Preparation method of electrostatic self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel

Abstract

The invention discloses a preparation method of a halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel modified by electrostatic self-assembly, comprising the steps of: in an electrostatic assembly mode, surface modification of halloysite nanotubes is performed using a cationic polyelectrolyte to obtain a modified halloysite nanotube having a positive zeta potential; in an electrostatic assembly mode, negatively charged phytic acid is modified on the surface of the modified halloysite nanotube having a positive zeta potential to obtain a phytic acid-functionalized halloysite nanotube; under alkaline conditions, using 1,3-diepoxybutane as a cross-linking agent, a three-dimensional network structure is formed by cross-linking reaction of polyvinyl alcohol and phytic acid-functionalized halloysite nanotubes, and finally the reaction solution is freeze-dried, and then the alkaline substance is removed to obtain an electrostatic self-assembly modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel. The present invention solves the problem that polyvinyl alcohol easily remains in the solution and that halloysite nanotubes are not easily recycled and reused, and improves the adsorption performance of heavy metal ions by synergistic enhancement.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogels, in particular to a method for preparing an electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel. Background Art

[0002] Polyvinyl alcohol (PVA) contains numerous hydroxyl functional groups on its molecular chains. These groups can form stable bonds with heavy metal ions through mechanisms such as hydrogen bonding and complexation reactions, thereby effectively adsorbing heavy metals. However, when used alone as an adsorption material, due to the strong hydrophilicity of its molecular chains and the dissociation of some hydroxyl groups in aqueous solution, the PVA may swell or partially dissolve during the adsorption process, resulting in material loss and residue in the water.

[0003] Halloysite nanotubes (HNTs), used as adsorption materials, easily form a stable suspension state in water, but also have the problem of being difficult to recycle and reuse. Summary of the Invention

[0004] In response to the above-mentioned defects in the prior art, the present invention provides a method for preparing an electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel, which achieves efficient adsorption of heavy metal ions while solving the problems of polyvinyl alcohol easily remaining in the solution and halloysite nanotubes being difficult to recycle and reuse.

[0005] The technical solution of the present invention is as follows: a method for preparing an electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel, comprising the following steps:

[0006] (1) Using an electrostatic assembly method, the surface of halloysite nanotubes was modified with cationic polyelectrolytes to obtain modified halloysite nanotubes with a positive zeta potential.

[0007] (2) modifying the surface of the modified halloysite nanotubes with a positive zeta potential obtained in step (1) with negatively charged phytic acid by electrostatic assembly to obtain phytic acid-functionalized halloysite nanotubes;

[0008] (3) Under alkaline conditions, 1,3-diepoxybutane was used as a crosslinker to form a three-dimensional network structure by cross-linking polyvinyl alcohol with phytic acid functionalized halloysite nanotubes. Finally, the reaction solution was freeze-dried and the alkaline substance was removed to obtain an electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel.

[0009] Furthermore, the step (1) specifically comprises ultrasonically dispersing the halloysite nanotubes in deionized water, then adding sodium chloride and polydimethyldiallylammonium chloride, stirring at room temperature, filtering, washing, and drying to obtain modified halloysite nanotubes with a positive zeta potential.

[0010] Furthermore, the mass ratio of the halloysite nanotubes, sodium chloride and polydimethyldiallylammonium chloride is 1: (0.6-1.2): (0.3-0.7).

[0011] Furthermore, the step (2) specifically comprises slowly dropping the phytic acid aqueous solution into the deionized water dispersion of the modified halloysite nanotubes with a positive zeta potential, stirring at room temperature, filtering, washing, and drying to obtain phytic acid functionalized halloysite nanotubes.

[0012] Furthermore, the concentration of the phytic acid aqueous solution is 50% or 70%, and the mass ratio of phytic acid to modified halloysite nanotubes with positive zeta potential is (0.5-2):1.

[0013] Furthermore, the step (3) specifically includes ultrasonically dispersing the phytic acid functionalized halloysite nanotubes in a polyvinyl alcohol aqueous solution, then adding sodium hydroxide to adjust the pH value of the solution to 8-9, and finally adding 1,3-diepoxybutane to carry out a cross-linking reaction.

