Triethanolamine-Modified CMPSF Anion Exchange Membranes for High-Efficiency Acid Recovery via Diffusion Dialysis

Abstract

Anion exchange membranes (AEMs) serve as critical components in diffusion dialysis (DD) systems due to their unique permselectivity. This study developed a series of triethanolamine (TEA)-functionalized chloromethylated polysulfone (CMPSF) AEMs via solution casting. The physical and chemical structural characterization through ¹H NMR, XPS, FTIR, and SEM proved successful membrane synthesis. The performances of the membranes, such as ion exchange capacity (IEC), water contact angle (WCA), water uptake (WU), chemical stability, and mechanical stability, were systematically evaluated. For HCl/FeCl₂ acid recovery (1 mol L⁻¹ HCl + 0.25 mol L⁻¹ FeCl₂), the optimal membrane (TEA-CMPSF-M50) demonstrated exceptional DD performance, with an acid dialysis coefficient (U_H⁺) of 47.9 × 10⁻³ m h⁻¹ and separation factor (S) of 3.87. Crucially, after 7-day immersion in acidic solution at 65 °C, the membrane maintained U_H⁺ and S values of CMPSF-M50 AEM of 45.4 × 10⁻³ m h⁻¹ and 4.02, respectively, confirming the outstanding acid resistance and thermal stability of TEA-CMPSF-M50 AEM. These results indicated that the TEA-functionalized AEMs developed in this work hold great promise for industrial acid recovery applications.

1. Introduction

In the production processes of fields such as steel smelting, electroplating, chemical industry, and drug synthesis, large quantities of industrial acidic wastewater are produced. This industrial acidic wastewater is characterized by strong acidity, high concentration of heavy metal ions, and complex composition, and its direct discharge can lead to environmental hazards [1,2,3,4]. Various treatment techniques are extensively employed for acid recovery such as neutralization precipitation, distillation, thermal decomposition, crystallization, etc. [5,6]. Although these methods are effective in treating acidic wastewater, they are plagued by limitations, including susceptibility to secondary contamination, high energy consumption, and poor applicability [7,8]. Therefore, highly efficient separation methods are essential for practical applications. Advanced membrane technologies such as diffusion dialysis (DD), nanofiltration (NF), and electrodialysis (ED) have attracted extensive research interest due to their significant advantages in high selectivity, cost-effectiveness, flexible adaptability, environmental friendliness, and resource recovery [9,10,11]. Among them, the DD process is an attractive technique for treatment of high-concentration acidic wastewater, distinguished by its high selectivity, absence of secondary pollution, and low energy consumption [6,12].

Anion exchange membranes (AEMs) are recognized as the pivotal core of the diffusion dialysis process for acid recovery. For this application, membranes should exhibit high proton permeability while ideally no leakage of other cations or salts. However, conventional AEMs are prone to functional group degradation in strongly acidic environments and suffer from low acid recovery efficiency and poor mechanical strength due to insufficient acid permeability. These issues have always been the key bottlenecks restricting DD technology from achieving industrial-scale applications [4,11,12,13,14,15]. Thus, the design of AEMs with great proton permeability/conductivity and superior stability stands as the key to current research. The main materials presently utilized in the fabrication of AEMs are polysulfone (PSF) type, brominated polyphenylene ether (BPPO) type, and polyvinyl alcohol (PVA) type [16,17,18,19,20]. These polymers typically exhibit a profile of clear advantages and distinct limitations. Polyvinyl alcohol (PVA) as a diffusion dialysis membrane material has the advantages of good hydrophilicity, high ion selectivity, and low cost but has the disadvantages of excessive swelling, poor long-term stability, and low permeation flux. Emmanuel, et al. varied the content of pyridinium salts and used PVA as a substrate to prepare pyridine-functionalized AEMs for diffusion dialysis, and the prepared AEMs showed that the acid dialysis coefficients ranged from 0.0174 to 0.0248 m h⁻¹, while the separation factors fell between 30.49 and 57.51 [7]. BPPO has advantages such as high chemical stability and good mechanical strength but suffers from disadvantages such as low permeation flux due to hydrophobicity and difficult processing. Khan et al. fabricated AEMs for DD via phase transition technique using trimethylamine and BPPO. These AEMs showed the acid dialysis coefficient was in the range of 0.0043–0.012 m h⁻¹ and separation factors were in the range of 13.14–32.87 [21].

