KAUST team create hydrogel combination to tap more water from air

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Efforts to tap more water from the air have moved forward with a hydrogel advance from KAUST that could bring a new source of drinking water to remote arid regions.

Renyuan Li pours the hydrogel into a petri dish and allows it to form to the mould (© 2018 KAUST)

The team at KAUST (King Abdullah University of Science & Technology) in Saudi Arabia said the device can capture its own weight in water from fresh air and then release it when warmed by sunlight.

The Earth’s air is estimated to contain almost 13 trillion tons of water but previous attempts to exploit the renewable resource have either been too inefficient, expensive or complex for practical use. Now, a prototype device developed by Peng Wang from the Water Desalination and Reuse Center and his team could change that.

Central to the device is calcium chloride, a form of salt that is cheap, stable, and non-toxic. This deliquescent salt has such a high affinity for water that it will absorb so much vapour from the surrounding air that eventually a pool of liquid forms, said Renyuan Li, a PhD student in Wang's team. "The deliquescent salt can dissolve itself by absorbing moisture from air.”

Calcium chloride has 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. "Systems that use liquid sorbents are very complicated," he said.

To overcome the problem, the researchers are said to have incorporated the salt into a hydrogel, which can hold a large volume of water and remain solid. They also added 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 35g of this material into a simple prototype device. Left outside overnight, it is said to have captured 37 grams 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 sorbed water was released and collected inside the device.

"The hydrogel's most notable aspects are its high performance and low cost," said Li. According to KAUST, if the prototype were scaled up to produce 3l 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.

Next steps involve fine-tuning the absorbent hydrogel so that it releases harvested water continuously rather than in batches, Wang said.

The research is detailed in Environmental Science & Technology.