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Preconcentration of Pb(II) by Magnetic Metal-Organic Frameworks and Analysis Using Graphite Furnace Atomic Absorption Spectroscopy

Preconcentration of Pb(II) by Magnetic Metal-Organic Frameworks and Analysis Using Graphite... Hindawi Journal of Analytical Methods in Chemistry Volume 2023, Article ID 5424221, 10 pages https://doi.org/10.1155/2023/5424221 Research Article Preconcentration of Pb(II) by Magnetic Metal-Organic Frameworks and Analysis Using Graphite Furnace Atomic Absorption Spectroscopy 1 1,2 1 Arman Sharifi, Rahman Hallaj , and Soleiman Bahar Department of Chemistry, University of Kurdistan, Sanandaj 66177-15175, Iran Research Center for Nanotechnology, University of Kurdistan, Sanandaj 66177-15175, Iran Correspondence should be addressed to Rahman Hallaj; rhallaj@uok.ac.ir and Soleiman Bahar; s.bahar@uok.ac.ir Received 31 July 2022; Revised 16 December 2022; Accepted 26 December 2022; Published 17 January 2023 Academic Editor: Luca Tortora Copyright © 2023 Arman Sharif et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In this study, a magnetic metal-organic framework (MOF) was synthesized based on magnetic Fe O , Cu(II), and benzene-1,3,5- 3 4 tricarboxylic acid (Cu-BTC) as a sorbent for solid phase extraction (SPE) of trace amounts of Pb(II) in water and lettuce samples. Pb(II) ion was adsorbed on the magnetic MOF and easily separated by a magnet; therefore, no fltration or centrifugation was −1 necessary. Te analyte ions were eluted by HCl 0.5 mol·L and analyzed via graphite furnace atomic absorption spectroscopy. Te prepared sorbent was characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and Fourier −1 transform-infrared (FT-IR) spectroscopy. Under optimal experimental conditions, the method had a linear range of 0.1–50 μg·L . −1 Te limits of detection and quantitation for lead were found to be 0.026 and 0.08 μg·L , respectively. Te results showed that the 2+ prepared sorbent has high selectivity for Pb even in the presence of other interfering metal ions. (ETAAS) [5], inductively coupled plasma optical emis- 1. Introduction sion spectroscopy (ICP OES) [6], and fame atomic ab- Lead is a very stable and nonbiodegradable element that sorption spectroscopy (FAAS) [7]. However, their accumulates in the environment [1]. It is considered as one selectivity and sensitivity are insufcient for direct de- of the toxic heavy metals which can cause damage to human termination of lead in real samples at very low concen- health, even at low concentrations. Many diseases such as trations, and on the other hand, most of these samples anemia, cardiovascular and developmental disorders, and have complex matrices [8]. In order to overcome these muscle paralysis are related to Pb(II) and can harm the liver, problems, preconcentration and separation procedures kidneys, the central nervous system, the endocrine system, such as solid phase extraction (SPE) [9, 10], liquid-liquid the hematopoietic system, and the reproductive system extraction [11], cloud point extraction [12], and ion [2, 3]. Humans are generally exposed to Pb through exchange [13] have been performed. breathing air, drinking water, and eating food. Te maxi- Among the mentioned methods, SPE is the most mum allowable level of lead in drinking water by the US common technique applied for the preconcentration and Environmental Protection Agency (EPA) and the European extraction of Pb(II) from environmental and food sam- −1 −1 Union (EU) is 15 μg·L and 10 μg·L , respectively. Con- ples. SPE has many obvious advantages, such as high sequently, developing an efective and highly efcient enrichment factor, high recovery, low consumption of method to monitor this hazardous element is necessary. organic solvents, and convenience of operation [14, 15]. It Numerous techniques have been used for the de- is usually acknowledged that sorbent plays a signifcant termination of lead, such as the electrochemical method role in the SPE technique because of analytical sensitivity, [4], electrothermal atomic absorption spectrometry precision, and selectivity. 2 Journal of Analytical Methods in Chemistry Table 1: Instrumental parameters and graphite furnace temper- ferric chloride, Cu(OAc) ·H O (98%), ethanol (C H OH), 2 2 2 5 ature conditions for Pb determination. dichloromethane (CH Cl ), N, N-dimethylformamide 2 2 (DMF), polyvinylpyrrolidone (PVP), citric acid (CA), and Instrumental parameters 25% ammonia solution was purchased from Merck Wavelength (nm) 283.3 (Darmstadt, Germany). Working reference solutions were Spectral bandwidth (nm) 0.5 prepared by stepwise dilution from the stock solution. Lamp current (mA) 10 Trimesic acid (H BTC) was purchased from Sigma-Aldrich. Sample volume (μL) 20 Integration mode Peak area All reagents and solvents were used in this work as received Background correction Deuterium without further purifcation. Graphite furnace temperature conditions Argon fow rate Step Temperature ( C) −1 2.2. Apparatus. All experiments were performed with the (L·min ) Varian spectrAA 220 (Australia) atomic absorption spec- Drying 80 3 trometer equipped with a deuterium background correction Drying 120 3 system and electrothermal atomizer, GTA-110. A hollow Pyrolysis 300 3 cathode lamp was used to determine lead at wavelength of Atomization 1900 0 283.3 nm and lamp current of 10.0 mA, with spectral bandwidth Cleaning 2100 3 of 0.5 nm. Te instrumental