Chemical cleaning reagent of sodium hypochlorite eroding polyvinylidene fluoride ultrafiltration membranes: Aging pathway, performance decay and molecular mechanism

In a practical project of polyvinylidene fluoride (PVDF) ultrafiltration membrane for polymer flooding wastewater treatment, the deterioration of membrane performance was observed after 4 years of operation. The aging effects of sodium hypochlorite (NaClO) on PVDF ultrafiltration membranes were systematically studied, including the membrane properties, filtration performance, and fouling behaviors. During the NaClO aging process, chemical transformations of polyvinylpyrrolidone (PVP) additives dominated the changes in membrane intrinsic characteristics, and the kinetic behavior of PVP decomposition was proposed, which was closely related to the evolution of the membrane pore size. For aged membranes, due to the changes in intrinsic characteristics, the antifouling performance significantly declined: the pseudo-stable flux with feeding solution and flux recovery after physical cleaning decreased by-55% and-62%, respectively, after 20 days of aging. The intermolecular forces in combination with the interfacial free energies were used to evaluate the fouling behaviors of aged membranes. The increased energies indicated the more spontaneous foulant-membrane adhesion and stabilized fouling layers, which deteriorated the irreversible fouling resistance, increased the initial fouling rates and reduced the flux recovery after physical cleaning. Compared to intermolecular forces, foulant-membrane interfacial free energies can serve as the more sensitive and reliable indicators of membrane fouling.