Enhanced selective removal of Cu(II) from aqueous solution by novel polyethylenimine-functionalized ion imprinted hydrogel: Behaviors and mechanisms

Accepted Manuscript Title: Enhanced selective removal of Cu(II) from aqueous solution by novel polyethylenimine-functionalized ion imprinted hydrogel: Behaviors and mechanisms Author: Jingjing Wang Zhengkui Li PII: DOI: Reference:

S0304-3894(15)00499-9 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.06.043 HAZMAT 16898

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

11-3-2015 15-5-2015 19-6-2015

Please cite this article as: Jingjing Wang, Zhengkui Li, Enhanced selective removal of Cu(II) from aqueous solution by novel polyethylenimine-functionalized ion imprinted hydrogel: Behaviors and mechanisms, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.06.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced selective removal of Cu(II) from aqueous solution by novel polyethylenimine-functionalized ion imprinted hydrogel: Behaviors and mechanisms

Jingjing Wanga, b, Zhengkui Lia, b* a

State Key Laboratory of Pollutant Control and Resource Reuse, Nanjing 210023, China; b

School of the Environment, Nanjing University, Nanjing 210023, China.

* Corresponding author. Prof. Dr. Zhengkui Li; School of the Environment, Nanjing University, 163 Xianlin Avenue, Nanjing, 210023, China; Tel.: +86-25-89680396; Fax: +86-25-89680396. Email: [email protected] Highlights

A novel ion-imprinted poly(Polyethylenimine/Hydroxyethyl acrylate) hydrogel was synthesized. The prepared hydrogel enhanced the selectivity of Cu(II) removal. The material had high adsorption capacity and excellent regeneration property for copper. The adsorption mechanism was the chelate interaction between functional groups

and Cu(II) ions. Abstract

A

novel

polyethylenimine-functionalized

(Cu(II)-p(PEI/HEA)) was newly synthesized by

60

ion-imprinted

hydrogel

Co-γ-induced polymerization for

the selective removal of Cu(II) from aqueous solution. The adsorption performances including the adsorption capacity and selectivity of the novel hydrogel were much better than those of similar adsorbents reported. The hydrogel was characterized via scanning electron microscope, transmission electron microscopy, Fourier transform infrared spectra, thermal gravimetric analysis and X-ray photoelectron spectroscopy to determine the structure and mechanisms. The adsorption process was pH and temperature sensitive, better fitted to pseudo-second-order equation, and was Langmuir monolayer adsorption. The maximum adsorption capacity for Cu(II) was 40.00 mg/g. The selectivity coefficients of ion-imprinted hydrogel for Cu(II)/Pb(II), Cu(II)/Cd(II) and Cu(II)/Ni(II) were 55.09, 107.47 and 63.12, respectively, which were 3.93, 4.25 and 3.53 times greater than those of non-imprinted hydrogel, respectively. Moreover, the adsorption capacity of Cu(II)-p(PEI/HEA) could still keep more than 85% after four adsorption-desorption cycles. Because of such enhanced selective

removal

performance

and

excellent

regeneration

property,

Cu(II)-p(PEI/HEA) is a promising adsorbent for the selective removal of copper ions from wastewater. Key Words: Ionic imprinted hydrogel; Selectivity; Polyethylenimine; Heavy metals; Adsorption mechanism

1. Introduction

Nowadays even trace amounts of heavy metal ions usually pose a serious threat to the environment and public health [1-3]. Copper (Cu(II)) is a common toxic contaminant in wastewater and excessive Cu(II) consumption could cause harmful effects on liver and kidney, making blood pressure and respiratory rates increased [4]. A variety of technologies have been developed to remove copper ions from wastewater [5, 6], for example, chemical precipitation [7], adsorption [4, 8-9], ion exchange [10], membrane filtration [11], and electrochemical treatment [12]. Among these techniques, adsorption is the attractive choice due to the low cost, easy operation, high efficiency and potential recovery of Cu(II) [13, 14]. Hydrogels, which are composed of hydrophilic homopolymer or copolymers, are regarded as promising adsorbents for copper ions removal, on account of the three-dimensional network structure and their capability of adding different chelating groups into the polymeric backbones [15, 16]. Various monomers containing functional groups such as amino (-NH2), acylamino (-CONH2), carboxyl (-COOH), and sulfonic (-SO3H) [17-21] that exhibit selected ability towards toxic metals have been used for the synthesis of copolymeric hydrogels. Polyethylenimine (PEI), attributed to the presence of a great number of primary and secondary amine groups on the macromolecular chains [22], displays excellent adsorption ability for Cu(II) [23, 24]. Owing to the water soluble nature, PEI, has to be crosslinked to provide an insoluble matrix. Plenty of adsorbent materials like biomass [25, 26], insoluble polymers [27, 28], silica [29], and cellulose [30, 31] have been selected to crosslink

