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Nanofiltration Membranes with Narrow Pore Size Distribution via Contra-Diffusion Induced Mussel-Inspired Chemistry Yong Du, Wen-Ze Qiu, Yan Lv, Jian Wu, and Zhi-Kang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10367 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 13, 2016

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ACS Applied Materials & Interfaces

Nanofiltration Membranes with Narrow Pore Size Distribution via Contra-Diffusion Induced Mussel-Inspired Chemistry Yong Du,a,b Wen-Ze Qiu, a,b Yan Lv, a,b Jian Wu,c and Zhi-Kang Xu*, a,b a

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of

Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China b

Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province,

Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China c

Department of Chemistry, Zhejiang University, Hangzhou, 310027, China

KEYWORDS polydopamine, mussel-inspired chemistry, pore size distribution, contra-diffusion, nanofiltration membrane.

ABSTRACT Nanofiltration membranes (NFMs) are widely used in saline water desalination, waste water treatment and chemical product purification. However, conventional NFMs suffer from broad pore size distribution, which limits their applications for fine separation, especially in complete separation of molecules with slight difference in molecular size. Herein, defect-free composite NFMs with narrow pore size distribution are fabricated using a contra-diffusion method, with dopamine/polyethylenimine solution on the skin side and ammonium persulfate solution on the other side of the ultrafiltration substrate. Persulfate ions can diffuse through the ultrafiltration substrate into the other side and in-situ trigger dopamine to form a co-deposited coating with polyethylenimine. The co-deposition is hindered on those sites completely covered

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by the polydopamine/polyethylenimine coating, while promoted at the defects or highly permeable regions because it is induced by the diffused persulfate ions. Such a “self-completion” process results in NFMs with highly uniform structures and narrow pore size distribution as determined by their rejection to neutral solutes. These near electrically neutral NFMs show high rejection to divalent ions with low rejection to monovalent ions (MgCl2 rejection = 96%, NaCl rejection = 23%), majorly based on steric hindrance effect. The as-prepared NFMs can be applied in molecular separation such as isolating cellulose hydrogenation products.

1. INTRODUCTION Recent years have witnessed the rapid developments of nanofiltration membranes (NFMs) because they have many celebrated features, such as commendable retention of multivalent ions and organic molecules (200–1000 Da), relatively high permeation flux under low operation pressure, as well as reduced operation and maintenance costs.1-3 Due to these characteristics, they have been widely applied in waste water treatment,1 seawater desalination,4 chemical product purification5 and food engineering.6 An important category of high-performance NFMs is thinfilm composite (TFC) membranes, which are composed of a selective layer on an ultrafiltration support.7-11 The properties of TFC NFMs are directly related to the structures of the selective layer. Their solute rejection increases with the effective membrane thickness, which is related to the physical thickness of the selective layer, as proved by both models based on extended NernstPlank equation and experimental results.12-15 On the other hand, it is revealed by computational simulations16,17 and microscopic observations18-20 that most TFC NFMs do not have a selective layer with perfectly uniform structures, which means that when solute or solvent molecules are permeating through different regions of the selective layer, their pathways will have different


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lengths. This results in different rejection of solutes at different regions of the selective layer since they have different effective membrane thicknesses. In order to obtain defect-free TFC NFMs with high rejection to target solutes, the overall thickness of the selective layer must be increased so many regions of the active layer may be over-thick, leading to relatively low permeation to untargeted solutes. For example, NFMs with high rejection to divalent ions always have > 40% rejection to NaCl.9,21-23 As a result, the separation performance of commercialized and reported TFC NFMs cannot fulfill the demands for sharp molecular separation, which is urgently needed by the biochemical and pharmaceutical industries.24-26 We suggest that fabricating membranes with more uniform structures may help us to address this challenge, which means the solute pathway is nearly constant at each region of the active layer. We have recently reported that nature-inspired TFC NFMs with highly controllable structures can be prepared via the co-deposition of polydopamine (PDA) and polyetheylenimine (PEI).27 And it has been proposed that thin films with extremely uniform structures can be formed via a contra-diffusion method based on the self-completion mechanism, which means that the film can grow only where the reactants on both sides are in contact.28,29 Here we demonstrate a novel method to fabricate TFC NFMs with highly uniform structures. In a home-made contradiffusion device, an aqueous solution of dopamine hydrochloride (DA)/PEI is placed on the skin side while an ammonium persulfate solution sits on the other side of the ultrafiltration substrate. Persulfate ions are able to diffuse into the DA/PEI side and rapidly oxidize dopamine into PDA,30 leading to the co-deposited PDA/PEI skin layer for the novel TFC NFMs. During the codeposition process, persulfate ions are easy to permeate through those defects and regions with high permeability. Therefore, these parts of the ultrafiltration substrate will be covered by PDA/PEI coatings predominantly; while those low permeability regions have relatively low


