Preparation and permeation recognition mechanism of Cr(vi) ion-imprinted composite membranes

Xin Wang; Peng Li; Guifang Wang; Li Zhao; Huiling Cheng

doi:10.1515/epoly-2022-0087

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Preparation and permeation recognition mechanism of Cr(vi) ion-imprinted composite membranes

Xin Wang , Peng Li , Guifang Wang , Li Zhao und Huiling Cheng

Veröffentlicht/Copyright: 24. November 2022

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Abstract

The Cr(vi) ion-imprinted composite membranes (Cr(vi)-IICMs) were prepared by using the surface imprinting method. The template ion was Cr(vi), the functional monomer was 4-vinylpyridine (4-VP), and the nylon filter membrane (nylon-6) was the support membrane. Non-imprinted composite membranes (NICMs) were prepared under the same conditions as the corresponding Cr(vi)-IICM. The adsorption effect of the imprinted membrane can reach 2.4 times that of the corresponding non-imprinted membrane. Meanwhile, the adsorption quantity of Cr(vi)-IICM was 34.60 μmol·g⁻¹. The physical characteristics of membranes were confirmed by Brunauer–Emmett–Teller and scanning electron microscopy. Inductively coupled plasma emission spectrometry was used to analyze their adsorption properties and permeation selectivity. Cr(vi)-IICM and NICM were both mesoporous materials from the structural characterization and performance test results. Their adsorption behavior conformed to the Langmuir isotherm adsorption model. The permeation recognition mechanism of Cr(vi)-IICM was the Piletsky’s gate model. The IICM still has excellent permeability selectivity to Cr(vi) in the presence of competitive ions. The results provided a reference for the isolation and enrichment of Cr(vi).

Keywords: chromium; ion-imprinted composite membrane; surface imprinting method; adsorption; permeation selectivity

1 Introduction

Chromium(vi) is a strong oxidizer that can directly act on the surface of the skin and cause skin diseases and even cancer (1,2). If it enters the respiratory system or blood, it can circulate and cause organ damage (3,4,5). The common methods to treat heavy metals in water include chemical precipitation (6,7), biological (8), ion-exchange (9,10), electrolysis (11), adsorption (12,13,14,15,16), and membrane separation (17,18,19) methods. However, these often require complex processes, high costs, and poor selectivity, or they easily cause secondary water pollution (20,21,22). This can aggravate clean water shortages, which directly pose health threats. Therefore, it is urgent to find highly selective adsorption and separation materials to remove Cr(vi) from wastewater.

Ion-imprinted composite membranes (IICMs) (23,24,25) have high fluxes and high selectivities because the support layer can be used for ultrafiltration or microfiltration. The template ions were bound to the functional monomers using chelation or electrostatic interaction. Due to the use of ion imprinting techniques in the preparation process, the material was provided with specifically identified cavities. IICM combine the advantages of membrane separation with the selectivity of ion-imprinting methods because they contain recognition sites on their surfaces, which improve the selection and permeation of the IICM (26,27). At the same time, since the action site was primarily on the surface of the basement membrane, the problem of too-deep encapsulation of the recognition site of the conventionally imprinted polymer was avoided. Compared with traditional particle polymers, IICM has unique advantages such as no abrasion and small diffusion resistance (28,29,30).

In 2005, Araki et al. (31) first applied the surface ion–imprinted technique for the preparation of membrane materials with high selectivity for ions. The polytetrafluoroethylene membrane (PTFE) membrane was used as a support membrane. The results showed that the ion-imprinted membrane had good permeation selectivity for Zn(ii). Vatanpour et al. (32) prepared Ni(ii) ion-imprinted membrane using dithizone as a functional monomer. In the permeation experiments with Co(ii) as the competing ion, the imprinted membrane still showed good selective recognition of Ni(ii). The separation coefficient of the imprinted membrane for Ni(ii) and Co(ii) was 2.6. The work also showed that in membrane permeation experiments, the adsorption of metal ions was the rate-control step of the process. Wang et al. (33) prepared a novel copper ion surface-imprinted membrane using layer-by-layer technology. The binding sites were distributed in a multilayer structure on the membrane surface. Amino and carboxyl groups were detected on the membrane surface by fourier transform infrared spectroscopy, and their effective adsorption was carried out by interaction with Cu(ii). Meanwhile Cu(ii) ion-imprinted membranes showed good reusability in six adsorption–desorption cycles.

