Abstract
Ion-imprinted polymer (IIP) technology has received considerable attention for its greatest potential application. In this work, a novel magnetic nano ion-imprinted polymer (MIIP) for the selective and sensitive pre-concentration of silver (I) ions were used. It was obtained using Fe3O4@SiO2@TiO2 nanoparticles as a magnetic support of adsorbent, Ag(I)-2,4-diamino-6-phenyl-1,3,5-triazine (DPT) complex as the template molecule and methacrylic acid (MAA), 2,2′-azobisisobutyronitrile (AIBN), ethylene glycol dimethacrylate (EGDMA), as the functional monomer, the radical initiator and crosslinker, respectively. The synthesized polymer nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) and Brunauer-Emmett-Teller (BET). Silver ions were separate from the polymer and measured by flame atomic absorption spectrometry (FAAS). The maximum adsorption capacity of the novel imprinted adsorbent for Ag(I) was calculated to be 62.5 mg g−1. The developed method was applied to the preconcentration of the analyte in the water, radiology film and food samples, and satisfactory results were obtained.
1 Introduction
Silver ion is an industrially important element. The widespread use of silver composites and silver containing procedures in medicine, jewelry, cloud seeding, industry and in the disinfection of drinking water has marked the larger silver content of environmental samples. It is used for the preparation and production of some corrosion resistant alloys, and silver compounds are used in the processing of drugs, foods, beverages, filters and other equipment used in water purification (1).
The separation and accurate determination of silver at low concentrations and in the presence of high concentration cations requires selective techniques. Due to this, trace determinations and analysis of silver ion in different water samples or other industrial and biological samples have become of increasing interest (2), (3). Flame atomic absorption spectrometry (FAAS) has been used due to its fast analysis time, simplicity, acceptable selectivity and inexpensive cost. So that determination of many trace elements in different samples has been carried out using it. Notwithstanding the improvement in analytical instrumentation, they cannot demonstrate enough sensitivity or selectivity for the monitoring of natural samples (4). On the other hand, the concentrations of natural samples, especially natural waters, are frequently lower than their limit of detection by these techniques; therefore a sometimes troublesome pre-concentration procedure needs to be carried out (5). Preconcentration techniques combined with FAAS are still necessary. Researchers have reported many techniques for the separation of metal ions from different matrixes and their preconcentration in the same or another phase such as coprecipitation (6), (7), liquid-liquid extraction (8), (9), dispersive liquid-liquid microextraction (10), (11), (12), (13), cloud point extraction (14), (15), (16) and solid phase extraction (SPE) (17), (18), (19), (20).
SPE is a successful technique for the preconcentration and separation of trace metals (20). The most important properties of material as an adsorbent in the SPE technique are its stability and capacity, accessibility for target molecule or atom, its solubility and regenerability. Therefore, many materials, such as ion exchange resins (21), functionalized alumina (22), functionalized activated carbon (23), metal oxides (24), functionalized graphen oxide (25) and ion imprinted polymer nanoparticles (26), (27), (28), (29), (30) have been widely employed as SPE adsorbents. Among these different adsorbents, ion imprinting technology such as highly selective ion-imprinted polymers (IIP) represents a novel class of sorbents possessing affinity and selectivity for separation, preconcentration or removal of target ions (30). Recently ion imprinted polymers with magnetic properties have attracted interest in the scientific community because of their special properties (31). The main characterization of MIIP (Fe3O4-SiO2-TiO2-IIP) as an adsorbent consists of high selective cavities, high adsorption capacities, high sensitivities, simple methodology and lower detection limits (31), (32). The main objective of this study is to develop a magnetic nano ion-imprinted polymer (MIIP) as a sorbent with the aforementioned properties for preconcentration by the SPE method and determination of silver ion in trace amounts from water and some other samples. The MIIP (Fe3O4@SiO2@TiO2-IIP) is a combined strategy involving using a Fe3O4-SiO2 core-shell as a nanomagnetic support for easy separation, TiO2 as an excellent substrate shell for polymer deposition an IIP shell as a selective material for the extraction of silver ion from complex systems. The principle of IIP is based on the complexation of Ag(I) ion with 2,4-diamino-6-phenyl-1,3,5-triazine (DPT) (as a template) and their polymerization with an appropriate monomer, methacrylic acid (MAA), a radical initiator, 2,2′-azobisisobutyronitrile (AIBN) and a cross-linking, ethylene glycol dimethacrylate (EGDMA) (30). The characterization of the synthesized material is confirmed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) and the Brunauer-Emmett-Teller (BET) theory. To the best of our knowledge, this methodology has not been employed previously for the separation and trace determination of silver in water, radiology film and food samples.
