Effective Adsorption of Pb2+ on Porous Carbon Derived from Functional Octadecahedron ZIF-8

An adsorbent ZO (oxidized ZIF-8-derived carbon) was prepared on the ZIF-8-derived carbon (ZC) by modified Hummer’s method. The removal rate and adsorption amount of Pb2+ were measured on the different molar ratios of 2-Hmim to 2, 2′-bipyridine in ZO, including 1:1 (1:1 ZO) and 1:2 (1:2 ZO). The adsorption experiments show that the best condition to adsorb Pb2+ in Pb2+ solution for 1:1 ZO is an adsorbent dosage of 20 mg, adsorption time of 16 h, initial Pb2+concentration of 15 mg/L, and pH = 3; that for 1:2 ZO is the adsorbent dosage of 15 mg, adsorption time of 18 h, initial Pb2+ concentration of 15 mg/L, and pH = 4. The adsorption data fits the quasi-second-order kinetics (R2 = 0.99998), indicating that chemical adsorption plays a leading role. The fitted isotherm adsorption curve is more consistent with the Langmuir adsorption model (1:1 ZO, R2 = 0.95058; 1:2 ZO, R2 = 0.97488). The competitive adsorption results show that the removal rate of Pb2+ by 1:1 ZO and 1:2 ZO is more than 98%, indicating that 1:1 ZO and 1:2 ZO have a superior selectivity for Pb2+ competing with Cu2+ and Fe2+. The maximum adsorption amount of Pb2+ is 15.52 mg/g by 1:1 ZO and 18.09 mg/g by 1:2 ZO. This study shows that 1:2 ZO is more helpful for the removal of Pb2+ than 1:1 ZO.


Introduction
Heavy metals (HMS) are the most common pollutants in sewage [1], including lead (Pb), cadmium (Cd), mercury (Hg), chromium (Cr), and arsenic (As). They are highly toxic and non-biodegradable and harmful to all organisms [2]. So, HMS removal has attracted much attention owing to the high requirement of ecological civilization and health. Pb 2+ as one of HMS is not only non-degradable but also the most toxic metal [3]. Even a small amount of Pb 2+ is toxic to plants and animals [4]. Thus, the emission of Pb 2+ has led to a serious environmental problem, and effective removal of Pb 2+ is essential.
Recently, a large number of scholars have studied the chemical precipitation method and the ion exchange method to remove Pb 2+ , demonstrating the high efficiency of those methods. Maria Teresa Alvarez et al. [5] have investigated the Pb 2+ precipitation by biologically produced H 2 S, achieving above 92% of the Pb 2+ removal rate. James P. Bezzina et al. [6] have reported that ion exchange removal of Pb 2+ from acid-extracted sewage sludge is highly effective. However, limitations owing to cost-effectiveness, incomplete removal of Pb 2+ , and high energy requirement determine the application of chemical methods is restricted [7]. Comparatively, the adsorption method is not only friendly environmental but inexpensive for removing Pb 2+ from sewage [8,9].
The adsorption method is widely used in research and industry for the advantages of strong operability and high efficiency [10]. Common adsorbents for the adsorption of Pb 2+ are zeolite [11], graphene oxide [12], and biomass [13]. Although zeolite, graphene oxide, and biomass are popular adsorbents due to the high Pb 2+ removal rate of almost 100%, they are of high cost for wide application [11]. Zeolitic imidazolate frameworks (ZIFs) as a nanoporous carbon (Nc) material are cost-effectiveness and easy to synthesize, and they have a large specific surface area and pore volume. Therefore, they can be a promising adsorbent for Pb 2+ removal in sewage [14].
Researchers have developed various ZIFs with zeolite or zeolite-like topological structures; they have strong chemical robustness and good thermal stability [15,16]. In particular, ZIFs with sodalite topology, such as ZIF-8, give the improvement of Pb 2+ removal a high possibility due to their porous structure [17]. The successful synthesis of ZIF-8 in concentrated ammonium hydroxide aqueous solutions at room temperature was reported by Ming He et al. [17]. ZIF-8 is a framework formed by zinc ions and imidazole ligands with albite topology and has been widely studied in this type of material [18]. It can be used for gas adsorption/separation, catalysis, etc. [19,20]. Common ZIF-8 shapes are cube [17], cuboid [21], dodecahedron [22], spherical [23], and leaf-like [24]. After the carbonization of ZIF-8 (zinc-based ZIF) powder in a nitrogen atmosphere, ZIF-8-Derived carbon can be obtained after washing with hydrochloric acid and drying. Related literature reported that the morphology of ZIF-8 before and after carbonization did not be changed [25]. Recently, researchers found that the porous carbon synthesized by the ZIF-8 carbonization method can be used as an efficient adsorbent to remove pollutants [26][27][28]. A number of researches show that the ZIF-8 membrane presents an advantage in the separation of H 2 from a mixture [16]. However, the Pb 2+ adsorption by ZIF-8-derived carbons has been rarely investigated.
Currently, Hummer's method is the most common method used for preparing graphene oxide [29]. Thus, most of the previous studies have focused only on the preparation of graphene oxide by the modified Hummer's method [30]. ZIF-8-derived carbon prepared at 900 °C has a certain degree of graphitization, so Hummer's method can be used to surface oxidize ZIF-8-derived carbon and increase the functional group of ZIF-8-derived carbon surface.
In this study, ZIF-8-derived carbon was oxidized by the modified Hummer's method and used to adsorb Pb 2+ in an aqueous solution. Then, SEM, XRD, FT-IR, zeta potential, and BET five methods were used to characterize the materials. The effects of adsorption time, pH, adsorbent dose, and initial concentration on the removal of Pb 2+ were studied, and the optimal parameters for material removal of Pb 2+ were determined. After that, the material was used to adsorb Pb 2+ solution containing other metal ions to explore the material's selectivity to Pb 2+ .

