Particle-Assisted Ion-Imprinted Cryogels for Selective CdII Ion

Jan 22, 2015 - Herein, we have reported particle-assisted ion-imprinted cryogels, which were synthesized by embedding finely crushed functional partic...
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Particle-Assisted Ion-Imprinted Cryogels for Selective CdII Ion Removal Bekir Tabaklı,† Aykut Arif Topçu,†,‡ Serhat Döker,§ and Lokman Uzun*,† †

Department of Chemistry, Hacettepe University, Ankara, Turkey Department of Biology, Kırıkkale University, Kırıkkale, Turkey § Department of Chemistry, Ç ankırı Karatekin University, Ç ankırı, Turkey ‡

S Supporting Information *

ABSTRACT: Herein, we have reported particle-assisted ion-imprinted cryogels, which were synthesized by embedding finely crushed functional particles into cryogel structure under semifrozen conditions. These cryogels showed high adsorption efficiency and affinity against CdII ions. CdII adsorption performances were evaluated by varying some effective factors. In order to analyze the data, we applied Langmuir and Freundlich adsorption isotherms while using three different kinetic models, pseudo-first-order, pseudo-second-order, and Weber−Morris as well. Also, the dimensionless equilibrium parameter (RL), initial adsorption rate, and half-adsorption time were calculated. The results revealed that ion-imprinted-polymer (IIP) cryogels have homogeneously distributed cavities, which were formed by a particle-assisted imprinting process, and the theoretical maximum adsorption capacity (Qmax, 35.97 μg/g) was very close to the experimental value (Qeq, 32.15 μg/g). In addition, RL values for both IIP and nonimprinted-polymer cryogels showed favorability of the adsorption process, while kinetic models indicated that there were no diffusion limitations during the adsorption process, which means that the rate-limiting step was chemosorption of heavy-metal ions on binding sites (imprinted cavities or functional groups) with a high initial adsorption rate and a low half-adsorption time. Desorption, reusability, and selectivity studies were also conducted to state the performance of the cryogels. In conclusion, this promising approach provides a novel methodology for selective CdII removal from water sources.

1. INTRODUCTION The presence of heavy-metal ions, as a group of persistent environmental pollutants, is one of the main technical challenges for real-time water monitoring and quality testing processes.1 Heavy-metal ions have been released by several industrial activities such as leaking of waste batteries and paints, mining activities, discharge from metal refineries, erosion of natural deposits, corrosion of galvanized pipes, and burning of fossil fuel and coal.2,3 Heavy-metal ions have several adverse effects on human health because of their relative ease of merging into the food chain at several points and cause serious health problems by poisoning the metabolism and inhibiting enzymatic regulators.4 For instance, cadmium (Cd), a toxic heavy metal found naturally in the environment and being released by several industrial activities, nephrotoxically effects the kidney and bone in two ways: direct effect on bone tissue or indirect effect by renal dysfunctionalization.5 Because of its density, which is the main identification criteria of heavy metals,6 Cd is another heavy metal7,8 having a long half-life that causes toxic effects like inhibition of some enzyme activities in plants7 and is harmful for kidneys and bone;8 however, it is still widely used in many areas such as coated materials for preventing corrosion,9 batteries and alloys,6 and agricultural areas.10 On the basis of importance, many approaches have been developed and reported in related literature for the removal of heavy-metal ions from water sources. Adsorption is the most studied method with respect to efficiency, ease-of-use, wide adsorbent spectrum, and operation cost.11−13 Several studies with different fundamentals have been reported for CdII ion removal from water in the literature with respect to © XXXX American Chemical Society

important terms of cost, efficiency, capacity, reusability, and applicability.2,4,14−20 Molecular imprinting is a versatile technique for creating specific binding regions for select (protein, metal ions, and pesticide) molecules.21 Basically, interested molecules, called targets, are interacted with functional monomer(s) before and then removed from the polymer matrix after the polymerization process. In this way, it is possible to create cavities in the polymeric network that have specific binding and selective recognition abilities against the target. Because of these properties, molecular imprinting polymers (MIPs) have been used for various applications such as chromatography, biosensors, solid-phase extraction, and affinity interactions.22−25 Ion-imprinted polymers (IIPs) are also interesting adsorbents that have been prepared for the specific removal of heavy-metal ions from wastewater effluents26−29 and were synthesized via the same approach with MIPs except using an ion as the template instead of a molecule.30 Although MIPs have excellent properties, they have some drawbacks, such as the formation of heterogeneous distribution of cavities, diffusion limitations of large molecules, and elution efficiency. However, there are some new approaches that have been developed to overcome these challenges, including imprinting on nanoparticles, surface imprinting, epitope imprinting, microcontact imprinting, and double imprinting via functional particles.31−35 Received: November 3, 2014 Revised: January 19, 2015 Accepted: January 22, 2015

