Modification method for industrial high-pressure reverse osmosis membrane

Abstract

The invention relates to the technical field of reverse osmosis membrane modification, and particularly discloses a method for modifying a high-pressure industrial reverse osmosis membrane into a household low-pressure reverse osmosis membrane. The separation layer structure of the original industrial reverse osmosis membrane is precisely regulated and controlled under the mild room temperature condition, and dual optimization of the porosity and hydrophilicity of the membrane is realized. The flux of the modified membrane is increased to 45.12 LMH or above from 15.74 LMH of an original industrial membrane under the 65psi household low-pressure working condition, the NaCl retention rate is stably maintained to be 96.5% or above, meanwhile, the membrane has the advantages that the process is simple, raw materials are easy to obtain, the modification cost is low, and the membrane is high in compatibility with existing household reverse osmosis equipment, conversion from the industrial membrane to the household membrane is efficiently achieved, and the membrane is suitable for large-scale popularization and application. The industrial problems of insufficient flux and high preparation cost of the household reverse osmosis membrane are solved, secondary utilization of industrial membrane resources is realized, and the method has large-scale production and popularization values.

Description

Technical Field

[0001] This invention belongs to the field of reverse osmosis membrane preparation technology for water treatment, and particularly relates to a modification method for industrial high-pressure reverse osmosis membrane sheets. The modified membrane sheets can be directly applied to the preparation of membrane elements for household low-pressure reverse osmosis water purification equipment, realizing the efficient conversion of industrial membrane resources to the household field. Background Technology

[0002] Reverse osmosis (RO) technology is the core technology of household water purification equipment. As a core functional component, the reverse osmosis membrane directly determines the water purification efficiency, effluent water quality, and user experience of household water purification equipment. Household reverse osmosis equipment is limited by household power supply conditions, equipment size, and installation space, and typically operates at a low pressure of 50-80 psi. Therefore, the core requirements for reverse osmosis membranes are high permeability and high salt rejection rate under low pressure.

[0003] Industrial reverse osmosis membranes are designed for high-pressure conditions exceeding 100 psi in applications such as industrial wastewater treatment and seawater desalination. Their separation layer utilizes a dense aromatic polyamide structure to ensure high-salt retention performance under high pressure. However, directly applying these industrial membranes to low-pressure household reverse osmosis systems presents a critical problem: insufficient flux at low pressures. The dense separation layer of industrial membranes results in high resistance to water molecule permeation, leading to fluxes typically below 20 LMH at 50-80 psi, far from meeting the actual daily water production requirements of 100-200 L for household systems.

[0004] To address the aforementioned issues, existing technologies have attempted to modify industrial diaphragms to suit low-voltage household applications, primarily employing single chemical etching or physical stretching methods. However, both methods suffer from significant drawbacks: 1. Single chemical etching often uses strong acids and strong bases as etching agents, which can easily damage the molecular skeleton of the aromatic polyamide separation layer of industrial membranes, causing the salt rejection rate of the membrane to drop sharply to below 90%, and the quality of the effluent cannot meet the standards for household water purification. 2. Physical stretching modification requires specialized large-scale stretching equipment, which is complex, has high equipment investment costs, and results in poor stability of the stretched membrane structure, which is prone to problems such as membrane cracking and pore deformation, making it difficult to achieve large-scale mass production.

[0005] Therefore, there is an urgent need in this field for an industrial reverse osmosis membrane modification method that can achieve a precise balance between increasing membrane flux and maintaining high salt rejection rate under low pressure, and that is simple in process, cost-controllable, and has good membrane stability, so as to promote the secondary utilization of industrial membrane resources and reduce the preparation and application costs of household reverse osmosis membranes. Summary of the Invention