The Cr(vi) exists as the Cr 2 O 7 2 − anion in water, and there are few reports on the study of anion-imprinted composite membranes (34,35,36). This work advanced the current research on anion-imprinted membranes with anions as the object of study. Based on the previous work, highly selective membrane materials were investigated. Also, a detailed study of the mass transfer mechanism was conducted based on the performance study in terms of membrane materials. To propose a reference for the research on anion-imprinted membranes, as well as selective materials, the Cr(vi)-IICM and corresponding non-imprinted composite membrane (NICM) were prepared by using the surface-imprinting method and thermal-initiation method with Cr(vi) as the template ion. The adsorption properties and competitive ion selectivity of Cr(vi)-IICM were studied. The results showed that Cr(vi)-IICM displayed favorable adsorption separation and permeation selectivity.

2 Materials and methods

2.1 Materials

α-Methacrylic acid (MAA), acrylamide (AM), 4-vinylpyridine (4-VP), ethylene glycol dimethacrylate (EGDMA), and 2,2-azobisisobutyronitrile (AIBN) were bought from Aladdin, Shanghai. Potassium dichromate (K₂Cr₂O₇), nickel chloride hexahydrate (NiCl₂·6H₂O), copper chloride (CuCl₂), cadmium chloride (CdCl₂), phosphoric acid, methanol, ethanol, acetonitrile, isopropanol, and glacial acetic acid were purchased from Tianjin Sailboat Chemical Reagent Technology Co., Ltd. Nylon membrane (nylon-6), PTFE, and polyvinylidene fluoride membrane (PVDF) were procured from Shanghai Xingya Purification Equipment Factory. The purity of all reagents was analytical grade. The water used in the experiment was deionized water.

2.2 Apparatus

The ultraviolet–visible spectrophotometer (UV-2500; Shimadzu, Japan) and inductively coupled plasma optical emission spectrometer (ICP-OES) (Prodigy; Leeman, USA) were used to detect ion concentrations. The surface morphology of the samples was detected with scanning electron microscopy (SEM) (MIRA LMS; TESCAN, the Czech Republic). The Brunauer–Emmett–Teller (BET) surface area analyzer (NOVA2000e, Quantachrome, America) was used to detect surface areas.

2.3 Preparation of membranes

The preparation of Cr(vi)-IICM is outlined in Table 1. About 0.10 mmol of K₂Cr₂O₇ was added to the Erlenmeyer flask and then dissolved with the corresponding pore-forming solvent. A certain amount of functional monomer was added, which was vibrated in a gas bath constant-temperature shaker for 3 h at room temperature. Then, EGDMA and 10.00 mg AIBN were added to the mixed solution, which was sonicated and deaerated for 5 min to form a pre-polymer solution. The support membrane was immersed in the pre-polymer solution for a period of time, followed by reacting at 60°C for 24 h and then eluting with methanol:glacial acetic acid (V:V = 9:1) to remove the template ion. Finally, IICM was eluted with pure methanol to neutral pH, and then membrane was dried for 12 h and obtained. The corresponding NICM was prepared under the same methods but without template ion.

