Comb-shaped anion exchange membrane to enhance phosphoric acid purification by electro-electrodialysis

Author’s Accepted Manuscript Comb-shaped anion exchange membrane to enhance phosphoric acid purification by electroelectrodialysis Xiaoling Duan, Cunwen Wang, Tielin Wang, Xiaolin Xie, Xingping Zhou, Yunsheng Ye www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(18)31657-0 https://doi.org/10.1016/j.memsci.2018.11.062 MEMSCI16664

To appear in: Journal of Membrane Science Received date: 16 June 2018 Revised date: 11 October 2018 Accepted date: 24 November 2018 Cite this article as: Xiaoling Duan, Cunwen Wang, Tielin Wang, Xiaolin Xie, Xingping Zhou and Yunsheng Ye, Comb-shaped anion exchange membrane to enhance phosphoric acid purification by electro-electrodialysis, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.11.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.

Comb-shaped anion exchange membrane to enhance phosphoric acid purification by electro-electrodialysis Xiaoling Duan1, Cunwen Wang2, Tielin Wang3, Xiaolin Xie1, Xingping Zhou1*, Yunsheng Ye1* 1

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry

of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China 2

Key Laboratory of Green Chemical Process of Ministry of Education, Wuhan

Institute of Technology, Wuhan 430073, China 3

School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology,

Wuhan 430073, China Correspondence to: XP Zhou (E-mail: [email protected]) and YS Ye (E-mail: [email protected]) Abstract: Comb-shaped polysulfone anion exchange membranes (AEMs) containing various pendant alkyl side chains were synthesized and characterized. Ionic clusters aggregation is observed in transmission electron microscopy (TEM) image and small-angle X-ray scattering (SAXS). The comb-shaped quaternized ammonium polysulfone (Cx-QAPSU) membranes show enhanced dimensional stability and ultra-low membrane area resistance. Notably, the C16-QAPSU with 16 carbon atoms has the best hydrophilic-hydrophobic phase separation structure and lowest membrane area resistance (Rm) of 2.5 Ω·cm2 despite low ion exchange capacity (IEC) of 1.55 mmol/g and swelling ratio of 5.3 %. Moreover, comb-shaped AEMs show much higher tensile strength (TS) compared to QAPSU membrane under the same IEC level. The C16-QAPSU with lowest Rm of 2.5 Ω·cm2 exhibits the highest TS of 16.7 MPa. These results indicate that the long alkyl side chains especially the length of the pendant side chain of Cx-QAPSU up to C16 achieve an electrochemical-mechanical balance of AEMs. In electro-electrodialysis systems, the comb-shaped AEMs exhibit higher purification efficiency of wet phosphoric acid than non-comb-shaped AEMs. Keywords: Anion exchange membrane; Comb-shaped; Electro-electrodialysis; Wet phosphoric acid 1

1 Introduction As an electrochemical membrane separation process, electrodialysis (ED) technology has been proven as an emerging technology to separate, concentrate and purify solution [1-4]. ED mainly divided into conventional electrodialysis (CED), bipolar membrane electrodialysis (BMED), selective electrodialysis (SED) and electro-electrodialysis (EED) [5]. CED is a separation process with conventional cation and anion exchange membranes [6]. Herranz et al. reported the concentration and recovery of sulfuric acid from acid mine drainage by CED [7]. BMED is an important membrane technology that simultaneously recycles specific acid and alkali from their corresponding salt solution without the introduction of any other substances by bipolar membrane [8]. Wu et al. employed BMED to produce lactobionic acid and sodium hydroxide from sodium lactobionic [9]. SED is a novel separation process for resources recovery with monovalent selective ion exchange membranes [10, 11]. Rapp et al. used SED with monovalent selective ion exchange membranes to selective eliminate chlorine in a solution containing multiple divalent anions [12]. EED separation technology with a single ion exchange membrane has various advantageous including low energy consumption, no waste generation and chemical consumption [13]. Luo et al. used EED for recovering and concentrating organic acids [14]. Mikołajczak et al. evaluated the five commercially membranes for production and concentration of sulfuric acid by EED [15]. Wu et al. used CED and EED coupling process to separate sodium hydroxide from sodium meta-aluminate solution [6]. Xu et al. used the EED process for producing lithium hydroxide from lithium contained brine [16]. Jaroszek et al. investigated the EED process during the processing of sodium sulfate waste solution [17]. The phosphoric acid solution, as the one of the most critical mineral acid, suffers from lots of metal impurities material by sulfuric acid leaching, resolving these purified issues more pressing [18, 19]. EED technique is feasible for wet phosphoric acid separation and purification by theoretical calculation [20]. Touaibia et al. used the EED technique with two different anion exchange membranes (AEMs) in wet phosphoric acid purification and obtained the high phosphoric acid yield [21]. Other researchers also studied the purification of industrial-grade phosphoric acid by EED 2

