Fast Removal of Cr(VI) from Aqueous Solution Using Cr(VI)-Imprinted

Article pubs.acs.org/IECR

Fast Removal of Cr(VI) from Aqueous Solution Using Cr(VI)-Imprinted Polymer Particles Delong Kong, Fan Zhang, Keyuan Wang, Zhongqi Ren,* and Weidong Zhang* Beijing Key Laboratory of Membrane Science and Technology, Beijing University of Chemical Technology, Beijing, People’s Republic of China 100029 ABSTRACT: A novel Cr(VI) ion imprinted polymer (IIP) was proposed for fast removal of Cr(VI) from aqueous solution. 4Vinylpyridine and N,N-diethylaminoethyl methacrylate were used as functional monomers, and ethylene glycol dimethacrylate and 2,2-azoisobisbutyronitrile were used as cross-linker and initiator in the presence of a binary porogenic solvent. The prepared Cr(VI)-IIPs were characterized by the Fourier transform infrared spectroscopy and scanning electron microscopy. The effects of pH, initial concentration of Cr(VI) in aqueous solution, temperature, and operating time on adsorption were investigated. At the low pH range of 1.5−2.5, the adsorption capacities were high. The influence of temperature was slight. The adsorption equilibrium time was within 3 min. The adsorption process followed the pseudo-second-order equation and the Langmuir isotherm model. The maximum adsorption capacity was up to 286.56 mg/g, and the selectivity factors of Cr(VI)/Cu(II), Cr(VI)/Cd(II), and Cr(VI)/Cr(III) were up to 135.78, 145.44, and 69.91, respectively.

1. INTRODUCTION There are large amounts of industrial wastewater generated in industries such as electroplating, metallurgy, and leather, in which there are many heavy metals, including chromium, copper, zinc, nickel, lead, and others, which are harmful to the environment and living organisms. The pollution of heavy metals has attracted more and more attention recently. Among these heavy metals, chromium(VI) is known for its high toxicity and can cause various symptoms, such as lung cancer, kidney damage, liver disease, vomiting, severe diarrhea, and so on.1,2 Therefore, it is necessary to remove Cr(VI) from wastewater before disposal. The Environmental Protection Agency (EPA) set its maximum concentrations in drinking water and industrial wastewater at 50 and 200 μg/L, respectively. The conventional methods for these purposes, such as chemical precipitation, electrolysis method, liquid−liquid extraction,3 liquid membrane separation,4 adsorption using active carbon,5 bioadsorbents,6 and chelating resins,7 etc., encounter various difficulties. Chemical precipitation may cause secondary pollution, and the electrolysis method is energy consuming and economically unfavorable. For liquid−liquid extraction and liquid membrane separation, a large amount of high purity organic solvents are needed, most of which are harmful to the environment and health. The method of adsorption using various kinds of sorbents is widely applied to heavy metal treatment due to its simplicity and efficiency, such as active carbon, bioadsorbents, chelating resins, etc. However, the selectivities and capacities of adsorption of these sorbents should be improved. In order to overcome these drawbacks, it is necessary to develop a kind of low cost and environmentally friendly sorbent with high adsorption capacity and selectivity. Molecular imprinting technology (MIT) is a method for separating a target molecule by creating matching space cavities with the memory of the shape, size, and functional groups of the template molecule in the imprinting materials.8,9 It has been widely studied in many areas, including separation, catalysis, and signal acquisition.10 For the MIT system, the target © 2014 American Chemical Society

