Ion-Selective Imprinted Beads for Aluminum Removal from Aqueous

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Ion-Selective Imprinted Beads for Aluminum Removal from Aqueous Solutions Mu1 ge Andac¸ ,† Evrim O 2 zyapı,† Serap S¸ enel,† Rıdvan Say,‡ and Adil Denizli*,† Department of Chemistry, Biochemistry DiVision, Hacettepe UniVersity, Ankara, Turkey, and Department of Chemistry, Anadolu UniVersity, Eskis¸ ehir, Turkey

The aim of this study is to prepare ion-imprinted polymers that can be used for the selective removal of aluminum ions [Al3+] from aqueous solutions. N-Methacryloyl-L-glutamic acid (MAGA) was chosen as the complexing monomer. In the first step, Al3+ was complexed with MAGA and Al3+-imprinted poly(hydroxyethyl methacrylate-N-methacryloyl-L-glutamic acid) (MIP) beads were synthesized by suspension polymerization. After that, the template ions (i.e., Al3+) were removed using 0.1 M EDTA solution. The specific surface area of the MIP beads was found to be 55.6 m2/g with a size range of 63-140 µm in diameter, and the swelling ratio was 102%. According to the elemental analysis results, the MIP beads contained 640 µmol of MAGA/g of polymer. The maximum adsorption capacity was 122.9 µmol of Al3+/g of beads. The applicability of two kinetic models including pseudo-first-order and pseudo-second-order models was estimated on the basis of comparative analysis of the corresponding rate parameters, equilibrium capacity, and correlation coefficients. Results suggest that chemisorption processes could be the rate-limiting step in the adsorption process. The relative selectivity coefficients of MIP beads for Al3+/Ni2+, Al3+/Cu2+, and Al3+/Fe3+ were respectively 1427, 14.8, and 6.2 times greater than that of the nonimprinted matrix. The MIP beads could be used many times without significantly decreasing in their adsorption capacities. Introduction About 8% of the Earth’s crust is comprised of aluminum. This element is the most abundant metal naturally present in air, soil, and water. Consequently, environmental exposure to aluminum is potentially possible. Its ingestion is unavoidable since aluminum compounds are added not only to most water supplies but also to many processed foods and medicines. Aluminum is a known neurotoxicant. It enters the brain, where it contributes to some neurodegenerative diseases including dialysis encephalopathy, osteomalacia, and osteodystrophy, in particular those related to dialysis treatment of uremic subjects.1 Even a small amount of Al3+ ions in dialysis solutions may cause these disorders. Aluminum may contribute to Alzheimer’s disease.2 Aluminum is also able to give rise to toxicity in the bones and hematopoietic system in humans.3 Positively charged aqua and hydroxy monomeric forms have been found to be the most toxic species of aluminum to living organisms in the terrestrial and aquatic environments.4 Generally, aluminum sulfate is used as a coagulant in the treatment of water to help in the removal of suspended matter and highly colored humic substances,5 thus reducing the dose of chlorine later required to ensure satisfactory microbiological quality. Hence, potable water often contains high aluminum levels of natural origin and/ or from the water purification process.6 The selective removal of aluminum ions has been extensively investigated by applying several techniques.7 Among them, the use of specific polymeric adsorbents has been considered one of the most promising techniques.8-10 Molecular imprinting is a technique for producing chemically selective binding sites, which recognize a particular molecule, in a macroporous polymer matrix.11 Molecular recognition based separation techniques have received much attention in various fields because of their high selectivity for target molecules.12 * To whom correspondence should be addressed. E-mail: [email protected]. † Hacettepe University. ‡ Anadolu University.