[0006] The technical problem this invention aims to solve is that existing technologies suffer from insufficient flux when adapting industrial reverse osmosis membranes to low-pressure household applications, significant drop in rejection rate due to existing modification methods, and complex processes that are difficult to scale up. To address these shortcomings, this invention provides a two-step synergistic modification method involving mild sodium hypochlorite etching and alcohol pore optimization. This method enables the efficient and stable conversion of industrial high-pressure reverse osmosis membranes to low-pressure household reverse osmosis membranes, while ensuring that the modified membranes maintain both high water permeability and high salt rejection rate under 50-80 psi low-pressure household conditions. Furthermore, the modification process is simple, the raw materials are readily available, and the compatibility is strong.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution: A method for modifying industrial high-pressure reverse osmosis membranes includes the following steps: S1: Pre-modification treatment: The original industrial reverse osmosis membrane is immersed in a sodium hypochlorite aqueous solution with a mass concentration of 0.4%-0.6% and treated at a temperature of 20-25℃ for 60-72 hours to obtain a primary modified membrane; S2: Post-optimization treatment: The modified membrane is rinsed with deionized water 3-5 times, each time for 5-10 minutes, to remove residual sodium hypochlorite on the surface. Then, it is soaked in an alcohol solution with a volume concentration ≥95% and treated at a temperature of 20-25℃ for 8-12 minutes to clear the membrane pores and adjust the hydrophilicity. After drying, a household reverse osmosis membrane is obtained. The household reverse osmosis membrane, under test conditions of 65 psi operating pressure, 25°C, and 500 ppm NaCl aqueous solution, has a flux ≥ 40 LMH and a NaCl rejection rate ≥ 96%.

[0008] As a preferred option, the original industrial reverse osmosis membrane described in S1 is an aromatic polyamide composite reverse osmosis membrane, which is suitable for industrial high-pressure conditions of 100 psi or above.

[0009] As a preferred embodiment, the sodium hypochlorite aqueous solution in S1 has a mass concentration of 0.5% and a treatment time of 64 hours; during the soaking process, there is no stirring or slight stirring is performed once every 12 hours, with a stirring rate of 40-60 r / min and a single stirring duration of 3-6 minutes.

[0010] As a preferred embodiment, the alcohol solution in S2 is a 95% ethanol solution or anhydrous ethanol solution, and the processing time is 10 minutes.

[0011] As a preferred option, the drying process described in S2 is either air drying or oven drying; the conditions for air drying are air drying at 20-25℃ in a ventilated environment for 2-3 hours, and the conditions for oven drying are oven drying at 30-40℃ in a ventilated environment for 1-1.5 hours. High temperatures should be avoided during the drying process to prevent deformation of the film structure.

[0012] As a preferred option, the deionized water after rinsing in S2 contains no free chlorine and tests negative using chlorine test paper.

[0013] The two-step synergistic modification process in this scheme is not a simple addition of reagents, but rather achieves precise control of the membrane structure and performance through the synergistic effect of sodium hypochlorite and alcohol. The core mechanism is as follows: Mechanism of action of sodium hypochlorite pre-modification Sodium hypochlorite, as a mild oxidant, can undergo a slight oxidation reaction with the amino groups (-NH2) in the aromatic polyamide separation layer of industrial membranes, forming uniform micro-etched pores on the membrane surface and in the internal channels. This effectively increases the membrane porosity, reduces the permeation resistance of water molecules, and initially improves the water flux of the membrane under low pressure. At the same time, by strictly controlling the mass concentration of the sodium hypochlorite solution (0.4%-0.6%) and the treatment time (60-72h), excessive oxidation can be avoided from damaging the molecular skeleton of polyamide, ensuring that the membrane rejection rate only decreases slightly and maintaining basic salt rejection performance.

[0014] Mechanism of action optimized by alcohol Alcohol solutions with a volume concentration ≥95% exhibit excellent solubility and polarity. After rapidly penetrating the micropores of the membrane, they achieve two main effects: first, they dissolve trace amounts of residual polyamide degradation products within the pores, clearing "clogging pores" caused by etching and further reducing the permeation resistance of water molecules; second, through their polarity, they regulate the hydrophilicity of the membrane pores and surface, reducing the retention resistance of water molecules within the pores and achieving a secondary increase in flux. Simultaneously, short-term alcohol treatment (8-12 min) effectively inhibits excessive swelling of the aromatic polyamide separation layer, preventing excessively large pores that lead to a decrease in rejection rate, and may even slightly increase the membrane's rejection rate, ultimately achieving the optimal balance between "water permeate flux and salt rejection rate."

