Preparation, Characterization of Cd(II) Ion-Imprinted Microsphere and Its Selectivity for Template Ion

Abstract

Cadmium is one of the many toxic elements for humans even at low concentrations, and it could exist in the environment for a long time. The ion imprinting technique has gained much attention due to its selective recognition performance. In this study, a cadmium ion imprinted maleic acid-co-acrylonitrile polymeric microsphere (Cd-I-MA-co-AN) was synthesized via precipitation polymerization using Cd(II) as a template ion, acrylonitrile and maleic acid as functional monomers, divinylbenzene as a cross-linker, and potassium persulfate as an initiator. UV–vis, SEM and FTIR were used for characterization, and the adsorption conditions were observed and optimized. The adsorption capacity and selectivity of Cd-I-MA-co-AN for Cd(II) were analyzed by flame atomic absorption spectrometry (FAAS). The results documented that the optimal pH, flow rate and eluent were 6, 2 mL min⁻¹ and 1 mol L⁻¹ nitric acid, respectively. Compared with the non-ion imprinted maleic acid-co-acrylonitrile polymeric microsphere (NI-MA-co-AN), Cd-I-MA-co-AN had a higher adsorption capacity. The saturated adsorption capacities of Cd-I-MA-co-AN and NI-MA-co-AN were 20.46 mg g⁻¹ and 7.64 mg g⁻¹, respectively. The adsorption behavior of Cd-I-MA-co-AN fitted with the Freundlich isotherm model. The relative selectivity coefficients of Cd-I-MA-co-AN for Cd(II) in the presence of Cu(II), Mn(II), Ni(II) and Pb(II) were 3.79, 3.39, 3.90 and 3.31, respectively. The Cd-I-MA-co-AN showed good selectivity for Cd(II). In addition, a reusability study showed that Cd-I-MA-co-AN can be recycled ten times and has high recovery in natural water samples.

1. Introduction

Cadmium is regarded as an environmental pollutant, released into the environment by industrial and agricultural production, such as the electroplating industry, pesticides, paint and plastic production. Cadmium enters the body through water and crops, and is hard to degrade, meaning that cadmium could exist in the environment for a long time. The World Health Organization (WHO) determined the allowed cadmium levels in drinking water to be 0.003 mg L⁻¹. Since cadmium is not an essential element in the human body, its presence harms human’s physical fitness, leading to liver damage, bone softening and kidney injury. Cadmium has been classified as a human carcinogen by the International Agency for Research on Cancer (IARC) [1,2,3,4,5,6]. Hence, it is essential to remove cadmium from the living environment. However, due to the complexity and diversity of the environment, selectively eliminating and separating cadmium ion at trace-level concentrations can be challenging. The common adsorbents have poor selectivity for trace heavy-metal ions, such as activated carbon, which has no specific selectivity for molecules, and metal ions will spill into the liquid [7]. Thus, there are difficulties in separating and enriching them. Molecular imprinting technology is the polymerization of a cross-linking agent and template molecule; then, the template molecule is removed. There is a synergistic effect between the functional group and template molecule, so the template molecule shows good selectivity [8]. Molecular imprinting technology supplies a wide range of applications in many fields, such as sensors, catalysis and sorbents. Bolukbasi et al. [9] reported a molecularly imprinting electrochemical paraoxon sensor based on MoS₂NPs@MWCNTs nanocomposite for the detection of organophosphorus pesticides. Xu et al. [10] reported a novel SiO₂-coated molecularly imprinted sensor based on Si quantum dots for selective detection of catechol in river water samples. Xie et al. [11] reported molecularly imprinted MOFs for the degradation of sulfamethoxazole in wastewater. Chen et al. [12] prepared molecularly imprinted polymers for the removal of kitasamycin from the environment.

