Effective remediation of Pb2+ polluted environment by adsorption onto recyclable hydroxyl bearing covalent organic framework

The removal of heavy metal ions from wastewater has attracted considerable interest because of their toxicity. Adsorption is one of the most promising methods for the removal of heavy metal ions due to its simplicity and effectiveness. Recently, covalent organic frameworks (COFs) have become promising adsorbents for effective wastewater remediation. However, many building blocks have been developed, and the design of COFs with high adsorption efficiency remains a challenge. Here, a covalent organic framework (DHTP-TPB COF) decorated with hydroxyl groups was developed for the efficient removal of Pb2+ ions. The DHTP-TPB COF showed excellent performance in adsorbing Pb2+ from aqueous solution. More importantly, DHTP-TPB COF exhibited high selectivity for Pb2+ compared to other competing ions, capturing Pb2+ ions with a removal efficiency of over 96% at pH 4. The results show that the DHTP-TPB COF exhibits excellent adsorption capacity at pH 4 of up to 154.3 mg/g for Pb2+ ions; the value is comparable to many previously reported COFs. Moreover, the adsorbed Pb2+ ions could be easily eluted with a 0.1 M EDTA solution, and the DHTP-TPB COF can be reused for more than five adsorption–desorption cycles without significant loss of adsorption capacity. Moreover, the adsorption mechanism was revealed using XPS analysis, indicating the formation of strong coordination-bonding interactions between hydroxyl and Pb2+ ions. Therefore, the DHTP-TPB COF prepared herein has high potential for the treatment of Pb2+-contaminated wastewater and is promising for the adsorption of Pb2+ ions in practical applications.


Introduction
Water is polluted with a large amount of heavy metals due to the growing population and rapid industrialization (e.g., metal coatings, mining, paper industry, fertilizers, batteries and pesticides) (Azad et al. 2021;Khan et al. 2021;Li et al. 2019;Shannon et al. 2010). Environmentally hazardous heavy metals can accumulate in living organisms and cause nervous, circulatory and immune system disorders, posing a serious threat to human life even in very low concentrations (Huang et al. 2020;Ragheb et al. 2022; Wang and Zhuang processes used to remove heavy metal ions (Gendy et al. 2021;Li et al. 2020;Liu et al. 2021;Wang and Zhuang 2019). Adsorption is one such process that can be used to remove heavy metals from water and plays a unique role in wastewater treatment because it is easy to develop, affordable, effortless to use, very powerful and widely applicable (Verma et al. 2022;Zhang et al. 2022;Zhu et al. 2021). There are many types of adsorbents, such as mesoporous silica (Mahmoud and Seliman 2014), clay adsorbents (Mahmoud et al. 2017;Rashad et al. 2016), metal oxides (Mahmoud et al. 2019), graphene-based materials (Gendy et al. 2021;Wang et al. 2015), and metal-organic frameworks (MOFs) (Li et al. 2018) have been developed. However, the fabrication of carbon-based nanostructures such as graphene and carbon nanotubes is expensive and time-consuming. The chemical stability of most MOFs is low. Despite the chemical stability of clay minerals, their adsorption capacity is low. Therefore, for the removal of Pb 2+ in practical applications, the development of novel materials that are highly effective and environmentally friendly is essential. A new class of crystalline porous organic materials with ordered porosity structures, tunable functionalities, and durable frameworks is known as covalent organic frameworks (COFs) (Abuzeid et al. 2021) which has attracted scientific interest for a number of applications such as adsorption (Gendy et al. 2021;Xiao et al. 2021), catalysis (EL-Mahdy et al. 2020;Elewa et al. 2022Elewa et al. , 2021, photoelectrics, etc. In addition, COFs have tremendous potential for adsorption of metal ions due to their sufficient functional properties. Because of their ability to adapt target skeletons or functional groups to bind specific pollutants, COFs are excellent adsorbents for a variety of pollutant removal processes. Amide-based covalent organic frameworks materials with an adsorption capacity of 185.7 mg/g for Pb 2+ were reported, using a polymerization reaction of acyl chloride and amino groups by mechanical ball milling (Li et al. 2019). Liu and co-workers reported an adsorption capacity of 476 mg g −1 for a COF containing triazine and hydroxyl groups, effectively removing Pb 2+ ions (Xu et al. 2019). In addition, Huang and co-workers reported covalent organic framework with EDTA for the removal of heavy metal ions with an adsorption capacity of > 50 mg g −1 (Jiang et al. 2019). Recently, Wang and co-workers reported amidinothiourea-linked COFs for the removal of heavy metal ions with an adsorption capacity of 100.76 mg g −1 (H. Wang et al.). Many other COFs have also been reported, demonstrating the stability and functionality of COFs for the removal of heavy metals by adsorption from wastewater.
