3.1. 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 N1i) from the two vim, four oxygen atoms (O1, O1i, O2, and O2i) 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.
In the complex, the Cd1—O1 and Cd1—O2 bond lengths are 2.524(2) and 2.373(3) Å, the Cd1—O3W bond length is 2.334(4) Å, and the Cd1—N1 bond length is 2.289(2) Å. Further, N1-Cd1-N1i bond angle is 169.77(14)°, O1—Cd1—O1i and O2—Cd1—O2i bond angles are 174.81(12)° and 81.30(12)°, respectively (Table S1., see Supplementary Data). The values of the complex are normal and comparable with the complexes in the literature [38-40].
Table 1. Crystal data and refinement parameters for [Cd(maa)2(vim)2H2O]·H2O complex
Crystal data
|
|
Empirical formula
|
C18H24CdN4O5·H2O
|
Formula weight
|
506.83
|
Temperature (K)
|
296
|
Crystal system
|
Orthorhombic
|
Space Group
|
Pnna
|
a, b, c (Å)
|
8.7850 (5), 15.2082 (9), 17.2115 (10)
|
Volume (A˚3), Z
|
2089.0(4), 4
|
Radiation type
|
Mo Kα
|
Crystal size (mm)
|
0.19 × 0.15 × 0.13
|
(D celcld, (g cm-3))
|
1.576
|
Absorpt. coeff. (μ (mm-1))
|
0.99
|
|
|
Data collection
|
|
Diffractometer
|
Bruker APEX-II CCD
|
Absorption correction
|
Multi-scan Bruker
|
Tmin, Tmax
|
0.618, 0.746
|
No. of measured, independent and
observed [I > 2σ(I)] reflections
|
47097, 2859, 2144
|
Rint
|
0.055
|
(sin θ/λ)max (Å−1)
|
0.667
|
|
|
Refinement
|
|
R[F2> 2σ(F2)], wR(F2), S
|
0.046, 0.082, 1.31
|
No. of reflections
|
2859
|
No. of parameters
|
142
|
No. of restraints
|
2
|
H-atom treatment
|
H atoms treated by a mixture of independent and constrained refinement
|
Δρmax, Δρmin (e Å−3)
|
0.41, −0.51
|
There are strong intra- and intermolecular O—H···O and weak C—H···O and C—H···π interactions in the crystal packing of complex (Fig. 3(a)). Intra molecular hydrogen bond is between H atom of uncoordinated water and O atom of maa and the distance O4···O2iii is 2.787 Å (Fig. 3(b)). Intermolecular hydrogen bond O3—H3···O1ii giving rise to R22(8) ring motifs is between aqua and maa (Fig. 3(b)). Another intermolecular hydrogen bond C5—H5···O1i is between vim and maa, where the vim donate H atoms to the neighboring maa O atoms (Fig. 3(c)). The distances O3···O1ii and C5···O1i 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 H-bonds and van der Walls interactions. Fig. 3(a) shows the packing structure of the complex along the b direction.
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)2H2O]·H2O complex, maaH, and vim ligand are shown in Fig.5. The narrow band at 3464 cm-1 is attributed to υ(O–H) stretching of uncoordinated water and broadband at 3222 and 3147 cm-1 are assigned to υ(O–H) of coordinated aqua ligand [41-42]. The peaks corresponding to asymmetric and symmetric υ(C=O) stretching bands of the methacrylic acid were observed at lower frequencies as 1527 and 1416 cm-1 for the complex indicating that the methacrylic acid coordination with the Cd(II) ion via the oxygen atoms of the carboxylate group. The difference (∆υ) between the asymmetric and symmetric carboxylate stretching vibrations is used to determine the coordination type of carboxylate group. The calculated (∆υ) value [υasym(COO) - υsym(COO)] of 111 cm-1 for the complex is suitable for bidentate coordination (∆υ < 200 cm-1), which is in line with reported previously [38, 43-44]. The peaks at 3113, 3011 and 2923cm-1 in the complex attributed to the aromatic, vinylic and aliphatic υ(C-H) stretching vibrations of ligands respectively [45-46]. The absorption band at 1647 cm-1 in the complex belongs to C=C band.
The thermal behavior of the complex, whose TGA curves are shown in Fig. 6, was investigated by simultaneous TG-DTA in the temperature range 30–800 ℃ under the nitrogen atmosphere. The Cd(II) complex decomposes in three main steps. The first stage associated with the loss of the aqua ligand and the uncoordinated water molecule occurs between 47 and 104 ℃ (DTGmax: 50 and 91 ℃) with a mass loss of 7.64% (Calcd: 7.11%). After this step is related to the degradation of two vim ligand and two maa ligand in the second (104-248 ℃, DTGmax: 169 ℃) and third step (248- 511℃, DTGmax: 408 ℃) with a mass loss of 66.85% (Calcd: 67.56 %). The total remaining weight calculations (25.51%) suggested that the final product at 700 ℃ was identified as CdO (Calcd: 25.34%).
3.2. 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 non-eluted 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 ℃ (DTGmax=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 ℃ (DTGmax=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.
3.3. Batch Adsorption Experiments
3.3.1. 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.
3.3.2. 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 defined by the equilibrium adsorption isotherm, expressed by certain constants related to the surface properties and affinity of the adsorbent. The adsorption isotherms were investigated using the Langmuir and Freundlich isotherm models which are given in the supplementary file. (Table S2, Fig. S1-S2, see Supplementary Data). The results indicate that the adsorption of the Cd(II) ions onto Cd(II)-IIP fitted well the Freundlich adsorption isotherm model.
3.3.3. 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 coefficients (k) were calculated using the equation (2) and (3). As shown in Table 2., the Kd value of the Cd(II) ion is greater than that of the other ions for Cd(II)-IIP. Also, the selectivity coefficients of Cd(II)-IIP for Cd2+/Pb2+, Cd2+/Ni2+ and Cd2+/Zn2+ were 2.260, 4.414, and 3.361, respectively.
Table 2. Selective adsorption properties of Cd-IIP and NIP
|
Cd(II)-IIP
|
NIP
|
Ions
|
Kd (L/g)
|
k
|
Kd (L/g)
|
k
|
Cd(II)
|
0.166
|
|
0.018
|
|
Pb(II)
|
0.074
|
2.260
|
0.051
|
0.360
|
Ni(II)
|
0.038
|
4.414
|
0.012
|
1.539
|
Zn(II)
|
0.046
|
3.361
|
0.013
|
1.381
|
According to these results, it is observed that Cd(II)-IIP exhibits a higher adsorption capacity for Cd(II) than the other ions. This is mainly because the ionic recognition and selectivity of the IIPs influenced by the nature of the metal ion, its coordination geometry, and number [47-48]. Therefore, the size and shape of the imprinted sites of Cd(II)-IIP are suitable for Cd(II) ions, and Cd(II)-IIP has a specific selectivity for Cd(II) in the presence of various competitor metal ions.