Thin-film nanocomposite polyamide membrane doped with imine-based covalent organic frameworks nanosphere for precise ion sieving

Nanomaterials have created many opportunities for the exploitation of novel thin-film nanocomposite (TFN) membranes. However, their poor compatibility at the organic-inorganic interface between the nanomaterials and the polyamide layers is still a challenge in hindering the development of TFN membranes. Herein, we firstly prepared a stable suspension of covalent organic frameworks (COFs) nanospheres as aqueous-phase nanofillers. The amino and hydroxyl groups in the COFs nanospheres form a large amount of hydrogen bonds with the piperazine molecules, the diffusion rate of the piperazine into the interface was reduced as a result, leading to the generation of thinner and more wrinkled polyamide layers. Furthermore, the exceptional dispersion characteristics of the COFs nanospheres contribute to reducing the number of defects in the resulting polyamide layer. Moreover, the intrinsic pore size of COFs provides rich channels for lowering the resistance for water molecules to transfer. These properties collectively enhanced the separation performance of the COF-doped TFN membranes, resulting in optimized COF-doped TFN membranes that exhibited a pure water permeance of up to 37.1 L‧m⁻²‧h⁻¹‧bar⁻¹, with 99.0 % and 95.9 % rejection of Na₂SO₄ and MgSO₄, respectively. Furthermore, the membrane demonstrated remarkable efficacy in the separation of mono/divalent salts and mono/divalent anions, with $S_{NaCl/{Na}_2{SO}_4}$ and $S_{{Cl}^{-}/{SO}_4^{2-}}$ values of 92.3 and 105.1, respectively. Additionally, the COF-doped TFN membrane exhibits consistent nanofiltration performance across a range of salt concentrations, a broad operating pressure range, and under prolonged operational conditions. Furthermore, the membrane exhibits excellent resistance to contamination. The outstanding performance of COF-doped TFN membranes paves the way for precise ion sieving in the field of water treatment, particularly in the related areas of seawater softening, brine refining, and salt mixture separation.