Intercalation of zwitterionic surfactants dramatically enhances the performance of low-pressure nanofiltration membrane

Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.

Mechanism of Permselectivity Enhancement in Polyelectrolyte-Dense Nanofiltration Membranes via Surfactant-Assembly Intercalation

Enhancing the water permeance while maintaining the solute rejection of a nanofiltration (NF) membrane can potentially result in significant cost-reduction for NF-a membrane process that excels in several unique environmental applications of growing interests. In this work, we demonstrate for the first time that intercalation of surfactant self-assemblies in the polyelectrolyte multilayer (PEM) can lead to significant performance enhancement of salt-rejecting dense NF membranes fabricated using layer-by-layer assembly of polyelectrolytes. Specifically, the intercalation of sodium dodecyl sulfate (SDS) bilayers in a PEM comprising poly(diallyldimethylammonium chloride) (PDADMAC) and poly (sodium 4-styrenesulfonate) (PSS) resulted in a decrease in PEM thickness, increase in pore size, and a smoother and more hydrophilic surface. The water permeance of the resulting PEM NF membrane increased by 100% without compromising the rejection of Na₂SO₄. Experiments with a quartz crystal microbalance also provide direct evidence that the intercalation of the surfactants substantially reduces the subsequent adsorption of the polyelectrolytes of a similar charge. Based on its mechanism of performance enhancement, surfactant intercalation may become a universally applicable and highly cost-effective approach for dramatically enhancing the performance of PEM NF membranes.