Facile Separation of Cu²⁺ from Water by Novel Sandwich NaY Zeolite Adsorptive Membrane

Abstract

Polyethersulfone-sulfonated polyethersulfone (PES-SPES)/NaY zeolite/nylon sandwich structure membranes were prepared and used to adsorb Cu²⁺ from water. The adsorption kinetics, adsorption isotherm, dynamic adsorption experiment, and reusability were discussed. The experimental data showed that the Langmuir isotherm model, Dubinin–Radushkevich (D-R) isotherm model, and the pseudo-first-order kinetic model can well represent the adsorption of Cu²⁺ on the membrane, indicating an ion exchange mechanism, with the maximum adsorption capacity of 111.25 mg·g⁻¹. Repeatability experiments show that the sandwich film still has good adsorption performance after five times of adsorption and desorption. The as-prepared membrane showed considerable separation performance in removing Cu²⁺ from aspirin solution, providing a feasible method to remove heavy metals from drugs.

1. Introduction

People are paying more and more attention to drug quality and safety, and the pharmaceutical industry has become one of the most regulated industries [1]. Since the World Health Organization (WHO) first implemented good manufacturing practice of medical products (GMP) in the 1960s, the rules and regulations of various countries for drug safety production have been continuously improved and adjusted [2], and the International Conference on Harmonization (ICH) proposed the guideline for metallic impurities (Q3D) to ensure the quality of drugs [3].

The application of metal catalysts in drug synthesis is becoming more and more general, resulting in the residue of heavy metals in the final product [4]. For example, palladium carbon and Raney nickel are employed in the synthesis of mepivacaine hydrochloride and rivaroxaban, respectively, and the bentonite loaded with CuCl₂ is used as the catalyst in the synthesis of aspirin [5,6,7]. Furthermore, concentrated sulfuric acid or phosphoric acid is often also used as a catalyst in the industrial production of aspirin [8]. The corrosive properties of these two acids can leach metals such as copper from stainless steel equipment into the drug [9]. At present, heavy metal pollution in water has become a global environmental problem. ICH-Q3D stipulates that the permitted daily exposure (PDE) of copper is 3 mg/day (assumes 10 g oral or parenteral dose) [10]. Heavy metals such as Cu, Pb, Hg, Zn, etc., have obvious effects on human metabolism and normal physiological functions [11]. For example, heavy metals can cause harm to mental and central nervous function, lungs, kidneys, and other organs [12,13,14]. If copper in the human body exceeds the standard, it can cause symptoms such as nausea and vomiting, abdominal pain and diarrhea, and acute hepatotoxicity [15,16]. Alzheimer’s disease (AD) is also related to the imbalance of copper in the body [17,18]. On the other hand, heavy metals can have adverse effects on the ecological environment. When released into the environment, they do not biodegrade and instead accumulate through the food chain. It has been reported that heavy metals are highly toxic and most of them even cause cancer [19,20].

At present, the methods of removing heavy metals mainly include alcohol precipitation, molecular imprinting, superfluid, and adsorption, etc. [21,22]. There are many methods to remove heavy metal pollution which have problems such as high treatment cost, difficult reuse, complicated process, and may also produce secondary pollution and waste water resources. In contrast, the adsorption method has advantages such as wide application range, fast reaction speed, adaptability to different reaction conditions, and environmental friendliness, which have attracted high attention from researchers. Commonly employed adsorbents are hydrogel [23], cation exchange resin [24], zeolite [25], and acticarbon [26], etc. The exchangeable cations in zeolite skeleton can exchange with heavy metals in water, and the large specific surface area has a unique adsorption function for heavy metals [27]. Therefore, zeolite has been drawing much attention because of its low price, unique mechanical stability, and fast regeneration speed [28]. Olegario, etc., found that Philippine natural zeolite showed good adsorption capacity and ion selectivity for heavy metals such as As⁵⁺, Cu²⁺, and Ni²⁺ [29]. Fan’s team found that the maximum adsorption capacity of NaX zeolite for Cd²⁺, Cu²⁺, and Pb²⁺ reached 2.14, 1.87, and 2.38 mmol g⁻¹, respectively [30]. The tetrahedrons of silica and alumina in zeolite are connected to the pores by sharing all oxygen atoms, forming a three-dimensional network structure. The Na⁺, K⁺, and Ca⁺ ions present in the structural channels of zeolite can be replaced by other metal cations such as Cu²⁺, Pb²⁺, Zn²⁺, and Cd²⁺, causing heavy metals in the water to adsorb on the zeolite, while the zeolite structure remains almost unchanged [31]. The exchangeable cations in zeolite skeleton can exchange with heavy metals in water. The large specific surface area and three-dimensional pore structure of zeolite make it widely used in the process of metal ion adsorption and removal. However, nanomaterials are difficult to separate from the mother liquor after adsorption, which restricts their practical application.

