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

High chemical stability is key for nanofiltration (NF) membranes used in challenging separation processes. An especially challenging stream to treat is one where high salinities and extreme pH values are combined, as extreme pH values can chemically degrade the cross-links that ensure membrane stability at high salinity. In contrast, polyelectrolyte multilayer (PEM) membranes based on PDADMAC (poly-(diallyldimethylammonium chloride)) and PSS (poly-(sodium 4-styrenesulfonate)) exhibit excellent stability at extreme pH, but they lack stability at high salinity. This study systematically investigates the stability of PDADMAC/PSS PEM NF membranes under the aforementioned harsh conditions, where an extremely low or high pH is combined with high salinities. Both non-cross-linked and DAS (Disodium 4,4'-diazidostilbene-2,2'-disulfonate tetrahydrate)-cross-linked membranes were tested under combined extreme pH (pH 0 or 1 and pH 14) and high salinity (up to 4.5 M NaCl). Stability was assessed through short-term dynamic filtration and long-term (4 months) exposure experiments. Cross-linked membranes showed excellent stability across all harsh conditions, maintaining consistent permeability, salt retention, molecular weight cutoff (MWCO), and membrane charge during short- and long-term experiments. Non-cross-linked membranes displayed unexpectedly high stability during long-term immersion, preserving NF-level performance despite changes in membrane charge. However, during short-term dynamic filtration, non-cross-linked membranes suffered from significant performance deterioration, including increased permeability, reduced salt rejection, and increased MWCO due to defect formation. By combining short-term filtration and long-term immersion tests, this study provides a comprehensive evaluation of PEM membrane stability under aggressive chemical conditions, while also highlighting the critical role of dynamic filtration in assessing membrane stability with practical relevance. After all, the DAS-cross-linked PDADMAC/PSS membranes exhibit high stability in both testing scenarios, showing their strong potential for applications in harsh environments.