A method for preparing a highly stable and anti-salt scale composite distillation membrane based on surfactant regulation

Abstract

A method for preparing a highly stable, anti-salt scale composite distillation membrane based on surfactant regulation belongs to the field of high-salt water treatment. The method comprises: using a high-molecular-weight, highly hydrolyzed alcohol polymer as the solute, preparing an aqueous solution with a mass concentration of 1-3%; adding an aldehyde crosslinker, controlling the mass ratio of aldehyde to polymer to be 1-5:10, and regulating the solution pH to ≤1; adding a surfactant, controlling the surfactant molar concentration to be 1-4 mM; rapidly coating the resulting solution on the surface of a hydrophobic microporous membrane and placing it in a well-ventilated area for drying and crosslinking; and post-treating the dense gel layer on the composite membrane surface with 60°C hot water for 2 hours to obtain the composite distillation membrane. The composite distillation membrane's surface gel layer and hydrophobic separation layer form a highly mechanically stable, embedded, interlocking structure. The membrane surface is smooth and non-porous, exhibiting high resistance to adhesion and abrasion against crystalline salt scales such as calcium sulfate and sodium chloride. It also exhibits high water flux, enabling efficient and stable extreme concentration of high-salt water and high-quality water production.

Description

Technical Field

[0001] The invention belongs to the field of high-salt water treatment, and in particular relates to a method for preparing a highly stable salt-scale-resistant composite distillation membrane based on surfactant regulation. Background Art

[0002] High-salinity wastewater is currently a difficult problem in the field of water treatment. The currently widely used reverse osmosis desalination technology can only treat wastewater with a salinity of no more than 100,000 mg / L, and will also produce a large amount of concentrated water with a higher salinity that is difficult to dispose of. Although thermal desalination technology (such as multi-effect distillation, etc.) can deeply concentrate high-salinity wastewater to zero emissions, its high land occupation and energy consumption have limited its further promotion and application. Membrane distillation is a technology that combines thermal and membrane methods. It uses the vapor pressure difference on both sides of the hydrophobic membrane as a driving force to complete the concentration of wastewater and produce high-quality water. Therefore, membrane distillation has the advantages of being able to utilize low-grade heat sources (such as solar energy, geothermal energy, industrial waste heat or waste heat, etc.), high salt tolerance, and good water quality. It has broad application prospects in the field of high-salinity wastewater treatment.

[0003] However, during the membrane distillation process for treating high-salt wastewater, as the salinity of the wastewater increases, the precipitation of inorganic salts in the wastewater can easily cause clogging of the membrane pores. At the same time, due to the presence of micropores on the surface of conventional hydrophobic membranes, salt crystals deposited on the membrane surface will also grow into the membrane pores. The huge crystal growth pressure formed will further destroy the membrane pore structure, and eventually the hydrophobic separation membrane will lose its interception efficiency. In order to improve the stability and long-term effectiveness of the membrane distillation process for treating high-salt wastewater, a composite structure distillation membrane with a dense anti-fouling layer has been developed. The dense anti-fouling layer on the surface of the composite membrane (such as a polyvinyl alcohol gel layer, etc.) can intercept pollutants and prevent the hydrophobic separation layer from being polluted. In order to ensure the water permeability of the membrane, the dense surface layer generally has a higher hydrophilicity, which also leads to poor compatibility between the hydrophilic surface layer and the hydrophobic separation layer. During long-term operation, the surface layer is easily detached and damaged, which seriously affects the stability of the membrane performance. In addition, the density of the surface layer will usually increase the water mass transfer resistance, causing a large loss of water flux in the formed composite membrane. The development of highly stable, highly permeable and salt-scale resistant composite distillation membranes can greatly advance the industrial application of membrane distillation and facilitate the low-carbon recycling of high-salt wastewater. Summary of the Invention

[0004] The purpose of the present invention is to solve the problem of poor compatibility between the hydrophobic separation layer and the hydrophilic surface anti-fouling layer of a composite structure distillation membrane, and to provide a method for preparing a highly stable anti-salt scale composite distillation membrane based on surfactant regulation. The method uses a surfactant to regulate the surface tension of a precursor liquid of a surface dense anti-fouling layer, so that the precursor liquid of the anti-fouling layer partially penetrates into the membrane pores before solidification. After solidification, the anti-fouling layer forms an embedded mechanical interlocking structure at the surface pores of the hydrophobic separation layer, significantly improving the mechanical stability of the surface anti-fouling layer. At the same time, the incorporation of surfactant molecules can also significantly reduce the water mass transfer resistance of the surface dense layer, specifically by reducing the water diffusion mass transfer distance, increasing the proportion of intermediate water at the evaporation interface, and reducing the water evaporation enthalpy, thereby accelerating the evaporation process of the interfacial water. Ultimately, the surfactant-regulated composite distillation membrane has high salt scale resistance while also having high water flux (which can be about 25% higher than the hydrophobic membrane base) and water quality (conductivity less than 2μS cm -1 The surfactant-regulated composite distillation membrane prepared by the present invention can achieve deep concentration and reuse of high-salt water in an efficient and stable manner, and has broad application prospects.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing a highly stable, anti-salt scale composite distillation membrane based on surfactant regulation, the method comprising:

[0007] Step 1: Using a high molecular weight and high alcoholysis degree alcohol polymer (molecular weight: 20k-200k, alcoholysis degree 80%-99%) as a solute, preparing a 1-3% mass concentration aqueous solution as a surface dense gel layer precursor, and ultrasonically mixing for 1-5 minutes;

[0008] Step 2: Add an aldehyde crosslinking agent to the solution prepared in step 1, controlling the mass ratio of the crosslinking agent to the polymer to be 1 to 5:10, using concentrated sulfuric acid to adjust the solution pH to ≤ 1, and ultrasonically mix for 1 to 3 minutes;

[0009] Step 3: Add the surfactant to the solution obtained in step 2, control the surfactant concentration to be 1-4 mM, and ultrasonically mix for 10-30 seconds;

[0010] Step 4: Apply the solution obtained in step 3 quickly and evenly on the surface of the hydrophobic microporous membrane within 3 minutes, control the liquid layer thickness to 10-20 microns, place it in a ventilated place, and carry out natural air drying and cross-linking process for 12-24 hours. The thickness of the dense surface gel layer formed is about 0.5-1.5 μm; the solution prepared in step 3 will begin to gel within about 3 minutes, and the viscosity of the solution will increase significantly, thereby affecting the uniformity of the coating and the degree of penetration of the surface gel layer into the membrane pores, which is not conducive to the formation of a composite membrane with a stable structure. Therefore, it must be applied quickly and evenly within three minutes.

[0011] Step 5: Float the dense gel layer on the surface of the composite membrane prepared in step 4 downward in 60°C hot water for post-treatment for 2 hours to remove the unreacted substances remaining in the surface layer and at the same time improve the hydration degree of the polymer chains in the dense gel layer on the surface, thereby obtaining a highly stable, highly permeable and salt-scale-resistant composite distillation membrane.

[0012] Furthermore, in step 1, the polymer is polyvinyl alcohol, with a molecular weight of 50k to 200k and a degree of alcoholysis of 80% to 99%.

[0013] Furthermore, in step 2, the aldehyde cross-linking agent is glutaraldehyde or glyoxal.

[0014] Furthermore, in step three, the surfactant is one of sodium alkylbenzene sulfonate, quaternary ammonium compound or Tween.

[0015] Furthermore, in step 4, the hydrophobic microporous membrane is prepared from polyvinylidene fluoride, polytetrafluoroethylene, and polypropylene by a phase inversion method or an electrospinning method, and has an average pore size of 0.1 to 1 micron.

[0016] The beneficial effects of the present invention compared to the prior art are:

[0017] 1. Under the regulation of surfactants, the dense anti-fouling layer on the surface of the composite membrane prepared by the present invention will partially penetrate into the membrane pores, forming an embedded mechanical interlocking structure with the microporous hydrophobic matrix, significantly improving the mechanical stability of the surface layer, avoiding detachment and breakage problems during membrane distillation operation, and maintaining the long-term and stable effect of membrane distillation in treating high-salt wastewater.

[0018] 2. The composite membrane prepared by the present invention has a higher water flux (about 25%) than the hydrophobic matrix. The main reasons are: (1) the concave structure formed by partial penetration of the surface layer significantly reduces the diffusion and mass transfer distance of water molecules and weakens the diffusion and mass transfer resistance of water; (2) the embedded morphology produces a larger evaporation interface. At the same time, the doping of surfactant causes a partial transformation of the state of water molecules at the gel interface, significantly increases the proportion of intermediate water, and reduces the evaporation enthalpy of water molecules. The combined effect of the two improves the evaporation efficiency of water molecules at the surface layer / hydrophobic separation layer interface.

[0019] 3. The composite membrane produced by this invention has a smooth, non-porous surface and exhibits effective resistance to crystalline scale deposits such as calcium sulfate and sodium chloride. It also significantly reduces mechanical damage to the membrane surface caused by solid scale deposits. This allows for efficient and stable extreme concentration of high-salinity water and high-quality water production. Furthermore, the preparation process is simple, cost-effective, and amenable to industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1Scanning electron microscope images of the surface and cross-section of the PVDF hydrophobic microporous membrane, the PVA dense gel composite membrane without surfactant regulation, and the composite membrane after surfactant regulation in Example 1 of the present invention;

[0021] Figure 2 Graphs showing tensile test of the PVDF hydrophobic microporous membrane, the PVA dense gel composite membrane without surfactant regulation, and the composite membrane after surfactant regulation in Example 1 of the present invention;

[0022] Figure 3 Schematic diagram of the surface water contact angles of the PVDF hydrophobic microporous membrane, the PVA dense gel composite membrane without surfactant regulation, and the composite membrane after surfactant regulation in Example 1 of the present invention;

[0023] Figure 4 The attenuated total reflection-Fourier transform infrared spectra of the PVDF hydrophobic microporous membrane in Example 1 of the present invention, the PVA dense gel composite membrane without surfactant regulation, and the composite membrane after surfactant regulation;

[0024] Figure 5 Surface Raman spectra of the PVA dense gel composite film without surfactant regulation and the composite film after surfactant regulation in Example 1 of the present invention, as well as the ratio of intermediate water to free water;

[0025] Figure 6 Graph showing the change in water flux over time for the PVDF hydrophobic microporous membrane, the PVA dense gel composite membrane without surfactant regulation, and the composite membrane after surfactant regulation in treating simulated high-salt wastewater (30 mM CaCl2 and 30 mM Na2SO4) in Example 1 of the present invention;

[0026] Figure 7 Graph showing the change in conductivity of produced water during treatment of simulated high-salt wastewater by the PVDF hydrophobic microporous membrane, the PVA dense gel composite membrane without surfactant regulation, and the composite membrane after surfactant regulation in Example 1 of the present invention;

[0027] Figure 8 These are scanning electron microscope morphologies of the membrane surface and cross-section of the PVDF hydrophobic microporous membrane in Example 1 of the present invention, the PVA dense gel composite membrane without surfactant regulation, and the composite membrane after surfactant regulation after treating simulated high-salt wastewater. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0029] Example 1:

[0030] Step 1: Prepare a surface dense anti-fouling gel layer precursor solution using polyvinyl alcohol with a molecular weight of about 22,000 and a degree of alcoholysis of about 99% in a 3% aqueous solution, and ultrasonically mix for 1 minute;

[0031] Step 2: Add glutaraldehyde to the solution prepared in step 1, control the mass ratio of crosslinker to polymer to be 1:10, use concentrated sulfuric acid to adjust the solution pH to 1, and ultrasonically mix for 1 minute;

[0032] Step 3: Add sodium dodecylbenzenesulfonate to the solution obtained in step 2, control the surfactant concentration to 4 mM, and mix by ultrasonication for 30 seconds;

[0033] Step 4: The solution obtained in step 3 was evenly coated on the surface of a PVDF hydrophobic microporous membrane (average pore size 0.45 μm) within 2 minutes, with the liquid layer thickness controlled to 15 μm. The membrane was placed in a ventilated place and allowed to air dry and crosslink for 12 hours, forming a dense surface layer with a thickness of about 1 μm.

[0034] Step 5: Float the composite membrane prepared in step 4 with the surface layer facing downward in 60°C hot water for post-treatment for 2 hours to remove the unreacted substances remaining in the surface layer and at the same time increase the hydration degree of the polymer chains in the surface dense layer, thereby obtaining a highly stable, highly permeable, and salt-scale-resistant composite distillation membrane.

[0035] The surface and cross-sectional morphologies of the PVDF hydrophobic microporous membrane in Example 1, the PVA dense gel composite membrane without surfactant regulation (obviously obtained by omitting step S3), and the composite membrane after surfactant regulation were characterized by scanning electron microscopy. Figure 1 It can be found that the surface of the composite membrane prepared in steps S1 to S5 has a dense gel layer of 1 μm. Through the tensile test, it can be found that the regulation of the surface layer by the surfactant can significantly increase the tensile strength of the membrane ( Figure 2 ). Water contact angle tests show that surfactant regulation can also significantly increase the hydrophilicity of the composite membrane surface layer ( Figure 3 ), which is beneficial to the mass transfer of water into the surface layer. The infrared spectrum can further confirm the integration of some surfactant molecules into the surface layer ( Figure 4 ). Raman spectroscopy was performed on the surface of the composite membrane after water swelling, and the results were as follows Figure 5As shown, the intervention of surfactants can increase the ratio of intermediate water to free water at the gel layer interface, which will be beneficial to the evaporation process of water at the interface. Membrane distillation experiments were carried out on PVDF hydrophobic microporous membranes, composite membranes coated with PVA dense gel layers, and composite membranes regulated by surfactants. The feed liquid used a high salt solution containing 30mM CaCl2 and 30mM Na2SO4, and the condensate used pure water. The feed liquid and condensate temperatures were controlled at 60℃ and 20℃ respectively. The solutions on both sides of the membrane flowed cross-currently at a flow rate of 0.1m / s. Water flux ( Figure 6 ), water quality ( Figure 7 ) and the surface morphology of the treated membrane ( Figure 8 The results show that the composite membranes with dense PVA gel layer have strong anti-gypsum salt scaling ability, can always keep the membrane surface clean during the treatment process, maintain the stability of water flux, and ensure high-quality water production (conductivity <2μS cm -1 The composite membrane formed under surfactant control exhibited a higher water flux (25% higher, approximately 25 LMH) than the original PVDF membrane substrate. The experimental results fully demonstrate the remarkable effect of surfactants on the surface anti-fouling dense layer structure, physicochemical properties, and membrane performance during the preparation of composite distillation membranes.

[0036] Example 2:

[0037] Step 1: Prepare a surface dense gel layer precursor solution with a total mass concentration of 2% using polyvinyl alcohol with a molecular weight of about 50,000 and a degree of alcoholysis of about 88%, and mix it by ultrasonic mixing for 1 minute;

[0038] Step 2: Add glyoxal to the solution prepared in step 1, control the mass ratio of crosslinker to polymer to be 1:5, use concentrated sulfuric acid to adjust the solution pH to 1, and ultrasonically mix for 3 minutes;

[0039] Step 3: Add Tween 80 to the solution obtained in step 2, control the surfactant concentration to 2 mM, and mix by ultrasonication for 20 seconds;

[0040] Step 4: The solution obtained in step 3 was evenly coated on the surface of a polypropylene hydrophobic microporous membrane (average pore size 0.22 μm) within 2 minutes of mixing, with the liquid layer thickness controlled to 20 μm. The membrane was placed in a ventilated place for drying and cross-linking for 24 hours, and the thickness of the formed surface dense layer was about 1.2 μm.

[0041] Step 5: Float the composite membrane prepared in step 4 with the surface layer facing downward in 60°C hot water for post-treatment for 2 hours to remove the unreacted substances remaining in the surface layer and at the same time increase the hydration degree of the polymer chains in the surface dense layer, thereby obtaining a highly stable, highly permeable, and salt-scale-resistant composite distillation membrane.

[0042] The surface of the composite membrane prepared in Example 2 has a gel-type functional layer with a thickness of 1.2 μm, which is dense and non-porous. The regulation of the surface layer by Tween can increase the tensile strength of the membrane by more than 3 times. A membrane distillation experiment was carried out on the composite distillation membrane regulated by Tween. The feed liquid used a sodium chloride solution with a mass concentration of 30%, and the condensate used pure water. The feed liquid and condensate temperatures were controlled at 60°C and 20°C, respectively. The solutions on both sides of the membrane flowed in a cross-current at a flow rate of 0.1 m / s. The results show that the composite distillation membranes regulated by Tween have a strong ability to resist sodium chloride scale. The membrane surface can always be kept clean during the treatment process, and the water flux is maintained stable (about 20% higher than the water flux of the original PVDF membrane substrate, about 24 LMH), while ensuring high-quality water production (conductivity <2 μS cm -1 ).

Claims

1. A method for preparing a highly stable and anti-salt scale composite distillation membrane based on surfactant regulation, characterized in that: The method is: Step 1: Using a high molecular weight, high alcoholysis degree alcohol polymer as a solute, preparing a 1-3% aqueous solution as a precursor for a dense surface gel layer, and ultrasonically mixing for 1-5 minutes; the molecular weight of the polymer is 20k-200k, and the alcoholysis degree is 80%-99%; Step 2: Add the aldehyde crosslinker to the solution prepared in step 1, control the mass ratio of crosslinker to polymer to be 1-5:10, use concentrated sulfuric acid to adjust the solution pH ≤ 1, and ultrasonically mix for 1-3 minutes; Step 3: Add the surfactant to the solution obtained in step 2, control the surfactant concentration to be 1-4 mM, and mix by ultrasonic mixing for 10-30 seconds; Step 4: The solution obtained in step 3 is quickly and evenly coated on the surface of the hydrophobic microporous membrane within 3 minutes, and the thickness of the liquid layer is controlled to be 10-20 microns. The membrane is placed in a ventilated place and naturally air-dried and cross-linked for 12-24 hours to form a dense surface gel layer with a thickness of 0.5-1.5 μm. Step 5: Float the composite membrane prepared in step 4 with the dense gel layer on the surface facing downward in 60° C. hot water for post-treatment for 2 hours to obtain a highly stable and anti-salt scale composite distillation membrane.

2. The method for preparing a highly stable and anti-salt scale composite distillation membrane based on surfactant regulation according to claim 1, characterized in that: In step 1, the polymer is polyvinyl alcohol, with a molecular weight of 50k~200k and a degree of alcoholysis of 80%~99%.

3. The method for preparing a highly stable and anti-salt scale composite distillation membrane based on surfactant regulation according to claim 1, characterized in that: In step 2, the aldehyde cross-linking agent is glutaraldehyde or glyoxal.

4. The method for preparing a highly stable and anti-salt scale composite distillation membrane based on surfactant regulation according to claim 1, characterized in that: In step 3, the surfactant is one of sodium alkylbenzene sulfonate, quaternary ammonium compound or Tween.

5. The method for preparing a highly stable and anti-salt scale composite distillation membrane based on surfactant regulation according to claim 1, characterized in that: In step 4, the hydrophobic microporous membrane is prepared from polyvinylidene fluoride, polytetrafluoroethylene, and polypropylene by a phase inversion method or an electrospinning method, and has an average pore size of 0.1 to 1 micron.

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