Ion-Imprinted Composite Hydrogels with Excellent Mechanical

Article pubs.acs.org/IECR

Ion-Imprinted Composite Hydrogels with Excellent Mechanical Strength for Selective and Fast Removal of Cu2+ Jingjing Wang*,† and Xianning Li‡ †

School of Energy and Environment, Southeast University, Nanjing 210096, China, and Department of Polymer Materials and Engineering, School of Material Engineering, Yancheng Institute of Technology, Yancheng 224051, China ‡ School of Energy and Environment, Southeast University, Nanjing 210096, China ABSTRACT: In the present study, Cu2+-imprinted composite hydrogel (Cu2+-ICH) has been prepared by in situ free-radical polymerization. The investigation of mechanical property indicated that the compression strength of the Cu2+-ICH was significantly enhanced by the introduction of silica nanoparticles. The ability of the Cu2+-ICH to adsorb and remove Cu2+ from aqueous solutions was assessed using batch adsorption technique. The adsorption amount was pH dependent, and the maximum adsorption capacity was observed at pH 5.0. The adsorption process could be well described by the Langmuir isotherm. The adsorption equilibrium was achieved within 20 min, and the kinetics of adsorption followed a pseudo-second-order rate equation. The selectivity coefficient of Cu2+-ICH for Cu2+ was 5 times greater than that of the nonimprinted composite hydrogels (NICH), indicating that the Cu2+-ICH had strong ability to selectively adsorb Cu2+ from several heavy metal ions present in aqueous solutions. Regeneration studies suggested that metal rebinding capacity of the Cu2+-ICH did not change significantly through repeated applications compared with the first run. This suggests that the Cu2+-ICH is a promising sorbent material for the selective removal of Cu2+ from aqueous solutions.

1. INTRODUCTION The presence of heavy metal ions in water streams has readily increased as a result of industrialization and urbanization. In the past decades, several techniques, such as chemical precipitation, solvent extraction, micellar ultrafiltration, and organic and inorganic ion exchange have been employed for the removal of heavy metal ions from aqueous solutions.1,2 Most of these techniques suffered from technical, economic, environmental, and health problems related to low efficiency, long time of processing, high energy consumption, and the large quantity of hazardous materials used.3 However, adsorption is an attractive method because of its high efficiency, ease of handling, and the availability of different adsorbents. Various kinds of new adsorbents for removing and recovering heavy metal ions have been reported. Among them, polymeric hydrogels are considered to be particularly effective because of their chemical stability and high selectivity.4−6 Hydrogels containing amide, amine, carboxylic acid, and ammonium groups can bind with heavy metal ions by virtue of the functional groups, and function as good adsorbents in water purification processes.7 Other important properties of hydrogels are their ability to control the diffusion process, their swelling response to ionic strength, and pH.8 Additionally, the ease of operation, regeneration, and high efficiency for these materials encourage their applications in wastewater treatment. Up to now, the removal of various kinds of heavy metal ions, including Cd2+, Pb2+, Cu2+, and Mn2+ based on hydrogels has been reported.9 However, hydrogels prepared from either natural or synthetic sources usually exhibit poor mechanical properties.10 The introduction of an inorganic phase inside the polymer matrix in the form of nanoparticles has been proved to be an effective way to improve the mechanical properties of hydrogels.11,12 © 2012 American Chemical Society

Silica sol is a dispersion system of amorphous silica particles in water, which can be divided into acid and basic sol according to the pH value. Due to the strong surface activity of hydroxyl groups on the surface of silica particles, it can mix and react with organic polymer. Thus, it has been widely applied in organic and inorganic composites.13 Generally adsorbents could not show specific selectivity to certain individual heavy metal ions. Selectivity is achieved only by adsorbents with specific affinity to definite metal ions or groups of metals. Molecularly imprinted adsorbents (MIA) represent a new class of materials possessing high selectivity and good affinity for the target molecule.14 As a branch of MIA, ion-imprinted adsorbents have shown considerable promise as a method for preparing materials which are capable of ion recognition.15 To the best knowledge of the author, there have been no reports about the silica based ion-imprinted composite hydrogels used for selective metal ion adsorption. In the current research, Cu2+-imprinted composite hydrogel (Cu2+ICH) has been prepared by in situ free-radical polymerization for selective and fast removal of Cu2+. The influence of pH value of the feed solution on the adsorption amount of Cu2+ was studied. Then the adsorption kinetic and isotherm of the imprinted composite hydrogels were investigated. Finally, the selective adsorption and regeneration abilities of the imprinted composite hydrogels were evaluated. Received: Revised: Accepted: Published: 572

