Nanofoamed Polyamide Membranes: Mechanisms, Developments, and Environmental Implications

Thin film composite (TFC) polyamide membranes have been widely applied for environmental applications, such as desalination and water reuse. The separation performance of TFC polyamide membranes strongly depends on their nanovoid-containing roughness morphology. These nanovoids not only influence the effective filtration area of the polyamide film but also regulate the water transport pathways through the film. Although there have been ongoing debates on the formation mechanisms of nanovoids, a nanofoaming theory─stipulating the shaping of polyamide roughness morphology by nanobubbles of degassed CO₂ and the vapor of volatile solvents─has gained much attention in recent years. In this review, we provide a comprehensive summary of the nanofoaming mechanism, including related fundamental principles and strategies to tailor nanovoid formation for improved membrane separation performance. The effects of nanovoids on the fouling behaviors of TFC membranes are also discussed. In addition, numerical models on the role of nanovoids in regulating the water transport pathways toward improved water permeance and antifouling ability are highlighted. The comprehensive summary on the nanofoaming mechanism in this review provides insightful guidelines for the future design and optimization of TFC polyamide membranes toward various environmental applications.