Layered Double Hydroxides Derived from MIL-88A(Fe) as an Efficient Adsorbent for Enhanced Removal of Lead (II) from Water

The efficient removal of lead (II) from aqueous solution remains a big problem and the development of novel nanomaterials as adsorbents by various technologies to solve this problem is promising. This study contributed a novel nanostructure of MIL-88A-layered double hydroxides (LDHs) as the adsorbent for Pb2+, which was synthesized by a two-step solvothermal method with MIL-88A(Fe) as the precursor. The as-prepared material featured a chestnut-like core-shell structure, and exhibited excellent removal performance towards Pb2+ from water in comparison to MIL-88A(Fe) and LDHs (directly synthesized). The adsorption of Pb2+ by the MIL-88A-LDHs conformed to the pseudo-second-order kinetic model and the Langmuir and Freundlich isotherm models. The maximal adsorption capacity was 526.32, 625.00, and 909.09 mg g−1 at 278, 298, and 318 K, respectively. The thermodynamic parameters suggested that the adsorption was an endothermic, entropy-increasing, and spontaneous reaction. X-ray photoelectron spectroscopy (XPS) analysis indicated that the surface complexation was mostly responsible for Pb2+ elimination. The MIL-88A-LDHs can be readily regenerated and showed good cyclic performance towards Pb2+. Thus, the as-prepared MIL-88A-LDHs may hold promise for the elimination of aqueous heavy metals.


Introduction
Water pollution caused by heavy metals is of increasing concerned all over the world because of their easy accumulation and non-degradable properties in environmental media [1]. Once heavy metals are released into the environment and assimilated by life beings, their toxicity and carcinogenicity would be durable. Thus, efficient control of heavy metals pollution and reduction of their risks to human health is of importance for sustainable development of society and harmonious ecosystem. In general, metals with a density over 5 g/cm 3 can be regarded as heavy metals [2]. There are a variety of heavy metals in the environment, such as Co, Ni, Cu, Zn, Cd, Hg, Cr, and Pb. Of them, Pb has been receiving much attention due to its high toxicity towards organisms and easy mobility in the media. Research has suggested excessive Pb 2+ in blood would cause severe damage to the nervous system and sensory disturbance [3]. It is therefore imperative to enhance Pb 2+ cleanup from water.
Various methods and technologies have been reported for the removal of Pb 2+ , mainly including chemical precipitation, membrane filtration, ion-exchange, adsorption and electrochemical technology. Among them, adsorption has been deemed as a popular and efficient method for removal of heavy metals thanks to the low cost, high efficiency, abundant sources of adsorbents, and easy manipulation. Until now, many materials have been designed and fabricated as adsorbents towards Pb 2+ . These adsorbents can be categorized into the following  Figure  2. It is easily found that LDHs consisted of many nanoplates, and these nanoplates suffered from a heavy agglomeration effect (Figure 2A,B). Distinct from LDHs, the pristine MIL-88A(Fe) had a hexagonal structure and topology with a length of ~2.2 µm and a width of ~1.2 µm ( Figure 2C,D). In terms of morphology, MIL-88(A)-LDHs showed a unique hierarchical structure with a core of MIL-88A(Fe) and a shell of LDHs ( Figure 2E,F). The yellow circle area may represent a top-view image of MIL-88(A)-LDHs subunit. Noticeably, the LDHs as shells were made up by much thinner nanosheets (~30 nm) than LDHs sheets (direct synthesis). This morphology may result from the role of MIL-88(A) template. During the synthesis process, the LDHs shells were gradually formed around the external surface of templates accompanied by the dissolution of MIL-88A(Fe). Influenced by steric hindrance, a hybrid with a unique core-shell structure was thus formed.  Figure 2. It is easily found that LDHs consisted of many nanoplates, and these nanoplates suffered from a heavy agglomeration effect (Figure 2A,B). Distinct from LDHs, the pristine MIL-88A(Fe) had a hexagonal structure and topology with a length of~2.2 µm and a width of~1.2 µm ( Figure 2C,D). In terms of morphology, MIL-88(A)-LDHs showed a unique hierarchical structure with a core of MIL-88A(Fe) and a shell of LDHs ( Figure 2E,F). The yellow circle area may represent a top-view image of MIL-88(A)-LDHs subunit. Noticeably, the LDHs as shells were made up by much thinner nanosheets (~30 nm) than LDHs sheets (direct synthesis). This morphology may result from the role of MIL-88(A) template. During the synthesis process, the LDHs shells were gradually formed around the external surface of templates accompanied by the dissolution of MIL-88A(Fe). Influenced by steric hindrance, a hybrid with a unique core-shell structure was thus formed.
As shown in Figure 3, the TEM images showed a clear core-shell structure, in which the residual MIL-88A was a core (corresponding to the dark region) and the newly formed LDHs was a shell (corresponding to the light region) ( Figure 3A-C). In the high-resolution TEM ( Figure 3D), we can identify that interplanar spacing was approximately 0.742 nm, which roughly corresponded to the lattice plane of (003) of LDHs. This phenomenon was also identified by the PXRD analysis, and verified the formation of the LDHs phase.