August 16, 2012 November 20, 2012 November 25, 2012 November 26, 2012 dx.doi.org/10.1021/ie3022016 | Ind. Eng. Chem. Res. 2013, 52, 572−577

Industrial & Engineering Chemistry Research

Article

2. EXPERIMENTAL SECTION 2.1. Materials. Acrylamide (AM) and silica nanoparticles (10 nm in size) were purchased from Sigma-Aldrich Chemicals. N,N′-Methylene bisacrylamide (MBA) and ammonium persulfate (APS), used as a cross-linking agent and an initiator, were purchased from Sigma-Aldrich Chemicals and used without further purification. Copper nitrate [Cu(NO3)2·3H2O)] used in sorption experiments was purchased from Shanghai Chemical Reagents Co., China. To improve the metal ion adsorption capacity, silica was modified by the amination reaction using the method as described in the literature.16,17 The structure of the modified silica (SiO2−NH2) is shown in Figure 1.

for different periods of time, respectively. V (L) was the volume of the solution added, and m (g) was the amount of sample used. 2.5. Adsorption Kinetics. Batch studies were carried out using the Cu2+-ICH to determine the effect of time duration on the chelation of Cu2+ at 25 °C. The Cu2+-ICH was added into the Cu2+ solution (0.005 mol/L) at pH = 5. At regular intervals, amount of adsorbed Cu2+ was determined as mentioned above. 2.6. Equilibrium Adsorption Isotherm. The studies were carried out using the Cu2+-ICH to determine the effect of the initial metal ion concentration, which varied from 0.0005 to 0.005 mol/L, on the adsorption capability. The pH and temperature of incubation were 5.0 and 25 °C, respectively. 2.7. Selective Adsorption Studies. The selectivity of the Cu2+-ICH and NICH for Cu2+ over other heavy metal ions was evaluated from the selectivity coefficient (βCu2+/M2+),18 which was defined as

Figure 1. The structure of the modified silica (SiO2−NH2).

βCu2+/M2+ =

2.2. Preparation of Cu2+-Imprinted Composite Hydrogels. The Cu2+-imprinted composite hydrogel (Cu2+-ICH) was synthesized by in situ free-radical polymerization with MBA as a cross-linker and Cu2+ as the template. The detailed synthesis route was described as follows. A mixture of 1.5 g of AM, 0.5 g of SiO2−NH2, and 0.44 g of Cu(NO3)2·3H2O were added into distilled water. And the mixture was stirred vigorously for 1 h. Then 0.075 g of MBA (5% w/w based on AM) and 0.015 g of APS (1% w/w based on AM) were added into the solution as the cross-linker and initiator. The solution was heated for 24 h in a constant temperature water bath at 65 °C until the monomer AM polymerized completely. Then the product was grinded and treated with 1.0 mol/L HCl to completely leach Cu2+. At last, the composite hydrogel was filtered with distilled water to neutralization, and dried at 60 °C under vacuum, resulting in the desired Cu2+-ICH. By comparison, the nonimprinted composite hydrogel (NICH) was similarly synthesized in the absence of Cu(NO3)2·3H2O. 2.3. Mechanical Strength Studies. The compressive strength measurements were performed by using an Instron1121 tensile-compressive tester. The cylindrical sample of 14 mm diameter and 10 mm thickness was set on the lower plate and compressed by the upper plate, which was connected to a load cell, at a strain rate of 2 mm/min. The mechanical strength reported here was an average value of at least three measurements. 2.4. Adsorption Capability Studies. Adsorption studies were carried out in magnetically stirred (160 rpm), thermostated (25 °C) cylindrical glass vessels in batch conditions. The sample was added into the heavy metal ion solution (0.005 mol/L metal ion, 500 mL) to determine the metal ion adsorption capacity of the samples under noncompetitive conditions. The pH of the metal feed solutions was adjusted before the hydrogels were applied for the adsorption process. Amount of the residual metal ion in the solution was determined using a Thermo Elemental-X Series inductively coupled plasma-mass spectrometer (ICP-MS) after 24 h.5 Amount of adsorbed metal ion (Q, mmol/g), was calculated from the following equation:

Q=

DCu2+ D M2+

where DCu2+ and DM2+ were the distribution ratios of the Cu2+ and other coexistent heavy metal ions, respectively. The distribution ratio (D) was calculated by using the following expression: D=

