Constructing of stable intermediate layers among the positively charged composite nanofiltration membranes for Mg2+/Li+ efficient separation

In recent years, many researchers have focused on incorporating functional nanomaterials into thin-film composite (TFC) membranes to synthesize thin-film nanocomposite (TFNi) nanofiltration (NF) membranes with intermediate layers. The stability of the intermediate layer was very important for the TFNi NF membranes. This study constructed a unique structure of ZIF-8@CNTs-OH@PEI nanocomposite intermediate layers on the polysulfone (PSF) supporting membranes, where hydroxyl-functionalized carbon nanotubes (CNT-OH) captured 2-methylimidazole through its porous structure and hydrogen bonding with hydroxyl groups, enabling zeolitic imidazolate framework-8 (ZIF-8) to grow on CNT-OH. In addition, polyethylenimine (PEI) could act as the “adhesive” among ZIF-8 and CNT-OH due to the strong adhesion of hydrogen bonding with abundant –NH 2 groups. Thus, the nanocomposite particles could form a stable intermediate layer on the surface of PSF membrane. Furthermore, the positively charged PA/ZIF-8@CNTs-OH@PEI/PSF composite NF membranes with strong interlayer bonding were synthesized through further interfacial polymerization reactions on the intermediate layer. In particular, the synthesized nanocomposite suspension of ZIF-8@CNTs-OH@PEI address the particle aggregation and poor dispersibility. Moreover, the structure of intermediate layer features abundant voids, which can reduce the transmembrane resistance of water and enhance permeation performance. Compared to the PA/M−0 without the intermediate layer, the PA/M−30 exhibited a significantly improved flux of 70.6 L m -2 h −1 , that was 2.6 times higher compared to that for PA/M−0. Additionally, the optimized PA/ZIF-8@CNTs-OH@PEI/PSF composite NF membranes achieved Li + and Mg 2+ retention rates of −25.5 % and 95.2 %, respectively, with a Mg 2+ /Li + separation factor (SLi, Mg) equal to 29.9, which showed good long-term operational stability.