MORE THAN 800 MILLION people worldwide lack access to safe drinking water. Over half the U.S. population drinks from water with detectable lead levels, studies suggest. And even as the pandemic reminded us of the importance of frequent handwashing with soap and water, three in 10 people around the globe can’t do that in their own homes. The World Health Organization and UNICEF warn that these numbers are likely to get worse in the next decade unless societies create and improve water infrastructure—a vast and expensive proposition.
A new tool for meeting this challenge has emerged from the laboratory of Princeton University chemical engineers Rodney Priestley and Xiaohui Xu. They have created a material that removes impurities in drinking water, requires no additional energy source beyond sunshine and is potentially cheap to manufacture.
The two engineers stumbled on their new approach while working on a project to make artificial skin to help heal wounds. Skin typically acts as a selective shield—keeping pathogens out of the body, while still permitting water to pass through it. To make artificial skin, they created a hydrogel (a complex polymer that will not dissolve in water) with a molecular structure that would permit the passage of water and block contaminants.
As they developed and tested this material, Xu realized the hydrogel might potentially have another application: water purification. That spurred a new project, in which the two researchers modified their hydrogel in a few key ways so it would not only filter impurities, but actively draw water in as well.
They designed their hydrogel so that it acts as a heat-sensitive sponge. At room temperature, compounds in the hydrogel attract water molecules. When heated, the molecular structure changes and the gel releases water. “Inside it is highly porous so it can store the water,” explains Xiaohui Xu. “When you heat it, the volume of the material will shrink [and] all of the water inside will be released.”
Then they covered the layer of spongy hydrogel with different polymer that acts as a filter. As the sponge draws in the water, the outer layer keeps impurities from entering. These layers sandwiched together form a thin, sheetlike “membrane.” In their testing to date, they’ve found the combined layers can block problem particles like lead and nitrates from agricultural runoff. “It effectively just sucks in all of the pure water while leaving out all of the contaminants,” Priestley explains.
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