A novel nanofiltration membrane with [MimAP][Tf2N] ionic liquid for utilization of lithium from brines with high Mg2+/Li+ ratio

In order to utilize lithium from brines with high Mg2+/Li+ ratio, a positively charged nanofiltration (NF) membrane was prepared by grafting amine-functionalized ionic liquid 1-(3-aminopropyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([MimAP][Tf2N]) onto the surface of polyamide (PA) membrane. The effects of [MimAP][Tf2N] contents on the chemical composition, surface morphology, surface charges and separation properties of the modified PA membranes were discussed. The results of chemical composition analyses indicated reaction of the amine group of [MimAP][Tf2N] with the residual acyl chloride groups, which was existed on the surface of the nascent PA membrane. The surfaces of all the modified PA membranes were positively charged at pH = 6.4 due to the quaternary ammonium group of [MimAP][Tf2N]. In the separation process, the resulting membrane presented high water permeance (37.8 L/(m2⋅h)) and mono-/divalent cation selectivity (Li+ and Mg2+) with much higher MgCl2 rejection (83.8%) than LiCl rejection (24.4%). For a simulative brine with a Mg2+/Li+ ratio of 20, the rejections to Mg2+ and Li+ by the optimized membrane (NF-IL-2.0%) were around 81.9% and −45.2%, respectively, where the permeate with the Mg2+/Li+ ratio of 3.5 was obtained and the selectivity (SLi,Mg) was 8.12. Additionally, the optimized membrane exhibited a stable performance for Mg2+/Li+ separation. Therefore, there is a very possibility for utilization of lithium from brine with high Mg2+/Li+ ratio by using the optimized NF membrane.