PEI to avoid its leaching during adsorption process. However, the polymerization methods for materials with polyethylenimine were mainly chemical and complicated [32-35]. Currently, gamma radiation technology has received growing concern since it is considered as a promising method for synthesis of polymers (e.g. hydrogels). Compared with chemical polymerization, there is no necessity to add any initiators or crosslinkers into the reaction system through gamma radiation synthesis. Thus, the synthesized products are more pure and the process could be easily controlled with no waste [36, 37]. Furthermore, hydrogels have seldom been synthesized by gamma radiation using PEI as one monomer and information of their application in Cu(II) removal from wastewater is limited [38, 39]. Since many competitive metal ions present in contaminated water could extremely compete for active sites of the adsorbents, it is essential to improve the selectivity toward certain metal ion. Recently, ion-imprinted polymers have received much concern as these new sorbents achieved higher selectivity coefficients compared with non-imprinted sorbents [40]. Moreover, lots of active sites and large binding surface area produced by ion imprinting procedure enable faster binding kinetics and higher adsorption capacity [41, 42]. Singh et al. [43] prepared a novel ion imprinted polymer by using 2-hydroxyethyl methacrylate as monomer, and the polymer was tested for removal of nickel ions with high selectivity. Jiang et al. [44] investigated the removal of copper ions by Cu(II) ion-imprinted porous methacrylic acid/vinyl pyridine (MAA/VP) particles. They found that the adsorption capacity of copper ions showed much higher than that of other metal ions. He et al. [45] developed a novel magnetic

ion-imprinted polymer using Cu(II) as the template. The results revealed that the adsorbent could be applied to rapid extraction of copper ions and exhibited high selectivity toward Cu(II). Despite of quantities of studies on ion-imprinted adsorbents, imprinting Cu(II) ion on a novel multi-amine hydrogel polymerized by gamma radiation has not been reported to date. Additionally, it would be significant and indispensable to develop a novel adsorbent with both outstanding adsorption capacity and high selectivity toward copper ions. In this study, a novel Cu(II) ion-imprinted poly (Polyethyleneimine/Hydroxyethyl acrylate)

(Cu(II)-p(PEI/HEA))

hydrogel

was

prepared

by

60

Co-γ-induced

copolymerization, using Cu(II) as the template. The hydrogel was characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectra (FTIR), thermal gravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). Furthermore, the selectivity of the new ion-imprinted material toward Cu(II) was evaluated, as well as the isotherms and kinetics during adsorption process.

2. Experimental 2.1. Materials Branched polyethyleneimine (PEI) and 2-hydroxyethyl acrylate (HEA) were purchased from Sigma-Aldrich, USA. Cu(NO3)2·3H2O, Pb(NO3)2, Cd(NO3)2·4H2O, Ni(NO3)2·6H2O, ethylene diamine tetraacetic acid (EDTA) and glutaraldehyde (25% aqueous solution), all of analytical grade, were purchased from Sinopharm (Shanghai,

China). 2.2. Preparation of Cu(II)-p(PEI/HEA) hydrogel Fig. 1 illustrates the radiation reaction during polymerization of PEI and HEA

monomers and detailed preparation processes can be represented as the following steps: (1) PEI and HEA mixture was prepared with deionized water at a fixed monomers ratio 1:3 (mol/mol) in a 125 ml brown sealed bottle. Before the radiation reaction, the solution was then purged with nitrogen gas for 20 min to get rid of any dissolved oxygen. (2) The prepared mixtures were irradiated with 60Co gamma source for 24 h to a total dose of 20 kGy. The

60

Co-γ ray source is installed in Nanjing

Radiation Center. (3) The obtained polymerized hydrogels were cut into 5 × 5 × 5 mm cubes and washed thoroughly with deionized water. 5.0 g of swollen hydrogel cubes was mixed with 100 ml of 7.0 g/L Cu(NO3)2 solution for 2 h to make the template Cu(II) ions loaded onto p(PEI/HEA) hydrogel through chelation with amino groups [46]. (4) p(PEI/HEA)-Cu was transferred into a three-necked flask containing 75 ml of 1%(wt) glutaraldehyde solution for 3 h. (5) p(PEI/HEA)-Cu-GA particles were fully treated with EDTA to eliminate the template Cu(II) ions, then washed and dried. 2.3. Characterization methods SEM (S-3400N II, Hitachi, Japan) was used to observe the surface morphology. TEM was operated on JEM-200CX (JEOL, Japan) at the accelerating voltage of the electron beam 200 kV. FTIR was carried out on Nexus 870 (NICOLET, USA) in the range of 400–4000 cm-1. XPS analysis was performed on a PHI-5000 spectrometer (ULVAC-PHI, Japan) in the range of 0-4000 eV. PerKinElmer TGA system

