Magnetic metal-organic frameworks for efficient removal of cadmium(II), and lead(II) from aqueous solution

Efficient and convenient methods for the removal of toxic heavy metal ions especially Cd(II) and Pb(II) from aqueous solutions is of great importance due to their serious threat to public health and the ecological system. In this study


Introduction
Over the last decade, numerous environmental pollutants have been released into the environment due to the accelerating pace of industrialization [1,2].Among environmental contaminants, toxic heavy metal ions are considered as a collection of hazardous and destructive pollutants [3].Due to their high solubility, they are easily absorbed by aquatic species, resulting in increased concentration [4].Toxic heavy metals such as cadmium (Cd) and lead (Pb) have a long biological half-life of 7-16 years and 150 years in the human body, respectively, and can cause harmful effects [5,6].Various techniques for the removal of Cd(II) and Pb(II) from aqueous solutions including precipitation [7], membrane [8], ion-exchange [9], solid phase extraction [10], phytoremediation [11], and electrochemical based methods [12], have been investigated over the last decades, however, these techniques have various disadvantages such as special working conditions, and complex contaminants removal processes.In contrast, adsorption techniques for the removal of contaminates from aqueous solutions stands out considering its valuable properties such as high efficiency, simple design, easy regeneration, good selectivity, and low operational cost [13,14].Many adsorbents including molybdenum disulfide nanosheets decorated with cerium oxide nanoparticles [15], biomass [16], magnetic nanoparticles [17], and polymers [18], have been extensively investigated for the removal of Cd(II) or Pb(II) from aqueous solutions; however, these materials usually suffer from low removal efficiencies or low adsorption capacities, which have largely limited their effectiveness.Furthermore, they require a tedious process for separation and lack of recyclability [19].Recently, metal-organic frameworks (MOFs) have been explored for the removal of heavy metal ions from aqueous solutions, due to their many attractive characteristics, such as high surface area, large pores, high chemical, and solvent stability [20][21][22][23].However, MOFs separation after the adsorption process is still a challenge due to the need to use a filter or centrifuge [20].Therefore, an easier separable design of MOFs adsorbents for the removal of heavy metal ions contaminants from aqueous solutions, that can overcome the weakness associated with the existing adsorbents, is essential for widespread applications.Magnetic MOFs have received considerable interest in contaminants removals since they can easily be separated from the aqueous solutions using an external magnet field, thus avoiding extra experimental steps, such as filtration of centrifugation [24,25].Composite MOF materials show great promise for many applications however their synthesis can be challenging, especially when considering process scale up [26].The method chosen should produce a material of sufficient quality for the specific application.
In this work, we report the synthesis and full characterization of two magnetic nanocomposites, namely Fe 3 O 4 @ZIF-8 (zeolitic imidazolate frameworks-8) [27] and Fe 3 O 4 @UiO-66-NH 2 (UiO refers to university in Oslo) [28].The synthesized magnetic nanoparticles Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 offer high adsorption efficiencies for Cd(II), and Pb (II) from aqueous solutions.The influence of key factors to be able to design an efficient adsorption process, including, pH value of the metal solution, adsorption contact time, adsorption capacity, effect of temperature, and adsorbent recyclability were investigated.Furthermore, kinetics, thermodynamics, and the mechanisms of the adsorption processes were discussed.

