10.1016/j.jcis.2019.10.078 YJCIS 25572

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

20 August 2019 18 October 2019 20 October 2019

Please cite this article as: D. Guo, Y. Xiao, T. Li, Q. Zhou, L. Shen, R. Li, Y. Xu, H. Lin, Fabrication of highperformance composite nanofiltration membranes for dye wastewater treatment: mussel-inspired layer-by-layer self-assembly, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.10.078

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Fabrication of high-performance composite nanofiltration membranes for dye wastewater treatment: mussel-inspired layer-by-layer self-assembly

Dongxue Guo, Yirong Xiao, Tong Li, Qingfeng Zhou, Liguo Shen, Renjie Li, Yanchao Xu*, Hongjun Lin*

College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, China

*Corresponding

author. Tel.: +86 0579 82282273, Email address: [email protected]

*Corresponding

author. Tel.: +86 0579 82282485. E-mail address: [email protected]

1

Abstract Inspired by the mussel adhesion mechanism, plant polyphenol tannic acid (TA) with abundant catechol groups and hydrophilic Jeffamine (JA) containing amino groups were used in a layer-by-layer (LBL) process to fabricate composite nanofiltration (NF) membranes in this study. Alternately immersing a polyacrylonitrile substrate into individual TA and JA buffer solutions could readily construct a NF membrane selective layer without any pre-treatment to the substrate. The optimised membrane showed a high pure water permeance of 37 L m-2 h-1 bar-1 whilst maintaining rejections higher than 90% towards various dyes with molecular weights ranging from 269 to 1017 g moL-1. Particularly, the obtained membrane exhibited excellent anti-fouling and long-term performance attributed to the hydrophilic membrane surface and covalent bonds in the selective layer. The novel strategy inherited the advantages of a mussel-inspired dopamine material but overcame its disadvantages. The results disclosed in this study not only provide a novel strategy to prepare composite NF membranes, but also facilitate the mussel-inspired LBL design of advanced materials for environmental applications.

Keywords: tannic acid, hydrophilicity, layer-by-layer self-assembly, nanofiltration, dye wastewater treatment

2

1. Introduction The ever-rising global demand for high quality and sustainable water accelerates the concerns about the eco-friendly purification and reuse of wastewater [1-4]. Dye wastewater generated by the textile industry is rated as the most aggressive pollutions in all the industrial sectors [5-7]. Many dyes in the wastewater are extremely toxic and even carcinogenic and can cause permanent damage to the receiving waters if discharged untreated. Various technologies, such as biological degradation [8], adsorption [9], coagulation [10] and advanced oxidation [11], have been developed to treat dye wastewater. Of these technologies, biological degradation has a poor effect on stable aromatic and azo dyes. Meanwhile, adsorption and coagulation usually result in an intractable sludge that requires significant land use, and advanced oxidation tends to use oxidizing agents resulting in secondary pollution. As a typical pressure driven membrane separation process, nanofiltration (NF) can readily fractionate solutes with molecular weights in the range of 200 to 1000 g mol-1 from aquatic systems, and is especially suitable for dye wastewater treatment [5, 12-14]. Compared to the conventional technologies, NF has the advantages of low separation cost, low energy consumption, and no secondary contaminants in the environment [15, 16]. Thin film composite (TFC) membranes consist of a dense and ultrathin selective layer on a porous ultrafiltration (UF) support [17-19]. Due to the fact that the selective layer and support are prepared separately, it allows the independent optimisation of both parts to obtain an excellent comprehensive membrane performance and has been widely studied in the field of NF. Several approaches such as interfacial polymerization, dip-coating, surface grafting, and layer-by-layer 3

(LBL) self-assembly, have been utilised to prepare TFC NF membranes. Of these various approaches, LBL assembly is known as a simple and versatile strategy to construct ultrathin selective layers with precise control of layer composition and thickness, and thus has attracted much attention [20, 21]. During the preparation process, a charged UF support is alternately immersed into solutions containing cationic and anionic polyelectrolytes, and a selective layer is deposited onto the support surface driven by electrostatic interaction. The support surface has to be pre-treated with an extra complicated modification to generate the requisite net charge before assembly, so as to adsorb the first layer. Besides, the formation of LBL selective layers driven by covalent bonding has also been developed. Gu et al. [22] dissolved m-phenylenediamine and trimesoyl chloride in toluene respectively, and alternately immersed a modified polyacrylonitrile (PAN) support into the above solutions to fabricate a high-performance polyamide TFC NF membrane. However, it still requires the hydrolysis of the support before LBL assembly and involves a toxic solvent during assembly. In addition, the scale of the LBL process is a challenge in applying this technology to real applications due to the substantial fabrication time for multiple layers. One potential solution to overcome this is to decrease the multiple layer number via optimising the preparation process. Natural materials have always been a source of inspiration for innovation and provide solutions to deal with tough, practical issues. Interface interaction plays an important role in membrane surface engineering [23, 24]. The mussel-inspired dopamine material is especially attractive due to the potential for fabricating multifunctional coatings onto nearly any substrate surface using a simple and effective approach [25]. Since then, the mussel-inspired chemistry 4

