Given the substantial environmental pollution from industrial expansion, environmental protection has become particularly important. Nowadays, anion exchange membranes (AEMs) are widely used in wastewater treatment. With the use of polyvinyl alcohol (PVA), ethylene-vinyl alcohol (EVOH) copolymer, and methyl iminodiacetic acid (MIDA), a series of cross-linked AEMs were successfully prepared using the solvent casting technique, and the network structure was formed in the membranes due to the cross-linking reaction between PVA/EVOH and MIDA. Fourier transform infrared spectrometer, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy were used to analyze the prepared membranes. At the same time, its comprehensive properties which include water uptake, linear expansion rate, ion exchange capacity, thermal stability, chemical stability, and mechanical stability were thoroughly researched. In addition, diffusion dialysis performance in practical applications was also studied in detail. The acid dialysis coefficient (UH+) ranged from 10.2 to 35.6 × 10−3 m/h. Separation factor (S) value ranged from 25 to 38, which were all larger than that of the commercial membrane DF-120 (UH+: 8.5 × 10−3 m/h, S: 18.5). The prepared membranes had potential application value in acid recovery.

  • The preparation method of the membranes is simple.

  • The membrane is based on PVA/EVOH/MIDA.

  • The raw materials are cheap and easy to obtain.

  • The acid recovery performance is excellent by diffusion dialysis.

  • The prepared membranes had potential application value in acid recovery.

In the process of industrial development, the environmental problems from industrial emissions are intensifying (Wang et al. 2022; Saravanan et al. 2021; Rathod et al. 2023). For example, sulfide minerals generate large amounts of wastewater during mining and transportation (Dudeney et al. 2012). Owing to the industrial wastewater having enormous utilization value, many researchers seek different technologies to recover resources. The traditional wastewater treatment technologies that include evaporation (Chen et al. 2020a), neutralization (Hu et al. 2021), crystallization (You et al. 2022), and thermal decomposition (Chen et al. 2020b) are slightly inadequate (Lu et al. 2017). Due to the significant environmental friendliness and low operating costs, diffusion dialysis (DD) stands out among numerous wastewater treatment technologies (Wei et al. 2010; Luo et al. 2011; Mao et al. 2014; Lu et al. 2017; Pawar et al. 2023).

In the DD process, ion exchange membranes (IEMs) are essential components. IEMs mainly consist of three parts, which include polymer matrixes, immobilized ion-functionalized groups, and movable counter-ions. IEMs are categorized into anion exchange membranes (AEMs) and cation exchange membranes (Ran et al. 2017). Here, the focus of research is on AEMs for acid wastewater recovery. AEMs with excellent performance should possess high ion conductivity, robust chemical and structural integrity, suitable water uptake, robust mechanical strength, and economic viability (Li et al. 2011b; Afsar et al. 2018; Lin et al. 2023). The design and preparation of high-performance AEMs are the core of the whole problem.

Polymer matrix materials are the most important part of AEMs, which have a significant effect on membrane performance (Lee et al. 2016). Currently, the polymer materials used for preparing AEMs mainly include polyvinyl alcohol (PVA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polyethersulfone (PES), and polystyrene (Xiao et al. 2018). These polymeric membranes such as PVA, PVC, PVD, and PES are also widely used in filtration applications (Acarer 2023). The PVA structure comprises a hydrogen bonding network, which greatly promoted the transport of H+ ions in DD. In addition, PVA with hydrophilic properties was widely studied owing to its low cost, excellent chemical stability, good flexibility, and strong surface adhesion (Ng et al. 2013). Nevertheless, due to the extreme hydrophilicity of PVA, the dimensional stability of the membranes based on PVA was poor in the practical application process (Huang et al. 2010; Du et al. 2018).