parameters and graphite furnace temperature conditions are presented in Table 1. Te pH of all Metal-organic frameworks (MOFs) are employed as solutions was measured with a pH-meter model 713 from excellent novel adsorbents owing to their signifcant char- Metrohm. Te FT-IR spectrometer (Vector-22 Bruker spec- acteristics such as high specifc surface area, high thermal trophotometer, Switzerland) was used for functional groups of and chemical stability, adjustable pore sizes, and rich magnetic MOFs. Te scanning electron microscopy (SEM) and functionalities [16, 17]. MOFs are porous crystalline ma- energy-dispersive spectroscopy (EDS) images were obtained terials that are constructed from metal ions and organic with Mighty-8 instrument (TSCAN Company, Prague). linkers via strong coordination bonds and arranged in the form of a network structure [18]. MOFs have signifcant 2.3. Sample Preparation. Te prepared sorbent was applied to applications in the feld of gas storage [19], drug delivery the determination of Pb in several real samples. Tap and mineral [20], catalysis [21], and adsorption [22]. MOFs have shown water samples were prepared from Sanandaj in Iran. According desirable adsorption for heavy metals and drugs [23, 24]. to the optimized experimental conditions, the pH of the sample Filtration and centrifugation are the methods used for the was adjusted at 5.5 and analyzed without pretreatment or fl- recovery of MOFs. But these methods, because of the slow tration. For preparing spiked samples of lettuce, 0.5 g of lettuce speed, high cost, and cumbersome operating steps, limit the was digested after the addition of 10 mL of HNO (65%). Ten, large-scale application of MOFs. Terefore, preparing the the mixture was centrifuged and the supernatant was fltered MOF material that can be separated easily is essential. through a flter (0.45 μm). Te residue solution was evaporated Magnetic MOFs, due to their superparamagnetic to dryness and then redissolved in 50 mL of double-distilled properties, can be easily separated from the matrix by water, and the pH of the sample was adjusted to 5.5 using employing a strong external magnetic feld and redispersed −1 NaOH 0.1 mol·L . Te analysis was carried out as indicated in in the eluent once the external magnetic feld is removed. the procedure section. Magnetic MOFs avoid taking steps such as fltration and centrifugation. In the current work, we have synthesized magnetic 2.4. Synthesis of Carboxyl Functionalized Fe O Nanoparticles. 3 4 MOFs by assembling Cu(II) and benzene-1,3,5-tricarboxylic Fe O nanoparticles were prepared by a hydrothermal method 3 4 acid (Cu-BTC) thin layers bonded through carboxyl groups [25]. Briefy, 4.44 g of FeCl .6H O and 1.73 g of FeCl .4H O 3 2 2 2 on the surface of magnetic Fe O nanoparticles for the 3 4 were dissolved in 80 mL of water. Ten, in the refux conditions, extraction of trace Pb(II) ion followed by graphite furnace under N2 protection and stirring at 1000 rpm, the temperature atomic absorption spectroscopy. Tese Cu-BTC@Fe O 3 4 was slowly increased to 70 C. After stirring for 30 min, 20 mL of nanocomposites have large pores and cavities that can the ammonia solution was added to the mixture, and we kept signifcantly increase the surface area. Te super- ° stirring the solution for another 30 min at 70 C. Ten, 4 mL of −1 paramagnetic properties of Fe O contribute to the rapid 3 4 the aqueous solution of the citric acid (0.5 g·mL ) was added to separation of the adsorbent from the matrix solution. Te ° the mixture and the temperature was set to 90 C under refux carboxyl groups present in Cu-BTC provide more bonding and reacted for 60 min with continuous stirring. Ten, it was sites for the lead ion. Based on these considerations, the cooled to room temperature and the black precipitate was preconcentration and determination of lead ions in the isolated using an external magnetic feld and washed with diferent real samples can be readily achieved. ethanol and water. 2. Experimental 2.5. Synthesis of Cu-BTC@Fe O Nanocomposite. 3 4 –1 2.1. Materials. Te stock standard solution of 1000 mg L Synthesis of nano-scaled core-shell Cu-BTC@Fe O was 3 4 lead for atomic absorption spectroscopy, ferrous chloride, achieved by a one-pot strategy [26]. At frst, 0.200 g of PVP Journal of Analytical Methods in Chemistry 3 2+ CA PVP Cu BTC 2+ Cu Scheme 1: Route of synthesis of Cu-BTC@Fe O nanocomposite. 3 4 and 0.100 g of Cu(OAc) ·H O were dissolved in 90 mL of Te surface morphology of the prepared Cu-BTC@ 2 2 mixed solvent of DMF/C H OH/H O (1 :1 :1) under me- Fe O nanocomposite was investigated using scanning 2 5 2 3 4 chanical stirring. Ten, 0.200 g of carboxyl functionalized electron microscopy (SEM). As shown in Figure 1(b), the Fe O was added to the mixture and kept for 10 min with prepared sorbent has nanobelt morphology and smooth 3 4 vigorous stirring at 900 rpm. Ten, 0.300 g of trimesic acid surface with lengths of 10–20 μm. After adsorption of Pb(II), and another 0.100 g of Cu(OAc) ·H O were added to the surface became rougher, indicating that Pb(II) ion was 2 2 reaction mixture and stirred for more than 12 h; the obtained adsorbed on magnetic MOF (Figure 1(c)). Te EDS spec- products were washed with DMF/C H OH/H O (1 :1 :1) trum of Cu-BTC@Fe O nanocomposite is shown in 2 5 2 3 4 and ethanol for three times. Finally, the black powder was Figure 1(d). Te existence of Fe, O, Cu, and C elements dried at 60 C for 3 h. Te route for the synthesis of Cu-BTC@ confrms successful synthesis of Cu-BTC@Fe O nano- 3 4 Fe O nanocomposites is shown in Scheme 1. composite. EDS spectrum (Figure 1(e)) and the mapping 3 4 analysis (Figure 2) ensure that Pb(II) ions are adsorbed on Cu-BTC@Fe O nanocomposite. 