Table 1

Preparation of membranes

Membranes	Functional monomer	Support membrane	Dosage of the functional monomer (mmol)	Crosslinking agent (mmol)	Porogen solvent (V:V)	Soaking time (s)
1	4-VP	PTFE	0.4	2	1:1 Isopropyl alcohol:water	180
2	MAA	—	—	—	—	—
3	AM	—	—	—	—	—
4	4-VP	—	—	—	1:1 Methanol:water	—
5	—	—	—	—	1:1 Ethanol:water	—
6	—	—	—	—	1:1 Acetonitrile:water	—
7	—	PVDF	—	—	1:1 Isopropyl alcohol:water	—
8	—	Nylon-6	—	—	—	—
9	—	—	0.2	—	—	—
10	—	—	0.6	—	—	—
11	—	—	0.8	—	—	—
12	—	—	0.6	1	—	—
13	—	—	—	3	—	—
14	—	—	—	4	—	—
15	—	—	—	5	—	—
16	—	—	—	2	—	30
17	—	—	—	—	—	60
18	—	—	—	—	—	120
19	—	—	—	—	—	1,800
20	—	—	—	—	—	3,600
21	—	—	—	—	1:2 Isopropyl alcohol:water	180
22	—	—	—	—	2:1 Isopropyl alcohol:water	—
23	—	—	—	—	1:3 Isopropyl alcohol:water	—
24	—	—	—	—	7:3 Isopropyl alcohol:water	—

2.4 Adsorption experiments

ICM or NICM (20.00 mg) was dispersed in 10.00 mL of Cr(vi) solution with 10.00–75.00 mg·L⁻¹. After shaking and adsorbing at 25°C for 12 h, they were filtered to a constant volume. The solutions before and after adsorption were analyzed using UV spectrophotometer. No chromogenic agent was added during the analysis. The maximum absorption wavelength of Cr(vi) was found to be about 350 nm by the UV spectrophotometer, and the subsequent adsorption experiments were performed at this wavelength.

The equilibrium adsorption amount Q and the imprinting factor α were obtained from the changes in the concentration before and after adsorption. The equations are shown in Eqs. 1 and 2, and the adsorption isotherm was drawn as follows:

(1) Q = ( C 0 − C e ) ⋅ V ⋅ 1 , 000 m

(2) α = Q Cr ( VI ) -IICM Q NICM

where Q (μmol·g⁻¹) is the equilibrium adsorption quantity; C _e (mmol·L⁻¹) is the concentration at adsorption equilibrium; C ₀ (mmol·L⁻¹) is the initial concentration; m (g) represents the mass of Cr(vi)-IICM or NICM; and V (L) represents the volume of the adsorption solution.

2.5 Permeability experiments

The permeation device shown in Figure 1 was used, where the effective membrane permeation area was 1.54 cm², and the membrane thickness was 0.015 cm. Cr 2 O 7 2 − , Cd²⁺, Cu²⁺, and Ni²⁺ mixed solutions (150 mL) were added into the supply tank with 1,000 mg·L⁻¹. and HPO 4 2 − mixed solution (150 mL) both with 5.00 mg·L⁻¹ were used. Deionized water (150 mL) was added to the receiving tank. After stirring, the solution in the receiving tank was taken, and the content of each ion in the sample was determined using ICP-OES. The permeability J _i (mg·h⁻¹·cm⁻²), permeability coefficient P _i (cm²·h⁻¹), and permeability selectivity coefficient β were calculated according to the following equations:

(3) J i = Δ C i V Δ t A ( i = Cr 2 O 7 2 − , Cd 2 + , Cu 2 + , Ni 2 + , HPO 4 2 − )

(4) P i = J i d ( C F i − C R i ) ( i = Cr 2 O 7 2 − , Cd 2 + , Cu 2 + , Ni 2 + , HPO 4 2 − )

(5) β = P Cr(VI) P i ( i = Cd 2 + , Cu 2 + , Ni 2 + , HPO 4 2 − )

where ΔC _i/Δt is the change in the concentration for each ion in the receiving tank; A (cm²) is the efficacious membrane area; V (mL) is the volume of the tank solution; d (cm) is the thickness of the membrane; C _Fi is the concentration for each ion in the supply tank; and C _Ri is the concentration for each ion in the receiving tank.

Figure 1

Permeability device.

