Modified Cyclodextrin/mesoporous Silica for Adsorbing Pb and Cd and Application Thereof

Abstract

The disclosure provides a modified cyclodextrin/mesoporous silica for adsorbing Pb and Cd and application thereof, belonging to the technical field of adsorbent materials. By using surface modification, chloroacetic acid is used as anions, and a modified cyclodextrin is grafted onto a surface of a cyclodextrin/mesoporous silica by nucleophilic substitution to prepare the modified cyclodextrin/mesoporous silica adsorbent material. The modified cyclodextrin/mesoporous silica adsorbent material prepared in the disclosure has the advantages of simple preparation method, strong adsorbability, easy separation, good biocompatibility and the like. When the material is used as an adsorbent to adsorb heavy metal ions Pb.sup.2+ and Cd.sup.2+, maximum removal rates can reach 97.8% and 81.29% respectively. Therefore, the material has wide application prospects in removal of heavy metals in sewage and the like, thereby improving the water environment that people depend on for survival.

Claims

1. A method for quickly and selectively adsorbing Pb, comprising preparing modified cyclodextrin/mesoporous silica by carboxymethyl modification on EA-CD-Si@Si by using chloroacetic acid, and using the modified cyclodextrin/mesoporous silica as an adsorbent to adsorb Pb in sewage; wherein adsorption conditions are at a pH of 4-6, and an adsorption time is 5 minutes-2 hours.

2. A method for quickly and selectively adsorbing Pb and Cd, comprising preparing modified cyclodextrin/mesoporous silica by carboxymethyl modification on EA-CD-Si@Si by using chloroacetic acid, and using the modified cyclodextrin/mesoporous silica as an adsorbent to selectively adsorb Pb.sup.2+ and Cd.sup.2+ in sewage; wherein adsorption conditions are at a pH of 4-6, and an adsorption time is 5 minutes-2 hours.

3. The method according to claim 2, wherein the preparing modified cyclodextrin/mesoporous silica further comprises the following steps: (1) dissolving EA-CD-Si@Si and NaOH in distilled water, adding a chloroacetic acid solution, stirring the mixture thoroughly and uniformly, reacting the reaction system at 40-80° C. for 6-18 h, and performing cooling; and (2) adjusting a pH of the reaction system after the reaction in step (1), precipitating the product with a methanol solution, filtering the mixture, washing the product, and drying the product at 40-60° C. to obtain the modified cyclodextrin/mesoporous silica.

4. The method according to claim 3, wherein a mass ratio of the EA-CD-Si@Si to the NaOH in step (1) is 0.5:1-2.5:1.

5. The method according to claim 3, wherein an amount of the distilled water in step (1) is 1-5 times a total mass of the EA-CD-Si@Si and the NaOH.

6. The method according to claim 3, wherein a concentration of the chloroacetic acid in step (1) is 5%-25%, and an amount is 0.5-2.5 times a total mass of the EA-CD-Si@Si and the NaOH.

7. The method according to claim 3, wherein in step (2), the reaction system is adjusted to the pH of 5-9.

8. The method according to claim 3, wherein the preparing modified cyclodextrin/mesoporous silica further comprises the following steps: (1) dissolving EA-CD-Si@Si and NaOH in distilled water, adding a chloroacetic acid solution, stirring the mixture thoroughly and uniformly, reacting the reaction system at 40-80° C. for 6-18 h, and performing cooling; wherein a mass ratio of the EA-CD-Si@Si to the NaOH is 0.5:1-2.5:1; a concentration of the chloroacetic acid is 5%-25%, and an amount is 0.5-2.5 times a total mass of the EA-CD-Si@Si and the NaOH; and (2) adjusting a pH of the reaction system after the reaction in step (1), precipitating the product with a methanol solution, filtering the mixture, washing the product, and drying the product at 40-60° C. to obtain the modified cyclodextrin/mesoporous silica; wherein the reaction system is adjusted to the pH of 5-9.

9. The method according to claim 3, wherein the preparing modified cyclodextrin/mesoporous silica further comprises: adding 5.0 g of EA-CD-Si@Si and 4.65 g of NaOH into 18.5 mL of ultrapure water, stirring the mixture at room temperature, adding 13.5 mL of a 16.3% chloroacetic acid solution, reacting the reaction system at 60° C. for 6 h, performing cooling, adding 36% HCl to adjust a pH of the solution to 7, precipitating the product with methanol, filtering the mixture, washing the product, and performing vacuum drying on the sample to obtain CM-EACD@Si.

