Selection of High Flux Membrane for the Effective Removal of Short-Chain Perfluorocarboxylic Acids

Nowadays persistent perfluorohexanoic acid (PFHxA) ubiquity in the environment is a result of its increasing use as substitute of longer chain perfluorocarboxylic acids. In this study, five nanofiltration (NF270, NF90) and reverse osmosis (XLE, BW30 and SW3OXLE) membranes were analyzed in the separation of PFHxA at elevated concentrations (100 mg L-1) relevant in industrial effluents of the production of fluorinated polymers. Experiments assessed the influence of the operation pressure, pH, and ionic strength. The NF270 membrane achieved the highest permeate flux, although PFHxA rejection was typically the lowest of all membranes and was negatively affected in the acidic pH range. The NF90, XLE, and BW30 membranes achieved PFHxA observed rejections over 99%, but only NF90 and XLE membranes attained suitable water permeability. Finally, both XLE and NF90 membranes were selected to provide the lowest PFHxA concentration in the permeate (0.2 mg L-1 at 10 bar). However, when the ionic strength was further increased the electrostatic repulsion to perfluorohexanoate was weakened and the size exclusion mechanism contribution increased, thus making the membranes more permeable to PFHxA. Our results demonstrate that proper selection of NF/RO membranes is a critical issue for the design of treatments for industrial effluents containing short chain PFCAs.