New salty gel can harvest pure water from dry desert air

So, even in desert conditions with 30% relative humidity, the material can pull in what little moisture there is in the air and accumulate it so that it can then be extracted as ultrapure liquid water. This can be critical for drought-prone areas where naturally occurring sources of drinking water might be scarce, and might even provide a safe alternative for drinking water than potentially contaminated rivers or groundwater.

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

Approaching the challenge of water vapor absorption

Given the incredible potential for a super-absorbent material like this, Díaz-Marin and his MIT colleagues weren't the first to explore the potential of salt-infused hydrogels.

Hydrogels are made from interlaced fibers (either natural or synthetic) that form pockets that are able to contain water. This produces a stretchy gel that is commonly used in disposable diapers among other products since their ability to stretch to accommodate more volume makes them an ideal way to accumulate water.

But the challenge of pulling water from the air needs more than just a hydrogel. Salts are powerful desiccants that can pull water vapor out of the air, and lithium chloride is among the most powerful desiccants known, capable of absorbing 10 times its mass in moisture from the air around it.

On its own, however, the water it pulls out of the air simply pools around it as it has no way to actually hold onto the water it is absorbing. The process of infusing the hydrogels with lithium chloride is also a time-consuming one. Previous attempts at infusion reported very little in terms of salt uptake by the hydrogels after 48 hours, typically only 4 to 6 grams of salt for each gram of polymer in the hydrogel.

The MIT researchers suspected that this wasn't a problem with the process itself, but rather the impatience of the researchers who were giving up after only a couple of days. By allowing samples of the hydrogel polyacrylamide to soak in lithium chloride solutions with different salt concentrations for up to 30 days, the MIT team was able to infuse 24 grams of salt for every gram of hydrogel polymer.

When they then tested the moisture-absorbing potential of the newly infused material, they far exceeded previous experiments at every level of humidity tested. At just 30% relative humidity, roughly what you'd find in a typical desert at night, the team reports that it recovered a "record-breaking" 1.79 grams of water for every gram of salt-infused hydrogel material.

“The big, unexpected surprise was that, with such a simple approach, we were able to get the highest vapor uptake reported to date,” paper co-author Gustav Graeber, who is now a principal investigator at Humboldt University in Berlin, said. “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.”