10. The method according to claim 3, wherein the preparing modified cyclodextrin/mesoporous silica further comprises: dissolving 10 g of EA-β-CD@Si and 5 g of NaOH particles in 10 mL of distilled water, adding 10 mL of a 10% chloroacetic acid solution, reacting the reaction system at 60° C. for 10 h, performing cooling, adding 36% HCl to adjust a pH of the solution to 6, precipitating the product with 50 mL of methanol, filtering the mixture, washing the product, and performing vacuum drying at 40° C. to obtain CM-EACD@Si.

11. The method according to claim 3, wherein the preparing modified cyclodextrin/mesoporous silica further comprises: dissolving 5 g of EA-β-CD@Si and 2.5 g of NaOH particles in 10 mL of distilled water, adding 10 mL of a 16.3% chloroacetic acid solution, reacting the reaction system at 80° C. for 6 h, performing cooling, adding 36% HCl to adjust a pH of the solution to 7, precipitating the product with 50 mL of methanol, filtering the mixture, washing the product, and performing vacuum drying at 40° C. to obtain CM-EACD@Si.

Description

BRIEF DESCRIPTION OF FIGURES

[0041] FIG. 1 is a schematic diagram of preparation of CM-EACD@Si.

[0042] FIG. 2 is an infrared spectrogram of EA-β-CD@Si and CM-EACD@Si.

[0043] FIG. 3 shows influence of pH value on HMs adsorption on CM-EACD@Si.

[0044] FIG. 4 shows influence of adsorption time on HMs adsorption on CM-EACD@Si.

[0045] FIG. 5A shows quasi-second-order kinetic fitting; and FIG. 5B shows particle internal diffusion fitting.

[0046] FIG. 6 shows removal effects under a multielement composite system.

[0047] FIG. 7 shows influence of interfering ions on removal effects.

[0048] FIG. 8A shows desorption results under three different types of desorption solutions; and FIG. 8B shows effects of three times of desorption on Pb.sup.21 in a 0.1 mol/L HNO.sub.3 solution.

DETAILED DESCRIPTION

[0049] The disclosure will be specifically described below in conjunction with the accompanying drawings and examples.

[0050] 1. Test Method of Adsorption Capacity and Removal Rate

[0051] 10 mg, 20 mg, 40 mg, 60 mg, 80 mg and 100 mg (different mass) of EA-β-CD@Si and CM-EACD@Si are respectively weighed and placed in 50 mL of a 50 mg/L HMs (Pb.sup.2+, Cd.sup.2+ and Cu.sup.2+) solution, the mixture is shaken at a constant temperature of 30° C. for 2 h and centrifuged at 10000 rpm for 15 min, the supernatant is taken and measured by an ultraviolet spectrophotometer, and corresponding HMs concentrations of Pb.sup.2+, Cd.sup.2+ and Cu.sup.2+ are measured by an atomic absorption spectrophotometer at 283.3 nm, 228.8 nm and 324.8 nm respectively. The adsorption capacity (Qe) and removal rate (E) are calculated by using the standard curve, formula (2-2) and formula (2-3):

Qe=(C.sub.0−C.sub.e)×V/m

E (%)=(C.sub.0−C.sub.e)/C.sub.0×100

[0052] where Qe: removal capacity of β-CD for HMs (Pb.sup.2+, Cd.sup.2+, Cu.sup.2+ and Ca.sup.2+), mg; V: volume of HMs (Pb.sup.2+, Cd.sup.2+, Cu.sup.2+ and Ca.sup.2+), mL; C.sub.0: initial mass concentration of HMs (Pb.sup.2+, Cd.sup.2+, Cu.sup.2+ and Ca.sup.2+), mg/L; E: removal rate, %.

[0053] 2. The cyclodextrin/mesoporous silica (EA-CD-Si@Si) mentioned in the disclosure is prepared with reference to the patent publication No. CN107597070A.

Example 1: Preparation of CM-EACD@Si

[0054] A preparation method of a modified cyclodextrin/mesoporous silica adsorbent material for selectively adsorbing Pb.sup.2+ and Cd.sup.2+ included the following steps:

[0055] Cyclodextrin/mesoporous silica powder (EA-β-CD@Si 10 g) and NaOH particles (5 g) were dissolved in distilled water (10 mL), 10 mL of a 10% chloroacetic acid solution was added, the reaction mixture was reacted at 60° C. for 10 h, cooling was performed, 36% HCl was added to adjust a pH of the solution to 6, the product was precipitated with 50 mL of methanol, the mixture was filtered, the product was washed, and vacuum drying was performed at 40° C. to obtain 8.89 g of CM-EACD@Si. The specific reaction principle is shown in FIG. 1.

