Three-dimensional solar evaporation material and preparation method and application thereof

Abstract

The application provides a three-dimensional solar evaporation material and a preparation method and application thereof. The preparation method of the application takes sponge as a raw material, and the sponge is loaded with polypyrrole by a simple solution reaction through preparation with a pyrrole monomer and an iron salt, so that the light absorption performance of the material is improved and the light-heat conversion capacity thereof is improved. The three-dimensional solar evaporation material prepared by the application has a low-tortuosity pore (i.e. vertical pore) water guide structure, can rapidly fill the material interior with water through capillary action, so that salt deposition in the pore interior is prevented in the seawater desalination process, the salt self-cleaning function is realized, and stable seawater desalination capacity is ensured. The preparation method of the three-dimensional solar evaporation material has the characteristics of high efficiency and low cost, can be mass-produced, and provides a solution to the global freshwater shortage, energy crisis and environmental pollution problem.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of interfacial evaporation materials, in particular to a three-dimensional solar evaporation material and a preparation method and application thereof. BACKGROUND

[0002] Currently, humanity is facing urgent challenges, including freshwater scarcity, energy crisis, and environmental pollution. According to statistical data, seawater accounts for 97% of the Earth's water resources. In order to alleviate the problem of global water scarcity, seawater desalination technology is widely recognized as an effective solution. However, traditional seawater desalination methods such as thermal distillation and reverse osmosis consume a large amount of energy and produce carbon dioxide, further exacerbating the energy crisis and environmental pollution. In order to address these problems, an interfacial solar-driven seawater desalination system was proposed in 2014, which has attracted widespread attention due to its high evaporation rate and environmental friendliness. The interfacial solar-driven seawater desalination system is usually composed of a photothermal material, a thermal insulation layer, and a water transport device. The role of the photothermal material is to absorb sunlight and convert it into heat energy to promote water evaporation. The role of the thermal insulation layer is to limit the dispersion of heat energy from the photothermal material to the large amount of water in the water vapor interface. The water transport device is used to transport water to the surface of the photothermal material for the evaporation process. Photothermal materials are mainly divided into four categories: plasmonic, semiconductor, carbon material, and polymer. These materials have their own unique characteristics and advantages, and play an important role in the interfacial solar-driven seawater desalination system. As a black polymer, polypyrrole has excellent sunlight absorption capacity, environmental friendliness, easy loading on the surface of other materials, and low price, so it is often used to prepare photothermal conversion materials.

[0003] Currently, solar-driven interfacial water evaporation faces three problems: (1) high evaporation efficiency; (2) achieving self-cleaning and constant evaporation efficiency during seawater desalination; (3) low price to meet the needs of practical applications. Therefore, designing a solar evaporator that can overcome these three problems is still a great challenge.

[0004] In recent years, in order to improve the evaporation rate, researchers have proposed a variety of innovative design schemes. Compared with two-dimensional evaporation surfaces that rely on solar energy, three-dimensional evaporators can capture the energy of the surrounding environment, thereby promoting the evaporation process and significantly improving the evaporation efficiency. Therefore, using three-dimensional evaporator design to achieve high-efficiency evaporation is considered an effective approach. In the field of seawater desalination, three-dimensional interfacial solar evaporation materials have shown great potential, thanks to their excellent evaporation efficiency. However, in the process of water and salt separation based on interfacial photothermal evaporation, there is a serious problem of salt crystallization and scale accumulation, which leads to the saturation of salt concentration. This phenomenon will hinder the absorption of sunlight and limit the sufficient supply of water, thereby causing the photothermal conversion efficiency to rapidly decrease, and even the device to fail.

[0005] Based on the defects of the three-dimensional interface solar evaporation material at present, it is necessary to improve it. SUMMARY

[0006] Therefore, the three-dimensional solar evaporation material, the preparation method and the application thereof are provided to solve the defects in the prior art.

[0007] In a first aspect, the application provides a preparation method of a three-dimensional solar evaporation material, comprising the following steps:

[0008] adding an iron salt into water to obtain a first solution;

[0009] adding a pyrrole monomer into water to obtain a second solution;

[0010] dropping the first solution into the second solution and stirring under an ice water bath to obtain a third solution;

[0011] immersing a sponge in the third solution and reacting under an ice water bath to obtain the three-dimensional solar evaporation material.

