Hydrophobic Side Chains Impart Anion Exchange Membranes with

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Hydrophobic side chains impart anion exchange membranes with high monovalent-divalent anion selectivity in electrodialysis Muhammad Irfan, Liang Ge, Yaoming Wang, Zhengjin Yang, and Tongwen Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06426 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Sustainable Chemistry & Engineering

1

Hydrophobic side chains impart anion exchange membranes with high

2

monovalent-divalent anion selectivity in electrodialysis

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Muhammad Irfan,a Liang Ge,a Yaoming Wang,a Zhengjin Yang,a,* Tongwen Xu a,*

4 5

aCAS

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for Energy Materials, School of Chemistry and Material Science, University of Science and

7

Technology of China, No.96 Jinzhai Road, Baohe district, Hefei, Anhui Province, 230026,

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People's Republic of China

Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry

9 10 11

*Corresponding author. Tel: +86(551)-63601587; Fax: +86(551)-63602171

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Email: [email protected] (Tongwen Xu), [email protected] (Zhengjin Yang)

13 14 15 16 17 18 19 20 21 22 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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ABSTRACT

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Developing anion exchange membranes that have high anion selectivity and can avoid

3

membrane swelling is a critical, but challenging target for electrodialysis, which has witnessed

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great success in a wide range of applications including saline wastewaters treatment, and resource

5

recovery. We found in this work that by tuning the hydrophobicity of alkyl spacers which connect

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the positively-charged ammonium groups and polymeric backbones of the anion exchange

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membrane, high anion selectivity and low membrane swelling of anion exchange membrane can

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be achieved. The membranes developed here are capable of discriminating monovalent and

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divalent anions, with a selectivity for Cl-/SO42- reaching up to 13.07, and our results suggest the

10

chain length of the spacers determines the anion selectivity of membrane. We attribute such

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phenomenon to the change in side chain hydrophobicity. This is a particularly interesting finding

12

that could facilitate the development of advanced anion exchange membranes for electrodialysis.

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KEYWORDS: Anion exchange membranes, Electrodialysis, Alkyl spacers, Wastewater treatment,

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Monovalent selectivity, Hydrophobicity

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2 ACS Paragon Plus Environment

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INTRODUCTION

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Electrically driving the movement of anions towards a specific direction is critical in many

3

fields, including saline wastewaters treatment, resource recovery and water treatment/pretreatment.

4

1-3

5

including tannery, pharmaceutical, petroleum and paper mills.4, 5 These saline wastewaters usually

6

comprise high concentrations of sulfates and chlorides.6, 7 The direct discharge of these wastewater

7

without pre-treatment would severely affect the soil, aquatic life, surface and ground water.5, 8 The

8

treatment of these saline effluents has become a major concern nowadays. Conventional

9

techniques such as physicochemical, biological and chemical precipitation technologies were

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previously used to treat these wastewaters.5, 8, 9 Among them, electrodialysis (ED) is considered as

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one of the most sustainable and cost effective method to reduce the harmful effluents and achieve

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the salt resource recycling. The recycled Na2SO4 and NaCl solution can be used to produce NaOH

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and HCl/H2SO4 via bipolar membrane ED.10 Thus, chloride and sulfate separation is the initial

14

step for successful recycling.11

For instance, high saline wastewaters are produced in large amounts in many industries

15

For a mixture of different anions, discriminating one specific anion from the rest will lead to

16

high product purity and improved system efficiency. This is exemplified by ED, a process which

17

takes advantage of the selective transportation of ions (anions or cations) across ion exchange

18

membranes (including anion exchange membrane and cation exchange membrane). Compared

19

with the selective transport of cations across a cation exchange membrane, the discrimination of

20

anions by an anion exchange membrane in ED remains more challenging.

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membrane (AEM) is a critical component in ED and its selectivity determines the separation

22

efficiency of entire system.


12

Anion exchange


1

Balancing water permeation between the dilute compartment and concentrated compartment

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in an ED process is another challenge to be addressed. Anions are hydrated in aqueous solution

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(for example, the hydration numbers of Cl- and SO42- are 8 and 14, respectively 13) and thus, their

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transport across an anion exchange membrane is accompanied by the migration of water, resulting

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sometimes in imbalance in solution volume between different compartments. That will hinder

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further removal of salts and cause pressure increase inside the membrane stack. Hong et al.14

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reported that water electro-transport with hydrated cations in ED is an important phenomenon and

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cannot be ignored. They suggested that IEMs (ion exchange membranes) should be designed to

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mitigate water electro-transport by reducing membrane hydrophilicity or increasing membrane

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cross-linkage. However, in another study it was observed that selective separation properties was

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independent of membrane cross-linkage.15 Ion exchange membranes absorb water due to

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hydrophilicity of ion conducting groups in the membrane matrix. Water uptake certainly lowers

13

the dimensional stability and mechanical strength but it is necessary to maintain sufficient IEC

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(ion exchange capacity) for required ionic flux. The water electro-transport is highly related with

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the membrane swelling of the membranes.16 Previous researchers 17-19 investigated that too many

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hydrophilic material in membrane enhances water electro-transport, membrane swelling and

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reduces membrane selectivity. It has been observed that the inherent hydrophobicity of the matrix

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material would reduce membrane swelling and water electro-transport through the membranes.

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To address the challenges, i.e. improving membrane selectivity and reducing membrane

20

swelling, we propose and demonstrate here by regulating the hydrophobicity of spacers between

21

membrane backbone and positively charged ammonium groups. High membrane selectivity can

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be achieved at high IEC values even the prepared membranes have more water uptakes (32.6-

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71.7%) compared to commercially available monovalent selective ACS membrane (Wu = 24.8%).


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This is based on reasonable analysis on anion properties and its transfer behavior across an anion

2

exchange membrane. Moreover, Cl- and SO42- have hydrated size 0.332 nm and 0.379 nm20 and

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thus it is not practical to discriminate them by size-sieving. However, the hydration energy of Cl-

4

and SO42- is -317 kJ mol-1 and -1000 kJ mol- 115, suggesting that SO42- binds strongly with water.

