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.

Green Solvent Cleaning Removes Irrecoverable Foulants from End-of-Life Membranes in Membrane Bioreactors: Efficacy and Mechanisms

“…Moreover, the EOL membranes have a higher water contact angle than the new membrane (Figure 2e), possibly attributing to the combined effect of hydrophobic foulant accumulation and the hydrophobic polymer matrix induced by long-term periodic cleaning−fouling cycles. 34,35 Efficacy of conventional cleaning and green solvent cleaning on the EOL PVDF membranes was evaluated. MDMO was selected as the representative green solvent for cleaning the EOL membranes because of its better cleaning efficacy over DMI and Cyrene (Section S7).…”

“…The F 1s content increased, and the O 1s content decreased in the GSC membrane compared to the EOL membrane (Figure 2b). The O 1s content can somehow reflect the content of the oxygencontained foulant on the surface of the membrane, 34 and thereby, a lower O/F here represents a better cleaning efficiency. 36 The porosity profile also shows a consistent tendency validating the efficacy of GSC over CC cleaning (Figure 2c).…”

“…It was reported that long-term exposure to a cleaning agent could result in the enlarged membrane pores of PVDF membrane. 34 Therefore, the higher permeance of the GSC membrane should be ascribed to the exposure of enlarged membrane pores after the foulant removal by green solvent cleaning. Simultaneously, the rejection of sodium alginate as the model pollutant by the GSC membrane was not compromised compared with that of the new membrane (Figure 2d).…”

“…Currently, to the best of our knowledge, information on the detailed composition of the irrecoverable foulant is almost missing in the literature. The FTIR spectra show that in comparison with the pristine solvent, the used solvent presented new emerging peaks at 1505 and 1649 cm −1 attributed to the aromatic ring stretching vibrations 39 and amino group, 34 as well as 1727 cm −1 attributed to oxygen-containing groups 40 (Figure 3a). The peaks at 1236 and 825 cm −1 suggested the presence of carboxyl groups 41 and −CH near the double bond, 42 respectively.…”

Rapid Upcycling of End-of-Life Microfiltration Membrane Mediated by the Healing of Metal–Organic Complex

“…The downcycling process has been thoroughly investigated, and strategies using sodium hypochlorite (NaOCl) exposure , and polyelectrolyte layer-by-layer deposition have been developed and some of these strategies have been pilot-tested. , Conversely, the upcycling process, for example, recycling EOL MF membranes as NF or RO membranes, remains in its infancy. Recently, a cleaning–healing–interfacial polymerization strategy using polydopamine as the healing agent was reported for upcycling EOL poly­(vinylidene fluoride) (PVDF) MF membranes to obtain new NF membranes . A cleaning step was responsible for removing foulants from EOL MF membranes, and a healing step was employed to heal the deteriorated structure of the membrane surface and reverse the hydrophobic nature of the surface induced by long-term cleaning cycles during service.…”