[0012] Preferably, the preparation method of the three-dimensional solar evaporation material, the sponge has a plurality of vertical pores inside, and the pore size of the pores is 10-500 μm.

[0013] Preferably, the preparation method of the three-dimensional solar evaporation material, the concentration of the iron salt in the first solution is 5-15 g / L.

[0014] The concentration of the pyrrole monomer in the second solution is 1-5 g / L.

[0015] Preferably, the preparation method of the three-dimensional solar evaporation material, in the step of dropping the first solution into the second solution, the volume ratio of the first solution to the second solution is (10-30):(400-600).

[0016] Preferably, the preparation method of the three-dimensional solar evaporation material, the iron salt comprises at least one of ferric chloride, ferric sulfate and ferric nitrate.

[0017] Preferably, the preparation method of the three-dimensional solar evaporation material, the first solution is dropped into the second solution, and the third solution is obtained by stirring under an ice water bath for 1-3 h.

[0018] The sponge is immersed in the third solution and reacted under an ice water bath for 3-9 h to obtain the three-dimensional solar evaporation material.

[0019] In a second aspect, the application further provides a three-dimensional solar evaporation material prepared by the preparation method.

[0020] In a third aspect, the present application further provides the application of the three-dimensional solar evaporation material prepared by the preparation method or the three-dimensional solar evaporation material in the preparation of a solar evaporator.

[0021] In a fourth aspect, the present application further provides a solar evaporator, comprising:

[0022] a substrate;

[0023] at least one three-dimensional solar evaporation material prepared by the preparation method or the three-dimensional solar evaporation material;

[0024] The three-dimensional solar evaporation material is fixed on the substrate.

[0025] In a fifth aspect, the present application further provides the application of the three-dimensional solar evaporation material or the solar evaporator in the desalination of seawater.

[0026] The three-dimensional solar evaporation material, the preparation method and the application thereof have the following beneficial effects compared with the prior art:

[0027] 1. The preparation method of the three-dimensional solar evaporation material of the present application uses sponge as a raw material, and the sponge is loaded with polypyrrole by a simple solution reaction through reaction with a pyrrole monomer and an iron salt, so as to improve the light absorption performance of the material and improve the light-heat conversion ability thereof; the three-dimensional solar evaporation material prepared by the present application has a low-tortuosity pore (i.e. vertical pore) water guide structure, can rapidly fill water into the inside of the material through capillary action, and thus prevents salt from depositing in the pores during the seawater desalination process, realizes the self-cleaning function of salt, and ensures the stable seawater desalination ability. In addition, due to the existence of the low-tortuosity pore water guide structure inside, the three-dimensional solar evaporation material can resist 20wt% salt water. The preparation method of the three-dimensional solar evaporation material of the present application is prepared through a simple solution reaction, has the characteristics of high efficiency and low cost, can be produced on a large scale, and provides a solution to the problems of global freshwater shortage, energy crisis and environmental pollution;

[0028] 2、The solar evaporator has the advantages that the light absorption performance of the polypyrrole-loaded sponge is obviously improved, the evaporation efficiency of the polypyrrole-loaded sponge with an exposure height of 6 cm can reach 3.67 kg / m2 / h, the low-tortuosity pore structure in the polypyrrole-loaded sponge enables the polypyrrole-loaded sponge to resist 20 wt% salt water, and the evaporation rate can reach 3.00 kg / m2 / h, the polypyrrole-loaded sponge is simple to prepare and can be mass-produced, and meets the actual application requirements, the solar evaporator uses waste mop sponge as raw material, and can desalinate seawater while relieving the environmental pressure, the material provides a promising solution to the global freshwater shortage, energy crisis and environmental pollution, the polypyrrole-sponge three-dimensional solar evaporator is prepared by using a simple solution soaking method, can be mass-produced, has good stability, is low in price, and can meet the market demand. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0030] Figure 1 The scanning electron microscope images of the sponge used in Example 1 and the finally prepared three-dimensional solar evaporation material;

