Incorporating arginine-FeIII complex into polyamide membranes for enhanced water permeance and antifouling performance

Journal Pre-proof III Incorporating arginine-Fe complex into polyamide membranes for enhanced water permeance and antifouling performance Jingyuan Guan, Lin Fan, Ya-nan Liu, Benbing Shi, Jinqiu Yuan, Runnan Zhang, Xinda You, Mingrui He, Yanlei Su, Zhongyi Jiang PII:

S0376-7388(19)33583-5

DOI:

https://doi.org/10.1016/j.memsci.2020.117980

Reference:

MEMSCI 117980

To appear in:

Journal of Membrane Science

Received Date: 23 November 2019 Revised Date:

22 January 2020

Accepted Date: 16 February 2020

Please cite this article as: J. Guan, L. Fan, Y.-n. Liu, B. Shi, J. Yuan, R. Zhang, X. You, M. He, Y. Su, Z. III Jiang, Incorporating arginine-Fe complex into polyamide membranes for enhanced water permeance and antifouling performance, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/ j.memsci.2020.117980. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Polyamide

Arginine-FeIII

Incorporating arginine-FeIII complex into polyamide membranes for enhanced water permeance and antifouling performance Jingyuan Guana,b, Lin Fana,b, Ya-nan Liua,b, Benbing Shia,b, Jinqiu Yuana,b, Runnan Zhanga,b, Xinda Youa,b, Mingrui Hea,b, Yanlei Sua,b∗, Zhongyi Jianga,b∗ a

Key Laboratory for Green Chemical Technology of Ministry of Education, 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

∗

Corresponding author: Tel: +86-22-27406646; fax: +86-22-23500086. E-mail address: [email protected], [email protected]

Abstract Nanofiltration

membranes

featuring

excellent

separation

and

antifouling

performances are highly desired for applications such as water purification. In this study, inspired by the phenomenon of coordination-driven self-assembly and the structures of aquaporins in nature, an arginine (Arg)-FeIII complex was designed and incorporated into the polyamide layer during interfacial polymerization, endowing the resultant nanofiltration membranes with enhanced water permeance and prominent antifouling performance. Owing to the active amine groups, the Arg-FeIII complex could react with trimesoyl chloride (TMC) and thus act as the aqueous additive in company with piperazine (PIP), which improved the compatibility between the organic phase and aqueous phase. The interfacial channels between the Arg-FeIII complex and the polyamide matrix as well as the hydrogen bonds interaction between water and the Arg-FeIII complex facilitated the rapid water transfer. When the content of the Arg-FeIII complex reached 0.3 g/L, the water permeance was elevated significantly, twice than that of the pristine polyamide membrane. Meanwhile, the rejections for Na2SO4 and dyes were close to those of the pristine polyamide membrane. Furthermore, the resultant membranes displayed exceptional antifouling performance.

Introducing

the

metal-organic

complex

into

the

interfacial

polymerization process proves an effective and promising attempt to fabricate membranes with high performance for water resource reclamation. Keywords: Arginine-FeIII

complex;

Interfacial

polymerization;

Nanofiltration performance; Antifouling performance.

Polyamide

membrane;

1

1. Introduction

2

The population explosion and environment degradation are increasing the demand

3

for drinking water resources. As an emerging technology for water treatment,

4

nanofiltration has drawn remarkable attention in industrial wastewater purification

5

and brackish water desalination owing to its distinct merits such as facile operation,

6

low energy consumption

7

polymerization (IP) is a commercially predominant technology to fabricate

8

nanofiltration membranes [4, 5]. During the IP process, aqueous monomers react fast

9

with organic monomers at the biphasic interface, and a polyamide (PA) layer with the

10

thickness varying from tens to hundreds nanometers in seconds can be formed [6].

11

Despite the aforementioned advantages, two bottlenecks restrict the development of

12

nanofiltration membranes. To be specific, the trade-off between water permeance and

13

dye/salt rejections, which is improving the water permeance while retaining the solute

14

rejections, is an obstacle confronted by nanofiltration membranes in practical

15

applications. Another intractable bottleneck is the membrane fouling arising from

16

inevitable physical or chemical interactions between foulants and the membrane

17

surface, resulting in the decreased water permeance and shortened lifetime [7, 8]. To

18

tackle the issues above, numerous strategies have focused on introducing hydrophilic

19

inorganic materials into the organic polymeric matrix [9-11], such as silica [12],

20

titanium dioxide [13] and carbon nanotubes [14] to alleviate the membrane fouling.

21

Nevertheless, the polymer-filler interfacial compatibility and filler agglomeration are

22

still burning challenges to obtain defect-free nanofiltration membranes. The

23

differences in physical and chemical properties, such as density and polarity as well as

24

high surface energy of the inorganic materials with small size make the inorganic

25

materials agglomerate and aggravate the phase separation, leading to the formation of

26

non-selective defects and weakened membrane performances.

and

environmental

friendliness

[1-3].