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In this circumstance, by slightly increasing the hydrophobicity of membranes, the transport of

6

SO42- will be impeded. On the contrary, because of the weak binding with water, Cl- could still

7

permeate after dehydration. The Cl-/SO42- selectivity of the anion exchange membrane is then

8

increased. In the meantime, because of the dehydration of anions, imbalanced water migration can

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thus be mitigated. The previous studies suggested that hydration energy is the most important

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characteristic for ion permeation through the hydrophobic zone in membrane. The hydrophobic

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domains may hinder the permeation of strongly hydrated anions while less hydrated anions can

12

easily permeate through membrane.21 The hydrophobicity in membrane can be induced by the

13

incorporation of long alkyl side chain.22, 23 Moreover, ions with smaller dehydration energy possess

14

soft hydration shells and lose water or rearrange the hydration shell during membrane transport. 24

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To tune the hydrophobicity of membranes, we designed and synthesized a series of

16

quaternized poly (2,6-dimethyl phenylene oxide)s containing alkyl spacers of varied chain length.

17

The alkyl spacers connect the polymer backbone of the membranes and the nitrogen-centered

18

functional groups and its length determines the membrane hydrophobicity among the series. To

19

test our hypothesis, we measured the anion selectivity of the as-prepared AEMs in ED with a

20

Cl−/SO42- mixture. We found that AEM with the longest hydrophobic alkyl spacer shows the

21

highest selectivity implying that enhanced membrane hydrophobicity significantly increased the

22

monovalent anion selectivity. Our study provides an effective and well-controlled strategy for

23

developing monovalent selective AEMs in wastewater treatment.



1

MATERIALS AND METHODS

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Materials:

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2-Aminomethylpyridine, 3-bromo-1-proponal, 6-bromo-1-hexanol, and 11-bromo-1-

4

undecanol were purchased from Aladdin Industrial Corporation (Sanghai, China) and were used

5

without further purification. Formaldehyde, formic acid, sodium hydroxide, anhydrous

6

magnesium sulfate, tetrahydrofuran, sodium chloride, sodium sulfate, dichloromethane, silver

7

nitrate, N-methyl-2-pyrrolidone (NMP) and sodium carbonate were purchased from Sinopharm

8

Chemical Reagent Co. Ltd (Sanghai, China). Bromomethylated (2,6-dimethyl-1,4-phenylene

9

oxide) (BPPO) with benzyl bromide content of 52% (-CH2-Br) was obtained from Tianwei

10

Membrane Corporation Ltd. (Shandong, P.R. China). These chemicals were of analytical grade

11

and used without further purification. Deionised water (DI water) was used throughout the

12

experiments. The Neosepta cation and anion exchange membranes (AMX and CMX respectively

13

from ASTOM, Tokuyama, Co., Japan) and Neosepta ACS (monovalent selective membrane from

14

ASTOM, Tokuyama, Co., Japan) were used in electrodialysis tests.

15

General Characterization Methods:

16

NMR spectra of the samples were collected on a Bruker DMX-300 NMR instrument at 300

17

MHz. The static water contact angles (WCA) of the fabricated membranes were measured with an

18

optical contact angle and interface tension meter, SL200KS (KINO, USA), using the sessile drop

19

method at room temperature. A water drop with the volume of 2 µL was placed on the membrane

20

surface with a micro-syringe in air and the values were recorded after 10 s. Mechanical strength

21

of prepared membrane samples were investigated with a Q 800 dynamic mechanical analyzer

22

(DMA, TA Instruments, USA) in controlled force mode at a stretch rate of 0.5 N min-1 at room

23

temperature. The stress-strain curves were recorded at room temperature. Membrane morphology


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1

was recorded on a field emission scanning electron microscopy (FE-SEM, Sirion 200, FEI

2

Company USA). The SEM samples for surface imaging were prepared by cutting a neat membrane

3

into small pieces, whereas samples for cross-sectional scans were prepared by freezing and

4

breaking the samples in liquid nitrogen. Tapping-mode atomic force microscopy (AFM) analysis

5

was carried out with a vecco dilnnova SPM USA), using microfabricated cantilevers with a

6

constant force of approximately 20 N m-1. The fluorescein isothiocyanate (FITC) analysis was

7

performed for the ACS and QP-11-1 membranes before and after ten sequential cycles of ED

8

operation to investigate fouling on chosen membranes. Fluorescence images were recorded with

9

optical microscope Olympus BX81 (Olympus, Japan) equipped with a halogen lamp, filter U-

10

MNG2 (λexit = 470-490 nm, λemit >510 nm) and camera type DP72. The relative amount of fouling

11

was examined based on the color intensity of fluorescence images.

12

Synthesis of 2-(N,N-Dimethylamino) Methylpyridine:

13

2-(N,N-Dimethylamino) methylpyridine was synthesized according to Figure 1. A mixture

14

of 2-picolylamine (4.33 g, 0.04 mol), formic acid (8.976 g, 0.195 mol) and formaldehyde solution

15

(3.68 g, 0.12 mol) was refluxed under nitrogen for 18 h. The reaction mixture was then cooled and

16

treated with 2 M NaOH solution to liberate the free base.25 The resulting solution was extracted

17

by methylene chloride and the organic phase was dried over MgSO4. Concentrating with a rotary

18

evaporator yields the product, as a dark brown oil (3.05 g, 56%). 1H NMR (CDCl3, 400 MHz,

19

Figure 2a): δ 2.30 (s, 6H), 3.58 (s, 2H), 7.22-6.99 (m, 1H), 7.38 (d, J = 7.8 Hz, 1H), 7.68-7.56 (m,

20

1H), 8.56 (d, J = 4.20 Hz, 1H).

21

Synthesis of QPP, QHP and QUP:

22

A typical synthetic procedure for QPP, QHP or QUP was described as follows (Figure 1).

23

A mixture of 2-(N,N-Dimethylamino) methylpyridine (2.72 g, 0.02 mol) and 3-bromo-1-proponal



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(2.03 g, 0.02 mol) in tetrahydrofuran (THF, 50 mL) was heated at 60 oC for 24 h. The reaction

2

mixture was then concentrated by a rotary evaporator and the resultant product was dried in oven,

3

yielding 3.4 g of QPP (87 % yield). Similarly, QHP and QUP were synthesized by reacting 2-

4

(N,N-Dimethylamino) methylpyridine with 6-bromo-1-hexanol and 11-bromo-1-undecanol in

5

THF, respectively. 1H

6

NMR of QPP (400 MHz, CDCl3, Figure 2b) δ 8.66 (dd, J = 15.40, 4.20 Hz, 1H), 8.07

7

(s, 1H), 7.86 (d, 1H), 7.41 (s, 1H), 4.95 (d, 2H), 4.47 (d, 1H), 3.94 (d, 2H), 3.72 (,d,2H), 3.43-3.23

8

(m, 6H), 3.02-2.84 (m, 2H).