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deposition degree of PDA/PEI. The total process can lead to a continuous decrease in the permeability of persulfate ions through the membrane until these ions can permeate through nowhere of the formed selective layer. This work studies the contra-diffusion process and discusses the resulted pore size distribution and nanofiltration performance of the as-prepared TFC NFMs. 2. RESULTS AND DISCUSSION Fabrication of the TFC NFMs. The pH value of the buffer solution is set at 7.0 in the fabrication process to make sure that the co-deposition of PDA/PEI is totally induced by persulfate ions. In this case, the oxidation of DA by air is neglected while persulfate ions can still efficiently oxidize DA into PDA.30 The oxidative polymerization takes place near the ultrafiltration substrate surface, resulting in a co-deposited coating of PDA/PEI on the skin surface of the support, as schematically illustrated in Figure 1. One can speculate that the codeposition of PDA/PEI is proportional to the amount of persulfate ions that permeate through the support pores, which increases with the pore size. The pre-formed coating is not able to fully cover these pores when the contra-diffusion process is carried out for a short time. Therefore, persulfate ions will continuously permeate through some highly permeable regions or defects into the DA/PEI solution, and in situ form PDA/PEI coatings until these defects are completely sealed. Such a self-completion process will finally lead to a defect-free selective layer with highly uniform structures, when persulfate ions can no longer permeate through anywhere of the membrane. Persulfate ions are negatively charged with a hydrated radius of 0.422 ± 0.055 nm (Figure S1 in Supporting Information), which is approximate to other divalent ions such as Mg2+ and SO42-.25 As a result, the fabricated TFC membranes should show good rejection performance in the range of nanofiltration.


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Figure 1. Schematic illustration for the fabrication process of the TFC NFMs via contra-diffusion induced codeposition of PDA/PEI.

The diffusion and oxidation process can be monitored by spectrometric study35 of the DA/PEI solution. The UV-vis spectra change rapidly in the first 20 min of contra-diffusion (Figure S2 in Supporting Information), with a new peak arising at 346 nm indicating the oxidation of DA and its reaction with PEI. The variation in spectra slows down after 40 min because the formed PDA/PEI coating gradually covers the pores of the ultrafiltration substrate, hindering the permeation of persulfate ions. The spectra have almost no change after 80 min contra-diffusion. It seems that persulfate ions can hardly permeate through the membrane after this time. The chemicals were also directly added into one solution and the spectra were measured for comparison. They show much higher absorbance than the formers mentioned above, suggesting that only a small proportion of persulfate ions can permeate through the substrate and they are quickly reduced in the DA/PEI solution in our contra-diffusion process. The permeation rate of ammonium persulfate was measured at different stages of the diffusion process to further evaluate our speculation. In these cases, PDA/PEI co-deposited membranes were fabricated with different co-deposition times via the contra-diffusion method. Then, the permeation rate of persulfate ions through these membranes was measured. Figure 2 indicates that the ammonium persulfate diffusion rate decreases with the co-deposition time, which is similar to the spectrometry results, indicating the hindrance of PDA/PEI co-deposition layer to the diffusion of


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persulfate ions. 6

Ammonium persulfate -2 -1 permeation rate (mg m s )

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5 4 3 2 1 0 0

20

40

60

80

100

120

140

Deposition time (min) Figure 2. Permeation rate of ammonium persulfate through the TFC NFMs fabricated via contra-diffusion method (DA/PEI mass ratio = 1:2) with different deposition times (in the same contra-diffusion device).