Interfacial

27

Biological membranes, ubiquitous in cells, exhibit superior permeability and

28

selectivity owing to the specialized membrane proteins responsible for transferring

29

water and excluding angstrom-scale solutes. In addition, cell membranes display

30

self-cleaning characteristics, not adsorbing peptides or other foulants although cells 1

1

exist in tissue fluid and blood comprised of proteins and inorganic salts. This

2

marvelous fouling-resistance performance can be attributed to the hydrophilic

3

compositions [15]. Hydrophilic surface tends to form hydration shell via binding with

4

water, preventing the contact of the foulants with the membrane surface. A class of

5

membrane proteins with highly permeable and prominently selective capability is the

6

water channel proteins, aquaporins (AQPs) [10, 16]. For AQP1, one of the AQPs, the

7

aromatic/arginine (Arg) selectivity filter where the interactions between water and

8

amino acid constitutes the narrowest part, only filtering water. Arg can bind water to

9

form strong hydrogen bonds, endowing AQP1 with superior water transfer ability [17,

10

18]. Currently, concurrent with the study on AQP1, much attention has been aroused

11

on mimicking them to construct artificial water channels [19]. In other words,

12

introducing aquaporin-inspired structures or compositions into nanofiltration artificial

13

membranes can be a promising attempt for endowing membranes with fast water

14

transfer and antifouling performances.

15

Self-assembly of biomolecules is ubiquitous in living organisms, providing a

16

favorable guarantee to achieve the functionality of the organisms. For instance,

17

peptide chain are folded to constitute the protein structures. Coordination bonds, one

18

of the interactions involved in the molecular self-assembly [20], have triggered

19

growing interest especially in protein assembly which are significant building blocks

20

for constructing complicated architectures [21]. In nature, almost one-third of all the

21

proteins rely on coordination with metal ions to perform their biological function [22].

22

Therefore, the rational incorporation of coordinated structures is favorable for

23

intensifying the membrane comprehensive properties.

24

Herein, inspired by the coordination-driven self-assembly and the aquaporins in

25

nature, we incorporated the Arg-FeIII complex into the separation layer for fabricating

26

the PA membranes with enhanced separation and prominent antifouling performance.

27

This Arg-FeIII complex, as an additive in aqueous phase, was designed and introduced

28

into the PA layer along with piperazine (PIP) via the interfacial polymerization.

29

During the formation of PA separation layer, the Arg-FeIII complex participated in the

30

interfacial polymerization process, improving the compatibility between the organic 2

1

phase and aqueous phase. Moreover, the interfacial channels between the Arg-FeIII

2

complex and the PA matrix facilitated the rapid water transfer. Meanwhile, the strong

3

hydrogen bonds between the Arg-FeIII complex and water also endowed the

4

membranes with better hydrophilicity, conducive to the water permeance and

5

antifouling performance. The resultant membranes displayed increased water

6

permeance as well as almost unchanged salt and dye rejections. Apart from the

7

mentioned separation performance above, the resultant membranes also exhibited

8

excellent antifouling performances against bovine serum albumin (BSA), emulsified

9

oil and humic acid (HA) representing protein, natural organic matter and the

10

hydrocarbon, respectively.

11

3

1

2. Experimental

2

2.1 Materials

3

Polythersulfone (PES MW = 29000) was purchased from Dingguo (Beijing, China).

4

L-Arg, PIP, TMC and ferric trichloride (FeCl3) were obtained from Aladdin (Shanghai,

5

China). Tris-HCl buffer solution (pH = 7.4, 50 mM) was purchased from

6

AKZ-Biotech (Tianjin, China). Other chemicals, such as polyethylene glycol (PEG,

7

MW = 2000), inorganic salts (NaCl, Na2SO4, MgCl2, MgSO4), organic dyes (Orange

8

GII, MW = 452.4; Conge red, MW = 696.7), simulated foulants (HA: humic acid,

9

BSA: bovine serum albumin), sodium dodecyl sulfate (SDS), and organic solvents

10

(n-heptane, ethyl alcohol, N, N-dimethylformamide) were purchased by Sigma

11

(Shanghai, China). Oil was the soybean oil specifically. Deionized water (15.0 MΩ)

12

was produced by the Millipore Water Purification System (U.S.A.).

13

2.2 Preparation and characterization of the Arg-FeIII complex

14

The Arg-FeIII complex was synthesized by the simple coordination of the L-Arg and

15

FeIII ions [23]. 1.394 g L-Arg and 1.081 g FeCl3·6H2O solid power were added in the

16

mixed solution (20 mL) of Tris-HCl (pH = 7.4, 50 mM) and NaCl (100 mM). After

17

reacting at 25 oC for 30 min under vigorous stirring, the Arg-FeIII complex was

18

precipitated using ethanol. The suspension was centrifuged at 3500 r/min and washed

19

twice with ethanol to obtain the Arg-FeIII complex. Finally, the Arg-FeIII complex was

20

dried at 60 oC for at least 24 h before use.

21

Samples of the L-Arg and the Arg-FeIII complex were dried at 60 oC for 12 h to

22

further investigate the chemical composition and structural morphology. Fourier

23

transform infrared (FT-IR, BRUKER, TENSOR II, Germany) spectroscopy was

24

utilized to analyze the functional groups of the samples. X-ray photoelectron

25

spectroscopy (XPS, Perkin Elmer Phi 1600 ESCA system) was taken to probe the

26

surface elemental composition of this complex. Surface structural morphology was

27

observed by Transmission electron microscopy (TEM, Tecnai G2 F20) micrographs.

28

Dynamic light scattering (DLS) was utilized to characterize the size of the Arg-FeIII

29

complex in the aqueous phase. Meanwhile, X-ray powder diffraction (PXRD,

30

D/MAX-2500), UV−vis spectrometer (UV-vis, Hitachi U-3010) and thermal 4

1

gravitational analysis (TGA, TG209 F3) were also employed to obtain more

2

supplementary information.