3 Results

3.1 Optimization of preparation conditions

In this study, the experimental conditions such as the functional monomer, pore-forming solvent, type of base membrane, dosage ratio of imprinted ions to the functional monomer, and crosslinking agent were optimized. Table 2 shows the experimental results. The functional monomer was 4-VP; the support membrane was nylon-6; the pore-forming solvent was a mixture of isopropanol and water with a volume ratio of 1:1; the molar ratio of the imprinted ion, functional monomer, and crosslinker was 1:6:20; and the soaking time of the base membrane in the pre-polymer solution was 180 s. The adsorption effect of the imprinted membrane can reach 2.4 times that of the corresponding non-imprinted membrane. The adsorption quantity of Cr(vi)-IICM was 34.60 μmol·g⁻¹.

Table 2

Adsorption properties of membranes

Membranes	Q _IICM (mg·g⁻¹)	Q _NICM (mg·g⁻¹)	α (Q _IICM/Q _NICM)
1	23.19	15.53	1.49
2	25.56	18.14	1.41
3	23.24	23.10	1.01
4	11.13	9.14	1.22
5	8.72	7.42	1.18
6	18.41	17.46	1.05
7	21.55	13.78	1.56
8	25.82	14.5	1.78
9	21.63	13.86	1.56
10	34.60	14.42	2.40
11	19.47	15.53	1.25
12	19.23	16.87	1.14
13	28.30	16.00	1.77
14	27.83	13.00	2.14
15	19.15	17.43	1.10
16	32.55	26.72	1.22
17	30.16	28.85	1.05
18	33.43	27.17	1.23
19	31.57	30.97	1.02
20	31.29	30.18	1.37
21	11.47	5.43	2.11
22	36.42	26.38	1.38
23	30.29	26.64	1.14
24	34.83	30.41	1.15

3.2 SEM analysis

Cr(vi)-IICM, NICM, and support membrane were characterized using SEM. Figure 2 shows the results. Compared with the support membrane, the surface of composite membranes was rougher. This indicated that cross-linking occurred on the surface of the composite membrane, which also led to changes in the membrane pore structure. In addition, Cr(vi)-IICM contained more pores and a more uniform structure, while NICM had fewer pores and an uneven structure. Compared with NICM, Cr(vi)-IICM may have a templating effect due to the addition of imprinted Cr(vi) ions during preparation. Imprinted holes matching Cr(vi) remained after elution, while NICM had a smooth surface without imprinted holes. These results suggest that there may be imprinted pores produced by interactions with Cr(vi) in Cr(vi)-IICM.

Figure 2

SEM images of IICM (a), NICM (b), and nylon-6 membrane (c).

3.3 BET analysis

Figure 3 shows the BET isotherms, wherein Cr(vi)-IICM and NICM had similar shapes except for the slope. They are both class IV isotherms in the international union of pure and applied chemistry (IUPAC) classification, indicating that they are typical mesoporous materials (37).

Figure 3

BET isotherms of IICM (a) and NICM (b).

Table 3 shows the BET parameters. The parameters of NICM were smaller than the ones of Cr(vi)-IICM, including specific surface area, pore volume, and pore size. Cr(vi)-IICM and NICM have pore size between 2 to 50 nm. According to IUPAC, they are both mesoporous materials.

Table 3

BET parameters

	Specific surface area (m²·g⁻¹)	Pore volume (cm³·g⁻¹)	Pore size (nm)
Cr(vi)-IICM	9.911	0.034	11.760
NICM	8.758	0.030	11.525

3.4 Adsorption isotherm

The isothermal adsorption of Cr(vi) by imprinted composite membranes is usually analyzed by the Freundlich isothermal adsorption, Langmuir isothermal adsorption, and Scatchard models. Their respective fitting equations are shown in the following equations:

(6) ln Q e = ln K f + ln C e n

(7) 1 Q e = 1 Q max b C e + 1 Q max

(8) Q e c e = Q max − Q e K d

where C _e is the equilibrium concentration (mg·L⁻¹); Q _e is the adsorption quantity of the membrane (μmol·g⁻¹); K _f and n are Freundlich constants; Q _max is the maximum adsorption quantity (μmol·g⁻¹); b is the Langmuir constant; and K _d is the dissociation constant.

