Facile synthesis of hierarchically structured MIL-53(Al) with superior properties using an environmentally-friendly ultrasonic method for separating lead ions from aqueous solutions

The present study aims at investigating sonochemically synthesized MIL-53(Al) and its applications in adsorption lead ions from aqueous solution. XRD, FESEM, BET, and FTIR analyses were employed to identify and characterize MIL-53(Al). The ultrasonic-assisted synthesis procedure results in reducing the synthesis time to 24 h; however, the conventional synthesis of MIL-53(Al) takes 3 days. Applying ultrasonic waves also leads to increase of the specific surface area up to 50% more than that of synthesized by the conventional method, as well as creating the hierarchical MIL-53(Al) structure which reduces the mass transfer limitation of ions into internal micropores. The optimum conditions for removing lead ions are pH of 6, Pb+2 ion concentration of 20 mg/L, contact time of 60 min, adsorbent dose of 0.04 g, and temperature of 318 K with the removal efficiency of 97.63%. The experimental adsorption equilibrium and kinetic data fit the Langmuir isotherm and pseudo-second-order kinetic models, respectively. Moreover, the usage of sonochemically synthesized MIL-53(Al), for the first time as an adsorbent in heavy metal removal points to the great potential of this new environmentally-friendly adsorbent in removing lead ions from aqueous solutions


Preparation of hierarchically structured MIL-53(Al). The present study employed a new environmen-
tally-friendly method for ultrasonic-assisted synthesis (UAS) of MIL-53(Al). According to this method, (6.5 g) Aluminum (III) nitrate nonahydrate (Al-(NO 3 ) 3 ·9H 2 O) (98% purity) and (1.44 g) 1,4-BenzeneDiCarboxylic (BDC) acid (98% purity) with 25 ml of D.I water were stirred for 10 min at room temperature until complete dissolution is achieved. The mixture was then exposed to ultrasonic waves of 24 kHz and 100% intensity for 30 min after dissolving and then, it was transferred to 100 ml autoclave with black Teflon liner steel. Finally, an oven was used to heat the sample to 220 °C for 24 h. The autoclave was cooled down and the solid phase was separated using a centrifuge, and 35 ml of DMF solution (solid solvent) was added to remove unreacted terephthalic acids trapped in the pores of the samples. The autoclave was put back in the oven to be heated to 150 °C for 12 h. Finally, the solid phase was separated by centrifuge and washed with methanol; then, 40 ml of methanol was added to the sample, and the sample was dried in an oven at 150 °C overnight. The result is a solid white powder subjected to different analyses used for identifying and ensuring the correct formation of MIL-53(Al). In the end, the synthesized crystals were calcined at 650 K in the air for 5 h. In order to synthesis MIL-53(Al) with the conventional (CS) method, the primary mixture, including water, BDC, and Aluminum(III) nitrate nonahydrate with the same amount as ultrasonic synthesis, was stirred for 1 h at room temperature until complete dissolving. It then transferred to 100 ml autoclave with black Teflon liner steel and used the oven to heat the sample to 220 °C for 72 h. Finally, the autoclave was let to be cooled down. All the other experimental conditions took place the same as the ultrasonic synthesis method.
Characterization. The morphology and composition characteristics of MIL-53(Al) were evaluated using Field Emission Scanning Electron Microscopy (FESEM) and a Seron AIS2100. In addition, X-Ray Diffraction (XRD) was employed to examine the crystalline structure recorded by the PW1730 diffractometer with λCuKα = 1.54056 A˚ and a step size of 0.05 at 40 kV and 30 mA. The BELSORP MINI ll instrument measured the Brunauer-Emmett-Teller (BET) analysis within the nitrogen adsorption-desorption isotherm method at (77 K) in order to detect structural properties of MIL-53(Al). Fourier-Transformed Infrared spectroscopy (FT-IR) was obtained using the PERKIN EKMER instrument at a resolution of 400-4000 cm −1 . Energy-dispersive X-ray analysis (EDX) was used to determine the elemental composition of the UAS MIL-53(Al) samples. The analysis was performed using a TECSCAN MIRA ll field emission scanning electron microscope equipped with a SAMX detector (France). Further, AAS was used to measure the lead concentration using Model 3110 Perkin Elmer Atomic Absorption Spectrometer. The pH meter model, combined with a pH electrode, was of the 827 pH Lab (Metrohm, Swiss).
