Effect of Flux (Transmembrane Pressure) and Membrane Properties on Fouling and Rejection of Reverse Osmosis and Nanofiltration Membranes Treating Perfluorooctane Sulfonate Containing Wastewater

TLDR

Perfluorooctane sulfonate (PFOS) is an emerging environmental contaminant of significant concern. The study evaluates the removal of PFOS from wastewater using reverse osmosis and nanofiltration membranes. Five RO and three NF membranes were tested at 10 ppm PFOS over four days, with systematic assessment of PFOS rejection and permeate flux. PFOS rejection exceeded 99 % for RO and 90–99 % for NF, correlated with NaCl rejection, and improved over time with only modest flux decline, likely due to PFOS accumulation and entrapment in the polyamide layer, with higher initial flux or pressure enhancing rejection and fouling, while rougher membranes experienced greater flux reduction.

Abstract

Perfluorooctane sulfonate (PFOS) is an emergent contaminant of substantial environmental concerns. In this study, reverse osmosis (RO) and nanofiltration (NF) membranes were used to remove this toxic and persistent compound from PFOS-containing wastewater. Five RO membranes and three NF membranes were tested at a feed concentration of 10 ppm PFOS over 4 days, and the PFOS rejection and permeate flux performances were systematically investigated. PFOS rejection was well correlated to sodium chloride rejection. The rejection efficiencies for the RO membranes were > 99%, and those for the NF membranes ranged from 90-99%. Improvement in PFOS rejection, together with mild flux reduction (< 16%), was observed at longer filtration time. Such shifts in rejection and flux performance were probably due to the increased PFOS accumulation at longer duration, as shown by X-ray photoelectron spectroscopy and liquid chromatograph and tandem mass spectrometry results. A fraction of PFOS molecules might be entrapped in the polyamide layer of the composite membranes, which hindered the further passage of both water and other PFOS molecules. In a similar fashion, PFOS rejection and fouling were enhanced for greater initial flux and/or applied pressure, where PFOS accumulation was promoted probably due to increased hydrodynamic permeate drag. Flux reduction was also shown to correlate to membrane roughness, with the rougher membranes tend to experience more flux reduction than the smoother ones.