Layer-thickness gradient nanofiltration membrane constructed by alternating assembly of two-dimensional MXene nanosheets and polyamide layer for dye and antibiotic wastewater purification

Membrane-based separation technology is playing a pivotal role in addressing challenges such as sustainable water purification, ion sieving, resource extraction and environmental protection. However, fabricating of the membrane integrates high water permeability, high selectivity and good durability has remained challenging due to the “trade-off” of perm-selectivity. Herein, a gradient membrane structure with gradually decreased layer thickness from the bottom to top surface is designed. The thickness-gradient multilayer structure is constructed by alternating assembly of two-dimensional Ti 3 C 2 T x MXene layer and polyamide (PA) layer. The importing of MXene layer could increase the adsorption of piperazine molecules and improve the interfacial properties of support, which greatly help to optimize the interfacial polymerization reaction-diffusion process and form a more uniform and defect-free PA separating layer. More importantly, the 2D-stacking format of MXene as well as layer-thickness gradient structure substantially reduce hydraulic resistances, promoting water molecules to flow rapidly through the membrane. As a result, the pure water permeability was elevated to ~20.4 L m −2  h −1  bar −1 (nearly 3-folds of the individual PA membrane) and a 97.3 % rejection of Na 2 SO 4 is obtained. Meanwhile, the resultant MXene/PA-2 exhibits outstanding rejection to methyl blue of ~99.9 % and nystatin of ~100.0 %. The membrane also retained 99.5 % of MB rejection and a water permeability of ~18.6 L m −2  h −1  bar −1 after 80 h cross-flow filtration, demonstrating its long-term operation stability. This work provides a rationally structural designing and a promising alternative route of the advanced nanofiltration membranes for high-efficient purification of organic wastewater.