Chemicals and Materials
The details of chemical materials used in the research are listed in Table 1. Deionized water (DW) used in all experiments was made in a laboratory. All chemical reagents were purchased without further purification.

Synthesis of ZIF-8
Zn (NO 3 ) 2 ·6H 2 O at 1.8 g was added to 90 mL CH 3 OH, and 0.5 g 2-Hmim was added to 45 mL NH 3 ·H 2 O. Then, the zinc ion-containing solution was slowly added to the above solution. Next, the solution was stirred for 5 h at 5 °C and centrifuged at 8000 rpm for 10 min and wash with methanol three times. Drying at 80 °C in a vacuum drying oven overnight, ZIF-8 powder was obtained. Finally, the molar ratios of 2-Hmim and 2, 2′-bipyridine in ZIF-8 include 1:1 (symbol 1:1 ZIF-8) and 1:2 (symbol 1:2 ZIF-8). (1) where R is the removal rate of Pb 2+ (%), C 0 and C t are the initial and equilibrium concentration of Pb 2+ (mg/L), Q t is the adsorption amount when the adsorption reaches equilibrium (mg g −1 ), V is the volume (L) of the adsorbed solution, and M is the mass (mg) of 1:1 ZO and 1:2 ZO used for adsorption.
Each group of the adsorption experiment was measured in parallel 3 times, the average value and standard deviation were calculated, and the error bars were made in the adsorption experiment diagram.

Adsorption Isotherm Experiment
In the experiment, 20 mg of 1:1 ZO and 15 mg of 1:2 ZO were added into 20 mL of Pb 2+ simulated waste liquid with a concentration of 15 mg/L. After stirring at room temperature for 16 h and 18 h, the obtained solution was filtered with a syringe filter. After filtration, 5 mL of the supernatant was taken and its concentration was determined using atomic absorption at a wavelength of 283.3 nm. Two isotherm adsorption models, Langmuir and Freundlich, were used to analyze the adsorption data of 1:1 ZO and 1:2 ZO on Pb 2+ .

Coharacterization
Scanning electron microscopy (SEM, SIGMA + X-MaxN, Germany) was used to observe the morphology of materials. X-ray diffraction (XRD, X'PertPowder, Netherlands) was used to study the crystal structure and phase of the materials, with the scanning range of 3 ~ 90° at the scanning speed of 2.4 s/step, and the tube pressure and tube flow were 40 kV and 20 mA, respectively. Functional groups can be identified by analyzing Fourier transform infrared (FT-IR) spectra (VERTEX 70) in the wavenumber range of 500 ~ 4000 cm −1 . Nitrogen adsorption-desorption isotherm analysis can be used for the structure analysis of porous materials (Micromeritics). Nitrogen gas was degassed at 120 °C for 6 h and then specific was determined at 77 K using a specific surface area and porosity analyzer. The N 2 adsorption and desorption curves of the materials were measured by Quantachrome AUTOSORB-1. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method in the range of partial pressure (P/P 0 ) from 0.02 to 0.22. Pore volume and pore size distribution were calculated by BJH (Barrett-Joyner-Halenda) theory. A thermogravimetric analyzer (TGA-DSC) STA 449C Jupiter thermal analyzer (Germany NETZSCH company) was employed to measure the weight loss of the ZIF-8 in the temperature range of 30 ~ 1000 °C with a heating rate of 10 °C min −1 under a nitrogen stream. Zeta potentials of the nanoparticles were determined by dynamic light scattering (Beckman, USA).

