Efficient Adsorption of Pb(II) from Aqueous Solutions by Metal Organic Framework (Zn-BDC) Coated Magnetic Montmorillonite

Composite adsorption materials combine the advantages of various adsorptive materials and compensate for the defects of single adsorbents. Magnetic montmorillonite (MMMT) shows good adsorption properties for Pb(II). In order to further improve the adsorption properties of MMMT, in this work, Zn-BDC, a kind of metal–organic framework (MOF), was modified onto the surface of MMMT by in situ polymerization. The composite material MMMT@Zn-BDC was characterized by Zetasizer, SEM, TEM, FTIR, XRD, VSM, and XPS. The influence of adsorption conditions on the adsorption capacity of MMMT@Zn-BDC for Pb(II) was examined, including the adsorbent dosage, pH of Pb(II) solution, initial concentration of Pb(II), and the temperature and adsorption time. Also, the adsorption mechanism was studied. The results of this study show that MMMT@Zn-BDC adsorbs Pb(II) via chemisorption. In addition, MMMT@Zn-BDC exhibits good potential for adsorbing Pb(II), including its high adsorption capacity (724.64 mg/g) and good recyclability.


Introduction
Heavy metal pollution in water is becoming a more serious problem with increasing industrialization. Heavy metal pollution is a serious threat to the human body, ecological environment, and economic development [1,2]. Heavy metals are highly toxic, are easily enriched in organisms, and cannot be degraded naturally [3]. Trace heavy metals cause serious harm to microorganisms, aquatic plants, and animals. They can also enter the human body through the food chain or drinking water and cause serious injury and disease [4,5]. Lead is one such heavy metal. Lead and its compounds are frequently used as raw materials for modern industrial transportation, they are also used in metallurgy, printing, military and electronics. Incidents of excessive Pb(II) in liquid food and water have been reported many times [6,7]. Lead poisoning is a kind of accumulative poisoning. With the increasing accumulation of lead in the human body, people have a higher risk of kidney damage, hematopoietic dysfunction, and mental impairment [8].
Heavy metal wastewater is usually treated by ion exchange, biological treatment, precipitation, membrane permeation and adsorption [9,10]. The adsorption method [11,12] is widely used because of its performance efficiency, simple operation, selectivity, lack of pollution and low cost. Magnetic adsorbent materials represent a new type of adsorbent and have been shown to effectively adsorb lead ions [13,14]. More importantly, applying a magnetic field can easily separate the adsorbent from the water, which is beneficial for reusing the materials and concentrating heavy metals during treatment.
the pH was adjusted to 9-10 using ammonia. After stirring for 30 min, the products were separated by external magnetic field. Next, the separated MMMT products were washed with alternating deionized water and ethanol, and finally, the product was dried in a freeze-drying box. Using this same method but without adding MMT, the resulting weight of Fe3O4 would be 1 g.

Preparation of MMMT@Zn-BDC
First, MMMT (1 g) was added to a round-bottomed flask containing 33 mL of DMF. Then, Zn(CH3CO)2 (1.699 g) was dissolved in this dispersion, and BDC (0.506 g) was separately dissolved in 33 mL of DMF. After stirring for 1 h, the dispersion and BDC solution were mixed. TEA (0.85 mL) was slowly added to the resulting suspension; then, after continuously stirring for 2 h, the reaction was stopped. This synthesis process is shown schematically in Figure 1. The products were separated and washed using the same process as used in the preparation of Zn-BDC, and finally, they were dried in a freeze-drying box.

Characterization
The surface morphology was recorded by scanning electron microscopy (SEM, S4800, Hitachi, Tokyo, Japan). A transmission electron microscope (TEM, JEM-2100, Lorentz, Tokyo, Japan) and energy dispersive spectrometer (EDS, GENESIS, EDAX, Mahwah, NJ, USA) were used to analyze the internal structures and composition of the materials. Nitrogen adsorption isotherms were obtained using Micromeritics ASAP 2020M. Functional groups of the adsorbent were determined using a Fourier transform infrared spectrometer (FTIR, Bruker, Karlsruhe, Germany). The crystallinity was determined by X-ray diffraction (XRD, Bruker, Karlsruhe, Germany). The magnetic properties were evaluated by vibrating sample magnetometry (VSM) analysis (SQUID-VSM, Quantum Design, Santiago, CA, USA). The elements of the adsorbent were analyzed by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, Shimadzu, Tokyo, Japan).

