Preparation of a Cation Exchange Membrane by a Sol-Gel Method-Based Polyvinyl Alcohol to Improve Alkali Recovery via Diffusion Dialysis in the Textile Industry

Abstract

In this work, a novel silane coupled cationic precursor (SAGS) was synthesized by 3-glycidyloxypropyltrimethoxysilane and sodium 2-((2-aminorthyl)amino) ethanesulfonate. A series of cation exchange membranes were prepared with poly(vinyl alcohol) (PVA) and SAGS by a sol-gel-based process. The structure of the prepared membranes were characterized by Fourier transform infrared spectrum (FTIR) and scanning electron microscopy (SEM), and its properties were studied by water uptake (W_R), cation exchange capacity (CEC), linear expansion ratio (LER), alkali stability, thermogravimetric analysis (TGA), mechanical properties, and diffusion dialysis performance. FTIR and X-ray photoelectron spectroscopy (XPS) confirmed the successful preparation of SAGS membranes, and SEM images showed that the prepared membranes were dense and uniform. The W_R values of the SAGS membranes were in the range of 91.49–122.39%, and the LER values were 17.65–28.21%. In addition, the SAGS membranes had suitable CEC value, good alkali resistance, and thermal stability which ensured the application of membranes in the field of diffusion dialysis (DD) for alkali recovery. In the DD test, the dialysis coefficients of NaOH (U_OH) ranged from 0.012 mm/h to 0.023 mm/h, and the separation factors (S) was in the range of 30.77–16.43. In conclusion, the prepared CEM containing silicon oxygen bonds by PVA and SAGS reaction has the advantages of low price, friendly environment, good alkali resistance, simple preparation process, and great application potential in the textile manufacturing wastewater recovery.

1. Introduction

In the textile industry, sodium tungstate (Na₂WO₄) is commonly used as fabric weighting agent in the production process of fireproof artificial silk [1] and waterproof artificial cotton [2], which can effectively improve the waterproof and fireproof performance of artificial silk or cotton [3,4,5]; however, it is accompanied by the production of a large amount of Na₂WO₄ and sodium hydroxide (NaOH) wastewater [6,7,8]. Due to massive NaOH in the wastewater, the recovery and reuse of NaOH have significant economic and social significance [9,10,11,12]. Diffusion dialysis is one of the most effective ways to treat the wastewater, which contains Na₂WO₄ and NaOH [13,14,15,16]. Diffusion dialysis has the advantages of low energy consumption, good separation effect, and a simple process [15,16,17]. The key core of recovery NaOH via diffusion dialysis is to prepare high-performance cation exchange membranes (CEM) [18,19,20,21,22,23]. The cation exchange membrane (CEM) consists mainly of fixed groups (such as sulfonate or carboxyl groups) and and dissociable ions, which can be used for the transport of cations [20,21,22,23]. The preparation of high-performance cation exchange membranes has been the unremitting pursuit of many scholars [21,22,23,24,25,26].

CEM can be divided into two main types, the heterogeneous ion exchange membrane, such as the semi-interpenetrating network structure of the cation exchange membrane and the blending polymer cation exchange membrane [27,28,29,30]; and the homogeneous ion exchange membrane, such as sulfonated polyether ether ketone [31,32], sulfonated polyphenyl ether [33,34], and perfluorosulfonic acid membrane (Nafion) [35,36]. The performance requirements of CEM should contain the following three characteristics. 1. High ion permeability flux and selectivity. This requires that the membrane not only has appropriate water content, but also has good barrier properties to anion [26,37,38]. 2. Outstanding thermal stability, mechanical stability, and low swelling degree. The performance of membrane decreased in the long-time application process, so it was necessary to maintain the performance stability of the separation process over a long period of time [27,39]. 3. Good alkali resistance. For diffusion dialysis, cation exchange membranes adopting traditional polymers as the backbone can no longer meet the requirement of long-term stability under strong alkaline condition, which was also difficult for commercialization of recovery alkaline via diffusion dialysis [40]. High-performance cation exchange membranes had many problems, such as cumbersome preparation methods, unfriendly environment, high price and limited application environment [23,24,25,26]. Hence, it was the pursuit of the majority of ion exchange membrane researchers to prepare cation exchange membranes with simple preparation, environmentally friendly process, and excellent performance.

