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Adsorption of copper ions by ion-imprinted simultaneous interpenetrating network hydrogel: Thermodynamics, morphology and mechanism
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Adsorption of copper ions by ion-imprinted simultaneous interpenetrating network hydrogel: Thermodynamics, morphology and mechanism Jingjing Wang ∗ , Liang Ding, Jun Wei, Fang Liu Department of Polymer Materials and Engineering, School of Material Engineering, Key Laboratory for Ecological-Environment Materials of Jiangsu Province, Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 21 October 2013 Received in revised form 10 March 2014 Accepted 17 March 2014 Available online xxx Keywords: Ion-imprinted hydrogel Interpenetrating polymer network (IPN) Metal ion adsorption Thermodynamics Mechanism
a b s t r a c t Cu(II) ion-imprinted hydrogel [Cu(II)-IIH] with interpenetrating polymer network (IPN) structure was prepared and its application to adsorb Cu(II) ions from aqueous solution was studied. The Cu(II)-IIH was prepared by UV-initiated simultaneous free radical/cationic hybrid polymerization. The adsorption capacity of the Cu(II)-IIH increased with the initial pH value of the solution, but decreased as the temperature rose from 303 to 323 K. Thermodynamic parameters such as the Gibbs free energy (G◦ ), enthalpy (H◦ ) and entropy (S◦ ) for the Cu(II) ions adsorption were evaluated. It was suggested that the adsorption was a spontaneous, exothermic process with further decrease in the degree of freedom at the solid-solution interface due to the negative S◦ value. The morphology study indicated that the copper adsorption caused significant changes to the hydrogel structure. Finally the adsorption mechanism was studied by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The results indicated that copper adsorption was mainly through interactions with the amide and ether groups. © 2014 Elsevier B.V. All rights reserved.
Introduction Environmental contamination by toxic metal ions in aquatic ecosystems has been increasing with industrial growth and development in recent years [1]. Among all toxic metals, copper could cause serious environmental problems because it is widely used in paints and pigments, paper and pulp, fertilizer manufacturing, copper polishing, etc. Excessive uptake of copper can cause a lot of serious health problems such as damage of liver and kidney, anemia, immunotoxicity, etc. [2,3]. Therefore, in order to protect human health and the environment, it is very important to remove or minimize excess of copper in industrial effluents before discharging it into surface water, as well as into groundwater. Compared with the conventional physicochemical technologies, the adsorption is demonstrated to be promising for heavy metal removal because of its low cost, high adsorption efficiency for low concentration metal ions [4,5]. It is based on utilization of various sorbents, such as agricultural wastes, biomass, clays,
∗ Corresponding author. Tel.: +86 051588298872. E-mail address: [email protected] (J. Wang).
hydroxyapatite, silica gels and zeolites, as well as natural and synthetic polymers [6–8]. Nowadays, a special attention has been given to hydrogels which possess ionic functional groups that can absorb and trap metal ions from wastewater [9–11]. Compared with conventional solid sorbents, the main advantages of hydrogels are desired tailored swelling and mechanical properties, easy handling and reusability [11]. Taking into account all the hydrogel adsorbents reported in the literature, the hydrogels based on ion imprinting technology have received considerable attention [12]. Ion-imprinted hydrogels are a class of materials with predetermined selectivity for analytical separation. In the process of ion imprinting, a functional monomer and a crosslinker are polymerized in the presence of a template ion. Then the template is extracted leaving sites which are complementary in both shape and chemical functionality to those of the template [13]. This hydrogel then becomes capable of selectively adsorbing the template species. Such an imprinted polymer shows an affinity for the template ion over other coexisting metal ions. To develop an adsorption process by using the ion-imprinted hydrogels for industrial applications, it is desirable to better understand the metal uptake process. Advanced instruments such as X-ray photoelectron spectroscopy (XPS) and Fourier transform
http://dx.doi.org/10.1016/j.apsusc.2014.03.102 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Wang, et al., Adsorption of copper ions by ion-imprinted simultaneous interpenetrating network hydrogel: Thermodynamics, morphology and mechanism, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.102
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infrared (FTIR) spectroscopy can provide good tools to achieve the goal because they can provide insight into environmental processes at the molecular level [14,15]. In our previous study [16], simultaneous interpenetrating network technique was employed to prepare Cu(II) ion-imprinted hydrogel [Cu(II)-IIH] with improved mechanical strength. The adsorption kinetics and isotherm, selective adsorption ability and reusability were investigated. The present work was in continuation to our previous study. In the current report, the Cu(II)-IIH was characterized using thermogravimetric analysis (TGA) and scanning electron microscopy/energy dispersive X-ray (SEM/EDX). Several factors affecting the sorption of Cu(II) ions, such as pH, initial concentration and temperature were investigated. Finally the adsorption mechanism was better elucidated via FTIR in combination of XPS.
Thermogravimetric analysis The thermal stability was evaluated by thermogravimetry using a TA Instruments TGA 2950 thermogravimetric analyzer in the temperature range of 30–800 ◦ C, at a scanning rate of 10 ◦ C/min. Surface morphology study SEM/EDX was used to investigate the cross-section morphology of the prepared hydrogels. Measurements were taken on a FEI QUANTA 200 scanning electron microscope. Before SEM measurements, hydrogels were lyophilized. Then the freeze-dried samples were fractured carefully in liquid nitrogen, and then fixed on tubs with sputter coated with gold before observation. Adsorption mechanism study
Experimental Materials Acrylamide (AAm), ethylene glycol dimethacrylate (EGDMA) and triethylene glycol divinyl ether (DVE-3) were purchased from Sigma–Aldrich Chemicals and used without further purification. Free radical photoinitiator (Darocur 1173 from Ciba Specialty Chemicals) and a diaryliodonium hexauorophosphate salt (DAI from Ciba Specialty Chemicals) were respectively used to initiate the free radical and cationic polymerization. Copper nitrate [Cu(NO3 )2 ·3H2 O)] was purchased from Shanghai Chemical Reagents Co., and was heated in order to remove crystal water before used.
Preparation of Cu(II) ion-imprinted hydrogel [Cu(II)-IIH] The Cu(II)-IIH was prepared according to our previous report [16]. Briefly, a mixture of 0.96 g AAm, 1.44 g DVE-3, and 1.86 g Cu (NO3 )2 were first dissolved in ethanol. Then, 0.048 g crosslinker EGDMA, 0.0192 g free radical photoinitiator, and 0.0288 g cationic photoinitiator were added. The mixture was introduced between two glass plates and cured under a high-pressure mercury lamp for 1 h. Subsequently, the film was treated with 1.0 mol/L HCl to completely leach Cu(II) ions. At last, the hydrogel was washed to neutralization, and dried at 60 ◦ C under vacuum, resulting in the desired Cu(II)-IIH.
Adsorption capacity study Adsorption study was carried out in magnetically stirred, thermostated cylindrical glass vessels in batch conditions. The sample was added into the heavy metal ion solution to determine the metal ion adsorption capacity of the samples under non-competitive conditions. The pH of the metal feed solutions was adjusted before the sample was applied for the adsorption process. Amount of the residual metal ion in the solution was determined using an Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) after 24 h [11]. Amount of adsorbed metal ion (Q, mmol/g) was calculated from the following equation: Q =
(C0 − C)V m
where C0 (mmol/L) and C (mmol/L) were the metal ion concentrations before and after adsorption respectively. V (L) was the volume of the solution added and m (g) was the mass of the xerogel.
