Enhancing Nanofiltration Rejection of Perchlorate by Suppressing Co-Ion Competition for Water Treatment: The Role of Membrane Structural Rigidity

Perchlorate removal by nanofiltration (NF) remains challenging, largely due to an incomplete mechanistic understanding under realistic ion-competition conditions. Here, we identify the hidden key membrane properties governing perchlorate rejection by using four commercial NF membranes in natural and synthetic waters. Perchlorate rejection was generally low, as it is intrinsically disadvantaged in inevitable co-ion competition with dominant anions, particularly sulfate and chloride, owing to its low hydration energy and typically much lower concentration. Mitigating such co-ion competition is therefore central to enhancing perchlorate rejection. Reducing membrane negative charge density partially alleviated co-ion competition, as evidenced by an increase in perchlorate rejection from ∼20% to ∼48% for the small-pore DK membrane when natural water pH decreased from 8.8 to 6.4. Beyond pore and charge effects, however, ion-membrane interactions were found to be comparably decisive. Specifically, the highly rigid and weakly hydrophilic active layer of the DK membrane impeded perchlorate adsorption, suppressing competitive permeation of monovalent anions and thereby enhancing perchlorate rejection. These findings indicate that attenuating ion-membrane interactions through enhancing structural rigidity offers a viable strategy for achieving higher removal of perchlorate and other weakly hydrated anions, providing new mechanistic insights for NF membrane design.