Polysulfone (PSF), as a commonly used membrane material, has been the focus of research as one of the more studied polymers in recent decades [22,23]. Owing to its high ionic conductivity, facile functionalization, thermal stability, mechanical strength, pH tolerance, and good film-forming properties, PSF-based AEMs represent promising choices for acid recovery application [24,25,26,27,28]. However, the poor hydrophilicity of PSF limited its practical applications. To enhance PSF-based AEM hydrophilicity, Gu et al. developed a GO-based anion-exchange membrane for acid recovery by intercalating cross-linkable imidazole cations into GO lamellae and cross-linking them via electron-beam irradiation, achieving a H⁺ ion permeability coefficient (U_H⁺) of 9.7 × 10⁻³ m h⁻¹ [29]. It can be found that the hydrophilicity of CMPSF membranes can be enhanced by introducing functional groups to increase proton permeability.

Note that triethanolamine (TEA) demonstrates notable hydrophilicity and chemical stability due to its unique combination of ammonium groups and polyhydroxy structures. In this study, we produced a series of triethanolamine-modified chloromethyl polysulfone (CMPSF) AEMs using CMPSF as the polymer matrix and TEA as the functional modifier. TEA underwent simultaneous quaternization and cross-linking with chloromethylated polysulfone chains during membrane formation. The impact of synthesis conditions on membrane properties was systematically investigated through CMPSF modification with varying TEA concentrations. This approach facilitated comprehensive evaluation of hydrophilicity, ion-exchange capacity, diffusion dialysis performance, mechanical strength, and thermal stability. By comparing the performance changes of membranes under different modification conditions, the optimal modification strategy was explored to provide practical guidance for both the applicability of CMPSF materials in AEM development and their application in acidic wastewater recovery.

2. Results and Discussion

2.1. Membrane Characterizations

2.1.1. ¹H NMR and XPS

In this work, CMPSF was dissolved in deuterated chloroform (CDCl₃) and analyzed by ¹H NMR spectroscopy using a Bruker Avance III HD NMR spectrometer. The spectrum is shown in Figure 1. The results indicated that, aside from the peak at 7.20 ppm (a) corresponding to residual protons in CDCl₃, the peak near 1.67 ppm (c) is attributed to the hydrogen atoms of the −(CH₃)₂ group [30]. A characteristic peak observed at 4.56 ppm (b) was attributed to the hydrogen atoms of the −CH₂Cl group. This confirmed the successful functionalization of PSF with chloromethyl groups.

The chemical compositions of the CMPSF membrane substrate and CMPSF AEMs (i.e., M50) were explored by XPS analysis, and the results are shown in Figure 2. Five characteristic peaks were observed in Figure 2a at binding energies of 533.1, 401.3, 284.5, 200.0, and 168.4 eV. These peaks correspond to the elements O 1s, N 1s, C 1s, S 2p, and Cl 2p, respectively [31]. Compared with the CMPSF film substrate, the peak intensities of the elemental N were enhanced in the CMPSF-M50 film, while the peak intensities of the Cl element were weakened. It could also be seen from

Table 1. Atomic percentages of CMPSF substrates and CMPSF-M50 AEMs.

Atomic	CMPSF Substrate (%)	CMPSF-M50 AEM (%)
C	81.55	78.89
O	14.66	15.48
S	2.47	2.53
N	0.70	2.84
Cl	0.62	0.26

that the atomic percentages of N and Cl in the CMPSF substrate are 0.70% and 0.62%, respectively. Following reaction with triethanolamine, the atomic percentage of N in the CMPSF-M50 film increases to 2.84%, whereas that of Cl drops to 0.26%. This reaction occurs during the preparation of CMPSF-based AEMs via the grafting of ammonium groups onto the CMPSF-M50 film, leading to the depletion of −CH₂Cl groups in the substrate. The phenomenon primarily arises from the nucleophilic substitution reaction between the −CH₂Cl groups in the CMPSF substrate and the amine groups in triethylamine (TEA).

This was further evidenced by the high-resolution N 1s spectra (Figure 2b). The CMPSF-M50 membrane exhibited a peak at 402.1 eV, attributed to the introduction of the quaternary ammonium group. In contrast, this peak was absent in the CMPSF substrate [32]. This confirms the successful grafting of TEA onto the CMPSF-based AEMs.

