Improving Li+/Mg2+ separating performance of polyethylene/polyamide membranes under high Mg(II)/Li(I) ratios: Reverse interface polymerization modulation and Mg(II)-Cyclen interaction

To simultaneously increase the Mg 2+ /Li + separating performance under high Mg 2+ /Li + ratios of polyethylene/polyamide (PE/PA) thin film composite (TFC) nanofiltration (NF) membranes, 1,4,7,10-tetraazacyclododecane (Cyclen) was added in the aqueous phase with polyethyleneimine (PEI) monomer. During reverse interface polymerization (RIP) on polyethylene (PE) substrates, phthaloyl chloride (PC) was added to the organic phase with terephthaloyl chloride (TPC) monomer. It was found that the addition of Cyclen and/or PC could both increase the hydrophilicity, surface roughness and average pore sizes of formed virgin PE/PA TFC NF membranes, leading to increased water permeability. The Li + /Mg 2+ separating factor S Li/Mg first increased and then decreased with maximum points at the TPC/PC ratio of 5:1 and Cyclen addition level of 0.02 wt%. Cation absorption, especially Mg(II)-Cyclen interactions in the PA layers under higher Mg 2+ /Li + ratio led to increased surface Zeta potential, decreased average pore diameters μ p, and greatly increased Li + /Mg 2+ separating factor (S Li/Mg ). S Li/Mg value of PEI-TPC/PC 5:1 -Cyclen 0.02 membranes could be increased from 5.0 to 55.47, keeping the water permeability around 6.5 L m −2  h −1  bar −1 when Mg 2+ /Li + ratio increased from 1 to 100 in the brine.