3

2.3 Simulation

4

The coordination interaction between Arg and metal ions was simulated with

5

Material Studio software. Force field method was utilized to optimize the structure

6

followed by the calculation of the total energy. Since the carboxyl group and the imine

7

group can participate in the coordination with FeIII, the charges of the O atom in the

8

carboxyl group and N atom in the imine group were set as -1 for coordination. The

9

expression of the binding energy (∆Ebinding) between Arg and metal ions was obtained

10

as follows:

11

∆Ebinding= E(Arg/metal ion) − E(Arg) − E(metal ion)

(1)

12

In addition, to further evaluate the stability of the Arg-FeIII complex, the bond

13

length between Arg and FeIII was calculated in the GAUSSIAN 09W package at the

14

DFT/B3LYP level.

15

2.4 Preparation and Characterization of nanofiltration membranes

16

The pristine PA and the resultant Arg-FeIII/PA membranes possessed the typical

17

asymmetric structure, composed of IP active layer and PES support layer. The PES

18

support layer was ultrafiltration membrane with the pure water permeance of

19

1500-2000 L/ (m2·h·MPa). The active layer was formed through the IP process. The

20

Arg-FeIII complex was added into the PIP aqueous phase with different content

21

(0.1-0.35 g/L). Similar to our previous study [24], the immersion time of PIP (0.1

22

wt %) aqueous phase and TMC (0.1 wt %) organic phase solutions were 10 min and 2

23

min, respectively under ambient temperature. For obtaining stable PA structure, the

24

resultant membranes were post-treated at 60 oC for 15 min and then stored in the

25

deionized water before use. The resultant PA membranes were named as shown in

26

Table 1.

27

For characterizing the chemical composition and structural morphology of the

28

composite nanofiltration membranes, Fourier transform infrared (FT-AIR, BRUKER,

29

TENSOR II, Germany) spectroscopy was utilized to testify the chemical structure of

30

the membranes in this study. The surface and cross-section morphologies were 5

1

detected by Scanning Electron Microscope (SEM, Nova Nanosem 430, FEI Co.,

2

USA), and atomic force microscopy (AFM, Nanoscope-IIIA, USA) was taken to

3

measure the surface roughness. All the samples were freeze-dried for 6 h before

4

testing. For characterizing the hydrophilicity and charges of the membrane surface,

5

contact angle goniometer (JC2000D2, POWEREACH, China) and zeta-potential

6

analyzer (SurPASS) were directly employed to measure the static water contact angle

7

and zeta potential, respectively. The elementary distribution of resultant PA

8

membranes was detected by Energy-dispersive X-ray spectroscopy (EDX, Genesis

9

XM2 APEX 60SEM, USA). X-ray diffractometer (XRD, D/MAX-2500) was taken to

10

characterize the interference on the packing of PA chains from the Arg-FeIII complex.

11

The pristine PA membrane and the resultant Arg-FeIII/PA membrane were immersed in

12

the N, N-dimethyl formamide and the free-standing membranes were obtained to

13

eliminate the interference of the support membranes. Meanwhile, pore size

14

distribution and the surface elemental composition of the free-standing pristine PA

15

membrane and the resultant Arg-FeIII/PA membrane were obtained through N2

16

adsorption isotherms by the gas adsorption analyzer (BELSOPR-Max-I, Microtrac

17

BET, Japan) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi),

18

respectively. The membrane mechanical performance was also measured by an

19

electronic tensile machine (Yangzhou Zhongke WDW-02) with the stretching rate 2

20

mm/min at the room temperature (20 oC) for more supplementary information.

21

Table 1. The content of monomers and the Arg-FeIII complex of different PA membranes.

Number of membranes

PIP in aqueous phase (wt %)

TMC in organic phase (wt %)

Arg-FeIII complex in aqueous phase (g/L)

1#

0.1

0.1

0

2#

0.1

0.1

0.10

3#

0.1

0.1

0.15

4#

0.1

0.1

0.20

5#

0.1

0.1

0.25

6#

0.1

0.1

0.30

7#

0.1

0.1

0.35

22 6

1

The water uptake (WU) of the resultant PA membranes was determined by

2

measuring the weight change between wet and dry membranes. The membranes were

3

first dried for 24 h at 60 oC to measure the dry weight denoted as Ws. Afterwards, the

4

membranes were immersed in the water for 48 h at 25 oC to be saturated. Finally, after

5

taking out the membranes and removing the water droplets on the surface, the

6

membranes were re-weighed with the result denoted as Wu. Final values of the water

7

uptake were obtained by equation (2) with the average of three measurements:

8

WU =

Wu - Ws × 100% Ws

(2)

9

To verify whether there is the leakage of metal ions and the stability of the resultant

10

Arg-FeIII/PA membranes, 10 mL deionized water was filtrated through the resultant

11

Arg-FeIII/PA membranes and then collected. The amount of FeIII in the collected

12

permeate water was probed by inductively coupled plasma (ICP, Leeman Prodigy,

13

USA).

14

2.5 Separation and antifouling performance evaluation

15

For nanofiltration membranes, both separation and antifouling performances are

16

critical for their application. Based on the experimental conditions and evaluation

17

methods, the dead-end filtration apparatus (Millipore, 8200, USA), an extreme

18

operation condition causing more serious fouling, was selected to assess the

19

separation and antifouling performances of the membranes. The effective area of the

20

membrane cell was 28.7 cm2.

21

2.5.1 Separation performance evaluation

22

The separation performance of the membranes was assessed by the pure water

23

permeance and rejections of inorganic salts and organic dyes. In the separation

24

experiment, the membranes were initially prepressed with deionized water at 0.25

25

MPa for 30 min to obtain a stable water permeance. Subsequently, a series of

26

inorganic salts (NaCl, Na2SO4, MgCl2, and MgSO4, 1.0 g/L) and organic dyes (Congo

27

red and Orange GII, 0.1 g/L) were respectively filled into the membrane cell to test

28

different rejections.