9

QHP (2.85 g, 60%), 1H NMR (400 MHz, DMSO, Figure 2c) δ 8.67 (s, 1H), 7.96 (s, 1H),

10

7.77 (s, 1H), 7.54 (s, 1H), 4.63 (d, 2H), 3.62-3.23 (m, 4H), 3.29-2.88 (m, 5H), 2.75 (s, 1H), 1.89

11

(d, 2H), 1.58-1.17 (m, 6H).

12

QUP (2.83 g, 46%), 1H NMR (400 MHz, CDCl3, Figure 2d) δ 8.54 (,d, 1H), 8.10 (s, 1H),

13

7.75 (s, 1H), 7.25 (s, 1H), 4.95 (s, 1H), 3.60 (d, 2H), 3.61-3.23 (m, 6H), 1.75 (d, 2H), 1.46 (d, 2H),

14

1.40 – 1.11 (m, 18H).

15

(a)

16

NH2

N

Fomaldehyde Formic acid

N

N

17 18

(b) N

N

HO

n

Br

THF N

19

N

n

OH

20 (c)

O

21

O

x

y

N

N

O

OH 11

Br

22

O

x

y

Br

Br

N (a) n=3, QP-P3-x (b) n=6, QP-P6-x (c) n=11, QP-P11-x


N OH

n

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1

Figure 1. Synthesis of (a) 2-(N,N-Dimethylamino) methylpyridine, (b) QnP (n=3, QPP; n=6,

2

QHP; n=11, QUP) and (c) QP-Pn-x AEMs

3 4

Figure 2.1H NMR spectra of (a) 2-(N,N-Dimethylamino methylpyridine), (b) QPP, (c) QHP, and

5

(d) QUP

6 7

Membrane Preparation:

8

To a NMP solution of BPPO (10 wt%) was added QPP, QHP or QUP. The resulting

9

mixture was stirred at 50 oC for 24 h. The obtained ionic polymer solutions were then cast on clean

10

glass plates and dried at 60 oC for 24 h to evaporate the solvents. The resulting membranes were



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1

peeled off from the glass plates and converted to the Cl– form prior to further characterizations

2

according to our previous study. 26 The synthesized membranes were referred to as QP-Pn-x, where

3

n denotes the length of the alkyl spacer. IEC values of these membranes were tuned by varying the

4

amount of QPP, QHP or QUP and the fabricated membranes were designated as QP-Pn-x, where

5

variable x was numbered according to the ranking of IEC values. Details of these membrane

6

samples are summarized in Table 1.

7

Membrane Water Uptake, Swelling, Ion Exchange Capacity and Fixed Charge

8

Concentration:

9

Water uptake (WU) of the QP-Pn-x membrane samples was measured to investigate the

10

hydrophobicity of the membranes. WU of the as-prepared membrane was measured according to

11

the difference in mass of wet membrane sample and dried membrane sample. First, the as-prepared

12

membrane samples were dried in oven at 60 oC and weighed accurately. Then, these membranes

13

were immersed in deionized water for 24h at room temperature and were weighed again after

14

gently removing the surface water with a piece of tissue paper. WU was calculated according to

15

the following equation.27

16

𝑊𝑈 =

(𝑊𝑤 ― 𝑊𝑑 ) × 100%

(1)

𝑊𝑑

17

where Wd and Ww represent the mass of dry and wet membrane samples, respectively.

18

Linear swelling ratios (LSR) of the prepared membrane samples were measured as follows:

19

The membrane sample was cut into 2 × 2 cm2 piece and its length was measured in dry state (Ld).

20

Then, it was placed in de-ionized water at room temperature for 24 h. After that, the membrane

21

sample was taken out and excess water was removed with tissue paper. Length of hydrated

22

membrane (Lw) was measure and LSR was determined by using Eq. (2). 28

23

𝐿SR =

(𝐿𝑤 ― 𝐿𝑑 ) × 100% 𝐿𝑑


(2)

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1

Mohr’s method was utilized to measure the ion exchange capacity (IEC) of the prepared

2

membrane samples. Briefly, dry membrane samples were weighed and placed in 1.0 M NaCl

3

solution for 24 h at room temperature. Then, the membrane samples were thoroughly washed with

4

DI water and soaked in 0.5 M Na2SO4 solution for 24 h to release Cl- ions. IEC was measured

5

according to the released amount of Cl-, which was titrated with 0.05 M AgNO3 using K2CrO4 as

6

an indicator. The IEC (in mmol g-1) was calculated according to the following equation. 29

7

IEC =

V(AgN𝑂3) C (AgN𝑂3)

(3)

W𝑑

8

where, V(AgNO3), C(AgNO3) and Wd represent the consumed volume of AgNO3 during titration,

9

the concentration of the AgNO3 solution and the dry membrane mass, respectively.

10

The ion exchange capacity and water uptake were used to determine the membrane fixed charge

11

concentration as given below. 30

12

𝐶𝑚 𝐴 ≈

IEC × 𝜌𝑤

(4)

𝑊𝑈

13

IEC and WU are calculated using eqtuations (3) and (1) respectively. The 𝐶𝑚 𝐴 is expressed

14

as moles of fixed charge per liter of sorbed water and 𝜌𝑤 is density of water in grams per mL. The

15

values of 𝐶𝑚 𝐴 for fabricated membranes are presented in Figure 6 (b).

16 17

Membrane Transport Number and Current-Voltage Curve:

18

We obtained the membrane transport number in a two-compartment cell by measuring the

19

membrane potential. The membrane sample was placed in the cell and NaCl concentration gradient

20

across this membrane was set at 5 (C1=0.05, C2=0.01). A multimeter (VC 890C+, Shenzhen Victor

21

Hi-Tech. Ltd, China) connected to Ag/AgCl electrodes was used to measure the potential across

22

the membrane. The solution was circulated by a peristaltic pump to minimize the concentration



1

and polarization. The transport number (𝑡𝑖) was determined by using the following modified Nernst

2

equation. 31

3

𝐸𝑚 =

𝑎1

RT

( ) zF 2𝑡𝑖 ― 1 ln (𝑎2)

(5)

4

Where, 𝐸𝑚 is the membrane potential, R is the universal gas constant, F is Faraday constant,

5

T is temperature, z is the charge number of counter ions, 𝑎1/𝑎2 is the mean activity ratio of the

6

electrolyte solutions. In calculation, we used concentration ratio instead of activity due to the low

7

concentration.

