Long-Term Stable Cross-Linked Polyelectrolyte Multilayer Nanofiltration Membranes at Extreme pH and High Salinity

Area of Science:

• Materials Science
• Chemical Engineering
• Environmental Science

Background:

• High chemical stability is crucial for nanofiltration (NF) membranes in challenging separations.
• Extreme pH and high salinity environments degrade conventional NF membranes.
• Polyelectrolyte multilayer (PEM) membranes offer pH stability but lack salinity tolerance.

Purpose of the Study:

• Investigate the stability of poly-(diallyldimethylammonium chloride) (PDADMAC)/poly-(sodium 4-styrenesulfonate) (PSS) PEM NF membranes under combined extreme pH and high salinity.
• Evaluate the performance of both non-cross-linked and DAS-cross-linked membranes.
• Determine the role of dynamic filtration versus long-term immersion in stability assessment.

Main Methods:

• Systematic testing of PDADMAC/PSS PEM NF membranes (non-cross-linked and DAS-cross-linked).
• Exposure to combined extreme pH (0, 1, 14) and high salinity (up to 4.5 M NaCl).
• Assessment via short-term dynamic filtration and long-term (4 months) immersion experiments.

Main Results:

• DAS-cross-linked membranes maintained consistent permeability, salt retention, MWCO, and charge under all harsh conditions.
• Non-cross-linked membranes showed good long-term immersion stability but significant performance loss during dynamic filtration due to defects.
• Dynamic filtration revealed critical performance deterioration in non-cross-linked membranes not apparent in static immersion.

Conclusions:

• DAS-cross-linked PDADMAC/PSS PEM membranes demonstrate robust stability for applications in aggressive chemical environments.
• Dynamic filtration is essential for a practically relevant assessment of membrane stability.
• The study provides a comprehensive evaluation of PEM membrane stability under combined chemical stresses.