[22-24]. These suggest that the EED technique can be applied in wet phosphoric acid purification. Electrode, operating voltage and AEM are the key factors that influence the phosphoric acid purification in EED [25]. As the primary component of EED, the AEM need to have the essential qualities, including excellent ion transport efficiency, high dimensional stability and mechanical strength [26]. Ideally, any perfect AEM should have intact structure, continuous ionic transportation channels and low membrane area resistance for EED operation. To some extent, the improvement of ion exchange capacity (IEC) can decrease the membrane area resistance [27]. However, high IEC level is always accompanied by excessive water swelling for the resultant AEMs, which sacrifice mechanical performance and the lifetime of the AEM in EED operation [28, 29]. Therefore, it is a challenging task to prepare AEMs which possess low water swelling and membrane area resistance simultaneously. Herein, the comb-shaped structure with pendant side chains in AEMs, which inspired by Nafion, could effectively improve the connectivity of ionic channels and reconcile the tradeoff between membrane area resistance and water swelling while maintaining mechanical stability [30, 31]. Nowadays, a large number of research efforts have been devoted to the design of long side chains to form a phase separation structure to achieve high conductivity and dimensional stability. Li et al. synthesized long aliphatic chains (from C6 to C16) poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) AEMs, which improved micro-phase separation and ionic channels connectivity, thereby resulting in higher ionic conductivity and dimensional stability than non-comb-shaped polymer [31-35]. Zhuang et al. synthesized comb-shaped polysulfone with ion-aggregating structures, the resulting ordered structure not only provided high conductivity but also improved the dimensional, mechanical and chemical stabilities significantly by theoretically calculated and experimental results [36-38]. Other comb-shaped AEMs with PPO, polysulfone and poly(arylene ether ketone) backbone have been successfully prepared for ED and fuel cells application, the synthesized AEMs exhibit high ionic conductivity, low membrane area resistance, suppressed water swelling, good mechanical and chemical stabilities at low IEC values [39-46]. In this study, a series of comb-shaped quaternized ammonium polysulfone (Cx-QAPSU) AEMs with pendant side chains of a specific length (4, 8 and 16 carbon atoms) were synthesized. The effect of side chain length on the physiochemical and 3

electrochemical properties of Cx-QAPSU was investigated. The micro-phase separation structure of Cx-QAPSU was described. The obtained Cx-QAPSU AEMs achieve an electrochemical-mechanical balance, especially the length of the pendant side chain of Cx-QAPSU up to C16 with IEC of 1.55 mmol/g exhibits the lowest Rm of 2.5 Ω·cm2 and the highest tensile strength of 16.7 MPa. Then, the EED performances such as voltage drop, phosphoric acid concentration, metal ion removal rate, current efficiency and energy consumption for phosphoric acid solution purification with Cx-QAPSU and QAPSU membranes have further been demonstrated.

2 Experimental 2.1 Materials Polysulfone (PSU) was obtained from Badische Anilin-und-Soda-Fabrik (BASF) and dried at 45 oC under vacuum overnight before use. Trimethylamine (TMA) and N,N-dimethylformamide (DMF) were provided by Shanghai Sinopham Chemical Reagent Co. Ltd. (China). N,N-dimethylbutylamine and N,N-dimethyloctylamine were

obtained

from

Aladdin

Industrial

Co.

Ltd.

(China).

