A novel loose nanofiltration membrane with high permeance and anti-fouling performance based on aqueous monomer piperazine-2-carboxylic acid for efficient dye/salt separation

Dye effluent is regarded as one of the most hazardous industrial effluents. Although loose nanofiltration (NF) membranes have been considered as an effective alternative technique for treating dye effluent, their development is still constrained by subpar permeance, separation efficiency and anti-fouling performance. In this work, a novel loose polyamide (PA) NF membrane was designed employing a conventional interfacial polymerization process using piperazine-2-carboxylic acid (PIP-COOH) and trimesoyl chloride as aqueous and organic phase monomers, respectively. The presence of unique electron-absorbing carboxyl groups on the PIP-COOH monomer not only provided abundant negative charge on the membrane surface, but also greatly reduced the reactivity, diffusion rate and solubility in n-hexane of the aqueous monomer, allowing for the formation of a more relaxed and smoother active layer. The balanced combination of pore size screening and Donnan effect achieved precise separations with high permeability. Moreover, the interaction of the carboxyl groups with the backbone structure increased the free volume of the PA layer and improved permeability significantly. The PIP-COOH/TMC membrane prepared under the optimal conditions exhibited high water permeance of 598.1 L·m−2·h−1·MPa−1, high reactive red rejection (99.3%) and low NaCl rejection (5.4%). Furthermore, the membrane possessed exceptional anti-fouling performance thanks to the ultra-smooth surface with crucial antifouling index flux recovery ratio as high as about 99%, which showed unexpected application possibilities. In conclusion, this study offers a potential novel membrane material for the development and application of loose NF membranes in the treatment of colored effluent.