Photoelectric-thermal super-hydrophobic anti-pollution composite membrane as well as preparation method and application thereof

Abstract

The invention relates to the technical field of seawater desalination and chemistry and chemical engineering, in particular to a photoelectric-thermal super-hydrophobic anti-pollution composite membrane as well as a preparation method and application thereof. The photoelectric-thermal super-hydrophobic anti-pollution composite membrane comprises a PVDF (Polyvinylidene Fluoride) base membrane layer, an adhesion layer and a fluorinated multi-walled carbon nanotube membrane layer, the PVDF base film layer is adhered to the fluorinated multi-walled carbon nanotube film layer through the adhesion layer. According to the invention, the PVDF base membrane is used as a substrate, and the fluorinated hydrophobic modified multi-walled carbon nanotube is plated on the PVDF base membrane, so that the membrane preparation process is simpler, less equipment is needed, and the cost is lower; the PVDF component is not added into the fluorinated multi-walled carbon nanotube film layer of the composite film, so that the composite film has good conductivity, and not only can be used for photo-thermal film distillation in seawater desalination, but also can be used for electrothermal film distillation and photoelectric coupling film distillation.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of seawater desalination and chemistry and chemical industry, and particularly relates to a photoelectric-thermal super-hydrophobic anti-pollution composite membrane and a preparation method and application thereof. BACKGROUND

[0002] With the increasing shortage of global freshwater resources, seawater desalination technology has become one of the important ways to solve the water resource crisis. Although traditional seawater desalination methods have been widely used, such as reverse osmosis (RO) and multi-stage flash distillation (MSF), they still face problems such as high energy consumption, equipment corrosion and concentrated salt water discharge. Therefore, developing new seawater desalination technologies with high efficiency, low energy consumption and environmental friendliness has become a research hotspot.

[0003] Currently, photothermal membrane distillation (PMD) is an innovative technology that combines solar light-to-heat conversion with membrane distillation, and has broad application prospects. This technology uses photothermal materials to absorb solar energy and convert it into heat, which locally heats the membrane interface, promoting water evaporation and selectively separating water vapor through a hydrophobic microporous membrane, and finally condensing fresh water. Existing photothermal materials include plasmonic nanoparticles, carbon-based materials or semiconductor polymers, etc. Compared with traditional membrane distillation, PMD technology has the following advantages: (1) low energy consumption: directly driven by solar energy, reducing dependence on electricity and steam; (2) high efficiency: localized heating of photothermal materials can improve heat utilization efficiency and reduce heat loss; (3) modular design: compact system structure, suitable for distributed and small-scale freshwater supply; (4) environmentally friendly: reducing fossil energy consumption and carbon emissions, in line with the concept of sustainable development.

[0004] However, photothermal membrane distillation technology still faces challenges such as photothermal material stability, membrane fouling and long-term durability. Future research will focus on optimizing photothermal membrane material design, improving solar energy utilization efficiency and developing low-cost large-scale preparation processes to promote the practical application of this technology. For example, most of the components of the membrane used in photothermal membrane distillation are composed of carbon-based materials, which have hydrophilic properties and are therefore easily contaminated by seawater. Although hydrophobically modified multi-walled carbon nanotubes have been used in membrane distillation technology, PVDF (polyvinylidene fluoride membrane) is used as a solvent to mix with multi-walled carbon nanotubes, and then reformed into a membrane, as described in patent application number CN202510180642.2 and patent application number CN201510278362.1. However, this method of preparing the membrane has the disadvantage of poor electrical conductivity due to the introduction of PVDF into the carbon nanotubes, which does not have electric heating capability. In view of this, the present application provides a photoelectric-thermal super-hydrophobic anti-pollution composite membrane and a preparation method and application thereof. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a photoelectric thermal super-hydrophobic anti-pollution composite film and a preparation method and application thereof.

[0006] The technical scheme for solving the above technical problem is as follows: In a first aspect, the photoelectric thermal super-hydrophobic anti-pollution composite film comprises a PVDF base film layer, an adhesive layer and a fluorinated multi-walled carbon nanotube film layer; the PVDF base film layer is bonded to the fluorinated multi-walled carbon nanotube film layer through the adhesive layer.