29

The water permeance was calculated by following expression: 7

V A∆tP

(3)

1

J=

2

where V (L) is the permeating volume; A (m2) is the effective membrane area; ∆t (h)

3

is the permeating time; P (MPa) is the operation pressure; J (L/(m2 ·h·MPa)) is the

4

water permeance.

5

The rejections were obtained by following expression:

6

 Cp  R = 1 −  ×100%  C  f  

7

where Cp (g/L) and Cf (g/L) are the concentration of permeation and feed solutions, R

8

(%) is the rejection. The concentration of solutions was measured by electrical

9

conductivity (for inorganic salt solutions) and ultraviolet–visible spectrophotometer

10

(for organic dye solutions), respectively. After the separation process, the membranes

11

were washed by deionized water with robust stirring for 1 h under no-pressure

12

condition. Subsequently, the membranes could be utilized for further antifouling

13

performance evaluation.

14

2.5.2 Antifouling performance evaluation

(4)

15

Antifouling performance of the resultant PA membranes was estimated by the

16

representative foulants solutions (BSA, emulsified oil and HA aqueous solutions, 1.0

17

g/L). Three permeance parameters were obtained by three-period evaluation

18

experiment. Firstly, the membranes were prepressed with deionized water for 30 min

19

under 0.25 MPa. Then the operation pressure was decreased to 0.20 MPa for 30 min

20

to get a steady water permeance, recorded as Jw1. Secondly, BSA, HA, and emulsified

21

oil aqueous solutions were separately filled into the filtration cell, replacing the

22

previous deionized water to measure the change of permeance during fouling. After

23

12 h, the permeance was decreased to a steady value, recorded as Jw2. Next, the

24

foulants aqueous solutions were replaced by deionized water again to wash the

25

membranes with intense stirring for 1h under no-pressure condition. Finally, the

26

steady permeance of pure water was measured at 0.20 MPa for 1 h, recorded as Jw3.

27

Four parameters, total permeance decline (DRt), the reversible permeance decline

28

ratio (DRr), the irreversible permeance decline ratio (DRir) and permeance recovery

29

ratio (FRR) were utilized to denote the antifouling performance of the membranes. 8

1

Commonly, high FRR and low DRt represent good antifouling performance of the

2

membranes [25]. These parameters were determined as follows:

3

DRt =

4

DRr =（

5

J w1 − J w2 ×100% J w1

(5)

Jw3 - Jw2 ）×100% Jw1

(6)

DRir =（

Jw1 - J w3 ）×100% Jw1

(7)

6

FRR =

J w3 × 100% J w1

(8)

7

2.5.3 Stability performance evaluation

8

The membranes for the long-term stability test were first pressurized under 0.25

9

MPa with the deionized water and then filtrated by Na2SO4 solution (1000 ppm) for

10

240 h continuously. The water permeance and the rejections of Na2SO4 was obtained

11

every certain time.

12

3. Results and discussion

13

3.1 Characterization of the Arg-FeIII complex

14

FT-IR was utilized for characterizing the chemical compositions of the Arg and the

15

Arg-FeIII complex. Due to the coordination between Arg and FeIII, the -NH2 stretching

16

vibration peak deviated in the Arg-FeIII complex compared with the Arg as shown in

17

Fig. 1(a), whereas two peaks appeared at 3183 cm-1 and 3067 cm-1 in the Arg-FeIII

18

complex and Arg respectively, accordant with the results in the literature [23]. Notably,

19

the stretching vibration peak of the imine peak disappeared in the FT-IR spetra of the

20

Arg-FeIII complex in comparison with that of Arg, which indicated the formation of

21

the Arg-FeIII complex was via the sp2 hybrid orbitals of N atom. Meanwhile, the

22

characteristic peak of C=O shifted to lower wavenumber compared with that of the

23

pristine PA membrane, indicating that the –COOH was also involved in the

24

coordination. The results above verified the successful preparation of the Arg-FeIII

25

complex. Moreover, the size of the Arg-FeIII complex was about 10-20 nm according

26

to Fig. 1 (b). The elemental compositions of the Arg-FeIII complex were detected and

27

the results were shown in Fig. 1 (c) and (d). From the wide-scan spectra, four peaks

28

appeared at the binding energies of 287.0 eV, 402.3 eV, 533.7 eV and 713.8 eV, 9

1

representing C 1s, N 1s, O 1s and Fe 2p regions, respectively, which was in accord

2

with the literature [28]. Through calculation, the atomic mass ratios of C, O, N and Fe

3

elements were about 70.66%, 18.08%, 8.64% and 2.62%, respectively.

4 5

Fig. 1. (a) FT-IR spectra of the Arg-FeIII complex. (b) TEM image of the Arg-FeIII complex. (c)

6

and (d) XPS spectra of the Arg-FeIII complex.

7 8

3.2 Simulation

9

A binary system consisting of the Arg and the metal ion was constructed to reveal

10

the binding energy between the Arg and metal ion by molecular dynamics simulation,

11

further verifying the stability of the Arg-FeIII complex structure. Sodium ions were

12

ubiquitous in brackish water or industrial wastewater, so the sodium ion was selected

13

as a comparison. As shown in Fig. 2 (a), the binding energy of the Arg-FeIII complex

14

was almost 4.5 times higher than that of Arg-NaI, which indicated that the

15

coordination interaction between Arg and FeIII was much stronger than the

16

electrostatic interaction. The GAUSSIAN result at the DFT/B3LYP level was shown

17

in Fig. 2 (b). The bond length between carboxyl and FeIII was about 2.05 Å while that 10

1

between the imine group and FeIII was approximately 1.84 Å.

