Cleaning-healing-interfacial polymerization strategy for upcycling real end-of-life polyvinylidene fluoride microfiltration membranes

Abstract

Polyvinylidene fluoride (PVDF) microfiltration (MF) membranes, which are widely applied in the fields of wastewater and water treatment, would inevitably reach their end-of-life (EOL) after numerous fouling–cleaning cycles. The lack of recycling strategy for the EOL PVDF MF membranes impedes the sustainability of membrane technology. In this study, we proposed a cleaning–healing–interfacial polymerization (IP) strategy to upcycle real EOL PVDF MF membranes from a membrane bioreactor for fabricating nanofiltration (NF) membranes. The cleaning step was responsible for removing most organic and inorganic foulants from the membrane, with a following healing step of membranes using polydopamine (PDA). After healing, a continuous and intact polyamide (PA) layer can be formed on the surface of the healed membrane via IP reaction between piperazine (PIP) and trimesoyl chloride. The PA NF membrane upcycled from the healed substrate (NF-healed) membrane had a pure water permeance of 20.2 ± 1.1 L m–2 h–1 bar–1 and a Na2SO4 rejection of 92.4 ± 1.2%. The Na2SO4 permeability of the NF-healed membrane showed an approximately 2 orders of magnitude reduction in contrast to the NF membranes upcycled from the cleaned PVDF MF substrate, highlighting the critical role of PDA healing in conditioning the substrate. Detailed mechanistic investigation reveals that the PDA healing layer can avoid unfavorable growth of the PA layer on the hydrophobic PVDF substrate or cleaned substrates with low PIP uptake. The healing layer constructed a favorable hydrophilic platform for connection between PA oligomers and subsequent continuous growth of the PA layer. This study provides an effective and robust protocol to upcycle EOL low-pressure membranes and reduce their environmental footprint.