[0056] In order to determine whether carboxymethylation modification of EA-β-CD@Si was successful, FT-IR was used to characterize the infrared spectra of the EA-β-CD@Si and the CM-EACD@Si. The results are shown in FIG. 2. By comparison, it can be found that the broad peaks at 3430 cm.sup.−1 were stretching vibration absorption peaks of O—H bond and N—H bond, and the wave number band at 1000-1200 cm.sup.−1 belonged to stretching vibration absorption peaks of C—O bond, Si—O bond and C—O—C bond with obviously enhanced intensity, and the peak at 945 cm.sup.−1 was the flexural vibration peak of R-1,4-bond in β-CD preserved. In addition, the CM-EACD@Si had a new characteristic absorption peak at 1718 cm.sup.−1, which was mainly due to the presence of the stretching vibration absorption peak of C═O in the carboxylate ion. This could also prove that COO.sup.− group was introduced to the surface of the EA-β-CD@Si, that is, the carboxymethylation-modified material was successfully obtained and further modification was realized.

[0057] The adsorption rates of the modified cyclodextrin/mesoporous silica adsorbent material prepared in Example 1 for adsorption in a 20 mg/L Pb.sup.2+ and Cd.sup.2+ solution were 80.6% and 67.7%.

Example 2: Preparation of CM-EACD@Si

[0058] A preparation method of a modified cyclodextrin/mesoporous silica adsorbent material for selectively adsorbing Pb.sup.2+ and Cd.sup.2+ included the following steps:

[0059] Cyclodextrin/mesoporous silica powder (5 g) and NaOH particles (4.65 g) were dissolved in distilled water (18.5 mL), 13.5 mL of a 16.3% chloroacetic acid solution was added, the reaction mixture was reacted at 50° C. for 6 h, cooling was performed, 36% HCl was added to adjust a pH of the solution to 7, the product was precipitated with 50 mL of methanol, the mixture was filtered, the product was washed, and vacuum drying was performed at 40° C. to obtain 4.55 g of CM-EACD@Si.

[0060] The adsorption rates of the modified cyclodextrin/mesoporous silica adsorbent material prepared in Example 2 for adsorption in a 20 mg/L Pb.sup.2+ and Cd.sup.2+ solution were 95.6% and 80.7%.

Example 3: Preparation of CM-EACD@Si

[0061] A preparation method of a modified cyclodextrin/mesoporous silica adsorbent material for selectively adsorbing Pb.sup.2+ and Cd.sup.2+ included the following steps:

[0062] Cyclodextrin/mesoporous silica powder (5 g) and NaOH particles (2.5 g) were dissolved in distilled water (10 mL), 10 mL of a 16.3% chloroacetic acid solution was added, the reaction mixture was reacted at 80° C. for 6 h, cooling was performed, 36% HCl was added to adjust a pH of the solution to 7, the product was precipitated with 50 mL of methanol, the mixture was filtered, the product was washed, and vacuum drying was performed at 40° C. to obtain 3.68 g of CM-EACD@Si.

[0063] The adsorption rates of the modified cyclodextrin/mesoporous silica adsorbent material prepared in Example 3 of the disclosure for adsorption in a 20 mg/L Pb.sup.2+ and Cd.sup.2+ solution were 83.6% and 72.2%.

Example 4: Application of CM-EACD@Si as Adsorbent in Single Metal Ion System

[0064] Using the CM-EACD@Si prepared in Example 2 as an adsorbent, Pb.sup.2+, Cd.sup.2+ and Cu.sup.2+ were selected as typical HMs to research removal effects of EA-β-CD@Si and CM-EACD@Si on HMs. Corresponding HMs concentrations at 283.3 nm, 228.8 nm and 324.8 nm were respectively measured by an atomic absorption spectrophotometer.

[0065] 1. Influence of pH on Removal Effect:

[0066] Different pH of the solution will influence distribution of surface charges of the material, resulting in different electrostatic attraction between the material and the HMs. A 20 mg/L HMs (Pb.sup.2+, Cd.sup.2+, Cu.sup.2+ and Ca.sup.2+) solution was prepared. 50 mL of the solution was taken, and 20 mg of CM-EACD@Si was weighed and placed in the solution. The pH of the solution was adjusted to 2-6 with NaOH/HCl. The mixture was shaken at a constant temperature of 30° C. for 2 h, and the HMs concentrations of the supernatant were measured. The results are shown in FIG. 3.

