Antibacterial polyamide nanofiltration membranes with intermediate layers of quaternized cellulose nanocrystal for Mg2+/Li+ separation

Positively charged and non-toxic intermediate layers are highly desirable for the construction of nanofiltration membranes in the field of Mg2+/Li+ separation. Herein, we have prepared dual-electric layer nanofiltration membranes via introducing positively charged quaternized cellulose nanocrystals (Q-CNC) beyond the negatively charged polyamide skin layers. On the one hand, the strong interactions between Q-CNC and piperazine (PIP) could regulate the thickness, roughness, and residual groups of the polyamide (PA) layers, resulting in improved water flux. On the other hand, the Q-CNC has lots of positive charges which are beneficial for enhancing the rejections of negatively charged polyamide skin layers towards divalent cations since the Donnan and electrostatic effects are long-range forces. Moreover, this effect could be amplified by reducing the thickness of skin layers or decreasing the contents of residual carboxyl groups via adjusting the monomer concentrations. The obtained nanofiltration membranes possess high water flux of 51 L/m2/h/bar, and rejection for MgCl2 of 86 %. The separation factor of Mg2+/Li+ is 29 when the Mg2+/Li+ mass ratio in the feed solution is 20:1. Moreover, the nanofiltration membrane with Q-CNC intermediate layers (QNFM) has good antibacterial properties, showing great potential in the fields of lithium extraction from salt lakes.