FTIR spectra were recorded on a Bruker Vector 22 FT spectrometer in the range 4000–400 cm−1 . XPS spectra were recorded by a Perkin-Elmer PHI 550 ESCA/SAM spectrometer using Mg K␣ X-ray source. Results and discussion Preparation of Cu(II) ion-imprinted hydrogel [Cu(II)-IIH] Free radical/cationic hybrid polymerization was usually employed to prepare simultaneous interpenetrating polymer network (IPN) materials [17,18]. In the present work, template Cu(II) ions firstly coordinated to the NH2 from acrylamide (AAm) and O from triethylene glycol divinyl ether (DVE-3). Then the Cu(II)-IIH was synthesized by UV-initiated free radical/cationic hybrid polymerization of complexes Cu(II)/AAm and Cu(II)/DVE-3 with EGDMA as a crosslinker. In the polymerization system, Cu(II)/AAm polymerized by free radical mechanism, and Cu(II)/DVE-3 polymerized by cationic mechanism simultaneously. Finally Cu(II) ions were leached from the hydrogel leaving behind some cavities with predetermined orientation and special size of templates. The obtained Cu(II)-IIH consisted of two polymer networks. One was poly(acrylamide) (PAAm), and the other was poly(triethylene glycol divinyl ether) poly(DVE-3). The proposed preparation process was shown in Fig. 1. Adsorption equilibrium study Effect of pH on adsorption Metal ion sorption on sorbent is influenced by pH value due to the competition between the metal ions and H+ for active sorption sites, so the pH value is one of the most important factors affecting the sorption of Cu(II) ions from aqueous solutions. As shown in Fig. 2, the removal percentage of metal ions increased sharply along with the increase of the pH value. That was to say, the adsorption capacity of the Cu(II)-IIH to Cu(II) ions in weak acidic solution was higher than that in strong acidic solution. The reason could be explained that, under strong acidic conditions, the Cu(II)-IIH surface was completely covered with H+ , and Cu(II) ions could not compete with them for adsorption sites [19]. However, with the increase of pH value, the competition from H+ decreased and Cu(II) ions could be adsorbed on the adsorbent quite easily. The similar results were obtained by other researchers as reported elsewhere [20,21]. Experiments were not carried out at pH values above 5.0 due to the fact that Cu(II) ions precipitation occurred at higher pH values. In order to avoid the formation of precipitation in the aqueous solution, the pH of 5.0 was selected as the initial pH value of Cu(II) ions solution for subsequent adsorption experiment.
Please cite this article in press as: J. Wang, et al., Adsorption of copper ions by ion-imprinted simultaneous interpenetrating network hydrogel: Thermodynamics, morphology and mechanism, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.102
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Removal percent, %
100 80 60 40 20 0
2.5
4.5 3.5 pH value
5
Fig. 2. Influence of pH values on the adsorption capacity of Cu(II)-IIH.
Effect of temperature on adsorption The adsorption amount of Cu(II) ions from its aqueous solution with various concentrations at 303, 313 and 323 K is shown in Fig. 3. The results clearly showed that with an increase in the initial metal ion concentration, the adsorption capacity of the Cu(II)-IIH increased significantly. It was illustrated that the adsorption sites on the sorbent were sufficient at low concentration, and the amount of adsorption in this case was dependent on the number of metal ions that were transported from the bulk solution to the surface of the hydrogel. With the increase of the metal ion concentration, adsorption sites were gradually occupied and the adsorption
Cu(II) ion adsorption capacity, mmol/g
Fig. 1. Scheme for the preparation of Cu(II) ion-imprinted hydrogel [Cu(II)-IIH].
303 K 0.6
313 K 323 K
0.5
0.4
0.3 0.000
0.001
0.002
0.003
0.004
0.005
Initial concentration of Cu(II) ions, mol/L Fig. 3. Effect of initial Cu(II) ion concentration on the adsorption capacity of Cu(II)IIH at 298, 308 and 318 K.
amount achieved a saturated value [22]. The maximum sorption capacity was 0.64 mmol/g. It was reported that the sorption capacities of Cu(II) ions on ion-imprinted chitosan/Sargassum sp. and polymeric nanoparticles were 1.08 and 0.346 mmol/g [20,21]. The sorption capacity of Cu(II)-IIH was compared with those of other sorbents in Table 1. It was clearly indicated that the ion-imprinted hydrogel sorbent developed in this study had a great potential for the treatment of heavy metal wastewater from various industries. From Fig. 3, it was also found that the equilibrium adsorption capacity decreased when the temperature increased from 303 to
Table 1 Comparison of the developed sorbent with others reported in the literature. Matrix
Metal ions
Sorption capacity (mmol/g)
Reference
Support: ion-imprinted polymers acrylamide and triethylene glycol divinyl ether Chitosan/Sargassum sp. composite 1,4-Dihydroxy-9,10-anthraquinone Support: calcined phosphate Support: hazelnut shell activated carbon Support: 717 anion exchange resin thiacalix[4]arenetetrasulfonate
Cu2+ Cu2+ Cu2+ Pb2+ /Cu2+ /Zn2+ Cu2+ Cu2+ /Pb2+ /Cd2+
0.64 1.08 0.346 0.414/0.466/0.317 0.91 0.334/0.231/0.401
This work [20] [21] [7] [8] [19]
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0.0
1.7
303 K 313 K
-0.5
1.6
-1.0
1.5
ln K
ln qe/Ce, L/g
323 K
-1.5
1.4 1.3 1.2
-2.0
1.1 0.00312
-2.5 15
20
25
30
35
0.00318
qe, mg/g
0.00324
0.00330
1/T
40
Fig. 5. Plot of ln K versus 1/T for the adsorption of Cu(II) ions by the Cu(II)-IIH.
Fig. 4. Plots of ln (qe /Ce ) as a function of qe for the adsorption of Cu(II) ions by the Cu(II)-IIH.
100 80 Weight, %
323 K. This result indicated the exothermic nature of Cu(II) ions adsorption onto the Cu(II)-IIH. The obtained results in the current research were in agreement with the previous report [19]. A decrease in the uptake value with the rise in temperature may be due to either the serious influence on ligand coordination geometry of the adsorbent or increasing tendency to desorb Cu(II) ions from the interface to the solution [22].
60 40 20 0
Thermodynamic parameters of adsorption
0
200
400
600
800
o
Temperature, C
As related to temperature effect, the thermodynamic parameters have been calculated for this adsorption system. The free
Fig. 6. TG curves for the Cu(II)-IIH.
Fig. 7. SEM images of the Cu(II)-IIH before (a) and after (b) adsorption; EDX spectra for the free (a ) and copper-loaded (b ) Cu(II)-IIH.
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J. Wang et al. / Applied Surface Science xxx (2014) xxx–xxx Table 2 Thermodynamic parameters of Cu(II) ions adsorption onto the Cu(II)-IIH. Temperature 303 K 313 K 323 K
ln K
K
G◦ (kJ/mol)
1.664 1.378 1.145
5.280 3.967 3.142
−4.192 −3.586 −3.075
H◦ (kJ/mol)
entrapment between the crosslinked networks of poly(AAm) and poly(DVE-3) took place [17].
