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Anti-fouling and high flux sulfonated polyamide thin-film composite membrane for nanofiltration Zhiwei Lv, Jiahui Hu, Junfeng Zheng, Xuan Zhang, and Lianjun Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00409 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016

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Anti-fouling and high flux sulfonated polyamide thin-film

composite

membrane

for

nanofiltration Zhiwei Lv a,b, Jiahui Hu a, Junfeng Zheng a, Xuan Zhang a* and Lianjun Wang a* a

Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse,

School of Environmental and Biological Engineering, Nanjing University of Science & Technology, Nanjing 210094, China. b

Department of Municipal engineering, Hebei University of Engineering, Handan,

Hebei 056038, China.

Corresponding Author: X. Zhang: [email protected] L. Wang: [email protected]

1


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Abstract A

new

sulfonated

aromatic

diamine

monomer,

potassium

2,5-bis(4-aminophenoxy)benzenesulfonate (BAPBS), was synthesized and employed to develop a series of thin-film composite (TFC) nanofiltration membranes with trimesoyl chloride (TMC) on a polysulfone (PSF) substrate by an interfacial polymerization (IP) technique. The TFC membrane performed a high water flux of 72.8 L m-2 h-1 and a rejection of 92.5% to Na2SO4 at 0.6 MPa. The surface hydrophilicity of the as-prepared sulfonated polyamide (SPA) membrane was greatly improved by the introduction of sulfonic acid groups, as confirmed by the much reduced contact angle value. Moreover, the membrane also exhibited good antifouling ability with water flux recovery ratio (FRR) and total flux decline ratio (DRt) of about 88% and 18%, respectively. Molecular dynamics simulation was investigated to obtain an in-depth understanding of the transport behaviors of water molecules through the SPA polymers. The results clearly illustrated that the diffusion coefficient of water molecules in the sulfonated membrane matrix was about 21% greater than that in the non-sulfonated one. Overall, the combined results suggest that this type of SPA nanofiltration membrane is a promising candidate for water softening and water purification applications.

Keywords:

Nanofiltration; thin-film composite; sulfonated polyamide; high

separation performance; molecular dynamics simulation

2



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1. Introduction Drinking water supplies are increasingly limited worldwide, and potable water is especially scarce in dry locations. Desalination of salty water is an alternative option being widely researched for providing potable water.1 Nanofiltration (NF) is a pressure driven filtration technique using membranes with a molecular weight cut-off (MWCO) ranging from 200 to 1000 Da, which is between that of ultrafiltration (UF) and reverse osmosis (RO).2,3 Therefore, NF technique is being used for water purification, removal of hazardous substance, treatment of industrial sewage, etc.4-7 To date, most membranes used for NF are polyamide thin-film composites (TFC) which are prepared by interfacial polymerization (IP) technique.8,9 However, these membranes have several disadvantages such as relatively low water flux and poor antifouling properties, which limit their application. Therefore, there has been a strong research focus on development of new and improved polyamide NF membranes that can overcome these drawbacks.10 An effective approach was to introduce hydrophilic polar groups (e.g., hydroxyl, carboxylic acid and sulfonic acid functional groups) into the polymer matrix.11-16 For instance, Tang et al. developed a novel TFC composite membrane using trimesoyl chloride (TMC) and triethanolamine (TEOA).14 The introduction of hydroxyl groups enhanced the water flux from 2.8 to 13.5 L m-2 h-1. Zinatizadeh’s group also prepared a series of composite NF membranes using chitosan coated Fe3O4 nanoparticles. With the aid of free carboxylic acid groups in the outer layer of the chitosan, the flux dramatically improved from 9.2 to 36 L m-2 h-1.15 More recently, a partially sulfonated polyamide TFC membrane was prepared from 3