Adsorption Experiment
The mother Pb(II) solution was prepared by Pb(NO3)2, and the resulting Pb(II) concentration was 1000 mg/L. Various Pb(II) solutions were diluted from this mother liquor. A Pb(II) solution (20 mL, 200 mg/L) was placed into several flasks, and various dosages of MMMT@Zn-BDC from 5 to 20 mg were separately added to these flasks. HNO3 and NaOH (0.1 mol/L) were applied to adjust the pH of the Pb(II) solution from 2 to 6. Then, the flasks were placed in an oscillator; the speed of revolution was 150 rpm. After the adsorption finished, the adsorbent was recovered using magnetic field. The concentration of residual Pb(II) solution was detected with an atomic absorbance spectrophotometer. The adsorption capacity of MMMT@Zn-BDC for Pb(II) at a certain time (qt) was calculated by Equation. 1, Equation 2 was used to calculate the adsorption capacity of MMMT@Zn-BDC for Pb(II) to be in equilibrium (qe).

Characterization
The surface morphology was recorded by scanning electron microscopy (SEM, S4800, Hitachi, Tokyo, Japan). A transmission electron microscope (TEM, JEM-2100, Lorentz, Tokyo, Japan) and energy dispersive spectrometer (EDS, GENESIS, EDAX, Mahwah, NJ, USA) were used to analyze the internal structures and composition of the materials. Nitrogen adsorption isotherms were obtained using Micromeritics ASAP 2020M. Functional groups of the adsorbent were determined using a Fourier transform infrared spectrometer (FTIR, Bruker, Karlsruhe, Germany). The crystallinity was determined by X-ray diffraction (XRD, Bruker, Karlsruhe, Germany). The magnetic properties were evaluated by vibrating sample magnetometry (VSM) analysis (SQUID-VSM, Quantum Design, Santiago, CA, USA). The elements of the adsorbent were analyzed by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, Shimadzu, Tokyo, Japan).

Adsorption Experiment
The mother Pb(II) solution was prepared by Pb(NO 3 ) 2 , and the resulting Pb(II) concentration was 1000 mg/L. Various Pb(II) solutions were diluted from this mother liquor. A Pb(II) solution (20 mL, 200 mg/L) was placed into several flasks, and various dosages of MMMT@Zn-BDC from 5 to 20 mg were separately added to these flasks. HNO 3 and NaOH (0.1 mol/L) were applied to adjust the pH of the Pb(II) solution from 2 to 6. Then, the flasks were placed in an oscillator; the speed of revolution was 150 rpm. After the adsorption finished, the adsorbent was recovered using magnetic field. The concentration of residual Pb(II) solution was detected with an atomic absorbance spectrophotometer. The adsorption capacity of MMMT@Zn-BDC for Pb(II) at a certain time (q t ) was calculated by Equation (1), Equation (2) was used to calculate the adsorption capacity of MMMT@Zn-BDC for Pb(II) to be in equilibrium (q e ).
C 0 (mg/L) is the initial concentration of Pb(II), C t (mg/L) is the concentration of Pb(II) at a certain time during the process of adsorption, V (mL) is the volume of the Pb(II) solution, m (mg) is the dosage of adsorbent. C e is the remaining Pb(II) concentration after the adsorption process reaching equilibrium. Finally, Equation (3) shows the calculated method of removal ratio (R) of MMMT@Zn-BDC for Pb(II):

Reutilization Test
HCl (0.1 mol/L) was used to elute Pb(II) loaded in MMMT@Zn-BDC. The elution process was as follows: Pb(II)-loaded MMMT@Zn-BDC and the eluent were added into a conical flask, and the concentration was maintained at 1 g/L. Then, the flask was placed in the oscillator, the eluent was replaced every 3 h until Pb(II) could no longer be detected in the eluent. After elution, the adsorbent was freeze-dried and then reused four times to evaluate the relationship between reuse times and the adsorption capacity.

