- Email: [email protected]
Highly selective ion-imprinted particles for solid-phase extraction of Pb2+ ions
Materials Science and Engineering C 29 (2009) 2464–2470
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Highly selective ion-imprinted particles for solid-phase extraction of Pb2+ ions Cem Esen a, Muge Andac b, Nilay Bereli b, Rıdvan Say c, Emür Henden a, Adil Denizli b,⁎ a b c
Department of Chemistry, Ege University, İzmir, Turkey Department of Chemistry, Hacettepe University, Ankara, Turkey Department of Chemistry, Anadolu University, Eskişehir, Turkey
a r t i c l e
i n f o
Article history: Received 22 January 2009 Received in revised form 5 June 2009 Accepted 14 July 2009 Available online 22 July 2009 Keywords: Ion-imprinted particles Selective recognition Lead determination Solid-phase extraction Molecular imprinting
a b s t r a c t The Pb2+-imprinted (PHEMAC-Pb2+) particles were prepared by bulk polymerization as a solid-phase extraction (SPE) adsorbent. N-methacryloyl-(L)-cysteine (MAC) was used as functional monomer to have a well-shaped molecular geometry between MAC monomer and Pb2+ ions that provide molecular recognition based on well fitted cavities for Pb2+ ions after removal of template ions. The PHEMAC-Pb2+ particles were characterized and the applicability of these particles was investigated for the solid-phase extraction of Pb2+ ions from aqueous solutions and environmental samples. The PHEMAC-Pb2+ particles with a size range of 50–200 µm have a rough surface and macropores in bulk structure. The adsorption capacity of the PHEMAC-Pb2+ particles is relatively low (2.01 mg/g). However, the high selectivity towards competitive ions (Cd2+, Ni2+ and Cu2+) promises the PHEMAC-Pb2+ particles an alternative SPE adsorbent in literature. The relative selectivity coefficients of PHEMACPb2+ particles for Pb2+/Ni2+, Pb2+/Cd2+ and Pb2+/Cu2+ were almost 71, 117 and 192 times greater than that of non-imprinted (PHEMAC) particles, respectively. Moreover, the reusability of the PHEMAC-Pb2+ particles was tested for several times and no significant loss in adsorption capacity was observed. The accuracy of the proposed procedure was also verified by the determination of Pb2+ ions in the certified reference material, LGC 6137 Estuarine sediment. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Contamination of water by heavy metal ions is of great concern due to their associated ecological and health toxic effects even at very low concentrations. Among them, lead is one of the most environmental pollutant, ubiquitous found in soil, water and air with no biological function, but many toxicological effects to human life [1–3]. Thus, the removal of lead from environmental, biological, food and geological samples is often required. However, the determination of lead is difficult due to its low concentrations in environmental samples, which fall below the detection limit of conventional analytical techniques such as flame atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry [4–7]. Solid-phase extraction (SPE) is one of the most widely used separation technique for isolation, preconcentration, clean-up and medium exchange [8]. SPE is based on the separation of selected analytes (lead ions) from a liquid sample by a solid-phase adsorbent. The basic approach is the contact of the selected analytes through a solid matrix containing an adsorbent that retains the analytes. The retained analytes are then recovered by elution using an appropriate solvent [9]. There are several SPE adsorbents used for metal ion preconcentration and determination in literature [10–12], but, these adsorbents lack of selectivity. The selectivity of solid-phase extraction
⁎ Corresponding author. E-mail address: [email protected] (A. Denizli). 0928-4931/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2009.07.012
can be enhanced by using molecularly imprinted polymers (MIPs), which have synthetic recognition sites with high selectivity and affinity for a selected analyte or a template (i.e. metal ions, molecules) [13–15]. The application of these MIPs as adsorbents allows not only preconcentration and cleaning of sample but also selective extraction of the target analyte (template), which is important, particularly when the sample is complex and impurities can interfere with quantification [16]. SPE preconcentration of trace metals or separation from environmental sources by ion-imprinted polymer can lead to selective removal of analytes from matrix that cannot be achieved by the conventional methods [17–20]. Moreover, MIPs are easy to prepare, demonstrate physical and chemical stability under harsh conditions and can be used repeatedly several times without loss in capacity. Because of these advantages, MIPs have been used successfully as SPE adsorbents [21–24]. In the present study, the novel Pb2+-imprinted particles were prepared for selective extraction of Pb2+ ions from aqueous solutions. The choice of complexing monomer is derived from the high affinity of side chain sulfhydryl groups in the functional monomer (Nmethacryloyl-(L)-cysteine (MAC) towards Pb2+ ions. In a typical ion imprinting process, the functional monomer MAC was complexed with the template ion (Pb2+) prior to polymerization process to form a noncovalent coordination complex. The Pb2+-imprinted poly (hydroxylethyl methacrylate-N-methacryloyl-(L)-cysteine) (PHEMAC-Pb2+) particles were prepared by bulk polymerization as a solid-phase adsorbent for selective extraction of Pb2+ ions from aqueous solutions with higher selectivity than other SPE adsorbents.
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470
2. Materials and methods 2.1. Instrumentation A GBC 904 PBT model atomic absorption spectrometer equipped with a deuterium background corrector was used. All measurements were carried out in an air/acetylene flame. The instrumental parameters were recommended by the manufacturer. The wavelength selected for the determination of lead was 283.3 nm. A Jenway 3040 Model pH meter was employed for measuring pH values in the aqueous phase. 2.2. Reagents The monomers, 2-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-Cysteine and methacryloyl chloride were purchased from Sigma (St. Louis, MO). Pb (NO3)2 was used as the source of Pb2+ ions. All other chemicals were of analytical 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 NANOpure® organic/colloid removal and ion exchange packed-bed system. All the plastic and glassware were cleaned by soaking in dilute HNO3 (%10) and were rinsed with distilled water prior to use. In order to evaluate recoveries of the present work, LGC 6137 Estuarine sediment certified reference material was analyzed for lead. 2.3. Preparation of PHEMAC-Pb2+ particles The synthesis of N-methacryloyl-(L)-cysteine (MAC) monomer was adapted from the procedure reported by [25]. 2 mmol MAC solution in ethanol was treated with 1 mmol of Pb2+ solution for 2 h at room temperature forming metal–monomer complex (MAC–Pb2+). The 2hydroxyethyl methacrylate (HEMA) (4 mmol) and MAC–Pb2+ coordinate complex (1 mmol) in hydroxyl ethyl piperazine ethane sulfonic acid (HEPES) (500 µL, 0.09 g in 1 mL) solution were copolymerized in the presence of ethylene glycol dimethacrylate (EGDMA) (1 mmol) as crosslinker by bulk polymerization. Toluene (750 µL) and potassium persulfate (KPS) (16 mg) were used as the pore maker and the initiator, respectively. The polymerization was allowed to proceed at 75 °C for 1 h in 100 × 10 mm ID of plastic syringes after purged with nitrogen for 15 min. Non-imprinted (NIP) control PHEMAC particles were also prepared at the same polymerization conditions in the absence of Pb2+ ions. Both imprinted and non-imprinted PHEMAC particles were washed extensively with ethanol solution (50% v/v) to remove unreacted monomers. The template (Pb2+ ions) was removed using 5% thiourea in 1.0 M HCl until Pb2+ ions cannot be detected in the atomic absorption spectrometer. The template removed bulk polymer was grinded and sieved after drying in a vacuum oven at 55 °C for 48 h. 2.4. Characterization of PHEMAC-Pb2+ particles The specific surface area of the particles was measured by Brunauer– Emmett–Teller (BET) model using single point analysis and a Flowsorb II 2300 from Micromeritics Instrument Corporation (Norcross, USA). Water uptake ratios of both MIP and NIP particles were determined in deionized water. The weight ratio of dry and wet samples was recorded. The water content of MIP and NIP particles was calculated using the weights of particles before and after uptake of water. The surface morphology of the particles was examined using scanning electron microscope (SEM) (JEOL, SEM 1200 EX, Tokyo, Japan). The surface of the sample was scanned at the desired magnification to study the morphology of the MIP particles. To evaluate the degree of MAC incorporation for both MIP and NIP particles, they were subjected to
2465
elemental analysis using a Leco Elemental Analyzer (Model CHNS-932). FTIR spectrometer was used in the range 4000–400 cm− 1 to study surface chemistry of MAC monomer, MAC–Pb2+ complex and MIP particles in the solid state (FTIR 8000 Series, Shimadzu, Japan). 2.5. Adsorption of Pb2+ions from aqueous solutions The adsorption of Pb2+ ions from aqueous solutions on the PHEMAC-Pb2+ particles was determined in a batch system. The effect of the pH of medium was studied in the range of pH 3.0–6.0 adjusted with universal buffer solutions. The effects of initial Pb2+ concentration and temperature on the adsorption rate and adsorption capacity were also studied. For this purpose, 10 mL volumes of aqueous solutions containing different amounts of Pb2+ ions (in the range of 5–100 ppm) were treated with 100 mg of the particles at room temperature and magnetically stirred at a speed of 300 rpm. After the desired treatment periods, the concentration of the Pb2+ ions in the aqueous phase was measured by using a flame atomic absorption spectrophotometer (FAAS) with deuterium background correction. The instrument response was periodically checked with known Pb2+ solution standards. The experiments were performed in replicates of three. The amount of Pb2+ adsorption per unit mass of the particles was evaluated by using the mass balance. 2.6. Selectivity studies The selectivity of the PHEMAC-Pb2+ particles towards Pb2+ (Mw: 207.2 g/mol, ionic radius: 120 pm) in the presence of competitive ions Cd2+ (Mw: 112.4 g/mol, ionic radius: 114 pm), Ni2+ (Mw: 58.7 g/mol, ionic radius: 69 pm) and Cu2+(Mw: 63.5 g/mol, ionic radius: 71 pm) was evaluated in a batch system with a 10 mL of solution containing 30 mg/L of competitive metal ions at a pH 5.0 at room temperature, in the beaker stirred magnetically at 300 rpm. After a competitive adsorption equilibrium was reached, the concentration of Pb2+, Cd2+, Ni2+ and Cu2+ ions in the remaining solution was measured by FAAS. The distribution coefficients (Kd) for Cd2+, Ni2+ and Cu2+ with respect to Pb2+ were calculated by Eq. (1): Kd = ½ðCi − Cf Þ = Cf × ðV = mÞ:
ð1Þ
In which, Kd represents the distribution coefficient for the metal ion (mL/g); Ci and Cf are the initial and final concentrations of metal ions (mg/mL), respectively. V is the volume of the aqueous solution (L) and m is the weight of the particles used in the column (g). The selectivity coefficient (k) for the binding of Pb2+ in the presence of the competing species (Cd2+, Ni2+ and Cu2+) was determined by Eq. (2), 2+ m+ = Kd X k = Kd Pb
ð2Þ
where k is the selectivity coefficient and Xm+ is the competitive metal ion. A comparison of the k values of imprinted polymers allows an estimation of the effect of imprinting on selectivity. The relative selectivity coefficient is an indicator to express metal adsorption affinity of recognition sites to the imprinted Pb2+ ions. The relative selectivity coefficient, which was used to estimate the effect of imprinting on selectivity, can be defined in Eq. (3) kV = kimprinted = kcontrol
ð3Þ
in which kimprinted is for the PHEMAC-Pb2+ particles, and kcontrol is for the PHEMAC particles in the presence of competitive ions.