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(FTIR) spectroscopy (PerkinElmer, Spectrum One, Waltham, MA) for structural characterization, zeta-size measurements (NanoS, Malvern Instruments, London, U.K.) for dynamic particle size, and elemental analysis for chemical composition were conducted to these particles for characterization purposes. 2.2.2. Synthesis and Characterization of CdII-Imprinted Composite Cryogels. Before the synthesis of composite cryogels, we performed CdII ion adsorption onto finely crushed poly(HEMA−MAC) particles. For this aim, we interacted 100 mg of particles with 25 mL of 100 ppm of a CdII ion solution at a stirring rate of 125 rpm for 2 h. After that, these particles were filtered and washed with 25 mL of deionized water to remove nonselectively adsorbed heavy-metal ions. Then, particles were incorporated into the cryogel structure after being completely dried in an oven at 50 °C. The amount of CdII ions adsorbed onto the particle surface was calculated with respect to measurements performed from the initial, final, and washout solutions. IIP cryogels were then synthesized as follows: 4.35 mL of HEMA and 1.90 g of MBAAm were dissolved in 10 mL of deionized water by applying ultrasound sonication at 45 °C until a clear solution obtained. After that, this solution was diluted to 30 mL to adjust the final monomer concentration to 25% (w/v). Then, this solution was treated with a nitrogen gas to remove dissolved oxygen for 5 min. After cooling, this solution was placed in an ice bath for 20 min, and 25 mg of APS was then dissolved. Subsequently, 75 μL of TEMED was added as a redox initiator couple; meanwhile, 50 mg of CdIIadsorbed poly(HEMA−MAC) particles was added and rapidly mixed. The final monomer solution was poured between two glass plates (25 cm × 25 cm) with three edges closed with spacers (thickness of 0.25 mm). Cryogelation was achieved at −12 °C for 24 h, whereas thawing was performed at room temperature along with washing with 20 mL of deionized water. The composite cryogel obtained was cut into small circles (1 cm o.d.) using a perforator, and extensive cleaning was conducted using water, an ethanol/water mixture, and water. In order to remove CdII ions from the cryogel structure, a Na2EDTA solution was used. Nonimprinted-polymer (NIP) cryogels were also synthesized through the same method except using poly(HEMA−MAC) particles without the CdII ion adsorption step. Scanning electron microscopy (SEM) was conducted to measure the structural properties of the cryogels. 2.2.3. CdII Ion Adsorption Studies. In this part, IIP and NIP cryogels were prepared and characterized. By the presence of poly(HEMA−MAC) particles, the functional thiol groups were incorporated into the cryogel structure in order to coordinate metal ions and to obtain well-distributed imprinted cavities that allow selective and high metal adsorption ability. Herein, it should be mentioned that polymer activation and immobilization stages were eliminated owing to the addition of the functional monomer directly into the polymeric chain. Thereafter, CdII ions were chosen as model heavy-metal ion because of their high toxicity and abundance in surface water samples. The effects of the pH (3.0−7.0), initial concentration (5−200 mg/L), and contact time (5−180 min) on the adsorption performance of the as-synthesized cryogels were investigated through a batch adsorption mode. In order to evaluate the selectivity of the cryogels, PbII and ZnII ions were used as competitor ions with respect to their ionic radii and Lewis acid characters. During the experiment, metal-ion concentration in aqueous solutions was measured with flame atomic absorption spectrometry (FAAS; AAnalyst 800, PerkinElmer, ABD). The adsorption capacity of cryogels was

In this study, we have focused on a particle-assisted imprinting approach to synthesize IIPs for selective CdII adsorption from aqueous solutions. We have tried to combine the structural advantages of cryogels with the selectivity/ specificity of IIPs in order to obtain composite polymers. In this way, we formed homogeneously distributed cavities and obtained fast adsorption kinetic by shortening the diffusion pathway. Herein, we first chose N-methacryloyl-L-cysteine methyl ester (MAC) as a functional monomer with respect to selective Lewis acid−base interaction between thiol groups and CdII ions. Then, MAC-incorporated particles have been prepared and used for Cd II ion coordination during cryogelation. After that, CdII ions were adsorbed onto the particles, and they were subsequently embedded into the cryogel matrix and synthesized according to bulk polymerization under semifrozen conditions at −12 °C. The assynthesized composite cryogels were used for CdII ion adsorption from aqueous solution to assess the effects of particle assistance on the cryogel performances.