Ion-imprinted polymers are developed on the basis of molecular imprinting technology. Compared with the molecular-imprinted polymers, ion-imprinted polymers are prepared by metal coordination, using ion as the template ion. Ion imprinted polymers have excellent adsorption properties, due to the synergistic effect of functional groups and imprinted holes, which selectively recognizes metal ions and does not interfer with other elements in the sample. The advantages of ion-imprinted polymers include their adsorption capacity, recognition, reapplication, and stability [13,14,15,16,17,18,19,20,21].

Although great strides have been made in the use of degrading cadmium ion in the environment using the ion-imprinting technique, there are few previously published papers on the preparation and application of Cd(II) ion imprinted polymers for the extraction and separation of cadmium ions. Luliński et al. [22] synthesized cadmium(II)-imprinted poly (1-allyl-2-thiourea-co-ethylene glycol dimethacrylate) particles using the bulk polymerization technique. In this work, cadmium nitrate was used as the template. They selected the following four different functional monomers: viz. allylurea, 1-vinylimidazole, acrylamide and 1-allyl-2-thiourea. Ethylene glycol dimethacrylate was applied as a cross-linker. Li and co-workers [23] employed a surface-imprinting technique to prepare a Cd(II) ion-imprinted polymer. Cadmium chloride and allyl thiourea were applied as a template metal ion and functional monomer, respectively. Ethylene glycol dimethacrylate (EGDMA) and azodiisobutyronitrile (AIBN) were taken as the cross-linker and initiator, respectively. Li et al. [24] applied the inverse emulsion polymerization method to synthetize a Cd(II) imprinted polymer. In this work, they used Cd(II) as a template, used acrylamide and β-cyclodextrin as functional monomers, and adopted epichlorohydrin as a cross- linker and ammonium persulfate as an initiator. The imprinted polymer had a good selectivity performance for Cd(II).

Although previous research has shown suitable properties, further studies are still necessary. In this paper, the separation and enrichment system was established using an ion-imprinted polymer microsphere, which can recognize cadmium ions as a solid-phase extraction material. This was used together with FAAS to improve the sensitivity and selectivity of the analysis method. We developed an easy synthesis route to prepare the cadmium imprinted maleic acid-co-acrylonitrile (Cd-I-MA-co-AN) using precipitation polymerization. A cadmium ion was used as a template ion, divinylbenzene as a cross-linker and potassium persulfate as an initiator. Maleic acid and acrylonitrile, which are cheap and readily available, have not been collectively applied in the preparation of cadmium ion-imprinted polymers to date. Therefore, we chose maleic acid and acrylonitrile as functional monomers. In addition, the characterization, adsorption and selectivity properties of the Cd-I-MA-co-AN were investigated.

2. Materials and Methods

2.1. Reagents

Sodium dodecyl benzene sulfonate (SDBS) was purchased from Tianjin Hedong Hongyan Reagent Factory (Tianjin, China). Potassium persulfate (K₂S₂O₈) was purchased from Beijing Chemical Plant (Beijing, China). Cadmium chloride (CdCl₂) and chloride or nitrate salts of other metal ions were purchased from Tianjin Beilian Fine Chemicals Development Co., Ltd. (Tianjin, China). Divinylbenzene (DVB) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃) were purchased from Urumqi Dicheng Chemical Co., Ltd. (Urumqi, China). Acrylonitrile (AN) was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China). Maleic acid (MA) was purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China).

2.2. Apparatus

The UV–vis spectra of the Cd(II) solution, maleic acid, acrylonitrile and Cd(II)–MA/AN complex were recorded with an ultraviolet absorption spectrometer (Puxi T6, New Century, Beijing, China). The concentrations of ions were measured using an atomic absorption spectrometer (Analyst300, Perkin Elmer, Waltham, MA, USA). The infrared spectra of Cd-I-MA-co-AN and NI-MA-co-AN were obtained using a Fourier transform infrared spectrometer (FTIR) (EQUINOX55, Bruker, Karlsruhe, German). The surface morphology was observed by using a scanning electron microscope (SEM) (LEO1430VP, ZEISS, Oberkochen, German).