2466
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470
2.7. Desorption and reusability of PHEMAC-Pb2+ particles Pb2+ ions bound to the PHEMAC-Pb2+ particles were desorbed by 5% of thiourea solution in 1.0 M HCl. In a typical desorption experiment, Pb2+ bound PHEMAC-Pb2+ particles is magnetically stirred at a speed of 300 rpm with 50 mL of desorption solution for 2 h at room temperature. The Pb2+ concentration in desorption solution was measured by FAAS. The desorption ratio was then determined by dividing the amount of Pb2+ detected in the desorption medium by the initial amount of Pb2+ adsorbed on the PHEMAC-Pb2+ particles. In order to test the reusability of the PHEMAC-Pb2+ particles, Pb2+ adsorption–desorption cycle was repeated ten times by using the same imprinted particles. 2.8. Sample treatment LGC 6137 Estuarine sediment certified reference material (100 mg) was accurately weighed into a beaker and a mixture of concentrated hydrochloric acid (24 mL) and nitric acid (8.0 mL) was added. The beaker covered with a watch glass was evaporated on a hot plate almost to dryness. Then, 8.0 mL of mixed acid solution was added to the residue. After cooling, the mixture was filtered through a glass filter. The sample was diluted to 25 mL with deionized water. The pH of the appropriate aliquots of the solution was adjusted to 5.0. The surface river water sample was collected from Buyuk Menderes River in Aydin, Turkey. The bottle filled with the water sample was cleaned with deionized water, dilute nitric acid, and deionized water, respectively. The sample was acidified with HNO3 and stored at 4 °C. Before the analysis, the sample was filtered and adjusted to pH 5.0. All samples were analyzed by the general procedure given in Section 2.5. 3. Results and discussion
Fig. 1. (A) The molecular formula of MAC monomer and (B) the molecular structure of MAC–Pb2+ complex monomer.
Fig. 2. Fig. 2A illustrates the PHEMAC-Pb2+ particles surface morphology in the size range of 50–200 µm that has a rough surface due to the large pores, which formed during the polymerization process rather than PHEMAC particles. As clearly seen in Fig. 2A, the photograph in Fig. 2A on the upper side shows the presence of macropores within the bulk structure. The PHEMAC-Pb2+ and PHEMAC particles are crosslinked hydrophilic matrices. The incorporation of MAC monomer into PHEMAC-Pb2+ and PHEMAC particles enhanced the hydrophilicity due to the carboxyl groups of MAC monomer. The MAC content of PHEMAC-Pb2+ and PHEMAC particles were found to be 321.4 µmol g− 1 and 242.8 µmol g− 1 polymers, respectively, by using nitrogen stoichiometry. Similar swelling degrees were obtained for PHEMAC-Pb2+ particles (78.3%), which swell more, compared to the PHEMAC particles (63.1%) due to the ion cavities in the PHEMAC-Pb2+ particles formed by Pb2+ (template) ions. Moreover, the specific surface areas of PHEMAC-Pb2+ and PHEMAC
3.1. Characterization studies Owing to the high affinity ligand–metal ion interaction between the sulfhydryl groups and Pb2+ ions, the N-methacryloyl-(L)-cysteine (MAC) was used as complexing monomer. MAC was synthesized by the reaction between L-cysteine and methacryloyl chloride to form a functional monomer as described by Andaç et al. [25]. The molecular formula of MAC monomer and the molecular structure of MAC–Pb2+ complex monomer are given in Fig. 1. In order to confirm complex formation between Pb2+ and MAC monomer, FTIR spectroscopy was performed. The IR band at 1611 cm− 1 of MAC was assigned the characteristic stretching vibration amidecarbonyl absorption. The N–H bending peak appears at 1453 cm− 1 and is associated with the amide vibration of MAC. The two peaks at 1115 cm− 1 and 1031 cm− 1 are also characteristic group frequencies for C–H groups and result from bending vibrations in the molecule. The FTIR result of MAC showed that the characteristic absorbance peak at 2535 cm− 1 ascribed to –SH stretching vibrations. The sulfhydryl groups could donate a lone electron pair for the empty orbit of metal ions alone [26]. For the characteristic determination of complex, due to linear coordinate covalent complex formation, the characteristic weak sulfhydryl stretching vibration band at 2535 cm− 1 slips to the higher frequency field at 2519 cm− 1, as a result of decreasing the electron density of sulfhydryl group of MAC monomer. The MAC–Pb2+ monomer complex was copolymerized with HEMA monomer by radical bulk polymerization in the presence of crosslinker EGDMA as described in Section 2.3. The FTIR result of PHEMACPb2+ particles ascribed to the characteristic stretching vibration band of hydrogen bonded alcohol, O–H, around 3440 cm− 1, carbonyl at 1732 cm− 1, amide I and amide II absorption bands at 1652 and 1488 cm− 1, respectively. The surface morphology and bulk structure of PHEMAC-Pb2+ and PHEMAC particles are shown by the scanning electron photographs in
Fig. 2. SEM photographs of (A) PHEMAC-Pb2+ and (B) PHEMAC particles: surface morphology and bulk structure.
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470
2467
particles were found to be 35.8 and 24.9 m2 g− 1, respectively, which also indicate the presence of gaps in the PHEMAC-Pb2+ particles which are replaced by Pb2+ ions (Table 1). 3.2. Adsorption of Pb2+ ions from aqueous solutions 3.2.1. Effect of time The time dependence of the adsorption capacity values of Pb2+ ions on the PHEMAC-Pb2+ particles is an important parameter, since the adsorption rate controls the order of reaction kinetics. As seen in Fig. 3, adsorption amount of Pb2+ increases with the time during the first 45 min and then reached to the equilibrium adsorption within 60 min due to the high complexation and geometric shape affinity (or memory) between Pb2+ ions and Pb2+ ion cavities in the PHEMACPb2+ particles. It is well-known that removal of the template from the polymeric matrix leaves cavities of complementary size, shape, and chemical functionality to the template [27]. The maximum adsorption capacity for Pb2+ ions was 2.01 mg/g of dry weight of particles. 3.2.2. Effect of pH The effect of pH of the Pb2+ ions adsorption on PHEMAC-Pb2+ particles is shown in Fig. 4. The pH range was adjusted in the range of 3.0 and 6.0. The binding affinity of PHEMAC-Pb2+ particles is highly dependent to pH of the medium, thus the pH of the solution is another important parameter for the adsorption process. The PHEMAC-Pb2+ particles show low affinity in acidic conditions (pH 3.0), whereas Pb2+ ions show higher affinity at pH 5.0. As seen in Fig. 4, the optimum pH for maximum Pb2+ adsorption capacity is at pH 5.0. No further adsorption studies were followed above pH 6 due to the saturation in adsorption capacity. The increasing pH of the solution favors complex formation between the sulfhydryl groups of MAC monomer and Pb2+ ions. 3.2.3. Effect of equilibrium concentration As shown in Fig. 5, effect of equilibrium concentration on the adsorption of Pb2+ ions was studied for both PHEMAC-Pb2+ and PHEMAC particles. The adsorption amount of Pb2+ ions increased with increasing concentration, and a saturation value is achieved at Pb2+ ion concentration of 30 mg/L, which represents saturation of the active binding ionic cavities on the PHEMAC-Pb2+ particles. The maximum adsorption capacity was found to be 2.01 mg/g dry weight of PHEMACPb2+ particles. In this study, the adsorption capacity is relatively low compared to the literature [21,28,29], however, such an adsorption process, adsorption capacity is dependent to the stirring rate in the aqueous phase, structural properties of the adsorbent (e.g., surface topography, porosity, swelling degree), amount of the adsorbent, metal ion properties (e.g., hydrated ionic radius, coordination complex number), initial concentration of metal ions, chelate-formation rate between the ligand and the metal ions, and the existence of other heavy metal ions. It should be noted that, all individual experimental parameters published in the literature have been performed at different conditions. Two important physico-chemical aspects for evaluation of the adsorption process as a unit operation are the kinetics and the equilibrium of adsorption. Modelling of the equilibrium data has been done using the Langmuir and Freundlich isotherms [30]. The Langmuir and
Fig. 3. Time dependence of adsorption amount of Pb2+ ions on PHEMAC-Pb2+ particles; 30 ppm of Pb2+ solution (10 mL), pH: 5.0, 100 mg polymer and T: 25 °C.