Adsorption Kinetics
As shown in Figure 4A, MIL-88A-LDHs exhibited the best adsorption performance in comparison to Ni-Fe-LDHs and MIL-88A(Fe). Pb(II) concentration declined from 53.15 to 0.02 mg L −1 , while for Ni-Fe-LDHs and MIL-88A(Fe), the concentration only decreased to 48.77 and 51.25 mg L −1 , respectively. The remarkable capability of Pb(II) removal may have mainly originated from the unique core-shell structure of MIL-88A-LDHs. The shell enabled abundant active sites to be exposed to the metal ions. As shown in Figure 3, the TEM images showed a clear core-shell structure, in which the residual MIL-88A was a core (corresponding to the dark region) and the newly formed LDHs was a shell (corresponding to the light region) ( Figure 3A-C). In the high-resolution TEM ( Figure 3D), we can identify that interplanar spacing was approximately 0.742 nm, which roughly corresponded to the lattice plane of (003) of LDHs. This phenomenon was also identified by the PXRD analysis, and verified the formation of the LDHs phase.

Adsorption Kinetics
As shown in Figure 4A, MIL-88A-LDHs exhibited the best adsorption performance in comparison to Ni-Fe-LDHs and MIL-88A(Fe). Pb(II) concentration declined from 53.15 to 0.02 mg L −1 , while for Ni-Fe-LDHs and MIL-88A(Fe), the concentration only decreased  As shown in Figure 3, the TEM images showed a clear core-shell structure, in which the residual MIL-88A was a core (corresponding to the dark region) and the newly formed LDHs was a shell (corresponding to the light region) ( Figure 3A-C). In the high-resolution TEM ( Figure 3D), we can identify that interplanar spacing was approximately 0.742 nm, which roughly corresponded to the lattice plane of (003) of LDHs. This phenomenon was also identified by the PXRD analysis, and verified the formation of the LDHs phase.

