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 by Penn State University and the University of Texas at Austin applied such design strategies to desalination polyamide membranes.
This 3D model of a polymer desalination membrane shows water avoiding dense spots in the membrane and slowing flow; red above the membrane shows water under higher pressure and with higher concentrations of salt; the gold, granular, sponge-like structure in the middle shows denser and less-dense areas within the salt-stopping membrane; silver channels show how water flows through; and the blue at the bottom shows water under lower pressure and with lower concentrations of salt. 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 crucial for 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 water flow through 3D models of the membranes, allowing detailed comparative analysis of why some membranes performed better than others.
“The simulations were able to tease out that membranes that are more uniform — that have no ‘hot spots’ — have uniform flow and better performance. The secret ingredient is less inhomogeneity,” said Professor Baskar Ganapathysubramanian, a researcher at Iowa State University.
“Just take a look at the image we created with assistance from the Texas Advanced Computing Center,” added Biswajit Khara, a doctoral student at Iowa State University.
“We’re showing how water concentration changes across the membrane,” Professor Ganapathysubramanian said.
“This is beautiful. It has not been done before because such detailed 3D measurements were unavailable, and also because such simulations are non-trivial to perform.”
“The simulations themselves posed computtional challenges, as the diffusivity within an inhomogeneous membrane can differ by six orders of magnitude,” Khara said.
The key to better desalination membranes is figuring out how to measure and control at very small scales the densities of manufactured membranes.
Manufacturing engineers and materials scientists need to make the density uniform throughout the membrane, thus promoting water flow without sacrificing salt removal.
“These simulations provided a lot of information for figuring out the key to making desalination membranes much more effective,” Professor Ganapathysubramanian said.
The team’s work appears in the journal Science.
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Tyler E. Culp et al. 2021. Nanoscale control of internal inhomogeneity enhances water transport in desalination membranes. Science 371 (6524): 72-75; doi: 10.1126/science.abb8518
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