Polymer−Clay Nanocomposite Iron Traps Based on Intersurface Ion

Eskisehir, Turkey. We combined the ion-imprinting technique and the binding ability of Fe3+ ions to organosmectite to create the inorgano-organo Fe3+ ...
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Ind. Eng. Chem. Res. 2008, 47, 2258-2264

MATERIALS AND INTERFACES Polymer-Clay Nanocomposite Iron Traps Based on Intersurface Ion-Imprinting Muharrem Karabo1 rk,† Arzu Erso1 z,† Adil Denizli,‡ and Rı´dvan Say*,†,§ Department of Chemistry, Anadolu UniVersity, Eskisehir, Turkey, Department of Chemistry, Hacettepe UniVersity, Ankara, Turkey, and Plant, Drug and Scientific Researches Center, Anadolu UniVersity, Eskisehir, Turkey

We combined the ion-imprinting technique and the binding ability of Fe3+ ions to organosmectite to create the inorgano-organo Fe3+ ions imprinted polymer-nanocomposite traps with the goal of preparing a solid phase that has high selectivity for Fe3+ ions. In the first step, the intercalation of quartamine cations was conducted by an ion-exchange process between the smectite host and an aqueous quartamine solution and Fe3+ ions were complexed with methacryloylamidoantipyrine (MAAP). In the second step, quartamine cations were exchanged with a preorganized metal-chelate complex monomer for the preparation of polymer nanocomposite traps based on the intersurface ion-imprinting. After that, the template ions (i.e., Fe3+) were removed using 4.0 M HNO3 solution. Fe3+-imprinted nanocomposites were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), swelling studies, and elemental analysis. Maximum binding capacity, optimum pH, and equilibrium binding time were found to be 78.5 mg/g, pH 3.0, and 10 min, respectively. The relative selectivity coefficients of the imprinted nanocomposite traps for Fe3+/Al3+, Fe3+/Cu2+, Fe3+/ Co2+, and Fe3+/Zn2+ were 5.28, 11.4, 15.8, and 72.6 times greater than the nonimprinted nanocomposites, respectively. The Fe3+-imprinted nanocomposite traps could be used many times without decreasing in their adsorption capacities significantly. Introduction It is well-known that iron plays an important role in almost all organisms.1-3 Many symptoms of iron toxicity, for example, heart attacks, diabetes, arthritis, depression, and liver failure, arise from the absorption of iron in unacceptably high concentrations because of a genetic failure or by accidental ingestion.4 Iron becomes extremely dangerous for cells and tissues because of generated free radicals. In chronic iron overload, a small nonspecific iron pool exists, but most of the iron is deposited in the organs, especially in the spleen, liver, and heart, causing widespread organ damage.5 The only available supportive treatment is chelation therapy, and the only available clinical drug for this treatment is desferrioxamine B (DFO), a linear hydroxamate, a natural siderophore.6 The use of DFO has already been shown to result in prolonged life expectancy, reduced liver iron, and the establishment of negative iron balance. However, the major limitation to the use of DFO is its lack of effectiveness when administered orally, the short half-life time in plasma, and its potential toxicity when present in high concentrations.7 DFO is also highly expensive. For this reason, a number of orally active iron chelators are being tested, but still none of them are satisfactory.8,9 To overcome the drawbacks of soluble iron chelators in the treatment of iron overload, attachment of ironchelating ligands has been studied. Comparing to soluble iron chelators, iron-chelating resins might have advantages in stability, reusability, and minimal damage to biological substances. * Corresponding author. E-mail: [email protected]. † Department of Chemistry, Anadolu University. ‡ Hacettepe University. § Plant, Drug and Scientific Researches Center, Anadolu University.