2.3. Synthesis of Cd(II) Imprinted Polymeric Microsphere

2.3.1. Preparation of Cd(II)–MA/AN Complex

Maleic anhydride (2.0 g, 20 mmol), acrylonitrile (5.5 mL, 83.6 mmol), and cadmium chloride (3.66 g, 20 mmol) were added to a single-neck round-bottom flask. Then, 0.05 g of sodium dodecyl benzene sulfonate was added and diluted to 50 mL with deionized water at 30–40 °C; the mixture was stirred for 30 min.

2.3.2. Preparation of Cd-I-MA-co-AN

The preparation of Cd-I-MA-co-AN was carried out using precipitation polymerization method. Divinylbenzene (0.79 mL, 5.6 mmol) was added to the above mixture, then heated to 60 °C under nitrogen atmosphere; 0.05 g of sodium dodecyl benzene sulfonate was added to the complex solution. Then, 0.3 g of potassium persulfate was added to the mixture at a constant temperature under nitrogen atmosphere. After stirring for 6 h, polymer precipitate was formed. The resulting polymer was cooled to room temperature and washed with deionized water. After filtering and drying, 1 mol L⁻¹ nitric acid was used to remove the Cd(II) until no cadmium ion signal was detected in the solution. Finally, a polymeric microsphere was dried under vacuum at 50 °C for 6 h. NI-MA-co-AN was simultaneously prepared under the same experimental conditions, except Cd(II) was not added during the complexation process.

2.4. Adsorption Experiments

2.4.1. Adsorption and Desorption Properties of Cadmium Ions

The adsorption and desorption of the Cd(II) on Cd-I-MA-co-AN and NI-MA-co-AN were carried out using dynamic experiments. Cd(II) solution was passed at an opposite flow rate through the separation and enrichment column, which used a certain amount of Cd-I-MA-co-AN or NI-MA-co-AN as an adsorbent; quantitatively collected Cd(II) solution that flowed out of the column. Then, the adsorbent was washed with acid solution and the desorption solution was quantitatively collected. The ion concentrations were determined by FAAS under the optimum determination conditions. The adsorption capacity, adsorption efficiency, and desorption efficiency were calculated by using Equations (1)–(3), which are as follows:

$$\mathrm{Q}=\frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})\mathrm{V}}{\mathrm{W}}$$

(1)

$$\mathrm{E}(\%)=\frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})}{{\mathrm{C}}_0}$$

(2)

$$\mathrm{B}(\%)=\frac{{\mathrm{C}}_{\mathrm{d}}{\mathrm{V}}_{\mathrm{d}}}{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})\mathrm{V}}$$

(3)

where C₀ is the initial concentration of Cd(II) (mg L⁻¹), C_e is the equilibrium concentration of Cd(II) (mg L⁻¹), and C_d is the concentration of Cd(II) in desorption solution (mg L⁻¹). V is the volume of the Cd(II) solution (mL), and V_d is the volume of the desorption solution (mL). W is the quality of the dry adsorbent (g) [25].

2.4.2. Adsorption Selectivity Experiment

Due to the presence of competitive ions, such as Cu(II), Mn(II), Ni(II) and Pb(II), which had the similar ionic radius and charge as Cd(II), these were used to investigate the selective property of Cd-I-MA-co-AN to Cd(II). The adsorption selectivity experiment was carried out using dynamic adsorption experiment. The ion concentrations were determined by FAAS. The selective adsorption property of polymeric microsphere was calculated by using Equations (4)–(6), which are as follows:

$$\mathrm{D}=\frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)\mathrm{V}}{{\mathrm{C}}_{\mathrm{e}}\mathrm{W}}$$

(4)

$$\mathsf{\alpha }=\frac{{\mathrm{D}}_{\mathrm{Cd}\left(\mathrm{II}\right)}}{{\mathrm{D}}_{\mathrm{M}\left(\mathrm{II}\right)}}$$

(5)

$$\mathrm{k}=\frac{{\mathrm{D}}_{\mathrm{Cd}\left(\mathrm{II}\right)}}{{\mathrm{D}}_{\mathrm{M}\left(\mathrm{II}\right)}}$$

(6)

where D is the distribution coefficient (mL g⁻¹), α is the selectivity coefficient, k is the relative selectivity coefficient, and M(II) represents Cu(II), Mn(II), Ni(II), and Pb(II) [26,27].

