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

Improving 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(Ca2+) coordinated polyamide(PA) NF membrane was prepared by directly adding CaCl2 to the piperazine(PIP) aqueous solution during the interfacial polymerization process. Due to the coordination interaction between Ca2+ 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 CaCl2 concentration, the pure water flux of TFC NF increased significantly while the rejection of Na2SO4 decreased sightly. Compared with TFC NF membrane prepared without CaCl2, the permeate flux of the Ca2+ coordinated polyamide NF membrane prepared under optimal conditions was increased by 3–4 folds with Na2SO4 rejection of 95.26%. Meanwhile, such a Ca2+ coordinated PA NF membrane showed a better SO42−/Cl− selectivity.