Licheng Ma, Qi Zheng

1 State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

2 Rare Metals and Metallurgy Materials Research Institute, General Research Institute for Nonferrous Metals, Beijing 100008, China

Keywords:Ion imprinted techniques Selective recognition Ion-imprinted magnetic chitosan beads Kinetic Thermodynamics

A B S T R A C T Heavy metal ion is one of the major environmental pollutants.In this study,a Cu(II)ions imprinted magnetic chitosan beads are prepared to use chitosan as functional monomer, Cu(II) ions as template, Fe3O4 as magnetic core and epichlorohydrin and glutaraldehyde as crosslinker, which can be used for removal Cu(II) ions from wastewater. The kinetic study shows that the adsorption process follows the pseudosecond-order kinetic equations.The adsorption isotherm study shows that the Langmuir isotherm equation best fits for the monolayer adsorption processes. The selective adsorption properties are performed in Cu(II)/Zn(II),Cu(II)/Ni(II),and Cu(II)/Co(II)binary systems.The results shows that the IIMCD has a high selectivity for Cu(II) ions in binary systems. The mechanism of IIMCD recognition Cu(II) ions is also discussed. The results show that the IIMCD adsorption Cu(II) ions is an enthalpy controlled process. The absolute value of ΔH (Cu(II)) and ΔS(Cu(II)) is greater than ΔH (Zn(II), Ni(II), Co(II)) and ΔS (Zn(II), Ni(II), Co(II)), respectively, this indicates that the Cu(II) ions have a good spatial matching with imprinted holes on IIMCD. The FTIR and XPS also demonstrates the strongly combination of function groups on imprinted holes in the suitable space position. Finally, the IIMCD can be regenerated and reused for 10 times without a significantly decreasing in adsorption capacity. This information can be used for further application in the selective removal of Cu(II) ions from industrial wastewater.

1. Introduction

As the rapid growth of industry and increase the human population, the water pollution has become a major problem of the word [1-3]. Heavy metal ions are one of the most contaminations in water [4-6], because they can serious damage to the ecological and threaten human health even at low concentrations[7,8]. Copper and its compounds are common pollutants in wastewater.The potential sources of copper from industrial effluents include mineral processing, metal fabrication, mechanical manufacturing [9-11]. Although copper is one of the essential elements for human body, excessive intake of copper results in an accumulation in human body [12,13]. Therefore, the separation and purification of copper in wastewater needs much more attention [14-17].Adsorption has considered to be the most effectively methods for removal metal ions from aqueous solution [18]. Attentions has been focused on preparation adsorbents which have a high adsorption capacity for the removal of metals from contaminated water.

Molecular imprinting technique is a strategy for producing selectivity recognition sites in synthetic materials [19,20]. In the process of molecular imprinting,appropriate functional monomers are introduced to interact with template molecules, and then the functional groups on the monomers are fixed with chemical crosslinker. Finally, after removing the template molecule from the imprinted polymers, the recognition sites can be used to bind the template or its analogue selectively. Ion imprinting polymers are similar to molecular imprinting polymers, but they recognize metal ions after imprinting and retain all virtues of molecular imprinting polymers[21-23].Its recognition is based on the specification of the ligand, on the coordination geometry, coordination number, charge, and size of the ions.

Chitosan is a sort of alkaline polysaccharide with natural polymers, which has a rich yield, low price, biocompatibility,biodegradability. It has both amino and hydroxyl can be chelated with heavy metal ions [24,25]. Thus, chitosan has widely application prospects in concentration of heavy metal ions,environmental protection and wastewater treatment. Several cross-linking methods such as cross-linked,modified,graft have been used to modify both the chemical and physical properties of chitosan to improve its chemical stability,mechanical strength,hydrophilicity and biocompatibility [26-29]. Therefore, chitosan as a functional monomer can be used for removal heavy metal ions from wastewater.

In this study, a Cu(II) ions imprinted magnetic chitosan were prepared to use chitosan as functional monomer, Cu(II) ions as template, Fe3O4as magnetic core and epichlorohydrin and glutaraldehyde as crosslinker, which are used for removal of Cu(II)ions from aqueous solutions. As magnetic property, Fe3O4is used in Cu(II) ions imprinted polymer, which can easy separation of adsorbents from the system after the adsorption, and the adsorbents could be reused for further application [30,31]. Then, the adsorptions behaviors including Kinetics, isotherms and regeneration were performed. The effect of ionic strength and selective adsorptions behaviors were performed in Cu(II)/Zn(II), Cu(II)/Ni(II), and Cu(II)/Co(II) binary systems. Finally, recognition mechanisms of Cu(II) ions imprinted magnetic chitosan beads were presented and discussed.