Adsorption Kinetics
As shown in Figure 4A, MIL-88A-LDHs exhibited the best adsorption performance in comparison to Ni-Fe-LDHs and MIL-88A(Fe). Pb(II) concentration declined from 53.15 to 0.02 mg L −1 , while for Ni-Fe-LDHs and MIL-88A(Fe), the concentration only decreased to 48.77 and 51.25 mg L −1 , respectively. The remarkable capability of Pb(II) removal may have mainly originated from the unique core-shell structure of MIL-88A-LDHs. The shell enabled abundant active sites to be exposed to the metal ions. For insight into adsorption behavior, three kinetic models were used to fit the experimental data. The models included pseudo first-order model (PFO) (Equation (1)), pseudo second-order model (PSO) (Equation (2)), and intraparticle diffusion model (Equation (3)), and the linear equations are listed as follows [20,21]: (1) where qt and qe (mg g −1 ) in Equations (1) Figure 4B,C show the fitting curves of two types of kinetic models and the corresponding parameter values are listed in Table 1. It can be easily found that both PFO and PSO fitted well with a high R-square of 0.9443 and 0.9736, respectively. PSO showed a better fitting result, indicating that the adsorption of Pb(II) by MIL-88A-LDHs might be dominated by chemisorption. It usually occurs via the ions exchange or the chemical reactions sharing electrons between adsorbents and adsorbates. The intraparticle diffusion model was used to fit the adsorption process to identify the rate-limiting step. It was divided into two portions ( Figure 4D). The first stage revealed the film diffusion through the boundary layer in the solid-liquid interface, while the second depicted the particle diffusion across the internal hydrated interlayer of adsorbents. The diffusion rate constants were 51.29 and 3.13 mg (g h −0.5 ) −1 , respectively. These results suggested that the first stage was mostly controlled by the film diffusion process (a very small intercept) and the second almost depended on the intraparticle diffusion (a big intercept).   For insight into adsorption behavior, three kinetic models were used to fit the experimental data. The models included pseudo first-order model (PFO) (Equation (1)), pseudo second-order model (PSO) (Equation (2)), and intraparticle diffusion model (Equation (3)), and the linear equations are listed as follows [20,21]: where q t and q e (mg g −1 ) in Equations (1)-(3) represent the adsorption amount of Pb(II) at time of t and equilibrium, respectively.
are the rate constants of the pseudo first-order, the pseudo-second-order and the intraparticle diffusion models in Equations (1)-(3). The C (mg g −1 ) in Equation (3) reflects the effect of boundary layer on the adsorption. Figure 4B,C show the fitting curves of two types of kinetic models and the corresponding parameter values are listed in Table 1. It can be easily found that both PFO and PSO fitted well with a high R-square of 0.9443 and 0.9736, respectively. PSO showed a better fitting result, indicating that the adsorption of Pb(II) by MIL-88A-LDHs might be dominated by chemisorption. It usually occurs via the ions exchange or the chemical reactions sharing electrons between adsorbents and adsorbates.
The intraparticle diffusion model was used to fit the adsorption process to identify the rate-limiting step. It was divided into two portions ( Figure 4D). The first stage revealed the film diffusion through the boundary layer in the solid-liquid interface, while the second depicted the particle diffusion across the internal hydrated interlayer of adsorbents. The diffusion rate constants were 51.29 and 3.13 mg (g h −0.5 ) −1 , respectively. These results suggested that the first stage was mostly controlled by the film diffusion process (a very small intercept) and the second almost depended on the intraparticle diffusion (a big intercept).

Adsorption Isotherms
The adsorption isotherms were investigated by changing the initial concentrations of Pb(II) at three temperatures (278 K, 298 K and 318 K). The curves are shown in Figure 5A. Obviously, the adsorption amount of Pb(II) increased with the equilibrium concentration of Pb(II) at each temperature level. Moreover, the adsorption amount showed an increasing trend as the temperature rose from 278 to 318 K. This implied that the adsorption was an endothermic reaction.  Note: KL: (L mg −1 ), KF: ((mg g −1 )(L mg −1 )1/n), and qm (mg g −1 ). To further investigate the thermodynamic features, the thermodynamic functions, such as enthalpy change (ΔH0, kJ/mol), entropy change (ΔS0, J/(mol·K)), and Gibb's free energy change (ΔG0, kJ/mol), were calculated from Equations (6) and (7): where R is the ideal gas constant, 8.314 J/mol/K; T is the Kelvin temperature, K; K0 represents the thermodynamic equilibrium constant and lnK0 can be obtained by the extrapo- Moreover, we used the well-known isotherm models of the Langmuir (Equation (4)) and Freundlich (Equation (5)) equations to fit the adsorption data, and the linear form equations are given as follows [22]: C e /q e = 1/(q m K L ) + C e /q m (4) lnq e = 1/n·lnC e + lnK F where C e (mg L −1 ) and q e (mg g −1 ) are the equilibrium concentration of Pb(II) and the corresponding equilibrium adsorption amount, respectively. K L (L mg −1 ) andK F ((mg g −1 ) (L mg −1 )1/n) are the constants of the Langmuir model and the Freundlich model, respectively. The fitting curves are presented in Figure 5B,C and the calculated parameters are listed in Table 2. It was clear that the Langmuir model was better for fitting the adsorption of Pb(II) than the Freundlich model. This suggested a monolayer adsorption may occur between Pb(II) and MIL-88A-LDHs, which was driven by the chemical interactions [23]. Note: K L : (L mg −1 ), K F : ((mg g −1 )(L mg −1 )1/n), and q m (mg g −1 ).