[0015] Compared with the prior art, the two-step synergistic modification method of the present invention has the following significant advantages: 1. High performance adaptability, balancing water production efficiency and water safety. The modified membrane has a permeate flux of ≥40 LMH (45.12 LMH under optimal parameters) under low-pressure domestic conditions of 50-80 psi, which is more than 186% higher than the original industrial membrane, meeting the daily water production needs of 100-200L for domestic equipment; at the same time, the NaCl rejection rate is stable at ≥96.5%, far exceeding the industry standard of 95% for domestic reverse osmosis membranes, ensuring the safety of the output water quality.

[0016] 2. The process is simple and controllable, suitable for large-scale production. The entire modification process only involves the steps of "soaking-washing-soaking-drying", requiring no special large and complex equipment and can be completed at room temperature and normal pressure. Furthermore, by strictly controlling the reagent concentration and treatment time, precise control of the membrane structure can be achieved, resulting in uniform modification effects. The performance deviation of each part of the large-size membrane is ≤0.5%, making it suitable for industrial mass production.

[0017] 3. Significant cost advantages, enabling resource reuse Sodium hypochlorite and ethanol used in the modification are common chemical raw materials that are readily available and inexpensive. The entire modification process involves no high-cost reagents or equipment, and the modification cost is only 1 / 3 to 1 / 2 of that for directly preparing household reverse osmosis membranes. At the same time, it realizes the secondary use of industrial high-pressure membranes, improves the utilization rate of industrial membrane resources, and reduces the overall preparation cost of household reverse osmosis membranes.

[0018] 4. Strong device compatibility, lowering the application threshold. The modified membrane has a standardized structure and can be directly installed in existing household reverse osmosis water purification equipment without modifying the equipment's pressure system, pipeline structure, or control program. It is compatible with mainstream household water purification equipment on the market, lowering the threshold for membrane replacement and equipment upgrades, and facilitating rapid promotion and application in households. Detailed Implementation

[0019] The technical solution of the present invention will be described in detail below with reference to specific embodiments. All embodiments were carried out at room temperature (20-25℃) and normal pressure. The original industrial reverse osmosis membranes used were commercially available aromatic polyamide composite reverse osmosis membranes (suitable for industrial high pressure conditions above 120psi, with a measured flux of 15.74 LMH and a NaCl rejection rate of 99.19% under the conditions of 65psi, 25℃, and 500ppm NaCl aqueous solution). All reagents used were of analytical grade. The testing equipment included an electronic balance, a constant temperature immersion bath, a low-pressure reverse osmosis performance testing device (adjustable pressure up to 65psi), a conductivity meter (for determining the NaCl rejection rate), and chlorine test paper (for detecting free chlorine).

[0020] All performance tests for the embodiments were conducted under the conditions of 65 psi operating pressure, 25°C, and 500 ppm NaCl aqueous solution. These conditions are fully compatible with the actual operating conditions of household reverse osmosis equipment. The test indicators were permeable flux (LMH) and NaCl rejection rate. Example

[0021] This embodiment describes a modification method under preferred parameters, and the specific steps are as follows: S1. Pre-modification treatment: Weigh 5g of sodium hypochlorite solid and dilute it with deionized water to 1000mL to prepare a sodium hypochlorite aqueous solution with a mass concentration of 0.5%; completely immerse a 10cm×10cm original industrial membrane in the solution and let it stand at 23℃ for 64h, stirring slightly once every 12h at a stirring rate of 50r / min for 5min; after the treatment, take out the membrane and rinse it with deionized water 4 times for 8min each time. After chlorine test paper is used to detect that there is no free chlorine in the water after rinsing, a first-modified membrane is obtained.