N,N-dimethylhexadecylamine was purchased from J&K Scientific Ltd. Simulated wet phosphoric acid solution (H3PO4=0.52 mol/L, Fe=0.02 mol/L, Mg=0.03 mol/L, Ca=0.01 mol/L) was used by self-made. 2.2 Anion exchange membrane (AEM) preparation The chloromethylated polysulfone (CMPSU) with different degree of chloromethylation (DCM) ranging from 0.43 to 1.23 were prepared according to our previous work [47]. A 10 wt% homogeneous CMPSU/DMF solution was obtained at 40

°C.

Subsequently,

N,N-dimethylbutylamine

(molar

ratio

of

N,N-dimethylbutylamine to chloromethyl groups is 2) was added into CMPSU solution and stirred at 50 °C for 24 h. The C4-QAPSU membranes were obtained by drying at 60 °C for 12 h and then dried in a vacuum oven at 80 °C for 24 h. The preparation

process

of

C8-QAPSU

and

C16-QAPSU

membranes

with

N,N-dimethyloctylamine and N,N-dimethylhexadecylamine is similar to described 4

above. Moreover, quaternized ammonium polysulfone (QAPSU) with varying DCM were synthesized with TMA ethanol solution for contrast. After immersing in 0.5 mol/L NaCl solution for 24 h, the AEMs were washed with deionized water and then kept in deionized water. The thickness of dried AEMs is about 50-70 μm. 2.3 Characterizations 2.3.1 Structure and morphology. 1

using

H NMR spectra were measured at 300 MHz on a Bruker AV 400 spectrometer deuterated

dimethyl

sulfoxide-d6

(DMSO-d6)

as

the

solvent

and

tetramethylsilane (TMS) as the internal standard. The morphologies and microstructures of AEMs were studied by transmission electron microscopy (TEM, JEOL JEM-2010FEF) at 200 kV. The Cx-QAPSU and QAPSU at 0.5 wt% were cast to form a thin film on a Cu grid, respectively. And then the films were immersed in 1 mol/L KI solution for 48 h. After washing with deionized water, the samples were dried at room temperature for 24 h. Small angle X-ray scattering (SAXS) measurements of dry Cx-QAPSU and QAPSU membranes were performed using a SAXS LAB ApS system (JJ-Xray, Denmark). The scattering vector (q) and Bragg spacing (d) were calculated as follows: q

d

4 sin 

(1)

 2 q

(2)

where θ is the scattering angle and λ is the scattering wave length (0.154 nm). 2.3.2 Ion exchange capacity, water uptake, hydration number and swelling ratio. The dry membranes were immersed in 0.5 mol/L NaSO4 standardized solution, and then the solution was titrated with 0.1 mol/L AgNO3 solution. The ion exchange capacity (IEC) was calculated as follow: IEC 

VAgNO3  CAgNO3 m

100%

(3)

where VAgNO3 , CAgNO3 and m are the consumed volume, the concentration of AgNO3 (aq) and the weight of the dry membrane, respectively. The water uptake (WU), the swelling ratio (SR) and hydration number (λ) of the 5

AEMs were determined by investigating weight, length and width of membrane samples (4 cm × 1 cm) before and after hydration. WU, SR and λ were calculated as follows: WU 

SR 



Wwet  Wdry Wdry

Lwet  Ldry Ldry

100%

(4)

100 %

(5)

WU  1000 IEC  18  100

(6)

where Wwet and Wdry are the mass of hydrated and dried membranes (dried until constant weight at 60 oC); Lwet and Ldry are the plane dimension of hydrated and dried membranes, L  Llength  Lwidth . 2.3.3 Membrane area resistance and transport number. The membrane area resistance (Rm) of AEMs were determined according to the AC impedance method [4]. The resistances of conductivity cell with the membrane (Rm1) and without membrane (Rm2) were measured in a 0.5 mol/L NaCl solution. Rm was calculated as follow:

Rm  ( Rm1  Rm 2 )  S

(7)

where S is the effective area of membrane (3.14 cm2). The transport number (t-) was determined using the same device for Rm with a constant current in 0.1 mol/L and 0.2 mol/L KCl solution. The relationship between tand the potential difference across the membrane (Em) can be calculated using the following formula:

Em  (2t   1)