8

The current-voltage (I-V) curves were measured at room temperature in a four-

9

compartment cell (Figure 3) according to the previous studies. 32, 33 0.3M Na2SO4 solution was fed

10

in the electrode compartments, while mixed solution of 0.05M NaCl and 0.05 M Na2SO4 was fed

11

into the two central compartments on the both sides of investigated membrane. The solutions in

12

all the compartments were circulated at a flow rate of 10 mL/min using peristaltic pumps (BT600L-

13

2*YZ15, Baoding Lead Fluid Technology Co., Ltd., China). I-V measurements were performed

14

using four-electrode mode under constant current density supplied by the working electrodes

15

which were made of titanium coated stainless steel sheets. The tips of reference electrodes

16

(Ag/AgCl) were brought close to the testing membrane surfaces to record the voltage drop across

17

the membrane, which has an effective area of 7.07 cm2. A stepwise increased current was supplied

18

by a direct current power supply (Beijing Hanshengpuyuan Technology Co., Ltd) between two

19

electrodes made of titanium coated with ruthenium. The corresponding steady-state voltage drop

20

across the membrane was measured by a digital multimeter (VICTOR, VC890C+, VICTOR®

21

YITENSENTM) coupled with Ag/AgCl electrodes located very close to the investigated membrane

22

surface. All membrane samples were equilibrated in the test solution for 24 h before measurement.


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1 2

Figure 3. Schematic illustration for four compartment cell

3 4

Electrodialysis Experiment:

5

The separation of Cl-/SO42- were conducted to investigate the ion flux and membrane

6

selectivity in an ED cell, which has four compartments (cathode compartment, diluted

7

compartment, concentrated compartment and anode compartment), separated by two pieces of

8

commercial CMX membranes and one piece of the as-prepared membrane, as shown in Figure 4.

9

The membranes have an effective area of 7.07 cm2. The anode and the cathode were made of

10

titanium-coated ruthenium.

11

200 mL of 0.3 mol L-1 Na2SO4 solution was fed to the electrode compartments, while the

12

diluted compartment was fed with 200 mL of mixed solution containing 0.05 mol L-1 NaCl and

13

0.05 mol L-1 Na2SO4. The concentrated compartment was fed with 200 mL of 0.05 M Na2CO3

14

solution to be initially free from investigated anions concentration and the same strategy was 13 ACS Paragon Plus Environment


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1

adopted in previous study.34 Solutions in the compartments were circulated by peristaltic pumps

2

at a flow rate of 56 mL min-1. The experiment was conducted at a current density of 3.54 mA cm-

3

2.

4

samples was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES,

5

Optima 7300 DV, USA), while the Cl- ion concentration was measured by titration according to

6

Mohr’s method. Three parallel measurements were carried out to ensure reproducible data. Anion

7

flux across the membrane and the membrane selectivity were determined from the concentration

8

change of anions in the concentrated compartment after electrodialysis for a period of time, t.35

Samples were collected from the concentrated compartment and SO42- ions content in these

9 10

𝐽𝐶𝑙 ― =

(𝐶𝑡 ― 𝐶𝑜)𝑉

(6)

𝐴𝑚𝑡

11 12

𝐽𝑆𝑂4―2 =

(𝐶𝑡 ― 𝐶𝑜)𝑉

(7)

𝐴𝑚𝑡

13

Where, 𝐶𝑜 and 𝐶𝑡 represent anion concentration at time 0 and t in the concentrated

14

compartments, v is the volume of the solution in the concentrated compartment, 𝐴𝑚 is membrane

15

effective area (7.07 cm2), and t is time. The membrane selectivity (𝑃𝐶𝑙 ) is determined by the 𝑆𝑂24 ―

16

following equation. 17, 36

―

𝑡 ― 𝐶𝑙

17

― 𝑃𝐶𝑙 𝑆𝑂24 ―

=𝐶

𝐶𝑙 ―

𝑡 2― 𝑆𝑂4 𝐶 2― 𝑆𝑂4

𝐽 ― 𝐶𝑙

=

𝐶 ― 𝐶𝑙

𝐽 2― 𝑆𝑂4

=

𝐽𝐶𝑙 ― . 𝐶𝑆𝑂2 ― 4

𝐽𝑆𝑂2 ― . 𝐶𝐶𝑙 ―

(8)

4

𝐶 2― 𝑆𝑂4

18

Where ti is the transport number of ions, Ji is the flux of ions through membrane expressed

19

in mol/m2 s and C is the concentration of ions in the concentrated compartment, expressed in mol/L.

20

The effect of CO2 absorption on membrane performance was reduced by sealing the vessels at


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1

upstream and downstream solutions. Before running the solutions in ED stack, the deionized water

2

was circulated in the compartments by peristaltic pumps at a flow rate of 56 mL min-1. Thus, air

3

bubbles were removed from the system that developed the continuous flow of solution in ED stack.

4

In addition, the overall concentrations of NaCl and Na2SO4 suggest that salt permeability and

5

sorption are not seriously affected by the ambient CO2.37

6

Figure 4. Proposed schematic experimental setup for ED test

7 8

RESULTS AND DISCUSSION

9

NMR Analysis:

10

The anion exchange membranes were prepared by quaternizing BPPO with QPP, QHP or

11

QUP in a nucleophilic substitution reaction and the corresponding polymer products were analyzed

12

by 1H-NMR, as shown in Figure 5. The new emerging peaks at 3.5-1.9 ppm were assigned to the

13

propyl side chain protons of QPP showing the successful grafting of QPP onto the PPO backbone

14

(Figure 5b). In the case of hexyl side chain of QHP, these peaks were shifted to the up-field (3.5-

15

1.4 ppm) as shown in Figure 5c. Similarly, the more up-field chemical shift (3.5-1.2) was observed

16

during the introduction of undecyl side chain of QUP in PPO (Figure 5d). Hence, we can



1

differentiate the introduction of these amine in BPPO on the basis of characteristic aliphatic signals

2

in the 1H-NMR spectra. Furthermore, the success in synthesis of AEMs is further confirmed by

3

emerging the new peaks at δ = 7.2-8.8 ppm are assigned to the instigation of pyridine protons.