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TMC, m-phenylenediamine (MPD), and m-phenylenediamine-5-sulfonic acid (SMPD) using IP technique. In this study, the flux was improved from 30.0 to 55.0 L m-2 h-1 by increasing the mass ratio of SMPD/MPD from 0 to 1. However, the NaCl rejection decreased from 99% to ca. 50% due to the presence of mostly linear polymers in the barrier layer, thus displaying a lower crosslinking density.16 It is worth noting that, most of the surface modification using polar molecules have been mainly reported on the aforementioned two hydrophilic moieties, i.e., “-OH” and “-COOH” groups. There is still limited data available on the use of the “-SO3H” group in modification of NF membranes. Moreover, the water transport behavior in such a membrane matrix is also yet unclear and needs to be systematically explored. Based on the above considerations, a new sulfonated aromatic monomer, potassium 2,5-bis(4-aminophenoxy)benzenesulfonate (BAPBS), has been designed to fabricate a series of improved TFC membranes. The detailed properties of the new TFC membranes are reported here, including water flux, rejection ability, Zeta potential, contact angle and antifouling properties. Furthermore, in order to obtain an insightful understanding of the membrane properties after the introduction of “-SO3H”, an efficient method was used to build an atomistic model of the as-prepared TFC membranes. Structural and dynamic properties of confined water and free water passing through the membrane were investigated using a molecular dynamics (MD) simulation method.

2. Experimental 4



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2.1. Materials Polysulfone ultrafiltration membranes sheets (MWCO: 35,000 Da) were purchased from Hangzhou Water Treatment Technology Development Center Co. Ltd. and served for a substrate. Trimesoyl chloride (TMC, purity > 98%) was purchased from

Meryer

Chemical

Reagent

Co.

Ltd.

(Shanghai,

China).

Potassium

2,5-dihydroxybenzenesulfonate and 4-fluoronitrobenzene was purchased from Energy Chemical Co. Ltd. (Shanghai, China). De-ionized (DI) water used in all the experiments was purified using Millipore water purification system with a minimum resistance of 18 MΩ. All other reagents including inorganic salts and organic dyes of analytical grade were used without purification.

2.2 Synthesis of potassium 2,5-bis(4-aminophenoxy)benzenesulfonate (BAPBS) BAPBS was synthesized via a nucleophilic substitution from potassium 2,5-dihydroxybenzenesulfonate and 4-fluoronitrobenzene, followed by a reduction of the resulting potassium 2,5-bis(4-nitrophenoxy)benzenesulfonate (BNPBS) in the presence of Pd/C catalyst. A typical BAPBS synthesis procedure was as follows (Scheme 1).

Scheme 1. Synthetic route of BAPBS.

To a 250 mL three-necked flask, equipped with a N2 inlet and outlet, were 5


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charged with 10.0 g (43.8 mmol) of potassium 2,5-dihydroxybenzenesulfonate, 12.4 g (87.6 mmol) of 4-fluoronitrobenzene and 6.10 g (43.2 mmol) of potassium carbonate. After that, 115 mL of DMAc and 60 mL of toluene were added. The mixture was stirred and the temperature was kept at 145 oC overnight. The resulting mixture was poured into de-ionized water and washed by water and isopropanol several times. The powder (BNPBS) was dried under vacuum at 80 oC (Yield: 95 %). FTIR (KBr): ν 1021, 1079 (S=O); 1192 (C-O-C); 1469 (C=C); 1580, 1335 cm-1 (N-O). 1

H NMR: (DMSO-d6, ppm) 8.28 - 8.30 (d, J =5.0 Hz, 2H); 8.19 - 8.21 (d, J = 5.0 Hz,

2H); 7.51 - 7.52 (d, J = 2.5 Hz, 1H); 7.24 - 7.27 (m, J = 7.5 Hz, 1H); 7.19-7.22 (d, J = 7.5 Hz, 2H); 7.18-7.19 (d, J = 2.5 Hz, 1H); 7.03 - 7.04 ppm (d, J = 2.5 Hz, 2H). To a 250 mL three-neck flask, equipped with a N2 inlet and outlet, were charged with 10.0 g (21.2 mmol) of BNPBS and 0.5 g Pd/C. After that, 50 mL of ethanol was added. The mixture was heated to 60 oC until the color of the solution changed to greenblack (about 20 min), and then 80% hydrazine hydrate (7.9 mL, 127 mmol) was added dropwise. The mixture was refluxed overnight. Finally, the residue was obtained by filtration, evaporation and recrystallized from a mixture of ethanol/water and dried under vacuum at 80 oC (Yield: 75%). M.p., 266.0±1.0 oC. FTIR (KBr):