2. MATERIALS AND METHODS 2.1. Materials. The basic monomer, 2-hydroxyethyl methacrylate (HEMA), cross-linkers, ethylene glycol dimethacrylate (EGDMA) and N,N′-methylenebis(acrylamide) (MBAAm), and initiators, azobis(isobutyronitrile) (AIBN) and ammonium persulfate (APS), and the redox couple N,N,N′,N′-tetramethylethylenediamine (TEMED) were purchased from Sigma-Aldrich Chem. Co. (St. Louis, MO). The functional monomer N-methacryloyl-L-cysteine methyl ester (MAC) was synthesized according to a procedure given elsewhere36 using cysteine methyl ester and methacryloyl chloride as reactants (Sigma-Aldrich Chem Co.). All other chemicals and glasswares were purchased from local suppliers in reactant-grade purity if not mentioned. Deionized waters obtained using a Barnstead ROpure LP reverse osmosis unit (Dubuque, IA) were used for the preparation of of all solutions. All glasswares were cleaned with a sodium hydroxide solution and dried in a dust-free environment before use. 2.2. Methods. 2.2.1. Synthesis and Characterization of Poly(HEMA−MAC) Particles. Poly(HEMA−MAC) particles were synthesized via the crushing of monolithic polymers prepared through bulk polymerization. Herein, 1 mL of HEMA and 2 mL of EGDMA were dissolved in 3 mL of toluene and magnetically stirred at 150 rpm for 15 min. Then, 25 mg of the functional monomer MAC was added to this solution and kept stirring until a homogeneous monomer mixture was obtained. This mixture was then treated with nitrogen gas to remove dissolved oxygen from the media. To initiate the reaction, 5 mg of AIBN was added to the mixture after it was poured into a plastic tube. Finally, polymerization was conducted by applying a two-step temperature control, at 65 °C for 4 h and at 75 °C for 2 h. The as-synthesized polymer was then crushed into small pieces using an appropriate mortar. In order to obtain fine particles in narrow size distribution, a wet-milling process was applied using a ball-miller (Retsch, Mixer Mills MM 200, Haan, Germany) at 250 rpm for 30 min (repeated three times) using stainless steel balls (1 cm o.d., 5 g) and stainless steel jars as well. For this purpose, 0.5 g of roughly crushed polymers was put into jar containing 5 mL of pure ethanol, which was a protective solvent for further oxidation and burning of the polymeric particles formed. The finely grounded particles were first dried with a nitrogen gas stream and then left in an oven at 50 °C until complete dryness. Fourier transform infrared B

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Figure 1. (a) Particle size distribution for finely crushed poly(HEMA−MAC) particles and SEM images of (b) IIP and (c) NIP cryogels.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Polymers. The chemical and physical properties of poly(HEMA−MAC) particles were analyzed by conducting measurements such as FTIR, zeta-sizer, and elemental analysis. The FTIR spectrum of poly(HEMA−MAC) particles contained specific vibrational bands stemming from −OH, C−H (aliphatic), CO, amide I/ II, ester (−COO−), and −SH functional groups at 3383, 2950, 1728, 1658, 1531/1454, and 890 cm−1, respectively (spectra not given). The particles size and size distribution of finely crushed particles were found to be 997 nm with a polydispersity index value of 0.242 (Figure 1a). Elemental analysis conducted based on sulfur stoichiometry indicated that the MAC incorporation efficiency was achieved with a high rate of 93%, which was calculated from the ratio of elemental analysis/ feeding amount. The MAC content of the poly(HEMA−MAC) particles was calculated as 41.01 μmol/g. The surface morphologies of both IIP and NIP cryogels were investigated by SEM, and the images are shown in Figure 1b,c. According to the SEM images, both cryogels have interconnected macropores, as expected, in the diameter range of 2−100 μm. This feature allows the analyte ions to easily flow through the polymer and limits backpressure and diffusion problems. Besides, it enhances the adsorption kinetics and flow dynamics even when working with large molecules like cell and bulk media like wastewater and/or serum samples.37−40 Herein, it should be noted that embedded particles were well distributed through the polymeric structure on the surface or near the inside of the surface, which caused a very short diffusion pathway and a rapid adsorption rate. In this way, higher adsorption capacities could be achieved. 3.2. CdII Ion Adsorption from Aqueous Solutions. 3.2.1. Effect of the pH. In order to investigate the effect of the pH on IIP and NIP cryogels, the pH values ranged between 3.0 and 7.0 and were adjusted using HNO3/NaOH solutions