Membrane adsorption technology possesses both adsorption and filtration functions and is widely used in water treatment, drug purification, chemical industry, and other fields [32]. Yurekli et al. dispersed zeolite in polysulfone to make a mixed matrix membrane (MMM) and the adsorption capacity of Ni²⁺ and Pd²⁺ was 122 and 682 mg g⁻¹, respectively [33]. However, the coating of the zeolite by the polymer can lead to a longer equilibrium time. In addition, the adsorbent may fall off from the membrane during the adsorption/desorption process, resulting in secondary pollution [34]. The sandwich adsorption membrane is composed of three parts: the base membrane, adsorption layer, and surface layer. First, the nylon microfiltration membrane is used as the base membrane to filter the adsorbent particles, and then the adsorption layer of micron thickness is obtained. Finally, the hydrophilic polymer solution is scraped and coated on the surface of the adsorption layer, and the asymmetric porous polymer surface membrane was obtained by the immersion precipitation phase conversion method.

The sandwich membrane structure has the following advantages: 1. The polymer solution can penetrate into the microporous membrane where there is no adsorbent at the edge of the membrane, playing the anchoring role and improving the strength and stability of the membrane. 2. The nanoparticles are fixed between the base membrane and the surface layer to avoid secondary pollution caused by the adsorbent falling off. 3. The adsorbent nanoparticles were prepared in a sandwich membrane, which improved the adsorption efficiency and was easily separated from the mother liquor and easy to recover. 4. The cation exchange property of zeolite makes the metal ions adsorbed, and drug molecules pass through the gap of nanoparticles without passing through the three-dimensional pore of zeolite, so as to achieve the purpose of separating drugs and heavy metals, which is conducive to industrial applications. In this paper, NaY zeolite was synthesized by a hydrothermal method and used as an adsorbent to prepare the sandwich adsorptive membrane. The kinetics, adsorption isotherm, dynamic adsorption, and reusability for Cu²⁺ adsorption were studied, and the dynamic adsorption of Cu²⁺ in aspirin aqueous solution was conducted.

2. Experimental Part

2.1. Materials

Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were bought from Jinan Jiewei Chemical Technology Co., Ltd., Jinan, China. N-methylpyrrolidone (NMP) and ethanol (EtOH) were bought from Shanghai Gaoshen Chemical Co., Ltd., Shanghai, China. Sodium metaaluminate (AlNaO₂) and sodium silicate solutions were all analytical reagents and bought from Shanghai McLean Biochemical Technology Co., Ltd., Shanghai, China. Aspirin was purchased from China National Pharmaceutical Group Co., Ltd., Beijing, China. Copper standard solution was bought from Beijing Zhongke Quality Control Technology Co., Ltd., Beijing, China. Sodium chloride (NaCl) was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd., Tianjin, China. Polyethersulfone (PES) was bought from Shanghai Zhuoying plasticization Co., Ltd., Shanghai, China, and nylon membranes (Φ25 mm) were purchased from Dalian Shizhou Filter Equipment Co., Ltd., Dalian, China.

