Multi-stage solar distillation desalting device and application thereof in saline-alkali soil treatment

Abstract

The invention discloses a multi-stage solar distillation desalting device and application thereof in saline-alkali soil treatment, the device comprises seven stages, and each stage comprises a heating layer, a capillary layer, an air gap layer and a condensation layer from top to bottom; each stage is stacked from top to bottom, and a heat dissipation aluminum grating is placed at the lower part of the seventh stage; the device is of a multi-stage structure with the optimized heat insulation performance, so that energy loss is reduced, latent heat recovery is achieved, and the heat efficiency is improved; fresh water generated by condensation is collected to irrigate soil, so that water is recycled; the water absorption channel is optimized, and the capillary layer of the U-shaped structure is designed, so that continuous water supply of thick soil and large-scale soil is achieved, and the method has high application potential in large-scale salinized soil remediation and saline-alkali soil treatment.

Description

Technical Field

[0001] This invention belongs to the field of solar energy device technology, specifically relating to a multi-stage solar distillation desalination device and its application in saline-alkali land management. Background Technology

[0002] Soil salinization poses a global ecological challenge, affecting approximately 1 billion hectares. Saline soils severely hinder plant growth and interfere with water absorption processes by creating highly permeable environments, ultimately leading to stunted growth and reduced crop yields. To address this, researchers have developed various saline soil remediation strategies. Irrigation leaching is the most widely used method, involving the injection of large amounts of freshwater to leach soluble salts from the root zone down into deeper soil layers or remove them from the field, thereby reducing soil salinity. However, irrigation leaching consumes large amounts of freshwater, and saline-alkali lands are often water-scarce, exacerbating the pressure on water-land relationships. Furthermore, the discharge of saline wastewater generated during irrigation leaching poses a significant and unavoidable environmental challenge.

[0003] In recent years, interfacial solar evaporators have demonstrated great potential in soil remediation. This technology converts sunlight into heat energy through photothermal materials and concentrates the heat energy at the evaporation interface, thus ensuring high photothermal conversion efficiency. When applied to saline soil remediation, the absorbent material transports brine from the soil to the evaporation interface through capillary action. As the water evaporates, the salt in the saline soil accumulates within the evaporator, thereby reducing the soil salinity. Compared to traditional irrigation and leaching methods, interfacial solar evaporation technology can efficiently remediate saline sites in situ while significantly reducing freshwater consumption. However, the inherent high latent heat of vaporization in the thermal evaporation process limits the evaporation rate, even under high photothermal conversion efficiency conditions. Since water is the primary medium for salt transport, the limited evaporation rate directly reduces the salt flux in the soil. Continuous evaporation of water into the atmosphere requires a continuous freshwater input to maintain system balance, leading to a significant increase in freshwater consumption. Furthermore, as an emerging technology, its scalability and adaptability have not yet been quantitatively assessed under real-world conditions—including spatially varying soil thickness and actual-scale remediation needs. Summary of the Invention