RT C1 ln F C2

(8)

where R is gas constant; T is absolute temperature; F is Faraday’s constant; C1 and C2 are the concentration of the KCl solutions, respectively. 2.3.4 Thermal and mechanical stability. The thermal stability of the CX-QAPSU and QAPSU membranes were tested by a thermal gravity analysis analyzer (Perkin-Elmer TGA7) within 30-700 oC with a 6

heating rate of 10 oC/min under a nitrogen atmosphere. The mechanical stability of fully hydrated AEMs was determined by an electric universal tensile machine (CM4104, China) at room temperature. The membrane samples (4 cm × 1 cm) was tested at the tensile speed of 5 mm/min. 2.4 Electro-electrodialysis (EED) process The EED stack for purifying wet phosphoric acid in our previous report includes: (1) an AEM with active area of 20 cm2; (2) two Plexiglas half-cells; (3) two titanium electrodes were iridium and tantalum coated (7 cm × 6 cm ×2 mm); (4) two channel peristaltic pump to supply the solution flow of 4.5 L/h; (5) a direct current power to supply constant current of 40 mA/cm2; (6) two reservoirs (500 cm3) used to store the cathode and anode compartments solution [47]. A 0.52 mol/L simulated wet phosphoric acid solution and the pure phosphoric acid solution was pumped to the anode and cathode compartments, respectively. The concentration of H3PO4 and metal ions (Fe, Mg, Ca) were tested by NaOH titration and atomic absorption. All the measured data were collected three times. The current efficiency (η) and energy consumption (E) are calculated as follows:

（C f aV f a  Ci aVi a ) F = 100% It

(9)

UIdt 0 98  (C V a  C aV a ) f f i i

E

t

(1

a

0) where t is the operating time; Ci a , Cf a , Vi a and Vf a are the concentration and volume of anolyte at the initial and final, respectively; U and I are the voltage drop and current applied in the EED.

3 Results and discussion 3.1 Membrane fabrication The chloromethylated polysulfone (CMPSU) with different degree of chloromethylation (DCM) were prepared according to our previous report using paraformaldehyde, chlorotrimethylsilane and tin chloride anhydrous [47]. Then, a 7

series of comb-shaped quaternized ammonium polysulfone (Cx-QAPSU, x = 4, 8, 16) with varying DCM and side chain length were synthesized in Scheme 1 by reacting CMPSU

with

N,N-Dimethylbutylamine,

N,N-Dimethyloctylamine

and

N,N-Dimethylhexadecylamine using Menshutkin reaction, respectively. Moreover, quaternary ammonium polysulfone (QAPSU) with varying DCM were synthesized with trimethylamine ethanol solution for contrast.

Scheme 1 Schematic diagram and synthetic route of QAPSU and Cx-QAPSU. 1

H NMR was used to examine the products in Fig. 1. The almost disappearance

of the signal peak for chloromethyl at 4.53 ppm and the appearance of the signal peak for methyl at 3.00 ppm indicate that the quaternary ammonium (QA) groups were formed successfully in QAPSU and Cx-QAPSU [35]. The proton signals of the long alkyl side chains at 0.83 ppm and 1.22 ppm demonstrate that the Cx-QAPSU membranes have been synthesized successfully [32]. The peak intensity of methylene at 1.22 ppm strongly dependent on the length of pendant side chains, is increased with the increase of side chain length. Furthermore, there was no byproduct found in the synthetic process of Cx-QAPSU and QAPSU in Fig. 1.

8

Fig. 1 1H NMR spectra of QAPSU and Cx-QAPSU.

3.2 Membrane morphology The anion exchange membrane (AEM) performances such as water uptake (WU) and ion transport are influenced by hydrophilic-hydrophobic micro-phase separation structure [48, 49]. The phase separation structure of the QAPSU and Cx-QAPSU membranes was investigated by TEM and SAXS. The four images in Fig. 2 present the TEM micrographs of (a) QAPSU, (b) C4-QAPSU, (c) C8-QAPSU, and (d) C16-QAPSU membranes where the lighter parts represented hydrophobic moieties while the darker regions represent localized hydrophilic ionic cluster (stained by KI). As shown in Fig. 2a, QAPSU membrane possesses a uniform ionic cluster of a few nm in size. Comparatively, the Cx-QAPSU membranes exhibit much better micro-phase separation structure in Fig. 2b-d, when they have the similar ion exchange capacity (IEC) value to the QAPSU membrane. The side chain length determines the hydrophobicity and flexibility of the Cx-QAPSU membranes [32]. The morphological structure of the C4-QAPSU with relatively short alkyl side chain may lead to low phase separation degree and small hydrophilic region. Moreover, the hydrophilic ionic clusters in the Cx-QAPSU became large and distinct with the increase of side chain length. When the length of pendant side chain of Cx-QAPSU up to C16, the membrane exhibits largest hydrophilic region and best phase separation 9

structure, which benefits to produce best-connected ion clusters and continuous ion transport pathways [50].

