A few parched middle-eastern countries already rely on removing salt from seawater to satiate their thirst. Many others might have to turn to desalination as they deal with increasing populations and climate change. But the technology is still expensive and requires a lot of energy.
Today’s foremost desalination method, reverse osmosis, uses membranes that block salt and impurities as seawater is pushed through them. Better membranes translate to more energy-efficient, less costly desalination.
To help design improved membranes, a team of researchers has used electron microscopy and 3D computational modeling for a nanoscale understanding of how water flows through the barriers. It turns out that uniform membrane density down to the nanometer scale, and not the thinness of the membranes, is crucial for improving water flow. Improving uniformity could increase efficiency by over 30 percent, the team, from the University of Texas, Penn State, and DuPont, reported in the journal Science last week.
The team used common reverse osmosis polymer membranes made by DuPont Water Solutions. Think of these solid membranes as tangled mats of polymer strings. Water flows through the tiny, Angstrom-scale voids formed between the polymer strings.
Membrane manufacturers change the internal structure and thicknesses of these polymer membranes to increase the flow of water. But it has been difficult to pinpoint exactly which parameter affects performance. DuPont researchers, for instance, recently found that making some of the company’s membranes thicker counterintuitively increased the amount of water that flowed through.
To get to the bottom of this paradox, Manish Kumar at the University of Texas, Enrique Gomez at Penn State, and their collaborators at DuPont used a transmission electron microscopy technique called electron tomography to get a 3D view of membranes at nanoscale. It involves scanning a high-intensity electron beam across the surface of the membrane to make several 2D images that are angled and put together to create a 3D image. “This gives you information on the density of the polymer,” Kumar says. “Now you can find the density of each cubic nanometer of membrane to see how much is occupied by polymer and how much with free space.”
Next, they fed the image data from the nanometer-scale structures comprising the membrane into the supercomputer at the Texas Advanced Computing Center. They were able to run large-scale computer simulations that revealed the pathways water took through the membranes.
Water, of course, chooses the path of least resistance. But even in the thinnest membranes, the simulations showed that densely packed polymers could form structures that obstruct water and make it take a longer route, Kumar says. So membrane density at the nanoscale rather than thickness, the team found, is the chief parameter that affects water transport. The most permeable membrane was the least dense and its density did not fluctuate much across the membrane.
If manufacturers could similarly evaluate their reverse osmosis membranes at the nanoscale and figure out how to use the right chemistry and processing techniques to make more uniformly dense membranes, Kumar says, that would make desalination more cost-effective.