ν 1021, 1079 (S=O); 1192 (C-O-C); 1544, 3406,

3324 cm-1 (N-H). 1H NMR: (DMSO-d6, ppm) 7.20 (s, 1H); 6.77 - 6.79 (m, J = 5.0 Hz, 1H); 6.68 - 6.71 (d, J = 7.5 Hz, 2H) 6.7 - 6.73 (d, J = 7.5 Hz, 2H); 6.58 - 6.60 (d, J = 5.0 Hz, 1H); 6.55 - 6.57 (d, J = 5.0 Hz, 2H); 6.54 - 6.52 (d, J = 5.0 Hz, 2H) 4.84 4.94 (d, 4H). 6



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2.3 Fabrication process of TFC membrane The BAPBS/TMC TFC membranes were prepared by the IP technique on a PSF substrate and the process was similar to our previous report.17 In brief, the detailed procedure is described as follows. Firstly, the pH of aqueous phase (100 mL) containing BAPBS (0.05 to 1.0% (w/v), 0.12-2.40 mmol) was adjusted to 11.0 by triethylamine (TEA). Then the solution was poured onto the PSF substrate surface (ca. 78.5 cm2, 10 cm in diameter) and allowed to keep for at least 3 min. After removal the excessive solution, the membranes were dried and tissue off at ambient condition until no liquid remained. The 50 mL n-hexane solution containing TMC ranging from 0.05-0.12% (w/v) (0.19-0.45 mmol) was then poured onto the surface of amine-saturated membranes for 2 min. After removal the excessive solution again, the membranes were cleaned with fresh n-hexane solution (50 mL) and then cured in an oven at 60 oC for 5 min. Finally, the as-prepared membranes were washed by DI water and stored wetly until they were used.

3. Results and discussion The nomenclature for BAPBS/TMC TFC membranes is as follows. XaYb, where X and a refer to BAPBS and concentration in aqueous solution, respectively; Y and b stand for TMC and concentration in an organic solution, respectively. 3.1 Synthesis and characterization

7


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(a) g a d

e

f b c

(b) g a

h

f b e

c d

8.5

8.0

7.5

7.0

6.5 ppm

6.0

5.5

5.0

4.5

Figure 1. 1H NMR spectra of (a) BAPBS and (b) BNPBS in DMSO-d6.

BAPBS

was

synthesized

by

the

condensation

of

potassium

2,5-dihydroxybenzenesulfonate and 4-fluoronitrobenzene, followed by reduction of BNPBS in the presence of hydrazine hydrate and Pd/C, as shown in Scheme 1. The two-step synthesis provided the BAPBS product in high yields, suggesting its scalability. The chemical structures of both BNPBS and BAPBS were determined by IR and 1H NMR spectroscopy. The IR spectrum of BNPBS exhibited characteristic absorption bands at 1580 and 1335 cm-1, which corresponded to the asymmetric and symmetric stretching of the nitro groups. These absorption bands disappeared in the spectrum of BAPBS, indicating the reduction of nitro groups to amino groups, which was further confirmed by the observation of the typical absorptions of amino groups at 3406 and 3324 cm-1 (assigned to N-H stretching), and 1544 cm-1 (assigned to N-H 8



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deformation). All peaks in the 1H NMR spectra of BNPBS and BAPBS matched well with the expected structures (Figure 1). In Figure 1(b), the two peaks at 8.29 and 8.20 ppm were assigned to the ortho- aromatic protons next to the nitro-group in BNPBS; these peaks shifted to 6.72 and 6.69 ppm in BAPBS, respectively, due to the electron-donating effect of the amino groups (Figure 1(a)). Moreover, the new characteristic amino protons of BAPBS appeared at 4.84 ppm. These spectral data clearly confirm the formation of BNPBS and BAPBS.

3.2 Formation of sulfonated polyamide layer

-1

1544 cm N-H

-1

1021 cm O=S=O

-1

1645 cm C=O

-1

1544 cm N-H

PSF X0.75Y0.1

1800 1700 1600 1500 1400 1300 1200 1100 1000 1540 -1 Wavenumber / cm Figure 2. FTIR spectra of X0.75Y0.1 barrier layer and PSF substrate.

