1. Characteristics of Strongly Alkaline Water Quality

1.1 pH Range and Common Sources of Strongly Alkaline Water Quality

2. Basic Principles and Classification of Nanofiltration Membranes

2.1 Filtration Mechanism of Nanofiltration Membranes

• Solution-Diffusion Mechanism: When a solution comes into contact with the surface of a nanofiltration membrane, solute molecules first dissolve on the membrane surface and then transport from the high-pressure side to the low-pressure side through diffusion within the membrane. Since nanofiltration membranes have different affinities for various ions, they can selectively permeate certain ions, thereby achieving separation.
• Pore-Flow Mechanism: Nanofiltration membranes have nanoscale pores that allow water molecules and some small molecules to pass through while retaining larger molecules or ions. This mechanism enables nanofiltration membranes to effectively remove hardness components, heavy metal ions, and some organic substances from water.
• Charge Effect: Nanofiltration membranes usually carry a charge, which can interact electrostatically with ions in the solution, further enhancing their selective permeability. For example, in strongly alkaline water, the negative charge of the nanofiltration membrane can repel cations in the solution, thereby increasing the retention rate of cations.

2.2 Main Types of Nanofiltration Membranes

• Polymeric Nanofiltration Membranes
  • Materials and Structure: Polymeric nanofiltration membranes are the most common type, primarily made from materials such as polyamide, polyethersulfone, and polysulfone. These membranes typically consist of a thin active layer and a porous support layer, with the active layer responsible for selective permeation and the support layer providing mechanical strength.
  • Performance Features: Polymeric nanofiltration membranes have good hydrophilicity and certain chemical stability, allowing them to operate within a wide pH range. However, their chemical stability is somewhat compromised under strongly alkaline conditions, especially for polyamide membranes, which may undergo hydrolysis reactions, leading to a decline in membrane performance.
  • Application Scenarios: For mildly alkaline water quality (pH values between 11 and 12), polymeric nanofiltration membranes can effectively remove hardness components and some organic substances from water. However, in highly concentrated alkaline water quality, the membranes need to be replaced regularly or chemically cleaned to maintain their performance.
• Inorganic Nanofiltration Membranes
  • Materials and Structure: Inorganic nanofiltration membranes are mainly made from materials such as ceramics and metal oxides, featuring high chemical and thermal stability. Their structure is usually a porous ceramic membrane or a metal oxide coating membrane.
  • Performance Features: Inorganic nanofiltration membranes exhibit excellent chemical stability in strongly alkaline water quality, capable of withstanding highly concentrated alkaline solutions without undergoing hydrolysis or corrosion. Moreover, their pore size distribution is uniform, providing stable filtration performance.
  • Application Scenarios: In the treatment of strongly alkaline water quality, especially for highly concentrated alkaline water quality with pH values between 12 and 14, inorganic nanofiltration membranes are an ideal choice. They can operate stably over the long term, effectively removing heavy metal ions, hardness components, and organic pollutants from water.
• Composite Nanofiltration Membranes
  • Materials and Structure: Composite nanofiltration membranes are composed of two or more materials, combining the advantages of different materials. For example, combining polymeric membranes with inorganic materials can enhance the chemical stability of polymeric membranes while maintaining their hydrophilicity.
  • Performance Features: Composite nanofiltration membranes offer better overall performance, providing good filtration effects and a certain degree of resistance to corrosion by strongly alkaline water quality. Their performance depends to some extent on the proportion and structure of the composite materials.
  • Application Scenarios: Composite nanofiltration membranes are suitable for treating moderately strong alkaline water quality, effectively removing a variety of pollutants from water while offering a longer service life and lower maintenance costs.

3. Characteristics of Nanofiltration Membranes Suitable for Strongly Alkaline Water Quality

3.1 Requirements for Alkali-Resistant Materials

• Chemical Stability of Materials: Hydroxide ions in strongly alkaline water quality are highly corrosive and can chemically react with certain materials. For example, polyamide membranes are prone to hydrolysis reactions under strongly alkaline conditions, leading to increased pore size, reduced mechanical strength, and decreased selective permeability. Studies have shown that when the pH value exceeds 12, the hydrolysis rate of polyamide membranes significantly increases, and their service life may be shortened to less than one year. In contrast, inorganic materials such as ceramics and metal oxides exhibit excellent alkali resistance and can remain stable in highly concentrated alkaline solutions. For instance, an alumina ceramic membrane remains almost unaffected in structure and performance after being soaked in a strongly alkaline solution with a pH value as high as 14 for several months.
• Hydrophilicity and Anti-Fouling Properties of Materials: In addition to alkali resistance, the hydrophilicity of nanofiltration membrane materials is also crucial. Hydrophilic materials can reduce the interfacial tension between the membrane surface and the solution, minimizing the adsorption and deposition of pollutants on the membrane surface, thereby enhancing the membrane's anti-fouling performance and flux stability. In strongly alkaline water quality, some organic pollutants and inorganic particles tend to form fouling layers on the membrane surface, affecting filtration efficiency. Hydrophilic materials can effectively mitigate this issue. For example, introducing hydrophilic groups or modifying the surface of polymeric membranes to be hydrophilic can significantly improve the membrane's anti-fouling performance. Studies have found that hydrophilically modified polyethersulfone membranes have a flux decline rate in strongly alkaline water quality that is about 30% lower than that of unmodified membranes.