3 4 2.6. Measurement Procedure. 2 mL of the Pb(II) solution was added to diferent doses of magnetic MOFs (1− 4 mg) in 3.2. Optimization of the Efective Variables. Several param- a 10 mL glass vial with magnetic stirring at 1000 rpm. Te −1 eters that may afect the preconcentration and extraction pH of Pb(II) solution was adjusted with HCl (0.1 mol·L ) −1 process, such as pH of the sample solution, adsorption time, and NaOH (0.1 mol·L ) from 2 to 10. Ten, the solution was efect of the type, concentration of eluent, desorption time, stirred for 5 min. Magnetic MOFs were separated from the sorbent amount, and reusability of the adsorbent, were sample solution using a magnetic feld, and supernatant optimized. Te optimization was carried out on 2 mL of water was decanted. Finally, the sorbent was washed with −1 −1 50 μg·L lead aqueous solution. deionized water and eluted using 500 μL of HCl 0.5 mol·L at a stirring rate of 1000 rpm. Ten, analyte ions in the elution solutions were determined by GF AAS. Te mea- 3.2.1. Efect of Sample Solution pH. Te pH has a signifcant surement procedure of the proposed strategy is shown in role in the SPE studies of metal ions. Terefore, the Scheme 2. pH aqueous sample solution on the preconcentration of lead ions was changed in the range of 2.0–10.0. As shown in 3. Results and Discussion Figure 3(a), the absorbance of Pb increased with the increase of sample pH up to 5.5 and then decreased at high pH. 3.1. Characterization of Cu-BTC@Fe O Nanocomposites. 3 4 However, for pH> 5.5, due to the formation of Pb(OH) , Figure 1(a) shows the FTIR spectra of Cu-BTC@Fe O 3 4 − absorbance was decreased. At pH< 5.5, the COO ions –1 nanocomposite. Te broad band at 3450 cm is assigned present on the surface sorbent can bind positive Pb(II) ions to the stretching vibration of OH groups, and the band at through electrostatic interactions, but at very low pH, hy- –1 1643 cm is attributed to the C�O stretching vibration of drogen cations can interact with the oxygen electrons. the carbonyl group in H3BTC. Te band located at −1 Terefore, pH 5.5 was selected as the optimum pH. 1440 cm could be ascribed to the C−C frame vibration of the aromatic nucleus conjugated with C�O. Te peaks −1 at 1374 cm can be attributed to the C�C stretching 3.2.2. Efect of Adsorption Time. Te efect of the extraction −1 vibration in H BTC. Te band at around 575 cm can be time on the adsorption of Pb(II) was examined in the range attributed to the Fe–O stretching vibrational mode of of 3 to 45 min. As presented in Figure 3(b), the results Fe O . showed that after 5 min, absorbance reaches a maximum and 3 4 HCI Elution 4 Journal of Analytical Methods in Chemistry Magnetic Separation Adsorption Adsorbent pb (II) Other substrant With Preconcentration Injection Readout Without Preconcentration 2+ Scheme 2: Schematic diagram of the proposed strategy for preconcentration of Pb . remains almost constant from 5 to 45 times, indicating that 3.2.4. Efect of Sorbent Amount. Te efect of sorbent the process of sorption is very quick. Finally, 5 min was used amount on the preconcentration of lead was investigated in for further experiments. the range of 1.0–4.0 mg. As shown in Figure 3(e), absorbance increased to 3.0 mg and remained constant. Tis develop- ment is due to an increase in the surface area and available 3.2.3. Efect of the Type, Concentration of Eluent, and De- sites for the adsorption of the analytes. Terefore, 3.0 mg was sorption Time. Eluent solution and desorption time as used in all subsequent experiments. essential factors in the preconcentration and extraction process were studied. Eluent solution must be able to dissolve the target analyte and overcome the bond be- 3.2.5. Reusability of the Adsorbent. Repeated experiments tween the analyte and the adsorbent. According to the were performed to check the reusability of Cu-BTC@Fe O 3 4 results of the efect of pH, acid solution may be a good nanocomposites. After collecting the used adsorbent, Pb(II) choice as an eluent. Terefore, a series of acidic solutions was desorbed from the adsorbent by treatment with HCl −1 of HCl and HNO with diferent concentrations (0.01, 0.1, 0.5 mol·L . As shown in Figure 3(f), the adsorbent stability −1 0.3, 0.5, and 1.0 mol·L ) were employed. As shown in was good and no signifcant change was observed for the −1 2+ Figure 3(c), when HCl 0.5 mol·L was used, the highest absorbance of Pb up to 5 adsorption-desorption cycles. desorption efciency was achieved. Finally, HCl Tus, these results demonstrate that Cu-BTC@Fe O 3 4 −1 0.5 mol·L was chosen as the eluent to ensure complete nanocomposites are efcient and cost-efective adsorbents desorption in subsequent studies. In order to complete with good potential for reuse. desorption of lead ions from the surface of magnetic MOFs, desorption time was examined. As shown in Figure 3(d), 5 min was chosen as the optimal 3.3. Efect of the Interfering Ions. Te efect of metal ions on desorption time. the signal intensity was studied under optimal conditions. In Journal of Analytical Methods in Chemistry 5 350 1350 2350 3350 -1 Wavenumber (cm ) (a) (b) CuLα C Kα O Kα FeLα CuKα FeKα FeKβ CuKβ leV 0 5 10 (c) (d) O Kα CuLα C Kα FeLα PbMα CuKα FbMβ FeKα FeKβ CuKβ leV 01 5 0 (e) Figure 1: (a) FTIR spectra of Cu-BTC@Fe O nanocomposite. (b, c) SEM images of magnetic MOF before and after adsorption. (d, e) EDS 3 4 images of Cu-BTC@Fe O nanocomposite before and after adsorption. 