In this study, we designed and synthesized crystalline porous COF (DHTP-TPB COF) with a well-defined structure by solvothermal condensation of 1,3,5-tris(4-formylphenyl)benzene (TPB-3CHO) and 2,5-dihydroxyterephthalohydrazide (DHTP) and investigated its performance for lead ion removal from wastewater. The synthesized COF (DHTP-TPB COF) has even better properties, such as good crystallinity and surface area. More crucially, the dense hydroxyl groups of the DHTP-TPB COF skeleton (OH) increase the density of coordination sites, resulting in high metal loading capacity and affinity.

Adsorption experiments
The adsorption experiments for lead (Pb) were performed in a batch procedure, as shown in Fig. S7. For the later experiments, a stock solution of lead in water (1000 mg/L, pH 2.7) was prepared. DHTP-TPB COF was placed in a 20-mL glass bottle containing 5 mL of Pb 2+ solution at a specified initial concentration (C 0 ). Adsorption studies were performed at a temperature of 25 °C for 24 h, and the pH was adjusted with 1 M HCl/NaOH. Then, 2 ml of the solution was withdrawn and filtered through a 0.22 μm filter membrane and diluted to 10 ml with water. All experiments were performed twice, and the average value was used as the final result. Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to analyze the filtrate to determine the residual concentration of Pb 2+ ions. The following Eqs. (1)-(3) were used to determine the percent removal (R %), equilibrium adsorption capacity (q e ), and distribution coefficient (K d ), respectively.
where C 0 (mg/L) is the initial concentration, C e (mg/L) is the equilibrium concentration, V (L) is the volume of the solution, and w (g) is the weight of the adsorbent.

The point of zero charge (pH PZC )
The point of zero charge (pH PZC ) was determined from the zeta potential measured by dynamic light scattering (DLS). A series of a mixture of 10 mg adsorbent and 50 ml 0.01 M KNO 3 solution was stirred at room temperature for 24 h to reach equilibrium. The initial pH was changed from 1.73 to 9.96 with 0.1 M HCl and 0.1 M NaOH solutions. The final pH of the solution was recorded at equilibrium. The supernatant liquid was collected, then the zeta potential was measured at 25 °C with 700 μl supernatant in the sample cuvette and the corresponding data were recorded. Plotting the zeta potential (mV) with the initial pH gives the intersection point on the horizontal axis, and this point is the point of zero charge for the adsorbent.

The reusability DHTP-TPB COF
A column was used to study the reusability of DHTP-TPB COF; the column (1 cm inner diameter and 20 cm length) was filled with 10 mg of DHTP-TPB COF. The column was then filled with 10 mL of Pb 2+ solution (20 ppm) at pH 4.0 at a flow rate of 0.5 mL min −1 . The Pb 2+ ions were eluted with 0.1 M EDTA. The column was washed with 10 mL of deionized water after each loading/elution cycle to clean the packed DHTP-TPB COF. To determine the Pb 2+ content, the elution was collected and analyzed by ICP-OES.