(TGA-Pyris 1, USA) was selected for thermal gravimetric analysis with the temperature range from 25 to 700 °C at a heating rate 20 °C/min. The concentrations of heavy metal ions were detected by the atomic adsorption spectroscopy (Hitachi Z-8100, Japan). 2.4. Batch adsorption experiments Based on the pH study, the best adsorption performance was achieved at pH 5.5. Adsorption isotherms of Cu(II) on the prepared sorbent were implemented by adding 0.1g dried hydrogel into 50 ml of different initial concentrations of Cu(II) (0-200 mg/L) at pH 5.5. The mixtures were stirred for 24 h at 298 K. For determination of adsorption kinetics, 0.1 g dried sorbent was added to 50 ml of 64 mg/L Cu(II) solution at pH 5.5. The mixtures were shaken at 298 K and collected at different contact time (0.1-72 h). The adsorption capacity Q (mg/g) was calculated as follows:

Q=

V(C0－C1 ) W

(1)

where C0 and C1 are the concentrations of metal ions before and after adsorption, respectively (mg/L), V is the volume of solution (L) and W is the hydrogel mass (g). 2.5. Effect of pH and temperature on adsorption The pH effect on metal adsorption was examined by mixing 0.1 g of the dried hydrogel samples with 50 ml of 64 mg/L Cu(II) ion solution for 24 h at 298 K. The pH values of the Cu(II) solutions were adjusted to 1.0-5.5 by hydrochloric acid or sodium hydroxide. Similar work was performed by varying the temperatures of Cu(II)

solutions (288-328 K) at pH 5.5 to analyze the effects of temperature on metal adsorption. 2.6. Selective adsorption studies The selectivity of adsorption was determined in such binary adsorption systems as Cu(II)/Pb(II), Cu(II)/Ni(II) and Cu(II)/Cd(II) solutions under initial pH 5.5 at 298 K. In detail, 0.1 g of the hydrogel sample was added into 50 ml of the metal ion solutions containing Cu(II), Cd(II), Ni(II) or Pb(II) at equal initial concentration. The selectivity of non imprinted hydrogel was also measured for comparison. 2.7. Regeneration studies For repeated adsorption and desorption studies, the Cu(II) loaded hydrogels were gently washed with deionized water to remove any unabsorbed Cu(II) and dried in an oven, and then mixed with 50 ml of 33.6 g/L EDTA. To examine the reusability of the ion imprinted hydrogel, the adsorption-desorption cycle was repeated four times.

3. Results and discussion 3.1. Characterization 3.1.1. Morphology analysis The

synthesized

p(PEI/HEA)

and

Cu(II)-p(PEI/HEA)

adsorbents

were

characterized using SEM and TEM to observe the surface morphology. As shown in Fig. 2 (a) and (b), the p(PEI/HEA) hydrogel presented a three dimensional network microstructure, with irregularly shaped micropores, ranging from 10 to 20 µm in the size. However, after imprinting Cu(II) ion onto the p(PEI/HEA) sorbent (Fig. 2 (c)

and (d)), the network surface turned rough. Additionally, the pore size of ion imprinted hydrogel was slightly increased. Such differences between the images of p(PEI/HEA) and Cu(II)-p(PEI/HEA) adsorbents indicated that the Cu(II) imprinting process caused significant changes to the matrix structure. The TEM images of p(PEI/HEA) and Cu(II)-p(PEI/HEA) in Fig. 2 (e) and (f) exhibited porous structure of both hydrogels and the pores were irregularly shaped. This result was in agreement with the SEM result but different with several reported articles (e.g. hexagonal uniform arranged mesopores) [4, 8-9, 13-14], and the difference could be attributed to the network microstructure produced by polymerization of two monomers. 3.1.2. FTIR spectra The FTIR spectrums of p(HEA), p(PEI/HEA) and Cu(II)-p(PEI/HEA) hydrogels were investigated to identify the presence of expected functional groups (Fig. 3). As depicted in Fig. 3a, two strong peaks at 3408 cm-1 and 1733 cm-1 should correspond to the stretching vibration of hydroxyl (O-H) and the carbonyl (C=O) bonds in HEA, respectively. Moreover, the peaks appearing at 1168 cm-1 and 1078 cm-1 were assigned to the carbon-oxygen stretching vibration (C–O) and the peak at 893 cm-1 also demonstrated the existence of carbon–oxygen. Other apparent peaks occurring at 2954cm-1 and 2883cm-1 were attributed to aliphatic C-H stretching vibrations [47]. Compared to the spectrum of p(HEA), the spectrum of p(PEI/HEA) in Fig. 3b expressed some different but crucial messages. The broad band within 3100–3700 cm-1 could be associated to the stretching vibration of O-H in HEA and N-H in PEI

