Graphene oxide modified porous P84 co-polyimide membranes for boron recovery by bipolar membrane electrodialysis process

Separation and Purification Technology 232 (2020) 115963

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Graphene oxide modified porous P84 co-polyimide membranes for boron recovery by bipolar membrane electrodialysis process ⁎

Mengjie Suna, Meng Lia, Xu Zhanga, Cuiming Wua, , Yonghui Wub,

T

⁎⁎

a

Anhui Key Lab of Controllable Chemical Reaction & Material Chemical Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, PR China b School of Chemistry and Environmental Engineering, Yancheng Teachers University, Yancheng 224002, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Bipolar membrane electrodialysis P84 co-polyimide Graphene oxide Phase inversion Boron removal

Composite anion-exchange membranes with porous structures have been successfully prepared by incorporating quaternized graphene oxide (QGO) into P84 co-polyimide, and then undergoing the phase inversion, amination and quaternization processes. The membranes’ porous structures can be readily adjusted by the category of nonsolvent during the phase inversion process. Large finger-like voids are present in the lower parts of the membrane cross sections by using water or 50% isopropanol aqueous solution, which facilitate the entrance of polyethylenimine chains into the membrane matrix, leading to higher degree of amination and quaternization. The incorporation of QGO does not influence the membrane morphology obviously, but can further enhance the ion exchange capacity (1.23–1.65 mmol/g) and decrease the membrane area resistance (1.6–1.9 Ω cm2). The QGO-P84 composite membranes are used in bipolar membrane electrodialysis (BMED) for the removal of boron from synthetic model solutions (Na2B4O7·10H2O, 1000 mg B/L). The finger-like pores decrease the steric resistance of boron transport, while the incorporation of QGO improves the membrane electro-chemical properties and thus the BMED performances. The separation efficiency is 76.6% after running 3 h under 30 V, the current efficiency is 94.9% and the energy consumption is 26.16 kW h/kg by using the optimal composite membrane. The BMED performances are better than those of commercial membrane CJMA-3 (separation efficiency of 51.6%, current efficiency of 81.2%, and energy consumption of 30.56 kWh/kg). Hence, the QGO-P84 composite membranes are effective for removal and recovery of boron from aqueous solution through BMED method.

1. Introduction Boron is one of the useful elements for different types of industries, and the demand for boron has been increasing in recent decades, especially for glass, ceramics, detergents and semiconductors manufacturing industries. Therefore, recovering boron from its natural and industrial sources is widely researched [1–3]. Boron can exist at low concentration in nature as undissociated orthoboric acid, partially dissociated borate anions in the form of polyborates, complexes of transition metals and fluoroborate complexes [4]. Higher concentrations of boron are often found in anthropogenic wastewater, including domestic, industrial and agricultural wastewater. For instance, boron could amount to average concentration of 500 mg L−1 in low level radioactive wastewaters (LLRWs) produced from the nuclear power plants [5]. The boric acid in the brines of a gasfield was around 2.5 g/l

⁎

and then decreased to 2.0 g/l after the traditional precipitation step [6]. If the irrigation water contains boron with high concentration, it will have toxic effects on most plants [7]. Excessive intake of boron through drinking water for a long time is extremely harmful to human and animal health [8]. The World Health Organization recommends that the boron concentration in drinking water should be less than 2.4 mg/L [9]. Hence, boron removal from industrial wastewaters, which can not only gain useful boron resources but also eliminate the health harm of the wastewater, has received a lot of attention in recent years. In the past few decades, several types of methods are proposed for removing boron from industrial wastewaters, such as adsorption, membrane processes (reverse osmosis [10] and nanofiltration [11]), ion exchange [12], electrocoagulation [13,14], Donnan dialysis [15,16], chemical coagulation and hybrid process [17]. There have been some laboratory trials and reports, but these methods are difficult to apply

Corresponding author. Corresponding author. E-mail addresses: [email protected] (C. Wu), [email protected] (Y. Wu).

⁎⁎

https://doi.org/10.1016/j.seppur.2019.115963 Received 24 April 2019; Received in revised form 19 August 2019; Accepted 19 August 2019 Available online 20 August 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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Nomenclatures Codes GO QGO IPA PEI ED BMED

DD AEM IEC WR Rm Se CE E

full name or meaning graphene oxide quaternized graphene oxide isopropanol polyethylenimine electrodialysis bipolar membrane electrodialysis

membrane dissociates water into H+ and OH− ions in the presence of an electrical field. The generated OH− ions are continuously fed into the feed solution, so that the solution pH can be controlled constantly at high value and boron can exist in the form of B(OH)4−, which is crucial for separation and recovery of boron. The B(OH)4− ions then migrate through anion exchange membrane (AEM) to the concentrate compartment to combine with the H+ generated by the bipolar membrane. In the above process, the migration of B(OH)4− ions through AEM is also a key factor influencing the BMED efficiency. High stability and permeability of the AEM are required for the transport of B(OH)4− ions in the BMED process. P84 co-polyimide has excellent chemical and mechanical stabilities, and can undergo easy and controllable quaternization [23]. The P84 co-polyimide has been used to prepare porous AEMs, which have the ion exchange capacities (IECs) of 0.83–0.86 mmol/g. The membranes are applied in BMED process to produce N-2-hydroxyethylpiperazine- N′-2-ethanesulfonic acid (HEPES). The high recovery ratio and current efficiency prove the advantages of porous AEMs for production of high molecular organic acids. The membranes’ pore structure is a key factor to improve the transport of HEPES−. For boron recovery or removal, however, the porous structure needs to be modified, as boric acid has lower molecular weight. What’s more, the stability and electro-chemical properties, including the IECs need to be further improved to promote the BMED performances. Membrane stabilities including dimensional stability and mechanical strength can be improved by formation of three-dimensional network through cross-linking. The three-dimensional composite structure may be achieved by the cross-linking of graphene oxide (GO), since GO contains plenty of –OH and –COOH groups, which can form hydrogen bonding, ionic bonding or covalent bonding with other polymers [24,25]. Bai et al. prepared the nanohybrid membranes by incorporating phosphorylated graphene oxide (PGO) nanosheets into

practically because of specific disadvantages in removing boron to an acceptable level. For instance, adsorption will produce a large amount of sludge to give a rise to a secondary pollution. Reverse osmosis or nanofiltration is usually insufficient in the rejection of boron near neutral pH, leading to insufficient recovery ratio. Ion exchange is uneconomical because of the large consumption of acids and bases in the regeneration of resins. Overall, most of the conventional methods consume extra chemical reagents or lack efficiency. Hence, environmental-friendly and effective methods need to be developed for removal of boron from wastewater. Electrodialysis (ED) has also been proposed as an alternative method for boron removal [18–20]. During the conventional ED treatment process, the boron removal depends largely on the solution pH and it is possibly effective only under alkaline condition. This is because boric acid (H3BO3), which is the major chemical species of boron in aqueous solution, is a very weak acid with a pKa of 9.14. The predominance of boron species (B(OH)4− and H3BO3) is governed by the solution pH according to their equilibrium distribution in aqueous solution: B(OH)3 + H2O ↔ B(OH)4− + H+

diffusion dialysis anion exchange membrane ion exchange capacity water uptake membrane area resistance Separation efficiency current efficiency energy consumption