S◦ (J/mol K) −55.9 −56.1 −55.9
−21.131
energy change of adsorption G◦ was calculated by using the equation: G◦ = −RT ln K where R was the universal gas constant and T was the Kelvin temperature. K was the thermodynamic equilibrium constant for the adsorption process. It was determined using the method of Wu and Yu [23] by plotting ln (qe /Ce ) versus qe at 303, 313, and 323 K and extrapolating to zero qe as suggested (Fig. 4). The other thermodynamic parameter, such as the enthalpy change H◦ was calculated from the slope of the straight line which was plotted by ln K against 1/T (Fig. 5) according to the equation: −H ◦ ln K = + constant RT H◦
The negative value of as shown in Table 2 suggested that the adsorption process was exothermic. The standard change in entropy can be calculated from the following equation: S ◦ =
5
Morphology study SEM images of free and copper-loaded Cu(II)-IIH are shown in Fig. 7. The Cu(II)-IIH exhibited three-dimensional porous structure before adsorption (Fig. 7a). While the copper adsorption caused significant changes to the hydrogel structure. The copperloaded Cu(II)-IIH became rougher and the pore size became smaller (Fig. 7b). In order to confirm the adsorption of Cu(II) ions onto the hydrogel, EDX spectra of the unloaded and copper-loaded Cu(II)-IIH were investigated (Fig. 7a and b ). The EDX spectrum for the Cu(II)-IIH before adsorption did not show the characteristic peak of copper. On the other hand, the peak of copper was clearly shown in the EDX spectrum of copper-loaded Cu(II)-IIH. Therefore, the existence of Cu(II) ions on the hydrogel was confirmed by EDX spectra. Adsorption mechanism FTIR spectrum has been widely used for the structural investigation of an adsorbent because it can provide considerable insight into the various functional groups in the adsorbent. As an important method for chemical characterization, FTIR spectrum has also been used to investigate the interactions between adsorbent and metals ions.
H ◦ − G◦ T
The negative value of S◦ might be associated with the adsorption of Cu2+ to the adsorbent, which resulted in a decrease in freedom degree of the systems during the adsorption [23].
O 1s
(a)
C 1s
Thermogravimetric analysis Thermogravimetric analysis is useful in obtaining enough information on the relative thermal stability, and it helps with the selection of materials for analytical purposes focused on adsorption studies. As shown in Fig. 6, according to the thermogravimetric analysis, the Cu(II)-IIH supported a 65% weight loss at about 30–100 ◦ C, which was ascribed to the removal of physically adsorbed water. During the next stage of heating (200–440 ◦ C), a 28% weight loss was observed, which was attributed to the amide bond decomposition and triethylene glycol divinyl ether segment degradation. The high thermal stability might be explained by the interpenetrating network structure, where the physical
N 1s
0
200
400
600
800
1000
Binding energy, eV
(b)
a
Cu 2p
O 1s Transmittance, %
1119 1643
1111
C 1s
b
N 1s
1608
3454 1385 3415 0
1000
2000
3000
4000 -1
Wavenumber, cm
Fig. 8. FTIR spectra of (a) free and (b) copper-loaded Cu(II)-IIH.
0
200
400
600
800
1000
Binding energy, eV Fig. 9. XPS wide-scan spectra of (a) Cu(II)-IIH and (b) copper-loaded Cu(II)-IIH.
Please cite this article in press as: J. Wang, et al., Adsorption of copper ions by ion-imprinted simultaneous interpenetrating network hydrogel: Thermodynamics, morphology and mechanism, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.102
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(e)
(c)
(a) C-O
C-N
(285.4)
C-C
(399.4)
C-O
(284.3)
C=O
C=O
(532.5)
(531.5)
(286.3)
C-N (287.6)
282
284 286 288 Binding energy, eV
290
394
396
398 400 402 Binding energy, eV
C-C
528
530 532 534 Binding energy, eV
C-N
C-O
C-O
(400.0)
(286.3)
536
(f)
(d)
(b)
404
(533.0)
C=O
(284.3)
C=O
(532.1)
(287.2)
C-N (288.6)
282
285 288 Binding energy, eV
291
396
398
400 402 Binding energy, eV
404
528
530 532 534 536 Binding energy, eV
538
Fig. 10. XPS spectra of (a) C 1s of Cu(II)-IIH; (b) C 1s of copper-loaded Cu(II)-IIH; (c) N 1s of Cu(II)-IIH; (d) N 1s of copper-loaded Cu(II)-IIH; (e) O 1s of Cu(II)-IIH; (f) O 1s of copper-loaded Cu(II)-IIH.
Fig. 8 shows the FTIR spectra of the free and copperloaded Cu(II)-IIH. Several peaks, characteristic for poly(AAm) and poly(DVE-3), were observed in the spectrum of the free hydrogel. A characteristic peak from poly(AAm) appeared at 1643 cm−1 , which was assigned to the C O stretching vibration. The intense band around 3454 cm−1 should be assigned to the stretching vibration of N H. The bands which appeared at 1119 cm−1 corresponded to the asymmetric stretching vibration of the C O C in poly(DVE-3). After adsorption, the bands observed at 1385 cm−1 , assigned to the stretching vibration of O Cu, confirmed that Cu(II) ions were really adsorbed onto the hydrogel [24]. In addition, the absorption intensity and frequency of some bands changed. A broad peak at 3415 cm−1 was caused by the symmetrical vibration of NH2 groups, indicating that the groups were possibly involved in the sorption. As known, the metal ions with empty orbital could be capable of accepting electron pairs, while the NH2 groups having non-shared electron pairs, could donate their electron pairs. The strong absorption band at 1608 cm−1 , assigned to C O stretching vibration from poly(AAm), was obviously weakened. This behavior reflected the interaction between Cu(II) ions and C O. The absorption band at 1119 cm−1 , which corresponded to the absorption bands of C O C groups, was also shifted to lower wave numbers (1111 cm−1 ). According to the observed changes in FTIR spectra of the free and copper-loaded hydrogels, it seemed that NH2 , C O C and C O groups were involved in the Cu(II) ions adsorption. XPS was further performed to elucidate the adsorption mechanism. Binding energy profiles of Cu 2p3/2 , N 1s, C 1s and O1s in Cu(II)-IIH before and after copper sorption are shown in Fig. 9. Comparing the wide scans results of Cu(II)-IIH and copper-loaded Cu(II)-IIH, the spectrum of Cu 2p3/2 at 934.0 eV can be clearly observed in the latter one, indicating the accumulation of copper on Cu(II)-IIH. The C 1s spectra of Cu(II)-IIH and copper-loaded Cu(II)-IIH comprised four peaks with differentiated BE values via deconvolution,