molecule is a template molecule, functional monomers are used to form interactions with the template before polymerization, and a cross-linking agent is used to ensure the intensity and stability of the imprinting cavity by cross-linking the polymerization reaction.11−13 In general, there are three steps to synthesizing molecular imprinting polymers (MIPs):14 (1) Functional monomers and template molecules are assembled by covalent and nocovalent interactions. (2) Polymerization reaction is carried out to fix the interactions between the template molecules and functional monomers. (3) Templates are removed from polymers to produce imprinting sites toward template molecules. It is observed that MIPs possess many good characteristics, such as simple preparation, low cost, high adsorption capacity and selectivity, etc. Ion-imprinting technology (IIT) is derived from MIT.15 Currently, applications of metal ion imprinted polymers (IIPs) to remove heavy metal ions have been reported. Say et al.16 prepared IIPs to selectively remove Cu(II) from aqueous solution; the maximum adsorption capacity of Cu(II) onto IIPs was about 48 mg/g. Sun et al.17 synthesized a new Cd(II)imprinted thiocyanato-functionalized silica gel sorbent; the maximum adsorption capacity was 49.3 mg/g. Singh et al18 also reported Ni(II)-IIPs by the copolymerization of 2-hydroxyethyl methacrylate (HEMA) with nickel vinylbenzoate complex; the adsorption capacity of IIPs was 1.51 mmol/g. Tan et al.19 developed a surface molecular-imprinted bioadsorbent for the removal of Ag(I); the maximum adsorption capacity was up to 199.2 mg/g. Current studies showed that the focus of IIPs was on the metal cation, and there were few reports about ionimprinting polymers used to remove the metal anion. Gao et al.20 prepared IIP-PEI/SiO2 for the adsorption on Cr(VI) by Received: Revised: Accepted: Published: 4434

October 16, 2013 January 4, 2014 February 24, 2014 February 24, 2014 dx.doi.org/10.1021/ie403484p | Ind. Eng. Chem. Res. 2014, 53, 4434−4441

Industrial & Engineering Chemistry Research

Article

Figure 1. Synthesis of Cr(VI)-imprinted polymer.

temperature, followed by purging with nitrogen for 10 min to remove the oxygen. The volume ratio of 4-VP, DEAEM, and EGDMA was 2.8/1.1/1, and the amount of AIBN was 50 mg. The prepared solution was distributed into weighing bottles, which were placed in a drying oven. The polymerization reaction was carried out at 60 °C for 24 h. After polymerization, the ion-imprinted polymers were dried in the oven. The resulting polymers were ground and sieved to form fine particles. To remove unreacted monomers and other ingredients, polymers were washed with acetone for 6 h at room temperature. Then, the template ions were removed from polymers with a NaOH solution (0.1 mmol/L) in a conical flask under magnetic stirring. The polymers were washed until template ions could not be detected. Prepared polymers were dried at 50 °C in the oven and then applied to the study on adsorption performance. 2.3. Characterization of Cr(VI)-IIPs. Fourier transform infrared spectroscopy (FTIR 8000 Series, Shimadzu) of IIPs before and after elution was conducted. The Cr(VI)-IIPs were mixed with KBr and pressed into a pellet for measurement. The morphology of the particles was evaluated from scanning electron microscopic (SEM) images (S-4700, Hitachi Ltd.). 2.4. Adsorption of Cr(VI)-IIPs. The adsorption performance of Cr(VI) from aqueous solution on IIPs was assessed in batch experiments. A 0.1 g sample of Cr(VI)-IIPs was dispersed in 100 mL of Cr(VI) anion aqueous solution. K2Cr2O7 solutions in a concentration range of 100−1000 mg/L were prepared, in which the pH was adjusted to 1.5−8.0 with dilute aqueous solution of HCl or NaOH. The effect of temperature on the adsorption capacity was investigated in the temperature range of 15−40 °C at different concentrations, which was applied to calculating adsorption thermodynamics basis data. Kinetic properties of adsorption process were studied at pH 2.0 with an initial concentration of 200 mg/L. Meanwhile, 0.1 g of Cr(VI)-IIPs was added to the solution at 25 °C by stirring. At a suitable time interval, a certain volume of the solution was withdrawn and centrifuged for analysis. The adsorption capacity of IIPs was calculated as follows:

surface imprinting. The adsorption equilibration time was 2 h, and the selectivity coefficient for PO43− was 12.23. Chimuka et al.21 synthesized ion-imprinted polymers for the selective removal of chromium(VI) from other metal anions. The adsorption capacity was 37.58 mg/g, and the selectivity for other anions was unsatisfied. Bayramoglu et al.22 prepared a high-performance Cr(VI) ion imprinted polymer, but the maximum adsorption capacity of IIPs was only about 170 mg/ g, and meanwhile the adsorption rate was slow. In this paper, a novel Cr(VI)-imprinted polymer (Cr(VI)IIP) was developed to improve the capacity, selectivity, and adsorption rate of Cr(VI) anion. The Cr(VI)-IIPs were prepared using 4-vinylpyridine (4-VP) and N,N-diethylaminoethyl methacrylate (DEAEM) as functional monomers and ethylene glycol dimethacrylate (EGDMA), and N,N-azoisobisbutyronitrile (AIBN) as cross-linker and initiator. The effects of pH, initial concentration of Cr(VI) anion aqueous solution, temperature, and adsorption time on absorption performance were investigated. Furthermore, thermodynamic and kinetic properties of the adsorption process using the new IIPs were studied to explore the mechanism of adsorption.