Synthesis of the magnetic Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 nanocomposites
The magnetic nanoparticles were synthesized using a coprecipitation method with a slight modification [29][30][31].Typically, an aqueous solution of FeSO 4 (0.63 g) and Fe(NO 3 ) 3 (1.73 g) were prepared using deionized water (25 mL).Then a solution of NH 4 OH (25 mL, 30%) was added and the solution was left for 1 h at 90 • C. The synthesized magnetic nanoparticles were separated using an external magnet.The materials were washed several times using deionized water (2 ×30 mL).The magnetic nanoparticles were then again collected using an external magnet and the samples were dried under vacuum at 85 • C overnight.
UiO-66-NH 2 material was synthesized using a solvothermal method with modifications [32].First, ZrCl 4 (37 mmol) and BDC-NH 2 (33 mmol) were dissolved in acetic acid (10 mL) and DMF (25 mL).The reaction container was heated at 120 • C for 24 h.Subsequently, the product was collected via centrifugation (12000 rpm), washed three times with DMF, and soaked in methanol at 60 • C for three days with replacing the soaking solvent every 24 h to exchange DMF, and finally dried at 100 • C overnight.
Zeolitic imidazolate framework (ZIF-8) particles are synthesized according to our previously reported procedure [33].Briefly, in a glass scintillation vial, TEA (0.70 mmol) was added to Zn(NO 3 ) 2 ⋅6 H 2 O solution (0.67 mmol) followed by addition of 2.3 mL of the Hmim solution (48.0 mmol).The reaction mixture was stirred for 30 min.The product was collected using centrifugation (12000 rpm).The materials were washed several times using deionized water (3 ×50 mL), and finally dried at 85 • C overnight.

Characterization of materials
The phases of the formed materials were identified using X-ray diffraction (PANanalytical X'Pert PRO diffractometer coupled with Cu K α1 radiation, λ = 1.5406Å.The morphology and particle size of Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 were investigated using transmission electron microscopy (TEM, TEM-2100, JEOL, Japan, accelerating voltage 200 kV.Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) for elemental analysis and mapping were measured using TM-3000 (Hitachi).N 2 adsorption− desorption isotherms (77 K) were recorded using a Micromeritics ASAP (Micromeritics Ltd., England).The samples were evacuated at 120 • C for 3 h in vacuum before measurement.The pore volumes were calculated at P/P 0 of 0.98 of the N 2 sorption isotherms at 77 K.The pore size, the micropore volumes and external surface area were calculated using the t-plot method.Pore size distribution of the materials was determined using a DFT method [34].The total concentrations of metal ions were determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Fisher iCAP 7400, USA).The pH value of the solution was measured by a pH-meter (ORION Star A211, Thermo-Scientific™, USA).Zeta potential of Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 were determined using a Zetasizer Nano ZS (Malvern, UK).The elemental composition of samples and information about the bonding environment was obtained by X-ray photoelectron spectroscopy (XPS, Physical Electronics Quantum 2000, Al K α X-ray source).Powder samples were spread and adhered on adhesive carbon tape.The spectral energies were calibrated by setting the binding energy of the C-C as a reference at 285.0 eV.Data analysis was performed using the MultiPak software (Physical Electronics).

Batch adsorption experiments
Stock solutions of Cd(II) and Pb(II) were prepared using Cd(NO 3 ) 2 ⋅4 H 2 O and PbCl 2 salts.The pH of the stock solutions was adjusted using NaOH (0.1 M) and HCl (0.1 M).Adsorption experiments were carried out by adding the desired dosage of Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 to 10 mL of Cd(II) and Pb(II) solutions at a given concentration and the suspensions were stirred at a constant speed of 300 rpm for a certain amount of time.The magnetic framework composites Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 were separated from the aqueous solution with an external magnet, and the concentration of Cd(II) and Pb(II) in the supernatant was determined using ICP-OES.The adsorption capacity (q e , mg⋅g − 1 ), and the removal percentage (%) were calculated as follows: A.F. Abdel-Magied et al.
where C 0 and C e (mg⋅L − 1 ) are the initial and equilibrium concentration in aqueous solution, respectively, V (mL) is the volume of the aqueous phase, and m (g) is the mass of adsorbent.The adsorption experiments were performed in duplicate/triplicate and the average values of total adsorption are reported, with relative errors less than 5%.

Desorption experiments
The regeneration and recyclability of the Fe 3 O 4 @ZIF-8 and  To regenerate the materials, the loaded Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 were stirred in acetonitrile solution for 10 h, then separated from the aqueous solution via an external magnet, and finally dried at 85 • C overnight in an oven for the next usage.