[0004] The purpose of this invention is to provide a multi-stage solar distillation desalination device and its application in saline-alkali land remediation. For a solar-powered evaporative desalination system, the following factors should be considered in its design: 1. Stable evaporative desalination. 2. Water recycling to reduce freshwater consumption. 3. High evaporative desalination rate. 4. Effective application to practical soil desalination, especially for thick soil layers and large-scale soil operations.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A multi-stage solar distillation desalination device comprises seven stages, each consisting of a heating layer, a capillary layer, an air gap layer, and a condensation layer from top to bottom. Each stage is stacked from top to bottom, with a heat-dissipating aluminum grid placed at the bottom of the seven stages. To reduce radiative heat loss in the heating layer, a low-emissivity photothermal material is used, such as a blue film, to convert solar energy into heat energy for heating and evaporating the salt solution. The capillary layer is made of non-woven fabric with numerous capillaries, ensuring the fabric's water absorption and transport capabilities. The left side of the non-woven fabric is inserted into the soil to absorb the salt solution. The middle section, serving as the evaporation zone, is placed directly below the heating layer, receiving heat to evaporate the salt solution and generate water vapor. The right side of the non-woven fabric extends outside the distillation device, contacting the air, where the salt solution crystallizes. The air gap layer consists of gaps formed by an insulating foam frame around its perimeter, through which the water vapor generated in the evaporation zone is transported to the condensation layer. The condensation layer, from top to bottom, comprises non-woven fabric and aluminum sheets. The nonwoven fabric is T-shaped. Water vapor condenses into liquid water on the nonwoven fabric in the condensation layer. The condensate flows back to the soil through the T-shaped protrusion of the nonwoven fabric, thus maintaining soil moisture. Water vapor releases heat during condensation, which in turn heats the aluminum plate. The aluminum plate, heated by the condensation heat, acts as the heating layer for the next stage of the device. This multi-stage design fully utilizes the condensation heat to improve thermal efficiency. To reduce convective heat loss between the heating layer and the air, plastic wrap and insulation cotton are used to create an air gap between the external air and the heating layer, preventing direct convective heat dissipation from the heating layer to the external air. The second, third, and seventh stages follow the same pattern. A heat-dissipating aluminum grid is placed at the bottom of the seventh stage (the lowest stage). The left side of each layer of nonwoven fabric is inserted into the soil (it can be grouped together on the left side and inserted into the soil, or inserted individually). Through capillary action, the salt solution existing in the soil capillary gaps is transported to the evaporation area of ​​the evaporator capillary layer for thermal evaporation. The resulting water vapor is transported downwards through the air gap layer and cooled into fresh water in the condensation layer. This fresh water is used to irrigate saline-alkali soil. After being heated and concentrated in the evaporation zone, the salt solution continues to be transported to the extension zone. In the extension zone, salt ions in the solution crystallize at the extension end once saturation is achieved. These salt crystals are collected at the extension end, thus removing salt from the soil. Crops can then grow normally in the remediated soil.

[0006] This invention demonstrates through experiments using a multi-stage distillation device for deep soil desalination that the device can stably and efficiently remediate highly saline soils. Furthermore, it verifies the practical application potential of multi-stage solar distillation devices in large-scale outdoor soil desalination for the remediation of saline soils. Therefore, this device can be applied to the treatment of saline-alkali land, providing a new approach for saline-alkali land management.

[0007] Advantages of this invention: This device addresses the low thermal efficiency of interfacial solar evaporation by leveraging multi-stage condensation heat, significantly improving thermal efficiency. Regarding the difficulty in steam collection, multi-stage solar distillation, with its reverse condensation design, efficiently converts steam into liquid water, solving the steam collection problem. During prolonged evaporation, crystalline salts easily accumulate within the evaporator; the capillary layer extension design gives multi-stage solar distillation the ability to repel salts, particularly allowing salt crystallization at the extended ends.

[0008] This device can achieve: 1. Stable evaporation desalination. 2. Water recycling, reducing freshwater consumption. 3. High evaporation desalination rate. 4. Effective application to thick soil layers and large-scale soil applications. In summary, the device features a multi-stage structure with optimized insulation performance to reduce energy loss and achieve latent heat recovery, thereby improving thermal efficiency; it achieves water recycling by collecting the freshwater generated by condensation to irrigate the soil; and it optimizes the water absorption channel to achieve continuous water supply for thick soil layers and large-scale soil applications. Attached Figure Description

[0009] Figure 1 : A multi-stage solar distillation device for desalinating saline soil. a: Schematic diagram of the solar distillation device operating on saline soil. The brine in the soil is desalinated through the multi-stage solar distillation device, and the salt is enriched at the edge of the evaporator, thereby reducing the salinity of the saline soil. b: Schematic diagram of salinity distribution within the non-woven fabric of the evaporation layer. c: Schematic diagram of the water circulation loop for rinsing the soil with desalinated water. d: The treated saline soil is now suitable for plant cultivation.

[0010] Figure 2 Material characterization. ad: Photographs of Mode I, Mode II, Mode III, and the multi-stage distillation apparatus, respectively. e: Scanning electron microscope (SEM) image of the nonwoven fabric. f: Scanning electron microscope (SEM) image of the CNT nonwoven fabric. g: Scanning electron microscope (SEM) image of porous soil. h: Absorption spectra of the blue film and CNT nonwoven fabric in the wavelength range of 300 nm to 2500 nm. i: Water absorption height of the fabric in soils with different moisture contents.

