Newswise — MIT scientists have created a highly absorbent substance capable of extracting an unprecedented volume of moisture from the atmosphere, even under arid circumstances.
While absorbing water vapor, the substance expands to accommodate additional moisture. It demonstrates remarkable performance in extremely arid environments, maintaining its integrity without any leakage even when exposed to 30 percent relative humidity. Subsequently, the collected water can be heated, condensed, and gathered as exceptionally pure water.
Crafted from hydrogel, a naturally absorbent substance employed in disposable diapers, this transparent and flexible material has been imbued with lithium chloride—a potent desiccant. This infusion has substantially bolstered the material's absorbency capabilities.
Through their investigation, the scientists made a significant discovery—by surpassing the salt infusion levels achieved in prior studies, they were able to witness the hydrogel absorbing and retaining an unparalleled quantity of moisture. This remarkable attribute was observed across various humidity levels, even in extremely arid environments where conventional material designs have been constrained.
If manufactured rapidly and on a large scale, this highly absorbent gel holds the potential to serve as a passive water harvester, specifically in desert and drought-stricken areas. The material's ability to continually extract vapor from the atmosphere, subsequently condensing it into potable water, could prove invaluable. Furthermore, the researchers envision integrating the material into air conditioning units, where it could function as an energy-efficient and dehumidifying component.
Carlos Díaz-Marin, a mechanical engineering graduate student and a member of the Device Research Lab at MIT, emphasizes that their research has primarily centered around the material's fundamental characteristics, without specific applications in mind. However, they have now begun delving into diverse areas, such as enhancing air conditioning efficiency and water harvesting. The material's exceptional attributes, coupled with its affordability, render it incredibly promising and open up a multitude of possibilities for practical utilization.
The findings of Díaz-Marin and his colleagues have been published in a recent issue of Advanced Materials. The paper, authored by the MIT team, includes Gustav Graeber, Leon Gaugler, Yang Zhong, Bachir El Fil, Xinyue Liu, and Evelyn Wang as co-authors.
“Best of both worlds”
Within MIT's Device Research Lab, scientists are actively engaged in developing innovative materials to address global energy and water-related issues. In their quest to find suitable substances for atmospheric water harvesting, the team turned their attention to hydrogels—a type of slippery and elastic gel primarily composed of water and a small portion of cross-linked polymers. Hydrogels have long been employed as absorbent materials in diapers due to their ability to expand and efficiently absorb significant quantities of water upon contact.
Díaz-Marin explains that their primary question was how to adapt hydrogels to effectively absorb vapor from the atmosphere. They aimed to harness the material's absorbency properties and extend its functionality to encompass moisture absorption from the air.
Upon thorough research, Díaz-Marin and his team examined existing literature and discovered previous attempts to combine hydrogels with different types of salts. It was noted that certain salts, including the kind used to de-ice roads, exhibit remarkable efficiency in absorbing moisture, including water vapor. Among these salts, lithium chloride stood out as the most exceptional, capable of absorbing more than ten times its weight in moisture. However, when left alone, lithium chloride would only attract vapor without a mechanism to retain the absorbed water, resulting in the moisture pooling around the salt.
So, researchers have attempted to infuse the salt into hydrogel — producing a material that could both hold in moisture and swell to accommodate more water.
"It presents an optimal fusion," remarks Graeber, presently an investigating principal at Humboldt University in Berlin. "The hydrogel possesses substantial water retention capabilities, while the salt exhibits impressive vapor absorption capacities. Therefore, it naturally follows that amalgamating the two is desirable."
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However, the MIT team discovered that other researchers encountered a constraint in terms of the quantity of salt they could incorporate into their gels. The hydrogels with the most impressive performance thus far were those infused with 4 to 6 grams of salt per gram of polymer. Under dry conditions with a relative humidity of 30 percent, these samples absorbed approximately 1.5 grams of vapor for every gram of material.
In prior studies, researchers typically prepared samples by immersing hydrogels in saline water and allowing the salt to permeate the gels over a period of 24 to 48 hours. However, the process proved to be excessively sluggish, yielding minimal salt absorption within the gels. Consequently, when these samples were evaluated for their capacity to absorb water vapor, they exhibited limited effectiveness due to their inadequate salt content, thereby impeding moisture absorption.
If the material synthesis process were extended for days or even weeks, it is plausible that a hydrogel could absorb a greater amount of salt given enough time. To investigate this, the MIT team conducted experiments using polyacrylamide, a commonly used hydrogel, and lithium chloride, a highly absorbent salt. Following conventional mixing techniques, the researchers formed tubes of hydrogel and subsequently cut them into thin disks. Each disk was then immersed in a solution of lithium chloride with varying concentrations of salt. Every day, the disks were removed from the solution, weighed to determine the infused salt quantity within the gels, and then returned to their respective solutions for further absorption.
Ultimately, the researchers discovered that with an extended duration of soaking, hydrogels were able to absorb more salt. After immersing the hydrogels in a saline solution for a period of 30 days, they successfully incorporated up to 24 grams of salt per gram of polymer, surpassing the previous record of 6 grams. This prolonged exposure allowed for a significantly higher salt uptake by the hydrogels.
Subsequently, the research team subjected different specimens of the salt-infused gels to absorption tests under various humidity conditions. Remarkably, the samples exhibited the ability to expand and absorb greater amounts of moisture across all humidity levels without any leakage. Notably, in extremely arid conditions with a relative humidity of 30 percent, the gels achieved a remarkable feat by capturing an unprecedented "record-breaking" 1.79 grams of water for every gram of material, highlighting their exceptional water-absorption capabilities.
Díaz-Marin emphasizes the potential of this material by stating, "Considering the low relative humidity experienced in deserts during the night, it is conceivable that this material could produce water in such environments." Presently, Díaz-Marin is actively exploring methods to enhance the superabsorbent properties of the material, aiming to accelerate its water-absorption capabilities. The goal is to further optimize its performance for practical applications in water generation and related areas.
Graeber expresses their astonishment, stating, "The significant and unexpected surprise was that with such a straightforward method, we achieved the highest vapor absorption ever reported." They further explain that the current focus of their research is on improving the kinetics of the material, aiming to enhance its speed of water uptake. By achieving faster water absorption, it would enable frequent cycling of the material, potentially enabling water harvesting up to 24 times a day, rather than the previous once-a-day recovery. This emphasis on enhancing the material's efficiency and cycle rate has become the primary objective for future advancements in the field.
This research received partial support from the U.S. Office of Energy Efficiency and Renewable Energy and the Swiss National Science Foundation.
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