2.2. Preparation of NaY Zeolite

NaY zeolite was preparation by a hydrothermal method [35] with a molar composition of 12.8 (Al₂O₃):17.1 (Na₂O₃):1 (SiO₂):675 (H₂O). During this procedure, the identified amounts of NaOH, AlNaO₂, and DI water were mixed; then, sodium silicate solutions were added drop-wise and stirred until completely dissolved. After aging for 12 h, the mixture was moved to a polytetrafluoroethylene (PTFE) bottle and then kept at 90 °C for 12 h. After it cooled down, the solid was filtered and washed with DI water.

2.3. Preparation of PES-SPES/NaY Zeolite/Nylon Sandwich Adsorptive Membrane

SPES was synthesized according to the reference [36,37]. The nylon membrane (Φ 25 mm) was soaked in EtOH for 2 min and washed with DI water. Then, 100 mg NaY zeolite was dispersed in 100 mL EtOH and ultrasoniced for 10 min. A certain volume of the suspension was filtered with the nylon membrane to obtain the NaY zeolite/nylon membrane. Finally, the NMP casting solution of SPES-PES (10 wt%, SPES/PES = 1/4) was casted on the NaY zeolite/nylon membrane with a scraper (spacing of 365 μm), then soaked in pure water for 8 h to obtain the PES-SPES/NaY zeolite/nylon sandwich adsorptive membrane (Figure 1). The sandwich adsorption membrane is composed of three parts: the base membrane (nylon membrane), adsorption layer (NaY zeolite), and surface layer (PES-SPES hydrophilic polymer layer).

2.4. Static Adsorption

The adsorption performance of the sandwich adsorption membrane was investigated by the stirring method. The concentration of Cu²⁺ aqueous solution (100 mL) was prepared in the conical flask, and the adsorption membrane was added. The pH value of the aqueous solution was adjusted by 0.1 mol·L⁻¹ HCl and 0.1 mol·L⁻¹ NaOH. The concentration of Cu²⁺ in the conical flask solution was measured at room temperature by stirring at a low speed (40 rpm) under the action of a magnetic agitator. The saturated adsorption capability (Q_e, mg g⁻¹) was expressed as Equation (1):

$$Q_e=\frac{c_0-c_e}{\mathsf{\Delta }m}\times v$$

(1)

where c_e and c₀ are the saturated and initial concentrations of Cu²⁺ solution (mg L⁻¹), respectively, Δm is the mass of NaY zeolite (g), and v is the volume of aqueous solution (L). The removal ratio (Re, %) and desorption ratio (DR, %) were calculated as Equations (2) and (3), respectively:

$$Re=\frac{c_0-c_e}{c_0}\times 100\%$$

(2)

$$DR=\frac{c_1\times v_1}{(c_0-c_e)\times v}$$

(3)

where v₁ and c₁ are the volume of eluent (L) and the concentration of Cu²⁺ in the eluent (mg L⁻¹), respectively.

2.5. Dynamic Adsorption and Desorption

During the dynamic filtering experiment, the sandwich membrane (Φ 25 mm) was transferred to the filter, pre-treated with 20 mL pure water, and then the Cu²⁺ solution was flowed through the sandwich adsorptive membrane with the rate of 0.68 mL min⁻¹. During the desorption experiment, the eluent (saturated NaCl solution) was filtered at a rate of 0.8 mL min⁻¹ [38]. The pure water flux was expressed as Equation (4) [39]:

$$J_0=\frac{V}{A\times \mathsf{\Delta }t\times P}$$

(4)

where Δt is the filtration time (h), V and A are the volume of filtered water (L) and the effective area (m²), respectively, and J₀ and P are the pure water flux (L m⁻² h⁻¹ bar⁻¹) and test pressure (1 bar = 100 KPa).

