Efficient cation sieving and recovery via synergistic nanoconfinement and ionic anchoring in engineered PSS-functionalized nanochannel membranes.

Cation exchange membranes (CEMs) enabling selective monovalent cation transport are of critical importance for sustainable water treatment and resource recovery processes such as energy-efficient desalination and lithium recovery. However, achieving high monovalent selectivity remains challenging due to insufficient nanoscale control over ion-membrane interactions. In this study, we developed a high-performance monovalent CEM featuring biomimetic nanochannels via an in situ "seeding-counterdiffusion-growth" strategy. A positively charged polyaniline (PANI) layer and a ZIF-8 framework were constructed on a commercial cation-exchange membrane, followed by incorporating poly(4-styrenesulfonate) (PSS) into the ZIF-8 cavities to tailor the ion transport pathways. The resulting ZIF-PSS modified membrane demonstrated exceptional monovalent selectivity, with permselectivity values of [Formula: see text] = 47.7 and [Formula: see text] =36.6, respectively, while maintaining high ion fluxes (JNa⁺ = 2.8 × 10⁻8 mol·cm⁻2·s⁻1; JLi⁺ = 2.3 × 10⁻8 mol·cm⁻2·s⁻1). Electrochemical analyses (I-V and EIS) revealed that PSS incorporation promotes Mg2⁺ accumulation and electric double layer overlap within the nanochannels, increasing interfacial resistance toward divalent ions. Gaussian simulations further indicated that sulfonic groups reduce the diffusion energy barrier for monovalent ions while hindering Mg2⁺ transport via a spatial confinement-ionic anchor mechanism. The modified membrane presented high mono-/divalent cation permselectivity among state-of-art CEMs and demonstrated long-term performance stability. This work provides an applicable design principle and fundamental insights of biomimetic ion-selective membranes for highly efficient mono/divalent ion separation in desalination and resource recovery.