Recently, one of the most promising techniques for blood detoxification is extracorporeal affinity adsorption. So far, only a few affinity adsorbents were reported in literature.10-16 However, selectivity still remains a great problem in these systems. Molecular imprinting is a method for making selective binding sites in synthetic polymers by using a molecular template. Metal cations can be used as templates for imprinting cross-linked polymers. After the removal of template (i.e., ion), the remaining polymer is more selective. The selectivity of the polymer depends on various factors like the charge of the ion, the size of the ion, the specificity of the interaction of the ligand, the coordination geometry, the rebinding interactions, the size and shape of the cavity, and the number of the ions.17 Noncovalent interactions such as hydrogen bonding,18-20 covalent interactions,21-24 π-π bonding and hydrophobic interaction,19,25,26 electrostatic interactions,27 and metal ion coordination28-30 can be exploited to organize the organic functional monomers around the template. Molecularly imprinted polymers (MIPs), which were prepared by ordered inorganic hosts, were synthesized for diverse applications.31-34 The dispersion of clay composites in a monomer or polymer matrix can result in the formation of three types of composite materials: conventional composites,35 intercalated polymer-clay nanocomposites,36 and exfoliated polymer-clay nanocomposites.37,38 The objective of this study was to determine the feasibility of the inorganic/organic hybrid imprinting approach using methacryloylamidoantipyrine-Fe3+ ((MAAP)2-Fe3+) complex monomer and quartamine modified smectite organoclay. We combined ion-imprinting with the binding ability of Fe3+ to organosmectite to create inorgano-organo Fe3+-imprinted poly-

10.1021/ie070885o CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008

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Figure 1. FTIR spectra of (A) MAAP and (B) (MAAP)2-Fe3+.

mer-nanocomposite traps with the goal of preparing a solid phase that has high selectivity for Fe3+ ions. Fe3+-ions adsorption and selectivity studies of Fe3+ versus other metal ions, such as Al3+, Co2+, Zn2+, and Cu2+, were reported here. Finally, use of Fe3+-imprinted nanocomposite hybrid traps for the separation and preconcentration of Fe3+ was also discussed. Materials and Methods Chemicals. 4-Aminoantipyrine and methacryloylchloride were supplied by Sigma (St. Louis, MO) and used as received. Ethylene glycol dimethacrylate (EDMA) was obtained from Fluka A.G. (Buchs, Switzerland), distilled under reduced pressure in the presence of hydroquinone inhibitor, and stored at 4 °C until use. Azobisisobutyronitrile (AIBN) was also obtained from Fluka (Switzerland). All other chemicals were of reagent grade and were purchased from Merck AG (Darmstadt, Germany). All water used in the 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 NANO pure organic/ colloid removal and ion-exchange packed-bed system. The concentrations of Fe3+ and other ions in the aqueous phase were measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES) (Perkin-Elmer 4300 D/V model). The working wavelengths for Fe3+, Al3+, Co2+, Zn2+, and Cu2+ ions were 238.204, 396.153, 228.616, 206.200, and 327.393 nm, respectively. A Fisher Scientific, Accumet Basic AB15 pH-meter was used to measure pH values. Synthesis of the MAAP Monomer. The following experimental procedure was applied for the synthesis of MAAP:25 4-Aminoantipyrine (0.5 g; 2.463 mmol) and pyridine (0.2 mL; 2.46 mmol) were dissolved in 100 mL of dry CHCl3, and this solution was cooled to 0 °C. Then, methacryloyl chloride (0.26 mL; 2.46 mmol) was poured slowly into this solution and stirred magnetically at room temperature for 2 h. At the end of the chemical reaction period, the solution was washed with 50 mL of dilute HCl and 50 mL of dilute NaOH. Then, the organic phase was evaporated in a rotary evaporator, and the residue was crystalized in petroleum benzene-ethyl acetate mixture.