In the ion-imprinting process, the selectivity of a polymeric adsorbent is based on the specificity of the ligand, on the coordination geometry and coordination number of the ions, on their charges, and on their sizes. Different approaches have been reported so far for metal ion imprinted resins,13-18 but no studies concerning aluminum ion removal from aqueous solutions using ion-imprinting materials have been reported in the literature. In this paper, we have described a process for the preparation of aluminum-imprinted polymer beads and their characterization and application for the selective removal of aluminum from aqueous solutions. By this aim, N-methacryloyl-L-glutamic acid (MAGA) was chosen as the complexing monomer. Usually, molecularly imprinted polymers are prepared by the bulk polymerization method. The disadvantage of this method is that the obtained block polymers should be crushed, ground, and sieved to produce packing materials. However, in this study, Al3+-imprinted poly(HEMA-MAGA) [MIP] beads were produced by suspension polymerization. After removal of Al3+ ions, MIP beads were characterized. The MIP beads were used for the removal of Al3+ ions from aqueous solutions. To show Al3+ specificity of the MIP beads, competitive adsorptions were performed with different metal ions (Fe3+, Ni2+, and Cu2+). Finally, repeated use of the MIP beads from aqueous solutions is also discussed. Experimental Section Materials. Hydroxyethyl methacrylate (HEMA) and ethylene glycol dimethacrylate (EGDMA) were obtained from Fluka A.G. (Buchs, Switzerland), distilled under reduced pressure in the presence of hydroquinone inhibitor, and stored at 4 °C until use. L-Glutamic acid hydrochloride and methacryloyl chloride were purchased from Sigma (St. Louis, MO). Benzoyl peroxide (BPO) was obtained from Fluka (Switzerland). Poly(vinyl alcohol) (PVAL; MW 100.000, 98% hydrolyzed) was supplied by Aldrich Chemical Co. (USA). All other chemicals were of analytical grade and all solvents were of HPLC grade and were purchased from Merck AG (Darmstadt, Germany). All water

10.1021/ie0512338 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/26/2006

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used in the adsorption experiments was purified using a Barnstead (Dubuque, IA) ROpure LP reverse osmosis unit with a high-flow cellulose acetate membrane (Barnstead D2731) followed by a Barnstead D3804 NANOpure organic/colloid removal and ion exchange packed-bed system. Aluminum concentration was less than 0.003 ppm in the resulting deionized water. Preparation of Al3+-Imprinted Poly(HEMA-MAGA) Beads. The synthesis of N-methacryloyl-L-glutamic acid (MAGA) monomer was adapted from the procedure reported elsewhere.19 To prepare MAGA-Al3+ complex, solid N-methacryloyl-Lglutamic acid (MAGA) (0.430 g, 2.0 mmol) was added slowly to 10 mL of ethanol and then treated with aluminum nitrate [Al(NO3)3‚9H2O (0.375 g, 1.0 mmol)] at room temperature with continuous stirring for 3 h. Then, the formed Al3+-MAGA complex was filtered, washed with 96% ethanol, and dried in a vacuum oven. 2-Hydroxyethyl methacrylate (HEMA) (8.0 mL) and MAGA-Al3+ (500 mg) were polymerized in suspension medium using benzoyl peroxide (BPO) (100 mg) and poly(vinyl alcohol) (200 mg) as the initiator and stabilizer, respectively. Toluene (6.0 mL) and EGDMA (4.0 mL) were included as the diluent (as a pore former) and cross-linker, respectively. The NIP beads were synthesized in a similar way, but in the absence of aluminum nitrate. After the completion of the polymerization reaction, soluble components were removed from the polymer by repeated decantation with water and ethyl alcohol. To remove unreacted monomers and other ingredients, the beads were extensively washed with methanol/water solution (60/40 v/v) for 24 h at room temperature. After the cleaning procedure, the template was removed from the polymer beads using 25 mM EDTA. The MIP beads were added to the 25 mM EDTA solution for 48 h at room temperature. The template-free polymers were cleaned with 0.1 M HNO3 in a magnetic stirrer for 3 h. When not in use, the resulting beads were kept under refrigeration in 0.02% NaN3 solution for prevention of microbial contamination. Characterization of Beads. The average size and size distribution of the NIP and MIP beads were determined by screen analysis performed by using Standard Test Sieves (Retsch GmbH & Co., Germany). The specific surface area of the beads was measured according to the Brunauer-Emmett-Teller (BET) model using single point analysis and a Flowsorb II 2300 from Micromeritics Instrument Corporation, Norcross, GA. Water uptake ratios of nonimprinted and imprinted beads were determined in deionized water. The weight ratio of dry and wet samples was recorded. The water content of nonimprinted and imprinted beads was calculated using the weights of beads before and after uptake of water. The surface morphology of the polymeric beads was examined using scanning electron microscopy (SEM; Model Leitz-AMR-1000, Raster Electronen Microscopy, Germany). The surface of the sample was scanned at the desired magnification to study the morphology of the MIP beads. To evaluate the degree of MAGA incorporation, both NIP and MIP beads were subjected to elemental analysis using a Leco Elemental Analyzer (Model CHNS-932). Fourier transform infrared (FTIR) spectra of MAGA monomer and MIP beads were obtained by using a FTIR spectrophotometer (FTIR 8000 Series, Shimadzu, Japan). Background corrections of MAGA monomer and MAGA-Al3+ complex solutions were made with 96% ethanol. The 1H NMR spectrum of the MAGA monomer was taken in CDCl3 on a JEOL GX-400 300 MHz instrument. The residual nondeuterated solvent (CHCl3) served as an internal reference. Chemical shifts are reported in ppm (δ) downfield relative to CHCl3. Fluorimetric measurements

