A high-efficiency salt-rejecting solar evaporator for high-salt water treatment and its preparation method

Abstract

The present invention relates to a highly efficient, salt-rejecting solar evaporator for treating high-salinity water. The solar evaporator is characterized by being a three-dimensional aerogel comprising carbon nanofibers and graphene oxide. In the solar evaporator, graphene oxide sheets support the carbon nanofibers to form a three-dimensional space. The carbon nanofibers in the solar evaporator partially penetrate between the graphene oxide sheets and are partially embedded in the graphene oxide sheets. The channels constructed by the carbon nanofibers and graphene oxide sheets in the solar evaporator are continuous, hierarchical channels. The solar evaporator's material composition and structure are easily controllable, its mechanical properties are stable, and it can reject salt while maintaining efficient evaporation. It exhibits significant application potential in the field of high-salinity water treatment.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment, in particular to the field of high-salt water treatment based on solar thermal evaporation. Background Art

[0002] Water is a vital resource for the survival and development of human society. Large-scale water pollution and water shortages are seriously affecting people's lives. In order to alleviate the water shortage crisis, various water treatment methods have emerged. In contrast, solar-driven evaporation is a sustainable water treatment technology. It is green and economical, applicable to various high-salinity water bodies, and has attracted widespread attention. However, salt crystals precipitated on the surface during evaporation will significantly affect light absorption. Although some evaporators can achieve salt rejection, their evaporation rate still needs to be improved. Therefore, constructing an efficient salt-rejecting solar evaporator has important practical significance in the treatment of actual water bodies, especially high-salinity water.

[0003] Electrospinning technology is a method of making a charged polymer solution (or melt) flow and deform in an electrostatic field, solidify by solvent evaporation or melt cooling, and thus obtain a fibrous material. Electrospinning nanofiber technology can integrate multiple components into the same fiber to prepare composite nanofibers, and can realize the preparation of polymer / polymer, polymer / inorganic and inorganic / inorganic composite fibers. Electrospun nanofibers have a large specific surface area, high porosity, and easy-to-control structure. In addition, the interconnected channels formed by continuous electrospun nanofibers are conducive to rapid water transport and salt diffusion, making them an ideal choice for solar evaporators and attracting widespread attention in the field of solar-driven evaporation. For example, patent CN113023809 A discloses a two-dimensional planar membrane of polylactic acid / polylactic acid-hydroxypropyl cellulose / titanium carbide with a multi-level pore structure. Due to the small evaporation area and poor thermal insulation of the two-dimensional electrospun nanofiber membrane, the evaporation rate is only 1.3 kg m -2 h -1 To improve evaporation, patent CN111282443A discloses a laser-induced porous evaporation material. Laser ablation technology is used to form a porous three-dimensional triangular array on the membrane surface, converting two-dimensional evaporation into three-dimensional evaporation, effectively increasing the evaporation area, and thus achieving 1.595 kg m -2 h -1 However, most 3D electrospun evaporators are composed solely of pure fibers, lacking mechanical support. This reduces porosity and pore connectivity, hindering the diffusion of salt ions. While some research has attempted to enhance salt ion diffusion by adding materials such as silica nanofibers to support the evaporator, the lack of precise control over the evaporator's shape and internal structure results in significant heat loss, and the evaporation effect remains unsatisfactory. Summary of the Invention

[0004] To address the shortcomings of current solar evaporators, which cannot achieve both salt rejection and efficient evaporation, as well as the poor mechanical properties of three-dimensional electrospun nanofibers, the present invention discloses a highly efficient salt-rejecting solar evaporator for high-salt water treatment. This method easily regulates the composition and structure of the material, not only improving the mechanical strength of the material but also constructing hierarchical channels for rapid water evaporation and salt diffusion. Even during treatment with near-saturated brine (24.0 wt% sodium chloride solution) and actual concentrated industrial high-salt wastewater (pH 0.5-8.5), no salt crystals were found on the evaporator surface, demonstrating great application potential in the field of high-salt water treatment.

