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2.1. Materials
The stock standard solution of 1000 mg L–1 lead for atomic absorption spectroscopy, ferrous chloride, ferric chloride, Cu(OAc)2·H2O (98%), ethanol (C2H5OH), dichloromethane (CH2Cl2), N, N-dimethylformamide (DMF), polyvinylpyrrolidone (PVP), citric acid (CA), and 25% ammonia solution was purchased from Merck (Darmstadt, Germany). Working reference solutions were prepared by stepwise dilution from the stock solution. Trimesic acid (H3BTC) was purchased from Sigma-Aldrich. All reagents and solvents were used in this work as received without further purification.
2.2. Apparatus
All experiments were performed with the Varian spectrAA 220 (Australia) atomic absorption spectrometer equipped with a deuterium background correction system and electrothermal atomizer, GTA-110. A hollow cathode lamp was used to determine lead at wavelength of 283.3 nm and lamp current of 10.0 mA, with spectral bandwidth of 0.5 nm. The instrumental parameters and graphite furnace temperature conditions are presented in Table 1. The pH of all solutions was measured with a pH-meter model 713 from Metrohm. The FT-IR spectrometer (Vector-22 Bruker spectrophotometer, Switzerland) was used for functional groups of magnetic MOFs. The scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) images were obtained with Mighty-8 instrument (TSCAN Company, Prague).
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Table 1
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2.3. Sample Preparation
The prepared sorbent was applied to the determination of Pb in several real samples. Tap and mineral water samples were prepared from Sanandaj in Iran. According to the optimized experimental conditions, the pH of the sample was adjusted at 5.5 and analyzed without pretreatment or filtration. For preparing spiked samples of lettuce, 0.5 g of lettuce was digested after the addition of 10 mL of HNO3 (65%). Then, the mixture was centrifuged and the supernatant was filtered through a filter (0.45 μm). The residue solution was evaporated to dryness and then redissolved in 50 mL of double-distilled water, and the pH of the sample was adjusted to 5.5 using NaOH 0.1 mol·L−1. The analysis was carried out as indicated in the procedure section.
2.4. Synthesis of Carboxyl Functionalized Fe3O4 Nanoparticles
Fe3O4 nanoparticles were prepared by a hydrothermal method [25]. Briefly, 4.44 g of FeCl3.6H2O and 1.73 g of FeCl2.4H2O were dissolved in 80 mL of water. Then, in the reflux conditions, under N2 protection and stirring at 1000 rpm, the temperature was slowly increased to 70°C. After stirring for 30 min, 20 mL of the ammonia solution was added to the mixture, and we kept stirring the solution for another 30 min at 70°C. Then, 4 mL of the aqueous solution of the citric acid (0.5 g·mL−1) was added to the mixture and the temperature was set to 90°C under reflux and reacted for 60 min with continuous stirring. Then, it was cooled to room temperature and the black precipitate was isolated using an external magnetic field and washed with ethanol and water.
2.5. Synthesis of Cu-BTC@Fe3O4 Nanocomposite
Synthesis of nano-scaled core-shell Cu-BTC@Fe3O4 was achieved by a one-pot strategy [26]. At first, 0.200 g of PVP and 0.100 g of Cu(OAc)2·H2O were dissolved in 90 mL of mixed solvent of DMF/C2H5OH/H2O (1 : 1 : 1) under mechanical stirring. Then, 0.200 g of carboxyl functionalized Fe3O4 was added to the mixture and kept for 10 min with vigorous stirring at 900 rpm. Then, 0.300 g of trimesic acid and another 0.100 g of Cu(OAc)2·H2O were added to the reaction mixture and stirred for more than 12 h; the obtained products were washed with DMF/C2H5OH/H2O (1 : 1 : 1) and ethanol for three times. Finally, the black powder was dried at 60°C for 3 h. The route for the synthesis of Cu-BTC@Fe3O4 nanocomposites is shown in Scheme 1.
2.6. Measurement Procedure
2 mL of the Pb(II) solution was added to different doses of magnetic MOFs (1−4 mg) in a 10 mL glass vial with magnetic stirring at 1000 rpm. The pH of Pb(II) solution was adjusted with HCl (0.1 mol·L−1) and NaOH (0.1 mol·L−1) from 2 to 10. Then, the solution was stirred for 5 min. Magnetic MOFs were separated from the sample solution using a magnetic field, and supernatant water was decanted. Finally, the sorbent was washed with deionized water and eluted using 500 μL of HCl 0.5 mol·L−1 at a stirring rate of 1000 rpm. Then, analyte ions in the elution solutions were determined by GF AAS. The measurement procedure of the proposed strategy is shown in Scheme 2.
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3.4. Analytical Performance of the Method
Under the optimized conditions, the performance of the developed method was investigated for the determination of lead according to the measurement procedure. As shown in Figure 4, the calibration curve was linear in the range of 0.1–50 μg·L−1. The linear regression equation was A = 0.0178C + 0.158, where A is the absorbance value of the eluent and C is the concentration of lead (ppb) with a correlation coefficient square (r2) of 0.9943. The limit of detection (LOD) was 0.026 μg·L−1 and calculated using equation LOD = 3Sb/m, where Sb and m represent the standard deviation of three replicate blank signals and slope of the calibration curve, respectively. This LOD was lower than that of the US Environmental Protection Agency (EPA) and the European Union (EU). The limit of quantification, defined as LOQ = 10Sb/m, was found to be 0.08 μg·L−1. The preconcentration factor, defined as the ratio of Pb(II) ion concentrations after extraction to concentrations before extraction, was 3.9. In addition, the comparison of the proposed method with the other reported preconcentration methods for extracting Pb(II) ion from water samples is shown in Table 3.
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