2.6. Characterization

The structures and morphologies of ZaY particles and the sandwich membrane were under observation by field emission scanning electron microscope (SEM, S-480003040155, Hitachi, Tokyo, Japan). The membrane structures were investigated by viewing the surface and cross section of the sandwich membrane. The membrane was freeze dried in liquid nitrogen and then quenching to observe the cross section of the membrane. Quantitative analysis of Cu²⁺ in water (λ_max = 547 nm) was measured by the colorimetric method with a UV-vis spectrophotometer (UV, Lambda 35, PerkinElmer, Waltham, MA, USA) [40]. The pore characteristics and BET surface areas of the samples were determined using N² adsorption/desorption analysis performed at 200 °C on a specific surface area analyzer (BSD-660, BSD Instrument, Beijing, China). Specimens of the zeolites were measured by an X-ray powder diffraction technique (XRD, XRD-6000, Shimadzu, Kyoto, Japan).

3. Results and Discussion

3.1. Characterization of Sandwich Membrane

Figure 2 shows the SEM images of the sandwich adsorptive membrane clearly. The surface is smooth and contains a large number of nanopores (Figure 2b), which can filter out the impurities in the solution and prevent blocking of the membrane. The PES-SPES layer has a finger-like asymmetrical cross section to reduce flow resistance during filtration. The total thickness of the sandwich adsorptive membrane is 129.5 μm, and the thickness of the PES-SPES polymer layer and substrate membrane are 32.3 μm and 78.9 μm, respectively. The adsorbent layer forms a compact particle accumulation with a thickness of 18.3 μm. The total mass of the membrane is 52.0 mg, and the percentage of adsorbent in the total mass of the membrane is 7.7 wt%. The water flux of the sandwich adsorptive membrane is 47.2 L m⁻² h⁻¹ bar⁻¹. Figure 2d is the characterization of NaY zeolite. The XRD pattern is consistent with the Y-type zeolite (2θ = 6.2°, 15.6°, 23.6°, and 31.36°) [41]; the crystal is uniform, with a diameter of approximately 0.84 μm.

Figure 2e shows the N₂ adsorption/desorption isotherm of the sandwich adsorptive membrane. The adsorption branch and desorption branch in the curve do not coincide, forming a narrow hysteresis ring, indicating that there is a mesoporous structure in the membrane [42]. The BET specific surface area is 50.9 m² g⁻¹. Figure 2f displays the pore size distribution curve of the sandwich adsorptive membrane. The pore diameter is mostly distributed within the scope of 2–10 nm, with an average size of 6.6 nm (in the range of mesoporous pore) [43].

3.2. Adsorption Isotherm

To explore the effect of pH, sandwich adsorptive membranes were added to 5 mg L⁻¹ of Cu²⁺ aqueous solution (100 mL) with different pH (2.0, 3.0, 4.0, 5.0, and 6.0) at room temperature under stirring (40 rpm). It can be seen from Figure 3a that the Q_e of the sandwich adsorptive membrane was obviously affected by the solution pH. With the increase in pH from 2.0 to 6.0, the Q_e increased significantly from 0 to 71.75 mg g⁻¹. NaY is a kind of porous aluminum silicate material with a three−dimensional pore structure. Na⁺ and K⁺ in its skeleton are the main exchangeable cations, and it is easy to have ion exchange with other cations in aqueous solution. In acidic water, a large amount of H⁺ will form ionic competition with Cu²⁺, resulting in the reduction in adsorption capacity [44]. With the increase in pH, the competition between H⁺ and Cu²⁺ is gradually weakened, and the adsorption capacity of Cu²⁺ increases gradually, reaching the maximum at pH 6.0. With the increase in pH, the Q_e gradually increases. In order to avoid the hydrolysis of Cu²⁺ under alkaline conditions or neutral, pH = 6.0 was chosen as the optimal reaction condition.

