(573b) Electron Tomography Reveals Details of the Internal Microstructure of Desalination Membranes

As water availability becomes a growing challenge in various regions throughout the world, desalination and wastewater reclamation through technologies such as reverse osmosis (RO) are becoming more important. Nevertheless, many open questions remain regarding the internal structure of thin-film composite RO membranes. In this work, fully aromatic polyamide films that serve as the active layer of state of the art water filtration modules were investigated using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) tomography. Reconstructions of the 3D morphology reveal intricate aspects of the complex microstructure not visible from 2D projections. We find that the polyamide top surface areas, believed to be a major contributor to the overall flux of RO membranes, normalized to projected areas are 3.48 ± 0.15 and 3.07 ± 0.04 for the seawater and brackish water desalination membranes, respectively, which are more than double the values measured by techniques such as atomic force microscopy (AFM). This is because of the limited accessibility of the AFM tip to the overall highly heterogonous and three-dimensional polyamide top surface. Furthermore, the internal voids of the active layer of compressed commercial membranes account for less than 0.2% of the total polymer volume, contrary to previously reported values that are two orders of magnitude higher. Thus, 3D reconstructions and quantitative analyses will be crucial to characterize the complex morphology of polymeric membranes used in next-generation water purification membranes.