Optimize the Preparation Conditions of MMMT@Zn-BDC
We studied the effects of adding MMT and Zn-BDC on the adsorption capacity and the binding rate (BR) of MMT and Zn-BDC coated onto the adsorbent to the actual weight added in the reaction solution. The results are shown in Table 1.
where m (g) is the mass of MMT and Zn-BDC loaded onto the adsorbent, and M (g) is the weight of MMT and Zn-BDC inserted in the reaction solution. According to Table 1, when the mass ratio of MMT to Fe 3 O 4 is 1:1, the BR and the adsorption capacity showed a good result. Based on the optimal ratio (1:1) of MMT to Fe 3 O 4 , we further optimized the amount of added Zn-BDC. Considering both BR and the adsorption capacity, we chose 1:1 as the reaction mass ratio of MMMT and Zn-BDC.  Figure 2. In Figure 2a, the morphology of magnetic nanoparticles is agglomerated spherical. After MMT was coated onto Fe 3 O 4 by co-precipitation method, as shown in Figure 2b, irregular sheet MMT adheres to surface of Fe 3 O 4 . In Figure 2c, Zn-BDC shows an uneven crystal shape and size because it was prepared at room temperature [35]. In Figure 2d, after Zn-BDC was coated onto MMMT via in situ polymerization, short, rod-shaped crystals of Zn-BDC appear on the rough surface of the MMMT. The introduction of MMMT changed the size of the Zn-BDC crystal. Fe3O4. In Figure 2c, Zn-BDC shows an uneven crystal shape and size because it was prepared at room temperature [35]. In Figure 2d, after Zn-BDC was coated onto MMMT via in situ polymerization, short, rod-shaped crystals of Zn-BDC appear on the rough surface of the MMMT. The introduction of MMMT changed the size of the Zn-BDC crystal.

TEM Analysis
TEM was used to analyze the internal structures and composition of MMMT, Zn-BDC, and MMMT@Zn-BDC. As shown in Figure 3a, when the amplification size is 500 nm, Fe3O4 particles are nearly spherical, while the irregular crystals are MMT. Fe3O4 and MMT are distributed almost in the same plane. It was confirmed that MMMT had been prepared successfully. In Figure 3b, the Zn-BDC crystals are also irregular, and it shows a slender lamellar structure. After synthesizing Zn-BDC on the MMMT surface, a slender lamellar structure appeared as can be seen in Figure 3c. It was concluded that a slender lamellar structure had been prepared successfully.

TEM Analysis
TEM was used to analyze the internal structures and composition of MMMT, Zn-BDC, and MMMT@Zn-BDC. As shown in Figure 3a, when the amplification size is 500 nm, Fe 3 O 4 particles are nearly spherical, while the irregular crystals are MMT. Fe 3 O 4 and MMT are distributed almost in the same plane. It was confirmed that MMMT had been prepared successfully. In Figure 3b, the Zn-BDC crystals are also irregular, and it shows a slender lamellar structure. After synthesizing Zn-BDC on the MMMT surface, a slender lamellar structure appeared as can be seen in Figure 3c. It was concluded that a slender lamellar structure had been prepared successfully.
TEM was used to analyze the internal structures and composition of MMMT, Zn-BDC, and MMMT@Zn-BDC. As shown in Figure 3a, when the amplification size is 500 nm, Fe3O4 particles are nearly spherical, while the irregular crystals are MMT. Fe3O4 and MMT are distributed almost in the same plane. It was confirmed that MMMT had been prepared successfully. In Figure 3b, the Zn-BDC crystals are also irregular, and it shows a slender lamellar structure. After synthesizing Zn-BDC on the MMMT surface, a slender lamellar structure appeared as can be seen in Figure 3c. It was concluded that a slender lamellar structure had been prepared successfully.  Figure 4a shows the results of N 2 adsorption and desorption. The adsorption isotherm belongs to the type II isotherm. With increasing adsorption pressure, the first inflection point of the isotherm rapidly appears, indicating that monolayer adsorption is complete. As the adsorption pressure continues to increase, multi-layer adsorption gradually occurs, and when the saturated vapor pressure is reached, the adsorption layer is infinite. The surface area of MMMT@Zn-BDC was calculated to be 74.35 m 2 /g by the multipoint BET (Brunauer-Emmett-Teller) method [36], and the pore volume and pore size were 0.27 cm 3 /g and 15.48 nm, respectively.  shows the results of N2 adsorption and desorption. The adsorption isotherm belongs to the type II isotherm. With increasing adsorption pressure, the first inflection point of the isotherm rapidly appears, indicating that monolayer adsorption is complete. As the adsorption pressure continues to increase, multi-layer adsorption gradually occurs, and when the saturated vapor pressure is reached, the adsorption layer is infinite. The surface area of MMMT@Zn-BDC was calculated to be 74.35 m 2 /g by the multipoint BET (Brunauer-Emmett-Teller) method [36], and the pore volume and pore size were 0.27 cm 3 /g and 15.48 nm, respectively.  Figure 4b is the FTIR spectra of MMMT. The peaks at 811 cm −1 , 848 cm −1 and 1046 cm −1 belong to the asymmetric stretching of O-Si-O, deformation vibration of Al-OH-Mg, and stretching vibration of Si-O. These three characteristic peaks belong to MMT [37]. For MMMT prepared by the co-precipitation method, the stretching vibration of Fe-O caused a peak at 576 cm −1 [38], and the peak at 3439 cm −1 is the stretching vibration of O-H, which together show that MMMT was synthesized successfully. The band between 1376 cm −1 and 1589 cm −1 was attributed to the stretching vibration of the carbonyl group in BDC [39]. After Zn-BDC was coated onto the surface of MMMT, the characteristic absorption band remained, which shows that Zn-BDC was successfully synthesized on the surface of MMMT.

