Investigation of the rejection of mixed salt solutions using a g-C₃N₄ functionalized nanofiltration membrane

The use of nanofiltration technologies for seawater desalination is one of the most effective ways to solve the shortage of water resources. Salt ions in seawater are complex and diverse, and the traditional nanofiltration membrane has a high rejection rate for a single salt solute. However, it is difficult to maintain good rejection rates for a variety of salt ions in mixed or even multi-salt solutions while ensuring a good permeation flux. Therefore, it is necessary to prepare a nanofiltration membrane with good filtration performance in mixed solutions. In this research manuscript, we used the interfacial polymerization method to prepare a polyamide nanofiltration membrane with a small amount of charge on the surface using 1,4-bis (3-aminopropyl)-piperazine and introduced graphite phase carbon nitride treated using solvent green 7 intercalation into the separation layer. The hydrophilicity of the prepared composite membrane improved greatly, and the surface charge distribution changed. Due to the special charge distribution, the dielectric repulsion in the mass transfer process of the nanofiltration membrane was enhanced. Compared with the traditional commercial method, the rejection of different salt ions in the mixed solution system (Na₂SO₄-NaCl, CaCl₂-NaCl) increased by 50%~100%, and the permeation flux exceeded the commercial membrane by 2~10 times, reaching 50.76 L m⁻²h⁻¹. Through model fitting, the important role of dielectric effect in the mass transfer process was verified, and the difference in the mass transfer results between single/mixed solutions was explained. In addition, the composite membrane maintained good anti-fouling performance in bovine serum albumin and humic acid solutions. The research results provide research ideas and directions for seawater desalination pretreatment.