To solve this problem, the common method is to blend PVA with other hydrophobic polymers. However, the selection of blending materials is extremely stern. If the material selection is not appropriate, it will lead to poor coordination of the membranes. After comprehensive consideration, ethylene-vinyl alcohol (EVOH) with similar properties to PVA is introduced into the membrane preparation process. Initially, EVOH exhibits unique chemical composition, consisting of hydrophobic ethylene segments and hydrophilic vinyl alcohol segments (Du et al. 2018). This combination, particularly the hydrophobic ethylene content, significantly mitigates membrane expansion in the presence of solvents (Simari et al. 2021; Lin et al. 2023). At the same time, EVOH has similar structural characteristics to PVA, which indicates that these two materials have good compatibility. In addition, EVOH has excellent mechanical strength and film-forming performance (Luzi et al. 2018). Therefore, EVOH is suitable as a material for blending with PVA during the membrane preparation process.

In this work, methyl iminodiacetic acid (MIDA) was introduced in membrane preparation. Due to the presence of two carboxyl groups (–COOH) in MIDA, the network structure was formed in the membranes by the reaction between the carboxyl group and hydroxyl group, which greatly limited the swelling rate of the membranes. Through the quaternization reaction of MIDA and methyl iodide (CH3I), the ion exchange sites were introduced into the membranes. Meanwhile, MIDA, PVA, and EVOH were easily dissolved in dimethyl sulfoxide (DMSO), so the membrane preparation process was simple. Five different AEMs were successfully prepared by changing the content of MIDA, and the structure and comprehensive performance were studied. The application performance was also researched. It was proved that the prepared membranes had good application prospects.

Materials

PVA with a molecular weight of 1,750 ± 50 was purchased from Sinopharm Chemical Reagent Co., Ltd (China). EVOH with an ethylene molar content of 27% was supplied by Kuraray Co., Ltd (Tokyo, Japan). MIDA (98% purity) and methanol (MeOH, 99.5% purity) were both from Shanghai Maclean's Biochemical Technology Co., Ltd (China). DMSO analytical reagent (AR) and hydrochloric acid (HCl, AR) were from Sinopharm Group Chemical Reagent Co., Ltd (China). Methyl iodide (CH3I, 98% purity) was provided by Shanghai Yien Chemical Technology Co., Ltd (China) as a quaternization reagent. Deionized water (DI water) was used throughout the preparation and testing process.

2.2. Preparation of MIDA membranes

The preparation process is exhibited in Figure 1. Here, membrane A was taken as an example to introduce the detailed preparation process.
Figure 1

Experimental process diagram.

Figure 1

Experimental process diagram.

Close modal

First, 0.1 g EVOH and 0.4 g PVA were dissolved in 10 mL DMSO, and the PVA/EVOH/DMSO solution was stirred at 100 °C for 2 h to ensure that the mixture was completely dissolved. Then, 0.1 g MIDA was dissolved in 10 mL DMSO, and the MIDA/DMSO solution was stirred at 70 °C for 1 h. After cooling to room temperature, the MIDA/DMSO solution was added to the PVA/EVOH/DMSO solution, and 3–5 drops of HCl were added to the mixture solution (Emmanuel et al. 2016b). The mixture solution reacted at 100 °C for 24 h.

Following this step, the mixture solution was spread onto flat glass and dried in an oven at 60 °C for 48 h. The membranes were peeled off from the glass plate. The membranes need to undergo gradual temperature rise from 60 to 120 °C with heating rate of 10 °C/h. The membranes were then held at 120 °C for 4 h to achieve thermal cross-linking. Afterward, the membranes were cleaned with DI water 3–4 times. The membranes were dried in a vacuum oven at 60 °C for 24 h.

Ultimately, the membranes were immersed in DI water for 48 h with the water being refreshed every 8 h. Subsequently, the membranes were placed in 0.5M CH3I/MeOH solution for 48 h to undergo the quaternization reaction. The membranes were removed from the solution and thoroughly washed with DI water. It was dried in an oven set at 60 °C. The preparation process of membrane A was completed.