Structural Characterization
Scanning electron microscopy images of the material 1:1 ZO and 1:2 ZO are shown in Fig. 1. 1:1 ZO and 1:2 ZO is octadecahedral, and the particle is uniform and has a good dispersion. The particle size of 1:1 ZO is smaller than that of 1:2 ZO [14].
TGA curves of 1:1 ZO and 1:2 ZO are shown in Fig. 2. When the temperature is below 150 °C, the weight loss of 1:1 ZO and 1:2 ZO are minimal, attributed to the evaporation of water molecules adsorbed in the material's pore. There is no significant weight loss observed in the temperature range of 150 ~ 400 °C. 5.5% (1:1 ZO) and 10.84% (1:2 ZO) of the weight loss at 400 ~ 600 °C can be due to the decomposition of the carbon-containing frame into gaseous products, such as nitric oxide, carbon monoxide, and the formation of metal oxides (zinc oxide). The weight loss of 10.94% (1:1 ZO) and 11.94% (1:2 ZO) after 900 °C can be attributed to the generation of nitric oxide, carbon monoxide, and the precipitation of Zn during the reduction of ZnO. The final weight of 1:1 ZO retains about 45% of the initial mass, while the final weight of 1:2 ZO is only 33% of the initial mass.
The results indicate that the thermal stability of 1:1 ZO is higher than that of 1:2 ZO, which is related to the presence of more 2-Hmim in 1:1 ZO [17]. Table 2 shows the pore parameters of 1:1 ZO and 1:2 ZO. It can be seen that the specific surface area, pore volume, and pore size of 1:1 ZO are all larger than that of 1:2 ZO. This is related to the higher oxidation degree of 1:2 ZO. The results confirm the conclusions obtained from the nitrogen adsorption and desorption curves [11]. The nitrogen adsorption-desorption curves in Fig. 3a show that the adsorption isotherms of 1:1 ZO and 1:2 ZO have the characteristics of type IV isotherm. The adsorption of nitrogen by nitrogen adsorption and desorption isotherm at low pressure indicates a certain micro-porous structure in the material [23]. Figure 4a is an XRD diagram of 1:  [34].
As shown in Fig. 4b, the Fourier transform infrared spectra of 1:1 ZO and 1:2 ZO have the same trend. Both have  absorption peak at 1578 cm −1 , 1252 cm −1 , and 1720 cm −1 . Among them, the 1252 cm −1 peak corresponds to C-N, and 1578 cm −1 peak belongs to C = N and N-H groups. The peak at 1720 cm −1 corresponds to the stretching vibration peak of -COOH, indicating that 1:1 ZO and 1:2 ZO have been successfully oxidized [12]. According to Fig. 5a, both 1:1 ZO and 1:2 ZO are negatively charged, in which 1:1 ZO has a negative charge of 18.44 mV, and 1:2 ZO has a negative charge of 20.66 mV. The generation of these negative charges indicates that the surface of ZO material contains a large number of negatively charged functional groups. As shown in Fig. 5b, the pH PZC of 1:1 ZO is 2.38. So, when pH < 2.38, 1:1 ZO has a positive charge; and when pH > 2.38, 1:1 ZO is negatively charged. The pH PZC of 1:2 ZO is 3.87. So, when pH < 3.87, 1:2 ZO is positively charged, and pH > 3.87, 1:2 ZO has a negative charge [8].

