Alkaline pre-treatment enables controllable downcycling of Si-Al fouled end-of-life RO membrane to NF and UF membranes

Environmental and economic concerns necessitate the recycle of end-of-life (EOL) reverse osmosis (RO) membranes. Controlled downcycling of EOL RO membrane to nanofiltration (NF)-like or ultrafiltration (UF)-like membrane through regulating sodium hypochlorite (NaClO) exposure doses has been developed in recent years. However, the impact of foulants on the chlorination-based membrane downcycling is often overlooked. An alkaline-NaClO treatment strategy was employed to downcycle Si-Al complex-fouled EOL RO membrane. Alkaline cleaning not only partially removed Si-Al complex foulants via dissolution, but also converted Al 2 O 3 ·SiO 2 to Al-O-Si amorphous oxides . Subsequent NaClO treatment gradually deconstructed the Si-Al complex fouling layer by dissolving Al contained in Al-O-Si amorphous oxides and degraded the polyamide (PA) layer with extended exposure time, resulting in membrane with loose NF-like characteristic. Moreover, the conversion of Al-contained component with alkaline pre-treatment prevented Al 3+ -catalyzed degradation of PA layer and ensure controlled downcycling. For the downcycling to UF-like membrane, complete removal of PA layer with sufficient chlorine oxidation was essential while alkaline pre-treatment process was optional. Both downcycled loose NF-like and UF-like membranes exhibited satisfactory filtration and antifouling performance. Our study presents an effective and robust protocol for controllable downcycling of EOL RO membrane with alkaline pre-treatment based on their fouling characteristic.