[0007] On the basis of the above technical scheme, the present application can also be improved as follows.

[0008] Further, the thickness of the PVDF base film layer is 180-250 microns; the thickness of the fluorinated multi-walled carbon nanotube composite film layer is 120-180 microns; the thickness of the adhesive layer is 10-18 microns; preferably, the thickness of the PVDF base film layer is 216 microns; the thickness of the fluorinated multi-walled carbon nanotube composite film layer is 147.1 microns; and the thickness of the adhesive layer is 14 microns.

[0009] In a second aspect, a preparation method of a photoelectric thermal super-hydrophobic anti-pollution composite film comprises the following steps: coating an adhesive on a PVDF base film; preparing fluorinated multi-walled carbon nanotube solution; applying the fluorinated multi-walled carbon nanotube solution to the PVDF base film coated with the adhesive, and obtaining a PVDF-fluorinated multi-walled carbon nanotube composite film after drying, which is a photoelectric thermal super-hydrophobic anti-pollution composite film.

[0010] Further, the adhesive comprises at least one of PVA (polyvinyl alcohol), PEG (polyethylene glycol) and PAA (polyacrylic acid). The weight ratio of the PVDF base film to the adhesive is 20:0.5-2, preferably 20:1.

[0011] Further, the mass concentration of fluorinated multi-walled carbon nanotubes in the fluorinated multi-walled carbon nanotube solution is 0.095wt%-0.12wt%. The solvent in the fluorinated multi-walled carbon nanotube solution is at least one of ethanol, isopropyl alcohol and tetrahydrofuran. The weight ratio of the PVDF base film to the fluorinated multi-walled carbon nanotubes is 8:2-4, preferably 8:3.

[0012] Further, the fluorinated multi-walled carbon nanotube solution is plated on the PVDF base film coated with an adhesive agent by vacuum filtration, and dried at a temperature of 60-100 DEG C for 3-12 hours to obtain a PVDF-fluorinated multi-walled carbon nanotube composite film.

[0013] Further, the fluorinated multi-walled carbon nanotube is prepared by the following steps: The hydroxylated multi-walled carbon nanotube and 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane are added to tetrahydrofuran, and reacted under stirring, and then the fluorinated multi-walled carbon nanotube is obtained by separation.

[0014] Further, the weight ratio of the hydroxylated multi-walled carbon nanotube to the 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane is 1.5-6:4. The ratio of the amount of the hydroxylated multi-walled carbon nanotube to the tetrahydrofuran is 1.2-6 mg:3 mL.

[0015] Further, the reaction conditions are as follows: the rotation speed is 8000-10000 rpm, and the reaction is carried out at room temperature for 12-36 hours.

[0016] The third aspect is the application of the photoelectric-thermal super-hydrophobic anti-pollution composite film to seawater desalination.

[0017] The present application is characterized in that the hydroxylated multi-walled carbon nanotube solution is fluorinated to change its hydrophilicity to super-hydrophobicity, and then plated on a PVDF base film by filtration, and the composite film is mainly composed of two layers: a PVDF base film and a modified carbon nanotube super-hydrophobic layer, so that the modified composite film has the functions of photo-thermal, electric-thermal and super-hydrophobicity, and becomes a very excellent membrane distillation material for seawater desalination.

