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-(NH 2 ) 2 into the negatively charged polyamide layer. Thanks to the customizable nanochannels of metal-organic frameworks, the amino-rich nanochannels of UiO-66-(NH 2 ) 2 could be fabricated through a ligand replacement strategy. UiO-66-(NH 2 ) 2 played a crucial role in the enhanced rejection of divalent cations, meanwhile negatively charged polyamide surface guaranteed the efficient Na 2 SO 4 rejection. Satisfactory rejections of CaCl 2 (96.07 %) and Na 2 SO 4 (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-(NH 2 ) 2 rejected Ca 2+ efficiently since the migration energy barrier across UiO-66-(NH 2 ) 2 was an order of magnitude higher than that of UiO-66-NH 2 and UiO-66. Moreover, the desirable rejections were beyond 94 % towards many divalent cation salts including MgCl 2 , NiCl 2 , PbCl 2 , MnCl 2 , CdCl 2 and ZnCl 2 . Thus, this work provided an efficient way to realize the synergistic removal of both divalent cations and anions.