4 5 6

Figure 5. 1H NMR spectra of (a) BPPO, (b) QP-P3-4 , (c) QP-P6-4 and (d) QP-P11-3 AEMs IEC, WU and LSR:

7

IEC and WU are crucial properties for AEMs in ED. High IEC can sometimes lead to high

8

water content, resulting in poor dimensional stability and unwanted membrane swelling and

9

thereby poor selectivity. IEC values of the as-prepared membranes were tuned by varying the

10

molar ratio of QPP, QHP or QUP to benzyl bromide groups in BPPO, as shown in Figure 6a and 16 ACS Paragon Plus Environment

Page 16 of 48

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1

Table 1. The LSR was measured as well to investigate the dimensional stability of corresponding

2

membranes. WU for membranes with the same pendant chain structure is increased with an

3

increase in IEC value, while for membranes with different alkyl chain structure, the increase in

4

alkyl chain length suppressed the water adsorption of the as-prepared membranes provided the

5

same IEC value is maintained. For instance, if the IEC value was the same, the QP-P11 membrane

6

with the longest side chain, displayed the lowest water content values, implying long hydrophobic

7

alkyl side chains restrict the water adsorption. We attribute this to the increase in hydrophobicity

8

induced by the insertion of long alkyl side chain.

9

Ion sorption in charged polymers is usually governed by the concentration of fixed charge

10

groups and the water content of the polymer. Thus, it needs the knowledge of polymer fixed charge

11

concentration. The membrane fixed charge concentration played an important role during its

12

performance, and may determine from membrane water uptake and ion exchange capacity. 30 The

13

fixed charge concentration 𝐶𝑚 𝐴 is very important to keep balance between IEC and WU for ion

14

flux. Herein, fixed charge concentration 𝐶𝑚 𝐴 of the prepared AEMs was investigated (Figure 6b).

15

Considering the values of 𝐶𝑚 𝐴 , QP/P3-2, QP/P6-2 and QP/P11-2 were selected for comparison of

16

membrane performance (Figure 10a) because of nearly same 𝐶𝑚 𝐴 values.

17

This is proved by water contact angle measurements. A dramatic increase in water contact

18

angle was observed for QP-P11 membrane with same 𝐶𝑚 𝐴 value and longest alkyl chain that

19

suppress the water absorption and promote hydrophobicity (Figure 6b). The increase in water

20

contact angle also reduces membrane swelling (Table 1). This is due to the increase in

21

hydrophobicity caused by insertion of long alkyl side chain that improved membrane swelling

22

resistance and leads better dimensional stability. Table 1 showed the reduction in membrane

23

swelling with increased in hydrophobicity caused by insertion of long alkyl side chain in BPPO 17 ACS Paragon Plus Environment


Page 18 of 48

1

polymer matrix. Similarly, figures 9 and 10a demonstrated increased in membrane selectivity with

2

the increase in membrane hydrophobicity.

3 4

Figure 6. (a) IEC and WU (b) fixed charge concentration 𝐶𝑚 𝐴 and contact angles of as-prepared

5

AEMs

6 7

Table 1. Composition, LSR, transport number, and mechanical stability of QP /Pn-x AEMs Membranes

Molar ratio of

LSR

amine to BPPO

Transport

TS

Number

(MPa)

Eb (%)

IEC (mmol/g)

QP/P3-1

1/3

12.2

0.89

28.76

10.32

0.48

QP/P3-2

2/3

18.7

0.91

26.18

21.71

0.95

QP/P3-3

4/5

25.4

0.92

23.05

17.94

1.38

QP/P3-4

1

32.5

0.94

23.62

25.27

1.61

QP/P6-1

1/3

11.7

0.92

26.66

15.47

0.40

QP/P6-2

2/3

16.3

0.94

26.09

20.13

0.79

QP/P6-3

6/7

21.8

0.95

11.11

14.11

1.36

QP/P6-4

1

26.9

0.96

9.89

46.91

1.49

QP/P11-1

1/3

9.2

0.93

26.63

70.54

0.33

QP/P11-2

2/3

12.9

0.96

16.88

14.94

0.68


Page 19 of 48 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


QP/P11-3

1

19.2

0.97

14.28

38.29

1.35

1 2

Mechanical Strength:

3

Mechanical stability is among the most important properties for membranes to withstand

4

under the stress condition in ED stack imposed by the screws and nuts. 34, 38 Thus, the mechanical

5

strength of the prepared AEMs was evaluated in fully hydrated state at room temperature (Table

6

1). The tensile stresses of fabricated AEMs ranges 9.89-28.76 MPa and the elongation at break is

7

between 10.32% and 70.54%. The tensile strength shows a decreasing trend with increase in length

8

of alkyl side chain due to incompatibility of main chain and alkyl side chain.22 Increasing IEC

9

value of membranes with the same alkyl side chain results in decreased tensile strength.34 The

10

elongation at break shows an increasing trend with the increase in length of alkyl side chain,

11

implying an increase in membrane flexibility, which benefits its application in ED.22, 23 The QP/P3-

12

1 AEM has the highest tensile strength value of 28.76 MPa among the prepared membranes. This

13

TS value is much better than those of previous reported membranes SBQAPPO AEMs (2.6-6.4

14

MPa) 34 and QPPO membranes (8.3-18.3 MPa) 39 used in ED. Hence, it suggests that the tensile

15

strength of as-prepared membranes is sufficient for ED applications.

16

SEM and AFM Morphology:

17

Representative membranes including QP-P3-3, QP-P6-3 and QP-P11-3 were chosen for

18

morphological studies. The cross-sectional and surface images of the selected membranes were

19

observed by SEM (Figure 7). These membranes were prepared at identical conditions. SEM

20

images show that there are no cracks on the membrane surface, and the surface as well as cross-

21

section images of all the membranes are homogeneous and dense in nature.