[0014] Furthermore, the usage ratio of the phytic acid functionalized halloysite nanotubes, polyvinyl alcohol and 1,3-diepoxybutane is 1 g: (1-3) g: (7-15) mL.

[0015] Furthermore, the cross-linking reaction in step (3) is stirred at 30-50° C. for 10-16 hours.

[0016] Furthermore, the freeze drying in step (3) is to freeze dry the reaction solution at -50 to -40°C for 30 to 50 hours.

[0017] Furthermore, in step (3), the alkaline substances are removed by soaking the freeze-dried sample in water at 30 to 50° C. for 8 to 12 hours, and changing the water every 1 to 3 hours.

[0018] The present invention first utilizes the negative surface charge of halloysite nanotubes (HNTs) to modify their surfaces with a cationic polyelectrolyte (polydimethyldiallyl ammonium chloride) layer through electrostatic interaction, forming modified halloysite nanotubes (p-HNTs) with a positive zeta potential. Subsequently, phytic acid molecules rich in phosphate groups are adsorbed onto the p-HNT surfaces, again based on the principle of electrostatic adsorption, to form phytic acid-functionalized halloysite nanotubes (PA-p-HNTs). This composite nanostructured material (PA-p-HNTs) is then blended with an aqueous solution of polyvinyl alcohol (PVA), and 1,3-diepoxybutane is introduced as a crosslinker. In an alkaline solution, the epoxy groups in the 1,3-diepoxybutane react with the phosphate and hydroxyl groups on the PA-p-HNT surfaces and the hydroxyl groups on the PVA molecular chains to form a crosslinked composite aerogel material with high porosity, excellent structural stability, and high adsorption properties.

[0019] The advantages of the present invention compared with the prior art are:

[0020] (1) The composite aerogel prepared by the present invention not only has a rich pore structure, but also integrates the unique structural characteristics of halloysite nanotubes, the charge regulation ability of cationic electrolytes, the excellent adsorption performance of phytic acid and the rich active hydroxyl groups of polyvinyl alcohol, and benefits from the excellent interfacial compatibility. The four form a deep combination and synergistic enhancement in adsorption performance, greatly improving its adsorption performance for heavy metal ions.

[0021] (2) The present invention introduces 1,3-diepoxybutane as a cross-linking agent into the composite system, and forms a three-dimensional network structure through a chemical cross-linking reaction. It not only introduces more hydroxyl groups, but also optimizes the interface pore structure, and can improve the structural stability of the aerogel, effectively solving the problem that polyvinyl alcohol easily remains in the solution and halloysite nanotubes are difficult to recycle and reuse. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The zeta potential curves of original HNTs, p-HNTs and PA-p-HNTs prepared in Example 1 in aqueous dispersion with a pH of 7.

[0023] Figure 2 This is an SEM image of the electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel prepared in Example 1. DETAILED DESCRIPTION

[0024] The present invention will be further described below with reference to the examples, but are not intended to limit the present invention.

[0025] Example 1

[0026] A method for preparing an electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel comprises the following steps:

[0027] (1) 1g of halloysite nanotubes (Xianfeng Nano, CAS No.: 1332-58-7, Product No.: 103763) were ultrasonically dispersed in deionized water, and 0.6g of sodium chloride and 0.3g of polydimethyldiallylammonium chloride (Wuxi Tianxin Chemical Co., Ltd., product model: PDLS41) were added in sequence. Stirring was continued for 2h at room temperature, and then filtered, washed, and dried to obtain p-HNTs. Figure 1 It can be seen that the surface of HNTs carries a negative charge, but after the cationic polyelectrolyte is adsorbed on its surface, the surface of p-HNTs becomes positively charged.

[0028] (2) Ultrasonic dispersion of 1g of the prepared p-HNTs in deionized water, then dropwise addition of 1g of 50% phytic acid aqueous solution, stirring continuously at room temperature for 5h, filtration, washing, and drying to obtain PA-p-HNTs. Figure 1 It can be seen that the surface of PA-p-HNTs has negative charge due to the introduction of phytic acid.