2 Experimental
2.1 Materials
EGDMA, AIBN, MAA, ethanol, AgNO3 and nitrate or chloride salts of other cations were purchased from Merck (Darmstadt, Germany). DPT, as a ligand, was purchased from Merck. A stock standard solution of Ag+ (1000 μg ml−1) was prepared by dissolving an appropriate amount of AgNO3 in deionized water containing a 1 ml of concentrated nitric acid (HNO3). FeCl2·4H2O and FeCl3·6H2O, tetraethyl orthosilicate and titanium oxide were purchased from Merck. The working reference solutions were obtained daily by stepwise dilution from stock solution with deionized water. Sodium hydroxide (NaOH) and HNO3 with the highest purity available were purchased from Merck.
2.2 Apparatus
A 350 atomic absorption spectrometer (Analytik Jena AG, Jena, Germany) equipped with a silver hollow cathode lamp was used for absorbance measurements at a wavelength of 328.1 nm. An air-acetylene flame was used for the determination of silver ions and their flow rates were 10 l min−1 and 0.8 l min−1, respectively. In order to do pH measurements, a 780 digital pH Meter (Metrohm), equipped with a combined Ag/AgCl glass electrode was used for the pH adjustments at room temperature. An ultrasonic bath with a temperature control model LBS2 was used (FALC instruments S.V.I, Treviglio, Italy). FT-IR analysis of the synthesized material was conducted using an infrared spectrometer (Bruker FTIR Vertex 70) over the wavenumber range 400–4000 cm−1 in KBr. High angle XRD patterns were collected at ambient temperature using Cu-Kα radiation on a Philips-PW 17 C diffractometer. SEM-EDS was done on an ESEM-EDX Quanta 400-ESEM with EDAX-FEI (The Netherlands). TEM analyses were conducted using a JEM-2010 (HT) microscope at an accelerating voltage of 200 kV. The magnetic property was analyzed using a VSM (Meghnatis Daghigh Kavir Co., Kashan, Iran) with a maximum field of 18 kOe at room temperature. The surface area of Ag(I)-IIP was investigated and N2 adsorption isotherms were measured on the MIIP using a Micromeritics ASAP 2020 gas adsorption apparatus (USA).
2.3 Synthesis of Fe3O4 nanoparticles
The Fe3O4 nanoparticles were synthesized by coprecipitation of a stoichiometric mixture of FeCl2, 4H2O and FeCl3, 6H2O (molar ratio 1:2) in an ammonium hydroxide solution (33) under nitrogen atmosphere with dynamic stirring. The Fe3O4 nanoparticles were collected using a magnet and thoroughly washed with double distilated water to eliminate excess amounts of ammonium hydroxide. Then, the magnetic nanoparticles were dried in an oven for 5 h at 70°C.
2.4 Preparation of Fe3O4@SiO2 nanoparticles
The core-shell Fe3O4@SiO2 microspheres were prepared according to a previously reported method (34). The 0.50 g of Fe3O4 nanoparticles were dissolved in 50 ml 0.5 mol·l−1 HCl aqueous solution by ultrasonication for 10 min. The magnetite particles were separated, washed with deionized water, and homogeneously dispersed in a mixture of deionized water, ethanol (1:4 v/v) and concentrated ammonia aqueous solution (1 ml, 28 wt%), followed by the addition of tetraethyl orthosilicate (TEOS; 0.12 g). Then, stirred at room temperature for 6 h, the Fe3O4-SiO2 microspheres were separated, washed with deionized water, the Fe3O4-SiO2 nanoparticles were dried in an oven for 5 h at 60°C.