The sandwich adsorptive membrane (containing 4 mg adsorbent) was placed in Cu²⁺ aqueous solution (100 mL) with different concentrations of 1, 2, 4, 5, 6, 8, 10, 16, and 16 mg L⁻¹, respectively (pH 6.0, 25 °C), for 8 h. As shown in Figure 3b, with the increase in Cu²⁺ aqueous concentration (c_e), the adsorption capacity of Cu²⁺ on the sandwich adsorption membrane also increases, and the growth trend shows that the adsorption capacity of Cu²⁺ rises rapidly first and then gradually flattens. This may be due to the limited ions available for exchange in NaY zeolite, and the adsorption capacity reaches the maximum when the concentration of Cu²⁺ solution reaches a certain amount. In order to explain the adsorption mechanism of the sandwich adsorptive membrane toward Cu²⁺, the adsorption isotherms date was fitted to Freundlich, Langmuir, and D-R adsorption models [45,46,47]. The Langmuir adsorption model indicates that the adsorption processes are chemical adsorption and monolayer adsorption, and there are limited homogeneous adsorption sites. The Freundlich adsorption isotherm model is fit for multilayer adsorption, and calculated by the following Equations (5) and (6):

$$Q_e=k_F{c}_e{}^{\frac{1}{a}}$$

(5)

$$\log Q_e=\log k_F+\frac{1}{a}\log c_e$$

(6)

where k_F is the Freundlich parameter (mg g⁻¹) (L mg⁻¹)^1/a, indicating the affinity between adsorbents and adsorbents, and a is the adsorption strength [45]. The Langmuir adsorption isotherm model is based on chemisorption monolayer and expressed as Equation (7):

$$\frac{1}{Q_e}=\frac{1}{Q_m}+\frac{1}{b{Q}_m}\frac{1}{c_e}$$

(7)

where b and Q_m are the Langmuir parameter (L mg⁻¹) and the saturated adsorption capacity (mg g⁻¹), respectively, and b reflects the stability of the combination of adsorbent and adsorbate [46]. It can be seen from Figure 4 and

Table 1. Adsorption isotherm parameter for Cu²⁺ adsorption on sandwich adsorptive membrane.

Isotherm Model	Parameters	−
Freundlich	1/a	0.5792
	kF (mg g−1) (L mg−1)1/n	2.976
	R2	0.899
Langmuir	Qm (mg g−1)	111.3
	b (L mg−1)	0.922
	R2	0.974
D-R	k	0.00201
	E (kJ mol−1)	15.77
	R2	0.869

that the Langmuir adsorption isotherm model (R² > 0.974) displays a more satisfactory description than the Freundlich adsorption isotherm model, which indicates that the adsorption of Cu²⁺ on the sandwich adsorptive membrane is chemical adsorption, and the Q_m of 111.3 mg g⁻¹ is higher than that recorded in the other literatures (

Table 2. Comparison of Cu²⁺ adsorption by different adsorbents.

Adsorbents	Concentrations (mg L−1)	Adsorption Time (min)	Saturated Adsorption Capacity (mg g−1)	Reference
Natural zeolites from the Philippines	136	360	14.65	[29]
bagasse pith grafted copolymer (BPGC)	6	700	105	[49]
Chitosan/hydroxyapatite/nano-magnetite (Fe3O4) composite	1–5	100	3.65	[50]
Chitosan blended with polyvinyl alcohol (PVA) membrane	20–100	720	35.71	[51]
PES-SPES/NaY zeolite/nylon sandwich membrane	1–25	480	111.3	This work

). To further describe the adsorption mechanism, the D-R model is analyzed as Equations (8)–(11) to evaluate the average adsorption energy:

$$Q_e=Q_{m}exp(-k{\epsilon }^2)$$

(8)

$$ln{Q}_e=ln{Q}_m-k{\epsilon }^2$$

(9)

$$\epsilon =RTln(1+\frac{1}{c_e})$$

(10)

$$E=\frac{1}{{(2k)}^{1/2}}$$

(11)

where ε is the Polanyi potential, and R and T are the ideal gas parameter (8.314 kJ mol⁻¹ K⁻¹) and the degree kelvin (K), respectively. The average adsorption free energy (E, kJ mol⁻¹) is used to judge whether the adsorption is physical adsorption or ion exchange adsorption [47]. As shown in the Table 1, E is 15.77 kJ mol⁻¹, between 8–16 kJ mol⁻¹, proving that is an ion exchange process [48]. Metal cations enter the three-dimensional pore of the zeolite skeleton and exchange with the cations of the zeolite skeleton itself.

