Mono-/monovalent ions separation through an electro-nanofiltration

The separation of mono-/monovalent ions posed a long-standing challenge due to nearly identical physicochemical properties. To address the challenge, an electro-nanofiltration (ENF) strategy was proposed to separate K + /Li + with the assistance of multivalent cations. Experimental results demonstrated that an acceptable K + /Li + separation factor was achieved under the synergistic effect of multivalent cations and electric field (EF). Through an external EF, the multivalent cations were accumulated on the membrane surface to enhance the positive charge density and created a strong near-surface EF. Under the synergistic effects of the near-surface EF and the external EF, low-hydration-energy hydrated ion (K + ·nH 2 O) underwent preferential dehydration. The separation could occur between hydrated (Li + ·nH 2 O) and dehydrated (K + ) ions, resulting in a S K,Li value of 3.2. Meanwhile, the ENF system exhibited good stability. The difference of activation energy (Δ E a) of transport process between K + and Li + increased significantly from −0.4 ​kJ ​mol −1 under NF to 8.0 ​kJ ​mol −1 under ENF, which was believed to be the kinetic reason for achieving K + /Li + separation. Additionally, the separation of K + /Na + or Na + /Li + systems was achieved. Compared with existing mono-/monovalent ions separation strategies, the ENF strategy offered enhanced scalability and provided new insights into selective monovalent ion separation.