Preparation of lead solutions. Millipore/D.I. water and lead (II) nitrate were used to prepare the lead (II) stock solution with a concentration of 1000 mg/L. Other lead (II) solutions with different concentrations were freshly prepared for each experiment using the stock solution.
Evaluation of adsorbents performance. MIL-53(Al) adsorption capacities for Pb(ll) were presented at different dosages with known initial concentrations for fixed values of time, temperature, and pH. When the solution reached equilibrium, two solid and liquid phases were divided through filtration and then, Atomic Absorption Spectrometer (AAS) was employed to determine the remaining concentration of the Pb +2 ions in the solution. The pH of point of zero charges (pHpzc) of MIL-53(Al) was calculated by the method proposed by Faria et al. 16 . The effect of initial pH on lead ion adsorption was also examined at different pHs ranging from 2 to www.nature.com/scientificreports/ 7. The pH of the solution was fixed by adjusting HCl and NaOH 0.1 and 0.01 M to the solutions. Then, 0.04 g of MIL-53(Al) was added to every 40 ml of solution, and the agitation samples were placed on an incubator shaker at a speed of 315 rpm for three hours at room temperature. Eventually, the final concentration of lead(II) ion was analyzed using AAS. Further, Pb +2 adsorption isotherm and other practical parameters were studied in the following experimental conditions: the concentration of 20-130 (mg/L), time range of 5-180 min, and different temperatures of 288, 298, 308, and 318 K. The adsorbent dosage ranges from 0.001 to 0.04 g. The kinetics investigations for Pb(ll) removal were evaluated through the analysis of Pb +2 adsorption in different time periods. The adsorption efficiency% and adsorption capacity of Pb(II) at equilibrium q e (mg/g) were determined using Eq. (1) and Eq. (2), respectively 17 : where V(ml) is the volume of lead(II) ion solution; m(g) is the amount of MIL-53(Al); and C 0 and C e are the initial and equilibrium concentrations (mg/L) of lead(II) ion adsorbed, respectively.

Result and discussion
MIL-53(Al) characterization. The XRD pattern of the synthesized MIL-53(Al) is presented in Fig. 1. These patterns are very similar to 18 . The XRD pattern of the UAS and CS MIL-53(Al) in this study shows the prominent characteristic peaks at 2θ = 9.38°, 12.58°, 17.93°, 23° 48, 25.28°, 27.33°, which match the peaks reported in 18 and (CCDC file no. 220477), indicating that MIL-53(Al) is well crystallized and synthesized. Also, the difference in peak intensities is due to higher ultrasonic energy as a driving force, increasing nucleation rate and leading to a considerable amount of cavitation bubbles 19 .
The morphology of UAS and CS MIL-53(Al) samples with different magnifications were examined using FESEM, the results of which are presented in Fig. 2. According to this figure, the samples, which were synthesized through the sonochemical method, have octahedral shapes with crystal-like structures. Such structures are not aggregated; instead, they have a more uniform particle size which is smaller than the sample obtained by the conventional hydrothermal method. According to the standard conditions required for MOF crystal growth, faster nucleation rates result in smaller crystal sizes 20 . Following the FESEM characterization results, the sonochemical synthesized MIL-53(Al) has smaller uniformly distributed nanoparticles mainly because of the accelerated nucleation and shorter synthesis time required for sonochemical synthesis.
The chemical composition of MIL-53(Al) was determined using energy-dispersive X-ray analysis (EDX). The presence of C(56.34%), O(37.53%), and Al(6.13%) in the Al-MOF is confirmed as demonstrated in Fig. 3.
