Quaternary ammonium salts modification preparing charged Janus nanofiltration membrane for the simultaneous separation of divalent anions and cations

Simultaneous separation of divalent co-ions and counter-ions from mixed ions water environment was urgently needed to improve separation efficiency and reduce energy consumption. The charged Janus nanofiltration membranes (NFMs) came into being based on the Donnan effect, however, its practical application was still limited by complex preparation, low permeability or poor ion selectivity. Here, with carboxylated polysulfone (PSF–COOH) ultrafiltration membrane as the support layer, a newly discovered quaternary ammonium salt containing diamine group (ethidium bromide, EB) was used to modify pristine polyamide nanofilm to prepare a novel charged Janus NFM. Structural characterization and molecular dynamics simulation indicated that the difference in electrostatic force of PSF-COOH with piperazine (PIP) and EB resulted in a difference of at least 20 times between their diffusion rates and hence achieved their regular distributions in the oil-water interface. As a result, EB modification induced the structure heterogeneity (charge and morphology) of the polyamide layer, which allowed the selectivity of both monovalent/divalent anions and cations to be higher than 45, while ensuring a permeability of up to 56.2 L/m2⋅h. Moreover, other analogues of EB were applied and proved the generality of the mechanism for preparing charged Janus NFMs in this study, which represented a method for the scalable preparation of ultra-selective membranes with heterostructure for simultaneous separation of divalent co-ions and counter-ions.