2. Materials and Methods

2.1. Materials and instruments

Chitosan with a deacetylation 80%-95% was purchased from Sinopharm Chemical Reagent Co. Ltd (China), Formaldehyde,Epichlorohydrin (ECH), Glutaraldehyde (GLA) Cupric sulfate,Sodium Hydroxide,Sulfuric acid, Zinc sulfate, Cobalt sulfate, Nickelous sulfate and other chemicals were purchased from Beijing Chemical Works (China), all the chemicals were of analyticalreagent graded. Fe3O4was prepared from our laboratory.

The IR spectra were measured by FTIR (Nexus 670, USA). Ion concentration was measured by inductively coupled plasma spectrometer(ICAP-9000,USA),pH-values were measured with a PHSC pH-meter meter(Shanghai,China).The binding energy was measured by X-ray photoelectron spectroscopy (ESCALAB MK II, UK).

2.2. Preparation of ion-imprinted magnetic chitosan beads (IIMCD)

2 g chitosan was dissolved in 80 ml of acetic acid (2%, volume)and stirred at room temperature. 0.2 g Fe3O4was added slowly to the solution and stirred for 0.5 h.The mixture solution was sprayed into 2 mol NaOH,which neutralized the acetic acid within the chitosan gel thereby coagulated the chitosan gel to spherical uniform magnetic chitosan beads. The obtained magnetic chitosan beads were added to a 200 ml aqueous solution of formaldehyde (10%).The mixture solution then stirred for 3 h at 50-60 °C. Then 5 ml of 0.8 mmol epichlorohydrin solution was added to complete the crosslinking reaction. The process was followed by filtering and intensive washing of the crosslinked magnetic chitosan beads with distilled water to remove an unreacted formaldehyde and epichlorohydrin. After vacuum filtration, the crosslinked magnetic chitosan beads were again stirred with 1 mol H2SO4solution to remove the formaldehyde. The crosslinked magnetic chitosan beads were extensively rinsed with distilled water to remove any H2SO4, filtered and air dried to remove the water from the pore structure.

The procedure of Cu(II) ions imprinted crosslinked magnetic chitosan beads were given as follow:The crosslinked magnetic chitosan beads were added to a 200 ml aqueous solution of 1000 mg·L-1CuSO4solution, then stirred for 8 h at room temperature in order to reach adsorption equilibrium. After filtering this solution, the obtained Cu(II) ions magnetic chitosan beads were added to a 200 ml aqueous solution of 0.25 mol GLA and stirred for 3 h at 70°C.When vacuum filtration,the crosslinked magnetic chitosan beads were washed with 1 mol H2SO4solution and distilled water several times to remove the Cu(II) ions until the Cu(II) ions was not detected.The Cu(II)ions imprinted magnetic chitosan beads were dried in a vacuum oven for 6 h. the resulting material produced Cu(II)ions imprinted crosslinked magnetic chitosan beads with diameters about 1 mm.

The FTIR spectroscope of complex formation between functional monomer and crosslinking in the process of preparation was shown in Fig. 1. It can be seen from Fig. 1a, the IR spectrum of Fe3O4has shown the characteristic absorption peak at 3200-3400 cm-1, which can be assigned to O-H stretching vibrations.It indicated that certain hydroxyl existed on the surface of Fe3O4.Characteristic absorbance peaks at 565.77 cm-1were ascribed to Fe-O stretching vibration. The Fig. 1b of the chitosan showed absorption at 3423.06 cm-1, assigned to the O-H and N-H stretching vibration, 2912.30 cm-1and 2875.86 cm-1ascribed to C-H stretching vibration, respectively. 1421.98 cm-1ascribed to C-O stretching vibration, 1600.43 cm-1ascribed to N-H stretching vibration, 1030.16 cm-1and 1087.79 cm-1ascribed to O-H stretching vibration.Comparing with Fig. 1c,the intensity of characteristic adsorption peaks at 3423.06 cm-1was weakened, however, 1600.43 cm-1and 1030.16 cm-1absorption peaks were decreased significantly and shifted to low frequency, respectively.This could be attributed to the part of amino and hydroxyl groups reacted with Fe3O4and crosslinker. At the same time, the Fe-O bending vibration peaks shifted to the low frequency and weakened. This concluded the Fe3O4was embedded by chitosan.