Freundlich isotherms are represented as shown in Eqs. (4) and (5), respectively. q = qmax KD Ce = ð1 + b Ce Þ
ð4Þ
q = KF Ce 1 = n
ð5Þ
where, q is the Langmuir monolayer adsorption capacity (mg/g), Ce is the equilibrium Pb2+ concentration (mg/mL), KD is the constant related to the affinity binding sites, KF is the Freundlich constant, and n is the Freundlich exponent. The experimental data is in conformity with Langmuir adsorption isotherm rather than Freundlich isotherm. Since, the correlation coefficient (R2) was high (0.99). The KD constant for Langmuir isotherm was calculated as 438.12 g/mg. Fig. 6 compares the experimental adsorption behavior with Langmuir and Freundlich adsorption isotherms. The maximum adsorption capacity (2.01 mg/g) obtained from experimental results is also very close to the calculated Langmuir adsorption capacity (2.07 mg/g). It is clearly seen that the adsorbed Pb2+ ions onto the PHEMAC-Pb2+ particles show a monolayer adsorption behavior. The kinetic models were investigated by experimental data to evaluate the adsorption mechanism such as mass transfer and chemical reaction. The kinetic models (pseudo-first-order and pseudo-secondorder equations) can be used in this case assuming that the measured concentrations are equal to adsorbent surface concentrations. The firstorder rate expression of Lagergren is one of the most widely used for the adsorption of solute from a liquid solution [31]. The pseudo-first-order kinetic model is expressed by Eq. (6): logðqe − qt Þ = logðqe Þ −ðk1 t Þ = 2:303
ð6Þ
where qe is the experimental amount of Pb2+ adsorbed at equilibrium (mg/g); qt is the amount of Pb2+ adsorbed at time t (mg/g); k1 is the rate constant of the pseudo-first-order adsorption (1/min). A straight
Table 1 The characterization table of PHEMAC-Pb2+ and PHEMAC particles. Notations for particles
Specific surface area (m2 g− 1)
Swelling degree (%) by volume
MAC content (µmol g− 1)
PHEMAC-Pb2+ PHEMAC
35.8 24.9
78.3 63.1
321.4 242.8
Fig. 4. Effect of pH on the adsorption of Pb2+ on the PHEMAC-Pb2+ particles; 30 ppm of Pb2+ solution (10 mL), polymer amount: 100 mg, t: 60 min and T: 25 °C.
2468
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470 Table 2 The first- and second-order kinetic constants for the PHEMAC-Pb2+ particles. Initial conc.
Experimental First-order kinetic
Second-order kinetic
(mg/L) qex (mg/g)
k1 (1/min) qe (mg/g)
R2
30
0.012
0.15 0.064
2.01
3.72
k2 (g/mg min) qe (mg/g) R2 2.13
0.99
3.3. Selectivity studies
2+
2+
Fig. 5. Effect of equilibrium Pb concentration on adsorption of Pb on the PHEMACPb2+ particles at pH: 5.0; polymer amount: 100 mg, t: 60 min, T: 25 °C.
line of log (qe − qt) versus t suggests the applicability of this kinetic model. The pseudo-second-order kinetic model is expressed by Eq. (7): 2 ðt = qt Þ = 1 = k2 qe + ð1 = qe Þt
ð7Þ
where k2 is the rate constant of the pseudo second-order adsorption (g/mg min). If the pseudo-second-order kinetics is applicable, the plot of t/q versus t should show a linear relationship [32]. In Table 2, the correlation coefficients (R2) for the pseudo-firstorder equation is lower than 0.50. The theoretical qe value is slightly more different from the experimental value (3.72 mg/g). These values show that this adsorbent system is not so well described by pseudofirst-order kinetic model. However, the theoretical qe value calculated from the pseudo-second-order kinetic model is very close to the experimental value and the correlation coefficient is much higher than the pseudo-first-order kinetic model. By these results, the pseudosecond-order adsorption mechanism is predominant for the PHEMACPb2+ particles and that the overall rate of the Pb2+ adsorption process appeared to be controlled by chemical process. 3.2.4. Effect of temperature Temperature dependence of Pb2+ adsorption on PHEMAC-Pb2+ particles was investigated at different temperatures from 5 to 40 °C. As shown in Fig. 7, the adsorption capacity of PHEMAC-Pb2+ particles increased as the temperature increases. The thermodynamic equilibrium constant (KD) was used to calculate all other thermodynamic parameters such as free energy change (ΔG0) and entropy change (ΔS0). The negative change in free energy (ΔG0, − 468.8 J/mol) indicated that the adsorption of Pb2+ ions on the PHEMAC-Pb2+ particles is a thermodynamically favorable process. The ΔS value for the adsorption of Pb2+ ions to PHEMAC-Pb2+ particles was calculated as 336 J/mol K.
Fig. 6. Experimental data of absorbed Pb2+ ions on the PHEMAC-Pb2+ particles compare to Langmuir and Freundlich isotherms; polymer amount: 100 mg, t: 60 min, T: 25 °C.
The selectivity of PHEMAC-Pb2+ particles in the presence of competitive ions such as Pb2+/Cd2+, Pb2+/Ni2+ and Pb2+/Cu2+ was studied in a batch system. The competitive metal ions have the same ionic charges and different ionic radius (Cd2+ = 114 pm, Ni2+ = 69 pm, Cu2+ = 71 pm,) with respect to Pb2+ (Pb2+ = 120 pm) [33]. The comparison for the selectivity between PHEMAC-Pb2+ and the PHEMAC particles was listed in Table 3. The distribution coefficient (Kd) of Pb2+ ions for PHEMAC-Pb2+ particles is significantly greater than the competitive ions. The relative selectivity coefficient is an indicator to express metal binding affinity of recognition sites (i.e., molecular cavities) to the imprinted Pb2+ ions. The results showed that relative selectivity coefficients of the PHEMAC-Pb2+ particles for Pb2+/Ni2+, Pb2+/Cd2+ and Pb2+/Cu2+ were almost 71, 117 and 192 times greater than the PHEMAC particles, respectively (Table 3). Although these ions have similar chemical property, the competitive adsorption capacity of the PHEMAC-Pb2+ particles for Pb2+ ions is higher than PHEMAC particles. The high selectivity of the PHEMACPb2+ particles is due to the well designed coordination geometry of incorporated MAC molecules and Pb2+ ions. In addition to these results, Fig. 8 illustrates the adsorbed template and competitive ions both on PHEMAC-Pb2+ particles and PHEMAC particles. As clearly seen in Fig. 8, the competitive adsorption amount for Pb2+ ions in the PHEMAC-Pb2+ particles is 2.01 mg/g polymer in the presence of competitive ions (Cd2+, Ni2+ and Cu2+). It can be concluded that the PHEMAC-Pb2+ particles show the following metal ion affinity order under competitive adsorption conditions: Pb2+ N Ni2+ N Cd2+ N Cu2+. In literature, different NIP and MIP polymeric adsorbents with a wide range of adsorption capacities for Pb2+ ions are published [21,28,29]. But no one have such a high selectivity in the presence of competitive ions. In this study, the molecular geometry of Pb2+ ions was well fitted in the Pb2+-imprinted polymer cavities and hence the newly synthesized Pb2+-imprinted particles presented are promising for the solid-phase extraction of Pb2+ ions from aqueous solutions. 3.4. Desorption and reusability The PHEMAC-Pb2+ particles have reversible adsorption dynamics where it can also be desorbed by using a desorbing agent. This is another important factor for the PHEMAC-Pb2+ particles because the
Fig. 7. Temperature dependence of Pb2+ ions adsorption on the PHEMAC-Pb2+ particles; 30 mg/L of Pb2+ ions in 10 mL of aqueous solutions, pH: 5.0, t: 60 min.