has been widely used in membrane surface engineering [26]. The exact reaction mechanism underlying the dopamine self-polymerization has long been the topic of scientific debate due to the complex oxidation-reduction process, and the involvement of a series of intermediates [27, 28]. However, it is generally accepted that the covalent reactions between quinone intermediates and amine groups via Michael addition and Schiff base reactions, the self-crosslinking reaction of quinone, and the non-covalent interactions such as π-π stacking, hydrogen bonding, and charge transfer interactions, contribute to the deposition of a polydopamine coating onto material surfaces [29-32]. It has been proved that the catechol and amine

groups

in

the

dopamine

molecule

play

crucial

roles

during

dopamine

self-polymerisation, and any system containing both catechol and amine groups can mimic dopamine and be co-deposited onto various substrates by simulating a similar polymerization mechanism [33, 34]. Plant-derived polyphenols such as tannic acid (TA), gallic acid (GA), and epigallocatechin gallate (EGCg), extracted from various plant tissues, have also been demonstrated to be capable of constructing a substrate-independent coating [35, 36]. Due to the abundant catechol groups in their structure, plant polyphenols have been mixed with polymer or organic containing amine groups to mimic dopamine self-polymerisation and fabricate NF membrane selective layers via a co-deposition strategy. For example, Cheng et al. have fabricated a loose NF membrane selective layer via the co-deposition of GA and branched polyethylenimine (PEI) [37]. Xu’s group reported the preparation of TFC NF membranes via tea catechins and chitosan [38]. Zhang et al. have demonstrated the co-deposition of EGCg and PEI to prepare TFC NF membrane selective layers [39]. However, the co-deposition process is 5

uncontrollable due to the rapid reactions in the mixed system. In addition, only a few raw materials are deposited on the substrate surface, while the rest are aggregated and precipitated from solution as particles. Herein, we demonstrated the fabrication of TFC NF membranes based on the low cost and environmental-friendly TA and a hydrophilic Jeffamine (JA) via LBL assembly on PAN supports. PAN supports can be directly utilised for the current LBL assembly without any pre-treatment due to the substrate-independent coating behaviour of TA. JA is a polyethylene glycol (PEG) like water-soluble compound possessing excellent hydrophilicity and flexible long chains, with amino groups at both ends. The middle polymer segment in JA not only interacts with the hydroxyl groups in TA via hydrogen bonding and drives the LBL process [40], but also improves the surface hydrophilicity and prevents the adhesion of hydrophobic molecules on the resultant membrane surface. The amino groups at both ends of JA can react with the quinone intermediate from TA oxidation, via Michael addition or Schiff base reactions, to form covalent bonds and render the JA molecules firmly embedded in the selective layer. In addition, the application of the TFC NF membrane in dye wastewater separation is investigated in detail.

2. Materials and methods 2.1 Materials Polyacrylonitrile (PAN) powder containing methyl methacrylate sequence (Mw= 75,000 g/mol, 96%) was obtained from the Shanghai petrochemical company. Jeffamine (JA, 6

O,Oˊ-Bis(2-aminopropyl)

polypropylene

glycol-block-polyethylene

glycol-block-polypropylene glycol 400, 99%) was bought from Sigma-Aldrich. Tannic acid (TA, 99%) was received from the Aladdin Industrial Corporation. Polyethylene glycol (PEG, 800 g/mol, 99%) was purchased from the Xilong Chemical Industrial Company. 1-methyl-2-pyrrolidone (NMP, 98%), NaH2PO4 (99%), Na2HPO4 (99%), bovine serum albumin (BSA), ethanol (99.7%) and n-hexane (97%) were bought from the Sinopharm Chemical Reagent Co., Ltd. Methyl orange (MO, 96%), Rose bengal sodium salt (RB, 90%), Methylene blue (MB, 99%), Congo red (CR, 99%), Rhodamine B (RhB, 99%) and Methyl red (MR, 99%), were purchased from the Tianjin Jinbei Fine Chemical Co., Ltd. Ultrapure water was used throughout the experiments.