[0031] Figure 2 The light absorption performance result graphs of the polypyrrole-waste mop sponge prepared in Example 1 and the waste mop sponge used in Example 1;

[0032] Figure 3 The mass change curve graphs of the polypyrrole-waste mop sponge and the waste mop sponge under 1 solar intensity;

[0033] Figure 4 The evaporation rate graphs of the solar evaporator in Example 1 for desalinating 20 wt% salt water on the first day and the seventh day;

[0034] Figure 5 The photos of the solar evaporator in Example 1 for desalinating 20 wt% salt water on the first day, at the first hour and the eighth hour;

[0035] Figure 6 The outdoor graph of the solar evaporator in Example 1, the polypyrrole-waste mop sponge for desalinating 20 wt% salt water with an exposure height of 6 cm;

[0036] Figure 7Figure 1 shows the ion concentration of the 20wt% brine before and after desalination by the solar evaporator of Example 1. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be clearly and completely described in combination with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0038] It should be noted that the sequence of the following embodiments is not limited as the preferred sequence of the embodiments. In addition, in the description of the present application, the term "comprising" means "including but not limited to". Various embodiments of the present application can exist in a range form; it should be understood that the description in a range form is only for the convenience and brevity, and should not be understood as a hard limitation on the scope of the present application; therefore, it should be considered that the described range has been specifically disclosed all possible sub-ranges and single values in the range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers in the range, such as 1, 2, 3, 4, 5 and 6, which is applicable to any range. In addition, whenever a numerical range is indicated in the present application, it means that any cited number (fraction or integer) in the indicated range is included.

[0039] The embodiment of the present application provides a preparation method of a three-dimensional solar evaporation material, comprising the following steps:

[0040] S1, adding an iron salt into water to obtain a first solution;

[0041] S2, adding a pyrrole monomer into water to obtain a second solution;

[0042] S3, adding the first solution into the second solution dropwise, and stirring under an ice water bath to obtain a third solution;

[0043] S4, soaking a sponge in the third solution, and reacting under an ice water bath to obtain a three-dimensional solar evaporation material.

[0044] The preparation method of the three-dimensional solar evaporation material of the present application uses sponge as raw material, the sponge has low twist hole structure, and is prepared by reaction with pyrrole monomer and iron salt. The sponge is loaded with polypyrrole through simple solution reaction, so as to improve the light absorption performance of the material and improve its light-heat conversion capacity. The three-dimensional solar evaporation material prepared by the present application has low twist hole water guide structure, can rapidly fill the material inside through capillary action, so as to prevent salt from depositing in the hole during the seawater desalination process, realize the salt self-cleaning function, and ensure the stable seawater desalination capacity. In addition, due to the existence of the low twist hole water guide structure inside, it can also resist 20wt% salt water. The preparation method of the three-dimensional solar evaporation material of the present application is simple solution reaction, has the characteristics of high efficiency and low cost, can be mass-produced, and provides a solution to the global freshwater shortage, energy crisis and environmental pollution problem.

[0045] In some embodiments, the sponge can be a waste mop sponge, and the sponge is a porous sponge with low twist hole structure, that is, the sponge has a plurality of vertical or through holes (i.e. the holes are low twist holes or holes) inside, specifically, the hole diameter of the holes (or holes) on the sponge is 10-500μm.

[0046] In some embodiments, the sponge can be a commercially available sponge, for example, a PU (polyurethane) sponge, and in some embodiments, the sponge can be a waste mop sponge, specifically, the mop is a David mop, and the sponge used in the present application is a waste David mop sponge, product name David M12 cotton mop, model DJ-M1204, cotton head material: PVA cotton, the sponge used in the present application has a low twist hole structure with a pore size of 10-500 microns, for more specific information, please refer to the following website:

[0047] https: / / detail.tmall.com / item.htm?ali_trackid=2:mm_28347190_2425761_109058700251:1689575435_001_924098931&bxsign=tbkedgYL8pa07pewaQKMvhlaW2-9KhFyA2eAMDU4NMYIMbNNasf3AVAbcGbfesxq-4Z4acN6fwQOM4BB7t0hgQ65uk PiIRJr-nuFANQXFDpgc6gyCUzT2HQigvh7JDi8XM4&id=680560880258&spm=a2e1u.27655827.d1661933647166.5&union_lens=lensId:OPT@1689575430@0b13eb3d_0b00_189628b1ab8_955d@01@eyJmbG9vcklkIjo2MTc4NH0ie;recoveryid:201_33.60.109.167_271717_1689575426101;prepvid:201_33.53.204.179_205239_1689575430140.