3. Results and Discussion

3.1. Characterization Studies

3.1.1. UV–Vis Adsorption Spectra

The UV–vis adsorption spectra of the Cd(II) solution, MA, AN and Cd(II)–MA/AN complex are displayed in Figure 1. The major adsorption peaks for the Cd(II) solution, MA, and AN occur at 225, 235, and 254 nm, respectively. The major adsorption peak for the Cd(II)–MA/AN complex appears at 264 nm. According to the superposition principle of ultraviolet adsorption spectrum, if AN and MA do not coordinate with the Cd(II), the adsorption peak of the complex will overlap with that of the Cd(II) solution, AN and MA, and there will be no new adsorption peak. The results show that a new adsorption peak appears in the long-wavelengths range, proving that the ligands coordinate with the central ion Cd(II). This can be attributed to the empty orbital of Cd(II), the lone pair electron on nitrogen atoms of AN and the hydroxyl oxygen atom of MA. The decreased density of the electron cloud around the functional monomer is caused by the coordination bond among Cd(II), AN and MA. When illuminated by ultraviolet light, the electron transition can occur at lower energies and generate an adsorption spectrum [28,29]. Thus, the adsorption peak of the Cd(II)–MA/AN complex appears at long wavelengths.

3.1.2. SEM

The surface morphologies of Cd-I-MA-co-AN before and after elution were characterized using SEM. Figure 2a,b show the Cd-I-MA-co-AN before elution, while Figure 2c,d show the Cd-I-MA-co-AN after elution. Phenomena worth noting are the fact that the surface of the Cd-I-MA-co-AN before elution was plat and smooth and showed a more compact surface. The surface of Cd-I-MA-co-AN was relatively rough after elution, resulting in an increase in specific surface area. This indicates that the template ions had been eluted. The synergy between holes left after eluting template ions and functional groups is beneficial for increasing the adsorption capacity and enhancing the selectivity performance.

3.1.3. FT-IR Spectra

The FTIR spectra of Cd-I-MA-co-AN before and after Cd(II) elution are shown in Figure 3. In the Cd-I-MA-co-AN spectrum before elution, bands of around 3400–3700 cm⁻¹ are attributed to the hydroxyl group in –COOH, the C–H bond of the backbone appears at 2930, 2872 and 1450 cm⁻¹, respectively. The peak at 2241 cm⁻¹ can be attributed to the –CN group of AN, the peak at 1708 cm⁻¹ is ascribed to the C=O bond stretching vibrations of the MA. In the Cd-I-MA-co-AN spectrum after elution, the hydroxyl group in –COOH is around 3400–3700 cm⁻¹, with the C–H bond of the backbone at 2930, 2872 and 1450 cm⁻¹, respectively. The peak at 2280 cm⁻¹ is ascribed to the –CN bond of AN. The peak at 1738 cm⁻¹ can be assigned to the C=O stretching vibrations of the MA, showing the formation of Cd-I-MA-co-AN [30]. There is a peak at 2241 cm⁻¹, which shifted to 2280 cm⁻¹, verifying that the N groups were related to Cd(II) adsorption. The reason for the change in wavenumber is that electron transition occurs when N supplies electrons to the Cd(II) [27]. The peak at 2241 cm⁻¹ on the Cd-I-MA-co-AN shifted to 2280 cm⁻¹ before elution. This further evidences the occurrence of the imprinting process [31]. These results suggest that Cd-I-MA-co-AN contained the functional groups –COOH and –CN from MA and AN, which are successfully immobilized in the microsphere.