(1)

Therefore, non-charged B(OH)3 species is predominant in boroncontaining solution at pH lower than 9.14. Only under alkaline condition (pH > 9.14), B(OH)4− is the predominant form and can be responsive to electric field-driven transport during ED. To overcome the above shortcoming of conventional ED, bipolar membrane electrodialysis (BMED) has been adopted for boron removal [21]. BMED is the combined technology of electric field-enhanced water dissociation in bipolar membranes and the ion separation in conventional ED [22]. A simple but typical BMED stack for boron removal is shown in Fig. 1. During the BMED process, a bipolar

Fig. 1. Configuration of BMED experimental setup, including the membrane stack for the recovery of B(OH)4−. 2

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modified Hummers method [29]. The GO (150 mg) was dispersed in anhydrous THF (500 mL) and sonicated to obtain a homogeneous dispersion. Then APTMS (1.5 mL) was added, and the reaction mixture was refluxed for 15 h at 80 °C. After cooled to 30 °C, the mixture was filtered, then rinsed several times with THF and dried under vacuum overnight to yield gray-black A-GO. The A-GO (0.3 g) was dispersed in methanol (100 mL) and added with methyl iodide (10 mL). The mixture was stirred for 24 h at 30 °C, then filtrated, washed several times with methanol, and dried under vacuum overnight to obtain QGO. The reaction route is illustrated in Fig. 2.

chitosan matrix [26]. The PGO-filled membranes achieve higher thermal and mechanical stabilities due to the strong electrostatic interactions between the groups of -PO3H and –NH2. Zhang et al. prepared high temperature stable proton exchange membranes based on the cross-linking between phosphoric acid doped quaternized poly (ether ether ketone) and sulfonic groups functionalized grapheme oxide [27]. The obtained composite membranes exhibit improved mechanical strength and oxidative stability. Zhao et al. [28] reported that the ionic cross-linking networks were implemented by the interactions between the sulfonic acid groups of sulfonated polyethersulfone (SPES) and the quaternary ammonium groups of QGO. The composite membranes showed enhanced mechanical properties and improved oxidative stabilities. Inspired by the previous literatures, QGO is prepared in the present work and then incorporated into P84 co-polyimide, so that a series of novel cross-linked polymer composite AEMs are obtained. Besides, the membrane pore structure is controlled and modified for recycling and removing boron from aqueous solutions. The membranes’ structure and physico-chemical properties are correlated with the BMED performances to explore the optimum composite AEMs and achieve optimized boron removal performances.

2.3. Preparation of QGO-P84 composite anion-exchange membranes QGO was dispersed in NMP under a continuous stirring and ultrasound, and then added with P84 powder to form a homogeneous casting solution. The concentration of P84 in the solution was 23 wt%, while the QGO weight percent with respect to P84 was varied from 0, 1 to 2 wt%. The casting solution underwent phase inversion, amination with PEI and quaternization by ethylation through procedures similar to our previous work [23]. However, different coagulation baths were used during phase inversion so that the membrane pore morphologies could be changed. The coagulation bath was composed of water and IPA, with IPA concentration increasing from 0 wt%, 50 wt% to 100 wt %. The preparation conditions for different membranes are summarized in Scheme 1. Membranes M0, M50 and M100, which are P84 co-polyimide AEMs with no dosage of QGO, are obtained mainly for investigating the influence of the non-solvent category. M50-QGO1 and M50-QGO2 are QGO-P84 composite membranes. During the above preparation processes, part of the large number of –NH2 groups in PEI chains can induce the cross-linking of P84 chains, while the remaining may form quaternary ammonium groups after the ethylation step. The incorporation of QGO can enhance the crosslinking of P84 further since functional groups of QGO such as –NH2 and –OH can form covalent and hydrogen bonding with the imide groups of P84. The reactions above could result into AEMs with three-dimensional network, as illustrated in Fig. 3.

2. Experimental 2.1. Materials P84 co-polyimide with a molecular weight (Mw) of 153 kDa was obtained from HP polymer GmbH (Austria). Isopropanol (IPA), graphene, (aminopropyl)-trimethoxysilane (APTMS), tetrahydrofuran (THF), methanol and N-methylpyrrolidone (NMP) were purchased from Sinopharm Chemical Reagent Co, Ltd (China). THF was dehydrated by sodium before use. Polyethylenimine (PEI, Mw of 10,000) was supplied by Aladdin Industrial Corporation (China). All the chemicals were analytical grade and used as received. Deionized water was used throughout the experiments. Two kinds of commercial membranes, ie. bipolar membrane FBM from Fumatech (Germany) and anion-exchange membrane CJMA-3 from Hefei ChemJoy Polymer Materials Co., Ltd (China) were used.

2.4. Characterizations 2.2. Synthesis of A-GO and QGO Fourier transform infrared (FTIR) spectroscopy was recorded using FTIR spectrometer (Nicolet 67, Therom Nicolet) with a resolution of

GO was prepared by oxidizing the natural graphite powder through OH HO

O

H3C

HO O

H3C

OH

O

O O

NH2

Si O

H3C

HO O

OH

OH

THF, 80

GO

CH3I CH3OH, 30

N+ I-

N+

, 24h

I

-

N+ I-

N+ IN+ I-

QGO

A-GO

Fig. 2. Synthetic reaction route for the preparation of QGO from GO. 3

=

, 15h

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P84 co-polyimide

QGO

stirring to dissolve

P84-QGO-1 wt%

P84-QGO-0 wt%

P84-QGO-2 wt%

Phase inversion in isopropanol aqueous solution

0 wt% IPA

100 wt% IPA

50 wt% IPA

50 wt% IPA

50 wt% IPA

Amination

Amination

Amination

Amination

Amination

Quaternization

Quaternization

Quaternization

Quaternization

Quaternization

Mem M50

Mem M0

Mem M100

Mem M50-QGO1

Mem M50-QGO2

Scheme 1. The preparation procedures for membranes M0 to M50-QGO2.