[0011] Figure 3 Design evolution of solar evaporators for saline soil remediation. ac: Schematic diagrams of Model I, Model II, and Model III. Concentration distribution in the fabric is shown at the bottom. d: Evaporation rate over time during soil desalination in Model I without additional water supply. e: Water production rate in the first hour during soil desalination in Model II. f: Water production rate curve during soil desalination in Model III (without additional water supply). Note: Water collected during the test was circulated and reinjected into the soil every hour. g: Salinity distribution in the nonwoven fabric of Models I, II, and III.

[0012] Figure 4Desalination performance of a multi-stage solar still. a: Schematic diagram of a multi-stage solar still for soil desalination. b: Temperature of each evaporation layer under single-day illumination during the desalination process of saline soil. c: Water production of the multi-stage solar still during the desalination process of saline soil. d: Changes in soil salinity during the remediation process. e: Comparison of water consumption for soil desalination using irrigation leaching and a multi-stage solar still. f: Comparison of crop growth in conventional soil, remediated saline soil, and untreated saline soil.

[0013] Figure 5 Multi-stage solar still for the remediation of thick saline soil. a: Schematic diagram of the multi-stage solar still for desalination, using U-shaped tubes to reduce the effective water absorption height to treat 30 cm thick saline soil. The numerical sequence indicates the order in which water flows into the fabric. b: Water production of the solar still. c: Changes in soil salinity at different depths during the remediation process. d: Crops growing in the remediated saline-alkali soil.

[0014] Figure 6 Outdoor large-scale saline soil remediation using a multi-stage solar distiller. a: On-site photos of outdoor large-scale saline soil desalination treatment. b: Temperature curves of the absorber and condenser of the solar distiller during outdoor testing. c: Solar radiation intensity and water production during the outdoor desalination process of the solar distiller. d: Changes in soil salinity during the large-scale soil remediation process. Detailed Implementation

[0015] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer with the description. However, unless otherwise specified, the specific experimental methods involved in the following embodiments are conventional methods or implemented according to the conditions recommended in the manufacturer's instructions.

[0016] Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art. Unless otherwise specified, the experimental methods in the following embodiments are all conventional methods. Unless otherwise specified, the reagents and materials used can be purchased commercially.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as are familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be used in this invention. The preferred embodiments and materials described herein are for illustrative purposes only. Example 1

[0018] One embodiment of the present invention is: a multi-stage solar distillation desalination device ( Figure 1a) The device comprises seven stages, each consisting of, from top to bottom, a heating layer 1, a capillary layer 2, an air gap 3, and a condensation layer 4. The heating layer, the first stage, is a blue film that acts as a photothermal material, absorbing sunlight and converting it into heat. The heating layer has an area of ​​5 cm × 5 cm. The capillary layer is a non-woven fabric. The left side of the non-woven fabric is inserted into the soil to absorb salt solution. The middle part of the non-woven fabric serves as the evaporation zone, with an area of ​​5 cm × 5 cm. This evaporation zone is located below the heating layer and is in direct contact with it. The heat required for evaporation is provided by the heating layer, and the steam generated during evaporation diffuses downwards to the condensation layer. The right side of the non-woven fabric extends outside the device as an extension zone, with an area of ​​4 cm × 5 cm. Because this extension zone is in direct contact with the air, the steam generated during evaporation directly escapes into the air. The air gap layer consists of a 5 cm × 5 cm spacer with a thickness of 4 mm, and the insulating foam is 2 cm wide and 4 mm thick. Water vapor generated in the evaporation zone diffuses into the condensation layer through the air gap. The condensation layer, from top to bottom, comprises non-woven fabric (untreated non-woven fabric), an aluminum sheet, and a T-shaped structure. The non-woven fabric on the aluminum sheet has a surface area of ​​5 cm × 5 cm, the protruding end has a surface area of ​​1 cm × 15 cm, and the aluminum sheet itself has a surface area of ​​7 cm × 7 cm. Water vapor condenses into liquid water on the non-woven fabric in the condensation layer. The condensate flows back to the soil through the T-shaped protruding end of the non-woven fabric, thus maintaining soil moisture, or flows into the distillation apparatus. The water vapor releases heat during condensation, thereby heating the aluminum plate. The aluminum plate, heated by the condensation heat, acts as the heating layer for the next stage of the apparatus. Therefore, starting from the second stage, the heating layer is the aluminum sheet of the condensation layer from the previous stage.

