Cationic covalent organic framework defluorination adsorbent and its preparation method and application

Abstract

The present invention discloses a cationic covalent organic framework defluorination adsorbent, its preparation method, and application. The method comprises the following steps: subjecting a phenolic hydroxyl-functionalized covalent organic framework (COF) to a solvent-free grinding reaction with a brominated alkylammonium salt compound in the presence of a base at room temperature to prepare a cationic covalent organic framework defluorination adsorbent PS-COF with an ammonium salt grafted structure. The present invention utilizes the electrostatic interaction between the grafted ammonium salt COF and fluoride ions to significantly improve the adsorbent's defluorination performance, achieving rapid fluoride ion removal within 2 hours. The adsorbent also has a high adsorption capacity and excellent defluorination effect.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment, and in particular to a cationic covalent organic framework defluorination adsorbent, a preparation method thereof, and applications thereof. Background Art

[0002] Fluoride (mainly in the form of fluoride ion F - ) is an essential component for humans and animals, preventing tooth decay and promoting the mineralization of hard tissues. The World Health Organization stipulates that the optimal level of fluoride in drinking water is 0.5 to 1.5 mg·L -1 Fluoride concentrations above 1.5 mg / L can cause dental fluorosis and skeletal fluorosis. China's Drinking Water Quality Standard (GB5749-2006) stipulates that the fluoride content in drinking water should not exceed 1.0 mg / L. Fluoride-containing wastewater discharged from the electronics, glass and ceramics production, steel, electroplating, and semiconductor industries pollutes the environment and causes excessive fluoride concentrations in drinking water. Therefore, the treatment of fluoride-containing wastewater is urgent.

[0003] Currently, the main methods for removing fluoride include precipitation and flocculation, membrane technology, ion exchange technology, and adsorption technology. Adsorption technology offers the advantages of low energy consumption, environmental friendliness, safety, efficiency, wide application range, and simple operation. However, existing defluoridation adsorbents suffer from issues such as low adsorption capacity, slow adsorption rate, secondary contamination, and poor regeneration performance.

[0004] Covalent organic frameworks (COFs) are a new type of ordered porous crystalline materials. Compared with traditional materials, COFs have advantages such as good chemical stability, high specific surface area and more active sites. The functionalization of COFs can be achieved mainly through pre-modification and post-modification strategies. Compared with the disadvantages of pre-modification such as complex synthesis process and high cost, post-modification is simpler and more efficient.

[0005] Currently, COFs materials are widely used in the field of water treatment, but the use of COFs materials to remove fluoride from water has never been reported. Therefore, it is of great value to develop a defluorination adsorbent with high adsorption capacity and fast adsorption rate. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the above background technology and provide a cationic covalent organic framework defluorination adsorbent and its preparation method and application to improve the defluorination performance.

[0007] In order to solve the above technical problems, the technical solution proposed by the present invention is:

[0008] A method for preparing a cationic covalent organic framework defluorination adsorbent comprises the following steps:

[0009] The phenolic hydroxyl functionalized covalent organic framework (COF) was subjected to solvent-free grinding reaction with a brominated alkylammonium salt compound in the presence of a base to prepare a cationic covalent organic framework defluorination adsorbent PS-COF with an ammonium salt grafted structure.

[0010] The phenolic hydroxyl functionalized covalent organic framework COF has the following structure:

[0011]

[0012] The structural unit Ar1 is derived from any of the following amino monomers:

[0013]

[0014] The structural unit Ar2 is derived from any one of the following phenolic hydroxyl functionalized aldehyde monomers:

[0015]

[0016] Wherein, the brominated alkylammonium salt compound is selected from any one of the following compounds:

[0017]

[0018] As a further improvement, the phenolic hydroxyl functionalized covalent organic framework COF is composed of two structural units, Ar1 and Ar2, which are alternately connected to form a periodic hexagonal framework. Ar1 is located at the vertex of the hexagon, has uniform hexagonal channels, and the phenolic hydroxyl functional groups are evenly distributed on the channels.

[0019] As a further improvement, the phenolic hydroxyl functionalized covalent organic framework COF is a COF prepared by reacting 2,4,6-tris(4-aminophenyl)-1,3,5-triazine with 2,5-dihydroxyterephthalaldehyde. TAPT-DHTA ; The bromoalkylammonium salt compound is (2-bromoethyl)trimethylammonium bromide, (3-bromopropyl)trimethylammonium bromide, 1-(2-bromoethyl)pyrrolidine hydrochloride or (3-bromopropyl)triethylammonium bromide.