3 4 Transmittance (%) 6 Journal of Analytical Methods in Chemistry 2+ Figure 2: Elemental mapping of Cu-BTC@Fe O after adsorption of Pb . 3 4 −1 these experiments, 10 μg·L of the lead standard solution of the calibration curve, respectively. Tis LOD was lower was spiked with various concentrations of interfering ions than that of the US Environmental Protection Agency (EPA) 2+ 3+ 2+ 2+ 2+ 3+ 2+ 2+ 2+ such as Cd , As , Hg , Ni , Cu , Fe , Co , Mn , Ca , and the European Union (EU). Te limit of quantifcation, 2+ + + 2+ −1 Mg , K , Na , and Zn . Ions were considered to interfere defned as LOQ � 10S /m, was found to be 0.08 μg·L . Te when the deviation of the recovery was more than ±5%. Te preconcentration factor, defned as the ratio of Pb(II) ion results in Table 2 show that the ions do not interfere in the concentrations after extraction to concentrations before determination of lead. Tus, the proposed method is robust extraction, was 3.9. In addition, the comparison of the for lead determination. proposed method with the other reported preconcentration methods for extracting Pb(II) ion from water samples is shown in Table 3. 3.4. Analytical Performance of the Method. Under the op- timized conditions, the performance of the developed method was investigated for the determination of lead 3.5. Analysis of Real Sample. Te method was used for the according to the measurement procedure. As shown in determination of lead in tap water, mineral water, and Figure 4, the calibration curve was linear in the range of lettuce samples according to the standard addition method. −1 0.1–50 μg·L . Te linear regression equation was Diferent concentrations of lead were spiked and analyzed A � 0.0178C + 0.158, where A is the absorbance value of the using the proposed method. As shown in Table 4, the re- eluent and C is the concentration of lead (ppb) with coveries in the range of 98.84–101.30% and RSD in the range of 0.25–3.02 were obtained by the proposed method. Tese a correlation coefcient square (r ) of 0.9943. Te limit of −1 detection (LOD) was 0.026 μg·L and calculated using results indicate that the proposed method is suitable for the equation LOD � 3S /m, where S and m represent the separation and preconcentration of lead from water and b b standard deviation of three replicate blank signals and slope other samples. Journal of Analytical Methods in Chemistry 7 1.4 1.2 1.15 1.1 1.2 1.05 0.95 0.8 0.9 0.8 0.85 A A 0.6 0.8 0.6 5.3 5.4 5.5 5.6 5.7 pH 0.4 0.4 0.2 0.2 0 0 0 1020304050 02468 10 12 pH Adsorption time (min) (a) (b) 1.4 1.4 1.2 1.2 1 1 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 HCl HCl HCl HCl HCl HNO3 0 1020304050 (0.01 ) (0.1 ) (0.3 ) (0.5 ) (1.0 ) (0.01 ) Desorption time (min) -1 Eluent (mol L ) (c) (d) 1.2 1.2 1 1 0.8 0.8 A 0.6 A 0.6 0.4 0.4 0.2 0.2 0 0 Recycles Sorbent amount (mg) (e) (f) Figure 3: (a) Efect of sample solution pH. (b) Efect of adsorption time. (c) Efect of the type and concentration of eluent. (d) Efect of desorption time. (e) Efect of sorbent amount. (f) Reusability of the adsorbent. 8 Journal of Analytical Methods in Chemistry −1 Table 2: Efect of interfering ions in the presence of 10 μg·L lead. Interfering ions Interferent/Pb(II) ratio Recovery (%) 2+ Cd 100 96 3+ As 300 98 2+ Hg 200 97 2+ Ni 200 100 2+ Cu 200 99 3+ Fe 400 97 2+ Co 150 98 2+ Mn 200 99 2+ Ca 150 98 2+ Mg 150 97 Na 200 99 K 200 97 2+ Zn 100 96 1.2 1 y = 0.0178x + 0.158 R = 0.9943 0.8 A 0.6 0.4 0.2 0 10 20 30 40 50 60 -1 Concentration (µg L ) 2+ Figure 4: Calibration curve for Pb ion determination. Table 3: Comparison of the proposed method with some diferent methods for determination of lead. −1 −1 Preconcentration technique Detection method LOD (μg·L ) Linear range (μg·L ) Reference SPE FAAS 0.29 1.0–20 [27] SPE FAAS 0.7 3.0–100 [28] CPE UV-vis 3.9 5.0–100 [29] SPE GF AAS 0.0008 0.005–0.5 [30] SPE GF AAS 0.11 10–250 [5] DLLME GF AAS 0.003 0.01–3 [31] SPE GF AAS 0.026 0.1–50 Tis work CPE: cloud point extraction; DLLME: dispersive liquid-liquid microextraction. Table 4: Determination of lead in real samples. −1 −1 −1 Samples Measured (μg·L ) Added (μg·L ) Found (μg·L ) Recovery (%) RSD (n � 3) 5 5.24 98.87 0.50 Tap water 0.30 10 10.27 99.71 0.25 5 5.01 100.2 1.2 Mineral water ND 10 10.13 101.3 3.02 5 5.30 99.06 1.89 Lettuce 0.35 10 10.32 99.7 1.55 ND (not detected) � below the limit of detection. BTC@Fe O nanocomposite signifcantly enhanced the 4. Conclusions 3 4 sensitivities of lead. By applying an external magnetic feld, In this work, magnetic MOFs were applied for the separation magnetic MOFs are separated from solution which avoids and preconcentration of trace amounts of lead from water fltration and centrifugation steps. Te adsorbed ion of and other samples. Te carboxyl groups present in Cu- Pb(II) was ready to be desorbed with HCl solution followed Journal of Analytical Methods in Chemistry 9 of trace heavy metals in natural water,” Te Analyst, vol. 140, by GF AAS analysis. 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Preconcentration of Pb(II) by Magnetic Metal-Organic Frameworks and Analysis Using Graphite Furnace Atomic Absorption Spectroscopy