were taken with a Shimadzu spectrofluorometer (Tokyo, Japan) with 1 cm quartz cells. Adsorption of Al3+ Ions from Aqueous Solutions. Adsorption of Al3+ ions on both the NIP and MIP beads from aqueous solutions were studied in batches. Nitrate salt was used as the source of Al3+ ions. Effects of the initial Al3+ concentration and pH of the medium on the adsorption rate and adsorption capacity were studied. For this purpose, 50 mL volumes of aqueous solutions containing different amounts of Al3+ ions (in the range of 0.5-50 ppm) were treated with 100 mg of the beads at room temperature and magnetically stirred at a speed of 600 rpm. Different pH ranges (3.0-5.3) were adjusted with universal buffer solutions. After the desired treatment periods, the concentration of the Al3+ ions in the aqueous phase was measured by using the spectrofluorometer. Morin is a reagent with minimized disturbance, and the fluorescence from Almorin measured was directly related to a parameter comprising Al3+ and Al-OH complexes.20 Solution for analysis was prepared by successive addition of an aliquot of Al3+ ion solution, 1.25 mL of 1.0 mol/dm3 NH4Ac-HAc buffer solution (pH 4.0 for Al), and 2.50 mL of 1.00 × 10-4 mol/dm3 morin (3,5,7,2′,4′-pentahydroxyflavone) solution in ethanol to a 25 mL volumetric flask. Then, ethanol was added to a mark and the solution was sonicated for 5 min. At λex and λem of 430 and 505 nm, respectively, the fluorescence intensity was measured against a corresponding reagent blank. Monochromatic readings were taken from a digital display with a 0.25 s time constant and with a 3 nm bandwidth on the excitation side and a 3 nm bandwidth on the emission side. The initial calibration was carried out with a standard solution of aluminum-morin complex, buffered pH 4.0, with 430 nm flourecence excitation wavelength and 505 nm emission wavelength. The instrument response was periodically checked with known Al3+ solution standards. The experiments were performed in replicates of three, and the samples were analyzed in replicates of three as well. For each set of data present, standard statistical methods were used to determine the mean values and standard deviations. Confidence intervals of 95% were calculated for each set of samples to determine the margin of error. The amount of Al3+ adsorption per unit mass of the beads was evaluated by using the mass balance. Selectivity Experiments. To measure aluminum specificity (Al3+; at. wt 26.98 g/mol, ionic radius 67.5 pm) of the MIP beads, competitive adsorptions (i.e., iron (Fe3+; at. wt 55.847 g/mol, ionic radius 73.8 pm), copper (Cu2+; at. wt 63.546 g/mol, ionic radius 87 pm), and nickel (Ni2+; at. wt 58.70 g/mol, ionic radius 83 pm) were also studied. The concentration of competitive ions (i.e., Fe3+, Cu2+, and Ni2+) was 10 mg/L. The MIP beads were treated with these competitive ions. After adsorption equilibrium, the concentration of Fe3+, Cu2+, and Ni2+ ions in the remaining solution was measured by BIO-LC (Dionex BIOLC and Post Column Reactor). The column was an ion pack CG5A Guard (P/N 046104) and CS5A Analytical Column (P/N 046100). The eluent was MetPac PDCA Eluent Concentrate (P/N 046088). The flow rate was 1.2 mL/min, and the expected back-pressure was 1.700-1.900 psi. The postcolumn reagent was 0.5 mM 4-(2-pyridylazo)resorcinol (PAR, P/N 39672) in MetPac PAR Postcolumn reagent diluent (P/N 046094), while the postcolumn reagent flow rate was 0.6 mL/min. The distribution and selectivity coefficients of Fe3+, Cu2+, and Ni2+ with respect to Al3+ can be obtained from equilibrium binding data according to eqs 1 and 2.

