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Free radical graft copolymerization strategy to prepare catechin modified chitosan loose nanofiltration (NF) membrane for dye desalination Songbai Liu, Zhan Wang, and Peng Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04699 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Sustainable Chemistry & Engineering

1

Free radical graft copolymerization strategy to prepare catechin modified

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chitosan loose nanofiltration (NF) membrane for dye desalination

3

Songbai Liu1, Zhan Wang*, Peng Song 2

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1, 2) Beijing Key Laboratory for Green Catalysis and Separation, Department of

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Chemistry and Chemical Engineering, Beijing University of Technology, Beijing

6

100124 P.R. China

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Corresponding

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Department of Chemistry and Chemical Engineering, Beijing University of

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Technology, Chaoyang District, Beijing 100124 P.R. China

author at: Beijing Key Laboratory for Green Catalysis and Separation,

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Tel: 86-10-67396186; Fax: +861067391983;

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E-mail addresses: wangzhan3401@ 163.com.

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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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ABSRTACT

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In this study, a novel free radical graft copolymerization strategy was applied to

14

prepare the catechin modified chitosan loose NF membrane for dye desalination.

15

Firstly, catechin, the eco-friendly natural material, was grafted onto chitosan through

16

free radical reaction and self-crosslinking. Secondly, the catechin-grafted-chitosan

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conjugates were assembled onto the surface of the hydrolyzed polyacrylonitrile

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ultrafiltration (UF) membrane. Finally, the prepared membrane was characterized by

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FT-IR, XPS, SEM, AFM, electrokinetic analyzer and contact angle goniometer. The

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results showed that the prepared membrane exhibited a high rejection for dyes (i.e.

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99.6% for congo red, 98.7% for acid fuchsin and 98.5% for crystal violet) and a low

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retention for inorganic salts (i.e. 4.8% for Na2SO4, 12.5% for NaCl, 15.8% for MgSO4

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and 16.2% for MgCl2). Especially, the Na2SO4 rejection was 2~3 times lower than

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that of reported loose NF membranes. Meanwhile, a high removal rate of 82% for

25

NaCl with dye loss rate of 10% was also observed in constant-volume batch dye

26

desalination process. Moreover, the membrane also possessed good dye anti-fouling

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ability with a flux recovery ratio of 87.8% and an irreversible fouling ratio of 12.2%.

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Hence, this catechin modified chitosan loose NF membrane show a promising

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application for dye desalination.

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KEYWORDS: Catechin, Free radical graft copolymerization, Dye desalination,

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Loose NF membrane

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2


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INTRODUCTION

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In China, more than 2.37 billion tons of dye wastewater with high salinity (~6.0

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wt.% NaCl or ~5.6 wt.% Na2SO4) is directly discharged annually and it causes serious

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environment pollution and resource waste. 1 , 2 Consequently, finding effective

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approaches for dye reuse and inorganic salt removal from dye wastewater are

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imperative. Unfortunately, conventional technologies, such as oxidation, adsorption,

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chemical degradation, coagulation and ultrafiltration, are inadequate for this purpose.3

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In addition, the addition of chemicals in the most methodologies not only will

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increase the operation cost, but also cause secondary pollution.4

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Nanofiltration (NF) technology becomes an attractive alternative in dye

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desalination and purification owing to its environmental friendliness and low energy

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consumption. 5 Traditional commercially available NF membranes prepared by

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interfacial polymerization was negatively charged owing to the hydrolyzation of

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residual acyl chloride groups.6 This membrane could reject both organic dyes and

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multivalent ignorant salts (i.e. sulfate), which might not only lower the quality of

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rejected dyes but also cause a big loss of valuable salts.7 For example, commercial

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DK (Osmonics) NF membrane possessed a 96.0% rejection for reactive black 5 and

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21.1% retention of NaCl, but with above 98% rejection of Na2SO4 in the dye

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desalination process.8 In this situation, the preparation of loose NF membrane that

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have low retention of ignorant salts (i.e. chloride and sulfate) and high dye rejection

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became the focus of most researches.9-12 A poly (m-phenylene isophthalamide) loose

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NF membrane with 98.0% rejection for eriochrome blue black B and 5.0% retention

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of NaCl was prepared by phase inversion method using polyvinyl pyrrolidone (PVP) 3



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and lithium chloride (LiCl) as additive.9 By blending with poly (sodium 4-styrene

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sulfonate) grafted SiO2 via SI-ATRP, a negatively charged loose NF membrane with