[0019] The specific experimental steps of this invention are as follows: 1. Materials and Methods 1.1 The blue membrane was provided by Dezhou Jinheng New Energy Co., Ltd. Non-woven fabric and thermal insulation foam were purchased from Taobao. Saline-alkali soil was sourced from Dongying City, Shandong Province, China (118.9°E, 37.6°N). Carbon nanotube (S) powder was provided by Chengdu Organic Chemical Co., Ltd.

[0020] 1.2 Material Characterization The morphological characteristics of the samples were characterized using scanning electron microscopy (SEM, Hitachi Regulus 8100). The light absorption of the samples in the wavelength range of 250–2500 nm was quantitatively analyzed using a Hitachi UH4150 spectrophotometer. X-ray diffraction (XRD) patterns of the saline soil were obtained using a Bruker AXS D8 Advance diffractometer.

[0021] 1.3 Preparation of CNT nonwoven fabric A black nonwoven fabric (CNT nonwoven fabric) was prepared by immersing a nonwoven fabric in a 10 mg / mL carbon nanotube suspension for 5 minutes and then drying it in an oven at 60°C for 30 minutes. This process was repeated twice to ensure uniform loading of carbon nanotubes onto the nonwoven fabric. After repeated immersion, carbon nanotubes were uniformly loaded onto the surface of the nonwoven fabric at a loading rate of 1.5 mg / cm².

[0022] 1.4 Soil Preparation Soil samples obtained from saline-alkali land were dried in an oven at 60 degrees Celsius and then sieved through a 2 mm nylon mesh sieve. Since the average soil moisture content in most areas is typically between 2% and 40%, four soil samples with moisture contents of 10%, 20%, 30%, and 40% (mass fraction) were prepared by adding deionized water to the dried soil.

[0023] The above four soil samples were used when measuring the water absorption capacity of nonwoven fabrics. Dry soil was used in conventional irrigation experiments. A 40% (mass fraction) soil sample was used in laboratory-scale thin-layer soil desalination experiments, laboratory-scale thick-layer soil desalination experiments, and large-scale outdoor thick-layer soil desalination experiments.

[0024] In traditional irrigation experiments, 180 g of soil was placed in an acrylic cube container measuring 6 cm × 6 cm × 5 cm. In laboratory-scale thin-layer soil desalination experiments, 180 g of soil was placed in an acrylic cube container measuring 6 cm × 6 cm × 5 cm. In laboratory-scale thick-layer soil desalination experiments, 1080 g of soil was placed in an acrylic cube container measuring 6 cm × 6 cm × 30 cm. In large-scale outdoor thick-layer soil desalination experiments, 17000 g of soil was placed in a container measuring 55 cm × 18 cm × 17 cm.

[0025] 1.5 Assembly of Solar-Powered Desalination Unit We designed four types of solar-driven soil desalination devices, including three single-stage devices and one multi-stage device, all of which desalinate the soil through solar-driven evaporation. The three single-stage solar-driven soil desalination devices are: an interfacial evaporation desalination device (Mode I), a single-layer distillation desalination device with no capillary extension (Mode II), and a single-layer distillation desalination device with an extended capillary layer (Mode III). To improve the evaporation desalination rate and the device's thermal efficiency, we designed Mode III as a multi-stage device, namely a multi-stage solar distillation desalination device (MSDD).

[0026] Model I is a black CNT nonwoven fabric with numerous capillaries. The left side of the nonwoven fabric is inserted into the soil to absorb salt solution. The middle part of the nonwoven fabric serves as the evaporation zone, with an area of ​​9 cm × 5 cm. The heat required for evaporation is provided by the black CNT nonwoven fabric itself. Because the evaporation zone is in direct contact with the air, the vapor generated during evaporation escapes directly into the air.