Fig. 1. FTIR spectra of Fe3O4 (a), chitosan (b) and IIMCD (c).

Non-imprinted magnetic chitosan beads (NIIMCD) were also prepared for the same polymerization condition in the absence of Cu(II) ions.

2.3. Adsorption kinetic study

Adsorption kinetic studies were accomplished by mixing 0.5 g(dry)IIMCD and NIIMCD with 25 ml of 300 mg·L-1Cu(II)ions solution at pH 5.0 shaking at 25 °C. In the process of adsorption, the concentration of metal ions is measured by ICP-AES every 60 min. The adsorption capacity was calculated by Eq. (1).

wherec0andceare the initial and final concentration of metal ions,mg·L-1, respectively.Vis volume of metal ions solution, L.Wis the mass of the adsorbent (dry).

2.4. Adsorption isotherm

Adsorption of metal ions on the IIMCD and NIIMCD were studied in batch experiments. Equilibrium studies of the IIMCD and NIIMCD were performed by 25 ml of Cu(II)ions solution with initial concentration in the range of 50-900 mg·L-1at pH 5.0 shaking at 25°C for 2.0 h.The adsorbents were separated from solution by magnetic separation and the concentrations of metal ions were determined by ICP-AES. The adsorption capacity of metal ions was calculated using Eq. (1).

2.5. Selective sorption study

To measure the selectivity of IIMCD, Batch adsorption studies were conducted in 25 ml of binary systems containing Cu(II)/Zn(II),Cu(II)/Ni(II),and Cu(II)/Co(II)ions,respectively.The concentration of solution was 200 mg·L-1,in each binary systems,the initial concentration of Cu(II)ions in the range of 10-140 mg·L-1,the initial concentration of competitive ions(Zn(II),Ni(II)or Co(II))in the range of 190-600 mg·L-1. After 6 h reach to adsorption equilibrium,the concentration of Cu(II),Zn(II),Ni(II),and Co(II)ions were determined by ICP-AES.The adsorption capacity was calculated by Eq.(1).The distribution coefficients(Kd)was calculated by Eq.(2):

wherec0is the initial concentration of metal ions,ceis the equilibrated concentration of metal ions on the absorbent.The selectivity adsorption coefficient (K) for the binding of Cu(II) ions in the presence of competitor species can be obtained from equilibrium binding data according to the Eq. (3):

whereKis the selectivity coefficient,M represents Ni(II),Zn(II)or Co(II)ions.The selectivity adsorption coefficient of theKvalues of the imprinted beads with those metal ions allows an estimation of the effect of imprinting on selectivity.The relative selectivity coefficient is an indicator to express metal adsorption affinity of recognition sites to the imprinted Cu(II) ions.

The relative selectivity coefficient, which was used to estimate the effect of imprinting on selectivity,can be calculated by Eq.(4):

2.6. Regeneration and reuse

After Cu(II)ions were adsorbed onto the IIMCD,the adsorbed Cu(II) ions IIMCD were desorption by 1.0 mol·L-1Sulfuric acid solution.The Cu(II)ions adsorbed on IIMCD was stirred at 200 r·min-1with 100 ml of desorption solution at room temperature for 2 h.After desorption, IIMCD were washed with distilled water to neutrality and dried in vacuum oven at 60 °C. In order to test the reusability of IIMCD, Cu(II) ions adsorption-desorption procedure was repeated 10 times using the same absorbents.

3. Results and Discussion

3.1. Adsorption kinetics

In order to investigate the mechanism of sorption and potential rate controlling steps, the sorption kinetics for Cu(II) ions on the IIMCD and NIIMCD were presented in Fig. 2. The adsorption rate of IIMCD was faster than NIIMCD. The adsorption capacity increased during the first 120 min and gradually approached to the limiting adsorption capacity after 180 min. This might be attributed to the sorption sites on the IIMCD were gradually covered by the Cu(II)ions,slowing down the ion exchange action from bulk solution.

Fig. 2. Adsorption kinetics of IIMCD and NIIMCD.

The pseudo-first-order and pseudo-second-order kinetics equations are a basic model for describing the adsorption rate.