2. EXPERIMENTAL SECTION 2.1. Reagents. 4-Vinylpyridine (4-VP), N,N-diethylaminoethyl methacrylate (DEAEM), ethylene glycol dimethacrylate (EGDMA), and N,N-azoisobisbutyronitrile (AIBN) were obtained from China National Pharmaceutical Group Corp. (Beijing). Acetone, ethanol, potassium dichromate (K2Cr2O7), sodium hydroxide (NaOH), 1,5-diphenylcarbazide (Guangfu Refinement Chemical Institute, Tianjin City, China), copper chloride (CuCl2), cadmium chloride (CdCl2), chromium chloride (CrCl3), and other chemicals were all of analytical grade. 2.2. Preparation of Cr(VI)-IIP. As shown in Figure 1, Cr(VI)-IIP was synthesized through a thermally induced free radical polymerization. 4-VP, DEAEM, and Cr(VI) anion solution were dissolved in solvent (acetone/ethanol) and then were stirred for 1 h, forming a Cr(VI) anion−monomer complex. Certain amounts of EGDMA and AIBN were added to the well-mixed solution and stirred for 15 min at room

Q= 4435

(C0 − Cf )V m

(1)

dx.doi.org/10.1021/ie403484p | Ind. Eng. Chem. Res. 2014, 53, 4434−4441


Article

vibration adsorption of C−O, while 1750 and 1400 cm−1 belong to the characteristic adsorptions of CO and C−N, respectively. On the other hand, a new band was observed at 1300 cm−1, which could be assigned to the characteristic stretching vibration of the pyridine group after complexation of Cr(VI) anions. For Cr(VI)-IIPs that had been eluted, the Cr(VI) anion was eluted completely. These results indicated that the synthesis of Cr(VI)-IIPs was successful. 3.1.2. SEM. The morphology of the particles was evaluated from SEM images. From Figure 3a, the size of particles was in

where Q is the adsorption capacity, mg/g; C0 and Cf are the initial and equilibrium concentrations of the metal ion in aqueous solution, respectively, mg/L; V is the volume of added solution, L; and m is the weight of the Cr(VI)-IIPs, g. The selectivity of the IIPs for Cr(VI) anion was evaluated with batch experiments. A solution (100 mL) containing 200 mg/L Cr(VI) anion and 200 mg/L other ions was contacted with 0.1 g of IIPs at pH 2.0 and 25 °C. Distribution and selectivity coefficients of Cr(VI) with respect to Cu(II) or Cd(II) could be calculated as follows. Kd =

k=

C0 − Cf (V / m ) Cf

Kd(template metal ion) Kd(interferent metal ion)

(2)

(3)

where Kd and k represent distribution and selectivity coefficients; C0 and Cf represent the initial and equilibrium metal ion concentrations in aqueous solution, mg/L; V is the volume of the solution, mL; and m is the mass of Cr(VI)-IIPs, g. 2.5. Analytical Method. A pH meter (Denver UB-7) was used to determine the pH of the aqueous solution. An ultraviolet spectrophotometer (Model UV-1800, Shimadzu) was used to determine the concentration of Cr(VI) anion at 540 nm in the presence of 1,5-diphenylcarbazide.23 The concentrations of Cd(II), Cu(II), and the total amount of Cr(VI) and Cr(III) ions in the remaining solution were measured by inductively coupled plasma atomic electron spectroscopy (ICP-AES; ICPS-7500, Shimadzu).