Fig. 2 TEM images of AEMs. (a) QAPSU, (b) C4-QAPSU, (c) C8-QAPSU, and (d) C16-QAPSU.

SAXS was further carried out to verify the sizes of ionic clusters in Fig. 3. There is no notable scattering peak in the QAPSU membrane, whereas the scattering peak for the Cx-QAPSU membranes appears in 2.61, 1.33 and 0.87 nm-1, respectively. Moreover, the scattering peak becomes narrow with the increase of side chain length. Meanwhile, the Bragg spacing was calculated to be 2.4, 4.7 and 7.2 nm in Table 1, respectively. The calculated values of Bragg spacing are in good agreement with the results of TEM in Fig. 2. We speculated that the hydrophobic side chain enhances the ionic clusters aggregation, ion channels interconnection and the nanoscale ionic domains formation. Therefore, the Cx-QAPSU membranes may possess lower WU and higher ion transport compared to the QAPSU membrane at similar IEC value [28, 51].

10

Intensity

0.87 nm-1 C16-QAPSU -1

1.33 nm

C8-QAPSU 2.61 nm-1

C4-QAPSU QAPSU

0.5

1.0

1.5

2.0 2.5 q (nm-1)

3.0

3.5

4.0

Fig. 3 SAXS profiles of CX-QAPSU and QAPSU membranes in the dry state. Table 1. The scattering vector and Bragg spacing of Cx-QAPSU and QAPSU membranes. AEMs

q (nm-1)

d (nm)

C4-QAPSU

2.61

2.4

C8-QAPSU

1.33

4.7

C16-QAPSU

0.87

7.2

QAPSU

-

-

3.3 Ion exchange capacity, water uptake, hydration number and swelling ratio It could be observed that the IEC values of QAPSU and Cx-QAPSU membranes increase with increasing of DCM in Fig. 4, due to the resultant AEMs with a large number of QA groups [52]. Even with the same DCM, the Cx-QAPSU membranes show a lower IEC than QAPSU membrane, and their IEC decreases with the increase of side chain length, which results from the increasing of alkyl side chain length in the resulting membranes leading to increasing the polymer mass, thereby reducing the number of functional groups per unit mass.

11

2.0 1.8

IEC (mmol/g)

1.6 1.4 1.2 1.0 0.8

C4-QAPSU C8-QAPSU C16-QAPSU QAPSU

0.6 0.4 0.3

0.5

0.7

0.9

1.1

1.3

DCM

Fig. 4 IECs of Cx-QAPSU and QAPSU AEMs as a function of DCM.

Water in the AEM is essential for ion transport, which can reduce the electrostatic interactions between the anions and cations [4]. However, excessive WU reduces the service life of AEM and decrease the current efficiency of electro-electrodialysis (EED) experiment [53]. In general, the WU and swelling ratio (SR) of AEM should be less than 50 % and 15 % in EED application, respectively [4, 29]. As shown in Fig. 5, the WU, hydration number (λ) and SR of all AEMs dramatically increase as IEC value increases by introducing more hydrophilic QA groups in the membranes. The Fig. 5 also shows that the Cx-QAPSU membranes with long alkyl chains have lower WU, λ and SR than QAPSU membranes at similar IEC value. Moreover, these values for the Cx-QAPSU membranes decrease with the increase of side chain length. It is assumed that the pendant alkyl chains show strong hydrophobicity which effectively restricts the water absorption and membrane swelling with micro-phase separation structure [28]. This result suggests that all the fabricated Cx-QAPSU membranes exhibit excellent swelling resistance (SR is less than 10 %), which can be applied in EED. The C16-QAPSU membranes with the most extended alkyl side chain have the best swelling resistance, which indicates that the hexadecyl chain has the most efficiently in ionic domains aggregation and anti-water absorption [32, 33].