Adsorption Thermodynamic
To further investigate the thermodynamic features, the thermodynamic functions, such as enthalpy change (∆H 0 , kJ/mol), entropy change (∆S 0 , J/(mol·K)), and Gibb's free energy change (∆G 0 , kJ/mol), were calculated from Equations (6) and (7): where R is the ideal gas constant, 8.314 J/mol/K; T is the Kelvin temperature, K; K 0 represents the thermodynamic equilibrium constant and lnK 0 can be obtained by the extrapolation of ln(q e /C e ) vs. C e . The Van Der Hoff curve is shown in Figure 5D and the thermodynamic parameters are provided in Table 4. The negative values of ∆G 0 indicated the adsorption of Pb(II) by MIL-88A-LDHs was spontaneous. In addition, the absolute value of ∆G 0 increased with the elevated temperature, revealing that high temperature can boost the adsorption reaction. The values of ∆H 0 and ∆S 0 were calculated to be 14.03 kJ/mol and 75.91 J/mol/K respectively, based on the intercept and slope of Van Der Hoff equation ( Figure 5D). The positive value of ∆H 0 suggested that Pb(II) adsorption by MIL-88A-LDHs was an endothermic process, whereas the positive value of ∆S 0 revealed an increase in randomness at interface. Thus, the adsorption was an endothermic, entropy-increasing, and spontaneous process [3,29]. Table 4. Adsorption thermodynamic parameters.

Effects of Adsorbent Dosage and pH
The effects of adsorbent dosages on Pb(II) removal were investigated and the results are shown in Figure 6A. The removal percentages increased with increasing the dosage. When the dosages varied from 0.1 to 0.5 g/L, the Pb(II) removal percentages increased from 64.92 to 98.16%. This could be due to the presence of more available active sites at a higher dosage in the solution. However, the unoccupied adsorption sites would no longer make contributions to the adsorption when the dosage exceeded the optimal value [30]. The dosage of 0.4 g/L was fixed in this study, since the removal percentage can reach 96.66% at the dosage. and the removal percentage obviously increased from 42.77 to 99.59% when pH increased from 1.0 to 5.0. Additionally, the point of zero charge (pHzpc) of MIL-88A-LDHs was about 4.6 ( Figure 6C). A relatively high removal efficiency (~90.68%) was achieved at pH < pHzpc, and this phenomenon suggested the electrostatic interaction was not the only driving force. Given the constituents of MIL-88A-LDHs, it can be inferred that the precipitation between lead ions and anions (OH − or CO3 2− ) may make an important contribution to the adsorption. As shown in Figure 6B, the Pb(II) adsorption depended heavily on the solution pH, and the removal percentage obviously increased from 42.77 to 99.59% when pH increased from 1.0 to 5.0. Additionally, the point of zero charge (pHzpc) of MIL-88A-LDHs was about 4.6 ( Figure 6C). A relatively high removal efficiency (~90.68%) was achieved at pH < pHzpc, and this phenomenon suggested the electrostatic interaction was not the only driving force. Given the constituents of MIL-88A-LDHs, it can be inferred that the precipitation between lead ions and anions (OH − or CO 3 2− ) may make an important contribution to the adsorption.