The chemical structures of the TFC membranes were characterized by FTIR spectroscopy (Figure 2). Compared with the PSF substrate, the spectrum of the TFC 9


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membrane showed a characteristic absorbance band at 1645 cm-1, which came from the stretching vibration of amide carbonyl bond. Furthermore, two new bands were observed at 1544 and 1021 cm-1 corresponding to the stretching vibration of N-H in the amide bond and the O=S=O stretching vibration of sulfonic acid group, respectively, indicating the formation of sulfonated polyamide barrier layer.13,16

3.3 Separation properties of TFC membranes The effect of BAPBS concentration on the performance of the TFC membranes was evaluated at TMC concentration of 0.1% (w/v) under 0.6 MPa, as is shown in Figure 3 (a). Initially, a model IP experiment was performed with BAPBS concentration ranging from 0.01% to 0.04% (w/v). However, applying those conditions only results in the membranes with rather low salt rejections. Therefore, the threshold concentration of BAPBS was selected to 0.05%, which resulted in a moderate rejection of 57.8% towards Na2SO4. The rejection of Na2SO4 increased as the BAPBS concentration increased from 0.05% to 0.25% (w/v), after which it remained constant even when the BAPBS concentration was increased to 0.75% (w/v). Further increase in BAPBS concentration to 1.0% (w/v) led to a small decease in the Na2SO4 rejection. The water flux property was at the lowest point at BAPBS concentration of 0.25% (w/v), after which the flux steadily increased until the highest feed ratio of 1.0% (w/v). These behaviors may be explained by considering the stoichiometry of the monomers involved in polycondensation, which is a crucial factor

for

obtaining

a

high-molecular-weight

polymer.

10


During

interfacial


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polymerization, amines in the aqueous phase pass into the organic solution near the interface and react with acyl chlorides. The molar balance of both monomers is not very critical in this type of IP as compared to a typical homogenous polycondensation reaction. However, there is still a small effect of non-stoichiometry on the polycondensation which should be considered. Firstly, the poor performance of the membrane formed at the BAPBS concentration of 0.05% (w/v) could be explained by the low extent of crosslinking and unreacted carboxylic acid groups in the polyamide, which in turn are due to the excess amount of TMC relative to BAPBS. Secondly, at the BAPBS concentration of 0.25% (w/v), there is an almost equivalent molar ratio of TMC and BAPBS for a homogeneous state. Therefore, the high rejection of Na2SO4 and low flux at the BAPBS concentration of 0.25% (w/v) are possibly due to this near-stoichiometric ratio of the reagents which results in a high crosslinking density of the resulting polyamide layer. Thirdly, in the BAPBS concentration range of 0.25% to 0.75% (w/v), it is not easy to enter into the organic phase for BAPBS as expected, due to large molecular weight of BAPBS and the resistance of the formed SPA. Thus, while the extent of crosslinking may not change much, the content of linear amide units in the polyamide increases quite slowly. Accordingly, the salt rejection remains constant, while the flux increases gradually with the BAPBS concentration over this range. Finally, until the maximum concentration of 1.0% (w/v), the crosslinking density of the polymer might be too low to form a robust network, which would cause the pores to be enlarged under the hydraulic pressure, thus leading to decreased rejection ability. The optimum separation performance was found for the TFC 11


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membrane X0.75Y0.1, which showed a high Na2SO4 rejection of 92.5% and a high flux of 72.8 L m-2 h-1.

(a) 100

180 160

80

140

60

Flux

120 100

40

80

-2 -1

Rejection / %

Rejection

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


60

20

40 0 0.00

0.25 0.50 0.75 1.00 Concentration of BAPBS / % (w/v)

12


20


110

90

100

80

90 80

70

70 60 60 Rejection

50

40

Flux

40

30 20 0.04

-1

50

-2

Rejection / %

(b) 100

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

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30 0.06 0.08 0.10 0.12 Concentration of TMC / % (w/v)

20

Figure 3. Flux and rejection of Na2SO4 solution for TFC membranes as a function of (a) BAPBS (TMC concentration was fixed at 0.1% (w/v)) and (b) TMC concentration (BAPBS concentration was fixed at 0.75% (w/v)). The seperation test was operated at 0.6 MPa, 25 oC, salts concentration was 1.0 g L-1).

