A functionalized metal–organic framework decorated with O– groups showing excellent performance for lead(ii) removal from aqueous solution

A novel MOF decorated with O– groups was elaborately constructed and showed excellent performance for Pb2+ removal.


Introduction
Lead (Pb 2+ ), a prevalent and bio-accumulative heavy metal, has been recognized as one of the most toxic metals. 1 With the rapid growth of industrialization and urbanization, Pb 2+ discharge has become a serious environmental concern. 2 At present, available clean water is limited and 90% of the available fresh water will be consumed by 2025. 3 Almost all countries strive to remove Pb 2+ from potable water in order to reduce levels of elevated blood Pb 2+ in children. 4 Owing to this fact, the removal of Pb 2+ from water is critical in terms of the protection of public health and the environment. Many efforts have been dedicated to purify wastewater to get more available clean water. However, an efficient, cost-effective, robust and handy technology for the decontamination of water is urgently needed. The adsorption technique for removing heavy metal ions has gained extensive attention considering the easy operation, eco-friendliness and cost-effectiveness. 1,5 Some effective adsorbents have been continuously developed and improved, including zeolites, 6 carbon materials, 7 clay minerals, 8 nanomaterials 9,10 and chelating polymers. 11 Meanwhile, those sorbents face some challenges, such as a low capacity, a moderate affinity/ selectivity, difficulties in separation, and a lack of structural and functional tunability, 12 which have largely limited the effectiveness for Pb 2+ removal. Therefore, there is an increasing interest in developing more efficient absorbents for the removal of Pb 2+ from aqueous solution.
Herein, a functionalized MOF decorated with negatively charged O À groups was designed and applied to the removal of Pb 2+ from aqueous solution (Scheme 1). Based on the design, the negative charges of the O À groups generate electrostatic interaction with Pb 2+ , which acts as a driving force in the adsorption process. Secondly, the O À groups also worked as active sites to form coordination bonds with Pb 2+ . Thirdly, as a Lewis acid, Pb 2+ has a lower hydration energy and a larger ionic radius, and could readily accept electrons from the Lewis base of the O À groups. As a result, the O À groups show a signicant affinity and high selectivity for Pb 2+ . Fourthly, the multiple porosity densely populated with O À groups would endow Pb 2+ with fast sorption and a high removal efficiency. In this study, we synthesized a three-dimensional (3D) porous framework {[Zn 3 L 3 (BPE) 1.5 ]$4.5DMF} n (1, H 2 L ¼ 4,4 0 -azoxydibenzoic acid, BPE ¼ bis(4-pyridyl)ethylene, DMF ¼ N,Ndimethylformamide) functionalized with O À groups for the removal of Pb 2+ . The activated MOF material [Zn 3 L 3 (BPE) 1.5 ] n (1a) exhibited an ultrahigh uptake capacity (616.64 mg g À1 ), a very high affinity (K d 10 6 mL g À1 ) and a high removal efficiency for Pb 2+ . The adsorption mechanism was revealed by zeta potential, FT-IR and XPS studies. To the best of our knowledge, we are the rst to introduce functional groups with negative charges into the pores of MOFs for the removal of heavy metal ions with excellent performance.

Materials and apparatus
The ligand H 2 L was synthesized according to the literature method. 30 All of the other reagents and chemicals were of an analytical grade and obtained from commercial sources. Powder X-ray diffraction (PXRD) data were collected on a PANalytical X'Pert PRO MPD system (PW3040/60). Fourier transform infrared (FT-IR) measurements were conducted on a Thermo Nicolet iS50 spectrometer. Scanning electron microscopy (SEM) images were taken on a Hitachi SU8010 instrument. X-ray photoelectron spectroscopy (XPS) data were obtained with a Thermo Escalab 250 spectrometer with monochromated Al-Ka excitation. The zeta potentials were determined using dynamic light scattering (DLS) on a Malvern Instruments Nanosizer-ZS. Thermogravimetric analysis (TGA) was carried out on a Netzsch STA-449F3 thermogravimetric analyzer under a nitrogen atmosphere at a heating rate of 10 C min À1 . Simultaneous inductively coupled plasma optical emission spectrometry (ICP-OES) on a PerkinElmer Optima 8000 instrument was used to determine the metal ion concentration in aqueous solution.