Notably, DA and PEI can also permeate inside the support membrane and even through it into the ammonium persulfate solution, leading to the co-deposition of PDA/PEI in the pores of the ultrafiltration support as well as on the macroporous side of the support, which can be verified with energy dispersive X-ray spectroscopy (EDX) (Figure S3 in Supporting Information). It shows that polydopamine structure distributes all through the NFM cross-section, although it is more concentrated on the NFM skin layer. However, deposition on the regions other than the skin layer should not have any detectable influence on the rejection performance of the as-prepared TFC NFMs since it is dominated by the selective layer. Surface structures of the TFC NFMs. FT-IR/ATR and XPS were used to analyze the chemical structures of the TFC NFMs, as shown in Figure 3a. The ultrafiltration support has characteristic peaks at 2242 cm-1 and 1733 cm-1, which are due to the stretching vibrations of


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cyano and hydrolyzed cyano groups, respectively. New peaks arising at 1562 cm-1 and 1642 cm-1 can be ascribed to the N-H, and C=N stretching vibrations in the PDA/PEI coating. New peaks at 1307 cm-1 and 1404 cm-1, are assigned to the C-N stretching vibrations when C is aliphatic or aromatic, respectively. These results corroborates with our previous study.3,

27

The elemental

composition

(a)

PAN substrate 1562 1307 2242

TFC NFMs

1733 1642 1404

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) C1s

(b) N1s O1s PAN substrate

TFC NFMs 600

500

400

300

200

100

Binding energy (eV) Figure 3. FT-IR/ATR (a) and XPS (b) spectra of the substrate and the TFC NFMs. The deposition is performed with a DA/PEI mass ratio of 1:2 and a deposition time of 120 min at room temperature in the contra-diffusion


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device.

of the co-deposited coating (Table 1) is also similar to that of the previous report,27 as calculated from the XPS spectra in Figure 3b. The chemical structure of the NFM selective layer is further studied with high-resolution C 1s and N 1s XPS spectra (Figure S4 in Supporting Information). The peaks of C 1s at the binding energy of 285.4 eV and 287.5 eV can be assigned to O-C and O=C species, suggesting that both phenol and quinone groups exist in the NFMs. The peaks of N 1s at the binding energy of 399.1 eV and 399.8 are ascribed to primary, secondary and tertiary amines, while the peak at binding energy of 401.6 is ascribed to C=N, indicating PEI reacts with PDA via both Michael addition and Schiff base reaction. These surface analyses suggest that the co-deposited coating formed via contra-diffusion is quite similar in chemical structures to that induced by air oxidation.27,36-37 Table 1. Surface compositions of the substrate and the TFC NFMs prepared with a DA/PEI mass ratio of 1:2 and a deposition time of 120 min.

Sample

C 1s (%)

O 1s (%)

N 1s (%)

PAN substrate

72.73

7.00

20.27

TFC NFMs

65.12

19.03

15.85

Morphologies of the TFC NFMs. Figure 4 shows the surface and cross-section morphologies of the TFC NFMs fabricated under various conditions. It can be seen that the DA/PEI mass ratio has tremendous influence on the surface morphology of the TFC NFMs fabricated with a fixed co-deposition time. Small pores are clear on the surface of the nascent support (Figure 4a). These pores can still be seen (Figure 4b, c) when the support is co-deposited with a DA/PEI mass ratio of 1:0 or 1:0.5. Then, they are partially covered by the PDA/PEI coating (Figure 4d) with the mass ratio of DA/PEI decreased to 1:1. These pores can no longer be observed with the mass


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ratio of 1:2 (Figure 4e). However, many pores cannot be covered by the PDA/PEI coating with a mass ratio of 1:4 (Figure 4f). These results are consistent with previous studies27, 36-38 that proper amount of PEI promotes the co-deposition of PDA/PEI; however, too much PEI will result in more soluble aggregates, which are difficult to be deposited on the substrate surface. It is interesting to find that the optimized DA/PEI mass ratio is 1:2 in this study, rather than 1:1 in our

Figure 4. Surface morphologies of the support (a) and the TFC NFMs fabricated with DA/PEI mass ratio of 1:0 (b), 1:0.5 (c), 1:1 (d), 1:2 (e) and 1:4 (f) (deposition time is fixed as 120 min) and the membrane cross section with deposition time of 0 (g), 60 (h), 80 (i), 100 (j), 120 (k) and 140 (l) min (DA/PEI mass ratio is fixed as 1:2). The inserted scale bars are 1 µm.


