Synergistic regulation of macrocyclic polyamine-based polyamide nanofiltration membranes by the interlayer and surfactant for divalent ions rejection and mono-/di-ions sieving

For nanofiltration membranes, the selective separation of divalent/monovalent cations with high permeance is a desirable yet challenging demand due to the trade-off phenomenon between the perm-selectivity and permeance. In this study, a novel macrocyclic polyamine 1, 4, 7, 10-tetraazacyclododecane (Cyclen) is used as the aqueous phase monomer to construct a nanofiltration (NF) membrane with high permeance without sacrificing the perm-selectivity. The large molecular size of Cyclen greatly limits its interfacial mass transfer across the water/oil interface, resulting in the formation of many defects in the separation layer. Based on the widely applied interlayer technology in literature, a surfactant-assembly regulated interfacial polymerization (SARIP) is adopted to intensify the phase transfer process of the Cyclen. The interlayer accommodates enough Cyclen monomers and the surfactant acts as the phase transfer catalyst, which jointly enhance the monomer's phase transfer rate. The resultant polyamide membrane yields a uniform sub-nanometre pore size, and the combination of reduced pore size and charge distribution within the pores achieves high rejection for divalent ions and low rejection of monovalent ions, with the rejection order of LiCl (18.9 %) < NaCl (24.8 %) < MgSO4 (96.2 %) < MgCl2 (96.3 %) < Na2SO4 (97.0 %). The corresponding water permeance for MgCl2 reaches up to 18.5 L·m−2·h−1·bar−1 with the desirable Li+/Mg2+ and Na+/Mg2+ perm-selectivity of 21.9 and 20.3, respectively.