Mix-charged nanofiltration membrane: Engineering charge spatial distribution for highly selective separation

Steric and charge exclusions are mainly responsible for the separation selectivity of nanofiltration (NF) membranes, thereinto membrane charge spatial distribution plays an important role in filtration behavior of charged solutes, yet doesn't get enough attention. In this review, we suggest that NF membranes with particular charge spatial distribution can be called mix-charged membranes. First, the roles of Donnan effect on NF separation selectivity as well as antifouling ability for mix-charged membranes are discussed from practical application point of view. Second, the effect of horizontal/longitudinal charge distribution on the NF performance and corresponding six characterization methods are critically reviewed from the perspective of fundamental research. Third, three fabrication technologies for mix-charged NF membranes including interfacial polymerization (five strategies), sequential coating and self-assemble are summarized, focusing on the strategies to regulate charge proportion and polarize charge distribution. We find that there are still many unclear issues in the separation mechanisms of mix-charged NF membranes, such as micro-electric filed, electrostatic neutralization and ion dehydration, and deeper understanding on the structure-performance relationship and more precise control the charge spatial distribution need a quantitative index of charge distribution and higher-resolution characterization methods. This review not only intends to emphasize the importance of charge spatial distribution of NF membranes, but also offers guidance for designing and developing advanced mix-charged membranes for specialty separations.