CN103537200B - A kind of Cellulose acetate forward osmotic membrane and preparation method thereof - Google Patents

Abstract

The invention relates to a cellulose acetate forward osmosis membrane and a preparation method thereof. The preparation is based on the ratio of adding 20-40ml of acetone, 50-70ml of 1,4-dioxane, and 7-10ml of methanol per 3-5g of cellulose acetate to configure the raw materials, and add 5-10ml of pore-forming agent lactic acid or 0.8-28g of zinc chloride; mix the raw materials evenly, stir at room temperature to completely dissolve the cellulose acetate, and let the casting solution stand for more than 12 hours; soak in sodium hydroxide solution; soak in the casting solution for 30-60s, After pulling once, soak in deionized water for 2-6 hours to carry out phase inversion to obtain a cellulose acetate forward osmosis membrane, the middle basement membrane is a polysulfone membrane, and form attached cellulose acetate membranes on both sides of the basement membrane; the preparation method of the present invention The process is simple, and the prepared forward osmosis membrane can effectively prevent solutes from entering the porous support layer, and has good chemical stability, high salt rejection rate and membrane flux.

Description

A kind of cellulose acetate forward osmosis membrane and preparation method thereof

technical field

The invention relates to a forward osmosis membrane, in particular to a cellulose acetate forward osmosis membrane and a preparation method thereof, belonging to the technical field of polymer separation membranes.

Background technique

Rapid population growth and depletion of fossil fuels have made water and energy two of the world's most important resources. Water shortages and energy crises plague many parts of the world, and it is reported that more than 1.2 billion people in the world lack clean and healthy drinking water. Membrane separation technology operates at room temperature and has no phase change. It is a promising technology for regenerating drinking water.

The forward osmosis technology developed in recent years is a technology that uses the principle of osmosis for membrane separation. The difference in osmotic pressure of the solution on both sides of the membrane is used to make water molecules spontaneously pass through the semi-permeable membrane, from the raw material solution with high water chemical potential to the In the draw solution with low water chemical potential, it can be used to extract fresh water directly from sea water. At present, the only commercial forward osmosis membrane is the cellulose triacetate membrane (CTA) of HTI Company in the United States, which is composed of an active dense layer and a polyester support layer, and has high membrane flux and salt rejection rate.

Theoretically, forward osmosis can achieve higher membrane flux than reverse osmosis by drawing liquid with high osmotic pressure. However, the research found that the actual membrane flux is much smaller than the theoretical value. Symmetrical structure, the dense structure of the active layer leads to a serious internal concentration polarization phenomenon in the forward osmosis process, resulting in a significant decrease in the effective driving pressure, thereby significantly reducing the membrane flux. Since the internal concentration polarization exists in the membrane pores, it can only be alleviated by improving the membrane structure and membrane performance, and the internal concentration polarization is the root cause of the drastic decline in the flux of the forward osmosis membrane, which often reduces the membrane flux by 80%.

At present, for the phenomenon of internal concentration polarization, effective mitigation methods include increasing the pore size of the support layer, using hydrophilic materials to prepare the support layer, or preparing dual active layers. The polysulfone ultrafiltration support base membrane has a finger-like structure, which is beneficial to reduce the internal concentration polarization phenomenon, and the modified hydrophilic polysulfone membrane can improve the membrane flux. Hydrophilic cellulose acetate (CA) is used to prepare the active layer, and the cellulose acetate forward osmosis membrane prepared by using lactic acid as a pore-forming agent has a high salt rejection rate and membrane flux.

