High flux Mg2+/Li+ nanofiltration membranes prepared by surface modification of polyethylenimine thin film composite membranes

A quaternary ammonium salt with bulk size and contorted configuration was effectively anchored to surfaces of polyethylenimine thin-film composite membranes, leading to 3 times higher water peremability coupled with good Magnesium/Lithium selectivity. • Thin-film composite membranes were surface modified with a quaternary ammonium salt. • The membrane shows highly improved water permeability and good Mg 2+ /Li + selectivity. • The membrane exhibits stable performance in separating Mg 2+ /Li + mixture at 6 bar pressure. Nanofiltration plays emerging roles in lithium extraction from salt-lake brines with high magnesium-to-lithium (Mg 2+ /Li + ) ratio, yet current lithium extraction nanofiltration membranes show relatively low water permeability. A new monomer (1-(2-hydroxyethyl)-1,3,5,7-tetraazaadamantane-1-ium bromide, HMTAB) was designed to improve membrane separation performances by means of surface modification. Infrared characterizations show that the HMTAB monomer was anchored the pristine polyethylenimine (PEI) thin film composite membrane through esterification condensation. Average roughness of the HMTAB membrane is relatively small (5.7 nm), while the thickness of the top-layer was increased by ∼ 20%, i.e., from 90 to 110 nm. Water flux of the HMTAB membrane in filtrating Mg 2+ /Li + mixture (2000 ppm, Mg 2+ /Li + : 100) at 6 bar pressure is 97.9 L m -2 h −1 , which is 3.6 times as high as the pristine PEI membrane. Meanwhile, the Mg 2+ /Li + ion selectivity of the HMTAB membrane is maintained, and the membrane reduced the high Mg 2+ /Li + ratio (50) in the feed to a relatively low value (4.9) in the permeate. Last, the modified membrane exhibits stable performance in 40 h’ continuous nanofiltration.