[0029] (3) 1 g of polyvinyl alcohol (Shanghai Sangon Biotechnology Co., Ltd., molecular weight 65,000-75,000) was stirred and dissolved in deionized water, and then 1 g of the prepared PA-p-HNTs was added and ultrasonically dispersed uniformly. Sodium hydroxide was added to adjust the pH value of the solution to 8, and then 7 mL of 1,3-diethoxybutane was slowly added dropwise. The reaction was stirred at 30 ° C for 16 h. The reaction solution was freeze-dried at -40 ° C for 50 h, and the sample was immersed in warm water at 30 ° C for 12 h. The water was changed every 3 h, and the sample was taken out and dried to obtain an electrostatic self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel. Its SEM image is shown in FIG. Figure 2 As shown, it can be seen that it has a rich pore structure.

[0030] The adsorption performance experiment of the electrostatic self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel prepared in Example 1 is as follows:

[0031] A 100 mL aqueous solution of 100 mg / L lead ion (lead nitrate as reagent) was prepared in a flask. After adjusting the pH of the solution to 7 with 0.1 M NaOH, the prepared electrostatic self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel was added. o C constant temperature oscillator, adsorption at 200 rpm for 3 hours. After adsorption, the supernatant was taken and the lead ion concentration in the supernatant was determined by ICP-OES. The adsorption amount of lead ions by the adsorbent material was calculated by the following formula ( Q , mg / g), and the results are listed in Table 1.

[0032] Q =( C 0 - C t ) V / M

[0033] Q is the adsorption capacity (mg / g); C 0 is the initial concentration of heavy metal ions (mg / L); C t is the concentration of heavy metal ions after adsorption (mg / L); V is the volume of the solution (L); M is the mass of the adsorbent (g).

[0034] Example 2

[0035] A method for preparing an electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel comprises the following steps:

[0036] (1) 1 g of halloysite nanotubes (Xianfeng Nano, CAS No.: 1332-58-7, Product No.: 103763) were ultrasonically dispersed in deionized water, and 0.9 g of sodium chloride and 0.5 g of polydimethyldiallylammonium chloride (Wuxi Tianxin Chemical Co., Ltd., product model: PDLS41) were added in sequence. The mixture was stirred for 3 h at room temperature, filtered, washed, and dried to obtain p-HNTs.

[0037] (2) 1 g of the prepared p-HNTs was ultrasonically dispersed in deionized water, and then 2.5 g of a 50% phytic acid aqueous solution was added dropwise. The mixture was stirred at room temperature for 6.5 h, filtered, washed, and dried to obtain PA-p-HNTs.

[0038] (3) 2 g of polyvinyl alcohol (Shanghai Sangon Biotechnology Co., Ltd., molecular weight 65,000-75,000) was stirred and dissolved in deionized water, and then 1 g of the prepared PA-p-HNTs was added and ultrasonically dispersed uniformly. Sodium hydroxide was added to adjust the pH value of the solution to 8.5, and then 11 mL of 1,3-diethoxybutane was slowly added dropwise. The reaction was stirred at 40 °C for 13 h. The reaction solution was freeze-dried at -45 °C for 40 h, and then the sample was immersed in warm water at 40 °C for 10 h. The water was changed every 2 h, and the sample was taken out and dried to obtain an electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel. Its adsorption properties were measured according to the adsorption properties experiment of the product in Example 1. The results are listed in Table 1.

[0039] Example 3

[0040] A method for preparing an electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel comprises the following steps:

[0041] (1) 1 g of halloysite nanotubes (Xianfeng Nano, CAS No.: 1332-58-7, Product No.: 103763) were ultrasonically dispersed in deionized water, and 1.2 g of sodium chloride and 0.7 g of polydimethyldiallylammonium chloride (Wuxi Tianxin Chemical Co., Ltd., product model: PDLS41) were added in sequence. The mixture was stirred for 4 h at room temperature, filtered, washed, and dried to obtain p-HNTs.

[0042] (2) 1 g of the prepared p-HNTs was ultrasonically dispersed in deionized water, and then 2.9 g of a 70% phytic acid aqueous solution was added dropwise. The mixture was stirred at room temperature for 8 h, filtered, washed, and dried to obtain PA-p-HNTs.

