Rapid and selective extraction of trace amount of Pb(II) in aqueous samples using a magnetic ion-imprinted polymer and detection by flame atomic absorption spectrometry

A magnetic Pb(II) ion-imprinted polymer was synthesized using magnetic Fe3O4@SiO2 nanospheres as supporter, n-benzoyl-n-phenyl hydroxylamine as ligand, 4-vinyl pyridine as monomer, ethylene glycol dimethacrylate as crosslinker and 2,2′-azobis(isobutyronitrile) as the initiator. The template was removed from the polymer using 4 mL of HCl (2 mol L-1). The chemical structure, morphology-particle size, elemental analysis, thermal behavior and magnetic properties of the sorbent were evaluated using Fourier-transform infrared spectroscopy, scanning electron microscopy & ImageJ software - Energy dispersive X-ray system, thermal gravimetric analysis and vibrating sample magnetometry. Various parameters such as pH, equilibrium extraction time, temperature and the eluent were optimized. The results showed the sorbent adsorption capacity was 92.3 mg g-1 in pH 6.0 media, equilibrium time of 7 min at 40 °C. Calibration linearity was 2-150 µg L-1 with detection limit of 0.3 µg L-1. The prepared sorbent had high selectivity and was successfully applied to the removing of lead in real samples.


Introduction
It is clear that heavy metals contamination represents serious hazards to the ecosystem and especially to humans because of the complex toxicological effects on living beings 1 . Lead has been accepted as an important toxic heavy metals resulting from mining, acid battery factories 2 , metal plating 3 , printing 4 , textile 5 , photographic materials 6 , ceramic and glass industries as well as explosive manufacturing 7 . Through the joined system of water-plant-animal-human, Pb(II) is transferred into human body and causes to various severe health problems in vital organs of humans, such as damage to kidney, liver, blood composition, central nervous system (CNS) due to binding sites on cerebellar phosphokinase C at trace amounts and also retarding the reproductive system in mental function [8][9][10] . The maximum permitted amount of lead ions by world health organization (WHO) in drinking water fixed as 10.0 µg L -1 and for the U.S. environmental protection agency (USEPA) is 15.0 µg L - 1 11 . Consequently, the developments of reliable and highly sensitive techniques to the determination and then removal of lead in water samples as the pollutant sources are crucially necessary.
Nowadays flame atomic absorption spectrometry (FAAS) because of its analytical abilities has been confirmed for trace analysis of ABSTRACT: A magnetic Pb(II) ion-imprinted polymer was synthesized using magnetic Fe3O4@SiO2 nanospheres as supporter, n-benzoyl-nphenyl hydroxylamine as ligand, 4-vinyl pyridine as monomer, ethylene glycol dimethacrylate as crosslinker and 2,2′-azobis(isobutyronitrile) as the initiator. The template was removed from the polymer using 4 mL of HCl (2 mol L -1 ). The chemical structure, morphology-particle size, elemental analysis, thermal behavior and magnetic properties of the sorbent were evaluated using Fourier-transform infrared spectroscopy, scanning electron microscopy & ImageJ software -Energy dispersive Xray system, thermal gravimetric analysis and vibrating sample magnetometry. Various parameters such as pH, equilibrium extraction time, temperature and the eluent were optimized. The results showed the sorbent adsorption capacity was 92.3 mg g -1 in pH 6.0 media, equilibrium time of 7 min at 40 °C. Calibration linearity was 2-150 µg L -1 with detection limit of 0.3 µg L -1 . The prepared sorbent had high selectivity and was successfully applied to the removing of lead in real samples. metal ions but lead concentration in environmental waters is usually lower than FAAS detection limit 12 . Therefore, a separation and enrichment pretreatment is necessary before to Pb(ΙΙ) determination at the µg L -1 levels. Several methods have been developed for Pb 2+ ions separationpreconcentration, such as liquid-liquid extraction 13 , liquid-phase microextraction 14 , solid phase extraction (SPE) 15 , coprecipitation 16 and cloud point extraction 17 which among them SPE is more common due to the higher enrichment factor, minimal costs, environment friendly and simple automation.
Different solid phases have been applied as sorbents which among them, molecularly imprinted polymers (MIPs) have been widely utilized for selective separation of organic and also inorganic compounds from aqueous samples 18 . Similar sorbents to MIPs are ion imprinted polymers (IIPs) that produced by chelating metal ions with the relevant ligands and then polymerizing with a monomer. Ion imprinted polymers (IIPs) are more attractive for separation of inorganic ions due to their high selectivity, high preconcentration factor and chemical stability 19 . In this manner Liu 20 synthesized a surface-imprinted mesoporous sorbent for Pb(II) ion by the postsynthesis method and the adsorption capacity reached to 36.6 mg g -1 . Ghoohestani 21 synthesized Pb(II)-IIP by polymerization of chitosan on the surface of MCM-41 with adsorption capacity of 57.7 mg g -1 . Fan 22 synthesized silica-supported sorbent functionalized with Schiff base by coupling a surface imprinting technique with a sol-gel process for removal of Pb(II) ions with adsorption capacity of 54.9 mg g -1 .
In present study, a novel magnetic Pb-IIP containing of n-benzoyl-n-phenyl hydroxylamine was synthesized on the surface of Fe3O4@SiO2 by a surface imprinting technique for the selective and efficient preconcentration of lead ions in various aqueous samples. The adsorption experiments coupled with FAAS system was used to the analyte quantification at µg L -1 levels.

