Preparation method and application of porous structure sodium alginate / carbon nanotube composite hydrogel

Abstract

This invention discloses a method for preparing and applying a porous sodium alginate / carbon nanotube composite hydrogel, belonging to the technical field of solar interfacial evaporation and seawater desalination materials. Sodium alginate is used as the hydrophilic gel matrix, and modified carbon nanotubes are used as the photothermal conversion material. The method involves introducing a pore-forming agent into a sodium alginate / carbon nanotube precursor solution, followed by film formation and Fe... 3+ Ion crosslinking and washing processes are used to construct a composite hydrogel with a connected porous structure. This invention provides a simple, mild, and low-cost preparation method. The resulting composite hydrogel exhibits excellent water transport capacity and cycle stability, achieving a high evaporation rate under single sunlight exposure and effectively controlling Na+ in seawater. + K + Mg 2+ Ca 2+ Plasma has a good removal effect and can be used in solar seawater desalination, brine purification and distributed freshwater production.

Description

Technical Field

[0001] This invention belongs to the fields of functional hydrogel materials, solar interfacial evaporation and seawater desalination technology, specifically relating to a method for preparing a porous sodium alginate / carbon nanotube composite hydrogel and its application as a solar evaporator. Background Technology

[0002] Freshwater scarcity has become a critical global issue that urgently needs to be addressed. While traditional seawater desalination technologies such as thermal distillation, reverse osmosis, and electrodialysis can purify brine, they generally suffer from high energy consumption, complex equipment, high operating costs, and demanding maintenance requirements, limiting their widespread application in remote areas, island and reef regions, and distributed water supply scenarios.

[0003] Solar interfacial evaporation technology utilizes photothermal materials to convert solar energy into thermal energy and localizes the heat at the water-air interface, thereby achieving efficient water evaporation. Compared with traditional bulk heating methods, solar interfacial evaporation has advantages such as low energy consumption, simple system, and environmental friendliness, making it a promising green freshwater production technology.

[0004] Hydrogel materials are widely used in the construction of solar evaporators due to their hydrophilic three-dimensional network structure, high water content, and tunable pore structure. Sodium alginate, in particular, is widely available, inexpensive, and can form a stable gel network through cross-linking with multivalent metal ions, making it an ideal matrix material for constructing hydrophilic evaporators. Carbon nanotubes possess broad-spectrum absorption capabilities and excellent photothermal conversion performance; their introduction into sodium alginate hydrogels can significantly enhance the material's ability to absorb and convert sunlight.

[0005] However, existing composite hydrogels still suffer from problems such as insufficient controllability of pore structure, inadequate water transport channels, limited vapor escape paths, and susceptibility to salt crystallization under high-salt environments. If the pores are too small, the water replenishment rate is limited, easily leading to water shortage and salt accumulation at the evaporation interface; if the pores are too large, heat may be lost to the bulk water phase, reducing thermal localization efficiency. Therefore, how to control the internal pore structure of composite hydrogels through simple, safe, and scalable methods to improve water transport capacity and salt crystallization resistance while ensuring thermal localization performance is a key problem that needs to be solved in the field of solar interfacial evaporation materials. Summary of the Invention

[0006] This invention provides a method for preparing and applying a porous sodium alginate / carbon nanotube composite hydrogel. The method involves introducing a pore-forming agent into a sodium alginate / carbon nanotube precursor solution, followed by film formation and Fe... 3+ Crosslinking and washing processes are used to construct a composite hydrogel with an interconnected porous structure, which is then used as a solar evaporator. This hydrogel exhibits high water transport capacity, photothermal evaporation efficiency, and salt resistance.

[0007] The technical solution of this invention is as follows: A method for preparing a porous sodium alginate / carbon nanotube composite hydrogel includes the following steps: (1) Preparation of carbon nanotube dispersion Add 0.01-0.2g of modified carbon nanotubes to 100mL of deionized water and disperse for 5-15min to obtain a uniform modified carbon nanotube dispersion. (2) Preparation of sodium alginate / carbon nanotube / porogen precursor solution Add 2-8g sodium alginate and 0.5-15g pore-forming agent to the modified carbon nanotube dispersion in step (1), heat and stir to fully dissolve sodium alginate and mix it evenly with carbon nanotubes and pore-forming agent to obtain precursor solution; (3) Drying to form a film The precursor solution is poured into a molding mold and dried at 40-50℃ for 18-30h to obtain a composite membrane. During the drying process, the pore-forming agent induces phase separation, particle occupancy, or network rearrangement in the precursor system, thereby forming a porous structure inside the composite membrane. (4) Fe 3+ Ionic crosslinking The composite membrane was immersed in a ferric chloride solution for cross-linking, allowing Fe... 3+ Coordination and crosslinking with the carboxyl groups on the sodium alginate molecular chain form a stable three-dimensional hydrogel network to obtain a crosslinked membrane. The crosslinked membrane is repeatedly washed with deionized water to remove residual Fe³⁺, soluble salts and pore-forming agent residues, resulting in a porous sodium alginate / carbon nanotube composite hydrogel.