2.2 Preparation of PAN UF membranes PAN UF membranes were prepared by a non-solvent induced phase inversion process. The PAN powder was dried in an oven at 60 oC under vacuum for 12 hours to remove the moisture. 19 g PAN powder and 1 g PEG 800 were dissolved in 80 g NMP and stirred at 60 oC for 12 hours. Then the solution was allowed to stand at 60 oC for 10 hours to remove any bubbles in the solution. After cooling to room temperature, the polymer solution was poured on a glass plate and the membrane was scraped off, using a 200 μm scraper, and immediately placed in deionised water at room temperature. After the phase inversion was completed, the membrane was transferred into a second deionised water to remove any residual solvent and was then stored for further modification.

7

2.3 Preparation of (TA/JA)n/PAN membrane 10 - 50 mg of TA and JA were dissolved in a 100 mL NaH2PO4/Na2HPO4 buffer solution with designed pH value, respectively. A PAN UF membrane was immersed in the TA solution for 5 - 45 min. Then, the membrane was removed and washed with deionised water for 5min. After that, the membrane was immersed in the JA solution for 5 - 45 min. Again, the membrane was taken out and washed with deionised water for 5min. Thus, one layer of LBL self-assembly was achieved. The immersion and washing steps were cycled so as to deposit the desired number of bilayers on the PAN membrane surface. Note that the solute concentration, buffer solution pH, and membrane deposition time in the JA solution was identical with that in the TA solution for each self-assembly. For convenience, the resultant membranes were designated as (TA/JA)n/PAN membranes, where n is the number of bilayers. The schematic diagram of the composite NF membranes via the LBL assembly process is shown in Fig. 1.

Fig. 1. The schematic diagram of LBL assembly process of the multilayer membrane.

2.4 Characterisations of PAN and modified membranes The chemical structure of the membrane was characterised by using Fourier transform infrared spectroscopy (FT-IR, NEXUS 670, USA). The chemical element composition of the 8

membrane surface was measured by an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA). The surface morphology of the membrane was characterised by a scanning electron microscope (SEM, S-4800, Japan). The contact angles of the different membranes were tested by a contact angle meter (Kino Co., Ltd., USA). Membrane surface-energies were calculated according to a reported method [32].

2.5 Separation performance of the membranes The dye solution was used as the feed solution to evaluate the separation performance of the modified membranes. The filtration experiments in this experiment were carried out on a dead-end stirred cell filtration device at room temperature, and a low pressure of 2 bar. The effective area of each membrane was 44 cm2 and at least three tests of each membrane were performed. The membranes were pre-pressed for 30 min, under a pressure of 2 bar, to achieve a steady stable pure water permeance. Then, a 35 μM dye solution was poured into the cell as a feed and stirred at 700 rpm to diminish the possible concentration polarisation. The membrane permeance was calculated as shown in Eq. (1):

P

V A  t  p

(1)

where P is the permeance, V is the solution volume (L) through the membrane in time t, A is the effective area (m2) of the membrane, t is the filtration time (h), and Δp is the filtration pressure (bar). The rejection of the dye was determined as shown in Eq. (2):

9

R  (1 

Cp Cf

) 100%

(2)

where R is the rejection, Cp is the concentration in the permeate, and Cf is the concentration of the feed solution. The concentration of the dye solution was measured by using an ultraviolet spectrophotometer (Beijing Persee, T6).

2.6 Anti-fouling performance experiment The anti-fouling performance of the pristine membrane and the modified membrane were evaluated by using BSA as a model protein. In this study, three cycles of filtration experiments were carried out. Each cycle filtration experiment was divided into three steps: (1) permeation of deionised water for 30 min at 2 bar, P1; (2) filtration of BSA solution (1 g/L) for 1 h at 2 bar, P2; (3) the membrane was rinsed with pure water for 20 min, then permeation of pure water for 30 min, P3. The flux recovery rate (FRR) was calculated using Eq. (3):

FRR(%) 

P3 100% P1

(3)

Generally, the FRR is used to characterise the anti-fouling properties of the membrane. A higher FRR value means a stronger membrane anti-fouling ability. In addition, the anti-fouling parameters, including total fouling ratio (Rt), reversible fouling ratio (Rr) and irreversible fouling ratio (Rir), were calculated according Eqs. (4)-(6), to study the anti-fouling performance of the modified membrane [41]:

Rt  (1 

P2 ) 100% P1

(4)

10

Rr  (

P3  P2 ) 100% P1

(5)

Rir  (

P1  P3 ) 100% P1

(6)

where Rt is the sum of Rr and Rir, Rir is the fouling caused by the adsorption or deposition of macromolecules on the membrane surface, and Rr is the fouling caused by concentration polarisation.