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previous work.27 This is because the DA oxidation induced by persulfate ions is much faster than that caused by air.30 As a result, more PEI is needed to enhance the Michael addition and Schiff base reactions for matching the oxidation process, so as to reach the optimized PEI content in the co-deposited coating. The thickness of the co-deposited coating can be measured from the cross-sectional SEM images shown in Figure 4h-l. At first, the coating thickness increases from ~40 nm to ~90 nm with the deposition time varying from 60 min to 100 min. During this time, the support membrane is not fully covered by the coating, with many defects. Persulfate ions can still efficiently permeate through the membrane (Figure 2) and in situ induce the co-deposition of PDA/PEI, which causes the increase in the coating thickness. There is no obvious change after 100 min because the deposition rate decreases with the decreased permeability of persulfate ions through the membrane. The selective layer is much thinner than the TFC NFMs fabricated by the air-induced co-deposition of PDA/PEI,27 indicating that much thinner selective layer is capable to reject divalent ions, if the coating has a highly uniform structure. Surface charges of the TFC NFMs. The surface charges are measured for the studied membranes fabricated with different deposition times (Figure 5a). The substrate surface is highly negatively charged. Therefore, PEI can be gradually adsorbed to the substrate surface via electrostatic interaction, which results in an increase of the surface charge. The adsorbed PEI will be cross-linked and stabilized by PDA. The surface charge decreases slightly with the codeposition time from 40 min to 140 min. The surface charge varies with pH and the NFMs are slightly positively charged under the test condition (Figure 5b).


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20

40

(a)

(b)

30

Zeta potential (mV)

10

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


0 -10 -20 -30

20 10 0 -10

-40

-20

-50

-30 0

20

40

60

80

100

120

140

3

4

Deposition time (min)

5

6

7

8

9

pH

Figure 5. Surface zeta potential of (a) the TFC NFMs with different deposition times at pH 5.8 (fabrication condition is the same as that in Figure 2 and (b) the TFC NFMs fabricated under the optimized condition at different pH values.

Performance and pore size distribution of the TFC NFMs. The nanofiltration performances are characterized with a cross-flow module for the TFC NMFs fabricated under various conditions, using MgCl2 as the solute since the membranes are slightly positively charged under the test condition. Figure 6a shows the influence of DA/PEI mass ratio on the membrane performance. The nanofiltration performances of the NFMs with DA/PEI mass ratio of 1:0, 1:0.5 and 1:4 are all very poor because the support pores are not fully covered, as demonstrated in Figure 4b, c, f. The solute rejection is only 50% for the TFC NFMs prepared with a mass ratio of 1:1, since the co-deposited coating seems not dense enough (Figure 4d). The support pores can be effectively covered with a dense co-deposited coating only at the proper mass ratio of 1:2, and TFC NFMs with satisfactory performance are obtained with a high retention rate of ~96% and water flux of ~30 L m-2 h-1. Figure 6b shows the influence of deposition time on the nanofiltration performance. The retention rapidly increases, while the water flux decreases with deposition time varying from 60 to 120 min, which is in the same trend as the changes in thickness of the selective layer. After that, the retention and water flux have only slight change because almost no persulfate ions can permeate through the TFC NFMs. Therefore, the optimized


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fabrication condition is determined as co-deposition for 120 min, with DA/PEI mass ratio of 1:2. 100

100

1400

20

200

0

0 1:0.5

1:1

1:2

60

80

40

60 40

20 20 0

0

1:4

-2 -1 h )

100

Water flux (L m

400

80

Rejection (%)

40

Water flux (L m

1200

60

-2 -1 h )

120

80

1:0

140

(b)

(a) 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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60

80

100

120

140

Deposition time (min)

Mass ratio of DA/PEI

Figure 6. Effects of DA/PEI ratio (a) and deposition time (b) on nanofiltration performance of the TFC NFMs. The fabrication condition is the same as that in Figure 4. Test conditions: MgCl2 concentration = 1000 mg L-1, pH = 5.8, T = 303 K, P = 0.6 MPa, cross-flow rate = 30 L h-1.

We also fabricated TFC NFMs by directly adding the same amount of ammonium persulfate as that in the contra-diffusion method in DA/PEI solution with various mass ratios and codeposited on the support surface for 2 h as control. Although more aggregates are formed in the solution and the solution has much higher light absorbance (Figure S2 in Supporting Information), the as-prepared TFC NFMs under optimized condition show a low rejection (48.0%) to MgCl2 (Table S1 in Supporting Information). Prolonging the deposition time increases the rejection and reduces the water flux. However, even when the water flux is the same as the NFMs prepared via contra-diffusion method, the control NFMs still have much lower salt rejection. These results indicate that the co-deposition NFMs prepared without contra-diffusion method tend to have many defects. Therefore, we can draw the conclusion that the selfcompletion process is very important to fabricate defect-free selective layers for TFC NFMs with highly uniform structures. The pore size distribution is analyzed via measuring the rejection of neutral solutes of our TFC NFMs fabricated under the optimized condition. Figure 7a indicates that the pore size mainly