2 3

Fig. 2. (a) Binding energy between the Arg and metal ions. (b) GAUSSIAN result of the bond

4

length between Arg and FeIII.

5 6

3.3 Membrane preparation and characterization

7

Owing to the active amine groups, the Arg-FeIII complex could react with trimesoyl

8

chloride (TMC), acting as the aqueous additive in company with piperazine (PIP).

9

Fabrication process of the resultant Arg-FeIII/PA membranes via interfacial

10

polymerization was shown in Fig.3. The FT-ATR spectrometer was utilized to analyze

11

the functional groups on the surface of the resultant Arg-FeIII/PA membranes. Since

12

amine groups of both the Arg and the PIP could react with acyl chloride groups in the

13

TMC, forming amide bonds, the amide bonds served as the indication of IP process.

14 15

Fig. 3. Schematic illustration of the fabrication process of the resultant Arg-FeIII/PA membrane and 11

1

the interaction between Arg-FeIII complex and PA.

2

As shown in Fig. 4 (a), the peaks in PES at 1242 cm-1, 1487 cm-1 and 1578 cm-1

3

were attributed to the stretching vibration of the aromatic ether, C-C and the benzene

4

ring, respectively [26]. As a comparison, a divergent peak at 1640 cm-1 notably

5

appeared in the spectra of the resultant Arg-FeIII/PA membranes which was ascribed to

6

the stretching vibration band C=O of the amide bonds groups [27], indicating the

7

successful formation of the PA selective layer via the IP process. In order to further

8

testify the successful incorporation of the Arg-FeIII complex, EDX Fe element

9

mapping of the resultant Arg-FeIII/PA membrane was proceeded and simultaneously

10

indicated a uniform distribution of the Arg-FeIII complex on the membrane surface as

11

shown in Fig. 4 (b).

12 13

Fig. 4. (a) FT-ATR spectra of the pristine PA membrane (1# membrane) and the resultant

14

Arg-FeIII/PA (3#、6#) membranes. (b) EDX Fe element mapping of the resultant Arg-FeIII/PA

15

membrane.

16 17

The SEM characterization result was shown in Fig. 5, which revealed the

18

morphology of the surface and the cross section of the pristine PA and the resultant

19

Arg-FeIII/PA membranes. The pristine PA membrane exhibited representative surface

20

and cross-section structures fabricated by IP. In the meantime, the dispersed

21

nodular-like structure was exhibited on account of the cross-linking between the

22

monomers, PIP and TMC. As shown in Fig. 5 (a), when the PA layer was incorporated

23

with the Arg-FeIII complex, the rugged and nodular-like structure became more 12

1

obvious at the same magnification compared with the pristine PA membrane.

2

Moreover, with the increasing content of the Arg-FeIII complex, this typical

3

morphology with the embossments on the rough surface could be observed more

4

clearly. The surface roughness was confirmed by AFM measurement, as shown in Fig.

5

5 (b). The pristine PA membrane (1# membrane) possessed the lowest roughness

6

(Rrms=5.97 nm). With the increment of the Arg-FeIII complex, the surface roughness

7

increased. When the content of the Arg-FeIII complex reached 0.3 g/L, the

8

Arg-FeIII/PA membrane exhibited the roughest surface with the value of 13.4 nm for

9

Rrms. The change of the membrane surface roughness was ascribed to the incremental

10

content of the Arg-FeIII complex. The cross-section structure as shown in Fig. 5 (c)

11

indicated that the resultant Arg-FeIII/PA membranes possessed the typical

12

asymmetrical structure including the finger-pore structure of PES support layer and

13

the compact separation PA layer.

14 15

Fig. 5. (a) SEM surface morphologies, (b) AFM, (c) SEM cross-section morphologies of the

16

pristine PA membrane (1#) and the resultant Arg-FeIII/PA membranes (3# and 6# membranes).

17 13

1

Surface zeta potential was employed to further investigate the surface charge

2

performance of the pristine PA and the resultant Arg-FeIII/PA membranes. As shown in

3

Fig. 6 (a), an initial value -28.18 mV for the pristine PA membrane surface was

4

detected. With the ever-increasing content of the Arg-FeIII complex, the surface zeta

5

potential exhibited a downward trend gradually which means that more negative

6

charges emerged on the membrane surface. While the content of the Arg-FeIII complex

7

increased from 0.1 g/L to 0.3 g/L, the values of the surface zeta potential decreased

8

from -31.10 mV to -54.27 mV. This aforementioned result could be accounted for the

9

incremental negative carboxyl groups. During the IP process, most acyl chloride

10

groups in TMC participated in the reaction with amine groups, forming amide bonds.

11

Simultaneously, the disturbance from the Arg-FeIII/PA complex mentioned above

12

caused more PA network terminals derived from the unreacted acyl chloride groups,

13

easily hydrolyzed to carboxyl groups. Furthermore, the inherent α-carboxylic acid

14

group in the Arg-FeIII complex also intensified the amount of the carboxyl groups.

15

Accordingly, the incorporation of the Arg-FeIII/PA complex increased the carboxyl

16

groups in the PA layer, endowing the membrane surface with more negative charges.

17

Although there were unreacted amine groups from the Arg-FeIII complex in PA

18

separation layer, abundant carboxyl groups were able to shield the influence of the

19

positive charges from the Arg-FeIII complex. Abundant carboxyl groups also endowed

20

the membrane surface with higher hydrophilicity [28].