[0027] Mode II consists of, from top to bottom, a heating layer, a capillary layer, an air gap, and a condensation layer. The heating layer is a blue film, which acts as a photothermal material, absorbing sunlight and converting it into heat. The heating layer has an area of ​​5 cm × 5 cm. The capillary layer is a black CNT nonwoven fabric. The left side of the nonwoven fabric is inserted into the soil to absorb salt solution. The middle part of the nonwoven fabric serves as the evaporation zone, with an area of ​​9 cm × 5 cm. This evaporation zone is located below the heating layer and is in direct contact with it. The heat required for evaporation is provided by the heating layer, and the vapor generated during evaporation diffuses downwards to the condensation layer. The air gap is a void formed by an insulating foam frame. The air gap has an area of ​​5 cm × 5 cm and a thickness of 4 mm, while the insulating foam is 2 cm wide and 4 mm thick. Water vapor generated in the evaporation zone diffuses into the condensation layer through the air gap. The condensation layer is composed of nonwoven fabric and an aluminum sheet. The nonwoven fabric has an area of ​​5 cm × 5 cm, and the aluminum sheet has an area of ​​7 cm × 7 cm. Water vapor condenses into liquid water on the nonwoven fabric of the condensation layer, and the liquid water flows out of the distillation apparatus through a conduit.

[0028] Mode III consists of, from top to bottom, a heating layer, a capillary layer, an air gap, and a condensation layer. The heating layer is a blue film, which acts as a photothermal material, absorbing sunlight and converting it into heat. The heating layer has an area of ​​5 cm × 5 cm. The capillary layer is a black CNT nonwoven fabric. The left side of the nonwoven fabric is inserted into the soil to absorb salt solution. The middle part of the nonwoven fabric serves as the evaporation zone, with an area of ​​9 cm × 5 cm. This evaporation zone is located below the heating layer and is in direct contact with it. The heat required for evaporation is provided by the heating layer, and the vapor generated during evaporation diffuses downwards to the condensation layer. The right side of the nonwoven fabric extends outside the device as an extension zone, with an area of ​​4 cm × 5 cm. The heat required for evaporation is provided by the black CNT nonwoven fabric itself. Because the extension zone is in direct contact with the air, the vapor generated during evaporation directly escapes into the air. The air gap is a void formed by a surrounding insulating foam frame, with an area of ​​5 cm × 5 cm and a thickness of 4 mm. The insulating foam is 2 cm wide and 4 mm thick. Water vapor generated in the evaporation zone diffuses into the condensation layer through the air gap. The condensation layer is composed of non-woven fabric and aluminum sheet, with the non-woven fabric having an area of ​​5 cm × 5 cm and the aluminum sheet having an area of ​​7 cm × 7 cm. Water vapor condenses into liquid water on the non-woven fabric of the condensation layer, and the liquid water flows out of the distillation apparatus through the conduit.

[0029] The multi-stage solar distillation desalination device consists of seven stages. Each stage, from top to bottom, comprises a heating layer, a capillary layer, an air gap, and a condensation layer. The first stage's heating layer is a blue film, which acts as a photothermal material, absorbing sunlight and converting it into heat. This heating layer has an area of ​​5 cm × 5 cm. The capillary layer is a black non-woven fabric. The left side of the non-woven fabric is inserted into the soil to absorb the salt solution. The middle part of the non-woven fabric serves as the evaporation zone, with an area of ​​5 cm × 5 cm. This evaporation zone is located below the heating layer and is in direct contact with it. The heat required for evaporation is provided by the heating layer, and the steam generated during evaporation diffuses downwards to the condensation layer. The right side of the non-woven fabric extends outside the device as an extension zone, with an area of ​​4 cm × 5 cm. The heat required for evaporation is provided by the black non-woven fabric itself. Because the extension zone is in direct contact with the air, the steam generated during evaporation directly dissipates into the air. The air gap is a void surrounded by an insulating foam frame, with an area of ​​5 cm × 5 cm and a thickness of 4 mm. The insulating foam is 2 cm wide and 4 mm thick. Water vapor generated in the evaporation zone diffuses into the condensation layer through the air gap. The condensation layer consists of non-woven fabric (5 cm × 5 cm) and aluminum sheets (7 cm × 7 cm). Water vapor condenses into liquid water on the non-woven fabric of the condensation layer, and the liquid water flows out of the distillation apparatus through the guide tube. The water vapor releases heat during condensation, which in turn heats the aluminum sheet. The aluminum sheet, heated by the condensation heat, acts as the heating layer for the next stage of the apparatus. Therefore, starting from the second stage, the heating layer is the aluminum sheet of the condensation layer from the previous stage.

