Salty gel could harvest water from desert air

MIT hydrogel discs swollen in water. Image: Gustav Graeber/Carlos D. Díaz-Marín

Engineers have developed a super-absorbent material that can soak up a record amount of moisture from the air, even in desert-like conditions.

The Massachusetts Institute of Technology (MIT), US, which led the study, said the super-absorbent gel could be used as a water harvester, particularly in desert and drought-prone regions, where it could continuously absorb vapour that can be condensed into drinking water.

The transparent, rubbery material is made from hydrogel, a naturally absorbent material that is also used in disposable nappies. The team enhanced the hydrogel’s absorbency by infusing it with lithium chloride — a type of salt that is known to be a powerful dessicant, meaning it can absorb moisture from air.

As the material absorbs water vapour, it can swell to make room for more moisture. Even in very dry conditions, the material can pull vapour from the air and hold in the moisture without leaking.

The water can then be heated and condensed, then collected as ultrapure water.

"This material, because of its low cost and high performance, has so much potential. Conceivably, it could generate water in the desert."

Carlos Díaz-Marin, Massachusetts Institute of Technology

Hydrogels have been used for years as absorbent material in nappies because they can swell and soak up a large amount of water when it comes in contact with the material.

Carlos Díaz-Marin, a mechanical engineering graduate student at MIT, said, “Our question was, how can we make this work just as well to absorb vapour from the air? This material, because of its low cost and high performance, has so much potential.”

The team observed the salt-loaded gel absorbed and retained an unprecedented amount of moisture, across a range of humidity levels, including very dry conditions that have limited other material designs. At very dry conditions of 30% relative humidity, the gels captured a “record-breaking” 1.79g of water per gramme of material.

“Any desert during the night would have that low relative humidity, so conceivably, this material could generate water in the desert,” says, Díaz-Marin, who is now looking for ways to speed up the material’s super-absorbent properties.

Image: Ivars Krutainis, Unsplash

The study's co-author Gustav Graeber, who is now a principal investigator at Humboldt University in Berlin, said, “It’s the best of both worlds. The hydrogel can store a lot of water, and the salt can capture a lot of vapor. So it’s intuitive that you’d want to combine the two.

“The big, unexpected surprise was that, with such a simple approach, we were able to get the highest vapor uptake reported to date. Now, the main focus will be kinetics and how quickly we can get the material to uptake water. That will allow you to cycle this material very quickly, so that instead of recovering water once a day, you could harvest water maybe 24 times a day.”

This research was supported, in part, by the US Office of Energy Efficiency and Renewable Energy and the Swiss National Science Foundation. It was published in journal Advanced Materials.