[0020] As a further improvement, the base is sodium hydroxide or sodium hydride.

[0021] As a further improvement, the mass ratio of the phenolic hydroxyl functionalized covalent organic framework COF, the brominated alkylammonium salt compound, and the base is 5:(6-9):(1-4).

[0022] As a further improvement, the grinding reaction temperature is 20-35° C., and the reaction time is 1-4 h.

[0023] The present invention also provides a cationic covalent organic framework defluorination adsorbent, which is prepared by adopting the method described above.

[0024] The present invention also provides an application of the cationic covalent organic framework defluorination adsorbent in defluoridation of water bodies.

[0025] As a further improvement, the fluoride ion removal temperature is set at 25°C to 45°C.

[0026] As a further improvement, after the adsorbent adsorbs fluoride ions, it is treated with Ca(OH)2 to regenerate the adsorbent.

[0027] The present invention uses aldehyde-based building blocks bearing phenolic hydroxyl groups to prepare COF. The phenolic hydroxyl functional groups are evenly distributed throughout the hexagonal pores. Through a simple post-modification strategy involving room-temperature grinding, the phenolic hydroxyl groups react with a series of bromoalkylammonium salt compounds to achieve ammonium salt functionalization, resulting in a cation-functionalized COF material. This COF, grafted with ammonium salts, utilizes electrostatic interaction between the COF and fluoride ions, significantly improving the adsorbent's fluoride removal performance.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) Unlike the existing multi-step synthesis method of ionic COF, the present invention adopts room temperature grinding followed by modification of phenolic hydroxyl COF for preparation, which has a high degree of modification, is easy to operate, and does not require the use of any organic solvent.

[0030] (2) The prepared cationic COF maintains a high degree of crystallinity and has the characteristics of good hydrophilicity, multiple adsorption sites and large specific surface area. It can quickly remove fluoride ions within 2 hours and has a high adsorption capacity and good fluoride removal effect.

[0031] (3) Cationic COFs are linked by covalent bonds, have strong structural stability, can be recycled, and are not prone to secondary pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0033] Figure 1 It is a polymer COF TAPT-DHTA and cationic COF TAPT-DHTA (PS-COF TAPT-DHTA a) Fourier transform infrared spectrum, b) X-ray diffraction pattern and simulation diagram;

[0034] Figure 2 COF TAPT-DHTA and PS-COF TAPT-DHTA Scanning electron microscope images of

[0035] Figure 3 COF TAPT-DHTA and PS-COF TAPT-DHTA Optical image of

[0036] Figure 4 PS-COF TAPT-DHTA Energy dispersive X-ray spectroscopy (EDS) spectrum;

[0037] Figure 5 COF TAPT-DHTA and PS-COF TAPT-DHTA Water contact angle diagram;

[0038] Figure 6 COF TAPT-DHTA and PS-COF TAPT-DHTA N2 adsorption-desorption curve;

[0039] Figure 7 PS-COF TAPT-DHTA Adsorption kinetics at 25 °C;

[0040] Figure 8 PS-COF TAPT-DHTA Adsorption isotherms at 25°C, 35°C, and 45°C. DETAILED DESCRIPTION

[0041] To facilitate understanding of the present invention, the present invention will be described in more comprehensive and detailed form below in conjunction with the accompanying drawings and preferred embodiments. However, the protection scope of the present invention is not limited to the following specific embodiments.

[0042] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0043] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0044] The preparation method of the cationic covalent organic framework defluorination adsorbent according to some specific embodiments of the present invention comprises the following steps:

[0045] The phenolic hydroxyl functionalized covalent organic framework (COF) is subjected to solvent-free grinding reaction with a brominated alkylammonium salt compound in the presence of a base to prepare a cationic covalent organic framework defluorination adsorbent PS-COF with an ammonium salt grafted structure.

[0046] Among them, the phenolic hydroxyl functionalized covalent organic framework COF has the following structure:

[0047]

[0048] It is composed of two structural units, Ar1 and Ar2, which are alternately connected to form a periodic hexagonal framework, with Ar1 located at the vertices of the hexagon. It has a uniform hexagonal channel in the center, and the phenolic hydroxyl functional groups are evenly distributed in the channel.