The effect of TMC concentration on the property of the TFC membrane was evaluated at a fixed BAPBS concentration of 0.75% (w/v), and the results are shown in Figure 3 (b). At a low TMC concentration (0.05% (w/v)), the membrane exhibited low Na2SO4 rejection, which was mainly attributed to the low cross-linking degree and a resultant loose polymeric structure. Then, as the TMC concentration enhanced from 0.05 to 0.12% (w/v), the rejection of Na2SO4 dramatically increased; however, the water flux gradually decreased over this same range. The results indicate that a certain degree of crosslinking is of great importance in providing the unique membrane active layer, which can endure the hydraulic pressure while maintaining 13


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the high solute rejection. However, higher crosslinking degree negatively affects the water flux property since the barrier layer becomes thicker and more compact.18 The effect of pH of amine aqueous solution on the performance of TFC membranes was also evaluated, under 0.6 MPa, and the results are shown in Figure S1. In the preparation of TFC membranes, the pH condition plays a key role since it directly determines the reactivity of amino groups. The salt rejection is rather low when the pH of feed aqueous solution is kept at 9.0, indicating the insufficient nucleophilicity of aromatic amino groups towards the acid chloride. The reason is likely due to the low pKa value of the BAPBS monomer (4.32, determined by UV/VIS spectrophotometry) itself. The highest rejection by the TFC membrane is obtained when the interfacial polymerization is performed at pH of 11.0. A strong interaction of aromatic amino groups with TEA significantly increases the nucleophilicity. However, when the pH further increases to 12.0, the separation performance becomes worse since the rapid hydrolysis of the acid chloride predominantly occurred in contact with the highly alkaline solution. This results in incomplete reaction and low extent of crosslinking.19 To evaluate the performance of our self-made TFC membranes in comparison with commercial NF membranes and other lab-scale TFC membranes (Table 1), membrane X0.75Y0.1 was selected as the representative.20-26 The flux was normalized in order to obtain a qualitative comparison of all the membranes. Among them, X0.75Y0.1 exhibited the highest salt rejection while the water flux was also at the higher end of the range for the NF series. The introduction of the sulfonic acid 14



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group greatly enhances the membrane separation performance due to the strong hydrophilicity as well as anionic nature of these groups in the neutral state. The highly negatively charged membrane surface effectively rejects anion species in the solution.

Table 1. Performance comparison of X0.75Y0.1 with commercial and lab-scale NF membranes. Membrane

Rejection %

Flux L m h-1 bar-1

Feed system

Ref

X0.75Y0.1 SCMC/PSF NS-300 PDMSPS PDADMAC/PSS Dopamine/TMC NH2-MWCNTs/PES PEI/PAN

92.5 92.6 78.0 88.5 91.0 63.2 65.0 92.8

12.1 6.6 5.05 3.82 6.0 6.9 5.93 1.63

1.0 g L-1 Na2SO4 1.0 g L-1 Na2SO4 1.0 g L-1 Na2SO4 1.0 g L-1 Na2SO4 1.0 g L-1 Na2SO4 1.0 g L-1 Na2SO4 0.2 g L-1 Na2SO4 2.0 g L-1 Na2SO4

This work 20 21 22 23 24 25 26

-2

The rejection and flux of membrane X0.75Y0.1 for different inorganic salts (Na2SO4, MgSO4, NaCl, and MgCl2) and dyes are shown in Figure 4. It can be seen that rejection of the inorganic salts followed the order: Na2SO4 > MgSO4 > NaCl > MgCl2, while the flux remained almost constant for the different salts. At the feed solution pH of 6.8, the X0.75Y0.1 composite membrane has negatively charged surface, therefore the rejection is not only depended on size exclusion but also the Donnan’s effect. Considering the unique rejection sequence of Na2SO4 > MgSO4 and NaCl > MgCl2, the relatively lower rejection of the Mg salts is mainly due to adsorption of divalent cations, which “shields” the negative charges on the membrane surface, and thus decreases the absolute surface potential with a much weakened 15


Page 17 of 34

electrostatic repulsion.27-29 Meanwhile, X0.75Y0.1 also exhibits a promising removal rate for the dye solutions, that is, 99.8, 82.3 and 73.8% for CR, RB and MB, respectively.

