1. Overview of the Chemical Corrosion Resistance of Nanofiltration Membranes

 1.1 Material Characteristics of Nanofiltration Membranes

Nanofiltration membranes, which are pressure-driven membrane separation processes between ultrafiltration and reverse osmosis, have core material characteristics that determine their chemical corrosion resistance.

- Material Classification: Nanofiltration membranes are mainly divided into organic and inorganic types. Organic nanofiltration membranes, usually composed of polymers like polyamide, have good film-forming properties and certain chemical stability, but their pH tolerance is relatively narrow, generally between pH 4 and 9. They are not resistant to strong acids, strong alkalis, and oxidants. Inorganic nanofiltration membranes, such as ceramic membranes made of alumina and zirconia, have superior chemical stability, a wider pH range, and can withstand higher temperatures, making them suitable for harsh chemical environments.

- Pore Structure and Surface Properties: With a pore size of 1-2 nm, the surface of nanofiltration membranes is usually charged, affecting separation not only by sieving but also by electrostatic interactions. In high ionic strength solutions, the surface charge may be shielded, impacting separation performance and anti-pollution ability.

- Chemical Stability in Practical Applications: In water treatment, nanofiltration membranes face complex chemistries. For example, organic membranes may degrade under chlorinated water, while inorganic membranes show better chlorine resistance. Their acid and alkali tolerance is crucial when treating acidic or alkaline wastewater. Some modified membranes, introduced with acid/alkali-resistant monomers or improved surface properties, significantly enhance stability in extreme pH conditions.

 2. Corrosion Resistance of Nanofiltration Membranes in Different Chemical Environments

 2.1 Acidic Environment

The corrosion resistance of nanofiltration membranes in acidic environments shows significant material differences:

- Organic Nanofiltration Membranes: Traditional organic membranes, like polyamide-based ones, have poor stability in acidic conditions. When pH is below 4, the amide bonds hydrolyze, causing flux decline and worse retention. For example, in pH 2 sulfuric acid, the water permeation flux of polyamide membranes dropped by 50% in 24 hours, and the divalent ion rejection fell from 95% to 70%.

- Inorganic Nanofiltration Membranes: Inorganic membranes, such as ceramic ones, have better acid resistance. Ceramic membranes have a pH range of 2-12 and maintain stable performance in strong acids. For instance, alumina ceramic membranes only saw a 10% flux drop in pH 1 hydrochloric acid after 72 hours, with sulfate rejection above 98%.

- Modified Nanofiltration Membranes: Modifications can enhance the acid resistance of organic membranes. For example, introducing acid-resistant monomers via interfacial polymerization or improving surface properties allowed polyamide membranes to perform well in pH 3 conditions. After 48 hours in pH 3 hydrochloric acid, the flux only dropped by 20%, and monovalent and divalent ion rejections stayed above 90%.

 2.2 Alkaline Environment

The corrosion resistance in alkaline environments also varies by material:

- Organic Nanofiltration Membranes: Polyamide-based membranes have poor stability in alkaline conditions. When pH exceeds 9, amide bond hydrolysis accelerates, degrading performance. For example, in pH 12 sodium hydroxide, the flux dropped by 60% in 12 hours, and sodium chloride rejection fell from 90% to 50%.

- Inorganic Nanofiltration Membranes: Inorganic membranes show good stability in alkaline environments. Zirconia ceramic membranes have a wide pH range and maintain performance in strong alkalis. For example, after 96 hours in pH 13 sodium hydroxide, the flux only dropped by 15%, and calcium ion rejection stayed above 95%.

- Modified Nanofiltration Membranes: Modifications can improve the alkali resistance of organic membranes. For example, polyelectrolyte layer-by-layer assembly enhanced polyamide membrane performance in pH 11 conditions. After 72 hours in pH 11 sodium hydroxide, the flux only dropped by 25%, and sulfate rejection stayed above 92%.

 2.3 Organic Solvent Environment

The corrosion resistance in organic solvents is key for specific industrial applications:

- Organic Nanofiltration Membranes: Traditional organic membranes have poor stability in organic solvents. For example, polyamide membranes showed significant flux decline and structure swelling in ethanol, worsening retention. After 24 hours in ethanol, the flux dropped by 70%, and small organic molecule rejection fell from 85% to 50%.

