Electro-nanofiltration membranes with positively charged polyamide layer for cations separation

High performance and low-cost monovalent cation perm-selective membranes (MCPMs) are extremely desirable since the widespread demand for monovalent/multivalent cations separation. Herein, we developed a simple fabrication method for the preparation of electro-nanofiltration membranes (ENFMs), which were used as MCPMs for cations separation. The ultra-thin polyamide layer of ENFMs could offer selective channels for the transport of monovalent cations. Meanwhile, the positive charges in the polyamide layer act as useful barriers to divalent cations via electrostatic repulsion. The results show that the ENF-Q3 membrane with the highest quaternization degree owns outstanding perm-selectivity for Li+/Mg2+ system (PMg2+Li+=11.3), which is 7 times greater than that of the commercial CSO membrane, and high Li+ flux (JLi+=5.79×10-8mol·cm-2·s-1) at the current density of 10 mA cm−2. Moreover, the ENFMs exhibit high limiting current density and excellent long-term stability, indicating that the developed strategy is promising to scale up for industrial applications. A series of ENFMs with positively charged polyamide layer were prepared by constructing an ultra-thin polyamide layer on the porous substrate surface and following quaternization. The dense polyamide layer could offer selective channels for the transport of monovalent cations. Meanwhile, the positive charge in polyamide layer acts as a useful barrier to divalent cations. The synergistic effect of pore size sieving and electrostatic repulsion endowed the ENFMs excellent ion separation performance. [Display omitted] •ENFMs with positively charged polyamide layer are prepared.•The ENFMs show excellent perm-selectivity and flux for monovalent cations.•The ENFMs exhibit high limiting current density and low area resistance.