Preparation of highly selective nanofiltration membranes by moderately increasing pore size and optimizing microstructure of polyamide layer

The enhancement in selectivity of nanofiltration (NF) membranes is of great significance for extending the application areas and improving the process economy, especially for the separation of monovalent/divalent ions. The characteristics of moderately increased pore size, fewer nonselective defects and a more uniform structure are conducive to NF membrane exhibiting higher rejection of divalent ions but lower rejection of monovalent ions. Nonylphenol polyoxyethylene ether (NP-10) was introduced in interfacial polymerization process in this study to optimize the microstructure and surface properties of polyamide (PA) layer to manufacture highly selective NF membranes. The hydrophobic benzene ring and aliphatic chain in NP-10 could form a local space that restricted the interfacial polymerization reaction on the side of the oil phase, thereby appropriately increasing the pore size of the PA layer. It was advantageous to the permeation of water molecules and monovalent ions with a smaller radius. The long hydrophilic oxyethylene group as a guide could facilitate the uniform diffusion of the monomer to the water/hexane interface. NP-10 as an amphiphilic substance would form a self-assembled network at the water/hexane interface, which was expected to optimize the distribution of aqueous amine monomers at the phase interface. Meanwhile, introducing NP-10 can form a stable interface and reduce the interface disturbance, thus inhibiting the formation of non-selective macropores (or “defects”) in PA layer, which could improve the separation selectivity of monovalent/divalent ions. In addition, the hydrophilicity and electronegativity were also enhanced. The results showed that the MWCO of NF membrane increased from 210.1 Da (TFC-0) to 269.6 Da (TFC-0.25), and the average pore radius increased from 0.248 nm to 0.276 nm. The water flux of optimized membrane was doubled compared to the control membrane. Most important, the optimized exhibited higher rejection of divalent Na2SO4 (from 98.07% to 99.43%), and meanwhile lower rejection of monovalent NaCl (from 54.07% to 27.52%). NaCl/Na2SO4 separation factor reached up to 130.53, which was about four times of TFC-0 membrane.