[0008] Step (1) The modified carbon nanotubes are prepared by adding 0.01-0.50g of carbon nanotubes and 8-16g of ammonium persulfate to 100mL of deionized water and ultrasonically dispersing them. After filtration, washing, drying and grinding, the modified carbon nanotubes are obtained.

[0009] Step (2) The porogen is sodium chloride, ethanol or polyethylene glycol 400. When the porogen is sodium chloride, the amount of sodium chloride added is 2-10g; when the porogen is ethanol, the amount of ethanol added is 5-15g; when the porogen is polyethylene glycol 400, the amount of polyethylene glycol 400 added is 0.5-2g.

[0010] Step (2) involves heating and stirring at 60-90℃ for 8-14 hours.

[0011] In step (4), the concentration of the ferric chloride solution is 0.3-0.8 mol / L, and the cross-linking time is 8-16 h.

[0012] The present invention also provides the composite hydrogel prepared by the above preparation method as an application of solar evaporator, mainly in solar interface evaporation, seawater desalination, brine purification or distributed freshwater preparation.

[0013] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses a pore-forming agent to regulate the internal structure of sodium alginate / carbon nanotube composite hydrogel. The preparation process is simple and the conditions are mild. It does not require a complex template removal process or a high-energy-consuming freeze-drying process, which is conducive to large-scale preparation.

[0014] (2) By regulating the pore structure through pore-forming agents, the present invention can obtain porous networks with different morphologies. Among them, sodium chloride can form particle dissolution pores, ethanol can form phase separation-induced crack-like pore structures, and polyethylene glycol 400 can form a uniformly connected sponge-like porous network, thereby providing a structural basis for water transport, vapor escape and localized thermal regulation.

[0015] (3) The composite hydrogel obtained by controlling polyethylene glycol 400 in this invention has good pore uniformity.

[0016] (4) The composite hydrogel obtained by the present invention has good ion removal ability as an evaporator, and can effectively remove ions such as Na⁺, K⁺, Mg²⁺ and Ca²⁺ in brine, and is suitable for seawater desalination and brine purification.

[0017] (5) The composite hydrogel obtained by the present invention has good cycle stability and outdoor application potential as a solar evaporator. It can maintain stable evaporation behavior under natural light fluctuation conditions, providing a feasible material solution for distributed freshwater preparation. Attached Figure Description

[0018] Figure 1 Scanning electron microscope images of composite hydrogels obtained by different porogens; Figure 2 The evaporation performance of composite hydrogels obtained by different porogens under one sun irradiation is shown in the figure. Figure 3 The diagram shows the cyclic evaporation stability of the composite hydrogel. Figure 4 The image shows the anti-salt crystallization and ion removal effect of the composite hydrogel. Figure 5 This is a test diagram for the outdoor evaporation application of composite hydrogels. Detailed Implementation

[0019] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments. Any equivalent substitutions or conventional adjustments made to the raw materials or methods based on the concept of the present invention should fall within the scope of protection of the present invention.

[0020] Example 1 (1) Carbon nanotube pretreatment 0.5g of multi-walled carbon nanotubes and 10g of ammonium persulfate were added to 100mL of deionized water and ultrasonically dispersed for 12h. The mixture was then filtered, washed with deionized water, dried, and ground into powder to obtain modified carbon nanotubes. (2) Preparation of carbon nanotube dispersion Weigh 0.10 g of modified carbon nanotubes and add them to 100 mL of deionized water. Disperse the mixture for 10 min using a cell disruption method to obtain a uniform carbon nanotube dispersion. (3) Preparation of precursor solution Add 4g sodium alginate and 2g sodium chloride to the carbon nanotube dispersion obtained in step (2), and stir overnight (12h) at 80℃ to obtain a uniform sodium alginate / carbon nanotube / sodium chloride precursor solution. (4) Film formation and crosslinking The precursor solution obtained in step (3) was poured into a glass mold and dried at 45°C for 24 hours to form a film. Then, the resulting composite film was immersed in a 0.5 mol / L ferric chloride solution for crosslinking for 12 hours. The ferric chloride solution needed to cover the composite film to obtain a crosslinked film. (5) Washing The cross-linked membrane was repeatedly washed with deionized water to remove residual sodium chloride and iron ions, resulting in a porous sodium alginate / carbon nanotube composite hydrogel with NaCl pores.