[0005] In order to achieve the above object, the present invention adopts the following steps:

[0006] Step a) preparing an electrospinning solution;

[0007] Step b) using the graphene oxide dispersion to directly receive the electrospun nanofibers to obtain a nanofiber-graphene oxide dispersion;

[0008] Step c) freezing the nanofiber-graphene oxide dispersion in a suitable mold as needed, and drying it in a freeze dryer to obtain a three-dimensional nanofiber-graphene oxide composite aerogel;

[0009] Step d) pre-oxidizing the three-dimensional nanofiber-graphene oxide composite aerogel in an oven at 250° C. for 1 hour, and carbonizing the three-dimensional carbon nanofiber-graphene oxide composite aerogel in a box furnace at 800° C. for 1.5 hours under nitrogen protection to obtain a three-dimensional carbon nanofiber-graphene oxide composite aerogel.

[0010] In some embodiments of the present invention, the specific operation of step b) is to obtain a uniformly dispersed nanofiber-graphene oxide dispersion in one step, so that the fibers maintain good continuity to construct continuous fiber channels.

[0011] In some embodiments of the present invention, the content and ratio of the graphene oxide receiving liquid and nanofibers in step b) can be adjusted to construct different hierarchical channels. When the content or ratio of graphene oxide is low, it exhibits a fluffy, highly interconnected porous structure, with carbon nanofibers and graphene oxide sheets evenly distributed. The insertion of graphene oxide sheets constructs multi-sized hierarchical channels, which is conducive to rapid water transport and salt diffusion. When the content or ratio of graphene oxide is high, it exhibits a tightly packed hierarchical porous structure, which is conducive to suppressing heat conduction loss to water.

[0012] In some embodiments of the present invention, the solar evaporator is a three-dimensional aerogel comprising carbon nanofibers and graphene oxide; it is characterized in that the graphene oxide sheets support the carbon nanofibers to form a three-dimensional space; it is characterized in that the carbon nanofibers partially penetrate between the graphene oxide sheets and partially are embedded in the graphene oxide sheets, and the carbon nanofibers and the graphene oxide sheets construct a continuous and continuous hierarchical channel; it is characterized in that the density of the solar evaporator is 3 mg / mL~19 mg / mL, and the mass ratio of carbon nanofibers to graphene oxide is 1:1~8:1.

[0013] In some embodiments of the present invention, the solar evaporator is an inverted cone, the tip of the cone is in contact with the surface of the salt water, and the immersion depth is 0.1 cm, which is beneficial to reduce the contact between the evaporator and the water, reduce the heat conduction loss to the water body, and improve the evaporation rate and energy conversion efficiency.

[0014] In some embodiments of the present invention, when the solar evaporator uses a density of 3.4 mg / mL, the fiber pores are large, the water transmission flux is large, and the salt can be redissolved to ensure that there is no salt crystallization on the surface of the evaporator. However, excessive water in the evaporator will also take away heat, reducing the evaporation rate and energy conversion efficiency.

[0015] In some embodiments of the present invention, when the solar evaporator uses a density of 6.2 mg / mL, the fiber pores become smaller, the water transmission flux becomes smaller, the heat conduction loss to the water during evaporation becomes smaller, and the evaporation rate and energy conversion efficiency are improved. At the same time, the water flux is still sufficient to ensure the redissolution of salt, and efficient and stable evaporation can be maintained in sodium chloride solutions with a concentration of up to 24.0 wt%.

[0016] In some embodiments of the present invention, when the solar evaporator adopts a density of 8.4 mg / mL, the fiber pores continue to become smaller, the water transmission flux becomes smaller, the heat conduction loss to the water during evaporation becomes smaller, and the evaporation rate and energy conversion efficiency continue to improve. At the same time, it can maintain efficient and stable evaporation in sodium chloride solutions with a concentration of up to 16.7 wt%, actual concentrated thermal power plant spray wastewater, and coal chemical wastewater.

[0017] In some embodiments of the present invention, when the solar evaporator uses a density of 11.5 mg / mL, the fiber pores are further reduced, the water transmission flux is reduced, the heat conduction loss to water during evaporation is reduced, and the evaporation rate and energy conversion efficiency are further improved.

[0018] In some embodiments of the present invention, when the solar evaporator uses a density of 18.7 mg / mL, the fiber pores become further smaller, the water transmission flux becomes smaller, the heat conduction loss to the water during evaporation becomes smaller, and the evaporation rate and energy conversion efficiency reach the highest level. At the same time, efficient and stable evaporation can be maintained in simulated seawater (3.5 wt% sodium chloride solution) and actual concentrated thermal power plant desulfurization wastewater.