[22, 48]. Furthermore, a characteristic peak of amine groups in PEI was observed at 1592 cm-1 [27]. The band appearing at 2954 cm-1 of p(PEI/HEA) was attributed to the C-H groups. The information obtained from the spectrum of p(PEI/HEA) confirmed that the radiation synthesis of PEI and HEA monomer was successful and the p(PEI/HEA) hydrogel was indeed a copolymer of HEA and PEI. As shown in Fig. 3c, the spectrum of Cu(II) ion imprinted hydrogel was similar to that of non imprinted hydrogel, demonstrating that Cu(II)-p(PEI/HEA) hydrogel maintained the copolymer features of HEA and PEI. 3.2. Adsorption studies 3.2.1. Effect of pH The pH value of aqueous solution is a significant parameter affecting the adsorption procedure. Metal ions appear in different forms by varying the aqueous solution pH and the Cu(II) precipitate in the forms of metal oxides or hydroxides at pH>6 [49]. Thus, the effect of pH on the adsorption capacity of Cu(II)-p(PEI/HEA) and p(PEI/HEA) hydrogel was performed at pH=1-5.5. Fig. 4 illustrates that the adsorption capacity for both copolymer hydrogels barely increased with the increase of pH from 1 to 2, but increased remarkably with pH values varying from 2 to 5.5. The reason was that the amino groups (-NH2) on the PEI chains were protonated to –NH3+ at low pH values (pH≤2) which led to electrostatic repulsion with divalent copper ions, decreasing the amount of binding sites available for Cu(II) uptake [50, 51]. However, when the pH values were improved (pH>2), the -NH2 groups could capture Cu(II) ions through chelation [52], creating a high uptake. The basic reactions

involved in the solution are shown as follows: R－NH 2 + H + ⇔ R－NH 3+

(2)

R－NH 2 + Cu 2+ ⇔ R－NH 2 Cu 2+

(3)

3.2.2. Effect of temperature Fig. 5 depicts the effect of temperature on the adsorption capacity of

Cu(II)-p(PEI/HEA) and p(PEI/HEA) hydrogel. In general, the adsorption procedure of Cu(II) was enhanced by increasing temperature. Thermodynamic parameters such as free energy change (∆G°), enthalpy change (∆H°) and entropy change (∆S°) were calculated as follows [18]:

∆G o = ∆H o－T∆S o

(4)

∆G o = －RT ln Kc

(5)

Kc =

CAc Ce

ln Kc =

∆S o ∆H o － R RT

(6)

(7)

where Kc is the equilibrium constant; CAc and Ce are equilibrium concentrations (mg/L) of Cu(II) on the hydrogel and in the solution, respectively; T is the absolute temperature (K) and R is the universal gas constant. The values of ∆H° and ∆S° obtained from the slope and intercept of Von’t Hoff plot of lnKc versus 1/T were recorded in Table 1. The ∆G° decreased with the increase of temperature indicated that the Cu(II) adsorption on hydrogels was spontaneous and the spontaneity increased with the temperature [18]. Positive value of ∆H° and ∆S° reflected endothermicity and randomness at the solid-solution interface during adsorption,

respectively [14]. 3.2.3. Adsorption isotherms As shown in Fig. 6, the adsorption capacity of ion-imprinted hydrogel and non-imprinted hydrogel for copper ions evidently both increased with increasing the initial concentration of Cu(II). Moreover, it could also be seen that the adsorption capacity of both hydrogels heightened swiftly in the low concentration range (C0 < 60 mg/L) but the changes of the adsorption capacity then leveled off as the initial concentration raised to higher values (60 mg/L < C0 < 200 mg/L). In order to deeply explore the above trends, Langmuir and Freundlich isotherm models were applied to analyze the experiment data [53, 54]:

Ce 1 1 = Ce + Qe Qmax K L Qmax

lg Qe =

1 lg Ce + lg K F n

(8)

(9)

where Ce is the equilibrium concentration of Cu(II) in solution (mg/L); Qe is the amount adsorbed (mg/g); Qmax is the maximal adsorption capacity (mg/g); and KL is a Langmuir binding constant; KF and n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively. Fig. 7 presents the fitting results of Langmuir and Freundlich models for adsorption of Cu(II) onto Cu(II)-p(PEI/HEA) and p(PEI/HEA). Table 2 lists the parameters and correlation coefficients (R2) calculated from corresponding models. As presented in Table 2, the Langmuir model behaved better descriptions of the experimental data (R2 > 0.99) than the Freundlich model, implying that the adsorption of Cu(II) on both

