To coordinate the trade-off between the separation and permeation of the nanofiltration membrane for the separation of Mg2+/Li+, we regulated poly(ethyleneimine)/piperazine interface polymerization parameters to construct a positively/negatively charged ultrathin Janus nanofiltration membrane at a free aqueous–organic interface. At the optimized interfacial polymerization parameters, 0.03 wt % of piperazine reacted with trimethylbenzene chloride prior to poly(ethyleneimine), forming a primary polyamide layer with fewer defects or limiting large-scale defects of the polyamide layer. The controlled subsequent reaction of poly(ethyleneimine) and trimethylbenzene chloride results in a Janus nanofiltration membrane, with one side enriched with the carboxyl groups, the other side enriched with the amine groups, and a dense polyamide structure in the middle. Under the optimum conditions, the positive potential of the rear surface of the prepared membrane was 14.57 mV, and the water contact angle reached 71.31°, while the negative potential of the front surface was −25.48 mV, and the water contact angle was 12.93°, confirming a Janus membrane with opposite charges and large hydrophilicity differences in the front and rear surfaces. With a high cross-linking degree, a 40 nm thick polyamide layer is 29.09% more thinner than the traditional polyamide membrane. The ultrathin Janus nanofiltration membrane showed an excellent separation factor (SLi,Mg of 18.26), stability, and water permeability flux (10.6 L·m–2·h–1·bar–1). The rejections to MgCl2, CaCl2, MgSO4, and Na2SO4 are measured above 90% at a nearly constant permeability of 10.6 L·m–2·h–1·bar–1, particularly stable rejections to MgCl2 and Na2SO4.
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/m 2 ⋅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.
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