21

For water treatment process, surface hydrophilicity is of critical significance for

22

nanofiltration membranes because higher affinity for water molecules represented

23

better water solubility, thus contributing to higher water permeance and better

24

antifouling capability [29]. The apparent water contact angles (θ) were influenced by

25

both the abundant hydrophilic functional groups on the surface and the membrane

26

roughness. The effect of roughness could be described by the Wenzel model,

27

represented by cosθ' = cosθ/r, where θ' was the corrected water contact angles and r

28

was the ratio of membrane surface area to membrane projected area obtained by AFM

29

[30]. Thus, the corrected water contact angles (θ') were merely related to the abundant

30

hydrophilic functional groups. As the content of the Arg-FeIII complex increased from 14

1

0.0 to 0.30 g/L, the corrected water contact angles decreased from 45.9 ± 0.4° to 22.2

2

± 0.5°. The decreased water contact angles indicated the elevated wettability of the

3

membranes.

4

As mentioned above, abundant carboxyl groups could explain this enhanced

5

hydrophilicity reflected in the decreased water contact angle, also consistent with the

6

larger negative values of the surface zeta potential. Naturally, XPS characterization

7

was carried out to further reveal these carboxyl groups on the surface of the pristine

8

PA membrane and the resultant Arg-FeIII/PA membranes. As shown in Fig. S2, the

9

peaks of C 1s at 284.4, 284.9, 286.0, and 288.0 eV represented the binding energy of

10

C–C, Cα–N, carboxyl groups and the C atom in guanidine groups [31-34],

11

respectively. Furthermore, the ratio of the carboxyl groups on the surface of the

12

pristine PA membrane and the resultant Arg-FeIII/PA membranes was calculated. The

13

amount of the carboxyl groups increased from 12.7% of the pristine PA membrane to

14

20.0% of 6# membrane, which increased by 57.5 %. The result was consistent with

15

the result of surface zeta potential and corrected water contact angles. Water uptake is

16

another crucial parameter for membranes utilized in water treatment which is quite

17

related to the water permeance. Due to the enhanced hydrophilicity of the PA layer,

18

the resultant membranes exhibited elevated value of the water uptake. As shown in

19

Fig. 6 (c), as the content of the Arg-FeIII complex increased from 0.1 g/L to 0.3 g/L,

20

water uptake of the resultant membranes improved accordingly. By contrast, the

21

pristine PA membrane displayed the minimum value. This result was consistent with

22

the above-mentioned enhancement of the membrane hydrophilicity, resulting in the

23

enhanced water permeance of the resultant Arg-FeIII/PA membranes.

24

15

1 2

Fig. 6. (a) Surface zeta potential of the pristine PA and the resultant Arg-FeIII/PA membranes at

3

pH=6.0±0.2. (b) Corrected water contact angles of the pristine PA and the resultant Arg-FeIII/PA

4

membranes. (c) Water uptake of the pristine PA and the resultant Arg-FeIII/PA membranes. (d)

5

XRD patterns of the free-standing pristine PA membrane and the resultant Arg-FeIII/PA membrane.

6 7

3.4 Separation performance of membranes

8

In practical applications, permeability and selectivity are two crucial performances

9

parameters for nanofiltration membranes, represented by permeance and rejections,

10

respectively. The water permeance and dye/salt rejections were evaluated, as shown in

11

Fig. 7. The pristine PA membrane exhibited the water permeance of 92.5 L/

12

(m2·h·MPa) with the operation pressure 0.20 MPa. With the content of the Arg-FeIII

13

complex increased from 0.1 g/L to 0.3 g/L, the water permeance enhanced from 113.0

14

L/(m2·h·MPa) to 185.0 L/(m2·h·MPa). Notably, when the content of the Arg-FeIII

15

complex reached 0.3 g/L, the water permeance doubled compared with that of the

16

pristine PA membrane. The enhanced water permance could be ascribed to three

17

aspects. Primarily, the BET surface area improved according to the N2 adsorption

18

isotherms in Fig. S5 (b) which means that the higher porosity of the membranes was

19

achieved. This increased porosity could be ascribed to the interference from the 16

1

Arg-FeIII complex on the packing of PA chains. For PA membranes, the interfacial

2

compatibility between the additives and the PA chains influences the packing of the

3

PA chains and is crucial to obtain the defect-free membranes [35-37]. In order to

4

testify the interference on the packing of PA chains from the Arg-FeIII complex, XRD

5

characterization of the pristine PA membrane and the resultant Arg-FeIII/PA membrane

6

were carried out as shown in Fig. 6 (d). Generally, narrow diffraction peaks of the

7

XRD patterns are associated with the favorable degree of the crystallinity (i.e. full

8

width at half maximum, FWHM≈0.2; the broader ones with FWHM≥1 for the

9

semi-crystalline/amorphous behavior) [38]. Specifically, the material with crystallinity

10

will have the sharp X-ray diffraction peaks with high intensities while the diffraction

11

peaks of the semi-crystalline or the amorphous material will be broader [39]. In this

12

study, the incorporation of the Arg-FeIII complex disturbed the motion and ordered

13

packing of the PA network compared with the pristine PA membrane which weakened

14

the membrane crystallinity. As mentioned above, higher crystallinity could be

15

reflected by the smaller FWHM value. Therefore, the resultant Arg-FeIII/PA

16

membrane exhibited higher FWHM value than that of the pristine PA membrane. This

17

interference was conducive to the increased porosity, leading to the enhanced water