VSM Analysis
The magnetic properties of MMMT and MMMT@Zn-BDC were studied by VSM analysis, and the results are shown in Figure 3d. The saturation magnetization of MMMT was 31.88 emu/g. After MMMT was coated onto Fe 3 O 4 , this value decreased to 10.09 emu/g. The decrease in magnetic properties indicated that Zn-BDC successfully covered MMMT. Good magnetic properties enable MMMT@Zn-BDC to accumulate rapidly under the external magnetic field. As shown in the inset of Figure 4d, MMMT@Zn-BDC could be separated by applying an external magnet for 20 s. Figure 5a is the full XPS spectrum of MMMT@Zn-BDC before and after adsorption. The peak of Fe 2p belongs to Fe 3 O 4 , the peaks of Si 2p, Al 2p, Mg(KL 23 L 23 ) originate from MMT [40], and the Zn 2p peak is from Zn-BDC. Meanwhile, the Pb4f peak appeared only after loading MMMT@Zn-BDC with Pb(II), demonstrating that MMMT@Zn-BDC is a good adsorbent for Pb(II). In Figure 5b, the peaks at 725.12 eV and 711.08 eV are characteristic of Fe 2p 1/2 and Fe 2p 3/2 . Notably, the satellite peak of Fe 2 O 3 at 714 eV did not appear in Figure 4b; this is because the peak is covered by the peaks of Fe 2p 1/2 and Fe 2p 3/2 , which also proves Fe 3 O 4 was successfully prepared [41]. In the C 1s spectrum in Figure 5c, the peaks at 288.75 eV, 286.14 eV and 284.80 eV belong to C=O, C-O and C-C, respectively [42]. However, as shown in Figure 5d, after loading MMMT@Zn-BDC with Pb(II), the above three peaks are moved to 288.45 eV, 285.96 eV, and 284.75 eV, respectively. The hydroxyl and carboxyl groups can complex with Pb(II), and caused the move of the binding energy. Figure 5c, the peaks at 288.75 eV, 286.14 eV and 284.80 eV belong to C=O, C-O and C-C, respectively [42]. However, as shown in Figure 5d, after loading MMMT@Zn-BDC with Pb(II), the above three peaks are moved to 288.45 eV, 285.96 eV, and 284.75 eV, respectively. The hydroxyl and carboxyl groups can complex with Pb(II), and caused the move of the binding energy.