In the current invention patent, some exploratory and comparative experiments have been done on various film-forming solutes, solvents, additives, etc. The film-making method used is to scrape the film with a scraper, and the film-forming solutes include CA, CTA, and cellulose acetate butyrate. Su et al., their experimental method is more complicated. For example, the patent number CN201210453553.3 has done exploratory tests on various additives and film-forming solute solvents. The obtained film was obtained when deionized water was used as the raw material solution and 1mol/L sodium chloride was used as the draw solution. The throughput is not very high, the highest can reach 13.12L/m ² .h, and the back-mixing flux of salt can reach 3.32g/m ² .h. In the patent number CN201210137147.6, highly toxic raw materials are used, such as polyamine solution and polyacyl chloride solution to make molds. When pure water is used as the raw material solution and 1mol/L sodium chloride solution is used as the drawing solution, the obtained The membrane flux of the hollow fiber membrane is only 5.97L/m ² .h. In Patent No. CN201310092067.8, the polysulfone is firstly modified to prepare a hydrophilic carboxylated polysulfone ultrafiltration membrane, and then in-situ polymerized on the ultrafiltration membrane to form a polyamide forward osmosis membrane. This method still uses highly toxic reagents. Phenylenediamine and trimesoyl chloride are used as raw materials to synthesize polyamide forward osmosis membranes, which is not conducive to environmental protection, and under the experimental conditions of using 2g/L sodium chloride as the raw material solution and 2mol/L glucose as the draw solution, the membrane pass The amount is 13.9L/m ² .h, and the salt interception rate is only 96.12%. The ultra-thin double-skin forward osmosis membrane prepared by Patent No. CN201210578252.3 uses environmentally friendly cellulose triacetate and cellulose diacetate as film-forming agents. When mol/L sodium chloride is used as the raw material solution and 4mol/L glucose is used as the draw solution, the membrane flux is 12.8L/m ² .h, and the salt cut-off rate reaches 96.8%.

Contents of the invention

One of the objects of the present invention is to provide a cellulose acetate forward osmosis membrane with good positive hydrophilicity, high membrane flux, good salt cut-off rate and good mechanical properties. Another object of the present invention is to provide a preparation method of the cellulose acetate forward osmosis membrane.

The film-making solute used in the present invention is simple cellulose acetate with a molecular weight of 40,000-50,000, the solvent is a mixed solvent mixed with 1,4-dioxane and acetone in a certain proportion, the additive is a single methanol, and a pore-forming agent Different materials were selected, mixed and dissolved in a certain proportion to defoam evenly, and a simple impregnation method was used to form a film with a uniform thickness. When liquid is used, the membrane flux that can be obtained reaches 17.17L/m ² .h, and the salt interception rate reaches over 99.999%, which is very suitable as a forward osmosis membrane required for seawater desalination. Moreover, if the forward osmosis membrane prepared in the present invention is properly post-treated in the later stage of film formation, such as soaking with different concentrations of non-ionic surfactants, the membrane flux can be significantly improved.

The object of the invention is achieved through the following technical solutions:

A preparation method of cellulose acetate forward osmosis membrane, comprising the steps of:

1) Preparation of raw materials: Prepare raw materials according to the ratio of adding 20-40mL of acetone, 50-70mL of 1,4-dioxane, and 7-10mL of methanol for every 3-5g of cellulose acetate, and add 5-10mL of pore-forming agent lactic acid or 0.8-28g of zinc chloride;

2) Mix the raw materials evenly, stir at room temperature to completely dissolve the cellulose acetate to obtain a casting solution, and let the casting solution stand for more than 12 hours until it completely defoams;

3) Soak the porous polysulfone membrane with a sodium hydroxide solution with a mass concentration of 8%-12% for 30-70min to remove impurities adsorbed on the surface, then wash it with deionized water, and use it as a support layer after drying. use;

4) Soak the support layer obtained from the pretreatment in step 3) in the casting solution in step 2 for 30-60 seconds, pull it once and then soak it in deionized water for 2-6 hours to perform phase inversion to obtain cellulose acetate with uniform pore size Forward osmosis membrane, the base membrane in the middle is a polysulfone membrane, which is a porous support layer, and cellulose acetate membranes are formed on both sides of the base membrane, which is the active layer;

5) Put the forward osmosis membrane obtained in step 4) into a 40-90 degree water bath for heat treatment, wash it and store it in a sodium bisulfite solution with a mass concentration of 0.3%-0.8% for low temperature preservation.

In order to further realize the purpose of the invention, the molecular weight of cellulose acetate in the active layer of the invention is preferably 40000-50000. The stirring is preferably mechanical stirring. The phase inversion time is 3-5h. The heat treatment time is preferably 5-60 min.

A cellulose acetate forward osmosis membrane is prepared by the above preparation method.

Compared with the prior art, the present invention has the following advantages:

1) The cellulose acetate forward osmosis membrane prepared in the inventive method takes cellulose acetate as main material, and cellulose acetate has good hydrophilicity and film-forming property, and specific surface area is big, and adsorption capacity is low, and can have better cut-off. Salt effect, compared with the existing technology, the salt interception rate has been significantly improved.