[0048] In some embodiments, the sponge is cuboid, and the length, width and height thereof are 2.5 cm, 2.5 cm and 10 cm respectively.

[0049] In some embodiments, the concentration of the iron salt in the first solution is 5-15 g / L.

[0050] In some embodiments, the concentration of the pyrrole monomer in the second solution is 1-5 g / L.

[0051] In some embodiments, in the step of dropping the first solution into the second solution, the volume ratio of the first solution to the second solution is (10-30):(400-600).

[0052] In some embodiments, the iron salt comprises at least one of ferric chloride, ferric sulfate and ferric nitrate.

[0053] In some embodiments, the first solution is dropped into the second solution, and stirring is performed under ice water bath (i.e. at 0℃) for 1-3 h to obtain a third solution.

[0054] The sponge is immersed in the third solution, and reaction is performed under ice water bath for 3-9 h to obtain the three-dimensional solar evaporation material.

[0055] Based on the same inventive concept, the application further provides a three-dimensional solar evaporation material prepared by the above preparation method.

[0056] Based on the same inventive concept, the application further provides a three-dimensional solar evaporation material prepared by the preparation method, or application of the three-dimensional solar evaporation material in preparation of a solar evaporator.

[0057] Based on the same inventive concept, the application further provides a solar evaporator, comprising:

[0058] a substrate;

[0059] at least one three-dimensional solar evaporation material prepared by the preparation method or the three-dimensional solar evaporation material;

[0060] The three-dimensional solar evaporation material is fixed on the substrate.

[0061] In some embodiments, the substrate is a foam substrate.

[0062] In some embodiments, the three-dimensional solar evaporation material penetrates through the substrate and is fixed thereto, and the height of the three-dimensional solar evaporation material above the substrate is 4-10 cm, preferably 6 cm; by adjusting the exposed height of the three-dimensional solar evaporation material, more ambient energy can be obtained, and thus a higher evaporation rate can be obtained. Advantages of the solar evaporator of the application include: 1) the light absorption performance of the polypyrrole-loaded sponge is significantly improved; 2) the evaporation efficiency of the polypyrrole-loaded sponge with an exposed height of 6 cm can reach 3.67 kg / m2 / h; 3) the low-tortuosity pore (i.e. vertical pore) structure inside the polypyrrole-loaded sponge makes it resistant to 20wt% salt water, and the evaporation rate can reach 3.00 kg / m2 / h; 4) the preparation method of the polypyrrole-loaded sponge is simple, and large-scale production can be realized to meet the actual application requirements; 5) the polypyrrole-loaded sponge mop uses waste as raw material, desalination of seawater while also alleviating environmental pressure. This material provides a promising solution to the global freshwater shortage, energy crisis and environmental pollution problems. The polypyrrole-sponge three-dimensional solar evaporator prepared by the simple solution immersion method of the application can realize large-scale production, has good stability, is low in price and can meet market demand.

[0063] Based on the same inventive concept, the application further provides application of the three-dimensional solar evaporation material or the solar evaporator in desalination of seawater.

[0064] The three-dimensional solar evaporation material, its preparation method and application of the present application are further illustrated in specific embodiments below. This part further illustrates the content of the present application in combination with specific embodiments, but should not be understood as a limitation of the present application. If not specifically stated, the technical means adopted in the embodiments are conventional means familiar to those skilled in the art. Unless specifically stated, the reagents, methods and equipment adopted by the present application are conventional reagents, methods and equipment in the art.

