Calcium Ion Coordinated Polyamide Nanofiltration Membrane for Ultrahigh Perm-selectivity Desalination

I mproving the permeate flux but retaining the rejection of thin-film composite(TFC) polyamide nanofiltration(NF) membrane is a high requirement for desalination. In this work, a calcium ion(Ca 2+ ) coordinated polyamide(PA) NF membrane was prepared by directly adding CaCl 2 to the piperazine(PIP) aqueous solution during the interfacial polymerization process. Due to the coordination interaction between Ca 2+ and the amide bond in PA active layer, the number of hydrogen bonds in the PA active layer was reduced, causing in turn the decrease of physical cross-linking degree. As a consequence, the pore of the PA active layer was enlarged, prominently enhancing the water permeance of NF membrane. With the increase of CaCl 2 concentration, the pure water flux of TFC NF increased significantly while the rejection of Na 2 SO 4 decreased sightly. Compared with TFC NF membrane prepared without CaCl 2 , the permeate flux of the Ca 2+ coordinated polyamide NF membrane prepared under optimal conditions was increased by 3–4 folds with Na 2 SO 4 rejection of 95.26%. Meanwhile, such a Ca 2+ coordinated PA NF membrane showed a better SO 4 2− /Cl − selectivity.