An inexpensive hydrogel-based material efficiently captures moisture even from low humidity air, releasing it on demand.
A simple device that can capture its own weight in the fresh air water and release the water when heated by sunlight can provide a new safe source of drinking water in remote and arid regions, suggests a new KAUST survey.
Globally, Earth's air contains almost 13 trillion tons of water, a vast renewable reservoir of drinking water. Tests of many materials and devices developed to explore this source of water have shown that each of them is very inefficient, expensive or complex for practical use. A prototype developed by Peng Wang of the Water Reuse and Reuse Center and his team could finally change that.
At the heart of the device is cheap, stable and non-toxic salt, calcium chloride. This deliquescent salt has such a high affinity for water that it will so absorb so much vapor from the surrounding air that it will eventually form a liquid, says Renyuan Li, Ph.D. student in Wang's team. "Deliquescent salt can dissolve by absorbing moisture from the air," he says.
Calcium chloride has a great potential for capturing water, but the fact of turning a solid into a salty liquid after absorbing water has been a major obstacle to its use as a water capture device, Li says. liquid sorbents are very complicated, "he says. To overcome the problem, the researchers incorporated the salt into a polymer called a hydrogel, which can hold a large volume of water while remaining solid. They also added a small amount of carbon nanotubes, 0.42% by weight, to ensure that the vapor of captured water could be released. Carbon nanotubes absorb sunlight very efficiently and convert captured energy into heat.
The team incorporated 35 grams of this material into a simple prototype. Left outside at night, it captured 37 grams of water in one night when relative humidity was around 60%. The next day, after 2.5 hours of natural irradiation from the sun, most of the sorbed water was released and collected inside the device.
"The most notable aspects of the hydrogel are its high performance and low cost," says Li. If the prototype was enlarged to produce 3 liters of water per day – the minimum water requirement for an adult – the cost of hydrogel material adsorbent would be as low as half a cent per day.
The next step will be to adjust the absorbent hydrogel so as to release the collected water continuously, rather than in batches, says Wang.