Synthesis and characterization
DHTP-TPB COF was synthesized from TPB-3CHO and DHTP in organic solvent containing acetic acid (6 M) under solvothermal conditions, as shown in Fig. 1a. The crystalline structures of DHTP-TPB COF were investigated by PXRD. DHTP-TPB COF shows an ordered 2D hexagonal mesophase as shown in Fig. 1b, with diffraction peaks at 2θ = 2.16°, 3.79°, 4.40°, and 5.83° in the PXRD small-angle pattern corresponding to (100), (110), (200), and (210) facets, respectively. Theoretical simulations were performed using Materials Studio to obtain more details about the crystal structures. The PXRD patterns were simulated for two types of stacking models, eclipsed stacking (AA) and staggered stacking (AB), as shown in Fig. 1b. The experimental values of eclipsed AA stacking agreed well with the simulated values. The schematic representation of Pb 2+ adsorption on DHTP-TPB COF is shown in Fig. 1c.
The molecular structure of DHTP-TPB COF was verified by nuclear magnetic resonance (NMR) spectroscopy and FTIR. The FTIR spectrum confirmed the formation of DHTP-TPB COF ( Fig. 2a) due to the presence of the C = N peak (1602 cm −1 ), the absence of the characteristic peaks of the primary amine (3396, 3308 cm −1 ) of the NHNH 2 group in DHTP, and the lack of evidence of the C = O group (at 1685 cm −1 ) of TPB-3CHO, indicating the complete consumption of these monomers and the subsequent successful synthesis of DHTP-TPB COF. Moreover, the condensation between DHTP and TPB-3CHO to form DHTP-TPB COF was also confirmed by 13 C NMR solid-state spectroscopy (Fig. 2b). The absence of a signal for the aldehydic carbon of TPB-3CHO (at 194 ppm) and the appearance of new signals for the C = O and C = N groups at 168 and 152 ppm, respectively, indicate that the DHTP-TPB COF was successfully formed.
Nitrogen sorption studies at 77 K were performed to determine the permanent porosity of DHTP-TPB COF. As shown in Fig. 2c, the Brunauer-Emmett-Teller surface area (BET) of DHTP-TPB COF was estimated to be 360 m 2 g −1 . The adsorption/desorption curve of the DHTP-TPB COF is type IV isotherm as shown in Fig. 2c, indicating that the product has a typical mesoporous structure (Hazra et al. 2018). The average pore size of DHTP-TPB COF calculated using nonlocal density functional theory (NLDFT) was approximately 3.77 nm (Fig. 2c, inset), suggesting that our synthesized DHTP-TPB COF is a mesoporous material. The large surface area and ordered mesoporous distribution enabled the even dispersion of the high density of chelating sites throughout the channel surface of DHTP-TPB COF. Thermogravimetric analysis (TGA) is used to monitor weight changes of COF as a function of temperature. Weight changes of COF can occur due to decomposition and oxidation reactions as well as physical processes such as sublimation and evaporation (Ng et al.). Thermogravimetric analysis was performed to evaluate the thermostability of DHTP-TPB COF (Fig. 2d). The weight loss in the first stage (6.86%) was attributed to the evaporation of adsorbed water.
The results showed that DHTP-TPB COF was stable even up to 300 °C (the mass loss is 7.07% between 30 and 300 °C), with decomposition temperatures (T d10 ) of 322 °C in air atmosphere. The significant weight loss of DHTP-TPB COF occurred in the range of 300-450 °C, which was probably related to the destruction of imine bonds and decomposition of COF structures into oligomers and structural monomers (Cao et al. 2020). As shown in Fig. S9, FE-SEM and TEM were used to analyze the morphology of DHTP-TPB COF. The crystalline DHTP-TPB COF underwent self-assembly during its synthesis, resulting in micrometer-sized, uniform tubes, as shown in Fig. S9a. Most of these DHTP-TPB COF tubes were assembled due to the strong hydrogen bonding between the COF surfaces. The micrometer-sized tubular shape of DHTP-TPB COF was established by TEM examination, which also showed that the average length and diameter of the tubular DHTP-TPB COF were 295 and 750 nm, respectively (Fig. S9).