[0018] The present application has the following advantages: (1) Unlike the existing super-hydrophobic photo-thermal film, the PVDF and multi-walled carbon nanotube are dissolved together and then re-filmed, while the present application uses a PVDF base film as a substrate, and fluorinated hydrophobic modified multi-walled carbon nanotubes are plated on the PVDF base film, so that the film forming process is simpler, the required equipment is less, and the cost is lower. (2) Since the fluorinated multi-walled carbon nanotube film layer (hydrophobic layer) of the composite film of the present application does not contain PVDF, it has good electrical conductivity, and can not only be used for photo-thermal membrane distillation in seawater desalination, but also for electric-thermal membrane distillation and photoelectric coupling membrane distillation. (3) In the performance test of the composite film in the present application, the surface temperature of the film under electric-thermal conditions can reach 120 DEG C, which can make the film have very strong membrane distillation performance and achieve very high flux. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the preparation process of the composite membrane of the present invention. Figure 2 This is a test state diagram of the photothermal performance of the composite film of the present invention; Figure 3 This is a graph showing the photothermal performance data of the composite film of the present invention; Figure 4 This is a test state diagram of the electrothermal performance of the composite film of the present invention; Figure 5 This is a graph showing the electrothermal performance data of the composite film of the present invention; Figure 6 This is a contact angle diagram of the composite film of the present invention; Figure 7 This is a comparison image of the membrane before and after modification according to the present invention; Figure 8 This is a test state diagram of the photothermal performance of the PVDF base film of the present invention; Figure 9 This is a graph showing the photothermal performance data of the PVDF base film of this invention; Figure 10 This is a diagram showing the test status of the electrothermal performance of the PVDF base film of this invention; Figure 11 This is a graph showing the electrothermal performance data of the PVDF base film of this invention; Figure 12 This is a contact angle diagram of the PVDF base film of the present invention; Figure 13 This is a 250x scanning electron microscope image of the present invention; Figure 14 This is a detailed image of the composite membrane of the present invention under a 500x scanning electron microscope. Figure 15 This is a schematic diagram of the composite membrane of the present invention used in a seawater desalination experiment. Detailed Implementation

[0020] The principles and features of this invention are described below. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they should be performed according to the techniques or conditions described in the literature in this field, or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased through legitimate channels.

[0021] Description of the source of materials and reagents: The PVDF base film was provided by Aladdin Biochemical Technology Co., Ltd. (Shanghai, China); the PVA (polyvinyl alcohol) was provided by Maclean's Reagent Co., Ltd.; and the hydroxylated multi-walled carbon nanotubes were provided by Maclean's Reagent Co., Ltd.

[0022] Example 1 The embodiment relates to a photoelectric-thermal super-hydrophobic anti-pollution composite film, which comprises a PVDF base film layer, a bonding layer and a fluorinated multi-walled carbon nanotube film layer; the PVDF base film layer is bonded with the fluorinated multi-walled carbon nanotube film layer through the bonding layer.

[0023] Preferably, the thickness of the PVDF base film layer in the embodiment is 180-250 microns; the thickness of the fluorinated multi-walled carbon nanotube composite film layer is 120-180 microns; the thickness of the bonding layer is 10-18 microns; specifically, the thickness of the PVDF base film layer is 216 microns; the thickness of the fluorinated multi-walled carbon nanotube composite film layer is 147.1 microns; and the thickness of the bonding layer is 14 microns. Figure 13 And 14 ).

[0024] Embodiment 2 The embodiment relates to a preparation method of a photoelectric-thermal super-hydrophobic anti-pollution composite film, which comprises the following steps (as shown in Figure 1 ): coating a bonding agent on a PVDF base film; preparing fluorinated multi-walled carbon nanotubes into a fluorinated multi-walled carbon nanotube solution; applying the fluorinated multi-walled carbon nanotube solution to the PVDF base film coated with the bonding agent, and drying to obtain a PVDF-fluorinated multi-walled carbon nanotube composite film, that is, a photoelectric-thermal super-hydrophobic anti-pollution composite film.

[0025] Preferably, the bonding agent in the embodiment comprises at least one of PVA (polyvinyl alcohol), PEG (polyethylene glycol) and PAA (polyacrylic acid); and the weight ratio of the PVDF base film to the bonding agent is 20:0.5-2, specifically 20:1.

[0026] Preferably, the mass concentration of the fluorinated multi-walled carbon nanotubes in the fluorinated multi-walled carbon nanotube solution in the embodiment is 0.095wt%-0.12wt%; the solvent in the fluorinated multi-walled carbon nanotube solution is at least one of ethanol, isopropyl alcohol and tetrahydrofuran; and the weight ratio of the PVDF base film to the fluorinated multi-walled carbon nanotubes is 8:2-4; specifically 8:3.

[0027] Preferably, the fluorinated multi-walled carbon nanotube solution is applied to the PVDF base film coated with the bonding agent through vacuum filtration in the embodiment, and the PVDF-fluorinated multi-walled carbon nanotube composite film is obtained by drying at a temperature of 60-100 DEG C for 3-12 hours.

