Oxidation resistances of polyamide nanofiltration membranes to hydroxyl and sulfate radicals

The oxidation compatibility with polyamide membranes is an important feature of nanofiltration (NF) technology in practical water treatment processes. In this study, we systematically investigated the oxidation resistance of fully aromatic (NF90) and semi-aromatic (NF270) polyamide NF membranes to reactive oxidative species including SO₄^•- and HO^• in terms of membrane surface properties and filtration performance. SO₄^•− and HO^• exhibit different oxidative behaviors in NF membranes with different polyamide structures. The HO^•-dominated oxidation significantly damaged the membrane integrity of NF90 and NF270, with notably varied surface morphology, hydrophilicity, and charge. Further analysis of the chemical property on membrane surfaces revealed that the SO₄^•−-dominated oxidation was more reactive to the N moieties (N–H or N–C) of the NF90 membrane, whereas HO^• unselectively reacted with polyamides. The radicals also impact the permselectivity (water permeability and salt rejection) of the NF90 and NF270 membranes in different ways. The NF90 membrane after SO₄^•−-exposure exhibited higher water permeability and slightly decreased salt rejection, whereas the permselectivity of the NF270 membrane was more significantly affected by HO^•-exposure. To further reveal the underlying oxidation mechanisms, polyamide monomers were selected to react with the peroxodisulfate solutions. The results demonstrated that a fully aromatic polyamide membrane was more resistant to oxidative radicals than a semi-aromatic polyamide membrane. This study will facilitate the development of novel antioxidant membranes and provide information for selecting appropriate membrane materials in specific water treatment processes.