X-ray data collection and structure determination
Single X-ray diffraction intensities of crystals were collected on a CCD diffractometer at 153 K. All diffractometers were equipped with graphite monochromated Mo-Ka radiation (l ¼ 0.71073). The structure was solved by a direct method and expanded with the Fourier technique. All of the calculations were performed with the SHELXL-97 package. 31 In 1, two azoxy groups of the L 2À ligand were found to be disordered over two positions. All H atoms in 1 were placed in geometrically idealized positions and constrained to ride on their parent atoms. Moreover, the diffused electron densities resulting from these residual solvent molecules were removed from the data set using the SQUEEZE routine of PLATON and rened further using the data generated. 32 The formula of {[Zn 3 L 3 (BPE) 1.5 ]$ 4.5DMF} n was derived from thermogravimetric characterization. The crystal data for 1 are summarized as follows:

Adsorption studies
All of the adsorption experiments were carried out at 25 C using 10 mg of MOF 1a and 80 mL of Pb 2+ standard solution. The Pb 2+ solution was prepared by dissolving Pb(NO 3 ) 2 in deionized water and diluting to the desired concentration. To study the adsorption kinetics, adsorption experiments were performed at pH 6.0 (10 ppm Pb 2+ solution) under continuous stirring. The Pb 2+ concentrations of samples were measured at given time intervals using an ICP-OES spectrometer. The adsorption isotherm experiments were investigated by adding 10 mg MOF 1a into 80 mL Pb 2+ solutions with different concentrations to reach adsorption equilibrium within 180 min. The adsorption capacity q t (mg g À1 ) and the removal efficiency were obtained from the following equations: where C 0 (mg L À1 ) and C e (mg L À1 ) are the initial and equilibrium Pb 2+ concentrations, respectively; C t (mg L À1 ) is the Pb 2+ concentration at time t (min); V (L) is the volume of the solution, and m (g) is the dry weight of adsorbent. Furthermore, the desorption test was carried out by immersing the Pb 2+ loaded MOF in 80 mL of HNO 3 solution (0.1 mmol L À1 ) at 25 C for 24 h. Then it was washed three times with water and dried under vacuum at 140 C for 24 h. The desorbed Pb 2+ was evaluated by the same method used in the adsorption studies.

Characterization of 1 and 1a
The single-crystal X-ray diffraction study indicates that MOF 1 crystallizes in the C2/c monoclinic space group. The framework is composed of paddlewheel dinuclear Zn 2 (COO) 4 secondary building units that are bridged by L 2À ligands and further pillared by BPE to construct an interpenetrating 3D framework ( Fig. 1). Although interpenetration occurs, two kinds of pore are still present in MOF 1 ( Fig. 1c and d), which are calculated by the Platon program 31 to have an effective solvent accessible volume of 8462.8Å 3 per unit cell (47.2% of the total cell volume). As expected, the pores in MOF 1 are decorated by a large number of O À sites, resulting in the functionalized pores. These functionalized pores will certainly be favorable for the capture of heavy metal ions.
