Polyethyleneimine (PEI) based positively charged thin film composite polyamide (TFC-PA) nanofiltration (NF) membranes for effective Mg2+/Li+ separation

To effectively separate Mg2+ and Li+ from high Mg2+/Li+ ratio salt lake brine, positively charged thin film composite polyamide (TFC-PA) nanofiltration (NF) membranes were prepared by the interface polymerization involving polyethyleneimine (PEI) with benzenetricarbonyl chloride (TMC). In this work, the structures and performances of PEI based TFC-PA NF membrane prepared on polysulfone (PSF) ultrafiltration substrates from the normal interface polymerization process PSF(NIP), was compared with those on the same substrates from the reverse-phase interface polymerization (RIP) process PSF(RIP), and those on polyethylene substrates from RIP process PE(RIP). Results showed that PSF(RIP) showed improved Mg2+/Li+ separation factor SLi/Mg from PSF(NIP) with an increased surface positive charge, but with greatly reduced water flux values due to reduced surface roughness, increased surface hydrophobicity, and decreased MWCO Stokes pore radius. PE(RIP) NF membrane showed not only higher Mg2+/Li+ separation factor SLi/Mg, but also higher water flux values due to increased surface roughness, decreased PA cross-linking degree, reduced PA separation layer thickness, and increased MWCO Stokes pore radius. PE(RIP) NF membrane also showed high rejection rates to a variety of divalent metal salts with high separation factors SLi/Mg of 40–49, high brine permeability values of 10.5 L·m−2·h−1·bar−1, and excellent chemical and operational stability.