calculated by applying proper mass balance. The FAAS measurements were repeated in triplicate of three parallel experimental setups. We used standard heavy-metal ion solutions to continuously check the calibration of the instrument. 2.2.4. Desorption and Reusability Studies. Desorption of metal ions from cryogels was also investigated batchwise. After the adsorption process, the cryogels were interacted with 10 mL of a 0.1 M HNO3 solution at 150 rpm at room temperature for 1 h. The amount of heavy-metal ion concentration in the desorption solution was also determined by FAAS. After each desorption step, cryogel samples were washed with 10 mL of deionized water and 10 mL of a 50 mM NaOH solution in order to regenerate them. Finally, cryogels were reequilibrated with deionized water before the next adsorption step. 2.2.5. Selectivity Studies. The selectivity of composite cryogels toward to CdII ions was investigated in accordance with PbII and ZnII ions. To achieve this, imprinted and nonimprinted cryogels were interacted with a heavy-metal ion solution under uncompetitive conditions. Cryogel samples were separately put into 10 mL of heavy-metal ion solutions (20 ppm) and allowed to interact at 125 rpm for 2 h. The amount of each heavy-metal ion was determined using AAS measurements. To find the selectivity of the cryogels, distribution (Kd) and selectivity coefficients (k) and the imprinting factor (k′) were calculated according to the following equations: Kd = [(C i − Cf )/Cf ](V /m)

(1)

k = Kd,imprinted /Kd,competitor

(2)

k′ = kIIP/kNIP

(3)

where Ci, Cf, V, and m were the initial and final heavy-metal ion concentration (ppm, mg/L), volume of the solution (L), and weight of the cryogel (g), respectively. C

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3.2.3. Adsorption Rate. Figure 4 showed the effect of the contact time between CdII ions and cryogels. As seen in the

(Figure 2). As mentioned before, we selected amino acid, a cysteine-based functional monomer, to coordinate/adsorb

Figure 4. Effect of the contact time on CdII ion adsorption onto the IIP and NIP cryogels: CCdII, 20 ppm; pH, 6.0; temperature, 25 °C.

Figure 2. Effect of the pH on CdII ion adsorption onto IIP and NIP cryogels: CCdII, 20 ppm; contact time, 2 h; temperature, 25 °C.

figure, the adsorption process of CdII ions on both the IIP and NIP cryogels was very fast, and only 15 min was enough to reach half of the desorption capacity. This fast adsorption kinetic then slowed down and reached equilibrium around 45− 60 min. The fast adsorption kinetics may depend on some features of the cryogels developed here. First, cryogels have large pores and interconnected flow channels, which allow the whole solution to flow easily through the polymeric structure. Second, embedded particles located near the inside of the polymeric wall, which minimize diffusion limitations and pathways while enhancing the interaction between the analyte and active sites in the polymeric structure. Third, particles embedding into a cryogel network caused an increase in the specific surface area, which is one of the most efficient parameters on the adsorption capacity.4,41,42 Finally, the combination of these features formed a synergy to get higher adsorption capacities with fast and efficient adsorption kinetics. 3.2.4. Mathematical Approaches. In order to describe the adsorption process of CdII ions on both the IIP and NIP cryogels, we applied two adsorption isotherms and three kinetic models to the data. Langmuir and Freundlich isotherms, which are mostly employed, were used to describe the adsorption dynamics, and the dimensionless equilibrium parameter (RL) was also calculated in accordance with the data obtained. In addition, we have analyzed the adsorption kinetics of the IIP and NIP cryogels by applying three different kinetic models including pseudo-first and -second-order kinetic and Weber− Morris models to assess the rate-limiting parameter of either diffusion or chemical recognition during the adsorption process. In this respect, the intraparticle diffusion coefficient, initial adsorption rate, and half-adsorption time were also calculated from the obtained parameters. To achieve these mathematical approaches, the linearized forms of the respective equations were used as follows:

target CdII ions. As is well-known, the charge of amino acid directly depends on the pH of the solution. As seen in the figure, the adsorption capacities of both cryogels increased with increasing pH values because of deprotonation of the functional monomer, which enhanced its Lewis base character. Lower pH values caused lower adsorption capacity, while no significant increases were obtained after pH 6.0. Therefore, this value was accepted as an optimal value and used for further studies. Hereby, we did not increase the pH values higher than 7.0 to avoid precipitation of CdII ions as their hydroxide salts. 3.2.2. Effect of the Initial Concentration. Figure 3 showed the effect of the initial Cd(II) concentration on the IIP and NIP