Thermogravimetric analysis (TGA) of 1 displays a major weight loss (20.2%) of the DMF guest molecules at 40-222 C (Fig. S1 †). So, the as-synthesized 1 was heated at 140 C under vacuum for 24 h to generate the activated sample 1a. The TGA of 1a showed that the DMF molecules were completely removed ( Fig. S1 †), which suggested it would be a potential sorbent. The TGA, together with the elemental analysis, conrmed that the chemical formula of 1a was [Zn 3 L 3 (BPE) 1.5 To examine the permanent porosity of 1a, gas sorption isotherms were investigated (Fig. S2 †). Unexpectedly, there is almost no N 2 adsorption at 77 K compared to CO 2 adsorption at 195 K, which can possibly be attributed to the different kinetic diameters of N 2 (3.64Å) and CO 2 (3.30Å which results in easier diffusion into the micropores). Similar N 2 and CO 2 sorption behaviors have been observed in many other exible MOF materials. [33][34][35][36][37][38] The Brunauer-Emmett-Teller surface area of 1a is calculated to be 82.5 m 2 g À1 based on the CO 2 adsorption isotherm. This is probably due to a higher diffusion barrier imposed on the 1D pore for the exible MOF aer desolvation and the corresponding structural contraction. 33,34 As illustrated in Fig. S3, † the X-ray diffraction peaks obtained from 1a were sharp, indicating the crystalline nature of the desolvated phase. The low-angle Bragg's reection in 1 vanished in 1a. According to the literature, there is a slight shrinkage of the framework of 1a aer guest molecule removal from 1. [38][39][40][41] Soaking 1a in DMF for 24 h generated 1 0 , the low-angle Bragg's reection reappeared and the PXRD pattern was almost the same as that of 1 (Fig. S3 †), which indicates that 1a is a exible MOF and the framework did not collapse during this process. These results can be explained by the breathing behavior which has been extensively studied over the past few decades (Scheme S1 †). 34,35,41,42 For the exible interpenetrated frameworks, subtle differences of guest content and composition will lead to different structures, and this transformation is reversible. 33,36,39,43,44 Interestingly, a slight difference was observed in the PXRD data simulated from the single crystal data of 1 at 153 K and collected at 298 K (Fig. S3 †). It can be concluded that the temperature induced structural changes in the exible interpenetrated frameworks 34,39 and also resulted in the different unit cell parameters (153 K, a ¼ 19. To elucidate the solvent induced breathing behavior, 33 1 was immersed in water (24 h), and the single crystallinity of the resulted H 2 O-exchanged 1 (1-H 2 O) was not good. However, by immersing 1-H 2 O in DMF for 24 h, 1-H 2 O-DMF with better crystal quality was obtained and the unit cell parameters were very similar to those of 1 (298 K, Table S1 †). So the structural transformation is reversible, and it is directly evidenced by the single crystal images (Fig. S4 †). To reveal the structural change in the process of solvent-exchange, we tried to determine the crystal structures of various solvent-exchanged MOFs. Fortunately, we got the single-crystal structure of CHCl 3 -exchanged 1 (1-CHCl 3 ) at 153 K (Table S2 †), which retained the original metal-ligand connectivity. Meanwhile the total cell volume decreased from 17 929Å 3 for 1 (153 K) to 17 753Å 3 for 1-CHCl 3 (Table S2 †), indicating a slight contraction of the square grid (Fig. S5 †). Aer soaking 1-CHCl 3 in DMF, the unit cell parameters of the generated 1-CHCl 3 -DMF were almost the same as those of 1 (298 K). These results further conrmed that the MOF is dynamic in nature and the transformation of the exible framework is reversible, 33,37,39,43 which is also supported by the PXRD patterns. As shown in Fig. S6, † the PXRD patterns of solvent-exchanged 1 are different from those of 1, but return to 1 aer immersion in DMF for 24 h (Fig. S7 †).

Pb 2+ sorption studies
The pH value greatly inuences the adsorption performance of Pb 2+ , so the effect of the pH ranging from 3.0 to 7.0 on the adsorption of Pb 2+ was investigated. As shown in Fig. S8, † the removal efficiency of 1a was very low at pH 3.0 and it increased dramatically with the increasing of the pH and achieved the largest signal at pH 6.0. Further increasing the pH resulted in a decline of the removal efficiency. This is due to the protonation effect on the surface of the MOF adsorbent and excess H + competing for the sorption sites at a low pH. When the pH is higher than 6.0, Pb 2+ hydroxide precipitation may occur. 45 Accordingly, the optimum pH value of 6.0 was selected for the subsequent adsorption experiments.