- Inorganic Nanofiltration Membranes: Inorganic membranes have better stability in organic solvents. For example, silicon carbide ceramic membranes only saw a 10% flux drop in ethanol and acetone after 72 hours, with dye molecule rejection above 95%. This is due to their chemical inertia and mechanical stability.

- Modified Nanofiltration Membranes: Introducing hydrophobic groups or using nano-composite technology can enhance organic membrane stability in organic solvents. For example, nano-silica modified polyamide membranes only had a 30% flux drop in ethanol after 48 hours, with small organic molecule rejection above 80%. These modifications improved hydrophobicity and mechanical strength, suppressing swelling and enhancing solvent corrosion resistance.

 3. Factors Affecting the Chemical Corrosion Resistance of Nanofiltration Membranes

 3.1 Chemical Stability of Membrane Materials

The chemical stability of membrane materials is a key factor in determining corrosion resistance. Different materials show significant differences in acid, alkali, oxidant, and organic solvent environments.

- Organic Nanofiltration Membrane Materials: Organic membranes, made of polymers like polyamide, have chemical stability depending on molecular structure and bond strength. Polyamide membranes have good performance in neutral conditions but hydrolyze in strong acids/alkalis, degrading structure and performance. For example, in pH below 4 or above 9, flux and rejection significantly decrease. They also swell in organic solvents, affecting performance.

- Inorganic Nanofiltration Membrane Materials: Inorganic membranes, like ceramic ones made of alumina and zirconia, have superior chemical stability. These materials have strong chemical bonds, tolerating a wide pH range and higher temperatures. For example, alumina ceramic membranes only had a 10% flux drop in pH 1 hydrochloric acid after 72 hours, with sulfate rejection above 98%. They also show good stability in organic solvents, maintaining high performance after immersion in ethanol and acetone.

 4. Strategies to Improve the Chemical Corrosion Resistance of Nanofiltration Membranes

 4.1 Material Modification

Material modification is an important method to enhance corrosion resistance. Choosing suitable materials or chemically modifying existing ones can significantly improve stability in harsh chemistries.

- Modification of Organic Nanofiltration Membranes:

  - Introducing Chemically Resistant Monomers: Using monomers resistant to acids, alkalis, and oxidants in interfacial polymerization can improve membrane stability. For example, sulfonic acid group-containing monomers enhanced polyamide membrane stability in strong acids by over 30%. This method strengthens chemical bonds, inhibiting amide bond hydrolysis.

  - Nano-Composite Technology: Adding nano-materials like nano-silica and nano-alumina to organic membranes enhances mechanical and chemical stability. Nano-particles improve anti-swelling performance and act as physical barriers against chemical erosion. For example, nano-silica modified polyamide membranes had a 30% flux drop in ethanol after 48 hours, with small organic molecule rejection above 80%.

  - Crosslinking Modification: Chemical crosslinking enhances the chemical stability and mechanical strength of organic membranes. Crosslinked polyamide membranes showed significantly improved stability in strong alkalis, with a 20% flux drop in pH 12 sodium hydroxide after 24 hours and sodium chloride rejection above 85%.

- Modification of Inorganic Nanofiltration Membranes:

  - Composite Materials: Inorganic membranes can be combined with other materials to improve corrosion resistance. For example, silicon carbide-alumina composite ceramic membranes showed excellent stability in acidic and alkaline conditions. In pH 1 hydrochloric acid, flux only dropped by 5%, and sulfate rejection stayed above 99%.

  - Surface Coating Technology: Coating inorganic membranes with chemically resistant materials protects the main structure. For example, PTFE-coated ceramic membranes had better stability in organic solvents, with a 10% flux drop in ethanol after 72 hours and dye molecule rejection above 95%.

 5. Application Case Analysis of the Chemical Corrosion Resistance of Nanofiltration Membranes

 5.1 Industrial Wastewater Treatment

Nanofiltration membranes are widely used in industrial wastewater treatment, especially for complex wastewaters with various chemicals, where corrosion resistance is crucial.

- Acidic Wastewater Treatment:

  - Case Background: Acidic wastewater from an electronics plant containing sulfuric and hydrochloric acids (pH 2-3) with heavy metal ions and organic pollutants.