12

50

a

45 40

WU (%)

35 30 25 20 15 C4-QAPSU C8-QAPSU C16-QAPSU QAPSU

10 5 0 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

IEC (mmol/g)

a 14

b 12

λ

10

8

C4-QAPSU C8-QAPSU C16-QAPSU QAPSU

6

4 0.4

0.6

0.8

a

1.0 1.2 1.4 IEC (mmol/g)

1.6

1.8

2.0

14 12

c

SR (%)

10 8 6 4 C4-QAPSU C8-QAPSU C16-QAPSU QAPSU

2 0 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

IEC (mmol/g)

Fig. 5 (a) WU, (b) λ, and (c) SR of the QAPSU and Cx-QAPSU AEMs as a function of IEC.

13

3.4 Membrane area resistance and transport number The membrane area resistance (Rm) and transport number (t-) are the two most crucial parameters in AEM for EED operation, which dominate the efficiency and ion selective permeability of the resulting membranes [54, 55]. Generally speaking, the Rm and t- of the AEM should be <15 Ω·cm2 and >0.9 to meet the demand of EED application, respectively [29]. The Rm of AEMs is strongly dependent on their active functional groups and WU [56], thus leading us to investigate the impact of both IEC and WU on Rm. As shown in Fig. 6a and Fig. 6b, the Rm of these hydrated membranes decreases with the increase of IEC and WU values. The comb-shaped membranes show much lower Rm compared to QAPSU membranes in the same condition. It is believed that the Cx-QAPSU AEMs with micro-phase separation structure can utilize water molecules more efficiently for ion transport [52]. In particular, the C16-QAPSU with IEC of 1.55 mmol/g and WU of 19.4% exhibits the lowest Rm of 2.5 Ω·cm2. The micro-phase separation structure of Cx-QAPSU membranes can effectively promote counter-ion transport compared with the QAPSU membrane, which drives by the alkyl chains attached to the QA groups [42]. Fig. 7 showed that the t- of Cx-QAPSU and QAPSU membranes increases with enhanced IEC value, which can be attributed to the suppressing diffusion for co-ions across the membrane with increasing QA groups in the resultant membrane [40, 57]. It can be clearly observed that Cx-QAPSU membranes show higher t- than QAPSU membranes under the same IEC. The C8-QAPSU5 and C16-QAPSU5 achieve the maximum t- of 0.98, which is similar to commercial AEM. Therefore, the Cx-QAPSU membranes possess low Rm and high ion selective permeability for EED application.

14

25

C4-QAPSU C8-QAPSU C16-QAPSU QAPSU

a

Rm (Ω·cm2)

20

15

10

5

0 0.4

25

0.6

0.8

1.0 1.2 1.4 IEC (mmol/g)

1.6

2.0

C4-QAPSU C8-QAPSU C16-QAPSU QAPSU

b

20

Rm (Ω·cm2)

1.8

15

10

5

0 0

5

10

15

20 25 30 WU (%)

35

40

45

50

Fig. 6 Rm of the QAPSU and Cx-QAPSU AEMs as a function of (a) IEC and (b) WU.

15

1.00 0.98

t-

0.96 0.94 0.92 C4-QAPSU C8-QAPSU C16-QAPSU QAPSU

0.90 0.88 0.4

0.6

0.8

1.0 1.2 1.4 IEC (mmol/g)

1.6

1.8

2.0

Fig. 7 t- of the QAPSU and Cx-QAPSU AEMs as a function of IEC.