3.3. Adsorption Kinetics

The kinetics of the sandwich adsorptive membrane were conducted in Cu²⁺ aqueous solution (5, 10 mg L⁻¹, pH 6.0, 100 mL) under stirring (40 rpm) at room temperature. Figure 5a shows the change curve of the adsorption capacity with time. It can be seen that, with the growing time (0–300 min), the adsorption capacity increases rapidly with time. This is because there are more exchangeable Na⁺ and K⁺ ions in the NaY zeolite skeleton at the initial stage of adsorption. Then, the NaY zeolite skeleton exchangeable ions gradually become less, so the adsorption capacity growth rate also becomes slow, until it reaches a stable rate.

In order to clarify the control mechanism and velocity limiting steps in the adsorption process, three commonly used kinetic models were selected to analyze the experimental data. The pseudo-first-order adsorption kinetics model represents the adsorption reaction caused by boundary diffusion, and the pseudo-second-order adsorption kinetics model represents the adsorption process controlled by chemisorption, and expressed as Equations (12) and (13), respectively [52]:

$$\log (Q_e-Q_t)=\log Q_e-\frac{k_{1}t}{2.303}$$

(12)

$$\frac{t}{Q_t}=\frac{1}{k_2{Q}_e{}^2}+\frac{t}{Q_e}$$

(13)

where Q_t (mg g⁻¹) is the adsorption capacity at a time t, and k₂ (g mg⁻¹ min⁻¹) and k₁ (min⁻¹) are the pseudo-second-order and pseudo-first-order rate parameter, respectively. The model of intra-particle diffusion is commonly used to describe the rate-limiting step in the adsorption process [53] and expressed as Equation (14):

$$Q_t=k_i{t}^{\frac{1}{2}}+C$$

(14)

where C (mg g⁻¹) is the intercept of the curve in the graph and k_i is the diffusion rate parameter (mg g⁻¹ min^−1/2).

Table 3. Kinetic adsorption model parameters of Cu²⁺ by sandwich adsorptive membrane.

Kinetic Models	Parameters	5 mg L−1	10 mg L−1
The pseudo first order	Slope	−0.0042	−0.0048
	Intercept	2.286	2.160
	k1 (min−1)	0.0097	0.011
	R2	0.850	0.941
The pseudo second order	Slope	0.0017	0.0043
	Intercept	7.151	4.416
	k2 (g mg−1 min−1)	4.04 × 10−7	4.20 × 10−6
	R2	0.120	0.454
Intra-particle diffusion model	Slope (ki (mg g−1 min−1/2))	4.005	4.371
	Intercept (C (mg g−1))	−29.028	−22.071
	R2	0.974	0.949

shows that the R² of pseudo-first-order kinetics is preferable to the pseudo-second-order kinetics at different initial concentrations (5 and 10 mg L⁻¹) and indicates that the rate-controlling step is the transport of Cu²⁺ to the membrane surface [27]. The fitting line of Q_t vs. t^1/2 does not pass through the origin in Figure 5d, indicating that intra-particle diffusion is not the only rate-limiting process of Cu²⁺ adsorption by the sandwich adsorptive membrane and that the effects of surface diffusion are also important [54].

3.4. Dynamic Adsorption and Desorption

During dynamic adsorption, 10 mg L⁻¹ and 5 mg L⁻¹ Cu²⁺ aqueous solutions (100 mL, pH 6.0) were filtered with the sandwich membrane at 25 °C. As shown in Figure 6a, for 5 mg L⁻¹ Cu²⁺ aqueous solution, in the initial process (v < 7.8 mL), c/c₀ = 0, indicating that all Cu²⁺ are adsorbed; then, c/c₀ increases rapidly, and tends to be stable. For 10 mg·L⁻¹, the adsorption curve grows faster when the filtration volume is less than 30 mL, and then the growth rate slows down and gradually reaches equilibrium. c/c₀ = 0.8 and c/c₀ = 0.1 are the saturation point and breakthrough point, respectively, and the corresponding solution volumes are the saturation volume (v_s) and breakthrough volume (v_b), respectively [55]. v_s and v_b are 60.2 and 9.4 mL, respectively. For the 10 mg L⁻¹ Cu²⁺ aqueous solution, v_s and v_b are 33.6 and 2.6 mL, respectively. The dynamic adsorption capacity (DC) is calculated by the following Equation (15):

