Novel insight into dual-polyamide layer composite nanofiltration membrane toward enhanced ion selective performance

High-performance thin-film composite (TFC) nanofiltration (NF) membranes featuring suitable pore size and appropriate charge density are critical for achieving high ion selectivity. Herein, a secondary interfacial polymerization (SIP) strategy was proposed to prepare TFC NF membrane with dual-polyamide (PA) layer structure using polyethylenimine (PEI) and piperazine (PIP) as aqueous monomers in sequence. The first PEI-PA layer with a smoother surface and uniform pore size distribution serves as an interlayer for the SIP, contributing to the uniform distribution of PIP monomers and producing a controllable regulation of the SIP reaction. The resultant NF membrane featured thinner thickness, smoother surface, denser pore size, and asymmetric charge properties, achieving greater than 99% rejections of five heavy metals (Cu 2+ , Mn 2+ , Zn 2+ , Ni 2+ , Co 2+ ) during mixed salts solution filtration test at concentration range of 100-2000 mg L -1 . More importantly, the unique dual-PA layer NF membrane achieved an excellent selectivity of Li + and Mg 2+ of 95.08 under the combined effects of pore size exclusion and charge repulsion. Interestingly, its pure water permeance (13.5 L m -2 h -1 bar -1 ) was increased by about 35% due to the "gutter effect" between the bilayers in comparison with the monolayer PA membrane. Moreover, the transition state theory was introduced to analyze the partition and diffusion energy barriers of salts during filtration. Our work demonstrates that the SIP strategy is promising for realizing ultra-high solute-selective NF membrane.