Advances in desalination maximize flow for cheaper water filtration

This 3D model of the polymer desalination membrane shows the flow of water – 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

Nature has figured out how to make excellent membranes.

Biological membranes inject the right things into the cells while leaving out the wrong ones. And, as the researchers point out in a recent article in the journal Science, are outstanding and suitable for their work.

But they are not necessarily suitable for high-volume industrial jobs, such as pushing salt water from a membrane to remove salt and make fresh drinking water, irrigate crops, irrigate livestock, or generate energy.

Can we learn from these high-performance biological membranes? Can we apply nature’s homogeneous design strategies to manufactured polymer membranes? Can we quantify whether some of these membrane membranes work better than others?

Researchers from Iowa State University, Penn State University, Texas at Austin University, DuPont Water Solutions and Dow Chemical Co. – led by Penn State’s Enrique Gomez and Texas’s Manish Kumar – used transmission electron microscopy and computational 3D modeling for the answers.

Baskar Ganapathysubramania of Iowa State, Joseph C. and Elizabeth A. Anderlik, Professor of Engineering in the Department of Mechanical Engineering, and Biswajit Khara, PhD in Mechanical Engineering, Applied Mathematics, High Performance Computing, and 3D Modeling.

Researchers have found that creating a uniform membrane density up to a trillion-meter nanoscale is key to maximizing the performance of reverse osmosis and water-filtering membranes. Their findings have just been published online by the magazine Science and will be the cover paper for the January 1, 2021 print edition.

Working with measurements from the Penn State transmission electron microscope of four different polymer membranes used to desalinate water, Iowa State engineers predicted water flow through 3D membrane models, allowing a detailed comparative analysis of how some membranes worked better than others.

“Simulations have been able to test that membranes that are more uniform (those that do not have‘ hot spots ’) have a uniform flow and better performance,” Ganapathysubramanian said. “The secret component is less homogeneity.”

You just have to look Science cover image created by Iowa State researchers with the help of the Texas Center for Advanced Computing, Khara said: Red on the membrane shows water with higher pressure and higher salt concentrations; in the middle a structure like gold, granular, and sponge shows a dense and not-so-dense area within the membrane to stop the salts; the silver channels show how the water passes through; and the blue bottom shows water at lower pressure and with lower salt concentrations.

“You can see a lot of variation in the internal flow characteristics of 3D membranes,” Kharak said.

The most significant are the silver lines that the water moves around the dense places of the membrane.

“We’re showing how the water concentration changes across the membrane.” Ganapathysubramanian said about the models that required high-performance computing for the solution. “This is beautiful. It has not been done before because accurate 3D measurements like this were not available, and these simulations are not insignificant. “

Khara added: “The simulations themselves posed computational challenges because the diffusivity within a non-homogeneous membrane can differ by six orders of magnitude.”

The paper concludes that the key to better desalination membranes is to know how to measure and control the densities of membranes manufactured on very small scales. Manufacturing engineers and materials scientists need to make a uniform density across the membrane so that they can promote the flow of water without sacrificing salt removal.

Ganapathysubramanian is an example of laboratory computing work that helps solve a very fundamental but practical problem.

“These simulations provided a lot of information on the key to making salting membranes much more effective,” said Ganapathysubramanian, whose project work is supported by two grants from the National Science Foundation.

Reference: 31 December 2020, Science.
DOI: 10.1126 / science.abb8518

The project was attended by Enrique Gomez, a professor of chemical and materials engineering and engineering at Penn State University, and Manish Kumar, an associate professor of Civil, Architectural and Environmental Engineering at the University of Texas.

Also from Iowa State University: Biswajit Khara, Baskar Ganapathysubramanian; From Penn State: Tyler Culp, Kaitlyn Brickey, Michael Geitner, Tawanda Zimudzi, Andrew Zydney; From DuPont Water Solutions: Jeffrey Wilbur, Steve Jons; and Dow Chemical Co.: Abhish Roy, Mou Paul.

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