Nanofiltration membranes fabricated by plant polyphenol-intermediated MXene and polyethyleneimine layer-by-layer self-assembly for efficient dye/salt separation

The traditional layer-by-layer (LBL) self-assembly method used to prepare nanofiltration (NF) membranes with polyelectrolytes often suffers from low permeability caused by the restricted free volume of the selective layer. Furthermore, substrate surfaces typically require rigorous chemical treatment with potent acids or bases to achieve the necessary charge for adsorbing the initial polyelectrolyte layer. To address these issues, a green and straightforward aqueous immersion method was utilized in this investigation to form a coating of plant polyphenol tannic acid (TA) on the substrate surface. The TA coating served as an intermediary for the LBL self-assembly of negatively charged 2-dimensional material MXene nanosheets and positively charged polyethyleneimine (PEI), resulting in a well-structured NF membrane. By utilizing a 4-bilayer number, 0.05 mg/mL MXene concentration, 1.0 mg/mL PEI concentration, and PEI molecular weight of 1800 g mol−1, an NF membrane with an ultrahigh pure water permeance of 204.0 L m−2 h−1 bar−1 and 99.9% Evans Blue rejection was produced. Furthermore, the membrane possesses an ultra-high dye/salt separation factor of 758.1, surpassing most of the previously reported data. The innovative plant polyphenol-mediated MXene and PEI LBL self-assembly strategy proposed can be applied not only in NF membrane preparation but also in inspiring the design of advanced MXene composite materials for environmental and energy applications.