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Improving permeation and antifouling performance of the polyamide nanofiltration membrane through incorporating arginine Lin Fan, Qi Zhang, Zhen Yang, Runnan Zhang, Ya-nan Liu, Mingrui He, Zhongyi Jiang, and Yanlei Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2017 Downloaded from http://pubs.acs.org on April 5, 2017

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ACS Applied Materials & Interfaces

Improving permeation and antifouling performance of the polyamide nanofiltration membrane through incorporating arginine Lin Fana,b, Qi Zhanga,b, Zhen Yanga,b, Runnan Zhanga,b, Ya-nan Liua,b, Mingrui Hea,b, Zhongyi Jianga,b, ∗, Yanlei Sua,b,∗

a

Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology,

Tianjin University, Tianjin 300072, China b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University,

Tianjin 300072, China

Keywords: arginine, hydrophilicity, negative charge, nanofiltration membrane, antifouling Abstract: Inspired by the hydrophilicity effect of arginine (Arg) in water channel aquaporins (AQPs), Arg

∗

Corresponding author. School of Chemical Engineering and Technology, Tianjin University, No. 92,

Weijin Road, Nankai District, Tianjin 300072, China. Tel: +86-22-27406646. Fax: +86-22-23500086. E-mail address: [email protected] ∗

Corresponding author. School of Chemical Engineering and Technology, Tianjin University, No. 92,

Weijin Road, Nankai District, Tianjin 300072, China. Tel: +86-22-23500086. Fax: +86-22-23500086. E-mail address: [email protected]

1 ACS Paragon Plus Environment


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was incorporated into polyamide layer during interfacial polymerization, in order to enhance the permeation and antifouling performance of the nanofiltration (NF) membranes. Due to the presence of active amine groups, Arg became another aqueous phase monomer except piperazine (PIP) to react with trimesoyl chloride (TMC) during interfacial polymerization, which was incorporated into polyamide network. The resulting polyamide NF membranes were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), atomic force microscopy (AFM), static water contact angle, zeta potential, and positron annihilation spectroscopy (PAS) measurement. The effects of incorporating Arg in aqueous phase on water permeability, and rejection of dyes and inorganic salts of the NF membranes were studied, respectively. Similar to its function in AQPs, Arg apparently increased the hydrophilicity and the negative charges of membrane surface, consequently the permeation performance. When the addition of Arg reached 40% to PIP, the water flux was doubled and the rejection ratios of Congo red and Orange GII were still over 90%. Meanwhile, the antifouling experiments verified that the modified polyamide NF membranes possessed excellent fouling-resistant performance for negatively charged foulants of BSA, emulsified oil droplet, and HA. The flux was decreased below 15% and recovered even rose to 89%.


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1. Introduction Nanofiltration (NF), as a kind of efficient separation process operated under room temperature and low pressure, could implement purification/separation by exploiting different physicochemical properties

of

the

molecules

in

the

mixture

to

be

separated:

size,

charge,

hydrophilicity/hydrophobicity 1. Great efforts by researchers in different fields have provided diverse design ideas on nanofiltration, such as nature inspired strategies 2, functionalized polymers or hydroid materials

3-5

, and membrane preparing methodologies 6. Among the methods of NF

membrane preparation, interfacial polymerization has been most commonly used owing to the facile and versatile features. Usually, interfacial polymerization happens quickly between aqueous phase monomer and organic phase monomer at the interface of the two phases

7-8

. Furthermore, most

commonly used aqueous phase monomer include piperazine (PIP) and m-phenylene diamine (MPD), while organic phase monomer is trimesoyl chloride (TMC). The polyamide membranes prepared by interfacial polymerization are the most widely used NF membranes

9-10

. However, in many

applications, the polyamide layer of NF membranes is easy to be fouled, attributing to the multiple interactions between membrane surface and foulants, which constitutes a major obstacle for operation and cost efficiency 11-13. Therefore, it becomes a research focus to fabricate the antifouling polyamide membranes for practical applications. Different kinds of hydrophilic organic or inorganic particles and molecules, such as silica, titanium dioxide, MOFs (metal−organic frameworks) and carbon materials, have been utilized to enhance the hydrophilicity of the membranes surface, consequently increase the antifouling performance and permeation

flux

14-17

.

For

hydrophilic

macromolecules,

biomacromolecules, and zwitterionic polymers

18-22

such

as

PEG-like

polymers,

, have also been chosen to incorporate in



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polyamide layer, establishing an expedient, effective, and most common method 23. An et al. 19 added polyvinyl alcohol (PVA) into aqueous phase monomer during interfacial polymerization. The introduction of PVA increased the membrane hydrophilicity and decreased the surface roughness, which improved the antifouling performance of polyamide NF membranes. Ma et al.