Pseudo-first-order kinetics equations was obtained from the following Eq. (5)

Pseudo-second-order kinetics equations was obtained from the following Eq. (6)

Qeis equilibrium adsorption capacity, mg·g-1;Qtis the adsorption capacity at timet, mg·g-1;K1is constant of pseudo-first-order rate equilibrium, min-1,K2is constant of pseudo-second-order rate equilibrium, g· (mg· min)-1.

Linear plots ln(Qe-Qt)versus twas employed to determine the value ofQeandK1, andt/Qt versust were employed to determine the value ofQeandK2. The results of Linear regression analysis are shown in Table 1. It can be seen from Table 1 that pseudosecond-order kinetics equations of the linear correlation coefficientR2were close to 1. These values showed that the adsorbents system was well described by the pseudo-first-order kinetic equations. These results showed that the adsorption rate was greatly influenced by the change of the concentration,Adsorption process was mainly the chemical adsorption control. Thus, the adsorption process can be described as the external surface mass transfer that controls the early stages of the adsorption process,this may be followed by a reaction rate stage, and finally by a diffusion stage where the adsorption process slows considerably.

Table 1 The parameters of adsorption kinetics

3.2. Adsorption isotherm

Fig. 3 shows IIMCD and NIIMCD adsorption isothermal curve.The adsorption isothermal curve of IIMCD and NIIMCD were similar in shapes.With the initial concentration increasing,the adsorption capacity firstly increased sharply, then increased slightly, and approached a maximum at last.The maximum adsorption capacity for Cu(II) ions on IIIMCD and NIIMCD were 78.1 mg·g-1and 64.6 mg·g-1, respectively. The maximum adsorption capacity of IIMCD was 20.9% higher than NIIMCD. There were likely to be removal of template Cu(II) ions revealed recognition cavities to recognize Cu(II) ions, which rebind the Cu(II) ions efficiently.

Fig. 3. Adsorption isotherm of IIMCD and NIIMCD.

Fig. 7. The recognition behavior of IIMCD under the different ions concentration ration of Cu(II)/Zn(II) binary systems.

Fig. 8. The recognition behavior of IIMCD under the different ions concentration ration of Cu(II)/Ni(II) binary systems.

Fig. 9. The recognition behavior of IIMCD under the different ions concentration ration of Cu(II)/Co(II) binary systems.

The Langrmuir adsorption isotherm and Freundlich adsorption isotherm were used to evaluate adsorption properties.

Langrmuir isotherm equation was given as the following Eq.(7)

whereceis the equilibrium concentration of Cu(II) ions in solution,mg·L-1,Qeis the equilibrium adsorption capacity,mg·g-1,Qmis the maximum adsorption capacity, mg·g-1,Klis the affinity constant,L·mg-1,Kfis the Freundlich constant,nis the constant.

Linear plots ofCe/Qeversus Cewere employed to determine the value ofQmandKl, and lnQeversuslnCewere employed to determine the value ofnandKf.The results of Linear regression analysis were shown in Table 2.In the metal ion concentration in the range of 50-900 mg·L-1, the equilibrium adsorption capacity of two kinds of adsorption materials were 85.1 mg·g-1and 70.72 mg·g-1,respectively. The correlation coefficient for Langmuir adsorption isothermal model was greater than the Freundlich adsorption isothermal model.The maximum adsorption capacity(78.1 mg·g-1and 64.6 mg·g-1) obtained from experimental results were very close to the calculated Langmuir adsorption capacity(85.10 mg·g-1and 70.72 mg·g-1),respectively.These values showed that the two kinds of adsorption materials can be used to describe by Langmuir adsorption isothermal model. And it was clearly seen that the adsorption of Cu(II) ions onto IIMCD and NIIMCD show a monolayer adsorption behavior.

Table 2 Adsorption isotherm parameters for Cu(II) adsorption on IIMCD

3.3. Selective sorption

3.3.1. The recognition behavior of IIMCD under the same ions concentration ration

Figs.4-6 shows the adsorption capacity and relative selectivity coefficient of Cu(II)ions on the IIMCD in the same ions concentration ration of Cu(II)/Zn(II),Cu(II)/Ni(II),and Cu(II)/Co(II)binary systems,respectively.It can be seen that the adsorption capacity of Cu(II) ions on the IIMCD was increased with increasing concentrations of metals ions, and a saturation adsorption capacity was achieved at ion concentration of 180 mg·L-1, which represented saturation of the active binding cavities on the IIMCD.The adsorption capacity of Zn(II),Co(II)and Ni(II)ions on the IIMCD was very low and remained unchanged with increasing concentrations of metals ions. The IIMCD shows a high selectivity adsorption for Cu(II) ions in Cu(II)/Zn(II), Cu(II)/Ni(II), and Cu(II)/Co(II) binary systems.