Adsorption Mechanism
The SEM-EDX spectrum and elemental mapping are shown in Figure 7. After adsorption, the Pb signals have been detected in the used adsorbent surface and the contents were approximately 1.0 wt%. In addition, the elemental distributions were almost identical. These findings evidenced that Pb(II) had successfully attached to the adsorbent surface.
XPS can provide the information of chemical states of surface elements for the purpose of unveiling the adsorption mechanism. As depicted in Figure 8A The high-resolution C 1s spectra of MIL-88A-LDHs and MIL-88A-LDHs/Pb 2+ were deconvoluted into two peaks, i.e., C=O and C=C, as shown in Figure 8C1,C2. It can be found that the relative contents (peak area) changed and peak locations slightly shifted to low binding energy orientation (from 288.20 and 284.83 eV to 288.00 and 284.80 eV, respectively). These phenomena resulted from the formation of PbCO 3 . CO 3 2− went through the internal pore to reach the interface and encountered Pb(II) to form precipitate. Thus, the contents of surface Pb(II) obviously increased and the peak movement may be caused by the strong interaction between Pb(II) and CO 3 2-. This observation further verified that Pb(II) was removed by the formation of precipitate.

Adsorption Mechanism
The SEM-EDX spectrum and elemental mapping are shown in Figure 7. After adsorption, the Pb signals have been detected in the used adsorbent surface and the contents were approximately 1.0 wt%. In addition, the elemental distributions were almost identical. These findings evidenced that Pb(II) had successfully attached to the adsorbent surface. XPS can provide the information of chemical states of surface elements for the purpose of unveiling the adsorption mechanism. As depicted in Figure 8A, the main elements included Ni, Fe, O, N, and C in the survey spectra of two samples. Particularly, the Pb 4f characteristic peak was found in the MIL-88A-LDHs/Pb 2+ . The Pb chemical states can be divided into two main species PbCO3 and Pb-O in the high resolution Pb 4f spectrum Moreover, the high-resolution O1s spectra of two samples were divided into three peaks. As shown in Figure 8D1,D2, the relative contents of −OH species heavily decreased to 50.00% from 71.74% (before adsorption), while those of H 2 O and O 2− species showed an increasing trend (up to 25.00% from 17.39% and 10.87%, respectively). This result suggested that there were abundant OH− (LDHs structural constituents) or surface hydroxyls consumed during the Pb(II) adsorption process, and confirmed that the surface hydroxyls also served as adsorption sites for Pb(II) cleanup, besides the precipitation of CO 3 2− [31][32][33]. By the analysis above, we can draw a scheme of adsorption mechanisms (Figure 9). Both CO 3 2− and surface hydroxyls made contributions to Pb(II) elimination. After adsorption, Pb(II) existed as PbCO 3 , Pb(OH) 2 , and Pb-complex on the adsorbent surface.
peaks. As shown in Figure 8D1,D2, the relative contents of −OH species heavily decreased to 50.00% from 71.74% (before adsorption), while those of H2O and O 2− species showed an increasing trend (up to 25.00% from 17.39% and 10.87%, respectively). This result suggested that there were abundant OH− (LDHs structural constituents) or surface hydroxyls consumed during the Pb(II) adsorption process, and confirmed that the surface hydroxyls also served as adsorption sites for Pb(II) cleanup, besides the precipitation of CO3 2− [31][32][33]. By the analysis above, we can draw a scheme of adsorption mechanisms (Figure 9). Both CO3 2-and surface hydroxyls made contributions to Pb(II) elimination. After adsorption, Pb(II) existed as PbCO3, Pb(OH)2, and Pb-complex on the adsorbent surface.

Cycling Performance of MIL-88A-LDHs
The cycling performance of MIL-88A-LDHs has been investigated. As shown in Figure 6D, the Pb(II) removal efficiency dropped from 97.64% at the first cycle to 62.35% at the fourth cycle. The as-prepared adsorbent only maintained moderate adsorption ability since some active sites occupied by Pb(II) could not be regenerated. Particularly, the PbCO3 precipitation would consume CO3 2− . The exploration of a regenerable adsorbent with excellent adsorption capacity is still ongoing.

Cycling Performance of MIL-88A-LDHs
The cycling performance of MIL-88A-LDHs has been investigated. As shown in Figure 6D, the Pb(II) removal efficiency dropped from 97.64% at the first cycle to 62.35% at the fourth cycle. The as-prepared adsorbent only maintained moderate adsorption ability since some active sites occupied by Pb(II) could not be regenerated. Particularly, the PbCO 3 precipitation would consume CO 3 2− . The exploration of a regenerable adsorbent with excellent adsorption capacity is still ongoing.