[0028] Preferably, the fluorinated multi-walled carbon nanotubes of the present embodiment are prepared by the following steps: adding hydroxylated multi-walled carbon nanotubes and 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane into tetrahydrofuran, and reacting under stirring, and then separating to obtain fluorinated multi-walled carbon nanotubes.

[0029] The weight ratio of the hydroxylated multi-walled carbon nanotubes to the 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane is 1.5-6:4, for example, 1.5:4, 3:4, 6:4, and the like. The ratio of the amount of the hydroxylated multi-walled carbon nanotubes to the tetrahydrofuran is 1.2-6 mg:3 mL, for example, 1.2 mg:3 mL, 3 mg:3 mL, 6 mg:3 mL, and the like.

[0030] Preferably, the reaction conditions of the present embodiment are as follows: the rotation speed is 8000 rpm-10000 rpm, for example, 8000 rpm, 10000 rpm, and the reaction is carried out at room temperature for 12-36 hours.

[0031] Specifically, a preparation method of a photoelectric-thermal super-hydrophobic anti-pollution composite film includes the following steps: (1) adding 30 mg of hydroxylated multi-walled carbon nanotubes and 40 mg of 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane into a beaker, and adding 30 ml of tetrahydrofuran thereto, and stirring vigorously in a magnetic stirrer for 24 hours; (2) centrifuging the solution obtained after magnetic stirring in a centrifuge at a speed of 10000 revolutions for 10 minutes, and separating the supernatant from the precipitate; (3) discarding the supernatant in (2), and washing the obtained precipitate by ultrasonic dispersion in deionized water for 30 minutes, and then repeating the steps in (2) again, discarding the supernatant to obtain the precipitate; then adding anhydrous ethanol to the precipitate, and washing by ultrasonic dispersion for 30 minutes, and then repeating the steps in (2) again, discarding the supernatant to obtain the precipitate, and placing the precipitate in a vacuum drying oven, and drying at 80°C for 12 hours, to obtain fluorinated hydroxylated multi-walled carbon nanotubes, i.e., fluorinated multi-walled carbon nanotubes; (4) adding 30 mg of the fluorinated hydroxylated multi-walled carbon nanotubes into a beaker, and adding 30 ml of anhydrous ethanol, and ultrasonic dispersing for 2 h, so that the fluorinated hydroxylated multi-walled carbon nanotubes are completely dissolved in the anhydrous ethanol to form a fluorinated modified hydroxylated multi-walled carbon nanotube original solution; (5) coating a layer of 1 ml of a 0.85%wt concentration PVA (polyvinyl alcohol) solution on the PVDF base film, and the solution is used to strengthen the binding of the fluorinated modified hydroxylated multi-walled carbon nanotubes and the PVDF base film; (6) The fluorinated modified hydroxylated multi-walled carbon nanotube original solution in (4) is slowly deposited onto the treated PVDF base film in (5) by a vacuum filtration device. The ratio of the fluorinated modified hydroxylated multi-walled carbon nanotube original solution to the PVDF film is 300:1, and a wet PVDF-fluorinated multi-walled carbon nanotube composite film is obtained. (7) Place the moist PVDF-fluorinated multi-walled carbon nanotube composite membrane obtained in (6) into a vacuum drying oven and dry it at 80°C for 6 hours to obtain a PVDF-fluorinated multi-walled carbon nanotube composite membrane that can be used for seawater desalination membrane distillation.

[0032] Test case 1. Performance testing of photo / electrothermal superhydrophobic antifouling composite membrane.

[0033] (1) Photothermal properties: By irradiating the PVDF-fluorinated multi-walled carbon nanotube composite film prepared in Example 2 with a solar irradiance of 1000 candela, the surface temperature of the film reached 75°C; as a control group, PVDF ( Figure 7 The base film can only reach a temperature of 50℃ under the same light irradiation, but its photothermal temperature can be increased by 25℃, exhibiting excellent photothermal properties (such as...). Figures 2-3 (and 8-9); (2) Electrothermal effect: When a voltage of 5V is applied to the PVDF-fluorinated multi-walled carbon nanotube composite film prepared in Example 2, the current reaches 0.37A, and the temperature on the film surface reaches 120°C, exhibiting very strong electrothermal properties. The PVDF-based film, serving as a control group, does not generate current under the same voltage and does not show any electrothermal effect (e.g., ...). Figures 4-5 and 10-11).

