Electric field-assisted ultra-thin MXene conductive nanofiltration membrane for efficient dye/salt separation

Two-dimensional MXene membranes have garnered significant attention in the field of water treatment. Nevertheless, achieving a balance between permeability and selectivity remains a fundamental challenge in MXene membrane separation. Herein, a novel electrically active nanofiltration (NF) membrane (CNT/MXene-PEI) with an ultra-thin separation layer was prepared through vacuum filtration of MXene and cross-linking of polyethyleneimine (PEI) on the CNT substrate. By controlling the thickness of the MXene separation layer to reduce mass transfer resistance and applying a negative voltage to enhance the electrostatic effect between the conductive membrane and charged pollutants, the dye rejection rate and anti-pollution performance were improved. At an optimal MXene loading of 15.625 mg/m2, the thickness of the separation layer was about ∼47 nm. Under these conditions, the pure water flux achieved was 156.11 LMH/bar. The rejection rate of methylene blue (MLB) increased from 26.2 % to 97.9 % under an applied voltage of −2 V, while the rejection rate for inorganic salts was 9 %. The dye/salt separation factor was approximately 11. Furthermore, the flux recovery rate of the CNT/MXene-PEI membrane was at 90 % after 12 h of operation with applied voltage of −2 V, indicating excellent anti-pollution performance and long-term operational stability. These results demonstrate the combined effect of structural regulation and electric field enhancement in the membrane's filtration performance. This study offers a new approach for overcoming the trade-off effect between permeability and selectivity in dye/salt separation processes.