Removal mechanisms of perfluorinated compounds (PFCs) by nanofiltration: Roles of membrane-contaminant interactions

Abstract

Perfluorinated compounds (PFCs)-membrane interactions play important roles in the removal of PFCs by membrane separation, especially in their adsorption onto membrane interfaces and membrane rejection. In this work, a loose nanofiltration (NF) membrane and a tight NF membrane were used to remove six PFCs that have different head groups and different C-F chains. The roles of typical PFCs-membrane interactions, including electrostatic repulsion, hydrophobic interactions and hydrogen bonding, on membrane adsorption and rejection were quantitatively evaluated. The membrane adsorption capacities to the six PFCs were determined by static adsorption experiments. Influences of the PFCs-membrane interactions mentioned above were quantified with six negatively charged PFCs by comparing the adsorption and rejection at neutral pH with those at the isoelectric points (IEPs) of the two membranes. Results showed that electrostatic repulsion caused that the rejections by the loose membrane and tight membrane increased by 2.2%–36.0%, and 0.8%–7.4%, respectively. Non-electrostatic interactions (hydrophobic interactions and hydrogen bonding) decreased rejections of the tight membrane and the loose membrane by 9.5%–23.6% and 1.2%–10.3%, respectively. Interactions between the PFCs and the membranes were further calculated by density functional theory (DFT). DFT analysis and Gibbs free energy changes suggest that PFCs could interact with membranes in thermodynamics. The head groups are away from the membrane surface because of electrostatic repulsion and the tail groups close to the membrane surface due to hydrophobic interactions and hydrogen bonding.