Figure 4 shows the adsorption isotherm, Freundlich fitting curves, Langmuir fitting curves, and Scatchard fitting curves of Cr(vi)-IICM. The Scatchard fitting curve in Figure 4d shows that Cr(vi)-IICM had specific adsorption and non-specific adsorption.

Figure 4

Adsorption isotherms of IICM (a), Freundlich fitting curve (b), Langmuir fitting curve (c), Scatchard fitting curve (d).

Table 4 shows the parameters obtained from the Cr(vi)-IICM fitting curves. The linear correlation coefficient of the Freundlich isothermal adsorption model was 0.9745, while that of the Langmuir isothermal adsorption model was 0.9917. The Langmuir isothermal adsorption model was better suited to represent the adsorption behavior of Cr(vi)-IICM. The adsorption was monomolecular adsorption. The Scatchard fitting curve showed that the sum of the maximum specific and non-specific adsorption capacities was 228.32 μmol·g⁻¹.

Table 4

Fitting parameters of the Langmuir, Freundlich, and Scatchard adsorption models

Langmuir		Freundlich		Scatchard
Q _max (μmol·g⁻¹)	50.00	K _f	0.24	Q _max (μmol·g⁻¹)	65.82
Q _max (μmol·g⁻¹)	50.00	K _f	0.24	Q _max (μmol·g⁻¹)	162.50
b	6.84 × 10⁻³	1/n	1.19	K _d	41.66
b	6.84 × 10⁻³	1/n	1.19	K _d	250.00
R ²	0.9917	R ²	0.9745	R ²	0.9778
R ²	0.9917	R ²	0.9745	R ²	0.9502

3.5 Adsorption kinetics analysis

Figure 5a shows the change in the adsorption quantity of Cr(vi)-IICM over time. The data were also fitted using Lagergren’s first- and second-order dynamics models. The fitting equations are Eqs. 9 and 10, and the results are shown in Figure 5b.

(9) log ( Q e − Q t ) = log Q e − k 1 2.303 t

(10) t Q t = 1 k 2 Q e 2 + t Q e

where Q _e represents the equilibrium adsorption quantity (μmol·g⁻¹); Q _t represents the adsorption quantity at time t (μmol·g⁻¹); t represents the adsorption time (min); k ₁ and k ₂ represent the first- and second-order dynamics constants, respectively.

Figure 5

Adsorption quantity of composite membranes over time (a); kinetic fitting curve (b).

The kinetic fitting parameters of Cr(vi)-IICM are shown in Table 5. The equilibrium adsorption quantity fitted by the first-order kinetics model was 21.38 μmol·g⁻¹, which was significantly lower than the experimental value of 78.28 μmol·g⁻¹. The linear correlation coefficient was 0.9735. The equilibrium adsorption quantity Q _e fitted by the second-order kinetics model was 100.00 μmol·g⁻¹. This result was similar to the value obtained in the experiment. The linear correlation coefficient was 0.9978. The experimental results suggested that the kinetic adsorption of Cr(vi)-IICM was more in line with the second-order kinetics model. Chemisorption was dominant in this process.

Table 5

Kinetic fitting parameters of Cr(vi)-IICM

First-order dynamic model			Second-order dynamic model
K ₁	Q _e (μmol·g⁻¹)	R ²	K ₂	Q _e (μmol·g⁻¹)	R ²
0.05	21.38	0.9735	0.005	100.00	0.9978

3.6 Adsorption thermodynamic analysis

The temperature was a factor that had a strong influence on the amount of Cr(vi)-IICM adsorption. The temperature range selected for this study was 298–328 K. Figure 6a displays the outcomes. The thermodynamic adsorption of Cr(vi)-IICM was analyzed by the following thermodynamic equations:

(11) ln Q e C e = Δ S R − Δ H R T

(12) Δ G = − R T ln K

where C _e denotes the concentration after adsorption equilibrium (mmol·L⁻¹); Q _e is the adsorption quantity of composite membrane (μmol·g⁻¹); ΔS is the entropy variation in the adsorption process (kJ·K⁻¹·mol⁻¹); R is the molar gas constant; ΔH represents the enthalpy change during the adsorption process (kJ·mol⁻¹); K represents the Langmuir adsorption constant; and ΔG represents the change of the Gibbs free energy (kJ·mol⁻¹).