Fourier transform infrared (FTIR) spectra of MAAP is given in Figure 1: FTIR (KBr, cm-1): 710-770 cm-1 (monosubstitued benzene ring), 1500 and 1580 cm-1 (conjugation in benzene ring), 1642 cm-1 (amide carbonyl band), 1730 cm-1 (carbonyl band in cyclic ketone), 2975 and 2925 cm-1 (CsH band), 3260 cm-1 (NsH band); 1H-NMR (CHCl3): 2.05 ppm 3H singlet (sCdCsCH3, vinyl methyl), 3.0 ppm 3H singlet (sCsCH3), 3.35 ppm 3H singlet (sNsCH3), 5.5 ppm 1H singlet (sCHadCs), 5.8 ppm 1H singlet (sCHbdCs), 7.258.80 ppm 4H multiplet (aromatic, CDCl3 peak is also observed at 7.3 ppm with aromatic peaks), 8.80 ppm 1H singlet (aromatic), 9.1 ppm 1H singlet (NsH). Synthesis of (MAAP)2-Fe3+ Chelate Monomer. MAAP (5.0 mmol) in 20 mL of ethanol and 2.5 mmol of FeCl3‚6H2O were mixed together and stirred for 2 h until the solution of reddish-black colored (MAAP)2-Fe3+ chelate monomer was obtained. In FTIR spectrum of (MAAP)2-Fe3+, the band that appeared at 472 and 647 cm-1 might be assigned to stretching Fe-O modes. The 768 cm-1 band might be assigned to the monosubstituted benzene ring. The 1642 cm-1 amide carbonyl band shifted to 1617 cm-1, which indicated that the oxygen atom of CdO (amide) took part in coordination to the metal. The 1730 cm-1 carbonyl band at cyclic ketone position shifted to 1672 cm-1, which indicated that the oxygen atom of CdO took part in coordination to the metal. As a result, when the possible interactions between Fe3+ and O atoms were considered, it was concluded that Fe3+ ions mainly coordinated to the O atoms of the carbonyl (CdO) groups of MAAP, because the considerable changes in the infrared frequencies were observed only for those bands containing (CdO) groups. The sharp ν(OH) vibration band that apperared at 3553 cm-1 is an indication of coordination of H2O molecules to Fe3+ ion. Preparation of Fe3+-Imprinted Polymer-Clay Nanocomposite. The organoclay was prepared in a similar way as described by Say et al.37 The smectite (20 g) was dispersed in deionized water (500 mL) at 80 °C. A solution of quartamine [dimethyl(dihydrogenated tallow)ammonium chloride] (0.05 mol) and concentrated HCl (5 mL) in deionized water (100 mL)

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Figure 2. Imprinting mechanism employed in intersurface imprinting using the quartamine-modified organosmectite hosts.

were added, heated, and stirred for 3 h. The suspension was filtered, and the solid residue was washed with hot distilled water until no chloride was left. The product was dried at 55 °C for several days in a fan oven and then dried under vacuum for 24 h, yielding the quartamine modified smectite (QS). The imprinting mechanism employed in intersurface imprinting using the quartamine modified organosmectite hosts is shown in Figure 2. The intercalation of quartamine cations was conducted by an ion-exchange process between the smectite host in an aqueous quartamin solution. To exchange quartamine cations with preorganized metal-chelate complex and for the preparation of polymer nanocomposite traps, the following experimental procedure was applied: The resulting quartamine-modified smectite (5.0 g) was suspended in 80 mL of an acetonitrile solution containing a mixture of 2.5 mmol of preorganized complex monomer, 2.5 mL of EGDMA (cross-linking agent), and 0.05 g of AIBN (initiator). The reaction flask was suspended and stirred in a thermostatted water bath at 65 °C for 6 h and then at 80 °C for 3 h. After the polymerization, the polymer/ nanocomposite traps were separated from the polymerization medium. The residuals (e.g., unconverted monomer and initiator) were removed by a cleaning procedure (ethanol, water-ethanol (1:1) mixture, and then deionized water, respectively) and dried in a vacuum oven at 70 °C for 48 h. The polymer nanoocomposites were suspended and refluxed with 4 M HNO3 to remove Fe3+ ions and then extensively washed with hot distilled water until no more Fe3+ ions released. Nonimprinted nanocomposites were also prepared by using MAAP and EGDMA. Characterization Studies. The Fourier transform infrared (FTIR) spectra of nanocomposites were obtained through the use of a FTIR spectrophotometer (Perkin-Elmer model 2000). X-ray diffraction (XRD) patterns were performed using Cu KR radiation (n ) 1.5406 Å) on a Rigaku Rint 2000 diffractometer operating at 40 kV and 30 mA between 5 and 40° (2θ). The interlayer distances of smectite, quartamine-modified smectite, and Fe3+-imprinted polymer/clay nanocomposites were calculated from the (001) peak using the Bragg equation.