Through adjustment of the MIDA content, five different membranes were prepared. These membranes were designated as A–E and had their respective compositions detailed in Table 1. The reaction equation is presented in Figures 24. The equation for the quaternization reaction is presented in Figures 57.
Table 1

Compositions of membranes A–E

MembranePVA (g)EVOH (g)DMSO (mL)MIDA (g)The ratio of MIDA to PVA/EVOH (%)
0.4 0.1 10 0.1 20 
0.4 0.1 10 0.15 30 
0.4 0.1 10 0.2 40 
0.4 0.1 10 0.25 50 
0.4 0.1 10 0.3 60 
MembranePVA (g)EVOH (g)DMSO (mL)MIDA (g)The ratio of MIDA to PVA/EVOH (%)
0.4 0.1 10 0.1 20 
0.4 0.1 10 0.15 30 
0.4 0.1 10 0.2 40 
0.4 0.1 10 0.25 50 
0.4 0.1 10 0.3 60 
Figure 2

The reaction equation between PVA and MIDA.

Figure 2

The reaction equation between PVA and MIDA.

Close modal
Figure 3

The reaction equation between EVOH and MIDA.

Figure 3

The reaction equation between EVOH and MIDA.

Close modal
Figure 4

The reaction equation between PVA/EVOH and MIDA.

Figure 4

The reaction equation between PVA/EVOH and MIDA.

Close modal
Figure 5

The quaternization reaction of CH3I, PVA, and MIDA.

Figure 5

The quaternization reaction of CH3I, PVA, and MIDA.

Close modal
Figure 6

The quaternization reaction of CH3I, EVOH, and MIDA.

Figure 6

The quaternization reaction of CH3I, EVOH, and MIDA.

Close modal
Figure 7

The quaternization reaction of CH3I, PVA/EVOH, and MIDA.

Figure 7

The quaternization reaction of CH3I, PVA/EVOH, and MIDA.

Close modal

Measurement of membrane properties

Fourier transform infrared spectrometer

The prepared membranes were determined by the use of Fourier transform infrared (FT-IR) spectrometer (Nicolet 6700, Thermo Scientific). The testing procedure was as follows. Before analyzing the membrane sample, cut the membrane into 1 × 1 cm samples and scan the blank infrared window to obtain the background spectrum. The membrane samples were placed in the sample tray, and the detection part was fully in contact with the membrane sample. The infrared absorption spectra were recorded in the range of 4,000–500 cm−1.

X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy

Before conducting X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) tests, the membrane samples were washed and dried. Then, the membrane was cut into small pieces and placed on the sample board for fixation and other operations. XPS (Axis Ultra, Kratos, Manchester, UK) was employed to obtain the membrane spectra. SEM (Hitachi Regulus 8230) was utilized to obtain cross-sectional images. In addition, the phase separation was examined using TEM (JEM-2100, Japan).

Water uptake and linear expansion ratio

Water uptake (WR) was a significant parameter for evaluating the hydrophilic membranes. WR was measured using the following method. Initially, the dry membranes were soaked in DI water at ambient temperature for 48 h. The water-absorbed membranes were taken from DI water. Excess water on the membrane surface was eliminated by using absorbent paper. The wet weight (Wwet) of the membranes was recorded. Following that, the membranes were subjected to drying in an oven at 60 °C until their weight no longer fluctuated. The final dry weight, noted as Wdry, was then ascertained and recorded. WR values were the result of Equation (1).
formula
(1)
Linear expansion ratio (LER) was to study the dimensional stability of the membranes under certain conditions. The experimental process was as follows. The membranes were cut into 20 × 40 mm sizes and immersed in DI water for 24 h. The dimensions of the samples in their wet state were documented. The lengths of both the dry and wet membranes throughout the experiment were designated as Ldry and Lwet, respectively. Through Equation (2), LER for the membranes was computed for both the dry and wet states.
formula
(2)

Thickness

A screw micrometer was used to measure the thickness of the membranes.

Thermogravimetric analysis and chemical stability

The thermal stability of the prepared membranes, pure PVA, and EVOH film was examined under an N2 atmosphere by using a thermogravimetric analysis (TGA) analyzer (TGA 550). The temperature during the heating process varied between 50 and 600 °C, with constant heating increments of 10 °C/min.

All membranes were dried at 100 °C for 8 h before testing to remove excess moisture.

