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

In this study, a magnetic metal-organic framework (MOF) was synthesized based on magnetic Fe3O4, Cu(II), and benzene-1,3,5-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 filtration or centrifugation was necessary. The analyte ions were eluted by HCl 0.5 mol·L−1 and analyzed via graphite furnace atomic absorption spectroscopy. The prepared sorbent was characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and Fourier transform-infrared (FT-IR) spectroscopy. Under optimal experimental conditions, the method had a linear range of 0.1–50 μg·L−1. The limits of detection and quantitation for lead were found to be 0.026 and 0.08 μg·L−1, respectively. The results showed that the prepared sorbent has high selectivity for Pb2+ even in the presence of other interfering metal ions.


Introduction
Lead is a very stable and nonbiodegradable element that accumulates in the environment [1]. It is considered as one of the toxic heavy metals which can cause damage to human health, even at low concentrations. Many diseases such as anemia, cardiovascular and developmental disorders, and muscle paralysis are related to Pb(II) and can harm the liver, kidneys, the central nervous system, the endocrine system, the hematopoietic system, and the reproductive system [2,3]. Humans are generally exposed to Pb through breathing air, drinking water, and eating food. Te maximum allowable level of lead in drinking water by the US Environmental Protection Agency (EPA) and the European Union (EU) is 15 μg·L −1 and 10 μg·L −1 , respectively. Consequently, developing an efective and highly efcient method to monitor this hazardous element is necessary.
Numerous techniques have been used for the determination of lead, such as the electrochemical method [4], electrothermal atomic absorption spectrometry (ETAAS) [5], inductively coupled plasma optical emission spectroscopy (ICP OES) [6], and fame atomic absorption spectroscopy (FAAS) [7]. However, their selectivity and sensitivity are insufcient for direct determination of lead in real samples at very low concentrations, and on the other hand, most of these samples have complex matrices [8]. In order to overcome these problems, preconcentration and separation procedures such as solid phase extraction (SPE) [9,10], liquid-liquid extraction [11], cloud point extraction [12], and ion exchange [13] have been performed.
Among the mentioned methods, SPE is the most common technique applied for the preconcentration and extraction of Pb(II) from environmental and food samples. SPE has many obvious advantages, such as high enrichment factor, high recovery, low consumption of organic solvents, and convenience of operation [14,15]. It is usually acknowledged that sorbent plays a signifcant role in the SPE technique because of analytical sensitivity, precision, and selectivity.
Metal-organic frameworks (MOFs) are employed as excellent novel adsorbents owing to their signifcant characteristics such as high specifc surface area, high thermal and chemical stability, adjustable pore sizes, and rich functionalities [16,17]. MOFs are porous crystalline materials that are constructed from metal ions and organic linkers via strong coordination bonds and arranged in the form of a network structure [18]. MOFs have signifcant applications in the feld of gas storage [19], drug delivery [20], catalysis [21], and adsorption [22]. MOFs have shown desirable adsorption for heavy metals and drugs [23,24]. Filtration and centrifugation are the methods used for the recovery of MOFs. But these methods, because of the slow speed, high cost, and cumbersome operating steps, limit the large-scale application of MOFs. Terefore, preparing the MOF material that can be separated easily is essential.
Magnetic MOFs, due to their superparamagnetic properties, can be easily separated from the matrix by employing a strong external magnetic feld and redispersed in the eluent once the external magnetic feld is removed. Magnetic MOFs avoid taking steps such as fltration and centrifugation.
In the current work, we have synthesized magnetic MOFs by assembling Cu(II) and benzene-1,3,5-tricarboxylic acid (Cu-BTC) thin layers bonded through carboxyl groups on the surface of magnetic Fe 3 O 4 nanoparticles for the extraction of trace Pb(II) ion followed by graphite furnace atomic absorption spectroscopy. Tese Cu-BTC@Fe 3 O 4 nanocomposites have large pores and cavities that can signifcantly increase the surface area. Te superparamagnetic properties of Fe 3 O 4 contribute to the rapid separation of the adsorbent from the matrix solution. Te carboxyl groups present in Cu-BTC provide more bonding sites for the lead ion. Based on these considerations, the preconcentration and determination of lead ions in the diferent real samples can be readily achieved.

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. Te instrumental parameters and graphite furnace temperature conditions are presented in Table 1. Te pH of all solutions was measured with a pH-meter model 713 from Metrohm. Te FT-IR spectrometer (Vector-22 Bruker spectrophotometer, Switzerland) was used for functional groups of magnetic MOFs. Te scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) images were obtained with Mighty-8 instrument (TSCAN Company, Prague).

Sample Preparation.
Te 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 fltration. For preparing spiked samples of lettuce, 0.5 g of lettuce was digested after the addition of 10 mL of HNO 3 (65%). Ten, the mixture was centrifuged and the supernatant was fltered through a flter (0.45 μm). Te 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 . Te analysis was carried out as indicated in the procedure section.

Synthesis of Carboxyl Functionalized Fe 3 O 4 Nanoparticles.
Fe 3 O 4 nanoparticles were prepared by a hydrothermal method [25]. Briefy, 4.44 g of FeCl 3 .6H 2 O and 1.73 g of FeCl 2 .4H 2 O were dissolved in 80 mL of water. Ten, in the refux 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. Ten, 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 refux and reacted for 60 min with continuous stirring. Ten, it was cooled to room temperature and the black precipitate was isolated using an external magnetic feld and washed with ethanol and water.