Figure 6

Temperature dependence of the adsorption quantity of Cr(vi)-IICM (a), thermodynamic fitting curve (b).

Using Eq. 11, the change in the adsorption quantity Q _e of Cr(vi) with temperature T was fitted. Figure 6b shows the fitting curve. The corresponding ∆H and ∆S values were obtained as the intercept and slope of the fitting curve. According to Eq. 12, ∆G at different temperatures was obtained, and the calculated results are shown in Table 6. The ∆H was negative. This indicates that the adsorption process is an exothermic reaction. The negative value of ∆G and the positive value of ∆S indicate that adsorption was a spontaneous process with an increase in entropy.

Table 6

Thermodynamic fitting parameters of Cr(VI)-IICM

T (K)	∆H	∆S	∆G	R ²
293	−13.97	5.40	−15.56	0.949
298			−15.59
303			−15.61
308			−15.64
313			−15.66
318			−15.69
323			−15.72

3.7 Effect of pH on adsorption

The pH affected the form of Cr(vi) present in the aqueous solution and the recognition site of Cr(vi)-IICM. Therefore, the adsorption performance of Cr(vi)-IICM was investigated in the pH range of 1.5–8 in this experiment. The variation of Cr(vi)-IICM adsorption quantity on Cr(vi) with pH is shown in Figure 7. Cr(vi)-IICM has a high adsorption quantity when the pH was less than 2.5. As the pH exceeded 2.5, the adsorption quantity gradually decreased with the increased pH. This suggested that acidic conditions favored the adsorption of Cr(vi)-IICM. The acidic conditions caused the nitrogen atoms in the pyridine ring to be protonated and adsorbed Cr(vi).

Figure 7

Effect of pH on adsorption.

3.8 Permeation selectivity

3.8.1 Permeation selectivity of metal ions

The permeability of Cr(vi)-IICM to various metal ions is shown in Table 7. The permeability of Cr(vi)-IICM to Cr 2 O 7 2 − was greater than that toward Cd²⁺, Cu²⁺, or Ni²⁺. The results indicated that the binding sites in Cr(vi)-IICM matched Cr 2 O 7 2 − , which promoted the transfer of Cr 2 O 7 2 − from one binding site to another. However, there were no Cd²⁺, Cu²⁺, and Ni²⁺ imprinting recognition sites in Cr(vi)-IICM, and these ions were blocked on the membrane surface. This indicated that the permeation recognition mechanism of Cr(vi)-IICM follows Piletsky’s gate model (38).

Table 7

Permeability parameters of Cr(vi)-IICM to cations

	Ion	J _i (mg·h⁻¹·cm⁻²)	P _i × 10⁻⁶ (cm²·h⁻¹)	β
Cr(vi)-IICM	Cr(vi)	0.42	6.80	—
	Cd(ii)	0.19	2.99	2.27
	Cu(ii)	0.14	2.17	3.13
	Ni(ii)	0.12	1.83	3.72

3.8.2 Permeation selectivity of anionic

Cr(vi) mainly exists as Cr 2 O 7 2 − in solution, so HPO 4 2 − was used as a competitive anion to analyze the permeability selectivity. The resulting permeability flux is shown in Figure 8. Upon increasing the time, the osmotic increase of Cr(vi)-IICM to Cr 2 O 7 2 − was much greater than that of HPO 4 2 − . Compared with Cr(vi)-IICM, NICM showed almost no permeability difference between Cr 2 O 7 2 − and HPO 4 2 − , which further reflects the excellent permeability selectivity of Cr(vi)-IICM. Table 8 shows the permeability parameters of Cr(vi)-IICM. The selectivity factor of Cr(vi)-IICM at 12 h was 3.08, which also indicates that the osmotic separation mechanism of Cr(vi) by Cr(vi)-IICM followed Piletsky’s gate model.