2) The cellulose acetate forward osmosis membrane prepared by the inventive method has a hydrophilic support layer and two layers of hydrophilic active layers, and the membrane flux that can be achieved in the experimental process has 17L/m ² .h, and the salt cut-off rate Both have reached more than 99.99%, while other forward osmosis membranes can reach a membrane flux of 13L/m ² .h, and a salt cut-off rate of 97%. The cellulose acetate forward osmosis membrane prepared by the method of the present invention has a membrane With the characteristics of high flux and good salt interception rate, it can better solve the problem of internal concentration polarization and can be used in seawater desalination, sewage treatment and food processing and other fields.

3) The hydrophilic polysulfone membrane of the present invention can provide a good support layer, has high thermal stability, is resistant to hydrolysis, and has a strong binding force with cellulose acetate without affecting the membrane flux.

4) The present invention uses lactic acid or zinc chloride as the pore-forming agent, which has a small molecular weight and can ensure a high salt-cutting effect.

detailed description

In order to better understand the present invention, the present invention will be further described below in conjunction with examples, but the embodiments of the present invention are not limited thereto.

Example 1.

Step 1: Prepare raw materials: 3.0 g of cellulose acetate, the molecular weight of cellulose acetate is 40320 g/mol, 52.5 mL of 1,4-dioxane, 22 mL of acetone, 8.2 mL of methanol, and 6.6 mL of lactic acid;

Step 2: Add the solid and liquid weighed in step 1 into a round-bottomed flask and mix evenly, mechanically stir at room temperature to completely dissolve the cellulose acetate to obtain a cellulose acetate casting solution, and let the casting solution stand for more than 12 hours to use its complete defoaming;

Step 3: Soak the porous polysulfone membrane JQE-3631-130326 produced by Shenzhen Jiaquan Membrane Filtration Equipment Co., Ltd. in a sodium hydroxide solution with a mass concentration of 10% for 30 minutes to remove impurities adsorbed on the surface. Then wash and dry with deionized water for later use;

Step 4: Soak the polysulfone membrane support layer obtained from the pretreatment in step 3 in the casting solution in step 2 for 60 seconds, pull it once and then soak it in deionized water for 6 hours; it can be seen by electron microscope that the membrane is divided into Three layers, with a porous support layer in the middle and cellulose acetate active layers formed on both sides;

Step 5: heat-treat the membrane obtained in Step 4 in a water bath at 60°C for 10 minutes to obtain a cellulose acetate forward osmosis membrane with uniform pore size.

The forward osmosis membrane was tested with 3.5% sodium chloride solution as the drawing solution and deionized water as the raw material solution.

The test result of the forward osmosis membrane is that the membrane flux is 13.1L/m ² .h, and the salt cut-off rate is 99.999%. Compared with the prior art, the test results show that the salt interception rate has been significantly improved under the condition of equivalent membrane flux.

Example 2.

Step 1: Preparation of raw materials: 3.3g of cellulose acetate, the molecular weight of cellulose acetate is 40320g/mol, 52.5mL of 1,4-dioxane, 22mL of acetone, 8.2mL of methanol, 6.6mL of lactic acid;

Step 2: Add the solid and liquid weighed in step 1 into a round-bottomed flask and mix evenly, mechanically stir at room temperature to completely dissolve the cellulose acetate to obtain a cellulose acetate casting solution, and let the casting solution stand for more than 12 hours to use its complete defoaming;

Step 3: Soak the porous polysulfone membrane with 10% sodium hydroxide solution to remove the impurities adsorbed on the surface, then wash it with deionized water and dry it before use;

Step 4: Soak the support layer obtained from the pretreatment in step 3 in the casting solution in step 2 for 60 seconds, pull it once and then soak it in deionized water for 6 hours for phase inversion;

Step 5: heat-treat the membrane obtained in Step 4 in a water bath at 60°C for 10 minutes to obtain a cellulose acetate forward osmosis membrane with uniform pore size.

The forward osmosis membrane was tested with 3.5% sodium chloride solution as the drawing solution and deionized water as the raw material solution.