100

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

Flux / L m h

80 -1

90

-2

90

100

Flux Rejection

0

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


0 Na2SO4 MgSO4 NaCl MgCl 2

CR

RB

MB

Figure 4. Rejections and flux to different salts and dyes for the X0.75Y0.1 composite membrane. (The seperation test was operated at 0.6 MPa, 25 oC, salts concentration was 1.0 g L-1, dyes solution concentration was 0.1 g L-1).

16



(a)

100

Solute rejection, RT / %

X0.75Y0.1 80 60 40 20 0 0.20

(b)

0.25

0.30 0.35 Stokes radius, rs / nm

0.2

0.3 0.4 0.5 0.6 Pore radius, rp / nm

0.40

-1

1.0 Probability density function / nm

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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0.5

0.0

0.0

0.1

0.7

0.8

Figure 5. (a) Probability plots of the effective rejection curves (solute rejection vs. Stokes radius); (b) probability density function curves of the X0.75Y0.1 membrane.

The plot of the four solute rejections for X0.75Y0.1 versus the Stokes radius of the solutes is shown in Figure 5(a), from which a high correlation coefficient (r2>0.98)

17


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was obtained. The TFC membrane processed a broad pore size distribution with most of the pore radii in the range of 0.10 - 0.44 nm, as shown in Figure 5(b).

3.4 Surface morphology of TFC membrane The morphologies of the TFC membranes prepared at different BAPBS concentrations were characterized by FE-SEM, and the results are shown in Figure 6. Compared to the virgin PSF (Figure 6 (a)), no clear pores were found on the surface images after the interfacial polymerization (Figure 6 (b-d)). Different from previous reports, in which clear peaks and valleys could be identified, the relatively smooth surface structure in our case might be attributed to the lower reaction rate of the sulfonated diamine monomer compared to MPD or piperazine. 12-13,16 As a result of the lower reaction rate, the overall amount of polyamide participating in the interfacial polymerization was lower. The AFM topographies provided another evidence to help understand the membrane structure. The root-mean-square (RMS) roughness values are in the narrow range of 6.33-8.10 nm (Figure S2 (b-d)). By comparing the total thickness values of the PSF and X0.75Y0.1, the barrier layer is estimated to be 200 nm in thickness, which is comparable with other NF membranes,13,16,19 as shown in Figure 6 (e, f).

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Figure 6. FE-SEM surface image of (a) PSF support membrane, (b) X0.25Y0.1, (c) X0.5Y0.1, and (d) X0.75Y0.1, and cross-section of (e) PSF, (f) X0.75Y0.1. 3.4 Contact angle and antifouling performance The surface hydrophilicity is estimated by evaluating the contact angle of water between membrane surface and air interface (Figure S3). It is known that a lower contact angle reveals higher hydrophilicity and a greater tendency to wet the membrane surface. It could be observed that the contact angles of our TFC membranes follow the order of PSF > X0.05Y0.1 > X0.25Y0.1 > X0.5Y0.1 > X0.75Y0.1 > X1.0Y0.1. The results clearly reveal that the surface hydrophilicity is 19


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related to the BAPBS concentration. Considering the separation properties (Figure 3 (a)), the water flux dramatically increases with increasing BAPBS concentrations, which is explained by the greatly enhanced surface hydrophilicity. Notably, although the membrane X1.0Y0.1 showed the lowest contact angle and highest water flux, its salt rejection ability was unsatisfactory, indicating the imperfect formation of polymer matrix on the PSF substrate. Antifouling property of X0.75Y0.1 membranes was studied using 0.1 g L-1 BSA as a feed solution at pH 7.0. Filtration tests were implemented for 5 cycles. After the test period, the water flux recovery ratio (FRR) and total flux decline ratio (DRt) reached to ca. 88% and 18%, respectively, indicating excellent anti-fouling performance of the membrane (Figure 7). On one hand, the surface hydrophilicity is of great importance since the strongly bonded water layer can provide a hydraulic resistance to the organic pollutants. On the other hand, the amount of the hydrophilic precursors should be controlled within a certain range as excess feed amounts can affect the IP process and lead to collapse of the polymer matrix.