2.5 Preparation of Fe3O4@SiO2@TiO2 nanoparticles
The 0.5 g Fe3O4-SiO2 nanoparticles were added to 15 ml titanium oxide and sonication for 10 min. A mixture of water and ethanol (1:5 v/v) was added gently with a dropper into this mixture. Then, the mixture was stirred further for 1 h. Finally, after separating and washing the residue with ethanol, the obtained powder was dried in the oven at 200°C for 5 h (31) .
2.6 Synthesis of magnetic Ag(I)-ion imprinted polymer
Fe3O4@SiO2@TiO2-IIP was prepared with a sol-gel method (31). One millimole Ag(NO3) (0.17 g) and 2 mmol (0.3744 g) DPT were dissolved in 50 ml ethanol as a porogenic solvent. Then, the mixture was stirred using a magnetic stirrer bar for 2 h under reflux conditions, the ternary complex was formed. Then Fe3O4-SiO2-TiO2 (0.5 g), 4.7 mmol (0.4 ml) MAA, 30 mmol of EGDMA and 0.4 mmol of AIBN as a free radical initiator were added to the mixture. Then, the mixture was transferred into the oil bath at 60°C and stirred using a magnetic stirrer bar under a nitrogen atmosphere for 24 h. Finally, the product was separated with the help of an external magnetic force, and washed with water and ethanol. In order to remove the imprinted Ag(I) it was leached with 3×50 ml of 50% (v/v) HNO3 by continuous stirring for 6 h. The obtained Fe3O4@SiO2@TiO2-IIP was washed with water, and then dried in a vacuum oven at 60°C for 24 h. The magnetic non-imprinted polymer (Fe3O4@SiO2@TiO2-NIP) was prepared following the same procedure without adding Ag(NO3). Figure 1 shows the schematic of synthesis of IIP (which is carried out on the surface of the magnetic nanoparticles).
The schematic of synthesis of IIP (on the surface of magnetic nanoparticles).
2.7 General procedure
The sorption and desorption studies of the Ag(I) ions with the prepared MIIP were carried out by batch experiments. Firstly, the pH of the analyte solutions was adjusted to 6 by adding NaOH (0.1 mol·l−1) and HNO3 (0.1 mol·l−1). Then, 40 mg of MIIP or magnetic NIP particles were added to 25 ml of these solution that contained 1 μg ml−1 of Ag(I), and stirred for 20 min at room temperature. Then, the beakers were placed on the magnet and the nanoparticles were collected. After decanting the supernatant solution, the collected MIIP were washed with a mixture of 5 ml of 2 mol·l−1 HNO3 solution in order to elute the adsorbed silver ions. Then, silver ion in the eluent was determined by FAAS.
The extraction percent of Ag(I) ions can be obtained by using the following equation 1:
Ci (mg l−1) and Cf (mg l−1) are the concentrations of silver ion in solution before and after extraction, respectively.
Adsorption capacity Q (mg g−1) was calculated as follows (equation 2):
V volume of solution (l) M indicated the mass of the absorbent material (mg); Instead of using “Ci–Cf” the concentration of silver ions can be used in the desorption solution that is determined by FAAS. The distribution ratio (ml g−1) of Ag(I) was defined based on equation 3:
Equations 4 and 5 were used to calculate the selectivity coefficient and relative selectivity coefficient (K′) of Ag(I) ions with respect to other ions that are possibly present in the solution.
2.8 Samples preparation
Tap, well and waste water samples were collected in acid leached polyethylene bottles. Tap and well water samples were collected from our university (Islamic Azad University, Ilam Branch, Ilam, Iran). Wastewater samples were from (Ilam, Iran) and the urban waste water was from Chalesara (Ilam, Iran). The only pretreatment was acidification to pH 2 with HNO3, which was performed immediately after collection, in order to prevent adsorption of the metal ions on the flask walls. The samples were filtered before analyses through a cellulose membrane of 0.45 μm pore size (Millipore).