0.09 cm−1 and a spectral range of 4000–500 cm−1. Thermal stability was investigated by the thermogravimetric analysis, which was conducted by a Shimadzu TGA-50H analyzer under air flow with a heating rate of 10 °C/min. Membrane morphologies were observed with field emission scanning electron microscopy (FESEM, SU8020 Hitachi). Membrane samples were fractured in liquid nitrogen, dried under vacuum at room temperature and then coated with gold before FESEM observation. Ion-exchange capacities (IECs) were determined using the Mohr method [23]. Dry membrane was sampled and accurately weighed, then was converted to Cl− form in 1.0 mol/L NaCl solution for 48 h. Excess NaCl was washed off with water for 4–6 times within one day. Then the sample was immersed in 0.5 mol/L Na2SO4 for 48 h. The IEC value was obtained by determining the amount of exchanged Cl− through potentiometric titration using 0.1 mol/L AgNO3 as titration

reagent. Here, a potentiometric titrator (ZDJ-400, Beijing Pioneer Weifeng Technology Development Co., Ltd., China) was used instead of manual operation, so that the accuracy of the results could be improved.

IECA (mmol/g) =

P84 NH2

O

O

+

NH2

+

+

+

HO NH2

Blending Solution Casting

PEI:

+

+

O

NH

+ C

O

+

+

+

+

+ NH2

+

NH2 NH 2

NH2

HO

QGO

+

+

O

O OH

NH2

+

+

HO

N

Quaternization

NH2

+

+

+

different groups or chains

C NH O N

H N

O

Amination

(2)

m1

where VAgNO3 (mL) is the consumed volume of AgNO3, CAgNO3 (mol/L) is the concentration of standard AgNO3 solution, and m1 is the weight of membrane sample. Water uptake (WR) was measured to investigate the membrane hydrophilicity. The membrane samples were dried and weighed. Then the samples were immersed in water at room temperature for 48 h and weighed after removal of the surface water with filter paper. The WR was calculated as the relative weight gain per gram of the dry OH

NH

VAgNO3 × CAgNO3

+

+

+

+

NH 2

-N(CH3CH2)4+ O

CH3 O

H C H

O N

N O

20%

N

O 80%

H N x

different interactions

C

H N

covalent bonding

O OH

NH

y

NH2

Fig. 3. Schematic illustration of the construction procedure of QGO-P84 composite membranes. 4

hydrogen bonding

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be freshly prepared before use, so that the experimental detection error is less than 3%.

GO A-GO QGO

2.5.2. Data analyses and calculations The BMED performance was evaluated in terms of current efficiency, energy consumption and separation efficiency. The current efficiency (CE) is the ratio of the stoichiometric number of electrical charges required for boron production to the overall electrical charge employed in the BMED stack [34]:

1720

3460-3300

1616

1160 1035

1560

2925

1193 1480

915

CE (%) =

750

1132

3430

4000

3500

3000

2500

2000

1500

1000

500

E (kW ·h/ kg ) =

Fig. 4. FT-IR spectra of GO, A-GO and QGO.

× 100% (4)

t ∫0 UIdt

m

(5)

where U is the voltage drop across the BMED stack (V), I is the electrical current (A), m is the weight of recovered boric acid. Separation efficiency (Se) of boron is calculated by Eq. (6):

membrane sample [30]. The membrane area resistance was measured using a specific measuring cell as reported in previous literature [31]. The membranes were immersed in 0.5 mol/L NaCl solution for 12 h, and then inserted into a clip cell (effective area 7.065 cm2) with applying the current at 0.05A. The Rm (Ω cm2) was determined by the following Eq.

U − U0 ×S I

t

where Δn is the mole amount of recovered boron (mol); Z is the ion’s absolute valence; F is the faraday constant (96,485 C mol−1); N is number of repeating units (N = 2); I is the current, and t is the operation time. The integral energy consumption E (kW h/kg) was defined as [34]:

Wavenumber (cm-1 )

Rm =

ZF Δn N ∫0 Idt

Se =

(C0 V0 − Ct Vt ) × 100% C0 V0

(6)

where C0, Ct are the concentrations of boron at time 0 and t respectively in the dilute compartment, V0, Vt are the solution volumes in the dilute compartment.

(3)

where U0 (V) is the gap voltage drop without membrane, U (V) is the membrane voltage drop, I (A) is the applied current, S (cm2) is the effective membrane area.

3. Results and discussion 3.1. Membrane composition by FTIR

2.5. Bipolar membrane electrodialysis (BMED) The FTIR spectra of GO, A-GO, and QGO are shown in Fig. 4. For the spectrum of GO, the strong and broad absorption band at 3460–3300 cm−1 is due to the OeH stretching vibrations, and the peaks at 1720 cm−1 and 1616 cm−1 are ascribed to the C]O stretching vibrations and C]C vibrations [35,36]. The shoulders at 1160 cm−1 and 1035 cm−1 are the characteristic absorption for the CeO of epoxy/ ether [37]. The characteristic absorption peaks described above confirm the successful preparation of GO. New peaks arise for the A-GO. The absorption band at 2925 cm−1 is for the –CH2 stretching vibration and the weak absorption band at 1480 cm−1 is for the –OCH2 deformation and wagging vibration. The peaks at 1560 cm−1 and 750 cm−1 arise due to the –NH2. The stretching vibrations at 1132 and 915 cm−1 are attributed to the SieC, SieOeSi and SieOH. As for QGO, the broad peak at 3430 cm−1 is indicative of NeH of amino groups, the SieOH and/or the OeH stretching vibration, while the strengthened peak at 2925 cm−1 should be due to quaternary ammonium groups. The FTIR spectra of the P84 composite membranes are illustrated in Fig. 5. As previously reported, P84 base membrane should have characteristic bands of imide groups, including C]O asymmetric stretching at 1775 cm−1, C]O symmetric stretching at 1718 cm−1 and CeN stretching at 1360 cm−1 [23]. After modification by PEI, the -CO-NHstructure of the amide group emerges as indicated by the bands at 1640 and 1540 cm−1, which correspond to C]O and CeN stretching bands, respectively. The peaks at 2850–3015 cm−1 indicate the presence of -CH2-CH3, which is associated with -N+(CH2CH3)3Br− group. Hence, quaternization by ethylation has successfully taken place to obtain AEMs.