[0019] In some embodiments of the present invention, the solar evaporator may further comprise other materials, such as carbon black, nano-gold, carbon nanotubes, hydrogel, titanium oxide, polydopamine, polyvinyl alcohol, chitosan, and the like.

[0020] The advantages of the present invention are:

[0021] By adjusting the graphene oxide content, a rich layer of interconnected pores is constructed for rapid water transport and salt diffusion, improving the evaporator's mechanical properties for long-term stable operation. By optimizing the evaporator's immersion depth and density, the contact between the evaporator and water and the pore structure are controlled, suppressing heat conduction losses to the water and reducing the water's evaporation enthalpy, maintaining efficient evaporation while preventing salt formation. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will be further described below with reference to the accompanying drawings.

[0023] Figure 1 Schematic diagram of the preparation process of the solar evaporator.

[0024] Figure 2 Electron micrographs of pure carbon nanofiber aerogel (CNFA) and carbon nanofiber / graphene oxide composite aerogel (CNF / GOA) solar evaporators (original density of 2.5 mg / mL) with different ratios (4:1, 2:1, 1:1).

[0025] Figure 3 The salt tolerance test graph of CNFA and CNF / GOA solar evaporator (original density is 2.5 mg / mL) with different ratios (4:1, 2:1, 1:1). CNF / GOA4 represents carbon nanofiber / graphene oxide = 4:1, CNF / GOA2 represents carbon nanofiber / graphene oxide = 2:1, and CNF / GOA1 represents carbon nanofiber / graphene oxide = 1:1.

[0026] Figure 4 This is the evaporation performance test curve of CNF / GOA4 solar evaporator (original density is 2.5 mg / mL) at different immersion depths.

[0027] Figure 5 Electron microscopy images of CNF / GOA4 solar evaporators with different densities.

[0028] Figure 6 The evaporation performance test curve of CNF / GOA4 solar evaporator with different densities. DETAILED DESCRIPTION

[0029] The present invention will be described in detail below.

[0030] Most existing three-dimensional electrospun solar evaporators are composed solely of pure fibers, lacking mechanical support and preventing long-term stable operation. Furthermore, after thermal stabilization, the fibers may collapse, reducing porosity and pore connectivity, blocking salt diffusion and causing surface salt deposition. To improve long-term stability and salt tolerance, auxiliary technologies such as fiber freeze-molding have been introduced to create nanofiber evaporators with excellent mechanical properties. However, due to a lack of heat loss management, they cannot simultaneously reject salt and maintain efficient evaporation.

[0031] To address these shortcomings, the present invention utilizes liquid-phase assisted collection-electrospinning technology to fabricate a highly efficient, salt-rejecting solar evaporator for high-salinity water treatment. This method easily regulates the material's composition and structure, improving its mechanical strength while also enabling the construction of hierarchical channels for rapid water evaporation and salt diffusion. Even during treatment with near-saturated brine (24.0wt% sodium chloride solution) and actual concentrated industrial high-salinity wastewater (pH 0.5-8.5, with a conductivity more than four times that of seawater), no salt crystals were observed on the evaporator surface, demonstrating significant potential for high-salinity water treatment. Example

[0032] To make the present invention easier to understand, the present invention will be further described in detail below with reference to the following examples. These examples are merely illustrative and are not intended to limit the scope of application of the present invention. The raw materials or components used in the present invention can be obtained by commercial routes or conventional methods unless otherwise specified.

[0033] Example 1: Preparation of CNFA and CNF / GOA solar evaporators with different ratios

[0034] Polyacrylonitrile powder was dispersed in N,N-dimethylformamide solution (mass fraction of 12 wt%) and magnetically stirred at 60°C for 4 hours to completely dissolve it. The dissolved polyacrylonitrile / N,N-dimethylformamide solution was placed in a 10 mL syringe, connected to a 19-gauge stainless steel needle, and a positive voltage of +15 kV and a negative voltage of -5 kV were applied between the needle tip and the graphene oxide-water dispersion for spinning, respectively. The pushing speed was controlled at 1.5 mL / h. After a specific electrospinning time, the original density of 2.5 and 3.5 were obtained. mg / mL of pure polyacrylonitrile nanofiber dispersion and polyacrylonitrile nanofiber / graphene oxide dispersions of different ratios (4:1, 2:1, 1:1); pour the obtained dispersion into a conical mold, freeze it in a refrigerator (-22°C) for 4 hours, and then freeze-dry it in a freeze dryer for 48 hours to obtain pure polyacrylonitrile nanofiber aerogel and three-dimensional polyacrylonitrile nanofiber / graphene oxide composite aerogels of different ratios (4:1, 2:1, 1:1); pre-oxidize the obtained aerogel in a 250°C oven for 1 hour, and then carbonize it in an 800°C box furnace for 1.5 hours; after naturally cooling to room temperature in the box furnace, take it out to obtain CNFA and CNF / GOA of different ratios, and use them as solar evaporators. The electron microscope images of the CNFA and CNF / GOA solar evaporators of different ratios are shown in the figure below. Figure 2 shown.