[0030] The blue membrane of the multi-stage solar distillation desalination device is designed with a transparent cover air gap to prevent the added layer from convecting and dissipating heat to the air. The transparent cover air gap is a gap formed by a heat insulation foam frame on all sides, and is covered with a transparent plastic wrap. The transparent cover air gap has an area of ​​5 cm × 5 cm and a thickness of 4 mm, and the heat insulation foam is 2 cm wide and 4 mm thick.

[0031] 1.6 Solar-Powered Soil Desalination A desalination device was placed above the soil in the laboratory, and then non-woven fabric was inserted into the soil to provide moisture, completely sealing the soil. The evaporation process was then measured under illumination from a uniform xenon lamp light source system (CEL-PF3000-TEB) equipped with a CEL-AM 1.5 filter. A CEL-NP2000 optical power density meter was used to determine the light intensity on the evaporation surface and calibrated by current changes. Temperature was recorded throughout using an infrared camera (FLIRE 64501). Temperature was continuously monitored using thermocouples and recorded at one-second intervals using a data logger (TC-08). Mass changes were precisely measured using a Mettler Toledo analytical balance (model ME204) and recorded every 5 seconds. All indoor evaporation tests were conducted under laboratory environmental conditions (approximately 25°C and approximately 50% relative humidity). All water used in this experiment was deionized (DI) water.

[0032] 1.7 Measurement of Salt Concentration Before assembling the solar-powered salt extractor, the quality and dimensions of the fabric or carbon nanotube fabric were rigorously evaluated. After the salt extractor operated for a specific period, the fabric or carbon nanotube fabric was carefully cut into uniformly sized samples along its length. The area of ​​these samples was then precisely measured, followed by weighing and drying.

[0033] Salt concentration (ω) f The calculation formula for ) is as follows:

[0034] Where m0 is the weight of the fabric (grams), m1 is the weight of the sample after evaporation (grams), m2 is the weight of the sample after drying (grams), A is the area of ​​the sample (square centimeters), and A0 is the area of ​​the fabric (square centimeters).

[0035] 1.8 Measurement of Soil Salinity To measure soil salinity, a 1 cm × 1 cm × 1 cm soil sample was collected from near a cotton fabric and dried in an oven. The soil was then mixed with water at a 1:5 ratio. After allowing sufficient settling time, a portion of the supernatant was collected and dried. Soil salinity (ω) was then measured. s The calculation method for () is as follows:

[0036] Where m1 represents the total mass of the dried supernatant and the petri dish, m0 represents the mass of the petri dish, m represents the mass of the dried soil sample, and D represents the mass ratio of total water to the extracted supernatant.

[0037] 1.9 Thermal efficiency of solar multi-stage distillation desalination unit

[0038] in, h represents the water production rate. fg Indicates latent heat, Q sun This represents optical power density.

[0039] 2 Results and Discussion 2.1 Characterization Figure 2 The ad showcased real-world photos of the equipment used for saline soil remediation. All desalination devices used non-woven fabric to absorb salt solution from the soil; Modes II, III, and the multi-stage distillation unit used virgin non-woven fabric. Figure 2 e) The nonwoven fabric is made of cellulose fibers with a diameter of approximately 10 μm, which enhances the transport of salt solutions. Mode I uses CNT nonwoven fabric ( Figure 2 f), the CNT-treated nonwoven fabric has 2-5 micrometer CNT particles attached to the cellulose fibers. Mode I uses CNT nonwoven fabric as the light-absorbing material, while Modes II, III, and the multi-stage distillation unit use blue film as the light-absorbing material, achieving solar energy absorption rates of 96% and 95%, respectively. Figure 2 Soil is a porous material, mainly composed of SiO2, Al2(SO4)3 and CaCO3 (h). Figure 2 g). The porous soil and fabric form continuous capillary bridges, enabling the nonwoven fabric to absorb salt solution from the soil. Soil moisture content affects the nonwoven fabric's ability to absorb water from the soil (Figure 2i). As soil moisture content decreases, the amount of salt solution within the soil capillaries decreases, and the water absorption height of the CNT nonwoven fabric decreases due to the reduced water supply. Subsequent desalination experiments were conducted in soil with saturated moisture content (40%).