2.5.1. Experimental set-up The BMED membrane stack configuration was BP-A-BP-A-BP, which mainly contained two pieces of anion exchange membrane (CJMA-3 or our porous P84 membrane), and three pieces of bipolar membrane (FBM). The membrane stack configuration contains two repeating units. Each membrane had an effective area of 20 cm2. The membranes were arranged alternatively between the anode and the cathode, as shown in Fig. 1. The membranes in the stack were separated by plexiglas spacer with a thickness of around 10 mm and silicon rubber to form four compartments for passing the following three solutions: (1) an electrode solution (0.1 mol/L Na2SO4) for connecting anode and cathode; (2) the initial solution for the dilute compartment of Na2B4O7·10H2O (1000 mg B/L); (3) the initial solution for concentrate compartment of H3BO3 (100 mg B/L). Each solution (250 mL) was pumped into a closed loop at a flow rate of 300 mL/min. Each compartment was connected to an external reservoir to allow a continuous recirculation. The solution was circulated in the system for 10 min to eliminate the gas bubbles before the current was applied, since the presence of gas bubbles may increase the total resistance of the stack, voltage drop and energy consumption [32]. Applied voltages were 20 and 30 V, since the applied voltages for recovering boron in previous researches were generally in the range of 10–35 V [1,21,33]. Solutions in the dilute and concentrate compartments were sampled every 0.5 h to evaluate the BMED performance. The pH of the solutions was measured with a pH meter (METTLER TOLEDO Co., Ltd., Switzerland). The boron concentration was analyzed by Spectrophotometry (UV-2401PC, Shimadzu Ltd., Japan) using azomethine-H as the colour developing reagent. The prepared boron solution needs to be stored strictly in the dark, and azomethine-H should

3.2. Thermal stability TGA curves, together with DrTGA graphs, are used to evaluate 5

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M. Sun, et al. O

controlled by the type of non-solvent during phase inversion [41]. The porous structure will influence the membrane separation performances and applications. Finger-like pores should have less ions resistance and thus may be more suitable for separating organic acids of high molecular weight. Sponge-like pores are advantageous for application in diffusion dialysis (DD) to recover small acids such as H2SO4 [42], and high selectivity can be obtained. Some of the previous works about P84 porous membranes are summarized in Table 1 for better understanding. Overall, the physiochemical and ion-selective properties of the AEMs can be adjusted according to their porous structure.

O

C

C

N

NH

C O

M0 M50 M100

M50-QGO1

3.4. Membrane properties 1775 1718

2850-3015

4000

3500

3000

2500

1640 1540

2000

Wavenumber

1500

Table 2 summarizes the membrane properties including thickness, IEC, water uptake (WR) and area resistance. The data of commercial membrane CJMA-3 are also shown as comparison. Although the amination and quaternization reactions are carried out under identical conditions for all membranes, membranes M0 and M50 have higher IEC values than that of M100. This is because large finger-like voids in M0 and M50 can facilitate better the entrance of PEI chains into the membrane matrix, leading to higher degree of amination and quaternization. Previous researches have also revealed the influence of the membranes’ structure on the IEC values. For instance, amination of porous brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) is easier, so that higher IEC values (2.47 mmol/g [45] or 2.13 mmol/g [46]) can be obtained as compared with the value of dense BPPO-based AEMs (around 1.96 mmol/g). Membrane M50-QGO1 has higher IEC than other membranes, indicating that the addition of QGO also increases the content of ion exchange groups in the membranes. However, further increase of QGO for M50-QGO2 lowers the IEC somehow, which may be attributed to the cross-linking effect between QGO and the P84 polymer chains. The cross-linking strengthens the three-dimensional network and thus hinders the following quaterization process. Overall, membrane M50-QGO1 with 1 wt% QGO has the highest IEC of ~1.65 mmol/ g. Water uptake (WR) as an important factor in ions and water transfer, is mainly influenced by the porous structure and the functional groups [47]. M0 has the highest WR and M100 has the lowest value among M0, M50 and M100, confirming that the porous structure with large fingerlike voids can contain more water and the functional groups of -N+(C2H5)3Br− is highly hydrophilic. As for M50, M50-QGO1 and M50QGO2, the WR values gradually decrease, which may be due to the gradually strengthened three-dimensional network of the membranes as the dosage of QGO increases. The area resistance of membrane M0 is only ~2.9 Ω cm2, which is attributed to its big finger- like pores and high hydrophilicity. Previous

1360

1000

(cm-1 )

Fig. 5. FTIR spectra of membranes M0 - M50-QGO2.

short-term thermal stability and provide information on the degradation patterns, as shown in Fig. 6. The AEMs show three similar weight loss peaks. The peak lower than 100 °C is attributed to the release of absorbed water [38]. The second peak in the range of 200–290 °C is attributed to the degradation of quaternary ammonium groups [39]. The low weight loss (7.83%) of membrane M100 in this stage indicates lower content of quaternary ammonium groups, which can be confirmed by its lower IEC value as will be discussed later. Degradation of the polymer main chain occurs at 460 °C at the third peak. As comparison, previous research reports that degradation of the original P84 membrane starts at temperature higher than 400 °C [38]. Modification by PEI can increase the thermal stability of P84 main chain to around 420 °C [23], and here, the degradation temperature is further increased to around 460 °C. The enhanced thermal stability of the polymer main chains should be due to the formation of the threedimensional network by the incorporation of PEI and QGO (Fig. 3). Similar findings have been reported previously. For instance, the SPESQGO composite membranes show better thermal stabilities and higher residues at 800 °C than the pristine membrane, since the ionic crosslinking structure can hold the polymer chain together, restricting the chain from decomposition and thereby improving the membranes’ thermal stabilities [28]. Overall, the formation of the three-dimensional network increases the membrane thermal stabilities. 3.3. Membrane morphology by FESEM

M0

100

The FESEM images are shown in Fig. 7. Membrane M0, which was obtained by phase inversion in water, contains a number of large parallel finger-like pores throughout the lower part of the cross-section. The result indicates that an instantaneous phase separation occurred during the precipitation step due to the strong mutual affinity between water and NMP [40]. Membrane M100, obtained by phase inversion in IPA, exhibits a sponge-like porous structure, which is attributed to the relative low rate of the liquid-liquid demixing between IPA and NMP. The magnified graph also shows a large number of micronsized pores separated by ultra-thin walls (Fig. 7c). Membranes M50, M50-QGO1 and M50-QGO2, obtained by phase inversion in 50% IPA aqueous solution, are very similar in structures and features, having less big finger-like pores as comparison with membrane M0. The result indicates that on one hand, the addition of QGO does not obviously change the internal structure and pores size; on the other hand, as the IPA concentration increases in the coagulation bath, the proportion of the finger-like structure decreases while the spongelike structure predominates. Hence, the porous structure is mainly

M50

Weight Loss percent (%)

80

M100 M50-QGO1

60

M50-QGO2

40 20

DrTGA

M50-QGO2

0

0

0

200

100

400

200

600

300

400

800

500

600

700

800

o

Temperature ( C) Fig. 6. TGA and DrTGA thermograms of membranes M0 - M50-QGO2. 6

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(a) cross-section of M0 membrane