18

permeance. Simultaneously, since Arg-FeIII complex possessed abundant amino and

19

carboxyl groups and could form hydrogen bonds with water, water transfer through

20

the PA separation layer was promoted [24, 40]. Finally, the rougher surface endowed

21

the membrane surface with a larger contact area which was also conductive to the

22

water penetration [41, 42]. In summary, the Arg-FeIII complex played a pivotal role in

23

elevating the water permeance of the membranes. When the content of the Arg-FeIII

24

complex continuously increased to 0.35 g/L, the decreased water permeance was

25

probably ascribed from the increased membrane thickness. Generally, membrane

26

thickness is positively proportional to the mass transport resistance reflected as the

27

mass transport pathway across the membranes, further affecting the water permeance

28

[43]. After incorporating Arg-FeIII complex, the water permeance enhanced owing to

29

more water channels provided. When the content of the Arg-FeIII complex increased

30

to some extent, the water permeance was dominated by the membrane thickness. The 17

1

SEM cross-section morphology of the 7# membrane with the content of the Arg-FeIII

2

complex 0.35 g/L was shown in Fig. S11. It indicated that the thickness of the 7#

3

membrane was much greater than that of the 6# membrane with the Arg-FeIII content

4

0.3 g/L. Consequently, the enhancement of the water permeance was counteracted due

5

to the increased hydrodynamic resistance. Hence, the water permeance exhibited a

6

decreasing trend when the content of the Arg-FeIII complex reached 0.35 g/L.

7

For the rejections towards dyes (Properties of the dye molecules utilized in this

8

study were shown in Table S1.), the dye rejections followed the order of small-size

9

dye (< 1nm, OG) < (>2 nm, CR), which manifested that the size exclusion played a

10

significant role during the filtration. As shown in Fig. 7 (a), the rejections of the

11

Congo red and Orange GII remained more than 98.9% and 95.9%, respectively.

12

Therefore, it can be concluded that the water permeance increased continuously while

13

the rejections was maintained, demonstrating that the trade-off limit between

14

permeability and selectivity was broken. The salt rejections of the membranes obeyed

15

the order: Na2SO4 > MgSO4> MgCl2 > NaCl. According to the Donnan exclusion

16

theory, the negative charges on the membranes would repulse the anions from the

17

membranes while the cations would be retained due to the electroneutrality

18

requirements. Subsequently, the high-valent anions would be repulsed and high-valent

19

cations would be attracted. Thus, the salt with multivalent anions and monovalent

20

cations would be highly rejected by the resultant PA membranes [44, 45]. Hence, the

21

resultant membranes would exhibit high rejection of SO42- owing to the negative

22

charges on the surface as shown in Fig. 7 (b). As for the salt NaCl with monovalent

23

cations, the rejection towards it was lower than that of MgCl2. This phenomenon was

24

attributed to the steric effect. The hydrated ions radii of Na+ (0.358 nm) was smaller

25

than that of Mg2+ (0.428 nm) [46] which made them more difficult to reject by PA

26

layer, which manifested that the steric effect would also affect the desalination process

27

[47]. Besides, as the content of the Arg-FeIII complex increased from 0.1 g/L to 0.3

28

g/L, the rejection for Na2SO4 was not sensitive towards the variation of content of the

29

Arg-FeIII complex, which maintained as high as 90.4%-91.9%. When the content of

30

the Arg-FeIII complex reached 0.35 g/L, the salt rejections of the resultant Arg-FeIII/PA 18

1

membranes dropped. This phenomenon could be interpreted as the formation of

2

defects arisen from the aggregation of the Arg-FeIII complex in the resultant

3

Arg-FeIII/PA membrane. As shown in Fig. S4, the DLS result indicated that the size of

4

the Arg-FeIII complex with the content 0.35 g/L was about 30 nm, larger than that of

5

the Arg-FeIII complex with the content 0.30 g/L. Hence, the optimal content of the

6

Arg-FeIII complex was 0.3 g/L which was denoted as 6# membrane. In addition, the

7

rejection efficiency of the resultant Arg-FeIII/PA membrane under different salt and

8

dye concentrations was further carried out as shown in Fig. S8 and Fig. S9. The

9

resultant Arg-FeIII/PA membrane exhibited maintained Na2SO4 under different salt

10

concentrations from 500 ppm to 5000 ppm as shown in Fig. S8, which demonstrated

11

that the resultant Arg-FeIII/PA membrane exhibited superior stability when applied in

12

the desalination process. Meanwhile, the resultant Arg-FeIII/PA membrane displayed

13

stable dye rejections under different dye concentrations from 50 ppm to 500 ppm

14

shown in Fig. S9, also indicating the excellent stability of the resultant Arg-FeIII/PA

15

membrane. Separation performances can also be expressed in the MWCO values. The

16

MWCO values of the pristine PA membrane and the resultant Arg-FeIII/PA membranes

17

were taken as the molecular weight cut-off of PEG at 90%. The rejections of the

18

pristine PA and the resultant Arg-FeIII/PA membranes to five kinds of PEG molecules

19

with molecular weights 200, 400, 600, 800, 1000 and 2000 Da were shown in Fig. S6.

20

With the content of the Arg-FeIII complex increased, the PEG rejections remained

21

stable.

19

1 2

Fig. 7. (a) Water permeance and dye rejections of the pristine PA and the resultant Arg-FeIII/PA

3

membranes. (b) Salt rejections of the pristine PA and the resultant Arg-FeIII/PA membranes. (c)

4

Water permeance and salt rejections towards Na2SO4 of 6# membrane in this study and other

5

literatures (for details in Table S2). (d) Antifouling performance (denoted by FRR) against BSA

6

foulants of 6# membrane in this study and other literatures (for details in Table S3).

