Material Basis and Structural Characteristics of Acid-Resistant Nanofiltration Membranes
The core advantages of acid-resistant nanofiltration membranes lie in their chemical resistance and structural stability. According to the referenced document, these membranes are mainly fabricated using polysulfonamide (PSA) and polyvinylidene fluoride (PVDF), forming a composite structure through interfacial polymerization. The sulfonamide bonds in PSA exhibit strong hydrolysis resistance under highly acidic conditions, allowing the membranes to maintain separation performance after soaking in solutions with pH 1–2. For example, the rejection of MgSO₄ can reach 93.5%, while Na₂SO₄ remains at approximately 89.2%.
Typical nanofiltration membranes have pore sizes of 0.5–2 nm. Through a combination of pore-size sieving and surface-charge effects, they effectively retain multivalent ions while permitting partial passage of monovalent ions. This structural configuration ensures that acid-resistant nanofiltration membranes maintain both high flux and high selectivity in strong acid environments, forming the foundation for their widespread use in processing acidic industrial wastewater and resource recovery.
Performance and Durability of Acid-Resistant Nanofiltration Membranes
The chemical stability of acid-resistant nanofiltration membranes is one of the key performance indicators. As reported in the document, PSA-modified PVDF membranes soaked in pH 1 sulfuric acid for 72 hours show only a slight flux decrease-from 20 L/m²·h to 18 L/m²·h-indicating excellent structural integrity. Test results for PTFE-based membranes also demonstrate minimal change in Mg²⁺ and Na⁺ rejection, confirming their strong acid resistance.
Mechanical strength is equally important for long-term industrial operation. Membranes made from PVDF and PPSU show tensile strengths above 30 MPa and maintain structural stability after 50 hours of dynamic pressure cycling. This combination of chemical durability and mechanical robustness allows acid-resistant nanofiltration membranes to operate continuously under harsh conditions involving strong acids and high pressure.
At the membrane-element level, the choice of acid-resistant potting material, end-cap structure, and outer shell is crucial, as highly acidic wastewater may cause corrosion-induced leakage or structural failure. Potential issues include early-stage flux decline, changes in surface charge affecting selectivity, and pressure-drop increases caused by acidic fouling or metal deposition. Understanding these characteristics is essential for designing stable and long-lasting membrane systems.
Industrial Value of Acid-Resistant Nanofiltration Membranes in Acidic Wastewater Treatment
Acid-resistant nanofiltration membranes are widely used in metal-processing wastewater, acid recovery in the food industry, and electroplating waste treatment-three categories of wastewater characterized by high acidity, high metal concentration, and corrosiveness.
In the metal-processing sector, the referenced membrane achieves a Cu²⁺ rejection rate of 98% and Cr³⁺ rejection of 97%, and maintains stable performance during 500 hours of continuous operation at pH 3. This high efficiency makes it a valuable tool for metal recovery and wastewater minimization.
In the food industry, the membrane can recover valuable organic acids such as citric and malic acid. Reported citric-acid rejection reaches 96%, with a flux exceeding 80 L/m²·h, allowing efficient recycling of acid-containing by-products.
In electroplating wastewater treatment, the membrane shows Cr³⁺ and Ni²⁺ rejection rates of 95% and 94%, respectively, maintaining stable performance over 300 hours of operation. This significantly reduces environmental risks while improving heavy-metal recovery efficiency.
These application cases highlight not only the membrane's stability in strong acid conditions but also its economic and environmental benefits, including resource recovery, chemical consumption reduction, and lower pollution discharge.
Common Issues in Nanofiltration Membrane Elements and Key Technical Considerations
During engineering operation, nanofiltration elements may encounter challenges such as acidic fouling layers, surface-charge degradation, and localized structural fatigue caused by prolonged exposure to strong acids. High concentrations of metals (such as iron and aluminum) may form deposits on the membrane surface, impacting flux and increasing pressure drop. If the potting or end-cap materials are not sufficiently acid-resistant, leakage or edge-corrosion failures may occur. Additionally, uneven mechanical stress or pressure fluctuations within the system may lead to channel deformation or localized compaction of the rolled membrane structure.
To address these issues, several technical practices are essential:
– Adequate pretreatment to reduce particulates and deposition-prone metals.
– Maintaining stable operating pressure.
– Using cleaning chemicals that are compatible with acid-resistant membranes and avoiding oxidizing agents.
– Selecting membrane elements with acid-resistant potting materials and corrosion-resistant housings.
Proper control of these details directly determines the service life and stable performance of acid-resistant nanofiltration membranes.