This selectivity adsorption of Cu(II) ions in Cu(II)/Zn(II), Cu(II)/Ni(II),and Cu(II)/Co(II)binary systems is most probably due to high complexation and geometric shape affinity between Cu(II)ions and Cu(II)cavities in the IIMCD structure.It is well known that removal of the template from the polymeric leaves cavities of complementary size, shape and chemical functionality to the template. Cu(II)ions can’t completely complexing with Cu(II)cavities in the IIMCD structure at the lower ions concentration ration of Cu(II) ions,which leads to the Zn(II), Co(II) and Ni(II) ions complexation with surplus cavities in the IIMCD.The specific recognition of Cu(II)ions on the IIMCD was enhanced with increasing concentrations of metals ions. Otherwise,in the process of IIMCD polymerization, a part of functional monomer was not complexation with Cu(II)ions and formed nonspecific binding sites, which can be specific binding of Cu(II)ions,and also can be nonspecific binding of Zn(II),Co(II)and Ni(II) ions. Thus, the IIMCD cannot realize the absolute specific selectivity for Cu(II)ions in mixtures system,and relative selectivity coefficientK’increase slowly or reaches a certain value.

3.3.2. The recognition behavior of IIMCD under the different ions concentration ration

Figs.7-9 shows the adsorption capacity and relative selectivity coefficient of Cu(II)ions on the IIMCD in the different ions concentration ration of Cu(II)/Zn(II), Cu(II)/Ni(II), and Cu(II)/Co(II) binary systems, respectively. It can be seen that the adsorption capacity of Cu(II) ions on the IIMCD was increased with increasing concentrations of Cu(II) ions in Cu(II)/Zn(II), Cu(II)/Ni(II), and Cu(II)/Co(II)binary systems, respectively. The adsorption capacity of Zn(II), Co(II) and Ni(II) ions on the IIMCD was very low and remained unchanged. The IIMCD shows a high selectivity adsorption for Cu(II) ions in Cu(II)/Zn(II), Cu(II)/Ni(II), and Cu(II)/Co(II) binary systems is most probably due to high complexation and geometric shape affinity between Cu(II) ions and Cu(II) cavities in the IIMCDstructure.Cu(II)ions can’t completely complex with Cu(II)cavities in the IIMCD structure at the lower ions concentration ration of Cu(II) ions, which leads to the Zn(II),Co(II) and Ni(II) ions complexation with surplus cavities in the IIMCD.Otherwise,in the process of IIMCD polymerization,a part of functional monomer was not complexation with Cu(II) ions and formed nonspecific binding sites,which can be specific binding of Cu(II) ions, and also can be nonspecific binding of Zn(II), Co(II) and Ni(II) ions. Thus, the IIMCD cannot realize the absolute specific selectivity for Cu(II) ions in mixtures system, and relative selectivity coefficientK’increase slowly or reaches a certain value.

Fig.5. The recognition behavior of IIMCD under the same ions concentration ration of Cu(II)/Ni(II) binary systems.

Fig.6. The recognition behavior of IIMCD under the same ions concentration ration of Cu(II)/Co(II) binary systems.

3.4. The mechanism of IIMCD recognition Cu(II) ions

3.4.1. Thermodynamics of IIMCD recognition Cu(II) ions

It is observed that the template ion is able to form complexes with functional monomer molecules in the preparation of ion imprinted polymer process, which was a dynamical equilibrium.The stability of complexes was controlled by change of Gibbs free energy. Nicholls [32] also reported the recognition of molecular imprinted polymer with template was controlled by the laws of thermodynamics. In order to determine thermodynamic parameters, adsorption experiments of Cu(II)/Zn(II), Cu(II)/Ni(II), and Cu(II)/Co(II)from their binary mixture was also investigated in different temperature.The distribution coefficientKdand relative selectivity coefficientK’was expressed as follows:

Linear plots lnKdversus1/Twere employed to determine the value of -ΔH/Rand ΔS/R, and linear plots lnK’ versus1/Twere employed to determine the value of -ΔΔH/Rand ΔΔS/R.The data obtained with the change of Gibbs free energy-ΔGand-ΔΔGwas listed in Tables 3-6.