as shown in Fig. 10a and b. These peaks could be assigned to C C, C O, C O and C N bonds, respectively. Among them, the C O, C O and C N bonds could be assigned to ether, amino and carbonyl groups. After copper adsorption, the binding energy of C O, C O and C N shifted from 285.4, 286.3 and 287.6 eV to 286.3, 287.2 and 288.6 eV, respectively. This result indicated that carbonyl, ether and amino groups in Cu(II)-IIH got involved in copper sorption, in which oxygen and nitrogen atoms donated electrons to metal ions and thus the electron density at the adjacent carbon atom decreased. Fig. 10c and d revealed that the N 1s bands in copper-loaded Cu(II)-IIH increased from 399.4 eV and 400.0 eV, indicating the formation of R-NH2 Cu2+ complexes, in which a pair of lone electrons from the N atoms was shared with the Cu2+ , and hence the electron cloud density of the nitrogen atom was reduced, resulting in a higher BE peak. Seen from Fig. 10e and f, the O 1s peaks of CS and copper-loaded Cu(II)-IIH comprised two peaks: C O and C O. The BE values of C O and C O groups increased from 531.5 and 532.5 eV to 532.1 and 533.0 eV, respectively. Therefore, copper sorption was followed by charge transfer from these groups to copper ions, which confirmed that C O and C O groups acted as the sorption sites. In general, the XPS spectral changes in the C 1s, N 1s, and O 1s peaks of free and copper-loaded Cu(II)-IIH were consistent with the observations in FTIR analysis. Conclusions In the present study, Cu(II) ion-imprinted hydrogel [Cu(II)-IIH] with interpenetrating polymer network (IPN) structure was prepared by free radical/cationic hybrid photopolymerization. The adsorption capacity of the Cu(II)-IIH increased with the initial pH value of the solution, and the maximum adsorption capacity was observed at pH 5.0. It was also found that the equilibrium adsorption capacity decreased when the temperature increased from 303 K to 323 K. From thermodynamic parameters, it was
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concluded that the adsorption was a spontaneous, exothermic process. Thermogravimetric analysis indicated the Cu(II)-IIH exhibited high thermal stability due to the interpenetrating network structure. The Cu(II)-IIH exhibited three-dimensional porous structure before adsorption. The copper-loaded Cu(II)-IIH became rougher and the pore size became smaller. The FTIR and XPS studies revealed that NH2 , C O and C O groups were involved in the sorption of copper and complexation dominated the sorption process. Acknowledgements The project was supported by Jiangsu Provincial Natural Science Foundation (No. BK2012251), Open Project of Key Laboratory for Ecological-Environment Materials of Jiangsu Province (No. EML201202), research fund of Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (No. AE201069) and Research Fund of Yancheng Institute of Technology (No. XKY2013062). References [1] J. Gong, X. Wang, X. Shao, S. Yuan, C. Yang, X. Hu, Adsorption of heavy metal ions by hierarchically structured magnetite-carbonaceous spheres, Talanta 101 (2012) 45–52. [2] S. Sun, A. Wang, Adsorption properties of carboxymethyl-chitosan and crosslinked carboxymethyl-chitosan resin with Cu(II) as template, Sep. Purif. Technol. 49 (2006) 197–204. [3] H. Qiu, S. Zhang, B. Pan, W. Zhang, L. Lv, Effect of sulfate on Cu(II) sorption to polymer-supported nano-iron oxides: behavior and XPS study, J. Colloid Interface Sci. 366 (2012) 37–43. [4] R.S. Azarudeen, R. Subha, D. Jeyakumar, A.R. Burkanudeen, Batch separation studies for the removal of heavy metal ions using a chelating terpolymer: synthesis, characterization and isotherm models, Sep. Purif. Technol. 116 (2013) 366–377. [5] Z.H. Xiao, R. Zhang, X.Y. Chen, X.L. Li, T.F. Zhou, Magnetically recoverable Ni@carbon nanocomposites: solid-state synthesis and the application as excellent adsorbents for heavy metal ions, Appl. Surf. Sci. 263 (2012) 795–803. [6] A. Demirbas, Heavy metal adsorption onto agro-based waste materials: a review, J. Hazard. Mater. 157 (2008) 220–229. [7] A. Aklil, M. Mouflih, S. Sebti, Removal of heavy metal ions from water by using calcined phosphate as a new adsorbent, J. Hazard. Mater. 112 (2004) 183–190.
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[8] E. Demirbas, N. Dizge, M.T. Sulak, M. Kobya, Adsorption kinetics and equilibrium of copper from aqueous solutions using hazelnut shell activated carbon, Chem. Eng. J. 148 (2009) 480–487. [9] N. Kawasaki, H. Tominaga, F. Ogata, K. Kakehi, Removal of cadmium and copper by vegetable biomass treated with hydrochloric acid, Chem. Eng. J. 157 (2010) 249–253. [10] V. Bekiari, M. Sotiropoulou, G. Bokias, P. Lianos, Use of poly(N,N-dimethyl acrylamide-co-sodium acrylate) hydrogel to extract cationic dyes and metals from water, Colloids Surf. A: Physicochem. Eng. Asp. 312 (2008) 214–218. [11] H. Kas¸göz, A. Durmus¸, A. Kas¸göz, Enhanced swelling and adsorption properties of AAm-AMPSNa/clay hydrogel nanocomposites for heavy metal ion removal, Polym. Adv. Technol. 19 (2008) 213–220. [12] M.G. Segatelli, V.S. Santos, A.B.T. Presotto, I.V.P. Yoshida, C.R.T. Tarley, Cadmium ion-selective sorbent preconcentration method using ion imprinted poly(ethylene glycol dimethacrylate-co-vinylimidazole), React. Funct. Polym. 70 (2010) 325–333. [13] L. Zhang, S. Yang, T. Han, L. Zhong, C. Ma, Y. Zhou, X. Han, Improvement of Ag(I) adsorption onto chitosan/triethanolamine composite sorbent by an ionimprinted technology, Appl. Surf. Sci. 263 (2012) 696–703. [14] L. Dambies, C. Guimon, S. Yiacoumi, E. Guibal, Characterization of metal ion interactions with chitosan by X-ray photoelectron spectroscopy, Colloids Surf. A: Physicochem. Eng. Asp. 177 (2001) 203–214. [15] J.P. Chen, L. Yang, Study of a heavy metal biosorption onto raw and chemically modified Sargassum sp. via spectroscopic and modeling analysis, Langmuir 22 (2006) 8906–8914. [16] J. Wang, F. Liu, Enhanced and selective adsorption of heavy metal ions on ionimprinted simultaneous interpenetrating network hydrogels, Des. Monomers Polym. 17 (2014) 19–25. [17] J. Wang, F. Liu, UV-radiation curing of simultaneous interpenetrating polymer network hydrogels for enhanced heavy metal ion removal, Mater. Sci. Eng. B 177 (2012) 1633–1640. [18] J. Wang, X. Li, Preparation and characterization of interpenetrating polymer network silicone hydrogels with high oxygen permeability, J. Appl. Polym. Sci. 116 (2010) 2749–2757. [19] X. Hu, Y. Li, Y. Wang, X. Li, H. Li, X. Liu, P. Zhang, Adsorption kinetics, thermodynamics and isotherm of thiacalix[4]arene-loaded resin to heavy metal ions, Desalination 259 (2010) 76–83. [20] H. Liu, F. Yang, Y. Zheng, J. Kang, J. Qu, J.P. Chen, Improvement of metal adsorption onto chitosan/Sargassum sp. composite sorbent by an innovative ion-imprint technology, Water Res. 45 (2011) 145–154. [21] M. Shamsipur, A. Besharati-Seidani, Synthesis of a novel nanostructured ionimprinted polymer for very fast and highly selective recognition of copper(II) ions in aqueous media, React. Funct. Polym. 71 (2011) 131–139. [22] S. Chatterjee, S.H. Woo, The removal of nitrate from aqueous solutions by chitosan hydrogel beads, J. Hazard. Mater. 164 (2009) 1012–1018. [23] J. Wu, H.Q. Yu, Biosorption of 2,4-dichlorophenol from aqueous solution by phanerochaete chrysosporium biomass: isotherms, kinetics and thermodynamics, J. Hazard. Mater. 137 (2006) 498–508. [24] N.T. Hoai, D. Kim, Synthesis, Structure, and selective separation behavior of copper-imprinted microporous polymethacrylate beads, AIChE J. 55 (2009) 3248–3254.
