Introduction

The entry of effluents containing lead into freshwater is a threat to the environment, and accordingly, the removal of heavy metals from industrial wastewater is of great importance [1]. Numerous methods, such as chemical deposition and membrane processes, as well as electrochemical, evaporation, minerals, etc., are utilized in the removal and separation of heavy metal ions from aqueous solutions; all have disadvantages, such as high cost of chemicals and equipment, the impossibility of recycling the adsorbent and adsorbate, and low efficiency. Adsorption processes are one of the effective methods in the removal of lead. Among the adsorbents, mesoporous silica and MCM molecular sieves, due to the high surface area, high porosity, spatial heterogeneity of pore-size distribution, and controllable surface area, are considered. These features make such materials a good choice for the adsorbent. Changes can be made to the channel walls or bound of different groups to their surfaces for increasing and improving these features. Besides, high efficiency, nanostructured adsorbents, with a large specific surface area and higher adsorption potential, produce less waste and are recoverable and reusable, which reduce cost and make the process cost-effective [2]. Shahbazi et al., in a recent study, used magnetic iron nanoparticles for the removal of lead and its recycling [3]. Alizadeh et al., investigated the synthesis of 2-mercaptobenzothiazole functionalized magnetic Fe to remove heavy metals, such as lead [4]. Despite many advantages, none of these methods had a maximum efficiency in the removal of heavy metals, such as lead [5]. Metal-organic frameworks, as a new class of advanced porous materials, have wide applications in the processes of adsorption and separation of heavy metals. They are coordination polymer compounds using a metal as a node and organic ligands as a binder. The size and shape of cavities in these crystalline and porous compounds can be engineered. These frameworks can be synthesized by binding metal clusters, as coordination centers, to organic ligands, as binders of mineral metal ions with specific physical and chemical properties. The first metal-organic framework, called five, was introduced (1999) and synthesized (2001) by Yaghi et al [6]. They synthesized it by the solvent, thermal method using zinc as a metal, carboxylic acid as a binder, and dimethylformamide as a solvent. This compound decomposes at above 350-400 °C, indicating its high thermal stability. Metal-organic frameworks also have high selectivity, poor guest-host interaction (desorption at lower temperatures), and high chemical stability. The 1,3,5-benzenetricarboxylate is an organic ligand, which its structure is shown in Figure 1 [7].

Figure 1: Chemical structure of trimesic acid (C₉H₆O₆) BTC^3-

In the present study, due to the low toxicity and high biocompatibility of iron, lead adsorption was performed using self-assembled iron metal-organic frameworks. Fe-BTC, a semi-amorphous non-crystalline material with photocatalytic properties, is the first synthesized framework, in which the effects of ultrasonic temperature (50 and 70 °C) and irradiation time (90 and 120 minutes) were investigated on its synthesis. It is synthesized similar to iron (III). The synthesized Fe-BTC nanoparticles show good photodissociation dynamics, and its 70 and 120 specimens have the highest photocatalytic activity. The second framework (MIL-100 (Fe)) is an octahedral zeolite equivalent to iron trimesate. Observations indicated that the catalytic activity of Fe-BTC is significantly higher [8].

Material and methods

Materials

The chemicals used in the present study included:

FeCl₂4H₂O, FeCl₃6H₂O, Pb (NO₃)₂, NH₄OH, HNO₃, HCl, C₂H₅OH, Cu₃(BTC)₂, and pH regulators, including NaOH, HCl, and H₃BTC (trimesic acid)

Equipment

Magnetic stirrer (IKA RH basic2), vibration magnetometer (Egg Princeton Applied VSM), atomic absorption spectrometer (Perkin-Elmer), scanning electron microscopy (CambridgeS-360), thermogravimetric analyzer under N₂ atmosphere (Meter), Fourier-transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), ultrasonic bath, vacuum oven, a micro-analytical balance with 0.1 g accuracy, XRD device (Philips X-ray diffractometer with cobalt and copper generators), and desiccator were the equipment used in the study.

Methods

Synthesis of the metal-organic framework

Synthesis of MIL-100 (Fe) (Lavazza Institute)

Two primary solutions were used for the synthesis: a) a solution with pH = 11, containing 1.676 g (7.6 mM) trimesic acid dissolved in 23.72 g, NaOH 1 M (22.8 mM), and b) a solution with pH =2.7, prepared by dissolving 2.26 g (11.4 mM) of FeCl₂4H₂O in 97.2 g of water (Figure 2). The first solution was added dropwise to the second one while stirring, and a mixture with a molar ratio of Fe 1.5/H₂ O880/NaOH 3/H₃BTC 1 was synthesized (pH = 5.2). Stirring at room temperature was continued for 24 hours. The formed crystalline precipitates were separated by centrifugation at 3700 rpm, followed by washing in triplicates with water and once with ethanol. The specimen was dried at room temperature, and accordingly, 4.01 g of MIL-100 (Fe) powder with 76% efficiency was produced [9].