Radiology film samples were dissolved according to the following method (35). Briefly 1 g of radiology film was placed in a beaker and 10 ml concentrated HNO3 was added with heating on a water bath for 20 min. The resulting solution was then transferred into a 100 ml volumetric flask and diluted to mark with deionized water. Finally, the solution was taken for analysis according to our methods.
One gram of each sample (rice, potato and tomato) was placed in a beaker and 10 ml concentrated HNO3 was added with heating on a water bath. The solutions were cooled, and dissolved in 2 ml of 30% H2O2. pH was adjusted to 6 by NaOH and HNO3 and the solution was diluted to 100 ml with deionized water in calibrated flasks. Then, 25 ml of the sample solution was taken individually and silver ion was determined by the extraction procedure.
3 Results and discussion
3.1 Characterization studies
FT-IR spectra of unleached, leached MIIP and MNIP are shown in Figure 2. To get further insight into the mechanism of the metal atom interaction with the ligand surface, FTIR spectroscopy was carried out on the synthesized nano-particles. The typical band in 1732.19 cm−1 is related to the banding vibration of the N-H bond in leached polymer. The shifting of bending vibration of the N-H bond to a lower frequency region in unleach confirming the N-H bond was coordinated to the metal atom. Similar spectra for all polymers indicate the polymeric network is stable over leaching process.
FT-IR spectra of unleached, leached and magnetic NIP Ag(I)-IIP.
The crystallinity of (a) pure Fe3O4, (b) Fe3O4@SiO2 and (c) Fe3O4@SiO2@TiO2 structure and their phase purity were determined by powder XRD, as shown in Figure 3A–C. The XRD study shows that the as-synthesized Fe3O4 nanoparticles have a face-centered cubic (FCC) structure. The FCC structure has atoms located at each of the corners and the centers of all the cubic faces. Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells (Figure 3A). As shown in Figure 3, six diffraction peaks in the 2θ range of 10°–80°. (2θ=29.5°, 34.95°,43.00°, 53.64°, 57.23° and 62.66°) were observed for Fe3O4 (a), Fe3O4@SiO2 (b) and Fe3O4@SiO2@TiO2-IIP (c). The peak positions could readily be indexed to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0), respectively [JCPDS card (19-0629)] which matched well with the results of Deng et al. (36). Silica-coated magnetite showed the same diffraction peaks (Figure 3B) of the uncoated one which confirms that the crystalline nature of the Fe3O4 is retained after the formation of the SiO2 shell. The XRD pattern of the Fe3O4@SiO2 sample shows almost the same feature as pure Fe3O4, except that a broad peak centered at 220 of 2θ corresponding to SiO2 was observed, indicating that the prepared SiO2 is amorphous (37). In addition, Fe3O4@SiO2@TiO2 is composed of anatase TiO2 (JCPDS card No. 21–1272) along with the magnetic phase (Figure 3C). The diffraction peaks of Fe3O4 were reduced in intensity after coating TiO2 layer which can be attributed to the shielding effect of silica and titania layers. The XRD patterns of Fe3O4-SiO2-TiO2-IIP were similar to that of Fe3O4@SiO2@TiO2; thus, the coating procedure did not cause a structural change in it. The crystallite size was calculated from the Scherrer equation [6] (38):
XRD patterns of (A) Fe3O4, (B) Fe3O4-SiO2, (C) Fe3O4-SiO2-TiO2, and (D) Fe3O4-SiO2-TiO2-IIP.
where ε is crystallite size and β1/2 is the peak width (integral or full width at half maximum) in radians. The crystallite size of Fe3O4@SiO2@TiO2-IIP powder prepared in this study was 73 nm, and the error was 3.12%.
The surface morphologies of the synthesized Ag-IIP and Ag-NIP (bulk polymers) were studied using SEM (Figure 4A and B). The SEM pattern of Fe3O4@SiO2@TiO2-IIP and Fe3O4@SiO2@TiO2-NIP illustrate particles with the size of about 45–75 nm. The cross-sectional surface of the nanobeads is shown in Figure 4C and D, and as can be seen, the polymer structure immobilized on the nanoparticles has a much higher porosity than the bulk structure. The porous structure shows an important role in the improvement of the adsorption capacity in association with increased surface.