3.5 Thermal and mechanical stability Thermal stability of Cx-QAPSU and QAPSU membranes was investigated by thermogravimetrical analysis (TGA) in Fig. 8, and the thermal decomposition temperatures at 5 wt% loss (Td,5%) are listed in Table 2. The Cx-QAPSU and QAPSU membranes exhibit two weight loss steps except for the weight loss of absorbed water and residual solvent. The first weight loss step that occurred at 200-300 oC is mainly attributed to the decomposition of QA groups, and the second weight loss observed over 400 oC is ascribed to the polysulfone main chain decomposition. Cx-QAPSU membranes show a slightly lower thermal decomposition temperature (197-204 oC) than QAPSU (252 oC) in Table 2. However, the thermal stability of all Cx-QAPSU membranes below 100 oC is sufficient for membrane preparation and EED operation. Mechanical stability of Cx-QAPSU and QAPSU membranes in hydrate state is shown in Fig. 9 and Table 2. Fig. 9 shows a decreasing trend of tensile strength (TS) with the increase of IEC value, probably because the interaction between polymer chains decreases by the plasticizing effect of QA groups [58]. The Cx-QAPSU membranes show higher TS and lower elongation at break (Eb) than QAPSU membranes under the similar IEC in Table 2, resulting from the lower WU and SR of the Cx-QAPSU membranes [59]. All the Cx-QAPSU membranes with TS from 13.2 MPa to 16.7 MPa and Eb from 12.2 % to 8.8 % in Table 2 show good toughness and strength properties, which are suitable for application in EED process. 16

110 QAPSU4

Weight percentage (%)

100 90

C4-QAPSU4

80 70

C8-QAPSU5

60 50 40 30 C16-QAPSU5

20 10 0 0

100

200

300 400 Temperature (°C)

500

600

700

Fig. 8 TGA curves of QAPSU and Cx-QAPSU AEMs. 35

C4-QAPSU C8-QAPSU C16-QAPSU QAPSU

30

TS (MPa)

25 20 15 10 5 0 0.4

0.6

0.8

1.0 1.2 1.4 IEC (mmol/g)

1.6

1.8

2.0

Fig. 9 TS of the QAPSU and Cx-QAPSU AEMs as a function of IEC. Table 2. Properties of Cx-QAPSU and QAPSU membranes.

a

AEMs

IEC (mmol/g)

WU (%)

SR (%)

Rm (Ω·cm2)

t-

Td,5% a (oC)

TS (MPa)

Eb (%)

C4-QAPSU4

1.51

25.1

6.8

7.0

0.95

204

13.2

12.2

C8-QAPSU5

1.66

23.9

7.0

3.0

0.98

199

13.5

11.4

C16-QAPSU5

1.55

19.4

5.3

2.5

0.98

197

16.7

8.8

QAPSU4

1.61

34.4

9.4

6.0

0.95

252

10.6

33.4

Measured by TGA under N2.

17

3.6 AEM for phosphoric acid solution purification The Cx-QAPSU and QAPSU membranes that meet the requirements of EED in Table 2 have been served as separator under the similar IEC. Fig. 10 shows the results of the purifying phosphoric acid solution with different AEMs as a separator by EED under a constant current of 40 mA/cm2. As shown in Fig. 10a, the phosphoric acid concentration of different AEMs in anode and cathode compartments increases and decreases as time elapsed, respectively. This attributes to the fact that H2PO4- ions in the cathode compartment pass through the AEM into anode compartment under the electric field [21]. In addition, all the comb-shaped membranes show higher phosphoric acid concentration than QAPSU membrane in the anode compartment and lower in the cathode compartment. It is easy to understand that the micro-phase separation structure and continuous ionic domains can be formed by introducing alkyl chains in Cx-QAPSU membranes which promote counter-ion transport and result in higher migrating H2PO4- capacity as compared to the membranes without alkyl chains functionalization [60]. It is noteworthy that the phosphoric acid concentration increases and decreases with increasing of the pendant side chain length. The enhancement effect can be attributed to that the increase of side chain length reduces membrane area resistance and facilitates the hydrophilic ionic clusters channel formation, thereby promoting the H2PO4- migration through the membrane. These results indicate that the C16-QAPSU5 membrane exhibits the best phosphoric acid concentration performance in the same condition. Voltage drop can determine the energy consumption and the working life of AEM, which depend on the resistance of solution and membrane [61]. The voltage drop decreases rapidly at the beginning of the test (Fig. 10b), which results from the high resistance of the phosphoric acid solution. Moreover, the decrease of the solution resistance and voltage drop is also due to the increase of the phosphoric acid migration and dissociation as the time prolongs [47, 62]. Subsequently, the voltage drop is stable when the electrical resistance of the phosphoric acid solution at low levels [63]. As compared to QAPSU membrane, it can be seen that the Cx-QAPSU membranes exhibit lower voltage drop, and their voltage drop decreases from C4-QAPSU4 to C16-QAPSU5, which can be ascribed to the lower membrane area resistance. In this study, the Cx-QAPSU membrane with hexadecyl chains has the 18