3. RESULT AND DISCUSSION 3.1. Characterization Studies. 4-VP and DEAEM were selected as functional monomers, because they both contain nitrogen groups which can form complexation interactions between the Cr(VI) anion and protonated nitrogen of Cr(VI)IIPs. 3.1.1. FTIR. FTIR spectroscopy was used to determine complex formation between Cr(VI) anions and monomers. The FTIR spectra of IIPs (before elution and after elution) are shown in Figure 2. The comparison of these IR spectra showed that these polymers had a similar backbone. The band at 3400− 3500 cm−1 could be assigned to the stretching vibration for −OH of IIPs. The range 1100−1200 cm−1 corresponded to the

Figure 3. SEM images of surface particles: (a) IIPs (magnification to 200 μm); (b) IIPs (magnification to 1 μm); (c) NIP (magnification to 1 μm).

the range 50−100 μm. The SEM images of IIPs (Figure 3b) exhibited a very rough surface, while that of non-imprinted polymer (NIP, Figure 3c) showed a smoother surface. Due to the nature of bulk polymerization, the particles of IIPs displayed irregular shapes. The surface of IIPs exhibited some degree of porous surface that would benefit the adsorption process. 3.2. Effect of pH on Absorption. It is well-known that pH in aqueous solution is one of the key factors of adsorption performance. The effect of pH on the adsorption process was studied in the range 1.5−8.0. As shown in Figure 4, the

Figure 2. FTIR spectra of (a) IIPs before elution and (b) IIPs after elution. 4436



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Figure 4. Effect of pH on adsorption capacity (100 mL of 200 mg/L Cr(VI) anion solution, 0.1 g of IIPs, 10 min, 25 °C).

Figure 5. Effect of initial Cr(VI) concentration on adsorption capacity (100 mL of Cr(VI) solution, 0.1 g of IIPs, 10 min, pH 2.0, 25 °C).

adsorption capacity reached the maximum value at pH 2.0. At the low pH range of 1.5−2.5, the adsorption capacities maintained a high level and decreased quickly when the pH was beyond 2.5. It is mainly because the surface of Cr(VI)-IIPs is more positively charged due to the combination of nitrogen in the polymer matrix and protons at lower pH in aqueous solution, which would more easily combine with Cr(VI) anions. When the pH was higher than 2.5, the amount of protonated nitrogen decreased quickly, which was not beneficial for the adsorption of Cr(VI) anion. The adsorption capacity declined slightly from pH 2.0 to 1.5, and the results were attributed to the speciation of Cr(VI) such as H2Cr2O7, HCrO4−, and Cr2O72− at different pHs. According to the literature24 and the following balance expressions, HCrO4− and Cr2O72− predominate in acidic environment at pH 2−6 and CrO42− exists at pH >7; also H2Cr2O7 appeared at pH 1−2.

Table 1. Comparison of Different Adsorbents’ Performances

HCrO4 − ↔ CrO4 2 − + H+

(4)

H 2CrO4 − ↔ HCrO4 − + H+

(5)

Cr2O7 2 − + H 2O ↔ 2HCrO4 −

adsorbent

equilib time

qmax (mg/g)

ref

Cr(VI)-IIPs (4-VP/HEMA) IIP chitosan GMA, amino functionalized aniline formaldehyde based silica gel chitosan based on perlite black carbon poly(ethylene terephthalate) fiber poly(EGMA-co-VI) functionalized pyridine copolymers Cr(VI)-IIPs (this study)

40 min 24 h 30 min 90 min 300 min 4h 150 min 20 min 120 min 3 min

172.12 50.96 109.72 65.00 153.8 21.34 81.0 108.68 94.34 286.56

22 26 27 28 29 30 31 32 33 this work

maximum adsorption capacity of Cr(VI) ion imprinted polymers was up to 286.56 mg/g, which was far higher than those of other adsorbents listed in Table 1. The enhanced adsorption performance is caused by more imprinted sites created in the polymers, which also indicates that the selected monomers are easier to get complexation with Cr(VI) anion. 3.4. Adsorption Kinetics of Cr(VI)-IIPs. The adsorption rate of Cr(VI) was also studied, the results are shown in Figure 6. The adsorption rate was quite fast, and adsorption

(6)

