Desalination can help provide much-needed freshwater to communities lacking access, but the process can be difficult at scale. Now, scientists have discovered one of the reasons behind that difficulty.
Roughly 80% of the world’s desalination involves the use of reverse osmosis — a water-purification process utilizing a semi-permeable membrane to separate water molecules from other undrinkable substances. Engineers and scientists around the world have leveraged thin-film-composite reverse-osmosis membranes, essential in reverse osmosis, for more than 40 years.
A multi-institutional research team from the UCLA Samueli School of Engineering, the University of Wisconsin-Madison, Yale University and the University of Connecticut has recently published a study investigating the behavior of the membranes as governed by their viscoelastic properties.
Published in the Journal of Membrane Science, the study looked at the compaction (the process by which the polymer membranes increase in density under pressure) and relaxation (the corresponding decrease in density when pressure is relieved) involved in reverse osmosis.
Ying Li
The researchers found that by increasing the crosslinking-degree, or density of polymers in the polyamide layer, the membrane will exhibit decreased compaction and increased relaxation, thereby improving water permeability and salt rejection. Polyamide, known for its use as a versatile material, is a type of synthetic polymer composed of long chains of molecules linked together by specially arranged atoms known as an amide group.
Findings from the research have the potential to alleviate some operational challenges of the desalination industry by highlighting the importance of developing new reverse-osmosis membrane materials with enhanced compaction resistance to improve long-term desalination performance.
Erik Hoek, a professor of civil and environmental engineering at UCLA Samueli, and Ying Li, an associate professor of mechanical engineering at the University of Wisconsin-Madison, are the co-corresponding authors.
“Mechanistic studies like this one can provide practical insights not just about how a reverse-osmosis membrane can be damaged at high applied pressure, but also how new membrane materials might be designed to withstand such harsh operating conditions,” says Hoek, who directs UCLA’s Nanomaterials & Membrane Technology Research Laboratory, which explores membrane formation and application for water treatment.
To examine the polyamide layer’s role in compaction, the team created five reverse-osmosis membranes with varying degrees of crosslinking in the polyamide film. These membranes, all supported by identical base layers, demonstrated that higher crosslinking resulted in lower water permeability, higher salt rejection, less compaction and better recovery of permeability when pressure was reduced.
The study’s primary authors are Jishan Wu — a postdoctoral researcher in Hoek’s lab — and Jinlong He of the University of Wisconsin-Madison. Other authors on the paper are members of Hoek’s lab — postdoctoral researchers Derrick Dlamini and Javier Alan Quezada, graduate students Xinyi Wang and Minhao Xiao, as well as undergraduate students Kay Au, Kevin Guo and Jason Le. They are joined by Hanqing Fan, Kevin Pataroque and Menachem Elimelech from Yale University, as well as Yara Suleiman and Sina Shahbazmohamadi from the University of Connecticut.
The study was funded by the National Alliance for Water Innovation through the U.S. Department of Energy’s Industrial Efficiency and Decarbonization Office, the UCLA Samueli School of Engineering and the UCLA Sustainable LA Grand Challenge.
A version of this story was previously published by the UCLA Samueli School Of Engineering.
Featured image caption: A schematic from a study investigating the compaction and relaxation behavior of composite reverse osmosis polyamide selective layers. Credit: Jishan Wu/UCLA.