Pb 2+ adsorption studies
The pH of the solution, which affects the adsorption behavior, is one of the most important factors. The effect of solution pH on lead adsorption efficiency was studied using 1 mg DHTP-TPB COF in 5 ml of a 20 mg/L Pb 2+ solution at various initial solution pH values (pH i ) ranging from 2.08 to 6.02 (see Fig. 3a), with higher pH values prohibited to avoid precipitation of Pb 2+ ions as hydroxide species. Dilute solutions of HCl or NaOH were used to adjust the pH of the solutions. In the pH range studied, the adsorption percentage increases with the increase of pH from 2.0 to 4.0 and remains at a high level at pH 4.0-6.0. This is because the adsorption of Pb 2+ at the adsorption sites of COF is inhibited by high concentrations of H + at low pH. Consequently, the higher the pH, the greater the Pb 2+ adsorption at the surface of DHTP-TPB COF. In addition, the final pH of the solution (pH f ) was measured, and the relationship between the initial and final pH of the solution for DHTP-TPB COF is shown in Fig. 3a. At low pH (below pH 3.0), the final pH after adsorption remained practically constant, but at pH 3.0, the final pH after adsorption increased significantly. This means that fewer H + ions compete with Pb 2+ ions for the active sites. As a result, larger removal percentages are achieved when the pH increases rapidly, especially at pH 4.0. In order to interpret well the obtained adsorption data of Pb 2+ on DHTP-TPB COF at the studied pH values, the pH of point zero charge (pH pzc ) of the adsorbent was determined experimentally to estimate the surface charge of DHTP-TPB COF in solution. The pH pzc (point of zero charge) value of DHTP-TPB COF is about 3.3 (Fig. 3b). At pH < pH pzc , the surface charge Fig. 1 a The synthesis of DHTP-TPB COF; b experimental PXRD patterns of DHTP-TPB COF (black), Pawley refinement (blue), their difference (red), simulated profiles using AA-stacking (green), and AB-stacking (purple) modes, c Schematic Pb 2+ adsorption processing onto DHTP-TPB COF of DHTP-TPB COF is positive, while at pH > pH pzc , the surface of COF is negatively charged, which is consistent with the above result.
On the other hand, Fig. 3c shows the various species of lead ions Pb(II) as a function of solution pH. In general, changing the pH of a solution changes the speciation (or the charges of the metal ions in the water), which affects the interactions between the predominant metal species and the adsorbent. The speciation diagram of the Pb 2+ ion shows that Pb 2+ is the dominant species at pH values below 6.0. Considering the relationship between the pH of the initial solution and the corresponding pH of the final solution (Fig. 3a) and the Pb 2+ speciation diagram (Fig. 3c), the next Pb 2+ adsorption experiments were performed at DHTP-TPB COF at an initial pH of 4.0 to avoid Pb 2+ precipitation.
Another important criterion for adsorbents is their adsorption capacity. To evaluate the total adsorption capacity of DHTP-TPB COF for Pb 2+ , the adsorption isotherm was determined with different initial concentrations of aqueous Pb 2+ solutions at pH 4.0 and room temperature. DHTP-TPB COF showed a high affinity for Pb 2+ at low concentrations, which decreased with increasing initial Pb 2+ concentration, with a maximum value of 152.7 mg/g at pH 4.0, as shown in Fig. 4a. The excellent DHTP-TPB COF adsorption capacity of Pb 2+ is superior to other reported adsorbents (Table S3) We note that many papers have used a pH greater than 5.0, as shown in previous reports. However, it is known that as the metal concentration increases, the precipitation of metals is accelerated, so that some of the metal removal is by precipitation (Marchioretto et al. 2005). Therefore, metal adsorption is better at pH less than 5.0, especially at higher metal concentrations to avoid metal precipitation. Accordingly, the adsorption isotherm to evaluate the adsorption capacity of DHTP-TPB COF for Pb 2+ was performed at pH 4.0 (see Fig. 4a) to avoid precipitation of Pb 2+ at higher concentration. On the other hand, the adsorption isotherm was performed at a pH of 6.0 (see Fig. 4a) to allow comparison with previously published work. As shown in Fig. 4b, the adsorption capacity of DHTP-TPB COF for Pb 2+ is 419.6 mg/g, but we observed that the Pb 2+ is precipitated at higher concentrations, as shown in Fig. S9. Activated carbon (AC) is a solid adsorbent commonly used to purify wastewater. Since different test conditions in different laboratories can give different results for the same adsorbents, the isothermal experiment on AC was conducted under similar experimental conditions. DHTP-TPB COF outperforms the adsorbent AC in terms of adsorption capacity (Fig. 4a).
