Recent advances of loose nanofiltration membranes for dye/salt separation

Loose nanofiltration (NF) is a promising method for the fractionation of dyes and salts due to its excellent selective separation performance. Such type membrane is a potential candidate that can realize the transformation and upgrading for efficient recovery and cyclic utilization of high value-added components (i.e., dyes and salts). In this review, we first discuss and determine the concept of “loose nanofiltration” from the perspective of functionality orientation and briefly expound separation mechanisms and applications of loose NF. Then mainly summarize the recent advances in design and preparation technology towards fabricating highly selective membranes for dye/salt separation, including interfacial polymerization, phase inversion (blending nanofillers), surface locating. Furthermore, analyze different types of materials such as nanoparticles (NPs), two-dimensional materials (2DMs), ionic liquids (ILs), zwitterionic compounds, biomaterials (e.g. dopamine, tannins etc.,) and advanced porous materials such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), which are attractive candidates used in membrane technology. Finally, state briefly the inherent restrictions and challenges of loose nanofiltration membranes for future practical applications including membrane fouling, stability and precise separation, outlook and future directions toward loose nanofiltration are additionally discussed.