HCrO4−

When the pH is in the range from 2.0 to 6.0, and Cr2O72− predominate. With decreasing pH, H2Cr2O7 is detected.25 H2Cr2O7 is not beneficial for the adsorption of Cr(VI) from aqueous solution, because H2Cr2O7 molecules in aqueous solution cannot be combined with protonated nitrogen of the polymer matrix by electrostatic interaction, and also cannot match the cavity with the memory of the shape, size, and functional groups of the template ion. 3.3. Effect of Initial Cr(VI) Concentration in Aqueous Solution on Absorption. The experiments were carried out in the initial Cr(VI) anion concentration range 100−1000 mg/ L at pH 2.0. As shown in Figure 5, the adsorption capacity increased with the increase of the initial concentration of Cr(VI) anion in aqueous solution, and it stepped to a plateau after the initial concentration of Cr(VI) anion reached 600 mg/ L. This is mainly because of the adsorption driving force caused by the concentration difference of the Cr(VI) anion between the bulk solution and the surface of the IIPs. In the Cr(VI) concentration range 100−600 mg/L, the adsorption driving force and the adsorption capacity increased with the increase of the Cr(VI) concentration. When the Cr(VI) concentration in aqueous solution was higher than 600 mg/L, the adsorption capacity was close to the saturation value and the adsorption capacity of Cr(VI)-IIPs did not change evidently. There were several adsorbents reported for the adsorption of Cr(VI) anion, and the results are listed in Table 1.22,26−33 In our studies, the

Figure 6. Effect of time on adsorption capacity (100 mL of 200.0 mg/ L Cr(VI) anion solution, 0.1 g of IIPs, 25 °C, pH 2.0).

equilibrium was reached within 3 min, which was faster than the other sorbents listed in Table 1. This is mainly because of the high complexation performance between Cr(VI) anions and functional monomers. Therefore, IIPs prepared in our studies could be considered as a good adsorbent that could be used in the analysis and detection of Cr(VI) anion in the future. In order to obtain the adsorption rate constants, the pseudofirst-order and pseudo-second-order kinetic models were 4437



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adopted to fit the experimental data, which can be defined by the following expressions: ⎛ k ⎞ log(qe − qt ) = log qe − ⎜ 1 ⎟t ⎝ 2.303 ⎠

Freundlich isotherm equation were used to calculate thermodynamic data as follows: ce c 1 = + e qe qmax b qmax (9)

(7)

t 1 t = + qt qe k 2qe 2

where qe is the adsorption capacity of Cr(VI) on IIPs, mg/g; ce is the concentration of Cr(VI) at equilibrium, mg/L; qmax is the maximum adsorption capacity, mg/g; and b is the Langmuir constant.

(8)

where qt is the adsorption capacity at time t, mg/g; k1 is the rate constant of the pseudo-first-order equation, min; and k2 is the rate constant of the pseudo-second-order equation, g/(mg· min). The best fit model is chosen according to linear regression correlation coefficients and the calculated qe values. Corresponding curves of both the pseudo-first-order equation and the pseudo-second-order equation are shown in Figure 7. The kinetic parameters are summarized in Table 2.

ln qe = ln k f +

ln ce n

(10)

where kf and n are the Freundlich constants, which are indicators of the adsorption capacity and intensity, respectively. The adsorption isotherms are shown in Figures 8 and 9. Relative calculation data, including the Langmuir and

Figure 8. Langmuir adsorption isotherm of Cr(VI)-IIPs (100 mL of Cr(VI) anion solution, 0.1 g of IIPs, 10 min, 25 °C, pH 2.0). Figure 7. Kinetic fitting plots of pseudo-first-order and pseudosecond-order equations (100 mL of 200 mg/L Cr(VI) anion solution, 0.1 g of IIPs, 25 °C, pH 2.0).

The calculated adsorption capacity from the pseudo-secondorder equation was 172.41 mg/g, which was in accordance with the experimental value, 173.49 mg/g. The correlation coefficient was 0.9998. The results indicated that these kinetic data agreed with the pseudo-second-order equation, not the pseudo-first-order equation. The adsorption behavior of IIPs was similar to chemical adsorption.34 The comparison of the pseudo-first-order and pseudosecond-order adsorption rate constants for Cr(VI)-IIPs (4VP/HEMA)22 and Cr(VI)-IIPs (4-VP/DEAEM) is listed in Table 2. Both the pseudo-first-order and pseudo-second order adsorption rate constants of Cr(VI)-IIPs (4-VP/DEAEM) in this study were greater than those of Cr(VI)-IIPs (4-VP/ HEMA). In further experiments, the adsorption time of 10 min is selected to ensure the adsorption equilibrium. 3.5. Adsorption Isotherms. To evaluate the adsorption performance of Cr(VI)-IIPs, adsorption isotherms of IIPs were investigated at different concentrations with 0.1 g of IIPs for 10 min at 25 °C and pH 2.0. The Langmuir isotherm equation and

Figure 9. Freundlich adsorption isotherm of Cr(VI)-IIPs (100 mL of Cr(VI) anion solution, 0.1 g of IIPs, 10 min, 25 °C, pH 2.0).