21

reported

electro-neutral NF membranes by using a novel zwitterionic amine monomer polymerized with TMC, which successfully built an antifouling surface with high water flux. However, based on the incorporation molecules method, there were still two aspects limiting the broad utilization of hydrophilic macromolecules: (1) for the method of simple physical incorporating or macromolecule polymerization, the weak interaction or stereo-hindrance effect might generate non-ideal effects, such as pore defects which were disadvantageous to rejection performance; (2) for the most commonly used chemical grafting method, the graft density, length and regularity of grafted chains were still difficult to control

24

. To avoid such non-ideal effects, as well as effectively control the

incorporation process of hydrophilic molecules, small hydrophilic molecules may become competitive candidates 25-28. Arginine (Arg), as one of twenty kinds of natural amino acids, contains an α-amino group, an α-carboxylic acid group, and a side chain of 3-carbon aliphatic straight chain capped by a complex guanidinium. For human bodies in alkaline environment, Arg exists as cation owing to the composition of multiple amine groups. Meanwhile, Arg becomes an important amino acid source to build up water channels. When water molecules transport through water channel AQPs, amino acids of AQPs and neighboring water molecules both affect the transport process. Water–amino acid interactions occur primarily in two characteristic domains: the asparagine–proline–alanine (NPA) motif and the aromatic/Arg selectivity filter (ar/R region)

29

. For AQP1 (one of AQPs), the ar/R


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region constitutes the narrowest part of the pore to only filter water molecules, which is formed by phenylalanine-56 (Phe-56), histidine-180 (His-180), and Arg-195. This region possesses the weakest water-water hydrogen bonds, attributing to that Phe-56 enhances the hydrogen bonds between water and Arg-195, as well as between water and His-180. Meanwhile, positively charged Arg generates strong electrostatic repulsion to protons and other positively charged ions

30

. Both structure and

composition make the ar/R region transport only water in single file. The strong hydrogen bonds between Arg and water make AQP1 transport water smoothly, which demonstrates a highly efficient utilization of the high hydrophilicity of Arg. Inspired by the hydrophilicity effect of Arg in AQPs, we incorporated Arg into polyamide layer to enhance the hydrophilicity, as well as the consequent antifouling performance and permeation performance of the polyamide NF membranes. Arg contained multiple amine groups, which were able to take part in polyamide formation through reaction with acyl chloride groups. Therefore, in this study, different ratio of Arg to PIP was introduced into aqueous phase, which reacted with TMC to complement interfacial polymerization. The fabricated polyamide NF membranes were analyzed by measuring the surface composition, hydrophilicity, morphology, roughness, and zeta potential. The separation performance was evaluated by filtrating pure water, as well as dye and inorganic salt solutions. The antifouling performance and permeation performance were evaluated by three batches of fouling experiments using negatively charged foulants including BSA, emulsified oil droplet and HA. 2. Experiments 2.1. Materials Polyethersulfone (PES Ultrason E6020P with Mw = 29,000 g/mol), from BASF Co. (Germany),



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was dried at 110 oC for 48 hours and stored at 50 oC before used. TMC was purchased from Jkchemical Technology Co. (Beijing, China). Arg was purchased from Institute of Hematology, Chinese Academy of Medical Science (Tianjin, China). Polyethylene glycol (PEG, Mw = 2000) was received from Sigma-Aldrich Co. (Shanghai, China). N, N-dimethylformamide (DMF), PIP, n-heptane, TMC, sodium dodecyl sulfate (SDS), inorganic salts were purchased from Jiangtian Chemical Technology Co. Ltd. (Tianjin, China). Bovine serum albumin (BSA) and humic acid (HA) were supplied by Aobox Biotechnology Co. (Beijing, China). Oil was the edible soybean oil. Methyl orange (C14H14N3NaO3S, Mw = 327.3), Orange GII (C16H10N2Na2O7S2, Mw = 452.4), and Congo red (C32H22N6Na2O6S2, Mw = 696.7) were purchased from Guangfu Fine Chemical Technology Co. (Tianjin, China). Ultrapure water (pH = 6.0±0.2) was used throughout all experiments. 2.2. Preparation of membranes The polyamide NF membranes were constituted by PES support and thin active layer. The PES support membrane was prepared by the method of non-solvent induced phase separation (NIPS), which was described in our previous works

31

. The active layer was formed by interfacial

polymerization, which was implemented by the reaction between aqueous phase monomer and organic phase monomer. PIP was dissolved in water with the concentration of 0.1 wt%. Arg was dissolved in PIP aqueous solution with different weight rate to PIP respectively, which was given in Table 1. TMC was dissolved in n-heptane solution with the concentration of 0.1 wt%. The PES support membrane was first immersed in aqueous solution for 10 min to absorb PIP and Arg molecules on the PES membrane surface. Then the PES membrane was taken out and dried off with filter paper to remove excess water. Next, the PES membrane was immersed in TMC solutions to conduct the interfacial polymerization. This reaction took place between the amine groups from PIP