The following was the chemical stability testing procedure. The membranes were dried to consistent mass and recorded. The membranes were subjected to immersion in 2 M HCl aqueous solution for durations of 7 and 14 days, respectively. Subsequently, they were extracted from the solution and rinsed with DI water. They were recorded again after drying these membranes to consistent mass. The chemical stability can be measured using mass maintenance based on the mass recorded before and after.

Mechanical property

With the use of a stretching tester (CMT4304, MTS Industrial Systems Co., Ltd), the mechanical properties of the dry membranes were measured with the stretch rate of 25 mm/min at 25 °C.

Ion exchange capacities

Ion exchange capacities (IECs) of the membranes were determined using the Mohr method. Initially, the dried membranes were immersed in 1 M NaCl solution for 36 h so that the membranes could be completely converted into Cl ion form. The membranes were next removed from the 1 M NaCl aqueous solution and cleaned repeatedly with DI water to completely rinse the NaCl solution from the membrane surface. The membranes were later immersed in 0.05 M Na2SO4 aqueous solution for 48 h, so that Cl ions could be completely released from the membranes and replaced by . Finally, the chloride ions in the final sediment were determined by titration with 0.05 M AgNO3, using K2CrO4 as the colorimetric indicator. IEC was calculated using Equation (3).
formula
(3)

Among them, the quantities of AgNO3 solution consumed, its concentration, and the weight of the dry membrane are denoted by and , respectively.

DD performances

As depicted in Figure 8, the DD experiment was carried out in a device that consisted of two independent chambers. The two chambers were separated by AEMs, which had an effective area of 5.5 cm2. They were labeled as the feed side and the permeate side, respectively. This setup aimed to investigate ion transport across the membranes under simulated acidic conditions. Before the experiment, the membranes were put in simulated acid wastewater (1 M HCl and 0.2 M FeCl2) for 4 h. Into the feed compartment, 100 mL of simulated acidic effluent was introduced, while the permeate compartment was supplied with 100 mL of DI water.
Figure 8

Experimental device for DD.

Figure 8

Experimental device for DD.

Close modal
During the DD experiment, mechanical stirring was implemented in both compartments to mitigate the effects of concentration polarization. The experiment lasted for 45 min. Subsequently, 10 mL of both feed liquid and penetrant liquid were measured from either side of the DD device. Utilizing methyl orange as the pH indicator, the HCl concentration of both sides was tracked via the 0.05 M Na2CO3 aqueous titration method. The concentrations of FeCl2 were assessed through the 0.002 mol/L KMnO4 aqueous titration method. All experiments were performed at room temperature. The dialysis coefficient (U) can be determined using Equation (4).
formula
(4)
In this formula, M serves as the representative for the transported component (mol), and A denotes the effective membrane area and symbolizes DD time (h). ΔC represents the logarithm of the average component concentration and is determined by Equation (5).
formula
(5)
In this equation, denotes the initial feed concentration at time 0, while signifies the feed concentration at later time t. refers to the concentration of the dialysate at time t. S value (separation factor) was used to measure the efficiency of the membrane separation during DD. According to the above data, S value can be expressed using Equation (6).
formula
(6)

S is the ratio of to . represents the diffusion rate of hydrogen ions (H+) through the membranes. expresses the diffusion rate of iron ions (Fe2+) through the membranes.

The above experimental results were the average of three test results.

FT-IR analysis of the prepared membranes

The prepared membranes were analyzed by FT-IR, and the results are shown in Figure 9. In Figure 9(a), the broad band which located at 3,500–3,200 cm−1 corresponded to the hydroxyl groups (–OH) in PVA and EVOH (Cheng et al. 2014). The strong peak at 2,950–2,850 cm−1 was attributed to symmetric stretching of –CH2– groups in PVA, EVOH, and MIDA (Zhang et al. 2009; Guan et al. 2017; Chen et al. 2021). The peak at 1,650 cm−1 was attributed to the C–N bond stretching, which was accounted for the quaternization reaction of MIDA and hydroxyl groups (–OH) (Lin et al. 2023; Pawar et al. 2023). The band at 1,100 cm−1 was identified as the C(=O)–O–C bond stretching (Irfan et al. 2018; do Nascimento et al. 2020). In Figure 9(b), the peak at 1,745 cm−1 came from the vibration of the ‘C = O’ group by obtaining the reaction of MIDA and PVA/EVOH (Emmanuel et al. 2016b; Vidhyeswari et al. 2021). Through the above analysis, the successful reaction between MIDA and PVA/EVOH was proven.
Figure 9

(a) FT-IR spectra of membranes A–E and PVA/EVOH film. (b) FT-IR spectra of membrane C and PVA/EVOH film.