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Adsorption of copper ions by ion-imprinted simultaneous interpenetrating network hydrogel: Thermodynamics, morphology and mechanism Jingjing Wang ∗ , Liang Ding, Jun Wei, Fang Liu Department of Polymer Materials and Engineering, School of Material Engineering, Key Laboratory for Ecological-Environment Materials of Jiangsu Province, Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 21 October 2013 Received in revised form 10 March 2014 Accepted 17 March 2014 Available online xxx Keywords: Ion-imprinted hydrogel Interpenetrating polymer network (IPN) Metal ion adsorption Thermodynamics Mechanism
a b s t r a c t Cu(II) ion-imprinted hydrogel [Cu(II)-IIH] with interpenetrating polymer network (IPN) structure was prepared and its application to adsorb Cu(II) ions from aqueous solution was studied. The Cu(II)-IIH was prepared by UV-initiated simultaneous free radical/cationic hybrid polymerization. The adsorption capacity of the Cu(II)-IIH increased with the initial pH value of the solution, but decreased as the temperature rose from 303 to 323 K. Thermodynamic parameters such as the Gibbs free energy (G◦ ), enthalpy (H◦ ) and entropy (S◦ ) for the Cu(II) ions adsorption were evaluated. It was suggested that the adsorption was a spontaneous, exothermic process with further decrease in the degree of freedom at the solid-solution interface due to the negative S◦ value. The morphology study indicated that the copper adsorption caused significant changes to the hydrogel structure. Finally the adsorption mechanism was studied by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The results indicated that copper adsorption was mainly through interactions with the amide and ether groups. © 2014 Elsevier B.V. All rights reserved.
Introduction Environmental contamination by toxic metal ions in aquatic ecosystems has been increasing with industrial growth and development in recent years [1]. Among all toxic metals, copper could cause serious environmental problems because it is widely used in paints and pigments, paper and pulp, fertilizer manufacturing, copper polishing, etc. Excessive uptake of copper can cause a lot of serious health problems such as damage of liver and kidney, anemia, immunotoxicity, etc. [2,3]. Therefore, in order to protect human health and the environment, it is very important to remove or minimize excess of copper in industrial effluents before discharging it into surface water, as well as into groundwater. Compared with the conventional physicochemical technologies, the adsorption is demonstrated to be promising for heavy metal removal because of its low cost, high adsorption efficiency for low concentration metal ions [4,5]. It is based on utilization of various sorbents, such as agricultural wastes, biomass, clays,
∗ Corresponding author. Tel.: +86 051588298872. E-mail address: [email protected] (J. Wang).
hydroxyapatite, silica gels and zeolites, as well as natural and synthetic polymers [6–8]. Nowadays, a special attention has been given to hydrogels which possess ionic functional groups that can absorb and trap metal ions from wastewater [9–11]. Compared with conventional solid sorbents, the main advantages of hydrogels are desired tailored swelling and mechanical properties, easy handling and reusability [11]. Taking into account all the hydrogel adsorbents reported in the literature, the hydrogels based on ion imprinting technology have received considerable attention [12]. Ion-imprinted hydrogels are a class of materials with predetermined selectivity for analytical separation. In the process of ion imprinting, a functional monomer and a crosslinker are polymerized in the presence of a template ion. Then the template is extracted leaving sites which are complementary in both shape and chemical functionality to those of the template [13]. This hydrogel then becomes capable of selectively adsorbing the template species. Such an imprinted polymer shows an affinity for the template ion over other coexisting metal ions. To develop an adsorption process by using the ion-imprinted hydrogels for industrial applications, it is desirable to better understand the metal uptake process. Advanced instruments such as X-ray photoelectron spectroscopy (XPS) and Fourier transform
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infrared (FTIR) spectroscopy can provide good tools to achieve the goal because they can provide insight into environmental processes at the molecular level [14,15]. In our previous study [16], simultaneous interpenetrating network technique was employed to prepare Cu(II) ion-imprinted hydrogel [Cu(II)-IIH] with improved mechanical strength. The adsorption kinetics and isotherm, selective adsorption ability and reusability were investigated. The present work was in continuation to our previous study. In the current report, the Cu(II)-IIH was characterized using thermogravimetric analysis (TGA) and scanning electron microscopy/energy dispersive X-ray (SEM/EDX). Several factors affecting the sorption of Cu(II) ions, such as pH, initial concentration and temperature were investigated. Finally the adsorption mechanism was better elucidated via FTIR in combination of XPS.
Thermogravimetric analysis The thermal stability was evaluated by thermogravimetry using a TA Instruments TGA 2950 thermogravimetric analyzer in the temperature range of 30–800 ◦ C, at a scanning rate of 10 ◦ C/min. Surface morphology study SEM/EDX was used to investigate the cross-section morphology of the prepared hydrogels. Measurements were taken on a FEI QUANTA 200 scanning electron microscope. Before SEM measurements, hydrogels were lyophilized. Then the freeze-dried samples were fractured carefully in liquid nitrogen, and then fixed on tubs with sputter coated with gold before observation. Adsorption mechanism study
Experimental Materials Acrylamide (AAm), ethylene glycol dimethacrylate (EGDMA) and triethylene glycol divinyl ether (DVE-3) were purchased from Sigma–Aldrich Chemicals and used without further purification. Free radical photoinitiator (Darocur 1173 from Ciba Specialty Chemicals) and a diaryliodonium hexauorophosphate salt (DAI from Ciba Specialty Chemicals) were respectively used to initiate the free radical and cationic polymerization. Copper nitrate [Cu(NO3 )2 ·3H2 O)] was purchased from Shanghai Chemical Reagents Co., and was heated in order to remove crystal water before used.
Preparation of Cu(II) ion-imprinted hydrogel [Cu(II)-IIH] The Cu(II)-IIH was prepared according to our previous report [16]. Briefly, a mixture of 0.96 g AAm, 1.44 g DVE-3, and 1.86 g Cu (NO3 )2 were first dissolved in ethanol. Then, 0.048 g crosslinker EGDMA, 0.0192 g free radical photoinitiator, and 0.0288 g cationic photoinitiator were added. The mixture was introduced between two glass plates and cured under a high-pressure mercury lamp for 1 h. Subsequently, the film was treated with 1.0 mol/L HCl to completely leach Cu(II) ions. At last, the hydrogel was washed to neutralization, and dried at 60 ◦ C under vacuum, resulting in the desired Cu(II)-IIH.
Adsorption capacity study Adsorption study was carried out in magnetically stirred, thermostated cylindrical glass vessels in batch conditions. The sample was added into the heavy metal ion solution to determine the metal ion adsorption capacity of the samples under non-competitive conditions. The pH of the metal feed solutions was adjusted before the sample was applied for the adsorption process. Amount of the residual metal ion in the solution was determined using an Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) after 24 h [11]. Amount of adsorbed metal ion (Q, mmol/g) was calculated from the following equation: Q =
(C0 − C)V m
where C0 (mmol/L) and C (mmol/L) were the metal ion concentrations before and after adsorption respectively. V (L) was the volume of the solution added and m (g) was the mass of the xerogel.