[0022] S2. Post-optimization treatment: The modified membrane was completely immersed in 95% ethanol solution and left to stand at 23°C for 10 minutes; the membrane was then removed and placed in a fume hood at 25°C to air dry naturally for 2 hours to obtain the modified household reverse osmosis membrane.

[0023] S3. Performance test: The modified membrane was tested and the results were: water flux 45.12 LMH, NaCl rejection rate 96.70%. Example

[0024] This embodiment examines the modification effect of sodium hypochlorite solution with a mass concentration of 0.4%. The specific steps are as follows: S1. Pre-modification treatment: Weigh 4g of sodium hypochlorite solid and dilute it with deionized water to 1000mL to prepare a sodium hypochlorite aqueous solution with a mass concentration of 0.4%; completely immerse a 10cm×10cm original industrial membrane in the solution and let it stand at 20℃ for 72h without stirring; after the treatment, take out the membrane and rinse it with deionized water 3 times, 10min each time. After chlorine test paper, there is no free chlorine in the water after rinsing, and a first-modified membrane is obtained.

[0025] S2. Post-optimization treatment: The modified membrane was completely immersed in anhydrous ethanol solution and left to stand at 20°C for 12 min; the membrane was then removed and dried in a ventilated environment at 30°C for 1.5 h to obtain the modified household reverse osmosis membrane.

[0026] S3. Performance test: The modified membrane was tested and the results were: water flux 40.25 LMH, NaCl rejection rate 97.20%. Example

[0027] This embodiment examines the modification effect of sodium hypochlorite solution with a mass concentration of 0.6%. The specific steps are as follows: S1. Pre-modification treatment: Weigh 6g of sodium hypochlorite solid and dilute it with deionized water to 1000mL to prepare a sodium hypochlorite aqueous solution with a mass concentration of 0.6%; completely immerse a 10cm×10cm original industrial membrane in the solution and let it stand at 25℃ for 60h, stirring slightly once every 12h at a stirring rate of 40r / min for 3min; after the treatment, take out the membrane and rinse it with deionized water 5 times for 5min each time. After chlorine test paper is used to detect that there is no free chlorine in the water after rinsing, a first-modified membrane is obtained.

[0028] S2. Post-optimization treatment: The modified membrane was completely immersed in 95% ethanol solution and allowed to stand at 25°C for 8 minutes; the membrane was then removed and dried in a ventilated environment at 40°C for 1 hour to obtain the modified household reverse osmosis membrane.

[0029] S3. Performance test: The modified membrane was tested and the results were: water flux 46.88 LMH, NaCl rejection rate 96.50%. Example

[0030] This embodiment examines the modification effect of sodium hypochlorite treatment for 60 hours and alcohol treatment for 12 minutes. The specific steps are as follows: S1. Pre-modification treatment: Prepare a 0.5% sodium hypochlorite aqueous solution, completely immerse a 10cm×10cm original industrial membrane in the solution, and let it stand at 22℃ for 60h. During this period, stir gently once every 12h at a stirring rate of 60r / min for 6min. After the treatment, rinse with deionized water 4 times for 7min each time. No free chlorine is detected by chlorine test paper, and a modified membrane is obtained.

[0031] S2. Post-optimization treatment: The modified membrane was immersed in 95% ethanol solution and treated at 22℃ for 12 min; after being removed, it was naturally air-dried in a ventilated environment at 23℃ for 3 h to obtain the modified household reverse osmosis membrane.

[0032] S3. Performance test: The modified membrane was tested and the results were: water flux 42.56 LMH, NaCl rejection rate 96.95%. Example

[0033] This embodiment examines the modification effect of sodium hypochlorite treatment for 72 hours and alcohol treatment for 8 minutes. The specific steps are as follows: S1. Pre-modification treatment: Prepare a sodium hypochlorite aqueous solution with a mass concentration of 0.5%, and completely immerse the original industrial membrane with a mass of 10cm×10cm in the solution. Let it stand at 24℃ for 72h without stirring. After the treatment, rinse with deionized water 4 times for 9min each time. No free chlorine is detected by chlorine test paper, and a modified membrane is obtained.