[0021] The composite hydrogel obtained in this embodiment was tested by scanning electron microscopy (SEM), and the results are as follows: Figure 1 As shown, when the amount of NaCl added is 2g, the surface of the hydrogel is relatively dense, with only a small number of nanoscale fine undulations and no obvious interconnected macroporous structure. In this embodiment, after the sodium chloride is dissolved, it can form pores inside the hydrogel, which is beneficial to improving porosity and water transport capacity.

[0022] The amount of sodium chloride added as the pore-forming agent in this embodiment was adjusted to 0g, 4g, 6g, 8g, and 10g, while other steps remained the same, to prepare different composite hydrogels. Evaporation performance was then tested (e.g., Figure 2 As shown in the figure, under the condition of 2g of NaCl and under sunlight, its evaporation rate is the highest, approximately 1.6kg·m³. -2 ·h -1 The evaporation efficiency did not fluctuate much with the amount of sodium chloride added as a pore-forming agent, and the evaporation efficiency was highest when the amount added was 2g.

[0023] Example 2 The difference between this embodiment and embodiment 1 is that in step (3), sodium chloride is adjusted to 10g ethanol, and other operations and raw materials are the same as in embodiment 1, so as to obtain a porous sodium alginate / carbon nanotube composite hydrogel with ethanol-induced pores.

[0024] The composite hydrogel obtained in this embodiment was subjected to SEM testing, and the results are as follows: Figure 1 As shown, when the amount of ethanol added is 10g, the pore size increases significantly, and fissures and pores coexist and are distributed relatively evenly. In this embodiment, ethanol forms a fissure-like pore structure through induced phase separation.

[0025] In this embodiment, the amount of porogen ethanol added was adjusted to 0g, 5g, and 15g, while other steps remained the same, to prepare different composite hydrogels. Evaporation performance was then tested (e.g., Figure 2 As shown in the figure, under sunlight conditions, when the amount of ethanol added is 10g, its evaporation rate is relatively high, approximately 1.3kg·m³. -2 ·h -1 However, excessive amounts can interfere with the gel network structure, leading to pore collapse or structural inhomogeneity. The high surface roughness of the ethanol-induced porous composite hydrogel obtained in this embodiment is beneficial for enhancing light scattering, and its anisotropic slit-like pore structure helps to regulate the water transport path.

[0026] Example 3 The difference between this embodiment and embodiment 1 is that the sodium chloride in step (3) is adjusted to 1g of polyethylene glycol 400, and the other operations and raw materials are the same as in embodiment 1, so as to obtain a porous sodium alginate / carbon nanotube composite hydrogel with polyethylene glycol 400 pores.

[0027] The composite hydrogel obtained in this embodiment was subjected to SEM testing, and the results are as follows: Figure 1 As shown, when the amount of pore-forming agent added is 1g of PEG-400, the resulting hydrogel exhibits a typical sponge-like three-dimensional porous structure with uniform pore size, excellent connectivity, thick pore walls, and a stable structure. The pore structure is the most uniform and continuous. In this embodiment, the polyethylene glycol molecular chains interact with the polymer matrix through hydrogen bonds, forming a uniformly connected sponge-like porous network. After 10 cycles of evaporation testing (e.g., ...), the composite hydrogel obtained in this embodiment... Figure 3 As shown), the evaporation rate remained at 2.16-2.31 kg·m³. -2 ·h -1 Within the specified range, the fluctuation amplitude is less than 5%, demonstrating its excellent long-term service performance; for simulated seawater Na... + K + Mg 2+ Ca 2+The removal rate of plasma exceeded 98% (e.g.) Figure 4 As shown); 10-hour outdoor test (e.g.) Figure 5 As shown in the figure, the cumulative mass loss reached 9.63 kg·m. -2 The corresponding freshwater production is approximately 9.63 L·m³. -2 The evaporation rate can be adaptively adjusted according to the light intensity, showing good potential for practical applications.

[0028] In this embodiment, the amount of porogen polyethylene glycol added was adjusted to 0g, 0.5g, 1.5g, and 2g, while other steps remained the same, to prepare different composite hydrogels. Evaporation performance was then tested (e.g., Figure 2 As shown in the figure, under sunlight conditions, when the amount of polyethylene glycol added is 1g, its evaporation rate is relatively high, approximately 2.2 kg·m³. -2 ·h -1 When 0.5g of polyethylene glycol 400 is added, its evaporation rate is approximately 1.7kg·m. -2 ·h -1 This indicates that even using the lower limit of PEG-400 dosage, a composite hydrogel with interconnected pores and good evaporation performance can still be obtained; when the amount of polyethylene glycol 400 added is 2g, its evaporation rate drops to approximately 1.5kg·m³. -2 ·h -1 This indicates that excessive PEG-400 addition will lead to excessively thin pore walls, decreased structural stability, and reduced evaporation performance. The PEG-400 pore-forming composite hydrogel obtained in this embodiment has the best overall performance and is the best pore-forming agent of this invention.