To evaluate the effectiveness of Pb 2+ removal from water, 1a was placed in a Pb 2+ solution of 10 ppm at pH 6.0. The Pb 2+ loaded 1a (1a-Pb) was isolated and washed with water to remove the residual Pb 2+ on the exterior of 1a-Pb. Then it was examined by energy-dispersive X-ray spectroscopy (EDS) and this conrmed the existence of Pb (Fig. S9 †). As shown in Fig. 2, 1a can rapidly capture Pb 2+ , and remove 98.12% Pb 2+ within 7 min. Aer 1 h, the concentration of the residual Pb 2+ reduced to 0.035 ppm, that is, 99.65% Pb 2+ was removed. The fast kinetics and high efficiency for Pb 2+ removal could be attributed to the high affinity of 1a.
To assess the sorbent's affinity for Pb 2+ , the distribution coefficient K d (mL g À1 ) was calculated as follows: where C 0 (mg L À1 ) and C e (mg L À1 ) are the initial and equilibrium concentrations of Pb 2+ ; V is the volume of the treated solution (mL); m is the dry weight of adsorbent (g). K d represents an important aspect of the sorbent performance. In general, a material with a K d value above 10 4 mL g À1 is considered to be an excellent adsorbent. 46,47 The K d value of 1a for Pb 2+ is 2.3 Â 10 6 mL g À1 , surpassing that of various reported benchmark materials, for examples, biochars (10 3 to 10 4 mL g À1 ), 48 mesoporous carbon (6.82 Â 10 5 mL g À1 ), 49 commercial resins (10 4 to 5.1 Â 10 5 mL g À1 ), 50 lignin functionalized carbon nanotubes (3.6 Â 10 5 mL g À1 ), 51 MoS 4 -LDH (2.6 Â 10 5 mL g À1 ), 47 and pampeano aquifer (7.5 Â 10 3 to 1 Â 10 4 mL g À1 ). 52 Fig. 2 The kinetics and efficiency of 1a for Pb 2+ removal with an initial concentration of 10 ppm.
The adsorption isotherms were investigated to estimate the maximum adsorption capacity of 1a by varying the Pb 2+ concentrations from 5 ppm to 200 ppm at pH 6.0. As shown in Fig. 3a, the value of q e (equilibrium adsorption capacity) increased with the increasing Pb 2+ concentrations and nally reached the maximum value of 616.64 mg g À1 . In addition, we also considered the removal efficiency at different Pb 2+ initial concentrations (Fig. S10 †). More than 90% Pb 2+ can be removed in the concentration range from 1 ppm to 75 ppm. The equilibrium adsorption isotherm data was tted by the Langmuir model, yielding a relatively high correlation coefficient of 0.9997. The maximum adsorption capacity of 1a for Pb 2+ was calculated to be 613.50 mg g À1 , which closely matched with the experimental equilibrium value of 616.64 mg g À1 . We reasoned that such an excellent lead adsorption capacity could be stemmed from the highly accessible O À groups densely populated throughout the pores of 1a.
