Janus membrane with tailored upper and lower surface charges for ion penetration manipulation in high-performance nanofiltration

Conventional homogeneously charged nanofiltration (NF) membranes showed limited capabilities in ion transmembrane behavior control, which hindered the ion separation performances. In this study, novel Janus membranes were developed to manipulate the penetration of monovalent and divalent ions in NF. Graphene oxide nanosheets with enhanced positive charge (PGO) and negative charge (NGO) were successfully synthesized and assembled in designed orders and proportions to construct the Janus membranes with desired upper and lower surface charges. The positively charged upper surface and the negatively charged lower surface of the PN Janus membrane showed high retention of divalent cations but high permeation of monovalent cations; when the mass ratio of PGO to NGO was 1–5, the PN-1/5 membrane indicated a significant Li+/Mg2+ separation factor at 23.5. On the other hand, the NP-4/2 Janus membrane composed of negatively charged upper layer and positively charged lower layer indicated the most promising indiscriminate divalent ion removal efficiency (>80%) as compared with the literature. The molecular dynamics simulation theoretically proved the ion penetration behavior manipulation by the Janus membranes were feasible. This study proposed a facile strategy to dramatically improve the performances of NF in Mg2+/Li+ separation and indiscriminate divalent ion removal.