[0065] Example 1

[0066] The present application provides a preparation method of a three-dimensional solar evaporation material, comprising the following steps:

[0067] S1, iron trichloride is added to water to obtain a first solution; the concentration of iron ions in the first solution is 10 g / L;

[0068] S2, pyrrole monomer is added to water to obtain a second solution; the concentration of pyrrole monomer in the second solution is 2 g / L

[0069] S3, 20 mL of the first solution is added dropwise to 500 mL of the second solution, and stirred under ice water bath for 1 h to obtain a third solution;

[0070] S4, the waste mop sponge (length, width and height are 2.5 cm, 2.5 cm and 10 cm respectively, the sponge has a low-tortuosity pore structure (i.e. vertical pore channel) structure, and the pore diameter is between 10-500 microns) is soaked in the third solution, and reacted under ice water bath for 6 h, and then ultrasonically cleaned until no black substance falls off, to obtain a polypyrrole-sponge, i.e. a three-dimensional solar evaporation material.

[0071] The present application further provides a solar evaporator, comprising:

[0072] It comprises a foam substrate and the three-dimensional solar evaporation material prepared in Example 1 fixed on the foam substrate, specifically, the dispersed black-polyester fiber solar evaporation material penetrates through the foam substrate and is fixed therewith, and the height of the three-dimensional solar evaporation material above the foam substrate is 6 cm, specifically, the height is the exposure height mentioned below.

[0073] Performance test

[0074] Figure 1 The scanning electron microscope images of the waste mop sponge used in Example 1 and the three-dimensional solar evaporation material finally prepared; Figure 1 (a) and (b) are scanning electron microscope images of the sponge used at different magnifications, and (c) and (d) are scanning electron microscope images of the polypyrrole-waste mop sponge (i.e. the three-dimensional solar evaporation material finally prepared) at different magnifications.

[0075] Figure 1 (a, b) are SEM images of the waste sponge, from Figure 1 As can be seen in (a), the sponge has a low tortuosity pore (i.e. vertical channel) structure with pore diameters between 10-500 microns (i.e. vertical channel diameters between 10-500 microns), which is beneficial for salt self-cleaning during the process of seawater desalination, ensuring stable seawater desalination performance. Figure 1 As can be seen in (b), the surface of the waste sponge is free of any material loading. Figure 1 (c) is an SEM image of the waste sponge loaded with polypyrrole. Figure 1 (c) shows that the waste sponge loaded with polypyrrole has a similar low tortuosity pore as Figure 1 (a). Figure 1 The surface of the sponge in (c) is rougher than Figure 1 (a), due to the loading of polypyrrole on the surface of the sponge. Figure 1 In (d), polypyrrole loading on the surface of the waste sponge can be more directly observed.

[0076] Further, the light absorption performance results of the polypyrrole-waste sponge (i.e. the three-dimensional solar evaporation material finally prepared) prepared in Example 1 and the waste sponge used in Example 1 are shown in Figure 2 .

[0077] In order to preliminarily evaluate the light-heat conversion ability of the material, by measuring the ultraviolet-visible-near infrared spectrum of the waste sponge and the polypyrrole-waste sponge (i.e. the left side of Figure 2 ), it is found that the light-heat conversion ability of the waste sponge loaded with polypyrrole is increased from 82.27% to 98.92% (i.e. the right side of Figure 2 ), close to 100%. The improvement of the light absorption performance of the material is beneficial to enhance the evaporation efficiency of the material.

[0078] The solar evaporator is assembled according to the method in Example 1, and the height of the three-dimensional solar evaporation material (i.e. the polypyrrole-waste sponge prepared in Example 1) above the foam substrate is 0 cm, 3 cm, 6 cm, respectively. Similarly, the solar evaporator assembled with the waste sponge is used as a comparison, and the height of the waste sponge above the foam substrate is 0 cm, 3 cm, 6 cm, respectively. The evaporation rate of the underground water of the polypyrrole-waste sponge and the waste sponge at different exposure heights is tested, and the results are shown in Figure 3 .