Cu(II)-p(PEI/HEA) and p(PEI/HEA) hydrogels were monolayer adsorption. This could be attributed to the chelating action between the -NH2 groups and the Cu(II) ions on the hydrogel surface [53]. Additionally, the Qmax for Cu(II) onto Cu(II)-p(PEI/HEA) and p(PEI/HEA) hydrogels were 40.00 and 29.41 mg/g, respectively, clearly indicating the imprinting effect. Furthermore, the separation factor (RL) for Langmuir isotherm model can be used to estimate the adsorption system “favorable” (0 < RL < 1), “linear” (RL = 1), “unfavorable” (RL > 1), or “irreversible” (RL = 0), and is defined as below [18]. RL=

1 1+K L C0

(10)

where C0 (mg/L) is the initial concentration of Cu(II) and KL is the Langmuir adsorption equilibrium constant. The calculated RL values given in Table 2 were all within the range of 0 < RL < 1, indicating that the Cu(II) adsorption on both Cu(II)-p(PEI/HEA) and p(PEI/HEA) hydrogels were favorable. 3.2.4. Adsorption kinetics It is essential to determine the rate-limiting step including mass transport and chemical reaction procedure through adsorption kinetics study in a given system. Fig. 8 depicts the results of adsorption kinetics experiments on Cu(II)-p(PEI/HEA) and p(PEI/HEA) hydrogels. The adsorption rate increased rapidly at the pre-adsorption stage (0–4 h) and then slowed down from 12 to 72 h until the adsorption equilibrium reached at 36 h for Cu(II)-p(PEI/HEA) and 48 h for p(PEI/HEA), revealing that the ion imprinting reaction improved the adsorption rate to some extent. This result

agreed with the SEM result indicating that the pore size of ion-imprinted polymer was slightly larger than that of non-imprinted one. Thus, the Cu(II) ions could easily transport from aqueous solution onto the surface of ion imprinted hydrogels and approached to equilibrium quickly. Besides, the kinetic data were examined using the pseudo-first-order and pseudo-second-order rate equation [55, 56]:

lg(Qe－Qt ) = lg Qe－

K1t 2.303

t 1 t = 2 + Qt K 2 Qe Qe

(11)

(12)

where Qe (mg/g) and Qt (mg/g) are the amount adsorbed at equilibrium and at any time t (h), respectively; K1 and K2 are the adsorption rate constants. Fig. 9 and Table 3 show the fitting graphics and the kinetic parameters of pseudo-first-order and pseudo-second-order models, respectively. According to the correlation coefficients (R2 > 0.99) and the comparison between the experimental adsorption capacity (Qe,e) and the calculated equilibrium adsorption capacity (Qe,c), the uptake–time curves of both Cu(II)-p(PEI/HEA) and p(PEI/HEA) hydrogels were satisfactorily correlated to the pseudo-second-order kinetic model, suggesting the chemical adsorption as the rate control step [57]. 3.3. Selectivity experiments The selectivity of the hydrogels for Cu(II) over other metal ions can be appraised by the selectivity coefficient (βCu2+/M2+), which is expressed as [58]: D β

Cu 2 + /M 2 +

=

Cu 2 +

D

M 2+

(13)

where DCu2+ and DM2+ are the distribution ratios (D) of the Cu(II) and other coexisted heavy metals, respectively. The distribution ratio could be calculated by the equation below:

D=

C0－C e V × Ce W

(14)

where C0 and Ce are the concentrations of metal ions before and after adsorption, respectively (mg/L). V is the volume of solution (L) and W is the hydrogel mass (g). In addition, the selectivity of imprinted material for the template ion with respect to non imprinted material can be identified by the relative selectivity coefficient (βr), which is defined as follow:

βr =

βim βnon

(15)

where βim and βnon are the selectivity coefficients of the imprinted and non imprinted hydrogels, respectively. Table 4 displays the distribution ratios and selectivity coefficients towards other competitive metal ions by Cu(II)-p(PEI/HEA) and p(PEI/HEA) hydrogels. It was clearly that the distribution ratio of Cu(II)-p(PEI/HEA) hydrogel for Cu(II) achieved about 3 times higher than that of p(PEI/HEA) hydrogel, while it showed almost equal for other competitive metals. The selectivity coefficients of ion-imprinted hydrogel for Cu(II)/Pb(II), Cu(II)/Cd(II) and Cu(II)/Ni(II) were 55.09, 107.47 and 63.12, respectively, which were 3.93, 4.25 and 3.53 times greater than those of non-imprinted hydrogel, respectively. Moreover, the relative selectivity coefficients of Cu(II)-p(PEI/HEA) hydrogel for each specific metal ion exhibited far greater than 1.

