Adsorption Performance of Heavy Metal Ions under Multifactorial Conditions by Synthesized Organic-Inorganic Hybrid Membranes

A series of hybridized charged membrane materials containing carboxyl and silyl groups were prepared via the epoxy ring-opening reaction and sol–gel methods using 3-glycidoxypropyltrimethoxysilane (WD-60) and polyethylene glycol 6000 (PEG-6000) as raw materials and DMF as a solvent. Scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR), and thermal gravimetric analyzer/differential scanning calorimetry (TGA/DSC) analysis showed that the heat resistance of the polymerized materials could reach over 300 °C after hybridization. A comparison of the results of heavy metal lead and copper ions’ adsorption tests on the materials at different times, temperatures, pHs, and concentrations showed that the hybridized membrane materials have good adsorption effects on heavy metals and better adsorption effects on lead ions. The maximum capacity obtained from optimized conditions for Cu2+ and Pb2+ ions were 0.331 and 5.012 mmol/g. The experiments proved that this material is indeed a new environmentally friendly, energy-saving, high-efficiency material. Moreover, their adsorptions for Cu2+ and Pb2+ ions will be evaluated as a model for the separation and recovery of heavy metal ions from wastewater.


Introduction
With the rapid development of the global economy, mankind's best efforts to exploit the Earth's environmental resources have also made our living environment increasingly harsh [1][2][3]. Heavy metal cations and certain toxic anions can cause serious pollution and hazards in the water environment, affecting fisheries and agriculture directly, and these ions are either directly or indirectly harmful to human health. For example, a single dose of 60-l00 mg/kg body weight of lead will stimulate the digestive system, causing abdominal pain, nausea, vomiting, and gastroenteritis, and a long-term overdose can lead to liver cirrhosis. Membrane separation is a new separation technology that emerged in the early 20th century and became increasingly popular after the 1960s. Membrane separation technology has been widely used in food, medicine, biology, environmental protection, the chemical industry, metallurgy, energy, petroleum, water treatment [4], electronics, bionics, and other fields, has produced huge economic and social benefits, and has become one of the most important methods in separation science today. Nowadays, membrane separation technology is widely used in water treatment [5,6] and the chemical, biological, and pharmaceutical fields at an industrial scale, showing its broad application prospects. Polyethylene glycol (PEG) is an excellent biodegradable and water-soluble biopolymer. Due to its environmentally friendly properties, PEG and its derivatives have been used extensively in the manufacture of PEG-based polymeric membranes or materials In this paper, a novel negatively charged siloxane hybrid membrane material is prepared by epoxy ring opening and sol-gel reactions to achieve the efficient and optimized separation of heavy metals at the molecular level. For this study, we chose Pb 2+ and Cu 2+ ions as typical adsorbed species because of their water polluting properties. The aim of this study is to optimize the molar ratio of organic and inorganic components, which is the most important factor affecting the degree of thermal stability and the separation effect. On this basis, the membrane material's heavy metal ion adsorption properties were then investigated. The best adsorption pH, adsorption temperature, Cu 2+ /Pb 2+ concentration, and adsorption time were identified, respectively, via the single variable method.

Preparation of Hybrid Membranes
This class of hybrid membrane materials was prepared using N,N-dimethylformamide (DMF) as the solvent, and charged hybrid membrane materials containing acidic groups were prepared via the epoxy ring opening reaction and sol-gel methods. Four different ratios of materials were designed as presented in Table 1. We started the DF-101S magnetic stirrer with a stirrer (900 rpm), thermometer, and heating device, increased its temperature to 80 • C, added 6.00 g of polyethylene glycol 6000 (PEG-6000) to the beaker, and then added 20 mL of DMF. We placed the beaker in the magnetic stirrer with heat collection in a water bath to dissolve it and then waited for it to cool for a while before using it. After adding 1.00 g of maleic acid and dissolving it in a water bath, we added an equal amount of WD-60 and heated the mixture in a DF-101S magnetic stirrer for 8 h. After the reaction was complete, the mixture was extracted and cooled to room temperature (27 • C). Then, we used a glass rod to coat it evenly on a Teflon board. We made sure to remove any air bubbles from the solution before coating, and we pumped it with a vacuum pump if required. We left it for 2-3 days at room temperature. After the material had been produced, we baked it for one day in a constant-temperature oven at 35 • C initially, then raised the temperature by 5 • C every other day. At 50 • C, the material was fully dry (usually 5-6 days). The samples were put in self-sealing bags and stored for testing after they had dried completely.

