A method for surface modification of a membrane material to improve membrane performance

Abstract

The present invention belongs to the technical field of polyamide membranes. By modifying the membrane material to improve the performance of the membrane, a method for improving the performance of nanofiltration membranes and thus enhancing the operating efficiency of the membrane system is provided. Under alkaline conditions, the membrane material is chlorinated, which can significantly increase the water flux while maintaining or even enhancing the rejection rate. On this basis, a certain concentration of ferric ions is added, further improving the performance of the membrane, that is, the volume of leachate permeated through the membrane per unit area per unit time is more. The above-mentioned method for modifying the membrane material can significantly improve the operating efficiency of the entire membrane system, achieving the purpose of energy conservation and consumption reduction, and has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanofiltration membranes, and particularly relates to the modification of polyamide membrane materials to improve their performance. Background Art

[0002] Fresh water resources in our country are scarce, and the total amount of sewage discharged each year is large, with complex components and high treatment difficulty. The water environment quality is closely related to human health, so the requirements for water treatment are getting higher and higher. Membrane separation technology has been widely used in the treatment of various water bodies, including desalination, preparation of pure water and ultrapure water, wastewater treatment, desalination of seawater and brackish water, etc., due to its advantages such as good effluent quality, low energy consumption, and high efficiency, significantly promoting the development of water treatment technology. Aromatic polyamide membrane composites have characteristics such as high flux, stable chemical properties, and good heat resistance, and are one of the most widely used nanofiltration and reverse osmosis membrane materials in the fields of water treatment and seawater desalination. However, during long-term operation, polyamide membranes usually suffer from membrane fouling problems such as organic, inorganic, and biological fouling. Among them, biological fouling is difficult to remove through conventional pretreatment and cleaning processes. Therefore, in order to prevent and treat membrane biofouling, disinfectants are usually added for pretreatment at the front end of the membrane treatment process, or disinfectants are intermittently added for cleaning during the operation of the membrane system. Currently, commonly used disinfectants include active chlorine, chloramine, hydrogen peroxide, etc., and active chlorine is widely used due to its advantages such as low price and mature process.

[0003] The chlorine resistance of polyamide membranes is poor and they are extremely vulnerable to attack by active chlorine, resulting in damage to the surface and internal structures. Therefore, the long-term exposure of the polyamide layer to active chlorine is one of the main reasons for the decline of the membrane separation function. In view of this, reducing agents such as activated carbon or sodium metabisulfite are usually added at the front end of the membrane process to remove residual chlorine. However, the remaining residual chlorine may still come into contact with the polyamide membrane material, and after long-term operation, the membrane material will still degrade to a certain extent. Existing research generally believes that active chlorine will attack the amide bonds and benzene rings in the polyamide chain, causing the membrane to be chlorinated; in addition, the chlorinated polyamide chain may also undergo hydrolysis, and the hydrolysis reduces the crosslinking degree of the membrane, resulting in changes in the membrane separation performance. In addition to active chlorine exposure, the chlorine resistance of polyamide membranes may also depend on other actual water quality conditions.

[0004] Iron is ubiquitous in the water environment. The iron content in the groundwater of the Songnen Plain and the Baihe River Basin reaches 3 and 7 mg / L respectively. Iron-containing chemicals are commonly used in water treatment, such as polyferric sulfate as a coagulant to remove colloidal particles, and ferrous sulfate as a Fenton reaction catalyst to remove refractory organic pollutants. In addition, equipment corrosion is common in the water treatment process, and the presence of chloride ions greatly enhances the degree of corrosion. For example, a membrane filtration system composed of all-steel materials in 10 mM Cl -Under the condition of only running for eight days, the iron generated in the system reached about 0.2 mg / L. Existing studies have pointed out that when hydrogen peroxide and iron coexist, hydroxyl radicals will be formed through the Fenton-like reaction. Hydroxyl radicals have strong oxidizing properties and will accelerate the degradation of polyamide membranes, which is the main reason for the change in membrane separation performance. Compared with hydrogen peroxide, active chlorine has the advantages of low price and mature technology and is a more commonly used disinfectant. Existing studies have preliminarily reported the effect of fixed-concentration iron on membrane chlorination. However, there is still a lack of comprehensive evaluation of the coexistence system of iron and active chlorine on membrane performance. Furthermore, it is of great practical significance to explore the coexistence exposure conditions of iron and active chlorine that can significantly improve membrane performance. Summary of the Invention

[0005] The purpose of the present invention is to provide a technology for improving the performance of membranes through membrane material modification, and to provide a method for significantly increasing the water flux under the condition of unchanged or even enhanced rejection rate, thereby improving the operating efficiency of the entire membrane system.

[0006] The present invention takes the typical fully aromatic polyamide nanofiltration membrane NF90 as the research object and uses the coexistence environment of sodium hypochlorite and ferric sulfate as the reaction system.

[0007] The present invention modifies the polyamide membrane material, performs chlorination treatment on the membrane material under alkaline conditions, significantly increases the water flux under the condition of unchanged or even enhanced rejection rate, thereby improving the operating efficiency of the entire membrane system.