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470
2469
Table 3 Kd, k, and k′ values of Cd2+, Ni2+ and Cu2+with respect to Pb2+. Metal ion Pb2+ Ni2+ Cd2+ Cu2+
PHEMAC-Pb2+ particles (imprinted)
PHEMAC particles (non-imprinted) Kd (mL/g)
k
Kd (mL/g)
k
k′
6.38 6.62 9.58 11.16
– 0.96 0.67 0.57
203.03 2.98 2.61 1.85
– 68.09 77.80 109.58
– 70.63 116.80 191.66
prepared MIPs should also be used several times without decreasing adsorption capacities, which play an important role on economics. The main advantage of the PHEMAC-Pb2+ particles as an SPE adsorbent is the regeneration and reusability with no significant losing in adsorption capacity. The different concentrations of HCl, HNO3 and 5% thiourea in 1.0 M HCl solution (acidic thiourea) were applied for desorption of Pb2+ ions from the PHEMAC-Pb2+ particles after the binding step. The desorption ratios of Pb2+ ions were not quantitative enough with HCl and HNO3 solutions. The highest desorption ratio (97.1%) was observed with 10 mL of 5% thiourea in 1.0 M HCl solution. This means that 5% thiourea in 1.0 M HCl solution breaks down the interaction forces between Pb2+ ions and binding sites in the cavities of the PHEMAC-Pb2+ particles. In this study, the desorption time was 2 h at 25 °C with a stirring rate of 300 rpm. In order to show the reusability of the PHEMAC-Pb2+ particles, adsorption–desorption cycle was repeated 10 times by using the same PHEMAC-Pb2+ particles. Adsorption–desorption cycle of PHEMAC-Pb2+ particles is shown in Fig. 9. The adsorption capacity of the recycled PHEMAC-Pb2+ particles can still be maintained at 95% level at the tenth cycle. 3.5. Analytical performance of the method The preconcentration of Pb2+ ions was performed in 100 mL of the aqueous solution containing 200 ng/mL of Pb2+ ions by treating with 100 mg of PHEMAC-Pb2+ particles at pH 5.0 for 60 min at room temperature. The Pb2+ ions were then desorbed by 10 mL of 5% thiourea in 0.1 M HCl solution. The concentration of Pb2+ ions in desorption medium was determined by FAAS. The characteristic performance data for the off-line preconcentration procedure can be given as follows. The precision of the Pb2+ determination method for 200 ng/mL of standard Pb2+ ions was evaluated as the relative standard deviation (R.S.D., n = 8), which was found to be 3.8% for Pb2+ ions. The limit of detection defined as the concentration of analyte giving signals equivalent to three times the standard deviation of the blank plus the net blank intensity for 100 mL of sample volume was 50.2 ng/mL. Determination after the preconcentration and elution procedures showed a linear calibration curve within the concentration range from 0.05 to 2.5 µg/mL. The regression equation was obtained as A = 0.0121 ⁎CPb (R2 = 0.9952).
Fig. 9. Adsorption–desorption cycle of the PHEMAC-Pb2+ particles; desorption agent: 5% thiourea in 1.0 M HCl solution; 30 mg/L of ions in 10 mL of aqueous solutions, polymer amount 100 mg, t: 120 min, T: 20 °C.
3.6. Determination of Pb2+ in certified reference material and river water sample The accuracy of the method was evaluated by the determination of Pb2+ ions in the certified reference material, LGC 6137 Estuarine sediment from the Laboratory of the Government Chemist, UK. The results are given in Table 4. As seen in Table 4, the relative standard deviation of the adsorption was under 5% for 10 experiments. Therefore corrected values can be obtained for recovery values by multiplying the found values with the reverse of removal percent. It can be concluded that the corrected values obtained for recoveries are 112% and 118%, respectively for the recovery percents. 4. Conclusion The newly synthesized MIPs were obtained by imprinting Pb2+ ions in functional MAC monomer incorporated PHEMAC-Pb2+ particles as highly selective SPE adsorbent in application for preconcentration and determination of Pb2+ ions in aqueous solutions and environmental samples. The removal of template Pb2+ ions was carried out by 5% thiourea in HCl solution. The characterization and the application of the PHEMAC-Pb2+ particles were investigated. The results show that the PHEMAC-Pb2+ particles in the size range of 50–200 µm have bulky rough surface and macropores within the bulk structure. The adsorption rate was found to be relatively fast. The time required to reach equilibrium conditions was about 60 min. The maximum adsorption capacity of the PHEMAC-Pb2+ particles for Pb2+ was 2.01 mg/g of the dry weight of the polymer. It can be used in the presence of competitive ions (Cd2+, Ni2+ and Cu2+) with high selectivity compared to the literature, suggesting that the fast adsorption equilibrium reached is most probably due to a high binding affinity between Pb2+and the Pb2+ binding sites in the polymer structure. Moreover, the PHEMAC-Pb2+ particles can be used several times without decreasing in adsorption Table 4 Determination of lead in certified reference material and river water sample (n = 5). Certified
LGC 6137 River water Fig. 8. Adsorbed Pb2+ and competitive ions both in PHEMAC-Pb2+ and PHEMAC particles. 30 mg/L of ions in 10 mL of aqueous solutions, pH: 5.0, polymer amount 100 mg, t: 60 min, T: 25 °C.
Found
Added
Found
Recovery
Corrected recovery
(µg/g)
(µg/g)
(µg/mL)
(µg/mL)
(%)
(%)
73.0 ± 3.6 –
62.2 ± 2.9 –
– 0 20 40
– N.D. 15.2 ± 1.5 32.1 ± 2.7
– – 76 80
– – 112 118
N.D.: Not detected.
2470
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470
capacity. The reusability is the main advantage of an SPE adsorbent, like the PHEMAC-Pb2+ particles, considering the SPE adsorbents that provide cost effective separations are the most promising polymers. Although, it has comparable detection limit and linear calibration range with literature [21], the results obtained in this work testify to the applicability of off-line preconcentration to FAAS for the selective determination of Pb2+ by using the PHEMAC-Pb2+ particles as SPE adsorbent, even in the presence of complicated matrices like river water, sea water and mineral waters, so that salt effect on lead determination by AAS can be eliminated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
X.M. Zhana, X. Zhao, Water Res. 37 (2003) 3905. H.A. Godwin, Curr. Opin. Chem. Biol. 5 (2001) 223. S.K. Hall, Environ. Sci. Technol. 6 (1) (1972) 30. M.G.A. Korn, J.B. Andrade, D.S. Jesus, V.A. Lemos, M.L.S.F. Bandeira, W.N.L. Santos, M.A. Bezerra, F.A.C. Amorim, A.S. Souza, S.L.C. Ferreira, Talanta 69 (2006) 16. F.A.C. Amorim, S.L.C. Ferreira, Talanta 65 (2005) 960. K. Ndung'u, R.P. Franks, K.W. Bruland, A.R. Flegal, Anal. Chim. Acta 481 (2003) 127. J.H. Wang, E.H. Hansen, J. Anal. At. Spectrom. 17 (2002) 1284. C.F. Poole, Trends Anal. Chem. 22 (2003) 362. V. Camel, Spectrochim. Acta Part B. 58 (2003) 1177. M. Ghaedi, F. Ahmadi, Z. Tavakoli, M. Montazerozohori, A. Khanmohammadi, M. Soylak, J. Hazard. Mater. 152 (2008) 1248.
[11] [12] [13] [14] [15] [16] [17]
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
M. Soylak, M. Tuzen, I. Narin, Quim. Nova 29 (2006) 203. P.T. Rao, R. Kala, S. Daniel, Anal. Chim. Acta 578 (2006) 105. M. Kempe, K. Mosbach, J. Chromatogr., A. 694 (1995) 3. H. Nishide, J. Deguchi, Chem. Lett. 2 (1976) 169. M.J. Whitcombe, M.E. Rodriguez, P. Villar, E.N. Vulfson, J. Am. Chem. Soc. 117 (1995) 7105. F. Qiao, H. Sun, H. Yan, K.H. Row, Chromatographia 64 (2006) 625. R.A. Bartsch, M. Maeda (Eds.), ACS Symposium Series 703: Molecular and Ionic Recognition with Imprinted Polymers, American Chemical Society, Washington DC, 1998, p. 29. D. Kriz, O. Ramstrom, K. Mosbach, Anal. Chem. 69 (1997) 345. I. Ferrer, D. Barceló, Trends Anal. Chem. 18 (1999) 180. B. Sellergren, Trends Anal. Chem. 18 (1999) 164. J. Pan, S. Wang, R. Zhang, Int. J. Environ. Anal. Chem. 86 (2006) 855. T.P. Rao, S. Daniel, J.M. Gladis, Trend Anal. Chem. 23 (2004) 28. R. Say, E. Birlik, A. Ersöz, F. Yılmaz, T. Gedikbey, A. Denizli, Anal. Chim. Acta 480 (2003) 251. A. Ersöz, R. Say, A. Denizli, Anal. Chim. Acta 502 (2004) 91. M. Andaç, R. Say, A. Denizli, J. Chromatogr. B 811 (2004) 119. H. Shindo, T.L. Brown, J. Am. Chem. Soc. 87 (1965) 1904. H. Yavuz, R. Say, A. Denizli, Mater. Sci. Eng. C 25 (2005) 521. S. Sun, A. Wang, Sep. Pur. Technol. 51 (2006) 409. J.C.Y. Ng, W.H. Cheung, G. Mckay, Chemosphere 52 (2003) 1021. G.M.S. Finette, M. Qui-Ming, M.T.W. Hearn, J. Chromatogr. A 763 (1997) 71. C.W. Cheung, J.F. Porter, G. Mckay, Water Res. (2001) 605. Z.P. Liang, Y.Q. Feng, Z.Y. Liang, S.X. Meng, Biochem. Eng. J. 24 (2005) 65. R.D. Shannon, Acta Cryst. A 32 (1976) 751.