At present, CEMs based on polyvinyl alcohol (PVA) are studied intensively [41,42]. For example, Y H. Wu et al. prepared cation exchange hybrid membranes with blending sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) in PVA/SiO₂ matrix [43], the PVA improved the hydrophilicity of the hybrid membrane, and the ion flux of CEM increased. Chunhua Dai et al. prepared hybrid cation exchange membrane by combining benzaldehyde disulfonic acid disodium salt with PVA [44], the results showed that the prepared cation exchange membranes had good performance [11]. There is also a lot of literature on the preparation of CEMs with PVA. The main reason for this is that polyvinyl alcohol is a new green material with a low price, environmental friendliness, wide compatibility, and good alkali resistance, which have attracted the interest of many researchers. Therefore, PVA has often been used to prepare CEM.

In this paper, a silane coupling agent containing an epoxy group was used to react with an amino-containing sulfonate to synthesize a silane coupling agent with sulfonate group (SAGS) [45], and then the cation exchange membrane was prepared with PVA and SAGS by sol-gel reaction. The preparation process was carried out in the aqueous phase, which was environmentally friendly. The preparation process of the membrane uses silane coupling agents to connect the corresponding polymer main chains, making the prepared cation exchange membrane environmentally friendly, cost-effective, and suitable for sustainable industrial development. At the same time, the performance of the prepared membrane was comprehensively studied, which provided technical support for recovery NaOH via diffusion dialysis in the recycling wastewater in the textile industry.

2. Materials and Methods

2.1. Materials

Polyvinyl alcohol (PVA) and dimethyl sulfoxide (DMSO) were purchased from Sinopharm Chemical Reagent (Shanghai, China), and the average degree of PVA was 1750 ± 50. 3-glycidyloxypropyltrimethoxysilane (97%) and sodium 2-((2-aminorthyl)amino)ethanesulfonate (50% in H₂O) were supplied by Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China).

2.2. Methods

2.2.1. Synthesis of SAGS

First, 1.36 g (0.005 mol) 3-glycidyloxypropyltrimethoxy-silane and 16.64 g DMSO were loaded in a dried 50 mL round-bottomed flask with adequately stirring at 80 °C in order to completely mix. After 30 min, 2.00 g 2-((2-aminorthyl)amino)-ethanesulfonate was slowly added dropwise into the mixed solution and then the mixture was continually stirred at 80 °C for 6 h to fully react. The product was a colorless transparent viscous solution in DMSO. The reaction synthesis process of SAGS is shown in Figure 1.

2.2.2. Preparation of SAGS-X Cation Exchange Membrane

First, PVA was completely dissolved using DMSO at 90 °C to prepare 5 wt% PVA solution. Second, the SAGS solution was added into the PVA solution to according to different mass ratios (SAGS to PVA: 16%, 32%, 48%, 64% and 80%). The sol-gel reaction lasted for 24 h at 60 °C. The solution after the sol-gel reaction was cast on a clean glass plate and dried in a vacuum oven at 60 °C to form membrane. After the solvent had fully evaporated, the membrane was detached from the plate and then the membrane was heated in vacuum oven at 110 °C for 4 h to completely cross-link the membrane. Finally, the membrane was stored in deionized water for further testing. The prepared membranes were named SAGS-X, where X represented the mass percentage of SAGS in PVA. The preparation process of SAGS-X cation exchange membrane is shown in Figure 2.

