May 19, 2021. By Kolemann Lutz
A desalination membrane can filter perchlorate salts from clean water that is later pumped for disinfection, storage, and distribution. The desalination membrane thickness and nanoscale design complexities have baffled scientists for decades. As one of the most widely used techniques for the desalination of water, reverse osmosis (RO) membranes can remove 99%of contaminants from water, including chemicals such as chlorine, heavy metals, and pesticides. Although, many improvements since the 1980s have developed without an understanding on how water moves through the RO membrane on a nanoscale level. Reverse Osmosis supports lower mass infrastructure and requires an immense amount of energy although improving membrane desalination efficiency could reduce the power demands.
Researchers recently published a key finding about how membranes actually filter minerals from water in January 2021 in Science Journal.
"We found that how you control the density distribution of the membrane itself at the nanoscale is really important for water-production performance", said Enrique Gomez, Professor of chemical engineering and materials science and engineering at Penn State, who led the research study.
Electron microscopy was utilised to image atomic scale particles and chemical composition to determine that most desalination membranes are inconsistent in density and mass. The researchers devised a 3D map of the density variations in polymer film with a spatial resolution of approximately one nanometer, which is less than half the diameter of a DNA strand.By combining electron tomography, nanoscale 3D polyamide density mapping, and modeling of bulk water permeability, researchers quantified the effect of 3D nanoscale variations in polymer on water transport within the polyamide membrane, which is key in understanding the role of density in filtration 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."
Density fluctuations severely hinder the filtration and transport of H2O, which makes systematic control over nanoscale membranes a key component to maximize water permeability loss of salt selectivity in desalination membranes.
Traditionally, the belief was 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 as their scientists found that thicker membranes were more permeable.
The researchers found that avoiding highly dense nanoscale regions, or “dead zones” with consistent density is more effective than membrane thickness for maximizing water production. Better synthesis methods could also improve performance without affecting selectivity. The research team believes that this understanding could increase membrane efficiency by 30% to 40%, resulting in more water filtered with less energy and cost savings over current desalination processes. They are also examining how to engineer sustainable, durable membranes for specific materials to prevent bacterial growth.
Amid climate change and water-energy sustainability issues, nano-porous membrane centrifuges fabricated for industrial scale desalination operation offers great potential to provide clean potable drinking water.
In a separate 2020 study on Biomimetic artificial water channel membranes, an international team of researchers from Saudi Arabia, Italy, and France developed a hybrid strategy, which consists of combining a polyamide matrix and artificial water channels into a single structure. Their membranes, which take the form of a sponge superstructure, have been tested to outperform conventional membranes. The membrane structure flow is 75% greater than current industrial membranes and uses 12% less energy for desalination.
On Mars, most of the potable drinking water can be produced from combining atmospheric oxygen and imported methane into a hydrogen fuel cell. One of the key components of the initial uncrewed and crewed missions will be to identify and access the native water sources in underground water reservoirs. From dirty water brine in storage tanks, potable drinking water can pass through filtration such as a desalination membrane or reverse osmosis
Culp TE, Khara B, et al. Nanoscale control of internal inhomogeneity enhances water transport in desalination membranes. Science. 2021 Jan 1;371(6524):72-75. doi: 10.1126/science.abb8518. PMID: 33384374.