A bifunctional janus membrane for multi-component seawater separation and recovery

Abstract

The application relates to a bifunctional Janus membrane for multi-component seawater separation and recovery, and belongs to the field of membrane separation technology. The novel porous Janus photothermal separation membrane has a reverse super-wetting design (one side super-hydrophobic / super-oleophilic, and one side super-hydrophilic / super-oleophobic), has a dual function of solar seawater desalination (for separating and purifying water and salt crystals) and oil-water separation (for separating light oil and heavy oil), and can realize efficient and stable separation and recovery of four components in oil-contaminated seawater. Meanwhile, the application proposes a novel method for reducing graphene oxide on the surface of a porous material by using an ultrashort pulse femtosecond laser. The method can realize reduction of graphene oxide and effectively control the surface morphology of the graphene oxide, so that the separation performance of the graphene oxide is more excellent. The application has great practical application prospect in the field of contaminated seawater separation and recovery.

Description

A bifunctional Janus membrane for multi-component seawater separation and recovery Technical Field

[0001] This invention relates to a bifunctional Janus membrane for the separation and recovery of multi-component seawater, belonging to the field of membrane separation technology. Background Technology

[0002] Freshwater scarcity has become the second leading environmental problem after global warming. Currently, there are many commercially available seawater desalination methods, such as distillation, seawater freezing, electrodialysis, and reverse osmosis. However, these methods suffer from high energy consumption, enormous costs, and significant environmental damage. Solar energy is a green, abundant, and sustainable energy source. Solar-powered interfacial seawater evaporation, through localized heating of the water-air interface, greatly improves evaporation efficiency. Porous photothermal films, composed of a substrate and photothermal materials, are the core component of interfacial evaporators due to their rich functionality, low cost, and simple fabrication.

[0003] Due to oil spills from ships and the discharge of industrial and domestic sewage, seawater contains a large amount of oil-water mixture. As oil-containing seawater separation proceeds, excess components (such as inorganic salts and oil) gradually accumulate, inevitably forming crystals or scale, leading to capillary gap blockage and membrane corrosion. This hinders light absorption, water transport, and steam escape, ultimately causing evaporator failure after long-term operation. Simultaneously, volatile oils can also enter the purified water along with water vapor, resulting in impure collected water. Therefore, traditional single-wetting photothermal membranes struggle to efficiently and stably separate high-quality pure water.

[0004] The effective combination of photothermal and wetting materials represents a future trend in solar-powered seawater desalination. By designing and adjusting the interfacial wettability of porous photothermal films, richer functionalities can be achieved. Furthermore, the recovery of excess components also offers significant economic benefits; it is meaningful to recover other destructive components (such as inorganic salts and oils) while simultaneously recovering pure water. However, oil-contaminated seawater contains four components: pure water, salt crystals, light oil (less dense than water), and heavy oil (more dense than water). Spontaneously separating and recovering these four components using current material systems is extremely difficult.

[0005] Preparing porous photothermal films with varying wettability is a challenge in this field. Coating the surface of a porous film with reduced graphene oxide (PBO) can effectively control surface wettability, but directly coated PBO does not bond firmly to the substrate. Therefore, a common approach is to first coat the porous film with PBO, ensuring a strong bond, and then reduce it back to PBO. Current reduction methods primarily include high-temperature reduction, microwave reduction, and chemical reduction. While these methods can effectively reduce PBO, PBO is a multilayer sheet structure. Coating it onto a porous substrate inevitably leads to pore blockage, affecting the inherent porous properties of the material. Furthermore, the photothermal and wettability properties of the porous photothermal film are closely related to its surface morphology. Therefore, a novel method is urgently needed for this composite material surface that can both effectively reduce PBO and control its surface morphology. Summary of the Invention

[0006] The purpose of this invention is to solve the technical problem of spontaneous separation and recovery of multiple components in oil-polluted seawater. It provides a porous Janus photothermal membrane with reverse wetting properties. This membrane is superhydrophobic / superoleophilic on one side and superhydrophilic / superoleophobic on the other side. It has dual functions of solar-powered seawater desalination and oil-water separation. It can use solar energy to spontaneously achieve efficient and stable separation and recovery of four components in oil-polluted seawater.

[0007] The objective of this invention is achieved through the following techniques.