[0067] It can be seen from FIG. 3 that the overall HMs adsorption capacity increases as the pH value increases, and the adsorption effect of the CM-EACD@Si is higher than that of the EA-β-CD@Si. This is because the carboxyl deprotonation degree on the material surface increases, the surface charge density increases, and HMs are removed through electrostatic or ionic interaction between the HMs (positively charged) and COO.sup.− (negatively charged); and when the pH<6, the HMs mainly exist as M.sup.2+ and M(OH).sup.+, and the variation of the adsorption capacity is mainly related to the isoelectric point. At a pH higher than the isoelectric point, the carboxylate ions (negatively charged) have a strong coordination affinity to the metal ions (positively charged), and the attraction also makes the carboxylate ions trap M.sup.2+ through surface complexation, thereby forming a chelate whose complexation degree increases with the increase of pH. At a pH lower than the zero point charge, there is repulsive force between positive charges on the surface of the CM-EACD@Si and the HMs, so that the overall removal effect is weak. The maximum adsorption effect is at pH=6. The overall adsorption effect of the CM-EACD@Si on HMs is: Pb.sup.2+ (92.02%)>Cd.sup.2+ (80.29%)>>Cu.sup.2+ (23.4%)>Ca.sup.2+ (15.42%), that is, in the whole process, the removal effect on the first two ions is obviously better than the removal effect on the last two, and the removal effect on the Ca.sup.2+ is the weakest.

[0068] 2. Influence of Adsorption Time on Removal Effect:

[0069] A 50 mg/L HMs (Pb.sup.2+, Cd.sup.2+ and Cu.sup.2+) solution was prepared, 20 mL of the solution was taken respectively, 20 mg of CM-EACD@Si was weighed and placed in the solution, the mixture was shaken at a constant temperature of 30° C. for adsorption for 1 h, sampling was performed at regular intervals, the sample was shaken at a constant temperature of 30° C. for 2 h, and the HMs concentration of the supernatant was measured. The results are shown in FIG. 4.

[0070] FIG. 4 shows a relation diagram of Pb.sup.2+/Cd.sup.2+/Cu.sup.2+ adsorption capacity (Qt, mg/g) of CM-EACD@Si as a function of contact time in a 50 mg/L HMs solution with a pH of 5.5-6 and a temperature of 30° C. It can be seen from the figure that the Pb.sup.2+/Cd.sup.2+/Cu.sup.2+ adsorption capacity of the CM-EACD@Si increases with the increases of the contact time, and the overall adsorption rate is high. The CM-EACD@Si is in a quick adsorption state within 0-5 min, and has achieved an equilibrium state within 10-15 min. Compared with currently reported materials, for example, an activated carbon/chitosan composite material, a β-CD/graphene oxide composite material and a β-CD/SiO2 composite material, the use of the CM-EACD@Si to adsorb HMs requires shorter time to achieve the equilibrium, which is mainly because abundant COO.sup.− distributed on the surface may quickly electrostatically interact with the HMs (positively charged) and unreacted hydroxyl groups in the β-CD molecule may quickly chelate with the HMs, thereby realizing quick adsorption.

[0071] 3. Adsorption Kinetic Experiment

[0072] A 1000 mg/L HMs (Pb.sup.2+, Cd.sup.2+ and Cu.sup.2+) stock solution was prepared. At 25° C., an adsorption kinetic experiment was respectively performed in 50 mL of a 50 mg/L HMs solution containing 50 mg of EA-β-CD@Si and CM-EACD@Si. The pH was controlled with HNO.sub.3/NaOH. After the adsorption was completed, the supernatant was taken, and filtered through a filter membrane with a pore size of 0.22 μm for separation, so as to measure the residual HMs concentration. The results are shown in FIG. 5 and Table 1.

