Mechanism Analysis of PFHxS Purification in Water Using Nanofiltration under the Coexistence of Sodium Alginate and Ca2+ Based on DFT

Abstract

Perfluorohexanesulfonic acid (PFHxS) is considered a persistent and harmful substance to ecosystems and human health and should thus be necessarily removed. In this paper, the influence of working pressure, temperature, pH value, initial concentration, and the coexistence of sodium alginate (SA) and calcium ions on removing PFHxS using nanofiltration were hereby studied through a self-made hyaluronic acid-interlayered thin-film composite nanofiltration membrane (HA-TFCiNFM). When the substances SA or/and calcium ions (Ca2+) co-existed, the retention rate of the HA-TFCiNFM to PFHxS exceeded that in the single PFHxS system and increased with SA and Ca2+ concentrations. By filtering a single PFHxS, PFHxS/SA, and PFHxS/SA/Ca2+ water sample system, it was observed that the HA-TFCiNFM had the highest membrane fouling degree in the PFHxS/SA/Ca2+ system. The mechanisms of membrane separation and fouling when SA and Ca2+ co-existed were explored based on density functional theory (DFT). Building on DFT, the PFHxS structure was constructed, and when Ca2+ co-existed, Ca2+ complexed two PFHxS molecules to increase the complex compound size, making the sieving effect notable. When SA co-existed, although PFHxS and SA could not react with each other, the presence of SA would block the membrane pore and enhance the negative charge on the membrane surface, thereby strengthening the sieving and Donnan effects. When the substances Ca2+ and SA co-existed, Ca2+ complexed a PFHxS molecule and a SA molecule to form a larger complex compound so that the sieving effect could be enhanced. From the molecular perspective, the intermolecular interaction relationship between pollutants and the HA-TFCiNFM was explored based on DFT, and weak interactions, such as hydrogen bonding and van der Waals interactions, were clarified as the main mechanisms to control the membrane fouling. These results provide insight into the purification mechanism of PFHxS in water using nanofiltration and highlight the usefulness of DFT in exploring membrane separation and fouling mechanisms.