Electrochemically tailored ion-trapping nanoarchitectures in COF membranes for selective monovalent/divalent ion sieving

The efficient separation of monovalent and divalent ions remains a core challenge in desalination and ion resource recovery. Here, we propose an electrochemical confinement strategy to spatially organize EDTA into aligned arrays within the one-dimensional transport channels of TFP-Tag membrane with a pore size of 1.1 nm. This structure establishes a biomimetic, gradient-functionalized conical nanochannel. The negatively charged upper channel regions capture and block divalent ions, while the lower regions with their larger pore size and an anion-affinitive local environment facilitate monovalent ion transport, thereby achieving highly selective monovalent/divalent ion separation. The membrane exhibits outstanding monovalent/divalent ion separation performance in multi-ion mixed solutions, with Na⁺ flux reaching 10.6 mmol m⁻² hour⁻¹ and the selectivity of Na⁺/Mg²⁺ exceeding 500. This breakthrough separation behavior is attributed to an induced transport lag effect arising from the synergy between the COF structure and the chelation-assisted retardation of divalent ions by EDTA. This study establishes a theoretical framework for the rational design of ion-separation membranes exhibiting simultaneous high-selectivity and high-flux performance.