Ion Imprinted Polymers Prepared with a Novel Cd(II) Methacrylate Monomer Complex with 1- Vinylimidazole for Selective Removal of Cd(II) Ions


 A novel [Cd(maa)2(vim)2H2O]·H2O monomer complex was synthesized using methacrylic acid (maaH) and 1-vinylimidazole (vim) that are suitable ligands for polymerization with cadmium central atom. Cd(II)-IIP was prepared by precipitation polymerization technique using monomer complex, EGDMA, and AIBN as functional monomer, crosslinker, and initiator, respectively. The structure of the monomer complex was elucidated by single-crystal X-ray diffraction method. Infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA) methods were used for characterization both of the monomer complex and Cd(II)-IIP. Scanning electron microscopy / energy-dispersive X-ray spectroscopy (SEM/EDX) methods were used for observation surface morphology and content of the polymer surfaces. The adsorption performance and selectivity properties of Cd(II)-IIP were also investigated. The maximum adsorption capacity of Cd(II)-IIP was 43.0 mg/g with 250 mg/L initial Cd(II) concentration at pH:6.0, and the selectivity was higher for Cd2+ ions than that of Pb2+, Ni2+, and Zn2+ as competitor ions. The Langmuir and Freundlich isotherm models were applied comparatively to experimental results. Cd(II) ion content of the solutions determined by inductively coupled plasma mass spectrometry (ICP-MS).


Introduction
Cadmium is one of the heavy metals on the Environmental Protection Agency's (EPA) list of primary pollutants due to its high toxicity [1][2][3][4]. Unlike copper and zinc, cadmium is very harmful to human health even at excessively low concentrations [5][6][7]. Cadmium is classi ed as carcinogenic by the International Cancer Research Agency (ICRA) [8][9][10]. Since cadmium has a high solubility in water, it can discharge into the environmental waters via the waste materials from the Cd-Ni battery producing factories, anthropogenic sources, power stations, and metal planting [11][12]. Thus, it can easily transfer from soil to plants, from plants to the human body [13][14]. The recommendation of World Health Organization (WHO) for Cd(II) ions in drinking water is 3 ng/mL [15]. Therefore, selective separation and elimination of cadmium ions from wastewater have great signi cance. In recent years, a wide range of chemical and physical treatment methods such as adsorption, ion exchange, membrane treatment, chemical precipitation and electrocoagulation methods have been reported for selective removal of Cd(II) ions from wastewaters [16][17]. Among these methods, the adsorption technique is a highly effective and economical one. Also, many adsorbents such as clays, minerals (calcite), calcareous aggregates, zeolites, dead biomass, or mineral wool were used for the removal of Cd(II) ions from various samples [13,[18][19][20][21].
Selective determination of metal ions in real samples is di cult for lots of applications. Over the past decade, ion-imprinted polymers (IIPs) have been developed especially for the recognition of metal ions [22][23][24][25]. Among them, the researchers paid special attention to the studies on the development of Cd (II) ion-imprinted polymer, due to the harmful effects of cadmium on living things [2, 4-7, 9-10, 17, 26-30].
The polymer prepared by the ion-imprinting technique possesses good recognized ability to the template ion. The ion-imprinting method has three process steps as complexation, polymerization, and removal of template ions from the polymer. The coordination geometry, coordination number, charges and sizes of metal ions, and the speci city of ligand are important speci cations for the selectivity of polymer as adsorbent [26]. In recent years unsaturated carboxylic acids (acrylic, methacrylic, fumaric, maleic, oleic, crotonic, vinyl benzoic acids) are frequently used as monomers in the imprinted polymer method. The molecules containing vinyl group (-CH=CH 2 ) such as methacrylic acid (maaH) and 1-vinylimidazole (vim) are used as monomers in the polymerization processes. These polymerizable ligands are used in the synthesis of the metal-containing monomers. The metal-containing monomers are used in the synthesis phase of the polymer to be prepared. Existing carboxylic acid groups increase the variety of metal coordination environments and facilitate the design of new advanced materials [31][32].
In this study, a novel [Cd(maa) 2 (vim) 2 H 2 O]·H 2 O complex containing 1-vinylimidazole and methacrylic acid was synthesized as a monomer for preparation of Cd(II) ion-imprinted polymer (Cd(II)-IIP). Then, Cd(II)-IIP was prepared by precipitation polymerization method, in which the monomer complex as a monomer. Except for the synthesized complex, no other monomer was used in the polymer synthesis process The crystal structure of the monomer complex was elucidated by X-ray single crystal technique.
Thermal behavior of the complex and the polymers were determined by TG, DTG, and DTA techniques.
Metal adsorption performance and selectivity properties of Cd(II)-IIP were also investigated.
SPC Science brand standards were used for adsorption and pH studies.