Kd ) [(Ci - Cf)/Cf](V/m)

(1)

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Figure 2. Proposed structure of MAGA-Al3+ complex.

Figure 1. SEM micrograph of MIP beads.

In eq 1, Kd represents the distribution coefficient; Ci and Cf are the initial and final concentrations of metal ions, respectively. V is the volume of the solution (mL); m is the mass of beads used (g).

k ) Kd(Al3+)/(Kd(Xm+)

(2)

k is the selectivity coefficient, and Xm+ represents Fe3+, Cu2+, and Ni2+ ions. A comparison of the k values of the imprinted beads with those metal ions allows an estimation of the effect of imprinting on selectivity. A relative selectivity coefficient k′ (eq 3) can be defined as

k′ ) kimprinted/kcontrol

(3)

Desorption and Repeated Use. Desorption of Al3+ ions was studied with two different desorption agents: 0.1 M HNO3 and 25 mM EDTA solution. The MIP beads were placed in this desorption medium and stirred continuously (at a stirring rate of 600 rpm) for 1 h at room temperature. The desorption ratio was calculated from the amount of Al3+ ions adsorbed on the beads and the final Al3+ ion concentration in the desorption medium. To test the reusability of the MIP beads, the Al3+ ion adsorption-desorption procedure was repeated five times by using the same polymeric sorbent. To regenerate the desorption medium, the beads were washed with 0.1 M HNO3. Results and Discussion Characterization of Al3+-Imprinted Beads. The MIP beads were spherical in shape with a size range of 63-140 µm in diameter mostly. The specific surface area of the NIP beads was found to be 29.3 m2/g. The specific surface area of the MIP beads, before and after template removal, was also measured and obtained as 41.1 and 55.6 m2/g, respectively. The MIP beads are cross-linked hydrophilic matrixes. The equilibrium swelling ratios of the NIP beads and the MIP beads are 66% and 102% w/w, respectively. Compared with the NIP beads, the water uptake ratio of the MIP beads increases due to formation of metal ion cavities in the polymer structure. This introduces more hydrodynamic volume into the polymer chain. The rough surface morphology and microporous interior of the MIP beads are exemplified by the electron micrographs in Figure 1. The roughness of the bead surface should be considered a factor providing an increase in the surface area. In addition, these micropores reduce diffusional resistance and facilitate mass transfer because of high internal surface area.

Figure 3. FTIR spectra of MAGA and MAGA-Al3+ complex.

The incorporation of MAGA by using nitrogen stoichiometry was found to be 489 and 640 µmol/g polymer for the NIP beads and MIP beads, respectively. Note that HEMA and other polymerization ingredients do not contain nitrogen. This nitrogen amount determined by elemental analysis comes from only incorporated MAGA groups into the polymeric structure. The FTIR spectra of MAGA and MAGA-Al3+ complex are shown in Figure 3. The IR bands of MAGA at 1712 and 1631 cm-1 were assigned the characteristic stretching vibration carboxyl-carbonyl and amide-carbonyl absorptions, respectively. The N-H bending peak appearing at 1524 cm-1 is associated with the amide vibration of MAGA. The two peaks about at 1363 and 1421 cm-1 are also characteristic group frequencies for C-H groups and result from bending vibrations in the molecule. For the characteristic determination of complex, due to linear coordinate covalent complex formation, the characteristic strong carbonyl stretching vibration band at 1631 cm-1 slips to the higher frequency field at 1709 m-1, as a result of decreasing the electron density of carbonyl group of MAGA monomer. This is the confirmation of the complex formation

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Figure 4. FTIR spectrum of MIP beads. Figure 6. Effect of pH on adsorption of Al3+ ions on MIP beads: Vtotal, 20 mL; Al3+ concentration, 20 mg/L; polymer amount, 50 mg; T, 20 °C.