Reagents and materials
Ferric chloride, sodium acetate, ammonium hydroxide and tetraethyl orthosilicate (TEOS) were used for synthesis of Fe3O4@SiO2 nanoparticles and for the functionalization.

Apparatus
The measurements were performed using flame atomic absorption spectrometer (Varian, Model Spectra A-20 Plus) with hollow cathode lamp (10 mA current, spectral bandwidth and wavelength of 0.5 and 283.3 nm respectively). The infrared spectra of the IIP samples were obtained using a Shimadzu 8400S FTIR spectrometer. A scanning electron microscopy along with ImageJ software (Ver. 5.1.1)-energy dispersive X-ray (Joel-JSMT 300A) were used for morphology-particle sizeelemental analysis. TGA studies were carried out with Stanton Redcroft-STA-780 system applying heating rate of 10 C min -1 in a temperature range of 50-600 °C, under air atmosphere with a flow rate of 50 mL min -1 . Magnetic properties were analyzed using a vibrating sample magnetometer (Microsense-EZ VSM). The pH was determined using a Metrohm pH meter (M. Lab-827) with a combined glass-calomel electrode.

Synthesis of magnetic nanospheres (MNSs)
The MNS was synthesized by a solvothermal reduction method 23 . Typically, 1.35 g FeCl3.6H2O was dissolved in ethylene glycol (40 mL) to form a clear solution, followed by the addition of sodium acetate (3.6 g) and polyethylene glycol (1.0 g). The mixture was vigorously treated by ultrasonic bath for 30 min. After that it was refluxed at 180 °C for 8 h, and then allowed to cool down to room temperature. The black product was washed several times with ethanol and deionized water. Then it was dried at 60 °C for 6 h.

Synthesis of SiO2 coated MNSs (Fe3O4@SiO2)
These nanospheres were prepared according to a previously reported method with minor modifications 24 . Typically, 0.5 g of the MNS was dispersed in 60 mL ethanol and 10 mL of deionized water, and treated by sonication for 15 min, followed by the addition of 1.0 mL ammonium hydroxide (25%) and 3.0 mL TEOS sequentially. The mixture was reacted for 12 h at room temperature under continuous stirring. The resultant product was collected by an external magnetic field and rinsed six times with ethanol and water. Subsequently, 8.0 g Fe3O4@SiO2 were mixed with 100 mL of HCl (2 mol L -1 ) and refluxed for 6h. the activated microspheres were recovered and washed several times. Finally, the obtained activated Fe3O4@SiO2 was dried under vacuum at 60 °C for 3 h.