Apparatus
A Costech ECS 4010 model instrument was used for elemental analysis of the monomer complex. FT-IR analysis was conduct by using a Jasco FT/IR-6600 typeA model spectrophotometer. Thermogravimetric analyzes were carried out using a TA Instruments SII O-EXSTAR 6000 model thermogravimetric/differential thermal analyzer (TG/DTA). The re ection intensities of the monomer complex were collected by using a Bruker D8 QUEST model diffractometer. SEM-EDX analysis was performed with a Hitachi SU 1510 model instrument. A Bruker brand 820 MS model ICP-MS spectrometer was used for metal determinations. A Denver instruments UB-7 (USA) model pH meter was used for pH measurements.

Preparation of Cd(II)-IIP
The preparation scheme of Cd(II)-IIP was shown in Fig. 1. Primarily Cd(II)-methacrylate monomer complex 250 mg was dissolved in 6 mL of methanol. 10 mL of 2:3 (v/v) EGDMA toluene solution was added to rst solution. 40 mg of AIBN was dissolved in the polymerization mixture deoxygenated with N 2 gas for 15 min. N 2 gas was passed through another balloon containing 30 mL of methanol for 15 min to remove the oxygen and heated to 60 ℃ in a thermostatic regulated oil bath. The polymerization mixture was added into the ask containing methanol and the polymerization was completed at 60 ℃ under magnetic stirring for 24 h. After ltration, the polymer beads were washed with 1:1 (v/v) ethanol:water solution. Then, dried polymer was washed with 0.5 M HCl to ensure the removal of the Cd(II) ions from the polymer. Finally, the polymer (Cd-IIP) was washed with deionized water, until the ltrate was neutralized and dried at room temperature. Also, NIP particles were prepared in the absence of monomer complex.

Characterization methods of Cd(II)-methacrylate complex and Cd(II)-IIP
FT-IR and TGA methods were used for characterizations of both the monomer complex and Cd(II)-IIP. Also, single-crystal X-ray diffractometry and SEM-EDX methods were speci cally used for characterizations of the monomer complex and Cd(II)-IIP, respectively. The morphologies of the Cd(II)-IIP particles were observed by SEM, and also the determination of the presence of Cd(II) ion in Cd(II)-IIP was performed using EDX methods. TGA experiments were carried out from 30 to 800 ℃ (heating rate: 10 ℃ min -1 , sample weight: 3 mg, N 2 atmosphere). The re ection intensities of the complex were collected at 296 K with graphite-monochromated Mo-Kα radiation (l=0,71073 Å). The structure was solved using SHELXT [33] by direct methods, and all non-hydrogen atoms were re ned with anisotropic displacement parameters by full-matrix least-squares methods on F 2 using SHELXL [34] from within the WINGX [35] . The MERCURY program was used for molecular graphics [36]. Supramolecular analyses were made with the PLATON [37]. The details about the crystal data and structure determination are summarized in Table 1.
Selected bond lengths, bond angles and hydrogen-bond geometry are given in Table S1. 2.6. Adsorption procedure The adsorption performances of the Cd(II)-IIP were investigated via batch sorption experiments. The initial concentration and pH effect on the adsorption were investigated. Standard solutions (50 mg/L concentration of Cd(II) ions) were adjusted to the demanded pH levels (pH range 2.0−7.0) by using a pHmeter with the additions of HNO 3 or NH 3 solutions. 50 mg of Cd(II)-IIP was added into a conical ask that contained 20 mL of Cd(II) solution with 50 mg/L concentration. The mixture was mechanically shaken for 24 h, at 25 ℃. The absorbance of the ltrate was measured at ICP-MS and the amount of metal ion adsorbed by the polymer was calculated according to the quantity of Cd(II) left in the ltrate.
The adsorption capacity of Cd(II)-IIP was also investigated by batch experiments for different initial Cd(II) ion concentrations ranging from 10 to 300 mg/L at pH:6,0. The adsorption capacity of Cd(II)-IIP was calculated from the following equation: where Q is the adsorption capacity of Cd(II)-IIPs, mg/g. C 0 and C are the initial and nal concentrations of cadmium ions in aqueous solution, mg/L, respectively. V represents the volume of aqueous solution, L; m represent the weight of Cd(II)-IIP, g.
Competitive adsorptions of Pb(II), Ni(II), and Zn(II) in the presence of Cd(II) were investigated by batch experiments. 50 mg of Cd(II)-IIP or NIP were treated with 20 mL (pH 6.0) of a solution of these competitive ions (100 mg/L for each one). The distribution ratio and the selectivity factor of the ions were calculated using the following equations: where M 2+ represents competitive metal ions, K d and k are distribution ratio and selectivity factor, respectively.