[0008] The technical solution of the present invention is as follows:

[0009] A method for modifying a nanofiltration membrane material, which includes the following modification steps:

[0010] (1) First, the cut NF90 membrane is rinsed and soaked in deionized water for more than 24 h. The membrane treatment process is carried out in a 250 mL brown glass bottle.

[0011] (2) Prepare an active chlorine solution with a volume of 200 mL and a concentration of 400 mg / L in the glass bottle. Use ferric sulfate (Fe2(SO4)3·xH2O) to control the content of ferric ions in the influent water to be 10 mg / L, and adjust the pH to 10.0 or 12.0 using 1 mol / L NaOH solution according to the experimental design.

[0012] (3) After adding the membrane, move the glass bottle to a constant temperature shaker, adjust the temperature of the shaker to 25 °C, the rotation speed to 120 r / min, and the reaction time to 24 h.

[0013] (4) After the reaction time ends, take out the membrane and rinse both sides of the membrane 6 times with deionized water to remove the residual active chlorine and ferric ions on the membrane surface.

[0014] After standing for a period of time, the nanofiltration membrane material can be prepared.

[0015] The beneficial effects of the present invention are as follows:

[0016] According to the present invention, chlorination treatment of the membrane material under alkaline conditions can significantly increase the water flux while maintaining or even enhancing the rejection rate, that is, more volume of leachate permeates through the membrane per unit area per unit time, thereby improving the operating efficiency of the entire membrane system. Specific embodiments

[0017] The present invention will be further described below through specific examples, but the protection scope of the present invention is not limited thereto. All variations within the content and scope of the present invention should be included in the technical scope of the present invention.

[0018] Static experiment: A static experiment means that first, according to the experimental design, the membrane is prepared in a glass bottle under relevant designed conditions and then transferred to a constant temperature shaking incubator for reaction. After a certain period of time, the membrane is taken out and subjected to pressure membrane testing in a membrane filtration device.

[0019] The present invention takes the typical fully aromatic polyamide nanofiltration membrane NF90 as the research object and uses the coexistence environment of sodium hypochlorite and ferric sulfate as the reaction system. Under static experimental conditions, Fe 3+ is selected as the exogenous iron supplement to systematically compare the filtration performance of the polyamide membrane after chlorination degradation, and ferric sulfate (Fe2(SO4)3·xH2O) is used to control the content of ferric ions in the influent water.

[0020] Example 1. pH = 10.0

[0021] (1) 400 mg / L active chlorine solution

[0022] The cut NF90 membrane is rinsed and soaked in deionized water for more than 24 h. The membrane treatment process is carried out in a 250 mL brown glass bottle. A certain volume of 200 mL of 400 mg / L active chlorine solution is prepared in the glass bottle. According to the experimental design, 1 mol / L NaOH solution is used to adjust the pH to 10.0. After adding the membrane, the glass bottle is transferred to a constant temperature shaking incubator, and the temperature of the shaking incubator is adjusted to 25 °C, the rotation speed is 120 r / min, and the reaction time is 24 h. After the reaction time ends, the membrane is taken out and rinsed on both sides 6 times with deionized water to remove the residual active chlorine on the membrane surface. Finally, the membrane is installed in a membrane filtration device for filtration performance testing. The feed liquid in the system contains 10 mM NaCl for determining the rejection rate, and the system pressure is adjusted to 100 psi, the flow rate is 1 L / min, the temperature is 25 °C, and the operation time is 24 h.

[0023] Implementation result: The water flux of the original membrane is 40.25 L / (m 2h), the rejection rate is 93%, and the water flux of the chlorinated membrane is 43 L / (m 2 h), the rejection rate is 98.5%, that is, the water flux of the chlorinated membrane increases by 7%, and the rejection rate increases by 6%.

[0024] (2) 400 mg / L active chlorine solution, 10 mg / L ferric ion

[0025] Rinse and soak the cut NF90 membrane in deionized water for more than 24 h. The membrane treatment process is carried out in a 250 mL brown glass bottle. Prepare a certain amount of 200 mL active chlorine solution with a concentration of 400 mg / L in the glass bottle. According to the experimental design, use 1 mol / L NaOH solution to adjust the pH to 10.0, then add ferric sulfate to the solution to make the ferric ion concentration reach 10 mg / L. After putting the membrane in, move the glass bottle to a constant temperature shaker, adjust the temperature of the shaker to 25 °C, the rotation speed to 120 r / min, and the reaction time to 24 h. After the reaction time ends, take out the membrane and rinse both sides of the membrane 6 times with deionized water to remove the residual active chlorine and ferric ion on the membrane surface. Finally, put the membrane into a membrane filtration device for filtration performance testing. The feed liquid in the system contains 10 mM NaCl for measuring the rejection rate. Adjust the system pressure to 100 psi, the flow rate to 1 L / min, the temperature to 25 °C, and the running time to 24 h.

[0026] Implementation result: The water flux of the original membrane is 40.25 L / (m 2 h), the rejection rate is 93%, and the water flux of the chlorinated (iron-containing) membrane is 59.71 L / (m 2 h), the rejection rate is 93.36%, that is, the water flux of the chlorinated membrane increases by 48%, and the rejection rate increases by 0.4%.

