Rapid transport of water and monovalent ions through ultrathin polyamide nanofilms for highly efficient mono/bivalent ions separation

Abstract

Defect-free ultrathin polyamide nanofilms enable superior separation performance, which can be prepared by interfacial polymerization (IP) at free oil–water interface (also known as support-free IP). However, the current free IP methods usually suffer from complicated process, or complex equipment. Herein, an easily-manufactured plate-and-frame assembly for free IP was designed for more efficient fabrication of ultrathin membranes. Piperazine (PIP)-based negatively-charged NF membranes were fabricated by this method, combining with introduction of a rigidly-contorted spirobisindane monomer that simultaneously enhanced the mechanical strength, microporosity, and electronegativity of membrane. The resulting defect-free ultrathin membranes not only exhibit super-high Na2SO4 rejection (∼99.7 %), and allow rapid transport of water and monovalent ions, thus leading to a Cl-/SO42- selectivity of up to 240. Furthermore, the polymerization of acyl chloride and macromolecule polyethyleneimine (PEI) at free oil–water interface was also investigated. The resulting positively-charged ultrathin membranes show outstanding calculated monovalent/bivalent cation selectivity (54.8 for Li+/Mg2+, 43.2 for Na+/Mg2+), further demonstrating the effectiveness of ‘thickness reduction’ strategy to enhance ions selectivity. In addition, the cause of different surface charge properties for PIP and PEI membrane system was revealed by molecular simulation.