$Figure 8 Permeation flux changes of Cr(vi) and HPO 4 2 − {\text{HPO}}_{4}^{2-} on IICM (a); permeation flux changes of Cr(vi) and HPO 4 2 − {\text{HPO}}_{4}^{2-} on NICM (b).$

Figure 8

Permeation flux changes of Cr(vi) and HPO 4 2 − on IICM (a); permeation flux changes of Cr(vi) and HPO 4 2 − on NICM (b).

Table 8

Permeability parameters of Cr(vi)-IICM to anions

	Ion	J _i × 10⁻³ (mg·h⁻¹·cm⁻²)	P _i × 10⁻⁶ (cm²·h⁻¹)	β
Cr(vi)-IICM	Cr(vi)	1.48	9.93	—
Cr(vi)-IICM	HPO 4 2 −	0.52	3.22	3.08

3.9 Comparison with other methods

Table 9 shows the works (39,40,41,42,43) with good effects in the adsorption of Cr(vi). Many different types of materials were included in these jobs. These materials could be applied to different environments and had reference values for the treatment of Cr(vi). The adsorption quantity of the Cr(vi)-IICM in this work was not the best. However, the Cr(vi)-IICM was a membrane material with better selectivity, which was more advantageous for the adsorption and recovery of single ions in mixed samples. It was easier to separate from the solution after adsorption than conventional adsorbents and had the same advantages for sample recovery and processing.

Table 9

Comparison of adsorption quantity by different materials

Adsorbents	Q (mg·g⁻¹)	Ref.
Ion imprinting polymer@graphene oxide-Fe₃O₄	8.50	(39)
Olive stones	4.11	(40)
Artemisia monosperma plant powder	36.90	(41)
Magnetite–humic acid	3.17–25.06	(42)
Attapulgite–Fe₃O₄	10.49	(43)
Cr(vi)-IICM	4.07	This work

4 Conclusions

Cr(vi)-IICM was one useful material for removing the Cr(vi) from the wastewater. Structural characterization and performance test results showed that both ion-imprinted membranes and non-imprinted membranes were mesoporous materials, and imprinted pores were generated after ion-imprinted membranes interacted with the imprinted ion. The adsorption behavior of Cr(vi)-IICM was in accordance with the Langmuir isothermal adsorption model. This suggested that the whole process was monomolecular adsorption. Kinetic and thermodynamic studies showed that the adsorption behavior was exothermic chemical adsorption that included both specific and non-specific adsorption. Meanwhile, the adsorption of Cr(vi) is more favorable in an acidic environment. The osmotic separation mechanism of Cr(vi)-IICM followed Piletsky’s gate model, which had excellent osmotic selectivity. As a material with high adsorption quantity and high selectivity, Cr(vi)-IICM has a promising research potential for the separation and enrichment of Cr(vi). By comparing with other adsorbents, the imprinted membrane in this work has advantages such as better selectivity and easy recovery. Of course, improving the adsorption quantity while maintaining high selectivity is also the focus of our next research.

Acknowledgment

The authors acknowledge the financial support provided by the National Natural Science Foundation of China.

Funding information: This work was supported by the National Natural Science Foundation of China (No. 21764008).
Author contributions: Xin Wang: methodology, writing – original draft, data curation; Peng Li: software; Guifang Wang: investigation, data analysis; Li Zhao: investigation; Huiling Cheng: supervision, project administration, funding acquisition, writing – review and editing.
Conflict of interest: The authors state no conflict of interest.
Data availability statement: The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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Received: 2022-08-22

Revised: 2022-10-30

Accepted: 2022-11-02

Published Online: 2022-11-24

This work is licensed under the Creative Commons Attribution 4.0 International License.

Artikel in diesem Heft

https://doi.org/10.1515/epoly-2022-0087

Schlagwörter für diesen Artikel

chromium; ion-imprinted composite membrane; surface imprinting method; adsorption; permeation selectivity

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