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1.0 0.9 Normalized flux / Jt/J0

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0.8 0.7 0.6 0.5 0.4 0.3

0

200

400 600 800 Running time / min

1000

Figure 7. Normalized flux of X0.75Y0.1 membrane with a feed solution of 0.1 g L-1 bovine serum albumin (BSA) at 0.6 MPa.

3.5 Zeta potential Zeta potential of the BAPBS/TMC composite membrane and PSF support membrane at a wide pH range are presented in Figure S4. Generally, the membrane showed amphoteric nature with an isoelectric point of nearly 4.0. The Zeta potential of X0.75Y0.1 was found to be positive at pH values lower than 4.0 and negative at pH higher than 4.0. This is likely due to a combining effect both on the protonation of free amine groups in the polyamide matrix and the presence of carboxylate and sulfonate anions. The negatively charged surface of the TFC membrane is responsible for the high rejection ability towards anions, particularly for the multivalent ions.30-33

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3.6 Molecular dynamics simulation

Figure 8. Snapshot of the initial configuration of the water-membrane model. The water molecules are presented in ball/stick model and the cross-linked polyamide membrane is colored by the molecules in the center of the model. A snapshot of the initial configuration of the membrane is shown in Figure 8. The system consisted of the cross-linked polyamide and water phases, and periodic boundary conditions were used in the three directions. The density of the final hydrated crosslinked membrane was found to be 1.32 g cm-3, which is in good agreement with the reported values,34,35 indicating the reliability of our simulation method.

Typically,

a

reference

monomer

without

sulfonic

acid

group

(1,4-bis(4-aminophenoxy)benzene, BAPB) was used to establish the polymer structure in a similar manner, for comparison purposes. The movements of the water molecules both in the bulk and confined in the membrane were recorded by means of the diffusion coefficients obtained from the mean square distance (MSD) parameter in the Einstein equation (1). 1 d Nα D = lim ∑ | Ri (t ) − Ri (t0 ) |2 6 t → x dt i

(1) 22



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where Nα is the total number of atoms, Ri(t0) and Ri(t) are the initial and final positions of the centre of mass of particle i over the simulation time t that was used.

Figure 9. The mean square displacement (MSD) of water molecules as a function of time in bulk phase and membrane barrier layer.

Figure 9 shows the variation of the MSD of water molecules with time. The self-diffusion coefficients (Ds) of water molecules for the BAPBS and BAPB systems were calculated to be 4.38 x 10-9 and 4.83 x 10-9 m2 s-1, respectively, which were slightly greater than the reported data.34,36-38 The reason might be the relatively lower crosslinking degree initially set for the polymer matrix. Although it is considered that the extent of crosslinking of rather stiff polymers such as fully aromatic polyamide is likely to impact the water dynamics,35 there is insufficient information available regarding this, and further studies are warranted. The simulations show a fairly strong 23


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impact on the confined water diffusion; the Ds inside the membrane is dramatically decreased to 2.8 x 10-9 and 2.3 x 10-9 m2 s-1 for both models. It is worth noting that the Ds of water molecules is nearly 21% greater in the sulfonated polymer matrix than in the non-sulfonated one, which meets our expectations. Because of the strong hydrophilicity of the sulfonic acid groups, water molecules can be easily pulled away from the bulk to the sulfonated polyamide membrane surface resulting in better permeation. Further research in our laboratory is in progress to obtain a more in-depth understanding of the interactions between the SPA membrane surface and the solutes. Nonetheless, it is evident from this study that these new membranes display very effective filtration performance due to the introduction of the “-SO3- M+” groups.