[0034] S2. Post-optimization treatment: The modified membrane was immersed in anhydrous ethanol solution and treated at 24°C for 8 min; after removal, it was dried in a ventilated environment at 35°C for 1.2 h to obtain the modified household reverse osmosis membrane.

[0035] S3. Performance test: The modified membrane was tested and the results were: water flux 43.89 LMH, NaCl rejection rate 96.80%. Example

[0036] This embodiment represents an attempt at large-scale modification of a large-size membrane, with a membrane size of 50cm × 50cm. The specific steps are as follows: S1. Pre-modification treatment: Prepare 50L of sodium hypochlorite aqueous solution with a mass concentration of 0.5%, and completely immerse the original industrial membrane (50cm×50cm) in the solution. Treat at 23℃ for 64h, stirring slightly once every 12h at a stirring rate of 50r / min for 5min. After treatment, rinse with deionized water 4 times for 10min each time. No free chlorine is detected by chlorine test paper, thus obtaining a modified membrane.

[0037] S2. Post-optimization treatment: Prepare 50L of 95% ethanol solution, immerse the modified membrane in it, and treat it at 23℃ for 10min; after taking it out, let it air dry naturally in a ventilated room at 25℃ for 2.5h to obtain the modified household reverse osmosis membrane.

[0038] S3. Performance test: Five test points were selected at different locations on the membrane (center, four corners, and edge) for testing. The average results were: water flux 44.95 LMH, NaCl rejection rate 96.75%, and performance deviation of each test point ≤0.5%, indicating that the modification effect is uniform and suitable for large-scale production.

[0039] Comparative Example 1 This comparative example is a single sodium chlorate modification (optimized without alcohol). The specific steps are the same as the pre-modification treatment in Example 1. The resulting membrane was tested and the results were: water flux 30.81 LMH and NaCl rejection rate 96.55%.

[0040] Comparative Example 2 This comparative example directly applies the original industrial membrane to household applications without any modification. The performance test results are: water flux 15.74 LMH, NaCl rejection rate 99.19%.

[0041] Comparison of results between the examples and the comparative examples The performance test results of each embodiment and comparative example are summarized in Table 1: Table 1 Performance test results of each diaphragm As shown in Table 1: 1. The flux of the unmodified original industrial membrane is only 15.74 LMH, which cannot meet the needs of household use; although the flux is improved by sodium hypochlorite modification alone, it does not reach 40 LMH, and is still not suitable for household use. 2. The membranes treated by the two-step synergistic modification method of the present invention have a flux ≥40LMH and a NaCl rejection rate ≥96.5%, which fully meets the industry standard of "flux ≥35LMH and rejection rate ≥95%" for household reverse osmosis membranes; 3. The optimal modification effect is achieved under the optimized parameters (0.5% sodium hypochlorite for 64h + 95% ethanol for 10h), with a flux of 45.12 LMH and a retention rate of 96.70%. Moreover, the modification effect is uniform for large-size membranes, making it suitable for large-scale mass production.

[0042] The modification method of the present invention is not only applicable to the 10cm×10cm and 50cm×50cm membranes in the above embodiments, but can also be adapted to industrial aromatic polyamide composite reverse osmosis membranes of any size according to the actual needs of household reverse osmosis equipment. It is only necessary to adjust the amount of sodium hypochlorite and alcohol solution according to the ratio to ensure that the membrane is completely submerged.

[0043] The stirring rate and duration of each stirring during the soaking process can be flexibly adjusted within the range of 40-60 r / min and 3-6 min. The core purpose is to ensure uniform etching of all parts of the membrane and avoid excessive etching in certain areas. Effective modification can also be achieved without stirring, although the etching uniformity is slightly lower than under the conditions of slight stirring.