2.1.2. FTIR Spectra

The FTIR spectra of CMPSF and its series of CMPSF AEMs are shown in Figure 3. The absorption peak at 2967 cm⁻¹ was ascribed to the stretching vibration of the C-H bond on the benzene ring. Multiple peaks between 1409 and 1583 cm⁻¹ arose from benzene ring skeletal vibrations. A peak at 1235 cm⁻¹ corresponded to the antisymmetric vibration of the ether bond, while characteristic sulfone group (O=S=O) vibrations were observed at 1320 cm⁻¹ for symmetric stretching and at 1146 cm⁻¹ for antisymmetric stretching [33,34]. Notably, a new absorption peak at 930 cm⁻¹, attributed to C-Cl stretching vibrations, appeared in CMPSF but gradually disappeared in the CMPSF-series membranes. This provided preliminary evidence for successful chloromethylation and quaternization. Concurrently, a peak at 1010 cm⁻¹ emerged in the CMPSF-series membranes, assigned to C-N stretching vibrations, thus confirming the introduction of ammonium groups [35]. A strong, wide absorption band centered at 3380 cm⁻¹ primarily stems from O-H stretching vibrations of triethanolamine and potential bound water. This further indicated enhanced hydrophilicity in the AEMs following triethanolamine grafting.

2.1.3. Microscopic Morphology of CMPSF Membranes

As revealed in Figure 4, all prepared TEA graft-modified CMPSF membranes exhibited a typical sponge-like morphology. The surface of the CMPSF-M20 membrane appeared relatively smooth. However, increasing the TEA content progressively induced surface roughness and wrinkle-like structures. This morphological change is attributed to enhanced membrane hydrophilicity resulting from triethanolamine grafting [36]. Concurrently, pore structures emerged on the membrane top surfaces. Notably, the porosity of these top surfaces increased with higher TEA content. This phenomenon likely arose from the introduction of quaternary ammonium and hydroxyl groups, which enhanced membrane hydrophilicity, promoting internal swelling and consequently leading to pore enlargement [31]. Furthermore, cross-sectional morphology analysis indicated that the porous structure within the cross-section became progressively denser as the triethanolamine percentage increased, suggesting an elevated degree of membrane phase separation. The cross-sections also revealed that the membranes had an average thickness of approximately 35 to 40 μm.

2.2. IEC, WCA, WU, and LER

IEC is a critical parameter for evaluating membrane properties and plays an important role in the DD process [37,38]. The IEC values of AEMs were measured via the titration method, with the results presented in Figure 5. The IEC of the prepared AEMs rose from 0.62 mmol g⁻¹ to 1.19 mmol g⁻¹ with increasing TEA proportion in the polymer matrix, confirming the quaternization reaction between TEA and CMPSF.

The hydrophilicity of the TEA-CMPSF-AEMs was evaluated via measurements of water contact angle (WCA), water uptake (WU), and linear expansion rate (LER). As shown in Figure 6, the WCA gradually decreased from 85.8° for CMPSF-M20 AEM to 66.8° for CMPSF-M50 AEM as the triethanolamine (TEA) content increased. This reduction is attributed to the higher concentration of hydrophilic functional groups introduced at greater TEA loadings. Correspondingly, as shown in Figure 7, the WU of the CMPSF membranes exhibited an opposite trend to the WCA. WU increased from 25.9% to 51.9% as the TEA content rose, consistent with the membrane’s enhanced hydrophilicity and increased anion exchange capacity. This improvement in hydrophilicity arose from the incorporation of hydroxyl groups and quaternary ammonium via TEA grafting. Furthermore, as shown in

Table 2. Composition and LER of different TEA-CMPSF AEMs.

Membrane Samples	CMPSF (g)	TEA (g)	LER (%)
TEA-CMPSF-M20	0.9	0.20	7.7
TEA-CMPSF-M30	0.9	0.30	12.8
TEA-CMPSF-M40	0.9	0.40	20.9
TEA-CMPSF-M50	0.9	0.50	25.6

, the LER increased from 7.7% for CMPSF-M20 AEM to 25.6% for CMPSF-M50 AEM as the TEA content increased. This increase in LER positively correlates with the observed rise in WU [39]. The greater swelling occurs because the quaternary ammonium groups generated during the reaction impart increased chain flexibility. Importantly, despite the increase in LER, the membranes retained favorable dimensional stability. Since LER quantified the swelling degree, the results demonstrated that the prepared AEMs exhibited favorable anti-swelling properties relative to their high WU. This characteristic, stemming from the enhanced chain flexibility, made them highly appropriate for prolonged use in DD processes.