Figure 9

(a) FT-IR spectra of membranes A–E and PVA/EVOH film. (b) FT-IR spectra of membrane C and PVA/EVOH film.

Close modal

XPS analysis of the prepared membranes

The elemental composition of the membranes was analyzed by XPS, and the corresponding data are illustrated in Figure 10 (Lin et al. 2016). According to Figure 10(a), the peak at 528 eV (O 1s) and 280 eV (C 1s) were observed at membranes A–E, pure PVA, and pure EVOH films. While compared to pure PVA and EVOH films, additional peaks at 400.0 eV (N 1) can be observed in membranes A–E (Pan et al. 2021). This was mainly attributed to the quaternization reaction of MIDA and CH3I. Through these data, the reaction between MIDA and PVA/EVOH is further proved successfully.
Figure 10

(a) XPS spectra of membranes A–E and PVA/EVOH film. (b) High-resolution N 1s spectra of membrane C.

Figure 10

(a) XPS spectra of membranes A–E and PVA/EVOH film. (b) High-resolution N 1s spectra of membrane C.

Close modal

Furthermore, to prove the success of the quaternization reaction, the N 1s peak was analyzed in detail. Taking membrane C as an example, the XPS signal was clearly deconvolved into two peaks in Figure 10(b). The peak at 398 eV was derived from the C–N bonds in MIDA (Afsar et al. 2018). The peak located at 402.7 eV was related to the quaternary N atoms (C–N+), which successfully confirmed the quaternization reaction (Lin et al. 2023).

Thermal stability

In the actual environment, thermal stability was one of the comprehensive properties of AEMs. The TGA test performance results for membranes A–E are shown in Figure 11 (Wei et al. 2005).
Figure 11

TGA curves of membranes A–E and PVA/EVOH film.

Figure 11

TGA curves of membranes A–E and PVA/EVOH film.

Close modal

From TGA curves, it was observed that the weight loss of the prepared membranes was divided into three stages. In the first stage, within the temperature span of 60–150 °C, the evaporation of absorbed and bound water molecules resulted in weight loss (Tsai et al. 2015). When the temperature was in the range of 240–340 °C, the second stage was caused by the breakdown of MIDA in membranes (Emmanuel et al. 2016b). When the temperature is at 400–500 °C, the third stage was linked to the decomposition of the polymer main chain (Emmanuel et al. 2016a). The tested temperature was much higher than the actual temperature, which indicated that the prepared membranes had excellent thermal stability.

According to the experimental results, the thermal degradation temperature (Td, here it was defined as the temperature resulting in 5% weight loss) of the prepared membranes was about 300 °C, while the Td of PVA and EVOH were around 245 and 350 °C, respectively.

Membrane morphology

The morphology of AEMs had an important effect on acid recovery efficiency. Figure 12 depicted SEM images of the membrane surface. It was observed that the membrane surface was uniform and compact, and no holes or cracks were observed on the surface of the prepared membranes (Chen et al. 2023). It was observed that the surface was smooth from membranes A to B, which indicated favorable compatibility between different materials (Kadanyo et al. 2022), whereas it exhibited slight surface phase separation in membranes C–E, and slight phase separation was observed on the membrane surface, which had a positive effect on DD.
Figure 12

SEM images of membranes A–E.

Figure 12

SEM images of membranes A–E.

