Mechanistic insights to the reversibility of membrane wetting in membrane distillation

Membrane wetting is an inevitable challenge in membrane distillation (MD). To overcome this challenge, a few dewetting techniques have been developed, but the reversibility of membrane wetting induced by various contaminants has not been well understood. Herein, we evaluate the reversibility of membrane wetting induced by different types of contaminants and elucidate the underlying mechanisms. From direct contact MD experiments and membrane wettability characterizations, membrane wetting induced by amphiphilic molecules, low-surface-tension liquids, and highly soluble salts can be easily reversed by 20-min DI water rinsing and oven drying, while to reverse membrane wetting induced by sparingly soluble salts, 3-h DI water rinsing and oven drying are required. Based on microscopic imaging and elemental mapping, the membrane wetting processes did not induce severe damage to membrane structure, and thus the hydrophobicity of the membrane can be restored after the contaminants were removed from the membrane. Furthermore, the mechanisms of the removal of different contaminants are elucidated. Specifically, after membrane rinsing and drying, the amphiphilic molecules automatically desorb from the dried membrane due to the disappearance of the underwater hydrophobic-hydrophobic interaction, while the low-surface-tension liquids leave the membrane pores because of evaporation. Additionally, during the membrane rinsing process, the highly soluble salts inside the membrane pores are dissolved, while the sparingly soluble salts detach from the membrane due to the hydrodynamic disturbance. The insights from this study shed lights on the development of effective dewetting techniques for hydrophobic membranes, which benefits not only MD but also membrane contactor applications.