Synthesis of functional Fe3O4@SiO2 microspheres
Two grams of Fe3O4@SiO2 activated microspheres were added to 50 mL of anhydrous toluene, followed by an addition of moderate TEA and 12 mL APS. The mixture was incubated at 110 °C for 12 h in nitrogen atmosphere. Then the Amino modified microspheres (Fe3O4@SiO2-APS) were magnetically separated, rinsed with toluene and vacuum dried at 25 °C. To prepare the Fe3O4@SiO2-APS-MA particles, 1.5 g Fe3O4@SiO2-APS and 2.5 g MA were added to 50 mL DMF, then the mixture reacted for 24 h at room temperature. The solid separated with magnetic field, rinsed with DMF and ethanol and then dried under vacuum at 25 °C for 2 h.

Synthesis of Pb 2+ imprinted-polymer magnetic adsorbent (MIIP)
First, 2 mmol of Pb(NO3)2 was dissolved in 25 mL DMF under ultrasonic oscillation. Then 4 mmol BPHA, 8 mmol 4-vynil pyridine and 25 mL methanol were added to the solution and stirred overnight at room temperature 25 . After that, 0.5 g Fe3O4@SiO2-APS-MA, 8 mmol EGDMA and 0.1 g AIBN were added to the solution and reacted at 60 °C for 24 h under nitrogen atmosphere. The final product was collected with magnetic field and washed with DMF and methanol. The lead ions in polymer were removed by an HCl solution (2 mol L -1 ). The removal of the template was determined by FAAS. Schematic diagram of this procedure is shown in Fig. 1.
Magnetic non-ion imprinted polymer (MNIIP) were prepared by same process exception of Pb(NO3)2 addition for the purpose of contrast test.

Determination procedure
The pH of sample solution containing Pb(II) was adjusted to 6 by drop-wise additions of 0.1 mol L −1 of NaOH or HCl. Then 0.2 g of the sorbent was added into 100 mL of the solution and shake for 7 min via 40 °C. After adsorption equilibrium, these magnetic nanoparticles were separated by a magnet and the adsorbed Pb 2+ ions were eluted with 4 mL HCl (2 mol L -1 ) for 15 min. Now, the analyte in the supernatant and the eluent were determined by FAAS.
Percent extraction of lead (E%) and adsorption capacity (q) of magnetic ion-imprinted polymer (MIIP) was calculated via the following formulas (Eq. 1 and 2): Where: Ci (mg L -1 ) and Ce (mg L -1 ) are the Pb 2+ concentrations before and after adsorption in the solution respectively. V (L) is the volume of added solution, m (g) is the mass of MIIP in dry form.

Sample preparation
Environmental and wastewater samples were collected from Tehran, Polur (Anahita Co.) and an ammonium perchlorate factory in Iran respectively. Prior to sampling, the bottle was cleaned with deionized water, diluted nitric acid and deionized water in succession. The samples were filtered through a cellulose filter membrane (0.45 µm) and their pH was adjusted before the determination.

Structure investigation with FT-IR
The FT-IR spectra of Fe3O4@SiO2 and MIIP showed in Fig. 2. The peak at 1150 cm -1 is corresponding to Si-O-Si stretching that observed in both of samples, which indicates SiO2 is encapsulated correctly inside the MIIP. The carbonyl peak at 1730 cm -1 , O-H peak at 3205 cm -1 and C-N peak at 1450 and 1380 cm -1 showed the presence of ligand (BPHA) in the MIIP structure.

Morphology analysis
The morphology and particle size histograms of the synthesized adsorbents were analyzed via SEM system and ImageJ software. The results of the MNIIP and MIIP nanoparticles are shown in Fig.  3(a, b). SEM image of Fig. 3a shows a homogenous and smooth surface for MNIIP nanospheres with particle size range of 30-40 nm that obtained from ImageJ software usage. As shown in SEM image of Fig. 3b, in the presence and then removing of Pb(II), a porous structure was resulted for MIIP which was attributed to imprinted cavities. This rough structure of this surface on the surface of MIIP is favor to mass transfer and the formation of three-dimensional recognition sites. In this manner the particle size range of MIIP was obtained 20-30 nm.