Characterization of Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2
Two MOFs, UiO-66-NH 2 , and ZIF-8, were used to be conjugated with Fe 3 O 4 magnetic nanoparticles (MNPs).The phases of the materials were identified by XRD (Fig. 1).The collected data showed that the XRD patterns of the prepared materials are in good agreement with the simulated pattern, confirming the formation of ZIF-8, and UiO-66-NH 2 .
The XRD patterns show no extra phases and confirm that the materials have high purity.The XRD for the prepared Fe 3 O 4 @ZIF-8 shows high stability after soaking in water for 3 days (Fig. S1).The morphology and particle size of the formed composite was evaluated using TEM (Fig. (2ab)).TEM images show distribution of magnetic nanoparticles on the crystal surfaces of ZIF-8 and UiO-66-NH 2 .Porosity of the materials was determined using N 2 adsorption-desorption isotherms (Fig. 2(c-d), Table S1).The collected data showed Brunauer-Emmett-Teller (BET) specific surface area of 160 m 2 ⋅g − 1 , and 287 m 2 ⋅g − 1 for Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 , respectively.The Langmuir surface areas were determined to be 213 m 2 ⋅g − 1 , and 368 m 2 ⋅g − 1 for Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66− NH 2 , respectively.The analysis reveals an external surface area of 116 m 2 ⋅g − 1 , and 103 m 2 ⋅g − 1 with pore volume of 0.33 and 0.29 cm 3 ⋅g − 1 for Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66− NH 2 , respectively (Table S1).Pore size distribution shows the presence of mesopores with pore volume of 0.21-0.31cm 3 ⋅g − 1 that were created due to the interparticle pore between MOFs and MNPs, as can be seen in the TEM images (Fig. 2(d)).SEM image and EDX analysis of the prepared composites Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 are shown in Fig. S2, and  S3, respectively.The FT-IR spectra for UiO-66-NH 2 , ZIF-8, Fe 3 O 4 MNPs, and their magnetic nanocomposite are shown in Figure S4.They showed characteristic bands at a wavenumber of 541 cm − 1 corresponding to Fe-O bond.The FT-IR spectrum of UiO-66-NH 2 shows characteristic bands at wavenumber the peak at 751 cm − 1 that can be assigned to the symmetric vibration of O-Zr-O.The FT-IR spectrum of ZIF-8 displays a characteristic band at 421 cm − 1 corresponding to the Zn-N bond [35,36].The spectra of the composite show the characteristic feature of both components.

Effect of initial pH on Cd(II) and Pb(II) removal
The adsorption of metal ions onto Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 are strongly dependent on solution pH and the Zeta potential of the materials.The adsorption of Cd(II), and Pb(II) was studied for initial pH values in the range of 4-6.Cadmium and lead will precipitate at pH > 8 and > 6, respectively [37], moreover, the structure of Fe 3 O 4 @ZIF-8 in aqueous solutions is only stable at pH > 2.5 [25].The effect of pH value on the competitive adsorption capacity of Cd(II), and Pb(II) using Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 is shown in Fig. 3(a) and (b), respectively, and the removal percentage for Cd(II) and Pb(II) using Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 is shown in Fig. S5(a) and (b), respectively.As expected, the pH had a significant influence on the adsorption capacity of Cd(II) and Pb(II) onto Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 .At higher pH values, higher adsorption capacities for Cd(II) and Pb(II) were obtained.Such behavior can be attributed to the isoelectric point (IEP) for Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 as shown in Fig. S6.The measured IEP values for Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 were 5.01 and 4.98, respectively.When the pH of the solution is higher than the IEP, the magnetic MOFs composites surface is negatively charged, and this may enhance the electrostatic interactions between Cd(II) and Pb(II) and Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 , leading to higher adsorption capacities.On the contrary, at lower pH values (pH<IEP), the surface charge of Fe 3 O 4 @-ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 is positive, restricting the approach of Cd(II) and Pb(II), leading to low adsorption capacities.The effect of pH on the adsorption of Cd(II) and Pb(II) is in agreement with previously reported findings [38,39].In consideration of high adsorption capacities, pH 6 was selected for subsequent studies.