$$DC=\frac{{\displaystyle {\int }_0^v(c_0-c)dv}}{\mathsf{\Delta }m}$$

(15)

where c is the concentration of Cu²⁺ in the filtrate (mg L⁻¹), and v is the filtration volume (L). The DC_0.1 and DC_0.8 of the membrane are 1.80, 24.05 mg g⁻¹ for the 5 mg L⁻¹ Cu²⁺ solution, and 2.25 and 35.53 mg g⁻¹ for the 10 mg L⁻¹ Cu²⁺ solution, respectively, indicating that the sandwich membrane has good adsorption capacity for Cu²⁺.

In industrial production, the reusability of the membrane is very important; the excellent performance of the membrane can save production costs and greatly improve economic benefits. The reusability of the sandwich adsorptive membrane was investigated by a dynamic adsorption/desorption experiment. A total of 5 mg L⁻¹ Cu²⁺ aqueous solutions (100 mL, pH 6.0) was filtered with the sandwich membrane at 25 °C at a rate of 0.68 mL min⁻¹. After adsorption, 20 mL saturated NaCl solution was filtered as eluent at a rate of 0.25 mL min⁻¹, and then 10 mL DI water was filtered to remove the residual NaCl for the next cycle. It can be seen from Figure 6b that the Q_e in the five cycles are 72.5, 70.0, 65.3, 62.5, and 50.4 mg g⁻¹, respectively, indicating that the sandwich membrane has good regeneration and reuse performance.

In order to meet the standards of GMP and ICH for trace meta l impurities in drugs, the mixed aqueous solution (100 mL, pH 6.0) of aspirin (20 mg L⁻¹) and Cu²⁺ (1 mg L⁻¹) was filtered with the sandwich adsorptive membrane at 25 °C. As shown in Figure 6c, the removal ratio of Cu²⁺ by the sandwich adsorptive membrane reaches 100% when v < 10 mL, complying with the ICH-Q3D standard (PDE ≤ 3 mg·day⁻¹); then, the removal ratio decreases gradually with the increased volume, and remains above 70% when v < 100 mL. On the contrary, the removal ratio of aspirin is almost zero, indicating that the sandwich adsorptive membrane can effectively remove Cu²⁺ from the aspirin solution. Aspirin has a large molecular diameter and exists as an anion in water, so it cannot be exchanged with the cation in the zeolite skeleton, thus achieving a good separation effect with Cu²⁺.

4. Conclusions

PES-SPES/NaY zeolite/nylon sandwich adsorptive membranes were successfully fabricated. It can be observed that the base membrane plays a supporting role, the adsorption layer ensures sufficient adsorption capacity, and the surface layer makes the membrane more stable. The Langmuir isotherm model, D-R isotherm model, and the pseudo-first-order kinetic model can well represent the adsorption of Cu²⁺ on the membrane, indicting a chemisorption adsorption and ion exchange reaction process, with the maximum adsorption capacity of 111.25 mg·g⁻¹. The sandwich adsorptive membranes exhibited good reusability in the cycling test. Contrary to the adsorption mechanism of Cu²⁺, the diameter of aspirin molecules is large, and they cannot be allowed to enter the nanoparticle gap. Moreover, it ionizes into anion form in water and cannot exchange with the cation in the NaY zeolite skeleton. Therefore, it is of guiding significance to apply the three-dimensional nanomaterials in the separation of heavy metals in drugs.