3. Results and discussions 3.1 Chemical characterisation of membranes As shown in Fig. 2(a), the sole TA treated membrane TA/PAN is white and appears unchanged after TA deposition. However, it changed to dark upon treatment with aqueous AgNO3 as a result of silver nanoparticle formation via a redox couple between Ag+ and the phenol groups, demonstrating the presence of polyphenolic coating on the membrane surface [35, 36]. This polyphenolic coating provides a secondary interaction platform for the following JA assembly onto the membrane surface. Fig. 2(b) shows the ATR-FTIR spectra of the pristine PAN membrane and the (TA/JA)2/PAN membrane. Compared with the pristine PAN membrane, a broad peak around 3300 cm-1 attributed to the stretching vibration of the hydroxyl groups can be found in the (TA/JA)2/PAN membrane [42, 43]. Meanwhile, the intensity of peaks at 1190 cm-1, assigned to the Ar-OH stretching vibration and 1370 cm-1 related to the ester C-O stretching vibration, is significantly enhanced [44, 45]. These results further confirm the successful deposition of TA on the membrane surface. In addition, the peaks at 2926 cm-1 11

and 2858 cm-1, related to -CH3, -CH2- and -CH- groups, become more intense and a characteristic peak at 1505 cm-1, associated with the deformation vibration of N-H, emerges after the self-assembly suggesting success of the deposition of JA [46]. The enhanced peak intensity at 1080 cm-1 related to the C-O stretching vibration could be due to a combined contribution of Ar-OH in TA and C-O-C in JA [47]. Moreover, the enhanced peak at 1630 cm-1 due to the -C=N stretching vibration indicates the Schiff base reaction between TA and JA [48]. The construction of covalent bonds would favour the membrane stability. (b)

Counts

1630 3300

758

1505

2926

3000

2500

2000

1500

6000 4000 2000 402

(e)

400

398

3000

538

396

Binding Energy (eV)

-1

(c)

-C=O -C-OH

2000

0 404

1000

Wavenumber (cm )

3500

2500

1080 1190

1370

3500

4000

-C ≡ N Counts

Transmittance (a.u.)

8000

(TA/JA)2/PAN

5000 4500

10000

4000

Ag/TA/PAN

(f)

(d) 12000

PAN

TA/PAN

(g)

6000

536

534

532

530

528

526

Binding Energy (eV)

20000

C 1s 5000

O 1s N 1s Counts

(TA/JA)2/PAN

16000

-C≡N

12000

4000 3000

PAN

Counts

(a)

-C=N-NH-

-C=O

8000 4000

2000 900 800 700 600 500 400 300 200 100

-C-OH

404

402

Binding Energy (eV)

400

398

Binding Energy (eV)

396

0 538

536

534

532

530

528

526

Binding Energy (eV)

Fig. 2. (a) Digital images of TA/PAN membrane and AgNO3 treated TA/PAN membrane, (b) ATR-FTIR and (c) XPS spectra of pristine PAN membrane and (TA/JA)2/PAN membrane, (d) N 1s fitting spectra of PAN membrane, (e) N 1s fitting spectra of (TA/JA)2/PAN membrane, (f) O 1s fitting spectra of PAN membrane, and (g) O 1s fitting spectra of (TA/JA)2/PAN membrane.

XPS was further used to quantitatively characterise the membrane surface chemistry. As shown in Fig. 2(c) and Table 1, after the LBL self-assembly, the O content increased from 6.74% to 27.14% while the N content decreased from 20.05% to 8.08%. Since both TA and JA 12

have a relatively higher O content than PAN, it is rational that the O content increased after TA and JA were self-assembled onto the PAN membrane surface. Similarly, TA contains no N content, and JA has a much lower N content than that of PAN, so the N content significantly decreases after self-assembly. The N 1s deconvolution in the PAN membrane shows a single peak at 399.04 eV, which is related to the -C≡N groups (Fig. 2(d)), while two new peaks at 399.4 eV and 401.7 eV emerge in the (TA/JA)2/TA membrane, as shown in Fig. 2(e). The former is attributed to the -C=N- bonds, while the latter is assigned to -NH- bond. The existence of the -C=N- bond on the membrane surface corresponds to the ATR-FTIR results and further confirms the Schiff base reaction between TA and JA. The PAN membrane shows two O 1s fitting peaks at 531.09 ev and 532.3 ev, which can be assigned to -C=O and -C-O-, respectively (Fig. 2(f)). For the (TA/JA)2/PAN membrane, the ratio of the peak assigned to -C-O- increases significantly owing to the abundant -C-OH groups in TA and the -C-O-C groups in JA, as shown in Fig. 2(g). Table 1. Surface chemical composition of PAN membrane and (TA/Jeffamine)2/TA membrane obtained from XPS spectra. Three tests of each membrane were performed to obtain an average value.