2.3. DD Performance

The acid dialysis coefficient (U_H⁺) and separation factor (S) are the key performance indicators for evaluating the recovery of spent acid by AEMs via DD [6,38]. U_H⁺ represents the diffusion rate of H⁺, which reflects the acid permeation rate; S indicates the membrane’s selectivity for acid over metal ions [38]. To assess the DD performance of the prepared membranes, a simulated waste acid solution (1 M HCl + 0.25 M FeCl₂) was used. As shown in Figure 8, U_H⁺ for the TEA-CMPSF AEMs increased progressively from 8.3 × 10⁻³ m h⁻¹ (M20) to 47.9 × 10⁻³ m h⁻¹ (M50) with the increase in TEA content. The observed increases in U_H⁺ and U_Fe²⁺ can be attributed to several factors: ① Enhanced ion exchange sites: higher TEA content increased the density of quaternary ammonium groups, which acted as ion exchange sites facilitating anion (Cl⁻ and OH⁻) transport [39]. To maintain electroneutrality, cation transport (H⁺ and Fe²⁺) also increased. However, H⁺ permeation increased more significantly than Fe²⁺ due to its smaller ionic radius and lower charge density [40]. ② Elevated ion exchange capacity: membranes with higher IEC (e.g., M50) promoted anion transport, further driving cation counter-transport. ③ Increased hydration and channel formation: higher quaternary ammonium and hydroxyl content enhanced water uptake. Increased water content facilitated ion migration channels and raised the concentration of H⁺ and Fe²⁺ within the membrane. This amplified the transmembrane concentration gradient, boosting the diffusion of both ions [41]. ④ Morphological changes: SEM analysis indicated that phase separation increased from M20 to M50, forming larger ion channels. This reduced ion transport resistance (H⁺ and Fe²⁺), particularly facilitating the passage of the larger Fe²⁺ ion, thereby increasing both U_H⁺ and U_Fe²⁺.

Defined as the ratio of the H⁺ dialysis coefficient (U_H⁺) to the Fe²⁺ dialysis coefficient (U_Fe²⁺) is the separation factor (S). As shown in Figure 8, the separation factor exhibited a gradual decline from 21.26 for M20 to 3.87 for M50. This reduction correlated with the enhanced IEC and WU. The higher concentration of ion exchange sites promoted greater phase separation within the membrane matrix. While this accelerated the diffusion rates of both Fe²⁺ and H⁺ ions, it concurrently diminished the membrane’s selective permeability. The increased hydration and degree of phase separation reduced the barrier to Fe²⁺ transport more significantly relative to H⁺, thereby lowering the U_H⁺/U_Fe²⁺ ratio [7,19]. Despite this trade-off, the TEA–CMPSF AEMs demonstrated better DD performances compared to other reported dense AEMs in the recovery of acid from HCl/FeCl₂ mixtures, as summarized in

Table 3. DD performance of the TEA-CMPSF AEMs compared to those reported in the literature measured at 25 °C using the simulated HCl/FeCl₂ solution.

Membrane	UH+ (×10−3 m h−1)	S (UH+/UFe2+)	Ref.
TEA–CMPSF	8.3–47.9	3.87–21.26	This work
DF-120	9	18.5	[7]
Pyridinium salt and PVA	17.4–24.8	30.49–51.51	[7]
HTA and quaternized brominated polyphenylene oxide	11–33	≥35.6	[19]
TMA and BPPO	4.3–12	13.14–32.87	[21]
Quaternized diaminobutane and PVA	18.6–29.5	24.7–44.1	[39]
Quaternized poly epichlorohydrin and PVA	11.1–30.0	24.79–42.24	[41]
EPTAC and PVA	11.0–18.0	18.5–21.0	[42]
Quaternized aromatic amine and PVA	17.2–25.2	14.0–21.0	[43]
Quaternized poly epichlorohydrin blending with PVA	8.56–27.33	2.55–39.27	[44]
Triphenylphosphine (TPP) and BPPO	6.7–26.3	27–49	[45]

[7,19,21,39,41,42,43,44,45,46]. Furthermore, all prepared CMPSF membranes maintain a competitive advantage over the commercial benchmark DF-120 membrane (U_H⁺ = 9 × 10⁻³ m h⁻¹, S = 18.5), exhibiting significantly higher U_H⁺ values while retaining substantial separation factors. These results indicated promising application prospects for CMPSF membranes in acid recovery.