C0 − Ce V × Ce W

where C0 and Ce were the concentrations of metal ions in the initial solution and equilibrium solution (mol/L), respectively, V was the volume of the aqueous solution (L), and W was the mass of dry composite hydrogel (g). The effect of imprinting on selective adsorption was evaluated with the relative selectivity coefficient βr, which can be defined as the following expression: βr =

βimprinted βnonimprinted

where βimprinted and βnonimprinted were selectivity coefficients of the imprinted and nonimprinted composite hydrogels, respectively. 2.8. Regeneration Studies. The metal ions were freed from the metal-complexed Cu2+-ICH by treating with 1 mol/L HCl. The acid-treated Cu2+-ICH was filtered and washed with distilled water to remove acid. The concentration of desorebed metals was determined as mentioned above. The desorption ratio was calculated as follows: desorption ratio = (amount of metal ion desorbed into solution/amount of metal ion bound to Cu2+-ICH) × 100%. A second metal binding cycle was repeated with the regenerated Cu2+-ICH. Recovery ratio = (amount of Cu2+ adsorpted in the second run/amount of Cu2+ adsorpted in the first run) × 100%.

3. RESULTS AND DISCUSSION 3.1. Mechanical Property. The study of mechanical property was conducted to investigate the effect of modified silica (SiO2−NH2) on the compression strength. From the results, it was found that the polyacrylamide (PAM) hydrogel broke at a stress of 0.4 MPa, whereas Cu2+-ICH could sustain the stress of 7.8 MPa, which was more than 19 times that sustained by the PAM hydrogel matrix. It was obvious that the compression strength of the Cu2+-ICH could be significantly

(C 0 − C )V m

where C0 (mmol/L) and C (mmol/L) were the metal ion concentrations in the initial solution and after the adsorption 573

dx.doi.org/10.1021/ie3022016 | Ind. Eng. Chem. Res. 2013, 52, 572−577


Article

enhanced by the introduction of silica nanoparticles.12 As for the composite hydrogels, the cross-linked PAM chains acted as the network and the nanoparticles filled in the gap of the network. The two components of the composite hydrogels could be connected via both hydrogen bonds and physical interaction, which was same as for the interpenetrating polymer network (IPN) hydrogels.19,20 The distribution of the modified silica and the hydrogen bonding linkages within the hydrogel frame was shown in Figure 2. When the composite hydrogels

Under acidic conditions, the Cu 2+-ICH surface was completely covered with H+ ions and the Cu2+ could not compete with them for adsorption sites. However, with increasing pH value, the competition from the H+ ions decreased and Cu2+ could be adsorbed on the adsorbent.23 To obtain the optimum adsorption amount and avoid hydrolyzation or precipitation, an initial pH value of 5.0 was selected as the experimental condition. 3.3. Kinetics of Cu2+ Adsorption. To achieve the proper design of an adsorbent, the adsorption equilibrium needs to be supplemented with adsorption kinetics, which offers information on the rate of metal adsorption. The time required to achieve adsorption equilibrium for Cu2+ from aqueous solutions was determined for the Cu2+-ICH. The relationship between adsorption capacity and adsorption time is described in Figure 4. As seen, the initial adsorption of Cu2+ was rapid, and the

Figure 2. Schematic illustration of the distribution of the modified silica and the hydrogen bonding linkages within the hydrogel frame. Figure 4. The time dependence of adsorption capacity of Cu2+ by the Cu2+-ICH.

were compressed, the PAM network was destroyed while the silica component still remained intact, which kept the crack from growing to a macroscopic level.21 Thus, the compression strength of composite hydrogels was far higher than that of PAM hydrogel. 3.2. Effects of Initial pH on Cu2+ Adsorption. The aqueous solution pH value is probably the most important parameter affecting the adsorption process, because it affects the solubility of the metal ions, and the degree of ionization of the sorbent during the process. The distribution of metal species in aqueous solution as a function of pH reveals that Cu2+ precipitate in the forms of metal oxides or hydroxides at pH > 6.22 Hence, the experiments were conducted at pH = 2− 5. Figure 3 shows the effect of pH on the adsorption of Cu2+ by