Composition (atomic%) Membrane C

N

O

PAN

73.2±1

20.1±0.2

6.7±0.4

(TA/JA)2/PAN

64.8±2

8.1±0.2

27.1±0.3

13

Accordingly, we suggested a possible reaction mechanism of TA and JA, as shown in Fig. 3. Under weak alkaline conditions, the TA was oxidised to quinoid form and gradually deposited onto the PAN membrane surface via the plant polyphenol inspired coating ability. Then, the covalent reactions between the amino groups in JA and the quinoid form in TA via the Michael addition reaction or Schiff base reaction, as well as the hydrogen bonding reaction between the phenolic hydroxy group and the ether group, contribute to the deposition of JA onto the membrane surface. OH

OH OH

OR O

OR O

RO

O O

O OR

O

Oxidation

OH

RO

OR O

O O

O

O OR

O

O OR

OR RO O

O

O O O

O

O

O

O

O

OR OR

O

R

NH2

JA

O

RO

RO

OR O

O

OH OR O

O

Self-polymerization

O

OR O

OH O O

OR

O

OH

TA

O

OR

OH

OH O

OR

O

O

OR O

O

O

OR O

OH O

O OR

OR O

RO

O

O O

O OR

O

OH OH

O

Schiff base reaction

NH2

OH O

RO

O

O

OH OR

O O

O

O

Mechael addition reaction

Hydrogen bonding interaction

Fig. 3. Possible reaction mechanism between TA and JA.

3.2 Surface properties of membranes Hydrophilic membrane surfaces have been proved to form a strong interaction with water molecules, conducive to the formation of a tightly bound water molecular layer and fouling repellence during filtration applications. The pristine PAN membrane showed a water contact angle of 55o (Table 2). In comparison, the water contact angle of the (TA/JA)2/PAN membrane was reduced down to 44.5o, indicating the improved hydrophilicity after the LBL self-assembly process. This could be attributed to the synergic effect of hydrophilic groups, such as the 14

hydroxyl and ether groups on the membrane surface, and the improved membrane surface roughness. Membrane surface hydrophilicity was further elucidated by surface energy, and the results can be found in Table 2. It can be seen that the (TA/JA)2/PAN membrane possesses an increased total surface energy than that of the pristine PAN membrane, and the main contribution to the increment is from the polar component, highlighting the role of polar groups in the improved hydrophilicity. Fig. 3 shows that the membrane water contact angle gradually decreases with the increase in bilayer number. Since both TA and JA are highly hydrophilic molecules, the decreased water contact angle could be mainly related to the increased membrane roughness, as proved in Fig. 6 and will be discussed later.

Table 2. Contact angle and surface-energy data of the PAN membrane and the (TA/JA)2/PAN membrane Surface-energy components (mJ m-2)

Contact angle (°) Membrane Pure water

Glycerol

Diiodomethane

γp

γd

γ

PAN

55±1.2

39.6±0.6

13.2±1.4

8.2

49.5

57.7

(TA/JA)2/PAN

44.5±0.5

65.2±1.2

23.5±0.4

15.9

46.7

62.6

15

65

Water contact angle (°)

60 55 50 45 40 35 30

0.5

1.0

1.5

2.0

2.5

Number of bilayer Fig. 3. Water contact angle of modified membranes with different bilayer numbers. Three tests of each membrane were performed to obtain an average value. 3.3 Morphologies of membranes Fig. 4 shows the SEM images of the pristine PAN membrane and the self-assembled membranes prepared under different buffer solution pH values. The pristine PAN membrane shows a clear surface with obviously visible pores (Fig. 4(a)). After LBL self-assembly by TA and JA at various pHs, the pores on the membranes disappear, indicating a new layer fully covering the PAN membrane surface. As shown in Fig. 4 (b)-(d), the self-assembled membrane surfaces prepared at pH 6 and 7 are relatively clear and smooth, while the membrane surface obtained at pH 8 shows massive nanoaggregates. Under weak alkaline conditions, the phenols in TA molecules can be readily oxidised to highly reactive quinones. Meanwhile, the self-crosslinking reactions of quinones contributes to the covalent binding among aryl rings and leads to the formation of nanoaggregates [44]. As shown in Figs. 4(f)-(h), cross-sectional 16

morphologies show that a top layer has formed on the PAN membrane surface after self-assembly, and the top layer thickness is strongly influenced by the buffer solution pH during the self-assembly process. The top layer obtained under pH 7 exhibits a thickness of 260 nm, which is higher than that prepared under pH 6 (120 nm) and pH 8 (220 nm). In fact, the TA coating process is highly dependent on the solution pH values and the optimal value, under which TA would form a thicker coating than other pH values, is pH 7 [36]. Considering this, its rational that the thickest top layer was formed under a pH of 7 during the LBL self-assembly process. (a)

PAN

(b)

1 ηm (e)

PAN

pH 6

(c)

1 ηm (f)

pH 6

pH 7

(d)

pH 8

1 ηm (g)

pH 7

1 ηm (h)

pH 8 220 nm

120 nm 260 nm

1 ηm

1 ηm

1 ηm

1 ηm

Fig. 4. Surface and cross-sectional SEM images of membranes prepared under different buffer solution pH values: (a), (e) the pristine PAN membrane; (b), (f) pH 6; (c), (g) pH 7; (d), (h) pH 8.