Characterization of Cd(II)-methacrylate monomer complex
According to the single crystal X-ray diffraction analysis, the complex crystallized in the orthorhombic system with space group Pnna (Table 1.). The crystal structure of the monomer complex with the atom labeling is shown in Fig. 2. The two maa acts as a bidentate ligand and vim is monodentate ligand coordinated to the metal via its tertiary nitrogen atom. The Cd(II) ion has seven coordinates by the two nitrogen atoms (N1 and N1 i ) from the two vim, four oxygen atoms (O1, O1 i , O2, and O2 i ) from two maa, and one oxygen (O3w) from an aqua ligand. Thus, Cd(II) ion display distorted pentagonal bipyramid geometry. In crystallization, an uncoordinated water molecule is also part of the unit cell.
Another intermolecular hydrogen bond C5-H5···O1 i is between vim and maa, where the vim donate H atoms to the neighboring maa O atoms (Fig. 3(c)). The distances O3···O1 ii and C5···O1 i are 2.732 and 3.260 Å, respectively. The C8-H8···π interaction is between H atom of vim and the neighboring imidazole rings [C8-H8···π, d = 2.911 Å and 150°] (Fig. 4). The 3D crystal structure of the complex is formed by Hbonds and van der Walls interactions. Fig. 3(a) shows the packing structure of the complex along the b direction. respectively [45][46] . The absorption band at 1647 cm -1 in the complex belongs to C=C band.

FT-IR spectral analysis provides important information for rationalizing the mechanism of the interactions between ligands and metal ion. FT-IR spectrums of [Cd(maa) 2 (vim) 2 H 2 O]·H
The thermal behavior of the complex, whose TGA curves are shown in Fig. 6

Characterization of Cd(II)-IIP
The surface structure image of the Cd(II)-IIP and NIP were evaluated by SEM. Fig. 7 show structural differences of the before and after elution IIP and NIP, respectively. According to the SEM images shown in Fig. 7 (a) and (b), it was observed that the eluted Cd(II)-IIP had a rougher surface area than the noneluted Cd(II)-IIP. In addition in Fig. 7(c) NIP particles are quite small in size and have dust form in contrast to the IIP particles.
The energy dispersive X-ray (EDX) analysis of Cd(II)-IIP before and after elution and NIP are shown in Fig.  7. The EDX analysis was used to determine the content of the polymer surfaces, and complete removal of Cd(II) ion from Cd(II)-IIP. The datas in Fig. 7(a) approve the entity of C, O, and Cd in the polymer structure.
According to Figure 7 (b), it was observed that the Cd(II) ion was not present in the structure of the Cd(II)-IIPs after elution, showing that the elution of the Cd(II) ion was successfully. Fig. 8 represents FT-IR spectrums of the Cd(II)-IIP's before and after elution. As can be seen from Fig. 8, both spectrums are similar, indicating that the elution did not cause deterioration on the polymer. The strong bands observed at 1726 and 1151 cm-1 for Cd(II)-IIP before elution, corresponding to C=O and C-O groups, respectively ( Fig.8(a)). These peaks moved to 1719 and 1137 cm -1 after elution. It means that the Cd(II) ions have been removed successfully. Additionally, when the FT-IR spectrums of the complex (Fig.  5) and polymers (Fig. 8) are compared, it can be seen that the peaks of the carboxyl groups are different. It means that the coordination interaction of Cd(II) ion and carboxyl group strongly changed when Cd(II)-IIP formed.
The thermal behaviors of the Cd(II)-IIP were investigated by TG analysis. As can be seen from TGA curves, degradation of the Cd(II)-IIP before elution ( Fig. 9(a)) occur in one step, indicating that 96% of the polymer was decomposed in the range of 152-715 ℃ (DTG max =408 ℃). Similarly, degradation of the Cd(II)-IIP after elution ( Fig.9(b)) occur in one step, indicating that 100% of the polymer was decomposed in the range of 229-503 ℃ (DTG max =425 ℃). The Cd(II)-IIP before and after elution showed different residue yields of 4% and 0%, respectively. The residue yield of Cd(II)-IIP before elution was higher than that of Cd(II)-IIP after elution, clearly indicating that the completely removal of Cd(II) ions from the polymer.

