Researchers have created a superabsorbent hydrogel that absorbs water from the air in greater quantities than previously reported materials, even under desert conditions. This new material opens the door to creating an effective, sustainable solution to the important problem of water scarcity.
Water is key to human survival, energy, food production and healthy ecosystems. At the same time, climate change increases the burden of maintaining global water and energy supplies due to changing environmental conditions.according to unicefNearly two-thirds of the world’s population experiences severe water scarcity for at least one month each year.
In the search for innovative materials capable of trapping water, hydrogels that absorb moisture from the air — hygroscopic hydrogels — caught the attention of researchers. To be effective and usable in various environmental situations, these hydrogels need to be low-cost, scalable and sustainable, and provide a high degree of water vapor absorption.
Researchers at MIT have developed a superabsorbent hydrogel that meets all these requirements, even in desert-like conditions. The key to the material’s absorbency is the addition of a special salt, lithium chloride, to the hydrogel.
After reading other studies using a mixture of hydrogel and salt, the researchers decided to use lithium chloride, which is highly hygroscopic. It is capable of absorbing more than 10 times its mass in water. But it needs a material that can hold the moisture from the salt that collects from the air. That’s where hydrogels come in.
“It’s the best of both worlds,” said the study’s lead author, Gustav Graeber. “A hydrogel can store a lot of water, and a salt can trap a lot of water vapor. So it’s pretty intuitive that you’d be tempted to combine the two.”
The researchers performed experiments by dropping hydrogel discs into solutions containing different concentrations of lithium chloride salt. Every day they were weighed to see how much salt had been infused into the hydrogel. After soaking for 30 days, the researchers found that the hydrogel absorbed 0.8 ounces (24 grams) of salt per gram of gel. Previous studies have achieved up to 0.2 ounces (6 grams) of salt absorption, but the hydrogel didn’t stay in the saline solution for that long.
Saline-loaded gels were tested under various humidity conditions. The researchers found that at humidity levels of 30%, 50%, and 70%, the hydrogel absorbed water without leaking out. According to the researchers, even at 30 percent relative humidity (lower than that seen at night in the desert), the hydrogel absorbed 0.06 ounces (1.79 grams) of water per gram of material, 15 percent more than previously tested hydrogels . Water can be heated, condensed and collected as ultrapure water.
“Any desert at night would have that low relative humidity, so it’s conceivable that this material could generate water in a desert,” said Carlos Diaz-Marin, one of the study’s co-authors.
The next challenge for the researchers was to speed up the water absorption process.
“The biggest, unexpected surprise was that with this simple method, we were able to obtain the highest vapor uptake reported to date,” Graeber said. “Right now, the main focus will be on the kinetics and how quickly we can get the material to absorb water. This will allow you to recycle this material very quickly so you can collect water 24 times a day instead of recycling it once a day. “
Because it can be fabricated quickly and on a large scale, the researchers are considering other applications for their salty hydrogel besides serving as an efficient water collector.
“We’ve been application-agnostic because we’ve focused on the fundamental properties of the material,” Díaz-Marín said. “But now we are exploring very different problems, such as how to improve the efficiency of air conditioning and how to harvest water. This material has great potential due to its low cost and high performance.”
The study was published in the journal advanced materials.