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

Abstract

Abstract 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 (ESNA1-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 N a + 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.