Desalination breakthrough could lead to cheaper water filtration

UNIVERSITY OF TEXAS AT AUSTIN

 NEWS RELEASE 31-DEC-2020

Research News

Producing clean water at a lower cost could be on the horizon after researchers from The University of Texas at Austin and Penn State solved a complex problem that has baffled scientists for decades, until now.

Desalination membranes remove salt and other chemicals from water, a process critical to the health of society, cleaning billions of gallons of water for agriculture, energy production and drinking. The idea seems simple -- push salty water through and clean water comes out the other side -- but it contains complex intricacies that scientists are still trying to understand.

The research team, in partnership with DuPont Water Solutions, solved an important aspect of this mystery, opening the door to reduce costs of clean water production. The researchers determined desalination membranes are inconsistent in density and mass distribution, which can hold back their performance. Uniform density at the nanoscale is the key to increasing how much clean water these membranes can create.

"Reverse osmosis membranes are widely used for cleaning water, but there's still a lot we don't know about them," said Manish Kumar, an associate professor in the Department of Civil, Architectural and Environmental Engineering at UT Austin, who co-led the research. "We couldn't really say how water moves through them, so all the improvements over the past 40 years have essentially been done in the dark."

CAPTION

Paper co-author Kaitlin Brickey, a Penn State graduate student in chemical engineering, stands in front of the scanning electron microscope that allowed researchers to examine how dense pockets in membranes could hinder efficient water filtration efforts.

CREDIT

Tyler Henderson/Penn State

The findings were published today in Science.

The paper documents an increase in efficiency in the membranes tested by 30%-40%, meaning they can clean more water while using significantly less energy. That could lead to increased access to clean water and lower water bills for individual homes and large users alike.

Reverse osmosis membranes work by applying pressure to the salty feed solution on one side. The minerals stay there while the water passes through. Although more efficient than non-membrane desalination processes, it still takes a large amount of energy, the researchers said, and improving the efficiency of the membranes could reduce that burden.

"Fresh water management is becoming a crucial challenge throughout the world," said Enrique Gomez, a professor of chemical engineering at Penn State who co-led the research. "Shortages, droughts -- with increasing severe weather patterns, it is expected this problem will become even more significant. It's critically important to have clean water availability, especially in low-resource areas."

The National Science Foundation and DuPont, which makes numerous desalination products, funded the research. The seeds were planted when DuPont researchers found that thicker membranes were actually proving to be more permeable. This came as a surprise because the conventional knowledge was that thickness reduces how much water could flow through the membranes.

The team connected with Dow Water Solutions, which is now a part of DuPont, in 2015 at a "water summit" Kumar organized, and they were eager to solve this mystery. The research team, which also includes researchers from Iowa State University, developed 3D reconstructions of the nanoscale membrane structure using state-of-the-art electron microscopes at the Materials Characterization Lab of Penn State. They modeled the path water takes through these membranes to predict how efficiently water could be cleaned based on structure. Greg Foss of the Texas Advanced Computing Center helped visualize these simulations, and most of the calculations were performed on Stampede2, TACC's supercomputer.

CAPTION

The density of filtration membranes, even at the atomic scale, can greatly affect how much clean water can be produced.

CREDIT

Enrique Gomez/Penn State

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Producing clean water at a lower cost could be on the horizon after researchers from The University of Texas at Austin and Penn State solved a complex problem that has baffled scientists for decades, until now. Desalination membranes remove salt and other chemicals from water, a process critical to the health of society, cleaning billions of gallons of water for agriculture, energy production and drinking. The idea seems simple -- push salty water through and clean water comes out the other side -- but it contains complex intricacies that scientists are still trying to understand. The research team, in partnership with DuPont Water Solutions, solved an important aspect of this mystery, opening the door to reduce costs of clean water production. The researchers determined desalination membranes are inconsistent in density and mass distribution, which can hold back their performance. Uniform density at the nanoscale is the key to increasing how much clean water these membranes can create.

Researchers measure, model desalination membranes to maximize flow, clean more water

IOWA STATE UNIVERSITY

NEWS RELEASE 31-DEC-2020

Research News

 

IMAGE: THIS 3D MODEL OF A POLYMER DESALINATION MEMBRANE SHOWS WATER FLOW -- THE SILVER CHANNELS, MOVING FROM TOP TO BOTTOM -- AVOIDING DENSE SPOTS IN THE MEMBRANE AND SLOWING FLOW. view more 

CREDIT: IMAGE BY THE GANAPATHYSUBRAMANIAN RESEARCH GROUP/IOWA STATE UNIVERSITY AND GREGORY FOSS/TEXAS ADVANCED COMPUTING CENTER.

AMES, Iowa - Nature has figured out how to make great membranes.

Biological membranes let the right stuff into cells while keeping the wrong stuff out. And, as researchers noted in a paper just published by the journal Science, they are remarkable and ideal for their job.

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

Can we learn from those high-performing biological membranes? Can we apply nature's homogenous design strategies to manufactured, polymer membranes? Can we quantify what makes some of those industrial membranes perform better than others?