The above results suggested that the binding ability of Cu(II)-p(PEI/HEA) hydrogel for Cu(II) was stronger than that of p(PEI/HEA) hydrogel. The reason may be that the Cu(II)-p(PEI/HEA) hydrogel created specific recognition cavities for Cu(II) ions through ion imprinting procedure [45]. Besides, the cavities developed by Cu(II) ion imprinting procedure were not suited to Pb(II), Cd(II) and Ni(II) in shape, size and dimensional arrangement of action sites, so the selectivity of Cu(II)-p(PEI/HEA) hydrogel for Cu(II) was much higher than that for other metal ions [58]. It could also be seen that the selectivity coefficient of Cu(II)/Cd(II) was nearly double greater than that of Cu(II)/Pb(II) and Cu(II)/Ni(II). The reason was that the affinity of non-imprinted hydrogel for Pb(II) and Ni(II) was larger than that for Cd(II) according to the different selectivity coefficients of non-imprinted hydrogel for Cu(II)/Pb(II), Cu(II)/Cd(II) and Cu(II)/Ni(II). Besides, the ion imprinting procedure effectively enhanced the affinity of hydrogel for Cu(II), causing the further reduction of adsorption capacity for Cd(II). The high selectivity of adsorption in such binary adsorption systems as Cu(II)/Pb(II), Cu(II)/Ni(II) and Cu(II)/Cd(II) solutions all indicated that the Cu(II)-p(PEI/HEA) hydrogel had forceful selectively and affinity for Cu(II) in several competitive metal ions existed solutions. 3.4. Comparison with other Cu(II) ion imprinted adsorbents Adsorption capacity of Cu(II) on Cu(II)-p(PEI/HEA) hydrogel and the selectivity coefficient (βCu2+/M2+) in this research were compared with other forms of adsorbents in recent literatures. As shown in Table 5, the adsorption capacity and selectivity of Cu(II)-p(PEI/HEA) hydrogel were higher than most of the reported sorbents and

relatively lower than some of those published adsorbents possibly due to the different matrixes, structural features, chelating groups and polymerization methods of materials. The comparison confirm that the ion-imprinted hydrogel has certain application prospect for selective Cu(II) removal. 3.5. Adsorption mechanisms 3.5.1 Thermal gravimetric analysis Fig. 10 illustrates the TGA analysis of HEA, PEI monomer, p(PEI/HEA) hydrogel

and Cu(II)-p(PEI/HEA) hydrogel. It was obvious that the pure HEA curve displayed only one sharp loss at 180 °C and the pure PEI curve showed a major weight loss at 390 °C. However, both the p(PEI/HEA) hydrogel and Cu(II)-p(PEI/HEA) hydrogel exhibited higher thermal stability than pure HEA, but lower than pure PEI, and also demonstrated one major weight loss between 420 and 450 °C owing to the backbone degradation [59]. This result verified that the copolymer hydrogel was indeed a copolymer rather than a simple mixture of the two monomers. The thermal gravimetric analysis of Cu(II)-p(PEI/HEA) hydrogel before and after Cu(II) adsorption were compared so as to identify the effect of the adsorbed metal ion on the thermal stability of the copolymer hydrogel. As shown in Fig. 11, the presence of Cu(II) decreased the initial decomposition temperature but increased the decomposition temperature in the range of 450 to 600 °C. The decline in the thermal stability of Cu(II) chelated hydrogel was probably attributed to the lost of water which was coordinated to the Cu(II), leading a little higher weight loss in the initial stages of decomposition [59]. However, the improvement in the thermal stability of the Cu(II)

adsorbed hydrogel at high temperature stages could possibly be result of the strong interaction between Cu(II) ion and the hydrogel [60]. 3.5.2 XPS spectra To better understand the elemental speciation on the surface of hydrogels, XPS measurement of the Cu(II)-p(PEI/HEA) hydrogel before and after heavy metal adsorption was performed. The low-resolution and high-resolution XPS spectrums are illustrated in Fig. 12 and 13, respectively. From Fig. 12, besides the characteristic peaks of C1s (284eV), N1s (399eV) and O1s (531eV) for the Cu(II)-p(PEI/HEA) hydrogel, Cu2p (932eV) peak was clearly detected for the Cu(II)-adsorbed hydrogel, clarifying that the Cu(II) ions were certainly attached onto the hydrogel. Fig. 13a exhibits the typical N1s XPS spectra of the Cu(II)-p(PEI/HEA) hydrogel with and without adsorbed Cu(II) ions. The 3 peaks at 398.5eV, 399.8eV and 400.7eV before adsorption may be caused by the presence of N atoms in tertiary amine, -NH and -NH2 groups of Cu(II)-p(PEI/HEA) hydrogel, respectively [25, 26, 61]. After the adsorption of Cu(II), in addition to the peaks at around 399eV standing for the three kinds of nitrogen-containing groups above, a new peak was observed in the N1s spectra at a binding energy of about 401.4eV. This could result from the formation of N-Cu(II) complexes, in which a lone pair of electrons in the N atom was donated to the shared bond between N and Cu. Consequently, the electron cloud density of N atom was decreased, causing a higher BE peak [62, 63]. Thus, the XPS spectra of N1s effectively substantiated that one of the adsorption mechanisms was the chelation between amine groups and Cu(II). The XPS spectra of O1s on Cu(II)-p(PEI/HEA)