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distribute in the range from 0.2 to 0.4 nm with a geometric standard deviation of 1.28. This pore size distribution seems to be obviously narrower than those NFMs reported in literatures.39-42 Although it is difficult to exactly compare the pore size distribution of our TFC NFMs with all literature data, it is reasonable for us to compare the monovalent ion/divalent ion selectivity.25 High selectivity can be obtained with NFMs with narrow pore size distribution and high surface charge.25 Table 2 lists the monovalent ion/divalent ion selectivities of various NFMs in literatures. It can be seen that the pore size distribution of our TFC NFMs is much narrower than the NFMs with similar chemical structures,27 due to the self-completion process. Furthermore, although the TFC NFMs in this work have very low absolute value of surface zeta potential (as shown in Figure 5b), the Na+/Mg2+ selectivity reaches 20.6, which is among the highest in literature and comparable to those defect-free MOF membranes with theoretically mono-poresize-distribution.47 100

50

(a)

(b) 40

60

30

40

40

20

20

20

10

0

0 0.2

0.4

0.6

0.8

Pore radius (nm)

-2

Rejection (%)

60

0.0

80

-1

80

Water flux (L m h )

100

Cumulative probability (%)

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


0 MgCl2

CaCl2

MgSO4

Na2SO4

NaCl

Figure 7. Cumulative distribution of pore size (a) and nanofiltration performance (b) for different salts of the NFMs fabricated under the optimized condition (test conditions: salt concentration = 1000 mg L-1, pH = 5.8, T = 303 K, P = 0.6 MPa, cross-flow rate = 30 L h-1).

Figure 7b shows the rejection of different salts with the optimized TFC NFMs. The retention ratio of various salts follows the order: MgCl2 ≈ MgSO4 > CaCl2 > Na2SO4 >> NaCl. The TFC NFMs exhibit high rejection to various divalent ions but low rejection to monovalent ions


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because they have a relatively narrow pore size distribution between the size of hydrated monovalent ions and divalent ions and the size sieving effect plays the dominant role. Besides, the Donnan exclusion also has minor influence on the rejection performance since the TFC NFMs are slightly positively charged under the test condition. The retention ratios for MgCl2 and MgSO4 are similar because they are mainly based on the rejection of Mg2+ with the slightly positively charged selective layer. The possible adsorption of SO42- does not have obvious influence on Mg2+ rejection because the rejection is mainly based on size sieving effect. The retention ratio for CaCl2 is slightly lower than that of MgCl2 since Ca2+ has a slightly smaller hydrated radius25. The retention ratio for Na2SO4 is also slightly lower because the Donnan exclusion of the positively charged NFMs does not work on negatively charged divalent ions. Table 2. Comparison among monovalent ion/divalent ion selectivity of various NFMs prepared by different methods. Zeta potential (mV)

Monovalent /divalent ion pair

Monovalent ion rejection (%)

Divalent ion rejection (%)

Monovalent ion/divalent ion selectivity

Ref.

Polyamide NFMs via interfacial polymerization

~-46

Cl-/SO42-

40.8

95.4

12.9

43

Homogeneous polysulfone NFMs

-60

Cl-/SO42-

15

87

6.5

44

Poly(Nvinylimidazole)gelfilled NFMs

~18

Na+/Mg2+

80.3 ± 7.6

91.1 ± 3.2

2.9 ± 1.9

40

Cross-linked PEI NFMs

not given

Na+/Mg2+

27.3

89.3

6.8

45

Reduced graphene oxide NFMs

-43

Cl-/SO42-

~42

~60

1.4

46

PDA/PEI codeposition NFMs after cross-linking

4.8 ± 0.6

Na+/Mg2+

~47.9 ± 0.4

~95.6 ± 0.2

11.85 ± 0.65

27

MOF UiO-66 NFMs

not given

Na+/Mg2+

47.0

98.0

26.5

47

Membrane


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PDA/PEI codeposition NFMs

1.8 ± 0.4

Na+/Mg2+

23.4 ± 6.1

95.8 ± 1.3

20.6 ± 7.8

this work

The water permeation of the optimized NFMs is 5.0 L m-2 h-1 bar-1. The performance of the NFMs are compared with NFMs in the literature in Table 3. The NFM performance is fair, considering the low molecular weight cut off (MWCO, < 200 Da), although the water permeation is not as high as some of the high-performance NFMs prepared via interfacial polymerization or layer-by-layer assembly. Table 3. Comparison among divalent ion rejection, MWCO and water permeation of various NFMs prepared by different methods. Retention for MgSO4 (%)

MWCO (Da)

Water permeation (L m-2 h-1 bar-1)

Ref.