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Highly selective ion-imprinted particles for solid-phase extraction of Pb2+ ions Cem Esen a, Muge Andac b, Nilay Bereli b, Rıdvan Say c, Emür Henden a, Adil Denizli b,⁎ a b c
Department of Chemistry, Ege University, İzmir, Turkey Department of Chemistry, Hacettepe University, Ankara, Turkey Department of Chemistry, Anadolu University, Eskişehir, Turkey
a r t i c l e
i n f o
Article history: Received 22 January 2009 Received in revised form 5 June 2009 Accepted 14 July 2009 Available online 22 July 2009 Keywords: Ion-imprinted particles Selective recognition Lead determination Solid-phase extraction Molecular imprinting
a b s t r a c t The Pb2+-imprinted (PHEMAC-Pb2+) particles were prepared by bulk polymerization as a solid-phase extraction (SPE) adsorbent. N-methacryloyl-(L)-cysteine (MAC) was used as functional monomer to have a well-shaped molecular geometry between MAC monomer and Pb2+ ions that provide molecular recognition based on well fitted cavities for Pb2+ ions after removal of template ions. The PHEMAC-Pb2+ particles were characterized and the applicability of these particles was investigated for the solid-phase extraction of Pb2+ ions from aqueous solutions and environmental samples. The PHEMAC-Pb2+ particles with a size range of 50–200 µm have a rough surface and macropores in bulk structure. The adsorption capacity of the PHEMAC-Pb2+ particles is relatively low (2.01 mg/g). However, the high selectivity towards competitive ions (Cd2+, Ni2+ and Cu2+) promises the PHEMAC-Pb2+ particles an alternative SPE adsorbent in literature. The relative selectivity coefficients of PHEMACPb2+ particles for Pb2+/Ni2+, Pb2+/Cd2+ and Pb2+/Cu2+ were almost 71, 117 and 192 times greater than that of non-imprinted (PHEMAC) particles, respectively. Moreover, the reusability of the PHEMAC-Pb2+ particles was tested for several times and no significant loss in adsorption capacity was observed. The accuracy of the proposed procedure was also verified by the determination of Pb2+ ions in the certified reference material, LGC 6137 Estuarine sediment. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Contamination of water by heavy metal ions is of great concern due to their associated ecological and health toxic effects even at very low concentrations. Among them, lead is one of the most environmental pollutant, ubiquitous found in soil, water and air with no biological function, but many toxicological effects to human life [1–3]. Thus, the removal of lead from environmental, biological, food and geological samples is often required. However, the determination of lead is difficult due to its low concentrations in environmental samples, which fall below the detection limit of conventional analytical techniques such as flame atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry [4–7]. Solid-phase extraction (SPE) is one of the most widely used separation technique for isolation, preconcentration, clean-up and medium exchange [8]. SPE is based on the separation of selected analytes (lead ions) from a liquid sample by a solid-phase adsorbent. The basic approach is the contact of the selected analytes through a solid matrix containing an adsorbent that retains the analytes. The retained analytes are then recovered by elution using an appropriate solvent [9]. There are several SPE adsorbents used for metal ion preconcentration and determination in literature [10–12], but, these adsorbents lack of selectivity. The selectivity of solid-phase extraction
⁎ Corresponding author. E-mail address: [email protected] (A. Denizli). 0928-4931/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2009.07.012
can be enhanced by using molecularly imprinted polymers (MIPs), which have synthetic recognition sites with high selectivity and affinity for a selected analyte or a template (i.e. metal ions, molecules) [13–15]. The application of these MIPs as adsorbents allows not only preconcentration and cleaning of sample but also selective extraction of the target analyte (template), which is important, particularly when the sample is complex and impurities can interfere with quantification [16]. SPE preconcentration of trace metals or separation from environmental sources by ion-imprinted polymer can lead to selective removal of analytes from matrix that cannot be achieved by the conventional methods [17–20]. Moreover, MIPs are easy to prepare, demonstrate physical and chemical stability under harsh conditions and can be used repeatedly several times without loss in capacity. Because of these advantages, MIPs have been used successfully as SPE adsorbents [21–24]. In the present study, the novel Pb2+-imprinted particles were prepared for selective extraction of Pb2+ ions from aqueous solutions. The choice of complexing monomer is derived from the high affinity of side chain sulfhydryl groups in the functional monomer (Nmethacryloyl-(L)-cysteine (MAC) towards Pb2+ ions. In a typical ion imprinting process, the functional monomer MAC was complexed with the template ion (Pb2+) prior to polymerization process to form a noncovalent coordination complex. The Pb2+-imprinted poly (hydroxylethyl methacrylate-N-methacryloyl-(L)-cysteine) (PHEMAC-Pb2+) particles were prepared by bulk polymerization as a solid-phase adsorbent for selective extraction of Pb2+ ions from aqueous solutions with higher selectivity than other SPE adsorbents.
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470
2. Materials and methods 2.1. Instrumentation A GBC 904 PBT model atomic absorption spectrometer equipped with a deuterium background corrector was used. All measurements were carried out in an air/acetylene flame. The instrumental parameters were recommended by the manufacturer. The wavelength selected for the determination of lead was 283.3 nm. A Jenway 3040 Model pH meter was employed for measuring pH values in the aqueous phase. 2.2. Reagents The monomers, 2-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-Cysteine and methacryloyl chloride were purchased from Sigma (St. Louis, MO). Pb (NO3)2 was used as the source of Pb2+ ions. All other chemicals were of analytical 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 NANOpure® organic/colloid removal and ion exchange packed-bed system. All the plastic and glassware were cleaned by soaking in dilute HNO3 (%10) and were rinsed with distilled water prior to use. In order to evaluate recoveries of the present work, LGC 6137 Estuarine sediment certified reference material was analyzed for lead. 2.3. Preparation of PHEMAC-Pb2+ particles The synthesis of N-methacryloyl-(L)-cysteine (MAC) monomer was adapted from the procedure reported by [25]. 2 mmol MAC solution in ethanol was treated with 1 mmol of Pb2+ solution for 2 h at room temperature forming metal–monomer complex (MAC–Pb2+). The 2hydroxyethyl methacrylate (HEMA) (4 mmol) and MAC–Pb2+ coordinate complex (1 mmol) in hydroxyl ethyl piperazine ethane sulfonic acid (HEPES) (500 µL, 0.09 g in 1 mL) solution were copolymerized in the presence of ethylene glycol dimethacrylate (EGDMA) (1 mmol) as crosslinker by bulk polymerization. Toluene (750 µL) and potassium persulfate (KPS) (16 mg) were used as the pore maker and the initiator, respectively. The polymerization was allowed to proceed at 75 °C for 1 h in 100 × 10 mm ID of plastic syringes after purged with nitrogen for 15 min. Non-imprinted (NIP) control PHEMAC particles were also prepared at the same polymerization conditions in the absence of Pb2+ ions. Both imprinted and non-imprinted PHEMAC particles were washed extensively with ethanol solution (50% v/v) to remove unreacted monomers. The template (Pb2+ ions) was removed using 5% thiourea in 1.0 M HCl until Pb2+ ions cannot be detected in the atomic absorption spectrometer. The template removed bulk polymer was grinded and sieved after drying in a vacuum oven at 55 °C for 48 h. 2.4. Characterization of PHEMAC-Pb2+ particles The specific surface area of the particles was measured by Brunauer– Emmett–Teller (BET) model using single point analysis and a Flowsorb II 2300 from Micromeritics Instrument Corporation (Norcross, USA). Water uptake ratios of both MIP and NIP particles were determined in deionized water. The weight ratio of dry and wet samples was recorded. The water content of MIP and NIP particles was calculated using the weights of particles before and after uptake of water. The surface morphology of the particles was examined using scanning electron microscope (SEM) (JEOL, SEM 1200 EX, Tokyo, Japan). The surface of the sample was scanned at the desired magnification to study the morphology of the MIP particles. To evaluate the degree of MAC incorporation for both MIP and NIP particles, they were subjected to
2465
elemental analysis using a Leco Elemental Analyzer (Model CHNS-932). FTIR spectrometer was used in the range 4000–400 cm− 1 to study surface chemistry of MAC monomer, MAC–Pb2+ complex and MIP particles in the solid state (FTIR 8000 Series, Shimadzu, Japan). 2.5. Adsorption of Pb2+ions from aqueous solutions The adsorption of Pb2+ ions from aqueous solutions on the PHEMAC-Pb2+ particles was determined in a batch system. The effect of the pH of medium was studied in the range of pH 3.0–6.0 adjusted with universal buffer solutions. The effects of initial Pb2+ concentration and temperature on the adsorption rate and adsorption capacity were also studied. For this purpose, 10 mL volumes of aqueous solutions containing different amounts of Pb2+ ions (in the range of 5–100 ppm) were treated with 100 mg of the particles at room temperature and magnetically stirred at a speed of 300 rpm. After the desired treatment periods, the concentration of the Pb2+ ions in the aqueous phase was measured by using a flame atomic absorption spectrophotometer (FAAS) with deuterium background correction. The instrument response was periodically checked with known Pb2+ solution standards. The experiments were performed in replicates of three. The amount of Pb2+ adsorption per unit mass of the particles was evaluated by using the mass balance. 2.6. Selectivity studies The selectivity of the PHEMAC-Pb2+ particles towards Pb2+ (Mw: 207.2 g/mol, ionic radius: 120 pm) in the presence of competitive ions Cd2+ (Mw: 112.4 g/mol, ionic radius: 114 pm), Ni2+ (Mw: 58.7 g/mol, ionic radius: 69 pm) and Cu2+(Mw: 63.5 g/mol, ionic radius: 71 pm) was evaluated in a batch system with a 10 mL of solution containing 30 mg/L of competitive metal ions at a pH 5.0 at room temperature, in the beaker stirred magnetically at 300 rpm. After a competitive adsorption equilibrium was reached, the concentration of Pb2+, Cd2+, Ni2+ and Cu2+ ions in the remaining solution was measured by FAAS. The distribution coefficients (Kd) for Cd2+, Ni2+ and Cu2+ with respect to Pb2+ were calculated by Eq. (1): Kd = ½ðCi − Cf Þ = Cf × ðV = mÞ:
ð1Þ
In which, Kd represents the distribution coefficient for the metal ion (mL/g); Ci and Cf are the initial and final concentrations of metal ions (mg/mL), respectively. V is the volume of the aqueous solution (L) and m is the weight of the particles used in the column (g). The selectivity coefficient (k) for the binding of Pb2+ in the presence of the competing species (Cd2+, Ni2+ and Cu2+) was determined by Eq. (2), 2+ m+ = Kd X k = Kd Pb
ð2Þ
where k is the selectivity coefficient and Xm+ is the competitive metal ion. A comparison of the k values of imprinted polymers allows an estimation of the effect of imprinting on selectivity. The relative selectivity coefficient is an indicator to express metal adsorption affinity of recognition sites to the imprinted Pb2+ ions. The relative selectivity coefficient, which was used to estimate the effect of imprinting on selectivity, can be defined in Eq. (3) kV = kimprinted = kcontrol
ð3Þ
in which kimprinted is for the PHEMAC-Pb2+ particles, and kcontrol is for the PHEMAC particles in the presence of competitive ions.