FTIR spectra were recorded on a Bruker Vector 22 FT spectrometer in the range 4000–400 cm−1 . XPS spectra were recorded by a Perkin-Elmer PHI 550 ESCA/SAM spectrometer using Mg K␣ X-ray source. Results and discussion Preparation of Cu(II) ion-imprinted hydrogel [Cu(II)-IIH] Free radical/cationic hybrid polymerization was usually employed to prepare simultaneous interpenetrating polymer network (IPN) materials [17,18]. In the present work, template Cu(II) ions firstly coordinated to the NH2 from acrylamide (AAm) and O from triethylene glycol divinyl ether (DVE-3). Then the Cu(II)-IIH was synthesized by UV-initiated free radical/cationic hybrid polymerization of complexes Cu(II)/AAm and Cu(II)/DVE-3 with EGDMA as a crosslinker. In the polymerization system, Cu(II)/AAm polymerized by free radical mechanism, and Cu(II)/DVE-3 polymerized by cationic mechanism simultaneously. Finally Cu(II) ions were leached from the hydrogel leaving behind some cavities with predetermined orientation and special size of templates. The obtained Cu(II)-IIH consisted of two polymer networks. One was poly(acrylamide) (PAAm), and the other was poly(triethylene glycol divinyl ether) poly(DVE-3). The proposed preparation process was shown in Fig. 1. Adsorption equilibrium study Effect of pH on adsorption Metal ion sorption on sorbent is influenced by pH value due to the competition between the metal ions and H+ for active sorption sites, so the pH value is one of the most important factors affecting the sorption of Cu(II) ions from aqueous solutions. As shown in Fig. 2, the removal percentage of metal ions increased sharply along with the increase of the pH value. That was to say, the adsorption capacity of the Cu(II)-IIH to Cu(II) ions in weak acidic solution was higher than that in strong acidic solution. The reason could be explained that, under strong acidic conditions, the Cu(II)-IIH surface was completely covered with H+ , and Cu(II) ions could not compete with them for adsorption sites [19]. However, with the increase of pH value, the competition from H+ decreased and Cu(II) ions could be adsorbed on the adsorbent quite easily. The similar results were obtained by other researchers as reported elsewhere [20,21]. Experiments were not carried out at pH values above 5.0 due to the fact that Cu(II) ions precipitation occurred at higher pH values. In order to avoid the formation of precipitation in the aqueous solution, the pH of 5.0 was selected as the initial pH value of Cu(II) ions solution for subsequent adsorption experiment.
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Removal percent, %
100 80 60 40 20 0
2.5
4.5 3.5 pH value
5
Fig. 2. Influence of pH values on the adsorption capacity of Cu(II)-IIH.
Effect of temperature on adsorption The adsorption amount of Cu(II) ions from its aqueous solution with various concentrations at 303, 313 and 323 K is shown in Fig. 3. The results clearly showed that with an increase in the initial metal ion concentration, the adsorption capacity of the Cu(II)-IIH increased significantly. It was illustrated that the adsorption sites on the sorbent were sufficient at low concentration, and the amount of adsorption in this case was dependent on the number of metal ions that were transported from the bulk solution to the surface of the hydrogel. With the increase of the metal ion concentration, adsorption sites were gradually occupied and the adsorption
Cu(II) ion adsorption capacity, mmol/g
Fig. 1. Scheme for the preparation of Cu(II) ion-imprinted hydrogel [Cu(II)-IIH].
303 K 0.6
313 K 323 K
0.5
0.4
0.3 0.000
0.001
0.002
0.003
0.004
0.005
Initial concentration of Cu(II) ions, mol/L Fig. 3. Effect of initial Cu(II) ion concentration on the adsorption capacity of Cu(II)IIH at 298, 308 and 318 K.
amount achieved a saturated value [22]. The maximum sorption capacity was 0.64 mmol/g. It was reported that the sorption capacities of Cu(II) ions on ion-imprinted chitosan/Sargassum sp. and polymeric nanoparticles were 1.08 and 0.346 mmol/g [20,21]. The sorption capacity of Cu(II)-IIH was compared with those of other sorbents in Table 1. It was clearly indicated that the ion-imprinted hydrogel sorbent developed in this study had a great potential for the treatment of heavy metal wastewater from various industries. From Fig. 3, it was also found that the equilibrium adsorption capacity decreased when the temperature increased from 303 to
Table 1 Comparison of the developed sorbent with others reported in the literature. Matrix
Metal ions
Sorption capacity (mmol/g)
Reference
Support: ion-imprinted polymers acrylamide and triethylene glycol divinyl ether Chitosan/Sargassum sp. composite 1,4-Dihydroxy-9,10-anthraquinone Support: calcined phosphate Support: hazelnut shell activated carbon Support: 717 anion exchange resin thiacalix[4]arenetetrasulfonate
Cu2+ Cu2+ Cu2+ Pb2+ /Cu2+ /Zn2+ Cu2+ Cu2+ /Pb2+ /Cd2+
0.64 1.08 0.346 0.414/0.466/0.317 0.91 0.334/0.231/0.401
This work [20] [21] [7] [8] [19]
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0.0
1.7
303 K 313 K
-0.5
1.6
-1.0
1.5
ln K
ln qe/Ce, L/g
323 K
-1.5
1.4 1.3 1.2
-2.0
1.1 0.00312
-2.5 15
20
25
30
35
0.00318
qe, mg/g
0.00324
0.00330
1/T
40
Fig. 5. Plot of ln K versus 1/T for the adsorption of Cu(II) ions by the Cu(II)-IIH.
Fig. 4. Plots of ln (qe /Ce ) as a function of qe for the adsorption of Cu(II) ions by the Cu(II)-IIH.
100 80 Weight, %
323 K. This result indicated the exothermic nature of Cu(II) ions adsorption onto the Cu(II)-IIH. The obtained results in the current research were in agreement with the previous report [19]. A decrease in the uptake value with the rise in temperature may be due to either the serious influence on ligand coordination geometry of the adsorbent or increasing tendency to desorb Cu(II) ions from the interface to the solution [22].
60 40 20 0
Thermodynamic parameters of adsorption
0
200
400
600
800
o
Temperature, C
As related to temperature effect, the thermodynamic parameters have been calculated for this adsorption system. The free
Fig. 6. TG curves for the Cu(II)-IIH.
Fig. 7. SEM images of the Cu(II)-IIH before (a) and after (b) adsorption; EDX spectra for the free (a ) and copper-loaded (b ) Cu(II)-IIH.
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J. Wang et al. / Applied Surface Science xxx (2014) xxx–xxx Table 2 Thermodynamic parameters of Cu(II) ions adsorption onto the Cu(II)-IIH. Temperature 303 K 313 K 323 K
ln K
K
G◦ (kJ/mol)
1.664 1.378 1.145
5.280 3.967 3.142
−4.192 −3.586 −3.075
H◦ (kJ/mol)
entrapment between the crosslinked networks of poly(AAm) and poly(DVE-3) took place [17].