LBL polyelectrolyte NFMs

96

not given

7.1

34


85-90

not given

~15

49


97.4

296

13.8

50


82

not given

2.4

51

NFMs with ultrathin zirconia selective layer

~80

1150

10

3


~65

~700

2.9

52


not given

330

16.6

53


95.0

~300

11.0

54


40.5

880

11.9

43

PDA/PEI co-deposition NFMs after cross-linking

92

not given

1.8

27

PDA/PEI NFMs

96

190

5.0

this work

Membrane

co-deposition

The narrow pore size distribution is very useful in the separation of chemical products. For


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example, in cellulose hydrogenation, glucose (molecular weight (Mw) = 180) is the main product, with glycerol (Mw = 92) and ethylene glycol (EG, Mw = 62) as byproducts.48 These byproducts are toxic to microbes or harmful to the following handling process of glucose. Mixtures of EG/glucose or glycerol/glucose can be efficiently separated by our TFC NFMs with high rejection difference, as shown in Table 4. Table 4. Molecular separation performance of the TFC NFMs fabricated under the optimized condition. Test condition: solute concentration = 1500 mg L-1, T = 303 K, P = 0.6 MPa, cross-flow rate = 30 L h-1. Larger molecule rejection

Smaller molecule rejection

Water flux (L m-2 h-1)

EG/glucose

89.3 ± 4.01%

11.2 ± 3.46%

30.5 ± 1.08

Glycerol/glucose

85.9 ± 3.96%

35.0 ± 3.39%

29.4 ± 1.78

Mixture

The as-prepared NFMs show excellent stability in long-time operation tests for ~200 h, with little change in salt rejection and water flux (Figure S5 in Supporting Information). The NFMs are also quite stable, keeping over 90% salt rejection capability after treated with a wide range of pH value from 3 to 11 (Figure S6 in Supporting Information). However, they show moderate stablility under high temperature (Figure S6 in Supporting Information), possibly due to the relatively low glass transition temperature of PAN in the support membrane. 3. CONCLUTION In conclusion, we demonstrate a contra-diffusion method to fabricate TFC NFMs based on the mussel-inspired PDA chemistry. The as-prepared TFC NFMs possess highly uniform structures and narrow pore size distribution due to the self-completion process. The TFC NFMs have high rejection to various divalent ions but low rejection to monovalent ions mainly based on size sieving. Molecules with slight difference in Mw (< 100 Da) can be sharply separated (rejection


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difference > 50%) with the TFC NFMs. These TFC NFMs can be hopefully used in many molecular separation processes, such as isolating cellulose hydrogenation products. EXPERIMENTAL SECTION Materials. Polyacrylonitrile (PAN) ultrafiltration membranes (MWCO = ~10 kDa) were obtained from AMFOR INC (USA). Dopamine hydrochloride (DA) was purchased from SigmaAldrich (USA). Polyethyleneimine (PEI, Mw = 600 Da) and silver nitrate were procured from Aladdin (China). Other chemicals, including tris(hydroxymethyl) aminomethane (Tris), hydrochloric acid solution (18 M), sodium hydroxide, ammonium persulfate, potassium persulfate, polyethylene glycol (PEG) with the molecular weight of 200 (PEG200), 400 (PEG400), 600 (PEG600), chemicals with the formula H-(O-CH2CH2)n-OH, which is called here EG (n = 1), DEG (n = 2), TEG (n = 3), glycerol, glucose, potassium iodide, and other inorganic salts, were obtained from Sinopharm Chemical Reagent Co. Ltd (China). Ammonium persulfate was dissolved in water at 40 °C and recrystallized at room temperature before use. Potassium persulfate was dissolved in water at 40 °C and recrystallized at 4 °C. All of the other chemical agents were used as received without further treatment. Ultrapure water (18.2 MΩ) was produced by an ELGA Lab Water system (France). Hydrated radius measurement. The identification of persulfate ion hydrated radius was achieved based on Stokes’ publication,31 with an electrical conductivity meter (METTLER TOLEDO, FE30, China). The hydrated radius of ion B, rB can be calculated as:

rB =

0.820 z

λ0Bη 0

(1)


