Effects of Physicochemical Properties of Nanofiltration Membranes on the Rejection of Small Organic DBP Precursors

Abstract

In this study, two types of commercial nanofiltration (NF) membranes (NF70 and NF270) were chosen to remove four model disinfection by-product (DBP) precursors (resorcinol, phloroglucinol, 3-hydroxybenzoic acid, and tannic acid) with different functional groups. The clean NF membranes were also characterized by physicochemical properties of membrane surface charge (determined by streaming potential measurement), hydrophobicity [determined by contact angle measurement), membrane surface morphology (determined by scanning electron microscopy and atomic force microscope (AFM)], surface roughness (determined by AFM), and surface composition (X-ray photoelectron spectroscopy) to validate the NF rejection mechanisms of the DBP precursors. The filtration experiments of the model DBP precursors were assessed under various pH values (3–10), and the removal efficiencies were good at high pH values for both membranes (84–99% at pH 9 and 89–100% at pH 10). Electrostatic repulsion is the prevailing mechanism for the model compounds with negatively ionizable functional groups rejected by the negatively charged NF membranes at high pH values, whereas steric hindrance exclusion and adsorption are controlling factors for the rejection of unionized small organic molecules. For the examination of membrane surface roughness, it is essential to use the same scan area when comparing the surface roughness of different membranes. Furthermore, NF70, with a rougher surface, reasonably posed a higher fouling potential, which makes the flux decline more serious than that of NF270 under the same operation conditions.