3.2. Adsorption and Desorption Experiments

3.2.1. Effect of pH

The effect of varying pH values on the adsorption efficiency of Cd-I-MA-co-AN for Cd(II) was investigated in the 1–10 pH range (adjusted using 1 mol L⁻¹ HCl and NaOH). Figure 4 shows that the efficiency of Cd(II) adsorption on the microsphere was increased with increasing pH values up to 6, after which they gradually decreased. This is because, under acidic conditions, the imprinted site was occupied by protons rather than cadmium ions. When the pH increased, the protonation of ligating atoms becomes weak and the coordination between functional groups and Cd(II) at the surface binding sites of Cd-I-MA-co-AN becomes dominant. The maximum adsorption of Cd-I-MA-co-AN was observed at pH 6. After pH 6, Cd(II) began to hydrolyze; with the formation of precipitate, the adsorption rate decreased [32,33]. Hence, pH 6 was chosen for further experiments.

3.2.2. Effect of Flow Rate

To determine the optimal flow rate, Cd(II) solution at pH 6 was applied to through the separation and enrichment column. Cd-I-MA-co-AN was used as an adsorbent, changing the flow rate between 0.5 and 5 mL min⁻¹. Figure 5 shows the flow rate’s effect on the adsorption efficiency for Cd(II). The results show that the adsorption efficiency reached 94.3% when the flow rate was 0.5–2 mL min⁻¹. However, there was a decrease in adsorption efficiency when the flow rate of sample solution passed 2 mL min⁻¹. This can be attributed to the Cd(II) solution, which could not sufficiently contact the adsorbent [34]. Hence, the flow rate of 2 mL min⁻¹ was chosen for the column adsorption process.

3.2.3. Effect of Eluent

To find a suitable eluent, different concentrations of hydrochloric acid, sulfuric acid, and nitric acid were provided through the microcolumn at a certain flow rate to elute Cd(II) adsorbed on Cd-I-MA-co-AN. The Cd(II) concentration was determined by FAAS. As shown in Figure 6, the desorption efficiency of nitric acid was better than that of sulfuric acid and hydrochloric acid under the same concentration. The desorption efficiency of 1 mol L⁻¹ nitric acid was 94%. At low acid concentrations, cadmium ions are not completely eluted. When the acid concentration is higher than a certain value, the imprinted sites are completely protonated. The structure of Cd-I-MA-co-AN may be destroyed, decreasing the interaction with Cd(II) [35]. Therefore, 1 mol L⁻¹ nitric acid was chosen for eluting.

3.2.4. Adsorption Capacity

Figure 7 shows the effect of various concentrations on the adsorption capacity of Cd-I-MA-co-AN and NI-MA-co-AN for Cd(II). As presented in the figure, the adsorption capacity increased with the increasing concentration of Cd(II) before reaching saturation. The saturated adsorption capacities of Cd-I-MA-co-AN and NI-MA-co-AN were 20.46 and 7.64 mg g⁻¹, respectively. Cd-I-MA-co-AN has a higher adsorption capacity for Cd(II). The saturated adsorption capacity of the Cd-I-MA-co-AN was almost three times that of NI-MA-co-AN. This is due to the synergistic effect on imprinted ions and functional groups [36].

3.2.5. Adsorption Isotherm

The Langmuir isotherm model and Freundlich isotherm model were used to analyze the adsorption interaction between the adsorbate and adsorbent. The Langmuir isotherm model and Freundlich isotherm model are expressed by Equations (7) and (8).

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{Q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{KQ}}_{\mathrm{m}}}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{Q}}_{\mathrm{m}}}$$

(7)

$${\mathrm{lnQ}}_{\mathrm{e}}=\frac{1}{\mathrm{n}}{\mathrm{lnC}}_{\mathrm{e}}+{\mathrm{lnK}}_{\mathrm{f}}$$

(8)

where Q_e denotes the adsorption capacity of the polymeric microsphere at equilibrium (mg g⁻¹), C_e denotes the equilibrium concentration of Cd(II) solution (mg L⁻¹), Q_m denotes the maximum adsorption capacity of the polymeric microsphere (mg g⁻¹), K is the Langmuir constant, K_f is the Freundlich constant and 1/n is the adsorption index [37].

