Detection of desalination maximizes the flow for cheaper water filtration

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

Nature has learned how to make great membranes.

Biological membranes leave the right things in the cells, while keeping the wrong things. And, as the researchers noted in a paper just published by the journal Science, are remarkable and ideal for their profession.

But they are not necessarily ideal for high-volume industrial jobs, such as pushing salt water through a membrane to remove salt and make fresh water for drinking, irrigating crops, watering animals or creating energy.

Can we learn from those high-performance biological membranes? Can we apply homogeneous design strategies of the nature of manufactured polymeric membranes? Can we quantify what makes some of these industrial membranes work better than others?

Researchers at Iowa State University, Penn State University, Texas University of Austin, DuPont Water Solutions and Dow Chemical Co. – led by Enrique Gomez of Penn State and Manish Kumar of Texas – used transmission electron microscopy and 3D computer modeling to look for answers.

Baskar Ganapathysubramanian of Iowa, Joseph C. and Elizabeth A. Anderlik, a professor of mechanical engineering, and Biswajit Khara, a doctoral student in mechanical engineering, contributed their expertise in applied mathematics, high-performance computing, and 3D modeling to the project.

The researchers found that creating a uniform membrane density up to the nanoscale of billions of meters is crucial for maximizing the performance of reverse osmosis water filtration membranes. Their discovery has just been published online by the journal Science and will be the cover paper of the January 1, 2021 print edition.

Working with measurements at Penn State’s electron transmission microscope of four different polymer membranes used to desalinate water, Iowa engineers predicted water flow through 3D models of membranes, allowing detailed comparative analyzes of why some membranes had results. better than others.

“The simulations managed to eliminate the fact that membranes that are more uniform – that do not have ‘hot spots’ – have a uniform flow and better performance,” said Ganapathysubramanian. “The secret ingredient is less inhomogeneity.”

Take a look at Science the cover image, Iowa researchers created with the help of the Texas Center for Advanced Computing, said Khara: Red above the membrane shows water under higher pressure and higher salt concentrations; the golden, granular structure, similar to a sponge in the middle, has denser and less dense areas in the membrane that stops the salt; the silver channels show how the water flows; and the blue at the bottom has water under lower pressure and lower salt concentrations.

“You can see huge amounts of variation in the flow characteristics in 3D membranes,” Khara said.

The most telling are the silver lines that show the water moving around dense spots on the membrane.

“We show how the concentration of water on the membrane changes.” Ganapathysubramanian said about the models that needed high-performance calculations to solve them. “This is nice. It hasn’t been done so far, because such detailed 3D measurements were not available, and also because such simulations are not trivial.”

Khara added: “The simulations themselves were computational challenges, because the diffusivity in an inhomogeneous membrane can differ by six orders of magnitude.”

So, the conclusion of the paper, the key to a better desalination membrane is to find how to measure and control at very small scales the densities of manufactured membranes. Manufacturing engineers and materials scientists must make the density uniform across the membrane, thus promoting the flow of water without sacrificing salt removal.

It is another example of computational work in Ganapathysubramanian’s laboratory that helps solve a very fundamental but practical problem.

“These simulations provided a lot of information to find out the key to making desalination membranes more efficient,” said Ganapathysubramanian, whose work on the project was partially supported by two grants from the National Science Foundation.

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

The project was led by Enrique Gomez, a professor of chemical engineering and materials science and engineering at Penn State University, and Manish Kumar, an associate professor of civil, architectural and environmental engineering at the University of Texas at Austin.

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 from Dow Chemical Co.: Abhishek Roy, Mou Paul.