Controlling the nanoscale structure of membranes is key to clean water, researchers say

A desalination membrane acts as a filter for salt water: push the water through the membrane, get clean water suitable for agriculture, energy production and even drinking water. The process seems simple enough, but it contains complex intricacies that have puzzled 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 today (Dec. 31) an important finding about how membranes actually filter minerals from water. Science. The article will be featured on the cover of the print edition, due out tomorrow (January 1).

“Despite their use for many years, there is much we do not know about how water filtration membranes work,” said Enrique Gomez, a professor of chemical technology and materials science and engineering at Penn State who led the study. “We found that the way you control the density distribution of the membrane itself at the nanoscale is very important for the performance of water production.”

Co-led by Manish Kumar, associate professor in the Department of Civil, Structural and Environmental Engineering at UT Austin, the team used multimodal electron microscopy, which combines the detailed atomic-scale 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 about one nanometer – that’s less than half the diameter of a DNA strand. According to Gomez, these technological advancements were key to understanding the role of density in membranes.

“You can see by your eye how some places are more or less dense in a coffee filter,” Gomez said. “It looks the same in filtration membranes, but it’s not at the nanoscale, and how you control that mass distribution is really important to 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 part of DuPont Water Solutions, which makes numerous desalination products, partnered with the researchers and funded the project because their in-house scientists discovered that thicker membranes actually turned out to be more permeable.

The researchers found that thickness doesn’t matter as much as avoiding very dense nanoscale regions or “dead zones.” In a way, more consistent density throughout the membrane is more important than thickness for maximizing water production, Gomez said.

This insight, the researchers said, could increase membrane efficiency by 30% to 40%, which would result in more filtered water with less energy – a possible cost-effective update to current desalination processes.

“Reverse osmosis membranes are so often used for cleaning water, but there is still a lot we don’t know about them,” said Kumar. “We couldn’t really tell how water moves through it, so all the improvements over the past 40 years have been essentially done in the dark.”

Reverse osmosis membranes work by applying pressure to one side. The minerals stay there while the water flows through them. While this is more efficient than desalination processes without membranes, it still takes a tremendous amount of energy, the researchers said, but improving the efficiency of the membranes could reduce that load.

“Freshwater management is becoming a critical challenge around the world,” said Gomez. “Shortages, droughts – with increasing severe weather patterns, this problem is expected to get worse. Having clean water available is critical, 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 investigating how to develop the best membranes for specific materials, such as durable but tough membranes that can prevent bacterial growth from forming.

“We continue to push our techniques with more high-performance materials with the aim of clarifying the critical factors of efficient filtration,” said Gomez.