Materials and Reagents
All chemicals were of analytical grade. Dimethylformamide (DMF), ethanol, fumaric acid (FA), ferric chloride hexahydrate (FeCl 3 ·6H 2 O), nickel nitrate hexahydrate (Ni(NO 3 ) 2 ·6H 2 O), urea, citric acid, and lead nitrate (Pb(NO 3 ) 2 ) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Typically, 0.928 g of FA and 1.296 g of FeCl 3 ·6H 2 O were dissolved in 80 mL DMF. The mixture in a Teflon-lined stainless-steel autoclave (100 mL) was transferred to an electric oven to react at 100 • C for 8 h. After reaction, the precipitate was collected by centrifugation and washed by absolute ethanol and ultrapure water repeatedly. Finally, the product was dried overnight at 80 • C for further use.

Synthesis of Layered Double Hydroxides (LDHs)
Typically, 1.5 g of ferric chloride hexahydrate, 0.52 g of nickel nitrate hexahydrate, and 1.0 g of urea were dissolved in 60 mL deionized water and reacted at 90 • C for 12 h. After reaction, the precipitate was collected by centrifugation and washed by ultrapure water repeatedly. Finally, the product was dried overnight at 80 • C for further use.

Synthesis of LDHs Derived from MIL-88A(Fe)
The scheme of synthesis of LDHs derived from MIL-88A(Fe) is shown in Figure 10. Firstly, 80 mg of MIL-88A(Fe) was dissolved in 48 mL ethanol, labelled as solution A. Then 1.2 g of nickel nitrate hexahydrate and 0.8 g of urea were dissolved in 32 mL deionized water, labelled as solution B. Solution A and B were mixed to form a homogeneous mixture. The mixture was poured into a Teflon-lined stainless-steel autoclave (100 mL) and transferred to an electric oven to react at 95 • C for 6 h. After reaction, the precipitate was collected by centrifugation and washed by absolute ethanol and ultrapure water repeatedly. Finally, the product was dried overnight at 80 • C for further use. The product was denoted as MIL-88A-LDHs. water, labelled as solution B. Solution A and B were mixed to form a homogeneous mixture. The mixture was poured into a Teflon-lined stainless-steel autoclave (100 mL) and transferred to an electric oven to react at 95 °C for 6 h. After reaction, the precipitate was collected by centrifugation and washed by absolute ethanol and ultrapure water repeatedly. Finally, the product was dried overnight at 80 °C for further use. The product was denoted as MIL-88A-LDHs.

Characterization of Materials
Powder X-ray diffraction (XRD) patterns of the samples were recorded with a X'Pert Pro diffractometer (PANalytical, Eindhoven, Holland) using Cu Kα radiation (40 kV, 40 mA, λ = 0.15406 nm). The surface morphology of the materials was observed using a scanning electron microscope (SEM, Hitachi SU-8010, Tokyo, Japan) equipped with an energy dispersive X-ray spectrometer (EDS). The structure and the elemental distribution were determined using a transmission electron microscope (TEM, JEOL, JEM-2100F, Tokyo, Japan) operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed in a PHI 5000C ESCA system (PHI, Louisiana, USA). The lead contents in the samples were measured quantitatively by atomic absorption spectroscopy (AAS) by using a HITACHI ZA3000 instrument (Tokyo, Japan).

Characterization of Materials
Powder X-ray diffraction (XRD) patterns of the samples were recorded with a X'Pert Pro diffractometer (PANalytical, Eindhoven, Holland) using Cu Kα radiation (40 kV, 40 mA, λ = 0.15406 nm). The surface morphology of the materials was observed using a scanning electron microscope (SEM, Hitachi SU-8010, Tokyo, Japan) equipped with an energy dispersive X-ray spectrometer (EDS). The structure and the elemental distribution were determined using a transmission electron microscope (TEM, JEOL, JEM-2100F, Tokyo, Japan) operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed in a PHI 5000C ESCA system (PHI, Louisiana, USA). The lead contents in the samples were measured quantitatively by atomic absorption spectroscopy (AAS) by using a HITACHI ZA3000 instrument (Tokyo, Japan).