2.3. SEM, TGA, FTIR, XPS

The chemical functional groups information of the SAGS-X cation exchange membranes was obtained by Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, MA, Thermo Fisher Scientific of American). FTIR condition was set with a resolution of 0.1 cm⁻¹ and a wavenumber range of 4000–400 cm⁻¹. The surface elemental compositions of prepared membranes were analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Waltham, MA, USA, Thermo Fisher Scientific) measurements. The cross-section microstructure analysis of the SAGS-X cation exchange membranes was performed by scanning electron microscopy (SEM, JSM-7500F, Tokyo, JEOL of Japan). Thermal stability of the SAGS-X cation exchange membranes was obtained by thermogravimetric analysis (TGA, TGA-50H, Tokyo, Shimadzu of Japan). TGA condition was set with a constant heating ratio of 10 °C per minute from room temperature to 600 °C under N₂ atmosphere.

2.4. Water Uptake (W_R)

Water uptake (W_R) was measured by the standard method described below. Primarily, the SAGS-X CEM was dried to a constant weight, and we recorded the weight in the dry state (W_dry) by analytical balance, and then the membrane was completely immersed in deionized water for 48 h at room temperature. After wiping off the surface moisture, the membrane was weighed again and recorded the weight (W_wet) of in a fully absorbed moisture state. Water uptake (W_R) that was based on the data obtained above was calculated, and the formula is as follows:

$${\mathrm{W}}_{\mathrm{R}}=\frac{{\mathrm{W}}_{\mathrm{w}\mathrm{e}\mathrm{t}}-{\mathrm{W}}_{\mathrm{d}\mathrm{r}\mathrm{y}}}{{\mathrm{W}}_{\mathrm{d}\mathrm{r}\mathrm{y}}}\times 100$$

(1)

2.5. Cation Exchange Capacity (CEC) and the Thickness of the Membrane

The cation exchange capacity (CEC) of the SAGS-X CEM was determined by studying the content of -SO₃H groups in membrane under dry weight conditions. The theoretical value of CEC (CEC_T) was obtained through theoretical simulation calculation. The experimental value of CEC (CEC_E) was characterized by a titration method. First, the dried SAGS-X CEM was accurately weighed by analytical balance (W_Dry). Then the membrane was equilibrated with 1 M HCl aqueous solution for 48 h to convert to H⁺ form. Afterwards the membrane was washed with DI water three times in order to completely remove the residual acid, and finally put into 0.04 M NaCl aqueous solution for 48 h. The content of H⁺ which was released by the membrane was tested through titration with 0.04 M NaOH aqueous solution to determine cation exchange capacity. The CEC (mmol·g⁻¹) was calculated as follows,

$$\mathrm{C}\mathrm{E}\mathrm{C}=\frac{{\mathrm{C}}_{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}{\mathrm{V}}_{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}}}{{\mathrm{W}}_{\mathrm{D}\mathrm{r}\mathrm{y}}}$$

(2)

where C_NaOH was the concentration of the NaOH aqueous solution, and V_NaOH was the volume of the NaOH aqueous solution.

The thickness of the membrane used spiral micrometer to measure.

2.6. Linear Expansion Ratio (LER) and Alkali Stability

Linear expansion ratio (LER) was important properties to evaluate dimensional stability of cation exchange membrane. The membrane in the dry state was cut into a rectangular shape with a length of 5 cm and a width of 1 cm. LER test was similar to the W_R test, the membrane lengths in both dry and wet states were recorded before and after the test. LER was calculated as follows:

$$\mathrm{L}\mathrm{E}\mathrm{R}=\frac{{\mathrm{L}}_{\mathrm{w}\mathrm{e}\mathrm{t}}-{\mathrm{L}}_{\mathrm{d}\mathrm{r}\mathrm{y}}}{{\mathrm{L}}_{\mathrm{d}\mathrm{r}\mathrm{y}}}\times 100$$

(3)

Alkali stability was a key property of CEM to estimate membrane application performance. The weight of the membrane in the dry state was recorded. After that, the membrane was immersed into 2 M NaOH aqueous solution for 7 days at room temperature. Finally, the membrane was washed, dried, and weighed again. The weight maintenance rate was calculated as the ratio of the mass of the membrane before and after the acid resistance test which was a main indicator to evaluate the alkali stability of the membrane.