Figure 5. Time-dependent adsorption of Al3+ ions on MIP beads: Vtotal, 20 mL; Al3+ concentration, 20 mg/L solution; pH 5.0; polymer amount, 50 mg; T, 20 °C.

of Al3+ ions with carbonyl groups. Note that the characteristic stretching vibration band of hydroxyl groups, O-H, around 3000 cm-1, does not appear because of ethanol background correction. The proposed structure of the synthesized MAGAAl3+ complex is shown in Figure 2. MAGA-Al3+ complex was polymerized with the HEMA monomer by radical suspension polymerization. The FTIR spectrum of the MIP beads has the characteristic stretching vibration band of hydrogen bonded alcohol, O-H, around 3353 cm-1 and carbonyl absorption bands at 1735 cm-1, respectively. The N-H bending peak appearing at 1530 cm-1 is associated with the amide vibration of MAGA (Figure 4). 1H NMR was used to determine the characterization of the MAGA structure. The 1H NMR spectrum of MAGA indicates the characteristic peaks from the groups in the MAGA monomer. These characteristic peaks are as follows: 1H NMR (DMSO) δ 1.19-1.23 (m; 2H, CH2), 1.50-1.55 (m; 2H, CH2), 1.90 (s; 3H, CH3), 4.49-4.59 (m; 1H, CNH), 5.33 (s; 1H, H2CdC), 5.73 (s; 1H, H2CdC), 6.87 (δ; 1H, NH), 9.58 (δ; 6s, 2H, OH). Adsorption of Al3+ from Aqueous Solutions. (A) Effect of Time. Figure 5 shows the time dependence of the adsorption capacity values of Al3+ ions on the MIP beads. It can be seen from Figure 5 that the adsorpion of Al3+ ions is initially fast, with the most aluminum being adsorbed within the first few minutes, and a complete equilibrium between the two phases was established in 15 min. This fast adsorption equilibrium is most probably due to high complexation and geometric shape affinity (or memory) between Al3+ ions and Al3+ cavities in the MIP bead structure. It is well-known that removal of the template from the polymeric matrix leaves cavities of comple-

Figure 7. Effect of equilibrium Al3+ concentration on adsorption of Al3+ ions on MIP beads: Vtotal, 20 mL; pH 5.0; polymer amount, 50 mg; T, 20 °C.

mentary size, shape, and chemical functionality to the template.21 The maximum adsorption capacity for Al3+ ions was 122.9 µmol/g of dry weight of imprinted beads. (B) Effect of pH. Metal ion adsorption onto specific adsorbents is pH dependent. Complexing agents, called ligands, capable of satisfying the coordination number of a metal, form complexes with metals in solution. When these ligands are not present or their concentration is low, metal ions are normally hydrated with water molecules and combined with hydroxyl ions in the form of hydroxyl complexes such as Al(OH)2+, Al(OH)2+, Al(OH)3, and Al(OH)4-. At high metal ion concentration and high pH, polynuclear hydroxyl complexes are expected to be present.22 Hydration of Al3+ ions becomes significant at approximately pH above 5.0. Therefore, in the present paper, the adsorption capacities of Al3+ ions on the MIP beads were determined at the range of pH 3-5.0 of buffered solutions. The adsorption results are given in Figure 6. In all of the cases the adsorption increases with increasing pH, reaching a maximum value around pH 5.0. At low pH values, e.g., pH 3.0, the adsorption capacities are lower. At higher pH values, i.e., above pH 5.25, aluminum ions precipitate as Al(OH)3. (C) Effect of Initial Concentration of Al3+ Ions. Figure 7 shows the initial concentration of metal ion dependence of the adsorbed amount of the Al3+ onto the MIP beads. The adsorption values increase with increasing concentration of Al3+ ions, and a saturation value is achieved at ion concentration of 20 mg/L, which represents saturation of the active binding cavities on

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Table 1. Kinetic Constants of Langmuir and Freundlich Isotherms Langmuir adsorption isotherm

