Positively-charged PEI/TMC nanofiltration membrane prepared by adding a diamino-silane coupling agent for Li+/Mg2+ separation

Positively charged nanofiltration membrane has gained significant interest in magnesium and lithium separation due to its separation mechanism, especially in salt lake-based lithium industry. To obtain a high-density positive charged surface and high Li+/Mg2+ selectivity, a novel thin-film nanocomposite (TFN) nanofiltration (NF) membrane for recovery of Li+ from Mg/Li mixtures with Mg2+/Li+ ratios of 20 was successfully prepared by using polymeric amine polyethyleneimine (PEI) and monomeric amine silane coupling agent (3-diamino-methyl-cyclohexyl triethoxysilane, DTES). Then the effect of DTES on the surface properties and performance of the prepared DTES/PEI/TMC TFN membranes were investigated. Chemical composition analysis revealed that DTES not only hydrolyzed to Si–OH and then self-polymerized to Si–O–Si, but also reacted with the self-carrying acyl chloride of 1,3,5-benzene trichloromethane (TMC). Moreover, SiO2 nanoparticles were in-situ generated from DTES inside the PA layer with high compatibility. Due to the compatible properties with the positively charged PEI polymer, the reacted DTES was distributed uniformly in the formed TFN selective layer with no noticeable aggregation. The doping of SiO2 into the polyamide network gave rise to the enlargement of the membrane pore size, enhanced the surface hydrophilicity, and reinforced the surface positive charges. The water flux of the DTES/PEI/TMC membrane improved from 36.9 L m−2 h−1 to 49.58 L m−2 h−1 compared to the PEI/TMC membrane. Additionally, the Mg2+ and Li + rejection of the optimized membranes was 91.46% and −10.5%, respectively, with permeability and selectivity (SLi, Mg = 12.95) superior to that of the commercial NF membranes. Considering the commercial availability of DTES, this study not only offers a high-performance nanofiltration membrane for recovering lithium from brines but also provides a feasible strategy for industrial separation of other monovalent and divalent ions.