2.7. Diffusion Dialysis (DD)

At first, the prepared SAGS-X CEM was immersed in feed solution (1 M NaOH + 0.1 M Na₂WO₄), which was used to simulate wastewater, for 2 h. Next, the membrane needed to be washed carefully several times with the use of DI water. Then, the membrane was placed in the experimental device, which was the combination of two compartments. These two compartments were separated by the prepared membrane, and the effect area between the two parts was 5.5 cm². Based on the work above, 100 mL DI water was poured into one compartment and 100 mL feed solution was put into the other compartment. After that, in order to minimize concentration polarization, both compartments needed to be strongly stirred for 60 min. Later, 10 mL solution from each compartment of the experimental device was removed. OH⁻ concentration in the solution was determined by titration with HCl solution, while WO₄²⁻ concentration was determined by thiocyanate spectro photometric method. The calculation of dialysis coefficients (U) and separation factors (S) was calculated as follows,

$$\mathrm{U}=\frac{\mathrm{M}}{\mathrm{A}\mathrm{t}∆\mathrm{C}}$$

(4)

where M was the amount of component transported in moles, A was the effective area in square meters, t was the time in hours, and ∆C was the logarithmic mean of the concentration difference between the two chambers. ∆C can be calculated as below,

$$∆\mathrm{C}=\frac{{\mathrm{C}}_{\mathrm{f}}^0-\left({\mathrm{C}}_{\mathrm{f}}^{\mathrm{t}}-{\mathrm{C}}_{\mathrm{d}}^{\mathrm{t}}\right)}{\ln \left[{\mathrm{C}}_{\mathrm{f}}^0-\left({\mathrm{C}}_{\mathrm{f}}^{\mathrm{t}}-{\mathrm{C}}_{\mathrm{d}}^{\mathrm{t}}\right)\right]}$$

(5)

where ${\mathrm{C}}_{\mathrm{f}}^0$ and ${\mathrm{C}}_{\mathrm{f}}^{\mathrm{t}}$ were the feed concentrations at time 0 and t, respectively, and ${\mathrm{C}}_{\mathrm{d}}^{\mathrm{t}}$ was the dialysate concentration at time t.

The separation factor (S) of membrane was calculated using the ratio of dialysis coefficients (U) of the two species (WO₄²⁻ and OH⁻) presented in the solution. S can be calculated as follows:

$$\mathrm{S}=\frac{{\mathrm{U}}_{\mathrm{O}\mathrm{H}}}{{\mathrm{U}}_{\mathrm{W}{\mathrm{O}}_4^{2-}}}$$

(6)

The data obtained from W_R, LER, CEC, DD performance, and film thickness had been tested for an average of three times.

3. Results and Discussion

3.1. FT-IR

The chemical structure information of membranes SAGS-16, SAGS-32, SAGS-48, SAGS-64, and SAGS-80, which was studied by FT-IR (Figure 3). The large and wide bands at 3600–3200 cm⁻¹ was the tensile vibration peaks of hydroxyl in PVA and SAGS [45]. The peaks that appeared at 2937 cm⁻¹ and 2907 cm⁻¹ were due to the symmetric and asymmetric stretching vibration peaks of methylene [46]. The absorption peak at 1646 cm⁻¹ was ascribed to the bending vibration peak of the hydroxyl groups on the sulfonic groups [44]. The uptake peaks at 1420 cm⁻¹ and 1323 cm⁻¹ were due to the bending vibration peaks of methylene and the rocking vibration peaks of hydroxyl groups, respectively [44]. The absorption peak at 1191 cm⁻¹ was assigned to the S=O asymmetric vibration peak of -SO₃H groups [46]. The S-O stretching vibration peak of -SO₃H groups was also clearly observed at 1041 cm⁻¹ [46]. The presence of -SO₃H groups in the membranes could be fully confirmed by the characteristic absorption peaks of -SO₃H groups which appeared at 1191 cm⁻¹ and 1041 cm⁻¹. The stretching vibration of Si-O-Si and Si-O-C appears at 1018 cm⁻¹, and these groups mainly formed by the sol-gel process.