(b) cross-section of M50 membrane

(c) cross-section of M100 membrane

(d) cross-section of M50-QGO1 membrane

(e) cross-section of M50-QGO2 membrane Fig. 7. SEM graphs of membrane cross-sections.

researches have shown that the space inside the membrane and the hydrophilicity are closely related with Rm. For instance, poly(vinyl alcohol) (PVA)/polystyrene sul-fonic acid-co-maleic acid (PSSA-MA) membranes with relatively loose structures exhibit highly waterswollen properties and remarkably low electrical resistances for large molecular cations [48]. The values of M50-QGO1 and M50-QGO2 (1.6–1.9 Ω cm2) are lower than that of M50 (~2.5 Ω cm2), suggesting

that the incorporation of QGO can decrease the area resistance. The functional groups including –OH, –NH2 and quaternary ammonium groups on QGO are in favor of the ions transport and lowering the membrane area resistance. Membrane M100 has a relatively high area resistance (3.5 Ω cm2), which should be attributed to the sponge-like structure and lower IEC value. But the value is still lower than that of commercial membrane CJMA-3 (6.0 Ω cm2), which is advantageous for 7

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production of lactobionic acid (LBA) by BMED removing boron by BMED

finger-like

finger-like or spongelike finger-like or spongelike

Membrane type

Thickness/mm

IEC/mmol g−1

WR/%

Rm/Ω cm2

M0 M50 M100 M50-QGO1 M50-QGO2 CJMA-3a

0.16 0.11 0.10 0.13 0.20 0.15

1.09 ± 1.14 ± 0.71 ± 1.65 ± 1.23 ± 0.5–0.6

104 ± 9 94 ± 6 74 ± 8 80 ± 2 63 ± 5 15–20

2.9 2.5 3.5 1.6 1.9 6.0

a

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

0.02 0.01 0.02 0.02 0.01

0.02 0.02 0.03 0.01 0.01

The data of membrane CJMA-3 are collected from the product brochure.

their application in BMED process. In summary, membrane M50-QGO1 with 1 wt% QGO dosage has the highest IEC value and the lowest area resistance. Hence the optimized physico chemical properties can be obtained with proper dosage of QGO into P84 membrane matrix.

3.5. Removal and recovery of boron with BMED 3.5.1. Comparison of porous P84 membranes Membranes M0, M50 and M100 were firstly compared to investigate the influence of membrane morphologies. The experiments were performed under a constant voltage of 20 V. Dilute compartment used 1000 mg B/L Na2B4O7 with initial pH of 9.14, while concentrate compartment used 100 mg B/L H3BO3 as the initial solution. The boron concentration in the dilute compartment continually decreases over time, as shown in Fig. 8, suggesting the feasibility of BMED process to remove boron. As shown in Fig. 9, the pH in the dilute compartment continues to rise because of the release of OH− ions by the bipolar membrane. Hence, most of the boron exists as the form of anion (B(OH)4−) and can smoothly migrate through the AEM to enter the concentrate compartment. The above process needs no external addition of chemical reagents, and therefore BMED recovery of boron is a clean and effective process. Membranes M0 and M50 yield higher separation efficiency of boron than membrane M100, which is attributed to their big finger-like pores, higher hydrophilicity and lower area resistance. Membrane M50 shows a quite similar concentration of boron with membrane M0 during initial 1 h, but can get a lower boron concentration after longer time of operation. The separation efficiency is higher (54.8% in Fig. 8), and the pH increasing is more significant at the end of BMED (Fig. 9). Membrane M50, though with fewer finger-like pores than M0, has higher IEC and lower area resistance (Table 2). The IEC and area resistance seem to take a dominant position so that better boron separation efficiency is obtained for membrane M50. Another possible factor that may influence 80

Boron concentration (mg/L)

Water and/or IPA P84, PEI, QGO This work

water or IPA P84, diamines [44]

water P84, PEI [23]

water or IPA P84, diamines [42]

IPA P84, diamines [43]

Non-solvent for phase inversion Starting material

± ± ± ± ±

70 60

900

50

800

40

700

30

M0 M50

600

20

M100

Separation efficiency (%)

production of HEPES by BMED

finger-like or spongelike

Sponge-like membrane (Mem-I-46) shows lower permeability but significantly higher selectivity than fingerlike membranes, including H+ transport coefficient of 0.0069 m/h, separation factor of 53.8, and water osmosis of 27 mL. HEPES of 0.0574–0.0693 mol/L with water permeation of 20–30 mL are obtained after running 6 h under 30 V. Compared with sponge-like porous membranes, the finger-like porous membranes produce more LBA under 8 V, but less LBA under 15 V or 20 V due to serious water permeation (80–145 mL). Membrane M50-QGO1 from phase inversion in 50% IPA has the separation efficiency of 68.1% under 20 V and 76.6% under 30 V, with water permeation of 42 mL and 77 mL respectively.

Free-flow isoelectric focusing system for protein separation recovering acid from H2SO4/FeSO4 mixture by DD sponge-like

The myoglobin flux can be 10 times higher than that of the original non-charged P84 membrane.

Application Membrane morphology

Performances

Table 2 Membrane physico-chemical properties including thickness, IEC, WR and area resistance.

1000

Refs.

Table 1 Summary of some researches about the P84-based porous membranes, including their preparation, morphologies and applications.

M. Sun, et al.

10

500

0

0

1

Time / h

2

3

M0

M50

M100

Membranes

Fig. 8. Change of boron concentration in the dilute compartment over time and the separation efficiency at the end of BMED running. 8

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10.8

pH in the dilute compartment

dilute the concentrate, hindering the production of concentrated solutions. Previous researches have shown that water permeation is mainly caused by osmosis and electro-osmosis when there are no obvious hydrodynamic pressure differences across the membranes [49]. When the concentration gradient is below 1 mol L−1, water permeation by osmosis is at least eight times smaller than that by EO [50] and accordingly, the EO phenomenon is mainly responsible for water permeation. Namely, water molecules can combine with ions in solution to form hydrated ions and pass through the membrane under the action of an electric field. In this study, the water permeation increases the volume of concentrate compartment and dilutes the recovered boron, which may reduce the production efficiency and increase the energy consumption in industrial production. Water permeation of porous membranes might be more serious, as the hydrated ion migration is promoted through the abundant membrane pores, inducing more obvious electro-osmosis phenomenon. The water permeation of membrane M0, M50 and M100 are 14 mL, 26 mL, and 5 mL, respectively (Fig. 10). Though membrane M50 has the largest water permeation, the value is still much lower than previous values (140–145 mL) by using finger-like and tear-like porous P84 co-polyimide membranes after the same running time (3 h) under 20 V [44]. Therefore, the water permeation here is acceptable. The results above show that M50 is the optimized membrane, which was obtained from phase inversion in 50 wt% IPA aqueous solution. Therefore, composite membranes M50-QGO1 and M50-QGO2 with dosing of 1 wt% and 2 wt% QGO were prepared with 50 wt% IPA as the non-solvent, and their performances are shown in the following sections, with M50 and commercial membrane CJMA-3 as comparisons.