[0040] 2.2 Design Evolution of Solar Evaporators for Saline Soil Remediation In the traditional interface solar evaporator mode I (Figure 3a), during soil desalination, the brine in the soil is transported to the evaporation interface and evaporates directly into the air under sunlight. As the water evaporates, the salt accumulates on the evaporator surface, thus achieving continuous soil desalination. Figure 3 The bottom of a shows the simulated salinity distribution, compared with the experimental salinity distribution. Figure 3 g) Presented side-by-side. Mode I achieves an evaporation rate of 0.88 kg m³ in the first hour. -2 h -1 As soil moisture content continues to decrease, the evaporation rate gradually decreases. Figure 3 d). Therefore, this mode requires a continuous supply of fresh water to maintain stable operation. To reduce water consumption, water vapor needs to be collected, which led to the design of Mode II (Figure 3b). Mode II achieved a water production rate of 0.925 kg m³ in the first hour. -2 h -1 ( Figure 3 e) During evaporation, salt deposits in the inner fabric of the distiller, leading to salt blockage. Figure 3 b and Figure 3 g). For example Figure 3 As shown in diagram c, to improve operational stability, the fabric extends outwards from the still. This structure allows salt to deposit at the edges of the fabric, preventing clogging inside the still. Figure 3 a and Figure 3 g). Mode III achieved a water production rate of 0.89 kg m³ in the first hour. -2 h -1 The collected water can be recycled into the saline soil to maintain its moisture content, ensuring stable operation of the distillation process for eight consecutive hours. Figure 3 f).

[0041] 2.3 Multistage distillation unit for soil desalination To further improve the desalination rate of Mode III, a multi-stage solar-powered distillation unit was constructed to recover the latent heat of vaporization of water, thereby increasing the water production. The increased water production rate of the distillation unit leads to a greater volume of water available for soil leaching. A schematic diagram of the seven-stage distillation desalination device of this invention (Figure 3a) was shown. Inserted into a 5cm thick layer of soil, under one solar irradiance, we measured the temperature distribution and freshwater collection rate in the sealed evaporation zone of the multi-stage distillation device. The temperature distribution is as follows: Figure 4 As shown in b, the temperature of the first-stage evaporation layer stabilizes at around 70℃ after 1 hour. Without external water supply, the device achieves a freshwater collection rate of 3 kg / m³ over 8 hours. -2 h -1 (Figure 4c) A 200% thermal efficiency was achieved in the soil. After six days of continuous operation of the multi-stage distillation unit, the soil salinity decreased from 22.5 g / kg to 1.49 g / kg (Figure 4d), a reduction of 93.4%. This is significantly lower than other desalination methods, such as leaching, which consumes 97.2 g / kg of water. -2 The multi-stage distillation unit consumes only 16.7 gm³ of water. -2 (Fig. 4e). Under the same breeding and irrigation conditions, the germination rate and growth of ryegrass in the treated saline-alkali soil were comparable to those in ordinary soil, while no vegetation growth was observed in the untreated saline-alkali soil (Fig. 4f). These results indicate that multi-stage solar distillers can achieve stable and efficient salt extraction with extremely low water consumption.

[0042] 2.4 Multistage distillation unit for deep soil desalination Deep soil desalination is more challenging than thin soil desalination, mainly for two reasons. First, water has difficulty penetrating downwards in the soil. Second, due to limitations in the water absorption capacity of materials, increasing soil thickness is equivalent to increasing the water supply height, which makes it difficult for brine from thick soil layers to reach the evaporation zone and extract salt from the thick soil, resulting in inconsistent salt content along the vertical direction of the soil. To address these issues, a U-shaped water supply method using a capillary layer (the capillary layer can be made of non-woven fabric; seven layers of non-woven fabric are joined together and inserted into the soil from one end, then wound into a U-shape and extended to the soil surface) is proposed. Figure 5 As shown in Figure a, the left side of the U-shape is sealed with a polytetrafluoroethylene (PTFE) membrane, while the right side is exposed in the soil. This allows water to penetrate deep into the soil layer via the right-end capillary tube under gravity, while also utilizing the principle of communicating vessels to reduce the water absorption height, thus achieving deep soil treatment. The device of this invention was inserted into the soil to a thickness of 30 cm, and the freshwater collection rate of the sealed evaporation zone of the multi-stage distillation device was measured under 1 solar radiation intensity. The freshwater collection rate of the multi-stage distillation device also reached 3 kg / m³ when desalinizing deep soil. -2 h -1 (Figure 5b). After 12 days of continuous operation, the soil salinity at three different locations decreased from 22.5 g / kg to 2.33 g / kg, 2.68 g / kg, and 3.35 g / kg, respectively (Figure 5c), representing reductions of 89.6%, 88.1%, and 85.1%, with an average desalination rate of 87.6%. The germination rate and growth of ryegrass in the treated saline-alkali soil were comparable to those in ordinary soil (Figure 5d). This demonstrates that the equipment can stably and efficiently remediate highly saline soils.