lowest voltage drop during the EED operation. Fig. 10c shows the change in current efficiency by Cx-QAPSU and QAPSU membranes. The current efficiency decreases along with the running time, which can be explained by the proton leakage through the AEM [21]. Moreover, the Cx-QAPSU membranes show much higher current efficiency than the QAPSU membrane, and the current efficiency increases with the side chain length, which is consistent with the previous study on the micro-phase separation structure.

a 1.0

0.5

0.9

0.4

0.8

0.3

0.7

0.2 C4-QAPSU4 C8-QAPSU5 C16-QAPSU5 QAPSU4

0.6

0.1

0.5 0

80

160

240

320

400 0

80

160

240

320

Time (min) 11

b

C4-QAPSU4 C8-QAPSU5 C16-QAPSU5 QAPSU4

9

7

5

3

1 0

50

100

150 200 250 Time (min)

19

300

350

400

0.0 400

H3PO4 concentration in cathode (mmol/L)

0.6

Voltage drop (V)

H3PO4 concentration in anode (mmol/L)

1.1

100

C4-QAPSU4 C8-QAPSU5 C16-QAPSU5 QAPSU4

c

η (%)

80

60

40

20 0

50

100

150 200 250 Time (min)

300

350

400

Fig. 10 (a) H3PO4 concentration in the anode and cathode compartments, (b) voltage drop, and (c) current efficiency of QAPSU and Cx-QAPSU AEMs as a function of operation time.

The eventually metal ions removal rate and energy consumption of AEMs were tested after running 6 h. All the metal ions purification rate of Cx-QAPSU is more than 68 wt% and higher than the QAPSU membrane in Table 3. Moreover, the longer side chain leads to the higher metal ions removal rate of Cx-QAPSU. Because the decreasing WU effectively restrains the diffusion of co-ions at the similar IEC value [40, 57]. All the energy consumption is lower than 2.7 kw·h/kg after 6 h treatment. Like voltage drop, the energy consumption decreases with the increase of side chain length. In this study, C16-QAPSU5 membrane shows the highest purification efficiency and lowest energy consumption than other AEMs, which can prolong the EED operation life [9]. Table 3 Metal ions removal rate and energy consumption of different AEMs. Metal ions removal rate (%) AEMs

E (kw·h/kg)

Ca2+

Fe2+

Mg2+

C4-QAPSU4

75.0

68.8

78.0

2.6

C8-QAPSU5

79.7

72.5

83.4

2.0

C16-QAPSU5

83.4

78.0

85.6

1.5

QAPSU4

73.4

65.3

76.1

2.7

20

4 Conclusions We have synthesized comb-shaped AEMs with alkyl side chains from C4 to C16 to form hydrophilic-hydrophobic micro-phase separation structure and organize ionic domains. The comb-shaped structure can mitigate the swelling and improve anion transportation along with good mechanical properties of the AEMs. Notably, the C16-QAPSU membranes with hexadecyl chains have the best dimensional and mechanical stabilities and lowest membrane area resistance at the similar IEC. The C16-QAPSU5 exhibits the best concentration effect and highest metal ions removal rate in phosphoric acid purification at 1.55 mmol/g. We consider that the C16-QAPSU5 membrane can meet the demanding challenges of EED application in phosphoric acid purification and concentration.

Acknowledgment The authors gratefully are grateful for financial support from the National Science Foundation of China (51273073) and Collaborative Innovation Center of Hubei Province of China (E201103). The instrumental facilities of Analysis and Testing Center of HUST are also acknowledged.

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Highlights: 

Comb-shaped polysulfone anion exchange membranes were synthesized by Menshutkin reaction.



Hydrophilic-hydrophobic phase separation structure was designed to enhance the performance of the comb-shaped polysulfone membranes.



The comb-shaped polysulfone membranes exhibit low membrane area resistance and swelling ratio.



The comb-shaped polysulfone membranes exhibit high phosphoric acid purification efficiency in electro-electrodialysis.

25