[0034] (3) Hydrophobicity: The PVDF-fluorinated multi-walled carbon nanotube composite film prepared in Example 2 was found to have a contact angle of 160°, exhibiting very strong hydrophobicity, preventing water molecules from penetrating into the film and achieving superhydrophobicity. In contrast, the PVDF-based film used as a control group had a contact angle of only 114°, failing to achieve superhydrophobicity (e.g., ...). Figure 6 and 12 ).

[0035] 2. Application of this photoelectric and thermal superhydrophobic and antifouling composite membrane in seawater desalination.

[0036] (1) The performance of the composite membrane of the present invention was tested using a photothermal film distillation apparatus. Figure 15A solution containing 3.5 wt% sodium chloride was used as the feed liquid. The feed liquid flowed through the fluorinated multi-walled carbon nanotube layer of the composite membrane, directly contacting and being heated. Water vapor generated by the evaporation of the feed liquid during heating permeated through the composite membrane under osmotic pressure and was condensed and collected below the membrane. The light intensity on the membrane surface was 1 kW·m². -2 The membrane size exposed to light was 10 × 10 cm. The photothermal desalination performance of the hydrophobic carbon nanotube composite polyvinylidene fluoride photothermal hydrophobic membrane obtained in Example 1 was tested, and the results were obtained at 1 kW·m -2 Under light intensity, 150 grams of fresh water were collected within 10 hours. The calculated permeation flux of the hydrophobic carbon nanotube composite polyvinylidene fluoride photothermal hydrophobic membrane was 1.5 kg / (m²). 2 (·h), the salt interception rate can reach 99.5%.

[0037] (2) The performance of the composite membrane of the present invention was tested using an electrothermal film distillation apparatus. Figure 15 A solution containing 3.5 wt% sodium chloride was used as the feed liquid. The feed liquid flowed through the fluorinated multi-walled carbon nanotube layer of the composite membrane, directly contacting and being heated. Water vapor formed by the evaporation of the feed liquid due to heating was driven by osmotic pressure, permeated through the composite membrane, and was condensed and collected below the membrane. A voltage of 9 V was applied to the membrane surface, and the membrane electrothermal size was 10 × 10 cm. The photothermal desalination performance of the hydrophobic carbon nanotube composite polyvinylidene fluoride photothermal hydrophobic membrane obtained in Example 1 was tested. Under 9 V voltage, 250 grams of fresh water were collected in 10 hours. The permeation flux of the hydrophobic carbon nanotube composite polyvinylidene fluoride photothermal hydrophobic membrane was calculated to be 2.5 kg / (m²). 2 (·h), the salt interception rate can reach 99.5%.

[0038] (3) The performance of the composite membrane of the present invention was tested using a photoelectric and thermal simultaneous action membrane distillation device. Figure 15 A solution containing 3.5 wt% sodium chloride was used as the feed liquid. The feed liquid flowed through the fluorinated multi-walled carbon nanotube layer of the composite membrane, directly contacting and being heated. Water vapor formed by the evaporation of the feed liquid due to heating was driven by osmotic pressure, permeated through the composite membrane, and was condensed and collected below the membrane. A 9V voltage and a light intensity of 1 kW·m were applied to the membrane surface. -2 The membrane size exposed to light was 10 × 10 cm. The photothermal desalination performance of the hydrophobic carbon nanotube composite polyvinylidene fluoride photothermal hydrophobic membrane obtained in Example 1 was tested. The results were obtained at 9 V voltage and 1 kW·m -2 Under light intensity, 340 grams of fresh water were collected within 10 hours. The calculated permeation flux of the hydrophobic carbon nanotube composite polyvinylidene fluoride photothermal hydrophobic membrane was 3.4 kg / (m²). 2 (·h), the salt interception rate can reach 99.5%.