  - Application: Modified polyamide membranes, with acid-resistant monomers introduced via interfacial polymerization, were used. In operation, heavy metal ion rejection was above 95%, organic pollutant removal over 80%, and flux only dropped by 20%. Treated wastewater met discharge standards.

  - Conclusion: Modified polyamide membranes showed good corrosion resistance and separation effect in acidic wastewater treatment, effectively removing harmful substances and reducing costs.

- Alkaline Wastewater Treatment:

  - Case Background: Alkaline wastewater from a paper mill (pH 11-12) containing lignin, cellulose, and alkaline salts.

  - Application: Zirconia ceramic membranes were used. In operation, lignin rejection was 98%, alkaline salt rejection over 90%, and flux only dropped by 15%. Treated wastewater was reused in production.

  - Conclusion: Inorganic membranes showed excellent corrosion resistance and separation ability in alkaline wastewater treatment, reducing treatment costs and improving water recycling.

 5.2 Separation and Purification in Chemical Production

Nanofiltration membranes face complex chemistries in chemical production separation and purification, directly affecting production efficiency and product quality.

- Organic Solvent Separation:

  - Case Background: Separating target products from mixtures of ethanol, acetone, etc., in fine chemical production.

  - Application: Nano-silica modified polyamide membranes, enhanced via nano-composite technology, were used. In operation, target product rejection was 90%, organic solvent separation was significant, flux only dropped by 30%, and solvents were recovered.

  - Conclusion: Modified organic membranes showed good corrosion resistance and separation effect in organic solvent separation, improving production efficiency and reducing costs.

- Acid and Alkali Separation and Recovery:

  - Case Background: Separating and recovering valuable acids and alkalis from mixed wastewater in a chemical enterprise.

  - Application: Composite ceramic membranes, made of silicon carbide and alumina, with excellent acid/alkali resistance, were used. In operation, sulfuric acid rejection was 99%, sodium hydroxide rejection over 95%, flux only dropped by 10%, and acid/alkali solutions were recovered.

  - Conclusion: Composite ceramic membranes showed outstanding corrosion resistance and separation ability in acid/alkali separation and recovery, improving resource recovery and reducing environmental risks.

 6. Conclusion

The chemical corrosion resistance of nanofiltration membranes is key to their wide application. Studying their performance in different chemistries and factors affecting corrosion resistance provides important insights for optimizing membrane design and application.

Organic and inorganic nanofiltration membranes differ significantly in corrosion resistance. Organic membranes, though good for film formation, are unstable in strong acids/alkalis and organic solvents, prone to hydrolysis or swelling. Inorganic membranes, like ceramics, with superior chemical stability and wide pH tolerance, excel in harsh chemistries, withstanding high temperatures and complex erosion.

Membrane fabrication processes deeply impact corrosion resistance. Interfacial polymerization, by selecting suitable monomers and optimizing conditions, can adjust pore distribution and surface chemistry, enhancing corrosion resistance. Post-treatment processes like surface modification and crosslinking further improve mechanical and chemical stability.

To enhance corrosion resistance, material modification and membrane surface treatment are key strategies. Material modification includes introducing chemically resistant monomers, nano-composite technology, and crosslinking, significantly improving organic membrane stability. Membrane surface treatment, via plasma treatment, polyelectrolyte layer-by-layer assembly, surface grafting, and coating, changes surface chemistry and structure, enhancing corrosion resistance.

In practice, nanofiltration membranes' corrosion resistance is well-validated. In wastewater treatment, modified membranes effectively remove harmful substances from acidic and alkaline wastewaters, reducing costs and maintaining high flux and rejection. In chemical production separation and purification, membranes excel in organic solvent separation and acid/alkali recovery, improving efficiency, reducing costs, and enabling resource recycling.

In summary, the chemical corrosion resistance of nanofiltration membranes is closely related to material properties, fabrication processes, and modification strategies. By selecting suitable materials, optimizing fabrication, and using effective modifications, membrane performance in harsh chemistries can be significantly improved. This expands their application in water treatment, chemical, and pharmaceutical industries, enhancing lifespan, economic benefits, and providing strong technical support for solving industrial problems.