Synergistic regulation of nanofiltration membrane pore structure and surface charge by a novel bis-quaternary ammonium salt for efficient Li+/Mg2+ separation

A novel bis-quaternary ammonium salt was synthesized. • The bis-quaternary ammonium salt was used as an aqueous co-monomer to enhance the size sieving and charge exclusion effects of the nanofiltration membrane. • The Li + /Mg 2+ separation factor of the optimized membrane reached 73.3, which is 5.4 times that of the pristine membrane. Efficient separation of Li + and Mg 2+ from salt lakes is important for the application and recovery of lithium resources. In this work, a novel bis-quaternary ammonium salt (QTMPDA) was synthesized and utilized as a co-monomer in the polyethyleneimine aqueous solution for interfacial polymerization (IP), synergistically regulating the pore structure and surface charge of the nanofiltration membrane and improving its Li + /Mg 2+ separation performance. The QTMPDA monomer features a linear short-chain diamino structure, enabling it to participate in the IP, increase the crosslinking density of the polyamide network, and form a more concentrated pore size distribution, thereby enhancing the membrane's size sieving effect. Meanwhile, the two quaternary ammonium cations in QTMPDA can effectively increase the membrane surface electropositivity, strengthening the electrostatic repulsion of Mg 2+ . Relative to the unmodified membrane, the zeta potential of the optimized membrane was elevated from −5.9 mV to 20.2 mV at pH 7 and the MgCl 2 rejection was increased from 93.3% to 99.2%. Moreover, the membrane demonstrated an excellent Li + /Mg 2+ separation factor of 73.3 (mass ratio of Mg 2+ /Li +  = 100). This work offers a novel approach for regulating IP process via functional aqueous co-monomers to optimize membrane structure and performance. Download: Download high-res image (139KB) Download: Download full-size image