Membrane Characterization
To examine the microstructures, surface and cross-sectional morphology of the membranes, SEM (400 FEG, FEI) was performed. FTIR spectra were recorded on a Shimadzu FTIR-8400S spectrophotometer to determine the chemical compositions of the membranes. Solid samples were subjected to the KBr tablet method. All infrared spectra were accumulated 32 times with a 4 cm −1 resolution. The samples were analyzed thermally using a Netzsch STA 409 PC/PG integrated TGA/DSC (differential scanning calorimetry) thermal analyzer protected by nitrogen with a ramp-up rate of 10 • C/min; the temperature was increased from room temperature to 500 • C.

Adsorption Experiment
The adsorption experiment of organic-inorganic hybrid membranes for Cu 2+ /Pb 2+ ions was conducted in a similar way as described in the previous articles [32][33][34], in which Cu 2+ /Pb 2+ ions were used as the adsorption medium [35]. The detailed test procedure can be described in the Supplementary Materials (File S1). The adsorption capacity (q Cu 2+ ) of Cu 2+ ions (mmol/g) is calculated as follows (Equation (1)): where V is the volume of aqueous Cu(NO 3 ) 2 solution (mL), C 0 and C R are the concentrations of initial and remaining Cu(NO 3 ) 2 , respectively (mol/L), and W is the weight of hybrid membrane (g). The adsorption capacity (q Pb 2+ ) of Pb 2+ ions (mmol/g) can also be calculated by Equation (1), where V should be changed to the volume of aqueous Pb(NO 3 ) 2 solution (mL), C 0 and C R are the concentrations of initial and remaining Pb(NO 3 ) 2 , respectively (mol/L), and W is the weight of hybrid membrane (g) (File S2). The effect of adsorption pH, adsorption temperature, Cu 2+ /Pb 2+ concentration, and adsorption time were performed to determine optimal conditions. The macroscopic morphology of the prepared hybrid membrane materials before and after adsorption can be visualized in Figure 1. days at room temperature. After the material had been produced, we baked it for one day in a constant-temperature oven at 35 °C initially, then raised the temperature by 5 °C every other day. At 50 °C, the material was fully dry (usually 5-6 days). The samples were put in self-sealing bags and stored for testing after they had dried completely.

Membrane Characterization
To examine the microstructures, surface and cross-sectional morphology of the membranes, SEM (400 FEG, FEI) was performed. FTIR spectra were recorded on a Shimadzu FTIR-8400S spectrophotometer to determine the chemical compositions of the membranes. Solid samples were subjected to the KBr tablet method. All infrared spectra were accumulated 32 times with a 4 cm −1 resolution. The samples were analyzed thermally using a Netzsch STA 409 PC/PG integrated TGA/DSC (differential scanning calorimetry) thermal analyzer protected by nitrogen with a ramp-up rate of 10 °C/min; the temperature was increased from room temperature to 500 °C.