Adsorption kinetics
The effect of contact time on the competitive adsorption of Cd(II) and Pb(II) onto Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 is presented in Fig. S7(a) and (b), respectively.A series of identical experiments were carried out varying the contact time (5 min− 24 h).The adsorption amount (q t , mg⋅g − 1 ) increased rapidly in the first 2 h and about 87% of Cd(II) and 79% of Pb(II) can be adsorbed in case of Fe 3 O 4 @ZIF-8, while for Fe 3 O 4 @UiO-66-NH 2 , 76% of Cd(II) and 77% of Pb(II) were adsorbed.Then, for adsorption onto Fe 3 O 4 @ZIF-8 equilibrium is reached after 5 h for the adsorption of Cd(II) reaching a q e of 27 mg⋅g − 1 , and after 8 h for adsorption of Pb(II), reaching a q e of 37 mg⋅g − 1 .In case of Fe 3 O 4 @UiO-66-NH 2, 35 mg⋅g − 1 of Cd(II) and 40 mg⋅g − 1 of Pb(II) were absorbed after 5 and 8 h, respectively.
To describe the kinetics of Cd(II) and Pb(II) adsorption on the  3) and (4), respectively.
where k 1 (min − 1 ) and k 2 (g⋅mg − 1 ⋅min) are the pseudo-first order and pseudo-second order rate constants of adsorption and q t (mg⋅g -1 ) is the adsorption capacity at a given time t.The calculated kinetic parameters are listed in Table 1.Adsorption of Cd(II) and Pb(II) using Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 follows the pseudo-second order model, since the non-linear regression correlations (R 2 ) were higher than those obtained from the pseudo-first-order model.In addition, the calculated q e,cal values obtained from the pseudo-second order model are much closer to the experimental q e,exp , which further demonstrate that the pseudo-second order model is more appropriate for describing the adsorption process using Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 , suggesting that the rate limiting step in governed by surface adsorption [13,14,20].

Adsorption isotherms
The adsorption capacities of Cd(II) and Pb(II) using Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 are shown in Fig. 5(a) and (b), respectively.The adsorbed amount of Cd(II) and Pb(II) onto Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 at equilibrium increase gradually with increasing initial concentrations of the ions, then reaching a plateau where after the respective maximum adsorption capacity under the applied conditions are reached.
The obtained data for the adsorption of Cd(II) and Pb(II) onto Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 were analyzed using two different isothermal adsorption models, namely Langmuir isotherm and Freundlich isotherm models, respectively (Fig. 5).The calculated adsorption coefficients and the nonlinear regression coefficient (R 2 ) values for each model are shown in Table 2.The nonlinear Langmuir and Freundlich isotherm models can be described as shown in Eqs. ( 5) and (6).
where q max (mg⋅g -1 ) is the maximum adsorption capacity, k L (L⋅mg -1 ) is the Langmuir equilibrium constant, K f (mg 1− n ⋅L n ⋅g -1 ) and n are constants for given adsorbate and adsorbent at a particular temperature.The correlation coefficient (R 2 ) values of the Langmuir isotherm model are higher than that of the Freundlich isotherm model, suggesting that a monolayer of Cd(II) and Pb(II) is formed in the adsorption onto Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 respectively.The maximum adsorption capacities (q max ) for Cd(II) and Pb(II) onto Fe 3 O 4 @ZIF-8 calculated from the Langmuir isotherm model are 370 and 666.7 mg⋅g − 1 , respectively.For Fe 3 O 4 @UiO-66-NH, a q max of 714.3 and 833.3 mg⋅g − 1 for adsorption of Cd(II) and Pb(II) respectively, were obtained.