7 8

3.5 Antifouling performance of membranes

9

For nanofiltration membranes, antifouling performance has been regarded as the

10

critical surface property of the membranes. Membrane fouling was driven by the

11

inevitable interaction between foulants and the membrane surface, including hydrogen

12

bond

13

electrostatic interaction. Inhibiting the interaction between foulants and the membrane

14

surface was an efficient approach to fabricating antifouling membranes. We tested the

15

antifouling performance using BSA, HA and emulsified oil as raw material solution

16

representing the fouling of protein, natural organic matter and the hydrocarbon.

interaction,

Vander

Waal

force,

hydrophobic

interaction

and

17

An imitated fouling process was carried out. For BSA foulants, when the content of

18

the Arg-FeIII complex reached 0.3 g/L, DRt reduced to 12.0% and FRR was elevated to 20

1

100% after the water rinsing. Moreover, three-cycle filtration experiment was carried

2

out to further evaluate the antifouling performance of the pristine PA membrane and

3

the resultant Arg-FeIII/PA membranes as shown in Fig. 8. The FRR value still

4

maintained over 90% for BSA and HA foulants after three cycles of separation

5

experiment. As a comparison, the pristine PA membrane exhibited inferior fouling

6

resistance performance embodied as the smaller FRR value. This enhanced antifouling

7

performance could be ascribed to the elevated membrane surface hydrophilicity

8

reflected by the decreased static water contact angle and the increased negative

9

charges confirmed by the surface zeta potential characterization. On one hand, the

10

resultant Arg-FeIII/PA membranes were equipped with an effective hydrated layer with

11

large steric hindrance to resist the foulants from adhering on the membranes. On the

12

other hand, since the surface of the resultant Arg-FeIII/PA membranes became more

13

electronegative with the incremental content of the Arg-FeIII complex and the foulants

14

utilized in this paper all carried negative charges, the strong negative charges

15

endowed the resultant Arg-FeIII/PA membrane with a reinforced antifouling surface

16

towards the negative foulants by electrostatic repulsion. Therefore, the resultant

17

Arg-FeIII/PA membranes exhibited lower DRt after fouled by the foulants and higher

18

FRR after rinsing by the deionized water than that of the pristine PA membrane. To

19

summarize, the resultant Arg-FeIII/PA membranes exhibited significantly intensified

20

fouling resistance performance. In Fig. 7 (c) and (d), the separation performance and

21

the antifouling performance of the 6# membrane were compared with those of other

22

nanofiltration membranes in the literatures. Notably, the resultant Arg-FeIII/PA

23

membranes in this study exhibited comparable separation performance and antifouling

24

performance. (The specific data were listed in Table S2 and Table S3.)

21

1 2

Fig.8. Time-dependent permeance of the pristine PA and the resultant Arg-FeIII/PA membranes (1#

3

and 6# membranes) for the three-cycle separation experiment during filtrating the foulant

4

solutions.

5 6

For water treatment applications, tackling heavy metal ions in the wastewaters has

7

been a special concern [48], FeIII is one of the toxic metal ions which tend to

8

accumulate in the organisms, resulting in numerous disorders and diseases [49]. As a

9

consequence, the amount of FeIII in the permeate was detected and no FeIII leaked

10

during the filtration process (as shown in Table S4). Besides, the resultant Arg-FeIII/PA

11

membranes exhibited prominent long-term stability during the filtration process of

12

Na2SO4 solution (1000 ppm) under 0.2 MPa. No obvious decline in the water

13

permeance was observed during the filtration process and the rejections towards

14

Na2SO4 also remained stable as shown in Fig. S10.

15

22

1

4. Conclusions

2

In this study, inspired by the coordination-driven self-assembly phenomenon in

3

nature and the structures of aquaporins, an Arg-FeIII complex was designed and

4

introduced into polyamide layer via the interfacial polymerization process, which

5

endowed the resultant Arg-FeIII/PA membranes with doubled water permeance

6

compared with that of the pristine PA membrane. Simultaneously, the salt and dye

7

rejections remained almost unchanged. Moreover, the elevated negative charges and

8

hydrophilicity contributed to the superior antifouling performance of the resultant

9

Arg-FeIII/PA membranes. High separation performance, excellent antifouling

10

performance, along with the facile and controllable fabrication process manifest that

11

incorporating the versatile metal-organic complex into interfacial polymerization is an

12

effective strategy to manufacture the membranes with high performance for water

13

resource reclamation.

14 15 16

Acknowledgements This study is supported by National Natural Science Foundation of China (No.

17

21621004,

21878217)

18

(18JCZDJC36900).

and

Natural

Science

23

Foundation

of

Tianjin

City

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Highlights 1. Arg-FeIII complex was designed as aqueous additive in interfacial polymerization. 2. The Arg-FeIII/polyamide nanofiltration membranes were in-situ fabricated. 3. The membranes exhibited enhanced water permeance while retaining high rejections. 4. The membranes exhibited excellent antifouling performances.

Author statement Jingyuan Guan: Research work design and implementation, paper writing Zhongyi Jiang: Project administration, research work design and supervision, paper design and polishing Lin Fan: Experimental investigation Ya-nan Liu: Theoretical analysis Benbing Shi: Figure drawing guidance Jinqiu Yuan: Data analyisi Runnan Zhang: Data analysis Xinda You: Research work suggestion Mingrui He: Research work suggestion Yanlei Su: Research work suggestion

Conflict of interest We declare that we do not have any conflict of interest existed in this manuscript.