Adsorption Experiment
The adsorption experiment of organic-inorganic hybrid membranes for Cu 2+ /Pb 2+ ions was conducted in a similar way as described in the previous articles [32][33][34], in which Cu 2+ /Pb 2+ ions were used as the adsorption medium [35]. The detailed test procedure can be described in the Supplementary Materials (File S1). The adsorption capacity (qCu 2+ ) of Cu 2+ ions (mmol/g) is calculated as follows (Equation (1)): where V is the volume of aqueous Cu(NO3)2 solution (mL), C0 and CR are the concentrations of initial and remaining Cu(NO3)2, respectively (mol/L), and W is the weight of hybrid membrane (g). The adsorption capacity (qPb 2+ ) of Pb 2+ ions (mmol/g) can also be calculated by Equation (1), where V should be changed to the volume of aqueous Pb(NO3)2 solution (mL), C0 and CR are the concentrations of initial and remaining Pb(NO3)2, respectively (mol/L), and W is the weight of hybrid membrane (g) (File S2). The effect of adsorption pH, adsorption temperature, Cu 2+ /Pb 2+ concentration, and adsorption time were performed to determine optimal conditions. The macroscopic morphology of the prepared hybrid membrane materials before and after adsorption can be visualized in Figure 1.

Characterization of Hybrid Membranes
The microscopic surface structure and cross-sectional morphology of the organicinorganic hybrid membrane were clearly characterized in scanning electron microscopy Membranes 2023, 13, 531 5 of 13 (SEM) micrographs. As shown in Figure 2, it is obvious to find that the surface of the hybrid membrane (sample A) exhibits a larger pore-size structure [36].