Freundlich adsorption constants and the corresponding correlation coefficients, are listed in Table 3. The results Table 3. Isotherm Parameters for the Adsorption of Cr(VI) on Cr(VI)-IIPs Langmuir model

Freundlich model

qmax (mg/g)

b (L/mg)

R2

kf

n

R2

294.12

0.0685

0.9997

34.12

4.2

0.8166

Table 2. Comparison of Pseudo-First-Order and Pseudo-Second-Order Adsorption Rate Constants and Adsorption Capacities first-order rate constant Cr(VI)-IIPs Cr(VI)-IIPs (4-VP/HEMA)22

−1

second-order rate constant 2

qe (mg/g)

k1 (min )

qe (mg/g)

R

173.49

0.5189 0.4080

16.95

0.8037

4438

k2 (g/(mmol·min))

qe (mg/g)

R2

0.437 0.172

172.41

0.9998



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physical absorption, because traditional physical adsorption is usually exothermic. This is also consistent with the results of adsorption kinetics. ΔS can characterize the randomness of the system. The positive value of ΔS indicates the adsorption of the Cr(VI) anion on IIPs makes the randomness increase on the surface. 3.7. Selectivity of IIPs. To investigate the adsorption selectivity of Cr(VI)-IIPs, the binary mixtures of Cr(VI)/ Cu(II), Cr(VI)/Cd(II), and Cr(VI)/Cr(III) were considered . A comparison of the selectivity factor of IIPs is listed in Table 5.

showed that the adsorption process was more similar to the Langmuir isotherm adsorption than the Freundlich isotherm; the correlation coefficients, R2, were 0.997 and 0.8166, respectively. The calculated maximum adsorption capacity of IIPs was up to 294.12 mg/g based on the Langmuir adsorption equation, which was very close to the experimental data. Then, it showed that the adsorption occurred in a monolayer and at a fixed number of identical adsorption sites.29 Furthermore, the Freundlich constant n value was 4.20, which was far greater than 1 as listed in Table 3. This further confirmed that the adsorption process could be considered as Langmuir adsorption. 3.6. Thermodynamic Parameters. The temperature is an important parameter for the adsorption of Cr(VI) on IIPs. As shown in Figure 10, the adsorption capacity of the Cr(VI)

Table 5. Selectivity of Cr(VI)-IIPs Cr(VI)-IIPs Cr(VI) Cu(II) Cr(VI) Cd(II) Cr(VI) Cr(III)

anion did not change in the range from 288 to 313 K evidently. In other words, the effect of temperature on adsorption was not significant. To further study the mechanism of adsorption, some thermodynamic parameters can be calculated using the following equations:22 (11)

ΔG = ΔH − T ΔS

(12)

where R is the universal gas constant; K is the Langmuir constant (b); ΔG is the Gibbs free energy change, kJ/mol; ΔH is the adsorption heat, kJ/mol; and ΔS is the adsorption entropy change, kJ/(K·mol). The thermodynamic parameters are listed in Table 4. The negative values of ΔG indicate the adsorption process for IIPs Table 4. Thermodynamic Parameters for the Adsorption of Cr(VI)-IIPs T (K)

ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (kJ/K·mol)

298 308 318

−20.26 −21.05 −22.06

6.56 6.56 6.56

0.09 0.09 0.09

Kd (L/g)