Effect of the pH of Pb(II) Solution
A NaOH and HNO 3 (0.1 mol/L) solution was used to adjust the pH of the Pb(II) solution from 2 to 6. When the pH is greater than 7, Pb(II) precipitates in the form of Pb(OH) 2 . Therefore, the pH was adjusted to 2−6 in this study. The effect of pH value on the adsorption capacity is shown in Figure 6a. MMMT@Zn-BDC (20 mg) was added to 20 mL Pb(II) with a concentration of 200 mg/L. The solution was agitated in a rocking bed. The adsorption capacity of MMMT@Zn-BDC to Pb(II) was then measured at each pH value. When the pH was between 2 and 4, the adsorption capacity increased rapidly with increasing pH. However, between 4 and 6, the adsorption capacity increased slowly. The pH of the Pb(II) solution can affect the surface charge of the adsorbent. At a lower pH, the surface negative charge of MMMT@Zn-BDC is relatively less; thus, its capacity to electrostatically adsorb Pb(II) is poor. With the increasing pH, the amount of surface negative charge of MMMT@Zn-BDC increases, which makes it favorable to positively charged Pb(II). This also proves that electrostatic adsorption is one way that the adsorbent adsorbs Pb(II). Because the initial pH of the 200 mg/L Pb(II) solution was 5.45, when the pH was not adjusted the adsorption capacity was 195.33 mg/g, which is close to that of the adsorption capacity with a pH of 6. So, there is no need to further adjust the pH.
negative charge of MMMT@Zn-BDC is relatively less; thus, its capacity to electrostatically adsorb Pb(II) is poor. With the increasing pH, the amount of surface negative charge of MMMT@Zn-BDC increases, which makes it favorable to positively charged Pb(II). This also proves that electrostatic adsorption is one way that the adsorbent adsorbs Pb(II). Because the initial pH of the 200 mg/L Pb(II) solution was 5.45, when the pH was not adjusted the adsorption capacity was 195.33 mg/g, which is close to that of the adsorption capacity with a pH of 6. So, there is no need to further adjust the pH.

Effect of the Adsorbent Dose
Various amounts of MMMT@Zn-BDC (from 10 mg to 40 mg) were separately dispersed in 20 mL of Pb(II) solution with a concentration of 200 mg/L. Figure 6b shows the effect of the dosage of MMMT@Zn-BDC on the adsorption capacity. As the dosage of MMMT@Zn-BDC increased from 10 mg to 40 mg, q e decreased from 326.99 to 99.03 mg/g. At the same time, the removal ratio increased from 81.74% to 99.03%. When the dosage of the adsorbent was low, the amount of Pb(II) was excessive for the available adsorption sites. At this time, the probability that adsorption sites were occupied was high, so the adsorption capacity was high. When the dosage of the adsorbent increased, sufficient adsorption sites were available for Pb(II), so the removal ratio increased while q e decreased because of a large number of unoccupied adsorption sites exist in the adsorbent. When the adsorbent dosage was 1.0 mg/mL, the removal ratio reached 97.67%. As the adsorbent dosage increased, the change in the removal ratio was not obvious. Thus, considering the adsorption capacity, we chose 1 mg/ml as the dosage of the adsorbent for further experiments.

Effect of the Pb(II) Concentration
The initial concentration of Pb(II) can have serious implications for the adsorption capacity of MMMT@Zn-BDC. Therefore, we investigated the adsorption capacity of the solution with concentrations ranging from 10 to 1000 mg/L. Similarly, 20 mg of MMMT@Zn-BDC was separately dispersed in 20 mL different concentrations of Pb(II) solutions, the results are shown in Figure 6c. Even if the concentration reached 1000 mg/L, the adsorption was still not saturated but the removal rate decreased to 71.7% at this concentration. In practical applications, the removal ratio is an important factor for evaluating the adsorption properties of an adsorbent, so we did not continue to increase the Pb(II) concentration. With the increase in Pb(II) concentration from 10 to 1000 mg/L, the adsorption capacity increased from 10 mg/g to 717.01 mg/g. This is because at a higher concentration, excessive Pb(II) is available for the adsorption sites of MMMT@Zn-BDC, thus, it is easy to capture Pb(II) for the adsorbent sites. When the concentration of Pb(II) was below 200 mg/L, the lead ions were almost completely removed, and the removal ratio reached 97.67%. So, the Pb(II) concentration of 200 mg/L was chosen as the experimental concentration for further experiments.

Effect of Adsorption Time
The relationship between adsorption time and adsorption capacity was also investigated in this study. MMMT@Zn-BDC (20 mg) was dispersed in 20 mL of Pb(II) solution, and the concentration remained at 200 mg/L. The adsorption capacities at different times are shown in Figure 6d. During the primary stage of the adsorption process, the increase in the adsorption capacity is an almost logarithmic relationship. As adsorption proceeded between 0 and 20min, a large number of free adsorption sites could rapidly bind Pb(II). As the adsorption time was prolonged, the adsorption sites gradually become occupied by Pb(II), and the increase in the adsorption capacity started to slow down. After 90 min of the adsorption process, the adsorption capacity was 195.33 mg/g, and the removal rate was 97.67%. Beyond this adsorption time, the adsorption capacity remained unchanged. Thus, the time that the adsorption reached equilibrium was 90min, at the concentration of 200 mg/L.

