Fabrication of positively charged nanofiltration membrane with uniform charge distribution by reversed interfacial polymerization for Mg2+/Li+ separation

Nanofiltration is considered a promising technology for separation of multivalent/monovalent ions such as extracting lithium from Salt Lake brine with high Mg2+/Li+ ratio. However, the nanofiltration membranes fabricated via conventional interfacial polymerization process generally possess negatively charged surface and have low Mg2+/Li+ selectivity. Herein, we proposed a new and facile method, i.e., reversed interfacial poly-merization (RIP), to prepare nanofiltration membranes with positively charged surface and uniform charge distribution. The sequence of amine (polyethyleneimine, PEI) and acyl chloride (trimesoyl chloride, TMC) monomers on polysulfone support was reversed compared to the conventional interfacial polymerization (IP) process. Moreover, the solvent of amine monomer was acetone instead of water. The nanofiltration membranes fabricated by this method exhibited higher water flux (from 4.88 to 22.25 Lm(-2)h(-1)) and MgCl2 rejection (from 84.36% to 97.71%). Meanwhile, LiCl/MgCl2 separation factor in single salt solution reached up to 13.93, which was almost quadruple compared to the conventional control membrane. In addition, the developed nanofiltration membrane also exhibited an excellent separation selectivity (S-Li,S- Mg = 9.22) in mixture system (Mg2+/Li+ mass ratio of 20), which had good application potential in extracting lithium resources from Salt Lake brine.