TABLE-US-00001 TABLE 1 Quasi-second-order kinetic parameters of different HMs Fitting equation R.sup.2 K.sub.2/min.sup.−1 Qe, cal/(mg/g) R.sup.2 Qe, exp/(mg/g) Pb.sup.2+ 0.02106x + 0.00576 0.9994 0.0206 47.9846 0.9966 46.4124 Cd.sup.2+ 0.05085x + 0.02767 0.9975 0.0509 19.6657 0.9896 30.4405 Cu.sup.2+ 0.10295x + 0.0879  0.9986 0.0752 11.3645 0.9931 10.3672

[0073] FIG. 5A and Table 1 show the quasi-second-order kinetic fitting graph of CM-EACD@Si for the three HMs and the corresponding kinetic parameter results. It can be seen from the figure and the table that the fitting linear correlation coefficients R.sup.2 of the three HMs are all greater than 0.997, which are very consistent with the quasi-second-order kinetic fitting with chemical adsorption as the dominant influencing factor, and the maximum adsorption capacities for the corresponding three HMs are respectively Pb.sup.2+ (46.4124 mg/g), Cd.sup.2+ (30.4405 mg/g) and Cu.sup.2+ (10.3672 mg/g), which are very close to the calculated results by fitting.

[0074] FIG. 5B shows fitting of the particle internal diffusion model. The fitting means that in a uniformly mixed solution, the particle internal diffusion rate is the main rate control step, and if there is an external resistance or chemical action, the fitting result is linear without passing through the origin. It can be seen from the figure that for the three HMs with an initial concentration of 50 mg/L, qt of the CM-EACD@Si is not linearly related to t %, but is synthesized by two straight lines and does not pass through the origin either. This indicates that the whole process involves 2 steps, and each step has a different control mechanism: the first stage may be attributed to HMs diffusing to the outer surface of the material through solution and to the inner surface of the material through the boundary layer, and the second stage is attributed to HMs then diffusing in the particles. Generally, since the HMs diffuse into the internal structure of the material, as the diffusion velocity of diffusion pores becomes smaller, the diffusion rate will decrease slowly. Based on this, it can be inferred that the adsorption process of CM-EACD@Si on HMs is mainly surface adsorption, and the surface adsorption rate is obviously higher than the internal diffusion rate.

Example 5: Application of CM-EACD@Si as Adsorbent in Composite Metal Ion System

[0075] In order to test the interactive competitive effect of certain HMs in a multi-metal solution, removal efficiencies of the CM-EACD@Si on the HMs in a single-element system, a two-element system and a three-element system were compared.

[0076] Using the CM-EACD@Si prepared in Example 2 as an adsorbent, in order to research the influence of other interfering ions on the CM-EACD@Si while adsorbing the HMs, the adsorption effects of several HMs in different mixed systems were compared. Each of the HMs was made into a mixed solution with the same mass concentration (20 mg/L), the mixed solution was shaken for 2 h, and the HMs concentration of the supernatant was measured. The results are shown in FIG. 6.

[0077] FIG. 6 shows removal effects of the CM-EACD@Si on the HMs in different systems. It can be seen that the removal rates for the single-element systems are respectively 97.80% for Pb.sup.2+, 81.29% for Cd.sup.2+ and 24.00% for Cu.sup.2+. In a two-element system containing Pb.sup.2+, the presence of Cd.sup.2+ or Cu.sup.2+ slightly reduces Pb.sup.2+ adsorption (91.17% and 90.05% respectively). However, in the same two-element system, the removal rate of Cd.sup.2+ or Cu.sup.2+ is significantly reduced to 39.09% and 9.50% respectively, which indicates that Pb.sup.2+ will be preferentially adsorbed on the surface of the CM-EACD@Si while Cd.sup.2+ or Cu.sup.2+ has weaker affinity. In the two-element system (Cd.sup.2+, Cu.sup.2+), the removal rates of Cd.sup.2+ and Cu.sup.2+ are reduced to 55.49% and 15.1% respectively, indicating the mutual competitiveness of the two ions. In a three-element mixed system, the removal rate of Pb.sup.2+ by the CM-EACD@Si is slightly reduced (88.57%), and the removal rates of Cd.sup.2+ and Cu.sup.2+ are lower than those in the single-element or two-element system (being only 28.69% and 7.30%), that is, the adsorption capacity of the CM-EACD@Si in the multielement system mixed solution will be lower than that in the single-element system, which can be attributed to the reduced availability of binding sites. In the multielement system solution, metals with greater affinity can replace other metals with weaker affinity. After various comparisons, it is found that the removal rates of three HMs are in the order of Pb.sup.2+>Cd.sup.2+>Cu.sup.2+, and even in the three-element composite system, the CM-EACD@Si still has a removal effect of more than 80% on Pb.sup.2+.