pH Effect
The pH is an important parameter effected on the amount of the adsorption in aqueous solution. The pH effect on the adsorption capacity of Cd(II)-IIP has been examined for Cd(II) solutions over the pH range of 2.0 to 7.0. The maximum adsorption value was observed at pH 6.0 (Fig. 10.). The pH effect was not studied over pH 8.0 because of the formation of cadmium hydroxide precipitate [17]. According to the results, adsorption of all pH values was observed, but the highest adsorption was at pH: 6.0 for Cd(II) ion in Cd(II)-IIP particles. For this reason, 6.0 was chosen as optimum pH value for other parameters.

Concentration Effect
The initial concentration effect on adsorption capacity of Cd(II)-IIP was investigated. The adsorption capacities increased gradually from 10 to 250 mg/L initial Cd(II) concentrations (Fig. 11.). At 300 mg/L, adsorption capacity of Cd(II)-IIP decreased due to the driving force effect and three-dimensional network structural expansion [29]. The maximum adsorption capacity of Cd(II)-IIP was calculated as 43.0 mg/g. The results suggest that the Cd(II)-IIP had a high adsorption capacity.
The adsorption capacity of imprinted polymers can be de ned by the equilibrium adsorption isotherm, expressed by certain constants related to the surface properties and a nity of the adsorbent. The adsorption isotherms were investigated using the Langmuir and Freundlich isotherm models which are given in the supplementary le. (Table S2, Fig. S1-S2, see Supplementary Data). The results indicate that the adsorption of the Cd(II) ions onto Cd(II)-IIP tted well the Freundlich adsorption isotherm model.

Selectivity of Cd(II)-IIP
In order to investigate of selectivity of Cd(II)-IIP; Pb(II), Ni(II), and Zn(II) ions are chosen as the competitor ions. The selectivity factor coe cients (k) were calculated using the equation (2) and (

Conclusions
In this work, a novel [Cd(maa) 2 (vim) 2 H 2 O]·H 2 O monomer complex containing polymerizable ligands (maa and vim), and its Cd(II)-ion imprinted polymer were successfully synthesized and characterized by different methods. The structure of the monomer complex was determined in detail by the X-ray diffraction method and FT-IR. The complex crystallizes in the orthorhombic system with space group Pnna. Cd(II) ion display distorted pentagonal bipyramid geometry [CdO 5 N 2 ] in the crystal structure. The complex molecules show three-dimensional supramolecular networks by C-H···O, O-H···O, N-H···O, and C-H···π interactions. The SEM-EDX and FT-IR were employed to elucidate the morphology, content of surfaces, and bonding of Cd(II)-IIP. In addition, the thermal behaviors of both the complex and the polymer were analyzed by TGA.
In order to determine the maximum adsorption capacity of the prepared polymer, appropriate pH and initial concentration were investigated. The results indicating that Cd(II)-IIP had maximum adsorption capacity at pH:6, as 43 mg/g. The adsorption isotherms were investigated using the Langmuir and Freundlich isotherm models. The experimental data was described the Freundlich adsorption isotherm model well. The selectivity of Cd(II)-IIP was higher for Cd 2+ ions than that of Pb 2+ , Ni 2+ , and Zn 2+ as competitor ions. Consequently, the Cd(II)-IIP obtained in this study can be regarded to be a suitable sorbent for the selective removal of Cd(II) ions in wastewater. Figure 11 Effect of initial Cd(II) concentration (50 mg Cd-IIP, 24 h, 25 ℃)

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