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above 90% rejection for reactive black 5 and 17.2% retention of Na2SO4 was

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obtained.10 Another loose NF membrane with 95% rejection for congo red and 17.0%

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retention of Na2SO4 was fabricated through interfacial polymerization with in-situ

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generated TiO2 nanoparticles. 11 By the interfacial polymerization reaction of

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isophorone diisocyanate (IPDI) with graphene oxide quantum dots (GOQDs), a thin

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film nanocomposite membrane with 97.6% rejection for methylene blue and 17.2%

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retention of NaCl was prepared.12 Although above mentioned loose NF membranes

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possessed high dye rejection and low removal rate of inorganic salts for dye/salt

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mixture solution, the generous use of alkane solvents will cause environmental issues

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in subsequent preparation process.13 Besides, diafiltration of dye/salt mixture solution

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to obtain the pure dye solution and inorganic salts have not been investigated.

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As bio-inspired materials, dopamine, tannin acid, catechol, gallic acid, catechin,

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Vitamin C and chitosan are obtained from nature and had potential application in

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surface modification of the polymer membrane. For instance, the co-deposition of

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dopamine with PEI,14 the interfacial polymerization with tannic acid and trimesoyl

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chloride,15 the self-polymerization of polyamines and catechol,16 gallic acid grafted

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onto PEI,17 as well as the cross-link of chitosan with glutaraldehyde (GA),18 have

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been applied to prepare the composite membrane. Moreover, the water-soluble

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catechin, which is a natural material and widely present in tea, fruit, chocolate and

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wine19 could provide the possibility of avoiding the use of alkane solvents in the 4


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membrane preparation process. However, the graft of catechin onto chitosan through

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free radical copolymerization for the preparation of the loose NF membrane has never

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been reported in the literature.

81

In this study, a novel free radical graft copolymerization strategy was applied to

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prepare the catechin modified chitosan loose NF membrane for dye desalination.

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Hydroxyl radical (HO•) is generated by the reaction of Vitamin C with H2O220 and

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then initiated catechin grafted onto chitosan (Figure 1). Meanwhile, chitosan,

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amino-containing polymer, was co-deposited onto the HPAN membrane by the

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covalent connection between the catechin and amino groups.

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properties and separation performances of the obtained catchin/chitosan composite

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membrane were investigated. After that, the anti-fouling performance for dyes and

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long-term stability in dye/salt mixture solution were evaluated. Finally, the constant

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volume batch dye desalination test is carried out to investigate the dye desalting effect.

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The detailed membrane preparation process, properties of dyes (Table S1) and

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permeation test were shown in Supporting Information.

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

Then, the surface

RESULTS AND DISCUSSION

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Surface Modification with Catechin/chitosan by Graft Copolymerization.

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Figure 2a revealed that the color of catechin-chitosan mixed solution turned brown

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after graft copolymerization initiated by Vitamin C and H2O2 redox pair for 18 h and

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gradually deepened with the increase of the pH from 2.0 to 5.0. However, the color

5



98 99 100

Figure 1. Possible graft copolymerization mechanism of catechin and chitosan, and the preparation process of the catechin/chitosan loose NF membrane.

6


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(a)

(b)

(c)

Figure 2. Color change of catechin-chitosan mixed solution at different pH values after 18 h graft copolymerization (a); UV-vis spectra of catechin solution, chitosan solution and catechin-chitosan mixed solution at different pH values (b); The PAN UF substrate and the catechin/chitosan loose NF membrane prepared using 10 g L-1 chitosan and 0.7 g L-1 catechin aqueous solution at pH value of 4.0 (c). 101

became light with further increasing the pH. In addition, the viscosity and zeta of

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catechin-chitosan mixed solution respectively decreased to 30.0 mPa s and 24.5 mV

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compared to pure chitosan solution (Table S2). This can be explained by that the

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formation of catechin-grafted-chitosan conjugates weakens the intermolecular and

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intra-molecular hydrogen bonds of chitosan and decreases the amount of protonated

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amine groups.21 As presented by a UV–Vis spectrophotometer (Figure 2b), the pure

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catechin solution show a narrow absorption peak at 292 nm. 22 After the

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catechin-chitosan mixed solution was initiated by Vitamin C and H2O2 couple for 18 h,

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a new peak at 450 nm appeared owing to the successful formation of catechin-grafted

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conjugates and its intensity was significantly strengthened due to the increase of

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synthetic conjugates with increasing the pH of mixed solution from 2.0 to 6.0.