The linear spot and isotherm constant values of the Langmuir isotherm are presented in Figure 8 and

Table 1. Langmuir isotherm constants for adsorption of Cd-I-MA-co-AN and NI-MA-co-AN.

Adsorbent	Langmuir Isotherm
	K	R2
Cd-I-MA-co-AN	3.732 × 10−3	0.7867
NI-MA-co-AN	8.819 × 10−3	0.7567

. The values of K and R² of Cd-I-MA-co-AN are 3.732 × 10⁻³ and 0.7867, and the values of K and R² of NI-MA-co-AN are 8.819 × 10⁻³ and 0.7567. The Langmuir model shows a poor fitness of the adsorption isotherm. The linear spot and isotherm constant values of the Freundlich isotherm are presented in Figure 9 and

Table 2. Freundlich isotherm constants for adsorption of Cd-I-MA-co-AN and NI-MA-co-AN.

Adsorbent	Freundlich Isotherm
	$\frac{1}{\mathit{n}}$	lnKf	R2
Cd-I-MA-co-AN	0.959	−1.3816	0.9564
NI-MA-co-AN	0.7545	−1.3983	0.9128

. The values of R² and 1/n of Cd-I-MA-co-AN are 0.9564 and 0.959, and the values of R² and 1/n of NI-MA-co-AN are 0.9128 and 0.7545. The adsorption index of Cd-I-MA-co-AN is in the 0–1 range, proving that the adsorption reaction is favorable. Therefore, the Freundlich isotherm model evaluates the adsorption behavior well. Compared with the NI-MA-co-AN, the adsorption behavior of Cd-I-MA-co-AN was found to be better fitted with the Freundlich isotherm model. This indicates that the adsorbed sites on the surface of the imprinted polymeric microsphere are not energetically equivalent to cadmium ions and that the surface is heterogeneous [38].

3.2.6. Adsorption Selectivity Study

To evaluate the selective property of Cd-I-MA-co-AN and NI-MA-co-AN, various coexisting ions, such as Cu(II), Mn(II), Ni(II) and Pb(II) with a similar charge and similar ionic radius to Cd(II), were used as competing ions. The distribution coefficient (D), selectivity coefficient (α) and relative selectivity coefficient (k) are presented in

Table 3. Adsorption selectivity parameters of Cd-I-MA-co-AN and NI-MA-co-AN.

Metal Ion	Distribution Coefficient D		Selectivity Coefficient α		Relative Selectivity Coefficient k
	Cd-I-MA-co-AN	NI-MA-co-AN	Cd-I-MA-co-AN	NI-MA-co-AN
Cd(II)	3.58	1.18
Cu(II)	0.236	0.294	15.2	4.01	3.79
Mn(II)	0.873	0.975	4.10	1.21	3.39
Ni(II)	0.389	0.500	9.20	2.36	3.90
Pb(II)	1.19	1.30	3.01	0.908	3.31

. The distribution coefficient reveals that Cd-I-MA-co-AN has a better adsorption performance for Cd(II) than that of Cu(II), Mn(II), Ni(II), and Pb(II). This is due to the size and shape of the holes left by the ion imprinted polymer after eluting, which matched Cd(II) and the imprinted polymeric microsphere containing active sites that allow for them to interact with cadmium ions. Cd-I-MA-co-AN has a stronger affinity for Cd(II) with an ionic radius of 95 pm than for Cu(II) (73 pm), Mn(II) (83 pm), Ni(II) (69 pm), and Pb(II) (119 pm) [39]. Thus, Cd-I-MA-co-AN has high selectivity for Cd(II).