The Langmuir-Freundlich isotherm model (Fig. S11) and Freundlich isotherm model (Fig. S12) were used to fit the experimental Pb 2+ adsorption data in this study. The Langmuir and Freundlich isotherms are the most comprehensive and simplest isotherm equations to explain the adsorption equation. The linear forms of the Langmuir and Freundlich models are given by Eqs. (4) and (5), respectively, and the corresponding parameters are listed in Table 1. (4) C e q e = 1 q max × K L + C e q max Fig. 3  where C e is the equilibrium concentration of Pb 2+ (mg/L), q e is the equilibrium amount of Pb 2+ adsorbed by DHTP-TPB COF (mg/g) and q max is the maximum adsorption capacity of DHTP-TPB COF (mg/g). The isotherm constants of Langmuir (K L , L/mg) and Freundlich (K F , mg 1−n L n /g) are related to the adsorption energy and adsorption capacity, respectively. The Langmuir isotherm model has a higher correlation coefficient (R 2 , 0.998) than the Freundlich isotherm model (R 2 , 0.742), suggesting that Pb 2+ adsorption by DHTP-TPB (5) log q e = logK F + 1 n logC e COF is monolayer adsorption. Using the Langmuir equation, the maximum adsorption capacity of Pb 2+ for DHTP-TPB COF is estimated to be 154.3 mg/g, which is closer to the experimentally determined value (152.7 mg/g). This value is comparable to many previously reported COFs, as shown in Fig. 4b and Table S3. As shown in Fig. 4c, the impact of contact time on Pb 2+ adsorption by DHTP-TPB COF with solution pH 4.0 was investigated. The high number of adsorption sites offered by DHTP-TPB COF caused the adsorption percentage for Pb 2+ onto DHTP-TPB COF to grow quickly, reaching roughly 48% within the first 5 min. After that, the adsorption capabilities gradually increased with increasing contact time until they reached equilibrium, since metal ion migration into pores and adsorption by the inner surface are slow processes after practically all surface adsorption sites of DHTP-TPB COF were occupied.
Adsorption kinetics tests of Pb 2+ were conducted on DHTP-TPB COF to examine the Pb 2+ adsorption behavior on the material and examine its potential for heavy metal management. The adsorption data have been fitted with the common two kinetic equations, pseudo-first-order Eq. (6) (Fig. S13) and pseudo-second order Eq. (7) models (Fig. S14), in addition to the intraparticle diffusion model Eq. (8) (Fig. 4d), the kinetic parameters of Pb 2+ adsorption on DHTP-TPB COF are included in Table 2. In comparison to the correlation coefficient of pseudo-first-order model Fig. 4 a Adsorption isotherms of Pb 2+ using DHTP-TPB COF and AC, b the comparison adsorption capacity of Pb 2+ between our COF and the previously reported adsorbents with experimental pH, note that the adsorption isotherm study at pH 6.0-6.5 is unfavorable to avoid Pb 2+ precipitation at higher concentration, c effect of contact time, and d the intraparticle diffusion model plots for adsorption of Pb 2+ onto DHTP-TPB COF (R 2 = 0.959), the pseudo-second-order kinetic model has a stronger correlation coefficient (R 2 = 0.996), suggesting that the Pb 2+ adsorption behaviors on DHTP-TPB COF could well be best defined by the pseudo-second-order kinetic model, implying that Pb 2+ adsorption on DHTP-TPB COF is chemisorption through strong chemical forces.
where w e (mg/g) and q t (mg/g) represent the amount of Pb 2+ adsorbed onto DHTP-TPB COF at equilibrium and time t, respectively. k 1 (min −1 ) and k 2 (g/mg min) are the pseudo-first-order and pseudo-second-order rate constants, respectively. k i stands for the intraparticle diffusion rate constant (mg/g min 0.5 ), and C is the intercept (mg/g) proportional to the boundary layer thickness.