1

It is assumed that the incorporation of long alkyl side-chains into AEMs can promote the

2

microphase separation between hydrophilic and hydrophobic domains. To validate this, atomic

3

force microscopy (AFM) analysis were performed for the selected membranes and the results are

4

shown in (Figures 7g,h,i). The darker regions show the hydrophilic domains while the brighter

5

regions represent the hydrophobic domains.40 We found that membrane QP-P11-3 showed more

6

distinct and interconnected microphase separation morphology than the rest. This microphase

7

separation is due to the insertion of long alkyl side chain connected to QA (quaternary ammonium)

8

group. In addition, an increase in immiscibility promotes microphase separation and facilitates the

9

formation of larger and interconnected hydrophilic ionic channels.22 The connectivity of the

10

hydrophilic domain played an important role in ED performance for anions permeation. When we

11

compare the AFM morphologies of the membrane samples, we conclude that the hydrophobicity

12

and interconnected ionic channels in QP-P11-3 and QP-P6-3 can enhance the flux of Cl− ions as

13

compared to that of SO42- ions. Moreover, the difference in Gibbs hydration energy also played an

14

important role as less hydrated anion (Cl−) easily permeates while it is hard for strongly hydrated

15

anion (SO42-) to permeate. 21

16

Transport Number:

17

Transport number is an important parameter in ED to interpret the fraction of total current

18

carried by counter ions passing through membrane. Investigating the transport number of

19

fabricated membranes for Cl-/SO42- system is very important to examine the transportation of

20

chloride ions. The influence of junction potentials on anion exchange membrane potential

21

measurements is usually negligible for Ag-AgCl electrodes 41, and the same procedure was utilized

22

by many previous studies to measure the transport numbers. 31, 34, 38 The transport numbers of the

23

prepared membranes are shown in Table 1. We found transport numbers of Cl- ions were increased


Page 20 of 48

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1

with an increase in IEC or an increase in length of alkyl side chain. The increase in transport

2

number of Cl- ions with an IEC increase is due to the increasing amount of quaternary ammonium

3

groups in membrane matrix.42 Whereas, the increase in transport number of Cl- ions with an

4

increase in length of alkyl side chain is due to the hydrophobic nature of chloride ions, which can

5

also permeate through hydrophobic regions of the as-prepared membranes.43 Hence, the results

6

suggest that membranes with higher IEC and longer alky side chain length tend to selectively

7

transport Cl- ions.

8

9 10

Figure 7. Surface SEM images (a, b, c), cross sectional SEM images (d, e, f), and AFM phase

11

images (g, h, i) of QP-P3-3, QP-P6-3, and QP-P11-3 AEMs respectively



1

Current-Voltage Curve:

2

Current voltage curves (I-V curves) of QP-P3-2, QP-P6-2, QP-P11-2 and ACS membranes

3

are measured (Figure 8). The QP-P3-2, QP-P6-2, and QP-P11-2 were selected and they have

4

similar fixed charge concentration. Typically, the ion exchange membrane exhibits three regions.

5

At low current densities, the potential drop across the membrane was linearly increased with

6

current (ohimic region). When, the current density reached to diffusion-limited value, ion

7

concentration near the interface decreases rapidly causing an increase in resistance. Thus, a large

8

potential drop that gives rise to a smaller slope or a plateau. The diffusion-limited value is known

9

as limiting current density (Ilim). The value of (Ilim) is determined by the intersection of linear

10

region and plateau region, while membrane resistance is determined by the inverted slope of linear

11

region. These are very important parameters for the smooth and efficient operation of ED. Further

12

increase in current density provides less-dramatic changes in membrane voltage drop. This is due

13

to water splitting or electroconvection that brings additional ions to the membrane interface and

14

adversely affect the membrane selectivity. 44-47

15

The values of limiting current density, resistance in linear region and plateau length (∆E)

16

for the as-prepared membranes and ACS are summarized in Table 2. Results suggest that the

17

increase in alkyl side chain length increased limiting current density and reduced membrane

18

resistance. It is interesting to note that limiting current density of membranes QP-P11-2 and QP-

19

P6-2 were significantly higher as compared to ACS membrane. The high limiting current density

20

and low area resistance combined to suggest that the as-prepared membranes have better

21

application potential for ED than ACS membrane.


Page 22 of 48

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1 2

Figure 8. IV curves of QP-P3-2, QP-P6-2, QP-P11-2 and ACS membranes

3 4

Table 2. ilm, ∆E, and Ror of ACS and representative membranes Membranes

ilm (mA cm-2)

∆E (V)

Ror (Ω cm2)

QP-P3-2

7.3

0.58

51.77

QP-P6-2

17.9

0.95

67.60

QP-P11-2

21.7

0.83

53.14

ACS

17.60

0.97

54.26

5 6

Mono/Multi-valent Anion Selectivity:

7

Effect of IEC on Selectivity:

8

Removal of sulfate and chloride anions from high saline wastewater is considered a crucial

9

pretreatment. 8 Thus, ion flux and selectivity of the as-prepared membranes were tested for chloride

10

and sulfate anions. Three different kinds of AEMs were fabricated by changing the length of side

11

chain. To investigate the effect of IEC on each type of membrane, AEMs with different IEC were 23 ACS Paragon Plus Environment


1

prepared for each kind of membrane. The as-prepared membranes were tested for ED in a Cl−/SO42-

2

system. ACS membrane was also evaluated for comparison (Figure 9). The three kinds of

3

membranes showed improved Cl- and SO42- permeability with increasing ion exchange capacity

4

due to the electrostatic attraction force between anions and fixed positive charged groups.

5

Typically, the Cl- ions have more permeability compared to SO42- ions, as SO42- ion has a larger

6

hydration energy compared to Cl- ion. 48 However, with the increase in ion exchange capacity of

7

as-prepared membranes, the permeability of SO42- ions was increased compared to Cl- ions due to

8

the strong electrostatic attraction between the high densities of quaternary ammonium (QA) groups

9

and divalent ions as shown in Figure 9.

10

The general principle of mono/divalent ionic flux and separation attributes to two

11

mechanisms, i.e., steric-hindrance and electrostatics repulsion.49 The higher hydration number

12

causes the larger hydrated ionic size which not only influence ED process but also significantly

13

affect the ionic selectivity of ion exchange membrane in view of steric hindrance effect. Moreover,

14

steric hindrance effect made it more difficult to penetrate the organic ions through coated

15

membranes in ED process.17 Moreover, the separation factor is mostly depended on the

16

hydrophilicity of membrane, charge on membrane surface and the hydrated size of the ions. 21 To

17

enhance the ionic flux and reduce membrane resistance, the synthesized cationic moieties such as

18

QPP, QHP and QUP were introduced in the BPPO polymer matrix to fabricate QP-Pn-x AEMs.

19

These hydrophilic precursors provide additional water channels which facilitate ions transport.

