Mix-charged polyamide membranes via molecular hybridization for selective ionic nanofiltration

Nanofiltration membranes that can selectively remove multivalent ions from salty water hold great promise in water treatment and salt utilization. However, developing nanofiltration membranes with high rejection to both divalent anions and cations remains a grand challenge. Herein, we design a kind of mix-charged polyamide membranes (MCPMs) via molecular hybridization to achieve the removal of versatile divalent ions. Amino-functionalized polyhedral oligomeric silsesquioxane (NH2-POSS) was employed as molecular filler added into piperazine and reacted with trimesoyl chloride at the aqueous-organic interface to generate cross-linked polyamide-NH2-POSS mixed network. The protonated amino groups from NH2-POSS offered positively charged sites for enhanced divalent cation repulsion while the deprotonated carboxyl groups from polyamide network can afford negatively charged sites for divalent anion repulsion. The optimized membrane displayed high rejections to both divalent anionic and cationic ions (Na2SO4 rejection of 98.2%, MgCl2 rejection of 98.0%), together with high mono-/divalent salt selectivity (NaCl/Na2SO4 of 48.2, LiCl/MgCl2 of 43.9) that outperformed the benchmark nanofiltration membranes. This molecularly-hybridized charge modulation method may enlighten the structural design of membranes for efficient ionic separations.