Close modal
Furthermore, the phase separation was directly observed by TEM. As shown in Figure 13, with the MIDA content increased, the membranes C–E exhibited significant phase separation. Here, the dark area corresponded to the hydrophilic domains in the membrane matrix, which consisted of quaternary ammonium groups and hydrophilic ethylene alcohol units in PVA and EVOH (Yadav et al. 2021). The bright area represented the hydrophobic units in PVA and EVOH (Al Munsur et al. 2022). The incorporation of hydrophilic and hydrophobic components in the prepared membranes had a beneficial effect on the formation of ion transport channels, which enhanced the acid recovery performance of the membranes (Kadanyo et al. 2022).
Figure 13

TEM images of membranes A–E.

Figure 13

TEM images of membranes A–E.

Close modal

Mechanical stability

To assess the mechanical characteristics of the membranes, the tensile strength (TS) and elongation at break (Eb) data are graphically represented in Figure 14. It was visible that the Eb value decreased from 160.5 to 21.82%, TS value increased from 15 to 28.94 MPa. Usually, these two followed opposite trends. The main reason for the increasing trend of TS values was that the cross-linking degree increased, which enhanced the structural network of the prepared membranes (Irfan et al. 2017). The prepared membranes had more excellent mechanical properties compared with some based on PVA membranes (TS was 8.3–20.3 MPa, TS was 9.1–26.0 MPa) (Gu et al. 2012; Tong et al. 2017).
Figure 14

TS and Eb values of membranes A–E.

Figure 14

TS and Eb values of membranes A–E.

Close modal

Chemical stability

In the process of acid recovery, good chemical stability was crucial to the performance of the membranes, which also affected the actual application process. To simulate actual application conditions, the prepared membranes were placed in a 2.0 M aqueous HCl solution. The mass maintenance after 7 and 14 days at 25 °C was recorded in Table 2. It can be clearly seen that the mass maintenance remained above 97%, which proved that the prepared membrane had excellent acid stability. The experimental results were mainly attributed to the excellent chemical stability of PVA and EVOH (Lv et al. 2006). The excellent acid stability proved the feasibility of using prepared membranes for DD acid recovery.

Table 2

Mass maintenance of the prepared membranes A–E

MembraneABCDE
Mass maintenance (%) 7 days 98.1 97.9 98.7 99.5 98.4 
14 days 98.0 97.3 98.1 98.9 98.2 
MembraneABCDE
Mass maintenance (%) 7 days 98.1 97.9 98.7 99.5 98.4 
14 days 98.0 97.3 98.1 98.9 98.2 

WR, LER, IEC, and thickness

The WR of the membranes was closely related to its hydrophilicity, which played an important role in the membrane during the DD process (Li et al. 2011a). As shown in Figure 15(a), WR showed a decreasing trend from 100 to 46.2%. As a hydrophilic material, the PVA content continued to decrease from membranes A to E, which resulted in a decrease in WR. Compared to some membranes based on PVA, the WR of the prepared membranes were controlled within better range (63–205%, 196–267%) (Wu et al. 2013; Cheng et al. 2014).
Figure 15

(a) WR and LER of membranes A–E. (b) IEC of membranes A–E.

Figure 15

(a) WR and LER of membranes A–E. (b) IEC of membranes A–E.

Close modal

Furthermore, the dimensional stability of the membranes can be better understood through the LER value. According to Figure 15(a), LER followed the same trend as WR and the value decreased from 30 to 22.5% from membranes A to E. In the process of membrane formation, different network structures were constructed in the membrane matrix. These network structures intertwined and formed complex and stable structures (Xu et al. 2019). Meanwhile, the EVOH network structure construction was more stable in contrast with PVA; thus, the swelling of PVA was effectively controlled. The LER was greatly controlled within a certain range, which indicated that the prepared membrane had good dimensional stability.

The IEC for different MIDA content is shown in Figure 15(b). The IEC value increased from 0.54 to 1.89 mmol/g with increasing MIDA content. Numerous quaternary ammonium groups were introduced into the membranes, which provided enough ion exchange sites. All prepared membranes were controlled at around 100 μm with the use of membrane scraper.