hydrogel before and after heavy metal adsorption were displayed in Fig. 13b. Two peaks at about 531.3eV and 532.7eV were observed owing to the existence of O-containing groups (C-O-C, C=O and C-OH) in Cu(II)-p(PEI/HEA) [17]. However, a new peak in the O1s spectra at a BE of about 533.2eV was detected after the metal adsorption, indicating that another adsorption mechanism was the chelate interaction between O and Cu(II) same as N and Cu(II) [64]. In conclusion, adsorption mechanisms of Cu(II) on Cu(II)-p(PEI/HEA) hydrogel could mainly be the chelation between N-containing groups and Cu(II) as well as O-containing groups and Cu(II). 3.6. Regeneration studies The adsorption process was repeated to evaluate the potential capability of Cu(II)-p(PEI/HEA) hydrogel in practical applications. As shown in Fig. 14, the adsorption capacity of Cu(II)-p(PEI/HEA) hydrogel could still keep more than 85% after four adsorption-desorption cycles and slightly decreased after several cycles. The reason was probably that the amount of Cu(II)-containing hydrogel was not weighed after sorption and small amount of hydrogel was losing during several sorption and regeneration operations [13]. The regeneration studies suggest that the novel ion imprinted hydrogel has potential applications for selective Cu(II) ions removal from wastewater.

4. Conclusion

In this study, a novel polyethylenimine-functionalized ion imprinted hydrogel (Cu(II)-p(PEI/HEA)) was successfully prepared by

60

Co-γ-induced copolymerization

for selective removal of Cu(II) from aqueous solution. The ion-imprinted hydrogel presented a three dimensional network microstructure and had many irregularly shaped pores. FTIR spectra and TGA analysis confirmed that Cu(II)-p(PEI/HEA) was really a copolymer of HEA and PEI with the functional groups featured of amine groups and hydroxyl groups. The adsorption process was pH and temperature dependent, better fitted to pseudo-second-order equation, and was Langmuir monolayer adsorption. The maximum adsorption capacity for Cu(II) was 40.00 mg/g. XPS spectra analysis revealed that the adsorption mechanisms could mainly be the chelate interaction between N-containing and O-containing groups and Cu(II). Furthermore, the selectivity coefficients of ion-imprinted hydrogel for Cu(II)/Pb(II), Cu(II)/Cd(II) and Cu(II)/Ni(II) were 55.09, 107.47 and 63.12, respectively. The adsorption performances including the adsorption capacity and selectivity of Cu(II)-p(PEI/HEA) were much better than those of similar adsorbents reported. Besides, the adsorption capacity of Cu(II)-p(PEI/HEA) could still keep more than 85% after four adsorption-desorption cycles. So, Cu(II)-p(PEI/HEA) hydrogel is a promising adsorbent for the selective removal and recovery of Cu(II) ions from wastewater.

Acknowledgments

This study was financially supported by Research projects of the Major State Water Pollution

Control

and

Treatment

Technique

2012ZX07101006, 2013ZX07101014-001).

Programs

of

China

(Nos.

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Table 1

Thermodynamic parameters at different temperatures for the adsorption of Cu(II) on hydrogels. Temperature

Cu(II)-p(PEI/HEA)

p(PEI/HEA)

(K) 288 298 308 318 328

∆H° (kJ/mol)

27.31

∆S° (kJ/mol/K)

∆G° (kJ/mol)

0.096

-0.16 -1.68 -2.63 -3.38 -4.08

∆H° (kJ/mol)

18.01

∆S° (kJ/mol/K)

∆G° (kJ/mol)

0.007

2.04 1.04 0.60 0.23 -0.33

Table 2

Langmuir and Freundlich isotherm parameters for Cu(II) adsorption. Langmuir model constants Adsorbent

Freundlich model constants

R2

Qmax (mg/g)

KL

RL

R2

n

KF

p(PEI/HEA)

0.996

29.41

0.0195

0.204-0.893

0.981

1.53

0.998

Cu(II)-p(PEI/HEA)

0.998

40.00

0.0553

0.083-0.747

0.879

1.52

2.061

Table 3

Rate constants, calculated and experimental adsorbed amounts of Cu(II) on p(PEI/HEA) and Cu(II)-p(PEI/HEA) hydrogels.