Figs. 5(a)-(d) shows the morphologies of membranes prepared at various TA and JA concentrations. It can be seen that with the increase of monomer concentration, the membrane surface became rough, and some significant nanoaggregates appeared on the membrane surface when the concentration reached 0.3 g/L. Cross-sectional images of the selective layers in Figs. 17

5(e)-(h) indicate that the layer thickness increased with the monomer concentration. This could be due to the deposition behaviour being enhanced at higher monomer concentrations, leading to rougher surfaces and thicker top layers. (a)

0.1 g/L

(b)

0.2 g/L

(c)

(e)

0.1 g/L

(f)

0.2 g/L

(g)

200 nm

0.3 g/L

(d)

0.5 g/L

0.3 g/L

(h)

0.5 g/L 220 nm

110 nm

1 ηm

1 ηm

1 ηm

1 ηm

Fig. 5. Surface and cross-sectional SEM images of membranes prepared with different monomer concentrations: (a), (e) 0.1 g/L; (b), (f) 0.2 g/L; (c), (g) 0.3 g/L; (d), (h) 0.5 g/L.

The membrane surface nanoaggregates structure became more significant and the top layer thickness inceased with the increase of bilayer number, as shown in Fig. 6. The nanoaggregates increase the membrane roughness and contribute to the membrane hydrophilicity, which is in agreement with the water contact angle results.

18

(a)

1.0

(b)

(d)

1.0

(e)

200 nm

2.0

(c)

2.5

2.0

(f)

2.5 250 nm

100 nm

1 ηm

1 ηm

1 ηm

Fig. 6. Surface and cross-sectional SEM images of membranes prepared with different bilayer numbers: (a), (d) 1.0; (b), (e) 2.0; (c) (f) 2.5.

3.4 Separation performance of membranes The effect of deposition time on the LBL self-assembly membrane separation performance is shown in Fig. 7(a). The monomer concentration was fixed at 0.5 g/L, the pH of the phosphate buffer solution was fixed at 7, and the bilayer number was 2. It can be found that with the increase of deposition time, the water permeance of the assembled membrane decreased, while the rejections of MO and RB gradually increased. These could be due to the fact that the thickness of the assembled selective layer increases with the deposition time, which results in an increased membrane permeation resistance. When the coating time was prolonged from 5 min to 15 min, the water permeance of the membrane decreased from 40.7 L m-2 h-1 bar-1 to 4.5 L m-2 h-1 bar-1, while the MO rejction increased from 24.5% to 71.2%, and the RB rejection increased from 83.5% to 95.8%. Further increases in deposition time would significantly decrease the water permeance while the dye rejection remained nearly unchanged. 19

So the coating time was fixed at 15 min for the next tests.

40

80

(b)

50

100

40

80

30

60

20

40

20

40

10 0

-1

-1

60

Water Permeance MO Rejection RB Rejection

20 0 50

40

30

20

10

0

-2

30

Rejection(%)

-2

-1

Water Permeance (L m h bar )

100

-1

50

10

Water permeance MO rejection RB rejection

0

6.0

6.5

7.5

8.0

0

150

100

120

80

90 60

Water Permeance 60 MO Rejection RB Rejection 40

30

20

-1

-1

-1

80 60 60 40

40

Water Permeance MO Rejection RB Rejection

20 0.1

0.2

0.3

20

0.4

0.5

0

-2

-1

Water Permeance (L m h bar )

(d)

100

80

Rejection(%)

-2

7.0

Monomer Concentration (g/L)

0

0.5

1.0

1.5

2.0

2.5

Rejection(%)

Water Permeance (L m h bar )

20

pH

Coating Time (min)

(c)

Rejection (%)

Water Permeance (L m h bar )

(a)

0 3.0

Bilayer Number

Fig. 7. Membrane separation performance in terms of (a) coating time, (b) solution pH, (c) monomer concentration, and (d) bilayer number. Three tests of each membrane were performed to obtain an average value.