20

Consequently, different ionic flux of as-prepared AEMs was obtained with various quantity of

21

synthesized cationic moiety for each type of membrane. This is associated to the increased ion

22

exchange content in membrane matrix that increased the rate of ionic diffusion in the membrane

23

matrix. Consequently the ionic flux were increased in the membrane matrix. The similar results


Page 24 of 48

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32, 38, 50

1

were observed by the previous researchers

for their fabricated membranes that ionic

2

diffusion in the membrane matrix were increased with the increased in IEC values although the

3

elctrodialysis tests were performed under a constant current mode. One of the recent studies He et

4

al., 34 investigated the mono/divalent ions separation for the same membranes having different IEC

5

values under a constant electric current condition. During that study, it was observed that the

6

reported membrane SBQAPPO-2 showed higher ionic flux and low selectivity compared to

7

SBQAPPO-1 membrane due to higher IEC value, greater transport number and higher swelling

8

ratio. Considering those previous studies, our prepared membranes also showed the similar

9

behaviors for ions separation in electrodialysis. On the other hand, too many hydrophilic groups

10

cause the failure of perm-selectivity.51 Similarly, the ionic flux and perm-selectivity can be

11

controlled through migration speed of the respective ions and affinity of ions with membrane.21, 52,

12

53

13

synthesized cationic moiety content, fixed charge concentration and hydrophobicity of fabricated

14

membranes (Figure 9). Previous studies15, 17, 36, 48, 54, 55 were performed to separate the chloride and

15

sulfate by ED under the constant electric current condition for a series of fabricated membranes.

16

Each study observed different ionic flux and permselectivity although they have performed the ED

17

tests at constant electric current. One of the recent studies

18

commercially available monovalent selective membrane ACS. Therefore, we also compared our

19

results with ACS membrane (Figure 9). The selectivity of all the three kind’s membranes were

20

decreased with increased in ion exchange capacity. The membranes QP-P11 showed more

21

selectivity compared to the other two kinds of membranes. This is due to the longer alkyl side

22

chain that drive membrane self-assembly, forming interconnected hydrophilic channels and

23

provide highway for anion transport.22, 56

Thus, the flux and selectivity of ions were varied across the membrane due to the difference in

17


has compared their results with


1

The solute concentration in the membrane is associated to the concentration in fluid

2

adjacent to membrane surface by assuming thermodynamic equilibrium between membrane

3

material and fluid at the membrane fluid interface. The direction and driving force of mass transfer

4

through diffusion is governed by thermodynamic with typical limitation of equilibrium.57 The

5

various anion exchange groups have significant impact on membrane selectivity. The membrane

6

performance is related to the hydration energy of anions and hydrophilic/hydrophobic atmosphere

7

around the ion exchange sites in membrane. The ionized fixed groups within membrane are

8

surrounded by water and have hydration shells through strong coordination of water molecules.

9

These hydration shells reduce the possibility of Coulombic interaction between fixed ionic groups

10

and mobile ions. Thus, affecting the membrane selectivity toward the desired ions.56, 58 Similarly,

11

Yaroslavtsev et al. have performed calculation according to quantum-mechanics and predicated a

12

substantially weakened in Coulombic interaction between mobile ions and fixed ionic groups when

13

water molecules are present.59 Thus, interaction between fixed groups and counter-ions are

14

affected by both hydration effect of ions and columbic force which substantially affected the

15

membrane performance.60 Consequently, ionic mobility and thermodynamic selectivity of as-

16

prepared membranes for chloride/sulfate are adversely affected.


Page 26 of 48

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1 2

Figure 9. The ion flux and permselectivity of (a) QP-P3 (b) QP-P6 and (c) QP-P11 and ACS

3

AEMs

4

The previous study

61

observed the diffusion selectivity of chloride and sulfate based on

5

concentration gradient and ionic size. However in ED, the permeability of ions across the

6

membrane is driven by electric field, and the ionic transport is negligible due to concentration

7

gradient.52, 60 Moreover, the transport of anions relative to chloride ions is not decided by the

8

difference of hydrated ion size but by the difference of Gibbs hydration energies as observed in

9

the previous studies. 21, 62

10



Page 28 of 48

1

Table 3. An overview of monovalent selective AEMs operational conditions and

2

permselectivity for Cl− and SO42- reported in recent literature Membranes

Operating conditions

Current

Permselecti

Density

vity,𝑷𝑪𝒍 𝑺𝑶𝟐𝟒 ―

Ref.

―

(mA/cm2) 0.01 M Na2SO4, 0.01 M NaCl

2.0

1.8

54

PDA modified AEMs

0.02 M Na2SO4, 0.02 M NaCl

5.0

2.2

58

Multilayer

0.02 M Na2SO4, 0.02 M NaCl

5.0

2.9

36

PEI-QPPO membrane

0.05 M Na2SO4, 0.05 M NaCl

5.1

4.3

63

LbL of PSS/PDDA

0.01 M Na2SO4, 0.01 M NaCl

1.6

11.5

17

PEI multilayers AEM

0.05 M Na2SO4, 0.05 M NaCl

10.0

2.9

15

anion 0.05 M Na2SO4, 0.05 M NaCl

10.0

1.8

64

PSS-top-layer membrane

polyelectrolyte modified AEM

Modified

exchange membrane QP-P3-1

0.05 M Na2SO4, 0.05 M NaCl

3.5

6.7+0.3

This work

QP-P6-1

0.05 M Na2SO4, 0.05 M NaCl

3.5

9.3+0.5

This work

QP-P11-1

0.05 M Na2SO4, 0.05 M NaCl

3.5

13.1+0.5

This work

3 4

Effect of Hydrophobic Side Chains on Selectivity:

5

To investigate the effect of hydrophilic side chains on membrane performance, three

6

membranes of different alkyl side chain length and nearly similar 𝐶𝑚 𝐴 values were selected and

7

tested for chloride and sulfate anion separation, as shown in Figure 10a. As we proposed, that

8

selective separation of Cl− and SO42- was mostly based on membrane fixed charge concentration

9

and the difference in anion dehydration. We found that Cl−/SO42- selectivity is increased with the

10

increase in chain length of the alkyl spacer although 𝐶𝑚 𝐴 values were nearly same for the

11

membranes. We attribute this to the increased hydrophobicity, which hinders the transport of SO4228 ACS Paragon Plus Environment

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1

ion because it can bind water much stronger than Cl− ion does. 15, 21 Permeation of hydrophilic ions

2

is difficult through hydrophobic membrane.43 In this case, dehydration dominates the transport

3

process. As a consequence, selectivity of as-prepared membranes was increased with an increase

4

in side chain hydrophobicity. Consequently, higher selectivity for QP-P11-2 membrane was

5

observed compared to QP-P6-2 membrane although they have nearly same 𝐶𝑚 𝐴 values. Similarly,

6

QP-P11-2 membrane also showed higher selectivity than QP-P3-2 and ACS membranes (Figures

7

9c and 10a).