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Moreover, the peak at 292 nm was obviously broadened and its intensity increased

113

owing to the self-crosslinking reaction of catechin molecules via covalent binding 7



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among aryl rings. This was similar to the self-polymerization of dopamine and

115

catechol.23,24 In addition, the π-π stacking and hydrogen bonding between aryl rings

116

contribute to the aggregation of catechin in the solution. As a result, a brown mixed

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solution was obtained for the preparation of the melanin-like catechin/chitosan loose

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NF membrane (Figure 2c).

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Characterization of the Catechin/chitosan Loose NF Membrane. As shown in

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Figure 3a, there are three typical peaks at 2243 cm-1 (C≡N), 1405 cm-1 and 1568 cm-1

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(COOH) in the HPAN UF membrane.25 After coating the pure chitosan layer, two

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new characteristic peaks at 1035 cm-1 (C-O) and 1164 cm-1 (C-N) indicated the

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saccharide structure of chitosan.26 Then, free radical graft copolymerization route

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was carried out to form catechin-grafted-chitosan conjugates and two new adsorption

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bands appeared in the catechin/chitosan composite membrane. The one peak at 1258

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cm-1 (C-O-C)27 indicated the self-polymerization of catechin molecules and another

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peak at 1612 cm-1 (C=N)28 proved the successful formation of conjugates between

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free amine groups in chitosan chain and phenolic functional groups in catechin. In

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addition, the intensity of peaks at 3300–3500 cm−1

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increase of hydroxyl groups of catechin molecules in the active layer.

29

became strengthened due to the

131

Figure 3b demonstrated that three types of membrane surfaces had the same

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binding energy peaks for carbon, nitrogen and oxygen, but different element ratios

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(Table S3). The lowest N/C ratio (0.13) and the highest O/C ratio (0.50) were

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observed for the catechin/chitosan loose NF membrane and revealed the existence of

135

vast oxygen-containing groups including –OH and -COOH, which could improve the 8


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hydrophilicity of the membrane surface.17 Furthermore, the N1s core peaks of the

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membranes in Figure 3b were analyzed (Figure 3c-e). The clear peak at 399.38 eV

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(C≡N)30 was observed for the HPAN UF membrane. After coating the chitosan layer,

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the peaks at 398.52 eV (C-N) and 401.20 eV (–NH2)31 indicated the saccharide

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structure of chitosan. For the catechin/chitosan loose NF membrane, a new peak at

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399.24 eV (C=N) proved the successful synthesis of catechin-grafted-chitosan

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conjugates by graft copolymerization and this result was consistent with FT-IR

143

analysis. The possible mechanism for the synthesis of catechin-grafted-chitosan

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conjugates through free radical graft copolymerization was illustrated in Figure 1. The

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HO•, generated by the oxidizing reaction of Vitamin C with H2O2, attacked the amine

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and hydroxyl groups of chitosan for the formation of chitosan radicals.20 Afterwards,

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the formed chitosan radicals acted as a powerful nucleophile32 and readily reacted

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with catechin to synthesize catechin-grafted-chitosan conjugates via C=N bond.

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Compared to other preparation method for chitosan conjugates, such as activated

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ester-mediated graft using EDC−NHS 33 and the enzyme-mediated route using

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tyrosinase or laccase, 32 the free radical graft copolymerization using Vitamin C and

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H2O2 as initiator shown unique advantages with low energy consumption, no

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generated toxic products and low cost.

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As illustrated in Figure 3f, the HPAN UF membrane possessed a negative charge of

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-35.0 mV. After assembling with catechin-grafted-chitosan conjugates, the membrane

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surface was reversed to positive charge of 14.85 mV owing to the protonated amino

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groups in chitosan. The formed conjugates with vast –OH and –NH2 groups tend to 9



a

b

c

d

e

f

158 159 160 161

Figure 3. The FT-IR spectra (a) and XPS wide-scan spectrum (b) of the membrane; The N1s spectra of the HPAN UF membrane (c), the chitosan-HPAN UF membrane (d) and the catechin/chitosan loose NF membrane (e); The zeta potential and water contact angle of the membrane surface (f).

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attract water molecules and form a thin hydration layer on the membrane surface via

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hydrogen bond,34 which led to the decline of water contact angle. In addition, the

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rough membrane surface (Ra=45.6 nm) also increased its wettability (Figure 4).35 10


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Thus, the finally formed catechin/chitosan loose NF membrane show a low water

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contact angle of 54.0° and good hydrophilicity.