M0

10.6

M50

10.4

M100

10.2 10.0 9.8 9.6 9.4 9.2 9.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time / h Fig. 9. Change of pH value with time in the dilute compartment.

the pH change is leakage of H+ and OH−: The H+ ions which are generated by the bipolar membranes may leak through the porous AEM and go into the feed solution, while the OH− ions in the feed solution may leak and go into the concentrate compartment, so that the increase of pH in the feed solution is restrained. The H+ and OH− leakage for M0 may be more serious as compared with M50 due to the presence of more finger-like pores, leading to less significant pH increase in later BMED running stage. Energy consumption and current efficiency are presented in Fig. 10. The energy consumption decreases and the current efficiency increases in the sequence of membranes M100, M0 and M50. Membrane M50 shows the energy consumption of 27.94 kW h/kg and the current efficiency of 88.8%. The reasons are that presence of finger-like voids in the lower part of the membrane cross-sections of M0 and M50 decreases the steric resistance of boron transport, while the higher IEC and lower area resistance of M50 can further improve the BMED performances. Water permeation, which is usually caused by water pressure, water concentration difference and electro-osmosis (EO), should also be considered during the BMED process. Serious water permeation will 100

3.5.2. Effect of the QGO contents and applied voltage The separation efficiency of boron is still relatively low for membrane M50, and hence membranes M50-QGO1 and M50-QGO2 were prepared with 1 wt% and 2 wt% QGO dosages, correspondingly. The BMED performances in terms of current, pH, separation efficiency, water permeation, and energy consumption were investigated by change of the voltage (20 or 30 V). The data of CJMA-3, which were obtained working in similar conditions as the self-prepared membranes, are also shown as comparison. Fig. 11 illustrates the pH of the dilute compartment increases with 50

Current efficiency Energy consumption

a

b Water permeation 30 40

Energy consumption (kW h/kg)

Current efficiency (%)

80

60

30

20

40

20

10

10

20

0

0

M0

M50

0

M100

M0

Membranes

M50

M100

Membranes

Fig. 10. Energy consumption, current efficiency of whole BMED stack and water permeation under the voltage of 20 V. Note: Water permeation is measured as the volume increase in the concentrate compartment after running 3 h. 9

Separation and Purification Technology 232 (2020) 115963 M50

20V

M50-QGO1

12.0

b

1000

CJMA-3

11.5

11.5 11.0

10.5

10.5

10.0

10.0

9.5

9.5

9.0

9.0

pH

pH

11.0

0

30 60 90 120 150 180

0

30 60 90 120 150 180

900

50

800

40

700

30 600 20 500

10 0

400 1

2

Boron concentration (mg/L)

0.14

M50-QGO1 (b) 30V

0.12

M50-QGO2

0.10

0.08

0.08

0.06

0.06

0.04

0.04

0.02

0.02

0.00

0.00 0

1

2

3

0

1

2

Current / A

Current / A

CJMA-3

0.10

M50

M50-QGO1 M50-QGO2 CJMA-3

Membranes 80

M50

1000

(a) 20V

3

Time / h

Fig. 11. pH of the dilute compartment with respect to the time under 20 or 30 V.

0.12

60

CJMA-3

0

M50

70

M50-QGO2

T/min

0.14

(a) 20 V

M50-QGO1

12.0

M50-QGO2

80

M50

12.5

30V

M50-QGO1

(b) 30 V

70

M50-QGO2

900

60

CJMA-3 800

50

700

40

600

30

500

20

400

10 0

300

0

1

2

3

Time / h

M50

M50-QGO1 M50-QGO2 CJMA-3

Membranes

Time / h

3

Separation efficiency (%)

a

Boron concentration (mg/L)

12.5

Separation efficiency (%)

M. Sun, et al.

Fig. 13. Boron concentration and separation efficiency under (a) 20 V and (b) 30 V.

Fig. 12. Current of the dilute compartment with respect to the time under 20 or 30 V.

80

time, and the increasing trend is more obvious as the applied voltage increases. This is because the water splitting by the bipolar membrane is strengthened under higher applied voltage, introducing higher amount of OH− ions into the dilute compartment. Fig. 12 shows when the operation voltage is fixed, the current increases with time gradually and then shows a tendency to decrease. The initial increase is caused by the increasing pH of the dilute solution, which leads to more formation of B(OH)3 into B(OH)4−. Accordingly, the system resistance decreases and the current increases. At later stage, most of B(OH)4− ions transfer from the dilute compartment to the concentrate compartment and combine with H+ ions produced by bipolar membrane to form H3BO3 and H2O [34]. H3BO3 has much lower conductivity than B(OH)4− ions and hence the system resistance increases. Comparison of Fig. 12(a) with Fig. 12(b) also shows that as the applied voltage increases, the current increases. This is because a higher current can lead to more rapid migration of ionic species such as B(OH)4− toward the concentrate compartment. The change of boron concentration in the dilute solution and the separation efficiency are shown in Fig. 13. The separation efficiency increases as the applied voltage increases from 20 V to 30 V. Taking membrane M50-QGO1 as an example, the separation efficiency increases from 68.1% to 76.6%. Commercial membrane CJMA-3 shows the lowest boron separation efficiency, such as 47.8% at 20 V, which is attributed to its low IEC (0.5–0.6 mmol/g), high area resistance (6.0 Ω cm2) and especially its dense structure. The dense structure will incur a higher resistance for the passage of boron. So the performance of the P84 co-polyimide porous membranes has an obvious advantage than commercial membrane CJMA-3.

Water permeation / mL

20 V 30 V 60

40

20

0

M50

M50-QGO1

M50-QGO2

CJMA-3

Membranes Fig. 14. Water permeation under 20 or 30 V, which is the increased volume of the concentrate compartment after running 3 h.