Freundlich adsorption isotherm

qmax ) 130.4 mg/g b ) 0.46 R2 ) 0.996

KF ) 0.895 n ) 0.449 R2 ) 0.830

the MIP beads. Mass transfer limitations were also overcome by high driving force, which was the concentration difference of Al3+ between the liquid and solid phases, in the case of high Al3+ concentration. The maximum adsorption capacity is 122.9 µmol/g. Adsorption Isotherm. The Langmuir model assumes monolayer coverage of adsorbate over a homogeneous adsorbent surface. The binding sites are also assumed to be energetically equivalent and distant from each other, so there are no interactions between molecules adsorbed on adjacent sites. The Langmuir model can be described by the equation

qe ) qmaxbCe/(1 + bCe)

(4)

where qe is the amount of adsorbed Al3+ in the adsorbent (mg/ g), Ce is the equilibrium ion concentration in solution (mg/L), b is the Langmuir constant, and qmax is the maximum adsorption capacity (mg/g). The Freundlich expression is an exponential equation that describes reversible adsorption and is not restricted to the formation of the monolayer. This empirical equation takes the form

qeq ) KF(Ceq)(1/n)

(5)

where KF and n are the Freundlich constants; Ceq is the equilibrium ion concentration in solution (mg/mL). Table 1 shows the kinetic constants of the Langmuir and Freundlich isotherms. According to the correlation coefficients of isotherms, the Langmuir adsorption model is more favorable. Adsorption Dynamics. To examine the controlling mechanism of adsorption processes such as mass transfer and chemical reaction, kinetic models were used to test experimental data. The kinetic models (pseudo-first-order and pseudo-second-order equations) can be used in this case assuming that the measured concentrations are equal to the adsorbent surface concentrations. The pseudo-first-order equation is given as follows:23

log(qe- qt) ) log(q1,calc) - (k1t)/2.303

(6)

where qe is the experimental amount of Al3+ adsorbed at equilibrium (mg/g); qt is the amount of Al3+ adsorbed at time t (mg/g); k1 is the equilibrium rate constant of first-order adsorption (1/min); q1,calc is the adsorption capacity calculated by the pseudo-first-order model (mg/g). The rate constant for the second-order adsorption model could be obtained from the following equation:

(t/qt) ) (1/k2q2,calc2) + (1/q2,calc)t

(7)

where k2 is the equilibrium rate constant of pseudo-second-order adsorption (g/mg‚min); q2,calc is the adsorption capacity calculated by the pseudo-second-order kinetic model (mg/g). According to the values in Table 2, the optimum results are for both the second- and first-order models, with the secondorder mechanism R2 values being the highest (0.99). The theoretical qe value estimated from the pseudo-second-order kinetic model was very close to the experimental value. However, the correlation coefficient for the linear plot of

Table 2. First- and Second-Order Kinetic Constants for MIP Beads initial concn (mg/L) experimental const qe (µmol/g) first-order kinetic consts qe (µmol/g) k1 (1/min) R2 second-order kinetic consts qe (µmol/g) k2 (g/µmol‚min) R2

20 122.9 52.5 0.122 0.70 133.3 0.013 0.99

Table 3. Kd, K, and k′ Values for Fe3+, Cu2+, and Ni2+ with Respect to Al3+ nonimprinted beads metal ion

