“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.”
“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.
Finding the path to better desalination
Polyamide membranes have been used in large-scale desalination for decades. However, because of the thinness of the membranes and their internal variability, it has been hard to determine which aspects of the membranes most affect their performance. Culp et al. combined electron tomography, nanoscale three-dimensional (3D) polyamide density mapping, and modeling of bulk water permeability with zero adjustable parameters to quantify the effect of 3D nanoscale variations in polymer mass on water transport within the polyamide membrane (see the Perspective by Geise). They found that variability in local density most affects the performance of the membranes. Better synthesis methods could thus improve performance without affecting selectivity.
Biological membranes can achieve remarkably high permeabilities, while maintaining ideal selectivities, by relying on well-defined internal nanoscale structures in the form of membrane proteins. Here, we apply such design strategies to desalination membranes. A series of polyamide desalination membranes—which were synthesized in an industrial-scale manufacturing line and varied in processing conditions but retained similar chemical compositions—show increasing water permeability and active layer thickness with constant sodium chloride selectivity. Transmission electron microscopy measurements enabled us to determine nanoscale three-dimensional polyamide density maps and predict water permeability with zero adjustable parameters. Density fluctuations are detrimental to water transport, which makes systematic control over nanoscale polyamide inhomogeneity a key route to maximizing water permeability without sacrificing salt selectivity in desalination membranes.
SOURCES- Penn State, The University of Texas at Austin, Iowa State University, Dow Chemical Company and DuPont Water Solutions, Science
Written by Brian Wang, Nextbigfuture.com