Diffusion dialysis

and S values were the most vital criteria to evaluate the performance of AEMs in acid recovery applications. The relevant data are presented in Figure 16.
Figure 16

(a) and of membranes A–E. (b) S value of membranes A–E.

Figure 16

(a) and of membranes A–E. (b) S value of membranes A–E.

Close modal

The values ranged from 10.2 to 35.6 × 10−3 m/h and were influenced by the following factors. First, PVA and EVOH contained a large amount of hydroxyl groups (–OH). These hydroxyl groups had high hydrophilicity, which greatly enhanced H+ ions transport through hydrogen bonding (Wu et al. 2013). Then, the increased IEC was also beneficial for H+ ions transport. High IEC prompted counter-ions to pass through ion sites, which allowed more anions to be transported to the dialysis side (Zhou et al. 2014; Rathod et al. 2023). To satisfy the electrical neutralization of the solution, a large amount of H+ was also transported to the dialysis side (Chen et al. 2023). Finally, through the morphological analysis in Section 3.5, the degree of phase separation gradually increased from membranes A to E, which led to a significant reduction in the resistance of ion transport (Emmanuel et al. 2017). Therefore, the transport of H+ also increased.

was 0.26–1.42 × 10−3 m/h in Figure 16(a). It was observed that Fe2+ and H+ follow the same increased trend. The above reasons for explaining the increasing trend of also apply to . However, the Fe2+ ions had a larger radius compared to H+ and carried more positive charge, which resulted in less efficient transport of Fe2+ than H+ (Rathod et al. 2023).

The separation factor (S) was calculated by the ratio of to . Based on the above data, the S value was shown in Figure 16(b) and ranged from 38 to 25. Both and showed an increasing trend. However, the effect of phase separation on promoting Fe2+ transport was higher than that of H+, which led to faster growth rate of than (Zuo et al. 2022). Therefore, the S value showed a downward trend. However, the prepared membranes exhibited better performance compared to other conventional materials (Table 3). The prepared membranes in the DD process had notably progressed, which demonstrated its substantial application value in the field of acid recovery.

Table 3

Comparison of present data with reported

Membrane typeSReference
BPPO 5.6–10.4 21.9–38.8 Khan et al. (2016)  
PVA 10.2–35.6 24–38 This work 
DF-120 18.5 Cheng et al. (2014)  
TPSF 4.2–6.5 24.5–34 Lin et al. (2017)  
PVC 12–40 36–61 Cheng et al. (2015)  
Membrane typeSReference
BPPO 5.6–10.4 21.9–38.8 Khan et al. (2016)  
PVA 10.2–35.6 24–38 This work 
DF-120 18.5 Cheng et al. (2014)  
TPSF 4.2–6.5 24.5–34 Lin et al. (2017)  
PVC 12–40 36–61 Cheng et al. (2015)  

In this work, five AEMs were successfully prepared based on PVA/EVOH and MIDA. The membrane preparation process was simple. Through TGA and chemical stability tests, the prepared membranes exhibited good thermal stability (Td value surpassed 300 °C) and acid stability. TS (15–28.94 MPa) and Eb values (160.5–21.82%) proved that the membranes had excellent mechanical stability compared to the previous study (TS: 8.3–20.3 and 9.1–26.0 MPa). Through SEM and TEM, the membranes were uniformly dense, and phase separation continuously increased. With the enhancement of cross-linking, WR (100–46.2%) and LER (30–22.5%) showed a downward trend, but IEC increased from 0.54 to 1.89 mmol/g. In the DD testing experiment, the increased from 10.2 × 10−3 to 35.6 × 10−3 m/h, S value was in the range of 25–38, which was higher than DF-120 (: 0.009 m/h; S:1 8.5). In summary, the prepared membranes showed certain application value in acid recovery.

This work was supported by the Natural Science Foundation of Anhui Provincial, China (1908085MB55), Natural Science Foundation of Anhui Provincial Education, China (KJ2020ZD44), and Science and Technology Projects of Anhui Province, China (202003a05020045).

All relevant data are available from an online repository or repositories: https://webofscience.clarivate.cn/wos/woscc/basic-search.

The authors declare there is no conflict.

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