Preparation of pH-responsive PVDF membrane and investigation of the synergistic anti-fouling and self-cleaning mechanism via “hydration layer & steric hindrance” and “acid washing-alkaline washing”

Abstract

In this study, the polyvinylidene fluoride (PVDF) membrane with hydrophilicity and pH-responsiveness was prepared by grafting chitosan (CS). The pH-responsiveness refers to a property of the membrane to undergo controlled and reversible changes in its structure, physical, and chemical properties in response to variations in the environmental pH. For comparison, polyethyleneglycol 400 (PEG400), which is hydrophilic but not pH-responsive, was introduced onto the membrane surface as a control. The differences in parameters were compared among the modified membranes, including surface charge, hydration layer properties and anti-fouling performance against calcium sulfate (CaSO₄). The effect of the conformation change in CS on foulant removal was investigated in the pH range of 2–12. The results showed that the thickest hydration layer (205.43 nm) was formed on the surface of G3, which effectively inhibits the adhesion of foulants to the membrane surface. In addition, G3 exhibited superior anti-fouling and self-cleaning performance during the “acid washing (pH = 2)-alkaline washing (pH = 12)” process, as evidenced by the flux recovery rate of 98.95 %, the total flux decline rate of 18.74 % and the irreversible fouling rate of only 1.05 %. Based on differences in CaSO₄ agglomeration behavior and removal efficiency among the membranes, the synergistic mechanism of anti-fouling and self-cleaning based on “hydration layer & steric hindrance” and “acid washing-alkaline washing” was proposed. This study laid the foundation for the preparation of membranes with high anti-fouling performance using CaSO₄ as a model foulant. The proposal of the synergistic mechanism can provide new insights into the efficient removal of foulants.