The N 2 adsorption-desorption isotherm of MIL-53(Al) is exhibited in Fig. 4. The synthesized sample resembles an IUPAC-type IV isotherm with a type H4 hysteresis loop, illustrating hierarchical structure including  www.nature.com/scientificreports/ mesoporous and microporous in the sample while conventional synthesis MIL-53(Al) only possesses a microporous structure 21 . Furthermore, BET data of MIL-53(Al) are given in Table 1. The high crystallinity and porosity of the sample yield a high BET surface area of 1538.6 m 2 /g and a vast average pore size of 1.74 nm. BET surface area and average pore size are greater than those conventionally synthesized MIL-53(Al) introduced in the literature [21][22][23] ; it could be explained by the effect of ultrasound waves on the crystalline structure of MIL-53(Al). Ultrasonic intensity as a driving force ends up in a substantial quantity of cavitation bubbles, and increases in crystallinity, leading to a higher BET expanse in line with the XRD pattern 19,24 . Moreover, the high intensity of ultrasonic waves creates mesopores in the structure of MOFs 25 , leading to a hierarchical ((meso-and micro-) pores) structures attending to a more significant average pore sizes 21,26 . Figure 5 illustrates the FT-IR spectrum of the synthesized UAS MIL-53(Al) which also shows the vibration bands in the wavelength range of 1700-1400 cm −1 , indicating that the carboxylic functional groups are attached to aluminum. According to this figure, the firm observed peaks at 1507.87 and 1579.56 cm −1 belong to the (−COO) asymmetric stretching, and the characteristic peak at 1413.15 belongs to (−COO) symmetric stretching of the carboxyl vibration. No additional peak is observed at a wavenumber of 1700 cm −1 , indicating that the free BDC www.nature.com/scientificreports/ acid molecules are completely removed from UAS MIL-53(Al) pores 18 . The peaks at 1631.9 cm −1 and vibration bands ranging from 3700 ± 3400 cm −1 confirm the observation of the bending and stretching modes of water as well as the signature of the hydroxyl group that links the aluminum particles 27 . The peak at 989.59 may relate to the bending vibrations of the hydroxyl group in the octahedral AlO 4 (OH) 2 with trans corner-sharing 28 . The FTIR peaks of UAS MIL-53(Al) samples are in contant with those of referenced MIL-53(Al) 27 . Following the adsorption of Pb(II) ions, some bands in the FTIR spectra of UAS MIL-53(Al) shift to lower or higher wave numbers.  www.nature.com/scientificreports/ For example, the bandwidth at 3456.17 cm −1 assigned to O-H moves to 3469.48 cm −1 revealing that stretching of the hydroxyl group was responsible for Pb(II) ion binding to the adsorbent. A minor shift in peak location is also noticed, moving from 1579.56 to 1582.22 cm −1 which could be related to the creation of co-ordinate bonds during the adsorption process 29 . The hydroxyl group band at 1631.9 cm −1 broadens after Pb(II) ions adsorption. Transformations in peak sizes and locations were seen in the UAS MIL-53(Al) spectra following adsorption of Pb (II). As illustrated in Fig. 5, it was discovered that the (−COO), (C-H), and (C-O-H) bends were also responsible for the efficient removal of Pb(II) ions.

Adsorption studies UAS MIL-53(Al) adsorption performance in removing of different heavy metal ions.
In order to discover the selectivity of UAS MIL-53(Al) in removing the heavy metals from water, an adsorption experiment was carried out to measure the remaining concentration of these metals such as Pb 2+ , Ni 2+ , Cu 2+ in the solution after adsorption. Three adsorption experiments, each containing 20 ppm of intended heavy metal, 0.03 g UAS MIL-53(Al), and 40 ml solution, were carried out to determine the selectivity of the adsorbent. According to Fig. 6, UAS MIL-53(Al) as an adsorbent can perfectly remove lead(II) from the solution since it comprises carbonyl groups. Therefore, the following tests were carried out using lead(II).

Effect of pH on Pb(II) adsorption.
