Fig. 1. Researchers at Penn State, The University of Texas at Austin and DuPont Water Solutions found that the density of filtration membranes, even at the atomic scale, can greatly affect how much clean water can be produced. Credit: Enrique Gomez/Penn State and Greg Foss/UT Austin.Fresh, clean water is in increasingly short supply around the globe for agriculture, livestock, and drinking. Membrane filtration, distillation, and ion exchange are all used to purify water, and reverse osmosis is becoming more and more important in the recycling and recovery of wastewater. But all these techniques to remove solutes, especially salt, require energy to force water through the membrane. Despite their widespread use, it is still not clear how membrane morphology affects performance, limiting efforts to optimize the process, including energy use.
“Freshwater management is becoming a crucial challenge throughout the world,” says Enrique D. Gomez of The Pennsylvania State University. “It’s critically important to have clean water available, especially in low resource areas.”
He has co-led a team using high-resolution electron microscopy to identify tiny variations in membrane morphology that can be linked to filtration performance [Culp et al., Science 371 (2021) 72–75, https://doi.org/10.1126/science.abb8518].
“Reverse osmosis membranes are widely used for cleaning water but there is still a lot we don’t know about them,” adds Manish Kumar of the University of Texas, Austin, who co-led the effort. “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.”
Together with colleagues at Iowa State University, DuPont Water Solutions and the Dow Chemical Company, Gomez and Kumar took a close look at typical polyamide membrane films produced using a conventional polymerization reaction in a commercial pilot-scale manufacturing line. A powerful combination of energy-filtered transmission electron microscopy (TEM) and electron tomography revealed nanoscale inconsistencies in the density and mass of the membrane material. Bringing together atomic-scale imaging and chemical composition analysis allows variations in the density of the membrane material, which affect the transport of water through the membrane, to be mapped in three dimensions and at a resolution of around 1 nm.
“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,” explains Gomez. “We found that how you control the density distribution of the membrane itself at the nanoscale is really important for water-production performance.”
It had been thought, for example, that thicker membranes should be less permeable. Scientists at DuPont Water Solutions, which makes desalination products, however, had found the opposite, with thicker membranes proving more permeable. Thickness appears to be much less important for the transport of water through membranes than highly dense, nanoscale “dead zones”. Water molecules take the “path of least resistance”, diffusing more readily through regions of low density than high density dead zones. The most permeable membrane, therefore, will be one with the lowest average density and the least variation in density. By minimizing fluctuations in mass, a membrane that maximizes permeability while retaining its selectivity is conceivable. Producing more homogenous, uniformly dense membranes would maximize water transport and could increase membrane efficiency by 20–30%.
There are more questions to answer, however, according to the researchers, who are also looking at the chemical reactions involved in the desalination process and the best membrane materials for specific situations, such as membranes that limit bacterial growth.
“We’re continuing to push our techniques with more high-performance materials with the goal of elucidating the crucial factors of efficient filtration,” says Gomez.
This article was originally published in Nano Today 37 (2021) 101114.