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as well as Arg and acyl chloride groups from TMC. After the polymerization for 2 min, the membranes were dried by air and subsequently post-treated at 60 oC for 15 min to stabilize the structure. Finally, the formed polyamide NF membranes were stored in pure water before using. Table 1. Static water contact angle of the PIP-TMC interface polymerization NF membranes. The number of membranes 1# 2# 3# 4# 5# 6# 7#

PIP in water phase (wt%) 0.1 0.1 0.1 0.1 0.1 0.1 0.1

TMC in organic phase (wt%) 0.1 0.1 0.1 0.1 0.1 0.1 0.1

Arg in water phase (wt%) 0 0.010 0.015 0.020 0.025 0.030 0.040

Ratio of Arg to PIP (%) 0 10 15 20 25 30 40

Static water contact angle (o) 32.2±0.6 28.8±1.2 26.6±0.4 23.3±0.5 22.3±1.1 21.1±1.0 19.7±0.5

2.3. Characterization of membranes The functional groups of membrane surface were analyzed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Vectot 22 FTIR spectrometer, Bruker Optics). Transmittance spectra were obtained in the region of 4000-400 cm-1 with resolution of 4 cm-1 for 64 scans. The experiments were run with air as the background. Field emission scanning electron microscope (FESEM, Nanosem 430) was chosen to investigate the surface and cross section morphologies of the polyamide NF membranes. All the samples were freeze-dried for 12 h and broken in liquid nitrogen. Atomic force microscopy (AFM, Multimode 3, Bruker Co.) in a contact mode was used to detect the surface roughness of the membranes. All the samples were freeze-dried for 24 h and then tested in a scan size of 5 µm × 5 µm. The root-mean-squared roughness (Rrms) was measured at four locations at one sample to get an average value. The static water contact angle of the membranes was measured by contact angle goniometer 7 ACS Paragon Plus Environment


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(JC2000C Contact Angle Meter, Powereach Co., Shanghai, China). The membranes as the samples were dried off with filter paper. The tiny drop of deionized water with volume of 3.0 µL was dropped on the surface of samples. The water contact angle was measured at least six locations on one surface in order to get a reliable value. To measure the surface zeta potential of the membranes, a SurPASS Electrokinetic Analyzer (Austria/Anton Paar KG, Austria) was used at the temperature of 25.0±0.5 °C. The membrane samples were immersed into 1 mM KCl solution at the pH of 2.5 to 10.5 in the test. Positron annihilation spectroscopy (PAS) was used to measure the cross-linking degree of different membranes with Arg. The

22

Na was chosen as the positron source and the energy of the implanted

positrons was continuously varied in the range of 0.18−20.18 keV. The samples were freeze-dried and measured at room temperature. 2.4. Separation performance evaluation The separation performance of the prepared membranes was analyzed by using a laboratory-scale dead end filtration system. The main part of the system was a filtration cell with a volume capacity of 210 mL (Model 8200, Millipore Co., USA), the membranes with surface area of 28.7 cm2 were placed in the cell for filtration. Before the evaluation of permeation flux and salt rejection ratios, membranes needed to be prepressed with water at pressure of 0.25 MPa for 30 min to get a stable permeation flux. The permeation flux and rejection ratios were evaluated by 0.1 g/L organic dye and salt solutions. The permeation flux was obtained by J=

V A∆ t

(1)

where J (L/m2h) and V (L) were permeation flux and volume, A (m2) was effective membrane area (28.7 cm2), △t (h) was operation time.


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The salt rejection was obtained by  C  R = 1− p  ×100%  C  f  

(2)

where Cp (g/L) and Cf (g/L) were concentration of permeation and feed solutions, R (%) was the salt rejection ratio. 2.5. Antifouling performance evaluation To evaluate the antifouling performance of the polyamide NF membranes, we chose three different foulants feed solutions (BSA, emulsified oil droplet and HA, 1.0 g/L) to simulate pollution condition. BSA and HA were directly dissolved in ultrapure water used (pH = 6.0±0.2), respectively, while the solution of emulsified oil droplet solution was surfactant-stabilized oil-in-water emulsion (0.9 g/L soybean oil and 0.1 g/L SDS mixing under high-speed stir). Firstly, a stable pure water flux (Jw1) of the prepressed membranes was measured for about 30 min under the operation pressure of 0.20 MPa. Then, the pure water was replaced by the foulants feed solutions for 24 h to measure the flux (Jp) under the pollution condition at 0.20 MPa. Next, the membranes were washed by pure water under the robustly stir for about 30 min. Finally, the pure water was pressed into the cell filtration system again to measure the flux (Jw2) of the washed membranes for 1 h at 0.20 MPa. Furthermore, the antifouling experiments were also operated at 0.15 MPa, 0.20 MPa and 0.25 MPa with BSA foulants solutions for 4 hours to study the antifouling performance of the membranes at different operation pressure. The antifouling parameters of flux recovery ratio (FRR) and total flux decline ratio (DRt) were determined as follows:

FRR =

J w2 ×100% J w1 9

ACS Paragon Plus Environment

(3)


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DRt =

J w1 − J p J w1

× 100%

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(4)

In fact, the two parameters of FRR and DRt were commonly used index to evaluate the antifouling performance of the membranes

21, 32-33

. The higher value of FRR and lower value of DRt meant the

better antifouling performance of the membranes. 3. Result and discussion 3.1. Membrane characterization As a natural amino acid containing multiple amine groups, Arg could take part in the polyamide network formation through the reaction with TMC. Schematic illustration of the interfacial polymerization in the preparation process of the Arg/PIP-TMC NF membranes was shown in Figure 1. The FTIR spectrometer was first used to measure the surface groups of the prepared membranes. The amine groups of PIP and Arg both reacted with the acyl chloride groups of TMC to form amide bonds. Therefore, the interfacial polymerization could be confirmed by the peaks of amide bonds. Figure 2 showed the FTIR spectra of the PES membrane, PIP-TMC NF membrane (1# membrane), and Arg/PIP-TMC NF membrane (4#, 7# membrane). As respected, the prominent peaks in PES at 1578cm-1, 1487cm-1 and 1242cm-1 were attributed to the stretching vibration of benzene ring, C-C bond and aromatic ether bond, respectively

34-35

. Compared with the PES membrane, a new peak at

1640 cm-1 obviously appeared in the spectra of PIP-TMC and Arg/PIP-TMC NF membranes, attributing to the stretching vibration band of C=O in the -CO-NH- groups 36. Due to the low addition amount, the characteristic peaks of Arg were not observed, which might be covered by the strong peaks of PES support membrane and polyamide active layer. We also used X-ray photoelectron spectroscopy (XPS) to analyze the surface composition, but there was no useful evidence to prove the existence of Arg. Therefore, the results shown in Figure 2 illustrated that the polyamide layer was 10 ACS Paragon Plus Environment

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successfully formed on the membrane surface.

Figure 1. Schematic illustration of the interfacial polymerization in the preparation process of the Arg/PIP-TMC NF membranes

-1

1640 cm 7#

Transmittance

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


4#

1# PES support

4000

2000

1500

1000

500

-1

Wavenumber (cm )

Figure 2. FTIR spectra of the PES support, PIP-TMC (1# membrane in table 1) and Arg/PIP-TMC NF (4#, 7# membrane in table 1) membranes. The FESEM measurement revealed visible images of the surface and cross-section structure of the membranes, which were shown in Figure 3 (a, c). The PIP-TMC NF membranes showed a typical



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surface and cross-section structure of polyamide NF membranes prepared by interfacial polymerization. The dense and rugged surface with finely dispersed nodular-like structure was formed by crosslinking PIP and TMC, which was driven by crystallization nucleation and hydrogen bonding 19. Compared with the PIP-TMC NF membranes, the Arg/PIP-TMC NF membranes existed more rugged surface with more apparent, denser and bigger nodular-like structure. This structure was also strengthened with the increasing addition of Arg. When the ratio of Arg to PIP reached 40%, the surface of the membranes appeared irregular rock-like structure. It seemed to be caused by continuous denser and bigger spherical globules. While the introduction of unsymmetrical aliphatic chain produced strands of the polymers by reacting with TMC, which could gradually form the ridge-and-valley structure 37. Therefore, the final surface morphology was generated by the two types of structure together. However, the addition of Arg did not change the cross-section structure of the membranes, which still displayed the typical asymmetric structure with a dense skin layer coated on top of the porous support. AFM measurement was used to detect the surface roughness of the membranes. As the results shown in Figure 3 (b), the PIP-TMC NF membrane presented the smoothest surface (Rms = 5.21 nm). With the increasing ratio of Arg to PIP, the surface roughness gradually increased. When the addition ratio reached 40%, the surface roughness finally rose to the highest value (Rms = 10.66 nm). As a result, the roughest membranes possessed the largest surface area to contact more water molecules, attributing to the highest water permeability

38

. The change of surface roughness was

caused by chemical crosslinking degree of Arg and TMC. In the interfacial polymerization process, the polyamide structure was significantly influenced by the monomer diffusion coefficient, which was dependent on the monomer type and monomer concentration under the same operation


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conditions 7. The addition of Arg constituted an obstacle to the structure formation of the PIP-TMC network, since Arg participated in polymerization but could not form the dense network structure by itself. Meanwhile, the different amine groups in Arg might endow the polyamide network with multiple extension directions, which increased the surface roughness of the membranes.

