A cheap hydrogel material effectively captures moisture even from low-humidity air and then releases it on demand.
A simple device that can capture its own weight in fresh air and then releases that water when heated by sunlight could provide a fresh source of drinking water in distant arid regions, suggests new research from KAUST.
Globally, Earth's air contains nearly 13 trillion tons of water, a vast renewable pool of clean drinking water. The attempts of many materials and devices developed to reach this source of water have shown everyone either too inefficient, expensive or complex for practical use. A prototype device developed by Peng Wang from the Water Desalination and Recycling Center and his team could eventually change that.
In the heart of the device is cheap, stable, non-toxic salt, calcium chloride. This delicious salt has such a high affinity for water that it will absorb so much vapor from the surrounding air that ultimately a pool of liquid forms, says Renyuan Li, a doctorate. student in Wang's team. "Deliquescent salt can be dissolved by absorbing moisture from the air," he says.
Calcium chloride has a great potential for water harvesting, but the fact that it turns from a solid liquid into a salty liquid after water absorption has been a major obstacle to its use as a water capture device, Li said. "Systems that use liquid sorbents are very complicated," he says. To overcome the problem, researchers have incorporated salt into a polymer called hydrogel, which can maintain a large amount of water, while remaining solid. They also added a small amount of carbon nanotubes, 0.42% by weight, to ensure that captured water vapors could be released. Carbon nanotubes absorb the sun very efficiently and convert heat captured into heat.
The team has incorporated 35 grams of this material into a simple prototype device. Left overnight, he captured 37 grams of water one night when the relative humidity was about 60%. The next day, after 2.5 hours of natural irradiation of 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 scaled to produce 3 liters of water per day – the minimum water requirement for an adult – the material cost of the adsorbent hydrogel would be as low as half a cent a day.
The next step will be to adjust the absorbent hydrogel so as to continuously release the harvested water, rather than in batches, says Wang.