Dually charged polyamide nanofiltration membrane incorporated UiO-66-(NH2)2: Synergistic rejection of divalent cations and anions

Traditional polyamide nanofiltration membranes suffer from permeability-selectivity trade-off limitation and unsatisfactory rejection for divalent cations. Herein, a novel dually charged membrane was constructed by incorporating positively charged UiO-66-(NH2)2 into the negatively charged polyamide layer. Thanks to the customizable nanochannels of metal-organic frameworks, the amino-rich nanochannels of UiO-66-(NH2)2 could be fabricated through a ligand replacement strategy. UiO-66-(NH2)2 played a crucial role in the enhanced rejection of divalent cations, meanwhile negatively charged polyamide surface guaranteed the efficient Na2SO4 rejection. Satisfactory rejections of CaCl2 (96.07 %) and Na2SO4 (90.62 %) were achieved simultaneously by optimal TFN-D5. Meanwhile, TFN-D5 owned the greatly-improved water permeance of 19.44 LMH/bar, which was about 1.5 times better than that of the TFC-control. Calculations based on density functional theory further confirmed UiO-66-(NH2)2 rejected Ca2+ efficiently since the migration energy barrier across UiO-66-(NH2)2 was an order of magnitude higher than that of UiO-66-NH2 and UiO-66. Moreover, the desirable rejections were beyond 94 % towards many divalent cation salts including MgCl2, NiCl2, PbCl2, MnCl2, CdCl2 and ZnCl2. Thus, this work provided an efficient way to realize the synergistic removal of both divalent cations and anions.