Two-dimensional MXene membranes with biomimetic sub-nanochannels for enhanced cation sieving

TLDR

High‑permeability, highly selective membranes are sought for water treatment, resource extraction, and energy storage. This study aims to build MXene‑based cation‑sieving membranes using EDTA molecules, inspired by the KcsA K⁺ channel. The membranes combine EDTA’s negatively charged oxygen atoms with MXene nanosheets’ 6.0‑Å sub‑nanopores to create biomimetic channels with tunable charge density. The resulting membranes achieve a K⁺/Mg²⁺ selectivity of 121.2, surpassing existing membranes, and the high selectivity is attributed to EDTA‑mediated cation recognition, partial dehydration, and enhanced local charge density, offering a new design strategy for ion‑separation membranes.

Abstract

Membranes with high ion permeability and selectivity are of considerable interest for sustainable water treatment, resource extraction and energy storage. Herein, inspired by K+ channel of streptomyces A (KcsA K+), we have constructed cation sieving membranes using MXene nanosheets and Ethylenediaminetetraacetic acid (EDTA) molecules as building blocks. Numerous negatively charged oxygen atoms of EDTA molecules and 6.0 Å two-dimensional (2D) sub-nanochannel of MXene nanosheets enable biomimetic channel size, chemical groups and tunable charge density for the resulting membranes. The membranes show the capability to recognize monovalent/divalent cations, achieving excellent K+/Mg2+ selectivity of 121.2 using mixed salt solution as the feed, which outperforms other reported membranes under similar testing conditions and transcends the current upper limit. Characterization and simulations indicate that the cation recognition effect of EDTA and partial dehydration effects play critical roles in cations selective sieving and increasing the local charge density within the sub-nanochannel significantly improves cation selectivity. Our findings provide a theoretical basis for ions transport in sub-nanochannels and an alternative strategy for design ions separation membranes.