The test result of the forward osmosis membrane is that the membrane flux is 17.17L/m ² .h, and the salt rejection rate is 99.998%. The experimental results show that, under the same other conditions, appropriately increasing the amount of cellulose acetate will increase the membrane flux of the forward osmosis membrane, but the salt rejection rate will basically remain unchanged. Compared with the prior art, the test result has significantly improved the salt interception rate.

Example 3.

Step 1: Preparation of raw materials: 3.3g of cellulose acetate, the molecular weight of cellulose acetate is 40320g/mol, 52.5mL of 1,4-dioxane, 22mL of acetone, 8.2mL of methanol, 6.6mL of lactic acid;

Step 2: Add the solid and liquid weighed in step 1 into a round-bottomed flask and mix evenly, mechanically stir at room temperature to completely dissolve the cellulose acetate to obtain a cellulose acetate casting solution, and let the casting solution stand for more than 12 hours to use its complete defoaming;

Step 3: Soak the porous polysulfone membrane with 10% sodium hydroxide solution to remove the impurities adsorbed on the surface, then wash it with deionized water and dry it before use;

Step 4: Soak the support layer obtained from the pretreatment in step 3 in the casting solution in step 2 for 60 seconds, pull it once and then soak it in deionized water for 6 hours for phase inversion;

Step 5: heat-treat the membrane obtained in Step 4 in a water bath at 60°C for 20 minutes to obtain a cellulose acetate forward osmosis membrane with uniform pore size.

The forward osmosis membrane was tested with 3.5% sodium chloride solution as the drawing solution and deionized water as the raw material solution.

The test result of the forward osmosis membrane is that the membrane flux is 14.65L/m ² .h, and the salt rejection rate is 99.998%. The test results show that when other conditions are the same, appropriately increasing the heat treatment time will reduce the membrane flux of the forward osmosis membrane, but the salt rejection rate will basically remain unchanged. Compared with the prior art, the test result has significantly improved the salt interception rate.

Example 4.

Step 1: Preparation of raw materials: 3.3g of cellulose acetate, the molecular weight of cellulose acetate is 40320g/mol, 52.5mL of 1,4-dioxane, 22mL of acetone, 8.2mL of methanol, 6.6mL of lactic acid;

Step 2: Add the solid and liquid weighed in step 1 into a round-bottomed flask and mix evenly, mechanically stir at room temperature to completely dissolve the cellulose acetate to obtain a cellulose acetate casting solution, and let the casting solution stand for more than 12 hours to use its complete defoaming;

Step 3: Soak the porous polysulfone membrane with 10% sodium hydroxide solution to remove the impurities adsorbed on the surface, then wash it with deionized water and dry it before use;

Step 4: Soak the support layer obtained from the pretreatment in step 3 in the casting solution in step 2 for 60 seconds, pull it once and then soak it in deionized water for 6 hours for phase inversion;

Step 5: heat-treat the membrane obtained in step 4 in a water bath at 70°C for 10 minutes to obtain a cellulose acetate forward osmosis membrane with uniform pore size.

The forward osmosis membrane was tested with 3.5% sodium chloride solution as the drawing solution and deionized water as the raw material solution.

The test result of the forward osmosis membrane is that the membrane flux is 7.5L/m ² .h, and the salt cut-off rate is 99.995%. The experimental results show that when other conditions are the same, appropriately increasing the heat treatment temperature will significantly reduce the membrane flux of the forward osmosis membrane, and the salt rejection rate will basically remain unchanged. Compared with the prior art, the test result has significantly improved the salt interception rate.

Example 5

Step 1: Preparation of raw materials: 3.3g of cellulose acetate, the molecular weight of cellulose acetate is 40320g/mol, 52.5mL of 1,4-dioxane, 22mL of acetone, 8.2mL of methanol, 3.0g of zinc chloride;

Step 2: Add the solid and liquid weighed in step 1 into a round-bottomed flask and mix evenly, mechanically stir at room temperature to completely dissolve the cellulose acetate to obtain a cellulose acetate casting solution, and let the casting solution stand for more than 12 hours to use its complete defoaming;

Step 3: Soak the porous polysulfone membrane with 10% sodium hydroxide solution to remove the impurities adsorbed on the surface, then wash it with deionized water and dry it before use;

Step 4: Soak the support layer obtained from the pretreatment in step 3 in the casting solution in step 2 for 60 seconds, pull it once and then soak it in deionized water for 6 hours for phase inversion;

Step 5: heat-treat the membrane obtained in Step 4 in a water bath at 60°C for 20 minutes to obtain a cellulose acetate forward osmosis membrane with uniform pore size.