SEM images of IIP unleached (A), NIP (B), unleached Fe3O4@SiO2@TiO2-IIP (C), Fe3O4-SiO2-TiO2-NIP (D).
The EDX study of magnetic-Ag-IIP confirms the presence of silver, titanium, silica and iron nanoparticles in the structure of the material Figure 5.
EDS images of Fe3O4-SiO2-TiO2-IIP.
The magnetic hysteresis curve of the products were investigated by a VSM. Figure 6 shows the magnetization curves measured at 300 K. The magnetic saturation (Ms) values are 76.3, 64.3, 22.4 emu g−1 for Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2@TiO2-IIP, respectively. These results indicated that the magnetic property of Fe3O4 is reduced by coating the SiO2 and TiO2-non-magnetic imprinted polymer but still are separable under application external magnetic field. Significantly, the low value of remanent magnetization (Mr) and coercivity (Hc) indicate; that the prepared samples exhibit superparamagnetic behaviors at room temperature.
VSM curve of Fe3O4@SiO2@TiO2-IIP.
The TEM images of Figure 7 show the Fe3O4@SiO2@TiO2-IIP composites after direct deposition of IIP nanoparticles on the surface of the TiO2 shell. As indicated, the IIP nanoparticles were primarily distributed on the edge and junction sites of the interlaced TiO2 nanoparticles, with a mean diameter of 50 nm.
TEM images of Fe3O4 (A), Fe3O4-SiO2 (B), Fe3O4-SiO2-TiO2 (C).
All the samples were degassed at 180°C prior to nitrogen adsorption measurements. The BET surface area was determined by a multipoint BET method (39) using the adsorption data in the relative pressure (P/P0) range of 0.05–0.3. The adsorption branch of nitrogen adsorption-desorption isotherms was used to determine the pore size distribution by the Barrett-Joyner-Halenda (BJH) method (40), assuming a cylindrical pore model.
Pore size distribution was determined according to the International Union of Pure and Applied Chemistry (IUPAC standard using BET) (41). Pore size are classified in the following order; micropores [diameter (d)<20 Å), mesopores (20 Å<d<500 Å) and macropores (d>500 Å). Micropores can be divided into ultra-micropores (d<7 Å) and super-micropores (7 Å<d<20 Å) (42). The textural properties and porosity characteristics of the Fe3O4@SiO2@TiO2-IIP and Fe3O4@SiO2@TiO2-NIP composites were evaluated by N2 physisorption analysis as shown in Figure 8. As shown in Figure 8, the nitrogen adsorption-desorption isotherms were classified as type IV according to the IUPAC standards (43), with H1-hysteresis loops which indicates the presence of textual mesopore. The BET surface area, centered pore size, and total pore volume are calculated from the obtained isotherms, and are summarized in Table 1. Low pressure hysteresis is observed extending to the low pressures (P/P0<0.4), which can often be observed in systems containing micropores. The hysteresis loops appear in low pressure range may be associated with the irreversible uptake of N2 molecules in pore (or through pore entrances), because the size of micropore is about the same width as that of the adsorbate molecules. The hysteresis loop in the relative pressure range between 0.4 and 0.9 is probably related to finer intra-aggregated pores formed between intra-agglomerated primary particles. The high pressure part of the hysteresis loop (0.9<P/P0<1) is probably associated with larger inter-aggregated pores produced by inter-aggregated secondary particles. The pore size distribution is further confirmed by the corresponding pore size distribution. Pore size distribution curve is calculated by using the adsorption branch of the isotherm. The obtained surface area for the IIP was only slightly higher than the NIP one, most likely due to the higher pore volume.
(A) Nitrogen adsorption-desorption isotherms for Fe3O4@SiO2@TiO2-IIP and Fe3O4@SiO2@TiO2-NIP, (B) the corresponding pore size distributions of Fe3O4@SiO2@TiO2-IIP and Fe3O4@SiO2@TiO2-NIP.