The pH of the mixture is considered an essential parameter to be examined during the adsorption studies since changing pH can alter the surface charge of the adsorbent. Also, pH can strongly influence the solubility of metal ion 30 . First, in order to choose the pH range of the solution, the pH pzc of the adsorbent should be analyzed. The pH pzc of the solution which is the surface charge of the adsorbent is zero. In case pH pzc > pH, the surface has a more positive charge by decreasing pH, and in case pH pzc < pH, the surface has a more negative charge by increasing the pH. Figure 7 shows the pH pzc in this study. The removal efficiency of Pb(II) ions were investigated in different initial pH ranges (2-7), as shown in Fig. 8, which were set using the required amount of HCl or NaOH solutions. The pH range in this study was selected from (2-7) because at pH ≥ 7, lead(II) ions precipitated as Pb(OH) 2 . While pH pzc 's adsorbent charge, at pH above 5 is negative, it is positive at lower pH 13,31,32 . As observed in Fig. 7, upon increasing the pH from 2 to 7, at first, the removal of adsorption on UAS MIL-53(Al) increases from 19 to 86%, meaning that the surface charge of adsorbent is adsorption efficiency was studied in the range of 0.001 to 0.04 g. This study was conducted at a temperature of 298 K and pH of 6 with an initial Pb(II) concentration of 20 ppm for about 60 min. As shown in Fig. 9, the adsorption rate of Pb(II) increases from 27.1 to 86% upon increasing UAS MIL-53(Al) dose, thus increasing the number of available adsorption sites on the UAS MIL-53(Al) surface for binding Pb(II) ions. The significance of the adsorbent dosage indicates its adsorption capacity at a specific initial concentration. It is expected that upon increasing the amount of the adsorbent, the adsorption efficiency will increase due to the increasing amount of adsorbent. Because of the large surface area of the sample, the number of active sites for complexing metal ions increases and the adsorption process accelerates 33 . As the adsorbent reaches its maximum efficiency, the number     Fig. 11, the adsorption efficiency increases rapidly upon increasing the time in the first 30 min. Finally, by occupying all the active sites on the adsorbent, the adsorption speed decreases considerably and reaches equilibrium in 60 min. Hence, the time limit decreases from 180 to 60 min for all the other experiments. During the first 30 min of the adsorption, the process accelerates owing to the large number of adsorption sites available for metal ions and the adsorbent pores are swiftly filled by the adsorbed ions. Over time, increased adsorption slows down due to such constraints as repulsive forces between the adsorbed metal ions on the surface of UAS MIL-53(Al) and metal ions in the liquid; of note, the adsorption sites remain intact. Moreover, the metal ions are forced to move more profoundly and longer to capture the pores, thus reducing the adsorption rate within minutes. The adsorption process continues until reaching the equilibrium time for adsorption. Equilibrium time is defined as when the adsorption process reaches equilibrium and saturation and the adsorption rate does not change much with time.

The adsorption kinetics of Pb(II) on UAS MIL-53(Al).
The kinetic studies were conducted using 0.04 g of UAS MIL-53(Al) as an adsorbent in the condition characterized by different times (5-180 min), pH of 6, and temperature of 298 K using 20 ppm of the initial concentration to remove lead(II) ion from 40 ml solution. To  www.nature.com/scientificreports/ determine the effect of contact time on the adsorption rate and present valuable information about the process mechanism, equilibrium time, and rate control levels, kinetic models were used to test experimental data. The adsorption kinetics of Pb(II) on UAS MIL-53(Al) are shown in Fig. 12. The figure shows the amount of adsorption capacity (mg/g) versus contact time. As discussed before the adsorption capacity swiftly increases in the first 30 min and then the changes were gradually until the equilibrium was reached after 60 min. In order to analyze the data and evaluate the adsorption quality different kinetic models including pseudo-first-order and pseudosecond-order as well as intra-particle diffusion kinetic models are employed 34 .
Pseudo first-order kinetic model. The Pseudo first-order rate is presented in 35 Eq. (3).
in which q e (mg/g) is the amount adsorbed equilibrium, q t (mg/g) the amount adsorbed at any time, and K 1 (min −1 ) a pseudo-first-order rate constant. The k 1 and ln q e of the pseudo-first-order model can be obtained from the slope and intercept of ln (q e − q t ) vs. time plot ( Fig. 13; Table 2). The theoretical plot of qt vs. time using pseudo first-order is depicted in Fig. 12, indicating that the theoretical values of q t do not match the experimental data. Therefore, this kinetic model does not adequately describe the adsorption process. The theoretical data's R 2 and q e values are shown in Table 3.
Pseudo-second order kinetic model. The pseudo-second-order rate is determined using 34 Eq. (4).
