Researchers from Penn State observed the inner structure of reverse osmosis membranes using electron tomography and quantitative analysis of 3D models
Reverse osmosis membranes are widely used for salt water desalination, wastewater recycling, and residential use. These membranes are layers of material with an active aromatic polyamide layer that allows water molecules through while screening out most of the salt. The researchers observed the internal structure of the polyamide film using High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) tomography. The image intensity of HAADF-STEM is directly proportional to the density of the material that facilitates mapping of the material to nanoscale resolution.
The researchers constructed 3D models of the membrane’s internal structure using HAADF-STEM. The models assist to analyze the structural components and determine the characteristics required for the membrane to function. Moreover, the model manipulates those characteristics that could be manipulated to improve membrane longevity, antifouling, and enhance water recovery. The HAADF-STEM revealed another characteristic of the absence of previously reported enclosed voids. As opposed to the team’s notion, the 3D models showed that there are few closed voids in the novel material. Local variations in properties of liquids such as porosity, density, and surface area lead to heterogeneity in flux within membranes. Therefore, 3D reconstructions are required from techniques such as electron tomography to connect chemistry, microstructure and performance of membranes for reverse osmosis, ultrafiltration, virus and protein filtration, and gas separations.
“We don’t know if sub nanometer pores exist in these materials and we want to be able to push our techniques to see whether these channels exist,” said Enrique Gomez, professor of chemical engineering, Penn State. The researchers aim to map the flow through active aromatic polyamide layers to directly determine the effect of microstructure on water flow. This can be achieved by marking or staining the membrane with special compounds that can flow through the membrane and be visualized in the electron microscope. The research was published in the journal Proceedings of the National Academy of Sciences (PNAS) on August 28, 2018.