Adsorption thermodynamics
The adsorption capacities for Cd(II) and Pb(II) at different temperatures are represented in Fig. S8.The q e values increased with increasing the adsorption temperature due to the higher energy of the system, facilitating the adsorption process, and thus, the adsorption of Cd(II) and Pb(II) are endothermic processes.
The change in q e as a function of temperature allow us to determine the thermodynamic parameters (enthalpy change (ΔHᵒ), entropy change (ΔSᵒ) and standard free energy (ΔGᵒ)) of the adsorption process using Eqs.7 and 8.
where R is the universal gas constant (8.314J⋅mol -1 ⋅K -1 ), and T (K) is the absolute temperature, and K d is the equilibrium constant.When using Van't Hoff's equation for calculating the thermodynamics parameters, a very important remark is that K d must be dimensionless.Thus, the Langmuir constant, k L , was corrected according to Eq. 9 to adjust K e [40,41] (Fig. 6, and Table 3).
where k L (L⋅mg -1 ) is the Langmuir equilibrium constant, M a (g⋅mol -1 ) is the molar mass of adsorbate, M 0 (mol⋅L -1 ) is the standard concentration of adsorbate (usually used as 1.0 mol⋅L -1 for diluted solutions), γ e is the chemical activity of the adsorbate (which can be assumed to be 1.0 for dilute solutions).The calculated values of ΔGᵒ at each temperature are negative, suggesting that the adsorption processes of Cd(II) and Pb(II) are spontaneous.For ΔHᵒ the values are positive, in combination with the fact that the adsorption capacities of Cd(II) and Pb(II) increases with increasing the temperature, this demonstrates that the adsorption process is endothermic.In addition, the positive values of ΔSᵒ indicates that the adsorption of Cd(II) and Pb(II) using Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 is entropy driven, and rising the temperature will increase the

Langmuir model
Freundlich model Metals q max,cal (mg⋅g − 1 )  disorder of the whole system.

Effect of competing cations
Alkaline divalent cations including Ca(II) and Mg(II) exist in wastewater and natural water in high concentrations and may compete with toxic heavy metal ions for the available binding sites of the adsorbent materials.Therefore, evaluating the adsorption of Cd(II) and Pb(II) in the presence of other competing cations such as Ca(II) and Mg (II) using Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 at various concentrations were assessed.As can be seen from the tabulated results in Table 4, the removal efficiencies of Cd(II) and Pb(II) using Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 were slightly influenced by adding Ca(II) and Mg (II) as competing cations.Thus, Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 are promising adsorbents for the removal of Cd(II) and Pb(II) from aqueous solutions, even in the presence of Ca(II), and Mg(II) at high concentrations.