in which K 2 (g/g min) is a pseudo-second-order constant. The k 2 and q e of the pseudo-second-order model can be obtained by plotting the t/q t vs. time plot (Fig. 13; Table 2). The values of 1/q e and 1/k 2 q e 2 can be obtained from the slope and intercept, respectively. The theoretical plot of q t vs. time using pseudo-second-order is shown by dash line in Fig. 12. The consistency between the experimental and theoretical data regarding pseudo-secondorder kinetic model can be confirmed from Fig. 12. The R 2 and q e values of pseudo-second-order are given in Table 3; The q e values calculated from pseudo-second-order kinetic (18.51 mg/g) is more consistent with the experimental q e value found at 17.35 mg/g than that calculated from the pseudo-first-order model, showing that the pseudo-second-order kinetic model described the adsorption process better. (5) defines the intra-particle diffusion rate to detect the diffusion mechanism and rate control steps of the adsorption process 36 . in which K i is the intra particle diffusion rate constant (mg/(g.min 0.5 )) and C(mg/g) is the intercept value (Table 3; Fig. 13). The theoretical plot of q t versus time using intra particle diffusion kinetic model is shown by dash line in Fig. 12. Commonly, the overall adsorption process may be controlled by several steps, e.g., adsorbent mass transfer occurs across the boundary layer (film diffusion), pore diffusion, surface diffusion, and intra particle

Intra-particle diffusion kinetic model. Equation
(5) q t = K i t 0.5 + c  www.nature.com/scientificreports/ diffusion. The slowest stage is used to manage the overall rate of the adsorption process 37 . Intra particulate diffusion predominantly contributes to the rate-limiting step if the correlation diagram of adsorbed ions (q t ) versus (t 0.5 ) gives a straight line through the origin. If not, the plot may present a multi-linearity demonstrating that multiple steps, such as boundary layer diffusion or other processes. Moreover, the intercept value (C) represents the boundary layer thickness, and a higher C value specifies a thicker boundary layer. As illustrated in Fig. 13, adsorption occurs in two stages, the first and final stages. It is assumed that the initial stage with a sharper slope indicates that the adsorption follows diffusions with the particles or mass transfer. The second section, where slope k is close to zero, is the gradual adsorption stage with controlling intra-particle diffusion 37,38 . The R 2 , C, and K i values are obtained from the second stage (intra particle diffusion), and the results are given in Table 2. Figure 13 indicates that intra particle diffusion is not the only rate-limiting step since the straight lines do not pass from the origin 39 .
Adsorption isotherms. The adsorption isotherm models proposed by Langmuir 40 , Freundlich 40 , Temkin 40 were applied at a temperature of 298 K with different lead(II) concentrations of 20, 50, 70, 100, and 130 ppm, 0.04 g of UAS MIL-53(Al), and pH of 6 for 60 min to investigate the interaction between equilibrium concentration data and lead(II) ion adsorption.
Langmuir isotherm. This isotherm is employed to adsorb a dynamic equilibrium surface on perfectly homogeneous surfaces, assuming a monolayer coating onto the adsorbent surface area. In addition, due to the occupation of the adsorption sites, the adsorbed molecules do not interact with each other. The parameters of the Langmuir isotherm can be calculated through plotting its linear diagram in Eq. (6): where C e (mg/L) is the metal ion concentration, q e (mg/g) the number of metal ions adsorbed in the equilibrium phase, q m the maximum monolayer adsorption (mg/g), and k l (L /mg) the Langmuir constant related to the adsorption energy. The values for the Langmuir constants k l and q m were achieved from slope and intercept of C e versus C e /q e plot given in Table 4 and Fig. 14. In case R 2 is more than 0.99, the surface assimilation of the method follows the Langmuir isotherm. The dimensionless equilibrium parameter R L is defined to explain the type of isotherm (if R L = 0, irreversible; R L > 1, unfavorable; 0 < R L < 1, favorable; and R L = 1, linear) in the adsorption process obtained from 41 Eq. (7).
where C 0 is the initial concentration (mg/L). When the value of R 2 is 0.9965, the Langmuir isotherm fits the adsorption data and since 0 < R L < 1, the type of Langmuir isotherm is favorable. The maximum monolayer adsorption (q m ) can be achieved by linear Langmuir isotherm that is estimated to be 48.076 mg/g near the values of experimental maximum adsorption capacity (44.85 mg/g).
Freundlich isotherm. Freundlich equilibrium isotherm model can illustrate the concept of heterogeneous surfaces and assume adsorption on the multilayer surfaces. The parameters of this model (k f and n) can be calculated from the intercept and slope of ln q e versus ln C e diagram, respectively, using Eq. (8).
where K f is an adsorption isotherm constant that signifies an approximate adsorption capacity and n is the Freundlich equilibrium constant related to the adsorption intensity. If the value of n ranges from one to ten, www.nature.com/scientificreports/ the adsorption is assumed favorable. In the case of n = 3.6941, the adsorption process is favorable. The values of Freundlich constants are given in Table 4 and Fig. 14.
