Polyelectrolyte Multilayer-Based Nanofiltration Membranes with Tunable Performance for Target Pollutants

Abstract

This study demonstrates the preparation of nanofiltration (NF) membranes using the high degree of tunability of an asymmetric polyelectrolyte multilayer (PEM) coating. The developed membranes are much more open with a high molecular weight cutoff (MWCO ∼1000 Da) compared to typical PEM NF membranes (MWCO ∼300–500 Da). A layer-by layer (LbL) assembly approach is applied to prepare PEM coatings to serve as the active separation layer of membranes. This approach allows additional control over the fine-tuning of the membrane’s effective pore size and surface chemistry (charge density, hydrophilicity), which are important aspects in membrane separation processes. The membrane selectivity can be further tailored by utilizing asymmetric PEM coatings, where the first support pores are coated with loose PEM followed by the application of a denser and thinner PEM separation layer. Following this approach, PEM-based NF membranes were prepared using poly(allylamine hydrochloride) (PAH)/poly(styrenesulfonate) (PSS) on a charged flat-sheet polyether sulfone (PES) membrane support prepared via a non-solvent induced phase inversion (NIPS) process. Coating with PAH/PSS quickly filled the support pores, and the selectivity was further enhanced by subsequent coating by replacing PSS with poly(acrylic acid) (PAA) to form (PAH/PAA) multilayers, followed by cross-linking. This led to significantly improved membrane performance while maintaining the thin film composite design. Further, it is shown that the PEM structure can itself be carefully tuned toward the removal of specific pollutants from specific feeds. The resulting membranes are highly hydrophilic, as confirmed through contact angle results, and rejection is governed by Donnan and size exclusion mechanisms, with retentions of divalent ions (up to 80%), dyes (100%), and neutral solutes (∼90%). These membranes can be an excellent choice for the simultaneous treatment of water and resource recovery applications in the textile industry to retain/recover dyes and heavy metal ions.