Adsorption Research
To investigate the effects of the adsorbent dosage on the removal rate and adsorption amount of Pb 2+ , 5 mg, 10 mg, 15 mg, 20 mg, and 25 mg of 1:1 ZO and 1:2 ZO were added into 20 mL and 15 mg/L Pb 2+ solutions for adsorption  observation. The obtained removal rate and adsorption amount are shown in Fig. 6.
As shown in Fig. 6, with the increase in adsorbent dosage (5 ~ 25 mg), the removal rate of Pb 2+ increases from 20.62 to 96.62% by 1:1 ZO and from 21.41 to 98.21% by 1:2 ZO. The Pb 2+ adsorption amount of 1:2 ZO first increases from 12.8 to 16.76 mg/g, and then decreases to 11.79 mg/g, while that of 1:1 ZO firstly increases from 12.37 to 13.36 mg/g and then decreases to 11.59 mg/g. Thus, 1:1 ZO and 1:2 ZO have the same trend in the removal rate and adsorption amount. Reaching the maximum adsorption amount, the dosage of 1:2 ZO (15 mg) is less than that of 1:1 ZO (20 mg). Under this condition, the removal rate of Pb 2+ by both materials can reach more than 90%.
The reason for the low removal rate when a small amount of adsorbent was added is the insufficient adsorption binding site which leads to incomplete adsorption. When a sufficient amount of adsorbent was added, the provided active sites are enough to adsorb more Pb 2+ , leading to an increased removal rate. Further to this, increasing the amount of adsorbent to a state where the target pollutants were almost adsorbed, results in a low adsorption quantity so that the adsorption cannot be saturated. Therefore, it is necessary to choose a suitable dose of adsorbent. When the dosage is less than a certain value (15 mg of 1:2 ZO and 20 mg of 1:1 ZO), the active sites on the adsorbent are insufficient; the adsorption does not reach a saturated state, and the removal rate is low. On the contrary, the adsorption is saturated and the adsorption amount decreases.
To research the effects of the initial concentration in Pb 2+ solution on the removal rate and adsorption amount, Fig. 7 shows 1:1 ZO and 1:2 ZO adsorption following the concentrations of Pb 2+ solution. The volume of the Pb 2+ solution is 20 mL and the concentrations include 5 mg/L, 10 mg/L, 15 mg/L, 25 mg/L, and 35 mg/L. 20 mg of 1:1 ZO and 15 mg of 1:2 ZO were used as the adsorbent in the experiment.
As shown in Fig. 7, with the increase of the initial concentration of Pb 2+ (5 mg/L ~ 25 mg/L), the adsorption amount of Pb 2+ by 1:1 ZO (Fig. 2a) first increases from 4.81 to 12.01 mg/g. Then, it decreases to 7.17 mg/g and finally increases to 9.67 mg/g. The removal rate continues to decrease (96.35 ~ 27.62%). For comparison, the adsorption amount of Pb 2+ by 1:2 ZO (Fig. 7b) first increases from 6.32 to 12.81 mg/g. Then, it decreases to 12.38 mg/g and finally increases to 16.82 mg/g. The removal rate continues to decrease by 58.71%. Thus, the optimum initial concentration of Pb 2+ to obtain the maximum adsorption amount and the peak removal rate is 15 mg/L for 1:1 ZO and 1:2 ZO. Significantly, the adsorption amount and removal rate of 1:2 ZO are higher than those of 1:1 ZO at the same initial Pb 2+ concentration. So, 1:2 ZO has a better removal effect of Pb 2+ .  For the same adsorbent, when the concentration of Pb 2+ in the solution is less than 15 mg/L, it is easy to chelate with Pb 2+ or produce electrostatic adsorption due to enough carboxyl groups. However, if the concentration of Pb 2+ is greater than 15 mg/L, the carboxyl groups available for adsorption are insufficient, and sufficient active sites could not be provided for the adsorption of Pb 2+ , so the removal rate decreases.
