Effect of quaternary ammonium salt structures on Mg2+/Li+ separation performance of modified membranes

Surface modification is an effective strategy to improve separation performance of Mg 2+ /Li + nanofiltration membranes, but less is known about how chemical structures of modification molecules affect separation performances of resultant membranes. In this work, three types of quaternary ammonium salts with modulated structures were designed to modify polyethyleneimine-trimesoyl chloride (PEI-TMC) membranes. The surface and cross-section morphologies, mean pore sizes, charge and hydrophilicity properties of the modified membranes were characterized. These quaternary ammonium salts can tune membrane properties effectively, and impart good separation performance to modified membranes. Molecular structures of quaternary ammonium salts affect the diffusion of the monomer into the organic phase and the reactivity with the acyl chloride. The flux of modified membranes are 99.6 Lm −2  h −1 , 112.2 Lm −2  h −1 , and 128.8 Lm −2  h −1 , and the rejection of MgCl 2 are 93.2 %, 94.9 %, and 92.8 %, respectively. The modified membranes also show good retention for heavy metal ions (e.g., Cu 2+ , Cr 3+ , Zn 2+ , Co 2+ , and Ni 2+ ).