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A Solar-Powered Hydrogel Cleans Water Fast

A Solar-Powered Hydrogel Cleans Water Fast

Fresh water is getting harder to come by. But now this hydrogel will simply, quickly, and cheaply turn filthy water clean.
There’s plenty of fresh water all over the world, but much of it is so befouled by plastics, oils, heavy metals, and bacteria that it’s not safe for consumption. A cheap simple method for removing those contaminants would be a life saver for the quarter of the world’s population that has no regular access to clean drinking water.
Now researchers out of Princeton University have created a solar-powered gel that can turn polluted water potable. Toss their gel into a puddle of dubious water, pull it out, put it in the sun, and it will release clean water ready for quaffing, and hold back the dangerous stuff.

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This was not the Princeton team’s first attempt to make such a water purifying hydrogel. But the previous incarnation, though it effectively produced clean water, did so slowly. Its pores were closed, regular, and honeycombed. “Because it has this structure, when you heat it, you can see that many water bubbles are formed on the surface,” said Xiaohui Xu, a postdoctoral research associate in the university’s chemical and biological engineering department. “We call it a skin layer, and it slows down the process.”
In a Princeton University study, researchers placed the gel in lake water where it absorbed pure water, leaving contaminants behind. The researchers then placed the gel in the sun, where solar energy heated up the gel, causing the discharge of pure water. Credit: Xiaohui Xu, Princeton University
Xu and her team turned to the loofah gourd—beloved by bathers, and those in need of exfoliation, all over the world—for inspiration. Its fibrous bread-like construction would release water faster, just as it does in the tub.
So, to open up their gel’s pores in imitation of the vegetable, they turned to ethylene glycol. It’s the active ingredient of many antifreezes and they were using it in a separate project where they were trying to make a gel infused with antifreeze. Those efforts produced an opaque gel, unlike the usual transparent gel they had made previously. Thinking that the color indicated a structural change, Xu had a closer look and discovered that the gel had the more open spongier geometry they were after. “With the new gel, there are no bubbles, no skin layer at all,” Xu said. “When you heat it, it just immediately releases the water, without any skin layer formation.”
After five minutes at the right temperature, their previous gel had released only 5 percent of the water it held. The new one releases 70 percent. And the temperature cut off is 91 °F, perfect for activation by sun rays. Below 91 °F the gel can suck up water and hold it, above 91 °F it lets the water go. The resultant paper, “Quick-Release Antifouling Hydrogels for Solar-Driven Water Purification,” appeared in ACS Central Science.
The actual mechanism of purification is a combination of chemistry and filtration. Large contaminants can’t get through the pores, and bacteria, repelled by the super hydrophilic makeup of the gel, can’t latch on. For pollutants caught in the fibers of the gel, a simple washing with diluted acid will remove them, regenerating the gel and allowing it to be used again. But it will take several befoulings before such a cleaning is needed. “We tried it 10 times and there was no decrease in performance,” Xu said.
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The production of the gel is a straightforward, inexpensive business. The monomers and other ingredients are all chemical staples, available at low cost. “The preparation is quite simple,” Xu said. “You mix everything together and then pour it into a container.”
Currently, Xu and her colleagues are working with a designer to create a device that can purify water automatically and continually. They may include some kind of heating element so it can keep the water flowing when the sun’s in hiding. And, of course, they are looking to make larger gels for water purification at a larger scale.
It’s a simple solution with a potentially massive benefit. With the de-fouling gel, households worldwide may very soon have a cheap way to turn questionable water drinkable.
Michael Abrams is a technology writer in Westfield, N.J.

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