Figure 3. Effect of the initial concentration on CdII ion adsorption onto the IIP and NIP cryogels: pH, 6.0; contact time, 2 h; temperature, 25 °C.

cryogels. As seen in the figure, the adsorption capacities increased with increasing initial concentration, as expected. Then, the IIP and NIP cryogels reached plateau values of 32.2 and 16.5 μg/g, respectively, because of occupation of all accessible interaction points, imprinted cavities, and/or functional −SH groups. The concentration difference between the aqueous phase and cryogel surface played a major role for obtaining this tendency during the adsorption process. The increase in the initial concentration caused an increase in the concentration difference as well, so higher adsorption capacities were obtained at higher initial concentration.

Isotherms: Langmuir Ceq /Q eq = 1/Q maxb + Ceq /Q max

(4)

Freundlich ln Q eq = ln KF + (1/n) ln Ceq

(5)

dimensionless equilibrium constant RL = 1/(1 + bCo) (6) D

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μg/g), which indicated no steric hindrance, no diffusion limitations, and efficient binding abilities of the imprinted cavities. The dimensionless equilibrium parameters (RL) for both the IIP and NIP cryogels were between 0 and 1, which showed the favorability of the adsorption process. Because these values were very close to zero, it should be clearly seen that adsorption was achieved with high affinity. The kinetic models applied to the data showed that a secondorder kinetic model was best fitted with respect to the correlation coefficient (R2) values (Figure S2 in the SI). The first-order kinetic model assumes that some diffusion limitations control the adsorption rate, whereas the secondorder kinetic model assumes that chemical adsorption/binding controls the adsorption rate without any diffusion problems. In addition, the Weber−Morris model assumes no diffusion limitations from the solution to the adsorbent surface, but there are some diffusion restrictions through the intraparticle. As mentioned earlier, cryogels have large pores and interconnected flow channels, so they could be preferred as an adsorbent for adsorption/removal of analyte molecules even at high viscosity and high flow rates. With these structural features, the analyte solution could easily diffuse into the polymeric network without any limitations and backpressure. The macroporous nature of the cryogels shortens the diffusion path as well. Because diffusion is not the rate-limiting step for a process in which macroporous adsorbents like cryogel were used, second-order adsorption kinetics is a more coherent model to follow as expected. The results indicated that there is no diffusion limitation during the adsorption process, and the rate-limiting step was chemosorption of heavy-metal ions on binding sites, imprinted cavities, or functional groups. In addition, the high initial adsorption rate and low halfadsorption time proved these provisions, as given in Table 2.

Models: pseudo‐first‐order log(Q eq − Q t) = log(Q eq) − k1t /2.303

(7)

pseudo‐second‐order t /Q t = 1/k 2Q eq 2 + (1/Q eq)t (8)

Weber−Morris (intraparticle Q t = kdt 0.5 + C WM diffusion)

(9)

initial adsorption rate h = k 2Q eq 2

(10)

half‐adsorption time t1/2 = 1/k 2Q eq

(11)

where Ceq (mg/L) and Co (mg/L) were analyte concentrations at equilibrium and at the highest feeding concentration, respectively. Qt (μg/g) and Qeq (μg/g) were adsorption capacities at contact time t and at equilibrium, respectively, whereas Qmax (μg/g) was the theoretical maximum adsorption capacity. b (L/mg), KF, and CWM (μg/g) were specific constants of the Langmuir and Freundlich isotherms and Weber−Morris model. 1/n was the Freundlich exponent called the surface heterogeneity index, which should be in the range of 0−1, with closer to 1 indicating monolayer coverage. t (min) and t1/2 (min) were the contact time and half-adsorption time, respectively. The constants k1 (1/min), k2 (g/μg.min), and kd (μg/g.min0.5) were first-order, second-order, and intraparticle diffusion rate constants, respectively. RL was a dimensionless equilibrium parameter, which reflects the nature of the adsorption process. There are four different conditions with respect to the RL value: RL = 0; 0 < RL < 1; RL = 1; RL > 1, which indicate that the isotherm shapes are irreversible, favorable, linear, and unfavorable, respectively.43 The adsorption isotherms are given in Figure S1 in the Supporting Information. As seen in the figure, the results obtained are well-fitted to the Langmuir isotherm, in accordance with the correlation (R2) values. The Langmuir adsorption isotherm assumes monolayer coverage of the surface, no interanalyte adsorption, no lateral interactions and hindrances, and homogeneously distributed binding sites with equal energies, whereas the Freundlich isotherm does not have monolayer adsorption constraints.42−44 In this context, the IIP cryogels have homogeneously distributed cavities that were formed by a particle-assisted imprinting process (Table 1). In addition, the theoretical maximum adsorption capacity (Qmax, 35.97 μg/g) was very close to the experimental data (Qeq, 32.15