2466
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470
2.7. Desorption and reusability of PHEMAC-Pb2+ particles Pb2+ ions bound to the PHEMAC-Pb2+ particles were desorbed by 5% of thiourea solution in 1.0 M HCl. In a typical desorption experiment, Pb2+ bound PHEMAC-Pb2+ particles is magnetically stirred at a speed of 300 rpm with 50 mL of desorption solution for 2 h at room temperature. The Pb2+ concentration in desorption solution was measured by FAAS. The desorption ratio was then determined by dividing the amount of Pb2+ detected in the desorption medium by the initial amount of Pb2+ adsorbed on the PHEMAC-Pb2+ particles. In order to test the reusability of the PHEMAC-Pb2+ particles, Pb2+ adsorption–desorption cycle was repeated ten times by using the same imprinted particles. 2.8. Sample treatment LGC 6137 Estuarine sediment certified reference material (100 mg) was accurately weighed into a beaker and a mixture of concentrated hydrochloric acid (24 mL) and nitric acid (8.0 mL) was added. The beaker covered with a watch glass was evaporated on a hot plate almost to dryness. Then, 8.0 mL of mixed acid solution was added to the residue. After cooling, the mixture was filtered through a glass filter. The sample was diluted to 25 mL with deionized water. The pH of the appropriate aliquots of the solution was adjusted to 5.0. The surface river water sample was collected from Buyuk Menderes River in Aydin, Turkey. The bottle filled with the water sample was cleaned with deionized water, dilute nitric acid, and deionized water, respectively. The sample was acidified with HNO3 and stored at 4 °C. Before the analysis, the sample was filtered and adjusted to pH 5.0. All samples were analyzed by the general procedure given in Section 2.5. 3. Results and discussion
Fig. 1. (A) The molecular formula of MAC monomer and (B) the molecular structure of MAC–Pb2+ complex monomer.
Fig. 2. Fig. 2A illustrates the PHEMAC-Pb2+ particles surface morphology in the size range of 50–200 µm that has a rough surface due to the large pores, which formed during the polymerization process rather than PHEMAC particles. As clearly seen in Fig. 2A, the photograph in Fig. 2A on the upper side shows the presence of macropores within the bulk structure. The PHEMAC-Pb2+ and PHEMAC particles are crosslinked hydrophilic matrices. The incorporation of MAC monomer into PHEMAC-Pb2+ and PHEMAC particles enhanced the hydrophilicity due to the carboxyl groups of MAC monomer. The MAC content of PHEMAC-Pb2+ and PHEMAC particles were found to be 321.4 µmol g− 1 and 242.8 µmol g− 1 polymers, respectively, by using nitrogen stoichiometry. Similar swelling degrees were obtained for PHEMAC-Pb2+ particles (78.3%), which swell more, compared to the PHEMAC particles (63.1%) due to the ion cavities in the PHEMAC-Pb2+ particles formed by Pb2+ (template) ions. Moreover, the specific surface areas of PHEMAC-Pb2+ and PHEMAC
3.1. Characterization studies Owing to the high affinity ligand–metal ion interaction between the sulfhydryl groups and Pb2+ ions, the N-methacryloyl-(L)-cysteine (MAC) was used as complexing monomer. MAC was synthesized by the reaction between L-cysteine and methacryloyl chloride to form a functional monomer as described by Andaç et al. [25]. The molecular formula of MAC monomer and the molecular structure of MAC–Pb2+ complex monomer are given in Fig. 1. In order to confirm complex formation between Pb2+ and MAC monomer, FTIR spectroscopy was performed. The IR band at 1611 cm− 1 of MAC was assigned the characteristic stretching vibration amidecarbonyl absorption. The N–H bending peak appears at 1453 cm− 1 and is associated with the amide vibration of MAC. The two peaks at 1115 cm− 1 and 1031 cm− 1 are also characteristic group frequencies for C–H groups and result from bending vibrations in the molecule. The FTIR result of MAC showed that the characteristic absorbance peak at 2535 cm− 1 ascribed to –SH stretching vibrations. The sulfhydryl groups could donate a lone electron pair for the empty orbit of metal ions alone [26]. For the characteristic determination of complex, due to linear coordinate covalent complex formation, the characteristic weak sulfhydryl stretching vibration band at 2535 cm− 1 slips to the higher frequency field at 2519 cm− 1, as a result of decreasing the electron density of sulfhydryl group of MAC monomer. The MAC–Pb2+ monomer complex was copolymerized with HEMA monomer by radical bulk polymerization in the presence of crosslinker EGDMA as described in Section 2.3. The FTIR result of PHEMACPb2+ particles ascribed to the characteristic stretching vibration band of hydrogen bonded alcohol, O–H, around 3440 cm− 1, carbonyl at 1732 cm− 1, amide I and amide II absorption bands at 1652 and 1488 cm− 1, respectively. The surface morphology and bulk structure of PHEMAC-Pb2+ and PHEMAC particles are shown by the scanning electron photographs in
Fig. 2. SEM photographs of (A) PHEMAC-Pb2+ and (B) PHEMAC particles: surface morphology and bulk structure.
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470
2467
particles were found to be 35.8 and 24.9 m2 g− 1, respectively, which also indicate the presence of gaps in the PHEMAC-Pb2+ particles which are replaced by Pb2+ ions (Table 1). 3.2. Adsorption of Pb2+ ions from aqueous solutions 3.2.1. Effect of time The time dependence of the adsorption capacity values of Pb2+ ions on the PHEMAC-Pb2+ particles is an important parameter, since the adsorption rate controls the order of reaction kinetics. As seen in Fig. 3, adsorption amount of Pb2+ increases with the time during the first 45 min and then reached to the equilibrium adsorption within 60 min due to the high complexation and geometric shape affinity (or memory) between Pb2+ ions and Pb2+ ion cavities in the PHEMACPb2+ particles. It is well-known that removal of the template from the polymeric matrix leaves cavities of complementary size, shape, and chemical functionality to the template [27]. The maximum adsorption capacity for Pb2+ ions was 2.01 mg/g of dry weight of particles. 3.2.2. Effect of pH The effect of pH of the Pb2+ ions adsorption on PHEMAC-Pb2+ particles is shown in Fig. 4. The pH range was adjusted in the range of 3.0 and 6.0. The binding affinity of PHEMAC-Pb2+ particles is highly dependent to pH of the medium, thus the pH of the solution is another important parameter for the adsorption process. The PHEMAC-Pb2+ particles show low affinity in acidic conditions (pH 3.0), whereas Pb2+ ions show higher affinity at pH 5.0. As seen in Fig. 4, the optimum pH for maximum Pb2+ adsorption capacity is at pH 5.0. No further adsorption studies were followed above pH 6 due to the saturation in adsorption capacity. The increasing pH of the solution favors complex formation between the sulfhydryl groups of MAC monomer and Pb2+ ions. 3.2.3. Effect of equilibrium concentration As shown in Fig. 5, effect of equilibrium concentration on the adsorption of Pb2+ ions was studied for both PHEMAC-Pb2+ and PHEMAC particles. The adsorption amount of Pb2+ ions increased with increasing concentration, and a saturation value is achieved at Pb2+ ion concentration of 30 mg/L, which represents saturation of the active binding ionic cavities on the PHEMAC-Pb2+ particles. The maximum adsorption capacity was found to be 2.01 mg/g dry weight of PHEMACPb2+ particles. In this study, the adsorption capacity is relatively low compared to the literature [21,28,29], however, such an adsorption process, adsorption capacity is dependent to the stirring rate in the aqueous phase, structural properties of the adsorbent (e.g., surface topography, porosity, swelling degree), amount of the adsorbent, metal ion properties (e.g., hydrated ionic radius, coordination complex number), initial concentration of metal ions, chelate-formation rate between the ligand and the metal ions, and the existence of other heavy metal ions. It should be noted that, all individual experimental parameters published in the literature have been performed at different conditions. Two important physico-chemical aspects for evaluation of the adsorption process as a unit operation are the kinetics and the equilibrium of adsorption. Modelling of the equilibrium data has been done using the Langmuir and Freundlich isotherms [30]. The Langmuir and
Fig. 3. Time dependence of adsorption amount of Pb2+ ions on PHEMAC-Pb2+ particles; 30 ppm of Pb2+ solution (10 mL), pH: 5.0, 100 mg polymer and T: 25 °C.