The effect of phosphate buffer solution pH on the LBL self-assembly membrane separation performance is shown in Fig. 7(b). The monomer concentration was fixed at 0.5 g/L, coating time was fixed at 15 min, and the number of bilayers was 2. As shown in Fig. 7, the water permeance of the assembled membrane tends to decrease at first, and then increase with the pH rise. The rejection of MO and RB shows a reverse trend. The lowest water permeance is obtained at pH 7. The optimal coating pH of TA, under which the polyphenol 20

coating is thicker than that formed at other solutions, is pH 7. This results in the thickest selective layer at pH 7 (as proved by the SEM images in Fig. 3) and the lowest permeance, due to the highest permeate resistance. When the solution pH is enhanced to 8, the water permeance of the membrane increases to 20 L m-2 h-1 bar-1, and the rejection of MO and RB increases to 91% and 98%, respectively. Therefore, the solution pH was fixed at 8 for the following tests. Figs. 7(c) and 7(d) shows the increase of the MO and RB rejections, and the decrease of water permeance with the increase in monomer concentration and bilayer number. This phenomenon can be attributed to the fact that a low monomer concentration, or few bilayer numbers, will restrict the formation of an integrated selective layer, while an excessive monomer concentration, or bilayer numbers, may result in too thick a selective layer. Herein, the optimal membrane was fabricated using a monomer concentration of 0.3 g/L and a bilayer number of 2. This membrane showed an MO rejection of 90.5% and RB rejection of 99.5%, with a water permeance up to 36 L m-2 h-1 bar-1. The separation performance of the optimal membrane towards different dyes was further investigated, and the results can be seen in Fig. 8(a). It shows that the dye rejection increases with its molecular weight. The dyes with relatively higher molecular weight were almost completely rejected. Some dyes with lower molecular weight, such as MR and MO, were partly rejected, but still held rejections higher than 90%. These results demonstrate the sieving effect in filtration processes. However, it cannot exclude the existence of charge effects during filtration. 21

(a)

(b)

100

-1

-1

Permeance (L m h bar )

40 20 0

45 90 40 80

-2

60

100

35 Permeance RB Rejection

25 20

MR MO MB RhB CR RB 269 479 327 374 697 1018 -1 Dye and its molecular weight (g mol )

70

30

0

10

20

RB Rejection(%)

Rejection (%)

80

50

60

30

40

50 50

Time (hour)

Fig. 8. (a) Rejection of (TA/JA)2/PAN for various dyes; (b) long-term separation performance of (TA/JA)2/PAN membrane. Three tests of each membrane were performed to obtain an average value. The stability of the (TA/JA)2/PAN membrane was characterised by a long-term separation performance using RB solution as a feed. As shown in Fig. 8(b), the RB rejection was stable all through the filtration, and the water permeance did not change after an initial stage of membrane compaction. These results demonstrate that the membrane has good stability over a long-term filtration while still maintaining a remarkable separation performance. Membrane fouling is one of the most trickiest problems in membrane processes [49-54] and it results in many drawbacks such as permeance decline, increase in operational costs, and membrane degeneration. Therefore, lots of efforts have been made to enhance the membrane anti-fouling ability. In this study, the anti-fouling ability of both the pristine PAN membrane and the (TA/JA)2/PAN membrane was evaluated with three cycle filtration tests using BSA as a model protein. Two kinds of filtration models, time-dependent filtration, and filtrate volume-dependent filtration, were used to measure the membrane anti-fouling performance, as 22

shown in Figs. 9(a) and 9(b). It can be seen that both membranes exhibit a lower BSA flux than the pure water permeance due to protein fouling, while in both models, the water permeance of the (TA/JA)2/PAN membrane can be completely recovered after cleaning.

-1

80

) -1

100

PAN (TA/JA)2/ PAN

-2

-2

60 40 20 0

120

Permeance (L m h bar

-1

-1

80

Permeance (L m h bar

100

(b)

PAN (TA/JA)2/ PAN

120

)

(a)

0

40

80

120

160

200

240

60 40 20 0

280

0

100

Time (min)

(c)

100

400

100

500

600

PAN (TA/JA)2/PAN

80 Percentage (%)

Percentage (%)

(d)

60 40

60 40 20

20 0

300

Filtrate volume (mL)

PAN (TA/JA)2/PAN

80

200

FRR

Rt

Rr

0

Rir

FRR

Rt

Rr

Rir

Fig. 9. (a) time-dependent permeance and (b) permeate volume-dependence of the pristine PAN membrane and the (TA/JA)2/PAN membrane under three cycles of BSA solution filtration tests, (c) FRR, Rt, Rr and Rir from time-dependent filtration, and (d) FRR, Rt, Rr and Rir from permeate volume-dependent filtration. Three tests of each membrane were performed to obtain an average value. To further investigate the anti-fouling performance of the membranes, four parameters including FRR, Rt, Rr and Rir were calculated and are shown in Figs. 9(c) and 9(d). It is apparent that the FRR value of the (TA/JA)2/PAN membrane is higher than that of the pristine 23