To research the effects of the pH in Pb 2+ solution on the removal rate and adsorption amount, 20 mg 1:1 ZO and 15 mg 1:2 ZO were added into 20 mL of simulated waste liquid with a Pb 2+ concentration of 15 mg/L. 1 M NaOH and 1 M HCl were used to adjust the pH value containing 2, 3, 4, 5, 6, 7, and 8. The experiment results are shown in Fig. 8.
The pH value affects the interaction between adsorbates and adsorbents by changing the charge distribution on the surface of the adsorbates and adsorbents [35]. When the pH < 5, lead mainly exists in Pb 2+ and Pb(OH) + , and when 5 < pH < 10, lead mainly exists in the form of Pb(OH) 2 , Pb(OH) 4 2− , and Pb(OH) 3 − . As shown in Fig. 8, the adsorption amount and removal rate of 1:1 ZO and 1:2 ZO for Pb 2+ are first increased and then decreased with the increase of pH. The Pb 2+ adsorption amount (14.57 mg/g) and the Pb 2+ removal rate (97.13%) of 1:1 ZO reach the peak at pH = 3, shown in Fig. 8a. Because pH PZC of 1:1 ZO is 2.38 (shown in Fig. 5), when pH < 2.38, 1:1 ZO is positively charged, there are Pb 2+ and Pb(OH) + exist, and there is electrostatic repulsion between them. When 2.38 < pH < 5, the negative charge of 1:1 ZO is enhanced, and Pb 2+ has a positive charge and attracts each other.
As shown in Fig. 8b, both the Pb 2+ adsorption amount (19.52 mg/g) and the Pb 2+ removal rate (97.58%) of 1:2 ZO reach the peak at pH = 4. Since pH PZC of 1:2 ZO is 3.78 (shown in Fig. 5), when pH < 3.78, the 1:2 ZO is positively charged. At this time, lead mainly exists in Pb 2+ and Pb(OH) + , while the Pb 2+ and Pb(OH) + are mutually repellent to the 1:2 ZO. When 3.78 < pH < 4, 1:2 ZO is negatively charged, it electrostatically attracts Pb 2+ and Pb(OH) + . When 5 < pH, 1:2 ZO is negatively charged. The adsorption amount and the removal rate significantly decrease due to that lead mainly exists in the form of Pb(OH) 2 and Pb(OH) 4 2− , and mutual repulsion occurs between them. To investigate the effects of the adsorption time on the Pb 2+ removal rate and the Pb 2+ adsorption amount, 20 mg 1:1 ZO and 15 mg 1:2 ZO were added to 20 mL of simulated waste liquid with a Pb 2+ concentration of 15 mg/L, and the solutions were shaken at room temperature (200 rpm Fig. 9. As shown in Fig. 9, the Pb 2+ removal rate of 1:1 ZO and 1:2 ZO gradually increases when increasing the adsorption time. The adsorption time of 1:1 ZO and 1:2 ZO to reach the saturation (13.81 mg/g for 1:1 ZO and 18.09 mg/g for 1:2 ZO) of Pb 2+ adsorption is 16 h and 18 h, respectively.  Significantly, at the same adsorption time, the adsorption amount of 1:2 ZO is much higher than that of 1:1 ZO; that is, the adsorption effect of 1:2 ZO is better.
At the beginning of adsorption, there are a large number of active sites on the surface of the adsorbent. So, 1:1 ZO and 1:2 ZO can well combine with Pb 2+ , resulting in a high removal rate. With the decrease of active sites, the adsorption rate gradually slows down, and the adsorption reaches saturation after a certain time.
In addition, the experiment also compared the adsorption of Pb 2+ by the prepared material and other materials, and the results are shown in Table 3. It can be seen that the saturation adsorption amount of 1:1 ZO and 1:2 ZO to Pb 2+ prepared is higher than that of some other adsorbents, such as activated bentonite and magnetic graphene, indicating that 1:2 ZO is a more effective adsorbent for Pb 2+ .