The average cluster sizes of clay, organoclay, and nanocomposites were estimated by applying Scherrer’s formula.

D ) 0.9λ/β1/2 cos θB

(1)

where D represents the average cluster size, λ is the wavelength of Cu KR, β1/2 is the full-width at half-maximum, and θB is the diffraction angle.40 To evaluate the degree of imprinting, the nanocomposites were subjected to elemental analysis using a Vario el III CHNS elemental analyzer. The specific surface areas of the nanocomposites were determined by Nova 2200 surface area and pore size analyzer. Water uptake properties of nanocomposites were determined by volumetric method. In this method, the dry nanocomposites (100 mg) were placed in cylindrical tubes, and the top point of the tube was marked. Then, the tube was filled with distilled water and the nanocomposites were allowed to swell at room temperature. The heights of the nanocomposites were marked every 30 min. The height of swollen nanocomposites in the tube was used to calculate the swelling ratio by using the following formula,

swelling ratio (%) ) [(hswollen - hdry)/hdry] × 100

(2)

where hswollen is the height of the swollen nanocomposites and hdry is the height of the dry nanocomposites. Adsorption-Desorption Studies. Adsorption of Fe3+ ions from aqueous solutions was investigated in batch experiments. Effects of the initial Fe3+ ion concentration, pH of the medium on the equilibrium adsorption time, and adsorption capacity were studied. The suspensions were brought to the desired pH by adding sodium hydroxide and nitric acid. The pH was maintained in a range of (0.1 units until equlibrium was attained. In all experiments, polymer concentration was kept constant at 25 mg/25 mL. Chloride salt (FeCl3‚6H2O) was used for Fe3+ ion source. The concentrations of the metal ions in the

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Figure 3. FTIR spectra of (A) smectite, (B) organosmectite, (C) nanocomposite before leaching, and (D) nanocomposite after leaching.

aqueous phases after desired treatment periods were measured by using an ICP-AES. The instrument response was periodically checked with known ICP Fe3+ solution standards. The experiments were performed in replicates of five, and the samples were analyzed in replicates of five 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 in order to determine the margin error. Adsorption values (mg/g) were calculated as the difference in Fe3+ ion concentration of the pre- and post-adsorption solutions divided by the weight of dry nanocomposites. Adsorbed Fe3+ ions were desorbed by treatment of nanocomposites with 4 M HNO3 solution. The Fe3+-imprinted nanocomposites were placed in the desorption medium and stirred continuously at 600 rpm at room temperature for 2 h. The final Fe3+ ion concentration in the aqueous phase was determined by ICP-AES. The desorption ratio was calculated from the amount of Fe3+ ions adsorbed on the imprinted nanocomposites and the final Fe3+ ion concentration in the desorption medium. In order to test the reusability of Fe3+imprinted nanocomposites, the Fe3+ adsorption-desorption procedure was repeated ten times by using the same imprinted nanocomposites. Selectivity Experiments. The batchwise selective adsorption experiments of Al3+, Co2+, Zn2+, and Cu2+ ions with respect to Fe3+ ions were conducted using the imprinted and nonimprinted nanocomposites. The nanocomposites (25 mg) were added to 25 mL of aqueous solution containing 25.0 mg/L Fe3+/ Al3+, Fe3+/Cu2+, Fe3+/Co2+, and Fe3+/Zn2+ and placed in a sealed test tube. A solution (25 mL) containing 25.0 mg/L from each metal ion was treated with the Fe3+-imprinted nanocom-

posite traps at a pH of 4.0 at room temperature, in the flasks stirred magnetically at 600 rpm. Chloride salts were used for metal ion source. After adsorption equilibrium, the concentration of each ion in the remaining solution was measured by an ICPAES. The effect of imprinting on selectivity was defined as given in eq 3.