[0049] Ar1 is derived from one of the following amino monomers:

[0050]

[0051] Ar2 is derived from one of the following phenolic hydroxyl functionalized aldehyde monomers:

[0052]

[0053] Wherein, the brominated alkylammonium salt compound is selected from any one of the following compounds:

[0054]

[0055] In some specific embodiments, the covalent organic framework COF is prepared by reacting 2,4,6-tris(4-aminophenyl)-1,3,5-triazine with 2,5-dihydroxyterephthalaldehyde (COF TAPT-DHTA ); the bromoalkylammonium salt compounds are (2-bromoethyl)trimethylammonium bromide, (3-bromopropyl)trimethylammonium bromide, 1-(2-bromoethyl)pyrrolidine hydrochloride and (3-bromopropyl)triethylammonium bromide.

[0056] In some specific embodiments, the base can be sodium hydroxide or sodium hydride.

[0057] In some specific embodiments, the mass ratio of COF, bromoalkylammonium salt compound, and base is 5:(6-9):(1-4), more preferably 5:(6-9):(1-2.5).

[0058] In some specific embodiments, the grinding reaction is carried out at room temperature, the reaction temperature is 20-35° C., and the reaction time is 1-4 h.

[0059] In some specific embodiments, COF, bromoalkylammonium salt compounds, base and other reaction materials are stirred and mixed uniformly at room temperature before grinding and reacting.

[0060] Example 1: Cationic PS-COF TAPT-DHTA Preparation and characterization of fluoride removal adsorbent

[0061] 1.COF TAPT-DHTA Preparation

[0062] At room temperature, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (A3, 0.08 mmol) and 2,5-dihydroxyterephthalaldehyde (B1, 0.12 mmol) were added to a microwave reaction tube, and 0.5 mL each of mesitylene and dioxane was injected into it. After ultrasonic dispersion, acetic acid solution (3M, 0.2 mL) was added thereto. After sealing, it was placed in a microwave reactor and reacted at 80°C for 2 hours. After cooling to room temperature, the precipitate was collected by vacuum filtration, washed with ethanol, and extracted in tetrahydrofuran for 24 hours. The product was dried in a vacuum drying oven at 90°C and taken out after 24 hours to obtain a bright red powder with a yield of 84%. The reaction formula is as follows:

[0063]

[0064] 2. Cationic PS-COF TAPT-DHTA Preparation

[0065] At room temperature, COF was added to a dry reaction bottle. TAPT-DHTA (100 mg), (2-bromoethyl)trimethylammonium bromide (135 mg) and sodium hydroxide (40 mg) were stirred at room temperature for 2 h to make them uniformly dispersed, and then ball milled at 30 ° C for 4 h. After the reaction, it was washed with ethanol and tetrahydrofuran and dried in vacuum at 90 ° C for 24 h to obtain reddish brown powder PS-COF TAPT-DHTA , yield: 50%. The reaction formula is as follows:

[0066]

[0067] FTIR spectroscopy (VARIAN 1000) was used to characterize the COF TAPT-DHTA and cationic COF TAPT-DHTA Characterization was performed with a scanning range of 4000-400 cm -1 The X-ray diffraction (XRD) patterns were collected using a Bruker Advanced D8 diffractometer with a step size of 0.01° and a scanning range of 2° to 30°. The COF was observed using a scanning electron microscope and an energy dispersive spectrometer (SEM, EDS, FEI SIRION200, America). TAPT-DHTA and cationic COF TAPT-DHTAMicromorphology and surface elemental analysis of the samples. Nitrogen adsorption and desorption isotherms and pore size distribution curves of the porous materials were measured at 77 K using a Quanta Instruments Autosorb-iQ (Quantachrome) surface area and pore size analyzer. All samples were first degassed at 100°C under high vacuum for 12 hours, and the pore size distribution was calculated using nonlocal density functional theory (NL-DFT).

[0068] Figure 1 -a is 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) 2,5-dihydroxyterephthalaldehyde (DHTA), COF TAPT-DHTA and PS-COF TAPT-DHTA The infrared spectra of the graphs show that cationic PS-COF with COC and CN is formed. TAPT-DHTA .

