Ion Transport Mechanism in the Sub-Nano Channels of Edge-Capping Modified Transition Metal Carbides/Nitride Membranes

Edge-capping modified MXene membranes with new channels created by lateral nanosheets are of great research significance. After introducing tripolyphosphate (STPP) to Ti edges of Ti3C2Tx nanosheets and fabricating the STPP-MXene membranes edge-capping method, this research investigated the performance optimization mechanism of STPP-modified MXene membranes in terms of salt permeability (NaCl, Na2SO4, MgCl2, and MgSO4) and transmembrane energy barriers (Esalt) through the concentration gradient permeation test. Experimental results demonstrated an approximately 1.86-fold enhancement in salt flux (Js) compared to the MXene membranes. The solution–diffusion model was also introduced to evaluate the salt solubility (Ks) and diffusivity (Ds) during permeation. Furthermore, analysis of transmembrane energy barriers revealed that STPP modification induced significantly larger reductions in activation energy for magnesium salts (MgSO4: 55.1%; MgCl2: 47.4%) compared to sodium salts (NaCl: 30.5%; Na2SO4: 30.9%). This phenomenon indicated the weakened electrostatic interactions between high-valent Mg2+ and the modified lateral membrane Ti edges, whereas the limited charge density of Na+ resulted in relatively modest optimization. The results highlight the contribution of STPP capping on the edges of adjacent lateral nanosheets. Therefore, the modification increased the transportation rate of cations across the MXene membrane by more than twice, thus advancing the application of 2D MXene membranes in resource recovery.