Kd

Al3+ Fe3+ Cu2+ Ni2+

164.8 70.6 35.2 0.8

imprinted beads

k

Kd

k

k′

2.33 4.68 206

570.8 92.0 38.6 0.4

6.20 14.8 1427

2.66 3.16 6.93

-log(qeq - qt) vs t for the pseudo-first-order equation is lower than 0.90. This value show that this adsorbent system is not well described by the pseudo-first-order kinetic model. These results suggest that the pseudo-second-order mechanims is predominant and that chemisorption might be the rate-limiting step that controls the adsorption process. The rate-controlling mechanism may vary during the course of the adsorption process: three possible mechanisms may be occurring.24 There is an external surface mass transfer or film diffusion process that controls the early stages of the adsorption process. This may be followed by a reaction or constant rate stage, and finally by a diffusion stage where the adsorption process slows considerably.25 Selectivity Experiments. Competitive adsorptions of Fe3+/ Al3+ and Cu2+/Al3+ from their mixtures were also studied in a batch system. The Fe3+ ion was chosen as the competitor species because it has the same charge and nearly identical size and also binds well with the carboxylate groups. Ni2+ and Cu2+ were chosen because of their similar ionic radii. Table 3 summarizes Kd, and k, values of F3+, Cu2+, and Ni2+ with respect to Al3+, i.e., iron (Fe3+; at. wt 55.847 g/mol, ionic radius 73.8 pm), copper (Cu2+; at. wt 63.546 g/mol, ionic radius 87 pm), nickel (Ni2+; at. wt 58.70 g/mol, ionic radius 83 pm), and (Al3+; at. wt 26.98 g/mol, ionic radius 67.5 pm). A comparison of the Kd values for the MIP samples with the control (i.e., NIP) samples shows an increase in Kd for Al3+ while Kd decreases for Fe3+, Cu2+, and Ni2+. The relative selectivity coefficient is an indicator to express metal adsorption affinity of recognition sites to the imprinted Al3+ ions. These results show that the relative selectivity coefficients of imprinted beads for Al3+/Fe3+, Al3+/Cu2+, and Al3+/Ni2+ were respectively 6.2, 14.8, and 1427 times greater than for the nonimprinted matrix (Table 3). Figure 8 shows adsorbed template (i.e., Al3+) and competitive ions (Fe3+, Cu2+, and Ni2+) in both MIP and NIP beads. The maximum adsorption capacities of the MIP beads are 87.1 µmol/g for Al3+, 13.4 µmol/g for Fe3+, 5.5 µmol/g for Cu2+, and 0.07 µmol/g for Ni2+, which correspond to an initial concentration of 20 mg/L. The maximum adsorption capacity of the metal ions occurred in the order Al3+ > Fe3+ > Cu2+ > Ni2+. The same trend was observed for the NIP beads. It should be mentioned that the adsorption capacities for MIP beads are higher than those for NIP beads. Desorption and Repeated Use. The regeneration of the adsorbent is likely to be a key factor in improving separation process economics. Desorption of the Al3+ ions from the MIP

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the pores, which formed during the polymerization procedure. The MIP beads were cross-linked hydrophilic matrixes. High complexation and geometric affinity between Al3+ ions and Al3+ cavities in the bead structure resulted in faster adsorption rates and higher active binding cavities. The Langmuir adsorption model can be applied in this affinity adsorbent system. The competitive studies showed that the selectivity of the MIP beads was significantly higher than that of the corresponding control polymers. Finally, the MIP beads can be used many times without decreasing their adsorption capacities significantly. From the results obtained in this study, it is concluded that the new MIP beads produced are promising for the selective adsorption of Al3+ ions from aqueous media. Literature Cited Figure 8. Adsorbed template and competitive ions in both MIP and NIP beads: Al3+ concentration, 20 mg/L; Fe3+, Cu2+, and Ni2+ concentration, 10 mg/L; pH 5.0; Vtotal, 20 mL; polymer amount, 50 mg; T, 20 °C.

Figure 9. Adsorption-desorption cycle of MIP beads. Adsorption conditions: Al3+ concentration, 20 mg/L; pH 5.0; Vtotal, 20 mL; polymer amount, 50 mg; T, 20 °C.

beads was performed in a batch experimental setup. Various factors are probably involved in determining rates of Al3+ desorption, such as the extent of hydration of the metal ions and the polymer microstructure. However, an important factor appears to be binding strength. In this study, desorption time was chosen to be 30 min. Desorption ratios are high (up to 85%) for both desorption agents (i.e., 0.1 M HNO3 and 25 mM EDTA solution). To obtain the reusability of the MIP beads, adsorption-desorption cycles were repeated five times by using the same imprinted beads. The adsorption capacity of the recyled MIP beads can still be maintained at 90% of its original value at the fifth cycle (Figure 9). It can be seen concluded that the MIP beads can be used many times without decreasing their adsorption capacities significantly. The removal percent of Al3+ ions was also given in Figure 9. As seen in this figure, the removal percent of Al3+ ions is not significantly changed with increasing reuse number. The removal percent decreased from 84% to 79% with the increase of the reuse number from 1 to 5. Conclusions The selectivity and specificity of MIP beads have been demonstrated. The MIP beads were easily applied and particularly well suited. The suspension polymerization procedure provided the cross-linked MIP beads, which were spherical in shape with a size range of 63-140 µm in diameter mostly. The polymeric beads had a spherical form and rough surface due to

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ReceiVed for reView November 8, 2005 ReVised manuscript receiVed December 14, 2005 Accepted December 28, 2005 IE0512338