Polyelectrolyte-modified covalent organic framework membranes for multivalent cation removal

Covalent organic framework (COF) has become a promising membrane material in chemical separations. However, the application of COF membranes for ion separation is still challenging, due to the mismatch between the pore size of COFs (1–5 nm) with the diameter of hydrated ions (<1 nm). Herein, a cationic polyelectrolyte surface modification strategy of COF membranes was proposed, by assembling a cationic polyelectrolyte (PEI, PDDA, PAH) layer on the surface of an anionic COF (TpPa-SO₃H) membrane via dip coating. The cationic polyelectrolytes on the membrane surface offered abundant positive charges and reduced the surface pore size, leading to the synergistic enhancement of the size sieving and the electrostatic repulsion. Accordingly, effective rejection of multivalent cations was realized with a significantly increased rejection rate of 97.1 % for MgCl₂, compared to that of TpPa-SO₃H membrane (10.3 %). Furthermore, the polyelectrolyte-modified COF membrane exhibited desirable separation performance for heavy metal ions in water. The rejection rates for heavy metal salts, including CrCl₃, CdCl_2, CuCl₂, PbCl₂ and NiCl₂, reached 98.5 %, 97.3 %, 97.0 %, 96.2 %, and 96.0 %, respectively, and remained above 95 % during the 7-day long-term operation test.