S◦ (J/mol K) −55.9 −56.1 −55.9
−21.131
energy change of adsorption G◦ was calculated by using the equation: G◦ = −RT ln K where R was the universal gas constant and T was the Kelvin temperature. K was the thermodynamic equilibrium constant for the adsorption process. It was determined using the method of Wu and Yu [23] by plotting ln (qe /Ce ) versus qe at 303, 313, and 323 K and extrapolating to zero qe as suggested (Fig. 4). The other thermodynamic parameter, such as the enthalpy change H◦ was calculated from the slope of the straight line which was plotted by ln K against 1/T (Fig. 5) according to the equation: −H ◦ ln K = + constant RT H◦
The negative value of as shown in Table 2 suggested that the adsorption process was exothermic. The standard change in entropy can be calculated from the following equation: S ◦ =
5
Morphology study SEM images of free and copper-loaded Cu(II)-IIH are shown in Fig. 7. The Cu(II)-IIH exhibited three-dimensional porous structure before adsorption (Fig. 7a). While the copper adsorption caused significant changes to the hydrogel structure. The copperloaded Cu(II)-IIH became rougher and the pore size became smaller (Fig. 7b). In order to confirm the adsorption of Cu(II) ions onto the hydrogel, EDX spectra of the unloaded and copper-loaded Cu(II)-IIH were investigated (Fig. 7a and b ). The EDX spectrum for the Cu(II)-IIH before adsorption did not show the characteristic peak of copper. On the other hand, the peak of copper was clearly shown in the EDX spectrum of copper-loaded Cu(II)-IIH. Therefore, the existence of Cu(II) ions on the hydrogel was confirmed by EDX spectra. Adsorption mechanism FTIR spectrum has been widely used for the structural investigation of an adsorbent because it can provide considerable insight into the various functional groups in the adsorbent. As an important method for chemical characterization, FTIR spectrum has also been used to investigate the interactions between adsorbent and metals ions.
H ◦ − G◦ T
The negative value of S◦ might be associated with the adsorption of Cu2+ to the adsorbent, which resulted in a decrease in freedom degree of the systems during the adsorption [23].
O 1s
(a)
C 1s
Thermogravimetric analysis Thermogravimetric analysis is useful in obtaining enough information on the relative thermal stability, and it helps with the selection of materials for analytical purposes focused on adsorption studies. As shown in Fig. 6, according to the thermogravimetric analysis, the Cu(II)-IIH supported a 65% weight loss at about 30–100 ◦ C, which was ascribed to the removal of physically adsorbed water. During the next stage of heating (200–440 ◦ C), a 28% weight loss was observed, which was attributed to the amide bond decomposition and triethylene glycol divinyl ether segment degradation. The high thermal stability might be explained by the interpenetrating network structure, where the physical
N 1s
0
200
400
600
800
1000
Binding energy, eV
(b)
a
Cu 2p
O 1s Transmittance, %
1119 1643
1111
C 1s
b
N 1s
1608
3454 1385 3415 0
1000
2000
3000
4000 -1
Wavenumber, cm
Fig. 8. FTIR spectra of (a) free and (b) copper-loaded Cu(II)-IIH.
0
200
400
600
800
1000
Binding energy, eV Fig. 9. XPS wide-scan spectra of (a) Cu(II)-IIH and (b) copper-loaded Cu(II)-IIH.
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(e)
(c)
(a) C-O
C-N
(285.4)
C-C
(399.4)
C-O
(284.3)
C=O
C=O
(532.5)
(531.5)
(286.3)
C-N (287.6)
282
284 286 288 Binding energy, eV
290
394
396
398 400 402 Binding energy, eV
C-C
528
530 532 534 Binding energy, eV
C-N
C-O
C-O
(400.0)
(286.3)
536
(f)
(d)
(b)
404
(533.0)
C=O
(284.3)
C=O
(532.1)
(287.2)
C-N (288.6)
282
285 288 Binding energy, eV
291
396
398
400 402 Binding energy, eV
404
528
530 532 534 536 Binding energy, eV
538
Fig. 10. XPS spectra of (a) C 1s of Cu(II)-IIH; (b) C 1s of copper-loaded Cu(II)-IIH; (c) N 1s of Cu(II)-IIH; (d) N 1s of copper-loaded Cu(II)-IIH; (e) O 1s of Cu(II)-IIH; (f) O 1s of copper-loaded Cu(II)-IIH.
Fig. 8 shows the FTIR spectra of the free and copperloaded Cu(II)-IIH. Several peaks, characteristic for poly(AAm) and poly(DVE-3), were observed in the spectrum of the free hydrogel. A characteristic peak from poly(AAm) appeared at 1643 cm−1 , which was assigned to the C O stretching vibration. The intense band around 3454 cm−1 should be assigned to the stretching vibration of N H. The bands which appeared at 1119 cm−1 corresponded to the asymmetric stretching vibration of the C O C in poly(DVE-3). After adsorption, the bands observed at 1385 cm−1 , assigned to the stretching vibration of O Cu, confirmed that Cu(II) ions were really adsorbed onto the hydrogel [24]. In addition, the absorption intensity and frequency of some bands changed. A broad peak at 3415 cm−1 was caused by the symmetrical vibration of NH2 groups, indicating that the groups were possibly involved in the sorption. As known, the metal ions with empty orbital could be capable of accepting electron pairs, while the NH2 groups having non-shared electron pairs, could donate their electron pairs. The strong absorption band at 1608 cm−1 , assigned to C O stretching vibration from poly(AAm), was obviously weakened. This behavior reflected the interaction between Cu(II) ions and C O. The absorption band at 1119 cm−1 , which corresponded to the absorption bands of C O C groups, was also shifted to lower wave numbers (1111 cm−1 ). According to the observed changes in FTIR spectra of the free and copper-loaded hydrogels, it seemed that NH2 , C O C and C O groups were involved in the Cu(II) ions adsorption. XPS was further performed to elucidate the adsorption mechanism. Binding energy profiles of Cu 2p3/2 , N 1s, C 1s and O1s in Cu(II)-IIH before and after copper sorption are shown in Fig. 9. Comparing the wide scans results of Cu(II)-IIH and copper-loaded Cu(II)-IIH, the spectrum of Cu 2p3/2 at 934.0 eV can be clearly observed in the latter one, indicating the accumulation of copper on Cu(II)-IIH. The C 1s spectra of Cu(II)-IIH and copper-loaded Cu(II)-IIH comprised four peaks with differentiated BE values via deconvolution,