The intra-particle diffusion model was utilized to match the adsorption kinetic data in order to comprehend the diffusion process. Figure 4d depicts the relation of q t with t 1/2 of the intraparticle diffusion model, and three phases can be seen on the adsorption curve. External surface adsorption is responsible for the initial linear step. Intraparticle diffusion might cause the second stage. The third stage is related to equilibrium, during which intraparticle diffusion slows down because there are fewer Pb 2+ ions left in the solution and fewer interior active sites. The fact that the straight regression line that fits the first stage data did not pass through the origin indicates that there are other rate-limiting phases besides intra-particle diffusion. Table 2 shows the values of K i and C computed from the second linear stage's slope and intercept.
(6) ln q e − q t = lnq e − k 1 t (7) t q t = 1 k 2 q 2 e + t q e (8) q t = k i t 0.5 + C Furthermore, adsorption of Pb 2+ onto DHTP-TPB COF at pH 4.0 has been studied at different temperature (30-60 °C), because of Brownian movement, the adsorption process may be influenced directly by temperature. The adsorption of Pb 2+ onto DHTP-TPB COF increases dramatically as the temperature rises from 30 to 70 °C. The various thermodynamic parameters for adsorption of Pb 2+ onto DHTP-TPB COF were calculated using Eq. (9), (10), and (11): enthalpy change (ΔH o ), entropy change (ΔS o ), and free energy (G o ).
where C i and C f represent the concentrations of Pb 2+ solution before and after the adsorption process, respectively, V represents the volume of the solution (mL), M represents the DHTP-TPB COF weight (g), R represents the ideal gas constant (R = 0.008314 kJ/mol), and T represents the absolute temperature (K). The slope and intercept of the linear plot of lnKd vs 1/T (Fig. S15) were used to calculate the values of ΔH o and ΔS o , respectively. Table 3 summarizes the thermodynamic values for Pb 2+ adsorption onto DHTH COF. The negative ΔG o values for DHTP-TPB COF at various temperatures imply that Pb 2+ adsorption is possible and spontaneous. The observation that ΔH o is positive indicates that the adsorption process is endothermic. The positive value of ΔS o suggests randomness during the adsorption of Pb 2+ on DHTP-TPB COF.
Furthermore, as shown in Fig. 5a, the influence of various coexisting ions such as Na + , Ca 2+ , and Mg 2+ on the removal efficiency of Pb 2+ onto DHTP-TPB COF is investigated in concentration ranges of 20-300 ppm. According to the findings, the presence of mono-cations (Na + ) has no influence on the efficacy of Pb 2+ removal at all concentrations investigated. While the efficiency of Pb 2+ removal in the presence of Mg 2+ was marginally decreased at higher concentration. On the other hand, the efficiency of Pb 2+ removal onto DHTP-TPB COF significantly decreases in the presence of Ca 2+ ions especially at higher concentration. The competition for adsorption on the active sites of the DHTP-TPB COF is responsible for the reduction in Pb 2+ removal efficiency as the concentration of coexisting ions rises. But there is still no effect on the efficiency of Pb 2+ removal on DHTP-TPB COF up to about 50 ppm at present from three coexisting ions.