[0069] like Figure 1 -b, COF TAPT-DHTA The X-ray diffraction spectrum shows that it is a highly crystalline material with a periodic hexagonal framework. TAPT-DHTA The main peak still retains a certain intensity, indicating that it is still crystalline after modification. The 2θ value of the first intensity peak after modification is 2.6°, which is consistent with the diffraction spectrum simulated using the overlapping superposition model and the diffraction spectrum obtained experimentally.

[0070] like Figure 2 As shown, COF was observed using scanning electron microscopy. TAPT-DHTA and PS-COF TAPT-DHTA Microscopic morphology of the material. a) COF obtained TAPT-DHTA PS-COF after modification (b) presents interlaced strip-like fibers TAPT-DHTA The shape is coral-like.

[0071] Figure 3 COF before and after modification TAPT-DHTA Optical images of cationic COF (a) before modification (red), and (b) after modification TAPT-DHTA Appears reddish brown.

[0072] Figure 4 This is the EDS spectrum. The appearance of the Br peak indicates that the grafting of (2-bromoethyl)trimethylammonium bromide is successful.

[0073] like Figure 5 As shown, a) COF before modification TAPT-DHTA The water contact angle is 65°, b) PS-COF after modification TAPT-DHTA The water contact angle decreased to 58.5°, and the hydrophilicity of the material was improved.

[0074] like Figure 6 As shown in a), c), the COF specific surface area before modification is 1476m 2 ·g -1 (BET specific surface area was calculated using the BET calculation model based on the nitrogen adsorption-desorption curve). The main pore size distribution is 1.69 / 2.36 / 2.60 nm. b) PS-COF TAPT-DHTA The specific surface area is 306m 2 ·g -1 The main pore size distribution is 1.49 / 1.69 / 2.60 / 3.20nm.

[0075] Cationic PS-COF of Example 1 TAPT-DHTA Fluoride removal performance test

[0076] The fluoride ion concentration determination method is as follows:

[0077] 1. Preparation of main reagents

[0078] Total ionic strength adjustment buffer (TISAB I): Dissolve sodium citrate and sodium nitrate in ultrapure water, adjust the pH of the solution to 5-6 with 1 M hydrochloric acid and sodium hydroxide, and transfer the solution to a 500 mL volumetric flask using a glass rod for dilution.

[0079] Preparation of fluoride stock solution (100 mg·L -1 ) Take 0.1105g of sodium fluoride that has been dried in an oven at 115℃-120℃ for 2 hours, add it to a 500mL volumetric flask, dilute it to the mark with water, and store it in a polyethylene bottle.

[0080] Preparation of fluorine-containing standard solution (10 mg·L -1 ):10mL F - The stock solution was added to a 100 mL volumetric flask and diluted to the mark with ultrapure water to obtain 10 mg·L -1 of NaF(aq). This solution must be prepared and used immediately.

[0081] 2.F - Drawing of the standard curve

[0082] Use a non-gradient pipette to draw 1.00, 3.00, 5.00, 10.0, and 20.0 mL of fluoride standard solution respectively, place it in a 50 mL volumetric flask, add 10 mL of total ionic strength adjustment buffer solution, dilute with water to the mark, shake well, and inject it into 100 mL polyethylene cups respectively. Place a magnetic stirring bar in each cup, and insert the electrodes in order from low to high concentration. Stir the solution continuously. After the potential stabilizes, read the potential value E under stirring. Before each measurement, rinse the electrode with water and dry it with filter paper. The concentration log C F -The horizontal axis is the potential value E and the vertical axis is the potential value E. The standard curve of fluoride solution is obtained by plotting and fitting.

[0083] 3.F - Determination of potential value

[0084] Use a non-gradient pipette to draw an appropriate amount of solution and place it in a 50mL volumetric flask. Adjust it to near neutral with hydrochloric acid, add 10mL of total ionic strength adjustment buffer solution, dilute it to the mark with water, shake well, and inject it into a 100ml polyethylene cup. The specific measurement method is the same as above. According to the measured millivolts, calculate F from the standard curve. - concentration.

