Polyethylenimine Deposited on Phosphomolybdic-Acid-Loaded Polyacrylonitrile Membranes for Enhanced Dye/Salt Separation

Rationally designing membranes for efficient fractionation of dyes and salts is a great challenge in the membrane community. Here, a composite loose nanofiltration membrane (NF) was fabricated by depositing polyethylenimine (PEI) onto a phosphomolybdic acid (PMo12)-loaded polyacrylonitrile substrate followed by cross-linking with glutaraldehyde. The physical and chemical properties of the loose NF membrane were systematically investigated. Compared with the unloaded membrane, the introduction of PMo12 regulates the deposition of PEI, leading to a smaller pore size, higher hydrophilicity, and stronger charge of the loose NF membrane. The optimized loose NF membrane presented an extremely low rejection of <2.00% to inorganic salts sodium sulfate (Na2SO4), magnesium sulfate, magnesium chloride, and sodium chloride, while maintaining a high rejection of >99.00% to organic dye Congo red (CR). In addition, the loose NF membrane is capable of fractionating CR/Na2SO4 mixtures with an astonishing separation factor of 59.18. Moreover, the stability of the loose NF membrane is qualified for a 100 h period filtration. This work provides an efficient strategy to fabricate loose NF membranes with high separation performance for the fractionation of dyes or salts.