Structure evolution of MXene membranes mediated by ultralong HAP nanowires: from 2D plains to 3D mountains for ultrafast dye/salt separation

The development of high-performance nanofiltration membranes for dye/salt separation remains challenging due to the ubiquitous permeability–selectivity trade-off. Conventional MXene membranes suffer from structural compactness caused by irreversible nanosheet restacking, which severely limits their water permeability and separation efficiency. Herein, we propose a structural engineering strategy using ultralong hydroxyapatite (HAP) nanowires as nanoscale spacers to transform conventional 2D MXene laminates into 3D mountain-like architectures. The ultralong HAP nanowires (length: tens of micrometers; diameter: <100 nm) with ultrahigh aspect ratios (>1000) serve as effective structure mediators, creating abundant water transport channels while preventing MXene nanosheet restacking. Comprehensive characterization reveals that the optimized HAP/MXene membrane exhibits significantly expanded structural parameters, including a 98.31 % increase in surface roughness, 12.05 % rise in porosity, and 26.05 % enhancement in specific surface area compared to pristine MXene membranes. These structural advantages translate into exceptional separation performance: the membrane achieves an unprecedented water flux of 608.50 LMH/bar (1023.73 % improvement) while maintaining high dye rejection rates (>98 % for Congo Red) and minimal salt rejection (<15 % for various salts). Furthermore, the membrane demonstrates excellent stability in mixed dye/salt solutions, with consistent performance across different solute concentrations and long-term operation. This work not only overcomes the long-standing permeability–selectivity dilemma in MXene membranes but also establishes a generalizable paradigm for designing next-generation loose membranes through rational structural engineering.