[0043] (3) 3 g of polyvinyl alcohol (Shanghai Sangon Biotech Co., Ltd., molecular weight 65,000-75,000) was stirred and dissolved in deionized water, and then 1 g of the prepared PA-p-HNTs was added and ultrasonically dispersed uniformly. Sodium hydroxide was added to adjust the pH value of the solution to 9, and then 15 mL of 1,3-diethoxybutane was slowly added dropwise. The reaction was stirred at 50 °C for 10 h. The reaction solution was freeze-dried at -50 °C for 30 h, and then the sample was immersed in warm water at 50 °C for 8 h. The water was changed every 1 h, and the sample was taken out and dried to obtain an electrostatic self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel. Its adsorption properties were measured according to the adsorption properties experiment of the product in Example 1. The results are listed in Table 1.

[0044] Comparative Example 1

[0045] 1 g of polyvinyl alcohol (Shanghai Sangon Biotech Co., Ltd., molecular weight 65,000-75,000) was stirred and dissolved in deionized water. Then, 1 g of original HNTs (Xianfeng Nano, CAS No.: 1332-58-7, Product No.: 103763) was added and ultrasonically dispersed uniformly. Sodium hydroxide was added to adjust the pH of the solution to 8. Then, 7 mL of 1,3-diepoxybutane was slowly added dropwise. The reaction was stirred at 30°C for 16 h. The reaction solution was freeze-dried at -40°C for 50 h. The sample was then immersed in warm water at 30°C for 12 h, and the water was changed every 3 h. The sample was removed and dried to obtain an HNTs / PVA composite aerogel. The adsorption properties of the aerogel were measured according to the adsorption properties experiment of the product in Example 1. The results are listed in Table 1.

[0046] Comparative Example 2

[0047] 1 g of polyvinyl alcohol (Shanghai Sangon Biotech Co., Ltd., molecular weight 65,000-75,000) was stirred and dissolved in deionized water, and then 1 g of p-HNTs (prepared according to step (1) in Example 1) was added and uniformly dispersed by ultrasonication. Sodium hydroxide was added to adjust the pH value of the solution to 8, and then 7 mL of 1,3-diepoxybutane was slowly added dropwise. The reaction was stirred at 30°C for 16 h. The reaction solution was freeze-dried at -40°C for 50 h, and then the sample was immersed in warm water at 30°C for 12 h. The water was changed every 3 h, and the sample was taken out and dried to obtain p-HNTs / PVA composite aerogel. Its adsorption properties were measured according to the adsorption properties experiment of the product in Example 1. The results are listed in Table 1.

[0048] Comparative Example 3

[0049] 1 g of polyvinyl alcohol (Shanghai Sangon Biotech Co., Ltd., molecular weight 65,000-75,000) was stirred and dissolved in deionized water, and then 1 g of the prepared PA-p-HNTs (prepared according to steps (1) and (2) in Example 1) was added and uniformly dispersed by ultrasonication. The dispersion was freeze-dried at -40°C for 50 h to obtain PA-p-HNTs / PVA composite aerogel. Its adsorption properties were measured according to the adsorption properties experiment of the product in Example 1. The results are listed in Table 1.

[0050] Table 1 Adsorption capacity of lead ions by aerogels prepared in Examples 1-3 and Comparative Examples 1-3

[0051]

[0052] The results show that the electrostatic self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel prepared in the embodiment of the present invention, on the one hand, achieves a perfect combination and synergistic effect of the unique structural characteristics of halloysite nanotubes, the excellent adsorption performance of cationic electrolytes and phytic acid, and the rich active hydroxyl groups of polyvinyl alcohol. On the other hand, it achieves effective cross-linking between halloysite nanotubes and polyvinyl alcohol, which not only optimizes the pore structure of the aerogel, but also introduces more active hydroxyl groups with adsorption properties, while promoting the dispersibility of halloysite nanotubes. Therefore, the prepared aerogel has excellent adsorption performance for lead ions, with a maximum adsorption capacity of up to 339 mg / g.

