Efficient removal of perfluorooctane sulphonate by nanofiltration: Insights into the effect and mechanism of coexisting inorganic ions and humic acid

The removal of perfluorooctane sulphonate (PFOS) with coexisting substances is very important. Inorganic ions and humic acid (HA) commonly coexist and may influence PFOS removal performance. However, there is little information on the capacity for systematic PFOS removal in the presence of inorganic ions and HA during nanofiltration (NF) process. In this paper, the impact of the coexistence of anions, cations, and HA on PFOS removal were investigated by NF. More specifically, the effect and interaction of anions, cations, and HA on PFOS removal performance and mechanism were examined. The PFOS removal rejection increased from 92% to 99% when HA was present together with both cations and anions. The higher the valence of the inorganic ion was, the greater the PFOS rejection was. The sieving effect was dominant in the improvement of PFOS removal by the presence of cations. The electrostatic repulsion played an important role in the improvement of PFOS removal by the presence of anions. Furthermore, density functional theory (DFT) was applied to calculate the interaction between PFOS molecules, cations, and HA. Corresponding DFT structures and thermodynamics parameters were obtained. It showed that the Ca²⁺ ions could more easily bridge two PFOS than one PFOS, HA could more easily coordinate with both Ca²⁺ and PFOS molecules. The increase of PFOS size after the coordination reaction results in the improvement of the sieving effect. Additionally, other analyses including X-ray photoelectron spectroscopy, atomic force microscopy, and the surface zeta potential also demonstrated the sieving effect mainly governed PFOS removal in coexistence with HA and cations. The electrostatic repulsion was dominant in PFOS separation performance in coexistence with HA and anions.