Aging Behavior and Mechanism of Polyvinylidene Fluoride Membrane by Intensified UV Irradiation and NaOCl: A Comparative Study

Membrane technology plays a critical role in solving worldwide water scarcity and security issues. However, membrane aging causes deterioration or complete failure of membrane performance. In this work, polyvinylidene fluoride (PVDF) membrane aging induced by ultraviolet (UV) and NaOCl were investigated and compared. The skin layer was completely destroyed in 2d by UV irradiation under a dry state, which further caused deformation in the sublayer and pore shrinkage. The water layer on the membrane surface protected the skin layer from damage by UV till 8d under the wet state. NaOCl had a moderate influence, the flux was firstly decreased as the hydrophobicity of the membrane increased, and then increased due to pore enlargement. Membrane aging also resulted in a lower virus rejection, which highlighted the biosafety concern in membrane-based wastewater treatment. Infrared and two-dimensional correlation spectroscopy confirmed that the hydrophilic additive was first oxidated in both processes. β-PVDF segment changed before α-PVDF under UV, during which the CC bond was formed by the dehydrofluorination reaction. But the α-PVDF segment was attacked firstly by NaOCl, the radical reaction occurred both at CH2-CF2 and CH=CF segments. This work studied two aging patterns in the membrane lifecycle and demonstrated the aging mechanism, which shed light on membrane development and maintenance in the future.