Although the presence of perfluorooctane sulfonate (PFOS) in environmental systems has generated widespread interest, little information is available on PFOS removal from surface water by membrane technology. Herein, a novel nanofiltration membrane (NF270) was investigated so as to remove PFOS. More specifically, the interaction and effect of Mg2+ ion and/or humic acid (HA) in the feed solution on PFOS removal efficiency and total permeate flux were examined. Under 0.8 MPa, an improvement in PFOS rejection efficiency from 94.3% to 99.5% was observed when the concentration of Mg2+ ion was increased from 0-2 mM in the feed solution. This enhancement is attributed to enhanced size exclusion because PFOS molecules are linked together by Mg2+ ion, as shown by Density Functional Theory (DFT) calculation. The result shows that Mg2+ ion can more easily neutralize two PFOS anion than one PFOS anion on the basis of the Gibbs free energy changes. Conversely, the presence of HA had minimal improvement on PFOS rejection because HA had much weaker ability to bind with PFOS comparing to Mg2+. Consequently, The further interaction between Mg2+ and the carboxylic groups of HA molecule, carboxylic group or amino group of NF270 membrane was also calculated via DFT theory. Corresponding DFT structures and calculation parameters of Mg2+ with different groups were obtained. These findings indicate that the size exclusion mechanism primarily governs PFOS removal by the NF270 membrane. Flux results showed that the co-presence of Mg2+ ion and HA in the feed solution resulted in the most flux reduction, by 70%, that was associated with the thickest fouled layer. (C) 2016 Elsevier B.V. All rights reserved.