Water. desalination + reuse

water d+r December 2018

Water. Desalination + reuse

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December 2018 Water.desalination+reuse Far Site 29 Penn State models inside membranes with microscopy A team of researchers at Penn State Uni- versity has used the latest in microscopy to model the internal structure of a desali- nation membrane, seeking to improve the technology. The team led by professor of chemical engineering Enrique Gomez investigated the internal structure of polyamide • lm using high-angle annular dark • eld scan- ning transmission electron microscopy (HAADF-STEM) tomography. The image intensity of HAADF-STEM is directly pro- portional to the density of the material, enabling it to be mapped to nanoscale resolution. "We found that the density of the polyamide layer varies throughout the • lm. This potentially changes the way engineers think about how water moves through the material, because resist- ance to † ow is not homogeneous, and is highest at the membrane surface," Gomez says. HAADF-STEM enabled the researchers to construct a three- dimensional model of the membrane's internal structure, and to analyse the structural components, determining which charac- teristics might be manipulated to improve membrane life, reduce resistance to foul- ing, and enhance recovery. "Knowing what the mate- rial looks like on the inside, and understanding how this microstructure aŠ ects water transport properties, is crucial to designing next- generation membranes with longer operational lifetime that can func- tion under a diverse set of conditions," he adds. Void theory disproven The research further shows that the • ne structure of the membrane contained fewer enclosed voids than had been suspected. "Lo- cal variations in porosity, density, and surface area will lead to heterogeneity in † ux within membranes, such that connecting chemistry, microstructure, and performance of the membranes — for reverse osmosis, ultra• ltration, virus and protein • ltration, and gas separa- tions — will require three-dimensional reconstructions from techniques such as electron tomogra- phy," says Gomez. The next step is to push the resolution of the technique to below 1 nanometer in resolution. "We don't know if sub-nanometer pores exist in these materials and we want to push our techniques to see whether these channels exist. We want to map how † ow moves through these materials to connect how the microstruc- ture aŠ ects water † ow, marking or staining the membrane with special compounds that can † ow through the membranes and be visualised in the electron microscope," he says. The other Penn State researchers working on the project are graduate students in chemical engineer- ing Tyler Culp, Yue-xiao Shen, and Michael Geitener; and Manish Kumar, associate pro- fessor of chemical engineer- ing, who co-led the work. The Dow Chemical Company and National Science Foundation sponsored the work, which was a collaboration between Penn State and Dow Chemical Company. Up close: The electron tomography of isolated polyamide fi lm, shown here, could change the way engineers think about the movement of water through a membrane. We don't know if sub- nanometer pores exist in these materials and we want to push our techniques to see whether these channels exist. Enrique Gomez, professor of chemical engineering, Penn State

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