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

To effectively separate Mg 2+ and Li + from high Mg 2+ /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 Mg 2+ /Li + separation factor S Li/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 Mg 2+ /Li + separation factor S Li/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 S Li/Mg of 40–49, high brine permeability values of 10.5 L·m −2 ·h −1 ·bar −1 , and excellent chemical and operational stability.