Conclusions A novel aromatic sulfonated diamine monomer, BAPBS, was synthesized and employed to develop TFC membranes by the IP technique. FTIR analysis verified the presence of the sulfonated polyamide barrier layer. SEM and AFM images demonstrated that the BAPBS/TMC composite membranes had a smooth surface. Zeta potential curves revealed that the obtained TFC membranes were amphiphilic in nature, with an isoelectric point of nearly 4.0. The hydrophilicity of the active barrier layer was significantly improved by the introduction of sulfonic acid groups, as confirmed by the measured contact angles. Membrane X0.75Y0.1 exhibited the best separation performance, with a high Na2SO4 rejection rate of 92.5% and a high water flux of 72.8 L m-2 h-1 at 0.6 MPa. The rejection ability towards various salts followed 24



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the order: Na2SO4 > MgSO4 > NaCl > MgCl2 in the neutral condition, which is consistent with Donnan’s effect. Antifouling test showed that the organic pollutants were not readily absorbed by the TFC membranes, with a high FRR of ca. 88% and CRt of ca. 18%. With the aid of molecular dynamics simulation, the diffusion coefficient of the water molecules in the SPA matrix was calculated to 2.8 x 10-9 m2 s-1, which was 21% greater than that of the non-sulfonated system. Overall, the results of this study are encouraging towards the development of these novel sulfonated polyamide TFC membranes for various nanofiltration applications.

ASSOCIATED CONTENT Supporting Information Characterization methods and the molecular dynamics simulation details. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Xuan Zhang ([email protected]) Lianjun Wang ([email protected]) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT 25


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This work was financially supported by NSFC (21406117), Natural Science Foundation of Jiangsu Province (BK20140782), National Science Foundation for Post-doctoral Scientists of China (2014M561652), Jiangsu Planned Projects for Postdoctoral Research Funds (1401045B), PAPD and Fundamental Research Funds for the Central Universities (30915011306).

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Figure Captions Scheme 1. Synthetic route of BAPBS. Figure 1. 1H NMR spectra of (a) BAPBS and (b) BNPBS in DMSO-d6. Figure 2. FTIR spectra of X0.75Y0.1 barrier layer and PSF substrate. Figure 3. Flux and rejection of Na2SO4 solution for TFC membranes as a function of (a) BAPBS (TMC concentration was fixed at 0.1% (w/v)) and (b) TMC concentration (BAPBS concentration was fixed at 0.75% (w/v)). The seperation test was operated at 0.6 MPa, 25 oC, salts concentration was 1.0 g L-1). Figure 4. Rejections and flux to different salts and dyes for the X0.75Y0.1 composite membrane. (The seperation test was operated at 0.6 MPa, 25 oC, salts concentration was 1.0 g L-1, dyes solution concentration was 0.1 g L-1). Figure 5. (a) Probability plots of the effective rejection curves (solute rejection vs. Stokes radius); (b) probability density function curves of the X0.75Y0.1 membrane. Figure 6. FE-SEM surface image of (a) PSF support membrane, (b) X0.25Y0.1, (c) X0.5Y0.1, and (d) X0.75Y0.1, and cross-section of (e) PSF, (f) X0.75Y0.1. Figure 7. Normalized flux of X0.75Y0.1 membrane with a feed solution of 0.1 g L-1 bovine serum albumin (BSA) at 0.6 MPa. Figure 8. Snapshot of the initial configuration of the water-membrane model. The water molecules are presented in ball/stick model and the cross-linked polyamide membrane is colored by the molecules in the center of the model. Figure 9. The mean square displacement (MSD) of water molecules as a function of time in bulk phase and membrane barrier layer. 32



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Figure S1. Effect of pH of amine aqueous solution on the performance of the TFC membranes. (BAPBS and TMC concentrations were fixed at 0.75 and 0.1% (w/v), respectively, the seperation test was operated at 0.6 MPa, 25 oC, Na2SO4 salts concentration was 1.0 g L-1). Figure S2. AFM images of (a) PSF support membrane, (b) X0.25Y0.1, (c) X0.5Y0.1, and (d) X0.75Y0.1. Figure S3. Contact angle of the TFC membranes and PSF. Figure S4. Zeta potential of the X0.75Y0.1 and PSF substrate as a function of pH.

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Antifouling and High Flux Sulfonated Polyamide ... - ACS Publications

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