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Hindawi Publishing Corporation
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2090-8865
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2090-8873
DOI
10.1155/2023/5424221
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Abstract

Hindawi Journal of Analytical Methods in Chemistry Volume 2023, Article ID 5424221, 10 pages https://doi.org/10.1155/2023/5424221 Research Article Preconcentration of Pb(II) by Magnetic Metal-Organic Frameworks and Analysis Using Graphite Furnace Atomic Absorption Spectroscopy 1 1,2 1 Arman Sharifi, Rahman Hallaj , and Soleiman Bahar Department of Chemistry, University of Kurdistan, Sanandaj 66177-15175, Iran Research Center for Nanotechnology, University of Kurdistan, Sanandaj 66177-15175, Iran Correspondence should be addressed to Rahman Hallaj; rhallaj@uok.ac.ir and Soleiman Bahar; s.bahar@uok.ac.ir Received 31 July 2022; Revised 16 December 2022; Accepted 26 December 2022; Published 17 January 2023 Academic Editor: Luca Tortora Copyright © 2023 Arman Sharif et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In this study, a magnetic metal-organic framework (MOF) was synthesized based on magnetic Fe O , Cu(II), and benzene-1,3,5- 3 4 tricarboxylic acid (Cu-BTC) as a sorbent for solid phase extraction (SPE) of trace amounts of Pb(II) in water and lettuce samples. Pb(II) ion was adsorbed on the magnetic MOF and easily separated by a magnet; therefore, no fltration or centrifugation was −1 necessary. Te analyte ions were eluted by HCl 0.5 mol·L and analyzed via graphite furnace atomic absorption spectroscopy. Te prepared sorbent was characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and Fourier −1 transform-infrared (FT-IR) spectroscopy. Under optimal experimental conditions, the method had a linear range of 0.1–50 μg·L . −1 Te limits of detection and quantitation for lead were found to be 0.026 and 0.08 μg·L , respectively. Te results showed that the 2+ prepared sorbent has high selectivity for Pb even in the presence of other interfering metal ions. (ETAAS) [5], inductively coupled plasma optical emis- 1. Introduction sion spectroscopy (ICP OES) [6], and fame atomic ab- Lead is a very stable and nonbiodegradable element that sorption spectroscopy (FAAS) [7]. However, their accumulates in the environment [1]. It is considered as one selectivity and sensitivity are insufcient for direct de- of the toxic heavy metals which can cause damage to human termination of lead in real samples at very low concen- health, even at low concentrations. Many diseases such as trations, and on the other hand, most of these samples anemia, cardiovascular and developmental disorders, and have complex matrices [8]. In order to overcome these muscle paralysis are related to Pb(II) and can harm the liver, problems, preconcentration and separation procedures kidneys, the central nervous system, the endocrine system, such as solid phase extraction (SPE) [9, 10], liquid-liquid the hematopoietic system, and the reproductive system extraction [11], cloud point extraction [12], and ion [2, 3]. Humans are generally exposed to Pb through exchange [13] have been performed. breathing air, drinking water, and eating food. Te maxi- Among the mentioned methods, SPE is the most mum allowable level of lead in drinking water by the US common technique applied for the preconcentration and Environmental Protection Agency (EPA) and the European extraction of Pb(II) from environmental and food sam- −1 −1 Union (EU) is 15 μg·L and 10 μg·L , respectively. Con- ples. SPE has many obvious advantages, such as high sequently, developing an efective and highly efcient enrichment factor, high recovery, low consumption of method to monitor this hazardous element is necessary. organic solvents, and convenience of operation [14, 15]. It Numerous techniques have been used for the de- is usually acknowledged that sorbent plays a signifcant termination of lead, such as the electrochemical method role in the SPE technique because of analytical sensitivity, [4], electrothermal atomic absorption spectrometry precision, and selectivity. 2 Journal of Analytical Methods in Chemistry Table 1: Instrumental parameters and graphite furnace temper- ferric chloride, Cu(OAc) ·H O (98%), ethanol (C H OH), 2 2 2 5 ature conditions for Pb determination. dichloromethane (CH Cl ), N, N-dimethylformamide 2 2 (DMF), polyvinylpyrrolidone (PVP), citric acid (CA), and Instrumental parameters 25% ammonia solution was purchased from Merck Wavelength (nm) 283.3 (Darmstadt, Germany). Working reference solutions were Spectral bandwidth (nm) 0.5 prepared by stepwise dilution from the stock solution. Lamp current (mA) 10 Trimesic acid (H BTC) was purchased from Sigma-Aldrich. Sample volume (μL) 20 Integration mode Peak area All reagents and solvents were used in this work as received Background correction Deuterium without further purifcation. Graphite furnace temperature conditions Argon fow rate Step Temperature ( C) −1 2.2. Apparatus. All experiments were performed with the (L·min ) Varian spectrAA 220 (Australia) atomic absorption spec- Drying 80 3 trometer equipped with a deuterium background correction Drying 120 3 system and electrothermal atomizer, GTA-110. A hollow Pyrolysis 300 3 cathode lamp was used to determine lead at wavelength of Atomization 1900 0 283.3 nm and lamp current of 10.0 mA, with spectral bandwidth Cleaning 2100 3 of 0.5 nm. Te instrumental