[0027] Example 2. pH = 12.0

[0028] (1) 400 mg / L active chlorine solution

[0029] The cut NF90 membrane was rinsed and soaked in deionized water for more than 24 h. The membrane treatment process was carried out in a 250 mL brown glass bottle. A certain amount of 200 mL active chlorine solution with a concentration of 400 mg / L was prepared in the glass bottle. According to the experimental design, 1 mol / L NaOH solution was used to adjust the pH to 12.0. After adding the membrane, the glass bottle was transferred to a constant temperature shaking incubator, and the temperature of the shaking incubator was adjusted to 25 °C, the rotation speed was 120 r / min, and the reaction time was 24 h. After the reaction time ended, the membrane was taken out and rinsed 6 times on both sides with deionized water to remove the residual active chlorine on the membrane surface. Finally, the membrane was installed in a membrane filtration device for filtration performance testing. The feed liquid in the system contained 10 mM NaCl for determining the rejection rate, and the system pressure was adjusted to 100 psi, the flow rate was 1 L / min, the temperature was 25 °C, and the running time was 24 h.

[0030] Implementation results: The water flux of the original membrane was 40.25 L / (m 2 h), and the rejection rate was 93%. The water flux of the chlorinated membrane was 51.21 L / (m 2 h), and the rejection rate was 96.96%. That is, the water flux of the chlorinated membrane increased by 27%, and the rejection rate increased by 4%.

[0031] (2)400 mg / L active chlorine solution, 10 mg / L ferric ion

[0032] The cut NF90 membrane was rinsed and soaked in deionized water for more than 24 h. The membrane treatment process was carried out in a 250 mL brown glass bottle. A certain amount of 200 mL active chlorine solution with a concentration of 400 mg / L was prepared in the glass bottle. According to the experimental design, 1 mol / L NaOH solution was used to adjust the pH to 12.0, and then ferric sulfate was added to the solution to make the ferric ion concentration reach 10 mg / L. After adding the membrane, the glass bottle was transferred to a constant temperature shaking incubator, and the temperature of the shaking incubator was adjusted to 25 °C, the rotation speed was 120 r / min, and the reaction time was 24 h. After the reaction time ended, the membrane was taken out and rinsed 6 times on both sides with deionized water to remove the residual active chlorine and ferric ion on the membrane surface. Finally, the membrane was installed in a membrane filtration device for filtration performance testing. The feed liquid in the system contained 10 mM NaCl for determining the rejection rate, and the system pressure was adjusted to 100 psi, the flow rate was 1 L / min, the temperature was 25 °C, and the running time was 24 h.

[0033] Implementation results: The water flux of the original membrane was 40.25 L / (m 2 h), and the rejection rate was 93%. The water flux of the chlorinated (iron-containing) membrane was 79 L / (m 2 h), and the rejection rate was 93.55%. That is, the water flux of the chlorinated membrane increased by 96%, and the rejection rate increased by 0.6%.

[0034] In summary, at pH = 10.0 and pH = 12.0, under the exposure condition of a single 400 mg / L Cl2, both the water flux and the rejection rate increased, rising by 7%, 27% and 6%, 4% respectively. After adding 10 mg / L Fe 3+ , the water fluxes increased by 48% and 96% respectively, and the rejection rate was comparable to that of the original membrane. The above research results show that under alkaline conditions, treating the membrane surface with a certain concentration of active chlorine can improve the membrane filtration performance; after adding a certain amount of Fe 3+ , the water flux of the polyamide membrane can be significantly increased without changing the membrane rejection rate.

Claims

1. A method for modifying a nanofiltration membrane material, characterized in that, The modification steps include: (1) Rinse and soak the cut NF90 membrane in deionized water for more than 24 hours. The membrane treatment process is carried out in a brown glass bottle with a capacity of 250 mL; (2) Add deionized water to a brown glass bottle, add NaOH, and adjust the pH to 10.0 or 12.0; add sodium hypochlorite with an active chlorine concentration of ≥7.5% to the solution, the active chlorine concentration is calculated as Cl2, and the active chlorine content of the solution is 400 mg / L, and the solution volume is 200 ml; (3) Add ferric sulfate to the solution to make the concentration of trivalent iron ions reach 10 mg / L, mix thoroughly, and then place the polyamide membrane; (4) Move the glass bottle to a constant temperature oscillating box, adjust the temperature of the oscillating box to 25°C, the speed to 120 r / min, and the reaction time to 24 h; after the reaction time is up, take out the membrane and rinse both sides of the membrane with deionized water for 6 times to remove the residual active chlorine and trivalent iron on the membrane surface; (5) After standing for a period of time, the nanofiltration membrane material can be prepared.

2. A nanofiltration membrane, comprising a nanofiltration membrane matrix layer, wherein the nanofiltration membrane matrix layer is a polyamide thin film layer, characterized in that, The polyamide film layer is modified by the nanofiltration membrane material modification method according to claim 1.