The boron separation efficiency increases in the sequence of membranes M50, M50-QGO2 and M50-QGO1. For instance, the separation efficiency of membranes M50, M50-QGO2 and M50-QGO1 are 54.8%, 57.4% and 68.1% under 20 V, respectively. Similarly, the separation efficiency of membranes M50, M50-QGO2, and M50-QGO1 are 61.9%, 62.7%, and 76.6% under 30 V, respectively. Therefore, the introduction of QGO is effective for improving the membranes separation 10

Separation and Purification Technology 232 (2020) 115963

Current efficiency Energy consumption

20V

30

Current efficiency (%)

80

60

20

40 10 20

0

M50-QGO1

M50

b

80

30V

80 60 60 40 40 20 20

0

0

M50-QGO2

Current efficiency Energy consumption

100

Current efficiency (%)

a

Energy consumption (kW h/kg)

100

CJMA-3

Energy consumption (kW h/kg)

M. Sun, et al.

0

M50

Membranes

M50-QGO1

M50-QGO2

CJMA-3

Membranes

Fig. 15. Current efficiency and energy consumption of whole BMED stack under the voltage of (a) 20 V and (b) 30 V. Table 3 The performances of different ion exchange membranes for BMED recovering of boron. Ion exchange membranes

Feed solution

AMV, CMV

Na2B4O7 (100 mg B/L)

a

b

Applied voltage (V)

Separation efficiencies

10, 15, 20

Boron concentration decreases to 4–40 mg/ L after 1 h 51.0–90.0% 61.6–74.7% 91.6–99.1% Boron concentration decreases to 9.8 mg/L after 1 h 68.1–76.6%

CMB, AHA PC SK, PC Acid 60 CMB, AHA AMV

Li2B4O7·5H2O (1000 mg B/L) Li2B4O7·5H2O (850 mg B/L) Li2B4O7·5H2O (1000 mg B/L) Na2B4O7·10H2O (100 mg B/L)

15, 20, 30 10, 13, 15, 18 25, 28, 30, 35 12.5

M50-QGO1

Na2B4O7·10H2O (1000 mg B/ L)

20, 30

current efficiencies

energy consumption

Refs.

< 10%

13–16 kWh/m3

[53]

– – 36.1–42.6% –

1.6–7.9 kWh/m – 13.4–24.0 Wh/L –

[21] [1] [33] [54]

79.1–94.9%%

26.2–47.0 kW h/kg

this work

3

a

The initial pH of the feed solution has obvious influence on the BMED performances and hence the data under pH of 9.1 were listed here for better comparison with the present work. b These are the data based on the boron concentration change in the dilute compartment for better comparison with the present work.

performances. As membranes M50, M50-QGO1 and M50-QGO2 have similar membrane morphology, some other factors are influencing the membrane performances. On one hand, the functional groups of GQO such as –OH, -NH2 and quaternary ammonium groups may accelerate the transport of B(OH)4− ions by hydrogen and electrostatic interaction. Previous research also reveals that QGO demonstrates an excellent electrochemical performance and is used to improve ion selectivity of composite membranes [36]. On the other hand, membrane M50-QGO1 has the highest IEC and thus the highest membrane surface charge, the lowest area resistance (1.6 Ω cm2) and thus the least hindrance to ions, both of which accelerate the membrane flux [51]. The water permeation values, as shown in Fig. 14, become more obvious as the applied voltage increases. For instance, use of membrane M50, M50-QGO1 or M50-QGO2 yields 41 mL, 77 mL, 44 mL water permeation under 30 V, respectively. The data are higher than that of the CJMA-3 membrane, but still much lower than that in our previous work [44]. Some other data of our research group can be further compared: The water permeation was 28–63 mL after 3 h ED running under 60 mA cm−2 to separate NaOH from the sodium aluminate solution using commercial dense membranes (CMV/AMV, CJ-EDC/EDA or JCM/ JAM) [52]; The water permeation was 20–30 mL after 6 h BMED running under 30 V to produce HEPES using finger-like porous P84-based AEMs [23]. Therefore, the water permeation is also influenced by the operation conditions such as the category of hydrated ions, voltage drop and running time. Overall, membrane M50-QGO1 with 1 wt% QGO has significant advantage for removal of boron ions. Though the membrane has higher water permeation, its high electro-chemical properties guarantee the most rapid permeation of boron and the highest removal efficiency.

Table 4 An estimation of the process cost. Operation conditions Voltage drop Operation time Effective membrane area Fluid flow rate Concentration of Na2SO4 Initial concentration of dilute compartment Initial concentration of concentrate compartment Stack configuration Energy consumption

30 V 3h 0.002 m2 18 L/h C0(Na2SO4) = 0.1 mol/L C0(Na2B4O7·10H2O) = 1000 mg B/L C0(H3BO3) = 100 mg B/L BP-A-BP-A-BP 47.04 kW h/kg

Investment cost Membrane life Membrane price (mono-polar) Membrane price (bipolar-polar) Cost of membrane Cost of stack Peripheral cost Total investment cost

5 years 117.31 $/m2 1513.72 $/m2 9.55 $ 14.33 $ 21.49 $ 35.82 $

Energy cost Electricity charge Energy cost for production Energy cost peripheral equipment Total energy cost

0.09 $/kW h 4.234 $/kg 0.212 $/kg 4.446 $/kg

Total fixed cost Total process cost

13.61 $/year 38.09 $/kg H3BO3

11

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can guarantee that most of the boron exists as an anion (B(OH)4−) and can smoothly migrate through the AEM to enter the concentrate compartment. The porous P84 co-polyimide membranes show more excellent BMED performances than commercial membrane CJMA-3. The membrane from phase inversion in 50% IPA aqueous solution can yield higher boron separation efficiency (54.8% under 20 V), and incorporation of proper QGO (1 wt%) can further increase the BMED performances including increased boron separation efficiency (68.1% under 20 V and 76.6% under 30 V), enhanced current efficiency and lower energy consumption. Water permeation for the optimal membrane is enhanced at the same time (42–77 mL under 20–30 V). Overall, the QGO-P84 composite porous AEMs can be successfully utilized in BMED process to remove boron from aqueous solutions.

