Increasing population demands, growing agricultural needs, and global economy are all putting strains on Earth's limited freshwater supply, increasing the risk of drought and water wars. Some 2 billion people live in countries with significant "water stress," according to the UN.
Desalination processes, which apply pressure to convert salt water from the ocean into drinkable freshwater through a process known as reverse osmosis, could be the solution to providing clean water for our growing human population.
But we still know relatively little about the membrane structures — specifically, their thickness — that allow for reverse osmosis to successfully occur in water filtration.
Until now. New research published in the journal Science reveals a different way of looking at reverse osmosis so we can better understand how water passes through these membranes, paving the way for more efficient water filtration.
This fix could dramatically improve the lives of those two billion people, by upgrading the desalination technology that brings them freshwater.
What they found — Using a combination of two technologies — electron microscopy and conventional modeling — the researchers were able to create highly specific three-dimensional maps of these membranes down to the nanometer.
These maps allowed scientists to analyze reverse osmosis via these membranes in greater detail than ever before.
Previously, scientists thought that thicker membranes would allow less water to filter through. Scientists have approached water filtration with this assumption in mind.
But this study examines reverse osmosis to reach a surprisingly counterintuitive conclusion. It turns out that the thickness of the membrane matters less than the density of the membrane.
Thickness and density are not the same things. For example, a sponge might be thick, but lacking in density.
The study compared water filtration in a thin, but dense portion of the membrane to an area that was thicker, but less dense.
They found that less water filtered through the dense surface of the membrane. The opposite occurred in the less dense region of the membrane, where more water was more easily filtered through.
Why it matters — For years, researchers have been puzzled by the complex nature of the membrane in water filtration.
Lead author Enrique Gomez, who is also a professor of chemical engineering and materials science and engineering at Penn State, said this of the finding: "Despite their use for many years, there is much we don't know about how water filtration membranes work."
But their research shows that if you can control the density distribution of the membrane, then you can generate more efficient water filtration.
"We found that how you control the density distribution of the membrane itself at the nanoscale is really important for water-production performance," Gomez says.
This discovery could be crucial to improving processes in desalination plants, which are on the rise. Yet, the technology remains prohibitively expensive.
More efficient water filtration could be a gamechanger for the desalination business — and for the future of clean water, which is already in jeopardy.
"Freshwater management is becoming a crucial challenge throughout the world," Gomez says. "Shortages, droughts -- with increasingly severe weather patterns, it is expected this problem will become even more significant. It's critically important to have clean water available, especially in low resource areas."
What's next — There are still a few more molecular processes the researchers need to uncover before we can fully apply these findings to desalination techniques.
For example, the scientists acknowledge that they still need to do more research on how salt molecules pass through these inconsistently dense layers.
Yet, the implications of this new scientific revelation are pretty groundbreaking. Researchers believe they can apply their findings to similar systems using reverse osmosis and improve their design.
The study concludes that these findings can help "improve design strategies for various applications, including gas and hydrocarbon separations, carbon capture, blue energy production, and desalination."
They'd like to construct better and sturdier membranes that won't degenerate and can withstand bacterial growth. According to the study, these findings can help assess the efficacy of "membrane surface treatments that are commonly used to prevent biofouling or reduce membrane degradation."
Ultimately, the researchers are looking into chemical processes that will help them build even better membranes and generate more efficient water filtration.
"We're continuing to push our techniques with more high-performance materials with the goal of elucidating the crucial factors of efficient filtration," Gomez says.
Abstract: 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.
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