PAN membrane, indicating its stronger anti-fouling ability. The (TA/JA)2/PAN membrane has a much lower Rir than the pristine PAN membrane, suggesting that the pristine PAN membrane is prone to irreversible fouling. The Rr of the (TA/JA)2/PAN membrane is relatively higher, indicating that reversible fouling is dominant for the membrane. In fact, the reversible fouling can be readily removed by simple physical cleaning, which is beneficial for efficient filtration processes. It has been shown that the hydrophilic surface contributes to the resistance to BSA absorption [55]. The previous analysis (Table 2) shows that the (TA/JA)2/PAN membrane is more hydrophilic than the pristine PAN membrane, and the highly hydrated polypropylene glycol segments from the JA structure would take up a large amount of free water to form a water molecule layer on the (TA/JA)2/PAN membrane outer surface, and so reduce, or even prevent, the direct contact of BSA with the membrane [56]. This characteristic is helpful to mitigate membrane fouling, which is generally considered as the major limitation of membrane technology [57-62]. Table 3 lists the separation performance of some TFC NF membranes reported in the literature and in this study. (TA/JA)2/PAN membranes prepared in this study had a much higher pure water permeance whilst possessing a similar, or higher, dye rejection than the other TFC NF membranes including dopamine, TA, and EGCg based membranes fabricated via interfacial polymerisation or co-deposition methods. The significantly higher permeance of the (TA/JA)2/PAN membrane could be mainly attributed to the fact that the hydrophilic segment of the selective layer facilitates water molecule permeation. Table 3. Dye removal performance comparison of some TFC NF membranes in reference to 24

(TA/JA)2/PAN prepared in this study a Pure water Membrane

Preparation method

permeance (L m-2 h-1

Dye rejection (%)

Reference

bar-1)

(PDDA/GO)4/PAN

LBL

5.8

CR, 99.9

[

63]

(PS/PDMAEMA)/PSf

LBL

8

CR, 99

[

64]

(CMCNa/PEI)/PP

LBL

5.7

CR, 99.4; RhB, 98

[

65]

(Dopamine/TMC)/PES

Interfacial polymerization

30

CR, 95

[

66]

(DETA/TMC)/PES

Interfacial polymerization

17

MO, 81; CR, 97

[

67]

(TA/TMC)/PES

Interfacial polymerization

23.4

MB, 71.2

[

68]

Atomic layer deposition

8

RB, 96

[

69]

Co-deposition

19

CR, 99

[

39]

LBL

37

TiO2/Ceramic (EGCg/EPI)/PES

(TA/JA)2/PAN a

MO, 91; MB, 98; RhB, 99; CR, 99.5; RB, 99.5

This work

PDDA, GO, PS, PDMAEMA, PSF, CMCNa, PEI, PES, DETA, TMC are abbreviations of

poly(diallyldimethyl ammoniumchloride), graphene oxide, polystyrene, poly(N,N-dimethylaminoethyl methacrylate),

polysulfone,

sodiumcarboxymethylcellulose,

polyethylenimine,

polyethersulfone,

diethylenetriamine, trimesoyl chloride.

4. Conclusions Inspired by the mussel adhesion mechanism, where catechol and amino groups interact via various covalent and non-covalent bonds, a strategy which alternately assembles a plant polyphenol tannic acid (TA) with abundant catechol groups and hydrophilic Jeffamine (JA) containing amino groups, onto a polyacrylonitrile (PAN) substrate membrane, was developed 25

in this study. Accordingly, novel (TA/JA)n/PAN membranes were successfully fabricated via this novel mussel-inspired LBL strategy. Compared with the previously reported mussel-inspired co-deposition strategy, which is uncontrollable and generates a lot of waste in solution [37-39], the LBL process in this study is controllable and has a high atomic economy. The hydrogen bonding interactions and covalent reactions between TA and JA acted as the driving force of the LBL process. The optimal membrane can be obtained under a deposition of 15 min, a buffer solution pH of 8, a monomer concentration of 0.3 g moL-1 and a bilayer number of 2. The resulting membrane showed a high pure water permeance of 37 L m-2 h-1 bar-1 and dye rejection, toward various dyes, of higher than 90%. In addition, the membrane exhibited an excellent anti-fouling performance due to the hydrophilic membrane surface. Most importantly, the prepared membrane demonstrated good stability during a long-term separation process which could be attributed to the covalent bonding between TA and JA in the selective layer. This mussel-inspired LBL strategy also can be used to fabricate high performance ultrafiltration or microfiltration in further work. The novel, mussel-inspired, LBL assembly method described here offers a new incentive not only to nanofiltration membrane fabrication, but also to the surface engineering of advanced materials in various applications.

Acknowledgements This study was financially supported by the Natural Science Foundation of Zhejiang province, China (No. LQ19B060008) and the National Natural Science Foundation of China (Nos. 51978628, 51578509).

26

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36

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

37

Graphical abstract

dye solution TA

JA

38