adsorption equilibrium was gradually achieved within 20 min. It can be explained that during the initial stage in the process, a greater number of adsorption sites were available to the Cu2+, enabling them to interact readily with the adsorbent and hence leading to a high adsorption rate. The slow adsorption rate observed during the latter stage of the process may be attributed to the slower rate of diffusion of the solute into the interior of the adsorbent particles.20 In fact, the adsorption rate was dependent on the rate at which the metal ions were transported from the bulk liquid phase to the actual adsorption sites. To examine the controlling mechanism of an adsorption process such as mass transfer and chemical reaction, the kinetic data were fitted by the pseudo-first-order and pseudo-secondorder kinetic models,24 which were respectively expressed as ln(qe − qt) = ln qe − k1t t 1 t = + 2 qt qe k 2qe

where t was the contact time (min), qt and qe were the amount of Cu2+ adsorbed at an arbitrary time t and at equilibrium (mg/ g) respectively, and k1 (min−1) and k2 (g mg−1 min−1) were the rate constants of adsorption, respectively. The plot of ln(qe − qt) versus t, and t/qt versus t for the adsorption of Cu2+ were respectively shown in Figures 5 and 6. The results showed that the correlation coefficient for the pseudo-first-order kinetic model was relatively low (R2 = 0.954). However, the pseudo-second-order rate equation for adsorption of Cu2+ onto Cu2+-ICH agreed well with the data (R2 = 0.998). It could be said that the pseudo-second-order kinetic model provided a good correlation for the adsorption of

Figure 3. Influence of pH values on the adsorption capacity of Cu2+ICH and NICH.

imprinted and nonimprinted composite hydrogels. It could be seen that the adsorption capacity for both composite hydrogels increased remarkably with increasing pH values up to 5.0. The highest adsorption amount for Cu2+ onto Cu2+-ICH and NICH were 8.48 and 4.96 mmol/g, respectively, from which the imprinting effect was clearly observed. 574



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adsorption approached saturation. The maximum static adsorption capacity of Cu2+-ICH was 8.48 mmol/g . The data for the uptake of Cu2+ on Cu2+-ICH were then fitted to the Langmuir and Freundlich isotherm equation in the linearized form,25 which were respectively expressed as Ce C K = + e qe qm qm

ln qe = Figure 5. Pseudo-first-order kinetic for adsorption of Cu2+ by the Cu2+-ICH.

1 ln Ce + ln KF n

where qe was the equilibrium metal ion adsorption amount (mol/g), Ce was the equilibrium metal ion concentration in the solution (mol/L), qm represented the maximum amount of metal ions that could be adsorbed on the Cu2+-ICH (mol/g), and K was a constant of the Langmuir model (mol/L). KF (mol/g) and n were the Freundlich constants depicting the adsorption process. The plot of Ce/qe against Ce and ln qe versus ln Ce for the experimental data were respectively shown in Figures 8 and 9.

Figure 6. Pseudo-second-order kinetic for adsorption of Cu2+ by the Cu2+-ICH.

Cu2+ onto Cu2+-ICH in contrast to the pseudo-first-order model. The pseudo-first-order model considers that the adsorption process may proceed by diffusion of Cu2+ through the boundary layer at the sorbent surface and this may be the rate determining step of the overall process. The pseudosecond-order model assumes that the chemical adsorption may be the rate controlling step, which involves valence forces through sharing electrons between metal ions and adsorbent. From the results of the correlation coefficients, it was indicated that chemical adsorption could be the rate-limiting step for the adsorption process of Cu2+ onto Cu2+-ICH.24 3.4. Evaluation of Adsorption Isotherm Models. The equilibrium isotherm is fundamental to describe the interactive behaviors between the solutes and adsorbents. The Cu2+ adsorption isotherms of the Cu2+-ICH were investigated at pH 5.0 with the initial Cu2+ concentration in the range 0.0005− 0.005 mol/L. As shown in Figure 7, the adsorption amount increased sharply in the initial concentration, but after the concentration of 0.004 mol/L, it reached a plateau because the

Figure 8. Illustration of the experimental adsorption isotherm data presented in terms of the linearized Langmuir model.

Figure 9. Illustration of the experimental adsorption isotherm data presented in terms of the linearized Freundlich model.