Effect of the Adsorption Temperature
The adsorption capacity of MMMT@Zn-BDC for Pb(II) was measured from 293.2 K to 308.2 K. The dosage of the addition of MMMT@Zn-BDC in Pb(II) solution was 1 mg/mL. The effects of temperature on the adsorption capacity are shown in Figure 6e. With increasing temperature, the adsorption capacity increased from 194.34 mg/g to 198.21 mg/g. At the same time, the removal rate increased from 97.17% to 99.11%. The results show that the adsorption process of MMMT@Zn-BDC for Pb(II) is an endothermic process. Therefore, increasing the temperature is beneficial to the adsorption process.

Effect of the Recycling Process
Dilute HCl (0.1 mol/L) was used to desorb the Pb(II) loaded on the adsorbent and after detachment the adsorbent was repeatedly applied to remove Pb(II). The effects of reuse cycles on adsorption capacity and removal ratio are shown in Figure 6f. With the increasing number of reuses, the adsorption capacity decreased gradually. After being reused five times, the adsorption capacity was 160.90 mg/g, that is, the adsorption capacity declined. However, the adsorption rate remained at 80.45%, which shows that MMMT@Zn-BDC has good recyclability.

Adsorption Mechanism
The adsorption isotherm was determined by the Langmuir and Freundlich models. The adsorption data were taken from Figure 5c. In general, the Langmuir isotherm model usually assumes that the adsorption capacity of the adsorption site is uniform, and this isotherm model is often applied to describe monolayer adsorption processes. On the other hand, the Freundlich isotherm model is used to describe the effect of the adsorbate concentration on the adsorption capacity and whether adsorption is prone to occur. The two equations of the adsorption isotherms are as follows: Langmuir equation : Freundlich equation : where the maximum adsorption capacity (q m , mg/g) can be obtained from the Langmuir model, b and n are the constants of Langmuir and Freundlich isotherm models, respectively, and K F is a constant representing the adsorption capacity. R L is a dimensionless constant for Langmuir isotherm model, and it is used to determine whether the adsorption behavior is preferential adsorption. The equation is as follows: Figure 7a,b shows the curves fitted by the Langmuir and Freundlich isotherm models, Table 2 shows all of the related parameters while the correlation coefficients (R 2 ) fitted by the Langmuir and Freundlich isotherm models are 0.9850 and 0.9239, respectively. According to its higher correlation coefficient, the adsorption process of MMMT@Zn-BDC for Pb(II) is better described by the Langmuir isotherm model, which indicates that Pb ions do not interact. The adsorption capacity of each site on the adsorbent is the same, and monolayer adsorption accords with the adsorption process. Moreover, q m reached 724.64 mg/g. In Table 3, the calculated R L values are all less than 1, therefore, it can be concluded that this adsorption process is favorable [43]. Diffusion is the main adsorption mode for adsorption processes conforming to a pseudo-first-order kinetics model. The adsorption process that accords with this model, only have one kind of binding site while the adsorption process that accords with the pseudo-second-order model is chemical adsorption, which involves the sharing or transfer of electron pairs between the adsorbent and the adsorbate.
Pseudo − sec ond − order equation : k 1 (1/min) and k 2 (g/mg·min) are the constants in these two models. The linear equation fitted by the above two models is shown in Figure 7c,d, and the parameters were calculated and are shown in Table 4. The correlation coefficient (R 2 ) of the pseudo-second-order kinetics model is 0.9999, and the q e,cal is 194.93 mg/g; this result is similar to the experimental result (195.33 mg/g). Meanwhile, the R 2 of the pseudo-first-order kinetics model is 0.9566, and the q e,cal is 8.71.Therefore, we can conclude that the adsorption process of MMMT@Zn-BDC on Pb(II) fits the pseudo-second-order kinetics model, combined with the hypothetical premise of the pseudo-first-order kinetics, the adsorption process of MMMT@Zn-BDC for Pb(II) is chemical adsorption [44]. qe,cal is 194.93 mg/g; this result is similar to the experimental result (195.33 mg/g). Meanwhile, the R 2 of the pseudo-first-order kinetics model is 0.9566, and the qe,cal is 8.71.Therefore, we can conclude that the adsorption process of MMMT@Zn-BDC on Pb(II) fits the pseudo-second-order kinetics model, combined with the hypothetical premise of the pseudo-first-order kinetics, the adsorption process of MMMT@Zn-BDC for Pb(II) is chemical adsorption [44].  investigated. These data were used to study the adsorption thermodynamics, and the relevant 329 parameters were calculated by Equation (10): where ΔG (kJ/mol), ΔH (kJ/mol) and ΔS (kJ/mol) represent Gibbs free energy, enthalpy and entropy, 331 respectively. T (K) is the thermodynamic temperature, and R is the gas constant. Table 5 shows the 332 calculated results.  Moreover, the adsorption capacity changes in the range of 293.2 K to 308.2 K were also investigated. These data were used to study the adsorption thermodynamics, and the relevant parameters were calculated by Equation (10):