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

(3)

where Kd is the distribution coefficient and Ci and Cf represent the initial and final solution concentrations, respectively. V is the volume of solution used for the extraction, and m is the mass of nanocomposites used for extraction. The selectivity coefficient (k) for the binding of a specific metal ion in the presence of competitor species can be obtained from equilibrium binding data according to eq 4.

k ) Kd (template metal cation)/Kd (competing metal cation)

(4)

The relative selectivity coefficient was given in eq 5.

k′ ) kimprinted/kcontrol

(5)

Results from the comparison of the k values of the imprinted nanocomposites with nonimprinted nanocomposites allow an estimation of the effect of imprinting on selectivity. Results and Discussion Characterization Studies. The FTIR spectra of smectite, organosmectite, and polymer-clay nanocomposite are shown

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Figure 4. XRD spectra of (A) smectite, (B) organosmectite, (C) nanocomposite before leaching, and (D) nanocomposite after leaching. Table 1. Elemental Analysis Results nanocomposites

%C

%H

%N

before leaching after leaching

8.306 10.46

1.112 1.441

0.889 1.320

in Figure 3. As seen from this figure, the smectite showed 529, 796, 920, 1042, 1642, 3430, and 3626 cm-1 bands. The spectra of the organosmectite show the presence of the bands at about 1475, 2855, 2929, and 3260 cm-1. The bands correspond to the symmetrical methyl stretching band at 2855 cm-1 and -CH2- scissor vibration band at 1475 cm-1, respectively.41 The 2928 cm-1 peak was assigned to the -CH- stretching band, and the 3444 cm-1 peak was assigned to the intercalated quarternary ammonium band. The spectrum of the nanocomposites with Fe3+ (before leaching) showed the presence of the bands at about 700 and 1580 cm-1. The 1580 cm-1 band might be assigned to conjugated benzene ring of MAAP. In unleached nanocomposite, two weak bands at about 700 and 475 cm-1 were obtained because of the complexing of MAAP with Fe3+ inside the galleries of organosmectite. The Fe-O band of MAAP-Fe3+ in leached nanocomposite cannot be seen because Fe3+ ions were removed from the structure. So, it can be said that the imprinting process was done succesfully. The XRD patterns of original smectite and each phase are shown in Figure 4. As seen from Figure 4, a strong peak is present between 4 and 6° (2θ) for smectite, organosmectite, and leached and unleached Fe3+-imprinted polymer/clay nanocomposites. Figure 4 shows the increase in the basal spacing from 15.07 to 17.49 Å for quartamine-modified smectite. This XRD trend showed that tallow C-17 molecule chains entering the interlayer of smectite and Na+ ions were successfully replaced by quartamine ions. After polymerization of MAAP-Fe3+ with EGDMA cross-linker, the position of the 001 peak angles shifted to a higher angle (34.09 Å) and the intensities of the peaks decreased significantly. These results showed that the imprinted polymer of EGDMA/(MAAP)2-Fe3+ has intercalated the interlayers of quartamine-modified smectite (nanocomposite). After leaching of Fe3+ ions from the structure of nanocomposite, the position of the 001 peak angles shifted to a lower angle (28.57 Å), indicating that the high complexation and functionalization position of poly(EGDMA/ MAAP)-imprinted polymer inside the galleries of organosmectite is dictated by Fe3+ template (Figure 4).