3.2 Matching of Membrane Structure and Performance

• Pore Size Distribution and Selectivity: The pore size distribution of nanofiltration membranes plays a key role in their separation performance. In the treatment of strongly alkaline water quality, the membrane should effectively retain heavy metal ions, hardness components, and organic pollutants in the water while allowing water molecules and some small molecules to pass through. Studies have shown that nanofiltration membranes with pore sizes around 1 nanometer can meet this requirement well. For example, inorganic nanofiltration membranes typically have a uniform pore size distribution, and the pore size can be precisely controlled through the preparation process. When treating strongly alkaline water quality, a zirconia nanofiltration membrane with a pore size of 1.5 nanometers can achieve a heavy metal ion retention rate of over 95% and a hardness component retention rate of more than 90%. In contrast, polymeric nanofiltration membranes have a relatively wider pore size distribution, and their selective permeability mainly relies on the charge effect on the membrane surface and the solution-diffusion mechanism. Under strongly alkaline conditions, the interaction between the charge on the membrane surface and the ions in the solution is enhanced, further improving their selectivity.
• Support Layer and Mechanical Strength: The support layer of a nanofiltration membrane provides mechanical support for the active layer, ensuring that the membrane can withstand certain pressures during the filtration process. In the treatment of strongly alkaline water quality, the alkali resistance and mechanical strength of the support layer material are equally important. The support layer of inorganic nanofiltration membranes is usually made of porous ceramic materials, which have high strength and good alkali resistance, ensuring stable operation of the membrane in highly concentrated alkaline solutions. For polymeric nanofiltration membranes, the alkali resistance and mechanical strength of the support layer need to be improved through optimized material formulations and preparation processes. For example, using high-strength polysulfone material as the support layer and enhancing its alkali resistance through special preparation processes can improve the mechanical properties and service life of polymeric nanofiltration membranes in strongly alkaline water quality.
• Surface Modification and Performance Optimization: To further enhance the performance of nanofiltration membranes in strongly alkaline water quality, the membrane surface can be modified. Surface modification can introduce new functional groups or structures on the membrane surface through physical or chemical methods, thereby improving the membrane's hydrophilicity, anti-fouling properties, and selectivity. For example, coating the membrane surface with a layer of hydrophilic polymer or inorganic nanoparticles can significantly improve the membrane's hydrophilicity and anti-fouling performance. Studies have found that hydrophilically modified polymeric nanofiltration membranes have a flux recovery rate in strongly alkaline water quality that is about 40% higher than that of unmodified membranes, and their pollutant retention rate is also increased. Additionally, surface modification can introduce specific chemical groups to enhance the membrane's selective permeability. For example, introducing negatively charged groups on the membrane surface can further increase its retention rate of cations, thereby better adapting to the treatment requirements of strongly alkaline water quality.
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4. Specific Applicable Nanofiltration Membrane Types and Case Studies

4.1 Applicability Analysis of Polyamide Composite Nanofiltration Membranes

• Performance Advantages: Polyamide composite nanofiltration membranes have good hydrophilicity and high selective permeability. In the treatment of mildly alkaline water quality (pH values between 11 and 12), they can effectively remove hardness components and some organic substances from water. For example, in the treatment of mildly alkaline wastewater from a paper mill, polyamide composite nanofiltration membranes achieved a retention rate of calcium and magnesium ions of over 85% and an organic substance removal rate of about 70%.
• Alkali Resistance Challenges: However, in highly concentrated alkaline water quality, the alkali resistance of polyamide composite nanofiltration membranes is poor. Studies have shown that when the pH value exceeds 12, the hydrolysis rate of polyamide membranes significantly increases, and their service life may be shortened to less than one year. This is mainly because polyamide materials are prone to hydrolysis reactions under strongly alkaline conditions, leading to increased pore size, reduced mechanical strength, and decreased selective permeability.
• Modification and Optimization: To enhance the alkali resistance of polyamide composite nanofiltration membranes, researchers have attempted various modifications. For example, introducing hydrophilic groups or modifying the surface to be hydrophilic can significantly improve the membrane's anti-fouling performance and alkali resistance. Studies have found that hydrophilically modified polyamide composite nanofiltration membranes have a flux decline rate in strongly alkaline water quality that is about 30% lower than that of unmodified membranes. Additionally, coating the membrane surface with a layer of inorganic nanoparticles can improve the membrane's alkali resistance and mechanical strength.