[0035] Example 2: Salt tolerance test of CNFA and different ratios of CNF / GOA solar evaporators

[0036] The CNFA and CNF / GOA solar evaporators prepared in Example 1 were respectively inserted into a circular polyethylene hydrophobic foam with a hole in the middle and a thickness of 1 cm, and then floated in a 500 mL beaker filled with sodium chloride solution of different concentrations. A xenon lamp with a spot diameter of 59 mm was used as a simulated light source. The vertical distance between the simulated light source and the top surface of the evaporator was set to 12 cm (i.e., one light intensity), and a light shield with holes was placed between the two so that the light was only irradiated on the evaporator. The irradiation was continued for 8 hours, and the surface salt formation of the CNFA and CNF / GOA solar evaporators with different ratios was observed to obtain their tolerable salt concentration. The results are shown in FIG. Figure 3 shown.

[0037] Example 3: Evaporation performance test of CNF / GOA4 solar evaporator (original density 2.5 mg / mL) at different immersion depths

[0038] The CNF / GOA4 solar evaporator prepared in Example 1 was inserted into a circular polyethylene hydrophobic foam with a thickness of 1 cm and a hole in the middle, and then floated in a 500 mL beaker filled with 3.5 wt% sodium chloride solution. The immersion depth of the solar evaporator in the salt water was controlled to be 2 cm (complete immersion), 1 cm, and 0.1 cm (the tip of the evaporator was in contact with the salt solution). A xenon lamp with a spot diameter of 59 mm was used as the simulated light source, and the vertical distance between the simulated light source and the top surface of the evaporator was set to 12 cm (i.e., one light intensity), and a light-blocking plate with holes was placed between the two to allow light to only illuminate the evaporator. During the evaporation process, the mass change was monitored in real time by an electronic balance connected to a computer, thereby obtaining the mass loss and evaporation rate of the CNF / GOA4 solar evaporator at different immersion depths. At the same time, a thermal infrared imager was used to record the real-time temperature throughout the evaporation process, including point temperature, regional maximum temperature, regional minimum temperature, and regional average temperature. Once the evaporation experiment was completed, the temperature of the bottom water was immediately measured with a thermometer to obtain the energy conversion efficiency of the CNF / GOA4 solar evaporator at different immersion depths. The results are shown in Figure 2. Figure 4 shown.

[0039] Example 4: Preparation of CNF / GOA4 solar evaporators with different densities

[0040] The preparation process is basically the same as that of Example 1, except that the ratio of polyacrylonitrile nanofibers and graphene oxide dispersion is fixed at 4:1. After a specific electrospinning time, polyacrylonitrile nanofiber / graphene oxide dispersions with original densities of 2.5 mg / mL, 5 mg / mL, 7.5 mg / mL, 10 mg / mL, 12.5 mg / mL and 15 mg / mL are obtained respectively; after thermal stabilization treatment, CNF / GOA4 with final densities of 3.4 mg / mL, 6.2 mg / mL, 8.4 mg / mL, 11.5 mg / mL, 15.4 mg / mL and 18.7 mg / mL are obtained respectively and used as solar evaporators. The electron microscope images of the CNF / GOA4 solar evaporators with different densities are shown in FIG. Figure 5 shown.