Characterization of Hybrid Membranes
The microscopic surface structure and cross-sectional morphology of the organic-inorganic hybrid membrane were clearly characterized in scanning electron microscopy (SEM) micrographs. As shown in Figure 2, it is obvious to find that the surface of the hybrid membrane (sample A) exhibits a larger pore-size structure [36]. PEG/Si-O-Si organic-inorganic hybrid materials were synthesized via an epoxy ring-opening reaction, as shown in Figure 3a. Figure 3b illustrates the FTIR spectra of the hybrid membrane materials, which all exhibited Si-O-Si stretching vibration absorption peaks at 1107 cm −1 ; in the range of 2700-2900 cm −1 , there were larger broad peaks, primarily methylene and methylene absorption peaks, demonstrating that the carbon chain was not broken in the reaction; 2900 cm −1 was the asymmetric stretching vibration peak of -CH2; 3437 cm −1 was the -OH stretching and bending vibration peak; and the characteristic absorption peak of -COOH appeared at 1718 cm −1 , and the peak increased as the ratio increased, indicating that the content of -COOH also increased [19,20,28]. In addition, the spectra of the other three ratios showed an increase with the amount of WD-60. The characteristic absorption peak for epoxy was near 962 cm −1 , showing that there were epoxy group residues in the system, but the peak area was tiny, indicating that there were fewer epoxy residues in the system. PEG/Si-O-Si organic-inorganic hybrid materials were synthesized via an epoxy ringopening reaction, as shown in Figure 3a. Figure 3b illustrates the FTIR spectra of the hybrid membrane materials, which all exhibited Si-O-Si stretching vibration absorption peaks at 1107 cm −1 ; in the range of 2700-2900 cm −1 , there were larger broad peaks, primarily methylene and methylene absorption peaks, demonstrating that the carbon chain was not broken in the reaction; 2900 cm −1 was the asymmetric stretching vibration peak of -CH 2 ; 3437 cm −1 was the -OH stretching and bending vibration peak; and the characteristic absorption peak of -COOH appeared at 1718 cm −1 , and the peak increased as the ratio increased, indicating that the content of -COOH also increased [19,20,28]. In addition, the spectra of the other three ratios showed an increase with the amount of WD-60. The characteristic absorption peak for epoxy was near 962 cm −1 , showing that there were epoxy group residues in the system, but the peak area was tiny, indicating that there were fewer epoxy residues in the system. Figure 4a shows TGA curves with different ratios of silica-based hybrid membranes. As can be seen from the figure, there were a total of three weight loss processes with an onset temperature of 44 • C. The first weight loss process occurred between 44 • C and 300 • C, with a mass reduction of approximately 10% of the total initial mass of the specimen. Significant weight loss occurred between 300 and 440 • C, with a mass reduction of approximately 50% of the initial total mass of the specimen, indicating that the thermal stability of the hybrid material could reach above 300 • C. With an increase in inorganic WD-60, the thermal stability of the hybrid material increased. The third, which occurred between 440 • C and 500 • C, was a plateau process with essentially constant mass, where an increase in the inorganic WD-60 meant that the amount of inorganic material remaining in the hybrid material also increased. Meanwhile, the weight loss beyond 330 • C can be ascribed to the further degradation of organic groups and the production of crystallized silica [35]. It can be seen from Figure 2a that the thermal stability of the hybridized membrane material is best when the ratio is 6:1; the weight loss around 330 • C was 7.4%. This sample, D, demonstrates a relatively higher compatibility between the inorganic and organic ingredients [35].
The DSC curves for the various ratios of the hybridized membrane materials are shown in Figure 4b. The glass transition temperature of the hybridized material, where the absorbing group exerted its heat, was around 63 • C, where a downward peak can be seen. The peak shifted to the right as the ratio rose at about 380 • C, showing that the stability of the material rises in step with the ratio. Figure 4a shows TGA curves with different ratios of silica-based hybrid membranes. As can be seen from the figure, there were a total of three weight loss processes with an onset temperature of 44 °C. The first weight loss process occurred between 44 °C and 300 °C, with a mass reduction of approximately 10% of the total initial mass of the specimen. Significant weight loss occurred between 300 and 440 °C, with a mass reduction of approximately 50% of the initial total mass of the specimen, indicating that the thermal stability of the hybrid material could reach above 300 °C. With an increase in inorganic WD-60, the thermal stability of the hybrid material increased. The third, which occurred between 440 °C and 500 °C, was a plateau process with essentially constant mass, where an increase in the inorganic WD-60 meant that the amount of inorganic material remaining in the hybrid material also increased. Meanwhile, the weight loss beyond 330 °C can be ascribed to the further degradation of organic groups and the production of crystallized silica [35]. It can be seen from Figure 2a that the thermal stability of the hybridized membrane material is best when the ratio is 6:1; the weight loss around 330 °C was 7.4%. This sample, D, demonstrates a relatively higher compatibility between the inorganic and organic ingredients [35].
The DSC curves for the various ratios of the hybridized membrane materials are shown in Figure 4b. The glass transition temperature of the hybridized material, where the absorbing group exerted its heat, was around 63 °C, where a downward peak can be seen. The peak shifted to the right as the ratio rose at about 380 °C, showing that the stability of the material rises in step with the ratio.    Figure 5a demonstrates that there was no significant change of Cu 2+ adsorption effect. As the amount of WD-60 increased, the adsorption effect slightly increased, and the qCu 2+ was about 0.33 mmol/g, which was three times higher than in previous studies (0.075 mmol/g) in Table 2 Figure 5a demonstrates that there was no significant change of Cu 2+ adsorption effect. As the amount of WD-60 increased, the adsorption effect slightly increased, and the q Cu 2+ was about 0.33 mmol/g, which was three times higher than in previous studies (0.075 mmol/g) in Table 2 Figure 5a demonstrates that there was no significant change of Cu 2+ adsorption effect. As the amount of WD-60 increased, the adsorption effect slightly increased, and the qCu 2+ was about 0.33 mmol/g, which was three times higher than in previous studies (0.075 mmol/g) in Table 2. The Cu 2+ adsorption schemes of the hybrid membranes (A, B, C, and D) under multifactorial conditions are shown in Figure 5b.    The constant temperature vacuum drying oven was set at 30, 35, 40, 45, and 50 • C. The adsorption capacity of the hybrid membrane material for Cu 2+ ions fluctuated with temperature, as can be seen in Figure 6a, with the best adsorption effect occurring at 45 • C. The optimal temperature for Cu 2+ adsorption by the hybrid membrane material was therefore determined to be 45 • C. The same conclusion can be found in the previous articles for the zwitterionic hybrid adsorbents [32].

