Electron Tomography Reveals Details of the Internal Microstructure of Desalination Membranes

Reverse osmosis (RO) membranes are the most widely used technique for desalination and reuse of wastewater, treating tens of billions of gallons of water per day. However, the internal structure of fully-aromatic polyamide films used in RO is largely unknown. In this work, the internal structure of the active layer of RO membranes was investigated using scanning transmission electron microscopy (STEM) tomography. Quantitative analysis of the 3D models of the active layer of compressed commercial membrane samples revealed that internal voids account for less than 0.3% of the total polyamide volume, contrary to previous reports that report values that are two orders of magnitude higher. We also measured that the normalized surface area of the polyamide top surface to be 3.48 and 3.07 for the seawater desalination and brackish water desalination membranes examined, respectively. These surface area values are more than double of what the conventional technique, atomic force microscopy (AFM), reports for such measurements. The reported procedures for obtaining a high-resolution 3D model of aromatic polyamide membranes and the obtained structural information can be used for further studies which could inform development of the next generation of water purification membranes.