Perfluorooctane sulfonate removal by nanofiltration membrane the role of calcium ions

Perfluorooctane sulfonate (PFOS) is a persistent and emergent contaminant in aqueous environments that is a problem of global concern. Accordingly, its removal from surface water is critical to drinking water security. In this study, the NF270 membrane was used to remove PFOS from simulated surface water containing calcium ions. The effects of PFOS concentration, calcium concentration and pH on PFOS rejection were investigated. Additionally, permeate flux performance and membrane morphology were also evaluated based on SEM, AFM, contact angle and PFOS accumulation. The results showed that PFOS rejection increased with PFOS concentration in the range of 10-500 ppb when 0.1 mM Ca2+ and 1 mM Ca2+ were present in the feed solution. An increase of calcium chloride concentration enhanced PFOS rejection from 94.0% to 99.3% for the feed of 100 ppb PFOS with calcium concentration from 0 to 2 mM at 0.4 MPa owing to the increasing permeation of the smaller anions, as well as calcium-bridging and pore blockage. Density functional theory (DFT) calculations furthermore allowed quantifying the calcium-bridging mechanism and qualitatively interpreting the improvement of the PFOS rejection efficiency. pH had a significant effect on PFOS rejection, with an increase in pH from 3 to 9 leading to an increase in PFOS rejection from 86% to 95% and 93% to 97% in the presence of 0.1 mM Ca2+ and 1 mM Ca2+ at 0.4 MPa, respectively. The permeate flux demonstrated that the addition of calcium decreased the permeate flux compared to the single PFOS system. With the calcium concentration increase, the images of SEM and AFM exhibited that the membrane surface had more precipitation and higher surface roughness, and PFOS accumulation on the membrane increased, all off which correspond to flux decline.