Figure 5. Effect of time on Fe3+ adsorption; pH ) 4.0; Fe3+ concentration ) 1.0 mg/L; T ) 25 °C. Each point is an average of five parallel studies.

The final cluster size of nanocomposite after leaching (22.05 nm) was larger than that of nanocomposite before leaching (21.83 nm), indicating that Fe3+ ions removed from nanocomposite interlayer and Fe3+ template ions were frozen inside the galleries of the nanocomposite (Figure 2). This result was also supported by the elemental analysis results (Table 1). As seen from the table, when Fe3+ ions were removed from the nanocomposite structure, the amounts of C, H, and N were found to be high, as expected. The specific surface areas were found to be 74.31 m2/g for unleached and 132.5 m2/ g for imprinted nanocomposites (after leaching of Fe3+ ions). The equilibrium swelling ratios of the nonimprinted and imprinted nanocomposites are 60% and 65%, respectively. Compared with nonimprinted nanocomposites, the water uptake ratio of the Fe3+-imprinted nanocomposites increased. Formation of Fe3+ ion cavities in the polymer structure inside the galleries of nanocomposites introduced more hydrodynamic volume into the polymer chains, which can result in uptake of more water molecules by the polymer matrix. Adsorption Studies. Figure 5 shows the time dependence of the adsorption capacities of Fe3+ ions on the Fe3+-imprinted nanocomposites. As seen here, Fe3+ adsorption increases with the time during the first 10 min and then levels off as equilibrium is reached. This fast adsorption equilibrium is most probably due to high complexation and geometric shape affinity (or shape memory) between Fe3+ ions and Fe3+ cavities in the nanocomposite structure. It is well-known that removal of the template from the polymeric matrix leaves cavities of complementary

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Figure 6. Effect of equilibrium concentration on Fe3+ adsorption; pH ) 4.0; T ) 25 °C. Each point is an average of five parallel studies. Figure 8. Effect of pH on the Fe3+ adsorption; 1.0 mg/L; T ) 25 °C. Each point is an average of five parallel studies. Table 2. Kd, k, and k′ Values of Al3+, Co2+, Zn2+, and Cu2+ with Respect to Fe3+ nonimprinted nanocomposites

Figure 7. Linear representation of Langmuir equation of Fe3+-imprinted nanocomposites.

Fe3+

size, shape, and chemical functionality to the template (i.e., ions). Figure 6 shows the dependence of the equlibrium concentration on the adsorbed amount of the Fe3+ ions onto the Fe3+imprinted nanocomposites. The adsorbed amount increased with increasing concentration of Fe3+ ions, and a saturation value was achieved at an Fe3+ ion concentration of 100 mg/L, which represents saturation of the active binding cavities on the Fe3+imprinted nanocomposites. The maximum adsorption capacity for Fe3+ ions was 78.5 mg/g of dry weight of Fe3+-imprinted nanocomposites. An adsorption isotherm is used to characterize the interactions of each molecule with adsorbents. This provides a relationship between the concentration of the molecules in the solution and the amount of ion adsorbed on the solid phase when the two phases are at equlibrium. The Langmuir adsorption model assumes that the molecules are adsorbed at a fixed number of well-defined sites, each of which is capable of holding only one molecule. These sites are also assumed to be energetically equivalent and distant from each other so that there are no interactions between molecules adsorbed on adjacent sites. During the batch experiments, an adsorption isotherm was used to evaluate adsorption properties. The maximum adsorption capacity (Qmax) for the adsorption of Fe3+ ions was obtained from experimental data (Figure 7). The correlation coefficient (R2) was 0.9902. It should be also noted that the maximum adsorption capacity and the Langmuir constant were found to be 79.3 mg/g and 0.7 L/mg, respectively. For the systems considered, the Langmuir model was found to be applicable in interpreting Fe3+ adsorption on the Fe3+-imprinted nanocomposites. Metal ion adsorption onto specific adsorbents is pH-dependent. In the absence of complexing agents, the hydrolysis and precipitation of the metal ions are affected by the concentration and form of soluble metal species. The effect of pH on the Fe3+ ion adsorption of nanocomposites is shown in Figure 8. The Fe3+-imprinted nanocomposites exhibited a low affinity in acidic concentrations (pH < 3.0) and a high affinity at pH 4.0.