Selectivity and reusability
The selectivity of adsorbent has been studied, and it should be noted that other heavy metal ions found in wastewater include Co 2+ , Ni 2+ , Fe 2+ , Cu 2+ , Zn 2+ , potentially causing competing adsorption. Therefore, to evaluate the potential of DHTP-TPB COF to remove Pb 2+ at real conditions, an aqueous solution containing 20 ppm of Co 2+ , Ni 2+ , Fe 2+ , Cu 2+ , Zn 2+ and Pb 2+ was prepared. As shown in Fig. 5b, the DHTP-TPB COF absorbed the Pb 2+ ions with a removal efficiency of over 96%, indicating that the DHTP-TPB COF has a high selectivity for Pb 2+ ions compared the other cations. Whereas the metal ions examined carry the same charge; therefore, the selectivity was probably governed by its ionic radii and electronegativity (Oladipo et al. 2019 The adsorbent reusability is a crucial factor in determining the commercial applications viability. As shown in Fig. 5c, d, the adsorption and desorption experiments of Pb 2+ onto DHTP-TPB COF were performed five times, for testing the reusability of DHTP-TPB COF. For the desorption of Pb 2+ ions from DHTP-TPB COF, 0.1 M EDTA was used. The results indicated that DHTP-TPB COF has a high reusability greater than 96% recovery after cycles. As a result of its high reusability and Pb 2+ removal effectiveness, DHTP-TPB COF might be used in wastewater treatment.

Adsorption mechanism
The results indicate that Pb 2+ adsorption on DHTP-TPB COF was pH-dependent and fit pseudo-second-order kinetics, suggesting that chemical adsorption influenced the process (Huang et al. 2022a). Additionally, the adsorption of Pb 2+ on DHTP-TPB COF increases fast during the first three contact Fig. 5 a Study the effect of different concentrations of co-existing ions on the adsorption of Pb 2+ on DHTP-TPB COF, b adsorption efficiency of DHTP-TPB COF to absorb Pb 2+ ions in the multi-ion system at pH 4.0, c schematic diagram of the adsorption and desorption process, and d recycling for adsorption of Pb 2+ onto DHTP-TPB COF hours and subsequently stabilizes as contact time increases. Chemical adsorption or strong surface complexation, rather than physical sorption, appears to play a major role in Pb 2+ adsorption on the DHTP-TPB COF, according to the results (Yang et al. 2011). To gain a thorough understanding of the mechanism of Pb 2+ adsorption onto DHTP-TPB COF. In addition to the findings from the parameters studied in this study, the XPS characterization of the DHTP-TPB COF before and after adsorption also prove that Pb 2+ is successfully adsorbed as shown in Fig. S16. The high-resolution XPS spectra of C1s, N 1 s, O 1 s, and Pb 4f of DHTP-TPB COF before and after Pb 2+ adsorption were analyzed as shown in Fig. 6. There is no change in the spectra of C 1 s and N 1 s before and after Pb 2+ adsorption, where the C 1 s spectrum of DHTP-TPB COF has three different peaks, and their binding energies are 283.7 eV for C = O, 284.9 eV for C-C/C-H, 286.9 eV for N-C = O, as well as the N 1 s spectrum of DHTP-TPB COF has two different peaks, and their binding energies are 398.7 eV for C = N, and 399.5 eV for N-C = O. On the other hand, a new peak of the O 1 s spectra appeared after Pb 2+ adsorption at 531.8 eV attributed to C-O-Pb. as well as the new peak of Pb 4f appears on the XPS patterns of DHTP-TPB COF after the adsorption, indicates that Pb 2+ successfully adsorbs chemically on DHTP-TPB COF as shown in Fig. 6.

Conclusion
In summary, we synthesized crystalline porous COF, DHTP-TPB COF, under solvothermal conditions as efficiency adsorbent for Pb 2+ ion removal. The adsorption behaviors of Pb 2+ onto DHTP-TPB COF were investigated using batch adsorption studies, and the results were analyzed using several models. The Langmuir isotherm and pseudo-second-order kinetics better represented the uptake of Pb 2+ ions than other models investigated. The DHTP-TPB COF displayed excellent performance in Pb 2+ adsorption capability and the removal efficiency is > 95%, due to the strong chelating effect of hydroxyl groups, and the maximum adsorption capacity for Pb 2+ is 154.3 mg g −1 at pH 4.0, which makes DHTP-TPB COF a promising material for the adsorption of Pb 2+ ions in practical application. More interestingly, this work provided a correction to the way of the isotherm study that has been frequently followed before.