Scientists have developed a simple device that can capture water from thin air, and release it when warmed by sunlight. The advance could provide a secure new source of drinking water in remote arid regions, researchers said.
Globally, Earth's air contains almost 13 trillion tonnes of water, a vast renewable reservoir of clean drinking water.
Trials of many materials and devices developed to tap this water source have shown each to be either too inefficient, expensive or complex for practical use.
The prototype device, developed by researchers at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, makes use of a cheap, stable, nontoxic salt, calcium chloride.
The salt has high affinity for water and will absorb so much vapour from the surrounding air that eventually a pool of liquid forms, said Renyuan Li, a PhD student at KAUST.
"The deliquescent salt can dissolve itself by absorbing moisture from air," he said.
Calcium chloride has great water-harvesting potential, but the fact it turns from a solid to a salty liquid after absorbing water has been a major hurdle for its use as a water capture device, said Li.
To overcome the problem, the researchers incorporated the salt into a hydrogel which can hold a large volume of water while remaining a solid.
They also added a small amount of carbon nanotubes, 0.42 per cent by weight, to ensure the captured water vapour could be released.
Carbon nanotubes very efficiently absorb sunlight and convert the captured energy into heat.
The team incorporated 35 grammes of this material into a simple prototype device. Left outside overnight, it captured 37 grammes of water on a night when the relative humidity was around 60 per cent.
The following day, after 2.5 hours of natural sunlight irradiation, most of the water was released and collected inside the device.
"The hydrogel's most notable aspects are its high performance and low cost," said Li.
If the prototype were scaled up to produce 3 litres 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 per day.
The next step will be to fine tune the absorbent hydrogel so that it releases harvested water continuously rather than in batches, Wang said.