Pore structure parameters of magnetic NIP and IIP.
| Sample | Surface area (m2 g−1) | Total pore volume (cm3 g−1) | Average pore diameter (nm) |
|---|---|---|---|
| Fe3O4-SiO2-TiO2-NIP | 356 | 0.467 | 7.91 |
| Fe3O4-SiO2-TiO2-IIP | 379 | 0.492 | 7.12 |
3.2 Optimization studies
3.2.1 pH response
As the pH of sample solutions is an important analytical factor in SPE, the adsorption of Ag(I) by sorbent was investigated at the pH range of 3–10. The pH of sample solutions was adjusted by the drop wise addition of (0.1 mol·l−1) of NaOH or (0.1 mol·l−1) HNO3 solutions and measured by a pH meter. The results are given in Figure 9 and showed that with the increase of pH, the adsorption efficiency of the analyte quantitative recovery at pH 4–6 were obtained. When the pH of the solution <4, the complex between Ag(I) ions and ligand starts to break down and the pH of the solution more than 6, the hydroxide of silver ions may be formation. Therefore, pH 6 was chosen for further experiments.
Effect of the pH on the extraction of Ag(I) ions.
3.2.2 Effect of MIIP weight
The weight of sorbent is another vital parameter that affects the recovery (%). A quantitative retention is not obtained when the amount of sorbent is less. For this purpose, different weights of the MIIP (10–70 mg) were studied. The results in Figure 10, showed that quantitative recoveries of the metal ions were obtained when the sorbent quantity was greater than 30 mg. With 40 mg of the adsorbent, the highest recovery was obtained. Therefore, 40 mg was chosen for further experiments.
Effect of IIP weight on the extraction percentage of Ag(I) ions.
3.2.3 Selection of the best eluent
In order to choose the most effective eluent, various acidic solutions were studied as an eluent for desorption of the silver ions from the surface of the magnetic polymeric network. The results show that quantitative recoveries for silver ions were obtained with a HNO3 and therefore it was selected as the eluent for further applications. After the findings above, the experiments were carried out for studying the concentration of HNO3 solutions. For this purpose various concentrations (0.5–5 mol·l−1) were studied for desorption of the silver ions. As can be seen, the recovery percent of the silver ions increased with the increase in concentration of HNO3 solutions from 0.5 to 2 mol·l−1 and then remained constant. Therefore, 2 mol·l−1 HNO3 solution were selected as the eluent for the desorption of the analyte ions. The influence of the volume of 2 mol·l−1 HNO3 between 0.5 and 6 ml was also examined. The results show that quantitative recovery for silver ion was obtained with of 5 ml of 2 mol·l−1 HNO3 and therefore it was selected as the eluent for further applications.
3.2.4 Effect of adsorption and desorption times
Forty milligrams of sorbent were added in 25 ml solution containing 5 μg of silver ions, then stirred at different times from 5 to 40 min. The results obtained are shown and show that the optimum time found is 10 min. To understand the effect of desorption time we use the same method but we investigated the desorption time in the range of 5–40 min. These results show the data indicated that 15 min is optimum time.
3.2.5 Breakthrough volume
The breakthrough volume is another factor that influences the preconcentration factor and reliability of analytical results of an SPE technique. It is very important to get acceptable recoveries for the analytes from a large volume of the sample solutions. Breakthrough volume depends on the nature of the sorbent material and the type and concentration of sample constituents. The effect of sample volume on the retention of silver from the sample solution was investigated. For this purpose, 25–400 ml of the sample solutions containing 25 μg of silver ions was subjected to the extraction procedure. The results in Figure 11 show that quantitative extraction of Ag(I) ions (>95%) were obtained up to 250 ml of the sample solution. When the sample volume is increased, so analyte (Ag+) concentration comes down. Also, when the distance between the analyte ions is increased, the amount of sorbent is fixed so effective collision is reduced between the analyte and sorbent. As a result, the percent recovery is reduced above 250 ml of the sample volume. The preconcentration factor is defined by the ratio of the highest sample volume for analyte (250 ml) and the lowest final eluent volume (5 ml). Therefore, a preconcentration factor of 50 for silver ions was obtained.