2.4. Thermal Stability, Acid Resistance, and Mechanical Properties

Thermal stability is critical for the practical implementation of AEMs. Thermogravimetric analysis (TGA) of the optimal CMPSF-M50 membrane (Figure 9) revealed three distinct weight loss stages between 20 °C and 800 °C. An initial slight mass loss below 114.91 °C corresponded to the evaporation of water adsorbed by quaternary ammonium groups, and the weight loss rate reached the maximum at 58.74 °C, confirming membrane hydrophilicity [45,46]. The second stage of weight loss was at 114.91–408.6 °C and the maximal weight loss rate is at 216.60 °C. This stage primarily involved the degradation of quaternary ammonium groups [46]. Significant weight loss above 500 °C was attributed to the degradation of the polymer backbone and the maximal weight loss rate is at 529.70 °C [47]. Notably, the obtained membranes had superior thermal stability, ensuring reliable thermal performance in diffusion dialysis applications.

For acid recovery applications, membrane stability in acidic environments is equally critical as DD performance. To systematically evaluate the acid resistance of the fabricated membranes, the CMPSF-M50 AEM which demonstrated superior DD performance was selected for stability testing. The membrane was immersed in a simulated waste solution (HCl/FeCl₂) at 65 °C for 7 days, with subsequent routine DD evaluations. As shown in Figure 10 and Figure 11, the results revealed minimal mass loss. For the CMPSF-M50 AEM treated under the above conditions, its acid recovery and separation performance are as follows: U_H⁺ = 45.4 × 10⁻³ m h⁻¹ and S = 4.02. These findings confirmed that the TEA-CMPSF-M50 AEM possesses excellent acid resistance and thermal stability, demonstrating its suitability for practical acid recovery applications.

The mechanical properties of AEMs served as a crucial indicator for their practical application in DD. The tensile strength (TS) and elongation at break (E_b) were tested at room temperature. Figure 12 reveals that the mechanical properties exhibited variations across membranes M20 to M50, with TS values ranging from 20.8 to 34.6 MPa and E_b values varying between 13.3% and 22.7%. The incorporation of TEA was found to intensify phase separation within the membrane matrix, which adversely affected membrane compatibility and continuity, and thereby decreasing TS [41]. Concurrently, increased hydrophilic groups elevated water uptake (see Figure 7) and linear expansion rate (see Table 2). The absorbed water acted as a plasticizer, further reducing TS while enhancing chain mobility to increase E_b [48]. Despite these trends, all membranes exhibited robust mechanical properties meeting commercial DD requirements. This strength–flexibility balance ensures operational durability while accommodating necessary swelling for efficient ion transport.

3. Materials and Methods

3.1. Materials

Polysulfone (PSF), polyformaldehyde (POM), trimethylsilyl chloride [(CH₃)₃SiCl], and tin (IV) chloride were purchased from Shanghai Maclean Biochemical Technology Co., Ltd. (Shanghai, China). Dichloromethane (DCM), N-methyl-2-pyrrolidone (NMP), triethanolamine (TEA), ferrous chloride (FeCl₂·4H₂O), ethanol, sodium hydroxide (NaOH), methyl orange, sodium chloride, potassium bromide, hydrochloric acid (HCl), 1,10-phenanthroline (C₁₂H₈N₂), standard solution of iron (II), and sodium carbonate (Na₂CO₃) were provided by Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China). All reagents in the experiment were analytical grade, and deionized (DI) water was used throughout the requirement.

3.2. Chloromethylation of Polysulfone (CMPSF)

On the basis of prior studies and validated methods, this research details the successful synthesis of CMPSF [49]. The specific preparation method was as follows: 3 g of PSF was placed into a three-necked flask equipped with a magnetic stirrer and condensation reflux device, then 150 mL of DCM was added, and stirring was continued until the PSF was completely dissolved. Afterwards, POM and (CH₃)₃SiCl were added sequentially, and, after mixing well, an appropriate amount of anhydrous tin tetrachloride was added as catalyst slowly and dropwise. The reaction system was refluxed under N₂ protection at 40 °C for 48 h. At the end of the reaction, the reaction solution was poured into 600 mL of anhydrous ethanol and a white flocculent precipitate was precipitated by stirring. The precipitate was filtered and washed three times with anhydrous ethanol and finally dried in a vacuum drying oven at 80 °C for 24 h. CMPSF precipitated as a white solid material.