Absorption Properties of Cu 2+ by Hybrid Membrane Materials
Different concentrations of 0.005 mol/L, 0.01 mol/L, 0.03 mol/L, 0.05 mol/L, 0.1 mol/L, 0.2 mol/L, 0.3 mol/L, and 0.5 mol/L of Cu(NO 3 ) 2 were prepared, and their pH was adjusted to pH = 4. The volume of Na 2 S 2 O 3 consumed can be used to compute the amount of sample adsorbed. The adsorption of the membrane materials all increased with the concentration of Cu 2+ ions, as can be seen in Figure 6b,c. The reason for this is that as the Cu(NO 3 ) 2 solution's concentration increased, meaning that more Cu 2+ ions engaged in ion exchange, which increased the amount of Cu 2+ ions that the hybrid polymer was able to adsorb. Figure 6d shows that the hybrid membrane material's adsorption amount gradually increased over time and eventually tended to be constant, indicating saturation adsorption. This is because the ion exchange capacity of the hybrid material, which was correlated with adsorption, progressively rose over time and eventually approached saturation at the optimum adsorption duration. The equilibrium adsorption time was 9 h, at which time the membrane's adsorption capacity was essentially saturated. ymer was able to adsorb. Figure 6d shows that the hybrid membrane material's adsorption amount gradually increased over time and eventually tended to be constant, indicating saturation adsorption. This is because the ion exchange capacity of the hybrid material, which was correlated with adsorption, progressively rose over time and eventually approached saturation at the optimum adsorption duration. The equilibrium adsorption time was 9 h, at which time the membrane's adsorption capacity was essentially saturated. The material adsorbed Cu 2+ ions very quickly, and Table 3 demonstrates that the amount of adsorption progressively increased with time and that the rate of adsorption was speedy, with the first 9 h accounting for 99.3% of the maximum adsorption of the entire process.