as shown in Fig. 10a and b. These peaks could be assigned to C C, C O, C O and C N bonds, respectively. Among them, the C O, C O and C N bonds could be assigned to ether, amino and carbonyl groups. After copper adsorption, the binding energy of C O, C O and C N shifted from 285.4, 286.3 and 287.6 eV to 286.3, 287.2 and 288.6 eV, respectively. This result indicated that carbonyl, ether and amino groups in Cu(II)-IIH got involved in copper sorption, in which oxygen and nitrogen atoms donated electrons to metal ions and thus the electron density at the adjacent carbon atom decreased. Fig. 10c and d revealed that the N 1s bands in copper-loaded Cu(II)-IIH increased from 399.4 eV and 400.0 eV, indicating the formation of R-NH2 Cu2+ complexes, in which a pair of lone electrons from the N atoms was shared with the Cu2+ , and hence the electron cloud density of the nitrogen atom was reduced, resulting in a higher BE peak. Seen from Fig. 10e and f, the O 1s peaks of CS and copper-loaded Cu(II)-IIH comprised two peaks: C O and C O. The BE values of C O and C O groups increased from 531.5 and 532.5 eV to 532.1 and 533.0 eV, respectively. Therefore, copper sorption was followed by charge transfer from these groups to copper ions, which confirmed that C O and C O groups acted as the sorption sites. In general, the XPS spectral changes in the C 1s, N 1s, and O 1s peaks of free and copper-loaded Cu(II)-IIH were consistent with the observations in FTIR analysis. Conclusions In the present study, Cu(II) ion-imprinted hydrogel [Cu(II)-IIH] with interpenetrating polymer network (IPN) structure was prepared by free radical/cationic hybrid photopolymerization. The adsorption capacity of the Cu(II)-IIH increased with the initial pH value of the solution, and the maximum adsorption capacity was observed at pH 5.0. It was also found that the equilibrium adsorption capacity decreased when the temperature increased from 303 K to 323 K. From thermodynamic parameters, it was
Please cite this article in press as: J. Wang, et al., Adsorption of copper ions by ion-imprinted simultaneous interpenetrating network hydrogel: Thermodynamics, morphology and mechanism, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.102
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concluded that the adsorption was a spontaneous, exothermic process. Thermogravimetric analysis indicated the Cu(II)-IIH exhibited high thermal stability due to the interpenetrating network structure. The Cu(II)-IIH exhibited three-dimensional porous structure before adsorption. The copper-loaded Cu(II)-IIH became rougher and the pore size became smaller. The FTIR and XPS studies revealed that NH2 , C O and C O groups were involved in the sorption of copper and complexation dominated the sorption process. Acknowledgements The project was supported by Jiangsu Provincial Natural Science Foundation (No. BK2012251), Open Project of Key Laboratory for Ecological-Environment Materials of Jiangsu Province (No. EML201202), research fund of Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (No. AE201069) and Research Fund of Yancheng Institute of Technology (No. XKY2013062). References [1] J. Gong, X. Wang, X. Shao, S. Yuan, C. Yang, X. Hu, Adsorption of heavy metal ions by hierarchically structured magnetite-carbonaceous spheres, Talanta 101 (2012) 45–52. [2] S. Sun, A. Wang, Adsorption properties of carboxymethyl-chitosan and crosslinked carboxymethyl-chitosan resin with Cu(II) as template, Sep. Purif. Technol. 49 (2006) 197–204. [3] H. Qiu, S. Zhang, B. Pan, W. Zhang, L. Lv, Effect of sulfate on Cu(II) sorption to polymer-supported nano-iron oxides: behavior and XPS study, J. Colloid Interface Sci. 366 (2012) 37–43. [4] R.S. Azarudeen, R. Subha, D. Jeyakumar, A.R. Burkanudeen, Batch separation studies for the removal of heavy metal ions using a chelating terpolymer: synthesis, characterization and isotherm models, Sep. Purif. Technol. 116 (2013) 366–377. [5] Z.H. Xiao, R. Zhang, X.Y. Chen, X.L. Li, T.F. Zhou, Magnetically recoverable Ni@carbon nanocomposites: solid-state synthesis and the application as excellent adsorbents for heavy metal ions, Appl. Surf. Sci. 263 (2012) 795–803. [6] A. Demirbas, Heavy metal adsorption onto agro-based waste materials: a review, J. Hazard. Mater. 157 (2008) 220–229. [7] A. Aklil, M. Mouflih, S. Sebti, Removal of heavy metal ions from water by using calcined phosphate as a new adsorbent, J. Hazard. Mater. 112 (2004) 183–190.
7
[8] E. Demirbas, N. Dizge, M.T. Sulak, M. Kobya, Adsorption kinetics and equilibrium of copper from aqueous solutions using hazelnut shell activated carbon, Chem. Eng. J. 148 (2009) 480–487. [9] N. Kawasaki, H. Tominaga, F. Ogata, K. Kakehi, Removal of cadmium and copper by vegetable biomass treated with hydrochloric acid, Chem. Eng. J. 157 (2010) 249–253. [10] V. Bekiari, M. Sotiropoulou, G. Bokias, P. Lianos, Use of poly(N,N-dimethyl acrylamide-co-sodium acrylate) hydrogel to extract cationic dyes and metals from water, Colloids Surf. A: Physicochem. Eng. Asp. 312 (2008) 214–218. [11] H. Kas¸göz, A. Durmus¸, A. Kas¸göz, Enhanced swelling and adsorption properties of AAm-AMPSNa/clay hydrogel nanocomposites for heavy metal ion removal, Polym. Adv. Technol. 19 (2008) 213–220. [12] M.G. Segatelli, V.S. Santos, A.B.T. Presotto, I.V.P. Yoshida, C.R.T. Tarley, Cadmium ion-selective sorbent preconcentration method using ion imprinted poly(ethylene glycol dimethacrylate-co-vinylimidazole), React. Funct. Polym. 70 (2010) 325–333. [13] L. Zhang, S. Yang, T. Han, L. Zhong, C. Ma, Y. Zhou, X. Han, Improvement of Ag(I) adsorption onto chitosan/triethanolamine composite sorbent by an ionimprinted technology, Appl. Surf. Sci. 263 (2012) 696–703. [14] L. Dambies, C. Guimon, S. Yiacoumi, E. Guibal, Characterization of metal ion interactions with chitosan by X-ray photoelectron spectroscopy, Colloids Surf. A: Physicochem. Eng. Asp. 177 (2001) 203–214. [15] J.P. Chen, L. Yang, Study of a heavy metal biosorption onto raw and chemically modified Sargassum sp. via spectroscopic and modeling analysis, Langmuir 22 (2006) 8906–8914. [16] J. Wang, F. Liu, Enhanced and selective adsorption of heavy metal ions on ionimprinted simultaneous interpenetrating network hydrogels, Des. Monomers Polym. 17 (2014) 19–25. [17] J. Wang, F. Liu, UV-radiation curing of simultaneous interpenetrating polymer network hydrogels for enhanced heavy metal ion removal, Mater. Sci. Eng. B 177 (2012) 1633–1640. [18] J. Wang, X. Li, Preparation and characterization of interpenetrating polymer network silicone hydrogels with high oxygen permeability, J. Appl. Polym. Sci. 116 (2010) 2749–2757. [19] X. Hu, Y. Li, Y. Wang, X. Li, H. Li, X. Liu, P. Zhang, Adsorption kinetics, thermodynamics and isotherm of thiacalix[4]arene-loaded resin to heavy metal ions, Desalination 259 (2010) 76–83. [20] H. Liu, F. Yang, Y. Zheng, J. Kang, J. Qu, J.P. Chen, Improvement of metal adsorption onto chitosan/Sargassum sp. composite sorbent by an innovative ion-imprint technology, Water Res. 45 (2011) 145–154. [21] M. Shamsipur, A. Besharati-Seidani, Synthesis of a novel nanostructured ionimprinted polymer for very fast and highly selective recognition of copper(II) ions in aqueous media, React. Funct. Polym. 71 (2011) 131–139. [22] S. Chatterjee, S.H. Woo, The removal of nitrate from aqueous solutions by chitosan hydrogel beads, J. Hazard. Mater. 164 (2009) 1012–1018. [23] J. Wu, H.Q. Yu, Biosorption of 2,4-dichlorophenol from aqueous solution by phanerochaete chrysosporium biomass: isotherms, kinetics and thermodynamics, J. Hazard. Mater. 137 (2006) 498–508. [24] N.T. Hoai, D. Kim, Synthesis, Structure, and selective separation behavior of copper-imprinted microporous polymethacrylate beads, AIChE J. 55 (2009) 3248–3254.
Please cite this article in press as: J. Wang, et al., Adsorption of copper ions by ion-imprinted simultaneous interpenetrating network hydrogel: Thermodynamics, morphology and mechanism, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.102