Figure 2: (Up) synthesis of MIL-100(Fe) and Fe-BTC [14]; (down) synthesis and the three-dimensional structure of MIL-100(Fe)

Figure 3: MIL-100 (Fe) structure by scanning tunneling electron microscope; a) at magnification

Synthesis of Fe-BTC

The semi-amorphous Fe-BTC is prepared similar to Basolite 300F, a commercial material. It was synthesized using two solutions: a) a colorless solution with pH ~ 11 that was prepared by dissolving 0.263 g of trimesic acid in 10/150 g of an aqueous solution sodium hydroxide (1M) and b) a yellowish-orange solution with pH 1.8 prepared by dissolving 0.508 g of FeCl₃(H₂O)₆ in 10 g of water; then the second solution was added dropwise, under a magnetic stirrer, to the first one (Figure 2). A solid brown-orange precipitate of Fe-BTC was produced. The suspension (pH ~ 1.2) was left for 10 minutes under the stirrer at room temperature (23 °C). The molar composition of the final mixture was exactly similar to that of MIL-100 (Fe) (1.5 Fe, 1H₃BTC, 3NaOH, 880 H₂O). The brown precipitate was separated by centrifugation and then washed with deionized water and ethanol and left to dry at room temperature.

After the identification of features and characterization of structures, the synthesized compound was utilized for the removal of lead at different concentrations. The concentration of lead was measured at 520 nm after and before the adsorption process by an atomic absorption spectrometer. The adsorbent value was within the range of 0.2-0.5 mg/L; the lead concentration was 50-150 mg/L, the temperature was 10-75 °C, and the pH was 3.5-12.5 (Figure 3) [10].

Result and Dissection

The specific area and diameter of pores in the adsorbent were investigated. Considering many unsaturated surfaces and active sites in the adsorbent, the lead adsorption rate was high at the beginning of the adsorbent and the lead cation contact processes. There is a significant amount of hydronium ion in the environment. On the other hand, the surfaces of synthesized adsorbent have benzenetricarboxylate ligands with three negative charges and a high affinity to neutralize, which due to the acidity of the environment, neutralize negative charges on the adsorbents and make the adsorbent stable. Besides, due to the presence of an electron-donating group on the benzene ring in the adsorbent ligand and higher electronegativity of lead than iron, the lead metal was trapped (electronegativity of lead is higher than that of iron (2.33 vs. 1.83); therefore, the removal of lead was non-electrostatic. The adsorption of lead continued until saturation and then it was fixed, which was due to the full removal of lead ions from the solution. Therefore, the efficiency of the process was highly dependent on the lead concentration in the solution.  Increased removal rate following the increase in adsorbent amount was due to more availability of surface areas on the adsorbent and more effective contacts between the vacant sites and lead cations in the aqueous solution; the removal rate even increases with increasing the active surface of the catalyst. The reason for this is the overlap of adsorption removal sites in the first phase, and then their synergy to make it stable in terms of the number of adsorption sites and adsorption rate.

Magnetization

The magnetization of the synthesized Fe-BTC metal-organic framework was calculated based on the Faraday law with a measurement range of zero Oe, using the Langevin equation (cm/g/s). According to Figure 4, the synthesized framework has magnetic properties, increasing with an increase in the magnetic field.

Figure 4: Hysteresis curve by Oe  [16]

Langevin equation (1) is as follows:

                                      (1)

Since the curve passed through the origin, coercivity field and magnetization hysteresis were not observed. So, the framework synthesized at room temperature was superparamagnetic particles (Table 1).

Table 1: Magnetic parameters of the framework

The lead absorption rate (mg/g) was calculated by equations (2) and (3), where Ce is the equilibrium and C0 the initial concentration of lead (ppm), M the result of dividing the weight of the adsorbent (g) by solution (L), and qe the absorption capacity (mg/g) [11].

Adsorption efficiency                                       (2)

The amount of lead absorbed                         (3)

Intensive distribution of particle sizes

According to the results, the synthesized specimen was 30-36 nm on average (a 50

Figure 5: Intensive distribution of particle sizes

Scanning tunneling electron microscopy

This microscope is used for imaging the external surfaces, including the shape and size of particles. It was used for direct examination and measurement of crystals, and the results were consistent with those of an X-ray analyzer. The images taken by this microscope showed that the synthesized Fe-BTC had a stable structure and consisted of amorphous quasi-cluster units in terms of internal morphology (Figure 7) [12].

Figure 6: Scanning electron microscope Fe-BTC; a) scanning electron, b) scanning tunne

Gravimetry

In terms of thermogravimetric analysis, the structure of MIL-100 (Fe) was compared with that of the commercially available compound (Figure 7). Analysis of an organic ligand is a one-step process; an ignorable difference was observed between them that can be attributed to the uncontrolled atmosphere of synthesis and the presence of ions (fluoride) in the commercial framework, replaced by hydroxide at room temperature in the synthetized specimen. Two-stage analysis of Fe-BTC at 307 °C and weight loss was exclusively attributed to the decomposition of H₃BTC and the residual of Fe₂O₃. The second stage of weight loss was related to the destruction of structures and conversion to iron (II, III) oxide. Removal of the organic ligand of MIL-100 (Fe) and synthetic MIL-100 (Fe) were observed in synthetic Fe-BTC at 403, 383, and 346 °C. The weight loss to the residual weight ratio were 1.53 and 1.68 in MIL-100 (Fe) and synthetic MIL-100 (Fe), respectively; it was also 2.22 for synthetic Fe-BTC, which was due to the excess amount of ligand in the structure of Fe-BTC. The thermal and structural resistance of the synthesized material was compared with its commercial form, using the same method. Observations indicated the excellent thermal resistance of structures.