[0085] The adsorption kinetics experiment is as follows:

[0086] Dissolve 1.105 g of pre-dried NaF in 500 mL of ultrapure water to obtain 1000 mg·L -1 NaF standard stock solution, take 1000 mg·L -1 The NaF standard stock solution was prepared to obtain 10 mg·L -1 of NaF solution. -1 NaF, PS-COF TAPT-DHTA The adsorbent concentration was 3 g·L -1 The polypropylene bottle of the solution with pH 4 was placed in a water bath constant temperature oscillator. One bottle of solution was taken out at 5min, 10min, 15min, 20min, 40min, 60min, 80min, 120min, 160min, and 200min after the adsorption reaction to detect F. - concentration.

[0087] The adsorption thermodynamics experiment is as follows:

[0088] Take 1000mg·L -1 The NaF standard stock solutions were prepared to obtain 4 mg·L -1 , 6mg·L -1 , 8mg·L -1 、10mg·L -1 , 12mg·L -1 , 14mg·L -1 , 16mg·L -1 , 20mg·L -1 、30mg·L -1 , 70mg·L -1 , 120mg·L -1 , 200mg·L -1 NaF solution, PS-COF TAPT-DHTA The concentration of adsorbent is 3 g·L -1The solution pH was 4, and the adsorption equilibrium was detected after constant temperature oscillation at 298K, 308K and 318K for 2h. - concentration.

[0089] Figure 7 is the initial concentration C0(F - ) is 10 mg·L -1 The adsorption kinetics curve showed that F - Adsorption can reach equilibrium within 2 hours. The pseudo-first-order and pseudo-second-order kinetic models were used to fit the curves. The fitting parameters of the two models are shown in Table 1. The R 2 All were greater than 0.90, and the pseudo-first-order kinetic curve R 2 The value of 0.95048 is greater than the pseudo-second-order kinetics and the pseudo-first-order fitting degree is higher, which indicates that the cationic PS-COF TAPT-DHTA Adsorption F - The process may be physical adsorption. In addition, the K1 value is 0.0633, indicating that the adsorption rate is very fast.

[0090] The results show that the F - The concentration is 4 mg·L -1 ~200mg·L -1 , the adsorbent concentration is 3 g·L -1 The fluoride removal effect was investigated by fitting the Langmuir adsorption isotherm model and the Freundlich adsorption isotherm model respectively to explore the adsorption mechanism. Figure 8 PS-COF TAPT-DHTA Adsorption isotherms at different temperatures (298K, 308K, 318K). Table 2 shows the fitting parameters of the two models and the correlation coefficient R of the Langmuir model. 2 are greater than the Freundlich model. The maximum adsorption capacity Q calculated by this model is m 36.96 mg·g -1 .

[0091] As can be seen from Table 2, with the increase of temperature, PS-COF TAPT-DHTA Adsorbent for F - The maximum adsorption capacity Q m From 36.96 mg g -1 Dropped to 8.67 mg·g -1 .

[0092] Table 3 is PS-COF TAPT-DHTA Adsorption F - Thermodynamic parameters, △G at temperatures of 298K, 308K, and 318K 0 -10.06 kJ·mol -1、-9.40kJ·mol -1 、-8.68kJ·mol -1 , both less than 0.

[0093] Table 4 shows that the PS-COF of the present invention TAPT-DHTA The fluorine removal performance of the adsorbent is much better than that of other known adsorbents.

[0094] Table 1 PS-COF at 25℃, 35℃, and 45℃ TAPT-DHTA Adsorption F - Isothermal model fitting parameters

[0095]

[0096] Table 2 PS-COF TAPT-DHTA Adsorption F - Kinetic parameters

[0097]

[0098] Table 3 PS-COF TAPT-DHTA Adsorption F - Thermodynamic parameters

[0099]

[0100] Table 4 PS-COF TAPT-DHTA Comparison of fluorine removal performance of adsorbent with other adsorbents

[0101]

[0102] Example 2

[0103] At room temperature, COF was added to a dry reaction bottle. TAPT-DHTA (100 mg), (3-bromopropyl) triethylammonium bromide (144 mg) and sodium hydride (26 mg), stirred at room temperature for 2 h to make it uniformly dispersed, and then ball milled at 30 ° C for 3 h. After the reaction, it was washed with ethanol and tetrahydrofuran and dried in vacuum at 90 ° C for 24 h to obtain cationic PS-COF TAPT-DHTA -1.

