Effects of acid acceptor and pH on the structure and filtration performance of nanofiltration membrane

The thin film composite nanofiltration (TFC-NF) membranes with various properties were prepared by changing type and concentration of acid acceptors in the piperazine (PIP) solution. The influences of acid acceptors on interfacial polymerization (IP) process depend on the pH of PIP solution and the properties of acid acceptors. Na2CO3 and TEA provide positive effects at a pH range of 11.2-12.0. Especially for Na2CO3, water permeability and ion selectivity of the membrane simultaneously enhanced. The pH values reached above 12.8 with NaOH, and negative effects on permeance and selectivity were detected. The pH values ranged 10-11 after adding Na3PO4 and NaHCO3. The rejections of both Cl- and SO42- were high, especially for Cl- rejection increasing to above 73%. The pH values got to around 7 with NaH2PO4. The IP reaction was retarded and defects were detected on the membrane with quite low SO42- rejection. Protonation of PIP happens at low pH, while hydrolysis of acyl chloride of trimesoyl chloride happens at high pH. These changes will slow down the deprotonated PIP diffusion and the IP reaction. It is a feasible way to fabricate tailored NF membranes by changing acid acceptors and adjusting pH value of the PIP solution.