This salty gel can harvest water from the desert air
(Nanowerk News) MIT engineers have synthesized a super absorbent material that can absorb large amounts of moisture from the air, even in desert-like conditions.
Because the material absorbs moisture, it can swell to give the room more moisture. Even in extremely dry conditions, with a relative humidity of 30 percent, the material can draw in moisture from the air and hold in moisture without leaking. The water can then be heated and condensed, then collected as ultrapure water.
The transparent rubber material is made from hydrogel, a naturally absorbent material that is also used in disposable diapers. The team increased the absorbency of the hydrogels by infusing them with lithium chloride – a type of salt known as a strong desiccant.
The researchers found that they were able to infuse the hydrogel with more salt than was possible in previous studies. As a result, they observed that the salt-containing gel absorbed and retained unprecedented amounts of moisture, across a wide range of humidity levels, including the very dry conditions that limit other material designs.
If they can be made quickly, and on a large scale, superabsorbent gels can be used as passive water harvesters, especially in desert and drought-prone areas, where they can continuously absorb vapor, which can then be condensed into drinking water. . The researchers also envision that the material could be attached to air conditioning units as an energy-efficient dehumidifier.
“We’ve been application-agnostic, in the sense that we’ve mostly focused on the fundamental properties of materials,” says Carlos Díaz-Marin, a mechanical engineering graduate student and member of the Device Research Lab at MIT. “But now we are exploring very different issues like how to make air conditioning more efficient and how can you harvest water. This material, because of its low cost and high performance, has so much potential.”
Díaz-Marin and colleagues have published their results in a paper at Advanced Materials (“Extreme Water Absorption of Hygroscopic Hydrogels through Maximized Swelling-Induced Salt Loading”). The MIT study’s co-authors are Gustav Graeber, Leon Gaugler, Yang Zhong, Bachir El Fil, Xinyue Liu, and Evelyn Wang.
“Best of both worlds”
At MIT’s Device Research Laboratory, researchers design new materials to address the world’s energy and water challenges. In search of a material that could help harvest water from the air, the team focused on hydrogels – slick, stretchy gels made mostly of water and a few interlocking polymers. Hydrogels have been used for many years as absorbent diapers because they swell and absorb a lot of water when they come into contact with the material.
“Our question was, how can we make this work properly to absorb moisture from the air?” said Díaz-Marin.
He and his colleagues dug into the literature and found that others had experimented with mixing the hydrogels with various salts. Certain salts, such as the rock salt used to melt ice, are very efficient at absorbing moisture, including moisture. And the best of them is lithium chloride, a salt capable of absorbing moisture more than 10 times its own mass. Left alone in piles, lithium chloride can draw vapor from the air, though moisture will just pool around the salt, with no way of holding back any absorbed water.
So researchers have tried incorporating salt into hydrogels – resulting in a material that can hold moisture and expand to hold more water.
“It’s the best of both worlds,” said Graeber, who is now a principal investigator at Humboldt University in Berlin. “Hydrogel can hold a lot of water, and salt can trap a lot of vapor. So intuitively you want to combine the two.”
Time to load
But the MIT team found that other people were hitting limits on the amount of salt they could put in their gels. The best performing samples to date are hydrogels infused with 4 to 6 grams of salt per gram of polymer. This sample absorbs about 1.5 grams of vapor per gram of material under dry conditions at 30 percent relative humidity.
In most studies, researchers have previously synthesized samples by immersing the hydrogels in salty water and waiting for the salts to seep into the gel. Most trials ended after 24 to 48 hours, because the researchers found the process was too slow, and not enough salt had gotten into the gel. When they tested the resulting material’s ability to absorb moisture, the samples absorbed very little, because they contain very little salt to absorb moisture.
What would happen if the synthesis of matter was allowed to continue, say, for days, even weeks? Could the hydrogel absorb more salt, given enough time? As an answer, the MIT team conducted experiments with polyacrylamide (a common hydrogel) and lithium chloride (a superabsorbent salt). After synthesizing the hydrogel tubes via a standard mixing method, the researchers sliced the tubes into thin discs and dropped each disc into lithium chloride solutions with different salt concentrations. They took a disk from the solution each day to weigh it and determine the amount of salt that had been incorporated into the gel, then returned it to their solution.
Ultimately, they found that, over time, the hydrogel absorbed more salt. After soaking in a salty solution for 30 days, the hydrogels were loaded up to 24, compared to the previous record of 6 grams of salt per gram of polymer.
The team then put various salt-laden gel samples through absorption tests in various humidity conditions. They found that the samples could expand and absorb more moisture at all humidity levels, without leaking. Most notably, the team reported that under very dry conditions with a relative humidity of 30 percent, the gel captured a “record breaking” 1.79 grams of water per gram of material.
“Every desert at night will have a low relative humidity, so you can imagine this material can produce water in the desert,” says Díaz-Marin, who is currently looking for ways to accelerate the material’s superabsorbent properties.
“The big and unexpected surprise was that, with such a simple approach, we were able to get the highest vapor absorption reported to date,” said Graeber. “Right now, the main focus is kinetics and how fast can we get the material to absorb water. That will allow you to recycle this material very quickly, so instead of recovering water once a day, you can harvest water maybe 24 times a day.