Proposed adsorption mechanism
Based on the adsorbents, there are several proposed mechanisms for the removal of target metal ions.Complexation with the frameworks, electrostatic interactions, and ion exchange may take place during the adsorption process.To gain further insight into the possible mechanism for the adsorption of Cd(II) and Pb(II) onto Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 , XPS analysis of the MOFs after adsorption were carried out.The XPS survey spectrums demonstrates that peaks of Zn, Fe, O, N, and C are found in Fe 3 O 4 @ZIF-8 (Fig. 7(a)), while peaks of Zr, Fe, O, N, and C are found in Fe 3 O 4 @UiO-66-NH 2 (Fig. 7(d)).After adsorption of Cd(II) and Pb(II), the high-resolution XPS spectrum of Fe 3 O 4 @ZIF-8 shows peaks at binding energies of 412.1 eV (corresponding to Cd 3d 3/2 ) (Fig. 7(b)), 137.2 and 142.1 eV, (corresponding to Pb 4 f 7/2 , and Pb 4 f 5/2 , respectively) (Fig. 7(c)), indicating that both Cd (II) and Pb(II) were adsorbed to the Fe 3 O 4 @ZIF-8 sample.For the Fe 3 O 4 @UiO-66-NH 2 sample, peaks of Cd 3d 3/2 , Pb 4 f 7/2 , and Pb 4 f 5/2 can be found in the high-resolution XPS spectrums of Fe 3 O 4 @UiO-66-NH 2 after adsorption of Cd(II) and Pb(II).The binding energies of Pb4f 7/2 and Pb4f 5/2 in Pb(NO 3 ) 2 are 139.6 eV and 144.5 eV, respectively, while in this study the binding energies of Pb 4 f 7/2 and Pb 4 f 5/2 are approximately 137 eV and 142 eV, respectively, in both Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 samples.Thus, compared with the binding energies of Pb 4 f in Pb(NO 3 ) 2, those of Pb 4 f in Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 samples significantly shift to lower range (approximately 2 eV shift).This significant binding energies shift of Pb 4 f can be attributed to the formation of strong affinities between Pb(II) and the prepared materials.The energy separation of 4.9 eV and 5.2 eV between the Pb 4 f 5/2 and 4 f 7/2 for adsorption onto Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 , respectively, confirmed the coordination interaction between Pb(II) and the adsorbent.The obtained XPS analysis for Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 before and after the adsorption process, indicates that the used magnetic MOFs in this study can adsorb Cd(II) and Pb(II) via a coordination interaction between Cd(II) and Pb (II) and the imino groups of 2-methylimidazole of the ZIF-8 shell of Fe 3 O 4 @ZIF-8 [25], and via a coordination interaction between Cd(II) and Pb(II) and the amino group of Fe 3 O 4 @UiO-66-NH 2 [42].No extra peaks after the adsorption for the possible formation of new solid phases could be observed by XPS analysis, which demonstrates that the adsorption of Cd(II) and Pb(II) from aqueous solutions using Fe 3 O 4 @-ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 is mainly attributed to coordination reactions.

Recycling and desorption studies
To evaluate the potential reusability of the synthesized magnetic Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 composites for Cd(II) and Pb(II) removal, the regeneration process was carried out using acetonitrile solution for 10 h.As shown in Fig. S9, after four-cycles of regeneration, the Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 still exhibits almost the same initial removal percentage, indicating the good recycling ability of Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 .The results confirmed the high stability and reusability of the synthesized magnetic Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 adsorbents.

Comparison with other adsorbents
The q max determined in this study for adsorption of Cd(II) and Pb(II) onto both Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66− NH 2, compare well with other MOFs lacking MNPs (Table 5).However, the composite magnetic materials investigated in the present study can be separated easily from aqueous solutions using an external magnet, which is favorable for industrial implementation.Furthermore, no extra reagents is required during adsorption [43].The adsorption and desorption conditions are simple and can be easily scaled-up.Magnetic MOFs show high performance compared to various materials such as Fe 3 O 4 magnetic nanoparticles modified with 3-aminopropyltriethoxysilane (APS) and copolymers of acrylic acid (AA) and crotonic acid (CA) [44], and Fe 3 O 4 @Cu 3 (btc) 2 [45].

Conclusions
In this work, two magnetically-separable metal-organic frameworks, Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 , have been synthesized and fully characterized.The magnetic Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-  66-NH 2 exhibits excellent adsorption performance for Cd(II) and Pb(II) removal from aqueous solutions.In addition, the Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 adsorbent showed excellent reusability, being highly efficient and effective after at least four consecutive adsorptiondesorption cycles, indicating its impact for the removal of hazardous metal ions from aqueous solutions.Mechanism investigation revealed that coordination reactions were the main mechanisms involved in the removal of Cd(II) and Pb(II) by Fe 3 O 4 @ZIF-8, and Fe 3 O 4 @UiO-66-NH 2 from aqueous solution.Thus, our results may pave a way for developing an easily separable adsorbents for efficient removal of Cd(II) and Pb(II) from aqueous solutions.

Table 4
Cd(II) and Pb(II) removal percentages using Fe 3 O 4 @ZIF-8 and Fe 3 O 4 @UiO-66-NH 2 in the presence of Ca(II) and Mg(II) as competing cations.

Table 5
Comparison of the adsorption of Cd(II) and Pb(II) from aqueous solution using various magnetic adsorbents.