[0044] The drying method can be flexibly selected according to the production scenario: natural air drying can be used for small-batch preparation in the laboratory, while drying can be used for large-scale industrial production. It is only necessary to ensure that the drying environment is well-ventilated and the temperature does not exceed 40°C to prevent the membrane structure from deforming due to high temperature and affecting the membrane performance.

[0045] The alcohol solution used in this invention is an ethanol solution with a volume concentration of ≥95%, including 95% ethanol and anhydrous ethanol. Both can achieve the effects of unblocking pores and adjusting hydrophilicity. The optimization effect of anhydrous ethanol is slightly better than that of 95% ethanol, and the choice can be made flexibly according to cost requirements.

[0046] The modification method of this invention realizes the efficient conversion of industrial high-pressure reverse osmosis membranes into household low-pressure membranes. It not only solves the industry pain points of insufficient flux and high manufacturing cost of household reverse osmosis membranes, but also provides a new path for the secondary utilization of industrial membrane resources, which is in line with the development concept of green environmental protection and resource recycling.

[0047] The modified membrane can be directly applied to mainstream household reverse osmosis water purifiers such as under-sink reverse osmosis water purifiers, countertop water purifiers, and wall-mounted water purifiers, as well as small commercial water purification equipment (such as office and convenience store water purifiers), making it suitable for a wide range of applications. Furthermore, the modification process of this invention is simple, low-cost, and highly feasible for large-scale production, enabling mass modification of industrial membranes and promoting cost reduction and efficiency improvement in the household reverse osmosis membrane market, thus possessing significant economic and social value.

[0048] This invention is not limited to the above embodiments. Based on the technical solutions disclosed in this invention, those skilled in the art can make some substitutions and modifications to some of the technical features without creative effort, and all such substitutions and modifications are within the protection scope of this invention.

Claims

1. A method for modifying industrial high-pressure reverse osmosis membranes, characterized in that: It includes the following steps, S1: Pre-modification treatment: The original industrial reverse osmosis membrane is immersed in a sodium hypochlorite aqueous solution with a mass concentration of 0.4%-0.6% and treated at a temperature of 20-25℃ for 60-72 hours to obtain a primary modified membrane; S2: Post-optimization treatment: The modified membrane is rinsed with deionized water 3-5 times, each time for 5-10 minutes, to remove residual sodium hypochlorite on the surface. Then, it is soaked in an alcohol solution with a volume concentration ≥95% and treated at a temperature of 20-25℃ for 8-12 minutes. After drying, a household reverse osmosis membrane is obtained. The household reverse osmosis membrane, under test conditions of 65 psi operating pressure, 25°C, and 500 ppm NaCl aqueous solution, has a flux ≥ 40 LMH and a NaCl rejection rate ≥ 96%.

2. The modification method for industrial high-pressure reverse osmosis membranes according to claim 1, characterized in that: The original industrial reverse osmosis membrane described in S1 is an aromatic polyamide composite reverse osmosis membrane, suitable for industrial high-pressure conditions above 100psi.

3. The modification method for industrial high-pressure reverse osmosis membranes according to claim 1, characterized in that: The sodium hypochlorite aqueous solution in S1 has a mass concentration of 0.5% and a treatment time of 64 hours. During the soaking process, there is no stirring or slight stirring is performed once every 12 hours. The stirring rate is 40-60 r / min and the duration of each stirring is 3-6 minutes.

4. The modification method for industrial high-pressure reverse osmosis membranes according to claim 1, characterized in that: The alcohol solution mentioned in S2 is a 95% ethanol solution or anhydrous ethanol solution, and the treatment time is 10 minutes.

5. The modification method for industrial high-pressure reverse osmosis membranes according to claim 1, characterized in that: The drying process described in S2 is either air drying or oven drying; air drying is performed at 20-25℃ in a ventilated environment for 2-3 hours, while oven drying is performed at 30-40℃ in a ventilated environment for 1-1.5 hours.

6. The modification method for industrial high-pressure reverse osmosis membranes according to claim 1, characterized in that: There was no free chlorine in the deionized water after rinsing in S2, and the test result using chlorine test paper was negative.