imprinted nanocomposites

metal ion

K

k

K

k

k′

Fe3+

5 913 10 234 12 498 32 236 16 612

0.58 0.47 0.18 0.35

53 602 17 519 10 027 18 828 2 109

3.1 5.4 2.9 25.4

5.3 11.4 15.8 72.6

Al3+ Cu2+ Co2+ Zn2+

Desorption of Fe3+ ions from imprinted nanocomposites was performed using 4.0 M HNO3. It can be observed that Fe3+ ions could be quantatively desorbed with HNO3 solution (97.8 ( 3.6%). When the HNO3 is used as the desorption agent, the coordination spheres of chelated Fe3+ ions are disrupted and, subsequently, Fe3+ ions are released from the Fe(III) cavities into the galleries of nanocomposites into the desorption medium. In order to show the reusability of the Fe3+-imprinted nanocomposites, the adsorption-desorption cycle was repeated 10 times using the same imprinted nanocomposites. The results showed that the Fe3+-imprinted nanocomposite traps can be used repeatedly without losing their adsorption capacities significantly. Selectivity Studies of Fe3+ Synthetic Traps. An important parameter to consider is the selectivity of binding. For this reason, competitive adsorption of Fe3+/Al3+, Fe3+/Cu2+, Fe3+/ Co2+, and Fe3+/Zn2+ from the binary solutions was studied in a batch system. The reason for choosing Al3+ and Cu2+ is the selectivity of these ions to MAAP. Because of the interference properties of the other ions, they were chosen as competitive ions. Table 2 summarizes Kd, k, and k′ values of Al3+, Co2+, Zn2+, and Cu2+ with respect to Fe3+. The relative selectivity coefficient is an indicator to express an adsorption affinity of recognition sites to the imprinted Fe3+ ions. These results showed that the relative selectivity coefficients of the ionimprinted nanocomposite traps for Fe3+/Al3+, Fe3+/Cu2+, Fe3+/ Co2+, and Fe3+/Zn2+ were 5.3, 11.4, 15.8, and 72.6 times greater than that of the nonimprinted matrix, respectively. This finding is consistent with fact that the ionic radii of Al3+, Co2+, and Fe3+ are similar size, 51, 72, and 64 pm, respectively, while Zn2+ and Co2+ possess a greater ionic radius, 88 and 96 pm, which renders these ions less suitable in size for the binding sites on nanocomposite structure. Conclusions The selectivity and specificity of the polymer-clay nanocomposite iron traps toward iron(III) ions have been demon-