To highlight the contribution of the O À groups to the Pb 2+ uptake, we synthesized a similar MOF to 1 but without O À groups, [Zn(ADC)(BPE) 0.5 ] n (ADC ¼ 4,4 0 -azobenzenedicarboxylate), 37 and investigated its adsorption isotherms for Pb 2+ . Results show that the activated [Zn(ADC)(BPE) 0.5 ] n displayed a lower uptake capacity of 473.92 mg g À1 . Meanwhile, we calculated the theoretical uptake capacity of 1a based on the molecular formula, pore volumes and one O À group per L 2À adsorbing one Pb 2+ ion by a coordination interaction. The calculated theoretical uptake capacity is 470.11 mg g À1 , which is obviously lower than the experimental equilibrium value (616.64 mg g À1 ) of 1a. The higher experimental adsorption capacity for 1a could be attributed to the various adsorption modes formed by the O À groups, which offered not only coordination interactions but also electrostatic attractions between the O À groups and Pb 2+ . To the best of our knowledge, the Pb 2+ uptake capacity of 1a is the highest among the reported MOF adsorbents (Table S3 †), and also exceeded that of other Pb 2+ adsorbents. 51,[53][54][55][56][57] These results suggest that 1a is a prospective adsorbent for Pb 2+ removal due to its large adsorption capacity and high removal efficiency.
The selective removal of heavy metal ions will facilitate environmental protection and allow the reuse of heavy metals. 53 However, a lot of adsorbents for Pb 2+ removal exhibit low selectivity. An absorbent with high selectivity is required to separate heavy metal ions from wastewater. Selectivity tests were performed in a mixed solution containing Na + , Mg 2+ , K + , Ca 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cd 2+ and Pb 2+ with the concentration of 10 ppm for  each metal ion. The adsorption ability of 1a toward Pb 2+ is considerably higher than that for other ions (Fig. 4a). In the mixed solution, 99.27% Pb 2+ was removed, while 17.46% Cd 2+ was absorbed. In contrast, other background metal ions such as Na + , Mg 2+ , K + , Ca 2+ , Mn 2+ , Co 2+ , and Ni 2+ do not quite bind to 1a, with an equilibrium removal efficiency of less than 8%. This phenomenon can be explained by the HSAB principle. 58 The negatively charged O atoms of the azoxy groups have the properties of a borderline base, which could readily bind with the metals Co 2+ , Ni 2+ , and Pb 2+ (borderline acids) and Cd 2+ (so acid), and bind difficultly with Na + , Mg 2+ , K + , Ca 2+ , and Mn 2+ (hard acids). Compared to the borderline acid Pb 2+ , the relatively low removal efficiencies for the borderline acids Co 2+ and Ni 2+ are attributed to their fairly high hydration energies (1915 kJ mol À1 for Co 2+ , 1980 kJ mol À1 for Ni 2+ , and 1425 kJ mol À1 for Pb 2+ ). 59 It is difficult for Co 2+ and Ni 2+ to detach water molecules and further interact with the negatively charged O atoms. So the MOF material has a strong preference and fairly high selectivity for Pb 2+ against other metal ions, highlighting its potential in selectively removing Pb 2+ from real wastewater.
Ca 2+ and Mg 2+ are common cations that always ubiquitously co-exist in the natural water environment, and the same positive-charge and high level concentration might lead to competitive adsorption with Pb 2+ . 2,60,61 The so acid Cd 2+ , which has a comparable ionic size and charge density to Pb 2+ , and oen co-exists with Pb 2+ , 62 can strongly compete for the selectivity of sorbents. Thus, it is essential to investigate the effect of the competing ions Ca 2+ , Mg 2+ and Cd 2+ on the adsorption of Pb 2+ . As shown in Fig. 4b, outstanding Pb 2+ removal efficiencies were obtained in the presence of competing ions of Ca 2+ and Mg 2+ . With the molar ratios of Ca 2+ /Pb 2+ and Mg 2+ /Pb 2+ ranging from 1 to 50, there were only little decreases for Pb 2+ removal efficiencies, changing from 99.82% to 99.21% with Ca 2+ and 99.65% to 99.31% with Mg 2+ . A large removal capacity (96.43%) was still obtained at a molar ratio of 10 for Cd 2+ /Pb 2+ . When the ratio was increased to 50, the Pb 2+ removal efficiency reduced to 56.31%. The concentration of Cd 2+ in the natural water environment can hardly reach such a high ratio, thus Cd 2+ could not affect the Pb 2+ removal performance in a practical application.