[0043] 2.3 Multistage distillation unit for large-scale outdoor soil desalination To further investigate the performance of a large still (50 cm × 50 cm in area) under real weather conditions, an outdoor experiment was conducted on a sunny day. Figure 6 As shown in Figure a, the experimental setup is located on the third-floor corridor of the School of Chemistry and Chemical Engineering at Jinan University.

[0044] A large-scale still was used for soil desalination to test its evaporative desalination performance. An outdoor experiment was conducted on October 14, 2025, where we measured the freshwater collection rate in the sealed evaporation zone of the large-scale still. The daily freshwater collection rate reached 4080 kg / m³. - 2 h -1 (Figure 6d). After 14 days of continuous operation, the soil salinity decreased from 22.5 g / kg to 2.3 g / kg, a reduction of 90% (Figure 6e). These results confirm the practical application potential of multi-stage solar distillation in the remediation of large-scale saline soils.

[0045] The embodiments described above are merely preferred embodiments of the present invention and are only used to explain the present invention. They are not intended to limit the scope of the present invention. For those skilled in the art, other implementation methods can be easily made by substitution or modification based on the technical content disclosed in this specification. Therefore, all changes and improvements made on the principle of the present invention should be included within the scope of the patent application of the present invention.

Claims

1. A multi-stage solar distillation desalination device, characterized in that, The device comprises seven stages, each stage consisting of a heating layer (1), a capillary layer (2), an air gap layer (3), and a condensation layer (4) from top to bottom. Each stage is stacked from top to bottom, with a heat dissipation aluminum grid placed at the bottom of the seventh stage. The heating layer (1) converts solar energy into thermal energy to heat and evaporate the salt solution. One end of the capillary layer (2) is inserted into the soil to absorb the salt solution from the soil. The air gap layer (3) is a porous structure with a heat-insulating foam frame on all sides. The condensation layer (4) consists of non-woven fabric (41) and aluminum sheet (42) from top to bottom.

2. The multi-stage solar distillation desalination device according to claim 1, characterized in that, The heating layer uses a low-emissivity photothermal material to absorb sunlight and convert it into heat.

3. The multi-stage solar distillation desalination device according to claim 2, characterized in that, The photothermal material is a blue film.

4. The multi-stage solar distillation desalination device according to claim 1, characterized in that, The capillary layer (2) is a non-woven fabric with capillary pores, which ensure the non-woven fabric's water absorption and water transport capabilities.

5. The multi-stage solar distillation desalination device according to claim 1, characterized in that, The nonwoven fabric is divided into a left part, a middle part, and a right part. The left part of the nonwoven fabric is inserted into the soil, the middle part is placed directly below the heating layer as an evaporation area to receive heat from the heating layer and evaporate the salt solution to produce water vapor; the right part extends out of the distillation device and comes into contact with the air, where the salt solution crystallizes at the extended end.

6. The multi-stage solar distillation desalination device according to claim 1, characterized in that, The capillary layer (2) is designed as a U-shaped structure (21), with the left side of the U-shaped structure (21) sealed with a polytetrafluoroethylene film and the right side of the U-shaped structure (21) exposed in the soil.

7. The multi-stage solar distillation desalination device according to claim 1, characterized in that, The nonwoven fabric (41) is T-shaped and is used to return condensed liquid water to the soil.

8. The application of the multi-stage solar distillation desalination device according to claims 1-7 in the treatment of saline-alkali land.

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