Salt-modified MXene membrane for ultrafast and efficient cationic and anionic dyes removal

MXene, the two-dimensional (2D) transition metal carbides/nitrides, have gained much attention in the application as membranes since the stacking of MXene nanosheets could realize hydrophilic, nanoscale interlayer spacing with remarkable separation performance. Herein, we reported the simple modification of MXene membranes using chloride salts (NaCl, KCl, and MgCl₂), which could substantially improve the performance in the separation of cationic (methylene blue, MB) and anionic (congo red, CR) dyes from aqueous solutions. The salts could reside in the membrane surface and penetrate the interlayer galleries as well as the space within the adjacent MXene nanosheets. As a result, the membrane could possess modified nanochannels and altered surface chemistry that improve water transport and dye molecules' retention. In the case of MB filtration, MXene membrane modified with KCl (MM-KCl) exhibits the highest flux (141 L m⁻² h⁻¹) and a complete rejection, attributed to the weaker interaction between the water molecules and potassium ion. On the other hand, MM-MgCl₂ demonstrates the best performance (flux of 117 L m⁻² h⁻¹ and a ∼100 % rejection) in the CR filtration. This could be related to the stronger interaction of anionic dye molecules with the magnesium cation that has the highest positive charge density. All MXene membranes have high flux-recovery ratios and low reversible and irreversible fouling, showing desirable anti-fouling properties.