Oxygen Plasma-Activated PES Ultrafiltration Substrates for Constructing High-Performance Mg2+/Li + Separation Nanofiltration Membranes

Abstract

Abstract Positively charged polyamide (PA) nanofiltration (NF) membranes hold great potential for Mg2+/Li+ separation, but their performance is strongly affected by the surface properties and pore structures of the substrate. In this study, a PES ultrafiltration substrate was pretreated by ethanol natural evaporation and then activated by oxygen plasma prior to interfacial polymerization to fabricate composite NF membranes. The pretreatment effectively suppressed irreversible pore shrinkage of the PES substrate during drying, enabling the E-PES-UF substrate to maintain a high water flux of 228 LMH. Subsequent oxygen plasma activation improved the hydrophilicity and interfacial wettability of the substrate through mild etching and the introduction of polar oxygen-containing groups, thereby promoting the uniform distribution of PEI aqueous-phase monomers on the activated substrate surface. As a result, the optimized E-30-20-NF membrane exhibited a uniform PA layer with a thickness of only 37 nm and an isoelectric point of pH 8.85. It achieved a Mg2+ rejection of 99.3%, a separation factor of 52.8, and a permeation flux of 21.1 LMH. During the 120 h continuous-filtration test, the Mg2+ rejection remained above 98.9%, while the Mg2+/Li+ mass ratio in the permeate was maintained at 0.28–0.40. The membrane also exhibited a good antifouling performance.