Ozone compatibility with polymer nanofiltration membranes

Abstract

Ozone is a strong oxidant applied in water treatment for disinfection and organic and inorganic pollutants removal. It can be coupled with membrane processes as a pre-treatment or post-treatment as well as in a hybrid configuration. In this study, we investigated the resistance of three commercial polymer nanofiltration membranes (NP10, NF90 and NF270) in contact with ozone (10 ppm for 1 h) at pH 3 and 7 to assess the influence of the ozone to hydroxyl radical concentrations balance. The surface properties of membranes were characterized before and after ozonation by means of various techniques, i.e. Fourier transform infrared spectroscopy in attenuated total reflectance mode (ATR-FTIR), zeta potential, water contact angle, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM). For all membranes, the impact of ozonation on pure water permeability was greater at pH 7 than pH 3 due to the faster decomposition of ozone at pH 7 leading to the formation of more free radicals. A decrease in the NP10 membrane permeability (up to 25%) was obtained after ozonation. ATR-FTIR, zeta potential and SEM revealed a fairly good resistance of the polyethersulfone (PES) matrix to ozonation (thanks to the protective effect of electron-withdrawing sulfone groups) under the exposure conditions of this study but the polyvinylpyrrolidone (PVP) additive was substantially oxidized. XPS indicated that the degraded PVP was not released from the PES matrix. It was suggested that the decrease in the NP10 membrane permeability might result from a cross-linking process between macroradicals of degraded PVP chains. In contrast to what was observed with the NP10 membrane, the pure water permeability of the thin-film composite polyamide (PA) membranes dramatically increased after ozonation. The fully aromatic NF90 membrane appeared to be even more sensitive to ozone than the semi aromatic NF270. The different resistances of NF90 and NF270 membranes were attributed to the different amine monomers used for the synthesis of their active layer. Indeed, m-phenylenediamine used in interfacial polymerization of the NF90 active layer is an aromatic amine (aromatic rings are sensitive to ozonation) and is less basic than the non-aromatic piperazine used to develop the NF270 membrane (protonation of amines contributes to protect them from electrophilic attacks). For both PA membranes, ATR-FTIR and SEM indicated severely damaged active layers. The very sharp increase in the NF90 and NF270 permeabilities was attributed to the removal of active layer fragments, which was found compatible with both zeta potential and water contact angle measurements.