Biological membranes can achieve remarkably high permeabilities while maintaining ideal selectivities by relying on homogeneous internal structures in the form of membrane proteins. In new research, a team of scientists led from Penn State University and the University of Texas at Austin applied such design strategies to desalination of polyamide membranes.
This 3D model of a polymer desalination membrane shows that water avoids dense spots in the membrane and slows the flow; red above the membrane shows water under higher pressure and with higher salt concentrations; the golden, grainy, spongy structure in the center shows denser and less dense areas within the salt stop membrane; silver channels show how water flows through them; and the blue at the bottom shows water under lower pressure and with lower salt concentrations. Image credit: Ganapathysubramanian Research Group / Iowa State University / Gregory Foss, Texas Advanced Computing Center.
Dr. Enrique Gomez, Dr. Manish Kumar and their colleagues from Iowa State University, Penn State University, the University of Texas at Austin, DuPont Water Solutions, and Dow Chemical Co. found that creating a uniform membrane density down to the nanoscale of billionths of a meter is critical to maximizing the performance of reverse osmosis water filtration membranes.
Using transmission electron microscope measurements of four different polymer membranes used for water desalination, they predicted the water flow through 3D models of the membranes, allowing for detailed comparative analysis of why some membranes performed better than others.
“The simulations showed that membranes that are more uniform – that do not have ‘hot spots’ – have a uniform flow and better performance. The secret ingredient is less inhomogeneity, ”said Professor Baskar Ganapathysubramanian, a researcher at Iowa State University.
“Just look at the image we took with the help of the Texas Advanced Computing Center,” added Biswajit Khara, a doctoral student at Iowa State University.
“We show how the water concentration changes through the membrane,” said Professor Ganapathysubramanian.
“This is great. It has not been done before because such detailed 3D measurements were not available, and also because such simulations are not trivial to perform.”
“The simulations themselves presented a computational challenge, as the diffusivity within an inhomogeneous membrane can vary by six orders of magnitude,” said Khara.
The key to better desalination membranes is figuring out how to measure and control the density of fabricated membranes on a very small scale.
Manufacturing engineers and materials scientists need to make density uniform throughout the membrane, promoting water flow without sacrificing salt removal.
“These simulations provided a lot of information to figure out the key to making desalination membranes much more effective,” said Professor Ganapathysubramanian.
The team’s work appears in the diary Science.
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Tyler E. Culp et al. 2021. Nano-scale control of internal inhomogeneity improves water transport in desalination membranes. Science 371 (6524): 72-75; doi: 10.1126 / science.abb8518