Absorption Properties of Pb 2+ by Hybrid Membrane Materials
The 6:1 material has the highest adsorption capacity of 5.012 mmol/g, as can be seen in Figure 7a. In the following experiments, we looked at the degree of Pb 2+ ion adsorption for materials A, B, C, and D at various times, concentrations, and pH values (Figure 5b).
As can be seen in Figure 5a, the adsorption capacity of the material for Pb 2+ ions became stronger as the organic composition increased. tion for materials A, B, C, and D at various times, concentrations, and pH values ( Figure  5b). As can be seen in Figure 5a, the adsorption capacity of the material for Pb 2+ ions became stronger as the organic composition increased.
The theoretical explanation for this phenomenon is that the amount of adsorption of the hybrid material may be related both to the charging characteristics of the hybrid material and to the network structure formed by the hybrid material. Different structures also show different adsorption for different metal ions. For the adsorption of Pb 2+ ions, which was mainly based on ion exchange, a large number of hydrogen ions were exchanged with Pb 2+ ions in solution [14,19,28], so the larger the WD-60 ratio, the greater the amount of adsorption (Figure 7b). At a pH of 5, the four materials were best at adsorbing Pb 2+ . The optimal pH for Pb 2+ adsorption by materials A, B, C, and D hybrids was therefore determined to be 5 ( Figure  8a). The maximum adsorption capacity (qPb 2+ ) for the four materials was reached at 21 h. (Figure 8b). Because ion exchange adsorption was the primary cause of Pb 2+ ion adsorption, the best adsorption effect of the hybrid material was obtained at a Pb 2+ ion concen- The theoretical explanation for this phenomenon is that the amount of adsorption of the hybrid material may be related both to the charging characteristics of the hybrid material and to the network structure formed by the hybrid material. Different structures also show different adsorption for different metal ions. For the adsorption of Pb 2+ ions, which was mainly based on ion exchange, a large number of hydrogen ions were exchanged with Pb 2+ ions in solution [14,19,28], so the larger the WD-60 ratio, the greater the amount of adsorption (Figure 7b).
At a pH of 5, the four materials were best at adsorbing Pb 2+ . The optimal pH for Pb 2+ adsorption by materials A, B, C, and D hybrids was therefore determined to be 5 ( Figure 8a). The maximum adsorption capacity (q Pb 2+ ) for the four materials was reached at 21 h. (Figure 8b). Because ion exchange adsorption was the primary cause of Pb 2+ ion adsorption, the best adsorption effect of the hybrid material was obtained at a Pb 2+ ion concentration 0.2 mol/L, as can be seen in Figure 8c. The adsorption performance of the hybrid material was subsequently improved by increasing the amount of WD-60 at various concentrations. The adsorption capability of the hybrid membrane material for Pb 2+ ions fluctuated with temperature, as can be seen in Figure 8d, with the best adsorption occurring at 40 • C. Therefore, 40 • C is the ideal temperature for the hybrid membrane material to adsorb Pb 2+ ions. The theoretical explanation to this phenomenon can be ascribed to the electrostatic attraction between Pb 2+ ions and carboxylic groups on the polymer chains [36].
Commonly, when metal ions are adsorbed by an adsorbent, the metal ions transport from the solution through the interface between the solution and adsorbent into the pores of the network structure. To have an insight into the effect of intraparticle diffusion on adsorption rate, sample A-D can be calculated based on the relationship of the adsorption capacity and time, which usually is expressed as (Equation (2)) [32,36]: where q t is the adsorbed amount (mmol/g) at time t, k p is the intraparticle diffusion rate constant, and x i is the intercept of the straight line, which is related to the boundary layer thickness. It is now well accepted that if the plot of q t vs. t 0.5 gives a straight line, the adsorption process is solely controlled by intraparticle diffusion. If the data exhibit multilinear plots, however, two or more steps will influence the adsorption process [32,36].
material was subsequently improved by increasing the amount of WD-60 at various concentrations. The adsorption capability of the hybrid membrane material for Pb 2+ ions fluctuated with temperature, as can be seen in Figure 8d, with the best adsorption occurring at 40 °C. Therefore, 40 °C is the ideal temperature for the hybrid membrane material to adsorb Pb 2+ ions. The theoretical explanation to this phenomenon can be ascribed to the electrostatic attraction between Pb 2+ ions and carboxylic groups on the polymer chains [36]. Commonly, when metal ions are adsorbed by an adsorbent, the metal ions transport from the solution through the interface between the solution and adsorbent into the pores of the network structure. To have an insight into the effect of intraparticle diffusion on adsorption rate, sample A-D can be calculated based on the relationship of the adsorption capacity and time, which usually is expressed as (Equation (2)) [32,36]: (2) where qt is the adsorbed amount (mmol/g) at time t, kp is the intraparticle diffusion rate constant, and xi is the intercept of the straight line, which is related to the boundary layer thickness. It is now well accepted that if the plot of qt vs. t 0.5 gives a  Figure 9 illustrated the intraparticle diffusion curves for Cu 2+ and Pb 2+ ions adsorption in this case. As shown in Figure 9, it can be found that the adsorption of Cu 2+ and Pb 2+ ions on samples A-D is non-linear over the range of adsorption time and three steps are involved: the rapid interface diffusion from 0 to 3 h; subsequently, the intraparticle diffusion (increased slowly and approached equilibrium), suggesting that Cu 2+ and Pb 2+ adsorption on samples A-D is not governed by intraparticle diffusion; diffusion-controlled adsorption mechanisms might be the major process, as reported in other articles [32,36]. Table 4 below shows the adsorption capacity (q Pb 2+ ) percentage of the first 9 h to 21 h (maximum adsorption capacity). Table 4 shows that the rate of adsorption was quick and that the amount of adsorption gradually increased over time, with the first 9 h accounting for a maximum of 93.0% of the entire process. straight line, the adsorption process is solely controlled by intraparticle diffusion. If the data exhibit multilinear plots, however, two or more steps will influence the adsorption process [32,36]. Figure 9 illustrated the intraparticle diffusion curves for Cu 2+ and Pb 2+ ions adsorption in this case. As shown in Figure 9, it can be found that the adsorption of Cu 2+ and Pb 2+ ions on samples A-D is non-linear over the range of adsorption time and three steps are involved: the rapid interface diffusion from 0 to 3 h; subsequently, the intraparticle diffusion (increased slowly and approached equilibrium), suggesting that Cu 2+ and Pb 2+ adsorption on samples A-D is not governed by intraparticle diffusion; diffusion-controlled adsorption mechanisms might be the major process, as reported in other articles [32,36].  Table 4 below shows the adsorption capacity (qPb 2+ ) percentage of the first 9 h to 21 h (maximum adsorption capacity). Table 4 shows that the rate of adsorption was quick and that the amount of adsorption gradually increased over time, with the first 9 h accounting for a maximum of 93.0% of the entire process.