k

156.22 11.56 144.54 8.1 173.79 29.76

7.972 0.059 8.971 0.062 10.33 0.148

135.78 145.44 69.91

The maximum selectivity factors of Cr(VI)/Cu(II), Cr(VI)/ Cd(II), and Cr(VI)/Cr(III) were up to 135.78, 145.44, and 69.91, respectively. In addition, the adsorption experiments were conducted in a mixed system comprising potassium sulfate and potassium phosphate (concentration 200 mg/L for anions). The results showed that the adsorption capacity of Cr(VI)-IIPs in the system was up to 153.20 mg/g, and it had a slight decrease compared with that in the one-component system. In other words, the prepared Cr(VI)-IIPs in our work had good adsorption selectivity for Cr(VI) in the presence of other anions, such as SO42− and PO43−. Then, the results indicated that Cr(VI)-IIPs had a high selectivity because of the effect of imprinting. That is, the created cavities of IIPs matched the Cr(VI) ion in size, shape, and coordination. Also, the monomers could provide required interaction groups for the chelation of the Cr(VI) anion. On the other hand, the protonated nitrogen of the IIP matrix weakened its chelation interactions with other metal ions. 3.8. Desorption and Repeated Use. The stability and regeneration of adsorbents are important factors in the industrial application of Cr(VI)-IIPs. The elute solution was 0.1 M NaOH solution. A 0.1 g sample of IIPs with saturation adsorption of Cr(VI) was eluted in a bottle containing 100 mL of elute solution under stirring. Until the Cr(VI) could not be detected, the regenerated IIPs were reused the next time. In order to show the reusability of the Cr(VI)-IIPs, the adsorption−desorption cycle was repeated five times using the same Cr(VI)-IIPs. The results are shown in Figure 11. The adsorption capacity of Cr(VI)-IIPs decreased only about 5% at the fifth cycle. The results showed that the Cr(VI)-IIPs could be reused without a significant decrease in their adsorption capacity. The results showed that Cr(VI)-IIPs had good stability and regeneration, which could be used in practical wastewater treatment.

Figure 10. Effect of temperature on adsorption capacity (100 mL of 200 mg/L Cr(VI) anion solution, 0.1 g of IIPs, 10 min, pH 2.0).

ΔG = −RT ln K

Q (mg/g)

is spontaneous. The absolute value of ΔG is greater than 20 kJ/ mol, which suggests that monomers employed can complex with the template ion easily. The Gibbs free energy decreases with the increase of temperature from 298 to 318 K. This shows that the adsorption process is more spontaneous at higher temperatures. A positive value of ΔH indicates that the process is endothermic; this is in accordance with the reported results.30 It also demonstrates that the adsorption process is not

4. CONCLUSIONS The metal ion imprinted technology is a promising method for the selective removal of metal ion with high adsorption capacity and selectivity. In this paper, 4-vinylpyridine and N,Ndiethylaminoethyl methacrylate were used as functional monomers and ethylene glycol dimethacrylate and 2,24439



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REFERENCES

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Figure 11. Stability and regeneration of Cr(VI)-IIPs (100 mL of 200 mg/L Cr(VI) anion solution, 0.1 g of IIPs, 10 min, pH 2.0).

azoisobisbutyronitrile were used as cross-linker and initiator in the presence of a binary porogenic solvent. A novel Cr(VI)-IIP was prepared. FTIR and SEM were used to confirm the complex formation and the morphology of the particles. The optimal pH of the adsorption process was 2.0, and the maximum adsorption capacity was up to 286.56 mg/g at the initial Cr(VI) concentration of 1000.0 mg/L, which was much higher than those of other sorbents. The dynamic study results agreed with the pseudo-second-order kinetic equation. The Langmuir isotherm model was adopted for the description of this adsorption process. The results also showed that Cr(VI)IIPs had high selectivity, good stability, and regeneration.

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ΔG = Gibbs free energy change [kJ/mol] ΔH = adsorption heat [kJ/mol] ΔS = adsorption entropy change [kJ/(K·mol)]

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (Nos. 21076011 and 21276012) and the Program for New Century Excellent Talents in University (No. NCET10-0210). The authors gratefully acknowledge these grants.

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LIST OF SYMBOLS Q = adsorption capacity [mg/g] C0 = initial solution concentration of Cr(VI) or Cu(II) ion [mg/L] Cf = equilibrated solution concentration [mg/L] C = concentration of ion in aqueous solution [mg/L] V = volume of the solution [mL] m = mass of IIPs [g] Kd = distribution coefficient k = selectivity coefficient qe = equilibrated capacity of adsorption [mg/g] qt = capacity of adsorption at time t [mg/g] ce = equilibrated concentration [mg/L] qmax = maximum capacity of adsorption [mg/g] k1 = rate constant of pseudo-first-order equation [min] k2 = rate constant of pseudo-second-order equation [g/(mg min)] kf = Freundlich constant n = Freundlich constant R = universal gas constant K = Langmuir constant (b) 4440



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Fast Removal of Cr(VI) from Aqueous Solution Using Cr(VI)-Imprinted

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