parameters and graphite furnace temperature conditions are presented in Table 1. Te pH of all Metal-organic frameworks (MOFs) are employed as solutions was measured with a pH-meter model 713 from excellent novel adsorbents owing to their signifcant char- Metrohm. Te FT-IR spectrometer (Vector-22 Bruker spec- acteristics such as high specifc surface area, high thermal trophotometer, Switzerland) was used for functional groups of and chemical stability, adjustable pore sizes, and rich magnetic MOFs. Te scanning electron microscopy (SEM) and functionalities [16, 17]. MOFs are porous crystalline ma- energy-dispersive spectroscopy (EDS) images were obtained terials that are constructed from metal ions and organic with Mighty-8 instrument (TSCAN Company, Prague). linkers via strong coordination bonds and arranged in the form of a network structure [18]. MOFs have signifcant 2.3. Sample Preparation. Te prepared sorbent was applied to applications in the feld of gas storage [19], drug delivery the determination of Pb in several real samples. Tap and mineral [20], catalysis [21], and adsorption [22]. MOFs have shown water samples were prepared from Sanandaj in Iran. According desirable adsorption for heavy metals and drugs [23, 24]. to the optimized experimental conditions, the pH of the sample Filtration and centrifugation are the methods used for the was adjusted at 5.5 and analyzed without pretreatment or fl- recovery of MOFs. But these methods, because of the slow tration. For preparing spiked samples of lettuce, 0.5 g of lettuce speed, high cost, and cumbersome operating steps, limit the was digested after the addition of 10 mL of HNO (65%). Ten, large-scale application of MOFs. Terefore, preparing the the mixture was centrifuged and the supernatant was fltered MOF material that can be separated easily is essential. through a flter (0.45 μm). Te residue solution was evaporated Magnetic MOFs, due to their superparamagnetic to dryness and then redissolved in 50 mL of double-distilled properties, can be easily separated from the matrix by water, and the pH of the sample was adjusted to 5.5 using employing a strong external magnetic feld and redispersed −1 NaOH 0.1 mol·L . Te analysis was carried out as indicated in in the eluent once the external magnetic feld is removed. the procedure section. Magnetic MOFs avoid taking steps such as fltration and centrifugation. In the current work, we have synthesized magnetic 2.4. Synthesis of Carboxyl Functionalized Fe O Nanoparticles. 3 4 MOFs by assembling Cu(II) and benzene-1,3,5-tricarboxylic Fe O nanoparticles were prepared by a hydrothermal method 3 4 acid (Cu-BTC) thin layers bonded through carboxyl groups [25]. Briefy, 4.44 g of FeCl .6H O and 1.73 g of FeCl .4H O 3 2 2 2 on the surface of magnetic Fe O nanoparticles for the 3 4 were dissolved in 80 mL of water. Ten, in the refux conditions, extraction of trace Pb(II) ion followed by graphite furnace under N2 protection and stirring at 1000 rpm, the temperature atomic absorption spectroscopy. Tese Cu-BTC@Fe O 3 4 was slowly increased to 70 C. After stirring for 30 min, 20 mL of nanocomposites have large pores and cavities that can the ammonia solution was added to the mixture, and we kept signifcantly increase the surface area. Te super- ° stirring the solution for another 30 min at 70 C. Ten, 4 mL of −1 paramagnetic properties of Fe O contribute to the rapid 3 4 the aqueous solution of the citric acid (0.5 g·mL ) was added to separation of the adsorbent from the matrix solution. Te ° the mixture and the temperature was set to 90 C under refux carboxyl groups present in Cu-BTC provide more bonding and reacted for 60 min with continuous stirring. Ten, it was sites for the lead ion. Based on these considerations, the cooled to room temperature and the black precipitate was preconcentration and determination of lead ions in the isolated using an external magnetic feld and washed with diferent real samples can be readily achieved. ethanol and water. 2. Experimental 2.5. Synthesis of Cu-BTC@Fe O Nanocomposite. 3 4 –1 2.1. Materials. Te stock standard solution of 1000 mg L Synthesis of nano-scaled core-shell Cu-BTC@Fe O was 3 4 lead for atomic absorption spectroscopy, ferrous chloride, achieved by a one-pot strategy [26]. At frst, 0.200 g of PVP Journal of Analytical Methods in Chemistry 3 2+ CA PVP Cu BTC 2+ Cu Scheme 1: Route of synthesis of Cu-BTC@Fe O nanocomposite. 3 4 and 0.100 g of Cu(OAc) ·H O were dissolved in 90 mL of Te surface morphology of the prepared Cu-BTC@ 2 2 mixed solvent of DMF/C H OH/H O (1 :1 :1) under me- Fe O nanocomposite was investigated using scanning 2 5 2 3 4 chanical stirring. Ten, 0.200 g of carboxyl functionalized electron microscopy (SEM). As shown in Figure 1(b), the Fe O was added to the mixture and kept for 10 min with prepared sorbent has nanobelt morphology and smooth 3 4 vigorous stirring at 900 rpm. Ten, 0.300 g of trimesic acid surface with lengths of 10–20 μm. After adsorption of Pb(II), and another 0.100 g of Cu(OAc) ·H O were added to the surface became rougher, indicating that Pb(II) ion was 2 2 reaction mixture and stirred for more than 12 h; the obtained adsorbed on magnetic MOF (Figure 1(c)). Te EDS spec- products were washed with DMF/C H OH/H O (1 :1 :1) trum of Cu-BTC@Fe O nanocomposite is shown in 2 5 2 3 4 and ethanol for three times. Finally, the black powder was Figure 1(d). Te existence of Fe, O, Cu, and C elements dried at 60 C for 3 h. Te route for the synthesis of Cu-BTC@ confrms successful synthesis of Cu-BTC@Fe O nano- 3 4 Fe O nanocomposites is shown in Scheme 1. composite. EDS spectrum (Figure 1(e)) and the mapping 3 4 analysis (Figure 2) ensure that Pb(II) ions are adsorbed on Cu-BTC@Fe O nanocomposite. 