Figure 3. Morphologies of the fabricated polyamide NF membranes (1#, 2#, 4# and 7# membrane). The hydrophilicity has a significant effect on permeation and antifouling performance of the membranes. Generally speaking, high hydrophilic surface gives rise to high water flux and enhance



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antifouling performance

39-41

. In this study, the static water contact angle was measured to evaluate

the membrane surface hydrophilicity. Arg contained abundant amine groups and carboxyl groups, which could not only react with TMC, but also introduce more hydrophilic groups into the polyamide layer. Particularly, the most striking feature of Arg was the self-association property. Some researchers used molecular simulation to exactly confirm that the Arg molecules showed a marked tendency to form clusters with head to tail hydrogen bonding 42-44. When the pH was below 12.1, Arg molecules existed as Arg cations, and guanidinium existed as guanidinium cation (Gdm+). The Gdm+ could pair with each other in water despite the obvious electrostatic repulsion between them 45. The Gdm+-Gdm+ pairs stably existed in water, owing to the strong hydrogen bonds to water molecules. This existing was also benefit to improve the hydrophilicity of the membranes. As the results showed in Table 1, when the ratio of Arg to PIP increased from 0 to 40%, the static water contact angle decreased from 32.2±0.6o (1#) to 19.7±0.5o (7#). Arg was successfully introduced into the water channel and improved the hydrophilic of polyamide layer, which was the similar function of Arg-195 to the ar/R region of AQP1, attributing to the stronger hydrogen bonds between water and Arg. Meanwhile, the introduced Gdm+-Gdm+ bonded with abundant water molecules, which could strengthen the water retention capacity inside the membranes, and be helpful to from hydration layer on the membrane surface. The surface zeta potential of fabricated membranes was measured and the results were shown in Figure 4. With increasing pH value of the test solution, the negative charges continually increased of both pristine polyamide NF membranes and modified membranes. The introduction of Arg distinctly made the membranes surface possess more negative charges. Furthermore, with the increasing addition of Arg, the negative charges of the membrane surface gradually increased, which was


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clearly showed in Figure 4 (b). Accurately, the negative charge was derived from the negative carboxyl groups

46

. During the interfacial polymerization, most of acyl chloride groups from TMC

reacted with amine groups to form amide bonds, while part of acyl chloride groups became the polyamide network terminal, easily hydrolyzed into carboxyl groups. Arg disturbed the formation of continuous PIP-TMC network, causing more polyamide network terminal and carrying more negative charges. Although unreacted amine groups from Arg carried positive charges into the membranes, but the relatively low addition of Arg was not much enough to shield the influence of more carboxylic groups with negative charges. Meanwhile, every Arg molecule had an α-carboxylic acid group, which also increased the surface negative charges. Therefore, the introduction of Arg apparently increased the carboxyl groups in polyamide layer, causing the membrane surface possessing more negative charges. Ample carboxyl groups also increased the surface hydrophilicity of the membranes, which was consistent with the decreasing of static water contact angle in Table 1.

(b)

0 PIP-TMC NF membrane Arg/PIP-TMC NF membrane

-20

Zeta Potential (mV)

(a) 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


-40 -60 -80 -100

-20 Arg/PIP-TMC NF membranes

-30 -40 -50 -60 -70

2

4

6 pH

8

0

10

10

20

30

40

Additon rate of Arg to PIP (%)

Figure 4. (a) Surface zeta potential of PIP-TMC NF membrane (1#) and Arg/PIP-TMC NF membrane (4#), (b) Surface zeta potential of Arg/PIP-TMC NF membranes with different Arg addition at pH = 6.0±0.2. In order to determine the cross-linking degree of different membranes with Arg, the positron



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annihilation Doppler broadening energy spectra were obtained by using PAS. S parameter is defined as the rate of the photon annihilation count within the range of 510.2−511.8 keV to that within the range of 499.5−522.5 keV, which is correlated to the changes of the positron and positronium states resulting from free volume changes and chemical structure

47-48

. In other words, for polyamide

membranes prepared by interfacial polymerization, S parameter could reflect the compactness of active layer. S value will decrease with the increasing of active layer compactness

48-50

. The mean

implantation depth (nm) of the energetic positrons could be expressed by experiential formula:  40  Z ( E+ ) =   E+1.6  ρ 

(5)

where Z is the mean implantation depth (nm), ρ is the density (g/cm3), and E+ is the positron incident energy (keV). Upon the density, similar report

51

and the thickness of the active layer, the

region of dense active layer was roughly labelled in Figure 5. As shown in Figure S1, S near the surface increased sharply before a fluctuant levelling off with the increasing of E+. S value had an order of 7# membrane >2# membrane >4# membrane (in the same depth) in the region of dense active layer. The lower S value indicated compacter polyamide active layer structure 51-52. When the ratio of Arg to PIP reached 20%, the membrane possessed the most compact active layer structure, while the ratio of Arg to PIP reached 40%, the membrane possessed the loosest active layer structure. The influence of compactness of the active layer on the separation performance will be discussed in the next section.