[0079] Figure 3The groundwater quality change curve of the poly-pyrrole-waste mop sponge with different exposure heights, the waste mop sponge under 1 sun intensity, was found to increase with the increase of the exposure height of the poly-pyrrole-waste mop sponge. When the exposure height was 6 cm, the evaporation efficiency of the groundwater reached 3.67 kg / m2 / h.

[0080] Figure 4 The evaporation rate of the solar evaporator in Example 1 for desalination of 20wt% salt water; Figure 4 The preparation method of 20wt% salt water in Example 1 was to dissolve NaCl (76.34 g), KCl (2.06 g), MgCl2 (18.21 g) and CaCl2 (3.31 g) in 500 ml of deionized water;

[0081] The specific test method of the evaporation rate was to place the solar evaporator in Example 1 in the salt water, and the solar evaporator absorbed heat to cause the salt water to evaporate, and the evaporation rate was tested, and the calculation formula was: v = dm / (S·dt);

[0082] Wherein, v is the evaporation rate, m is the change of the mass of the salt water before and after evaporation, S is the light area at the top of the three-dimensional solar evaporation material, and t is the illumination time.

[0083] Figure 4 The change of the mass of the salt water with the illumination time was shown, and specifically, Figure 4 In (a) and (b), the change of the mass of the salt water with the illumination time on the first day and the seventh day, respectively.

[0084] From Figure 4 It can be seen from the above that the evaporation rate of the solar evaporator in Example 1 for desalination of 20wt% salt water can reach 3.00 kg / m2 / h, and after seven days of desalination experiment, it was found that the material had good stability.

[0085] Figure 5 The first hour and the eighth hour photos of the solar evaporator in Example 1 for desalination of 20wt% salt water on the first day.

[0086] Figure 6 The outdoor graph of the solar evaporator in Example 1, the poly-pyrrole-waste mop sponge with an exposure height of 6 cm for desalination of 20wt% salt water.

[0087] Figure 7 The ion concentration graph of the solar evaporator in Example 1 for desalination of 20wt% salt water after 1 hour of illumination before and after desalination of the salt water. Figure 7 In the above, Before desalination represents the ion concentration of the salt water before desalination, and After desalination represents the ion concentration of the evaporated water after desalination.

[0088] After 1 hour of illumination, water droplets were attached to the top of the evaporator, and the ion concentration of the desalinated seawater was detected by ICP-MS, and it was found that the ion concentration of the desalinated brine met the standard of the World Health Organization (WHO).

[0089] The above description is merely preferred embodiments of the present application, but not to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a three-dimensional solar evaporation material, characterized in that, The method comprises the following steps: adding iron salt into water to obtain a first solution; adding pyrrole monomer into water to obtain a second solution; adding the first solution into the second solution to obtain a third solution under stirring in an ice-water bath; immersing a sponge into the third solution to react under an ice-water bath to obtain a three-dimensional solar evaporation material; the sponge has a plurality of vertical channels inside, and the pore size of the channels is 10-500 μm; the concentration of the iron salt in the first solution is 10 g / L; the concentration of the pyrrole monomer in the second solution is 2 g / L; in the step of adding the first solution into the second solution, the volume ratio of the first solution to the second solution is 20:500; the iron salt is ferric chloride; immersing a sponge into the third solution to react under an ice-water bath for 6 h to obtain a three-dimensional solar evaporation material.

2. The method of claim 1, wherein the three-dimensional solar evaporative material is prepared by, adding the first solution into the second solution to obtain a third solution under stirring in an ice-water bath for 1-3 h.

3. A three-dimensional solar evaporation material, characterized in that, obtained by the preparation method of any one of claims 1-2.

4. The use of the three-dimensional solar evaporation material obtained by the preparation method of any one of claims 1-2 or the three-dimensional solar evaporation material of claim 3 in the preparation of a solar evaporator.

5. A solar evaporator characterized by, comprise: a substrate; at least one three-dimensional solar evaporation material obtained by the preparation method of any one of claims 1-2 or the three-dimensional solar evaporation material of claim 3; the three-dimensional solar evaporation material is fixed on the substrate.

6. The use of the three-dimensional solar evaporation material of claim 3 or the solar evaporator of claim 5 in the desalination of seawater.

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