Table 2. Kinetic Parameters for the IIP and NIP Cryogels IIP First-Order 9.33 k1, ×10−2 min−1 Qeq, μg/g 34.24 R2 0.8464 Second-Order k2, ×10−2 g/μg·min 1.36 Qeq, μg/g 21.46 R2 0.9503 Weber−Morris (Intraparticle Diffusion) kd, μg/g·min0.5 1.41 CWM, μg/g 2.32 R2 0.7744 Initial Adsorption Rate h, μg/g·min 6.26 Half-Adsorption Time t1/2, min 3.43

Table 1. Isotherm and Dimensionless Equilibrium (RL) Parameters Calculated for the IIP and NIP Cryogels

Langmuir Qmax, μg/g b, ×10−2 L/mg R2 Freundlich KF 1/n R2 RL

IIP

NIP

y = 0.0278x + 0.528 35.97 5.26 0.9784 y = 0.5474x + 1.0104 2.76 0.547 0.819 0.0868

y = 0.0562x + 1.2627 17.79 4.45 0.9782 y = 0.529x + 0.2966 1.34 0.529 0.877 0.101

NIP 8.43 5.68 0.8991 7.60 6.91 0.9534 0.40 0.20 0.5665 3.63 1.90

3.2.5. Selectivity Studies. Herein, we have performed the adsorption of heavy-metal ions (CdII, PbII, and ZnII) onto both the IIP and NIP cryogels to demonstrate the selectivity and imprinting efficiency. Although the embedded particles had thiol functionalities for CdII ion coordination, the responses of both cryogels against the variation affecting factors such as the pH, initial concentration, and temperature looks similar to significant decreases in the adsorption capacities of the NIP cryogels. In order to compare the CdII ion recognition ability of E

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Figure 5. Selectivity of IIP for CdII ions with respect to PbII and ZnII ions: Cmetal(II), 20 ppm; pH, 6.0; contact time, 2 h; temperature, 25 °C.

each cryogel, we have conducted selectivity studies using PbII and ZnII ions as competitors, which were selected according to their Lewis acid characters and ionic radii. The results and selectivity parameters are given in Figure 5 (and inset table). As seen in the figure, the NIP cryogel has a lower CdII adsorption capacity than the IIP cryogel. The result may depend on the possible multipoint coordination of CdII ions due to the absence of a CdII adsorption step before cryogelation. In the case of the IIP cryogels, CdII adsorption during precomplexation may prevent this multipoint coordination and cause a higher adsorption capacity. Lower adsorption capacities for PbII and ZnII ions of the IIP cryogel prove this explanation and show the formation of specific cavities well-matched to the CdII ionic radius and coordination sphere. The selectivity coefficient (k) and imprinting factor (k′) for the IIP cryogel are also higher than those for the NIP cryogel. The IIP cryogel has shown selectivity coefficients of 3.76 and 7.20, whereas the NIP cryogel has shown selectivity coefficients of 0.79 and 2.26 against CdII ion in accordance with PbII and ZnII ions, respectively. The imprinting factors in this study were 4.76 and 3.18 for CdII/PbII and CdII/ZnII pairs, which reflect the high efficiency and selectivity gained by the particle-assisted imprinting process. These results indicated the high selectivity of the IIP cryogel against CdII ions with respect to both competitors and the NIP cryogel and emphasized that the IIP cryogels recognized CdII ions with higher selectivity. 3.2.6. Desorption and Reusability. The reusability of the adsorbent is one of desired features with respect to operation cost. In this context, we have performed 10 consecutive adsorption−desorption cycles. After each desorption period, cryogel were sterilized by 10 mL of a 50 mM NaOH solution. Using proper mass balances, we calculated the actual adsorption capacity at each cycle and retained the adsorption capacity that was the ratio of the actual adsorption capacity at each cycle and the adsorption capacity at the first cycle (Figure 6). As seen in the figure, the adsorption capacity values of the cryogels decreased from 17.24 to 14.50 μg/g for the IIP cryogels and from 6.25 to 4.65 μg/g for the NIP cryogels. After the 10th cycle, the IIP cryogels retained 84.1% of their first adsorption