Synthesis of Cu-BTC@Fe 3 O 4 Nanocomposite.
Synthesis of nano-scaled core-shell Cu-BTC@Fe 3 O 4 was achieved by a one-pot strategy [26]. At frst, 0.200 g of PVP and NaOH (0.1 mol·L −1 ) from 2 to 10. Ten, the solution was stirred for 5 min. Magnetic MOFs were separated from the sample solution using a magnetic feld, 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. Ten, analyte ions in the elution solutions were determined by GF AAS. Te measurement procedure of the proposed strategy is shown in Scheme 2. Te surface morphology of the prepared Cu-BTC@ Fe 3 O 4 nanocomposite was investigated using scanning electron microscopy (SEM). As shown in Figure 1(b), the prepared sorbent has nanobelt morphology and smooth surface with lengths of 10-20 μm. After adsorption of Pb(II), surface became rougher, indicating that Pb(II) ion was adsorbed on magnetic MOF (Figure 1(c)). Te EDS spectrum of Cu-BTC@Fe 3 O 4 nanocomposite is shown in Figure 1

Optimization of the Efective Variables.
Several parameters that may afect the preconcentration and extraction process, such as pH of the sample solution, adsorption time, efect of the type, concentration of eluent, desorption time, sorbent amount, and reusability of the adsorbent, were optimized. Te optimization was carried out on 2 mL of 50 μg·L −1 lead aqueous solution.

Efect of Sample Solution pH.
Te pH has a signifcant role in the SPE studies of metal ions. Terefore, the pH aqueous sample solution on the preconcentration of lead ions was changed in the range of 2.0-10.0. As shown in Figure 3(a), the absorbance of Pb increased with the increase of sample pH up to 5.5 and then decreased at high pH. However, for pH > 5.5, due to the formation of Pb(OH) 2 , absorbance was decreased. At pH < 5.5, the COO − ions present on the surface sorbent can bind positive Pb(II) ions through electrostatic interactions, but at very low pH, hydrogen cations can interact with the oxygen electrons. Terefore, pH 5.5 was selected as the optimum pH.

Efect of Adsorption Time.
Te efect of the extraction time on the adsorption of Pb(II) was examined in the range of 3 to 45 min. As presented in Figure 3(b), the results showed that after 5 min, absorbance reaches a maximum and remains almost constant from 5 to 45 times, indicating that the process of sorption is very quick. Finally, 5 min was used for further experiments.

Efect of the Type, Concentration of Eluent, and Desorption Time.
Eluent solution and desorption time as essential factors in the preconcentration and extraction process were studied. Eluent solution must be able to dissolve the target analyte and overcome the bond between the analyte and the adsorbent. According to the results of the efect of pH, acid solution may be a good choice as an eluent. Terefore, a series of acidic solutions of HCl and HNO 3 with diferent concentrations (0.01, 0.1, 0.3, 0.5, and 1.0 mol·L −1 ) were employed. As shown in Figure 3(c), when HCl 0.5 mol·L −1 was used, the highest desorption efciency was achieved. Finally, HCl 0.5 mol·L −1 was chosen as the eluent to ensure complete desorption in subsequent studies. In order to complete 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 desorption time.

Efect of Sorbent Amount.
Te efect of sorbent amount on the preconcentration of lead was investigated in the range of 1.0-4.0 mg. As shown in Figure 3(e), absorbance increased to 3.0 mg and remained constant. Tis development is due to an increase in the surface area and available sites for the adsorption of the analytes. Terefore, 3.0 mg was used in all subsequent experiments.

Reusability of the Adsorbent.
Repeated experiments were performed to check the reusability of Cu-BTC@Fe 3 O 4 nanocomposites. After collecting the used adsorbent, Pb(II) was desorbed from the adsorbent by treatment with HCl 0.5 mol·L −1 . As shown in Figure 3(f ), the adsorbent stability was good and no signifcant change was observed for the absorbance of Pb 2+ up to 5 adsorption-desorption cycles. Tus, these results demonstrate that Cu-BTC@Fe 3 O 4 nanocomposites are efcient and cost-efective adsorbents with good potential for reuse.

Efect of the Interfering Ions.
Te efect of metal ions on the signal intensity was studied under optimal conditions. In  these experiments, 10 μg·L −1 of the lead standard solution was spiked with various concentrations of interfering ions such as Cd 2+ , As 3+ , Hg 2+ , Ni 2+ , Cu 2+ , Fe 3+ , Co 2+ , Mn 2+ , Ca 2+ , Mg 2+ , K + , Na + , and Zn 2+ . Ions were considered to interfere when the deviation of the recovery was more than ±5%. Te results in Table 2 show that the ions do not interfere in the determination of lead. Tus, the proposed method is robust for lead determination.

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 . Te 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 coefcient square (r 2 ) of 0.9943. Te limit of detection (LOD) was 0.026 μg·L −1 and calculated using equation LOD � 3S b /m, where S b and m represent the standard deviation of three replicate blank signals and slope of the calibration curve, respectively. Tis LOD was lower than that of the US Environmental Protection Agency (EPA) and the European Union (EU). Te limit of quantifcation, defned as LOQ � 10S b /m, was found to be 0.08 μg·L −1 . Te preconcentration factor, defned 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.

Analysis of Real Sample.
Te method was used for the determination of lead in tap water, mineral water, and lettuce samples according to the standard addition method. Diferent concentrations of lead were spiked and analyzed using the proposed method. As shown in Table 4, the recoveries 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 results indicate that the proposed method is suitable for the separation and preconcentration of lead from water and other samples.

Conclusions
In this work, magnetic MOFs were applied for the separation and preconcentration of trace amounts of lead from water and other samples. Te carboxyl groups present in Cu-BTC@Fe 3 O 4 nanocomposite signifcantly enhanced the sensitivities of lead. By applying an external magnetic feld, magnetic MOFs are separated from solution which avoids fltration and centrifugation steps. Te adsorbed ion of Pb(II) was ready to be desorbed with HCl solution followed

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.