Adsorbent

Qe,e(mg/g)

p(PEI/HEA) Cu(II)-p(PEI/HEA)

Pseudo-first-order values

Pseudo-second-order values Qe,c(mg/g)

K2

R2

0.920

16.67

0.0212

0.994

0.989

24.39

0.0158

0.998

Qe,c(mg/g)

K1

R

16.43

13.40

0.0829

23.40

18.88

0.0967

2

Table 4

Selective adsorption of Cu(II), Pb(II), Cd(II) and Ni(II) on p(PEI/HEA) and Cu(II)-p(PEI/HEA) hydrogels. (The concentrations of Cu(II), Pb(II), Cd(II) and Ni(II) were all equal to 1 mmol/L.)

Metal ions

Ionic charge

Ionic radius (Ǻ)

Cu(II)

2

Pb(II)

Distribution ratio (L/g)

Selectivity coefficient βCu2+/M2+

Relative selectivity coefficient βr

Cu(II)p(PEI/HEA)

p(PEI/HEA)

Cu(II)p(PEI/HEA)

p(PEI/HEA)

0.73

0.9870

0.3218

-

-

-

2

1.19

0.0179

0.0229

55.09

14.03

3.93

Cd(II)

2

0.95

0.0092

0.0127

107.47

25.28

4.25

Ni(II)

2

0.69

0.0156

0.0180

63.12

17.87

3.53

Table 5

Adsorption capacities and selectivity coefficients from several reported polymers.

Adsorbent Cu(II)-imprinted Poly(methacrylic acid/vinyl pyridine) polymer Fe3O4@SiO2-Cu(II)-imprinted polymer Cu(II)-imprinted Poly(chitosan/attapulgite) Cu(II) ion-imprinted Poly(methacrylic acid/vinyl pyridine) micro-particles Cu(II)-imprinted Poly(copper methacrylate/ ethylene glycol dimethacrylate) Copper-imprinted polymethacrylate porous beads Novel ligand modified composite adsorbent Ligand modified mesoporous adsorbent A novel facial composite adsorbent An efficient mesoporous adsorbent Tailor-made composite adsorbent Ligand based dual conjugate adsorbent Novel meso-adsorbent double-imprinted polymer Cu(II)-p(PEI/HEA) hydrogel

Adsorption capacity of Cu(II) ion (mg/g) 22.40 24.20 35.20 15.04 21.18 2.00 182.15 145.98 176.27 182.39 200.80 199.20 175.75 47.63 40.00

Selectivity coefficient βCu2+/M2+ Cu(II)/Ni(II) Cu(II)/Cd(II) Cu(II)/Zn(II) Cu(II)/Cd(II) Cu(II)/Ni(II) Cu(II)/Zn(II) Cu(II)/Ni(II) Cu(II)/Zn(II) Cu(II)/Ni(II) Cu(II)/Zn(II) Cu(II)/Ni(II) Cu(II)/Pb(II) Cu(II)/Cd(II) Cu(II)/Ni(II)

11.55 27.51 91.84 82.44 43.48 42.48 41.30 44.60 7.45 5.68 2771 55.09 107.47 63.12

References [44] [58] [65] [66] [67] [68] [4] [8] [9] [13] [14] [69] [70] [71] This work

Fig. 1. Scheme for synthesis of p(PEI/HEA) hydrogel.

Fig. 2. SEM images of p(PEI/HEA) hydrogel ((a) and (b)), and Cu(II)-p(PEI/HEA) hydrogel ((c) and (d)) with different magnifications; TEM images of p(PEI/HEA) and Cu(II)-p(PEI/HEA) hydrogel ((e) and (f)).

Fig. 3. FTIR spectra of p(HEA) (a), p(PEI/HEA) (b) and Cu(II)-p(PEI/HEA) (c).

Fig. 4. Effect of pH on the uptake of Cu(II) ions.

Fig. 5. Effect of temperature on adsorption of Cu(II) ions.

Fig. 6. Effect of initial concentration on the adsorption of Cu(II) ions onto hydrogels.

Fig. 7. Langmuir (a) and Freundlich (b) plots and linear fits of the experiment data.

Fig. 8. Effect of time on the adsorption of hydrogels for Cu(II) ions.

Fig. 9. Pseudo-first-order (a), pseudo-second-order (b) plots and linear fits of the adsorption data.

Fig. 10. TGA thermograms of the monomers (PEI, HEA) and the copolymer hydrogels p(PEI/HEA) and Cu(II)-p(PEI/HEA).

Fig. 11. TGA thermograms of Cu(II) chelated Cu(II)-p(PEI/HEA) hydrogel.

Fig. 12. Low-resolution XPS spectra of Cu(II)-p(PEI/HEA) hydrogel before and after Cu(II) adsorption.

Fig. 13. High-resolution XPS spectra of N1s (a) and O1s (b) of Cu(II)-p(PEI/HEA) hydrogel before and after Cu(II) adsorption.

Fig. 14. Repeated adsorption of Cu(II) by Cu(II)-p(PEI/HEA) hydrogel.