Freundlich isotherms are represented as shown in Eqs. (4) and (5), respectively. q = qmax KD Ce = ð1 + b Ce Þ
ð4Þ
q = KF Ce 1 = n
ð5Þ
where, q is the Langmuir monolayer adsorption capacity (mg/g), Ce is the equilibrium Pb2+ concentration (mg/mL), KD is the constant related to the affinity binding sites, KF is the Freundlich constant, and n is the Freundlich exponent. The experimental data is in conformity with Langmuir adsorption isotherm rather than Freundlich isotherm. Since, the correlation coefficient (R2) was high (0.99). The KD constant for Langmuir isotherm was calculated as 438.12 g/mg. Fig. 6 compares the experimental adsorption behavior with Langmuir and Freundlich adsorption isotherms. The maximum adsorption capacity (2.01 mg/g) obtained from experimental results is also very close to the calculated Langmuir adsorption capacity (2.07 mg/g). It is clearly seen that the adsorbed Pb2+ ions onto the PHEMAC-Pb2+ particles show a monolayer adsorption behavior. The kinetic models were investigated by experimental data to evaluate the adsorption mechanism such as mass transfer and chemical reaction. The kinetic models (pseudo-first-order and pseudo-secondorder equations) can be used in this case assuming that the measured concentrations are equal to adsorbent surface concentrations. The firstorder rate expression of Lagergren is one of the most widely used for the adsorption of solute from a liquid solution [31]. The pseudo-first-order kinetic model is expressed by Eq. (6): logðqe − qt Þ = logðqe Þ −ðk1 t Þ = 2:303
ð6Þ
where qe is the experimental amount of Pb2+ adsorbed at equilibrium (mg/g); qt is the amount of Pb2+ adsorbed at time t (mg/g); k1 is the rate constant of the pseudo-first-order adsorption (1/min). A straight
Table 1 The characterization table of PHEMAC-Pb2+ and PHEMAC particles. Notations for particles
Specific surface area (m2 g− 1)
Swelling degree (%) by volume
MAC content (µmol g− 1)
PHEMAC-Pb2+ PHEMAC
35.8 24.9
78.3 63.1
321.4 242.8
Fig. 4. Effect of pH on the adsorption of Pb2+ on the PHEMAC-Pb2+ particles; 30 ppm of Pb2+ solution (10 mL), polymer amount: 100 mg, t: 60 min and T: 25 °C.
2468
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470 Table 2 The first- and second-order kinetic constants for the PHEMAC-Pb2+ particles. Initial conc.
Experimental First-order kinetic
Second-order kinetic
(mg/L) qex (mg/g)
k1 (1/min) qe (mg/g)
R2
30
0.012
0.15 0.064
2.01
3.72
k2 (g/mg min) qe (mg/g) R2 2.13
0.99
3.3. Selectivity studies
2+
2+
Fig. 5. Effect of equilibrium Pb concentration on adsorption of Pb on the PHEMACPb2+ particles at pH: 5.0; polymer amount: 100 mg, t: 60 min, T: 25 °C.
line of log (qe − qt) versus t suggests the applicability of this kinetic model. The pseudo-second-order kinetic model is expressed by Eq. (7): 2 ðt = qt Þ = 1 = k2 qe + ð1 = qe Þt
ð7Þ
where k2 is the rate constant of the pseudo second-order adsorption (g/mg min). If the pseudo-second-order kinetics is applicable, the plot of t/q versus t should show a linear relationship [32]. In Table 2, the correlation coefficients (R2) for the pseudo-firstorder equation is lower than 0.50. The theoretical qe value is slightly more different from the experimental value (3.72 mg/g). These values show that this adsorbent system is not so well described by pseudofirst-order kinetic model. However, the theoretical qe value calculated from the pseudo-second-order kinetic model is very close to the experimental value and the correlation coefficient is much higher than the pseudo-first-order kinetic model. By these results, the pseudosecond-order adsorption mechanism is predominant for the PHEMACPb2+ particles and that the overall rate of the Pb2+ adsorption process appeared to be controlled by chemical process. 3.2.4. Effect of temperature Temperature dependence of Pb2+ adsorption on PHEMAC-Pb2+ particles was investigated at different temperatures from 5 to 40 °C. As shown in Fig. 7, the adsorption capacity of PHEMAC-Pb2+ particles increased as the temperature increases. The thermodynamic equilibrium constant (KD) was used to calculate all other thermodynamic parameters such as free energy change (ΔG0) and entropy change (ΔS0). The negative change in free energy (ΔG0, − 468.8 J/mol) indicated that the adsorption of Pb2+ ions on the PHEMAC-Pb2+ particles is a thermodynamically favorable process. The ΔS value for the adsorption of Pb2+ ions to PHEMAC-Pb2+ particles was calculated as 336 J/mol K.
Fig. 6. Experimental data of absorbed Pb2+ ions on the PHEMAC-Pb2+ particles compare to Langmuir and Freundlich isotherms; polymer amount: 100 mg, t: 60 min, T: 25 °C.
The selectivity of PHEMAC-Pb2+ particles in the presence of competitive ions such as Pb2+/Cd2+, Pb2+/Ni2+ and Pb2+/Cu2+ was studied in a batch system. The competitive metal ions have the same ionic charges and different ionic radius (Cd2+ = 114 pm, Ni2+ = 69 pm, Cu2+ = 71 pm,) with respect to Pb2+ (Pb2+ = 120 pm) [33]. The comparison for the selectivity between PHEMAC-Pb2+ and the PHEMAC particles was listed in Table 3. The distribution coefficient (Kd) of Pb2+ ions for PHEMAC-Pb2+ particles is significantly greater than the competitive ions. The relative selectivity coefficient is an indicator to express metal binding affinity of recognition sites (i.e., molecular cavities) to the imprinted Pb2+ ions. The results showed that relative selectivity coefficients of the PHEMAC-Pb2+ particles for Pb2+/Ni2+, Pb2+/Cd2+ and Pb2+/Cu2+ were almost 71, 117 and 192 times greater than the PHEMAC particles, respectively (Table 3). Although these ions have similar chemical property, the competitive adsorption capacity of the PHEMAC-Pb2+ particles for Pb2+ ions is higher than PHEMAC particles. The high selectivity of the PHEMACPb2+ particles is due to the well designed coordination geometry of incorporated MAC molecules and Pb2+ ions. In addition to these results, Fig. 8 illustrates the adsorbed template and competitive ions both on PHEMAC-Pb2+ particles and PHEMAC particles. As clearly seen in Fig. 8, the competitive adsorption amount for Pb2+ ions in the PHEMAC-Pb2+ particles is 2.01 mg/g polymer in the presence of competitive ions (Cd2+, Ni2+ and Cu2+). It can be concluded that the PHEMAC-Pb2+ particles show the following metal ion affinity order under competitive adsorption conditions: Pb2+ N Ni2+ N Cd2+ N Cu2+. In literature, different NIP and MIP polymeric adsorbents with a wide range of adsorption capacities for Pb2+ ions are published [21,28,29]. But no one have such a high selectivity in the presence of competitive ions. In this study, the molecular geometry of Pb2+ ions was well fitted in the Pb2+-imprinted polymer cavities and hence the newly synthesized Pb2+-imprinted particles presented are promising for the solid-phase extraction of Pb2+ ions from aqueous solutions. 3.4. Desorption and reusability The PHEMAC-Pb2+ particles have reversible adsorption dynamics where it can also be desorbed by using a desorbing agent. This is another important factor for the PHEMAC-Pb2+ particles because the
Fig. 7. Temperature dependence of Pb2+ ions adsorption on the PHEMAC-Pb2+ particles; 30 mg/L of Pb2+ ions in 10 mL of aqueous solutions, pH: 5.0, t: 60 min.