Effect of sample volume on the pre-concentration of silver ions.
3.2.6 Adsorption capacity
Sorption capacity determines the amount of the ion imprinted polymers required for quantitative determination of analytes in a given solution. In order to consider the adsorption capacity factor, 40 mg of MIIP particles was added to a solution containing 10–100 μg ml−1 of Ag(I) under optimum conditions. The adsorption capacity of MIIP and MNIP was calculated to be 62.5 and 12.5 mg g−1, respectively, by using equation [2] noted in the extraction method. As expected, the absorption capacity of the MIIP particles is much higher than the MNIP and confirmed that the imprinting procedure plays an important role in the adsorbent behavior. In MNIP particles, the active site on matrix of polymer are dispersed very randomly (35).
3.2.7 Regeneration and reusability
From an economic point of view, for the prepared polymer reusability, the times that a sorbent used in adsorption-desorption processes without significant changing in the absorption capacity, is an important feature. To evaluate this factor, 40 mg of Ag(I)-MIIP was applied in the adsorption-desorption cycle for 8 times under optimum condition. The resultant data are shown in Figure 12. The prepared MIIP could be reused in six adsorption-desorption cycles and no significant change was observed in the absorption capacity. Calculations show that the prepared MIIP after 6 times using, the absorption capacity will be reduced about 6%.
Reusability of the Fe3O4@SiO2@TiO2-IIP.
3.2.8 Effect of foreign ions
In view of the high selectivity provided by FAAS, the only interference may be attributed to the separation step. To perform this study, the selectivity of the synthesized Ag-MIIP, during several batch experiments, pairs of silver and coexisting cations were extracted under optimum conditions. Competitive adsorption of silver ion over the selected inorganic ions for MIIP and MNIP particles from their cations mixture were investigated and then the distribution ratios (kd), selectivity coefficients (K) and relative selectivity coefficients (K′) for Ag+ ion relative to potentially interfering ions were calculated using equations [2–4], respectively. The results are summarized in Table 2. The results were shown that the selectivity coefficients of MNIP for Ag(I) with respect to potentially interfering ions are very low; the selectivity coefficients of Ag(I)-MIIP for Ag(I) with respect to potentially interfering ions are very high; the relative selectivity coefficients indicate adsorption affinity and selectivity of imprinting material for the template with respect to non-imprinting material. Compared to other cations, the Ag(I) fits better into the imprinted cavities and has higher affinity with the Ag-MIIP.
Distribution ratio (kd), selectivity coefficient (K) and relative selectively coefficient (K′) values of magnetic IIP and magnetic NIP material for different cations.
| Cation | kd (IIP) (ml g−1) | kd (NIP) (ml g−1) | K (IIP) | K (NIP) | K′ |
|---|---|---|---|---|---|
| Ag+ | 61,875.0 | 2437.5 | – | – | – |
| K+ | 1329.9 | 366.7 | 46.5 | 6.65 | 7.0 |
| Na+ | 1019.7 | 158.2 | 60.7 | 15.4 | 3.9 |
| Cd2+ | 827.2 | 91.2 | 74.8 | 26.7 | 2.8 |
| Pb2+ | 933.7 | 86.3 | 66.3 | 28.2 | 2.3 |
| Ni2+ | 792.2 | 72.0 | 78.1 | 33.84 | 2.3 |
3.2.9 Calibration, precision and detection limits
The calibration graphs Ag(I)-IIP particles were linear in the range of 0.001–0.5 μg ml−1. The line equations for Ag(I) was A=1.3484 C+0.0006 (R=0.9995). In these equations, A is the absorbance value, C is the concentration of analyte ions (μg ml−1) and R is the correlation coefficient. The detection limit (according to IUPAC) is the smallest concentration or absolute amount of analyte that has a signal significantly larger than the signal arising from a reagent blank. The limits of detection based on 3Sb for Ag(I) ions were 0.5 and ng ml−1. Eight replicate determinations a mixture of 0.2 μg ml−1 of Ag(I) gave a mean absorbance of 0.126 with relative standard deviations of 2.4%.