[0008] A bifunctional Janus membrane for multi-component seawater separation and recovery is a porous Janus photothermal membrane with reverse wetting properties. The membrane is superhydrophobic / superoleophilic on one side and superhydrophilic / superoleophobic on the other side, and has dual functions of solar seawater desalination and oil-water separation. It can achieve efficient and stable separation and recovery of four components in oily polluted seawater.

[0009] The Janus membrane uses a glass fiber membrane as a substrate, and graphene oxide is uniformly sprayed onto it using a spray gun. A photothermal layer is prepared by reducing the graphene oxide using a femtosecond laser.

[0010] The surface morphology of graphene oxide can also be effectively controlled when it is reduced by femtosecond laser.

[0011] A method for preparing a bifunctional Janus membrane for multi-component seawater separation and recovery, the specific steps of which are as follows;

[0012] Step 1: Select a glass fiber membrane as the substrate (superhydrophilic / superoleophobic) and spray graphene oxide onto its surface using a spray gun. At this point, one side of the glass fiber membrane consists of multi-layered sheet-like graphene oxide.

[0013] Step 2: Use a femtosecond laser to generate Gaussian femtosecond laser pulses, and use a cylindrical mirror to spatially shape the beam into a line-focused beam (to improve processing efficiency).

[0014] Step 3: Apply a shaping femtosecond laser to the reduced graphene oxide coating obtained in Step 1, adjusting the laser energy to near the ablation threshold of the graphene oxide (which is much higher than the reduction threshold). At this point, the energy distribution on the undulating graphene oxide surface varies. Utilizing this energy difference, selective ablation is performed on the graphene oxide while reducing it, controlling the pore size and shape on the photothermal film surface. This side then exhibits superhydrophobic / superoleophilic properties. It is worth noting that the energy used in this method is much lower than the ablation threshold of the high-strength glass fiber film; therefore, this method does not damage the substrate.

[0015] Step 4: The Janus membrane obtained in Step 4 is used to form a system for the separation and recovery of oil-contaminated seawater. The system is divided into oil-water separation and seawater desalination areas.

[0016] Step 5: In the oil-water separation zone, when the oily seawater passes through the Janus membrane, the light oil, seawater, and heavy oil separate from top to bottom. The heavy oil is separated and recovered after contacting the reduced graphene oxide zone. The remaining mixture contacts the next-stage inverted Janus membrane, where the light oil and seawater separate from top to bottom. The seawater contacts the glass fiber membrane zone and enters the next-stage seawater desalination zone, where the light oil is separated and recovered.

[0017] Step six: Seawater comes into contact with a suspended Janus membrane in the desalination area and evaporates on both sides under solar power for purified water recovery. Salt crystallizes on the lower side of the Janus membrane and is then recovered. At this point, purified water, salt crystals, light oil, and heavy oil are separated and recovered separately.

[0018] Beneficial effects

[0019] 1. In terms of functional design, this invention innovatively designs a novel porous Janus photothermal membrane with a reverse wettability design (one side is superhydrophobic / superoleophilic, and the other side is superhydrophilic / superoleophobic). This Janus membrane simultaneously possesses dual functions of solar-powered seawater desalination (for separating pure water and salt crystals) and oil-water separation (for separating light and heavy oil), and can spontaneously achieve efficient and stable separation and recovery of four components in oil-contaminated seawater.

[0020] 2. Regarding material preparation, this invention proposes a novel method for reducing graphene oxide on the surface of porous materials using an ultrashort pulse femtosecond laser. This method effectively reduces graphene oxide while simultaneously controlling its surface morphology. Specifically, porous materials are heterogeneous, with varying thicknesses of the graphene oxide coating on their surfaces, while the femtosecond laser focus is fixed in the vertical direction. Therefore, the laser energy can be controlled near the ablation threshold of graphene oxide (much greater than the reduction threshold), utilizing the energy differences across regions to achieve selective ablation of the graphene oxide, thereby enabling the control of its surface morphology.

[0021] 3. This method of reducing graphene oxide morphology can effectively prevent pore blockage caused by traditional methods, and improve photothermal performance (enhanced solar photon absorption) and wettability by controlling its surface morphology.