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

Iowa State's Baskar Ganapathysubramanian, the Joseph C. and Elizabeth A. Anderlik Professor in Engineering from the department 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 down to the nanoscale of billionths of a meter 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 Jan. 1 print edition.

Working with Penn State's transmission electron microscope measurements of four different polymer membranes used for water desalination, the Iowa State engineers 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," Ganapathysubramanian said. "The secret ingredient is less inhomogeneity."

Just take a look at the Science cover image the Iowa State researchers created with assistance from the Texas Advanced Computing Center, said Khara: 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.

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

Most telling are the silver lines showing water moving around dense spots in the membrane.

"We're showing how water concentration changes across the membrane." Ganapathysubramanian said of the models which required high-performance computing to solve. "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."

Khara added, "The simulations themselves posed computtional challenges, as the diffusivity within an inhomogeneous membrane can differ by six orders of magnitude"

So, the paper concludes, 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.

It's one more example of the computational work from Ganapathysubramanian's lab helping to solve a very fundamental yet practical problem.

"These simulations provided a lot of information for figuring out the key to making desalination membranes much more effective," said Ganapathysubramanian, whose work on the project was partly supported by two grants from the National Science Foundation.

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The research team

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.

Controlling the nanoscale structure of membranes is key for clean water, researchers find

PENN STATE

Research News

 

UNIVERSITY PARK, Pa. -- A desalination membrane acts as a filter for salty water: push the water through the membrane, get clean water suitable for agriculture, energy production and even drinking. The process seems simple enough, but it contains complex intricacies that have baffled scientists for decades -- until now.

Researchers from Penn State, The University of Texas at Austin, Iowa State University, Dow Chemical Company and DuPont Water Solutions published a key finding in understanding how membranes actually filter minerals from water, online today (Dec. 31) in Science. The article will be featured on the print edition's cover, to be issued tomorrow (Jan. 1).

"Despite their use for many years, there is much we don't know about how water filtration membranes work," said Enrique Gomez, professor of chemical engineering and materials science and engineering at Penn State, who led the research. "We found that how you control the density distribution of the membrane itself at the nanoscale is really important for water-production performance."

Co-led by Manish Kumar, associate professor in the Department of Civil, Architectural and Environmental Engineering at UT Austin, the team used multimodal electron microscopy, which combines the atomic-scale detailed imaging with techniques that reveal chemical composition, to determine that desalination membranes are inconsistent in density and mass. The researchers mapped the density variations in polymer film in three dimensions with a spatial resolution of approximately one nanometer -- that's less than half the diameter of a DNA strand. According to Gomez, this technological advancement was key in understanding the role of density in membranes.

"You can see how some places are more or less dense in a coffee filter just by your eye," Gomez said. "In filtration membranes, it looks even, but it's not at the nanoscale, and how you control that mass distribution is really important for water-filtration performance."

This was a surprise, Gomez and Kumar said, as it was previously thought that the thicker the membrane, the less water production. Filmtec, now a part of DuPont Water Solutions, which makes numerous desalination products, partnered with the researchers and funded the project because their in-house scientists found that thicker membranes were actually proving to be more permeable.

The researchers found that the thickness does not matter as much as avoiding highly dense nanoscale regions, or "dead zones." In a sense, a more consistent density throughout the membrane is more important than thickness for maximizing water production, according to Gomez.

This understanding could increase membrane efficiency by 30% to 40%, according to the researchers, resulting in more water filtered with less energy -- a potential cost-saving update to current desalination processes.

"Reverse osmosis membranes are so widely used for cleaning water, but there's still a lot we don't know about them," Kumar said. "We couldn't really say how water moves through them, so all the improvements over the last 40 years have essentially been done in the dark."

Reverse osmosis membranes work by applying pressure on one side. The minerals stay there, while the water passes through. While more efficient than non-membrane desalination processes, this still takes an immense amount of energy, the researchers said, but improving the efficiency of the membranes could reduce that burden.

"Freshwater management is becoming a crucial challenge throughout the world," Gomez said. "Shortages, droughts -- with increasing severe weather patterns, it is expected this problem will become even more significant. It's critically important to have clean water available, especially in low resource areas."

The team continues to study the structure of the membranes, as well as the chemical reactions involved in the desalination process. They are also examining how to develop the best membranes for specific materials, such as sustainable yet tough membranes that can prevent the formation of bacterial growth.

"We're continuing to push our techniques with more high-performance materials with the goal of elucidating the crucial factors of efficient filtration," Gomez said.

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Other contributors include first author Tyler E. Culp, Kaitlyn P. Brickey, Michael Geitner and Andrew Zydney, all of whom are affiliated with the Penn State Department of Chemical Engineering; Biswajit Khara and Baskar Ganapathysubramanian, both with the Department of Mechanical Engineering at Iowa State University; Tawanda J. Zimudzi of the Materials Research Institute (MRI) at Penn State; Jeffrey D. Wilbur and Steve Jons, both with DuPont Water Solutions; and Abhishek Roy and Mou Paul, both with Dow Chemical Company. Gomez is also affiliated with MRI. The microscopic work was conducted on electron microscopes in the Materials Characterization Lab in MRI. DuPont and the National Science Foundation funded the research.