3.2. Thermal Stability

Thermal stability of the SAGS-X CEMs was evaluated using TGA and DTG. According to Figure 4, the weight loss process of the SAGS-X CEM was divided into three decomposition stages. In the first decomposition stage, the main loss of membranes was the free water and bound water in the membrane in the temperature range of 80–120 °C [13]. The second decomposition stage occurring from 280 °C to 310 °C was mainly the degradation of grafted side chain functional groups, such as -SO₃H groups [47]. The mass loss in the last decomposition temperature which was from 370–480 °C was mainly due to the degradation of the polymer backbone [48]. According to the TGA curve, the residual mass of the prepared membranes gradually increased from SAGS-16 to SAGS-80. Therefore, the addition of SAGS which could increase the overall crosslinking degree of the membrane significantly improved the thermal stability of the prepared membranes. The initial decomposition temperature (IDT) of the SAGS membranes was 266.6–282.5 °C, and thermal degradation temperature (T_d) was in the range of 278.5 °C to 283.9 °C from

Table 1. IDT and T_d values of the SAGS-X cation exchange membranes.

Membrane	SAGS-16	SAGS-32	SAGS-48	SAGS-64	SAGS-80
IDT (°C) a	266.6	268.4	271.5	266.8	282.5
Td (°C) b	283.9	278.5	280.3	278.7	282.5

^a IDT is the initial decomposition temperature determined from TGA thermograms. ^b Thermal degradation temperature (T_d) is defined as the temperature at which the weight loss becomes 5% in TGA thermograms.

. So that the IDT and T_d values of the SAGS-X CEMs were completely higher than the operating temperature of the CEM.

3.3. XPS Spectra of Membranes

In order to obtain more comprehensive structural information of prepared membrane, the surface chemical component of the membrane was determined by XPS. The test results of membranes are shown in Figure 5.

The peak at 537–526 eV is O 1s. The peak in this region is related to -OH on the PVA chain and -O- in the silane coupling agent [49]. For O 1 s, it was divided into two subspecies, including -OH (located at ca. 536 eV) and O-Si (located at ca. 528 eV). This indicated the presence of hydroxyl and silicon oxygen bonds in the membrane, which was also confirmed by infrared detection. The peak at 102 eV was Si 2p, which confirmed the presence of the Si bond in the membrane [49]. The tensile vibration of Si-O-Si and Si-O-C at 1018 cm⁻¹ in FTIR spectra also confirmed the existence of Si.

In addition, the success synthesized of membrane can be supported by the high-resolution spectra of S 2p. taking membrane SAGS-16 as an example, the XPS signal can be clearly deconvolved into two peaks in Figure 5B. The two peaks detected at 168.2 eV and 163.5 eV in Figure 5B were related to the S atom in the sulphonic group [50]. The characteristic absorption peaks at 1191 cm⁻¹ and 1041 cm⁻¹ in FTIR spectra also confirmed the presence of sulfonic acid groups.

3.4. Water Uptake (W_R), Linear Expansion Ratio (LER) and Cation Exchange Capacity (CEC)

The water uptake of the SAGS CEM was tested to collect data, which is shown in

Table 2. W_R, LER and CEC of the SAGS-X cation exchange membranes.