The porous membranes have lower energy consumption and higher current efficiency than membrane CJMA-3, as shown in Fig. 15. For instance, the values are 26.16 kW h/kg and 94.9% under 20 V for membrane M50-QGO1, which are superior to the performances of membrane CJMA-3 (30.56 kW h/kg and 81.2%). The higher BMED performances indicate the porous P84 composite AEMs provide sustainable and feasible alternative for removing boron from aqueous solutions. In addition, membrane M50-QGO1 also has higher performances than membrane M50, indicating the addition of QGO can improve the membrane performances. The data of some previous researches regarding BMED removal of boron are listed in Table 3, together with those of M50-QGO1 membrane. Comparison of the different data shows that M50-QGO1 membrane can yield excellent current efficiencies and separation efficiencies. The energy consumption is relatively higher than some reported data. Feed concentration may also influence the energy consumption significantly. For instance, BMED removal of boron ions using AMV/CMV membranes shows the current efficiency and energy consumption of < 10% and 15.5 kW h/m3 when the voltage was 20 V, the initial boron concentration was 100 mg/L [53]. Here the current efficiency and energy consumption of membrane M50-QGO1 are 94.9% and 14.81 kW h/m3 respectively under 20 V, with initial boron concentration of 1000 mg B/L. Overall, the BMED operation in this work can be comparable, or superior to previous reports.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21476056 and 21606063). References [1] S. Bunani, M. Arda, N. Kabay, K. Yoshizuka, S. Nishihama, Effect of process conditions on recovery of lithium and boron from water using bipolar membrane electrodialysis (BMED), Desalination 416 (2017) 10–15. [2] A. Shahmansouri, J. Min, L.Y. Jin, C. Bellona, Feasibility of extracting valuable minerals from desalination concentrate: a comprehensive literature review, J. Clean. Prod. 100 (2015) 4–16. [3] P. Loganathan, G. Naidu, S. Vigneswaran, Mining valuable minerals from seawater: a critical review, Environ. Sci. Water Res. Technol. 3 (2017) 37–53. [4] R.P. Aj Wyness, C. Neal, A summary of boron surface water quality data throughout the European Union, Sci. Total Environ. 314 (2003) 255–269. [5] X. Wen, F.Z. Li, X. Zhao, Removal of nuclides and boron from highly saline radioactive wastewater by direct contact membrane distillation, Desalination 394 (2016) 101–107. [6] M.L. Tang, T.L. Deng, M.X. Miu, Research of recovery of boric acid by precipitation from the mother liquor after desalting, J. Salt Chem. Ind. 23 (5) (1994) 17–19. [7] U. Roessner, J.H. Patterson, M.G. Forbes, G.B. Fincher, P. Langridge, A. Bacic, An investigation of boron toxicity in barley using metabolomics, Plant. Physiol. 142 (2006) 1087–1101. [8] P.A. Fail, R.E. Chapin, C.J. Price, J.J. Heindel, General, reproductive, developmental, and endocrine toxicity of boronated compounds, Reprod Toxicol. 12 (1998) 1–18. [9] W.H. Organization, Guidelines for Drinking-water Quality 4th Ed, (2011). [10] H. Hyung, J.H. Kim, A mechanistic study on boron rejection by sea water reverse osmosis membranes, J. Membr. Sci. 286 (2006) 269–278. [11] L.A. Richards, M. Vuachère, A.I. Schäfer, Impact of pH on the removal of fluoride, nitrate and boron by nanofiltration/reverse osmosis, Desalination 261 (2010) 331–337. [12] L.A. Melnik, Y.V. Babak, V.V. Goncharuk, I.K. Chepurnaya, Application potential of boron-selective sorbents of different nature for water conditioning in terms of the boron content, J. Water Chem. Technol. 37 (2015) 25–31. [13] E.H. Ezechi, M.H. Isa, S.R.M. Kutty, A. Yaqub, Boron removal from produced water using electrocoagulation, Process. Saf. Environ. 92 (2014) 509–514. [14] M.A. Sari, S. Chellam, Mechanisms of boron removal from hydraulic fracturing wastewater by aluminum electrocoagulation, J. Colloid Interface Sci. 458 (2015) 103–111. [15] E. Kir, B. Gurler, A. Gulec, Boron removal from aqueous solution by using plasmamodified and unmodified anion-exchange membranes, Desalination 267 (2011) 114–117. [16] D.E. Akretche, H. Kerdjoudj, Donnan dialysis of copper, gold and silver cyanides with various anion exchange membranes, Talanta 51 (2000) 281–289. [17] S. Samatya, P. Köseoğlu, N. Kabay, A. Tuncel, M. Yüksel, Utilization of geothermal water as irrigation water after boron removal by monodisperse nanoporous polymers containing NMDG in sorption–ultrafiltration hybrid process, Desalination 364 (2015) 62–67. [18] N. Kabay, O. Arar, F. Acar, A. Ghazal, U. Yuksel, M. Yuksel, Removal of boron from water by electrodialysis: effect of feed characteristics and interfering ions, Desalination 223 (2008) 63–72. [19] L. Melnyk, V. Goncharuk, I. Butnyk, E. Tsapiuk, Boron removal from natural and wastewaters using combined sorption/membrane process, Desalination 185 (2005) 147–157. [20] M. Turek, B. Bandura, P. Dydo, Electrodialytic boron removal from SWRO permeate, Desalination 223 (2008) 17–22. [21] D. İpekçi, E. Altıok, S. Bunani, K. Yoshizuka, S. Nishihama, M. Arda, N. Kabay, Effect of acid-base solutions used in acid-base compartments for simultaneous recovery of lithium and boron from aqueous solution using bipolar membrane electrodialysis (BMED), Desalination 448 (2018) 69–75. [22] C.H. Huang, T.W. Xu, Electrodialysis with bipolar membranes for sustainable development, Environ. Sci. Technol. 40 (2006) 5233–5243.

3.5.3. Process economics The process cost is estimated on the basis of the laboratory scale experimental equipment under the conditions shown in Table 4. The total cost of BMED comprises the investment cost and energy consumption in electrodialytic water dissociation. The total investment cost is the sum of the stack and peripheral equipment cost. The stack cost is proportional to the membrane cost and is estimated to be 1.5 times the total membrane cost. The peripheral equipment cost (pumps, monitoring, control panes, etc.) is estimated at 1.5 times the stack cost. It can be observed that the process cost for the recycling of 1 kg H3BO3 from the model aqueous solution is around USD 38.09. The cost is much higher than the price of H3BO3 for common usage, but similar to or even lower than that of H3BO3 for electrophoresis or molecular biology usages. The high cost of the present work is mainly because the annual production of H3BO3 produced by the BMED process is low. It can be expected that the process cost becomes lower in a pilot scale or industrial scale because both the energy cost and investment cost will decrease with production capacity. In addition, the BMED technology can provide products of higher quality, higher recovery and are more environmentally friendly. The productiono of relatively pure byproducts will also make up for the cost disadvantage. It is reasonable to assume that BMED is promising due to the increase in environmental awareness and raw material cost. 4. Conclusion Quaternized graphene oxide (QGO) is synthesized and incorporated into P84 co-polyimide, followed by phase inversion, amination and quaternization processes so that porous AEMs are prepared. The phase inversion process is varied to control membrane porous structure. The finger-like pores decrease while the sponge-like pores predominate as the isopropanol concentration increases from 0%, 50% to 100% in the coagulation bath. The addition of QGO introduces functional groups such as –OH and –NH2, which can improve the membrane physicochemical properties and thus are advantageous for their application in BMED process. QGO-P84 composite AEMs show high hydrophilicity and IEC (1.23–1.65 mmol/g), low area resistance (1.6–1.9 Ω cm2) and high short-term thermal stability (above 250 °C). The porous membranes are used in BMED process to remove boron from aqueous solution. The pH in the dilute compartment continues to rise because of the release of OH− ions by the bipolar membrane, which 12

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