3.2.7. Analysis of Real Sample

The columns with microspheres prepared as a solid phase were applied to the preconcentration and determination of trace cadmium ions in natural water samples. Tap water samples were collected from our research laboratory. Spring water samples were collected from the Baicheng county of Aksu in Xinjiang, China. River water samples were collected from the Aksu River in Xinjiang, China. The enrichment factor reached 20 times by the columns. For the analysis of natural water samples, the standard addition method was used; the results are given in

Table 4. Analytical results of trace Cd(II) in natural water samples.

Sample	Added/μg L−1	Found/μg L−1 (n = 3, p = 90%)	Recovery/%
Tap water	0	-	-
	1.00	0.98 ± 0.06	98.0
	5.00	4.94 ± 0.12	98.8
Spring water	0	0.26 ± 0.09	-
	1.00	1.22 ± 0.10	96.2
	5.00	5.13 ± 0.13	97.4
River water	0	0.31 ± 0.11	-
	1.00	1.29 ± 0.08	97.6
	5.00	5.26 ± 0.15	98.6

. The recovery of the added 1 and 5 μg L⁻¹ Cd(II) in tap water, spring water and river water exceeded 96%, indicating that Cd-I-MA-co-AN can be applied to preconcentrations of Cd(II) from natural water.

3.2.8. Reusability Study

Reusability is one means of improving the economic efficiency of adsorbents when treating wastewater. The reusability of Cd-I-MA-co-AN is shown in Figure 10. In adsorption–desorption experiments, with the use of 1 mol L⁻¹ nitric acid on Cd-I-MA-co-AN, the adsorption rate began to decrease after ten cycles. Therefore, Cd-I-MA-co-AN has good reusability.

3.2.9. Comparative Study

The prepared Cd-I-MA-co-AN was compared with the previously reported Cd(II) imprinted polymers for the adsorption capacities of Cd(II) on the base of functional monomers and polymerization techniques. The results are given in

Table 5. Comparative study.

Functional Monomers	Polymerization Techniques	Adsorption Capacity	References
2-(p-Sulphophenylazo)-1,8-dihydroxy-3,6 naphthalene disulphonic acid trisodium salt	Copolymerization	270 μg g−1	[15]
Methacrylic acid and vinylimidazole	Bulk polymerization	3.3 mg g−1	[40]
Acrylonitrile	Aqueous suspension polymerization	0.018 mg g−1	[41]
3-Mercaptopropyltrimethoxysilane	Surface imprinting technology	4.8 mg g−1	[42]
1-Vinylimidazole	Suspension polymerization	4.73 mg g−1	[43]
3-Mercaptopropyltrimethoxysilane	Surface imprinting technique	5.5025 mg g−1	[44]
2-Vinylpyridine	Precipitation polymerization	16.52 mg g−1	[45]
Carboxymethyl chitosan	Surface imprinting technique	20.7 mg g−1	[46]
Chitosan	Surface imprinting technique	18.2 mg g−1	[47]
3-Mercaptopropyltrimethoxysilane	Surface ion imprinting technique	11.64 mg g−1	[48]
Maleic acid and acrylonitrile	Precipitation polymerization	20.46 mg g−1	This work

. The comparison table showed that Cd-I-MA-co-AN demonstrates higher adsorption capacity than most of the Cd(II) ion-imprinted polymers.

4. Conclusions

In the current work, Cd-I-MA-co-AN was synthesized using the precipitation method. Cd(II) was used as a template ion; acrylonitrile and maleic acid were chosen as functional groups. The surface and composition of Cd-I-MA-co-AN were characterized with UV–vis, SEM and FTIR, which demonstrated that Cd-I-MA-co-AN was successfully prepared. The effect of parameters such as pH, flow rate and the type and concentration of eluent were investigated. The results showed that the adsorption capacity of Cd-I-MA-co-AN under optimal experimental conditions was 20.46 mg g⁻¹. The adsorption behavior of Cd-I-MA-co-AN obeyed the Freundlich isotherm model. Cd-I-MA-co-AN was shown to be promising for the determination of cadmium ions in water samples. In conclusion, the Cd-I-MA-co-AN exhibited a good adsorption capacity and specific selectivity for Cd(II).