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Morphological Structure. As shown in Figure 4, the HPAN UF membrane

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possessed a porous surface and some defects were observed on the membrane surface

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(Figure 4a). After coating with pure chitosan layer, there were massive flaky polymers

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on the membrane surface owing to the strong interaction force between chitosan

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molecules and this resulted in the aggregation of chitosan (Figure 4b).21 As for the

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catechin/chitosan loose NF membrane, the flaky polymer was softened and became

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smaller (Figure 4c). Meanwhile, some long strips of crystals were formed (Figure 4d).

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This revealed that interaction force of chitosan molecules had been significantly

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declined after free radical graft copolymerization, which was also supported by

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decreased viscosity and zeta potential (Table S2). With further increasing the catechin

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graft

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self-polymerization of catechin (Figure 4e). The thickness of the active layer

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increased from 312 nm to 487 nm according to the cross-sectional SEM images

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(Figure 4f-i). Consequently, the fabricated catechin/chitosan loose NF membrane

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(Ra=45.6 nm) exhibited a rougher surface compared with the pure chitosan-HPAN UF

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membrane (Ra=27.4 nm) and the HPAN UF membrane (Ra=12.5 nm) (Figure 4j-l).

concentration,

more

long

strips

of

crystals

11


appeared

due

to

the


Figure 4. SEM images of the HPAN UF membrane (a), the chitosan-HPAN UF membrane (b) and the catechin/chitosan loose NF membrane with different catechin concentrations (0.4 g L-1 (c), 0.7 g L-1 (d), 1.0 g L-1 (e)) at chitosan concentration of 10 g L-1; The cross-section images of the HPAN UF membrane (f) and the catechin/chitosan loose NF membrane prepared at different catechin concentration (0.4 g L-1 (g), 0.7 g L-1 (h), 1.0 g L-1 (i)); The AFM images of the HPAN UF membrane (j), the chitosan-HPAN membrane (k) and the catechin/chitosan loose NF membrane (l). 183

The Optimization of Membrane Preparation Conditions. To systematically

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investigate the effect of preparation conditions on the membrane separation

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performance, the permeate flux and rejection for dyes were measured as follows with 12


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varying the chitosan concentrations, catechin concentrations and pH of catechin-

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chitosan mixed solution.

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At the chitosan concentration of 0 g L-1, the water flux was as high as 1234 L m-2

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h-1 MPa-1, whereas the membrane had no rejection for acid fuchsin and crystal violet.

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This was ascribed to that without chitosan graft, the catechin layer was relatively

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loose since it was formed by long strips of crystals under self-polymerization of

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catechin (Figure 4e). With increasing the chitosan concentration from 6 g L-1 to 10 g

193

L-1, the rejection for crystal violet increased from 70.5% to 98.6% while the permeate

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flux decreased from 87 L m-2 h-1 MPa-1 to 72 L m-2 h-1 MPa-1 and the similar variation

195

trend was also observed for acid fuchsin (Figure 5a). The further increase of the

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chitosan concentration made two kinds of dyes completely rejected (rejection ≥

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98.5%). These variations could be rationalized that more reaction sites for catechin

198

graft was provided at high chitosan concentration and the increased activated chitosan

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initiated by vitamin C and H2O2 couple would promote the self-crosslinking of

200

catechin onto the catechin-grafted-chitosan conjugates. Thereby, more cross-linked

201

conjugates and flaky chitosan were assembled onto the membrane surface and this led

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to the formation of a denser and thicker active layer. In view of permeation and

203

rejection, the optimal chitosan concentration was determined to be 10 g L-1. In this

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condition, the permeate flux for two water-soluble dyes was 66 L m-2 h-1 MPa-1 (acid

205

fuchsin) and 72 L m-2 h-1 MPa-1 (crystal violet), along with the rejection of 98.7% and

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98.5%, respectively.