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λ0AB = λ0A + λ0B

Page 18 of 30

(2)

where z is the charge number, η0 represents the viscosity of water, λ0A, λ0B, λ0AB represent the limiting molar conductivity of ion A, B and the electrolyte as a whole, respectively. λ0K+ = 73.5 cm2 Ω-1 equiv.-1.31 The limiting molar conductivity of potassium persulfate can be obtained as the intercept by linear fitting the molar conductivity of potassium persulfate aqueous solution, ɅPPS to the square root of its molar concentration, c0.5 as the following equation:

Λ PPS = Λ0PPS − k c

(3)

where k is a constant depending on the ion species and test environment. Membrane Fabrication. PAN ultrafiltration membranes were hydrolyzed according to our previous study.27 Briefly, at first the membranes were stewed in 1.5 M sodium hydroxide solution for 1 h at 50 °C, and then immersed into 1 M hydrochloric acid solution for 1 h at room temperature for protonation. The hydrolyzed PAN membranes were washed with ultrapure water for at least three times and then used as support membranes. A round piece of support membrane was held in a home-made contra-diffusion device (membrane area that contact with solution was ~18.8 cm2), with 35 mL phosphate-citric buffer solution (pH = 7.0) on each side of the membrane. 70 mg of DA and various amount of PEI (0~280 mg) were added in the solution on the skin side of the membrane. 74 mg of ammonium persulfate was added in the solution on the other side (certain amount of NaCl was also added into the solution on either side to balance the osmosis pressure). Then the contra-diffusion device was incubated at room temperature for different diposition time (60~140 min). Finally, the membrane was taken out from the device and washed with ultrapure water for several times and stored in ultrapure water.


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Characterization. UV-vis spectra and absorbance of various solutions were measured by an ultraviolet spectrophotometer (UV 2450, Shimadzu, Japan). Surface chemical structures of the NFMs were identified by Fourier transform infrared-attenuated total reflection spectrometer (FTIR/ATR Nicolet 6700, Thermo Fisher Scientific, USA). The spectra were collected from 400 cm1

to 4000 cm-1 by cumulating 32 scans at a resolution of 4 cm−1. Atomic components were

analyzed by X-ray photoelectron spectrometer (XPS, PerkinElmer, USA) using Al Kα excitation radiation (1486.6 eV). The spectra were collected ranging from 0 to 1000 eV with a survey depth of ~10 nm. Surface morphologies and cross-sections of the membranes were observed with field emission scanning electron microscopy (FESEM, Hitachi, S4800, Japan). Energy dispersive Xray spectroscopy (EDX) mapping of the NFM cross sections were run at the same FESEM equipped with an energy dispersive X-ray spectroscope. The NFM sample for EDX was prepared under the optimized condition, labeled with 50 mM AgNO3 aqueous solution for 24 h and washed for several times before drying in vacuum. A streaming potential method was applied to detect the surface charge of the NFMs using an electro kinetic analyzer (SurPASS Anton Paar, GmbH, Austria) with 1 mM KCl solution as electrolyte solution. NMR spectra were recorded on a 400 MHz NMR spectrometer (Bruker Avance III, Bruker Corporation, Germany). Measurement of ammonium persulfate diffusion rate. TFC NFMs fabricated with deposition time of 0, 20, 40, 60, 80, 100, 120 and 140 min were fixed in a home-made contra-diffusion device (membrane area that contact with solution was ~18.8 cm2), respectively, with 35 mL ultrapure water on each side of the membrane. Then 74 mg ammonium persulfate was added in the solution on the support side of the membrane and 16 mg NaCl was added in the solution on the other side to balance the osmotic pressure. After stewed for 20 min, solution on the skin side was taken for persulfate ion concentration measurement, according to Liang and coworkers’


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Page 20 of 30

method.32 Briefly, 30 µL sample solution was added into 2970 µL KI aqueous solution (100 g L-1, containing 5 g L-1 NaHCO3). Then the solution was subjected to UV-vis spectrometric test. Ammonium persulfate concentration was calculated with absorbance at 352 nm based on the standard curve. Each diffusion experiment was repeated for three times. The permeation rate, Pr can be calculated as: ܲ‫ܪ = ݎ‬/‫ݐ ∙ ܣ‬