3.2.10 Accuracy of the method
A 25 ml of the pretreated sample solution was taken individually and silver ion was determined by the proposed technique. The recovery of silver ions from food samples spiked with silver ions was also calculated. The related results are presented in Table 3. According to this table, the added silver ions can be quantitatively recovered from the food samples by the analytical procedure. These results demonstrate the applicability of the analytical procedure for silver ions determination in food samples.
Determination of silver ion in food samples.
| Sample | Silver amount (μg g–1) | Recovery% | |
|---|---|---|---|
| Added | Founda | ||
| Rice | 0 | NDb | – |
| 20 | 19.8±0.1 | 99 | |
| Potato | 0 | ND | – |
| 20 | 20.1±0.3 | 100.5 | |
| Tomato | 0 | ND | – |
| 20 | 19.5±0.2 | 97.5 | |
| Radiology film | 0 | 1.5±0.3 | – |
| 20 | 21.5±0.1 | 100 | |
aMean±standard deviation (n=3).
bNot detect.
The extraction procedure has been applied to the determination of silver ions in different water samples and radiology film . The results were given in Table 4. The recovery of silver ions from water samples and radiology film spiked with silver ions was also studied. According this table, the added silver ions can be quantitatively recovered from the water samples and radiology film by the proposed method. These results demonstrate the applicability of the proposed technique for silver ion determination in water samples and radiology film.
Determination of silver ion in real samples.
| Sample | Silver amount (ng ml–1) | Recovery% | |
|---|---|---|---|
| Added | Founda | ||
| Tap water | 0 | 8.1±0.3 | – |
| 10 | 18.1±0.1 | 100 | |
| Well water | 0 | 9.2±0.2 | – |
| 10 | 19.1±0.4 | 99.5 | |
| Waste water | 0 | 6.5±0.1 | – |
| 10 | 16.5±0.33 | 100 | |
aMean±standard deviation (n=3).
3.2.11 Comparison with other methods
A comparison of the extraction procedure with the other reported preconcentration procedures (3), (44), (45) for silver ions were given in Table 5. Based on the results shown in Table 4, the linear range of the extraction procedure was wider than all of the other reported methods. The RSD% (relative standard deviation) of the extraction procedure was lower than some of the other reported methods. The proposed method provides lower or similar limit of detection (LOD) in comparison to prior methods.
Comparison of the proposed procedure with other reported procedures for preconcentration of Ag(I).
| Method | Linear range (ng ml−1) | BV (ml)a | PFb | LOD (ng ml−1)c | RSD (%) | Ref. |
|---|---|---|---|---|---|---|
| SPE-FAAS | 10–1000 | 50 | 10 | 3.9 | 4.4 | (44) |
| SPE-FAAS | 2–100 | 500 | 250 | 0.56 | 3.1 | (45) |
| CPE-FAAS | 3–200 | 10 | 43 | 0.56 | 2.14 | (15) |
| SPE-FAAS | 1–500 | 250 | 50 | 0.5 | 2.4 | This work |
aBreakthrough volume.
bPreconcentration factor.
cLimit of detection.
4 Conclusion
Novel nanostructured MIIP were successfully synthesized by sol-gel polymerization method for the selective SPE of Ag(I) ions. The synthesized adsorbent was characterized by FT-IR spectra, SEM images, TEM, VSM, BET, EDS and XRD analysis. Fast of absorption, high absorption capacity, high selectivity, stability and easy preparation makes a lot of attention to MIIP. The prepared nanostructured MIIP has an increased selectivity toward silver ion over a range of potentially interfering ions with the same charge and similar ionic radius. In this study, the adsorption capacity of Ag(I) ions on the prepared MIIP under optimum conditions was obtained 62.5 mg g−1. Ag(I)-MIIP particles was shown an extraordinary selectivity to silver ions than to other cations. Quantitative enrichment factor of silver from dilute water solutions was achieved with Ag(I)-MIIP particles. Finally, the developed method was excellent candidate in the extraction of trace amounts of Ag(I) in various samples with high selectivity.
Acknowledgements
The authors are grateful to the Islamic Azad Ilam University Research Council for financing the project.
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