Adsorption Experiments
The adsorption behavior of MIL-LDHs was evaluated by batch adsorption experiments to determine the kinetics, equilibrium and thermodynamic experimental results. All the experiments were conducted in 50 mL plastic vials with each containing 25 mL solution. The dosages of MIL-88(A), LDHs, and MIL-LDHs used in Pb(II) adsorption experiments were set as 0.4 g L −1 . All solution samples were shaken in a ZHWY-2102C incubator shaker (ZhiCheng, Shanghai, China) with the desired temperature of 25 • C and fixed rotating speed of 250 rpm in the sorption process.
The adsorption kinetic experiments of Pb(II) on MIL-88(A), LDHs, and MIL-LDHs were all performed at an initial concentration of 53.15 mg L −1 and shaken for 12 h in the incubator shaker. At predetermined time intervals, replicate vials were sacrificially sampled to measure the residual concentration of Pb(II) in the liquid phase. The pseudo-first-order model and the pseudo second-order model were employed to fit kinetic data for further analysis.
The adsorption isotherm experiments of Pb(II) on MIL-LDHs at three temperatures (5, 25 and 45 • C) were all established with concentrations ranging from 85 to 280 mg L −1 after a 24 h oscillation. The Langmuir and Freundlich models were used to fit isotherm data for further analysis.
By varying from 0.1 to 0.5 g L −1 , effects of adsorbent dosages on Pb(II) removal efficiency were performed. The initial Pb(II) concentration was 88 mg L −1 and the solution pH was adjusted to 3.0. The pH effects on Pb(II) removal efficiency were investigated at a pH range of 1.0-5.0. The adsorbent dosage was 0.4 g L −1 .

Cycling Experiments
The adsorption experiments were performed under the condition of the initial Pb(II) concentration of 20 mg L −1 and the dosage of 0.4 g L −1 . After equilibrium, the used MIL-LDHs were recovered by filtration, and were redispersed in the citric acid solution (0.02 mol/L) for desorption. The MIL-LDHs were rinsed by ethanol and deionized water successively. After drying treatment (105 • C, 8 h), the regenerated MIL-LDHs were used for next cycle.
All collected filtrates were measured by polarized Zeeman Atomic Absorption Spectrophotometer ZA3000 Series (AAS, Hitachi, Tokyo, Japan). In this study, all adsorption experiments were conducted in duplicate and the average data were reported. The removal percentage (R) and the adsorption amount were calculated from the following Equations (8) and (9): where C 0 , C t , and C e (mg L −1 ) are the Pb(II) initial concentration, the instant concentration at time t and the equilibrium concentration in the solution, respectively. V (mL) represents the volume of the Pb(II) solution, and m (mg) the mass of the adsorbents.

Conclusions
This study contributed a novel adsorbent for Pb 2+ of MIL-88A-layered double hydroxides (LDHs), which was synthesized by a two-step solvothermal method with MIL-88A(Fe) as the precursor. The as-prepared material featured a chestnut-like unique structure and exhibited excellent removal performance towards Pb 2+ from water in comparison to MIL-88A(Fe) and LDHs (directly synthesized). The adsorption of Pb 2+ by the MIL-88A-LDHs was well fitted by the pseudo-second-order kinetic model and the Langmuir and Freundlich isotherm models. The maximal adsorption capacity was 526.32, 625.00, and 909.09 mg g −1 at 278, 298, and 318 K, respectively. The thermodynamic parameters suggested that the adsorption was an endothermic, entropy-increasing, and spontaneous reaction. XPS analysis indicated that the surface complex model was mostly responsible for Pb 2+ elimination. The MIL-88A-LDHs can be readily regenerated and showed good cyclic performance towards Pb 2+ . Thus, the MIL-88A-LDHs may be taken as a promising adsorbent for removing Pb 2+ and other heavy metals from water.