Figure 6. Desorption and reusability of IIP and NIP cryogels: CCdII. 20 ppm; pH. 6.0; contact time. 2 h; temperature. 25 °C; desorption agent. 0.1 M HNO3; regeneration agent. 50 mM NaOH.

capacity, while NIP cryogels retained only 74.4% of their first adsorption capacity. These results proved that (i) cryogels developed in this study could repeatedly be used without a significant decrease in their adsorption ability, (ii) they could be classified as cost-friendly, efficient, and promising adsorbents for CdII ion removal, (iii) they could be used as alternative adsorbents for selective CdII ion removal, (iv) they have high affinity against CdII ions, and (v) the adsorption process has a favorable nature.

4. CONCLUSIONS Herein, we focused our attention on combining excellent flow dynamics, structural, and osmotic features of cryogels with the powerful selectivity of IIPs.41 By means of particle assistance, the adsorption capability and selectivity against template molecules were improved. Under optimum conditions, the maximum adsorption capacity for the NIP cryogel was 16.3 μg/ g, whereas that for IIP was 32.2 μg/g as well. Also, particle assistance helps us to obtain well-distributed ion-imprinted cavities on the surface or near the inside of the polymeric walls, F

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Industrial & Engineering Chemistry Research

(10) Bester, K. P.; Lobnik, F.; Erzen, I.; Kastelec, D.; Zupan, M. Prediction of Cadmium Concentration in Selected Home-Produced Vegetables. Ecotoxicol. Environ. Saf. 2013, 96, 182. (11) Denizli, A.; Sanli, N.; Garipcan, B.; Patir, S.; Alsancak, G. Methacryloylamidoglutamic Acid Incorporated Porous Poly(Methyl Methacrylate) Beads for Heavy-Metal Removal. Ind. Eng. Chem. Res. 2004, 43, 6095. (12) Yang, Y.; Chen, T.; Li, P.; Liu, H.; Xie, J.; Xie, Q.; Zhan, X. Removal and Recovery of Cu and Pb from Single-Metal and Cu−Pb− Cd−Zn Multimetal Solutions by Modified Pyrite: Fixed-Bed Columns. Ind. Eng. Chem. Res. 2014, 53, 18180−18188. (13) Fan, H. T.; Liu, J. X.; Yao, H.; Zhang, Z. G.; Yan, F.; Li, W. X. Ionic Imprinted Silica-Supported Hybrid Sorbent with an Anchored Chelating Schiff Base for Selective Removal of Cadmium(II) Ions from Aqueous Media. Ind. Eng. Chem. Res. 2014, 53, 369−378. (14) Zhao, X.; Holl, H. W.; Yun, G. Elimination of Cadmium Trace Contaminants from Drinking Water. Water Res. 2002, 36, 851. (15) Li, H. Y.; Wang, S.; Luan, Z.; Ding, J.; Xu, C.; Wu, D. Adsorption of Cadmium(II) from Aqueous Solution by Surface Oxidized Carbon Nanotubes. Carbon 2003, 41, 1057. (16) Xu, M.; Hadi, P.; Chen, G.; McKay, G. Removal of Cadmium Ions from Wastewater Using Innovative Electronic Waste-Derived Material. J. Hazard. Mater. 2014, 273, 118. (17) Balasubramanian, R.; Perumal, S. V.; Vijayaraghavan, K. Equilibrium Isotherm Studies for The Multicomponent Adsorption of Lead, Zinc, And Cadmium onto Indonesian Peat. Ind. Eng. Chem. Res. 2009, 48, 2093. (18) Singh, V.; Sharma, A. K.; Maurya, S. Efficient Cadmium(II) Removal Ffrom Aqueous Solution Using Microwave Synthesized Guar Gum-Graft-Poly(Ethylacrylate). Ind. Eng. Chem. Res. 2009, 48, 4688. (19) Bodagh, A.; Khoshdast, H.; Sharafi, H.; Zahiri, H. S.; Noghabi, K. A. Removal of Cadmium(II) from Aqueous Solution by Ion Flotation Using Rhamnolipid Biosurfactant as An Ion Collector. Ind. Eng. Chem. Res. 2013, 52, 3910. (20) Wan, S.; Ma, Z.; Xue, Y.; Ma, M.; Xu, S.; Qian, L.; Zhang, Q. Sorption of Lead(II), Cadmium(II), and Copper(II) Ions from Aqueous Solutions Using Tea Waste. Ind. Eng. Chem. Res. 2014, 53, 3629. (21) Ge, Y.; Turner, A. P. F. Too Large to Fit? Recent Developments in Macromolecular Imprinting. Trend Biotechnol. 