Ion-Selective Separation Using MXene-Based Membranes: A Review

Global lack of clean water makes it essential that new technologies are developed for separation of pollutants from raw water. Membrane separation has arisen as a promising solution among other conventional liquid-separation processes, such as evaporation, distillation, and crystallization. Synthetic polymer membranes have been emphasized as an important component in liquid separation processes due to their outstanding water/salt selectivity and ionic conductivity. However, their inherent trade-off between selectivity and permeability has remained a challenge for polymeric membranes. Recently, twodimensional nanomaterials have been actively sought after as an alternative to polymeric membranes. MXene, a new type of nanomaterial, has been attracting considerable attention as a building block for nanostructured separation membranes. Due to several appealing features, such as their surface functional groups with negative charge, they have been used in size-, charge-, and adsorption-selective separation for different ions or molecules. This article reviews recent progress in MXene-based membranes, with a particular emphasis on ion-selective separation and their applications for water purification including salt rejection, reverse electrodialysis, or heavy metal adsorption. Lastly, the challenges and future directions of MXene-based ionselective membranes are discussed.