Membrane	SAGS-16	SAGS-32	SAGS-48	SAGS-64	SAGS-80
WR	91.49	94.34	96.67	112.31	122.39
LER	17.65	24.42	26.51	27.32	28.21
CECT	0.36	0.64	0.85	1.03	1.17
CECE	0.25	0.42	0.57	0.72	0.84
Thickness (μm)	99	91	103	112	107

. The W_R value of the membrane was 91.49–122.39%. The increased W_R value was due to the strong hydrophilicity of the sulfonic acid group. The content of SAGS in the membrane increased, the number of sulfonic acid groups also enhanced, which resulted in an increase in the water uptake of the membrane. The increased content of SAGS made polar groups increase, and resulted in the formation of larger ion clusters, which would cause the membrane to absorb more water and increased the W_R value. The LER value of the membrane was 17.65–28.21% (Table 2). The enhanced LER was due to the SAGS content increase, which improved the crosslinking degree of the membrane and enhanced the dimensional stability [46]. The number of ion exchange groups of the prepared membrane was determined by the CEC value. The CEC of the prepared membrane was estimated by using the titration method, and the results were shown in Table 2. The CEC value was 0.25–0.84 mmol/g. The CEC value mainly depends on the number of sulfonic acid groups present in the membrane. With the increased content of SAGS, the number of sulfonic acid groups increased, and the CEC value of the membrane increased. The thickness of the SAGS cation membrane was 91–112 μm. Compared with other membranes, as shown in

Table 3. Ion exchange capacity (CEC), water uptake (W_R), and linear expansion ratio (LER) values of different membranes.

Membrane	CEC (mmol/g)	WR (%)	LER (%)	Ref.
SAGS-X	0.25–0.84	91.49–122.39	17.65–28.21	This work
PVA/TiO2	0–0.0157	90.9–101.7	187.2–206.5	[51]
PVA/CBACS	0.0147–0.0518	122.6–150	222.6–241.9	[52]

, the experimental membrane had good CEC value and lower LER value.

3.5. Mechanical Properties and Alkali Resistance

Mechanical properties were the material properties of CEMs. The TS and E_b values were shown in Figure 6. The tensile strength (TS) of the membrane was 20.1–30.8 MPa, and elongation at break (E_b) value was 92.3–107.2%. Due to the stronger molecular force caused by the cross-linking of the polymer chains, which improved the mechanical properties of the membrane, enhanced the value of TS and E_b of the membrane [53,54]. For SAGS-80, TS and E_b values of the membrane decreased attribute to excessive crosslinking of the membrane, which lead to uneven stress distribution of the membrane and excessive hardness of the local membrane, thereby affecting the TS and E_b values of the membrane [54,55]. Compared with other reported membranes, the prepared membranes had good TS and E_b values. The relevant data were shown in

Table 4. TS and E_b values of different membranes.

Membrane	TS (MPa)	Eb (%)	Ref.
SAGS-X	20.1–30.8	92.3–107.2	This work
PVA/SSS/γ-MPS	9.1–26.0	12.4–21.1	[56]
PVA/SPPO	12–13	27–49	[54]
PVA/MA/γ-MPS	14.2–28.3	18.8–67.3	[57]

.

3.6. Alkali Resistance

The alkali resistance of the membrane reflected its potential application in the field of diffusion dialysis for alkali recovery. The membrane was immersed in a 2 M NaOH environment at 25 °C for 7 days [58], and weight maintenance of the membrane was tested. The relevant data were shown in

Table 5. Weight maintenance of the SAGS-X cation exchange membranes.

Membrane	SAGS-16	SAGS-32	SAGS-48	SAGS-64	SAGS-80
Weight maintenance (%)	91.4	94.3	88.3	90.2	91.9

, and it can be observed clearly that the weight maintenance of SAGS-X CEM was above 88%. The data showed that the SAGS-X CEM had good alkali resistance that was suitable for application in the field of diffusion dialysis for alkali recovery.

3.7. Membrane Morphologies

With the use of SEM, the homogeneity and uniformity of five prepared membranes could be researched in detail. The cross-sectional morphologies of obtained membranes were presented in Figure 7. According to the SEM images, it was not difficult to observe that the membranes were dense and compact without any holes and cracks, which was mainly attributed to the cross-linking between PVA and SAGS. With the increase in SAGS content in the membrane, the degree of crosslinking between SAGS and PVA also increased, which resulted in an increased compactness of the membrane. In addition, the phenomenon of particle aggregation was also observed in Figure 7. The white dots that appear in the images are the silica particles in the membrane [54]. Taking SAGS-80 as an example, the membrane with highest SAGS content, showed the most obvious aggregation due to the accelerated hydrolysis of silica precursor. In addition, the SAGS-64 and SAGS-80 showed the slight phase separation, which was of great help to improve the diffusion dialysis performance for alkali recovery [45].

