Membrane Density Key to Water Desalination

By Iva Fedorka

Worldwide, agriculture, livestock, and energy applications consume the majority of fresh water. As sources of drinking water are compromised or harder to find, desalinating sea water can help produce a potable supply when other water treatment options are too expensive or endanger the environment.

Researchers have used a combination of electron microscopy techniques and simulations to discover that uniform density is critical to the performance of reverse-osmosis membranes.

Desalination

Desalination is the process of removing excess salts and minerals from water. Most commonly, desalination is achieved via thermal distillation or membrane distillation. In the United States, the latter is preferred.

Reverse osmosis (RO) comprises about two-thirds of all membrane distillation. The method uses a pressure gradient to force feed water through membranes that filter out salt ions.

RO membranes continue to evolve, and highly permeable desalination membranes have been manufactured by controlling internal morphology, thickness, and feed surface area of the active aromatic polyamide (PA) layer. However, how the morphology was linked to performance was unknown.

New Findings

Professors Baskar Ganapathysubramanian, Enrique Gomez, Manish Kumar, and other colleagues from Iowa State and Penn State Universities and the University of Texas at Austin used assessments of existing desalination membranes and 3D models to predict water flow. They conducted a complicated and detailed comparative analysis to understand 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 Ganapathysubramanian of Iowa State University.

“We’re showing how water concentration changes across the membrane,” 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 key to better desalination membranes is control during manufacturing of the membrane densities. If manufacturing engineers and materials scientists can create a uniform density throughout the membrane, water flow will increase without reducing the amount of salt removed.

“These simulations provided a lot of information for figuring out the key to making desalination membranes much more effective,” Ganapathysubramanian said.

This new research methodology quantifies membrane structure and property relationships and expands our understanding of water diffusion mechanisms and transport rate predictions. The same methods can be used for other molecular separation and polymeric systems. Improved designs can be applied not only to desalination but also to gas and hydrocarbon separations, carbon capture, and blue energy production.

Their work appears in the January 1, 2021 issue of the journal Science.