Temkin isotherm. Temkin isotherm is elaborated in Eqs. (9) and (10). Through this model, the effect of the indirect interaction of adsorbent on the heat reduction of the adsorption can be clarified 39 .
B T (J/mol) constant denotes the heat of the adsorption coverage, and K T (L/g) is a constant of the binding energy, indicating the significant connection between the adsorbate and adsorbent 39 . Moreover, the Temkin isotherm (K T and B T ) constants can be measured by plotting ln q e versus ln C e diagram from the intercept and slope, respectively. The isotherm model plots are given in Fig. 14, and all the constant values are summarized in Table 4.
Comparison of different adsorbents performances in Pb(II) removal. Table 5 compares the lead (II) removal data using various adsorbents. It was found that a significantly small amount of UAS MIL-53(Al) had a moderately better adsorption ability than other samples in a short time. The difference in adsorption rate and capacity of the listed adsorbents lies in different conditions of the adsorption performance, such as temperature, the mass of adsorbent, contact time, the volume of the solution, and difference in surface areas and functional groups. Principally, UAS MIL-53(Al) managed to have a very high adsorption efficiency of 97.63% in comparison to some other adsorbents at pH 6, due to its hierarchical structure and vaster surface area. Two key www.nature.com/scientificreports/ factors influence the adsorption performance the most, including specific surface area and adsorbent-adsorbate interactions, which can be optimized by doing some pre/post-treatments. As shown in Table 1, UAS MIL-53(Al) has a greater surface area comparing the conventional one CS MIL-53(Al), 1538.6 and 1274.3 (m 2 /g), respectively. Although the nature of both adsorbents and so their interactions to the adsorbate is the same, the one with the more specific surface area has a greater adsorption performance of 97.63%, which shows the importance of the surface area in providing the more accessible active sites for surface adsorption. Besides, comparing with MIL possessing functional groups 13 , although the adsorbent has a low surface area it shows the great adsorption performance because of adding functional groups which resulted in more adsorbent-adsorbate interactions.
Regeneration of adsorbent. The regeneration efficiency of UAS MIL-53(Al) as an adsorbent was evaluated with various elements such as H 2 SO 4 , and HCl. HCL 0.1 M as a strong acid was discovered to be the best desorption of Pb +2 ion to wash the adsorbent with it, rewash it with distilled water, and dry and check its recyclability adsorbent. 0.1 M HCl was used to wash up the Pb(II) for one cycle. The Comparison of adsorption for Pb(II) on Conversion and Ultrasonic synthesis and regeneration of adsorbent in one cycle has shown in Fig. 15.

Mechanism of adsorption.
According to the pseudo-second-order kinetic model, the adsorption mechanism is chemical. Figure 16 proposes the adsorption mechanism schematically. It is observed that MIL-53(Al) has oxygen atoms with free negative charges, which attracted positively charged Pb +2 . Thus, chemo sorption occurred as a result of an electrostatic attraction between electronegative oxygen atoms and electropositive Pb +2 .

Conclusion
In this study, ultrasonic assisted synthesis (UAS) of MIL-53(Al) and its adsorption performance in removing of lead (II) ions from aqueous solution were examined. Comparing the conventional synthesis, the sonochemical synthesis leads to shorten the synthesis time from 3 days to 1 day, resulting in energy consumption reduction, faster reaction rate. The usage of ultrasound in MIL-53(Al) synthesis additionally creates the hierarchical www.nature.com/scientificreports/ structure, increases the relative crystallinity and specific surface area of the sample up to 1538.6 m 2 /g. The optimal conditions for lead(II) adsorption were achieved using 0.04 g of UAS MIL-53(Al) adsorbent, 60 min of contact time as equilibrium time, an initial concentration of 20 mg/L, and maximum pH of 6. UAS MIL-53(Al) confirmed the adsorption efficiency of 97.63% at 315 K. The kinetic data followed a pseudo-second-order rate equation where lead(II) ions were adsorbed onto various sites at different points. Also, the equilibrium data fit the Langmuir isotherm model. In summary, it is expected to use UAS MIL-53(Al) as a promising adsorbent for lead(II) removal from aqueous solutions.