The greater of absolute value of ΔH, it indicates that the template ions have a stronger affinity with ion imprinted polymer IIMCD and form a stable complexes. The ΔSabsolute value larger,indicating the orderly degree increases when complexes of is formed by template ions and molecularly imprinted polymer IIMCD.The greater of the absolute value of ΔΔH,indicating differences of affinity intensity increases between template ions and molecularly imprinted polymer IIMCD, which exhibits a strong selectivity. The greater of the absolute value of ΔΔS, indicating the difference of orderliness increases when complexes of is formed by template ions and molecularly imprinted polymer IIMCD. The greater of absolute value of ΔΔG298, indicating the selective separation at this temperature may be easier.

The results in Tables 3-6 shows that the value of ΔHand ΔSis negative, indicating IIMCD adsorption Cu(II) ions is an enthalpy controlled process. For template ion, the acting force with functional monomer arranged in imprinted holes is determined by types of force and interaction distance. For Cu(II), Zn(II), Ni(II), Co(II) ions which have the same charge number and ionic radius,the types of interaction force with IIMCD is the same.So the causes of acting force difference derive from the different of spatial matching between ions and imprinted holes on IIMCD. Imprinted holes on IIMCD have the ability of predetermined recognize the template ion. Cu(II) ions have a good spatial matching with imprinted holes on IIMCD,which can strongly combine with function groups on imprinted holes in the suitable space position.However, the combine of Zn(II), Ni(II) and Co(II) ions with function groups on imprinted holes is weakly,due to poor spatial matching with imprinted holes on IIMCD. Consequently, the total force between Cu(II) and IIMCD is greater than that of Zn(II), Ni(II), Co(II). the absolute value of ΔH(Cu(II)) is greater than ΔH(Zn(II),Ni(II), Co(II)).

Table 3 The parameters of thermodynamics of IIMCD recognition in Cu(II)/Zn(II) binary systems

Table 5 The parameters of thermodynamics of IIMCD recognition in Cu(II)/Co(II) binary systems

Table 6 The parameters of thermodynamics of NIIMCD recognition in Cu(II)/Zn(II), Cu(II)/Ni(II), and Cu(II)/Co(II) binary systems

ΔSreflects increase of order degree caused by different matching between ions and imprinted holes when they interacted. Similarly, spatial matching between Cu(II) and imprinted hole of IIMCD was better than that of Zn(II), Ni(II), Co(II). Combination of Cu(II) and imprinted hole is favorable, the resulting order degree increased. So the absolute value of ΔS(Cu(II)) is always greater than ΔS(Zn(II), Ni(II), Co(II)). The symbol of ΔΔG298showed that the separation is spontaneous at 25 °C under experimental conditions.This is because imprinted Cu(II)and IIMCD hole has complementary three-dimensional structure, so the formation of compound between them is easier and more stable, although Cu(II), Zn(II), Ni(II), Co(II) have the same charge number and ionic radius as well as types of interaction force. In contrast, in NIIMCD no complementary hole can complement Cu(II),Zn(II),Ni(II),Co(II),thus steric hindrance information of compound is enormous,resulting in higher energy and worse stability of the compound.

3.4.2. FTIR analyze

The FTIR spectrum for IIMCD and IIMCD-Cu complex was shown in Fig. 10. As indicated in Fig. 10a, the characteristic broad absorption bands at 3423.06 cm-1represented multi-overlapped absorption peaks of hydroxyl groups (O-H) and amido groups(N-H) stretching vibration, which might be related to the strong inter- and intramolecular hydrogen bonding. The absorption band at 1600.43 cm-1was due to the N-H stretch vibration peaks.Comparing with Fig. 10b, the station of absorption bands due to the O-H and N-H stretch (at 3423.06 cm-1) hardly changed, but the intensity of the peaks was weakening. The facts suggested there was an interaction of the hydroxyl groups, amido groups and Cu(II) ions. This could be attributed to the depletion of all groups in the binding reaction. For the characteristic determination of IIMCD-Cu(II) complex, due to linear coordinate covalent complex formation, the characteristic weak N-H stretching vibration band at 1600.43 cm-1shifts to down field at 1596.32 cm-1amido groups could donate a lone electron pair for the empty orbit of metal ions alone. There is no other significant change between the two IR spectra in the range 500-4000 cm-1.