333
where ∆G (kJ/mol), ∆H (kJ/mol) and ∆S (kJ/mol) represent Gibbs free energy, enthalpy and entropy, respectively. T (K) is the thermodynamic temperature, and R is the gas constant. Table 5 shows the calculated results. The negative ∆G indicates that the adsorption process of MMMT@Zn-BDC for Pb(II) is endothermic and spontaneous under constant pressure. Meanwhile, ∆H is greater than 40 kJ/mol, indicating that the adsorption process is chemisorption. This conclusion is in agreement with the adsorption kinetics model. Therefore, a higher temperature is beneficial to the adsorption process.
In general, based on the results of the optimization process of the adsorption conditions, the adsorption capacity of MMMT@Zn-BDC for Pb(II) suffers most with changes in the pH. The hydroxyl and carboxyl groups of the adsorbent surface are not easily dissociated when the pH is lower than 2. Moreover, a high concentration of hydrogen ions is not conducive to ion exchange between MMMT and Pb(II). At a higher pH, hydroxyl and carboxyl groups are easier to dissociate, and these dissociated hydroxyl and carboxyl groups easily form complexes with Pb(II). The study of adsorption kinetics and adsorption thermodynamics confirmed that the adsorption process is chemisorption, as shown schematically at different pH values in Figure 8. In this study, MMMT@Zn-BDC is a type of composite material. In order to further understand the advantages of MMMT@Zn-BDC for the adsorption of Pb(II), we compared MMMT@Zn-BDC with the qm and equilibrium time of the adsorption of several reported adsorbents (Table 6).

Conclusion
For the first time, we prepared a magnetic composite material by combining Fe3O4, MMT and Zn-BDC using a co-precipitation method and in situ polymerization. This magnetic compound was applied to adsorb Pb(II). Based on the results of adsorption isotherm and adsorption kinetics, the adsorption behavior is in accordance with the Langmuir isotherm model and pseudo-second-order kinetics model, and the process of MMMT@Zn-BDC adsorbing Pb(II) is endothermic. The time required to reach adsorption equilibrium is 90 min. In addition, MMMT@Zn-BDC also exhibited excellent recycling performance. The removal rate remained at 80.45% after recycling five times. Its easy separation and sustainability make MMMT@Zn-BDC a superior adsorbent for removing Pb(II).  In this study, MMMT@Zn-BDC is a type of composite material. In order to further understand the advantages of MMMT@Zn-BDC for the adsorption of Pb(II), we compared MMMT@Zn-BDC with the q m and equilibrium time of the adsorption of several reported adsorbents (Table 6).

Conclusions
For the first time, we prepared a magnetic composite material by combining Fe 3 O 4 , MMT and Zn-BDC using a co-precipitation method and in situ polymerization. This magnetic compound was applied to adsorb Pb(II). Based on the results of adsorption isotherm and adsorption kinetics, the adsorption behavior is in accordance with the Langmuir isotherm model and pseudo-second-order kinetics model, and the process of MMMT@Zn-BDC adsorbing Pb(II) is endothermic. The time required to reach adsorption equilibrium is 90 min. In addition, MMMT@Zn-BDC also exhibited excellent recycling performance. The removal rate remained at 80.45% after recycling five times. Its easy separation and sustainability make MMMT@Zn-BDC a superior adsorbent for removing Pb(II).
Author Contributions: X.-k.O. and N.W. conceived and designed the experiments; J.S. and Y.G.W. performed the experiments; J.S. and N.W. analyzed the data; D.Y. and X.-k.O. contributed reagents/materials/analysis tools; N.W. wrote the paper.