8

The performance of as-prepared membranes is better than that of previously reported

9

monovalent selective AEMs, as shown in Table 3. This increase in selectivity was due to the

10

increase in permeation of hydrophobic anions (Cl−), which was facilitated by the incorporation of

11

long hydrophobic side chains. In meantime, the permeation of hydrophilic anions (SO42-) became

12

difficult which lead to higher selectivity.43 Thus, increasing the hydrophobicity is an effective

13

method to manipulate the selective discrimination of monovalent and divalent anions in an AEM.

14

Operational Stability:

15

The operational stability for QP-P11-1 and commercially available monovalent selective

16

ACS membrane was investigated in ten sequential cycles of ED and shown in Figure 10b. For

17

operational stability, each cycle was run for 60 minutes as reported previously.31, 33 The decrease

18

in selectivity for ACS is slightly obvious as compared to the as-prepared membrane (QP-P11-1).

19

To investigate the decrease in selectivity with time, the FITC analysis was performed and the

20

results are shown in Figures 10c-f. Figures 10c and 10d showed the ACS and QP-P11-1

21

membranes respectively before the ten sequential cycles of ED operation, while Figures 10e and

22

10f showed the ACS and QP-P11-1 membranes respectively after ten sequential cycles of ED. The

23

intense fluorescence (appearing black) was observed for ACS and QP-P11-1 membranes in



1

Figures 10e and 10f respectively, suggesting the fouling on the surface of membranes after ten

2

sequential cycles of ED operation. ACS membrane shows substantially increased fouling

3

compared to QP-P11-1 membrane that affected its ionic selectivity (Figures 10e-f). These results

4

suggest that the synthesized membrane can be used in long-term ED operation.

5 6

Figure 10. (a) Comparison of AEMs with different alkyl chain of nearly same 𝐶𝑚 𝐴 values (b)

7

operational cycles of QP-P11-1 and ACS membranes, and fluorescence microscopy for (c) ACS


Page 30 of 48

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1

membrane before cycle operation (d) QP-P11-1 membrane before cycle operation (e) ACS after

2

ten sequential cycles of ED (f) QP-P11-1 after ten sequential cycles of ED

3

CONCLUSIONS

4

Anion exchange membranes with tunable side chain hydrophobicity were fabricated via a

5

facile and efficient method. The membranes displayed high selectivity for Cl−/SO42- mixture. Our

6

results proved that selectivity for Cl−/ SO42- system can be increased with an increase in the chain

7

length of alkyl side chain. We attribute this to the change in membrane hydrophobicity and the

8

difference in dehydration of Cl− and SO4−2. QP-P11-1 showed higher selectivity, improved ion flux

9

and low water content compared to commercial monovalent anion selective ACS membrane. The

10

fabricated membranes show a reasonable long term stability for commercial applications. The

11

methodology developed here is versatile and can be utilized to fabricate AEMs from many other

12

polymer backbones and varied functional groups. We conclude here that regulating the

13

hydrophobicity of alkyl side chain of an anion exchange membrane is a promising strategy to tune

14

the monovalent-divalent anion selectivity in ED.

15

ACKNOWLEDGEMENTS

16

The research was supported by the National Natural Science Foundation of China (Nos.

17

21490581, 21506200), and K.C. Wong Education Foundation (2016-11). The first author would

18

like to appreciate the CAS-TWAS Presidential Fellowship authorities for the scholarship award.

19

We gratefully acknowledge to Prof. Yanmei Wang (Department of Polymer Science and

20

Engineering) for static water contact angles analysis.

21 22 31 ACS Paragon Plus Environment


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1 2 3

Synopsis

4 5

The hydrophobicity of alkyl side chains provide anion exchange membranes with high

6

monovalent-divalent anions selectivity to treat wastewater by electrodialysis.

7 8

Abstract Graphic

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1

Nomenclature AEM

anion exchange membrane

J

flux (mol/m2 s)

AFM

atomic force microscopy

Ld

length of dry membrane (m)

Am

membrane effective area (m2)

LSR

linear swelling ratio

BPPO

bromomethylated (2,6-dimethyl-1,4phenylene oxide)

Lw

length of hydrated membrane (m)

NMP

N-methyl-2-pyrrolidone

C

concentration (mol/m3)

NMR

nuclear magnetic resonance

𝑪𝒎 𝑨

membrane fixed charge concentration (mol/m3 Kg)

P

membrane selectivity

QA

quaternary ammonium

DI

deionised water

R

universal gas constant (J/mol K)

DMA

dynamic mechanical analyzer

Ror

membrane resistance (Ω m2)

Eb

elongation at break

T

temperature (K)

ED

electrodialysis

t

time (s)

Em

membrane potential (V)

THF

tetrahydrofuran

F

faraday constant (C/mol)

ti

transport number

FE-SEM

field emission scanning electron microscopy

TS

tensile strength (Pa)

V

volume (m3)

FITC

fluorescein isothiocyanate

WCA

static water contact angle (o)

ICPAES

inductively coupled plasma atomic emission spectroscopy

Wd

mass of dry membrane (Kg)

WU

water uptake

IEC

ion exchange capacity (mol/Kg)

Ww

mass of wet membrane (Kg)

IEM

ion exchange membrane

z

charge number of counter ions

Ilim

limiting current density (A m-2)

∆E

plateau length (V)

I-V

current-voltage

𝜌𝑤

density of water (Kg/m3)

2



Graphical abstract

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Figure 1 114x73mm (300 x 300 DPI)



Figure 2 99x70mm (300 x 300 DPI)


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Figure 3 272x169mm (120 x 120 DPI)



Figure 4 278x162mm (120 x 120 DPI)


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Figure 5 88x78mm (300 x 300 DPI)



Figure 6


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Figure 7 100x73mm (300 x 300 DPI)



Figure 8 279x215mm (150 x 150 DPI)


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Figure 9



Figure 10 441x486mm (120 x 120 DPI)


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Hydrophobic Side Chains Impart Anion Exchange Membranes with

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