[0041] Example 5: Evaporation performance test of CNF / GOA4 solar evaporator with different densities

[0042] The CNF / GOA4 solar evaporators prepared in Example 4 were respectively inserted into a circular polyethylene hydrophobic foam with a hole in the middle and a thickness of 1 cm, and then floated in a 500 mL beaker filled with 3.5 wt% sodium chloride solution, and the immersion depth of the solar evaporator in the salt water was controlled to be 0.1 cm; a xenon lamp with a spot diameter of 59 mm was used as a simulated light source, and the vertical distance between the simulated light source and the top surface of the evaporator was set to 12 cm (i.e., one light intensity), and a light baffle with a hole was placed between the two so that the light was only irradiated on the evaporator; during the evaporation process, the mass change was monitored in real time by an electronic balance connected to a computer, and the mass loss, evaporation rate and energy conversion efficiency of the CNF / GOA4 solar evaporators with different densities were obtained. The results are shown in FIG. Figure 6 shown.

[0043] It should be noted that the embodiments described above are only used to explain the present invention and do not constitute any limitation of the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory words, rather than restrictive words. The present invention may be modified as specified within the scope of the claims of the present invention, and the present invention may be revised without departing from the scope and spirit of the present invention. Although the present invention described therein relates to specific methods, materials and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein. On the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A high-efficiency salt-rejecting solar evaporator for high-salt water treatment, characterized in that The solar evaporator is a three-dimensional aerogel containing carbon nanofibers and graphene oxide; in the solar evaporator, the graphene oxide sheets support the carbon nanofibers to form a three-dimensional space; in the solar evaporator, the carbon nanofibers partially penetrate between the graphene oxide sheets and partially are embedded in the graphene oxide sheets; the channels constructed by the carbon nanofibers and the graphene oxide sheets in the solar evaporator are continuous hierarchical channels; the mass ratio of the carbon nanofibers to the graphene oxide in the solar evaporator is 1:1 to 8:1; the solar evaporator is an inverted cone with a density of 3 mg / mL to 19 mg / mL.

2. A high-efficiency salt-rejecting solar evaporator for high-salt water treatment according to claim 1, characterized in that When the salt concentration of the treated brine is lower than 5.0wt%, the evaporator density is 15mg / mL to 19mg / mL; when the salt concentration of the treated brine is between 5.0wt% and 16.7wt%, the evaporator density is 8mg / mL to 15mg / mL; when the salt concentration of the treated brine is higher than 16.7wt%, the evaporator density is 3mg / mL to 8mg / mL.

3. A high-efficiency salt-rejecting solar evaporator for high-salt water treatment according to claim 1, characterized in that Polyethylene foam was used as a supporting layer to float the inverted cone evaporator in the treated water sample for thermal evaporation. The tip of the inverted cone was in contact with the surface of the brine with an immersion depth of 0.1 cm.

4. A high-efficiency salt-rejecting solar evaporator for high-salt water treatment according to claim 1, characterized in that The specific preparation method of the solar evaporator is as follows: Step a) preparing an electrospinning solution; Step b) using the graphene oxide dispersion to directly receive the electrospun nanofibers to obtain a nanofiber-graphene oxide dispersion; Step c) freezing the nanofiber-graphene oxide dispersion in a suitable mold as needed, and drying it in a freeze dryer to obtain a three-dimensional nanofiber-graphene oxide composite aerogel; Step d) pre-oxidizing the three-dimensional nanofiber-graphene oxide composite aerogel in an oven at 250° C. for 1 hour, and carbonizing the three-dimensional carbon nanofiber-graphene oxide composite aerogel in a box furnace at 800° C. for 1.5 hours under nitrogen protection to obtain a three-dimensional carbon nanofiber-graphene oxide composite aerogel.

5. A high-efficiency salt-rejecting solar evaporator for high-salt water treatment according to claim 4, characterized in that The specific steps of step b) are to obtain a uniformly dispersed nanofiber-graphene oxide dispersion in one step, so that the fibers maintain good continuity to construct interconnected channels.

6. A high-efficiency salt-rejecting solar evaporator for high-salt water treatment according to claim 1, characterized in that The mass ratio of carbon nanofibers to graphene oxide in the solar evaporator is 4:

1.

7. A high-efficiency salt-rejecting solar evaporator for high-salt water treatment according to claim 1, characterized in that A layer of polydopamine or polyvinyl alcohol is in situ polymerized on the evaporator surface to further enhance thermal evaporation.

8. The high-efficiency salt-rejecting solar evaporator for high-salt water treatment according to claim 1, characterized in that The evaporator further comprises at least one of carbon black, nano-gold, carbon nano-tubes, hydrogel, titanium oxide, polyvinyl alcohol, polydopamine and chitosan.

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