Bifunctional polyhedral oligomeric silsesquioxane engineered polyamide membrane for efficient Li+/Mg2+ separation

Nanofiltration membrane technology holds great promise in extracting lithium (Li) from salt-lake brines. However, designing nanofiltration membranes with both high water permeance and ion selectivity remains a grand challenge. Here, amino-functionalized polyhedral oligomeric silsesquioxane (8NH2-POSS) nanocage is designed as a bifunctional molecular filler into polyamide (PA) layer during the interfacial polymerization of polyethyleneimine (PEI) and trimesoyl chloride (TMC). The 8NH2-POSS not only provides abundant amino groups to increase the membrane electropositivity from +4.5 mV to +33.2 mV (pH = 6.0), but also enlarges the pore size of PA matrix and constructs additional water channels to facilitate water transport. The optimal POSS-PA membrane has a rejection rate of 98.1 % to 1 g/L MgCl2, a pure water permeance of 12.0 L m-2h−1 bar−1 and an exceptional separation factor (SLi,Mg) of 13.9 at an ultralow Li+/Mg2+ mass ratio of 1:150, surpassing the benchmark PA membranes.