3.3. Membrane Preparation

In this study, AEMs based on CMPSF were successfully fabricated via solution casting [50]. The reaction equation is shown in Figure 13. First, 0.9 g of CMPSF was dissolved in 10 mL of N-methyl-2-pyrrolidone (NMP) solvent to prepare a 9 wt% casting solution. Subsequently, triethylamine (TEA) was added to individual casting solutions in quantities of 0.20 g, 0.30 g, 0.40 g, and 0.50 g, respectively, yielding AEMs with distinct physicochemical properties designated as M20, M30, M40, and M50. After the mixture was continuously stirred at 45 °C overnight to ensure complete reaction, the solution was poured onto a prepared glass plate and dried at 80 °C for 2 h to evaporate the solvent partially. Subsequently, the membrane was carefully peeled off from the glass plate and immersed in deionized water (acting as a non-solvent coagulation bath) to complete the solidification process via non-solvent-induced phase separation (NIPS). The resulting membrane was stored in deionized water.

3.4. Membrane Characterization

3.4.1. ¹H NMR, XPS, FTIR Spectroscopic, and Microscopic Studies

¹H NMR spectra were obtained from Bruker Avance III HD ¹H NMR spectrometer (Bruker BioSpin, Billerica, MA, USA). The composition of CMPSF AEMs was observed by X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha, Thermo Fisher Scientific, Inc., Waltham, MA, USA). FTIR spectra were recorded in KBr pellets with a Fourier transform infrared spectrometer (Thermo Fisher Scientific Nicolet iS20, Thermo Nicolet Corp., Madison, WI, USA). In addition, the microstructures of the prepared AEMs were characterized by scanning electron microscopy (FE-SEM, Hitachi Regulus 8100, Tokyo, Japan).

3.4.2. Ion Exchange Capacity

IEC was measured using the acid–base titration [36]. First, a dried membrane sample was accurately weighed (m, g). This sample was then immersed in 0.1 mol L⁻¹ NaOH solution at 25 °C overnight to ensure complete Cl⁻/OH⁻ exchange. Subsequently, the membrane was thoroughly rinsed with deionized water to remove residual surface OH⁻. Next, the membrane was equilibrated in 1 mol L⁻¹ NaCl solution at 25 °C overnight, facilitating OH⁻ replacement by Cl⁻. The released OH⁻ was then titrated with 0.05 mol L⁻¹ HCl standard solution using methyl orange indicator. The IEC (mmol g⁻¹) was calculated using Equation (1):

$$\mathrm{IEC}={\displaystyle \frac{V_{HCl}{C}_{HCl}}{m}}$$

(1)

where C_HCl is the HCl concentration (mol/L), V_HCl is the consumed HCl volume (L), and m is the dry membrane mass (g).

3.4.3. Water Uptake, Water Contact Angle, and Linear Expansion Rate

The water uptake (WU) was measured using the weight difference method; the membranes were soaked in deionized water at 25 °C for 2 days; then, the membranes were removed from the deionized water. The wet weight ($W_{wet}$) and dry weight ($W_{dry}$) of the membrane were measured and recorded separately: first, the excess water on the membrane surface was gently wiped to obtain the $W_{wet}$; subsequently, the membranes were dried in a vacuum oven at 60 °C to a constant weight to determine the $W_{dry}$. WU was calculated using Equation (2):

$$WU={\displaystyle \frac{W_{wet}-W_{dry}}{W_{dry}}}\times 100\%$$

(2)

The water contact angle (WCA) was obtained using a contact angle measuring instrument (OCA 20, DataPhysics Instruments, Filderstadt, Germany).