[0022] 4. In addition, replacing traditional Gaussian laser shaping with line-focused laser greatly improves the processing (reduction) efficiency of graphene oxide. Attached Figure Description

[0023] Figure 1 shows the processing optical path diagram of femtosecond laser for spatial shaping (point focusing to line focusing);

[0024] Figure 2 is a flowchart of the Janus membrane preparation process;

[0025] Figure 3 is a schematic diagram of the bifunctionality of the Janus membrane;

[0026] Figure 4 is a flowchart of solar-powered seawater separation process;

[0027] Figure 5 is a schematic diagram of the oil-water separation process;

[0028] Figure 6 is a flowchart of the separation and recovery of multi-component polluted seawater;

[0029] Figure 7 shows the physical images of the components collected after the separation experiment.

[0030] In the diagram: 1-Femtosecond laser, 2-Aperture, 3-Attenuator, 4-Imaging system, 5-White light, 6-Beam splitter, 7-Dichroic mirror, 8-Switch, 9-Objective lens, 10-Control computer, 11-Translation stage. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0032] This invention proposes a method for preparing bifunctional Janus membranes based on femtosecond laser technology, and applies it to the separation and recovery of multi-component seawater. The specific steps are as follows:

[0033] Step 1: Construct a femtosecond laser processing and observation system, as shown in Figure 1. This system mainly includes a femtosecond laser (1), an aperture (2), an attenuator (3), an imaging system (4), a white light source (5), a beam splitter (6), a dichroic mirror (7), a switch (8), an objective lens (9), a control computer (10), and a translation stage (11). Then, turn on the laser (center wavelength 800nm; pulse width 35fs; repetition frequency 1kHz). At this point, the beam focused by objective lens 9 is a point-focused Gaussian beam. Finally, replace the ordinary objective lens with a cylindrical lens. This reshapes the ordinary Gaussian beam into a line-focused beam, enabling high-efficiency processing.

[0034] Step 2: Figure 2 shows the preparation flow chart of the bifunctional Janus membrane.

[0035] 2.1 Substrate selection: High temperature and corrosion resistant glass fiber membrane (SiO2 content >99.9%) is selected as the substrate, and the substrate has excellent mechanical properties and can maintain its original stable state under suspension conditions;

[0036] 2.2 Preparation of graphene oxide: 3 g of natural graphite (325 mesh) and 60 mL of concentrated sulfuric acid (98 wt.%) were thoroughly mixed and stirred at 0 °C for 30 min. Then, 9 g of potassium permanganate was added to the mixture several times. The reaction mixture was transferred to a water bath and heated to 90 °C. 500 mL of deionized water was added, followed by 15 mL of hydrogen peroxide (30 wt.%) until the solution turned orange-brown. The solution was thoroughly filtered and washed with 200 mL of hydrochloric acid (3.7 wt.%). The dispersed solution was thoroughly stirred for 12 h, and centrifuged to remove unexposed graphite. Next, the dispersed solution was placed in a dialysis bag for 7 days to remove residual ions, and the resulting graphene oxide was obtained by centrifugation and compression.

[0037] 2.3 Graphene oxide coating: Dilute graphene oxide to 2 mg / mL -1 The material is then introduced into a spray gun, and graphene oxide is uniformly coated onto the glass fiber membrane. At this point, due to the interaction of oxygen-containing functional groups in the material, the graphene oxide and the glass fiber membrane are firmly bonded together.

[0038] 2.4 Graphene Oxide Reduction: A linearly focused femtosecond laser was applied to graphene oxide, causing it to change from brown to black. The laser flux at this point was 20 J / cm². -2 The scanning speed is 100 μm / s;

[0039] At this point, the Janus membrane with a specific morphology has been prepared.

[0040] Step 3, as shown in Figure 3, the Janus membrane has dual functions: solar seawater desalination (for separating purified water and salt crystals) and oil-water separation (for separating light oil and heavy oil).

[0041] 3.1 Solar energy is required to drive the desalination process. In this invention, the reduced graphene oxide on the Janus membrane is a photothermal material, which can achieve excellent photothermal conversion performance;

[0042] 3.2 In this invention, the reduced graphene oxide on the Janus membrane is a hydrophobic material, and the rough structure after femtosecond laser processing is significantly improved to superhydrophobic properties; the glass fiber membrane side is a superhydrophilic material.

[0043] 3.3 Therefore, in the solar-powered seawater desalination process, as shown in Figure 4, a high-mechanical-performance glass fiber membrane is stably suspended in the center of the water tank, while hydrophilic fibers below pump water via capillary action. Solar energy is converted into heat energy to provide the power for seawater evaporation. At this time, water vapor escapes from both the top and bottom sides and is recovered as pure water. Because the top side is superhydrophobic, salt can only accumulate at the bottom. Over time, the salt is crystallized into blocks and then recovered.