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With the increase of the catechin concentration from 0.4 g L-1 to 0.8 g L-1, the 13



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208

rejection was significantly improved from 78.5% to 99.6% (acid fuchsin)/ from 75.4%

209

to 99.1% (crystal violet), while the permeate flux declined from 79 L m-2 h-1 MPa-1 to

210

60 L m-2 h-1 MPa-1 (acid fuchsin) / from 81 L m-2 h-1 MPa-1 to 63 L m-2 h-1 MPa-1

211

(crystal violet) (Figure 5b). Clearly, vast reaction sites for chitosan graft were

212

provided

213

catechin-grafted-chitosan conjugates at higher catechin concentration, which formed a

214

denser active layer and resulted in high rejection with low permence. This could be

215

proved by SEM images (Figure 4e), at higher catechin concentration, more long strips

216

of crystals were formed and more flaky chitosan with big pore was softened and

217

became smaller. Therefore, the catechin concentration of 0.7 g L-1 was selected in the

218

subsequent study.

and

the

catechin

could

polymerize

to

form

a

cross-linked

219

The effect of pH value of catechin-chitosan mixed solution on the separation

220

performance was investigated (Figure 5c). When the pH increased from 2.0 to 6.0, the

221

permeate flux first decreased from 136 Lm-2 h-1 MPa-1 to 62 L m-2 h-1 MPa-1 and then

222

increased to 102 L m-2 h-1 MPa-1 for crystal violet, whereas the rejection show the

223

opposite trend. This was ascribed to that at pH 2.0, vast amine groups of chitosan

224

were protonated and became positively charged,36 which was not beneficial for the

225

formation of catechin-grafted-chitosan. In contrast, when the pH rose from 2.0 to 5.0,

226

the protonation degree of chitosan decreased and its amine groups became

227

reactive. 37 Hence, more chitosan were grafted onto catechin and promoted the

228

synthesis of catechin-grafted-chitosan conjugates. Moreover, the self-polymerization

229

of catechin was also enhanced at this pH range (Figure 2b) and the dense structure of 14


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(a)

(b)

(c)

Figure 5. The optimization of membrane preparation conditions: effect of chitosan 15



concentration on the separation performance (a), effect of catechin concentration on the separation performance (b) and effect of pH value of catechin-chitosan mixed solution on the separation performance (c). (Membrane preparation condition: 6-14 g L-1 chitosan aqueous solution, 0.4-0.8 g L-1 catechin aqueous solution with free radical graft times of 18 h at pH value of 2.0-6.0, deposition at 30 °C for 4 h, heat treatment at 45 °C for 1 h. Test conditions: 0.1 g L-1 acid fuchsin solution, 0.1 g L-1 crystal violet solution, 0.2 MPa, at 25 °C.) 230

the membrane surface was formed, which led to a low permeate flux. However, as the

231

pH value increased to 6.0, chitosan got deprotonated and became insoluble.38 This

232

decreased the assembly amount of catechin-grafted-chitosan conjugates onto the

233

HPAN UF membrane surface and promoted the formation of relatively loose

234

architectures with a high permeance. Therefore, pH 4.0 was the optimal value for the

235

membrane performance.

236

Molecular Weight Cut-off (MWCO) and Separation Performance. The

237

catechin/chitosan loose NF membrane was prepared under optimal conditions (10 g

238

L-1 chitosan aqueous solution, 0.7 g L-1 catechin aqueous solution with free radical

239

graft times of 18 h at pH value of 4.0, deposition at 30 °C for 4 h and heat treatment at

240

45 °C for 1 h) to investigate its MWCO and separation performance towards different

241

dye aqueous solutions and salt solutions.

242

Base on the rejection curve for various molecular weights PEG (Figure 6a), the

243

MWCO of the membrane was determined to be 720 Da and the calculated mean pore

244

size of the membrane was 1.24 nm according to equation (1) of Supporting

245

Information. As illustrated in Figure 6b, the membrane showed the permeate flux of

246

64 (congo red), 66 (acid fuchsin), 72 (crystal violet) and 80 L m-2 h-1 MPa-1 (methyl

247

orange), along with the rejection of 99.6%, 98.7%, 98.5%, 56.3%, respectively. Note 16


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248

that the membrane achieved a higher permeance but relatively low rejection for dyes

249

with a smaller molecular weight than the larger ones: 99.6% (Mwcongo red=696.7 Da) >

250

98.7% (Mwacid fuchsin=585.5 Da) > 98.5% (Mwcrystal violet=408.0 Da) > 56.3% (Mwmethyl

251

orange=327.3

252

Dyes with larger molecule weight (congo red and acid fuchsin) would suffer from

253

greater steric hindrance in comparison with smaller dye molecules (crystal violet and

254

methyl orange) when passing through the membrane, leading to a low permeance.39

Da) (Table S1). These could be explained by the steric hindrance effect.