Example 6: Application of CM-EACD@Si as Adsorbent in Actual Sewage

[0078] Considering that not only some HMs in actual sewage, but also pollutants such as electrolytes and organic matters will influence the adsorption behavior, certain competitive adsorption behaviors will occur in aqueous solutions. Therefore, it is necessary to study the competitive effect of coexisting anions and cations. A series of different concentrations (20 mg/L and 50 mg/L) of Ni.sup.2+, Mg.sup.2+, Ca.sup.2+, Na.sup.+, K.sup.+, SO.sub.4.sup.2− and NO.sub.3.sup.− were selected and added to 50 mL of different concentrations (20 mg/L and 50 mg/L) of Pb.sup.2+ solutions, the mixture was shaken for 2 h, and the HMs concentration of the supernatant was measured.

[0079] After adding mixed system solutions containing one or more ions respectively to the solution containing Pb.sup.2+, the influence of multiple interfering ions on the Pb.sup.2+ removal effect was inspected, as shown in FIG. 7. It can be seen from FIG. 7 that when the Pb.sup.2+ is mixed with multiple other ions, the adsorption effect of the CM-EACD@Si on Pb.sup.2+ is inhibited to varying degrees, and especially Ni.sup.2+, SO.sub.4.sup.2− and NO.sub.3 have significant influence on the adsorption effect. In addition, the removal effect of the CM-EACD@Si on Pb.sup.2+ is slightly inhibited with the increase of concentrations of K.sup.+, Na.sup.+, SO.sub.4.sup.2− and NO.sub.3.sup.−, and is significantly inhibited with the increase of concentrations of Ni.sup.2+, Mg.sup.2+ and Ca.sup.2+, which is mainly due to the difference in the ionic radius of the cations and the different affinities for different ions. The main influence of anions is that under acidic conditions, functional groups on the surface of the CM-EACD@Si are easily protonated, which will bind with SO.sub.4.sup.2− and NO.sub.3.sup.− existing in the solution, and the active sites on the surface of the CM-EACD@Si are definite, which will make the removal of SO.sub.4.sup.2− and NO.sub.3.sup.− by CM-EACD@Si compete with the removal of Pb.sup.2+, so that part of the active sites are occupied by other anions, thereby inhibiting the adsorption on Pb.sup.2+.

Example 7: Reproducibility and Reusability

[0080] Saturated CM-EACD@Si was added to three desorption solutions (0.1 mol/L nitric acid, sodium ethylene diamine tetraacetate and phosphoric acid) for desorption study. After the mixture was shaken at 230 rpm for 2 h, the HMs concentration in the supernatant was measured, and the adsorption-desorption process was carried out three cycles to verify the reusability.

[0081] From an economic point of view, reproducibility and stability are the two major concerns of adsorbent materials. In view of the reusability for HMs, three types of desorption solutions (a 0.1 mol/L HNO.sub.3 solution, a H.sub.3PO.sub.4 solution and a Na.sub.2EDTA solution) were selected for inspection in this study. The desorption results of the three different types of desorption solutions are shown in FIG. 8A. It can be found that the HNO.sub.3 and Na.sub.2EDTA solutions have better desorption effects on Pb.sup.2+ (84.51% and 80.75%), and the H.sub.3PO.sub.4 has better desorption effects on Cd.sup.2+ and Cu.sup.2+ (55.48% and 66.89%), indicating that: the binding of the active sites of the CM-EACD@Si with the HMs cannot be maintained for a long time under acidic conditions, and H.sup.+ protonates the surface of the material, which is more beneficial to the reproduction of carboxyl (COO.sup.−), so that the positively charged HMs adsorbed on the surface of the material are more easily desorbed. In addition, the desorption effects of the Na.sub.2EDTA on the three HMs are in an order of Pb.sup.2+>Cd.sup.2+>Cu.sup.2+, which is because the stronger ligand in the Na.sub.2EDTA will form a stronger bond with Pb.sup.2+, making Pb.sup.2+ more easily desorbed from the CM-EACD@Si. In addition, the effects of three times of desorption on Pb.sup.2+ in the 0.1 mol/L HNO.sub.3 solution were also researched, as shown in FIG. 8B. It can be seen that the removal rate of Pb.sup.2+ by CM-EACD@Si can still reach about 71% after three times of desorption, which is mainly because the result reveals the stability and recoverability of CM-EACD@Si in practical application when pH is reached.

[0082] Although the disclosure has been disclosed as above in the preferred examples, it is not intended to limit the disclosure. Anyone familiar with this technology can make various changes and modifications without departing from the spirit and scope of the disclosure. Therefore, the protection scope of the disclosure should be as defined in the claims.