Polyamide nanofiltration membrane modified with defective covalent organic framework interlayer for selective ion sieving

Generally, thin-film composite (TFC) membranes prepared using piperazine (PIP) and trimesoyl chloride (TMC) exhibit good Na 2 SO 4 rejection, but their MgCl 2 rejection is not satisfactory. In this study, a defective covalent organic framework (COF) interlayer with residual unreacted amine groups was fabricated via interfacial polymerization (IP) by changing the catalyst concentration. Introduction of the COF layer into a polyamide (PA) separation membrane modified the pore size of the PA layer, thereby improving the rejection of divalent cations. The pore size of the PA layer decreased and the surface morphology of the membrane was converted from nodular to a hybrid morphology of “ridge-and-valleys’’ and nodules, indicating that the residual unreacted amine groups on the COF interlayer successfully reacted with the acyl chloride groups. The optimal membrane (PIP/COF-A/PES) exhibited high rejection of divalent ions (Na 2 SO 4 of 98.3% and MgCl 2 of 94.3%), and the membrane showed excellent capability for separating mono/divalent ions. Overall, a method of preparing nanofiltration membranes by modifying the PA layer to enhance the rejection of divalent cations was demonstrated.