The stability of the membranes was evaluated via linear expansion rate (LER). Membrane samples (4 × 4 cm) were immersed in deionized water at 25 °C for 12 h to reach swelling equilibrium. After surface blotting to remove excess water, the length of the membrane was immediately measured ($L_{wet}$). Subsequently, the membrane sample was placed in a vacuum oven at 60 °C for 12 h drying and the length of the membrane was measured ($L_{dry}$). The linear expansion rate was calculated using Equation (3):

$$LER={\displaystyle \frac{L_{wet}-L_{dry}}{L_{dry}}}\times 100\%$$

(3)

3.4.4. Diffusion Dialysis Test

The diffusion dialysis (DD) performance of the AEMs was evaluated using a static two-compartment apparatus [51] with an effective membrane area of 6.16 cm². Prior to testing, membranes were pretreated by immersion in a mixture of HCl (1 mol L⁻¹) + FeCl₂ (0.25 mol L⁻¹) for 12 h to simulate actual cold-rolled steel pickling liquor composition, followed by thorough rinsing with deionized water until the pH turned neutral. The preconditioned membrane was gently placed between the two compartments, with 100 mL of a mixed solution consisting of 1 mol L⁻¹ HCl with 0.25 mol L⁻¹ FeCl₂ on the feed side and 100 mL of deionized water on the permeate side. The solutions in both the diffusion and permeation chambers were stirred at the same rate to minimize concentration gradients. The DD experiment was carried out for 1 h. After 1 h of operation at 25 °C, 10 mL aliquots were sampled from each compartment. H⁺ concentration was determined by titration with an aqueous Na₂CO₃ solution at a concentration of 0.05 mol L⁻¹, while the Fe²⁺ concentration was determined by o-phenanthroline UV spectrophotometry at 510 nm. All the tests were carried out at a temperature of 25 °C. The dialysis coefficient ($U$) was calculated using Equation (4):

$$U={\displaystyle \frac{M}{At\mathsf{\Delta }C}}$$

(4)

where M denotes the amount of transferred component (mol), A is the effective area of the membrane (m²), t is the time (h), and $\mathsf{\Delta }C$ is the logarithmic mean concentration of the two chambers (mol m⁻³). $\mathsf{\Delta }C$ is calculated using Equation (5):

$$\mathsf{\Delta }C={\displaystyle \frac{C_f^0-(C_f^t-C_d^t)}{In[C_f^0/(C_f^t-C_d^t)]}}$$

(5)

where $C_f^0$ and $C_f^t$ are the feed concentrations at time 0 and t, respectively, and $C_d^t$ is the permeated concentration at time t. The separation factor (S) can be calculated by the following Equation (6):

$$S={\displaystyle \frac{U_{H^{+}}}{U_{F{e}^{2+}}}}$$

(6)

where $U_{H^{+}}$ denotes the dialysis coefficient of H⁺ and $U_{F{e}^{2+}}$ denotes the dialysis coefficient of Fe²⁺.

3.4.5. Thermal Stability

Thermal stability analysis was performed via a thermogravimetric analyser (PerkinElmer STA 6000, PerkinElmer Inc, Waltham, MA, USA) in nitrogen atmosphere and the heating rate was set at 10 °C/min with the temperature range of 30 °C to 800 °C. For acid resistance testing, the membranes were immersed in 1.0 M HCl/0.25 M FeCl₂ solution at 65 °C for 7 days in sealed containers. The weight loss and DD properties of the samples were evaluated to evaluate the thermal and chemical stability.

4. Conclusions

Herein, we presented an efficient membrane fabrication approach for the synthesis of AEMs, which exhibit exceptional DD acid recovery properties. Multiple TEA-CMPSF AEMs were fabricated by modifying PSF substrates via nucleophilic substitution, quaternization reactions, and cross-linking, using CMPSF as the base polymer and TEA as the graft modifier. Comprehensive characterization (XPS and FTIR) confirmed successful TEA-CMPSF formation. The obtained membranes exhibited favorable solubility, as well as the stability. With the increase in TEA content in CMPSF membranes, the IEC rose from 0.62 to 1.19 mmol g⁻¹, WU from 25.9% to 51.9%, and LER from 7.7% to 25.6%, while TS values ranged from 34.6 to 20.8 MPa. The membranes demonstrated exceptional DD performance in HCl/FeCl₂ systems, with simulated waste acid testing (1 M HCl/0.25 M FeCl₂) yielding outstanding acid dialysis coefficients (U_H⁺: 8.3–47.9 × 10⁻³ m h⁻¹) and separation factors (S = 3.87–21.26). These results verified the feasibility of applying TEA-CMPSF AEMs in the recovery of industrial acidic wastewater.