Modification to the polyamide TFC RO membranes for improvement of chlorine-resistance

Most current high-performance composite membranes comprise aliphatic or aromatic amines condensed with acyl chlorides or other reactive groups that usually contain substituted amide linkages. Aromatic rings bonded to the N–H group of amide linkages are sensitive to attack by chlorine radicals because of their high electron density. Consequently, the N–H group is converted to an N–Cl group by N-halogenation. This causes the failure of the polyamide reverse osmosis (RO) membrane resulting in decreased salt rejection and increased water flux after exposure to chlorine. In this study, the performance of a silane-coated RO membrane was investigated and its surface analyzed using field emission-scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The stability of the coated membranes in the presence of chorine was also studied in order to ascertain the effect of incorporating different silane compounds with alkyl, aryl and vinyl substituents. In the uncoated membrane, degradation of the polyamide network of the RO membrane by free chlorine in the feed solution resulted in decreased salt rejection and increased water flux after 15,000 ppm h. However, the silane-coated membrane maintained a salt rejection of above 99.0% even after 25,000 ppm h.