3.8. Diffusion Dialysis (DD)

The performance of diffusion dialysis was vital for anion exchange membranes, which was about whether the prepared membranes could be used in industrial production and daily life. Thus the diffusion dialysis experiment was carried out by taking NaOH/Na₂WO₄ mixture (1.0 M NaOH/0.1 M Na₂WO₄) as the feed solution, and relevant experimental results were shown in Figure 8.

From the collected data, U_OH⁻ value was increased from 0.012 mm/h to 0.023 mm/h with the increase in SAGS content. That was mainly due to the fact that -SO₃H groups carried the negative charge, so OH⁻ and WO₄²⁻ groups which carried the same charge were hardly able to pass through the cation exchange membranes [52]. On the contrary, those cations in the feed solution can be easily transported to the side. For the sake of meeting the requirement of electric neutrality, anions also must be migrated. Compared with WO₄²⁻, OH⁻ had less charge and the size of OH⁻ was smaller, which resulted in the gradual increase of the U_OH⁻ value. Secondly, the increased CEC and W_R had an effect on the performance of diffusion dialysis [45]. Both of these showed an upward trend, which had a positive effect on ion migration, hence the increase in U_OH values.

The U_WO4²⁻ value ranged from 0.39 × 10⁻³ mm/h to 1.40 × 10⁻³ mm/h. Under the condition that the membrane structure became more and more dense, the reason for the increase in U_WO4²⁻ value was phase separation [55]. Based on the previous discussion, larger ion channels appeared on the membrane because of phase separation, which indicated that WO₄²⁻ groups could pass through the membrane easily. The increased IEC and W_R also promoted the migration of WO₄²⁻ same as OH⁻ groups; however, this migration of WO₄²⁻ was not conducive to membrane selectivity, which caused the decrease in S value.

The S value was the ratio of U_OH⁻ and U_WO4²⁻, and the value was in the range of 30.77–16.43. Compared with some membranes previously reported, the prepared membranes showed the higher separation factor (13.6–18.1) [53], (16.9–18.5) [57]. In order to better prove the changes in different ion transport properties caused by the changes in the internal structure of the membrane, the internal transport mechanism diagram based on the corresponding data was drawn and shown in Figure 9 [54,58,59]. Compared with other membranes, the prepared membrane had good S value. The relevant data were shown in

Table 6. The U_OH and S values of different membranes.

Membrane	UOH (10−3 m/h)	S	Ref.
SAGS-X	12–23	16.43–30.77	This work
PVA/SSS	11–19	13.6–18.1	[53]
PVA/γ-MPS/SSS	10.2–11.1	16.9–18.5	[57]
PVA/THOPS	11–22	11.6–20.6	[59]

.

4. Conclusions

A series of long chain PVA-based cation exchange membranes had been successfully prepared by sol-gel process. The CEM containing silicon oxygen bonds prepared by reaction of PVA and SAGS had the advantages of low price, environmentally friendly, and simple preparation process. The SAGS membrane had moderate W_R and LER, suitable CEC value, and excellent alkali stability. Moreover, the SAGS membrane had extremely high thermal stability and a microstructure conducive for diffusion dialysis. In the DD test, the U_OH and S value of the SAGS membrane were in the range of 0.012–0.023 mm/h and 16.43–30.77, respectively. When the content of SAGS reached 80%, the membrane had a maximum U_OH value (0.023 mm/h) and a minimum S value (16.34). Hence, these good properties indicated that the membrane had certain application potential in the field of DD for alkali recovery.