This salty gel could harvest water from desert air

Engineers at MIT have created a novel material with "record-breaking" vapor absorption.

Engineers at MIT have created a superabsorbent substance that, even under desert-like circumstances, can absorb a record quantity of moisture from the atmosphere.

The material may expand to accommodate additional moisture as it takes up water vapor. With a relative humidity of 30%, the material can draw vapor from the air and retain moisture without leaking even in extremely dry situations. After being heated and condensed, the water might then be collected as ultrapure water.

The translucent rubber-like substance is formed of hydrogel, a naturally absorbent substance often utilized in disposable diapers. The scientists made the hydrogel more absorbent by adding lithium chloride, a salt that is well-known for being an effective dessicant.

More salt could be infused into the hydrogel than was previously conceivable, according to the researchers. They discovered that the salt-loaded gel absorbed and held onto a record amount of moisture in a variety of humidity levels, including extremely dry circumstances that have previously restricted the development of other material designs.

The superabsorbent gel may be utilized as a passive water harvester if it can be produced fast and in large quantities, especially in desert and drought-prone areas where the substance might continually collect vapor and subsequently condense it into drinkable water. The material might also be attached to air conditioners as a dehumidifying, energy-saving component, according to the researchers.

"We've been application-agnostic, in the sense that we mostly focus on the fundamental properties of the material," says Carlos Dáz-Marin, a doctoral student in mechanical engineering and part of the Device Research Lab at MIT. But right now, we're looking at quite different issues, including how to capture water and improve the efficiency of air conditioning. This material has so much promise because of its inexpensive cost and good performance.

In an article that will be published in the journal Advanced Materials today, Dáz-Marin and his colleagues have published their findings. Yang Zhong, Bachir El Fil, Xinyue Liu, Evelyn Wang, Gustav Graeber, Leon Gaugler, and others from MIT are the study's other co-authors.

The "best of both worlds"

Researchers at MIT's Device Research Lab are creating new materials to address the world's energy and water problems. The scientists focused on hydrogels, which are slick, stretchy gels consisting primarily of water and a little amount of cross-linked polymer, when searching for materials that can aid in the extraction of water from the atmosphere. Since they can expand and absorb a lot of water when it comes into touch with them, hydrogels have been utilized for years as an absorbent material in diapers.

How can we make this operate just as effectively to capture vapor from the air was our main concern. Dáz-Marin claims.

He and his colleagues looked through the literature and discovered that others had tried combining different salts with hydrogels. Certain salts are highly effective in absorbing moisture, including water vapor, such as the rock salt used to melt ice. The finest of them is lithium chloride, a salt that can take up more moisture than ten times its own mass. Lithium chloride, if left in a pile by itself, might draw vapor from the air, but the moisture would just collect around the salt and there would be no way to store the absorbed water.

In order to create a polymer that could both hold in moisture and expand to contain additional water, researchers have tried to infuse salt into hydrogel.

"It's the best of both worlds," Graeber, who is currently a lead investigator at Berlin's Humboldt University, explains. "The salt can absorb a lot of vapor, and the hydrogel can hold a lot of water. Therefore, it seems sense that you would want to mix the two.

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However, the MIT team discovered that some researchers had to stop adding more salt to their gels. The most effective examples to yet were hydrogels that had 4 to 6 grams of salt per gram of polymer injected into them. In dry settings with a 30% relative humidity, these samples absorbed around 1.5 grams of vapor per gram of material.

The majority of the investigations used samples that were created by soaking hydrogels in salty water and waiting for the salt to dissolve into the gels. Most trials came to an end after 24 to 48 hours because scientists discovered the process to be extremely sluggish and that the gels only contained little amounts of salt. The samples absorbed very little water vapor when the material's capacity to absorb it was tested since there wasn't enough salt in them to begin with to absorb the moisture.

What would happen if the process of creating the materials was permitted to continue for a few days or even weeks? If given enough time, might a hydrogel absorb even more salt? The MIT researchers experimented with lithium chloride, a superabsorbent salt, and polyacrylamide, a popular hydrogel, to find a solution. Hydrogel tubes were created by the researchers using conventional mixing techniques. The tubes were then cut into thin disks, and each disk was deposited into a solution of lithium chloride with a variable salt content. Each day, they weighed the disks to establish how much salt had absorbed into the gels, then put them back in their solutions after weighing.

In the end, they discovered that hydrogels did actually absorb more salt when given additional time. Compared to the previous record of 6 grams of salt per gram of polymer, hydrogels integrated up to 24 after 30 days of soaking in a salty solution.

The scientists next conducted absorption experiments on numerous samples of the salt-filled gels under a variety of humidity levels. They discovered that the samples could expand and take in more moisture without leaking at all humidity levels. The gels caught a "record-breaking" 1.79 grams of water per gram of material in extremely dry circumstances of 30 percent relative humidity, according to the research team.

According to Daz-Marin, who is currently working on ways to accelerate up the material's superabsorbent capabilities, "Any desert during the night would have that low relative humidity, so theoretically, this material could generate water in the desert."

"The big, unexpected surprise was that, with such a simple approach, we were able to get the highest vapor uptake reported to date," adds Graeber. "At this point, kinetics—how rapidly we can get the material to absorb water—will be the key emphasis. That will enable you to cycle this material fast, allowing you to perhaps capture water up to 24 times each day rather of just once.

The Swiss National Science Foundation and the American Office of Energy Efficiency and Renewable Energy both provided some funding for this study.