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0.485 0.480 S Parameter

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0

Mean Depth (nm) 436 1447 3045

5362

dense active layer

0.475 2# membrane 4# membrane 7# membrane

0.470 0.465 0.460

0

5 10 15 Incident Positron Energy (keV)

20

Figure 5. S parameter as a function of the positron energy for 2#, 4# and 7# membranes (the ratio of Arg to PIP was 10%, 20% and 40%, respectively). 3.2. Separation performance For NF membranes, the permeability and selectivity were the two major performance parameters in practical application, which were evaluated by the permeation flux and rejection ratio, respectively. Pure water flux was related to hydrophilicity and porosity of thin polyamide layer covered on the support, since the uncharged water molecule (0.27 nm) was much smaller than the pore size of polyamide separation layer

36

. Figure 6 (a) showed pure water flux and dye rejection ratios of the

membranes with different addition of Arg. With the increasing ratio of Arg to PIP, pure water flux apparently showed increasing. The pure water flux of the pristine PIP-TMC NF membrane was 17.31 L/m2h at 0.20 MPa. When the ratio of Arg to PIP increased from 10% to 40%, the pure water flux rose from 18.56 L/m2h to 35.12 L/m2h. Particularly, when the ratio of Arg to PIP reached 40%, the introduction of Arg doubled the flux of pure water, compared with the pristine polyamide NF membrane. There were two main aspects how Arg affected water permeation through the membranes. On one hand, the hydrophilicity of the membranes enhanced with the increasing addition of Arg, 17 ACS Paragon Plus Environment


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corresponding to the reducing of static water contact angle

53

. The higher hydrophilicity rendered

higher water permeation 54-55. Owing to the strong hydrogen bonds between Arg and water, the inner Arg assisted water molecules to transport quicker through the dense polyamide layer, which played the similar function to Arg195 in the ar/R region of AQP1

29-30

. On the other hand, Arg disturbed the

formation of PIP-TMC network and resulted in a looser thin polyamide layer. Meanwhile, the higher surface roughness was also another reason of influencing the water permeation in a degree. Therefore, Arg played a significant role in improving the permeation performance of the membranes. In the aqueous solution (pH = 6.0±0.2), Arg molecules homogeneously dispersed and freely moved. Gdm+ of Arg could stack parallel to each other, which was derived from a combination of cavitation effects, quadrupole-quadrupole, and dispersion interactions, despite the electrostatic repulsion between them

42, 56

. The stable existing of Gdm+-Gdm+ pairs in water was benefited from

strong hydrogen bonds between Gdm+ and water molecules. Eventually, The Gdm+-Gdm+ paired with increasing number of water molecules, forming a globally stable cluster with 12 water molecules 57. Researchers found that Gdm+-Gdm+ pairing occurred whenever two side chains had a chance to interact with each other 57. That is to say, when increasing the amount of Arg in aqueous solution, more Gdm+ had the possibility to interact with each other, attributing to more Gdm+-Gdm+ pairs. During the interfacial polymerization, the single and paired Arg molecules both constituted an obstacle to the fully developed PIP-TMC network, which disturbed the formation of dense polyamide layer, subsequently influenced the separation performance of the membranes. As for the separation performance of NF membranes, dye rejection ratio was mainly related to the pore size of the separation layer, while salt rejection ratio was not only related to the pore size, but also the surface charge of membranes. As shown in Figure 6 (a), the dye rejection ratios presented a


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trend of decrease-increase-decrease with the increasing addition of Arg. When introducing the lowest amount of Arg into the PIP aqueous solution (10% to PIP), Arg participated in polymerization and consequently disturbed the formation of PIP-TMC network. This disturbance made a relatively loose active layer, in accordance with the low S parameter in Figure 5, consequently decreased the rejection ratios of all the dyes used. With the increasing addition of Arg, more Arg molecules participated in polymerization. Compared with the previous relatively loose active layer, the active layer became more compact. As a result, when the ratio of Arg to PIP reached 20%, the dye rejection ratios gradually increased and showed similar value to the pristine PIP-TMC NF membranes. Meanwhile, the S parameter of the membrane reached the highest value among the membranes incorporated in Arg molecules (see in Figure 5). But when Arg addition was unceasingly increased, the formation of Gdm+-Gdm+ pairs gradually increased, which decreased Arg molecules diffusing to react with TMC 57. Increasing Gdm+-Gdm+ pairs and excess Arg molecules caused the active layer getting loose again, sequentially decreased the rejection ratios of dyes. When the ratio of Arg to PIP reached the highest value (40%), the membrane possessed the loosest structure and resulted in the lowest rejection ratios, in accordance with the results in Figure 5. Although the addition of Arg molecules influenced and disturbed the polyamide network formation, it did not give rise to evident structure defect of the active layer. Therefore, the dye rejection ratios still kept a high level when the ratio of Arg to PIP reached the highest value, showing rejection of 91.9%, 96.9% and 75.8% to Orange GII, Congo red, and Methyl orange, respectively.