4.2 Case Studies of Other Alkali-Resistant Nanofiltration Membrane Materials

• Inorganic Nanofiltration Membrane Case Study: Inorganic nanofiltration membranes (such as ceramic nanofiltration membranes) exhibit excellent performance in the treatment of strongly alkaline water quality. For example, in the treatment of highly concentrated alkaline wastewater (pH value of 13) from a chemical plant, an alumina ceramic nanofiltration membrane was used. After several months of operation, the membrane's structure and performance remained almost unaffected. The membrane achieved a heavy metal ion retention rate of over 98% and a hardness component retention rate of more than 95%. The high alkali resistance and stable filtration performance of inorganic nanofiltration membranes make them an ideal choice for treating highly concentrated alkaline water quality.
• Composite Nanofiltration Membrane Case Study: Composite nanofiltration membranes, which combine the advantages of polymeric membranes and inorganic materials, are suitable for treating moderately strong alkaline water quality. For example, in the treatment of moderately strong alkaline wastewater (pH value of 12) from a dyeing factory, a composite nanofiltration membrane made of polyethersulfone and inorganic nanoparticles was used. The membrane demonstrated good filtration performance and a long service life during operation. The composite nanofiltration membrane achieved an organic substance removal rate of over 80% and a hardness component retention rate of about 90%. By optimizing the proportion and structure of the composite materials, the performance and alkali resistance of composite nanofiltration membranes can be further enhanced.
• New Alkali-Resistant Nanofiltration Membrane Materials: In recent years, some new alkali-resistant nanofiltration membrane materials have been studied and applied. For example, graphene-based nanofiltration membranes have shown excellent alkali resistance and filtration performance. In laboratory studies, graphene nanofiltration membranes remained stable in performance after being soaked in a strongly alkaline solution with a pH value of 14 for several months. Additionally, graphene nanofiltration membranes achieved a retention rate of over 95% for small organic molecules and heavy metal ions, demonstrating great potential for the treatment of strongly alkaline water quality.

5. Considerations for Selecting Nanofiltration Membranes

5.1 Cost and Cost-Effectiveness

• Initial Investment Cost: Inorganic nanofiltration membranes (such as ceramic nanofiltration membranes) usually have a higher initial investment cost, which is about 3 to 5 times that of polymeric nanofiltration membranes. This is because the preparation process of inorganic materials is complex and requires high-energy-consuming processes such as high-temperature sintering. For example, the preparation cost of alumina ceramic nanofiltration membranes is high, but they have excellent alkali resistance and long-term stability, making them suitable for treating highly concentrated alkaline water quality.
• Operating Costs: The operating costs of polymeric nanofiltration membranes are relatively low, especially in the treatment of mildly alkaline water quality. However, in highly concentrated alkaline conditions, they require frequent chemical cleaning or replacement, which increases operating costs. In contrast, although inorganic nanofiltration membranes have a high initial investment, their long service life and strong alkali resistance result in lower long-term operating costs.
• Cost-Effectiveness Assessment: When selecting a nanofiltration membrane, it is necessary to conduct a cost-effectiveness assessment based on specific water quality conditions and treatment requirements. For mildly alkaline water quality (pH values between 11 and 12), polymeric nanofiltration membranes may be a more cost-effective choice, as they can achieve good filtration effects at a lower cost. For highly concentrated alkaline water quality (pH values between 12 and 14), inorganic nanofiltration membranes are more advantageous due to their long-term stable operating performance, which can reduce overall operating costs.

5.2 Filtration Performance and Stability

• Filtration Performance: The filtration performance of nanofiltration membranes is mainly reflected in the retention rate of pollutants and flux stability. Studies have shown that inorganic nanofiltration membranes typically achieve a retention rate of over 95% for heavy metal ions and hardness components, while polymeric nanofiltration membranes can achieve an organic substance removal rate of about 70% in mildly alkaline water quality. In the treatment of strongly alkaline water quality, inorganic nanofiltration membranes, with their uniform pore size distribution and excellent chemical stability, can more effectively remove a variety of pollutants.
• Stability: In strongly alkaline water quality conditions, the stability of nanofiltration membranes is crucial. Inorganic nanofiltration membranes exhibit excellent stability in highly concentrated alkaline solutions, with their structure and performance remaining almost unaffected. For example, alumina ceramic nanofiltration membranes maintain stable performance after being soaked in a strongly alkaline solution with a pH value as high as 14 for several months. In contrast, polymeric nanofiltration membranes may undergo hydrolysis reactions under strongly alkaline conditions, leading to a decline in membrane performance. Surface modification techniques can enhance the stability of polymeric nanofiltration membranes, but their long-term stability in highly concentrated alkaline water quality is still not as good as that of inorganic nanofiltration membranes.