[0044] 3.4 As shown in Figure 5, the Janus membrane's reduced graphene oxide region has hydrophobic and oleophilic properties, enabling the separation of heavy oil and pure water; while the glass fiber membrane region has superhydrophilic / superoleophobic properties, enabling the separation of light oil and pure water.

[0045] Step 3, as shown in Figure 6, involves the design and fabrication of a simple multi-component integrated separator for polluted seawater. This separator has two functional modules: solar desalination (located in the middle) and oil-water separation (located on either side). The selected oil-polluted seawater contains four components (pure water, salt crystals, light oil, and heavy oil). First, the oil-polluted seawater is introduced into the oil-water separation module. In this module, stratification occurs due to differences in density, with heavy oil being separated first due to its highest density. Subsequently, the remaining mixture is sent to the next zone, where light oil, due to its lowest density, is separated first. Finally, the remaining seawater is sent to the solar desalination module. Water vapor escapes under solar energy and is recovered as pure water. Salt crystals are also gradually recovered. At this point, all four components of the oil-polluted seawater are efficiently separated and recovered.

[0046] As shown in Figure 7, after the separation experiment, the components in the polluted seawater and the Janus membrane were collected. The concentration of ions (Na+) in the oil-contaminated seawater before separation is shown in Figure 7. + K + Ca 2+ Mg 2+ The ion content after separation is very high, and the ion content is far below the standard set by the World Health Organization (WHO); the Janus membrane surface is clean and undamaged, and can be reused; the blocky salt crystals, light oil and heavy oil are separated and recycled respectively.

[0047] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A bifunctional Janus membrane for multi-component seawater separation and recovery, characterized in that: It is a porous Janus photothermal membrane with reverse wetting properties. One side of the membrane is superhydrophobic / superoleophilic and the other side is superhydrophilic / superoleophobic. It has dual functions of solar seawater desalination and oil-water separation, and can achieve efficient and stable separation and recovery of four components in oil-polluted seawater. The Janus membrane uses glass fiber membrane as substrate, and graphene oxide is uniformly sprayed by spray gun. The photothermal layer is prepared by reducing graphene oxide with femtosecond laser.

2. The bifunctional Janus membrane for multi-component seawater separation and recovery as described in claim 1, characterized in that: The surface morphology of graphene oxide can also be effectively controlled when it is reduced by femtosecond laser.

3. The bifunctional Janus membrane for multi-component seawater separation and recovery as described in claim 2, characterized in that: The preparation method includes the following steps: Step 1: Spraying graphene oxide onto the surface of a glass fiber membrane with a spray gun, at which point one side of the glass fiber membrane consists of multilayer sheet-like graphene oxide; Step 2: Generating Gaussian femtosecond laser pulses using a femtosecond laser, and spatially shaping the beam using a cylindrical mirror to form a line-focused beam; Step 3: Applying the line-focused beam to the graphene oxide coating obtained in Step 1, adjusting the ablation threshold of the laser energy to be much greater than the reduction threshold; At this point, the energy distribution on the undulating graphene oxide surface varies; by utilizing this energy difference, graphene oxide is selectively ablated while being reduced, thereby controlling the pore size and shape of the photothermal film surface and forming a superhydrophobic / superoleophilic surface.

4. The application of the bifunctional Janus membrane for multi-component seawater separation and recovery as described in claim 1, characterized in that: Step 1: Assemble a Janus membrane system for the separation and recovery of oily polluted seawater. This system is divided into oil-water separation and seawater desalination zones. Step 2: In the oil-water separation zone, as the oily seawater passes through the Janus membrane, light oil, seawater, and heavy oil separate from top to bottom. The heavy oil is separated and recovered after contacting the reduced graphene oxide zone. The remaining mixture contacts the next-stage, rotating Janus membrane, where light oil and seawater separate from top to bottom. The seawater contacts the glass fiber membrane zone and enters the next-stage seawater desalination zone, where light oil is separated and recovered. Step 3: In the seawater desalination zone, the seawater contacts the suspended Janus membrane and evaporates on both sides under solar power for pure water recovery. Salt crystallizes on the lower side of the Janus membrane and is recovered. At this point, pure water, salt crystals, light oil, and heavy oil are separated and recovered separately.

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