[0039] From the above, the modified composite membrane has the functions of photothermal, electrothermal and superhydrophobicity, and becomes a very excellent membrane distillation material for seawater desalination.

[0040] Although the embodiments of the present application have been shown and described above, it should be understood by those skilled in the art that the above embodiments are exemplary and cannot be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application.

Claims

1. A photoelectric, thermal, superhydrophobic, and antifouling composite membrane, characterized in that, The photoelectric and thermal superhydrophobic antifouling composite membrane includes a PVDF base film layer, an adhesive layer, and a fluorinated multi-walled carbon nanotube film layer; the PVDF base film layer is bonded to the fluorinated multi-walled carbon nanotube film layer through the adhesive layer.

2. The photoelectric and thermal superhydrophobic antifouling composite membrane according to claim 1, characterized in that, The thickness of the PVDF base film is 180-250 micrometers; the thickness of the fluorinated multi-walled carbon nanotube composite film is 120-180 micrometers; and the thickness of the adhesive layer is 10-18 micrometers.

3. A method for preparing a photoelectric, thermal, superhydrophobic, and antifouling composite membrane according to any one of claims 1 to 2, characterized in that, Includes the following steps: An adhesive is coated onto the PVDF base film; Fluorinated multi-walled carbon nanotubes were prepared into a fluorinated multi-walled carbon nanotube solution. The fluorinated multi-walled carbon nanotube solution is deposited onto the PVDF base film coated with an adhesive, and after drying, a PVDF-fluorinated multi-walled carbon nanotube composite film is obtained, which is a photoelectric, thermal, superhydrophobic, and antifouling composite film.

4. The method for preparing a photoelectric, thermal, superhydrophobic, and antifouling composite membrane according to claim 3, characterized in that, The adhesive includes at least one of PVA, PAA, and PEG; The weight ratio of the PVDF base film to the adhesive is 20:0.5~2.

5. The method for preparing a photoelectric, thermal, superhydrophobic, and antifouling composite membrane according to claim 3, characterized in that, The fluorinated multi-walled carbon nanotube solution has a fluorinated multi-walled carbon nanotube mass concentration of 0.095 wt% to 0.12 wt%. The solvent in the fluorinated multi-walled carbon nanotube solution includes at least one of ethanol, isopropanol, and tetrahydrofuran. The weight ratio of the PVDF base film to the fluorinated multi-walled carbon nanotubes is 8:2~4.

6. The method for preparing a photoelectric, thermal, superhydrophobic, and antifouling composite membrane according to claim 3, characterized in that, The fluorinated multi-walled carbon nanotube solution was deposited onto the PVDF base film coated with an adhesive by vacuum filtration, and then dried at 60℃~100℃ for 3~12 hours to obtain a PVDF-fluorinated multi-walled carbon nanotube composite film.

7. The method for preparing a photoelectric, thermal, superhydrophobic, and antifouling composite membrane according to claim 3, characterized in that, The fluorinated multi-walled carbon nanotubes were prepared by the following steps: Hydroxylated multi-walled carbon nanotubes and 1H,1H,2H,2H-perfluorodecyltrichlorosilane were added to tetrahydrofuran and reacted under stirring. Fluorinated multi-walled carbon nanotubes were then obtained by separation.

8. The method for preparing a photoelectric, thermal, superhydrophobic, and antifouling composite membrane according to claim 7, characterized in that, The weight ratio of the hydroxylated multi-walled carbon nanotubes to the 1H,1H,2H,2H-perfluorodecyltrichlorosilane is 1.5~6:

4. The ratio of the hydroxylated multi-walled carbon nanotubes to the tetrahydrofuran is 1.2~6 mg:3 mL.

9. The method for preparing a photoelectric, thermal, superhydrophobic, and antifouling composite membrane according to claim 7, characterized in that, The reaction conditions are: a rotation speed of 8000 rpm to 10000 rpm, and a reaction time of 12 to 36 hours at room temperature.

10. An application of a photoelectric, thermal, superhydrophobic, and antifouling composite membrane, characterized in that, The photoelectric and thermal superhydrophobic and antifouling composite membrane according to any one of claims 1 to 2 is used in seawater desalination.

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