[0053] The aerogel obtained in Comparative Example 1 lacks the synergistic adsorption of cationic electrolytes and phytic acid, and the halloysite surface hydroxyl groups are relatively few, resulting in an inability to form an effective cross-linked structure between it and polyvinyl alcohol. The synergistic adsorption of phytic acid is lacking in Comparative Example 2, and effective cross-linking cannot be formed between the halloysite nanotubes and the resin. In Comparative Example 3, although the synergistic enhancement between the halloysite nanotubes, cationic electrolytes, phytic acid and polyvinyl alcohol is achieved, no cross-linking agent is added, and no cross-linking reaction is achieved between the filler and the resin. Cross-linking modification is not achieved in Comparative Examples 1-3, which not only cannot give full play to the synergistic adsorption between the components, but also reduces the porosity of the aerogel and lacks the abundant active hydroxyl groups introduced by cross-linking. It can be seen that the aerogels obtained in Comparative Examples 1-3 cannot be compared with the cross-linked aerogels obtained in Example 1 in terms of porosity, interface bonding and synergistic enhancement, so their adsorption performance is far less than that of the embodiment of the present invention.

Claims

1. A method for preparing electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel, characterized in that: The following steps are involved: (1) Using an electrostatic assembly method, the surface of halloysite nanotubes was modified with cationic polyelectrolytes to obtain modified halloysite nanotubes with a positive zeta potential. (2) modifying the surface of the modified halloysite nanotubes with a positive zeta potential obtained in step (1) with negatively charged phytic acid by electrostatic assembly to obtain phytic acid-functionalized halloysite nanotubes; (3) Under alkaline conditions, 1,3-diepoxybutane was used as a crosslinker to form a three-dimensional network structure by cross-linking polyvinyl alcohol with phytic acid functionalized halloysite nanotubes. Finally, the reaction solution was freeze-dried and the alkaline substance was removed to obtain an electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel.

2. The preparation method of the electrostatic self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel according to claim 1, characterized in that: The step (1) specifically comprises ultrasonically dispersing the halloysite nanotubes in deionized water, then adding sodium chloride and polydimethyldiallyl ammonium chloride, stirring at room temperature, filtering, washing, and drying to obtain modified halloysite nanotubes with a positive zeta potential.

3. The preparation method of the electrostatic self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel according to claim 2, characterized in that: The mass ratio of the halloysite nanotubes, sodium chloride and polydimethyldiallylammonium chloride is 1: (0.6-1.2): (0.3-0.7).

4. The method for preparing the electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel according to claim 1, characterized in that: The step (2) specifically comprises slowly dropping the phytic acid aqueous solution into the deionized water dispersion of the modified halloysite nanotubes with a positive zeta potential, stirring at room temperature, filtering, washing, and drying to obtain the phytic acid functionalized halloysite nanotubes.

5. The method for preparing the electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel according to claim 4, characterized in that: The concentration of the phytic acid aqueous solution is 50% or 70%, and the mass ratio of phytic acid to modified halloysite nanotubes with positive zeta potential is (0.5-2):

1.

6. The method for preparing the electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel according to claim 1, characterized in that: The step (3) specifically includes ultrasonically dispersing the phytic acid functionalized halloysite nanotubes in a polyvinyl alcohol aqueous solution, then adding sodium hydroxide to adjust the pH value of the solution to 8-9, and finally adding 1,3-diepoxybutane to carry out a cross-linking reaction.

7. The method for preparing the electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel according to claim 6, characterized in that: The usage ratio of the phytic acid functionalized halloysite nanotubes, polyvinyl alcohol and 1,3-diepoxybutane is 1 g: (1-3) g: (7-15) mL.

8. The method for preparing the electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel according to claim 6, characterized in that: The cross-linking reaction in step (3) is carried out by stirring at 30-50° C. for 10-16 hours.

9. The method for preparing the electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel according to claim 1, characterized in that: The freeze drying in step (3) is to freeze-dry the reaction solution at -50 to -40°C for 30 to 50 hours.

10. The method for preparing the electrostatically self-assembled modified halloysite nanotube-polyvinyl alcohol cross-linked composite aerogel according to claim 1, characterized in that: In step (3), the alkaline substances are removed by soaking the freeze-dried sample in water at 30 to 50° C. for 8 to 12 hours, and changing the water every 1 to 3 hours.

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