3 4 2.6. Measurement Procedure. 2 mL of the Pb(II) solution was added to diferent doses of magnetic MOFs (1− 4 mg) in 3.2. Optimization of the Efective Variables. Several param- a 10 mL glass vial with magnetic stirring at 1000 rpm. Te −1 eters that may afect the preconcentration and extraction pH of Pb(II) solution was adjusted with HCl (0.1 mol·L ) −1 process, such as pH of the sample solution, adsorption time, and NaOH (0.1 mol·L ) from 2 to 10. Ten, the solution was efect of the type, concentration of eluent, desorption time, stirred for 5 min. Magnetic MOFs were separated from the sorbent amount, and reusability of the adsorbent, were sample solution using a magnetic feld, and supernatant optimized. Te optimization was carried out on 2 mL of water was decanted. Finally, the sorbent was washed with −1 −1 50 μg·L lead aqueous solution. deionized water and eluted using 500 μL of HCl 0.5 mol·L at a stirring rate of 1000 rpm. Ten, analyte ions in the elution solutions were determined by GF AAS. Te mea- 3.2.1. Efect of Sample Solution pH. Te pH has a signifcant surement procedure of the proposed strategy is shown in role in the SPE studies of metal ions. Terefore, the Scheme 2. pH aqueous sample solution on the preconcentration of lead ions was changed in the range of 2.0–10.0. As shown in 3. Results and Discussion Figure 3(a), the absorbance of Pb increased with the increase of sample pH up to 5.5 and then decreased at high pH. 3.1. Characterization of Cu-BTC@Fe O Nanocomposites. 3 4 However, for pH> 5.5, due to the formation of Pb(OH) , Figure 1(a) shows the FTIR spectra of Cu-BTC@Fe O 3 4 − absorbance was decreased. At pH< 5.5, the COO ions –1 nanocomposite. Te broad band at 3450 cm is assigned present on the surface sorbent can bind positive Pb(II) ions to the stretching vibration of OH groups, and the band at through electrostatic interactions, but at very low pH, hy- –1 1643 cm is attributed to the C�O stretching vibration of drogen cations can interact with the oxygen electrons. the carbonyl group in H3BTC. Te band located at −1 Terefore, pH 5.5 was selected as the optimum pH. 1440 cm could be ascribed to the C−C frame vibration of the aromatic nucleus conjugated with C�O. Te peaks −1 at 1374 cm can be attributed to the C�C stretching 3.2.2. Efect of Adsorption Time. Te efect of the extraction −1 vibration in H BTC. Te band at around 575 cm can be time on the adsorption of Pb(II) was examined in the range attributed to the Fe–O stretching vibrational mode of of 3 to 45 min. As presented in Figure 3(b), the results Fe O . showed that after 5 min, absorbance reaches a maximum and 3 4 HCI Elution 4 Journal of Analytical Methods in Chemistry Magnetic Separation Adsorption Adsorbent pb (II) Other substrant With Preconcentration Injection Readout Without Preconcentration 2+ Scheme 2: Schematic diagram of the proposed strategy for preconcentration of Pb . remains almost constant from 5 to 45 times, indicating that 3.2.4. Efect of Sorbent Amount. Te efect of sorbent the process of sorption is very quick. Finally, 5 min was used amount on the preconcentration of lead was investigated in for further experiments. the range of 1.0–4.0 mg. As shown in Figure 3(e), absorbance increased to 3.0 mg and remained constant. Tis develop- ment is due to an increase in the surface area and available 3.2.3. Efect of the Type, Concentration of Eluent, and De- sites for the adsorption of the analytes. Terefore, 3.0 mg was sorption Time. Eluent solution and desorption time as used in all subsequent experiments. essential factors in the preconcentration and extraction process were studied. Eluent solution must be able to dissolve the target analyte and overcome the bond be- 3.2.5. Reusability of the Adsorbent. Repeated experiments tween the analyte and the adsorbent. According to the were performed to check the reusability of Cu-BTC@Fe O 3 4 results of the efect of pH, acid solution may be a good nanocomposites. After collecting the used adsorbent, Pb(II) choice as an eluent. Terefore, a series of acidic solutions was desorbed from the adsorbent by treatment with HCl −1 of HCl and HNO with diferent concentrations (0.01, 0.1, 0.5 mol·L . As shown in Figure 3(f), the adsorbent stability −1 0.3, 0.5, and 1.0 mol·L ) were employed. As shown in was good and no signifcant change was observed for the −1 2+ Figure 3(c), when HCl 0.5 mol·L was used, the highest absorbance of Pb up to 5 adsorption-desorption cycles. desorption efciency was achieved. Finally, HCl Tus, these results demonstrate that Cu-BTC@Fe O 3 4 −1 0.5 mol·L was chosen as the eluent to ensure complete nanocomposites are efcient and cost-efective adsorbents desorption in subsequent studies. In order to complete with good potential for reuse. desorption of lead ions from the surface of magnetic MOFs, desorption time was examined. As shown in Figure 3(d), 5 min was chosen as the optimal 3.3. Efect of the Interfering Ions. Te efect of metal ions on desorption time. the signal intensity was studied under optimal conditions. In Journal of Analytical Methods in Chemistry 5 350 1350 2350 3350 -1 Wavenumber (cm ) (a) (b) CuLα C Kα O Kα FeLα CuKα FeKα FeKβ CuKβ leV 0 5 10 (c) (d) O Kα CuLα C Kα FeLα PbMα CuKα FbMβ FeKα FeKβ CuKβ leV 01 5 0 (e) Figure 1: (a) FTIR spectra of Cu-BTC@Fe O nanocomposite. (b, c) SEM images of magnetic MOF before and after adsorption. (d, e) EDS 3 4 images of Cu-BTC@Fe O nanocomposite before and after adsorption. 