The forward osmosis membrane was tested with 3.5% sodium chloride solution as the drawing solution and deionized water as the raw material solution.

The test result of the forward osmosis membrane is that the membrane flux is 9.7L/m ² .h, and the salt cut-off rate is 99.997%. The experimental results show that when other conditions are the same, the forward osmosis membrane prepared by using zinc chloride instead of lactic acid as a pore-forming agent also has a certain membrane flux and a high salt interception rate. Compared with the existing technology, the salt interception rate has been improved. There is a significant improvement.

Example 6.

Step 1: Prepare raw materials: 3.3g of cellulose acetate, the molecular weight of cellulose acetate is 40320g/mol, 52.5mL of 1,4-dioxane, 22mL of acetone, 8.2mL of methanol, 4.0g of zinc chloride;

Step 2: Add the solid and liquid weighed in step 1 into a round-bottomed flask and mix evenly, mechanically stir at room temperature to completely dissolve the cellulose acetate to obtain a cellulose acetate casting solution, and let the casting solution stand for more than 12 hours to use its complete defoaming;

Step 3: Soak the porous polysulfone membrane with 10% sodium hydroxide solution to remove the impurities adsorbed on the surface, then wash it with deionized water and dry it before use;

Step 4: Soak the support layer obtained from the pretreatment in step 3 in the casting solution in step 2 for 60 seconds, pull it once and then soak it in deionized water for 6 hours for phase inversion;

Step 5: heat-treat the membrane obtained in Step 4 in a water bath at 60°C for 20 minutes to obtain a cellulose acetate forward osmosis membrane with uniform pore size.

The forward osmosis membrane was tested with 3.5% sodium chloride solution as the drawing solution and deionized water as the raw material solution.

The test result of the forward osmosis membrane is that the membrane flux is 14.6L/m ² .h, and the salt cut-off rate is 99.999%. The experimental results show that when other conditions are the same, appropriately increasing the amount of pore-forming agent zinc chloride can increase the membrane flux and salt rejection rate of the forward osmosis membrane; improve.

Claims (4)

1. the preparation method of a Cellulose acetate forward osmotic membrane, it is characterised in that comprise the steps:

1) preparation raw material: add the proportions raw material of the acetone of 20-40mL, the Isosorbide-5-Nitrae-dioxane of 50-70mL, 7-10mL methanol according to every 3-5g cellulose acetate, and add the lactic acid of pore creating material 5-10mL or the zinc chloride of 0.8-28g；

2) by raw material mix homogeneously, stirring at normal temperatures makes cellulose acetate be completely dissolved, and obtains casting solution, stands more than casting solution 12h, to complete froth breaking；

3) by the sodium hydroxide solution immersion treatment 30-70min that polysulfone membrane mass concentration is 8%-12% of porous, remove the impurity that adsorbs of surface, then with after deionized water cleaning, drying as supporting layer, stand-by；

4) by step 3) in the supporting layer that obtains of pretreatment be immersed in step 2) in casting solution in 30-60s, lift is once dipped into afterwards in deionized water and carries out inversion of phases 2-6h, obtain the Cellulose acetate forward osmotic membrane of uniform pore diameter, middle basement membrane is polysulfone membrane, it it is porous support layer, the cellulose acetate membrane in attachment is formed, for active layer in basement membrane both sides；Described active layer cellulose acetate molecular weight is 40000-50000；

5) by step 4) obtain forward osmosis membrane and put in the water-bath of 40-90 degree and carry out heat treatment, it is placed on cryopreservation in the sodium sulfite solution that mass concentration is 0.3%-0.8% after cleaning；The described heat treated time is 5-60min。

2. the preparation method of a kind of Cellulose acetate forward osmotic membrane according to claim 1, it is characterised in that: described stirring is mechanical agitation。

3. the preparation method of a kind of Cellulose acetate forward osmotic membrane according to claim 1, it is characterised in that: the described inversion of phases time is 3-5h。

4. a Cellulose acetate forward osmotic membrane, it is characterised in that its preparation method described in any one of claim 1-3 prepares。