All of these results indicate that 1a has a strong preference and an ultrahigh selectivity for Pb 2+ sorption. The particular selectivity can be ascribed to the following reasons: (1) the densely populated O À groups of the functionalized MOF provided strong interactions between O À and Pb 2+ ; (2) adsorbents preferentially adsorb divalent cations with a lower hydration energy in general, as metal ions have to detach a large share of hydrated water before they enter the smaller channels of the adsorbents. 59,60,63,64 Compared with Ca 2+ (1505 kJ mol À1 ), Mg 2+ (1830 kJ mol À1 ) and Cd 2+ (1755 kJ mol À1 ), the lowest Gibbs free energy of hydration was observed for Pb 2+ (1425 kJ mol À1 ), indicating Pb 2+ preferential adsorption than other divalent cations; 59 (3) it has been reported that a larger ionic radius would be favorable for the interactions between metal ions and functional groups of the adsorbent. [65][66][67] The radiuses of Ca 2+ , Mg 2+ and Cd 2+ are 0.100 nm, 0.072 nm and 0.095 nm, respectively, while the radius of Pb 2+ is 0.119 nm. The larger radius would endow 1a with quite good selectivity for Pb 2+ adsorption.

Stability and reusability study
Stability and reusability are two important factors in the application of an adsorbent. To verify the chemical stability of 1a, we performed in situ PXRD measurements to evaluate the crystalline structure. In Fig. 5 and S11, † aer soaking 1a in water for 300 min, and even soaking in Pb 2+ solution for more than 300 min, it still holds a high crystalline form and retained the structure of 1a, indicating the good chemical stability of 1a in water and Pb 2+ solution. On further immersion of 1a in aqueous solution of pH 3.0-10.0 (300 min), its high crystalline form was also preserved (Fig. S12 †). Meanwhile, SEM investigations were carried out to examine the surface morphology of 1a before and aer loading Pb 2+ . As shown in Fig. S13b, † the crystals of 1a did not collapse during the sorption process. 36 The adsorption isotherm of 1a-Pb exhibits a negligible uptake for CO 2 (Fig. S14 †). The different adsorption capacities and behaviors of 1a and 1a-Pb are clearly caused by loading Pb 2+ . To get single crystals with better quality, we soaked 1-Pb in DMF for 24 h, and determined the unit cell parameters of 1-Pb-DMF (Table S1 †). By comparison of the unit cell parameters with 1 (298 K), we can affirm that the adsorbent still maintains the single crystallinity even aer loading Pb 2+ .
From the consideration of economical and practical purposes, it is of great signicance to study the desorption and regeneration of an adsorbent. As shown in Fig. 6, 1a exhibited a favorable cycle performance for Pb 2+ adsorption, and the removal efficiency remained almost unchanged in the rst two cycles and only slight fading was observed in the following cycles. The slight decrease may result from the loss of the adsorbent during cycling. Similar results were also reported in previous studies. 55,68 Mechanism of Pb 2+ removal Generally, the adsorptive modes for metal ion removal mainly include electrostatic adsorption, coordination adsorption and ion exchange. To investigate whether there is electrostatic attraction in the sorption process of Pb 2+ , zeta potential measurements were performed. When the pH was above 4.0, the surface of 1a was negatively charged, and the density of the negative charge increased remarkably with increasing pH value (Fig. S15 †). Meanwhile, the removal efficiency exhibited a similar variation tendency along with the change of pH value. The zeta potential of 1a-Pb appeared to be less negative in the pH range of 5.0-7.0 due to charge neutralization. Hence, it is believed that the electrostatic attraction is the main force causing the adsorption of Pb 2+ onto 1a. When the solution pH was below 4.0, the surface charge on 1a turned positive, resulting in electrostatic repulsion between 1a and Pb 2+ . However, 1a still exhibited a removal efficiency of 19.68% at a pH of 3.0, which demonstrates that there are other interactions besides the electrostatic effect.
