This salty gel could harvest water from desert air

A new hydrogel material developed by MIT engineers exhibits “record-breaking” vapour absorption to harvest water.

MIT engineers have synthesized a superabsorbent material that can soak up a record amount of moisture from the air, even in desert-like conditions.

As the material absorbs water vapour, it can swell to make room for more moisture. Even in very dry conditions, with 30 per cent relative humidity, the material can pull vapour from the air and hold in the moisture without leaking. The water could then be heated, condensed, and collected as ultrapure water.

Hydrogel to harvest water – but how?

The transparent, rubbery material is made from hydrogel, a naturally absorbent material used in disposable diapers. The team enhanced the hydrogel’s absorbency by infusing it with lithium chloride — a type of salt known to be a powerful desiccant.

The researchers found they could infuse the hydrogel with more salt than in previous studies. As a result, they observed that 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.

If it can be made quickly and at a commercial scale, the superabsorbent gel could be used as a passive water harvester, particularly in the desert and drought-prone regions. The material could continuously absorb vapour, condensing into drinking water. The researchers also envision the material fitting onto air conditioning units as an energy-saving, dehumidifying element.

“We’ve been application-agnostic, in the sense that we mostly focus on the material’s fundamental properties,” said Carlos Díaz-Marin, a mechanical engineering graduate student and member of the Device Research Lab at MIT. “But now we are exploring widely different problems like how to make air conditioning more efficient and how you can harvest water. This material has so much potential because of its low cost and high performance.”

Díaz-Marin and his colleagues have published their results in a paper appearing today in Advanced Materials. The study’s MIT co-authors are Gustav Graeber, Leon Gaugler, Yang Zhong, Bachir El Fil, Xinyue Liu, and Evelyn Wang.

“Best of both worlds”

MIT’s Device Research Lab researchers are designing novel materials to solve the world’s energy and water challenges. In looking for materials that can help harvest water from the air, the team zeroed in on hydrogels — slippery, stretchy gels mostly made from water and a bit of cross-linked polymer. Hydrogels have been used for years as absorbent material in diapers because they can swell and soak up a large amount of water when it comes in contact with the material.

“Our question was, how can we make this work just as well to absorb vapour from the air?” Díaz-Marin said.

He and his colleagues dug through the literature and found that others had experimented with mixing hydrogels with various salts. Certain salts, such as the rock salt used to melt ice, are very efficient at absorbing moisture, including water vapour. And the best among them is lithium chloride, a salt capable of absorbing over 10 times its own mass in moisture. Left in a pile on its own, lithium chloride could attract vapour from the air, though the moisture would only pool around the salt, with no means of retaining the absorbed water.

So, researchers have attempted to infuse the salt into hydrogel — producing a material that could hold in moisture and swell to accommodate more water.

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

Time to load

But the MIT team found that others reached a limit to the amount of salt they could load into their gels. The best-performing samples to date were hydrogels infused with 4 to 6 grams of salt per gram of polymer. These samples absorbed about 1.5 grams of vapour per gram of material in dry conditions of 30 per cent relative humidity.

In most studies, researchers had previously synthesized samples by soaking hydrogels in salty water and waiting for the salt to infuse into the gels. Most experiments ended after 24 to 48 hours, as researchers found the process was too slow, and not very much salt ended up in the gels. When they tested the resulting material’s ability to absorb water vapour, the samples soaked up very little, as they contained little salt to absorb the moisture in the first place.

What would happen if the material synthesis was allowed to go on for days and even weeks? Could a hydrogel absorb even more salt if given enough time? For an answer, the MIT team carried out experiments with polyacrylamide (a common hydrogel) and lithium chloride (a superabsorbent salt). After synthesizing hydrogel tubes through standard mixing methods, the researchers sliced the tubes into thin disks. They dropped each disk into a lithium chloride solution with a different salt concentration. They took the disks out of the solution each day to weigh them and determine the amount of salt infused into the gels, then returned them to their solutions.

Results of water harvest by hydrogel

In the end, they found that, given more time, hydrogels took up more salt. After soaking in a saline solution for 30 days, hydrogels incorporated up to 24, versus the previous record of 6 grams of salt per gram of polymer.

The team then put various samples of the salt-laden gels through absorption tests across a range of humidity conditions. They found the samples could swell and absorb more moisture at all humidity levels without leaking. Most notably, the team reports that at very dry conditions of 30 per cent relative humidity, the gels captured a “record-breaking” 1.79 grams of water per gram of material.

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

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

This research was supported, in part, by the U.S. Office of Energy Efficiency and Renewable Energy and the Swiss National Science Foundation.