Study on the effects of cations and anions on the removal of perfluorooctane sulphonate by nanofiltration membrane

Previously, the nanofiltration (NF) membrane has been applied to remove perfluorooctane sulphonate (PFOS), a persistent pollutant from drinking water by our group. However, the effects of co-existence ions on the PFOS removal have not been systematically studied. For instance, cations such as Ca2+ and Fe3+ are found to coordinate with -SO3 groups of PFOS molecule and subsequently influence the rejection properties of NF membrane. In this study, a commercial NF membrane (ESNAl-K1) was utilized to separate PFOS compounds in the existence of three cations including Na+, Ca2+ and Fe3+, as well as three anions including Cl-, SO42- and PO43-. The PFOS rejection increased from 92.65% to 94.74%, 97.14%, and 97.94%, respectively, with 2 mM Na+, Ca2+ and Fe3+, respectively. As the concentrations of anions including SO42- and PO43- increased to 2 mM, the PFOS rejection increased to 94.74% and 97.60%, respectively. The density functional theory (DFT) was applied to analyze the interaction between three different cations with PFOS molecule. It indicated that one Na+ ion could only bind with one PFOS molecule, but every Ca2+ or Fe3+ ion preferred to coordinate with two PFOS molecules. Based on the analysis results of the NF membranes while using the atomic force microscopy (AFM), surface zeta potential (electro-osmosis) and X-ray photoelectron spectroscopy (XPS), the major separation mechanisms of PFOS in the existence of other cations and anions were the sieving effect, and the electrostatic repulsion, respectively.