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strated. The XRD findings showed the intercalations of ionimprinted nanocomposites between the interlayers of quartaminemodified smectite. The ion-imprinted nanocomposites are crosslinked hydrophilic matrixes. Formation of Fe3+ ion cavities in the polymer structure inside the galleries of nanocomposites introduced more hydrodynamic volume into the polymer chains, which can result in more uptake ofwater molecules by the polymer matrix. High complexation and geometric affinity between Fe3+ ions and Fe3+ ion-cavities in the nanocomposite structure resulted in faster adsorption rates and higher active binding cavities. The Langmuir adsorption model can be applied in the ion-imprinted nanocomposite adsorbent system. The competitive adsorption studies showed that selectivity of the ion-imprinted nanocomposite was significantly higher than the corresponding nonimprinted nanocomposite (control polymer). From the results obtained in this study, it is concluded that the new polymer-clay nanocomposite system is promising for the selective adsorption of Fe3+ ions from aqueous media. Literature Cited (1) Crichton, R. R. Inorganic Biochemistry of Iron Metabolism; Ellis Horwood Limited: U.K., 1991. (2) Arena, J. M. Poisoning; Thomas Springfield: Illinois, 1970. (3) Kontoghiorghes, G. J.; Pattichi, K.; Hadjigavriel, M.; Kolnagou, A. Transfusioal iron overload and chelation therapy with deferoxamine and deferiprone. Transfusion Sci. 2000, 23, 211. (4) Martin, R. B. The chemistry of aluminum as related to biology and medicine. Clin. Chem. 1986, 32, 1797. (5) Litovitz, T. L.; Schmitz, B. F.; Matyunas, N.; Martin, T. G. 1987 annual report of the American Association of Poison Control Centers National Data Collection System. Am. J. Emerg. Med. 1988, 6, 479. (6) Mahoney, J. R.; Hallway, P. E.; Hedlund, B. E.; Eaton, J. W. Acute iron poisoning. Rescue with macromolecular chelators. Am. Soc. Clin. InVest. 1989, 8, 1362. (7) Jacops, A. Tissue Changes in Iron Deficiency. Br. J. Haematol. 1979, 43, 1. (8) Whitten, C. F; Gibson, G. F.; Good, M. H.; Goodwin, J. F.; Brough, A. J. Studies in acute iron poisoning. I. Desferrioxamine in the treatment of acute iron poisoning: Clinical observations, experimental studies, and theoretical considerations. Pediatrics 1965, 36, 322. (9) Horowitz, D.; Margel, S.; Shimoni, T. Iron detoxification by haemoperfuion through deferoxamine conjugated agarose polyacrolein microsphere beads. Biomaterials 1985, 6, 9. (10) Ibrahim, E. H.; Denizli, A.; Bektas, S.; Genc¸ , O ¨ .; Piskin, E. Cadmium removal from human plasma by Cibacron Blue F3GA and thionein incorporated into polymeric microspheres. J. Chromatogr., B 1998, 720, 217. (11) Denizli, F.; Arica, Y.; Denizli, A. Cadmium detoxification from human plasma by thionein incorporated pHEMA membranes. React. Funct. Polym. 2000, 44, 207. (12) Denizli, A.; Salih, B.; Piskin, E. New chelate forming polymer microspheres carrying dyes as chelators for iron overload. J. Biomater. Sci. Polym. Ed. 1998, 9, 175. (13) Yavuz, H.; Arica, Y.; Denizli, A. Therapeutic affinity adsorption of iron(III) with dye- and ferritin-immobilized PHEMA adsorbent. J. Appl. Polym. Sci. 2001, 82, 186. (14) O ¨ zcan, A. A.; Say, R.; Denizli, A.; Erso¨z, A. L-Histidine imprinted synthetic receptor for biochromatography applications. Anal. Chem. 2006, 78, 7253. (15) Yavuz, H.; Say, R.; Denizli, A. Iron Removal Human Plasma Based On Molecular Recognition Using Imprinted Beads Mater. Sci. Eng., C 2005, 25, 521. (16) Yavuz, H.; Andac¸ , M.; Uzun, L.; Say, R.; Denizli, A. Molecular recognition based iron removal from human plasma with imprinted membranes, Int. J. Artif. Organs 2006, 29, 900. (17) Masque, N.; Marce, R. M.; Borrull, F.; Cormack, P.; Sherington, D. C. Synthesis and evaluation of a molecularly imprinted polymer for selective on-line solid-phase extraction of 4-nitrophenol from environmental water. Anal. Chem. 2000, 72, 4122. (18) Caro, E.; Masque, N.; Marce, R. M.; Borrull, F.; Cormack, P.; Sherington, D. C. J. Non-covalent and semi covalent molecularly imprinted polymers for selective on-line solid phase extraction of 4-nitrophenol from water samples. J. Chromatogr., A 2002, 963, 169.

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ReceiVed for reView June 28, 2007 ReVised manuscript receiVed November 5, 2007 Accepted January 26, 2008 IE070885O