39

100

36

95

33

90

30

85

27

80

24

75

100 90

21 18 15

(b)

1#

2#

3#

4#

5#

Water 70 Orange GII Congo red 65 Methyl Orange 60

6#

80

Rejection (%)

2

Flux (L/m h)

(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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70 60 NaCl Na2SO4

50 40

MgCl2

30

MgSO4

20 10 0

7#

1#

2#

3#

4#

5#

6#

7#

Membranes

Membranes

Figure 6. (a) Pure water flux and dye rejection ratios of the polyamide NF membranes at 0.20 MPa, (b) Salt rejection ratios of the polyamide NF membranes at 0.20 MPa. Besides, the change trend of salt rejection ratio was similar to the dye rejection ratio, as the results shown in Figure 6 (b). The modified Arg/PIP-TMC NF membranes possessed high rejection of SO42-. Especially when the ratio of Arg to PIP reached 20%, the rejection ratio of Na2SO4 and MgSO4 reached 92.11% and 73.53%, respectively, which was higher than the PIP-TMC NF membranes. The high rejection of SO42- was attributed to the strong electrostatic repulsion between SO42- and negative charge on the membrane surface. Because of the excess TMC, amine groups on the surface sufficiently reacted with acyl chloride groups. This polymerization conferred a strong negative charge surface of the polyamide NF membranes, which was consistent with the zeta potential results. As we all know, the salt rejection ratio was related to the charge of the membrane surface, but the size, charge category, and quantity of the salt were also important factors. Therefore, the change trend of the rejection ratio was still mainly controlled by the structure change of polyamide layer, showing the unusual trend of decrease-increase-decrease in Figure 6 (b). Meanwhile, the flux of all dyes and salts solutions used for the polyamide NF membranes were examined, and the results were shown in Figure S2. The flux of dyes and salts solutions was decreased within 20% compared with


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the flux of pure water. On one hand, the decreasing flux was attributed to the electrostatic repulsion between negative charge of the membrane surface and negative charge in salts and dyes solutions. On the other hand, the effect of concentration polarization also decreased the flux of salts and dyes solutions. 3.3. Antifouling performance The antifouling performance of polyamide NF membranes was investigated by feed solutions of BSA, HA, and emulsified oil droplet, respectively. The fouling was driven by non-specific interactions between the foulants and the membrane surface, including hydrophobic interactions, van der Waals interactions, hydrogen bonding and electrostatic interactions. BSA and HA foulants carried negative charges at the operation pH of 6.0±0.2. For emulsified oil droplet foulants, the oil drops were dispersed and stabilized by anionic surfactant (SDS), rendering them negative charges. These conditions were to prove that the Arg/PIP-TMC NF membranes possessed antifouling performance toward negatively charged foulants. Simulated fouling procedure experienced three periods and the results were showed in Figure 7 and Table 2. It was clearly to find that the introduction of Arg enhanced the antifouling performance toward all the three foulants. For BSA foulants, DRt and FRR of the PIP-TMC NF membranes was 20.0% and 90.0%, respectively. After introducing Arg into polyamide layer, DRt decreased by 50.0% and the FRR increased to 95.0%, which showed the lowest DRt and the highest FRR among all the three foulants. For emulsified oil droplet foulants, DRt decreased from 28.0% to 12.8%, FRR increased from 73.0% to 89.0%, showing more apparent enhancing of antifouling performance. Similarly, the introduction of Arg also strengthened the fouling-resistant and fouling-release properties for HA foulants. Compared to the DRt (23.8%) and FRR (86.0%) of the PIP-TMC NF



membranes, the DRt of the modified membranes decreased to 15.0% and FRR increased to 92.0%, respectively. Compared to the pristine PIP-TMC NF membranes, the Arg/PIP-TMC NF membranes possessed excellent fouling-resistant and fouling-removed properties. 1.0 0.9

Normalized flux (J/J0)

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Page 22 of 33

0.8 0.7

Foulant solutions

Water

Water

PIP-TMC membrane with 1g/L BSA

0.6

Arg/PIP-TMC membrane with 1g/L BSA PIP-TMC membrane with 1g/L Oil

0.5

Arg/PIP-TMC membrane with 1g/L Oil

0.4

PIP-TMC membrane with 1g/L HA Arg/PIP-TMC membrane with 1g/L HA

0.3

0

5

10

15

20

25

Time (h) Figure 7. Time-dependent flux for the polyamide NF membranes (1# and 4# membranes) during filtration of BSA, emulsified oil droplet and HA solutions at 0.20 MPa.

