Difunctional Janus membrane for separating and recycling multi-component seawater

Abstract

The invention relates to a difunctional Janus membrane for separating and recycling multi-component seawater, and belongs to the technical field of membrane separation. The novel porous Janus photo-thermal separation membrane has a reverse super-wettability design (one surface is super-hydrophobic / super-oleophylic, and the other surface is super-hydrophilic / super-oleophobic), and has dual functions of solar seawater desalination (used for separating purified water and salt crystals) and oil-water separation (used for separating light oil and heavy oil); the efficient and stable separation and recovery of four components in the oil-containing polluted seawater can be realized. Meanwhile, the invention provides a novel method for reducing graphene oxide on the surface of the porous material by using ultrashort pulse femtosecond laser. According to the method, the surface appearance of the graphene oxide can be effectively regulated and controlled while reduction of the graphene oxide is achieved, and the more excellent separation performance of the graphene oxide is achieved. The method has a great practical application prospect in the field of polluted seawater separation and recovery.

Description

Technical Field

[0001] The invention relates to a dual-function Janus membrane for separating and recovering multi-component seawater, belonging to the technical field of membrane separation. Background Art

[0002] The scarcity of fresh water resources has become the second largest environmental problem after global warming. There are many commercial desalination methods, such as distillation, seawater freezing, electrodialysis and reverse osmosis. However, these methods have high power consumption, huge costs and huge environmental damage. Solar energy is a green, abundant and sustainable energy source. Solar interfacial seawater evaporation greatly improves the evaporation efficiency by locally heating the water-air interface. The porous photothermal membrane is composed of a substrate and a photothermal material. Due to its rich functions, low cost and simple preparation, it is the core component of the interfacial evaporator.

[0003] Due to crude oil leakage from ships and the discharge of industrial and domestic sewage, there is a large amount of oil-water mixture in seawater. As the separation of oil-containing seawater proceeds, excess components (such as inorganic salts, oil) will gradually accumulate and inevitably form crystals or scale, leading to capillary gap blockage and membrane corrosion. This will hinder light absorption, water transmission and vapor escape, and eventually lead to evaporator failure after long-term operation. At the same time, volatile oil can also enter the purified water together with water vapor, resulting in impure collected water. Therefore, it is difficult for traditional single wettability photothermal membranes to efficiently and stably separate high-quality pure water.

[0004] The effective combination of photothermal materials and wetting materials is the future trend in the field of solar desalination. Richer functions can be achieved by designing and adjusting the wettability of the porous photothermal membrane interface. In addition, the redundant components also have considerable economic benefits. It is also meaningful to recycle other destructive components (such as inorganic salts and oil) while recycling pure water. However, there are four components in seawater contaminated by petroleum, including pure water, salt crystals, light oil (less dense than water) and heavy oil (more dense than water). It is extremely difficult to spontaneously separate and recycle these four components in the current material system.

[0005] The preparation of porous photothermal membranes with different wettability is a difficulty in this field. Coating reduced graphene oxide on the surface of the porous membrane can effectively regulate the surface wettability, but the directly coated reduced graphene oxide is not firmly connected to the substrate. Therefore, the commonly used method is to first coat graphene oxide on the surface of the porous membrane so that it is firmly bonded to the substrate, and then reduce it to reduced graphene oxide. The current reduction methods are mostly high-temperature reduction, microwave reduction, chemical reduction and the like. Although these methods can achieve the effective reduction of graphene oxide, graphene oxide is a multilayer sheet structure. When it is coated on a porous substrate, it will inevitably cause pore blockage and affect the inherent porous properties of the porous material; at the same time, the photothermal properties and wettability of the porous photothermal membrane are closely related to the surface morphology. Therefore, on the surface of this composite material, a new method that can both effectively reduce graphene oxide and regulate its surface morphology is extremely needed. Summary of the invention

[0006] The purpose of the present invention is to solve the technical problem that multiple components in oil-contaminated seawater are difficult to spontaneously separate and recover, and to provide a porous Janus photothermal membrane with reverse wettability. The membrane has one side that is super hydrophobic / super oleophilic and the other side that is super hydrophilic / super oleophobic. It has the dual functions of solar seawater desalination and oil-water separation, and can spontaneously realize the efficient and stable separation and recovery of four components in oil-contaminated seawater using solar energy.

[0007] The objects of the present invention are achieved through the following technologies.

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

[0009] The Janus membrane uses a glass fiber membrane as a substrate, graphene oxide is evenly sprayed by a spray gun, and a femtosecond laser is used to reduce the graphene oxide to prepare a photothermal layer.

[0010] Femtosecond laser reduction of graphene oxide can also effectively control its surface morphology.

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

[0012] Step 1: Select a glass fiber membrane as a substrate (super hydrophilic / super oleophobic), and spray graphene oxide on its surface through a spray gun. At this time, one side of the glass fiber membrane is multi-layered graphene oxide.

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

[0014] Step three: Apply a shaped femtosecond laser to the reduced graphene oxide coating obtained in step one, and adjust the laser energy to be near the ablation threshold of graphene oxide (much greater than the reduction threshold). At this point, the energy distributed on the surface of graphene oxide with undulating surfaces is different. This energy difference is used to selectively ablate graphene oxide while reducing it, and to regulate the size and shape of the pores on the surface of the photothermal film. At this point, this side exhibits superhydrophobic / superoleophilic properties. It is worth noting that the energy used in this method is much smaller than the ablation threshold of the high-strength glass fiber membrane, so this method will not cause damage to the substrate.

[0015] Step 4: The Janus membrane obtained in step 4 is used to form a system for separating and recovering oil-contaminated seawater, which is divided into oil-water separation and seawater desalination areas.

[0016] Step 5: In the oil-water separation area, when the oil-containing seawater passes through the Janus membrane, the light oil, seawater and heavy oil are stratified from top to bottom, and the heavy oil contacts the reduced graphene oxide area and is separated and recovered; the remaining mixture contacts the next-level flipped Janus membrane, at which time the light oil and seawater are stratified from top to bottom, and the seawater contacts the glass fiber membrane area and enters the next-level seawater desalination area, at which time the light oil is separated and recovered.

[0017] Step 6: Seawater contacts the suspended Janus membrane in the desalination area, and evaporates on both sides under the solar energy drive for pure water recovery, while salt is recovered after crystallization on the lower side of the Janus membrane. At this point, pure water, salt crystals, light oil and heavy oil are separated and recovered.

[0018] Beneficial Effects

[0019] 1. In terms of functional design, the present invention innovatively designs a new porous Janus photothermal membrane with reverse wettability design (one side is super-hydrophobic / super-oleophilic, and the other side is super-hydrophilic / super-oleophobic). The Janus membrane has the dual functions of solar desalination (for separating pure water and salt crystals) and oil-water separation (for separating light oil and heavy oil), and can spontaneously achieve efficient and stable separation and recovery of four components in oil-contaminated seawater.

[0020] 2. In terms of material preparation, the present invention proposes a new method for reducing graphene oxide on the surface of porous materials using ultrashort pulse femtosecond laser. This method can effectively control the surface morphology while achieving effective reduction of graphene oxide. Specifically, the porous material is a non-homogeneous material, and there is a certain difference in the thickness of the graphene oxide coated on the surface, while the focus of the femtosecond laser 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), and the energy difference between each region can be used to achieve selective ablation of graphene oxide, thereby achieving the control of its surface morphology.

[0021] 3. Through this controllable reduction method of graphene oxide morphology, the pore blockage caused by traditional methods can be effectively prevented, and the photothermal performance (enhanced solar photon absorption) and wetting properties can be improved by controlling its surface morphology.

[0022] 4. In addition, shaping the traditional Gaussian laser into a line-focused laser greatly improves the processing (reduction) efficiency of graphene oxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the processing optical path diagram of the spatial shaping (from point focus to line focus) femtosecond laser;

[0024] Figure 2 The preparation flow chart of Janus membrane;

[0025] Figure 3 Schematic diagram of the dual functionality of the Janus membrane;

[0026] Figure 4 This is a flow chart of solar seawater separation;

[0027] Figure 5 Schematic diagram of the oil-water separation process;

[0028] Figure 6 Flow chart for separation and recovery of multi-component contaminated seawater;

[0029] Figure 7 This is a physical picture of the components collected after the separation experiment.

[0030] In the figure: 1- femtosecond laser, 2- aperture, 3- attenuation plate, 4- imaging system, 5- white light, 6- beam splitter, 7- dichroic mirror, 8- switch, 9- objective lens, 10- control computer, 11- translation stage. DETAILED DESCRIPTION

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

[0032] The present invention proposes a method for preparing a dual-function Janus membrane based on femtosecond laser, and applies it to multi-component seawater separation and recovery. The specific steps are as follows:

[0033] Step 1: Build a femtosecond laser processing and observation system, such as Figure 1 As shown, the system mainly includes a femtosecond laser 1, an aperture 2, an attenuation plate 3, an imaging system 4, white light 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 the laser is turned on (central wavelength is 800nm; pulse width is 35fs; repetition frequency is 1kHz), and the beam focused by the objective lens 9 is a point-focused Gaussian beam. Finally, the ordinary objective lens is replaced with a cylindrical lens, and the ordinary Gaussian beam is shaped into a line-focused beam, which can achieve high-efficiency processing.

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

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

[0036] 2.2 Preparation of graphene oxide: 3 grams of natural graphite (325 mesh) and 60 milliliters of concentrated sulfuric acid (98 wt.%) were thoroughly mixed and stirred at 0°C for 30 minutes. 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 milliliters of deionized water were added, followed by 15 milliliters of hydrogen peroxide (30 wt.%) until the solution turned orange-brown. The solution was thoroughly filtered and washed with 200 milliliters of hydrochloric acid (3.7 wt.%). The dispersed solution was thoroughly stirred for 12 hours, and the unexposed graphite was removed after centrifugation. Next, the dispersed solution was placed in a dialysis bag for 7 days to remove residual ions, and the obtained graphene oxide was compressed by centrifugation;

[0037] 2.3 Graphene oxide coating: Dilute graphene oxide to 2 mg mL -1 It is then introduced into a spray gun, and then graphene oxide is evenly coated on the glass fiber membrane. At this time, due to the mutual bonding of oxygen-containing functional groups in the material, graphene oxide and the glass fiber membrane leaf are firmly bonded together;

[0038] 2.4 Graphene oxide reduction: The graphene oxide is treated with line-focused femtosecond laser, and the graphene oxide changes from brown to black. The laser flux is 20 J cm -2 , scanning speed is 100 μm / s;

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

[0040] Step 3, such as Figure 3 As shown, the Janus membrane has the dual functions of solar 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 provide driving force in the process of solar desalination. The reduced graphene oxide on the Janus membrane of the present invention is a photothermal material, which can achieve excellent photothermal conversion performance;

[0042] 3.2 In the present 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 super hydrophobic properties; the glass fiber membrane side is a super hydrophilic material.

[0043] 3.3 Therefore, in the process of solar desalination, if Figure 4 As shown, a high mechanical performance glass fiber membrane is stably suspended in the center of the water tank, and the hydrophilic fiber capillary action below pumps water. Solar energy is converted into thermal energy to provide the source power for seawater evaporation. At this time, water vapor escapes from both sides and is recovered as pure water. Because the upper side is super hydrophobic, salt can only gather at the lower side. Over time, the salt is crystallized into blocks and then recovered.

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

[0045] Step 3, such as Figure 6 As shown in the figure, a simple multi-component integrated separator for polluted seawater was designed and prepared. The separator has two functional modules: solar desalination (located in the middle) and oil-water separation (located on both sides). The selected oil-contaminated seawater contains four components (pure water, salt crystals, light oil and heavy oil). First, the oil-contaminated seawater is introduced into the oil-water separation module, where stratification occurs due to different densities, and the heavy oil is separated first due to its highest density. Subsequently, the remaining mixture is sent to the next area, and the light oil is separated first due to its lowest density. Finally, the remaining seawater is sent to the solar desalination module. Driven by solar energy, water vapor escapes and is recovered in the form of pure water. Salt crystals are also gradually recovered. At this point, all four components in the oil-contaminated seawater are efficiently separated and recovered.

[0046] like Figure 7 As shown in the experimental figure, after the separation experiment, the components in the polluted seawater and the Janus membrane were collected. The ion content (Na + , K + , Ca2+ Mg 2+ ) is very high, and the ion content after separation is far below the standards set by the World Health Organization (WHO); the surface of the Janus membrane is clean and undamaged and can be reused; the blocky salt crystals, light oil and heavy oil are separated and recovered respectively.

[0047] The specific description above further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above 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 in 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 wettability. One side of the membrane is superhydrophobic / superoleophilic, and the other side is superhydrophilic / superoleophobic. It has the dual functions of solar desalination and oil-water separation, and can achieve efficient and stable separation and recovery of four components in oil-contaminated seawater.

2. A method for preparing a bifunctional Janus membrane for multi-component seawater separation and recovery as claimed in claim 1, characterized in that: The Janus membrane uses a glass fiber membrane as a substrate, graphene oxide is evenly sprayed by a spray gun, and a femtosecond laser is used to reduce the graphene oxide to prepare a photothermal layer.

3. The method according to claim 2, characterized in that: Femtosecond laser reduction of graphene oxide can also effectively control its surface morphology.

4. The method according to claim 2 or 3, characterized in that: The preparation method comprises the following steps: Step 1: Spray graphene oxide on the surface of the glass fiber membrane with a spray gun. At this time, one side of the glass fiber membrane is multi-layered graphene oxide. Step 2: Use a femtosecond laser to generate a Gaussian femtosecond laser pulse, and use a cylindrical mirror to spatially shape the beam into a line-focused beam; Step 3: Applying a line focused beam to the graphene oxide coating obtained in step 1, and adjusting the ablation threshold of the laser energy to be much larger than the reduction threshold; At this time, the energy distributed on the undulating graphene oxide surface is different; this energy difference is used to selectively ablate the graphene oxide while reducing it, thereby regulating the pore size and shape on the surface of the photothermal film to form a superhydrophobic / superoleophilic surface.

5. The use of a bifunctional Janus membrane for multi-component seawater separation and recovery as claimed in claim 1, characterized in that: Step 1: The Janus membrane is assembled into a system for separation and recovery of oil-contaminated seawater, which is divided into oil-water separation and seawater desalination areas; Step 2: In the oil-water separation area, when the oil-containing seawater passes through the Janus membrane, the light oil, seawater and heavy oil are separated from top to bottom, and the heavy oil contacts the reduced graphene oxide area and is recovered after separation; the remaining mixture contacts the next-level flipped Janus membrane, at which time the light oil and seawater are separated from top to bottom, and the seawater contacts the glass fiber membrane area and enters the next-level seawater desalination area, at which time the light oil is separated and recovered; Step 3: In the desalination area, seawater contacts the suspended Janus membrane and evaporates on both sides under the drive of solar energy for pure water recovery. The salt is recovered after crystallization on the lower side of the Janus membrane. At this time, pure water, salt crystals, light oil and heavy oil are separated and recovered.

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