Conclusions
Using epoxy ring opening and sol-gel reactions, a new siloxane hybrid membrane material has been devised to enhance the heavy metal ion separation capabilities of organic-inorganic membranes under multifactorial reaction conditions. FTIR results proved that sol-gel technology was suitable for the preparation of organic-inorganic hybrid materials and the PEG and inorganic Si-O-Si components were bonded together through covalent bonds, thus successfully preparing a PEG/Si-O-Si system of ion-exchange membrane materials. TGA/DSC measurements revealed significantly improved thermal stability following the hybridization of these membranes, with the increased amount of inorganic material (WD-60) contributing to the molecular structural stability of the hybrid materials. The hybrid membrane material surfaces displayed high

Conclusions
Using epoxy ring opening and sol-gel reactions, a new siloxane hybrid membrane material has been devised to enhance the heavy metal ion separation capabilities of organicinorganic membranes under multifactorial reaction conditions. FTIR results proved that solgel technology was suitable for the preparation of organic-inorganic hybrid materials and the PEG and inorganic Si-O-Si components were bonded together through covalent bonds, thus successfully preparing a PEG/Si-O-Si system of ion-exchange membrane materials. TGA/DSC measurements revealed significantly improved thermal stability following the hybridization of these membranes, with the increased amount of inorganic material (WD-60) contributing to the molecular structural stability of the hybrid materials. The hybrid membrane material surfaces displayed high and efficient separation activity against Cu 2+ and Pb 2+ with varying degrees of heavy metal wastewater pollution. This study describes a low-cost but effective approach to preparing membrane separation surfaces and solves the problem of multifactorial reaction conditions and complicated operations from the perspective of industrial controllability. The best adsorption pH (Figure 5b), adsorption temperature, adsorption concentration, and adsorption time (Figure 7b) were found in turn via the single variable method, suggesting that these materials are likely to be widely used in water treatment and the chemical, biological, and pharmaceutical fields at an industrial scale with broad application prospects. In the practical condition, the membrane has to face the complicated feed solution. Future research will continue to investigate the microscopic properties of membrane materials to adapt to complex environments.
Author Contributions: C.W.: conceptualization, theoretical analysis, writing-review and editing, writing-original draft, supervision, funding acquisition, visualization, and project administration. J.Z.: methodology, investigation, validation, formal analysis, data curation, and visualization; L.H.: resources, investigation, and theoretical analysis. All authors have read and agreed to the published version of the manuscript.
Funding: This research was sponsored by"Chenguang Program" supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission. The funding number is 21CGB13.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.