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 (S
Li,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.