Evaluating the effect of the background electrolyte on Pb 2+ sorption is an effective macroscopic method to understand the adsorption mechanism. 69 One can see that the background electrolyte had little effect on Pb 2+ adsorption (Fig. S16 †). Even at a high concentration of 500 ppm, a relatively high removal efficiency (98.39%) for Pb 2+ was still obtained. Therefore, the mechanism of Pb 2+ removal does not involve ion exchange behavior.
To further investigate the mechanism of Pb 2+ sorption, FT-IR spectra of 1a and 1a-Pb were studied. In Fig. 7, a new peak at 581 cm À1 for 1a-Pb is a typical characteristic stretching vibration of Pb-O, 70 conrming the fact of Pb 2+ loading. The bands at 1378 cm À1 and 798 cm À1 occur aer Pb 2+ adsorption, implying the presence of NO 3 À , for charge balance. 47,71,72 There are strong interactions between Pb 2+ and the O À groups that could limit N-O stretching vibrations and consequently decrease their vibrational frequency, giving rise to a red shi from 1223 cm À1 to 1218 cm À1 . 25 For 1a, the peaks at 1608 cm À1 , 1579 cm À1 and 1375 cm À1 are assigned to the characteristic groups of the aromatic C]C and C-N vibrations, 73 whereas these peaks shied to 1601 cm À1 , 1573 cm À1 , and 1363 cm À1 , respectively, aer loading Pb 2+ . This might give further evidence of coordination interactions between Pb 2+ and O atoms.
XPS spectra were also employed to provide more information on the interactions between 1a and Pb 2+ . As shown in Fig. 8a, the appearance of the Pb 4f, Pb 4d and Pb 4p peaks veries that Pb 2+ is undoubtedly loaded on 1a. A more detailed structure of the Pb species could be obtained in the high resolution XPS spectrum of Pb 4f. In Fig. 8b, there are two peaks at 143.3 eV and 138.4 eV, corresponding to Pb 4f 5/2 and Pb 4f 7/2 . Compared with the Pb 2+ binding energies of puried Pb(NO 3 ) 2 at 144.5 eV for Pb 4f 5/2 and 139.6 eV for Pb 4f 7/2 , 60 a remarkable shi of 1.2 eV to a lower binding energy for Pb 4f can be observed in 1a-Pb, which reveals the formation of strong affinities between Pb 2+ and 1a. 2 The energy separation of 4.9 eV between the Pb 4f 5/2 (143.3 eV) and Pb 4f 7/2 (138.4 eV) peaks further conrms that the  coordination interaction, and not merely the electrostatic interaction, accounts for the mechanism of Pb 2+ sorption. 1,74 These results are consistent with the FT-IR analyses and zeta potential measurements.

Conclusions
In summary, a novel Zn(II)-based MOF functionalized with O À groups was elaborately constructed for the removal of Pb 2+ from aqueous solution. The activated MOF showed excellent performance for Pb 2+ removal with a record-high uptake capacity of 616.64 mg g À1 among MOF adsorbents, a high removal efficiency in a wide range (>90% at 1-75 ppm) and an exceptional distribution coefficient value of 2.3 Â 10 6 mL g À1 , making the activated MOF one of the most promising materials for eliminating Pb 2+ pollution from water. Notably, the activated MOF can selectively capture Pb 2+ with a high efficiency even in the presence of a high concentration of other metal ions. Also, the adsorbent can be readily regenerated and recycled without a signicant decrease of the removal efficiency. The densely populated and highly accessible O À groups with their remarkable affinity for Pb 2+ are responsible for the impressive results. This work therefore lays a foundation for introducing charged groups into MOFs as a new platform for efficiently removing contaminants from aqueous solution.

Conflicts of interest
There are no conicts to declare.