Elemental analysis
The EDX spectrums of MIIPs after adsorption and then elution of Pb(II) are shown in Fig. 4(a, b) respectively. As seen in Fig. 4a the spectrum of the no leached MIIP revealed that significant amount of the analyte was adsorbed in the sorbent from the initial solution. Also Fig. 4b related to the leached MIIP confirmed the elimination of lead in the sorbent after rinsing with the selected eluent.

Thermal studies by TGA
The amount of materials that modified the surface of nanoparticles can be valued by TGA. The TGA behavior of Fe3O4@SiO2 and MIIP are shown in Fig. 5. The thermogram in Fig. 5a shows that the Fe3O4 has good thermal stability up to 500 °C with only a little loss of about 3.9%. As shown in Fig. 5b, when the temperature was changed from 280 to 350 °C, a 72.3% reduction in (a)

Magnetism analysis
The magnetic properties of the, Fe3O4@SiO2, Pb(II)-MIIP and Fe3O4 were characterized using vibrating sample magnetometry (VSM) technique in the magnetic field of -10,000 to +10,000 Oersted at 25 °C. As shown in Fig. 6, the hysteresis curve of these samples is fully reversible that means nanomaterials have superparamagnetic properties and could be easily and quickly separated from a suspension. The saturation magnetization value of Fe3O4, Fe3O4@SiO2 and MIIP was about 59, 23 and 9 emu g -1 , respectively. The value of saturation magnetization was decreased by increasing modifying steps. The decrease in magnetization value can be attributed to the existence of IIP on the surface of the Fe3O4@SiO2 nanoparticles. Also, this relationship can be seen in the comparison between Fe3O4@SiO2 and Fe3O4. However, the MIIP with less magnetite encapsulation possesses enough magnetic response to meet the need of magnetic separation.
Also, from inset of Fig. 6, it is seen that in the absence of an external magnetic field, a yellow homogeneous dispersion was existed. When an external magnetic field was applied, the yellow particles were attracted to the wall of vial in a short time (about 50 s).

Optimization of extraction conditions
Adsorption of Pb(II) from solution was performed in a batch method. Effects of media pH, eluent solvent (type, volume and concentration), adsorption-desorption time, and equilibrium temperature were studied. Adsorbent addition was set to 0.2 g in 100 mL solution and initial Pb(II) concentration was 1 mg L -1 . After adsorption equilibrium, the adsorbent was separated by external magnetic field and the analyte quantities in the eluents were determined by FAAS.

Effect of media pH
The solution pH is one important parameter that can effect on metal ion adsorption. In order to figure out of pH effect on Pb(II) adsorption, variety of pH were studied in the range 3.0 to 8.0. For prevention from sedimentation, increasing pH not continued and the results are illustrated in Fig. 7. The results show that with the increase of pH to 6.0, percent recovery continually increased and then decreased. Probably the pH effects on functional groups of IIP or the chemistry of Pb 2+ ions. At lower pH than 6.0 and presence of higher concentration of H + , the active sites of the MIIP become protonated and their abilities for interaction with Pb(II) decreased. By increasing pH more than 6.0, Pb 2+ ions can react by hydroxyl groups and the adsorption process would be decreased.

Selection of eluent solvent
To achieve the eluent for complete Pb(II) removing from MIIP, a series of commercial solutions of HNO3 2 mol L -1 , HCl 2 mol L -1 , acetic acid (HOAC) 2 mol L -1 , HOAC-HCl (2:1 mol L -1 ), HNO3-HOAC (2:1 mol L -1 ) were studied. As shown in Fig. 8a, HCl is the best eluent that can extract Pb(II) ions from MIIP. In the next step, diverse concentrations of HCl were investigated and as it can be seen in Fig. 8b, the best solution is HCl 2 mol L -1 . In the last section of eluent optimization, the volume of it optimized and as shown in Fig. 8c, 4 mL of HCl 2 mol L -1 was chosen as optimum condition of eluent for the experiments.

