Evaluation of nanofiltration and reverse osmosis membranes for efficient rejection of organic micropollutants

Nanofiltration (NF) and reverse osmosis (RO) membranes have revolutionized water treatment processes. However, emerging separations, such as the removal of organic micropollutants (OMPs), pose a challenge for traditional RO and NF membranes due to either low water permeance (RO) or inadequate OMP rejection (NF). In this work, we investigated the rejection of thirteen frequently detected OMPs at environmentally relevant low feed concentration of 12 μg/L by using a wide range of commercial RO and NF polyamide thin-film composite (TFC) membranes and an in-house prepared NF membrane. The membranes were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), contact angle, and surface charge measurements. We investigated removal capabilities of OMPs at standard brackish water RO operating conditions (i.e., 15.5 bar transmembrane pressure, pH 8) and also examined the effect of feed solution transmembrane pressure on membrane performance. The rejection of ionic OMPs was strongly influenced by electrostatic repulsion effects, whereas rejection of non-ionic solutes was primarily driven by size exclusion and hydrophobic/hydrophilic interactions. The best performing TFC membranes — a lab-made semi-aromatic piperazine-based NF polyamide membrane and a fully aromatic commercial polyamide NF membrane (NF1) — displayed excellent rejection of OMPs comparable to a commercial RO seawater membrane (RO4), but exhibited ∼2-fold and ∼6-fold higher water permeance, respectively.