(4)

where H is the total mass of permeated ammonium persulfate, A is the effective membrane area, and t is the diffusion time. Measurement of the pore size distribution. Evaluation of the NFM pore size distribution was achieved based on G.-Martín and coworkers’ study,33 using a pore-flow model. Briefly, EG, DEG, TEG, PEG200, PEG400 and PEG600 were used as solutes. The observed retention can be expressed as:

 C  R0 = 1 − p 0  ×100%  C  f0  

(5)

where Cp0 and Cf0 are the solute concentrations in permeate and feed, respectively. The relationship between R0, true retention (Rt), water permeation (Jv) and cross flow rate (ω) can be expressed as follows:

1 − R0 ln  R0

  1 − Rt  = ln   Rt

 1  Jv  +  α  A ω

  

(6)

where A is a constant only related to the cross-flow flat membrane module, α is a parameter that depends on the configuration of the experimental setup. R0 and constant A can be determined by


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linear fitting ln((1-R0)/R0) to Jv/ωα. The cumulative distribution of pore size can be obtained by fitting solute radius, r to Rt with a sigmoid curve:

Rt =

100 1 + a b − cr

(7)

where the geometric standard deviation of the pore size, σg can be derived with the following equation:

n

σg = e

A

∑i=1 pi (ln Aai )

2

(8)

where pi is the proportion of the pores with radius Ai, Aa is the number average radius of the pores. Nanofiltration performance evaluation. The nanofiltration performance of the TFC NFMs was evaluated using a laboratory scale cross-flow flat membrane module under 0.6 MPa at 30 °C with the effective area of 7.07 cm2 for each sample. Each sample was pre-compacted under 0.6 MPa for 2 h before performance evaluation. Various salt solutions including Na2SO4, MgSO4, MgCl2, CaCl2 and NaCl at concentration of 1000 mg L-1 were used as feed solutions with a fixed crossflow rate of 30 L h-1. Long-term stability test was also conducted under the same condition, using MgCl2 as solute in a long-term operation process of 200 h. The stability of the NFMs under various conditions was identified by measuring the change in MgCl2 retention ratio and water flux after the NFMs were heated at 70 °C in pure water, or incubated in NaOH or HCl solutions with pH value of 11 or 3, respectively at room temperature for 12 h. Water flux (Fw, L m-2 h-1) and solute rejection (R, %) were calculated as follows:


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Fw =

Q A⋅t

Page 22 of 30

(9)

 C  R = 1 − p  ×100%  C  f  

(10)

where Q, t and A represent the volume of permeated water, the permeation time, and the effective membrane area, respectively. Cp and Cf are the solute concentrations in permeate and feed, respectively, which are proportional to the solution conductivity, detected by an electrical conductivity meter (FE30, METTLER TOLEDO, China). All results presented were average values obtained from at least three different membranes prepared under the same condition. In mixture separation experiments, 500 mg EG or glycerol was mixed with 1000 mg glucose in aqueous solution. Then the mixture was subjected to a nanofiltration process under the same condition mentioned above. Measurement of the concentration of each chemical in both feed and permeate solutions was achieved based on the H1 NMR spectra. All results presented were average values obtained from at least three different membranes prepared under the same condition. A Selectivity of solute A to solute B, α B , is defined the same as previous report. 34

α BA =

100 − R A 100 − R B

(11)

where RA and RB represent the retention of solute A and solute B, respectively.

ASSOCIATED CONTENT


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Supporting Information. Measurement of potassium persulfate molar conductivity, UV-vis spectra, EDX results, high-resolution XPS spectra, stability test and the nanofiltration performance of the TFC NFMs for control. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS 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). REFERENCES (1) Mohammad, A.W.; Teowa, Y. H.; Ang, W. L.; Chung, Y. T.; Oatley-Radcliffe, D. L.; Hilal, N. Nanofiltration Membranes Review: Recent Advances and Future Prospects. Desalination 2015, 356, 226-254. (2) Luo, J.; Wan, Y. Effects of pH and Salt on Nanofiltration-a Critical Review. J. Membr. Sci. 2013, 438, 18-28. (3) Lv, Y.; Yang, H.-C.; Liang, H.-Q.; Wan, L.-S.; Xu, Z.-K. Novel Nanofiltration Membrane with Ultrathin Zirconia Film as Selective Layer. J. Membr. Sci. 2016, 500, 265-271.


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