Effects of adsorption-desorption times
The adsorption and desorption times for Pb(II) were investigated under optimized conditions. As shown in Fig. 9a, it appears that MIIP showed good performance in adsorption during first 5 min and reached to equilibrium after 7 min and excess time does not provide any significant change in recovery. This time, probably due to plenty cavities that imprinted on the surface of thin MIIP that provide low diffusion resistance, is reliable and it can be stated that the process has fast equilibrium.
For desorption time optimization, different times (1-21 min) were studied and as it can be seen from Fig. 9b

Effect of equilibrium temperature
In IIP techniques temperature has a noticeable effect on the adsorption behavior and analyte recovery. The effect of equilibrium temperature on the extraction behavior of Pb(II) on MIIP was shown in Fig. 10. As shown by increasing temperature from 25 to 40 °C, the recovery yield is increased thermodynamically. It can be seen that Pb(II) extraction in the range of 40 to 50 °C drastically decreased. This manner may be related to swelling of polymer. Probably in elevated temperature the size and geometry of cavities will be changed and could not be suitable for adsorption of Pb 2+ as target ions.

Maximum adsorption capacity
For determination of maximum adsorption capacities of the ion imprinted polymer and nonimprinted polymers, 50 mg of each sorbents separately placed in a solution of 200 mgL -1 Pb(II) in pH 6.0 and the quantity of adsorbed ions were determined in both initial and eluent solutions. From three replicates, sorption capacities of MIIP and NIIP were calculated of 92.3 and 21.3 mg g -1 respectively. The results showed that MIIP compare to NIIP have a higher adsorption capacity for lead ions. Table  1 summarizes the analytical characteristics of the MIIP under the optimal conditions. The LOD of the proposed method was determined by 7 times placing of 0.2 g MIIP in 100 mL blank solutions and it was obtained to 0.3 µg L -1 . The precision (RSD, %) for 7 replicate uptaking of 10 µgL -1 Pb(II) was 2.4%. Also, the linear dynamic range of the calibration curve was obtained in 2-150 µg L -1 range.  merits of the proposed method (e.g. adsorption time, adsorption capacity, detection limit and precision) are comparable and in general it is better than with those published reports. The main advantages of this method are more adsorption capacity and lower LOD.

Effect of interfering ions
In order to prove specific recognition of magnetic ion imprinted polymer towards Pb(II) in the presence of common interfering ions, the selectivity of the prepared MIIP was investigated in binary systems 36 . The competitors examined were Na + , K + , Co 2+ , Ni 2+ , Ca 2+ and Mg 2+ . In this approach, each interfering ion was added to 100 mL of 10 µg L −1 Pb(II) solution under optimum conditions. After preconcentration by MIIP, the tolerable amount of each ion was determined separately. The tolerable amount is defined as the maximum concentration that could cause a change of less than 5% in signal compared to the initial signal of Pb 2+ uptaking without any interference. As shown in Table 3, this Pb-MIIP have high selectivity in the existence of Co 2+ , Ni 2+ , Ca 2+ and Mg 2+ that probably was obtained in order to the use of BPHA as a new ligand.

Method application
The proposed procedure was applied for determination of lead content in two environmental samples and a waste water (from an ammonium perchlorate factory) solution. The amounts of Pb(II) ions were determined in the final eluents and the results are listed in Table 4. As seen, the mean recoveries for different spiking of Pb 2+ are in the range 96.6-99.7%. Therefore, the proposed MIIP can be applied for the determination of lead in various samples.

Conclusions
In this research a new magnetic ion imprinted polymer (IIP@Fe3O4@SiO2-APS-MA) for Pb 2+ target was prepared by combination of surface imprinting technique and magnetic separation. The polymer synthesis and its characterization were proved by FT-IR, SEM-ImageJ, EDX, TGA and VSM techniques. The adsorption experiments combined to FAAS system showed that this sorbent have good analytical characteristics and high selectivity with maximum adsorption capacity of 92.3 mg(Pb) g -1 (MIIP) at 40 °C and pH 6.0. Also, this adsorbent has physical and chemical stability, low cost and simple-fast operation. A comparison of the presented work with the previously reports for lead extraction ( Table 2) indicates that the proposed Pb-MIIP due to the use of n-benzoyl-nphenyl hydroxylamine, provides better analytical merits of adsorption capacity and LOD. From the results obtained, it can be concluded that this polymer can be used as an excellent substrate for the adsorption of lead ion in real aqueous solutions.