Ultrathin polyamide nanofiltration membranes with tunable chargeability for multivalent cation removal

Positively charged nanofiltration membranes are promising in water softening and heavy metal ion removal. However, facile modulation on their chargeability remains a great challenge. Here, we proposed a charged monomer-engineered interfacial polymerization toward positively charged polyamide membranes. In particular, branched amino macromolecules (BAMs) with different charged group numbers and molecular sizes were selected as aqueous monomers, allowing for wide-range-tunable membrane chargeability. We found that larger BAMs tend to form intramolecularly crosslinked networks with more amino residues, conferring membrane chargeability up to +5.53 mC m(-2). Besides, the slower diffusion of larger BAMs also led to ultrathin membranes down to 9.0 nm in thickness. The optimal composite nanofiltration membrane displayed a high rejection to multivalent cations (e.g., MgCl2 rejection of 98.7%) with ultrahigh pure water permeance of 31.5 L m(-2) h(-1) bar(-1), which was around 2-10 times higher than that of the reported positively charged nanofiltration membranes. Our monomer design strategy for interfacial polymerization may evolve into a facile approach to constructing advanced charged membranes.

Automated summary

The study proposed a method of using charged monomers for interfacial polymerization to successfully prepare positively charged polyamide nanofiltration membranes. By selecting branched amino macromolecules with different charged group numbers and molecular sizes as monomers, a tunable membrane chargeability was achieved, showing excellent rejection to multivalent cations and high pure water permeance.