2008, 26, 218. (22) Sener, G.; Ozgur, E.; Rad, A. Y.; Uzun, L.; Say, R.; Denizli, A. Rapid Real-Time Detection Of Procalcitonin Using A Microcontact Imprinted Surface Plasmon Resonance Biosensor. Analyst 2013, 138, 6422. (23) Bakas, I.; Oujji, N. B.; Moczko, E.; Istamboulie, G.; Piletsky, S.; Piletska, E.; Ait-Ichou, I.; Ait-Addi, E.; Noguer, T.; Rouillon, R. Molecular Imprinting Solid Phase Extraction for Selective Detection of Methidathion In Olive Oil. Anal. Chim. Acta 2012, 734, 99. (24) Szekely, G.; Fritz, E.; Bandarra, J.; Heggie, W.; Sellergren, B. Removal of Potentially Genotoxic Acetamide and Arylsulfonate Impurities from Crude Drugs by Molecular Imprinting. J. Chromatogr. A 2012, 1240, 52. (25) Karimian, N.; Turner, A. P. F.; Tiwari, A. Electrochemical Evaluation of Troponin T Imprinted Polymer Receptor. Biosens. Bioelectron. 2014, 59, 160. (26) Ozkutuk, E. B.; Ersoz, A.; Denizli, A.; Say, R. Preconcentration of Phosphate Ion onto Ion-Imprinted Polymer. J. Hazard. Mater. 2008, 157, 130. (27) Birlik, E.; Ersoz, A.; Acikkalp, E.; Denizli, A.; Say, R. Cr(III)Imprinted Polymeric Beads: Sorption and Preconcentration Studies. J. Hazard. Mater. 2007, 140, 110. (28) Ersoz, A.; Say, R.; Denizli, A. Ni(II) Ion-Imprinted Solid-Phase Extraction and Preconcentration In Aqueous Solutions by Packed-Bed Columns. Anal. Chim. Acta 2004, 502, 91. (29) Esen, C.; Andac, M.; Bereli, N.; Say, R.; Henden, E.; Denizli, A. Highly Selective Ion-Imprinted Particles for Solid-Phase Extraction of Pb2+ Ions. Mater. Sci. Eng., C 2009, 29, 2464.

which enhanced adsorption kinetics while shortening the diffusion pathway and increasing the specific surface area of the cryogels.4,41,42 The good correlation between the R2 values of the Langmuir isotherm (0.9784) and pseudo-second-order kinetic model (0.9503) indicated these features and showed fast adsorption kinetics besides the data calculated for models including Weber−Morris, initial adsorption rate, and halfadsorption rate of the CWM value as 2.32 μg/g, h as 6.26 μg/g· min, and t1/2 as 3.43 min for the IIP cryogel, respectively, and those of the CWM value as 0.20 μg/g, h as 3.63 μg/g·min, and t1/2 as 1.90 min for the NIP cryogel. In light of the data reported here, these cryogels could be classified as a potential solution to overcoming challenges in the monitoring and removal of heavy-metal ions in environmental sources.

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ASSOCIATED CONTENT

S Supporting Information *

Figures including Langmuir and Freundlich isotherms and pseudo-first-order, pseudo-second-order, and Weber−Morris kinetic models for CdII adsorption on IIP and NIP cryogels. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +90312 297 7337. Fax: +90312 299 2163. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.U. warmly thanks to Prof. Dr. Adil Denizli for his valuable discussion and sharing facilities. B.T. thanks to The Scientific and Technological Research Council of Turkey for their financial support under Program 2209, Research Fellowship Programme for Undergraduate Students.

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REFERENCES

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DOI: 10.1021/ie504312e Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/ie504312e Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX