Forward osmosis membrane based on heat-assisted surface segregation method and preparation method of forward osmosis membrane

Abstract

The preparation method comprises the following steps: dissolving a membrane-forming polymer in an organic solvent to obtain a primary membrane casting solution, adding a hydrophilic polymer, continuously stirring for a certain time, and then carrying out vacuum defoaming to obtain a clear and transparent membrane casting solution; fixing the non-woven fabric on a clean and smooth glass plate, and scraping the membrane to obtain a hydrophilic ultrafiltration membrane; performing heat treatment on the hydrophilic ultrafiltration membrane, improving the forward osmosis performance of the membrane through heat assistance, and changing the heat treatment time and the heat treatment temperature of the membrane to finally obtain the high-performance forward osmosis membrane. The surface property of the forward osmosis membrane is regulated and controlled by regulating and controlling the content and segregation speed of the hydrophilic polymer with negative charges on the membrane surface, and the interception performance of the membrane is improved, so that the high-performance forward osmosis membrane is obtained.

Description

Technical Field

[0001] The invention relates to a forward osmosis membrane based on a heat-assisted surface segregation method and a preparation method thereof, belonging to the field of water treatment and membrane separation science and technology. Background Art

[0002] Membrane separation technology is an efficient, fast and energy-saving separation technology, which is widely used in water treatment, air separation, biological separation and other fields. Forward osmosis (FO) is an emerging membrane separation process that has attracted great attention from membrane scientists in recent years. It is one of the current technologies for water purification, wastewater reuse and seawater desalination. Compared with traditional pressure-driven membrane separation technologies such as reverse osmosis (RO) and nanofiltration (NF), the forward osmosis process uses the osmotic pressure difference to drive water molecules to spontaneously penetrate from the feed liquid side with high water chemical potential / low osmotic pressure to the draw liquid side with low water chemical potential / high osmotic pressure. The FO process provides many advantages, including reduced energy consumption and equipment costs, reduced pollution tendency, a wide range of potential applications such as power generation, liquid food processing, and trace organic matter removal, which is also the main exploration direction of the FO process. The forward osmosis membrane is the core of the forward osmosis process. The development of a forward osmosis membrane with high water flux and low reverse salt flux is an urgent problem to be solved in this field.

[0003] An ideal forward osmosis membrane should have the following characteristics: a porous and thin support layer, small structural parameters, which can reduce concentration polarization; low reverse salt flux; good hydrophilicity to increase water flux and reduce membrane fouling; and a certain resistance to acid and alkali corrosion. Although FO has received more and more attention, concentration polarization, membrane fouling, reverse solute diffusion, etc. are still problems that need to be solved in the FO process, and it is still necessary to explore forward osmosis membranes with good performance.

[0004] Common surface modification methods for membranes include post-modification methods represented by surface grafting and surface coating, and in-situ modification methods represented by surface segregation. Surface segregation is a phenomenon that is widely present in nature and has irreplaceable advantages in membrane manufacturing because it is an in-situ method for synchronously modifying the membrane and pore surfaces during membrane formation. During the surface segregation process, charged hydrophilic groups can be enriched on the membrane surface during the phase separation process. Correspondingly, external conditions will also affect the surface energy, leading to changes in the thermodynamic steady state of the material surface, thereby changing the surface composition and affecting the surface properties and applications. However, the main drawback of in-situ modification of the membrane surface is that the coverage of the modifier on the membrane surface is insufficient and the modification efficiency is poor. Therefore, how to increase the content of the modifier on the membrane surface and enhance the segregation behavior of the modifier on the membrane surface has become one of the hot topics in the preparation of high-performance membrane materials.

[0005] The polyamic acid (PAA) polymer contains a large number of hydrophilic polar functional groups, carboxyl and amine groups, a large number of secondary amine groups and primary amine groups at the ends of the molecules, which significantly improve the hydrophilicity of the membrane surface, thereby promoting water transfer, and the PAA molecules are insoluble in water and are not easily lost in water. As described above, the present invention provides a new idea for developing a simple and feasible method for modifying forward osmosis membranes, wherein the hydrophilic polymer polyamic acid PAA and the base membrane material are blended to form a casting liquid to prepare an ultrafiltration membrane, and the segregation of the polyamic acid is strengthened by heat treatment, thereby increasing the enrichment of PAA on the membrane surface, increasing the hydrophilicity and negative charge of the membrane, and enhancing the electrostatic repulsion between the membrane and ions, thereby greatly reducing the reverse diffusion of solutes, and being able to achieve high permeability and high separation performance, with good forward osmosis performance. Summary of the invention

[0006] The purpose of the present invention is to provide a forward osmosis membrane based on a heat-assisted surface segregation method and a preparation method thereof, wherein the membrane has good hydrophilicity and pure water flux and low reverse salt flux. The method is simple and easy to operate, highly efficient and environmentally friendly, and easy to scale up.

[0007] In order to achieve the above object, the first aspect of the present invention provides a method for preparing a forward osmosis membrane based on a heat-assisted surface segregation method, comprising the following steps:

[0008] Step 1): Preparation of high-throughput hydrophilic ultrafiltration membrane by in-situ polycondensation: dissolving a membrane-forming polymer in an organic solvent, heating and stirring to obtain a preliminary casting solution, cooling to room temperature, adding a hydrophilic polymer and continuing to stir for a certain period of time, then vacuum degassing to obtain a clear and transparent casting solution, and then fixing the non-woven fabric on a clean and smooth glass plate for film scraping, and placing it in a 20°C water bath for phase transformation to obtain a hydrophilic ultrafiltration membrane;

[0009] Step 2): Heat treatment: The hydrophilic ultrafiltration membrane is heat treated to improve the forward osmosis performance of the membrane through heat assistance, and the heat treatment time and heat treatment temperature of the membrane are changed to finally obtain a high-performance forward osmosis membrane.

[0010] In non-solvent induced phase separation, the choice of polymer is an important factor, and the choice of polymer directly affects the membrane performance, because the inherent membrane separation characteristics depend on the chemical structure of the polymer. Membrane main polymers such as polysulfone PSF, polyethersulfone PES, polyacrylonitrile PAN, polyvinylidene fluoride PVDF and polyvinyl chloride PVC are currently commonly used polymers in industry for preparing membranes by non-solvent induced phase inversion method. Due to the high commercial availability, easy processing, good selectivity and permeability of PSF and PES, they are often selected in industry. Therefore, the inventors of this application followed the research results and selected polyethersulfone and polysulfone as base membranes for further research.

[0011] Preferably, in step 1), the raw materials of the casting solution are 10-25 parts by weight of film-forming polymer, 70-85 parts by weight of organic solvent, 0.2-5 parts by weight of monomer A, and 0.2-5 parts by weight of monomer B.

[0012] Preferably, the organic solvent in step 1) is any one of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc).

[0013] Preferably, the monomer A in step 1) is any one of 4,4'-diaminodiphenyl ether (ODA), 4,4'-diaminodiphenyl sulfone (DDS), p-phenylenediamine (PDA), and 4,4'-diaminodiphenylmethane (MDA); the monomer B is any one of pyromellitic anhydride (PMDA), diphenyl ether anhydride (ODPA), and 3,3',4,4'-biphenyltetracarboxylic anhydride (BPDA). The molar ratio of monomer A to monomer B is 1:1-1.02.

[0014] Preferably, the heat treatment method in step 2) is one of vacuum heat treatment, steam bath heat treatment and water bath heat treatment.

[0015] Preferably, the water bath heat treatment solvent in step 2) is one of water, ethanol, ethylene glycol or a mixed solution of any two.

[0016] Preferably, the heat treatment temperature in step 2) is 30-100° C., and the heat treatment time is 5-100 min.

[0017] Preferably, the forward osmosis membrane in step 2) is placed in a forward osmosis test device, 1 mol / L sodium chloride solution is selected as the draw liquid, deionized water is selected as the raw material liquid, and the flow rate of the raw material liquid and the draw liquid is controlled by a flow meter to be maintained at 150 mL / min.

[0018] Another technical solution of the present invention is the forward osmosis membrane obtained by the above preparation method.

[0019] The advantages of the forward osmosis membrane preparation method based on the heat-assisted surface segregation method proposed in the present invention are:

[0020] (1) The preparation method adopted by the present invention regulates the content of the negatively charged hydrophilic polymer on the membrane surface and the segregation rate by changing the temperature and time of the heat treatment, thereby regulating the surface properties of the forward osmosis membrane. The method is simple and feasible, efficient and environmentally friendly, has excellent performance, and is conducive to promotion in production practice.

[0021] (2) The present invention utilizes heat treatment membrane materials to modify the membrane, thereby accelerating the enrichment of the hydrophilic polymer polyamic acid to the membrane surface. The polyamic acid contains polar functional groups, such as carboxyl and amine, with strong hydrophilic properties. The hydrophilicity of the modified forward osmosis membrane is improved, which is beneficial to the penetration of water. At the same time, the negative charge density on the membrane surface is increased, the negative charge is enhanced, and the retention performance of the membrane is improved, thereby obtaining a high-performance forward osmosis membrane.

[0022] (3) The preparation method adopted by the present invention is an in-situ modification method, which realizes all-round modification of the membrane surface and pores, avoiding the disadvantages of complex surface coating and surface grafting and clogging of membrane pores. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The following is a further detailed description in conjunction with the accompanying drawings and embodiments of the present invention.

[0024] Figure 1 This is a scanning electron microscope image of the outer surface of the polyethersulfone membrane containing the hydrophilic polymer polyamic acid prepared in Comparative Example 2.

[0025] Figure 2 This is a comparison chart of the pure water flux of the forward osmosis membrane based on heat-assisted surface segregation prepared in Comparative Example 2 and Examples 3 and 4.

[0026] Figure 3 This is a comparison chart of the reverse salt flux of the forward osmosis membrane based on heat-assisted surface segregation prepared in Comparative Example 2 and Examples 3 and 4.

[0027] Figure 4 Zeta potential diagram of the forward osmosis membrane based on heat-assisted surface segregation prepared in Comparative Example 2 and Examples 3 and 4. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be described below in conjunction with exemplary implementations or embodiments, but the content of the present invention is not limited to the following embodiments. It is hereby stated that these examples only provide partial descriptions and do not limit the present invention.

[0029] Performance Testing

[0030] 1. Forward osmosis performance test: 1mol / L sodium chloride solution is used as the draw liquid, and deionized water is used as the raw material liquid. The raw material liquid pool and the draw liquid pool are placed on an electronic balance respectively. The flow rate of the raw material liquid and the draw liquid is controlled by a flow meter to be maintained at 150mL / min, and the stable operation time is constant to ensure that the flow rate on both sides of the system remains stable during the test. The mass value of the raw material liquid is recorded every minute by an electronic balance to track the amount of permeated water. At the same time, a conductivity meter is used to monitor the changes in the ion concentration of the raw material liquid in real time. Among them, the water flux (Jw) is used to characterize the water permeability of the membrane, which indicates the volume of water passing through the unit membrane area per unit time; the reverse salt flux (Js) is used to measure the retention performance of the membrane, which indicates the mass of salt passing through the unit membrane area per unit time; the relevant calculation equations are as follows:

[0031]

[0032] Where Jw is the water flux of FO, in L·m -2 ·h -1 ; Δm is the mass change difference of the raw material liquid, in g; ρ is the density of the raw material liquid, in g / ml; Am is the effective area of ​​the membrane, in cm 2 ; Δt is the test time, in h; Js is the reverse salt flux of FO membrane, in g·m -2 ·h -1 ; V0 and V t They are the volume of the raw material liquid at the beginning of the test and the volume of the raw material liquid after the test is completed, in L; C0 and C t They are the salt concentration of the raw material solution at the beginning of the test and the salt concentration of the raw material solution after the test is completed, in g / L.

[0033] 2. Zeta potential analyzer: Zeta potential analyzer is used to analyze the electronegativity of the membrane surface. The Zeta potential of the membrane surface can reflect the positive and negative charge and size of the membrane. The change in Zeta potential can reflect the change in the functional groups contained in the membrane material.

[0034] Comparative Example 1

[0035] Preparation of polyethersulfone membrane without hydrophilic polymer: Weigh 15 parts by mass of polyethersulfone resin in N,N-dimethylformamide organic solvent, heat and stir at 80°C to completely dissolve it, and obtain a uniform and transparent casting solution. After degassing the casting solution in a vacuum dryer for 24 hours, fix the non-woven fabric on a clean and smooth glass plate for film scraping, and phase invert in a 20°C water bath to obtain the forward osmosis membrane, and immerse the membrane in deionized water for standby use. The water flux of the prepared membrane is 12.30L·m -2 ·h -1 , the reverse salt flux is 13.96 g·m -2 ·h -1 .

[0036] Comparative Example 2

[0037] Preparation of polyethersulfone membrane containing hydrophilic polymer polyamic acid: Weigh 15 parts by mass of polyethersulfone resin in N,N-dimethylformamide organic solvent, heat and stir at 80°C to completely dissolve it, and obtain a uniform and transparent preliminary casting solution. After cooling to room temperature, add 0.8 parts by mass of monomer 4,4'-diaminodiphenyl sulfone (DDS) and continue to stir with a stirrer to dissolve it and mix it evenly, then add the monomer pyromellitic dianhydride (PMDA) with a molar ratio of 1:1.02 to DDS, continue to stir with a stirrer for 4 hours to react the two monomers to obtain a mixed casting solution, place it in a vacuum dryer for degassing for 24 hours, fix the non-woven fabric on a clean and smooth glass plate for film scraping, phase invert in a 20°C water bath to obtain the forward osmosis membrane, and immerse the membrane in deionized water for standby use.

[0038] The prepared membrane surface is a porous structure (such as Figure 1 The water flux of the membrane is 22.17 L·m -2 ·h -1 , the reverse salt flux is 2.91 g·m -2 ·h -1 , showing that the introduction of hydrophilic polymers significantly increased the water flux of the membrane, while the reverse salt flux decreased significantly.

[0039] Example 3

[0040] The forward osmosis membrane based on the heat-assisted surface segregation method was prepared. The steps of Example 3 were the same as those of Comparative Example 1, except that after the membrane was prepared, the membrane was placed in a hot water bath at 90°C for 10 minutes. The water flux of the prepared membrane was 0.88 L·m -2 ·h -1 , the reverse salt flux is 3.92 g·m -2 ·h -1 It can be seen that the performance of the polyethersulfone membrane prepared by the casting solution without polyamic acid becomes worse after heat treatment, and the water flux drops sharply and is almost lost.

[0041] Example 4

[0042] A forward osmosis membrane based on the heat-assisted surface segregation method was prepared. The steps of Example 4 were the same as those of Comparative Example 2, except that after the membrane was prepared, the membrane was placed in a hot water bath at 90° C. for water bath heat treatment for 5 minutes.

[0043] Example 5

[0044] A forward osmosis membrane based on the heat-assisted surface segregation method was prepared. The steps of Example 5 were the same as those of Comparative Example 2, except that after the membrane was prepared, the membrane was placed in a hot water bath at 90° C. for water bath heat treatment for 100 min.

[0045] Figure 2 The pure water flux of Comparative Example 2, Example 4 and Example 5 is shown. Figure 3 The reverse salt flux of Comparative Example 2, Example 4 and Example 5 is shown. From the comparison between Comparative Example 2 and Example 4 in the figure, it can be seen that the water flux of the membrane containing polyamic acid is significantly improved after heat treatment, and at the same time, the pure water flux of the forward osmosis membrane based on heat-assisted surface segregation increases with the increase of heat treatment time, and the reverse salt flux decreases with the increase of heat treatment time, and the permeability and selectivity are improved simultaneously. Figure 4 The Zeta potential of the forward osmosis membrane based on heat-assisted surface segregation and the membrane that has not been heat treated is shown. The Zeta potential of the heat-treated membrane is lower than that of the non-heat-treated membrane. The reason is that polyamic acid contains a large number of carboxyl groups, and heat treatment enhances the enrichment of polyamic acid on the membrane surface, thereby enhancing the negative charge of the membrane surface.

[0046] Example 6

[0047] The forward osmosis membrane based on the heat-assisted surface segregation method was prepared. The steps of Example 6 were the same as those of Comparative Example 2, except that after the membrane was prepared, the membrane was placed in an oven at 50° C. for air heat treatment for 10 minutes.

[0048] Example 7

[0049] The forward osmosis membrane based on the heat-assisted surface segregation method was prepared. The steps of Example 7 were the same as those of Comparative Example 2, except that after the membrane was prepared, the membrane was placed in an oven at 80° C. for air heat treatment for 10 minutes.

[0050] Example 8

[0051] A forward osmosis membrane based on the heat-assisted surface segregation method was prepared. The steps of Example 8 were the same as those of Comparative Example 2, except that after the membrane was prepared, the membrane was placed in an oven at 100° C. for air heat treatment for 10 minutes.

[0052] The forward osmosis membranes prepared in Examples 6 to 8 and Comparative Example 2 were subjected to flux tests, and the results are shown in Table 1.

[0053] Table 1 Effect of different air heat treatment temperatures on forward osmosis membrane performance

[0054]

[0055]

[0056] According to the above test results, as the air heat treatment temperature increases, compared with Comparative Example 2 which has not undergone temperature enhanced segregation, the water flux of the embodiment does not decrease much, but the reverse salt flux decreases significantly. It can be seen from the data of the embodiment and the comparative example that the preparation method of the present invention enables the forward osmosis membrane to have higher selectivity through the modification of the membrane.

[0057] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for preparing a forward osmosis membrane based on a heat-assisted surface segregation method, characterized in that The following steps are involved: Step 1): Preparation of high-throughput hydrophilic ultrafiltration membrane by in-situ polycondensation: dissolving a membrane-forming polymer in an organic solvent, heating and stirring to obtain a preliminary casting solution, cooling to room temperature, adding a hydrophilic polymer and continuing to stir for a certain period of time, then vacuum degassing to obtain a clear and transparent casting solution, and then fixing the non-woven fabric on a clean and smooth glass plate for film scraping, and placing it in a 20°C water bath for phase transformation to obtain a hydrophilic ultrafiltration membrane; Step 2): Heat treatment: The hydrophilic ultrafiltration membrane is heat treated to improve the forward osmosis performance of the membrane through heat assistance, and the heat treatment time and heat treatment temperature of the membrane are changed to finally obtain a high-performance forward osmosis membrane.

2. The method for preparing a forward osmosis membrane based on a heat-assisted surface segregation method according to claim 1, characterized in that: The raw materials of the casting solution in step 1) are 10-25 parts by weight of film-forming polymer, 70-85 parts by weight of organic solvent, 0.2-5 parts by weight of monomer A, and 0.2-5 parts by weight of monomer B.

3. The method for preparing a forward osmosis membrane based on a heat-assisted surface segregation method according to claim 2, characterized in that: The organic solvent in step 1) is any one of N-methylpyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide.

4. The method for preparing a forward osmosis membrane based on a thermally assisted surface segregation method according to claim 2, characterized in that: The monomer A in step 1) is any one of 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone, p-phenylenediamine, and 4,4'-diaminodiphenylmethane; the monomer B is any one of pyromellitic acid dianhydride, diphenyl ether dianhydride, and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride; and the molar ratio of monomer A to monomer B is 1:1 to 1.

02.

5. The method for preparing a forward osmosis membrane based on a heat-assisted surface segregation method according to claim 1, characterized in that: The heat treatment method in step 2) is one of vacuum heat treatment, steam bath heat treatment and water bath heat treatment.

6. The method for preparing a forward osmosis membrane based on a heat-assisted surface segregation method according to claim 5, characterized in that: The water bath heat treatment solvent in step 2) is one of water, ethanol, ethylene glycol or a mixed solution of any two.

7. The method for preparing a forward osmosis membrane based on a heat-assisted surface segregation method according to claim 1, characterized in that: The heat treatment temperature in step 2) is 30-100°C, and the heat treatment time is 5-100 minutes.

8. The method for preparing a forward osmosis membrane based on a heat-assisted surface segregation method according to claim 1, characterized in that: The forward osmosis membrane described in step 2) is placed in a forward osmosis test device, 1 mol / L sodium chloride solution is selected as the draw liquid, deionized water is selected as the raw material liquid, and the flow rate of the raw material liquid and the draw liquid is controlled by a flow meter to be maintained at 150 mL / min.

9. A forward osmosis membrane obtained by the method for preparing a forward osmosis membrane based on a heat-assisted surface segregation method according to any one of claims 1 to 8.

Images