3 4 Transmittance (%) 6 Journal of Analytical Methods in Chemistry 2+ Figure 2: Elemental mapping of Cu-BTC@Fe O after adsorption of Pb . 3 4 −1 these experiments, 10 μg·L of the lead standard solution of the calibration curve, respectively. Tis LOD was lower was spiked with various concentrations of interfering ions than that of the US Environmental Protection Agency (EPA) 2+ 3+ 2+ 2+ 2+ 3+ 2+ 2+ 2+ such as Cd , As , Hg , Ni , Cu , Fe , Co , Mn , Ca , and the European Union (EU). Te limit of quantifcation, 2+ + + 2+ −1 Mg , K , Na , and Zn . Ions were considered to interfere defned as LOQ � 10S /m, was found to be 0.08 μg·L . Te when the deviation of the recovery was more than ±5%. Te preconcentration factor, defned as the ratio of Pb(II) ion results in Table 2 show that the ions do not interfere in the concentrations after extraction to concentrations before determination of lead. Tus, the proposed method is robust extraction, was 3.9. In addition, the comparison of the for lead determination. proposed method with the other reported preconcentration methods for extracting Pb(II) ion from water samples is shown in Table 3. 3.4. Analytical Performance of the Method. Under the op- timized conditions, the performance of the developed method was investigated for the determination of lead 3.5. Analysis of Real Sample. Te method was used for the according to the measurement procedure. As shown in determination of lead in tap water, mineral water, and Figure 4, the calibration curve was linear in the range of lettuce samples according to the standard addition method. −1 0.1–50 μg·L . Te linear regression equation was Diferent concentrations of lead were spiked and analyzed A � 0.0178C + 0.158, where A is the absorbance value of the using the proposed method. As shown in Table 4, the re- eluent and C is the concentration of lead (ppb) with coveries in the range of 98.84–101.30% and RSD in the range of 0.25–3.02 were obtained by the proposed method. Tese a correlation coefcient square (r ) of 0.9943. Te limit of −1 detection (LOD) was 0.026 μg·L and calculated using results indicate that the proposed method is suitable for the equation LOD � 3S /m, where S and m represent the separation and preconcentration of lead from water and b b standard deviation of three replicate blank signals and slope other samples. Journal of Analytical Methods in Chemistry 7 1.4 1.2 1.15 1.1 1.2 1.05 0.95 0.8 0.9 0.8 0.85 A A 0.6 0.8 0.6 5.3 5.4 5.5 5.6 5.7 pH 0.4 0.4 0.2 0.2 0 0 0 1020304050 02468 10 12 pH Adsorption time (min) (a) (b) 1.4 1.4 1.2 1.2 1 1 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 HCl HCl HCl HCl HCl HNO3 0 1020304050 (0.01 ) (0.1 ) (0.3 ) (0.5 ) (1.0 ) (0.01 ) Desorption time (min) -1 Eluent (mol L ) (c) (d) 1.2 1.2 1 1 0.8 0.8 A 0.6 A 0.6 0.4 0.4 0.2 0.2 0 0 Recycles Sorbent amount (mg) (e) (f) Figure 3: (a) Efect of sample solution pH. (b) Efect of adsorption time. (c) Efect of the type and concentration of eluent. (d) Efect of desorption time. (e) Efect of sorbent amount. (f) Reusability of the adsorbent. 8 Journal of Analytical Methods in Chemistry −1 Table 2: Efect of interfering ions in the presence of 10 μg·L lead. Interfering ions Interferent/Pb(II) ratio Recovery (%) 2+ Cd 100 96 3+ As 300 98 2+ Hg 200 97 2+ Ni 200 100 2+ Cu 200 99 3+ Fe 400 97 2+ Co 150 98 2+ Mn 200 99 2+ Ca 150 98 2+ Mg 150 97 Na 200 99 K 200 97 2+ Zn 100 96 1.2 1 y = 0.0178x + 0.158 R = 0.9943 0.8 A 0.6 0.4 0.2 0 10 20 30 40 50 60 -1 Concentration (µg L ) 2+ Figure 4: Calibration curve for Pb ion determination. Table 3: Comparison of the proposed method with some diferent methods for determination of lead. −1 −1 Preconcentration technique Detection method LOD (μg·L ) Linear range (μg·L ) Reference SPE FAAS 0.29 1.0–20 [27] SPE FAAS 0.7 3.0–100 [28] CPE UV-vis 3.9 5.0–100 [29] SPE GF AAS 0.0008 0.005–0.5 [30] SPE GF AAS 0.11 10–250 [5] DLLME GF AAS 0.003 0.01–3 [31] SPE GF AAS 0.026 0.1–50 Tis work CPE: cloud point extraction; DLLME: dispersive liquid-liquid microextraction. Table 4: Determination of lead in real samples. −1 −1 −1 Samples Measured (μg·L ) Added (μg·L ) Found (μg·L ) Recovery (%) RSD (n � 3) 5 5.24 98.87 0.50 Tap water 0.30 10 10.27 99.71 0.25 5 5.01 100.2 1.2 Mineral water ND 10 10.13 101.3 3.02 5 5.30 99.06 1.89 Lettuce 0.35 10 10.32 99.7 1.55 ND (not detected) � below the limit of detection. BTC@Fe O nanocomposite signifcantly enhanced the 4. Conclusions 3 4 sensitivities of lead. By applying an external magnetic feld, In this work, magnetic MOFs were applied for the separation magnetic MOFs are separated from solution which avoids and preconcentration of trace amounts of lead from water fltration and centrifugation steps. Te adsorbed ion of and other samples. Te carboxyl groups present in Cu- Pb(II) was ready to be desorbed with HCl solution followed Journal of Analytical Methods in Chemistry 9 of trace heavy metals in natural water,” Te Analyst, vol. 140, by GF AAS analysis. 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Journal of Analytical Methods in ChemistryHindawi Publishing Corporation

Published: Jan 17, 2023

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