[0104] Example 3

[0105] At room temperature, COF was added to a dry reaction bottle. TAPT-DHTA (100 mg), 1-(2-bromoethyl)pyrrolidine hydrochloride (128 mg) and sodium hydroxide (40 mg) were stirred at room temperature for 2 h to make them uniformly dispersed, and then ball milled at 30 ° C for 4 h. After the reaction, it was washed with ethanol and tetrahydrofuran and dried in vacuum at 90 ° C for 24 h to obtain cationic PS-COFTAPT-DHTA -2.

[0106] Example 4

[0107] At room temperature, COF was added to a dry reaction bottle. TAPT-DHTA (100 mg), (3-bromopropyl) triethylammonium bromide (168 mg) and sodium hydroxide (40 mg) were stirred at room temperature for 2 h to make them uniformly dispersed, and then ball milled at 30 ° C for 4 h. After the reaction, it was washed with ethanol and tetrahydrofuran and dried in vacuum at 90 ° C for 24 h to obtain cationic PS-COF TAPT-DHTA -3.

[0108] Fluorine removal performance test of Examples 2, 3, and 4

[0109] The materials obtained in Examples 2, 3, and 4 were used as adsorbents and the fluoride ion removal test was carried out in a similar manner to that in Example 1, wherein the initial fluoride ion concentration was 200 mg·L -1 , and the adsorption capacity was obtained as shown in Table 5.

[0110] Table 5 F of the materials obtained in Examples 2, 3, and 4 - Adsorption capacity (318K)

[0111]

[0112] The adsorbent after adsorbing fluoride ions can be regenerated by treating it with Ca(OH)2. - After the adsorbent powder was added, it was added to 0.1 mol·L -1 The Ca(OH)2 aqueous solution was stirred at room temperature for a certain period of time to allow it to desorb.

[0113] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any form. Therefore, any simple modifications, equivalent variations, and modifications to the above embodiments that do not depart from the technical solution of the present invention and are based on the technical essence of the present invention shall fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing a cationic covalent organic framework defluorination adsorbent, characterized in that: The following steps are included: The phenolic hydroxyl functionalized covalent organic framework (COF) is subjected to a solvent-free grinding reaction with a brominated alkylammonium salt compound in the presence of a base to prepare a cationic covalent organic framework defluorination adsorbent PS-COF with an ammonium salt grafted structure; the grinding reaction temperature is 20 to 35° C., and the reaction time is 1 to 4 hours; The phenolic hydroxyl functionalized covalent organic framework COF has the following structure: The structural unit Ar1 is derived from any of the following amino monomers: The structural unit Ar2 is derived from any one of the following phenolic hydroxyl functionalized aldehyde monomers: Wherein, the brominated alkylammonium salt compound is selected from any one of the following compounds:

2. The preparation method according to claim 1, characterized in that The phenolic hydroxyl functionalized covalent organic framework COF is composed of two structural units, Ar1 and Ar2, which are alternately connected to form a periodic hexagonal framework. Ar1 is located at the vertex of the hexagon. It has uniform hexagonal channels, and the phenolic hydroxyl functional groups are evenly distributed on the channels.

3. The preparation method according to claim 1, characterized in that The phenolic hydroxyl functionalized covalent organic framework (COF) is a COF prepared by reacting 2,4,6-tris(4-aminophenyl)-1,3,5-triazine with 2,5-dihydroxyterephthalaldehyde. TAPT-DHTA ; The bromoalkylammonium salt compound is (2-bromoethyl)trimethylammonium bromide, (3-bromopropyl)trimethylammonium bromide, 1-(2-bromoethyl)pyrrolidine hydrochloride or (3-bromopropyl)triethylammonium bromide.

4. The preparation method according to claim 1 or 3, characterized in that The alkali is sodium hydroxide or sodium hydride.

5. The preparation method according to claim 4, characterized in that The mass ratio of the phenolic hydroxyl functionalized covalent organic framework COF, the brominated alkylammonium salt compound and the base is 5: (6-9): (1-4).

6. A cationic covalent organic framework defluorination adsorbent, characterized in that: The invention discloses a method for preparing the invention according to any one of claims 1 to 5.

7. Use of the cationic covalent organic framework defluorination adsorbent according to claim 6 in defluoridation of water.

8. The use according to claim 7, characterized in that The defluorination temperature is set at 25℃~45℃.

9. The use according to claim 7 or 8, characterized in that After the adsorbent has adsorbed fluoride ions, it is regenerated by treating it with Ca(OH)2.

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