Adsorption Kinetics
The pseudo-first-order kinetic model (3) and the pseudosecond-order kinetic model (4) were used to calculate the adsorption rate. The equations are as follows: where q e and q t (mg/g) are the equilibrium adsorption amount and adsorption amount at time t, respectively; K 1 (min −1 ) and K 2 (min −1 ) are the pseudo-first-order kinetic constant and the pseudo-second-order kinetic constant, respectively.
The fitting curves and the related parameters are shown in Fig. 10 and Table 4. It can be clearly seen from Table 4 that the fitting effect of the quasi-second-order kinetic has a fitting coefficient of 0.99998, which is significantly higher than that of the quasi-first-order kinetic. Moreover, the maximum adsorption amount calculated by the quasi-second-order kinetics equation is closer to the experimental value. Thus, it  is indicated that the pseudo-second-order kinetics can better describe the adsorption kinetics of 1:1 ZO and 1:2 ZO for Pb 2+ , so the adsorption process is mainly a chemisorption adsorption process.

Adsorption Isotherm
The adsorption isotherm is employed to evaluate the adsorption characteristics of an absorbent. In this work, the Langmuir adsorption isotherm (5) and the Freundlich adsorption isotherm (6) were used to understanding the Pb 2+ adsorption behavior of 1:1 ZO and 1:2 ZO. The equations of the Langmuir model and the Freundlich model are as follows: where q e (mg/g) is the equilibrium adsorption amount of 1:1 ZO and 1:2 ZO; C e (mg/L) is the concentration of the Pb 2+ when the adsorption is in equilibrium; q m (mg/g) is the maximum adsorption amount of 1:1 ZO and 1:2 ZO; K L is the Langmuir constant; K f and n are the Freundlich constant, which is related to the adsorption strength of the adsorbent; C 0 is the initial concentration of the Pb 2+ (mg/L). Figure 11 shows the Langmuir adsorption isotherm equation and the Freundlich adsorption isotherm equation. As shown from Adsorption thermodynamics can be used to calculate the driving force of the adsorption process. The enthalpy change (ΔH 0 ) and the entropy change (ΔS 0 ) can be obtained by multiplying the slope to the intercept of the lnK versus 1/T fitted curve (Eqs. 7 and 9) with the gas molar constant where K d is the adsorption equilibrium constant (L/kg); R is the gas molar constant (8.314 J/mol/K); T is the absolute temperature (K); ΔS 0 (kJ/mol), ΔH 0 (kJ/mol), and ΔG 0 (kJ/ mol) are entropy change, enthalpy change, and Gibbs free energy, respectively. According to the thermodynamic analysis in the temperature range (298 K, 308 K, 318 K), the adsorption amount data at each concentration can be calculated, and a scatter plot of lnK vs. 1/T is fitted shown in Fig. 12. From the slope and the intercept of the fitted curve to the gas mole constant, the value of ΔH 0 and ΔS 0 can be obtained, as shown in Table 6. It can be seen from Fig. 12 that as the temperature of the system increases, the adsorption amount of Pb 2+ by 1:1 ZO and 1:2 ZO both decrease, and the △H 0 is a negative value in Table 6, indicating that the adsorption of Pb 2+ by 1:1 ZO and 1:2 ZO is an exothermic process. In addition, △G 0 is a negative value, indicating that the adsorption process of Pb 2+ by 1:1 ZO and 1:2 ZO is spontaneous. A negative value of ΔS 0 indicates that the disorder of the solid-liquid interface is reduced during the adsorption process.

Competitive Adsorption
In the experiment, Cu 2+ and Fe 2+ were selected as heavy metal ions to compete with Pb 2+ during adsorption. The adsorption results of 1:1 ZO and 1:2 ZO for Pb 2+ in different mixtures are shown in Fig. 13. When Cu 2+ and Fe 2+ exist simultaneously, the removal rates of Pb 2+ by 1:1 ZO and 1:2

Conclusion
In this work, 1:1 ZO and 1:2 ZO were prepared and used to adsorb Pb 2+ . The best conditions for 1:1 ZO to adsorb Pb 2+ are m (adsorption dosage) = 20 mg, t (adsorption time) = 16 h, C 0 (initial concentration) = 15 mg/L, and pH = 3. The best conditions for 1:2 ZO to adsorb Pb 2+ are m = 15 mg, t = 18 h, C 0 = 15 mg/L, and pH = 4. It shows that 1:2 ZO is more helpful for the removal of Pb 2+ than 1:1 ZO. The adsorption mechanism of 1:1 ZO and 1:2 ZO on Pb 2+ has been discussed. The Pb 2+ adsorption process of ZO accords with the quasi-second-order kinetics (R 2 = 0.99998), indicating that chemical adsorption plays a leading role. The fitted isotherm adsorption curve is more consistent with the Langmuir adsorption model (1:1 ZO: R 2 = 0.95058; 1:2 ZO: R 2 = 0.97488). The maximum adsorption amount of Pb 2+ is 15.52 mg/g by 1:1 ZO and 18.09 mg/g by 1:2 ZO. Thermodynamic analysis shows that the Pb 2+ adsorption process is spontaneous. Competitive adsorption experiments show that the removal rate of Pb 2+ by 1:1 ZO and 1:2 ZO is more than 98% in the presence of other heavy metal ions, which indicates that the investigated materials have a high selectivity for Pb 2+ .