The adsorption of Cu2+ on Cu2+-ICH was well fitted to the Langmuir isotherm model (R2 = 0.999). However, the correlation coefficient of Freundlich isotherm model was low (R2 = 0.980). The values of KF and n were calculated to be 0.0234 mol/g and 5.35. The theoretical qm value (9.14 mmol/ g) calculated from the Langmuir adsorption model was close to the experimental value (8.48 mmol/g). As a result, the Langmuir adsorption model, which was considered to be the formation of monolayer coverage of analytes at the surface of the sorbent, can be applied in the adsorption process of Cu2+ onto Cu2+-ICH.25 3.5. Evaluation of the Selective Adsorption. To evaluate the selectivity of Cu2+-ICH, the selective adsorption

Figure 7. Adsorption capacities for Cu2+ at various concentrations on the Cu2+-ICH at pH = 5. 575



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studies were conducted with binary mixtures containing Cu2+, Pb2+, Cd2+, or Ni2+ at equal concentration under initial pH 5.0. The distribution ratios and selectivity coefficients with respect to other heavy metal ions (that are likely to coexist with Cu2+ in natural sources) using Cu2+-ICH and NICH are shown in Table 1. It can be seen that the distribution ratio of Cu2+-ICH

Table 2. Desorption and Recovery Ratios of the Cu2+-ICH, % cycle I desorption ratio, % recovery ratio, %

2+

Table 1. Selective Adsorption Property of Cu -ICH and NICH

ions Cu2+ Pb2+ Cd2+ Ni2+

distribution ratio, mL/g

selectivity coefficient βCu2+/M2+

Cu2+-ICH NICH

Cu2+-ICH NICH

1562 280 258 265

302 271 255 260

5.58 6.05 5.89

1.11 1.18 1.16

cycle IV

cycle V

cycle VI

98.9

cycle II cycle III 98.7

98.3

98.1

98.0

97.8

98.1

97.7

97.5

97.2

97.1

96.8

efficiency was quite high and the adsorption capacity was affected very little by the cycles. In all, due to the high recycling efficiency, the composite hydrogels were qualified for practical application.

relative selectivity coefficient βr

4. CONCLUSIONS In the current study, Cu2+-imprinted composite hydrogel (Cu2+-ICH) has been prepared by in situ free-radical polymerization. The batch experiment showed that the Cu2+ adsorption on Cu2+-ICH was pH dependent, and the maximal Cu2+ uptake achieved at pH 5.0. The adsorption equilibrium was achieved within 20 min, and the kinetics of adsorption followed a pseudo-second-order rate equation. The adsorption isotherm was better fitted by the Langmuir equation, indicating the formation of monolayer coverage of Cu2+ on the surface of Cu2+-ICH. An overall selectivity for Cu2+ was observed showing that Cu2+-ICH could be used effectively to remove and recover Cu2+ from aqueous solutions. Cu2+ adsorbed on Cu2+-ICH could be easily desorbed at low pH values and the regenerated Cu2+-ICH could be reused almost without any loss of adsorption capacity for a few cycles.

5.03 5.13 5.08

for Cu2+ was 5-fold greater than that of NICH, whereas it was almost equal for other heavy metal ions. Furthermore, the relative selectivity coefficient of Cu2+-ICH for each individual heavy metal ion was far greater than 1. The obtained results were in agreement with the previous reports about the selective adsorption ability of the Cu2+-imprinted sorbent.26,27 These observations were attributed to the specific recognition cavities for Cu2+ created in Cu2+-ICH, which were developed by ion-imprinting. On the basis of the results shown in Table 1, it was evident that the Cu2+-ICH had strong ability to selectively adsorb Cu2+ from several heavy metal ions present in aqueous solutions. 3.6. Desorption and Reusability Study. Desorption experiments were conducted to regenerate Cu2+ loaded Cu2+ICH. The pH effects from 1.0 to 10.0 on desorption of Cu2+ are shown in Figure 10. The desorption of Cu2+ from Cu2+-ICH

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This project was supported by China Postdoctoral Science Foundation (No. 2012M520979) and Jiangsu Postdoctoral Science Foundation (No. 1201009B).

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REFERENCES

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Figure 10. Effect of pH value on percentage of desorbed Cu2+.

was easier at lower pH than at higher pH. Furthermore, at higher pH range (pH > 4.0), the percentage of Cu2+ desorption was near zero, but about 99% at pH < 2.0. Therefore, the Cu2+ loaded Cu2+-ICH could be regenerated at lower pH. These results also agreed with a previous report on the desorption study of the Cu2+-imprinted polymeric nanoparticles.26 The adsorption and desorption processes were repeated to examine the potential of the Cu2+-ICH in practical applications. Table 2 shows the experimental results of the percentages of desorption and recovery of the Cu2+-ICH in six consecutive adsorption−desorption cycles. It was found that desorption 576



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Ion-Imprinted Composite Hydrogels with Excellent Mechanical

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