255

Furthermore, the catechin/chitosan loose NF membrane had relative low rejection

256

for inorganic salts and the sequence was: RNa 2 SO 4 (4.8%) < RNaCl (12.5%) < RMgSO 4

257

(15.8%) < RMgCl 2 (16.2%) (Figure 6c). The relative loose catechin-grafted-chitosan

258

and poly-catechin layer on the HPAN UF membrane surface was responsible for the

259

low salt rejection, which could provide more salt pathways. In addition, a higher

260

rejection for divalent salts was obtained in comparison with monovalent salt due to

261

the combination of Donna effect and steric hindrance.40 The positive charge of the

262

membrane surface offered stronger electrostatic repulsion to Mg2+ than that to Na+,

263

resulting in higher Mg2+ rejection.41 In contrast, it was easier for SO42- to pass

264

through the membrane than that for Cl- owing to great affinity of multivalent anions

265

onto the positively charged membrane surface.42 Besides, the divalent ions (Mg2+ and

266

SO42-) with larger hydrated radius exhibited lower diffusion coefficient compared to

267

monovalent ions (Na+ and Cl-) (Table S4), which would more likely suffer from steric

268

hindrance.43,44 Therefore, the MgCl2 rejection (16.2%) was the highest while the

269

rejection (4.8%) of Na2SO4 was the lowest. Moreover, this catechin/chitosan loose NF 17



270

membrane was used to separate dye/salt mixture solution and this was discussed

271

detailedly in Figure S1 of Supporting Information.

(a)

(b)

18


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(c)

Figure 6. Effects of various molecular weight PEG (a), different dye (b) or inorganic salt solutions (c) on the separation performance. (Test conditions: 0.1 g L-1 PEG solution, 0.1 g L-1 dye aqueous solution, 1 g L-1 inorganic salts solution, 0.2 MPa) 272

Dye Purification Process. The constant volume batch dye purification test was

273

conducted using the catechin/chitosan loose NF membrane at 0.2 MPa and 25 °C. The

274

permeate flux tended to decline with operating time in the first cycle (Figure 7a).

275

Nevertheless, after the addition of 0.15 L water to the retentate, the permeate flux

276

largely recovered and then it declined again with operating time. This variation trend

277

was observed in each cycle of dye purification process. Finally, the permeate flux was

278

reduced to 67 L m-2 h-1 MPa-1 after 4 cycles of dye purification process, which

279

approximately decreased by 6.95% compared to the initial flux of 72 L m-2 h-1 in the

280

first cycle. On the other hand, the dye concentration in the retentate in each cycle

281

increased significantly with operating time as a result of the high rejection for dye,

282

whereas it abruptly decreased to half after 0.15 L DI-water was introduced into the

283

retentate (Figure 7b). In each cycle, the dye rejection increased slightly with operating

284

time due to the formation of the fouling layer consisting of rejected dye molecules,

285

which served as additional resistance toward to permeance.45 After the introduction 19



286

of DI-water, dye rejection declined a little but it was still above 98%. As for dye loss

287

rate, it increased with augmenting the number of dye purification and only reached 10%

288

after 4 cycles.

289

The NaCl concentration in the retentate and NaCl rejection showed reciprocating

290

variation during the entire dye purification process (Figure 7c). In each cycle, the

291

NaCl concentration in the retentate rose smoothly with operating time, whereas it

292

showed a great tendency to decrease during the entire process. On the contrary, the

293

NaCl rejection show opposite variation trend, which was ascribed to the fact that

294

more Na+ could pass through the membrane due to the enhanced “shielding effect” at

295

higher NaCl concentration.46 After the addition of 0.15 L pure water, the NaCl

296

rejection was abruptly improved due to the weakness of “shielding effect” and

297

membrane fouling caused by decreased NaCl and dye concentrations45 and

298

continuous removal of NaCl from dye/salt mixture solution can be achieved. As a

299

result, the removal rate of NaCl was up to 82% during the entire dye purification

300

process. Moreover, the relatively small deviation (less than 10 %) between calculated

301

and experimental NaCl concentrations revealed that the prepared membrane

302

possessed an unconspicuous effect of concentration polarization due to the relatively

303

high NaCl permeation in the range of 0.5 g L-1 to 2.5 g L-1 (Figure 7d).47 Additionally,

304

the good anti-fouling performance was also observed according to the low deviation

305

(

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