Controlling the Nanoscale Structure of Desalination Membranes is essential to obtain clean water

This 3D model of the polymer desalination membrane shows the flow of water – the silver channels, moving from top to bottom – avoiding dense stains on the membrane and slowing down the flow. Credit: Image of the Ganapathysubramanian Research Group / Iowa State University and Gregory Foss / Texas Advanced Computing Center

A desalination membrane acts as a filter for salt water: water enters through the membrane, obtaining clean water that is also suitable for agriculture, energy production and drinking. The process may seem simple enough, but it has so many complex difficulties that scientists have confused over the decades so far.

Researchers at Penn State, the University of Texas at Austin, Iowa State University, the Dow Chemical Company and DuPont Water Solutions have released a key finding today (December 31) to find out how membranes filter minerals from water. Science. The article will appear on the cover of the printed edition, which will be published tomorrow (January 1).

“Although they have been around for many years, we don’t know much about how water-filtering membranes work,” said Enrique Gomez, a professor of chemical and materials science and engineering at Penn State, who led the research. “We found it very important to control how you control the nanoscale of the membrane to achieve water production performance.”

Led by Manish Kumar UT, associate professor in the Austin Department of Science, Architecture and Environmental Engineering, the team used multimodal electron microscopy, combining accurate atomic-scale imaging with techniques that reveal chemical composition to determine desalination membranes to match density and mass. Researchers have mapped the density variations of a polymer film in three dimensions with a spatial resolution of one nanometer – less than half the diameter DNA thread. According to Gomez, this technological advancement was key to understanding the role of density in membranes.

“You can see with your own eyes that some of the places in the coffee filter are more or less dense,” Gomez said. “It looks paired in the filtration membranes, but it’s not on the nanoscale, and how you control that mass distribution is very important for water filtration performance.”

Gomez and Kumar said it was a surprise because it was previously thought that the thicker the membrane, the less water it produced. Filmtec, now part of DuPont Water Solutions, which makes a wide range of desalination products, collaborated with researchers and funded the project because local scientists were proving that thicker membranes were more permeable.

Researchers have found that thickness is not as important on a large scale as avoiding nano-regions or “dead zones”. In a sense, a more consistent density across the membrane is more important than thickness to maximize water production, according to Gomez.

According to the researchers, this understanding could increase the efficiency of the membrane by 30% to 40%, resulting in more water filtered with less energy – possibly a potential upgrade to save on the costs of current desalination processes.

“Reverse osmosis membranes are widely used to clean water, but we still don’t know much about them,” Kumar said. “We couldn’t tell how the water moves through them, so all the improvements that have been made over the last 40 years have been made in the dark.”

Reverse osmosis membranes work by applying pressure on one side. The minerals remain there as the water passes through them. Although they are more efficient than non-membrane desalination processes, researchers still say a huge amount of energy is needed, but improving the efficiency of membranes can reduce that burden.

“Freshwater management is becoming a crucial challenge worldwide,” Gomez said. “Deficiencies, droughts – as severe weather patterns are on the rise, this problem is expected to be even more pronounced. It is very important to have clean water available, especially in low-resource areas.”

The group continues to study the structure of membranes, as well as the chemical reactions caused by the desalination process. They are studying how to develop the best membranes for specific materials, such as durable but hard membranes that can prevent bacterial growth.

“We continue to push our techniques with more high-performance materials with the goal of shedding light on the crucial factors of efficient filtration,” Gomez said.

More information about this research:

References: Tyler E. Culp, Biswajit Khara, Kaitlyn P. Brickey, Michael Geitner, Tawanda J. Zimudzi, Jeffrey D. Wilbur, Steven D. Jons, Abhishek Roy Mou Paul, Baskar Ganapathysubramanian, Andrew L. Zydney, Manish Kumar and Enrique D. Gomez, 31 December 2020, Science.
DOI: 10.1126 / science.abb8518

Other contributors include first author Tyler E. Culp, Kaitlyn P. Brickey, Michael Geitner, and Andrew Zydney, all affiliated with the Penn State Department of Chemical Engineering; Biswajit Khara and Baskar Ganapathysubramanian, both with a degree in Mechanical Engineering from Iowa State University; Tawanda J. Zimudzi of the Penn State Material Research Institute (MRI); Jeffrey D. Wilbur and Steve Jons, both with DuPont Water Solutions; and Abhish Roy and Mou Paul, both with the Dow Chemical Company. Gomez is also linked to MRI. The microscopic work was performed on electron microscopes in the MRI material characterization laboratory. The research was funded by DuPont and the National Science Foundation.

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