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470
2469
Table 3 Kd, k, and k′ values of Cd2+, Ni2+ and Cu2+with respect to Pb2+. Metal ion Pb2+ Ni2+ Cd2+ Cu2+
PHEMAC-Pb2+ particles (imprinted)
PHEMAC particles (non-imprinted) Kd (mL/g)
k
Kd (mL/g)
k
k′
6.38 6.62 9.58 11.16
– 0.96 0.67 0.57
203.03 2.98 2.61 1.85
– 68.09 77.80 109.58
– 70.63 116.80 191.66
prepared MIPs should also be used several times without decreasing adsorption capacities, which play an important role on economics. The main advantage of the PHEMAC-Pb2+ particles as an SPE adsorbent is the regeneration and reusability with no significant losing in adsorption capacity. The different concentrations of HCl, HNO3 and 5% thiourea in 1.0 M HCl solution (acidic thiourea) were applied for desorption of Pb2+ ions from the PHEMAC-Pb2+ particles after the binding step. The desorption ratios of Pb2+ ions were not quantitative enough with HCl and HNO3 solutions. The highest desorption ratio (97.1%) was observed with 10 mL of 5% thiourea in 1.0 M HCl solution. This means that 5% thiourea in 1.0 M HCl solution breaks down the interaction forces between Pb2+ ions and binding sites in the cavities of the PHEMAC-Pb2+ particles. In this study, the desorption time was 2 h at 25 °C with a stirring rate of 300 rpm. In order to show the reusability of the PHEMAC-Pb2+ particles, adsorption–desorption cycle was repeated 10 times by using the same PHEMAC-Pb2+ particles. Adsorption–desorption cycle of PHEMAC-Pb2+ particles is shown in Fig. 9. The adsorption capacity of the recycled PHEMAC-Pb2+ particles can still be maintained at 95% level at the tenth cycle. 3.5. Analytical performance of the method The preconcentration of Pb2+ ions was performed in 100 mL of the aqueous solution containing 200 ng/mL of Pb2+ ions by treating with 100 mg of PHEMAC-Pb2+ particles at pH 5.0 for 60 min at room temperature. The Pb2+ ions were then desorbed by 10 mL of 5% thiourea in 0.1 M HCl solution. The concentration of Pb2+ ions in desorption medium was determined by FAAS. The characteristic performance data for the off-line preconcentration procedure can be given as follows. The precision of the Pb2+ determination method for 200 ng/mL of standard Pb2+ ions was evaluated as the relative standard deviation (R.S.D., n = 8), which was found to be 3.8% for Pb2+ ions. The limit of detection defined as the concentration of analyte giving signals equivalent to three times the standard deviation of the blank plus the net blank intensity for 100 mL of sample volume was 50.2 ng/mL. Determination after the preconcentration and elution procedures showed a linear calibration curve within the concentration range from 0.05 to 2.5 µg/mL. The regression equation was obtained as A = 0.0121 ⁎CPb (R2 = 0.9952).
Fig. 9. Adsorption–desorption cycle of the PHEMAC-Pb2+ particles; desorption agent: 5% thiourea in 1.0 M HCl solution; 30 mg/L of ions in 10 mL of aqueous solutions, polymer amount 100 mg, t: 120 min, T: 20 °C.
3.6. Determination of Pb2+ in certified reference material and river water sample The accuracy of the method was evaluated by the determination of Pb2+ ions in the certified reference material, LGC 6137 Estuarine sediment from the Laboratory of the Government Chemist, UK. The results are given in Table 4. As seen in Table 4, the relative standard deviation of the adsorption was under 5% for 10 experiments. Therefore corrected values can be obtained for recovery values by multiplying the found values with the reverse of removal percent. It can be concluded that the corrected values obtained for recoveries are 112% and 118%, respectively for the recovery percents. 4. Conclusion The newly synthesized MIPs were obtained by imprinting Pb2+ ions in functional MAC monomer incorporated PHEMAC-Pb2+ particles as highly selective SPE adsorbent in application for preconcentration and determination of Pb2+ ions in aqueous solutions and environmental samples. The removal of template Pb2+ ions was carried out by 5% thiourea in HCl solution. The characterization and the application of the PHEMAC-Pb2+ particles were investigated. The results show that the PHEMAC-Pb2+ particles in the size range of 50–200 µm have bulky rough surface and macropores within the bulk structure. The adsorption rate was found to be relatively fast. The time required to reach equilibrium conditions was about 60 min. The maximum adsorption capacity of the PHEMAC-Pb2+ particles for Pb2+ was 2.01 mg/g of the dry weight of the polymer. It can be used in the presence of competitive ions (Cd2+, Ni2+ and Cu2+) with high selectivity compared to the literature, suggesting that the fast adsorption equilibrium reached is most probably due to a high binding affinity between Pb2+and the Pb2+ binding sites in the polymer structure. Moreover, the PHEMAC-Pb2+ particles can be used several times without decreasing in adsorption Table 4 Determination of lead in certified reference material and river water sample (n = 5). Certified
LGC 6137 River water Fig. 8. Adsorbed Pb2+ and competitive ions both in PHEMAC-Pb2+ and PHEMAC particles. 30 mg/L of ions in 10 mL of aqueous solutions, pH: 5.0, polymer amount 100 mg, t: 60 min, T: 25 °C.
Found
Added
Found
Recovery
Corrected recovery
(µg/g)
(µg/g)
(µg/mL)
(µg/mL)
(%)
(%)
73.0 ± 3.6 –
62.2 ± 2.9 –
– 0 20 40
– N.D. 15.2 ± 1.5 32.1 ± 2.7
– – 76 80
– – 112 118
N.D.: Not detected.
2470
C. Esen et al. / Materials Science and Engineering C 29 (2009) 2464–2470
capacity. The reusability is the main advantage of an SPE adsorbent, like the PHEMAC-Pb2+ particles, considering the SPE adsorbents that provide cost effective separations are the most promising polymers. Although, it has comparable detection limit and linear calibration range with literature [21], the results obtained in this work testify to the applicability of off-line preconcentration to FAAS for the selective determination of Pb2+ by using the PHEMAC-Pb2+ particles as SPE adsorbent, even in the presence of complicated matrices like river water, sea water and mineral waters, so that salt effect on lead determination by AAS can be eliminated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
X.M. Zhana, X. Zhao, Water Res. 37 (2003) 3905. H.A. Godwin, Curr. Opin. Chem. Biol. 5 (2001) 223. S.K. Hall, Environ. Sci. Technol. 6 (1) (1972) 30. M.G.A. Korn, J.B. Andrade, D.S. Jesus, V.A. Lemos, M.L.S.F. Bandeira, W.N.L. Santos, M.A. Bezerra, F.A.C. Amorim, A.S. Souza, S.L.C. Ferreira, Talanta 69 (2006) 16. F.A.C. Amorim, S.L.C. Ferreira, Talanta 65 (2005) 960. K. Ndung'u, R.P. Franks, K.W. Bruland, A.R. Flegal, Anal. Chim. Acta 481 (2003) 127. J.H. Wang, E.H. Hansen, J. Anal. At. Spectrom. 17 (2002) 1284. C.F. Poole, Trends Anal. Chem. 22 (2003) 362. V. Camel, Spectrochim. Acta Part B. 58 (2003) 1177. M. Ghaedi, F. Ahmadi, Z. Tavakoli, M. Montazerozohori, A. Khanmohammadi, M. Soylak, J. Hazard. Mater. 152 (2008) 1248.
[11] [12] [13] [14] [15] [16] [17]
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
M. Soylak, M. Tuzen, I. Narin, Quim. Nova 29 (2006) 203. P.T. Rao, R. Kala, S. Daniel, Anal. Chim. Acta 578 (2006) 105. M. Kempe, K. Mosbach, J. Chromatogr., A. 694 (1995) 3. H. Nishide, J. Deguchi, Chem. Lett. 2 (1976) 169. M.J. Whitcombe, M.E. Rodriguez, P. Villar, E.N. Vulfson, J. Am. Chem. Soc. 117 (1995) 7105. F. Qiao, H. Sun, H. Yan, K.H. Row, Chromatographia 64 (2006) 625. R.A. Bartsch, M. Maeda (Eds.), ACS Symposium Series 703: Molecular and Ionic Recognition with Imprinted Polymers, American Chemical Society, Washington DC, 1998, p. 29. D. Kriz, O. Ramstrom, K. Mosbach, Anal. Chem. 69 (1997) 345. I. Ferrer, D. Barceló, Trends Anal. Chem. 18 (1999) 180. B. Sellergren, Trends Anal. Chem. 18 (1999) 164. J. Pan, S. Wang, R. Zhang, Int. J. Environ. Anal. Chem. 86 (2006) 855. T.P. Rao, S. Daniel, J.M. Gladis, Trend Anal. Chem. 23 (2004) 28. R. Say, E. Birlik, A. Ersöz, F. Yılmaz, T. Gedikbey, A. Denizli, Anal. Chim. Acta 480 (2003) 251. A. Ersöz, R. Say, A. Denizli, Anal. Chim. Acta 502 (2004) 91. M. Andaç, R. Say, A. Denizli, J. Chromatogr. B 811 (2004) 119. H. Shindo, T.L. Brown, J. Am. Chem. Soc. 87 (1965) 1904. H. Yavuz, R. Say, A. Denizli, Mater. Sci. Eng. C 25 (2005) 521. S. Sun, A. Wang, Sep. Pur. Technol. 51 (2006) 409. J.C.Y. Ng, W.H. Cheung, G. Mckay, Chemosphere 52 (2003) 1021. G.M.S. Finette, M. Qui-Ming, M.T.W. Hearn, J. Chromatogr. A 763 (1997) 71. C.W. Cheung, J.F. Porter, G. Mckay, Water Res. (2001) 605. Z.P. Liang, Y.Q. Feng, Z.Y. Liang, S.X. Meng, Biochem. Eng. J. 24 (2005) 65. R.D. Shannon, Acta Cryst. A 32 (1976) 751.