Fig.10. The FTIR spectra for IIMCD and IIMCD-Cu complex,a:IIMCD,b:Cu-IIMCD.

Fig. 11. The XPS spectrum of the binding energy for C atoms in IIMCD and Cu-IIMCD. a: IIMCD, b: Cu-IIMCD.

Fig. 12. The XPS spectrum of the binding energy for N atoms in IIMCD and Cu-IIMCD. a: IIMCD, b: Cu-IIMCD.

Fig. 13. The XPS spectrum of the binding energy for O atoms in IIMCD and Cu-IIMCD. a: IIMCD, b: Cu-IIMCD.

Fig. 14. Adsorption-desorption cycle of Cu(II)-IMCD.

3.4.3. XPS analyze

The XPS spectrum of the binding energy for C,O and N atoms in IIMCD was given in Figs.11-13,respectively.It can be seen that the C,O and N atoms were in the same chemical environment,and the binding energy were 285.93 eV, 532.38 eV and 399.34 eV, respectively.Cu(II)ions were adsorbed on the IIMCD,the XPS spectrum of the binding energy for C and O atoms remained unchanged, and the binding energy of N atoms were increasing to 400.09 eV. It can be considered that the N atoms were involved in the coordination reaction of Cu(II) ions.

According to the N atoms of binding energy,its electronic lost or lone pair electrons tend to be shared after ligand with ions which lead the binding energy increased. For the N atoms structure, N atoms outer has 5 electronic,three of them have been pair bonding and left a pair of lone pair electrons less likely to lose electron,and it’s easy to provide a lone pair electrons to form complexes. The structure of Cu(II) ion showed that the outer layer is d9, one of the electrons in the chemical reaction process is easy arouse to 4p, forming dsp2 hybrid orbitals, which leaving an empty track can easy accept the lone pair electrons. In conclusion, N atoms of NH2in the IIMPs cavities provided a lone pair electrons complex with dsp2 hybrid orbitals of Cu(II) ion forming a chelate with regular tetrahedron configuration,which ligand coordination number is 4.

3.5. Regeneration and reuse

Adsorbent regeneration and reuse were beneficial to shorten the production cycle, reduce the production cost and economic benefits. In order to obtain the reusability of the IIMCD, adsorption-desorption cycles was repeated 10 times by using the same IIMCD. The adsorption capacity of the recycled IIMCD was 8% loss of its original value at the tenth cycle(Fig. 14).It can be seen conclude that the IIMCD can be regenerated and repeatedly used 10 times without significantly decreasing in adsorption capacity.

4. Conclusions

In this study, the IIMCD were prepared using chitosan as function monomer, using chitosan as functional monomer, Cu(II) ions as template, Fe3O4as magnetic core and epichlorohydrin and glutaraldehyde as crosslinker and used for removal of Cu(II)ions from aqueous solutions.A study of the maximum adsorption capacity of Cu(II)ions was 78.1 mg·g-1,the adsorption isotherm study showed the Langmuir isotherm equation best fitted for the monolayer adsorption processes. The kinetic study showed that follow the pseudo-second-order kinetics equations in the adsorption process.Compared with the NIIMCD,the IIMCD indicated a high selectivity towards Cu(II) ions in the competitive ions. The mechanism of IIMCD recognition Cu(II) ions shows that the IIMCD adsorption Cu(II) ions is an enthalpy controlled process. The absolute value of ΔH(Cu(II)) and ΔS(Cu(II)) is greater than ΔH(Zn(II), Ni(II), Co(II)) and ΔS(Zn(II), Ni(II), Co(II)), respectively, it indicates that the Cu(II) ions have a good spatial matching with imprinted holes on IIMCD. The FTIR and XPS is used to demonstrate the strongly combine with function groups on imprinted holes in the suitable space position. Finally, the IIMCD can be regenerated and reused for 10 times without a significantly decreasing in adsorption capacity. Furthermore, The IIMCD can be regenerated and reused 10 times without significantly decreasing in adsorption capacity.As a result, the IIMCD can be used for separated the Cu(II) ions from mixing solution. This information can be used for further application in the treatment of wastewater.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.