[0029] Example 4 (1) Carbon nanotube pretreatment 0.01 g of carbon nanotubes and 8 g of ammonium persulfate were added to 100 mL of deionized water and ultrasonically dispersed for 12 h. The mixture was then filtered, washed with deionized water, dried, and ground into powder to obtain modified carbon nanotubes. (2) Preparation of carbon nanotube dispersion Weigh 0.01g of modified carbon nanotubes and add them to 100mL of deionized water. Disperse the mixture for 5min using a cell disruption method to obtain a uniform carbon nanotube dispersion. (3) Preparation of precursor solution Add 2g of sodium alginate and 3g of sodium chloride to the carbon nanotube dispersion obtained in step (2), and stir at 60°C for 14h to obtain a uniform sodium alginate / carbon nanotube / sodium chloride precursor solution. (4) Film formation and crosslinking The precursor solution obtained in step (3) was poured into a glass mold and dried at 40°C for 30 hours to form a film. Then, the resulting composite film was immersed in a 0.3 mol / L ferric chloride solution for crosslinking for 16 hours. The ferric chloride solution needed to cover the composite film to obtain a crosslinked film. (5) Washing The cross-linked membrane was repeatedly washed with deionized water to remove residual sodium chloride and iron ions, resulting in a porous sodium alginate / carbon nanotube composite hydrogel with NaCl pores.

[0030] Example 5 (1) Carbon nanotube pretreatment 0.4 g of multi-walled carbon nanotubes and 16 g of ammonium persulfate were added to 100 mL of deionized water and ultrasonically dispersed for 12 h. The mixture was then filtered, washed with deionized water, dried, and ground into powder to obtain modified carbon nanotubes. (2) Preparation of carbon nanotube dispersion Weigh 0.2g of modified carbon nanotubes and add them to 100mL of deionized water. Disperse the mixture for 15min using a cell disruption method to obtain a uniform carbon nanotube dispersion. (3) Preparation of precursor solution Add 8g of sodium alginate and 1g of polyethylene glycol 400 to the carbon nanotube dispersion obtained in step (2), and stir at 90°C for 8h to obtain a uniform sodium alginate / carbon nanotube / polyethylene glycol 400 precursor solution. (4) Film formation and crosslinking The precursor solution obtained in step (3) was poured into a glass mold and dried at 50°C for 18 hours to form a film. Then, the resulting composite film was immersed in a 0.8 mol / L ferric chloride solution for 8 hours for crosslinking. The ferric chloride solution needed to cover the composite film to obtain a crosslinked film. (5) Washing The cross-linked membrane was repeatedly washed with deionized water to remove residual polyethylene glycol 400 and iron ions, resulting in a porous sodium alginate / carbon nanotube composite hydrogel with polyethylene glycol 400 pores.

[0031] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for preparing a porous sodium alginate / carbon nanotube composite hydrogel, characterized in that, Includes the following steps: (1) Add 0.01-0.2g of modified carbon nanotubes to 100mL of deionized water and disperse for 5-15min to obtain a uniform modified carbon nanotube dispersion. (2) Add 2-8g sodium alginate and 0.5-15g pore-forming agent to the modified carbon nanotube dispersion in step (1), and heat and stir to obtain a precursor solution; (3) Pour the precursor solution into a molding mold and dry it at 40-50℃ for 18-30h to obtain a composite membrane; (4) The composite membrane is immersed in ferric chloride solution for cross-linking to form a stable three-dimensional hydrogel network and obtain a cross-linked membrane. The cross-linked membrane is repeatedly washed with deionized water to obtain a porous sodium alginate / carbon nanotube composite hydrogel.

2. The method for preparing the porous sodium alginate / carbon nanotube composite hydrogel according to claim 1, characterized in that, Step (1) Preparation method of modified carbon nanotubes: 0.01-0.50g of carbon nanotubes and 8-16g of ammonium persulfate are added to 100mL of deionized water and ultrasonically dispersed. After filtration, washing, drying and grinding, modified carbon nanotubes are obtained.

3. The method for preparing the porous sodium alginate / carbon nanotube composite hydrogel according to claim 1, characterized in that, The pore-forming agent in step (2) is sodium chloride, ethanol, or polyethylene glycol 400.

4. The method for preparing the porous sodium alginate / carbon nanotube composite hydrogel according to claim 1, characterized in that, Step (2) involves heating and stirring at 60-90℃ for 8-14 hours.

5. The method for preparing the porous sodium alginate / carbon nanotube composite hydrogel according to claim 1, characterized in that, In step (4), the concentration of the ferric chloride solution is 0.3-0.8 mol / L, and the cross-linking time is 8-16 h.

6. The composite hydrogel prepared by the method of claim 1 is used as a solar evaporator.

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