Table 2. Antifouling indexes (DRt and FRR) of PIP-TMC (1#) and Arg/PIP-TMC (4#) NF membranes during filtration of BSA, emulsified oil droplet and HA solutions. BSA DRt FRR

1# 20.0% 90.0%

4# 10.0% 95.0%

Emulsified oil droplet 1# 4# 28.0% 12.8% 73.0% 89.0%

HA 1# 23.8% 86.0%

4# 15.0% 92.0%

While the antifouling performance of the membranes was decreased with increasing the operation pressure, which could be seen from Figure S3 and Table S1. At 0.15 MPa and 0.20 MPa, the modified polyamide NF membranes possessed better antifouling performance than the pristine


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polyamide NF membranes, attributing to the higher hydrophilicity and negative Zeta potential. While at 0.25 MPa, the modified polyamide NF membranes showed worse antifouling performance, because of the higher surface roughness. The negative influence of the surface roughness was sharply enlarged at 0.25 MPa. The foulants were easier to attach on the membrane surface attributed to higher operation pressure

58

. Furthermore, the foulants could deposit in the “valleys”, resulting in

faster accumulation of the foulants in these “valleys”

59

. Although the antifouling performance was

decreased at high pressure, the modified membranes could show excellent antifouling performance at low pressure. The enhancing of antifouling performance could be illustrated by the increasing of hydrophilicity and negative charges. Schematic illustration of the antifouling performance was shown in Figure 8. On the one hand, the addition of Arg strengthened binding capacity to water molecules both within and around the polyamide layer, forming an effective hydrated layer to resist foulants. On the other hand, the strong negative charges also constituted a reinforced antifouling surface for the electron-negative foulants by electrostatic repulsion. The resulting hydrophilic surface with negative charges could suppress the non-specific interactions between the membrane surface and foulants, as well as prevent the foulants from attaching onto the membrane surface, consequently implementing excellent fouling-resistant performance 32. The modified polyamide NF membranes in this work were compared with several polyamide NF membranes modified by small molecules in the literatures (Table S2). The polyamide NF membranes modified by Arg could be operated under very mild pressure, and possessed good water flux and DRt, as well as moderate Na2SO4 rejection and FRR. To some extent, the improved performance was inspired by the AQPs, which appropriately used the strong interaction between Arg and water



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molecules. We expected it could be further developed to fabricate NF membranes with high performance in practical application. Furthermore, we also hope the great progress will be achieved in quantitative analysis of the formation of polamide active layer, as well as in increasing the membrane performance by more rationally regulating the structure of the membranes.

Figure 8. Schematic illustration of the antifouling performance of the membranes. 4. Conclusions In this study, inspired by the strong hydrogen bonds between Arg and water in AQPs, Arg was introduced into PIP aqueous solution, which became another aqueous phase monomer and participated in interfacial polymerization with TMC. The introduction of Arg molecules apparently increased the amount of carboxyl groups and improved the water retention capacity of polyamide layer, which enhanced the negative charge and hydrophilicity of the polyamide NF membranes. As a result, 1) the water flux was increased, while the dye and salt rejection ratios exhibited decrease-increase-decrease trend; 2) when the ratio of Arg to PIP reached 40%, the water flux was doubled and the rejection ratios of Congo red and Orange GII were still over 90%; 3) the membranes presented excellent antifouling performance for foulants of BSA, emulsified oil droplet and HA, showing that DRt all decreased below 15% and FRR even rose to 89%.


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ASSOCIATED CONTENT Supporting Information. The SEM cross-section images of the modified polyamide NF membranes (2#, 4#, and 7# membrane); the flux of all the dyes and inorganic salts solutions used for the polyamide NF membranes (1# and 4# membranes) at 0.20 MPa; time-dependent flux for the PIP-TMC (1#) and Arg/PIP-TMC NF membranes (4#) during filtration of BSA solutions at different pressure; antifouling indexes (DRt and FRR) of PIP-TMC (1#) and Arg/PIP-TMC (4#) NF membranes during filtration of BSA solutions at different pressure; the inﬂuence of long filtration time on the water flux and rejection of the modified polyamide NF membranes; various modified small molecules for the fabrication of NF membranes and their performance. Acknowledgement. This research is supported by Tianjin Natural Science Foundation (No. 13JCYBJC20500, 14JCZDJC37400), National Science Fund for Distinguished Young Scholars (No. 21125627), National Natural Science Fund of China (No. 21490583, 21621004), and National key research and development program-China (No. 2016YFB0600503). References (1) Marchetti, P.; Jimenez Solomon, M. F.; Szekely, G.; Livingston, A. G. Molecular Separation with Organic Solvent Nanofiltration: A Critical Review. Chem. Rev. 2014, 114 (21), 10735-10806. (2) Wen, L.; Tian, Y.; Jiang, L. Bioinspired Super-Wettability from Fundamental Research to Practical Applications. Angew. Chem. Int. Ed. Engl. 2015, 54, 2-15. (3) Bechelany, M.; Drobek, M.; Vallicari, C.; Abou Chaaya, A.; Julbe, A.; Miele, P. Highly Crystalline MOF-Based Materials Grown on Electrospun Nanofibers. Nanoscale 2015, 7, 5794-5802.



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